New Polyurethane Varnish Coating Technology for Plastic Automotive Parts

An Effective Way to Achieve a Glossy Surface

Innovative technology that enables coating of buffers, ceiling modules and other plastic accessories much more efficiently at temperatures below 100°C. The varnish surface created with this technology cures 30 percent faster than standard two-component polyurethane systems; the same glossy appearance is achieved. The varnish is the most important aspect of an automobile's external appearance, representing its aesthetics and value. This appearance influences customers' purchasing decisions and is regularly evaluated in market research surveys. Varnish, as the final applied layer, creates the glossy appearance of the automobile. This typically involves coating metal body parts at 140°C. However, temperatures at this level cause plastic parts to deform, so automobile manufacturers typically coat plastic parts at 80°C. Plastic and composite materials are increasingly used in the production of accessories and structural parts because they help make automobiles lighter and thus reduce fuel consumption and CO2 emissions.

The Contradiction

Of course, it is desired that plastic bumpers, mirror housings, spoilers, handles, trunk lids and ceiling modules maintain the same appearance quality as the steel body throughout their entire service life. Therefore, the coating applied to plastic parts must be resistant to weather conditions, cleaning agents, chemicals such as petrol, mechanical loads and many other effects. Two-component polyurethane varnishes meet these requirements and have also proven suitable for use on metal and plastic surfaces. However, previously, plastic part coatings achieving superior final properties at low temperatures took a long time. Suppliers and automobile manufacturers initially cured these parts in an oven at 80°C for half an hour. Even after this, despite the heat treatment, the polymers in the coating had still not completed cross-linking and surfaces were very sensitive, requiring extremely careful handling of the parts. This delays continuation of other processing and transport of the parts. Additionally, the parts must be stored using special methods. Complete curing of the varnish can take several days. Catalysts have been used for some time to accelerate varnish curing, but this method has a major disadvantage: in a two-component spray system, the moment polyol and polyisocyanate mix with each other, a reaction accelerated by the catalyst begins between them. As a result, after application the coating cannot flow freely. The resulting surface is rough (orange peel effect) and uniform gloss cannot be achieved. In coating processes based on systems without catalysts, there is a visibly discernible difference. To measure this, special equipment ("wave scan") can be used that detects wavy and dull spots. Such equipment is usually used to automatically compare the appearance of coated surfaces with automobile manufacturer requirements. Attempts to achieve rapid hardening without compromising appearance have so far always failed.

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1. Principle of two-component polyurethane varnish coating with BLULOGIQ technology

Solution

Thanks to the new technology from Covestro, formerly known as Bayer Material Science, it is now possible to achieve excellent appearance together with rapid curing. This technology was officially introduced at the European Coating Show 2015 in Nuremberg under the name BLULOGIQ. The main feature of the product is a thermo-latent hardener that becomes active only after solvent has evaporated from the coating and the temperature has reached approximately 70°C. As a result, film formation and curing occur in separate processes. After application, the coating initially flows unobstructed on the plastic surface, creating an even and smooth film. During heat treatment in the oven, the activated hardener causes the coating to cure rapidly. In Figure 1, the principle behind this new technology is shown representationally. The basic chemical structure of the thermo-latent hardener resembles standard hardeners used today. Therefore, it can take their place in existing formulations without requiring major changes. For this reason, there are no obstacles to using this new technology in series production coating of plastic automobile parts. Additionally, it can be used for efficient high-gloss coating of other molded plastic parts.

Fast Curing, Glossy Appearance

Figure 2 compares pendulum damping data for various two-component polyurethane systems. Even after 30 minutes in an 80°C oven, the polymers in catalyst-free coatings (black line) have undergone only a limited amount of reaction, as evidenced by the approximately 10-second pendulum damping value. Standard two-component polyurethane coatings (blue line) for plastics in coating lines contain a small amount of catalyst. For this reason, pendulum hardness is achieved 15 to 20 seconds after spraying. To increase this value to 180 seconds, several days must pass at 23°C after curing. By contrast, coatings based on the new technology (green line) have 70-second pendulum hardness when removed from the 80°C oven after 30 minutes. Therefore, parts coated this way can continue with subsequent processing much faster and much more easily after spraying. Faster and more energy-efficient drying offers an advantage from another perspective; even after only 20 minutes in the oven, the pendulum damping value is higher than standard systems processed for 30 minutes. The faster curing with the new technology does not significantly reduce open time compared to standard systems

(Figure 3). The reduction in open time has a noticeable effect on the appearance of the resulting coating.

Figure 4 shows how coatings based on BLULOGIQ technology (green line) reach the same high level of visual properties as catalyst-free systems (blue line). Visual gloss is much better than catalyzed systems (gray line). In the diagram, "structural spectra" are shown as a result of wave scan measurements, indicating dull and wavy areas. Dullness measurements record the light scattering effect caused by surface structures smaller than 0.1 mm. In automobile coatings, the wavy structure is approximately in the range of 0.1 mm to 30 mm. To account for the eye's resolution capacity at different distances from the coating surface, the wave scan device mathematically divides wavy structure data into several parts (for example; Wa = 0.3 mm, We = 10 to 30 mm).

Technology Potential

Compared to the best processing methods in use today, this new technology results in 15 percent energy savings and 10 percent reduction in CO2 emissions in the initial coating of an automobile. This result was obtained through joint research conducted by Covestro, one of the world's largest polymer companies, an independent automobile manufacturer, coating experts, an equipment manufacturer and a company specializing in sustainability certification. BLULOGIQ technology reveals new revolutionary potential in terms of reducing vehicle weight in series automobile production. In the medium term, it will enable all initial coatings applied in large-scale series production of light vehicles to be performed at low temperatures. The first step is low-temperature varnish application; the next step will be to add raw material development for primer technology, underbody protection, general insulation and adhesive application to the same stage. As a result, it may become possible to coat plastics, composites and metals together. The most important way to reduce weight in automotive construction is probably multi-material vehicle bodies. Plastic components are currently coated off-line with separate coating equipment and are later added to vehicle bodies. For this reason, it is difficult to achieve perfect color matching between metal and plastic parts and consequently there are many disadvantages such as affecting the overall appearance of the vehicle. The new technology can promote the use of alternative energy sources in the coating process. Thanks to the curing temperature below 100°C, for example, heating the oven with central heating to dry the coating could be a practical application. Waste heat generated in other production stages can also be utilized. Dilek Sivri / Customer Representative - Covestro Hare Yılmaz / Customer Representative - Covestro