Basic HTML Version
lower density compared to PVC.
Wear resistance and flexing
endurance are usually good or
excellent and attractive cellular
versions, which save weight and cost,
are also available. It is suitable for
heavy-duty
footwear
such
as
workboots and hiking boots, and is
usually oil resistant. However, its slip
resistance is only moderate. Unlike PVC
or TR, it is quite commonly used as the
wearing surface on dual-density soles,
usually in conjunction with a
lightweight microcellular PU midsole.
This mitigates the high cost factor while
taking advantage of its tough nature.
One drawback is that TPU, if polyester-
based, is potentially vulnerable to
hydrolysis in exactly the same way as
pure polyester PU. This breakdown is
due to attack by moisture and humidity,
promoted by warmth, over a period of
several years, depending on the storage
and service climate. It is a point that
must be watched.
THE FUTURE FOR PVC
The use of PVC remains widespread
and highly significant. At present, there
is no substantiated environmental
reason why its use for footwear solings
should not continue as these are
relatively longer-life products which
are stable on disposal. When taking
into account cost, ease of use and
performance compared to the
alternatives currently available, PVC is
in fact hard to beat. While its long-term
future—like that of anything else in
life—cannot be guaranteed, there is still
some way to go before this ubiquitous
soling material can be written off.
31
PVC: CONFRONTING THE ENVIRONMENTAL CHALLENGE
NOVEMBER/DECEMBER 2012 | www.footwearbiz.com
TR
TPU
Compact
Cellular
Compact
Cellular
Form
Direct moulded and unit soles
Unit soles and wear
surface of dual density
soles
Unit soles
Density range
0.9 - 1.1
0.8 - 1.0
1.15 - 1.25
0.7 - 1.0
Wear resistance*
Moderate - good
Fair - good
Excellent
Good - excellent
Hardness**
Soft - hard
Soft
Medium- very hard
Medium
Crack resistance***
Good - best at low temperature
Good including harder
grades
Good
Slip
resistance
Potentially very good but limited
tread life - wet grip can be low at
heel on soft compounds
Potentially good on softer
grades with long tread life
Hard grades can be
slippery
Potentially good with
long tread life
Other properties
Sensitive to oils or solvents and sti!
relative to hardness
High friction can melt surface in
vigorous use
Oil-resistant versions
available
Local density
depends on proximity
to injection point
Other comments
Cheaper lighter alternative to
vulcanised rubber but not tough
enough for heavy duty use and
prone to "ow lines - thin coring
ribs can collapse
Expensive but
performance and
appearance very good -
can be blended in any
ratio with PVC
High cost only partly
o!set by expansion
to medium density.
Applications
Casual and fashion footwear,
children's shoes, winter boots,
slippers
Women's top-pieces (hard
grades), industrial and
premium casual shoes
(soft grades)
Premium casual
shoes
*
Assessment given is based on resistance to loss of substance by normal abrasive wear.
** The following arbitrary ranges have been used: Very soft (<45 IRHD); Soft (45 - 54 IRHD); Medium (55 - 74 IRHD);
Moderately hard (75 - 84 IRHD); Hard (85 - 95 IRHD); Very hard (>95 IRHD). IRHD—International Rubber Hardness Degrees and
similar to Shore A.
*** Purely mechanical forms of cracking are considered here, not cracking due to chemical attack.
TR and TPU compared
Injection moulding PVC is both rapid and economical.