CRYSTAL CLEAR, New PBT with Strongly Improved Laser Transparency

Experts agree that polybutylene terephthalate (PBT) cannot, as a rule, be properly laser-welded. BASF researchers have recently disproved this with the new Ultradur® LUX. Through aclever modification of thepolyester's morphology, they succeeded in increasing its laser transparency from 30 to 60 percent. A PBT with such a high and constant laser transparency is unique and has not been available on the market until now.

Thanks to the improved laser transparency, markedly higher welding speeds are now possible, and the process window is considerably wider. Using the standard PBT with a 1064-nanometer laser, a welding speed of only 5-12 millimeters per second was possible; with the new Ultradur LUX, speeds of 10-70 millimeters per second can now be achieved.

The greater laser transparency also provides a variety of other benefits. For instance, it allows much thicker parts to be welded and applications that previously required other joining methods are now accessible. It is also possible to work with lower laser power, which extends the life of the laser. The laser-transparent Ultradur LUX not only opens up new possibilities, but also contributes greatly to the efficiency of the welding process.

Ultradur LUX is initially being offered in black or uncolored, with 20 or 30 percent glass fiber reinforcement. An expanded product line, including other colors, will be considered as needed.

LASER WELDING – CLEAN JOINING TECHNOLOGY

Compared to other joining techniques, laser welding offers the following advantages: no other materials – such as glue and primer – need to be stored, no particles are rubbed off (as with friction welding and ultrasonic welding) and there is no excessive vibration. Vibration-free joining is particularly important when delicate components are integrated into one of the two parts being joined. The benefits of laser welding are thus very important in the case of small parts requiring the use of flexible geometric design and a clean work method. This applies in particular to electronic and medical applications such as automobile ECU housings or assemblies with sensors.

The principle of laser welding is that laser beams pass through a lasertransparent joining partner and melt the underlying component, which absorbs the laser beams. The melted plastic transfers the heat to the laser-transmitting material, thereby creating a weld joint. Thus, a basic prerequisite for laser welding is the pairing of a laser-transmitting and a laser-absorbing material.

While there are special Ultradur grades in use today that exhibit the specified characteristics in regard to laser welding, these solutions still leave something to be desired; not only because of the intrinsically low laser transparency of the material, but also because of the difficult-to-control consistency of this property.

THE PROBLEM WITH THE TRANSPARENCY

Too low a laser transparency can lead to a prolonged cycle time, result in defective parts, or even make laser welding impossible. To a certain degree, this can be compensated for by increasing the welding time. However, a longer welding time increases the likelihood of the material burning or degrading.

When the average laser transparency is at a low level, fluctuations can have an especially negative impact to the point where the welding process can no longer be kept within an acceptable process window. Damage may also result. As laser welding is usually the last step in the production sequence, defective parts at this stage mean the total loss of the product's value. For components containing high-quality electronic modules, this can easily amount to 100 Euros per piece. The lost value then exceeds the material costs for the laser-transparent plastic by about a factor of 1000.

The goal was to develop, within the Ultradur line, a product with higher and more constant laser transparency. This task required the combined expertise of a large team of physicists, chemists, engineers and market experts.

STRONG SCATTERING BY LARGE SPHERULITES

Laser welding of semi-crystalline thermoplastics is fundamentally more difficult than of amorphous products, since the laser beam is scattered by the spherulites. That problem, which is common to all semi-crystalline plastics, is quite pronounced in PBT.

Compared to a polyamide 6 plaque of equal thickness, PBT lets much less laser light through. This is because the back-scattering portion is greater due to the pronounced scattering. In addition, the beam that passes through is widened more. From physics, we know that deflection of light rays is particularly low when the scattering centers are smaller than the light's wavelength. In the Nd-YAG laser, the most common type of laser, these are about 1000 nanometers (i.e. 1 micron / ?m). The solution was therefore to limit the spatial growth of the spherulites to a maximum size of one micron.

After numerous experiments, the BASF research team finally achieved the desired modification. The new Ultradur, labeled LUX, has a finer structure and, consequently, a significantly greater and much more constant laser transparency than other PBT grades available in the market. Its mechanical properties are at a comparable level to those of established Ultradur products.

HUGE IMPROVEMENT IN OPTICAL PROPERTIES

The huge improvement in the PBT's optical properties is clearly evident from the transmission curves. At the wavelength of the Nd-YAG laser, Ultradur LUX lets through about twice as much light as the standard PBT.

Not only is the laser transparency much better, the quality of the transmitted laser beam is increased significantly as well. By means of scattering experiments using a socalled integrating sphere, it was demonstrated that conventional unreinforced PBT allows virtually no light in the wavelength range relevant for laser-welding to pass through directly; all the rays are scatteredtoagreaterorlesser extent. In contrast, for Ultradur LUX direct transmission of about 50 percent and a significantly less widening of the laser beam are obtained at the wavelength of the Nd-YAG laser.

The practical importance of these theoretical values is exemplified clearly by holding the sample plaques up to the sun. The increased light transmission of Ultradur LUX is already evident here. Considering that the new material's transparency to laser light is much higher than to daylight (380-780 nanometers), the large step forward which is represented by the quality of new plastics becomes plausible.