PHYSICAL PROPERTIES REQUIRED OF A TREAD COMPOUND The physical properties of a rubber compound have high relevancy in predicting service performance of retreaded tyres. However, there are limitations to the scope of physical tests; that is, almost no physical test gives a 1:1 correlation to service performance. The above aspect notwithstanding evaluation of physical properties do help in identifying rubber compounds with the desired service properties. The purpose of this write-up is to give a brief idea about the important physical properties required of a tread rubber compound and the mode of their assessment.TENSILE STRENGTH The Tensile Strength of a rubber compound, is it’s resistance to rupture under tension. It is measured as strength-at-break and is expressed in pounds per squire inch or Kilograms per centimeter of cross section. Good quality tread compounds should have a tensile strength ranging between 2750 to 3250 pounds per square inch. Compounds having low tensile strength are indicative of filling with cheap non-carbonaceous fillers like china clay or of use of high amounts of reclaimed rubber. A tread stock with poor dispersion will also exhibit a low tensile strength. But tensile strength tests lose significance when comparing compounds based on different rubbers. For example, a combination of natural rubber and synthetic rubber may give a lower tensile strength, but this definitely is not an indication of poor quality, since in this case, the over all properties including tread-wear-resistance may still be better. Compounds which require superior cut-growth and crack resistance should have high tensile strength and such compounds are the best suited for O.T.R. treads. Due to this reason, tyre treads for O.T.R. are based generally on natural rubber. In short, tensile strength is a relevant parameters in testing physical properties of tread rubber and can be considered an index of quality. MODULUS OR STIFFNESS The term ‘modulus’ is used to denote resistance to being stretched. It is defined as the force in pounds or kilograms necessary to stretch a piece of rubber of one square inch cross section to a specified elongation. This amount of stretch is normally expressed as a percentage of the original length, and the stress as PSI at the fixed elongation. Modulus is probably one of the most important properties of a tread compound as it plays a major role in tyre wear, and is involved in heat build up. ELONGATION Elongation or strain is defined as the ability of a rubber compound to stretch without breaking. Elongation at break or the ultimate elongation is the elongation at the moment of rapture. It is expressed numerically as a percentage of the initial length, taken at the moment of rupture. Elongation at break is one of the main requirements of the rubbery state is used to indicate changes in the rubbery character during ageing. HARDNESS Hardness of a rubber compound can be described as the resistance to indentation. Probably this is the most often measured property of tread rubber using Durometer. Hardness measurements lack in precision since it is very common to find a minimum of 5 points difference in readings made by various people using different Durometers on the same piece of rubber. Hardness is only an approximate measure and it is not an accurate index of stiffness or load-carrying capacity. Lower hardness may be due to several reasons like improper reinforcement, usage of non-carbonaceous fillers for cheapening purpose etc. Such tread compounds will be lacking very much in abrasion resistance. RESILIENCE Resilience is the energy of recovery from a deformation. A resilient piece of rubber is to said to have a ‘fast come back’. Resilient rubber compounds generate lesser heat when flexed, and so are desirable for tyres and tyre treads, where high temperature due to generated heat tend to deteriorate the rubber quality rapidly. ABRASION RESISTANCE Abrasion resistance may be defined as the resistance to wearing away by rubbing or impact with running surface in service. This essential property required in a tyre tread is often considered as a measure of reinforcement of the tread compound. In laboratory, abrasion tests are carried out by using either abrasive paper or an abrasive wheel to abrade the rubber sample. But these tests will hold good only for making comparative studies as the measurements thus obtained, cannot be correlated with actual performance on the road, the reason being that it is difficult to reproduce similar abrading surface or severity in a laboratory, as in actual running surfaces. In addition to the above, compounds containing wax or excess oil will have a tendency to smear the paper or abrasive wheel giving false measurements. Due to such pit-falls in testing for abrasion resistance in laboratory, one would think that road tests for tyre abrasion would be the ultimate. However, road tests also have their diadvantages and give no correct picture by themselves, one of the main disadvantages is the difficulty in duplicating severity of services as seen in different terrain. It has been amply proved by research workers of B.F Goodrich research centre and Philips petroleum company both of U.S.A that depending on the severity of services, wear rates vary so widely that it is very difficult to design a road test which will give a single answer to wear resistance.The above fact has been amply substantiated by these scientists by conducting road test studies. In their study, in one group of identical test, tyre of the same make and specification were made placed on three private cars for test; and it was observed that the wear rates were 7,000, 20,000 and 30,000 miles to baldness. All three of these drivers were well experienced in their profession. They did not feel that they cornered or braked hard. But it is obvious that the driver who wore out the identical tyres in 7000 miles imposed quite a bit more severe wear on his tyres than did the 30000 miles driver. In a road test the severity can be varied by changing many factors which affect the results, like the surface, temperature of the road as well as the attained temperature of the tyre, the speed at which the vehicle is driven, the load, the road-surface roughness, the amount of cornering, braking and acceleration, wet and dry road and vehicle factors such as wheel alignment. It has been demonstrated that concrete roads are as much as five or six times as abrasive when they are first laid, as they are after several years of service. The amount and severity of cornering is probably the biggest factor in total severity in running. Abrasion under wet conditions is almost nil and dry roads give higher wear. The dust on pavements serve as a lubricant thereby reducing tread wear. When the same is washed off by rain, rate of wear increases. Geographical and climatic changes also have an effect on wear. Mountain regions are found to be areas of higher wear. In addition to local terrain difference, road construction materials also play an important role in the wear. All these factors which are encountered in the abrasion of the treads are difficult to be duplicated in the laboratory abrasion testing machines. Besides these tests are almost always designed to be more severe than the conditions encountered in normal road service, in order to accelerate testing. VISCOCITY OR PLASTICITY In the case of fluids, viscosity can be defined as resistance to flow. Viscosity or plasticity as used in rubber compound is the tendency to take on a permanent deformation or flow under the influence of continuous deforming force. Viscosity of rubber compounds is measured in Mooney Viscometer. Tread compounds should have sufficient pliability for fixing them on to the buffed tyre. Tread Rubber with high viscosity will be difficult to handle for retreading. SCORCH Scorch means premature cure and a scorched stock is a wasted stock. Scorch time is that time in the heat history of a rubber compound when the effects of vulcanisation can just be detected. In the laboratory, scorch time is measured using Mooney Viscometer. As tread compounds should have a shelf life of a minimum period of 6 months, the scorch safety as measured in a Mooney Viscometer of the compound should be minimum 15 minutes at 120 degree C. Otherwise mooney scorch for 5 pt rise at 120 degree C should be a minimum of 15 minutes. A lower scorch rate than 10 minutes indicates poor shelf life of the compound which is likely to get partially or fully cured on continued storing for a few weeks, rendering it unfit for use. When a rubber compound is vulcanised, it becomes insoluble. Therefore, a simple scorch test can be conducted by putting the sample tread compound into a good solvent for rubber. If it dissolves, it is certain that the stock has not scorched. If scorch has occured, the sample piece will swell and it will not dissolve. SPECIFIC GRAVITY As a means for saving maximum quantity of rubber used for retreading, the retreaders will insist on more length per kg of rubber supplied. For this purpose, they will compare the length of a certain size of rubber of specified weight to those supplied by various other manufacturers. Subsequently they brand the rubber with less length as of high specific gravity. Although this is a simple method by which one can get an idea of the specific gravity of rubber, there are loop holes in this kind of sampling also. As for example, take the case of a rubber 70-18 size. When one takes samples of this size of rubber from various suppliers, it can be seen that there are variations in the exact dimensions, namely thickness and width. While some parties supply under gauged rubber, that of some others may be over gauged. In such a case, the difference seen in length cannot be taken as a criteria for making studies regarding high or low specific gravity. In addition to this, tread compounds with severe porosity will also show less specific gravity.Formerly, when tread mixes, were done exclusively using Natural Rubber, such a standard tread used to show a specific gravity of 1.12 to 1.13. With the advent of the synthetic rubbers which are used along with Natural Rubber to improve the abrasion resistance, thermal stability, age resistance, resistance to chipping etc. of tread compounds, specific gravity of such compounds tend to be slightly on the higher side on account of the fact that these blends of NR and Synthetic rubber require more amount of reinforcing filters in them for giving optimum properties. Specific gravity higher than those mentioned above are indicative of adulteration of compounds with large quantities of reclaimed rubber and cheap mineral fillers like china clay. Even though a small percentage of reclaimed rubber is admissible in tread compounds on account of some technical merits, non-carbonaceous fillers like china clay can be used in tread compounds only at the cost of its quality. |