Page 27 - Plastics Engineering

10 General Properties of Plastics

Table 1.1
Comparison of structural foams based on various grades of polypropylene with some traditional
materials

40% 30%
Unfilled talc-filled coupled glass- Chip- Alumi- Mild
copolymer homopolymer reinforced board Pine nium steel

Solid Foam Solid Foam Solid Foam
Flexural modulus
MN/mz 1.4 1.2 4.4 2.5 6.7 3.5 2.3 7.9 70 207
Specific gravity 0.905 0.72 1.24 1.00 1.12 0.90 0.650 0.641 2.7 7.83
Relative thickness
at equivalent
rigidity 1 1.05 0.68 0.81 0.59 0.74 0.85 0.56 0.27 0.19
Relative weight
at equivalent
rigidity 1 0.84 0.94 0.90 0.74 0.73 0.61 0.40 0.81 1.65

manner and have sufficient flexibility to allow the material to undergo very large
deformations. In the green state the rubber would not be able to recover fully
from large deformations because the molecules would have undergone irre-
versible sliding past one another. In order to prevent this sliding, the molecules
are anchored together by a curing (vulcanisation) process. Thus the molecules
are cross-linked in a way similar to that which occurs in thermosets. This
linking does not detract from the random disposition of the molecules nor their
coiled and twisted nature so that when the rubber is deformed the molecules
stretch and unwind but do not slide. Thus when the applied force is removed
the rubber will snap back to its original shape.
Vulcanised rubbers possess a range of very desirable properties such as
resilience, resistance to oils, greases and ozone, flexibility at low tempera-
tures and resistance to many acids and bases. However, they require careful
(slow) processing and they consume considerable amounts of energy to facil-
itate moulding and vulcanisation. These disadvantages led to the development
of thermoplastic rubbers (elastomers). These are materials which exhibit the
desirable physical characteristics of rubber but with the ease of processing of
thermoplastics.
At present there are five types of thermoplastic rubber (TPR). Three of
these, the polyurethane, the styrenic and the polyester are termed segmented
block copolymers in that they consist of thermoplastic molecules grafted to
the rubbery molecules. At room temperature it is the thermoplastic molecules
which clump together to anchor the rubbery molecules. When heat is applied
the thermoplastic molecules are capable of movement so that the material may
be shaped using conventional thermoplastic moulding equipment.
The olefinic type of TPR is the latest development and is different in that it
consists of fine rubber particles in a thermoplastic matrix as shown in Fig. 1.1.