Abstract:
This invention relates to a process for minimizing or reducing the amount of residual free hydrazine in polymer latices. The process is particularly useful in the manufacture of hydrogenated nitrile rubber which is resistant to oxidative degradation at high temperatures, as well as resistant to corrosive environments such as acid environments. Nitrile rubbers are useful in the manufacture of fan belts, seals, gaskets, and hoses in increasingly small and hot-running car engines.

Description:
This application is a continuation-in-part of U.S. Ser. No. 664,146 filed Mar. 4, 1991 U.S. Pat. No. 5,171,833 which is a continuation-in-part of U.S. application Ser. No. 07/352,782 filed on May 16, 1989, now U.S. Pat. No. 5,068,313. 
    
    
     BACKGROUND OF THE INVENTION 
     Various forms of hydrogenated nitrile rubbers have recently been introduced into the marketplace. Hydrogenated nitrile rubber has the advantage of being resistant to oxidative degradation at high temperature, as well as being resistant to corrosive environments such as acid environments. These materials have found utility in the manufacture of fan belts, seals, gaskets, and hoses in increasingly small and hot-running car engines. 
     One process for the production of hydrogenated latex polymers, particularly hydrogenated latex rubber, utilizes hydrazine and an unsaturated polymer as the starting materials. This process is more particularly described in U.S. Pat. No. 4,452,950, assigned to the Goodyear Tire and Rubber Company. However, the product produced in accordance with the process described in the &#39;950 patent suffers from the disadvantage of containing residual unreacted hydrazine. This residual hydrazine is considered undesirable from an environmental and/or toxicity standpoint. 
     In view of the above, it would be highly desirable to provide a process for the elimination and/or reduction of the residual hydrazine in the hydrogenated polymer product mixture. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention relates to a process for the hydrogenation of a polymer latex and the reduction or elimination of residual free hydrazine from the polymer latex characterized by: 
     (a) contacting said polymer latex with hydrazine and peroxide at a temperature of between about 60° C. and about 70° C., in the presence of a metal ion initiator selected from the group consisting of copper sulfate, ferrous sulfate, and combinations thereof, in order to effect a reaction of said polymer latex with said hydrazine and said peroxide, using a reaction time of between about 1 hour and about 5 hours, thereby producing a hydrogenated polymer latex, and 
     (b) contacting free hydrazine in said polymer latex with a hydrazine scavenger selected from the group consisting of isocyanates, alkylene oxides, acrylates, methacrylates, acrylic acids, acrylonitrile, methacrylic acids, ketones, diketones, aldehydes, and combinations thereof, in a scavenging effective amount of between 1 and 10 times the number of moles of hydrazine present in said purified mixture in order to bind at least a portion of said free hydrazine, thereby providing a hydrogenated polymer latex containing a reduced amount of free hydrazine therein. 
     These and other aspects of the present invention will become apparent upon a reading of the following detailed description of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The polymer/hydrazine mixture generally contains a major amount (i.e. typically greater than 70 weight percent) of water, saturated polymer (typically about 5 to about 25 weight percent), a minor amount of unsaturated polymer (typically between about one and five weight percent), and a minor amount (typically between about 0.1 and about 3 weight percent) of hydrazine, wherein the weight percents are based upon the total weight of the latex. For most applications, it is preferred that the amount of hydrazine be minimized in the mixture. Reduction or elimination of the hydrazine in the mixture provides a mixture having reduced toxicity. 
     The hydrazine scavenger useful in the processes of the present invention acts to bind at least some portion of the free hydrazine in the purified latex. Useful hydrazine scavengers are compounds that react with hydrazine and include the following classes of compounds: isocyanates (such as tolylene diisocyanate, phenyl isocyanate and methylene dipenyl isocyanate (MCI)); alkylene oxides (such as ethylene oxide, propylene oxide, isobutylene oxide, styrene oxide, and polymeric epoxides); acrylates and methacrylates (such as methyl methacrylate); acrylonitrile, acrylic acids and methacrylic acids; ketones and diketones (such as acetyl acetone); aldehydes (such as benzaldehyde); and combinations thereof. 
     The hydrazine scavenger is employed in a &#34;scavenging effective amount&#34;. As used herein, the term &#34;scavenging effective amount&#34; designates an amount of hydrazine scavenger sufficient to react with and bind at least some amount of the free hydrazine in the latex mixture. Preferably, the hydrazine scavenger is employed in an amount of between about 1 and about 10, more preferably between about 1 and about 5 times the theoretical molar amount required to react with the free hydrazine present in the hydrazine/latex mixture. 
     In a preferred embodiment of the two-step process of the present invention, step (a) of the two-step process is utilized to hydrogenate the polymer latex with hydrazine and peroxide, and then step (b) is employed to reduce the amount of free hydrazine in the mixture to 25 ppm or less. 
     The polymer/hydrazine mixture in latex form is prepared by hydrogenation of unsaturated polymers. Prior to hydrogenation, the unsaturated polymers are typically composed of 5 to 100 percent by weight of a conjugated diene monomer unit and 95 to 0 percent by weight of an ethylenically-unsaturated monomer unit. Specific examples of the conjugated diene monomer are 1,3-butadiene, 2,3-dimethylbutadiene, isoprene, and 1,3-pentadiene, specific examples of the ethylenically unsaturated monomer include unsaturated nitriles such as acrylonitrile and methacrylonitrile, monovinyl aromatic hydrocarbons such as styrene, (o-, m-, and p-) alkylstyrenes, divinyl aromatic such as divinylbenzene, dialkenyl aromatics such as diisopropenylbenzene, unsaturated carboxylic acids and the esters thereof such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate and methyl methacrylate; vinyl pyridine; and vinyl esters such as vinyl acetate. 
     The conjugated diene polymer may be one prepared by any method of preparation, such as emulsion polymerization, solution polymerization or bulk polymerization. Specific examples of the conjugated diene polymer include polyisoprene, polybutadiene, a styrene/butadiene (random or block) copolymer, an acrylonitrile/butadiene (random or blocked) copolymer, a butadiene/isoprene copolymer, and an isoprene/isobutylene copolymer. 
     Suitable peroxides include organic peroxides, inorganic peroxides, and combinations thereof, such as, for example, hydrogen peroxide, alkali metal peroxides such as sodium peroxide and potassium peroxide, peracetic acid, benzoyl peroxide, alkali metal persulfates, and the like. Preferred peroxides are hydrogen peroxide and sodium peroxide, most preferably hydrogen peroxide. The peroxide is preferably employed in the presence of a metal ion initiator, such as copper sulfate or ferrous sulfate. The reaction time for the process of the present invention can vary over a wide range, but is preferably between about 1 hour and about 50 hours, more preferably between about 1 hour and about 25 hours, most preferably between about 1 hour and about 20 hours. 
     The following examples are intended to illustrate, but in no way limit the scope of, the present invention. 
     EXAMPLE 1 
     Part A--Hydrogenations of a Latex 
     Run (1)--Hydrogenation at 45° C. to 50° C. 
     An acrylonitrile/butadiene rubber with 66.2 weight percent butadiene and an average molecular weight of about 200,000 was hydrogenated in latex form with a mixture of hydrazine and hydrogen peroxide in a manner approximately as described in U.S. Pat. No. 4,452,950, without a metal ion initiator at a temperature of 45° C. to 50° C. After achieving about 88 percent conversion of the C═C double bonds, the run was interrupted. At this point in the reaction, the latex contained 4.05 weight percent unreacted hydrazine, based on the weight of the aqueous phase of the latex. 
     Run (2)--Hydrogenation at 60° C. 
     80 g (0.314 mole double bond) of an acrylonitrile/butadiene lates, with 61.1 weight percent butadiene, (34.8% rubber) charged to flask. Antifoam and CuSO 4  catalyst added followed by 21.3 mls 64.3% N 2  H 4  (0.441 mole) with an exotherm to 30° C. The reaction was heated to 60° C. and 36 mls of 49.4% H 2  O 2  were added over 4.2 hours. The reaction was post reacted 1 hour, then allowed to cool to room temperature over 2 hours. The final latex was 96.7% hydrogenated with no residual H 2  O 2  and 125 ppm residual N 2  H 4  based on the weight of the aqueous phase of the latex. 
     Run (3)--Hydrogenation at 70° C. 
     75 g (0.162 mole double bond) of an acrylonitrile/butadiene latex, with 63 weight percent butadiene, (18.5% rubber) charged to flask. Antifoam and 19.3 mls 64.2% N 2  H 4  (0.4 mole) charged to flask with an exotherm to 27° C. The reaction was heated to 70° C., and 39.8 mls 30.8% H 2  O 2  (0.4 mole) added over 4.5 hours. The reaction was post reacted 3 hours at 70° C., then cooled and left stirring overnight. No unsaturation was detected in the final latex by NMR. 1.11% residual N 2  H 4  and no residual H 2  O 2  were found based on the weight of the aqueous phase of the latex. 
     Part B--Binding of Free Hydrazine in Product of Run (1) 
     To a portion of this hydrogenated latex was added an excess of methyl acrylate and stirred at room temperature for 1.5 hours. During this period, there was a noticeable exotherm. The final hydrazine content was about 470 ppm, based on the weight of the aqueous phase of the latex, which represents about a 99 percent removal of hydrazine from the latex. 
     Another Illustration of Binding of Free Hydrazine 
     To a portion of a latex hydrogenated in a manner analogous to Run 2 above, but containing 58 ppm of residual free hydrazine, was added a molar excess of 2,4-pentanedione relative to the amount of free hydrazine present in the latex. The mixture was stirred at 45° C. for 2.5 hours. The hydrazine level was reduced from 58 ppm to less than 0.5 ppm. This represents more than a 99% removal of the free hydrazine from the latex.