Urethane adhesives plastic bonding

The morphology of a typical urethane adhesive was previously shown in Fig. 3. The continuous phase usually comprises the largest part of the adhesive, and the adhesion characteristics of the urethane are usually controlled by this phase. From a chemical standpoint, this continuous phase is usually comprised of the polyol and the small amount of isocyanate needed to react the polyol chain ends. A wide variety of polyols is commercially available. A few of the polyols most commonly used in urethane adhesives are shown in Table 2. As a first approximation, assuming a properly prepared bonding surface, it is wise to try to match the solubility parameters of the continuous phase with that of the substrate to be bonded. The adhesion properties of the urethane are controlled to a great extent by the continuous phase. Adhesion to medium polarity plastics, such as... 

These reactive hybrid epoxy-urethane adhesives were developed initially for bonding to oily cold-rolled steel, but they have given good results on other substrates as well. It is believed that the oil contaminant on the substrate is adsorbed into the uncured adhesive and acts as a plasticizer. Table 7.9 shows tensile shear values obtained after curing 20 min at 177°C on various untreated substrates. The epoxy-urethane adhesives resist hydrolysis and give useful bond strengths up to about 100°C. 

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. 

They are easily bonded with epoxy or urethane adhesives however, the temperature resistance of the adhesives do not match the temperature resistance of the plastic part. No special surface treatment is required other than abrasion and solvent cleaning. Polyetherimide (ULTEM ), polyamide-imide (TORLON ), and polyethersulfone can be solvent cemented, and ultrasonic welding is possible. 

With plasma treatment, surface wettability can be readily induced on a variety of normally non-wettable materials as shown in Table P. 5. Certain polymeric surfaces, such as the polyolefins, become crosslinked during plasma treatment. The surface skin of polyethylene, for example, will become crosslinked so that if the polymer were placed on a hot plate of sufficient heat, the interior would turn to a molten liquid while the crosslinked outer skin held a solid shape. Other polymers have their critical surface energy affected in different ways. Plasma-treated polymers usually form adhesive bonds that are two to four times the strength of nontreated polymers. Table P.5 presents bond strength of various plastic adherends pretreated with activated gas and bonded with an epoxy or urethane adhesives. 

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. 

The main adhesives for bonding plastics to rubbers include cyanoacrylates, two-part urethanes, two-part epoxies, hot melt reactive urethane prepolymers, heat reactive contact cements and silane treatments. 

Urethanes Urethane adhesion promoters are used widely to bond PVC to polyester, nylon, and other fabrics, including sized glass fiber. In multicoat operations, the additive is used only in the first coat. With, polyester fabric, the isocyanate component in urethane formation can be used alone. Solvent or plasticizer solutions of oligomerized toluene diisocyanate (TDl) are available under the trade names Vulcabond (Akzo Nobel), Vanchem HM-50 (Vanderbilt), Thanecure T9 (TSE Industries), and Desmodur (Bayer). For improved resistance of the bond to heat and moisture, products based on bisphenyl diisocyanate (MDl) are also used. These include Vanchem HM-4346 and Rubinate 9480 (Huntsman). These are typically used at 3-7 phr levels in plastisols for coating polyester fabric. The second or subsequent coats should contain titanium dioxide and UV light stabilizer for outdoor service. With translucent products, an aliphatic isocyanate such as Desmodur N-lOO or TMl META (Cytec) should be used. 

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. 

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 ... 

For bonding molded parts of polycarbonate to other plastics, glass, wood, aluminum, brass, steel, and other materials, a wide variety of adhesives can be used. Generally, the best results are obtained with solventless materials, such as epoxies and urethanes. 

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