The density of a material is the mass divided by the volume of a sample of the material; it is often expressed in gram per cubic centimetre or gr/cc. The density of most plastics is about the same as the density of water, which is 1 gr/cc. The density of various plastics at room temperature is shown in this table.
PLASTICS DENSITY IN GR/CC
LDPE 0.92
HDPE 0.95
PVC 1.40
ABS 1.02
PP 0.91
NYLON-6 1.13
PETP 1.35
PS 1.06
FEP 2.15
The density can also be described by the term specific volume. The specific volume is the volume divided by the mass of a sample of a material; it is the reciprocal of the density and can be expressed in cc/gr, The density or the specific volume is affected by temperature and pressure. The mobility of the plastics molecules increases with higher temperatures. As a result, the specific volume increases with increasing temperature, as illustrated in this figure (vol_t.cvs) for HDPE.
The specific volume increases rapidly as the plastics approaches the melting point. Beyond the melting point, the slope changes abruptly and the volume increases more slowly. A diagram that shows the effect of pressure and temperature on the specific volume of a plastics is called a P-V-T diagram.
MELTING POINT
The melting point is the temperature at which the crystallites melt. Since amorphous plastics do not have crystallites, there is no melting point for amorphous plastics, only for semi-crystalline plastics. Semi-crystalline plastics are usually processed about 50 degrees C above the melting point. If the plastics is susceptible to degradation, the processing temperature should be as low as possible. When the plastics has a high viscosity, the processing should be as high as possible, without degrading the plastics.
GLASS TRANSITION TEMPERATURE
Plastics at low temperature are rigid and stiff. At higher temperatures, plastics become soft and ductile because the molecules become flexible and can change conformation. The transition between the two states is called the glass transition temperature.
When the glass transition temperature is above room temperature, the plastics will be hard and brittle at room temperature - for example, polystyrene.
When the glass transition temperature is below room temperature, the plastics will be soft and flexible at room temperature. Melt and glass transition temperatures for some plastics are shown in this table:
Plastics Glass Transition
Temperature [° C]
Melting Point
polystyrene (PS) 101 -
polyvinylchloride (PVC) 80 -
polymethyl methacrylate (PMMA) 105 -
acrylonitrile butadiene styrene (ABS) 115 -
polycarbonate (PC) 115 -
low density polyethylene (LDPE) -120/-90 120
high density polyethylene (HDPE) -120/-90 130
polypropylene (PP) -10 175
nylon-6 (PA-6) 50 225
polyethylene terephthalate (PET) 70 275
polybutylene terephthalate (PBT) 45 250
polyvinylidene fluoride (PVDF) -40 170
tetrafluoroethylene/
hexafluoropropylene copolymer (FEP) 70 275
