L. V . Cooper
Fir e s t o n e Tir e & Ru b b e r Co m p a n y, Ak r o n, Oh io
M
A N Y factors are involved in the proper compounding of rub
ber stock. In tread com
pounding wear resistance is the major consideration, but other factors c lo s e ly ap
p r o a c h it in importance.
Among these is the ability
to withstand repeated flexing without cracking. Flex cracking produces an unsightly appearance which conveys the idea of inherent weakness in the tire, and also, if the flex cracking be
comes very pronounced, the tire may fail from carcass break at that point. A satisfactory tire must not flex-crack.
Flex-cracking resistance is dependent on two things— the stock itself and the stress under which it is working. The design of the tire— that is, size and shape of the buttons, the size and position of the side ribs, and the amount of lettering on the side— regulates the working stress under which the sidewall and tread are operating. This accounts for the fact that two tires made by different companies may be different in flex-cracking resistance although of very similar or even identical compounding.
Various reasons have been advanced as to why one stock is better than another in regard to flexing resistance. A few of these reasons are: (1) better dispersion of the pigments, (2) better surface continuity, (3) fewer stock folds during forming and curing, and (4) better stress-strain properties.
Arguments for and against these reasons have been advanced without any definite conclusion as yet.
Laboratory duplication of road-test conditions is the goal of all rubber technologists, as road testing is costly and time
consuming. A short study of the stresses under which the stocks in a tire work should tell us some things concerning what the laboratory machine for evaluating flex-cracking resistance should be like. A normally inflated tire is under less than 5 per cent elongation. If the tire were of uniform thickness and construction from bead to bead, the elongation would be uniform. However, such is not the case and there
fore the tire has points of greater and lesser stress. As a result some part of the tread or sidewall may be subjected to an elongation of possibly 10 per cent as it makes a revolution under load. Some parts of the tire are under compression during road contact, but inasmuch as rubber is only as
com-1 Received June 2, com-1930. Presented at the m eeting o f the Akron R u b ber Group, M a y 12, 1930.
pressible as bronze (1) the movement is practically all elongation, and therefore flex cracking may be considered as the result of elongation.
Therefore, a machine which subjects the rubber stock to repeated elongation should produce flex cracks.
Since the rigidity of a tire is due chiefly to the carcass fabric, the conditions under which the tread and sidewall operate are almost independent of the stress-strain properties of these two stocks. A laboratory evaluating machine should there
fore work to a definite elongation and not to a definite load.
Ordinarily flex cracking does not become noticeable on road test, even in poor tires, until 4000 or more miles have been run. If the machine duplicated road conditions, one
F ig u re 1— A p p a ra tu s fo r E v a lu a tio n o f F le x -C r a c k in g R e s is ta n c e
test would last 100 or more hours, or the equivalent of about 12 working days. The ideal machine would be one that could be set to duplicate road conditions and also one that could be so changed as to give accelerated tests.
A m achine has been designed for laboratory evalua
tion o f the flex-cracking resistance of any stock from which a du m b -b e ll strip m ay be cu t. The conditions under which the stock is tested m ay be varied at the will of the operator. The results obtained on this m achine on tread and sidewall stocks have always evaluated the flex-cracking resistance of stocks in the sam e order that actual road tests have.
0 to 100% 0 to 7 5%
A N A L Y T IC A L EDITION
0 to 50%
Vol. 2, No. 4
3 Hours 6 Hours 6 Hours
F ig u re 2— E ffect o f V a r y in g W o r k in g E lo n g a t io n
42>/i lbs. o f Black 50 lbs. o f Black
Eength o f test, 21/ « hours, elongation 0 to 100 per cent F ig u re 3— E ffect o f In cr e a s e d P ig m e n t a tio n
Clay Reclaim Control
1 Hour 2 Hours 2 Hours
Elongation, 0 to 100%
F ig u re 4— E ffe ct o f A d d in g F ille r P ig m e n t a n d C la y to T re a d S to c k s
Description and Operation of Testing Apparatus For several years a machine for evaluating flex cracking has been in use in the Firestone laboratories. This machine has won the confidence of the organization because the results obtained with it and the results obtained by actual road test have always been in accord. This machine is illustrated in Figure 1. The test strip is the dumb-bell strip used in stress- strain testing, eight of which may be tested at one time on this machine. The upper beam oscillates vertically, and as the cams which actuate the driving arm and the sides of the upper beam are slotted, it is possible to obtain any initial setting between 1 and 4V i inches and to vary the magnitude of the oscillation from practically 0 to 3 inches. The power is sup
plied by a Vvhorsepower motor. The pulleys are of such size that the upper beam oscillates about 400 times per minute.
The strips are fastened to the lower beam b y means of a bar which is held down by four bolts with wing nuts. The upper beam is then set at the top of its stroke and each strip is individually secured to the top beam by means of a special screw clamp. These clamps arc flattened eyelets and secure the strip by pulling it up into a slot in the upper beam. The slot in the upper beam is so tapered that strips of any thick
ness may be securely fastened. The strip is so placed in the upper beam that it is under maximum desired elongation.
Control 1% Diphcnylam ine
October 15, 1930 IN DU STRIAL AN D ENGINEERING CHEMISTRY 393
F ig u re 5— E ffect o f A d d in g 1 P er C e n t D ip h e n y la m in e
This elongation is measured in a manner similar to its measure
ment on a stress-strain machine. By adjusting the position o f the driving arm in the cams at one end and in the upper beam at the other end, it is possible to subject the strip to any elongation desired.
Early experimental work with this machine established the fact that strips must be allowed to return to a position of 0 stress in each oscillation if any rapidity of checking is to be secured. After studying various conditions of test, the follow
ing procedure was adopted:
M inim um distance between beam s... 2*/« Inches M axim um distance between beam s... 5 l/ j inches M axim um elongation of the strip under test... 100 per cent
Under these conditions it is possible to obtain flex cracks on the average present-day tread stock in about 8 hours with the machine making 400 flexes per minute, which is the number of flexes to which a tire in actual service is subjected when run
ning 35 to 40 miles an hour. In running a test on a series of strips the test is continued only long enough definitely to rank the several stocks under test, and it is desirable, if possible, to stop the test before any strip actually breaks.
Effect of Varying W orking Elongation
The effect of varying the working elongations under which the strip is flexed on this machine is shown in Figure 2. The stock used in this test was a tread stock containing 42‘/ j pounds of channel black per 100 pounds of rubber; formula used is No. 1 in the accompanying table.
F o r m u la s U sed In V a r io u s T es ts
Pig m e n t N o. 1 N o. 2 N o . 3 N o . 4 N o. 5
Smoked sheet 100 100 100 100 100
Channel black 42*/i 50 4 2 l/ i 421/* 42*/i
Z inc oxide 5 5 5 5 5
Sulfur * 3 3 3 3 3
Stearic a d d 3 3 3 3 3
Pine tar 4 4 4 4 4
Captax 1 1 1 1 1
Clay . . . . . . 40 . . . . . .
W hole-tire reclaim . . * . . . . . . 20 . . .
Diphenylamine . . . . . . . . . . . . 1
The strips on the left were subjected to 100 per cent elonga
tion for 3 hours, at the end of which time they were loosened from the upper beam and allowed to hang free while the test was continued on the other four strips. After 6 hours the strips under a maximum elongation of 75 per cent were in a condition similar to those which were stressed at 100 per cent maximum elongation and loosened after 3 hours. The strips which were flexed at only 50 per cent elongation were still in very good condition. T o obtain flex cracks on the two strips
shown at the right, it would have been necessary to run the test 10 to 12 hours longer.
Effect of Increased Pigm entation
Figure 3 shows the effect of increased pigmentation. The three strips at the left contain A2l/i pounds of channel black and are of the same stock used in the preceding test. The three strips at the right are identical, except that 50 pounds of channel black to 100 pounds of rubber were used (Formula 2). The detrimental effect of the increased pigmentation is very evident, and this is one of the reasons why more carbon black on the rubber is not used commercially.
Effect of Adding Filler Pigm ent and Reclaim The results of the third test (Figure 4) illustrate the effect of adding to a tread stock filler pigment and reclaim, respec
tively. A t present quotations whole-tire reclaim and the better grades of clay are about identical in cost on a volume basis. Although clay costs less than reclaim on a pound basis, the gravity of this material is twice that of whole-tire reclaim.
Formula 1 was again taken as the basis of the test, and the strip on the left is the compound to which 40 pounds of clay have been added. The middle strip is from a compound made up by adding 20 pounds of whole-tire reclaim to Formula 1 (Nos. 3 and 4 in table). The volume costs on compounds 3 and 4 are identical and are about 10 per cent cheaper than that of the control, Formula 1. After 1 hour of flexing the clay stock was in very poor condition and was loosened from the upper beam, and the test continued on the other two stocks. After 2 hours the stock containing the reclaim began to show serious checking, and so the test was stopped. The control stock on the right showed only a very few minor checks.
Effect of Adding D iphenylam ine to Stock
Figure 5 shows the effect on flex checking of adding 1 per cent diphenylamine to the control stock. The new formula is Formula 5. The beneficial effect of this amine is outstand
ing and would be of considerable commercial importance, ex
cept for the fact that other materials are available on the market which have the same property and other desirable properties as well. Among these materials are many of the antioxidants.
Com parison with Other Flex-Testing M achines Several other flex-testing machines have been designed, and one which is highly regarded is the so-called India machine
394 A N A L Y T IC A L EDITION Vol. 2, No. 4 (2). This machine consists of a rotating disk inside of an
off-center ring. In the rotating disk are cut radial slots in which specially cured test samples are inserted. As the disk rotates the free end of each test strip is bent down so that its plane is at approximately a 90-dcgree angle to the plane of that part of the strip which is secured in the radial slot.
In comparing the two machines, several desirable features are evident in the Firestone machine. With this machine it is possible to evaluate the flex-checking resistance of any stock from which a dumb-bell strip may be cut. This makes the machine valuable to compounders who are working with stocks that are air-cured. The conditions of test may be the working elongation. Also, this machine can be mounted in the direct sunlight and run under conditions which are very comparable to actual road test, a feature which recent ex carbon tetrachloride or p-di- chlorobenzene or both, and
termination of the two compounds separately have been de
scribed, some of them require special apparatus or reagents, and do not appear to be suited to the separation of the two constituents.
Review of M ethods
G f . n f . h a l M e t h o d s— The ultimate standard for halogen determination is the Carius method, but many laboratories do not possess the equipment required for heating under pres
sure. The Parr bomb ignition with sodium peroxide and starch has been adapted to halogen determination (17) in
cluding fluorine (IS). Marcusson and Doscher (20) ignite the compound in an atmosphere of oxygen and subsequently absorb the inorganic halide in alkali. When liquid ammonia is available the halogen compound may be determined ac
cording to Clifford (4) or Dains and Brewster (7) by decom
position with sodium. Several investigators have described methods involving catalytic reduction with various metals and hydrazine (2, IS, 23). Stepanow (25) has described a general method based upon the action of a twenty-five fold excess of sodium upon the compound in ethyl alcohol solution.
V o l a t i l e Com pounds— The combustion method of Plimp
ton and Graves (24) appears to be well adapted to the treat
ment of volatile compounds, although a rather elaborate
1 R eceived June 4, 1930.
pound with ethanolic potash under a trap of glass beads.
The writers, working independently, were unable to obtain complete recovery by this means, even when a slow counter
current of solvent down the condenser was used. The “ over
laying” procedure described below seems, however, to be effec
tive in preventing the escape of the volatile compound.
S p e c i a l M e t h o d s— Krishna and Swarup (15) have adapted Kux’s iodine method (1) to the determination of certain li
able halogens such as occur in acyl chlorides and chloramines, involving decomposition in alkaline solution. The separa
tion of /j-dichlorobenzene from certain other halogenated com
pounds by fractional distillation has been described by several investigators (6,1 2 ,1 4 ). The authors were able to detect as little as 2 per cent p-dichlorobenzene in a kerosene insecticide by suitable fractionation followed by the chilling of the 165
175° C. fraction to —10° C., but failed in the case of a solvent naphtha preparation, the amount of petroleum hydrocarbon distilling with the halogenated one being too great to permit separation of the latter. Cappenberg (3) has described a method suitable in certain cases in which the compound is decomposed by potassium hydroxide in methanolic solution.
Principles Involved
Of the above listed methods that of Stepanow, in conjunc
tion with the A. O. A. C. alcoholic potash saponification, ap
peared to offer the most promise. A large number of trials of various modifications finally led to the development of the The Stepanow m ethod for organic halogen is m od i