Effective acceleration.

Effective acceleration Hydrated silica fillers have been used in rubber formulations for several decades with wide success. They provide a high degree of reinforcement, improve tear strength and are very useful to all compounders in developing compounds with specific properties.

However, they have had drawbacks. When first introduced, it was hoped that these products would be effectively "white" carbon black. As we all know, they are not. Among the problems faced by the white fillers were problems with compression set, tension set and heat generation in dynamic applications.

There have been various ways to work around some of these problems, including use of co-agents. However, most of these "fixes" were less than totally effective.

Recently, however, new information was published by PPG regarding modifications that can be made to cure systems to improve the compression set and dynamic properties associated with silica filled compounds. PPG refers to the new acceleration systems as "effective acceleration."

What is effective acceleration?

Effective acceleration (EA) can best be described as an acceleration system that elevates the level of acceleration to one that is effective at overcoming deficiencies typical in silica filled rubber formulations.

Virtually all compounders are familiar with efficient vulcanization systems where accelerator levels are increased while sulfur levels are dropped, yielding compounds with low compression set and improved heat resistance. EA is similar in that accelerator levels are dramatically increased. Also, only one accelerator is used rather than a combination of two. Also, sulfur levels are not decreased. The result is a compound that closely approaches black filled compounds as far as compression set and heat build-up properties.

So far, most of the work has been done using natural rubber as the base polymer. The specific formulation used is shown in table 1. Accelerators used in work so far are also shown in table 1.

The activity of accelerators is determined to a great degree by the rate and extent of crosslink formation. While swell data is frequently used to determine crosslink density information, for this work, PPG decided to use the data available from oscillating disc rheometer curves. The ODR data can be obtained more rapidly and involves less filler-polymer bonding. Using the difference between minimum and maximum torque values, the number of crosslinks (not length of crosslinks) can be compared between the different iterations of the base formula. Normally, the rheometer data can also be expected to have a linear relationship to 300% modulus of the compound.

Figure 1 shows the ODR torque difference results for the different accelerators used in this study. The greatest number of crosslinks was achieved by using one of the sulfenamides, TCS, TBBS or MBS, at levels of 3.0 to 4.0 parts. The TCS is faster than the other two but, as shown in figure 2, it also has the greatest reversion. Figure 3 shows that the 300% modulus for the compounds virtually duplicates the ODR torque results.

One point of significance is the fact that total crosslinks are not necessarily related to the rate at which they develop. Cure rates, as might be expected, were fastest for TMTM, ZBDC and TBTD. However, none of these produced high levels of crosslinking.

TCS was next fastest in cure rate and also produced a high state of cure. Examination of the structural differences between the different accelerators suggests that curing speed is associated to the dithiocarbamate structure [Mathematical Expression Omitted] while the greatest degree of crosslinking is associated with the sulfenamide group [Mathematical Expression Omitted]

This same relationship is reported to occur with black filled compounds. However, scorching of these compounds at this level of acceleration prevents effective examination.

The effectiveness of TCS in providing both speed and degree of crosslinking can be explained by the fact that it combines both these structures in one material. [Mathematical Expression Omitted]

One exception to the cure rate/structure relationships is ZEPDC. It is conjectured that the phenyl group on this molecule hinders the crosslinking mechanism.

What about processability?

As we all know, there is more to a good cure system than just cure rate and state. Before any of that comes into play, we've got to get the material through a mixer, mills, extruders and a variety of other pieces of equipment. Two prime concerns are viscosity and scorch.

Regarding viscosity, there are significant differences between different accelerator types in silica filled compounds. This has been known for some time. Amine type accelerators and zinc salts result in significantly lower viscosities and greater nerve because of their de-agglomerating effect on the filler.

Scorch is the other major process consideration. Resistance to pre-cure in process must always be considered along with reactivity at curing temperatures. For the work in this study, the T2 value from ODR curves at 302[degrees]F were used for evaluation. Typically, a value of 4-5 minutes would be considered equivalent to Mooney scorch values of 15-25 minutes at 250[degrees]F and would be considered minimum acceptable under these conditions.

Results of the DETU, DPG, TMTM, TBTD and ZBDC were all below this value. Only the MBS, MBTS, TBBS, TCS AND ZEPDC were found to have adequate scorch safety for normal factory processing.

What about compression set?

With acceptable accelerators for processability determined, it's time to look at performance of fully cured products.

In sulfur cured compounds, performance of compounds is determined by the length of sulfur bond as well as the number of crosslinks. Shorter bonds (mono- and di-sulfide) produce materials with lower compression set, better heat aging and reduced heat build-up than longer bonds (polysulfide).

Figure 4 shows compression set as a function of level of the accelerators determined to be acceptable for processing. Figure 5 shows the same relationship for heat build-up

Both graphs show that MBS, MBT and ZEPDC have a continuous and nearly linear improvement in both compression set and heat build-up as accelerator level increases to 4.0 parts. For TCS and TBBS, improvement continues to about 3.0 parts and then levels out. Of particular note, heat build-up of the TBBS compound dropped 72[degrees]F as the accelerator level was increased from 2 to 3 parts. Tan delta values are reported to show the same relationship.

What about other properties?

Whenever a compound has a tear problem, one of the first solutions looked at is the use of silica fillers. The fine particle size silicas have been used to improve tear strength in compounds for years. While trying to improve properties such as compression set and heat build-up, it would not be advantageous to lose that benefit.

Results of this study showed that while the increased accelerator levels did reduce the tear strength in the silica compound, they still remained significantly higher than the control compound using N220 black. Trouser tear of the control was approx. 25 pli. Values for TCS cured material dropped from 140 pli at the 2.0 part level to 95 pli at 4 parts. For TBBS cured material, the tear dropped from 105 pli at 3 parts to 50 pli at 4 parts.

Heat resistance of the silica filled formulas using the effective acceleration systems appeared significantly better than the black filled control. Using a 4 part level of TBBS, the compound retained 53% of its original elongation and 50% of its original tensile strength after aging 7 days in open air at 212[degrees]F. This compares to values of 16% and 14% for the black filled control. Using TCS produced results intermediate between these two.

Summary

It's unlikely that silica will ever totally replace carbon black as a filler in rubber products. However, being able to compound silica filled formulas to approach black filled formulas offers a number of new possibilities for high performance compounds.

Problems with compression set and heat generation of silica filled rubber compounds have limited their effectiveness in many applications. Often, compounders have been torn between using silica to achieve performance benefits versus not using it because of performance problems. By using this type of "effective acceleration," it appears that a number of these benefits can now be obtained without suffering the penalties of the past.

One caution is bloom. While bloom has not occurred on shelf aging of the vulcanizates made, it should be evaluated in any formulations where accelerator levels are raised as they are here. Reported low cyclohexane solubility values for TCS and MBTS indicate that this could be a problem. However, adsorption effects from the active silica surface may well alter bloom and migration phenomena.

While all the data currently available are based on natural rubber formulas, PPG reports that similar effects have been seen in other elastomers as well, including SBR and EPDM.