Cure system and carbon black effects on NR compound performance in truck tires.

There are many different strategies in designing a rubber compound in order to meet end-product performance requirements. In general, the rubber and filler are the two major ingredients in a rubber compound and have the most dominant impact on compound performance. The selection of type of rubbers, type and amount of fillers and blending of more than two rubbers and/or fillers are critical considerations. Although other ingredients are relatively minor components, they do have their specific functions, and they act either as processing aids, antioxidants, antiozonants or anti-aging reagents. They often also show the side-impact on compound performance, which can be desirable or undesirable. However, unlike other minor ingredients, the cute package (curatives and accelerators) is necessary and essential for forming the rubber network, and it gives the unique elastomeric properties from other materials such as plastics, metal and ceramic, etc. Since the cure package controls the rubber network structure, its role is also critical to the compound performance.

A sulfur-based cure package is still the most widely used method for crosslinking most rubber compounds. It is well known that the ratio of sulfur to accelerator in sulfur vulcanization systems has the significant influence on the rubber network structure. Depending on the ratio, the sulfur vulcanization can be classified into three types: Conventional, semi-efficient and efficient systems (refs. 1-3). Each system can result in a network structure with different combinations of poly-sulfidic, di-sulfidic and mono-sulfidic bonds, which have a significant effect on typical compound properties such as tensile, fatigue, reversion and aging, etc.

For the rubber compounds used in truck/bus tires, the trade-off challenge is to optimize the performance of tread wear, heat build-up and cut-and-chip. Due to superior physical strength, NR is the most common rubber used in truck/bus tread compounds. Reinforcing carbon blacks such as N234, N220 and N231 with various loadings are used to enhance the treadwear and cut-and-chip resistance. There has been a number of papers written on how to optimize overall truck/bus tread compounds by manipulating the carbon black loading and type (refs. 4 and 5). This study will investigate how the cure package can play a role in a compounding trade-off strategy.

Although the effects of the sulfur caring network structure on NR compound performance has been reported previously (refs. 1-3 and 6), the results were limited. The key performance requirements for truck/bus tires such as tread wear, heat build-up and cut-and-chip were not reported in their studies. This study will investigate the combination of cure system and carbon black morphology impact on NR compound properties, particularly on abrasion, were; cut-and-chip and heat built-up, which are the important performance requirements in truck tire applications.

Experimental

Experimental design and materials

As shown in table 1, the sulfur to accelerator ratio generally classifies cure systems as conventional, semi-efficient and efficient systems (refs. 1-3). In this study, three ratios of sulfur to accelerator (5, 1 and 0.1) were selected to represent conventional, semi-efficient and efficient cure systems, respectively. The most commonly used carbon blacks in truck/bus tire compounds (N234, N220 and N231) at three different loadings were included. As shown in table 2, a generic truck/bus NR formulation was selected in this study with different combinations of three cure systems, three types of carbon blacks and three levels of carbon black loadings. A total of 27 compounds was prepared according to a four stage mixing procedure (ref. 7).

Compound testing

The network structure (types and amount of mono-sulfidic, di-sulfidic and poly-sulfidic crosslinks) has been characterized by the method described in reference 7. Curing characteristics of unvulcanized rubber compounds were measured by an oscillating disk rheometer (ASTM D 2084). Mooney scorch of the rubber compound, T5, was measured using a Mooney viscometer (ASTM D 1646).

Compound hardness was measured by an automated A2 durometer (ASTM D 2240). Tensile properties were characterized by using a six-station tester. Six samples of each compound were tested. The 100% and 300% moduli, tensile strength and elongation at break were reported. Tear strength at both room temperature and 100[degrees]C using a tensometer (ASTM D624-54 modified and ASTM D624 modified).

An automated rebound tester was used to measure the compound rebound at room temperature (DIN 53512/ISO 4662). Dynamic properties were characterized using a servo-hydraulic tester. A typical temperature sweep from -20[degrees]C to 100[degrees]C was conducted at 10 Hz and 2% strain. Tan [delta] at 60[degrees]C was reported in this study for comparison.

The compound crack growth rate was measured using a rotary flexing fatigue tester (ref. 8). Seventy-two samples can be simultaneously tested under the same bending angle. In this study, 12 specimens of each compound were tested at a 45-degree flexing angle at a temperature of 100[degrees]C.

The compound heat build up characteristics were measured by using a computerized flexometer (ASTM D623) (ref. 9). The temperature rise of the rubber compound at the surface was monitored at 30 Hz and 212[degrees]F chamber temperature. The temperature at the center of the tested piece was also probed at the end of the test.

An angle abrader was used for characterizing compound wear resistance. The test was conducted at a 20-degree slip angle with 3 kg load at room temperature. Three sample wheels were prepared for each compound, and each sample wheel was abraded consecutively through two angle abraders to improve the data consistency. Wear rate was calculated based on the volume loss per revolution.

A custom made "off-the-road service simulator" (OTR-SS) (ref. 10) was used for evaluating the compound cut-and-chip resistance. The basic idea behind the OTR-SS is to produce cutting and chipping on a pneumatic tire operating under both vertical and breaking loads. Testing compounds were put on retreaded tires. In general, three compounds (sections) are on a testing tire. One of the compounds is the control compound. Each compound is run in triplicate on three different three-section tires. Statistical analysis was performed to analyze the cuts and chips on the tread block edges and surfaces.

Results and discussion

Crosslink structure of NR matrix

The crosslink structure of all 27 compounds was characterized by the method outlined in reference 7. The total crosslink density, and the percentage of mono-, di- and polysulfide are summarized in table 3. As expected, for NR compounds with the efficient cure system (samples 007-1 to 007-9), most of the crosslinks are mono-sulfides (average 65.13%): while only an average of 9.85% crosslinks are polysulfides, and 25.01% crosslinks are di-sulfides, On the other hand, for NR compounds with the conventional cure system (samples 007-19 to 007-27), only an average of 14.78% crosslinks are mono-sulfides: while an average of 64.12% crosslinks are polysulfides, Compounds with a semi-efficient cure system (samples 007-10 to 007-18) have a more balanced structure with 14.3% mono-sulfides, 31.2% di-sulfides and 54.49% polysulfides respectively. Also, the data indicate that carbon black type and loading has an insignificant effect on crosslink structure of the NR matrix. The total crosslink density of these three cure systems is very similar. However, as discussed in the following, NR compound properties can be significantly influenced by the network structure of the rubber matrix.

Cure characteristics

As shown in figure 1, compounds with the efficient cure system have the shortest Mooney scorch safety, T5. The compounds with the conventional cure system have shown the longest scorch safety. In general, T5 decreases with increasing carbon black loading for all of the three cure systems. Cure rate index (CRI) was calculated based on t90 and ts1. The higher CRI means the faster cure rate. It was unexpected that compounds with the semi-efficient cure system show the fastest cure rate. The efficient cure system has the slowest cure rate. The trend seems consistent for compounds with different types of carbon blacks and loadings. Depending on the ratio of sulfur and accelerator, it is suspected that there may be a maximum cure rate. The reversion phenomenon was observed for compounds with the conventional cure system (refs. 1-3 and 6). This is due to the breakdown of polysulfidic crosslinks at high temperature.

[FIGURE 1 OMITTED]

Hardness

Although compound hardness is most effectively controlled by adjusting the carbon black loading and type, the cure system does show its influence on compound hardness, as shown in figure 2. In general, the compounds with the semi-efficient cure system have the higher compound hardness; while the conventional cure system tends to have lower compound hardness. The difference can be up to five points.

[FIGURE 2 OMITTED]

Tensile properties

As shown in figure 3, compounds with the semi-efficient cure system show the highest 300% modulus (E300) among the three systems, which is consistent with the trend of compound hardness discussed above. The E300 of the efficient and conventional cure systems is similar. Tensile strength of compounds with the efficient cure system is much lower than that of compounds with the conventional and semi-efficient cure systems, as shown in figure 4. This is probably due to the fact that the majority of crosslinks of the efficient cure system are mono-sulfides, which are less flexible than either di- or polysulfides. For the same reason, the elongation-at-break (EB) of compounds with the efficient cure system is the lowest among these three cure systems.

[FIGURES 3-4 OMITTED]

Tear strength

The tear strength at room temperature and 100[degrees]C was measured. In general, tear strength at room temperature is higher than at 100[degrees]C. The tear strength of compounds with the efficient cure system is much lower than the other two. Again, it is due to the high percentage of mono-sulfide crosslinks in the rubber matrix of compounds with the efficient cure system, which are less flexible than either di- or polysulfides.

Rebound

Although compounds with the conventional cure system tend to show lower compound rebound, there is no significant difference in compound rebound at room temperature for these three cure systems. The difference is within 3-4 points at most. As expected, the rebound decreases with increasing carbon black loading.

Hysteresis (tan [delta] @ 60[degrees]C)

The tan [delta] increases with increasing carbon black loading. The tan [delta] of compounds with conventional and semi-efficient cure systems is very similar. However, compounds with the efficient cure system show higher tan [delta] than the other two systems. It implies that the crosslink structure of compounds with the efficient cure system in this study is most hysteretic, which is unexpected. The real reason needs to be further investigated. There is no significant difference between compounds with either the conventional or semi-efficient cure system.

Crack growth rate

The compound crack growth rate was measured using a rotary flexing fatigue tester. Twelve specimens of each compound weir tested at 100[degrees]C and a 45 degree flexing angle. As shown in figure 10, the carbon black loading and type have no significant effect on flexing fatigue crack growth rate in this study. On the other hand, a cure system can have the most significant impact on flexing fatigue crack growth rate. The crack growth rate of compounds with the efficient cure system is about twice as fast as that of the conventional and semi-efficient cure systems. Again, it is due to the majority mono-sulfide crosslinks in the rubber matrix of compounds with the efficient cure system, which are less flexible than either di- or polysulfides. Also, the crack growth rate of compounds decreases slightly with increasing carbon black loading.

[FIGURE 5 OMITTED]

Heat build-up and compression set

The temperature rise at the surface and the maximum temperature at the center of the tested sample characterized the compound heat build-up. As shown in figure 6, the temperature rise of the compounds with the conventional cure system is much higher than compounds with the efficient and semi efficient cure systems at high carbon black loading. The impact is less pronounced for carbon black N234. The temperature rise of compounds with efficient and semi-efficient systems is similar. Furthermore, the temperature rise of NR compounds increases with increasing carbon black loading. The similar trend for the maximum temperatures was observed. The flexometer also measured the permanent compression set. Compounds with the conventional cure system show a much higher compression set than the compounds with the other two cure systems. Compounds with the efficient cure system have the lowest compression set among three cure systems. This implies that mono-sulfidic crosslinks are much more stable than polysulfidic crosslinks under dynamic thermal aging conditions. In general, the compression set increases with increasing carbon black loading due to the high degree of breakdown of the filler-filler network at a higher carbon black loading.

[FIGURE 6 OMITTED]

Abrasion wear

As shown in figure 7, compound abrasion wear volume loss decreases with increasing carbon black loading within the range studied. Compounds with the efficient cure system show the highest abrasion wear loss, white compounds with the conventional cure system demonstrate the best wear resistance among these three systems. This trend seems consistent with the other destructive properties such as tensile strength, tear strength and crack growth rate. The polysulfidic crosslinks are more flexible to absorb the "fracture energy." In general, at the same carbon black loading, compounds with the carbon black N234 have the better wear resistance than the other two carbon blacks. Wear resistance of the carbon black N231 is the worst. This is probably due to the relatively lower carbon black structure and poor dispersion quality of carbon black N231.

[FIGURE 7 OMITTED]

Cut-and-chip resistance

The cut-and-chip resistance was evaluated for each carbon black in two ways:

* Cure system effect at 55 phr of carbon black loading; and

* carbon black loading effect with the semi-cure system.

The cure system effect on compound cut and-chip resistance is complicated, as shown in table 4. For compounds containing carbon black N234 at 55 phr loading, the efficient cure system has the worst cut-and-chip resistance, while the conventional system is the best. However, the difference between semi-efficient and conventional cure systems is not significant. For compounds containing carbon blacks N220 and N231, the cure system effect on cut-and-chip is not obvious. As shown in table 5, for the N234 carbon black, the higher carbon black loading seems better in cut-and-chip resistance. However, for both blacks. N220 and N231, the cut-and-chip resistance seems better at the lower carbon black loading. According to these results, the compound cut-and-chip resistance is affected by the carbon black type, carbon black loading and cure system in a complicated manner.

Summary

The cure system influence on NR compound properties is summarized in table 6. The results are based on the consistent trends observed from this study on NR compound performance with different carbon black types and loadings. With the exception of the compound cut-and-chip resistance (not included in the table), the trends of cure system impact on compound performance are not affected by carbon black type or loading. The conventional cure system shows the long scorch safety, medium cure rate, but with reversion. The efficient cure system has the shortest scorch safety, and slowest cure rate. The semi-efficient cure system demonstrates the fastest cure rate among these three systems. Both efficient and semi-efficient do not show the reversion behavior.

For the compound performance, the conventional cure system offers the better abrasion wear resistance, tensile properties, tear strength, crack growth resistance and lower hysteresis. but with poor compression set and heat build-up resistance. On the other hand, the efficient cure system behaves almost opposite to the conventional system. The semi-efficient system seems to show the most balanced overall performance. Comparison of compound performance due to different cure systems is shown in figure 8 for a compound with carbon black N234. Similar results were observed for compounds with carbon blacks N220 and N231.

[FIGURE 8 OMITTED]

As expected, the crosslink structure of the NR matrix plays an essential role to the above discussed cure characteristics and compound performances. For most of the properties with a destructive nature, such as tensile strength, tear strength, crack growth rate and abrasion wear, compounds with the efficient cure system demonstrate the poor performances, and this probably is due to the high portion of mono-sulfidic crosslinks in the rubber matrix.

Table 1 - sulfur and accelerator of different
vulcanization systems for NR

                      Sulfur (phr)   Accelerator (phr)   S/A ratio

Conventional (C)        2.0-3.5           1.2-0.4          10-1.5
Semi-efficient (SE)     1.0-1.7           2.5-1.2         1.5-0.4
Efficient (E)           0.4-0.8           5.0-2.0         0.4-0.08

Table 2 - NR formulations with different carbon blacks and cure systems

Cure system           Conventional

NR (RSS1)       100        100        100
N234          45/50/55
N220                     50/55/60
N231                                55/60/65
ZnO             4          4          4
SA              2          2          2
TMO             1.5        1.5        1.5
6 PPD           1.5        1.5        1.5
Wax             2          2          2
TBBS            0.5        0.5        0.5
Sulfur          2.5        2.5        2.5

Cure system           Semi-efficient

NR (RSS1)       100        100        100
N234          45/50/55
N220                     50/55/60
N231                                55/60/65
ZnO             4          4          4
SA              2          2          2
TMO             1.5        1.5        1.5
6 PPD           1.5        1.5        1.5
Wax             2          2          2
TBBS            1.5        1.5        1.5
Sulfur          1.5        1.5        1.5

Cure system              Efficient

NR (RSS1)       100        100        100
N234          45/50/55
N220                     50/55/60
N231                                55/60/65
ZnO             4          4          4
SA              2          2          2
TMQ             1.5        1.5        1.5
6 PPD           1.5        1.5        1.5
Wax             2          2          2
TBBS            5          5          5
Sulfur          0.5        0.5        0.5

Table 3 - crosslink structure of rubber matrix in NR
compound with different cure systems
(C: conventional, SE: semi-efficient, E: efficient),
carbon black types and loadings

                                Total     Mono-     Poly-       Di-
                             crosslink   sulfide   sulfide   sulfide
Sample    Description        (Mmole/g)      (%)       (%)       (%)

007-1     N234, 45 phr, E      0.10624     68.09      7.59     24.32
007-2     N234, 50 phr, E      0.11731     64.01     10.13     25.86
007-3     N234, 55 phr, E      0.12464     65.13     13.50     21.37
007-4     N220, 50 phr, E      0.10428     65.00      9.30     25.70
007-5     N220, 55 phr, E      0.10884     66.36     11.20     22.43
007-6     N220, 60 phr, E      0.11194     64.77      9.50     25.73
007-7     N231, 55 phr, E      0.09637     61.81      9.77     28.42
007-8     N231, 60 phr, E      0.09986     65.94      9.48     24.58
007-9     N231, 65 phr, E      0.10365     65.09      8.20     26.71
Average                        0.10813     65.13      9.85     25.01
SD                             0.00828      1.71      1.72      2.14
007-10    N234, 45 phr, SE      0.1223     12.36     51.17     36.47
007-11    N234, 50 phr, SE      0.1326     13.39     54.61     32.00
007-12    N234, 55 phr, SE      0.1383     14.56     55.68     29.76
007-13    N220, 50 phr, SE      0.1180     13.18     54.58     32.25
007-14    N220, 55 phr, SE      0.1228     14.60     54.19     31.21
007-15    N220, 60 phr, SE      0.1321     15.37     54.48     30.15
007-16    N231, 55 phr, SE      0.1112     13.86     54.05     32.08
007-17    N231, 60 phr, SE      0.1108     15.23     52.96     31.81
007-18    N231, 65 phr, SE      0.1082     16.22     58.70     25.08
Average                        0.12229     14.31     54.49     31.20
SD                             0.01068      1.22      2.03      2.99
007-19    N234, 45 phr, C       0.1163     14.29     63.82     21.89
007-20    N234, 50 phr, C       0.1270     14.75     57.98     27.27
007-21    N234, 55 phr, C       0.1347     17.53     61.52     20.94
007-22    N220, 50 phr, C       0.1086     14.72     66.16     19.12
007-23    N220. 55 phr. C       0.1213     14.11     64.25     21.64
007-24    N220, 60 phr, C       0.1168     15.56     64.35     20.09
007-25    N231, 55 phr, C       0.1025     13.57     66.24     20.19
007-26    N231, 60 phr. C       0.0966     13.36     66.86     19.78
007-27    N231, 65 phr, C       0.1104     15.17     65.91     18.92
Average                        0.11340     14.78     64.12     21.09
SD                             0.01101      1.25      2.83      2.53

Table 4 - cure system and carbon black effect
on cut-and-chip resistance at 55 phr of carbon
black loading

                Efficient   Semi-efficient   Conventional

N234 @ 55 phr      117           103              99
N220 @ 55 phr       97           100             102
N231 @ 55 phr      101            98              99

Table 5 - carbon black loading effect on
cut-and-chip resistance of compounds with
semi-efficient cure system

       45 phr   50 phr   55 phr   60 phr   65 phr

N234    109      104      103
N220              95      100      103
N231                       98       99      104

Table Table 6 - summary of cure system effects on NR
compound performance

                                Efficient     Semi-     Conventional
                                            efficient

Mooney scorch                     Short      Medium         Long
CRI                               Slow        Fast         Medium
Compound reversion                 No          No           Yes
Hardness                         Medium       High          Low
Rebound                           High       Medium         Low
E300                               Low        High          Low
Tensile strength                   Low        High         Medium
Elongation @ break                 Low       Medium         High
Tear strength @ RT                 Low        High          High
Tear strength @ 100[degrees]C      Low        High          High
Crack growth rate                 High         Low          Low
Tan [delta] @ 60[degrees]C        High         Low          Low
HBU                                Low         Low          High
Compression set (%)                Low       Medium         High
Abrasion wear resistance          Poor        Fair          Good

References

(1.) D.J. Elliott, "Developments in rubber technology," Vol. I, A. Whelan and K.S. Lee, eds., Applied Science Publishers. London, 1979.

(2.) Rubber Technology, 3rd Edition, edited by Maurice Morton, Van Nostrand Reinhold, 1987.

(3.) "Rubber compounding ingredients--need, theory and innovation Part 1," C. Hepburn, Rapra Report 79, Vol. 7, No.7, 1994.

(4.) R.A. Swor, Tire Technology International 1995, p. 48.

(5.) S. Laube, M. Curtin and G.E. Denstaedt, "Carbon black effect in truck tire components," Rubber Division Meeting, October 8-11, 1996.

(6.) R.N. Datta, Tire Technology International 1995, p. 100.

(7.) B. Chung, H.R. Tomlinson, Gwen Mouzon and Tommy Miller, "Cure system and carbon black effects on NR compound performance in truck tire applications," Rubber Division Meeting, October 16-19, 2001.

(8.) B. Chung and N. Tokita, "Carbon black effects on flexing fatigue crack growth in rubber compounds," International Rubber Conference 95, October 23-27, 1995.

(9.) D. W. Askea, "The BFGoodrich flexometer Model II, an improved design and a new method from a traditional rubber testing instrument," Rubber Division Meeting, May 1821, 1993.

(10.) G.E. Denstaedt and R.G. Hollatz, "Carbon black interaction with natural rubber," Education Symposium #35, Rubber Division Meeting, May 2-5, 1995.

This article is based on a paper given at the October 2001 Rubber Division meeting.

Bin Chung, H.R. Tomlinson, Gwen Monzon and Tommy Miller; Cheng Shin Rubber USA/Maxxis International, and Hung-yu Lin. Chin-fang Ho and Wally Chen, Cheng Shin Rubber