Recycling of Scrap Tyres in Metal-plastic Composites
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KeywordsCore Material Deep Drawing Face Sheet Core Layer Blank Holder Force
Hybrid metal-plastic composites with classic sandwich design offer a great potential for lightweight constructions. Owing to the modification of the polymeric core layer with fine grinded waste tyres, also known as ground tyre rubber (GTR), the material costs could be reduced significantly and a major contribution for resource efficiency could be made.
The treated Hybrid metal-plastic composites (MPC) are layered materials, with metallic cover sheets and a polymeric core material. These kind of sandwich materials offer a great lightweight potential. Due to the large area adhesive bonding between the thin cover sheets and the core layer, these structures have an improved bending stiffness compared to the monolithic materials. By influencing the polymeric core and the cover sheets material as well as the layer thicknesses the properties of the MPCs could be modified in a wide range.
The investigations made were focussed on the optimal use of the positive characteristics of the single components for developing composites with a high a bending stiffness and a high strength. In the field of lightweight constructions, MPCs with relative thick core layers (core layer thickness to cover layer thickness ratio > 1) are mainly used. These structures combine a high bending stiffness with a slight mass per area. For enabling this technology in high volume applications, the material costs must be reduced significantly. The modification of the core material with recycled tyre rubber offers an equally cost-effective as well as environmental sustainable solution.
Adapted manufacturing processes and robust forming processes are a necessary condition for the use of this innovative core material. The manufacturing of a MPC with thermoplastic core, which is modified with cost effective recycled tyre rubber, could be realised for the first time in a collaborative project between the Institute for Lightweight Structures and Polymer Technology of the TU Chemnitz and the Fraunhofer Institute for Machine Tools and Forming Technology. The mechanical properties as well as the forming process were investigated for the novel hybrid composite material.
Manufacturing of MPC with Modified Core
The annual elastomer consumption of 30 million tonnes with a growth rate about 4.3 % per year constitutes an emerging ecological challenge. More than two thirds of these elastomers are used in the tyre manufacturing industry . Fine grinding offers an ecological as well as economical lucrative concept of recycling worn-out tyres. This process generates fine powders with particle sizes between 200 and 400 μm, which were already examined as additive for bitumen in the middle of the 19th century. The effects of these recycled elastomers on the mechanical properties of thermoplastic compounds had been examined in recent years. For the first time such a compound with polyamide 6 (PA6) has now been used as core material in a MPC.
The steel face sheets with a thickness of 0.27 mm and coated with Delo Saco-Plus blasting material are subsequently pressed together with the foils to the finished MPC. To achieve a complete consolidation in the MPC, the pressing tool is heated up until it reaches the melting point of PA6, which causes the polymer to fuse. Depositors are used at the edges of the tool to ensure the MPC’s correct thickness and to avoid leakage of the melted core.
After cooling down the pressing tool, the thermoplastic polymer congeals and the finished composite can be demoulded. As shown in Figure 1, a reliable production could only be obtained for 10 and 30 % of elastomer. Using cores with 50 % reduces the adhesive strength between the core and the face sheets, which results in extensive delamination when demoulding the MPC from the pressing tool.
Microsections are examined and surveyed microscopically to verify the achieved thickness of the MPC. Averaged, the deviation was less than 10 %. The modification of the polymer with fine-grained elastomer results not only in different mechanical properties, but also reduces the costs of the composite.
When comparing composites with the same bending stiffness, the ones with modified core materials offer cost savings of 18 % for 0.5 mm core thickness and 27 % for 1.1 mm core thickness. Especially the low price of only 0.20 Euro/kg for the GTR enables these significant cost savings. The production costs of the composite are equal compared to unmodified cores, although there was an additional process step used in this study, in the industrial application the modification could be integrated in the extrusion process.
Mechanical Properties of the Modified MPC
Forming the MPC is Possible at Room Temperature
For the further processing of the semi-finished composites to complex parts conventional processes of sheet metal forming are used. However the exceeding thinning of the polymeric core layer and the interface adhesion are critical. Especially an interface failure generates a debonding of the layers and thereby the loss of the macroscopic mechanical properties of the part.
In respect to the further applications the forming behaviour of the developed MPC has been investigated in simple deep drawing and bending experiments. The bending behaviour was investigated by folding a specimen, which is clamped at one side. For rating the bending behaviour the springback and the interface integrity were analysed. The interface integrity was analysed by metallographic cross sections. The know-how gained within the basic trials was transferred afterwards to complex demonstrator parts.
The investigated MPCs had an overall thickness of 1 mm. The tests showed, that bending with a bending radius of rB = 2 mm and 30 % GTR content is possible without any delamination. In contrast, wide delaminations are detected for 50 % GTR content and a bending radius of rB = 10 mm. This phenomenon is the result of the reduced adhesion properties with the increasing GTR content. Further the tendency to interface failure increases with raising the core layer thickness. As investigated in  the minimal bending radii for commercial available MPCs is twice to three times greater than the overall thickness of the composite. Hence the minimal bending radii for the MPC with modified core are in the same range as for commercial available MPCs suggested.
For investigating the deep drawing behaviour of the novel MPC, square cup tests and the drawing of a three-dimensional freeform geometry were realised. As reference, the experiments were also performed with the monolithic sheet material.
The experiments showed that the crack formation occurs for the MPC at the same drawing depths and blankholder forces as at the monolithic sheet material. Hence the failure tearing is mainly affected by the forming limit of the cover sheets. Due the low compression modulus of the polymeric core layer the wrinkling tendency is increased at MPCs compared to monolithic sheet material. It was further found that a higher GTR content decreases the compression modulus of the core layer and therefore a further increasing of the wrinkling tendency could be seen.
The experiments show, that the novel hybrid composite can be manufactured at room temperature with conventional sheet metal forming processes to complex parts.
Finite Element Simulations for Process Design
Today, forming operations like deep and stretch drawing are simulated in finite element simulations prior production. Therefore, the expenses for prototypes and rejects are reduced. For simulating the forming process of the novel hybrid composite FE models were developed. In the first step the material models for the single materials were parameterised and validated with experimental results. This was realised with tension tests, compression tests and three-point-bending tests.
For the first time such a compound with polyamide 6 (PA6) has now been used as core material in a MPC.
The analysis of the springback angel determined by the FE simulation shows an increasing deviation with increasing GRT content. This could be caused by the strong hyper-elastic material behaviour of the core material with increasing GTR content and in consequence of this the inaccurate determination of the onset of yielding for the polymer.
As conclusion, the forming behaviour of the novel MPC can be predicted in good accordance to the experiments with the developed FE models.
The presented results show that the manufacturing und the further processing by forming of MPCs with modified core layers is possible. Nowadays, MPC with a maximal GTR content of 30 % can be manufactured repeatable. By using a GTR content of 30 % the material cost of the MPC could be reduced by 18 %. The mechanical characterisation of the novel MPC show, that the bending stiffness is not affected in consequence of the core modification. However the interface strength between the core layer and the metallic cover layers decreases with increasing GTR contents. Therefore the manufacturing of MPCs with GTR content higher than 30 % is actually not possible.
The forming experiments made at Fraunhofer IWU demonstrate a good forming behaviour of the MPCs at room temperature. Complex three-dimensional parts could be produced by deep drawing without any interface failure. Furthermore, it could be proven that the FE modelling of the forming process is possible in good accordance to the experiments. In the next step, the failure ‘delamination’ must be considered in the forming simulation to achieve more accurate results and predict this type of failure. Further investigations should address inncreasing the GTR content by an adopted surface treatment. In this way, a further reduction of the material costs is possible and the damping behaviour can be improved as well.
The project was funded as a project of industrial research under grant no AiF 17895BG of the Research Association EFB e.V. financed and supervised by the German Federation of Industrial Research Associations (AiF) within the framework of the program for the promotion of industrial research and development (IGF) by the Federal Ministry for Economic Affairs and Energy (BMWi).
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