Fraunhofer IWM, MicroTribology Centrum µTC, together with the Freiburg Materials Research Centre and the polyolefin manufacturer LyondellBasell, have now produced and qualified a sustainable All-Polyethylene composite.
Polyethylene would be an ideal material for lightweight construction, it’s energy-efficient, can be made from renewable raw materials and is almost residue-free recyclable, unfortunately, only Polyethylene components that are reinforced as composites for example with carbon or glass fibres are truly mechanically resilient.
Both Polyethylene and Polypropylene account for over half of all polymers produced worldwide. Polyethylene is found in many of the plastic products used every day and as a pure grade material, it’s infinitely reusable with the used products melted down and formed into new components with consistently good quality. Polyethylene is heated and converted back into raw materials that go back into the chemical industry or into building blocks for the production of hydrocarbon materials completely without residue. For this reason and because of their low weight, hydrocarbon materials are ideal for sustainable lightweight construction.
Up to this point, however, it hasn’t been possible to manufacture load-bearing components from regular Polyethylene because on its own it is not a strong enough material. Traditionally fillers like carbon or glass fibres have been used for reinforcement.
The addition of fillers does have a negative impact on the cost and the energy, raw material, environmental balance: production and recycling are considerably more difficult and expensive. So-called ultra-high molecular weight Polyethylene or UHMWPE for short, used as a high-performance material in medical implants such as acetabular cups or knee joints, offers an alternative. However, this pure, high-strength and abrasion-resistant material cannot be processed by injection moulding: It has to be pressed into a mould as a powder, sintered and then milled into the exact component in a complex and cost-intensive process. Although UHMWPE fibres can achieve the strength of steel, they are expensive and unsuitable for material recycling.
In the SusCOMP project, we carried out research on All-PE single component composites that can be processed by injection moulding and directly reinforce themselves. Of course, we were particularly interested in the mechanical properties of these composites. DSM already spins high-performance fibres from long UHMWPE molecular chains that orient themselves along the fibre direction, so-called “Dyneema” fibres. It would be technically possible to incorporate such fibres into PE as reinforcements, but this would involve a great deal of work and expense and would not be suitable for material recycling Raimund Jaeger, leader of the Polymer Tribology and Biomedical Materials group at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg
Prof. Dr Rolf Mülhaupt and his team at the Freiburg Materials Research Centre at the University of Freiburg found the solution to this challenge by finely distributing different catalysts, which can be used to produce PE in different chain lengths, along with the same catalyst carrier. In the subsequent synthesis of PE using ethylene polymerization, mixtures of low, medium and ultra-high molecular weight PE, known as reactor blends, are simultaneously produced on this catalyst.
With this trick, PE blends are produced directly during polymerisation that can be injection moulded without any problems, explains Prof. Dr Mülhaupt
The process avoids high viscosities, which are normally a challenge when a high proportion of UHMWPE molecular chains are to be processed in injection moulding. High shear currents, which occur during injection moulding in narrow injection moulds, cause fibre-like UHMWPE structures to form from the ultra-high molecular weight fraction via self-organization of the material. These fibres reinforce the composite and even orient themselves in the desired direction during injection moulding, thus ensuring mechanical stability. These components are also easy to recycle.
The scientists at the Fraunhofer IWM tested samples of this new high-performance material for their material properties. The mechanical properties show: many applications are conceivable, for example, long furniture parts as well as rail and shutter guides or parts for car interiors. In addition to their low weight, the components also have the advantage that water-based lubricants are very well tolerated.
The follow-up project, 3D-SusCOMP, now involves processing the material using a 3D printer. Previously, the good properties of All-PE composites could only be achieved if the polymers were oriented when injecting them into a narrow mould. However, the reinforcement by self-organization exclusively occurs in the direction specified by the moulding tool. This is already a major step forward, but other component shapes and composite materials, so-called multidirectional composites, are also desirable. The scientists found out: the fibre structures also form in the nozzle of a 3D printer. In contrast to injection moulding, however, their orientation in the component can be controlled by the movement of the print head. As a result, many new applications for this recyclable material are conceivable: in addition to lightweight gear wheels in automobiles or for the food industry, it is also possible to produce robot grippers which adapt to the shape of a part, medical orthotics or connectors from a “single mould”.