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however, is that it provides inferior
physical properties compared to
polyester
(Figure 2).
While the data shown here indicates
that the physical characteristics of
polyether are typically lower than
polyester, they do appear to meet the
basic requirements for the majority of
applications as far as footwear soling is
concerned. The exception is the
abrasion resistance of its outer skin
which, unless enhanced through
additives or other techniques, is barely
sufficient even for casual footwear. The
problem has always been how to bring
it up to an acceptable level for more
demanding footwear applications.
Chlorofluorocarbons (CFC) and
hydrochlorofluorocarbons (HCFC) used
as blowing agents did, before they were
banned, increase surface density
enough to give an acceptable level of
abrasion resistance. Legal alternatives
such as hydrofluorocarbons (HFC) and
hydrocarbons
provide
similar
characteristics but, due to cost (HFC
245fa), flammability (pentane) or
solubility (HFC 134a), their scope is
limited. A further concern is that as
soon as the skin is abraded, the core
still lacks the desired durability. The
advantage of water-blown poly-
urethane is that the core is almost as
tough as the outer surface, which
results in more uniform wear.
Abrasion resistance can also be
improved by the use of solid fillers and
waxes, such as fine particles of alumina
and silica. However, a relatively high
content is needed and this has a
negative impact on other physical
properties. Synthetic waxes including
polyethylene (PE) have produced better
results but their relatively low density,
melting point and poor slip resistance
also proved limiting.
A better solution was found in
polyethylene/polytetrafluoroethylene
(PE/PTFE) copolymers that have
density ranges closer to the resin
blends themselves. The combination of
higher density and small particle size
with PE/PTFE powder (typically less
than 10 microns) allows fine
dispersions to be produced that are
dimensionally stable and do not require
special handling or processing. The
amount of PE/PTFE copolymer
required has proved to be remarkably
small as an addition of only 1% by
weight based on total system
composition
improved
abrasion
resistance by over 75%
(Figure 3)
.
By gradual developments such as
these, polyether systems have become
a viable alternative to polyester ones in
many more soling applications and
enabled manufacturers to exploit the
lower cost, easy handling and
processing, and hydrolytic and
microbial stability of this polyether
technology. The sheer diversity of
potential for PU polymers continues to
be exploited in footwear and this has
brought them into use for more than
just their hard wearing properties.
PU FOOTWEAR
The microcellular nature of blown PU
makes it lightweight and the lower the
density, the lighter it obviously
becomes. Indeed, the maximum
density at which blown PU can be
effectively processed is 280kg/m
3
. The
same microcellular structure also
means it is an excellent insulator that is
particularly effective in cold conditions
down to as low as –30°C. Technical
advances in recent years by moulding
machine manufacturers have now
enabled complete boots or shoes to be
moulded using two densities of PU
(Figure 4)
.
The first application has been to
produce Wellington boots for industrial
and agricultural workers required to
spend the whole of their working day in
cold, wet and otherwise adverse
conditions. Increased comfort is
obviously beneficial as it reduces
fatigue and helps to improve
performance. At the same time, it is of
course essential that safety standards
are not jeopardised. PU boots can do
this in several ways.
28
WORLD FOOTWEAR | SEPTEMBER/OCTOBER 2012
Figure 2: Typical Physical Properties of Polyether versus Polyester Soling Materials
Type of PU
Blowing
Molded Density
Tensile
Elongation
Die “C”
Hardness
Taber Abrasion
Agent
(gr./cc)
(psi)
at Break (%)
(pli)
Shore A
mgr. loss
Polyester
Water
0.50
900
600
150
55
40
Polyether
FREON 11
0.50
540
410
100
55
75
Polyether
Water
0.50
500
400
90
55
150
Polyether
HFC 134a
0.50
450
425
55
75
190
Figure 3: Physical Properties of Polyether Polyurethane with and without Additive
Blowing
PE/PTFE
Molded
Tensile
Elongation
Die “C”
Hardness
Taber
Ross Flex
Agent
Additive
Density
(psi)
at Break
(pli)
Shore A
Abrasion
KC to Fail
(%)
(gr./cc)
(%)
mgr. loss
(-20 F)
HFC 134a
1
0.50
475
550
85
55
45
>100
HFC 134a
0
0.50
450
425
75
55
190
>100
Water
1
0.60
610
420
110
63
37
>100
Water
0
0.60
600
400
100
63
153
>100
CREDIT: BASF
CREDIT: BASF