P014_RPN_20140616.qxp 6/11/2014 3:50 PM Page 1

14 Rubber & Plastics News ● June 16, 2014 www.rubbernews.com Technical When do need nitrogen inflation?

Second of two parts the oxidative reaction at 60°C is about Phoenix, a would experience about into approximate thirds north to south. By John W. Daws 7.4 times the rate at 21°C. 2.4 times the level of oxygen permeation It can be seen that north of I-80, all Daws Engineering L.L.C. For the permeation process, the activa- as the same tire would in a year in De- permeation times are generally 0.7 or Much of the previous work involving ni- tion energies governing the permeation co- troit. This is the same as saying that the lower. South of I-40, ratios are typically trogen tire inflation cited initially was tar- efficients for oxygen and nitrogen are dif- rate of potential oxidative degradation greater than 0.9 (note that the source for geted at the issue of rapid aging of tires ferent, indicating that the permeation in Phoenix is about 2.4 times higher these data was quarterly average data, (i.e., the development of an aged-tire test). coefficients for oxygen and nitrogen will than that experienced in Detroit. while the source for Fig. 3 was monthly Some of the initial work in this area was change at different rates with temperature. Fig. 5 shows a similar analysis done average data, which explains the slight reported by H. Kaidou and A. Ahagon11. D.J. Santaler, et al.12 gives the activation using 30-year National Oceanic and At- differences in permeation levels). They found that the primary tempera- energies for permeation through natural mospheric Administration data for quar- In between I-40 and I-80, the perme- ture driver in tire oxidative degradation rubber as about 37.8 kJ/mole for nitrogen terly average temperatures for a larger ation times generally range between 0.7 was the environmental temperature and about 28.8 kJ/mole for oxygen. number of cities in the continental U.S. and 0.9, with the notable exception of rather than the operating temperature of Using equation 4, it can be shown that Again, the reference temperature was Las Vegas, which had a permeation time the tire. This is likely due to the fact that, the rate of the permeation reaction for arbitrarily selected to be 21°C (70°F). A of a little more than one year relative to oxygen at 60°C will be about 3.9 times tire in Detroit would have about 0.64 years the reference tire at 21°C. It follows that TECHNICAL NOTEBOOK faster that at 21°C. These values suggest of oxygen permeation compared to the ref- tires in service in areas south of I-40 in Edited by Harold Herzlich that the permeation rates of the gases in- erence of one year at 21°C. Phoenix would the U.S. would be more susceptible to crease more slowly with temperature than have experienced 11 percent more oxygen oxidative degradation, while tires in at least for passenger and light truck do the oxidation reaction rates, resulting permeation as does the reference tire at service in areas north of I-80 are likely tires, the tire operation time is a small in what has been termed in the literature 21°C. Compared to Detroit, the tire in to be least affected by this process. fraction of its overall lifetime. The rate of a “Diffusion Limited Oxidation,” or DLO. Phoenix experiences about 73 percent This chart assumes that there is suffi- the oxidation reaction of the tire’s rubber This indicates that as temperature more oxygen permeation in a year. Fig. cient oxygen to support the oxidative re- hydrocarbon was described in terms of climbs, the total oxidative reaction system 5 also shows two major east-west inter- action. However, the tire inflation gas the standard Arhennius relationship: in the rubber polymer is limited by the state routes, Interstate 40 and Inter- composition changes with time, so the amount of oxygen that flows into the tire. state 80. These roadways divide the U.S. analysis is not as simple as these rate This is consistent with improvements in tire inner liners causing improvements in Fig. 11. Casing depth evolution to 1 percent oxygen concentration in P235/75R15 long-term tire durability and increases in for inflation with air at different locations in the U.S. oxygen content of the inflation gas causing where K is the rate of the reaction, E is the a loss in durability. Equation 4 also holds activation energy for the material, R is the for the relationship between the rates of a ideal gas constant, T is the absolute tem- reaction at different temperatures. perature, and B is a material constant. For practical purposes, then, the envi- Kaidou and Ahagon found, for the belt ronmental temperature in which the skim materials in their experiments (pas- tire is immersed for its lifetime plays a senger tires), that the value of the acti- significant role in how rapidly the oxida- vation energy, E, was about 42 kJ/mole. tive degradation takes place. Oxidative Given a situation where a rubber degradation in an inflated tire takes sample is exposed to a certain tempera- place with time, not mileage. ture Tl in the laboratory, it experiences Equation 4 can therefore be used to a reaction at a rate Kl. It can be shown compute an effective permeation time in that the same material exposed to a the field from environmental data. If the field service temperature, Tf, would be average monthly temperatures for a few expected to undergo the same reaction cities in the U.S. are plotted, using De- at a rate Kf , given by: troit as a reference, the result is shown If Equation 15 is evaluated using a in Fig. 3. (Figs. 1-9 appeared in the June 2 edition of Rubber & Plastics News.) Note that Detroit has the lowest average annual temperature at 47.8°F, while Miami has the highest at 74.3°F. In Fig. 4, the potential permeation time for a tire in each city has been nor- laboratory (rapid aging) condition of malized to that computed for Detroit. 60°C, and 21°C is used as a field condi- The reference time is one year at 21°C. tion, it is easily shown that the rate of Fig. 4 shows that in one year in

Fig. 10. Oxygen concentration distribution through P235/75R15 casing for infla- tion with nitrogen at different locations in the U.S. Fig. 12. Casing depth evolution to 1 percent oxygen concentration in P235/75R15 for inflation with nitrogen at different locations in the U.S. P015_RPN_20140616.qxp 6/11/2014 3:51 PM Page 1

www.rubbernews.com Rubber & Plastics News ● June 16, 2014 15 Technical

and time calculations might indicate. threshold at which oxidative degradation idative reaction will be able to consume all for Detroit, Oklahoma City and Phoenix. Recall that Fig. 2 showed a comparison becomes “significant” to “catastrophic.” the oxygen that can flow into the tire. Fig. Clearly, the result of an increased tem- of the oxygen flow across a P235/75R15 For the P235/75R15 SL tire discussed 7 shows that the innermost layers of the perature environment is to encourage the SL tire at a constant temperature. Using previously, the total tire weight would tire casing have exceeded an oxygen con- more rapid permeation of oxygen into the Equation 4, the permeability increase be around 13 kg. Sperberg2 showed that centration of about 1 percent, but the steel layers of the tire. Fig. 9 shows that the with temperature can be estimated for the oxidation of the tire is focused on the belts, which begin between 2 mm and 3 oxygen content is at 1 percent to a depth both oxygen and nitrogen. The same sim- casing components, since they are the mm from the inner liner surface, and outer greater than 3 mm if the tire is in Phoenix ulation can therefore be done for a given closest to the source of the oxygen. sidewalls of the tire have not. Of course, for six years, but the corresponding depth location using monthly temperature data. A P235/75R15 SL tire has about 5 kg this analysis ignores the effect of antioxi- for the tire in Detroit for the same amount Fig. 6 shows the results of such a sim- of tread rubber, leaving the casing with dants built into the rubber polymers. of time is slightly greater than 2 mm. ulation for the same P235/75R15 SL tire a weight of around 8 kg. If the weights If the antioxidants are sacrificial in How does nitrogen tire inflation influ- in Phoenix. Note that there is a seasonal of the beads, the polyester sidewall ma- nature, then the effect would be to delay ence this picture? Fig. 10 shows the re- progression from winter to summer in terial, and the steel cords in the belts the oxidative reaction by some period of sults of the same model, except the levels the oxygen curves like that shown for are subtracted, the weight of rubber time (until the antioxidants have been have been chosen to match the average temperature in Fig. 3. Since nitrogen compound would be about 7.5 kg. consumed). If the antioxidants serve as concentrations of oxygen predicted by the and oxygen permeabilities do not change For an air-inflated tire over a period of a dilutant, then the oxidative process is six-year model with an initial nitrogen tire at the same rate with temperature, this six years at a constant 21°C, the simula- slowed proportionally. purity of 93 percent and routine top-off be- simulation predicts that the air-inflated tion predicts about 78,881 mg of oxygen, or Either way, the analysis presented ing done with air. tire and the nitrogen-inflated tire actual- about 1.05 percent of the casing compound here ignores the effect of the antioxi- This clearly shows that the casing lay- ly end up with the same inflation gas ni- weight to permeate into the tire. Sperberg2 dants, and therefore represents an ex- ers having oxygen concentrations greater trogen purity after six years. also made measurements showing that the tremely conservative approach. than 1 percent are limited to the body ply With the annual temperature variation, oxygen concentration as a function of dis- The simple model shown in Equation 8 region even in Phoenix. Based on this the simulation predicts 84,009 mg of oxy- tance from the inner liner surface into the can be used to evaluate the effect of tem- analysis, nitrogen tire inflation has signif- gen flowing into the tire over the six-year tire was exponential in nature. perature on the permeation of oxygen into icant potential to improve the long-term period for the air-inflated tire and 44,777 If a simple exponential function for the tire. Fig. 9 shows the results of such durability of tires. mg of oxygen for the same period for the the distribution of oxygen concentration an analysis for a P235/75R15 SL tire in- Another way to look at this type of data nitrogen-inflated tire. Again, the effect of in the tire casing material is assumed as flated with air, using the permeation ra- is to ask how long a period of time would the nitrogen inflation is to reduce the tios for various temperatures computed need to pass before the oxygen concentra- amount of oxygen flowing into the tire by using Equation 4. Fig. 9 includes curves See Nitrogen, page 16 about 47 percent over the six-year period. Tire manufacturers formulate rubber Fig. 14. Oxygen concentration distribution through 315/80R22.5 casing for inflation compounds for use in tires and incorpo- where is the ratio of the concentration with air at different locations in the U.S. rate various ingredients called antioxi- at some point in the thickness to the con- dants. The function of these compounds centration at the inner liner surface (the is to react with oxygen molecules perme- maximum concentration), is the ratio ating through the rubber before the oxy- of the distance from the inner liner sur- gen can react with the rubber itself. Un- face to the casing thickness, and k is a fortunately, there is a practical limit to fitting parameter. The details of this the amount of antioxidant that can be simple model are shown in Fig. 7. put into a rubber compound and still be Note that the average concentration is present after the vulcanization process. simply the integral of this concentration In addition, antioxidants can migrate function divided by the casing thickness, from one rubber compound in the tire’s t. From the permeation simulations de- structure to another. In general, the scribed previously, the average concen- amount of antioxidant incorporated is tration of oxygen in the casing can be thought to be sufficient to deal with the computed as the amount of oxygen that “useful lifetime” of the tire. The problem moves into the casing divided by the for tire buyers is that the useful lifetime mass of rubber compound in the casing. of a tire depends upon its tread life and Fig. 7 shows that for an average con- the number of miles of service per year centration the maximum concentration demanded. The longer the service life- at the inner liner can be computed as Cm. time, the more important the environ- If the concentration at the extreme casing mental oxidation rate becomes. thickness is assumed to be 10 percent of So, the question becomes one of the sig- the maximum oxygen concentration, then nificance of the amount of oxygen that can Fig. 8 shows a distribution of oxygen for a permeate into the tire and still have the casing thickness of 7 mm. The maximum tire serve its function. The real question is of this distribution (2.5 percent at the in- how much oxygen is too much oxygen. ner liner surface) yields an average con- As previously noted, Sperberg and Toki- centration of 1.05 percent, i.e., that pre- ta independently arrived at about 1 per- dicted for the tire at 21°C for six years. cent by weight of rubber compound as the This is a logical assumption since the ox- Fig. 15. Oxygen concentration distribution through 315/80R22.5 casing for inflation Fig. 13. Comparison of oxygen permeation in an air-inflated and a nitrogen-inflated with nitrogen at different locations in the U.S. 315/80R22.5 truck tire after six years at 21°C. Casing depth set at 18 mm. P016_RPN_20140616.qxp 6/11/2014 3:55 PM Page 1

16 Rubber & Plastics News ● June 16, 2014 www.rubbernews.com Technical

ger and light truck tires, even in the most That is to say that if the tire were worn tra-cost option when purchasing tires. This severe climates in the U.S. Again, the ef- out and retreaded annually, consistent cost is normally either about equal to or Nitrogen fect of antioxidants has been ignored. use of nitrogen tire inflation would re- slightly greater than tire balancing for pas- The physics of nitrogen tire inflation for duce the oxygen permeation through the senger car tires, so it is not an insignificant Continued from page 15 medium radial truck tires yield slightly tire to about 15 percent of its normal lev- percentage of the total cost of tires. The tion in the tire attained 1 percent, and different results from those of passenger el in an air-inflated tire. This would dra- question for a consumer is clearly “Does the then how would that progress through and light truck tires for several reasons. matically reduce the potential oxidative tire really need nitrogen inflation?” This the tire’s layers. Following the model First, since the inflation pressures of degradation of the tire over its lifetime. clearly depends on a number of factors in- shown in Fig. 8, an analysis was per- medium radial truck tires are typically Using the simple oxygen concentration volving service life, environment and poten- formed for both air inflation and nitrogen in excess of 100 psi, attaining 95 percent distribution model of Equation 5, Figs. 14- tially other issues. The following discussion inflation of a P235/75R15 SL tire using nitrogen purity in the tire with a single 17 show concentration distribution curves examines several types of tires and service. the average oxygen concentrations pre- inflation step using 98 percent purity in- for a 315/80R22.5 medium radial truck tire. dicted by the permeation model at the flation gas is straightforward. Fig. 14 shows the distribution of oxygen Passenger and light truck tires various city temperature distributions Second, tire volumes are much larger concentration for the tire over six years In-service passenger and light truck (Detroit, Oklahoma City and Phoenix). than passenger and light truck tire vol- with several geographic locations (again, tires represent a significant market for The results for the air-inflated tire are umes, and the tires are significantly thick- Detroit, Oklahoma City and Phoenix). nitrogen tire inflation. Such tires are shown in Fig. 11. In this figure, it can er. This means that the IPLR for medium Fig. 15 shows the same information for marketed with different levels of expected be seen that an air-inflated tire reaches radial truck tires is significantly lower the tire inflated initially with 95 percent tread life (the warranty mileage). The use an oxygen concentration of 1 percent be- than for passenger and light truck tires. purity nitrogen and then topped off rou- of the tire can lead to very different serv- fore the end of the second year of service As an example, a set of simulations was tinely with air. In the case of the medium ice lifetimes, however. For example, if a life in Phoenix and between three and performed, using the method described radial truck tire, the working belts (the consumer who averages about 10,000 four years of service in Detroit. earlier, for a 315/80R22.5 tire inflated to second and third belts in a four-belt con- miles per year purchases a 60,000 mile The oxygen concentration of 1 percent 110 psi. This tire has an internal volume of struction) begin at about 7 mm into the tire, the expected service lifetime of that reaches the bottom of the steel belts, lo- about 144.8 liters (as compared to the 51.7 casing thickness. tire would be six years. cated about 3 mm to 3.5 mm into the liter volume of a P235/75R15 tire) and an If the curves for Phoenix are com- If the consumer averages 20,000 miles casing, during the fifth or sixth year of average sidewall thickness of around 18 pared, it can be seen that the air-filled per year, the expected service lifetime service life in Phoenix and about at the mm. While the complete tire will weigh tire will develop a 1 percent oxygen con- would only be three years. Hence, annu- 12th year of service in Detroit. These re- around 68.2 kg, the weight of the rubber centration at the 7 mm depth at slightly al mileage accumulation per tire dra- sults are consistent with a conservative compound in the casing and belt system, more than six years of continuously-in- matically influences service life (this is six-year service life recommendation for excluding the tread, will be about 32.2 kg. flated service. In the nitrogen-filled tire, true for all tires). passenger and light truck tires using air The percentage concentration of oxygen the oxygen concentration does not reach Vehicle manufacturers have, by and inflation (recall that the effect of antiox- can therefore be estimated by assuming the 1 percent level anywhere in the tire large, started recommending that tires idants has been ignored). that all the oxygen that permeates into in this six-year period. be removed from service after six years. For nitrogen inflation, Fig. 12 sum- the tire is consumed in the casing rubber. Fig. 16 shows the casing depth to an This includes spare tires which may nev- marizes the results. With nitrogen tire Fig. 13 shows a comparison of the oxygen oxygen concentration of 1 percent by years er have been placed into service. Fig. 6 inflation, the tire begins to see 1 percent concentration across the tire inflated with of service life for the 315/80R225 tire ini- and Fig. 9 would generally be in accord oxygen concentration in the fourth to air and inflated with nitrogen initially at tially inflated with air. As before, the tire with this principle, since tires in areas fifth year of service life in Phoenix, in 95 percent purity, with the simulation be- in Phoenix will reach an oxygen concentra- having the highest permeation ratios can the 12th year of service life in Detroit. ing done at a constant 21°C. tion of 1 percent at 7 mm depth into the be expected to start attaining oxygen con- The oxygen concentration of 1 percent For a six-year lifetime, the simulation casing in about 6.5 years. This simulation centrations approaching 1 percent in the reaches the bottom of steel belts (around 3 predicts 263,107 mg of oxygen permeation is for a tire that is continuously inflated. If steel belt areas by the end of six years. mm to 3.5 mm) between eight and 10 years for the air-inflated tire and 71,569 mg oxy- the tire were demounted and re-inflated Clearly, this is an extremely conserva- of service in Phoenix, and well beyond 15 gen permeation for the nitrogen-inflated annually (after being retreaded due to high tive recommendation, since the effects of years in Detroit. This suggests the effect of tire, a reduction of about 73 percent. Since mileage accumulation service, for exam- antioxidants have been ignored in this nitrogen tire inflation is to shift the devel- many of these tires are operated in high- ple) the oxygen level in the casing after six analysis and service in the highest temper- opment of critical oxygen concentration by mileage service, the comparison of the years would be about 22 percent higher. ature environments are assumed. about 3.5 years at the onset. first-year improvement is interesting. Fig. 17 shows the same simulation re- This suggests that when tire service The time to attain 1 percent oxygen con- During the first 12 months of service, sults for the 315/80R22.5 tire initially in- life is maintained at less than six years, centration at 3 mm into the casing also is the model predicts about 54,379 mg of flated to 95 percent nitrogen purity and tires generally will not suffer extensive shifted by over three years in Phoenix oxygen will permeate the air-inflated topped off with air. Note that the 1 percent oxidative damage even without consid- when using nitrogen. This is consistent tire, while only 8,087 mg of oxygen will oxygen concentration level does not reach ering built-in internal protection in the with a conservative 10-year service life- permeate the nitrogen-inflated tire, a the depth of 7 mm until well beyond 15 form of antioxidants and inner liner ma- time when using nitrogen-inflated passen- reduction of 85 percent. years of service. The simulation also pre- terial and thickness. dicts that if the tire is demounted and re- Obviously, if the service life of the tires inflated with nitrogen annually (again, re- is anticipated to be significantly less than treading of high mileage accumulation six years, then the use of nitrogen tire in- fleets), the oxygen level would be about 30 flation to reduce the oxidative age of the percent lower. tire in service becomes less beneficial. Tires that are expected to wear out in Applications of nitrogen tire inflation three to four years likely will obtain no ox- In general, nitrogen tire inflation is an ex- idative benefit from nitrogen tire inflation.

Fig. 16. Casing depth evolution to 1 percent oxygen concentration in 315/80R22.5 for inflation with air at different locations in the U.S. P017_RPN_20140616.qxp 6/12/2014 3:25 PM Page 1

www.rubbernews.com Rubber & Plastics News ● June 16, 2014 17 Technical

If, however, the consumer purchases When the consumer lives north of I-80 generally live in areas where the perme- one way of making sure the tire will have tires that likely will last six years or more, in the U.S., it is not clear that nitrogen ation rate may be lower, but not necessarily sufficient durability so that the consumer and lives south of I-80 in the U.S., then ni- tire inflation will provide a noticeable (example: very little snow falls in Phoenix, can take advantage of the entire tread trogen tire inflation would improve the benefit in reducing oxidative degrada- but snow tires are sold in the area because life of the tire. This also has significant likelihood that the tires would not suffer tion in the tires unless the tires will ski resorts are within driving distance). societal benefits, including vehicle safety excessive oxidative degradation. have an exceptionally long service life However, snow tires are used in two pre- and reduced environmental burden from Recall that nitrogen tire inflation with (greater than 13 years). dominate ways: continuously mounted, and prematurely scrapped tires. subsequent air top-off reduces the lifetime As mentioned previously, not all tires demounted/remounted each season. permeation of oxygen through the tire by are created with the same levels of inner When the snow tires are mounted con- Medium radial truck tires about 50 percent at the six-year point. The liner permeability, oxidative resistance, tinuously on specially purchased Medium radial truck tires generally effect of nitrogen inflation is to shift the ox- and so on, so consumers should consider for this task, then the oxidative process fol- have utilizations that are very different idation curve by about four years. Since all the worst-case scenarios. lows that described in detail above for the from those found in passenger and light tires are not created equal in areas like an- P235/75R15 SL tire. When the tires are de- truck tires. In addition to accumulating tioxidants and inner liner thickness and Spare tires mounted at the end of a season, the oxygen significant mileage every year, these tires material composition, however, even tires Spare tires are rarely used unless they permeation process is stopped. When they can see service in all parts of the country. in service north of I-40 in the U.S. may are full-sized spares, in which case they are remounted for the next season, the They generally can be retreaded and may benefit from nitrogen tire inflation. may be pressed into service after a oxygen permeation process begins anew go through that process several times As time goes on, however, this advan- number of years because the vehicle own- but at its maximum level because the tire during their lifetimes. tage is decreased, since the air-inflated er decides to purchase three new tires gets a fresh charge of air (recall Fig. 1). Vehicle owners generally fall into two tire and the nitrogen inflated tire both and employ the as-yet unused . However, since the snow tires are in use camps with regard to tire retreading: converge on concentrations of nitrogen However, the service life of the tire be- in the winter months when the permeation those that own their own casings, and and oxygen in the inflation gas of about gins when it is mounted and inflated, re- rates are lowest, the amount of oxygen be- those that exchange casings when the 88 percent and 12 percent, respectively, gardless of whether or not it is used. ing pushed into the tire will be essentially tire wears out. Owners who exchange by the end of six years. Space-saver spares spend their lives in at a minimum. For example, in the case of casings generally will not be interested Therefore, as consumers try to extend the storage area of the vehicle (where the the P235/75R15 SL tire, the amount of in the benefits of nitrogen tire inflation tire service life beyond 10 years, the initial temperatures may be significantly higher oxygen expected to enter the air-inflated related to oxidation reduction because nitrogen tire inflation nets them no incre- than those in the outdoor environment) tire over a six-year period in Phoenix they would bear the cost of the process mental benefit. This is why Daws8 recom- and are hardly ever used. Vehicle manu- would be 84,843 mg, as noted previously. but not stand to reap the long-term ben- mended either topping off with nitrogen or facturers’ recommendations to dispose of For the same tire inflated with nitro- efits. Probably the largest field study of periodic recharging of tire nitrogen during any tire over six years old apply to spare gen would be expected to see 45,250 mg. nitrogen tire inflation in fleets was re- the life of the tire for long-life situations. tires, but these recommendations are not If the same tire is demounted every ported by Transport Canada.13 Consumers who live north of I-80 in the rigorously being followed in the field. spring and remounted in the fall (six full This study comprised about 110 million U.S. are not likely to obtain oxidative ben- Spare tires, then, are at risk for long months of service per year), the amount tire miles and involved 1,988 tire po- efits from nitrogen tire inflation unless lifetimes even with little or no mileage of oxygen expected to enter the air-in- sitions. Both fuel and tire savings were the service life of the tires in question ex- (the annual mileage accumulation is zero flated tire would be 48,338 mg, or essen- considered, but the study details suggest ceeds 12 to 13 years. Other benefits such or nearly zero in most cases). In addition, tially the same level as if the tire were that these comparisons were made against as inflation with a dry gas, oxidative pro- the environment surrounding spare tires inflated with nitrogen. a tire pressure maintenance cycle that was tection for the tire in the event of struc- may be considerably different in terms of This holds true for any location, al- coincident with oil changes. tural damage (i.e., punctures), and so on temperature from that experienced by though the effect in Detroit is not quite Obviously, the longer the period be- still may make nitrogen inflation an at- the in-service tires on the vehicle. as impressive (57,230 mg for air-inflated tween tire pressure checks, the larger the tractive option for those consumers. Therefore, spare tires likely to experi- tire, 20,431 mg for nitrogen-inflated tire difference likely to be found between air- This idea can be refined further by ence additional risk due to increased tem- and 27,279 mg for the air-inflated snow inflated and nitrogen-inflated tires. In ad- stating that, for consumers who live perature exposure and consequently high- tire mounted only in winter). dition, gas used in routine pressure main- south of I-40 in the U.S., nitrogen tire er permeation ratio. The risk here is that Therefore, if the is to be de- tenance (air or nitrogen) was not specified. inflation for tires that have expected an unused, but heavily oxidized, spare tire mounted/remounted annually, the tire will Consider high-mileage fleets that own service lifetimes exceeding six years can will have little visual evidence of even ex- not obtain significant benefits from nitro- their own casings, and assume that each be considered important. treme oxidative damage because it has not gen inflation unless the service life is ex- tire sees 100,000 miles per year. Since The higher the value shown in Fig. 5 been strained (i.e., loaded and run). pected to be extremely long. If the snow tire the cycle would be on the order for a given locale, the more important It is the strain of being loaded and op- is mounted on its own on a permanent of one to two years, and the owner would this purchase is likely to be. Also, the erated that generates the cracks that are basis, then the same logic outlined above accrue the benefits of significant oxida- longer the service lifetime, the more im- normally held to be evidence of oxidative for passenger and light truck tires applies. tive reduction over the tire’s life (on the portant nitrogen tire inflation becomes. aging. The use of nitrogen inflation in It is clear from the discussion that the order of 85 percent). Nitrogen tire inflation should be highly any spare tire would therefore be recom- value proposition for nitrogen tire infla- Since the number of possible recommended for passenger and light mended, regardless of the type of tire or tion in passenger and light truck tires is See Nitrogen, page 19 truck tires in these areas. the geographic location of the vehicle. When the consumer lives between I- 40 and I-80 in the U.S., then the useful- Snow tires ness of nitrogen tire inflation essentially Snow tires fall into a slightly different requires longer tire service lifetimes, category than passenger and light truck and the service could be recommended. tires discussed above. Snow tire consumers HEXPOL: A Global

Fig. 17. Casing depth evolution to 1 percent oxygen concentration in 315/80R22.5 Auto Industry for inflation with nitrogen at different locations in the U.S. Partner Automobile manufacturers around the world turn to HEXPOL for elastomer mixing and custom compounding expertise. Our consistent mixing ensures optimum automotive rubber products such as: A Legacy of Excellence Rubber Compounding: Burton Rubber Processing Seals and Gaskets Colonial Rubber Works Hoses Gold Key Processing HEXPOL Compounding Wire Harnesses Robbins LLC Vibration Mounts Roll Compounds: Belts Chase Elastomer Body Seals TPE: ELASTO Müller-Kunststoffe HEXPOL delivers the continual improvement OEMs and suppliers expect for their global automotive applications.

A Material Difference United States Mexico Europe Asia www.hexpolcompounding.com HEX-1300-14 P019_RPN_20140616.qxp 6/12/2014 3:27 PM Page 1

www.rubbernews.com Rubber & Plastics News ● June 16, 2014 19 Technical

and four years thereafter. in initial inflation. In addition, Transport The temperature effects discussed for Total accumulated mileage per tire Canada indicated that the improved pres- passenger and light truck tires apply to Nitrogen would not be a significant issue for casing sure retention may have resulted in im- medium radial truck tires as well. For mechanical durability, but oxidative proved tire wear mileage. See Nitrogen, page 21 Continued from page 17 degradation would, especially in environ- tends to be limited by the casing condi- ments where temperatures are high. The Fig. 18. Proposed flow chart for use in determining when a tire needs nitrogen inflation. tion (up to some service lifetime defined air-inflated tire would reach a 1 percent by the owner), savings could be obtained oxygen concentration in about six years, by permitting additional retread lives, or during the life of the original tread. Re- provided that nitrogen tire inflation in treading the tire would result in continu- this service can be done economically. ally-increasing oxygen concentrations and However, this is likely to be an issue, an increasing possibility of tire failure. since each nitrogen inflation for a tire of In this type of utilization, nitrogen tire this size would require about 43.4 stan- inflation would reduce the oxygen perme- dard cubic feet of nitrogen gas (at 98 ation through the tire by about 75 percent, percent purity). Suppose that nitrogen making the possibility of achieving a sec- tire inflation would permit one addition- ond retread life more realistic, even in high al retread life on every casing (some temperature regions. In this case, only two sources have suggested two to four addi- nitrogen tire inflation cycles would have tional retread lives might be possible). been consumed to get the second retread If a fleet is averaging three retread life (extending the tire’s service lifetime to lives for every casing, then the use of ni- fourteen years), making the economics of trogen tire inflation would potentially the transaction more palatable. yield four. To be practical in this case, The IPLR reduction available on medi- the cost of five nitrogen tire inflation cy- um radial truck tires is predicted to be cles (the original new tire inflation plus about 30 percent, as was found for pas- four retread inflations) would need to be senger and light truck tires. The IPLR significantly less than the value of the for an air-inflated tire would be about additional mileage on the fourth retread. 0.67 percent monthly and about 0.47 This value presumably would be the re- percent for the nitrogen-inflated tire. duced number of new tires being pur- This means that, with an inflation chased on an annual basis. However, if pressure of 110 psi, an air-inflated tire the tires are being worn out in a one year would lose about 0.7 psi per month ver- period, there is a serious risk that the cas- sus about 0.5 psi for a nitrogen-inflated ing will suffer significant mechanical fa- tire. Again, the difference is likely not no- tigue with the total accumulated mileage ticeable over a short period. However, in (first life at 150,000 miles plus four re- the Transport Canada study fuel savings tread lives at 100,000 miles yields 550,000 were found to be 4 percent compared to miles on the casing). That is, the mechani- air-inflation and third-party tire pres- cal fatigue durability of the casing, and sure maintenance, and an impressive 6 not the potential oxidative degradation, percent over air-inflation with driver tire likely controls the service life of the tire pressure maintenance. (see the Wohler curve in Fig. 1). This study indicated that some tire This area will require field testing to pressure checks were only being done at resolve. In the interim, for tires that oil change intervals, suggesting that the have their maximum service mileages improvement in pressure maintenance exhausted in about six years, the bene- has more significance when the time be- fits of nitrogen tire inflation would be tween pressure checks becomes very long. related to IPLR reduction. Also, the gas being used for pressure In contrast, consider the case of a re- maintenance was not specified. These re- gional or local delivery fleet where each sults suggest that nitrogen tire inflation tire sees about 25,000 miles per year. In has significant cost savings potential in this case, the retread cycle would be on general fleet use, especially if nitrogen is the order of six years for the first life used in pressure maintenance as well as

Release & Anti-Stick

888 Team McLube [email protected] P021_RPN_20140616.qxp 6/12/2014 3:29 PM Page 1

www.rubbernews.com Rubber & Plastics News ● June 16, 2014 21 Technical

ated with the temperature exposure and years and operated south of I-80 in the volves the potential for additional retread the service life. For medium radial truck U.S. should be inflated routinely with lives obtained by improving the long-term Nitrogen tires, mileage accumulation rate is the nitrogen, again for the oxidative benefit. casing durability. primary determinant. Spare tires, due to their potentially Again, benefits associated with IPLR Continued from page 19 Fig. 18 is designed to reflect when a long service lives and potential for expo- may result, but this will depend upon a tires used with a local geographical area, tire needs to have nitrogen inflation for sure to temperatures higher than the given fleet’s current tire pressure main- the oxidative acceleration due to temper- oxidative durability reasons. The pres- surrounding environment, should be in- tenance practices and what type of ature can be reasonably predicted. ent study has assumed that any nitro- flated with nitrogen. maintenance gas is used. However, the Regional fleets with low annual gen tire inflation is done with nitrogen From an oxygen permeation viewpoint, energy savings, as well as the reduction mileage accumulation per tire and oper- inflation gas having at least 98 percent there is little difference between leaving in emissions associated with using less ating in the southern U.S. likely have purity, and that only one inflate cycle to snow tires mounted all year and inflated fuel, when accumulated at the national the best opportunity to benefit from ni- the tire’s maximum inflation pressure, with nitrogen and demounting/remounting level may be significant. trogen tire inflation from the standpoint is done. Daws8 showed that this is suffi- them each season, especially if the service of extended tire durability. However, for cient to yield 93 percent nitrogen purity life of the tire is likely to extend beyond six References tires used in long-haul applications hav- in passenger car tires and 95 percent years. Snow tires anticipated to last over 1. Sperberg, L.R., “Tire Durability with Nitrogen In- purity in medium radial truck tires. 10 years and operated south of I-80 in the flation,” Rubber Age, 99 (11), 83 (1967). ing high annual mileage accumulations 2. Sperberg, L.R., “Million Mile Truck Tires – Avail- per tire, ultimate casing durability may Obviously, a consumer may want to U.S. should either be inflated with nitro- able Today,” January 1985, Stronger Longer Tires of govern the end of use life of the tire purchase nitrogen tire inflation in order to gen or demounted/remounted each season. El Paso, Inc. rather than effective oxidative age. reap other associated benefits. For exam- Medium radial truck tires represent a 3. Tokita, N., et al., “Long Term Durability of Tires”, ple, since nitrogen is a dry gas, nitrogen significant opportunity for nitrogen tire International Rubber Conference Proceedings,” Kyoto, Clearly, the value proposition for Japan, Oct. 15-18, 1985, Paper 18D17, pp. 672-679. medium radial truck tires exists when inflation will help to mitigate the effect of inflation in terms of reduced oxidative 4. Baldwin, J.M., Bauer, D.R., and Ellwood, K.R., the casings belong to the vehicle owner, water vapor in the tire in cold climates, re- degradation. Nitrogen tire inflation on “Effects of Nitrogen Inflation on Tire Aging and Per- and involve the possibility of additional sulting in lower pressure drops overnight. long haul tires that exhaust their tread formance,” presented at meeting of the Rubber Divi- The use of nitrogen inflation presum- lives in one to two years would reduce the sion, Grand Rapids, Mich., May 17-19, 2004. retreading steps on those casings, along 5. Karmarker, U., and Herzlich, H., “Effect of Nitro- with fuel savings and improved tire ably will provide additional protection oxygen permeation through those tires gen Purity on the Oxidation of Belt Coat Com- mileage. for the tire in the event of damage to the by around 85 percent over air inflation, pound”, Presented at a meeting of the International The benefits associated with fuel sav- tire’s structure by minimizing the oxida- but mechanical fatigue may be more sig- Tire Exhibition and Conference (ITEC), Akron, Sep- tive component associated with in- nificant than oxidative degradation in tember 2006, Paper No. 19B. ings and improved tire wear life ultimate- 6. MacIsaac, J., et al., “The Effects of Inflation Gas on ly depend on the IPLR improvement from creased intra-carcass pressurization. this type of service. Tire Laboratory Test Performance,” presented at a nitrogen tire inflation being sufficient On Fig. 18, then, the “Nitrogen Op- In regional haul service where annual meeting of the International Tire Exhibition and Con- over extended periods to generate them. tional” reflects that the tire, in that en- mileage accumulation is lower, the ox- ference (ITEC), Akron, Sept. 17, 2008, Paper No. 18C-1. vironment and utilization, does not need idative degradation of the casing likely 7. Waddell, W.H., et al., “Nitrogen Inflation of Tires,” The Transport Canada study was presented at the fall 174th technical meeting of the based on very long periods between tire the oxidative improvements available overwhelms it mechanical fatigue prop- Rubber Division, ACS, Louisville, Oct. 14-16, 2008. pressure maintenance, and it likely from nitrogen tire inflation, but the erties. In that case, nitrogen tire infla- 8 Daws, J.W., “Nitrogen Inflation for Passenger Car would not be representative of what IPLR reduction and other protection af- tion may have oxidative benefits in and Light Truck Tires,” Tire Science and Technology, forded by the procedure may still be at- terms of the number of retread opera- TSTCA, Vol. 39, No. 2, April-June 2011, pp. 125-160. would occur if a fleet was performing tire 9. Costemalle, B., “Tyre Pressure Loss and Intracar- pressure maintenance, even using air as tractive to the customer. tions attainable, although this has not cass Pressure Modeling,” Paper presented to the the top-off gas, on a more routine sched- The IPLR reduction benefits of nitro- been proven in field trials. , Akron, March 1992. ule. The possibility of additional retread gen tire inflation, however, may be more For passenger car and light truck tires, 10. Waddell, W.H., “10th Worldwide Tire Survey: significant to society as a whole. While an the need for nitrogen tire inflation is as- Replacement Tires,” presented to National Highway lives for a casing has not been document- Traffic Safety Administration, ExxonMobil Chemi- ed in published, large-scale testing. individual vehicle owner may be insensi- sociated with permitting the use of the cal, Washington, D.C., March 22, 2007. tive to the fraction of a percent of fuel tread wear life built into the tire. This ap- 11. Kaidou, H., and Ahagon, A., “Aging of Tire Parts When does the tire need nitrogen? economy lost with each increment of un- pears to be reasonable based on known During Service II, Aging of Belt-Skim Rubbers in derinflation (about 0.28 percent per psi of response of tires to both higher and lower Passenger Tires,” presented at a meeting of the Obviously, nitrogen tire inflation is an Rubber Division, ACS, Las Vegas, May 29-June 1, option that has the potential to provide pressure loss for a light vehicle), the levels of oxygen permeation. Improve- 1990. benefits related to wear, fuel economy, same cannot be said for the aggregate of ments associated with IPLR reduction 12. Santeler, D.J., et al., “Vacuum Technology and and so on.These benefits are related to all vehicles in operation. likely are to be within the normal varia- Space Simulation,” NASA SP-105, prepared under Evans14 suggested that, of the addition- tion of tire pressure for owners who prop- contract NASw-680 by Aero Vac Corp., 1996, P. 216. the reduction in IPLR that is obtained. 13. “Freight Transportation Case Studies: Effects of The magnitude and duration of this re- al 45 million barrels of oil consumed an- erly maintain tire pressure (compared to Nitrogen Tire Inflation on Canadian Long-haul Truck- duction over an air-inflated tire will be nually because of low tire pressures in the an air-inflated tire with dry air). ing,” Transport Canada, TP #14851 E, Drexan Corp. larger if tire pressure maintenance is U.S., about half could be saved by system- While desirable, many buyers will not be 14. Evans, L.R., “Tire Inflation Pressure and Fuel not being done currently or is only being atically using nitrogen inflation. Evans able to perceive the improvement. For Consumption,” Rubber & Plastics News, April 8, 15 2013, pp. 16-18. done in a haphazard fashion. and Harold Herzlich cite the associated medium radial truck tires, the value 15. Herzlich, H.J., “Benefit of Nitrogen on Tires on Plan- If the tire is inflated with nitrogen, and reduction in CO2 emissions as a result of proposition for nitrogen tire inflation in- et,” Rubber & Plastics News, May 6, 2013, pp. 17-21. then routine pressure maintenance is per- fuel efficiency increases (i.e., not burning formed using air as the fill gas, the IPLR that saved fuel) that would accompany reduction and its associated benefits grad- the use of nitrogen tire inflation. In fact, ually disappear. When routine tire pres- “Nitrogen Optional” for an individual con- sure maintenance is performed using ni- sumer ultimately may be counterproduc- trogen as the fill gas, Daws8 showed that it tive for the nation as a whole. is possible to reduce oxygen permeation into the tire to nearly zero. Conclusions Nitrogen’s ability to reduce the total The IPLR reduction available from ni- oxygen that permeates into a tire has the trogen tire inflation, while large in per- potential to improve the long-term dura- centage, is small enough that it is not like- bility of the tire. Even when the pressure ly to be noticed by the vehicle owner/user. maintenance is performed with air, sig- This assumes that the comparable air-in- nificant improvements are shown. The flated tire does not have water vapor, first issue, then, is when is the durability which can result in much larger pressure of the tire possibly at issue? swings especially in cold climates. The Fig. 18 shows a flow chart that encom- IPLR reduction from nitrogen tire infla- passes, in broad strokes, the lessons from tion is maximized when the tire is initially the present analysis. This chart takes inflated, and it gradually gets smaller over into account the environment by break- time. The impact on national oil consump- ing the U.S. into three geographic areas tion, however, could be significant. using the major east-west interstate Nitrogen tire inflation has the poten- routes I-40 and I-80. Obviously, areas tial to reduce the effective oxidative ag- like Las Vegas should be included in the ing of the tire over its lifetime. For pas- southern third of the country. As such, senger and light truck tires, the oxygen Fig. 5 represents a “first” approach, and permeation process could be reduced by should be further refined for wide use. a factor of 50 percent over six years. For Based on the analyses presented, tires medium radial truck tires, this figure is used in areas north of I-80 in the U.S. on the order of 73 percent. likely do not need nitrogen tire inflation. Passenger and light truck tires oper- Also, Fig. 18 incorporates the anticipat- ated over long service lives where the ed service life of the tire. Any passenger environmental temperatures are high car tire, for example, with an anticipated will have the most oxidative benefit service life of less than five years likely from nitrogen tire inflation. Tires antici- does not need nitrogen tire inflation. pated to last more than six years and op- On Fig. 18, it is recommended that erated south of I-40 in the U.S. should spare tires be inflated with nitrogen, be inflated with nitrogen for this reason. due primarily to the uncertainty associ- Tires anticipated to last more than 10