MANASI PANDEY, DR.ROBIN SANTRA*
Jubilant Industries Ltd.
R&D-I, C-26, Sector -59, Noida-201301
E-mail: r_santra@jubl.com
Introduction
Ester Gum, a pale, medium-hard thermoplastic resin, is the glycerol ester of wood rosin. It is produced by a special process that yields a low-odour, low-acid-number product.
Glycerol ester of wood rosin is a complex mixture of tri- and diglycerol esters of resin acids from wood rosin obtained by the solvent extraction of aged pine stumps followed by a liquid-liquid solvent refining process. The refined wood rosin is composed of approximately 90% resin acids and 10% neutrals (non-acidic compounds). The resin acid fraction is a complex mixture of isomeric diterpenoid monocarboxylic acids having the typical empirical formula C19H29COOH, of which the main component is abietic acid. The substance is purified by steam stripping or by counter-current steam distillation.
Common names for glycerol ester of gum rosin include: glycerol-modified gum rosin, rosin glycerol ester, glycerol abietate, glyceryl triabietate or ester gum.
Ester Gum occurs as white to yellow-whitish powder, light yellow to light brown glassy lumps, hard, to pale yellow amber-coloured solid or as a clear, viscous liquid. It is odorless or has a slight, characteristic odor.
Raw materials & their characteristic
The raw materials required for manufacture of ester gum are wood rosin and glycerol.
Rosin
Rosin is one of the oldest raw materials for adhesive industry, either as such or converted to rosin ester. Three types of rosin are used for resin manufacture, gum rosin, wood rosin and tall oil rosin, all generated from the pine tree.
Gum Rosin
Gum rosin was once the only commercial source of rosin. It is the oleoresin (pine gum) of the living pine tree. The harvesting of the oleoresin is simple, involving only periodic wounding of the tree and collecting of the exudates into cups.
Wood rosin
After harvesting pine trees the stump is allowed to remain in the ground for about ten years so that its bark and sapwood may decay and slough off to leave the heartwood rich in resin. Resinous material is extracted from stump.
Tall oil rosin
Tall oil rosin is obtained by distillation of crude tall oil (CTO); a by-product of the kraft sulphate pulping process. CTO contains 70-90% acidic material, which is composed essentially of fatty acid and tall oil rosin. Tall oil rosin has a tendency to crystallize and usually contains 200-600ppm sulphur.Highly distilled TOR can produce esters, which are competitive with gum and wood rosin derivatives [1].
The resin acid composition of wood rosin can vary considerably; however, the main resin acids in ester gum are abietic acids, with smaller contents of dihydroabietic and neoabietic acids; pimaric acids, including isopimaric and sandaracopimaric acids, levopimaric and palustric acid. The toxicology of glycerol esters of wood rosins harvested from the stumps of the pine tree is different from that of glycerol esters from tall-oil and gums, which are not used for the preparation of food additives.
There are many different rosins available to the market place. The main world producer is China (approx. 90% world production; 450,000-tonnes/year gum rosin)[2].
Rosin unlike hydrocarbon resins are not polymers. In fact they are a blend of different molecules viz.
Abietic type
Pimaric type
Rosin molecules have poor stability caused by unsaturation. Stability can be improved by various methods such as disproportionation and hydrogenation.
Glycerol
Glycerol is a highly hygroscopic polyalcohol (1,2,3-propanetriol) with a high viscosity and relatively high density and with several applications in the cosmetic, food, pharmaceutical and chemical industries (polymers, triacetin and more). However, the amount of application is relatively small.
Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids known as triglycerides. Glycerol is sweet-tasting and of low toxicity.
The colour bodies in technical glycerol make a detectable contribution to the colour of the gum, so for uniformity, C.P. grade glycerol was used.
Production of gum rosin
Gum rosin is obtained by extracting oleoresin gum obtained from living pine trees, followed by refining by washing, filtration and distillation to separate the rosin from other components in the oleoresin extract. After adding water to the oleoresin, the mixture is heated to 90oC to separate the unusable portion, followed by three washing and filtration steps of the remaining rosin mixture prior to direct treatment to separate the turpentine and gum rosin components. The gum rosin then undergoes countercurrent steam distillation that further separates the material into light fractions, the desired distilled gum rosin and heavy weight fractions. This final purification step distinguishes between gum and wood rosin glycerol esters intended for use in beverages, in comparison to esters used in chewing gum base, according to specifications in the Food Chemicals Codex.
Glycerol ester of wood rosin is purified by counter-current steam distillation for use in adjusting the density of citrus oils for beverages, while both glycerol esters of gum and wood rosin are purified only by steam stripping for use in chewing gum base.[4]
Manufacturing process
Synthetic ester gum is formed 
by the reaction of rosin with glycerol. As abetic acid and its anhydride are the main constituents of rosin, the reaction is essentially that of glycerol with abetic acid or its anhydride where by one molecule of water is eliminated for each molecule of the acid combined with the glycerol radical.[3]
During the synthesis of ester gum to avid discolouration all possibility of access of air to the reactor must be eliminated. Measurable colour improvement is effected by sparging the initial charge of rosin and glycerol with a slow stream of carbon dioxide or nitrogen for several hours prior to melting and cooking.
Heavy metal impurities should not be introduced, either directly from the materials of the reactor, or as impurities of the rosin or glycerol used. The presence of certain colourless additives, such as light coloured zinc or calcium rosinates, increases the danger of discolouration, but these materials could be tolerated and a gum of satisfactory colour can be produced if all precautions are taken to eliminate traces of atmospheric oxygen.
The direct use of lime where glycerol is present always leads to the production of darker products.
The distilled gum rosin prepared in the preceding step is pumped into a batch-type reactor and esterified
with food grade U.S.P. glycerin under an N
2 atmosphere. The temperature maintained during the reaction is 270
oC and is allowed to proceed until the desired product specifications are met. This ester is then purified by direct counter-current steam distillation and analyzed for acid number, softening point, and colour and is then deodorized. The production process is conducted within an inert nitrogen atmosphere using stainless steel equipment and is confined to an area exclusively for food products.
No two lots of a natural product such as rosin are necessarily identical and variations can be expected between materials of various grades and from different sources.
The reaction can be performed using an open kettle or a partially closed kettle, using using aluminum foil or phosphoric acid as catalyst [5]. When aluminum foil was used the temperature is maintained around 271oC and reaction mixture was held at about this temperature for 3hours. Sampling is performed every half-hour and tested for acid number. This gave a measure of the progress of the reaction.
A product of low acid number cannot be made in an open vessel, but is produced in a covered vessel with a short chimney to act as a condenser for the glycerol vapours. It has also been found that steel has the greatest catalytic effect over all other metals. In a steel reactor product of low acid value could be secured in the shortest time; however this advantage is offset by the disadvantages that the rosin acid attack the iron or steel and yield a very dark coloured product. It is also interesting to note that after a low acid value is obtained in the presence of steel, the material starts to break down again and the acid number increases.
Aluminum has a greater catalytic action on the reaction than copper and also the advantages of yielding a much lighter coloured product.
Block Diagram of Esteification
Critical aspect of manufacturing
Softening Point, Acid Value and Colour
Carboxylic acids can be converted to esters using various alcohols. The molecular weight of the alcohol determines the softening point of the ester. Glycerol and pentaerythritol are the most commonly used alcohols. Methanol and triethylene glycol are used to produce lower softening point esters.
In the preparation of ester gum the first objective is usually to meet an acid number specification with a clear product, which is free of unreacted glycerol. The kind of rosin chosen determines the softening range of an ester gum, but formulation and extent of processing are also important factors. The softening range will rise slowly during cooking, if proper conditions are maintained. The use of an unusually high temperature or an unsuitable catalyst in an effort to reduce processing time can result in a darker or softer product.
Colour may not always be an important consideration, but as it reflects care in the choice of starting materials and especially of conditions of processing, it may also be taken as an indication of overall gum quality.
Esterification is affected by metallic impurities and composition of mixtures
Certain grades of rosin contain small and variable amounts of dissolved metallic impurities, which exert a marked catalytic effect in esterifications. The esterification rate of WG and M grade (dark rosin) wood rosin with C.P. glycerol were studied. With M grade rosin, acid numbers of 14 and 11 were attained after 4-5 hour cooks at 290oC, but with WG grade wood rosin 5-7 hours cooks were required to produce products of similar acidities. A subsequent analysis of the two rosins showed that the darker rosin (M grade) left almost twice as much residue on ignition, 56 ppm vs 32 ppm, and contained approximately twice as much iron. The activity of iron as an esterification catalyst is well known[6].
Esterification is affected by composition of mixtures
The solubility of glycerol in hot rosin increases very rapidly at temperatures above 230oC. The mixture used for ester gum preparation becomes clear at about this point. In a rapidly heating charge of rosin, the following amounts of glycerol (moles per mole of rosin) could be dissolved to form homogeneous reaction mixtures: 0.80 (245oC); 0.90 (255oC); 1.0 (260-270oC); 1.1 (275oC). As esterification progressed, the glycerol solubility diminished rapidly and a heterogeneous reaction mixture was formed.
Disappearance of glycerol in a typical cook
In laboratory preparations of ester gum charges containing 1.20 equivalents of C.P. glycerol per equivalent of rosin (a 20% excess over theoretical requirement) were esterified to obtain products with acid number 7.0 to 11.0. At temperature above 230oC homogeneous reaction mixtures were obtained. The disappearance of glycerol during a 5.5 hour cooking schedule at 285oC was obtained.
Effect of catalyst
When larger amounts of an alkaline catalyst such as lithium naphthenate were added at the start of a cook, esterification proceeded at an extremely rapid rate and molar quantities of rosin and glycerol were reacted in homogeneous system. This was not possible in the absence of the catalyst. None of the unesterified glycerol was converted to diglycerol in this case.
Zinc oxide (or resinate) is an efficient catalyst. It is approximately three times as active as calcium oxide on a weight basis. However, when more than 0.03% is used, it tends to be destructive at 290oC. Both zinc iodide and zinc chloride were very destructive and even zinc acetate yielded a soft and relatively dark product.
Calcium or lime is one of the oldest and best known catalyst. One of the objections to its use has been its darkening effect on gums when it is added directly to a cook. By using it in the form of a light coloured resinate, light gums with no reduction of catalytic activity was obtained. Calcium chloride gave a dark-coloured, soft gum. Calcium carbonate was not sufficiently soluble to be useful.
The calcium salt of an acid petroleum sulphonate was prepared and used in a 0.1% amount in a 33% excess-glycerol formulation. The esterification rate was rapid and a gum of low acid number and high hydroxyl number was obtained. The gum obtained was softer than the regular uncatalyzed gum, but the colour of the gum was quite light. A calcium naphthenate solution (Nuodex, 6% calcium) was used in a 20% excess glycerol formulation in an amount equivalent to 0.013% calcium oxide. Some catalytic effect was observed, but a larger amount of the catalyst would appear to be much more effective. The gum obtained was darker coloured than usual. The use of calcium sulphite hexahydrate in an amount equivalent to 0.14% calcium oxide resulted in a rather dark gum. The esterification rate was very high as would be expected. A slight precipitate was present in the final product.
For the synthesis of soft, highly hydroxylated gums lithium naphthenate was used. In a typical ester gum cook (formulated with 20% theoretical excess of glycerol) the naphthenate equivalent of 0.013% lithium hydroxide was about as active as 0.013% calcium oxide.
With aluminum salts, at 270oC, with a 33.3% excess of glycerol, the time required to reach an acid number of 8.0 was increased by 3 hours in the presence of 0.1% basic aluminum acetate (equivalent to approximately 0.03% Al2O3). At 290oC with aluminum chloride hexahydrate, equivalent to approximately 0.02% Al2O3, a soft gum was produced.
Inorganic acids
The use of small amounts of sulphuric acid results in destruction of rosin and dehydration of the glycerol. When trace amounts were used (about 0.004%) these effects were not observed, but there is no significant increase in the esterification rate.
Phosphoric acid can be used in small amounts and it exhibits considerable catalytic activity. The gum obtained using phosphoric acid as catalyst have softening range of 82-97oC, very low hydroxyl value and show a very faint haziness.
Boric anhydride in amounts between 0.1 and 0.3% was not an effective catalyst, but gave a hard gum.
Petroleum sulphonates
Several fat-splitting catalysts and sodium sulphonates were found to exhibit good catalytic activity. The gum obtained was slightly lighter than those prepared without any catalyst.
Sulphonic acids from several commercial products were prepared as follows: Neutral sulphonates were dissolved in benzene, acidified with gaseous hydrogen chloride and filtered. The solution were triturated in a mortar with barium hydroxide and filtered again. After a second treatment with hydrogen chloride and a third filtration, solvent and excess hydrogen chloride was removed in a vacuum. The acids were obtained as dark, viscous liquids (or mineral oil solutions) with acid numbers between 31 and 167. Three of these showed a good ability to increase the rate of reaction, but small reductions (1o to 3oC) in the softening points of the resultant ester gums were always evident. 2-naphthalenesulfonic acid and methyl p-toluenesulfonate, were both too destructive to be considered.
Organic acids
Acetic acid owing to its low boiling point was not easily retained in the reaction flask and no catalytic activity was observed when it was used in amounts less than 1%. When used in approximately 4-7% , there was considerable catalytic action and clear, hard gums of low acid number and low hydroxyl number could be prepared. During the first two hour of cooking there was extremely rapid reduction of acidity. The acid value obtained at the end is 8. Most of the acetic acid used was found in the distillate. The gums were darker than those prepared without any calalyst.
Ester gums of zero hydroxyl number were prepared in an acid interchange reaction by heating mixtures of triacetin and rosin. The softening ranges of the gums were lower than those prepared in the usual way with glycerol.
Lactic acid exhibited some catalytic activity in 0.2% amount, but the esterification rate was not further increased by using five times as much.
The activity of 0.1% maleic anhydride was comparable to that of lactic acid. Pyromellitic anhydride was very insoluble in the esterification mixture and did not show any catalytic effect.
Other organic compounds
Formaldehyde, glycerol formal, triethanolamine and two nonionic detergents were also used as additives in ester gum preparations. None of them appreciably affected the course of the esterification. Formaldehyde was tried primarily to test its effect on colour of the ester gum, but no appreciable effect was noted.
Typical characteristics of ester gum
Acid value
In chemistry, acid value (or "neutralization number" or "acid number" or "acidity") is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance. The acid number is a measure of the amount of carboxylic acid groups in a chemical compound, such as a fatty acid, or in a mixture of compounds.
The acid number of ester gums and intermediate esterification products was determined by dissolving a 1gm to 5gm sample of material in 15ml of xylene, and titrating to a phenolphthalein end-point with alcoholic 0.1N potassium hydroxide.
Softening range
This is the temperature range in which material without a melting point goes from a rigid to a soft condition.
Hydroxyl Number
The hydroxyl number gives the hydroxyl content of a polyol, and is derived from a method of analysis by acetylating the hydroxyl and titrating the resultant acid against KOH. The hydroxyl number is the weight of KOH in milligrams that will neutralize the acid from 1 gram of polyol.
Colour
Colours of ester gums and rosins were measured directly in a spectrophotometer on samples poured or melted into 19 x 150 mm test tubes, about 17mm inside diameter.
Uses
Rosin (and most of its products) is used in the paper, coating (varnishes, wax and adhesives), polymer and food industries as well as a precursor for flux in soldering. Esterification with methanol, ethylene-glycol, glycerol and pentaerythritol yields esters for a variety of applications. These are used as tackfiers for hot melt and pressure adhesives, in solder fluxes, as crystallization promoters in the production of polypropylene, as neutralizers in paper industry, in the formulation of chewing gum and for the manufacture of polymeric coatings used in the controlled dispersion of drugs and fitosanitary products[7].
Glycerol ester of wood rosin serves as a natural alternative to brominated vegetable oil, particularly in citrus-flavored soft drinks.
The glycerol ester of hydrogenated rosin can be used as tackifier for cigarette filter adhesives.
Typical recipe of gum base for chewing gum
| Ingredients | Phr* |
| SBR | 5-10 |
| PIB | 8-10 |
| Talc | 6-10 |
| Ester-gum | 25-30 |
| PVA | 25-30 |
| Oils & Fats | 10-17 |
| Wax | 10-15 |
*Parts per hundred parts of resin
References
1. The Chemistry of Tackifying Resins - Part II , by C.Donker , Special Chem - Nov 11, 2002, pp 149–152.
2. S. Zhaobang , Production and Standards for Chemican Non-Wood Forest Products in China, CIFOR report no.6, 1995, ISSN0854-9818
3. Laboratory Experimental work on Ester Gum by F.M. Beegie , Industrial & Engineering Chemistry, 1924, 16 (9), pp 953–955
4. Generally recognized as safe (gras) notification for glycerol ester of gum rosin by Environ International J, June24, 2002, pp 10-12
5. Method for making oxidation stable light coloured glycerol ester of modified rosin by Roland P.F. Scharrer et al. USPTO 4,447,354 , May 8, 1984
6. Ester gum by esterification of rosin with glycerol by J.D. Hind, Industrial & Engineering Chemistry, Vol 46, No. 3, 1954, pp 441-452
7. Kinetic modeling of the esterification of rosin and glycerol : Application to industrial operation by Miguel Ladero et al., Chemical Engineering Journal, 169, 2011, pp 319-328.
