Urethane adhesives metal bonding

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. 

Urethane structural adhesives have been very popular for bonding of plastics to plastics and plastics to metal by the transportation industry. As an example, more than 90% of the bonds in Fords experimental graphite car were urethane adhesives rather than rivets or welds. Urethane structural adhesives have been used by the transportation industry since 1969. It is estimated that 5 million pounds of urethane structural adhesives were used in the transportation industry in the United States in 1979. Some of the advantages of urethane adhesives as used by the transportation industry are shown in Table IX. 

Polyurethanes were first discovered by Otto Bayer and coworkers at I. G. Farbenindustrie, Germany, in the late 1930s. The first products were obtained by reacting an aliphatic diisocyanate with an aliphatic diamine or diol. These materials soon found commercial uses and were marketed under the trade names of Irgamid U, for plastics, and Perlon U for synthetic fibers and bristles. Very soon after this, it was discovered that isocyanates could be used to bond rubber to metal, which in turn led to the development of urethane adhesives based on polyester diols these adhesives were commercialized under the trade name Polystal. For a more complete account of the history of polyurethanes, see refs. 2, 4, and 5. 

Also, as the thickness of the adhesive bondline increases, read through problems become worse. The heat resistance of urethane-bonded SMC composites has also been a problem. This is believed to be due to the out-gassing of volatiles from the plastic which decreases the bond integrity. This is evidenced in Table VI, which compares the heat resistance of metal-metal bonds and SMC composites prepared with urethane adhesives. It can also be seen that the composition of the SMC has a significant affect on the heat resistance. The urethane-modified SMC was also reported to give off less volatiles than the unmodified SMC. Recent work has indicated that certain primers may be able to protect the adhesive from the substrate. 

Aminoamides Polyaminoamides are used to improve bonding of PVC plastisols to metal surfaces. Used at 3-5 phr, generally dissolved in solvent or plasticizer, such products are available under the trade names Euretek (Schering) and Versamid (Henkel = Cognis). The bond formed is to the surface metal oxide layer. Combinations with urethane adhesion promoters should not be used, since the aminoamides are strong urethane catalysts. A topcoat (without adhesion promoter) is usually desirable it should be formulated to resist amine stain. Phenolic antioxidants used with aminoamides (or other amine additives) should have all ortho and para positions blocked to prevent color development on aging. Similarly, aliphatic phosphites are the best choice. 

Automotive Sealants In filling underbody dead spaces, foamed plastisols compete with urethanes and other products. Such plastisols are highly filled, often with over 100 phr calcium carbonate, and plasticizer levels of 100-140 phr. Adhesion promoters for metal bonding (Chapter 12) are used with 2-4 phr of 5-7 p,m AZO, yielding expansion factors of 1.5-2. 

Uses Adhesion pronnoler, bonding agent In metal/plastic composite stnjctures, textile and leather coatings, primers for rigid surface coatings adhesive for food pkg. Polyurethane polymer. See Polyurethane resin Po urethane prepolymer... 

These adhesives are structural adhesives and have found widespread use in both industrial and household environments. They are designed to bond metals, plastics, rubber, wood, glass and ceramics with thin or wide gaps. They display good water resistance and have good room temperature performance but are poor at elevated temperatures. The following is the simple urethane linkage structure ... 

Although the polyurethanes form useful adhesive bonds, particularly between metals and elastomers, their use in the aerospace industry for bonding purposes is limited. Because polyurethanes tend to depolymerize above 120 °C and are subject to hydrolysis, and because aromatic urethanes autoxidize when exposed to thermal or UV light (13). epoxies are the preferred bonding material. Recently they were studied as launch seals for both land and sea missile launch tubes and were found to be superior to seals based on neoprene rubber (14). The chemical reaction for this application is proposed to be that between isocyanates and amines (Reaction 3). 

The primary problem with epoxy resins appears to be the swellability of the resin in moisture (18). This is likely due to the preponderance of hydrophilic groups in the cured epoxy-adhesive. Urethanes have the additional problem that metal ion catalyzed hydrolysis of the urethane bond can take place. 

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