Rubber World — February 2013
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Talc for peroxide cured compounds
Oscar F. Noel, Gilles Meli, Imerys Talc

Oscar F. Noel, III, and Gilles Meli, Imerys Talc

Peroxide cures are becoming more common in the rubber industry in order to meet performance demands such as better thermal resistance and lower compression set. An additional incentive for a peroxide cure is the elimination of undesirable by-products such as nitrosamines. Unfortunately, peroxide cured compounds are generally inferior in mechanical properties to those cured with sulfur or sulfur-donor systems. In order to minimize the loss in properties, the chemist must consider formulations with a higher percentage of polymer and/or reinforcing fillers. This results in a more expensive compound. One approach to mitigate this involves the partial substitution of talc for the reinforcing fillers.

It has been demonstrated that talc exhibits a synergism in combination with carbon black, improving mechanical properties, thermal resistance, permeability, weathering and toughness (ref. 1). This results in improved performance and durability of the finished rubber goods in service. The partial substitution of talc for carbon black also results in significant improvements in processing, with faster mixing times, lower compound viscosity, improved mold flow, higher extrusion rates, and reduced nerve and heat generation. These expand the process window to provide latitude in manufacturing, such as reduction in temperature to avoid scorching. In addition, the partial replacement of carbon black with talc can not only lower raw material cost, but can result in higher production throughput to further reduce finished product cost.

Partial substitution of talc for precipitated silica reduces compound viscosity without sacrificing performance (ref. 2). In addition, talc increases the rate of incorporation of silica, thus, reducing mixing time and increasing output (ref. 3).

Talc is a unique mineral. It is hydrophilic on the edges of the platy particle. The planar surfaces, however, are hydrophobic and are preferentially wetted by organic substances such as rubber. This translates to rapid incorporation of talc in rubber compounds. Eldred (ref. 4) observed excellent adhesion between the talc and the elastomer in his crack diversion study. This adhesion is evident for both the treated and untreated talc in figure 1.

In the case of the untreated talc, the surface of the talc is coated and no interfacial debonding is apparent due to cryogenic fracture. For the silane-treated material, the adhesion is obvious. The talc used in this work is Mistron Vapor R, which is a high purity, microcrystalline talc. Its platy morphology is preserved by special grinding techniques in order to maintain the aspect ratio.

The environmental aspect is also a consideration. The production of ultra-fine talc requires significantly less energy than needed to manufacture carbon black. It is estimated that the amount of CO2 emitted during the production of talc is only 1/10 of that for carbon black. Therefore, this makes partial substitution of talc for carbon black environmentally attractive.

Although talc has a pH of approximately 9, it may retard the cure in silicone rubber where the peroxide concentration is less than 1 phr. There is little, if any, evidence that talc retards the cure in systems with >2 phr of peroxide, as shown in table 1.

The use of talc in peroxide cured systems for hose, ducts, seals and primary insulation is examined. The results presented in this article provide supporting evidence for the use of talc in peroxide cured rubber compounds to meet the demanding requirements of today.

Failure in peroxide cured hose is initiated by thermal/oxidative attack, not electrochemical degradation. This exposure results in a loss of mechanical properties such as elongation. The hose becomes brittle with time and small cracks develop, usually in the cover (ref. 5). These propagate through the wall, resulting in failure (ref. 5). Therefore, retention of elongation with heat aging and the crack growth resistance are critical to the performance of coolant hose. Talc imparts both of these attributes to rubber compounds (ref. 1).

Polysar/Bayer found that partial replacement of carbon black with silane-treated talc in EPDM coolant hose provides higher elongation retention after aging (ref. 6). Ultra-fine talc has also been shown to improve crack-growth resistance via crack diversion phenomenon and reduction of stress concentration at the crack tip (refs. 1 and 4).

To determine the significance of surface-treated talc substitution on the performance of peroxide-cured EPDM hose compounds, a fractional-factorial designed experiment was conducted. The following variables were considered to be important: 1) volumetric percentage of polymer, 2) filler ratio, i.e., ratio of carbon black to the total filler content, 3) coagent concentration, and 4) zinc oxide level. The oil loading was adjusted in an attempt to maintain the same durometer A hardness.

The assigned levels for the variables are given in table 2.
Nine compounds were sufficient for the design; however, three additional compounds were included to provide supplementary information on the interaction of variables in the presence of zinc oxide. The formulations are shown in table 13.

The statistical analysis of the tensile data is summarized in table 3 (ref. 7). The S indicates that the variable had a significant effect on the mechanical property. The + or - sign after the S designates whether it was a positive or negative influence. NS signifies that the influence of the variable is not significant. M indicates a marginal influence. Certain influential points were removed from the analysis to provide a more accurate assessment of the effect of the variables on a specific property.

Inspection of table 3 indicates that filler ratio is the most important variable affecting tensile properties; volumetric percentage of polymer is the second most influential variable; and zinc oxide and the coagent have a minor influence on all mechanical properties and are only important in compounds with marginal properties.

The significance of the different variables on thermal aging performance after exposure to 150°C air for 168 hours is summarized in table 4.

Both filler ratio and polymer content have a significant effect on elongation. The filler ratio, however, has a negative influence, as designated by the – sign; therefore, the retained elongation improves with a higher percentage of talc.

In regards to compression set, the testing was conducted at 150°C for 70 hours. The results are presented in table 5. The number in parenthesis gives the ranking of importance for that variable.

Polymer content ranked (1) is the most important variable. Increasing polymer content decreases compression set. Zinc oxide is second (2) in importance. The presence of ZnO increases compression set. As expected, the coagent reduces compression set and is slightly more influential at high filler ratios. The filler ratio is significant only because it affects the response due to zinc oxide and coagent. Therefore, the percentage of silane-treated talc in the formulation had little effect on the compression set.

The effect of the variables on die C tear results was also determined. Statistical analysis of the data indicated that the filler ratio was the most important variable. It has a negative exponential effect; therefore, the die C increases with a higher percentage of talc. Volume percentage of polymer is second in importance, i.e., die C improves with an increase in polymer content. The coagent (SR517) level is also important, but has a negative effect.

Since crack propagation can determine the life of a hose, it is important to understand the effect of the variables on crack growth. The significance of talc on crack propagation is demonstrated in table 6. The cracks are simulated with a razor cut across the face of the tensile specimens. The depth of the cut was measured with a microscope.

The energy to break (EB), which corresponds to toughness, is significantly higher in the compounds with talc than in the two black controls (C1 and C2). For example, at 55 volume percent polymer, the average EB of specimens containing talc is 3.62, compared to 1.05 for the control with carbon black only. EB is significantly affected by filler ratio (-) and volume percentage of polymer (+). Therefore, energy at break increases with higher talc concentration.

The tensile-fatigue performance was evaluated using the Monsanto fatigue to failure tester. To simulate service conditions, testing was done on samples that were exposed to 100°C for 168 hours and were cut with a razor to simulate a surface crack. The depth of the cut was measured with an optical microscope. The samples were stretch ed 40% at a rate of approximately 90 cycles per minute. The permanent set was measured after approximately 150,000 cycles. The summary of the statistical analysis is presented in table 7.

The cut depth proved to be the most significant variable in cut-fatigue resistance, as expected. The filler ratio was next. For example, the fatigue performance improves as the talc percentage increases. Polymer content and coagent level also have a negative influence on fatigue.

In the case of permanent set, volume percent polymer is the most significant variable. As it increases, the permanent set decreases. The concentration of coagent also has a significant effect on the set. The filler ratio has only a marginal influence, which would suggest that the partial replacement of carbon black would have little effect on hose dimensions.

Mechanical rubber goods
Talc is used in mechanical rubber goods such as seals, diaphragms, and gaskets. In elastomers such as HNBR, which are typically cured with peroxides, partial substitution of talc for carbon black can provide superior resistance to aggressive environments. This is illustrated in figures 2 and 3 by the change in tensile strength and elongation after immersion in two flex fuel mixtures. Fifty percent of the carbon black was replaced on an equal volumetric basis, i.e., 1.5 parts of talc per one part of carbon black with untreated talc and two silanetreated talcs. The peroxide cured formulation and complete data set are located in tables 14 and 15. It is apparent that the resistance to a severe environment is improved, even at 50% replacement of the carbon black with talc.

It is also interesting to compare the performance of ultra-fine talc to precipitated silica, as shown in table 8. The silica products ranged from semi-reinforcing to ultra-reinforcing, with surface areas from 35 to 255 mg2/gm. The peroxide cured HNBR formulation can be found in table 16.

Inspection of table 8 indicates the ultra-fine talc provided equivalent tensile and tear properties as achieved with the reinforcing precipitated silica. The talc-reinforced compound, however, had superior compression set. It should be noted that all reinforcements were surface treated with vinyl-silane via integral addition, i.e., in-situ.

The effect of heat aging is shown in figure 4. It is apparent from this figure that the talc-reinforced compound has not only higher initial elongation, but significantly higher elongation after aging for over 500 hours.

An additional example of the applicability of talc in peroxide cured systems is demonstrated by the performance of EPDM automotive air duct. The partial replacement of carbon black with talc resulted in the following:

• a significant increase in elongation at break;
• no reduction in tensile strength; and
• an increase in tear properties.

The elongation at break is 60% higher at 20% replacement of carbon black with talc, as shown in table 9. The die C and trouser tear also increase significantly with the equal volumetric substitution of talc for carbon black.

As with hose, the life of mechanical rubber goods is affected by surface cracks or cuts resulting from aging and/or abuse. Talc has been shown to increases the flaw resistance of both sulfur and peroxide cured compounds. This is demonstrated in table 10 by the energy at break of tensile specimens which were precut with a razor blade to simulate a surface flaw.

The energy at break is three times greater with 20% replacement of carbon black with talc. This translates to toughness and durability in the finished product.

Wire and cable
Talc has been used for years in wire and cable because it is electrically non-conductive. From the technical literature, butyl compounds reinforced with Mistron Vapor talc exhibited a significantly higher dielectric strength than calcined clay, ie., 1,150 vs. 650 volts/mil (ref. 8).

Peroxides are used to cure EPDM compounds in primary insulation for medium to high voltage cable. In a standard formulation, a silane-treated talc was determined to be suitable for 2-35 kV industrial wire applications according to the EM60 test, as shown in table 11. The testing was conducted on a single wire Cu conductor using a peroxide cured EPDM formulation (NDR 3728), shown in table 17. The treated talc was equivalent to the industry standard, i.e., silane-treated calcined clay, after 14 days exposure to 75°C water at 600 volts AC.

The long-term performance of silane-treated talc is demonstrated by the electrical results after 12 months of exposure to 90°C water and 600 volts, as shown in table 18. It was equivalent to that of treated calcined clay. It should be noted, however, that the silanetreated talc compound became unstable at higher electrical stresses (2,200 volts).

In addition, the surfacetreated talc exhibited similar cure rheology and superior mechanical properties to the silane-treated calcined clay, as shown in table 12.

Talc can be used in peroxide cured compounds to improve overall performance. This was demonstrated in an EPDM coolant hose formulation where the partial substitution of a surface-treated talc for carbon black was a significant factor in:

• increasing elongation and energy at break;
• improving thermal performance;
• increasing die C and flaw resistance; and
• enhancing fatigue.

The combination of tear resistance and insensitivity to flaws which results in increased toughness and durability provides superior performance in service. Talc substitution also had little effect on the compression set. These improvements in performance were also observed with untreated talc in a highly-filled (>250 phr) EPDM formulation.

In peroxide cured HNBR compounds, surface-treated talc was substituted for 50% of the carbon black in order to improve the resistance to flex fuels. In a second compound, an ultra-fine, silane-treated talc exhibited equivalent mechanical properties to a reinforcing precipitated silica. The talc product also provided superior thermal aging.

Surface-treated talc can be substituted for silane-treated calcined clay in primary insulation for medium voltage industrial cable. The superior mechanical properties, as well as improved thermal performance achieved with talc, provide the opportunity to increase the filler loading and reduce cost in wire and cable.

In addition to the above advantages, talc improves the process-ability of compounds, thus reducing any production problems and increasing output. Talc can also reduce raw material cost by substitution or partial replacement of expensive ingredients.