Coextrusion tie-layer

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. 

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 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. 

A very different type of coextrusion is used to produce opaque bottles with clear viewing stripes, so that the consumer can see the product level in the container (see Eig. 12.5). In this case, the separate layer, rather than extending around the container, occupies the whole wall thickness in a defined portion of the container. Typically, unpigmented resin is used, which is processed in a small satellite extruder and introduced into the die as a separate vertical stripe in the parison. Since the resin is the same type as the pigmented resin found in the rest of the bottle, no tie layer is needed. Such containers are used for motor oil, liquid laundry detergent, and other products. 

Uses Adhesive/sealant resin for use in blown and cast film extrusion applies, incl. meat/poultry/seafood/cheese pkg., cereal liners, medical/ pharmaceutical pkg., powd./granularfood and non-food pouches, carded display/skin packaging films edible oil, motor oil, other li prod, pouches, snack structures coextrusions with nylon or in other film structures as heat seal or tie layer Features High clarity grade Regulatory FDA 21 CFR 177.1330 compliant Properties Melt flow 1.5 dg/min f.p. 78 C m.p. 103 C Vicat soften, pt. 81 C ultimate tens. str. 33.1 MPa (MD) ultimate elong. 450% (MD) Spencer impact str. 29 J/mm Dart drop str. 7.9 g/pm Elmendorf tear str. 46.3 mN/pm 9% methacrylic acid... 

When considering the properties of the layered structures, adhesion between the layers becomes a critical issue. If the materials are highly incompatible, the adhesion will be weak at the interface, leading to delamination. To circumvent this problem, tie layers can be used to increase the interfacial toughness. In the case of polypropylene (PP) and polyamide coextrusion, maleated PP has been used to increase the interfacial toughness of the... 

Adhesion between the plies is partially mechanical since the materials are molten when combined, but the major bond has to be chemical (see Theories of adhesion). Where compatible materials are used, such as low-density polyethylene or high-density polyethylene (LDPE/HDPE), strong bonds can be achieved between the layers without additional means (see Compatibility). However, where the wider opportunities to use coextrusion are taken and multi-layers conceived with non-compatible materials such as polyamides, polyolefins, polycarbonate and polyethylene vinyl alcohol, adequate adhesion may be difficult to achieve. In these circnmstances, a specific polymeric layer may be used to act as a compatibilizing agent or Tie layer. A very thin layer of the extrudable adhesive has functional gronps to enhance the chemical bonding at the interface. 

A tie layer or coextrudable adhesive has the function of improving interfacial forces between dissimilar polymers in the Coextrusion process. The layer is used in cases where resins coextruded are so incompatible that effectively no bond develops between them, or where the bond between two polymers is so limited as to be impracticable (see Compatibility). A typical coextruded film stractuie may consist of five layers of the basic form sealant layer/adhesive tie/barrier/adhesive tie/outer layer. The tie layers will generally be only a few microns thick. 

Because of their adhesive properties, EMA copolymers are used in extrusion coating, coextrusions, and laminating applications as heat-seal layers. EMA is one of the most thermally stable of this group, and as such it is commonly used to form hot and dielectric seals, as well in multiextrusion tie layer applications. 

Plexar, Tie-layer resins for coextrusion, Equistar Chemicals... 

Description (EMAC) Used as an impact modifier and compatibilizer good tie-layer in coextrusion, with broad temperature range translucent/clear colour ... 

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. 

Economics. An advantage of coextrusion is the capability of combining layers of high performance resins with low cost resin layers to produce high per-formance/low cost composite structmes. 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 compatibiUty of polymers in the recycle layer. Often the tie-layer polymer acts as a compatibi-lizer for recycle. Recycle of incompatible polymers with different refractive indexes usually causes haziness and cannot be used when excellent optical properties are required. 

These requirements are met with a multilayer structure containing a thicker central layer of the copolymer with thin outer layers of polypropylene (17). Such film structures are made by lamination or coextrusion, respectively using appropriate adhesive or tie layers. Other uses for these films include packaging of personal care and consumer products as in, for instance, blister bottles. 

Multilayer structures can be formed by simultaneous coextrusion of different layers through a feed block die. The individual layers can be selected for their specific attributes, such as strength, barrier properties, etc, or for incorporation of recycled material. When the basic layers are incompatible, tie layers can also be included to form a cohesive composite structure. 

Coextrusion and lamination processes have been described extensively in the patent literature. The technology usually involves multilayer structures utilizing, as an adhesive layer, a compound of PVDF with another material, which provides the bonding to the substrate. In this way, the full benefits of a thin film of PVDF may be realized (121). Another approach using acrylic alloys with PVDF sometimes achieves adherence to various substrates without the tie layer necessary for the pure PVDF film (122). The full breadth of options available by these technologies to protect surfaces with weatherable PVDF surfaces has been reviewed (123). Surface PVDF layers on polyolefins has been a particular technology of interest with respect to suitable finishes in cost effective parts for exterior automotive and agricultural equipment applications (124). 

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. 

Multilayer configuration. Multilayer structures of two or more polymers are commonly produced by coextrusion, adhesion lamination, coating, extrusion coating, or their combination. In a lamination, the contribution of the tie layer to the barrier of the structure is often neglected, although in some cases it may have some effect. 

Coextrusion is the latest process by which a series of plies can be joined together in the hot state. This may involve direct adhesion between plies or the use of extruded tie or adhesive layers. These bonding layers are frequently only a few micrometres thick and involve relatively low grammage levels. However, coextrusion is only cost-effective as a long-run process since set-up times can be lengthy. 

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