- November 30, 2004, Edgar B. Gutoff, Consulting Chemical Engineer
Bubbles in coatings are a very common defect and are the bane of most coating operators. But be certain that what you think are bubbles actually are bubbles. Examine the defect under a low-power microscope (about 50–200x), because craters, dirt particles, bacterial colonies, and pick-off from a face roll all have been mistaken for bubbles when examined by the unaided eye.
Bubbles frequently arise from air introduced during solution preparation. Try to prevent this from happening (see "Coating Bubbles: Causes & Cures" sidebar below).
Even when there are no bubbles in the coating fluid, bubbles still can form from dissolved air coming out of solution. Liquids dissolve less air at elevated temperatures and reduced pressures. Usually the temperature cannot be varied, but where the pressure is reduced, bubbles can form. In flowing fluids, this occurs whenever the fluid speeds up due to a reduction in the cross-sectional area, as in a valve. The increase in kinetic energy due to the more rapidly moving fluid causes this pressure reduction. The pressure can drop below atmospheric. Bubbles can form in the valve. Thus it may be worthwhile to use de-gassed liquids in solution preparation. Liquids are de-gassed in de-bubblers that operate under vacuum.
Do your best to prevent air bubbles in your coating fluid by taking the precautions outlined in this article. Even when the coating fluids do not contain bubbles normally, sometimes processes go awry. Install de-bubblers just in case this happens.
You also may want to reduce the dissolved air by using a vacuum de-bubbler to reduce any tendency to form new bubbles.
The simplest type of de-bubbler is just a large, relatively shallow chamber where the incoming fluid enters near the top, the residence time is adequate for all the bubbles of interest to rise to the surface, and the take-off point is near the bottom. The rate of rise of bubbles is given by Stoke’s Law, and this rate increases with bubble diameter. Obviously, less residence time is needed when the liquid layer is thin and the bubbles do not have far to rise to reach the surface. Thus, liquid flowing downward onto a dome spreads out in a thin film and so needs much less residence time than liquid in a pool.
Pulling a vacuum (but not enough to boil the fluid) will increase the volume of a bubble (bubble volume is inversely proportional to pressure), allowing the bubble to rise much faster. The vacuum also removes some dissolved gas. However, it does complicate the design, because now the liquid will not flow out of the unit unless it is pumped, and then a level controller must be used to maintain the liquid level in the unit.
Ultrasonics help in de-bubbling, as the standing waves that form drive the bubbles to the nodes of the sound waves where they coalesce to form larger bubbles, which then rise faster to the surface. Ultrasonics also help in de-gassing, as the ultrasonic energy acts to nucleate bubbles, reducing the supersaturation of any dissolved air.
Some de-bubblers are sold by the companies that supply coating machines, and some are designed by the coating companies themselves. One company de-bubbles its coating solutions batchwise before coating, by just pulling a vacuum of about half an atmosphere on the coating fluid for a number of hours.
Bubbles and Coating Speed
Air also can be entrained to form bubbles during the coating operation itself. A principle of fluid mechanics is that there is no relative motion at an interface. Thus a thin film of air (a boundary layer) is carried along by the moving web as it approaches the coating bead.
At relatively low coating speeds, this air is completely rejected as the web meets the bead. At higher speeds, a very thin film of air is entrained under the coating. This air film quickly breaks up to form very small bubbles, with a diameter on the order of the thickness of the air film. However, just as small crystals have a higher solubility than large crystals (due to surface energy effects), very small air bubbles have a higher solubility than bulk air, and these tiny bubbles tend to dissolve even when the coating liquid already is saturated with air. The liquid may become supersaturated with respect to bulk air.
However, at still higher speeds, a thicker air film will be carried under the coating, only to form larger bubbles when the film breaks up, and these larger bubbles may not dissolve fully because their solubility is now closer to that of bulk air. Also, they dissolve slower. In this case the maximum coating speed has been exceeded.
In the dryer, the coating liquid is heated and the solvent evaporates. If the film temperature exceeds the boiling point, bubbles and blisters may form.
As drying proceeds, the concentration of solvent in the film decreases and the boiling point, which is a strong function of solvent concentration, increases and becomes much higher than the normal boiling point of the solvent.
Also in the dryer, as the coating liquid is heated, dissolved air comes out of solution and conceivably may form bubbles, although I have never observed this.
In conclusion, to avoid bubbles in coating operations, do your best to avoid introducing air into the coating fluid. You should always consider using a de-bubbler, even when the coating fluids normally do not contain bubbles.
Coating Bubbles: Causes & Cures
- Strong agitation can introduce air into a solution and disperse it as bubbles. Also, a central agitator in an unbaffled vessel can cause vortex formation, which, if strong enough, will suck air into the solution to form bubbles. To avoid this, the vessel should be baffled, or one should use an off-center propeller agitator that is tilted, as described in books on mixing and in Perry’s Chemical Engineers’ Handbook.
- When a solution flows vertically downward out of a feed pipe onto the surface of a liquid in a mixing vessel, the flowing liquid, if at a high enough velocity, entrains air, which forms bubbles in the vessel. The solution should not flow vertically downward through the air but should be introduced against the vessel wall, preferably tangentially against the wall, which should keep the solution velocity below the air entrainment velocity.
- If the incoming solution enters below the surface and the feed line initially is empty, all the air in the line will be blown into the solution in the vessel, forming bubbles. Feed lines should enter above the liquid level in the vessel and should direct the liquid tangentially against the vessel wall.
- Any air pockets in the feed line can entrap air, which will be carried gradually into the vessel. If possible, it is desirable to have the line slope upward so the initial air will be pushed out by the liquid. Often this is not possible. However, the line should have no more than one high point, and preferably the high point should have a bleed valve.
- Liquid flowing through an empty feed line can leave air bubbles attached to the pipe wall, which later can be carried into the mixing vessel. Bubbles of a given size are swept away by fluid with a certain minimum wall shear stress, and the wall shear stress for a given flow rate increases with increasing viscosity and decreasing pipe size. This, of course, requires a larger pressure drop.
- Centrifugal pumps with a leaking seal can suck air into the pump suction if, as is often the case, the pump suction is below atmospheric pressure. This is one cause of bubbles.
- When fine particulates such as a powder are added during solution preparation, the solids tend to carry in large quantities of air. A suggested procedure to disperse the particles while minimizing the entrained air is to add the particles slowly into a vortex, which itself is not strong enough to suck air into the liquid.
Edgar B. Gutoff has been consulting on coating and drying since 1988, when he left Polaroid after 28 years, and is an adjunct professor of chemical engineering at Northeastern Univ., Boston, MA, where he has been teaching part-time since 1981. He is a fellow of AIChE and IS&T, serves on the Technical Advisory Panel of AIMCAL, and has received the Tallmadge Award for contributions to coating technology. He has co-authored two books on coating and one on statistical process control and has published more than 40 papers. Contact him at 617/734-7081; email@example.com; coe.neu.edu/~ebgutof/.
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