Redispersible polymer 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. 

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. 

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 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. 

Butyl acrylate-methyl methacrylate copolymer latices with a core-shell structure were prepared by a sequential emulsion polymerisation technique. SEM and transmission electron microscopy studies undertaken on the polymer dispersions, powders obtained by spray drying and latices prepared by redispersing the powders in water revealed the influence of polymerisation parameters on the micromorphology of the starting latices, and correlations between the dimensional and micromorphological characteristics of the starting latices, the powders and the redispersed latices. 8 refs. 

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

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. 

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. 

The building/construction industry is the main industry for redispersible powders. Over the years the usage of dry mortar technology has been developed dramatically and modernized the way mortars are being used on a job-site. The invention of redispersible powders enabled the industry for the first time to produce pre-packed, polymer modified building materials that needed only the addition of water before application. These materials, known as dry mortar mixes guarantee defined and consistent performance of construction materials. 

A redispersible powder is by definition a polymer in a powdered form that can be redispersed by adding water to it. The resulting emulsion will fulfill the functionality of a polymeric dispersion binder, normally within a cementitious or gypsum based system. Redispersible powders are manufactured by spray drying an emulsion (Fig. 13-1). 

To guarantee the performance of a redispersible powder in its final application a protective colloid is added to the emulsion before the spraying process. The colloid protects the polymer particles from film forming during the spray drying process... 

The invention of redispersible powders by Wacker-Chemie in 195 3 made for the first time the production of polymer modified dry mix mortars possible, which are nowadays referred to as one pack or one component system ( bagged materials). New construction methods and building materials, which had the need for more safety, reliability, durability, efficiency and economy, have been achieved by using modern methods like the dry mix mortar technology. As a consequence worldwide the job-site mix technology and the modification of mortars with liquid polymers on job-sites were and are substituted by polymer modified dry mix mortars. The product characteristics are very well adapted to the requirements of modern construction technologies, materials and cHmates. 

The use of redispersible powders and therefore also the use of polymer modified powdered mortars is already for many decades standard in the construction industry in Europe and North America (predominantly in the USA). Other marketplaces all over the world like South America, Asia, Africa and Australia are in the process following that example. More and more environmental reasons ask also for the usage of dry mortars, since the recycling of buckets becomes more and more an issue. Dry mortars are also easy to store, hansport and do not require biocides. 

To meet today s technical requirements, almost aU dry mix mortars require polymer modification. Many cementitious mortars contain cellulose ethers as an additive to improve water retention and workabihty. However, after setting and drying they wiU adhere poorly or not at all to most of the substrates used in modern construction technology such as polystyrene panels, fiber panels, wood panels, closed and non-absorbent substrates or old tiles. In addition, cementitious mortars are very hard, brittle and inflexible materials, whereas for many apphcations flexible and deformable cementitious materials are essentiaL As a consequence for almost all apphcations in modern construction, the modification of cementitious mortars with polymers is a must In dry mix mortars the mineral binder, cement, and the polymer binder, redispersible powder, are ideal partners. The combination of both in a dry mix mortar... 

Simple tile adhesives have aheady a polymer modification of 1 to 1.5 % of a redispersible powder (calculated on total formulation). Such tile adhesives meet some parts of the mentioned national standards, but usually fulfill not all requirements. Only the usage of tiles with a medium porosity and small size could result in acceptable results with these types of adhesives. 

The adhesion of tiles to the substrate is certainly as important for a ceramic tile adhesive as the flexibihty. The European Norm uses a pull off test to determine the adhesion, where as the US standard ANSI 118.1 - 1999 prefers the shear bond test. A simple ceramic tile mortar with no polymer modification will fail in the adhesion test especially after heat aging or over wood (ANSI 118.11 - 1999). The same mortar modified with only 2 % of redispersible powder will pass both tests. With the puD-off... 

A sufficient high polymer modification of the ceramic tile adhesive is necessary especially when non-porous, highly vitrified tiles (low to no water absorption) are used. In this case, there will be no mechanical anchoring like described earher for porous tiles. The redispersible powder (chemical bonding), in this case, only provides the adhesion. This is, besides the outhned reasons for sufficient flexibihty, another important factor for a higher polymer modification. 

A ceramic tile adhesive that performs very well over almost aU substrates, with all types of tiles (size, water absorption) should contain at least 6 % of redispersible powder and the cement content should be limited to 30 to 35 %. An adhesive formulation that considers these two important components at the right amount is very hkely to pass all international standards. However, in an adhesive formulation has more to be considered than only the polymer and cement level. 

The SEM technology was also used to demonstrate that the redispersible powders continue to fulfil their functionality over an extended period of time. This is also shown by experiments to determine physical factors such as water absorption and water vapor permeability on defined test specimen after long-term exposure to outdoor weathering conditions. Figure 13-11 shows the capillary water absorption of test specimen after up to 6 years outdoor exposure at different polymer levels. 

The improvement of adhesion to concrete and steel, using a polymer modified reprofiling mortar, with and without applying a cementitious primer, is demonstrated in Fig. 13-13 Fig. 13-14 shows the improvement in flexural strength of a typical reprofiling mortar appHed by hand with and without different grades of redispersible powder. 

Standard or rigid mineral waterproofing slurries, which are polymer-modified, pre-packed dry mix mortars containing approx. 3 to 6 % of redispersible powder. They are used for mineral substrates, which are stable, sound and solid. There should be no risk for crack formation, movements or dimensional changes hke... 

The modification of dry mix mortars with dry polymers in the form of redispersible powders also significantly improves the technical performance of the mortars. The combination of the mineral binder with a polymeric binder in the form of an redispersible powder in dry mix mortars guarantees outstanding synergistic properties and characteristics, which cannot be achieved by either of the binders alone. The sufficient modification of mineral dry mix mortars by redispersible powders wiU improve workability, adhesion to various substrates, flexibility and deformability of... 

C Bindschaedler, R Gurny, E Doelker. Process for preparing a powder of water insoluble polymer which can be redispersed in a liquid phase, the resulting powder and utilization thereof. Swiss Patent 1497/88 (1988). 

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