Q&A About Polyurethanes
Question: Solventborne contact adhesives have a long history of forming high-performance adhesives that bond a variety of substrates, develop quick grab and form durable bonds. Are waterborne versions also available?
Answer: Waterborne polychloroprenes have been developed that provide similar performance to their solventborne counterparts. Polychloroprene latices are produced with high solids content (~55-58%). They are available in fast- and medium-crystallizing versions, similar to typical solid polychloroprene grades. A very slow crystallizing grade is also available. These different products may be blended in various ratios to adjust open time, tack and heat resistance properties.
Polychloroprene latices are compounded similarly to solventborne adhesives. Aqueous resins can be added to improve heat resistance (terpene phenolics) and tack and open time (rosin esters). Resins are typically added in a range of 10-30 parts resin/100 parts of polychloroprene latex. It is important to keep in touch with your resin suppliers to keep abreast of new products.
An additional standard compounding ingredient is a dispersion of zinc oxide, which improves the aging stability of the finished adhesive film. Antioxidants are also added to improve the tackifier resin’s aging properties and color stability. Waterborne polychloroprene adhesives are frequently used in wet-on-wet bonding applications on porous substrates, such as in foam-to-foam and foam-to-fabrics or foam-to-leather for furniture or automotive applications. A key benefit of these waterborne adhesives is their positive environmental impact - Greenguard-certified commercial adhesives are on the market.
Question: What options are available to improve the heat resistance of a waterborne polychloroprene-based adhesive?
Answer: As is the case with solventborne contact cements, isocyanates can be used to improve the heat resistance of the final bond. It is necessary to use special water-dispersible polyisocyanates with the waterborne polychloroprene to prevent coagulation. One specific commercial crosslinker for this use is a product based on a derivative of isophorone diisocyanate. This polyisocyanate crosslinker is most advantageously used in conjunction with recently developed polychloroprene latices that contain hydroxyl functionality, which serves as a point for crosslinking to occur. Polychloroprene latices with hydroxyl functionality are available in fast and slow crystallizing versions.
Another recent development to improve heat resistance is to compound the polychloroprene latex with nano particulate silica sol. This will increase heat resistance while speeding drying and increasing adhesion strength. Nano particulate silica can be added at 5–15 parts per 100 parts of polychloroprene latex.
For additional information on the topics addressed, or to ask another question, e-mail firstname.lastname@example.org 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.