Resistance of Shellac Films to Air of High :Humidity JOHNW. PAISLEY, Gillispie-Rogers-Pyatt Company, New York, N . Y .
T
HE value of a protective coating depends to no s m a l l d e g r e e upon such mechanical properties as its elasticity and flexibility. So well known is the use of plasticizers in nitrocellulose varnishes that little need be said concerning their value except to recall that the success of these coatings developed with the discovery of softening agents. On the other hand, spirit varnishes, (especially shellac) possess in themselves most of the mechanic a l properties d e m a n d e d of films, so that seldom, if ever, is anything added to them. It will b e s h o w n i n t h i s paper, however, that heneficial effects c a n be obtained b y a d d i n g s u b s t a n c e s such as tricresyl phosphate to these latter composition..
On the other hand, the requirements demanded in t h e preparation of films can be obtained by using a very simple and inexpensive method. This consists in pouring varnishes, whose viscosities have been previously equalized in Gardner-Holt viscosity tubes, upon the center of an inverted watch glass and all o w i n g t h e v a r n i s h to drain. The watch glass may be placed on any s u i t a b le support, such as a 4-ounce s h e l l a c s a m p l e b o t t l e or s i m i l a r wide-mouth bottle which will hold the watch glass in a h o r i z o n t a l plane. These bottles are machine-made and hence are u n i f o r m . The bottle may be placed in a Petri dish to catch the overflow varnish. A diagram of the set-up is shown in Figure 1. A hygrometer and thermometer are placed t o record beside the amaratus -the exact atmospheric conditions. If several films are being prepared, it is essential that watch glasses of uniform curvature and thickness be selected. By pouring a number of varnishes a t the same time, the effects of atmospheric conditions are equalized. This method of pouring films eliminates mechanical disturbances, and insures uniform flow and surface-tension effects. An ample amount of varnish should be used to provide an all-round overflow. A large piece of paper may be set around the apparatus to eliminate stray air currents. After films have dried, which requires a t least 24 hours, a centered circle which covers most of the glass is marked on the film, and the underrun and brim are carefully cut away with a razor blade up to this circle. The circumference of the circle is marked a t least 1 inch from the edge of the glass. In this way the irregular thick parts of the film are removed. The shellac film remaining on the watch glass is remarkably uniform. Shellac varnishes, approximating 5 pounds t o the gallon, gave films of 0.180 gram when poured on a 4-inch watch glass. Duplicate films did not show more than 0.001 gram average difference in weight. When these were tested with moisture, they became milky simultaneously and, when airdried, returned to transparency a t an equal rate. The weight of moisture taken up by each film or lost on air-drying was equal. Furthermore, the weight of different sections of a film were equal, indicating equal thickness.
T H E R E S U L T S which are obtained upon testing films are often materially influenced by factors entering into their preparation. Unless these factors are known and carefully controlled, films .from the same varnish will give widely varying results. A simple cmd inexpensive method for the preparation of uniform shellac films is described. This method gives films which can be easily duplicated, as judged both by weights and general behavior. A preliminary study of general factors affecting the structure and properties of films is discussed. This gives for the first time a means of rontrolling the method of preparation of films and of legitimate appraisal of results. As a result of these studies it has been possible to develop bleached shellac varnishes possessing increased water resistance. I t has been found that the use of 10 per cent tricresyl phosphate will imparl such properties.
FACTORS INFLUENCIKG STRUCTURE O F FILMS Before the effect of adding such a substance can be ascertained, it is necessary that films can be prepared which will give reliable results for comparison purposes. The behavior of films, especially those of shellac, depend to no small extent upon their method of preparation. It follows, therefore, that in such a study as this, it is absolutely essential that careful consideration be first given to this important subject. The most important factor affecting the behavior of films is their thickness. The thickness obtained will depend t o a large extent upon the ratio of solids to liquid and on the viscosity of the varnish. The viscosity in turn influences the rate of evaporation, the drying time, and the actual structure. Temperature plays a dual role affecting both viscosity and drying time. Mechanical disturbances, atmospheric conditions, and surface tension a t the edges of the test piece may alter the films. When comparing varnishes, such as those containing varying amounts of tricresyl phosphate, it is necessary that all test pieces be prepared under identical conditions; for comparison purposes this may be accomplished by preparing all the films simultaneously with recorded conditions.
PREPARATION OF DUPLICATE UNIFORMSHELLAC FILMS The common method of preparing films for comparison, by pouring the varnish upon a flat piece of glass inclined a t a specified angle, does not give satisfactory results with shellac varnishes. Such films are wedge-shaped, and, when subjected to the action of moisture, the degree of whitening varies with the thickness. Furthermore, duplicate results cannot be obtained. It was found that ten films prepared from the same varnish by this method under the most careful conditions gave widely varying results.
JUDGING THE WHITENING OF FILMS As can be seen from the tables, the degree of whitening of shellac films cannot be measured by determining the amount of water absorbed when films are exposed t o moisture, as Venugopalan and Rangaswami (4) have assumed. It was 163
164
INDUSTRIAL AND ENGINEERING CHEMISTRY
therefore necessary t o use some other measure for comparing the effects of moisture on different films. What will be termed “clearness” and “gloss” were employed for this purpose. These terms are purely empirical, as can be seen from the methods of determining them. In determining clearness, a clear shellac film was taken for 100 per cent clearness as one point on a standard scale, and a film which had been whitened to the degree where printed matter could not be read through it, when the watch glass was placed directly over the printing,
Vol. 24, No. 2
causing swelling which disrupts its compact structure; this film, when dried, would then possess a structure having larger pores, which would not show the same ability of absorption and capillary condensation toward moisture. TABLE I. INFLUENCE OF THICKNESS OF FILM ON RESISTANCE TO MOISTURE TREATMENT
WEIQHTOF FILM CLEARNESS GLOSS 3-lb. 3-lb. 4- 5- 3-lb. 4- 6Cut 4-lb. 6-lb. out lb. Ib. out lb. lb. Gram Gram Gram % % % % % % 0.082 0.112 0.186 90 95 85 95 98 95
Dry film After exposure to air of high humidity 0.084 0.112 0.192 After air-drying 0.081 0.112 0.180 After 2nd exposure to air of high humidity 0.081 0.109 0.178
90 90 80 90 90 75
95 95 75 95 95 95
75 75 50
80 75 60
RATE OF DRYING. When films are prepared in confined spaces where normal evaporation cannot take place, the slow eraporation causes the wax in a shellac film to separate. In this way permanently mottled films only are obtained, This separation of wax, although destroying the clarity of the film, does not appear to materially affect its behavior toward water. FIGURE1. DEVICEFOR POURING UNIFORM DUPLICATE SHELLAC FILMS
was taken as 0 per cent for the other point. An arbitrary scale for each 5 to 10 per cent was then set up by judging the maximum distance a t which the test pieces could be held and the print read. All observations were made by one operator to minimize the personal factor of judgment. Gloss was measured in a similar manner by noting the degree of reflection of the frame of a window when pieces were held in a position to reflect as much light as possible. No attempt has been made in this work to reduce these values to any absolute scale, nor is any claim made for great accuracy made in their determination. They are, however, sufficiently accurate for the purpose of such a preliminary study as this. It can be seen that results are sufficiently diverse for conclusions to be clearly drawn. For these reasons it was not felt necessary to develop other methods a t this time, since it is believed that more accurate measurements of degree of whitening will in no way modify the general conclusions drawn in regard to films in this paper. GENERALOBSERVATIONS ON SHELLAC FIms Having developed a method whereby uniform and duplicate films could be obtained and a means of appraising them, it was now possible to observe quantitatively some of the factors affecting shellac films. THICKNESS OF FILMS.The effect of thickness of films of shellac in their behavior toward moisture is clearly illustrated in Table I. Samples of 3-, 4-, 5-pound cuts of the same varnish were prepared simultaneously, air-dried for 12 hours, and then further dried in an oven a t 43” C. for 2.5 hours. All samples showed practically equal clearness and gloss before treatment with moisture. It can be seen that the amount of water taken up varies with the thickness of films, and that less water is absorbed on repeated contacts with moisture after air-drying. It can also be seen that the weight of moisture has no direct relationship to the degree of clarity or gloss. These observations are in accord with the conclusions of Gardner (3) that the absorption of moisture by shellac films is a physical phenomenon, and that the amount of moisture absorbed is related to the structure of the film and not to its composition, since in these experiments the same varnish is used throughout. The decrease which is observed in the amount of moisture that is taken up by the films on successive humidity treatments may be accounted for by making the following assumptions: Moisture is condensed in the film,
FIGURE2. FILTERING DEVICE FOR SHELLAC VARNISH
On the other hand, if films are subjected to moist conditions before they dry thoroughly they will become opaque more readily than films which have lost all their alcohol. This is illustrated in Table 11. If “green” films are not subjected to the action of moisture for too long a period before they are airdried, they will finally approach in resistance a state not far removed from that of the previously dried films. TABLE11. EFFECT OF ALCOHOL IN A FILMON RESISTANCE~TO MOISTURE TREATMENT After exposure to air of high humidity Air-dried After 2nd exposure to air of high humidity Air-dried 2nd time
CLEARNESS Dry film Green film
GLOSS Dry film Green film
%
%
95-75 90
75-50 95-90
75 80
75 95-75
85-75 75-60
75-60 75-60
75-60 75-60
60-40
c/o
%
60-40
DRYINGIN HUMIDAIR. These observations were further confirmed by comparing films dried under humid conditions with those dried in a desiccator. As can be seen from Table 111,there is little or no difference in the appearance of the two films when subjected to the different treatments, but the amount of water absorbed or lost is quite different. After the
February, 1932
I N D U S T R I A L A IV D E N G I N E E R I N G C H E M I S T R Y
165
both cases saturated humidity a t room temperatures has been used, I n the first case, films were brought in contact with moisture by simply placing themin a bell jar containing a This method had the disadvantage of perTABLE111. EFFECTOF DRYING UNDER HUMIDCONDITIOKSvessel of water. mitting actual dew formation on the film when the room WEIQHTOF FILM. CLEARNESS GLOSR Dried Dried Dried D:ied Dried Dr,ied temperature is slightly lowered. I n the second case, films in in in in in in desichudesiccahu- desichumidiwere placed in a humidity chamber where conditions were so TREATMENT fier tor midifier cator midifier cator arranged as to prevent this dew formation. This was acGram Gram % % % % complished by means of an arrangement such as is shown in Untreated 0.100 0.107 95 95 95 95 After exposure to air of Figure 3. Here the air is saturated by bubbling through high humidity 0,107 0.115 95 95 95 95 Air-dry 0.098 0.102 95 95 95 95 water and then through a vessel containing moist cotton to After 2nd exposure t o which a breathing tube is attached. air of high humidity 0.114 0.120 20 15 65 50 Afterzndair-drying 0.097 0.097 70 75 80 85
second moisture treatment and air-drying, the tn7o films are practically ident)ical.
It was observed that repeated treatments with moisture with intermittent drying had a far greater deteriorating effect on shellac films than continued contact with moisture. The method of repeated treatments used in this study may be considered as one of subjecting the films to the most severe conditions. The deterioration which eventually takes place may be looked upon as a result of fatigue from continual bending of the fine particles of the film by the swelling and contraction caused by the moisture. Conditions as severe as these are not usually encountered in the use of shellac varnishes. Drying films in an air oven a t 105' C. destroys the surface of the films, but films so treated are unquestionably free from all traces of alcohol. Such films were shown to absorb moisture to the same degree as the desiccator-dried films cited above. Water was taken up more slo~vly,however. EFFECTOF WAX. It is well known that the condition of wax in a shellac varnish affects the drying properties of its films. For example, when a varnish is stored near a radiator, some of the wax dissolves in the alcohol a t the elevated temperature. On cooling, this wax separates as a gel and yields a varnish which will not dry. In order to study the effect of the presence of wax of normal condition in a shellac varnish, 5-pound cuts of shellac were filtered in an apparatus such as is shown in Figure 2. This consists simply of an inverted funnel joined to another in a vertical position by means of a rubber band. The vertical funnel is set in a flask and a side arm is connected to the stem of the inverted funnel by means of a rubber tube. I n this manner heavy cuts of shellac may be filtered without loss of solvent during the long period necessary for such a filtration. It can be clearly seen from Table I V that heavier films are obtained with the original varnishes, resulting from the wax
FIGURE 3.
1 Gram Untreated air-dried 0.625 After ventilated-chamber treatment 0.609 treatment 0.638 After 2nd air-drying 0.595 1. 5-lb. cut superfine orange shellac varnish. 2. Same filtered free of wax. 3. 5-lb. cut bone-dry bleached shellac varnish. TREATMENT
2 Gram
0.198 0.186 0.198
...
3 Gram 0.375 0.369 0.421 0.365
FILM
4 Gram
0.196 0.192 0.219
0.188
imparting a greater viscosity to these solutions. Since the object of this experiment was to compare varnishes as they are commonly used, the viscosities were not equalized as was done in later experiments. The fact that the wax-free varnishes here appear t o show greater resistance to moisture may be explained on the basis that the films of the wax-free varnishes are thinner (Table I). METHODSOF TREATING F I L m WITH MOISTURE. The method of treating films with moisture will determine the degree of whitening obtained. In these studies two methods have been employed for treating films with moisture. In
HUMIDITY CH.4MBER
It was found that, when actual condensation took place on the shellac films, the effects were much more severe than when a ventilated humidity chamber was used. This can be seen by comparing the data given in Table IV. In these experiments the films were left in the ventilated chamber for 13 days. No great change in the various films took place. On the other hand, when they were placed in the closed bell jar in which water was placed a t 28-30' C. daily to insure condensation, the films went to pieces completely in 4 days. Unless otherwise stated, films were treated by subjecting them to this latter more severe manner. TRICRESYL PHOSPHATE IN BLEACHED SHELLAC VARNISHES. Because of its very high boiling point, tricresyl phosphate will be left in a dry shellac film. It is not unreasonable to expect that this substance might have some plasticizing effect upon these dry films of shellac from its marked behavior in this manner in nitrocellulose lacquers. If this were true, increased resistance t o water would be expected, since the above studies have indicated that films deteriorate most rapidly under conditions of repeated swelling and contraction. In-
TABLE Iv. EFFECT OF REMOVAL OF W A X FROM SHELL.4C \vEIGHT OF
VESTILATED
5
.
Gram
\'ARNISAES
~ C L E A R N E B S 1 2 3 4 6
%
%
%
%
%
1
2
GLoss 3
4
5
%
%
%
%
%
98 95 75
25
90 95
98 95 80
98 95 50 60
C -
98 90 98 95 98 95 90 95 95 98 5 75 10 35 5 50 60 60 50 60 60 25 4. Same filtered free of wax. 5. Same as 1, treated with a little oxalic acid
0.445 0.432 0.453 0.420
95
90
25
25
40
and centrifuged.
creased flexibility would therefore add increased life to the films. Furthermore, it is possible that liquids of this type, which absorb little or no water themselves, might fill any small pores of the films and to some extent reduce the amount of water absorbed by them. Finally, tricresyl phosphate has a refractive index of 1.56 ( 2 ) ,while bleached shellac has been shown to have a refractive index of 1.534 (1). Hence it would not be expected that the tricresyl phosphate held in the film would refract light to any appreciable extent, nor that films containing this liquid would have the same appearance as unplasticized films.
IKDUSTRIAL AND ENGINEERING CHEMISTRY
166
Vol. 24. No. 2
TABLE V. EFFECTOF ADDINGUP TO 10 PER CENTTRICRESYLof tricresyl phosphate had a far less tendency t o crack on PHOSPHATE TO SHELLAC VARNISHES repeated moisture treatment. The beneficial effects of tri(Films prepared from varnishes containing 45.4 grams bleached bone-dry cresyl phosphate are even more strikingly illustrated in shellac cut in 75 7 cc of 95 o alcohol and poured on 3-inch watch glasaes a t 56' F.'(13.3' c.j anh 5 1 7 humidity. lll/ltinch circles used for study.) Figure 4 when pictures of these films are compared. Film 1 contained no tricresyl phosphate Table VI shows that there is little to be gained by adding Film 2 contained 0.045 gram tricresyl phosphate 0.1 Film 3 contained 0.227 gram tricresyl phosphate 0.5 greater amounts of tricresyl phosphate than 10 per cent, Film 4 contained 1.454 grams tricresyl phosphate 1.0 Film 5 contained 1.908 grams tricresyl phosphate except possibly in a few cases where films are to be subjected 2.0 Film 6 contained 2.270 grams tricresyl phosphate 5.0 to very extreme conditions. Films containing as much as 25 Film 7 contained 4.540 grams tricresyl phosphate 10.0 CLEARper cent show little difference in behavior from those containFILM WEIQHT NESS GLOSS APPEARANCE ing 10 to 15 per cent. Glass Film Net and film only Grams Gram
...
1
,
3 4 5 6 7
2
.. ...... .. . . ,, .. ....
1 2 3 4 5 6 7
9.425 10.712 9.490 8.956 8.707 8.994 10.429
1
9.418
0.096
2
10.704
0.096
3
9.483
0.094
4
8.950
0.092
5 6 7
8.700 8.988 10.425
0.096 0.093 0.093
1 2 3
9.419 10.707 9.486 8.952 8.696 Broke 10.427
0.097 0.099 0.097 0.094 0.092
...,
0.097 0.098 0.096 0.093 0.098 0.095 0.094
change %
A I R DRY F I L M S
.. .
90 90 90 90 90 90 90
. .. ... .. .. .. ... ...
FIRST HUMID-CHAMBER
4
5 6 7
0.103 0.006 0.104 0.006 0.101 0.005 0.098 0.005 0.103 0.005 0.099 0.004 0.097 0.003
01095
1
9.414
0.092
2
10.702
0.094
3
9.481
0.092
4
8.946
0.088
8.691
0.08i
5
.. .
6
Broke
7
10,422 0.090
%
Gram
... . ..
15 15 20 20 20 30 50
95 95 95 95 95 95 95
-4 trace mjlky A trace milky A trace milky A trace milky A trace milky Almost clear Almost clear
TREATMENT, 94 H O U R S
95 95 95 95 95 95 95
Milky.no cracking Mi1ky:no cracking Milkyfno cracking Milky; no cracking Mi1ky;nocracking Milky; no crackmg Somewhat milky; no cracking
SECOND AIR-DRYING
80
75
85
80
Slightly milky; coarsely cracked all over A trace milky: coarsely cracked all over A trace milky. coarsely cracked all over, bud less than 1 A trace milky; coarsely cracked all over A trace milky: a few cracks only Clear; no cracking Clear: no cracking
. . . 85 85 . . . 85 85 .. . 95 90 90 98 ... ... 95 98 S E C O N D HUMID-AIR CHAMBER 50 75 Milky: cracked coarsely, densely ... ... ... 60 75 Milky: cracked coarsely, densely 60 75 Milky: cracked coarsely, densley . . . 65 75 Milky.cracked coarsely densely .. .. .. 75 75 Milky: cracked coarsely: densely 80 90 Milky, few cracks .. .
.. . ., .
... .. . .. . .. . ...
85
90
75
75
75
75
75
75
75
75
75
85
90
90
90
(Films prepared from varnish containing 22.7 grams of bleached refined shellac dissolved in the following: FILM WEIQET ALCOHOL TRICRESYL PRORPHATE Grama % Grams % 30.88 95 None 29.74 1.14 95 5 28.61 95 2.27 10 27.47 3.41 95 15 26.34 4.54 95 20 25.20 5.68 95 25 These were poured on 4-inch watch glasses and 3-inch circle of film used for teat. WEIQHT Glass and air-dry Film CLEARFILM film only NESS GLOSS h PPE A R A NC E Grams Gram % % AIR-DRY FILMS
1 2 3 4 5 6
Slightly milky; cracked densely, coarsely Sliehtlv milkv: cracked denselv. coarsely Slightly milky: cracked coarsely and profusely finely Slightly milky; cracked coarsely and profusely finely Slightly milky: cracked coarsely and profusely finely Nearly clear: 1-2 coarse cracks: profusely very finely cracked Clear: no cracks
Table V gives results for seven varnishes prepared from the same sample of bone-dry bleached shellac containing varying percentages of tricresyl phosphate from 0.1 to 10. It can be seen that the addition of this plasticizer gives very marked increased resistance to treatment with moisture, especially when added to the extent of 10 per cent. The addition of this reagent further enhances the general appearance of the films when they are dried under the slightly humid conditions with which these films were prepared. The plasticizer retarded absorption of water during the first humid-chamber treatment. The film containing 10 per cent tricresyl phosphate absorbed only half the amount of water as compared with that of the unplasticized film, On air-drying they returned t o within 1-2 mg. of their original weight. On second treatment in the humid chamber, less water was absorbed than in the first. This time they gained equally in weight and were within 1mg. of the original weight. However, the clearness fell to 50-85 per cent, and the gloss to 79-90 per cent, the low figures corresponding to films containing little or no plasticizer. The third air-drying caused films to lose 4-5 mg. in weight. Films containing less than 5 per cent tricresyl phosphate lost their clearness and gloss, whereas the 5 and 10 per cent plasticized films were little affected or not a t all. The plasticized films became milky more slowly, and those containing large amount
24.136 18.712 21.657 18.343 22.154 20,011
0.209 0.254 0.285 0.277 0.318 0.340
24.131
0.204
18.709 21.654 18.341 22,152 20,008
0.251 0.282 0.275 0.316 0,337
Clear Clear Clear Clear Clear Clear HUMID-CHAMBER TREATMENT, 13 D A Y E 90 Somewhat milky: profusely finely 90 (hair) cracked Clear. no cracks 95 98 Clear! no cracks 98 98 Clear: no cracks 98 98 Clear; no cracks 98 98 Clear: no cracks 98 98 98 98 98 98 98 98
VENTILATED
Milky, no cracks
T A I R D AIR-DRYINQ
60
TABLEVI. EFFECTOF ADDINGGREATER THAN 10 PER CENT TRICRESYL PHOSPHATE TO SHELLAC VARNISHES
98 98 98 98 98 98
STILL-AIR H E N I D - C H A M B E R T R E A T M E N T 4 DAYS. WITH DEU' FORMATION
1
24.162
0,235
10
2 3 4 5 6
18.742 21.683 18.370 22.189 20.039
0.284 0,311 0.304 0.353 0.368
10 10 15 15 25
50 (25) Opaque; cracked profusely, peeled a t orarks Opaque; no cracks, no peeling Opaque; no cracks, no peeling Opaque: no cracks, no peeling Opaque: no cracks, no peeling Opaque; no crackx, no peeling
1
....
,,,
50
0
T
18.703
0.245
60
60
3
21.648
0.276
85
90
4
18.338
0.272
95
95
5 6
22,149 20.008
0.313 0.337
95 95
95 95
1
....
,,
60 70 70 70 75
S E C O N D AIR-DRYINO A F T E R 30 H O U R S
.
Milky; most of coating has cracked and peeled off glass art still adhering has pearly 'sEeen but no gloss. what is left on glass ia craoked brofusely Somewhat miik,y. profusely cracked; no peeling A trace milky; sparingly coarsely grooved; no peeling Faint trace of separation: no cracking. no peeling Clear: no oraoking: no peeljng Clear: no cracking: no peeling
AIR-DRYING AFTER 11 DAYS
..
,
.
2
18.702
0.244
60
60
3
21.648
0.276
85
90
4
18.338
0.272
95
95
5 6
22.149 20,008
0.313 0.337
;I
95
Film mostly peeled off glass; appearance of remaining part same as on 3/31/31 Milky; profusely cracked; no peeling Slightly milky: sparingly coarsely cracked: no peeling Faint trace of separation in radial design resembling fingerprint; no cracks Same Clear: clear etching on surface; no cranking
Table VI1 summarizes a series of results using tricresyl phosphate in a number of bleached shellac varnishes selected from three sets of experiments. The films of the bleached bone-dry shellac were air-dried for 4 days, and then placed in a humid chamber containing still saturated air for 24 hours. They were then allowed to air-dry a t room temperature for 6 days and given B second humid treatment for 20.5 hours. The final observations given in the table were made after air-drying for 35 days.
167
1
2.
liune-dry bleaolied shellac Hoiie-dry blesahcd s l d l n ~
0% tricrwyi p1i"s~'llsi.e triornyi pliorplinie trirreiyl piwnphaie
;:g
S. Bone-dry blenched nhellsc
'r
ti^ 'IRU:WSJY~. Piiospii*r.r~:0 5 l h ; . < c t i c ! > Slrr,:l.r.
