Rubber compounding glossary
Plain definitions of the terms that appear on datasheets, test reports and formulations. Each links to the guide that explains it further.
- Acid acceptor
- A metal oxide or hydroxide, such as magnesium oxide or calcium hydroxide, added to fluoroelastomers and some other compounds to take up the acid released during cure and in service. The choice affects cure, compression set and resistance to water and steam. Read more
- Ageing
- Change in properties over time under heat, fluids, oxygen, ozone or stress. Tests accelerate it by raising temperature so that a short test predicts a longer life.
- AS568
- The US size standard for O-rings. A dash number, such as 2-325, gives the inside diameter and cross-section.
- Batch
- A single mix of compound made in one run. Also called a lot when it is the unit that is tested and shipped. Read more
- Bisphenol cure
- A cure system for FKM in which a bisphenol curative and an accelerator, usually with metal oxides, crosslink the polymer. It gives good compression set but is less resistant to steam, amines and bases than peroxide cure. Read more
- Bloom
- A film or powder that migrates to the surface of a rubber part after cure or storage, caused by an ingredient that is not fully soluble in the rubber.
- Carbon black
- A reinforcing or semi-reinforcing filler made from burnt hydrocarbons. Grades such as N550 and N990 (also written MT) differ in particle size and structure and so in stiffness, strength and processing. Read more
- CAS number
- A unique numerical identifier for a chemical substance, used to tell ingredients apart when names vary.
- Coagent
- A reactive additive, such as triallyl isocyanurate (TAIC), used with a peroxide to raise crosslinking efficiency and improve properties. Read more
- Compound
- A mixture of polymer and all the other ingredients needed to make a rubber part: filler, cure system, processing aids and so on. The same polymer can produce very different compounds. Read more
- Compression set
- The permanent deformation left after a rubber specimen has been held compressed under set conditions and then released. A low value means the seal keeps its shape and sealing force. Read more
- Crosslink
- A chemical bond between polymer chains formed during cure. Crosslinks turn a soft, flowing mass into an elastic solid.
- Curative
- The ingredient that does the crosslinking, such as a peroxide or a bisphenol.
- Cure
- The chemical change in which crosslinks form. Also called vulcanisation. Time and temperature control how far it goes. Read more
- Cure system
- The set of ingredients that bring about cure, such as peroxide plus coagent or bisphenol plus accelerator. Read more
- Density
- Mass per unit volume, reported in g/cm3. For a given compound it is a quick check of mixing accuracy. Also called specific gravity when compared with water.
- Durometer
- The instrument that measures indentation hardness of rubber. Shore A is used for most seals.
- Elastomer
- A polymer that returns to nearly its original shape after being stretched or compressed.
- Elongation at break
- How far a specimen stretches before it breaks, expressed as a percentage of its original length.
- FEPM
- Tetrafluoroethylene-propylene copolymer rubber. Resistant to steam, amines, bases and many chemicals. Sold under trade names such as Aflas, which is a trademark of its owner. Read more
- FFKM
- Perfluoroelastomer. A fully fluorinated rubber with the widest chemical and heat resistance and a high price.
- Filler
- A solid added to compound to raise stiffness, strength or lower cost. Carbon black, silica and mineral fillers are common. Read more
- FKM
- Fluoroelastomer. A family of rubbers with good heat, oil and fuel resistance. The family includes several types with different fluorine contents and cure systems. Read more
- Glass transition (Tg)
- The temperature range in which a rubber changes from rubbery to glassy. Seals usually stop working a little above it. Read more
- Grade
- A specific product from a polymer maker, such as one particular FKM gum with a given viscosity and cure system.
- Hardness
- Resistance to indentation. In seals, usually Shore A on a scale of 0 to 100. Read more
- HNBR
- Hydrogenated nitrile butadiene rubber. Nitrile rubber with most of its double bonds removed, giving much better heat and ageing resistance and strong mechanical properties. Read more
- IRM 903
- A standard reference oil used to test how rubber swells in oil. It replaced the older ASTM No. 3 oil. Read more
- Lot
- A quantity of material treated as one unit for testing and release. Read more
- MDR
- Moving die rheometer. A test instrument that measures how torque rises during cure and gives ML, MH, ts2 and t90. Read more
- MH
- The maximum torque in a rheometer test. It rises with crosslink density. Read more
- ML
- The minimum torque in a rheometer test. It reflects the viscosity of the uncured compound. Read more
- Modulus
- The stress needed to stretch rubber to a given elongation, such as 100%. It measures stiffness.
- Mooney viscosity
- A measure of how stiff an uncured rubber is, from the torque needed to turn a rotor in it. Used on raw polymer and on mixed compound. Read more
- NBR
- Nitrile butadiene rubber. The most widely used oil-resistant rubber.
- O-ring
- A ring of circular cross-section used as a seal.
- Peroxide cure
- Crosslinking with an organic peroxide, usually with a coagent. Gives good heat and chemical resistance, especially to steam and bases. Read more
- Phr
- Parts per hundred rubber. A way of writing a recipe in which the polymer is always 100 and every other ingredient is given relative to it. Read more
- Plasticiser
- A liquid added to soften a compound or improve low-temperature flexibility.
- Polymer
- A long-chain molecule made from repeating units. In compounding, the base rubber.
- Post-cure
- A second heating step, after moulding, that finishes cure and removes volatile material. In FKM and FEPM it matters for compression set. Read more
- Process aid
- A small addition that helps a compound mix, flow in the mould or release from it.
- Rapid gas decompression (RGD)
- Damage to a seal when gas dissolved in the rubber at high pressure expands as pressure drops quickly. Also called explosive decompression. Read more
- Reinforcement
- Improvement in strength and tear resistance from a filler that interacts strongly with the polymer.
- Reverse engineering
- Analysis of a finished part to find out what it is made of and how it behaves, so that an equivalent can be developed. The result is an estimate, not a recovered recipe. Read more
- Scorch
- Premature cure during mixing or processing. Scorch time (ts2) tells you how long you have before it starts. Read more
- Shelf life
- The period over which a material can be stored and still be used as intended. Depends on the material and storage conditions. Read more
- Shore A
- A hardness scale for rubber measured with a durometer.
- Sour service
- Exposure to fluids that contain hydrogen sulfide. Read more
- Swell
- Increase in volume of a rubber after absorbing a liquid. Large swell usually means loss of properties. Read more
- t90
- The time to reach 90% of the maximum torque rise in a rheometer test. A common measure of cure time. Read more
- Tear strength
- Resistance to the growth of a cut or tear.
- Tensile strength
- The highest stress a specimen carries before it breaks in a pull test.
- Toll mixing
- Having a compound mixed by another company's equipment to your recipe and specification, for a fee per kilogram. Read more
- TR10
- The temperature at which a frozen, stretched specimen has retracted by 10% as it warms. A common low-temperature screening figure. Read more
- ts2
- The time for torque to rise by 2 units above the minimum in a rheometer test. A measure of scorch safety. Read more
- Volume swell
- Percent change in volume after immersion in a fluid, measured by weighing in air and in water. Read more
- Vulcanisation
- Cure of rubber by crosslinking. The word is from sulfur cure of natural rubber, now used for all cure systems.
- Zinc oxide
- A common activator in sulfur-cured rubber. In fluoroelastomers it is usually avoided or limited because of its effect on chemical resistance.