Latex factors controlling

Dispersions of insoluble polymer particles form films by coalescence of the particles. The largest volume of such coatings use latexes as a binder. The lowest temperature at which coalescence occurs to form a continuous film is called its minimum film-formation temperature (MFFT). A major factor controlling MFFT is the Tg of the polymer particles. The MFFT of latex particles can be affected by water, which can act as a plasticizer (5). Most latex paints contain volatile plasticizers, coalescing solvents, to reduce MFFT. The mechanism of film formation from latexes has been extensively studied the papers in References 6-9 review various theories associated with it. Film formation occurs by three overlapping steps evaporation of water and water-soluble solvents that leads to a close packed... 

A number of factors control successful film formation. The polymer must be soft enough and have a sufficiently low glass transition temperature (Tg) to allow fusion. For this reason, many of the latex polymers are copolymers, which are usually softer than homopolymers. Coalescing agents are added as temporary plasticizers. As the water evaporates, they have an increasing softening effect on the polymer particles, helping them to coalesce more easily. 

The choice of coagulant for breaking of the emulsion at the start of the finishing process is dependent on many factors. Salts such as calcium chloride, aluminum sulfate, and sodium chloride are often used. Frequentiy, pH and temperature must be controlled to ensure efficient coagulation. The objectives are to leave no uncoagulated latex, to produce a cmmb that can easily be dewatered, to avoid fines that could be lost, and to control the residual materials left in the product so that damage to properties is kept at a minimum. For example, if a significant amount of a hydrophilic emulsifier residue is left in the polymer, water resistance of final product suffers, and if the residue left is acidic in nature, it usually contributes to slow cure rate. 

In an earlier paper (22) studies on a commercial latex as a model system indicated clearly no difference in F/T results when NH3 or NaOH was used to adjust pH. It is now observed that at the minimum acid levels, the quantity of alkali and not pH is the controlling factor. Thus, Table IV shows that less NaOH than NH3 is required to obtain pH 9.5, but an equal number of equivalents is necessary to achieve F/T stability. Moreover, as might have been expected, if more than the minimum acid content is present, F/T stability can be obtained at a lower pH so long as an equivalent amount of alkali is added—i.e., as long as the same concentration of carboxylic acids are converted to carboxylate ions. 

Emulsion polymerization reactions are sometimes carried out with small seed particles formed in another reaction system. A number of advantages can he derived from using seed particles. In a batch reactor seed latex can he helpful hi controlling particle concentration, polymerization rate, particle morphology, and particle size characteristics. In a CSTR the use of a feed stream containing seed particles can also help to prevent conversion and/or surface tension oscillations, which are caused by particle formation phenomena, This factor will be discussed in more detail later in this chapter. 

Other reactor design considerations may be necessary in special cases. Monomer mass transfer, not normally a problem, can he important if the monomer- aqueous phase interface is small. This is more likely in systems involving gaseous monomers in which the large surface area of the monomer emulsion is not present. In such cases special attention must he paid to gas dispersion and transport. Giher factors that can have a significant effect on reactor design include latex viscosity, heat transfer rates, reaction pressure, and control mechanisms. 

In addition, SBR latexes with less emulsifier and inorganic electrolyte as well as some with larger particles and modified particle size distributions were also prepared for controlled variations of these factors. 

The morphology of latex particles is controlled by the thermodynamic and kinetic factors. The thermodynamic factors determine the ultimate stability of the multiphase system, inherent in the production of a composite latex particle, while the kinetic factors determine the ease with which such a thermodynamically favored state can be achieved. The parameters affecting the thermodynamics of the system include the particle surface polarity, the relative phase volumes, and the core particle size. The parameters affecting the kinetics of the morphological development include the mode of monomer addition (monomer starved or batch) and the use of crosslinking agents. Of course, crosslinked core/shell latexes constitute IPNs, see Section 6.4.1. 

Similarly as in the core/shell latex particles, the LIPNs morphology is controlled by miscibility of the polymers, the volume fraction of each polymer, the crosslinking density of the polymers, and the sequence or the order of the synthesis (which polymer is synthesized first) [Sionakidis et al., 1979 Hourston and Satgurunathan, 1987]. The effects of those factors on the LIPN morphology are summarized as follows ... 

Two procedures described in BS4443, Part 2, 1988 [48] are suitable in particular for latex, PVC, and polyurethane foams. One is faster to carry out and can be used as a quality control method (Procedure A). Procedure B can be used to determine the load to give indentations of 25, 40, and 65% deflections and hence the sag factor can be determined. In addition, by measuring the load for specified indentations of the foam on loading (as with Procedure B), followed by measuring the indentations on unloading, a measure of the foam hysteresis can be determined. Hysteresis is a measurement of the energy absorbed by a foam when subjected to a deformation. 

Slump control factor (by volume)=Vp+Vw (Ifisn ) Vc, Vp, Va, Vw, Vs, Vg Volumes of cement, polymer, air, water, sand, and gravel per unit volume of latex-modified concrete, respectively (l/m )... 

The slump (St) of latex-modified concrete can be predicted with every polymer type and at each sand-aggregate ratio by using slump control factor (q>) as follows ... 

Effects of Control Factors for Mix Proportions. The binder of latex-modified mortar and concrete consists of polymer latex and inorganic cement, and their strength is developed as a result of an interaction between them. The polymer-cement ratio has a more pronounced effect on the strength properties than the water-cement ratio. However, this effect depends on polymer t3rpe, air content, curing conditions, etc. The relation between the strength properties and polymer-cement ratio has been discussed in a number of papers.P l 1 1 A general trend which summarizes the results obtained in these papers is presented in Fig. 4.19. 

In general, electrochemical methods are in competition with rehabilitation utilizing an overlay such as low-slump, high-performance, or latex-modified concrete. The deck condition is often the controlling factor in the selection of the rehabilitation method. In some instances, a combination of these methods is selected. For example, electrochemical removal of chloride followed by an overlay or an overlay in conjunction with CP to noitigate any further corrosion. 

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