Plastic recyclates – a new sustainable material

sustainability, plastic recyclates, recycling

The recycling of valuable materials and the responsible use of raw materials is part of our social responsibility. Plastics must be kept in circulation for as long as possible so that an old product can be made into a new one again and again. Researchers at Fraunhofer LBF have carried out an extensive material characterization of the long-term mechanical properties under static and cyclic strain. They can now make “a new sustainable material” available in the form of recyclates.

Plastics are omnipresent in our day-to-day lives. They fulfill a wide variety of needs, being used, for example, in food packaging, multimedia items or highly-stressed structural components in everyday technical products, as well as in mobility. Using injection molding, plastics can be processed extremely economically and with a high degree of reproducibility.

In addition to primary energy, which is bound into the plastic in the form of crude oil, the production of plastics also requires secondary energy for various conversion processes, namely auxiliary energy in the form of energy, heat, steam or coolant. As a result, depending on the type of plastics, a total of two to six kilograms of crude oil is required for each kilogram of plastic granulate. Consequently, these resources and raw materials must be treated carefully and in a sustainable manner. Due to their molecular structure, thermoplastics can be reversibly melted and can therefore be recycled. However, reversible melting cannot be carried out as often as required, due to the fact the mechanical properties of plastics change from those of the starting material as a result of repeated melting, shearing of the molecular chains and mixing of different plastic material streams of the same type. In addition, environmental influences during the service life change the properties, as additives and stabilizers are degraded or oxidation processes occur.

In order for recyclates to be used in new technical products, manufacturers and producers are faced with the following question: Can the new recyclate material meet the technical requirements?

In order to answer this question, researchers at Fraunhofer LBF have carried out an extensive material characterization of the long-term mechanical properties under static and cyclic strain. Here, benchmarking was carried out between the virgin material currently used and a recycled material.  Both materials consisted of olypropylene with the same molecular structure and talc reinforcement.

The yield stress, modulus of elasticity and fatigue strength properties of recycled material are up to 10% lower than the virgin material. The characteristic values for elongation at yield are up to 35% lower. The reduced properties are not solely due to the previous life of the component and multiple processing, but are also caused by material mixes from different waste streams consisting of short-chain and long-chain polypropylene. When subjected to 1,200 hours of temperature aging, virgin and recycled material show no reduction in mechanical properties. The recyclate shows a stronger post-crystallization due to aging because of the shorter molecular chains, so that the mechanical properties are slightly increased by temperature aging and become similar to virgin material.

In summary, recycled materials exhibit slightly reduced mechanical properties compared to virgin materials. These are not only caused by the previous life of the component and repeated processing, but also by the molecular structure of the starting materials from the respective waste material stream. Taking these properties into account, recyclates can be used in numerous applications.

Looking ahead, the extensive material characterization will be followed by the possibility of substituting virgin material with the recyclate in a large household appliance. With an annual volume of 5,000 tons of recyclate, this would make an important contribution to sustainable highly-stressed structural plastic components. In the future, recyclates will be available to users as “the new sustainable material”.

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Area Of Expertise Circular Economy