Lightweighting affects paint technology
The demand for lightweight cars has resulted in a trend towards wider use of glass-filled plastics and carbon-fibre-reinforced polymers (CFRP) composites in car exteriors.
This means that OEMs are now aiming to reduce the temperature needed to harden coatings. Here they want both to save money, and to allow online painting of plastic components, Kahl explained.
At present, plastic parts are mainly coated offline, adding a lot of process cost, he said.
The aim is to be able to lower the paint curing temperature to 100¡C, so that full plastics and carbon-fibre-reinforced bodies, and attached plastic parts, can be painted online.
Present conditions are 200¡C for the EDC of the car body's metal frame. A lower temperature, 140 °C, is used for curing the clear coat.
The ED curing temperature could be cut to 150¡C, which is still high, but the crosslinking technology is there.
For the other processes, primer surfacer, base coat and clear coat, there are options for curing at under 100ºC. But these will be not 1K, they are 2K-PU ones, Kahl emphasised.
Polyurethane clear coats are mainly 2K PU materials hardened at 140¡C for 20-25 min. This can be done at lower temperatures, but that needs catalyst addition to maintain high productivity, Kahl added.
Lower temperature curing by adding catalyst is also possible with base coats, and primer surfacers, he added.
There is one technology in the market, and in use online, where the base coat is water-borne 2K PU, and the OEMs are very interested in this, said Kahl. Also a line with solvent-borne 2K PU primer surface is operating. One aim now is to combine these technologies within a proper OEM paint process.
But it will be another five to six years before technology for curing at below 100¡C will be available on OEM lines, Kahl said.
Such technology will be important in future because of the strong push towards replacing metal structural and body parts with lighter weight options, Kahl said.
An important factor OEMs have to consider is that the painting process involves a lot of investment in plant and equipment. Normally plant lifetime is 20-30 years, Kahl indicated.
"I believe when the paint development work is done there will be a seven-year transition period to low-temperature curing automotive paints," he added.
And while there is a lot of talk about electric vehicles, and lightweight structures, "I don't see that there will be so much plastic cars or fibre-reinforced chassis over the next years," Kahl said. But he does see a move to "multiple mixed materials," light steel, aluminium and plastics over the next years.
"There is not a clear long-term strategy or exact solutions, but long term, there will be an absolute need for electric vehicles and hybrids," with lighter weight parts. "That is really a need for our future generations."
Kahl thinks BMW will "show it is possible with its i-vehicles with CFRP bodies, now being planned, with much technology development and production investment."
But a lot depends on costs, since CFRP materials are also expensive.
A re-assessment of UV curing
In the very long term, Kahl said, the automotive industry needs even shorter processes and quicker cures, and here he pointed out the possibilities for UV curing materials.
"There are UV hardening paints which cure in seconds," he said.
The vehicle industry has worked on this, cooperating with OEMs and paint and equipment suppliers, in consortia. But up to now this has not been successful, because while UV can cure flat, smooth parts, it is much harder to get an even cure on complex 3D parts.
Anything not in the line of the radiation will not cure, the paint will not harden sufficiently, and while this technology is being further developed, it is not ready for the market yet, according to the Bayer coatings expert.
By 2025, such an option is likely to be available, Kahl said.
Today UV-cured coatings are used for painting PC-based parts such as auto headlamps, for very quick hardening with a scratch-resistant coating, Kahl said.
Here PU is involved, to give flexibility to counteract the shrinkage that UV curing provokes, with attendant brittleness and cracking.
Kahl said UV-cured coatings are used widely in the wood coating sector, parquet and plastics coatings and the electronics industry.
Another potential curing route is by plasma, which is also high cost, as it needs special plasma chambers.
UV curing is also hindered or its reactivity reduced by oxygen, so that the device needs to be close to the surface or users need to somehow reduce oxygen levels at the surface for the most effective cure, Kahl pointed out.
Kahl said that in the US, a lot of industry consortia looked at powder coatings in the 1990s for auto clear coats. The only European company who used powder coatings was BMW in Germany, which is now changing again to solvent-borne 2K PU, said Kahl.
This gives more flexibility on the paint line and it is easier to deal with problems when they arise. Beside this it fits to the primerless paint-line strategy.
Powder has to be applied at higher film thickness of 60-70um while solvent-borne coatings are 40-45μm thick. That means more material is used, adding to costs.
Some 20-30 percent of the primer surfacer market in the US has gone over to powder coatings, using epoxy and also PU technology, but the higher film thickness remains a question mark, Kahl feels.
There is no specific powder strategy in the automotive sector, he commented.
In Europe the OEMs have "dipped their toes into powder coatings," for the primer surfacer, with 3-4 lines using it, Kahl said.
But these companies changed to water-borne materials, or primerless technology, after some years, he said.
Biomaterials - the green issue
Kahl's view is that there is a lot of discussion about sustainable raw materials in coatings. "People want to know the solutions that are available," and there are a lot of ideas about and a lot of development of basic raw materials made by bio-reactors, Kahl agreed.
But the auto sector needs 100 000 tonnes of raw material. Today the bio-reactor industry is not at that sort of level, Kahl said.
Basic raw materials such as diols, carboxylic acids and amines made from renewable resources exist. These can be used to make polyethers, polyesters, polyamides and polyisocyanates, Kahl said.
The other argument against biomaterials is the one on fair use of land resources. "If you use land to grow crops for industry you may be damaging the food chain, and to grow enough crops to make industrial scale products needs millions of acres," Kahl said.
Supply chain security is also vital for the auto sector, and drought in recent years in the US, along with floods in Asia, have raised further doubts about the wisdom of using agriculture as a supply source.
OEMs understand that the overall impact in sustainability just from coatings material is limited. Stronger sustainable effects are seen in changing the painting process to a primer-less one or to use low-temperature curing, said Kahl.
Use of natural oil polyols is not really an option for high-tech uses such as automotive coatings, Kahl said. Using polymeric materials made from monomers derived from natural sources is the most acceptable route to use, to get a consistent high-quality material, he added.
Interiors offer PU potential
In the car interior, there are a lot plastics parts where "we see a lot of potential for PU" to coat these components, Kahl said.
OEMs get good haptics with TPU on PU/PC surfaces. "When you see what is possible with the coating to modify the haptics to get special colour effects, that is really quite something," he added.
Direct coating -a similar process to direct skinning - can be done, and gives a high-quality surface with more or less a one-step process.
On the paint side, in Europe, the trend is towards water-borne coatings for auto interiors, but some 30 percent - 40 percent is still solvent-borne. Water-borne 2K PU coatings can cover the full range of haptics from soft and velvety to surfaces with a hard dry feel, along with the whole range of colour options, Kahl said.
There is competition between OEMs as they jostle to differentiate themselves, with the "personalised" car, Kahl noted.
Here the interior is very important, and Kahl is convinced much differentiation can be carried out with painting processes for the car interior.
Changes in clear coats
Polyurethane use in automotive top coats (a colour-pigmented single layer instead of base coat/clear coat combination) started in the early 1980s at Mercedes Benz, said Kahl. At one stage, wanting to reduce emissions in plants, paint makers came up with the idea of high-solids 2K PU technology.
With the introduction of water-borne primer surfacer and base coats in the 1980s/1990s the need to improve the overall appearance using the clear coat came in focus. Here the 2K PU gives a very brilliant surface - typically a wet-look coat, Kahl noted.
In the early 1990s the idea of raising the etch resistance of the coats came up, he continued.
This was at the time when environmental aspects came to the forefront and for car makers acid rain was an issue, Kahl said.
Polyurethane again proved equal to this task, with high resistance to acid rain and chemicals, as shown by exposure testing in Europe and the US, he added.
Step-by-step 2K PU_was used more and more in the early 1990s. Later on the wish for better scratch resistance and more gloss and good appearance came to the fore.
This gave the next push for PU technology in the clear coat, with development of the concept of self-healing and nano-modified clear coats.
"Step by step, new needs came and PU offered optimum solutions," Kahl commented.
How the paint is built up