Action of Light on Cellulose APPLICATION OF RALPH E. MONTONNA AND c. c. WINDING'
COPPER NUMBER DETERMINATIONS TO CELLULOSE ACETATE
University of Minnesota, Minneapolis, Minn.
A satisfactory method for determining the copper numbers of cellulose acetates has been developed. The results of determinations on various samples indicate a high degree of degradation in the lower-viscosity cellulose acetates. Light increases the copper number of cellulose acetate, exposed either in the solid phase or in solution. The presence of oxygen is not necessary in the atmosphere surrounding the exposed acetate, Water appears to be necessary to allow this degradation to take place.
OPPER numbers are commonly used for the qualitative determination of the amount of degradation of cellulose and cellulose products, but the merits of this determination are a subject of controversy. It is generally admitted that copper numbers must be regarded as only qualitative in significance and, even so, must all be made according to a rigid empirical method. In spite of its qualitative empirical status, this determination, in the hands of experts, is one of the most successful methods for discovering overbleached or overheated paper or fabrics. Work in this laboratory had shown that it was a convenient tool for following the action of light on cellulose; when additional methods of following changes in cellulose acetate were desired, a consideration of copper number suggested that this determination might be applied to cellulose acetate. The use of copper number determinations for cellulose acetate was mentioned as a possibility by Rinse (6) and Barthelemy (1) but neither gave detailed information as to method. The common methods of Schwalbe (7), Knecht and Thompson (4) and Braidy (8), as well as their various modifications, were unsatisfactory for this work because they require unnecessarily large samples. Heyes (3) published a method using a 0.25-gram sample, but the amount of solution is so small that it is applicable only up to copper numbers of 5. The method used here was developed by modifying a combination of the Braidy (a) and Heyes (3) determinations, employing a microsample of 0.25 gram; it is capable of giving copper numbers up to 50. The permanganate method is used for the cuprous copper determination, but the electrolytic method may be employed by those who favor it without changing the essentials of the procedure.
C
COPPER NUMBER DETERMINATION
One of the most important .parts of a copper number determination is the preparation of the sample. Not only must i t
be representative, but successive samples should have a uniform particle size and the individual particles should be as small as possible to facilitate the reaction between a solid and a solution. The best method of fulfilling these conditions is a rapid precipitation from solution into a nonaqueous precipitating agent. Approximately 0.6 to 0.7 gram of cellulose acetate is put into a solution in acetone. This amount is sufficient for duplicate determinations as each sample weighs 0.25 gram. Approximately three times the solution volume of isopropyl ether is brought to a boil on the steam bath, and the cellulose acetate solution is added slowly with stirring. The acetone flashes off almost instantly. The remaining suspension is concentrated to a volume of about 50 cc. The last of the isopropyl ether is removed in a slow current of warm air (35" to 40" C.). This procedure gives a finely divided dry sample which is stored in a desiccator over PzOs for 48 hours before analysis. The acetate is left unchanged by this procedure but must be handled carefully during weighing to prevent it from becoming electrically charged and consequently sticking to the walls of the weighing bottle. The solutions are those recommended by Braidy (8): 1. 180 grams sodium carbonate (anhydrous) and 50 grams sodium bicarbonate per liter of water. 2. 100 grams crystalline copper sulfate per liter of solution 3. 40 grams ferric sulfate and 100 cc. concentrated sulfuric acid per liter of water. 4. 0.04 N potassium permanganate.
Approximately 0.25 gram of cellulose acetate is weighed out and placed in a 150-cc. suction flask. Ninety-five cubic centimeters of solution 1 are quickly heated t o boiling, and 5 cc. of solution 2 are added. About 65 cc. of this solution are poured over the cellulose acetate, and the flask is put into a boiling water bath. A reflux condenser is attached, and nitrogen is blown slowly through the side arm of the flask during the entire 3 hours of heating. At the end of the first hour the remaining 30 cc. of the reaction mixture are poured down the condenser. This serves to wash down any fine particles that may tend to creep up the sides of the flask. After cooling rapidly, the solution is filtered off from the precipitated cuprous oxide and cellulose acetate by a sintered glass crucible. Care should be taken not to expose the cuprous oxide to the air unnecessarily. The crucible is transferred to a small suction flask, and the Erlenmeyer flask washed out with 15 CC. of solution 3; this solution is poured over the precipitate on the filter without applying suction, allowed t o remain for a few minutes, and finally removed by the application of vacuum. The same procedure is repeated using 10 cc. of solution 3, followed by three or four washings with 4cc. portions of distilled water. The titration with permanganate is carried out without removing the solution from the suction flask. I
582
Present address, Cornel1 University, Ithscs,N. Y.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1943
The blank determination is very small and can be neglected for the relatively large copper numbers of cellulose acetate. Table I shows the data obtained by this method. The duplicate results indicate the accuracy to be expected with careful manipulation. OF CELLULOSE ACETATES TABLEI. COPPERNUMBERS
Material
Copper No.
Acetyl Content,
[q]
%
41.6 1.12 Medium-viscosity acetate (12A) 2.18-2.18-2.37 41.4 I .23 Medium-viscosity acetate (107) 2.GO-2.70 1.35 Tigh-viscosity acetate (101) 1.95-2.15-2.00 39.5 0.77 Low-viscosity acetate (50A) 5.20-5.00-4.96 42.2 46.1a Low-viscosity triacetate 6 .0&6.14-6.90 High-viscosity triacetate 0.52-0.55 44.8 Abiorbent cotton 0.15-0.15 0 O.GO-0.65 0 Paper pulp a This value. exceeding the theoretjcal value for cellulose triacetate, has been carefully checked. I n conjunction with the high copper number, it indicates t h a t this sample is a n acetylated, partially degraded qellulose. Whether the cellulose was degraded before acetylatlon or degradation took place during the acetylation process, is not known.
COPPER NUMBERS OF ACETATES
The copper number, as used in the cellulose industry, is defined as the number of grams of copper reduced by 100 grams of cellulose, when an arbitrary procedure is followed exactly. It is necessary to specify a definite method because the oxidation of cellulose is very sensitive to conditions such as concentration, time, temperature, and the subdivision of the sample. The same definition has been used in this work on cellulose acetate. It should be remembered, in comparing these results with those obtained on cellulose, that the weight of the structural unit of the acetate is from 75 to 80 per cent greater than cellulose itself, so that these copper numbers are in reality that much greater than the numerical values indicate. The strong, hot alkaline solution hydrolyzes off the acetate groups, leaving regenerated cellulose. Blank runs with additions of sodium acetate showed that the acetate group had no effect on this method of determining copper number. An alternative method might involve hydrolyzing off the acetate groups before running the copper number determination, except that the regenerated cellulose obtained by hydrolysis is usually a hard, horny substance that resists attack by copper number solutions. No evidence was noted of any discrepancies in this method because of the use of the cellulose acetate in place of cellulose itself. I n addition, further unpublished work performed in this laboratory has shown that mannitol hexa-acetate does not give a copper number. This evidence indicates that primary alcohol groups, even though acetylated, do not enter into this reaction. The copper number is inversely proportional to the viscosity classification of the various samples, and the large copper numbers of the low-viscosity acetates show a high degree of degradation. The extremely low copper number of carefully prepared triacetates indicates that the acetylation reaction itself does not necessarily cause degradation, but the subsequent hydrolysis to produce commercial or secondary cellulose acetate involves considerable degradation. I n several instances the viscosity characteristics of triacetates were accurately predicted before being experimentally determined. EFFECT OF LIGHT ON COPPER NUMBER
The loss of tensile strength, discoloration, and general deterioration of cellulose and its derivatives by light is well known, but to study and follow this degradation are difficult because of the high molecular weights involved. After the method of determining the copper number of cellulose acetate had been developed, it was decided to study the effect of light
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on the acetate and to attempt to follow the degradation by means of the copper number determination. Since the copper number should indicate any depolymerization of cellulose, it appeared that this might be an independent method of following the effect of light, because the copper number is indicative of the number of free potential aldehyde “end groups” on the long straight-chain cellulose acetate particles. This relation may not be direct because of the complicated crystalline structure of the solid and the peculiar course of oxidation of carbohydrates in alkaline solutions. The one fact that has been generally accepted is that a free aldehyde group must be present to allow oxidation to occur. How much further the oxidation will proceed is not known, but from independent determinations of the molecular weight it is evident that a greater degree of oxidation occurs than would be predicted by the oxidation of each aldehyde group to the corresponding carboxyl group (6). Therefore, if the action of light causes depolymerization without oxidation, the increase in free aldehyde groups should be revealed by an increase in copper number. All exposures were made in clear quartz flasks. A n aircooled Cooper-Hewitt mercury vapor lamp was surrounded by a double-walled enclosure through which water was circulated for additional cooling. The flasks Were inserted in holes cut in this enclosure so that a definite area could be exposed at a distance 12 inches from the lamp. I n the case of the exposure of solids, the atmosphere in the flask was controlled as desired. When solutions were exposed, an atmosphere of solvent vapor was maintained above the solution. Table I1 shows the effect of light on the copper numbers of cellulose acetates and related substances.
TABLE 11. EFFECTOF LIGHTON COPPERNUMBER OF VARIOUS MATERIALS -Copper Before exposure 2.0 2.7 2.0 2.0
No.-
Material Medium-viscosity cellulose acetate
Exposure, Days 63 42 34 30
High-viscosity cellulose acetate
33
Acetone solution
1.7
11.03
oellulose acetate
94
Acetone solution
5.0
19.4
Low-viscosity
a 6 C
Method of Exposure Acetone solution Acetone solution Acetic acid solution Films, Na atmosphere
After exposure 13.45 10.00 13.40 12.10
Not a true copper number. relative only.
Reduced all available copier in 5 minutes. Dried over PaOa for 48 hours.
The cellulose acetates were representative samples of commercial acetate as manufactured by two different companies; the pulp was LL high-grade commercial alpha-cellulose. I n view of the modern conception of the structure of cellulose and the fact that a free aldehyde group is necessary to give a copper number, an increase in copper number must indicate that the long straight chains of CJIl,06 groups have been broken down into shorter chains, or that some of the primary alcohol groups have been oxidized to produce more aldehyde groups. The possibility of the oxidation of primary alcohol groups was eliminated in this work by the use of purified nitrogen atmospheres as well as the fact that glucose shows no increase in copper number upon exposure. Therefore, depolymerization must take place to account for the large increase in copper number. This degradation might also cause the decrease noted in the relative viscosity of the acetate solutions on exposure as well as the loss of tensile strength of cellulose and its derivatives.
