Agriculture Reference
In-Depth Information
4.2.1 Chemical and Physical Properties
Most commercial PP is isotactic (atoms arranged in a similar configuration) and has
no intermediate level of crystallinity between that of LDPE and HDPE. Usually, PP
is tough and flexible, especially if co-polymerised with ethylene. This allows PP to be
used as an engineering plastic material, competing with materials such as ABS. PP is
reasonably economical and can be made translucent if uncoloured but is not readily
made transparent as PS, acrylics or certain other plastics. PP has good resistance to
fatigue, is often opaque, and can be coloured by pigments.
The melting point of PP is ≈171.1 °C. Perfectly isotactic commercial grades of PP have
a range of melting points (160-171.1 °C) depending on the grade and crystallinity.
Syndiotactic PP with a crystallinity of 30% has a melting point of ≈129.4 °C. The melt
flow rate or melt-flow index (MFI) is a measure of the MW of PP. This index helps to
determine how easily the molten raw material will flow during processing. PP with a
higher MFI if used as a matrix for composite resins will fill a mould more easily and
faster and also facilitate easy and faster extrusion of composite profiles. As the melt
flow increases, however, some physical properties (e.g., impact strength) decrease.
There are three general types of PP: homo-polymer (single monomer), random
copolymer and block copolymer. The co-monomer is typically used with ethylene.
Ethylene-propylene rubber added to propylene homopolymer increases its low-
temperature impact strength. Randomly polymerised ethylene monomer added
to PP homopolymer decreases polymer crystallinity and makes the polymer more
transparent. PP is liable to chain degradation from exposure to heat and the ultraviolet
(UV) radiation present in sunlight. Oxidation usually occurs at the tertiary carbon
atom present in each repeat unit. A free radical is formed here and reacts with
oxygen, followed by chain scission to yield aldehydes and carbolic acid. In external
applications, it shows up as a network of fine cracks and crazes that become deeper
with prolonged exposure. To counter this effect in external applications, UV-absorbing
additives can be used. Carbon black also provides some protection from UV attacks,
but this use may be limited by the type of desired surface finish. The polymer can also
undergo oxidisation at high temperatures, a common problem during processing.
Antioxidants are usually added to prevent polymer degradation.
4.3 Polyvinyl Chloride
PVC is a thermoplastic resin. It is probably the third most widely produced plastic
resin. PVC is a white, brittle solid available as a powder or granules. It can be made
softer and very flexible by the addition of plasticisers, the most widely used group
Search WWH ::
Custom Search