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发表于 2009-4-29 14:14:23
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4 ACRYLIC RESINS
In the coatings and plastic fields, the term “acrylic” resin applies to the polymers and
copolymers of the esters of methacrylic and acrylic acids. Copolymers of these esters
with nonacrylic monomers such as styrene, butadiene, or vinyl acetate are also referred
to as acrylic resins.
Acrylic resins are very versatile and popular for industrial liquid coatings. They
provide toughness, good weathering ability, and resistance to abrasion and chemical
attack. They are also considered to be better than alkyd resins for gloss retention.
Acrylic thermosetting resins that are cross-linked with epoxy or amino resins are
used in the appliance industry because of their excellent physical and chemical
properties.
Solid and solution polymers are used in lacquer finishes for metal, wood, leather,
ceramic, and plastic surfaces. Acrylic emulsions, which are manufactured directly in
the latex form, are used in both indoor and outdoor paints. Along with the stability
of the acrylic resins and their toughness and chemical resistance, they also provide
an appealing, high-quality coating. The hardness and slip properties can be varied over
a wide range to suit the demands of the application. Adhesion and solvent resistance
can be built into acrylic coatings by adding other functional monomers into the resin
system by copolymerization.
Acrylic polymers are used for decorative and protective functions. For decorative
applications, they provide high gloss, good pigment binding characteristics,
and clarity. For protective applications, they provide good adhesion, hardness, and
durability.
Acrylic lacquers are used for both ferrous and nonferrous substrates. Lacquers
are solutions of resins in organic solvents that harden as the result of the evaporation
of the solvent. This can be forced or accelerated with heat. The acrylic can be either
the major or minor portion of the lacquer.
There are thermoplastic lacquers that allow thermal reflow and better dispersability
of metallic colors than thermoset enamels. Applied over a primer and baked, the
acrylic lacquers resist many years of outdoor exposure.
Harder acrylics made of almost straight polymethyl methacrylate resin systems are
used to make hard, durable finishes for toys. Automotive touch-up lacquers in aerosol
cans are another large market. Plastic substrates are a growing market for acrylic
coatings, especially since there are now more grades of acrylic polymers that are
soluble in solvents that do not attack the plastic. These solvents include ethanol,
isopropanol, naphtha, and straight-chain hydrocarbons. The resin systems are usually
low to medium molecular weight homopolymers and copolymers of ethyl and n-butyl
methacrylate.
Thermosetting acrylics are generally harder, tougher, and more resistant to heat
and solvents than the thermoplastics. They are less resistant to UVlight and of course,
they must be heat cured to obtain cross-linking. Thermosetting acrylics are easier
to apply. The relatively low molecular weight copolymers of methacrylate with
other acrylic or nonacrylic monomers constitute the uncured resin. These functional
monomers provide sites for subsequent cross-linking, usually by reaction with nonacrylic
additives. These low molecular weight copolymers make it possible to apply
high solids, low volatile organic compound (VOC) coatings, which will level much
better before cross-linking and result in a smoother coating.
Most thermosetting acrylic resins available for commercial coating applications
contain relatively high amounts of styrene, vinyl-toluene, epoxies, or amine resins to
enhance their in-use properties, to lower their cost, or to effect cross-linking. The term
“acrylic thermoset” is even applied to coatings that have only the acrylic monomer
present to establish the curing or cross-linking sites for the coating. Because of the
nonacrylic nature of these coatings, they may not exhibit the characteristic properties
normally sought in an acrylic coating. |
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