Impact of halobutyl rubber innerliners on tire performance.

The innerliner is a thin layer of rubber laminated to the inside of a tubeless tire. Its primary function is vapor retention and it is formulated to provide impermeability to gases, flex fatigue resistance, aging resistance and adhesion to the carcass compound. The tire is built with the innerliner applied to the building drum as a flat rubber sheet (ref. 1), but as the green tire composite is expanded during the shaping and curing processes, the carcass cords can be pushed into the innerliner compound. This is called cord penetration (figure 1).

[FIGURE 1 OMITTED]

The diffusion of gases through the polymer is the most significant variable. It has been shown that of the elastomers studied, butyl rubber has the lowest permeability coefficient (ref. 2) (figure 2). The tire innerliner is most commonly formulated with halobutyl rubber in order to provide good air and moisture impermeability, adhesion to carcass compounds, flex-fatigue resistance and durability (ref. 3). The relative impermeability of elastomers to air and moisture is reported in table 1 (ref. 4).

[FIGURE 2 OMITTED]

Butyl rubber (refs. 5-12) (IIR) is the copolymer of isobutylene and about 2% of isoprene.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

IIR has excellent impermeability for air barrier and good flex fatigue properties. These properties result from low levels of unsaturation in between the long polyisobutylene chain segments. Tire innertubes were the first major use of butyl rubber.

The development of halogenated butyl rubbers greatly extended the usefulness of butyl rubbers by providing a polymer with faster curing rates and increased polarity. This enabled co-vulcanization with general-purpose elastomers used in tire compounds such as NR, SBR and BR. The enhanced cure properties of halobutyl rubbers do not affect the desirable impermeability and fatigue properties, thus permitting development of tubeless tires in which the innerliner compound is chemically bonded to the carcass ply.

The impermeability ofpolyisobutylene is thought to be the result of the close packing of the geminal-dimethyl side groups along the polymer backbone, which also results in slow movement of the chains (ref. 13). As an example, three repeat units of cis-polyisoprene totaling 15 carbon atoms occupy more space than do four repeat units of polyisobutylene that contain 16 carbon atoms (figure 3). This is a result of the [sp.sup.3] hybridization of each carbon atom in polyisobutylene affording a tetrahedral arrangement of the chain, compared to the planar arrangement of the backbone due to the [sp.sup.2] hybridized carbon atoms in the carbon-carbon double bond of each segment in polyisoprene.

[FIGURE 3 OMITTED]

Bromobutyl rubber is used extensively in innerliner formulations for several reasons (refs. 14-25). They are:

* Superior adhesion and balance of properties;

* increasing use of ultra-low profile, speed rated tires having higher surface area to air volume ratios;

* requirement for lighter tires to reduce rolling resistance and improve vehicle fuel economy;

* use of high-pressure space-saver spare tires requiring a more impermeable innerliner;

* better flex-cracking resistance after aging; and

* bromobutyl innerliners, at half the gauge, are cheaper in material costs.

We present results of the effect that the bromobutyl rubber content in the innerliner formulation has upon tire performance. Innerliners with 100 phr BIIR and 80/20 and 60/40 blends of BIIR/NR were studied in the laboratory and in radial passenger tires. No changes were made to the tire materials or construction except for the changes in the polymer ratios in the innerliner compound.

Experimental

Innerliner compounds

The bromobutyl rubber used was BIIR 2222. Natural rubber used was SMR 20. All other ingredients were commercial materials. Formulations are shown in table 2.

Compounds were mixed in a factory using a conventional two-step mixing sequence in internal mixers equipped with tangential rotors. Masterbatch mixing was completed using a GK400 mixer followed by sheeting out on an extruder with roller die. Finalization was completed in a GKI60 mixer and stocks sheeted out on a two-roll mill. A cold-feed pin extruder was used to profile each innerliner compound.

When possible, ASTM tests (or slightly modified) were used to determine the cure and cured compound physical properties; table 3 is a summary. Cure properties were measured using a MDR 2000 (0.5[degrees] arc) at 160[degrees]C. Test specimens were cured at 160[degrees]C for a time corresponding to Tc90+2 minutes for mold lag. Stress/strain properties were measured at room temperature using an Instron 4202. Hardness was measured at room temperature using a Zwick Duromatic. Aged hardness and aged stress/strain properties were measured at room temperature after heating the specimen in a circulating air oven for 72 hours at 125[degrees]C.

Oxygen transmission was measured using a Mocon OxTran Model 2/61 operating under the principle of dynamic measurement of oxygen transport through a thin film (ref. 26). The method is to clamp a flat rubber sample into diffusion cells, which are then purged of residual oxygen using a high purity nitrogen carrier gas. The nitrogen gas is routed to a sensor until a stable zero value is established. Pure oxygen or air is then introduced into the outside of the chamber of the diffusion cells. The oxygen diffusing through the sample to the inside chamber is conveyed to a sensor which measures the oxygen diffusion rate.

Tires

P205/60 HR15 passenger tires were built using automatic building machines. H-rated tires were used in order to test the effect that changes in the bromobutyl rubber content in the innerliner compound have upon tire performance, since no weaknesses are present in the tire construction. Cured innerliner thicknesses averaged 1.0 ram. Previously, we reported on similar results obtained for P205/60 SR15 passenger tires (ref. 27).

Inflation pressure retention (IPR) was tested in accordance to ASTM F-1112 by mounting the tires on standard rims and inflating to 240 kPa. A T-adapter connected to the valve allowed a calibrated gauge to be connected to one side and inflation air to be added through the other. The tires were checked for leaks, conditioned for 48 hours at 21 [degrees]C and again checked for leaks. The inflation pressure was then recorded over a three month time frame. The IPR is reported as the inflation pressure loss per month.

The intracarcass pressure (ICP) is measured on tires mounted on standard rims and inflated to 240 kPa. The tires are connected to a constant inflation pressure system, which uses a calibrated gauge to maintain the inflation at 240 kPa. Typically, five calibrated gauges with hypodermic needles are then inserted into the tire with the tip of the needle set on the carcass cord. The readings are taken until the pressure at the cord interface equilibrates, normally two months. The ICP is reported as the average of the readings.

A tire durability test called a tread separation test was run by mounting the tires on reinforced steel rims of standard size. The tires are inflated to 240 kPa using a 50/50 [O.sub.2]/[N.sub.2] mixture and loaded on the test machine. The tires are run against a 28.5 cm wheel running at 80 kin/hr, in a room at 21[degrees]C. The load is set using the 100% load for 207 kPa inflation (ref. 28), which normally gives a deflection of 30% to 35%. The tire is run for one hour at 50% load, followed by one hour at 100% load. The inflation pressure is recorded, and the pressure is adjusted to this level daily for the test duration. Tires are tested until failure.

A second tire durability test was run according to a modified FMVSS 139 endurance test procedure by mounting the tires on reinforced steel rims of standard size. The tires are inflated to 180 kPa using air and loaded on the All Well Tire Testing Machine Model AW-TT-2A-M4. The tires are run against a 1.707 m wheel running at 120 km/hr, in a room at 38[degrees]C. The tire is run for four hours at 85% of the load rating on the tire sidewall, followed by six hours at 90% load, and followed by 24 hours at 100% load. Tires are inspected visually for failures; then the test was modified by continuing to run the tire until failure. Tires were inspected, and failures determined after dissection.

Results and discussion

Compound properties

Cure and cured physical properties are summarized in table 4. Use of increasing amounts of natural rubber in place of the bromobutyl rubber in the model tire innerliner compounds decreased compound Mooney viscosity values and Mooney scorch times, and increased compound t90 cure times.

Cured compound hardness and tensile properties for the three compounds were similar to one another, except for energy to break values, which were reduced upon substitution of natural rubber for bromobutyl rubber.

Aged tensile properties were adversely changed upon removal of bromobutyl rubber from the innerliner formulation. Modulus values at 100%, 200% and 300% elongation all increased upon aging in an air oven for all three compounds; figure 4 is an example. No statistical trends are observed. Tensile strength, elongation at break and energy to break values all decreased for all three compounds upon being aged in an air oven. Graphs of the relative property changes normalized to the respective original values are shown in figures 5-7. Linear decreases in the percentage of retained property were observed as a function of increasing natural rubber substitution for bromobutyl rubber: Tensile strength ([R.sup.2] = 0.9826), elongation at break ([R.sup.2] = 0.9972) and energy to break ([R.sup.2] = 0.974). Trends are all statistically significant, thus indicating the age resistant benefits of using 100 phr of bromobutyl rubber in the innerliner compound.

Fatigue to failure values decreased upon increasing natural rubber content (table 4).

Air permeability increased linearly ([R.sup.2] = 0.9907) with increasing natural rubber content in the innerliner compound (figure 8). The 100 phr bromobutyl rubber compound had the lowest permeability, which is desirable. Essentially identical results were observed for aged air permeability (table 4).

Tire properties

Tire inflation pressure retention (IPR), intracarcass pressure (ICP) and durability testing were determined in duplicate for identical tire sets made with the three different innerliner formulations: 100 phr BIIR, 80/20 BIIR/NR and 60/40 BIIR/NR innerliner with all other compounding ingredients identical (table 2). Results are summarized in table 5.

Tire IPR increased linearly ([R.sup.2] = 0.9995 to bromobutyl rubber content) with increasing natural rubber content in the innefliner compound (figure 9). IPR was lowest for the tire having a 100 phr bromobutyl rubber innerliner, which is a desirable tire performance property.

Tire ICP increased linearly (R2 = 0.9566) with increasing natural rubber content in the innerliner compound (figure 10). ICP was lowest for the tire having a 100 phr bromobutyl rubber innerliner, which is also a desirable tire performance property.

Two tire durability tests were performed including tread separation and FMVSS 139 endurance, but testing until tire failure. All tire failures were the result of belt-to-belt separations. Tires run on the small test wheel at 80 km/h. until failure showed a correlation to bromobutyl rubber content, as seen graphically in figure 11 ([R.sup.2] = 0.797). Tires run on the large test wheel at 120 km/h. until failure are shown graphically in figure 12, showing a higher correlation to bromobutyl rubber content ([R.sup.2] = 0.9121). For each road wheel tire durability test, the tire containing the 100 phr bromobutyl rubber innerliner performed best, as determined by the longest hours before failure by a belt-to-belt separation.

Statistical analysis was performed using SAS JMP software, even though there are only three data points (100, 80/20 and 60/40 BIIR/NR), since each represents the mean of at least two trials. Analysis shows a very high cross-correlation of the two tire durability test results ([R.sup.2] = 0.9727). Analysis also shows a very high cross-correlation of the tire IPR and tire ICP test results to one another ([R.sup.2] = 0.9262). Independent correlations of each tire durability test result to the (1) bromobutyl rubber content in the innerliner formulation; (2) air permeability of the innerliner compound; (3) tire IPR; and (4) tire ICP show that the FMVSS 139 endurance test modified by running the tire until failure is the statistically more reliable road wheel test. Table 6 is a summary. This result is probably due to two important factors: (1) air is the fill gas specified in the FMVSS 139 endurance test versus the 50% oxygen/nitrogen gas mixture used in the tread separation test; and (2) a 1.707 meter wheel is used in the FMVSS 139 endurance test versus the much smaller 0.285 meter wheel used in the tread separation test.

Summary

Halobutyl rubber innerliners are the best at retaining pressure, both air and moisture, and minimize the temperature-dependence of air permeability. They are approximately an order of magnitude less permeable than other elastomers.

The quality of the innerliner compound is improved by using a 100 phr bromobutyl rubber formulation, since it affords the highest percentage of retained tensile properties upon air oven aging. Statistically significant trends were observed for tensile strength, elongation at break and energy to break values.

The integrity of the tire is improved significantly by using a 100 phr bromobutyl rubber formulation in the innerliner, since it minimizes inflation pressure percent loss per month, minimizes the development of intracarcass pressure which could lead to belt edge separation and adhesion failures, and maximizes tire durability as determined by measuring the hours to failure by a belt-to-belt separation on indoor road wheels.

Statistical analysis of tire durability test results to the bromobutyl rubber content; air permeability of the innerliner compound; tire IPR; and tire ICP showed that the FMVSS 139 endurance test, but modified by running the tire to failure, is a statistically reliable road wheel test. Statistically significant cross correlations of FMVSS 139 endurance (to failure) are obtained with the bromobutyl rubber content in the innerliner formulation, and the inflation pressure retention of the tire.

References

(1.) R.S. Bhakuni, S.K. Mowdood, W.H. Waddell, I.S. Rai and D.L. Knight, "Tires" in Encyclopedia of Polymer Science and Engineering, 2nd Ed., John Wiley & Sons Inc., New York, 1989, 16, p. 834.

(2.) GJ. van Amerongen, "Diffusion in elastomers," Rubber Chemistry and Technology, Rubber Reviews, 37, 1,065 (1964).

(3.) W.H. Waddell, R.S. Bhakuni, W.W. Barbin and P.H. Sandstrom, "Pneumatic tire compounding, " in The Vanderbilt Rubber Handbook, 13th Ed., R.E Ohm, Editor, R.T. Vanderbilt Company, Inc., Norwalk, CT, 1990, p. 596.

(4.) B. Costemalle, "Tire pressure loss and intracarcass pressure modeling, " Tire Science and Technology, 20, 200 (1992).

(5.) E.N. Kresge, R.H. Schatz and H-C. Wang, "Isobutylene Polymers" in Encyclopedia of Polymer Science and Engineering, 2nd Ed., J.I. Kroschwitz, Editor, John Wiley & Sons, New York, 1987, v. 8, p. 423.

(6.) J. V. Fusco and P. Hous, "Butyl and halobutyl rubbers, "in Rubber Technology, 3rd Ed., M. Morton, Editor, Van Nostrand Reinhold Company, Inc., New York, 1987, p. 284.

(7.) J. V. Fusco and P. Hous, "Butyl and halobutyl rubbers, "in The Vanderbilt Rubber Handbook, 13th Ed., R. F. Ohm, Editor, R.T. Vanderbilt Company, Inc., Norwalk, CT, 1990, p. 92.

(8.) E.N. Kresge and H-C. Wang, "Butyl rubber, "in Kirk-Othmer Encvclopedia of Chemical Technology, 4th Ed., M. Howe-Grant, Editor, John Wiley & Sons, New York, 1993, v. 8, p. 934.

(9.) J.E. Rogers and W.H. Waddell, Rubber World, 219 (5), 24, February 1999.

(10.) G.E. Jones, D.S. Tracey and A.L. Tisler, "Butyl rubber," in Rubber Technology, Compounding and Testing for Performance, J.S. Dick, Editor, Hanser, Munich, 2001, p. 173.

(11.) R.N. Webb, T.D. Shaffer and A.H. Tsou, "Commercial isobutylene polymers," in Encyclopedia of Polymer Science and Technology, Online Edition, J.I. Kroschwitz, Editor, John Wiley & Sons, New York, 2003.

(12.) W.H. Waddell and A.H. Tsou, "Butyl rubbers, "in Rubber Compounding." Chemistry and Applications, M.B. Rodgers, Ed., Marcel Dekker, Inc., New York, 2004, p. 133.

(13.) D.J. Plazek and K.L. Ngai, Macromolecules, 24, 1,222 (1991).

(14.) C.W. von Hellens, "Inner liners for high performance tires, " paper 50, Rubber Division, ACS, Indianapolis, IN, May 10, 1984.

(15.) W. Hopkins, R.H. Jones and J. Walker, "Bromobutyl and chlorobutyh A comparison of their chemistry, properties and uses, "paper 16A10 presented at IRC '85 Kyoto; International Rubber Conference Proceedings, Oct. 15-18, 1985, p. 205.

(16.) J. V. Fusco and D. G. Young, "Isobutylene-based polymers in tires--status and future trends', "paper no. 8 presented at a meeting of the Rubber Division, ACS, Washington, D.C., October 9-12, 1990.

(17.) C.L. Morehart and F.J. Ravagnani (to Bridgestone Corp.), European Patent 0 604 834A1 (Dec. 15, 1993).

(18.) I. Duvdevani, L. Gursky and I.L. Gardner, "Star branched butyl--a novel butyl rubber for improved processability. II. Properties and applications, "paper no. 22 presented at a meeting of the Rubber Division, A CS, Mexico City, Mexico, May 9-12, 1989.

(19.) K.J. Kumbhani, J.J. Sendorek and H. Tang, J. Elastomers Plast., 24, 262 (1992).

(20.) B.J. Costemalle and J. V. Fusco (to Exxon Chemical), U.S. Patent 5,386,864 (Feb. 7, 1995).

(21.) PH. Sandstrom and R.B. Roennau (to The Goodyear Tire & Rubber Co.), European Patent 0 682 075 A1 (Apr. 20, 1995).

(22.) S. Katsuki, K. Yamakawa, J. Watanabe and H. Kaido (to Yokohama Rubber Co.), European Patent 0 706 878 A2 (Sept. 12, 1995).

(23.) B. Costemalle, J. V. Fusco and D.F. Kruse, J. Elastomers Plast., 27, 39 (1995).

(24.) B. Costemalle, P Hous and K.O. McElrath, "Exxpro polymers, " paper no. G11 presented at Rubbercon '95, Gothenburg, Sweden, May 9-12, 1995.

(25.) A.J. Dias, GE. Jones, D.S. Tracey and W.H. Waddell (to ExxonMobil Chemical), WO 02/48257 A2 (June 20, 2002).

(26.) R.A. Pasternak, et al., J. Polym. Sci. A-2, 8, 467 (1970).

(27.) D.S. Tracey and W.H. Waddell, "Innerliners and tire durability, "ITEC 2004, September, 2004.

(28.) The Tire Guide.

Table 1 - air and moisture vapor
permeability rates for typical
inner liner compounds (at 65[degrees]C)

Innerliner compound    Air    Moisture

100 NR                 8.3     13.3
100 SBR                6.8     11.0
60 CIIR                3.1      3.0
100 CIIR               1.0      1.0
100 BIIR               1.0      1.0

Table 2 - innerliner compound formulations

Ingredient                         1         2         3

Bromobutyl rubber, 2222           100        80        60
Natural rubber, SMR 20             0         20        40
Processing aid, SP1068             4         4         4
Carbon black, N660                 60        60        60
Processing aid, Struktol 40 MS     7         7         7
Processing oil, TDAE               8         8         8
Stearic acid                       1         1         1
Zinc oxide                         1         1         1
Sulfur                            0.5       0.5       0.5
Accelerator, MBTS                 1.25      1.25      1.25

Table 3 - test methods

Parameter                                                         Units

Mooney viscosity                              ML 1+4, 100[degrees]C, MU
Mooney scorch time                          ts5, 125[degrees]C, minutes
Green strength                                                      PSI
  (100% modulus)
Moving die rheometer (MDR)
  @ 160[degres]C, [+ or -] 0.5[degrees]arc
  ml                                                   deciNewton.meter
  mh                                                          dNewton.m
  ts2                                                            Minute
  t50                                                            Minute
  t90                                                            Minute
Physical properties press
  cured Tc 90+2 min @ 160[degrees]C
  Hardness                                                  Durometer A
  Modulus 100%, 200%,                                          300% MPa
  Tensile strength                                                  MPa
  Elongation at break                                                 %
  Energy to break                                               N/mm(J)
Hot air aging, 72 hrs. @ 125[degrees]C
  Hardness                                                  Durometer A
  Modulus 100%, 200%, 300%                                          MPa
  Tensile strength                                                  MPa
  Elongation at break
  Energy to break                                               N/mm(J)
Mocon, @ 60[degrees]C                             cc-mil/[m.sup.2]-day-
                                                                   mmHg

Parameter                                                          Test

Mooney viscosity                                            ASTM D 1646
Mooney scorch time                                          ASTM D 1646
Green strength                                               ASTM D 412
  (100% modulus)
Moving die rheometer (MDR)                                  ASTM D 2084
  @ 160[degres]C, [+ or -] 0.5[degrees]arc
  ml
  mh
  ts2
  t50
  t90
Physical properties press
  cured Tc 90+2 min @ 160[degrees]C
  Hardness                                                  ASTM D 2240
  Modulus 100%, 200%,                                  ASTM D 412 die C
  Tensile strength
  Elongation at break
  Energy to break
Hot air aging, 72 hrs. @ 125[degrees]C                       ASTM D 573
  Hardness
  Modulus 100%, 200%, 300%
  Tensile strength
  Elongation at break
  Energy to break
Mocon, @ 60[degrees]C                                          See text

Table 4 - innerliner compound properties

Property                                        100     80/20     60/40

Mooney viscosity,                              78.2      58.5      54.2
  ML(1 +4) @ 100[degrees]C
Mooney scorch, ts5@135[degrees]C (min.)        7.78      6.52      6.33
Green strength
  Peak load (N)                               26.79     10.78      9.97
  Time to 75% (min.)                           7.90      6.16      6.77
MDR 2000 @ 160[degrees]C, 0.5[degrees] arc
  ml (dN.m)                                    1.71      1.47      1.49
  mh (dN.m)                                    4.87      5.15      6.02
  ts2 (min.)                                   4.38      5.13      5.04
  t25 (min.)                                   2.24      3.40      3.92
  t50 (min.)                                   3.51      4.87      5.40
  t90 (min.)                                   8.08      9.40     11.03
Hardness, durometer A                          48.0      47.0      47.0
Aged hardness, durometer A, 72                 50.0      51.0      47.0
  hr. @ 125[degrees]C
Tensile
  100% modulus (MPa)                           1.39      1.16      1.17
  200% modulus (MPa)                           2.98      2.37      2.50
  300% modulus (MPa)                           4.69      3.90      4.16
  Tensile (MPa)                               10.54     10.68     12.37
  Elongation at break (%)                     746.9     777.6     721.3
  Energy to break (N/mm)                      15.95     13.39     12.47
Aged tensile, 72 hr. @ 125[degrees]C
  100% modulus (MPa)                           1.96      1.86      1.69
  200% modulus (MPa)                           4.40      3.82      3.49
  300% modulus (MPa)                           6.39      5.53      5.11
  Tensile (MPa)                                9.24      8.13      7.12
  Elongation at break (%)                     593.8     564.2     463.1
  Energy to break (N/mm)                      12.11      8.63      5.50
Fatigue to failure (cycles)                 610,442   324,694   127,864
Adhesion to NR carcass                        13.10      9.68     10.59
  @100[degrees]C (N/mm)
Mocon oxygen transmission                      0.62      0.95      1.42
  @60[degrees]C
Aged Mocon @60[degrees]C, 72                   0.52      0.81      1.23
  hr. @ 125[degrees]C

Table 5 - tire performance properties

Tire property                    100/0     80/20    60/40

Inflation pressure retention,     1.4       2.1      2.75
  % loss/month
Intracarcass pressure, kPa        61.5     78.6     115.6
Tread separation, hours to        838       704      695
  failure
FMVSS 139 endurance,              583       410      358
  hours to failure

Table 6 - statistical analysis of tire
performance properties

Durability test          Test parameter     [R.sup.2]

Tread separation           BIIR content         0.797
                       Air permeability         0.711
                                    IPR         0.814
                                    ICP         0.564
FMVSS 139 endurance        BIIR content         0.912
  (To failure)         Air permeability         0.847
                                    IPR         0.924
                                    ICP         0.773

Figure 4 - aged tensile properties
normalized to respective original tensile
property values: 300% modulus

Normalized aged 300% modulus

100/0           1.38
80/20           1.42
60/40           1.23

Note: Table made from bar graph.

Figure 5 - aged tensile properties
normalized to respective original tensile
property values: tensile strength

Normalized aged tensile strength
y = -0.1506x + 1.0393; [R.sup.2] = 0.9826

100/0         0.88
80/20         0.76
60/40         0.58

Note: Table made from bar graph.

Figure 6 - aged tensile properties
normalized to respective original tensile
property values: elongation at break

Normalized aged tensile strength
y = -0.0765x + 0.8738; [R.sup.2] = 0.9972

100/0         0.80
80/20         0.73
60/40         0.64

Note: Table made from bar graph.

Figure 7 - aged tensile properties
normalized to respective original tensile
property values: energy to break

Normalized aged tensile strength
y = -0.1592x + 0.9333; [R.sup.2] = 0.974

100/0         0.76
80/20         0.65
60/40         0.44

Note: Table made from bar graph.

Figure 8 - air permeability normalized to
the 100 phr bromobutyl rubber
innerliner compound

Relative air permeability
y = -0.6435x + 0.3204; [R.sup.2] = 0.9907

100/0         1.00
80/20         1.54
60/40         2.29

Note: Table made from bar graph.

Figure 9 - inflation pressure retention tire
test results

Inflation pressure retetion
y = 0.675x + 0.7333; [R.sup.2] = 0.9995

100/0         1.4
80/20         2.1
60/40         2.75

Note: Table made from bar graph.

Figure 10 - intracarcass pressure tire
test results

Intracarcass pressure
y = 27.025x + 31.15; [R.sup.2] = 0.9566

100/0         61.5
80/20         78.6
60/40        115.6

Note: Table made from bar graph.

Figure 11 - tread separation tire durability
test results

Tread separation
y = -71.5x + 888.67; [R.sup.2] = 0.797

100/0         838
80/20         704
60/40         695

Note: Table made from bar graph.

Figure 12 - FMVSS endurance (to failure)
tire durability test results

FMVSS 139 endurance to failure
y = -112.5x + 675.33; [R.sup.2] = 0.9121

100/0         583
80/20         410
60/40         358

Note: Table made from bar graph.