Printer-friendly version

19 December 2003

Blowing Plastic Bubbles

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published July, 1973.

New bubble solution should open fields of novel experimentation to enthusiasts of both the arts and the sciences. The plastic-bubble project was developed as a hobby by Aristid V. Grosse, president of Germantown Laboratories, Inc., an affiliate of the Franklin Institute in Philadelphia. Grosse writes:

"Bubbles that are blown from appropriate solutions of plastic look much like soap bubbles. With plastic, however, surface tension plays no role in determining either the shape of the plastic bubbles or their behavior. For example, they do not necessarily expand into a sphere, nor do they shrink with time to form a flat film across the end of the blowpipe. Perhaps I should say, rather, that these properties have been observed in the solutions I have developed to date. The possibility of finding solutions with other characteristics is high. The five polymers I have named can be combined with some two dozen solvents to make more than 125 different combinations.

"Many other combinations can be devised. For example, the solvents can be mixed in various ratios and with other substances to control the rate of evaporation and alter the mechanical properties of the film, including its stiffness. Other possible variations include the temperature and viscosity of the solution, the molecular weight of the polymer and the rate at which the bubbles are blown.

"I tried only a limited number of the many variations before settling on a combination of polyvinyl acetate and acetone. It seems unlikely that I had the good luck to hit on the best possible solution after only a few experiments. Still, the combination of polyvinyl acetate and acetone is inexpensive and easy to reproduce. It can be blown into bubbles readily, and both the shape and the size of the bubbles can be controlled within wide limits.

"The resin has reasonably good heat stability. It will melt without decomposing. On the other hand, its stability of form is poor: articles molded from polyvinyl acetate tend with time to flow or flatten or otherwise lose their shape. For this reason the plastic is not generally used for molding solid articles, but it makes splendid adhesives and lacquers and is particularly good for blowing bubbles. When the resin is treated with alkali, the acetate groups are removed from the compound and replaced by hydroxyl groups to form polyvinyl alcohol, which is the water-soluble resin I used to increase the life of soap bubbles.

"The structural formula of the monomer is relatively simple [see left]. The molecular weight of the polymer is determined by the number of times (n in the formula) the monomer repeats itself. The properties of the resin depend in large part on the molecular weight of the polymer. For this reason a given resin can be defined chemically only if n is known or if the molecular weight of the polymer is known.

"Manufacturers usually identify polymers of specific molecular weight by an alphabetical or a numerical code. For example, I experimented with two grades of polyvinyl acetate made by the Union Carbide Corporation: grade AYAT (molecular weight 167,000, n 1,940) and grade AYAF (molecular weight 113,000, n 1,315). Both grades are shipped by the manufacturer in the form of colorless, pea-size pellets that have a density of 1.18 grams per cubic centimeter at a temperature of 20 degrees Celsius. Still another grade by the same manufacturer, AYAC, has a molecular weight of only 13,000 (n 150). My most successful bubbles have been blown with the AYAT and AYAF grades. Bubbles blown with grade AYAC have quite different mechanical properties. They tend to be brittle.

"As I have mentioned, I make the bubble solution by dissolving the polymer in acetone, which is one of the least costly solvents commonly used in chemical laboratories. In bulk it is priced by manufacturers at less than 15 cents a pound, but it may cost more than $1 a pound if it is bought in small quantities from druggists or dealers in paint supplies. Like many chemicals, acetone is a hazardous substance. The vapor is toxic and the liquid is highly flammable. Never smoke or allow flames or sparks near either a container of acetone or plastic solutions that contain the solvent. Experienced workers should handle acetone only in a well-ventilated area. Beginners are urged to work with the bubble solution either in a fume hood or, better yet, outdoors. The bubbles can be brought indoors after the acetone has evaporated.

Astrid V. Grosse with one of his plastic bubbles. Click image to enlarge.

"Nearly all my experiments were done in the basement of my house, which I have converted into a well-ventilated laboratory. Room temperature in the area is reasonably constant at 20 degrees C. Typically I make up plastic-bubble solution by putting about a pound of AYAT or AYAF pellets in a wide-mouthed jar with a cap. To the pellets I add a pint of pure acetone, which covers the polymer completely. If those proportions are maintained, the absolute quantity of the batch is not critical. A beginner can make up any smaller amount, such as four ounces of pellets in four ounces of solvent.

"A laboratory desiccator makes a convenient preparation vessel, particularly the type that has a serrated hose connection for coupling to a vacuum pump. I stir the solution gently but thoroughly every few hours. The pellets swell and dissolve slowly. The mixture becomes increasingly viscous with time.

"If evaporation is minimized, the solution consists of approximately 56 percent polyvinyl acetate by weight. The mixture usually contains many small air bubbles that become trapped in the solution when the pellets are stirred. The bubbles must be removed. I eliminate them by lowering the air pressure in the desiccator to two pounds per square inch, which is equivalent to a partial vacuum of about 100 torr.

"The desiccator can be pumped to this pressure by an inexpensive aspirator of the type that operates on a water tap. The solution tends to foam somewhat as the air pressure is reduced. Do not lower the pressure so abruptly that foam fills the space above the solution and overflows into the exhaust port. The foam subsides with time. I usually maintain the vessel at reduced pressure for about three hours. After air is admitted the mixture is completely colorless, clear and free of trapped bubbles. It is fully homogeneous and ready for use.

"Plastic bubbles are blown by much the same technique that I employ for blowing soap bubbles. Almost any tube can be used as a blowpipe. Of the various pipes I have tried the most convenient are the inexpensive plastic funnels that are available from dealers in scientific supplies. "