Chemically modified rubber latices

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]. 

Bitumen modifiers can be synthetic polymers, natural rubber (latex) and some chemical additives such as sulfur and certain organo-metallic compounds. Fibres and fillers (inorganic powders) are not considered to be bitumen modifiers. Table 3.16 gives some typical bitumen modifiers, as well as significant improvements to asphalts. Polymers are the most common type of bitumen modifiers, with thermoplastic elastomers being the most popular polymer. 

Mondragon et al ° reported that unmodified and modified NR latex were used to prepare thermoplastic starch/NR/MMT nanoeomposites by twin-screw extrusion. After drying, the nanoeomposites were injection moulded to produce test specimens. SEM of fractured samples revealed that chemical modification of NR latex enhanced the interfacial adhesion between NR and thermoplastic starch (TPS), and improved their dispersion. X-ray diffraction (XRD) showed that the nanoeomposites exhibited partially intercalated/exfoKated structures. Surprisingly, transmission electron microscopy (TEM) showed that clay nanoparticles were preferentially intercalated into the rubber phase. Elastic modulus and tensile strength of TPS/NR blends were dramatically improved from 1.5 to 43 MPa and from 0.03 to 1.5 MPa, respectively, as a result of rubber modification. 

The polymers described in this chapter are industrial-grade materials, and consequently some of the examples may contain additives and/or may be chemically modified. Polymers in various morphological forms may be analyzed, and these include films, fibers, solid pelletized and powdered products, and dissolved/dispersed materials in liquids such as paints and latex products. Also, the same base polymer, such as a styrene-butadiene copolymer, for example, may exist in a rubber, a resin, or a plastic. In general, reference will not be made to the original source of the polymer samples. Because infrared spectroscopy is more widely used than the Raman method, the authors will focus more on the applications of this technique. However, the Raman method, which is complementary to the IR method, does have important and unique applications in the polymer analysis, especially with regard to the determination of the fundamental polymer structure and its... 

Apart from pre-vulcanized latex where the rubber molecules have been chemically crosslinked by sulfur, the chemical nature of the rubber molecules of the other latices described above remain chemically intact during and after the process. There are several other latexes available on the market in which the rubber molecules of the latex have been chemically modified. The chemical reactivity of the rubber molecules arises from the olefinic structure of the cis-1,4-isoprene unit within the molecule, which can undergo rapid reactions with, for example, halogens, ozone and hydrogen chloride. Some of these will be described in this section. They are prepared to serve niche applications. 

Rubber-modified plastics (e.g, ABS), which are available in latex form, have also been modified with thio antioxidants and UV stabilizers. The 2-hydrobenzophennone (IX) reacts to give a 30% concentrate of which 80% of the UV stabilizer used becomes chemically attached to the polymer (24). The concentrates can be used either as... 

Floor underlayments are utilized to produce a level and smooth surface before placing vinyl tile, asphalt tile, rubber tile, or other resilient flooring materials. Acrylic latex modified underlayments are ideally suitable for leveling floors prior to the installation of these surfacing materials, since these underlayments provide an excellent substrate for bonding purposes, as well as offering good chemical resistance to tile adhesives and solvents which some of them contain. 

The objectives of the chemical modification of natural rubber include the enhancement of chemical and physical properties. It is possible to predict both the conditions (reaction in latex or in dry rubber) and the scale of such modification (usually 1 mole and, from these criteria, to define possible modifying reagents. The thermal ene reaction is shown to be particularly valuable. A new cycloaddition reaction of aryl sulfonyl azides to natural rubber is also promising. 

The most common preformed rubber particles used as a toughening agent for epoxy polymers are the so-called structured, core-shell latex particles (Figure 4). These particles typically have a polybutadiene-based core and an acrylate-based shell. Such additives can be purchased as powders from Rohm and Haas or Elf-Atochem and can be purchased as epoxy concentrates from the Dow Chemical Company. The key parameter for these modifiers is the composition of the shell polymer, since the shell chemistry plays a crucial role in the overall blend morphology. It should be noted that it is possible to obtain commercial core-shell latex particles with reactive groups in the shell for improved dispersion of the rubber particles. 

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