Polymers in Sport and Leisure

Polymers in Sport and Leisure
Protective helmets used in motorsports, cycling and
horse riding activities are made from thermoplastics
or reinforced plastics. The design of motor sports
helmets is very sophisticated and they can command
prices of up to £900.
In a number of sports there is need for goggles, visors or
face masks. These utilise such materials as polycarbonate
for the lenses. There have been numerous attempts to
find a clear replacement for the metal mesh face masks
used in fencing. Recently, one company has been
successful using polycarbonate. The lens is made in two
layers, one about 3 mm thick and a top layer about 0.5
mm thick. The disposable outer layer protects the base
layer from possible environmental stress cracking and
from the stress concentrations which could be produced
by blade damage on the surface (a.1).
The single highest value sector of sports goods is
footwear, with the leading manufacturers competing
to produce more and more technically advanced
designs. In many sports the athlete’s feet are subjected
to high dynamic loads, as much as 8 times body weight
in basketball, and the footwear is critical in giving
protection, providing traction and optimising
performance. The forces involved on the heel differ
from those on the forefoot and the specific requirements
depend on the sport in question. It is not surprising
that a modern sports shoe is a complicated construction
with many components and that there are many
variations in design. Apart from performance
considerations, the sports footwear market requires
strong aesthetic appeal and it can be difficult for a non
expert to understand which features actually enhance
performance and which are purely for looks.
A variety of polymers are used in sports footwear.
Trainers commonly have a rubber outsole, which may
be multicoloured or include clear thermoplastic
elements. Common materials for midsoles are cellular
ethylene vinyl acetate and polyurethane, and there can
be inserts such as polyurethane air bags and torsion
bars made from thermoplastics or even carbon fibre
reinforced polyester. Uppers are often made from
polyvinyl chloride coated fabrics and there can be
additional components such as Goretex linings, foam
cushioning and polyurethane heel supports. Shoes for
such sports as football and running have semi-rigid
soles made from such materials as thermoplastic
polyurethane.
A design of hiking boot uses a Hytrel insole, while
nylon has been used for the sole plate of running shoes
and glass reinforced nylon for a moulded cycling shoe
(48). Ethylene vinyl acetate and polyurethane compete
to produce lighter midsoles and density down to 0.25
g/cm 3 is now obtained with polyurethane which is
comparable with remoulded ethylene vinyl acetate
(Phylon) (77). Boots for skiing and skating are based
on a thermoplastic shell with a variety of plastic and
metal attachments.
4 Ball Games
The rubber ball must be the earliest use of polymers in
sport, apparently being known to the inhabitants of
South America when Europeans first crossed the
Atlantic. It is certainly a very long time since golf balls
were made of anything other than polymers; the gutta
percha ball was introduced in the mid 19th century and
the wound rubber ball as early as 1898.
The ‘classic’ construction of the three-part golf ball is
a complicated process; polyisoprene thread is wound
over a small solid rubber core and the cover formed
from balata or Surlyn. The solid or two-part ball has a
solid rubber core with a Surlyn cover and can be
compression or injection moulded. One-part balls can
be of rubber, polyurethane or Surlyn but the optimum
performance characteristics cannot be achieved by this
construction and they are used by beginners to practice
and on driving ranges.
The somewhat conflicting requirements for a golf ball
(speed and distance through the air, behaviour on the
ground, control of spin and toughness) mean that all
properties cannot be optimised in one design.
Generally, the three-part ball gives better control at
the expense of distance for given power input. As well
as the materials, the design of the dimpled cover is
critical to performance.
Development of the golf ball is limited to a considerable
extent by the regulations enforced by the sport’s
governing body. If balls which could travel significantly
further were allowed, the golf courses, which represent
very large investment, would appear too small. The
aerodynamic design is also limited by the requirement
for it to be symmetrical.
This has not stopped there being intense competition
between manufacturers and continuing efforts to make
improvements. A recent development has been tungsten
filler which partly reduces the amount of traditional
fillers (108).
Tennis balls also require specialised production techniques
and are again regulated by a specification. The bounce
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