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发表于 2009-4-29 11:54:24
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4. Paint Additives
Paint additives are auxiliary products that are added to coatings, usually in small
amounts, to improve particular technical properties of the paints or coating films.
Paint additives are named in accordance with their mode of action.
Leveling agents promote formation of a smooth, uniform surface on drying of the
paint. Suitable materials include certain high-boiling solvents such as butyl ethers of
ethylene glycol, propylene glycol and diglycols, as well as cyclohexanone and alkylated
cyclohexanones, and in some cases aromatic and aliphatic hydrocarbons. Low
molecular mass resins (e.g., some polyacrylates and silicones) are also used. Solid
leveling agents, such as special low molecular mass resins, are also useful for improving
the surface properties of films produced from powder coatings. Flow agents act
by reducing the paint viscosity during drying. The effectiveness of a particular flow
agent depends on the type of binder and the drying or hardening temperature.
FilniTformation promoters, which are closely related to flow agents, reduce the
film-forming temperature for film formation from dispersions, leading to a surface
that is as pore-free and uniform as possible. Certain high-boiling glycol ethers and
glycol ether esters are used, often in combination with hydrocarbons.
Wetting Agents, Dispersants. and Antisefting Agents. Wetting agents from one of
the largest groups of coating additives. These are surfactants which aid wetting of the
pigments by the binders and prevent flocculation of the pigment particles. This leads
to the formation of a uniform, haze-free color and a uniformly high luster of the coating film. This group also includes the dispersants, which give good pigment
wetting and hence optimum dispersion of the pigments in the paint, thereby preventing
sedimentation particularly of high-density pigments. As well as good wetting
properties, some pseudoplasticity is also necessary. Antisetting agents have similar
characteristics to dispersants.
Ant$oaming agents are used to prevent foaming during paint manufacture and
application and to promote release of air from the coating film during drying.
Various products are used, including fatty acid esters, metallic soaps, mineral oils,
waxes, silicon oils, and siloxanes, sometimes combined with emulsifiers and hydrophobic
silicas.
Catalysts are added to paints to accelerate drying and hardening. They include
drying agents (driers, siccatives), which, in the case of the air-drying binders (including
some alkyd resins or unsaturated oils), accelerate decomposition of the peroxides
and hydroperoxides that form during the drying process, thereby enabling radical
polymerization of the binders to take place. The driers used are mainly metallic soaps
such as cobalt naphthenate; manganese, calcium, zinc, and barium salts; and zirconium
compounds.
Various products are used to catalyze the cross-linking of binder systems at room
temperature. For acid-catalyzed systems such as polyester - melamine resin systems,
free acids, their ammonium salts, or labile esters are suitable. while for base-catalyzed
systems such as polyester - isocyanate, tertiary amines or dibutyltin dilaurate
are used. The amount of catalyst used must be such that the pot life is not impaired.
Antifloating and antiflooding agents prevent horizontal and vertical segregation of
pigments with different densities and surface properties. This prevents differences in
the color and luster of the surface of the film, which can lead to a blotchy appearance.
Antiskinning agents are added to air-drying paints to prevent surface skin formation
caused by contact with atmospheric oxygen. In the film, they produce uniform
drying and prevent shrinkage (wrinkling). Chemically, these materials are antioxidants
such as oximes, which evaporate with the solvents during the drying process.
Matting agents are used to produce coatings with a matt, semi-matt, or silk finish.
They include natural mineral products such as talc or diatomites and synthetic
materials such as pyrogenic silicas or polyolefin waxes. Matting can also be obtained
by special formulations that exploit the incompatibility between binder components
and their cross-linked structures.
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.
Thickening agents control the rheological properties of paints of various types.
They include inorganic (mainly silicates), organometallic (titanium and zirconium
chelates), naturally occurring organic (mainly cellulose ethers) and synthetic organic
products (polyacrylates, polyvinylpyrrolidone, polyurethanes).
Preservarives (biocides, fungicides) prevent the attack of paint systems, principally
water-based, by microorganisms.
Corrosion inhibitors are used to prevent the formation of corrosion products when
waterborne paints are applied to metallic substrates (flash rust). They include oxidizing salts such as chromates, metaborates, nitrites, and nitrates; organic amines or
sulfur-containing products; and organic salts (benzoates, naphthenates, octoates). |
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