Research activities in the field of "Recycling of elastomers"
Contacts
Prof. Dr. Volker Herrmann
Motivation
Against the backdrop of finite raw material reserves, increasing CO2 pollution and rising mountains of waste, the topic of recycling is more important than ever before, especially with the current international increase in the number of cars (Fig. 1). The proportion of used tyres that are not reused for export, other uses or retreading (so-called "end-of-life tires") is around 3 million tons per year in Europe, and the trend is rising here too. Closed material cycles from post-consumer waste are an ideal goal that has already become a reality in a few areas of the plastics industry (dual system, window profiles, plastic pipes, drinks bottles, car bumpers etc...). While material recycling and reshaping are possible in principle with thermoplastics due to the possibility of melting, elastomers cannot be reshaped in this way due to the chemical cross-linking of the molecular chains. The material recycling of elastomers is therefore more difficult, but it is not impossible and is already widely practiced. Compared to the plastics industry, where thermal recycling (incineration) currently still predominates (Fig. 2), the percentage of recycling including retreading of used tyres in Germany is already quite respectable and continues to increase. In pure material recycling, rubber products are shredded into granulates and rubber powder and incorporated into rubber compounds as additives. This process is often referred to as "downcycling" and describes the fact that the resulting products are of a lower quality and are only suitable for use where no critical requirements are made (fall protection mats, flooring for sports fields, rubber floor coverings, anti-slip underlays, etc...).
Project
The effect of rubber powder particles in rubber compounds is analyzed in our own studies. The focus here is on the effects on the mechanical properties. The interaction of the rubber powder particles with the rubber matrix is to be investigated with the aim of gaining knowledge that can lead to an improvement in the properties of the end product, thus demonstrating a first step in the direction from "downcycling" to "upcycling". Thanks to the lump-sum research funding and financial support of the German Rubber Society (DKG), a sustainable and long-term commitment in this field, which will presumably become more topical as time goes on, is possible. With the help of final theses (diploma/bachelor's and master's theses), students support these activities through their work in the laboratories of the degree program and are thus confronted with this important topic at an early stage. Initial insights have already been gained through subsequent student work.
Student projects carried out
Hammer, R., "Investigations into the cross-linking density of elastomers with and without rubber powder", Bachelor's thesis, Würzburg-Schweinfurt University of Applied Sciences, Plastics and Elastomer Technology course, 2019
Prawitz, S., "Development of a method for comparative measurement of the crosslinking density of elastomers using the swelling method", Bachelor thesis, University of Applied Sciences Würzburg-Schweinfurt, Plastics and Elastomer Technology course, 2019
Neckermann, T., "Numerical simulation of sulfur diffusion in a rubber matrix", Master's thesis, Würzburg-Schweinfurt University of Applied Sciences, Plastics and Elastomer Technology program, 2019
Werner, D., "Investigations into the use of non-pure rubber powder in rubber compounds: SBR flour in an NR matrix", Bachelor thesis, Würzburg-Schweinfurt University of Applied Sciences, Plastics and Elastomer Technology degree program, 2020
Maslowski, Th., "Investigations into the use of non-pure rubber flour in rubber compounds: NR flour in an SBR matrix", Bachelor's thesis, Würzburg-Schweinfurt University of Applied Sciences, Plastics and Elastomer Technology degree program, 2020
Viering, A., Ruhl, M., Schülein, T., Dilje, E., Schidt, A., "Feasibility study on the recycling of car tyres by means of recycling rubber powder", cooperation project, Würzburg-Schweinfurt University of Applied Sciences, Plastics and Elastomer Technology course, 2020
First results
The first results were already presented at the International Rubber Conference irc2015 in Nuremberg. Using rubber compounds to which truck rubber tyre powders from old tryes (Fig. 3) were added, significant losses in properties (lower hardness, lower tensile strength, higher abrasion values) were verified. The reason for this lower product quality appears to lie in the interaction of the powder particles with the surrounding rubber matrix and is currently the focus of further investigations. As could be shown by means of dynamic indentation and local X-ray fluorescence, the addition of rubber powder apparently depletes the rubber matrix of crosslinking agent sulphur, resulting in lower hardness of the recycled compound (Fig. 4). This finding also explains the loss of properties mentioned above. It has now been verified that the cause of this effect is the diffusion of sulphur into the rubber powder particles. With the help of FEM, the diffusion can be simulated and possible weak points can be identified. In order to obtain further relevant results, new measurement methods and testing devices are being developed in some cases, such as the method of spatially resolved, dynamic indentation (Figure 5). An overview of the results to date is shown on the following poster.
