US2028296A - Method of removing water of gelation - Google Patents

Description

Patented Jan. 21, 1936 Milton J. Shoemaker, Madison, Wis alsignor, by mesne assignments, to Carbide and Carbon Chemicals Corporatio poration of New York Application November 24, 1931. serial N0. 577,159

N Drawing.

TCIaims.

This invention relates to a metflod for removing water rapidly and efliciently by a freezing process from solid cellulosic products which have been coagulated, regenerated or otherwise de- I posited from aqueous solutionsand especially from such products as freshly formed sheets of regenerated cellulose (as from viscose) and of alphyl hydroxy-ethers of cellulose such as ethyl hydroxy-ethers which have a limited solubility in caustic alkalies and to the product resulting therefrom. Some of the objects of this invention are to cheapen the cost of drying films, threads or the like, of celluloslc products which have been formed from water or water solutions, to cut.

the cost of the machinery necessary for drying, to simplify the drying operation by decreasing the shrinkage on drying, and to increase the strength of the materi 4 At the present time films of regenerated cellulose are 'made by coagulating a film of viscose in an acidified bath after which the film is led through a number of treating and washing baths.

a when the film is a continuous sheet, it finally 88 over a series of heated drums to dry the product to the desired moisture content. This latter operation must becarried out carefully on equipment whichordinarily is expensive, to produce a smooth, highly transparent, and lus- '80 trous article, such as is demanded by the trade.

The evaporating costs are high 'since the wet sheet enters the drying machine with a water content of 75% to 90% (if no emollient is used) and this water must be practically entirely re- 85 moved. If sheets of .certain derivatives of cellulose, such as the hydroxy-ethers as previously described, are made; approximately the same conditions obtain. During the evaporation of this large amount of water the sheet, if allowed to 40 contract freely, is subject to excessive shrinkage,

the average lineal shrinka e being about 30 to 35 per cent. This complicates the drying prob? lems. However, when a sheet is dried on continuous machines it is kept under tension so that there is little or no lineal shrinkage but there is some cross shrinkage. As a result the sheet is stretched, usually'more or-less unevenly, and there is considerable variation in the thickness and in the strength of the sheet both across and 60. in the machine direction. By my invention it is possible to eliminate over per cent of the evaporation cost and machinery for making a continuous sheet and simultaneously, cut the tendencytoshrinkduiingdryingtoaminimum.-

This lessened tendency to greatly dcn, New York, N. Y., a 1'? creases thestretchinthedriedsheetandmakes for more uniformity in thickness and in the strengthof the shectbothacross andinthemachine direction. In addition it is not necessary to handle-on the'driers, the very tender film 5 having a high water content. 'When .the before mentioned cellulosic products are coagulated from a'water solution, and after being further treated and/or washed to remove impurities in the form of other products of re- 10 action and coagulating acids and salts, the coag- I ulated material contains from 75% to 90% of water. This water may be described as water of gelation. All of this water of gelation, however, apparently is not held in the same way by the 15 coagulated cellulosic material. If about to of the water present in this coagulated material (containing about to of water) is removed. water will not again reenter appreciabLv the partially dehydrated film (con- 2 taining 60% to 70% of water) even on prolongedsoakinginwater. Thebalanceofthe water may be removed by drying but the dried material will again take up a like amount of water (60% to 70% of water) if soaked in 25 water. The first dehydration or de-gelation, therefore, is an irreversible phenomenomand I have called the water which is removed in this step non-reentrant water of gelation since such water cannot again enter the material after being 30 removed therefrom.

I have found that it is possible to remove'the non-reentrant water of gelation by a freezing operation. If the freshly coagulated and washed material, for example, in the form of a film, is 35 frozen and then thawed, water drains from the surface thereof, About 55% to 60% of the water content of the film is removed by this procedure. The lineal shrinkage during this operation is about 6% for the hydroxy-ether. The presence 40 ofan emollient such as slycerine reducesthis shrinkage. The-balance of the drying may be accomplished in'the usual way, the additional shrinkage beinxabout 8%. After thefilm is .frozen and thawed, and the non-reentrant water 5 removed asby draining, the-strength of the film is increased considerably over that of the initially coaculated film and its thickness is reduced. An emollient such as glycerine, is preferably introduced into the film after the non -re- 50 ventrant water has been removed. The film is run through the necessary bath of emollient in the usual manner. The freezing operation may be carried out by placing the film against a cold surface or sub- 55 of ordinary. salt (sodium If thefrolennlm-remains 2 iecting it to cold air or to any suitableeold liquid. The film, if it is to be frozen in the-air or any other gaseous fluid. is subjected to such g'as cooled to a temperature below the freezing point of, waters The rate of freezing depends upon the temperature and circulation of the air. If the gas circulates rapidly it freezes the film more rapidly, but it also causes cloudiness and for most this is objectionable.

An excellent freezing method is that inwhich the coagulated cellulosic material is immersed in a liquid bath which is held at a temp rature below the freezing ,point of water. Obviously the bath should be of such a materialthat it does not affect the dim chemically. Mercury is especially suitable. Although various organic liquids such as toluene, gasoline, alcohols, carbon tetrachloride, be

sufficient quantity which permit the solution to lee-cooled considerably below the freezing point ofwatermaybeusedtoadtagesincethefllms frozen therein remain clear and bright. solutions chloride) or calcium chloride are suitable. The bath is cooled to a tem-' perature from 5 to 15 degrees centigrade belowi the freezing point of water, and the eoagulated and washed film immersed therein. The ordinary fllm freezes quickly, ice forming a coating over the, surface and extending through it in 10 to 20 seconds. The film may now be removed from the cold-bath to awarm'er place, to allow the ice to melt, or the film may be allowed to remain in the bath for -a longer period of time. in a refriaeratlnz bath of a solution-of a salt inwater for a period of timethe melts and the water resulting therefrom gradually diffuses into the refrls rating solution sothat'thefllmthenmaybera moved with the ice completely liquefied. After be carried out in definitive action of liquids 'coagulatc from takes place by conversion of that portion, from water of g'elation to frozen non-reentrant water. The removal of the frozen non-reentrant water is not necessarily practiced by melting, and may numerous ways. Removal has been described by melting it, and also by dissolving it, as it were, by the diffusive or other thereon which are effective below the melting point of the ice, such as a salt solution in which the ice may be formed.

Other known methods of removing ice may be applied, such as evaporation of the ice without melting it.

It is also to-be understood that the invention may be practiced on materials from which part of'the waterhas already been dried, but in which a portion of the non-reentrant water yet remains. In commercial practice of the invention, the methods employed for storing, conveyins, or exposins the material after coagulafreezins may be intentionally or inadvertently conducive to dryina, and may effect partial drying. Such practice does not in any way mitigate the advantagesof the present invention nor depart therefrom.

While cellulose regenerated from viscose, and the hydroxy ethers of cellulose have been speciiically described, it will be understood that the functional property 'of hydration upon coasulation from water is common to cellulose and its derivatives, including the hydroxy ethers,

v conteniplatesallsuch materials which are charaeteriaed by this pr p rty.

It will be understood that numerous chang in and departures from the processes particularlydiscloeedhereinmaybemadewithoutdepartingfromthe spirltand tionasinthe appended claims.

Iclaim;

1. 'lhe'methodofsubstantially decreasingthe watercontentofacellulosicfilmwhichhssbeen aqueous solution-and which contains non-reentrant water of gelation,

prisuimmersinssaidiilniinanaqueousresolution of a salt which is at a perature below the freezing point of water,

non reentrant water of 'freeses'and into-said refrigeratins solufrom'sfld liqui scope of the inven been coagulated from aqueuos solution and which contains non-reentrant water of gelation, which comprises freezing said coagulation product in a solution, and removing the ice formed by said freezing process by the results of diflusion'between the solution and the product containing the ice crystals at a temperature which is below the freezingpoint of said ice.

4. As a new article of manufacture, a. watermoist film of a hydrated cellulosic product from which non-reentrant water of gelation has been removed, said film being characterized by uniform tensile strength in right-angular directions in the film.

5. As a new article of manufacture, a watermoist film of a hydrated cellulosic product from Y which non-reentrant water of gelation has been removed, said fllm being characterized by uniform shrinkage and tensile strength in rightangular directions in the film.

6. As a new article of manufacture, a watermoist film of regenerated cellulose from which non-reentrant water of gelation has been removed, said film. being characterized by the property of uniform shrinkage and tensile strength in right angular directions in the film.

'7. As a. new article of manufactures. watermolst fllm of alphyl-hydroxy ether of cellulose from which non-reentrant water of gelation has been removed, said film being characterized by the property of uniform shrinkage and tensile strength in right angular directions in the film. 15

MILTON J. SHOEMAKER.