US2300733A - Cellulose pulping system - Google Patents

Description

'Patented Nov. 3, 1942 CELLULOSE lPULPIN G SYSTEM Lyle Melvin Sheldon, Alton, Ill., assig'nor to The Cellulose Research Corporation, East Alton, lll., a corporation of Delaware Application May 21, 1937, Serial No. 143,967

2 Claims.

This invention relates to the production of cellulosic pulp from resinous woods and in particular to the removal of4 resinous materials therefrom.

A primary object of the invention is to provide a process for producing puried cellulosic pulp suitablefor conversion into cellulose esters, particularly acetate, ethers, Viscose and other derivatives from resinous woods, such as the southern pine species; said purified cellulosic pulp being capable of very close duplication with respect to its physical and chemical properties between successive batches.

Another object is the provision of an economical process employing selected wood from `the various species of trees known collectively as southern pines as a base raw material to. produce purified wood cellulosic pulp capable of acetylation, xanthation and conversion into other derivatives, which wood has heretofore been considered unsuitable for the satisfactory production of purified cellulosic pulp of the class described; said process providing means for the removal of impurities, such as resins, which are characteristic of said southern pine species.

A further object is to provide an economical process for producing sulte pulp for paper, employing the selected Wood for use from the various species of trees known collectively as southern pines as the base raw material, which woods have been heretofore regarded as unsuitable for this purpose; said process providing the means for removing impurities such as resins and pitch.

l In the accompanying drawing which illustrates several features of the present invention:

Figure 1 is a typical time-temperature impregnation and digestion schedule.

Figure 2 shows the relation of bleaching temperature to viscosity reduction. f

Figure 3 shows the relation of-bleacn1ng time to viscosity reduction and color of cellulose. i

The advisability of conning the raw material for the manufacture of chemical celluloseto one genus or to one species of trees from which the base wood material is derived has been suggested. In treating even single species of wood there is a tremendous distinction'within each species of the factors affecting pulping and purification of the wood, the amount of recoverable alpha cellulose and the economy with which the process may be carriedout.

In the process of the present invention there is employed a certain group of trees not herepulp suitable for derivative purposes. The primary advantage found in this group is the almost complete absence of heartwood. For example', the group of trees, known collectively as southern pine (which include the individual species .of Pinus palustrz'along leaf; Pinus taeda, loblolly; Pinus echinata, short leaf; and Pinus caribead, slash pine), grow in such a manner that if they are cut before they reach the age of about 25 to 35 years, they consist of substantially all sapwood. At .the age. of 29 years, for example, slash pine will produce logs in excess of 8"-10'? in diameter, which obviously may be economically chipped by the customary practices.

The southern pine wood species have heretofore proven unsatisfactory for pulping by the sulte processes owing to the serious operating difficulties encountered because of their high pitch or resin content'. The pitch and resin-content is frequently as high as 5%-10%'and when permitted to solidify before pulping, as by evaporation and prolonged storage, is quite difficult to remove. In the manufacture of pulp for paper purposes by the prior practices the resin exhibits a strong tendency to agglomerate, clog the screens and wires of the paper machines and form very undesirable defects in the paper.

I have discovered, however, that by selecting southern pine wood substantially free of-heartwood and defective material, including pitch pockets, before the pitch or resin content has had an opportunity to solidify and preferably before any substantial amount of fungus growth such as blue stain develops, a highly uniform cellulosic pulp suitable for conversion into ce1- lulose derivatives may be economically prepared toforeknown to be capable of yielding cellulosic according tothe process hereinafter set forth. I have also found that such selected woods may be satisfactorily and economically used for producing strong sulfite paper pulp.

The logs are debarked. washed and chipped by the customary practices. The chips are preferably of the order of M1" in length, but the standard 3A" to V8 chips may be employed, if desired.

The suitably divided wood is impregnated with the selected delignifying reagent. Normally, it is preferredin connection with the present invention to use a sulte delignifying solution, such as ammonium, calcium or sodium bisuliite. The chips'are impregnated and thoroughly saturated with the delignifying liquor at a temperature of C. or less, while at the same time substantially their entire air content is removed. At a temperature of the order of 100 C. there is but slight tendency for deligniflcation to take place, hence the saturation of the chips is `accomplished before the delignication of the wood proceeds. By this provision a far more uniform treatment of the woody material is insured.

Impregnation of the wood with the digesting liquor may be accomplished, for example, by boiling the chips in a freely vented tank or digester filled with the treating solution and then allowing the surrounding liquor to flow into the interstices of the wood either by increasing the pressure on the liquor or by cooling the mass slightly below 100 C. In either case, the steam within the interstices of the wood is condensed, thus forming a partial vacuum which draws the surrounding liquor into all the capillaries and interstices.

Removal of the air from the subdivided wood and impregnation with the digesting liquor may also be accomplished by a preliminary boiling of the wood with water to remove the air and then adding the delignifying liquid to the chips and providing a suiilcient time to permit diffusion of the delignifying chemical through the water saturated chips.

'Regardless of the particular digestion procedure, the character of the raw material employed together with the advantageous presaturation of the chips with the delignifying solution makes it possible to carry out the digestion under such mild and lenient conditions that substantially no chemical degradation of the cellulosic material in the wood is encountered an a high yield results.

The mild and lenient digestion results in the production of pulpY wherein the cellulosic fibers have a gradually increased porosity and permeability over their original condition and which are relieved of the greater portion of their ligneous impurities. The digestion moreover reduces the pitch content of the pulp to less than the order of about 1.5%.`

At the completion of the digestion, the treating solution is drained and the reaction products washed from the pulp. Preferably with hot water and with air excluded. This treatment not only improves the color of the pulp but also aids in removing a part of the residual pitch and other hot water soluble impurities from the fibers, such for instance as hydrolyzed hemicelluloses and mineral constituents composed mostlyA of calcium, magnesium, iron, manganese, sodium, potassium and silicon salts. The pitch content of the digested pulp may be reduced even further by adding 1/4% or less caustic soda, based on the dry weight of the pulp, to the hot water used in washing.

Upon the completion of the thorough washing of the crude pulp, it is screened to remove knots and other fragment material and then completely deflbered by any suitable mechanical means; the debered pulp may be screened, if necessary, to remove any neps or bundles of bers which may have survived the deflbering process.

Regardless of the ultimate use of the cellulosic pulp, whether forcellulose acetate, viscose products or for bleached paper pulp, the rst step in the purication is a uniform chlorination of the deilbered pulp. The` chlorination may be accomplished by introducing a known amount of chlorine gas into a water slurry of the pulp under highly efllcient agitation. -For economy and to insure complete deligniilcation of the pulp, the gas is added to the pulp slurry in as short a time as possible consistent with uniform distribution., An amount of chlorine in pounds for less.

each 100 pounds of pulp (oven dry basis), equivalent to 14%18% of theV bleach value of the pulp is employed. The control of the distribution and amount of chlorine 4results in a maximum purification of the pulp with an almost complete absence of chemical degradation.

Following the chlorination, the pulp is treated with either hot or-cold alkaline solutions to remove the reaction products. The result of the chlorination and causticizing treatments is to reduce the bleach value of the pulp to between the order of 1.0% and-5%, depending upon thekind of alkaline treatment given, and to reduce the pitch content to less than the order of 0.3%.

Regardless of the bleachability of the washed pulp after the chlorination and causticizing treatment within the range of 1% to 5%, it is bleached in a dilute hypochlorite solution containing 1% to 6% bleaching powder based on the pulp. A sufiicient amount of alkali is added tc the bleach solution to bring its pH value to approximately 8.0 or above. Under these treating conditions the residual oxidizable impurities remaining in the pulp are reacted upon'in the early stages oi' the bleaching, for example, in from l to 11/2 hours or less.

The treatment is continued for an additional time, usually from 2 to 3 hours, to reduce the cuprammonium viscosity of the cellulosic pulp to the desired value. By reference to Figure 3 it will be observed that the cuprammonium viscosity of the pulp is rapidly reduced during the early stages of the bleaching treatment. This is followed by a period in which the rate of reduction is relatively small. For example, it will be seen that the viscosity is reduced from 42 centipoises to about 39.0 centipoises .in 1 hour, but in the succeeding 3 hours the value is reduced only an additional 2.2 centipoises. Thus by clontinuing the hypochlorite treatment until the relatively level portion of the viscosity curve is reached, the treatment may be readily terminated at the vdesired viscosity level, since the rate of reduction is relatively small and the desired time to give the required value may be calculated with precision.

'I'he desired reduction in the cuprammonium viscosity of the pulp may also be effected by varying the temperature of the hypochlorite bleach treatment and holding the time, concentration, and consistency of the solution constant.

Upon the completion of the bleaching treatment the pulp is thoroughly washed with water until free of available chlorine, preferably until neutral.

At the end of the bleaching treatment the cellulosic pulp will have an alpha cellulose content of about %94%, depending upon the time, temperature and concentration of alkali used in the treatment after chlorination. The resin content will be reduced to the order of 0.3% or The material is very satisfactory for use in preparing'viscous products and is characterized by its nearly invariable physical and chemical properties between successive batches. It is also very satisfactory for paper purposes since it possesses unusually high strength properties, a high white color, and the resin content is in such condition and of such low value as to cause none of the usual clogging of the screens or lulose content.

The washed pulp may be given a second hypochlorite bleach employing less than 1% bleachsuitable port above the water level. This forces the water surrounding the chips out of the di' gester. After all of the water has been drained from the chips in this manner, the water outlet ing powder based on the dry weight of the pulp.

The time, temperature and consistency are controlled substantially as described for the rst hypochlorite treatment. The pulp is again washed until free of available chlorine, prefer- 'ably lmtil neutral.

The. cellulosic pulp at this stage of its purification is of exceptionally high quality for use in the preparation of viscose products. The alpha cellulose content is approximately 95%-97%, its soda solubility less than 4% and the resin content is reduced to the order of 0.25% or less. It may also be used to advantage in the preparation of acetate where exceptionally low color and haze in solvent solutions are not essential.

When the cellulosic pulp is intended for conversion into cellulose acetate of high quality. the pulp obtained by the treatments above is mercerized in a strong vsodium hydroxide solution.

Upon the completion of the mercerization it is important to wash the caustic reagent from the cellulose as rapidly as possible and in the absence of atmospheric oxygen. lWith suitable equipement, this may be and is desirably accomplished in 30 minutes or less.

Cellulose acetate of lower cost may be obtained from the use ofthe puried pulp of the present invention since the base raw wood material is not only cheaper than other available supplies of raw cellulosic material, but its ready response to chemical treatment permits important economy in thel process of its conversion to acetylatible quality.

In vorder to more particluarly describe the present invention and manner of' attaining uniformity in the reactions, the following example sets forth a typical embodiment thereof:

Example A conditions. It is important that the chipsbe mosphere at the top of the digester is closed and the steam supply to the bottom ports is cut off.' A suitable outlet in the bottom of the digester for draining is then opened and steam is admitted at the top of the digester through a at the bottom and the steam inlet at the top of the digester are both closed. This leaves the digester and the voids around the chips, the capillaries and interstices within the chips completely filled with steam at approximately 100 C.

The ammonium bisulte liquor previously preheated to a temperature of the order of C. is then pumped into the digester in suiiicient quantity to attain a ratio of liquor tooven dry wood of the order of 6:1. When the liquor comes in contact with the chips the steam with which they are saturated is forced to condense. This creates a partial vacuum which acts to draw the treating solution at a temperature just under Steam is now admitted into the digester and the temperature raised at a uniform rate to the order of 1219-123" C. over a period of 3 hours, as shown by reference to curve AB', Figure 1. The temperature is held within this range for approximately 3 hours (curve B'iC', Figure l). During this period the sulfonation of the noncellulosic components of the wood takes place. Owing to the completeness with which the air has been evacuated from the digester and the chips before the temperature is raised to above 100 C., the pressure within the digester during the sulfonation cycle (curve B'C', Figure ,1) will be due solely to the heated treating solution and should not exceed 85 ypounds per square inch. Gas reliefs to prevent the pressure exceeding the customary limit in the conventional digester equipment are consequently usually unnecessary.

Thus, the usual variability in the concentration of the treating liquor during the sulfonation period, occasioned by the loss of sulfur dioxide gas by venting, is avoided and a treatment of the h woody material with a liquor of nearly invariable composition between successive batches is insured.'

At the end of the 3 hour sulfonatlon period, the temperature is raised to the order of C. at a uniform rate over a period of l hour (curve C'D, Figure 1) and maintained closely at this level for from 2-4 hours (curve D'E', Figure l), depending upon the bleachability and viscosity desired in the digested pulp. The eiect upon the bleachaoility and viscosity by varying the period of treatment at a temperature of the order -of 135 C. will be described later in this example.

Hydrolysis of the suli'onated, non-cellulosic material takes place satisfactorily during the treatment at temperatures4 in excess of 130 C., and in the present example the digestion is terminated at the end of 2 hours treatment at 135 C. Gas relief to control the pressure in the digester below the permissible limit of the equipment may be 'practiced during the hydrolyzing cycle. Care is exercised that the venting is carried out in such a manner as to avoid reducing the temperature appreciably below the preferred'level of the order of 135 C., since the rate of hydrolysis of the sulfonated, ligneous material is aected by variations in temperature. Short gas reliefs 'of Thus, by first less than 30 seconds each will accomplish the desired reduction in pressure without substantially lowering' the temperature, except in the -final stages of digestion where gas relief is practiced to effect as complete recovery of sulfur dioxide as possible.

Upon the completion of the digestion schedule, the pulp is -blown from the digester in the Vcustomary manner into a blow pit and washed with water, preferably as near 100 C. as possible until it is neutral to litmus. .The washed l pulp is screened through a 0.012"0.015" screen and passed over a riiile box for further removal of heavy foreign material, such as knots, particles of bark, dirt and the like.

By the foregoing digestion schedule, from 50-52 pounds or more of oven dry pulp are obtained from each 100 pounds of oven dry slash pine wood used. An actual analysis of a typical screened pulp showed the following:

It will be noted that the retention of the native alpha cellulose is 44-46 pounds of each 100 pounds of oven dry slash pine wood used. The resin content is reduced to the order of 1% and is in a condition which permits its ready removal to an inconsequential residue devoid of objectionable agglomerating tendency in the iirst step of purification to be presently described.

The digestion schedule just described is designed to produce a high yield of raw pulp of relatively high viscosity and bleachability. The same schedule and liquor composition applied to other coniferous woods such as spruce, or to decidous woods such as black gum, yellow birch and the like will result in raw pulps whose -viscosities and bleachabilities may vary to some extent 'from the abovevalues.

Consequently, it is usually necessary to determine the precise-adjustment of the time-'temperature schedule to give a. pulp of the desired bleach and viscosity characteristics by carrying out one or more experimental digestions on the quality of wood to be used in continuous operation. The adjustments in the digesting practice for controlling the bleachability and viscosity of the raw pulp are These variations will also` occur with wood of the same species obtain from different geographical areas and of vary-` ing growth conditions.

conveniently made by varying the time or the temperature, in some cases both. To obtain a maximum yield of raw pulp together with high cuprammonium viscosity, the average cooking temperature should be held as low as possible and the time of treatment prolonged by experiment to give a pulp that can be readily and completely deilbered.

High cuprammonium viscosity in the raw pulp is quite necessary where it is intended that the purified cellulose shall have a viscosity in excess of the order of 25 centipoises. There are certain types of cellulose acetate, however, for which it is necessary to provide a puried cellulose having a relatively low cuprammonium viscosity, for example, below the order of 10 centipoises. For

this purpose it is more convenient to produce a 7s 'the time being held constant.

' Ash raw pulp with a cuprammonium viscosity less than the order of 25-30 centipoises. To accomplish this the digestion schedule, represented by curve DE, Figure 1, is carried out at a somewhat higher-temperature, for example, 140-145 C., This treatment appreciably lowers the cuprammonium viscosity and also effects a considerably smaller proportional reduction in the bleach value. The purpose of providing a relatively low raw pulp viscosity as the starting material to produce puried cellulose of less than 10 centipoise viscosity will be explained in the description which follows of the hypochlorite bleach treatment.

After the washed, digested pulp has been thoroughly deiibered, the consistency is adjusted to 3 %l0% and the pulp slurry pumped to a gas tight chlorination tank equipped with an eilicient agitation device. The pulp is nowtreated with an amount of chlorine gas equal (in pounds per pounds of oven dry pulp) to 16% of the bleachability value of the pulp as determined by the permanganate number method. This amount will just satisfy the demand of the immediately reactive lignin and should be controlled within 3% or less. Larger amounts of chlorine are uneconomical and tend to cause chemical degradation of the cellulose, whil smaller amounts result in incomplete chlorination.

The required amount of chlorine gas is incorporated in the pulp slurry at a uniform rate of yiow in between 20 and 30 minutes. The chlorine will be substantially exhausted in an additional 5 to. 15 minutes but the treatment is allowed to continue for a total of 45 minutes (after all of the chlorine has been added) in order to allow the acids formed by the chlorination treatment to act upon the ash content of the pulp. The

chlorine should be added in a manner that will insure a uniform distribution of gas with the. unchlorinated fibers; otherwise an increased loss of chlorine may occur due to reaction with material already chlorinated and result in an incomplete chlorination of part of the fibers. It

has been-found desirable to effect the incorporationof'the required amount of chlorine gas into the pulp slurry in as short time as possible consistent with such uniform distribution as will avoid the `beforementioned undesirable localized action.

At the end of the 45 minute chlorination treatment, a suillcient amount of sodium hydroxide is added to the chlorinated pulp to .bring the concentration of the solution to about 1% and the I 'thoroughly with water, preferably until neutral.

A typical sample of the chlorinated and causticized pulp has the following analysis:

Per cent Alpha cellulose 94.9 Soda vsolubility 6.5 Cuprammonium viscosity (ctps.) 54.0 Bleachability 1.1 Pentosans 5.4 Resins 0.3

Ithas been' found that the viscosity of the finall purified cellulose will be approximately '75%-85% of its value at the end of the hypochlorite bleach treatment. In general, the higher the viscosity of the purified end product, the greater will be the spread between the vsicosity value at the end of the hypochlorite treatment and such nal viscosity. For example, if a viscosity of 30 cen-- tipoises for the nal puried cellulose is desired, the pulp viscosity should be reduced in the hypochlorite treatment to approximately 35 centipoises. If an end product of 18 centipoises is desired, the cuprammonium viscosity of the cellulose should be reduced by'means of the hypochlorite treatment to approximately 20 centipoises.

In carrying out the bleaching step of this example, the required volume of hypochlorite so.- lution to give. 2.5 pounds of bleaching powder (calculated on the basis of 35% available chlorine) for each 100 pounds of pulp is added to the pulp slurry which has previously been adjusted to a consistency of 2.7%. -This addition of reagent will give a concentration of bleaching powder in the treating solution at the start of the reaction of0.'06'l%. The hypochlorite is added in the form of a solution containing 60-'70 grams of bleaching powder per liter and saturated with lime. The bleaching reaction is allowed to proceed for 4 hours at a temperature of 25 C. which resultsl in a consumption of approximately 50% of the active reagent present. Control and close duplication of the pH conditions during the' bleaching step of successive batches is essential for reproduction of the rate and extent of viscosity reduction. The preferred practice is to have the pH at the start of the reaction at about 9.4 and allow it to decrease uniformly to a value about 8.7 at the end of the treatment. The lower limit of pH for satisfacviscosity. Variations such as the rate and degree of agitation, pH during bleaching, size of batch, direct or indirect heating, rate of heating, presence of metallic contaminants etc. are factors in the control of viscosity during bleaching. I have found a control viscosity determination to be a practical method for obtaining a substantially constant cuprammonium viscosity of successive .batches of pulp after the completion of the hypochlorite bleach. A curve showing the change in viscosity with time'of bleaching should first be determined on a number of successive lots from the same wood supply going through the process. This is desirable in order that the cumulative effects may be known ofthe natural variations in the raw material and the .minor unavoidable variations in the processing steps arising from the limitations of accuracy in measuring the weight and consistency of pulp, weight of reagents and the like. When these curves areA viscosity ofthe pulp upon the termination of 'the bleaching. Reference to Figure 3 shows that although in the last two hours oi the bleaching the change in viscosity with time -is small, proper adjustments in time can be made to readily overments in time may be made with no deteriment to the other pulp properties for the oxidizable material in the cellulose has been substantially tory bleaching 'of pulps lintended for use in derivatives is of the order of 8.0.

The bleaching treatment just described affects primarily the cuprammonium viscosity, the

color of the cellulose and the amount of oxidirar ble material present. g the course of the bleaching step, the cuprammonium viscosity will be reduced to about 43 centipoises, the color of the cellulose improved from a reflectance value of about 82.4% to a value of about 93.4% and the bleachability reduced to a value of about 0.4% f

or less. v

. If a greater reduction in the viscosity of the pulp is desired than illustrated by the present example. this may be eiected by employing higher bleaching temperatures or by adding agreater percentage of bleaching powder. However, either practice tends to undesirably impair the properties of the pulp and furthermore are lessv reacted with during the iirst hour of the treatment -andthere is practically no change in the alpha cellulose and soda soluble contents even though the bleaching treatment should be prolonged 2 hours or more beyondv the 4 hours preferably chosen and used in this example.

A typical analysis of the cellulose after bleach- Per cent Alpha cellulose '94.6 Soda solubility..-` .L '1.9 Cuprammonium viscosity ('ctpsJ. 43.0

Bleachability 0.35 Pentcsans 5.2 Resins A0.3

gld (based onoven dry.wood).. `4245, or moreA y The'cellulose at this stage of its puriiication may be used toprepare viscose products; however, to obtain a higher quality of cellulose further purincation treatments may be given. It is also of excellent quality for use in the manufacture of paper. It should be noted that the resin content of the pulp has been reducedto a negligible valueV and furthermore, it has been found that the condition in which the residue exists is such that it has notendency to-agglomerate and cause the objectionable clogging of the wires and felts, typical of pulps produced fromresinous woods by the prior practices when such pulps are used for the manufacture of paper.l

This is considered an important feature of the invention since it makes possible the use of the abundant supplies of low cost southern pines for paper purposes.

Aftervwashing until free of available chlorine, the consistency of the pulp slurry may be adjusted to about 4%-12% and the pulp treated with a '1% caustic soda solution for 11/2 to 3 "hours at about 100 C. under conditions which allow a minimum amount of atmospheric oxygen to come in contact with the pulp. Following this treatment the pulp is drained free of excess solution and washed thoroughly with water until the pH has been reduced to between 7.0 and 8.0

Yield (based on oven dry wood) 38-40, or more This cellulose may b e acetylated according to the practice .disclosed in Letters Patent No. 2,187,710 of Lionel Goff et al., granted January 16, 1940; or, it may be used as a high quality raw material for preparing vviscose products.

A further variation of the purification practice consists in-omitting the calcium hypochloritebleaching treatment and proceeding directly to the 7% sodium hydroxide boiling treatment as soon as the chlorinated compounds have been washed from the pulp.. In this case, after the caustic boiling treatment, the pulp is 'thoroughly washed in purified water, preferably until neutral, and given a 0.5% to 1.0% calcium hypochlorite bleach treatment in which the concentration of active reagent and/or temperature is adjusted to give the desired reduction in viscosity. 'I'he analysis ofthe pulp will be similar` tothat produced by the first described practice and is a particularly desirable quality for xanthation purposes, as well as for conversion into useful acetate and other esters".

Where the highest qualities of acetate or other esters are desired, particularly those acetates for use in film and sheeting, the `pulp is next merthe caustic soda still retained by the pulp washed out under carefully controlled conditions with puried water, preferably until neutral. 'Ihe ated even more economically and satisfactorily by the method set forth in copending application of Lyle Sheldon et al., S. N. 70,372, filed March 23, 1936, which has become abandoned. It should be noted that the resin content has been reduced to the low value of 0.1%.

A more thorough understanding of the advantages inherent in the use of sapwoods with substantially complete exclusion of heartwood and compression wood as a raw material for the production of chemical cellulose can possibly be derived by considering the morphological differences between sapwoods and heartwoods.

Sapwood may be considered as that portion of the xylem nearest to the bark which contains living cells and which serves for the conduction, support, and storage of food; while the heartwood functions mainly as mechanical support, after all active growing ceases in the sapwood and it vserves only as mechanical tissue, it becomes heartwood which is composed of dead wood cells and contains a greater amount of resin, gums, etc. than is present in sapwood.

Sapwood may be distinguished from heartwood i in most species of wood by the difference in color;

usually the latter is of a much darker color. 'I'he infiltration of the cell walls with such substances as oils, resins, and coloring matter and the plugging or-lling of the lamina of the cells with gums, resins, and waxes are largely responsible for the change in color. southern pines, particularly long leaf and slash pines, is impregnated with resin which solidifles in the -lumen of the tracheids, resin ducts, and parenchyma cells. The resin of the sapwood of these species is in the liquid state and is confined for the most part in the resin canals. In

and around injuries and knots, the'wood is often saturated with resin, even in the sapwood portion.

In the transforation of sapwood into heartwood a number of important changes occur; all

living cells losetheir protoplasts; the cell sap is withdrawn .and commonly the water content.

of the cell walls is greatly reduced; food material present in the living cells is removed; and

the partly lignifled walls or parenchyma cells properties'of the cellulose'are, especially affected by the procedure followed in removing the alkali from the cellulose at the completion of the mercerizing treatment. The time employed to' remove the caustic soda from the pulp should be no more than the order of 30 minutes andis preferably l5 minutes or less.

The'mercerization (and classification, if employed) completes the chemical purification of the cellulose, a typical sample of which will then analyze as follows:

I Per cent Yield (based on oven dry wood) 36738, or more The above highly purified cellulose can now be dried and acetylated by the wellknown methcds ordinarily practiced. Itis, however, preferably maintained in a wet condition u. :l acetylmay become more strongly lignifled. There are formed within, or brought into the changing cells, certain substances new to the tissuesl such as oils, gums, resins, tannin compounds, and various achromatic and coloring substances, the achromatic substances being such as the colorless salts of metals magnesium, silicon, calcium,

sodium and potassium, as wellas some colorless organic compounds. The so-called it .membranes, in the case of coniferous spec es, which act as valves permitting the flow of ,fluids to and fromthe interior of the individual wood or cell fibers becomes fixed in a closed position.

By virtue of having used only selected sapwood as the starting material in the present process and the careful control of the conditions under which each step of the digestion of the wood and the purification of the pulp are carried out, an

end cellulosic pulp product is produced having any desired degree of purity and distinguished by the nearly invariable chemical and physical properties between successive batches.

The control of the chemical treatment in each purication step almost completely avoids chemical degradation of the cellulosic fibers and effects a gradual increase in the purity of the product as measured by the alpha cellulose and soda soluble contents. At .the same time, pitch.. resins and The heartwood -of the other objectionable impurities are almost completely eliminated with the result that the cellulosic pulp product of the present invention possesses a homogeneity combined with high purity and permeability which renders it a nearly ideal product for conversion to cellulose derivatives.

The alkaline treatment of the chlorinated pulp is quite eiective in removing the residual pitch and resin compounds which have heretofore been a troublesome problem in the use of highly resinous wood pulps in -paper making. Furthermore, the process of the present invention not only removes these objectionable resinous materials, but produces substantially resin-free pulp of high whiteness with a yield considerably higher than that ordinarily obtained by the prior practices in producing similar pulp from the nonresinous wood species.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results obtained.

As many changes could be made in carrying out the above compositions and processes without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Since no published methods have been found which clearly dene certain measurements used herein, the following denitions of such treatments are given.

Soda. soluble material is dened as that portion of a cellulosic sample dissolved when it is sub- .jected to the action of l7.14% sodium hydroxide solution at the boiling point of water for 3 hours by a refinement of the method of Griiiin, Techl nical Methods of Analysis, 492 (1927 ed.), described in Letters Patent cf Sheldon et al. No. 2,185,776 granted January 2, 1940, 'page 2, column 2. v

Alpha cellulose is defined as that portion of a sample of cellulose material not dissolved by 17.5% sodium hydroxide solution at 20 C., determined by a refinement of the method described by H. F. Lewis in Technical Association Papers,

Series XVII, #1, 436 (1934) described in Letters Patent of Sheldon etal. No. 2,185,776 granted January 2, 1940, page 2, column 1.

Bleachability of cellulose, as used herein, is a measure of the materials oxidizable by potassium permanganate in the presence of an acid under specific conditions, and is expressed in terms of standard bleaching powder containing 35% available chlorine. method, including the conversion table for expressing the permanganate number in terms of per cent .bleaching powde` of 35% available chlorine, was published by T. A. P. P. I., Series XVII, #1, 146 (1934) Permanganate Number of Pulp by R. N. Wiles.

Cuprammomum viscosity, as' used herein,

means the viscosity number or value obtained according to the following method: The cuprammonium solution was prepared by the action of air on electrolytic copper in the presence of strong ammonia water. The copper concentration of the solutions employed for viscosity determinations was 30, i 2 g. per liter and the ammoniaI content Was 165 g., i 2 g. per liter. The concentraticn of cellulose employed was 0.6 g. (oven dry basis) per 100 cc. of cuprammonium solution. The cellulose sample for this determination was dried at C. to 4% moisture content. After weighing out 0.6 g. (oven dry basis), the sample A detailed description of the was moistened, squeezed to a uniform weight of tion in an atmosphere of hydrogen 'from which oxygen has been completely removed. The viscosity measurements were made at 25 C. with a modied Ostwald pipet, constructed according to the specifications of the American Chemical Society Committee on the Viscosity of Cellulose (Journal of Industrial & -Engineering Chemistry, I, #49; 1929). The time of flow in seconds was converted to centipoises on the basis of the calibration of the pipet with oils of known viscosity in centipoises obtained from the United States Bureau of Standards.

Colo@` and haze- The measurements referred to herein of the color and haze of the acetic acid and acetone solutions of the acetate were made by comparisons with standards of known color and also turbidity expressed in parts per million. The standards are those used for water analysis recommended by the American Public Health Association and consist of platinum cobalt (for color determinations) and fullers earth (for turbidity comparisons). With the materials recommended by thev American Public Health Association, a series of standard solutions ranging from 10 p. p. m. to 200 p. p. m. inclusive at' intervals of 10 p. p. m.'were prepared for both color and haze. cc. of each of these solutions was hervmetically sealed in an 8-ounce, wide-mouth. glass-stoppered bottle having an internal diamhour after the dope has been/killed? by the addition of the dilute acetic acid. At/-the time of measurement, the dope must bei'labsolutely free 1 from air bubbles and at a temperature of 25 C., and the quantity of dope under comparison in the bottle must not be less than 100 cc. It has been found that color and haze ratings by this method can be made to an accuracy of approximately 10 p. p. m.

Color of cellulosa-The color of the cellulose is measured by the amount of light reflected from a pad of cellulose. This is determined by using a photo-electric cell and is expressed as per cent of the range from black to the white of a magnesium carbonate block.

I claim:

1. The process of producing cellulose pulp from highly resinous wood such as, Southern' pine, comprising providing fresh Southern pine in desired subdivisions having their resin content in a liquid state in the natural resin solvent, impregnating the subdivisions with a sulphite di- .vesting liquor by 'heating the subdivisions to a temperature at which steam is formed Within the subdivisions. and condensing the steam while the subdivisions are submerged in a liquid solvent of the sulphite liquor, then progressively raising the temperature to the order of 121123 C., maintaining the temperature in the range below C. for at least'three hours with substantially no gas relief, thereafter raising the temperature above 130 C. and continuing the digestion for at least two hours at the higher temperature to 2,300,738 simultaneously reduce the resin content to less treatment, then treating the pulp in a. caustic solution to remove the reacted products of the chlorinaton and simultaneously reducing the resin content to 0.3% or less, and then bleaching said pulp to desired whiteness with hypochlorite.

`2. The process as set forth in claim 1 in which the hypochlorite bleach is followed by a treatment of the pulp with a 7% caustic soda solution at elevated temperature for from one to three hours further to reduce the soda soluble content of the pulp to less than 3%.

` LYLE MELVIN SHELDON.

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