4
A test run n-as made for a number of a c t i w t o i i t o determine the gelling time in sunlight. The results were as folloirs: benzophenone, less than 4.5 hr.; benzaldehyde, less than 4.5 hr.; carbon tetrachloride. ’7 hr .; benzoyl peroxide, 8 hr.; maleic anhydride, 14 hr.; chlorophyll. A 6 hr.; ~ n qiiinone, d 10s hr. -4 ‘rlimE I IvadiatioiL o j goldenrod rubber in abseiico of oxugeii ACTIVATOR
Iiows
Benzophenone.. . . . .
0
t
a f
1
c cii! i p o isps
0.700 0.681 O.il0 0.731 (incipient gelation) (gel)
I
j
~
~
1111
4.45 4.75 4.55 4.55
Carbon tetrachloride
0 -2 1 2 3 4 5 6 7
0.684 0.679 0.680 0.674 0.728 0.T28 0.736 0 . 765
4.45 4.45 4.40 4.30 4.30 4.30 4.35 4.30
Benzoyl peroxide.. . . . .
0 1 2 3 4 5 6 7 h 9
0.665 0.683 0.68,i 0.690 0.685 0.697 0.691 0.696 0.716 (gel)
5.05 4.90 4.85 4.80 4~75 4.70 4.70 4.65 4.70
Maleic anhydride. . . . , . , , . . .
0
0.694 0.679 0.689 0.695 0.710 (gel) 0.571 (gel)
4.40 4.50 4.65 4.60 4.65
1 2
1 3
5 7 10 15
___
4.70
blank containing no activator did not gel aftei 108 hr. I n a previous experiment, hon-ever, in n-hich a different goldenrod rubber solution v a s used, a blank gelled in 10 hr. A similar inconsistency in the behavior of various goldenrod rubber solutions has been noted in the irradiation experiments in the presence
IRRADIATIOX O F GOLDEXROD RUBBER
3
of oxygen. This inconsi5tency depended upon the active constituents that remained in the solution after the tieatmelit u i t h activated charcoal (6). X seiiw of tubes for each of a niiniber of these activators n a s exposed to sunlight for different peiiods of time, one tube of each set being kept in the dark as a blank. After exposuie the tube< nere stored in the dark until they were opened and tested. Six milliliters of each of these solutions was diluted to 100 nil. to produce a 0.175 per cent solution of rubber. I n the sample with carbon tetrachloride, 12 ml. v a s diluted t o 100 ml., to gile the correct final concentration Ailiquots11 cre then user1 foi the meayurement of the absolute viscosity at 25°C'. by means of u modified Oqt\\ald 1-ivornrter, and also for determination of the piecipitation 1alue. Thc precipitation value ( i )i5 a measure of the precipitability. being clefinctl a \ the niimbcr of milliliters of absolute ethyl alcohol required at :t given tcnipeiatiii,e to produce a cloud point in 10 ml. of a clarified benzene wlution containing 0.01 75 g. of iubher sample. The I P ~ l t 5arc gii en in table 1. The initial \-altie\ for the viscosity and presipitation value of the different solutions differed slightly, either because of the mere presence of the activator in the solution or liecauqe of some change caused by the activator during qtoiage in the dark. In each case, continued irradiation caused an increase in the viscosity and resulted finally in gelation. K i t h the exception of the benzoyl peroxide sample, this increase was preceded by an initial decrease in Tiscosity. I n the latter case this initial decrease might actually have occurred during the first hour, a t the end of which the first sample was taken. Since the gel nhich finally formed was practically insoluble in benzene, its viscosity and precipitation value could not be determined. Formation of rubber hydiocarbon polymers of higher molecular weight by irradiation ~ o u l dbe expected to result in a higher degree of precipitability; that is, the precipitation value would be expected to decrease as the viscosity increased. This seems to be the trend for the carbon tetrachloride and the benzoyl peroxide samples. I n the case of maleic anhydride, however, the precipitation value actually increased, though the absolute viscosity increased from 0.694 t o 0.771 centipoiye. This coiroborates the opinions of Stevens ( l l ) , Nnunton (3), and Farmer (1) that the fuimntion of photogels is not the result of :t ,iimple polymerization arid that it may wmetimes involve condensation 11ith the activator. hUMMARY
The viscosity of goldenrod rubber can be increased until gelation takes place by irradiation of its henzenc sohition in the piebence of catalysts and in the absence of oxygen. I3enzophenorit1, henzaltlehycle, and carbon tetrachloride seein to 1~ the h e i t of the activators tr-tcd.
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'l'h~:iutlioi :iw intlcbtecl to Florence I
