H a r o l d G r a y , H . S. K a r c h , a n d R . J . H u l l , B . F . G o o d rich C o m p a n y , A k ro n , O hio
T
H E initiation and growth of cracks are obviously im portant factors in the service rendered by tire treads.Several machines have been de
signed to g iv e c o m p a r a t i v e d ata on the flex resistance of various compounds, b u t in the experience of the present authors, none of the a v a i l a b l e m e t h ods for flex cracking compari
sons are satisfactory, since pre
dictions based upon them are n o t confirmed by road tests.
As a result, a great deal of tim e and money has been wasted in developing compounds in the laboratory only to meet with failure on the road.
Any new m ethod should perm it a fairly accurate means of estim ating th e tendency towards initiation of cracks and a quantitative measure of cracks and cuts once formed. In addition, it should be adapted to studying the effect on crack
ing of various compounding ingredients such as age resisters, of varying such physical properties as modulus, hardness, etc., of cure, methods of mixing, tread design, tread contour, etc. The m ethod should also perm it th e use of either labora
tory- or factory-mixed stock. Before the present work was completed, it was found possible to make fairly accurate studies of various construction details of the tire carcass on tread cracking as well as to compare the products of various manufacturers.
Definite knowledge on form ation of cracks due to flexing is necessary. On the other hand, in m any types of service, tires are subjected to the action of sharp objects, such as
cracked stones, which will cu t the tread. Such cuts are not always harmful. If there is a tendency to g ro w , h o w e v e r, the cu t will, in time, extend to th e carcass and serious trouble result. Some t r e a d d e s ig n s will pick up stones, which m ay c u t the undertread before they are e x p e l l e d . H e r e a g a i n , in m a n y cases, if th e cuts do n o t g ro w t o a n y e x t e n t , very little harm is done.
De s c r i p t i o n o f Te s t
Obviously, th e only way to test a tread th a t would answer all th e above qualifications would be on a tire. A 7.00-18, 6-ply tire was used exclusively for the preliminary work on laboratory-mixed batches. O ther sizes varying from a 5.25- 18, 4-ply, to a 10.50-22, 12-ply, have been satisfactorily used.
One standard and three experimental sections can be tested on a single tire. The raw materials for the four batches, except the particular items to be tested, are always taken from the same lot. If possible, the major portion of the batches are mixed in a master batch in the factory. The batches, generally 10 pounds, are mixed on a 24-inch laboratory mill. The stock Is sheeted off about 0.125 inch thick, and while still warm is cut and plied up to approximate the shape of the mold described in the next para
graph. Care must be taken not to trap air between the plies.
The plied-up stock is now ready to be molded. The mold used is merely a flat, plate, machined out to a cavity which in cross section equals the contour and gage of a tubed 7.00 tread. The length of the cavity is about 3 inches greater than one-fourth th a t of a 7.00-18 tread. A standard sheet-metal cover is used. The A method, fo r com paring the fle x cracking
properties o f tread stocks on standard tires w hich p erm its the use o f laboratory-m ixed batches is presented. T he m ethod can be adapted to the testing o f the effect o f carcass construction, tread design, etc., on fle x cracking. Com parative tests on tires o f d ifferent m ake can also be m ade.
Laboratory results check w ith in fo rm a tio n ob
tained fr o m the road.
A N A L Y T I C A L E D I T I O N Vol. 6, No. 4
perature of the stock as it comes from the mill.
A sheet of parchment paper is placed on each side of the built- up tread slab. The slab is then placed in position in the mold, and the mold placed in a press and cooled under pressure.
The standard and three experimental sections are cut to one- fourth the length of a 7.00-18 tread. The sections are spliced to form a full tread, built into a tire, cured, and are ready for teat.
As samples from factory-mixed batches can be used as well as laboratory batches, all desired variations in factory proc
essing can be studied. By using laboratory-mixed batches, it is possible to obtain information on materials available only in small quantities. T he use of both factory- and laboratory- mixed batches permits the study of all available materials.
Factory-mixed experimentáis are checked against factory- mixed standard and laboratory-mixed experimentáis against laboratory-mixed standard.
Molded treads have been checked against extruded treads and found to be equal in resistance to cracking. In testing this point, the stock in a given test was from the same fac
tory batch. P a rt of the batch was held out for the prepara
tion of two molded sections and the rest extruded in the
regu-Co m p o u n d CRATOa index cracks index cracks index cracks
A 100 0 100 1 100 186 large points, relative to the design. The blade of the knife is approxi
mately 0.031 inch thick and is exactly 0.25 inch long. The knife is fitted with a shoulder so th at the blade cannot penetrate the surface more than 0.016 inch. The cut formed is 0.25 inch long by 0.016 inch deep. After the tire has been run, the length of the cuts is measured. The increase over 0.25 inch multiplied by four gives the percentage increase. The average increase of the 12 cuts in a given section is used on the crack growth percent
age for the section.
After the cuts have been made, the tire is mounted and in
flated as desired, generally using loads and inflations recom
mended by the Tire and Rim Association. The tire is then run on a Bureau of Standards test wheel a t 45 miles per hour 2000 miles at room temperature, the tire is removed, and the increase in length of the cuts is recorded. Examination may reveal inde
pendently formed flex cracks with some tread compounds.
In the first part of the work on this problem, the tires, after the original 2000 miles, were replaced on the machine, using the same
July 15, 1931 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y and number of independent flex cracks were again recorded.
As the work progressed, it was found that the test was not drastic enough and it became necessary to develop an aging pro
cedure. The tires, after running the initial 2000 miles, are re heating would be. The tires are then run for 1500 miles at room temperature as described above. This modified test brings out will perform better on the road as far as cracking is concerned th a n those th a t show a larger num ber of cracks and greater crack growth on the indoor test.
A still further check on the laboratory procedure is to age tires with half-and-half treads for 7 days, as described, with
o ut previously running them on the wheel. If such tires are then run on a gravel or crushed stone road, the cracking tend
encies will check the results obtained on the indoor machine although percentage relationship will vary. Furtherm ore, such tires have all the appearance of having been run for a from table to table. Percentage relationships from compari
sons run on the road and in the laboratory are not exact b u t indicate greater growth. T he num ber of independent cracks formed is listed under “Flex Cracks.” In m ost cases, the
No attem p t has been made to compare the results obtained with those from various flexing machines available in the laboratory. Furtherm ore, it is not the interest of the authors to show how to compound to obtain maximum resistance to tread cracking.
Except where definitely indicated as “ R oad” the d ata are from tires run on the Bureau of Standards machine.
Co n c l u s i o n s
A practical m ethod of studying tread cracking in the labora
tory which gives d ata comparable with field service results is invariably in admixture with the heptafluoride. According to O. Ruff [Chem.-Ztg., 58, 449 (1934)] compounds with a lower proportion of fluorine could not be detected. In the presence of oxygen, however, or when the pentafluoride was exposed to mois
ture, iodine oxyfluoride ( I O F j ) could be detected.