Polymer-modified admixtures

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]. 

Polymer-modified or polymer cement mortar (PCM) and concrete (PCC) are a category of concrete-polymer composites which are made by partially replacing the cement hydrate binders of conventional cement mortar or concrete with polymers, i.e., polymeric admixtures or cement modifiers, thereby strengthening the binders with the polymers. Polymer-modified or polymer cement paste, which is prepared widiout any aggregate, is sometimes used. 

For the past seventy years or more, active research and development of polymer-modified mortar and concrete has been conducted around the world, resulting in products which are currently used as popular, important construction materials. To match the technical innovations in the construction industry in recent years, useful polymeric admixtures or polymer-modified mortar and concrete have been developed in advanced countries. There is currently great interest in using polymer-modified mortar and concrete as repair materials for deteriorated reinforced concrete structures. Polymer-modified concrete and mortar are promising construction materials for the future because of the good balance between their performance and cost compared to odier concrete-polymer composites. 

II. EPOXY POLYMERS MODIFIED WITH HETEROGENEOUS ADMIXTURES... 

Corrosion-inhibiting admixtures in conjunction with polymer-modified cementitious coatings applied to the rebar... 

Sprayed or trowelled concrete with or without admixtures or bonding aids. Polymer Modified Cement Concrete. Epoxide Resin Mortar. 

Ohama, Y.,Chapter 7. Polymer-modified mortars and concretes. In Concrete Admixtures Handbook Properties, Science and Technology.ed..V.S. Ramachandran, Noyes Publications, Park Ridge, New Jersey, 198i, pp.337-4.29. 

Thinset mortars are employed in demanding interior and exterior floor and waU applications where there may be standing water or high moisture exposure. Both one component polymer modified thinsets and two component cementitious adhesives are used. Ethylene vinyl acetate copolymers (EVA) are the predominant powder polymer base used in one component polymer modified thinsets, they are described in Chapter 13. Recently styrene acrylics or straight acrylics and styrene butadiene copolymer powders gain an increasing market share. Two component thinsets systems combine a cementitious powder mix and a separate polymer dispersion admixture. 

In 1971, Hiatt et al. found that polyethylene oxide (PEO) of molecular weight about 100000 prevented the adsorption of rabies virus to porous glass with an average pore diameter of 1250 A. The support was modified by passage of one void volume of 0.4% solution of the polymer in water, followed by 5 or more volumes of distilled water or buffered salt solution. The virus was effectively purified from the admixtures of brain tissue fluid by means of size-exclusion chromatography on the modified glass column [28]. 

Water, methanol, and n-hexane do not influence the photooxidation of PVC (43), but the photodegradation is accelerated by ferric chloride (70,71) and certain other compounds containing iron (70,71,72). Purification of the polymer might be expected to enhance its photostability by removing deleterious impurities such as iron compounds that are derived from metal equipment. This type of result was obtained in one recent study (58) but not in others (30,59). In contrast, the photo-oxidative degradation of PVC should be enhanced by admixture of the polymer with materials that are unusually susceptible to photooxidation themselves. Such behavior has been observed for impact-modified PVC containing polybutadiene-based polyblends (69,73). 

Homopolymers are sometimes modified by a mechanical admixture of another homopolymer. As only about 5 % of pairs of all known polymers are mutually miscible, compatibility may be a problem in mixtures (blends). Copolymerization is technically applied to overcome, for example, the brittleness of polystyrene, polypropylene and PVC. It is also applied for improving the curing properties and modifying the viscoelastic properties of rubbers. By copolymerization, the relation between the hydrophobic and hydrophilic properties of macromolecules can also be modified. Their resistance to solvents may be enhanced. 

Variation of structural and compositional parameters allow modification of the physical properties to tailor performance to specific application needs. For example, the effect of ingredients added to control the modulus of two UV curable silicone polymers is presented in Figure 1. Both Polymer A and Polymer B were polydiorganosiloxanes which contained terminal reactive unsaturation. Admixture with a photosensitization system and subsequent cure afforded soft, elastomeric products. Modifier A was incorporated into Polymer A to soften the system further. As the concentration of Modifier A increased, the modulus decreased, and the resultant composition became more gel-like. In a contrary fashion, Modifier C was added to Polymer B to provide reinforcement. As the concentration of Modifier C was increased, the modulus of the resultant cured film increased, the elongation decreased, and the tensile strength went through a maximum. 

Figure 4.44 shows the water permeability as measured parameter, time (s), by Figg s methodi 1 of dry-cured EVA-modified mortars with blended cements containing various mineral admixtures. According to this method, the measured parameter is the time taken for a standard volume of water to be permeated into the specimen through a 10 nun diameter x 40 mm deep hole.P 1 An increase in the measured parameter of EVA-modified mortars with the blended cements indicates a decrease in the water perme-ability, and increasing polymer-cement ratio decreases the water permeability by a factor of 10 or more. 

Abstract. Polyvinylalcohol (PVA) is a polymer soluble in hot water, it has the property of film formation and it can improve the concrete performance. The effects of PVA modified with nano clay on the cement hydration reaction have been investigated by means of semiadiabatic calorimeter, FTIR spectroscopy and SEM. FTIR spectroscopy was employed to monitor chemical transformation of cement. The morphology of the different samples was compared by means of SEM micrographs. With the semiadiabatic calorimeter the hydration kinetic was measured to compare the heat rate of the admixtures materials. Fixing the water-cement ratio, w/c, in 0,45, the ratio of polymer to cement (p/c) was 2 wt% and the ratio of clay to polymer was 4 wt% (0.8wt.% related to cement). The polymer and modified polymer admixtures produced a retardation effect on the kinetic of cement hydration, but the clay acts as nucleating agent. The increase of the temperature with time was measured and a new model with four parameters was employed and the kinetic parameters were determined for each sample. 

Polymers such as EVA, are used as admixture because it modify the elastic modulus, toughness, permeability and bond strength to various substrates in cement and mortars [10]. The polymer forms a film that creates a network inside the cement matrix, partially covering hydrated and anhydrous cement particles, sealing pores and bridging microcracks. Besides, this addition also changes the hydration rate. Silva et al [11] compare the effects of two polymers a water soluble polymer (HPMC — hydroxypropylmethylcellulose) and a latex [EVA-poly(ethylene-co-vinyl acetate)] on... 

Cyanoacrylates are rapidly polymerized by nucleophilic species to give linear polymers with high molecnlar weights. Unlike other adhesives, cyanoacrylates consist of pure monomer in admixture with low levels of stabilizers, accelerators and modifiers. The patent literature abounds with references to many different monomers, but only a restricted group are used in connnercial products. The structures of the most common monomers are shown below... 

Conformational studies on poly(decamethylene adipate) and poIy(tetra-methylene succinate) and the use of molecular orbital theory potential energy surfaces in determining polymer conformations for 2GT have also appeared. Crystallization, Crystallinity, and First and Second Order Transition Behaviour. The crystallization behaviour of 2GT, alone or in admixture with additives or modified by small amounts of co-monomers, continues to evoke considerable interest. Thus, X-iay fluorescence studies have been made on phenylmercurated... 

The effects of blended cements containing various mineral admixtures on the flexural strength of dry-cured EVA-modified mortars are illustrated in Fig. 4.17.12 1 The flexural strength of EVA-modified mortars with the blended cements increases with an increase in the polymer-cement ratio, and is similar to that of unmodified mortar except in a few cases. 

It has long been known that it is possible to modify the electrical properties of polymers by means of conductive admixtures of many kinds or, rather, to make them conductive. Despite this, electrically conductive polymers are still not of any great economic importance as materials. They do, however, play an indispensable role in many specialized technical applications. They are mostly manufactured by incorporating electrically conductive carbon black or carbon fibers in a variety of polymers, especially thermoplastic polymers. Other conductive additives, such as steel fibers, aluminum flakes, metal-coated carbon fibers, metal-coated hollow glass spheres, and low melting metallic alloys have not so far played any decisive part in the development of electrically conductive polymers. 

---