“More seeds for greater speed”

© Photo | BASF

Ultradur LUX is a plastic that is particularly suitable for laser welding. BASF product developer Peter Eibeck explains why crystalline seeds are the key to success.

Peter Eibeck, we already have a number of polybutylene terephthalate (PBT) polymers, so why develop another one specifically for laser welding?

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Peter Eibeck, Product developer at BASF Photo | BASF

Basically because conventional PBT is not particularly suitable for laser welding, even though laser light is often used to join PBT materials. At the moment industry mostly uses this plastic in automotive electrical systems. It’s a good choice for those applications because it can be used across a wide range of temperatures, it’s a good insulator, and it absorbs very little water. Manufacturers of high-end electronic components often enclose them in a PBT housing, which is sealed with a lid. For that joining process you need a method that is fast, non-aggressive and economical, and laser welding checks all those boxes. The advantage over an adhesive bond is that you don’t have to wait for the glue to set. And manufacturers are also keen to avoid the kind of mechanical and thermal stresses on electronics that are caused by vibration, ultrasonic and hot gas welding. What’s more, you don’t need any additional elastomer seals with laser welding, unlike clip or screw methods.

So what’s the problem with using a laser to join PBT components?

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Unlike amorphous thermoplastics, semi-crystalline thermoplastics feature a spherulite microstructure that contains phases with different refractive indices. This widens the laser beam and increases the amount of backscatter. Photo | BASF

If you use a conventional PBT, the laser can’t reach its full potential because the semi-crystalline structure of the PBT causes significant scattering of the laser beam when it passes through the ‘transparent’ mating part. As a result, only a fraction of the radiated energy reaches the mating part that should be absorbing it, so you have less energy available to form the weld seam. That’s why we decided to develop a PBT that would minimize beam deflection while still having a semi-crystalline structure, which is one of the fundamental characteristics of PBT. How did you manage it? PBT contains what are known as spherulites, and we made those smaller by introducing additives. We ended up with very many small scattering centers instead of several large ones, and that means less deflection of the laser beam, i.e. greater laser transparency.

How exactly does it work?

When semi-crystalline polymers crystallize, they form crystalline lamellae that are just a few nanometers thick. Starting from a tiny seed, spherulites grow to several micrometers in size. Inside these spherulites the crystalline lamellae are arranged in a certain alignment or orientation, interspersed by amorphous areas.

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Small scattering centers reduce the degree to which a beam of light is deflected. Photo | BASF

The combination of the size of the spherulites and their internal structure determines the optical properties of the PBT. Conventional PBT forms large spherulites. The spherulite size you end up with in a plastic is determined during the cooling of the polymer melt. This is the stage in which crystalline seeds form spontaneously and evolve into spherulites, which continue growing until they encounter an adjacent spherulite. The greater the number of seeds that form, the less the individual spherulites can expand, and the smaller the scattering centers in the PBT. By introducing additives, we encourage more seeds to form and ultimately get a plastic with an extremely fine structure. That minimizes the scattering of the laser beam as it passes through the transparent mating part, and this considerably expands the process window for laser welding. One example of the benefits you get is the welding speed, which can be twice or even four times as fast as with conventional PBT materials.

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A laser-welded air vent control unit made from PBT. Photo | BASF

What general trend do you see in terms of using lasers to join plastics?

Lasers are becoming an increasingly important tool. That’s partly due to their inherent advantages, the fact that they offer a fast, non-contact method that leaves sensitive components near the joining zone untouched. But it’s also because there are more and more plastics available that are suitable for laser welding. Researchers are also putting a lot of effort into finding new applications for laser technology, for example joining heterogeneous mating parts such as metal and plastic, which has traditionally been the domain of adhesive bonding.

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