Effect of Heat and Light on Nitrocellulose Films - American Chemical


Plasticized nitrocellulose films were subjected to the action of heat and light, and the effects of these agents on mechanical properties, water perme...
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I X D U S T R I A L A N D EXGIXEERIh'G CHEMISTRY

980

Vol. 23, No. 9

Effect of Heat and Light on Nitrocellulose Films' W. E. Gloor HERCULES

UNLIGHT is the agent most destructive to the protective properties of n i t r o cel lu 1ose lacquer. Not only does it subject the protecting film to the stresses incidental to normal temperature change, but it also promotes photochemical changes in the film itself. A study of the effects of light and of radiation, in general, upon the physical and chemical nature of the film should thus prove of value in developing nitrocellulose lacquer for all-round uses.

S

EXPERIMENT STATION,

WILMIXCTOX,

DEL.

Plasticized nitrocellulose films were subjected to the action of heat and light, and the effects of these agents on mechanical properties, water permeability, viscosity of the film material, acidity development, and nitrogen content of the films were determined. Heat is shown to produce softening and a gradual lowering of viscosity and moisture resistance: its effect on mechanical properties seems more pronounced with increasing amounts of plasticizer in the film. The added effect of light is to impart brittle properties to the film, to lower its viscosity and moisture resistance rapidly, and t o accelerate the development of acidity: its action is principally on the nitrocellulose in the film. The data indicate that the effects of heat are general, while those of light are localized at the surface exposed.

Treatment of Films with Ultra-Violet Light

Not all of the wave lengths present in sunlight are effective in promoting this photochemical decomposition. DeVore, Pfund, and Cofm$n (1) have shown that the wave lengths greater than 3130 A. have much lower quantum efficiencies than those below this length and that this particular wave length showed the highest quantum efficiency of the range investigated. With the sun a t the zeniih on a clear day, these efficient wave lengths (29OQ-3250 A.) carry 0.24 per cent of the total solar radiation, 0.86 per cent being transmitted by the range 3250-3500 8., while 2.8 per cent is borne by the range 3500-4000 8. (2). FIG 1

shows such a picture of a 10 1/2-secondtricresyl to 7.5 R.S. phosphate film. It is seen that the practical absorption limits of a film containing nitrocellulose o n l y , as s h o w n by t h i s methood,lie b e t w e e n 33402400A. T h e u p p e r l i m i t must lie close to the true absorption limit, for experience in this laboratory has shown that the Uviarc discoloration of nitrocellulose films takes place at half the usual rate when the wave lengths below 3130 are screened out by ordinary window glass. The lower wave length given is that of the lowest mercury-arc line of sufficient intensity to affect the film; as such low wave lengths are not present in sunlight, the lower limit seems of little practical importance. Further, these lines are a direct evidence of water sensitivity. To show the effect of radiation on water sensitivity, the water permeability of films given different treatments was measured on an R.S. 4-second nitrocellulose, of 12.0 per cent nitrogen content. The film composition was 10 parts nitrocotton and 7.5 parts tricresyl phosphate, dissolved in a 50-50 butyl acetate-toluol mixture. With films of equal thickness, a form of Fick's diffusion law should apply:

E f m C r OF LIGHT Oh'N/TROCELLULOSE RLV

dn

l f f T ~ N S l ~ l N O BY l~~ HEIGHT D OF LINE

dn where dt K

dc

=Kg&

water vapor penetrating in grams per hour diffusionor permeability constant q = area of film, square centimeters = concentration gradient of water vapor prevaildx ing in film, assuming it uniform for present purpose

Some idea of the extent of the decomposition obtained when a nitrocellulose film is irradiated by ultra-violet light is given by Figure la. Light from a Uviarc is directed through a Hilger E-26 quartz spectrograph, and is focused on a nitrocotton film spun on clear glass, in the plate holder. After an exposure of 24 hours the film is removed and is immersed for 2 minutes in a 2:l volume mixture of water and acetone. Upon drying, the film areas attacked by the various wave lengths show up as white lines whose position is easily determined by reference to a spectrum of the mercury arc. According to DeVore, Pfund, and Cofman, who reported the method ( I ) , this whitening is due to the increased sensitivity of the decomposed areas to water, with consequent precipitation of any undecomposed nitrocellulose. Later results will be presented which are obtained by exposing films containing various lacquer ingredients in this manner. Figure l b Presented before the Division of Colloid 1 Received April 14, 1931. Chemistry at the Slst Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931.

=

=

Permeability can thus be measured by separating a saturated and a completely dry atmosphere by a film, maintaining a constant temperature, and measuring the weight of water passing through the f ilmin a known time. This was done by putting water in vessels sealed with nitrocellulose films and placing them in a phosphorus, pentoxide desiccator at 25" C. Weighings were made and phosphorus pentoxide recharged daily. A convenient vessel for such work was found to be the top of a small-mouth 12-ounce (354-433.) bottle, cut off about one inch (2.5 cm.) below the flare, and inverted, thus giving a goblet-like cup. The mouth was closed with a clean rubber stopper and the cup filled with mercury to within 0.5 inch (1.27 cm.) of the top. Films 0.01 cm. thick were laid down on this surface from the above solutions and aged a week, and the mercury was removed from the bottom by suddenly pulling the stopper. Ten cc. of water were then added and the top again stoppered, weighed, and placed in the desiccator a t 25 O C. Table I gives the results of this work with irradiated films. It is seen that 4 hours of radiation at a temperature of 70 O C.

INDUSTRIAL A N D ENGI~VEERIA'GCHEMISTRY

September, 1931

has the same effect on permeability as 24 hours' heating, while 8 hours of irradiating nearly doubles the permeability. Table I-Water Permeability of Films 10 R.S. 4-second nitrocellulose, 7 . 5 tricresyl phosphate TREATMENT K Gramr/ctn./hr./cm.' None 1 . 0 9 x 10-4 1. 5 4 4 hours' irradiation, 70" C. S hours' irradiation, 70' C . 2.89 24 hours' irradiation, 70' C. 1.53

From these results it is evident that the idea of the destructive radiations $ken by the method of development used is a correct one, aside from the fact that they point out a distinct effect of ultra-violet light on film properties.

981

70' C. oven. Stress-strain measurements were made, using the h'ew Jersey Zinc machine (S), 14 days after laying down the films. h'itrogen and viscosity data were taken immediately after the completion of the exposures listed above. The viscosity change was obtained by dissolving untreated and treated film in the butyl acetatealcohol solvent to a 6 per cent nitrocellulose concentration, and determining the viscosity on a capillary tube viscometer. On some of the irradiated films it was found necessary to add the alcohol first to thoroughly wet the material and then the butyl acetate in order to get solution. Films that required this method of solution always gave small portions of insoluble matter easily seen in solution but not in sufficient volume to be examined for nitrogen content, etc. The results of the stress-strain test are showii in Figure 2. Irradiated films in most cases gave a curve only slightly below that of the original but with a marked decrease in breaking elongation, Films, given heat treatment only, showed a marked loss in tensile strength and an equally distinct increase in breaking elongation. Relative Effect of Heat and Light

*.aWrrrw

Due to the fact that films of nitrocellulose alone become very brittle after short periods of Uviarc irradiation, tensile properties together with the corresponding chemical changes were measured on films plasticized with tricresyl phosphate. A series of nitrocelluloses containing 12.00 * 0.03 per cent nitrogen, and varying in viscosity, was used. The effect of ultra-violet irradiation on the tensile strength of films made from nitrocellulose of the foregoing nitrogen content was measured on '/,-second, 4-second, and 70-second types, plasticized with tricresyl phosphate in various ratios. The change in viscosity of these films is shown in Table I1 together with their composition. Table 11-Change

To compare the relative effect of heat and light, it was considered preferable to compare these properties for a series of films treated by both agencies to the same viscosity in the solvent. From Table I1 it is seen that 4 hours of radiation a t 70" C. produces a viscosity reduction very near to that given

FG 3

I

I

i n Viscosity of Nitrocellulose

ORIGINAL

FILMCOMPOSITION

VISCOSITY

cp.

VISCOSITY AT 70' C. AFTER TREATMENT BELOW S 14 24 48 Hrs. Hrs. Hrs. Hrs cp. C P . C P . C P .

10 R.S. l/a-sec. 7 5 Tricresyl phosphate

10.2

10 R . S . 4-sec. 10 T.C.P.

56.1

2 5 . 6 19 S 33.8 . .

10 R.S. 4-sec 7 . 5 T.C.P.

52 9

20.6 20.7

10 R . S . 4-sec.

57.2

5 T.C.P.

10 R.S. 70-sec. 7 . 5 T.C.P.

S,56.0

9 2

..

TREATMENT

S.S...

...

Rayed Heated

12.1 32.0

..

Rayed Heated

18.8

,

7.4

..

44.2 37.0

Rayed Heated

2 5 . 2 20 9 ,.. ,.

14 4 . . . 4 6 . 4 31 8

Rayed Heated

lk7:3 68.0

202.8 1 4 1 . 5 50.3 . . .

Heated Rayed

,,

.

..

335.5

The films were laid down on mercury to a thickness of 0.01 cm. from solutions that contained 10 per cent nitrocellulose in a 50-50 butyl acetate-alcohol solvent. After dTing for 10 days, the required test strips were cut and given the treatments outlined in Table 11. It was desired to obtain the effect of both heat and light on films, so the materials were exposed to radiation in a cylindrical drum, 20 inches (50.8 cm.) in diameter and closed a t the ends, in the axis of which was placed a horizontal Uviarc, operating 4-5 amperes and 160-170 volts across the terminals. The temperature inside the drum varied from 70" to 75" C. The test strips mere laid on asbestos strips along the circumference of the drum and irradiated for the various times together with 4 by 4 inch (10.2 by 10.2 em.) strips for the viscosity work. Similar sets were given heat treatment in a

;4

%

8"

-

_n

by 24 hours' heating a t 70" C. A series of films as shown in Table I11 was given such treatment. The tensile strength data are shown in Figure 3. The nitrogen content was determined by the modified Gunning method, analyses being made on treated and untreated films. It was impossible to use the more rapid nitrometer method owing to side reactions with the plasticizer. Table 111-Change FILMCOMPOSITION

in Nitrogen C o n t e n t of F i l m s Treated t o

Equivalent Viscosity TREATMENT, VIS- NITRO70' C. COSITY G E N DIFFERENCE

cfi

77,

I _

10

h'one 4 hours, ray 24 hours, heat

9 . 2 6.48 S.6 5.96 7.7 6.06

0:52 0.42

10 R.S. 4-sec 7 . 5 T.C.P.

None 4 hours,ray 24 hours, heat

52.9 6 . 5 36.0 6 . 0 4 4 3 . 8 6.10

0.'46

10 R S. 70-sec. 7 . 5 T.C.P.

None 5 hours, ray 24 hours, heat

R . S . I/a-sec. 7 . 5 T.C.P.

335.5 192. 203.

0.40

6.47

..

6:39

0.08

Another point of great interest in this matter of relative stability is the effect of such treatments on the methyl-violet stability test. Table I V gives the methyl-violet tests shown by various nitrocelluloses on exposing them to heat or light when spread out in thin layers in weighing dishes, 60 by 30 mm ,

INDUSTRIAL A N D ENGINEERING CHEMISTRY

982 Table IV-Change

Vol. 23, No. 9

in Methyl-Violet Test of Various Nitrocelluloses after Heat and Light Exposures

nitrocellulose is changed upon exposure to light. The nature of this change leads to the conclusion that ultra-violet light METRYLVIOLETAT 70' C. APTER 48 HOURS causes a local denitration wherever a correctly energized Heat only Rayed quantum strikes the film, and this gives rise to insoluble bodies Mln. Min. 30 60 in the colloid owing to denitration and degradation. Polar24 60 1/1 63 ized light does not bring out such particles, however. 70 27 4 77 70 70 16 55 The effect of denitrated cellulosic bodies, visible at low magnifications and showing distinct orientation in polarized It is well to point out the differences between the effects of heat and light as found thus far. Heat a t 70" C. does cause light, on the stress-strain curve is shown in Figure 4. Here marked changes in viscosity, tensile properties, and nitrogen finely comminuted low-nitrogen nitrocotton and low-viscosity content; the added effect of ultra-violet light is to cause much cotton linters were incorporated in a formula to give the same more rapid loss of nitrogen and viscosity, to increase the nitrogen and solids as the irradiated 4-second film in Figure acidity development as shown by the methyl-violet test, and, 3. The decrease in elongation due to these particles is distinct, of greatest significance, to alter the tensile properties of the even though not as much as occurs on irradiating. It is found that on washing irradiated nitrocellulose which heated film remarkably and in a manner directly opposite to has been given treatment similar to that in Table IV, the the softening produced by heat. I n F i g u r e 3 t h e methyl-violet test goes back to its original value. Also, m ~ s m m mf -i m r m treated films of each lacquer films were irradiated 8 hours in the Uviarc and placed E m m7x mmt MAmm4Ls type are of approxi- in a weatherometer. After 9 cycles they showed a prominent mom.w,c~anrrn w~ mately the same vis- check failure; upon continuing exposure, it was noted that the cosity in solvents, yet failed surface washed off and the dulled under-surface was as as an untreated film. This points to surface . the irradiated samples protective show this same char- denitration and degradation. These data seem to indicate a pronounced local surface acteristicinspite of the d i s c r e p a n c y in denitration and degradation as the principal effect of ultratimes of t r e a t m e n t . violet light on nitrocellulose films, while that of heat is one of a The brittleness shown general denitration and degradation, coupled with a certain can be due to the ac- amount of continued colloiding action of the solvent plastitions either on plas- cizer on the nitrocellulose, this softening becoming more pro% BaVGLVfN ticizer or on n i t r o - nounced with increasing plasticization. cellulose. Merely Acknowledgment looking a t the differences in viscosity and strength obtained with different ratios of plasticizer is sufficient to see that it has The author wishes to express his appreciation of the helpful some effect on the degree of action obtained. But tests here advice and stimulating criticism of his colleagues a t the have shown the temperature of treatment to cause only an in- Experiment Station. significant evaporation of plasticizer. Also, irradiated triLiterature Cited cresyl phosphate does not lose its solvent properties for nitrocellulose appreciably, nor does it give films with brittle prop(I) DeVore, Pfund, and Cofman, J . Phys. Chem., 88, 1842 (1929). erties when mixed with untreated nitrocellulose. (2) Luckiesh, "Artificial Sunlight,'' p. 69, Van Nostrand, 1930. On the other hand, spectrographic work has shown that (3) Nelson and Rundle, Proc. Am. SOC.Tesling Materials, 21, 1111 (1921). ORIGINAL VISCOSITYOF METHYL NITROCOTTON VIOLET Scc. Min. 1/4 57

Prevention of Gelling of Bronze Lacquers' Charles Bogin, Vaughn Kelly, and William Maroney COMMERCIAL SOLVENTS CORPORATION, TERRBHAUTE,INDIANA

HE rapid gelling effect

Gelling of lacquers containing copper bronze can be long been interested in findof copper bronzes on prevented or greatly delayed by the use of certain ing some means of preventnitrocellulose solutions acids and salts, such as boric, citric, malic, tartaric, ing, or a t least delaying, the occurrence of this gelling. has b e e n k n o w n since the and lactic, in the proportion of 0.25 to 1.0 per cent. earliest uses of such lacquers Boric acid is preferable to all the others on account of Thereispracticallynoinforas coating materials and has its low price, minimum darkening and tarnishing mation on this subject availled to the practice of supplyeffect, and great uniformity of behavior. Proportions able in the literature. Woring the liquid vehicle and the of inhibitor necessary depend on the grade and concenden (2) states that cellulose bronze in separate containers tration of the bronze powder, on the purity and age of nitrate b r o n z i n g l a c q u e r s and mixingthe two just before the nitrocellulose, on the presence and proportions of containing aluminum tend to resins, and on the degree of protection desired. develop acidity and coaguuse. In many cases even this l a t e , especially if kept in remedy has been found insufficient, as some such mixtures gel appreciably in a few hours or metal cans. He describes a patent where the addition of overnight and any portion of the lacquer unused a t the time 8 ounces of sodium carbonate per gallon is recommended of gelling is irrevocably lost. The lacquer trade has, therefore, in cellulose acetate lacquers. The absurdly high proportion of salt used is commented upon, but the matter is not dis1 Received April 9, 1931. Presented before the Division of Paint and No mention is made Of the gelling Of nitrocussed Varnish Chemistry at the 81st Meeting of the American Chemical Society, cellulose lacquers. Wilson (1) states that free acid is detriIndianapolis, Ind., March 30 to April 3, 1931.

T