Improved processing stabilizer systems.

The rubber industry is frequently confronted with processing problems resulting in bad distribution of components, low quality or even unemployable rubber compounds. Reasons can be oxidative damage of the rubber because of extended storage or insufficient rubber stabilization, as well as oxidative cross-linking during mixing due to severe processing conditions. Standard rubber antidegradants, such as TMQ and PPDs, have a limited contribution to processing stability.

Specially optimized rubber processing stabilizers can reduce the problems caused by oxidative reactions during compounding. With these specially formulated antidegradant systems, it is possible to make black, natural or colored compounds with a reduced tendency of gel formation in a broader temperature range.

Additionally, these fully formulated antidegradant systems can be employed in silica-filled rubber compounds, e.g., tire treads, while broadening the processing window, reducing premature crosslinking and reducing compound viscosity and, hence, compound temperature during mixing.

This article covers new technology for improved processing and storage stability of rubber compounds, and offers information on recent advances in products that will allow for reduced scorching and higher temperature processing.

Antioxidants are used for synthetic rubber for two primary purposes, to stabilize the rubber system during the processing, and to provide protection to the rubber during storage and end use conditions. During the processing or compounding, the rubber is exposed to higher heat and shear, with oxygen present, and the stabilizer helps to maintain the needed properties of the rubber by preventing degradation upon production and auto-oxidation during storage, i.e., providing excellent shelf life. Generally, the antioxidants for this purpose are often referred to as basic stabilizers.

During mixing of rubber compounds, antioxidants are effective at avoiding free-radical induced oxidation, thus preventing undesirable crosslinking.

The first step toward avoiding processing problems is the use of effective antioxidants for basic stabilization of the synthetic rubber. This helps to prevent degradation during storage and transportation, which is occurring more frequently as a result of global commerce.

The effects of inadequate basic stabilization on processing can be minimized by the use of special processing stabilizers at the start of mixing. This undesirable and troublesome effect of oxidation crosslinking during processing can also be kept under control in mixtures at risk on account of the recipe or process used. Stabilizer systems that solve these problems are described in this article.

Long-term effect of basic stabilizers: Correlation between additive consumption or loss and residual stability of rubber

In order to simulate storage conditions of rubber, bales of synthetic rubber have been aged at 60[degrees]C for up to 24 weeks. To determine whether aging and additive reduction in the bale proceeds evenly, or whether a gradient can be observed in the cross section, the surface layer (10 mm thick) was cut off after aging and evaluated separately from the core.

Evaluation of three antioxidant systems

The highly volatile monophenol bis-tert.butyl-hydroxotoluene (BHT), sparingly volatile monophenol AO 1076 and multifunctional PSS2 were selected as representatives of different antioxidant classes used in the market and tested in high-cis and low-cis polybutadiene.

First, residual stabilizer concentration was determined as a function of oven aging time. Second, processing stability was evaluated in a Brabender measuring mixer by measuring the kneading time until the induction of a 1 Nm rise in torque, after going through a minimum torque reading. Third, we looked to determine whether a correlation exists between the measured residual antioxidant concentration after aging, and the subsequent processing stability.

Figure 1 shows the concentration of antioxidants in the top layer and in the center of the rubber bale throughout the aging period.

[FIGURE 1 OMITTED]

With AO 1076, the initial concentration of 0.23% is reduced by 10-15% during the 24 weeks storage at 60[degrees]C. There is very little difference between the concentration measured in the surface layer and the center of the bale.

With the multifunctional PSS2, however, the detectable content decreases much more quickly. Here, the reduction in the surface layer was greater than in the center.

An extremely marked decrease occurs with BHT, especially in the surface layer, where the initial stabilizer content of 1.2% drops by more than 90% after 24 weeks and by about 35% in the center of the bale.

The consequences of the varying degrees of reduction of aging systems on processing stability

Figure 2 shows the induction time in the Brabender test (110[degrees]C at 45 rpm) as a function of oven aging time at 60[degrees]C for high-cis BR. With AO 1076, the result is clearly a continuous decrease in induction time, and, hence, a progressive loss of processing stability in the surface layer and in the center. This reduction in processing stability is observed despite an almost constant stabilizer concentration.

[FIGURE 2 OMITTED]

This effect indicates that AO 1076 has a limited efficacy as a processing stabilizer in polybutadiene.

With PSS2, processing stability remains unaltered over a period of 24 weeks. This is somewhat surprising, as the measured concentration of PSS2 is decreasing over the duration of storage at the elevated temperature. Additionally, starting with only 0.12%, one-half the concentration of the AO 1076, the PSS2 still provides excellent processing stability.

Figure 3 summarizes the relative stabilizer concentration and the Brabender induction period after 24 weeks oven aging at 60[degrees]C.

[FIGURE 3 OMITTED]

With BHT, the processing stability is reduced as the additive concentration decreases. This is more pronounced in the surface layer than in the center. The residual processing stability is not, however, directly proportional to the residual additive content. AO 1076 shows a strong decrease of processing stability in the top and center of the rubber bale despite almost constant additive concentration. The PSS2 demonstrates the same processing stability, regardless of the measured reduction in additive concentration in both the top layer and the center of the bale.

Interpretation of the unexpected high performance of PSS2

PSS2 exhibits excellent processing stability, even when the additive content seems to be reduced or consumed during storage. This performance is related to its special, multifunctional chemical structure. This structure provides a protective mechanism, particularly under high temperature processing conditions, that is much more effective than that of conventional antioxidants.

In addition, the metabolites of PSS2 that are formed exhibit good antioxidant efficacy. Therefore, if the analytically determined decrease in concentration of PSS2 is due to this metabolite formation, it has only a minimal effect on residual stability. By measuring both the residual PSS2 and metabolites, we can account for nearly the entire, initial stabilizer concentration (figure 4).

[FIGURE 4 OMITTED]

The AO 1076 and BHT, not having the multifunctional capability of the PSS2, are not capable of giving the same protection to the rubber after extended storage conditions, regardless of whether the measured loss of these products over the duration of the aging is due to volatility or consumption via stabilization of the rubber. Analytical measurements of the residual stabilizer are not sufficient to determine whether the rubber is sufficiently stabilized. What may be an excellent stabilizer for freshly produced rubber may be inadequate, even at very high concentrations, to protect the rubber after storage and transport.

Decrease in processing stability of S-SBR during storage and transport

The first part of the article has demonstrated that the type of antioxidant has a strong impact on retained processing stability after long storage periods. The next question is whether standard tests such as Mooney viscosity or gel determination can clearly delineate the stability of the rubber for subsequent processing.

The following experiments describe whether, indeed, storage-induced damage is inflicted on the rubber that cannot be identified by specification data such as Mooney viscosity and gel content; damage that may change processing behavior and thus properties and the quality of compounds and the vulcanizates made thereof.

Experimental overview

The investigation was conducted with two samples of solution SBR, a commercially stabilized sample and a sample of the same S-SBR bale with an additional 0.1% of PSS2.

The rubber samples were aged as 10 mm thick plaques in circulating air ovens at 70[degrees]C to simulate extended storage. Specimens were taken from both the commercial S-SBR and the one with an additional 0.1% of PSS2 at various aging intervals. Then, two parameters frequently used in specifications were measured at the various aging times: Mooney viscosity and gel content (insoluble rubber in toluene at ambient temperature) (figures 5 and 6).

[FIGURES 5-6 OMITTED]

In the next step, the fresh and the pre-aged rubber samples were submitted to additional treatment in a Brabender in order to simulate internal mixing. The objective was to determine if Mooney viscosity and gel content measurements are sufficiently sensitive and reliable enough to reflect the processing behavior as seen in the Brabender.

The mixing process in an internal mixer was simulated by hot shearing tests in a laboratory Brabender kneader using the following conditions: Brabender chamber temperature 110[degrees]C; revolution of driven kneading blade 60 rpm; charge level, 100 %.

The definition of induction time is the kneading time in minutes to rise of torque of 1 Nm after going through the minimum (figure 7).

[FIGURE 7 OMITTED]

The last step involved the preparation of carbon black filled compounds in a laboratory kneader with all samples at differing pre-aged stages. Torque and temperature diagrams were recorded.

Objective of the investigations

The main objective of this investigation was to determine what level of additional aging of commercially available rubber changes the mixing behavior. And, whether this damage can be identified by measuring the rubber properties and/or whether additional high temperature tests permit such prediction. Can additional stabilizers be added to improve the stability and improve the processing of pre-aged, altered rubber?

Finally, clarification was sought whether the results of hot shearing tests permit conclusions regarding the rubber's behavior during mixing. Hot shearing tests are conducted with pure polymer only and at 100% charge level. During the mixing process, however, pure rubber is present only in the initial phase in a well-below-capacity-filled chamber. A charge level of nearly 100% chamber capacity is reached only after addition of other ingredients, i.e., essentially after the addition of carbon black. For this reason, it did not appear readily possible to predict the rubber's mixing behavior on the strength of results from hot shearing tests. In fact, hot shearing tests with an incompletely filled chamber may lead to deviations which can be explained by changing shear forces, oxygen exchange, etc. The experiments should permit clear answers to these questions. This article reviews only a small part of all data obtained in the study. It should be noted, however, that all results led to the same conclusions without contradictions.

Oven aging of raw rubber

Figure 5 illustrates the development of Mooney viscosity of the SSBR samples during oven aging at 70[degrees]C. Both formulations did not exhibit an increase in Mooney viscosity in the course of the first six weeks. The specimen with the additional stabilizer added had, as expected, better stability than the basic commercial sample itself throughout the duration of the aging period.

The measurements of gel content yielded the same result: No significant rise in gel content was observed in both the commercial SSBR and the one with an additional 0.1% PSS2 for up to six weeks aging at 70[degrees]C (figure 6).

After six weeks, the commercial SSBR without additional stabilizer showed a dramatic rise in the gel content. The SSBR with the additional stabilizer showed no increase in gel content, even up to 14 weeks aging at 70[degrees]C.

These aged samples were then subjected to hot shearing tests in a Brabender laboratory kneader at 110[degrees]C to determine changes in processing stability arising during aging. The resulting mass temperatures reached approximately 145[degrees]C, corresponding to those achieved during the preparation of compounds.

Figure 7 illustrates the development of torque of an unaged SSBR and the SSBR aged for five weeks at 70[degrees]C. The induction time of fresh, unaged commercial rubber was more than 30 minutes. After only five weeks pre-aging it dropped drastically to seven minutes, as evidenced by the torque reading. However, both samples had identical Mooney viscosities of 72.

Figure 8 demonstrates that the sample with an additional 0.10% of the proper stabilizer shows no detrimental effect after the five weeks aging at 70[degrees]C. The fresh, unaged samples and the aged sample with the 0.10% PSS2 additional stabilizer have equivalent Mooney viscosities and the time to induction of crosslinking remains beyond 30 minutes. These results indicate that there may be no or limited correlation between Mooney viscosity and actual hot shear induction time.

[FIGURE 8 OMITTED]

Figure 9 demonstrates that within five weeks, commercial SSBR displays a decrease of induction time from over 30 originally to approximately seven minutes after aging, although the Mooney viscosity of 72 remained unchanged. The sample stabilized with PSS2 showed no change in Mooney viscosity or in processing stability during the same time span of five weeks of aging. That means that samples stabilized with PSS2 fulfilled the requirements of the rubber specification and provided excellent processing stability.

[FIGURE 9 OMITTED]

The results of the raw rubber investigation can be summarized as follows: Rubber undergoes changes during storage that manifest itself in contradictory behavior in hot shearing tests, long before Mooney viscosity or gel content begin to rise. These storage-induced changes are stabilizer dependent and, therefore, oxidative in nature.

Preparation of carbon black filled S-SBR compounds

Carbon black containing compounds were prepared in the laboratory kneader with fresh and pre-aged samples. The resulting torque curves were plotted and evaluated. The objective was to determine whether the reduction of the induction time during the aged storage simulation permits conclusions regarding processability of the rubber and the quality of compounds. The development of torque during mixing is very characteristic and, within narrow fluctuations, well reproducible. Torque changes with increasing pre-aging are, consequently, caused by oxidative damage inflicted during the course of the simulated storage.

Figure 10 illustrates the mixing curves of fresh, unaged, commercial SSBR and SSBR after seven weeks pre-aging at 70[degrees]C. The difference between the two curves is obvious. Final torque, as well as mixing energy, calculated as the area below the curves, is substantially different, although the Mooney viscosities of 71 and 73 of the samples are hardly different.

[FIGURE 10 OMITTED]

When rubber stabilized with PSS2 was used, the curves of fresh and seven weeks pre-aged rubber were nearly identical, as seen in figure 11. If the results of these mixing tests are compared with those of the hot shearing tests, then the similarity of the findings becomes obvious: In both experiments, where the rubber is initially stabilized with PSS2, there is no increase of the torque, and that means there is no undesirable oxidation cross-linking.

[FIGURE 11 OMITTED]

Influence of IPPD on processing stability

As a rule, antioxidants based on aromatic amines (PPD) are components of most compound recipes. For this reason, a commercial SSBR was tested that contained IPPD in the recipe. It was found that the mixing curves of pre-aged rubber with or without the addition of 1 phr IPPD were equally poor, as seen in figure 12. Consequently, compounds of inferior quality resulted in both instances. The required energy to process the rubber containing the IPPD was significantly greater than the energy used when the SSBR contained PSS2, as seen by the torque measurement on the y-axis.

[FIGURE 12 OMITTED]

Smooth processing of damaged rubber using optimized processing stabilizers

Damaged rubber cannot be regenerated on a large scale through post addition of antidegradants. Therefore, tests were performed to ascertain whether the addition of stabilizers at the beginning of the mixing process can prevent oxidative crosslinking of damaged rubber and effectively consume hydroperoxides that have accumulated in the stored or pre-aged compound.

For this purpose, a commercially stabilized low-cis polybutadiene was first aged at 70[degrees]C for up to three weeks. The gel content and Mooney viscosity rose only minimally during this pre-aging time. On subsequent exposure of the rubber in silicone oil at 150[degrees]C for 20 minutes (gel sensitivity or James test), however, the gel content rose from 0.15% to 10%, and the Brabender induction time (at 130[degrees]C and 40 rpm) dropped in the same period from 36 to 5 minutes.

When the Brabender test was performed under slightly more rigorous conditions (150[degrees]C/40 rpm), induction time, without any stabilizer additions, was only three minutes (figure 13). With 2% 6PPD or 0.3% AO 1076 + 0.6% AO PS 800 (a thioether), minimal improvements were made, to 6.6 and 5.2 minutes, respectively.

[FIGURE 13 OMITTED]

However, with 0.2% PSS2, a multifunctional processing stabilizer, induction time was increased to >60 minutes, as shown in the torque curves in figure 13. A system such as this prevents processing problems arising from storage-related formation of hydroperoxides. The PSS2 stabilizer was far more effective at 0.2% than the other systems tested at much greater concentrations.

This processing benefit can only be recognized when the rubber is not already crosslinked. Pre-crosslinked rubber would be apparent via a Mooney viscosity or gel content test.

Prevention of oxidation crosslinking of rubber compounds in critical mixing processes

If undamaged rubber is used in mixing processes giving off high frictional heat, viscosity can increase as a result of oxidation crosslinking. New processing stabilizer systems have been developed that can prevent undesired crosslinking, and therefore keep the viscosity of the compound low, in some cases actually making mixing possible. This was demonstrated by tests run on a silica-filled SSBR mixture and EPDM carbon black masterbatches.

Table 2 shows the Mooney viscosity (MS 1+4) of the basic mixtures. With 0.5 and even with 2.0 phr 6PPD, viscosity scarcely differs from that of the stabilizer free reference.

With 2 phr AO 5057, a liquid diphenylamine, Mooney viscosity only reached 74, as opposed to 86 with the reference and 85 with 2 phr 6PPD. With only 0.5 phr of PSS2, a unique stabilizer, the viscosity was only 61; Mooney viscosity increase (crosslinking) was effectively prevented.

On the addition of 0.5 phr Ingralen 450, a naphthenic oil softener used as a reference, Mooney viscosity decreased from 86 to 81.

Summary

The proper protection of rubber during storage and processing is critical to prevent undesired oxidative crosslinking. The primary precaution that is most commonly used in the industry is to choose a natural or synthetic rubber that has a basic stabilizer added at the point of production. This stabilizer or antidegradant needs to be properly chosen in order to avoid the oxidative crosslinking associated with that elastomer in its likely storage and processing environment.

As was evident in the article, traditional means to measure the effective stabilizer concentration and performance do not always reveal the extent or the degradation to the rubber or how the rubber will subsequently process. Mooney viscosity, retained antioxidant and gel content tests can be of limited significance on measuring retained processing stability of the rubber.

It was demonstrated that PSS2, at very low levels, keeps the processing stability at a very high level, even after long rubber storage times. Thus, typical storage and transportation-related processing problems can be effectively eliminated. Even rubber that has seen extended storage can be compounded without problems if the material has been effectively stabilized with PSS2, either initially or added later at the beginning of the mixing step.

By adding PSS2, the undesirable effect of oxidative crosslinking during processing can also be kept under control in mixtures at risk on account of the recipe or process used.

Table 1--SSBR/silica mixture

Recipe:
SSBR Buna VSL 5025-0                   75
BR Buna CB 527T                        25
Extender oil, Ingralen 450             33
Ultrasil VN 3                          80
ZnO                                   2.5
Stearic acid                          1.0
Antilux 110                           1.5
Processing stabilizer            Variable
Mixing temperature [degrees]C         110
Rotor speed (rpm)                      75
Fill factor                          0.72
Mixing time (minutes)                  20

Table 2--influence of processing
stabilizers on Mooney viscosity of silica
filled S-SBR/BR compounds

Additive                         MS 1+4 (100[degrees]C)

Without                                              86
0.5 phr 6PPD                                         84
2.0 phr 6PPD                                         85
0.5 phr AO 5057                                      78
2.0 phr AO 5057                                      74
0.5 phr naphthenic oil                               81
0.5 phr PSS2                                         61