«Viscoelastic flow effects in multilayer polymer coextrusion Dooley, J. DOI: 10.6100/IR555718 Published: 01/01/2002 Document Version Publisher’s ...»
7. Debbaut, B. and Dooley, J., Secondary Motions in Straight and Tapered Channels:
Experiments and Three-dimensional Finite Element Simulation with a Multimode Differential Viscoelastic Model, J. of Rheol., 43, 6, 1525, November/December 1999.
xii Chapter 1 Overview of different multilayer extrusion processes 1 Overview of different multilayer extrusion processes
1.1 Introduction Multilayer coextrusion has developed into an important polymer fabrication process, providing large growth opportunities for the polymer industry. Coextruded multilayer polymers are challenging such traditional materials as metals, glass, paper, and textiles.
The attraction of coextrusion is both economic and technical. It is a single-step process starting with two or more polymer 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 (l). It is difficult to imagine another practical method of manufacturing these microlayer structures.
Layers may be used to place colors, bury recycle, screen ultraviolet 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, e.g. twoor 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 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.
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.
The choice of whether to use the blown or cast film process is normally dependent on the rate and final properties of the structure that are desired. Cast film lines can typically run at a higher rate than a blown film line because the cooling efficiency of a chill roll is higher than using air to cool a bubble. However, the cast film process produces a product with uniaxial orientation rather than the biaxial orientation produced with the blown film process. In many cases, biaxial orientation is preferred to produce a film with more balanced physical properties.
Coextrusion die sizes vary and are dependent on the application. Blown film dies for the agricultural market can be as large as 2 meters in diameter that would produce a film approximately 8 meters wide. Cast film dies for agricultural films have been made with widths as large as 10 meters.
In addition to uses in bags and packaging, coextruded structures are also used in many other areas. Many coextruded sheets are made for use in thermoforming operations to form specific package or container shapes. Coextrusion is also used in the profile market. Pipes as well as window profiles have been made from coextruded structures.
1.2 Blown film, single versus multi manifolds
Tubular coextrusion dies were the earliest dies used to make multilayer polymer 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 he 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 weldline problem.
The design most commonly used today is the multimanifold spiral mandrel tubular-blown film die (Fig. 1-1). 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 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. Whereas early designs were limited to two or three layers, dies containing seven or more layers are now offered commercially.
Figure 1-1. A three-layer blown film die
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 single-layer dies, are used to improve circumferential distribution. A well-designed spiral mandrel manifold can he 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 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 (Figure 1-2). 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. The major disadvantage for this style of die is that there is a large separating force between the plates and so many die bolts are required to hold the plates together. This means that the plates must be rather large in diameter in order to maintain structural integrity and this can produce longer flow paths and temperature differentials that can be detrimental to thermally sensitive polymers. Depending on the types of polymers to be used and the number of layers, the prices of the spiral mandrel and stacked plate dies are comparable.
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).
1.3 Sheet extrusion, single versus multi manifolds 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 250 µm, as well as in coextrusion coating processes (2), where a multilayer web is extrusion-coated onto a substrate such as paperboard, aluminum foil, polymer 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 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 tracks that diverge inside a temperaturecontrolled 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.
1.4 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.
The vast majority of multimanifold dies are internal combining rather than external combining. This is due to the fact that better adhesion between the layers is normally found with internally combined structures because they are in thermal contact for a longer period of time and can form products with better interfacial adhesion.
In principle, internal-combining dies are similar to multimanifold-tubular-coextrusion dies except that the manifolds are flat (Fig. 1-3). 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.
Figure 1-3. Cross-sectional view of three-layer internal-combining multimanifold flat film or sheet die A significant disadvantage of wide multimanifold dies is the difficulty in coextruding very thin layers, such as thin cap (surface) layers, or thin adhesive (tie) layers used to bond two dissimilar polymers. Frequently these thin layers represent only 1 or 2% of the total structure thickness and therefore are extruded at a relatively low rate. With wide dies it is difficult to obtain uniformity when extrusion 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.
1.5 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 as is shown in Figure 1-4 (12-13).
A layered melt stream, which is prearranged ahead of the die inlet by the feedblock, is extended the width of the die as it is reduced in thickness (Fig. 1-5). 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 1-6. 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 the flow of each material to specific locations and proportions required by the product. This technique can also be used to minimize edge waste through tuning of the feedblock to produce the desired structure across the desired width of the film or sheet.
Figure 1-5. The principle of the feedblock for coextruding multilayer film or sheet. Number of layers is equal to number of feedports 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 it does not contact the die walls, thus reducing residence time.