The paint delivery from spray guns, one of the most important factors of workability of paint spraying process, has been studied in relation with the type of flow, apparent viscosity measured with a Ford cup No. 4, the type and the nozzle size of spray gun, air pressure and the clearance of needle valve. For Newtonian fluids the delivery was inversely proportional to the viscosity, but this was not the case with non-Newtonians. The stochastical analysis of the results showed that all of the above the factors and some of their interactions were highly significant, the sum of the coefficient of determination reaching to 99%. The apparent viscosity of the non-Newtonians had a lower coefficient of determination, which is probably caused by an incomplete breakdown of the thixotropic structure in the spray gun. For the Newtonians it was observed that Q=kΔp/η where Q : delivery of paint from a spray gun, Δp : pressure difference between atmosphere and paint delivery pipe, η:viscosisty k : a constant related to the operating conditions. The apparent viscosities of the non-Newtonian samples were calculated from the above equation. They, however, did not equal the viscosities measured by other viscometers, and no regularity in the difference between these values was found. This is caused again by the incomplete breakdown of thixotropic structure.
As the accuracy of paint colour demanded becomes high, it becomes necessary to adopt a numerical and objective method in place of the conventional sensuous and subjective methods of colour determination. In the present report several problems pertaining to colour mesurements were discussed, and the possibility of the application of colour difference meters was studied. Although it is impossible to measure colour itself with a colour difference meter, it is possible to represent numerically the colour difference between two similar colours. It was found that the instrument has to be warmed up sufficiently before any measurements are made, and that the kinds of substrate and the conditions of film preparations have to be specified in order to obtain reproducible and results. In view of the practical aspects of paint application and of colour difference measurements, it appears that the reasonable value of colour diffence to standardize, is E-2.
The degree of sagging of paints during the process of spraying depends on the colour even if the paint compositions are the same otherwise, since the flow properties as well as hiding power are affected by the colour. Although a number of test methods of the degree of sagging have been reported, none of these appears to be satisfactory. In the present paper a new quantitative method was investigated, which provided very practical results. The results were in good accord with the results theoretically calculated from changes in wet film thickness, specific gravity, apparent viscosity and thixotropic recovery during solvent evaporation. Paints were sprayed at a constant speed on the panels laid on a drum which rotated at a specified speed. By changing the rotating speed of the drum, film thickness was changed. Thereby the critical film thickness above which the sagging appeared was determined. The extent of sagging was greater, the greater the apparent viscosity drop at high rate of shear was, and the slower the viscosity recovery was. The results which were obtained with the conventional applicator method did not agree with the results which were obtained with the above method. In the former method the solvent content immediately after spraying, thixotropic breakdown, its recovery and the wettability of new surfaces are not playing parts.