Chapter 24 - Thermoplastic Elastomers

Chapter 24

Thermoplastic Elastomers Dr. Declan Whelan

24.1 HISTORY The growth and development of thermoplastic elastomer (TPE) materials began as early as the late 1950s when both B.F. Goodrich (Schollenberger, 1959) and Mobay Chemical (Wagner, 1961) introduced thermoplastic polyurethane elastomers to the market place. The change was from the conventional chemical cross-linking approach to what was termed in its day “virtual cross-linking.” By a combination of block copolymer chemistry and careful selection of copolymer constituents, it was shown that the high glass transition (Tg) urethane-rich blocks would aggregate on cooling to form hard regions with high levels of hydrogen bonding or high levels of crystallinity, producing a type of cross-linking effect. The softer low Tg segments with low urethane content remained mobile lending flexibility and mobility to the polymer chains which when combined led to an elastomeric-type material. In the 1960s, Shell Chemical Company (Holden and Milkovich, 1966) introduced the precursors for what has now become the basis for an ever widening range of TPE compounds. These materials were based on styrene butadiene styrene and styrene isoprene styrene (SBS and SIS, respectively) block copolymers and sometime later KratonÒ, a styrene ethylene butylene styrene (SEBS) block copolymer. The principle was the same, phase separation on cooling into hard styrenic agglomerations producing pseudo cross-links and softer rubbery olefinic segments. As the market for these materials grew in the 1970s, other TPE types began to emerge. In 1972, DuPont marketed HytrelÒ, the first polyester block Elastomer (Sheridan, 1988). Uniroyal TPRÒ emerged at this time patented (De and Bhowmick, 1990) by the Uniroyal Chemical Company and was the first commercially available mechanical blend of polypropylene (PP) and EPDM rubber (which was not cross-linked) later to be classed with others of its type, using a variety of thermoplastics and thermoset rubber combinations, as TPO materials, thermoplastic olefin (TPO) elastomers. The 1980s saw another breakthrough in TPE technology. Thermoplastic vulcanisates (TPVs or elastomeric alloys (EAs)-TPV or dynamic vulcanisates (DVs)) became commercially available. Monsanto Chemical Company introduced to the marketplace in 1981 a two-phase olefinic-based product, SantopreneÔ Thermoplastic Rubber. This consisted of a PP matrix into which had been dispersed a fully cross-linked EPDM rubber using their patented technology. Their manufacturing process technology produced an olefinic TPE with much enhanced properties over TPO materials. In the mid-1980s DuPont also introduced their melt processable rubbers (MPRs) sold under the trade name of AlcrynÒ TPE. These products are partially cured single-phase alloys of ethylene interpolymers and chlorinated polyethylenes (PEs). The functionality on the ethylene interpolymers allows hydrogen bonding with the halogenated polyolefin giving miscibility and hence a claimed single-phase system. Finally, a high-performance subgroup of TPE, the thermoplastic polyamide elastomers, EstamidÒ, PebaxÒ, and VestamidÒ also appeared.

24.2 GENERAL DEFINITION OF THERMOPLASTIC ELASTOMER TPE is the generic term used to describe a family of polymeric materials that can be processed as a thermoplastic, but display a number of characteristics normally associated with traditional thermoset rubbers. This family of materials, certainly those that are commercially available, can be divided into eight main classes (Figure 24.1) based on their chemistry and morphology.

Brydson’s Plastics Materials. http://dx.doi.org/10.1016/B978-0-323-35824-8.00024-4 Copyright © 2017 Elsevier Ltd. All rights reserved.

653

654 Brydson’s Plastics Materials

Thermoplastic Elastomer or Rubber (TPE)

Thermoplastic Olefins (TPO) (Mechanical Blends)

Metallocene

Styrenics (Block Copolymers) Styrene Isoprene Styrene (SIS) Styrene Butadiene Styrene (SBS) Styrene Ethylene-Butylene Styrene (SBS)

Ionomer

Thermoplastic Polyester (Block Copolymer)

Thermoplastic Vulcanisate (TPV) Thermoplastic Polyurethane (Block Copolymer) Or Elastomeric Alloy (EA) Or Dynamic Vulcanisate (DV)

Thermoplastic Polyamide (Block Copolymer)

Melt Processible Rubber (MPR)

FIGURE 24.1 Subclassification of the thermoplastic elastomer (TPE) family.

Increasing Cost

Polyamides Copolyesters

Urethanes Elastomeric Alloys/ TPV’s TPO’s Styrenic’s Commodity

FIGURE 24.2

Increasing Performance

Specialties

Cost/performance of thermoplastic rubber (De and Bhowmick, 1990).

24.3 GENERAL DEFINITION OF THERMOPLASTIC ELASTOMER To characterize these classes further, Rader and Walker in their introduction to TPEs (Sheridan, 1988) use cost and performance as the differentiator, De and Bhowmick (1990) and Coran (1978e1982) also use these criteria as a distinguishing factor between TPE classes. What is clear from the simple schematic of cost performance comparison, Figure 24.2 (De and Bhowmick, 1990), is that there is an overlap in the general performance among classes, so material cost and specific material performance characteristics in relation to the application become the deciding factor in material selection. As discussed previously, the focus of the use of these materials is in the replacement of thermoset rubber in what have been traditional thermoset rubber applications. In order for this to occur it is important to understand how these materials compare. The polymer industry has traditionally used the heat and oil resistance of materials to produce a comparative matrix of performance (Figure 24.3). This method of comparison is a carry-over from the automotive industry, where the use of a materials selection system called “line call outs” enables engineers to quickly identify groups of materials that have properties to suit the proposed application. The performance matrix does show clearly that TPEs span to a great extent, the performance window of thermoset rubbers. As heat and oil resistance of a material increases, in general so does the cost. Figures 24.2 and 24.3 can be superimposed.

24.4 COMPARISON OF THERMOPLASTIC ELASTOMERS 24.4.1 Basic TPE Chemistry and Synthesis Routes Apart from the materials constructed by the mechanical blending of two primary polymer types (TPOs, MPR, and TPVs) the remaining materials (thermoplastic polyurethane, polyester, polyamide, styrenic- and the newer metallocene-based olefinic elastomers (VistamaxxÔ , VersifyÔ ), and Plastomers of which ExxonMobil ExactÔ and QueoÔ ) are examples, that derive their properties and characteristics from the block copolymerization of two or more segments of molecularly dissimilar polymers. One produces a “hard” segment in the polymer chain and the other a “soft” rubbery segment. Both segments have a direct influence over the final product properties. When heated or solvated, the hard segment becomes

Thermoplastic Elastomers Chapter | 24

655

FIGURE 24.3 Performance matrix comparing thermoset and thermoplastic rubbers (Sheridan, 1988).

Chemical

Strong Elastic Solids

Weak, Processable Fluids

Heat

Conventional Vulcanised Rubber

Conventional Unvulcanised Rubber

Time

Physical

Heat Fluid Melt Cool

Thermoplastic Elastomer

Dissolve Low Viscosity Solution Evaporate FIGURE 24.4 Chemical and physical changes (Holden, 1995).

mobile and polymer flow results. This reversible melting/reforming of physical cross-links is distinctly different from the chemical cross-linking in conventional rubbers (see Section 2.3.3) (Figure 24.4). The chemical composition of each phase strongly influences the performance of the TPE. The main factors affected are the thermal performance, physical and chemical resistance properties, and weatherability as a direct result of each phase’s glass transition temperature (Tg), melt transition temperature (Tm), and any chain inconsistencies. Other material properties are adjusted via the further addition of other additives and polymers as a secondary compounding operation. The following section briefly describes the fundamental differences in approach to the production of TPE materials. Block copolymer molecules tend to be linear di- or triblock structures for the production of these TPE materials, although other variants do exist, radial (Kraton Website) being an example. The simplest arrangement is an alternating triblock copolymer configuration (Figure 24.5) of the type AeBeA where A is the hard segment and B is the softer elastomer.

24.4.1.1 Styrenic-Based Thermoplastic Elastomers These materials are two-phase structures of the type SeEeS. The simplest arrangement is an alternating triblock copolymer configuration (Kraton Website) of the type AeBeA where A is the hard segment and B is the soft (Figure 24.5). The elastomers used commercially are; isoprene, butadiene, ethylene-co-butylene, and ethylene-copropylene.


Di Block

A

B

Tri Block

Radial

FIGURE 24.5 Linear diblock, triblock and radial styrenic structures (Kraton Website).

In order to display the characteristics of an elastomer, the hard phase must be at the extremities of the chain to enable the formation of hard domains during melt cooling, the “physical cross-links.” Typically, each block segment may contain more than 100 monomer units. The resulting structure forms a three-dimensional network with spherical styrenic domains (Figure 24.6). These styrenic end segments as they reform from a melt or solvated state are immiscible with the “rubber” phase and therefore there is a high probability of agglomeration into various styrenic domains. This effectively structurally reinforces the continuous elastomeric phase. It is this structure that produces the performance characteristics associated with styrenic TPE, strength, and elasticity. If the elastomer segment constituted the chain ends, or indeed a diblock structure was used, a material with the characteristics of a nonvulcanized synthetic rubber would result. The structures of commercial examples of this family of TPE are shown in Figure 24.7. The beginnings of this family of styrenic triblock TPEs started as early as the mid-1950s with work done by Firestone researchers on lithium metal catalysis for high cis polybutadiene (PB). This was a continuation of the use of anionic “living” polymerization of polymers developed by Ziegler and others in the 1920s to 1930s. Other researchers, in particular Milkovitch of Shell during the mid-1950s, revisited this type of polymerization technique. It resulted in the patenting of triblock styrenic TPE (SeBeS, SeIeS) in 1962 (Holden and Milkovich, 1966) and a further variant, KRATON G (SeEBeS) in 1972. The synthesis (anionic polymerization) of styrenic block copolymers has been described in detail (De and Bhowmick, 1990; Making SBS Rubber; Holden and Legge, 1987). The fundamental mechanism for this polymerization route is via anionic initiation of vinyl polymers. There are several potential mono and difunctional initiators but the organolithium most often described is butyl lithium. This compound (Figure 24.8) is preferred as it is seen to reduce the induction period for the onset of the initiation of the polymerization sequence, and the initiation rate is substantially higher than subsequent polymerization. There are three basic routes to synthesis described in literature, briefly they are; 1. Coupling reaction, where the initiation occurs at both ends of the molecule which is then subsequently joined by a coupling or linking agent, for example esters, organo-halogens or silicon halides. 2. Sequential, where the polymerization begins at one end of the molecule and progresses to the other end, is then terminated by a proton donor, for example an alcohol. 3. Multifunctional initiation, here the initiation occurs at the center of the molecule and progresses outward from both ends. Using SBS as an example, the basic reactions (Making SBS Rubber; Holden and Legge, 1987) are as follows.


657

Styrenic Domain [A] Rubber Domain [B]

FIGURE 24.6 Schematic of styrenic TPE morphology showing two-phase separation into hard glassy polystyrene and soft elastomeric domains.

(CH2 CH)x (CH2 CH CH CH2)n (CH2 CH)x

(CH2 CH)x (CH2 C CH CH2)n (CH2 CH)x CH3

(CH2 CH)x (CH2 CH2 CH2 CH2)n (CH2 CH)Y

Z

(CH2 CH)x

CH2 CH3 FIGURE 24.7 Chemical structures of commercial styrenic block TPE.


CH3 CH2 CH2 CH2 Li FIGURE 24.8 Structure of butyl lithium.

H CH3 CH2 CH2 C :- + Li+

CH3 CH2 CH2 CH2 Li

H FIGURE 24.9

Organolithium dissociation.

CH3 CH2 CH2 CH2 Li

+

CH2 CH

+

CH2 CH

H CH3 CH2 CH2 C :- Li+ H

H CH3 CH2 CH2 CH2 CH2 C:

FIGURE 24.10 Chain initiation sequence.

24.4.1.1.1 Initiation Under the initiation conditions, the lithium disassociates from the molecule producing a positively charged lithium cation and negatively charged butyl anion (carbanion), Figure 24.9: The free electrons on the butyl anion when coming into contact with a monomer molecule, in this case, styrene monomer, are shared with the adjacent carbon atom of the carbon: carbon double bond. This in turn produces a free radical site at the other end of the molecule, Figure 24.10: 24.4.1.1.2 Propagation This reaction mechanism can now occur whenever a monomer molecule is present. In this way the chain length increases (propagation). This stage is the “living” part of the reaction sequence, Figure 24.11 and will continue as long as monomer molecules are present: If butadiene monomer is now added (Figure 24.12) to the reaction vessel, it too will link to the living chain by the same mechanism producing an elastomeric segment. The level of monomer present and the number of initiation reactions control H

CH3 CH2 CH2 CH2 CH2 CH CH2 C: n

+

Li+

FIGURE 24.11 Chain growth of polystyrene segment.


659

H CH3 CH2 CH2 CH2 CH2 CH CH2 C: Li+ n

+

CH2 CH CH CH2

Butadiene Monomer

Living Polystyrene Chain

H CH3 CH2 CH2 CH2 CH2 CH

n

CH2 CH CH CH2

n

CH2 CH CH

C: Li+ H

Styrene - Butadiene Block Copolymer FIGURE 24.12 Synthesis of styrene butadiene block copolymer.

chain length. In order to ensure that the molar mass distribution of the second addition of styrene is controlled within the range required, a solvating agent is used, such as ether. The point of addition of the solvent is critical as this can affect the microstructure of the polydiene segment changing the ratios of 1,4- and 1,2-enchainments. One can see that by the addition of yet more styrene monomer a second block can be polymerized onto the living chain, however this is not the case. For some reason, the styrene monomer will not add to the anion end of a growing PB chain, so in order to achieve a triblock structure, the chain end has to be modified/terminated with a functional end cap. 24.4.1.1.3 Termination Termination of chain growth, Figure 24.13, is possible at the appropriate stage in chain growth by the addition of a proton donor, such as an alcohol which will produce a styrene butadiene block copolymer rubber or by the addition of a suitable coupling agent, for example dichlorosilane, to produce the SeBeS triblock copolymer TPE. With both SeBeS and SeIeS triblock copolymers there remain within the chain structure levels of unsaturation. This limits, in particular, the resistance of these materials to thermo-oxidative degradation and UV resistance. As a consequence of this, two further material types were synthesized from the SeIeS and SeBeS precursors, styrene ethylene/butylene styrene (SeEBeS) and styrene ethylene/propylene styrene (SeEPeS). The route to achieve a saturated elastomer with both SeBeS and SeIeS triblock copolymers is via hydrogenation of the residual unsaturation within the elastomer chain segment as shown in Figure 24.14. Taking SeEBeS as an example (the most common commercial product), the butadiene monomer can polymerize through either the 1,4- or 1,2-double bonds. In order to have an amorphous elastic chain segment, the level of crystallinity present has to be minimized. In the case of SeEPeS, this is relatively easy as the hydrogenation of the isoprene segment produces in essence an alternating ethylene, propylene sequence with regular small side branches inhibiting chain crystallization. For SeEBeS, the case is more complex, because polymerization can proceed through 1,4- and 1,2-double

CH3

H R CH2 CH CH C:

+

Cl Si Cl

H

CH3 CH3

R CH2 CH CH CH2

n

Si

Cl

:C CH2 CH2 CH

+

CH3

n

CH3 R CH2 CH

n

CH2 CH CH CH2

n

Si CH3

C CH2 CH2 CH

n

+

2LiCl

FIGURE 24.13 Chain termination steps via introduction of silane coupling agent.


R

CH2 CH CH CH2 CH2 CH

H2

R

R

CH2 CH2 CH2 CH2 CH2 CH

CH

CH2

CH2 1,2 Isomer

1,4 Isomer

R

CH3 Butylene

Ethylene

Butadiene Chain Segment FIGURE 24.14 Structural change via hydrogenation (Halper, 1988).

bonds. It is critical to achieve the correct balance of random distribution of the segment structure. A very high proportion of 1,4-polymerization will yield a crystallizable PE structure or in the case of 1,2-polymerization an atactic poly 1-butene, neither of which impart the true elastomeric qualities needed. The balance between both types of isomer structures is critical to produce a saturated olefinic elastomer midblock that maintains the balance between ethylene and butylene characteristics.

24.4.1.2 Synthesis of Thermoplastic Polyesters Of this family of copolyester TPE perhaps the most well known are DuPont HytrelÒ and DSM ArnitelÒ. The chemical synthesis of this type of TPE has been researched and discussed extensively (Witsiepe, 1972, 1973; Aleksandrovic and Djonlagic, 2001; Adams and Hoeschele, 1987a; Kresge, 1992; Hoeschele, 1974). As with the TPU and ester amide TPE (Sections 24.4.1.4 and 24.4.1.3), this material derives its elastic properties from the block copolymerization of both hard crystalline and soft amorphous segments. In this case, reinforcement or “pseudo cross-linking” is achieved by phase separation of the hard crystalline portions of the polymer chains. The polymerization route first patented by Witsiepe (1972, 1973) describes the synthesis of copolyester TPE materials. In essence, a melt transesterification reaction is carried out to produce intralinear long chain and short chain ester units connected head to tail though ester linkages. The long chain reaction product (ester) of glycol and dicarboxylic acid forms the main repeat unit of these polyesters. A noted feature as claimed in the patent is that it is important to have two different short chain ester units and 65e85% of these must be identical. This is achieved by a transesterifcation reaction combining a poly (alkylene oxide) glycol, poly (tetramethylene ether) glycol with an aliphatic glycol, 1,4-butanediol, and dimethyl terephthalate. The hard dicarboxylic groups are then randomly distributed throughout the subsequent polymer, and depending on their relative ratio (Figure 24.15) to aliphatic chain segments produce the elastomeric properties of the material. Hoeschele (1974) has described this reaction in detail by both the melt and solid phase polycondensation routes. The reaction sequence is two stages, the production of low molar mass prepolymer followed by further polymerization to the final high molar mass polyester TPE. The basic reaction sequence is shown in Figure 24.15. The use of a catalyst usually an organic titanate, tetrabutyl titanate (Hofmann, 1989) or alkaline earth metal salt, magnesium alkoxide (Witsiepe, 1972) accelerates the ester exchange

HO CH2 CH2 CH2 CH2 OH +

1,4 Butanediol

CH3 O C

H

+

Poly(tetramethylene ether) glycol

C O CH3

O

HO CH2 CH2 CH2 CH2 CH2 O

O

Dimethyl Terephthalate

O (CH2)4 O C

C

O

O

n = 7-10 Hard Segment

Bu Titanate/

Alk. Earth Salt

CH2 OH

+

Methanol

n

O (CH2 CH2 CH2 CH2 O)x C

C

O

O

x = 12-16, y = 1-11 Soft Segment

FIGURE 24.15 Chemical structure of HytrelÒ Thermoplastic Elastomer (Holden, 1995).

y


HO

(O)n OH

Polyether or Polyester based Glycol

CO HN Soft Segment

+ OCN

NCO + HOOC R

CH2

4,4’ methylene diphenyl diisocyanate

661

COOH

Dicarboxylic Acxid

CO2

CH2

NHCO R

CO NH n

CH2

NHCO

Hard Segment

FIGURE 24.16 General reaction scheme for the production of PEA and PEEA (Farrissey and Shah, 1988).

reaction, so limiting the thermal degradation of the product during polymerization. It is recognized that the use of antioxidants (diamine types) is needed because of the reduced thermal stability of the monomer fragments and polymer at the reaction temperatures. The transesterification reaction is carried out between 150e250 C; reducing reactor pressure to less than 1 mm fractionally distils off the methanol by-product.

24.4.1.3 Synthesis of Thermoplastic Polyamide Elastomers As with the copolyester and thermoplastic polyurethane elastomers, the polyamides derive their elastomeric qualities from the combination of soft flexible polyether or polyester segments combined with hard rigid polyamide sequences within a regular linear chain structure. The condensation polymerization chemistry has been described in detail in literature (Chen et al., 1978; Nelb et al., 1981; Bhowmick, 2001; Judas et al., 1998; della Fortuna et al., 1980; Lohmar, 2000), and the use of different precursors to arrive at variations of polyamide TPEs (Farrissey and Shah, 1988; Nelb et al., 1987; Deleens, 1987). In essence, the synthesis produces a class of copolyetheresteramides (PEEA) or copolyesteramides (PEA) through condensations involving the reaction of an aromatic isocyanate with a carboxylic acid to form an aromatic amide. This in turn is reacted with aliphatic glycols to produce a thermoplastic block copolymer elastomer. A third variant patented by Atochem (Foy et al., 1980) and commercialized as PebaxÒ TPE is described as a polyetheresteramide block copolymer (PEebeA). The general reaction scheme for the production of PEA and PEEA is described in Figure 24.16. The Atochem patent (Foy et al., 1980) claims the production of a linear polyetheresteramide block copolymer using hexamethylene diamine or nonamethylene diamine with a dicarboxylic acid. A prepolymer is produced via a condensation reaction at temperatures above 230 C and a pressure of 2.5 MPa. The polyoxyalkylene glycol is then added in the presence of an alkylortho-titanate catalyst [Ti(OR)4], which accelerates the conversion to high polymer. This second stage reaction is conducted at between 230 and 280 C under a vacuum of 13.3e1330 Pa. It is quite clear that this type of synthesis route can produce a wide variety of elastomeric block copolymer variants by simply l l l l

changing the nature of the polyamide block, changing the nature of the polyether block, altering the block lengths, controlling the relative proportions of the blocks present.

24.4.1.4 Polyurethane TPE Early polyurethane elastomers were generally based around the use of bulky diisocyanates of the type, nitro-diisocyanate (Muller et al., 1944) and naphthalene-1,5-diisocyanate (NDI) (Petersen et al., 1944). The mechanism proposed (Bayer et al., 1950) was that specific linear hydroxyl end-capped polyester was reacted with excess diisocyanate to form a low molar mass prepolymer. A short chain diol, for example water, was added which reacted with the prepolymer causing chain extension via the formation of urea linkages. These urea linkages could in turn react further with free diisocyanates to produce a material with some elastomeric qualities. The downside was that side chain branching was difficult to control and the melting temperature of the elastomer was higher than the decomposition temperature of the urea linkage. This


O O C N

CH2

N C O + HO (CH2)4

OH

CH2

OCN

O

O

HO (H2)4 O CHN

CH2

N C O

Dimer ‘Hard’ Segment O HO (H2)4 O CHN

O

+ HO (CH2)4 O C (CH2)4 CO (CH2)4 O H 5

Poly(tetramethylene adipate) glycol O

CH2

NHC O (CH2)4

O

NHC O (CH2)4 O C (CH2)4 CO (CH2)4 O)5 H x

‘Soft’ Segment Typical Thermoplastic Polyurethane Repeat Unit FIGURE 24.17 Basic synthesis for the manufacture of thermoplastic polyurethane elastomers (Carvey and Witenhafer, 1965; Rausch and McClellan, 1972).

synthesis route was different to the polyurethane elastomers (chemically cross-linked) originally proposed by Bayer (1975) and formed the basis of the future development of the polyurethane TPEs we know today. Thermoplastic polyurethane elastomers are produced in a similar way to the polyester TPE (Section 24.4.1.2). They are divided into two types, polyether or polyester based, each producing specific property benefits, particularly in regard to chemical resistance. The synthesis route is not via a condensation reaction as in the polyesters but by “rearrangement” or addition polymerization, here no molecule is split out during the reaction process. The elastomeric properties are derived by the copolymerization of three major constituents; 1. Diisocyanates (hard segment) The most commercially important diisocyanates is 4,40 -diphenylmethane dissocyanate (MDI) although others have attracted interest (Meckle et al., 1987a). 2. Short chain diol (hard segment) These form the “hard segments” in the polymer chain when combined with the diisocyanate, Figure 24.17. The most commonly used are 1,4-butanediol and to a lesser extent, 1,6-hexanediol and 1,4-dihydroxyethoxybenzene. 3. Long chain diol (soft segment) There are two types, the hydroxyl-terminated polyesters and polyethers. These control mainly the low temperature performance and solvent resistance of the final polyurethane TPE. It is common to use mixtures of glycols to minimize the level of crystallinity and further develop the amorphous rubbery characteristics of the soft segment. Examples of typical polyester types are the polycarbonate and polycaprolactone glycols and in the case of the polyethers, poly (oxypropylene) and poly (oxytetramethylene) glycols. A basic synthesis route is shown in the following section. The two basic processes for the production of polyurethane TPEs have been described the “one-shot” (Schollenberger, 1955) method and the prepolymer route (Carvey and Witenhafer, 1965; Rausch and McClellan, 1972). In the “one-shot” method, all the ingredients are mixed in a reactor vessel in a single step. The resultant is a randomly polymerized elastomer. The second route involves the initial reaction of the selected disocyanate (in excess) with the polyester or polyether polyol to produce an isocyanate end-capped prepolymer. This is reacted with the chain extender, a short chain glycol to produce the block copolymer polyurethane TPE. This material can be produced via a semicontinuous route using twin-screw compounding (Rausch and McClellan, 1972; Erdmenger et al., 1974).

24.4.1.5 Ionomers The ionomer resins are more specialty polymers and it is a debating point whether they are classified as true TPEs. These materials are discussed in more depth in Section 10.11.3.

24.4.1.6 Metallocene-Catalyzed TPE The introduction of this new family of materials some of which display TPE characteristics occurred in the early 1980s. As with the ionomers, these materials are marketed as specialty products and are discussed in greater detail in Section 2.5.5.


CH2 CH2

x

CH2 CH

H C

y

CH3

HC

H C

CH2

H 2C

CH2 CH

n

CH3

CH C

663

Polypropylene

z n

CH CH3 Ethylene-propylene–diene Rubber (EPDM) ENB Type, Figure 24.22 FIGURE 24.18 Main polymer constituents of olefinic-based TPV.

24.4.1.7 Thermoplastic Vulcanisates or Elastomeric Alloys This class of TPE differs fundamentally from those discussed previously in that they derive their physical and elastomeric qualities from mechanically combining various thermoplastics with, typically, thermoset rubbers and not via chain segment structure as in the case of block copolymer TPE. These are essentially two-phase systems further modified by the incorporation of fillers and mineral oil plasticizers. The exception is the MPR, AlcrynÒ (Wallace, 1988) which is claimed to be a single-phase system. TPV materials can be viewed as a further development of mechanical blends of thermoplastic with rubber which have been in the market place as far back as 1947 with the introduction of Geon PolyblendÒ by Goodrich (Wolfe Jr, 1987). This material was a blend of polyvinyl chloride (PVC) and nitrile rubber (NBR), and since that time other workers have further developed this and many other TPE blends (De and Bhowmick, 1990; Coran and Patel, 1982). Of these blends, the most predominant in recent times have been the TPO materials or TPO rubbers, the development and history of these materials being described in depth in various literature (Choudhury et al., 1990). These are mechanical blends of semicrystalline olefinic polymers, mainly PP and PE with olefinic rubbers, most commonly ethyleneepropylene rubber (EPR), ethyleneepropyleneediene rubber (EPDM) (Easterbrook and Allen, 1995). Other rubbers have been used, for example, natural rubber and PP or PE blends, but these did not have a sufficient balance of properties so TPV versions were developed. Commercially these TPVs have had little significant impact. In the case of the first fully cross-linked TPV material to be commercialized, Monsanto SantopreneÔ Thermoplastic Rubber was a further development of the TPO-type materials. Coran and coworkers (Gessler, 1962;Fischer, 1973; Coran et al., 1978) built on previous research where TPO materials had been partially vulcanized improving certain physical properties, compression set, heat deformation, and oil resistance among them. This further improvement in properties is discussed in Section 24.4.5. Since that time, TPV development has seen the introduction of various fully vulcanized EPDM:PP TPV materials SarlinkÒ, ForpreneÒ, UnipreneÒ among others. New TPV products have also been introduced commercially, GeolastÒ (PP:NBR) and TrefsinÒ (PP:butyl rubber) from Advanced Elastomer Systems and DuPont AlcrynÒ MPR. Taking SantopreneÔ TPE as the example of this type of TPE material, the main polymer constituents are PP and EPDM, Figure 24.18. PP homopolymer is the most often used because of its physical and thermal properties. This is normally an isotactic semicrystalline material, typically 60e70% crystallinity with a crystalline melting point in the range of 155e170 C. The glass transition of these materials is typically in the range of 10 to 0 C dependent on molar mass and causes obvious limitations in lower temperature performance. This performance deficit can be overcome by the addition of an amorphous rubber (EPR and EPDM). For TPV, the PP selected is normally a high molar mass material with a melt flow index (MFI) less than 20 in order to maximize physical and thermal properties. However, the PP phase can be modified with other PP types, particularly with isotactic copolymer PP containing small fractions of ethylene comonomer. A random or block distribution of ethylene monomer can be used to alter physical and thermal properties as can selection by molecular weight, especially for modification of processing rheology. The addition of higher MFI, lower melting temperature copolymers (melting temperature 145e155 C) improves flow in injection-molding processes, but also contributes to a reduction in modulus (stiffness) of the resultant TPV. Recent developments in metallocene-derived PPs have opened up a new avenue of approach in PP modification in TPV materials. The soft domain in SantopreneÔ TPV is produced from EPDM rubber. This is an amorphous rubber. As illustrated in Figure 24.19, this is a terpolymer of ethylene, propylene, and a diene monomer. PP is added in relatively low levels as a random distribution essentially to disrupt the crystallinity of the PE segment inducing greater flexibility or rubber-like characteristics. Commercially, EPM content is reported in weight percentage ethylene and between 45% and 60%


1. Dicyclopentadiene (DCPD):3 CH H H

CH C H C H C C CH CH2 CH C

H H

2. Ethylidene norbornene (ENB): CH H H

C CH CH3 H C H C CH2 CH C

3. 1, 4 hexadiene (1, 4 HD):4 CH2

CH CH2 CH

CH CH3

FIGURE 24.19 Chemical structures of diene comonomer in EPDM.

ethylene the rubber is completely amorphous and nonreinforcing. With ethylene content greater than this (70e80%), the rubber becomes partially crystalline and hence self-reinforcing to a degree giving greater strength. However, the disadvantage is a drop in low-temperature performance and processability. The diene segment is most commonly ethylidene norbornene (ENB), 1,4-hexadiene, or dicyclopentadiene, the structures of which are given in Figure 24.19. The addition of the diene segment facilitates vulcanization of the EPR without the need to use peroxides and in the case of TPV the potential for a fully cross-linked discontinuous phase in the PP:EPDM blend. Without the diene segment, the EPR is fully saturated, and as such conventional curative systems based on sulfur or resin chemistry will not work. The structure of the unsaturated diene comonomer is such that there is a reactivity difference between the unsaturated sites which enables the comonomer to be grafted onto the ethyleneepropylene main chain using only one active site preferentially. This leaves the other pendant site free for further chemical cross-linking. In EPDM rubber, the level of residual unsaturation after polymerization is low compared to other rubbers. Natural rubber, SBR, and chloroprene rubber have an unsaturated site (C]C) every four carbons, which are located within the main chain, EPDM has a site every 50 carbons, which are located pendant to the main chain backbone. This has the effect of increasing the inherent heat, ozone, light, and oxygen resistance of the rubber. In regard to diene usage in TPV, ENB is the most widely used commercially. This is due to its ease of incorporation during copolymerization, greater reactivity in subsequent sulfur-based curing, and greater control of chain branching during polymerization. The process by which the TPV is produced is described as dynamic vulcanization. This differs from conventional vulcanization of rubber, which is a static process, in that the rubber phase in the TPV is chemically cross-linked during the melt mixing process. This melt mixing route has been described by Gessler (Coran and Patel, 1983), Fischer (Coran et al., 1985), and Coran (Gessler, 1962) in their patents on polymer blends containing proportions of partially or fully cured rubber. These describe the use of conventional two-roll mills, banbury mixers, and extruders both single- and twin-screw types. Also described are the use of torque rheometers for the evaluation and monitoring of the vulcanization process during melt mixing. The commercial dynamic vulcanization process involving either single-screw mixers or twin-screw compounders is proprietary. As such little information of the specific techniques employed is available. It is known to be a continuous process. In broad outline, the process involves the initial mastication and melt mixing of a rubber with a thermoplastic and fillers, if any, under strictly controlled conditions to achieve a uniform blend. Once the mix has been thoroughly blended, the vulcanization system is added at a point downstream in the extruder. Rapid cross-linking of the rubber occurs while the blend continues to be mixed. The two-phase morphology associated with TPV develops at this time and is crucially dependent on the process conditions employed. The curing systems employed with these TPV materials are typically sulfur-based, resin cure, and peroxide. The resultant TPV properties will be affected in part by the curing system employed and the addition levels (Rader and Abdou-


Tg ‘A’ PS 20 wt% PPO

PCIS 20 wt% PPO

40 wt% PPO

40 wt% PPO 60 wt% PPO

60 wt% PPO 80 wt% PPO

DH/DT

DH/DT

Single Tg

665

80 wt% PPO

PPO

PPO Tg ‘B’

350

400

450

500

550

350

400

T (K)

450

500

550

T (K) DSC Thermogram of PPO/PS System

DSC Thermogram of PPO/PS System

FIGURE 24.20 DSC Curves for compatible and incompatible binary blends of polyphenylene oxide (PPO), polystyrene (PS), and poly(p-chloro-vinyl benzene) (PCIS) (Coleman et al., 1990).

Sabet, 1990; Fried, 1983). Care must be given to peroxide curing as the peroxide can attack the tertiary hydrogen atom of the PP causing a reduction in molar mass and hence final product properties. A second and somewhat unique TPV claimed by the inventors DuPont to be a single-phase system called AlcrynÒ MPR. It is a mixed blend of ethylene interpolymers with chlorinated polyolefins, details of which remain proprietary, as does the method of manufacture. In broadest terms, MPR is described by Wallace as an amorphous partially cross-linked DV. The claim for a single-phase system as opposed to two phases is based on the miscibility of chlorinated polyolefins with a variety of ethylene interpolymers of differing structure. This claim is supported by DSC and Rheovibron data showing a single glass transition point for the TPV, indicating complete miscibility of both materials independent of temperature.

24.4.2 Polymer Miscibility Often described in literature and discussion is the compatible or miscible nature of block copolymers or polymer blends. The terms are often used interchangeably but are in fact two distinct characteristics. In broadest and simplest terms in order for a solvent: polymer mixture to be compatible, the forces of attraction between the solvent: polymer molecules must be equal to or greater than the forces of attraction between the solvent: solvent and polymer: polymer molecules. When the forces of attraction of polymer: solvent are insufficient, the strongest attractive forces cause those molecules to congregate forcing out the other molecules causing separation into two phases. The case for polymer: polymer mixture is more complex. A compatible polymer system is where two distinctly different polymers are able to coexist in separate phases within a polymer mixture below their glass transition temperatures, both lending characteristics to the blend specific to each polymer type. Often this type of system is described a semi- or partially miscible. This is seen in a Differential Scanning Calorimetry (DSC) thermogram as distinct glass transition temperatures (Tg), each specific to the individual polymer type Figure 24.20(b). For a polymer: polymer blend to be truly miscible, where there is a single Tg for the system (Figure 24.20(a)), the Gibbs free energy of mixing (DG) must be favorable (Eqn (24.1)) (see also Section 5.3.5): G ¼ DH TDS

(24.1)

where DH is change in enthalpy (heat content, exothermic or endothermic); DS is the change in entropy; T is absolute temperature. In this context, DS is defined as the state of chain disorder within a given polymer. An amorphous polymer would have a high entropy factor, a crystalline, chemically more structured polymer a lower level of entropy. In order to achieve total mixing (miscibility) DG must be negative, chain disorder within the blend must be greater than that of each constituent. However, enthalpy (heat content of the materials) must also be considered. The change in heat content can be by either


Two Phase T0

T Single Phase

UCST

T

LCST

T0

Single Phase

Two Phase

Increasing proportion of polymer ‘B’

Increasing proportion of polymer ‘B’

FIGURE 24.21 Schematic of the boundary temperatures for the production of a single-phase binary blend.

absorption (endothermic) or by release (exothermic). If a big enough change in enthalpy can occur in an otherwise low entropic state, a change can occur. Taking a simple binary blend as an example, above the melt transition of the blend that is in the melt state, polymer miscibility can be seen even with immiscible polymers. This is because the polymers now have a similar state of molecular disorder at a high enough temperature. As the melt cools, the differences in entropy (state of molecular disorder in each polymer) between the polymers causes them to separate into distinct and separate phases in the cooled blend. In the worst case, where the proportions of both polymers allow, this will be seen as a lamellar-type morphology with no association between individual layers and hence no property enhancement. The composition range over which phase separation occurs is not constant and will change with temperature. As temperature increases, the point at which a blend ratio becomes single phase is termed the upper critical solution temperature or UCST. This is illustrated in Figure 24.21(a). Dependent on the proportion of polymer “B,” the influence of temperature produces a parabola of ranges of singleversus two-phase states. The same can be seen for some polymer blends where decreasing temperature causes miscibility to occur (exothermic conditions), resulting in a lower critical solution temperature or LCST, Figure 24.21(b). However, superimposed on this is the stability of the single-phase system during the enthalpic change, and is defined as the spinodal curve. This defines the limits within which phase separation takes place and by which process it proceeds. In the metastable region between the binodal and spinodal curves, two-phase segregation progresses by nucleation and growth. Within the spinodal region, phase separation is seen to occur spontaneously. This phase diagram, Figure 24.22, differs from that proposed by FloryeHuggins whose theory only addresses UCTS boundaries based on chi (c) being proportional to 1/T. FloryeHuggins addressed the question of polymer: polymer miscibility based on the assumption that polymer chain segments would follow the same model as small molecules (solvent: solvent, polymer: solvent) and each

immiscible

miscible region

LCST

T Spinodal curve

UCST

metastable region

immiscible

ΦB FIGURE 24.22 Phase Diagram showing spinodal curve and metastable regions within the immiscible phase.


667

chain segment would fill one lattice of a lattice site in the same fashion. This theoretical modeling approach does not take into account the influence of any molecular interactions other than Van der Waals forces. Assuming that these chain segments are randomly distributed in the lattice for a mixture of two polymers “A” and “B,” an equation for assessing the level of miscibility was proposed (Eqn (24.2)): DGmix fA fB ¼ cFH fA fB þ (24.2) ln fA þ ln fB RT NA NB where DGmix is the Gibbs free energy of mixing; R is the gas constant; T is the temperature; cFH is the FloryeHuggins polymerepolymer interaction parameter; N is the degree of polymerization; f is the volume fraction of each polymer. In simple terms, the interaction parameter (c) can be determined from cohesive energy density measurement via Hildebrand solubility parameters using the following relationship (Eqn (24.3)): cFH ¼ Vseg

ðdA dB Þ2 RT

(24.3)

where c12 is the FloryeHuggins polymerepolymer interaction parameter; Vseg is the volume of one polymer segment; d values are the solubility parameters of the two components of the blend; R is the gas constant; T is the temperature. Case et al. (Case and Honeycutt, 1994; Paul, 1987) discuss alternative routes to determine the chi (c) value of AlcrynÒ MPR more in line with the more complex polymer: polymer interactions known to be present. Based on the premise that the FloryeHuggins model does not account for specific interactions ranging from dispersive forces to hydrogen bonding, Coleman et al. (Sengupta et al., 2003) proposed an additional term to account for this (Eqn (24.4)): DGmix fA fB DGH ¼ cFH fA fB þ (24.4) ln fA þ ln fB þ RT NA NB RT where DGmix is the Gibbs free energy of mixing; R is the gas constant; T is the temperature; cFH is the FloryeHuggins polymerepolymer interaction parameter; N is the degree of polymerization; f is the volume fraction of each polymer; DGH is the term describing favorable interactions other than Van der Waals interactions. This term accounts for favorable changes to the free energy of mixing arising from the contribution of specific intermolecular interactions, particularly hydrogen bonding, and offsets the unfavorable conditions arising from the combined contribution of proportions of polymers and the interaction parameter. There are occasions where the properties of a single polymer do not meet all the needed requirements. Rather than synthesize a special polymer, ideally, blending or mixing with a second polymer should achieve the target requirements, the resultant blend having the balance of properties needed. As discussed, this is more complex than at first apparent. Taking PE and PP as a simple example, both are nonpolar hydrocarbon polymers and on the basis of “like should dissolve like,” one would expect those two polymers to easily mix together. The reality in practice is that they do not. This situation is seen to be the case for many polymer blends. The question of why the case for mixing is different for polymers as compared to liquids (solvents) and polymer: solvent has already been discussed. The reality is that partially miscible or


Polymer Phase ‘A’

Phase Interface

Block Copolymer Segment ‘A’

Polymer Phase ‘B’

Block Copolymer Segment ‘B’

FIGURE 24.23 Schematic of interfacial adhesion improvement via block copolymer.

immiscible blends, provided there is a level of compatibility between phases, can provide useful materials. An early example of this was the modification of PS, a brittle thermoplastic. By blending with a small amount of PB rubber, highimpact PS can be produced. The relative levels of both polymers in the blend will change the phase morphology and hence the blend properties generated. As discussed in Section 24.4.3.3, SantopreneÔ TPE is a further development of this technology being a blend of PP and vulcanized EPDM rubber. However, in this case there is at least partial compatibility between the phases arising from the olefinic nature of both molecules, this is reflected in the higher-than-expected properties associated with a simple blend of these two materials. Having stated that most polymer blends are of the immiscible type and that the resultant properties are governed by the nature of the polymers, it is often necessary to modify the blend to maximize these properties. The method to achieve this is via the use of compatibilizers. In immiscible blends, the phases have very low interactions with each other, and maximizing physical properties is reliant on both phases being able to support any stresses or strains imposed on the material. Typically, the compatibilizers will be a block copolymer consisting of the same or similar components as is present in the blend. Taking a blend of PS and PE or PP as an example, this blend is immiscible; there is very little interaction between the phases, the entropic states of both polymers being very different from each other. The properties of the blend are considerably poorer compared with the properties of the individual polymers. By the addition of a small amount of an SEBS block copolymer, the interface between the separated phases can be modified to improve the interfacial adhesion and hence produce a general property improvement. The compatibilizer essentially acts as an emulsifying agent which sits at the phase interface with each segment of the block copolymer sitting within the phase most energetically suited to it, as illustrated in Figure 24.23. This bridge across the interface provides the mechanical enhancement to the blend and hence its improved properties. The use of grafted copolymers may also be useful in compatibilization. It would suggest that a PS-grafted PB main chain would successfully act as a phase interfacial enhancer with short chain PS branches being fully miscible in the styrenic phase of the blend. What has also been noted is that with this improvement at the interface, the physical size of the dispersed phase reduces, again improving physical properties.

24.4.3 Morphology of Thermoplastic Elastomers The macro-structure of TPE materials is dependent in part on the chemical nature, molecular structure, and level of miscibility of the block copolymer or polymer blend. It is the morphology of the polymer that dictates the mechanical performance observed. As discussed previously, most commercial TPE materials possess a distinct two-phase morphology, but several basic morphological types have been identified. Morphological states can be: discontinuous sphere, lamellar, or rod structures, co-continuous network structures including interpenetrating network (IPN) structures. In the typical morphology, one phase is hard, the other soft and rubbery at room temperature. The hard phase promotes strength in the TPE preventing flow of the soft elastomeric phase. The balance of hard phase to soft elastomeric phase governs the hardness of the material and the physical properties attainable. The combined chemical nature of each phase will affect the thermal, chemical, and processing behaviors of the TPE.


669

An exception to two-phase TPE types is AlcrynÒ TPE MPR. AlcrynÒ is produced by combining halogenated polyolefin with special ethylene-based interpolymers. In this case, due to chemical compatibility and strong hydrogen bonding between the mixed precursors, there is complete miscibility, and a single-phase material results. The supporting evidence to confirm this is a single glass transition temperature (Tg) between 0 and 20 C depending on grade and also a single tan delta maximum demonstrated by thermo-mechanical testing (Wallace, 1985). Sperling (1977) and Gergen et al. (1985) discuss IPN morphologies in PS with SBR and hydrogenated diene block styrene copolymers. In the first case, blends of SBR and PS are shown to develop co-continuous IPN morphologies by the introduction of cross-linking, either semi-IPN where only one phase is cross-linked, or full IPN where both phases undergo cross-linking. Domain size is dependent on phase compatibility, level of cross-link density, and degree of polymerization. The second example highlights the use of increased acrylonitrile content in PVC: poly (butadiene-co-acrylonitrile) (NBR) mechanical blends. The subsequent development of an IPN morphology, as acrylonitrile content is increased (40%), is enough to produce hydrogen bonding levels suitable for an almost single-phase morphology. The morphological structure can be changed by altering temperature and strain level. Co-continuous morphological structures of PS and poly (ethereester) block copolymer have been studied by Veenstra et al. (1999) who studied the influence of the orderedisorder transition (ODT) of the block copolymer and its influence on the development of co-continuous morphology when mixed with PS. The ODT is the temperature where the two-phase structure of the block copolymer is converted into a homogeneous melt and the influence of physical cross-links is reduced or lost. Although reference is made to the ODT and its range, no definitive value is given in this study. However, from the results, the ODT can be presumed to be below 200 C. They found co-continuous morphologies over the range of 30e80 volume percent provided the block copolymer was maintained below its ODT during melt blending. They defined the presence of a co-continuous morphology when the block copolymer phase of the blend was fully self-supporting once the PS phase had been extracted. The self-supporting structural form is interconnecting elongated thread-like, suggestive of fibers. Annealing of these elongated, thread-like structures above 200 C leads to retraction (elongational shrinkage) and an increase in phase size and assumed drop off in the co-continuous nature of the blend. If there is sufficient physical cross-linking present, this effect is prevented, and stable co-continuous morphologies are possible. Comparing the morphologies of the remaining TPE materials, it is clear that they can be divided into two basic types; two-phase morphologies arising from immiscibility levels and differences in entropy between block copolymer segments and immiscibility levels within polymer blends (with or without compatibilizer) and mechanical blends.

24.4.3.1 Styrenic-Based Block Copolymer TPE The morphology of this type of TPE is based on a hard amorphous PS block phase combined with a soft elastomeric phase typically, butadiene (SBS), isoprene (SIS), or ethyleneebutylene (SEBS), Figure 24.6. The influence of molecular architecture on morphology is shown in Figure 24.24. Dependent on the ratio of styrenic to elastomeric segments, the full range of TPE morphological types can and have been observed. This TPE material has been studied in some depth by many workers referenced in literature, particularly in the area of morphology: property relationships (Halper and Holden, 1985). The influence of triblock styrenic copolymer morphology on mechanical properties has been reported by Adhikari et al. (2001). They evaluated by scanning microscopy techniques the relationship between morphological state and property of a range of SBS block copolymers. By using three differing linear copolymer architectures (Figure 24.25), it was demonstrated that a good correlation between determined morphology and morphological types does exist. Illustrated is a conventional SBS with symmetric PS end blocks of equal

A Spheres

A Cylinders

A, B Lamellae

B Cylinders

B Spheres

Increasing A Content Decreasing B Content FIGURE 24.24 Stylized morphology of styrenic TPE-based alternating blocks (Halper and Holden, 1985).


samples LN1-S74

molecular structure PS

morphology

PB

LN2-S74

LN4-S65 FIGURE 24.25 Schematic of polymer architecture of SBS triblock copolymer (Halper and Holden, 1988).

length, SBS with a tapered PB transition to the PS block and a block copolymer having equal PS end blocks with a homogeneous random distribution of PB and PS in the midblock chain segment. In cases one and two, the PS content was 74% and in case three, 65%. Figure 24.26 shows the resulting morphologies determined by scanning force microscopy from toluene solvent cast samples. These were left under ambient conditions for two weeks to allow for the full development of phase-separated microstructures. These photomicrographs clearly show the differences in morphology between the differing molecular architectures. LN1-S74 displays typical cylindrical morphology, PB cylinders (dark areas) in a PS (light areas) matrix. LN2-S74 produces a lamellar structure and LN4-S65 a random distribution of PS cylinders in the PB/PS phase typical of TPE materials. The mechanical properties varied with molecular architecture and morphology. LN1-S74 had a low strain at break (20%) and high yield stress (27 MPa) attributable to the PS continuous phase. LN2-S74 also gave a yield stress point, but about 50% lower than LN1-S74 but with much higher elongation (437%) and tensile strength (34 MPa). The more TPE-like LN4-S65 gave no yield stress, had the highest elongation (556%), and a tensile strength similar to LN2-S74. A second common styrenic TPE is that based on styrene (ethyleneebutylene) styrene (SEBS) block copolymers. Typically, the base SEBS is further compounded to reduce cost and enhance physical and thermal properties. Common additives are PP thermoplastic and extender oil. Sengupta et al. carried out a comparative study (Sengupta et al., 2003) of the morphology and mechanical properties of a dynamically cured TPV based on EPDM:isotactic PP:paraffinic oil and SEBS:isotactic PP:paraffinic oil. Unfortunately, what is not clear from this work is whether the base formulations had been adjusted to give comparable hardness levels. This has an impact on the physical performance observed. They compared in part, the morphology differences of the SEBS compound when manufactured on a Brabender Plasticorder and co-rotating twin-screw extruder. Various scanning electron microscopy techniques were used to determine the morphology of ruthenium tetraoxide-stained compression-molded samples, Figure 24.27. They found that a more co-continuous morphology with larger domain size from Brabender mixing led to higher tensile properties compared with twin-screwcompounded SEBS.

FIGURE 24.26 SFM images of the developed morphology of cast samples (Halper and Holden, 1988).


671

FIGURE 24.27 SEM images of SEBS:PP:Oil blend (John, 1980).

24.4.3.2 Block Copolymer Polyurethane, Polyester, and Polyamide TPE Morphology The remaining block copolymer types, polyurethane (TPU), copolyester, and copolyamide derive their morphology in much the same way as the styrenics. The difference here is that hard phase is derived by crystallization of particular elements of the main chain (Figure 24.28). For TPU TPE, the hard phase consists of short chain diols and isocyanate dispersed in the long chain diol soft phase. The influence of hard segment on final properties is discussed by Meckle et al. (1987b). In the case of polyamide TPE, the hard phase is poly (11-aminoundecanoic acid) or laurolactam (Farrissey and Shah, 1988). Polyester-based TPE morphology is reviewed by Adams (Adams and Hoeschele, 1987b). Here it is believed that the basic morphology is a lamellar structure of co-continuous hard phase existing within a continuous soft amorphous phase, Figure 24.29.

24.4.3.3 Mechanically Blended TPE Morphology The final group of TPE materials are the mechanical blends, TPO elastomer, or TPV. Both types display a distinctive twophase morphology, the major difference being in the nature and form of each phase. In TPO compounds, the hard phase (in most commercial materials) is generally isotactic PP homopolymer or an isotactic low ethylene-containing copolymer PP. The soft phase is normally either uncured EPR or ethylenee propyleneediene monomer rubber (EPDM). The diene type, ENB, 1,4-hexadiene (1,4-HD), or dicyclopentadiene (DCPD), can influence the product properties in both TPO (Morris, 1977) and indeed TPV materials. Further compounding additives, carbon black, oil, fillers, and others tend to be in the rubber phase at room temperature but can migrate into the PP phase during processing, become trapped, and finally migrate to the surface as a bloom. The morphology of TPO can be


Crystalline domain Rubber Domain FIGURE 24.28 Schematic illustrating the phase morphology of polyurethane, polyester, and polyamide TPE.

somewhat complex depending on the rubber type, level of addition, and method of mechanical blending. Typically, the rubber sits within the PP matrix either as discrete rubber particles or more normally as a co-continuous phase as illustrated Figure 24.30. TPVs have been described (Sheridan, 1988; De and Bhowmick, 1990; Coran, 1978e1982; Holden, 1995; Holden & Legge, 1987) and investigated in some depth by various researchers. Typically, the TPV has been based on blends of EPDM of the ENB type and isotactic PP thermoplastic where the EPDM phase is dynamically vulcanized during the melt mixing of the rubber: plastic blend (SantopreneÔ TPE). Figure 24.30 shows schematically the morphology of TPV materials. However, other TPV materials are now commercially available, examples being TrefsinÔ PP:Butyl rubber TPV and GeolastÔ PP:NBR TPV from Advanced Elastomer Systems.

FIGURE 24.29 TEM of solvent cast polyester film (Adams and Hoeschele, 1987b).


673

FIGURE 24.30 SEM of fracture surface of 50:50 EPDM:PP blend (a) and stylized morphology schematic (b). By permission of ExxonMobil.

In the case of EPDM:PP TPV, both the cure system employed and processing methodology have an impact on subsequent morphology and final material properties. Using a sulfur cure system, it has been demonstrated (Lopez-Manchado et al., 2001) that the phase morphology does indeed undergo changes dependent on the manufacturing route. Here, three routes were evaluated using a Haake torque rheometer with high shear roller rotors for 50:50 blends of EPDM and PP. In the first case, a static rubber cure was used, the EPDM and curatives being blended at low rpm (20 rpm) and then cured under high shear (60 rpm) at 170 C for 15 min. The cured EPDM was then finely milled before adding to the PP and melt blended for 15 min at 170 C. The second route mimicked the first but the blended EPDM:curatives were not precured. Instead, the preblend was added to the molten PP in the mixer and the dynamic vulcanization of the EPDM progressed as the rubber mixed into the thermoplastic. In the final route, EPDM and PP were first melt blended under the high shear condition for 15 min at 170 C, the curatives were then added and vulcanization occurred during a further 15 min of high shear mixing. Scanning electron microscopy (SEM) was used to compare the morphologies obtained for each type of mixing/vulcanization method. The compared morphologies are shown in Figure 24.31 (SantropreneÔ , 1998); it is clear that for a given level of curative, the rubber particle size is influenced by the process route. Little difference in EPDM phase size or dispersion was seen between the two dynamically vulcanized TPV materials. Examination of the mechanical properties show that a TPV produced by firstly ensuring the curative package is preblended into the rubber before melt mixing promotes greater adhesion between the two phases and better properties overall. Goharpy et al. (2001) concluded that the impact of interfacial adhesion arising from level of cure and changes in surface tension of the rubber were crucial in obtaining maximum TPV properties. This work used a 60:40 blend of EPDM (ENB type) and PP (no clear details regarding type) as a mechanical blend (control) and vulcanized with an accelerated sulfur curing system. The materials were blended and vulcanized in a Haake torque rheometer, the addition sequence being clearly defined as was the process, but the type of rotor used was not discussed. Their approach was to remove material from the mixer at different points along the torque/time curve for the process and conduct SEM studies on the developed morphology at these time intervals.

FIGURE 24.31 SEMfFracture surfaces of dynamically cured 50:50 blends of EPDM:PP (SantropreneÔ , 1998).


FIGURE 24.32 Four-stage model of microstructural development in EPDM:PP TPV (Goharpy et al., 2001).

Based on this analytical study, they propose a four-stage model to describe the formation of a stable TPV morphology, Figure 24.32 (Goharpy et al., 2001). The influence of the surface tension of the rubber was concluded to reduce as vulcanization progresses; this is because of elastic shrinkage of the rubber as the cross-link density is increased. This combined with an increase in interfacial adhesion and further breakdown of the rubber agglomerates leads to a stable morphology. The main disadvantage of sulfur-cured TPV is the relatively poor processing characteristics observed. The polysulfidic cross-links undergo sulfur exchange reactions resulting in coalescing of the rubber particles and changes in the physical properties attainable. The use of a phenolic-based curing system gives a TPV with much improved dispersion of the rubber phase and improved compression set, chemical resistance, and processing characteristics. This forms the basis of the now familiar TPV, SantopreneÔ TPE. The influence of the level of phenolic curative on the phase morphology of TPV has also been reported by Ellul et al. (2001). It was found that as the cross-link density increased, the EPDM domain size reduced and the particle size distribution became narrower during dynamic vulcanization. Both factors confirmed observations by other workers that this is key to generating TPV materials with optimum properties.

24.4.4 Rheology of TPE The rheology of TPE materials is the study of how they flow under the conditions of pressure, temperature, and shear. All these conditions are present during the processing of TPE materials. Understanding how the material reacts (flows) to variations of these conditions during processing is key to obtaining maximum output with optimal quality and performance of the component produced. The main types of flow are illustrated in Figure 24.33. Basic rheology describes three principal types of flow: l

Dilatant flowdthe viscosity increases with increasing shear rate (e.g., beaten egg white).


675

Slope = 1

Lower Newtonian

Upper Newtonian

Slope = < 1

Log η

Log τ

η0

Pseudoplastic

η∞

Slope = 1 Log γ∙

A

B Chain Orientation

C

FIGURE 24.33 Pseudoplastic flow behavior (John, 1980).

l l

Newtonian flowdthe viscosity is independent of shear rate (water). Pseudoplastic flowdthe viscosity decreases with increasing shear rate (typical thermoplastic melt behavior).

In order to understand the concept of flow and flow differences in TPE, it should be noted that most flow is of a laminar type, typical of most thermoplastic materials. There is a distinct flow velocity change observed across the melt section during any processing technique as the melt layers are dragged between the metal surfaces. The equipment for determining the flow characteristics of materials over a range of shear rate conditions is most commonly a capillary rheometer, but other rheometric techniques can also be graphically represented and gives a linear relationship for the fluidity (the reciprocal of viscosity) of the polymer. TPE materials display non-Newtonian viscoelastic behavior in the melt state. Here, the rheology combines both viscous flow (e.g., water), the energy from which is dissipated as heat, and an elastic component where the energy from deformation is stored. The Newtonian model must be adjusted to account for this. The TPE material has inherent structure that needs to be overcome in order for flow to occur. This structure is primarily related to molar mass and weight distribution, level and length of chain branching, chain entanglement, segmental compatibility (Halper and Holden, 1988), and ease of rotation of main chain bonds, that is their flexibility. As the entangled chains are subjected to a stress, the chains extend (stored energy), then begin to disentangle and align (viscous flow). What is observed in the case of TPE is pseudoplastic flow behavior; the material viscosity reduces with increasing shear stress and shear rate, Figure 24.33. Pseudoplastic flow behavior can be viewed as a combination effect of time-independent Newtonian and plastic flow. The influence of temperature on the melt viscosity of TPE materials varies with the TPE type. Styrenic TPE materials have melt viscosities higher than the materials that constitute the chain blocks, PB and PS. In order to achieve a homogeneous melt condition, high levels of energy are needed to disrupt the PS domains (Halper and Holden, 1988). This is achieved by increasing temperature and shear, as illustrated in dynamic viscosity curves, Figure 24.34 (Halper and Holden, 1988) and is particularly pertinent to the SEBS block copolymers where segmental incompatibility is extremely high. This type of temperature influence is also relevant to other TPE types. TPU is more sensitive to extremes of temperature and shear which can cause voiding and degradation. Polyester-based TPE flow is claimed to be more dependent on temperature than shear. What is clear is that in the main, melt viscosity is less sensitive to temperature at higher shear rates. For TPV materials, the effect between temperature and shear on melt viscosity is shown in Figure 24.35. The influence of dynamic vulcanization on the melt of EPDM/PP and NBR/PP blends has been investigated and it was shown that the TPV melt behavior is similar to that of filled polymers and rubber-modified plastics (Chung et al., 1971). It was concluded that the level of dynamic vulcanization increased melt viscosity, and the selection of cross-linking agent was highly influential, TPV being more shear-sensitive than simple blends. Phase compatibility and oil extension were also shown to increase melt flow. Jayaraman et al. (2004) also investigated the influence of oil extension of EPDM:PP TPV where the extending oil soaks the EPDM particles, but is also present in the PP matrix at the EPDM/PP interface causing some swelling of both phases. It should be noted that there is a limit to the level of oil the PP is capable of accommodating before exudation occurs which relates to PP molar mass and level of crystallinity. The precise nature of the oil is not cited but is presumed to be a


106

105 Viscosity, η′, poise 150°C 170°C 180°C 200°C

104

103 10–2

10–1

102

1 10 Frequency, ω (sec–1)

103

FIGURE 24.34 Dynamic viscosity of SBS block copolymer at various temperatures.

mineral oil. The aim in this work was to evaluate the influence of the EPDM:PP:oil distribution on the subsequent viscosity of the PP:oil matrix and its effect on the TPV melt viscosity. This was identified as a key influence in rubber particle size generation during dynamic vulcanization and also governed the shape and level of rubber particle deformation during flow. The conclusion was that TPV viscosity reduced with increase in PP molar mass arising from its ability to cause greater deformation of the rubber particles. Based on this, it is possible to engineer TPV materials with a variety of flow characteristics to suit specific processes.

(a)

(b) 105

1.5

Calendaring

1.0

40D 87A .5

80A 73A 64A

Apparent Viscosity, Poise

Apparent Viscosity, 104 Poise

Extrusion 104

103

102 1 10

0 177

191

204

218

Temperature, °C

Injection Molding

203– 40 201– 87 201– 80 201– 73 201– 64 102

103

104

Apparent Shear Rate, S–1

FIGURE 24.35 Influence of temperature (a) and shear rate (b) on apparent viscosity of various SantopreneÔ TPV grades. By permission of ExxonMobil.


677

FIGURE 24.36 Effect of cross-link density on tensile strength and tensile set of TPV (Rader et al., 1988a).

24.4.5 StructureeProperty Relationships The ultimate properties achieved in any TPE material are governed by the chemistry, nature of the constituents, and its morphology. The aim is to produce a product that has some of the attributes of a thermoset rubber but with the processing characteristics of a thermoplastic. A specific property will vary with the relative proportions of hard and soft phases, so a range of TPE materials are available within each TPE group. Specific property values of commercially available TPE materials from SantopreneÔ specialty products are discussed and compared in Section 24.4.5.2. The styrenic, copolyester, urethane, and polyamide TPE groups all possess two-phase morphology derived from their copolymerization chemistry. The physical properties are dependent on the level of hard phase present, glassy domains in the styrenics, or strong hydrogen-bonded crystalline segments in the others. The mechanical strength and modulus (stiffness), abrasion, hardness (can be a limited range), compression and tension set, and tear resistance of the TPE above room temperature and below the softening point are significantly influenced by the hard phase. The elastic soft phase generates the rubber-like properties of elongation, flexure, low-temperature performance, dynamic properties, and to some extent tensile strength by virtue of strain-induced crystallization of chain segments. In the case of TPO and TPV materials, physical properties are governed in the main by the level of the thermoplastic phase, usually PP. The proportion of EPDM or EPR present controls the rubber-like characteristics of the blends, rubber-like qualities, and tensile properties being further enhanced by cross-linking (TPV) as shown in Figure 24.36. It has been clearly demonstrated that a reduction of the rubber particle size (Figure 24.37), in combination with good dispersion, adequate miscibility, and some alloying between the PP and EPDM rubber phases, leads to a physical property improvement far beyond a simple TPO blend or indeed a fine dispersion of micro-ground fully vulcanized EPDM mechanically blended into PP. Key to the performance of TPE is its thermal properties both in terms of its overall performance and ease of melt processing. The glass transition temperature (Tg) of the hard phase governs in part the mechanical performance at room temperature and above, while the soft phase controls the subroom temperature performance and brittle point. In the more polar TPE, a combination of hydrogen bonding and crystalline segments restricts overall chain mobility, raising the glass transition point. This is reflected in their higher heat distortion temperatures (HDT), melting temperatures (Tms), and service temperature compared with the styrenics. The styrenics (Tg 90 C) have an upper service performance limited to 90e100 C without further modification with other higher Tg thermoplastic materials (PPO), while the remaining copolymer TPE types have somewhat higher service temperatures. The PP-based TPO and TPV materials fall between the two, being reliant on the thermal characteristics of the PP (Tg about 0 C, Tm 160e165 C) to give a continuous service temperature up to 135 C (TPV). Chemical resistance is determined by the chemistry of the TPE and its morphology. Nonpolar amorphous TPE materials, styrenics, have somewhat limited chemical resistance to a broad range of solvents. This is due to the low solvent resistance of the amorphous PS, isoprene, and butadiene or to a lesser extent ethyleneebutylene phases. The more polar


FIGURE 24.37 Influence of rubber particle size on tensile properties of SantopreneÔ TPV.

copolymer TPE types, copolyester, polyamide, and urethane have better chemical resistance particularly to nonpolar solvents. To polar solvents, their resistance is reduced but is maintained to some extent by the levels of crystallinity present. Further improvement can be achieved in these TPE materials by selection of the soft segment, polyether or polyester, and molecular structure. TPO and TPV materials derive their chemical resistance from the nonpolar PP; this in essence provides a relatively inert protective layer around the EPR or EPDM rubber phase. Cross-linking of the rubber does provide some improvement in chemical resistance by restricting the level of swell in some solvents. Electrical insulation properties are dependent on the level of polarity present in the TPE. Most TPE materials will give a level of electrical insulation. Here, the essentially nonpolar olefinic TPO and TPV materials and SEBS TPE (dependent on other compounded polymer and additives) display good to excellent electrical insulation properties. They possess high resistivity, low dielectric constant, and very low power factor over a range of temperature, frequency, and humidity. Because of their inherent polarity, the other TPE types have what could be generally termed moderate electrical performance to some extent influenced by their more hygroscopic nature. The environmental resistance of TPE types is a key consideration especially for outdoor applications, particularly in the automotive sector. All the TPE family are susceptible to a greater or lesser extent to the effects of high-energy UV radiation. The effect can range from color changes to embrittlement and degradation. Because of this, carbon black and other UV energy absorbers are used to extend the service life of the material. Other considerations are the effect on properties/function of combinations of heat, oxygen, ozone, and water (or other chemicals). The polyurethane TPE is, for example, more susceptible to light and water. In TPV, the addition of carbon black and/or UV stabilizers produces materials that can withstand the vigorous Kalahari weathering testing (most SEBS compounds are seen to fail) and other accelerated weathering/aging tests. It should be noted, however, a continual development in stabilizer systems is changing this picture. Figure 24.38 shows the results of accelerated desert aging of a mid-range hardness (Shore A 73) SantopreneÔ TPV. The influence of carbon black as an energy absorber is clear; neither grade contained any other commercial UV stabilizers. Table 24.1 is an illustration of some of the performance characteristics for different types of TPE. The table has been constructed to reflect the chemistry and morphology of each TPE group. The data were obtained from various commercial sources and ranked based on comparative performance with other TPE types. It should be noted that the performance rating given is in essence a broad summation of performance, and individual TPE grades within a given family could give performance values beyond that suggested.

24.4.5.1 Properties of TPE Materials The data shown in the following tables are examples of the typical properties of TPE materials based on commercially available literature. The structureeproperties relationship was discussed in general terms in Section 24.4.5. In this section, a comparison of some of the key physical, thermal characteristics, and other properties of each type of TPE material is made. For all the groups in the family, a range of properties can be attained by the judicious use of additives, types of polymer selected in the case of compounded TPEs, or by selective use of polymer precursors in the case of TPEs produced by polymerization. Table 24.1 provides an illustrative summary of what might be expected from each family type.


679

140 120 Percent Retention

Ultimate Elongation 73A Black

100 80

Tensile Strength

60

Ultimate Elongation

40 73A Colorable

Tensile Strength

20 0 2

0

4

6 8 10 Time (Months)

12

14

FIGURE 24.38 Retention of tensile strength and ultimate elongation of Shore A SantopreneÔ TPV during accelerated desert aging (Rader et al., 1988b).

TABLE 24.1 Illustrative Summary of Some Performance Characteristics of TPE Polymer Blends

Block Copolymers

IPN

Physical/Mechanical Properties

Olefinic TPO

Olefinic TPV

Styrenics

COPE

TPU

Polyamide

MPR

Specific gravity

0.97

0.97

0.94

1.2

1.2

1.02

1.23

Hardness range (Shore AeD durometer)

60e90A

35Ae50D

20Ae40D

40e70D

60e90A

75Ae70D

60e80A

Compression set

F

EG

G

GeF

G

EP

F

Continuous service temperature ( C)

120

135

100

110

120

80

100

Lower service temperature ( C)

45

60

45

70

40

40

40

Processing (cycle time)

EG

EG

EG

EG

FeG

G

EG

Recyclable

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Environmental Resistance Ozone

E

EG

F

E

G

No data

EG

Weather (UV)

G

EG

GeF

EeG

G

GeF

E

Acids

E

EG

E

P

E

GeF

GeF

Alkalis

E

E

E

GeF

P

GeF

E

Lubricating oils

P

FeP

P

F

F

E

E

Gas permeability

F

F

F

F

G

No data

No data

E, excellent; G, good; F, fair; P, poor. Data sourced from references Sheridan (1988) and De and Bhowmick (1990) and ExxonMobil Product data sheets.

24.4.5.2 Comparison Between TPO and TPV Properties In this section, a comparison of some of the key physical, thermal characteristics of a mechanical blend of PP and EPDM TPO and a vulcanized TPV is made. Further, property data are also provided for SantopreneÔ TPV. The data shown in Tables 24.2e24.4 are examples of typical properties taken from commercially available literature. The respective test standard is quoted where available. Table 24.2 shows the level of tensile properties that can be generated by TPO and TPV materials.


A fundamental characteristic of this family of materials is their ability to provide some of the physical characteristics associated with thermoset rubbers. In the softer hardness grades, both TPO and TPV display typical rubber-like deformation behavior showing a lack of defined yield point. The co-continuous phase morphology and lack of cross-linking in the EPDM gives the TPO much improved strength and elongation properties at room temperature compared to a TPV of equivalent hardness. In the harder Shore D range, the TPO becomes distinctly more plastic-like in its deformation; at these hardnesses, the rubber content is reduced and has significantly less influence on any reaction to applied stresses. The PP is

TABLE 24.2 Tensile Property Comparison of TPO and TPV Materials Properties

Test Standard

Units

TPO

TPV

TPO

TPV

Hardness

ASTM D2240

Shore A/D

60A

64A

35D

40D

Strength at break

ASTM D412

Mpa

13.5

6.9

e

19

Strength at yield

ASTM D638SO ISO 527

MPa

e

e

10

e

Elongation at break

ASTM D412

%

790

400

e

600

Elongation at yield

ASTM D638SO ISO 527

%

e

e

29

e

Tensile stress (100% elongation)

ASTM D412

MPa

1.6

2.3

6

9.2

TABLE 24.3 Physical Property Comparison of TPO and TPV Materials Property

Test Standard

Units

TPO*

TPV

TPO

TPV

Shore hardness

ASTM D2240

A/D

60A

64A

36D

40D

0.91

0.97

0.95

0.94

23 C, 170 h

e

23

e

39

23 C, 170 h (method B)

70

e

e

e

e

36

e

65

17

10

e

48

33

25

90

65

e

10

e

36

Specific gravity (23 C)

ASTM D792

Compression set

ASTM D395

%

100 C, 170 h Tensile set

ASTM D412

%

Tear strength

ASTM D624

kN m1

23 C

100 C

TABLE 24.4 Comparison of Thermal Properties of TPO and TPV Property

Test Standard

Units

TPO

TPV

TPO

TPV

Shore hardness

ASTM D2240

A/D

60A

64A

36D

40D

ASTM D746

C

60

60

40

57

Continuous operating temperature

C

100

135

100

135

Melt flow rate

g/10 min 3.5

e

e

e

e

e

45

e

e

66e96

e

81e108

Brittle point

230 C, 2.16 kg

230 C, 5 kg a

LCR viscosity a

Capillary rheometer, shear rate ramp.

1

204 C, 1200 s

Pa s


Hardness

Permanent set

Operating Temperature

681

Tensile Strength

Poor

Excellent

Tear Strength

TPO

TPV

Chemical Resistance

Melt Flow Tension set

FIGURE 24.39 Property variation between TPV and TPO of similar hardness (VistaflexÔ , 1995).

Viscosity (Pa.s) 10,000 1,000 100

Santoprene™103-40 200°C 220°C

200°C

240°C

100

220°C 240°C

10 1

1 0.1 10

Santoprene™101-64 180°C

1,000

260°C

10

Viscosity (Pa.s) 10,000

100

1,000 10,000 Shear Rate (1/s)

100,000 1,000,000

0.1 10

100

1,000 10,000 100,000 1,000,000 Shear Rate (1/s)

FIGURE 24.40 Viscosity: shear rate curves for standard grades of SantopreneÔ TPV.

now the strongest influencer on performance. This is not the case with the TPV; the homogeneous distribution of discrete rubber particles allows the material to still display a more rubber-like behavior. A key attribute associated with rubber materials is their ability to recover from an imposed load. This is especially necessary in the area of sealing and stretching under a wide variety of service conditions. TPO materials while being flexible do not have good recovery properties. Both the lack of cross-linking in the rubber phase combining with the creep behavior of the PP causes permanent, nonrecoverable “set” in the material even at room temperature (Table 24.3). This precludes their use in many sealing and other application areas where recovery from applied load is important. The discrete rubber particles in the TPV cause a comparative drop in tear resistance compared to TPO but only under ambient conditions. TPV continues to give acceptable tear resistance even at elevated temperatures. The flex fatigue of TPV is excellent outperforming chloroprene, EPDM, and chlorosulphonated thermoset rubbers (SantropreneÔ , 1998). The thermal properties of TPO and TPV are compared in Table 24.4. The low temperature brittle point is comparable between TPV and soft TPO, but the stronger influence of the PP phase in the hard TPO raises the brittle point significantly. Although not shown, the low-temperature impact performance is good for both TPO and TPV. Because of the lack of cross-linking of the rubber in the TPO material, the upper service temperature is limited. A general overview of property variations between TPO and TPV is shown in Figure 24.39. Both TPO and TPV display linear shear thinning behavior in the melt condition. It is possible to generate melt flow rate MFR values for TPO but due to the different rheological characteristics of TPV, it is not possible to achieve a consistently repeatable MFR value. Because of this, TPV melt flow characteristics are defined by generating capillary rheometry curves over a range of shear rates at different temperatures, Figures 24.40e24.41. A standard temperature (204 C) is used for


1.00E+04

1.00E+04 714-B, T=250 °C 911-B1, T=250 °C Apparent Viscosity (Pa.s)

Apparent Viscosity (Pa.s)

714-B, T=215 °C 911-B1, T=215 °C

1.00E+03

1.00E+02

1.00E+01

1.00E+03

1.00E+02

1.00E+01 1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+00

1.00E+01

Shear Rate (1/s)

1.00E+02

1.00E+03

1.00E+04

Shear Rate (1/s)

FIGURE 24.41 Viscosity: shear rate curves for TPO.

quality assurance purposes and the viscosity measured at 1200 s1 is taken as the standard value, although it has little relevance to the extrusion process where shear rates are typically in the range of 80e220 s1 or injection molding where higher shear rates up to 100,000 s1 can be encountered. Melt temperature has much less influence on flow with TPV than TPO, Figures 24.40 and 24.41. Comparing a standard grade of Santoprene TPV with TPO, it can be seen that the TPO has a lower melt viscosity and higher melt flow capability. It is also seen that in the Santoprene TPV, there is a convergence of viscosity at lower shear rates independent of temperature. This effect is discussed further in Section 24.4.5.2. In order to improve the melt flow of Santoprene TPV for more demanding injection-molding applications (thin section or long flow lengths), it is necessary to modify/change the nature of the PP phase. Tables 24.5e24.7 compare some key properties and the differences (from published literature) between a mid-range Shore A hardness (rubber scale) SantopreneÔ TPV and an example of a Shore D hardness (plastic scale) grade. Where applicable, test methods used to generate the data are quoted. Both products are black grades. The purpose is to highlight the wide range of properties available by careful selection and manipulation of the TPV compound constituents. The TPV grades range from Shore A 25 to Shore D 50 hardness scales including both natural (light brown) and black, and using alternative cure chemistry (proprietary) white colorable SantopreneÔ 8000 grades. This wide range allows for grade positioning in all market areas including demanding static and semidynamic sealing applications in the automotive, construction, and medical markets where “rubber-like” properties are required. Table 24.5 illustrates that the influence of heat aging on SantopreneÔ TPV is largely independent of hardness. Heat resistance is related to the saturated chain structure of both the fully cured EPDM and the PP. At extended time periods above the recommended upper service temperature (135 C), in this case 150 C, the resistance performance drops and this can be attributed to thermo-oxidative degradation of the polymer arising from volatilization of antioxidants and oxidation of process oil. The inherent chemical resistance is very good across the hardness range, the reason being, the chemical resistance afforded by the continuous PP phase to the EPDM particles. As expected, this type of TPV is resistant to solvents of a polar nature and to a lesser extent nonpolar solvents. The degree of swell also is reflected in the proportion of EPDM present in a particular grade, the softer the grade the higher the EPDM content, therefore less PP and greater susceptibility to swell.

24.4.5.3 Thermoplastic Polyester Table 24.8 gives examples of the typical properties and the range of values that might be expected depending on grade selection. These will vary as previously discussed, depending on the copolymerization precursors and other additives selected by each manufacturer of these materials. The table includes the range of values for both polyester and polyether ester TPE materials. As with all of the TPE family of materials, continued development is producing new grades having enhanced higher temperature resistance, better abrasion resistance, increased flex fatigue resistance, and excellent strength properties over a wide range of temperatures. In more general terms, the key characteristics of these materials are: l l l l l l

excellent dynamic properties, for example creep and fatigue; exceptional resistance to oils and greases, good general resistance to chemicals; excellent strength over a wide range of temperatures; excellent heat resistance (long term 165 C); good electrical insulation properties; low moisture absorption, excellent dimensional stability.


683

TABLE 24.5 Heat Aging Comparison Between Shore A 64 and Shore D 40 SantopreneÔ TPV Property

Test Standard

Unit

SantopreneÔ TPV 101e64

ASTM D573 Heat Aging 168 h 100 C

1000 h 100 C

168 h 150 C

1000 h 150 C

SantopreneÔ TPV 103e40

% Change

Tensile strength

ASTM D412

%

99

96

Elongation at break

ASTM D412

%

105

100

100% Modulus

ASTM D412

%

98

106

Hardness

ASTM D2240

Shore unit

0

0

Tensile strength

ASTM D412

%

103

95

Elongation at break

ASTM D412

%

105

96

100% Modulus

ASTM D412

%

101

111

Hardness

ASTM D2240

Shore unit

2

0

Tensile strength

ASTM D412

%

116

94

Elongation at break

ASTM D412

%

111

74

100% Modulus

ASTM D412

%

104

119

Hardness

ASTM D2240

Shore unit

3

6

Tensile strength

ASTM D412

%

50

61

Elongation at break

ASTM D412

%

16

2

100% Modulus

ASTM D412

%

e

e

Hardness

ASTM D2240

Shore unit

9

10

However, this TPE does require the addition of UV stabilizer systems to protect for exterior use. Table 24.8 gives an overview of the property range that can be expected.

24.4.5.4 Thermoplastic Polyurethane TPU elastomer excels in offering a wide and effective combination of physical properties and attributes over a range of hardnesses. Properties of commercially available TPU elastomer include: l l l l

high abrasion resistance, low-temperature performance, high shear strength, high elasticity,


TABLE 24.6 Fluid Resistance Comparison Between Shore A 64 and Shore D 40 SantopreneÔ TPV Temperature (8C)

Solvent

Test Standard

Unit

SantopreneÔ 101 e64

Fluid Resistance L166 h Immersion

SantopreneÔ 103 e40

Weight Change

50% sodium hydroxide (NaOH)

23

ASTM D471

%

0

0

98% sulfuric acid (H2SO4)

23

ASTM D471

%

5

2

IRM 903 oil (aromatic)

100

ASTM D471

%

80

29

ASTM no. 1 oil (aliphatic)

100

ASTM D471

%

30

6

Extracted from Fluid Resistance brochure, by permission of ExxonMobil.

TABLE 24.7 Weathering and Electrical Properties Comparison Between Shore A 64 and Shore D 40 SantopreneÔ TPV Conditions

Property

Test Standard

Unit

SantopreneÔ 101e64

SantopreneÔ 103e40

Weathering e Outdoor Exposure 1987e1991 Florida exposure 6 months

Florida exposure 48 months

Arizona exposure 6 months

Arizona exposure 48 months

Tensile strength

ASTM D412

%

22

2

Ultimate elongation

ASTM D412

%

22

0

Delta E color shift

SAE J1545

e

6.25

2.67

Tensile strength

ASTM D412

%

27

2

Ultimate elongation

ASTM D412

%

22

12

Delta E color shift

SAE J1545

e

1.65

11.67

Tensile strength

ASTM D412

%

25

5

Ultimate elongation

ASTM D412

%

31

5

Delta E color shift

SAE J1545

e

5.22

3.06

Tensile strength

ASTM D412

%

25

4

Ultimate elongation

ASTM D412

%

27

7

Delta E color shift

SAE J1545

e

1.47

11.91

Dielectric constant

e

V mil

2.3

2.3

Dielectric strength (3.17 mm, 19.6 kV mm1)

e

500

500

Electrical Properties

By permission of ExxonMobil.

1


685

TABLE 24.8 Typical Properties of Polyester/Copolyether Ester-Based TPE Property

Unit

Test Method

Value

kg m3

ISO 1183

940e1480

Melt flow (2.16 kg/230 C)

g/10 min

ISO 1133

0.3e58

Water absorption

%

ISO 62

0.1e12

Hardness shore

A/D

ISO 868

65Ae80D

Tensile strength at break

MPa

ISO 527

0.56e104

Tensile strength at yield

MPa

ISO 527

1.18e72.2

Physical Density

Mechanical

Flexural modulus

GPa

ISO 178

0.013e9.03

Elongation at break

%

ISO 527

1.9e900

Elongation at yield

%

ISO 178

10e74

2

Notched Izod impact strength

kJ m

ISO 180/1A

3e9

Tear strength

kN m1

ISO 34

40e362

Compression set

%

ISO 8115

50e60

Abrasion resistance (Taber)

mg/1000 cycles

5e90

Thermal Melting temperature (10 C/10 min)

C

ISO 11357

145e223

Vicat softening point

C

ISO 306

42e213

Maximum service temperature

C

e

161

Brittleness temperature

C

e

100 to 40

Glass transition (Tg)

C

e

78 to 50

IEC 60695-11e10

HBeVO

UL 94 flammability Electrical Dielectric constant

l l l l l l l

IEC 60250

3.3e20

Dielectric strength

kV mm1

IEC 60243-1

11.8e34

Comparative tracking index

V

IEC 60112

300e600

transparency, oil and grease resistance, good compression set, impact resistance (toughness), tear resistance, hydrocarbon resistance (polyester type), hydrolytic resistance (polyether type).

Similar to the copolyester TPEs, the final product attributes can be manipulated by the selection of the polymerization precursor. TPU elastomers require stabilization for exterior use otherwise they tend to become brittle, losing mechanical properties, and also yellowing with aging. TPU elastomers can be divided mainly in two groups, based on soft segment chemistry: l l

polyester-based TPUs (mainly derived from adipic acid esters); polyether-based TPUs (mainly based on tetrahydrofuran (THF) ethers).


TABLE 24.9 Typical Properties of Polyether and Polyester-Based TPU Elastomer Property

Unit

Test Method

Value

kg3

ISO 1183

992e1530


g/10 min

ISO 1133

1.20e150

Maximum water content

%

e

0.02e0.05

Hardness shore

A/D

ISO 868

40Ae95D


MPa

ISO 527

1.4e72


MPa

ISO 527

1.1e66

Flexural modulus

GPa

ISO 178

0.017e1.31

Elongation at break

%

ISO 527

10e1580

ISO 180/1A

0.849e5340

Physical Density

Mechanical


1

Jm

1

Tear strength

kN

ISO 34

12.4e285

Compression set

%

ISO 815

12e90


mg/1000 cycles

0.7e350

Thermal Melting point (10 C/10 min)

C

ISO 11357

100e230


C

ISO 306

56e145


C

e

70 to 35


C

UL 94 flammability

e

47 to þ120

IEC 60695-11e10

HBeV0

IEC 60250

4.1e8

Electrical Dielectric constant 1

Dielectric strength

kV mm

IEC 60243-1

31e49


V

IEC 60112

600

The main differences between these two groups are the higher hydrolysis and microbial resistance of the polyetherbased TPU elastomers. Table 24.9 gives an overview of the property range that can be expected from this grade range.

24.4.5.5 Styrenic-Based TPEs At room temperature, styrenic block copolymers display more “rubber-like” characteristics than many other TPEs. To maximize these properties, the SBS or SEBS base needs compounding with other polymers, oil, and additives. All styrenic-based TPEs generally have good chemical resistance to water, acids, bases, and polar solvents but poor resistance to hydrocarbon-based oils, fuels, and solvents. Saturated styrenic-based TPEs exhibit improved oxygen, ozone, UV radiation, and general chemical resistance than the unsaturated versions. However, they still require additives to achieve this: l l l l l l

Hardness range from 20 Shore A to 50 Shore D. Excellent flexural fatigue resistance. Good tear and abrasion resistance. High impact strength. Good electrical properties. Possess low compression set.


687

TABLE 24.10 Typical Properties of Styrenic-Based TPE Property

Unit

Test Method

Value

kg m3

ISO 1183

870e1100


g/10 min

ISO 1133

0.1e84

Water absorption

%

ISO 62

0.05

Hardness Shore

A/D

ISO 868

6Ae69D


MPa

ISO 527

0.345e80


MPa

ISO 527

0.207e29

Modulus of elasticity

GPa

ISO 178

0.018e0.207

Elongation at break

%

ISO 527

3.8e1900

ISO 180/1A

0.641e5340

Physical Density

Mechanical

2


kJ m

1

Tear strength

kN m

ISO 34

2e113

Compression set (RT)

%

ISO 815

8e90


mg/1000 cycles

30e54

Thermal Maximum service temperature (air)

C

ISO 11357

50e170

Melt mass-flow rate (2.16 kg/230 C)

g/10 min

ISO 1133

0.1e84

Ring and ball softening point

C

ISO 306

128e135


C

e

65 to 21

IEC 60695-11e10

HBeV0

IEC 60250

2.1e2.8

UL 94 flammability Electrical Dielectric constant

l l

1

Dielectric strength

kV mm

IEC 60243-1

24e50


V

IEC 60112

550e600

Colorability. Resistance to low and high temperatures from 30 to þ110 C.

The typical range of properties achievable with styrenic-based TPEs is shown in Table 24.10. Due to the large number of compounders promoting these materials, the property range is illustrative of what might be expected. General chemical resistance is good, but resistance to hydrocarbons is poor to moderate. Table 24.10 gives an overview of the property range that can be expected from this range of TPE grades.

24.4.5.6 Polyether Block Amides Polyether block amides find a niche part of the market due to some of their unique properties. These properties find their value in films, sports equipment, and automotive applications. They do however have a narrower hardness range being limited to the harder Shore D hardness range. Like all the TPE family, these materials also need UV resistance additives to be incorporated. In general terms, these TPEs show the following property characteristics: l l l l

excellent low-temperature impact strength; high elasticity and good resilience; moderate hydrolysis resistance; very good heat resistance;


TABLE 24.11 Typical Properties of Polyether Amide-Based TPE Property

Unit

Test Method

Value

kg m3

ISO 1183

1000e1120

g/10 min

ISO 1133

4e10

%

ISO 62

11e120

Hardness Shore (15 s)

A/D

ISO 868

40De75D

Tensile stress at break

MPa

ASTM D638

32e56

Tensile stress at yield

MPa

ASTM D638

12e62

Flexural modulus

GPa

ISO 178

12e2200

Elongation at break

%

ASTM D638

10 to >750

Physical Density

Melt index (1 kg/235 C)

Water absorption saturation (24 h/23 C) Mechanical

Tensile strain at yield

%

ASTM D638

6e25

Notched Izod impact strength (23 C)

J m1

ASTM D256

NBe847P

Tear strength (notched)

kN m1

ISO 34

44e166

Compression set (70 h/23 C)

%

ISO 815

19e54


mm (1 kg/1000 cycles)

ISO 9352

54e99

ISO 11357

133e204

Thermal Melting temperature (10 C/10 min)

C

Melt index (1 kg/235 C)

g/10 min

ISO 1133

4e10


ISO 306

58e173

C

Electrical

l l l l l l

Surface resistivity

Ohm sq1

IEC3 60093

109

Volume resistivity

Ohm cm

IEC 60093

>109

good overall chemical resistance; good antistatic properties; good damping properties; can be transparent; excellent permeability/breathability in film; excellent bonding to polyamide engineering materials. Table 24.11 gives an overview of the property range that can be expected from this grade slate.

24.4.5.7 Melt Processable Rubber This TPE, today, is limited to the range of grades supplied by Advanced Polymer Alloys under the AlcrynÒ trade name. The properties shown by these materials lend themselves to applications that require enhanced grease, oil, and fuel resistance, and due to their higher compression set characteristics they are limited to the nonsealing application areas. Unlike the copolyester and polyether amide TPEs, the hardness range spans Shore A to Shore D. Key characteristics of these materials are: l l l l

good noise-dampening; similar stress relaxation properties to nitrile rubber; excellent chemical resistance; good physical properties;


689

TABLE 24.12 Typical Properties of MPR TPE Property

Unit

Test Method

Value

g cm3

ASTM D471

1.181.99


g/10 min

e

0.08e150

Water absorption

%

e

0.3e7

Hardness shore

A/D

ISO 188

64Ae80D


MPa

ASTM D412

3e44

Physical Density

Mechanical


MPa

e

8.314.0

Flexural modulus

GPa

e

0.0021e2.26

Elongation at break

%

ASTM D412

9.5e1660

Elongation at yield

%

ASTM D412

16e100

Tear strength (graves)

kN m1

ASTM D624

16e62

Compression set

%

ASTM D395

3.9e85


mg/1000 cycles

ASTM D3389

0.0013e581

Thermal Melting point (10 C/10 min)

C

e

33e140


C

e

38e93


C

ASTM D746

100 to 20


C

e

64 to þ7

e

18.3e767

UL 94 flame spread

l l l l l

1

mm min

wide range of densities; low water absorption; good low-temperature impact resistance; good tear strength; UL 94 flammability ranging from HB to V0. Table 24.12 gives an overview of the property range that can be expected from this grade slate.

24.5 PROCESSING OF TPE MATERIALS The TPE thermoplastic melt behavior of these materials allows them to be processed in existing conventional thermoplastic machinery. This enables plastics fabricators to produce parts with some of the rubber properties previously only seen in parts manufactured from thermoset rubber. Today, TPE is utilized in all the major fabrication processes, for example, injection molding, extrusion, blow molding, calendaring, film extrusion, and thermoforming. In most circumstances, the use of regrind is possible. Typically, levels of up to 30% are suggested by manufacturers. With all TPEs, it is important to consider the storage and drying processes when utilizing regrind with virgin material. Moisture absorption is increased as the material’s particle size reduces. Regrind should be used as soon after grinding as is practicable. Certain processing properties are not just associated to the material itself but also reliant on processing factors. In extrusion of profile for example, part cooling is related to the profile dimensions, especially thickness, the length of the cooling bath, water temperature, level of water agitation, and haul-off rate (profile stretch) can lead to dimensional control issues. Similarly, in injection and blow molding, process conditions control final part quality and dimensional control. Shrinkage of molded parts is influenced not only by the material itself but also by factors, such as:


l l l l

l

part design and complexity; wall thickness; gate design in molded components; processing conditions, in particular melt temperature, injection pressure, holding pressure, extrusion rate, haul-off rate, cooling efficiency; mold and die temperature.

Total shrinkage is a result of immediate molding or haul-off shrinkage or stretching in blow-molding shrinkage, and post-shrinkage which occurs not only during annealing, but also during longer-time storage of the parts. This is also reflective of hardness TPE grade selected. Shore D grades tending to display lower shrinkage values than softer Share A grades at equivalent wall thickness. Apart from mono-material injection molding, the following methods are suitable for combinations with other thermoplastic materials: l

l

l

Multicomponent injection molding of TPE elastomers with other compatible plastic materials creates good bonding without the need for additional bond promoting additives or relying on purely mechanical adhesion. The key criteria is to select grades designed to adhere to more rigid substrates by either their inherent chemistry being compatible or grades modified with additives to promote adhesion specific to dissimilar materials. Sandwich injection molding is a method of multicomponent injection molding using a core component in combination with a different plastic material which is molded in a complete outer skin layer. Besides the combination of different thermoplastics, it is possible to use cheaper regrind as the core component and virgin grades as the outer skin thus maintaining part appearance. It is also possible to reduce the weight of the part by the use of chemical foaming of the core material. Gas injection molding is in principle similar to sandwich molding. Gas is injected as core component to produce a hollow central section; this is most commonly used for weight reduction.

The purpose of Tables 24.13e24.15 is to give an appreciation of the range of processing condition by TPE type and process. Focus has been placed on the most used techniques where possible, principally injection molding and extrusion. In most cases, data shown is a compilation of various suppliers-published literature. TABLE 24.13 Typical Molding Conditions TPV Injection Molding Conditions (SantopreneÔ , 2011) Processing Parameter

Value

Barrel set temperature Rear

160e175 C

Middle

180e200 C

Front

195e210 C

Nozzle

200e230 C

Melt temperature

195e240 C

Injection speed

Fast-profiled

Injection time

0.5e1.5 s

Screw speed

100e200 rpm

Injection pressure

2.1e2.8 MPa

Hold pressure

w50% injection pressure

Hold time

2e3 s

Screw cushion

3e6 mm

Cooling time

20e120 s

Screw back pressure

0.2e0.5 MPa

Mold temperature

20e30 C

Mold clamp pressure

41e69 MPa


691

TABLE 24.14 Typical Extrusion Parameters for TPV Elastomers TPV Elastomer Extrusion Parameters (SantopreneÔ , 2011) Processing Parameter

Value

Barrel set temperature Feed

165e195 C

Zone 2

170e195 C

Zone 3

175e200 C

Zone 4

180e205 C

Head

190e210 C

Die

195e215 C

Melt temperature

190e215 C

Screw compression ratio

2.5e3.5

General purpose/barrier screw L/D

24e30:1

Head pressure (max)

5.0e20 MPa

24.5.1 Processing SantopreneÔ TPV The linear flow relationship of shear rate to viscosity does prove to be an advantage in processing Santoprene TPV. It enables the material to be processed equally well in low shear processes (10e220 s1), extrusion, blow molding, vacuum forming, or high shear injection molding (10,000e100,000 s1). The material is able to accommodate this wide range of processes because the melt viscosity responds more readily to work done than to a temperature increase as shown in Figure 24.42. For the low shear processes, the melt integrity is such that the material retains enough melt strength to enable profile, parison, and stretching to occur without melt tearing. The limiting factor in extrusion blow molding is the EPDM content, below about shore hardness A 73, the parison is prone to tearing with a blow ratio greater than 2. The two principal conversion techniques are injection molding and extrusion. For all conversion processes, there is a requirement to dry all SantopreneÔ TPV grades (exception being Santoprene 8000 Series) before processing. Santoprene TPV is slightly hygroscopic due to the resin cure chemistry employed. The chart in Figure 24.43 illustrates the influence of

TABLE 24.15 Typical Blow-Molding Conditions SantopreneÔ TPV Elastomer Blow Molding Parameters (Advanced Elastomer Systems) Processing Parameter

Value


170e195 C

Middle

180e200 C

Front

190e205 C

Head zone 1

195e215 C

Head zone 2

195e215 C

Die

195e220 C

Blowing/cooling time

25e120 s

Blow ratio

2e5 (grade dependent)

Melt temperature

200e215 C

Predrying

Air circulating or dehumidifier 2e4 h at 80 C


Apparent Viscosity, Poise

Apparent Viscosity, Poise 105

105

104

104

103

103

102

101

102 103 Apparent Shear rete, S’

102

104

190 205 220 Temperature,°C

FIGURE 24.42 Shear rate and temperature effects on SantopreneÔ TPV melt viscosity.

SANTOPRENE RUBBER 103-40 & 101-73 MOISTURE CONTENT, %

0.20

Dessicart drying @ 80°C.

73A

0.16

Maximum moisture level when packed

0.12 0.08

Maximum moisture level for optimal processing

40D

0.04 0.00 0

1

2

3

4

5

6

7

8

9

DRYING TIME, hours FIGURE 24.43 Drying time of SantopreneÔ TPV at the recommended 80 C.

drying time on the moisture content. By utilizing a desiccant type or air-circulating drier, it is possible to achieve low moisture contents. This helps to reduce or remove gate silvering, poor surface finish, and internal voids in injection molding and poor surface, voids, dimensional instabilities, and plate-out in extrusion. From the graph it can be seen that a minimum drying time of 3 h is required to reach the maximum moisture level suitable for all types of processing. SantopreneÔ TPV can be used to extrude parts ranging from simple single material extrusions to multimaterial/grade complex profiles and tubes. These materials have a good green strength lending stability to the extrudate and generally exhibit low levels of die swell. The softer grades below Shore A 87 do not need calibrating unless combined with for example, a PP in a co-extrusion situation. Blow molding Santoprene TPV is processed as single layer, multilayer, blow molding, sequential material moldings as well as by extrusion, three-dimensional flashless extrusion, injection, and press blow-molding techniques.

24.5.2 Copolyester-Based TPE This TPE is hygroscopic and as such the manufacturers recommend that they must be predried prior to their fabrication. Typical process conditions are given in Tables 24.16e24.18.

24.5.3 Thermoplastic Polyurethane Elastomers The two most important manufacturing methods with TPU elastomers are extrusion and injection molding. TPU elastomers can be used with most of the current injection-molding techniques previously discussed. Both the equipment and methods


TABLE 24.16 Typical Injection-Molding Conditions Copolyester Injection Molding Conditions (HytrelÒ, 2000) Processing Parameter

Value


165e245 C

Middle

190e245 C

Front

190e245 C

Nozzle

180e250 C

Melt temperature

180e250 C

Injection speed

Moderate to high

Screw speed

100 rpm

Hold pressure

4e8 s

Screw cushion

Yes

Back pressure

3e100 bar

Mold temperature

20e50 C

Drying temperature

70e110 C

Drying time

2e6 h

Mold clamp pressure

48e69 MPa

TABLE 24.17 Typical Extrusion Parameters for Copolyester Elastomer Copolyester Elastomer Extrusion Parameters (HytrelÒ, 2000) Processing Parameter

Value

Barrel set temperature Feed

140e220 C

Zone 2

220e250 C

Zone 3

235e240 C

Zone 4

235e240 C

Head

230e240 C

Die

230e240 C

Melt temperature

165e250 C

Cooling water

15e35 C

Compression ratio

3e3.5

Screw L/D

25:1 min

Screw type

Barrier preferred

Predrying

80e110 C/3e4 h

693


TABLE 24.18 Typical Blow-Molding Conditions Copolyester Elastomer Blow Molding Parameters (ArnitelÒ, 2014) Processing Parameter

Value


200e220 C

Middle

210e230 C

Front

220e240 C

Accumulator

220e240 C

Die

220e240 C

Blow ratio

Low

Melt temperature

230e250 C

Mold temperature

15e50 C

Predrying

Dehumidifier 3e4 h/110 C

TABLE 24.19 Typical Drying Conditions TPU Elastomer Drying Conditions (Mat Web Material Property Data) Temperature (8C) Grade Hardness

Air-Circulating Drier

Desiccant Drier

Drying Time (h)

< Shore A 90

100e110

80e90

2e4

> Shore A 90

100e120

90e120

2e4

normally used for the extrusion or injection molding of a conventional thermoplastic are generally suitable for TPU elastomers. In order to ensure optimal performance properties in the finished TPU elastomers parts, it is required that the material is dried before processing irrespective of the technique employed. TPU elastomers by their polar nature are hygroscopic. Drying conditions are shown in Table 24.19.

Injection Molding Injection-molding machines with single-flighted, three-zone screws with a maximum compression ratio of 1:2 are suitable for the processing of TPU elastomers. High shear screws are not recommended for TPU elastomers due to their shear sensitivity. (Typical injection molding conditions are shown in Table 24.20.)

Extrusion TPU elastomers are used to produce profiles, tubing, and cable sheathing products. Table 24.21 shows some typical examples of the extrusion parameters used to manufacture extruded parts.

24.5.4 Styrenic-Based TPEs This broad family of compounded TPE materials have a wide range of performance offerings with the further addition of polymers for example, PP, polyphenylene oxide (PPO), oils, and other extenders and fillers. This in turn leads to a wider variety of processing conditions than some of the other TPE materials. Compounds based on SBS and SEBS are commercially available but from a processing perspective, SBS-based compounds tend to display inferior thermal stability when overheated or if the material is too heavily worked.


695

TABLE 24.20 Typical Injection-Molding Conditions TPU Elastomer Injection Molding Conditions (Mat Web Material Property Data) Processing Parameter

Value


130e220 C

Middle

200 C

Front

205e210 C

Nozzle

205e240 C

Melt temperature

150e235 C

Injection speed

>10 mm s1

Cycle time

80e180 s

Screw speed

60e200 rpm

Back pressure

0.5e1.2 MPa

Mold temperature

16e70 C

Mold clamp pressure

0.6e1.2 MPa

Tables 24.22e24.25 provide a general guide to the typical processing parameters associated with SEBS-based materials for most used injection-molding and extrusion conversion processes.

24.5.5 Polyether Block Amides The example processing conditions are taken as a summary of the range across the grade range and are taken from the processing guidelines. These grades can be processed either as mono-material products or as a part of a multimaterial combination. Similar to other hygroscopic TPEs, drying and control of dried and undried materials is important in order to achieve optimal processing performance and part quality.

TABLE 24.21 Typical Extrusion Parameters for TPU Elastomers TPU Elastomer Extrusion Parameters (Mat Web Material Property Data) Processing Parameter

Value (Metric)


140e190 C

Middle

175e200 C

Front

175e205 C

Die

205e215 C

Screw-type 3 zone

Single screw or barrier


1.2e1.3

Screw L/D

25e30

Flight clearance

0.1e0.2 mm

Melt temperature

205e215 C

Adapter pressure (max)

30 MPa

Calibration die

Yes


TABLE 24.22 Typical Injection-Molding Conditions Styrenic Elastomer Injection Molding Conditions (Thermolast Product Properties and Processing; Injection Molding) Processing Parameter

Value


40e75 C

Middle

175e190 C

Front

190e200 C

Nozzle

200e220 C

Melt temperature

180e220 C

Injection speed

Fast-profiled

Injection time

1e5 s

Screw speed

25e150 rpm

Injection pressure

1.4e5.5 MPa

Hold pressure

40e60% injection pressure

Screw cushion

Yes

Cooling time

15e40 s

Back pressure

0.2e1 MPa

Mold temperature

5e50 C

24.5.6 Melt Processable Rubber The single supplier of MPR, AlcrynÒ is from Advanced Polymer Alloys. Tables 24.26e24.28 are a summary of selected values taken from their inline literature over their grade range. What is different is the lower process temperature range when compared generally with other TPE types. This is related in part to the presence of chlorinated polymers in the products.

TABLE 24.23 Typical Extrusion Conditions Styrenic Based Elastomer Extrusion Parameters (Thermolast Product Properties and Processing) Processing Parameter

Value


140e170 C

Middle

150e190 C

Front

160e200 C

Head

170e180 C

Die

180e220 C


2.5e4.5:1

Screw L/D

>24:1


TABLE 24.24 Typical Injection-Molding Conditions Polyether Amide Injection-Molding Conditions (Mat Web Material Property Data) Processing Parameter

Value

Processing temperature

220e260 C

Nozzle temperature

200e270 C

Melt temperature

210e270 C

Injection screw L/D

18e22

Compression ratio

2.2:1e2.8:1

Drying temperature

55e80 C

Drying time

4e8 h

Mold temperature

10e60 C

TABLE 24.25 Typical Extrusion Conditions Polyether Amide Elastomer Extrusion Parameters (Mat Web Material Property Data) Processing Parameter

Value (Metric)

Processing temperature

170e250 C

Drying temperature

60e80 C

Dry time

4e8 h

TABLE 24.26 Typical Injection-Molding Conditions MPR Injection Molding Conditions (AlcrynÒ) Processing Parameter

Value


171e177 C

Front

171e177 C

Nozzle

171e177 C

Melt temperature

170e190 C

Injection speed

16e49 mm se1

Injection time

0.5e2 s

Screw speed

50e100 rpm

Mold temperature

21e50 C

Injection pressure

4.83e8.27 MPa

Hold pressure

2e5.5 MPa

Cooling time

2e30 s

Back pressure

0.2e0.6 MPa

697


TABLE 24.27 Typical Extrusion Conditions MPR Extrusion Parameters (AlcrynÒ) Processing Parameter

Value

Barrel set temperature (Profiled) Rear

150e165 C

Middle

160e185 C

Die

163e180 C


2:1e3.5:1

Screw L/D

20:1e24:1

Melt temperature

170e190 C

Draw down