16 августа, 2015 
Malyar Paint imperfections or mechanical damage can occur post-manufacture. Due to the many heat sensitive parts on the car, (for example, tyres, interior plastic trimmings), it is not possible to repaint the vehicle with the standard paint and subject the whole vehicle to the original curing process, which took place at 120°C or above. Special repair coatings have therefore been developed to cope with this situation. Two routes are possible:
a) acid catalysis of the original top coat finish
b) use of a two component acrylic isocyanate coating (covered in the Chapter on 2K finishes).
The standard thermosetting acrylic can be catalysed with up to 3% of an organic acid such as p. toluene sulphonic acid or acid butyl phosphate. This is added to the paint immediately prior to repainting and reduces the curing threshold temperature from 120 to 85-90°C. Local infra red heaters are used to cure the repainted area. Alternatively, in extreme cases the whole vehicle can be repainted and put through a low bake oven.
Typical starting formulations for automotive finishes are given below :
FORMULATION 4-1:
BASIC THERMOSETTING ACRYLIC AUTOMOTIVE ENAMEL
|
60% titanium dioxide mill base |
22.50 |
|
Coloured mill bases |
3.34 |
|
Hydroxy acrylic (55% wt. solids) |
36.40 |
|
Butylated melamine formaldehyde resin (55% wt. |
|
|
solids) |
24.12 |
|
Butanol |
3.04 |
|
xyiene |
7.08 |
|
Dowanol PM acetate |
3.24 |
|
50% p-toluene sulphonic acid in xylene |
0.28 |
|
Total |
100.00 |
The above formulation should be thinned to spray application with around 10% by volume solvent to a viscosity of 25-30 secs Ford No. 4 Cup.
Cure schedule is 30 min. at 120°C ™ Dowanol is a registered trade name of Dow Chemical Co.
Colour matching requirements in automotive finishing are very exacting. Pigments are dispersed individually to produce a range of standard mill bases. These millbases are then mixed and let down with resin to produce the final colour. Addition of butanol improves the shelf life. High boiling Dowanol PM acetate is added to improve levelling. The amount of acid catalyst added should be kept to a minimum to prevent the reduction of film properties.
A typical metallic top coat formulation is given below:
FORMULATION 4-2:
BLUE METALLIC AUTOMOTIVE TOP COAT ENAMEL
|
Alcoa aluminium paste 726*1* Phthalocyanine blue ® |
1.3 |
|
0.7 |
|
|
60% solution hydroxy acrylic resin |
67.5 |
|
60% melamine formaldehyde resin |
15.0 |
|
Xylene |
11.0 |
|
n-Butanol |
4.0 |
|
Dowanol PM® acetate |
0.5 |
|
Total |
100.0 |
|
(1) Alcoa Ltd. (2’1 Irgalite Blue BCS Ciba Geigy Dowanol is a registered trade name of Dow Chemical Co. |
Thin to spray viscosity (20-25 secs Ford No 4 Cup) with xylene, butanol, butyl acetate solvent mixture
Cure schedule is 30 min. at 120°C
Alcoa aluminium paste 726 is a non leafing grade, acid resistant, tinting paste.
A typical formulation for a COB system is as follows :
FORMULATION 4-3:
CLEAR OVER BASECOAT SYSTEM — BLUE POLYCHROMATIC BASECOAT
|
Aluminium paste*1* |
6.10 |
|
Irgalite blue GLA *2* |
0.60 |
|
Saturated polyester resin*3* |
28.30 |
|
60% Iso butvlated melamine*4* formaldehyde resin CAB 5510.2’5* |
5.50 |
|
4.30 |
|
|
CAB 5510.01(5) |
4.30 |
|
Di butyl phthalate |
1.60 |
|
10% wax dispersion*6* |
34.19 |
|
Ethyl acetate |
15.11 |
|
Total |
100.00 |
Viscosity at 25°C approx 1 mPas
Vehicle solids content 36.2%
Total solids content 40.5
Pigment/Binder ratio 0.2/1
Thin to spray viscosity (15-20 secs Ford No. 4 Cup), with fast solvents (e. g. ethyl acetate/MEK blend)
^Sparkle silver 5262, Silberlene *2* Ciba Geigy
*3*e. g. Synolac 154X, Cray Valley *4*e. g. Dynomin MI-11-І, Dyno Industries ^Eastman Chemical *6*e. g. Cerafak 1000 (Cera Chemie)
A good basecoat is important in a COB system. Coloured metallic basecoats use strong, transparent, light fast pigments dispersed in an oil free polyester resin. As the film thickness is thin, relatively high levels of pigment are required to give deep metallic shades.
The melamine resin promotes hardness and adhesion. The combination of CAB’s, which are firstly dissolved in ethyl acetate, give a balance between solids content and the rate of setting and hardness. The plasticiser improves cold check resistance.
A wax dispersion is preferred over powdered waxes because of the difficulty in obtaining the required degree of dispersion from the latter. The wax controls the aluminium lay-down and improves lustre. It also protects the basecoat from attack by solvents in the overlacquer.
A typical formulation for an overlacquer might be:
FORMULATION 4-4: CLEAR AUTOMOTIVE OEM OVERLACQUER
|
Hydroxy functional acrylic(1) (60%) |
60.83 |
|
Butylated melamine formaldehyde resin solution (60%) |
26.20 |
|
Tinuvin 900[8]2* |
0.79 |
|
Tinuvin 292(2) |
0.43 |
|
Irganox 1010® |
0.12 |
|
Byk 3100’3) |
0.49 |
|
Xylene |
8.35 |
|
Butanol |
2.79 |
|
Total |
100.00 |
Viscosity at 25°C 3,500 mPaS
Vehicle solids content 53.2%
Thin to approximately 20 secs Ford No. 4 Cup with Xylene/Butanol/Dowanol PM acetate solvent mixture.
See list for suitable resins Ciba Geigy Byk additives
The pigmented basecoat is applied by 2 wet on wet passes, for better appearance, to a film thickness of about 15 to 20 jam. It is allowed to flash dry for 15 minutes prior to application of the clear overlacquer. The complete system is then cured for 30 min. at 120-130°C.
Air drying overlacquers suitable for refinishing are covered in the 2 pack isocyanate/acrylic chapter.
Thermosetting acrylics recommended for automotive OEM finishing include :
Synocryl 826S Synocryl 850ST* Crodaplast AC500 Crodaplast AC540 Synthacyl SC303 Synthacyl SC370 Setalux CI385BX51 Setalux CI389UU60 Setalux CI756UU65 Setalux C91389VX45 Setalux C91757VX60
Cray Valley Ltd.
Cray Valley Ltd.
Croda Resins Ltd.
Croda Resins Ltd. Resinous Chemicals Ltd. Resinous Chemicals Ltd. Synthese NV Synthese NV Synthese NV Synthese NV Synthese NV
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