Bonding mechanism during welding of radiation cross-linked thermoplastic polymers

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If plastic components cannot be manufactured in a single fabrication step because of their com-plexity or if this should not be an objective (for example) for economic reasons, it becomes neces-sary to utilize joining technologies. Welding yields the advantages of a material-locking and media-tight joint which permits a high load transmission without using any welding filler materials. The heat needed for the complete melting of the plastic can be input into the joining zone in different ways. For example, this is carried out using convection, radiation, heat conduction or friction. Vibration welding constitutes one process which makes use of the principle of heating by means of external friction.The entanglement of molecules at the interface by interdiffusion is recognized as the main cause for the high strength of weld joints. The forces transmitted by secondary valence bonds are too low to explain the bonding mechanisms in weld joints. The formation of new valence bonds across the interface is unknown for the welding of plastics. Accordingly, since as a bonding mechanism during welding of plastics merely an interdiffusion comes into question, most weld joints are made of plas-tics from the same type. In terms of a smart material mix in products joints made of different com-ponents, with locally to the specific requirements adapted plastics, are also of particular interest. Such joints, e.g. made of high-performance engineering thermoplastics and low cost polyolefins, are currently not or only limited fabricable in the vibration welding process.For plastics in different applications, increasingly higher requirements are placed on their properties, which often cannot be implemented directly in the production or by fillers and additives. One potential possibility of influencing the properties of thermoplastics in a targeted way is provided by the cross linking of the materials with high energy irradiation. In own studies on vibration welding of radiation cross linkable thermoplastics it could be shown, that the temperature influence of the welding process can possibly lead to a thermally induced cross linking across the interface in the weld area. Thus an additional bonding mechanism would be available during welding of plastics.The aim of the project is to work out fundamental knowledge about material and process related influences on the bonding mechanisms in welding of radiation cross linked thermoplastics. The focus is on the thermally induced cross linking in the weld area during the joining process. Based on the understanding of the fundamental mechanisms of thermally induced cross linking, joining strategies are to be derived in order to promote cross linking as an additional bonding mechanism in a targeted way. This gives the possibility to produce mixed material joints, which are currently not or only limited fabricable in a common welding process.