AACHEN, GERMANY (May 21, 12:45 p.m. ET) — Laser penetration welding is an established means of bonding laser-transparent and laser-absorbing versions of the same polymer.
Researchers at the IKV plastics processing institute at RWTH university in Aachen have now developed laser welding for dissimilar polymers. Details were revealed in a paper presented by research specialist Mathias Weber at the IKV Colloquium in March.
A challenge in developing two component laser welding is that the method leads to discoloration due to the presence of black laser absorbing pigments, such as carbon black, in at least one joined material.
Welding requires entanglement (diffusion) of molecular chains between polymers. This takes place under pressure applied externally and by expansion of the polymers as they melt.
A suitable two component technique is the use of a laser-absorbing film between laser-transparent polymers. This achieves 70-80 percent of conventional laser-welded bond strength, says IKV.
The IKV researchers investigated different film solutions. These included a three-layer, 50-150µm PP/PA6 film in transparent and partially carbon black pigmented versions, each with a 10µm maleic acid anhydride (MAH) grafted PP adhesion promoter core layer, and a monolayer MAH/PP or MAH/PA6 adhesion promotion film. The Borealis HD120MO grade of PP was used, while the MAH was Mitsui Chemicals' Admer QF551E.
With multilayer films produced by blown film extrusion at RKW, the MAH/PP adhesion promoter core layer binds to the 30-110µm PP layer physically, but chemically to the 10-30µm PA6 layer at 200ûC during film coextrusion. In the case of monolayer cast film produced at IKV, MAH becomes activated during welding.
In IKV's tests on transparent films used with laser-transparent/absorbing moldings, 100µm multilayer film gave higher bond strength (450Nm) than monolayer film (around 400Nm). But the results were lower and reversed in 150µm film (160Nm and 300Nm, respectively). In tests on pigmented multilayer film between laser-transparent plastics, 500Nm was achieved for 100µm film, and close to 500Nm for 50µm film, but only around 320Nm was achieved for 150µm film.
IKV also investigated the use of plasma pre-treatment. Variations of the technique involved using non-layer-forming oxygen-, nitrogen- and argon-based plasma pre-treatment to form functional groups in dissimilar polymer surfaces and to roughen surfaces. Oxygen was the most effective.
The researchers compared laser-welded bond strength of plasma-treated dissimilar polymers with results they obtained using 50µm intermediate multilayer film bonding. PA6 used as an absorbing partner did not exceed 60 percent of the best value obtained with multilayer film bonding, and PP as absorbing partner reached 73 percent of the best film-bonded strength. IKV is now investigating adhesion mechanisms.
Plasma pre-treatment allows wider laser welding process windows and higher maximum welding speeds in carbon black pigmented plastic of up to 150 mm/s with PA6 and 180 mm/s with PP, although there is a trade-off between speed and weld strength.
Film solutions showed highest bond strength with low laser power applied at low speed, since the film and dissimilar polymer surfaces have to melt. IKV is now looking at interfaces at which bonds fail.
IKV has focused on replacing laser welded PA6 with less expensive PA/PP or PA/PE combinations, for example in welding PA12 supports to automotive media ducts that often have an outer PE layer. But the institute has also bonded other semi-crystalline thermoplastics, PA12/PP and PA12/PBT, using BASF's Ultradur B3K grade of PBT.
The use of two component welding enables parts in dissimilar polymers can eliminate other joining techniques such as gluing, snap fitting, riveting or screwing.
Although Weber did not present details at the colloquium, IKV has also looked at laser penetration welding with laser-absorbing layers applied to one of the dissimilar polymers by two-component injection moulding.
IKV uses LDM 940nm wavelength lasers from Laserline in Mülheim-Kärlich for the two-component welding process.
Another part of RWTH university is working with the Fraunhofer ILT laser technology institute within the EU Polybright project, aimed at evaluating absorption behaviour at 1,500-1,900nm of different polymers for additive-free welding applications. IPG Laser, based in Burbach, Germany, announced at the end of March it has developed 1,567nm and 1,940nm lasers for Polybright.