Kiến thức sơn Acrylic

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Kiến thức sơn Acrylic

25/06/2018 - 09:46:09
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Paint is used to decorate, protect and prolong the life of natural and synthetic materials, and acts as a barrier against environmental conditions.

Paints may be broadly classified into Decorative paints, applied on site to decorate and protect buildings and other objects, and Industrial coatings which are applied in factories to finish manufactured goods such as cars.

The constituents of paint

Figure 1 Contents of a white gloss (alkyd) paint and a white matt emulsion (acrylic) paint.

Paints contain:

pigment(s) - prime pigments to impart colour and opacity
binder (resin) - a polymer, often referred to as resin, forming a matrix to hold the pigment in place
extender - larger pigment particles added to improve adhesion, strengthen the film and save binder
solvent (sometimes called a thinner) - either an organic solvent or water is used to reduce the viscosity of the paint for better application. Water-borne paints are replacing some paints that use volatile organic compounds such as the hydrocarbons which are harmful to the atmosphere.
additives - used to modify the properties of the liquid paint or dry film

The binder (resin) and solvent together are sometimes known as the vehicle. The binder may be dissolved as a solution or carried as a dispersion of microscopically small particles in a liquid.

Depending on the type of paint and intended use, additives may include:

dispersants - to separate and stabilise pigment particles
silicones - to improve weather resistance
thixotropic agents - to give paints a jelly-like consistency that breaks down to a liquid when stirred or when a brush is dipped into it
driers - to accelerate drying time
anti-settling agents - to prevent pigment settling
bactericides - to preserve water based paints in the can
fungicides and algaecides - to protect exterior paint films against disfigurement from moulds, algae and lichen

Paints are formulated according to their proposed use - primer, undercoat, special finishes (matt, gloss, heat resistance, anti-corrosion, abrasion resistance). The pigment powder is broken down into individual particles which are coated by and dispersed in the binder (resin) - known as 'wetting out'. Solvent is then added to give the required consistency. Each batch of ingredients is thoroughly mixed in large, stirred containers with the required additives (Figure 1). Amounts ranging up to 40 000 dm³ of paint may be made in a single batch.

This unit discusses the most commonly used binders followed by the pigments.

Binders in paints

The three most important binders (resins) used in modern paints are:

acrylic polymers (resins)
alkyd polymers (resins)
epoxy polymers (resins)

Acrylic polymers (resins)

The binder in many emulsion paints is based on homopolymers or co-polymers of ethenyl ethanoate (vinyl acetate) and a propenoate (acrylic) ester.

Ethenyl ethanoate is manufactured by passing a mixture of ethanoic acid vapour, ethene and oxygen over heated palladium(ll) and copper(ll) chlorides:

Ethenyl ethanoate and an acrylic ester (for example, methyl 2-methylpropenoate) are then co-polymerized to form a random array, in which these groups link into a linear chain: