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Creating superior fatigue and abrasion resistant rubber compounds through liquid phase mixing.

Mixing of reinforcing agents, such as carbon black (CB), into rubber is a critical step in determining the performance characteristics of a rubber compound. The primary functions of mixing are incorporation, dispersion and distribution of the reinforcing agent and other additives into the polymer matrix. Conventional compounding involves the use of batch mixers to incorporate and disperse reinforcing agents, usually in pelletized form, into a solid polymer block. The breakdown and dispersion of carbon black in the rubber matrix in such a process is limited by the amount of work done by the mixer on the compound. Although intense mixing can be helpful in improving the CB dispersion, it also results in breakdown of polymer chains and a compound with lower average molecular weight. Achieving satisfactory dispersion with conventional processes can become even more challenging in the case of high surface area and/or low structure grades of CB.

Over the past couple of decades, efforts have been made to produce reinforced masterbatches by mixing latex with a slurry of CB and then coagulating the mixture chemically. Such processes are often not continuous, but batch, and generally result in an improved CB dispersion in the polymer. However, such processes often have limited productivity due to the long mixing and coagulation time compared to that of a continuous mixing process, and can have products with reduced performance due to the presence of non-rubber materials such as proteins that can be adsorbed on the CB surface interfering with polymer-CB interaction.

Transfinity composites are NR-CB masterbatches produced with a unique continuous liquid phase mixing technology (refs. 1 and 2). CB slurry is intensively mixed and homogenized, resulting in breakdown of pellets and large agglomerates to a very fine aqueous CB dispersion which is then mixed with field latex. Such a process allows input of large amounts of energy targeted toward breakdown of agglomerates, and at the same time conserves the molecular weight of the NR polymer. Such a continuous process can achieve a very high quality of CB dispersion, even with hard-to-disperse CBs (high surface area and low structure CBs), that cannot be obtained with conventional mixing methods.

In this article, we describe the performance characteristics of compounds of Transfinity materials and of compounds derived from Transfinity materials. While the dramatically improved material characteristics find application in different fields (ref. 3), this study specifically focuses on compounds that are suitable for use in various components of rotary mills used in mining operations. These compounds are predominantly blends of NR and butadiene rubber (BR), and we describe how such blends can be derived from Transfinity products to achieve a good balance of the compound properties.

The wear mechanism of mill furniture components is not well understood, but probably comprises an impact component and a sliding abrasion component. The relative importance of these two probably depends on ore type, ore size, mill size, speed, media, etc., and is almost certainly different for different specific applications. Of standard lab tests available, tear strength is the best indicator of impact wear resistance, and DIN abrasion an indicator of sliding abrasion resistance. Mill furniture components (lifter bars and shell liners) wear due to impact of ores (impact abrasion), as well as due to sliding of ores past the rubber components (sliding abrasion). Therefore, it is essential for the rubber components to have the right balance of tear strength (related to impact abrasion resistance) and sliding abrasion resistance (as measured on a DIN abrader).

Experimental

All compounds in this study were based on N 134 carbon black and were either derived from a Transfinity product containing 100 parts NR, 50 parts N 134 and 1 part antioxidant (6 ppd), or were conventionally mixed. Four different compounds were studied: (1) 100% NR Transfinity N134 compound, (2) conventionally mixed 100% NR/N134, (3) 80/20 NR/BR blend Transfinity compound diluted with pure BR, and (4) 80/20 NR/BR masterbatch blend Transfinity compound blended with a 50 phr BR/N134 masterbatch. All compounds were mixed in two stages according to the formulation shown in table 1. The 50 phr BtUNI34 masterbatch used in compound 4 was prepared by mixing BR with N134 in a single stage internal mixer.

Abrasion rates were tested by a Cabot proprietary test method, SiC wear, in which a rubber sample wears against silicon carbide grit, and the rate of weight loss of rubber in mg/h is reported. The test mimics the wear mechanism of the rubber components in a rotary mill where hard ore particles both impact and slide past the rubber parts. We believe that the test is a good predictor for performance of rubber components in mining applications and has been shown to correlate with performance in field trials.

Demattia flex cracking cycles were measured according to the ASTM D430-06 test method. DIN abrasion resistance of compounds was measured according to the ASTM D5963 method. Durometer A hardness was measured according to the ASTM D2240 test method, and tensile strength was measured according to the ASTM D412 test method. Die C tear strength was measured according to the ASTM D624 method.

Results and discussion

This study was designed to test the effects of mixing method and to come up with a method to blend BR with Transfinity composites. In compounds 1 and 2, we made a direct comparison of conventionally mixed N134 50 and Transfinity N134 50 compounds. Knowing that BR is also added to improve frictional wear resistance for many mill furniture components, we wanted to determine how different approaches to blending BR would affect the performance of Transfinity composites. To do this, two additional compounds were tested. Compound 3 was produced by dilution of a Transfinity product with BR, resulting in a slightly soft compound, but intact CB dispersion. Compound 4 was produced by blending a 50 phr Transfinity product with a 50 phr BR masterbatch containing N134 to retain the CB loading (and thereby hardness). Previous work has shown the addition of CB to a Transfinity compound to be detrimental to the performance of the compound. We hypothesize that the CB that has been added separately to the Transfinity compound does not become as well dispersed as the CB present in the Transfinity product. These not-so-well dispersed CB agglomerates are sources of defects in the compounds and assist in the early onset of failure.

Hardness

Typically, the hardness of mill furniture compounds ranges from about 60 to 65 durometer A; however, it is not known how critical this range is to performance. Compounds 1 and 3 have hardnesses of 64 and 60 durometer A, respectively (table 1). Since compound 3 has been diluted by the addition of pure BR, the final compound has a lower loading of CB (40 phr), and therefore is lower in hardness. Compounds 1, 2 and 4 contain the same loading of CB, 50 phr, and have the same hardness of 64 durometer A.

Dispersion

The dispersion quality of the compounds was assessed by optical microscopy, and the optical reflec tion images acquired are shown in figure 1. Transfinity based N 134 50 and conventional N134 50 compounds are shown in images a and b of the figure, respectively, which clearly shows that the macrodispersion of CB in Transfinity compound is superior. Also shown in the figure are compounds 3 and 4 of the study in insets c and d of figure 1, respectively. Compound 3 (image c), which has been produced by dilution of Transfinity material with BR, appears to have very similar macrodispersion to compound 1 (image a). However, compound 4 (image d), which has been produced by addition of a B R masterbatch containing N134 CB, clearly shows a poor macrodispersion compared to compounds 1 or 3. Although the number of defects in compound 4 appears to be far fewer than those in image b (conventional NR/N134 50 compound), they are much greater than those in compounds 1 and 3. These fewer defects in compound 4 are thought to have come from the CB present in BR masterbatch. While good macrodispersion is essential to minimize defects in the system, excellent microdispersion is essential to improve mechanical strength of the compound. Electrical resistivity is a good proxy for estimating microdispersion quality of a CB filled polymer system. By looking at the electrical resistivities of compounds, shown in figure 2, it can be concluded that addition of additional CB (besides that already present in the Transfinity compound) in the system results in a reduction in microdispersion. We believe that the BR phase in compound 3 is mostly devoid of any CB, and that further breaks the conducting network of CB, resulting in the highest electrical resistivity among compounds studied.

Reinforcement index

Transfinity compounds have a more unagglomerated CB network in the polymer matrix and as a result a greater interfacial area between the polymer and CB. The improved polymerfiller interaction can be quantified by looking at the slope of the stress-strain curve between 100% and 300% strains. Figure 3 shows the ratio of M300/M 100, and it is evident that both compounds 1 and 3 have relatively higher values due to a high degree of CB microdispersion. Compounds 2 and 4, on the other hand, have a poor CB microdispersion and thus a shallower stress-strain curve.

Tensile and tear strengths

Tensile and tear strengths (Die C) measurements are shown in table 1. Compounds 1 and 2 have the best combination of tensile and tear strengths, reflecting the advantage of using NR. Compound 1 is marginally better than compound 2; the difference could be attributed to a better dispersion of CB in compound 1. Adding BR or BR masterbatch (compounds 3 and 4) to Transfinity does not deteriorate the strength much; however, in a separate study, we found that adding more BR than 20% affects the tensile and tear properties. With the addition of 40% BR/N 134 masterbatch to Transfinity N 134 50, the tensile and tear strengths drop to 27 MPa and 93 N/mm, respectively. As a reference point, the tensile strengths of mill furniture compounds commercially available typically range from 20 to 24 MPa. We believe that resistance to tear is an important material property required for mill furniture rubber components where large rocks of ore material are continually impacting the rubber surface.

Abrasion resistance

DIN abrasion is widely used in the industry as an indicator of wear life of rubber components used in mill furniture, but correlation with field performance has not been established. DIN abrasion tests frictional wear of a vulcanized elastomer, and therefore good performance of a material on the DIN abrader may not be a good predictor for impact wear resistance of the compound. A number of studies in our lab have shown that for compounds of similar hardness, DIN abrasion is strongly affected by BR content, and a higher BR content gives a lower abrasion rate. BR has a lower coefficient of friction compared to that of NR, and therefore an increase in percent of BR blend in a compound invariably results in an improved frictional wear resistance. In a rotary mill, we believe that wear is a combination of both frictional and impact components, and that the tear and tensile strengths of the compound are more important for the impact wear resistance.

A silicon carbide (SiC) abrasion test, proprietary to Cabot, seems to more closely mimic the wear mechanism (balance between frictional wear and impact wear) of rubber components used in mill furniture. In this test, the rubber component is made to wear against a silicon carbide grit, and the weight loss of the rubber component is monitored over a period of time. A comparison of abrasion resistance, as measured on a DIN abrader and the SiC abrader, is shown in figure 4. The wear rates are normalized with reference to a conventionally mixed N134 50/NR compound according to the formulation shown in table 1. Compounds 3 and 4 show a very similar DIN abrasion index, while compounds 1 and 2 also show a comparable DIN abrasion index. Compounds 3 and 4 have 20% BR in the polymer blend, whereas compounds 1 and 2 are 100% NR compounds. The DIN abrader, which measures frictional wear rate, shows a better performance of compounds that contain BR than compounds that are 100% NR based. The DIN abrader fails to distinguish between a very well dispersed compound (Yransfinity based) and a poorly dispersed compound (conventionally mixed). The SiC abrader, however, shows the importance of microdispersion (and the reinforcement index due to an improved polymer-CB interaction) of CB in the polymer matrix. Based on the SiC abrader test, we find that the 100% NR based Transfinity compound (compound 1) has the best abrasion resistance, which agrees with our observations from field trials of Transfinity compounds in mill furniture applications. The observation is more pronounced in severely abrasive conditions where impact wear resistance is equally or more important than the frictional wear resistance. We therefore recommend the use of 100% NR based Transfinity compounds in aggressive mills where impact wear is the dominant wear mechanism, and the use of up to 20% BR blends of Transfinity compounds in mills that are less aggressive, where frictional wear is also important.

Fatigue life

Fewer stress concentrations in the Transfinity compounds result in a substantially better resistance to crack initiation. Figure 5 shows the number of cycles taken to initiate and grow a crack to 0.5 mm in the vulcanized compounds. Compound 3, Transfmity N134 50 diluted with 20% BR, shows a significantly higher number of cycles to failure. We hypothesize that the primary cause of higher fatigue life is the fewer defects that are present in the Transfinity material. The BR phase, which we believe is devoid of any CB particles, further helps to arrest any crack growth that may have initiated in the NR phase at a defect.

Conclusions

This work shows that by attaining high quality dispersion of a reinforcing agent in the polymer, one can greatly enhance the abrasion resistance and fatigue properties of a compound. BR blends of Transfinity compounds can enhance the frictional wear of the compound, but only at the cost of tear and tensile strength; properties that may be important in applications where impact abrasion resistance is key. The study also presents an abrasion test method that can clearly show the impact that improved dispersion of CB in polymer has on the abrasion resistance of the compound. The SiC abrader seems to correlate strongly with field performance of rubber compounds in mill furniture. We believe that dispersed 100% NR based Transfinity compound is well suited for applications where wear rates are severe, and a blend of Transfinity N134 50 with 20% BR/N134 masterbatch is more suitable for less severe applications where frictional wear rate of compounds is also important.

References

(1.) T. Wang, M.J. Wang, J. Shell, Y.L. Wong and B. Vejins, proceedings of the 164th ACS Rubber Division Meeting, October 14-17, 2, 173 (2003).

(2.) T. Wang, M.J. Wang, J. Shell, J. Vejins, F. Yoo-ling, E Guoqiang and J. Zheng, Rubber World, 227 (6), 33 (2003).

(3.) D. Reynolds and V.R. Tirumala, Rubber World, 246 (1), 30 (2012).

by Anand Prakash, Michael Morris and Russell Coverley, Cabot Corporation


Table 1 - formulations and basic mechanical properties of
compounds in the study

                              Compound 1     Compound 2
                             Transfinity   Conventional
                                 N134 50        N134 50

NR/BR                              100/0          100/0
Transfinity N134 50 phr              151              -
NR                                    -             100
N134                                  -              50
BR                                    -               -
BR/N134 masterbatch 50 phr            -               -
Stage 1
ZnO                                    4              4
Stearic acid                           2              2
6 ppd                                  1            1.5
Stage 2
Sulfur                               1.2            1.2
TBBS                                 0.8            0.8
Properties
Hardness, durometer A                 64             63
Tensile strength, MPa                 31             29
Die C tear strength, N/mm            149            145

                               Compound 3    Compound 4
                              Transfinity   Transfinity
                             diluted with       blended
                                      BR        with BR
                                            masterbatch

NR/BR                               80/20         80/20
Transfinity N134 50 phr               121           121
NR                                      -             -
N134                                    -             -
BR                                     20
BR/N134 masterbatch 50 phr              -            30
Stage 1
ZnO                                     4             4
Stearic acid                            2             2
6 ppd                                   1             1
Stage 2
Sulfur                                1.2           1.2
TBBS                                  0.8           0.8
Properties
Hardness, durometer A                  60            64
Tensile strength, MPa                  28            31
Die C tear strength, N/mm             116           144
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Author:Prakash, Anand; Morris, Michael; Coverley, Russell
Publication:Rubber World
Date:Mar 1, 2013
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