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Lightweight Design worldwide

, Volume 10, Issue 1, pp 24–29 | Cite as

Recycling of Scrap Tyres in Metal-plastic Composites

  • Matthias Riemer
  • Dirk Landgrebe
  • Mario Wührl
  • Lothar Kroll
Materials Metal-Plastic Composites
  • 287 Downloads

Keywords

Core Material Deep Drawing Face Sheet Core Layer Blank Holder Force 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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 [1]. 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.

For the production of the innovative MPC, the complete process chain is shown in Figure 1. The first step is the fabrication of the compound with PA6 and elastomer which then is processed into granulate. Thereby, four different mass fractions are handled. In the ensuing injection moulding the compounds are further processed to test specimens to determine the mechanical properties. In a parallel process foils for the MPC are extruded. This foil production was limited up to 50 % elastomer due to the low fraction of polymer matrix in the compound.
Figure 1

Process chain for the production of MPCs with GTR modified core materials (© IWU)

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

Hereafter, the modified core materials are characterised under uniaxial quasi static tension according to DIN EN ISO 527-1 [2]. Therefore, test specimens were produced by injection moulding, Figure 1, for all compounds. Generally, the addition of recycled elastomer results in an almost mass proportional decrease of the elastic modulus. Thus, it decreases 32 % in average with a filler content of 30 % GTR. Furthermore, in Figure 2 it can be observed that the tensile strength as well as the elongation at break are reduced with an increasing fraction of elastomer.
Figure 2

Results of the tension test according to DIN EN ISO 527-1 [2] (© IWU)

For the production of the MPC at a later stage, the free surface energy of the core materials can be evaluated, which leads to an assumption about the bonding properties. The results are shown in Figure 3 and display a significant decrease of the disperse as well as the polar fraction of the surface energy. The measuring fluids used in this test were water, ethylene glycol and diiodomethane.
Figure 3

Free surface energy of the produced compounds (© IWU)

If these changes of mechanical and physical properties are also detectable in the composite, will be investigated by a shear tension and four-point bending test on MPCs with two different core thicknesses. The results prove the assumption from the surface energy measurements, that the bonding properties are decreased with an increasing fraction of elastomer. Regarding the four point bending test, the composites with thicker core materials show a higher bending stiffness, which is explained by their higher section modulus, Figure 4. An influence of the decreased stiffness of the modified core materials on the bending properties is not detectable. Delamination between core and face sheets was not observed on any of the investigated composites in this test.
Figure 4

Bending stiffness of the two investigated core thicknesses (© IWU)

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 [3] 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 analysing the springback behaviour the springback angle was used. This is the difference between the measured angle after the bending process and the nominal process bending angle (here: 90°). In general, the experiments show an increasing springback angel with an increasing core layer thickness and with increasing GTR contents. A machine housing part serves as demonstrator, Figure 5. This part was manufactured on a conventional CNC controlled stamping and bending machine.
Figure 5

Machine housing as demonstrator part made of MPC with 30 % GTR content (© IWU)

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.

To avoid wrinkling much higher blank holder forces must be used compared to the monolithic metal sheets. The interface between the core layer and the cover sheets was investigated with metallographic cross sections. The results show that no delaminations could be detected. Figure 6 shows the complex three dimensional part as well as selected cross sections.
Figure 6

Complex three-dimensional deep drawing part made of MPC with 30 % GTR content (© IWU)

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.

In general, different approaches for the modelling of metal-plastic composites are possible. The different concepts were compared and the best compromise between computing time and accuracy shows the modelling of the core layer with solid elements and the face layers with shell elements. The failure in consequence of delamination was not considered in the presented model. The simulation results for the bending process were verified by the comparison of the force-displacement curves and the springback angels of the experimental results. The FE model as well as the results of the simulation are shown in Figure 7. The results show a good agreement between simulation and experiment.
Figure 7

FE model (left) and force-displacement diagram for the bending process (© IWU)

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.

Furthermore a FE model of the forming process for the complex three-dimensional part was developed and the results are shown in Figure 8. The thinning of the lower cover sheet is depicted on the left-hand side. The cross check were carried out with the metallographic cross section and a deviation less than 10 % could be determined. Areas with a high wrinkling tendency could be identified by the local thickening (blue area). In comparison to the experimental results, wrinkling could be seen in this area.
Figure 8

Simulation result for the complex three-dimensional part (links) and experimental part (right) (© IWU)

As conclusion, the forming behaviour of the novel MPC can be predicted in good accordance to the experiments with the developed FE models.

Final Remarks

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.

Notes

Thanks

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).

References

  1. [1]
    Ramarad, S.; Khalid, M; Ratnam, C.T.; Luqman Chuah, A.; Rashmi, W.:. Waste tire rubber in polymer blends: A review on the evolution, properties and future. In: Progress in Materials Science, 72(0), pp. 100–140, 2015CrossRefGoogle Scholar
  2. [2]
    Norm DIN EN ISO 527-1:2012, Kunststoffe — Bestimmung der Zugeigenschaften — Teil 1: Allgemeine Grundsätze, Deutsches Institut für NormungGoogle Scholar
  3. [3]
    Nutzmann, M.: Umformung von Mehrschichtverbundblech für Leichtbauteile im Fahrzeugbau. In: Umformtechnische Schriften, Band 138. Aachen: Shaker-Verlag, 2008Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Matthias Riemer
    • 1
  • Dirk Landgrebe
    • 1
  • Mario Wührl
    • 2
  • Lothar Kroll
    • 2
  1. 1.ChemnitzGermany
  2. 2.TU ChemnitzChemnitzGermany

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