GLYCERIN-MAN'S MOST VERSATILE CHEMICAL SERVANT MILTON A. LESSER Glycerine Producers' Association, New York City
NOTlong ago several leading scientists from a dozen countries were invited to draw up lists of the chemical substances which they considered "invaluable" from the viewpoint of general industrial usefulness. One of the few chemicals on every list was glycerin. This unanimous verdict was not really surprising, for in the 170 years since its discovery by a small-town pharmacist in Sweden, glycerin has proved itself important and helpful in more than a thousand processes and products, touching almost every phase and department of modern living. The story of glycerin is studded with names that loom large in scientificprogress. Its Swedish discoverer, Karl Wilhelm Scheele, came upon it more or less by accident. While making a lead plaster, he produced a soap by mixing olive oil with litharge. Upon washing this soap with water, he obtained a thick solution which, after evaporation, left a sweet, viscous, heavy liquid. He called this substance "the sweet principle of fats" (principe doux des huiles). It was also known as "Scheele's sweet principle" or "oil sugar.'' In 1811, Chevreul studied the composition of this sweet liquid and gave it the name glycerin, after the Greek word "glykeros," meaning sweet. Pelouze soon afterward established the chemical formula of glycerin. Wurtz determined its exact chemical composition and showed its relationship to the aliphatic compounds. Other important studies were made by such famous chemists as Berzelius, Liebig, Berthelot, and de Luca. I n 1858, Pasteur found that glycerin is produced in fairly uniform concentrations during alcoholic fermentation. Most of the glycerin available today is obtained as a by-product of the soap industry, where it is separated during the saponification of fats and oils. Some glycerin is obtained more directly by fat-splitting processes. Less than a year ago, the production of synthetic glycerin from petroleum gases was begun on a commercial scale, thereby yielding additional supplies of this widely demanded material. The 1947 production of glycerin in the United States was over 207 million pounds. Many industries shared in using up this record production. A monograph available in most libraries catalogs 1583 uses for glycerin. Published about four years ago, the list is already out of date, because many new uses for glycerin have since been found, and industrial chemists are constantly extending the field. Specific data as to industrial products which use glycerin are available in government statistics. Credit
for the largest consumption of glycerin goes to the producers of synthetic (alkyd) resins and ester gum. Next comes tobacco, with third place to dynamite and nitroglycerin. After these three classifications the important uses of glycerin are: cellulose films and meat casings, dentifrices and toilet articles, drugs and pharmaceuticals, gaskets and cork packings, printers' rollers and supplies, margarine, shortening and other edibles; adhesives, textile processing, beverages, flavors, candy and gum, glassine paper, greaseproof paper and vegetable parchment, rubber processing, cleaning materials, paper other than glassine and greaseproof, and the manufacture of chemicals. Back of these major users of glycerin are several products of great importance to science, technology, and the arts; such as antifreeze fluids for various purposes, hydraulic fluids, electrolytic fluids, liquid coolants, embalming fluids, masking and shielding compounds, grinding compounds, soldering fluids and pastes, special cements, caulking compounds and packing materials, various types of lubricants and various products for insect and microorganism control. In addition to the thousands of pounds of glycerin that go into instrument and equipment manufacture, substantial quantities are also used as laboratory reagents and for research. Of course there are many reasons for this wide diversity of uses. They are to be found in the very unusual combination of physical and chemical properties inherent in Karl Scheele's clear, heavy sirupy fluid. It is because of these properties that glycerin is variously used as a humectant or hygroscopic agent, a vehicle, a solvent, a sweetening agent, an emollient, a chemically reactive material, a lubricant, a softening and demulcent agent, a penetrant, an antifreeze, a heattransmitting medium, a pressure-transmitting fluid, a refrigerant, an antiseptic and preservative, a suspending agent, and a blending and dispersing aid. Thus, to the manufacturer of explosives and synthetic resins glycerin is a valuable chemical substance; a cosmetician thinks of it as an emollient; a tobacco processor uses it as a humectant; to the cellulose film producer it is a plasticizer; while the baker values it .as a means of retaining freshness and palatability in cakes, cookies and other baked goods. All these and other users of glycerin know that it will "stay put" and continue to function effectively under conditions that render many other materials quite useless, mainly because of its high boiling point and low vapor tension. Another important factor in the employment of glycerin is that it can be used with entire safety; being
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nontoxic and absolutely wholesome, it may be handled, applied, and swallowed without harm. In foods, beverages and other edible products it is not an inert vehicle or solvent; on the contrary, as shown by studies at the University of Chicago, it is definitely nutritious, with a caloric value equivalent to that of a carbohydrate, and adds sweetness. Indeed, glycerin is a normal product of digestion, being liberated in the gastrointestinal tract. Glycerin's varied properties combined with the fact that it is a healthful food, give it a unique place in the manufacture of pharmaceutical preparations and manufactured foods. It is used in so many drugs and medicines that an average of more than three pounds of glycerin per hospital bed is required annually by American hospitals. An analysis of 15,000 physicians' prescriptions in a single American city showed that, with the sole exception of water, glycerin was the mostused liquid ingredient. Surgeons have long recognized its value in burn therapy and wound dressing. Food processors, from soup to nuts, avail themselves of the advantages of glycerin in many varied types of products. Peanut butter, for example, needs glycerin to prevent separation of an oily, unsightly layer, and to increase palatability and decrease cloying. Flavorings of all kinds require glycerin because of its high solvent powers, its effectiveness as an extractant, and its efficacy as a blending and smoothmg agent. Food wrapping materials such as cellophane, glassine, and greaseproof paper use glycerin, and it is now entering a new field of great promise in food-freezing processes based on immersion freezing. Although the greatest variety of glycerin uses stems from its physical properties, the largest volume of consumption is in processes dependent upon its chemical characteristics. As a trihydric alcohol, it has some interesting chemical properties, whereby either one, two or three of the hydroxyl groups can react with a given reagent. It is also possible for each of the three groups to react with a different substance to produce compounds of mixed type. Glycerin can form esters, ethers, inner ethers, acetals, amines, halohydrin, and metallic derivatives. It can be oxidized, reduced, dehydrated, and made to undergo various special reactions. The esters are unquestionably the most important of the glycerin derivatives. At the top of the important esters are nitroglycerin, alkyd resins, ester gum, acetins and long-chain fatty derivatives. Historically, nitroglycerin was the first of these compounds to assume importance. Over a century ago, in 1846, Sohrero reported the reaction of glycerin with nitric acid to form nitroglycerin. In 1863, Nobel demonstrated its value as an explosive. Five years later he found that kieselguhr would absorb large quantities of glycerin and that in this form it was safer to handle. Thus "dynamite" was born, a product which over the years has become vital to the l i e of the world. For example, agricultural uses such as ditching, land clearing, stump blasting and other farm activities consume about five million pounds
JOURNAL OF CHEMICAL EDUCATION
of dynamite each year. In passing, it should be noted that nitroglycerin is also an important medical agent in the treatment of heart disorders. Later, ester gum made its appearance. Formed by the reaction between glycerin and rosin, this material (called the first of the "synthetic" resins) started a revolution in the paint industry. I t was followed by the even more important synthetic resins of the alkyd type. They are described as the reaction product of polyhydric alcohol with a polybasic acid, and glycerin and phthalic acid (in the form of the anhydride) are the most important team of reactants in this group. Alkyd resins are usually modified with oils, fatty acids and other substances to yield compounds with properties suited to specific applications. Owing to their versatility and because they may be used advantageously with other resins (including the new silicone resins)-together with the fact that they produce finishes of outstanding durability, toughness, and beauty-alkyd resins now hold the dominant position among synthetic resins used in the manufacture of protective and decorative coatings of all kmds, including emulsion paints. While their use in paint comprises their greatest outlet, suitably modified alkyd resins are find'mg important applications in other fields as well. They are valuable as binding agents in the manufacture of electrical and radio equipment, in the production of optical goods and in the making of abrasives. Used in special coatings for paper, they are also important components of modem printing inks. The textile industry has found them suitable for making superior fabric finishes and better pigmented printing pastes. During recent years, increasing interest has been focused on a group of glycerin derivatives generally referred to as monoglycerides. They are formed, under controlled conditions, by heating excess glycerin with free fatty acids, or by heating a triglyceride (e. g., vegetable or animal fat or oil) with glycerin. The resulting compounds are used as emulsifiers and homogenizers in shortenings, in baked goods and various foods, and in certain industrial processes, like the production of foam rubber. Glyceryl monostearate is probably the best known of the monoglycerides. Made available in special edible grades, this compound has proved to be an effective staling retarder for use in cake and bread baking. In addition to maintaining freshness and improving the flavor of baked goods, it is also used by ice cream manufacturers as a basic ingredient in stabilizers. It is also one of the best known and most e5cient emulsifiers in cosmetic creams and dermatologic ointments. The potentialities of the monoglycerides are receiving concentrated attention in many quarters. Also attractive to medical chemists and pharmacologists is a group of glycerin ethers. These compounds show a muscle-relaxing action similar to curare but without the hazards of the arrow poison derivative. These newer glycerin derivatives are expected to open up new fields of usefulness in the domains of medicine and surgery.
