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The lacquer dry type acrylic, better known as thermoplastic acrylic or TPA is a polyacrylate.
It is formed of methyl methacrylate blended with ethyl acrylate and the properties of the coating depend upon the blend ratio.
MMA __________________ EA
Hard, brittle Soft, flexible
The copolymer is of large molecular structure and its high viscosity requires extensive thinning for application – at least volume for volume.
Soluble in esters, ketones and aromatic hydrocarbons, it has fast dry properties by evaporation only and, therefore, classed as a non-convertible coating.
It is compatible with NC, plasticisers and vinyl resins, but not with alkyds and drying oils.
The dry film will soften back when subjected to high temperature and at 160°C-180°C will reflow to give a good gloss.
The reflow technique has been employed by the motor industry as OE.
Known as bake, sand, bake, it required first a force dry, followed by a second bake to obtain reflow. It is now almost completely replaced by stoving acrylics requiring only one bake.
The reflow system is not applicable to the systems in refinish.
TPA characteristics are:
PLUS
Excellent durability.
Good optical properties – water white.
Non-yellowing in clearcoats.
Non-critical application.
Good metallic control.
Excellent polishability.
MINUS
Low solids at gun.
Low gloss from gun.
Low flash solvents.
Poor water resistance.
FORMATION OF ACRYLIC POLYMER
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Grey Acrylic Motor-Car Lacquer:
Rutile Titanium Dioxide
)
Pigment
11.83
Lamp Black
)
0.12
Cellosolve Acetate
Solvent
2.27
Paraloid A21LV
Polymer
12.10
Material loaded into Roller Mill (Grinde stage):
Paraloid A21LV
Polymer
53.32
Butyl Benzyl Phthalate
Plasticiser
8.39
Cellosolve Acetate
)
3.47
Methyl Ethyl Ketone
)-
Solvents
4.05
Toluene
)
4.05
100.00
Let Down Stage (canned paint):
Acetone
)
39.50
Toluene
)-
Thinners
43.40
Cellosolve Acetate
)
17.10
100.00
Thin to 11 seconds Ford 4 cup for spraying at 16% solids.
Paraloid A12LV is acrylic co-polymer, largely Polymethyl Methacrylate at 30% solids.
The convertible coating known as Thermo-setting Acrylic or TSA is based on the Methyl Methacrylate and Ethyl Acrylate blend to give optimum hardness and flexibility, plus some styrene to give a degree of resistance to mild chemicals. A hydroxyl monomer is introduced to provide cross-linking with an amino-resin hexamethoxymethyl melamine.
MMA
Hardness
EA
Flexibility
SYTRENE
Oil & Grease
Resistance
HYDROXYL
Cross-linking with Amino
The copolymer forms a coating that requires stoving at 120-130°C and when baked it becomes completely set.
Used mainly in the car industry, its characteristics compared with the amino alkyd type are:
PLUS
Good durability.
Good metal control.
Easy repairability on track.
Clearcoat capability.
MINUS
Poorer resistance to some strong organic solvents.
Weathered coatings prone to solvent crazing on repair with high levels of plasticiser when compared to some industrial finishes (due to migration of the plasticiser content).
SILVER GREY STOVING ENAMEL FOR MOTOR CARS
Pigments
)
Aluminum Paste 65% Al
2.18%
)-
Carbon Black Synthetic Yellow Iron Oxide
0.07%
)
Synthetic Yellow Iron Oxide
0.04%
Resins
)
Synedol 2263 XB
69.12%
)
Synresine ME 2070
16.42%
Solvents
)
Cellosolve Acetate
2.30
)-
Toluene
6.88
)
n-Butanol
2.30
Additive
-
1% Silicone Oil in Xylene
0.69%
100.00%
Thin Xylene/ n-Butanol/ Cellosolve Acetate 65:30:5.
Spray at 30% solids, stove 30 MTS at 127%.
Synedol 2263 XB = Acrylic Copolymer at 50% solids.
Styresine ME 2070 = lso-Butylated M/F Resin at 70% solids.
