Лако-красочные материалы — производство

Proper surface preparation is the key to obtaining good adhesive bonds having a predict­able service life. Substrate surfaces may have dirt, grease, mold-release agents, processing additives, plasticizers, protective oils, oxide scales, and other contaminants that will form a weak boundary layer. When the adhesive fails it is usually through this region, giving a low-strength bond. Some form of surface treatment is necessary to obtain optimum bond strength. The primary goal of surface treatment is to remove any weak surface boundary layer on the substrate [49]. A large number of surface treatments have been developed, with many targeted toward specific substrates. These would include mechanical abrasion, etching, solvent cleaning, detergent washing, flame treatments, chemical treatments, and corona discharges [19,50-55].

Primers are also used in conjunction with a surface treatment either to improve adhesive performance or to increase production flexibility in a bonding operation. Isocyanates have been used for over 50 years as primers on substrates such as rubber,

plastic, fibers, and wood [56]. Isocyanates will react with polar groups on the surface and promote bonding.

Silane coupling agents are commonly used as primers for glass, fiber composites, mineral-filled plastics, and cementacious surfaces. The silane coupling agents have been found to be especially effective with glass substrates. One end of the coupling agent is an alkoxysilane that condenses with the silanol groups on the glass surface. The other end of the coupling agent is an amino, mercapto, or epoxy functionality that will react with the isocyanate group in the adhesive. Epoxy silanes have also been used as additives to adhesives to improve water resistance [57]. Other organometallic primers are based on organotitanates, organozirconates, and some chromium complexes [49].

VIII. COMMON ADHESIVE TYPES

A. One-Component Adhesives

The oldest types of one-component polyurethane adhesives were based on di — or triiso­cyanates that cured by reacting with active hydrogens on the surface of the substrate or moisture present in the air or substrate. The moisture reacts with the isocyanate groups to form urea and biuret linkages, building molecular weight, strength, and adhesive proper­ties. Prepolymers are also used either as 100% solids or solvent-borne one-component adhesives. Moisture-cured adhesives are used today in rebonded foam, tire cord, furniture, and recreational vehicle applications.

A second type of one-component urethane adhesive comprises hydroxypolyurethane polymers based on the reaction products of MDI with linear polyester polyols and chain extenders. There are several commercial suppliers of these types of thermoplastic polyurethanes. The polymers are produced by maintaining the NCO/OH ratio at slightly less than 1:1 to limit molecular weight build to the range 50,000 to 200,000 with a slight hydroxy content (approximately 0.05 to 0.1%). These are typically formulated in solvents for applications to shoe soles or other substrates. After solvent evaporation heat is used to melt the polymer (typically 50 to 70° C; at these temperatures the polymers reach the soft, rubbery, amorphous state), so the shoe upper can be press fit to the sole. Upon cooling, the adhesive recrystallizes to give a strong, flexible bond [58]. More recently, polyisocyanates have been added to these to increase adhesion and other physical properties upon moisture curing. In Section IX. B we discuss this in more detail.

The use of waterborne polyurethane adhesives has grown in recent years as they have replaced solvent-based adhesives in a number of application areas. There are a number of papers and patents covering the use of waterborne polyurethanes in shoe soles, packa­ging laminates, textile laminates, and as an adhesive binder for the particleboard industry [59-62]. Because waterborne polyurethane adhesives have no VOC (volatile organic content) emissions and are nonflammable, they are more environmentally friendly. Typically they can be blended with other dispersions without problems and exhibit good mechanical strength. Water-based systems are fully reacted, linear polymers that are emulsified or dispersed in water. This is accomplished by building hydrophilicity into the polymer backbone with either cationic or anionic groups or long hydrophilic polyol segments or, less frequently, through the use of external emulsifiers. Figure 25 illustrates the more common functional groups that can be built into the urethane mole­cule that will confer hydrophilicity.

A typical example of how these groups are built into the polymer backbone is shown in Fig. 26. A urethane prepolymer is reacted with chain extenders containing either car — boxylates or sulfonates in a water-miscible solvent (e. g., acetone). The reaction product is an isocyanate-terminated polyurethane or polyurea with pendant carboxylate or sulfonate groups. These groups can easily be converted to salts, which as water is added to the prepolymer-solvent solution, allows the prepolymer to be dispersed in water. The solvent is then stripped, leaving the dispersed product. There are variations on this theme that allow lower solvent volumes to be used [63]. Long hydrophilic polyol segments can also be introduced. Chain extenders with hydrophilic ethylene oxide groups pendant to the back­bone are reacted with the prepolymer to form a nonionic self-emulsifying polyurethane. This reaction is also carried out in a water-miscible solvent that can later be stripped from the solvent-water solution.

Blocked isocyanates can also be considered a one-component adhesive. The use of a blocking agent allows the isocyanate to be used in a reative medium that can be heat activated. One-component adhesives based on blocked isocyanates are thus not amenable to room-temperature curing applications. The chemistry of these products is covered in more detail in Section VI. A.