Redispersible adhesion

Latexes are usually copolymer systems of two or more monomers, and their total solids content, including polymers, emulsifiers, stabilizers etc. is 40-50% by mass. Most commercially available polymer latexes are based on elastomeric and thermoplastic polymers which form continuous polymer films when dried [88]. The major types of latexes include styrene-butadiene rubber (SBR), ethylene vinyl acetate (EVA), polyacrylic ester (PAE) and epoxy resin (EP) which are available both as emulsions and redispersible powders. They are widely used for bridge deck overlays and patching, as adhesives, and integral waterproofers. A brief description of the main types in current use is as follows [87]. 

In contrast, most of the conventional foam separation techniques use large bubbles, requiring relatively high gas flow rates to generate sufficient interfacial area for adhesion of solid particles to bubbles. This causes turbulence at the foam/liquid boundary and, in order to prevent redispersion of floated particles, a rather tall foam column is required (ref. 36). 

Extraction, purification, and bleaching (with hydrogen peroxide) leave hemicellulose fibers softened, swollen, and with greatly enhanced amor-phism and adhesiveness, but with water retentivity (Gould et al., 1990) inferior to polysaccharides, although still quite high. The stability of hemicellulose dispersions is maximum at neutral pH. Dried fibers are difficult to redisperse. 

Primer A compatible coating designed to enhance adhesion. Redispersible powder Polymers typically produced by spray-drying isolation of the specially designed latex such as those of acrylics, vinyl acetate, etc. Depending on the nature of the resin type, a suitable amount of inorganic filler is added to the powders to prevent caking. 

The problems that arise when a dmg is dispersed in a liquid include sedimentation, caking (leading to difficulty in resuspension), flocculation and particle growth (through dissolution and recrystallisation). In practice we wish to avoid the problems of aggregation of particles in suspensions and in many lyophilised preparations and to ensure their efficient redispersion on reconstitution with water or other media. Adhesion of suspension particles to container walls has also been identified as a problem, particularly with low-dose drugs. 

Table 5.4 Ceramic Tile Adhesive Using Redispersible EVA Powder. ( ...

Similar to latex-modified systems, the properties of redispersible polymer powder-modified systems are improved in comparison with ordinary cement mortar and concrete, and these depend on the nature of polymer and polymer-cement ratio. Figs. 5.3 to 5.5i l represent the strengths, adhesion to cement mortar, water resistance, and water absorption of the redispersible polymer powder-modified mortars. The properties are improved with an increase in the polymer-cement ratio. This tendency is very similar to that of the latex-modified systems. In general, the redispersible polymer powder-modified mortars are inferior to SBR-modified mortar (control) in certain properties. VAA eoVa powder-modified mortars show tetter properties than EVA powder-modified mortars as seen in Fig. 5.5. The film formation characteristics of recent redispersible polymer powders for cement modifiers are improved, and continuous polymer films can be found in the redispersible polymer powder-modified systems as seen in Fig. 5.6. This contributes greatly to improvements in their properties. 

Figure 5.4 Water resistance of redispersible polymer powder-modified mortars, evaluated by adhesion change with water immersion.

Figure 5.8 Comparison of adhesion of latex- and redispersible EVA powder-modified mortars. ( 1993, ASTM, reprinted with permission.)...

Schneider, et examined the effect of glass transition temperature (Tg) on the adhesion (to plywood) and flexibility of redispersible EVA powder-modified mortars for plywood substrates, and found that EVA powder with a glass transition temperature of -10°C is more effective than one with a glass transition temperature of 10°C. 

Redispersible polymer powders are widely used to produce prepackaged-type products such as decorative wall coatings, ceramic tile adhesives, self-leveling floor overlays, and patching mortars for concrete structures. 

Yamamoto, T. Osaki, K. Wakasugi, M. Sugiura, A. Sakakibara, H. Adhesive Strength of Modified Cement Mortar with Redispersible Polymer Powders. IX. ICPIC 14. - 18. September Bologna (Italien) 1998 S. 255-261... 

Waterborne dispersed polymers include both synthetic polymer dispersions and natural rubber. Synthetic polymer dispersions are produced by emulsion polymerization. A substantial part of the synthetic polymer dispersions is commercialized as dry products these include SBR for tires, nitrile rubbers, about 10% of the total PVC production, 75% of the total ABS and redispersable powders for construction materials. Carboxylated styrene-butadiene copolymers, acrylic and styrene-acrylic latexes and vinyl acetate homopolymer and copolymers are the main polymer classes commercialized as dispersions. The main markets for these dispersions are paints and coatings, paper coating, adhesives and carpet backing. 

The Fantesk composite powders were reconstituted into dry-film lubricants by redispersing them in aqueous sucrose solutions. Sucrose was obtained from Fleming Companies, Inc. (Oklahoma City, OK) or purchased from a local supermarket (Domino, Extra Fine Granulated 100% Pure Cane Sugar, Domino Foods Inc., Yonkers, NY). Sucrose regulates the drying of the films on the metal and also improves the adhesion of the composite film to the metal surface. It was demonstrated that the concentration of suCTOse had only a very minor effect on COF [35]. The dry-film-lubricant formulations were then applied onto precleaned stainless steel fiat sheets using doctor-blade or spray application methods as described below. 

According to Minfs s theory, two different processes take place in each functioning layer of the sand filter (1) adhesion of sediment and colloidal particles onto filter grains and previously entrapped imparities (2) detachment of previously adhered particles by hydrodynamic forces and their redispersion into filtered water. Mathematically, these phenomena are described as the following ... 

The main markets for the dispersions are paints and coatings (26%), paper coating (23%), adhesives (22%), and carpet backing (11%) [2], They are also used in such niche applications as diagnosis, drug delivery, and treatment [3], The main dry products are styrene-butadiene rubber for tires, nitrile rubber, about 10% of the PVC production, acrylonitrile butadiene styrene (ABS), and redispersable powders for construction materials. 

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. 

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. 

Depending on the intended field of use, cement-based tile adhesives are modified with different amounts of redispersible synthetic resin powder such as co- or terpol-ymers based on vinyl acetate or acrylates. The recipe must be adjusted to the different types of tiles and natural stones. On account of the various types of tile adhesives corresponding standards may be found, for example in DIN, BSI, AFNOR (Association... 

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