Effect of increasing ethylene content in EPM rubber

In Buna® EP, distribution of the comonomer propylene can vary from random to alternating. If the ethylene and propylene contents are approximately equal, both monomers within the polymer molecule are evenly distributed, meaning the rubber is amorphous.

If the ethylene content is over roughly 65 weight %, ethylene sequences form in increasing number and length. These sequences are able to form crystallites. Propylene disrupts long ethylene sequences and prevents them from crystallizing:

• At low propylene levels (below 35 weight percent), a small amount of crystallinity is present which provides the EPM with good green strength.
• At high propylene levels (above 50 weight percent), only short ethylene sequences exist in the polymer chain so there is no crystallinity.

In the vulcanizate, a higher crystallinity of the polymer results in improved tensile strength and increased hardness, but also with a higher compression set at low temperatures.

Other effects of increasing ethylene content include:

• improved cold green strength
• good extrudability
• high filler and plasticizer loading capacity.

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