Improving Carcass Durability Through Filler Selection

Paper No. 2CRP-02-05IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONStephen Laube* and James ShellCabot Corporation, North American Carbon Black Division1095 Windward Ridge Parkway, Suite 200, Alpharetta, Georgia 30005, U.S.A.Ping ZhangCabot Corporation, Billerica Business & Technology Center157 Concord Road, Billerica, Massachusetts 01821, USAPresented at a Meeting of theRubber Division, American Chemical SocietySavannah, GAApril 29 - May 1, 2002* Speaker

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONABSTRACTCarcass durability is a complex function of compound selection, mixing/processing, final assembly,tire design, vehicle fitment, and ultimately, consumer use – or abuse. The formidable task facingtoday’s tire manufacturers is to design and build a carcass that can have a life of up to a million miles,as in the case of retreaded truck tires, for example.Recent advances in fillers that may allow the tire compounder and tire engineer to enhance carcassdurability include:• Low hysteresis carbon blacks, silicas, and, most recently, carbon-silica dual phase fillers for treadcompounds to reduce tire running temperature• Carbon black/natural rubber composites that have excellent (near “perfect”) dispersion,improved hysteresis, and much improved flex fatigue for higher durability• Carbon blacks that can be compounded to reduce innerliner permeability, preserving criticalinflation pressure while minimizing intracarcass pressure buildupThis paper will review these advances in filler technology and examine their potential for improvingcarcass durability.2

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONIMPORTANCE OF CARCASS DURABILITYTruck TiresThe economics of medium radial truck tires are such that retreading plays an important role in thelife cycle of the tire. A retreaded 11R22.5 tire can cost about 70% less than the price of a new one [1].Multiple retreadings are the goal, and the “million mile” carcass is frequently discussed. A paper by amajor tire manufacturer cites that 33% of the low profile truck radials in North America areretreaded once, 40% are retreaded twice, 22% three times, and 5% more than three times [2]. Thesame paper cites mileage of 270,000 kilometers for removal of low profile truck radials on drive axlesand 240,000 miles on trailers. Thus, carcass mileage in the range of 960,000 – 1,120,000 kilometers isalready a reality for some low profile truck tires in North America. As shown in FIGURE 1, thesituation in Europe is similar, with original mileage increasing by a factor of 5 over the last 50 years,and projected mileage of over 400,000 kMs by 2010 [3]. Carcass durability is thus a major concernand requirement of the OE producers of truck tires.Passenger TiresRetreading does not play a major role in the economics of light truck/passenger tires, but carcassdurability can loom large in consumer safety issues. In addition, OE customer satisfaction surveysweigh the importance of product quality at 39%, including the number of tires with a problem at16% [4]. By contrast, only 22% of customers listed long term performance including 10% who citedwear as critical to satisfaction. Brand loyalty is directly affected by the OE satisfaction index; a 25%increase in the OE satisfaction index corresponded to a 25 % increase in the percentage of customerswho expected to purchase the same brand. Thus, carcass durability improvements are continuouslysought in passenger tires for safety and marketing reasons.3

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONPARAMETERS AFFECTING CARCASS DURABILITYBasic FactorsCarcass life for both passenger and truck tires is determined through a complex interaction betweenthe tire manufacturer, vehicle manufacturer, and the consumer. Arguably, the consumer plays themost important role as s/he is responsible for maintaining the recommended inflation pressure overthe life of the tire, the load on the tire, the speed, and even the terrain over which the vehicle isdriven.The tire manufacturers’ role includes good design, use of quality raw materials, proper compounding,and precision assembly. Last, the vehicle manufacturer and the tire manufacturer interact to establishand recommend both the correct tire specification and inflation pressure for a given vehicle.Flexing and Heat GenerationIt is well documented that the two major enemies of carcass life are flexing and heat generation. Thetwo are directly related; flexing causes excessive heat build-up in the tire’s carcass components.Flexing: Is also directly related to inflation pressure – an under-inflated tire flexes more,generating more heat which in turn causes undesirable consequences such as acceleratedcompound aging and increased innerliner permeability, issues that are discussed below. Last,flexing of and by itself can lead to crack initiation and propagation in rubber compounds,especially at initiation points caused by improperly dispersed compound ingredients.Heat Generation: Even with the correct recommended inflation pressure, rubbercompounds, especially the tread compound, play an important role in heat build-up underdynamic conditions. In a heavy truck tire, 42% of the total composite rolling resistancecomes from the tread compound itself [5]. The filler chosen, usually carbon black, comprisesapproximately 30% of the weight of the tread compound, and in turn contributes heavily tothe hysteresis of the tread compound. The internal temperature of a truck tire duringoperation can easily reach 100° C. Belt edge temperatures can exceed well over 100°C inDOT FMVSS #119 testing [1]. At these high temperatures, aging of the unsaturatedelastomers occurs relatively rapidly [6]. These high temperatures also decrease thepermeability of the innerliner compound, allowing intracarcass pressure to further increase[7]. The net result creates a “double whammy” on the carcass compounds.Intracarcass PressureThe mechanism of carcass degradation has been described as including intracarcass pressure build-upand the resultant oxidative attack on the internal tire compounds [8]. Mileage on an indoor wheeldurability test was doubled by reducing the intracarcass pressure from 115 kPa (17 PSI) to 55 kPa (8PSI) through the use of a 75/15 CIIR/NR innerliner versus a 100 phr general purpose elastomerinnerliner. In another study, reducing the intracarcass pressure from 62 kPa (9.1 PSI) to 36 kPa (5.3PSI) by going to 100% BIIR in the innerliner from a 75 BIIR/25 NR blend, doubled the hours tofailure on a DOT FMVSS #109 high speed test from 25 to about 48 hours [7].4

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONAn older but comprehensive study concluded that belt edge separation in a radial tire was influencedequally by liner permeability and its gauge and the tire operating temperature. Wire wicking was not afactor unless the innerliner was highly permeable [9].FILLER ENHANCEMENTS TO DURABILITYHysteresis Reduction via Carbon Silica Dual-Phase Fillers (CSDPF)The target for the raw material producer and the tire manufacturer is to obtain the required tensile,tear, and abrasion resistance while minimizing potentially damaging heat build-up. With fillers, amajor trade–off is that taking the usual route of higher surface area to increase the tensile/tear/abrasion resistance also leads to higher heat build-up. Recent advances in silica technology andcompounding allow a reduction in hysteresis with relatively little sacrifice in abrasion resistance, andan increase in wet traction when using anti-lock braking systems under certain testing conditions isalso claimed [10]. The use of coupled silica is not without other problems including the high cost ofraw materials, and even higher compound costs due to the use of a high coupling agent level. Otherfactors associated with coupled silica include longer mixing cycles, the evolution of ethanol from thecoupling agent, molding problems, high electrical resistivity, and abrasiveness to process equipment.Another alternative to carbon black and silica for imparting reinforcement while minimizinghysteresis was recently introduced. Known as “Carbon Silica Dual-Phase Fillers (CSDPF),” theseproducts are made by a unique process in the “carbon black” reactor. These products represent anentirely new class of fillers having a surface with features of both silica and carbon black. Comparedto carbon black, the CSDPF’s features lower filler-filler interaction and increased polymer-fillerinteraction. The lower filler-filler and higher polymer-filler interaction is evident in their low PayneEffect [11], strong adsorption from polymer analogs on the filler surface [12], [13], and high boundrubber content even when used without silane coupling agents [14]. Laboratory wet skid resistancecorrelates directly with the silica surface coverage, and cost is directly related to the Si content. Thishas resulted in optimizing the co-fuming process to produce high surface coverage with minimal Si.For example, when only 10% Si is used in the process, the resultant CSDPF’s surface is about 55%silica and 45% carbon black.5

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONThe next two figures show the much reduced hysteresis at high temperatures exhibited by theCSDPF family vs. carbon black of a similar surface area. FIGURE 2 shows a typical “firstgeneration” product, CSDPF 2124, in natural rubber. FIGURE 3 shows the heat build-up curves forthe same compounds. FIGURE 4 shows a typical “second generation” product, CSDPF 4210, in anSSBR/BR compound. CSDPF 4210 gives rise to a tan delta vs. temperature curve similar to that of a“HD” silica based SSBR/BR compound when compared at equal filler volume [13]. Even at higherloading levels, CSDPF 4210 based compounds exhibit reduced hysteresis in comparison with carbonblack compounds [14].Preliminary tire testing has shown that the second generation CSDPF products give tractionon a par with silica with wear on a par with the underlying carbon black. Other advantages versussilica include less ethanol evolution and less abrasiveness to factory equipment.6

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONElastomer Composites for Flex Fatigue and Hysteresis ImprovementsThe Elastomer Composite (EC) process is the first truly continuous commercial process for mixingcarbon black and natural rubber in the liquid phase. Compounds produced using the ElastomerComposite process have the following characteristics:• Excellent dispersion, much better than normally experienced in conventional dry-phase mixing(FIGURE 5). This characteristic is independent of carbon black type• Excellent carbon black distribution, in which the distribution of the carbon black in the polymermatrix is much more uniform than in dry mixing (FIGURE 6).7

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONMolecular weight preservation—since the dispersion process happens in the liquid phase, and thecompound is subjected to a minimum of shear force during mixing and dewatering, ElastomerComposite materials of a given loading have greatly higher polymer molecular weight thanequivalent dry mixes, while retaining the dispersion advantage.• Intimate interaction between polymer and filler. Bound rubber values for Elastomer Compositecompounds are typically 20-50% higher than for equivalent dry mixes.RUBBER PROPERTY IMPROVEMENTSThe excellent dispersion allows a reduction in the number of mix cycles versus dry mix material, andyet physical properties are improved. Specifically, when not overmixed, EC compounds exhibit thefollowing improvements versus dry mixed compounds, achieved at the factory scale (FIGURE 7 andFIGURE 8):• Higher tensile strength• Lower hysteresis/higher rebound• Greatly improved flex life8

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONDynamic Hysteresis at High TemperatureOne of the most important features of Elastomer Composite vulcanizates is their lower hysteresis athigh temperatures, such as 50~80°C. Due to the excellent correlation between rolling resistance oftires and the hysteresis of their tread compounds at high temperature, lower tire rolling resistance andlower heat build-up are benefits of EC.FIGURE 9 summarizes the maximum tan delta results of 340 EC compounds containing 8 differentblacks with varying black and oil loadings. When the hysteresis of the compounds is compared pairby-pairbetween EC and their dry-mixed counterparts with the same formulations, the hysteresis ofEC vulcanizates is on average 7% lower. One of these blacks was a 200 surface area, 76 CDBPproduct that likely could not even be dispersed by conventional dry mixing techniques. The lowerhysteresis for EC compounds is mainly due to improved micro-dispersion of the carbon black, i.e.,depressed filler networking. This can be demonstrated by the Payne effect, i.e., the difference indynamic moduli measured at low strain amplitude and high strain amplitude. The Payne effect hasbeen widely used as a measured of filler networking. For all 340 compounds mentioned above, theaverage Payne effect measured at strain amplitude at 0.1% and 60% is about 8% lower for ECvulcanizates.9

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONIMPROVEMENTS IN TIRE DURABILITYCooler RunningThe higher rebound or lower hysteresis of EC compounds results in lower heat build-up. In a typicalendurance test for Elastomer Composite truck tires, compared to an existing commercial tire, therunning temperature was reduced by 10°C and endurance life increased by 17% for EC-treadcompounds prepared with a comparable formulation as the dry mix. (FIGURE 10)Improved Flex LifeMainly due to the excellent dispersion of the carbon black in EC, and maybe also due to lower heatbuild-up, a great benefit for fatigue resistance has been obtained from Elastomer Composites.Compared with the traditional compounds, the average fatigue life in compression mode increases bywell over 90% which will impart a significant improvement to service life of some rubber goods,such as anti-vibration products, wipers, belts and tire carcass compounds. FIGURE 11 shows onetypical example.10

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONLOW SURFACE AREA/STRUCTURE BLACKS FOR REDUCINGINNELINER AIR PERMEABILITYInflation Pressure: an Antidote to Flexing and Heat Generation“The only thing worse than inflation is underinflation, have you checked your tires this month?”[15]. This question appeared recently on a billboard in a major US city. Unfortunately, the answer tothe above question, in a word, is “no” 52% of the time [16]. The NHTSA recently stated that “morethan one in four cars and about one in three light-duty trucks, vans, and SUV’s on America’s roadsare riding on at least one tire that is severely underinflated [17].” NHTSA defines severelyunderinflated as “at least 54.4 kPa (8 psi) below the OEM’s recommended pressure.” A recent fieldsurvey by a major tire manufacturer showed that less than 25% of 1500 tires surveyed had properinflation pressure; nearly 60% were underinflated by 10% or more [18].As discussed, underinflation results in more flexing and more heat build-up, resulting in reduced fuelefficiency, loss of ride and handling characteristics, and potentially reduced carcass life. In addition, amajor tire manufacturer has shown that a reduction in wear also results from underinflation [2].In summary, maintaining the correct inflation pressure thus benefits the consumer in four ways:• treadwear is improved• flexing is reduced, minimizing heat build-up and thus maximizing fuel economy• reduced heat build-up extends carcass life and minimizes the chance of a failure• ride comfort and handling levels are maintained at the target levelsWith a tubeless tire, inflation pressure is primarily maintained through the use of a relativelyimpermeable membrane on the inside of the tire; the innerliner. This membrane is made of relativelyexpensive, relatively impermeable elastomers such as chlorobutyl or bromobutyl. In spite of thismembrane, consumer neglect, as noted above, results in a lot of underinflated tires.Fillers in general and carbon black in particular are much less permeable than even relativelyimpermeable elastomers such as the halobutyls currently used in most tire innerliners. Therefore,higher filler loadings in elastomeric components designed to minimize permeability of moisture andgases should lead to a reduction in permeability. Indeed, Cabot’s lab work has confirmed that thepermeability of a compound is correlated highly to a weighted average of the two major component’spermeability; the polymer and the filler. When using this approach, our work has shown thathalobutyl effectively has a permeability of about 120 and carbon black is zero [19]. Others haveobtained similar results [20]. In this reference, it is clearly shown that increasing the carbon blackloading from 60 to 90 phr decreases permeability of a low bromine innerliner significantly.11

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONIn order to be able to increase the filler loading, the filler needs to have minimal impact on propertiessuch as Mooney viscosity, hardness, and elongation and yet it must provide the necessaryreinforcement. In the case of carbon black, hardness and loading capability both vary with the twomajor morphological properties of surface area and structure: hardness varies directly, that is, highersurface area or structure causes higher hardness, while loading capability varies inversely; lowersurface area and structure both lead to higher loading capability. Thermal black has very lowstructure but only 10 square meters/ gram surface and can therefore be used at high loadings, butthermal black is virtually non-reinforcing.A unique (non-ASTM) grade has resulted from a manufacturing technology breakthrough. Theanalytical or morphological properties of this product, Regal 85 carbon black, approach those ofthermal black, allowing very high loadings. Regal 85 carbon black is about 10 square meters/ gramhigher surface area than thermal; that is, double. This higher surface area provides a level oftensile/tear not provided by thermal black.FIGURE 12 shows how the permeability of the 100% bromobutyl compound is decreased as theloading of Regal 85 carbon black is increased, even with some added oil.FIGURE 13 shows improved flex fatigue , and FIGURE 14 shows the good aged tensile andmodulus values of the highly loaded compounds.12

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONCONCLUSIONSNew advances in filler technology have lead to the introduction of three new products that can beused to improve the durability of tire carcasses by attacking the root causes of carcass failure:1. Hysteresis can be reduced while maintaining carbon black’s traditional level of abrasionresistance by using carbon silica dual-phase fillersHysteresis and flex fatigue resistance of key internal components such as the base stock and wireskim stock can be improved drastically via the use of Elastomer CompositesInflation pressure can be maintained at the proper level more consistently via the use of Regal 85carbon black in the tire innerliner13

IMPROVING CARCASS DURABILITY THROUGH FILLER SELECTIONREFERENCES(1) Marvin Bozarth, “The Effects of Casing Retreadability on New Tire Market Share” , ITECPaper #10 B, 1994C. G. Yurkovich, The Tire Industry Conference, by Clemson University, Hilton Head, March, 1996F. Aufauvre, International Polymer Sci. & Technol., T/20 (1999)Jeff Zupancic, The Tire Industry Conference, by Clemson University, Hilton Head, March, 2000M. B. Rodgers, S. Mezynski, ACS Rubber Division Paper #17, November 3-6, 1992Hiroyuki Kaidou and A. Ahagon, Rubber Chem. & Technol., 63,698 (1990)D. A. Paterson and K. M. Wolniak, ITEC Paper # 24A/B, 1994D. S. Tracey and G. E. Jones, ITEC Paper #13A, 1994N. Tokita, W. D. Sigworth, G. H. Nybakken, G. B. Ouyang, IRC Kyoto, Japan, 1985Roland Rauline (to Michelin & Co.), U.S. 5,227,425 (July 13, 1993)M. J. Wang, Y. Kutsovsky, P. Zhang, G. Mehos, L. J. Murphy, and K. Mahmud, Kautschuk GummiKunstoffe, 55 , 33 (2002)M. J. Wang, H. R. Tu, L. J. Murphy, and K. Mahmud, Rubber Chem. & Technology, 73, 666 (2000)M. J. Wang, Y. Kutsovsky, P. Zhang, L. J. Murphy, S. Laube and K. Mahmud, ACS Rubber DivisionPaper # 55, April 24-27, 2001P. Zhang, M. J. Wang, Y. Kutsovsky, S. Laube and K. Mahmud, ACS Rubber Division Paper # 94,October 16-19, 2001Rubber Industry Billboard in Washington, D. C. 2000Steve Butcher, “Efforts to Harmonize Tire Regulations” presented at the Clemson Tire IndustryConference, March , 2000NHTSA press release, Sept 10, 2001Michelin tire survey, April, 1999 as reported by Jeff Telman, ITEC,paper #8C, Sept 2000R. R. Juengel, D. C. Novakoski, S. G. Laube, ITEC Paper , October, 1994A. J. M. Sumner, R. Engehausen, ACS Rubber Division Paper # 185, Sept 199914