Extrusion Coating and Laminating
Extrusion coating and extrusion laminating operations are normally described together because the process equipment for accomplishing them is essentially the same. Extrusion coating is the operation by which a coating of a thermoplastic material is applied to a substrate web, such as paper. In extrusion lamination, a second web is incorporated and both webs are combined by the adhesive action of the extruded thermoplastic material. The processes are depicted. In operations for converting packaging materials, these processes are often combined in the same line to create a web such as paper/acid copolymer/foil/heat sealant layer.
In the first operation, the paper and the foil are extrusion laminated using an acid copolymer as the adhesive. Next, the sealant layer is applied as an extrusion coating. The operation can be accomplished in-two passes, or in a single pass in a tandem extrusion line as shown in Figure 8.2. In such a line, both extrusion heads are usually capable of coextrusion, as well. To understand the process of extrusion coating and laminating, it is necessary to start with the process in its simplest form, which, by the way, is how it is still practiced today by some converters.
Extrusion laminating and coating represent early applications of polymers in packaging. Polyethylene coated paper was substituted for waxed paper as a bread. overwrap in the 1950s. Since polyethylene is non-polar and paper is polar, there is little or no adhesion between the materials unless the PE can flow into the paper pores, or some modification is done to the PE. A solution to this problem was the discovery that allowing the web of molten PE to drop through the air for a certain distance causes the hot surface to oxidize slightly, creating carboxyl (-COOH) groups on the surface that are polar enough to bond to the paper fibers. To get the PE to oxidize quickly enough for the desired line speeds, the process is often run at a melt temperature of 3160C (6000F) or higher. One of the drawbacks to this approach is that the oxidation is not always confined to the surface, but can continue into the bulk of the coating. If too much oxidation occurs, off-odors may develop which can affect the product.
To reduce the need for such high temperatures, primers are used to improve adhesion. These primers are organic compounds that are coated onto the paper in line with the extrusion coating to provide non-polar groups to which the PE can adhere, as well as polar groups which can adhere to the paper. Polyethylene mine (PEI) is one such compound. These primers allow the melt temperature to be reduced t0 2880C (550 0F), so less oxidation occurs and there is less potential for off-odors.
Primers can be used on foil also, but another approach often used with foil is to use a polymer that has polar groups on the chain, such as an acid copolymer or an ionomer, for the coating. Polyethylene acrylic acid is a typical acid copolymer. Ionomers , are partially neutralized acid copolymers, of which Surlyn TM is the best known. Acid copolymers and ionomers can also be used as sealant layers.
Because the polymer melt is expected to drop though an open space before contacting the web, the polymer must have sufficient melt strength to support its weight; also the polymer curtain should not contract too much in the cross-direction (neck-in) during the operation (Fig. 8.3). Melt strength requirements are greater and neck-in is more of a problem in extrusion coating than in film extrusion because of the greater distance the polymer curtain travels without support. The distance between the die and the chill roll (the air gap) is typically 6 t0 8 inches in extrusion coating, while in film or sheet extrusion it is much less.
These special requirements must be provided by the polymer molecular architecture. A polymer used for extrusion coating and laminating needs to have reasonably high elongational viscosity, and the melt should exhibit strain hardening at high elongation rates. Strain hardening means the resistance to deformation increases at a more rapid rate as deformation continues (the slope of the stress-strain curve increases). Both of these characteristics keep the melt from pulling apart in the MD, and from necking-in too much in the CD. Not all polymers that one might want to use for this operation have these characteristics. However, if a polymer that has certain desirable properties does not have the necessary melt characteristics, using coextrusion it can be carried on another polymer layer that does have the required characteristics. LLDPE’s are examples of polymers with poor elongational melt characteristics, but they are desirable as sealant layers in many applications because of their hot tack. They can be used for extrusion coating in a coextruded structure where they are carried on a regular extrusion-coating-grade LDPE layer.
The tandem coating line seen in Figure 8.2 can be used to reduce the cost of producing products that require both laminating and coating, such as a paper/polyethylene/foil/sealant type of structure. If this product is made in two passes through the machinery, the cost is higher because of the increased cost of handling the materials twice and the increased start-up and shut-down waste.