One of the exciting attributes of polymers is that the properties of any polymer can be modified by tinkering with the basic molecular structure or incorporating a range of additives.  However, you have to be aware that altering one property may have a knock-on effect on other properties.

I recall one example where the designer was trying to push wall thicknesses down to 0.7 mm.  (This was in the days when 1 mm was considered daring).  As you can imagine mould filling proved a headache with the chosen material, polycarbonate.  The material supplier offered a low molecular weight grade, with lower melt viscosity and better mould filling.  Unfortunately, this grade had a slightly reduced impact resistance and did not pass the stringent impact test for the product.

To solve this problem? The material supplier added an impact modifier (a rubbery polymer) to restore impact strength, without affecting mould filling.  Everything in the garden looked rosy.

Unfortunately, they did not anticipate the moulder edging up the melt temperature beyond the recommended limit, to squeeze fractions from the cycle time and improve his margins.  He had applied this tactic with standard polycarbonate grades without any negative outcome. However, the impact modifier did not enjoy the dwell in the hot runner system at the higher temperature and incurred incipient degradation.  Although the mouldings looked no different cosmetically and met the impact standards, trouble lay ahead during service with embrittlement and stress cracking.

The moral of the story is to check that modifying one property does not lead to unwelcome consequences in another property.

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

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