Modifying a thermoplastic polyurethane for improving the bonding performance in an adhesive technical process

The present work describes pretreatment processes of thermoplastic polyurethane elastomer (TPU) material, used as an adhesive in powder form, that results in the increased peeling resistance of joints. The TPU in powder form is an environmentally-friendly adhesive and provides safer bonding conditions in the shoe manufacturing industry. TPU particles are soft and require an anti-blocking agent to prevent agglomeration. However, particles covered by such an anti-blocking agent may account for a low peeling resistance in the resulting joints. To overcome this disadvantage, two pretreatment processes for the TPU adhesive were evaluated. The results of a two-step wet washing process, representing a conventional solvent-based procedure that involves both the application of a solvent and a powder separation step, are compared to the performance of a one-step dry plasma treatment process. The process type did not affect the size and agglomeration behavior of the particles, and both processes were effective in improving joint strength according to peeling test results. Spectroscopic investigation of the particle surface after washing or plasma treatment indicated some zinc concentration, which is interpreted to result from the anti-blocking agent. The crystallinity of the TPU, investigated by thermal analysis, was higher after pretreatment. The obtained results indicate that TPU in powder form is a versatile adhesive for the shoe industry with a safer application compared to solvent based adhesive systems.