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
Ease of processing
Chemical resistance
Abrasion resistance
Electrical properties
Impact resistance
Low coefficient of friction
Near-zero moisture absorption
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
Lightweight
Formable
Impact Resistant
Excellent Electrical Properties
Machinable
Weldable
Fabrication
LDPE can be fabricated using the following techniques.
Hot gas welded
Fusion and butt welded
Ultra-sonic sealed
Die cut
Machined with wood or metal working tools
Vacuum formed
Thermo-formed
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
HDPE can be fabricated using the following techniques:
Hot gas welded
Fusion and butt welded
Ultra-sonic sealed
Die cut
Machined with wood or metal working tools
Vacuum formed
Thermo-formed
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.³/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
|
-
|
