Cross links cushion impacts

Tom Shelley reports on a novel material developed to protect sportsman from impacts but which also shows great potential for engineering A new polymer based material undergoes an almost complete transformation from a viscous state to an elastic state when subjected to impact. Incorporated into sports clothing, it is at the same time comfortable to wear while protecting against knocks. It also shows promise as a material for automotive suspension bushes and for sound insulation. The material is the invention of Dr Phil Green at the University of Hertfordshire in Hatfield, now technical director of d3o, the company set up in order to exploit it. Its Dr. Green explained, "I was sitting off the piste in the Alps in 1999 nursing a bruised elbow and it just occurred to me that all the systems around that are used for impact protection work on a macro scale. If I could get a system to work at a molecular level I knew we could have an amazing product. "The material evolved from several from several conceptual stages for a reactive system for impact protection which I worked on with the company founder and managing director Richard Palmer. As a material scientist I looked at the systems we had created for initial prototypes and embarked on replicating these types of systems on a micro scale. Many hours in lab and two years later we now have 'smart material' which does this." Dr Green describes d3o as, "An elastic cellular polymer containing transient dynamic cross links. The end product contains no liquid phase and is therefore very easy to process. The reactive inorganic constituent can be likened to a traditional dispersion dilatant, such as a corn starch suspension. However the polymer undergoes a near complete transformation from a viscous to an elastic state. The final product uses a low modulus elastic constituent to inhibit viscous flow of the reactive polymer but the presence of the dynamic transient cross links create a material which has an elastic modulus which increases according to the rate of deformation. This behaviour results from free radicals which form a temporary chemical bond during high rates of strain. "The material can be moulded into virtually any shape, which has enabled us to optimise the geometry for maximum conformity, flexibility and breathability for integration into garments. It can be either fabric backed for easy integration into clothing or moulded into discrete components. We currently supply a polo company, Casablanca, with moulded inserts for their polo knee protectors. "We have a number of automotive applications we want to explore, such as suspension bushes, pillar liners and sound insulation. Although the cost is not prohibitive and will come down with volume production it is a premium material that is high value add and not a commodity sheet material, although we are looking at continuous sheet production for the near future. The raw materials are not commercially available and have been formulated to our specification. "We have prototyping facilities based at the University of Hertfordshire that we are using to develop products with customers and we are developing relationships with manufacturing partners to provide significant quantities within the next four months." d3o web site Pointers * d3o material is able to flex at low strain rates, but becomes an elastic solid at high strain rates, and can thus protect against impact * It contains no liquid phase * The change in properties is due to dynamic cross linking which forms links at high rates of deformation which break when the material relaxes