Improved economics in the production of NR goods through the use of peptizers.

Improved economics in the production of NR goods through the use of peptizers

For blending with synthetic rubbers, and also to enable the necessary fillers and rubber chemicals to be incorporated and dispersed satisfactorily, we need natural rubber (NR) of defined viscosity.

The viscosity of NR can be reduced and made uniform by mastication, which shortens the molecule chains. Today mastication is mostly carried out in internal mixers and either precedes the actual mixing operation or constitutes a part of a one-step masterbatch process.

Will mastication continue to be necessary, or will natural rubber soon be available, like synthetic rubber, in a wide range of viscosities? From the type data and Mooney viscosity values of the NR exported by the main producer countries in 1988 it is clear that only about 10% of the total output has reduced viscosity levels, purity or other characteristics as a result of special treatment of the latex or control of the coagulation procedure. These operations are only possible at large plantations. Most of the NR produced consists of cup lumps, namely TSR 10-50, and the various RSS types. More than 60% of the total is accounted for by smaller operations without facilities for special treatment.

From the Mooney viscosities of the various types of NR (table 1) it is evident that mastication, though costly and time-consuming, will continue to be necessary in the processing of natural rubber. Conversion of the main types to lower viscosity levels is hardly conceivable in the short and medium terms, since it would have to be preceded by the establishment of additional large-scale plantations with suitably cloned rubber trees and relatively expensive latex preparation equipment. Thus the improvement of mastication on rubber processing machinery remains a worthwhile goal.

A fairly large number of publications exists on mastication in general and chemically accelerated mastication in particular[1-4]. One of the conclusions that may be drawn from this literature concerns the great importance of oxygen in mastication. The very rapid reaction of oxygen with carbon radicals formed through the breaking of chainlike molecules under strong shear forces prevents recombination; on the other hand, oxygen at elevated temperatures depolymerizes rubber through thermal oxidation. Mechanical mastication has a negative temperature coefficient because the shear forces decrease with rising temperature, whereas thermo-oxidative depolymerization becomes more intensive as the temperature rises. Because the two depolymerization mechanisms overlap, the curve representing mastication effect as a function of temperature passes through a minimum, which in the case of natural rubber is within the range 100-130 [degrees] C (figure 2).

Depolymerization at low temperatures, which results mainly from the mechanical stress, cannot be accelerated significantly with catalysts. Thermo-oxidative depolymerization, on the other hand, can be accelerated - and the start of mastication is shifted to lower temperatures when catalysts are present. Catalysts promote oxidative depolymerization by facilitating the formation of primary radicals or accelerating the decomposition of spontaneously formed peroxy radicals or hydroperoxides.

Many compounds, among them nitroso compounds, mercaptans and their zinc salts, thiocarboxylic acids and their salts, disulphides, sulphenamides, hydrazines, peroxides and metal complexes, are known to have depolymerizing effects[5]. Their degrees of effectiveness naturally depend to a large extent on the structure of the radicals R (figure 1).

The only peptizing agents now important industrially are pentachlorothiophenol and its derivatives, dibenzamidodiphenyl disulphide and complex iron compounds. The metal complexes known as boosters are used mainly to activate these latter organic molecules but are also used in pure form[6]. The commercial forms today consist of the peptizing agents and additives as fillers, waxes and oils that make them easier to handle or improve their dispersion, but have no peptizing effect themselves.

As peptizing agents catalyze the oxidative chain splitting, it is obviously conceivable that the chemically accelerated depolymerization could affect the properties and durability of the vulcanizates.

The investigation reported here was carried out in order to study the chemically accelerated mastication of pentachlorothiophenol derivatives and boosters, how carbon black affects depolymerization, to compare the respective vulcanizate properties in general and aging in particular, and to consider the economics of mixing in the presence of peptizing agents.

For all the investigations the commercially available active ingredient combinations of Renacit peptizers were used. The composition is given in table 2. The abbreviations represent the assay of the pentachlorothiophenol (P) or the corresponding zinc salt (ZP) as a percentage by weight and the assay of the proprietary booster (FeHe) in parts per hundred thousand. The mastication trials were performed with SMR 5, a natural rubber with very low impurity content, so that the influences of foreign matter on the mastication effect could be minimized.

The Renacit mastication agents are activated by a special and proprietary iron complex (FeHe). The activator content of the commercial ingredient combinations account for 0.2-0.7% of the organic peptizing agent content and is lower by the factor 2-4 than that of the pure booster, Renacit 8 (B 300).

The high-temperature depolymerizing effects (> approx. 100 [degrees] C) on natural rubber of the commercially available peptizing agents based either on pentachlorothiophenol or on the pure booster Renacit 8 (B 300) were adjusted to similar levels; this is shown for various mastication times in figure 3. In the case of differing pentachloro-thiophenol concentrations this adjustment was made via the amount of booster.

In mastication on a mixing mill (at temperatures below 100 [degrees] C) activated peptizing agents are more effective than the zinc salt of pentachlorothiophenol. Boosters are already active at temperatures considerably lower than those at which pentachlorothiophenol, for example, becomes active (figure 4).

The catalysis of oxidative chain splitting by peptizers is strongly influenced by various compounding ingredients. The effects of such rubber chemicals as accelerators, sulphur and antioxidants have been investigated already by H. Fries[7] and R.R. Pandit[8] In systems without fillers these chemicals act virtually as stoppers of peptizing agents.

The depolymerizing effects of boosters, on the one hand, and thiophenols, on the other, differ very much when carbon black is present. Figure 5 shows that in this case the booster alone is almost without effect and that, where combinations are used, much depends on their pentachlorothiophenol content. This is particularly important in one-step mixing, where carbon black is already added at the mastication stage. The effects of boosters are eliminated even at low carbon black contents. Hence only peptizing agents with high contents of organic active ingredient are suitable for one-step mixing procedures.

In one-step mixing the peptizing effect is greatest if the natural rubber and peptizing agent are put into the internal mixer first and then carbon black, zinc oxide, stearic acid, and possibly the other ingredients, are added after 60 seconds, for example. In this mixing cycle the effect of the peptizing agents is utilized most fully, as shown by the fact that the viscosity drop is very much greater here than in the blank test. Figure 6 completes the picture by showing the remilling behavior of base compounds containing carbon black, zinc oxide and stearic acid, in each case the mixing cycle being 150 seconds and the storage interval before remilling 24 hours.

The viscosity drop in the second or third mixing cycle is substantially the same with and without peptizing agents, and not, as might be expected, considerably greater. This suggests that in carbon-black-containing compounds and at the chosen low peptizing agent dosage the catalytic effect exhausts. We have observed similar behavior on a smaller scale in the case of compounds containing silica.

Now that the effects of peptizing agents on raw compounds and the interactions of these substances with fillers and rubber chemicals have been discussed, we must consider how the vulcanizate properties are affected. As catalysts of oxidative depolymerization, peptizing agents are of particular interest in connection with aging.

We determined the vulcanizate properties of NR compounds whose mastication was accelerated chemically and compared the data with those of equivalent compounds masticated entirely mechanically to the same viscosity level (ML-4 approx. 60).

As expected, no differences were seen in the static properties, such as tensile strength, elongation at break, modulus and hardness. The viscoelastic properties were likewise characterized by comparable storage modulus, loss modulus and tan [Delta].

The results just mentioned were supported by those of the Goodrich Flexometer test, where, comparable permanent set values were recorded for equal increases in temperature. The rebound resilience of natural rubber peptized to various degrees is shown in figure 7; to increase the statistical reliability of the results, mean values were calculated from the readings obtained for materials vulcanized for different times in 15 separate tests. Not until the compound was softened excessively (Defo hardness 450) was the rebound resilience seen to suffer, but this drop was unrelated to the presence of the peptizing agents.

Figure 7 shows both the influence on rebound resilience and also the disputed influence on tear strength. This property was measured on 25 ring specimens without a difference appearing at the 95% confidence level. In addition the fatigue resistance on the fatigue to failure instrument (Monsanto) was determined for the vulcanizates containing the peptizing agents Renacit 7 (P45-B75) and Renacit 9 (P25-B175). The results are comparable with those of the compounds peptized entirely mechanically.

The aging behavior of the vulcanizates was determined by several methods. Figure 8 shows the loss of tensile strength after different oxygen bomb aging times. Differences between the pure booster Renacit 8 (B300) and the variously activated pentachlorothiophenol peptizing agents were not found.

This result was supported by the results of hot air aging in the cell oven.

The results of the vulcanizate properties and aging tests show that the mastication of natural rubber accelerated by peptizing agents is no less favorable than purely mechanical mastication. This allows us to consider what economic advantages can be gained by the use of peptizing agents.

Where rubber is masticated in internal mixers, peptizing agents are now almost universally used. The high temperatures make thermo-oxidative depolymerization predominate. This process is accelerated greatly by all the peptizing agents considered in this article, which can be dosed sparingly at 0.05-0.2 phr.

Let us first consider mastication as a separate step in mixing. Figure 9 gives the power consumption curves with and without the peptizing agents in a 60-liter Werner and Pfleiderer GK-50 internal mixer. At the chosen dosage, 0.15 phr, all the peptizing agents were very effective. Relevant differences in Mooney viscosity were not seen.

The dumping temperatures of chemically masticated NR were about 20 [degrees] C lower than those of the rubbers masticated without peptizing agent. This presents a considerable advantage.

If mastication is carried out as a part of the one-step masterbatch operation, additional advantages are gained: the mixing time is shortened still further, energy consumption is minimized, the compound sheets are easier to handle and the logistics of intermediate storage are simplified.

In a previous investigation it was shown that the properties of pure NR vulcanizates are basically the same for both mixing procedures. In the masterbatch process in the presence of carbon black, the peptizer dosage must be increased a little bit over pure NR mastication.

From the mixing times needed with and without peptizing agents it is clear that the use of these products enables the capacity of a mixing line to be raised up to about 45%.

If we set the output of an internal mixer arbitrarily 100 for pure gum mastication in the presence of peptizers you get the output data given in figure 10. As a gum masticate needs an additional mixing cycle for the incorporation of fillers and other ingredients, in our experiments the output figure for this two step mixing cycle would be 50/36 with and without peptizers.

Summary

The mastication of natural rubber to technically desirable viscosity levels is accelerated greatly by peptizing agents based on pentachlorothiophenol and activated with the booster FeHe.

The time and energy savings achieved in mastication reduce processing costs and increase the capacity of processing equipment. Mixing is possible at considerably lower temperatures and the viscosity values of the compounds are more stable in storage.

Laboratory tests have shown that the vulcanizate properties and aging of natural rubber mixes masticated with the aid of peptizing agents are just as good as those of controls masticated entirely mechanically.

If carbon black is present during mastication, only active ingredient combinations with high pentachlorothiophenol contents re suitable. Mastication on mixing mills, is favored by relatively high booster contents, provided carbon black is not present. [Figures 1 to 10 Omitted] [Table 1 to 2 Omitted]

Acknowledgements

"Improved economics in the production of NR goods through the use of peptizers" is based on a paper given at the ACS Rubber Division meeting May, 1990.

"Post vulcanization stabilization for NR" is based on a paper given at the April, 1991 meeting of the Northeast Ohio Rubber Group.

"New tire black sidewall composition" is based on a paper given at the ACS Rubber Division meeting May, 1991.

"Development, characterization of a water based semi-permanent mold release agent" is based on paper given at the ACS Rubber Division meeting October, 1990.

References

[1]F.H. Cotton, Trans. IRI 6, 487 (1931) [2]W.F. Busse, Ind. Eng. Chem. 24, 140 (1932) [3]G.L. Bolland, G. Orr, Trans. IRI 21, 133 (1945) [4]P. Schneider, Kautsch, Gummi 6 WT 21 and 48 (1953) [5]Th. Kempermann, Gummi + Asbest, Kunst. 8, 566 (1974) [6]W. Redetzky, IRI Conference, Manchester Section, 22. -24.4.1969 [7]H. Fries, R.R. Pandit, Rubber Chem. Tech. 55, 309 (1982) [8]S.N. Chakravarty, R.R. Pandit, Kautsch. Gummi, Kunst. 29, 676 (1976) [9]A.G. Sears, NR Techn. 19, 68 (1988)