US2429643A - Art of esterification - Google Patents

Description

Patented Oct. 28, 1947 UNITED STATES PATENT OFFICE ART OF ESTERIFICATION William Beach Pratt, Boston, Mass. Annette Harris Pratt, administratrix of said William Beach Pratt, deceased, assignor, by mesne assignments, to Joseph G. Denny, Jr.

N Drawing. Application August 11, 1939, Serial N 0. 289,712,

7 Claims.

hydrate, such as dehydrated cellulose, with a substantially anhydrous alcohol, such as methanol, containing an esterification catalyst, such as a perchloric acid hydrate, forming a relatively fixed constant boiling acid soluble in methanol, and which is convertible by dehydration into a volatile oxide which is more effective as an esterification catalyst than the mineral acid itself.

The impregnation of the carbohydrate with alcohol places it on the same phase of reaction;

as a suitable substantially anhydrous dehydrating and esterifying reagent, such as acetic anhydride. The anhydride reacts with the hydrous catalyst to form a volatile and more effective catalyst,

such as chlorine heptoxide; reacts with the alco-,, hol to form water and a diluent of anhydridej such as methyl acetate, and reacts rapidly but uniformly with the carbohydrate to form an ester, such as acetyl cellulose. The catalyst thus formed in situ in the carbohydrate is unstable in the; presence of oxidizable material, such as cellulose,

and-is stabilized during esterification of the carbohydrate and thereafter evaporated from the ester, which consequently requires little or no washing to prevent degeneration. The stabilization of the unstable catalyst is preferably effected by maintaining oxygen or air in contact therewith.

Acetyl cellulose may be produced from natural fibres of normal cellulose, such as cotton or bast, fibres, by my method and retains the toughness,'

flexibility, elasticity, strength and uniformity, as well as the physical form, configuration and convolutions of the natural fibres. By normal cellulose, I mean cellulose which has not been degenerated by hydrolysis or oxidation or regenerated from solution. While hydrocellulose, oxycellulose and regenerated cellulose are more readily esterified than normal cellulose, such celluloses and the esters thereof lack the toughness, flexibility,

elasticity and strength ofnormal cellulose and esters made therefrom by my process. Yarn suitable for weaving or knitting may be spun entirely from acetyl cellulose fibres made from normal cellulose by my method.

My method also permits the esterification of natural fibres of normal cellulose after they have been spun into yarn or woven into fabric and results in an increase in the diameter and number of turns per inch in the yarn and the translucency or transparency of such yarns or fabric may be largely controlled by the tension applied thereto during esterification. Both the yarn and fabric have as much or more toughness, fiexi-- bility, elasticity and strength as similar yarns or fabrics of'untreated normal cellulose, and possess these qualities in much greater degree than yarns or fabrics made from hydrolized, degenerated, or regenerated cellulose either before or after esterification thereof.

My method of esterification avoids the localized reactions resulting in an irregular rate of esterification, accompanied by high temperatures and disintegration or possible degeneration of the cellulose, characteristic of esterification as heretofore practiced by reaction between cellulose material and esterifying acids in the presence of an esterification catalyst and unevenly distributed water. My invention obviates the need for the exacting thermal control ordinarily required in carrying on esterification.

My invention facilitates and unifies the esterification without the degeneration or degradation of the cellulose molecules before esterification, or the solvation or disintegration of the cellulose cells or fibres during or after esterification, and which result in loss of toughness, flexibility, elasticity, strength and uniformity in the resulting product as compared with the characteristics of natural fibres composed'of normal cellulose.

My improvements moreover eliminate the need for long after treatments of acetylated products for their hydrolization, purification, and removal of the esterification catalyst or reaction products thereof.

My improvements further provide esterified cellulose in the form of textiles or yarn composed entirely of esterified cellulose staple, which has been heretofore unknown, or in the form of a cellulose esterification.

3 liquid or plastic, which may be used without dissolution thereof in expensive solvents.

My improvements further permit the formation of yarns or textiles of various and contrasting colors or shades by dyeing or printing the cellulose fibres, yarns or fabrics with ordinary cotton dyes (such as those which are set or developed by acids), prior to the esterification thereof, thereby eliminating or minimizing the need of the special and expensivedyes ordinarily required for dyeing cellulose acetate.

I have discovered that localized and irregular reactions during esterification are due primarily to irregular distribution of the water occluded in, or physically or chemically combined with-the cellulose or the esterifying bath. While the presence of water is essential ,to the esterification of cellulose, I have found that its .action can be. effectively controlled only by the formation thereof by reaction in situ and that all the substances involved in the esterification-should be initially so anhydrous that their aggregate water content is initially insufficient to initiate esterification of the cellulose or local reaction.

The practice of my method therefore requires the exclusion of water to the utmost degree possible from the esterifying substances and the uniform diffusion of any minute quantity of such extraneous'water that may remain; and the generation,.by reaction, of Water in situ in the molecular structure of the cellulose sufficient to activate Such water of reaction is generatedbythe esterification of a water soluble alcohol which more readily esterifies than cellulose; such preliminary esterification being effected immediately prior to orconcurrently with the .esterification. of the cellulose and in intimate contact with the molecular structure thereof.

fI'heremoval of all free Water, including physically or chemically combined water, from the cellulose and the complete impregnation of its molecular structurewith alcohol places the cellulose upon. the same phase. of reaction as the waterfree esterifying bath. By thuseliminating any interfacial tension between the constituents of the esterifying bath and'the products to be esterified, the latter are completely, uniformly and substantiallyinstantaneously penetrated and impregnatedby the'former so that the esterifying reaction takes place uniformly throughout the '50 molecular structure.

I have further found that the mineral acids, such as. sulphuric acid and phosphoric acid, ordinarily used in practice as esterification catalysts, so combine with cellulose-that their removal entails long and difficult washing operations which result in an after-degeneration of the cellulose, and should any such acids remain in the fabric ityresults-in an ultimate weakening of the fibre or-ageing. Esterification is greatly accelerated by the use of chlorine heptoxide (C1207) as the esterification catalyst. Such chlorine .heptoxide hasno injuriousafter effects since it readily and completely volatilizes'fromthe cellulose after the esterification thereof is completed. The chlorine heptoxide is preferably formed in situ in the'cellulose and its tendency to instability may beovercome by maintaining oxygenin contact therewith during esterification. The formation of thechlorine heptoxide in situ is preferably effected by 70 the reaction of an esterifying anhydride or of phosphorous pentoxide on a perchloric acid hydrate containing '70 to 12% .of H0104 and two molecules of water, and which is capable of holdgerous and harmful.

"weight ofhygroscopic -72% perchloric acid.

There is no apparent reaction of the perchloric acid on the methanol or on the cellulose in this treatment even at the boiling point of methanol.

The-methanol uniformly disperses the catalyst throughout the cellulose fibres and removes or unifies the distribution of the last traces of water moisture from thecellulose, but does not remove the water of hydration from the perchloric acid.

The saturation point of the cellulose fibres is about 62% 'of their weight of methanol and about l'.5 of their weight of perchloric acid, which is not selectively absorbed from the methanol. Such a high methanol content is unnecessary and generally undesirable for esterification, since the amount of methanol carrying the catalyst left in the fibres should only be enough to form by reaction with anhydride sufficient water for esterification of the cellulose. Consequently the saturated cellulose, after being passed through squeeze-rolls,-is dried until the methanol content is reduced by evaporation to *between 10% and 30%, and preferably about 20% of the weight of the cellulose. The perchloric acid content is un- "affected by the drying and remains about 1.4% to 1.5% of the weight of the cellulose. The cellulose so dried is immersed in or covered by an esterifying bath, such as acetic anhydride alone or mixed with a diluent,such as anhydrous methyl or ethyl acetate, which is a non-solvent for cellulose acetate.

The penetration of the acetic anhydride is immediate and uniform throughout the molecular structure of'thecellulose and the reaction takes place progressively to form a mono-, di-, or triacetate according to the proportion of anhydride present to the cellulose and methanol.

The perchloric'acid is dehydrated by the acetic anhydride to form chlorine heptoxide (C1207) and. water; the anhydride entering this reaction only so far as to pick up the water from the perchloric acid and form acetic acid.

The amount of water released by the dehydration of the perchloric acid is negligible, but the reaction between the anhydride and the methanol in the fibres, in the presence of the catalyst, forms methyl acetate and water in situ in the fibres. This water converts a portion of the acetic anhydride to acetic acid and activates the acetylation of the cellulose in the presence of the chlorine heptoxide (anhydride of perchloric acid) which acts as the catalyst to the esterification of the cellulose. Normally chlorine heptoxide would break down in the presence of cellulose and give off chlorine dioxide (C102) and unknown oxychlorine compounds, which are dan- I prevent this breakdown of the chlorine heptoxidein the acetylating solution or in the cellulose by maintaining air containing oxygen in contact therewith.

The requisite contact of oxygen with the catalyst during esteriiication may be maintained by working in the open air, or blowing air onto the cellulose fibres where only the necessary amount ing 1.5 more molecules of water. Such perchloric ofacetic anhydride is used. Where a diluent is added to the anhydride and the cellulose is immersed in the bath during esterification, air should be blown through or occluded in the solution, as for instance by agitation thereof in closed containers. The maintenance of contact of oxygen with the catalyst during esterification and the subsequent evaporation of the catalyst is essential to securing the highest values in the esterified cellulose.

The ultimate volatilization of the chlorine heptoxide from the cellulose acetate leaves the latter free from any injurious mineral acids or residues such as are left when other inorganic acids are used as the catalyst. This renders possible the complete removal of any reagents left in the acetylated fibres by a wash in cold water for about five minutes.

While the best and most uniform results are obtained by the use as an esterification catalyst of chlorine heptoxide formed and stabilized in situ from perchloric acid diffused in the cellulose by the methanol, it will nevertheless be understood that the catalyst may be introduced directly into the acetic anhydride, and that small quantities of mineral acids, such as sulphuric, perchloric or phosphoric acids may be used as catalysts to cellulose esterification, in conjunction with novel steps herein described, if long washing, hydrolization, and less perfect products are tolerable.

When sulphuric acid is used as the esterification catalyst, 1.07% of such acid by weight relative to the weight of methanol has been found generally sufiicient, but additional amounts up to say 3 may be used without showing excessive ill effects when the methanol content of the cellulose is of the order of 25% by weight of the cellulose and the esters are long and thoroughly washed.

In lieu of methanol, other anhydrous but water soluble alcohols, which react with esterifying acids to form water and in which cellulose and cellulose esters are insoluble, may be used, as, for instance, other mono-hydric alcohols, such as dehydrated ethyl or butyl alcohol, di-hydric alcohol, such as glycol, and tri-hydric alcohol, such as glycerol. The hygroscopic nature of ethyl alcohol, however, renders more difficult the control of the reactions because of the tendency of ethyl alcohol to carry water into the fibres. Butyl alcohol and the like also tend to absorb water and even when free from water are slow in initiating reaction on introduction of the cellulose into the esterifying bath, and when the reaction occurs it is violent and causes a greater rise in temperature than where methanol is used. Glycerol and glycol are also generally less satisfactory than methanol because the reaction in the esterifyin bath lacks the uniformity of effect that is produced where methanol is used.

In lieu of acetic anhydride, other esterifying anhydrides or anhydrous fatty acids may be used such, for instance, as formic acid.

My method permits the use of amounts of esterifying acids or anhydrides so close to the amounts theoretically required for reaction with the alcohol and with the cellulose that uniform mono-, di-, or tri-esters of cellulose may be directly produced by properly proportioning the amount of the esterifying anhydridev or acid to the alcohol and cellulose, thereby avoiding the need of hydrolizing or saponifying the primary esters to secondary esters.

The factors governing the percentages of acetic anhydride necessary for the formation of a desired primary or secondary cellulose acetate, the reactions involved, and controls desirable in commercial practice are illustrated by the following table applicable to the introduction into acetic anhydride of pounds of cellulose fibres containing 30% of methanol and catalyst as hereinbefore set forth:

Pounds of water formed 8.46 Pounds of methyl acetate formed 69.3 Pounds of 100% acetic anhydride hydrolyzed by the water formed 48.0 Pounds of acetic acid formed from the hydrolysis of the acetic anhydride 56.4

To form cellulose triacetate under such conditions there should theoretically be initially present 293 pounds of acetic anhydride; or to form cellulose diacetate there should be initially present 227.3 pounds of acetic anhydride; or to form cellulose monoacetate there should be initially present 161.6 pounds of acetic anhydride. In commercial practice it is desirable to use approximately ten per cent in excess of the amounts of acetic anhydride theoretically indicated.

The uniform saturation of fibres with such small amounts of anhydride is diificult, hence it is generally desirable to add to the anhydride a suflicient quantity of diluent to facilitate and unify reaction. When the cellulose is simply passed through the esterification bath and the reaction completed during the movement of the cellulose through the air, only about one half as much diluent is required as when the cellulose is completely esterified in the bath.

The diluent used may be varied to meet variations in the form or condition of the cellulose or variations in the impregnating alcohol and in the esterification catalyst, variations in the equipment used, or variations in the product desired. The most advantageous diluent for the practice of my method under any given set of conditions is readily determinable by empirical tests, but they should be water insoluble and free from water, inert with respect to the impregnating alcohol, the esterifying acid and to the esterification catalyst, and when fibre structure is to be maintained the diluent should be inert to the cellulose ester formed. When the cellulose fibres to be treated have not been degummed it is desirable that the diluent, as well as the impregnating alcohol, be a solvent for the natural gums and waxes of the fibre.

Illustrative examples of suitable diluents are methyl acetate, toluol, benzol or xylol. The esterifying bath may also include or contain heavy mineral oils and coal tar derivatives which have no catalytic action but vary the physical characteristics of the fibres and preserve or accentuate desirable characteristics in the esterified fibres.

When it is desirable to form a cellulose ester solution rather than maintain the identity of the fibres, the diluent may comprise or contain a solvent for the particular cellulose ester being formed, as for instance, acetic acid may be used as the diluent when a solution of cellulose acetate is to be formed.

The surface only of a cellulose fabric may be dissolved as an incident of its conversion into cellulose acetate so as to provide a binder securing pile threads therein or other fabrics thereto.

When using perchloric acid, to the ultimate formation of chlorine heptoxide, as an esterification catalyst, methyl acetate (99.5% pure) was found to be the most satisfactory diluent for the acetic anhydride and to produce the most satisfactory results when solvation of the cellulose acetate was not desired. This diluent may be economically recovered due to its low boiling point and solubility to a degree in water.

Methyl acetate is not, however, a desirable dilucut for acetic anhydride where sulphuric acid is used as an esterification catalyst, and in such case toluol may be successfully employed as a diluent.

In the practice of my invention for the continuous production of cellulose acetate fabric, a cotton fabric, which has been degummed by a usual kier boil and bleached to produce standard or normal cellulose of great purity substantially free from degradation products, has its free and combined moisture evaporated therefrom by passage through a suitably ventilated section of a tcnter drier having a temperature sufficient to extract all the moisture but insufficient to cause any appreciable oxidation or decomposition of the cellulose. When the moisture content of the fabric has been reduced below 1%, the dehydrated fabric passes from the drier section through a bath consisting of anhydrous methanol and perchloric acid and is thoroughly impregnated therewith. The methanol and acid content of the soaked fabric may be reduced by passage of the fabric between squeeze rolls, and the methanol content of the fabric is further reduced substantially below itssaturation point by passing the fabric through a further drier section until the methanol content has been evaporated down to between and 30% and preferably 20% of the weight of the goods and the acid content is approximately 1 /2% of the weight of the goods. The evaporated methanol may be recovered from the dryer and condensed.

The fabric is cooled by evaporation of methanol therefrom and is fed through a solution containing 100 parts of acetic anhydride and 50 parts of methyl acetate. It is then passed, under tension, over rollers, in contact with a stream of air. The acetylation is completed in air and the chlorine heptoxide evaporated from the cellulose acetate in about ten minutes. The temperature of the bath does not require external thermal control as the rise in temperature is insufficient to degenerate the cellulose, and when the bath contains diluent equal to several times the weight of anhydride the temperature rise of the bath is negligible. The fabric is then fed into a washer and washed in water at a temperature not exceeding the estcrification temperature. Ordinarily any residual reagents may be completely washed out within ten minutes with water at a temperature of 60 to 80 F. The product is then in suitable condition for finishing as. a textile fabric.

Having described my invention, I claim:

I 1 In the art of acetylating cellulosic material, the steps which include dehydrating the cellulosic material below its normal water of condition; impregnating the cellulosic material while so dehydrated with a liquid impregnator comprising amixture of anhydrous methanol and a stable perchloric acid hydrate having a constant boiling point well above the boiling point of methanol; subjecting the cellulosic material, while impregmatedwith between 10 and 30% of methanol on the weight of cellulosic material and with said perchloric acid hydrate to the action of an acetylating bath containing acetic anhydride more than sufficient to both remove the water of hydration from the perchloric acid hydrate and to molecularly combine with the methanol present;

the aggregate of free water in the cellulosic material, in the impregnator, and in the bath being insufiicient when they are initially brought together to initiate appreciable acetylation of the cellulosic material by the anhydride in the absence of the methanol; and continuing such action for a time and at a reacting temperature, effecting relatively rapid reaction between the anhydride and cellulosic material without appreciable degradation of the latter.

2. In the art of acetylation, the steps which includesubjecting normal cellulose substantially freefrom degradation products to a dehydrating temperature insufficient to cause appreciable oxi" dation or degradation of the cellulose; impregnating the fabric when its moisture content is reduced below 1% with an impregnator which is substantially inert relatively thereto and comprising a liquid mixture of anhydrous methanol and perchloric acid hydrate; volatilizing from the fabric a portion of the methanol therein at a temperature below the boiling point of the perchloric acid hydrate until the methanol content is between 10% and 30% of the weight of the goods; subjecting the so impregnated cellulose to the action of an acetylating bath comprising acetic anhydride and methyl acetate for less than an hour at an acetylating temperature without appreciable degradation of the cellulose; andremoving any residual reagents.

3. In the art of acetylating cellulose, the steps which include uniformly dispersing in the cellulose perchloric acid by impregnating such cellulose with a perchloric acid hydrate dissolved in solvent therefor, and subjecting the impregnated cellulose to the action of an acetylating bath containing acetic anhydride in excess of that required to remove the water of hydration from the perchloric acid hydrate and form chlorine heptoxide and in contact with sufficient oxygen to maintain the stability of the chlorine heptoxide so formed.

4. The method of acetylation which comprises impregnating substantially anhydrous cellulose with substantially anhydrous methanol containing perchloric acid, treating the impregnated cellulose with sufficient acetic anhydride to form chlorine heptoxide by reaction with the acid, to form water by reaction with the methanol. and to form acetyls by reaction with the cellulose at reacting temperatures, stabilizing the chlorine heptoxide by oxygen during acetylation, and permitting the chlorine heptoxide to volatilize from the esters.

5. A process for the manufacture of cellulose esters which comprises conducting acetylation of the cellulose in contact with chlorine heptoxide and oxygen sufficient to stabilize the chlorine heptoxide.

6. In the art of acetylating cellulose, the steps which include uniformly dispersing in the cellulose a perchloric acid hydrate having a substantially constant boiling point dissolved in solvent therefor, converting said hydrate while dispersed in the cellulose into chlorine heptoxide by the removal of water of hydration therefrom by the action of acetic anhydride thereon, acetylating the cellulose by the action of acetic anhydride thereon during the dehydration of said hydrate, and stabilizing said chlorine heptoxide during acetylation of the cellulose by supplying oxygen thereto.

7. A step in a process for the manufacture of cellulose esters which comprises conducting acetylatlon of cellulose in contact with chlorine heptoxicle and oxygen free to gombine therewith.

WILLIAM BEACH PRATT. REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,236,578 Lindsay Aug. 14, 1917 2,008,021 Kenety July 16, 1935 2,064,384 Richter Dec. 15, 1936 2,087,036 Malm July 13, 1937 2,045,161 Muller June 23, 1936 Number Number