Crosslinking Fluorocarbon Compositions Using Polyallylic Esters of Polycarboxylic Acids

This invention relates to crosslinking compositions and more particularly to crosslinking compositions comprising blends of at least two polyallylic esters of polycarboxylic acids and to their use as crosslinkers for fluorocarbon polymers.

It has recently been discovered that certain diallylic esters of dicarboxylic acids have a remarkable combination of useful properties for both placticizing and crosslinking polymeric compositions, particularly high temperature processing fluorocarbons. The diallylic esters act to placticize the fluorocarbon polymers during their high temperature processing operations and, in addition, produce crosslinked polymers of outstanding chemical, electrical and mechanical properties. The esters of phenyl indan dicarboxylic acid are one such class of diallylic esters. These compounds and their use for this purpose are the subject of our copending U.S. patent application Ser. No. 243,052, filed Apr. 11, 1972, now U.S. Pat. No. 3,763,222. A second class of diallylic esters useful for this purpose are the esters of alkyl, cycloalkyl, mixed alkyl-cycloalkyl or aralkyl carboxylic acids and are shown on our abandoned U.S. patent application Ser. No. 489,697, filed July 18, 1974 which in turn is a division of abandoned U.S. application Ser. No. 401,973, filed Oct. 1, 1973.

The crosslinking of a limited group of fluorocarbon polymers such as polyvinylidene fluoride and certain lower temperature processing polyfluorocarbon elastomers, with triallyl esters of cyanuric and isocyanuric acid is known. In addition, the crosslinking of fluorocarbon polymers with the triallyl esters of aryl carboxylic acids is disclosed in our U.S. patent application Ser. No. 276,055 filed July 28, 1972, now U.S. Pat. No. 3,840,619.

The present invention is based upon the discovery that certain blends of polyallylic esters provide unusually excellent overall properties both in the crosslinked product and in its high temperature processing. The particular diallyl esters of our invention contribute outstanding plasticization during high temperature processing and combine with tri or tetrallyl esters to contribute outstanding high temperature mechanical properties and deformation resistance on crosslinking. The properties of the blend are unique and unpredictable from a knowledge of the properties of the unblended individual esters. For example, elongation, and hence flexibility of crosslinked polymers is usually improved only at the expense of deformation resistance in previously known systems. The crosslinker blends of the instant invention, however, produce a combination of elongation and deformation resistance which is considerably above the average of such properties obtained with the individual esters when used alone. Deformation resistance, particularly at elevated temperatures, is of critical importance in many applications, as for example, in wire and cable insulation where deformation is often the greatest cause of failure.

The compositions of the invention comprise a blend of at least two polyallylic esters of polycarboxylic acids, one of the esters being a diallylic ester of a dicarboxylic acid in which the carboxylic groups are connected by an alkylene, cycloakylene, mixed alkylene, cycloalkylene, mixed alkylene-cycloalkylene or aralkylene group, the other of the esters being a polyallylic ester of a carboxylic acid having at least three carboxylic groups connected by an aromatic or hetroaromatic group or a triallyl ester of isocyanuric acid. The blends are particularly useful for the plasticizing and crosslinking of high temperature processing fluorocarbon polymers.

The diester for use may be one of those diallyl esters of dicarboxylic acids set forth in our aforesaid abandoned application Ser. No. 489,697. As there set forth, these esters can be generally represented by the formula: ##STR1## wherein R is an organic radical containing from 4 to 20 carbon atoms and is selected from the group consisting of alkylene radicals, cycloalkylene radicals, mixed alkylene-cycloalkylene radicals and aralkylene radicals: R' and R" are independently selected from hydrogen, alkyl, cycloalkyl, aralkyl and aryl radicals and mixtures thereof; and the total carbon atoms in R, R' and R" is from 10 to 34.

In formula I, it is preferred, but not necessary for the carbonyl carbon of the allyloxy carbonyl group to be linked to an alkylene carbon. Additionally, when R in Formula I is an aralkylene, or when R' or R" are aryl or aralkyl radicals, it is preferred that the ratio of aromatic carbons to aliphatic (or alicyclic) carbons in R, R' and R" be less than about 3:1, and most preferably less than about 2:1.

Exemplary of preferred diallyl esters for use in the blends of the present invention are the diallyl esters of dodecandioic and brassylic acids. In the latter esters, R is a simple aliphatic chain containing 10 and 11 carbon atoms, respectively, and R' and R" are hydrogen atoms in Formula I above. Other examples of preferred diallyl esters for use in the present invention are those prepared from bis-(dicyclohexane carboxylic acid) where R is a bicycloalkyl radical containing 12 carbon atoms. Esters of "Dimer Acids" are also useful. These are acids produced by the acid catalyzed dimerization of natural 18 carbon fatty acids. In these compounds, R predominantly has a carbon-carbon double bond and an alicyclic ring in the otherwise aliphatic chain. The acid has the generalized structural formula: ##STR2## and an ester produced with the acid of Formula II would correspond to the structure of Formula I where R is: ##STR3## R' is --(CH.sub.2).sub.c --CH.sub.3 ; R" is --(CH.sub.2).sub.d --CH.sub.3 ; and the sum of a a, b, c and d is an integer from 4 to 28.

Other examples of compounds suitable for use in the present invention are the diallyl esters of hexadecanedioic acid; octadecanedioic acid; diundecylenic acid; tricyclodecane dicarboxylic acid; p-menthane dicarboxylic acid; 1,2-dicyclohexylethane-4,4'-dicarboxylic acid; 2,2,3,2', 2', 3'-hexamethyldicyclopentyl-3,3'-dicarboxylic acid; 1,2,2-trimethyl-cyclopentane carboxylic-3-.beta.-propionic acid; methyl homocamphoric acid; .beta.-ethyl homocamphoric acid; 2,6-dibutyl pimelic acid; .alpha.-n-octyl sebacic acid; .beta.-methyl-.alpha.-allyl adipic acid; 2,2,5,5-tetramethylhexene-3-dicarboxylic acid; 3,4-diisopropyl-hexene-3-.alpha., .omega.-dicarboxylic acid; .alpha., .alpha., .alpha.', .alpha.'-tetramethyl-.beta., .beta.'-diphenyl adipic acid; 2,2,3-trimethyl-3-carboxycyclopentylphenyl acetic acid; 2,6-dibenzylpimelic acid; 1,3-diphenylcyclobutane bis-(.alpha.-phenyl propionic acid); 1,4,1', 4'-tetramethyl-5,8,5', 8'-tetrahydrodinaphthyl-2,2'-di-propionic acid; and the like.

A preferred group of diallyl esters for use herein are the aralkyl dicarboxylic acid esters including the phenyl indan dicarboxylic acid esters of the type shown in our above-identified U.S. Pat. No. 3,763,222. A preferred class of such phenyl indan dicarboxylic acid esters is shown by the following structural formula: ##STR4## wherein R.sub.1 and R.sub.2 are independently selected from the group consisting of hydrogen and methyl radicals.

The polyallyl esters present in the blend are preferably triallylic esters of a carboxylic acid containing three or more carboxylic groups. However, tetraallyl esters such as the tetraallyl ester of 1,2,4,5-pyromellitic acid may be employed herein as the polyallyl constituent of the blend.

Illustrative preferred triallyl esters are triallyl esters of aryl polycarboxylic acids and are shown in our above identified U.S. Pat. No. 3,840,619. Examples are triallyl trimesate and triallyl trimellitate. Examples of other triallyl esters which may be used are triallyl isocyanurate; 1,3,5-tris(alloyloxycarbonyl) isocyanurate; 1,3,5-tris(allyloxy carbonylethyl) isocyanurate; triallyl esters of 1,3,5-trialkyl trimesic acids; triallyl ester of 5-carboxyl-m-phenylene diacetic acid; the triallyl ester of phenylene-1,3,5-triacetic acid; the triallyl ester of 1,3,5-tris(2-carboxyphenyl)benzene; the triallyl ester of 1,3,5-tris (4-carboxyphenyl)benzene; 4,6-bis(2-carboxyphenyl)-0-toluic acid; the triallyl ester of .alpha., .alpha.-bis(4-carboxyphenyl)propionic acid and the like.

We have found that blends of these compounds possess excellent properties as plasticizing and as crosslinking agents for high temperature processing fluorocarbon polymers, including homopolymers and copolymers such as ethylenetetrafluoroethylene copolymers, ethylene-chlorotrifluoroethylene copolymers, polyvinylidene fluoride homopolymers, tetrafluoroethylene-vinylidene fluoride copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, vinylidene fluoride-hexafluoropropylene copolymers, vinylidine fluoride-hexafluoropropylene-tetrafluoroethylene copolymers and the like and mixtures thereof. The crosslinking agents will normally be added to the fluorocarbon polymer in amounts of about 0.5 to 20 parts by weight by weight per 100 parts by weight of the polymer, preferably 0.5 to 10 parts and even more preferably, 2.5 to 10 parts of crosslinker blend to 100 parts by weight of polymer. The weight of diallyl ester in a preferred embodiment of our invention should be at least that of the tri- or tetraallyl ester. Normally, the ratio of diallyl ester to tri- or tetraallyl ester crosslinker will range from about 1:4 to 4:1, with a most preferred ratio being about 1:1 to 3.5:1 of diester to tri- or tetraester crosslinking agent.

The following examples illustrate the practice of the present invention. All parts are by weight unless otherwise indicated.

This example and the following examples illustrate the substantial and unexpected increase in efficiency of crosslinking achieved by employing blends of diallyl and triallyl esters as compared with use of the individual esters.

A polymeric composition (hereinafter Composition A) was prepared by blending an ethylene-tetrafluoroethylene copolymer with 3 parts per hundred parts copolymer of a triallyl trimellitate ester. A second polymeric composition (hereinafter Composition B) was also prepared by blending the ethylene-tetrafluoroethylene copolymer with 3 parts per hundred parts copolymer of a diallyl brassylate ester. As will be noted Compositions A and B were prepared as control samples. A third and fourth polymeric composition (hereinafter Compositions C and D) were similarly prepared by blending ethylene-tetrafluoroethylene with blends of triallyl trimellitate and diallyl brassylate. Composition C contained 1.5 parts trimellitate and 1.5 parts brassylate per hundred parts copolymer and Composition D contained 1.0 part trimellitate and 2.0 parts brassylate per hundred parts copolymer. Magnesium oxide in an amount of 1 part per hundred parts copolymer was blended into each of the four compositions.

Each of the blended mixtures was then compression molded with 5 minutes preheat followed by 4 minutes at about 520.degree.-530.degree. F at about 10,000 psi ram pressure. Irradiation of the compression molded compositions was performed to a 10 megarad dose with a 1.5 MEV electron beam accelerator to produce crosslinked polymeric compositions having the following mechanical properties at elevated temperature (250.degree. C).

Additional compositions were prepared by the procedure of Example I containing the diallyl ester of brassylic acid or phenyl indan dicarboxylic acid as the diallylic ester in blends with triallyl trimesate as the triallylic ester. As in Example I, the copolymer was ethylene-tetrafluoroethylene. All samples were compression molded and irradiation crosslinked at 10 megarad doses according to the procedure of Example I. The compositions prepared were:

Table II sets forth the mechanical properties of these crosslinked compositions.

Further compositions were prepared, compression molded and irradiation crosslinked by the procedures of Examples I and II. The compositions prepared were:

The mechanical properties of the resulting crosslinked compositions were as follows:

This example shows the effectiveness of crosslinked polymeric compositions of the present invention for use as insulation coatings for wires.

A sample comprising ethylene-chlorotrifluoroethylene copolymer and 5.0% triallyl trimellitate and another sample comprising ethylene-chlorotrifluoroethylene copolymer, 4.0% triallyl trimellitate and 1.0% diallyl brassylate were prepared by blending process similar to that shown in Example I. Magnesium oxide in the amount of 1.0% and titanium dioxide pigment in the amount of 0.3% were incorporated into each sample. The resulting samples in powdered form, were then extruded through an extruder having a head temperature of 500.degree. F to form rods. The rods were then pelletized and the pellets were extruded onto the surface of a 20 gauge tin coated copper wire. The extrusion conditions of the wire insulation were as follows:

The two insulated wires were then subjected to 10 and 20 megarad doses of irradiation with high energy electrons by a 1.0 MEV election beam accelerator. The resulting insulated wires with irradiation crosslinked coatings were then annealed and subsequently tested to determine their mechanical and electrical properties. The results of this testing are set forth in the following table:

Samples comprising ethylene-tetrafluoroethylene copolymer and varying amounts of blends of the diallyl ester of phenyl indan dicarboxylic acid and the triallyl ester of trimesic acid were prepared by a blending process similar to that shown in Example I. The resulting samples had the following compositions on a parts by weight basis:

These samples were then extruded through an extruder having a head temperature of 530.degree. F to form rods. The rods were then pelletized and the pellets were extruded onto the surface of tin coated copper wires. The extrusion conditions for the wire insulations were as follows:

Sample A was extruded onto an 18 gauge wire having an outside diameter of 0.085 inch and a wall thickness of .016 inch. Sample B was extruded onto 18 gauge, 0.087 inch O.D., .02 inch wall thickness wire and Sample C was extruded onto 20 gauge, 0.05 inch O.D., 0.006 inch wall thickness wire.

Each of the insulated wires were then subjected to irradiation with high energy electrons by a 1.0 MEV electron beam accelerator. The resulting insulated wires with irradiation crosslinked coatings were then annealed and subsequently tested to determine their mechanical and electrical properties. The results of this testing are set forth in the following table: