Spraying twin-feed

Because the reactants are not diluted in solventless coatings, pot-lives are short (0 5-2 hours). Because of short pot-lives, special twin-feed airless spray equipment is often used. Aromatic polyamines are preferred for curing, so that hardening times of 4-12 hours can be obtained even at 0-10° C. This better low temperature cure cannot be obtained in outdoor topcoats, where colour is important, since the aromatic polyamines discolour. 

In this method, the finish and activator are metered mechanically along separate feed lines and mix only at the instant of application, usually after both materials have left the application equipment. Special twin-feed spray-guns have to be used. The main objections are that sometimes mixing is not efficient enough and faults in the mechanical metering device can lead to insufficient or excess peroxide. If mechanical failure occurs, the defect will probably remain undetected until several objects have been coated and probably ruined. However, the latest equipment is quite good and the method is particularly suited to electrostatic spraying. 

The feed system consisted of a reservoir, a plunger-type feed pump, a twin-fluid atomising nozzle (giving a round spray of angle 13° in free air) and a mini-compressor. Inert gas for the twin-fluid nozzle was drawn between the outlet of the heat exchanger and the rotameter... 

The slurry is applied by direct dip coating of the twin drums in the feed tray at the bottom of the dryer (Figure 11.7) or by splash or spray feeders from a feed reservoir at the bottom of the dryer (Figure 11.8) on to the surface of the two steam-heated drums that are counter-rotating away from each other. - The sheet is formed by adhesion on to the drum surface and is held up against gravity by its surface tension. 

Spray impact is the impingement of a spray upon its target. It is a factor of droplet size, the feed pressure, the flow rate and the gas velocity in the case of twin-fluid atomizers. While impact is quantitatively measurable, it is more typically qualitatively measured. That is, testing with various nozzles and operating conditions can determine what impact is most effective for that spray application. 

To analyze the liquid fragmentation process of the PVP model solutions, a commercial lab-scale twin-fluid atomizer (Fig. 19.4) and the hot gas nozzle are used. The two atomizers mainly differ in their prefilming mechanism and the air-liquid ratio. Table 19.3 provides a brief overview of the atomizer parameters. The key parameter in this spray morphology study is the atomizer gas pressure, chosen that an optimum of the spray disintegration is foimd. The liquid feed rate and other parameters are kept constant during the experiments. The lab-scale atomizer is operated at a pressure of 5 bar absolute. 

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