PROCESS FOR TREATING ALGINIC MATERIAL

Abstract: Calcium alginate material in the form of fibres or fabric or film is treated by a process which increases its solubility in water or saline solutions to extend its applications of use in the medical field particularly for haemostatic dressings. The process comprises substituting a proportion of calcium ions by hydrogen ions by reaction with an acid, and then treating the resulting alginic material with a solution of an alkali metal or ammonium bicarbonate to substitute alkali metal or ammonium ions for a proportion of the hydrogen ions to produce an alginic material which has a pH in the range 3.5 to 7 after drying. The process minimizes washing requirements and the risk of gelation of the alginic material, and avoids the discolouration which alkaline conditions can produce.

Description: Process for treating alginic material This invention is concerned with a process for increasing the solubility of alginic material for the purpose of extending its uses in the medical field, for example in the area of haemostatic dressings. Alginic material is usually produced in fibre or film form by spinning sodium alginate solution into an aqueous bath containing calcium ions where it is converted to the insoluble calcium alginate. Commonly 95 per cent of the carboxyl groups are in the calcium form, the remainder being in the sodium form. Calcium alginate has haemostatic properties and is used in the form of gauze or staple fibre wool for wound dressings. Its solubility in body fluids and in water or saline solutions is limited, and for some purposes a more soluble form is preferred. An increase in solubility is effected by substituting some of the calcium ions by, usually-,—sodium ions and a number of processes have been used or proposed for this purpose. One such process has been described in British Patent No. 1,231,506 and comprises successive treatments with acid and base solutions to strip off some of the calcium ions and replace them with sodium ions. The titration with the base is time-consuming and produces a product with an alkaline pH which is prone to discolouration. A modification of this process is described in British Patent No. 1,375,572 in which the pH is maintained on the acid side, but the titration is still time-consuming. Moreover, both processes require partial desiccation with alcoholic solution prior to heating to complete the drying; otherwise gelation of the product can occur. Another process which dispenses with the acid treatment is described in British Patent No. 1,394,741. This process effects a direct exchange of calcium for solubilis- ing cation by treating the calcium alginate with an appro¬ priate salt of an organic acid such as sodium acetate. However, the advantage of direct exchange is offset by the need to wash out the sodium acetate from the product, the need to reject the acetate bath as calcium salts build up in it, and the need to partially desiccate the product with alcohol to prevent gelation during drying. The washing and desiccation steps in the processes referred to are carried out with alcoholic solutions, and in the interest of economy, recovery of the alcohol by dis¬ tillation is required. Of course, such a recovery is itself an expense, and it is a concern of the present invention to provide a process for solubilising alginic material which can avoid the need for such a recovery process. According to the invention, a process for increasing the solubility of alginic material comprises reacting a cal¬ cium alginate material with an acid to substitute hydrogen ions for a proportion of the calcium ions attached to the carboxyl groups, treating the resulting alginic material with a base solution and subsequently drying the treated alginic * material, and is characterised in that said treat¬ ment is effected using a solution of an alkali metal or ammonium bicarbonate to substitute alkali metal or ammonium ions for a proportion of the hydrogen ions to produce an alginic material having a ' pH in the range 3.5 to 7 after drying. The use of the bicarbonate anion gives an alginic material product which is low in occluded alkali metal or ammonium bicarbonate, particularly if the pH of the product is below 6 as is preferred. What appears to happen is that carbon dioxide is given off during the subsequent drying of the product with the result that the neutralisation reaction moves near to completion. This minimising of impurities in the product reduces or even removes the need to wash the product after the bicarbonate treatment and prior to it being dried. As the product is water-soluble at this stage, any washing needs to be done with a solution which 5 does not swell the alginic material, such as an aqueous alcoholic solution. The avoiding or reducing of washing, minimises the use of, say, alcohol in the process and makes a recovery process unnecessary. The solution of bicarbonate may be an aqueous -10- . alcoholic solution, for example aqueous industrial methylated spirits (I.M.S.), or indeed a solution in any suitable solvent which does not substantially swell the treated alginic material. The solution may be used for repeated treatments, being topped up with bicarbonate and 15 small quantities of alcohol to compensate for the low volumes of water taken into the treatment . bath by the alginic material itself. Excess bath may be discarded, the volumes being sufficiently low to make recovery unnecessary. The reaction of the alginic material with the 20 bicarbonate is relatively quick and convenient. Solid bicarbonate may be added to the treatment bath containing the alginic material and reaction proceeds as the bicarbonate is dissolving. Usually this takes no longer than 10 to 15 minutes at a bath temperature of 30°C. After 25 all solid has dissolved, another 30 minutes usually will see the reaction reaching equilibrium. The neutralisation reaction is not stoichiometric, and so for a target pH above about 4.5, excess bicarbonate over the stoichiometric quantity is required. Once the amounts 30 needed have been determined by trial, the process can be operated repeatedly to give alginic material of the desired pH. Product samples may be tested for pH at regular intervals and the bicarbonate treatment adjusted to compensate for any variation from the target pH. O PI The pH of the alginic material is measured by immersing 1 gm of alginic material in 100 ml of distilled water at room temperature, and 'measuring the pH of the solution using a pH meter after one hour's immersion. After the -bicarbonate treatment, the alginic material may be centrifuged to remove excess liquid and is then dried. The preferred method of drying is a heat treatment which is carried out at least initially at a temperature below 50°C, preferably 35°C. to 40°C, in order to avoid ^fehe risk-of -gelation at the surfaces of the alginic material — as moisture comes out of the material. The low temperature may be compensated for by carrying out drying in a forced draught of air so as to reduce the drying time. The temperature may be raised for the later stages of drying when the small quantity of liquid left gives little risk of gelation. The reaction of the calcium alginate material with an acid to substitute hydrogen ions for a proportion of the calcium ions attached to the carboxyl groups may be carried out in the conventional way using hydrochloric acid as the preferred reagent. Other suitable acids include sulphuric, phosphoric and acetic acids. The product of this reaction has both calcium ions and hydrogen ions attached to its carboxyl groups and is still insoluble in water. It may be washed substantially free of the acid anion using water and then centrifuged prior to the bicarbonate treatment. The calcium alginate starting material used for the process of the invention is preferably in a substantially unswollen state. So-called air-dry material with its normal moisture regain at ambient temperature is suitable in this regard. The point of starting with unswollen material arises from the fact that once swollen the material is difficult to dry without using, for example, neat alcohol as a desiccant. .lso the risk of gelation is high. When the material is in fibre form such swelling can result in the fibres becomming stuck together and therefore difficult to process. In the previous processes referred to, the calcium . alginate material is scoured and bleached before the acid treatment, and these treatments inevitably produce a swollen material. It is preferred to avoid the need for scouring by keeping the material free from oily finishes. Bleaching is required for many medical end-uses where a good white appearance is expected,--_ and - this can-—be- done_ -without substantially swelling the material by bleaching it at the same time as it is reacted with the acid. Hypochlorite added to the acid solution will generate free chlorine which is a strong bleach. Any excess chlorine is readily removed in the post-acid washing and there is no need for an antichlor treatment as with normal hypochlorite bleaching. The calcium alginate starting material may be in any suitable for , including the form of fibres, fabric or film. Staple fibres may be subjected to the various stages of the process of the invention in a batch operation or alternatively in a continuous process. The latter may be carried out. on a bed of fibres on a conveyor as in the processing of viscose rayon fibres. Continuous calcium alginate filaments may be treated by the process of the invention whilst in the form of a continuous filament tow or in the form of a wet-laid web of filaments as described in European Patent No. 0,027,113. If a knitted gauze is required, then the calcium alginate filaments or yarns used to knit the gauze preferably have a finish which does not interfere with the process of the invention and so does not need to be removed by scouring. A fibre finish, for example based on glycerol or sorbitan esters, may be applied to the alginic fibres or fabric by incorporating it in the bicarbonate solution. This avoids a subsequent application of finish which would need to be carried out from alcoholic solution or its equivalent to avoid gelation of the alginic material. The invention includes alginic material which has been treated by the process of the invention, particularly in the form of a fibrous dressing. This invention is illustrated by the following Examples in which percentages are by weight:- Example 1 Air-dry calcium alginate in staple fibre form was analysed and found to have its carboxyl groups occupied*96.6 per cent by calcium, 3.0 per cent by sodium, and 0.4 per cent by hydrogen. Its moisture content was about 30 per cent by weight based on the bone dry material. An acid treatment/bleaching bath was made by adding sodium hypochlorite to 2.5 litres of water in amount equal to 0.75 gm available chlorine. Dilute hydrochloric acid was added to this bath until its pH was brought down to 7, and then 270 ml of M/l hydrochloric acid was added, the bath volume finally being brought up to 3 litres with water. 14.0.5 gms of the alginate fibre were added to this bath which was at a temperature of 24°C. The fibre was gently agitated in the bath for 30 minutes, removed from the bath and washed free from chloride with water. The washed fibres were wrapped in a cloth and centrifuged for 5 minutes in a commercial machine giving 650 g acceleration. Analysis of the fibre prior to washing indicated that O Pi_ all the sodium and 47.5 per cent calcium had been removed by the acid treatment. i ■ A stock solution for a bicarbonate treatment bath was made by mixing 4 litres of water with 6 litres of I.M.S. (95 5 per cent) and adding 40 gms of Tween 20, a commercial fibre finish comprising essentially sorbitan monolaurate. 22.9 gms (108 per cent of the stoichiometric quantity) of Analar sodium bicarbonate powder were added to 3 litres of this stock solution, and 5 minutes later, after its temperature 10 had been raised to _ - . ° C _, the washed, acid-tr.eated alginate fibre was immersed in the solution and gently agitated. After 10 minutes all solid bicarbonate had disappeared. Agitation was continued for a further 30 minutes, and then the fibres were removed and centrifuged as 15 before. Analysis of the remaining liquor revealed that about 10 per cent of the bicarbonate was unused in the reaction. The fibres were dried in a current of air at a temperature of 37°C. They were not stuck together and 20 carded well to produce attractive webs of 100 gms/square metre density and of a good white colour. The pH of the fibre was measured as 5.0 and analysis gave carboxyl occupancy as 49.3 per cent calcium, 47.7 per cent sodium and 3.0 per cent hydrogen. The fibres contained no sodium 25 bicarbonate. Example 2 The procedure of Example 1 was repeated with the difference that the bicarbonate treatment bath contained 14.85 gms sodium bicarbonate (70 per cent of the 30 stoichiometric quantity) . OMPI Analysis of the remaining liquor after treatment revealed that only 0.9 per cent of the bicarbonate was unused in the reaction. The fibres were similar in appearance and process- ability to those produced by the process of Example 1. Their pH was measured as 4.0 and analysis gave carboxyl occupancy as 49.3 per cent calcium, 35.7 per cent sodium and 15.0 per cent hydrogen. The fibres contained no sodium bicarbonate. • Example 3 The procedure of Example 1 was repeated with the difference that the bicarbonate treatment bath contained 27.6 gms sodium bicarbonate (130 per cent of the stoichiometric quantify) . Analysis of the remaining liquor after treatment revealed that 24.0 per cent of the bicarbonate was unused in the reaction. The fibres were similar in .appearance and -process- ability to those produced in the process of Example 1. Their pH was measured as 6.5 and analysis gave carboxyl occupancy as 49.3 per cent calcium, 50.5 per cent sodium and 0.2 per cent hydrogen.

Claims: CLAIMS 1. A process for increasing the solubility of alginic material comprising reacting a calcium alginate material with an acid to substitute hydrogen ions for a proportion of the calcium ions attached to the carboxyl groups, treating the resulting alginic material with a base solution, and subsequently drying the treated alginic material, characterised in that said treatment is effected using a solution of an alkali metal or ammonium bicarbonate _ to substitute -alkali—metal or ammonium ions for a proportion of the hydrogen ions to produce an alginic material having a pH in the range 3.5 to 7 after drying. 2. A process as claimed in claim 1, characterised in that the pH of the dried alginic material is below 6. 3. A process as claimed in claim 1. characterised in that the treated alginic material is dried by a heat treatment which is carried out at least initially at a temperature of below 50 σ C, preferably 35°C. to 40°C. 4. A process as claimed in claim 1, characterised in that the calcium alginate starting material is in a substantially unswollen state. 5. A process as claimed in claim 1, characterised in that the alkali metal or ammonium bicarbonate is in solution in a solvent which does not substantially swell the treated alginic material. 6. A process as claimed in claim 1, characterised in that the treated alginic material is not washed prior to being dried. 7. A process as claimed in claim 1. characterised in that the calcium alginate material is bleached at the same time as it is reacted with the acid. 8. A process as claimed in claim 1, characterised in that after the acid treatment, the resulting alginic material is washed substantially free of acid anion and any bleaching agent before being treated with the bicarbonate solution. 9. A process as claimed in claim 1, characterised in that the calcium alginate material is in the form of fibres. _ __ 10. A_ proce_s,s._as claimed in claim..9., characterised in that a fibre finish material is applied to the fibres by incorporating it in the bicarbonate solution. 11. Alginic material, characterised by having been treated by a process as claimed in any of claims 1 to 10. 12. Alginic material as claimed in claim 11. characterised by comprising a fibrous dressing.

Inventors: ROSE LESLIE, ROSE, LESLIE
Applicants: COURTAULDS PLC
Application year: 1984
Main classification: C08B37
IPC codes: A61L15