LARGE BUBBLES that display iridescence and other properties of soap bubbles can be blown from a recently developed solution of clear plastic. Plastic bubbles last for years. Unlike soap bubbles, they do not collapse when touched. Moreover, they can be colored with dye and coated with ornamental particles. Metallic powders and chemicals similarly applied can make the bubbles electrically conductive and even magnetic. The bubbles can be blown in a variety of shapes. The thickness and flexibility of the films can be altered at will.

For these reasons the 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:

"The new bubble solution is an outgrowth of the soap-bubble solution I described in these columns four years ago [see "The Amateur Scientist," SCIENTIFIC AMERICAN, May, 1969]. During those experiments I increased the life of soap bubbles substantially by adding a small amount of polyvinyl alcohol to the soap solution. Polyvinyl alcohol is a water-soluble plastic.

"Subsequently it occurred to me that a number of other organic polymers of high molecular weight would form bubbles. Examples are polystyrene, polyvinyl chloride, polyvinyl acetate, polyurethanes and cellulose esters. Water is specifically excluded from these compounds. The result is a water-insoluble film.

"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 vinyl polymers have been known for about 60 years, although their versatility and breadth of application was not generally appreciated before World War II. The vinyl acetate monomer is made by reacting acetylene and acetic acid in the presence of a mercury catalyst. The resulting compound can be transformed into the plastic polyvinyl acetate by heating the monomer in the presence of benzoyl peroxide.

"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.

"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.

"I invert the funnel and clamp the spout to a horizontal rod that is attached to the vertical rod of an apparatus stand of the kind found in chemical laboratories. A hose for blowing air through the funnel is connected to the open end of the stem. Small bubbles can be blown by mouth. A filter that contains sodium hydroxide should be inserted in the air line followed by another that contains a desiccating agent such as anhydrous calcium sulfate to absorb the exhaled moisture and carbon dioxide.

"I tend to run out of breath when I try to blow large bubbles by mouth. To inflate bubbles a foot or more in diameter I recommend working with any compressor that is capable of delivering air at a pressure of at least two ounces per square inch, which is equivalent to the pressure exerted by a column of water about four inches high. I have successfully used a bicycle pump, a vacuum cleaner and even a hair drier. An air connection was made to the vacuum cleaner and the hair drier with a perforated rubber stopper having a serrated hose connection that is inserted into the exhaust port. I now use a compressor of the rotary type made by the Gast Manufacturing Corporation (P.O. Box 97, Benton Harbor, Mich. 49022).

"The size of the funnels that serve as blowpipes varies in rough proportion to the size of the bubble to be blown. My funnels are made of either polyethylene or polypropylene. The outside diameters of the cones are 50, 78, 108, 150, 168 and 200 millimeters (two to eight inches). In addition to being inexpensive, plastic funnels make attractive blowpipes for several reasons. They are relatively light. The stems can be softened and sealed with heat. The bubble film can be peeled from the cone easily when the acetone has evaporated, after which the funnel can be reused.

"To blow a bubble put a shallow container of solution under the inverted funnel and raise the container to dip the cone into the solution [see illustration at left]. If the blowtube is open to the air, solution will wet both the inner an the outer surface. If suction is applied to the blowtube at an appropriate rate, solution will be pulled into the cone and will wet only the inner surface. Conversely, solution can be excluded from the interior by applying appropriate pressure to the blowtube; solution will wet only the outer surface of the cone. In all cases a thick film of plastic clings to the wetted surfaces.

"The bubble is started by blowing into the funnel as the container of solution is lowered. A closed film of solution forms between the lower edge of the funnel and the surface of the solution. The volume enclosed by the film increases as the experimenter continues to blow and to lower the solution.

"The shape the film assumes depends on a number of variables, including the relative distribution of plastic on the inner and outer surfaces of the funnel, the velocity at which the solution is lowered, the rate at which the film is blown and the rate at which solvent evaporates from the film. Unlike soap films, which are influenced by surface tension, bubbles of plastic do not necessarily expand uniformly. The shape is determined by the distribution of plastic that adheres to the blowpipe and its subsequent manipulation by the experimenter.

"The initial film that is formed by blowing as the container is lowered must be detached from the solution, after which the film is expanded into the final bubble by additional blowing. The shape of the initial film strongly influences the shape of the final bubble. The final shapes of bubbles can range from perfect spheres to prolate and oblate spheroids. Indeed, with a bit of practice the experimenter can blow ellipsoids, hyperboloids and catenoids. A perfect cylinder can be formed by inflating a long hyperboloid.

"I have blown cylinders 30 inches long by using a funnel only 15 centimeters in diameter. Polygonal bubbles- cubes, hexagons, fluted shapes and so on-can be blown with molds that are easily improvised from heavy aluminum foil [see illustration above]. Edges of the foil can be fastened together with tape or with a stapler. I line the molds with wax paraffin paper to prevent freshly blown plastic from sticking to the metal.

"The solution is compounded to be stiff when the solvent evaporates by adding a minimum amount of plasticizer, a material that will be discussed below. The evaporation of the solvent can be accelerated by circulating fresh air through the interior of the plastic film while it is in the mold. I do so by blowing fresh air into the bubble through one hole of a two-hole rubber stopper that fits into the stem of the funnel. Acetone vapor that evaporates from the film is swept out of the bubble through the second hole.

"The shape of the initial film that is to be expanded into the desired bubble is controlled in part by the thickness of the plastic that adheres to the inner and outer surfaces of the funnel, by the relative distance between the rim of the funnel and the surface of the solution at which the film is separated and by the shape of the cut. I make most separations by cutting the film with a pair of scissors. Freshly made solutions tend to be quite sticky, and the film may adhere to the scissors. To separate films that are blown with freshly made solutions I place strips of tissue paper below the proposed cut on both sides of the film and press the strips together with my fingers. The paper prevents the tacky material from adhering to my fingertips. The film is allowed to dry for about 30 minutes. A straight or circular cut can then be made with a pair of sharp scissors

"Less tacky material can be separated by pressing the film together without the use of paper strips and cutting it immediately with scissors that have been lightly coated with paraffin oil. Films that are cut close to the funnel tend to expand into oblate spheroids. Those that taper toward the solution and are cut close to it expand into prolate spheroids. Cylinders expand into spheres [see illustration at left].

"The film can also be detached from the solution without cutting. By adjusting the rate at which the container is lowered and the rate at which one blows it is possible to pull a conical film with the apex at the solution. The film can be twisted apart at the narrowest region. It is also possible to gather a film by dipping the blowpipe into a relatively thin solution and withdrawing it sideways, as is done with a soap solution. Such a film contains a limited quantity of plastic, however, and cannot be blown into bubbles of the largest sizes. Surplus film in the form of a thread or a tab usually remains at the place where the material has parted. Fold the surplus against the adjacent film. It will adhere and stretch as the bubble is blown. With experience the experimenter can make seals that all but vanish.

"Beginners may have some difficulty blowing bubbles of uniform shape. Bulges tend to develop in areas where the film is thinnest. Simply stop blowing into the bubble and instead blow air across the thin area to accelerate the evaporation of solvent. The viscosity of solution in the thin area increases quickly because of the lowered temperature and the increase in the density of the plastic. Films of plastic must be inflated more slowly than those of soap solution. I often blow plastic bubbles in a series of partial expansions, allowing the plastic to flow and thus to adjust itself to local stresses between successive inflations.

"The thickness of films that have been dyed is much easier to judge by eye than the thickness of clear films. Bubbles of uniform shape can therefore be blown most easily from dyed solution. Various acetone-soluble dyes are made by Du Pont, Tennessee Eastman and many other companies. I use Du Pont's oil-blue-A and oil orange. The dye solution is made by dissolving one part dye in 99 parts acetone, by weight. I then mix 20 milliliters or more of this solution in 500 grams of clear plastic solution, depending on the intensity of the color that is wanted.

"In general bubbles can be blown to diameters about 10 times larger than the diameter of the funnel. For example, a 60-inch bubble can be blown with a 15-centimeter funnel if the experimenter blows slowly, takes care to gather an initial film of uniform thickness and maintains uniformity by accelerating local evaporation. The speed with which bubbles can be blown depends in part on the relative distribution of solution on the inner and outer surfaces of the funnel. Solvent evaporates readily from solution that adheres to the outer surface of the cone. The film stiffens quickly, which minimizes the required blowing time. Conversely, minimum evaporation occurs inside the funnel, where the atmosphere is saturated with acetone. The solution flows readily and so can 'feed' the expanding bubble for a considerable time.

"Internal coating is preferred for flowing extraordinarily thin films. Films that vary in thickness from 200 microns down to one micron can be blown with an eight-millimeter funnel that carries an internal coating of freshly made solution. (One micron is equal to about 40 millionths of an inch.) Thin bubbles must be blown quickly, as in the case of soap solution. The thinnest bubbles show rainbow colors. Hence, as Isaac Newton pointed out, the thickness of the films must approach the wavelength of light.

"The funnel can also be detached from the bubble, although frequently I just uncouple the blow hose and plug the stem with a stopper. The stopper can be removed to reinflate the bubble from time to time. To detach the funnel I make a tool by cutting a hole slightly larger than the diameter of the funnel in a sheet of either polyethylene or Teflon that is mounted on a flat backing of stiff cardboard or plywood. The diameter of the sheet should be several times the diameter of the hole. The plastic surface surrounding the hole is lubricated with a film of paraffin oil.

"The stoppered funnel is dropped through the hole so that the fully inflated bubble rests on and is supported by the upper surface of the lubricated plastic. I then exert a light, downward pull on the stem of the funnel and simultaneously rotate the stem two or three revolutions. This maneuver twists the film near the funnel into a short cord. I cut the cord with scissors close to the bubble and brush the tip lightly with acetone to ensure an airtight seal.

"Bubbles that have been sealed completely can be reinflated by thrusting a large hypodermic needle through a thick area of the film. The resulting hole can be sealed with a dab of viscous plastic solution. After drying for a few weeks bubbles tend to retain their initial shape even if they are not sealed.

"Bubbles blown from solutions that consist solely of polyvinyl acetate dissolved in acetone tend to become rigid and brittle when dry. This may be a desirable property if the bubble is not subjected to stress. The plastic can be made as flexible and tough as one wishes, however, by adding to the solution one or more plasticizers. A plasticizer is essentially a liquid that boils at about the same temperature as a heavy oil and is a good solvent for the material it is to plasticize.

"The substances known as phthalates (particularly dibutyl, diethyl, butyl benzyl and butyl octyl) have been found to be quite effective plasticizers for the vinyl plastics. After some experimentation I settled on dibutyl phthalate (which is also known as n-butyl phthalate) as a good plasticizer for bubble solution. I dissolve the oily liquid in an equal part by volume of acetone and, depending on the desired flexibility, add from 2 to 15 percent (by weight) of the mixture to the bubble solution. The optimum percentage of plasticizer must be determined by experiment. In general, however, a 2 percent solution results in bubbles that approach brittleness after they have fully dried, whereas bubbles blown with solutions that contain 15 percent plasticizer remain almost as flexible as rubber balloons.

"Occasionally the experimenter may wish to soften a bubble that has become too stiff. Such softening can be accomplished easily. Just as the film of the bubble hardens as the solvent evaporates, so can it be made pliable by allowing the plastic to absorb acetone. Do not apply liquid acetone to the bubble, however. The liquid will dissolve the plastic and make a hole. To soften the film displace the air inside the bubble with an atmosphere that is saturated with acetone vapor. I achieve such an atmosphere by bubbling air from a compressor through a closed container of acetone. Vapor from the container enters the bubble through a glass tube in the stem of the funnel. The spent air is exhausted from the bubble through a T fitting that surrounds the inlet tube coaxially [see illustration at left].

"With practice and patience the experimenter can blow plastic bubbles in a remarkable range of sizes. My largest plastic bubble measured just slightly less than 11 feet in horizontal diameter. In contrast, my largest soap bubble was just over four feet. I have also had a lot of fun blowing what I call 'microbubbles,' which are tiny spheres that show rainbow colors. To blow them I file the sharp end of a large hypodermic needle to a square tip. After dipping the tip in plastic solution I blow the bubble by gently pushing the lubricated plunger of the syringe. The bubbles range from .5 millimeter to 10 millimeters in diameter. They dry much more quickly than large bubbles and last for months. Incidentally, plastic bubbles will float in water indefinitely without damage.

"Polyvinyl acetate is available from a number of U.S. companies, including the Borden Chemical Division (Borden, Inc.), the Celanese Corporation, E. I. du Pont de Nemours & Company, Goodyear Tire & Rubber Company, Polyvinyl Chemicals Inc., the Sherwin-Williams Company and the Union Carbide Corporation The material currently costs about $1.50 a pound, but it is available from manufacturers only in a minimum quantity of 100 pounds. For a limited period experimenters can buy half pound packages of grade AYAT polyvinyl chloride from our laboratory for a handling charge of $2.50. Make checks payable to Germantown Laboratories (4150 Henry Avenue, Philadelphia, Pa. 19144). In addition the laboratory has assembled bubble-blowing kits in three sizes. The kits contain various dyes, plasticizers and related materials. Written material describing them will be forwarded on receipt of a self-addressed, stamped envelope. Acetone, which is a highly flammable solvent, cannot be mailed. It is available in small quantities from most druggists."

Bibliography

TRUE PLASTIC BUBBLES AND HOW TO BLOW THEM A. V. Grosse. Plastic Bubbles Report No. 1, GL 1970-4, Germantown Laboratories, Philadelphia, 1970.

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