Plasma polymerization of sulfur to decrease the blooming effect and its effect on vulcanization with different accelerators.

Sulfur vulcanization is widely used in the tire industry. Despite all the advantages of such processing methodology, there is an important aspect that must be taken into account when preparing the mixture to process. The main problem rubber mixers have to deal with is the poor miscibility of sulfur due to its polarity. Furthermore, even when a good homogeneity is obtained, in some cases it can occur that during the storage of the formulation before its vulcanization, sulfur blooms onto the surface. Although the mixing process homogeneously distributes the sulfur in the rubber mixture, during the non-vulcanized mixture, storage ruination of the sulfur takes place as a result of the low miscibility. This migration can create sulfur agglomerates and spots on the surface that added the final properties of the vulcanized product (ref. 1), including, among others, high scorching risk and domains with low adhesion (ref. 2).

Several projects have been developed in order to solve this problem. Basically, the solutions found until this moment have to do mainly with the following three options: sulfur substitution by sulfur donor compounds, chemical treatment of the sulfur and/or the improvement of the affinity between sulfur and the polymer matrix.

Regarding the chemical modification, one of the most used systems is the addition of diethylamine chlorhidrate to the mixture. This increases the thermal resistance and the blooming (ref. 3) at the same time. Another chemical used is pentachlorothiophenol, which reduces the blooming as well as the mastication time required. Although physical properties are not affected by the addition of these chemicals, the scorch time is reduced (ref. 4).

In order to improve the sulfur-polymer miscibility, different products can be introduced to coat the sulfur and increase its affinity with the polymer matrix. There are some interesting patented possibilities, such as the use of surfactants in the formulation (ref. 5) or the generation of core-shell structures by treating sulfur with [alpha]-methylstyrene and process oil (ref. 6). A more sophisticated option consists of microencapsulating sulfur with epoxy resins. The latter implies a noteworthy improvement compared to untreated sulfur, however, blooming reduction remains at a rate of 10 to 30% (ref. 7). Even though the problem has been partially solved, all the solutions presented are mainly based in dispersing agents and coating oils, which might always imply a significant increase in the amount of chemicals used in the formulation, with potential effect on the final mechanical properties of the rubber good.

Plasma polymerization of powder sulfur could be an efficient way to modify sulfur surface polarity without the necessity to add more ingredients to the rubber formulation. Plasma polymerization has been described as a technique that generates thin uniform layers capable of changing the surface properties of particles (refs. 8 and 9). In addition, this process is highly versatile due to the fact that different monomers can be used, and that several reactor conditions can be applied to create a specific chemical surface composition (refs. 10-12). However, it should be taken into account that surface modifications, initially meant to improve certain properties, will probably also modify the reactivity of the compound. For that Mason, it is necessary to study both the coating of the particles and its influence in the vulcanization process (ref. 13).

Thus, the aim of this present work is to study cold plasma polymerization on sulfur to modify the interaction of sulfur with the polymer matrix and consequently increase its solubility. The influence of plasma treated sulfur on rubber vulcanization will be studied to determine if the treatment influences the vulcanization kinetics, especially the length of scorch time (ref. 4).

Experimental

Materials

CBS (cyclobenzothiazole sulfenamide) and TMTD (tetramethylthiuramdisulfide) were provided by Flexsys: stearic acid, pyrrole and perfluorohexane by Aldrich; and micronized sulfur delivered by Jevsa S.A. Fluka provided high purity squalene.

Sample preparation and measurements

Sulfur samples were prepared in a downstream plasma reactor. Pyrrole, acetylene and perfluorohexane were used as monomers at 2.5 x [10.sup.-4] bar of partial pressure inside a high vacuum reactor. Plasma was generated using RF (14.6 MHz and 60 W) for a period of two hours. Pyrrole-acetylene treatment was generated at a lower RF power (5 W).

Vulcanization reactions were performed at 140[degrees]C, as described in a previous work (ref. 14). The mixture used is presented in table 1.

Results and discussion

As described in the previous section, sulfur was treated with three different monomers. The monomers were chosen regarding their polarity (pyrrole > acetylene > perfluorohexane) mad the possibility to obtain pinhole-free films on the sulfur surface. A combination of pyrrole and acetylene was also tested.

It should be pointed out that the objective of the treatment was to improve the sulfur miscibility within the rubber formulation. It is well known that polarity plays a very important role ha this feature. Therefore, the plasma film resistance to hydrophilic and hydrophobic solvents was evaluated using a simple approach. Sulfur and the different plasma treated sulfurs were poured on squalene simultaneously. Pyrrole and pyrrole-acetylene treated samples sink slightly faster than perfluorohexane treated and untreated sulfur. After ultrasonic stirring for 15 seconds, sulfur was dispersed in squalene. Untreated sulfur precipitated in less than one minute. Perfluorohexane-treated sulfur remained more than one minute in dispersion and pyrrole-treated sulfur precipitated after six minutes. Pyrrole-acetylene treated sulfur showed a slightly different behavior. It was observed that sulfur particles were separated from the polymeric coating and precipitated at the same time as untreated sulfur. The reason for this separation was attributed to the low cross-linking of the pyrrole-acetylene plasma polymer that covered the sulfur particles. Therefore, a higher RF power was determined to be needed to obtain resistant coatings on sulfur.

In order to better understand the effect of surface treatment on sulfenamide and thiuram vulcanization acceleration kinetics, sulfur treated with pyrrole was chosen to study its behavior and evolution during its incorporation in the polymer chain. Results were compared with those obtained on nontreated sulfur.

Sulfenamide vulcanization

The model compound vulcanization approach (MCV) was used to characterize the vulcanization kinetics in terms of evolution of accelerators, active sulfurating intermediates and sulfur. Squalene was used as the model molecule to simulate the behavior of real natural rubber.

Sulfur concentration evolution versus reaction time determined by RP-HPLC, is presented in figure 1. Differences in the kinetic behavior due to the surface treatment were detected. Untreated sulfur is consumed in 30 minutes, while the sulfur treated with pyrrole takes around 50 minutes.

[FIGURE 1 OMITTED]

The decomposition of the accelerator can also give information about the sulfur reactivity. In figure 2, CBS showed a slightly different behavior in the final decomposition minutes when treated sulfur was present in the mixture. As studied in a previous work (ref. 15), this fact could be related to the low accessibility between sulfur and CBS due to the separation produced by the plasma polymer film. However, this hypothesis needs further study to be confirmed.

[FIGURE 2 OMITTED]

The CBS decomposition leads to the MBTS (2-bis-benzothiazole-2,2'-disulfide) formation. This vulcanization intermediate product leads to MBTP (2-bis-benzothiazole-2,2'-polysulfide), which is the sulfurant active agents that introduce the sulfur into the polymer chain. As presented in figure 3, clear differences between MBTS evolution in time were observed for the treated sulfur. First, a delay in the MBTS rise related to the CBS decomposition delay was observed. Second, a higher MBTS concentration was reached. This higher concentration was due to the accumulation of this product caused by the impossibility of introducing sulfur into its organic structure. This accumulation of MBTS has been also reported in previous works when the reaction of sulfur with the accelerator is inhibited (ref. 16). When sulfur is delivered through the polymer film, MBTS accepts sulfur into the organic structure, giving rise to MBTP.

[FIGURE 3 OMITTED]

MBT (2-mercaptobenzothiazole) is the final reaction byproduct when CBS is used as an accelerator. As expected, figure 4 shows that when the treated sulfur is used, a delay is observed in MBT formation. This could be related to the fact that the formation of sulfurating active agents is controlled by sulfur accessibility. Thus, when a polymeric film covers the sulfur particles, it is more difficult for the accelerator to react with them, and the overall vulcanization is retarded.

[FIGURE 4 OMITTED]

Thiuram vulcanization

The vulcanization reaction using thiuram type accelerators was also studied in the presence of pyrrole-treated sulfur. Unlike with CBS, TMTD (tetramethylthiuramdisulfide) decomposition seemed to not be affected by the sulfur treatment (figure 5). However, following the sulfur content in the mixture (figure 6), the curve shape was different despite the apparent similar rate. Untreated sulfur concentration started decreasing at minute 3; while, when pyrrole-treated sulfur is used, it started at minute 10. This difference showed the two possible pathways that TMTD could use to donate sulfur to the polymer chain. First, sulfur can be introduced in its molecular structure in order to make the donation to the chain easier, just as with MBTS in sulfenamide vulcanization. On the other hand, since TMTD is a sulfur donor accelerator, it can directly donate the sulfur contained within the accelerator structure. Therefore, TMTD will use preferentially the introduction-donation mechanism when free sulfur is available, and will use the direct donation to the hydrocarbon chain when the previous pathway is blocked.

[FIGURES 5-6 OMITTED]

In order to observe the proposed TMTD behavior, TMTP (tetramethylthiurampolysulfide) was analyzed using both sulfur types. As it is well known, TMTP is the TMTD main intermediate vulcanization product formed by introducing sulfur in its molecular structure. The generation of this product was extremely fast at 140[degrees]C (figure 7). However, as sulfur availability in the mixture has a certain restriction, the generation rate of this intermediate clearly decreased. Considering the results obtained, it could be concluded that the pyrrole-based film resistance at these reaction conditions is around ten minutes.

[FIGURE 7 OMITTED]

Conclusions

Plasma polymerization on sulfur was used successfully to modify sulfur polarity. This modification has also changed the sulfur accessibility and consequently the vulcanization kinetics. Using CBS as accelerator, the squalene vulcanization seems to be affected by the plasma treatment, mainly in the sulfur introduction in the MBTS structure to generate the sulfurating active agents. This could be explained by the fact that plasma treatment generates a polymeric film, which insulates the sulfur from the reaction mixture. Concerning TMTD, a similar influence is observed in the TMTP formation. In this case, another reaction pathway is also possible, and the temporary sulfur protection can be by-passed.

Table 1--vulcanized mixture composition

Ingredients                  Amount (phr)

Squalene or natural rubber       100
CBS/TMTD                       1.2/0.6
Sulfur                            2
Zinc oxide                        5
Stearic acid                      2

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