Water sensitivity, latex film

The emulsifier provides sites for the particle nucleation and stabilizes growing or the final polymer particles. Even though conventional emulsifiers (anionic, cationic, and nonionic) are commonly used in emulsion polymerization, other non-conventional ones are also used they include reactive emulsifiers and amphiphilic macromonomers. Reactive emulsifiers and macromonomers, which are surface active emulsifiers with an unsaturated group, are chemically bound to the surface of polymer particles. This strongly reduces the critical amount of emulsifier needed for stabilization of polymer particles, desorption of emulsifier from particles, formation of distinct emulsifier domains during film formation, and water sensitivity of the latex film. 

Water Sensitivity. Improvement in the water sensitivity and improved barrier properties of latex film has been an important goal of many coatings chemists. One of the first publications (38) on HEUR-thickened... 

The application properties of many latexes are strongly affected by the chemistry of the surface of the polymer particles. Relatively small amounts (1-2 wt% based on monomers) of acidic monomers (e.g., acrylic acid, AA) are frequently used in the manufacture of latexes. Because this monomer is water-soluble, upon polymerization, most of the AA-rich polymer chains are located at the surface of the polymer particles. The presence of AA at the surface of the polymer particles is beneficial for both the stability of the latex [11] and the application properties (e.g., both the shear strength of the adhesives [ 12] and the pick strength of coated paper [9] increase with the AA content). In addition, the type and amount of surfactant affects application properties such as colloidal stability and water sensitivity of the film [13]. 

The dispersion of fibers in polymer latex to prepare composite has been reported for poly(6-hydroxyoctanoate) (PHO) [101, 102], polyvinylchloride (PVC) [103], waterborne epoxy [104] and polyvinyl acetate (PVAc) [94]. Most of the works focus on the use of non-polar, non-water-sensitive polymers, while keeping an aqueous media for the processing of the films to preserve the dispersion of the nanoparticles. In their pioneering work, Favier et al [94] adopted the technique of solvent casting using a synthetic latex obtained by the copolymerization between styrene (35 wt%) and butyl-acrylate (65 wt%) (poly(S-co-BuA)). Nanowhiskers were dispersed in the latex and evaporated. The nanocomposite films were obtained by water evaporation and particle coalescence at room temperature, that is at a temperature higher than Tg of poly(S-co-BuA), around 0 C. 

The main advantages of post-polymerization treatments are that they can be carried out either in the polymerization reactor or in the storage tank, and no additional equipment is needed. However, only the polymerizable residual volatiles can be eliminated, and in some cases new VOCs are produced from secondary reactions. Thus, formaldehyde is formed when sodium sulfoxylate formaldehyde is used as the reductant [66] and acetone and fert-butanol are formed when tert-butyl hydroperoxide is used as the oxidant [67]. In addition, inorganic water-soluble initiators may be deleterious to both stability and water sensitivity of the film formed with the latexes. 

Dry bonding adhesives are used to form bonds after the water has evaporated. They may also be classed as pressure-sensitive adhesives, because pressure is needed to achieve contact between the joining surfaces. The most simple case is the dried natural rubber latex film which will stick only to itself and requires that both surfaces be coated. At the other extreme is the material normally associated with the term pressure-sensitive adhesive, which will stick to a variety of surfaces. 

The key parameters to control are formulation water content, clay/latex ratio and resin/latex ratio, depending on the desired adhesive cost structure. Not surprisingly, too much filler or too little resin and/or oil will result in inferior properties. Excess water will result in slower drying, and probably the need for more water sensitive thickeners. Like many other SBR based adhesives, HSL based flooring mastics include in-can and dry-film preservatives as well as an antioxidant package to ensure long-term performance. 

The prevulcanization of natural rubber in latex form has also been a subject of much investigation. The cross-linking mechanism is not yet fully understood, but the water apparently plays a major role in it. Irradiation results in the cross-linking of the rubber molecules and in coarsening of the latex particles. A process of cross-linking of natural rubber latex has been developed to the point that it can be used for an industrial-scale application. The irradiation is performed in aqueous media by electron beam without a prorad (sensitizer) at a dose of 200 kGy (20 Mrad) or in the presence of n-butyl acrylate at considerably lower doses, typically 15 kGy. The cross-linked film exhibits physical properties comparable to those obtained from sulfur cured (vulcanized) film. As an alternative, the addition of a variety of chloroal-kanes makes it possible to achieve a maximum tensile strength with radiation doses of less than 5 Mrad (50 kGy). ... 

Associative thickeners are low-molecular-mass water-soluble polymers with at least two hydrophobes such as hydrophobically modified ethylene oxide-urethane block copolymers (HEUR) or hydrophobically modified hydroxyethylcellulose (HMHEC). The hydrophobes can associate with themselves or with hydrophobes on surfactant, cosolvent, latex, and pigment. This sets up a loose network that is sensitive to mechanical disturbance but re-forms quickly. The result is that pigment settling and film sag are reduced because the network structure increases the low shear viscosity, but the formulations show easy... 

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