Redispersions powders

Of the several types of the polymer-modified mortars and concretes used for various construction applications, latex-modified mortar and concrete are by far the most widely used materials. Latex-modified mortar and concrete are prepared by mixing a latex, either in a dispersed liquid or as a redispersible powder form with fresh cement mortar and concrete mixtures. The polymers are usually added to the mixing water just as other chemical admixtures, at a dosage of 5-20% by weight of cement. Polymer latexes are stable dispersions of very small (0.05-5 pm in diameter) polymer particles in water and are produced by emulsion polymerization. Natural rubber latex and epoxy latex are exceptions in that the former is tapped from rubber trees and the latter is produced by emulsifying an epoxy resin in water by the use of surfactants [87]. 

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]. 

These polymers are thermoplastic materials usually supplied as redispersible powders. They are widely used in proprietary prepackaged products such as tile grouts, self-leveling underlay and patching materials. 

Have a low air-entraining capability during mixing when a suitable antifoaming agent is used in the emulsion or redispersible powder. 

The polymers are usually applied either from aqueous or from organic solvent. More recently, aqueous dispersions or redispersible powders of the polymers have become available that ensure economical, fast, and environmentally safe processing of the film coatings. In some cases it might be necessary to prevent an interaction between the acidic functional groups and the drug. In these cases a subcoat (e.g., of hydroxypropylmethylcellulose [HPMC]) is recommended. 

K. Lehmann, Acrylic latices from redispersible powders for peroral and transdermal drug formulations, Drug Dev Ind Pharm 72 265-287 (1986). 

FIGURE 4.18 The hydrodynamic particle radii distribution of initial Sn02 sols and redispersed powder. 

Table 5.1 outlines useful properties, products and application areas of acrylic systems. Key market segments are coatings based on methacrylates as well as emulsions and water-based sealants. Acrylic emulsions and redispersible powders as cement modifiers offer many useful products for several end-uses. However, the high cost of acrylic has limited their use as cement modifiers. The use of water-based acrylic systems is increasing because of their non-toxic and environmentally friendly nature. 

These are redispersible powders of 100% acrylic composition and are readily redispersible in water. They are manufactured by spray drying of the appropriate latex. Addition of some inorganic fillers prevents the caking of these powders. 

The physical properties of acrylic redispersible powders are given in Table 5.8. Tsai et al. (1993) have reported that the particle size distribution of... 

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. 

Eudragit L 100-55 is an alternative to Eudragit L 30 D-55. It is commercially available as a redispersible powder. 

Acryl-EZE and Acryl-EZE MP are also commercially available as redispersible powder forms, which are designed for enteric coating of tablets and beads, respectively. 

The first dispersion marketed under the tradename Aquacoat by FMC (Princeton, NJ) is based on lOmPas ethylcellulose. A similar product is under development by Dow Chemical (Midland, MI)/Coiorcon (Orpington, U.K.). A cellulose acetate dispersion is presented under experimentation by FMC. Finally, FMC has commercialized a CAP redispersible powder for enteric coating (CAP hydrolyses slowly in an aqueous solution) as Aquateric (see Sect 5). 

The investigations were carried out using 2 polymers (Pol5mier 1 - redispersible powder with a film formation temperature of 5 °C, Polymer 2 - dispersion with a film formation temperature of 30 °C). Both based on styren-acrylic ester copolymer. 

Justnes, H. Reynaers, R. Van Zundert, W. The influence ofLatices and Redispersible Powders on Hydratation and Strength Development of Polymer Cement Mortars. IX. ICPIC, 14. -18. September, Bologna (Italien), 1998, S. 225-238... 

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. 

Poly(vinyl chloride) (10% of the total production) Redispersable powders (VAc copolymers, SB)... 

ABS, an elastomer modified thermoplastic used in electrical and electronic equipment, house and office appliances and in the automotive industry) and redispersable powders for construction materials. Natural rubber accounted for 6.5 million tonnes per year, including about 1 million tonnes that is commercialized as a waterborne dispersion. 

In sols where the particles are greater than S nm in diameter, and are relatively uniform with a normal distribution of sizes about an average value, determination of th.e specific surface area provides a rapid and dependable method for estimating average size. If, on the other hand, the sols contain particles varying widely In size, such as is often the case with sols made from pyrogenic silicas or other redispersed powders, the results may be misleading. 

Galenical polymer Methyl methacrylate plus (meth)acrylic acid Core methyl methacrylate Shell (meth)acrylic acid Redispersable Powder... 

Mortar Vinyl acetate plus various comonomers Poly(vinyl alcohol) as protective colloid Redispersable powder... 

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. 

The elasticity of the set films can be increased by plasticizers and by copolymerization. Redispersion powders, that is, spray-dried synthetic resin emulsions, are added to synthetic resin emulsions to increase their solids content and their setting rate. 

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. 

Lutz, H. and Hahner, C. (2002) Applications of redispersible powders. In D. Urban and K. Takamura (eds). Polymer Dispersions and Their Industrial Applications. WUey-VCH, Weinheim. 

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