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and comfortable to wear. It now
accounts for over 7% of all soling
materials used and, while this may
appear surprisingly small, it must be
remembered that a major proportion of
global footwear output is still low cost
sandals and flip flops.
The principal footwear application is
as microcellular compounds used to
produce single or dual density soling,
either as unit soles or by direct
moulding onto the lasted upper. The
compounds used can be based on
either polyester or polyether polyols,
with the former being dominant. They
are also used to produce midsoles,
which can be made as a separate unit
for subsequent bonding to the sole
and upper in a two-stage operation or
by direct moulding to attach a
vulcanised rubber or TPU outsole to
the shoe bottom.
POLYESTER PU
Polyester PUs offer the best physical
properties but are more difficult to
process and handle due to the fact that
most urethane grade polyester polyols
are either solid or highly viscous at
room temperature and therefore
require thawing and preheating prior to
use. The moulding process itself
involves similar steps and needs
special processing equipment, adding
to an high material cost. Polyester
products are also hydrolytically
unstable and, under some conditions,
subject to microbial degradation.
Polyester PU is therefore more
susceptible to deterioration due to
hydrolysis (the chemical action of
water) whereby it gradually loses
physical properties if stored or used in
moist conditions. This water does not
need to be in liquid form and the culprit
is more usually water vapour present in
humid air. The process causes
breakage of the polyurethane
molecular chain and, in seriously
affected material, may show obvious
weakening and softening, or to even
crack and crumble
(Figure 1)
.
The degree of deterioration depends
on temperature, humidity, exposure
time and whether the soles are in
contact with any materials that can act
as catalysts and accelerate the
reaction. In temperate conditions it can
be several years before a sole
deteriorates to a critical level.
Breakdown is much quicker in warmer
climates or under more challenging
usage when a sole can be significantly
weakened within 12 months. Even in
temperate climates, soles may suffer if
stored in warm conditions, particularly
in confined spaces without ventilation
if still damp from use. While more
hydrolysis resistant polyether PU would
be less affected, its inferior abrasion
and oil resistance render it less suitable
for industrial footwear which is where
the problem is most prevalent.
The reason this deterioration occurs
lies in the chemical reaction used to
create the material in the first place,
where a diacid reacts with a hydroxyl-
containing compound to produce
hydroxyl-polyester and water. These
are held in balance, so the water must
be removed in order to produce the
final compound. Exposing it to
moisture for a prolonged period simply
causes a reverse reaction.
Polyester PUs were traditionally
stabilised against hydrolysis by adding
from 0.5-2.0% carbodiimides and/or
polycarbodiimides. The main disad-
vantage of this approach was that it is
essentially a temporary solution where
the effect is limited to the volume
added which, in turn, is controlled by
cost and impact on the properties of the
PU itself. The past ten years have seen
new polyester systems where, instead
of containing additives, the building
blocks forming the backbone of the
polyol have been modified so that the
steric effect (spatial arrangement of
atoms in each molecule) and reduced
hydrophilicity (capacity to absorb
water) of the polymer chain protects
the ester bonds from water attacks. The
resulting materials are more hydrolysis
resistant and generally also have
enhanced tensile, tear and flex fatigue
properties.
The quest to use renewable materials
wherever possible has led to another
approach to the hydrolysis problem
where dimerised fatty acids are used in
the formulation of the polymers. Based
on soya bean oil or tall oil (a by-product
of timber pulping), they have an
irregular structure consisting of a large
number of isomers and carbon atoms
that
makes
them
extremely
hydrophobic. When dimer-based
polyester polyols are used instead of
conventional esters in PU systems, the
hydrophobic nature, closed cell
structure and high molecular weight of
the dimerised fatty acid result in greater
hydrolytic stability, microbial resistance
and increased sole life. Being plant
derived, the technology is also safe,
sustainable and not directly influenced
by oil prices.
POLYETHER PU
Polyether PUs, on the other hand, are
typically low viscosity allowing easier
handling and processing, and makes
them equally suitable for both low and
high-pressure processing equipment.
They are also hydrolytically stable and
not subject to microbial degradation. In
general, the material and processing
costs of polyether technology are
considerably lower compared to
polyester. Its main disadvantage,
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VERSATILE PU
SEPTEMBER/OCTOBER 2012 | www.footwearbiz.com
Fig 1 – Serious deterioration of a sole due to hydrolysis.
CREDIT: BASF