AGING OF TIRES (Mechanisms and Countermeasures)

Transcription

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

2 Sulfur crosslinks 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. 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. 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.

3 Structure of the rubber to steel cord bond 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 sometimes evident. Countermeasures to the degradation of the steel to rubber bond. Minimize the amount of oxygen and moisture entering the tire structure. Tire Innerliner.

4 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.

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

6 Innerliner rubber type and tire durability

7 Nitrogen inflation When nitrogen is used to inflate tires: Inflation pressure losses are reduced. Changes in rubber properties are significantly slowed down or even halted. Oxidation of the tire natural rubber casing components is reduced. The belt skim rubber degradation is significantly lower. Belt wedge rubber degradation is significantly lower. Summary Tire aging mechanisms are well known. Countermeasures are well known. Tires should be designed with these best countermeasures. 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 References used in preparing this presentation include: Effect of Crosslink Structures on the Fatigue Crack Growth Behavior of NR Vulcanizates with Various Aging Conditions, Sang Goo Kim, Hankook Tire Mfg. Co. Ltd., Jang-Dong, Yuseung-Gu, Suck-Hyun Lee, A-Jou University, Suwon, Korea, ACS V Antioxidant Efficiency of p-phenylene Diamines in NR, O Lorenz & C R Parks, Research Division, Goodyear Tire and Rubber, ACS V Oxidation and Antioxidant Action in Rubber Vulcanizates, J. Reid Shelton & William L. Cox, ACS V Investigations on Initial Stage of Aging of Tire Rubbers by Chemiluminescence Spectroscopy, Schuster, Giese, Santoso, Deutsches Institut fur Kautschuktechnologie e.v., Germany. ACS V Aging of Tire Parts During Service - Part II Aging of belt-skim rubbers in passenger tires, Kaidou & Ahagon, Yokohama Rubber Co Ltd, ACS V Field Aging of Tires, Part 1, Baldwin, Dawson, Hurley, Ford Motor Company, ACS, October 2003 Accelerated Aging of Tires, Part 2, Bauer, Baldwin, Ellwood, Ford Motor Company, ACS, May 2004

8 Field Aging of Tires, Part 3, Baldwin, Bauer, Hurley, Ford Motor Company, ACS, October 2005 Correlation of Rubber Properties between Field Aged Tires and Laboratory Aged Tires. Part 2, Bauer, Baldwin, Hurley, Ford Motor Company, ACS, May 2005 Correlation of Laboratory Tire Endurance and Rubber Aging, Bauer, Baldwin, Ford Motor Company, ACS, V EA00-023: Engineering analysis report and initial decision regarding Firestone Wilderness AT Tires, US Department of Transportation Office of Defects Investigation, Oct 2001 Early Stages of Oxidation in Rubber, J. W. Temple, Sidney M. Cadwell, and Morris W. Mead, Jr., ACS 1931 Aging and Oxidation of Elastomers, Shelton, ACS V Effect of Oxygen on Tear Strength of Elastomers, Gent & Hindi, University of Akron, ACS V Effects of aging on the steel cord-rubber interface., H Lievens, NV Bekaert, Belgium, 27 June 1985 Adhesion of steel cords in tyres., N.J. McCaffrey & O. Delatycki, Dept Industrial Science, University of Melbourne Mechanism and theories of rubber adhesion to steel tire cords- an overview, W J van Ooij. Akzo Research, ACS V Fundamental aspects of rubber adhesion to brass-plated steel tire cords, W J van Ooij, Akzo Research, ACS V Harmful effects of water on the adhesion of steel cord to rubber in tyres, W Coppens, R&D, Bekaert, Rubbercon 81 The fundamental aspects of adhesion of brass plated steel cord to rubber compounds, John M Swarts, Rubber World, February 2002 Steel cord skim compounds: the achievement and maintenance of maximum adhesion, R C Ayerst & E R Rodger, Monsanto Chemicals, ACS V Adhesion of steel cords in tyres, N.J. McCaffrey & O. Delatycki, Dept Industrial Science, University of Melbourne, 1985 Rubber - brass bonding, Wim van Ooij,Chapter 6, Handbook of Rubber Bonding, RAPRA Technology, 2001 Inflation pressure loss in tubeless tires effects of tire size, service and construction, D M Coddington. Exxon Chemical Co., ACS V Effect of intracarcass pressure build up on tubeless tire performance, S A Banks, F Breznk, J A Rae, and C S Hwa, Esso Research & Engineering, ACS V Factors in tubeless radial tire durability, D M Coddington, Rubber and Plastics News. Aug Tire innerliner technology unseen advances, D A Paterson and K M Wolniak, Miles Inc, ITEC Conference Sept 1994 Tire Application of Elastomers - 3. Innerliners, W. H. Waddell & M. B. Rodgers, ExxonMobil, ACS, May 2004 Impact of Halobutyl Rubber Innerliners on Tire Performance, W. H. Waddell, ExxonMobile Chemical Company, ACS, May 2005 Halobutyl innerliners offer best tire durability, Tracey & Waddell, ExxonMobile, Rubber & Plastics News, May 30, 2005 Tire durability with nitrogen inflation, L R Sperberg, ACS, V Effects of Nitrogen Inflation on Tire Aging and Performance, Baldwin, Bauer, Ellwood, Ford Motor Company, ACS, May 2004 Nitrogen Inflation of Tires, Waddell, Napier, Tracey, ExxonMobil, ACS, V