Urethane adhesives applications

The vast majority of reactive hot melts are moisture-curing urethane adhesives. Radiation (UV/EB) curable adhesives have been explored in the laboratory since the mid-1970s, but are only recently beginning to gain significant market penetration, particularly for PSA applications. The formulation and properties of these two classes of adhesives are discussed below. 

Several large applications for one-component moisture-cure urethane adhesives are available. Polymeric MDI is an exceptional binder for wood products, such as oriented strand board and particleboard. One-component urethane windshield adhesives are used almost exclusively in both the OEM and automotive aftermarket. One-part urethane adhesives are used to assemble the sidewalls for recreational vehicles (RV s), manufactured housing, and mobile homes. In construction applications, one-part urethanes are used to bond metal doors, hardwood flooring, panels, and partitions. 

Windshield sealants comprise some of the most significant applications of one-component urethane adhesives. Several years ago, the requirements for these sealants were increased. In an accidental rollover, in certain automobiles, the windshield becomes a structural part of the car. Therefore, the windshield adhesive, which holds the windshield in place, must have structural strength. An example of a one-component windshield sealant is shown below [48] ... 

Moisture-curing hot melts are a small but fast growing segment of the urethane adhesive market. They are used mostly in construction and furniture assembly applications. Recent applications include RV sidewall assembly and other OEM automotive applications. Smaller applications include bookbinding and footwear. A typical adhesive is shown below ... 

Applications for blocked urethane adhesives are small. However, they may be used as flocking adhesives or as crosslinkers for solvent-borne adhesives. Blocked urethane adhesives are also used as splicing adhesives for belts. Blocked isocyanates based on TDI, IPDI, and derivatives of hexamethylene diisocyanate are most commonly commercially available. 

Two-component urethane adhesives are used to bond sheet molding compound (SMC) panels for automotive OEM and aftermarket applications. Two-part urethanes are used as laminating adhesives in the RV industry. 

One application for a two-part urethane adhesive is a windshield adhesive/sealant... 

Two-component waterborne urethanes are the preferred choice to replace solvent-borne urethane adhesives, especially in the packaging and shoe industries. At this time, the packaging area is the largest application of two-part waterborne urethanes. Good strides have been made by the two-part waterborne urethanes in the shoe industry, especially in the athletic shoe market. Waterborne urethanes are also replacing the solvent-borne products in the OEM door panels. 

Certain fillers are commonly added to protect the urethane backbone from oxidative degradation. Carbon black and titanium dioxide are commonly used in conjunction with antioxidants to protect polyether polyurethanes in exterior adhesive applications that may be exposed to oxygen and light (Fig. 12). 

H. Lee and K. Neville, Epoxy Resins, Their Applications and Technology, McGraw-Hill, New York, 1957 Handbook of Epoxy Resins. New York McGraw-Hill, 1967 Encycl. Polym. Sci. Technol. 6, 209 (1967) I. Skeist, Epoxy Resins. Princeton, New Jersey Van Nostrand-Rheinhold, 1958 M. W. Ranney, Epoxy and Urethane Adhesives. New Jersey Noyes Data Corp., 1971 G. R. Somerville and P. D. Jones, Abstr. 168th Am. Chem. Soc. Meet., Atlantic City, N.J., 1974 Abstract ORPL, 146 in the Organic Coatings and Polymer Chemistry Division (1974). 

A second type of urethane adhesive is currently being introduced to the U.S. construction industry. The adhesive was originally developed in Japan. The system is two component. The isocyanate component is a modified polymeric MDI. The cross-linkable component contains water with water-soluble or water-emulsifiable resins. The adhesive produces strong, water-resistant bonds and has shown promise as a possible lower cost substitute for resorcinol formaldehyde resins in the adhesion of laminated beams. The product is now being used in Japan to bond wood and various other porous substrates at the rate of about 10 million pounds per year. The first U.S. application of the product has been the bonding of wood doors. The opportunities for products of this type should be excellent in view of the ever-increasing governmental pressures on solvents. 

In the case of urethane emulsions, many of the commonly employed thickeners for resin emulsions may be used, such as polyacrylic acids, polyacrylamides, etc. This is particularly useful in textile coating or adhesive applications where "hold-out" properties, for example, to permit the coating to stay on the surface rather than to penetrate the substrate, are required. 

Polyurethane adhesive consumption has been estimated at 217 million pounds (1991) having a value of approximately 301 million (see Fig. 2). Applications contributing to this volume are shown in Table 1. It is interesting to note that while the packaging market is the fourth-largest market in terms of pounds of urethane adhesives sold, it is substantially larger than the forest products market and the foundry core binder market in terms of... 

Polyols for adhesive applications can be generally broken down into three main categories (1) polyether polyols, (2) polyester polyols, and (3) and polyols based on polybutadiene. Polyether polyols are the most widely used polyols in urethane adhesives because of their combination of performance and economics. They are typically made from the ringopening polymerization of ethylene, propylene, and butylene oxides, with active proton initiators in the presence of a strong base as shown in Fig. 12. 

Polyester polyols are used widely in urethane adhesives because of their excellent adhesive and cohesive properties. Compared to polyether-based polyols, polyester-based polyol adhesives have higher tensile strengths and improved heat resistance. These benefits come at the sacrifice of hydrolytic resistance, low-temperature performance, and chemical resistance. One of the more important application areas for these products is in the solvent-borne thermoplastic adhesives used in shoe sole binding. These products are typically made from adipic acid and various glycols (see Fig. 15). 

The adhesives in Table 5 have many industrial applications. For example, EC-3535 B/A has been used to bond aluminum extrusions into auto roofs while EC-3532 B/A has been used to bond vinyl clad wire to printed circuit board. Urethane adhesives have also been us ed to bond polyester molded resin to the steel frame in Corvette auto bodies. 

Wattle tannins have been used as fortifiers in starch-based adhesives for corrugated carton manufacture in order to improve the moisture resistance of boxes used in shipment of fruit, for example (144, 217). Addition of 4% wattle tannin based on resin solids substantially improved the durability of these cartons. Wattle tannin urethane-based varnishes with excellent durability have been developed by Saayman (214). Other adhesive applications based on reactions of wattle tannins with isocyanates have been developed for specialty applications such as the bonding of aluminum (181). 

FireShield HPM Series High Purity antimony oxide is used as a catalyst, chemical Intermediate and flame retard2uit in specialty applications requiring low levels of lead, arsenic and other trace metals. FireShield HPM is available in tvro grades. The 1.0 to 1.6 micron average particle size is suitable for many sensitive electronic applications, while the 0.2 to 0.4 submicron grade is used in liquid systems such as epoxies, urethanes, adhesives, paints, solvents and colloids where dispersibility or surface area is ing>ortant. 

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