Glycerol or glycerine is a versatile chemical with three carbon atoms and three hydroxyl groups, one on each carbon atom. It is an extremely useful product with a large variety of applications because of its particular combination of physical and chemical properties. Lack of colour and odour, and non-toxicity are very important properties of glycerol. This simple chemical is important in pharmaceuticals, cosmetics, as well as the food industry because it is non-toxic and possesses good solvent properties for many organic as well as inorganic compounds. Due to its hygroscopicity, it keeps preparations and formulations moist and hinders unwanted crystallization. It can be used as a softener; it supplies moisture to the skin, and its high viscosity renders solutions syrupy. Its use in foods and cosmetics is growing.
Because of its three hydroxyl groups, glycerol is very reactive. This polyfunctional reactivity of glycerol is used in the production of alkyd resins and in cross-linked polyesters from mono- and di-carboxylic acids and polyols.
Glycerol is an excellent solvent for diverse materials like iodine, bromine, phenol, tannins, alkaloids, and mercury chloride.
Glycerol serves as a lubricant in the textile industry as well as in equipments and materials that come in contact with food, pharmaceuticals, cosmetics or skin. In special cases it is also employed as a hydraulic fluid.
Glycerol is used as a solvent, humectants, plasticizer, emollient, sweetener; in the production of nitroglycerine (dynamite), liquid soap, liquors, confectionery, blacking, printing and copying inks, elastic glues, lead oxide cements; to keep fabrics pliable to preserve printing on cotton, for printing rollers, hectographs; to keep frost from wind shields; as antifreeze in automobiles, gas meters and hydraulic jacks, in shock absorber fluids. In fermentation fluids, it is used as a nutrient.
Glycerol helps to increase the viscosity of liquid drug formulations, pharmaceutical syrups, etc. It also provides humidity or moisture to drug pills or tablets.
Glycerol is used in cement compounds, caulking compounds, and pressure media. It is also used in embalming fluids, masking and shielding compounds, soldering compounds, and compasses; wetting agents, emulsifiers, skin protective products, asphalt, ceramics, photographic products, leather, and wood treatments, and adhesives.
Glycerol mixed with red lead (i.e., litharge) sets to a hard, slightly elastic, and resistant cement. Mixtures of glycerine and red lead were widely used in pipe work for joints and foundations till the advent of new adhesives and technologies.
Glycerol is often used for extracting essential oils from flowers; enzymes from organic tissues and in preparing fluid extracts, solutions, tinctures, elixirs, syrups, pills and ointments.
Glycerol is employed in the electrochemical industry as a compound of brightening, particularly in the electroplating of stainless steels, and in the formulation of plating solutions.
The above list of diverse applications of glycerol is presented to highlight the wide range of end-uses of glycerol in different industry sectors. It is by no means an exhaustive one. It is very difficult to list all the uses of glycerol here. In 1945, Georgia Leffingwell and Milton Lesser listed 1583 different applications of glycerol in the book titled, “Glycerin”, spread over 27 chapters! These chapter titles or end-use applications include – Adhesives and cements, Cleaners and polishes, Electrical equipment, Explosives, Leather, Lubricants, Metals, Packaging materials, Paper, Photography, Plastics, Printing and lithography, Paints and protective coatings, Rubber, Textiles and dyes, Tobacco, Glass, Agriculture, Cosmetics, Beverages, Foods, Medicine and surgery, Orthodontia and Dentistry, Pharmaceuticals, Veterinary medicine, Optometry, and Glycerin derivatives.
The present article discusses only the major applications of glycerol – crude and pure – briefly in the following pages. Derivatives of glycerol are discussed separately.
With the development of biodiesel manufacture from oils and fats, the availability of crude glycerine increased exponentially across the globe. Therefore, many applications of this amply available material, somewhat dilute solution of glycerol, as available from the biodiesel production plants, were also developed. These applications of crude glycerol are also listed here.
Crude Glycerol
Crude glycerol is a major by-product during biodiesel manufacture. It consists of large amounts of several impurities inhibiting its direct usage or consumption in industries.
The purity of by-product crude glycerol from conventional industrial biodiesel plants is of key economic and technological concern. Small biodiesel plants usually discard the glycerol by-product as waste. The quality of crude glycerol depends on the biodiesel process characteristics followed at the manufacturing facility. Crude glycerol with 50% purity – containing methanol, mono-, di-, and tri-glycerides, methyl esters and other organic matter – has few direct uses and is of low value. Even its fuel value is marginal. Depending on the type of catalyst route and separation process used, the glycerol purity can reach 80 – 95%. In conventional industrial biodiesel processes using methanol transesterifiction of vegetable oils in the presence of a homogeneous catalyst (e.g. sodium hydroxide or sodium methoxide), sodium is recovered from the glycerol phase as sodium glycerate, from which glycerol is recovered by acid (HCl) neutralization. Glycerol is obtained as an aqueous solution containing sodium chloride or salt. Such crude glycerol is typically sold to industrial glycerol refiners at low cost.
Biodiesel production units generate substantial quantities of crude glycerol as a by-product – about one gallon of waste product containing glycerol for every five gallons of biodiesel produced. Typically, biodiesel production will generate about 10% wt/wt glycerol as the main by-product. In other words, every gallon of biodiesel produced generates approximately 1.05 pounds of glycerol. This indicates a 30-million-gallons-per-year biodiesel plant will generate about 11500 tonnes of 99.9% pure glycerol. For the purification of raw biodiesel into fuel biodiesel, huge amount of water is required and may generate as much as three gallons of wastewater for each gallon of fuel produced.
Due to the high energy content, crude glycerol can be used as a substitute for boiler fuel or cogeneration of electricity or thermal energy. In this case, crude glycerol can be directly used in large quantities without any refining process. The presence of water and methanol in crude glycerol restricts its usage as co-firing in thermal energy generation.
Crude glycerol may be used directly as a supplement energy source for cattle feed. At low concentration, crude glycerol can be used as a potential ingredient source for the production of broiler feed and pig feed. Glycerol has high absorption rates and is a good energy source. Once absorbed, it can be converted to glucose for energy production in the liver of animals by the enzyme glycerol kinase. Although crude glycerol can be added to animal feed, excess glycerol in the animal diet may affect normal physiological metabolism. Use of crude glycerol as an animal feed component has great potential for replacing corn in diets, and is gaining increasing attention. However, care must be taken to remove hazardous impurities like potassium and methanol from crude glycerol.
Crude glycerol can be used effectively as a dust depressor for dust control on roads. Once spread on the dirt roads, glycerol can be biologically degraded to an insignificant level with a few weeks, as it is biodegradable. However, crude glycerol from biodiesel production contains various impurities, which may cause some problems for human health and/or the environment.
In-depth investigative studies have been carried out to assess direct use of crude glycerol for the manufacture of triacetin, i.e., glycerol triacetate, via esterification. Reactive distillation is used to improve the triacetin production process. Three process designs have been proposed and investigated – (a) direct feed of crude glycerol to reactive distillation, (b) separation of crude glycerol coupled with reactive distillation, and (c) reactive separation of crude glycerol coupled with reactive distillation. Higher methanol content in the crude glycerol decreases the purity of triacetin produced. The conversion of glycerol and yield of triacetin obtained by using crude glycerol are slightly lower than those using pure glycerol. Removal of methanol from crude glycerol before feeding it to the reactive distillation process results in the highest purity of triacetin produced.
Production of butanol from crude glycerol obtained as by-product in biodiesel manufacture, through fermention, has been proposed. The fermentation of biodiesel-derived crude glycerol to produce butanol will improve the sustainability of the biofuels industry and will also advance the integrated biorefinery concept. From both technical and economic perspectives, biodiesel-derived crude glycerol appears to be preferable to sugars for butanol production, because it is a potentially no-cost or low-cost substrate, requires no pretreatment, and generates higher butanol yields. In additional, the use of this crude glycerol as a feedstock alternative to glucose and other corn-derived sugars will provide much-needed diversity and flexibility to the emerging biofuels industry.
Crude glycerol has also been reportedly found useful for the production of ethanol (ethyl alcohol) by fermentation with microorganisms.
The anaerobic fermentative production of 1,3-propanediol is the most promising option for biological conversion of crude glycerol and has been successfully demonstrated.
Crude glycerol has also been used successfully for citric acid biosynthesis.
The bacterium Rhodopseudomonas palustris was found to be capable of photofermentative conversion of crude glycerol to hydrogen. Nearly equal quantities were obtained from crude glycerol and pure glycerol. Up to six moles of hydrogen per mole of glycerol were obtained, which was 75% of theoretical. Both rates and yields of hydrogen production could be modified by changing concentration of added nitrogen. Crude glyceol, as a co-substrate, could be used to enhance hydrogen and especially methane production during the anaerobic treatment of different feedstocks including the organic fraction of municipal solid wastes, sewage sludge and slaughterhouse waste.
Poly(hydroxyalkanoates) (PHA) represent a complex class of naturally occurring bacterial polyesters (bio-polymers or bio-plastics), and have been recognized as good substitutes for non-biodegradable polymers produced from petrochemicals. Crude glycerol can be used as a feedstock to produce PHA, especially the PHB polymer, by microbial fermentation. It has been estimated that a 10 million gallons per year biodiesel plant would have the potential to produce 20.9 tonnes per year of PHB polymer.
Several research papers have been published on the production of docosahexanoic acid (DHA)-rich algae using crude glycerol, by fermentation of the alga Schizochytrium limacinum.
Crude glycerol has been reported to be useful as a component of culture media to produce a variety of chemicals such as lipids, carotenoids, succinic acid, bacterial cellulose (BC), glycolipid-type biosurfactants, fungal proteins, etc.
Monoglycerides could also be produced via glycerolysis of triglycerides with crude glycerol obtained as a by-product in biodiesel manufacture. The two-step process, involving removal of residual glycerol and crystallization, has been reported for purification of the monoglycerides produced from this glycerolysis. Approximately 99% pure monopalmitin has been produced.
Pure Glycerol
Applications of pure glycerol may be divided into direct applications and as derivatives.
Major applications of pure glycerol in different industry sectors are briefly described in the following paragraphs.
Direct Applications
Drugs and Pharmaceuticals
In medicines and drugs, glycerol is an ingredient of many elixirs and tinctures. It is also used in jellies and ointments. Glycerine is widely used in the preparation of cough remedies, both as syrup and lozenges. It acts as a dispersing and suspending agent and its warm sweetness is soothing to irritated membranes. It is employed in anesthetics, such as for ear treatments, eye lotions, and in bacteriological cultural media.
Its hygroscopic property is valuable in its use as humectants in pharmaceutical formulations including ointments. Glycerol also improves smoothness and provides lubrication in medicinal preparations. In tablets, it is used as a tablet holding agent.
Glycerol is used to make capsules for medicinal use, which are plasticized with glycerol. Other pharmaceutical applications of glycerol include suppositories, ear infection remedies, anaestheitcs, cough remedies, lozenges, gargles, and vehicles for antibiotics and antiseptics.
Pharmaceutical grade glycerol is used in wound and burn treatments due to its antimicrobial and antiviral properties.
Medically, glycerol serves as an emollient and demulcent in skin preparations and as an osmotic diuretic to manage cerebral edema, reduce cerebrospinal pressure and lower intraocular pressure.
Glycerol is also used to package donor skin used in skin grafts and in blood banks to preserve red blood cells prior to freezing.
It acts as a laxative when taken rectally. It can cause a rapid, temporary decrease in internal pressure of the eye when taken orally. Probiotics can be supplemented with glycerol to enhance the production of antimicrobial substances in the human gastrointestinal tract.
Glycerol is used as a marker in the measurement of liver disease.
Glycerol has been reported to be useful as a component of bio-ink formulations for bio-printing, i. e., 3D printing to fabricate biomedical parts.
Glycerine-phenol solutions are used for antiseptic treatment after oral surgery. Water dispersible multi-vitamin compositions can be made by incorporating the vitamins into solutions containing ethyl alcohol and glycerine, the latter serving as a blending agent.
Derivatives of glycerol are used in tranquilizers (e.g., glycerine guaiacolate) and reduce elevated blood pressure, and also as a heart stimulant in asthma, nephritis, Round’s syndrome, and to relieve pain in angia pectoris. The glyceride of hydrogen peroxide is effective in the treatment of infections, particularly tubercular abscesses and in the draining of emphysema cavities. Next to water, glycerol is used most widely for medicinal substances, for both internal and external use.
Cosmetics
Glycerine is one of the principal ingredients of toilet goods, cosmetics and personal care products, acting as a plasticizer, vehicle, solvent, penetrant, humectant, emollient and antifreeze. Glycerol is used in many cosmetics – creams and lotions – to keep the skin soft (emollient action), and replace skin moisture (hygroscopic property). It is also used in skin preparations, shaving preparations, deodorants and make-up formulations.
The most important use of glycerine in cosmetics is the preparation of toothpaste, to maintain the desired smoothness, viscosity, and lend a shine to the paste. It functions as a humectant as well as a sweetener. Glycerol prevents drying out and hardening in the tube and around the cap threads or at the opening of the pump-type dispenser. However, glycerol competes with sorbitol in this application based on economics.
Glycerol is used as an important component in the formulations of mouthwashes, skin care products, shaving creams, hair care products, soaps as well as water-based personal care lubricants.
Glycerol is used in glycerine soap, which is useful on sensitive, easily irritated skin as it prevents skin dryness due to its moisturizing property.
Glycerine derivatives such as glyceryl mono- and di-stearate, are used in brilliantines, make-up formulations, after-shave lotions, creams, etc. Glycerol esters are also used as replacements for waxes in lipsticks, in mascara, and in other non-greasy emulsions.
Foods
Glycerol as a food is easily digested and is non-toxic. Its metabolism places it with carbohydrates, although it is present in combined form in all vegetable and animal oils and fats. In flavouring and colouring products, glycerol acts as a solvent and its viscosity lends body to the product. Raisins saturated with glycerol remain soft when mixed with cereals. In foods and beverages, it is used as a solvent, a moistening agent or humectant, preservative, and an ingredient of syrups as a vehicle. In candies, glycerol retards crystallization of sugar. Glycerol is used as a humectants and softening agent in candies, cakes, and casings for meats and cheese. Glycerine is also used in dry pet foods to help retain moisture and enhance palatability.
Food grade glycerol is used as filler in commercial low-fat food products, such as cookies. It is used as a thickening agent in liqueurs. Glycerol is used as an additive along with polyol sweeteners like erythritol and xylitol; which have a cooling effect, due to the heating effect of glycerol in the mouth, especially if the cooling effect of polyol sweeteners is not desired.
Because of its antiseptic properties, glycerine has been used in the preservation of foods like meats, fruits, dried fruits, plant leaves, etc. Glycerol has proved useful in the preparation of ice-cream and other frozen milk products. In the manufacture of confectionery, glycerol lends smoothness to mixtures, helps uniform distribution of flavourings, contributes to palatability and improves keeping qualities.
Glycerol is used as a heat transfer medium in direct contact with foods in quick freezing and as a lubricant in machinery used for food processing and packaging. The polyglycerols and polyglycerol esters have increasing use in foods, particularly in shortenings and margarines.
Glycerol is the next most common alcohol in wine (after ethyl alcohol). Because of its low volatility, glycerol has no detectable odour. It possesses sweet taste, but it is so mild that it is likely to affect sweetness only in dry wines, if the concentration surpasses 5 gm/lit.
Another important use of glycerine in foods is through the applications of its esters, especially monoglycerides – glycerol esters of fatty acids. These monoglycerides, such as glycerol monostearate (GMS), glycerol monolaurate, glycerol monooleate, etc. are emulsifiers and stabilizers for many food products. These edible monoglycerides help maintain moisture balance in a product and permit richer formulations with longer shelf life when added to margarine to increase plasticity and to dough mixes to promote dispersion of fat. Monoglycerides are also used in salad dressings, frozen desserts, candy and food coatings.
Botanical Extracts
Glycerol is used to prepare tinctures (usually 10% solutions) of botanical extracts as it prevents precipitation of tannins in ethanol extracts of plant materials. Glycerol is also used as a solvent for preparing herbal extracts, instead of ethanol. Its extractive power is considered equivalent to that of alcohol and alcohol-water mixtures. Glycerol has the ability to extract many different constituents and complex compounds from various plant materials. In some cases, glycerol is added after extraction in herbal extracts prepared using hot water. Glycerol also acts as preserving agent for the botanical extracts.
Tobacco
Significantly large quantities of glycerol are used by the tobacco processing industry throughout the world in the manufacture of cigarettes, pipe mixtures, and chewing tobacco. Hygroscopicity of glycerol is used for moisture conditioning of tobacco. Glycerol is an important component of the casing solution sprayed on tobacco before the leaves are shredded and packed. Along with other flavouring agents, glycerol is applied at a rate of about 2 to 3 wt% of the tobacco to prevent the leaves from becoming friable and thus crumbling during processing. It keeps the tobacco moist and soft. By remaining in the tobacco as a humectant, glycerol helps to retain moisture and thus prevents drying out of the tobacco, and influences the burning rate of the tobacco. Presence of glycerol ensures freshness in packaged cigarettes and other tobacco products.
Glycerol is used in the processing of chewing and pipe tobaccos to add sweetness and prevent dehydration. It is also used as a plasticizer in cigarette papers.
Triacetin, i.e., glycerol triacetate, acts as a plasticizer for cellulose acetate in the manufacture of cigarette filter tips.
Electronic Cigarettes
Glycerol is used as a component of e-liquid, along with propylene glycol, used in electronic cigarettes. This e-liquid is heated in the atomizer of electronic cigarettes producing the aerosol that delivers nicotine to the smoker.
Antifreeze
Glycerol was used earlier as antifreeze in automobiles. It was replaced by ethylene glycol, which has a lower freezing point than that of glycerol. However, glycerol is non-toxic and bio-based chemical, and therefore, is being re-considered for antifreeze applications.
Glycerol is commonly used as a component of solvents for enzymatic reagents stored at temperatures below 0 deg. C due to the depression of the freezing temperature. It is also used as a cryoprotectant for laboratory organisms such as fungi, bacteria, etc. These are stored in a mixture of glycerol and water which prevent or reduce formation of ice crystals, thus protecting the organisms from damage.
Lubricants
Glycerol is an important lubricant in many applications because of its stability over a braod range of temperatures and pressures. In addition, its non-toxic nature makes it suitable for lubrication of food and other machinery where product purity is of paramount importance.
Glycerol can be used as a lubricant in applications where oil cannot be used. Glycerol is a recommended lubricant for oxygen compressors because it is more resistant to oxidation than mineral oils. It is also used to lubricate pumps and bearings exposed to fluids such as gasoline and benzene, which would dissolve oil-based lubricants. Glycerol is also used instead of oil, in the manufacture of foods, pharmaceuticals and cosmetics, where the material being processed comes in contact with the lubricant.
Glycerol is used in the textile industry to minimize thread breakage and in dyeing, printing and finishing. It is a textile conditioning agent and is widely used for lubrication. It is frequently used in textile oils, in spinning, twist setting, knitting, and weaving operations. Its hygroscopic property is taken advantage of in the formulation of textile auxiliaries for sizing. Glycerine has a softening effect on yarn and fabric; it increases the pliability and acts as a surface lubricant. Its effectiveness in these and other similar applications is due mainly to its viscosity and hygroscopicity.
Glycerol is commonly used as a lubricant due to its high viscosity, and its ability to remain fluid at low temperatures makes it valuable without any modification. Finely divided graphite is dispersed in glycerol to increase its lubricating power. Its viscosity may be decreased by addition of water, alcohol, or glycols, and increased by polymerization or mixing with starch. Pastes of such compositions may be used in packing pipe joints, in gas lines, or in similar applications. For use in high pressure gauges and valves, soaps are added to glycerol to increase its viscosity and improve its lubricating ability.
In the rubber industry, glycerine is used in vulcanizing, surface lubrication, and as preservative.
Gaskets and Corks
Sheets and gaskets made with ground cork and glue require a plasticizer that has some humectants action in order that they may be pliable and tough. Glycerol is used because it has low vapour pressure, is not readily extractable by oils and greases, is readily absorbed by the cork, and is compatible with glue. With crown sealers and cork stoppers that come into contact with foods, it fulfills the additional requirement of non-toxicity.
Papers - Wrapping and Packaging Materials
Glycerol is used in the manufacture of papers as a plasticizer, humectants, and lubricant. In addition to the softening effect of retained moisture, it also reduces shrinkage. Since many papers are used as food wrappers or in sanitary products, glycerol’s essential non-toxicity, freedom from odour, and stability are very useful in these applications.
Meat casings, and special types of papers such as grease-proof paper, need plasticizers to give pliability and toughness. Glycerol is used since it satisfies the requirements. Glycerol is completely compatible with the base materials used, is absorbed by them, and does not crystallize or volatilize appreciably.
Glycerol is used to keep cellophane and special, high quality papers flexible and tough.
Pressure Gauges
Glycerol is used as fill in pressure gauges to damp vibrations. External vibrations from compressors, engines, pumps, etc. produce harmonic vibrations within Bourdon gauges that can cause the needle to move excessively, giving inaccurate readings. The excessive swinging of the needle can also damage the internal gears and / or other components of the pressure gauges, resulting in premature wear. Glycerol, when poured into these gauges to replace the air space, reduces the harmonic vibrations that are transmitted to the needle, increasing the life and reliability of these gauges.
Fuels
Glycerol is used as fuel in internal combustion engines. It is used to power diesel generators supplying electricity for the FIA Formula E series of electric race cars.
Glycerol acetate, an ester with acetic acid, is a potential fuel additive.
Miscellaneous Uses
Glycerol is used on the film industry sets while filming scenes involving water prevent an area meant to look wet from drying out too quickly.
It is also used in the generation of theatrical smoke and fog as a component of the fluid used in the fog machines as a replacement for glycol, which is an irritant on prolonged exposure.
Glycerol can be used as an ultrasonic (ultrasound) couplant or coupling agent. It is used as replacement for water in ultrasound testing as it has favourably higher acoustic impedence than water, while being relatively safe, non-toxic, non-corrosive and has relatively low cost.
Glycerol, when used in combination with other chemicals, yields many useful products. For example, glycerol and ethylene glycol together can be used as a solvent for alkaline treatment of polyester fabrics, as a dielectric medium for compact pulse power systems, and as a medium in electro-deposition of indium-antimony alloys from chloride tartrate solutions. Glycerol is widely used for the manufacture of electrolytes for electrolytic condensers. It is also used in the treatment of metals.
Glycerol Derivatives
Due to the presence of three hydroxyl groups, glycerol is very much reactive and forms many different chemical derivatives, including acetals, amines, esters and ethers. Of these, the glycerol esters are the most widely used.
Glyceryl trinitrate or nitroglycerine is produced in a mixture of nitiric acid and sulphuric acid. Nitroglycerine is an essential component of various explosives such as dynamite, gelignite, and propellants like cordite. Dynamite, as it is manufactured today, is a mixture based on an explosive compound, usually nitroglycerine, mixed with an absorbent, usually diatomaceous earth, in a proportion of about 3:1 nitroglycerine to the absorbent. As natural glycerol was in limited supply earlier, synthetic glycerine processes were developed to meet the high demand of explosives during wars.
In addition to its use as an explosive in the form of dynamite, nitroglycerine is also an important therapeutic agent – coronary vasodilator – used in the treatment of angina pectoris, and as a heart stimulant. Glycerol has also been used therapeutically for canine bronchial asthma.
Alkyd resins, an important class of resins, are esters of glycerol and phthalic anhydride. These alkyd resins are widely used in paints, surface coatings, printing inks and varnishes. Short oil length and medium oil length, air drying, as well as, stoving alkyd resins, are made by using glycerol. Glycerol, due to its chemical versatility and process advantages, has been a standard component in the manufacture of alkyd resins. They may be modified to meet a wide range of coating applications and demanding conditions. However, pentaerythritol is being used in place of glycerol nowadays to a large extent. Oil varnishes prepared from glycerol and rosin show better solvent release properties, are harder-drying, and also show water resistance.
Glycerol forms esters with both inorganic and organic acids. Depending on reaction conditions and degree of esterification, these can be mono-, di- or tri-esters or –glycerides. The above-mentioned phthalic acid and nitric acid esters, fatty acids and acetic acid esters important ones. Estergums (rosin acid ester of glycerol) used as emulsifier; and the salts of glycerophosphoric acid which are used medicinally, should also be mentioned here.
Glycerol-terephthalic acid polyester coatings are widely used in wire coating formulations by the electrical insulation industry. These coatings exhibit exceptionally good electrical insulation and mechanical properties, and have more or less replaced other types of wire coatings.
Many different organic esters of glycerol – mostly with short- and long-chain fatty acids – are used in pharmaceuticals as well.
Glycerol can be used as a feedstock for the production of ethanol (ethyl alcohol), acroelin, propylene glycol, epichlorohydrin, dihydroxyacetones, glyceric acids, succinic acid, and 1,3-propanediol (1,3-PD).
It is interesting to note here that the synthetic glycerine was produced in the past from epichlorohydrin and acrolein. But nowadays, due to increased availability of glycerol as a by-product from biodiesel manufacture, the trend has reversed and those intermediates are being produced from glycerol!
Major esters and other chemical derivatives are discussed in detail in the following paragraphs.
Fatty Acid Esters
Triglyceride esters of fatty acids are major components of naturally occurring fats and oils. Mono- and di-glycerides esters of fatty acids occur naturally in fats and oils that have become partially hydrolyzed. These partial glycerides are obtained by reactions of fatty acids with excess glycerol or by interesterifiction or transesterifiction of fats and oils with glycerol. Commercial glycerides are mixtures of mono- and di-esters, with a small percentage of the tri-ester. They also contain small amounts of free glycerol and free fatty acids. High-purity monoglycerides are prepared by molecular or short-path distillation of glyceride mixtures.
The long-chain fatty acids mono- and di-glycerides are oil soluble and water insoluble. They are all edible, except the ricinoleate and arsenate, and find their greatest use as emulsifiers in foods and in the preparation of baked goods., ice cream and dairy products. Mixed fatty acid glycerides with citric, lactic or acetic acid are also used for this purpose. A mixture of mono-, di- and tri-glycerides is manufactured in large quantities for use in super glycerinated shortenings.
The emulsifying properties of these partial glycerides are used in the cosmetics and pharmaceutical industries. The range of emulsifying and stabilizing properties can be increased further by replacing some of the glycerol in the fatty esters with polyglycerol or ethoxylated glycerol. These esters are used in the production of fat-containing baked goods or as re-greasing agents in shampoos and foam baths.
Tailored glycerides with unique nutritional properties have grown in importance in recent years. These are produced from glycerol esterification with specific high-purity fatty acids. These are mostly made from the medium-chain length fatty acids – caprylic (C8), capric (C10) and lauric (C12) acids. They are called as medium chain triglycerides or MCTs. These MCTs are being increasingly used in a variety of applications. A triglyceride, consisting primarily of caprylic (C8), capric (C10) and behenic (C22) fatty acid chains, called as “coprenin” has been misquoted as a low calorie substitute for cocoa butter. By starting with behenic monoglyceride made from glycerol and behenic acid, the shorter caprylic (C8) acid and capric (C10) acid can be attached to the behenic monoglyceride to deliver a triglyceride having only one long fatty acid (behenic (C22) acid) chain.
Partial glycerides are important modifying agents in the manufacture of alkyd resins, detergents, and other surface active agents. They are also employed as lubricants for working plastics and serve as components for special lubricant formulations. The monoglycerides are also used in the preparation of pigments, floor waxes, synthetic rubbers, coatings, textiles, etc.
Glycerol monostearate (GMS) and medium chain triglycerides (MCT) are commercially the most important fatty acid esters of glycerol.
Glycerol monostearate (GMS) is mainly used as a food additive. It is used as thickening, emulsifying, anticaking, and preservative agent. GMS is used to emulsify oils, waxes and solvents; as a protective coating for hygroscopic powders; as a solidifier and controlled release agent in pharmaceuticals; and as a resin lubricant. It is also used in cosmetics and personal care products. It is also used as a polymer additive – as antifogging and antistatic agent – in plastic packaging films.
Medium chain triglycerides (MCTs) are being increasingly consumed in baby foods, special dietary formulations, pharmaceuticals or drugs delivery systems, etc. MCTs are useful in the treatment of a variety of malabsorption ailments due to their ability to be absorbed rapidly by the body. Medium chain triglycerides are also used as carrier oils or solvents for flavours, vitamins and several oral medicines.
Acetins
The acetins arecolourless mono-, di- and tri-acetates (esters) of glycerol that are formed when glycerol is heated with acetic acid, acetic anhydride or both. They are the most important esters of glycerol from short-chain carboxylic acids.
Monoacetin, i.e., glycerol monoacetate, is a thick hygroscopic liquid and is sold for use in the manufacture of explosives, in tanning, and as solvent for dyes.
Diacetin, i.e., glycerol diacetate, is a hygroscopic liquid, and is sold in a technical grade for use as a plasticizer and softening agent and as a solvent.
The biggest use is of triacetin, i.e., glycerol triacetate. It has a very slight odour and a bitter taste. Its primary use is as a cellulose plasticizer and stabilizer in the manufacture of cigarette filters, and its second-largest use is as a component in binders for solid rocket fuels. Smaller amounts are used as a fixative in perfumes, in the manufacture of cosmetics, in the production of vinyl polymers and rubbers, and as a carrier in fungicidal compositions. Tiacetin can be used as a fuel additive, as an antiknock agent. It is also useful in improving cold properties and viscosity of biodiesel. Triacetin is a possible source of food energy (i.e., it can be used as a food) in artificial food regeneration systems on long space missions. It is a safe food alternative to obtain over half of one’s dietary energy.
Direct utilization of crude glycerol for the manufacture of triacetin has been investigated, as mentioned earlier.
Polyglycerols
Polyglycerols are ethers of glycerol and have many properties of glycerol. These polyglycerols are produced industrially by base catalyzed condensation of glycerol, and as by-products of epichlorohydrin hydrolysis. Separation and isolation are achieved by treatment with dimethoxypropane or acetone (ketalization), fractional distillation of the diisopropylidene derivatives, and finally, acid-catalyzed hydrolysis.
Diglycerol is a viscous liquid, about 25 times as viscous as glycerol. Diglycerol and the higher oligomers, as synthetic building blocks to glycerol, can be converted to several interesting products.
Polyglycerols offer greater flexibility and functionality than glycerol. Polyglycerols upto and including triacontaglycerol (30 condensed glycerol molecules) have been prepared commercially. The higher forms are solid.
Polyglycerols are soluble in water, alcohol, and other polar solvents. They act as humectants, similar to glycerol, but have progressively higher molecular weights and boiling points. Products based on polyglycerols are useful as surface active agents in emulsifiers, plasticizers, adhesives, lubricants, antimicrobial agents, medical specialties and dietectic foods. For example, fatty acid esters of polyglycerol offer a larger range of properties than the simple glycerides. Because of their lipophilic and hydrophilic properties, they have a wide range of applications as emulsifiers in the cosmetics and food industries.
The following novel applications of polyglycerol have also been suggested –
- Low fat cookies
- Smokeless tobacco to give tobacco flavour withless second-hand smoke
- Improved fabric softener which is environmentally friendly
- Spot drinks for improved hydration during exercises
- Environment-friendly drilling fluids
Polyether Polyols for Urethane Polymer (Polyurethanes)
Urethane polymers is an important application of glycerol. It is used as the fundamental building block in polyether polyols used in making polyurethanes. In this use, it is the initiator to which propylene oxide, alone or with ethylene oxide, is added to produce tri-functional polymers which, on reaction with diisocyanates (MDI and TDI) produce flexible urethane foams. These flexible polyurethane foams resulting from glycerine based polyether polyols have superior properties with respect to humid ageing and resilience.
Glycerol-based polyethers have found some use, too, in rigid polyurethane foams and particularly, in urethane coatings.
Polyhydroxyalkanoates (PHA)
Polyhydroxyalkanoates (PHA) can be produced from glycerol by mixed culture fermentation. In fact, other raw materials such as glucose, sucrose as well as vegetable oils can also be used in the same fermentation process instead of glycerol to produce PHAs.
PHAs are used to produce polymers that have good flexibility and strength, desirable for making packaging materials. Most importantly, PHA is made from glycerol, which is renewable, natural chemical; and PHAs are biodegradable polymers.
Epichlorohydrin
Epichlorohydrin is an organochlorine compound having an epoxide group. It is an intermediate in one of the synthetic routes to produce glycerol. However, due to ample availability of glycerol in recent years due to biodiesel production, several commercial plants have been built to produce epichlorohydrin from glycerol.
In the process developed by Dow Chemical, glycerol is first reacted with hydrogen chloride to replace two hydroxyl groups, thus producing an intermediate having two chloride groups and on hydroxyl group. This is the same intermediate formed in the allyl chloride – hypochlorous acid process to manufacture synthetic glycerol. This is treated with alkali (sodium hydroxide) to form epichlorohydrin.
Major application of epichlorohydrin is in the manufacture of epoxy resins, after conversion to bisphenol A diglycidyl ether.
Succinic Acid
Glycerol can be converted to succinic acid by biological or fermentation processes. However, such processes have not been commercialized yet.
Succinic acid is an important chemical intermediate which is used to produce several downstream products in addition to its direct applications. Succinic acid is used as a flavouring agent for foods and beverages. Polyesters, resins, 1,4-butanediol (1,4-BDO) are some of the important derivatives of succinic acid. All these derivatives have diverse applications in various industries.
Dihydroxyacetone (DHA)
Dihydroxyacetone (DHA) is a simple three-carbon, non-toxic sugar. It can be produced from glycerol using either anaerobic or aerobic biological or fermentation processes as well as chemical processes. DHA can be prepared from glycerol by mild oxidation with hydrogen peroxide and a ferrous salt as a catalyst, with glyceraldehydes (a structural isomer of DHA) as a co-product. DHA can also be produced from glycerol in high yield and selectivity at room temperature using cationic palladium-based catalysts with oxygen, air or benzoquinone acting as co-oxidants.
DHA is used as a (sunless) tanning agent in cosmetics. It is also used in wine during sugar fermentation due to its anti-microbial activity.
Glyceric Acid (GA)
Glyceric acid (GA) is a natural three-carbon sugar acid obtained by glycerol oxidation. Esters of glyceric acid are called as glycerates, which are a family of chemicals having a wide range of applications. GA also has a great potential as an important ingredient for cosmetics and pharmaceuticals.
1,3-Propanediol (1,3-PD)
Glycerol is converted to 1,3-propanediol (1,3-PD) by biological or fermentation processes as well as chemical or catalytic synthetic processes. The fermentation process utilizes bacteria Clostridium diolis and Enterobacteriaceae. However, this process has not been commercialized yet.
1,3-Propanediol is used for producing composites, adhesives, laminates, powder and UV-cured coatings, moldings, novel aliphatic polyesters, co-polyesters, solvents, antifreeze, and others. One of the most successful applications has been in the formulation of the polymer of Sorona (by DuPont) or Corterra (by Shell) – chemical name polytrimethylene terephthalate (PTT) – which is used for making carpets of domestic and industrial grades. DuPont Tate & Lyle biological process uses genetically modified E. coli to convert glucose (corn sugar) to 1,3-propanediol.
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