Polyurethane Dispersion Adhesives

In contrast to solvent-based systems, the PUR dispersions (Section 5.4) are characterized by their incombustibility and thus by considerably smoother processing. They are high molecular weight hydroxyl polyurethanes disperged in water. Apart from the physically setting one-component systems, two-component systems are also applied, with component B containing special polyisocyanates that react with the -O-H-groups of the hydroxyl polyurethane in aqueous solution. 

The described polyurethane adhesives are applied depending on the materials to be bonded and the given application conditions in the different fields of industry, 

Special advantages of polyurethane adhesives are their excellent adhesion strength on many surfaces even on those otherwise difficult to bond, for example, flexible 

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BajdiydroL [Miles] Polyurethane dispersion adhesion promoters in metal/plas-tic composites, textile and leather coatings, primers for rigid surface caodngs. 

Polyurethane dispersions (PUD s) are usually high-performance adhesives based on crystalline, hydrophobic polyester polyols, such as hexamethylene adipate, and aliphatic diisocyanates, such as methylene bis(cyclohexyl isocyanate) (H12MDI) or isophorone diisocyanate (IPDI). These PUD s are at the more expensive end of the waterborne adhesive market but provide excellent performance. 

For dispersion adhesives, the most important polymers are polyvinyl acetate, acrylate, rubbers, polyurethane and polychloroprene. 

Sometimes plasticizer is deliberately added to a product to make it less sensitive to external plasticizer. This laminating adhesive is used in conjunction with PVC " Polyurethane dispersion 17.5 wt%... 

Water-based dispersions or emulsions such as polyvinyl acetate, acrylics, polyvinyl chloride and polyvinyl alcohol with plasticizers and tackifiers. In addition, this range can include urea formaldehyde and phenolic adhesives, resins, natural adhesives produced from starch, dextrin, casein, animal glues (see Polyvinyl alcohol in adhesives, Phenolic adhesives single-stage resoles. Phenolic adhesives two-stage novolacs. Animal glues and technical gelatins) and rubber latex (see Emulsion and dispersion adhesives). Solvent-free 100% solids such as polyurethane. Hot melt adhesives include Ethylene-vinyl acetate copolymers, polyolefins, polyamides, polyesters with tackifiers and waxes. More recent additions include cross-linkable systems. 

The general formula of an emulsified polyurethane or aqueous dispersion used for adhesive purposes is shown in Fig. 8.11. Aqueous polyurethane dispersions usually have 40% solids of a thermoplastic type (for aqueous inertness), aliphatic diisocyanate polymer (for light stability) and an anionic surfactant is used. However, if a polyurethane ionomer is prepared this does not contain added emulsifier, the polyurethane itself having its own emulsifier function. The incorporation of this function in the polyurethane chains as (a) sodium sulphonate or (b) sodium carboxylate groups is seen in Fig. 8.11. 

Polyurethane dispersions Excellent cohesive and adhesive properties. Good toughness and abrasion resistance. Fabric, elastomers, metal, plastic Medium to high... 

Solutions (5-25% solids) of fully reacted polyurethanes based on MDI or IPDI are also used in coating applications. These coatings are used mostly as finish coats on fiexible substrates. Because of concern over airborne solvents aqueous polyurethane dispersions are used today. These polymers are prepared by incorporating ionic groups into their backbone, and both cationic and anionic groups are utilized in the preparation of the ionomers. The solvent and water resistance of films formed from aqueous dispersions can be improved by cross-linking. Ionomers are also used in the formulation of speciality adhesives. 

Dispersion adhesive containing high molecular weight polyurethane with ionic groups dispersed in water. 

Bonding Soles to the Lasted Uppers. Polychloroprene and polyurethane solvent adhesives, occasionally also dispersions of the same polymers, are used for this purpose. Various pretreatments are used to improve the bond application of a thin first coat of adhesive, especially for absorbent surfaces priming, in particular the halogenation of vulcanized rubber and thermoplastic rubber soling the use of base-coat primers for nylon and other synthetics, as well as UV-curing primers for EVA and other substrates in athletic footwear. 

Aqueous dipersion adhesives are high-molecular-weight polyurethanes dispersed in water. They are applied and used in exactly the same manner as solvent-borne systems with the benefit of not using hazardous or flammable solvents. 

The preparation of polyurethane aqueous dispersions can be accomplished in several ways. The procedure claimed to yield at present the highest quality products involves the reaction of an isocyanate-terminated prepolymer in a water-miscible solvent, e.g., acetone, with a diamine carboxylate or sulfonate. The resultant polyurethane ionomer solution is mixed with water which forms the dispersion and further chain-extends the polymer. Finally, the organic solvent is removed by distillation and can be recycled. As in the case of the thermoplastic polyurethane elastomer adhesives, dispersions can also be prepared with different degrees of crystallinity in the polyurethanes depending on the nature of the prepolymer. 

One-component water-borne polyurethane systems can be derived from polyurethane dispersions or blocked polyisocyanates (refer to Section 3.2.1). Blocked isocyanates are added to the co-reactant resins providing one-component systems with excellent shelf life. This type of adhesives is principally used for non-porous materials and the bonding of unlike metals such as aluminium to steel, and stainless steel to mild steel. They are also usefiil in bonding some of the high pressure laminates such as those based on phenoUcs and melamine [39]. Systems based on water-borne blocked polyisocyanate crosslinkers and suitable waterborne polymers approach the performance levels previously obtained only by solvent-borne systems [16]. 

Water-based dispersion adhesives consist of oligomers dispersed in water with surfactants as dispersing agents and stabilizers. Before dispersions, natural rubber latex and casein were used to manufacture such adhesives. Currently, polyvinyl acetate (PVAc) and polyacrylate serve as synthetic base polymers for water-based PSA, while polychloroprene and thermoplastic polyurethane serve the same purpose for water-based contact adhesives. 

Shanghai Jiuyuan Petrochemical Co was founded in 1999. The company primarily produces polyurethane (PU adhesives and thermoplastic polyurethane and related polyurethane products). It also produces a range of dispersants, asphalts, additives, activated clays and hydrogen peroxide. 

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