August 1, 1921 



THE INDIA RUBBER WORLD 



807 



FABRIC 



Fabric is the foiindalioii upon which the tire is buih. It is 

 used to give stability and strength. There are two well-known 

 classes of tires, the square-woven fabric and the cord fabric. In 

 the square-woven fabric tire the threads in each ply run in both 

 directions, alternating over and under as in a piece of ordinary 

 cloth. In the cord fabric tire the threads or cords in each ply 

 run parallel with the exception of a few small cross threads used 

 simply to hold the cords in place while they are being impregnated 

 with the rubber compound. The life of a tire would be greatly 

 increased if internal friction could be eliminated. The internal 

 friction caused by intermittent distortion of the tire in use is the 

 result of the friction of the threads upon each other and the 

 strains and stresses set up in the rubber compound. Naturally 

 the fabric which gives the least amount of internal friction will 

 give the longest life to the tire. 



Since square-woven fabric cannot be thoroughly impregnated 

 with rubber compound, the places where threads cross will be left 

 bare and at these points fle.xing will cause a sawing action and 

 the generating of frictional heat. It has been demonstrated very 

 clearly by experiment that, when the temperature resulting from 

 mechanical action reaches 230 degrees F., vulcanized rubber ceases 

 to function as an adhesive compound, and crumbles into minute 

 particles which fail to resume their original condition, causing 

 the compound to lose its function in the tire. This causes separa- 

 tion, weakness, and finally a blow-out. It may be interesting to 

 know that 265 degrees F. is not an uncommon temperature 

 reached in a tire when driven at high speed over the road ; this 

 is particularly true of large truck tires. 



In the case of cord fabric each thread is embedded in the rubber 

 compound and the internal friction is reduced to a minimum. 

 The ideal condition would be to have each cotton fiber of thread 

 embedded in rubber but of course this is not practical and on 

 account of weaving difficulties this has not been accomplished. 

 Since cord fabric comes closer to the ideal condition, the time 

 is not far distant when it will entirely supplant square-woven 

 fabric in the carcass of the tire. A step in this direction will 

 be made when the cost of producing cord tires is sufficiently re- 

 duced to successfully compete with the square-woven tire. A 

 brief summary of the advantages derived from the use of cord 

 tires would include easier riding, due to greater resiliency; saving 

 of gasoline and oil ; saving of machinery and more miles per 

 dollar. 



Fabric is also used in the breaker and chafing strips. The 

 breaker fabric is covered with a rubber compound that will act 

 as a binder between the soft cushion stock and the stiff tread 

 stock. The breaker fabric is used to give this compound stability 

 and therefore decreases the separation between the tread com- 

 pound and the cushion. The chafing strips are used for pro- 

 tection and reinforcement. 



At this point something should be said about the mi.xing of the 

 rubber and ingredients, and the calendering or application of the 

 compound to the fabric. There are so many factors that enter 

 into the mixing that they can be only briefly described here. Break- 

 ing down of the rubber by mechanical action changes it from a 

 tough, hard state to a tacky, plastic condition. This influences 

 the impregnation of the fabric, tackiness, blooming and other 

 physical qualities, and also the vulcanization. The thoroughness 

 of incorporation of the compounding elements has an influence 

 upon uniform vulcanization and wearing conditions. In order to 

 eliminate to the highest degree the variable conditions inherent 

 to milling, calendering, building operations and vulcanizing, it is 

 necessary to have every process standardized and a rigid inspec- 

 tion to hold to a minimum the factor of the human element. 

 Therefore all reputable manufacturers analyze thoroughly all com- 

 pounding materials and rigidly inspect all fabric before these 

 elements enter into the tire, and also carefully control the degree 

 of vulcanization in the finished product. 



TIRE CONSTRUCTION 



Tire construction is an art in itself. It is like the building of a 

 machine, and just as much care must be used in designing a tire 

 as is used in designing a finely adjusted machine. .\s nearly every 

 tire is built on an iron core and vulcanized in a mold, the space 

 occupied by the tire is constant and is filled with a unit composed 

 of many variables. Therefore, when the fabric is frictioned and 

 skim-coated it is held to a gage of a maximum or minimum varia- 

 tion of two or three thousandths of an inch. Likewise all other 

 parts, as top cushion, breaker, tread, bead and side-wall, are held 

 to a maximum or minimum gage. The proportion of the fabric 

 to the rubber compound must be properly balanced. The addition 

 of an extra ply or the increase of the thickness of rubber com- 

 pound may destroy this balance and materially weaken instead of 

 strengthen the tire. It has been demonstrated many times by ac- 

 tual service tests that the correct distribution of rubber compound 

 in the tire will increase its life several thousand miles; or the 

 changing of the sulphur one-half of 1 per cent in a single com- 

 pound will cause an equal variation in the mileage. There are 

 many faults in a finished tire that may be attributed to improper 

 construction, as wide overflow, wrinkling of breaker and plies, 

 incompletely filled molds, and weakness in the beads. These are 

 usually remedied b\- changing the construction hut. in some in- 

 stances the proper results can be obtained by changing the com- 

 pound, the process of vulcanizing, or redesigning the equipment. 



VULCANIZATION 



There is probably no phase of tire manufacture that receives 

 more attention than the .vulcanization of the tire and still there is 

 no phase that is more problematical. The proper degree of vul- 

 canization IS an empirical condition existing in the various com- 

 ponents of a tire which is detennined by results obtained by road 

 tests. Either an undervulcanized or an overvulcanized tire 

 will give low mileage; even if a single part, such as cushion, 

 breaker, or tread compound, is over or undervulcanized, the en- 

 tire shoe will give poor results. 



The controlling factors in a compound to obtain this empirical 

 state are the sulphur, accelerators, rubbers, milling and calender- 

 ing, time and temperature of the cure. Any one of these condi- 

 tions will materially affect the state of vulcanization. The proper 

 manipulation of these variables is a chemist's job and requires 

 great care, thought and experience. It is absolutely essential 

 for the chemist to have a laboratory fully equipped to make com- 

 parative physical and chemical analyses, to develop new com- 

 pounds, and try out new compounding ingredients. Various types 

 of rubbers made by different methods of coagulating, washing and 

 dryin.g have diff'erent vulcanizing ranges and optinnnn cures, that 

 is, the state of maximum efliciency when vulcanized, therefore 

 great care must be exercised in their selection for a compound. 

 Furthermore, the optimum cures of all compounds must be so 

 adjusted that in the finished product every component of the tire 

 has simultaneously reached its maximum efliciency. Excessive 

 milling causes the compound to vulcanize more slowly but more 

 uniformly; decreases the tensile strength, and increases the stretch. 

 These actions can be explained by the breaking down of the 

 rubber molecule into its polymeric stages, each stage having its 

 own particular range and optimum cure with its corresponding 

 tensile and stretch. The slower a compound is vulcanized to its 

 optimum cure, the better resistance to aging it will have; there- 

 fore low vulcanizing temperature and long time is preferable for 

 quality of product, but owing to the demands of quantity produc- 

 tion, higher temperatures and shorter times are resorted to. The 

 scientific explanation of the effect of time, temperature and me- 

 chanical action upon quality of product is a problem for research 

 and it is high time that some of these problems were given proper 

 investigation by the scientific men of toda\-. 



USAGE 



If the tire is neglected and abused while in service, all the care 

 used in testing and selecting the rubbers and the compounding 



