Picture yourself driving down a country road in your dream convertible. Now, forget the usual clichés — top down, radio blasting your favorite tunes — and focus on the car itself.
As you’re buzzing down the road (no speed limits in imagination land, after all), you sink into your comfy custom interior and take in the glare of the sun on the dash, the comfortable grip of the steering wheel and the feel of the gear shifter in your hands — and you check on the fuel gauge, just to confirm that you’ve got several more miles to go before you need a top-off.
In the real world of automotive manufacturing, these details are closely related. More and more automotive interior manufacturers are turning to plastics, which offer better customization and lighter weight for improved fuel efficiency and reduced CO2 emissions.
With UV, you have the advantage of productivity and production efficiency. Parts can be curing in seconds or minutes — compared with other methods that could take hours or even days to produce a fully cured surface.
Technical Specialist, BASF
That said, the advent of plastics brings a new set of considerations around coatings and the curing process. Auto interior manufacturers are increasingly opting for UV curable coatings to leverage their flexibility, performance and cost savings benefits.
If you’re considering making the leap to UV curable coatings, here’s everything you need to know as they relate to auto interiors: what you should look for in a curing system, the additives at your disposal, how to ensure an effective cure, and an overview of the many benefits (and one challenge) with UV curing.
Don’t forget to buckle up.
Flexibility, matting and strong performance
What’s the first thing you should look for in an auto interior coating?
“For automotive interiors, the most important thing is that with all those plastics, you need good adhesion and flexibility,” says Ziniu Yu, Technical Specialist at BASF.
Because plastic parts are often shaped into a curved material, you can’t have a brittle resin because it will crack in the UV curing process. Yu mentions 100 percent solid UV systems, for instance, tend to crosslink too much during UV cure, creating cracks in the material.
“For that reason, we have tested a lot of water-based UV systems,” he says. Distinct from pure water or solvent-borne systems, which are not typically crosslinkable, water-based UV systems are also polymers that disperse water. “What happens is they have double-bond acrylic groups that allow them to crosslink or cure even after the water is dry. After it dries, it becomes a film on top of the coating, and then you can UV cure and crosslink it.”
Water-based UV systems convey the adhesion and flexibility required for auto interior applications, and they also allow for better gloss or matting compared to 100 percent solid UV systems. For automotive exteriors, glossing gives cars that “shiny, new” look, whereas interiors generally call for a matte finish to diffract the light and maintain a low-gloss effect.
According to Yu, it’s more difficult to achieve low gloss with a 100 percent solid, high-viscosity UV system, even when you add more of the matting agent. Water and solvents help because when they evaporate during UV curing, they essentially push the matting agent closer to the surface to create a better matte effect.
“For UV, if there’s no solvent and no water, you have trouble,” says Yu, referring to the solid UV systems. “Because there’s nothing to evaporate from the coating, you need to add a lot of matting agent. With water-based UV, you will have that evaporation step and get a very low gloss as a result.”
That’s not to say 100 percent solid systems don’t have their place — but when it comes to low gloss, low VOC, high flexibility and overall strong performance, water-based UV is often a better fit for automotive interiors.
Mix it up with additives
A matting agent is just one of many additives you might consider when formulating a coating for automotive interiors. The nature of the additive depends on your needs and application.
For instance, if you are not formulating a clear coating, you probably need an additive that adds pigment to your coating.
“Carbon black, iron oxide — there are all kinds of pigments you might choose,” says Yu. “You might also need an additive that acts as a flow and levelling agent.” As the name suggests, levelling agents help with surface levelling, keeping the coating surface smooth and even.
Wetting agents are another important piece of the coating formulation puzzle.
“Let’s say you’re working with PVCs or polycarbonate,” he explains. “With those plastics, you might find it difficult to get good adhesion or flow because your coating surface energy is higher than the substrate’s surface energy. It won’t form a good film as a result.”
In cases like these, you will need to add a wetting agent to lower the surface energy of the coatings to match the surface energy of the substrate.
“When you do,” says Yu, “it will form a very nice, wetted-out film before the UV cure.”
Again, there are several more additives to consider. BASF technical specialists like Yu typically advise customers with tailored recommendations for the right additives, equipment and packages to fit their needs and application.
Riding the wavelength: Factors to consider
Formulators working with UV technology should carefully consider two critical elements: the UV lamps and photoinitiator package.
“The most common is called a medium pressure mercury lamp, which will generate a UV spectrum in 200-450 nanometer wavelengths,” says Yu.
These lamps have mercury in the bulb, he explains, but they may be modified with doping elements to change the spectrum of ultraviolet radiation — thereby optimizing them for special applications.
To ensure your formulation cures properly, you must also select the right photoinitiator package.
“Let’s say you have pigment in the formulation,” says Yu. “The pigment itself will absorb UV, and so will your photoinitiator. If you add too many pigments, the photoinitiator won’t have enough light exposure to complete the polymerization process.”
What’s in it for you?
Why are more and more automotive manufacturers turning to UV curing for interiors?
Sustainability is one big driver: the use of UV reduces CO2 and eliminates volatile organic compounds (VOCs) from the curing process, helping to meet environmental regulations. UV also has higher crosslinking properties, which translates into better scratch resistance and mechanical performance.
UV coatings bring a range of benefits to automotive manufacturers, but they still present some roadblocks for others. The main issue, says Yu, is capital cost.
“Manufacturers must purchase expensive pieces of UV light equipment to fit their needs,” Yu explains.
He encourages manufacturers to look past the initial investment cost to see the overall return in cost savings over the life of the equipment.
“With UV, you have the advantage of productivity and production efficiency. Parts can be curing in seconds or minutes — compared with other methods that could take hours or even days to produce a fully cured surface.”
UV curing allows manufacturers to save time and money in every single instance of use. Based on the cost efficiency, sustainability and performance characteristics of water-based UV coatings, Yu sees a strong future for them in the automotive supply market.
“The high crosslink density will help water-based UV coatings pass some harsh chemical resistance tests, which will be very important for automotive interiors.”
Built to last
Provided they follow the right steps and work with a BASF expert like Yu to make effective formulations, auto interior manufacturers can reap several benefits from water-based UV coatings. There are always new coating and curing technologies coming down the road, but for now, UV cure seems well-positioned to outlast competing methods.