Where are engineering thermoplastics blends going?
This month, polymer expert and technical blog author, Dr Charlie Geddes, wonders where industry demand for increasingly versatile applications might take polymer blends.
The timeline of appearance of materials for the plastics industry can be viewed as several overlapping phases. The industry started with natural materials such as horn and bone, which can be thermoformed. Next came chemical modification of natural polymers, producing cellulose nitrate and cellulose acetate from cotton and wood fibre. The 20th century saw the appearance of wholly synthetic polymers (PVC, polystyrene, polyethylene etc). The era of seeking monomers to convert into useful polymers was followed by a period of creating copolymers to extend the choice of thermoplastics and the range of properties. This quest for new polymers and copolymers became curtailed because of the massive capital investment required.
The less expensive route of blending two existing polymers has become an attractive method of widening the properties profiles to satisfy diverse applications. Some polymers mix well with each other (miscible or compatible blends) but most pairs result in blends that literally fall apart or delaminate. The addition of a few percent of special polymers (compatibilisers) can encourage the two engineering thermoplastics to stick together and generate some interesting blends, which bring out the best properties of the two components in a synergistic way.
In PC/ABS, the most common blend, the polycarbonate adds fire performance to the properties of ABS and, in return, ABS contributes processability. Acrylonitrile-styrene-acrylic (ASA) and polymethylmethacylate (PMMA) improve weathering resistance in blends with polycarbonate or polyesters, while polycarbonate adds dimensional stability and flame resistance in blends with polyesters. Some blends, such as polypropylene/polyamide, give good mould detail for matt, scratch resistant finishes, obviating the cost of painting, while acrylics in blends with polycarbonate can achieve high quality, gloss, piano black finishes.
What will be the next development in thermoplastic blends in providing designers with broader palettes in matching thermoplastics with design requirements?