Ethylene vinyl acetate copolymers (EVA)
Many groups were involved in the development of vinyl acetate copolymers (see also polyvinyl acetate), with DuPont filing a patent in 1956 and introducing the Elvax range of materials in 1960. This is based on copolymerisation products of ethylene with vinyl acetate and are normally produced either from bulk continuous polymerisation or solution polymerisation. The former produces low molecular weight copolymers useful for coatings, hot melt adhesives, etc., whilst the latter yields high molecular weight products for tougher applications.
As the level of vinyl acetate in the copolymer increases so the level of crystallinity found in polythene alone reduces from about 60% to 10%. This yields products ranging from materials similar to low density polythene to flexible rubbers. Common grades can contain from 2% to 50% vinyl acetate. Clarity, flexibility, toughness and solvent solubility increase with increasing vinyl acetate content. Of particular note is the retention of flexibility of EVA rubber grades down to (-70C) and because they are copolymers, problems due to plasticiser migration are not experienced.
Good resistance to water, salt and other environments can be obtained but solvent resistance decreases with increasing vinyl acetate content. The copolymers can accept high filler and pigment loadings. Being thermoplastic EVA can be moulded by extrusion, injection, blow moulding, calendaring and rotational moulding. Crosslinking with peroxides can produce thermoset products.
Applications are diverse, such as flexible shrink wrap, footware soles, hot melt and heat seal adhesives, flexible toys, tubing, wire coatings, medical gloves, masks, babies’ dummies and bottle teats. Crosslinked foamed tyres have been used for tough service. Many grades and modifications now exist to meet modern demands from these versatile EVA copolymer types.
Ethylene vinyl acetate copolymers (EVA) admin 2016-12-06T05:35:22+00:00