The basic compound formula

The specific formulation in Flowing Table has 100 parts of raw gum elastomer and 160.15 parts of total material. After curing for 35 minutes at 140°C its vulcanized properties are indicated as 57 IRHD, with a tensile strength of 30 MPa and elongation at break of 645%. The specific formulation and properties are taken from 'The Natural Rubber Formulary and Property Index published by The Malaysian Rubber Producers Research Association (MRPRA) Rubber chemists use the term phr (parts per hundred rubber), meaning parts of any non rubbery material per hundred parts of raw gum elastomer (rubbery material). They prefer this rather than expressing an ingredient as a percentage of the total compound weight. Parts can mean any unit of weight (kg, lb, etc.) as long as the same weight unit is used throughout the formulation.

Specific Formulation
Material	PHR	For Example	PHR
Raw gum elastomer	100	SMR20	100
Sulfur	from 0 to 4	Sulfur	0.35
Zinc oxide	5	Zinc oxide	5
Stearic acid	2	Stearic acid	2
Accelerators	from 0.5 to 3	MBS	1.4
		TMTD	0.4
Antioxidant	from 1 to 3	HPPD	2
Filler	from 0 to 150	N330 Black	45
Plasticizer	from 0 to 150	Aromatic petroleum oil	4
Miscellaneous		None
		TOTAL	160.15
PHR is defined as parts by weight of ingredient per 100 parts of raw gum elastomer. The limits given are typical examples and are not intended to be absolute values.

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.

Trade names of Rubbers

Trade names

The following, Table is a listing of just a few elastomer types, and some trade names, mainly American and European.

Symbol	Generic name	Some trade names	Company
SBR	Styrene butadiene rubber Emulsion	Copo Cariflex Ameripol-Synpol	DSM Elastomers Shell Ameripol Synpol
SSBR	Solution	Duradene Soloflex Solprene	Firestone Soloflex Housmex
CR	Chloroprene rubber	Neoprene Baypren Denka	DuPont Dow Elastomers Bayer Denki Kabushiki Kaisha Kagaku Kogyo
NBR	Nitrile	Nipol Krynac Paracril Chemigum Perbunan N Nysyn	Zeon Bayer Uniroyal Goodyear Bayer DSM Copolymer
EPDM	Ethylene propylene diene rubber	Buna EP Nordel VistaIon Royalene Keltan	Bayer DuPont Dow Elastomers Exxon Uniroyal DSM Copolymer
IR CIIR BIIR	Butyl	Exxon Butyl Polysar Butyl	Exxon Bayer
MQ	Silicone elastomers	Elastosil Silopren	Wacker Chemie Bayer FKM (FPM)
HNBR	Highly saturated (hydrogenated) nitrile	Zetpol Therban	Zeon Bayer
FKM	Fluorocarbon	Fluorel Viton Tecnoflon	Dyneon DuPont Dow Elastomers Montedison
BR	Polybutadiene rubber	Taktene Budene Diene Solprene Intene Buna	Bayer Goodyear Firestone Negromex EniChem Hils GmbH
ACM	Polyacrylate	HyTemp Europrene AR	Zeon EniChem
ECO	Epichlorhydrin ethylene oxide	Hydrin C	Zeon
CSM	Chorosulfonated polyethylene	Hypalon	DuPont Dow Elastomers
EAM(EVM)	Ethylene vinyl acetate	Levapren	Bayer
AU	Urethane (ester) (see chapter 8)	Urepan Millathane Vibrathane	Bayer TSE Industries Uniroyal
EU	Urethane (ether) (see chapter 8)	Millathane Adiprene Vibrathane	TSE Industries Uniroyal Uniroyal

Some other Types of Rubbers

Polybutadiene rubber BR

Although this is a significant elastomer it is most commonly used as a blend with other rubbers. Grades are very much dependent on the architecture of the repeating unit in the polymer chain. BR is traditionally difficult to process on rubber machinery; this difficulty is not apparent when BR is blended with other non polar elastomers such as NR. BR vulcanizates confer high resilience, therefore low heat build up, and good abrasion resistance to blends with other rubbers (its resilience is excellent and it has a low temperature flexibility second only to silicone rubber). In view of the above properties its major application area is in tires. Other applications are golf ball centers, modification of polystyrene to make high impact polystyrene and miscellaneous products needing improvements in abrasion, low temperature and resilience.

Polyacrylate ACM

This family of polymers exhibit oil resistance. Their heat aging temperature limit is between 150°C and 175 °C. The major application areas are automotive engine and transmission seals, gaskets and O-rings. The low temperature properties are not good, although some grades are flexible to -40°C.

Epichlorohydrin ECO CO and GECO

These halogenated polyethers are available in three forms: a homo polymer (CO), a copolymer (ECO) and a terpolymer (GECO). Attributes found within this group are: extremely low gas permeability, good oil and ozone resistance, and a good low and high temperature range. The high temperature performance is better than that of nitrile. They are used for automotive air ducts, fuel line hose tube and cover and some oilfield applications.

Chlorosulfonated polyethylene CSM

Best known as Hypalon this material has excellent ozone, acid, and weathering resistance together with mild oil and heat aging resistance. It is used extensively for roofing, pond liners and applications needing resistance to strong mineral acids.

Polynorbornene

This rubber has an extremely high molecular weight, allowing it to absorb from 150 to 300 phr of plasticizer and still retain good physical properties in very low hardness compounds. It is used for soft feed rolls for copiers and as the tread for dragster tires.

Reference: An Introduction to Rubber Technology by Andrew Ciesielski 1999

Aflas TFE/IP & Kalrez FFKM

Aflas TFE/IP

This is a copolymer of tetrafluoroethylene and propylene. It is a fluoroelastomer and has many of the attributes of FKM. Aflas has generally better resistance to both high temperature steam, and bases such as amines and concentrated alkalis, but poorer resistance to benzene and chlorinated solvents than conventional FKM. Elastomers with a chemistry combining that of Aflas and FKM are available. Aflas has a specialized, small market consisting primarily of oil seals for the automotive industry, wires and cables and oilfield drilling (downhole).

Kalrez FFKM

To the chemist this material is a copolymer of perfluoromethyl vinyl ether and tetrafluoroethylene. The latter monomer is better known in the plastic material polytetrafluoroethylene (PTFE; Teflon is an example). FFKM (Perfluoroelastomer) has a chemical resistance close to the outstanding levels reached by PTFE. Its upper continuous dry heat aging temperature is about 260°C. Applications are those where all other elastomers are unsuitable. In terms of properties (chemical and heat resistance) FFKM is the closest thing to a universal elastomer. FFKM can be used for highly critical oilfield parts and in the chemical industry for parts which have to stand up to highly corrosive chemicals and extreme temperatures. The price, relative to other elastomers is extremely high and molding of Kalrez compounds is usually performed by specialists.

Reference: An Introduction to Rubber Technology by Andrew Ciesielski 1999

Hydrogenated Nitrile HNBR (HSN)

This is a relatively new elastomer, making its first appearance in 1984. The symbol for the generic material is HNBR, although HSN is sometimes used in literature, standing for highly saturated nitrile. It has all the attributes of NBR plus a very much higher heat resistance, dependent on the grade chosen. It also has very good weather and abrasion resistance, plus good mechanical strength. It is used in oilfields where it has resistance to amine corrosion inhibitors and better hydrogen sulfide resistance than NBR. It has established itself in automotive applications for timing belts, gaskets and o-rings, where higher temperature resistant elastomers are needed. Peroxide cured HNBR has heat aging resistance up to 150°C, based on around 1,000 hours, while sulfur donor cured HNBR temperature resistance might drop to 135°C. Cost is somewhat less than conventional fluorocarbon rubber (FKM) on a weight basis, also since the density (using g/cm', which approximates to specific gravity) of HNBR is about half that of FKM, more products can be made for the same weight purchased.

Reference: An Introduction to Rubber Technology by Andrew Ciesielski 1999

Fluorocarbon rubber FKM (FPM)

In the United States fluorocarbon rubber is well known by its trade name of Viton (Based on vinylidene fluoride and hexafluoro-propylene the grades available differ in the chemical building blocks which were used to construct the polymer. Like silicone rubber, FKM has excellent high temperature resistance with an upper continuous heat aging temperature limit of 205°C. DuPont literature quotes continuous dry heat service to be >3,000 hours at 232°C decreasing to >48 hours at 316 0C. At the opposite end of the scale Nagdi points out that conventional FKM is usually serviceable at temperatures down to -20°C in dynamic applications, while for static use the temperature can be lower, although this will depend on the grade chosen. A primary variable in FKM grades is the level of fluorine in the elastomer molecule, FKMs being fluorohydrocarbons. Terpolymers tend to have higher fluorine content than copolymers and therefore have better resistance to various media. In general, fluoroelastomers have excellent resistance to oxidation, ozone, fuels and petroleum oils and are resistant to most mineral acids at high concentrations. Although FKM has good resistance to many chemicals, excessive swelling occurs in some polar solvents such as low molecular weight ethers, esters and ketones. Chemicals such as alkalis and amines should be used with caution, with standard fluorocarbon grades, especially at higher temperatures because alkalis harden the general purpose FKM, which will eventually embitter and then crack. FKM has a tendency to self extinguish when a flame is removed. This is of benefit in situations where the results of a fire would be catastrophic, for example in a coal mine. Other elastomers might burn out of control, when the source of the originating flame (such as methane gas explosion) is removed. Applications for FKM include automotive fuel hose liners and seals and flue duct expansion joints, where high temperatures and acidic products from gas desulfurization are involved. The relative cost of FKM is high, also a high specific gravity (around 1.8) means less cured product (volume) per unit weight A recent addition to the FKM family is an 'alloy' of a polar ethylene copolymer with a fluoroelastomer which optimizes cost, oil and heat resistance.

Reference: An Introduction to Rubber Technology by Andrew Ciesielski 1999