Page 29 - Plastics Engineering

12 General Properties of Plastics

Table 1.2
vpical plastic alloys

Alloy Features
PVClacrylic Tough with good flame and chemical resistance
PVC/ABS Easily processed with good impact and flame resistance
Pol ycarbonate/ABS Hard with high heat distortion temperature and good notch
impact strength
ABSPolysulphone Less expensive than unmodified polysulphone
Polyphenylene oxide/HIPS Improved processability, reduced cost
S AN/olefin Good weatherability
Nylon/elastomer Improved notched impact strength
Modified amorphous nylon Easily processed with excellent surface finish and toughness
PolycarbonatePBT Tough engineering plastic

1.3.7 Liquid Crystal Polymers
Liquid crystal polymers (LCP) are a recent arrival on the plastics materials
scene. They have outstanding dimensional stability, high strength, stiffness,
toughness and chemical resistance all combined with ease of processing. LCPs
are based on thermoplastic aromatic polyesters and they have a highly ordered
structure even in the molten state. When these materials are subjected to stress
the molecular chains slide over one another but the ordered structure is retained.
It is the retention of the highly crystalline structure which imparts the excep-
tional properties to LCPs.
Typical Characteristics of Some Important Plastics
(a) Semi-crystalline plastics
Low density polyethylene (LDPE). This is one of the most widely used
plastics. It is characterised by a density in the range 918-935 kg/m3 and is
very tough and flexible. Its major application is in packaging film although its
outstanding dielectric properties means it is also widely used as an electrical
insulator. Other applications include domestic ware, tubing, squeeze bottles and
cold water tanks.
Linear Low Density Polyethylene (LLDPE). This new type of polyethylene
was introduced in 1977. LLDPE is produced by a low pressure process and
it has a regular structure with short chain branches. Depending on the cooling
rate from the melt, the material forms a structure in which the molecules are
linked together. Hence for any given density, LLDPE is stiffer than LDPE and
exhibits a higher yield strength and greater ductility. Although the difference
melt processing characteristics of LLDPE take a little getting used to, this new
material has taken over traditional LDPE markets.
High Density Polyethylene (HDPE). This material has a density in the range
935-965 kg/m3 and is more crystalline than LDPE. It is also slightly more