2,5-Dimercapto-1,3,4-Thiadiazole as a Cross-Linker for Saturated, Halogen-Containing Polymers

This invention relates to the cross-linking of halogen-containing polymers. More particularly, this invention relates to a method of cross-linking saturated, halogen-containing polymers with 2,5-dimercapto-1,3,4-thiadiazole and certain basic materials.

2,5-Dimercapto-1,3,4-thiadiazole is a known compound and has been reported in British Pat. No. 974,915 as a curing agent for certain unsaturated, halogen-containing polymers (i.e., chlorobutyl rubber and polychloroprene elastomers). However, the compositions disclosed in British Pat. No. 974,915 will not cross-link saturated, halogen-containing polymers.

It has now been discovered that saturated, halogen-containing polymers can be cross-linked to yield stable vulcanizates with good aging, compression set resistance, and insolubility in organic solvents using 2,5-dimercapto-1,3,4-thiadiazole, or a concentrate thereof in a binder material, in the presence of certain basic materials in normal vulcanization procedures. Accordingly, this invention relates to a process of cross-linking saturated, halogen-containing polymers with 2,5-dimercapto-1,3,4-thiadiazole in the presence of a basic material selected from (1) amines having a boiling point above about 110.degree. C. and a pK value below about 4.5; (2) salts of amines having pK values below about 4.5 with acids having pK values above about 2.0; (3) quaternary ammonium hydroxides and their salts with acids having pK values above about 2.0; (4) diphenyl- and ditolyl- guanidines; and (5) the condensation product of aniline and at least one mono-aldehyde containing one to seven carbon atoms, in combination with at least an equal amount of an inorganic base. The term "pK value" refers to the dissociation constants of bases and acids in aqueous solution. Representative values are shown in the Handbook of Chemistry and Physics, 45th Edition, The Chemical Rubber Co., page D-76 (1964).

The saturated, halogen-containing polymers to be cross-linked in accordance with this invention contain at least about 2%, most preferably about 5%, by weight of halogen and less than 0.1 mole % ethylenic unsaturation. Typical of the saturated, halogen-containing polymers are homopolymers of epichlorohydrin, copolymers of epichlorohydrin and ethylene oxide or propylene oxide, chlorinated high-density polyethylene, chlorosulfonated polyethylene, poly(vinyl chloride), poly(vinyl fluoride), poly(chloroalkylacrylates), poly(vinylidene chloride), and copolymers of vinylidene fluoride and hexafluoropropylene. In addition, the process of this invention may be used to cross-link blends of saturated, halogen-containing polymers.

Typical amines having a boiling point above about 110.degree. C. and a pK value below about 4.5 are the aliphatic and cycloaliphatic primary, secondary and tertiary amines having 5 to 20 carbon atoms, such as n-hexylamine, octylamine, dibutylamine, tributylamine, trioctylamine, di(2-ethylhexyl)amine, dicyclohexylamine, and hexamethylene diamine. Various amounts of amine can be used, depending upon the degree of cross-linking desired and the presence of inorganic basic material. In general, the amount of amine used will be from about 0.25% to about 10%, more preferably from about 0.5% to about 5%, most preferably from about 1.0% to about 3% based on the weight of the polymer. For economic reasons, it may be desirable to substitute less expensive inorganic basic material for some of the amine. However, at least 0.1% of amine based on the weight of the polymer must be present with any inorganic basic material to obtain cross-linking of the polymer.

Typical salts of amines having pK values below about 4.5 with acids having pK values above about 2.0 are n-butylamine acetate, dibutylamine sorbate, hexamethylenediamine carbamate, and the dicyclohexylamine salt of 2-mercaptobenzothiazole. A special type of salt of an amine with an acid, included in this definition, is the salt of an amine with 2,5-dimercapto-1,3,4-thiadiazole. Various amounts of the said salts can be used, depending upon the degree of cross-linking desired and the presence of inorganic basic material. In general, the amount of salt used will be from about 0.25% to about 10%, more preferably from about 0.5% to about 5%, most preferably from about 1.0% to about 3% based on the weight of the polymer. For economic reasons, it may be desirable to substitute less expensive inorganic basic material for some of the amine salt except for the amine salts of 2,5-dimercapto-1,3,4-thiadiazole. However, at least 0.1% of amine salt based on the weight of the polymer must be present with any inorganic basic material to obtain cross-linking of the polymer.

Diphenyl- and ditolyl-guanidine will be used in the same amounts as recited above for the amines and amine salts.

Typical condensation products of aniline with an acyclic aliphatic mono-aldehyde useful in combination with an inorganic base in this invention are the condensates of aniline with butyraldehyde, the condensates of aniline with heptaldehyde and the condensates of aniline with acetaldehyde and butyraldehyde. Typical inorganic bases used in combination with the above condensates are the alkali metal hydroxides, alkaline earth metal oxides, hydroxides and their salts with weak acids such as sodium hydroxide, sodium carbonate, potassium hydroxide, magnesium oxide, calcium oxide, calcium hydroxide, barium oxide, calcium carbonate, and magnesium carbonate. As stated above, the aniline-aldehyde condensates must be used in combination with at least an equal amount of an inorganic base. Usually the inorganic base will be present in an amount equal to 2 to 5 times the amount of condensate. The condensate will be used in the same amounts as recited above for the amines and amine salts.

In addition to the cross-linking agent and basic material, other ingredients can also be incorporated. The types of additives commonly used in rubber vulcanization can be used, as, for example, extenders, fillers, pigments, plasticizers, softeners, etc. The presence of a filler and, in particular, carbon black, is beneficial and, as in hydrocarbon rubber compounding, gives very advantageous results. There are many cases, however, in which a filler is not required or desired and excellent results are achieved when only the cross-linking agent and basic materials are added. Also, most of the saturated, halogen-containing polymers contain a small amount (i.e., from about 0.1% to about 2% by weight) of antioxidant, added at the time of their preparation. It may be desirable in some cases to add a small additional amount of antioxidant before or at the time of cross-linking the polymer. Exemplary of the preferable antioxidants are phenyl-.beta.-naphthylamine, di-.beta.-naphthyl-p-phenylenediamine, sym-di-.beta.-naphthyl-p-phenylenediamine, N-isooctyl-p-amino phenol, the reaction product of diphenylamine and acetone, polymerized trimethyldihydroquinoline, 4,4'-thio-bis-(6-tert-butyl-m-cresol), the reaction product of crotonaldehyde and 3-methyl-6-tert-butyl-phenol, nickel dibutyldithiocarbamate, the zinc salt of 2-mercaptobenzimidazole, and nickel dimethyldithiocarbamate.

Particularly in the case of epichlorohydrin polymers, it may be advantageous to add at least one carboxylic acid to the cross-linkable composition to act as a scorch retarder during the compounding step if calcium oxide or calcium hydroxide is used as an inorganic basic material. Malic acid and N-acetylanthranilic acid are especially useful.

The cross-linking agent, basic material and additives, if any are used, can be incorporated or admixed with the polymer in any desired fashion. For example, they can be uniformly blended with a polymer by simply milling on a conventional rubber mill or mixing in a Banbury mixer. By this means, the cross-linking agent and basic material are uniformly distributed throughout the polymer and uniform cross-linking is effective when the blend is subjected to heat. It is generally preferable to mill at temperatures within the range of from about 20.degree. C. to about 95.degree. C. However, the blends are generally scorch-resistant below about 120.degree. C., unless compounded with a large amount of organic basic material. Other methods of admixing the cross-linking agent with the polymer will be apparent to those skilled in the art.

The conditions under which the cross-linking is effected can be varied over a wide range. Cross-linking can be effected in minutes at elevated temperatures or in days at temperatures slightly above room temperature. In general, the cross-linking temperature will be within the range of from about 30.degree. C. to about 280.degree. C., more preferably from about 135.degree. C. to about 235.degree. C., and most preferably from about 150.degree. C. to about 205.degree. C. The time will vary inversely with the temperature and with concentration of basic material will range from about 30 seconds to 70 hours, preferably from about 30 seconds to about 120 minutes. While the cross-linking process can be conducted in air at normal atmospheric pressure, it will generally be conducted in a metal mold under a compression of at least about 50 p.s.i. or in a steam autoclave at the pressure required for the desired temperature.

For ease in incorporating the 2,5-dimercapto-1,3,4-thiadiazole into the cross-linkable polymer, and to avoid the use of powders in the compounding step, it may be desirable to prepare the 2,5-dimercapto-1,3,4-thiadiazole as a concentrate in a binder or carrier which can be added, along with the basic materials, in small amounts to the polymer composition without adverse effect on the properties of the cross-linked composition. Particularly advantageous binders or carriers are polymers which may or may not be cross-linkable by the cross-linking agent. Suitable materials, in addition to the cross-linkable polymers, are, for example, ethylene-propylene rubber, ethylene-propylene terpolymers, butadiene-styrene rubber, natural rubber, low-density polyethylene, amorphous polypropylene and polyisobutylene. Concentrations of the 2,5-dimercapto-1,3,4-thiadiazole in the binders can vary from about 15% to about 90%, preferably from about 30% to about 75%. Other materials which can advantageously be incorporated in the concentrates are scorch retarders, antioxidants and nonbasic fillers. It is normally undesirable to incorporate the basic material in the concentrate. These polymer concentrates are normally stored and used in the form of sheets, extruded pellets or rods. Other suitable binders or carriers for use in the preparation of such easily handled concentrates are waxes, resins, or other low-melting solids. Typical useful materials are paraffin wax, stearic acid, microcrystalline wax, rosin, rosin esters and hydrocarbon resins.

The cross-linked products of this invention can be utilized to make hoses, tubes, etc., for use as fuel lines carrying hydrocarbon fuels.

The following examples will illustrate the process of cross-linking in accordance with this invention. All parts and percentages are by weight unless otherwise stated.

This example illustrates the preparation of a concentrate of 2,5-dimercapto-1,3,4-thiadiazole in stearic acid.

The concentrate is prepared by dry blending the ingredients tabulated below, warming the concentrate blend to the melting point of the stearic acid, extruding the melted concentrate blend and cutting the extrudate into rods or pellets.

In the following examples a Farrel Size "B" Banbury mixer is used to compound the formulations. Ingredients are added to the mixer in the order listed in each example.

The above formulations are cross-linked by heating for 30 minutes at a temperature of 160.degree. C. in a steam autoclave. The cross-linked products have the following physical properties:

The above formulation is cross-linked by heating at 160.degree. C. for thirty minutes in an oscillating disc Rheometer (Americal Standard Testing Method D2705-68T). The properties measured are:

The above formulations are cross-linked by heating at 160.degree. C. for thirty minutes in a compression mold. The physical properties of the cross-linked products are:

The above formulation is heated at 135.degree. C. for thirty minutes in an oscillating disc Rheometer (American Standard Testing Method D2705-68T). Satisfactory cross-linking is obtained without noticeable discoloration.

The above formulation is heated at 160.degree. C. for thirty minutes in an oscillating disc Rheometer (American Standard Testing Method D2705-68T). Satisfactory cross-linking is obtained.

These examples illustrate the pre-reaction of 2,5-dimercapto-1,3,4-thiadiazole with an amine. The reaction product is then used to cross-link a saturated, halogen-containing polymer.

A mixture of 15 grams (0.1 mole) 2,5-dimercapto-1,3,4-thiadiazole and 37 grams (0.2 mole) tributylamine in 200 ml. tetrahydrofuran is heated at 45.degree. C. for 20 minutes. A yellow precipitate results, which is separated from the solution by crystallization, filtered and washed with methylene chloride. The product is dried overnight in a vacuum oven at 50.degree. C. and a pressure of about 18 mm mercury. Analysis of the product indicates a bis salt.

The reaction product obtained above is used to cross-link an epichlorohydrin-ethylene oxide copolymer by formulating as follows:

The above formulations are cross-linked by heating at 160.degree. C. for thirty minutes in an oscillating disc Rheometer (American Standard Testing Method D2705-68T). The properties measured are:

This example illustrates the cross-linking of chlorinated polyethylene in accordance with this invention in comparison with attempts to cross-link using compositions disclosed in British Patent Ser. No. 974,915.

These examples illustrate the cross-linking of chlorinated polyethylene using various organic amines.

A masterbatch was prepared of the following:

To portions of the masterbatch were added 2.66.times.10.sup.-2 moles per 100 parts of polymer of one of the following amines. To other portions of the masterbatch were added 4 parts of magnesium oxide plus the above amount of amine. Each formulation is then heated at 166.degree. C. for twelve minutes in an oscillating disc Rheometer (American Standard Testing Method D2705-68T). The amines, their boiling points and pK values and the torque (in inch-pounds) of each sample are tabulated below.

These examples illustrate the cross-linking of chlorinated polyethylene using various amine salts of 2,5-dimercapto-1,3,4-thiadiazole.

The above formulations are cross-linked by heating for twelve minutes at 166.degree. C. in an oscillating disc rheometer.

These examples illustrate the cross-linking of various saturated halogenated polymers in accordance with this invention.

The above formulations are heated at the temperature recited below in an oscillating disc rheometer.