INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1931
as the percentage of softener is decreased. Therefore the average tread-wear curves shown on Chart 4 might be displaced upward slightly a t their lower extremities, but such changes would be insufficient to alter the conclusions. The effect of stearic acid on tread wear may be seen in Table I. Table I-Effect of Stearic Acid on Tread Wear (Base stock plus 4 parts pine tar plns stearic acid) STEARIC ACID RATING---IN STOCK Av. Max. Min.
---
Parts
%
%
%
2 4
95 100 97
97 100 106
90 100 92
S
These data apply to the three stocks whose tensile properties are shown on Chart 3. From them we can conclude that the optimum ratio of stearic acid for this stock is approximately 4 parts per 100 of rubber. Abrasion Test Comparison
The abrasion test comparisons are shown in Chart 5. The various methods used have been fully described by Vogt (7) and therefore will be described only briefly here. The designation of the method coincides with that used on Chart 5. Method A-Regular Goodyear machine, 16-degree angle Method B-30 per cent slip machine Method C-20-degree angle machine Method D-du Pont-Grasselli abrader
Examination of this chart shows method B to produce data most comparable with the road-wear ratings. Method A in each case lies above the road wear, while methods C and D are considerably below. It might be remarked in this connection that Lambourn (4) has found a variable slip machine to give very good agreement with road-wear values and the present data lend support to such a conclusion. The data in Table I1 were obtained from the stearic acid stocks using methods A and D. Table 11-Effect of Stearic Acid on Abrasion Resistance (Base stock plus 4 parts pine tar plus stearic acid) STEARIC ACID Av. METHOD A METHOD B IN STOCK ROADWEAR
Parts 2 4 8
%
%
%
95 100 97
87 100 109
98 100 109
The high ratings of the stock containing 8 per cent stearic acid are, no doubt, due to the lubricating action of an excess of this material, which has been previously reported by North (6). Tensile Test Comparisons
These comparisons are shown on Chart 6. Curve A represents the average elongation of a series of five cures; curve B , the average tensile; curve C, the modulus of the 70-minute or best cure at 500 per cent elongation; and curve D, the modulus of the 140-minute cure at 500 per cent elongation. The elongation increases, as would be expected, with increased softener, while the modulus and tensile values are decreased, as indicated on Chart 2. The average tensile does not decrease so rapidly as the average road wear, but the modulus values drop off more rapidly. These values are not so widely divergent as the abrasion data, but no single one of them presents an accurate index of road-wear resistance. Tear Test Comparisons
On Chart 7 are shown the ratings of a tear test described in an unpublished work of G. J. Albertoni, of this laboratory. This test employs a rectangular test piece cut from standard laboratory test sheets. The dimensions are approximately
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7.62 by 5.08 cm. (3 by 2 inches) with the length in the direction of the grain. A cut is made with the shears 2.54 cm. (1 inch) long in the center of the test piece in the direction of the grain. Two jaws are then fastened to the sample approximately 1 mm. from the edge of the cut. One jaw is 2.54 cm. (1 inch) wide and extends over the length of the cut, while the other is 7.62 cm. (3 inches) wide and extends over the entire width of the test piece. The jaws are separated on a Cooey machine and the pull is recorded. The test is an effort to duplicate the conditions of hand tear. It will be seen on Chart 7 that the tear is measurably improved with the addition of softeners and that mineral rubber produced more improvement than pine tar. The durometer ratings of the tires after curing are also shown on this chart, The hardness of the stock containing no softener was 62 and that of the stock containing the maximum of softeners, 57. Economy
From an economic standpoint the savings to be effected by the use of softeners are dependent on a number of processing advantages which are impossible to evaluate in accurate cost figures. For this reason simple compound costs do not tell a complete story. However, if softeners are to be effective as a means of giving lower quality tread stocks a t a lower volume cost, the decrease in cost should be the same as the decrease in quality as the softener is increased. With the present prices on rubber and other compounding ingredients, this order does not hold. For example, with the addition of 16 parts of pine tar the compound cost is reduced to 93 per cent of that of the control, while the tread wear is reduced to 67 per cent. I n the case of mineral rubber the cost is reduced to 92 per cent while the tread wear is reduced to 75 per cent. Conclusions
I n view of these results, it may be concluded that in this tread stock the minimum amount of pine tar or mineral rubber commensurate with good factory processing is desirable if the maximum abrasion resistance for a given cost is to be obtained. This means that softeners which are effective, as such, in smaller amounts than those available a t present should h d a use in tread compounding. Stearic acid should be held to a ratio of approximately 4 parts per 100 of rubber. Most of the laboratory tests give indications in the same direction as the road wear, but none coincide exactly. Acknowledgment
The authors desire to acknowledge the work of C. R. Park, formerly of this laboratory, in the initial planning of these experiments, and to express their thanks to R. P. Dinsmore for permission to publish this paper. Literature Cited (1) (2) (3) (4) (5) (6)
Aultman and North, IND.ENG.CHEM.,15, 262-4 (1923). Burbridge, Trans. India Rubber I n s l . , 1, 429 (1926). Burbridge, I b i d . , 2, 256-66 (1926). Lambourn, Rubber Chen. Tech., 2, 166-92 (1929). Marzetti, Giorn. chim. ind. applicala, 5, 342 (1923). North, IND. END.CHEM., 21, 722-3 (1929). (7) Vogt, Ibid., 20, 140-9 (1928). (8) Williams, Ibid., 16, 362 (1924). (9) Zimmerman and Cooper, Ibid., 20, 812-3 (1928).
Correction In our paper on “Heat Transmission to Water Flowing in Pipes,” IND.ENG.CHEM.,23, 301 (1931),the constant in Equation 4, on page 306, should be 550 instead of 450. This is evident from Figure 4, where the ordinate at an abscissa of unity is 550. A. E. LAWRENCE T. K. SHERWOOD
