I-\‘DUSTRIAL A N D ENGINEERING CHEMISTRY
October, 1928
Figure 8
molecular layer may be conceived to be brought within the range of the attractive forces due to ethylene linkages, and in this event these forces of molecular attraction aid the external compressive force, with the result that crumpling of the monomolecular layer occurs a t force values which are lower for the more highly unsaturated acids and their esters. There are some d3culties in accepting the premise that
955
the molecules are oriented in such a way that the ethylene linkages are drawn down to the surface of the water or near to it. If this premise is accepted, the increase of area per molecule is explained, a t least in a qualitative way, but i t would then be expected that the force required to crumple a monomolecular film of unsaturated acid or its esters would be greater than that required to crumple a monomolecular film of a saturated acid. The reverse is very definitely true. Langmuir explains this by postulating an attractive force group and the double bonds, which uses between the -COOup part of the energy of each. Less force is then required to crumple the film in the case of the compounds containing double bonds. Answer t o the far-reaching question as to whether ethylene linkages are drawn down to the sarface and whether t h e 4 0 0 - groups on the end exert an attraction for ethylene linkages seems possible by a study of several different types of compounds. This will be attempted in connection with further work already started on pure acids and esters and on oxidized acids and their metallic salts. Castor oil and otherOH bodies are particularly suggestive in work done so far. Literature Cited (1) Adam, Proc. Roy SOL.(London),9 9 8 , 3 3 6 (1921); lOlA, 452,516 (1922). (2) Harkins, Davies, and Clark, J. Am. Chem. SOC.,S9, 541 (1917). (3) Langmuir, I b r d . , S9, 1848 (1917).
Cellulose Acetate Lacquers‘ Harry E. Hofmann and E. W. Reid A ~ E L L O NINSTITUTE OF INDUSTRIA&RESI$ARCH, UNIVERSITY OF PITTSBURGH, PITTSBURGIl, PA.
ROTECTIVE and This paper is intended to be the first comprehensive type. It was not until 1894, however, that the first proccontribution to the modern literature of cellulose acedecorative coatings in which the p r i n c i p a l tate coatings. A brief rbsumh is given of the earlier ess for preparing cellulose. steps leading up to the present-day cellulose acetate acetate was patented (4). A film-forming material is an lacquer, and also an outline of the usual method of great many patents have been ester or an ether of cellulose preparation of cellulose acetates. Solvents are disissued since that time, both in have become important articussed at some length-viz., ketones, esters, chlorinated this and in foreign countries, cles of manufacture in recent compounds, two-type solvents, and the common dealing for the most part with years, owing to their superior diluents, alcohols and hydrocarbons. The principal improvements in the process hardness and greater speed of properties of each of the solvents in tabulated form andwithnewcatalyticagents. drying. The two c e l l u l o s e The earliest mention of prod? esters that have found the and a few remarks on the general nature of cellulose widest application in prodacetate solvents are included. ucts resembling what we now The available plasticizers and resins for use in cellucall lacquers, and containing ucts of this type are the nilose acetate lacquers are mentioned, and then concellulose acetate, is made in, trate and the acetate, and while both have several shortsiderable attention is devoted to the formulation of the patent literature of 1910comings and disadvantages, cellulose acetate lacquers. Such topics as solubility 11. These products were al-. no satisfactory s u b s t i t u t e s of the acetate in various three-component systems, most all impractical and were. tolerance of diluents, blush resistance, evaporation suggested for the impregnahave yet been found. Nitroc e l l u l o s e lacquers are now rates, gloss, adhesion, flexibility, etc., are discussed tion of paper, cloth, etc., to, under this heading. Finally a few typical formulas produce waterproofness an& fairly well understood, and are manufactured i n l a r g e are given for practical lacquers, together with some t o r e d u c e inflammability. quantities for a wide variety suggestions on the incorporation of pigments. O n e of t h e s e w a s c a l l e & of uses, but the available in“ C e l l i t ” varnish (IR), an4 formation with regard to cellulose acetate lacquers is some- was said to be a solution of cellulose acetate in a mixwhat meager. They are considered by many to have certain ture of ethyl alcohol, ethyl acetate, ethyl acetoacetate, an& desirable properties, such as non-inflammability and greater camphor. Another (IO) was prepared by diluting a constability than nitrocellulose lacquers. centrated (25 to 33 per cent) solution of cellulose acetate in The first acetylated carbohydrate was described by Schut- acetone with a mixture (proportions not definitely stated) zenberger ( I d ) , and his communication precipitated a large of 90 per cent alcohol and benzene. To this a small amounb amount of investigational work (21) on compounds of this (2.5 per cent) of beta-naphthol or hexachloroethane was. added, presumably as a plasticizer. This method appears I Presented before the Division of Paint and Varnish Chemistry at the to be the forerunner Of modern practice in which liquids, 77th Meeting of the American Chemical Society, Columbus, Ohio, April, 29 to May 3,1929. called “non-solvents” or “diluents,” are used ip thq soluen$L
P
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
mixture which themselves are not active solvents for cellulose acetate. A large amount of experimental work on cellulose acetate compositions was done during the World War by the various governments involved, toward the development of witable non-inflammable protective coatings for airplane wings. These "dopes," as they were called, differed materially from the present-day lacquers, and will only be mentioned briefly. Cellulose acetate is, as its name implies, an ester of cellulose (which may be considered a polyhydric alcohol) and acetic acid. KO precise formula can be given for the ester, however, since the constitution of cellulose has not yet been definitely established, although the usual products are commonly considered to be triacetates, or hydroacetates. As in the case of ordinary alcohols, satisfactory acPtylation .of cellulose cannot be accomplished with acetic acid alone, and mixtures of acetic acid, acetic anhydride, and sulfuric acid or other catalyst (16) are generally used. Y'unizrous patents (20) have been obtained on processes for the acetylation of cellulose employing a great many variations in time, temperature, catalyst, composition of the acetylating bath, etc. The Dreyfus patents are probably the most important from a technical standpoint, and cover developments from 1911 to the present time.
Vol. 21, No. 10
made by the larger manufacturers, by means of various combinations of hydrolysis and acetylation under varying conditions. The esters of low viscosity are the ones of interest t o the lacquer industry. Five samples of cellulose acetate of different types were obtained from three different sources and a few of. their more important properties determined. These are shown in Table I and are representative of the products that are obtainable commercially a t the present time. The degree of esterification, or the saponification value, was determined by the method of Eberstadt. This consists of moistening a weighed sample of the ester (0.2 t o 0.3 gram) with about 5 cc. of alcohol, then adding 20 cc. of standard half-normal alkali and warming on a hot plate or steam bath for GO minutes. After that time the contents of the flask are cooled and titrated with standard acid, using phenolphthalein as the indicator. The viscosity was determined by preparing solutions of each ester in equal concentrations in the same solvent. For the purposes of this test, 10 per cent solutions were made in 1,4-dioxan which was chosen as an active solvent of only moderate volatility, and the viscosity measurements were made a t 25" C. with the 1Iac1Iichael viscometer, .previously calibrated a t 25' C. with standard samples of oil obtained from the Bureau of Standards.
of Cellulose Acetates 70% TOLUENE METHYL ETHYL CHLORO-ETHYL 1,4307, METHYL CELLOSOLVE TRIACETONE KETONE FORM ACETATEDIOXANALCOHOL CELLOSOLVEACETATE ACETIN Table I-Properties
h.f E T H Y L
VISCOSITY MOISTCRE HOAc Per cent P e r cent "Low" 9 46 57 7 S S "15,' 4 63 53 1 S i Lacquer 1 90 56 0 S h'ormal 2 21 52 5 S Hieh 1 99 54 5 S i S = soluble. i = insoluble pS = partially soluble " Viscosity of 10 per cent solution in dioxan at 25' C.
s
G
G S
G G G = swells
The raw materials in the manufacture of cellulose acetate are, therefore, cellulose, acetic acid, acetic anhydride, and sulfuric acid. The cellulose is in the form of cotton fibers, linters, or paper. I n any case the raw material must be of low moisture content, and should not contain any fats, ash, or soluble salts. The exact details of the acetylation processes actually used are considered trade secrets, and are consequently not divdged. However, it is understood that the general composition of the acetylating bath is approximately 30 t o 40 parts glacial acetic acid, 20 t o 30 parts acetic anhydride, and 1 to 2 parts concentrated sulfuric acid. Other variables in the process are ratio of cellulose t o acid, temperature, time, and ratio of catalyst to moisture in the cellulose. For products of high viscosity the acetylation is conducted at relatively low temperatures (0"to 20" C.), thus preventing excessive hydrolysis (15). Note-It is interesting to note that the earlier workers were interested in t h e acetates of higher viscosity, since they were somewhat stronger, but that the modern tendency is toward lower viscosities, so that coating compositions may be prepared with a maximum non-volatile content. This is similar to the trend in cellulose nitrate manufacture.
After the cellulose has completely dissolved in the acetylating bath, it is allowed t o remain for a time a t a slightly higher temperature (30-45" C. or higher) until the desired degree of solubility is produced. Before this "ripening" the cellulose acetates are soluble in chloroform (with the addition of a little alcohol), but not completely soluble in acetone. iifter mild hydrolysis they are completely soluble in acetone, and may even be rendered soluble in ethyl acetate k,y still further hydrolysis ( 3 ) . The limits of acetone solubility in the cellulose acetate have been found t o lie between 50.0 a n d 57.6 per cent of combined acetic acid (13). Many cellulose acetates with widely different properties are nom- being
s
BS
S
S S
i
S
S
i
PS PS PS
S S
I
S
S
S
S
S S
s i
S
i
s
VISCOSITY~
Poises 22 7 46 1 13
94 437
The solubility (qualitative) of each sample was tested in acetone, chloroform, toluene plus alcohol (70:30), methyl ethyl ketone, etc., with the results indicated in the table. The specific gravity of one sample of cellulose acetate was determined by the method used for pigments (A. S. T. M. Standard D153-27) and was found t o be 1.207 at 25" C. Solvents for Cellulose Acetate Lacquers
Cellulose acetate is, in general, soluble in fewer organic solvents than cellulose nitrate, and these are commonly considered more powerful solvents than those which dissolve the nitrate. Cellulose acetate is soluble in a few liquids that are not solvents for cellulose nitrate but with these few exceptions any compound that dissolves cellulose acetate is also a solvent for cellulose nitrate. The reverse is not true, however. The first solvent t o be widely used in connection with cellulose acetate was acetone, partly on account of its good solvent properties, and partly on account of its availability. Acetone was formerly obtained almost exclusively from the destructive distillation of wood, but a t present the greater part of the supply is produced by the fermentation of corn, although more recently a synthetic method has been developed and is now being operated on a large scale. Acetone is a very active solvent for cellulose acetate, although, as previously mentioned, some of the unripened forms are not soluble in acetone, and the solutions will tolerate considerable dilution with non-solvents such as toluene and alcohol. The principal objection to acetone in cellulose acetate lacquers is its low boiling point and consequent extremely rapid rate of evaporation. This property, coupled with the fact that water ir mircible with acetone in all proportions, causes
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I N D U S T R I A L A S D EAY‘GIAVEERISGC H E M I S T R Y
October, 1929
Table I1
SOLVENT FOR
WEIGHT
BOILING
KO.
SOLVEST
POINT
c.
80 83 85
0 792 0 964 1 499 (150 C ) 0 900 0 789 0 Si9 0 SO5 1 265 0 878
1 2 3
Acetone Methyl acetate Chloroform
E?61
4
E t h y l acetate E t h v l alcohol Benzene Methyl ethyl ketone Ethylene dichloride Dimethyl Cellosolve
77 78 80
6,
Q9
SP. GR.. 2o°C.
PER
GALLON, 200 C. Lbs. 6 60 8 04 12.48
7.50 6.57 7 32 6 72 10 53 7 32
REFRACTIVE
ISDEX, 200 C.
1.3691 1 3619 1.4449
CBLLU- MISCI-
FLASH POINT
CLOSED CVP F. “C < 3%
