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Microcellular Rubber
When rubber compound is added with blowing agent, fine cell structure is formed on the vulcanisate after curing, this rubber vulcanisate is called microcellular. The cell structure of microcellular is normally closed with gas trap within the cell. Since the blowing can be controlled the final products density varies, the bigger the cell size the lower the density and the softer the material. If the cell size is small the density is higher and the harder the product. The properties of microcellular rubber is normally inferior compared to solid vulcanisate, this is because the cell initiates failure. Some of the properties deteriorated are tensile properties, hardness, tear strength, abrasion and compression set. The most preferable properties of microcellular are low density, lightweight, high impact absorption and soft that assured comfort.

The application of microcellular rubber is very wide from toy to important engineering applications. Some of the applications are sound absorption, vibration isolation, insulation, padding, floor backing, wall lining, footwear components cushioning and many others. For applications where physical properties is critical and light weight is required then low density is normally used and for applications where some engineering requirements are needed then higher density is used. In footwear application abrasion is important, the microcellular used is normally medium density and of harder vulcanisates.

Since microcellular is a close cell it does not allow liquid to penetrate. These advantages have been explored to be used in floatation applications. Some of the applications are life jacket, buoyancy, fishing float and many other applications as float.
  Microcellular samples
Microcellular samples

Toy tyres
Toy tyres made of microcellular

Microcellular Rubber
Wall is padded with microcellular rubber to keep horses comfortable

Manufacturing Process

The formulation of microcellular is just like any solid rubber with an exception that the blowing agent is added. The blowing agents are inorganic and organic compounds that are stable at room temperature and decompose and liberate gas at suitable higher temperature. The blowing agents for rubber decompose either just before or during the cure. This gives the material a porous or cellular structure and a low specific gravity.

An idle blowing agent would be a product that has the following characteristic.
  • Disperse well in rubber

  • Has no effect on the compound's curing characteristic

  • Has a suitable decomposition temperature range, not too high and not too low

  • Liberates a large volume of gas per unit weight of the chemical

  • Decomposition products are not toxic and unpleasant smell

  • Causes no discolouration of the vulcanisate
There are basically two types of blowing agent, decompose at higher temperature and decompose at lower temperature. Azodicarbonamide groups are mainly decomposed at higher temperature.

Porofor ADC/R Decomposition temperature = 210°C
Porofor ADC/K Decomposition temperature = 165°C

Another group is sulphohydrazide this chemical decompose at lower temperature.

Porofor TSH Decomposition temperature + 105° C

Blowing agent decomposes at higher temperature are meant for rubber vulcanisate cured at higher temperature, such as EPDM and EVM. However ADC also suitable for NR for producing fine cell structure microcellular. The Porofor TSH is normally used in NR for producing large cell structure microcellular.

Apart from blowing agent other ingredients are also important to determine the final properties of the microcellular. The microcellular can be black and coloured vulcanisate.

Other Info
  Processing Flow Chart
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