CELLULOSE ETHERS

Variation of Physical Properties with Composition

E. J. LORAND Hercules Powder Company, Wilmington, Del.

Physical properties of cellulose ethers, such as solubility in organic solvents, softening on heating, moisture absorp- tion, etc., depend on the following fac- tors : (1) nature of the ether group (alkyl, aralkyl, hydroxyalkyl, other substituted alkyl, etc.) ; (2) relative numbers of etheri- fied and free hydroxyls; (3) chain length (viscosity) of the product; (4) uniformity as regards length and degree of substitu- tion of the single chains. The influence s f factor 1 may be related to the size and polarity of the substituent. I n a homolo- gous series, moisture absorption and sof-

HE literature on cellulose ethers is not very rich in data on physical properties (3). Moreover, the available information is often useless, since the products are in-

sufficiently characterized as to their chemical composition, method of preparation, etc. Bulletins of the manufacturing companies deal only with the few particular types actually produced. The published material is insufficient for any systematic presentation of the physical properties of cellulose ethers in relation to their chemical composition. At most, it enables us to point out some of the chemical characteristics which are likely to influence the physical properties:

(1) Nature of the substituent group (alkyl, aralkyl, hydroxy- alkyl, other substituted alkyls, etc.).

(2) Relative numbers of etherified and free hydroxyl groups (degree of substitution).

(3) Average chain length (viscosity) of the product. (4) Uniformity of the product as regards length and degree

of substitution of the single chains.

A systematic investigation of the physical properties of cellulose ethers as a function of these factors is confronted with great difficulties. It involves, among other things, the preparation of whole series of compounds-e. g., with slight variations in substitution. These products must be of good uniformity and practically of the same average chain length. Those familiar with the preparation of cellulose ethers will realize what such a task involves.

In general, cellulose is treated in the presence of aqueous caustic soda or the like, with esters of inorganic acids, such as alkyl sulfates and halides, or with other etherifying agents, such as alkylene oxides and chlorohydrins. Many of the pre- ferred agents-e. g., the alkyl chlorides and ethylene oxides- are volatile so that the reaction, in most cases, must be carried

T

tening point decrease with growing num- bers of carbon atom, while solubility in nonpolar solvents increases. Some of the properties show maxima or minima at cer- tain values of factor 2. Maximum solu- bilityand lowest softening temperature are reached at about the same degree of sub- stitution. Other properties, such as mois- ture absorption, seem to decrease (or in- crease) continually with the substitution.

The importance of uniformity and the secondary influences due to viscosity are pointed out, and solubility in solvent mix- tures is discussed.

out in suitably constructed autoclaves. The preparation of benzylcellulose is one notable exception. As the reactions are of the heterogeneous type (4) , the mechanical construction of the apparatus (in particular, the type of agitation and a provision for speed variation) is of major importance. I n the composition of the reaction mixture the ratios of alkali to cellulose, alkali to water, and cellulose to etherifying agent are the determining factors for the main reaction; the side reac- tions, such as direct hydrolysis, depend on the ratio of water to etherifying agent. All these ratios change in the course of the reaction so that either adjustments have to be made or the initial composition of the reaction mixture must be such as to provide for compensation of these changes. Both the main and the side reactions are greatly influenced by the tem- perature of treatment, and the nature of the effect changes a t the different stages of the reaction. Appropriate heating schedules have a beneficial effect on the uniformity of the ethers.

It is obvious that the general method must be adapted to each case, depending on the specific nature of the cellulose ether to be prepared, the degree of etherification and viscosity desired, and the properties of the etherifying agent. How- ever, the actual procedures have been worked out in detail only for the production of the commercial brands, such as dibenzylcellulose, ethylcellulose of 47 to 48 per cent ethoxyl content, and methylcellulose with 1.4 to 1.6 methoxyl groups per glucose anhydride unit. Minor variations in the relative proportions of the ingredients (e. g., caustic soda, water, etc.) are sometimes sufficient to produce a uniform product with a slightly altered degree of substitution. In most cases, how- ever, the whole set of details, such as initial composition of the reaction mixture and external reaction conditions, must be

527

528 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 30, NO. 5

freshly worked out to obtain the desired uniform product. This is especially difficult in cases-e, g., in the preparation of low-substituted ethers-where the reaction remains hetero- geneous to the very end. Fractionation (by selective solvent or precipitation) may yield products of satisfactory homo- geneity, but the method must be worked out for each com- pound separately.

In other cases-for example, amylation and to some ex- tent butylation-the only effective reaction conditions that have been found are so strenuous that the ethers obtained are of very low viscosity. In such cases it is imperative that new or strongly modified methods of preparation be devised.

When comparing ethers with different substituent groups, or even ethers with identical substituents but of different de- gree of substitution on the basis of practically equal average chain length, the latter requirement calls for some practical test. In spite of the long-standing controversy over the question as to whether the molecular weight (chain length) of highly polymerized compounds can be determined by vis- cosity measurements, there is no doubt that specific viscosi- ties, determined in very dilute solutions prepared with good solvents, give a reliable measurement of the relative molecu- lar size. However, difficulties of interpretation arise when, as a consequence of different solubility characteristics, the solvent or solvent mixture may have to be changed from one product to the other.

These are some of the reasons why, in spite of the sorely felt need, a systematic study of the physical properties of cellulose ethers, as determined by chemical composition, has not been undertaken. Although the present paper is not the outcome of such a systematic investigation, it is an attempt to give a preliminary picture of a few of the physical properties on the basis of data gathered from the writer's experience with a great variety of cellulose ethers. This presentation will by no means obviate the necessity for a well-planned systematic investigation in the above sense. The writer's purpose is to pave the way for such a fundamental study covering a greater variety of technically important properties.

Solubility, Softening, and Moisture Absorption Solubility, softening, and moisture absorption have been

chosen since most of the available data refer to these proper- ties. Although, in a few cases, quantitative data on solubili- ties (including dilution ratios) are also available, here only qualitative aspects, such as complete or partial solubility, swelling effect, and complete insolubility, are considered. Softening characteristics were determined by a simple practical test which has no claim to great accuracy. It is based on the observation of certain visible changes that take place when a small amount of the material is heated gradually. The actual test is carried out in a test tube about 15 mm. in diameter, containing about 0.3 gram of the cellulose ether and a ther- mometer which serves not only for temperature readings but also as a stirring rod. The tube is held in a 600-cc. beaker containing a transparent oil of high flash point (e. g., Crisco). Another thermometer measures the temperature of the bath. The whole is mounted on the same ring stand. Heating of the oil bath is carried out with a gas burner in such a way that the temperature of the oil bath is raised according to a fixed schedule-e. g., a t h s t a temperature rise of 5" C. per minute is effected. This rise is then gradually reduced to 2" C. per minute. This is a highly arbitrary procedure, but the obser- vations can be duplicated within a few degrees.

With gradually rising temperature, several more or less characteristic changes are observed :

(1) When the test sam le is in a fluffy form (e.g., due to precipitation with steam or toiling water), it may be packed down with the thermometer at a relatively low temperature.

(2) More general and characteristic is a phenomenon observed at a considerably higher temperature, when the packed material seems to yield under a pressure exerted through the thermometer. This point may be designated as yield point or point of incipient softening.

The most characteristic point appears at a slightly higher tem erature when the thermometer, under pressure, seems to sticf slightly to the packed material; i. e., withdrawal of the thermometer meets with some resistance. Since this point is that which can be best reproduced, it was chosen as the best softening point to characterize the various cellulose ethers in the present discussion.

On further heating and kneading, the sample may be formed into a single bead which comes loose from the wall and can be lifted out with the thermometer. This bead formation does not always take place.

At a higher temperature the sample turns glassy (tram- parent) without becoming fluid (fusion point or glassiness).

Change in color, probably due to decomposition more often than not, sets in before complete fluidity.

The latter occurs only in a few cases.

(3)

(4)

(5)

(6)

(7)

Moisture absorptions were determined by conditioning samples in powder or fine granular form a t 19" C. and 72 per cent relative humidity for 48 hours, then drying in a vacuum oven a t 90" C. for 15 hours, or correspondingly longer a t lower temperatures if the product is of low softening point. The samples were weighed after conditioning (immediately before drying) and immediately after drying. The difference in weight, expressed as per cent of the dry weight, is taken as the moisture absorption.

Considering the effect of the various composition factors, as listed above, on the properties selected, one is inclined to attribute more importance to the degree of substitution, or the proportions of substituted and free hydroxyl groups, even than to the nature of the substituent groups. On the other hand, chain length and uniformity of substitution, except in extreme cases, are only of secondary influence as compared with the first two factors.

Effect of Substitution Size and polarity seem to be the decisive characteristics of

the substituents. Benzylcellulose, as a phenyl-substituted methylcellulose, shows much lower softening points and mois- ture absorption values than either methyl- or ethylcellulose of the same degree of substitution. Solubility in nonpolar solvents is also more pronounced for benzylcellulose a t low degrees of substitution. In a homologous series, moisture absorption and softening points both decrease with growing number of carbon atoms in the substituent, while solubility in nonpolar solvents a t low degrees of substitution increases:

Subatitu- Softening Moisture Soly. in Ether tiona Point Absorption, Benzene

c. % Ethylcellulose 2 .15 158 3.001 Insol. Butylcellulose 2 .28 65 1.673 Sol. Amylcellulose 1.91 45 0.975 Sol.

The presence of large nonpolar groups, such as benzyl and even butyl, prevents water and alkali solubilities a t low de- grees of substitution. Alkali-soluble ethylcellulose contains about 0.5 ethoxyl per 6-carbon unit; water-soluble grades have substitutions ranging from 0.8 to I .3. BenzylcelIulose is insoluble both in alkali and water a t any degree of sub- stitution. The strongly hydrophobic character of the benzyl group explains the fact that even a relatively small number of such groups neutralizes the hydrophilic effect of the rest of the structure.

The fact that a t least some of the ethers of low sibstitution are soluble in water or aqueous alkalies seems to strengthen the theory that the chains in solid cellulose are held together by forces between their hydroxyl groups. In order to bring about solubility, the chains first have to be pried apart to

Average number of groups per Cs unit.

MAY, 1938 INDUSTRIAL AND ENGINEERING CHEMISTRY 529

break the bonds between the hydroxyls of neighboring chains. This may be done, for example, by etherifying some of the hydroxyls. It is not reasonable to assume that solubility is due to the hydrophilic nature of the groups introduced, as the hydroxyls, which had been thus replaced, were more hydro- philic. A more logical conclusion is that etherification has the effect of increasing the distance between the chains. t By introducing strongly polar groups containing hydroxyls or carboxyls, the softening point of lower alkyl ethers is raised appreciably. The following table shows the effect on ethyl- cellulose of further etherification with ethylene oxide and chloroacetic acid, respectively :

Ether Ethylcellulose Ethylhydroxyethylcellulose Cellulose glycolic acid

Softening Point, C.

142 152 170

In order to demonstrate the great effect of substitution on the solubility characteristics, the following table shows the different solubility types for ethylcellulose in relation to cer- tain approximate substitution ranges; accurate figures could be obtained only from homogeneous products, which are not available :

Substitution Range Solubility

About 0.5 Alkali (4-8y0 NaOH) 1.0 (0.8-1 3) Water 1.4-1.8 Increasing swelling in (organic) polar-nonpolar solvent

1.8-2.2 Increasing solubility in the above ty e of solvent mixtures 2.2-2.4 Increasing solubility in alcohol an8less polar solvents

mixtures

2.4-2.5 Maximum solubility 2.5-3.0 RaDid dror, in alcohol solubilitv: soluble only in nonDolar

There i s a shift in water solubility when alkyl cellulases in a homologous series are considered. Methylcellulose is soluble in water up to about 1.8, ethylcellulose only up to 1.2- 1.4, depending on uniformity. Butylcellulose is not soluble in water a t any substitution. It may be expected that there exists for propylcellulose a very narrow range of substitution (say 0.8-1.0) where a completely uniform product may be water soluble.

Solubility in organic solvents and softening (temperature) arenot linear functions of the degree of etherification; maxima and minima, respectively, occur at certain values of sub- stitution. These values are different for the various ethers, but i t is interesting to note that in each case maximum solu- bility and lowest softening temperature go together. Benzyl-

FIGURE 1. SOFTENING POINT OF CELLULOSE ETHERS

The curve for benzylcellulose is, to some extent, con- jectural. The position of the minimum is accurate, but the shape of the curve would be flatter if the results of actual measurements had been used. Arbitrary cor- rections were made to account for the lack of uniformity in both the low- and high-substitution samples. An- other source of error is peroxide formation, which takes place on standing under the influence of light. The peroxide is subsequently decomposed to benzaldehyde which affects the softening point, as does also the loss of benzyl groups involved. Brandt (9) gives a curve with much steeper branches. His "sintering" points do not correspond to the softening points as defined in this paper. This may be the explanation of the very

high figures given by Brandt.

cellulose reaches this point a t the disubstitution stage, while ethylcellulose has 2.4 to 2.5 ethoxyl groups per 6-carbon unit when the maximum-minimum effects occur (Figure 1).

An interesting example of the gradual increase of the solu- bility in nonpolar solvents is shown by benzylcellulose. The following table presents solubilities in benzene, toluene, and xylene. Slight differences in substitution give marked ef- fects. It would be hard to interpret such results without the chain theory and uniform substitution of the chains:

Substitution Benzene- per C6 Unit Alcohola Benzene Toluene Xylene

2.0 S PS sw s w 2.1 S S PS Sw 2.2 S S S PS 2.3 S S S S

S = soluble; PS = partially soluble; Sw = swelling.

Moisture absorption seems definitely linked with the num- ber of free hydroxyl groups, as it decreases continually with the substitution (Figure 2 ) .

FIGURE 2. MOISTURE ABSORPTION OF ETHYLCELLULOSE

Viscosity The effect of viscosity (chain length), as pointed out above,

is of lower magnitude and may be thought of as superimposed on those of the major factors. This effect is most pronounced on the softening point, which is considerably depressed by very low viscosity at the same degree of substitution:

530 INDUSTRIAL AND ENGINEEHING CHEMISTRY VOL. 30, NO. 5

Specific Softening Ether Substitution Viscosity Point, O C.

Ethylcellulose 2 .16 0.132 99 2 .14 1.267 162 2 . 5 2 0.229 112 2 . 5 0 2.847 144

Benzylcellulose 2 . 2 0.362 68 2 . 2 1.183 104

Uniformity of Substitution Three examples may show the importance of uniformity of

substitution: Bock (1) found that with an ethylcellulose pre- pared under homogeneous conditions by ethylating cellulose in a quaternary base (trimethylbenzylammonium hydroxide) solution, complete water solubility is obtained with 0.6 to 0.7 ethoxyl groups per 6-carbon unit; about 1.0 or more eth- oxy1 groups are required when the product is prepared by the heterogeneous “vapor-phase” ethylation of alkali cellulose. Similar results were previously reported by Traube and co- workers (6), who methylated a cellulose-copper hydroxide- sodium hydroxide complex and obtained a water-soluble methylcellulose containing only 0.8 to 0.9 methoxyl groups, whereas the methoxyl content of commercial products varies from 1.3 to 1.7.

Another example is the alcohol solubility of ethylcellulose. Uniform products are completely soluble over a much wider range (2.2 to 2.5 ethoxyl groups per 6-carbon unit) than the

less uniformly ethylated material. In the latter case alcohol solubility is often limited to the range of 2.4-2.5.

The third example deals with the great increase in softening temperature obtained by fractionating a sample of ethylcellu- lose. In the following table “untreated” designates a prod- uct obtained by ethylation and the usual purification, but without any additional treatment. The “fractionated” prod- uct was obtained by extracting a sample of ethylcellulose with a solvent mixture to remove the low-substitution (and, to some extent, the low-viscosity) fractions. Both products are mixtures of the same average substitution, but the con- stituents of the second are Limited to a narrower range:

Degree of Softening Product Substitution Point, a C .

Untreated 2 . 7 189 Fractionated 2 . 7 225

Literature Cited (1) Bock, L. H., IXD. ENQ. CREM., 29, 985 (1937). (2) Brandt, Karl, “Benzyloellulose,” dissertation, Univ. of Berlin,

(3) Hagedorn, Max, and Moeller, Paul, Ceellulosechem., 12, 29 (1931) (4) Lorand, E. J., and Georgi, E. A., J. Am. Chem. SOC., 59, 1166

(5) Traube, W., Piwonka, R., and Funk, A., Ber., 693, 1483 (1936). RECEIVED January 14, 1938. Presented before the Division of Cellulose Chemistry at the 94th Meeting of the American Chemical Society, Rochester, N. Y. , September 6 to 10, 1937.

1933.

(1 937).

X-Ray Diffraction Behavior of

Cellulose Derivatives WAYNE A. SISSON

Boyce Thompson Institute for Plant Research, Inc. , Yonkers, N. Y.

-RAY diffraction analysis has been especially useful in problems relating to fundamental structure, a knowl- edge of which is imperative in understanding the

variable physical and chemical properties of cellulose deriva- tives which are of growing industrial importance. It is the purpose of this paper to outline briefly the results which have been reported.

I n order to ensure simplicity, addition compounds (la, 47, loo), compounds with inorganic acids (3, 4 , 6), cuprammo- nium complexes (SO, 49, 98), xanthates (36, 61, 67, 74 ) , etc., which might be classified under the general heading of cellu- lose derivatives, are not included. Much of the data listed refer to the nitrates and acetates, but instead of discussing the subject under these or under the customary headings of esters and ethers, the data are outlined as follows: interpreta- tion of x-ray data regarding the mechanism of derivative formation, factors which affect the structure, types of orienta- tion, mechanism of swelling and dispersion in organic liquids, and mechanism of hydrolysis and structure of resulting products.

Format ion It is difficult to classify

satisfactorily the changes that take place in the x-ray din-

X

CHANGES IN X-RAY DIAGRAM.

gram paralleling derivative formation. The change may be specific for each derivative, for the nature and condition of the starting material, for the reagents used, and for the time, temperature, and other variables of the reaction. It is often possible, however, as in the typical case of acetylation, to dis- tinguish three more or less overlapping phases (36, 37, 41): Only the cellulose diagram is present; the interferences of the product of the reaction appear superimposed on the cellulose diagram; and finally the cellulose interferences disappear and only those of the product of the reaction remain.

I n many other cases the second phase of the reaction does not consist of two superimposed diagrams but of a disorgan- ized or amorphous x-ray diagram. For example, the dia- grams of nitrated ramie up to 7.5 per cent nitrogen are identi- cal with those of cellulose. Between 7.5 and 10.3 per cent nitrogen the pattern is indefinite or amorphous. Above 12.8 per cent nitrogen the definite pattern of the trinitrate is reached (61, 62).

During the early his- tory of x-ray diffraction analysis of cellulose derivatives, when the diagram of .the final product gradually displaced that of cellulose, the data were thought to indicate all of the lower derivatives to be mixtures of cellulose and its trisubstitution product. The various nitrates were also thought to be mix- tures of trinitrocellulose and unchanged cellulose (99). It was pointed out, however, that the amorphous diagrams seemed to infer different degrees of esterification, since a well-defined pattern was obtained only with completely sub- stituted products (63). The matter was further clarified,

EARLY EXPLANATION OF CHANGE.