Surface preparation emulsion adhesives

Pa.ints, Paints (qv) prepared from poly(vinyl acetate) and its copolymers form flexible, durable films with good adhesion to clean surfaces, including wood, plaster, concrete, stone, brick, cinder blocks, asbestos board, asphalt, tar paper, wahboards, aluminum, and galvani2ed iron (147). Adherence is also good on painted surfaces if the surfaces are free from dirt, grease, and mst. Developments in emulsion polymeri2ation for paint latices have been reviewed (148). 

The adsorption of surfactants at the liquid/air interface, which results in surface tension reduction, is important for many applications in industry such as wetting, spraying, impaction, and adhesion of droplets. Adsorption at the liquid/liquid interface is important in emulsification and subsequent stabilization of the emulsion. Adsorption at the solid/liquid interface is important in wetting phenomena, preparation of solid/liquid dispersions, and stabilization of suspensions. Below a brief description of the various adsorption phenomena is given. 

Surfactants find apphcation in almost all disperse systems that are utilised in areas such as paints, dyestulfs, cosmetics, pharmaceuticals, agrochemicals, fibres, and plastics. Therefore, a fundamental understanding of the physical chemistry of surface-active agents, their unusual properties, and their phase behaviour is essential for most formulation chemists. In addition, an understanding of the basic phenomena involved in the application of surfactants, such as in the preparation of emulsions and suspensions and their subsequent stabilisation, in microemulsions, in wetting, spreading and adhesion, is vitally important to arrive at the correct composition and control of the system involved [1, 2]. This is particularly the case with many formulations in the chemical industry mentioned above. 

Two other classes of silicones deserve mention. These are the water-based silicones that are used in sealant and coating applications and the silicone pressure-sensitive adhesives. Water-based silicones can be prepared by anionic polymerization of siloxanes in water using a surface-active catalyst such as dodecylbenzenesulfonic acid [4]. The resulting emulsion can then be cross-linked in several ways, including the use of alkoxysilane copolymerization or tin catalysts in conjunction with colloidal silica. The result is essentially an emulsion of cured PDMS in water. Various fillers and other components are added, resulting in a sealant composition. Upon evaporation of water. 

A very common copolymer of vinyl chloride is with vinyl acetate. Copolymerization with vinyl acetate improves stability and molding characteristics. The copolymers are also used as fibers and as coatings. Copolymers intended for use in moldings are usually prepared by suspension polymerization. Those intended for coating purposes are prepared by solution, emulsion, and suspension polymerizations. The copolymers used in molding typically contain about 10% poly(vinyl acetate). Copolymers that are prepared for coating purposes can contain from 10-17% poly(vinyl acetate). For coatings, a third comonomer may be included in some resins. This third component may, for instance, be maleic anhydride, in small quantities, like 1%, to improve adhesion to surfaces. 

Emulsions bond most materials which absorb water. Non-absorbent adherends may be bonded if the other surface provides an escape route for water. Thus, some adhesion can be obtained on most metal and plastics surfaces, though care should be taken to ensure that metals are adequately painted to prevent corrosion. Without special preparation, polyolefin plastics, other low surface-energy types such as PTFE and the silicone rubbers cannot be bonded. 

It is common to add a few percent of an alkyd-surfactant blend to latex paints to improve adhesion to chalky surfaces and, in some cases, to improve adhesion to metals. It is important to use alkyds that are as resistant as possible to hydrolysis. Hybrid alkyd-acrylic latices have been prepared by dissolving an oxidizing alkyd in the monomers used in emulsion polymerization, yielding a latex with an alkyd grafted on the acrylic polymer (28,29). Nonyellowing waterborne alkyds based on rosin-fatty acid modified acrylic latices have been reported (30). 

Several interfacial aspects must be considered when dealing with agrochemical formulations (i) Both equilibrium and dynamic aspects of adsorption of surfactants at the air/liquid interface. These aspects determine spray formation (spray droplet spectrum), impaction and adhesion of droplets on leaf surfaces as well as the various wetting and spreading phenomena, (ii) Adsorption of surfactants at the oil/water interface which determines emulsion formation and their stability. This subject is also important when dealing with microemulsions, (ill) Adsorption of surfactants and polymers at the solid/liquid interface. This is important when dealing with dispersion of agrochemical powders in liquids, preparation of suspension concentrates and their stabilization. 

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