MXPA97005548A

MXPA97005548A - Method for the preparation of nitr rubber

Info

Publication number: MXPA97005548A
Application number: MXPA/A/1997/005548A
Authority: MX
Inventors: Charles Grimm Donald
Original assignee: The Goodyear Tire & Rubber Company
Priority date: 1996-08-01
Filing date: 1997-07-22
Publication date: 1998-02-01

Abstract

The present invention relates to an improved method for recovering nitrile rubber from an emulsion containing nitrile rubber, this method is characterized by comprising the steps of: 1) adding an antioxidant to the emulsion containing nitrile rubber to produce a stabilized nitrile rubber, 2) add a salt and an acid to the stabilized nitrile rubber emulsion to produce a suspension of coagulated nitrile rubber, where the suspension of nitrilocoagulated rubber consists of whey and lumps of nitrile rubber, 3) Separate the lumps of nitrile rubber from the serum of the coagulated nitrile rubber suspension. 4) Mix the lumps of nitrile rubber in washing water to produce a new suspension of nitrile rubber. 5) Adjust the pH of the new rubber suspension. of nitrile so that it is within the range from about 5 to about 8, and 6) separating the lumps of nitrile rubber from the wash water of the new suspen nitri rubber

Description

METHOD FOR THE PREPARATION OF NITRILE RUBBER FIELD OF THE INVENTION The present invention relates, in general, to the production of crude crumb rubber. More specifically, the invention relates to the production of unvulcanized rubber lumps that can subsequently be vulcanized to form any number of rubber products, such as automotive parts, industrial products, household tools and utensils, as well as insulating and structural foams. .

BACKGROUND OF THE INVENTION Synthetic rubbers, which can be used in the manufacture of various products, are produced in commercial form from the emulsion polymerization. During the formation of the polymers by emulsion polymerization it is usually essential to control the molecular weight, crosslinking and branching of the finished polymer to produce a polymer having predictable and desired performance characteristics. For ple, the ability to form a useful vulcanized article may depend on the control of parameters such as polymer molecular weight and polymer uniformity. When provided as a non-vulcanized polymer, the rubber or elastomer can take any number of forms that facilitate processing to a vulcanized finished product. The non-vulcanized polymer may be provided in the form of lumps, in packaged form, in the form of a solid block or cube, or any number of other forms that serve the consumer. Although the shape of the non-vulcanized rubber or elastomer may be important to meet the needs of the final processor, other processing limitations are also important. Among these limitations are those processing parameters that affect the chemical and physical uniformity of unvulcanized rubber. A problem that can occur with unvulcanized rubbers or elastomers is the unexpected additional crosslinking during storage and before vulcanization. This additional undesirable crosslinking is commonly referred to as "gelling". One theory about this continuous crosslinking or gelation is that an excess of free radicals remains from the initial polymerization favoring additional undesirable reactions. Efforts have been made to avoid gel formation in the polymers. One of the most widely practiced methods includes the addition of antioxidants to the polymer. It is a common practice to add agents that stop the reaction in the polymer emulsions (latex) and then purify the monomers that did not react from the emulsion and then add an antioxidant. Antioxidants can prevent gel formation by reducing the level of free radicals present. Preferably, the antioxidant should provide protection up to or for one year after polymer formation, and even more preferably for two or more years after polymer formation. The choice of a suitable commercial antioxidant can be very difficult. The antioxidant must have adequate chemical and physical properties for the proposed end use for the polymer. The antioxidant must also be available in sufficient quantities and at a price that makes commercial production viable. Other aspects include regulatory or governmental approval for the final use of the product. The class of antioxidants known as alkylated aryl phosphites is favored, although these antioxidants have the disadvantage of hydrolyzing and thus lose their antioxidant behavior. It is also possible that additional problems arise from the salt-acid coagulation techniques that are used to recover the emulsion rubbers from the emulsions in which they were synthesized. The residual acid remaining in the polymer after coagulation can present significant problems, in which the increase in the corrosivity of the polymer is included. The presence of acid in the polymer can also adversely affect the degrees of vulcanization. Usually, polymers that are at low pH values are vulcanized at a slower speed than polymers that have a neutral or alkaline pH. The decreased vulcanization rate is undesirable because it increases the time needed to vulcanize the rubber, which is often the bottleneck in commercial operations. The uncertainty also arises, at least in part, from the fact that the amount of acid added to coagulate the emulsions may vary. This variation in the acid level in different batches of rubber can result in variations in the vulcanization rate of the rubber from one batch to another. Efforts have been made to improve the emulsion polymerization processes of elastomers and rubbers to produce a better polymer. For example, Japanese Patent No. 49, 066, 725 describes the gel inhibitors that are obtained by mixing solid phenyl diamine derivatives with liquid reaction products derived from aromatic amines and acetone. U.S. Patent Nos. 3, 984, 372 and 4, 168, 3387 describe polyphenol esters as polymerizable or integrated antioxidants. The compounds of these two US Patents are reaction products of a polyphenolic compound with an ester-forming compound. These patents describe the use of compounds such as 2- (2-hydroxy-2-butyl-5-methylbenzyl) -4-methyl-6-tert-butyl phenylmethacrylate as polymerizable antioxidants for the polymer compositions. There continues to be a need for a process whereby the stability and uniformity of a polymer can be improved easily and in a cost-effective manner. SUMMARY OF THE INVENTION Nitrile rubber can be synthesized by the copolymerization of acrylonitrile and 1,3-butadiene in an aqueous emulsion. These emulsion polymerization techniques are widely used in the commercial production of nitrile rubbers. After nitrile rubber has been prepared, of course, it is necessary to recover it from the aqueous emulsion. Nitrile rubber is usually recovered from the emulsion by coagulation. Normally the coagulation is carried out by adding a salt or an acid to the emulsion. A part of the acid that is used for the coagulation of the emulsion usually remains in the lumps of rubber that are recovered. The presence of this acid with the dry rubber can give rise to the instability of the polymer and can inhibit vulcanization during a subsequent vulcanization process.

By using the techniques of this invention, nitrile rubber can be prepared which exhibits stability and improved vulcanization characteristics. More specifically, this invention describes an improved method for recovering nitrile rubber from an emulsion containing nitrile rubber, this method comprises the steps of: (1) adding an antioxidant to the emulsion containing nitrile rubber to produce a stabilized nitrile rubber, (2) adding a salt and an acid to the stabilized rubber emulsion to produce a suspension of coagulated nitrile rubber, where the suspension of coagulated nitrile rubber contains the whey and lumps of nitrile rubber, ( 3) Separate lumps of nitrile rubber from the serum of the coagulated nitrile rubber suspension, (4) Mix the nitrile rubber lumps in wash water to produce a new suspension of nitrile rubber, (5) Adjust the pH of the new nitrile rubber suspension so that it is within the range of about 5 to about 8, and (6) separating the lumps of nitrile rubber from the wash water of the new suspension of nitrile rubber. Preferably, the antioxidant that is added in step (1) may be an antioxidant alkylated aryl phosphite, the antioxidant alkylated aryl phosphite will usually be a tris (alkylphenyl) phosphite. More preferably, a non-hydrolysable antioxidant is added to the emulsion, in addition to the antioxidant alkylated aryl phosphite. Thus, in step (1) a mixture of antioxidants consisting of (a) an alkylated aryl phosphite antioxidant (b) a non-hydrolysable antioxidant, more preferably will be added to the emulsion. The combination of the two antioxidants seems to have at least two positive effects. First, the presence of a non-hydrolysable antioxidant ensures that at least some of the antioxidant will exist in the coagulated polymer even if the antioxidant alkylated aryl phosphite is completely hydrolyzed. Second, the combination of the two antioxidants has an impeding effect on the hydrolysis of the antioxidant alkylated aryl phosphite. In this way, the increase in the concentration of the antioxidant alkylated aryl phosphite (which has not been hydrolyzed) provides additional protection against gelation for the coagulated polymer. It is preferable that the non-hydrolysable antioxidant is a hindered phenolic antioxidant. Tris (nonylphenyl) phosphite is a highly preferred alkylated aryl phosphite antioxidant and octadecyl-3,5-di-t-butyl-4-hydroxycinnamate is a highly preferred non-hydrolysable antioxidant. DETAILED DESCRIPTION OF THE INVENTION The nitrile rubber-containing emulsion useful in the process of this invention can be synthesized by standard emulsion polymerization techniques. This is synthesized by the copolymerization of acrylonitrile and 1,3-butadiene in an aqueous emulsion under conditions of free radical polymerization. This nitrile rubber in these emulsions, therefore, contains repeating units which are derived from monomer 1,3-butadiene and the acrylonitrile monomer. The repeating units derived from monomer 1,3-butadiene and acrylonitrile monomer differ from the monomers from which they were derived since double bonds are consumed during the polymerization. Nitrile rubbers usually contain from about 20% to about 45% repeating units that are derived from acrylonitrile and from about 55% to about 80% repeat units that are derived from 1,3-butadiene. Nitrile rubbers most commonly contain from about 50 to about 36% of repeating units that are derived from acrylonitrile and from about 64% to about 70% of units that are derived from 1,3-butadiene. The carboxylated nitrile rubbers containing repeat units derived from 1,3-butadiene, acrylonitrile and methacrylic acid can also be recovered from the emulsions using the techniques of this invention. These carboxylated nitrile rubbers are synthesized by the terpolymerization of the free radicals of the monomers 1, 3- butadiene, acrylonitrile and methacrylic acid under emulsion polymerization conditions. The carboxylated nitrile rubbers will usually have repeating units which are derived from about 5 percent by weight to about 80 percent by weight of the 1,3-butadiene monomer, from about 19 percent by weight to about 44 percent by weight of the acrylonitrile monomer, and from about 1 percent by weight to about 20 percent by weight of methacrylic acid. The carboxylated nitrile rubbers of this invention will generally have repeating units that are derived from about 4 percent by weight to about 70 percent by weight of monomer 1,3-butadiene, from about 20 percent by weight to about 32 percent. percent by weight of the acrylonitrile monomer, and from about 4 percent by weight to about 10 percent by weight of methacrylic acid. Conventional nitrile rubber grids usually have a solids content that is in the range of about 20 percent to about 30 percent. It is more common for standard nitrile rubber grids to have a solids content that is in the range of about 25 percent to about 28 percent. The emulsion polymerizations that are used during the synthesis of these nitrile rubber emulsions which are used in the practice of this invention generally use a charge composition consisting of water, the monomers, an initiator and an emulsifier (soap) . These emulsion polymerizations can be carried out over a wide range of temperatures, from about 0 ° C to as high as about 100 ° C. It is usually preferred to carry out the emulsion polymerization at a temperature that is within the range of about 5 ° C to about 60 ° C. In general, it is more preferable that the emulsion polymerization be carried out at a temperature ranging from about 5 ° C to about 30 ° C. The ratio of the monomers combined with the carboxylated nitrile rubber can vary from the proportion of the monomer charge used in the synthesis of the polymer due to the differences in the polymerization rates of the monomers. In this way, the carboxylated nitrile rubber must have a different ratio of repeat units that are derived from the different monomers that were used in the loading of monomers. The ratio of the monomers used in the composition of the filler may vary and, of course, will vary with the ratio of the desired combined monomers to the nitrile rubber to be synthesized. However, the composition of the monomer charge will normally contain from about 30 to about 80 percent by weight of 1,3-butadiene, from about 20 to about 70 percent by weight of acrylonitrile and from about 0 to about of 20 percent by weight of methacrylic acid. The charge composition that is used in the preparation of the nitrile rubber emulsions will contain a substantial amount of water. The ratio between the total amount of monomers present in the charge composition and the water can be between the range of 0.2: 1 and about 1.2: 1. It is generally preferred that the ratio of the monomers to the water in the charge composition is within the range of about 0.8: 1 and about 1.1: 1. For example, it is very convenient to use a ratio of the monomers to water in the charge composition of about 1: 1. Typically, the charge composition will also contain from about 0.5 cfm (parts per 100 parts by weight of the monomer) to about 6 cfm of at least the emulsifier. It is usually preferred that the emulsifier be present in the polymerization medium at a level in the range of about 1 cfm to about 5 cfm. Generally, more preferably, the charge composition will contain from about 2 to about 4 pcm of the emulsifier.

The emulsifiers that are used in the polymerization can be charged at the start of the polymerization or they can be added in increments or proportionate as the reaction proceeds. In general, the anionic emulsifier system gives good results, however, any of the general types of anionic, cationic or nonionic emulsifiers can be employed in the polymerization. Anionic emulsifiers that can be used in emulsion polymerizations include fatty acids and their alkali metal soaps such as caprylic acid, capric acid, pelargonic acid, lauric acid, undecylic acid, myristic acid, palmitic acid, margaric acid, stearic acid , arachidic acid and the like; amine soaps of fatty acids such as those formed from ammonia, mono-, di-alkylamines, substituted hydrazines, guanidine and the different low molecular weight diamines; fatty acid derivatives substituted in the chain as products having alkyl substituents; naphthenic acid and its soaps and the like; sulfuric esters and their salts, such as sulfate alcohol sulfates, coconut alcohol sulfates, fatty alcohol sulfates, such as oleyl sulfate, sodium lauryl sulfate and the like; sterol sulfates, alkyl cyclohexanal sulphates, sulfation products of lower ethylene polymers such as straight chain olefins of C 0 to C 0 and other mixtures of hydrocarbons, sulfur esters of aliphatic and aromatic alcohols having intermediate bonds, such as ether group, ester or amide such as the alcohols of (polyethyleneoxy) alkylbenzyl, the sodium salt of the tridecyl ether sulfate; sulphanates, ester and alkane salts, such as alkyl chlorosulfonates with the general formula RS02C1, wherein R is an alkyl group having from 10 to 20 carbon atoms and alkyl sulfonates with the general formula RS02-OH, wherein R is an alkyl group having from 1 to 20 carbon atoms; sulfonates with intermediate bonds such as esters and sulfonates with ester bonding such as those having the formula RCOOC2H4S03 and ROOC-CH2-S03H, wherein R is an alkyl group having from 1 to 20 carbon atoms, such as sulfosuccinates of dialkyl; the salts of the ester with the general formula: Where R is an alkyl group having from 1 to 30 carbon atoms; alkaryl sulfonates in which the alkyl groups preferably contain from 10 to 20 carbon atoms, for example, the dodecylbenzenesulfonates such as the sodium dodecylbenzenesulfonates; alkylphenol sulfonates; sulfonic acids and their salts, such as acids with the formula ROS ^ Na, where R is an alkyl and the like; sulfonamides; sulfamidomethylene sulphonic acids; rosin acids and their soaps; sulfonate derivatives of rosin and rosin oil; and lignin sulfonates and the like. The rosin acid soap has been used successfully at a concentration of about 5 percent by weight in the composition of the initial charge used in the synthesis of the carboxylated elastomers. Of the rosin acid, about 90 percent are isometric with abietic acid and the other 10 percent is a mixture of dehydroabietic acid and dihydroabietic acid. Polymerization can be initiated using free radical catalysts, light or ultraviolet radiation. To ensure satisfactory polymerization speed, uniformity and controllable polymerization, free radical initiators are generally used. The free radical initiators that are commonly used include various hydrogen peroxide compounds, such as potassium persulfate, ammonium persulfate, benzoyl peroxide, hydrogen peroxide, di-t-butyl peroxide, dicumyl peroxide, peroxide 2. , 4-dichlorobenzyl, decanoyl peroxide, lauroyl peroxide, eumeno hydroperoxide, p-menthane hydroperoxide, t-butyl hydroperoxide, acetyl acetone peroxide, methyl ethyl ketone peroxide, succinic acid peroxide, dicetyl peroxydicarbonate, peroxyacetate -butyl, t-butyl peroxymalonic acid, t-butyl peroxybenzoate, acetyl cyclohexyl sulfonyl peroxide and the like; the various azo compounds such as 2-t-butylazo-2-cyanopropane, dimethyl azodiisobutyrate, azodiisobutyronitrile, 2-t-butylazo-l-cyanocyclohexane, 1-t-amylazo-1-cyanocyclohexane and the like; the different alkylpercatales, such as 2, 2-bis- (t-butylperoxy) butane, 3, 3-bis (t-butylperoxy) ethyl butyrate, 1,1-di- (t-butylperoxy) cyclohexane and the like. The eumeno hydroperoxide is a highly preferred initiator. After having achieved a desired degree of monomer conversion, a conversion switch agent, such as hydroquinone, can be added to the polymerization medium to complete the polymerization. In general, polymerization will be allowed to continue until a high level of conversion has been achieved. In most cases, the monomer conversion will be at least about 75 percent, with monomer conversions of at least 80 percent being preferred. After the polymerization has been interrupted, it is usually desirable to remove monomers that did not react from the emulsion. This can be achieved using conventional techniques such as steam extraction. After performing any extraction operation, an antioxidant may be added to the emulsion containing nitrile rubber to produce a stabilized nitrile rubber. For this purpose almost any type of antioxidant can be used. For example, any anoxide capable of rendering the polymer less susceptible to oxidative attack can be used by chemically interrupting the autooxidation process, whereby the polymer is oxidatively degraded. More specifically, the antioxidant can be a chain-breaking antioxidant, an antioxidant that breaks down the peroxide or a UV light-protecting agent, a triplet retarder or a metal quencher. However, it is preferred that the antioxidant be an alkylated aryl phosphite. More preferably, the antioxidant should be a mixture of (a) an antioxidant alkylated aryl phosphite and (b) a non-hydrolysable antioxidant. The alkylated aryl phosphite antioxidants that can be used are the structural formula: wherein R represents an alkyl group containing from 1 to about 30 carbon atoms. It is preferred that the alkyl group contains from about 4 to about 20 carbon atoms and more preferably that the alkyl group contains from about 6 to about 12 carbon atoms. Tris (nonylphenyl) phosphite is a highly preferred alkylated aryl phosphite antioxidant which is commercially available from various suppliers. The hindered phenolic antioxidants that can be employed are usually alkyl-substituted phenol of the structural formula: wherein R1 and R2 represent alkyl groups containing from 1 to about 10 carbon atoms and wherein R3 represents a hydrogen atom or an organic radical containing from about 1 to about 30 carbon atoms. It is usually preferred that R1 and R2 represent tertiary alkyl groups containing from 4 to about 10 carbon atoms and R3 present an organic radial of the formula -CH2-CH2-C00R4 wherein R4 represents an alkyl group containing from about 12 to about 24 carbon atoms. Thiodiethylene bis (3, 5-di-t-butyl-4-hydroxy) hydrocinnamate and octadecyl 3- (3, 5-di-t-butyl-4-hydroxyphenyl) propionate are highly preferred phenol hindered antioxidants. Usually, 0.25 pcm will be added to about 3 pcm of the antioxidant to obtain a nitrile rubber with the desired degree of stability. Generally, from about 0.5 pcm to 2 pcm of the antioxidant that is added in step (1) is preferred. In most cases it is more preferred that the antioxidant be present at a level within the range of about 1 pcm to about 1.5 pcm. After the antioxidant has been added in step (1), the stabilized nitrile rubber emulsion is coagulated using a conventional salt / acid coagulation method. In other words, a salt and an acid are added to the stabilized nitrile emulsion to cause coagulation. The coagulation is achieved by adding to the latex at least one strong inorganic acid and one salt. During the coagulation of the nitrile rubber emulsion, coagulation aids can also be used. The strong inorganic acid will usually be sulfuric acid, hydrochloric acid or nitric acid, with sulfuric acid being preferred. A wide variety of salts can be used. Some representative examples of salts that may be used include sodium chloride, potassium chloride, calcium chloride, aluminum sulfate, magnesium sulfate, and quaternary ammonium salts. The amount of salt and acid needed to cause coagulation will vary with the specific emulsion and the type of salt used. Calcium chloride is a highly preferred salt and will normally be added in an amount within the range of about 13 pee to about 40 pee. After the nitrile rubber emulsion has coagulated, a suspension of coagulated nitrile rubber is formed. The suspension of coagulated nitrile rubber consists of serum and lumps of nitrile rubber. The serum is mainly the aqueous phase, the lumps of rubber being the solid phase. Of course, the serum is composed of water, emulsifier, acids, salts and other water soluble residual compounds. In general, the suspension of coagulated nitrile rubber is transferred to a conversion tank to complete the coagulation process. The lumps of nitrile rubber are then filtered through a shaking screen that collects lumps of nitrile rubber and deposits them in a tank for resuspension. Washing is commonly used to remove excess soap and / or electrolyte from the nitrile rubber. In the resuspension tank lumps of nitrile rubber are washed and shaken in clean wash water to produce a new suspension of nitrile rubber. The pH of the new nitrile rubber suspension is then adjusted so that it is within the range from about 5 to about 8. This neutralization step is performed by adding a base. The numerous bases known to those skilled in the art may be used., which includes calcium hydroxide, magnesium hydroxide, potassium hydroxide and sodium hydroxide. The pH of the new nitrile rubber suspension will preferably be adjusted within the range from about 5.5 to about 7.5 and more preferably will be adjusted to be within the range from about 6 to about 7. The whey that is obtained after of the screening usually goes back to recycling to the coagulator, allowing an efficient use of the coagulants. The lumps of nitrile rubber found in the tank of the new suspension then normally pass over a second mesh with agitation and are directed to an extractor, in which the water is removed from the polymer. The extractor usually consists of a screw that transports the rubber down a shaft of the extractor under conditions of increasing compression. The rotating drum of the extractor is aligned all along with narrow slits, the width of which decreases as the rubber moves through the extractor.

The extracted water exits through the slits while the rubber advances towards an open cone at one end located at the far end of the drum. The cone provides a back pressure for the dehydration screw. The dehydration force can be controlled by adjusting the parameters of the cone. This adjustment can vary with different types of nitrile rubber and can be modified during a finishing process to achieve the desired moisture content. The moisture content of the rubber that comes out of the extractor will usually be in the range from about 7 percent by weight to about 10 percent by weight. The dehydrated rubber then usually passes through a small cone port, at the end of the extractor shaft, like a long strand. This nitrile rubber strand can be cut into small pieces by rotating blades located near the hole in the cone. These pieces of nitrile rubber are then usually directed towards a Jeffery crusher, where the rubber is milled to a smaller size to facilitate drying. From the Jeffery mill, the nitrile rubber is transported, in the presence of air, to a cyclone, where it subsequently falls on a metal conveyor and continues through a conveyor belt dehydrator. The cyclone functions as an ejector vessel that separates the rubber from the air. Consequently, the rubber falls on the conveyor in a uniform and dispersed manner. The conveyor dryer is usually a one-step dryer that contains a series of hot zones that can each be set at specific temperatures. Hot air is directed through each zone at the specific temperature and removes moisture from the rubber. The maximum drying temperature is somewhat limited because the heat significantly affects the final properties of the rubber. The zone temperatures and the speed of the conveyor belt can vary to adjust the drying conditions inside the conveyor dryer. The moisture content of the finished rubber is preferably less than about 1 percent and more preferably down from about 0.7 percent.

When the rubber leaves the conveyor it is left to cool and it is taken to a packing machine. Dry rubber is usually weighed, compressed into 55-pound (20.5 kg) packages and wrapped in a film. The wrapped packages are usually packed in returnable containers or in cardboard boxes. The nitrile rubbers produced by the processes of the present invention can be used to produce various finished products. For example, it has been foreseen that these rubbers can be used for traditional applications such as the manufacture of gaskets, seals, adhesives, fiber ties, protective and decorative coatings, foams, paper coatings, carpets and upholstery, concrete modifiers and asphalt, fiber and textile modifiers. In addition, the most recent applications as protein immobilizers, electronic applications, as photoresistors for circuit boards, in accumulators, conductive paints and as compounds in molecular electronic devices. Ordinarily, nitrile rubber is vulcanized during the processes used in the manufacture of these products. The practice of this invention is further illustrated by following the examples that are proposed as representative and not as limiting the scope of the subject matter of the invention. Unless stated otherwise, all parts and percentages are given by weight.

Comparative Example 1 For this experiment 1.5 percent of the antioxidant tris (nonylphenol) phosphite was added to a nitrile rubber latex prior to coagulation. Then, the nitrile rubber latex was coagulated and the rubber was dehydrated and dried. The dry nitrile rubber with a content of 0.04 percent of unhydrolysed tris (nonylphenol) phosphite antioxidant and 0.54 percent of hydrolyzed tris (nonylphenol) phosphite antioxidant was determined.

Comparative Example 2 For this experiment the procedure described in Comparative Example 1 was repeated. However, in this experiment the dry nitrile rubber was determined with a content of only 0.01 percent of unhydrolyzed tris (nonylphenol) phosphite antioxidant and 0.77 percent of antioxidant tris (nonylphenol) hydrolyzed phosphite.

Comparative Example 3 In this experiment the procedure described in Comparative Example 1 was repeated. However, in this experiment the dry nitrile rubber with a content of only 0.02 percent of unhydrolysed tris (nonylphenol) phosphite antioxidant and 2.20 percent was determined. of antioxidant tris (nonylphenol) hydrolyzed phosphite.

Comparative Example 4 In this experiment, 0.8 percent of tris (nonylphenol) phosphite antioxidant and 0.2 percent of octadecyl 3, 5-di-t-butyl-4-hydroxyhydrocinemate antioxidant were added. Ultranox ™ 276 was added to a rubber latex of nitrile before coagulation. Then, the nitrile rubber latex was coagulated and the rubber was dehydrated and dried using the same procedure as in Comparative Examples 1-3. The dry nitrile rubber was determined with a content of 0.31 percent of unhydrolysed tris (nonylphenol) phosphite antioxidant and 0.71 percent of hydrolyzed tris (nonylphenol) phosphite antioxidant. In this way, the non-hydrolyzed tris (nonylphenol) phosphite antioxidant was present in the dry nitrile rubber 7 times more than in any of the rubbers recovered in Comparative Examples 1-3. This is very surprising, in particular in light of the fact that initially much less antioxidant phosphate tris (nonylphenol) was added to the latex.

COMPARATIVE EXAMPLE 5 In this experiment, the procedure described in Example 4 was repeated. However, in this experiment dry nitrile rubber with a content of only 0.37 percent of unhydrolysed tris (nonylphenol) phosphite antioxidant and 0.49 percent of antioxidant tris (nonylphenol) hydrolyzed phosphite. Thus, the non-hydrolyzed tris (nonylphenol) phosphite antioxidant was present in the dry nitrile rubber more than 9 times in any of the rubbers recovered in Comparative Examples 1-3. This experiment and Example 4 show that the presence of the octadecyl 3, 5-di-t-butyl-4-hydroxyhydrocinemate antioxidant greatly increases the level of the unhydrolyzed tris (nonylphenol) phosphite antioxidant present in the dry nitrile rubber .

Example 6 and Comparative Example 7 In this experiment 0.2 percent (octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinemate, Ultranox ™ 276, was added to a nitrile rubber latex before coagulation. nitrile rubber was subsequently coagulated by the addition of calcium carbonate and sulfuric acid to the latex In one case (Example 6) the pH of the nitrile rubber suspension in the tank of the new suspension was adjusted by adding hydroxide In the other case (Comparative Example 7), the pH of the nitrile rubber in the tank of the new suspension was not adjusted.The nitrile rubbers were then dehydrated and dried.The nitrile rubbers then recovered were matured to a temperature of 70 ° C for 28 days Mooney ML4 viscosities of nitrile rubbers were measured and recorded as samples of matured rubber.The results of this study are shown in Table I Table I MOONEY ML4 VISCOSITY COMPARED TO THE MATURATION TIME. Maturation time Example 6 Ex,. Comparative 7 0 days 66 69 3 days 68 73 5 days 67 72 7 days 67 72 14 days 67 75 21 days 66 76 28 days 69 80 Examples 8-12 and Comparative Example 13 In this series of experiments, 2 percent of the antioxidant tris (nonylphenyl) phosphite was added to a nitrile rubber latex before coagulation. The nitrile rubber latex was subsequently coagulated by the addition of a polyquarternary salt of ammonium and sulfuric acid in the latex. In Examples 8-12, the pH of the nitrile rubber suspension in the tank of the new suspension was adjusted by adding sodium hydroxide to the pH shown in Table II. In Comparative Example 13 the pH of the nitrile rubber in the tank with the new suspension was not adjusted. Then, the nitrile rubbers were dehydrated and dried. The recovered nitrile rubbers were then compounded with 5 parts of zinc oxide per 100 parts of rubber, 1 part of the accelerator 2-mercaptobenzothiazole Captax® per 100 parts of rubber, 0.5 parts of methyl zimate per 100 parts of rubber and 2 parts of sulfur per 100 parts of rubber. After being compounded, the nitrile rubber samples were subjected to the rheometer test at 250 ° F (138 ° C) to determine the characteristics of the vulcanizate. TABLE II Example pH TC501 TC90- '8 3. 0 5.8 min 7-8 min 9 4. 5 5.6 min 7.7 min 10 6. 0 5.3 min 7.5 min 11 7. 0 4.9 min 7.3 min 12 8. 0 5.2 min 7.4 min 13 _ 7.2 min 8.8 min -TC50 is the time, in minutes, that it takes to reach 50% of the maximum torque. 2TC50 is the time, in minutes, that it takes to reach 90% of the maximum torque. As can be seen in Table II, the vulcanization times were reduced when the pH of the new suspension was adjusted to 7.0. In this way, it seems desirable to add a sufficient amount of base to the new suspension to make it as neutral as possible. Although certain embodiments and representative details have been shown for the purpose of illustrating the subject of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the present invention.

Claims

An improved method for recovering nitrile rubber from an emulsion containing nitrile rubber, this method is characterized by comprising the steps of: (1) adding an antioxidant to the emulsion containing nitrile rubber to produce a stabilized nitrile rubber , (2) adding a salt and an acid to the stabilized nitrile rubber emulsion to produce a suspension of coagulated nitrile rubber, wherein the suspension of coagulated nitrile rubber consists of whey and lumps of nitrile rubber, (
3) Separate the lumps of nitrile rubber from the serum of the coagulated nitrile rubber suspension (
4) Mix the lumps of nitrile rubber in washing water to produce a new suspension of nitrile rubber (5) adjust the pH of the new suspension of nitrile rubber to be within the range of about 5 to about 8, and (6) separating lumps of nitrile rubber from the wash water of the new nitrile rubber suspension. The method, as specified in claim 1, is characterized in that the pH of the new nitrile rubber suspension is adjusted in step (5) by the addition of a base, and is characterized in that the pH is adjusted in the step (5) so that it is within the range from about 5.5 to about 7.
5 The method, as specified in claim 2, is characterized in that lumps of nitrile rubber are separated from the wash water of the new suspension of nitrile rubber in step (6) with a screen. The method, as specified in claim 3, is characterized by the dehydration and subsequent drying of the nitrile rubber lumps separated from the wash water of the new nitrile rubber suspension in step (6). The dry nitrile rubber, which is characterized by being made by the process specified in claim 4. The method, as specified in claim 4, is characterized in that the antioxidant is an antioxidant alkylated aryl phosphite wherein the Alkylated aryl phosphite antioxidant is of the structural formula:
wherein R represents an alkyl group containing from 6 to about 12 carbon atoms.
7. The method, as specified in claim 4, is characterized in that the antioxidant is a mixture of: (a) an alkylated aryl phosphite antioxidant and (b) a hindered phenol antioxidant, wherein the hindered phenol antioxidant is of the structural formula :
wherein R1 and R2 represent tertiary alkyl groups containing from 4 to about 10 carbon atoms and R3 has an organic radial of the formula -CH2-CH2-COOR4, wherein R4 represents an alkyl group containing from about 12 to about 24 atoms 8. The method, as specified in claim 7, is characterized in that the antioxidant alkylated aryl phosphite is tris (nonylphenyl) phosphite, and is characterized in that the antioxidant phenol hindered is 3- (3, 5- octadecyl di-t-butyl-4-hydroxyphenyl) propionate
9. The method, as specified in claim 8, is characterized in that the base is selected from the group consisting of calcium hydroxide, magnesium hydroxide and sodium hydroxide.
10. The method, as specified in claim 9, is characterized in that the base is sodium hydroxide and wherein the pH is adjusted in step (5) to be within the range of from about 6 to about 7.

SUMMARY OF THE INVENTION

The present invention relates to nitrile rubber which can be synthesized by the copolymerization of acrylonitrile and 1,3-butadiene in an aqueous emulsion. Of course, after these monomers have been polymerized it is necessary to recover the polymer from the aqueous emulsion. The present invention describes an improved method for recovering nitrile rubber from an emulsion containing nitrile rubber, this method comprises the steps of: (1) adding an antioxidant to the emulsion containing nitrile rubber to produce a nitrile rubber stabilized, (2) add a salt and an acid to the stabilized nitrile rubber emulsion to produce a suspension of coagulated nitrile rubber, where the suspension of coagulated nitrile rubber contains whey and lumps of nitrile rubber, (3) separate the lumps of nitrile rubber from the whey of the coagulated nitrile rubber suspension (4) mix the lumps of nitrile rubber in wash water to produce a new suspension of nitrile rubber (5) adjust the pH of the new suspension of nitrile rubber to be within the range of about 5 to about 8, and (6) separating lumps of nitrile rubber from the wash water of the new nitrile rubber suspension. Nitrile rubber that is recovered from emulsions using this technique usually has better stability and vulcanization characteristics.