Expert ForumsMixing

Topic: Re: What is the best way to minimize air bubbles?

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The answers to the following questions involve several assumptions about the type of mixing that might be involved.  In general, problems involving air bubbles being drawn into a liquid are less about the type of mixer and more about how it is being used. 1. How to minimize air bubble in Latex compounds. The mixing intensity or mixer mounting should not create a surface vortex that could draw air into the latex.  A surface vortex, especially a deep one is usually an indication of poor mixing, with little top-to-bottom motion and too much rotational flow (potentially just solid body rotation).  A center-mounted, top-etering mixer needs to have baffles in the tank.  An off-set, angle-mounted portable mixer should be aimed so that the axial flow from the impeller counteracts the inherant rotation created by the mixer rotation.
The simple and most effective answer is avoid drawing air into the mixer. 
2. What is the recommended stirrer/agitator type to avoid air bubble formation during mixing (compounding)?
The type of mixer and/or impeller is not as critical as the mounting or use of the mixer.  Reducing or eliminating a surface vortex is the first step.  A baffled tank is best.  One of the most critical factors in surface motion is how far from the surface the impeller is located.  Be sure that the impeller is located down in the batch, potentially no more than 1/2 the impeller diameter off the bottom of the tank.  Hydrofoil impellers are less likely to draw in air than pitched-blade turbines.  Adding powders always adds air bubbles, because bulk  powder is between 40% and 50% air. 
3. What is the effective de-aeration method for latex compounds?
The most effective de-aeration method is gentle mixing, just enough to keep the liquid moving, and operating under vacuum to reduce the quantity of dissolved gas.  With a lower saturation of gas in the liquid, some of the small bubbles can be re-dissolved when the vacuum is released.  Running and stopping a mixer may accumulate small bubbles in the impeller region, combine them into larger bubbles, and allow the release of the large bubbles when the mixer stops.
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