INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1942
It appears to be better, when studying synthetics, to use a higher speed of stretching than the usual 20 inches per minute. At higher temperatures and higher extensions synthetics flow at a rate depending upon the speed of the stretching. The rate of flow differs for each particular compound. Therefore fast stretching is necessary to eliminate the plastic flow. A detailed discussion of this new theory will be published elsewhere.
Henry
F. Palmer and Robert H. Crossley
1367
Literature Cited (1) Ferry, J. D., J. Am. Chem. Sac., 64, 1323 (1942).
(2) Guth, Kautschuk, 13, 201 (1937). (3) Guth and James, IND.ENG.CHEM.,33, 624 (19411. (4) Guth and James papers at Dii.isions of Rubber and Colloid Chem. of A. C. S.,Detroit, 1940. (6) Guth and Mark, Monatsh., 65, 93 (1934). (6) James and Guth, Phys. Rev., 59, 111 (1941). (7) Ibid., to be submitted for publication (1942).
ots of three typical reclaims were set aside to age, and at intervals acetone and chloroform extracts, alkalinities, milling tests, reclaim-sulfur tests, and tests i n typical test formulas were obtained. As the reclaims aged, they became less tacky and harder to break d o w n during milling. This appeared to be less true of reclaims of high alkalinity than of those w i t h l o w alkalinity. The acetone extracts remained constant w i t h increasing age, but the chloroform extracts decreased. These changes were accompanied by small variations i n physical properties in test formulas, but no significant changes in quality were observed in the reclaims u p to 18 months. A pile of scrap tires was set aside, and from them four separate lots of w h o l e tire reclaim were manufactured a t such intervals that fresh and aged reclaim could be tested simultaneously; these tests w e r e made in the laboratory in a typical tread formula and i n the treads OF tires which were roadtested. No significant difference was found in quality between tires containing reclaim 1 5 months or 3 months old.
L
Xylos Rubber Company, Akron, O h i o
WITH
the increasingly important role being played by reclaimed rubber in the war effort, it seems desirable to examine this raw material more closely than heretofore from the standpoint of maintenance of quality during natural aging. Some lots of reclaimed rubber in certain consumers' plants have attained greater age than usual because of unforeseen changes in production schedules, occasioned by the present situation. Moreover, the creation of a stock pile of reclaimed rubber from excess production, as an emergency supply, has been and may again be considered by the Government. Also, substantial amounts of reclaimed rubber have been exported and thereby have acquired more age than usual before use. Finally, certain manufacturers have desired that reclaimed rubber have a definite age from the standpoint of processing advantages; in some cases, an age of 6 months has been requested if possible. Reclaimed rubber undergoes certain changes upon aging. It becomes drier, nervier, and less tacky on the mill, and requires more mastication to achieve the desired workability. Palmer and Kilbourne (8) commented on this tendency and showed that these changes are accompanied by a reduction in the chloroform extract of the reclaim. The purpose of this paper is to show the effect of these changes in properties of reclaimed rubber during aging upon the quality of products in which it is used. Periodic Testing of Different Types of Reclaim
Approximately one ton of each of three commercial products (Table I) was set aside from regular production. Five slabs of each reclaim were chosen, and samples obtained by cutting a strip across the cut end of each slab so that the entire cross
Table 1. Reclaim Type Mfg. process
Analyses of Reclaims
A B Black passenger Black passenger inner tube inner tube Open-steam pan Alkali digestion
Analysis", % Acetone extract 7.32 6.37 Ash 26.32 22.87 Alkalinity 0.79 0.14 Total sulfur 1.75 1.66 6.70 Carbon black 4.67 Rubber content (by difference) 59.00 64.84 1.12 Specific gravity 1.17 0 Teats run when reclaims were two weeks old.
C
whole tire Blend of alkali digestion and opensteam 8.87 18.67 0.09 1.91 14.56 ~~
~~
56.00 1.18
section of the slab was included. These five strips were blended before testing to make one composite and representative sample of each reclaim. The same five slabs of each reclaim were periodically tested as they increased in age. Acetone extracts and chloroform (uncured) extracts were obtained by the method of Palmer and Kilbourne (8). Milling tests were made by the procedure suggested by Palmer, ' Miller, and Brothers (IO) and later described in detail by Palmer and Kilbourne (8). It consists in milling a sample of the reclaim on a laboratory-size mill under standardized conditions with respect to batch size, roll temperature, roll speed, and roll setting. The times necessary for the reclaim to knit to the slow roll, reach a definite degree of smoothness, and leave the slow roll and adhere to the fast roll are recorded. The alkalinity of the sample was determined by the benzenealcohol-water digestion method described by Palmer and Miller (9). Tensile tests of the reclaims were obtained in test formulas I and I1 (Table 11); these are the same as formulas IV and 111, respectively, proposed by Palmer and Crossley (7). The reclaim-sulfur tensile tests were obtained by using
INDUSTRIAL AND ENGINEERING CHEMISTRY
1368
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the standard mix of 100 parts reclaimed rubber and 5 parts sulfur. Artificial aging was done by the air pressure heat test, as recommended by the American Society for Testing Materials (1). The results of periodic chemical tests on reclaims A, B, and C are shown in Figure 1. I n all cases the chloroform extract decreased as the reclaim aged, while the acetone extract remained fairly constant. The alkalinity of reclaims B and C
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INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1942
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INDUSTRIAL AND ENGINEERING CHEMISTRY
final tensile was somewhat lower than its 2-week value. The final tensile of reclaim C was superior to that obtained when it was 2 weeks old. Elongations appeared t o be unaffected by the aging of the reclaims, and all reclaims seemed to improve with age in resistance to air bomb deterioration, although reclaim B was somewhat less consistent than the others. Similar trends were observed in formula I1 Figure 5. The normal tensile of all reclaims reached a maximum after 2 months, then decreased gradually up to an age of one year. After 18 months the tensile of reclaim A was somewhat lower than the 2-week value, while the tensiles of reclaims B and C rose to values equal to or higher than the 2-week values. As in formula I, elongations were not affected by the aging of the reclaims. Air bomb tests on all three reclaims improved as the reclaims increased in age up to one year but appeared to decrease somewhat after the reclaims had aged 18 months. Considerably more fluctuation from test to test was experienced with reclaims B and C than with A; it may be significant that the reclaims which behaved most erratically were of low alkalinity, mhile the one giving the most consistent results was high in alkalinity. This behavior suggests that the alkalinity of a reclaim may have a beneficial effect on the age resistance of compounds in which it is used as well as on the preservation of the processability of the reclaim itself. In any event, there is no evidence in the above data that the samples of reclaimed rubber deteriorated in quality after 18 months of aging. Comparison of A g e d and Fresh Reclaim
There are two ways in which this can be studied. First, a sample of reclaimed rubber may be set aside and tested a t any desired interval as it ages, as was done in the experimental work described above. A second method is to manufacture lots of the reclaim a t definite intervals and test them all simultaneously. It seemed desirable to use the second method to test the effect of natural aging of whole tire reclaim C on the quality of a tire tread compound, since this type of reclaim is widely used for the purpose. Accordingly, a pile of scrap tires large enough to make four separate lots of reclaim C was chosen, and four portions were reclaimed a t definite intervals and tested together as described below. These lots were called C-1, C-2, C-3, and C-4, to indicate the order in which they were made. Their phypical and chemical characteristics are shown in Table 111. All are similar in processability, and the greatest variation in chemical tests occurs between lots C-1 and C-4. The variation in ash and specific gravity is not unusual for this reclaim, for dusting pigment is used during the manufacturing process; which unavoidably influences these properties. Physical tests were obtained in a typical thirdline tire tread, formula I11 (Table 11),from which the antioxidant was purposely omitted in order to bring out any differences in aging characteristics which might occur. Reclaims C-1, C-2, and C-3 were made up at intervals so that when tested in formula 111 they were, respectively, 6 months, 6 weeks, and 1 week old. The results are shown in Figure 6. Normal physical tests indicate very little difference beyond a slight stiffening on the part of C-1. Air bomb aging tests (1) show C-1 to be slightly poorer although not seriously so, while abrasion resistance as determined by the du Pont abrader (1) was equal for all three reclaims. Their resistance to heat build-up and blowout or rupture during flexing was substantially the same as measured on the Firestone Flexometer. As described by Cooper @), this test consists in compressing, under definite load, a block of rubber between two plates; one is stationary while the other travels in a circular motion of definite magnitude at constant speed, Temperature rise measurements are made by inserting a
Vol. 34, No. 11
Table 111. Characteristics of Reclaims Tested Reclaim No. c-1 C-2 c-3 Date made0 11-15-40 5-17-40 11-8-40 Analysis, % Acetone extract 9.17 9.20 7.80 Ash 16.35 18.15 18.60 Alkalinity 0.10 0.11 0.12 6 ecifio gravity 1.15 1.17 1.17 Reefaim-sulfur test (25-min cure a t 287O F.) Stress at 300%, lb./sq. in. 490 405 460 Elongation, % 330 370 320 Tensile, lb./sq. in. 570 525 505 Milline teat AgePdays 0 3 8 Smooth time min. 4.8 5.0 5.0 1.0 1.3 Knit time, i n 1.6 5.0+ 5.0+ 5.0f Fast roll time, min. a
c-4 5-16-41
7.17 19.32 0.08 1.18
520 380 690 5
5.0 0.8
5.0+
Reclaims were approximately one week old when these tests were made.
thermocouple in the block after a certain interval, and the time necessary to rupture the block is measured. After reclaim C-1 was one year old, reclaim C-4 was made up and both were again tested in formula 111, with results as shown in Figure 7. Except for slightly poorer air bomb aging tests, the one-year-old reclaim C-1 is equivalent to fresh reclaim C-4. As a confirmatory test, reclaims C-1 and C-4 were compared in 22 per cent quantities in the treads of standard 6.00 X 16 tires under standard road test conditions. The reclaims were aged 3 and 15 months when the tires were built. The tires were run on test cars under identical conditions for approximately 10,000 miles; then they were removed and inspected for wear and cracking. The tires containing 15month-old reclaim C-1 were slightly superior to those containing 3-month-old reclaim C-4 in resistance to tread cracking and were approximately 6 per cent poorer in wear resistance. It was felt that these differences were no greater than the normal variation to be expected between any two lots of this reclaim, and only slightly greater than the normal variation between any two sets of test tires. Conclusions
As reclaimed rubber undergoes natural aging, it becomes less tacky and more resistant to breakdown during milling. This appears to be true to a lesser degree for reclaims having a high alkalinity than for those with low alkalinity. The acetone extract of reclaimed rubber remains constant as it ages, but its chloroform extract tends to decrease. These changes are accompanied by only small variations in physical properties, as shown by results obtained in typical test formulas. There is no significant change in quality when reclaim up to 18 months of age is used. This was true also of tire treads containing reclaim aged 15 months, as judged by actual road tests. Literature Cited Am. SOC.Testing Materials, Standards on Rubber Produots, D394-40 and D454-41 (1941). Cooper, L. V., IND. E N G .CHEX, ANAL.ED., 5, 350 (1933). Hurleston, E. H . , Trans. Inst. Rubber I n d . , 5, 348 (1929-30). EKG.CHEM.,ANAL.ED., 6, 56 (1934). Palmer, H. F., IND. Palmer, H. F., Rubber Age (N.Y.), 41,25-9, 93-6 (1937). Palmer, H. F., and Crossley, R. H., IND. ENG.CHEM.,32, 1366 (1940). Palmer, H. F., and Crossley, R. H., IND.ENQ.CHEM.,ANAL. ED., 13,154 (1941). ENG.CHEM.,32,512 Palmer, H. F., and Kilbourne, F. L., IXD. (1940). Palmer, H.F., and Miller, G. W., IND. ENG.C H E W ,ANAL.ED., 3,45 (1931). Palmer, H. F., Miller, G. W., a n d Brothers, J. E., IND.ENO. CHEM., 23, 821 (1931). Stafford, W. E., Trans. Inst. Rubber Ind., 5, 340 (1929-30). Winkelmann, H. A,, IND. EZG.CHEM.,18, 1163 (1926).
