OF ALL THE MATERIALS WITH WHICH we come in daily contact, rubber is unique in having a peculiar charm. Given half an opportunity, we almost instinctively play with it; we stretch it, bounce it, knead it. Another of rubber's engaging qualities is that it comes in an extraordinary variety of forms, from soft foam rubber to hard rubber. This very versatility probably discourages the amateur who thinks of performing chemical experiments with rubber or of mixing his own rubber. The variety of rubbers suggests that, to work with them requires a chemical manufacturing plant, or at least a well-equipped chemical laboratory.

Paul West of Park Ridge, III., has not found it so. With relatively simple equipment he compounds his own rubber in all sorts of interesting ways. He even makes special foam rubbers that can be molded into anti-vibration parts and can otherwise be applied in the home laboratory. West writes:

"Before dashing off to make a foam-rubber pillow, you may find it useful to review a few basic facts about the nature of rubber and to do some introductory experiments. The starting material is pure natural latex. It can be ordered, together with essential processing chemicals, through local dealers in chemical supplies or through druggists in most communities. Latex is a slightly viscous liquid drawn from the inner bark of such tropical trees as Hevea brasiliersis. It consists of a 35-per-cent dispersion of isoprene (the basic building block of rubber) in water, together with small quantities of fatty acids, proteins, resins, sugars and enzymes. For reasons of economy natural latex is usually concentrated for the market to an isoprene content of 62 per cent.

"The isoprene molecule (called a monomer) is a hydrocarbon consisting of 13 atoms; five of carbon and eight of hydrogen? arranged as shown in the accompanying structural formula [upper left in illustration]. When isoprene is placed in a suitable environment, either physical or chemical, the electron configuration of its molecule is changed as illustrated [upper right]; in this state it tends to attach itself to a similarly modified molecule in its vicinity. The united pair annex still other molecules to form a growing chain or fiber that may ultimately contain as many as 5,000 monomers. The result is a giant molecule of rubber (called a polymer). In the absence of external mechanical force the rubber molecule assumes a randomly coiled form. When the coil is stretched, it straightens into a long strand that snaps back into the coiled pattern when the force is removed. It is this property that enables rubber to stretch, flex and absorb vibration.

"Latex is physically unstable. The monomers easily acquire the electron configuration that encourages the growth of rubber molecules. To observe this reaction pour an ounce or two of latex on a level sheet of clean glass and let the water evaporate. Within a matter of three to 12 hours, depending upon the temperature, the latex will coagulate into a solid sheet of rubber that you can peel from the glass. Examination of the sheet will quickly disclose, however, that it does not exhibit the properties we usually associate with rubber. You will find that although the sheet can be stretched, it never quite returns to its original shape. It is soft; you will doubtless find your fingerprints on the surface, and the material will stick to itself when pressed together. This was the only kind of rubber known until 1889, when Charles Goodyear discovered a chemical reaction, known as vulcanization, that made rubber tough.

"The molecular structure of the rubber sheet formed on the glass may be imagined as a thick layer of coiled chains or fibers piled at random. When the sheet is stretched, some of the individual chains are straightened, but most of them merely slide past each other. When the force is removed, the straightened chains return to their coiled state, but chains that have shifted position remain at their new sites. This accounts for the distortion of the stretched sheet. Goodyear accidentally hit upon a way to stitch adjacent polymers together so that they could not slide past one another in the pile. He did not know why his process worked, but according to modern theory he took advantage of the fact that each isoprene unit of the polymer contains a pair of carbon atoms that are joined by the electrical forces of two pairs of electrons that continuously exchange orbits; in other words, between each pair of carbon atoms there is a double chemical-bond. When an atom of sulfur wanders into the vicinity of the rubber molecule, the double bond in effect opens up, permitting a three-way exchange of electrons that includes those of the sulfur atom. A bond or bridge is thus established between the polymer and the sulfur atom. The same sulfur atom can react similarly with a second polymer and thus link the two. Similar bonds form throughout the rubber, their number depending upon the number of sulfur atoms present. Thereafter they restrict the freedom of the polymers to coil and uncoil; the closer the stitching, the more rigid the rubber. Toy balloons have relatively few crosslinks; they normally contain about .5 per cent of sulfur by weight. Conversely, bowling balls and hard-rubber objects such as combs are closely linked; they contain as much as 47 per cent sulfur.

"To produce vulcanized rubber, then, why not simply dissolve the desired amount of sulfur in latex, permit the water to evaporate and place the resulting coagulated mix in a heated oven to hasten formation of sulfur bonds? Essentially this is what Goodyear did, though he started with coagulated rubber. The procedure is impractical. First, sulfur does not dissolve in latex; second, unless an 'accelerator,' or catalyst, is added to speed the reaction, vulcanization requires a very large amount of heat.

"Sulfur can be introduced into latex as a colloidal dispersion in water, however. It is reduced to this form in a ball mill, along with a dispersing agent (such as Darvan) and a protective colloid (casein). As prepared commercially, these ingredients are poured into a rotating drum containing spheres of porcelain and are reduced to the required fineness by the tumbling balls. Having no mill, I purchase commercially prepared sulfur dispersions. Most come in a concentration of 50 per cent. When computing the weight of sulfur needed for an experimental batch, it is therefore necessary for me to allow for the water in the commercial preparation.

"The heat required for vulcanization is minimized by adding an accelerator to the mix. Although many chemicals can act as accelerators (some induce complete vulcanization in a few minutes at temperatures below 200 degrees Fahrenheit), not all are soluble in latex. They must be milled and dispersed in water just as sulfur is.

"Accelerators that are insoluble in water are in general more satisfactory than the water-soluble accelerators, because they are not lost by leaching, particularly when the heat for vulcanization is applied by immersing the rubber in hot water. A water-soluble accelerator ('Butyl Zimate,' or zinc dibutyldithiocarbamate) is used in the next experiment I shall describe, however, because of its convenience. A relatively small amount of the chemical, something less than 2 per cent by weight of the other ingredients, produces the desired acceleration. When accelerators are used in excessive amounts, they thicken the latex, and the resulting product appears to have been scorched. The catalytic action of Butyl Zimate is quite marked. As little as 1 per cent will induce complete vulcanization within five days at room temperature.

"For maximum catalytic action another ingredient is usually added to the mix: an activator. This agent provides the mix with a source of zinc ions. These appear to increase the rate at which the accelerator opens carbon bonds to the sulfur atoms, although the precise nature of the reaction is not fully understood. Zinc oxide is commonly used as the activating agent.

"To make a sheet of pure vulcanized rubber, then, you need four basic materials: latex, colloidal sulfur, an accelerator and an activator. Starting with 161 parts of 62-per-cent latex (which equals 100 parts of pure rubber hydrocarbon), add two parts of sulfur, one part of Butyl Zimate and one part of zinc oxide. These proportions are in terms of weight and are added as dispersions. If a 50-per-cent dispersion is used, the measured amount will be doubled; obviously half of the weight is represented by water. The solution is mixed thoroughly and poured onto a carefully leveled plate of clean glass. After the water has evaporated, the coagulated sheet of rubber is carefully stripped from the glass and immersed in a container of boiling water for 15 minutes. When you remove the sheet from the water, you will find that it exhibits all the familiar characteristics of soft, highly elastic rubber. When it is stretched and released, it immediately snaps back into its original form. If the sheet is stored for several years, or exposed to contaminating substances such as copper or magnesium, it will lose some of its elasticity and perhaps show minute surface cracks. To retard this 'aging' still another substance should be added to the mix: an antioxidant. As its name implies, this additive retards chemical reaction between the rubber and the oxygen in the air. A convenient and inexpensive antioxidant for amateur use is de-betanaphtyl-p-phenyleadiamine, dispensed under the trade name of Agerite White.

"This antioxidant is used in the next experiment I shall describe-the compounding of foam rubber-along with still another ingredient, an agent that encourages the latex to gel or coagulate quickly. Such an agent is particularly desirable in the manufacture of foam rubber, because the reaction it promotes stiffens the foam before it can settle. The most popular gelling agent is sodium silicofluoride, an insoluble white powder that is added to the mix in the form of a dispersion. Gelling begins about six minutes after the agent is stirred into the other ingredients. The mechanism of gelation is not well understood, but sodium silicofluoride seems to combine with the water to form hydrofluoric acid, thus sending the latex into the highly unstable acid range. Moreover, hydrated silicic acid appears as a product of the reaction in the form of a gel that contributes to the stiffness of the cellular structure.

"In making foam rubber at home you should also add soap to the mix to encourage the formation of bubbles. An effective soap can be made by mixing together (by weight) 7.5 parts of potassium hydroxide, 37 parts of castor oil and 83 parts of water. The ingredients are then beaten (with an egg beater) until they react to form liquid soap. Three to seven parts (by weight) of soap are added to the latex mix. More will retard the gelling action and less will contribute little to the desired frothing.

"First combine all the ingredients listed under 'Phase 1' of the accompanying recipe [right]. An ordinary glass mixing-bowl will serve as a convenient vessel. When the batch is thoroughly mixed, it is allowed to set for 12 hours (to assure complete coagulation). The ingredients of Phase 2 are then added and beaten until the mix becomes frothy. If a household food-mixer is available for the job, set the motor for the highest speed. The resulting froth should occupy about eight times the volume of the unbeaten mix and resemble the color and consistency of a vanilla milkshake. The minute bubbles should be apparent only as a marked decrease in the density of the fluid. When it is judged that the beating is complete (by the eight-fold increase in volume), and before the mixer is turned off, add the gelling agent (Phase 3), beat for half a minute more and pour the contents into a mold.

"Almost any container with smooth sides will serve as a mold: a cake pan, a paper cup or a wax or plaster-of-Paris mold made in the shape of a desired part. The tendency of the rubber to adhere to the container can be minimized by applying a light coat of talcum powder to the mold. Gelling will be completed in about 30 minutes. In this unvulcanized state foam rubber, though firm, is tacky, and the inner walls of the bubbles will stick together if the foam is squeezed. If the rubber is to be vulcanized outside the mold, it must be removed gently.

"You have a choice of three simple methods of applying heat to the rubber during vulcanization: hot air, hot water or steam. A hot-air oven is easy to construct from a breadbox fitted with a 300-watt lamp or a conventional electric heating element. The oven must also be provided with a thermometer calibrated to at least 300 degrees F., and a fan such as an electric hair dryer for circulating the air. A thermostat is convenient but not essential. If the oven is to be used for a series of experiments you may wish to substitute for the 300-watt lamp six 50-watt lamps wired with individual switches. The temperature can then be regulated by turning on the lamps as desired. If the box is reasonably well insulated, it should reach a temperature of at least 250 degrees F. This method of vulcanization suffers from the disadvantage that air is not an effective medium for transferring heat. A sketch of my oven appears in the accompanying illustration [above].

"It is of course possible to vulcanize rubber in hot water, as demonstrated by the experiment described earlier. The method is convenient, but it has two drawbacks. At sea level boiling water reaches a maximum temperature of only 212 degrees. Moreover, if unvulcanized foam is immersed, it can easily be collapsed by the weight of the water.

"I prefer steam vulcanization. This medium has the convenience of hot water, transfers heat more effectively than hot air, yet exerts no more mechanical force on the rubber than does air. A simple boiler for home vulcanization is shown in the accompanying illustration [Figure 4]. To vulcanize the foam bring about an inch of water to a boil place the molded rubber on the wire grid and close the boiler with a lid. The time required for 'cure' will range from 10 minutes to an hour, depending upon the thickness of the rubber.

"Having made the basic experiments described above, you are prepared to delve into the variables of rubber technology. The broad field of rubber accelerators, for example, offers numerous experimental opportunities. The zinc salt of mercaptobenzothiazole is a particularly interesting accelerator. It is an insoluble yellow powder that reacts at low temperature, does not discolor latex and produces a foam of high quality that resists aging. Another is piperidine (pentamethylene dithiocarbonate), an extremely quick-acting accelerator that is also insoluble.

"The pore size of foam rubber varies with surface tension, viscosity, type of soap and method of beating. The higher the ratio of rubber to air, the denser the foam. A watery mixture can be thickened by adding casein in the form of a 10-per-cent solution, or by adding bentonite clay, sodium silicate, Karaya gum or starch. Thick solutions can be thinned by adding water.

"Foaming can be encouraged by injecting air into the mix during the beating operation. To inject air, substitute a thermoplastic vessel for the glass mixing-bowl and force a heated glass tube through the bottom. (Cement the tube in place.) The outer end of the glass tube is attached to a bicycle pump or other source of compressed air. It is also interesting to investigate the effect of different gases (such as the inert gases, the halogens and acid anhydrides) on the resulting product. The technique of making spongy foams is equally interesting. The formation of large bubbles requires a low-viscosity latex. After beating, the mix is strengthened with unfrothed latex to which a thickener has been added. The hardness of the foam can also be controlled. The foam is made harder by loading the mix with clays; softer by the addition of emulsified oils. Typical loading agents are mineral oil and china clay.

"Latex and the chemicals essential for processing it can be ordered through your local chemical supply dealer or druggist from such firms as E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.; the Chicago Latex Products, Chicago, III.; and the R. T. Vanderbilt Co., Inc., New York, N. Y. Your local dealer can also order latex from the Firestone Tire & Rubber Company, Akron, Ohio."

T. J. Gauss of the department of mathematics at the University of California has recently constructed several variations of Lord Kelvin's all but forgotten "water dropper," the primitive electrostatic machine on which the modern Van de Graaff generator is based. Both machines transform mechanical energy into electrical energy by employing mechanical force to separate electrical charges of unlike sign and to push like charges together. In other words, work is expended to overcome the mutual repulsion of like charges. The potential difference across the terminals of the machines is thereby increased.

In the Van de Graaff generator one set of charges is transported by a moving belt that runs between a second set of charges on the pulleys, the direction of the belt's travel being chosen so that mechanical work is expended in overcoming the electrical forces [see "The Amateur Scientist" April, 1955]. Lord Kelvin accomplished the same result a century ago by lifting water into a reservoir (thus doing work) and permitting it to fall as drops through conductors arranged so that the drops acted as carriers of charge.

"It is a bit hard to believe," writes Gauss, "that such a simple device, using no moving parts other than a pair of dripping nozzles, can generate enough charge to cause a small neon bulb to flash. My reservoir is a separatory funnel and the water drips into a pair of collectors that formerly held frozen orange juice. (An ambitious friend has suggested that I substitute oil drums for the tin cans!) The collecting cans are insulated from the laboratory bench by two blocks of paraffin. A piece of stiff wire is soldered to each can and supports a ring about three inches above the top of the other can. The wires also serve as the electrical connections between the respective rings and cans. The rings may be either a one-inch piece of one-inch brass tubing or the equivalent made from pieces of tin can. A small neon bulb (NE-2) is soldered to one of the conductors at the point where the two cross. A spark gap about a quarter of an inch wide is left between the other lead of the neon bulb and the second conductor.

"A reservoir made of a large tin can would work as well as a separatory funnel. I used the funnel merely because it chanced to be handy. You can substitute a tin can for the funnel by siphoning the water through a length of rubber tubing. Weight the end of the tubing with a nut to prevent it from slipping from the can, and rig a clothes pin with a bolt and wing-nut to serve as a pinch clamp as shown in the accompanying drawing [below]. The rate of flow is adjusted by turning the wing-nut. The nozzles must be positioned to direct the stream down the axis of the two metal rings and into the collecting cans below. To make the nozzles, heat a piece of quarter-inch glass tubing to the softening point in a gas flame and draw the ends apart quickly. Then nick the glass tips with a file and snap off the closed tips. The resulting orifices should be about a 16th of an inch in diameter. If you hesitate to tackle glass work, you can make adequate jets by crimping the end of a length of copper tubing over a shingle nail and then removing the nail. If you wish to avoid making the T-connection between the nozzles, use two supply tubes and equip each with a pinch clamp.

"The flow of water is adjusted so that the streams break into droplets inside the rings. Once the reservoir has been filled (ordinary tap water works well), this adjustment puts the machine into operation. The maximum voltage generated is limited by the spacing of the air gap between the free lead of the neon lamp and the conductor. Close spacing encourages small, frequent sparks to jump the gap, accompanied by weak lamp-flashes. With large gap-spacing the lamp flashes less frequently but more brightly. If the gap spacing is larger than a quarter of an inch, the electrical forces will alter the path of the falling drops. Instead of falling vertically they will veer toward the opposite collecting cans. This results in some pretty patterns of droplet flow, as well as a wet workbench! Moreover, fine spray will soon collect on the apparatus, and because the machine is sensitive to electrical leakage it will stop working.

"The device starts and continues to work because it is electrically unstable. Assume that at the beginning the can at right is a bit more positively charged than the one at left. The left ring will be at the potential of the right can and vice versa. Negative charges (naturally present in water) will be attracted into the water stream of the positively charged left ring. As the stream breaks apart, negative charges are trapped on the droplets and carried (by gravity) away from the attracting ring and into the negatively charged can against the force of electrostatic repulsion. The charge in the left can therefore becomes increasingly negative (electric charge as well as water collects in the can). The identical mechanism is at work on the other side of the machine, where the accumulating charge is of opposite sign. Thus we have a runaway condition. The potential difference increases without limit save for the leakage of the system, the sparking potential of the gap and the diversion of the droplets. I leave for you to explain the existence of the initial charge-separation that triggers the whole performance."

To demonstrate the conservation of angular momentum with the weight and string, hold the free end of the string in your right hand, hold the middle of the string loosely in your left, start the weight going in a circular orbit and then pull the string with your right hand (so as to shorten the radius of the orbiting weight). The Coriolis force, tending to speed up the weight, will be developed as a consequence of the inward radial motion imparted by the pull, just as a figure skater executing a spin with outstretched arms speeds up when he folds his arms against his body.

Bibliography

LATEX, NATURAL AND SYNTHETIC. Phillp G. Cook. Reinhold Publishing Corp., 1956.

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-877-527-0382 voice/fax

Internet: http://www.sas.org/