How Oxygen Effects the Integrity of Tire Compounds, The Work Being Done to Test for This Degradation and the Consequences on Tire Durability Bruce A. Currie, P.E. Currieengineeringllc.com August 13, 2010

A Tire is a Complex Product. Contains up to 20 Different Components. Components are: Rubber Only Rubber and Fabric Rubber and Steel Physical Properties of Rubber Change Over Time due to: Temperature Stress to the Rubber Age of the Rubber

Aging of Rubber Compounds is the Primary Topic of this Presentation. One Factor in Aging is the Effect Oxygen has on the Physical Properties of Rubber. Oxygen Degradation of Rubber Compounds can Lead to Tire Failure and is the Primary Mechanism Studied by NHTSA in the Ford/Firestone Rollover/Tire Failure Problem. NHTSA ’ s Work Lead to the Various Parts of the TREAD Act of 2000.

Oxygen Degradation of Rubber Compounds has Been Studied by the Tire Industry for Years. Introduction to Rubber Technology (1959) Stated: “ The attack of oxygen has a greater effect on rubber then any other degrading influence ”. Rubber Technology (1973) Stated: “… the age- resistors used must be capable of reacting with the agents causing aging … to prevent or slow the polymer breakdown ”.

Antioxidant Chemicals are used to Slow Oxygen Degradation. The Vanderbilt Rubber Handbook (1990) Stated: “ Antioxidant selection is based on anticipated service requirements ”. Science and Technology of Rubber (1994) Stated: “ Increase in service temperature will thus accelerate the degradation of rubber …”. The Quantity of Antioxidant Must be Sufficient to Protect the Tire Compounds for the Life of the Tire.

One Very Important Factor Concerning Oxygen Degradation is the Permeability of the Tire ’ s Inner Liner. All Tires Loss Air through Their Inner Liner. Oxygen Flow (and Degradation) Increases as: Inner Liner Gage Decreases Halobutyl Content Decreases Inflation Pressure Increases Pure Nitrogen Inflation Eliminates Oxygen Degradation, but How Practical is Nitrogen Inflation.

Tire Companies Have Tire Aging Procedures and Testing to Artificially Age Tires to Simulate the Effects of Aging on Tires in the Field. NHTSA, ASTM and Ford have Studied the Tire Aging Mechanism. ASTM is about to Issue a Tire Aging Procedure and Wheel Test that Most Likely Will be Used by NHTSA as Part of the TREAD Act. Ford has done the Most Extensive Amount of Research on the Tire Aging/Oxygen Degradation Topic and Ford ’ s Results Will be Discussed.

Ford Collected OE Tires of a Specific Size and Brand on Vehicles They Sold with Various Years of Exposure in the Field. Tires were Collected in Detroit, Phoenix, Los Angeles, Hartford, Miami and Denver. Ford Stated: “ A crucial part of developing an accelerated aging test is understanding what mechanism of aging occurs in field-used tires ”. Ford found that Tires made by Different Tire Companies age at Different Rates. This is due to Tire Compound and Construction Differences Between Tire Companies.

Ford ’ s Study was Extensive, Approximately 2500 Tires were Collected and 1500 were Dissected and Analyzed. Ford found that All Tires Aged Oxidatively in the Field Regardless of Geographic Location. From the Spare Tires Collected Ford found that Mechanical Fatiguing does not Impact Physical Property Changes in the Tire ’ s Rubber Compounds. In Later Comments Ford Indicated that Spare Tires Aged at about 80% Compared with Tires on the Ground from the Same Vehicles.

Ford also Looked at Repaired Tires. “ Of the 923 road tires analyzed, 73 had been repaired … No significant differences were found between the 73 repaired and 850 non-repaired tires. ” Ford also Stated: “ For a given tire, field aging rates are controlled by the ambient temperature. In North America, Pheonix serves as a reasonable ‘ worst-case ’ location ”. This Statement is Based on the Six Cities Tested. Ford Indicated that Tires Age Roughly Twice as Fast in Pheonix as They do in Detroit.

Ford Indicated The Rubber Properties for Which Aging Rates have been Measured Include: Peel Strength of Belt Skim Stock Crosslink Density of Belt Skim Stock Elongation to Break of Belt Wedge Stock Modulus of Belt Wedge Stock Ford Further Stated: “ Peel strength retention appears to be the most sensitive measure of tire aging and can be used to predict long-term field behavior. ” Based on a 25% Retention of Peel Strength in Pheonix Tires Ford Came Up with 6 Years as the Time Tires Need to be Replaced.

Based on Their Results Ford Proposed the Following Tire Aging Test. Inflate the Tire on a Wheel with a Mixture of 50% Nitrogen and 50% Oxygen, Oven Age at 65 Degrees C for 8 Weeks While Removing and Refilling Every Two Weeks. Next Wheel Test at 75 mph on a 1.7 m Test Wheel at 38 Degrees C at 26 psi at 85% Max Load for 4 Hours, 90% Max Load for 6 Hours and Then 100% Max Load for 24 Hours. This is a Total of 34 Hours. Increase Load an Additional 10% Every 4 Hours Until Tire Failure (Passenger Tire Test).

The Wheel Test to 34 Hours is Likely to be What NHTSA Specifies for Their Test. Ford Indicated That Most P-metric Passenger Tires Supplied to Them Could Meet This Test, but Light Truck Tires Will Need Some Work. Ford ’ s Age Limit of 6 Years is Based on 25% Peel Strength Retention. But This Value is Dependent Upon the Original Peel Strength in the Tire. Tires Manufactured by Different Companies have Different Peel Strength Values. What is More Important, the 25% Value or the Actual Numeric Value of the Aged Tire ’ s Peel Strength.

In Europe, the British Rubber Manufacturers ’ Association stated: “ BRMA members strongly recommend that unused tyres should not be put into service if they are over 6 years old and that all tyres should be replaced 10 years from the date of their manufacture. ” The NHTSA Website, Talks About Some Vehicle Manufacturers Cite 6 Years and Some Tire Companies Cite 10 Years. NHTSA States: “ Remember, it is always wise to err on the side of caution if you suspect your vehicle has tires that are over six years of age. ”

Most USA Tire Manufacturers do not Specify an Age Limit for Their Tires. It is Logical to Base an Age Limit on the Tires Serial Number. This Situation has Presented a Problem for Tire Manufacturers as to Whether They Specify a Tire Age Limit and if They do, What Should it be.