WHAT IS OXYGEN?
Air is about 21% oxygen.  Even though it is a necessary, life supporting gas, it is really a nasty,  aggressive, corrosive gas that vigorously attacks the rubber molecule at very low concentrations.  It also degrades the expensive antioxidant and antiozonant chemicals that manufacturers include in their highly secretive tire compound formulations. Rubber weakening oxidation reaction is rapid and aggressive.

WHAT IS VULCANIZATION?
Rubber is a soft material called an elastomer. It has little technical value until it is reacted with sulfur (vulcanization). Before vulcanization (curing), the rubber is like a bowl of spaghetti chains in constant microscopic motion slipping, sliding and entangling. It is a string like micro-structure of very long chains of repeating carbon molecules with many chemically reactive sites (unsaturation) in the chain backbone. These sites react with sulfur (heat) and crosslink (tie) the loose elastomer chains together into a rubbery network tough enough for a tire. What is important to note is the small amount of sulfur necessary to dramatically change rubber properties. Only a very small percentage of these highly reactive sites in the elastomer backbone are used up by the sulfur during the vulcanization reaction. A large number of these unused sites remain vulnerable to the chain cutting attack by traces of oxygen.

Charles Goodyear discovered this critical sulfur reaction in 1843 while experimenting at home 
(smelling up the house and ruining his wife’s oven).

WHAT’S THE BIG DEAL ABOUT OXYGEN WHEN INFLATING TIRES?
It takes very small amounts of oxygen to weaken the rubber holding the tire together.  Manufacturers address this concern with the expensive step of adding secretive antioxidant and antiozonant protection systems to their formulations. Antioxidants were an important tire reliability improvement step and recently,  high purity nitrogen inflation has been added to this powerful technology as the next step for fighting tire oxidative weakening.

Some manufacturers use 50% oxygen enriched inflation fill gas to accelerate research and development tire failures. In fact the NHTSA is seriously considering enriched oxygen fill gas to impose a more severe certification test for tires  No doubt about it, oxygen in the fill gas is not good for a tire.

WON’T THE ANTIOXIDANTS HANDLE THE OXYGEN WEAKENING PHENOMENA?
Under conditions of proper motorist inflation maintenance (don’t bet on it), antioxidants will continue to partially and sacrificially react with the oxygen diffusing (migrating)  through the tire structure as the inflation gas tries to permeate and escape to the outside. From the time the tire is initially installed, it’s chemistry is under attack from the pressurized migrating oxygen inflation gas.

Antioxidants can also migrate to where they are not needed such as the tire surface (ie: the  brown discoloration is not always dirt).  They are subject to many oxidative degradation steps. This leaves the tire’s complex structure less protected and more vulnerable to the oxidative weakening process.

WHY IS INFLATION PRESSURE MAINTENANCE SO IMPORTANT?
A properly inflated tire runs cooler and is not subject to excessive deflection that can cause  micro-tearing in it’s internal structure. All tires gradually lose pressure though a complex, multi step rubber/gas solution/permeation process. Nitrogen diffuses out of the tire slower than air.  Improved pressure retention saves fuel and repays the motorist for any initial nitrogen service fee.  After an oil change most people notice that the vehicle rolls easier.  They incorrectly attribute it to the cleaner oil when actually it is due to the soft tires being brought up to pressure.

An underinflated, hotter running tire will suffer a greatly accelerated oxidative weakening process. (ie: An added 18 degrees F about doubles the chemical degradation rate of oxidation.)

WHY IS NITROGEN BETTER FOR INFLATION MAINTENANCE
Pure nitrogen diffuses through rubber at a 30% slower rate than air;  making it the choice insofar as improved pressure maintenance is concerned.  Nitrogen is 50% less soluble in rubber than oxygen. Studies done by Bridgestone/Firestone in passenger tires showed that internal pressure loss rates were reduced 45% when using pure nitrogen inflation

WHY IS NITROGEN BETTER FOR TIRE DURABILITY?
Uniroyal reported in a Kyoto, Japan paper that air inflated tires failed on a test wheel at less than 250 hours while “nitrogen inflated tires did not fail at 600 hours”.  A Coddington paper (Exxonmobil)  also reported improved durability for nitrogen inflated tires.  Karmarker/Herzlich and Karmarker in two Akron papers showed rubber elongation and  rubber steel adhesion laboratory improvements. A newly developed “Stepped Angled Shearography Laser Interferometry Tire Test” at the highly regarded Akron Rubber Development Laboratory in combination with other physical tests clearly showed an advantage for the higher purity nitrogen.

PURITY, PURITY, PURITY.  WHAT PURITY LEVEL IS NECESSARY IN THE TIRE?
The scientific literature says to not let the tire go below 95% nitrogen purity to get the greatest benefit. This does not exclude the common sense probability that 100% purity is better than 95%. However 100% nitrogen purity is commercially impractical and in fact cannot be scientifically established due to the natural variability from tire to tire. 98% nitrogen generator capability now state of the art and makes the required 95% nitrogen tire inflation commercially and easily attainable. Less pure nitrogen generation (more oxygen inflation contamination) must be considered marginal by not giving the tire nitrogen’s scientifically proven potential.

When a tire is seated, but completely deflated, it actually contains 14.7 psi of air’s natural 79% nitrogen/21% oxygen mixture. Engineering air gauges such as we use, don’t show this 14.7 pounds background pressure and register as zero. The chemist, when calculating the makeup of a gas must include this naturally present pressure in the gas mixture. In reality, an 80 psi tire actually has 80 psi + 14.7 psi in it.  The 14.7 psi however is counteracted by the natural 14.7 psi pushing against the outside of the tire and the tire feels uninflated.

Nitrogen inflation during a tire installation requires very important controlled purging (removal of the oxygen from the atmospheric 14.7 psi of air) in the uninflated tire. Fewer purges are required to reach the +95% nitrogen purity when starting with a higher purity generator.

Peer reviewed studies reported by Dr J Baldwin of Ford Motor Company showed:
“It has been shown that property changes with greater than 95% purity in a tire are within the error limits inherent in tire cure variations.  Consequently our study indicates a practical minimum level of 95% purity of nitrogen is desirable.”

“The tires inflated with more than 95 percent nitrogen do not appear to change very much from the new tires even after 12 weeks in the oven, whereas the tires inflated with the oxygenated media change dramatically, even after three weeks in the oven.”Further analysis validated Baldwins’ conclusions that, “...tires inflated with more than 95 percent nitrogen do not appear to change very much from new tires.” After twelve weeks of  aging condition designed to replicate real world tire aging, Baldwin concluded that, “at 12 weeks tires inflated with 96 percent nitrogen......the beginning of oxidative degradation can be seen.  Nitrogen inflated tires, however degrade far slower than tires inflated with the oxygenated media.”.

Akron Rubber Development Laboratory also validated the required 95% tire inflation nitrogen purity minimum.

BELT EDGE DETERIORATION IN RADIAL STEEL BELTED TIRES was presented by Uday Karmarker (ARDL) at a recent Cincinnati meeting of the Rubber Division of the American Chemical Society.
“This study aims to develop a methodology to study tire behavior in a designed box of various external factors.  Improved compounds and design are internal factors that have been kept constant in this study and will influence the tire rank in this bench marking practice.  The external factors that influence tire belt edge deterioration are time, temperature and tire cavity partial pressure of oxygen” (ie: nitrogen purity in tire).

A similar paper by Karmarker/Herzlich was presented at the 2006 International Tire Exhibition and Conference in Akron confirming the “greater than 95% nitrogen purity” Ford work.

WHY IS NITROGEN INFLATION GAS PURITY SUCH A BIG DEAL?
The rubber molecule is a very long chain of repeating chemical units. It is easy to picture a chain being shortened and compromised by the cutting or weakening of only a few links in it. Oxygen rapidly attacks the long molecular backbone structure of rubber by chemically reacting and breaking chain links.

Even though you may have little interest in the chemistry of oxygen attack of the rubber in the tire, here it is from a chemists perspective.  Note the increasingly destructive set of chain reaction parallel attacks. This results in reduced (tire important) rubber properties such as  stretchability (elongation) and tear strength.

H-R-R-R-R-R-R-H   (rubber chain) = 6R* (very chemically reactive radical)
R* + O2 ( any oxygen in the inflation gas) = ROO*
ROO* + -R-R-R-H =  R* + ROOH
ROOH = RO* + OH
ROOH + RH = ROH + R* + HOH (water)
RO* + RH = ROH + R*
OH* +RH = HOH (water) + R*

Here we go again proving that a little oxygen can go a long way!
R* + Oxygen = ROO* etc, etc

As can be visualized in a link metal chain, this polymer chain degradation cutting reaction requires only a few strategic oxidative chemical breaks. The reaction can be partially interrupted by the expensive chemical antioxidants in the formulation that are sacrificially cannibalized by oxygen induced free radical reactions.  They are also lost to the environment.

In summary the oxidation weakening of rubber is a complex chain reaction that requires very little oxygen. High purity nitrogen inflation helps preserve the antioxidants that were originally formulated into the tire.

HOW DOES NITROGEN GENERATOR PURITY EFFECT THE NUMBER OF PURGES?
Low levels of oxygen degrade rubber. Since the seated, uninflated tire will always have that background 14.7 psi atmospheric pressure air, the purity of the nitrogen fill gas will determine the efficiency of the purge. The number of purges depend on the nitrogen purity coming from the generator. Common sense and simple arithmetic establish the fact that higher generator nitrogen purity reduces the number of purges required.  .

CHEMICAL LAWS MISAPPLIED (A TIRE IS NOT A SIMPLE MEMBRANE BALLOON)
There have been attempts to justify lower purity generator nitrogen fill gas that ignore real world tire application considerations such as the oxidation reactions that initiate as soon as the tire is installed.. The rational trying to justify lower generator and fill gas nitrogen purity  is scientifically unacceptable when considering the diffusion dynamics and chemical needs of a complex tire structure.

Daltons Law is a very simple ideal gas rule that states that the pressure of a mixture of gases is nothing more than the sum of the pressures of each separate gas in the mixture (ie: pressure of the nitrogen part of the mixture and the pressure of the oxygen part of the mixture equals the air pressure). Dalton does not address the immediate active oxidative chemistry weakening the tire from the moment it is inflated and put onto operation. A tire is not an inert diffusion membrane (balloon). It is a oxygen vulnerable structure through which any inflation gas oxygen impurity immediately starts reacting as it permeates to the outside.