Suitability of low iodine HAF for carcass.

In order to meet the extensive severity and retreadability during service life of the carcass of a tire, the suitability of HAF black has been explored in laboratory scale. A low iodine HAF black in comparison to the general carcass black (N660) was found to have better flex fatigue resistance that plays a vital role in the performance of a tire, with the ability to bend, stretch and shear repeatedly without failure. The compound with the developed black has better elasticity, so that heat generation due to repeated flexing is not excessive during tire service. The carcass compound modulus with the developed black is higher, which avoids large stress concentrations at the cord-rubber interface due to a large difference in modulus between the tire cord and the rubber compound. This black gives better retention of aged physical properties that is required for the retreadability compared to conventional black. Keeping in view the extent of severity in the end use of a tire, the evaluation was carried out in three different polymer blends (NR, NR+BR and NR+SBR). In addition to performance requirements, certain processing parameters like processing safety and faster cure time have been accomplished with this black. With lowering iodine values in the developed black compared to normal HAF black, the material benefit/cost ratios are also optimized.

The rubber exterior of the pneumatic tire is the flesh for a skeleton of fabric. This skeleton (or carcass) and the manner in which it is constructed are of fundamental importance in establishing the tire's performance characteristics (ref. 1). The pressure of the air within the pneumatic tire acts equally in all directions. Higher inflation pressure in the tire would deform the rubber structure unless supported by the carcass. The tire carcass is made of a number of rubberized warp sheets known as plies. The number of plies and number of cords in a ply are governed by tire service conditions like load bearing capacity, operating speed, permissible deflections, operating road conditions, vehicle design, riding comfort, road holding capacity, extended tread life and lateral stability (ref. 2).

Different carcass compounds are used, depending on the severity of the application. The benefit/cost ratio is optimized for each type of tire. Low severity applications, such as passenger and farm tires, have distinctly different recipes for carcass compounds as compared to recipes for high severity applications. High severity applications require compounds having better physical properties, such as higher tensile strength, higher tear resistance and lower hysteresis. Generally, a higher natural rubber content is found in high severity applications, while low severity applications contain more synthetic rubber. A more reinforcing carbon black (a tread black) is found in high severity applications, while low severity applications usually contain N660 black. Due to major growth in the development of radial tires, the performance expectation of individual tire compounds is higher compared to cross-ply compounds. Since the carcass compound plays a major role in the reinforcement contribution of a tire, the compounding aspects have to be improved in order to satisfy extensive severity conditions.

The present work aims to use a newly developed N330 black that has smaller particle size and moderate reinforcement compared to conventional carcass black (N660). Evaluation of the cost/performance properties like flex fatigue and heat build-up of carcass compounds have been studied by lowering the iodine number value from 84 (for N330) to 58 mg/gm in different polymer blends (NR, NR+BR and NR+SBR) in consideration of the severity of the final product requirements.

Experimental

Six carbon blacks (CB1 to CB6) were characterized physico-chemically following the ASTM Standard Methods. Table 1 shows the major properties of all the carbon blacks along with the test methods adopted for the analysis.

The selections of the formulations have been done on the basis of the common carcass formulations being practiced, as well as from the literature available, depending on end-use requirements. Details of the formulations and mixing conditions are given in table 2. We have used a 1.5 L internal mixer for both stages (masterbatch and final batch) of mixing, and final sheeting was done using an open two-roll mill.

Curing of the compounds was done after eight hours of cooling at room temperature (23 [+ or -] 3[degrees]C) in a 180 mt curing press using hard chrome plated molds of 152 x 152 x 1.90 mm size, as per ASTM D412.

The following are the basic equipment used for the physicochemical and compound characterization of carbon blacks and rubber vulcanizates: OAN/COAN (Brabender OAN machine model E, Germany, with DADS software from Hitec, Luxembourg), [N.sub.2]SA (Quantachrome, U.S.), aggregate size (Bi-DCP, Brookhaven Instruments, U.S.), tint (Erichsen Tint Tester, Germany), Mooney viscometer, (MV 2000, Alpha Tech. U. S.), Rheometer (MDR 2000, Alpha Tech. U.S.), tensile tester (Zwick Z010, Germany), Rebound Resilience (Zwick 5109, Germany), heat build-up at different temperatures (Goodrich Flexometer, Model II, U.S.) and fatigue to failure tester (Monsanto).

Results and discussion

The test results for the six carbon black colloidal properties have been compiled in table 1. From the table, it can be seen that CB6 is N660, and CB 1 to CB5 belong to the N330 series with varying iodine levels from 58 to 84 mg/g. The tint for all the HAF blacks is almost in an identical range, showing their uniformity in aggregate size. Both OAN and COAN are within the ASTM specified limits. The results will be discussed in detail.

The size of the aggregates in carbon black can be understood from many measurements, like iodine adsorption (ASTM D1510), nitrogen surface area (ASTM D6556), CTAB surface area measurement (ASTM D3765), aggregate size analysis (laser aggregate size analyzer) and tint strength (ASTM D3265).

Iodine adsorption number (IAN)

Iodine adsorption (expressed in mg/gm of carbon black) measures the amount of iodine, which can be adsorbed from a potassium iodide solution, on the surface of a given mass of carbon black. The iodine adsorption number is a primary indication of surface area for defining different carbon blacks. In the present study, it varies from 58 to 84 mg/gm for different HAF blacks; and for N660 it is 36.2 mg/gm.

Nitrogen surface area ([N.sub.2]SA)

Nitrogen surface area is a measurement of the amount of nitrogen that can be adsorbed on a given mass of carbon black, forming a monolayer. High surface area is associated with a high level of reinforcement and high surface energy, but at the expense of more difficult dispersion, processing and increased hysteresis. Nitrogen surface areas of the HAF blacks varied from 69 to 77 [m.sup.2]/gm. It is interesting to note that, although the iodine value for CB1 is very low (58.5), the [N.sub.2]SA value is very high, indicating its high surface activity.

Aggregate size

Clusters of fused particles of carbon black make up aggregates. The distribution of aggregates and the average aggregate size play major roles in determining the utility of a given carbon black grade in a rubber formulation. Smaller aggregates contribute to higher rubber reinforcement, but with difficult dispersability and lower resilience. Since aggregate size or surface area is the primary determinant of reinforcement, a black with larger aggregate size than HAF black was developed based on attaining a level of reinforcement without affecting important tire properties like tensile strength and tear resistance, etc. In the present study, the average aggregate size ([d.sub.50]) for the newly developed CB1 black has comparatively larger aggregate size (81 nm) compared to regular HAF blacks (68-75 nm) and lower compared to N660 (190 nm).

CTAB surface area

The CTAB surface area test was designed to overcome the problems with the iodine number and nitrogen surface area tests, in that the molecule to be adsorbed, cetyl-trimethyl-ammonium bromide, is far larger than the nitrogen molecule and is also a very effective wetting agent. The present study showed a good agreement of CTAB values with the aggregate size distribution, as observed by S.S. You and S.K. Choi (ref. 3).

The compounding formulations used for the present study and the mixing sequences are shown in tables 2 and 3, respectively.

Compound properties

For the evaluation of carcass compounds, the following performance properties were considered for all the HAF carbon blacks (CB1 to CB5) along with N660 (CB6).

Rheological properties

The rheological properties (figure 1) such as t5, t35 and t90 play a major role in deciding the processing of rubber compounds in downstream operations, like extrusions and calendering. Hence, it is necessary to have more scorch safety (t5 and t35). For improved productivity, fast curing of rubber compounds is required, which is achieved by low t90. The CB1 experimental black met these requirements, as shown in figure 1.

[FIGURE 1 OMITTED]

Modulus

Modulus is an expression of the force per cross-sectional unit area required to stretch a test piece to a given elongation. Modulus of the compound is dependent on crosslinking. The filler structure, both primary and secondary, controls the crosslinking when keeping other variables constant. OAN is an expression of both primary and secondary structure (ref. 4), but COAN reflects only the primary structure and has more relevance for modulus. The modulus test was carried out using a Zwick tensile tester as per ASTM D412.

The carcass compound modulus or stiffness must be high enough to avoid a large stress concentration at the cord-rubber interface, due to the large difference in modulus between the tire cord and the rubber compound. The higher the modulus differential between the cord and the carcass coat compound, the greater the stress concentrations at the cord-rubber interface. As expected, the modulus of HAF blacks (irrespective of IAN value) was higher compared to the N660 grade, irrespective of the formulation used.

Flex fatigue resistance

It is known that the two major parameters that affect carcass life are heat generation and flexing. These properties are inter-related, and flexing causes excessive heat to build-up in the tire carcass components (refs. 5 and 6). As the carcass deflects when the tire rolls down the road, the carcass coat must be able to bend, stretch and shear repeatedly without cracking. A high mileage tire may flex more than 100 million times during its service life. Flexing can lead to crack initiation and propagation in rubber compounds, especially at initiation points caused by improperly dispersed compound ingredients. As a tire rotates, the sidewall is deformed, and this deformation is very high when a vehicle comers hard or if the vehicle is driven with an under-inflated tire.

Fatigue measurements were made using a Monsanto fatigue to failure tester. The tests were carried out with specimens having no central cut, employing the procedures defined by Monsanto in the operation and service manual. Six specimens of each compound were tested and the average of these values was reported in kilocycles (KC). Results for all compounds were plotted in figure 2. It has been observed that the fatigue resistance with the experimental black (CB1) was better than conventional N660 black (CB6) in high severity applications (NR recipe). There was an improvement of 20% in flex fatigue of the experimental black in the NR recipe. Radialization has brought a change in carcass composition in favor of natural rubber because of the need for fatigue resistance at higher strength; and NR is again favored in the shaping of radial tires, because of its intrinsically high green strength.

[FIGURE 2 OMITTED]

In crystallizing rubbers, tearing generally occurs in a stick-slip manner, with the force increasing until a rapid failure point is reached. A tear test was carried out as per ASTM D624 using a type C die. A good correlation of tear strength with the FTFT values of the different formulations was noticed (figures 2 and 3). Tear strength with the experimental black (CB1) was found to be better than CB6 in all three formulations tested (figure 3), indicating its better suitability for the carcass compound of the tire, especially in high severity application areas.

[FIGURE 3 OMITTED]

Heat generation

The internal temperature of a tire during operation can easily reach 100[degrees]C. Belt edge temperatures can reach well over 100[degrees]C (ref. 7). At these high temperatures, aging of the unsaturated elastomers occurs relatively rapidly (ref. 8), and the permeability of the innerliner compound is affected (ref. 9), allowing intra-carcass pressure to increase further. This will result in a "double whammy" on the carcass compounds. The specimens were tested using a Goodrich flexometer as per the method ASTM D623, and the tests were carried out at different test temperatures, such as 23[degrees]C, 60[degrees]C and 100[degrees]C. But by considering the practical applicability, discussions are limited to 60 and 100[degrees]C only. During flexing, the carcass compound is deformed repeatedly. When the deforming force is removed, the carcass coat must be elastic and return to its un-deflected shape. Also, the compound must be elastic so that heat generation is not excessive during tire service. Excessive heat generation can reduce the strength of the compound and cause failure.

Compounds mixed with experimental black showed comparable heat build-up values, tested at 60[degrees]C and 100[degrees]C, to that of conventional black in all test formulations (figure 4). Although heat build-up is directly related to surface area of the carbon black and its loading, in the present study at identical loading the heat build-up values for the low iodine HAF blacks (CB1 and CB2) were comparable to N660 (although a little higher by 4-5[degrees]C) in all formulations studied. The surface area range (36-58 IAN) between CB1 and CB6 that was covered in the present work did not show a significant beating on heat build-up, indicating CB1 is suitable for carcass.

[FIGURE 4 OMITTED]

Adhesion to tire cord

It is well proven that excellent adhesion between the cord and the compound is extremely important, as slackness in adhesion can result in catastrophic tire failures.

Adhesion tests (ASTM D 4776) were carried out with all compounds, but the test results will not be discussed due to poor reproducibility and repeatability. However, it was found that there was no deterioration in compounds mixed with experimental black (CB1) compared to conventional black (CB6).

Degradation resistance

During service, the tire gets heated up and becomes exposed to oxygen. The compounds must be resistant enough to the degradative effects of heat and oxygen, which can ultimately cause tire failure. Ignatz-Hoover and To (ref. 10) suggested that compound reversion is a major factor affecting the tire carcass life cycle. The results of this are most often seen in tread separations or blowouts. The greatest heat development occurs at the mid-section of the shoulder and then extends downward into the tread/undertread-carcass interface, where tread separations can occur.

Considering the extensive severity and retreadability during service life of carcass compounds, aging tests were carried out at different test conditions, such as 70[degrees]C for 24, 72 and 96 hours. Properties like 100% modulus, tensile strength and elongation at break were measured. Aging properties of the carcass compounds are very important, because this will indicate the extent of retreadability and failures under extended service life. For easy understanding and the comparison of aging properties, the absolute values have been converted to an index, taking the unaged values in each formulation of N660 (CB6) as 100.

100% modulus

In all formulations, it was observed that the experimental black had superior retention of modulus values under different aging conditions (figure 5). Within the HAF series (CB1 to CB5), the decrease of IAN value did not show any specific trend with respect to retention of aging value, but the percentage of rate of retention of all carbon blacks varies from formulation to formulation. It has been observed that the experimental black had a higher percentage of retention of 100% modulus in the NR+SBR formulation, followed by NR and NR+BR.

[FIGURE 5 OMITTED]

Tensile strength

As the term implies, it is the strength borne by the crosslink network under uniaxial tension. This is related to strength added to the rubber chains through crosslinks and filler reinforcement. The crosslinkable sites in the rubber control crosslink bridges. Filler reinforcement depends on rubber-filler interactions, wettability, surface chemistry, etc. When the crosslinked network is strained, if tension is uniformly distributed (as in the case of polysulfidic linkages or filled with high surface area carbon black), then tensile strength is increased. In the same network, upon aging (under thermal influence), polysulfidic links are broken (the bridged network is broken), bringing down the tensile value. Tensile strength index values, before and after aging at different time intervals, for all six carbon black samples in the three recipes are shown in figure 6. For tensile values, a trend similar to modulus was noticed. In the NR+SBR and NR test recipes, the percentage retention of experimental black CB1 showed higher values in comparison with N660 in all test conditions. Possible reasons might be due to lower aggregate diameter, as well as high surface activity of the experimental black CB1 compared to conventional black N660.

[FIGURE 6 OMITTED]

Elongation

Elongation at break is defined as the ability of the crosslinked network to be stretched before rupture. This is controlled by the type and density of crosslinks, nature of filler and its content. If the mobility of rubber chains is restricted by any of these factors, then elongation would be affected. On aging, either with formation of additional crosslinks or breakdown of weak crosslinks, the network is affected. It has been found that on aging (under thermal influence) the change in elongation is accelerated. Of all the physical properties, elongation is found to vary linearly with thermal input. Elongation at break of the unaged compound with the experimental black was found to be comparable to N660. However, the retention percentage of the experimental black was better (figure 7), which is more important for repeated retreadability.

[FIGURE 7 OMITTED]

Summary

The main objective of the present work was to find an alternate black for high severity application of tire carcass due to global radialization, where the conventional carcass black (N660) has performance limitations like flex fatigue, modulus and retention of aged properties under service conditions including the retreadability. The purpose of selecting the N330 black is based on techno-commercial reasons and that it is a black with improved modulus in the hard black series. Also, it was kept in the mind that the surface area of the black directly controls the heat build-up of the compound. Therefore, the iodine value of N330 was reduced from 84 to 58 to determine the impact on the characteristic performance requirements of a carcass compound. Evaluations of the blacks (CB1 to CB6) were carried out in different polymer blends, taking into consideration the severity of the applications.

The experimental black (CB1) had better scorch safety and cure behavior in comparison with the conventional N660 grade (CB6) in all the tested formulations, which will yield better productivity. As required, the modulus of the experimental black was higher than the conventional black. Fatigue resistance of the experimental black (CB1) was better than the conventional black (CB6) in high severity applications (an improvement of 20% in flex fatigue in the NR recipe). Fatigue resistance was found to have a positive correlation with the tear strength of the compounds. Heat build-up of the low iodine HAF (CB1) was a little higher (4-5[degrees]C) in comparison to CB6 in all formulations studied. The adhesion properties were better for the experimental black. The better retention of aged stress-strain properties has clearly established the superiority and suitability of the experimental carbon black CB1 over the conventional black.

References

(1.) L.J.K. Setright, Automobile Tyres, Ch. 1, "The carcass," Chapman & Hall, 1972.

(2.) F.J. Kovac, Tire Technology, 5th Ed, Goodyear Tire & Rubber Co., Ch. 1, 1978.

(3.) S.S. You and S.K. Choi, "A new characterization method of tread carbon black by statistical regression treatment," DC Chemical Co. Ltd., Korea.

(4.) J. Frohlich, et al., "The effect of filler-filler and filler-elastomer interaction on rubber reinforcement," Applied Science & Manufacturing, Composites, Part A, pp. 1-12, 2004.

(5.) D. C. Novakoski and J.A. Shell, "Getting more truck tire mileage with advanced filler technology," paper 48, Rubber Division, ACS, 163rd Technical Meeting, April 2003.

(6.) S. Laube and J.A. Shell, "Improving carcass durability through filler selection," Rubber Division, ACS, April 2003.

(7.) M. Bozarth, "The effects of casing retreadability on new tire market share," ITEC 1994, paper 10-B, 1994.

(8.) Y. Bomal, Ph. Cochet, B. Dejean, I. Gelling and R. Newell, Kautschuk Gummi Kunststoffe, 51, p. 261 (1998).

(9.) H. Kaidu and A. Ahagon, "Aging of tire parts during service. Part II. Aging of belt-skim rubbers in passenger tires," Rubber Chemistry and Technology, 63, p. 698 (1990).

(10.) F. Ignatz-Hoover and B.H. To, "Increased truck tire durability through compound modifications," paper presented at the 155th Technical Meeting, Rubber Division, ACS, April 1999.

by D. Mahapatra, B. Arun and V. Taneja, Hi-Tech Carbon

Table 1 - physico-chemical properties

Parameters                 Test method

Iodine No., mg/g           ASTM D1510
[N.sub.2]SA, [m.sup.2]/g   ASTM D6556
CTAB, [m.sup.2]/g          ASTM D3765
Tint, %ITRB                ASTM D3265
[DELTA] d50, nm
DBPA No., cc/100gm         ASTM D2414
CDBPA No., cc/100gm        ASTM D3493

Parameters                            Experimental HAF
                               CB1          CB2       CB3

Iodine No., mg/g                58.50     68.30       75.80
[N.sub.2]SA, [m.sup.2]/g        69.90     70.90       76.90
CTAB, [m.sup.2]/g               61.60     66.50       70.03
Tint, %ITRB                    100.30    100.60      101.60
[DELTA] d50, nm                 81        72          68
DBPA No., ocl100gm             100.00    103.70      101.80
CDBPA No., cc/100gm             87.55     86.20       89.60

Parameters                         Regular HAF        N660
                                 CB4       CB5        CB6

Iodine No., mg/g                83.30     84.60       36.20
[N.sub.2]SA, [m.sup.2]/g        77.80     77.70       36.40
CTAB, [m.sup.2]/g               77.80     77.30       34.20
Tint, %ITRB                    102.30    102.30       35.00
[DELTA] d50, nm                 75        72         190
DBPA No., ocl100gm             101.60    102.55       90.30
CDBPA No., cc/100gm             87.70     85.45       72.50

Table 2 - detailed formulation

Ingredients, phr                  NR + BR
Formulation no.   F1       F2      F3      F4      F5      F6

CB: CB1            50      --      --      --      --      --
CB: CB2            --       50     --      --      --      --
CB: CB3            --      --       50     --      --      --
CB: CB4            --      --      --       50     --      --
CB: CB5            --      --      --      --       50     --
CB: CB6            --      --      --      --      --       50
NR                 50       50      50      50      50      50
BR                 50       50      50      50      50      50
SBR 1712           --      --      --      --      --      --
Zinc oxide        3.5      3.5     3.5     3.5     3.5     3.5
Stearic acid      2.0      2.0     2.0     2.0     2.0     2.0
TDQ A.O.           --      --      --      --      --      --
6PPD A.O.         1.0      1.0     1.0     1.0     1.0     1.0
Aromatic oil      3.0      3.0     3.0     3.0     3.0     3.0
CBS                --      --      --      --      --      --
Sulfur            2.0      2.0     2.0     2.0     2.0     2.0
TBBS              0.8      0.8     0.8     0.8     0.8     0.8

Ingredients, phr               NR + SBR
Formulation no.   F7       F8      F9     F10     F11     F12

CB: CB1            50      --       --     --      --      --
CB: CB2            --       50      --     --      --      --
CB: CB3            --      --       50     --      --      --
CB: CB4            --      --       --      50     --      --
CB: CB5            --      --       --     --       50     --
CB: CB6            --      --       --     --      --       50
NR                 70       70      70      70      70      70
BR                 --      --       --     --      --      --
SBR 1712         41.3     41.3    41.3    41.3    41.3    41.3
Zinc oxide        4.0      4.0     4.0     4.0     4.0     4.0
Stearic acid      2.0      2.0     2.0     2.0     2.0     2.0
TDQ A.O.           --      --       --     --      --      --
6PPD A.0.         1.0      1.0     1.0     1.0     1.0     1.0
Aromatic oil       --      --       --     --      --      --
CBS                --      --       --     --      --      --
Sulfur            2.5      2.5     2.5     2.5     2.5     2.5
TBBS              0.9      0.9     0.9     0.9     0.9     0.9

Ingredients, phr                     NR
Formulation no.  F13      F14     F15     F16     F17     F18

CB: CB1            50      --       --     --      --      --
CB: CB2            --       50      --     --      --      --
CB: CB3            --      --       50     --      --      --
CB: CB4            --      --       --      50     --      --
CB: CB5            --      --       --     --       50     --
CB: CB6            --      --       --     --      --       50
NR                100      100     100     100     100     100
BR                 --      --       --     --      --      --
SBR 1712           --      --       --     --      --      --
Zinc oxide        5.0      5.0     5.0     5.0     5.0     5.0
Stearic acid      1.0      1.0     1.0     1.0     1.0     1.0
TDQ A.O.          1.5      1.5     1.5     1.5     1.5     1.5
6PPD A.O.         2.0      2.0     2.0     2.0     2.0     2.0
Aromatic oil      4.0      4.0     4.0     4.0     4.0     4.0
CBS               1.0      1.0     1.0     1.0     1.0     1.0
Sulfur            2.5      2.5     2.5     2.5     2.5     2.5
TBBS               --      --       --     --      --      --

Table 3 - mixing conditions

Start temp: 50[degrees]C, RPM 70, fill factor 80%
First Stage:             Order of addition
0 - 01:00 min:sec        Add rubber
01:01 - 02:00            1/2 CB + ZnO + S. acid
02:01 - 03:00            Add remaining CB + oil
03:01 - 03:30            Sweep
03:40                    Dump and sheet it out

Dump temp: 125[degrees]C - 130[degrees]C
Maturation period. 2 hrs at 25 [+ or -] 3[degrees]C
Second Stage:            Order of addition
0 - 00:30 m:s            Warm master batch
00:31 - 01:30            Add curatives
01:31 - 01:40            Sweep
01:45                    Dump and sheet it out

Dump temp: 95[degrees]C - 100[degrees]C
Cool at room temperature (25 [+ or -] 3[degrees]C)