Polyurethane Structural Adhesive Applications

Although general purpose polyurethane adhesives have been used for over 40 years, it is only within the last two decades that structural adhesives based on polyurethanes have made significant inroads into the marketplace. This is due to the advances in urethane technology and to the advent of synthetic structural materials such as fiber-reinforced plastics (FRPs). These materials are readily bonded, with minimal surface treatment, using polyurethane structural adhesives. 

There are, however, certain limitations associated with polyurethane structural adhesives, the most significant being their elevated temperature performance. It is typical of such adhesives to lose 50%, or more, of their room temperature strength on aluminum or steel at 80-100°C as shown in Table I. Urethanes produce only moderate strength bonds to metals, and adhesives which contain free isocyanate groups suffer from limited shelf stability due to their moisture sensitivity. 

Polyurethane adhesives can be formulated as one- or two-part systems. These are also referred to as either one- and two-pack or one- and two-component systems. The two-part adhesives have been the more widely used of the two for structural applications, particularly for high-performance bonds. 

Primed CRS Primed aluminum Accuthane UR-1100 Sand-blasted CRS Etched aluminum Uralane 5738 A/B Etched aluminum 

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Most conventional low-modulus adhesives and sealants, such as polysulfides, flexible epoxies, silicones, polyurethanes, and toughened acrylics, are flexible enough for use at intermediate low temperatures such as -40°C. Low-temperature properties of common structural adhesives used for applications down to -129°C are illustrated in Fig. 15.9, and the characteristics of these adhesives are summarized in Table 15.12. 

Although the consumption of these materials in aerospace is but a small fraction of the resinous polymers used in industry, the impact of the needs and applications is large. Virtually all modern day structural adhesives have aerospace origins. The need for elevated temperature performance resulted in polymers useful for such diverse applications as electrical insulation and brake linings. The search for coatings to resist rain errosion and UV light contributed heavily to the technology of the polyurethanes. 

Polyurethanes are among the better adhesives for cryogenic applications. Room temperature-curing polyurethane adhesives in current use provide higher ultimate shear and tensile stress and higher peel and shock properties at -253°C than the earlier polyurethanes. This situation is the inverse of what happens to most structural adhesives. The polyurethane adhesives increase in strength at -253°C, but become weaker at ambient and higher temperatures, as shown in Table 10.3. ... 

The term Structural Adhesive covers many basic chemical classes of adhesives. Also, within chemical classes there may be both structural and non-structurai adhesives, e.g. not all polyurethane adhesives are structural some are sealants having some adhesion characteristics. As such, information is restricted to those types of adhesives used, or promoted for use, in structural applications. In some cases, the dividing line between sealant and adhesive becomes blurred . For example silicone-based materials may be used as adhesives or as sealants. In many applications, the ability of an adhesive to also seal a joint against the environment is an important advantage, avoiding leakage or crevices where corrosion could occur. 

Landscape of adhesives for structural automotive applications (PUR polyurethane]... 

The large volume applieations for moisture-euring polyurethanes are diseussed briefly in the applieations seetion below. These adhesive offer superior low temperature, high temperature, and speeifie adhesion performanee in comparison to conventional hot melts. These attributes have allowed them to excel in various product assembly applications, including structures such as windows, doors, furniture, and automotive headlamps and trim. Newer applications include bookbinding, fabric laminating, and assembly of athletic shoes. 

Foaming the polymer creates small, open cells that are able to hold fluids and the cell size may be controlled during the foaming process. The most common polymer used is polyurethane. Their structure and softness also provide a cushion that protects and contributes to thermal insulation of the wound. They also may be tailored for particular applications such as tracheostomy dressing without particle loss to the wound and with the retention of their conformable characteristics. The non-adhesive foams will require a secondary dressing. 

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