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What can imide links do that amide cannot?

Polymers with amide links have been around for some time but the close cousins, the imide links, are less well known. 

The original polyimides, first synthesised over 60 years ago, offered some amazing high-temperature performance up to 300oC but could be processed only by sintering, to produce film (‘Kapton’) and rod (‘Vespar’), which greatly limited their applications.

To produce a material that could be extruded or injection moulded, links with more ‘molecular flexibility’ had to be introduced. This was in order to reduce the melt processing temperature to manageable levels.  The most successful member of the imide family is polyetherimide (PEI). It incorporates ether links, which has high stiffness and strength and a heat distortion temperature in excess of 200oC, even without reinforcing filler.  The bonus is that PEI is amorphous and hence transparent.  Few thermoplastics can match the combination of transparency and high-temperature performance.

The other contributions from the imide links are hydrocarbon resistance, good fire performance, interesting electrical properties, weathering resistance, stress crack resistance and low mould shrinkage. These lead to diverse applications in the automotive, electronics and medical markets.  Perhaps not as high tech as some applications but the tough, lightweight spectacle frames appeal to me.

Although resistant to hydrocarbon solvents, alcohol and water, PEI is attacked by alkali and chlorinated solvents.  PEI also requires high processing temperatures (over 350oC), high mould temperatures and predrying.

This article was written by Dr.Charlie Geddes for Hardie Polymers.

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