Redispersible polymer

PVA is used for example as a water soluble dispersing agent for many different polymer dispersions used in the construction industry. An important property of PVA is the ability to redisperse polymer dispersions that have been dried to powders. 

Several types of polymer-modified mortars and concretes, i.e., latex-redispersible polymer powder-, water-soluble polymer-, liquid resin-, and monomer-modified mortars and concretes, are produced by using the polymers and monomers shown in Fig. 2.1. Of th, the latex-modified mortar and concrete are by far the most widely used cement modifiers. 

Redispersible Polymer Powders Water-Soluble Polymers Liquid Resins Monomers... 

The principle of modification of cement mortar and concrete with redispersible polymer powders is almost the same as that of latex modification except that it involves the addition of redispersible polymer powders. Mostly the redispersible polymer powders are used by dry mixing with the cement and aggregate premixtures, followed by wet mixing them with water. During the wet mixing, the redispersible polymer powders are reemulsified in the modified mortar and concrete, and behave in the same manner as the latexes for cement modifiers. 

Fabrication and Properties of Redispersible Polymer Powder-Modified Systems... 

Figure 5.1 Spray drying system for redispersible polymer powders. 1990, American Concrete Institute, reprinted with permission.)...

The fabrication technique of redispersible polymer powder-modified mortar and concrete is about the same as that for latex-modified mortar and concrete. The materials and mix prc rtions used in this nnodified mortar and concrete are the same as those used in the latex-modified systems except that the addition of the redispersible polymer powders is involved. At present, commercially available redispersible polymer powders as cement modifiers are classified in Fig. 5.2. Table 5.2 gives the properties of typical redispersible polymer powders. The rediqretsible polymer powders are usually free-flowing powders, and have ash contents of 5 to 15%, whidi primarily come from the anti-blockirig aids. When the polymer powders are placed in water under agitation, they redtsperse or re-emulsify easily, and provide the polymer latexes with polymer particle sizes of 1 to 10 pm. 

Figure 5.2 CommerciaUy available redispersible polymer powders for cement modifleis.

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 5.3 Polymer-cement ratio vs. flexural and compressive strengths of redispersible polymer powder-modified mortars.

According to a recent study, Bright, et al.1 1 compared the physical properties of various latex-modified mortars and redispersible EVA powder-modified mortars, and showed that the properti of EVA powder-modified mortars are at least equal to those of the latex-modified mortars as seen in Fi. 5.7 to 5.9i i Figure 5.10 exhibits the strength properties of patching mortars using redispersible polymer powders.i l... 

Lambe, et al.l It l reported that repair mortars using redispersible polymer powders for concrete structures show high resistance to the diffusion of chloride ions, oxygen and carbon dioxide, and also low shrinkage. 

Afridi, et al.l found that redispersible polymer powder-modified mortars provide a freeze-thaw durability similar to that of latex-modified mortars. Fig. 5,11 shows the polymer-cement ratio vs. freeze-thaw durability of redispersible polymer powder- and latex-modified mortars.I 1 Figure 5.12 sho % the weatherability of patching mortars using redispersible polymer powders. " ... 

Figure 5.11 Polymer-cement ratio vs. freeze-thaw durability of redispersible polymer powder-and latex-modified mortars.

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. 

Recently, redispersible polymer powder-modified mortar has been increasingly used in much the same applications as ordinary mortar and concrete. 

Proposal of a new integrated model by the modification of Ohama s three-step model in the microstructure formation process of polymer-modified mortar and concrete using polymer dispersions or redispersible polymer powders [58]... 

Development of the effective powdered shrinkage-reducing agents and antifoamers for one-packaged polymer-modified mortar (dry premix mortar or dry mix mortar) products using redispersible polymer powders [61]... 

Y. Ohama and S. Matsumoto Effect of type of powdered antifoamer on properties of polymer-modified mortars using redispersible polymer powders with powdered shrinkage-reducing agent, in Proceedings of the 4th Asia Symposium on Polymers in Concrete, Kangwon National University, Chuncheon, Korea (2003), pp.81-87. 

Y. Ohama, M. Ota and H. Tatematsu Properties of polymer-modified mortars using redispersible polymer powders with nitrite-type hydrocalumite (in Japanese). Cement Science and Concrete Technology No.59/2005 (2006), pp.461-468. 

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... 

In PMC, the polymers are added as polymer latexes, redispersible polymer powder, water soluble polymers and liquid polymers. The cement hydration process rather precedes the polymer film formation around aggregate grains and a kind of co-matrix is formed in which cement phase and polymer phase are interpenetrated (Ohama 1998). The formation of such a co-matrix is shown schematically in Figure 13.4. 

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