Page 294 - Plastics Engineering

Processing of Plastics 277

of reactive bonding processes in which the co-extruded layers are chemically
cross-linked together.
The main reason for producing multi-layer co-extruded films is to get mate-
rials with better barrier properties - particularly in regard to gas permeation.
The following Table shows the effects which can be achieved. Data on perme-
ability of plastics are also given in Figs 1.13 and 1.14.

Table 4.2
Transmission rates for a range of plastics

Transmission rates
Layer Oxygen
distribution (cm3/m2 Water vapour
Polymer (Pm) 24 hr atm) (glrn’ 24 hr)
ABS lo00 1050 30 2
UPVC lo00 1390 5 0.75
Polypropylene loo0 910 60 0.25
PET loo0 1360 1 2
LDPE lo00 920 140 0.5
HDPE loo0 960 60 0.3
PSEVOH*IPE 825/25/150 1050 5t 1.6
PSIPVdCIPE 8251501125 1070 1 0.4
PPEVO WPP 3001401660 930 It 0.25

‘EVOH ethyl vinyl alcohol.
tDepends on humidity.

(ii) Highly Oriented Grids: Net-like polymer grids have become an extremely
important development - particularly to civil engineers. The attraction in civil
engineering applications is that the open grid structure permits soil particles
to interlock through the apertures thus providing an extremely strong rein-
forcement to the soil. These geogrids under the trade name ‘Tensar’ are now
widely used for road and runway construction, embankment supports, landslide
repairs, etc.
The polymer grid achieves its very high strength due to the orientation of
the polymer molecules during its manufacture. The process of manufacture is
illustrated in Fig. 4.29. An extruded sheet, produced to a very fine tolerance
and with a controlled structure, has a pattern of holes stamped into it. The
hole shapes and pattern can be altered depending on the performance required
of the finished product. The perforated sheet is then stretched in one direction
to give thin sections of highly orientated polymer with the tensile strength
of mild steel. This type of grid can be used in applications where uniaxial
strength is required. In other cases, where biaxial strength is necessary, the
sheet is subjected to a second stretching operation in the transverse direction.
The advantages of highly oriented grids are that they are light and very easy to