Polymeric Particles adhesion

FIGURE 16.14 Adhesion force of polymeric particle on poly-Si as a function of oxidizer concentration. 

Aqueous dispersion of polymeric particles, a polymer emulsion, a product of emulsion polymerization used in paints, adhesives, coatings, etc. 

Values calculated for the force of adhesion of spherical polymeric particles to a steel surface, with allowance for deformation of the surfaces in accordance with Eq. (11.75), have been compared with experimental data [66] ... 

The adhesion of suspended particles to vessel walls likewise depends on the temperature of the liquid medium. This relationship, in the case of polymeric particles forming a suspension with a concentration of 15%, was found to be as follows [191] ... 

The adhesion of a layer of polymeric particles to a glass surface was found in [187] to vary along with the wetting angle in the following manner ... 

Fuchs kinetic Eq. (5.29) is commonly related to the initial aggregation of interacting particles even though it actually refers to particle collisions. Yet, one usually assumes that any collision coincides with adhesion and, thus, aggregation, because for most particle systems the short range interactions are dominated by the attractive van-der-Waals forces. However, in the presence of short-ranging repulsive forces, e.g., due to hydration interactions or polymeric layers, adhesion is not an inevitable result of particle contact. In this case, a suspension can appear stable... 

Moreover, intumescent char adhesion to the surface of the burning polymer prevents molten inflamed polymeric particles from dripping, thus avoiding a source of fire propagation which is typical of some materials. 

Latex dispersions have attracted a great deal of interest as model colloid systems in addition to their industrial relevance in paints and adhesives. A latex dispersion is a colloidal sol formed by polymeric particles. They are easy to prepare by emulsion polymerization, and the result is a nearly monodisperse suspension of colloidal spheres. These particles usually comprise poly(methyl methacrylate) or poly(styrene) (Table 2.1). They can be modified in a controlled manner to produce charge-stabilized colloids or by grafting polymer chains on to the particles to create a sterically stabilized dispersion. Charge-stabiHzed latex particles obviously interact through Coulombic forces. However, sterically stabilized systems can effectively behave as hard spheres (Section 1.2). Despite its simpHcity, the hard sphere model is found to work surprisingly well for sterically stabilized latexes. 

Uses Film-former, thickener, protective colloid, stabilizer, vise, modifier, suspending agent, and dispersant used for emulsion and suspension polymerizations, cosmetics, adhesives, sealants, paints, coatings, paper, detergents, glass fibers, inks, ceramics, nonphannaceutical tableting, photographic films hair fixative Properties Powd. or liq. > 99% > 250 pm particle size (powd.) sol. in alcohols, esters, ether alcohols, ketones, lactams, chlorinated hydrocarbons, nitroparaffins, acids, amines bulk dens. 0.4-0.5 g/cc (powd.) vise. 5.4 cps (5% in methanol) pH 3-7 (10%, po vd.), 7-10 (10%, sol n.) > 92% solids (powd.), 50% solids in water (sol n.)... 

Uses Emulsion polymerization surfactant imparts good stability and uniform particle size to emulsion polymers based on vinyl chloride, vinyiidene chloride, styrene, butadiene, and acrylic esters surfactant for fruit coatings, polymeric coatings, adhesives, paperboard, rubber goods food-pkg. adhesives, coatings, paper, cellophane, rubber articles, textiles, animal glue defoamer in food-contact paper/paperboard Features Low cloud pt. 

Although the principles of surface thermodynamics, the interfacial Iree energies, and the work of adhesion play an important role in models of particles adhesion to polymeric substrates the thermodynamic quantities associated with dividing lines between differing phases—the edge energy or line tension—have not been considered in these adhesion models. 

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). 

Suspension Polymerization. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

It may also be possible to crosslink the acrylic PSA with the help of multifunctional acrylates or methacrylates [87], These monomers can simply be copolymerized with the balance of the other monomers to form a covalently crosslinked network in one step. Since the resulting polymer is no longer soluble, this typ)e of crosslinking is typically limited to bulk reactions carried out as an adhesive coating directly on the article or in emulsion polymerizations where the crosslinked particles can be dried to a PSA film. 

Fumed silicas (Si02). Fumed silicas are common fillers in polychloroprene [40], natural rubber and styrene-butadiene rubber base adhesives. Fumed silicas are widely used as filler in several polymeric systems to which it confers thixotropy, sag resistance, particle suspension, reinforcement, gloss reduction and flow enhancement. Fumed silica is obtained by gas reaction between metallic silicon and dry HCl to rend silica tetrachloride (SiCU). SiC is mixed with hydrogen and air in a burner (1800°C) where fumed silica is formed ... 

Moreover, the interaction of the surface of the fillter with the matrix is usually a procedure much more complicated than a simple mechanical effect. The presence of a filler actually restricts the segmental and molecular mobility of the polymeric matrix, as adsorption-interaction in polymer surface-layers into filler-particles occurs. It is then obvious that, under these conditions, the quality of adhesion can hardly be quantified and a more thorough investigation is necessary. 

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