Лако-красочные материалы — производство

Dennis G. Lay and Paul Cranley

The Dow Chemical Company, Freeport, Texas, U. S.A.

I. INTRODUCTION

The development of polyurethane adhesives can be traced back more than 60 years to the pioneering efforts of Otto Bayer and co-workers. Bayer extended the chemistry of poly­urethanes initiated in 1937 [1] into the realm of adhesives about 1940 [2] by combining polyester polyols with di — and polyisocyanates. He found that these products made excel­lent adhesives for bonding elastomers to fibers and metals. Early commercial applications included life rafts, vests, airplanes, tires, and tanks [3]. These early developments were soon eclipsed by a multitude of new applications, new technologies, and patents at an exponential rate.

The uses of polyurethane adhesives have expanded to include bonding of numerous substrates, such as glass, wood, plastics, and ceramics. Urethane prepolymers were first used in the early 1950s [4] to bond leather, wood, fabric, and rubber composites. A few years [5] later one of the first two-component urethane adhesives was disclosed for use as a metal-to-metal adhesive. In 1957 [6] the first thermoplastic polyurethane used as a hot-melt adhesive (adhesive strips) was patented for the use of bonding sheet metal containers. This technology was based on linear, hydroxy-terminated polyesters and diisocyanates. Additional thermoplastic polyurethane adhesives began appearing in the 1958-1959 period [7,8]. During this period the first metal-to-plastic urethane adhesives were developed [9]. Waterborne polyurethanes were also being developed, with a polyurethane latex claimed to be useful as an adhesive disclosed in 1961 by du Pont [10]. A commercial urethane latex was available by 1963 (Wyandotte Chemicals Corporation) [11]. The adhesive properties of urethane latexes were explored further by W. R. Grace in 1965 [12]. In the early 1960s, B. F. Goodrich developed thermoplastic polyester polyurethanes that could be used to bond leather and vinyl [13]. In 1968 Goodyear introduced the first structural adhesive for fiberglass reinforced plastic (FRP), used for truck hoods [14].

Polyurethane pressure-sensitive adhesives began appearing in the early 1970s [15]. By 1978 advanced two-component automotive structural adhesives (Goodyear) were com­mercially available. Waterborne polyurethane adhesives received additional attention during this period [16]. In 1984, Bostik developed reactive hot-melt adhesives [17]. Polyurethane adhesives are sold into an ever-widening array of markets and products, where they are known for their excellent adhesion, flexibility, low-temperature
performance, high cohesive strength, and cure speeds that can readily be tailored to the manufacturer’s demands [18].

Urethanes make good adhesives for a number of reasons: (1) they effectively wet the surface of most substrates (the energy level of very low energy surfaces such as polyethy­lene or polypropylene must be raised before good wetting occurs) [19], (2) they readily form hydrogen bonds to the substrate, (3) their small molecular size allows them to permeate porous substrates, and (4) they form covalent bonds with substrates that have active hydrogens. Figure 1 shows the typical mechanism for a urethane adhesive bonding covalently to a polar surface.

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

dollars. Overall, the polyurethane adhesives market grew at an annual rate of approxi­mately 3% from 1986 to 1991. Specific market segments such as automotive and recrea­tional vehicles easily surpassed the gross national product (GNP) growth rate. In the next few years a number of specific market segments are expected to grow at about 5% per year. These would include vehicle assembly (automotive and recreational vehicles), elec­tronics, furniture, and curtain wall manufacture.