304
E. L. SKAU, IT-.
J. RUSCI~EL, F. B.
KREEGER, ASD 31. A . ~ U L L I Y A S
PHYSICAL CHEMICAL ISTFSTIGATIOSS O F GOLDEXROD RUBBER. 111 THE FRACTIOSATIOK
OF
GOLDESROD ASD
OTHER S A T U R A L
RUBBERS
ET-ALD L . SKAU, WILLIAM J. RUSCKEL, FLORESCE B. KREEGER. MARY A. SULLIVAN
ASD
S o u t h er n Regional Research Laboratory‘, Mew Orleans, Loui si ana Received J a n u a r y 30, 1945
I t is generally accepted that natural rubber consists essentially of a niixture of homologous polymeric polyprenes differing widely in molecular weight. It was shon-n by Fol (3) in 1909 that the more readily soluble parts have a lower viscosity than have the more difficultly soluble parts. This difference in solubility has been used by a number of investigators in the past fifteen years a. a h s i s for methods of separating rubber into a number of fractions of different ranges of molecular weights (1, 7 , 9, 10, 11, 12, 13, 14, 15, 16, 17, 20). Intrinsic viscosity can be used as a measure of the molecular weight ; the higher the intrir-sic viqcosity, the higher is the average molecular 11-eight (4, 7 , 19). The techniques of fractionation may be classified roughly as either di,bolution (diffusion) methods, in which successive fractions are leached out of the rubber by different solvents or solvent mixtures, or precipitation methods, in which the successive fractions are precipitated from solution by increasing amount, of a non-solvent. Fractionation of rubber by these procedures does not re-iilt in a sharp separation of polymers of definite molecular u-eight ; on the contrary, each fraction consists of a mixture of polymers over a n-ide range ot molecular weights, as can be shonn by its refractionation (5, 7 , 8, 11, 12, 13). In the course of the investigation of rubber obtained from goldellrod leaves (Solidago Zeavenwortlzii) on a pilot-plant scale by a ti\-o-stage extraction process, as part of the Emergency Rubber Project, fractionation wab employed as a means of characterization and evaluation of the rubber obtained find a190 of comparing it n-ith other natural rubberq. EXPCRIJIETTIL
Proccdiire
The method of fractionation employed in the present)investigation i.: :I slightly modified form of that used by Johnson (ti’). In brief it involves the folloning steps: a sample of about 3 g . of rubher is partially freed from resins .-t:l.nding in reagent-grade acetone in the d f i ~ ~ for k 18 t o 24 h r . The acetone i. decunted, . . and the lubber is dried in z’nciio. l’n-o grams of this $ample is veighed into n tared 500-nil. glase-stoppered flask with 2 per cent of its Tveight oi tii?Iio:;idant (Flectol H ) ; 200 ml. of i~c:!gent-gi’adcbenzene ia nddetl, and the flask 1. allon-et1 to stand in the dark for from it o 12 hr., or until solution of the rubher is com1 One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, United States Department of Agriculture.
PHYSICAL CHEMICAL IST'ESTIGATIOSS
OF GOLDESROD RUBBER.
111
305
plete. (In some cases it is necessary to clarify the solution as described in connection with the fractionation of other natural rubbers.) The solution is then brought to 25°C. in a constant-temperature bath, and successive definite quantities of methyl alcohol are added slowly n-ith snirling to precipitate a small portion of the rubber. The temperature is raised until the turbidity disappears, and then the solution is alloiyed to stand in the 25'C. bath with gentle swirling while the gelatinous precipitate gradually forms. This precipitate is allowed to settle, and the supernatant liquid is then decanted, brought to 25°C. again, and treated with an additional portion of methyl alcohol, as before, to get a second precipitate. This procedure is continued until a total of about 200 ml. of alcohol has been added, after which no precipitate is formed on further addition. The tinie necessary for settling is short for the first precipitate, and it increases as the fractionation progresses. The last precipitate, 11-hichin the case of goldenrod is fluid. is allo1ved to settle overnight. The size of the fractions obtained depends upon the amount of methyl alcohol added and i 5 influenced somewhat by the rates of precipitation, the amount of agitation, the time allowed for settling, and perhapc hy other variablcs in the technique. Each +iecebcive precipitatc iq dried i n vacuo, wighcd, and redissolved in benzene to make a 0.2 to 0.4 per cent solution. T ~ concentration P of this solution is accurately determined by a total-solids determination on an aliquot. Its relative viscoqity is then meabiired at 25°C. by means of an Ostn-ald viscometer. a' modified by Zeitfuchs (21). the technique of Craston being used ( 2 ) . The intiin4c iqcosity of the fiaction, 7 %is . then calculated from the formula 7
vi =
2.303 log qr C
nhere 7; i.- the relative viscosity (nith respect to pure benzene), and c is the concentration in grams per 100 ml. of 5olution. The higher the values of the intrinsic viqcobity, the higher is the average molecular n-eight of the sample. The successive volumes of methyl alcohol added to precipitate the various fraction. depend on the type of rubber being fractionated and sometimes must be determined by a preliminary experiment. The whole procedure, after the first precipitation is begun, usually requires 2 days, and it must be carried out as rapidly and with as little exposure to light and oxygen as possible during the handling of the sample and of the separate fractions so as to avoid degradation. As the precipitates probably contain only traces of antioxidant, they may be very sensitive to light and oxygen. The intrinsic viscosity is very susceptible to concentration error, and therefore the total solids of the solutions involved must be determined nith accuracy.
Typical data The results of experiment I , for a sample of goldenrod rubber from Pilot Plant Run KO. 17, are given in table 1. These are typical data obtained by the technique described.
306
E. L.
sur,
T. J. RUSCKEL, F. B. KREEGER, ASD N. A. SULLIVSS
Choice of method of graphical representation As mentioned below, it has been found that these data can best be represented as a form of histpgram. The intrinsic viscosity of each successive fraction precipitated is plotted as ordinate against the cumulative percentages precipitated as abscissa. The histogram corresponding to table 1 is shown in figure 1, experiment 1. T$BLE 1 Fractionation d a t a for goldenrod rubber from Pilot P l a n t Run iYo. 17 (All viscosities a t 25'C.) hiETEANOL
ADDED, FRACTION NO. CUMULA~
1
PRECIPITATED
PRECIPITATED, CUUUL4TWE
I ,
I ?a
'Ir
p e r cent
53 57 62 70 90 135 185
1 2 3 4 5 6
14.25 17.98 21.10 15.24 14.60 5.40
14.26 32.23 53.33 68 57 83.17 88.57 95.56
'
I CONCENTRATIOX
grams p e r 100 ml. ~
I
1.145 0 . 865 0 * 808 0.698 0.674 0.646 0.642
1.883 1.422
0.251 0.330
2 * 522 1.068 0.694 0.500 0.379 0.325 0.274
Unprecipitated. . . .4.18 (99.74) 2.00 Antioxidant added. . . . . __97.74 Accounted f o r . . . . . . . . .
35.93 19.20 14.64 7.62 6.52 1.76 1.92 86.60
1OO(qi of original sample) = 87.2.
Srea of histogram a measure of the average molecular weight It is already known that for rubber polymer mixtures intrinsic viscosity is an additive property and that therefore the intrinsic yiscosity of a mixture is the weighted average of the intrinsic viscosities of the constituents (17). That this additivity of intrinsic viscosities is valid for mixtures of goldenrod rubber polymers was confirmed by measurements of the intrinsic viscosities of binary mixtures of two fractions obtained from a fractionation. It thus follows that the total area under the histogram should be independent of the size and number of the individual fractions. That is, 2[qd(per cent precipitated)], the sum of the values in the last column of table 1, should be equal to 100 times the intrinsic viscosity of the original sample. In the case of the fractionation data of table 1, these two values, 86.6 and 87.2, are in excellent agreement. Since the intrinsic viscosity is a function of the arerage molecular weight, it follow that the larger the area of the histogram the larger the average molecular weight of the rubber sample. The difference between the histogram area and the area calculated from the intrinsic viscosity of the original sample can be taken as a measure of the experimental error in a fractionation. One source of error in obtaining the area of the
PHYSICAL CHEMIC-IL ISVESTIGATIOSS O F GOLDESROD KUBREZ.
I11
307
histogram is the fact that the total amount precipitated does not add up exactly to the original sample Iyeight. This is caused ( I ) by experimental error in estimating the weight of the fractions and ( 2 ) by the fact that all of the sample is not precipitated by an excess of the methyl alcohol, as shonn by the unprecipitated fraction of 4.18 per cent in table 1. The first of these errors usually amounts to less than 2 per cent and may be positive or negative. The second source of error varies with the sample. The small portion unprecipitated would, if it could be isolated, have a lower intrinsic viscosity than fraction 7, and therefore the area involved in this error, the product (7; X per cent precipitated), mould usually be very small. This error would always be negative. The combined effect of both these errors n-odd thus be expected to favor a slightly low value for the experimental area. Differences between the histogram areas for duplicate fractionations indicate the reproducibility of the fractionation data. RESULTS AND DISCUSSION
Experimental results obtained by the fractionation of a number of rubber samples are summarized in table 2, and they are represented graphically for some typical samples in figure 1. Column 3 of table 2 lists the areas of the histograms. The values in column 4 v-ere obtained by multiplying the intrinsic viscosities of the original samples by 100,and the deviations from these calculated values are given in column 5. Table 3 shows the differences between the histogram areas for duplicate experiments on a number of rubber samples.
The shape of the hisfogrant In general, the histogram is a stepu-ise curve descending from left to right. Experiments 13 and 14 in figure 1 are typical duplicate histograms for a goldenrod rubber sample. In this case the determinations vere run under identical conditions with the same yolurnes of methyl alcohol added for corresponding fractions. Experiments 15 and 16 in figure 1, on the other hand, shorn the histograms for duplicate samples where the sizes of the corresponding fractions were varied by using different amounts of methyl alcohol in their precipitation. As would be expected, the first step of the histogram is higher when the first fraction is small. Although the histograms for experiments 15 and 16 are quite different, their areas are essentially the same, 60.1 and 59.6, respectively (see table 2). This further corroborates the general applicability of the additivity of intrinqic viscosity.
Removal of resins Experiments were made to determine whether it is necessary to remove the resins from the goldenrod rubber by soaking n-ith acetone for 18 to 24 hr. in preparing the sample for fractionation. Duplicate runs were made on goldenrod rubber PP32 (resin content 15 per cent) in which the sample was prepared for fractionntion ( u ) by the L I Q U R ~procedure as above (experiments 9 and 10); ( b )
308
E. L . SICAU, IT-.
J. R ~ S C K C L , F. B . KREEGER,
ISD
11.A . S L - L L I V . ~
by acetone precipitation (experiments 11 and 12); and ( e ) by omitting the removal of resins and taking enough sample to give 2 g. of rubber, excluding resins, calculated on the basis of the resin content previously determined by analysis (experiments 13 and 14). The areas of the histograms obtained are shon-n in
-
'L
EXP. I PP 17
EXP. 2 4 HEVEA (Sm.Sh.)
EXP.23
L i EXP. I 3
1
EXP.14
L
EXP.22
pp32
pp32
EXP.15 PP n
1
EXP. 2 5 HEVEA CREPE EXP. 16
i-L.
PP n
2
I
7-
1
EXP. 18 PP 2
I 7
I
EXP. 21 I
50
75
HEVEA (Sm.Sh), MASTICATED
2
EXP.20
25
EXP. 3i KOK-SAQHYZ
7 25
50
75
0
25
50
75
I
% PRECIPITATED (CUMULATIVE)
FIG.1. Histograms corresponding to experiments in table 2
table 2. The areas for the acetone-precipitated sample, 65.2 and 63.8, 2 3 compared to the acetone-soaked sample, 59.9 and 62.2, indicate that precipitation of the goldenrod rubber from the benzene extract with an excess of acetone results in a slightly larger histogram area. On the other hand, the areas for the sample in which removal of resins Tms omitted, Ci2.i and 61.8, are the same,
TABLE 2 S u m m a r y of fractionation data f o r goldenrod a nd other natural rubber sampl e s
1
I
I
i 1
1
Goldenrod PP328 Goldenrod PP32S Goldenrod P P a Goldenrod P P a Goldenrod PP2d Goldenrod P P 2 7 Goldenrod PP27 Goldenrod PP4J[ Goldenrod PP4JI Goldenrod PP32 20QJo hevea smoked sheet-80yc goldenrod PP32 Hevea smoked sheet Hevea crepe Hevea crepe Hevea smoked sheet masticated Guayule (Sample 1) Guayule (Sample 2) Kok-saghyz
199.1 126.8 166.3
~
5 . . . . . . . Goldenrod 6 . . . . . . . Goldenrod i . . . . . . . . Goldenrod S . . . . . . . Goldenrod 9 . . . . . . . Goldenrod
1
PP26 PPd PP29 PPb PP32
graphically in figure 1. t Sample hard and almost non.sticky . $ Acetone-precipitated sample . S Resins not removed . 7 Sample very soft and sticky .
1
1
AREA
(CALCULATED)
-I 86.6 71.8 69.0 6S.8 68.8 64.1 62.6 62.5 59.9 62.2 65.2 63.8 62.7 61.8 60.1 59.6 53.0 34.6 30.6 44.7 44.7 62.2 198.5 626.2 199.3 213.3 104.5
4. . . . . . . . Goldenrod P P c
13" . . . . . . 14*. . . . . . 15*. . . . . . 16*. . . . . . 17. . . . . . . 18. . . . . . . 19" . . . . . . 20* . . . . . . 21. . . . . . . 22* . . . . . . 23* . . . . . . 24* . . . . . . 25* . . . . . . 26 . . . . . . . 27. . . . . . . 28* . . . . . . 29* . . . . . . 30 . . . . . . . 31* . . . . . .
ARE i (EXPERIXENT.AL)
3ARE.A
:
i
___
87.2 73.7 i 67.2 I 6i.3 68.8 71.5 63.6 1 62.1 I 63.6 63.6 63.6 63.6 63.6 63.6 59.4 59.4 55.5 37.3 37.3 ~
(58.4)
-1.4 +27.5 $26.2 -5.3 $8.3 $2.0 -14.3
63.6 600 205 205 102.5 565.0 120 171.5
,
* Represented
jl Fractionated
by dissolution method
.
I 9. 10. . . . . . . . . . . . . . . . .1 11, 12 . . . . . . . . . . . . . . . . , 13, 14 . . . . . . . . . . . . . . . . . 15, 16 . . . . . . . . . . . . . . . . . 18, 19 . . . . . . . . . . . . . . . . . 2 5 , 26 . . . . . . . . . . . . . . . . .
i
59.9, 62.2 65.2, 63.8 62.7, 61.8 60.1, 59.6 34.6, 30.6 199.3, 213.3
per cent
2.3
1.4 0.9 0.5 4.0 14.0 309
-0.6 -1.9 $1.8 $1.5 0.0 -7.4 -1.0 +0.4 -3.7 -1.4 $1.6 +0.2 -0.9 -1.8 +O.i +0.2 -3.0 -2.7 -6.7
3.i 2.2 1.4 0.8 12.2 6.S
$6.8 -5.2
310
E. L. SKAU, TT.
J. RCSCKEL,
F. B. K R L C G C R , .ISD
31. A. SULLIVAN
within the probable experimental error, as those for the iamples prepared by extracting with acetone. I t can be concluded, therefore, that if the resin content of the golden rod rubber sample is knonm, the fractionation may be carried out without removal of the reqins. In the expeiiments reported in table 2, holyever, unless otherwise indicated, the resins n-ere first remored by standing in acetone for 18 to 24 hr.
Comparison of pilot-plant goldenrod lubber samples The various samples of goldenrod rubber obtained from the pilot plant have been arranged in table 2 roughly in the decreasing order of the magnitude of their histograms (experiments 1 to 19). Most of these areas fall between GO and 70. Excluding PP17 (experiment 1) and PP2 (experiments 18 and 19), which are exceptional samples, the arerage area found for goldenrod rubber in these and other experiments made in connection with this investigation is about 63. The average for goldenrod rubber PP2 (32.4) indicates a much lower average molecular weight than usual. This sample resulted from the extraction of “goldenrod leaves from dead and field-dried stems” and v a s very much degraded, judging from the fact that it was very sticky and semi-liquid. On the other hand, the large area (87.2) found for goldenrod rubber PP17 showed that it had an abnormally high average molecular n-eight. In fact, this mas in a sense a partially fractionated rubber, since the acetone used for the initial extraction of resins from the leaves contained some 20 per cent of benzene, and it had been shon-n in previous experiments that a mixture of acetone and benzene will extract some of the lower molecular fractions from the rubber, leaving behind a rubber of higher average molecular weight. This fact is also discussed in connection uith experiments 20 and 21. Goldenrod rubber PP17 obviously had more body and practically no stickiness. CompaTison of hevea rubber samples A comparison of the areas for experiments 22, 23, 2 4 2 5 , and 27 in table 2 and an inspection of the corresponding histograms in figure 1 indicate a marked difference between the various hevea samples and afford an interesting comparison with goldenrod rubber. The histogram for herea smoked sheet, experiment 24, 5hon-s that heren rubber in this form has the highest average molecular weight (area = G2G). The duplicate histograms for the sample of crepe rubber investigated, experiments 25 and 26, indicate that hevea in this form has a much loTyer a\-erage molecular veight, as shovn by the average area, 206, than hevea in the iorm of emolied sheet. The figure for experiment 27, with an area of 104.5, shoi\-s the extent to which the hevea smolied sheet was broken down by mastication. It was estimated that this sample had been masticated roughly to the extent necessary for compounding. This histogram differs from the others obtained in the present investigation by being nearly horizontal, thus indicating that the sample was made up for the most part of rubber of approximately uniform niolecular weight. Experiment 23, figure 1, is the histogram of a binary mixture of 20 per cent
PHYSICAL CHEMICAL INVESTIGATIOSS OF GOLDESROD
RUBBER.
111
311
hevea smoked sheet (experiment 24) and 80 per cent of goldenrod rubber PP32 (experiment 22) made by mixing appropriate volumes of benzene solutions of these two samples of rubber. The histograms obtained in all three of these experiments are plotted together on the same coordinates in figure 1 for comparison. The presence of hevea in the goldenrod rubber sample is apparent by inspection. It is interesting to note that since intrinsic viscosity is an additive property it is possible to estimate the percentage of hevea rubber in the hevea-gcldenrod rubber mixture from the histogram areas. Thus, from the values given in table 2 for the areas of histograms of the hevea and goldenrod rubber samples, 626.2 and 62.2, respectively, and for the 20 per cent hevea mixture, 198.5, the following equation can be written:
n-here z is the percentage of hevea in the mixture. Solution of this equation gives x = 24.1 per cent hevea rubber.
Coinparison of samples of various natural rubbers Several different varieties of natural rubber were subjected to the fractionation procedure. In order to make the benzene solutions, the samples extracted with acetone and dried in the usual m y vere allowed to stand in benzene for several days Tvith occasional gentle swirling and then clarified. For clarification the solution was filtered through glass wool or cotton, or, if not too viwous, through filter paper. That this procedure is adequate to dissolve the high-molecularweight polymers is evidenced by the results obtained for hevea smoked sheet (experiment 24) and castilla (experiment 28). The procedure also gives reproducible results. This is shown by the results for duplicate samples of hevea crepe (experiments 25 and 26 in table 2), and by the agreement between the experimental areas of the histograms and their areas calculated from the intrinsic viscosity of the original sample usually determined independently. The conclusions that can be drawn from these fractionation results are, lion-ever, necessarily limited. In the first place, the history of the various specimens is unknown, and secondly, the samples were sometimes not completely soluble in benzene. Though the insoluble residue appeared to be foreign non-rubbery material, it may have consisted partially of rubber that had become insoluble owing to oxidation or some other transformation in the original sample. For this reason the results obtained and sumrnarizcd in table 2, experiments 24 to 31, and plotted in figure 1 are for the particular specimens of rubber, and may be considered as only roughly representative of the variety of rubber. TT'eiyht distribution curves
Differential weight distribution curves were plotted for the above fractionation data but were found less satisfactory and significant than the histograms in figure 1, because data for only seven fractions n-ere available in each case. The
312
E. L. SKAL-, TI-.
J. KUSCKEL, F. B. KREEGER, .ISD 11. -1.SULLIVAS
differential weight di-triliution curve i. obtained by hiqt plotting the integral weight distribution curx-e (cumulative per cent precipitated as ordinate against qi of the yuccessive precipitatc- as abscissr'i and then plotting the slope of this curve a t various values of 7%against theo? xalues of q z . -4smooth integral n-eight distribution curve can he clran-n accurntcly only if data for a large number of small fractions ha\ e been obtained. L r o r s in this plot are of course reflected in the differential weight distribution c i m - e . This becomes particularly apparent in experiment 23, table 2 and figure 1, a case in which the complete integral weight distribution curve could be dran-n from the given data only arbitrarily. From the histogram areas, on the other hand, it was even possible t o estimate the percentage of hevea present.
Fractionafion and the precipitation value The various fractions obtained in some of the above experiments were brought t o the appropriate concentration in benzene and titrated n-ith alcohol to deter-
TJ,
TABLE 4 Precipitation values of successive f r ac t i ons obtained i n f rac t i onat i on experiments = intrinsic viscosity a t 25°C.; P.V. = precipitation value at 25°C. in milliliters of alcohol EXPERIXENT 14 FRACTION
'li
1. . . . . . . . . . . . . . . . . . 2. . . . . . . . . . . . . . . . . . 3. . . . . . . . . . . . . . . . . . 4. . . . . . . . . . . . . . . . . . 5. . . . . . . . . . . . . . . . . 6. . . . . . . . . . . . . . . . . . i ..................!
1.860 1.052 0.517 0.413 0.334 0.256 0.211
Original sample
0 636
~
,
I
P.V.
I
ml.
,,
__-
EXPEBIXEST 7i
4.20 1.034 4.40 0.512 4.60 0.453 5 . 1 5 1 0.328 5.70 0.286 6.35 0.274 6.95 0.215
I
, ~
~
4.50
0 594
13
EXPERIXENT
P.V.
1
ml.
I
P.V.
'li
1
19
ml.
-
1
1
EXPERIMENT
Ti
_
I
I
_ ml.
~
I
,
1 ~-i j 0.373
P.V.
1.587 4.40 4.90 5.00 1 1.136 1 5.25 5.65 0.880 1 4.50 6.25 6.65 4.60
4.40 0.713 5.20 0.540 5.85 0.403 6.70 , 0.326 7.45 0.304 8.00 0.292 9.so 4.50
I
_
27
~
5.30
1
~
1.025
i
mine their pcecipitation values at 25OC. (18); Le., the number of milliliters of absolute ethyl alcohol necessary to produce a cloud point in 10 ml. of a clarified benzene \elution containing 0.0175 g. of rubber sample. Typical results are listed in table 4, Tvhich gives the intrinsic viscosities and precipitation values of the beparate fractions obtained in experiments 14, 15, 19, and 27, thus corresponding to the successive steps in these hidograms in figure 1. It i. evident that the precipitation value is aln-ays lowest for the first fraction and that it increases, TT hile the intrinsic T iscosity decreases mith each successive iraction. thus, for experiment 14 it increnies from 4.20 to 6.95 ml. from fraction 1 to fraction 7 . The first fraction usually has a l o m r precipitation value than that of the original sample, and the difference is more marked if the first fraction is small. For example, in espeiinient~14 and 15 the precipitation values of the original
PHTSIC-1L CHENIC.YL
1STESTIG.YTIOSS O F G O L D E S R O D HCBBER.
111
3 13
goldenrod rubber samples n-ere identical, 4.50 ml. As can be seen by referring to figure 1, the first fraction in experiment 15 n-as very large (37.7 per cent); still it liacl a precipitation value of 4.40 n i l , which na. lower than that of the original sample. The first fraction in experiment 14, on the other hand. TVRS re1:ttively small (10.3 per cent), and it had a much lon-er precipitation xalue, 4.20 nil The second fraction (17.0 per cent) had a precipitation value of 4.40 nil., ~ \ h i c l i\\a3 also lon-er than that of the original saniple. These differences are significant; with the fact that the precipitation T alue increases rapidly for bucce+ii e fractions, they indicate that the fractionation-by-precipitationtechnique sed is fairly efficient for goldenrod rubber. Since the presence of only a small amount of a higher polymer is effective in lon-ering the precipitation value (18),it can be concluded that very little of the high-polymer portion of the sample iq inc1:idetl in the later fiactions.
Fractzonatzon by dissolution Smie oi the first fractionations of rubber performed in this laboratory vere by the
