Industrial rubber products: carbon black and compound performance.

Industrial rubber products: Carbon black and compound performance

Industrial rubber products (IRP) are goods used in non-tire applications. This broad definition includes a variety of applications, such as hoses, belts, sealants and boots for automotive applications, recreational goods, appliance parts, apparel and many others. A myriad of demands are placed upon part performance, including environment (temperature, chemical resistance), severity of mechanical stress and complexity of geometry. Polymers ranging from low-priced SBS block polymers, which have thermoplastic processing characteristics, to exotic, expensive, halogenated, inorganic rubbers have been used. Each rubber system, coupled with the applications requirements, has placed different constraints on the performance of the filler systems used.

For carbon black filled systems, the compound requirements have led to two broad-based classes of carbon blacks, reinforcing grades and semi-reinforcing grades[1 and 2]. The reinforcing grades include those having ASTM classifications in the series N300, N200 or N100 ranges. These blacks are generally characterized by moderate to high surface area (small particle size) as measured by iodine adsorption. The reinforcing blacks can impart improved abrasion resistance, higher modulus and greater strength to a compound. A typical IRP application of reinforcing blacks is for conveying belts, particularly in mining applications, where the abrasion resistance is a critical performance property. Semi-reinforcing grades include those blacks which have ASTM classifications in the series N900, N700, N600 and N500. These blacks are generally characterized by low surface area (large particle size) as measured by iodine adsorption. These blacks are often selected for cost, ease of processing, ability to obtain high loadings and the balance of static and dynamic mechanical properties imparted. For many applications with semi-reinforcing blacks, the visual appearance of the surface is a critical performance property. This article is limited to results obtained with compounds using only semi-reinforcing carbon blacks.

Several rubber systems have been selected for study, including SBR-1500, NR, HNBR and EPDM. SBR and NR are widely used rubbers in belts, hoses and vibration isolation applications[3]. EPDM is widely used for hoses, sealants, gasketing, wire and cable. Hydrogenated nitrile rubber is a relatively new polymer which has made significant inroads in under-the-hood applications in the Japanese car market. It is a polymer of growing interest in Europe and a nascent interest in the U.S. The primary driving force for the use of HNBR, and even more exotic rubbers, is the increasing engine compartment temperature with the resulting increase in severity of heat aging requirements for the compounds.

Model compounds have been prepared in each of the rubber systems. The carbon black types and loading were selected to represent a range of applications, although commercial compounds are likely to have significant variations on these formulations, including other plasticizers, fillers, antioxidants, etc. A combination of commercial and experimental carbon blacks was used in the study

to enable a decoupling of area ([I.sub.2] No, CTAB) and structure (DBPA, CDBPA) effects. Most reports in the literature have been based on series of commercial blacks in which these two variables are strongly correlated[4 and 5]. A variety of loadings have been used to span the range of loadings normally seen in commercial compounds. The effects of carbon black characteristics at each loading on compound properties for each rubber system have been analyzed using regression analyses which include second order and interaction terms. The carbon black effect at each loading has been analyzed separately because of the substantial effect of loading alone.

Experimental

Nine commercial and experimental grades of carbon black in the class of semi-reinforcing blacks were used in this study. The analytical properties characterizing the blacks, summarized in table 1, are [I.sub.2] No (D1510) from 38 to 54 mg/g and CTAB (D3765) from 30 to 44 [m.sup.2]/g as measures of area (particle size), DBPA (D2414) from 65 to 140cc/100g and CDBPA (D3493) from 61 to 90cc/100g as measures of aggregate structure and tint (D3265) from 46 to 64% ITRB as a measure of aggregate size. Carbon black loadings in each rubber system were 0, 20, 40 and 60 phr, except for NR which was also tested at 80 phr.

Four rubber systems have been included in the study: SBR-1500, natural rubber, hydrogenated nitrile (Zetpol 2020) at 90% hydrogenation and EPDM (Nordel 1070). Both NR and SBR interact strongly with carbon black because of their relatively high degree of unsaturation. NR can strain crystallize while SBR, EPDM and HNBR do not. HNBR and EPDM have a relatively weak interaction with the carbon black because of their high degree of saturation. Compound formulations are summarized in table 2.

The combinations of carbon blacks and loadings were used to generate a statistically-designed D-optimal experiment based upon the experimental design package RS/Discover from BBN. A second-order regression model was used, including interaction terms between the independent variables for each rubber. The rubber base was used as a categorical variable. Thirty-two compounds, including seven replicates, were prepared and tested for each rubber system. All compounds were mixed using a mixer (Brabender) with a 300cc mixing chamber. A two-stage mixing procedure was used for the SBR, NR and HNBR systems. A single stage mix was used for EPDM. The procedures are summarized in appendix 1. Torque-time curves were recorded for each mix to characterize processing.

The mixing curves were analyzed for incorporation time (TI) and rate of incorporation (Thalf) where Thalf is the time for the torque curve to reach one-half the distance from the second power peak to the steady-state torque level. This value is inversely related to the rate of dispersion. TI is the time to the second power peak.

Bound rubber was calculated from the measured weight loss of an uncured sample of rubber immersed in a solvent for four days. Toluene was used for SBR, NR and EPDM. Acetone was used for HNBR. Except for HNBR, extrusion shrinkage was measured using a circular die with 5/1 L/D on a one-inch extruder (Brabender) run at 0.83 Hz and 100 [degrees] C. It was measured at 150 [degrees] C for HNBR. Viscosity and die swell were measured on a Monsanto MPT rheometer using a 16/1 L/D die at 80 [degrees] C and apparent shear rates from 10 to 150 1/s for NR, SBR and EPDM. For HNBR, the temperature was set at 120 [degrees] C.

Cure characteristics were measured using a Monsanto MDR curemeter at appropriate temperature (NR:155 [degrees] C, SBR:145 [degrees] C, HNBR:160 [degrees] C and EPDM:145 [degrees] C). Time and change in torque for 90% completion (t90 and DL90) are reported here.

Tensile properties were measured on an Instron 4204 tensile tester to obtain 100% and 300% moduli (E100, E300), tensile strength and elongation to break. Dynamic properties, where measured, were run on cured blocks tested in shear at 1 Hz, 70 [degrees] C using 0 mean strain and DSA from 0.5% to 40% with a servohydraulic Instron tester with computer controls and automated data analysis. Rebound was measured with a Zwick pendulum rebound tester.

Results and discussion

Natural rubber

The effect of carbon black variables on the processing and mechanical properties of NR compounds was complex, with significant differences in behavior depending upon the loading level. In general, the effect of loading on compound properties for a single black was greater than that for changes between blacks at a given loading level.

Figure 1 shows a comparison of the loading effect on DL90, the change in torque with cure, for two blacks which span the range of DBPA, NS and EX2 black. At the highest loading used, the difference in DL90 for the two blacks was 0.7 N-m compared to a change of 0.8 N-m for a change in loading of NS carbon black from 40 to 60 phr. Figure 2 shows a similar plot for the 300% modulus. The difference in modulus for the two blacks at 60 phr was 2.8 MPa compared to a change of 3.4 MPa for a change in loading from 40 to 60 phr for NS black. Figure 3 shows the effect of structure (DBPA) on E100 at each of the loadings. At the lowest loading, 20 phr, there was little or no carbon black effect on modulus. As the loading increased, the DBPA-dependence of modulus increased. Figure 4 shows the effect of loading on tensile strength for the same pair of blacks. There is clear indication of a maximum in tensile strength at relatively low loadings in NR. This effect may be caused by interference in the ability of the rubber to strain crystallize.

These results are typical for all of the NR rubber properties measured. At low loadings, changes in carbon black properties had little effect on compound properties. As the loading increased, the dependence of compound properties on carbon black properties became more important and the details of these effects became more complex. These results are summarized in table 3 which shows the significant terms in the regression analyses for compound properties at 20 phr and 60 phr. In general, structure dominated the tensile properties, with a secondary effect of tint (aggregate size) at higher loadings. Processing and cure properties showed a complex dependence on compounding variables, with only DL90 showing a consistently significant correlation. In general, DL90 increased with an increase in structure but at high loading, 60 phr, also was affected by particle and aggregate size. None of the dynamic properties showed a good correlation with carbon black variables at 20 phr. At higher loadings, such as 60 phr, particle size was a key variable, with structure having a significant influence on hysteresis. Higher strain dynamic modulus also depended on structure through interaction terms.

SBR-1500

The effect of carbon black variables on the processing and mechanical properties of SBR compounds is more complex than that found with NR. Carbon black analyticals have more of an effect at lower loadings and, in general, the regressions have a higher significance in SBR compounds. Particularly at higher loadings, the effect of changes in carbon black type can be greater than the effect of loading changes.

Figure 5 shows a comparison of the loading effect on DL90 in SBR for the same two blacks shown in figure 1 with NR. At each loading, the difference in DL90 is approximately 0.45 N-m compared to a change in DL90 of 0.8-0.9 N-m for a 20 phr change in carbon black loading. Figure 6 shows the effect of carbon black and loading on E300 in SBR for the same two blacks. At 20 phr, the difference in modulus for NS carbon black versus EX-2 is 1.1 MPa. At 60 phr, this difference has increased to 2.4 MPa. The change in E300 for an increase in loading from 40 to 60 phr was approximately 5.5 MPa for each carbon black. Figure 6 also compares the loading effect on tensile strength for the same pair of blacks. As in NR, there is an apparent maximum in the tensile strength with changes in loading, with the maximum occurring between 40 and 60 phr compared to 20 phr in NR.

Bound rubber is formed by the physical and chemical attachment of rubber molecules to the carbon black surface, possibly with additional entangled rubber trapped by the adsorbed rubber. Bound rubber depends upon a number of carbon black variables including particle size, structure and aggregate size as well as variables such as loading and temperature history of the compound. Figures on the effect of tint and structure for SBR at 40 phr are available. For the SBR compounds, bound rubber was a good predictor of many properties measured for formulations which varied loading, particularly at 40 and 60 phr, particle and aggregate sizes and structure over a wide range of values. The regression coefficients with bound rubber as the single independent variable were generally as good as using the underlying carbon black properties separately in a full interaction model. The overall results of the regression analyses for the carbon black effect on SBR compound properties are also available from the author.

HNBR

In general, the processibility of HNBR was different from most conventional rubbers. A relatively high processing temperature had to be used. The gum HNBR showed high nerve, so that smooth extrudates were obtained only for compounds with high carbon black loading, such as 60 phr in this study. The torque-time curves in mixing were substantially different for HNBR compared to NBR, reflecting the low degree of unsaturation with HNBR. Even with the relatively low carbon black-rubber interaction at the low unsaturation levels, carbon black properties had a substantial effect on compound properties, even at the lowest loading levels.

Figure 7 shows a comparison of the loading effect on DL90 in HNBR for the same two blacks shown in figures 1 and 5 with NR and SBR, respectively. The effect of change in black from 65 to 90 ml/100g DBPA at 60 phr gave a change in DL90 of 0.26 N-m compared to a change of 0.5 N-m for a change in loading from 40 to 60 phr. Figure 8 shows the effect of carbon black and loading on E300 in HNBR for the same two blacks. At 20 phr, the difference in modulus for NS carbon black versus EX-2 is 1.4 MPa. At 60 phr, this difference has increased to 2.2 MPa. The change in E300 for an increase in loading from 40 to 60 phr was approximately 5.5 MPa for each carbon black. Figure 9 compares the loading effect on tensile strength for the same pair of blacks. As in NR, there is an apparent maximum in the tensile strength with changes in loading, with the maximum occurring around 25 phr. There was relatively little effect of carbon black type compared to loading effects.

HNBR has a relatively weak interaction with carbon black. At low carbon black loading, e.g., 20 and 40 phr, the amount of bound rubber formed was insufficient to form a coherent gel. As a result, reliable bound rubber measurements could not be obtained. At 60 phr loading, the bound rubber was measurable only when the black had DBPA greater than 90 ml/100g. For these few blacks, the bound rubber value was comparable to that found with SBR compounds, varying from 11 to 22%. Bound rubber depended strongly on particle and aggregate size. There was insufficient data to test bound rubber as a predictor of compound properties. The overall results of the regression analyses for the carbon black effect on HNBR compound properties are available for 20 and 60 phr. At 60 phr loading, the interaction terms had no effect on most dynamic properties.

EPDM

Figure 10 shows that the DL90 in EPDM went through a maximum for increased loading for both NS and EX-2 carbon blacks, in contrast to the behavior shown in the other rubber systems. There was relatively little effect of loading on DL90. The 300% modulus showed a strong effect of loading for higher levels of carbon black. At the lowest level, there was little or no increase in modulus. By contrast, there was an increase in tensile strength, even at the lowest loadings of carbon black. In contrast to the other rubbers, there was no maximum in the tensile strength with an increase in carbon black loading. Although the combined effect of loading and structure on E300 was smaller for EPDM than with the other rubbers, there was the same general trend of an increase in modulus with an increase in loading and structure. The loading effect was significantly greater than the structure effect. The results of the testing in EPDM are available for 20 and 60 phr black compounds.

Conclusions

A comparison of the general effects of carbon black type and loading in a series of rubber types shows that the general effect of changes in carbon black are similar in all of the rubber compounds, although the magnitude of the effect depends strongly on the nature of the rubber. For most properties, the effect of loading and of carbon black type can be considered separately, using an expression of the form:

(1) P/Pgum = f(loading)f(carbon black, rubber) where the carbon black and rubber effects show up as affecting the coefficients in the function of loading. For example, the stress-strain measured in tension can be converted to true stress and Cauchy strain using standard expressions from the theory of rubber elasticity. The slope of the linearized plot yields a true modulus, E, which is essentially equal to the initial tangent modulus. Such a linearization for gum rubbers is shown in figure 11. Then the loading effect can be modeled empirically using:

(2) E/Egum = 1 + C*L*L where L = loading and C depends upon rubber type and carbon black type. The coefficient C depends upon the carbon black analyticals in a complex manner. With the limited loading data, the fit for the effect of loading cannot be distinguished from the Guth-Gold expression.

Future work will further explore the generalization of the relations shown in this work, including testing of analytical expressions for prediction of compound properties and the testing of the results against theoretical models, such as developed for stress-strain curves and reinforcement. Work is also underway for the measurement of crack initiation properties, such as tear strength and J-integral for these compounds to provide information on how carbon black affects fracture and fatigue properties of compounds. [Tabular Data 1 to 3 Omitted] [Appendix 1 Omitted] [Figures 1 to 11 Omitted]

References

[1]J-B. Donnet and A. Voet, "Carbon black," Marcel Dekker, Inc., New York, 1976. [2]J.T. Byers in "Rubber technology," M. Morton, Ed., Van Nostrand Reinhold Co., New York, 1987. [3]J.R. Dunn, presented at a meeting of the Rubber Division, American Chemical Society, Cincinnati, Ohio, October 18-21, 1988. [4]A.M. Gessler, W.M. Hess and A.I. Medalia, Plastics and Rubber: Processing, 3, 37, 109 (1978). [5]A.I. Medalia, Rubber Chem. Technol., 51, 437 (1978).

J.M. Funt and B. Chung, Cabot Corp.