Multi-layer films Coextrusion

Biaxial orientation is often used to improve the in-plane tensile strength and toughness of polymer films. The crystallinity of PET increases on stretching (Fig. 8.16), so the permeability decreases. For multi-layer films made by coextrusion or coating, the total permeability Ptot is related to the layer value... 

Typical processing methods - mono- and multi-layer film extrusion, blow molding, pipe coextrusion, extrusion coating, co-injection molding, lamination ... 

The objective of this study is to understand the role of the coextrusion process in governing the interlayer adhesive strength in multi-layer films. 3-layer flow... 

This chapter covers single layer, multi-layer and combination materials found as films, foils, laminations, coextrusions, coatings, etc. 

The above heading has been selected to include all variants which may be now used for both flexible and rigid applications. Whereas previously these may have been introduced under flexibles , film, foils and laminates , etc., the use of new technology such as coextrusions, metallisation and different coating techniques has produced some confusion as to what terminology should be employed, so the word multi-layer may be more appropriate. 

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. 

Interlayer adhesion between a polycarbonate (PC) layer and a PC-ABS blend layer, in 3-layer films made by melt coextrusion in a multi-manifold die, was analyzed using nonlinear viscoelastic (NLVE) die-flow simulations with POLYFLOW. These simulations showed significant extensional stresses in the interface vicinity where the two melt layers come into contact for the first time. A Viscosity Normalized Nonlinearity Ratio parameter was defined to correlate the simulated interfacial melt stresses and the observed adhesion behavior. 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. 

Coextrusion produces multilayer laminates in a single process step. Two or more extruders feed different molten polymers into a multi-manifold die which layers them directly, or into a modular feedblock which layers them before feeding them into the die. This is used primarily in the packaging field, to sandwich an impermeable barrier layer between two commodity outer-film layers, and often includes adhesive tie layers to bond the barrier layer to the outer layers. 

Coextrusion produces a multi-ply material directly from the individual resins. The method is limited to thermoplastic materials such as polythene, polypropylene and nylons. Thin layers of extruded bonding resins are necessary to combine many of the resins. Coextrudates have to be surface printed and the outer film cannot be reverse printed as it is often used with more conventional laminates with a film outer ply. 

The third coextrusion technique uses multi-manifold external-combining dies, which have completely separate manifolds for the different melt streams as well as distinct orifices through which the streams leave the die separately, joining just beyond the die exit. This technique is also referred to as multiple lip coextrusion. The layers are combined after exiting while still molten and just downstream of the die. For fiat film dies, pressure rolls are used to force the layers together. In blown... 

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