In coextruded structures

Ethylene vinyl alcohol barrier resins are never used as single layer films in packaging but rather are utilized in coextruded structures combined with less expensive structural polymers. 

HNR can aiso used in coextruded structures, particuiarly with poiyolefins, in sheet, Aim, or bottie form. The HNR provides gas barrier and chemicai resistance, while the polyolefin provides water vapor barrier. 

Trends in interlayer adhesion in coextruded structures have been correlated to the nonlinear viscoelastic normal stresses in the interface vicinity using an ordering parameter called Viscosity Normalized Nonlinearity Ratio . Larger deviation of this parameter from a value of 1.0 (large disparities in melt extensional configuration across the interface) corresponded to poorer observed peel strength. This parameter is of particular importance in the region where the melt layers come into contact for the first time, in scenarios where polymer relaxation times are of a similar order as the flow timescales. 

Recycled PVC supermarket trays have been used successfully in production-scale trials to make the foamed layer in coextruded cladding. The extruded product had satisfactory foam density, foam structure and colour. Impact properties were better than those of the control made from virgin PVC this is attributed to the high levels of impact modifier used in tray formulations. These trials demonstrated that PVC straight-on trays can be recycled into foamed extrusions for wood replacement products. 8 refs. 

In the final section water and oxygen permeation experiments were done on thin coextruded films to aid in understanding possible transport mechanisms in multilayer structures. An important conclusion is that water transport is important and must be understood before quantitative calculation of oxygen permeation will be possible. 

The principal polymers used in blown film production are polyolefins, although other polymers can also be used. The major applications are those that require biaxial strength, and include bags of all kinds, as well as agricultural and construction film. In food packaging, coextruded structures with three to five layers, or even more, are common, with major markets including packaging for cereal, meat, snacks, and frozen foods. 

The resin was utiiized as the ciing iayer in coextruded cast and biown 0.8 mii (21 mm), one-sided stretch ding fiim structures. 

One disadvantage is the ability to provide printed structures where the print layer is sandwiched as is often used with conventional adhesive lamination. A way around this is to use a combination of adhesive lamination and coextrusion coating such that the coextruded structure is assembled with a pre-printed web of film. In addition, other non-extrudable materials, such as paper and metal foils, can be combined. 

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... 

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). 

Developments using copolymer-based materials in foam applications are of particular note. Novel foam structures offer attractive properties and characteristics, including softness, aesthetics, and drape, for a wide range of thermoplastic and cross-linked foam applications. Other product technologies of interest are as injection-molded structural foams, as foamed layers in multilayer structures and as foamed blends of interpolymers with styrenic and olefinic pol5nners (Fig. 8) (84-86). Interpolymers also have potential for coextruded film and sheet applications. 

For this reason, most EVAL resins are used in multi-layer coextruded structures with polyolefin materials. These are usually five-layer structures consisting of polyolefin/adhesive/EVAL resin/adhesive/polyolefin. 

EVAL resins have little or no adhesion characteristics to most other polymers except nylon. Therefore, adhesive resins, usually modified polyolefins, have been developed for use in coextrusion applications. A typical coextruded structure is shown in Figure 6. While this is shown as five different materials, a minimum of three can De utilized. For example,... 

Many advantages exist in coextrusion. The different materials used in the coextruded structure meets different performance requirements based on their combinations. A single expensive plastic could be used to meet performance requirements such as permeability resistance. 

The viscosity data presented for the coextruded structures is one way to look at the processability of those structures. Another way to look at the processability of a coextruded structure is to look at the pressure drop produced by the structure as it flows through the processing equipment. For the circular (rod) die used in these experiments, the pressure drop created as the resins were extraded through the die was recorded at each flow rate and layer ratio. For example, the pressure drop for Resin A flowing through the circular die at a shear rate of 40 1/s was 13.1 MPa/m (48.2 psi/in). The pressure drop for Resin B flowing through the same die at the same shear rate was 26.9 MPa/m (99.1 psi/in). 

The recycled plastic will also have a degree of different contaminants that would eliminate its use in certain devices or products, such as in medicine, electronics, and food packaging. However, within these market applications there are acceptable designs with three-layer coextruded, coinjected, or laminated structures having the contaminated plastic as the center layer, isolated by clean plastics around it and no migration occurring. 

Viscoelastic flow effects in polymer coextrusion. In this example we will present work done by Dooley [7, 8] on the viscoelastic flow in multilayer polymer extrusion. Dooley performed extensive experimental work where he coextruded multilayer systems through various non-circular dies such as the teardrop channel presented in Fig. 9.39. For the specific example shown, 165 layers were coextruded through a feedblock to form a single multiple-layer structure inside the channel. 

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