Glycols and glycerols have numerous applications in industry because they are completely water-soluble organic solvents. Many of these compounds are used as solvents for dyes, paints, resins, inks, insecticides and pharmaceuticals. In addition, their two chemically reactive hydroxyl groups make the glycols important chemical intermediates. Among the many uses of glycols and polyglycols, major ones include being an additive for freezing-point depression, for lubrication and for solubilization. The glycols also serve as indirect and direct additives to foods and as ingredients in explosive and alkyd resin formulations, theatrical fogs and cosmetics.
Propylene glycol is used widely in pharmaceuticals, cosmetics, as a humectant in certain foods and as a lubricant. It is also used as a heat-transfer fluid in uses where leakage might lead to food contact, such as in coolants for dairy refrigeration equipment. It is also used as a solvent in food colours and flavours, an antifreeze in breweries and establishments, and an additive to latex paint to provide freeze-thaw stability. Propylene glycol, ethylene glycol and 1,3-butanediol are components of aircraft de-icing fluids. Tripropylene glycol and 2,3-butanediol are solvents for dye-stuffs. The butanediols (butylene glycols) are used in the production of polyester resins.
Ethylene glycol is an antifreeze in cooling and heating systems, a solvent in the paint and plastics industries, and an ingredient of de-icing fluid used for airport runways. It is used in hydraulic brake fluids, low-freezing dynamite, wood stains, adhesives, leather dyeing, and tobacco. It is also serves as a dehydrating agent for natural gas, a solvent for inks and pesticides, and an ingredient in electrolytic condensers. Diethylene glycol is a humectant for tobacco, casein, synthetic sponges, and paper products. It is also found in cork compositions, book-binding adhesives, brake fluids, lacquers, cosmetics and antifreeze solutions for sprinkler systems. Diethylene glycol is used for water seals for gas tanks, as a lubricating and finishing agent for textiles, a solvent for vat dyes, and a natural-gas dehydrating agent. Triethylene glycol is a solvent and lubricant in textile dyeing and printing. It is also used in air disinfection and in various plastics to increase pliability. Triethylene glycol is a humectant in the tobacco industry and an intermediate for the manufacture of plasticizers, resins, emulsifiers, lubricants and explosives.
Some measure of the versatility of glycerol can be gained from the fact that some 1,700 uses for the compound and its derivatives have been claimed. Glycerol is used in food, pharmaceuticals, toiletries and cosmetics. It is a solvent and a humectant in such products as tobacco, confectionery icing, skin creams and toothpaste, which would otherwise deteriorate on storage by drying out. In addition, glycerol is a lubricant added to chewing gum as a processing aid; a plasticizing agent for moist, shredded coconut; and an additive for maintaining the smoothness and moisture in drugs. It serves to keep frost from windshields and is an antifreeze in automobiles, gas meters and hydraulic jacks. The largest single use of glycerol, however, is in the production of alkyd resins for surface coatings. These are prepared by condensing glycerol with a dicarboxylic acid or anhydride (usually phthalic anhydride) and fatty acids. A further major use of glycerol is in the production of explosives, including nitroglycerine and dynamite.
Glycerol is a trihydric alcohol and undergoes reactions characteristic of alcohols. The hydroxyl groups have varying degrees of reactivity, and those in the 1- and 3- positions are more reactive than that in the 2- position. By using these differences in reactivity and by varying the proportions of reactants, it is possible to make mono-, di- or tri- derivatives. Glycerol is prepared either by the hydrolysis of fats, or synthetically from propylene. The chief constituents of virtually all animal and vegetable oils and fats are triglycerides of fatty acids.
Hydrolysis of such glycerides yields free fatty acids and glycerol. Two hydrolysis techniques are used—alkaline hydrolysis (saponification) and neutral hydrolysis (splitting). In saponification, fat is boiled with sodium hydroxide and sodium chloride, resulting in the formation of glycerol and the sodium salts of fatty acids (soaps).
In neutral hydrolysis, the fats are hydrolyzed by a batch or semi-continuous process in a high-pressure autoclave, or by a continuous countercurrent technique in a high-pressure column. There are two main processes for the synthesis of glycerol from propylene. In one process, propylene is treated with chlorine to give allyl chloride; this reacts with sodium hypochlorite solution to give glycerol dichlorohydrin, from which glycerol is obtained by alkaline hydrolysis. In the other process, propylene is oxidized to acrolein, which is reduced to allyl alcohol. This compound may be hydroxylated with aqueous hydrogen peroxide to give glycerol directly, or treated with sodium hypochlorite to give glycerol monochlorohydrin, which, upon alkaline hydrolysis, yields glycerol.
Glycerol has a very low toxicity (oral LD50 (mouse) 31.5 g/kg) and is generally considered harmless under all normal conditions of use. Glycerin produces only very slight diuresis in healthy individuals receiving a single oral dose of 1.5 g/kg or less. Adverse effects following oral administration of glycerin include mild headache, dizziness, nausea, vomiting, thirst and diarrhoea.
When present as a mist, it is classified by the American Conference of Governmental Industrial Hygienists (ACGIH) as a “particulate nuisance”, and as such a TLV of 10 mg/m3 has been assigned. In addition, the reactivity of glycerol makes it dangerous, and liable to explode in contact with strong oxidizing agents such as potassium permanganate, potassium chlorate and so on. Consequently it should not be stored near such materials.
Glycols and derivatives
The commercially important glycols are aliphatic compounds possessing two hydroxyl groups, and are colourless, viscous liquids that are essentially odourless. Ethylene glycol and diethylene glycol are of major importance among the glycols and their derivatives. The toxicity and hazard of certain important compounds and groups are discussed in the final section of this article. None of the glycols or their derivatives that have been studied have been found to be mutagenic, carcinogenic or teratogenic.
The glycols and their derivatives are combustible liquids. since their flashpoints are above normal room temperature, the vapours are liable to be present in concentrations within the flammable or explosive range only when heated (e.g., ovens). For this reason they present no more than a moderate fire risk.
Synthesis. Ethylene glycol is produced commercially by the air oxidation of ethylene, followed by hydration of the resulting ethylene oxide. Diethylene glycol is produced as a by-product of the production of ethylene glycol. Similarly, propylene glycol and 1,2-butanediol are produced by the hydration of propylene oxide and butylene oxide, respectively. 2,3-Butanediol is produced by the hydration of 2,3-epoxybutane; 1,3-butanediol is produced by the catalytic hydrogenation of aldol using Raney nickel; and 1,4-butanediol is produced by the reaction of acetylene with formaldehyde, followed by hydrogenation of the resulting 2-butyne-1,4-diol.
Hazards of Common Glycols
Ethylene glycol. The oral toxicity of ethylene glycol in animals is quite low. However, from clinical experience it has been estimated that the lethal dose for an adult human is about 100 cm3 or about 1.6 g/kg, thus indicating a greater toxic potency for humans than for laboratory animals. The toxicity is due to the metabolites, which vary for different species. Typical effects of excessive oral intake of ethylene glycol are narcosis, depression of the respiratory centre, and progressive kidney damage.
Monkeys have been maintained for 3 years on diets containing 0.2 to 0.5% of ethylene glycol without apparent adverse effects; no tumours were found in the bladder, but there were oxalate crystals and stones. Primary eye and skin irritation are generally mild in response to ethylene glycol, but the material can be absorbed through the skin in toxic amounts. Exposure of rats and mice for 8 hours/day for 16 weeks to concentrations ranging from 0.35 to 3.49 mg/l failed to induce organic injury. At the higher concentrations, mist and droplets were present. Consequently, repeated exposures of humans to vapours at room temperature should not present a significant hazard. Ethylene glycol does not seem to present a significant hazard from the inhalation of vapours at room temperatures or from skin or oral contact under reasonable industrial conditions. However, an industrial inhalation hazard could be generated if ethylene glycol were heated or vigorously agitated (generating a mist), or if appreciable skin contact or ingestion occurred over an extended period of time. The primary health hazard of ethylene glycol is related to the ingestion of large quantities.
Diethylene glycol. Diethylene glycol is quite similar to ethylene glycol in toxicity, although without production of oxalic acid. It is more directly toxic to the kidneys than ethylene glycol. When excessive doses are ingested, the typical effects to be expected are diuresis, thirst, loss of appetite, narcosis, hypothermia, kidney failure and death, depending on the severity of exposure. Mice and rats exposed to diethylene glycol at levels of 5 mg/m3 for 3 to 7 months experienced changes in central nervous and endocrine systems and internal organs, and other pathological changes. While not of practical concern, when fed at high doses to animals, diethylene glycol has produced bladder stones and tumours, probably secondary to the stones. These may have been due to monoethylene glycol present in the sample. As with ethylene glycol, diethylene glycol does not seem to present a significant hazard from the inhalation of vapours at room temperatures or from skin or oral contact under reasonable industrial conditions.
Propylene glycol. Propylene glycol presents a low toxicity hazard. It is hygroscopic, and in a study of 866 human subjects, was found to be a primary irritant in some people, probably due to dehydration. It might also cause allergic skin reactions in over 2% of people with eczema. Long-term exposures of animals to atmospheres saturated with propylene glycol are without measurable effect. As a result of its low toxicity, propylene glycol is used widely in pharmaceutical formulations, cosmetics and, with certain limitations, in food products.
Dipropylene glycol is of very low toxicity. It is essentially non-irritating to the skin and eyes and, because of its low vapour pressure and toxicity, is not an inhalation problem unless large quantities are heated in a confined space.
Butanediols. Four isomers exist; all are soluble in water, ethyl alcohol and ether. They have low volatility so inhalation is not a concern under normal industrial conditions. With the exception of the 1,4- isomer, the butanediols create no significant industrial hazard.
In rats, massive oral exposures of 1,2-butanediol induced deep narcosis and irritation of the digestive system. Congestive necrosis of the kidney may also occur. Delayed deaths are believed to be the result of progressive renal failure, while acute fatalities are probably attributable to narcosis. Eye contact with 1,2-butanediol may result in corneal injury, but even prolonged skin contact is usually innocuous with respect to primary irritation and absorption toxicity. No adverse effects of vapour inhalation have been reported.
1,3-Butanediol is essentially non-toxic except in overwhelming oral doses, in which case narcosis may occur.
Little is known abut the toxicity of 2,3-butanediol, but from the few animal studies published, it appears to lie between 1,2- and 1,3-butanediols in toxicity.
1,4-Butanediol is about eight times as toxic as the 1,2-isomer in acute toxicity tests. Acute ingestion results in severe narcosis and possibly renal injury. Death probably results from collapse of the sympathetic and parasympathetic nervous systems. It is not a primary irritant, nor is it easily absorbed percutaneously.
Glycols and glycerols tables
Table 1 - Chemical information.
Table 2 - Health hazards.
Table 3 - Physical and chemical hazards.
Table 4 - Physical and chemical properties.