1. AGING of TIRES Mechanisms & Countermeasures

2. Aging of Tires (Mechanisms and Countermeasures)  As a pneumatic tire grows older chemical changes take place in the components parts of the tire that affect their physical characteristics.  The changes in the physical characteristics of these components are detrimental to their performance.  Degradation of the steel belt structure takes place as tires age.  Degradation of the steel belt structure can lead to catastrophic tread separation failure.

3. Three main aging mechanisms 1)Oxidation of rubber. 2)Degradation of the steel to rubber bond. 3)Increase in sulfur crosslink density. 1) and 2) have been found to contribute most to aged deterioration and 3) the least.

4. Time vs. tread wear 1)Oxidation of rubber. 2)Degradation of the steel to rubber bond. 3)Increase in sulfur crosslink density.  Tire age is the predominant factor.  Tread wear has little effect.  Unused spare tires age by these mechanisms.

5. Changes in the sulfur crosslink density occurs over time. This causes: Increase in modulus. Increase in fatigue crack growth rate. Increased crosslink density

6. Vulcanization and sulfur crosslinks

7. Countermeasures to increasing crosslink density. EV and semi-EV curing systems are more resistant to increases in crosslink density than conventional curing systems. EV and semi-EV systems rely on lower sulfur levels and higher accelerator levels. Belt skim compounds require conventional curing systems to achieve the necessary bond between the rubber and brass plated steel belt cords. EV and semi-EV systems are not suitable for belt skim compounds. Increased crosslink density is less problematical than aerobic aging.

8. Oxidation of belt skim and wedge rubber.  Oxygen reacts with the polymer chain structure.  This causes chain scission.  Physical properties of the rubber degrade.  Elongation at break and tear strength are reduced.

9. Countermeasures to oxidation of rubber.  Minimize the amount of oxygen entering the tire structure.  Use appropriately high levels of antioxidants in belt skim and wedge rubbers.  Antioxidants are consumed as they react with oxygen so they do not protect indefinitely.

10. Structure of the rubber to steel cord bond

11. Degradation of the rubber to steel cord bond  Oxygen and water vapor reacts with CuZn brass layer.  Dezincification of brass occurs.  Breakdown of the rubber-brass adhesion system.  Debonding of steel belt cords.  Corrosion of steel belt cords sometime evident.

12. Countermeasures to the degradation of the steel to rubber bond. Minimize the amount of oxygen and water vapor entering the tire structure.

13. Tire Innerliner

14. Innerliner properties  All innerliners allow inflation gasses to permeate into the tire structure.  The most impermeable rubber is halogenated butyl (halobutyl).  Halobutyl rubbers are sometimes blended with NR and/or SBR to  reduce costs.  The best compound design for an innerliner is one of 100% halobutyl  rubber.  The thicker the innerliner the lower the rate of permeability.

15. Measurement of permeability - IPR (internal pressure retention)

16. Measurement of permeability - ICP - intracarcass pressure.  A pressure gradient exists between the inside and outside of the tire.  This drives internal gasses through the innerliner into the tire structure.  The polyester ply cords act as a reservoir where permeating gasses build up causing measurable pressure within the ply cords.  Measurement of the pressure in the ply cords is a method of determining the permeability of the innerliner.

17. Measurement of permeability - ICP - intracarcass pressure.

18. Innerliner rubber type and tire durability

19.  Inflation pressure losses are reduced.  Changes in rubber properties are significantly slowed down.  The belt skim rubber degradation is significantly lower.  Belt wedge rubber degradation is significantly lower. Nitrogen Inflation

20. Summary  Tire aging mechanisms are well known.  Countermeasures are well known.  Tires should be designed with best  countermeasures.

21. Conclusion Tires designed with the best countermeasures are:  More resistant to aging.  More resistant to belt failure.  More resistant to tread separation failure.  SAFER FOR THE CONSUMER