Presidential Green Chemistry Challenge: 2005 Designing Greener Chemicals Award
Archer Daniels Midland Company
Archer RCTM: A Nonvolatile, Reactive Coalescent for the Reduction of VOCs in Latex Paints
Innovation and Benefits: Latex paints require coalescents to help the paint particles flow together and cover surfaces well. Archer Daniels Midland developed Archer RCTM, a new biobased coalescent to replace traditional coalescents that are volatile organic compounds (VOCs). This new coalescent has other performance advantages as well, such as lower odor, increased scrub resistance, and better opacity.
Summary of Technology: Since the 1980s, waterborne latex coatings have found increasingly broad acceptance in architectural and industrial applications. Traditional latex coatings are based on small-particle emulsions of a synthetic resin, such as acrylate- and styrene-based polymers. They require substantial quantities of a coalescent to facilitate the formation of a coating film as water evaporates after the coating is applied. The coalescent softens (plasticizes) the latex particles, allowing them to flow together to form a continuous film with optimal performance properties. After film formation, traditional coalescents slowly diffuse out of the film into the atmosphere. The glass transition temperature of the latex polymer increases as the coalescent molecules evaporate and the film hardens. Alcohol esters and ether alcohols, such as ethylene glycol monobutyl ether (EGBE) and Texanol® (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), are commonly used as coalescents. They are also volatile organic compounds (VOCs). Both environmental concerns and economics continue to drive the trend to reduce the VOCs in coating formulations. Inventing new latex polymers that do not require a coalescent is another option, but these polymers often produce soft films and are expensive to synthesize, test, and commercialize. Without a coalescent, the latex coating may crack and may not adhere to the substrate surface when dry at ambient temperatures.
Archer RCTM provides the same function as traditional coalescing agents but eliminates the unwanted VOC emissions. Instead of evaporating into the air, the unsaturated fatty acid component of Archer RCTM oxidizes and even cross-links into the coating. Archer RCTM is produced by interesterifying vegetable oil fatty acid esters with propylene glycol to make the propylene glycol monoesters of the fatty acids. Corn and sunflower oils are preferred feedstocks for Archer RCTM because they have a high level of unsaturated fatty acids and tend to resist the yellowing associated with linolenic acid, found at higher levels in soybean and linseed oils. Because Archer RCTM remains in the coating after film formation, it adds to the overall solids of a latex paint, providing an economic advantage over volatile coalescents.
The largest commercial category for latex paint, the architectural market, was 618 million gallons in the United States in 2001. Typically, coalescing solvents constitute 2–3 percent of the finished paint by volume; this corresponds to an estimated 120 million pounds of coalescing solvents in the United States and perhaps three times that amount globally. Currently, nearly all of these solvents are lost into the atmosphere each year.
Archer Daniels Midland Company has developed and tested a number of paint formulations using Archer RCTM in place of conventional coalescing solvents. In these tests, Archer RCTM performed as well as commercial coalescents such as Texanol®. Archer RCTM often had other advantages as well, such as lower odor, increased scrub resistance, and better opacity. Paint companies and other raw material suppliers have demonstrated success formulating paints with Archer RCTM and their existing commercial polymers. Archer RCTM has been in commercial use since March 2004.