Coextrusion

Coextrusion is the process of forming an extru-date composed of more than one thermoplastic melt stream. The process came about because some service demands, particularly from the packaging industry, could not be satisfied by a single polymer although they could be met by a combination of polymers. Coextrusion was first practiced in the production of cast film and is now also used in blown film and sheet extrusion. The intention is normally to produce a laminar structure in which each layer contributes a key property to the overall product performance. Coextruded films may be very complex structures composed of many different functional layers, including tie layers whose purpose is to bond neighboring layers of limited compatibility. Five layers are not uncommon. However, side-by-side coextrusion is also possible. Fluoroplastics can be coextmded with other polymers such as ETFE and nylon. 

Coextrusion is the simultaneous extrusion of two or more polymers through a single die where the polymers are joined together such that they form distinct, well-bonded layers forming a single extruded product. Coextrusion has been applied in film, sheet, tubing, blown film, wire coating, and profile extrusion. 

Advantages of coextrusion are better bonds between layers, reduced materials and processing costs, improved properties, and reduced tendency for pinholes, delamination, and air entrapment between the layers. Another advantage of coextrusion is that it is often possible to reuse scrap material and locate it in an inside layer of the extruded product so that it does not affect the appearance of the product. An obvious disadvantage of coextrusion is that the tooling is more difficult to design and manufacture and, therefore, more expensive. Further, it requires at least two extruders, and it takes more operational skill to run a coextrusion line. 

Multi-manifold blown film die, three layers 

Coextrusion is practiced on a wide scale in blown film. There are many five-layer blown film coextrusion dies used in the industry five-layer films are now considered a commodity [41], Even seven-layer dies are not unusual. Some coextrusion dies use as many as 8 to 10 layers. Most of these multi-layer dies are used in high-barrier packaging for food. Conventional blown film coextrusion dies have a concentric arrangement of spiral mandrel manifolds. In some cases conical spiral mandrel sections are used, while in other cases the spiral mandrel section is machined into a flat horizontal surface. The latter arrangement is referred to as a pancake coextrusion die, because the different sections of the die are stacked like pancakes. A schematic of a pancake coextrusion system is shown in Fig. 9.39. 

The advantage of the pancake system is that many modules (disks) can be stacked together in a more or less modular fashion, allowing for as many as ten layers to be produced. Another advantage is the relatively compact design that keeps the space requirements to a minimum. 

A brief coverage of the topic of blown film coextrusion is provided in this chapter. Coextmsion is the process of feeding a single die with two or more different polymer melt streams. Within the die, the various flow streams are combined to form a single-ply film comprised of the individual layers (Fig. 5.1). Because of the high viscosity of polymer melts, the individual layers tend not to mix but to retain their positions within the combined flow stream. In some cases, over seven layers of polymer are extruded into a film. For each type of polymer layer in the final structure, a different extruder is connected to the die (Fig. 5.2). 

Coextrusion increases the cost and complexity of a blown film line considerably. Significantly higher hardware costs, increased operator training, and more sophisticated machinery and control systems are some of the demands of a coextrusion line. However, the gains resulting from increased product capabilities make most investments in coextrusion profitable. Improved film performance leads to increased market share and may open new markets. Also, material costs may be decreased through the use of thinner layers due to higher product performance characteristics. 

As discussed in chapter 3, there are two main types of coextrusion dies concentric and stack. Though both of these types are used in industry and most die manufacturers have a preferred design, there are several factors to consider with regard to the use of CO extrusion dies. 

Both concentric and stack dies possess a mix of positive and negative characteristics with regard to the above list of important factors. It is best to identify the key material (such as viscosities and melt temperature,) and processing (such as floor space and the need for layer rearrangement) characteristics of a specihc coextrusion system and discuss these with various die manufacturers. However, some general benehts of the two die types are that concentric dies tend to have minimal flow length of conjoined layers, and stack dies are easier to reconhgure and tend to have better layer temperature isolation. 

Dual Spiral Systems, Ontario, Canada, recently developed a novel approach to economically gaining the benehts of increased him layers [39]. The new die (Fig. 5.3) contains a how-splitting device in each melt stream that doubles the total number of layers in the hnal him structure. That is, every how stream is divided into two adjacent streams of identical polymer that have passed through opposite spiraling subchannels. The resulting 

Coextruded film may be a very complex structure composed of many layers to provide different functionalities and of different tie layers which improve the bonding between adjacent layers. Separate extruders are required for extrusion of the different layers. Apart from the functionality provided by the resin, the main issues involved with the selection of different layers for coextrusion are the viscosity of the resin and the adherence between layers. 

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Coextrusion. An increasingly popular technique to produce tailored film or sheet products is to coextmde one or more polymer types in two or more layers of melt (6). In this fashion the benefits of specific polymer types or formulations may be combined. Thus high cost barrier resins may be combined with a low cost thicker layer of standard resin to achieve an optimum barrier film at lower cost. Thin sUp-control layers may be used on the surface of a bulk layer of opticaUy clear resin to obtain an aesthetic film with good handleabUity. Lower melting outer layers may be used to provide heat sealing for polymers that seal with difficulty by themselves. 

Conventional extmsion or coextrusion may be performed on vertical or horizontal rotary or shuttle mold configurations. In shuttie blow mol ding the extmder and die are in fixed horizontal and vertical position two or more molds shuttie into and out of position beneath the die. By reciprocating in two planes, the mold may remove a parison and permit the extmder to function continuously. 

Strength they are of interest as a streteh film for meat packaging and for cling-wrap purposes. Some EVA is used in coextrusion processes for the manufacture of laminated film. 

With the growth of plastic use in containers and packages, requirements to make them more compatible or useful resulted in new developments occuring and continue to occur. The two major approaches for providing permeability resistance in plastic containers involve chemically modifying the plastics surfaces and, more important from a marketing standpoint, the use of barrier plastics with nonbarrier types to meet cost-to-performance requirements. This is achieved through coextrusion, coinjection, corotation, and other such processes (Chapter 8). 

In the past, a processor desiring to enter the field had little choice of equipment, but the increased interest in coextrusion has produced a proliferation of equipment. With rapidly changing market conditions and the endless... 

Table 8-17 Examples of compatibility between plastics for coextrusion...

Table 8-18 Comparison of feedblock and multimanifold coextrusion dies...

A number of techniques are available for coextrusion, some of them patented and available only under license. Basically, three types exist feedblock, multiple manifolds, and a combination of these two (Table 9-18). Productions of coextruded products are able to meet product requirements that range from flat to complex profiles. Figure 8-35 (a) shows a typical 3-layer coextrusion die and (b) examples of rather complex profiles that are routinely extruded. 

Coextrusion is the process of extruding two or more materials simultaneously or in tandem. It allows a combination of an ingredient such as wheat flour, which is inexpensive and easily enriched with vitamins and minerals, with dairy protein, which provides functionality and texture. For example, an early coextrusion of wheat flour and rennet casein was performed by van de Voort et al. (1984), who obtained products with varying characteristics depending on process parameters. 

Investigating coextrusion of com meal and WPI, Onwulata et al. (2003b) found that the melt temperature of the extmdate was more of an indicator of physical properties than specific mechanical energy. Quality attributes such as breaking strength, color, and expansion index were related to melt temperature measured at the die. 

Coenzyme A (CoA), 20 249—250. See also Ace to acetyl- Co A in citric acid cycle, 6 633 Coenzyme Q10, 17 673 Coercivity, ofM-type ferrites, 11 70 Coextruded food packaging, 18 44, 45 Coextrusion techniques, for gelatin capsule preparation, 11 549 Cofactors, 10 253 11 4 folic acid, 25 801-802 for enzymes, 3 672-673 protein, 20 828-829 vitamin B12, 25 804 vitamins as, 25 781 Coffea arabica, 7 250 Cojfea Canephora, 7 250 Coffea liberica, 7 250 Coffee, 2 108 6 366 7 250-271 biotechnology, 7 265-267 decaffeinated, 7 263 economic aspects, 7 263-264 estimated maximum oxygen tolerance, 3 381t... 

Novel laminates produced by coextrusion using bonding agents. 

Approximately 252 million dollars in counterfeit U.S. currency is recovered each year. While this is a small percentage of the 540 billion U.S. dollars that are assumed to circulate worldwide, there is clearly a need for more sophisticated security features. A possible solution is to use multilayer coextrusion technology to create an optical polymer substrate for U.S. currency. 

Multilayer coextrusion technology is used to create an optical polymer substrate for U.S. currency. The final product specifications of the optical currency are listed below ... 

The design of a die for an appiication is a very complex process. The design must be such that regions with long residence times do not exist, and it must provide uniform flow at the exit. Moreover, coextrusion dies must be designed with iayer uniformity and without process instabilities known as the wave instability and the zig-zag instability [i2, i3]. These instabilities are beyond the scope of this book and thus are not discussed further. Like transfer lines, the velocities in a die shouid be at least i.5 cm/s for thermally stable resins (with antioxidant systems) and at least 3 cm/s for resins that are not thermally stable. 

Ramanathan, R., Shanker, R., Rehg, T., Jons, S., Headley, S., and Schrenk, W.J., Wave Pattern Instability in Multilayer Coextrusion - An Experimental Investigation, SPE ANTEC Tech. Papers, 42, 224 (1996)... 

Tzoganakis, C. and Perdikoulias, J., Interfacial Instabilities in Coextrusion Flows of Low-Density Polyethylenes Experimental Studies, Polym. Eng. ScL, 40, 1056 (2000)... 

The production of foamed films and sheets from polypropylene and polystyrene is discussed, with particular reference to packaging applications. Advantages of foamed materials for this application are examined, and the chemical and physical foaming processes are described. Extrusion technology for film and sheet by chemical and physical foaming processes is discussed, and recent developments in the coextrusion of multilayer packaging trays for the food industry are considered. 

Multi-layer sheets for use in the mannfacture of food packaging containers were produced by the coextrusion of PP foam in combination with PE, an EVOH barrier layer and tie layers. Results are presented of studies of the mechanical and morphological properties, density and cell stmctnre of foam layers based on different types of PP. 8 refs. 

Laminar materials are produced by a variety of techniques. Coextrusion blow-molding produces a number of common food containers that consist of multilayers such as layers consisting of PP-adhesive-PVA-adhesive-adhesive-PP. 

Multilayered articles can be made by coinjection blow-molding or coextrusion methods. A three-layer system generally contains a barrier layer sandwiched between two exterior layers. These are actually laminar products. In the coextrusion sequence, several extruders can be used to place the material into the mold. The multilayer container is then produced from blowing air into the preform. 

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