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Improved off-the-road tire formulations with discrete surface functionalized multiwall carbon nanotubes: Correlating wear, abrasion and tear resistance with cut and chip to predict lifetime.

The off-the-road (OTR) tread is a mixture of crosslinked elastomers with filler network, as well as a reinforcing filler-elastomer network. Typically, carbon black (CB) (refs. 1 and 2) is used in tire formulations for reinforcing the elastomer matrix. Silica (refs. 3 and 4) with some combinations of silane coupling agents is used to enhance the wear and abrasion properties. However, the amount of fillers, as well as the coupling agents, while improving the properties, frequently hamper the processability of the compound. Since the discovery of carbon nanotubes in 1991 (ref. 5), many research groups have employed single- (SWCNT), double- (DWCNT) and multi-wall (MWCNTs) in various polymers and elastomers to achieve improvements in the mechanical properties based on theoretical calculations (refs. 6-9). However, due to their existence as bundles and the poor dispersion quality in the final matrix, the resultant properties are not as high as expected. The true potential of enhanced mechanical properties of elastomer composites with multiwall carbon nanotubes (MWCNTs) can be achieved when they are completely disentangled and dispersed in the host matrix. Molecular Rebar (MR) is completely disentangled, discrete, cleaned of catalyst residues and surface functionalized multiwall carbon nanotubes. The presence of surface functionalization facilitates good bonding between the rubber matrix and tube surface for improved load transfer, which results in higher tear and tensile properties, as well as enhancements in wear and tear resistance. In this article, we discuss the delivery of MR via elastomeric specific masterbatches, as well as via a naphthenic oil based dry liquid concentrate (DC) into an OTR formulation, and the resulting improved tear, wear and abrasion properties are evaluated.

Experimental

An OTR formulation comprised of a blend of natural rubber (NR) and styrene-butadiene rubber (SBR) is chosen for this study, and 1.25 parts per hundred rubber (phr) of MR is delivered to each formulation via different routes, (i.e., combinations of NR and SBR masterbatches and a naphthenic oil based DC), as shown in table 1. It can be noted that MR-SBR MB, MR-NR MB and a mixture of MR-NR and MR-SBR masterbatches were used in formulations 2, 3 and 4, respectively. Typical MR DC in naphthenic oil was used in formulation 5. Both NR and SBR masterbatches (MB) with MR were prepared using glacial acetic acid for coagulation. In formulations 1 through 4, all rubber lacking MR (rubber used for diluting the masterbatch) was coagulated from their respective lattices that were also used to prepare the MR MB. To compare the quality of coagulation from latex, Formulation 5 contains industrially produced bale rubber instead of coagulated rubber. There is no surfactant used in these masterbatches, and the MR is unaltered from Molecular Rebar design; completely disentangled MR slurry in H20 was used as a precursor. All masterbatches were washed with DI water at least four times to bring the final pH to 5.5. All masterbatches and coagulated rubber used in the experiment were masticated with 1 phr of antioxidant (6PPD).

Mixing

All formulations were mixed in a 1.6 liter internal mixer in two passes. Before mixing, the internal mixer was warmed up with a dummy batch of NR/SBR blend to 160[degrees]C (maximum allowed mixing temperature for this rubber blend in this study). Pass 1 mixing includes rubber, masterbatches, CB, silica and other chemicals, excluding the cure package. Temperature of the mix did not exceed 160[degrees]C. Mixing time was kept roughly to 6 minutes. The cure package was added in pass 2 and the maximum temperature was less than 105[degrees]C to avoid the onset of sulfur crosslinking.

Cure characteristics

A Monsanto R100 single frequency oscillating die rheometer (ODR) was used to determine the optimum cure time (t90) at 160[degrees]C. Angular frequency of oscillation was set to 1[degrees] and time was set to 30 minutes. Once both dies and rotors were preheated to 160[degrees]C and temperature stabilized, about 15 gm of material was cut from the pass 2 sheet and placed on the oscillating rotor. The dies were then closed. Scorch time (tsl), minimum (ML) and maximum (MH) torque were noted with the assistance of the ODR software as the vulcanization/crosslinking of the material occurred. The cure data are given in table 2 and all samples were cured with a cure time of t90 + 2 minutes. Slabs of 150 mm x 150 mm x 2 mm dimensions were pressed using a Carver compression press at 160[degrees]C and 45 tons of pressure.

Mechanical testing

Tensile testing was performed according to ASTM D412-16 using an Instron 3360. The die used to create the tensile specimens was DIN-53504-S2. Six specimens were produced from each formulation to be tested. Tear testing was performed by using two methods. One is a die C test according to ASTM D 624, and the second one is a lab based single edge notch constrained tear.

A pull rate of 500 mm/minute was used for the constrained tear test. Tear samples were die cut, making a rectangle 50.8 mm x 25.4 mm. A slice perpendicular to the side of the specimen was also made during die cutting. A total of six specimens were tested per sample compound. A typical single edge notch test specimen with dimensions is shown in figure 1.

Only samples "control" and MR-NR MB were tested for trouser tear resistance due to material and resource limitations. These samples were tested in accordance with the ISO 34-1 standard, and the specimens were cut by hand, rather than with a die.

Microtomy and TEM

A Leica Ultramicrotome UC7 fitted with an FC7 cryo-microtome was used to obtain approximately 40 nm thick sections of the cured composites. Specimens were frozen to -120[degrees]C, and sectioning was performed with a 45[degrees] angle wet cryo diamond knife. Sections are collected with a "perfect loop" dipped in a sugar-DI water mixture and placed on a 400-mesh sized copper grid. The grids with sections were later dipped in DI water to remove sugar deposited on the surface of the sections and air dried.

Dispersion quality of MR in the formulations was observed using a JEOL 71 OOF SEM equipped with a field emission scanning transmission electron microscope (STEM). Operating voltage is varied between 15-25 KeV, depending on the composition and thickness of the sections.

Dynamic mechanical analysis

Dynamic properties testing of cured slab strips is performed using a TA Instruments DMA Q800. Temperature sweep tests are carried out from 25[degrees]C to 105[degrees]C in tension mode (2% static, 1% dynamic and 1 Hz frequency). The ratio of loss modulus to storage modulus, value tan 8, is observed at 30/70/100[degrees]C. Tan [delta] at 30[degrees]C represents the wet grip, at 70[degrees]C relates to the rolling resistance and at 100[degrees]C correlates to heat build-up. For an improved tire, tan [delta] at 30[degrees]C should be a higher value (tacky) to get good wet grip, whereas at 70 and 100[degrees]C, the values should be lower than the control for improved rolling resistance and heat build-up properties, respectively.

Cut and chip and abrasion

Cut and chip resistance of the cured formulations was performed using a BFGoodrich cut and chip tester (Akron Rubber Development Laboratory). Cured formulations were made into wheels of 28.9 mm external diameter and 12.5 mm thickness. The specimens were rolled at 850 rpm, and the frequency of the impacting blade was set to 1 Hz. Time of the test was 20 minutes. Mass and diameter loss measurements were taken after the test time. Cut and chip resistance was calculated as the reciprocal of mass loss. A DIN abrasion test was done on a DIN abrasion tester for determining the abrasion resistance of compounds of vulcanized rubber according to ASTM D5963-96.

Results and discussion

A transmission electron micrograph of the formulation showing a typical dispersion of discrete MR in a carbon black filled formulation is shown in figure 2. It can be seen that the tubes are in the rubber phase as well as in between CB particles, suggesting the fillers can work synergistically to reinforce the matrix.

Tensile stress at break and modulus values measured at 100%, 200% and 300% elongation are shown in figures 3a and 3b, respectively. Irrespective of the route of addition of MR into these final formulations, ultimate stress values were found to be increased by at least 5% over the control formulation. Moduli measured at three elongations are also found to increase with the addition of MR, which is a direct indication of the reinforcing effect of MR on the rubber host matrix.

Maximum load per unit thickness values from the die C tear test and the area under the curve (toughness) values measured from a laboratory based constrained tear test described in the previous section are shown in figures 4a and b, respectively. The maximum load/thickness values from a die C test show no significant change in the values with the addition of 1.25 parts of MR. However, the toughness values measured by calculating the area under the curve from the single edge notch constrained tear test shows a significant increase in the property, but with a large standard deviation. From previous observations of improvements in formulations with elastomers such as acrylonitrile butadiene rubber (NBR) and styrene butadiene rubber (SBR) (refs. 10 and 11), where the standard deviations are very minimal, the improvements in NR/SBR systems are taken as considerable. Moreover, this constrained tear test is employed as a screening tool in the lab to observe improvements in toughness which correlate to macro-abrasive wear testing, such as a cut and chip test performed at Akron Rubber Development Laboratory.

The trouser tear test results are reported in the bar graph in figure 5. The local maxima and minima were calculated as follows: All maxima to minima drops of less than 10% were dismissed, as frequent chatter in the NR/SBR samples led to false "tears" being recorded. The maxima, or load at which tears began, were then averaged and standard error calculated. All data points outside of 1 a were then dismissed, as a form of eliminating outliers. The average was then calculated again, representing the maxima for the compound. The same procedure was used to calculate the minima, or arrest points, for each compound.

Cut and chip resistance and abrasion resistance, which are calculated as the reciprocals of the weight loss and volume loss, are shown in figures 6a and 6b, respectively. The weight and diameter losses due to the cutting and chipping are given in table 3. With the addition of MR, the formulations have greatly improved resistance to chipping, irrespective of the method of delivery, i.e., via a masterbatch versus an oil based dry liquid MR concentrate. Volume loss due to abrasion in compounds containing MR is lower than in the control formulation, and this points to MR greatly reducing the material loss from a different abrasion process. From these observations, MR is bridging the micron sized cracks in the CB filled elastomer, preventing coalescence into bigger cracks that lead to the catastrophic failure of the part.

Dynamic properties are measured as tan [delta] (ratio of loss modulus E" to storage modulus E') in tensile mode at temperatures of 30[degrees]C (wet grip), 70[degrees]C (rolling resistance) and 100[degrees]C (heat buildup), as shown in figure 7. It can be seen that these properties did not change drastically, and only slightly worsened with the addition of MR. This is attributed to the energy loss from the perturbations in elastomer chains near the tube surface and slippage of chains far from the surface. Recent lab studies have shown these properties can be optimized by changing the MWCNT to CB ratio, as well as oil content.

Correlation between tear tests and cut and chip resistance

By comparing all three bar graphs, it is visually evident that the die C tear test, the constrained tear test, and the cut and chip test have correlated results for this study. This can be verified through a simple Pearson correlation coefficient matrix, seen in table 4, which indicates the lowest correlation being a 0.66 between die C and cut and chip, with the highest correlation being 0.96 between tear toughness and cut and chip. All Pearson correlation coefficients fall in the range of 0.5-1.0, commonly known as being "large effect size" correlations (ref. 12).

The P-values, indicating statistical significance of the above correlations, are shown in table 5. None of the P-values for this data set are smaller than 0.05, indicating that none of these results is statistically significant. However, standard deviations in rubber testing are frequently broad (particularly so for tear testing and cut and chip testing); a range of two standard deviations can encompass a broad spectrum of data. As an example, no abrasion or physical tests had a coefficient of variance rated "good," or <5%, in the 1988 study by BF Goodrich (ref. 13). Within the context of the rubber industry, a 93% chance of observing toughness correlated to cut and chip with a coefficient of 0.96 is sufficient for some measure of confidence going forward with continued experiments. These massive improvements in cut and chip resistance can be discovered earlier in the development process through the use of a constrained tear test.

Summary

Discrete and surface functionalized multiwall carbon nanotubes, also known as Molecular Rebar (MR), are delivered into an OTR formulation consisting of a blend of natural rubber (NR) and styrene butadiene rubber (SBR) via masterbatches of NR and SBR, and a naphthenic oil-based MR dry liquid concentrate. Transmission electron micrographs of the formulations showed excellent dispersion of the tubes present in the rubber phase and within the CB filler network. With just 1.25 phr of MR, mechanical properties such as stress at break and modulus values increased when compared with the control formulation, showing the reinforcing effect of the tubes on the carbon black filled elastomer matrix. The area under the curve value, representing the toughness of the compounds, measured during a single edge notch constrained tear test, shows an increase in the toughness by at least 30%. The crack bridging effect of MR is investigated by cut and chip testing where improvements of at least 100% are observed over the control formulation. Lower values of volume losses in abrasion testing with the presence of MR show the effectiveness of the specialty filler against the harsh environments that an OTR tire encounters in field applications. Dynamic properties such as wet grip, rolling resistance and heat build-up do not change much with a loading of only 1.25 phr of MR. With the improvements in tear, cut and chip, and abrasion shown in this study, these OTR formulations can be further optimized for better improvements.

Using three critical tear tests (die C, academic constrained tear and trouser tear), the mechanisms of tear improvements can be gleaned in the laboratory. Tear initiation resistance, tear propagation resistance, and subsequent tearing re-initiation and arrest energies all affect the wear resistance of a material, and have different correlations to macro-abrasive wear, in particular. The use of a cut and chip tester can accurately predict field performance in OTR conditions, as proven by numerous published literature sources. By utilizing the die C tear test, the constrained tear test, and the cut and chip test, the risk of moving to a field trial can be mitigated with minimal capital costs and laboratory time. The constrained tear test, and specifically the resistance to crack propagation, correlates extremely well ([R.sup.2] = 0.96) with the cut and chip test, which in-turn correlates extremely well to OTR abrasive wear ([R.sup.2] > 0.9) (ref. 13). The use of Molecular Rebar products can improve crack propagation resistance by upwards of 20%, and improve cut and chip results by upwards of 200%. Based on literature sources reviewed here, this type of cut and chip resistance should lead to a minimum of doubling the tread life of an OTR tire in macro-abrasive conditions.

This article is based on papers presented at the 194th Technical Meeting of the Rubber Division, ACS, October 2018.

by Sateesh Peddini and August Krupp, Molecular Rebar Design, LLC

References

(1.) H. Greensmith, Journal of Polymer Science, 21, 175 (1956).

(2.) B. Boonstra and A. Medalia, Rubber Chemistry and Technology, 36, 115 (1963).

(3.) M. Wagner, Rubber Chemistry and Technology, 49, 703 (1976).

(4.) M. Wagner, Rubber Chemistry and Technology, 50, 356 (1977).

(5.) S. Iijima, Nature, 354, 56 (1991).

(6.) M.M.J. Treacy, T.W. Ebbesen and J.M. Gibson, Nature, 381, 678 (1996).

(7.) L. Bokobza and M. Kolodziej, Polymer International, 55, 1,090 (2006).

(8.) L. Bokobza, Polymer, 48, 4,907 (2007).

(9.) M. Bhattacharya, M. Maiti and A.K. Bhowmick, Rubber Chemistry and Technology, 81, 782 (2008).

(10.) S.K. Peddini, C.P. Bosnyak, N.M. Henderson, C.J. Ellison and D.R. Paul, Polymer, 55, 258 (2014).

(11.) S.K. Peddini, C.P. Bosnyak, N.M. Henderson, C.J. Ellison and D.R. Paul, Polymer, 56, 443 (2015).

(12.) "Effect Size," Emory College of Arts & Sciences, Access September 9, 2018 at URL: http://www.psychology.emory.edu/clinical/bliwise/Tutorials/SCATTERJscatterplots/effect.htm.

(13.) A.M. Fairlie, et al., "Systematic Investigation of Laboratory Abraders, " BFGoodrich, 1988.

Caption: Figure 1--tear testing specimen dimensions with notch

Caption: Figure 2--dispersion of MR in a CB filled NR/SBR OTR formulation

Caption: Figure 3--reinforcing effect of MR on the composites shown by the increase in ultimate stress (a), and moduli (b)

Caption: Figure 4--tear properties of composites showing a) maximum load/thickness measured from die C, b) area under curve measured from constrained tear test, respectively

Caption: Figure 5--a bar graph summarizes the maxima and minima average for the trouser tear test from this study

Caption: Figure 6--bar charts showing resistance to a) cut and chip, and b) abrasion, with the addition of MR

Caption: Figure 7--tan [delta] versus temperature plots measured at 30, 70 and 100[degrees]C representing various dynamic properties for the formulations
Table 1--off-the-road formulations with Molecular
Rebar (MR) delivered via different methods (NR, SBR
masterbatches and a dry liquid concentrate)

Ingredients                  1         2        3
                       Control    MR-SBR    MR-NR
                                      MB       MB

Natural rubber            49.6      49.6     49.6
Coagulated NR             40.4      40.4      0.0
MR NR MB                   0.0       0.0     41.7
SBR 1502                   2.9       2.9      2.9
Coagulated SBR             7.1       0.0      7.1
MRSBR MB                   0.0       8.3      0.0
N110/115 CB               27.0      27.0     27.0
N 326 CB                  27.0      27.0     27.0
Ultrasil VN3 silica       10.0      10.0     10.0
Naphthenic oil             5.0       5.0      5.0
Tackifying resin           2.0       2.0      2.0
Stearic acid               3.0       3.0      3.0
ZnO                        5.0       5.0      5.0
6PPD (a)                   1.5       1.5      1.5
TMQ                        1.5       1.5      1.5
MC Wax (b)                 2.0       2.0      2.0
MR 1420X DC                0.0       0.0      0.0
Sulfur                     1.5       1.5      1.5
TBBS (c)                   1.3       1.3      1.3
HMT (d)                      1         1        1

Ingredients                    4         5
                       MR-SBR MB/    1420X-
                         MR-NR MB     MR DC

Natural rubber               49.6      90.0
Coagulated NR                 4.0       0.0
MR NR MB                     37.5       0.0
SBR 1502                      2.9      10.0
Coagulated SBR                3.0       0.0
MRSBR MB                      4.2       0.0
N110/115 CB                  27.0      27.0
N 326 CB                     27.0      27.0
Ultrasil VN3 silica          10.0      10.0
Naphthenic oil                5.0       0.0
Tackifying resin              2.0       2.0
Stearic acid                  3.0       3.0
ZnO                           5.0       5.0
6PPD (a)                      1.5       1.5
TMQ                           1.5       1.5
MC Wax (b)                    2.0       2.0
MR 1420X DC                   0.0       6.3
Sulfur                        1.5       1.5
TBBS (c)                      1.3       1.3
HMT (d)                         1         1

(a) 6PPD = N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine

(b) MC wax = mixture of long straight chain and branched micro
crystalline wax

(c) TBBS = N-tert-butyl-benzothiazole sulfonamide

(d) HMT = Hexamethylenetetramine

Table 2--ODR cure characteristics calculated
at 160[degrees]C for 30 minutes

                           1          2          3
                     Control     MR-SBR      MR-NR
                                     MB         MB

[M.sub.L], (N-m)         0.9        1.1        1.0
[M.sub.H], (N-m)         4.1        4.6        4.4
[t.sub.s1], (min.)   02:32.3    02:37.7    02:40.0
[t.sub.90], (min.)   05:45.6    06:08.6    05:58.2

                             4          5
                     MR-SBR MB/     1420X-
                       MR-NR MB      MR DC

[M.sub.L], (N-m)            1.0        1.1
[M.sub.H], (N-m)            4.4        4.4
[t.sub.s1], (min.)      02:47.6    02:45.8
[t.sub.90], (min.)      06:07.2    05:52.8

Table 3--weight and diameter loss values
calculated by cut and chip tester

             Weight loss (g)    Diameter loss (mm)

Control               1.3615                 2.032
MR-SBR MB             0.4611                0.2032
MR-NR MB              0.8966                 1.143
MR-SBR/NR             0.5372                0.2794
MR-DC                 0.5359                0.2794

Table 4--Pearson correlation coefficient
matrix for die C tear test, tear toughness
from constrained tear test and cut and
chip test

PCC            Die C    Toughness    Cut and chip

Die C              1           --              --
Toughness       0.79         1.00              --
Cut and chip    0.66         0.96               1

Table 5--generated P-values from the data
sets, comparing die C tear test, tear
toughness from constrained tear test
and the cut and chip test

P-values        Die C    Toughness    Cut and chip

Die C              --           --              --
Toughness        0.17           --              --
Cut and chip     0.34         0.07              --
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Author:Peddini, Sateesh; Krupp, August
Publication:Rubber World
Date:Feb 1, 2019
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