Heat Sealable Compositions

Polymers. In combination with various metal salts, sorbitol is used as a stabilizer against heat and light in poly(vinyl chloride) (qv) resins and, with a phenohc antioxidant, as a stabilizer in uncured styrene—butadiene mbber (qv) compositions and in polyolefins (see Heat stabilizers Olefin POLYMERS Rubbercompounding). Heat-sealable films are prepared from a dispersion of sorbitol and starch in water (255). Incorporation of sorbitol in coUagen films gready restricts their permeabiUty to carbon dioxide (256). 

The commercial grades available in the 1970s used either zinc or sodium as the cross-linking ion and ranged in melt flow index from 0.4 to 14. The main application of the ionomer resins has been for packaging film. The polymer is particularly useful in composite structures to provide an outer layer with good heat sealability. The puncture resistance of film based on ionomer film has the puncture resistance of a LDPE film of twice the gauge. 

Table I shows the chemical composition limits of various aluminum alloys presently used for packaging applications (3). In general, these alloys have good corrosion resistance with most foods. However, almost without exception, processed foods require inside enameled containers to maintain an acceptable shelf life (4, 5). Moreover, when flexible foil packages are used for thermally processed foods, the foil is laminated to plastic materials that protect it from direct contact with the food and also provide heat sealability as well as other physical characteristics (6,7).

Properties of selected compositions are shown in Table 4.6. The compositions thus obtained have excellent mechanical properties including heat resistance, heat sealability, and impact resistance as well as satisfactory gas permeability. Therefore, the compositions are an appropriate material for producing various structural materials by injection molding. 

The compositions may also be extruded into films and laminate papers. The films have excellent mechanical properties, including impact resistance and tensile strength as well as satisfactory releasability and gas permeability. The films are also heat sealable and the heat sealed film has sufficient strength. Therefore, such films are quite appropriate for the use as vegetable wrapping films, industrial films, and bags for storing platelets and cells. 

The first successful one consisted of 90 percent vinyl chloride copolymerized with 10 percent vinyl acetate. It was dry-spun from acetone and given the trade name Vinyon by its producer, Union Carbide Corporation. (In 1960, vinyon was accepted as a generic name for fibers containing not less than 85 percent vinyl chloride.) It has never been produced in large volume it is used for heat-sealable compositions. 

PVC can be chlorinated (CPVC) and be alloyed with other polymers like ABS, acrylics, polyurethanes, and nitrile rubbers to improve its impact resistance, heat deflection, and processability. Although these vinyls differ in having literally thousands of varying compositions and properties, there are certain general characteristics that are common to nearly all these plastics. Most materials based on vinyls are inherently TP and heat sealable. The exceptions are the products that have been purposely compounded with TSs or crosslinking agents are used. 

The composition of vinyl chloride copolymers without functional groups influences their solubility behavior and compatibility with other paint binders. For example, copolymers with isobutyl vinyl ether or maleate esters dissolve in aromatic hydrocarbons, whereas copolymers with vinyl acetate merely swell in these solvents. Paint films formed from vinyl chloride copolymers without functional groups are heat sealable on account of their thermoplastic character. Since the films adhere poorly to nonabsorbing substrates such as metals, they are suitable as binders for strippable coatings. On account of their good chemical resistance, vinyl chloride copolymers are also extremely suitable as binders for exterior-use paints, traffic paints, and paper and foil lacquers their lack of taste and odor means that they can be used as pasteurization-resistant coatings for can interiors. 

Extrusion Casting A term sometimes employed in the industry for the process of extruding unsupported film, especially a composite of two or more integral resin layers formed by Coextrusion. Such extrusion-cast composite films possess desired properties on each of the respective sides, e.g., heat-sealability on one side and stiffness on the other, or different levels of slip, or different colors. 

Dissimilar materials can be joined, e.g., aluminum-to-paper, iron-to-copper. When two metals are bonded, the adhesive separates them and prevents corrosion. When the two adher-ends are markedly different in coefficient of thermal expansion, a flexible adhesive lessens the stress due to temperature change. Laminates of dissimilar materials can give combinations superior to either adhered alone for example, a polyethylene-cellophane composite has the heat-sealability and water-resistance of the former plus the grease-resistance and print-ability of the latter. 

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