December 17, 2006 THE SCIENCE OF NITROGEN TIRE INFLATION By: Harold Herzlich, BChE LET S TALK ABOUT THE SCIENCE OF NITROGEN TIRE INFLATION AND FORGET THE RAZZLE DAZZLE As the dealer trying to do the best for the customer, it is to your business advantage to be aware of the science behind Nitrogen as the tire fill gas instead of air. Your tire training, experience and common sense will be reinforced by the following overview. WHAT IS AIR? Air is a mixture of many gases. About 79% is Nitrogen and about 21% is Oxygen. WHAT IS AIR PRESSURE? Normal sea level pressure of air is 14.7 pounds per square inch. That is the weight of a one inch square column of air extending miles high to the upper reaches of the rarified atmosphere. WHY IS TIRE TEMPERATURE SO IMPORTANT? Unlike a solid or liquid, a gas has no shape and takes the shape of its container (tire). The gas molecules follow what is referred to as The Ideal Gas Law. Temperature, pressure and volume are mathematically interrelated (i.e. when a tire gets warmer, its pressure increases). As you squeeze (compress) more gas molecules into the tire, the pressure increases. Gas molecules are in constant high speed motion impacting all sides of the container (tire) equally. They are perfectly elastic (the golf ball we would all like to have) and keep bouncing and impacting the container (pressure). When heated, these gas molecules gain more energy, bounce harder and cause an increase in pressure (i.e. for every ten degrees Farenheight temperature increase in a tire, the pressure increases about 1 psi. Example: A vehicle tire specification decal calling for 26 psi (cold) would be about 5 psi under that when filling a 120 degree tire to 26 psi. Inflation specifications are set for cold. Besides temperature being an important inflation pressure factor, we know that rubber becomes weaker at higher temperatures. It is no wonder that many tire failure problems are directly related to hot weather/high load conditions. The manufacturer tries to address this through the compound and design features of the tire. Maintenance of proper inflation pressure is a critical contribution to lower operating temperature. Besides it s non-chemical reactivity advantage, Nitrogen solubilizes and diffuses out of the tire 30% slower than air; improving inflation pressure retention. WHAT IS INERT NITROGEN? Air is about 79% Nitrogen, a close cousin to Oxygen on the chemical periodic table. However, by virtue of its atomic structure, Nitrogen is chemically unreactive (inert) in rubber. Oxygen s atomic structure makes it aggressively reactive (i.e. during high quality welding or brazing operations, the area is often bathed in
Nitrogen to eliminate oxidation contamination. Nitrogen is blown into many packaged foods before the container is sealed to reduce oxidation (rancid change) of the contents. 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, its 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 (i.e. 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. 2
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 its 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 the correct pressure. An under-inflated, hotter running tire will suffer a greatly accelerated oxidative weakening process. (i.e. 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 is 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 un-inflated. 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 un-inflated tire. Fewer purges are required to reach the 95% + 3
nitrogen purity when starting with a higher purity Nitrogen 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 Baldwin s 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 (i.e. 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 chemist s perspective. Note the increasingly destructive set of chain reaction parallel attacks. This results in reduced (tire-important) rubber properties such as stretch-ability (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 4
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 AFFECT THE NUMBER OF PURGES? Low levels of Oxygen degrade rubber. Since the seated, un-inflated 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 depends 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 rationale of 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. Dalton s 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 (i.e. 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 an Oxygen-vulnerable structure through which any inflation gas oxygen impurity immediately starts reacting as it permeates to the outside. OTHER ADVANTAGES OF NITROGEN INFLATION Of course moisture in the fill gas is undesirable. Tire manufacturers use very special inner-liner compounds that are quite impermeable to moisture passage. Rusting of the steel belts is usually a phenomenon caused by outside moisture and contaminants from cuts and unplugged punctures. Reduced moisture is an additional Nitrogen inflation benefit (i.e. TPMS). Punctures and improper repairs can bring about the weakening oxidative/chemical weakening of the tire through a mechanism called Intra-Carcass Pressurization (ICP). The injury or repair (i.e. external plug only or excessive buff) can allow the casing to be pressurized by inflation gas leaking past the plug or damaged inner liner. (Warning: never plug repair from the outside only, a carefully applied inside patch is ALWAYS 5
required). High- purity Nitrogen inflation will minimize the ICP oxidative tire weakening component. Run flat tires can also benefit from Nitrogen tire fill gas inflation. If a run flat is operated at zero pressure, there is the danger of overheating and internal tire combustion. Giant earthmovers and large aircraft tires are inflated with Nitrogen specifically to reduce this possibility. THE REAL WORLD Nitrogen tire fill gas inflation will not make a tire bullet proof. Failure due to bad things such as road hazard damage, improper maintenance, etc. can always happen. Proper Nitrogen fill gas application will improve fuel economy and tire performance (inflation) while increasing tire reliability (reduced oxidative weakening) under certain real world abuse conditions. 6
CURRICULUM VITAE HAROLD HERZLICH, BChE HERZLICH CONSULTING, INC. TIRE ENGINEERING AND CHEMISTRY EDUCATION New York University, University Heights, Pre-Medical, College of engineering, chemical Engineering 56 Southern Connecticut College, Graduate Chemistry Program, Polymer Chemistry, ASM Corrosion University of Wisconsin, Instructions and Warnings, Nevada Community College Philosophy of Critical Thinking Quinnipiac College Graduate Business Program, Marketing Economics, Export Marketing, Statistical Decision Making, Private Investigator Skills and techniques, Private Security rules and Regulations, Private Security Investigator Ethics Alexander Hamilton Business Miscellaneous Tire Engineering, Rubber chemistry, Management Quality College Tire Mechanics and Vehicle Handling, Vehicle Rollover Mechanics, Aerospace Landing Gear Systems, Sudden Air Out, Wet Skid, Ice/Snow Traction, Tread Detachment, Mismounting Bead Rupture, Over Deflection Hysteresis, SEMA 93 04 PROFESSIONAL American Chemical Society Rubber Division Chairman ( 82); American Chemical Society Rubber Division Chairman-elect ( 81); American Chemical Society Rubber Division Treasurer ( 78 81); American Chemical Society Rubber Division Chairman, Membership Committee, Education Committee ( 94); Society of Automotive Engineers ( 87); Yale Medical School Medial Ethics Committee; American Chemical Society ( 66); Tire Society ( 83); American Academy of Forensic Sciences ( 88); American Association for the Advancement of Science ( 94); California Highway Patrol Tire Failure Expert/Lecturer; Charles Goodyear Award Committee, ASM ( 99); Technical Editor Tire Industry ITEC Select Reviewer J. Wiley, Chemical Technology Tire Cords; Connecticut Rubber Group Chair, ( 66), Treasurer, Director of Education; Invited Reviewer Anderson Publishing Tire Technology; Akron Rubber Group Charter Member, Ohio RP Group Tire Industry Association ( 05); Licensed Private Investigator Texas ( 02); Charter Member of State of Connecticut Highway Transport Institute; Technical Advisor Rubber Manufacturers Association; Reviewer Polyisoprene in Soviet Union; Chair, Belt Tear Pattern Expert Peer Review ( 04); Delivered papers and talks to Federal officials and industry including Rubber Division; AIChE; RMA; AAFS; Akron RG; Boston RG; Connecticut RG; Southern RG; Washington RG; Chicago RG; Philadelphia RG; NHTSA; Clemson University; University of Nevada; Yale University; Chemistry career talks local schools; International Tire Exhibition and Conference; American Chemical Society - Law Division, Rubber Division, Tire Topical Steering Committee; Patent on cartable tires; Patent application: Awling; Invented, developed and manufactured elastic asphalt composition for road repair; Peer reviews about 250 rubber-related technical papers a year; Selects and edits about 50 tire-related technical papers a year for presentation to the industry; Selects and edits rubber related technical papers for global publication; Pro Bono rubber-related technical advice to RPN generated rubber industry individual, state federal military, foreign inquiries; Symposia: Rubberized Asphalt for Roads, Medical Applications of Rubber, Creativity/Innovation/Excellence in Management, Rolling resistance in Tires, Mixing/Calendering Technology, Tire Safety, Rubber Industry Product Liability; Conference Chairman ITEC 1994, 1996, 2000, 2002, 2004, 2006; Lecturer Tire Industry Association 2005 (Customary and Usual Industry Practices), (Liability Risk Reduction); Lecturer: Community College of Southern Nevada (Management Practices), Lecturer: Southwestern Association of Technical Accident Investigators, 2001 (Tire Technology and Failure Analysis Protocols); Lecturer TIA 2001 (Tire Forensics); Technical Responder for RMA; Technical Tire Commentator for Public Radio Broadcasting; Petitioner to NHTSA, Quoted in the FEDERAL REGISTER, Department of Transportation, Tire Safety 49CFR. HONORS Who s Who in America American Men and Women of Science Trilogy Award Committee G.S. Whitby Award Nomination Honorary Life Member, American Chemical Society Rubber division Honorary Life Member, Connecticut Rubber Group INDUSTRIAL EXPERIENCE Goodyear Tire and Rubber Company Production Squadron, 7/56 Goodyear Tire and Rubber Company Process Development, 1/57 Armstrong Rubber Company Junior Product Compounder, 2/58 Armstrong Rubber Company Senior Compounder, 6/61, Division Compounder, 3/62 Armstrong Rubber Company Manager, Passenger Car Compound Development, 5/65 Armstrong Rubber Company Senior Research Chemist, 5/70 Armstrong Rubber Company - Manager, Compound Research, 1/73 Armstrong Rubber Company Manager, Compound Development, 7/75 Armstrong Rubber Company Director, Tire Engineering, Legal Matters and Product Reliability, 1/85 Pirelli-Armstrong Tire Company Director, Tire Engineering, Legal Matters and Product Reliability, 6/88 Elasphalt Corporation President Herzlich Consulting, Inc. CEO, 1/90 Rubber and Plastics News Technical Editor ITEC (International Tire Exhibition and Conference) Chairman and Conference Organizer MILITARY EXPERIENCE New York University, United States Army Reserve Officer Training Corp 53 United States Coast Guard, Critical Tire Engineering Skills enlistment Program 58 7