Selecting the Most Cost-effective Suitable Rubber
Two properties more than any other are dependent on the choice of rubber type - temperature range and fluid resistance. To a lesser degree, long-term weather and ozone resistance are also affected by the type chosen and this should be borne in mind if relevant.
To select the most suitable rubber the following information is required:
- What is the highest temperature likely to be encountered in service?
- What is the highest temperature at which continuous service will be required?
- What is the lowest temperature at which the component must remain operable?
- What fluids will be encountered in service and at what temperature?
- Is the frequency of contact with the fluid continuous, intermittent, or very occasional (e.g. accidental contamination)
- Is long-term weather or ozone resistance an important factor?
The chart below shows the different rubbers and their properties as related to these questions. The cost factor is also shown, with the rubbers arranged in order of increasing cost from left to right.
Starting at the left, check the properties of the rubber against your answers to the questions above. Keep moving to the right until a rubber is found that meets your need. (Where no particular temperature or fluid resistance is required Natural Rubber is the most widely used material and offers the greatest scope of compounds and properties coupled with the most economic cost).
If a rubber meets most of the requirements but is borderline on a particular property, Harboro should be consulted for further advice with a view to testing under the relevant conditions (if necessary).
Harboro can also offer advice and clarification if resistance to specific fluids is required. The company has detailed records and will carry out swell tests with the relevant fluid free of charge to establish a suitable rubber.
When a suitable rubber has been chosen, refer to the more detailed information contained in the Data Chart to check the suitability of the rubber for all aspects of the application.
Material Selection Chart
|Max Int Temp (°c)||105°c||150°c||125°c||130c||160°c||180°c||180°c||300°c||300°c||280°c||115°c|
|Max Cont Temp (°c)||75°c||130°c||95°c||100°c||130°c||150°c||150°c||205°c||205°c||200°c||85°c|
|Lowest Temp (°c)||-60°c||-50°c||-35°c||-20°c||-25°c||-20°c||40°c||-60°c
* -80° can be achieved with special silicones
Defining Basic Mechanical Properties
A set of hardness test pieces
may be obtained from Harboro
on request but the hardness
ranges may generally be
described as follows:
|Soft - Medium||50-60°|
Once a rubber type has been selected, the hardness range must be determined. Hardness is measured in degrees on the Shore “A” or IRHD scale (the values are similar although Shore “A” readings are usually one to three degrees higher than IRHD readings). Hardnesses are normally based on a nominal figure e.g. 50 ±5° or as a hardness range e.g. 50-60°.
Materials below 30° are extremely soft and comparable with foams. These are available but must be regarded as a special requirement.
Typical Tensile Strengths
Any specific requirements relating to tensile stress/strain properties should be defined by minimum tensile strength, minimum elongation at break and, where relevant, by modulus (i.e. minimum tensile stress at a given strain). Such limits should be derived by calculation based on the actual requirements of the application, by comparison with known values for similar applications or materials, or by testing a material which has been proved satisfactory by trial or experiment.
Typical Tensile Strengths
Many uses of rubber are compression applications and it may be necessary to define the maximum compression set taken on by a rubber when under load for a period of time. This is usually expressed as the percentage of the compression which is not recovered within a short time after release (30 minutes in BS 903 pt.A6).
Compression set limits should only be specified when necessary and must give the time and temperature as well as the amount of compression to be applied. Typical compression set values are shown below for 24 hours compression at 70°C. The test pieces were compressed to 75% of their original height.
Typical Compression Set Value
(24 hours @ 70°C)
Defining Other Mechanical Properties
Additional mechanical requirements should be specified only where necessary. They can be specified simply by description e.g. “good tear resistance required” or quantified by the use of a defined test method (such as ASTM or BS test methods) giving maximum or minimum values as appropriate. These values may be obtained by calculation based on the actual requirements of the application, by comparison with known values for similar applications or materials, or by testing a material which has been proved satisfactory by trial or experiment.
A list of properties which can be tested and defined by the use of the appropriate British Standard and ASTM tests.
Where requirements are considerably more complex than the scope of this publication, the ASTM or BS framework for specifying rubber materials (see below) can be consulted. Materials may also be selected from the BS range of standard and special application specifications, some of which are listed below.
If the properties required of the rubber are difficult to determine, the production of a prototype mould should be considered. A variety of rubber compounds can then be moulded and parts tested for suitability in the actual application. Once a material has been proved suitable, it can be tested and its properties appropriately defined.
Definining Electrical Properties
Rubber is often used as an electrically insulating material and, correctly formulated, it can offer outstanding properties in this respect. However, electrical properties are dependent on the materials used in compounding and requirements should be clearly defined.
In general, an insulating material can be defined as having an electrical resistance greater than 100 megohms per cm3. Materials for applications requiring extremely good insulating properties are usually defined by electric strength, measured in Kilovolts (KV) per mm thickness of material. This is calculated by placing a sheet of the material between two electrodes and applying an increasing voltage until electrical breakdown occurs.
Good insulation is achieved by the use of non-black fillers and compounds of any colour which are based on these fillers are suitable for insulation purposes.
For test methods and recommended resistance values for anti-static and conductive rubbers, please see BS 2050.
ASTM & BS Framwortk for Specifying Rubber Materials
Types of Rubber / Resistance
|Silver Loaded||Less than 1 ohm|
|Anti-static||104 - 106 ohms|
|Insulating||Over 108 ohms|
|Highest Insulation||1014 ohms|
A method for specifying rubber materials can be found in ASTM D2000 and BS 5176. These provide a full framework covering all rubbers and rubber properties. The relevant requirements for an application are selected from within the framework and are expressed in the form of a “line call out”. (The same framework is used in both ASTM D2000 and BS 5176).
An example of a line call out is BS 5176 1MBC514 F27Z1. This can be broken down as follows:
Line Call Out
|Basic Grade||Metric Figures||Mat’l Type||Mat’l Class||Hardness||Tensile||Low Temp Test||Special Requirements|
|-||-||Neoprene||Neoprene||50+5/-4 IRHD||14MPa Minimum||-40°c Flexibility||e.g. Colour Red|
The appropriate table for Grade 1 MBC materials shows the following basic requirements:
Elongation at Break
Heat Resistance (70 hrs @ 100°C)
T.S. ± 30% - E.B. -50% max - Hardness ±15° max
Oil Resistance (Oil no.3 70 hrs @ 100°C)
Volume change + 120% max
Compression Set (22 hrs @ 100°C)
F27 is a low temperature modulus test to BS 903 A13. For Grade 1 MBC materials the requirement is for a modulus of 70MPa at -40°C (i.e. rubber is still flexible at -40°C).
Z1 indicates an additional special requirement. This would be shown on the drawing e.g. Z1 = red colour required.
The following specifications cover rubber materials for general applications:
- BS 1154 Natural Rubber compounds
- BS 2751 Nitrile Rubber compounds
- BS 2752 Chloroprene (Neoprene) Rubber compounds
- BS 3222 Low compression set Nitrile compounds