Acrylic rubber

Acrylic rubber is a copolymer or terpolymer of ethyl acrylate and other acrylates, with a small amount of a vulcanisation-supporting monomer. It is produced by radically initiated emulsion polymerisation or by suspension polymerisation. Curing takes place by means of diamines, fatty acid soaps and peroxides, for example.

ACM is characterised by high oxygen and ozone resistance, thermal stability and good heat and chemical resistance, but it has poor hydrolysis resistance and relatively high water absorption.

The swelling resistance in the presence of mineral oils is better than that of ethylene acrylic rubber (AEM/Vamac®).

Acrylic rubber is mostly used in the automotive industry because it is resistant to engine and transmission oil and to automatic transmission fluids (ATF). The operating temperature range is between -20°C and +150°C, while the latest generation, HT-ACM (high-temperature-resistant polyacrylate elastomers), offers long-term thermal stability up to 175°C. ACM is used to manufacture components such as engine and transmission seals, turbocharger hoses, O-rings, diaphragms and hose applications. It is also used for roller coverings.

Production of acrylic rubber

Polyacrylate elastomers are based on various monomers, such as ethyl acrylate (EA), butyl acrylate (BA), methoxyethyl acrylate (MEA) and ethoxyethyl acrylate (EEA). These monomers are coupled with active crosslinking comonomers, typically 2-chlorovinyl ether, vinyl chloroacetate, allyl glycidyl ether and acrylic acid. The properties of ACM rubbers are correspondingly influenced by the structure of these monomers and comonomers. ACM rubbers are usually produced by radical emulsion polymerisation, the molar mass being adjusted by the amount of polymerisation initiator added.

Properties of acrylic rubber

The properties of acrylic rubber are influenced by the chemical structure of the ester groups and the constitution of the comonomers. For instance, the glass transition temperature drops markedly as the chain length of the ester group increases, while resistance to oils rises correspondingly. Acrylic rubber elastomers offer superb heat and hot oil resistance. In terms of hot oil resistance, acrylic rubber vulcanisates are far superior to those of NBR. ACM is resistant to engine oils with modern additives, transmission oils, ATF fluids and lubricating greases. It has good oxidation, ageing and ozone resistance. However, its low-temperature properties are not always adequate. The same is true of its strength, elasticity, water absorption and hydrolysis resistance.

Acrylic rubber is not resistant to water, steam, fuels and aromatic solvents.

Key properties of ACM at a glance:

  • Very good weather, ozone and hot air resistance
  • Moderate strength
  • Low elasticity
  • Thermal stability from -30 to +150°C, up to +160°C for short periods
  • Relatively poor low-temperature performance
  • Good resistance to mineral oils
  • Moderate resistance to fuels
  • Average rebound resilience

In comparison to most other rubber types, acrylic rubber is more resistant to swelling in animal, vegetable and mineral oils. This property reduces as the temperature rises. The dry-running performance of this material is poor. In addition, it is not suitable for use in water or at very low temperatures.

As ACM rubbers are amorphous, they require active fillers, such as moderate activity carbon blacks, to establish adequate mechanical properties. Depending on the comonomers used, diamines, soap/sulfur systems or triamines are used to cure ACM rubbers.

HT-ACM – special acrylic rubber for elevated temperatures

For tougher thermal stability requirements, HT-ACM grades have now been developed which cover a temperature range from -40 to +185°C, and even up to +200°C for short periods.

ACM rubbers can be processed by compression moulding (CM), transfer moulding (TM) or injection moulding (IM) methods. In addition, ACM mixes can also be vulcanised on continuous vulcanising lines. Over 90 percent of ACM products are used in the automotive industry. Acrylic rubber is a useful alternative, especially for high-volume parts. ACM is used in particular for radial shaft seal rings, housing covers, O-rings, oil sump gaskets and engine and transmission seals.

Rolf Müller
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