TWI Industrial Report Summary 917/2009
By U Ana and I A Jones
Background
Textiles are welded for a variety of reasons. There may be a need to provide a sealed seam such as in a waterproof garment, for an automotive airbag or for furniture used in hospitals. Welding might be introduced as a means of improving the degree of automation in manufacture of textile products. Laser welding of textiles using the transmission laser welding method with an absorber layer at the joint interface was first developed by TWI in 1998 (Jones and Wise, 1998), and provided a way of making a sealed seam without melting the outer surface of the fabric or using an adhesive film at the joint.
The aim of this work was to evaluate the performance of liquid absorbers with added polymer content, with the objective of increasing their viscosity. The absorber should sit more readily on the surface of the fabric and yet still be sprayable and easily applied along complex seam lines. Additionally, it should be possible to provide additional polymer suited to the fabric types being joined, with a controlled melting point and of the right volume to provide a controllable balance between melted fibres and polymer infiltration of unmelted fibres. The ultimate aim is improved performance and consistency.
Objectives
- Assess the effect of using polymer additives or a viscosity enhancing agent in the absorber coating used in laser welding of fabrics.
- Define processing conditions for delivering and welding with the coatings with the aim of improving the seam strength in different fabric types.
The higher viscosity coating remained on the surface of the fabric more readily than the low viscosity coating. However, this did not have a significant effect on the location or amount of melting or on the flow of melt into the fabrics.
The weld strength achieved in the woven nylon was similar to the results achieved in earlier work. The failure location was generally down the edge of the melted region, probably as a result of the large melt depth seen in these trials.
The second set of tests demonstrated a significant improvement in the weld strength, compared with previous experiments, and a change in failure mode from weld edge to weld interface failure. There were changes to the processing conditions that had less absorber coating applied and generally slower welding conditions, but inspection of the welds offered a possible explanation for the results.
The sections showed partial melting of the fabric with different amounts of melting in the warp and weft fibres. Generally the warp fibres were melted less than the weft fibres. The result is a weld that appears similar to a composite of nylon reinforcement fibres surrounded by a nylon matrix. As with the first set of trials the use of viscosity enhancer had no detrimental effect as long as the welding parameters were varied to re-optimise the weld strength.
No other polymer additives were found to improve the weld performance compared to the use of coating containing only dye and solvent. However, conditions were not tested in which a low melting point polymer additive melted preferentially and infiltrated the surrounding fibres.
All the strongest welds made in the polyester laminate failed by delamination of the polyurethane from the polyester fabric combined with rupture of the weld edge or laminated film.
No absorber coatings with polymer additives gave improvements over the coating containing just dye and solvent.