Elastomers and rubber materials provide a variety of properties.
Important specifications for elastomers and rubber materials include mechanical, thermal, electrical, optical, processing, and physical properties. Mechanical properties include tear strength, ultimate tensile strength (UTS), tensile modulus or modulus of elasticity, elongation, and impact toughness as measured with an Izod test and a notched sample. Thermal properties include maximum use temperature, glass transition temperature, thermal conductivity, and coefficient of thermal expansion (CTE). Electrical and optical properties include electrical resistivity, dielectric strength, dielectric constant or relative permittivity, index of refraction, and light transmission. Processing and physical properties include bulk or apparent density, water absorption, viscosity, process temperature, and melt flow index (MFI).
To make it possible to compare the different material characteristics several tests have been standardized. The material datasheets provide the results of these tests. Below the most important properties are explained in more detail.
Specific gravity
This property is fully defined by the composition of the material. Any other value will indicate another material or composition is on hand. The specific gravity is the mass per unit volume and is measured by weighing the sample in air and in water.
Specific gravity = weight in air / (weight in air – weight in water)
Standards: ISO 2871, ASTM D 1817
Hardness
Hardness represents the elasticity of the material. The lower the hardness the more elastic the material is. Two scales are normally used: Shore-A and micro-IRHD. They are roughly the same. The instruments used for the measurement are:
- Durometer, a pointed conical indentor when pressed against a sample, is pushed back into the case of the tester against a spring and this motion is translated into movement of the pointer on the dial. The harder the sample the farther it will push back the indentor point and the higher will be the numerical reading on the scale. The unit is Shore-A.
- IRHD tester, a dead-load is applied to the indentor for a specific time and the hardness is obtained from the depth of the indentation.
Shore-A micro IRHD
Standards: ISO 48, ISO 1400, ISO 1818, ASTM D 2240, ASTM D 1415
Tensile strength, elongation
Tensile strength is the maximum tensile stress reached in stretching a test piece, usually a flat dumb-bell shape, to its breaking point. By convention, the force required is expressed as force per unit area of the original cross section of the test length.
Elongation, or strain, is the extension between bench marks produced by a tensile force applied to the test piece and is expressed as a percentage of the original distance between the marks. Elongation at break, or ultimate elongation, is the elongation at the moment of rupture.
Standards: ISO 37, ASTM D 412
Compression set
Rubbers deform under load and rarely return completely to their original dimensions when the load is removed. The difference between the original and final dimensions is known as compression set.
Small cylindrical disks of 13 mm diameter and a thickness of 6 mm or 29 mm diameter and a thickness of 12.5 mm are being used to perform the tests.
The disks are compressed in such a way that the compression is 25% of the original height. This at a known temperature, often at 23°C (or between 70 and 250°C) with a duration of 24 or 72 hours.
At the end of the specified time, the test pieces are removed from the test jig and allowed to recover at 23°C for 30 minutes before the thickness is re-measured. The compression set is the difference between the original thickness of the test piece and that after recovery, expressed as a percentage of the initially applied compression.
As formula:
Original thickness – Thickness of the piece after recovery
Compression set = ---------------------------------------------------------------
Original thickness – Height of the compression
Standards: ISO 815, ASTM D 395
Resistance to heat aging:
The properties of an elastomer will generally change after prolonged exposure to high temperatures. Tests for heat aging are carried out for two reasons. Firstly, there are tests to establish the changes in physical properties at elevated service temperatures. Secondly, there are accelerated tests at high temperatures which attempt to predict the long-term life at lower temperatures. Tests are carried out in an air oven or an oxygen pressure chamber.
Standards: ISO 188, ASTM D 573
Resistance to weathering
Deterioration in physical properties can occur when elastomers are exposed to the weather. This deterioration can be observed as cracking, peeling, chalking, colour changes and other surface defects. By far the most important cause of deterioration by weathering is the presence of ozone. Less than one pphm of ozone in the atmosphere can severely attack non-resistant rubbers if they are in a slightly strained condition. The result are cracks sheer on the direction of the strain. Sunlight (UV), oxygen, moisture and temperature also affect elastomers.
Standards: 1431/1, ASTM D 1149
Resistance to low temperatures
All elastomers
undergo several kinds of change when they are exposed to low
temperatures. Some of the changes occur immediately,
others after prolonged exposure. All are reversible; the elastomer
regaining its original properties when it is returned to room
temperature. At low temperatures the material will become brittle and
shatter on sudden bending or impact.
The temperature at which
this occurs, when determined under certain prescribed testing
conditions, is called the brittle point. Another test, to measure the
modulus of the material, is the material retraction test. Generally
known as the TR test.
Standards: ISO R 812, ISO 2921, ASTM D 2137, ADTM D 1053, ASTM D 1329
Abrasion resistance
A test piece is pressed against a rotating drum covered with an abrasive cloth.
The loss in weight (volume) is measured after a certain number of revolutions and gives an indication of the abrasion resistance.
Standards: ISO 4649, ASTM D 394
Resistance to liquids
The action of liquids on elastomers may result in the absorption of liquid by the elastomer, extraction of soluble constituents from the elastomer or chemical reaction with the elastomer. Absorption is usually greater than extraction and there is a net increase in volume, generally known as swelling. For some products a decrease in volume or dimensions could be more serious than swelling and if there is a significant chemical reaction a low swelling may hide a large deterioration in physical properties. Consequently, although degree of swelling provides a good general indication of resistance it is also important to measure the change in other properties.
In general the following guide lines can be used:
0-5% swell; recommended, no or minor effect
5-10% swell; seal can be used in most cases, less to moderate effect.
10-20% swell; Seals only in static applications to be used, moderate to severe effect
> 20% swell; not recommended
Standards: ISO 1817, ASTM D 1817
Of course, many more properties can be tested. Depending on the application the most appropriate tests can be selected and carried out to give the best results to predict the life time of a product.