SCORCH RESISTANT COMPOSITIONS BASED ON WATER-CURABLE THERMOPLASTIC POLYMERS HAVING HYDROLYZABLE, PENDANT SILANE MOIETIES, AND ORGANO TITANATES

Abstract: Scorch resistant compositions comprising thermoplas­ tic polymers having hydrolyzable, pendant silane moieties, prepared in the presence of a free radical generating com­ pound and organo titanates, which are particularly useful as extrudates about wires and cables.

Description: This invention relates to scorch resistant compositions based on water-curable thermoplastic polymers having hydrolyzable, pendant silane moieties, prepared in the presence of a free radical generating compound, and organo titanates. The compositions of this invention are particularly useful in extrusion applications, being capable of extrusion about wires and cables and water-cured to cross-linked products. Currently, two major processes, so-called peroxide-curing and water-curing, are being employed in the application of protective coatings such as insulation and jacketing about wires and cables. The peroxide-curing process involves extruding compositions containing an organic peroxide about wires and cables and subjecting the resultant articles to elevated temperatures in order to cure the compositions to crosslinked products. The overall operation requires careful control of the process parameters in order to avoid undue heat and pressure build-up in the extruder. Undue heat and pressure build-up results in premature decomposition of the peroxides which in turn results in crosslinking of the compositions in the extruder. Crosslinking of the compositions in the extruder, commonly referred to as "scorch" necessitates, in extreme cases, stopping the operation and cleaning the extruder. In situations wherein "scorch" occurs but is not as severe, it has been found that the work-life of the ultimate coatings is relatively short. In addition to the processing difficulties of peroxide-curing, the peroxide containing compositions do not have that degree of resistivity to deformation, at normal peroxide loadings, demanded by many ultimate users of insulated and jacketed wire and cable articles. The water-curing process, on the other hand, involves compositions containing hydrolyzable, silane modified thermoplastic polymers and is more commercially attractive in that a wider latitude in processing conditions is possible. That is, compositions containing water-curable, silane modified polymers can be extruded at temperatures far in excess of maximum processing temperatures used in extruding peroxide containing compositions. Being capable of extrusion at higher temperatures, such compositions can be extruded at faster rates and under lower pressure and consequently are more cost effective. A disadvantage with respect to the so-called water-curing process is the water sensitivity of the hydrolyzable thermoplastic polymers. During normal handling and processing, these polymers tend to hydrolyze, determined by infra-red spectroscopic analysis; and in the presence of metal carboxylate silanol condensation catalysts tend to prematurely crosslink. As an illustration, water-curable compositions are generally formulated by admixing, in an extruder, a water-curable, silane modified thermoplastic polymer with a metal carboxylate silanol condensation catalyst such as dibutyltin dilaurate, extruding the resultant composition about a wire or cable and thereafter passing the coated wire or cable through a water bath in order to cure the coating to a crosslinked product. It has been found, however, that upon admixing the water-curable, silane modified thermoplastic polymer with a metal carboxylate silanol condensation catalyst, the resultant composition tends to prematurely crosslink in the extruder. This premature crosslinking, commonly referred to as "scorch" manifests itself in the extruded coating in the form of surface discontinuity and roughness. In some instances, excessive scorching, as previously stated, may cause enough of a pressure build-up in the extruder to require a cessation of the entire extrusion operation. Consequently, as a result of the "scorch" problem, the water-curing process has not found wide commercial acceptance in the United States. EP-A- 135923, a document relevant under Art. 54(3) EPC, discloses a composition comprising a preformed water-curable thermoplastic polymer having hydrolyzable pendant silane moieties prepared in the presence of a free radical generating compound as well as an organo titanate. The pendant silane moieties are introduced by grafting an unsaturated silane onto a basic thermoplastic polymer. The corresponding compositions are particularly useful, after crosslinking, as insulation about wires and cables. EP-A- 129 121, also a document relevant under Art. 54(3) EPC, is directed to compositions based on alkylene-alkyl acrylate copolymers and silanol condensation catalysts and the use thereof in the production of covered wires and cables. According to said document the silane-containing polymer is not preformed but prepared only after a silanol condensation catalyst and an organo titanate have been added to the basic alkylene-alkyl acrylate copolymer. According to the present invention, scorch resistant, water-curable compositions are provided comprising a preformed water-curable thermoplastic polymer having hydrolyzable, pendant silane moieties, and having been prepared by copolymerization of an olefinic monomer and an unsaturated silane in the presence of a free radical generating compound; and an organo titanate having the formula: Ti(OR²)₄ wherein each R² is a monovalent hydrocarbon radical having 1 to 18 carbon atoms inclusive, said organo titanate being present in an amount of at least about 0.05 percent by weight, generally 0.05 to 10 percent by weight and preferably 0.1 to 5 percent by weight, based on the weight of the thermoplastic polymer. More than about 10 percent by weight organo titanate can be used, but is not desirable from an economic standpoint. The presence of an organo titanate prevents premature cross-linking of the water-curable thermoplastic polymer without adversely affecting the "final cure" of the polymer. As a result, the compositions of this invention, unmodified or modified by the subsequent addition of metal carboxylate silanol condensation catalysts can be extruded about wires and cables providing coatings thereon which can be water-cured to crosslinked products. Suitable water-curable thermoplastic polymers having hydrolyzable, pendant silane moieties include polymers produced by a free radical catalyzed reaction of an olefinic monomer with an unsaturated silane as described in US-A-3,225,018 and 4,413,066. Preferred water-curable polymers are copolymers of ethylene and an unsaturated silane, particularly copolymers of ethylene and vinyltrimethoxysilane. The present invention also provides a crosslinked product of the composition defined above and a wire or cable having thereon said product or said composition. Further objects of the present invention are the following processes: (i) A process for the production of shaped articles which comprises forming the composition described above into a shaped article and water-curing the said shaped article. Preferably, a metal carboxylate silanol condensation catalyst, such as dibutyltin dilaurate, is added to the composition of said thermoplastic polymer and organo titanate prior to forming said composition into a shaped article. (ii) A process of applying a coating onto a wire or cable which comprises feeding a preformed, water-curable thermoplastic polymer having hydrolyzable, pendant silane moieties and having been prepared by copolymerization of an olefinic monomer and an unsaturated silane in the presence of a free radical generating compound, into an extruder wherein said polymer is melted, adding to said molten polymer at least 0.05 percent by weight of an organo titanate having the formula: Ti(OR²)₄ wherein each R² is a monovalent hydrocarbon radical having 1 to 18 carbon atoms inclusive, fluxing the resultant composition and thereafter extruding the resultant composition about a wire or cable. Preferably, a metal carboxylate, such as dibutyltin dilaurate, is added to the fluxed composition prior to extruding the composition about a wire or cable. Illustrative of monomers or mixtures of monomers which can be reacted with an unsaturated silane to produce the above polymers with pendant silane moieties are the following olefins: Suitable monoolefins have the general formula: FORMULA I C α H 2α wherein α has a value of at least 2. Exemplary of olefins falling within the scope of Formula I are: ethylene, propylene, butene-1, pentene-1, 4-methyl-pentene-1, hexene-1, heptene-1, octene-1 and the like. Suitable diolefins have the general formula: FORMULA II C β H 2β-2 wherein β has a value of at least 3. Exemplary of diolefins falling within the scope of Formula II are: 1,4-pentadiene, 1,3-hexadiene, 1,5-hexadiene, 1,4-octadiene, ethylidene norbornene and the like. Illustrative of monomers which can be polymerized with monoolefins and/or diolefins are styrene, p -methyl styrene, α-methyl styrene, p -chloro styrene, vinyl naphthalene and similar aryl olefins and substituted aryl olefins; nitriles such as acrylonitrile, methacrylonitrile, α-chloroacrylonitrile and the like; vinyl methyl ketone, vinyl methyl ether, vinylidene chloride; alkyl acrylates which fall within the scope of the following formula: wherein R⁶ is hydrogen or methyl and R⁵ is alkyl having 1 to 8 carbon atoms inclusive. Illustrative compounds encompassed by this formula are: methyl acrylate, ethyl acrylate, t -butyl acrylate, methyl methacrylate, n -butyl acrylate, n -butyl methacrylate, 2-ethylhexyl acrylate, chloroethyl acrylate and the like; provided that the olefinic content is at least about 0.1 percent by weight, preferably about 1 to about 50 percent by weight. It is to be understood that mixtures of reactants can be utilized to produce suitable polymers. Suitable unsaturated silane reactants have the general formula: wherein A is an unsaturated hydrocarbon radical or an unsaturated hydrocarbonyloxy radical and wherein other variables are as defined below. Particularly desirable unsaturated silanes fall within the scope of Formula V and Formula VI. wherein each V can be hydrogen, a hydrocarbon radical or a hydrolyzable group. Illustrative of suitable hydrocarbon radicals are alkyl radicals having 1 to 18 carbon atoms inclusive, preferably 1 to 6 carbon atoms inclusive such as methyl, ethyl, n-propyl, isopropyl, n-butyl, n-hexyl and the like; alkoxy radicals having 1 to 18 carbon atoms inclusive, preferably 1 to 8 carbon atoms inclusive, such as methoxy, ethoxy, propoxy, hexoxy, octoxy, dodecyloxy, methoxyethoxy and the like; aryl radicals having 6 to 8 carbon atoms inclusive such as phenyl, methylphenyl, ethylphenyl and the like; cycloaliphatic radicals having 5 to 8 carbon atoms inclusive such as cyclopentyl, cyclohexyl, cyclohexyloxy and the like. Z is a hydrolyzable group among which can be noted alkoxy radicals as previously described for V or oxyaryl radicals such as oxyphenyl and the like; oxyaliphatic radicals such as oxyhexyl and the like; halogens such as chlorine and the like and other hydrolyzable groups as further described in US-A- 3,408,420. wherein R¹ is an unsaturated monovalent hydrocarbon radical having a maximum of 18 carbon atoms as for example alkylene radicals having 2 to 18 carbon atoms inclusive, preferably 2 to 4 carbon atoms inclusive such as ethylene, propylene, isobutylene and the like. Other variables are as previously defined. Among suitable silanes falling within the scope of Formula IV can be noted vinyltrimethoxy silane, vinyltriethoxy silane, vinyltris-(n-propoxy) silane, vinylbis-(methoxy)methyl silane, vinylbis-(ethoxy)methyl silane, vinylbis-(n-propoxy)methyl silane, vinylmethoxydimethyl silane, vinylethoxydimethyl silane, allyltrimethoxy silane as well as the following silanes: Particularly desirable water-curable thermoplastic polymers for purposes of this invention contain about 0.01 to about 15 percent by weight unsaturated silane. Organo titanates for the compositions of this invention fall within the scope of Formula VII FORMULA VII Ti(OR²)₄ wherein each R², which can be the same or different, is a monovalent hydrocarbon radical having 1 to 18 carbon atoms inclusive, preferably 1 to 14 carbon atoms inclusive. Exemplary of suitable hydrocarbon radicals for R² are alkyl radicals such as methyl, ethyl, n-propyl, isopropyl, butyl, octyl, lauryl, myristyl, stearyl and the like; cycloaliphatic radicals such as cyclopentyl, cyclohexyl and the like; aryl radicals such as phenyl, methylphenyl, chlorophenyl and the like; alkaryl radicals such as benzyl and the like. Particularly desirable titanates falling within the scope of Formula VII are those wherein each R² is alkyl having 1 to 18 carbon atoms inclusive, preferably 1 to 14 carbon atoms inclusive, exemplified by tetrabutyl titanate, tetraisopropyl titanate, tetramethyl titanate, tetraethyl titanate and the like. The water-curable, thermoplastic polymers and organo titanates can be admixed in any suitable apparatus, such as a Brabender mixer, Banbury mixer, extruder, two-roll mill and the like wherein the polymers are melted and fluxed with the organo titanates. During the fluxing, temperatures of the compositions are maintained sufficiently low to prevent generation of water in the compositions due to the presence of water-containing additives. Also, compositions of this invention are utilized for the intended purpose immediately upon formulation. Aging of the compositions, reduces the efficacy of the organo titanate. The curing or crosslinking of the compositions of this invention is carried out by subjecting the compositions to a water bath which is generally at a temperature of about 70°C to about 100°C. The actual time of the curing cycle will depend on the temperature of the water bath and the thickness of the composition being crosslinked. Also, as previously stated, the crosslinking can be carried out in the presence of a metal carboxylate silanol condensation catalyst which serves to accelerate the crosslinking reaction. Among suitable metal carboxylate silanol condensation catalysts are the following: dibutyltin dilaurate, stannous acetate, stannous octoate, lead naphthenate, zinc octoate, iron-2-ethyl hexoate and the like. To the compositions of this invention can be added various other additives, in amounts well known in the art, such as fillers among which can be mentioned carbon black, clay, talc, talc coated with a metal salt of a fatty acid having 8 to 20 carbon atoms, calcium silicate, calcium carbonate, silica, aluminum hydroxide and the like. Other additives include silanes such as vinyl-tris-(2-methoxy) silane and other like surface treating agents. The compositions of this invention can be rendered flame retardant by the addition thereto of halogen containing flame retardants such as decabromodiphenyl oxide, ethylene(bis-tetrabromophthalimide), chlorinated polyethylene, polyvinyl chloride and halogenated paraffin waxes, alone or in admixture with inorganic compounds such as antimony oxide and/or alkaline earth metal oxides, carbonates, hydroxides and sulfates. Among such alkaline earth metal compounds can be noted calcium oxide, calcium carbonate, calcium hydroxide, calcium sulfate, magnesium oxide, magnesium carbonate, magnesium hydroxide and magnesium sulfate. Other additives such as smoke suppressants illustrated by zinc borate, molybdenum oxide and the like can also be used. The compositions of this invention, although described primarily as extrudates about wires and cables, can be used in molding applications in general such as roto-molding, injection molding and blow molding to form shaped articles such as film, pipe and the like. Also, if desired, the compositions of this invention can be formed into foamed products. The following examples further illustrate the present invention and are not intended to limit the scope thereof. Compositions, the formulations of which are set forth in Table I, were prepared as follows: Control 1 - an ethylene-vinyltrimethoxy silane copolymer was fluxed in a Brabender mixer for 5 minutes at a temperature of 160°C, under an argon gas atmosphere. The fluxed copolymer was discharged from the Brabender mixer, cooled to room temperature and stored under argon gas. Control 2 - prepared as described with respect to Control 1 with the exception that dibutyltin dilaurate was added over the last 30 seconds of the fluxing period. Composition A - an ethylene-vinyltrimethoxy silane copolymer was fluxed for two minutes at a temperature of 130°C, and an organo titanate added to the fluxed copolymer. Fluxing was then continued for an additional two minutes at a temperature of 130°C with dibutyltin dilaurate added over the last 30 seconds of the two-minute fluxing period. The fluxed copolymer was then discharged from the Brabender mixer, cooled to room temperature and stored under argon gas. Composition B was prepared as described with respect to Composition A with the exception that no dibutyltin dilaurate was added. Test plaques, 7.6 cm x 7.6 cm x 0.19 mm (3 inches by 3 inches by 0.0075 inch), were prepared from Compositions A-B and Controls 1-2 and the test plaques were subjected to the Monsanto Rheometer test, described in detail in US-A 4,018,852. Test plaques were prepared under the following conditions: temperature - 110°C - 115°C pressure -(5000 psig) 34.55 MPa time cycle - 5 minutes Plaques were tested initially, as prepared, and also tested after being placed in a water bath, which was at a temperature of 70°C for 16 hours. Initial and final rheometer readings are set forth in Table I. The data show that the presence of an organo titanate prevents premature cure without adversely affecting the final cure.

Claims: A composition comprising a preformed water-curable thermoplastic polymer having hydrolyzable, pendant silane moieties and having been prepared by copolymerization of an olefinic monomer and an unsaturated silane in the presence of a free radical generating compound; and an organo titanate having the formula: Ti(OR²)₄ wherein each R² is a monovalent hydrocarbon radical having 1 to 18 carbon atoms inclusive, said organo titanate being present in an amount of at least 0.05 percent by weight. A composition as defined in claim 1 wherein said preformed, water-curable polymer is a copolymer of ethylene and an unsaturated silane, preferably a copolymer of ethylene and vinyltrimethoxy silane. A composition as defined in any one of claims 1 and 2, wherein said organo titanate is present in an amount of 0.05 to 10 percent by weight, preferably 0.1 to 5 percent by weight. A composition as defined in any one of claims 1 to 3, wherein each R² of said organo titanate is a hydrocarbon radical having 1 to 14 carbon atoms inclusive, preferably an alkyl radical. A composition as defined in any one of claims 1 to 4, wherein said organo titanate is tetramethyl titanate, tetraethyl titanate, tetraisopropyl titanate or tetrabutyl titanate. A composition as defined in any one of claims 1 to 5, to which has been added a metal carboxylate silanol condensation catalyst, such as dibutyltin dilaurate. The crosslinked product of the composition defined in any one of claims 1 to 6. A process for the production of shaped articles which comprises forming the composition according to claim 1 into a shaped article and water-curing the said shaped article. A process as defined in claim 8, wherein a metal carboxylate silanol condensation catalyst, such as dibutyltin dilaurate, is added to the composition of said thermoplastic polymer and organo titanate prior to forming said composition into a shaped article. A process of applying a coating onto a wire or cable which comprises feeding a preformed, water-curable thermoplastic polymer having hydrolyzable, pendant silane moieties and having been prepared by copolymerization of an olefinic monomer and an unsaturated silane in the presence of a free radical generating compound, into an extruder wherein said polymer is melted, adding to said molten polymer at least 0.05 percent by weight of an organo titanate having the formula: Ti(OR²)₄ wherein each R² is a monovalent hydrocarbon radical having 1 to 18 carbon atoms inclusive, fluxing the resultant composition and thereafter extruding the resultant composition about a wire or cable. A process as defined in claim 10, wherein a metal carboxylate, such as dibutyltin dilaurate, is added to the fluxed composition prior to extruding the composition about a wire or cable. A wire or cable having thereon the composition or the crosslinked product of the composition defined in any one of claims 1 to 6.

Inventors: KEOGH, MICHAEL JOHN, WILLIAMS, GARY HARRY
Applicants: UNION CARBIDE CORPORATION
Application year: 1985
Main classification: C08L57
IPC codes: C08L51, C08K5, C08L101, H01B3, C08L57, C08L23