1. Field of the Invention
This invention is concerned with preparing alginic acid esters known as "alkylene glycol alginates" by a procedure for treating seaweeds of the Class Phaeophyceae. This novel and improved process involves treatment of the seaweed with an acid and thereby forming alginic acid therein and subsequently reacting the alginic acid content of the treated seaweed, with an alkylene oxide to form an alkylene glycol alginate in situ without prior isolation of alginic acid or its salts from the solid seaweed residue.
2. Description of the Prior Art
Alginic acid is a polyuronic acid generally believed to consist of two uronic acids; mannuronic acid and guluronic acid, the proportions of which vary depending on factors such as, for example, seaweed species, plant age and seasonal variation. Alginic acid in the form of mixed water insoluble salts, in which the principal cation is calcium, is present in seaweeds of the Class Phaeophyceae, typical examples of which are: Fucus vesiculosus, F. spiralis, Ascophyllum nodosum, Macrocystis pyrifera, Alaria esculenta, Laminaria longicruris, L. digitata, L. saccharina, and L. cloustoni.
Methods for the recovery of water insoluble alginic acid and its water soluble salts, particularly sodium alginate, are well known. The first extraction process was patented by Stanford in British Pat. No. 142 (1881). A series of variations of Stanford's method have subsequently been described in the patent literature. The most recent and familiar are the processes of Green, U.S. Pat. No. 2,036,934, and Le Gloahec and Herter, U.S. Pat. No. 2,128,551.
In Green's process, seaweeds of the Class Phaeophyceae are treated with dilute acid, such as dilute hydrochloric acid, followed by water washing. This converts the natural alginate salts present to alginic acid. Salts, residual hydrochloric acid, and water soluble organic materials are then removed by washing. The pretreated seaweed is then chopped or milled and an excess of sodium carbonate added together with a quantity of water to extract the water soluble sodium alginate formed. The mixture is filtered to recover the clarified sodium alginate solution to which a solution of calcium chloride is added to precipitate the alginate in the form of water insoluble calcium alginate. The highly hydrated precipitate is separated from the solution of sodium chloride, excess calcium chloride, and soluble color bodies, particularly phenolic compounds, usually present in the original extract. The precipitate may still retain some color bodies, even after water washing, and calcium hypochlorite is added as a bleaching agent. The precipitate is then treated with dilute hydrochloric acid to convert the purified calcium alginate to alginic acid. This gelatinous precipitate is washed with water to remove excess hydrochloric acid and calcium salts. It may be neutralized to provide the purified sodium alginate of commerce.
In the Le Gloahec and Herter process, seaweeds of the Class Phaeophyceae are treated with a solution of calcium chloride and the water soluble components of the seaweed are then extracted and removed by draining. The seaweed is then treated with dilute hydrochloric acid, drained, and water washed. Sodium carbonate is added and the seaweed mixture milled and diluted with water to extract the sodium alginate. The slurry is aerated to separate insoluble materials by a method of air flotation and the residual clarified solution is treated with a decolorizing agent. The purified sodium alginate could then be recovered by conventional means, as for example, by alcohol precipitation. To recover alginic acid, the solution is treated with dilute sulfuric acid, the precipitate being separated and pressed to remove the aqueous solution containing sodium sulfate and excess sulfuric acid. The precipitate is dehydrated with alcohol, washed with further quantities of alcohol, and then dried to produce the alginic acid of commerce.
Methods for preparing alkylene glycol alginates are also described in the patent literature. Steiner in U.S. Pat. No. 2,426,125 discloses a method for the manufacture of glycol alginates followed subsequently by Steiner et al in U.S. Pat. Nos. 2,494,911 and 2,494,912, Nielsen et al in Canadian Pat. No. 904,847, and Pettitt et al. in their related disclosures in Canadian Pat. No. 942,744 and U.S. Pat. No. 3,772,266. All of these patents describe, with variations in procedure, methods by which alkylene oxides are reacted with alginic acid.
The prior art teaches the preparation of glycol alginates by the reaction of alkylene oxides with the alginic acid produced by methods known in the art, such as those described above or those mentioned by Steiner in U.S. Pat. No. 2,426,125, viz. U.S. Pat. Nos. 1,814,981 to Thornley et al., 2,036,922 to Clark et al. and 2,036,934 to Green. It will be noted that Steiner U.S. Pat. No. 2,426,125 states (column 3, lines 3-6) "that the free acid from the above or other modifications of the general method may be used for my purpose provided only that the acid be in a state of commercial purity". The wet alginic acid is typically partially dried so as to contain approximately 50% by weight of water. This acid is milled to provide a large surface area; and either before, during, or after the drying process, it is partially neutralized with any suitable base so that between 5 and 30% of the carboxyl groups of the acid are combined with the base. It is essential that the base used be thoroughly disseminated through the acid so as to avoid the possibility of part of the acid being completely neutralized, and thus rendered unsuitable for the esterification reaction, while some of the acid remains free of partial neutralization. For this reason, Steiner describes several methods of partial neutralization. For example the base may be dispersed in a quantity of low boiling alcohol or ketone and added as a slurry to the stirred, wet, milled alginic acid. A wetting agent may also be added to aid dispersion.
Steiner's partially neutralized, finely divided, alginic acid is reacted with an alkylene oxide such as ethylene oxide or propylene oxide (1,2-epoxypropane). In commercial practice, the preferred glycol alginate is propylene glycol alginate formed by reaction of alginic acid with propylene oxide. The reaction proceeds with both the formation of the glycol alginate, and hydrolysis of the alkylene oxide due to the presence of water and the low pH of the mixture. Thus, a greater quantity of the alkylene oxide than the stoichiometric equivalent is required. It is believed that molar ratios of between 2:1 and 3:1 alkylene oxide: alginic acid are used, although Nielsen et al in Canadian Pat. No. 904,847 state (page 9, lines 17-18) that "the amount of propylene oxide may be from 1 mol to about 25 moles per mol alginic acid." In Nielsen's case, the reaction is carried out in a diluent, such as an alcohol or ketone, and a substantial portion of the oxide is thus not consumed in the esterification reaction. It is generally recognized that complete reaction of oxide and available carboxyl groups is not only difficult, requiring the use of large quantities of alkylene oxide, but unnecessary.
Said Pettitt et al patents disclose partly neutralized alginic acid of critical 65-78% solids content reacting with propylene oxide gas in the absence of air in 3 hours or less at 60.degree.-100.degree.C. In the examples, the oxide : acid molar ratios range from 2.8:1 to 15:1.
Wallerstein et al. in U.S. Pat. No. 2,478,988 teach the use of propylene glycol alginate as a foam stabilizing agent. According to Steiner U.S. Pat. No. 2,659,675, the glycol alginate used for the purpose of the Wallerstein patent was designed for use as an emulsifying agent in French dressings and the like and had the analysis: neutralization as ammonium alignate 30 to 40%; esterification 25 to 40%; and unreacted acidity 20 to 45%. In the same patent, Steiner teaches that an improved foam stabilizing agent has the following characteristics among others: neutralization as sodium alginate 15 to 20%; esterification 65 to 80%; and unreacted acidity 5 to 15%. Therefore, depending on the end use of the glycol alginate product, it appears that a fully satisfactory product may have any of the following characteristics: neutralization of between about 5 to 40% with any suitable base or mixture of bases; esterification of from about 25 to 80%; and unreacted acidity of from about 5 to 45%. In general the glycol alginate should be completely soluble in water giving a solution the pH of which is not less than about 3.5. Storage stability of the dry product may be impaired if the pH of its aqueous solution is below 3.5.
From the above description of the prior art, it is quite evident that prior art techniques for the preparation of glycol alginates have involved formation of alginic acid, separation thereof from seaweed residue and subsequent esterification of the alginic acid to produce the glycol alginate and have been rather complex, slow and costly in terms of chemical and processing requirements. Of the procedures described, the initial preparation of alginic acid appears the most complex, although separating the alginic acid in a form considered to be suitable for esterification, as described earlier, also appears to require a great deal of care and attention.
It has now been discovered that, contrary to previous belief, the alginic acid used for reactions, such as the esterification reaction described above, need not be in the extracted state of commercial purity. In other words, the present invention is based on the discovery that it is not necessary to isolate the alginic acid from the solid residue of the precursor seaweed in order to react the alginic acid with an alkylene oxide. In addition, the glycol alginate products of my novel process are capable of meeting the various commercial requirements and specifications such as those outlined above, together with color, purity, etc.