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MIXING AND PROCESSING
PVC and other compounding
ingredients are usually blended together
in a heated internal mixer such as a
‘Banbury’. This comprises a closed
chamber within which a rotor revolves
at speeds of 20-100 rpm. This process is
followed by further mixing in an
extruder/pelletiser. Alternatively, it can
be done in what is known as a ‘twin
screw compounding extruder’ where the
pre-mix is fed in at one end and
delivered as a complete compound at
the other in the form of small granules.
Injection moulding temperatures for
PVC are typically in the range of 160-
180ºC whilst the mould is kept at 30-
50ºC by water cooling. This allows a
short in-mould time of one to two
minutes and a rapid and economical
rate of production. Too low a
temperature will cause ‘cold flow’,
visually
poor
mouldings
and
performance problems such as
delamination. Overheating must also be
avoided to prevent degradation. Up to
10 percent of the weight of the moulded
material can comprise sprues and
similar waste. This can be granulated
and blended with virgin polymer without
causing problems, so eliminating it as
sheer waste. The proportion thus treated
must nevertheless be limited as
repeated heating can cause degradation
in PVC.
ADDITIONAL PROPERTIES
Plasticised PVC is suitable for unit
soles, direct moulded-on soles and all-
moulded footwear. It is tough and
durable in most respects, although it is
relatively easily cut and sensitive to
temperature, which limits its usage.
It softens with heating and stiffens with
cooling, so is prone to flex crack in
extremes of cold. It will also melt in
extremes of heat that might be found in
certain industrial situations such as
foundries and the laying down of tarmac
roads. As stated earlier, the softer grades
are the most abrasion, flex crack and,
subject to the correct tread pattern being
used for wet conditions, slip resistant.
The hardness spectrum among sole
materials in general is 30-95 IRHD; PVC is
generally best used between 50-80 IRHD.
ALTERNATIVES TO PVC
Alternatives are available for any
company seeking to reduce its usage of
PVC solings. The closest, from a
manufacturing perspective at least, are
TR (thermoplastic rubber) and TPU
(thermoplastic polyurethane). While
there is no single material capable of
doing everything PVC does, alternatives
can be chosen according to need and
depending on which properties are seen
to be the most important.
TR
has generally better slip and cold
flexing resistance than PVC, but
poorer wear resistance. Particular
care is also needed with the
adhesive bonding process when sole
attaching. Grades of TR with better
wearing properties have however
now been developed.
TPU
possesses good properties.
While it was once confined to hard
solings such as in sports boots, it is
now available in softer, rubbery
grades and also microcellular
versions. It is also much more
expensive than PVC.
TPO
(thermoplastic polyolefin)
materials can be used for all-moulded
boots and leisure footwear, but
adhesion difficulties have limited their
use in everyday footwear.
TR – ADVANTAGES AND
DISADVANTAGES
This is a good alternative material for
sole units, sharing the economy of PVC
and benefiting from more rubbery
aesthetics, better grip and lower
density.
However,
the
weight
advantage may be offset or lost by the
need to increase the thickness of
certain areas of the sole to ensure
adequate durability and structural
stability. For example, the wearing
thickness needs to be 1mm greater
than for PVC and the internal
ribbing/lightener bars by up to 2mm.
TR is stiffer than PVC, even though it is
often softer. It is usually also less
abrasion resistant making it generally
unsuitable for heavy-duty wear. On the
other hand, it does have better cold flex
crack resistance.
Greater care is needed with the
adhesion process than with PVC,
particularly where solvent-based primers
and adhesives are used due to its
inherent sensitivity to solvent.
Halogenation priming, gentle application
of primers and adhesives, and thorough
drying of both are all essential.
TR is a difficult material to direct
injection mould on to leather uppers
without special adhesives and is,
therefore, rarely practised. Like PVC, it
can be blown (made cellular), but not
quite to the same degree due to its
lower wear resistance.
TPU – ADVANTAGES AND
DISADVANTAGES
TPU is generally regarded as a
premium quality material and its price
reflects this, being relatively expensive.
This is its chief disadvantage as
otherwise its performance-related
physical properties are generally very
good. It is suitable for unit soles, and is
available in a range of hardnesses with
good aesthetics and similar or slightly
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WORLD FOOTWEAR | NOVEMBER/DECEMBER 2012
PVC compound is supplied in granular form.