US1838326A - Refined wood pulp of low pentosan content - Google Patents

Description

Patented Dec. 29, 1931 i UNITED "STATES OPATENT' oFFicE GEORGE A. RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE REFINE!) WOOD PULP OF LOW PENTOSAN CONTENT No Drawing.

low solution viscosity, if desired. Such a product, in addition to being an excellent papermakin-g material, is eminently suitable 'for use as a raw material for nitration, acetylation, or thelike, in cases'where it is desirable that the cellulose derivatives be soluble to form solutions of high clarity but of low viscosity. Especially-in the case of the acetates, itv has been found that the'presence of pentosans much above 1% in the fiber used as a raw material for acetylation is largely responsible for opacity or cloudinessin solutions prepared from the acetylated product, as the use of cellulose fibers of high alpha cellulose content and containing less than 1% of pentosans in the preparation of cellulose acetates makes possible the production of cellulose acetate solutions of clarity and sparkle.

In producing refined pulps from wood or kindred raw cellulosic materials, it has generally been the practice to prepare a raw wood pulp such as sulphite or kraft, and then to dissolve non-alpha cellulose components from such pulp by the use of alkaline liquors. Such a process doesnot result in a product of minimum pentosan content, as the usual cooking liquors employed for libcrating the fiber from wood are incompletely reactive upon the pentosans, and the resulting raw pulp, which contains a substantial residue of'pentosans is only partially freed from this impurity by alkaline liquors which, too, are only partly effective in their reaction thereupon.

I have found that if wood or kindred raw cellulosic material is treated or cooked in a liquor containing substantially only acid such as sulphurous and/or sulphuric, and is then cooked in an alkaline liquor containing comparatively strong alkalies such as caus-.

tic soda and/or sodium sulphide, it is possible to produce a final product of an alpha cellulose content of about 96% or greater and of a pentosan content of less than 1%. Such a product may be produced in a. form- Application filed liarch 14, 1930. Serial No. 435,962.

which is readily blcachable to high whiteness with a low bleach consumption; In bleached or unbleached condition, it may, if desired, be characterized by an exceedingly low solution viscosity, for instance, one which ranges from 0.2 to 1.0, as against a value of from 40 to 200 in the case of raw sulphite or kraft pulp. This low solution viscosity renders the product suitable for conversion into cellulose derivatives without necessitating one or more special treatments such as have heretofore been employed in lowering the solution viscosity of the cellulose fiber or the derivative prepared therefrom. Apparently the use of substantially only acid in the initial treatment results in an eil'ective hydrolysis cf'the pentosans into soluble sugars or other reaction products which are readily soluble 'in alkaline cooking liquors, but the initial treatment or cook need not be carried out to complete fiber liberation, even when thisis possible, as when sulphurous acid solutions are used, in order-to avoid chemical injury to the wood fibers such as takes place when they undergo a high temperature treatment in the form of pulp in a strongly acid liquor. Evidently the initial acid treatment transforms the alpha cellulose to a form profoundly different from that .present in ,wood pulp such as kraft or sulphite or raw cotton, for not only is the finalproduct of surprisingly low solution viscosity when tested by the usual cuprammonium cellulose solution standard, but other derivatives, such as the nitrocelluloses prepared therefrom,

have even lower solution viscosity than simi lar derivatives prepared from other fibers whose cuprammonium cellulose solution viscosity has been lowered by one or more special treatments to the same value as that possessed by a cuprammonium cellulose solution prepared therefrom.

ployed, however, are" comparatively strong or highly ionized in aqueous solution, and to this end mineral acids such as sulphuric, sulthe treatment may be carried out under atmospheric pressure conditions at temperatures of, say, from 20 to 40 (1., but the use of dilute acid solutions of, say, an acid content of about 1% makes necessary the use of much higher temperatures, which. may be attained by cooking the wood while confined in a digester under super-atmospheric pressure conditions. In any event, however, the initial treatment is preferably carried out under such conditions as not to effect fiber liberation, and when a sulphurous acid solution is emplo ed, it should be substantially free from acid sulphites which react upon the ligneous substance of the wood and tend to effect fiber liberation. It is possible to use an acid liquor containing, say, 1% free S0 and 2% to 4% sodium sulphate in the first step. When such a liquor is used, it is best to go to a temperature of 300 to325 F. If desired, however, acid salts such as sodium bis'ulphate may be added to a solution of sulphurous or sulphuric acid, as the acid constituent of such salt increases the acidity of the solution, while the salt constituent re; tards ionization of the sulphuric acid, which may be desirable, particularly when the treatment is carried out at elevated temperature. When comparatively involatile acids such as sulphuric are employed, the treatment ma be advantageously carried out in 0 en ta under atmospheric pressure, but t e. use of sulphurous acid solutions at elevated temperature requires that closed digesters be used for the treatment, in order to prevent the escape of sulphur dioxide from the solution.

The acid liquor associated with the wood after the treatment is a dark brown color, as a result of the solution of organic matter and the hydrolysis of pentosans present in the wood to soluble sugars. At this stage, the wood is lower in pentosans than is the raw wood, and the acid liquor contains appreciable amounts of reducin sugars which may have been formed by decomposition of pentosans. When a strong sulphuric or other acid solution is initially employed, it may be drained or otherwise separated from the wood and employed wholly or in part for the treatment of other raw wood, but if too dilute or contaminated for reuse, its sulphur content, in the case of sulphurous or sulphuric acid solutions, may be recovered by mixing it with the spent alkaline liquor recovered tives.

from the second cooking stepnlliefore the latter is subjected to the wellown recovery treatments practised in the kraft process. The acid-treated wood is of much darker color than the raw wood, usually being of a reddish hue. While the wood may be readily crushed at this stage into fragments, nevertheless it is so hi h in ligneous matter that it does not lend itself to ready disintegration into a pulp suitable for papermak ing purposes or for conversion into deriva- The wood is referably washed free of residual s nt acid liquor before it is subjected to coo 'n in an alkaline liquor which preferably conslsts of a solution of sodium hydroxide and sodium sulphide similar to that used in the kraft process. While it is possible to use a solution containing only sodium hydroxide, nevertheless it is preferable to use one containin additionallysodium sulphide, as the resu ting pulp has better physical characteristics and is more easily bleachable. It is necessary, however, to use a liquor of greater alkalinity than that employed in the usual refining processes, wherein a raw pulp is subjected to the action of an alkaline liquor, as the acid-treated wood contains a high proportion of ligneous matter, in addition to other non-alpha cellulose components, which must be reacted upon and removed by the alkali to effect fiber li eration and to produce a product of the desired high alpha cellulose content and freedom from pentosans.

While a great variet of procedures falling within the purview o the present invention are possible, only a typical example and cer tain modifications thereof will be described, as the present application is intended to cover the resulting product rather than the processes which are described and claimed in other atent applications.

pruce or other wood chips and an acid cooking liquor containin 5% free S0 are charged in the. usual aci digester, and the digester closed. The contents of the digester are heated to a temperature of from 225 F. to 275 F. in about three hours and are maintained at this temperature for two or three hours while the pressure above the solution is regulated by the usual relief valve so as to avoid a pressure of above 75 pounds. At the end of the cooking operation, the sulphur dioxide may be released from the digester and recovered, or the digester contents may be blown into the usual blowpit and the liberated sulphur dioxide may be recovered in a suitable system. The acid-cooked wood is still in chipped form, and while somewhat softer than the original wood, cannot readily be reduced to a pulp. It is preferable to wash the wood free of residual spent acid liquir so as to avoid consumption of alkali. The acidcooked chips are then cooked in a liquor containing about 1.5 pounds of alkali per cubic preferable to have sodium. sulphide fpresent,

because of the easier bleachability 0 the resulting pulp. The sulphidity of the alkaline liquor may amount to about 40%, which corresponds to the sulphidity of a typical kraft liquor. The alkaline cooking operation may be carried out in the usual alkaline digester and the digester contents may be heated to a maximum temperature of from 300 to 335 F. in two hours, after which the digester contents may be maintained at this temperature with the corresponding steam pressure of about 100 pounds for about two hours. At the end of the cooking operation, the wood has been converted into a pulp, so that the digest-er may be blown as usual, and the pulp Washed free of spent liquor. The pulp at this stage has an alpha cellulose content of about 96% to 98%, a pentosan content of from 0.3 to 1, and a viscosity of 0.5 to 10. \Vhen the alkaline liquor employed to effect fiber liberation corresponds in sulphidity to the typical kraft liquor, the pulp is bleachable and may be converted to a product of high whiteness by using a bleach liquor containing only about 5% hypochlorite bleach, based on the weight of pulp. In order to preserve the high alpha I cellulose content of the pulp during the bleaching operation, it is preferably to effect bleaching in a hypochlorite bleach liquor containing free alkali such as caustic soda .and to maintain such liquor at a temperature of chlorite bleach, other bleaching agents, such as permanganates, peroxides, perborates, or the like, may be employed.

A typical unbleached product produced as hereinbefore described has the following physical characteristics and composition as compared with the usual commercial unbleached sulphite and kraft pulps:

Unbleached Un- Un product of bleached bleached present krait sulphite invention pulp pulp Strength 90 to 130 150 120 Tear resistance 250 to 400 250 160 Alpha cellulose-(75) 94 to 98 88 to 89 84 to 86 Pentosans 0.2 to 1.0 8 to 11 3 to 5 Solution viscosity 0.5 to 10 40 to 200 40 to 200 The term solution viscosity," as herein applied to cellulose fiber, is an arbitrary one, being indicative of the v1s cosity of a cellulose derivative solution producible therefrom. The solution usually employed as a standard is a cuprammonium cellulose solution of definite cellulose concentration. the viscosity being determined by measuring the time of efflux of a definite volume of such solutions under standard conditions through an orifice of standard size.

. The solution viscosity of fiber is herein given in absolute c. ,z. s. units. and is determined by measuring the vis- ('nsity of a solution of 6 grams of dry fiber in a cuprammonium solution composed of 225 cc. of 28% ammonia water containing 9 grams of cuprous oxide. The c. g. s. unit is employed because it is definite, denoting a viscosity 100 times that of water at 20 C., Wherefore a cuprammonium cellulose solution prepared from a certain type of fiber an'd by means of which such. fiber is identified as having a solution viscosity of 10, is 1000 times as viscous as water at 20 C Glycerin which is often referred to as a standard when :dealing with viscosity, has a viscosity value of between 8 and 10 units.

. A typical bleached product produced as hereinbefore' described has the following physical characteristics and composition as compared with theusual bleached sulphite and typical refined wood pulps It is thus seen that pulps produced as herein described difi'er greatly from both sulphite pulp and kraft pulp in significant physical characteristics and in composition. Kraft pulp has lower tear resistance and is of lower alpha cellulose and much higher pentosan content. Sulphite pulp is much inferior in its physical characteristics, especially in its tear resistance, and has a distinctly lower alpha cellulose and higher pentosan content. Both sulphite and kraft pulp have a very much higher solution viscosity. Pulp produced as herein described is eminently suitable for use in the manufacture of high grade papers and in the preparation of various'cellulose derivatives, particularly the acetates, Where its substantial freedom from pentosans makes possible the production of cellulose acetate solutions of clarity and sparkle. between to 120 F. In lieu of hypo- In some cases, it may be preferable to treat the pulp chemically, before bleaching, in order to ensure a product of high whiteness and at the same time to preserve or to increase the high alpha cellulose content realized in previous processing. For instance, the unbleached pulp may be treated for four hours and at room temperature as a 1% stock suspension in chlorine water containing from 1% to 4% chlorine, based on the weight of dry fiber, whereupon the pulp may be washed free of chlorine and reaction products and stantially equivalent to the use of a considerably weaker solution at much higher temperature. After the treatment with an alkaline liquor, the stock may be washed free of such liquor and bleached to high whiteness with a very small amount of bleaching agent, for instance, a hypochlorite liquor containing only about 1% to 2% hyporchlorite, based on the weight of the dry pulp. Preferably, a small amount of free alkali such as caustic soda is added to the bleach liquor to reserve maximum alpha cellulose content in t e pulp.

In lieu of using a sulphurous acid solution in the initial treatment of the wood chips, an acid liquor containing 5% free S and from 1% to 2% sodium sulphate may be employed, or one containing free S0 and 0.5% sulphuric acid, in both of which cases substan tially the same cooking conditions previously given may be emplo ed. Or an acid liquor containing 1% free S 2 and 4% sodium sulphate may be employed, in which case cooking may be effected at 300 to 325 F. An

acid liquor containing 5% free S0 and from 0.1% to 0.2% combined SO in the form of sodium sulphite may be employed, but with disadvantage, as it is necessary to cook at a relatively low temperature for an excessively long period of time, say, at 225% F. for from twenty-four to thirty-six hours, at the end of which time a mass of pulp of fairly low pentosan content, say 1.5% to 2.0%, and having good papermaking characteristics is obtained. This last treatment differs from all the others in which the initially treated clips remain in chip form. If paper making characteristics are not sought, the cooking operation may be carried out at a higher temperature for a shorter period of time, under which conditions the strongly acid liquor attacks the liberated cellulose fibers and so injures them that the resulting pulp is of low tear resistance. The raw pulp obtained by such a cooking operation is of low pentosan content but not high in alpha cellulose content, which may amount to, say, 85% to 86%. When such raw pulp is cooked in alkaline liquors, for instance the usual kraft liquor, the prodnet is high in alpha cellulose and low in pentosan content. The cook may be carried out at above 100 C. and under super-atmos pheric pressure, but the desired purification may also be effected in alkaline liquor at from to 50 C., if the liquor is of suitable alka-- linity, say, containing 5% to 8% caustic soda or caustic soda equivalent in the form both of caustic soda and sodium sulphide. It may be desirable to treat the raw pulp with chlorine water or hypochlorite liquor before treating with the alkaline liquor, to dissolve ligneous matter present in the raw pulp or render suchmatter more easily soluble by the alkaline liquor. The pulp may be bleached to white; ness, as already described, after treatment with the alkaline liquor.

The pentosan determinations herein iven are based on the formation of fur iural (CJLOCHO or C H O by means of an acid hydrolysis of the pulp, the. distillation and recovery of the fur ural, and finally the determination of the amount of furfural formed (from a definite weight of dry pulp) by means of a condensation reaction of the same with phloroglucinol (C H CH) or C H O forming furfural phloroglucide (C H O an insoluble precipitate. The precipitate is filtered off, washed, dried, and weighed. The weight is calculated to pentosan by multiplying the vain! by 0.7852. The pentosan value is then reported as a percent. based on the original bone-dry pulp. The synopsis of the method which I employ is as follows. Add to a distilling flask fitted with a condenser, 2 grams of bone-dried shredded pulp and 100 cc. of 12% hydrochloric acid (sp. gr. 1.06). Distill slowly (rate of cc. per 10 minutes) into a graduated cylinder. As each 10-cc. portion comes over, add 30 cc. acid. Continue the distillation until 270 cc. of the distillate has come over. Dissolve the proper amount of phloroglucinol' (0.16 to 0.54 gram, depending upon the expected pentosan value of the pulp being tested) in cc. hydrochloric acid. Add the furfural containing distillate to the phloroglucinol solution and dilute the mixture with hydrochloric acid up to a volume of 400 cc. Digest at 8085 for two hours. Filter oil the precipitate of furfural phloroglucide in a tared Gooch crucible and wash it with 150 cc. of distilled water at.room temperature. Dry the crucible and contents for four hoursat 100 C., and weigh. VVci'ght of precipitate X 39.26 pentosans.

Using the method outlined in testing normal samples of various pulps, the following values might be expected:

Cotton 0.5

Unbleached sulphite pulp 4.0

Bleached sulphite pulp 3.5

Kraft 8.0

Groundwood 9.5

I claim:

1. A thoroughly bleached or whitened chemical wood pulp having an alpha cellulose content of above 96%, a pentosan content of less than 1%, and a tear resistance greater than that of unbleached kraft pu p derived from similar wood and subjected to the same degree of heating or hydration.

2. A thoroughly bleached or whitened chemical wood pulp having an alpha cellulose content of above 96%, a pentosan content of less than 1%, a tear resistance greater than that of unbleached kraft pulp derived from similar wood and subjected to the same degree of heating or hydration, and a solution viscosity markedly lower than that of a thoroughly bleached sulphite pulp derived from similar wood.

In testimony whereof I have allixcd my signature.

GEORGE A. RICHTER.