The Basic Rubber Compound
The rubber compound was first developed by Goodyear and Hancock and it continues to develop as new materials and new variations on old ones appear in the marketplace.
The compound we see everyday as rubber, such as in a tire or pencil eraser, is a mixture of a number of different ingredients.
It starts with the raw gum elastomer, supplied by the plantation owner as NR, or by the petrochemical complex converting petroleum products such as ethylene, propylene and butadiene into ‘raw’ bales or chips of rubbery polymers such as EPDM, BR, SBR, NBR or CR.
It is shipped to the rubber processor who blends it with various ingredients. The raw gum elastomer itself has very limited use, although adhesives provide one example. Most are mechanically weak and subject to significant swelling in liquids, and will not retain their shape after molding. Many of its other properties could also benefit from enhancement. It is at this point that the rubber compounder takes over, and all of his art and science is dedicated to modifying
the raw gum elastomer, changing it into a more useful material.
Silicone Rubber
Silicone rubbers are a group of synthetic elastomers noted for their
(1) resilience over a very wide temperature range,
(2) outstanding resistance to ozone and weathering, and
(3) excellent electrical properties.
COMPOSITION
The basic silicone elastomer is a dimethyl polysiloxane. It consists of long chains of alternating silicon and oxygen atoms, with two methyl (–CH3) side chains attached to each silicon atom. By replacing a part of these methyl groups with other side chains, polymers with various desirable properties can be obtained.
For example, where flexibility at temperatures lower than –57°C is desired, a polymer with about 10% of the methyl side chains replaced by phenyl groups
(–C6H5) will provide compounds with brittle points below -101°C. Sidechain modification can also be used to produce elastomers with lower compression set, increased resistance to fuels, oils, or solvents, or to permit vulcanization at room temperature.
Curing, or vulcanization, is the process of introducing cross-links at intervals between the long chains of the polymer. Silicone rubbers are usually cross-linked by free radical-generating curing agents, such as benzoylperoxide, which are activated by heat, or the cross-linking can be accomplished by high-energy radiation beams. Room temperature vulcanized compounds are cross-linked by the condensation reaction resulting from the action of metalorganic salts, such as zinc or tin octoates.
EPDM
EPM is a copolymer consisting of ethylene and propylene units as part of the main polymer chain. It can be cross-linked with peroxides or radiation but not sulfur. EPM is used as an ethylene based plastic impact modifier and as a viscosity index improver for lubricating oils.
When a non-conjugated diene is grafted on to the main polymer chain it becomes a terpolymer, ethylene propylene diene (EPDM) and interchain sulfur cross-linking becomes possible.
The names ethylene and propylene sound familiar because of their use in polyethylene and polypropylene (plastic) goods such as kitchenware. Raw polymer suppliers offer the usual viscosity variations plus different ethylene/propylene ratio grades. A higher ethylene content gives more green strength (high elongation in the uncured state) which can help the rubber processor in the mill mixing. However, a high ethylene content gives poorer low temperature properties. Raw polymer manufacturers use a variety of diene monomers unit grafted on to the main polymer chain, which, amongst other things, allows variation in the ease of vulcanization, depending on the unit chosen and the amount in the polymer.
Nitrile
Nitrile rubber is resistant to petroleum oils and aromatic hydrocarbons and is also highly resistant to mineral oils, vegetable oils, and many acids. In addition, nitrile has good elongation properties as well as adequate resilience, tensile and compression set.
To the chemist this rubber is known as acrylonitrile butadiene rubber, to others it is called Buna-N but to many people in the industry, simply, nitrile. It is the workhorse of the marketplace for its oil resistant properties. The grades offered differ in the percentage of acrylonitrile (ACN) in the polymer chain as well as the overall viscosity of the polymer.
The higher the amount of ACN in the elastomer the better the oil resistance; the lower end of the ACN distribution range being approximately equivalent to the oil resistance of CR and therefore only having a moderate level of oil resistance. NBR also has superior fuel resistance. The terms oil and fuel used here refers loosely to those products derived from petroleum.
The weather resistance of NBR is poor, similar to NR and SBR, although it can be enhanced by blending with the plastic, polyvinyl chloride (PVC), at some 'cost' to its low temperature properties. This latter attribute of NBR also varies with ACN content; the lower the percentage of ACN in the polymer, the better the low temperature flexibility, and the poorer the oil resistance.
A compound which has a nitrile raw gum elastomer in it, with a medium (33%) ACN content, would have good oil resistance and low temperature resistance down to the region of -40 °C. A low ACN (18%) nitrile would be useful down to -55 °C. NBR has better heat aging resistance than CR and is in the region of
107 °C for continuous (defined approximately as 1,000 hours) use. Special compounding ingredients can be added to increase heat aging resistance. Like SBR, NBR needs reinforcing fillers to give good mechanical properties.
Styrene butadiene rubber SBR for an emulsion
SBR and SSBR are derived from petroleum oil. This applies to most elastomers, with the obvious exception of NR. SBR represents half of all synthetic rubber production, and is much consumed in tires, where it competes with and complements NR. There are many subgroups of the raw gum elastomer, depending on the method of synthesis of the polymer, such as whether it is solution or emulsion polymerized, and the ratio of the two major chemical building blocks styrene and butadiene.
In comparison with natural and CR, gum vulcanizates made from SBR have poor mechanical properties. The raw gum elastomer must have reinforcing fillers , such as carbon black, in order to attain good mechanical strength and the filler increases hardness at the same time.
Elastomer selection to suit your needs
In elastomer selection it is usual to categorize the properties of elastomeric materials into mechanical aspects such as modulus, damping or fatigue behaviour and into the elastomer's tolerance to environmental factors such as high or low temperature, exposure to oils, ozone, or ionizing radiation.
The balance between the importance of mechanical properties and the degree of hostility of the environment, considered with the design of the component, will help determine whether an elastomer is suitable for a particular function. Simplified tables and guides to the performance of different elastomers may sometimes be misleading, as environmental factors can interact with each other and with mechanical conditions.
The rubber compound was first developed by Goodyear and Hancock and it continues to develop as new materials and new variations on old ones appear in the marketplace.
The compound we see everyday as rubber, such as in a tire or pencil eraser, is a mixture of a number of different ingredients.
It starts with the raw gum elastomer, supplied by the plantation owner as NR, or by the petrochemical complex converting petroleum products such as ethylene, propylene and butadiene into ‘raw’ bales or chips of rubbery polymers such as EPDM, BR, SBR, NBR or CR.
It is shipped to the rubber processor who blends it with various ingredients. The raw gum elastomer itself has very limited use, although adhesives provide one example. Most are mechanically weak and subject to significant swelling in liquids, and will not retain their shape after molding. Many of its other properties could also benefit from enhancement. It is at this point that the rubber compounder takes over, and all of his art and science is dedicated to modifying
the raw gum elastomer, changing it into a more useful material.
Silicone Rubber
Silicone rubbers are a group of synthetic elastomers noted for their
(1) resilience over a very wide temperature range,
(2) outstanding resistance to ozone and weathering, and
(3) excellent electrical properties.
COMPOSITION
The basic silicone elastomer is a dimethyl polysiloxane. It consists of long chains of alternating silicon and oxygen atoms, with two methyl (–CH3) side chains attached to each silicon atom. By replacing a part of these methyl groups with other side chains, polymers with various desirable properties can be obtained.
For example, where flexibility at temperatures lower than –57°C is desired, a polymer with about 10% of the methyl side chains replaced by phenyl groups
(–C6H5) will provide compounds with brittle points below -101°C. Sidechain modification can also be used to produce elastomers with lower compression set, increased resistance to fuels, oils, or solvents, or to permit vulcanization at room temperature.
Curing, or vulcanization, is the process of introducing cross-links at intervals between the long chains of the polymer. Silicone rubbers are usually cross-linked by free radical-generating curing agents, such as benzoylperoxide, which are activated by heat, or the cross-linking can be accomplished by high-energy radiation beams. Room temperature vulcanized compounds are cross-linked by the condensation reaction resulting from the action of metalorganic salts, such as zinc or tin octoates.
EPDM
EPM is a copolymer consisting of ethylene and propylene units as part of the main polymer chain. It can be cross-linked with peroxides or radiation but not sulfur. EPM is used as an ethylene based plastic impact modifier and as a viscosity index improver for lubricating oils.
When a non-conjugated diene is grafted on to the main polymer chain it becomes a terpolymer, ethylene propylene diene (EPDM) and interchain sulfur cross-linking becomes possible.
The names ethylene and propylene sound familiar because of their use in polyethylene and polypropylene (plastic) goods such as kitchenware. Raw polymer suppliers offer the usual viscosity variations plus different ethylene/propylene ratio grades. A higher ethylene content gives more green strength (high elongation in the uncured state) which can help the rubber processor in the mill mixing. However, a high ethylene content gives poorer low temperature properties. Raw polymer manufacturers use a variety of diene monomers unit grafted on to the main polymer chain, which, amongst other things, allows variation in the ease of vulcanization, depending on the unit chosen and the amount in the polymer.
Nitrile
Nitrile rubber is resistant to petroleum oils and aromatic hydrocarbons and is also highly resistant to mineral oils, vegetable oils, and many acids. In addition, nitrile has good elongation properties as well as adequate resilience, tensile and compression set.
To the chemist this rubber is known as acrylonitrile butadiene rubber, to others it is called Buna-N but to many people in the industry, simply, nitrile. It is the workhorse of the marketplace for its oil resistant properties. The grades offered differ in the percentage of acrylonitrile (ACN) in the polymer chain as well as the overall viscosity of the polymer.
The higher the amount of ACN in the elastomer the better the oil resistance; the lower end of the ACN distribution range being approximately equivalent to the oil resistance of CR and therefore only having a moderate level of oil resistance. NBR also has superior fuel resistance. The terms oil and fuel used here refers loosely to those products derived from petroleum.
The weather resistance of NBR is poor, similar to NR and SBR, although it can be enhanced by blending with the plastic, polyvinyl chloride (PVC), at some 'cost' to its low temperature properties. This latter attribute of NBR also varies with ACN content; the lower the percentage of ACN in the polymer, the better the low temperature flexibility, and the poorer the oil resistance.
A compound which has a nitrile raw gum elastomer in it, with a medium (33%) ACN content, would have good oil resistance and low temperature resistance down to the region of -40 °C. A low ACN (18%) nitrile would be useful down to -55 °C. NBR has better heat aging resistance than CR and is in the region of
107 °C for continuous (defined approximately as 1,000 hours) use. Special compounding ingredients can be added to increase heat aging resistance. Like SBR, NBR needs reinforcing fillers to give good mechanical properties.
Styrene butadiene rubber SBR for an emulsion
SBR and SSBR are derived from petroleum oil. This applies to most elastomers, with the obvious exception of NR. SBR represents half of all synthetic rubber production, and is much consumed in tires, where it competes with and complements NR. There are many subgroups of the raw gum elastomer, depending on the method of synthesis of the polymer, such as whether it is solution or emulsion polymerized, and the ratio of the two major chemical building blocks styrene and butadiene.
In comparison with natural and CR, gum vulcanizates made from SBR have poor mechanical properties. The raw gum elastomer must have reinforcing fillers , such as carbon black, in order to attain good mechanical strength and the filler increases hardness at the same time.
Elastomer selection to suit your needs
In elastomer selection it is usual to categorize the properties of elastomeric materials into mechanical aspects such as modulus, damping or fatigue behaviour and into the elastomer's tolerance to environmental factors such as high or low temperature, exposure to oils, ozone, or ionizing radiation.
The balance between the importance of mechanical properties and the degree of hostility of the environment, considered with the design of the component, will help determine whether an elastomer is suitable for a particular function. Simplified tables and guides to the performance of different elastomers may sometimes be misleading, as environmental factors can interact with each other and with mechanical conditions.
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O Rings / construction materials provide the best seal for dynamic, high temperature and caustic environments, and have become the most durable, dependable and robust solution to many applications in the automotive and manufacturing industries.
O Rings / construction materials are made from various elastomers, and the most commonly used are Nitrile (NBR). There are a variety of other compounds available depending on your application requirements.
One of the most important components of an O Rings / construction material is the elastomer material being used. When determining what elastomer is best suited for the environmental conditions in which the seal will be placed, the following properties are important to consider:
Good chemical resistance
Good elasticity
High resistance to water
Good resistance to heat and low temperature
Good resistance to ozone (UV) and weather aging
Low compression set
O Rings / construction materials are made from various elastomers, and the most commonly used are Nitrile (NBR). There are a variety of other compounds available depending on your application requirements.
One of the most important components of an O Rings / construction material is the elastomer material being used. When determining what elastomer is best suited for the environmental conditions in which the seal will be placed, the following properties are important to consider:
Good chemical resistance
Good elasticity
High resistance to water
Good resistance to heat and low temperature
Good resistance to ozone (UV) and weather aging
Low compression set
Product Application and Performance.
Performance Table and Compound Information
| Material |
Hardness ( Shore A ) | Temperature Range | Comments |
| Nitrile (Buna N) (NBR) |
40 ~ 90 | -40 F ~ 250 F |
Excellent general purpose material. A low cost compound, with good physical strength and good compression set. Good resistance to oil, fuel, water, and air. |
| Butyl (IIR) |
50 ~ 80 | -75 F ~ 225 F |
Excellent impermeability for vacuum service, non flammable phosphate ester fluid resistance, and good weathering, water swell, ozone, and chemical resistance, except with petroleum base derivitaves. Poor compression set. |
| Ethylene Propylene (EPT) (EPDM) |
40 ~ 80 | - 65 F ~ 300 F |
Excellent resistance to steam, aging, abrasion, and phosphate ester fluids. Good compression set, ozone, ketone and weather resistance. Poor petroleum base resistance. |
| Silicones |
30 ~ 80 | - 80 F ~ 450 F |
Standard high cost compounds, excellent water and dry heat resistance. Poor tensile, and abrasion, and poor oil, fuel and solvent resistance. |
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