PHT-:C.IL
CHEAIICIL ISVCSTIGATIOSS
OF GOLDESROD
RL-BBER.
11
293
PH17SICA4LCHEJIICLIL IST7ESTIGAATIOSSOF GOLDEKROD RG3BER. I1
THEI’KC :I‘IT.ITIOS
YALL-EIS .IFFECTED BY IRR ~ D I A T I O S IS THE PRESESCE OF T-.%RIoL-~ RL-BBER-~STIOSIDASTS ASD OTHER REAGCSTS
L ~ 7 ~ 4 1 L. - D SILiU, FLORCSCE B KREEGER,
ASD
WILLIAM J. R U S C K E L
,\ ,(iillLl1i Regional Research Laboratory,‘ .Vex Orleans, Loxisinnu Received J m i i u r y SO, 19.45 i i r r t i u n Trith the Emergency Rubber Project, goldenrod rubber has k e n I W 1i+rw1on a pilot-plant scale from the leaves of the goldenrod (Solidago lcni o ~ w o r - h7 at the Southern Regional Research Laboratory by a two-stage extmcricon prc)cess in which the leaves n-ere first extracted n-ith acetone to remove resins n n J then v i t h benzene to recover the rubber. The present publication is the 0[1igr() vtfi of an investigation of the effectiveness of various rubber antioxidant- when added to the benzene extracts so obtained. The results may find applicuiib)xi ’ . ) extracts of other natural rubbers that are obtained from leafy niatei~lal:tnd may contain chlorophyll. I t lis3 been -holm previously (11) that when a benzene solution of goldenrod rubber ii irradiated in a Pyrex vessel in the presence of oxygen the precipitation value incre-t-e-. the viscosity decreases, and the rubber undergoes degradation to a more !iquicl and sticky product. The precipitation value was defined as the number 01 milliliters of absolute ethyl alcohol required to produce a cloud point nt 25Y’. in 10 ml. of a clarified benzene solution containing 0.0175 g. of sample. The exact nature of this degradation is uncertain, but for hevea rubber it i q geaemlly considered that it involves some kind of oxidation (1, 3, 4, 5 , 9). -11$0, a . in the case of hevea, it may be inhibited by antioxidants and accelerated by actii-rt , A or.. In the present investigation the precipitation value has been used to investigate the light degradation of goldenrod rubber in benzene solution and especially to compare the effects of various commercial* rubber antioxidants and other reagents OR :he rate of such degradation. EXPERIAIESTAL
That goltlenrod rubber takes up oxygen on irradiation was shown by the folloning experiment: *A benzene solution of about 1 per cent concentration was clarifiell nith Nerck’s activated charcoal (11) and put into t v o stoppered Pyres Er1enn:eyPr flasks. One flask n-as kept in the dark, and the other was exposed to sunligl; for G hr. Aliquots of these solutions were brought t o a concentration of 0.175 g. total solids per 100 ml. of benzene solution, and 10-ml. portions 1 0 1 1 of ~ tke laboratories of the Bureau of Xgricultural and Industrial Chemistry, Agricultural Research .idministration, Vnited States Department of Agriculture. The use of trade names for identifying particular substances employed in this investigation does n3: constitute a recommendation of such substances or an endorsement of their manufaiiureis or distributors by the U. S. Department of .4griculture.
296
EV.4LD L. SK4E7FLOREXCE B. KREEGER A S D WILLIAM J. RUSCKEL
were then titrated to a cloud point with absolute alcohol to determine the precipitation value. This value n-asfound to be 4.40 nil. for the non-irradiated sample, the same as the original, while that for the irradiated sample had changed to 21 ml. Both irradiated and non-irradiated solutions were evaporated to dryness; benzene was removed in a nitrogen atniosphere and then in high vacuum; and the samples were subjected to combustion analysis. Empirical forniulas n-ere calculated from the data, five carbon atoms being assumed in the goldenrod isoprene unit and a trace of nitrogen disregarded. The results are tabulated below : SAXPLE
i Kon-irradiated Irradiated..
.
,
1
" " " '
., ..., , , , .,
'
-I per cent
86.0 86.1 78.6 i8.4
I
p e r cent
,
11.48 11.40
,
10.46 10.25
1
These data shov that a total of about 9 per cent of its n-eight of oxygen n-a% taken up by the goldenrod rubber during the irradiation, that the w n of the percentages of carbon and hydrogen Jyas changed by irradiation from 98 per cent to 89 per cent, and that the calculated formula had changed from (CbH; 9&)o to (CbH7,9200.~3),.Thus, 0 hr. of irradiation resulted in an increase of oxygen content corresponding to 0.4 atom of osygcn per isoprene unit.
Procedure f o r compahizg antzozidants Several common antioxidants and other reagents were tested by adding them t o clarified goldenrod rubber solutions, irradiating the solutions, and comparing the precipitation values obtained after irradiation. The solutions were benzene extracts of acetone-extracted goldenrod leaves and were prepared from henzene extracts obtained in the pilot plant. As these extracts were very highly pigmented, it n-as necessary to clarify them. They Tvere first brought to a concentration of about 1 g. of total solids per 100 ml., and an amount of Nerck'h activated charcoal equal to the n-eight of the rubber was added. The mixture \vas stirred well and allon-ed to settle. I t n-as then filtered through a layer of the charcoal and a filter aid (Hyflo Super-cel) on a Buchner funnel, brought to a concentration of 0.175 g. per 100 nil., and stored in the dark. During the preparation of the solution exposure to light iyas avoided as much as possible. This is the standard procedure and concentration used in the determination of precipitation value (11). Ten-milliliter aliquots of this stock solution IT ere pipetted into thc required number of 30-nim. test tubes. Each tube contained a 5-mg. portion oi a qpecific antioxidant. After solution was complete all the tubes were espoqed himultaneously to direct sunlight. Complete sets v-ere removed at definite interrals and placed in the dark. Each set included duplicate blanks containing no anti-
PHYSICAL CHEMICAL ISVESTIGATIOSS O F GOLDESROD RUBBER.
I1
297
oxidant. The precipitation value n-as then determined for each tube by titrating t o the cloud point n-ith absolute alcohol, and the temperature at the enti point Tyas noted. The precipitation values were corrected to 25OC. by subtracting (or adding) 0.05 nil. for each degree abo7.e (or belom-) that temperature. Precipitation values above 25 ml. are not measurable because of the high dilution involved. The presence of the added reagents did not affect the precipitation value of the stock solution appreciably. Table 1 shon-s the results of such an experiment. Irradiation of the blank (the stock solution) for GO min. changed its precipitation value from 4.43 nil. to 9.6 ml. On the other hand, no change in the precipitation value was noted after a GO-min. irradiation of the blank in low-actinic Pyrex glass that had :approximately the follon-ing tranmiission characteristics: 0 per cent at 3000 .L; 1 per T-IBLE 1 Stinlight i r r a d i a t i o n f o r 60 inin.; 5 m g . of reagent i n 10 nil. o j benzene solution c o n f a i n i n g 0.0175 g . of goldenrod rubber; precipitation value of original s a m p l e = 4.45 n!l. PRECIPITATIOP; VALUE
AFTER 60 X I S .
REAGEST ADDED I
IRR.AD1.ATIO.Y
ml.
................................. ....................... p-Nap hthylainine Pyrogallol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........................................... Catechol. . . . . . . . . . Hydroquinone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X o n e (blank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quinone ................................. Phenyl-@-naphthylamine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benzophenone.. ........... :........................................
'I
I
5.6 7.0 8.0 8.3 8.7 3.9
9.3 9.6
.I
12.0 15.0 Over 25.0
After 60 min. irradiation in loF-actinic red Pyrex glass tube, the titration value was unchanged, 4 45 ml.
cent a t 4000 -%.; 4 per cent a t 5000 8.; and 12 per cent a t 6000 8. I t is apparent that this sFecial glass filters out, for the most part, the wave lengths of light that are effective in the photochemical reaction involved. JZF, Flectol H (Sample l ) , Agerite Resin D, p-naphthylamine, p~iogallol, catechol. and hydroquinone inhibited the light reaction in lesser and leqser degree in the order named, ~vhilequinone, phenyl-p-naphthylamine, and henzophenone increased the rate of the reaction. Since 25 ml. is the highe-t precipitation value measurable, it is apparent that the benzophenone sample n i ~ yhave reached that ~ a l u ein condernbly less than GO min. I n general. the lower the precipitation value after irradiation, the gieater i. the a n t i o d a n t or inhibiting effect of the reagent added. i f the precipitation value after irradiation is higher than that of the irradiated blank, the added reagent can be said to enhance or activate rather than inhibit the light reaction.
298
EVALD L. SKAL-, FLORESCE B. KREEGER ASD s v I L L I m J. RUSCKEL
In table 1 and in all other tables, the reagents tested are arranged in the order of their apparent efficiencies as inhibitors; all above the blank are inhibitors, all below are activators. The resultc: of a similar experiment including additional commercial antiosidants are recorded in table 2. X number of apparent anomalies are disclosed by a comparison of these results with those of table 1. The commercial antioxidants, JZF, Flectol H (Sample l), and Agerite Resin D, show quite definite inhibiting effects on the reaction (table 1). Table 2, however, shows that JZF TdBLE 2 Sunlight irradiation; 5 mg. of reagent i n 10 ml. of benzene solution containing 0.0175 g. of goldenrod rubber; precipitation value of original sample = 4.45 ml.
p-Aminophenol . . . . . . . . . . . . . . . . . . . . . . I Stabilite Alba. . . . . . . . . . . . . . . . . . . . . . .................
.I
JZF . . . . . . . . . . . . BLE Powder.. . . . . . . . . . . . . . . . . . . . . . Agerite Resin D . . . . . . . . . . . . . . . . . .
PRECIPITATION VALLT. AFTER IRRADIATION FOR
,
R E 4GENT ADDED
, ,
Chlorocarvncrol (crude) . I Flectol H (Sample 2 ) . . , . Diphenylaini ne . . . . . . . . . . . . . . . . . . . . . . -4lbasan.. . . . . . . . . . . . . . . . . . . . . . . . . . . i .................. 1,4-Saphthaquinone. . . . . . . . . . . . . . . . . Phenyl-8-naphthylamine . . ~
~
MUF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stabilite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chlorothymol T.F. V I I . . . . . . . . . . . . . . BLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i Carvacrol C.P.. . . .
10 min.
70 min.
ml.
ml.
5.10 6.4 5.7 6.7 6.3 6.7 6.2 6.4 7.0 6.7 8.3 -
r
I . i
9. 7 11.0 11.0 12.5 23.0 25+ 25+ 25+
6.0 7.1 6.9 7.5 7.7 8.5 8.5 8.3 10.0 9.0 11.3 11.0 16.0 16.0 19.0 20.0 25s 25+ 25+ 25+
125 rnin.
tnl.
ml.
6.50 7.8 s.5 9.3 10.0 10.5 11.0
6.7 9.5 10.5 10.6 11.8 12.5
11 .0
23.0 13.0 13.5 24. 25 25425425+ 25f 25+ 25+ 25+ 25+
12.5 12.5 15.5 20.0 25 24. 25+ 25f 25+ 25f 25+ 25+
+
+
was less effective, Agerite Resin D had no effect, and Flectol H (Sample 2) had a slight activating effect. Furthermore, phenyl-/%naphthylamine, iigerite powder, and other commercial rubber antioxidants favored the light reaction strongly. The results of other similar irradiation experiments, for example those given in table 3, verified the effectiveness of Flectol H (Sample 1) and p-aminophenol as inhibitors. In this particular case, as shoim by the behavior of the blank, the stock solution used was obviously very much more sensitive to light than that used to obtain the results shon-n in tables 1 and 2. Examination of the stock solution revealed that it still contained considerable pigment, which indicated that the charcoal treatment for clarification had probably not been as effective as usunl.
.
. P H P E I C B L CHEMICAL ISVESTIGXTIOKS O F GOLDESROD RUBBER.
299
ZC
The discrepancy between Flectol H (Sample 1) and Flectol H (Sample 2) might be due either to a difference betn-een the tv-o samples or to 3 difference in the two stock solutions of goldenrod rubber used. The results given in table 4 n-ere obtained to determine the relative efficacy of Flectol H (Sample 1) and Flectol H (Sample 2 ) , and to determine the effect of residual pigmentation on the TABLE 3 S u n l i g h t i r r a d i a t i o i ~ ;5 wig. of reagent per 10 ml. of benzene solution c o n t a z t i i ~ yc 6175 g . o j golden? od rubber (sainple still slightly pigmented) PRECIPITATIOX VALL7E AFTER I R R A D I T I O S B 13 REAGEXT ADDED ~
Flectol H (Sample 1).
I
................ ~
Omin. 7121.
4.45 4.45
40 min.
'
. 1
1
60min.
mi.
I
ml.
9.5 9.7
1
23.0 23.0
K o n e (bla?ik).........................
4.45 Y.45
4.45
10.0 10.0 1
1
~
22.0 22.5 2 ~ +
25f
1
I
I
p-Aminophenol. .....................
3; min.
i
vi!.
25-k 25+ 2525-i2;"
I
TBBLE 4 S u n l i g h t irradiation; 5 m g . o j Flectol H p e r 10 m l . of benzene solution containz~t,nL J . < V ~ ?g~. of goldenlod rubber; rubber solution C was least colored I RUBBER SOLUTIOS
, 1
~
Omin.
I
~
-4
. I
PRECIPITATION VALLT AFTER IR2AD;AT;OS
FOR
R E A G E S T ADDED
Flectol H (Sample 1) Flectol H (Sample 2) il'one ( b l a n k )
'
1
ml.
4 60 4.50 4.60
~
30min.
70 min.
,
ml.
1
8.5 8.5 25+
15.0 14.0 ?5+
9.2 9.0 e5+
17.5 15.0
I
~
n!:.
I
B . . . ...
Flectol H (Sample 1) Flectol H (Sample 2) S o n e (blank)
c. . . . . . . . . . . ~
1
'
I
4.80 4.90 4.80
'
Flectol H (Sample 1) Flectol H (Sample 2) Xone (blank)
'
3.5+
I
I
4.40 4.50 4.85
1
1
6.00 6.00 6.5
5.G 3.5 10.2
results obtained. Three stock solutions, A, B, and C, prepared from different pilot-plant benzene extracts, n-ere so selected as to cover a range of degrees of pigmentation. Rubber solution A n-as slightly green; B, slightly yellow; and C, almost colorless. The results she\\- definitely that the two samples of Flectol H behax-ed essentially the same; they also indicate that the apparent effectiveness of Flectol H as an inhibitor of the light reaction clepends upon the amount of pigment left in the stock solution. In the case of rubber solution A, for example,
300
EVALD L. SIi.LU, FLOKESCE 13. KREEGER . i S D X I L L I A M 3 . RUSCKEL
after 30 niin. irradiation the Flectol H sample had reached a precipitation value of only 8.5 ml. \Then the value for the blank exceeded 25 ml. In the case of solution c', on the other hand, when the value for the sample in the Flectol H had reached 8.5 nil., that for the blank had reached only 10.2 ml. Thus the Flectol H apparently acted inore &icient!>- as an inhibitor in a colored solution than it did in a colorlesq solution. T-IBLE 5 ,Srcidight irradiation; 5 ? f i g . of reagent per 10 m l . of benzene solution containing 0.0175 g. of goldoirod rubber treated three t i m e s with carbon a n d practically colorless; precipitation calite of original solution = 4.45 m l .
,
P R E C I P I T . i T I O I I VALUE A F T E R 1RR.LDIATION F O R R E A G E N T ADDED
i ~
p-.hiinophenol. ....................
.I
13 min.
~
mi.
'
ml.
4.40
'
4.45 4.50
I
4.50
S o n e (biarik) . . . . . . . . . . . . . . . . . . . . . . .
.'
P.C.P. s o . 5 . . . . .
4.60 4.50
i
4.55 4.55
4 80 4.85
I
4.70 4.60
4.80 4.70
Stahilite .Ilbn. . . . . . . . . . . . . . . . . . . . . . .
.
~
'
I JZF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.65 4.65
~
!+.90
4.95 4 95 ~
1
I
5.00 4.80
I ~
ml.
4.70 4.70
5.10 5.20
1 1
1 ~
5.06 5.10
5.05 5.05 5.20 5.30
I 4.85 4.88
~
5.00 5.05
5.20 5.25
I
I
4.70 4.70
I
ml.
4.50 4.50
~
~Ieth~-luni'nelliferone .................
43 min.
33 min.
~
1.70 4.70
I
1
i
23 min.
4.90 4.90
'
5.40
'
5.20 5.20
,
5.30 5.30
I
AIgeriteResin D . . . . . . . . . . . . . . . . . . . .
.I
'
I
1
~
Flectol H \Sample 1). . . . . . . . . . . . . . . .
6.70 6.60
~
~
7.25 7.35
~
5.00 5.10
I '
- --
5.95 5.80
.
7.65
'
i.iJ
6.50 6.60
,
~
8.30 8.35
I
Phenyl-d-naphthylamine... . . . . . . . . . .
5.10 5.00
~
'
,
8.9 8.9
. ~
11.0 11.4
-1stock solution of goldenrod rubber that ivas practically colorless \\-as then prepared. This was acconiplished by three successive treatments, each n-ith n. fresh portion of Alerck's actirated ehni*coal. The various reagent's that had shown good mtioxidant or inhibiting effects (tables 1, 2 , and 3 ) were added to 10-nil. portions of this solution, and the irradiation nxs carried out'. =1 sample u-ith phenyl-&naphthylamine was inclucled. The results, given in table 5 , bring out :i nurnber of interesting facts. Of the mtiosiciants trsted, only p-aminophenol inhibited the light reaction.
PHYSICAL CHEJIIC.IL I X V E S T I G A T I O S 5 O F GOLDEXROD R U B B E R .
301
I1
P.C.P. So. 5, Stabilite -%ha,and methylumbelliferone had little or no effect, and JZF. .Agerite Resin D, and Flectol H (Sample 1) definitely activated the light reaction. Thus, by removal of the color from the rubber 3olution by successi\-r tr.e:itinents with activated charcoal the inhibiting effect of some of the antioxidant- was nullified, and in some cases it wis changed to an activating effect. TJG- exhaustive charcoal treatment also made the goldenrod rubber solution it-elf, n-ith no reagent added, much less sensitive t o light than were the solution- used in the previou:: experiments. Thiq iq shon-n by the fact that a 43-niin. irradiation of the blank had increased its precipitation value to only 5.10 1111. n- compared, for example, to an increase to more than 25 ml. for the corre-poncling blank in table 3, indicating that the charcoal treatment remove3 some conqtituent, or constituent., that activate the light reaction. T.1BLE 6 Z r i o d , o f ~ o ~C lI ~benzene solution containing 0.0175 g.
OJ
golderzrod rubber p e r 10 ml. with
added reagents PRECIPITATIOX V 4 L W A F T E R I R R A D I A T I O X FOR
RL'BBER S I N K SOLVIlCN
D ...., ... . D . . . , .. . . E....... .
E . .,. . .
,
.
R E A G E S T S ADDED PER 10 XL. SOLUTION
'
'
F. . . . . . . . . F . ,. , . , . . . .
Sone 5 mg. Flectol H 0.038 mg. chlorophyll 0.035 mg. chlorophyll and 5 nig. Flectol H 0.14 mg. chlorophyll 0.14 mg. chlorophyll and 5 mg. Flecto1 H
*
o min.
I
?d.
,
,
40 min.
ml.
1.60
5.20 6.00 25+ 9.2
4.70 4.70
25+ 25+
4.70 4.75 4.GO
I
1
' I
~
~~
80 min. ml.
5.90 9.1 25425+ PPt 25+
I
It i- Lite possible that the actiT-ating constituents in question are the pigments theni.eli e
