Waterborne paints binders

Possible combinations of red lead with various binder systems are listed in Table 39 [5.114]. Red lead is still used for heavy-duty anticorrosion applications, especially for surfaces bearing residual traces of rust. In waterborne paints, red lead has no advantages over zinc phosphate [5.149]. 

Waterborne paints are usually emulsions of polymer-based binder particles. They are made by making an emulsion of droplets of monomers in water, after which the monomers are polymerized to form solid particles. When applied, the water and possibly other solvents evaporate, and the binder particles fuse to form a solid layer. 

Neutralizing agents are used in waterborne paints to neutralize binders and stabilize the product. Ammonia and various alkylated aminoalcohols are used, depending on the type of binder and method of application. On hardening, the amines mainly evaporate along with the water. 

The increasing importance of environmental considerations places new requirements on paint resins and has broadened the range of paint systems. Paints are now required that have a low solvent content (medium-solids, high-solids coatings) or are solvent-free (powder coatings), that can be adjusted by dilution with water (waterborne paints), and that are thermoplastic or capable of undergoing cross-linking. All of these properties must be obtained via the polymer structure of the binders. Important parameters are described below. 

The high polarity of water is responsible not only for differences between the application behavior of waterborne paints and solvent-containing paints. It also means that the organic polymers used as binders for waterborne paints must have a different structure from those used in solventborne paints. 

Waterborne paints differ according to the nature of their stabilization in water the polymer molecules are dissolved in water or dispersed in water in the form of polymer dispersions or emulsion polymers. Recent developments include polymer particles formed in organic solvents and then emulsified in water with low or, more commonly, high molecular mass emulsifiers being used for internal or external emulsification. Internal emulsification denotes that part of the binder molecule functions as an emulsifying moiety, whereas for external emulsification separate emulsifiers are required. 

As a result of the particulate nature of the binders only a low gloss and, in some cases, only limited corrosion protection can be obtained. Waterborne paints based on dispersions can be applied by spraying, however they are of only limited use for electrostatic coating and dipping applications due to their rapid drying properties. 

Waterborne paints contain auxiliary solvents as solubilizers in amounts of 2-15%, depending on the binder. These solubilizers are water-miscible solvents or solvents that become water-miscible in all proportions in the presence of the binder [14.210], [14.211]. The most important are listed below ... 

Butanol by itself is not miscible in all proportions with water, but its water miscibility is unlimited in the presence of paint binders. Butanol is an extremely effective solvent in waterborne paints, although it has the disadvantage of a somewhat more pungent smell than glycol ethers. The auxiliary solvents in waterborne paints promote solubilization of the binder and water, reduce the viscosity maximum that occurs on dilution with water, and yield smooth-flowing, flawless paint surfaces [14.216]-[14.226]. 

Paint consists of a mixture of pigments which gives body and colour, and a resin or binder which is the actual film-forming component and acts as a glue to hold the pigment together and stick them to the surface. Binders include synthetic resins such as acrylics, polyurethanes, polyesters and epoxies and can be a combination of resins, e.g. epoxy/ acrylic and polyurethane/acrylic. To adjust the curing properties and reduce the viscosity so that the paint can be easily applied, a solvent or carrier is used. These evaporate after application and do not form part of the paint film. In waterborne paints, the carrier is water. With solvent-borne paints, also called oil-based paints, the carrier is a solvent such as acetone, turpentine, naphtha, toluene, xylene and white spirit. 

An overview is presented on the merits of polyurethane dispersions in waterborne paint formulations. In addition to the characteristics listed which they impart to coatings, such binders can offer an even wider range of properties by means of blending or copolymerisation, producing binders which are tailor made for specific applications, it is stated. 

Gruvberger B, Bruze M, Almgren G (1998) Occupational dermatoses in a plant producing binders for paints and glues. Contact Dermatitis 38 71-77 Hansen MK, Larsen M, Cohr K-H (1987) Waterborne paints, a review of their chemistry and toxicology and the results of determinations made during their use. Scand J Work Environ Health 13 473-485... 

The industrial reaction to this situation is one of compliance, with an increasing use of waterborne paint formulations based on the use of synthetic latices as binders especially in paints for domestic application. Such latices are usually polymeric colloids, of volume fraction from 0.2 to 0.5, dispersed in aqueous surfactant solution, prepared by a process of emulsion polymerization. These dispersions have a slightly turbid appearance, often with a low viscosity of order 1 mPa s. The latices can be readily prepared as near-monodisperse colloids. 

Paints are often classified by the type of binder they include. The most common classifications (with percent of total coatings used m 1985) are Latexes (31%) waterborne (10%) noil-aqueous dispersions (2%) solvent-borne (55%) and one hundred percent sohds coatings (2%) A small volume of paint is made with silane binders. 

Emulsion paints consist of polymer dispersions as binders, pigments, extenders, and small amounts of auxiliaries (in some cases < 1 %). Waterborne polymer dispersions are produced by emulsion polymerization monomer droplets are polymerized in water-containing surfactants and protective colloids. The size and size distribution of the dispersed polymer particles can be controlled by adjusting the stirring rate in the polymerization reactor and by selecting appropriate protective colloids. 

Fillers are applied with electrostatic spraying devices (fast-rotating bells) to give dry film thicknesses of about 40 pm. Waterborne fillers with polyester-melamine binders (primer surfacers) have been developed to reduce the volatile organic content. They yield a film thickness of 30 pm after a prereaction time of 10 min at lOO C and a reaction time of 20 min at 165 °C. The properties of the films are similar to those formed by solventborne paints. More recently, waterborne fillers based on blocked isocyanates have been developed. Field trials have shown that their mechan-ieal resistance is very good. 

For many years, paints used to coat cans contained considerable amounts of volatile organic solvents. Waterborne can coatings were developed to reduce solvent emissions and are used worldwide. Binders used in waterborne can coatings are modified epoxy resins (see Section 2.10). Acidic acrylate chains are grafted onto an epoxy molecule. After partially neutralizing with amines, the resins can be dispersed in water. 

Binders used in waterborne latex paints are prepared by emulsion polymerization of mixtures of monomers selected to give the optimum glass transition temperature, Tg [4(e)]. Low Tg contributes to good film formation so that the paint... 

The carrier is the vehicle in the uncured paint that carries the binder, the pigments, and the additives. It exists only in the uncured state. Carriers are liquids in the case of solvent-borne and waterborne coatings, and gases in the case of powder coatings. 

Whilst there are some thermoset acrylic emulsions cormnerdally available, the bulk of the thermoset resins, used as the main binder system, are produced in solution. Some may then be made waterborne by neutralisation and inversion (dispersion) into a water phase. Lower molecular weights favour this qrproach. The rate of conversion from solvent based to waterborne industrial thermoset coating systems has been, and is, much slower than the conversion from architectural alkyd paints to emulsion altonatives. There are two principle reasons for this. Firstly there are problems of application and substrate wetting of many waterborne systems. Secondly, the modifications frequently required to induce water dispersibility reduce one or more of the essential performance properties required from the cured film, compared to a solvent based system. Water resistance, with many films having an increased tendency for blushing is one example. However, for some applications, such as electrodeposition, only waterborne systems will work. 

Table 9.8 shows the composition (FP 2019/1) of waterborne acrylic paints to which varying amounts of glycerolpropylsilyl-modified silica dispersion (Bindzil CC301) have been added to the binder formulation, i.e., the letdown. Positions 9-13 in Table 9.8 constitute the letdown in the paint formulation, whereas positions 1-8 constitute the mill base of the paint formulation. The reference formulation in table 9.8 does not contain any glycerolpropylsilyl-modified silica dispersion in the letdown [68]. 

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