Thermoactivated hardener enables efficient coating of plastic add-on parts
Bayer MaterialScience has developed a unique technology for coating plastic parts on automobiles at low temperatures. Bumpers, mirror housings, spoilers, tailgates and roof modules are finished with their outer clearcoat at an energy- and cost-efficient temperature of just 80 °C. Although curing is as much as 30 percent faster than with proven two-component polyurethane coatings, appearance is still very good. In the medium term, this technology will offer the possibility of coating plastic, composite and metal automotive parts together for the first time.
A new car needs to look good and convey a sense of aesthetics and value. The clearcoat is responsible for the external appearance. It is the last layer to be applied to the body, and gives the vehicle its high-gloss finish. Two-component coatings formulated with polyurethane (PUR) raw materials from Bayer MaterialScience have proved invaluable for this. Along with their outstanding appearance, they also boast excellent resistance to weathering, chemicals and impact.
Although many bodywork parts are still made from sheet steel, plastics are increasingly used for add-on automotive parts. They are one way of reducing fuel consumption and CO2 emissions. To ensure that the coated plastic parts look just as good as the coated metal, they are coated in exactly the same way, but at a lower temperature.
Previously – focus on speed or brilliance
When using conventional, uncatalyzed coating technology, plastic parts generally need several days to dry completely after being coated with two-component polyurethane coatings. This leads to delays in further processing and requires special measures for storing the coated parts.
For some time, catalysts have therefore been used for curing. However, their use means that crosslinking begins immediately upon application. As a result, the coating cannot flow freely and does not achieve an optimal appearance. All previous attempts to satisfy the need for rapid curing without compromising on appearance have failed.
First film formation, then curing
At the heart of the new technology is a thermolatent hardener from Bayer MaterialScience that makes it possible to separate film formation and curing. “The coating initially flows smoothly on the substrate and forms an even film,” said Dr. Jan Weikard, Head of Application Technology in the Automotive/Transportation segment of the Coatings, Adhesives and Specialties Business Unit at Bayer MaterialScience. “Only when the temperature rises is the hardener present in the coating activated by a special latent catalyst. This ensures the coating dries rapidly on the plastic substrate.”
No significant changes to the coating formulation are required. Thermolatent two-component PU systems can therefore be used for coating plastic add-on parts in series production without any problems. Even in cases where the faster drying is not such an advantage, the new development still enables the parts to be processed with greater ease and speed after baking.
Promising prospects for energy-efficient cars
Constructing lightweight vehicles continues to be of great interest to the automotive industry. “Our technology opens up new and very promising opportunities for introducing lightweight construction concepts in mass production,” said Zivko Andelkovski, Head of Industrial Marketing Automotive at the Coatings, Adhesives and Specialties Business Unit. “Low-temperature clearcoat technology is the first milestone to achieving this. Further development of our raw materials in the fields of primer surfacer technology, underbody protection, seam sealing and adhesive bonding will make it possible in the medium term to complete the entire process at low temperatures and lead to a breakthrough for lightweight mass-produced vehicles.”
Compared to the best practice process available today, the technology allows energy savings of 15 percent and a reduction of CO2 emissions by 10 percent. This is the result of a joint study carried out by Bayer MaterialScience, a car manufacturer, a coatings formulator, a manufacturer of coating lines and a company for sustainability certification.
Thanks to the lower curing temperature, it will be possible in the medium term to coat plastics, composites and metals together. “This is a first for in-line coating,” explained Andelkovski.
Initial samples will be available to development partners starting the first quarter of 2015, with market launch to follow. The low curing temperature also offers future opportunities for using alternative energy sources. For example, district heating could be used to heat the ovens for drying the coatings. Another possibility would be to use cogeneration or waste heat from other manufacturing processes to produce the energy required.