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man-made substances containing chlorine are toxic and
persist in the environment, as was discovered with some
pesticides. There is concern that low levels of highly toxic
dioxins are generated when waste containing chlorinated
materials, especially packaging, is burned. However, dioxins
can arise from many materials and can be formed
irrespective of whether PVC is present or not. Moreover,
properly controlled incinerators working at high temperature
reduce the risks of dioxin being formed. This negative view of
certain chlorine-containing materials, such as PVC, has to be
countered by the benefits of others, most notably sodium
chloride (common salt). Indeed, chlorine itself is widely used
to kill harmful organisms in water.
POSITIVE ASPECTS
PVC does however have positive attributes to offset this
negative image.
•
Its manufacture requires less raw materials and fuel than
other plastics.
•
The chlorine used is generated as a by-product in the
manufacture of alkalis (such as sodium hydroxide) from salt.
•
It is the only major use of chlorine which leaves it in a more
inert form than the salt from which it came.
•
It is recyclable and easy to separate from mixed plastics
waste, though separation is more difficult with composite or
bonded PVC materials as found in footwear.
COMPOUNDING AFFECTS PROPERTIES
PVC polymer is hard and rigid so it must be compounded
with plasticisers to produce a softer and more flexible material
at ordinary temperatures. Most are high boiling-point liquids
whose molecules penetrate between and lubricate the PVC
polymer chains. This allows the chains to slip over each other
more easily, so enabling the material to be flexed and then slip
back into place as it is straightened again. They typically
comprise 35-50% by weight of a PVC soling compound and
are generally phthalate esters, sometimes extended with a
proportion of secondary plasticisers such as chlorinated
paraffins. However, C10-C13 chloroalkanes have been
identified as SVHCs (substances of very high concern) under
the REACH regulation.
Other compounding ingredients include stabilisers,
processing aids, pigments and fillers, together with blowing
agents for cellular compounds if required. These are
chemicals which mop up damaging degradation products
such as hydrogen chloride, caused by heat when the PVC is
being compounded, processed and moulded. They also
provide longer-term protection in service against the effects of
heat, light and oxidation.
Fillers are mainly used to add bulk. They are often
relatively cheap substances which can have the side effect of
hardening the material, thus countering the softening effect
of plasticisers. The range of minerals employed is similar to
those used in compounding rubber although they do not
usually have the reinforcing effect in PVC that some do in the
latter. Examples include calcium carbonate, silica, clays and
carbon black.
The density of the final compound is therefore reduced by
plasticisers and increased by fillers. PVC resin itself has a
specific gravity of 1.4 and the range encountered in
compounded solid (compact) PVC solings materials is
mainly between 1.15 and 1.35, with the lowest density ones
virtually unfilled. Modern blown PVCs with a uniform
microcellular structure do however offer specific gravities
down to around 0.75.
PVC blends also contain one or more secondary polymers
such as nitrile rubber and polyurethane at up to 30 parts per
hundred of PVC polymer. PVC/nitrile blends generally have
higher resistance to petroleum oils than PVC/polyurethane
blends but a lower resistance to animal fats so, at times,
blends of all three are used.
The amount of plasticiser contained in a PVC or PVC blend
compound controls not only its hardness but most physical
and wear properties as well. Its type determines specialist
properties such as low temperature and oil resistance.
Resistance to wear, flex cracking and slip are all dependent on
hardness and, therefore, on plasticiser content. Soft materials
perform the best and the typical hardness range is 50-80 IRHD
(international rubber hardness degrees).
Adhesion performance is influenced more by the type than
the quantity of plasticiser. Some plasticisers migrate into
adhesive films more rapidly than others, but generally there
are few adhesion problems with PVC soles provided the
proper preparation is given (such as wiping with MEK
solvent), and a polyurethane-based adhesive is used.
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PVC: CONFRONTING THE ENVIRONMENTAL CHALLENGE
NOVEMBER/DECEMBER 2012 | www.footwearbiz.com
PVC remains popular not only for low cost unit soles but also for
all–moulded boots and sandals.