Stage I: Charging
1.000 Zinc Naphthanate 6%
17.500 Soya Medium Oil Alkyd 60%
Stage II: Stabilization & Cleaning
10.000 Soya Medium Oil Alkyd 60%
Stage III: Adjustments
22.500 Soya Medium Oil Alkyd 60%
2.000 Melamine Formaldehyde MTO Tolerance a
0.080 Cobalt Drier 6%
0.400 Calcium Drier 6%
0.200 Lead Drier 18%
0.200 Zirconium Drier 6%
1.000 Marproof Agent PA 52
1.000 Anti Skinning Agent 10% Soln.
3.985 Adjustments MTO/ Alkyd 60%
This product comes under Metal Decorating for a Can coating, applied by roller to thin sheets. Many problems were involved in developing this formulation, and as a result a lengthy list of components was made up, which is not ordinarily a good formulating practice. However, the above formulation solved these problems.
Usually rubber rollers are used, hence we can not use aromatic solvents like Toluene etc.
After painting and coming out from oven hardly after 5 - 7 minutes they are stacked one above the other and they are heavy in weight also. Therefore waxes / Petroleum jelly helps a lot in keeping them tackfree and as a result they don't stick to each other.
The pigmentation levels in these formulations is very high as DFT is very less.
In roller-coating operation the sheets are fed flat through the rolls, where the coating is applied, passed for a few seconds flash-off and then lifted by wicket-type conveyor to a nearly vertical position and carried through the baking oven. This procedure sometimes causes the problem known as wicket marking, a gloss or shadow effect on the finished coating side showing the exact outline of the wicket touching the back or uncoated side.
The problem is eliminated by selecting the amine resin with lowest cure point. Since, the metal going through the baking oven does not reach its full temperature immediately, it is necessary to obtain initial cure at the earliest point possible in order to eliminate the wicket marking.
Normally travel time is 14 minutes out of which only 2 minutes peak temperature of 140°C is achieved.
Flow is of utmost importance in Roller coating, since only a fraction of time is allowed for flash-off. Therefore, 1% Zinc naphthanate is provided to keep the film open to avoid wrinkling and blisters.
However, too much flow will cause unsightly sagging as the sheets are raised to their vertical position. There are two kinds of flow, one external (on the surface), the other internal (within the pigment phase). It is possible to achieve absolute smoothness of surface and yet to obtain a mottled white appearance on a coated sheet. Only when the internal pigmented portion flows in the same manner as the film surface will the finish be acceptable. Naturally the solvent combination plays a very important role in achieving the optimum flow, in the above formulation Aromax is used for the purpose.
Petroleum jelly / Wax has been added to provide the “slip” required to prevent the sheets from “blocking” or sticking together when stacked. No recoating problems were encountered with this formulation when stenciling or printing operations are followed.
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