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MANUFACTURING & MATERIALS INNOVATION
P
olyurethane is a remarkable
material; it can be found in
the mattresses we sleep on,
the chairs we sit on, the cars
we drive, the lap-tops we work with
and the shoes we wear. To almost
everyone it is just another plastic and,
to a certain extent, this is probably true
in regard to most people working in
the footwear industry unless directly
involved with it on a day-to-day basis.
So, just how much do you know about
this versatile material?
BACKGROUND
Polyurethane is the result of a
polyaddition (step-growth) reaction
between diisocyanate and diols
discovered by Otto Bayer in the late
1930s, who is now regarded as the
father of the polyurethane industry. The
first commercial applications of
polyurethane polymers appeared after
the end of World War II and were
millable elastomers, coatings and
adhesives. By the late 1950s, both
flexible and rigid foams were available
leading to the use of the material in the
huge range of products and
applications we see today.
It is produced commercially by
creating a chemical reaction between
molecules which contain two or more
isocyanate groups with polyol
molecules having two or more hydroxyl
groups. The vast number of
polyurethanes now used are created by
combining a relatively small number of
basic isocyanates with a much broader
range of polyols possessing different
molecular weights and characteristics.
This simple fact is why polyurethane
can be regarded as the most versatile of
all plastic materials.
A further strength is that, unlike most
other plastic compounds, the properties
of the final product can be controlled or
changed during the actual production
process. This is made possible by the
nature of the process itself whereby the
component chemicals are mixed and
then allowed to react. Changes in the
balance of these components or the
introduction of additives enable varying
end polymers to be produced.
Further fine tuning is made possible
by altering mixing conditions and ratios
such that colours, for instance, can be
quickly changed as required. The fact
that the final polymerisation process
then immediately takes place is what
gives the material its versatility and,
coupled to its excellent physical
characteristics, has directly led to its
use in so many industrial applications
and products.
SOLING
Since its introduction in the late
1960s, polyurethane has established
itself as a major alternative to
traditional soling materials such as
leather and rubber. Here it competes
against other plastics such as
thermoplastic
rubber,
polyvinyl
chloride, ethylene vinyl acetate, nylon,
thermoplastic urethanes and other
polymers. Its big advantage however is
that it combines excellent physical
properties with light weight to produce
footwear that is both extremely durable
26
WORLD FOOTWEAR | SEPTEMBER/OCTOBER 2012
Versatile PU
Two-density PU boots.
CREDIT: KLÖCKNER DESMA
Footwear Fundamentals