Jun 25 2003
Background
Polyethylene terephthalate polyester (PETP) is the most common thermoplastic polyester and is often called just “polyester”. This often causes confusion - not only is the chemically similar polybutylene terephthalate (PBT) also a (thermoplastic) polyester, the most common resin system used in GRP is also a polyester system - and also often called just “polyester”. (In this latter case, however, the polyesters are chemically unsaturated and are “free-radical polymerised” into a thermoset).
General Properties
Polyethylene terephthalate polyester (PETP) is a hard, stiff, strong, dimensionally stable material that absorbs very little water. It has good gas barrier properties and good chemical resistance except to alkalis (which hydrolyse it).
Its crystallinity varies from amorphous to fairly high crystalline. Polyethylene terephthalate polyester (PETP) can be highly transparent and colourless but thicker sections are usually opaque and off-white.
Polyethylene Terephthalate Films
Polyethylene terephthalate polyester (PETP) is widely known in the form of biaxially oriented and thermally stabilised films usually referred to by their main brand names Mylar, Melinex or Hostaphan.
Strictly speaking, these names should be used only for this type of film whose properties are different from, and in several respects superior to, those of “ordinary” polyethylene terephthalate (PET) film.
Applications
These “Mylar®-type” films are used for capacitators, graphics, film base and recording tapes etc. Polyethylene terephthalate polyester (PETP) is also used for fibres for a very wide range of textile and industrial uses (Dacron®, Trevira®, Terylene®).
Other applications include bottles and electrical components.
Key Properties
The key properties of Polyethylene Terephthalate Polyester ( PETP ) are tabulated below.
Table 1. Key properties
| Chemical Resistance |
| Acids – concentrated |
Good |
| Acids – dilute |
Good |
| Alcohols |
Good |
| Alkalis |
Poor |
| Aromatic hydrocarbons |
Fair |
| Greases and Oils |
Good |
| Halogens |
Good |
| Ketones |
Good |
| Electrical Properties |
| Dielectric constant @ 1 MHz |
3.0 |
| Dielectric strength ( kV.mm-1 ) |
17 |
| Dissipation factor @ 1 kHz |
0.002 |
| Surface resistivity ( Ohm/sq ) |
1013 |
| Volume resistivity ( Ohm.cm ) |
>1014 |
| Mechanical Properties |
| Coefficient of friction |
0.2 - 0.4 |
| Hardness – Rockwell |
M94-101 |
| Izod impact strength ( J.m-1 ) |
13 - 35 |
| Poisson’s ratio |
0.37-0.44 (oriented) |
| Tensile modulus ( GPa ) |
2 - 4 |
| Tensile strength ( MPa ) |
80, for biax film 190 - 260 |
| Physical Properties |
| Density ( g.cm-3 ) |
1.3 - 1.4 |
| Flammability |
Self Extinguishing |
| Limiting oxygen index ( % ) |
21 |
| Refractive index |
1.58 - 1.64 |
| Resistance to Ultra-violet |
Good |
| Water absorption - equilibrium ( % ) |
<0.7 |
| Water absorption - over 24 hours ( % ) |
0.1 |
| Thermal Properties |
| Coefficient of thermal expansion ( x10-6 K-1 ) |
20 - 80 |
| Heat-deflection temperature - 0.45 MPa ( °C ) |
115 |
| Heat-deflection temperature - 1.8 MPa ( °C ) |
80 |
| Lower working temperature ( °C ) |
-40 to -60 |
| Specific heat ( J.K-1.kg-1 ) |
1200 - 1350 |
| Thermal conductivity ( W.m-1.K-1 ) |
0.15 - 0.4 @ 23 |
| Upper working temperature ( °C ) |
115 - 170 |
| Properties Polyethylene Terephthalate Film |
| Property |
|
Value |
| Dielectric Strength @ 25 µm thick |
KV.mm-1 |
300 |
| Dissipation Factor @ 1 MHz |
|
0.016 |
| Elongation at Break |
% |
60 - 165 |
| Initial Tear Strength |
g.µm-1 |
18 - 54 |
Permeability to Carbon Dioxide
@ 25 °C |
x10-13 cm3.cm.cm-2.s-1.Pa-1 |
0.07 - 0.11 |
| Permeability to Hydrogen @ 25 °C |
x10-13 cm3.cm.cm-2.s-1.Pa-1 |
0.45 |
| Permeability to Nitrogen @ 25 °C |
x10-13 cm3.cm.cm-2.s-1.Pa-1 |
0.0034 - 0.0038 |
| Permeability to Oxygen @ 25 °C |
X10-13 cm3.cm.cm-2.s-1.Pa-1 |
0.015 - 0.04 |
| Permeability to Water @ 25 °C |
X10-13 cm3.cm.cm-2.s-1.Pa-1 |
100 - 115 |
| Specific Heat |
KJ.kg-1.K-1 |
1.3 |
| Thermal Conductivity @ 23 °C |
W.m-1.K-1 |
0.13 - 0.15 |
| Properties Polyethylene Terephthalate Fibre |
| Property |
|
Value |
| Material |
|
Medium |
High tenacity |
| Specific Modulus |
cN/tex |
|
700 - 800 |
| Specific Tenacity |
cN/tex |
36 |
70 - 80 |
| Density |
g.cm-3 |
1.39 |
1.39 |
| Extension to break |
% |
36 |
13 - 16 |
| Modulus |
GPa |
|
9 - 11 |
| Shrinkage @ 100 °C |
% |
4 |
1.5 - 6 |
| Tenacity |
GPa |
0.5 |
0.9 - 1.1 |
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