Emulsion and dispersion adhesives

Water-based adhesives also encompass emulsion and dispersion adhesives in which polymer chains are dispersed and stabilized into tiny particles or spheres in aqueous medium with the aid of surfactants. Within this category there are low-cost poly(vinyl acetate) and acrylics, and more expensive higher-performance urethanes, which are typically made of two components. 

Polymer dispersions (in non-solvent liquids, usually water) have much lower viscosities than solutions with the same polymer content. So great is the disparity in viscosities that a 50% solids dispersion may have a considerably lower viscosity than a 10% solution. Such dispersion or suspension adhesives, often called emulsion adhesives or even more loosely water-based adhesives , combine the advantages of a high solids content with low inflammability and toxicity hazard, but suffer from long drying times (see Emulsion and dispersion adhesives). Their use on non-porous substrates is restricted in a similar way to that of solution adhesives. 

Further details appear in other articles. Emulsion and dispersion adhesives. Solvent-based adhesives, Adhesive classification. Hot melt adhesives, Rubber-hased adhesives. 

Following a period of sustained growth after their introduction many years ago, the use of emulsion and dispersion adhesives is now widespread in many industrial and commercial operations. This situation is considered very likely to continue owing to the many advantages offered by these products when compared to other types of adhesive, not least in terms of enviromnental considerations. 

Having dealt with the basic chemistry of emulsion and dispersion adhesives, the method by which they function should now be considered. 

The compositions in this article are expressed as percentages by weight.) Bulk polymerization is widely used for polymers of 40-50% VA, and an emulsion process for those over 60% VA. In the intermediate range (15-60%), solution polymerization may be used. Both low (10-40%) and high (60-95%) VA materials have adhesive applications, the former as Hot melt adhesives, the latter as emulsion adhesives (see Emulsion and dispersion adhesives). The term vinyl acetate-ethylene is sometimes used to describe the copolymers rich in VA. 

There are three basic categories of adhesive bonding used, each requiring specific equipment. Selection of a particular technique depends on the nature of the substrates used and final application. There are a considerable number of adhesive laminating materials and techniques or variants upon the theme, but it can be reduced to the three basic types of material, aqueous based (see Emulsion and dispersion adhesives), Soivent-based adhesives, solventless 100% solids or Hot melt adhesives, and to the two basic techniques, wet and dry lamination. 

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. 

Emulsion and dispersion adhesives harden by evaporation of the dispersion medium, which is usually water with porous substrates, hardening is again aided by capillary action. 

Generally, these adhesives are used where at least one of the substrates is porous or permeable to moisture vapour. The two substrates to be joined are laminated while the adhesive is still wet, and the water escapes either by initial absorption into one of the substrates or by evaporation due to permeability. See also Emulsion and dispersion adhesives. 

Emulsion and dispersion adhesives G C PARKER Polyvinyl acetate and copolymers, setting, advantages and disadvantages... 

Starch, often in its degraded form dextrin, and starch esters have long found application in general emulsion and dispersion technology, mainly in adhesives, construction materials or in pharmaceuticals for example, as aqueous coating material for controlled-release products [106]. 

Octtdite. [Tiarco] Antioxidant emulsions and dispersions for latex conq>ds., rubber, natural and synthetic adhesives, foams. 

Probably the most widely used industrial emulsion or dispersion adhesives are those based on poly(vinyl acetate), commonly referred to as PVA. These product are normally manufactured by emulsion polymerization whereby, basically, vinyl acetate monomer is emulsified in water with a suitable colloid-emulsifier system, such as poly(vinyl alcohol) and sodium lauryl sulfate, and, with the use of water soluble initiator such as potassium persulfate, is polymerized. The polymerization takes place over a period of four hours at 70°C. Because the reaction is exothermic, provisions must be made for cooling when the batch size exceeds a few liters. The presence of surfactants (emulsifiers) and water-soluble protective colloids facilitates the process resulting in a stable dispersion of discrete polymer particles in the aqueous phase. 

It has been reported in the literature that amphiphilic diblock copolymers can be synthesized by anionic polymerization by incorporating a new approach called oxyanion-initiated polymerization to combine tertiary amine methacrylates, such as 2-(N-morphoIino)ethyI methacrylate (MEMA), t-(butylamino)ethyl methacrylate (BAEMA), 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA), and 2-(N,N-diethylamino)ethyl methacrylate (DEAEMA). " " Such materials have been considered for various applications as adhesives, emulsion and dispersion stabilizers, wastewater treatment formulations, and biomaterials. 

Lubrication Adhesion Foams Wetting and waterproofing Control of rheological properties Emulsions Emulsion and dispersion polymerization Drilling muds Electrophoretic deposition... 

Pseudoplastic Refer to Fig. 3A and B. The most common pseudoplastic types are some water-based paints, adhesives, emulsions and dispersions. When you mix the material, it becomes more fluid in a way similar to yoghurt. 

Although they are a relatively small volume product—approximately 75,000 tons produced in 1949 (126)—interest in asphalt emulsion has continued at a high level. Abraham (6) has reviewed the patent literature relative to the types of emulsifying agents used, while commercial practice has been discussed by Day (16). The most common emulsifiers are sodium or potassium soaps of tall oil, abietic acid, or Vinsol resin, or colloidal clays such as bentonite for adhesive base emulsions. Lyttleton and Traxler (53) studied the flow properties of asphalt emulsions, and Traxler (122) has investigated the effect of size distribution of the dispersed particles on emulsion viscosity. A decrease in particle size uniformity was found to be accompanied by a decrease in consistency because particles of various size assume a more loosely packed condition than do those of the same size. 

Because of the widespread applications of surface chemistry, practically all industries, knowingly or otherwise, make use of the principles of surface chemistry. Countless cosmetic and pharmaceutical products are emulsions—lotions, creams, ointments, suppositories, etc. Food emulsions include milk, margarine, salad dressings and sauces. Adhesive emulsions, emulsion paints, self-polishing waxes, waterless hand cleaners and emulsifiable insecticide concentrates are commonplace examples of emulsions, which fall within the province of surface chemistry. Other products winch function in accordance with the principles of smface chemistry include detergents of every variety, fabric softeners, antistatic agents, mold releases, dispersants and flocculants. 

---