Crosslinkable rubber composition

Abstract: It is sometimes desirable to cure rubber compositions utilizing non-sulfur curing agents. The subject invention reveals rubber compositions which can be cured without utilizing sulfur or sulfur containing compounds. The subject invention more specifically discloses a rubber composition which can be crosslinked by heating which is comprised of (1) at least one rubber having pendant blocked isocyanate groups bound thereto; and (2) at least one compound which contains at least 2 Zerewitinoff active hydrogens. This invention also discloses crosslinking of elastomer compounds containing (1) at least one rubber having functionalized groups containing at least one Zerewitinoff active hydrogen and (2) at least one compound which contains at least two blocked isocyanate groups. This invention also discloses self-curing elastomers containing both blocked isocyanate groups and groups containing at least one active Zerewitinoff hydrogen.

Description: Background of the Invention Elastomers can be crosslinked by a number of techniques as described by A. Coran in "Science and Technology of Rubber" Chapter 7. Most elastomers are cured using sulfur or peroxide vulcanization although curatives such as phenolic resins, quinone derivatives, maleimide derivatives and metal oxides can also be employed in the crosslinking reaction. A novel crosslinking reaction was reported in Kautschuk Gummi Kunststoffe 8/83 which discloses a cure system consisting of the adduct of a diisocyanate with nitrosophenol. In this cure system, the nitrosophenol is released at cure temperature and reacts with the elastomer. Crosslinking occurs when the diisocyanate reacts with functionality on the polymer bound nitrosophenol. This blocked isocyanate curative is incorporated in a step separate from the polymerization. Polymer bound isocyanates have been used as part of a curing package of acrylate polymers. European Patent Application Publication No. 130,322 discloses that acrylate polymers containing m-isopropenyl α,α-dimethyl benzylisocyanate (TMI) can be cured and that the polymers can be used in coating applications. U.S. Patent 4,694,057 discloses that elastomers containing small amounts of 1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)-benzene (TMI) can be crosslinked by utilizing difunctional or polyfunctional active hydrogen compounds. For instance, U.S. Patent 4,694,057 discloses that elastomers which are difficult to cure with sulfur or sulfur containing compounds, such as polychloroprene, can be cured utilizing such a technique. However, neither the teachings of EP-A-130 322 or US-A-4 694 057 suggest any advantage that can be realized by blocking isocyanate groups in the elastomer. In fact, neither of the subject references disclose the possibility of curing such compounds which contain blocked isocyanate groups. Problems associated with premature crosslinking have hindered the commercial development of cure systems which rely on crosslinking through unblocked isocyanate groups in elastomers. EP-A-250 633 discloses package and tank stable, low temperature self-curing, cation-active aqueous soluble or dispersible coating compositions which are prepared by copolymerizing blocked meta-isopropenyl-α,α-dimethylbenzyl isocyanate (m-TMI) with various vinyl unsaturated monomers optionally in the presence of epoxy-amine adduct. It is reported that these cation-active polymers when acidified or partially acidified provide aqueous solutions or dispersions which are especially useful as low temperature curable vehicles in cathodic electrocoating. Summary of the Invention This invention discloses a technique for curing rubber compositions which does not rely upon sulfur or sulfur containing compounds. The cure systems of the subject invention rely upon the reaction between a blocked isocyanate group and an active Zerewitinoff hydrogen atom. The utilization of the process of this invention accordingly results in a urethane or urea type cure. One benefit realized by utilizing the process of this invention is that premature crosslinking of the elastomer is generally not a significant problem if the appropriate blocking group is used. The subject invention more specifically discloses a rubber composition which can be crosslinked by heating which is comprised of (1) at least one rubber having pendant blocked isocyanate groups bound thereto; and (2) at least one compound which contains at least 2 Zerewitinoff active hydrogens; characterized in that the pendant blocked isocyanate groups are derived from m-isopropenyl-α,α-dimethyl benzylisocyanate and are blocked with 2-pyrrolidone. The subject invention also reveals a rubber composition which can be crosslinked by heating which is comprised of (1) at least one rubber having Zerewitinoff active hydrogens bound thereto; and (2) at least one compound having at least 2 blocked isocyanate groups bound thereto; characterized in that the pendant blocked isocyanate groups are derived from m-isopropenyl-α,α-dimethyl benzylisocyanate and are blocked with 2-pyrrolidone. The subject invention further discloses a rubber composition which can be crosslinked by heating which is comprised of polymer chains having (1) pendant blocked isocyanate groups bound thereto; and (2) Zerewitinoff active hydrogens bound thereto; ; characterized in that the pendant blocked isocyanate groups are derived from m-isopropenyl-α,α-dimethyl benzylisocyanate and are blocked with 2-pyrrolidone. There are different requirements for each of the previous approaches. In all cases the polymer must not undergo premature crosslinking during the polymer drying step or when the polymer is mixed with other ingredients necessary for good elastomeric compounds. These polymers are generally prepared by an emulsion process and must be coagulated and dried. In commercial operations, emulsion polymers are dried at 93-110°C (200-230°F) for approximately 45 minutes. This polymer is then mixed with carbon black in a non-productive mix at 149-166°C (300-330°F) for 1-3 minutes. The curative is added to the rubber from the non-productive mix in a productive mix at 104°C (220°F). Thus, polymers containing the blocked isocyanate functionality cannot undergo extensive deblocking at drying conditions or during the non-productive mix or the material will become unworkable. In the case where the polymer contains the Zerewitinoff active hydrogen, premature vulcanization is not a problem. The blocked isocyanate compound curative must only be stable to the productive mix which takes place at 104°C (220°F) for 1-2 minutes. The following reaction depicts the curing of a rubber having pendant blocked isocyanate groups bound thereto with a curative which contains two Zerewitinoff active hydrogens. In the first step of the reaction, the blocking agent represented as X is removed from the isocyanate group by the action of heat as follows: wherein P represents polymer chains of the rubber. In the second stage of the curing reaction, the curative containing two active Zerewitinoff hydrogens reacts with the free isocyanate groups on two different polymer chains of the rubber being cured. This reaction is depicted as follows: wherein A represents an alkylene group or an arylene group. The same basic reactions are utilized in curing rubbers having active Zerewitinoff hydrogens bound thereto with curatives containing at least two blocked isocyanate groups. In such reactions, the heat utilized to cure the rubber causes the blocking group to be removed thereby creating a free isocyanate group which is then available to react with active Zerewitinoff hydrogens on the rubber. Similarly, identical reactions take place wherein the rubber being cured contains both pendant blocked isocyanate groups and active Zerewitinoff hydrogen atoms. In such cases, it is, of course, not necessary to utilize a separate curative. In other words, rubbers which contain both pendant blocked isocyanate groups and Zerewitinoff active hydrogen atoms have a built in cure package. Rubbers having pendant blocked isocyanate groups bound thereto can be prepared utilizing a wide variety of techniques. For instance, US-A-4,429,096 discloses a process wherein the isocyanate group on meta-TMI is blocked with a cationic carbamic structure and then polymerized into a polymer. The technique disclosed in US-A-4,429,096 is highly suitable for preparing rubbers having pendant blocked isocyanate groups which can be utilized in accordance with the process of this invention. US-A-4,604,439 also discloses a technique for incorporating blocked TMI into polymers utilizing emulsion polymerization. US-A-4,694,057 discloses a technique for polymerizing unblocked TMI into rubbers utilizing an emulsion polymerization technique. Such rubbers containing unblocked TMI can be blocked by reacting the rubber containing unblocked TMI with an appropriate blocking agent. In fact, any rubber containing pendant unblocked isocyanate groups can be blocked by reacting the unblocked isocyanate groups thereon with an appropriate blocking agent. In accordance with the process of this invention, 2-pyrrolidone is employed to block the isocyanate groups. 2-pyrrolidone has the structural formula: and a deblocking temperature which is within the range of 160°C to 190°C. A rubber having pendant blocked isocyanate groups bound thereto wherein 2-pyrrolidone is utilized as the blocking agent is depicted as follows: wherein P represents polymer chains of the rubber. 2-pyrrolidone is a particularly valuable blocking agent because it has a deblocking temperature which is within the range of 160°C to 190°C. When the blocked isocyanate is heated to the deblocking temperature, the blocking group is released exposing the free isocyanate. The free isocyanate then undergoes the curing reaction. In cases where the isocyanate is not blocked, premature crosslinking reactions occur making processing of the elastomer difficult if not impossible. If the rubber is processed at temperatures higher than the deblocking temperature, premature crosslinking or scorch of the rubber will occur. The higher the deblocking temperature is, the more latitude there is in processing of the rubber but cure temperatures must, of course, be higher in order for deblocking and subsequent crosslinking to occur. As the deblocking temperature is lowered, the rubber must be processed more gently but can be effectively cured at a lower temperature. Thus, the blocking group can be chosen to give the optimal mix of scorch safety and cure temperature. The deblocking temperature of 2-pyrrolidone has been found to be very good in some applications. Zerewitinoff active hydrogen is reactive as determined by the Zerewitinoff method as described in the Journal of the American Chemical Society, Vol. 49, page 3181 (1927). The Zerewitinoff active hydrogen will typically be present in a hydroxyl group, amine group, carboxyl group or thiol group. Zerewitinoff hydrogens which are present in hydroxyl groups are the most highly preferred. Zerewitinoff hydrogen atoms which are present in amine groups are also very good. However, amines react very readily with isocyanate groups which results in a very fast rate of cure. In fact, the rate of cure attained utilizing amines as the source of Zerewitinoff active hydrogen atoms can be too fast. The Zerewitinoff hydrogen present in carboxyl groups is far less active and promotes a much slower rate of cure. For this reason, carboxyl groups are not a preferred source of Zerewitinoff active hydrogen. The optimum rate of cure is believed to be attained when hydroxyl groups are utilized as the source of Zerewitinoff active hydrogen. Curatives can be utilized which contain at least two Zerewitinoff active hydrogen atoms. These compounds will typically have boiling points which are above the cure temperature of the rubber composition. In cases where the curative contains at least two blocked isocyanate groups, the blocking groups will also have a boiling point which is above the cure temperature utilized in crosslinking the rubber composition. Catalysts can be utilized in order to accelerate the reaction between the Zerewitinoff active hydrogen and isocyanate groups. Such catalysts are of particular importance in cases where the blocking agent has a very high deblocking temperature. For instance, the utilization of such catalysts is of particular value in cases where 2-pyrrolidone is utilized as the blocking agent. Catalysts capable of speeding up both the deblocking reaction and the reaction of the free isocyanate groups with the Zerewitinoff active hydrogen can be utilized. For example, tin salts, bismuth compounds, mercury compounds, tertiary amines, iron acetyl acetonate, cobalt acetyl acetonate and nickel acetyl acetonate can be utilized as the catalyst. Tin salts such as dibutyltin dilaurate, dimethyltin dilaurate, dibutyltin diacetate, and dimethyltin diacetate are most preferred. Dialkyltin sulfides are also highly preferred catalysts. The rubber compositions of this invention will typically contain from 0.001 moles to 0.4 moles of blocked isocyanate groups per 100 grams of polymer. The rubber compositions of this invention will preferably contain from 0.005 moles to 0.1 moles of blocked isocyanate groups per 100 grams of polymer. Such rubber compositions will more preferably contain from 0.01 to 0.03 moles of blocked isocyanate groups per 100 grams of rubber. The rubber compositions of this invention will typically have a molar ratio of Zerewitinoff active hydrogen atoms to blocked isocyanate groups of at least 0.5:1. Such rubber compositions will typically have a ratio of Zerewitinoff active hydrogen atoms to blocked isocyanate groups which is within the range of 0.6:1 to 2:1. The ratio of Zerewitinoff active hydrogen atoms to blocked isocyanate groups in the rubber composition will preferably be within the range of 0.7:1 to 1.4:1. More preferably, the ratio of Zerewitinoff active hydrogen atoms to blocked isocyanate groups will be within the range of 0.8:1 to 1.2:1. However, it should be noted that a very substantial excess of Zerewitinoff active hydrogen atoms over the amount of blocked isocyanate groups present typically is not detrimental in rubbers containing both pendant blocked isocyanate groups and Zerewitinoff active hydrogen atoms. In general, the elastomer containing one or both polymer bound curatives can be compounded using classical rubber compounding technology such as given by Long in his book "Basic Compounding and Processing of Rubber". There are several advantages associated with using the technology disclosed in the instant invention. In typical rubber formulations, sulfur, and at least one accelerator is commonly used to affect the cure. To activate the sulfur, zinc oxide and stearic acid are usually employed. In the cure system of the instant invention, one or both of the curatives is attached to the polymer. Thus, the total number of ingredients that must be added to the compound may be less with the instant invention than with a conventional sulfur cure. This leads to greater product uniformity because there are fewer items to weight and potentially fewer places to misweigh ingredients in the compounding step. Additionally, the non-sulfur cross links may have better stability to aging than the sulfur crosslinks. The following examples are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, parts and percentages are given by weight. Example 1 In this experiment a self-stabilized styrene-butadiene rubber containing both blocked isocyanate functionalities and hydroxyl functionalities was synthesized. The redox recipe utilized in the polymerization contained the following ingredients (shown in parts by weight): as an activator system: The emulsion made was polymerized in the absence of oxygen at 65°F (18.5°C). After allowing the polymerization to continue for 9.5 hours, 75% of the monomer charge was converted. A 10% aqueous solution containing 0.3 parts of sodium dimethyldithio carbamate and 0.04 parts of N,N-diethylhydroxyl amine was added to shortstop the reaction. The emulsion was coagulated and dried at 150°F (66°C) in a circulating air oven. The resulting dried polymer was soluble in methylene chloride and chloroform. Infrared analysis of a solution cast film of the polymer indicated that it contained hydroxyl, blocked isocyanate and antioxidant functionalities. Example 2 In this experiment an additional self-stabilized styrene-butadiene rubber containing both blocked isocyanate functionalities and hydroxyl functionalities was made. The emulsion utilized in the polymerization contained the following ingredients (shown in parts by weight): as an activator system: The emulsion was polymerized in the absence of oxygen at 65°F (18.5°C). After 3 hours of polymerization time, 75% of the monomer charge was converted to polymer. A 10% aqueous solution containing 0.3 parts of sodium dimethyldithiocarbamate and 0.04 parts of N,N-diethylhydroxylamine was added to shortstop the polymerization. The latex was coagulated and air dried at 150°F (66°C). Infrared analysis of a cast film of the polymer indicated the inclusion of hydroxyl, blocked isocyanate, and N,N-anilinophenyl methacrylamide functionalities.

Claims: A rubber composition which can be crosslinked by heating which is comprised of (1) at least one rubber having pendant blocked isocyanate groups bound thereto; and (2) at least one compound which contains at least 2 Zerewitinoff active hydrogens; characterized in that the pendant blocked isocyanate groups are derived from m-isopropenyl-α,α-dimethyl benzylisocyanate and are blocked with 2-pyrrolidone. A rubber composition which can be crosslinked by heating which is comprised of (1) at least one rubber having Zerewitinoff active hydrogens bound thereto; and (2) at least one compound having at least 2 blocked isocyanate groups bound thereto; characterized in that the pendant blocked isocyanate groups are derived from m-isopropenyl-α,α-dimethyl benzylisocyanate and are blocked with 2-pyrrolidone. A rubber composition which can be crosslinked by heating which is comprised of polymer chains having (1) pendant blocked isocyanate groups bound thereto; and (2) Zerewitinoff active hydrogens bound thereto; characterized in that the pendant blocked isocyanate groups are derived from m-isopropenyl-α,α-dimethyl benzylisocyanate and are blocked with 2-pyrrolidone. A rubber composition as specified in claim 1, 2 or 3 characterized in that said Zerewitinoff active hydrogens are present in a moiety selected from the group consisting of hydroxyl groups, amine groups, carboxyl groups, and thiol groups. A rubber composition as specified in any of the preceding claims characterized in that said Zerewitinoff active hydrogen is present in a hydroxyl group. A rubber composition as specified in any of the preceding claims characterized in that the rubber having pendant blocked isocyanate groups bound thereto has from 0.005 to 0.1 moles of blocked isocyanate groups per hundred grams of rubber; and in that the molar ratio of Zerewitinoff active hydrogen containing groups to blocked isocyanate groups in the rubber composition is within the range of 0.6:1 to 2:1. A process for curing the rubber compositions specified in any of the preceding claims which is characterized by heating said rubber composition to an elevated temperature above 160°C.

Inventors: COLVIN HOWARD ALLEN, PARKER DANE KENTON, WEINSTEIN ARTHUR HOWARD, COLVIN, HOWARD ALLEN, PARKER, DANE KENTON, WEINSTEIN, ARTHUR HOWARD
Applicants: GOODYEAR TIRE & RUBBER, THE GOODYEAR TIRE & RUBBER COMPANY
Application year: 1990
Main classification: C08J3
IPC codes: C08C19, C08J3, C08G18, C08L15, C08L21, C08F236