Question: Formaldehyde-based adhesives are traditionally used in a variety of wood-bonding applications. However, the media has reported concerns regarding formaldehyde exposure and current regulations. Are there alternatives to these adhesives?
Answer: A variety of adhesive types can be used for bonding wood, depending on the application and end-use requirements. Consumer applications for bonding and repairing wood can use standard polyvinyl acetate emulsions or moisture-curing polyurethanes to achieve higher performance. These materials are sold under a variety of brand names. Wood composites used in more demanding structural applications, such as plywood, OSB, particleboard and glulam, have traditionally used formaldehyde resin-based adhesives. Urea/formaldehyde resins have been used for interior applications, and phenol/formaldehyde or phenol/resorcinol/formaldehyde have exterior uses. Adhesives based on polyurethane chemistry are being used commercially in some wood-bonding applications as an alternative to formaldehyde-based products. Polymeric diphenylmethane diisocyanate (pMDI) is used in large volumes for the production of oriented strand board. These adhesives can be used to bond wood with high water content. Formaldehyde-free polyurethane adhesives based on one-component, moisture-curing polyisocyanate prepolymers are also used for engineered wood applications. Products of this type have been developed that pass stringent glulam performance standards. MDI-based polyurethane adhesives are also characterized as CARB NAF (no added formaldehyde) resins. -Dr. Jeffrey F. Dormish
Question: We want to develop a wood furniture coating with no added formaldehyde. What types of polyurethane technologies are available?
Answer: Polyurethane (PUR) technologies can provide the benefit of no added formaldehyde. In fact, the three polyurethane technologies most widely used to produce wood furniture coatings have no added formaldehyde: two-component solventborne PUR, two-component waterborne PUR, and one-component waterborne PUR. The latter two have the added benefit of low-VOCs.
Two-component solventborne PUR coatings are created through the reaction of a polyisocyanate and a polyol, or co-reactant. They harden by drying through solvent evaporation, and develop their performance properties through a chemical curing process. Polyurethanes yield excellent mechanical and chemical resistance, hardness, flexibility, and scratch resistance. What’s more, existing production lines can easily be converted to PUR coatings using the same application/curing equipment. This is especially important for smaller shops.
Two-component waterborne PUR coatings represent a growing technology. They are based on a combination of aqueous polyols with water-dispersible aliphatic polyisocyanates. Two-component waterborne PUR coatings are similar to two-component solventborne PUR, the main difference being the solvent is replaced by water, thus reducing VOCs.
One-component aqueous polyurethane dispersions (PUDs) are colloidal systems in which polyurethane particles are dispersed in a continuous phase (water). Dispersion compositions can be produced to optimize specific physical properties such as hardness, flexibility, and abrasion/chemical resistance. Self-crosslinking PUDs are produced by adding materials that cure by air oxidation. PUDs can be blended with waterborne acrylics to design specific coating property attributes. The major benefit of this technology is that it is a one-component coating with low VOCs. -Bob Wade
For additional information on the topics addressed, or to ask another question, e-mail jeff.dormish@bayermaterialscience.com with the subject line “Polyurethane Q&A."
Any views or opinions expressed in this column are those of the author and do not represent those of Adhesives & Sealants Industry, its staff, Editorial Advisory Board or BNP Media..
