Quantitative Estimation of GR-S in Rubber Reclaim D. S.
LE
BEAU
Midwest Rubber Reclaiming Company, Eust S t . Louis, I l l .
.4practical method for estimating the amounts of
accounts for the deviation normally encountered in natural rubber reclaim between the determinations of the hydrocarbon content by direct analysis and by difference. The factor was determined by the origin of the scrap from which the natural rubber reclaim had been prepared. A tentative chemical explanation of this deviation is given. Limits of error in the estimation are listed and two simple working examples illustrate the procedure.
GR-S or natural rubber reclaims in mixtures containing these two polymers has been worked out on the basis of determination of the rubber hydrocarbon content by difference and by direct analysis (chromic acid oxidation). A correction factor must be applied before estimation can be attempted for mixtures rich in natural rubber but not for reclaim mixtures rich in GR-S. This correction factor
T
HE identification of GR-S and its estimation in scrap rubber and reclaimed rubber have been the object of considerable research during recent years. Slthough it has become econoniically and practically impossible to se'gregate synthetic and natural rubber scrap, it still is of interest to determine the amount of GR-S in the finished reclaim for the purpose of evaluating its properties. Accurate methods have been worked out for the quantitative determination of GR-S in mixtures with natural rubber, but they require either a considerable amount of time or the use of expensive apparatus not accessible to the average reclaim nianufacturer. Several of these methods can be carried out only after the hydiocalbon polymers have been brought into solution, and this, as far as reclaim is concerned, introduces difficulties. The average reclaim manufacturer or compounder is in general not concerned in obtaining a highly accurate estimation of GR-S in reclaim mixtures, but rather prefers the application of a method which, while fairly easy to carry out, will permit estimation of GR-S within 5 parts of the total hydrocarbon present.
plete analysis of the sample and deducting the sum total of the acetone extract, ash, carbon black content, etc , from 100. The difference was considered to be the total rubber hydrocarbon content, which includes that of both the natural rubber and th'e GR-S. Inasmuch as the presence of the methyl group in the position as encountered in the isoprene unit is mandatory as far as the chromic acid oxidation method is concerned, it n-as not surprising that butadiene-styrene copolymers would not yield acetic acid by this method. Repeated analysis of GR-S yielded only a t the most 3 t o 47, of rubber hvdrocarbon, and values of 1 to 27, were found more generally. Reclaiming of vulcanized GR-S could not be expected to change this condition. Experiments have confirmed this point of vieiv. Therefore, it was felt that if the percentage of rubber hydrocarbon as obtained for any given sample by the above analysis were a direct function of either of the two kinds of polymers present, the proportions of each could be estimated by means of a simple plot based on the two rubber hydrocarbon content*, one obtained by difference and one by direct analysis. Inasmuch as RUBBER HYDROCARBON CONTENT GR-S practically does not enter into the formation of acetic acid For some time the evaluation of reclaim had been based on by chromic acid oxidation, the value obtained for the rubber either the direct or indirect determination of its rubber hydrohydrocarbon content by difference can be expected to represent carbon content. The direct determination was carried out by the sum total of the two polymers present, whereas the rubber chromic acid Oxidation ( I ) , which gives the quantity of natural hydrocarbon obtained by direct analysis would be a function of rubber hydrocarbon; the indirect, or difference method, by comthe content of natural rubber. Thus the values obtained by direct analysis of various mixtures containing GR-S would fall on a straight line proportional to the per *~ 70 cent of GR-S and natural rubber present in these mixtures. For this purpose GR-S and natural rubber reclaims 60 nere prepared separately, and carefullgand completely analyzed. Seven predetermined mixtures of these I 50 reclaims were then prepared with analytical accuJ e racy covering the whole range from 1 0 0 ~ oGR-S to '0 lO0yo natural rubber. The per cent of rubber hydro$40 carbon by direct analysis of these mixtures was determined. Figure 1 (dashed line) shows the actual data obtained for such a series plotted against the known proportions of GR-S and natural rubber for each mixture. Except for mixtures very high in GR-S a straight line was obtained. The aberration a t high GR-S contents is a consequence of the slight amount of rubber 10 hydrocarbon obtained for GR-S. This effect is bound to cause an error in the estimation of reclaims con0 taining 90% or more GR-S. Practically, however, 0 10 40 30 40 50 60 10 80 90 100 the straight line can be extrapolated to the zero point Parts of Natural Rubber Reclaim without changing the results to any greater extent in loo 90 ao 70 60 50 40 30 PO IO o Parts ol GR-S Reclaim mixtures less rich in GR-S. To apply this method for industrial use the estiFigure 1. Rubber Hydrocarbon in Mixtures
355
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ANALYTICAL CHEMISTRY
niation of a mixture of natural and synthetic rubber of unknown proportions will have to be based also on the percentage of total rubber hydrocarbon as determined by the difference method, because this latter value will represent the sum of both natural and synthetic rubber hydrocarbon. I n the case represented by Figure 1, this value was found to be 62.97,. -4s such it can be considered and recorded on the graph as 100 parts of polymers. This value must then be the end point of the straight line originating a t the zero point. The data obtained from the various mixtures analyzed were replotted on this basis and are also shown in Figure 1 (solid line). If the data found by direct rubber hydrocarbon analysis are placed on that line, the parts of GR-S and natural rubber reclaim which can now be derived from the abscissa of this 0 10 PO 30 40 50 60 70 EO 90 100 plot do not correspond to the actual analytical miuParts of Natural Rubber Reclaim tures prepared. A mixture actually containing 100% 100 90 80 10 60 50 40 30 PO 10 0 Parts of GR-S Reclaim of natural rubber reclaim seems to contain 88.555; 95% natural rubber reclaim, only 83.5%: 80y0 natFigure 2. Rubber Hydrocarbon in Mixtures ural rubber reclaim, 6gm0; 40% natural rubber reclaim, 34.5%: 20% natural rubber reclaim, 21.5%; and no natural rubber, 3CF. The greatest error III estimation quantitative chromic acid oxidation to acetic acid. The factor occurs for mixtures high in natural rubber. The less the amount of oxidation cannot be discounted. Burger (I) found the deviaof natural rubber, the smaller the error appears to be. Hontion much greater for reclaims than for vulcanizates not containever, the error introduced in this n a y was so great that unless ing reclaim. Experiments on a natural rubber gumstock, which the reason for it and some correction factor could be found, the did not contain antioxidant, showed a Considerable amount of method could not be considered useful. deviation when this stock was subjected to exaggerated reclaimThe rubber reclaiming industry ha> known for some time that ing conditions permitting considerable oxidation. The original the values obtained for the rubber hydrocarbon content of natvulcanized compound yielded 86% of natural rubber hydrocarbon ural rubber reclaim by direct analysis are in almost all cases as determined by direct analysis, but when subjected to the lower than those obtained for the same reclaims by the difference highly oxidizing reclaiming conditions yielded only 76% of method This difference is referred to below as deviation. natural rubber hydrocarbon. Saturally, such exaggerated conditions do not exist under actual reclaiming procedures. The DEVIATIONS scrap subjected to reclaiming contains antioxidant and in most I t has often been assumed that the deviation betv een the t b + o cases carbon black, both of TThich permit a certain amount of values of the natural rubber hydrocarbon determinations was due protection against oxidation. The conditions under which reto analytical errors and as such had to be considered entirely claiming is carried out usually do not permit any sizable amount accidental. However, if this were true, the rubber hydrocarbon of oxidation; therefore such large deviations as mentioned above content obtained by direct analysis should not be practically cannot be expected to occur. Inasmuch, however, as a deviation occurs at all times in natural always lower in value than that obtained by difference. The rubber reclaim, it will also play a part in any estimation of the sum total of errors introduced in the latter case nould be cumulaGR-S or natural rubber present in mixtures such as analyzed tive, owing to errors obtained in each single analysis of the various ingredients in the reclaim. Analytical errors, however, like above. If this is true, the error in the estimation of the components oY the mixtures will be the larger, the greater the deviaany other errors, can be positive as well as negative. Thus, if analytical errors were the sole cause for the deviation between the tion betm-een the rubber hydrocarbon content by difference and by direct analysis of the natural rubber reclaim present. This two rubber hydrocarbon determination methods, they should not cause the rubber hydrocarbon determination by difference to be deviation was considerable for the natural rubber reclaim used constantly of higher value than that obtained by direct analysis. in Figure 1. Furthermore, the error in the estimate of the components of the mixtures will be much greater if these mixtures The yield of acetic ac.d obtained by chromic acid oxidation is expressed in fractions of gram moles of acetic acid per gram mole contain considerable amounts of natural rubber. Figure 1 shows that the error in estimation decreases gradually as the amount of of methyl groups present. For natural rubber the yield amounts GR-S present in the mixture increases. to 0 75, which indicates that 4 isoprene units form 3 moles of Therefore, it was decided to examine this deviation for natural acetic acid. Combined sulfur can be expected to interfere n i t h rubber reclaims, such as peels, carcass, tire, and tube reclaims, in this reaction, necessitating the introduction of B correction factor. greater detail. Ample data for such critical scrutinization are Results obtained by the oxidation of hard rubber (3)have indiavailable in any control laboratory of the rubber reclaiming induscated that for about 3 atoms of combined sulfur 2 isoprene units try. These laboratories have carried out complete analyses of fail to yield acetic acid, or in other words, that for every 3 atoms natural rubber reclaims for product control and sales purposes of combined sulfur 1.5 moles of acetic acid are lost. over a number of years. Inasmuch as the combined sulfur is not removed during the For this purpose the various natural rubber reclaims were reclaiming procedure, part of the rubber hydrocarbon in reclaim grouped together according to the origin of the scrap from which is not available for further vulcanization. It is only logical to they had been prepared. The same kind of graphic representaassume that this is the same part of the rubber hydrocarbon that tion was used; the value of the rubber hydrocarbon content was does not enter into the chromic acid oxidation. assigned by difference on the ordinate of the system to a value Thus the devlztion between the two determinations of rubber equivalent to 100 parts of natural rubber reclaim on the abscissa. hydrocarbon niiuht be explrrined by the fact that a small amount of unsaturatioir IS lost during vulcanization of the original crude Inasmuch as the value of the rubber hydrocarbon by difference rubber and that twme unsaturation might also have been lost represented a t all times the total amount of polymers present, it could be considered as 100 parts of natural rubber. A straight through oxidation, both of which could be expected to disturb the '
V O L U M E 20, N O . 4, A P R I L 1 9 4 8
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line was then drawn through t,hc origin, assigning a zero rubber hydrocarbon content to a zero content of natural rubber reclaim. Each natural rubber reclaim was in this way represent,ed by a line, the slope of which was dettrniiricd by its natural rubber hydrocarbon content by difference. On each line t,he value for the ruhher hydrocarbon oht>ainedby direct anilysis for the particular rcclaim was recorded. The fact, that, in each case the parts of rubber which could he read in this way on the abscissa were less than 100, indicated that, the rubber h>-drocarbon conwas less thsn that obtained by t,ent, obtained by direct arialj difference analysis. -4great nuinbc~rof natural ruhher reclaims were plotted in this way. If this graphic representation is carried out separately for the reclaims originating from different kinds oi scrap ruhher, each kind of reclaim seems to possess a particular and characteristic range of deviation. For example, three natural rubher peel reclaims showed a range of deviation between 1.2 and 3% of natural rubber; three black carcass reclainis, one betnwn 6.0 and 8.07,; three light-colored carcass reclaims, one between 10 and 12.57,; and out, of the twelve first, second, and third line natural rubber tire reclaims, nine showed a range of deviation between 3.4 and 77,. The range of deviat,ion is characteristic for each kind of reclaim and seems to be a function of the origin of the reclaim more than of t,he reclaiming formulas used during the reclaiming process. This behavior of the reclaims is generally true, but it will fail in the case of certain specific compounding ingredients which interfere w-it,h the direct rubber hydrocarbon analysis. The difference in t,he deviation betiveen carcass and peel reclaims cannot be ascribed to errors in analysis due to the possible presence of cellulose or its decomposition products in the carcass reclaims. lteclainis prepared from cellulose-free scraps, such as tube reclaims and peel reclaims, also differ widely in their deviations; the tube reclaims exhibit much larger deviations than those encountereid for peel reclaims.
Table I.
Deviations of Reclaims
Reclaim Peel reclaim
Av. Deviation Found by Analysis over 2-4 Years
Mean Deviation
I
1
I1 111 IV Carcass reclaim I1 I11 IV V Firkt line whole tire reclaim
1.5
-..
?'-E
7.2
6.6
I1 111
tv i-I
VI1 VI11 IX X XI XI1 XI11 XIV XI'
rrinforcing agent, such as light-colored carcass reclaims, show the great'est. A tentat,ive explanation of this phenomenon can be attempted in regard to the loss of acet,ic acid due to vulcanization (loss of unsaturation) during the chromic acid oxidation. Furthermore, Thornhill and Smith (2) found that the smallest loss in unsaturation occurred during the vulcanization of highly reinforced carbon black compounds, which age bet,ter than nonreinforccd compounds. A drop in unsaturation ascribed to oxidation has been noted by several investigators during accelerat,ed aging experiments. The above seems t,o indicate that the deviation between the valucs of the rubber hydrocarbon by difference and by direct cannot be regarded as entirely accidcntal or due t o cumulative errors occurring during analysis. On the contrary, a certain amount of statistical order is evident,, indicating that these deviations are largely dependent on the compounds from which the reclaims have been prepared. If the original data obtained by analysis are replot,ted, taking t,he deviation for the kind of reclaim used into consideration, i t can be seen immediately that the magnit,ude of error, so predominant before this correct ion was applied, has become considerably smaller (Figure 2). S o w a mixture knoivn to contain 90% of natural rubber seems to contain 88%; 80% of natural rubber, 78y0;407, of natural rubber, 39%; and 20% of natural rubber, 24.5%. The higher the content of nat,ural rubber, the smaller the error seems to he. The biggest error is encountered in mixt,ures rich in GR-S. However, t,he magnitude of the error has decreased t,o such an extent t.hat the method becomes applicable for quantitative estimation of the components, at least for practical purposes. Correct,ion for the deviation in proport'ion to the amount of natural rubber present in each mixture is automatically obt,ained by the slope of the linp. Algebraic ca.lculation of the cont,ent of natural rubber or GR-S reclaim in their mixtures is equally possible. If no correction factor had to be taken into consideration, S = T - R, where S represented the per cent of GR-S, T the total rubber hydrocarbon content by difference, and R t'he per cent natural rubber. However, because of the slight amount of acetic acid obtained by chromic acid oxidation from GR-S, the experimental dat,a obtained for R would be too high. Therefore, the actual value for the nat,ural rubber present in the mixture can be expressed by R1 = R - U S ,where a is the amount of acetic acid obtained from GR-S. Introducing this correction into the first equation, we o b t a i n s = ___ 1-a' .4t t,he same t,ime, however, the yield of acetic acid from vulcanized natural rubber and rtsclaim prepared therefrom has been found to depend on the amount of combined sulfur and to a cert,ain extent on the oxidation of the rubber hydrocarbon. Therefore, another correction factor has t,ohe introduced.
3.8
Table I shows the average deviations of the three main groups of reclaims as found by actual analysis over a period of 2 to 4 years. The mean deviation of tire reclaims falls beth-een those of the peel and the carcass reclaims. This can hardly be called coincidence, but rather was to be expected. Tire reclaims must, after all, be basically considered as mixtures of peel and carcass reclaims. Reclaims prepared from highly reinforced compounds such as peel show the smallest deviation between the two hydrocarbon values, and reclaims which contain no carbon black &s
- bR S= T l-uv
would then constitute the correct scientific formulation for calculation of the content of synthetic rubber reclaim in mixture with natural rubber. The size of factor b would depend on t,he kind of scrap used (wholr tires, tube, etc.). Practically, however, factor a can i n most cases he neglecwd. The limits of accuracy of t,ht, carbon black and cellulose analysis, bot,h of which would enter into this formula by means of value T, are, in the case of reclaim, far from ideal. In detepination of the direct ruhber hydrocarbon content of reclaim, check analyses varying about 0.5%,are considered very good indeed. Thus, as the reclaim mixtures become richer in natural rubber the increment, of acetic acid due t,o the presence of GR-S will rapidly fall within the experimental limits of error, disqualifying the application of factor a. On the other hand, a mixture rich in GR-S will not just,ify the application of factor b. Practical experiments on such mixtures have shown that, the smallest amount of error was actually obtained if fact,or b was not used. Actually, the est,imation of GR-S reclaim could he much more accurate than *2.5(>
ANALYTICAL CHEMISTRY
358 if the limits of accuracy of the various analyses entering into the calculations were more favorable. Thus, it has been proved of practical expediency to use the correction for the deviation only in cases where direct analysis shows a comparatively large amount of rubber hydrocarbon, indicating the presence of mixtures rich in natural rubber. Mixtures containing 15% or less direct rubber hydrocarbon, or large amounts of GR-S, should be plotted without accounting for the deviation. Because only a small amount of natural rubber reclaim is present, the error due to the deviation between the rubber hydrocarbon content by difference and that by direct analysis is proportionately small and even compensates for the error due to the small amount of direct rubber hydrocarbon obtained from GR-S. Such mixtures are more acrurately determined if no correction for the deviation is applied. Figure 3 shows the simplicity of the method.
0
10
PO
100
90
80
PROCEDURE FOR ESTIMATING G R - S
Assuming that two unknown mixtures of tire reclaims have to be analyzed for their content of natural and synthetic polymer, the practical procedure for the estimation of GR-S would amount to the folloiving steps: 1. Determination of the rubber hydrocarbon by difference. 2. Determination of the direct rubber hydrocarbon by chromic acid oxidation. 3. Ascertaining the kind of reclaim under consideration (tire, peel, carcass, tube, etc.). 4. Ascertaining the average deviation for this kind of reclaim. 5. Drawing the graphic plot. Plotting the percentage rubber hydrocarbon on the ordinate, the parts of natural rubber reclaim on the abscissa.
Assuming for the sake of illustration that one of the unknown mixtures gave: 1. A total rubber hydrocarbon content by diffemnce of 60%. 2. A direct natural rubber hydrocarbon content of 30%. 3. First line tire reclaim. 4. Normal deviation for a natural rubber first line tire reclaim, 3.8. 5 . The direct natural rubber hydrocarbon content of 30% indicates a reclaim mixture fairly rich in natural rubber. Therefore the correction for the deviation should be applied (60 - 3.8 = 56.2) :56.27, of rubber hydrocarbon constitutes the anchor point of the line going through the zero point of the graph. That line is followed down to an ordinate reading of 307, natural rubber hydrocarbon, and the parts of GR-S reclaim are read at that point on the abscissa (see Figure 3).
Assuming for the sake of illustration that the second of the unknown mixtures gave:
A rubber total hydrocarbon content by difference of 60%. A direct natural rubber hydrocarbon content of 15%. Not necessary. Not necessary. 5. The direct natural rubber hydrocarbon content of 157, indicates reclaim mixtures rich in GR-S. Therefore, it is not necessary to apply any correction for the deviation. As 60% rubber hydrocarbon constitutes the anchor point of the line going through the zero point on the graph, that line is followed down to an ordinate reading of 15% hydrocarbon. The parts of GR-S reclaim are read a t that point on the abscissa (see Figure 3). Figure 3, as n-ell as these examples, must be considered a working example only. 1. 2. 3. 4.
First line tire reclaims show a smaller mean deviation than second or third line tire reclaims. Thus the estimation can be much closer if it is known whether the reclaim in question is classified as first, second, or third line reclaim. If the method is used by a reclaim manufacturer to determine the GR-S content on one of the reclaims of his line, still greater accuracy can be obtained because the history of the production of this reclaim is probably known over a number of years. In general, specific gravity and
Figure 3.
30 40 50 60 Parts of Natural Rubber Reclaim 70 60 50 40 Parts of G R - S Reclaim
70
80
90
100
30
PO
10
0
Rubber Hydrocarbon in Mixtiires
total rubber hydrocarbon content by difference will give the experienced worker some indications of t,he kind of reclaim with which he is working, and t'he correction for the deviation can thus be adapted properly. The method should not be expected to be more accurate than *2.5 parts of polymer, unless the origin of the reclaim and its behavior had been established. This is possible only if a reclaimer is trying to evaluate reclaim inixtures of his own production. The originators of the rubber hydrocarbon determination by direct analysis have pointed out that cert,ain substances, such as mineral rubber and greater amounts of cellulose, will obscure this analysis and render i t inaccurate. Consequently, the presence of such materials will naturally also affect the estimation of GR-S or nat.ura1rubber unless they are removed prior to determination of the rubber hydrocarbon content. CONCLUSION
The method is none too accurate above 90% of either natural or synthetic rubber present, unless the reclaim analyzed is well known. If the degree of deviation for a given reclaim is well established, it is possible to determine the mixtures up to 95% of either component. However, if the degree of deviation is an average figure only, derived by such critical scrutinization of a number of reclaims as shown in Table I greater accuracy cannot be expected in the estimation of the components of mixtures containing more than 90% of either natural or synthetic rubber. The GR-S or natural rubber content of reclaims containing a great variety of scraps, such as some of the so-called victory reclaims, can be estimated only with a lesser degree of accuracy than *2.5 parts. Difficulties in the estimation of the polymer components may also be encountered in light-colored reclaims However, such estimations are seldom necessary because of the peculiar compounding technique required by GR-S for proper physical performance. ACKNOWLEDGMENT
The author wishes to express appreciation to N. Novakovich, who prepared the reclaim mixtures and carried out the analysis. LITERATURE CITED
(1) Burger, V. L., et& Rubber Chem. Tech., 16,660 (1943). ( 2 ) Thornhill, F. S., and Smith, W. R., Ind. Eng. Chem., 34, 218 (1942). ( 3 ) Wake, TV. C., Trans. Inst. R u b b e r l n d . , 21, 160 (1945). RECEIVED May 13, 1947. Presented before the Division of Rubber Chemistry, AYERICAX CHEMICAL SOCIETY, Cleveland, Ohio, May 1947.
