US2030566A - Production of films - Google Patents

Description

Eek. H, 193 c. w. BONNIKSEN I PRODUCTION OF FILMS Filed Dec. ll, 1933 ALGINIC ACIQ WATER 11- NG-L 00 ALewATE 'SOLUTIO N HOPPER %1 8 WATER 13 0 DRYING CHAMBER TANNlNG Patented Feb. 11, 1936 UNITED STATES PRODUCTION OF FILMS Cyril Wilfred Bonniksen, Leamington Spa, England Application December 11, 1933, Serial No. 701,924

In Great Britain December 15, 1932 Claims.

This invention relates to a process for preparing flexible films and the like.

It is an object of this invention to provide an' inexpensive process of manufacture of flexible 5 films and the like which leads to the production of films similar to the well-known gelatine and cellulosic films commonly used for wrapping and decorative purposes and offering an alternative also to spun cellulosic filaments or threads as a 1 silk substitute.

I am aware that the manufacture of transparent soluble and. insoluble films has been suggested starting from alginic acid or its salts.

Alginic acid is a chemical substance which has some points of similarity with starch. It is insoluble in water, though prolonged steeping in water causes it to swell in a way which in some respects recalls gelatine. The substance is an acid and forms salts with the metals (and ammonia). Some of these salts are soluble and some insoluble in water, some are coloured and some colourless. The ammonium and magnesium and all the alkali salts are soluble in water to viscous solutions; these solutions are transparent and clear. Almost all the remaining salts are insoluble in water,

Although it is fairly easy to produce alginate threads or films which are smooth and. up to a point transparent yet the production of such films with a degree of transparency comparable with that commonly attained in the manufacture of viscose or cellulose ester films or threads is attended with many difficulties.

One difliculty in spinning or producing transparent and. clear films from alginic acid or its salts lies in the fact that these substances and especially their solutions offer a ready home for the growth of bacteria and degenerate so quickly that the spinning solution and the films produced therefrom either rapidly lose alginic acid or become discoloured by the bacteria and their byproducts or rendered opaque, irregular and. useless.

Another difiiculty lies in thelfact that alginate acid or its compounds can exist in more than one form, the forms difi'ering either chemically or colloidally and leading to the production of films, threads or filaments of different physical characteristics. Without expressing any final View on the exact reason for this difference it is convenient to regard it on the theory that alginic acid can undergo a transformation similar to what is known as a lactone transformation and the two resulting classes will herein be distinguished for convenience as the A form and the B form.

Broadly speaking it may be said that the A form, when spun into sheets, leads to opalescent or non-transparent products while the B form leads to clear and brilliant products if other conditions are suitable.

Based on these facts the present invention relates to the employment of certain precautions in the production of alginate films with the object of securing highly transparent, that is to say brilliant products.

These precautions are calculated to prevent bacterial action and also to prevent any change from the fB form (which the solutions initially have) to the A form. These precautions may be broadly stated as follows:

(a) Limiting the concentration of alginate in solution. Preferably not-more than 10% of alginic acid on the weight of the solution is employed, though larger amounts may be used if the other precautions are adequate.

(b) Forming into films or filaments as rapidly as possible after preparation of the solutions;

(0) Evaporating rapidly (for example to dryness in fifteen minutes);

(d) Keeping the solution acid or neutral, or at least only very weakly alkaline (for example of a pH value between 5 and 6) (6) Keeping the apparatus and solutions sterile for example by a sterilizing agent:

(1) Using ammonia along with alkali hydroxides or carbonates as the solvent.

One feature of the present invention comprises the manufacture of highly transparent alginate films consisting in producing an alkaline solution of an alginate, eliminating bacterial contamination, forming the solution into films and drying.

A further feature of the invention comprises the manufacture of transparent alginate films consisting in producingan alkaline solution of an alginate in the B form hereinbefore referred to, forming such solution into films or filaments and drying.

One of the factors which requires control and has an effect both upon the bacterial action and upon the question of whether the product exists in the A form or the B form is the nature and strength of the solution. Solution should be efiected by means of sodium or potassium hydroxides in combination together with ammonia, the resulting solution having preferably a concentration not exceeding 10% of alginate.

Very concentrated and neutral or alkaline solutions tend to revert to the A form and to produce opalescent films, although if spread into films immediately after making and quickly dried transparent films can be obtained from relatively strong solutions. Moreover, strong solutions do not keep. As time goes on films produced from them by evaporation will become more opalescent even though all the conditions of preparation remain otherwise unchanged. The more concentrated the solution the more rapidly will it change to a form producing opalescent films. An alginate solution may therefore produce transparent films if used immediately after it has been prepared, and twenty-four hours later it may be useless for the preparation of transparent films. If the strength of the solutions is kept down to about 10% of alginic material, however, and the solutions are sterilized as hereinafter described transparent films may be produced even after keeping for a month. It will be observed that the results of using a solution which has been allowed by time to change or revert to the "A" form are rather similar to results of using a solution which has been allowed by time to develop bacterial'action and it is not always easy or possible to distinguish between the results of bacterial action and the results of internal chemical or colloidal change in the material itself, but the various precautions herein set forth taken singly or together will suifice to overcome the various causes of nontransparency.

From what has been said it will be appreciated that sterilization per se may not always be necessary if the formation into films is carried out with adequate speed. Where bacterial action has occurred, however, the solution may be sterilized and/ or clarified by filtration prior to forming into The alkaline solution may be sterilized by the addition thereto of a suitable bactericide (e. g. a quantity of copper sulphate in a proportion such as to yield 1 part of copper ions by weight to yields an insoluble alginate, for example a solution of a calcium salt.

The following is a description by way of example of particular methods of carrying the invention into effect, but it is to be understood that the invention is not limited in its scope to the particular examples described:-

The accompanying drawing diagrammatically indicates the steps of the process. In the drawing l l represents a mixing bath where alginic acid, water and sodium carbonate or equivalent material are brought together and viscous alginate solution prepared. This passes to a hopper I2 which delivers a thin film through a slot in the bottom of the hopper to a travelling band 13 having a glazed surface, for example of cellulose acetate or polished metal. Steam coils l4 heat the band and warmed air is blown in at l5 over its surface so that the film dries quickly. It. is stripped at l6 under a roller l1 and passes successively through a tanning bath I8, a washing bath l9 and a drying chamber 20 to a rolling spool 2| where the product is wound up.

Source of alginic acid: The alginic acid used was extracted from wet laminaria digitata by treatment with sodium carbonate solution. After dilution and filtration the solution was bleached and sterilized by addition of sodium hypo'chlorite. Alginic acid jelly was then precipitated by adding dilute hydrochloric acid until the solution was one-sixtieth normal. The jelly was filtered off, washed with water and stored.

Example I 2700 grams of alginic acid jelly containing 12.7% alginic acid and ten days old, was stirred with 2500 cubic centimetres of water, 50 grams NazCOs and 130 cubic centimetres ammonia (tennormal). The resulting solution was spread and evaporated as before.

The products of these examples were clearand brilliant films showing no trace of opalescence' and comparable in transparency with the films obtainable from the spinning of viscose or cellulose acetate.

Further examples Solutions of alginic acid (containing 10% alginic acid) were made up with sodium hydroxide and ammonia in varying proportions; but such that the sum of sodium hydroxide and ammonia used in each case was chemically equivalent to the alginic acid content of the solution.

Proportions or sodium hydroxide and ammonia Solution No.

Sodium hydroxide Ammo Percent Percent I 15 II 30 70 III 60 40 IV 10 All these solutions gave clear films when spread within two hours of making. After 20 hours Solutions II and III still gave clear films but Solution IV gave an opalescent one and Solution I a clear film only where evaporation was very rapid. After 45 hours the opalescence produced in films made from Solutions I and IV was more pronounced. The films were cast on glass dried in about fifteen minutes and then stripped off.

To render these films insoluble they may be treated with calcium chloride solution so as to produce calcium alginate. It is important to appreciate that if a transparent film is required it must be in what has been called the B form because in some cases films in the A form appear almost transparent until they are tanned with the calcium bath, and. then turn opalescent. The film may remain objectionably opalescent when it has dried though it will be much less so than when wet. A 5% solution of calcium chloride tans a film on glass at room temperature in about ten minutes if it is about 0.035 to 0.040 millimetres thick. Stronger solutions tan more slowly but show a less tendency to make films become opalescent during tanning.

The following general remarks may prove useful as to the conditions of evaporation: It is reasonable to suppose that if evaporation is very slow a stage will be passed through (and maintained for a sufilcient time) during which the film is concentrated as just described, and thus aos sce 3 a change" to the A form may occur after the film has been formed, and clear films will not-be obtained. .It is necessary therefore in order to produce clear films by evaporation that the evaporation must be rapid "though high temperatures of evaporation tend to produce yellowv or discoloured films. .Thus in some cases dryingv under vacuum or a warm air blast may. be'.

requisite.

As to. the hydrogen ion concentration of the solution it is found that this must not be too high. If caustic soda. is used to dissolve the alginic acid a pH value of between and 6 should be observed, and though a slightly acid solution may yield transparent results if made into films quickly enough, say in half an hour, yet after two hours it may be useless for this purpose. The solution may, however, be made slightly alkaline with a volatile weak alkali such as ammonia and still be useful for the preparation of clear films.

Coming to the constitution of the solvent solutions, sodium carbonate or hydroxide used alone tend to give opalescent films of sodium alginate. This is presumably due to the sodium alginate remaining in the A" form during evaporation. Sodium hydroxide is preferred to sodium oarbonate in the preparation of solutions of sodium alginate to be used for producing transparent films but great care must be taken to avoid any excess of alkali and the solutions must be used almost immediately. Ammonia if used alone, will not easily give transparent films, and the type of non-transparency produced is different from the opalescence produced by an excess of sodium alkali. A mixture of sodium hydroxide or carbonate and ammonia gives very good results, as the ammonia evaporates quickly and leaves the film free from alkalinity and the presence of sodium alginate in sufiioient proportion seems to obviate the secondary effect of the ammonia referred to. The solution keeps better than plain sodium hydroxide solutions.

Clear films are stronger, especially when wet, than opalescent ones.

From what has been said on bacterial action it will be clear that if sterilization is only effected to a degree which merely retards the bacterial action but does not entirely prevent it, or if no sterilization is effected or if the solutions used are above a low degree of concentration the operations from clear solution to the dried product must proceed at an adequate rate in order to ensure that the product is not cloudy.

To ensure that no alginate remains in solution for more than this time entails far more, however, than attention to the time of operation. Absolute cleanliness must be adhered to. All tanks, pipe lines, filters in the circuit must be kept down to a minimum in size and must be cleared out daily. Provision must be made for rapid cleaning as Well as rapid rate of Working, both features peculiar to the use of algin for this purpose.

Alternatively methods have been discovered whereby alginate solutions may be sterilized and maintained sterile; in which case permanent clarity of solutions results.

(1) The initial solution may be sterilized by raising to 100 C. may be piped into sterile tanks through sterile pipes. The tanks are supplied with sterilized air and so on.

(2) The initial solution may have incorporated with it a suitable germicide. Not all well known germicides are suitable. A preferred strength of germicide isil part in ten thousand of copper ions in the solution. Copper sulphate may be the copper salt employed. v

I claim:-

.1. A process for producing from alginic material films freefrom opacity, consisting in preparing an aqueoussolution of an alkali-radicle alginate which .solution is substantially both sterile and clear and whose concentration (expressed in terms of alginiclacid) does not exceed of the weight of the solution, forming the said solution into a film before it has materially changed from the state in which it existed upon first formation and then drying the film in not more than fifteen minutes.

2. A process for producing from alginic material films free from opacity, consisting in preparing an aqueous solution of an alkaliradicle alginate which solution is substantially both sterile and clear and whose concentration (expressed in terms of alginic acid) does not exceed 10% of the weight of the solution, forming the said solution into a film before it has materially changed from the state in which it existed upon first formation and then drying the film in not more than fifteen minutes.

3. A process for producing from alginic material self-supporting films free from opacity consisting in preparing an aqueous solution of a mixture of alkali-metal and ammonium alginates which solution is substantially both sterile and clear and wherein the proportions (stated as equivalents) are such that ammonium constitutes not more than 85% of the total alkali-radicle present and the overall concentration of the mixed alginates (expressed in terms of alginic acid) does not exceed 10% of the weight of the solution, forming the said solution into a film before it has materially changed from the state in which it existed upon first formation and then drying the film in not more than fifteen minutes.

4. A process for the production of self -supporting films from alginic material by preparing an aqueous solution of an alginate, forming the solution into a film and drying the product, characterized by sterilizing the solution so that bacterial action during the process is obviated.

5. A process for the production of films from alginic material consisting in preparing an aqueous solution of a mixture of alkali-metal and ammonium alginates, the proportions (stated as equivalents) being such that ammonium constitutes not more than 85% of the total alkaliradicle present, sterilizing the solution so that bacterial action during the process is obviated, forming the sterile solution into a film and then drying the product.

6. A process for the production of films from alginic material as claimed in claim 4 wherein the solution is sterilized by the addition thereto of copper sulphate.

'7. A process for the production of films from alginic material as claimed in claim 5 wherein the solution is sterilized by the addition thereto of copper sulphate.

8. A process for the production of films from alginic material consisting in preparing an aqueous solution of a mixture of alkali-metal and ammonium alginates the proportions (stated as equivalents) being such that ammonium constitutes not more than 85% of the total alkaliradicle present and the overall concentration of the mixed alginates (expressed in terms of a1- ginic acid) not exceeding 10% of the weight of the solution, sterilizing the solution so that bacterial action during the process is obviated, forming the sterile solution into a film and finally drying the product.

9. A process for the production of films free from opacity as claimed in claim 1 wherein the pH value of the solution is maintained during the process between pHs and pHs.

10. A process for producing from alginic material continuous lengths of solidified material of small thickness free from opacity, consisting in preparing an aqueous solution of an alkali-radicle alginate which solution is substantially both sterile and clear and whose concentration (expressed in terms of alginic acid) does not exceed 10% of the weight of the solution, forming the said solution into a continuous length before it has materially changed from the state in which it existed upon first formation and then drying the length in not more than fifteen minutes.

CYRIL WILFRED BONNIKSEN.

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