Emulsion-based adhesives

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Solvent- and Emulsion-Based Adhesives Solvent-Based Adhesives. Solvent-based adhesives, as the name implies, are materials that are formed by solution of a high molecular weight polymer in an appropriate solvent. Solvent-based adhesives are usually elastomer-based and formulated in a manner similar to pressure-sensitive adhesives. 

Emulsion Adhesives The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)-poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vmyl acetate). This is also known as white glue. ... 

Hydroxyalkyl lignin derivatives were crosslinked with diisocyanates or with melamine in both solvent and aqueous emulsion-based adhesive formulations for wood products (51) (Figure 7). Adhesive performance was found to be related to component solubility and compatibility (51), The use of kraft lignin in aqueous alkali and that of lignin sulfonates in water has been explored in combination with emulsifiable diisocyanates (52) in wood adhesives. Satisfactory strength properties were reported. 

After introduction of emulsion-based adhesives, and later hot melts, animal glues were displaced in part or completely from their traditional uses, but they have retained a relatively small but significant share of the adhesive market due to their unique properties. It is estimated that U.S. consumption of animal glues is in the range 30 to 35 million pounds, and production worldwide is about 90 million pounds annually (W.E. Blair, Swift Adhesives, private survey, 1992). 

Uses Defoamer for emulsion paints, emulsion-based adhesives Properties Amber opaque liq. dens. 0.88-0.92 g/ml (20 C) vise. 500-1,400 mPa-s-100% act. 0.0-1.0% water Toxicology TSCA listed... 

Solvent-based adhesives The adhesive is a polymer solution, and again its low viscosity assists wetting of the surface. As the solvent evaporates, a tacky adhesive layer is left behind. Emulsion-based adhesives are similar to these but here droplets of adhesive stabilised by soap molecules are dispersed in water. A considerable disadvantage of solvent-based adhesives is the toxicity and flammability of many organic solvents, but to replace them by water is not a simple matter as the high heat of evaporation leads to slow solvent removal. 

Minimum film-forming temperature the temperature at which an emulsion-based adhesive just no longer dries to form a clear, homogeneous film. 

Standard contact adhesives and pressure-sensitive adhesives are made from SBR in solution. Styrene-butadiene rubber latices, in some cases containing carboxyl groups, are used for special emulsion-based adhesives. Thermoplastic elastomeric block copolymers of styrene with butadiene or isoprene are of increasing significance in contact adhesives and hot-melt adhesives. 

Poly(vlnyl esters) are one of the most important groups of raw materials for adhesives. Poly(vinyl acetates) are important in solvent adhesives. Synthetic resin emulsions, based on poly(vinyl esters) with a solids content of 50-70% are the principal raw materials for emulsion-based adhesives. 

Poly(vinyl acetate) emulsions for adhesives contain preferably poly(vinyl alcohol) as a protective colloid. Spray-dried poly(vinyl acetate) emulsions, known as redispersion powders, are used in combination with cellulose ethers as binders in floor-leveling compositions and for increasing solids content in emulsion-based adhesives. Polyfvinyl acetates) are used as solid polymers for formulating solvent adhesives. 

Poly(vlnyl alcohol) [9002-89-5] is used primarily as a protective colloid for poly(vinyI acetate) emulsions. Relatively small quantities are added to modify emulsion-based adhesives. Aqueous solutions of poly(vinyl alcohol), in some cases combined with fillers and emulsions, are used as paper glues and as labeling adhesives. 

Solvent adhesives and reactive adhesives are made from homo- and copolymers of methacrylates, generally methyl and ethyl methacrylate and, occasionally, butyl methacrylate. Monomeric (meth)acrylates are also used in reactive adhesive systems (polymerization adhesives). Poly(ethyIene glycol) dimethacrylates are the basis of anaerobically curing liquid resins (reactive adhesives). They also are added as adhesion promoters to plastisol adhesives. Acrylate-ethylene copolymers, in some cases with a small content of carboxyl groups, are used instead of ethylene-vinyl acetate copolymers as fusible polymers for special hot-melt adhesives. Salts of polyacrylate and acrylate - acrylic acid copolymers are used as thickeners for aqueous adhesive solutions and emulsion-based adhesives. 

Nonreactive resins with a variety of compositions are of considerable importance in the formulation of adhesives. In some cases, they are used on their own in solvent adhesives, so-called resin adhesives, but generally in combinations with polymers, in which they perform various functions. Above all, they increase tack, improve adhesion, influence viscosity, fluidity, and scalability, and, in some cases, also act as plasticizers. The most important applications for nonreactive resins are in pressure-sensitive adhesives, contact adhesives, hot-melt adhesives, solvent adhesives, and emulsion-based adhesives. In this context, the term resin covers materials differing very widely in their composition ... 

Plasticizers are used in adhesive formulations, above all for elastification of the polymer films in emulsion-based adhesives and also for increasing wet tack. They are used, e.g., in homopolymer emulsion-based adhesives. Phthalate plasticizers are the most widely used, diisobutyl phthalate being especially important for emulsion-based adhesives. In addition to phthalate and adipate plasticizers epo)q and phenol sulfonic amide plasticizers are used in PVC plastisols. Besides ester plasticizers, mineral oils and also chlorinated low molecular mass polyethylenes and low molecular mass hydrocarbon resins are used in rubber adhesives. 

Emulsion-based adhesives are adhesives that contain aqueous synthetic resin dispersions or emulsions or rubber latices as their principal raw material or active binder [41]. They are milky white, odorless hquids with solids contents of 40 - 60 %. In quantitative terms, emulsion-based adhesives are one of the most important groups of adhesives. They set by the release of water and the formation of a film fi om the emulsified or dispersed resin or rubber droplets. 

Case Making. In case making, when the covering material consists of textiles, plastics film, or paper, as in the backlining of books, hot animal glues are used because they set rapidly by gelatinization and rapidly bond the narrow overlap of the coating material around the board, even under the brief contact pressure normally applied in the machine. In special cases, also emulsion-based adhesives are also used. 

Bags, Sacks, Pouches, and Boxes. Starch-based adhesives and also emulsions of poly(vinyl acetate) homopolymers and copolymers or mixtures thereof are used in the manufacture of bags, pouches, and carrier bags, depending upon the type of materials (e.g., paper, plastic films, etc.), surface treatment, and the speed of the machine. Machine design factors necessitate the use of pastelike starch-based or emulsion-based adhesives for the bottom gluing of block-bottom or cross-bottom bags. 

Considerable recovery forces occur in the closing of prefabricated boxes, particularly in seaming machines, so that hot-melt adhesives often have to be used in such cases. Emulsion-based adhesives are normally used for the dust-tight sealing of cases. Slide boxes, deep-freeze packs, etc., can be line- or spot-sealed with hot-melt adhesives. 

Cartons are sealed with emulsion-based or hot-melt adhesives, depending on the length of the pressure belt and the quality of the cardboard. On modern, fast-running machines with up to 120 cases to be closed per minute, hot-melt adhesives are preferably used, due to their fast setting. An interesting combination is the simultaneous use of emulsion-based and hot-melt adhesives. In this case, a spot appfication of hot-melt adhesive performs the function of the pressure unit or compression section, whereas the actual bonding function is performed by emulsion-based adhesives, generally applied in line form. 

Bonding of Cellophane. Cellulose hydrate film is used for bag manufacturing or for wrapping operations. Because it readily takes up moisture in its unlacquered state, aqueous emulsion-based adhesives are used primarily for bonding. The addition of glycerol ensures they release their water only slowly so that the film stays flat. Lacquered cellophane is either bonded or sealed. The adhesives do not require any additives to keep the film flat because the film is protected by the lacquer. The adhesives are normally based on polyfvinyl acetate) copolymers or other flexible polymers. Where solvent-based or hot-melt adhesives are used, both types of film may be bonded with the same adhesive, because the absence of water eliminates the problem of keeping the film flat. 

Wallpapers include those with a PVC or metal surface and rough-textured plain wallpapers. They are applied with pastes of pre-gelatinized starch, starch ethers, carboxymethyl or methyl cellulose in the form of 2-10% solutions. For rough-textured plain wallpapers and heavy wallpapers, moisture resistance and tack are increased by utilizing poly(vinyl acetate) redispersion powders. High solid, emulsion-based adhesives, optionally in conjunction with resin-enhanced pastes (special purpose pastes), are used preferably for heavy vinyl- and metal-faced wallpapers. 

Light textile fabric wallcoverings are hung with special fabric wallcovering adhesives. Special-purpose pastes containing 10-20% of emulsion-based adhesives also are used. 

In the case of emulsion-based adhesives, the loss of a relatively small amount of water will cause the inversion of the emulsion and rapid setting of the adhesive. This contrasts favorably to solution-based adhesives, both water and organic solvent types, which require that most of their solvent be lost to the atmosphere and/ or absorbed into the substrate before they become tacky enough to hold the substrate together. In fact, even more solvent must be lost... 

When discussing adhesion of rod-applied coatings, we must consider that this coating process is normaliy used to coat solution or iow viscosity emulsion-based adhesives for pressure sensitive applications such as for paper-based labelstocks. To avoid the issue of contouring from wire rod coating, the process is more specifi-... 

Use Level 0.1-0.4% on finished paint 1-2% in emulsion-based adhesives Toxicology TSCA listed... 

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