US1860432A - Process of lowering the solution viscosity of cellulose fiber - Google Patents

Description

Patented May 31, 1932 UNITED STATES PATENT OFFICE GEORGE A.'RICH'IEB, mm, NEW HAIPBHIBE, ASSIGNOB 'IO BROWN OOHIANY, 0F

BERLIN, m HAIPBHIBE, A CORPORATION 01 PROCESS OF LOWEBIDTG 'IHI SOLUTION VISCOSITY 01 No Drawing. Application fled August 38,

In the manufacture of various cellulose products such as artificial silk, lacquers, and fibers, it is the practice to convert cellulose fiber'into a derivative such as xanthate, n1- trate, or acetate, and then to dissolve the derivative in a suitable solvent or mixture of solvents before forming into the ultimate product. In order to produce solutions of a viscosity sufiiciently low to permit spmmng into silk filaments, or use as lac uers, or forming into films, it is necessary eit or to depolymerize to lower the solution viscosity of the fiber used as a raw material, or to treat the derivative prepared therefrom in a manner to lower its solution viscosity. In the viscoserayon industry, for example, depolymerization of the fiber is effected by soaking sheets of fiber with a caustic soda solution of mercerizing strength, pressing the sheets free of excess solution and disintegrating them into a fluffy mass, and finally ageing the mass for a considerable eriod of time. Not only is such a process time-consuming and expensive,- but the ageing conditions must be carefully controlled to prevent degradation of the fiber into oxycelluloses and other undesirable reaction products. When the derivative is treated to lower its solution viscosity, as nitrocellulose sometimes is, under heat and pressure, not only must it be handled in comparatively small batches, but the treatment must cellulose fiber is an arbitrary one,

- herein given in also be carefully controlled to prevent decomposition from taking place.

The term solution viscosity as applied to being indicative of the viscosity of a cellulose derivative solution preparable therefrom. The solution employed as a standard is a cuprammonium cellulose solution of prescribed cellulose concentration, the viscosity being dedetermined by measuring the viscosity of a definite volume of such solution under standard conditions, through an orifice of standard size. The solution viscosity of fiber is absolute O. G. S. units, and is determined by measurin the viscosity of .a solution of 6 grams of fi r 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 mum:

OELLULOSE mean 192:. Serial a. scam.

because it is definite, denoting a viscosity 100 times that of water at 20 0., wherefore a cuprammonlum cellulose solution of standard composition identifying a fiber as having a solution viscosity of 10 is 1000 times as yiscous as water at C. Gylcerine, which is often referred to when dealing with viscoslty, for example, has a value of between 8 and 10 units. I One of the objects of the present invention s to provide a process for markedly and rapidly lowering the solution viscosity of cellulose fiber while it is in the form of an aqueous pulp of, say, about 2% to 10% fiber content, as this permits easy and continuous handlmg of large amounts of fiber by the use of comparatively inexpensive apparatus. Another object is to effect a lowering of the solutlon vlscosity of cellulose fiber substantially without affecting its alpha cellulose content so that the product is unimpaired in value for use as a raw material for the preparation of high grade derivatives. Still another object is to effect such solution viscosity lowering of cellulose fiber without impairing the capacity of such fiber undergoing hydration or gelatmization when beaten (when such impairment is undesirable), so that the resulting fiber may be successfully sheeted on paper machines into tissues or waterleaf papers suitable for nitration, acetylation, or other chemical conversion.

I have discovered that these objects may be attained by oxygenating cellulose fiber while suspended in an alkaline aqueous solution under a pressure of oxygenating gas materially greater than the vapor pressure of the solution. As a matter of ractice, the alkaline aqueous suspension of ber may be confined in a digester in contact with an atmosphere of oxygenating gas such as air or oxygen under the desired pressure. Preferably, the suspension is agitated during the treatment, as this ex oses fresh surfaces of fiber to the action 0 the gas and tends to produce a more rapid and uniform action. Apparently the comparatively high concentration of oxygen in the gas phase above the suspension brings about the desired results, for in the course of experiments leading up .sus nsion at elevated uni:

cannot obtain a product havin to the present discove it was found impossible to obtain a mar ed lowering of solution viscosity of the fiber by'digesting the temperatures in open or by digesting in closed digesters under steam pressure. Even when air or oxy-' gen is bubbled through the suspension in o 4 n tanks, reduction in the solution viscosity the fiber takes place so slowly that one a solution viscosit below 1, which is especially desired when t. e fiber is used as a raw material in the manufacture of cellulose products of the character described, in a reasonable and practical period of time.

In carrying out the process of the present invention, it is preferable to raise the temperature of the suspension, as this accelerates the reaction, but in so doing care must be exercised not to exceed the temperature at which the alpha cellulose content of thefiber is'lowered and the ield of product seriously diminished. Meta compounds, such as those of iron, nickel, cobalt, manganese and vanadium, when introduced into the alkaline solution, also tend to accelerate the reaction.

Cellulose fibers of various orlizgins may be employed as a raw material. or example, one may start with white wood fiber refined to an alpha cellulose content of about 94% and having a solution viscosity of about to 9, with cotton fiber as long staple, cotton linters, or rag pulp, and having a solution viscosity of 5 to 50, or with chemical Wood pulp such as bleached sulphite pulp hav1n an alpha cellulose content of about 86% an a solution viscosity of about 5 to 7. One may also emplo incom letely refined wood ulps bleached dried in bulk form,

such as un refined sulphite pu p or raw pulps such as unbleached unrefined sulphite or even kraft pulp.

A better appreciation of the process of the resent invention may be gained from the ollowing specific example. A refinedwood pulp having an alpha cellulose content of 94.5% and a solution viscosity of 6 was employed as a raw material. The pulp was mixed with a 1% caustic soda solution to produce a 4% pulp suspension, which was confined in a di ester under an air pressure of 100 unds. Vhile being maintained under the pressure, the suspension was subjected to agitation and its temperature raised to 130 F. At the end of a four-hour treat ment under these conditions, it was noted that the solution viscosity of the fiber had been lowered to about 0.6, while its alpha cellulose content had remained substantially unchan ed. The product may be washed free of al aline solution, after which it may be orbeaten and sheeted on a aper machine.

The foregoing example is illustrative of a rather wide range of conditions under which the treatment may be carried out, depending necessary in order upon the character of the raw material, the product which it is desired to produce, and the time in which it is desired to produce the product. For instance, the tein rature of treatment may be lower, say 100 but with lower temperature a longer treatment is 0 to attain the desired solut on viscosity of fiber. It is preferable to carry out the treatment below 150 F., in the case of a wood pulp of hi h alpha-cellulose content such as herein be ore described, for should this tem rature be exceeded, a dro in the yield an sometimes-in the al ha ce lulose content of the product is like to result. Thus, when a treatment was carried out substantially according to the example hereinbefore described, but at a temperature of 200 F., it was found that a reduction of the solution viscosity of the fiber to about 0.6 resulted in a product of an alpha cellulose content of about 92%. The air ressure under which the suspension is con ned may be as low as about 20 pounds, but a longer treatment is necessary. in order to attain the desired viscositiy. Oxy en as, when economically availab e, may e a vantageously used in place of air, as the desired solution viscosity of the fiber may be attained in a shorter period of time. The alkalinity of the solution and the density of the ul suspension ma be varied, but too ig a pulp density s ould be avoided so as to permit adequate contact of oxygen and fiber durin agitation. When mercerization of the fi r is not objectionable, as when the product is to be converted in bulk form into the xanthate the fibermay be suspended in solutions 0 mercerizing alkalinity, in which case the treatment may be effected at com aratively low temperature, say, 40 to to produce a product of a givenlow solution viscosity in a certain period of time. Caustic potash or other alkalies may be employed in place of caustic soda, but with weaker alalies such as sodium carbonate, tri-sodium phosphate, and lime, a longer treatment is necessary in order to effect a given viscosity lowering. I

The resulting product may have an alpha cellulose content higher than that of the original stock. Thus, if stocks, such as sulphite pulp are employed, it is likely that the fiber will undergo an increase in alpha cellulose content, owing to the solution of nonalpha cellulose com onents such as ligneous matter and beta an gamma celluloses from the pulp. The product may have'a solution viscosity of from 0.3 to 1.0, depending upon the condition of treatment, and when mercerized is especially suitable for conversion into viscose, as no ageing is required to depolymerize a soda cellulose prepared therefrom. The product also has a lower copper number and alower soda solubility (i. e., content of material dissolved by boiling in a 7% pur oses,

caustic soda solution for one hour), showin that the fiber, rather than being degrade is improved in its characteristics for conversion into cellulose derivatives. Thus, in the specific example hereinbefore given, the refined wood pulp used as a raw material had a copper number of 1.5 and a soda solubility of-9, and resulted in a product having, a copper number of 1.0 and a soda solubility of 5.

If the efl'ect of the treatment is accelerated by introducing compounds of iron into the alkaline solutlon in which the fiber is suspended, or if the treatment is carried out in iron receptacles, the fiber will be stained by iron compounds. If the fiber stained, as when the treatment is carried out at comparatively low temperatures in iron receptacles, treatment of the fiber with a dilute solution of sulphurous acid will restore the original "color of the fiber, but if more marke stained, the original color may be restored by digesting for a compara tivcily short time in a weak solution of oxalic aci When an unbleached wood fiber is .employed as a raw material, the bleaching of suc fiber may be advantageously combine with a treatment such as herein described. For instance, assuming unbleached, refined wood fiber, which requires a comparatively small amount of bleach to be whitened, to be the raw material, such fiber may be treate as a 4% to 8% pulp suspension in, say, a 1% caustic soda solution containing, say, about 2% bleach based on dry weight of fiber, the treatment being carried out at, or slightly above, room temperature, for about six to ten hours. The bleach is substantially consumed in this treatment, but the whitened stock remains in a distinctly alkaline condition, and when raised to a temperature of about 130 F. and subjected to an air pressure of about 100 pounds for about four to six hours, resulted in a product having a low solution viscosity, such as .hereinbefore described. One of the advantages of a treatment thus carried out is that the caustic soda serves two namely, that of maintaining the stoc in a. distinctly alkaline condition duringthe bleaching operation, and so avoiding the formation of oxycelluloses and other degradation products, and also that of maintaining the stock in the desired alkaline condition during the subsequent treatment for reducing its solution viscosity.

If a product of exceedingly low solution viscosity, say. 0.1, or lower, is desired, a treatment such as herein described may be advantageously combined with a process such as vdescribed in patent application Serial No. 132,319, filed August 28, 1926. According vto the} process, cellulose fiber is first depolymerize by first treating such fiber as in the j form of sheets with a-caustic soda solution of mercerizihg strength,

' The is but slightly d tent of about to 93% then pressing; free of excess solution, and allowing the fiber;(the so-called soda cellulose) to age until the desired depolymerizationand reduction in solution viscosity of fiber has taken place.

desira le water-soluble constituents therefrom, and preferably then. di sted in an alkaline solution to, remove un esirable water-insoluble constituents such as oxycelluloses. The process of that patent application may, for instance, be applied to convert a ,refined wood fiber having an alpha" cellulose content of, say, about 94% and a solution viscosity of about 6 into a final product having an alpha cellulose content of about 95 to 96% and a solution viscosity of about 0.4 to 0.5, but should ageing be efiected' under conditions resulting in a product having a solution viscosit as ow as .1, the al ha ce lulose content an yield of the pr not be seriously decreased. By further treatin the fiber oi low solution viscosit produce in accordance with the rocess t erein described, in accordance wit my present process these undesirable results may be avoi ed. For instance, a fiber having an are likely to d alpha cellulose content of about to 96% and a solution viscosity of about 0.4 to 0.5, when treated with 1% NaGEi solution at 30 F. and at 100 pounds air pressure, results in a final product having an alpha cellulose conand a solution viscosity oi about 0.1 or lower. Such a product is of advantage, for instance, when it is de sired to inoreasethe cellulose concentration in a viscose solution without increasingthe caustic soda required to maintain the cellulose in solution, thereby lowering the caustic soda consumption and the acid consumption it to use a 6.5%

in the filament setting baths. For instance,

whereas it may be necessa caustic soda solution tomaintain 7% cellulose in solution in usual practice, when a fiber having a solution viscosity ployed as a raw material, it may not necessar to use more caustic soda to keep in the or er of magnitude of 15% cellulose in solu-* tion.

There may be other changes or modifications in the process which will suggest themselves to those skilled in the art, without departing from the spirit or see e of invention as defined in the appende claims. I WhatIclaimis:-

1. A process? which comprises oxygenating a sus nsion of cellulose fiber in the-physical condition of pre-liberated wood pulp m an alkaline aqueous solution under a pressure oxygenating gas materially greater than t e vapor pressure of the so ution.

2. A process which 'com a sus nsion of cellulose fi r in the physical alkaline aqueous solution under a pressure of aged fiber is then washed to remove un-' hereinbefore described,

1 Ice of, say, .1 is emrises oxygenating;

dilute,

oxygenating gas materially greater than the vapor pressure of the solution while said suspension is being agitated.

3. A process which comprises oxygenating a suspension of cellulose condition of pre-liberated wood pulp in a heated alkaline aqueous solution under a pressure of oxygenating gas materially greater than the vapor pressure of the solution.

4. A process which comprises oxygenatin a suspension of cellulose fiber in the physica condition of pro-liberated wood pulp in an alkaline aqueous solution at elevated temperature under a pressure of oxygenating gas materially greater than the vapor pressure of the solution while said suspension is being agitated.

5. A process which comprises confinin a suspension of cellulose fiber in the physical condition of pre-liberated wood pulp in an alkaline solution in contact with an atmosphere of oxygenating gas under pressure until the solution viscosity of such fiber has been reduced to below 1.

6. A process which comprises confining a suspension of cellulose fiber in the physical condition of pre-liberated wood pulp in a caustic soda solution in contact with air under a pressure of about 20 to 100 pounds.

7. A process which comprises confinin a suspension of cellulose fiber in the physical condition of pre-liberated wood pulp in a heated, caustic soda solution in-contact with an atmosphere of oxygenating gas under pressure.

8. A process which comprises confining a suspension of cellulose fiber in the physical condition of pro-liberated wood pulp in a dilute caustic soda solution at temperatures below about 150 F in contact with an atmosphere of oxygenating gas under pressure.

9. A process which comprises confining cellulose fiber in the physical condition of pro-liberated wood pulp suspended in an al- 'aline solution containing metal compounds in contact with an atmosphere of oxygenating gas under pressure.

10. A process which comprises confining cellulose fiber suspended in an alkaline solution containing iron compounds in contact with an atmosphere of oxygenating gas under pressure.

11. A process which comprises confining cellulose fiber in the physical condition of Ere-liberated wood pulp suspended in an alaline solution containing iron compounds in contact with an atmosphere of oxygenating gas under pressure, and removing the iron stain from the resulting product.

12. A process which comprises confining .cellulose fiber in the physical condition of Ere-liberated wood pulp suspended in an alaline solution containing iron compounds in contact with an atmosphere of oxygenfiber in the physical 7 ating gas under pressure, and treating with oxalic acid solution to remove the iron stain from the resultin product.

13. A process w ich comprises suspending cellulose fiber in the hysical condition of pre-liberated wood pu p in an alkaline solution containing bleach, and when the bleach has been substantially consumed subjecting the: pulp suspension to the action of oxygenating gas under pressure.

14. A process which comprises treating unbleached pulp with a bleach liquor containing sufficient alkali to maintain a condition of distinct alkalinity throughout the bleaching operation, and after the bleach has been substantially consumed raising the temperature of the pulp suspension and subjecting it to the action of oxygenating gas under pressure.

15. A process which comprises treating cellulose fiber with a caustic soda solution, of mercerizing strength, ageing the treated fiber to effect a depolymerization thereof, washing the fiber, and oxygenating the product while suspended in an alkaline aqueous solution un er a pressure of oxygenating gas materially greater than the vapor pressure of the solution.

16. A process which comprises treating cellulose fiber with a caustic soda solution of mercerizing strength, ageing the treated fiber to e the fiber, digesting the fiber in an alkaline solution, ant oxygenating the product while sus ended in an alkaline aqueous solution un er a pressure of oxygenating gas materially greater than the vapor pressure of the solution. a

17. A process which comprises treating cellulose fiber with a caustic soda solution of mercerizing strength, ageing the treated fiber to effect a de olymerization thereof, washing the fiber, an oxygenating the product while suspended in a heated, alkaline aqueous solution under a pressure of oxygenating gas materially greater than the vapor pressure of the solution.

18. A process which comprises oxygenating a suspension of cellulose fiber in the physical condition of pre-liberated wood pulp in an alkaline aqueous solution under a pressure of oxygenating gas materially greater than the vapor pressure of the solution while maintaining the suspension in agitation during such oxygenation.

19. A process which comprises oxygenating a suspension of cellulose fiber in a mercerizing solution under a pressure of oxygenating gas greater than the vapor pressure of the solution.

20. A process which comprises suspending an unbleached cellulose pulp in a distinctly alkaline solution containing sufficient bleachto whiten the pulp, and after the bleach has been substantially consumed oxygenating the suspension under a pressure of oxygenattfect a depolymerization thereof, washing signature.

GEORGE A. RICHTER.