Laminates

Chapter 10

Manufacture of Films/Laminates The present chapter relates to films and laminates composed of biodegradable polymers and/or polymers derived from renewable resources.

10.1 FILMS Environmental problems arise with packaging films in that the environment is increasingly contaminated with plastic bags and plastic sheets that do not degrade, or degrade only slowly. The tendency is, therefore, to replace nonbiodegradable polymers derived from fossil fuel resources with biodegradable polymers for the formulation of packaging films. It is often required that a biodegradable film exhibits certain physical and thermomechanical properties, such as stiffness, flexibility, water resistance, tensile strength, elongation, temperature stability, or gas permeability. The intended application of a particular biodegradable polymer will often dictate which properties are necessary for a particular film manufactured there to exhibit the desired performance criteria. In the case of sheets and films suitable for use as packaging materials, desired performance criteria may include elongation, dead fold, strength, printability, imperviousness to liquids, breathability, temperature stability, and the like. It is often difficult, or even impossible, to identify one single biodegradable polymer or copolymer that meets all, or even most, of the desired performance criteria for a given application. This is particularly true in the area of packaging materials. Polymers that have a high glass transition temperature (Tg) are either difficult to blow into films on a mass scale or, at the least, tend to be too brittle for use as a packaging material, such as a wrap. On the other hand, polymers that have a low Tg also usually have relatively low softening and/or melting temperature (Tm) which makes them difficult to mass produce into sheets and films without the tendency of blocking, or self-adhesion. Moreover, such sheets and films may lack adequate strength, water vapor barrier, and/ or modulus to be suitable for certain applications, such as in the manufacture of wraps or laminate coatings. For these reasons, biodegradable polymers have found little use in the area of packaging materials, particularly in the field of wraps. Any known technique may be used to form a film from biodegradable polymers, including blowing, casting, and flat die extruding. Blown film is created by extruding melted polymer through an annular die, usually vertically, to form a thin walled tube, and forcing air via a hole in the center of the die to blow up the tube into a bubble. An Biopolymers: Processing and Products. http://dx.doi.org/10.1016/B978-0-323-26698-7.00010-6 Copyright © 2015 Elsevier Inc. All rights reserved.

air ring, mounted on top of the die, blows air onto the hot film to cool it down. The film then passes into a set of nip rolls, which collapse the bubble and flatten it into two flat film layers. Cast film is created by feeding a sheet of heated resin along a rolling path with chilled rollers. The cooling solidifies the film, and it is made into large rolls. Both techniques have advantages and disadvantages. Among the conventional biodegradable polymer films, films based on cellulose, starch, and aliphatic polyesters produced by microorganisms are unsatisfactory in barrier properties, mechanical properties (strength), and heat resistance and are difficult to melt process, so that their processing costs become high. The same applies mutatis mutandis to biodegradable polymer films based on synthetic type polyesters, such as poly(ethylene succinate) or poly(butylene succinate) (PBS) (1995, JPH07173271 A, MITSUI TOATSU CHEM INC), whereas the raw materials, succinic acid and butanediol, for making for example PBS, are considerably expensive (1997, EP0805175 A1, KUREHA CHEMICAL IND CO LTD).

10.1.1 Films by Chemical Type 10.1.1.1 Starch The use of starch to form film products is well known in the art. In the past, films have been prepared from amylosic materials by casting a solution of the amylosic material in a solvent on a suitable surface and then peeling the resulting film from the surface. In other instances, there have been suggestions of extruding amylosic materials, but invariably these extrusion processes also involved the use of a complete solution of the amylose in the carrier. Many of these techniques consisted of a chemical modification, followed by a regeneration of the amylose or starch, similar to the viscose process for cellulose. In any case, the films that resulted from such extrusion processes were frequently deficient in tensile strength, pliability, and transparency. GB965349 A (1964, DEPT OF AGRICULTURAL AND INSPECTION OF THE STATE OF NEBRASKA, USA) discloses the extrusion of amylose material without using solvents to form films claimed to have excellent tensile strength. Another film-forming operation using starch is shown in US3116351 A (1963, US AGRICULTURE), where an unsupported amylose film is made by extruding 361

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an aqueous alkali-amylose solution into a coagulation mixture of ammonium sulfate and sodium sulfate. EP0032802 A1 (1982, US AGRICULTURE) discloses a method of producing a flexible, self-supporting, and biodegradable film, wherein a mixture comprising a partially or completely gelatinized starchy material in an amount up to 60 wt% and an ethylene-acrylic acid copolymer is converted into a plasticized matrix and then shaped into a film comprising the following steps: (1) incorporating into said matrix a neutralizing agent selected from the group of aqueous ammonia and anhydrous ammonia; (2) adjusting the moisture content of said matrix to within the range of approximately 2–10 wt% based on the weight of the matrix; and (3) extrusion blowing said ammoniated and moistureadjusted matrix into a film. US2008147034 A1 (2008, KIMBERLY CLARK CO) discloses a film that is biodegradable and water sensitive (e.g., water soluble and water dispersible) in that it loses its integrity over time in the presence of water. The film comprises 1–50 wt% of at least one biodegradable polyester, and 50–99 wt% of at least one water-sensitive thermoplastic starch (TPS), wherein the TPS includes 40–95 wt% of at least one starch and 5–60 wt% of at least one plasticizer. The biodegradable polyester has a Tm of 50–180 °C and a Tg of approximately 25 °C or less, and is preferably an aliphatic-aromatic copolyester. The plasticizer is a polyhydric alcohol, such as sugar alcohol. The film has a thickness of approximately 50 μm or less, and is used for release liner, absorbent article, pouch, wrap, bag, diapers, training pants, absorbent underpants, incontinence articles, feminine hygiene products, swim wear, baby wipes, medical absorbent articles, food service wipers, and clothing articles. Figure 10.1 shows an embodiment of a method for forming a uniaxially oriented film by casting. KR20090008110 A (2009, YOU YOUNG SUN; KIM MAN SOO) and KR20090008111 A (2009, ENFORECO CO LTD; YOU YOUNG SUN) disclose a biodegradable film that contains starch (5–80 wt%) plasticizer (5–20 wt%), biodegradable material (5–50 wt%) and co-additive (3–55 wt%). The manufacture of the biodegradable film comprises the following steps: (1) mixing starch, inorganic filler, plasticizer, lubricant, and, optionally, an aliphatic polyester, in a highspeed mixer and removing moisture; (2) adding antioxidant, biodegradable material, and compatibilizer to the moisturefree mixture, and mixing the mixture at high speed; (3) manufacturing the mixture into the raw material pellet by using a twin extruder, wherein the twin extruder comprises a kneading part, at least one reverse direction screw, and at least one vent hole for ejecting volatile materials; and (4) processing the raw material pellet and, optionally, additive into a film.

10.1.1.2 Poly(Lactic Acid) A film made of poly(lactic acid) (PLA) is stiff and has little flexibility and adhesive properties at temperatures T ≤ 60 °C

FIGURE 10.1  Schematic diagram of an embodiment of the method for forming a water-sensitive biodegradable film (2008, US2008147034 A1, KIMBERLY CLARK CO). 10a, single-layered precursor film; 10b, resulting film; 60, take-up roll; 80, extrusion apparatus; 90, casting roll; 100, film-orientation unit or machine direction orienter (eight rolls).

(i.e., its glass transition temperature) but is too flexible to maintain its shape at temperatures T ≥ 60 °C, thus being difficult to use in practice. Although the temperature of air and water in nature do not often increase to 60 °C, for example, the interior space and windows of closed automobiles may be heated to such a temperature in midsummer. This significant change in the characteristics is attributable to the crystalline structure of PLA. More specifically, when cooled at a usual cooling rate after the melt-forming process, PLA is negligibly crystallized and a large portion thereof becomes solidified in an amorphous state. The crystallized portions of PLA, whose melting temperature (Tm) is as high as 160 °C, cannot easily melt, but the amorphous portions accounting for the major portion of the entire product start to move without restriction at temperatures close to 60 °C. Some early PLA films have been made by casting from solutions or by pressing, as disclosed already in some old patent publications (see US2703316 A (1955, DU PONT) and US4045418 A (1977, GULF OIL CORP)). PLA films are mostly manufactured by either blown film or casting method. The main drawback of thin PLA films obtained under normal conditions, by either the blown or cast method, is their low tear resistance. Blown films comprising PLA have proved difficult to manufacture. Indeed, currently available PLA blown films require the addition of additives, such as plasticizers, to enable their production. However, plasticizers are often undesirable for films with food-related applications; they are costly, and they seldom, if at all, are as environmentally friendly as PLA itself. To circumvent these issues, some manufacturers have resorted to manufacturing PLA film with casting method (e.g., cast

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and tenter). The cast film is generally better suited for certain end-user applications, such as those requiring film “sleeves” rather than “wrap-around” film. Furthermore, cast film generally has much better optical properties than a blown film and can be produced at higher line speeds. However, PLA films that are manufactured by current casting methods exhibit excessive shrinkage in the machine direction, which substantially contributes to curling and limits their range of application. In addition, casting technology produces films with limited applications and can be 5–10 times more costly than blown film processing (2006, US2006045940 A1, PLASTIC SUPPLIERS INC). US5443780 A (1995, SHIMADZU CORP) discloses the production of oriented films made from PLA. The process starts from a PLA melt, which is extruded and rapidly cooled. This prefilm can subsequently be subjected to a uniaxial stretching process or subjected to sequential or simultaneous biaxial stretching. The stretching temperature is between the Tg and the crystallization temperature (Tc1) of the PLA. The stretching results in increased strength and a higher Young’s modulus in the final film. If desired, the stretching is followed by heat setting. WO02087851 A1 (2002, TRESPAPHAN GMBH) discloses a film comprising at least one base layer containing PLA and a minority component of a polyolefin, such as polyethylene or polypropylene, in an amount of 0.2–1 wt% of the base layer. Such a formulation is particularly suitable for thermoforming or biaxial stretching by means of pneumatic drawing or other mechanical forming. JP2003103628 A (2003, TOYO BOSEKI) discloses a method for the manufacture of a biaxially stretched PLA film in which the difference between the heat of fusion ΔHm of the crystal of the film during a period from the point of time when biaxial stretching is completed to the start of thermal fixing treatment and the heat of crystallization ΔHc based on crystallization during a temperature increase is (ΔHm – ΔHc) ≥ 30 J/g. The disclosed PLA film is claimed to be excellent in stretchability and reduced in thickness irregularity, and to have an excellent winding state generating no winding shift or wrinkles. JP2004277682 A (2004, TOHO CHEM IND CO LTD) discloses a biodegradable film obtained by mixing 100 pbw of a PLA with 3–80 pbw of a rosin compound. The disclosed film is claimed to have improved flexibility and adhesive properties. However, the patent application does not discuss whether the shape of the film can be maintained at temperatures equal to or higher than the Tg of PLA. Furthermore, the flexibility of the film is also insufficient because the percentage elongation after fracture of the film is in the range of approximately 1.7–3.4% (2006, WO2006098159 A1, SUMITOMO ELEC FINE POLYMER INC). US2006045940 A1 (2006, PLASTIC SUPPLIERS INC) discloses PLA films substantially free of plasticizers and methods of manufacturing same by blown film process.

In an embodiment, a method of making a PLA blown film is provided comprising the following steps: (1) providing dry pellets of PLA; (2) melting the pellets to form a molten mass at a first desired viscosity value or range of values; (3) increasing the viscosity of the molten mass to a second desired viscosity value or range of values; (4) forming a heated bubble from the resulting molten mass; (5) collapsing the bubble to form a film, in which the PLA film is substantially free of plasticizer; and, optionally (6) annealing the film at a temperature range of approximately 120 °F (49 °C) to approximately 285 °F (141 °C). The first viscosity value is 1000–5000 P (100–500 Pa s) at 480 °F (249 °C) at an apparent shear rate of 55/s. The second viscosity value is 14,000–16,000 (1400–1600 Pa s) at approximately 375 °F (191 °C) at an apparent shear rate of 55/s. The viscosity increasing step may be performed most conveniently, but not exclusively, in a polymer cooling unit, and the step of forming a heated bubble may include a stretching step, which orients the film. Alternatively, the viscosity increasing step may be performed by internal cooling of the die mandrel (by air or liquid fluid), controlling the temperature of the die (by heated or cooled liquid fluid), and/or the addition of chemical viscosity enhancers, the latter being preferably added during or before the melting step. Preferred commercial PLA products are NatureWorks polymers grades, and especially 4060D, 4042D, and 4032D. The PLA blown films find applications in food packaging and labeling (e.g., envelopes and signage). WO2011082052 A1 (2011, 3M INNOVATIVE PROPERTIES CO) discloses a method for providing a semicrystalline PLA film, including the steps of providing a PLA composition that includes PLA, nucleating agent, and a plasticizer. Preferred plasticizers are as follows: acetyl tris3-methylbutyl citrate, acetyl tris-2-methylbutyl citrate, acetyl tris-2-ethylhexyl citrate, and acetyl tris-2-octyl citrate. The composition is extruded as a molten sheet, which is then cooled to crystallize PLA and provide the film (see also Chapter 5: Compounding; Section 5.4: Plasticizing). US2013004760 A1 (2013, AMPAC HOLDINGS LLC) discloses a PLA-based film composition comprising 90–99 wt% of one or more PLA and 1–10 wt% of one or more polyterpene resin additives (e.g., polymerized d-limonene and polymerized β-pinene) based on the total film composition. The film exhibits a low water vapor transmission rate (≤35 g/100 in2/day/mil). In one embodiment, the PLA-based film includes one or more coating layers chosen from PVDC, poly(vinyl alcohol) (PVOH), acrylic, or lowtemperature sealable coating. In another embodiment, the PLA-based film includes one or more vacuum-deposited aluminum layers. In a further embodiment, the PLA-based film includes a skin layer chosen from ethylene-propylene random copolymer, ethylene-propylene-butene-1 terpolymer, high-density polyethylene, medium-density polyethylene, low-density polyethylene (LDPE), linear LDPE,

364  Biopolymers: Processing and Products

propylene-butene-1 copolymer, ethylene vinyl alcohol copolymer, amorphous polyester, and ionomer. The PLAbased film is useful in frozen food packaging, snack food packaging, beverage packaging, labeling applications, and pet food packaging applications. JP2002146170 A (2002, UNITIKA LTD) discloses a film comprising a crystalline PLA, a plasticizer, and a nucleating agent as essential components having prescribed thermal properties. In addition, JP2006063308 A (2006, KASEI CO C I) discloses a method for performing a crystallization treatment on a film comprising a PLA composed of a weak crystalline or amorphous PLA and a crystalline PLA, a plasticizer, and an anti-clouding agent. According to WO2011162046 A1 (2011, TORAY INDUSTRIES), in the last two patent applications, the anti-blocking property and the anti-bleeding property are not sufficiently attained, and there is no disclosure at all with regard to a method for improving the tear resistance and impact resistance of the film. JP2009138085 A (2009, TORAY INDUSTRIES) discloses a film that is composed of a composition comprising a PLA and a plasticizer and has prescribed film elongation at break, thickness, and thermal contraction rate. The disclosed film is claimed to have excellent flexibility, impact resistance, and dimensional stability, as well as blocking resistance and bleeding resistance, especially of a good quality when produced by an inflation method. However, there is no disclosure at all with regard to a method for improving the tear resistance of the film (2013, WO2011162046 A1, TORAY INDUSTRIES). WO2013021772 A1 (2013, TORAY INDUSTRIES) discloses a biodegradable film comprising a lactic acid– based polymer (a) dispersed in a continuous phase comprising a biodegradable polyester (b). The dispersed phase is in the form of ellipse or layer, which is elongated in the length direction of the film, and has a thickness of 5–100 nm. The lactic acid–based polymer (a) comprises a block copolymer having a polyester-type segment and a PLA segment and/ or a block copolymer having a polyether-type segment and a PLA segment. The biodegradable polyester (b) is chosen from PBS, poly(butylene succinate adipate) (PBSA), and poly(butylene adipate terephthalate) (PBAT). The film has tearing strength of 5 N/mm or more (measured using trouser tear method based on JIS K7128-1, 1998) along length direction and width direction. The ratio of melt viscosity of PLA (a) and melt viscosity of biodegradable polyester (b), at 200 °C with shear rate of 100/s is 0.3–1. The biodegradable film further contains a compatibilizing agent. The weight ratio of lactic acid–type block copolymer and biodegradable polyester is 5/95–60/40. The disclosed biodegradable film is used for packaging material for food, garment, and industrial commodity, hand-carried bag for shopping, vegetable and fruit, agricultural mulch film, and sheet for pine beetle fumigation, manure bag, and garbage bag.

US2009326130 A (2009, FINA TECHNOLOGY) discloses a method of producing an oriented film comprising blending PLA (1–40 wt%) and polypropylene (51–99 wt%) and forming the polymer blend into a film, and orienting the film. The dispersed PLA phase in the blend functions as a cavitating agent. Herein, a cavitating agent refers to a compound(s) capable of generating voids in the structure of film during the film-making process. In an embodiment, the polypropylene/PLA blend further comprises a cavitating booster. The polypropylene/PLA blend displays desirable characteristics such as an increased strength and/or improved optical properties when compared to either polypropylene or PLA alone. In particular, the disclosed film has a haze of 10–95% and a gloss 45° from approximately 50 to approximately 125.

10.1.1.3 Poly(glycolic acid) US2676945 A (1954, DU PONT) discloses a biodegradable amorphous film based on polyglycolic acid (PGA), which is a synthetic aliphatic polyester. The film has a thickness of 3 mils (76.2 μm) and is biaxially stretched. The PGA polymer is obtained by directly polycondensing glycolic acid, and not by ring-opening polymerization of glycolide. However, the polycondensation process of glycolic acid includes heating and polycondensing glycolic acid for a long period (approximately 40 h) at a high temperature of at least 200 °C, and hence tends to involve side reactions such as decomposition reactions. This patent application describes, in its example, the melt viscosity of PGA as being approximately 2000 P (200 Pa s) at 245 °C measured at a shear rate of approximately 0/s. This melt viscosity value corresponds to a value extremely as low as approximately 20 P (2 Pa s) at 245 °C when converted into a value measured at a shear rate of 100/s. In addition, there is high possibility that this PGA may have an unstable structure because of adverse reactions. Therefore, oriented films formed from PGA obtained by such a direct polycondensation process have low mechanical strength and, hence, they are insufficient from the view point of practical use (1997, EP0805175 A1; 1997, EP0805176 A1, KUREHA CHEMICAL IND CO LTD). A biaxially oriented film of PGA obtained by ring-­ opening polymerization of glycolide is expected to enhance the gas barrier properties and mechanical properties thereof by the stretch processing. Therefore, there have been ­proposed various processes for producing a biaxially ­oriented film of PGA. In general, PGA is melt processed into the form of a film, and the resultant film is then subjected to stretch processing within the temperature range Tg