US2348001A

US2348001A - Acetylation of cellulose

Info

Publication number: US2348001A
Application number: US416376A
Authority: US
Inventors: Donald L Gibson; William M Shoemaker
Original assignee: National Vulcanized Fibre Co
Current assignee: National Vulcanized Fibre Co
Priority date: 1941-10-24
Filing date: 1941-10-24
Publication date: 1944-05-02
Anticipated expiration: 1961-05-02

Description

May 2, l944 GIBSON ETAL 2,348,001

.AACETYLATION OF CELLULOSE Filed OCT.. 24, 1941 Patented May 2, 1944 Q1 "2,348,001, l AcETYnATIoN oF-cnLLULosE. c f i Donald L. Gibson, west chester, and william M. Shoemaker, Kennett Square, Pa., assignors to YNational vulcanized Fibre a corporation of Delaware ton, Del.,

Company, Wilming- Application October 24, 1941, Serial No. 416,376 Y. r

1-2 claims. (o1. 8 121) This invention relates to the acetylation of cellulose and particularly to the continuous acetylation of a traveling web of cellulose material, such as paper or fabricated cellulose laments to effect a modication of the characteristics of the material uniformly and progressively and thereby to produce a new material capable of uses for which the original is not adapted.

While the invention is particularly useful in the preparation of acetylated materials from paper stock, it has been found that desirable products may be produced also from fabrics produced from cellulose materials by weaving or otherwise when such fabrics are subjected to the method as hereinafter described. It has been suggested that a web of paper may be subjected to acetylation by leading it over suitable rolls through an elongated bath containing an acetylating agent and a nonsolvent diluent. Nevertheless, acetylated paper in the form of webs or sheets has not heretofore been available as a commercial product, and" industry has not had an opportunity to utilize such material for the purposes for which it is particularly useful. In theory, such material may` be prepared without difficulty, but in practice there are many problems which heretofore have baled those who sought to prepare acetylated paper as a commercial product. Consequently it has not been possible to manufacture acetylated paper on a commercial and economical basis.

Foremost amongthe problems is that .of retaining the fibrous form of the paper or fabric in the acetylated product. Acetylation of cellulose is, if the acetylating agent is effective, a reaction which proceeds rapidly, and it is -very difficult to control this reaction which, if uncontrolled, results in complete disintegration lof the cellulose. acetylation results immediately and during the treatment withv the acetylating agent in substantial loss ofl tensile strength. In the absence of suitable precautions the material loses its fibrous structure and cannot be maintained in the formV of a sheet or web. A previously suggested method lof acetylating a web of paper produces complete degeneration and not a' web of acetylated paper as intended. i

Assuming a suitable bath which is properly adjusted to effect acetylation without disintegra tion, it is necessary, since the tensile strength of the material is materially reduced, to avoid any stress which would tear the web, and thus prevent continuous travel thereof. Moreover, the acetylation of the material rapidly exhausts the acetic anhydride which is the acetylating agent, and as the material advances the acetylation formula becomes progressively less' effective until it fails. Underthis condition, the product is non-uniform and useless.

'Ihere are numerous other factors Whichmust be considered. These will be referred to in more detail hereafter.v The continuous maintenance of the essential conditions for the acetylation of/Webs of cellulose material has not been possible heretofore since some of the conditions Awere Vnot recognized and no attempt had been made to employ suitable control in respect to other details.

It is the object of the present inventionftopro- .vide a novel method of yacetylating a web of cellulose material in a continuous operation which effects a uniform treatment of the material in a minimum of time, thus affording a practical commercial operation permitting the economical preparation of new products having advantageous properties and characteristics which make them useful for various purposes.

Another object of theinvention is the provision of a method wherein a web' of cellulose material is advanced continuously and subjected to uniform acetylation without the introduction of stresses which would break the web in its weakened condition. Another object of the invention is the provision 4of 'a methodwherein the web of cellulose mate- In the case of fibrous material,

-rial .is subjected to an acetylating formula capable of rapidly and effectively modifying the material as it advances, the. operation being controlled to avoid variations in the amount of acetylation effected in successive portions of the web. l

Another object of the invention is the provision of a method in which the uniformly acetylated web is purified continuously Vand effe'ctively to aiordnew and satisfactory products which will not deteriorate in use.

-val, purifying procedure and When all of these factors are properlycorrelated of uniform concentration. The

, itate purification.

' maintain it at its maximum concentration so Another object of the invention is ther provision of a method of effectively controlling the concentration ofthe acetylating agent so -that the invention is the proby reference to the following' specification and accompanying drawing, in which Fig. 1 is a diagrammatic illustration in longitudinal section of an apparatus adapted for the practice of the invention; and

Fig. 2 is-a section on the line 2-2 of Fig. 1. The apparatus hereinafter described is not in all of its details essential to the invention. vIt is.

therefore, merely illustrative of a preferred embodiment of practical means for accomplishing the desired results, which has been used successfully in a plant operation. `Various modifications of such an apparatus may be used to accomplish the purpose of the invention, provided the essential conditions of operation are observed.

In carrying out the invention, the following variables govern the treatment and the characteristics`of the nished product: type of material treated, conditionsof pretreatment with acetic acid including temperature and time of immersion of the paper therein, strength of acetic anhydride in the acetylating bath, percentage of catalyst in the bath, maintenance of uniform composition of the bath, composition of addition agents employed to maintain uniformity in the bath, time of immersion and temperature of the acetylating bath, ratio of the total acetylating bath to the amount of paper being treated at any interdrying procedure.

in accordance with their relative importance, it is possible to secure rapid and effective modification of the characteristicsof the cellulose material Without deterioration thereof and to maintain continuous operation. n

The procedure is conducted preferably in an apparatus in which subjected to a prelixnlnary treatment or soaking in acetic acid of suitable concentrationl and there'- after-subjected to the action of acetic anhydride of suitable concentration and .at a proper temperature in the presence of a catalyst and under conditions in which both sides of the web are subjected progressively to an acetylation formula web advances continuously, without being subjected to ariy substantial stress which .would tend totear it in its weakened condition, until it is delivered to a bath of water which promptly quenches the reaction andrestores partially the tensile strength of the paper. This water bath should be constantly renewed and may contain suitable agents to facil- The web is then led through a succession of water'baths which aresimilarly freshened and thence to the dryers.

To maintain uniformity of acetylation it is necessary to circulate constantly the acetylation formula, with suitable additions to fortify and the cellulose material is first each increment of the web as it comes from the soaking bath is uniformly aected by the acetylating formula at its maximum strength, that is to say, each increment of the web as it advances receives precisely the same treatment.

In order that the operation may be morefcl'early understood, reference is made to the accompanying drawing, in which 5 indicates a rollor cellulose material to be treated. which is mountedon a spindle 8. Unrolling of the web-is retardediby a suitablel brake indicated diagrammatieally'y by' the weight 1. The web 8 is delivered to areceptable 9 which may be made of any suitable material which is not affected by the soaking bath .I8 consisting of acetic acid. Provision is made for the addition and likewise to'v maintain the bath at a predetermined temperature. The web 8 passes under rollers II and I2 and is drawn through the soaking bath by driven rollers I3 and I4 which squeeze the excess of acetic acid from the paper as itadvarices. The web is fed by the rollers I3' and I4 to the acetylation chamber comprising an inclined plane surface I5 enclosed by a suitable hood I8 having an extension I1 covering the soaking bath Ill and an extension I8 which dips into the iirst water bath I9 so that fumes from the acetic acid and acetic anhydride are prevented from escaping to the surrounding atmosphere.

The angle of the inclined plane I5 is such that the web will travel downwardly thereover under the inuence of the acetylating agent which flows continuously with the web. The web advances continuously without being subjected to any stress which is likely to tear it, a condition which cannot be maintained if an attempt is made to draw the web through a bath by the application of tension. The angle of the inclined plane I5 may be varied considerably depending upon the nature of the material, but an angle of approximately 22 from the horizontal has been found to be satisfactory with most types of webs.

The acetylating agent flows with the web into a receptacle 20 from which it is withdrawn continuously through a` pipe 2I and delivered to a storage tank 22, provided preferably with a heat-A ing bath 22'. Water may be used for heating to a constant temperature. Suitable provision is made for the addition of materials to the storage tank from a tank 23' to maintain a precise composition of the acetylating agent at a uniform temperature. .From the tank 22 the acetylating agent is withdrawn continuously through a pipe 28 by a pump 24 and delivered by a pipe 25 to a trough 26 and by a branch 21' to a trough 28.

l The troughs 2i and 28 overilowcontinuously, supplying the acetylating agent beneath and above the web 8 as it advances to and down the inclined plane I5. This arrangement ensures that each increment of the web is subjected under precisely uniform conditions to an acetylating agent of exact formula. Also, the amount of acetylating agent'overowing from the

troughs

26 and 28, beneath and with which the liquid from those troughs flows down the inclined surface I5 is such as to maintain the web suspended inthe acetylating agent,

I- as shown in Fig. 1, so that both sides of the web are subjected to treatment with the acetylating agent that as it passes over the inclined surface and acetylation of the web proceeds uniformly and cannot vary substantially in respect to the predetermined conditions.

In the' receptacle 28 thus avoiding any possibility the web s forms 'a ioop. ze,

of substantial tenof acetic acid as required above the web, and the force causes the web to advance and at the same time' removes the excess of the acetylating agent. At this point there is, of course, a certain loss of the acetylating agent due to drag out, which loss must be takeninto account in maintaining the precise strength of the acetylating formula by suitable additions thereto.- The web passes into the Water in the receptacle I9 which promptly quenchesthe acetylating reaction. Passing under rollers 32 and 33, the web is drawn out of the bath I9 over rollers 34 land 35 and thence into successive Water baths 36 and 31 .under rollers 38 and 39, over rollers 40 and 4|, under rollers 42 and 43, over roller 44- and thence to a series of drying cans 45 where the web is subjected to drying under regulated conditions. As hereinbefore indicated, the water in the receptaclesi9, 36 and 31 is constantly renewed in order to elect rapid and complete purification, it being essential that all of the acetylating agent be removed from the paper together with any other impurities l which may be present.

'The conditions other than those already mentioned, under which the operation is conducted. involve particularly the character and strength of the treating solutions as well as the temperature and time of treatment.- For the soaking of the web preliminary to acetylation, weprefer to use commercial glacial acetic acid of a purity approximating 99.5%. It is believed that the presence of a minute amount of water equivalent to approximately 0.5 is necessary. Up to 2% of water in the acetic acid is permissible. The temperature of the pre-soaking bath is preferably between 80 and 110 F., or from 5 to 10 F. lower than the temperature maintained for the acetylating agent. The time of pre-soak will vary with the degree of acetylation desired, which is controlled by the speed of travel of the web. It will be usually one-third to one-half the time required for acetylation. As hereinbefore indicated, provision is 'made for the maintenance of the temperature and concentration of the presoaking lbath so that it will remain uniform during the entire operation, thus eifecting uniformity in the treatment of the web as it advances progressively.

The acetylation formula consists preferably of a liquid vhaving the following composition by weight:

Percent Acetic anhydride (97% technical grade) 50.75 Acetic acid (99.5% glacial) 49 Catalyst-perchloric acid (dry basis) 0.25

agent at uniform concentration. This will necessarily vary depending upon conditions, but a suitable strengthening formula consists of acetic anhydride 64.5%, acetic acid 35.25%, and perchloric acid 0.25%. The percentages mentioned are by weight, and the materials are those described in connection with the formulation of the acetylating agent. Obviously the acetylating agent must be subjected continuously to control by suitable analyses, and changes will be made in the strengthening formula depending upon the conditions observed. The time of treatment will vary, depending upon the degree of acetylation desired, from 2 minutes to approximately 12 minutes. Economy in operation, that is to say speed of treatment and corresponds to the speed of travel of the web, and it is therefore desirable to limit the time of treatment to the minimum.

Purification is effected in three or more con- 'secutive tanks, all supplied with circulating and refreshed water at approximately 100 F. The first purifying tank containing water stops thc reaction and coagulates or, hardens the material,

thus restoring its tensile strength. The second,

tank will include approximately 0.1% of a suitable wetting agent such as TergitoL a sodium secondary-alcohol sulphate (The Journal of Industrial and Engineering Chemistry, volume XXIX, 1937, page 1234). The function of the wetting agent is to facilitate the removal of impurities, and any othersuitable wetting agent may be used.- The third tank contains pure water to complete purification of the material.

` The web is purified to a pH of approximately 6.5,

it being important tor remove all free acid and ionizable salts which, if retained, would interfere with the electrical properties of the material under humid conditions. Any number of purifying tanks, required to effect the desired purification of the web,` may be used.

The web is dried continuously by passing over v' copper drying cans surfaced with a phenolic resin. It is dried at about 220 F. until optimum electrical values are reached. It is important to dry the material properly, since the bres seem to set with proper drying, producing high elec'- trieal values. The speed of drying and the drying temperature are limited by the size of the equipment and the temperature available. drying temperature is subject to wide variation and temperatures stated herein are merely illustrative of the preferred procedure. Higher temperatures could be used and the drying operation correspondingly limited with respect to time.

While we prefer to use perchloric acid as the catalyst, we may employ any ofthe known acetylation catalysts such as sulphuric, hydrochloric, hydrobromic and trichloracetic acids, salts such as copper chloride, copper sulphate, copper perchlorate, zinc chloride, zinc acetate, zinc sulphate and meta potassium periodate, and neutral esters such as dimethyl and diethyl sulphate. Certain of the catalysts mentioned, such as the sulphuric acid, perchloric acid and meta potassium periodate, are more active and. rapid than others. Sulphuric acid is so active and rapid that unless it is used in very small quantities and the conditions of the reaction are very carefully controlled as to time and temperature, the final product may not have good mechanical and electrical properties.

suitable in the case of cellulosic materials such as cotton rag papers and high alpha pulp papers. When meta potassium periodate is used, it should be mixed with an equivalent amount of a mineral acid to liberate meta periodic acid, which appears to be the active catalyst.

The procedure as described is adaptable for use The Zinc chloride is al .fairly slow-actingcatalyst and is particularly l E. Purifying with various types of papers, but is particularly useful inthe treatment of kraft papers and produces therefrom products of excellent quality. The most strikingv characteristic of the product produced by the method described for the treatmentof various kinds of papers is the relatively high volumetric resistivity of the paper under humid conditions which may vary from 5000 megohm inches up to or beyond 10,000,000 megohm inches. This remarkable increase in volumetric resistivity adapts the product to numerous applications in the electrical field, as for example in the production of condensers and other similar electrical devices.

To amplify the foregoing description, we have included the following examples. These examples aii'ord detailed description of procedures which we have followed successfully to produce satisfactory products in a continuous and readily maintainable operation. The examples, are illustrative and are not to be considered as limiting the disclosure of the invention.

Example I A. Paper A cotton paper made from uncooked raw rags.

Thickness-.0055" ,Basis weight-60# (24 x 36--500) Viscosity-66400 centipoises. (The test for viscosity is the standard method developed by Tappi.) pH-G Percentage of alkalinity in paperlow Bath used for treatment Acetic anhydride (97% technical grade)- 50.7 Acetic acid (99.5% glacial) 49% Catalystperchloric acid-0.25% (dry (This is percentage by weight) Strengthening formula added Acetic anhydride-64.5

` Acetic acid-35.25%

Perchloric acid-0.25%.

weight) Treatment conditions Presoak-l00% acetic acid at 80 to 85 F. Treating time-Zl/z minutes immersion in bath at 97 F.

basis) (Composition by 'I'he treated paper is dried continuously over copper drying cans surfaced with phenolic resin.

Paper is dried at about 220 F. until optimum electrical values are reached. d

G. Descriptiorrof the acetylated paper produced by the above method I'he paper was treated to a basis Weight (24 x 36-500) on a dry basis of 66-67 pounds,

B. Bath used for treatmentv Same as in Example 1 C. Strengthening formula added Approximately asin Example 1 D. Treatment conditions Presoak-100% acetic acid at (l0-85F. Treating time-3% minutes immersion in bath at 93 to 94 F.

E. Purifying Same as in Example 1 F. Drying conditions Same as in Example 1 G. Description of the acetylated paper produced and had a percentage combined acetic; acid of i 19-20 per cent. When tested in humid air at 90% R. H. and 25 C. the paper hada volumetric resistivityof 6,000 to 7,500 megohm-inches.

A. Paper Same as in Examplel l by the above method The paper was treatedto 70-71 pounds basis weight (24 X 36-500) on a dry basis and a percentage combined acetic acid of 231/2 to 241/2 per cent. The acetylated paper when exposed to humid airat R. H. at 25 C. has a volumetric resistivity of 60,000 megohm-inches.

Example 3 A. Paper A. saturating paper made from kraft (A11 composition by Paper was acetylated to 75 pounds basis weight (25 x 36-500) and a percentage combined acetic acid of 45 to 46 per cent. v'Ihe acetylated paper when exposed to humid air at 90% R. H. at 25 C. had a. volumetric resistivity of 1,250,000 and 1,500,000 megohm-inches.

(All composition by acetylated paper when exposed'to humid air atv 90% R. H. at 25 C. had a volumetric resistivity of 20,000 megohm-inches.

Example Paper A light weight paper made fibres (S424) Thickness-.003 Basis weight-30 to 31 pounds (24 x 36-500) Viscosity-44 centipoises pH-6 Percentage alkalinity-low B. Bath used for treatment Same as in Example 1 C. Strengthening formula. added Acetic anhydride-69.5%

Acetic acid-30.25%

Perchloric acid-0.25%. (All composition by Weight) v Treatment conditions Presoak--100% acetic acid at 85 to 88 F.

Treating time-10 minutes immersion in bath at 96 to 97 F.

Purifying Y Same as in Example 1 F. Drying conditions Same as in Example 1 G. Description of the acetylated paper produced by the above method The paper Was acetylated to a basis weight of 46 to 47 pounds (24 x 36-500) and a degree of combined acetic acid of 45 to 46 per cent. The acetylated paper when exposed to vhumid air at 90% R. H. at 25 C. had a volumetric resistivity of 500,000 megohm-inches.

Example 6 f from 100% kraft The paper was acetylated to a basis weight of 38 to 39 pounds (24 x 36-500) and to a per cent combined acetic acid of approximately 135%. When exposed to humid air at 90% R. H. at 25 C. the acetylated paper had a volumetric resistivity of' 10,000 megohm inches.

The treatment of cotton cloth in accordance with the invention produces equally desirable products. Following are examples of such treatment.

The runs were made making use of our standard metering device and standard amount of bath (160 lbs.). After treatment,'the material (All composition by was purified with water by using a system of'7`5 three tanks (with circulation) and 0.1% Tergitm solution. Purification was continued until a pH of six or better was reached. The acetylated cloth was dried in continuous rolls over can dryers, the drying being completed at temperature of 230 F. to aiord optimum electrical prop-- erties. After drying, the cloth was tested-for electrical properties. at relative humidity, 25 C., for 24 hours, and for mechanical properties.

Example 7 A. Cloth Puried cotton fabric (0-40) Thickness .008 2".0084

Weight 3.16 oas/sq. yd.

Count 80x 80 Viscosity to 179 cps.

Section A-2 minutes at 100 F. Section B-3 minutes at 100 F. Section C-31/2 minutes at 100 F. ,f Section D-41/ minutes at 110 F. Section E-8 minutes at 110 F. Section F-lll/z minutes at 110 F. Section G-2 minutes at 100 F.

Cloth was puried and dried.

Eample 8 Cloth- Same as in Example 7 Treating bath Per cent by weight (97.5% tech. grade) acetic anhydride- 49.25 (99.5% pure) glacial acetic acid 50.50 (Solid bis) perehloric acid 0.25

i i l 100.00 Treating conditions Ran 6 sections, designated A to F. Presoak and temperatures of presoak 1. A, B, C-% acetic acid at 110 F. 2. D, E, F-98% acetic acid and 2% Water' at F. Immersion time in bath and temperature of bath Section A--5l minutes at 110 F. Section B-8 minutes at 110 F. Section C-111/2 minutes at 110 F. Section D-5 minutes at 110 F. Section E -8 minutes at 110 F. Section F111/2 minutes at 110 F.

D. Cloth was purified anddried.

and directing it perior properties. The untreated cloth had volumetric resistivity in megohm-inches of 42, and after treatment a maximum of 3,500,000. The insulation resistance of the untreated cloth in megohms was 189, and after treatment reached a maximum of 3,000,000.

lThere was no substantial loss of' mullen or' tensile strength in any of the samples, and in most cases a gain in these factors. The acetylation in terms of per cent of combined acetic acid varied from 14.5 to 40, best results being indicated With acetylation in excess of 35%.

Various changes may be made in the apparatus and in the procedure as' described without departing from the invention or sacrificing the advantages thereof.

' We claim:

1. The method of acetylating cellulose material of the web to an acetylating agent of predetermined uniform composition, and maintaining uniformity of the acetylation of the web by continuously withdrawing and fortifying a -portion of the acetylating agent and returning said fortified portion to contact wtih successive increments of the non-acetylated portion of the web.

3. The method of acetylating cellulose material which comprises continuously advancing a web of the material, subjecting each increment of the web to initial contact with an acetylating agent of predetermined uniform composition owing in the direction of travel of the Web,

withdrawing and fortifying a portion of the acetylating agent, and returning the fortified portion of the acetylating agent for contact with l successive increments of the nonacetylated portion of the web.

4. The method of acetylating cellulose material which comprises continuously advancing a web of the material, subjecting it to treatment with acetic acid, thereafter applying an acetylating agent to each increment of the advancing web, floating the web with the owing lacetylating agent downwardly over an inclined surface thereby avoiding longitudinal stress in the web while it is subjected to the acetylating agent, maintaining the composition of the acetylating agent by adding fresh reagents thereto, ensuring .uniform treatment of the material by circulating the refreshed acetylating agent and directing it into contact with the non-acetylated incre' ment of the advancing web, and finally washing the web.

5. The method of acetylating cellulose material which comprises continuously advancing a web of the material, subjecting it to treatment with acetic acid, thereafter causing it to pass downwardly over an inclined surface, supplying lrial which comprises with such force as to maintain the web suspended in the acetylating agent as it passes downwardly over the inclined surface, and finally washing the web with water. l

6. The method of acetylating cellulose material which comprises continuously advancing a web of the material, subjecting it to treatment with acetic acid, thereafter causing it to pass downwardly over an inclined surface, supplying an acetylating agent containing at least 49% by weight of acetic anhydride both beneath and above the web as it passes downwardly over the inclined surface, the acetylating agent being supplied beneath and above the web in such amounts and with such force as to maintain the web suspended in the acetylatingv agent as it passes downwardly over the inclined surface, and finally washing the web with water.

7. The method of acetylating cellulose material which comprises continuously advancing a web of the material, subjecting it to treatment with .acetic acid, thereafter causing it to pass downwardly over an inclined surface, supplying an acetylating agent both beneath and above the web as it passes downwardly over theinclined'surface, the acetylating'agent being supplied beneath and above the web in such amounts and with such force as to maintain the web suspended in the acetylating agent as it passes downwardly over the inclined surface, and finally washing the web with water, and maintaining the composition of the acetylating agent by adding fresh reagents thereto.

8. The method of acetylating cellulose material which comprises continuously advancing a web of the materiaL'subjecting it to treatment with acetic acid, thereafter causing it vto pass downwardly over an inclined surface, maintaining the web in contact with an acetylating agent as it passes downwardly over the inclined surface, and finally washing the acetylated web with water, maintaining lating agent by adding fresh reagents thereto, and insuring uniform treatment of the material bycirculating the refreshed acetylating agent and directing it into contact with the lnonacetylated increment of the advancing web.

9. The method of acetylating cellulose matecontinuously advancing a web of the material, subjecting it to treatment with acetic acid, thereafter causing it to pass downwardly over an inclined surface, supplying an acetylating agent both beneath and above the web as it passes downwardly over the inclined surface, the acetylating agent being supplied beneath and above the web in such amounts and with such force as to maintain the web suspended in the acetylating agent as it passes downwardly over the inclined surface, and nally washing the` web with water, maintaining the'composition of the acetylating agent by adding fresh reagents thereto and insuring uniform treatment of the material by circulating the refreshed acetylating agent and directing it into contact with the nonacetylated increment of the advancing web.

10. The method of acetylating cellulose material which comprises continuously advancing a an acetylating agent both beneath and above y web of the materialysubjecting it to treatmentv with acetic acid, thereafter causing it to pass downwardly over an inclined surface,vsupplying an acetylating agent both beneath and above the web as it passes downwardly over the inclined surface,v the acetylating agent being supplied beneath and above the Yweb in such amounts and with such ,force as to maintain the web suspended the composition of the acetywith acetic acid, thereafter causing it to pass downwardly over `ari inclined surface, supplying an acetylating agent both beneath and above the web as it passes downwardly over the inclined surface, the aoetylating agent being supplied beneath and above the web in such amounts and with such torce as to maintain the web sus pended in the acetylating agent as it passes downwardly over-,the inclined surface. the ineline of the web during its passage over the inclined surface being such that it advances without longitudinal stress, and iinally washing the web with water, maintaining the composition of the acetylating agent by adding fresh reagents thereto and insuring uniform treatment of the material by circulating the refreshed acetylatinlg agent and directing it into contact with the nonacetylated increment o! the advancing web.

12. Ihe method of acetylating cellulose material which comprises advancing a web o! the material, subjecting it to treatment with acetic acid, thereafter causing the web to pass through an acetylating zone in a direction such that the force ot gravity normally would cause a 83881118 of the web, supplying an acetylating agent both beneath and above the web as it passes through said acetylating zone, the acetylatingagent being supplied beneath and above the web in such amounts and with such force as to maintain the web suspended in the acetylating agent as' it passes through the acetylating zone.