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. |