Transmission Efficiency of plastic Films Part 1

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TRANSMISSION EFFICIENCY OF PLASTIC FILMS In the first case, the aqueous or solvent-based laminating adhesive is applied in precise amounts to one web by means of direct gravure or reverse kiss coaters as already described under the heading of ‘Coating’. The coated web is then passed through an oven, to remove all water or solvent, and then combined with the other ply in a pressure nip, which may sometimes be heated. The set-up is shown schematically in Figure. Unlike wet bonding, this process is very suitable for the laminating of plastics films to other substrates. Factors affecting the success of this type of laminating when used for plastics films include tension control, accurate adhesive application and accurate control of drying. Film tension should normally be kept to a minimum and will depend on the distance the film has to be pulled through the laminating equipment and on the sharpness of any change in direction as it passes over the various rolls. Effective control of tension is easiest in those machines where both the unwind station and the rollers in the drying tunnel are in direct line. Where a reverse roll coating system is used the reverse roll should not run too fast otherwise drag will occur, thus introducing increased film tension. Smooth and accurate application of the adhesive is extremely important and failure in this respect will probably lead to delaminating. Both gravure coaters and reverse roll systems should give adequate control of adhesive application but adhesive viscosity is another factor and this should be kept as constant as possible. Drying oven performance is particularly important when laminating plastics film where solvents cannot be absorbed into the film. Excess solvent remaining in the adhesive at the nip stage is a major cause of delaminating. Films vary in respect of solvent absorption and cellulose acetate, for instance, is able to absorb a slight amount of solvent without serious effects during lamination, whereas oriented polypropylene is much more critical in this respect. The temperature stability of oriented polypropylene is lowered in the presence of certain adhesive solvents and shrinkage can occur at temperatures of around 100°C whereas in the absence of solvent it would withstand 130—140°C. For films such as polypropylene where drying is essential, the oven should have a maximum flow of air. 62
TRANSMISSION EFFICIENCY OF PLASTIC FILMS The second dry bonding method, involving the use of hot melts of one sort or another, can be further sub-divided into extrusion laminating and methods which apply the hot melt by reverse kiss, dip or curtain coating techniques. Extrusion laminating was mentioned briefly in the section on ‘Extrusion Coating’ flat die extruder discharges a molten curtain of polyethylene, polypropylene or some other thermoplastic into the nip between the two webs to be laminated. The heated adhesive is cooled by passing the laminated sheet over a specially designed combining and cooling roll section. This cooling section replaces the drying section generally needed in wet adhesive bonding. Extrusion laminates are good barriers to water and water vapor and are usually tough and flexible. Typical laminates produced by this method are paper! Polyethylene / aluminum foil or paper/polyethylene/regenerated cellulose film. The equipment used for roll application of hot melts. After the hot melt has been applied by a reverse-kiss coater to one of the webs, the two webs are brought together at the nip between a pneumatically loaded rubber roll and a fixed steel roll. The laminate again passes through a cooling section and is then wound up. The earliest hot melt adhesives were waxes of one sort or another and these are still widely used. Microcrystalline waxes are the most popular for this work but paraffin wax is still used. 63
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Transmission efficiency of Plastic
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C O N T E N T S S. No. CHAPTERS Pag
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The strategy for the project, TRANS
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TRANSMISSION EFFICIENCY OF PLASTIC
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Bags and Sacks 22% Bags and sacks 2
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2.1 EXTRUSION OF FILM: TRANSMISSION
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