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“Rubber” compounds are resilient (elastic) materials made from one or more cross-linked base polymers, reinforcing agents, processing aids, and performance-enhancing additives.
From Greek “Many Units”
o Polymers are long chains of repeating units. On the molecular level, they
resemble extremely long spaghetti strands.
Monomer = the unit that repeats in a polymer
Isomer = Polymer made from one monomer
Dimer or Copolymer = Polymer with two monomers
Trimer or Terpolymer = Polymer with three monomers
o Polymers tangle themselves together like in a large bowl of spaghetti
• Base polymer determines chemical resistance, rough temperature limits, and
rebound resilience
o In some materials, the high and low temp limits can be modified by other
compounding ingredients.
• Polymers provide “baseline” for abrasion resistance, compression set resistance,
permeability
o These can (and almost always are) modified – up or down – by other
compounding ingredients.
• Polymer chains must be “glued” together (cross-linked) to achieve resilience and
elasticity.
o Sulfur: simplest cure system, used in nitrile and EP
o Organic Peroxides: improved compression set in EP, improved
compression set & high temp limit in nitrile, standard cure system for
silicone.
o Bisphenol: best cure system available for fluorocarbon (specialty FKMs
need to be peroxide-cured, but it’s not the first choice.)
o Others: specialty materials have special cure chemistry
• Reinforcing agents add mechanical strength and resistance to abrasion,
permeation, and compression set
o Carbon black: standard for black compounds
o Silica: standard for non-black compounds
• Fillers lower the cost of a compound but reduce compression set resistance and
elongation
o Carbon black: lower grades or excessive amounts provide no performance
benefit for seals
o Clay: commonly used in “generic” seal compounds
• Oils and / or polymers used to lower the low temp limit of nitrile and make the
material flow better (see Process Aids, next)
o Reduce resistance to compression set
o In “generic” materials, they are used to offset the hardening influence
of high levels of filler
o Can extract into process fluids, resulting in seal shrinkage &
hardening
• Nitrile (NBR)
• Hydrogenated Nitrile (HNBR)
• Polycrylate (ACM)
• Vamac (AEM)
• Neoprene (CR)
• Ethylene-Propylene (EPR,EPDM)
• Butyl (IIR)
• Polyurethane (AU, EU)
• Flourocarbom (FKM)
• Tetrafluoroethylene-Propylene (TFE/P)
• Perfluoroelastomer (FFKM)
• Hiflour (FKM)
• Silicone (VMQ)
• Fluorosilicone (FVMQ)
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