Crosslinked polyethylene - NOPR

Indian Journal of Chemical Technology Vol. 11, November 2004, pp. 853-864

Crosslinked polyethylene S M Tamboli, S T Mhaske & D D Kale* Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, India Received 12 June 2003: revised received 22 July 2004; accepted 4 August 2004 Properties of polyolefins can be modified by crosslinking process. Different methods of crosslinking and effect of process parameters, selection of crosslinking agents and applications are briefly discussed. IPC Code: C 08 F 2/00 Keywords: Crosslinking, polyethylene, crosslinking agents.

Polyethylenes are commodity plastics. They account for more than 70% of total plastics market. Polyethylene is easily available, at relatively low cost and easily processable. It finds applications in household items, packaging, insulation, net ropes, fishing rods or medical applications, etc. Polyethylene is processed at temperature in the range 150-250°C1-3. Most polyethylene compounds contain reasonably good amount of fillers. Polyethylenes are thermoplastic in nature and therefore they can be reprocessed repeatedly. Polyethylene, however, will soften and flow, and lose critical physical properties at elevated temperature thereby limiting its applications4,5. Therefore, crosslinking of polyethylene is carried out to retain desirable properties at high temperature. Crosslinking will change the nature of polymer from thermoplastic to thermoset to yield a non melting, more durable polymer matrix. All types of important polyethylenes are crosslinked, like Linear low density polyethylene (LLDPE), Low density polyethylene (LDPE), High density polyethylene (HDPE) and Ethyl vinyl acetate copolymer (EVA) and Polyolefinic elastomer (POE). Branched structure is more suitable for crosslinking. Therefore, crosslinking of LLDPE and HDPE requires more attention. Crosslinking leads to the formation of insoluble and infusible polymers in which polymer chains are joined together to form three-dimensional network structure6-8. In thermoset, crosslinking (curing) takes place through reaction between polymer chains with _________ *For correspondence (E-mail: [email protected])

several functional groups. These functional groups are capable of forming chemical bonds to convert thermoplastics into thermosets9,10. McGrins11 has described various commercially important crosslinked thermoset materials and their curing reactions. These are not of much relevance in the present study. For thermoplastics, crosslinking is a process in which high molecular weight thermoplastics are converted into thermosets. Crosslinked polyethylenes are either extruded or injection moulded. When degree of crosslinking is deliberately maintained very low, the resulting compound is termed as crosslinkable polymer. Crosslinking can be combined with foaming also. Crosslinking of biopolymers and foaming is very common in food industry. Crosslinking for partially crosslinked extruded profile is commonly employed in furniture. Although crosslinking of thermoplastics such as nylon, polypropylene and styrenics has received attention in literature. Present review is directed to the crosslinking of polyethylenes only. Crosslinked polyethylene forms a dense network of high molecular weight, which improves impact strength, environmental stress crack resistance (ESCR), creep and abrasion resistance without influencing tensile strength and density to any appreciable extent. Crosslinked polyethylene finds wide applications in packaging and electrical insulation applications and rotomoulding 12,13 applications . The degree of crosslinking can change considerably from applications to applications. Some aspects of crosslinking are reviewed here.

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INDIAN J. CHEM. TECHNOL., NOVEMBER 2004

Crosslinking process

Crosslinking is a process in which carbon atoms of same or different polyethylene chains are joined together to form the three-dimensional network structure14-16. The crosslinking process essentially forms bonds between the polymer chains, which could be directly between carbon to carbon or a chemical bridge linking two or more carbon atoms18. The main difference between thermoplastic and crosslinked polymer is that, at temperature above the crystalline melting point crosslinked polymer behaves as a soft rubber while thermoplastic has no significant strength above melting temperature. The changes in the properties of polyethylene due to crosslinking have been compared and documented in literature16,17. Thus, crosslinking reduces the melt index and elongation at break, while improves the impact strength, creep resistance, resistance to slow crack growth and also environmental stress crack resistance (ESCR). The density and tensile strength of polyethylene are not influenced by crosslinking. The crosslinking of polyethylene takes place in four stages: initiation, propagation, branching and termination. The principal reaction involved in each step is discussed below. Initiation

The first step in crosslinking process is generation of free radicals, which can be through a chemical reaction or radiation energy. Decomposition of initiators which are normally peroxides, or highenergy radiations abstracts hydrogen atom from the backbone of polymer chain to produce free radicals. a) Peroxide decomposition ROOR RO* + PH b) High energy radiation hv PH

2 RO* ROH + P*

H* + P*

Propagation and branching

The free radicals react with atmospheric oxygen to generate peroxide radicals and through series of reaction crosslinking takes place, these are described by Peacock19. Crosslinking causes a dense network of different polymer chains through chemical bonding. O2 P* POO* POO*+PH POOH + P*

Fig.1⎯Schematic view of crosslinked and uncrosslinked polyethylene

Branching POOH PO* + PH

PO* + OH* POH + P*

When P* on two sites join, it leads to branching or network formation. Termination

Termination takes place by quenching of free radicals due to presence of additives, impurities etc. P* + P* POO* + POO* PO* + H*

P–P POOP + O2 POH

Presence of side branches in a polyethylene chain is a reason for variation in number of important physical properties such as density, hardness, flexibility and melt viscosity. Presence of branches is the point in the molecular network where oxidation may take place. Crosslinking takes place between carbon atom in neighboring chains or chain branches joined together with other branches of chain or with the same chain of polymer. This is depicted schematically in Fig. 1. The polyethylenes have different structures depending upon manufacturing process. Low density polyethylene is highly branched, while high density polyethylene and linear low density polyethylene are linear polymers. In general, branched polymers are easy to crosslink as compared to linear polymers, since formation of network is more probable for branched polymers20.

TAMBOLI et al.: CROSSLINKED POLYETHYLENE

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Table 2⎯Types of radiation sources Particulate

Non particulate

α - particles β - particles High energy electron Protons Deuterons Neutron

Microwave Infrared X-ray γ-ray Light energies (UV)

The relative scission to crosslinking ratio is given by 22 Fig.

β 1 G (S) = α 2 G (X)

2⎯Schematic representation of radiation crosslinking

Table 1⎯G(X) and G(S) values of some polymers Polymers

G(X)

G(S)

LDPE HDPE Atactic PP Isotactic PP Polyvinyl chloride Polypropylene oxide Nylon (6 & 6,6) Polyvinyl acetate Polybutadiene Polystyrene Polymethyl acrylate Polymethyl methacrylate

1.4 2.1 0.12 – 0.27 0.07 – 0.14 2.15 0.15 0.5 0.1 – 0.3 3.8 0.045 0.55 -

0.8 1.3 0.10 – 0.24 0.10 – 0.27 0.22 0.6 0.06 – 0.17 < 0.018 0.18 1.22 – 3.5

Crosslinking process is carried out by using (i) Physical or (ii) Chemical crosslinking methods. Physical crosslinking

In this method, crosslinking is obtained by free radical mechanism. The free radical is generated in polymer chain by using high energy radiations21. This process is shown schematically in Fig. 2. Thus, a free radical is generated by the high energy. Two or more chains, then, join together where the free radical is generated. High energy radiation on polymeric material gives chain scission or crosslinking. The changes in physical and chemical properties depend upon the efficiency of crosslinking reaction and its relative ratio with degradation. Table 1 shows the number of crosslinking and chain scission per 100eV radiant energy absorption for different polymers.

… (1)

Where, α = probability of crosslinking of chains after one electron volt of energy absorbed. β = probability of chain scission after one electron volt of energy absorbed. G(X) = number of crosslinking per 100eV radiant energy absorbed. G(S) = number of scission per 100eV of energy absorbed. It is known that bond energy for cleavage of C-H bond is 364 kJ/mol. The electron beam having energy sufficient to break C-H bond is suitable for crosslinking 23. Crosslinking of polymers by radiation and their technology involve four main variables. (i) (ii) (iii) (iv)

Type of radiation and its sources. The nature of polymer structure to be irradiated. Mechanism and theories of reactions. Physical, chemical and mechanical properties of network formation.

Some of these radiation crosslinking are described briefly. Radiation induced crosslinking of thermoplastics can be carried out using particulate or non-particulate radiations. These are listed in Table 2. Particulate radiation sources are not commercially used. Only non-particulate radiation sources are used for commercial crosslinking of thermoplastics by radiation. Crosslinking by radiation mainly depends upon photon energy of radiation sources. The higher the photon energy, the more the penetration taking place and higher crosslinking is obtained. The photon


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energy gained by polymer in UHF field is given by the following equation31,

Table 3⎯Wavelength and photon energy of some radiations Type of radiation

Wavelength (nm)

Photon [MeV]

Infrared UV Soft X - ray

1250 125 12.5 1.25 0.125 0.0125 0.001

100 101 102 103

X - ray γ - rays

energy

… (3)

Where,

104 105 1 2 × 106

energy of radiation is relatively dependent on wavelength. Table 3 depicts the relationship between radiation sources, wavelength and photon energies. Selection of radiation sources mainly depends upon availability, the radiation penetration rate required, the dose rate and impact on manufacturing process (product handling, shielding, safety, equipment cost and maintenance). The depth of high energy penetration is given by21, r = k c1/2 e–1.151εcx

N = E2. 2 f εt tan ζ

… (2)

Where, r: rate of crosslinking reaction, c: concentration of photoinitiator, ε: extinction coefficient of photoinitiator, x: thickness of reactive polymer layer. Thermoplastic crosslinking by UV radiation is a very slow process. Thermoplastic is mixed with photo-initiators, which makes it suitable to use UV light for crosslinking. UV radiation penetrates the polymer up to a depth of only a few millimeters24,25. Therefore UV light is used for crosslinking of thin parts only26. Ketones such as benzophone, and benzil dimethyl ketal are suitable photo initiator for crosslinking of mainly polyethylene27,28. Electron beams will penetrate up to few centimeters of thermoplastic polymers. The crosslinking of moulded parts having thick wall results in variable crosslink density. Therefore, this process is mainly used for thin wall products such as films, shrinkable insulating parts and crosslinking of insulating cables and foams 29,30. A microwave represents very high electromagnetic spectrum [109 to 1012 Hz]. Therefore, it is called as ultra high frequency (UHF) radiation source. Microwave crosslinking is independent on the part thickness. It is applicable to parts of any size. The

N = loss or gain of energy. E = field intensity. f = frequency (Hz) of the alternating field. εt = dielectric coefficient. tan ζ = dielectric loss factor. The drawback of microwave field is that, only components with polar group are excitable in this field. Thermoplastic such as polyethylene or polypropylene is non polar compound with very low tan ζ value. Therefore, crosslinking of polyethylene in UHF field becomes very difficult. Crosslinking of polyethylene in microwave field is possible only by using intensely polar additives such as carbon black, peroxide, metallic powders and triallyl oxy-s-triazine32,33. Most of the applications of radiation crosslinked polymers are in electrical insulation and packaging films. These are briefly described in Table 4. Advantages of radiation induced crosslinking

Advantages of radiation induced crosslinking are briefed below: (i) (ii) (iii) (iv) (v)

crosslinking reaction takes place at room temperature, reaction is completed in fraction of seconds, hence high output is obtained, reaction can take place without any additives, highly suitable for relatively thin insulating layers, crosslinking takes place in only one step.

Disadvantages of radiation induced crosslinking

Some of the disadvantages of radiation induced crosslinking are given below: (i) high capital cost, (ii) difficult to cross-link article with irregular shapes, (iii) Safety precautions are needed to protect operators from radiation.


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Table 4⎯Commercial uses of radiation-processing techniques Substrate

Radiation process

Commercial use

Polyolefins and PVC

Cross-linking with high-energy radiation sources in 0.4-3 Mev range. Cross-linking with high energy electron

Wire insulation for computers, and communication application Improved thermal stability for insulating and packaging application Conversions of waste Teflon material into easily moldable powder or waxes of commercial value No-wear high performance wood floors for high traffic areas Adhesive products for modification of wood, textiles, paper, film and metal substrates

Polyolefins and PVC foams Polytetra fluoroethylene (Teflon) Wood impregnated with acrylic or methacrylic monomers Curing of coating and adhesives

Degradation by high energy electron or cobalt-60 in 0.2-0.4 May Polymerization with cobalt-60 source Low energy electron processing equipment in 100-500 Kev range

Chemical crosslinking

Chemical crosslinking is a method, in which chemicals or initiators are used to generate free radicals, which in turns leads to crosslinking. In this method, crosslinking takes place through direct carbon-to-carbon bonds or through the chemical bridges which connect different polyethylene molecules34-36. Degree of crosslinking in thermoplastic resin varies according to crosslinking process. Chemical crosslinking by using peroxide gives highest and uniform degree of crosslinking as compared to physical crosslinking method. Kim and White have reported the difference in degree of crosslinking between physical and chemical crosslinking processes37. Accordingly, radiation crosslinking yields between 34-75% degree of crosslinking. In chemical crosslinking method, peroxide gives much high degree of crosslinking (up to 90%), while silane based crosslinking can be 45-70% degree of crosslinking. Peroxide initiated crosslinking process depends on several variables, namely operating temperature, type and concentration of peroxide, and molecular characteristics of virgin resin such as, molecular weight, molecular weight distribution, branch distribution and concentration of terminal vinyl groups. The two main chemical crosslinking methods are, (i) organic peroxide based and (ii) silane based (moisture cured). Crosslinking of thermoplastic by peroxide

Peroxide crosslinking has been in use for more than 40 years and is the most common method for crosslinking of thermoplastics especially polyethylenes. In this method, organic peroxide is used as initiator. Usually, organic peroxide is used in its original unprocessed structure. Downstream

Fig. 3⎯Schematic representation of crosslinking of polyethylene

processing equipment operates at higher temperature. The compounding of polyethylene and peroxide must be carried out at low temperature, below the peroxide decomposition temperature. Crosslinking is carried out in the downstream equipment at significantly higher temperature and pressure. The higher temperature decomposes the initiator and liberates a free radical that will abstract a hydrogen atom from polymer chain. This abstraction site then becomes reactive radical, forming a crosslinked bond with another reactive radical of same or different chain. This reaction occurs until all peroxide is consumed or the temperature falls below the decomposition point38,39. Schematic representation of this reaction is shown in Fig. 3. Elimination of hydrogen atom converts tertiary hydrogen atoms of polypropylene and polyethylene to tertiary radical chain with low reactivity. Tertiary radical sites are not very reactive and are not converted easily into more reactive secondary radicals. The shifting of the radical site along branched chain is hindered, and dimerization of chain radical becomes more difficult. Number of peroxides, which are suitable for crosslinking of thermoplastic and their dissociation temperatures, are listed in Table 5. Dicumyl peroxide (DCP) is widely used for crosslinking of thermoplastics, and crosslinking


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Table 5⎯Peroxide decomposition rates [kd] and curing temperatures Initiator

Solvent

Temperature (°C)

kd ( S-1)

Dicumyl peroxide

Benzene

115 130 145 112 132 154 128 138 148 138 158 100 115 130 108 128 150 125 115 130 145 115 134 156 115 130 145 120 141 164 100 115 130 85 100 115 75 80 100 70 80 91 80 80

2.05* 10-5 1.05* 10-5 6.86* 10-4 1.93* 10-5 1.93* 10-4 1.93* 10-3 8.75* 10-5 2.31* 10-4 5.37* 10-4 2.57* 10-4 1.52* 10-3 8.75* 10-7 5.66* 10-6 3.22* 10-2 1.93* 10-5 1.93* 10-4 1.93* 10-3 2.8* 10-5 1.15* 10-5 6.86* 10-5 4.75* 10-4 1.93* 10-5 1.93* 10-4 1.93* 10-3 3.91* 10-6 2.35* 10-5 1.14* 10-4 1.93* 10-5 1.93* 10-4 1.93* 10-3 5.83* 10-6 3.53* 10-5 2.91* 10-4 6.9* 10-6 5.05* 10-5 2.71* 10-4 2.62* 10-5 2.5* 10-5 2.28* 10-5 1.35* 10-6 4.64* 10-5 1.93* 10-4 2.53* 10-5 6.7* 10-4

Chlorobenzene Dodecane Cumene Di–t–butyl peroxide Di-t-amyl peroxide

2,5-Dimethyl-2, 5-di (t-butyl-peroxy) hexane

Benzene Chlorobenzene Decalin benzene Chlorobenzene

2,5-Dimethyl-2,5-di (t-butyl– peroxy) hexynes

Benzene Chlorobenzene

n-Butyl-4,4-bis (t-butyl peroxy) valerate

Dodecane

1,1-Bis (t-butyl peroxy)-3,3,5tri methylcyclohexane

Benzene

Benzoyl peroxide

Benzene Chlorobenzene Decane Dioxane

efficiency of DCP is more than the other peroxides. During reaction of dicumyl peroxide with polyethylene, gas generated in reaction contains 98% methane.

Curing temperature (°C)

160

175

185

195

160 150

175

Thermal decomposition of peroxide according to the following equation20,40,41, r = kd .[c] with kd = ko . e-E/RT Where,

Kinetics of crosslinking by peroxide

Decomposition of peroxide, that is, generation of free radical is slowest reaction and it is the ratedetermining step of crosslinking reaction of PE.

r = rate of decomposition kd = rate constant ko = rate constant at base temp.[0°C]

proceeds

… (4)


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Table 6⎯Various peroxides used for crosslinking Name

Dicumyl peroxide

Group

Half Life Data 10H 1H

Dialkyl peroxide

117 137 120 140 131 152

2,5-Dimethyl-2,5-di(t-butylperoxy) hexane 2,5-Dimethyl-2, 5-Di-(t-buytlperoxy) hexyane-3 Di-t-amyl peroxide Di-t-butyl peroxide 1,1-Di(t-butyl peroxy) 3,3,5 - trimethyl cyclohexane n-butyl 4, 4-bis (t-butylperoxy) valerate

Peroxy ketals

E = activation energy [c] = concentration of peroxide T= absolute temperature R = universal gas constant. Table 6 lists the curing parameter for various peroxides used for crosslinking alongwith their half life periods and active oxygen content. The crosslinking of PE depends upon the type of peroxide and the temperature. The reactions are of first order and depend also on the chemical nature of the polymer to be cross-linked. For LDPE, the value for the order of reaction is 0.90-0.99, for HDPE, 1.06 and for copolymer of ethylene between 0.88 and 1.22. The decomposition of peroxide is a rate-determining step. The cross-linking reaction is exothermic. The value of activation energy for various crosslinking reaction of different polymers such as LDPE, HDPE or copolymer is practically same if same peroxide is used. It has been shown that the order of reaction depends on the temperature and it increases at high temperatures. Number of bonds between PE chains as a function of peroxide content may be stoichiometrically calculated by assuming that one peroxide molecule independently is responsible only for one bond between two PE chains. The calculation is based on the following equation42, X .M pe … (5) A= M pr

% Active O2

Description

2.13- 5.92

Powder or flake Solid or liquid

5-10 10 – 10.6 5 – 5.36

123 143 129 149 96 115

92% liquid 45% solid on inert filler

8.8-9

96% liquid

10.8 9.73 4.1-4.34

98.5% liquid 92% liquid 40% solid on inert filler 40% solid on inert filler

109 129

Where, A = number of peroxide molecules per two PE chains, Mpe, Mpr = molecular weight of PE and peroxide respectively, X= thickness of reactive polymer layer OR the mass concentration of peroxide per one gram of PE. The decrease of XLPE density is due to additional branching introduced by peroxide and the maximum density can be obtained at 0.5% peroxide concentration. The drop in crystallinity as a function of peroxide concentration and increase in crosslinking impose some restraint on mobility of polymer chains in molten state preventing them from arranging into lamellae fold43. Relative degree of crosslinking and degradation of polymers is discussed by Kwei42 with the help of following equation, S + S0.5 = {P0 / Q0} + {1 / Q0 Yn D}

… (6)

Where, P0/Q0 = ratio of degradation to crosslinking D = radiation dose (Mrad) S = percent of soluble molecule in network Yn = initial number average molecular weight of polymer. Crosslinking of copolymers

Crosslinking of PE-PP has been studied by Braun44. They observed that PP did not crosslink and grafting

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of PP onto PE could take place45. They have reported that as PP content increased, the heat resistance of crosslinked compound decreased. Ethylene vinyl acetate (EVA) is blended with polyethylene quite widely. Ethylene vinyl acetate copolymer is more rapidly crosslinked and accepts high filler loading, without significant loss of physical properties, making them well suited for lower voltage applications and in automobile etc. Peroxide coagent crosslinking method

Coagents are low molecular weight molecules with two or more reactive double bonds. Coagents work in a free radical cure system. Free radicals are generated by using peroxide or high energy radiation sources, coagent reacts with these free radicals to increase the efficiency of cure i.e. to give more crosslinks. The presence or addition of a coagent in crosslinking provides more reactive sites where the crosslinking reaction can occur46. Addition of coagents reduces cure time, improves resistance to oils and fuels, improves heat aging, improves peroxide efficiency, improves flexibility and gives higher tensile strength and hardness47,48. Advantages of peroxide-coagent system

when exposed to excessive heat or light. Therefore, polyethylene compounds used for coating wire and cable application may contain anti-oxidants. Cables exposed to ultraviolet rays in sunlight must also contain carbon black or other UV inhibitors. Such additives also assure that the electrical and mechanical properties of the resin are preserved under the high temperatures prevailing in the extruder50,51. Carbon black of fine particle size can be used as a thermal antioxidant. Carbon black is used as a conducting filler for elastomer and plastics. Incorporation of carbon black increases brittle point and yields stress with concentration, without affecting tensile strength52. Crosslinking by silane

There are two types of processes for crosslinking of polyethylene by silane grafting: two-step process and one-step process. In two-step process, silane molecule such as vinyl trimethoxysilane (VTO) is grafted on polyethylene chain. For grafting, the peroxide such as dicumyl peroxide is mixed with the polyethylene in small percentage. The peroxide initially generates free radical on polyethylene chain. Grafting of silane takes

The advantages of peroxide-coagent system are: (i) excellent heat stability (ii) simple compounding (iii) better hot tear (iv) good shelf life stability. Disadvantages of peroxide-coagent system

The disadvantages of peroxide-coagent system are: (i) higher cost (ii) surface tackiness in presence of oxygen. Applications

Peroxide-coagent systems are widely used in automotive seals, automotive hoses, oil well packers, swab cups, golf balls, butterfly valves, belts, mats, shock absorber, cable, coating and foam packing. Some of the coagents used in crosslinking

Some of the coagents used in crosslinking are49, ethylene glycol dimethylacrylate, 1,3–butylene glycol dimethylacrylate, poly (ethylene glycol) dimethylacrylate and trially cyanurate dimethylacrylate.

Effect of anti-oxidants and carbon black fillers

The power factor of polyethylene may be unfavorably affected by oxidation of the insulator

Fig. 4⎯Schematic representation of two-step crosslinking process of polyethylene by using silane


place at the site where free radical is generated. The grafted polyethylene is mixed with silanol condensation catalyst such as dibutyltin dilauarate, and extruded or injection moulded profile is produced. Up to this stage the polyethylene retains the thermoplastic nature. Extruded or injection moulded article is crosslinked with the help of water or at elevated temperature or at room temperature53,54. The two-step silane crosslinking process is schematically represented in Fig. 4. In one-step process, a copolymer of silane and polyethylene may be formed or free radical generating and grafting take place in single step only. The chemical reaction of silane crosslinking is shown in Fig. 5. The reaction continues until all grafted copolymer is converted into crosslink chains. In the peroxide and irradiation cross-linking processes, the links between macromolecule consist of carboncarbon bond. The silane process gives -Si-O-Sicrosslinks. These siloxane bridges are weaker than carbon-carbon bond, and this will have effect on the attainable strength and long term chemical stability55,56. Silane grafting of polyethylene

The most common silane used for cross-linking of polyethylene is vinyl trimethoxysilane (VTO). The silane is introduced into polyethylene by melt grafting using peroxide as an initiator. The silane-grafted polyethylene is then crosslinked through hydrolyzation of the methoxysilane group with water followed by condensation of the hydroxyl group57,58. Various types of silanes used in crosslinking are listed in Table 7. Crosslinking of grafted polyethylene is completed only in presence of moisture. A catalyst is used to activate and speed up the crosslinking process. The crosslinking is also enhanced by high temperature, but silane crosslinking is usually performed at 50 to 80oC and at atmospheric pressure. Polypropylene is degraded due to peroxide and therefore, it cannot be crosslinked similar to

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polyethylene. The degradation is mainly due to βchain scission. Crosslinking of polypropylene is possible only by silane grafting method. Advantages of silane-grafted crosslinking

Various advantages of silane-grafted crosslinking are: (i) crosslinking can be done at room temperature (ii) low cost (iii) higher gel percentage obtained as compared to physical crosslinking. Disadvantage of silane-grafted crosslinking

Some disadvantages of silane-grafted crosslinking are given below: (i)

curing time is very high as compared to peroxide crosslinking (ii) extra downstream equipments are required (for condensation) (iii) bond strength of crosslinking is weaker than bond strength in peroxide crosslinking system. Effects of crosslinking

Polyethylene is crosslinked to improve its dimensional stability at elevated temperature, to

Fig. 5⎯Silane grafted polyethylene crosslinking reaction

Table 7⎯Various types of silane used for crosslinking Silane Tetramethoxysilane Tetraethoxysilane Methyltriethoxysilane Methyl tris methyltriethoxysilane

MolecularWeight

Colour

Boiling Point (°C)

Flash Point (°C)

152.2 208.3 136.3

Colourless Colourless Colourless Yellowish liquid

122 168 101

26 46 5

110

105

301.46


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Table 8⎯Comparison between crosslinking processes Crosslinking process

Peroxide

Silane

Radiation

No. of steps Crosslinking mechanism Gel % Curing mechanism

Two step Grafting > 65 Condensation reaction

One step Free radical >60

Curing temperature Curing time Additives Equipment cost Bond strength

One step Free radical > 75 Homolysis temperature of peroxide 150-160 °C Less No Medium Strong

Room temperature Very low Peroxide for initiating High Strong

Degree of crosslinking

Constant throughout the article

80-90 °C Very high Peroxide for grafting Low Weak as compared to peroxide Varies with residence time in water bath and temperature of bath

improve its impact resistance or to reduce its propensity to stress crack. Due to crosslinking polyethylene changes from ductile semi-crystalline solid to a non-crystalline elastomer59. As crosslinked density increases, a degree of crystallinity and crystallite thickness decreases. This was studied by Badar60. Reduction in crystallinity occurs because of crosslinking taking place in amorphous phase. There is also some breakdown of crystallinity. Decrease in degree of crystallinity and crystalline thickness, decreases Young’s modulus, yield stress, elongation at break and peak melting temperature of polyethylene61. Harper62 studied the effect of crosslinking on melt index and concluded that the melt flow index decreases uniformly with increase in degree of crosslinking. Crosslinking increases the impact strength, environmental stress crack resistance, creep resistance without affecting tensile strength and flexural modulus. Comparison of different crosslinking methods is presented in Table 8. Disadvantages of crosslinking

As mentioned earlier, crosslinking of polyethylene changes its nature from thermoplastic to thermoset. This enhances the viscosity to a very high value. To control the degree of crosslinking in a continuous process is very tricky. During extrusion of a profile, if excessive crosslinking takes place stresses are developed and, these do not get relaxed due to dense network of polyethylene. Selection of proper crosslinking agent is very critical. Blending uniformly crosslinking agents with polyethylene beds can lead to uneven distribution of crosslinking agent. Melting of

Varies with thickness of article

polyethylene prior to crosslinking is therefore, key function in processing. Due to its thermoset nature recycling of crosslinked polyethylene cannot carried out by melting it with virgin polyethylene, since crosslinked polyethylene does not melt. The crosslinking process decreased the crystallinity. Application of crosslinked polymers 63,64 Cable insulation

The most advanced area of application for crosslinked polyethylene is in the electrical cable industry. Crosslinking, whilst not interfering with the dielectric properties of polyethylene, introduces resistance to flow and permanent deformation above the softening point. This permits higher conductor operating temperature and reduces the level of short circuit and overload protection required. Flame retardant properties of pipe are also improved by irradiation (60% gel) by introducing double layer structure to the jacket, in which inner layer is adjacent to polyethylene insulation which is crosslinked more densely than the outer layer. The wire exhibits markedly improved resistance to flame and heat deformation. Crosslinked polyethylene pipes

The main applications for crosslinked polyethylene hot water pipe are: a) under floor or central heating b) domestic or portable water piping system. The benefits of crosslinking become obvious above ambient temperatures where a reduction in the rate of creep for the corresponding hoop stress is observed.


Injection and blow moulded articles

Specially moulded articles such as containers with significant improvement in ESCR and chemical resistance characteristics can be made using crosslinked polyethylene. Increasing the molecular weight by crosslinking increases its solvent and creep resistance. The increased dimensional stability at evaluated temperatures allows the article to come in contact with heated fluids. Crosslinked film

Crosslinked polyethylene in packaging applications is confined to multi-layer film constructions, in which the cross linked layer provides a number of specific effects including: increased temperature resistance especially for hot filled or heat sterilization applications; increased heat seal strength where a thermoplastic seal is subsequently crosslinked; increased impact, tear and abuse resistance capability especially for packing irregular shaped items to impart heat shrinking properties to the film.

Conclusions Crosslinking of polyethylene and polypropylene is practiced industrially for very interesting applications. Polyethylene can be crosslinked by radiation energy as well as by organic peroxide. Polypropylene can be crosslinked by silane method. Very little information exists on crosslinking of PVC. Various aspects of crosslinking process have been studied earlier and are reviewed in this article. References 1 2 3 4 5 6 7 8 9 10

Crosslinked polyethylene foam

Crosslinked polyethylene (XLPE) foams have various kinds of applications, such as thermal insulation, floatation, automotive trim, and sports goods. The advantages of crosslinking, prior to foaming are that high quality closed cell structure can be achieved which gives the following benefits a) increased mechanical properties, b) increased resistance and recovery from deformation, c) increased thermal resistivity, d) reduced vapour and liquid transport, e) to improve elevated temperature and load bearing characteristics of the foam, f) to permit secondary moulding and foaming without collapse of the cell structure.

11 12 13 14 15 16 17 18 19 20 21

Special application

High pressure crosslinked polyethylene tube shows water clear transparency on high heat, while lower heat produces translucency, but this effect of temperature is not obtained with untreated PE. High pressure crosslinked PE makes it feasible to use the material for applications such as automobile muffler tail pipe. Crosslinked polyethylene fibre used as a filler in denture base resin poly [methyl methacrylates], improves impact strength without adverse aesthetic effects.

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