While many paints and coatings undergo physical film formation following evaporation of solvent or water, others require some type of reaction between the polymer resins to establish the desired applied film properties. These reactions can be facilitated by heat and/or the use of a catalyst or curing/crosslinking agent. Because crosslinking agents are incorporated into the final polymer network, they can have a significant impact on the properties of the applied coating. Recently, research has focused on developing more sustainable crosslinking chemistries that still provide desirable coating performance. A few examples are noted herein.
Omnova Solutions has developed a self-crosslinking, one-component acrylic latex for the formulation of low-VOC garage floor and masonry coatings. The epoxy-modified acrylic copolymer can be formulated at <50 g/L VOC and provides 1K garage floor coatings with a desirable combination of fast cure time/easy application and film performance properties (UV and chemical resistance, hot tire pickup resistance, adhesion, hardness, etc.) similar to those observed for 2K systems, according to the company.
Evonik, meanwhile, has developed a range of urethane-alkoxysilane binders and crosslinkers for automotive and wood coatings with high levels of scratch resistance. This technology is isocyanate-free and is available in both ambient-temperature curing as well as thermosetting systems. In July 2017, the company formed a new Crosslinkers Business Line by combining its isophorone chemistry and epoxy curing agents business, which includes amine curing agents for ambient and heat cure applications. The move was made so that Evonik can offer its customers a larger number of solutions for epoxy and polyurethane applications under one roof, according to Dr. Claus Rettig, chairman of the Board of Management, Evonik Resource Efficiency GmbH.
Valspar’s Duramax® coatings are based on Paint + Primer with Ti3 Crosslinking Technology™. The Ti3 Crosslinking Technology™ ensures the formation of strong bonds throughout the polymer network for enhancing UV resistance combined with greater resistance to the growth of mold, mildew, and algae on the applied paint film. It also has early resistance to moisture, which means the weather has less of an impact.
Researchers at South China University of Technology as well as Guangzhou and Shenzhen University in China have applied click chemistry as a cross-linking strategy for waterborne polymers. Polymer films produced using the technology exhibited improved tensile strength, hardness, adhesion strength, and water/solvent resistance compared to traditional waterborne polymer films. The scientists also reported that because click groups are incorporated and preserved in the final polymer film, further functionalization is possible.
Scientists at the North Dakota State University (NDSU) Research Foundation have employed vanillin naturally derived from lignin as a biobased crosslinked for melamine-formaldehyde (MF) coatings. The vanillin is first subjected to a Knoevenagel Condensation with an acetoacetylated polyol to generate a modified polyol that can be crosslinked with MF resins for use in the formulation of a variety of coatings and other applications. The choice of initial acetoacetylated polyol allows for adjustment of final polymer properties (impact resistance, hardness, solvent resistance, etc.).
A photo-reversible crosslinking technology for acrylate/methacrylate coatings developed by researchers at Monash University allows their application at ambient temperature using UV light and “self-healing” of minor scratches with the application of specific UV radiation. The coatings are transparent and can be healed repeatedly at the same location. The technology is based on the use of special crosslinking agents that contain subunits that reversibly dissociated when exposed to certain wavelengths of UV light, allowing for polymerization/depolymerization as needed.
An interesting note about crosslinkers in pharmaceutical manufacturing: Protein crosslinking agents are used to covalently bind, or conjugate, biomolecules, such as peptides and proteins together, immobilization of proteins to a solid support, or in the case of advanced therapeutics such as antibody-drug conjugates, a small-molecule toxin to a monoclonal antibody engineered to target a specific type of cell, such as a cancer cell.