POLYETHYLENE
Polyethylene is used more than
any other thermoplastic polymer. There is a wide variety of grades and
formulations available that have an equally wide range of properties. In
general, the outstanding characteristics of polyethylene are:
Toughness
The three most commonly used
grades of polyolefin are: Low Density, High Density and Polypropylene.
LOW DENSITY POLYETHYLENE (LDPE)
LDPE, was the first of the polyethylenes to be developed. It is a corrosion
resistant, low density extruded material that provides low moisture
permeability. LDPE has a fairly low working temperature, soft surface and low
tensile strength. It is an excellent material where corrosion resistance is an
important factor, but stiffness, high temperature and structural strength are
not important considerations.
Features
Fabrication
Applications
-
Chemical resistant tank and containers
-
Food storage containers
-
Laboratory equipment
-
Disposable Thermo-formed products
-
Corrosion resistant work surfaces
-
Vacuum formed end caps and tops
-
Moisture barrier
HIGH DENSITY POLYETHYLENE (HPDE)
HPDE is more rigid and harder than lower density materials. It also has a higher
tensile strength, four times that of low density polyethylene, and it is three
times better in compressive strength. HDPE meets FDA requirements for direct
food contact applications. It also is accepted by USDA, NSF and the Canadian
Department of Agriculture.
Features
Abrasion Resistant
The extremely high molecular weight of HDPE combined with its very low
coefficient of friction provides and excellent abrasion resistant product
preventing gouging, scuffing and scraping.
Exceptional Impact Strength
HDPE is one of the highest impact resistant thermoplastics available and
maintains excellent machinability and self- lubricating characteristics.
Properties are maintained even at extremely low temperatures.
Chemical Resistant
HDPE has very good chemical resistance of corrosives as well as stress cracking
resistance (with the exception of strong oxidizing acids at elevated
temperatures). Certain hydrocarbons cause only a light surface swelling at
moderate temperature.
Water Resistant
Moisture and water (including saltwater) have no affect on HDPE. It can be used
in fresh and salt water immersion applications.
Fabrication
HIGH DENSITY POLYETHYLENE (HPDE)
APPLICATIONS
Food cutting boards
Corrosion resistant wall
coverings
Pipe flanges
Lavatory partitions
Man-hole covers in Chemical
Plants
Radiation shielding
Self supporting containers
Prosthetic devices
Typical properties of polyethylene
ASTM test |
Property |
Low density |
Medium density |
High density |
Ultrahigh molecular weight |
PHYSICAL
|
D792
|
Specific
gravity |
0.910-0.925 |
0.926-0.940 |
0.941-0.965 |
0.928-0.941 |
D792
|
Specific
volume (in.�/lb.) |
30.4-29.9
|
29.9-29.4
|
29.4-28.7
|
29.4
|
D570
|
Water
absorption, 24 hours, 1/8 inch thick (%) |
<0.01
|
<0.01
|
<0.01
|
<0.01
|
MECHANICAL
|
D638
|
Tensile
strength (psi) |
600-2,300
|
1,200-3,500 |
3,100-5,500 |
4,000-6,000 |
D638
|
Elongation (%) |
90-800
|
50-600
|
20-1,000
|
200-500
|
D638
|
Tensile
modulus (10~5 psi) |
0.14-0.38
|
0.25-0.55
|
0.6-1.8
|
0.20-1.10
|
D790
|
Flexural
modulus (10~5 psi) |
0.08-0.60
|
0.60-1.15
|
1.0-2.0
|
1.0-1.7
|
D256
|
Impact
strength, izod (ft-lb/in. of notch) |
No break
|
0.5-16
|
0.5-20
|
No break
|
D785
|
Hardness, Rockwell R |
10
|
15
|
65
|
67
|
THERMAL
|
C177
|
Thermal
conductivity (10~4 cal-cm/sec-cm~2-�C) |
8.0
|
8.0-10.0
|
11.0-12.4
|
11.0
|
D696
|
Coefficient of thermal expansion (10~5 in./in.-�F) |
5.6-12.2
|
7.8-8.9
|
6.1-7.2
|
7.8
|
D648
|
Deflection temperature (�F)
At 264 psi
At 66 psi |
90-105
100-121 |
105-120
120-165 |
110-130
140-190 |
118
170 |
ELECTRICAL
|
D149
|
Dielectric strength (V/mil) short time, 1/8-in. thick |
460-700
|
460-650
|
450-500
|
900 kV/cm
|
D150
|
Dielectric constant At 1kHz |
2.25-2.35
|
2.25-2.35
|
2.30-2.35
|
2.30-2.35
|
D150
|
Dissipation factor At 1kHz |
0.0002
|
0.0002
|
0.0003
|
0.0002
|
D257
|
Volume
resistivity (ohm-cm) At 73�F, 50% RH |
10~15
|
10~15
|
10~15
|
10~18
|
D495
|
Arc
resistance(s) |
135-160
|
200-235
|
-
|
-
|
OPTICAL
|
D542
|
Refractive index |
1.51
|
1.52
|
1.54
|
-
|
D1003
|
Transmittance (%) |
4-50
|
4-50
|
10-50
|
-
|
|