VISIOMER® - methacrylate monomers for Emulsifiers, surfactants & RHEOLOGY MODIFIERS
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Emulsion polymerization is a heterogeneous, free-radical polymerization process that has wide industrial application in the methacrylate-based production of polymer colloids or latexes. Emulsifiers play a major role in the emulsion polymerization process as well as in the formation and application of latexes. In emulsion polymerization, emulsifiers are key to solubilizing highly water-insoluble monomers and determining the mechanism of particle nucleation. Furthermore, they determine the number of nucleated particles, and thus the rate of polymerization. They also help maintain colloidal stability during the particle growth stage as well as control average particle size and size distribution of the final latex system. Emulsifiers are also essential in stabilizing latexes during post polymerization processes, such as stripping to remove residual monomers, formulation, storage, shipping and onward transformation. Emulsifier molecules comprise a hydrophobic ‘tail’ and a hydrophilic ‘head.’ These emulsifiers can be classified into four categories according to the charge of the head group: anionic emulsifiers, cationic emulsifiers, nonionic emulsifiers and ampholytic emulsifiers. Another classification is based on reactive emulsifiers and non-reactive emulsifiers according to their respective chemical properties. VISIOMER® MPEG methacrylates are often used as nonionic reactive emulsifiers. Like non-reactive emulsifiers, reactive emulsifiers are molecules that typically have a hydrophobic segment and an ionizable and/or polar group. The hydrophobic segment tends to adsorb onto the surface of the latex particle during – and following – particle polymerization. The hydrophilic group, on the other hand, extends into the aqueous solution phase, providing a steric barrier or charge repulsion against particle agglomeration and coagulation. Unlike their non-reactive counterparts, reactive emulsifiers also contain a reactive group on the hydrophobic segment that is capable of covalent bonding to the latex surface. Usually this is a moiety such as a methacrylate ester or a terminal olefin that can participate in free-radical emulsion polymerization reactions. When used in emulsion polymerization, a large fraction of the emulsifier molecules become irreversibly bound to the emulsion polymer chains and droplets. This can both improve latex stability and reduce foaming.
A dispersant is either a non-surface active polymer or a surface-active substance added to a suspension, usually a colloid, to improve particle separation and prevent settling or clumping. Dispersants normally consist of one or more emulsifiers. Polyacrylates represent one type of polymer dispersants. Monomers commonly used for these dispersants include VISIOMER® DMAPMA, MADAME™, TMAEMC, MAPTAC and MPEG methacrylates. Polymer dispersants are widely used in many products, for example in adhesive dispersions emulsions/dispersions, detergents, photographic solutions as well as in agriculture, cosmetics and pharmaceuticals. You will also find them in cleaning and polishing products.
Many aqueous systems utilize thickeners to provide a particular formulation with the desired viscosity. For example, certain aqueous thickeners are utilized in the formulation of oil field chemicals as well as polishing and cleaning compositions, and in the manufacture and/or preparation of personal care and food formulations. These types of thickeners are also used in specific pharmaceutical applications. In addition, thickeners are commonly used in latex paint formulations and in a variety of other coating compositions. There are many types of thickeners now used in paints and other coating formulations. Acrylic thickeners are commonly produced by emulsion polymerization. One of the most important monomers for this kind of thickeners is VISIOMER® GMAA. Usually, crosslinkers like VISIOMER® AMA or EGDMA are added in the formulation in a low concentration in order to increase the molecular weight of the produced polymer. Another type of thickener, also called associative thickener, is based on hydrophobically modified polymers that combine high thickening efficiency with a less marked shear-thinning viscosity profile. Here, viscosity/shear can be controlled by varying the length of the polymer’s hydrophobic groups and molecular weight. Evonik’s VISIOMER® C18PEG 1105 MA W was developed specifically with this purpose in mind and has become an industry reference. Associative thickeners have a strong thickening effect and deliver the required Stormer viscosity already at low addition levels. In most cases, their thickening efficiency is in fact comparable to that achieved with non-associative thickeners with a high molecular weight. Both high-shear and low-shear viscosity can be calibrated. Compared to the type of conventional thickeners with high molecular weight, associative thickeners have a much less pronounced shear thinning profile.