COEXTRUSION Introduction Multilayer coextrusion of thermoplastic film and sheet has developed into an important plastic fabrication process, providing large growth opportunities for the plastics industry. Coextruded multilayer plastics are challenging such traditional materials as metals, glass, paper, and textiles. The attraction of coextrusion is both economic and technical. It is a singlestep process starting with two or more plastic materials that are simultaneously extruded and shaped in a single die to form a multilayer sheet or film. Thus, coextrusion avoids the costs and complexities of conventional multistep lamination and coating processes, where individual plies must be made separately, primed, coated, and laminated. Coextrusion readily allows manufacture of products with layers thinner than can be made and handled as an individual ply. Consequently, only the necessary thickness of a high performance polymer is used to meet a particular specification of the product. In fact, coextrusion has been used commercially to manufacture unique films consisting of hundreds of layers with individual layer thicknesses less than 100 nm (1). It is difficult to imagine another practical method of manufacturing these microlayer structures. Layers may be used to place colors, bury recycle, screen uv radiation, provide barrier properties, minimize die-face buildup, and to control film-surface properties, for example. Additives, such as antiblock, antislip, and antistatic agents, can be placed in specific layer positions. High melt strength layers can carry low melt strength materials during fabrication. The largest market for coextruded films and sheets is in packaging applications, eg, two- or three-layer films for trash bags or five- to nine-layer structures for flexible and semirigid packages. As many as five different polymers may be used 1 Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
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to obtain heat sealability, barrier, chemical resistance, toughness, formability, and aesthetics. Coextrusion is also suitable for applying thin multilayer films as coatings on substrates (2). Growing applications for coextrusion are in automotive, construction, appliance, and food packaging markets (see PACKAGING, FLEXIBLE; BARRIER POLYMERS).
Methods of Coextrusion Coextruded films are produced by a tubular-blown film process and a flat-die, chill-roll casting process. Capital and operating costs for blown-film vs cast-film coextrusion lines are strongly dependent on product mix and utilization. Equipment suppliers provide comparative economic evaluations for specific products. Practical cast-film equipment has been discussed previously (3). Coextrusion dies are unique. Extruders used before the die and take-away equipment used afterwards are standard equipment for single-layer film manufacture of blown or cast film (see EXTRUSION). Tubular-Blown Film Process. This process is more flexible with regard to the permissible polymer viscosity mismatch, control of film orientation balance in the transverse and machine directions through blow-up ratio, and easy randomization of film-thickness variations. Production rates are limited by flow rates per circumferential length of die (pressure drop) and cooling rates (heat transfer). Casting Process. The flat-die, chill-roll, cast-film process is more suitable for high volume production on dedicated lines because of higher output rates obtained by wide dies and more efficient cooling on chill rolls. Cast films usually have better clarity than blown films because of rapid quenching, but uniaxial orientation can cause the film to split in the machine direction for some structures.
Coextrusion Dies Tubular-Blown Film Dies (Circular Dies). Tubular coextrusion dies were the earliest dies used to make multilayer plastic film. Successful design requires formation of uniform concentric layers in the annular die land formed by the mandrel and adjustable or nonadjustable outer die ring. Early designs included center-fed dies that had the mandrel supported by a spider (4). Feedports arranged a concentric melt stream that was pierced by the mandrel as it flowed to the die exit, forming annular layers. Limitations of this early design were discontinuity and nonuniformity caused by spider-induced weld lines in the layers. Another early design used stacks of toroidal distribution manifolds, so that as flow proceeded to the die exit, concentric layers were extruded on one another sequentially (5). The number of layers could be varied by changing the number of toroidal manifolds in the stack. The crosshead design of this die eliminated the spider support of the mandrel with its attendant weld-line problem. The design most commonly used today is the multimanifold spiral mandrel tubular-blown film die (Fig. 1a). This die consists of several concentric manifolds, one within the other. The manifolds are supported and secured through the base of the die. Each manifold consists of a flow channel that spirals around the mandrel,
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(a)
(b)
Fig. 1. (a) A three-layer blown film die and (b) a stackable blown film die.
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allowing polymer to flow down the channel or leak across a land area to the next channel. This flow pattern smoothes out the flow of the polymer and minimizes any weld lines in the final film. While early designs were limited to two or three layers, dies containing seven or more layers are now offered commercially. These dies must achieve uniform concentric flow of all layers because it is impractical to provide circumferential thickness adjustment for each layer. Most polymers are non-Newtonian, and polymer viscosity usually decreases with shear rate. Thus, rheological data obtained at the intended extrusion temperature and shear rate are needed to size manifolds and channels for layer uniformity and minimum pressure drop. Frequently, spiral mandrel manifolds, common in singlelayer dies, are used to improve circumferential distribution. A well-designed spiral mandrel manifold can be helpful, but streamlining is necessary to minimize stagnation, residence time, and purging. A manifold design is only optimum for a particular polymer. Employing a polymer with significantly different properties may require a different manifold insert in the die in order to obtain satisfactory layer distribution. Most tubular-blown film lines are designed for oscillation of the die or winder to randomize film thickness variations at the windup and avoid buildup of gauge bands, which can cause problems with film flatness. More layers complicate bearing and sealing systems in an oscillating die, but designs have now been refined to employ new sealing materials that minimize polymer leakage. New designs incorporate temperature control of individual annular manifolds to permit coextrusion of thermally sensitive polymers. Another style of tubular-blown film die is the stackable plate die (Fig. 1b). In this style of die, each layer is spread uniformly and formed into a tube in a single plate. Plates are then stacked on top of each other and the layers are added sequentially. This style of die is becoming popular for specific applications since the number of layers can be adjusted by simply changing the number of plates in the die. Tubular coextrusion dies are expensive, and care must be taken when disassembling and reassembling them to clean or change parts. Discussions of additional practical design, maintenance, and operating considerations have appeared (6–10). Flat Dies (Slit Dies). Flat dies, also called slit dies because the orifice is a wide rectangular opening, are used in chill-roll, cast-film coextrusion. These dies are used almost exclusively for multilayer coextrusion with sheet thickness >254 µm, as well as in coextrusion coating processes (2), where a multilayer web is extrusion-coated onto a substrate such as paperboard, aluminum foil, plastic foam, or textiles. Another commercial application for flat-die coextrusion is biaxially oriented multilayer films (11) made with the tentering process to improve mechanical properties. Tentered film is biaxially oriented by stretching in the longitudinal and transverse direction, either sequentially or simultaneously, at uniform optimum temperature. In sequential stretching, the multilayer extrudate is cooled to a suitable orientation temperature on a first set of rolls and then stretched in the machine direction between a second set of rolls which is driven faster than the first set. The uniaxially stretched film then enters a tentering frame, which has traveling clips that clamp the edge of the film. The clips are mounted on two
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A
B
C
Fig. 2. Cross-sectional view of three-layer internal-combining multimanifold flat film or sheet die.
tracks that diverge inside a temperature-controlled oven increasing film width to provide transverse stretch. The film is then heat set and cooled. Simultaneous tentering frames are also used which feature accelerating clips that stretch the film longitudinally as they diverge transversely. Two basic die types used in flat-die coextrusion systems are multimanifold dies and the feedblock/single-manifold die. A hybrid combines feedblocks with a multimanifold die. Multimanifold Dies. For each layer, these dies have individual manifolds that extend the full width of the die. Each manifold is designed to distribute its polymer layer uniformly before combining with other layers outside the die (external combining) or inside the die before the final die land (internal combining). External-combining dies are typically limited to two-layer coextrusion because two slit orifices must be individually adjusted with die-lip adjusting bolts. The webs are combined at the roll nip. In principle, internal-combining dies are similar to multimanifold-tubularcoextrusion dies, except that the manifolds are flat (Fig. 2). With these dies, it is possible to regulate flow across the width by profiling an adjustable restrictor bar in each manifold to help obtain uniform distribution. However, wide dies require numerous adjusting bolts on each layer manifold along with die-lip adjustment to control final thickness; this can make them difficult to operate. Multimanifold dies have been sold, capable of coextruding five and six layers; they are expensive and require skilled operators. The principal advantage of multimanifold dies is the ability to coextrude polymers with very different viscosities since each layer is spread independently prior to combining. A significant disadvantage of wide multimanifold dies is difficulty in coextruding very thin layers, such as thin cap (surface) layers, or thin adhesive (tie) layers used to bond two dissimilar polymers. Often these thin layers represent only 1 or 2% of the total structure thickness and are therefore extruded at a relatively low rate. With wide dies it is difficult to obtain uniformity when extrusion
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Direction of flow Transition channel Feedports meter layers of polymers
Layered sheet or film
Fig. 3. The principle of the feedblock for coextruding multilayer film or sheet. Number of layers is equal to number of feedports.
rate per width is very low. Also, it is difficult to coextrude thermally- sensitive polymers such as poly(vinyl chloride) (PVC) and poly(vinylidene chloride) copolymers (PVDC) in wide dies because slow-moving material near the walls greatly increases residence time and thermal exposure. Feedblock/Single-Manifold Dies. The feedblock method of flat-die coextrusion, originally developed and patented by The Dow Chemical Company, uses a feedblock before a conventional single-manifold die (12,13). A layered melt stream, which is prearranged ahead of the die inlet by the feedblock is extended to the width of the die as it is reduced in thickness (Fig. 3). Polymer melts from each extruder can be subdivided into as many layers as desired in the final product. Feedports arrange metered layers in required sequence and thickness proportions. A commercial feedblock/single-manifold die system is shown in Figure 4. Modular feedblock design similar to that illustrated can be used to change the number, sequence, or thickness distribution of layers by changing a flow programming module in the feedblock. Programming modules consist of machined flow channels designed to subdivide and direct flow of each material to specific locations and proportions required by the product. The shape of the multilayer melt stream entering the die inlet can be round, square, or rectangular, as long as the feedblock is properly designed to deliver the layers to the die with constant composition (14). Some feedblock suppliers prefer round die entry design for ease of machining or retrofitting to old dies. Others prefer square or rectangular die entries for ease of design and minimization of shape change as the layer interfaces are extended to the rectangular die orifice. A thermally sensitive polymer can be encapsulated by stable polymers so that it does not contact the die walls, thus reducing residence time. The fact that the multilayer stream at the die inlet is narrow (∼2.5–10 cm) compared to die width makes it relatively easy to meter thin surface or adhesive layers. The versatility of the feedblock has made it the most popular flat-die coextrusion method. Large numbers of layers may be coextruded, layer structure may be readily altered with interchangeable modules, and thermally sensitive
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Fig. 4. Exploded view of modular feedblock and single-manifold die for three polymers forming a five-layer coextrusion.
polymers may be protected by encapsulation. It is estimated that over 95% of flat-die coextrusion systems use a feedblock. One limitation of feedblocks is that polymer viscosities must be matched fairly closely because the combined melt stream must spread uniformly within the die. Severe viscosity mismatch results in nonuniform layers; the lower viscosity material tends to flow to the die edges. A crude rule of thumb is that polymer viscosities must be matched within a factor of 3 or 4, which is a reasonably broad range for many commercially important coextrusions. Layer uniformity may be adjusted by varying melt temperature within limits dictated by heat transfer. Increasing temperature decreases viscosity, and material moves from the center to the edges; decreasing temperature has the opposite effect. Typically, the individual polymer melt temperatures differ by as much as 30–60◦ C. Beyond that, heat transfer tends to nullify further adjustment by temperature variation. Often polymers are intentionally selected with a mismatch in viscosities to avoid flow instabilities. Viscosity mismatch of a factor of 10 or more may be necessary. Layer nonuniformity expected with the mismatch is compensated by using shaped feedport geometry; that is, the layers are introduced into the die nonuniformly so that uneven flow within the die results in a satisfactorily uniform
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Feed inlet
Distribution pin Selector plug
Adjustable vane Melt stream
Fig. 5. Feedblock with movable vane partitions that adjust positions for different polymer viscosities and/or layer-flow rates.
final distribution. Considerable art has been developed to extend the range of viscosity mismatch that can be accommodated in a feedblock system by using compensating feedport geometry. Some feedblocks are reportedly capable of coextruding polymers with viscosity mismatch of 100 or more (15). This style of feedblock has movable vanes that partition individual layers prior to combining (Fig. 5). The vanes may be freely floating, automatically seeking their equilibrium position on the basis of flow rates and viscosities. This self-adjusting feature can accommodate wide ranges in relative flow rates and viscosities while maintaining layer uniformity in the final product. The vanes may be rotated manually and locked into a nonequilibrium position to adjust uniformity further. Often distribution pins are used, or the vanes are profiled to compensate for nonuniform layers. Combined Feedblock/Multimanifold Dies. Combinations of feedblocks and a multimanifold die are also used commercially. The multimanifold die can incorporate the same design principles as the feedblock, ie, vanes separating individual manifolds within the die. In a sense, the multimanifold die is a wide feedblock. A feedblock may be attached to one or more manifold inlets, as shown in Figure 6. With this system, polymers with widely different polymer viscosities and processing temperatures may be coextruded. A very viscous or high temperature polymer may be extruded through one or more die manifolds, while a thermally sensitive or much lower viscosity polymer is coextruded with adhesive layers through a feedblock feeding another manifold. Combining of all layers occurs prior to the final die land.
Rheological Considerations Polymer rheology information is critical for designing coextrusion dies and feedblocks. The flow characteristics of the polymer must be considered when selecting materials for coextruded products (see RHEOLOGY).
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Distribution pin Selector plug
Melt stream Adjustable vane
Fig. 6. Combination feedblock and multimanifold die system. Feedblock feeds center-die manifold.
The best designed die or feedblock does not necessarily ensure a commercially acceptable product. Layered melt streams flowing through a coextrusion die can become unstable leading to layer nonuniformities and even intermixing of layers under certain conditions. The causes of these instabilities are related to nonNewtonian flow properties of polymers and viscoelastic interactions. Viscosities of non-Newtonian polymers are dependent on extrusion temperature and shear rate, both of which may vary within the coextrusion die. The shear rate dependence is further complicated in that it is determined by the position and thickness of a polymer layer in the melt stream. A polymer used as a thin surface layer in a coextruded product experiences higher shear rate than it would if it were positioned as a central core layer. There are several types of flow instabilities that have been observed in coextrusion. Interface Distortion from Viscosity Mismatch. The importance of viscosity matching for layer uniformity was first studied in capillary flow of two polymers in bicomponent fiber production (16–19). Two polymers introduced side by side into a round tube experience interfacial distortion during flow if the viscosities are mismatched. The lower viscosity polymer migrates to regions of highest shear (at the wall) and tends to encapsulate the higher viscosity polymer. It is possible for the low viscosity polymer to encapsulate the higher viscosity polymer totally. Nature seeks the path of least resistance. The degree of interfacial distortion due to viscosity mismatch depends on the extent of viscosity difference, shear rate, and residence time. Layer nonuniformities in feedblock fed flat dies occur for the same reason when there is a large enough viscosity mismatch. Low viscosity polymer migrates to wet the die wall. For unencapsulated layers, this migration starts in the die manifold as the layered stream spreads, resulting in increased layer thickness for low viscosity polymer at the edges of the film or sheet. If unencapsulated low viscosity polymer is a core layer, it not only becomes thicker at the edges but may even wrap around higher viscosity skin layers at the film edges. Tubular-blown- film dies are more tolerant of viscosity mismatch because the layers are arranged concentrically, ie, there are no ends. Since streamlines
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cannot cross each other, further migration cannot occur. However, good die design is required to obtain concentric layers. Interface Distortion from Viscoelasticity. While matching the viscosities of adjacent layers has proven to be very important, the effect of polymer viscoelasticity on layer thickness uniformity is also important (20–24). It has been shown that polymers that are comparatively high in elasticity produce secondary flows normal to the primary flow direction in a die that can distort the layer interface. This effect becomes more pronounced as the width of a flat die increases. Appropriate shaping of the die channels can minimize the effect of layer interface distortion due to elastic effects. Coextruding a structure that contains layers of polymers with low and high levels of elasticity can cause interface distortion because of the differences in elasticity between the layers in flat dies. The effect is typically not observed in tubular dies. Flow Instability. Interface distortion causes thickness nonuniformities, but the interface remains smooth. Other instabilities result in irregular interfaces and even layer intermixing in severe cases. These instabilities are related to viscoelasticity in non-Newtonian polymers. At low output rates, low amplitude waviness of the interface is observed, which is barely noticeable to the eye and may not interfere with the functionality of the multilayer film. At higher output rates, the layer distortion becomes more severe. If a large amplitude waveform develops in the flowing multilayer stream within the die, the velocity gradient can carry the crest forward and convert it into a fold. Multiple folding results in an extremely jumbled, intermixed interface. This type of instability has been observed in tubular-blown film dies, multimanifold dies, and feedblock/single-manifold dies. This instability develops in the die land, and its onset can be correlated with a critical interfacial shear stress for a particular polymer system (2). The most important variables influencing this instability are skin-layer viscosity, skin-tocore thickness ratio, total extrusion rate, and die gap. Although the interfacial shear stress does not cause instability, elasticity is related to shear stress, and interfacial stress is used to correlate variables for a particular system. Interfacial instability in a number of coextruded polymer systems has been experimentally correlated with viscosity ratios and elasticity ratios (25), and a simplified rheology review has been given (26). This type of interfacial instability can be reduced or eliminated by increasing skin-layer thickness, increasing die gap, reducing total rate, or decreasing skinlayer polymer viscosity. These methods may be used singly or in combination. These remedies reduce interfacial shear stress, and stable flow results when it is below the critical stress for the polymer system being coextruded. Most often skin-layer polymer viscosity is decreased. In feedblock coextrusion the resultant viscosity mismatch imposed by this remedy can cause variations in layer thickness as discussed earlier. Shaped skin layer feedslots are then used to compensate. Other types of instabilities may exist, for example, a problem has been observed in feedblock coextrusion of axisymmetric sheet (27). A wavy interface is also characteristic of this instability, but the wave pattern is more regular when viewed from the surface. The instability originates in the die, well ahead of the die land, and internal die geometry influences both the severity and pattern. For a given die geometry, the severity of instability increases with structure asymmetry and some polymers are more susceptible to unstable flow than others.
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It has been suggested that this type of instability may be related to the extensional rheological properties of the polymers used in the coextruded structure (28). No complete predictive theory exists for these complicated rheological interactions, but the accumulated experience of polymer producers, equipment suppliers, and experienced fabricators provides guidance in polymer selection.
Polymers for Coextruded Films Polymers are chosen for individual layers to achieve special combinations of properties, eg, mechanical strength and stiffness, gas and water-vapor barrier, oil and grease barrier, heat seal, hot tack, Adhesion, optics, formability, machinability, and economics. Individual layers may be pure polymers or blends, sometimes with regrind or recycled scrap. The polymer layers are often formulated with color, antislip, antiblock, antistats, processing aids, fillers, biocides, oxygen scavengers, antifogs, flame retardants, nucleating agents, and stabilizer additives to enhance layer characteristics (29–35) (see ADDITIVES). Adhesive polymers are used as tie layers to bond dissimilar polymers that do not normally adhere to each other (see ADHESIVE COMPOUNDS). Common polymers used in coextrusion applications are listed below with their abbreviations:
Name
Abbreviation
Low density polyethylene Medium density polyethylene High density polyethylene High molecular weight, high density polyethylene Linear low density polyethylene Ultralow density polyethylene Polyolefin plastomer Ethylene–styrene interpolymer Ethylene–vinyl acetate Ethylene–acrylic acid Ethylene–methyl acrylate Ethylene–ethyl acrylate Cyclic olefin copolymer Ethylene–n-butyl acrylate Ethylene–methacrylic acid copolymer Ethylene–methacrylic acid salts Ethylene–vinyl alcohol Poly(vinyl alcohol) Polyamide Poly(vinyl chloride) Poly(vinylidene chloride) copolymers Polypropylene Polybutylene Poly(ethylene terepthalate) Glycol modified polyester Poly(ethylene napthalate)
LDPE MDPE HDPE HMW-HDPE LLDPE ULDPE POP ESI EVA EAA EMA EEA COC EnBA EMAA Ionomer EVOH PVOH PA PVC PVDC PP PB PET PETG PEN
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Name
Abbreviation
Poly(methyl methacrylate) Poly(hydroxyaminoether) Polycarbonate Polystyrene High impact polystyrene Acrylonitrile–methyl acrylate copolymer Acrylonitrile copolymers Acrylonitrile–butadiene–styrene polymers Styrene–butadiene block copolymer
PMMA PHAE PC PS HIPS AN/MA AN ABS SB
See ETHYLENE POLYMERS, LDPE; ETHYLENE POLYMERS, HDPE; ETHYLENE POLYMERS, LLDPE; VINYL ALCOHOL POLYMERS; VINYL CHLORIDE POLYMERS; VINYLIDENE CHLORIDE POLYMERS (PVDC); POLYAMIDES, PLASTICS; POLYESTERS, THERMOPLASTIC; PROPYLENE POLYMERS (PP); METHACRYLIC ESTER POLYMERS; ACRYLONITRILE POLYMERS; STYRENE POLYMERS (PS); IONOMERS. An individual polymer usually can provide several functions, but selection for each layer is determined by the key property that the polymer can contribute to the total film for a specific application. Mechanical Properties. The coextruded film must have adequate tensile and impact strength, tear resistance, elongation, and puncture resistance for package integrity. LDPE, LLDPE, POP, HDPE, PP, nylon, ionomer, and EAA are typical resins used for toughness. Significant catalyst development has resulted in various forms of polyethylene copolymers. Ziegler–Natta catalysts produce linear copolymers with a broad short-chain branch distribution and a broad molecular weight distribution. Metallocene catalysts produce copolymers with a more homogeneous comonomer distribution and a narrower molecular weight distribution. Constrained geometry catalysts produce copolymers with a small but significant amount of long-chain branching, in addition to any short-chain branching from the comonomer, as well as with a more homogeneous comonomer distribution and a narrower molecular weight distribution. Metallocene and constrained geometry catalysts allow higher levels of comonomer and molecular tailoring for specific properties. In general, metallocene and constrained geometry catalysts have better optical properties, ESCR, impact strength, puncture strength, and tensile strength than Ziegler–Natta catalyst polymers. The lower density copolymers made possible with metallocene and constrained geometry catalysts, POP, demonstrate enhanced breathability for controlled atmosphere packaging applications. The lower density resins also have lower melting points and may be used as heat seal resins with good hot tack and seal integrity, allowing them to be substituted for EVA or ionomers. The open sites of the metallocene and constrained geometry catalysts also allow the copolymerization of different comonomers such as styrene and norbornene with ethylene. Metallocene technology has also been used for the manufacture of isotactic PP, syndiotactic PP, copolymers of PP with other olefins, and syndiotactic PS (36–45). Most published film data is derived from monolayer film and as a first approximation, tensile strength of a coextruded film may be estimated from the percentage of each polymer present in the film according to the law of mixtures,
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ie, the summation of the tensile strength per unit thickness of each layer, multiplied by its thickness, divided by total thickness. However, layer interactions can greatly influence mechanical behavior of composites. Favorable (synergistic) interactions provide mutual interlayer reinforcement (46,47), and the composite acquires better mechanical properties than expected from summation of the components as a blend (48–52). For example, when a normally brittle polymer layer is sandwiched between ductile layers which can inhibit crack propagation, the brittle layer may undergo greater deformation than is possible for it as an unsupported free film. Unfavorable layer interactions can lead to mutual interlayer destruction, ie, failure in one layer leads to premature failure in normally ductile layers, causing catastrophic failure of an entire composite. In this case, the cracked layer acts as a notch to localize stress concentration in adjacent layers. Layer multiplying enables structures with hundreds or thousands of layers to be produced. A layered melt stream from a feedblock is fed through layer multipliers. In each multiplier, the initial melt stream is divided vertically in two, spread horizontally, and then recombined, resulting in a doubling of the initial number of layers. Improved physical properties, mechanical, optical, barrier, and electronic, have been demonstrated. Synergistic combinations of the properties of the component polymers have also been reported. The failure mechanism of microlayered PC/SAN structures can be controlled, with layer thicknesses in the micron range demonstrating improved toughness and impact relative to conventional blends. Microlayering can also produce structures with aligned platelet fillers, demonstrating anisotropic physical, barrier, and electrical properties. Breathable films with high water vapor transition rates and good mechanical properties are obtained with a microlayered filled polypropylene/polyethylene oxide system. A number of companies have commercialized microlayer structures with unique optical properties. These structures have from 100 to 500 layers and total thickness from 1 to 2.5 mil. These films can be tuned to reflect or transmit different segments of the visible or near ir portions of the electromagnetic spectrum. Applications range from iridescent decorative films for packaging and labeling to metal free films that specularly reflect up to 98% of visible light from any incident angle for electronic display enhancement. Microlayer structures, with the ability to control the interface/volume ratio, have also been used for fundamental interdiffusion and adhesion studies (53–59). Molecular orientation is another important factor influencing mechanical properties of coextruded films (11). Biaxial orientation can greatly improve film strength. However, uniaxial or highly unbalanced orientation causes poor transverse properties, which result in easy splitting of coextruded films in the machine direction. This tendency may occur even when a relatively thin layer responds to unidirectional orientation and propagates failure to thicker adjacent layers. Therefore, although law of mixture calculations for multilayer films may be used for an approximate estimate of strength, it is inadequate for predicting layer interactions and ultimate film performance. Mechanical properties measured on coextruded films include the effect of orientation during processing, as well as polymer–layer interactions. Optimization of film properties by plotting property against percentage composition of polymer combinations yields the most comprehensive picture of film performance.
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Table 1. Oxygen Permeabilities Resina
nmol m·s·GPa
mL·mil m2 ·d·atm
mL·mil 100in·2 d·atm
EVOH PVDC Nitrile barrier resin Nylon-6 Polyester Rigid PVC Polypropylene HDPE PS LDPE
0.04 0.3 1.6 5.2 7 10–40 300 300 700 840
0.3 2.3 12.4 40.3 54 78–310 2300 2300 5400 6500
0.02b 0.15 0.8 2.6 3.5 5–20 150.0 150.0 350.0 420.0
a See b Dry
listed abbreviations in “Polymers for Coextruded Films” section. only.
Gas-Barrier Properties. Coextruded films are often used to provide a barrier to gases (oxygen, nitrogen, and carbon dioxide) and water vapor in packaging applications. PVDC, EVOH, acrylonitrile, nylon, PVC, and PET provide varying degrees of barrier to gases, flavor and aroma components, and organic fractions (60,61). A variety of nylon polymers are available including nylon-6, nylon-6,6, nylon-11, nylon-12, amorphous nylon, and MXD-6 (62) (see POLYAMIDES, PLASTICS). Coextruded films with liquid crystalline copolymers as well as with PCTFE have been reported (63–65). Poly(hydroxyaminoether) thermoplastics for barrier packaging have recently been commercialized (66). Table 1 gives comparative oxygen permeabilities for several plastics based on 25.4-µm thickness at 23◦ C. Various aspects of film barriers have been reviewed (67–70) (see BARRIER POLYMERS). At steady state, gas-transmission rate through a given layer is inversely proportional to its thickness. The total transmission rate through a multilayer film may be calculated by treating the contribution of each layer as resistances in series (71). For many coextruded films the overall transmission rate is controlled by the high barrier layer, ie, the layer with lowest transmission rate. Gastransmission rates through polymers increase with temperature. Some polymers, such as EVOH, are moisture-sensitive, and oxygen-transmission rate increases with relative humidity. Therefore, application of coextruded barrier films to packaging requires knowledge of the package environment during filling, processing, shipping, and storage. Data comparing EVOH and PVDC suggest an aggregate oxygen exposure index to evaluate barrier coextrusions under varying environmental conditions (72). Inadequate understanding of product barrier requirements poses a packagedesign problem in predicting adequate shelf life from gas-transmission data. Langmuir kinetic theory has been discussed for prediction of barrier requirements (73), but packagers still must rely on extensive shelf life testing of individual food products in candidate barrier films. Table 2 gives water vapor transmission rates for several polymers. The ranking of polymers for water vapor transmission is different from ranking them for
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Table 2. Water Vapor Transmission Rates Resina,b PVDC PP HDPE LDPE polyester rigid PVC nitrile barrier resin PS EVOH nylon
µmol m2 ·s
g h·m2
g 100in·2 ·d
1–1.5 2.5 3–4 10–15 10–13 9–51 50 70–100 130 160–220
0.065–0.097 0.16 0.23 0.81 0.8 0.6–3.3 3.2 4.5–6.5 8.4 10.3–14.2
0.1–0.15 0.25 0.3–0.4 1.0–1.5 1.0–1.3 0.9–5.1 5.0 7.0–10.0 13.0 16–22
(1 mil) film at 37.8◦ C and 90% rh. listed abbreviations in “Polymers for Coextruded Films” section.
a 25.4-µm b See
oxygen transmission. LDPE, HDPE, and PP are economical barrier polymers to water vapor. PVDC is one of the few polymers that provide an excellent barrier to both oxygen and water vapor. Barriers to aromas and flavors are very important in packaging and cannot be predicted from common gas-barrier data. Nylon has a good aroma barrier for certain snack-food packaging (74). Flavor scalping is also important in some applications. Oil and Grease Barrier. Oils and grease must be retained in the packaged product for product quality and must not degrade the package, its printing, or customer appeal. Nylon, EAA, and ionomers are good oil- and grease-barrier materials. Adhesion. Successful combination of coextruded film layers has been described in terms of rheological compatibility, surface tension and melt viscosity at processing temperature, interfacial behavior at the surfaces between layers, chemical interaction between two combined materials, crystallinity, and shear compatibility between components of the composite (67). Critical factors in layer bonding are polymer functionality (chemical composition), melt temperature, time in contact at temperature, viscosity of the joining layers, layer thickness, thermal stability, orientation, quench rate, and moisture sensitivity. No comprehensive theory exists for predicting interlayer adhesion in coextrusion (see ADHESION). Most knowledge comes from trial-and-error testing that has led to qualitative rating charts for polymer adhesion such as the one shown in Table 3. Polymer or copolymer functionality plays a strong role. Polar polymers tend to adhere to each other; adhesion is difficult between nonpolar polymers. Some polymers form covalent bonds in addition to H-bonding, acid–base, ionic, and dipole–dipole interactions (75–78). Tie layers are used to adhere incompatible layers of dissimilar polymers. EVA, EMA, EAA, EEA, EnBA, SB, and ionomer are frequently used copolymers. The importance of tie layers for coextrusion has led material suppliers to develop new chemically modified polymers for specific applications. Tie layers also contribute physical properties, optics water barrier, modulus, thermal resistance,
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Table 3. Qualitative Degree of Adhesion Between Resinsa,b
and toughness. Examples include a family of Plexar resins by Equistar (79), a series of CXA resins by DuPont (80), and Mitsui’s Admer adhesive resins. Greater development of extrudable adhesive polymers is expected. Heat Seal. Heat sealability characteristics of a structure are controlled by the outer surface layers and the type of seal geometry used to form a package. Heat-seal layers must fuse and adhere to themselves and other layers. Seal strength, heat-seal temperature range, and sealability through contaminants (81) are important to high fabrication rates and package integrity. LDPE, POP, EVA (82), EAA (83), ionomer (84), and LLDPE are common seal-layer materials. LDPE is the standard multiple-purpose layer, whereas EVA has outstanding low temperature seal and seal range. EVA is frequently coextruded with HDPE to prevent the puckering that occurs when HDPE is heated to its softening point to form a heat seal. The higher softening temperature of HDPE allows use of hotter heat-seal bars. EAA, POP, and ionomers are strong and seal through contaminants (85– 87). A resin’s hot tack, ie, the ability of a molten seal to resist separating force,
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determines the rate at which the polymer can be sealed. LDPE, LLDPE, POP, EVA, EAA, and ionomer are materials of choice. Machinability. Packaging films must run through printing presses and package-forming machinery without marring, hang-up, wandering, or deformation. Combinations of LDPE, PP, and PET are selected for structure-fabrication needs. PC is being investigated as a high modulus layer that can provide toughness without orientation and compete with oriented PET. Optical Properties. Many packages need sparkling clarity to display their contents (see OPTICAL PROPERTIES). Other packages have printing over nearly all of the surface area or the product must be protected from uv radiation. White, brown, and black pigmented layers are combined to protect light-sensitive products. The gloss and haze of a coextruded structure are modified by particular layers in the structure. The total haze exhibited by a film depends on the surface haze and the internal film haze. Placing a high haze layer on the inside of a film and low haze layers on the outside surfaces improves the haze of the buried material. The outside layer controls the gloss. Modification of haze gloss may be obtained with processing conditions, processing aids, nucleating agents, and other additives. In the case of cast films, a polished roll gives a high gloss, low haze film with tack. A matte chill roll produces a dull surface. High Temperature Dimensional Stability. Dimensional stability at elevated temperatures is important in high temperature filling, sterilization applications, and microwave oven lidding. Resistance to physical deformation under load is linked to physical distortion of the part or package. HDPE, PP, nylon, and PET are suitable for high temperature applications. Coextrusion of PC with barrier polymers is being developed for high temperature barrier packaging (88). Economics. An advantage of coextrusion is the capability of combining layers of high performance resins with low cost resin layers to produce high performance/low cost composite structures. The use of recycled and scrap resins in buried layers further improves economy. Multilayer extrusion economic considerations have been calculated (89). The effect of recycle on film properties, eg, tensile strength, impact, and elongation, depends on the degree of compatibility of polymers in the recycle layer. Often the tie-layer polymer acts as a compatibilizer for recycle. Recycle of incompatible polymers with different refractive indexes usually causes haziness and cannot be used when excellent optical properties are required. Aesthetics. Coextruded layers may be colored for appearance, light screening, or coding. White is used as a printing background or a cleanliness layer adjacent to the product. Colors are used to designate sterile and nonsterile surfaces; black and brown screen uv light. Coextruded film composed of over 100 layers of alternating materials with different refractive indexes produces a vivid iridescence used in decorative applications (1,90). If gloss is essential, a thin surface layer of a high gloss polymer can be coextruded to give the package sparkle for marketing appeal. Formability. Materials with a broad softening range, such as PS and PC, can be thermoformed into deep-draw package shapes. Crystalline polymers, eg, HDPE and PP, are more difficult to thermoform, but solid-phase forming processes below the melting point are possible. Multilayer semirigid barrier containers are made by Shell’s Solid Phase Pressure Forming Process and the patented Dow
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Scrapless Forming Process (91–94) that is being developed by Cincinnati Milacron. An advantage of the latter process is that round containers may be made from square blanks cut from sheet without the trim scrap normally associated with sheet forming.
Applications Flexible packaging is important in several markets. Stand-up pouches have grown significantly in the pet foods and agricultural chemical markets. In 2002, food packaging will be over half of the $9.7 billion converter sales in flexible packaging. Case ready meats, growing at 21% annually through 2002, is the fastest growing segment of the perishables market. Structures include clear antifog lidding material, high barrier films, and high abuse shrink films. High or moderate barrier materials will exhibit a 5% growth rate in flexible foodservice pouches. Dry foods markets include snacks and candy, cereals, pet food, and coffee, all of which are growing approximately 5% annually. Consumer and industrial markets include healthcare packaging growing (6% annual growth), medical disposables (8% annual growth), agricultural packaging (4% annual growth), and palletizing and unitizing (9% annual growth) (95–99). Future developments are expected to focus on high barrier coextruded films for longer shelf life and better control of product quality. Products now packaged in conventional materials such as glass and metal will be packaged in more efficient coextruded materials. Structure design requires careful selection and planning because of the multitude of possible choices regarding material combinations, layer placement, layer thickness, etc. Often a coextruded film is further processed by lamination and coating into more complex structures to meet specific product application requirements. Theoretical design and product testing must be combined to yield the most efficient film structure for a specific application. Table 4 shows common designs that have evolved for the principal markets (100–102). Agricultural films for mulch, greenhouse, and fumigation is illustrative. Films have been developed to capture a specific portion of the light spectrum. Films that absorb or block certain ir wavelengths and films with a specific color designed for a particular crop have been developed. Solarization films generate heat, raise soil temperature, and sterilize the soil. Light stabilizers are added to extend the life of greenhouse films on one hand while other films have been developed to minimize disposal costs at the end of the growing season. Coextruded barrier films that have 1000 times the methyl bromide barrier of a monolayer polyethylene film are also available (103). Commercial applications for coextruded multilayer sheet began later than for films, but volume has been growing rapidly and now exceeds that of films. The largest use of coextruded sheet is in thermoformed semirigid containers. The earliest applications were two- and three-layer sheet of similar polymers, such as PS two-color drinking cups, disposable dinnerware, and dairy tubs for cottage cheese and margarine. These dairy containers had a thin gloss layer of crystal PS coextruded on HIPS for marketing appeal. Because the layers were compatible in these easily thermoformed structures, problems with trim-scrap recycle were minimal. The disposable containers were usually coextruded as three layers with
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Table 4. Film Structures for Principal Markets Markets Trash bags Waste disposal
Stretch film
Shrink Film Tray wrap Meat packaging Primal, subprimal packaging
Bacon/luncheon/weiners Flexible, formed web
Flexible, nonformed webs
Rigid formed web Cooked ham Poultry Individual bags
Bulk barrier bag
Snack foods
Cookies/crackers, bakery
Typical structuresa LDPE/LLDPE/LDPE LDPE/LLDPE EVA/LLDPE LLDPE LLDPE/LDPE + regrind HDPE/regrind/LLDPE LDPE/LLDPE/LDPE EVA/LLDPE/EVA LDPE/LLDPE LDPE/LLDPE + regrind/LDPE EVA/LLDPE/EVA LDPE/EAA EVA/PVDC/EVA EVA/PVDC/EVA/ionomer Nylon/adhesive/EVA Nylon/ionomer Nylon/EVA/ionomer Nylon/PVDC/adhesive/EVA–ionomer Nylon/adhesive/EVOH/adhesive/ionomer PVC/PVDC/EVA Nylon/EVOH/EVA/ionomer PET/adhesive/EVA–ionomer PP/adhesive/EVA–ionomer PVC/PVDC/EVA AN, MA/adhesive/EVA–ionomer PETGb /adhesive/EVA–ionomer Nylon/adhesive/ionomer PP/adhesive/nylon/adhesive/ionomer LDPE/EVA HDPE/EVA HDPE/ionomer EVA/HDPE/LDPE/ionomer HDPE/EVA HDPE/ionomer + PVDC coating OPP, white LDPE, white HDPE/EVA–ionomer HDPE/adhesive/nylon/EVA HDPE/EVA–ionomer blend HDPE/ionomer/EVA HDPE/ionomer/nylon/ionomer White HDPE/brown HDPE/EVA HDPE/EVA HDPE/HDPE/EVA HDPE/MDPE/EVA PP/adhesive/EVA
19
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Table 4. (Continued) Markets Cereals Low barrier
Medium barrier Cheese
Frozen food
Heavy-duty bags Shipping bags
Medical
a See
Typical structuresa HDPE/EVA HDPE/HDPE/EVA HDPE/HDPE/EVA–ionomer blend HDPE/adhesive/EVOH/adhesive/EVA HDPE/adhesive/nylon/adhesive/EVA PP/PE Nylon, PVDC, LDPE/ionomer LDPE/PVDC/LDPE/ionomer EVA/PVDC/EVA LDPE/PVDC/LDPE/adhesive/EVA EVA/white LDPE/EVA EVA/white LLDPE/EVA PET/adhesive/LDPE–ionomer LDPE/PVDC/LDPE LDPE/EVA LDPE/LLDPE/EVA LDPE/HDPE/EVA LDPE/LLDPE Chlorinated PE/EVA White LDPE/black MDPE–EVA White LDPE/colored LDPE White ionomer/LDPE/colored EVA Plexar/nylon/LDPE/Plexar/nylon/Plexar PET/LDPE Nylon/LDPE LDPE/PVDC/LDPE
listed abbreviations in “Polymers forCoextruded Films” section. glycol-co-cyclohexane-1,4-dimethanol terephthalate).
b Poly(ethylene
mixed-color scrap as a buried layer. The marketing appeal and ease of fabrication of coextruded semirigid containers quickly saturated those market applications. The principal growth area is in high barrier, semirigid food packages for long shelf life applications in competition with metal and glass for unrefrigerated storage. These advanced coextruded structures were developed in Europe during the 1970s and combine as many as five dissimilar polymers into six- to -nine layer structures (Fig. 7). A relatively thin barrier layer is coextruded with one or more bulk structural layers of HDPE, PP, or HIPS to provide package strength and rigidity. Often a thin surface layer as the interior of the package provides heat sealability for the lid stock to the container flange. In other applications, a hermetic seal is made by double seaming a metal can end onto the plastic container body. Adhesive layers bond the barrier layer to the structural layers. Often a pigmented, light barrier layer is included as an inner layer for food products that are sensitive to uv light. Early European applications for barrier sheet were in form-fill-seal (FFS) packaging of soft cheese, fruit drinks, jams, jellies, and condiments, and high temperature-short time sterilized milk. FFS packaging (where roll stock is
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HDPE or PP Glue Saran Glue HDPE or PP (a) PE Glue Saran Glue HIPS (b) PE Glue Saran Glue Black HIPS or scrap HIPS (c)
Fig. 7. Typical coextruded sheet structures used for barrier food packaging. Functional properties: (a) retortability, high oxygen and moisture barrier (three extruders); (b) heat sealability, oxygen and moisture barrier, low taste and odor transfer, thermoformability (four extruders); (c) oxygen and moisture barrier, uv light protection, heat sealability, thermoformability (five extruders).
converted into packages, filled with product, and sealed in the customers plant) minimizes scrap recycle. Round FFS packages have square heat-seal flanges so that most of the material is used in the package. However, the desire to use round preformed containers and to make more economical use of materials has stimulated recycling of multilayer scrap of dissimilar materials as a buried core layer. Sometimes this recycle layer is a black, pigmented uv barrier layer. Selection of the bulk structural layers to be coextruded with the barrier polymer layer depends on package strength requirements, rigidity at minimal wall thickness, and package temperatures during product filling, thermal processing, and storage. In many FFS packaging applications, the food product is filled at ambient or low temperatures (74◦ C), and HIPS is chosen for its modulus and ease of forming. Most FFS machines are designed to form HIPS. Products requiring higher fill temperatures have layers of HDPE or PP. Heatsterilized containers heated up to 121◦ C employ PP, although HDPE can be used for retort temperatures up to about 112◦ C. HDPE has better low temperature impact properties than PP, which may be important in distribution and warehousing. HDPE and PP are more difficult to thermoform than HIPS; PP usually undergoes solid phase forming. Fillers are sometimes added to improve polyolefin stiffness and formability.
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An alternative to high temperature retort sterilization of shelf-stable foods in the package is aseptic packaging in which a sterile food is packaged in a commercially sterile environment. In one technique the container is sterilized with a hydrogen peroxide solution before it is filled and sealed in a safe environment. A second technique, invented by the French company, ERCA, and licensed to Continental Packaging, uses a novel sacrificial layer to provide a sterile package interior. Continental’s FFS machine, the Conoffast System, has the forming and filling stations in a safe environment. Roll stock of coextruded PP/PE/tie/PVDC/tie/black HIPS/HIPS is fed into the machine where the PP layer is separated, exposing a sterile PS layer. The barrier sheet is formed, filled, and then sealed with a second sacrificial-layer lid stock. Formed multilayer barrier packages, eg, trays, bowls, and cans from coextruded PP/PVDC/ PP, are being developed for soups and entrees that can be stored at ambient temperatures and heated to serving temperature in a microwave oven. Coextrusions of high performance, high temperature polymers, such as polyetherimide and polysulfone, for dual-oven containers capable of withstanding conventional bake-oven temperatures of 204–232◦ C are being tested. In addition to packaging markets, sheet coextrusion is applied in construction, recreational vehicle, and sanitaryware markets where weatherable or chemically resistant layers are coextruded with low cost polymers. Continuing development of coextruded film and sheet structures from combinations of high performance polymers with low cost polymers and improved adhesive resins will expand the market opportunities for plastics in competition with other materials. Easy combination of many layers and a better understanding of synergistic behavior in multilayer film and sheet will also lead to new applications.
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JOSEPH DOOLEY HARVEY TUNG The Dow Chemical Company
