RESEARCH

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Cotton Quality from W o o d Pulp? Borohydrîde reduction of the eîectrophîiic groups in w o o d pulp yields higher quali ty cellulose

JLHACTICAL conversion of wood pulp to a product that resembles cotton cel­lulose is a definite possibility, says Bengt G. Rânby of the State University College of Forestry at Syracuse Uni­versity. An important difference be­tween cotton and wood cellulose is the greater number of weak links in the latter, says Ranby. This would ac­count for the higher stability of cotton and the greater ease with which wood pulp hydrolyzes.

While both cotton pulp and wood pulp are used to make cellulose prod­ucts, such as rayon and acetate, cotton pulp is preferred for some premium products where the utmost s trength is required, such as sausage casings and some tire cord. Cotton pulp costs only about r>c,'< more than wood pulp , but the ability to upgrade wood pulp to cotton quality would have real signifi­cance.

Looking for the reason for weak links in wood pulp, Ranby, until recently at American Viscose, and R. H. Mar-chessault of Avisco, postulated that carboxyl, carbonyl, or other modified groups on the glucose ring exert an inductive effect on nearby linkages. Ranby tried to remove these groups by oxidation but couldn't do it. He then

tried to reduce them with borohydride and found that it removed most of t he aldehyde groups and cut the carboxyl content from 0.4 to 0.1 9^. This gave a product identical with cotton cellu­lose in stability against hydrolysis.

The present work is still in the lab­oratory stage and is not economically feasible because of the high price of borohydride. If the price of boro­hydr ide continues its downward trend, however, it won't be too main- years before its use in t reat ing wood pulp will offer real promise.

Ranby described this work at the Second Cellulose Conference of the Cellulose Research Inst i tute at the state forestry college at Syracuse. Some conferees expressed wonder at his using borohydride in an aqueous slurry such as pulp. Ranby pointed out that the reduction is done at p H 9 in a borate buffer. This was verified by the work of Ingvar Jullander of ^Mo Och Domsjo, Sweden. While sodium and potassium borohydrides are effec­tive reducing agents for carbonyl groups in cellulose pulps, says Jul­lander, most of the reagent is destroyed by the water. The piesence of lithiLim, magnesium, bar ium, or calcium salts cuts down this decomposition. Some

ions, such as aluminum, cadmium, and zinc, have an opposite effect, however.

• Weak Link. The term, "weak link," has been in disrepute, says Rânby, because it has, in the past, been re­lated to what is really a characteristic of crystallinity. To overcome this, Rânby treats the cellulose in homoge­neous solution. He predissolves pulp in cuprethylenediamine, precipitates to a cellulose gel (37c ) , and washes out all the ions. It is the gel form that is reduced. Then he dissolves the gel in 819r phosphoric acid. The result is a homogeneous solution.

The cellulose in fhese solutions shows the differences in reactivity character­istic of cotton and wood pulp. The difference, which Rânby ascribes to weak links, is measured oyer a period during which about 19c of all the bonds are hydrolyzed. Change in chain length of the cellulose is followed by viscosity and osmotic measurements After about 19c of the bonds are broken, the measurements become lesi precise and more difficult to interpret

The mechanism to explain weak link* is based on evidence that a carboxy group can stabilize the acetal bone in a disaccharide. This was describee by C . X . Richards of the British Rayor Research Association at the ACS meet ing in San Francisco last year. Rich aids interpreted the slower rate of hy­drolysis of an aldobiuronic acid as stem ming mainly from the inductive effec of the electrophilic carbonyl group ir the C-6 position:

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RESEARCH

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The more electronegative the group at C-6, the slower would be the rate of electron transfer at the acetal bond ( arrow ), which is the rate contr< g step in hydrolysis. Thus, a carbonyl or carboxyl group at C-6 stabilizes the bond against hydrolysis.

• Inductive Effect. Conversely, Rânby reasons, the inductive effect of a carboxyl group at C-6 would promote electron transfer at the preceding bond (Y) . Thus, while an electrophilic group at C-6 stabilizes one acetal bond, it weakens another. Rânby and Mar-chessault have found ample evidence for the inductive effects, both to weaken and to strengthen acetal bonds, in the chemical literature describing varying rates of hydrolysis with glycosides con­taining various substituants. Thus, methyl 3-deoxy-a:-D-glucoside and methyl 3-deoxy-a-D-mannoside hydro-lyze seven and five times as fast, respec­tively, as the normal glycosides. Also, β-phenyl-D-glucoside hydrolyzes nine times as fast as /5-methyl-D-glucoside.

In cellulose, the presence of "acti­vated" (weakened) links yields a higher initial rate of hydrolysis than for a polymer containing only normal link­ages. This rate decreases as these bonds are broken. When the average chain length gets small enough, the rate again increases, probably due to the activating effect of the terminal unit, which only becomes significant at a low degree of polymerization.

Branching of a polysaccharide should also cause inductive effects in hydroly­sis, both from the larger number of end groups and from the branch points. Wood contains such branched poly­saccharides as arabogalactans, xylans, and glucomannans, which all hydrolyze at a higher rate than cellulose.

There are, therefore, several causes for the greater predominance of weak links in wood pulp, which appears to be the principal difference between it and cotton pu lp . It is still not known to what extent the modified groups in wood pulps are native or introduced during the rather drastic treatments necessary to remove lignin and hemi-cellulose dur ing pulping.

SEA DISPOSAL. Solid wastes, incorporated iri concrete and sealed in these steel drums, are loaded aboard a Navy ship for transport to sea. The drums will be sunk at a depth of about 1000 fathoms—more than one mile deep

More Sec Studies NAS urges U. S. îo set up α long-range program to study radioactive waste disposal a t sea

J O M E positive statements on how to tackle the vast problems of radioactiv­ity in the seas are now being mulled over by many scientific leaders. In its latest report (No. 5) on oceanography, the National Academy of Sciences-Na­tional Research Council details two main aspects of this man-made prob­lem:

• Limit certain areas of the seas in which radioactive materials can be dumped.

• Keep the concentrations of these materials at "tolerable levels."

NAS goes on the books further to say that the U. S. should set up a long-range program in physics, chemistry, geology, and biology to study what is perl . ips the world's greatest disposal problem.

• One Agency. At present, several government agencies have the responsi­bility of protecting the public health

against the hazards of introducing ra­dioactive wastes into the seas. NAS shakes its head at this setup, believes

i l r l date this activity. And, it adds, that agency must have the authority to enforce its regulations. NAS's choice: the ever-expanding, multifunctional Atomic En­ergy Commission. The job of monitor­ing the seas, though, should go to either t he Coast and Geodetic Survey or the Public Health Service, NAS asserts.

• Research Needs. Many scientists believe that radioactive wastes should be dumped in only the deepest parts of the oceans. W e need to know, there­fore, how these materials, when in solu­tion, will disperse throughout the deep waters. Also, we must learn how much radioactivity will eventually enter the upper layers of the seas. Another im­portant fac4or: How much radioactiv­ity will enter sea organisms which are vital to man?

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