Esprit >> The Project

In Self-Reinforced Plastics (SRP), a polymer matrix is reinforced with high-tenacity fibres or tapes of the same polymer family, such as polypropylene-reinforced polypropylene, creating a material with typically 3-5 times the strength and stiffness of the unreinforced polymer, giving the ability to use less material for the same mechanical properties in a component. Additionally, unlike glass or carbon reinforced plastics, self-reinforced plastics are not contaminated with high levels of mineral fibres, so they have the same level of recyclability and density as the base polymer

However, current forms of self-reinforced plastics, especially in commodity and low-cost polymers, are only available in sheet or fabric form. This severely restricts the range and types of components that can be manufactured as the parts must have constant wall thickness and typically 30% of excess waste material must be trimmed away after forming.

The concept for this project is therefore to develop flowing versions of self-reinforced plastics from commodity polymers – polyolefins, polyamides and polyesters – by the development of techniques that will allow the selective melting of the polymer matrix (heating the matrix and not the reinforcement fibre), by materials developments and by process optimisation.

Firstly the matrix and reinforcement is combined. There are a number of ways to do this and for Esprit, the development has been in melt impregnation, commingling and powder impregnation. Melt impregnation uses a reinforcement yarn which is passed through a heated die into which a melted matrix polymer is injected, coating the yarn and resulting in a rod which can be chopped into pellets (for subsequent injection moulding). Commingling uses yarns for both matrix and reinforcement and combines them in an air jet, making a yarn with a controlled ratio of each. This yarn is then passed through a fie which forms the rods for subsequent chopping into pellets. Finally, powder impregnation has been successfully adapted to make chopped and continuous fibre pre-consolidated sheets (for subsequent compression moulding). It is possible to include the selective heating susceptors onto all of these techniques, creating a very diverse set of intermediate materials.

The technique of selective heating has been developed by Esprit partners. This innovation involves compounding additives into the matrix part of the composite which are susceptible to external energy sources, in this case electromagnetic, which allows the selective heating of just the matrix, with a reduced effect on the reinforcement element of the composite. This heating technique allows for quick, efficient heating of the polyer before moulding processes are used.

The research has shown which additives are most successful and also that homogenous distribution of the additives is critical. This distribution has been improved dramatically, even in difficult materials such as polypropylene. The result is that more even heating can be achieved, with lower percentages of additives and with less energy, using induction and microwave energy sources.

In its final stages, the project will be processing the materials in volume, to manufacture case study parts in a variety of techniques. The most challenging is injection moulding, however, through material developments and careful analysis of the processing system it is apparent that optimal successful injection (or LFT style) processing is possible with a package of material combinations, processing control and adapted machinery. Compression moulding is also being carried out on sheet materials and pellets. The sheet materials can be pre-consolidated, powder impregnated sheet or extruded sheets from SRP pellets.

In parallel to the moulding techniques, control systems are being improved to facilitate the use of these new materials. One of the possible processing routes for SRP composites is variothermic moulding technique where a tool is rapidly heated and cooled to form parts. The particular area of development in this technology for Esprit is a high-pressure, dual-circuit heating system for heating and cooling tools rapidly and with accurate control.

The characterisation of the SRP composites developed by the Esprit project is a key element to prove that these materials offer significant benefits. This graph shows the clear advantage SRP variants have in terms of tensile modulus (stiffness). The (self-reinforced) srPET variants are compared to the base (un-reinforced) matrix material. The additional stiffness allows less SRP composite material to be used, reduced energy is used in processing and less material is used in components which can result in long-term transport and in-use energy savings.