Self-crosslinking acrylics, advances in waterborne coatings, and the path to eco-friendly stain resistance

In the wood coatings market, sustainability and performance often seem to be at odds.

Government regulations have made it increasingly important to minimize the output of volatile organic compound (VOC) emissions in wood coatings, both for environmental and health-related reasons. As a result, manufacturers concerned with sustainability often turn to waterborne systems for a lighter emissions footprint.

Waterborne resins have undergone many changes in the past few decades. Though they’ve seen much more success in recent years, they’ve been working hard to overwrite a reputation bogged down by past failures — and a false perception about the superiority of solvent-borne coatings.

“Solvent-borne coatings are still prevalent, particularly in kitchen cabinets, wood components, and furniture in general,” says James Monroe, Market Segment Manager of Furniture and Flooring at BASF. “The perception is that if it’s waterborne, it’s no good, or it’s not going to be a very high-performing material.”

According to Monroe, the realistic outlook for waterborne is much brighter than the perception.

“Waterborne systems have come a long way since they were first introduced over twenty years ago,” he says. “There has been a lot of advancement, and there are high-performing coatings available today.”

In particular, recent developments in self-crosslinking technology have made it easier than ever to achieve the performance of a solvent-borne system with the low-emissions attributes of waterborne coatings.

That sounds like a valuable prospect — but before formulators can properly sink their teeth into self-crosslinking acrylics, it’s worth diving into the chemistry that makes these systems tick.

Low-VOC, low-hassle

Aditi Chavannavar, Technical Specialist from the Dispersions, Resins and Performance Additives unit at BASF, says self-crosslinking systems offer several advantages over previous systems. “They do not have to be mixed prior to application, they have a long pot-life, and they can be transported and stored easier than a 2K system. They are also less toxic and safer to use.”

“Once the formulation is applied,” Chavannavar explains, “and a key driver, such as heat or evaporation of water, is introduced, the crosslinker is activated and reacts with different functional groups on the polymer to form a crosslinked film.”

The result? You get a tougher film with better mechanical and chemical resistance properties. When waterborne elements are involved, you get a low-VOC product to boot.

That’s how one newly-released BASF product, Joncryl 541, was made to confer the high performance of solvent-borne coatings with a waterborne system. Though it was first introduced for industrial coatings, it has plenty of potential to be used in wood coatings, automotive manufacturing, and other markets as well.

Joncryl 541 is also fast-drying and has the ability to quickly develop its performance properties, so manufacturers using the system can expect better throughput and a faster return to service.

For manufacturers with different needs, Chavannavar and Monroe also recommend self-crosslinking systems like Joncryl 2980, a wet-look acrylic emulsion with good dirt pick up resistance, and Joncryl 2981, an acrylic dispersion that offers high chemical resistance with excellent weatherability.

Optimized for a performance boost

Along with the rise of self-crosslinking systems, certain advances have allowed waterborne coatings to achieve a comparable performance to solvent-borne systems. The crosslink density is one of these advances.

“New methods in the polymerization process have helped to increase crosslink density,” Monroe says. “This has ultimately led to higher performing products in the sense that fewer coalescing solvents are needed to aid in film formation.” Fewer coalescing agents ultimately means less VOC output.

He notes waterborne systems have also progressed aesthetically, gaining the ability to have pigmented coatings.

The main advantage to self-crosslinking, however, lies in its exceptional chemical resistance.

“Increasing crosslink density helps to create a more impenetrable network by linking polymer chains together,” Chavannavar explains. “This makes it difficult for water or other chemicals to pass through the coating and reach the substrate.”

She adds that because the polymer chains are linked to one another, it becomes difficult for chemicals to break the bonds and degrade the polymer. That’s how crosslinking boosts a coating’s resistance properties and overall performance.

Searching for the missing link

In particular, stain resistance has been a major driver for self-crosslinking systems — both in North America and in Europe.

Consumers have shown increasing interest in stain-resistant furniture and cabinetry. Household fixtures should be able to hold their own against stains from common items such as ketchup, mustard and coffee. Monroe adds that current design trends have also influenced the evolution of stain resistance.

For instance, recent market trends have shown a higher proportion of sales in white or very light-colored kitchen cabinetry. Because stains tend to show more on light paint, it becomes all the more important to use coatings with strong stain resistance.

The wood coatings market, and the industry at large, have also become increasingly interested in lowering the output of potentially harmful emissions.

“There are some really good products on the market, but those products demand higher levels of coalescing solvents,” says Monroe. As a result, the final formulation can end up emitting a higher percentage of VOCs.

As the industry and the market demand more sustainable coatings, the latest challenge lies in creating an acrylic material that formulates to lower VOC levels.

Some manufacturers have developed workarounds for the problem, which Monroe calls “tricks of the trade,” adding that they aren’t always as beneficial as they seem. For instance, manufacturers might add additional materials, like plasticizers, to offset the VOC content, but the trade-off often involves lowering performance characteristics and adding cost to the formulation.

Products like Joncryl 541 have successfully combined waterborne and self-crosslinking technologies to deliver a higher value overall. That said, formulators must also understand the general temperament of self-crosslinking resins before they dive into the system.

In particular, Chavannavar says there is a risk of having a self-crosslinking resin react while in storage.

“If the temperature is high or the pH is low, the crosslinking reaction may initiate prematurely,” she explains. “Because there are reactive species in the polymer backbone, these could also interact with additives in the formulation.”

To avoid a premature reaction, Chavannavar recommends testing formulated coatings for long-term storage stability and compatibility with additives. BASF also works with formulators to ensure nothing goes awry during the storage phase.

What’s next for self-crosslinking technology?

Self-crosslinking resins have been especially popular in challenging applications.

High-traffic areas such as garage floors, public parking lots, malls and retail stores demand good resistance against abrasion, as well as shoe and tire marks. Coatings for roofs, decks, and window trims must have strong water resistance to withstand the elements, while hospital floors need chemical resistance to hand sanitizers and disinfectants. Wood coatings, as well as interior coatings for cars and airplanes, should also have good chemical and stain resistance.

Chavannavar says self-crosslinking systems are in a particularly good position to offer the high-performance properties needed for all these applications.

Paired with the sustainable footprint of waterborne systems, self-crosslinking acrylics are poised to take the next big step into the future of wood coatings.

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