Novel approach of rubber-filler interaction through surface modification of carbon black.

Carbon black is known for its unique ability to enhance the dynamic mechanical properties of elastomers. This ability of carbon black is termed as reinforcement. Reinforcement leads to an increase in modulus, increased stress softening, and improved hysteresis and heat build-up properties. This also leads to an improvement of failure properties such as tensile strength, tear strength and abrasion resistance. Reinforcing fillers increase modulus at high extension in addition to the viscous contribution that is related only to suspended particulate matter. Due to the Mullin's Effect, there is an elastic contribution resulting from the strong bonding between rubber and carbon black. This strong bonding also acts as an additional crosslinking in the vulcanizate and is associated with stress softening, which reinforced vulcanizates exhibit when they are relaxed and then stretched for a second time.

The total surface area and surface activity property of carbon black is recognized to be due to the presence of various reactive functional groups on the surface. The reactive groups act as free radical acceptors formed by the chain scission of the long chain molecules partially due to the mechanical action during mixing or milling and also during partial vulcanization reaction. The surface activity of carbon black varies widely for different grades of blacks, and for the same grade it has a specific distribution, the activity varying from different particles, agglomerates and locations on the surface. The porosity of carbon black also influences the properties of the rubber compound. The total area of interphase depends on both filler loading and specific surface area of filler.

It may be noted that carbon black normally is of a turbostatic structure and the carbon is in a [s.sup.2][p.sup.2] state of hybridization, analogous to that of a crystalline graphite structure. However, the carbon black filled elastomeric compound attains a s[p.sup.3] hybridization during the course of processing and vulcanization, which leads to reinforcement (ref. 1).

Attempts are being made by several carbon black manufacturers and researchers to alter the morphology of the carbon structure, increase the crystalline site and the stress oriented active plane so as to obtain improved vulcanizate properties. Since the surface plays an important role in determining the vulcanizate property, suitable modification of the surface chemistry has produced interesting results, as the following cases would reveal. Carbon black having its surface modified by Si-69 (ref. 2), aliphatic dihydric and trihydric alcohol (refs. 3 and 4), imparts improved hysteresis and improved abrasion resistance properties at low and high temperatures compared to the native carbon black. It is reported that surface modification by silane coupling agent leads to lowering in reinforcement (ref. 5). It is also reported that carbon black with a phenolic plant product (ref. 6) gives higher flex crack growth resistance. Carbon black modified with vegetable oil (ref. 7) shows high tensile strength and elongation properties, low abrasion and hardness. Carbon black modified with primary amine showed improved extrusion properties (ref. 8) and that with diazonium salt is useful in plastic, rubber and textile compound applications (ref. 9).

This article deals with a short summary of studies with aromatic polydroxy compounds, which showed improved static and dynamic mechanical properties. The compounds that were selected for the purpose were resorcinol (ref. 10), phloroglucinol (ref. 11), catechol (ref. 12) and pyrogallol (ref. 12). This improvement can be attributed to a high charge density in surface due to the proportion and nature of surface groups present in surface modified blacks. The concentration of hydrogen ion is higher in the modified carbon black surface than in the normal carbon black. The change of surface chemistry is also confirmed from Fourier transformed infrared study.

Experimental

Materials used

The natural rubber (RMA-1X), zinc oxide and sulfur were of laboratory reagent grade; accelerator N-oxydiethylbenzthiazyl sulfenamide (NOBS), peptizer pentachlorothiophenol (PCTP) and N330 carbon black are commercially available. The catechol, resorcinol, pyrogallol and phloroglucinol were of laboratory grade.

Preparation of modified CB samples and their nomenclature

Three different strengths of aqueous solution (0.01%, 0.1% and 1.0%) of each of the surface modifiers were made and were treated suitably in the course of the carbon black manufacturing process. The modified CB samples were named A, B and C, respectively, while the untreated standard sample was named D in all cases. The colloidal properties were found to be unaltered before and after the addition of the chemicals.

Compound formulation and preparation

Each of the compounds contain (phr): NR-100; carbon black - 50; PCTP-0.15; NOBS-0.65; ZnO-3; Sulfur-2.5; stearic acid-1.5. The PCTP was used as a peptizer to facilitate the mixing process. Mixing was carried out on a laboratory two-roll mill (150 mm x 300 mm) at a friction ratio of 1:1.4 (front and back roll speed of 24 and 33 rpm, respectively) and a temperature of 70[degrees]C. Care was taken for complete dispersion of filler and the same time (17 minutes) was allocated for all the mixes to ensure the same level of polymer-filler interaction. The compounds were identified according to the identification of the carbon blacks. All the compounds were given a maturation time of 24 hours (h) at 25[degrees]C and were cured at their respective rheometric optimum cure times at 150[degrees]C in a steam heated hydraulic press.

Testing

After maturation of 24 hours, rheometer data were obtained at [+ or -] 3[degrees] arc and 100 range scale at temperatures of 140[degrees]C, 160[degrees]C and 180[degrees]C.

For mechanical properties, dumb-bell shaped samples were prepared from cured sheets which were subsequently tested for modulus, tensile strength, elongation and tear energy according to ASTM D412-51 using a universal testing machine at 27[degrees]C at a crosshead speed of 500 mm/min. A bench thickness gauge measured the thicknesses of the samples.

Abrasion loss ([mm.sup.3]) was obtained from an abrader as per DIN 53516.

The heat build-up was obtained from a flexometer using ASTM D623.

The hysteresis loss (%) was measured by a universal testing machine.

Scanning electron micrograph was taken of the gold coated fracture surfaces of the tensile strips and the gold coated abraded surfaces of the abraded specimens. The SEM was taken using an electron beam potential of 25 kV.

Dynamic mechanical property was measured in a dynamic mechanical thermal analyzer to obtain the tan [delta] values at a temperature of 25[degrees]C and 60[degrees]C at a frequency of 10 Hz.

A very thin section of the cured compound was analyzed in a FTIR analyzer. A resolution of 4 [cm.sup.-1] was given over 64 scans at a prism angle of 45[degrees].

X-ray diffraction (XRD) analysis was carried out for the section of the compound in unstretched and stretched conditions.

Results and discussion

Encouraging observations were obtained from the properties studied for the compounds containing the surface modified carbon blacks.

The difference of the maximum torque and the minimum torque, i.e., CT, obtained from rheometric data bears an interesting phenomenon since the value can somewhat be attributed to higher crosslink density. It is seen from figure 1 that in most cases the CT value is greater for the modified compounds than for the normal compounds, which gives an indication that some form of higher crosslink density has taken place.

[FIGURE 1 OMITTED]

In the cured compound, the modulus values at 100% elongation and 300% elongation were obtained, and the results (figures 2 and 3) were in analogy to the rheometric results. The modulus at 100% elongation as known bears a direct relationship to the crosslink density formation. Here again, these values were higher for the compounds employing surface modified blacks, indicating higher crosslink density formation. The modulus at 300% elongation also followed the same trend and analogy, very prominent in the case of resorcinol and phloroglucinol. The fact that there has been an improvement may be attributed to higher stress induced crystallization due to interactive sites in addition to the higher crosslink density formation. However, in the few cases where improved results were not obtained, the probable reasons for the same will be discussed later.

[FIGURES 2-3 OMITTED]

There was an improvement in the formation of crosslink density and in the compactness of the crystal structure, which was supported by reported improvement in the heat build-up properties (figure 4). The lowering of hysteresis loss (figure 5) also indicated an enhanced stress softening effect for the surface modified carbon black containing compounds. The improvement pattern was more prominent in the case of phloroglucinol and also in resorcinol. The results of these dynamic properties were in accordance with those of static properties.

[FIGURES 4-5 OMITTED]

Since the results obtained for phloroglucinol particularly were found to be more interesting, it was decided to carry out a dynamic mechanical analysis (DMA) study and a SEM analysis.

The DMA showed a lowering of tan [delta] value at 60[degrees]C (figure 6). The SEM (figures 7 to 10) of the tom tensile pieces showed higher deformation slips and larger shear displacement with sharp boundaries of dislocation for the surface modified compounds. These compounds also contained more knotty tears, which further supports better reinforcement.

[FIGURES 6-10 OMITTED]

From the above results, it has been seen that compound C incorporated with phloroglucinol modified carbon black gave relatively better overall compound properties. Wolff and Gorl (ref. 13) had ascribed such lowering of tan [delta] with Si69 modified carbon black, due to microdispersion of black and due to improved linkages between polymer and filler surfaces. Such improved linkages were expected to have occurred in the series of investigations also, since the additional -OH group attached to an aromatic ring might have increased the linkage between the carbon black and natural rubber. This in turn has possibly increased the crosslink density and the compact crystal structure.

To establish the above fact, characterization studies were carried out in the form of FTIR studies and XRD analysis. FTIR studies were carried out for the resorcinol modified compounds, which showed the presence of prominent hydroxyl groups bound in the surface of the rubber vulcanizate in the filler-elastomer matrix by having dual and deep peaks in these compounds (examples: figure 11--FTIR of modified Sample B and figure 12--FTIR of unmodified Sample D). It is believed that phloroglucinol will follow the same pattern (FTIR study of phloroglucinol is under preparation).

[FIGURES 11-12 OMITTED]

From the XRD data obtained for compound C of phloroglucinol and its normal compound in unstretched and stretched conditions (denoted by C' and D', respectively), it showed that compound C' gives more intense peak values compared to D', indicating more ordered crystal, resulting in stronger bond formation (table 1).

Thus it is possibly established that the -OH group in aromatic linkages increases its degree of nucleophilicity, which in turn increases its linkage with the polymer. In an earlier investigation, secondary ion mass spectroscopy (SIMS) on dry carbon black showed that complex hydrocarbon structures are suitable for chemical reactivity at the carbon black surface (ref. 14). These complex structures exist at the edges of the graphite-like layers that compose carbon black, and they provide an environment suitable for chemical reactivity through transition from [s.sup.2][p.sup.2] to the [sp.sup.3] state of hybridization. Further, a considerable amount of hydrogen is present at the surface as complex hydrocarbon structure, which supports the importance of hydrogen to the surface activity of carbon blacks. In this investigation, the introduction of the surface modifiers, with increased available hydrogen, thus helped in enhancing the surface activity.

A possible mechanism has been proposed which might have increased the elastomer-filler interaction (figure 13).

[FIGURE 13 OMITTED]

The discussion would probably be incomplete without considering the reason for the fall in the properties in certain cases. The decline is probably due to the formation of pseudo linkages resulting in higher cage effect above the optimum hydrogen bonding, coupled with the breakage of weak inter-particle bonds. Also, the results of the compounds modified with pyrogallol and catechol were not as encouraging, possibly due to the steric hindrance that might have resulted due to the -OH groups being attached to adjacent carbon atoms.

Conclusion

Surface modification by polyhydroxy benzene has led to improved rheological and thermo-mechanical properties. The improved phenomenon has been possibly due to increased filler-polymer interaction by enhanced chemical linkages. The results were more encouraging in the case of resorcinol and phloroglucinol due to the position of the hydrogen (of -OH) in these compounds relative to the carbon atoms.

Table 1--XRD peak intensities of first ten peaks

Peak            Theta/omega

         C        C'       D        D'

   1   7.4659   7.9115   8.4787   8.3292
   2   4.9393   5.3201   7.2372   5.9206
   3   4.6441   5.0995   4.9592   4.2557
   4   4.4662   4.4667   4.5810   4.0711
   5   4.0067   4.4313   4.3351   3.1255
   6   3.1503   4.2310   3.1925   2.9918
   7   2.8079   3.4000   2.8883   2.0538
   8   2.7167   3.1843   2.7512   2.0135
   9   2.6283   2.8830   2.6467   1.9515
  10   2.4613   2.0373   2.5659   1.9076

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