STABILIZATION OF HALOGENATED RUBBER

Abstract: Halogenated rubbery polymers are stabilized against discoloration with a weak base such as 100 to 500 ppm sodium bicarbonate. The additive is added to the rubbery polymers after dewatering in the finishing step of their manufacture. The curability of the stabilized rubbers is not adversely affected by the sodium bicarbonate additive.

Description: STABILIZATION OF HALOGENATED RUBBER Field of the Invention This invention relates to stabilizing halogenated rubbers such as halobutyl polymers and brominated isobutylene-p-methylstyrene copolymers, with a minor amount of a weak base. Background of the Invention Halogenated rubbers are generally prepared by halogenating a rubbery polymer such as isobutylene-isoprene copolymer or isobutylene-p-methylstyrene copolymer in an organic solvent. This produces a halogenated polymer solution containing hydrogen halide as a byproduct. The halide is typically neutralized with an aqueous base and the resulting mixture is stripped to obtain a neutralized halogenated polymer in the form of a wet crumb, typically containing about 50 percent by weight water. The wet crumb is then dewatered in a dewatering extruder, press, mill, dryer or the like to obtain a dewatered crumb usually less than about 10 - 20 weight percent water. The dewatered crumb is then finished in an extruder or its equivalent with any other additives such as antioxidants, UV stabilizers or the like to obtain a relatively dry halogenated rubber product which is typically pressed into bales for storage and shipment. In the case of the halobutyl polymers, these are known to undergo dehalogenation reactions under heat, during finishing or storage which can cause polymer degradation and discoloration. Typically, about 1-1.5 weight percent of an additive such as epoxidized soybean oil (ESBO) has been added to the halobutyl polymers during the finishing stage to prevent dehalogenation and discoloration during storage. However, ESBO is relatively expensive and tends to make the halobutyl polymers more tacky and lowers green strength. Owing to the higher stability of the brominated isobutylene-p-methylstyrene copolymers available under the trade designation EXXPRO™, and the undesirability of additives such as ESBO, additives such as ESBO are typically not used in the EXXPRO™ polymers. However, there are occasionally noted instances of orange discoloration of the EXXPRO™ polymers, particularly at high bromine levels, for example, 2 weight percent bromine. Accordingly, there remains a need to solve the discoloration of the EXXPRO™ polymers, and to find less expensive additives for the halobutyl polymers which do not affect the tackiness or green strength of the halobutyl polymers. Summary of the Invention It has been found that halogenated rubber can be effectively stabilized with a small amount of a base, preferably a weak base, without adversely affecting the curability, tackiness or green strength of the halogenated rubber. By adding the base, preferably a weak base, during the finishing stage, a stabilized halogenated rubber can be obtained which is basically free from discoloration and has virtually no change in physical or vulcanizable characteristics. In one aspect, the present invention provides an improvement in a method for making a halogenated rubber. The method includes the steps of halogenating a rubbery polymer in an organic solution to form a solution of halogenated rubber and hydrogen halide, neutralizing the hydrogen halide solution with an aqueous alkaline material, stripping the solvent from the neutralized solution to form a wet halogenated rubber crumb, dewatering the crumb to less than about 10 - 20 weight percent water, and finishing the crumb with one or more stabilizing additives to obtain a halogenated rubber product. The improvement is that the stabilizing additives in the finishing step include from 15 to 10,000 ppm of a weak metal base, preferably an alkali or alkaline earth metal base. In another aspect, the present invention provides a dry, intimate admixture of halogenated rubber and a stabilizing amount less than 1 weight percent of a base. In a further aspect, the present invention provides an intimate admixture of brominated isobutylene-p-methylstyrene copolymer and 15 to 10,000 ppm of a weak base comprising less than 0.3 weight percent water. Another aspect of the invention is the provision of an intimate admixture of halogenated rubber and 15 to 10,000 ppm of a weak metal base. Detailed Description of the Invention The halogenation of rubbery polymers is well known from, for example, U.S. Patents 4,384,072 to Newman et al.; 4,486,575 to Newman et al.; 4,501,859 to Newman et al.; 5,077,345 to Newman et al; 5,286,804 to Powers et al.; 5,087,674 to Gardner et al.; and 4,513,116 to Kowalski et al.; all of which are incorporated herein by reference. Particularly preferred halogenated rubbers according to the present invention include halobutyl rubbers and brominated isobutylene-p-methylstyrene copolymers. The base which is added as a stabilizing additive is preferably a weak base, i.e. a base having pK greater than 1, preferably greater than 6. Weak metal bases such as alkali and alkaline earth metal hydroxides, bicarbonates and carbonates are especially preferred. Sodium bicarbonate is exemplary of the preferred weak alkali metal bases. The base is used in an amount effective to stabilize the halogenated rubber, usually at least 15 ppm, preferably at least 50 ppm, and especially at least 100 ppm. To prevent discoloration of the halogenated rubbers, there is no particular upper limit of the amount of the base which may be used but it will generally be desirable to use 10,000 ppm or less to avoid adversely effecting the physical and chemical properties of the halogenated rubber such as curability, tackiness and green strength. Curable halogenated rubbers preferably contain less than 10,000 ppm of the base additive, more preferably less than 1,000 ppm, and especially less than 500 ppm. In a preferred embodiment, the stabilizing additive is sodium bicarbonate at a level of 100 - 500 ppm by weight of the halogenated rubber. The weak base additive is added to the halogenated rubber in the usual manufacturing process after the halogenated rubber has been dewatered, preferably to less than 20 percent water by weight, more preferably to less than 10 percent water by weight. The additive is added during the finishing operation in which the dewatered halogenated rubber is further dried to, for example, less than 0.3 weight percent water, in a finishing extruder, mill, dryer or the like. The additive is generally added in the form of a concentrated solution, for example, a 10 weight percent aqueous sodium bicarbonate solution. The additive could be added in the form of a dry powder, but this is generally more difficult to meter into the polymer. The additive should be added to the polymer after it is dewatered to avoid removing the additive when the water is removed. Any water that is included in the additive solution is generally removed in the finishing extruder. Generally, the finished polymer contains less than 1 weight percent water, preferably less than 0.3 weight percent water. It is only necessary that the additive be intimately mixed with the polymer, and thus the stage in the finishing extruder at which the additive is introduced is not particularly critical. The invention is illustrated by way of the following examples. Example 1. Effect of NaHCO 3 on Discolored EXXPRO™ Polymer This example describes the initial discovery of the effect of NaHCO 3 (sodium bicarbonate) and other bases on discolored EXXPRO™ polymers. In this experiment, 40 grams of discolored (orange) EXXPRO™ polymer (MDX 90-10) was milled in a Brabender mixer at around 180°C and 0.06 grams NaHCO 3 powder was added to the polymer melt and milling was continued for another 15 minutes. The polymer was then removed from the Brabender mixer and cooled to room temperature for inspection. It was observed that the orange discoloration was completely eliminated. Other bases like NaOH solution and DHT were also found to be effective to eliminate the orange discoloration in EXXPRO™ polymer. Similar results were observed via mixing of 200-1000 ppm of NaHCO 3 and discolored EXXPRO™ polymer (MDX 90-10) in a small laboratory Haake extruder at around 200°C. Both experiments indicated that addition of NaHCO 3 or other weak bases can eliminate the discoloration of EXXPRO™ polymers. Example 2. Adding 200 ppm NaHCO? to EXXPRO™ Polymer This example describes the addition of 200 ppm NaHCO 3 into the EXXPRO™ polymer and the impact on polymer vulcanization. During the normal production of an EXXPRO™ polymer, namely MDX 93-5, which contains about 0.49 mole percent benzylic bromine (about 0.9 wt% bromine), solid NaHCO 3 powder was added via a solid feeder to the polymer crumbs at about 200 ppm, based on polymer feed rate, just before the polymer hopper of the drying extruder. The finished polymer was then compounded with a standard recipe (100 phr polymer, 40 phr IRB #6 carbon black, 2 phr stearic acid, 1 phr zinc oxide and 1 phr zinc stearate). The compounded polymer was tested side-by-side with the control sample without NaHCO 3 in a standard Monsanto Rheometer ODR 2000E (160°C, 60 minutes, 3° arc) to evaluate the effect of 200 ppm NaHCO 3 on cure. The results are shown in the Table 1 below: TABLE 1 The data indicate that the presence of 200 PPM NaHCO 3 tends to slow down the cure rate of EXXPRO™ polymer slightly, but the impact on final cure state of EXXPRO™ polymer is insignificant. No discoloration was observed in the uncured EXXPRO™ polymer containing 200 ppm NaHCO 3 even after 18 months storage in the warehouse. This example confirms that the presence of 200 ppm level NaHCO 3 can effectively prevent the orange discoloration of EXXPRO™ polymer. Example 3. Adding 500 ppm NaHCO; to EXXPRO™ Polymer A similar experiment was carried out as Example 2 except that MDX 90-10 EXXPRO™ polymer (contains 2 wt% bromine on polymer) and 500 ppm NaHCO 3 was added to the polymer crumbs instead of 200 ppm NaHCO 3 as in Example 2. Similar evaluation on the effect of NaHCO 3 on cure versus a control sample without NaHCO addition was carried out using the same recipe and test conditions. The evaluation results are shown in Table 2 below TABLE 2 Again, the presence of 500 ppm NaHCO 3 has minimum impact on the cure rate and the final cure state of the EXXPRO™ polymer. No discoloration in the uncured polymer was observed even after 18 months of storage. Example 4. Adding 300 ppm DHT to EXXPRO™ Polymer Another experiment was carried out via similar conditions as Example 2 except that about 300 ppm of a different weak base, namely DHT (magnesium aluminum hydroxycarbonate), was used. Similar evaluation on the impact of DHT on cure versus the control sample without DHT was carried out using the same recipe and test conditions. The evaluation results are shown in Table 3 below. TABLE 3 Similar to NaHCO 3 , the presence of 300 ppm level DHT tends to retard the cure rate of EXXPRO™ polymer with minimum impact on the final cure state of the EXXPRO™ polymer. Again, no discoloration in the uncured polymer was observed even after 18 months of storage The invention is illustrated by way of the foregoing description which is intended to represent exemplary embodiments only, and not to limit the invention. Numerous changes and modifications will occur to those skilled in the art in view of the foregoing description. It is intended that all such variations and modifications within the scope and content of the appended claims be embraced thereby.

Claims: CLAIMS 1. A dry intimate admixture of halogenated rubber and a stabilizing amount less than 1 weight percent of a base. 2. The admixture of claim 1 comprising less than 1 weight percent water. 3. The admixture of claim 1 wherein the base comprises a weak alkali or alkaline earth metal hydroxide, bicarbonate, or carbonate. 4. The admixture of claim 1 wherein the halogenated rubber comprises halobutyl rubber. 5. The admixture of claim 1 wherein the halogenated rubber comprises isobutylene-co-methylstyrene-bromomethylstyrene terpolymer. 6. An intimate admixture of brominated isobutylene-p-methylstyrene copolymer and 15 to 10,000 ppm of a weak base comprising less than 1 weight percent water. 7. The admixture of claim 7 comprising 50 to 1,000 ppm alkali metal bicarbonate. 8. The admixture of claim 7 comprising 100 to 500 ppm sodium bicarbonate. 9. An intimate admixture of halobutyl rubber and 15 to 10,000 ppm of a weak metal base comprising less than 1 weight percent water. 10. The admixture of claim 10 comprising 50 to 1,000 ppm alkali metal bicarbonate. 11. The admixture of claim 10 comprising 100 to 500 ppm sodium bicarbonate. 12. In a method for making a halogenated rubber comprising the steps of halogenating a rubbery polymer in an organic solution to form a solution of halogenated rubber and hydrogen halide, neutralizing the hydrogen halide solution with an aqueous alkaline material, stripping the solvent from the neutralized solution to form a wet halogenated rubber crumb, dewatering the crumb to less than 20 weight percent water, and finishing the crumb with one or more stabilizing additives to obtain a halogenated rubber product, the improvement wherein the stabilizing additives in the finishing step include from 15 to 10,000 ppm of a weak metal base. 13. The improvement of claim 13 wherein the stabilizing additives include 50 to 1,000 ppm weak alkali metal hydroxide, bicarbonate or carbonate. 14. The improvement of claim 14 wherein the stabilizing additives comprise 100 to 500 ppm of the weak alkali metal hydroxide, bicarbonate or carbonate. 15. The improvement of claim 15 wherein the stabilizing additives include sodium bicarbonate. 16. The improvement of claim 13 wherein the weak metal base is added in the finishing step as a concentrated aqueous solution. 17. The improvement of claim 13 wherein the halogenated rubber comprises halobutyl rubber. 18. The improvement of claim 13 wherein the halogenated rubber comprises isobutylene-bromomethylstyrene copolymer. 19. The improvement of claim 19 wherein the copolymer comprises more than 1 mole percent bromomethylstyrene.

Inventors: CHEE CHIA SHIAN, CHUNG DAVID YEN-LUN, YEH RICHARD CHENG-MING, CHEE, CHIA, SHIAN, CHUNG, DAVID, YEN-LUN, YEH, RICHARD, CHENG-MING
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1998
Main classification: C08K3
IPC codes: C08K3, C08L15, C08L25