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Controlled sulfur vulcanization of NR.

Most of the rubber products currently applied consist of mixtures of polymers, fillers, plasticizers, stabilizers, crosslinking chemicals and corresponding accelerators that require a specific heat treatment in order to obtain the desired properties. Elevated temperatures are reached upon and/or required for mixing, processing and crosslinking. Upon mixing of all the ingredients, the temperature may increase to values as high as 120[degrees]C as a result of viscoelastic dissipation. In some cases, such an elevated temperature is required in order to obtain a good dissolution of the additives in the rubber matrix. The rubber compound thus obtained is processed into its final shape via, for example, extrusion or compression/injection molding. Finally, the rubber is vulcanized at high temperatures, typically at 150 to 200[degrees]C. A crosslinking system that is not active at 120[degrees]C and has a "normal" Arrhenius behavior only becomes active at temperatures around 180[degrees]C. This makes it very difficult to mix in crosslinking systems that react already at lower temperatures. No premature crosslinking (scorch) should occur during mixing because this negatively affects processability.

Keeping the vulcanizing chemicals inactive at the mixing temperature by encapsulating them in a (polymer) particle may be a possibility to circumvent the problem of premature crosslinking. The use of microspheres is well known in areas like food, pharmaceutical and crop protection (refs. 1 and 2). Recently, encapsulation of chemicals has been introduced in the field of polymer technology for the controlled release of stabilizers and other compounds (refs. 3-8). Application of such techniques in the field of crosslinking of rubber may lead to interesting results. Possibilities are one-component room-temperature vulcanization systems or systems that vulcanize on demand. Upon applying a suitable trigger, like an increase in temperature, the encapsulated chemicals can be released from the microspheres, enabling instantaneous crosslinking.

Figure 1 shows the schematic change in crosslinking rate when encapsulated curing or crosslinking chemicals are used. Such systems will allow an increase of the crosslinking rate and/or a decrease of the crosslinking temperature. The first case is attractive from an economical point of view; the extrusion throughput may be increased or the molding cycle time may be decreased. The second case is advantageous because the energy consumption may be decreased due to the lower reaction temperature. The decrease in processing temperature may also lead to less undesired side reactions, like degradation and oxidation.

[FIGURE 1 OMITTED]

The encapsulation of crosslinking chemicals is a generic concept that can be applied both for sulfur vulcanization (separate sulfur and accelerators) and for peroxide cure (separate peroxide and activator). The approach can be used for both high-temperature internal mixers, as well as low-temperature two-roll mills. In the latter case, encapsulated ultra-accelerators may allow crosslinking at low temperatures. Especially for this type of accelerators, compounding while preventing vulcanization is still difficult. Encapsulation of the accelerator may enable compounding without any premature vulcanization.

Several factors can influence the behavior of the microcapsule (ref. 9). Phenomena that have to be considered include:

* The capsule material must be non-reactive towards the core material and not interfere with the properties of the rubber;

* the "shell" must provide an adequate barrier against premature release of the core; and

* the release must be triggered under pre-selected conditions.

Moreover, parameters like particle size, composition of shell material and loading of the host are of major importance for the release behavior and, thus, in the case of encapsulating an accelerator on its release behavior and eventually on the curing characteristics of the rubber.

The objective of this article is to investigate the effect of controlled release of zinc isopropylxanthate (ZIX) and zinc diethyldithiocarbamate (ZDEC) encapsulated with polymethylmethacrylate (PMMA) on the characteristics of sulfur vulcanization.

Experimental

Encapsulation of ZDEC or ZIX in PMMA by means of solvent evaporation process

This is a variation of the procedure for encapsulating antioxidants for sustained release in polymers, in order to improve the stability of the polymer (ref. 3). 2.0 grams of ZDEC (or ZIX) accelerator was dispersed or dissolved in a solution of 5 g PMMA in 45 g methylene chloride. Subsequently, this was dispersed in 200 g water containing 2 g of a 10% polyvinyl alcohol solution as emulsifier. This dispersion was then mixed with an Ultra-Turrax stirrer at a speed of 9,500 rpm for 10 minutes at 39[degrees]C. Subsequently, the mixture was stirred for 24 hours at room temperature by means of a stirring bar. During this time, the methylene chloride evaporated, leaving solid polymer particles behind. The particles were washed and centrifuged twice with reverse-osmosis water and dried by means of freeze drying.

Compound testing

Compounds containing between 9 and 12 grams of liquid natural rubber (LNR), sulfur, stearic acid and zinc oxide were mixed with a speed-mixer (DAC150 FVZ, Synergy Devices Limited) for 5 minutes at 3,500 rpm. Subsequently, the encapsulated accelerator was added, and the compound was mixed for another minute at 3,500 rpm. The final formulation was: LNR 100 phr, sulfur 3 phr, zinc oxide 4 phr, stearic acid 3 phr and accelerator 2 phr.

Since temperature is a very important factor for the release of the encapsulated accelerator during the vulcanization, compounding has been performed at the lowest possible temperatures. Especially when the compound temperature comes close to the glass transition temperature ([T.sub.g]) of PMMA, which is about 100[degrees]C, the release of the accelerator from the microcapsule can be dramatically increased. A significant change in diffusion rate with passage of the [T.sub.g] will aid this (refs. 10 and 11). To prevent elevated temperatures during compounding, LNR has been used instead of "normal" NR. The use of LNR will minimize shear forces and, thus, temperature increases will be small.

Unfortunately, due to the low viscosity of the LNR, determination of the curing curve on a regular rubber rheometer is difficult. For this reason, a multi-purpose rheometer has been used to determine G' versus time (Physica UDS200 operating at 1 Hz).

Results and discussion

Encapsulation

Figure 2 shows the SEM micrographs of ZIX and ZDEC encapsulated in PMMA. The particle size has been estimated to be 5-30 [micro]m for ZIX in PMMA and 3-15 [micro]m for ZDEC in PMMA. A difference in particle size will affect the surface to volume ratio and, consequently, the release rate of the accelerator. Besides this effect, the solubility and the mobility of the accelerator in the polymer matrix and the loading of the particle will also affect the release rate.

[FIGURE 2 OMITTED]

Vulcanization characteristics--time scans

Rheometer curves were recorded at different temperatures for the sulfur vulcanization of LNR with (encapsulated) accelerators (figures 3 and 4). For ZIX as such (non-encapsulated), a modulus of 1 x [10.sup.5] MPa was reached after about 165 seconds at 120[degrees]C, while the encapsulated system required a time of 300 sec. to reach a similar modulus; which is about twice as long (figure 3). Upon decreasing the vulcanization temperature, the crosslinking was slowed down for ZIX (as such), as well as for encapsulated ZIX, as expected. However, the curve of the encapsulated system was shifted to much longer times, due to the presence of the PMMA shell. In this case, the final modulus of the encapsulated system was also found to be slightly lower compared to the non-encapsulated system. Further research is needed to exactly determine the reason for this. One also notices a small increase in modulus at 80[degrees]C at around 300-400 seconds of vulcanization, both for the encapsulated and non-encapsulated accelerator. This effect is due to a crosslinking reaction by the ZIX accelerator just by itself. If the same experiment is performed without the use of sulfur, the same small modulus increase at 350 seconds is found, but no vulcanization occurs.

[FIGURES 3-4 OMITTED]

In the case of ZDEC, the temperature dependence of the retarding effect is much clearer (figure 4). At temperatures around 120[degrees]C, the vulcanization curves for ZDEC as such and encapsulated ZDEC were similar, i.e., no significant delay was found. At lower temperatures, the effect of the encapsulation becomes very obvious, with the encapsulated system requiring much longer vulcanization times, especially at 80[degrees]C. With a further decrease in temperature, this effect will only become stronger. The decrease in temperature causes the diffusion of the accelerator from the particle to slow down. This in its turn will slow down the availability of the accelerator in the rubber matrix, which results in a slower rate of crosslinking. It is predicted that at room temperature the release of accelerator will be minimal, due to the very slow diffusion from the particles. A system like this would allow more effective compounding of room temperature vulcanization systems and circumvent the appearance of premature vulcanization.

In other words, rheometer curves for both ZDEC and ZIX show a shift to longer times upon encapsulating the accelerator, indicating a controlled release effect. This suggests that the release of the accelerator is the limiting factor instead of the crosslinking rate.

In order to confirm the effect of encapsulation, a control experiment was done in which just sulfur and empty PMMA shells, but no accelerator, were added to the LNR. In this case, no increase in modulus was found. Moreover, in the case of a control experiment with sulfur, unencapsulated accelerator and empty PMMA shells, the vulcanization curve was identical to the one with the unencapsulated accelerator. These reference experiments show that only capsules loaded with accelerator influence the vulcanization, while empty capsules do not influence the crosslinking.

The efficiency of the encapsulation process can be monitored by a comparison of the vulcanization times determined during the experiments. Table 1 shows the estimated onset and optimum vulcanization times. As one can see, at higher temperatures hardly any or no differences are found. At lower temperatures the retarding effects become more obvious. Encapsulation of ZIX and ZDEC results in delays of the onset time of 250 to 500%, respectively, at 80[degrees]C; and for the vulcanization time of 120 to 133%, respectively. Optimization of the system will enhance this effect. It is expected that the temperature, below which the differences between encapsulated and non-encapsulated accelerator become the most significant, is related to the [T.sub.g] of the capsule polymer. Since the [T.sub.g] of PMMA is 100[degrees]C, this explains why the largest transition is found when going from 100 to 80[degrees]C in our study. It should be noted that neither encapsulated accelerator system has been optimized. Optimization will certainly avoid problems, like having a certain percentage of particles not being encapsulated, and lead to an improved controlled release.

Vulcanization characteristics--temperature scans

In order to get a quick scan of the influence of temperature on the vulcanization rate, temperature scans instead of time scans have also been performed. Figure 5 shows the change of the modulus with an increase in temperature for (encapsulated) ZDEC. Initially, a drop in the modulus is found, due to the decrease in viscosity of the mixture upon increasing temperature. Around 80[degrees]C a sudden increase is found for the regular vulcanization experiment, comparable to the same effect as observed in the time scan at 80[degrees]C (figure 4). This effect is not visible in the temperature scan of the encapsulated accelerator. This is an indication that the accelerator is completely encapsulated in the PMMA and is, therefore, not able to initiate any premature reactions. Starting at 95[degrees]C, the modulus shows a sharp increase due to the start of the vulcanization. If this is compared to the temperature scan of the encapsulated accelerator, a shift of about 15[degrees]C is found. Most likely this effect is related to the [T.sub.g] of PMMA. The use of a shell polymer with a higher [T.sub.g] would increase this temperature shift. Actually, by selecting the right shell polymer, the vulcanization temperature could be tuned to a desired value. One should note that the speed of the temperature increase during the temperature scan can have a large effect on the obtained results.

[FIGURE 5 OMITTED]

In the case of ZIX, a comparable effect is found with some small differences (figure 6). First, the same decrease in modulus (read: viscosity) due to increasing temperature is found. The modulus after vulcanization was lower compared to the unencapsulated experiment, as was also observed for the time sweep at constant temperature. In the case of the encapsulated ZIX, the increase in modulus with temperature was less steep than in the case for ZDEC. This suggests that there is a difference in the release and/or reaction mechanisms of the two accelerators.

[FIGURE 6 OMITTED]

Conclusions

Controlled release of accelerators for sulfur vulcanization has been shown to have the potential for enhanced control. Even though further research is obviously needed, a system like this can be applied to a wide spectrum of crosslinking chemicals for both sulfur and peroxide systems. A short list of possibilities a system like this would offer:

* Possibility to compound room temperature vulcanization systems without premature curing;

* modification of the vulcanization rate without changing the accelerator;

* shorter cycle times and higher throughputs in batch and continuous processes, respectively, in combination with fewer side reactions;

* lower energy consumption; and

* prevention of blooming of accelerators.

A more thorough evaluation of the controlled release of accelerators on the final properties of the cured compounds is needed. The choice of an alternative shell polymer with a higher [T.sub.g] is needed to enable controlled release of accelerator at higher temperatures. Finally, the concept has still to be converted to high molecular weight rubber with reinforcing fillers and extender oil.

References

(1.) Gouin, S., "Microencapsulation: Industrial appraisal of existing technologies and trends," Trends in Food Science & Technology, 15, 330-347, 2004.

(2.) McKetta, J.J., Encyclopedia of Chemical Processing and Design; "Microencapsulation," Dekker: Basel, 1976.

(3.) Boersma, A., "Stabilizer encapsulation," EP 04075835, 2004.

(4.) Keen, F.E., Lehrle, R.S., Jakab, E. and Szekely, T., "The development of controlled-release antioxidants: A successful system demonstrated by its effect on the stabilization of rubber," Pol. Deg. Stab., 38, 219-227, 1992.

(5.) Leo, T.J. and Reynolds, M.J.; "Encapsulation of critical chemicals," U.S. 4,092,285, 1978.

(6.) Naoki, J. and Igarashi, Y., "Microcapsule incorporating vulcanization accelerators as core material therein," JP 60,262,838, 1985.

(7.) Evans, L.R., Benko, D.A., Gillick, J.G. and Waddell, W.H., "Microencapsulated antidegradants for extending rubber lifetime," Rubber Chem. Tech., 65, 201-210, 1992.

(8.) van Ooij, W.J., "Plasma polymerization of sulfur to decrease the blooming effect and its effect on vulcanization with different accelerators," Rubber World, 228-5, August 2003.

(9.) Frieberg, S. and Zhu, X.X.; "Polymer microspheres for controlled drug release," Int. J. Pharm., 282, 1-18, 2004.

(10.) van Krevelen, D.W., Properties of Polymers; 3rd ed.; Elsevier: Amsterdam, 1997.

(11.) Allen, G., Bevington, J.C., Booth, C. and Price, C., Comprehensive Polymer Science: The Synthesis, Characterization, Reactions and Applications of Polymers. Vol. 2. Polymer properties; Pergamon: Oxford, 1998.

by Ralf Heijkants (ralf.heijkants@tno.nl) and Arjen Boersma, TNO Science & Industry and Dutch Polymer Institute (DPI); Ben van Baarle, TNO Science & Industry, Innovative Materials Group; and Martin van Duin, DSM Research
Table 1 - estimated onset and vulcanization times

 Temperature [t.sub.onset] (sec.)
Encapsulated ([degrees]C) ZIX ZDEC

 + 120 125 325
 - 120 100 325
 + 100 100 700
 - 100 100 325
 + 80 250 1,600
 - 80 100 325

 [t.sub.vulc](sec.)
Encapsulated ZIX ZDEC

 + 425 570
 - 250 570
 + 900 1,700
 - 750 1,350
 + 2,000 4,800
 - 1,600 3,600
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Author:van Duin, Martin
Publication:Rubber World
Date:Nov 1, 2006
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