Reversible Hardening of Shellac in Storage L. MCCULLOCH Westinghouse Research Laboratories, East Pittsburgh, Penna. HELLAC is one of those useful natural products which have many substitutes but no duplicates. It is the basis of molded products in which it exhibits such a combination of useful properties that it has not been entirely displaced by the synthetic resins. Shellac was the first thermal-setting resin known, with the property of conversion by heat into an infusible, insoluble form. It has been of great aid to the electrical industry on account of its adhesive properties, and because a t oarbonizing temperatures it vaporizes without leaving an electrically conducting residue. At temperatures below 100" C. shellac is relatively nonvolatile and stable, not subject to change by oxidation. Its thermal hardening property is attributed to the presence of carboxyl and hydroxyl groups which react into cross linkages of several types, and cause the resin to harden and yield water. The purpose of this paper is to show that the reactions which lead to hardening are modified by the presence of water and may be reversed by it. Shellac is hydrophilic in nature; it is somewhat permeable to water, which it absorbs to the extent of 5 per cent a t saturation (W). The hardening of shellac takes place in two definite stages, and proceeds a t a rate dependent upon the temperature and upon the water content. During the first or fluid stage there is a progressive increase in viscosity and in the so-called melting point up to the point of gelation, which is the beginning of the second stage. In the gelled state the resin acquires a iixed shape which cannot be deformed beyond an elastic limit without fracture. Gelled shellac cannot be made fluid by heat below the decomposition temperature, and its structure is not broken down or dissolved by alcohol a t ordinary temperatures. After reaching gelation, the process of hardening may continue until the maximum strength is attained. The change proceeds by a "drying out" process, and the ultimate strength is not attained if the resin is confined (as in a mold) so that vapors cannot escape. The degree of hardness in the gelled resin is judged by a suitable test of mechanical strength-for example, by the shearing resistance. Thoroughly cured and hardened shellac remains slightly thermoplastic within its elastic limit, accommodating itself to the thermal, contraction of the surfaces to which it is adhering. This is one of the reasons for the adhesive strength of shellac. The reversible hardening of shellac by water has been demonstrated by treating the thoroughly hardened resin with water in an autoclave at 150" to 210" C. ( I ) . This treatment renders the resin again fusible and soluble. This soft resin can be rehardened by heat with drying substantially as before. These reversals can be carried through repeatedly. It is probable, however, that the resin recovered by the autoclave is not identical with the original shellac in all respects. Rangaswami and Aldis (I) reported : "Depolymerized shellac slowly repolymerizes. This effect is somewhat more rapid than with a normal shellac." Information is lacking as to the utility of such recovered resin.
S
STORAGE IN AMPOULES
To study the autoclave process of recovering shellac that had hardened in storage, a series of experiments was made with shellac and water in ampoules of Pyrex a t various temperatures. The ampoules were 1 inch in diameter and 6 inches in length, and contained 20 grams each of new orange shellac and water. They were enclosed in capped iron pipes for safety and were heated in controlled electric ovens. An unexpected difficulty had appeared while the writer was treating a pound of shellac in the autoclave. The center of the mass after cooling was found to be gelled and infusible, whereas all of it should have liquefied. The previous workers (1) also encountered this difficulty, remarking that "shellac can repolymerize within the autoclave on cooling". At first the writer attributed the gelation a t the center to evaporation of water from the hot center to the cooler walls of the autoclave. The experiments with the ampoules led, however, to another explanation; namely, gelation is reversed by changes in temperature alone, and the gel structure is stable a t lower temperature and unstable a t the temperature of the autoclave. The behavior of the shellac samples within the ampoules was as follows: Temp., O C. 70 90 100 110 125 150
160 200
Shellac Behavior Gelled in about 30 days 10 days 7 days 6 days 6 days 6 days Did not gel in 8 days 3 dnys
Shellac that had been liquefied a t 165" and a t 200" C. gelled in less than one day at 90°, although from the above table new shellac required 10 days to gel. After having gelled at 90" it liquefied in a few hours a t 165" and 200" C. The temperature of reversal lies between 150" and 165" C. Above this temperature there is liquefaction and below there is gelation. The samples were said to have gelled when no flow was seen within the ampoules after they had been inverted in the ovens for a few hours. EFFECT OF MOISTURE
I n another experiment in a flask of water boiling with reflux, some new shellac gelled after 7 days. It is well known, however, that shellac, baked in air at 100" C., gels or has a "life under heat" of about one day. Thus the effect of saturation with water is to retard gelation by at least seven times. In an experiment in a flask a t room temperature, shellac under water for 600 days is not yet infusible, although the fluidity is only a fraction of its former value. Unfortunately, shellac proceeds to harden slowly even at the storage temperatures and results in loss of usefulness from infusibility and insolubility. The following experimental findings lead to the conclusion that spoilage in storage can be lessened by keeping the shellac dry. Two samples 825
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Vol. 35, No, 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
of orange shellac were kept in glass jars at room temperature for months; one was stored over calcium chloride, the other over wet sodium chloride which gives 75 per cent humidity. After 1000 days the condition of the shellacs was as follows: Oiange Shellac High-flow Low-flow
Dry CaCll Still fusible
Over Wet NaCl Hardened
Over
Infusible
Stlll fuslhla
An experiment was commenced with orange shellac in t n o friction-top tin cans, covered with cloth to permit passage of moisture; one contained a small jar of calcium chloride, the other of 1%-etsodium chloride. The two cans were kept in an oven at 40" C. until the shellacs became infusible. Over dry calcium chloride the shellac was infusible after 400 days; mer wet sodium chloride, after 175 days. CONCLUSIONS
Shellac can best be preserved by storage in a dry atmosphere. Since, however, the fluidity and other molding properties vary with moisture content (2), the dry resin can be restored to the desired condition by adding a measured amount of water shortly before it is to be used.
Shellac at temperatures below 150" C. approaches the gelled state whether dry, moist, or wet. Shellac in storage should be kept dry to retard the reactions of hardening. At higher temperatures gelation is greatly retarded by water and does not occur in water above 150" C. Gelation under water is reversible between about 150" and 165" C. Since the effect of mater at room temperature is to hasten gelation and at higher temperatures to retard it, there must be an intermediate range where water is without effect upon the rate; this temperature seems to be between 4 0 " and 70" C. ACKEOW LEDG MENT
For criticisms of the manuscript, the writer expresses thanks to A. T. Krogh and L. R. Hill of this company, and to Paul F. Bruius of the Polytechnic Institute of Brooklyn. LITERATCRE CITED
(1) Rangaswami, M., and Aldis, R. W., Indian Lac Research I n s t . Bull. 419 (1934). (2) Townsend, R. V., and Clayton, W. R.. IND. ENG.CHEM.,A N ~ L . E D ,8. 108 (1936).
CORRESPONDENCE Calculation of Relative Volatilitv -J
SIR: A recent paper by John Griswold [IxD.ENG.CHEW,35, 247 (1943) ] shows convincingly the need for accurate relative volatility values in distillation calculations. There is, however, a slight erroi in his reference to my equation for the relative volatility of normal liquids at atmospheric pressure [ J . Int. Petroleum Tech., 2 5 , 558 (1939)l. The constant of 11.5 xas not derived from the Clausius-Clapeyron equation and Trouton's rule, but was a mean value obt,ained from consideration of a large number of hydrocarbon mixtures, mostly wide boiling. The constant obtained in the theoretical derivation was 11.1. Converting natural to common logarithms, this is nearly the same as the modified constant proposed in Griswold's Equation 4A. Thus it appears that the "constant" varies from 11.1 for close-boiling to 11.5 or more for wide-boiling mixtures. If the approximate Clausius-Clapeyron equation is combined with Kistiakowsky's equation instead of with Trouton's rule, the following is obtained: loga =
T - TA TB log T A +
where R
= gas
T
log T B
constant, (cc.) (atm.)
TB - TA +7 log R
(" C.) - 1 (mole) -1 =
82.048
P A P B = vapor pressures of components at abs. temp. T T A T B = atm. boiling points of components, abs. scale a = relative volatility at abs. temp. T
This equation applies only to nonpolar liquids at atmospheric pressure when the gas law and Raoult's law deviations are such = P A / P Bcan be assumed. It contains no that the relation empirical constant and is likely t o be more accurate than my earlier equation. Checked against the experimental results of Griswold and a few others, it gives values of (a - 1) from 4 t o 15 per cent low. (Y
R. EDGEWORTH-JOHSSTONE TRINIDAD LEABEHOLDB, LTD. POINT-A-PIERRE, TRINIDAD. B. W.I.
SIR: The new formula by Johnstone for calculating CYideal I t is probably as accurate as can be developed from theoretical considerations only without becoming complicated. For accuracy in wide-boiling hydrocarbon systems, both this formula and my Equation 6 leave something t o be desired. When the vapor pressure curve of each component fits the type form equation : ( = P A / P B )is a real contribution.
log P
= n
- C/l'
as is usually the case, then : log
(aided)
= iOg ( P A / P B ) = f l / T
-
"{
P and y are constants applicable over fairly wide ranges. Two vapor pressures for each component are needed to evaluate p and y, one of which may be the normal boiling point. When a vapor pressure curve has not been determined, a synthetic curve may be drawn through the boiling point on a suitable hydrocarbon vapor pressure chart, of which several are available. This procedure is recommended as the best at the present time for calculation of w e a l of wide-boiling hydrocarbon systems, and it is applicable to pressures other than atmospheric. THE UNIVERSITY o r TEXAS AUSTIN,TEXAS
JOHXGRISWOLD
...
SIR: I am obliged t o Griswold for his remarks on my formula for agree that with wide-boiling components for which vapor pressure curves are available or can be approximated, his method is probably better than any theoretical formula. With very closeboiling components it is possible that the formula might be more satisfactory, since atmospheric boiling points are often known with greater accuracy than vapor pressures at other temperatures. R. EDGEWORTH-JOHNSTONE a, and
