INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1928
137 EFFECTOF
35'C"RE
Figure 24
Figure 25
ACCELERATOR5
4
Figure 26
Figure 27
CK
35'CURf
35' C"RE
Figure 28
Figure 29
Safex were used as accelerators and the compounds were cured a t 287" F.(142" C.) and 267" F. (131" C.), respectively. We therefore have the effect of reenforcing pigments with accelerators a t both a high and moderately low temperature. The physical properties of the reclaim containing varying volumes of Kadox with diphenylguanidine as the accelerator are the same as those obtained when no accelerator is used, with the exception of 20 volumes of Kadox which shows a slightly higher stress-strain curve. (Figures 24 and 25) Safex a t 131' C. does not affect the rate of cure but increases the tensile strength (Figure 26) and resistance to abrasion. (Figure 27) Safex gives a very much higher stress-strain curve, accompanied by a lower elongation. It has no effect on the resistance to tear. Diphenylguanidine and Safex increase the rate of cure and improve the tensile strength of a whole-tire reclaim containing varying volumes of carbon black. (Figures 29 and 30) The resistance to tear (Figure 31) is slightly improved,
Figure 30
Figure 31
whereas the aceelerators giye a higher stress-strain curve with 10 volumes of carbon black (Figure 28), but with 20 volumes Safex does not give any higher stress-strain curve than when no accelerator is used. This is probably due to the degree of cure obtained a t the lower temperature. Safex in a whole-tire reclaim containing varying volumes of clay or whiting gives improved tensile strength, resistance to tear and abrasion, and a higher modulus. The rate of cure is increased. Diphenylguanidine has very little effect other than increasing the rate of cure. Effect of Low-Temperature Curing I t is logical to ask what benefits may be expected from low-temperature curing of a reclaim which in the course of its manufacture has been subjected to temperatures ranging from 350"to 385" F. (177" to 196" C.). The results of this study show that there is a distinct, benefit to be derived by curing at lower temperatures.
Effect of High vs. Low Sulfur in Vulcanizing Reclaim R. E. Cartlidge and H. L. Snyder THEAKRONRUBBERRECLAIMING Co., BARBERTON, OHIO
UCH progress has been made in determining what sulfur percentages will bring out the best quality of finished rubber, especially where organic accelerators are used, but this work has been done mostly with high-rubber stocks containing little or no rechim. Now that reclaim has effectively entered into rubber compounding we find that there is no definite idea as to just what percentage of sulfur will give the optimum results in a compound containing a mixture of crude and reclaimed rubber. The general notion has been that approximately 4 per cent of sulfur on the total rubber content of the batch would be satisfactory in any stock with any kind of rubber-reclaim mixture, since such a value has worked well with crude-
M
rubber compounds. Recognizing that such a rule is obviously untrustworthy for reclaim stocks, the authors have sought a more accurate method for computing sulfur percentages. They submit this paper as merely preliminary to a subject which is vital to all compounders. The rubber content of the reclaim is considered to be the difference between the total unit weight of the reclaim and the sum of the parts by weight of acetone extract, ash, total sulfur, and carbon black. Experiments with Straight Reclaim Stock
A sample of 45 kg. (100 pounds) of washed alkali tire reclaim was selected, and from it all the reclaim used in
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
70
VOl. 20, No. 2
-
0 -
2.8 kqjcrn?
1.5 2.5 4.0 5.0
P /. 5
the following tests was cut. By varying the sulfur percentages from 1.5 per cent to 6.0 per cent on the reclaim in straight reclaims u l f u r mixes one set of cures was obtained. These were made a t intervals of 5 minutes. All c u r e s were made a t 142" C. (287" F.) in the conventional type of horizontal press. Dumbbell test strips were cut from each slab and tested on a modified Olsen tensile machine, for loads a t chosen elongations as well as breaking tensiles and elongations. The results for the 25-minute cure indicate a surprising flatness after a minimum of 3.0 per cent of sulfur is added. Percentages less than that show undercure throughout. (Plate I-a) The only noticeable change in physical properties is in the stiffening of the modulus of rigidity progressively
5.5
3I.5
to the maximum percentage of sulfur present. This modulus also increases as the time of cure of any one sample increases. The ultimate tensiles of the 25-minute cure pass from a minimum a t 2 per cent sulfur, or below, to an approximately constant maximum from 3 per cent up to and including the maximum amount of sulfur added. Experiments with Compounded Stock
A typical tire-tread formula was used in making up the compounds for another set of cures to determine the effect of varying the sulfur in a compounded stock. Cures were made a t 15-minute intervals. Rubber Reclaim Carbon black Zinc oxide
Parts 35 0 35 0 17 0 5.0
FORMULA Stearic acid Mineral rubber Sulfur Accelerator
Parts 0.5 5.0
Variable 1.0
The sulfur percentages were varied from 1.5 per cent to 5 per cent on the reclaim. In all the compounds 4 per cent of sulfur was figured on the crude rubber present in addition to that added for vulcanizing the reclaim. Samples of the 60-minute cure for each compound were analyzed for
INDUSTRIAL A X D ENGINEERING CHEMISTRY
February, 1928
139
.$ h
t
2/u
-
XIV. -
% + l
-
/40
70
-
free and combined sulfur and acetone and chloroform extracts. (Table I) Table I-AnalyRis
of Tread Stocks Cured 60 Minutes at 2.8 kg. per sq. c m . (40 Ibs.) Pressure (Figures in per cent) ACETONE
SULFUR I N ACETOA-EEXTRACT TOTALCOMBINEDFREE RECLAIMEXTRACT(COR.) S U L F U R SULFUR SULFUR
CHCla EXTRACT
STOCK NO. 1
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
4.25 4.39 4.57 4.23 4.46 4.88 4.86 5.08
3.89 3.98 4.13 3.78 4.00 4.36 4.33 4.36
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
6.03 6.98 6.68 7.38 7.14 7.60 7.28 7.61
5.92 6.82 6.48 7.11 6.83 7.27 6.98 7.26
2.83 2.94 3.06 3.30 3.35 3.44 4.01 4.20
2.47 2.53 2.62 2.85 2.89 2.90 3.46 3.47
0.36 0.41 0.44 0.45 0.46 0.54 0.55 0.73
0.88 0.68 0.81 0.82 0.73 0.72 0.83 0.79
2.50 2.79 2.51 2.62 3.03 3.03 3.12 3.20
0.11 0.16 0.20 0.27 0.31 0.33 0.31 0.35
0.89 1.08 1.03 1.19 0.82 0.88 1.02
STOCK NO. 2
2.61 2.95 2.71 2.89 3.34 3.36 3.43 3.55
0.85
The effect of increasing curing time on the stress-strain curves is illustrated by Plates X, XI, I, XII, XIII, XIV, and XV in order mentioned. As the time of cure increases the stress-strain curves show considerable shortening and a decrease in the angle formed with the Y axis, except in case of low-sulfur compounds. Therefore, with a sulfur content of 1.5 per cent, 2.5 per cent on the reclaim, better curing conditions were obtained for a longer curing time. Larger percentages of sulfur give proportionately greater changes in the position of the stressstrain curve of the various cures. As the percentage of sulfur is increased to a maximum of 5 per cent on the reclaim, in addition to the 4 per cent added to the crude rubber, the ultimate tensile strength increases to a maximum, then decreases for all samples of each curing period. (Plate 11)
In the 30-minute cure the tensile strength increases with increase in sulfur content up to 4 per cent, but with further increase in sulfur it decreases again. In the 60-minute cure 2.5 per cent of sulfur gives the maximum tensile strength and remains in this position throughout the remainder of the cures. The tensile strength curve in this plate passes through a maximum as the percentage of sulfur increases and then decreases on further increase in sulfur. It also shows that this maximum value, 2.5 per cent, holds for a longer duration of curing time than for any other sulfur percentages used. As the percentage of sulfur increases the rate of cure increases. (Plate 111) The greatest tensile strength is given by 4.5 to 5 per cent of sulfur at 30-minute cure; 2.0 to 2.5 per cent gives the best results at 60-minute cure, and equally as good tensiles at the 90- and 120-minute cures. This indicates also, as was demonstrated before, that 2.0 to 2.5 per cent of sulfur on the reclaim content of the compound gives the best average tensiles over any chosen curing range, up to the point of overcure. Increasing sulfur content causes a corresponding decrease in the ultimate elongation in each cure examined. (Plate VII) Also the ultimate elongation decreases as the time of cure increases. (Plate IV) The loads a t 300 and 500 per cent elongation increase as the sulfur percentage increases. (Plates V and VI) On overcuring there is a noticeable drop, indicating reversion. There is a proportional stiffening on modulus up to a maximum a t the 90-minute cure, whence there is a slight falling off in load. I n Plate VI11 combined sulfur is plotted against percentage of sulfur added to reclaim, independent of that taken as necessary for vulcanizing the crude rubber present. The combined sulfur is directly proportional to the per cent of sulfur added to the reclaim. This indicates that all the above statements concerning percentage of sulfur added are proportionately true of combined sulfur percentages.
Vol. 20, No. 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
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The relation of combined sulfur to ultimate tensile strength is shown by Plate IX. Here the highest tensiles come within the range of 2.5 to 3 per cent of combined sulfur, which correspond to 1.5 to 3.0 per cent of sulfur added to reclaim. (Plate VIII) Further chemical analysis revealed that both the chloroform and acetone extracts of cured samples were constant for any given cure (Table I), and that the free-sulfur content of the cured sample increased as the percentage of sulfur in the reclaim was increased and was proportional to it.
Conclusions
To obtain the optimum results in curing compounded stocks containing whole tire reclaim, a minimum of 2.0 and a maximum of 2.5 per cent sulfur are required. This range of sulfur content, based on the total weight of reclaim in the compound besides the 4 per cent added for curing the crude rubber present, gives the flattest and longest curing range in conjunction with the highest physical qualities. Three per cent of sulfur on the straight reclaim-sulfur mix gives the maximum physical properties.
Value of the Rubber Hydrocarbon in Reclaimed Rubber w. w. vogt THEGOODYEAR TIRE & RUBBERCOMPANY, AKRON,OHIO
Tire-tread stocks containing reclaim were so compounded that their ultimate composition was the same except for the percentage of reclaimed-rubber hydrocarbon substituted for new-rubber hydrocarbon. Cures were adjusted to give the technical optimum in the same time of cure. The cured stocks were subjected to tensile and abrasion tests by five methods. The results show that the value of the reclaimed-rubber hydrocarbon varies from zero, when substituted in small percentages, up to a maximum of 50 per .cent of the value of new rubber, when compounded in large percentages.
I
N ORDER to determine the value of the hydrocarbon
present in reclaimed rubber compared with that in new rubber, it is necessary to compound the stocks in such manner that the percentage and kind of fillers, softeners, etc., are constant, the only variable being the percentage of the total rubber hydrocarbon which has been introduced in the form of reclaimed hydrocarbon. It is also necessary to adjust curing ratios in such fashion as to compensate for the accelerating or activating effects of the reclaim, so that all stocks reach technical best cure in the same length of curing time. Experimental
COMPOSITION AND PROPERTIES OF REcLAIhi-The reclaim used was a typical high-grade, alkali-process, whole-tire reclaim having the following characteristics: Acetone extract Total sulfur Free sulfur Free carbon Ash Specific gravity
Per cenl 10.4 3.0 0.08
7.5 19.3(ZnO, 10.5,insoluble, 8.8) 1.23
When cured in a formula consisting of 100 parts reclaim and 5 parts sulfur for 18 minutes at 2.8 kg. per sq. cm. (40 pounds) steam pressure (141.5' C.)>it gave a tensile of 55 kg. per sq. cm. (800 Ibs./sq. in.) and an elongation of 450 per cent. The insoluble matter was assumed to be 50 per cent clay and 50 per cent barytes. The acetone extract was assumed to be 50 per cent mineral rubber and 50 per cent thin pine tar. The hydrocarbon content of the reclaim was figured at 55 per cent. Note-These assumptions are in line with general experience with this type of reclaim. Furthermore, the exact ratio of clay to barytes makes very little difference in the ultimate physical properties of the compounds inasmuch a s neither contributes any marked qualities to the final rubber compounds. The same may he said of the acetone extract. Outside of effects on the rate of cure, which have been compensated for in another manner, the type of softener assumed is unimportant. The rather high percentage of pine tar assumed was based on the knowledge t h a t some of this material was used in the reclaiming process.
METHODOF CohfPonNDING-The lowest grade compound was designed to contain 40 parts of new rubber hydrocarbon and 60 parts of reclaimed hydrocarbon and therefore the rubber base would be 40 of new rubber and 108 of reclaim (= 60 of hydrocarbon). As a result 108 parts of reclaim would contain: 108 X 10.4 per cent = 11.2 parts acetone extract (5.6 mineral rubber, 5 . 6 thin pine tar) 108 X 7 . 5 per cent = 8.1 parts carbon black 108 X 1 0 . 5 per cent = 11.3 parts ZnO 108 X 8.8 per cent = 9 . 5 parts insoluble (4.8 clay, 4.7 barytes)
Hence, these quantities of the various fillers and softeners will be unavoidably introduced in the lowest grade compound. The base compound containing all new rubber must then be made to contain these additions, in order to secure constancy of composition over the series. It was known that the accelerating or activating effect of the reclaim could be duplicated by adding a small amount of litharge to the new-rubber base stock, so 0 . 5 part of the rubber was added to the base stock and this amount decreased in successive formulas as reclaim was added. This expedient gives a practically uniform rate of cure throughout the series. The complete formulas are given in Table I. Table I-Stock Stock yo Reclaim hydrocarbon Pale crepe Reclaim ZnO ~~. Gas black Clay Barytes Mineral rubber Pine t a r LithaFge Stearic acid Sulfur Captax Total
1 0 100 0 16.5 43.0 4.8 4.7 5.6 5.6 0.5 4.0 3.5 1.0
2 5 95.0 9.0 15.5 42.3 4.6 4.5 5.1 5.1 0.45 4.0 3.5 1.0
3
Formufas 4
10 90.0 18.0 14.6 41.6 4.4 4.3 4.6 4.6 0.4 4.0 3.5 1.0
20 80.0 36.0 12.7 40.3 3.2 3.1 3.7 3.7 0.32 4.0 3.5 1.0
5
6
7
30
45
fin
- -- ----_ 189.2 190.05 191.0 191.52 192.83 194.6 196.5
These stocks were milled in about 4-kg. batches on a 50cm. (20-inch) laboratory mill. The mineral rubber was added to the new rubber. The gas black and the accelerator were added in the form of a master batch. The new rubber containing the mineral rubber, the gas black master batch, and the separately broken-down reclaim were blended together and the rest of the ingredients added to the batch. Further compounds were made up in the following manner: New batches of stocks 1, 6, and 7 were milled and with the excess stock of the remaining batches were combined in such proportions by weight as to duplicate the chemical composition of stocks 1 to 7 (Table 11). By so doing it was thought that
