A solvent is a pure or mixed liquid that is used to make the paint flowable prior to its application. In practice, the term solvent, solvent blend, and thinner are often used interchangeably. Solvents are chemical substances that can dissolve, suspend, or extract other materials, usually without chemically changing either the solvents or the other materials. Solvents make it possible to process, apply, clean, or separate materials. Solvents work on the principle of “like dissolves like.” Therefore, for a solvent to work, it needs to have similar chemical characteristics to the substance that it is trying to dissolve.Water is also a solvent, which is described as “inorganic” (not containing carbon).
Some important terms relating to solvents are as follows:
• True solvent—Aliquid that can dissolve the binder.
• Diluent—Aliquid that cannot dissolve the binder by itself but can be added to a solution to increase its capacity for the binder.
• Latent solvent—A liquid that cannot dissolve the binder by itself but increases the binder's tolerance for a diluent.
• Thinner—Any pure or mixed liquid added to a paint to reduce its viscosity (make it more flowable).
• Front-end solvent—A fast evaporating solvent that leaves the paint very soon after application, usually before the part reaches the oven.
• Exempt solvent—Solvents that do not react with sunlight to form smog and whose use is not controlled by regulation. A few chlorinated compounds are now considered exempt as well as acetone and methyl acetate. Review of the status of solvents in ongoing.
• Middle solvent—A medium evaporation rate solvent that leaves the paint primarily during flash-off and oven warm-up.
• Tail-end solvent—Aslow evaporating solvent that leaves the paint primarily during the baking cycle.
• Retarder—A slow evaporating solvent added to a paint to prolong the drying time. Typically used to reduce orange peel or blushing.
• Solvent blend—The particular mixture of liquids that gives a paint the desired flow or evaporation properties. Skillful solvent blending allows a paint shop to adjust purchased paint for local variation in temperature, humidity, line speed, application equipment, and so forth. The selection of a solvent system is often a complex task, best handled by the paint vendor. Important considerations in blending solvents are solvency, viscosity, evaporation rate, safety, and cost.
• Solvency—The ability of the solvent to dissolve the binder. The solvent must be able to keep the binder in solution.
• Evaporation rate—The solvent must evaporate within the time allowed by the curing schedule. Available oven length and temperature, as well as production volume is important. If the solvent is a mixture of two or more liquids, their individual evaporation rates become very important. For example, if the solvent remaining on the workpiece should become excessively rich in diluent, the binder might come out of the solution as small sticky lumps, rather than the smooth even film that is
• Viscosity—The fluidity or flow-ability of the paint. Paint viscosity is adjusted to provide good atomization and flow-out.
The proper solvent for a given task will depend on the required properties of the formula and other considerations such as safety and cost.
Safety issues affect the solvents chosen for formulation. For example, naphthalike hydrocarbons are very flammable and chlorinated hydrocarbons are very toxic,so they must be used with extreme care.
Many times, the choice of a solvent system is dictated by simple economics. The purchase price of the solvent is not the only factor here. The cost of handling and disposal must also be considered. These factors help account for the current interest in waterborne and powder coating systems.
Solvents are volatile liquids added to paints to dissolve the binder (the resin component) and/or to modify the paint viscosity. The paint viscosity must be compatible with the storage and application methods.
The solvent must have an evaporation rate that works well in the application environment. Ideally, the solvent should also be nontoxic, of low cost, and have an acceptable odor.
The particular solvents that may work in a given formula will depend on their solubility characteristics. There are three classes of solubility: strongly hydrogen bonded, moderately hydrogen bonded, and weakly hydrogen bonded. Alcohols fit into the first category; ketones, ethers, and esters into the second; and hydrocarbons in the third class. The approximate ranges of these parameters for the various classes of solvent are given in Table 2.4.
The above selection criteria are valid for solvents used for convertible binders. In nonconvertible coatings, solvents have more complex functions, contributing to the coating application characteristics, the drying time, and the properties of the final film. In this case, a blend of solvents is used to dissolve the binder and provide a diluent. The diluent may not be a true solvent for the binder, but it can increase the capacity of the primary (or true) solvent to dissolve the binder. It also costs less than the true solvent so that its inclusion reduces the overall cost of the formulation.
Resins used as paint binders have a range of parameter values that affect solvent selection. The solvent and the binder must be compatible. Epoxy resins, which have a range of 8–13, are soluble in ketones, ethers, and esters but insoluble in hydrocarbons and alcohols because the values are zero.
If a coating needs to be solvent resistant, it will be formulated with a resin that has a solubility parameter significantly different from the solvent(s) that need to beresisted.
Typically, compatibility of two or more resins in a coating requires that there is no more than one unit difference in the midpoints of their ranges.
Usually, when selecting solvents for a particular resin, the solubility parameters of the solvents have to lie within the range of the resin. Other selection criteria include cost, paint viscosity, and drying time.Ablend of diluent with the primary solvent will usually create a mixture with a mean that is within the mid-80% of the resin solubility range.
The evaporation rate of a solvent is particularly important for nonconvertible coatings. Slow evaporating solvents may cause long drying times and create problems in batch coating operations or excessive flow and sag of the wet film. High evaporation rates may cause problems with flow and film integrity. Selection is affected by the method of application. Typically, slower evaporation rates are required for brushapplied coatings while faster rates are used for spray methods. Some properties of
some common solvents are given in Table 2.5.
Aliphatic and aromatic hydrocarbons are commonly used to formulate and reduce a paint material. They are complex mixtures of different compounds with different flash points, density, and other properties.
Toluene is an aromatic hydrocarbon used in solvent blends for air-drying vinyl and
chlorinated rubber coatings as well as a diluent in nitrocellulose paints.
1.2 Mineral Spirits
Mineral spirits are largely aliphatic (but containing 15–18% aromatics) blend of paraffins of variable composition. They are slow-evaporating and dissolves most natural resins, oleoresinous varnishes, and medium- and long-oil alkyd resins. Mineral spirit is commonly used as the main (often sole) solvent for brush-applied decorative and protective paints based on these binders. It is also used as a cleaning and degreasing solvent.
Xylene is an aromatic hydrocarbon widely used as a solvent for short-oil alkyd, chlorinated rubber, and polyurethane resins. The good solvating power and moderate evaporation rate permit xylene to be used for sag-resistant spray-applied coatings (even of relatively thick films) as well as for baking-cured coatings.
Oxygenated solvents include alcohols, glycol ethers, ketones, esters, and glycol ether esters that are synthesized from other chemicals to form a desired solvent that is typically 99% pure.
2.1 Butyl Alcohol
Butyl alcohol is a slow-evaporating solvent for a wide variety of oils and resins, particularly amino and acrylic resins and also, in solvent combinations for nitrocellulose resins.
2.2 Ethyl Alcohol
Ethyl alcohol is usually used admixed with methyl alcohol, dyes, and toxins in the form of industrial alcohol (methylated spirits). Ethyl alcohol, a fast-evaporating solvent, is used for poly(vinyl butyryl) as well as with other solvents for nitrocellulose.
2.3 Ethylene Glycol Monoethyl Ether
Glycol ether is a slow-evaporating solvent for many resins, which is often added to brush-applied formulations that cannot incorporate aliphatic hydrocarbons such as white spirit. The high-boiling ethers are widely used as coalescing aids (solvents) in emulsion paints.
Acetone is a fast-evaporating powerful solvent used for vinyl copolymers and nitrocellulose. It is also blended (at low addition levels) with many other solvents, when its high solvating power and evaporation rate modify the properties of the liquid paint and the film. They are treated just like water on the HEARS (Health, Environmental and Regulatory Services) program and will be reported as VOC-less exempt solvents.
2.5 Methyl Ethyl Ketone (MEK)
MEK is a powerful solvent with a fast evaporation rate, widely used for vinyl copolymers,epoxy, and polyurethane systems. It is often blended with less powerful solvents to modify the film-forming properties and application characteristics of coatings.
2.6 Methyl Isobutyl Ketone
Methyl isobutyl ketone is a fast-evaporating powerful solvent used in similar applications as MEK but where a somewhat slower evaporation rate is required. It is also used at low addition levels in solvent blends where its fast evaporation rate and high solvent power will benefit the properties of the liquid paint and film-forming characteristics.
2.7 Butyl Acetate
Butyl acetate is an ester solvent, with moderately fast evaporation rate, of general applicability. Formerly butyl acetate was the major solvent for nitrocellulose coatings; it is now used for a wide variety of synthetic resins but is less powerful than the ketone solvents.
2.8 Ethyl Acetate
Ethyl acetate is another ester solvent with rapid evaporation rate. The major application of ethyl acetate was formerly in nitrocellulose coatings, but it now has a somewhat wider application. It has a lower solvating power than the ketone solvents.
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