WO1991017206A1 - Stabilizing ethylene-tetrafluoroethylene copolymers - Google Patents

Abstract

The stability of a composition comprising an ethylene-tetrafluoroethylene copolymer; in particular, its stability when subjected to elevated temperature for a significant period of time is improved by incorporating therein a sulfide of a Group IIb metal. The composition may be crosslinked. Heat recoverable articles made of the composition are useful as wire and cable harnessing, boots, and sleeves.

Description

STABILIZING ETHYIENE-TETRAFLUOROE HYI.E E COPOLYMERS

This invention relates to a method of stabilizing ethylene- tetrafluoroethylene copolymers to a stabilized ethylene- tetrafluoroethylene copolymer composition and to a heat recoverable article made from said composition.

Ethylene-tetrafluoroethylene copolymers are well known. For certain uses of the copolymers in which the copolymer is subjected to elevated temperatures for a substantial period of time it has been found that the physical properties, in particular, the creep resistance, elongation and tensile strength of the polymer are undesirably affected.

There are several references to improving the heat resistance of ethylene-tetrafluoroethylene copolymers in the art. In general, this has been achieved by adding to the copolymer a metal oxide. The addition of the metal oxide is reported to prevent discoloration of the polymer during molding. The addition of metal oxides however does not solve the problem of poor retention of physical properties, such as creep, tensile strength and elongation, of the polymer when the polymer is subjected to elevated temperature over a substantial period of time.

It has now been unexpectedly discovered that the addition of a Group lib metal sulfide significantly improves the physical properties of an ethylene-tetrafluoroethylene copolymer-based composition when subjected to elevated temperatures compared to a similar composition which does not contain the metal sulfide.

One aspect of this invention comprises a composition comprising an ethylene-tetrafluoroethylene copαjβrmer comprising about 30 to about 60 mole percent of ethylene, about 6012) about 35 mole percent of tetrafluoroethylene, and about 0 to about 35 mole percent of one or more other comonomers, and a Group lib metal sulfide in an amount effective to stabilize the composition.

Another aspect of this invention comprises a method of stabilizing an ethylene-tetrafluoroethylene copolymer by incorporating therein a Group lib metal sulfide in an amount effective to stabilize the composition. Yet another aspect of this invention comprises a heat recoverable article made from a polymeric composition comprising an ethylene- tetrafluoroethylene copolymer about 30 to about 60 mole percent of ethylene, about 60 to about 35 mole percent of tetrafluoroethylene, and about 0 to about 35 mole percent of one or more other comonomers, and a Group lib metal sulfide in an amount effective to stabilize the composition.

Copolymers of ethylene, tetrafluoroethylene, and optionally a third monomer are known crystalline thermoplastic polymers. Typically such copolymers contain about 30 to about 60 mole percent of ethylene, about 60 to about 35 mole percent of tetrafluoroethylene, and about 0 to about 35 mole percent of one or more other comonomers. Preferably the third monomer is hexafluoropropylene; 3,3,3-trifluoropropylene-l; 2- trifluoromethyl-3,3,3-trifluoropropylene-l; or perfluoro(alkyl vinyl ether). Ethylene-tetrafluoroethylene copolymers are commercially available under the trademark "Tefzel" from E.I. duPont de Nemours and Company of Wilmington Delaware, and under the trademark "Neoflon" from Daikin Industries of Osaka Japan.

In accordance with this invention an ethylene-tetrafluoroethylene copolymer is stabilized by incorporating therein a Group lib metal sulfide in an amount effective to stabilize the composition. Group lib metal oxides include zinc, cadmium, and mercury (see the periodic table of elements in the Handbook of Chemistry and Physics, published by the Chemical Rubber Company, 50th edition 1970). Zinc sulfide is particularly preferred in the practice of this invention. The term stabilize is used to mean that the properties of the composition are stabilized or retained, particularly when the composition is subjected to heat aging, i.e. is exposed to elevated temperature for a significant length of time.

The Group lib metal sulfide is incorporated into the ethylene- tetrafluoroethylene copolymer in an amount effective to stabilize the ethylene-tetrafluoroethylene copolymer. Generally, the amount of Group lib metal sulfide used is about 0.05 to about 20 weight %, based on the weight of the ethylene-tetrafluoroethylene copolymer. Preferably the Group lib metal sulfide is used in an amount of from about 0.1 to about 10 weight % .and most preferably in an amount of from about 0.5 to about 6 weight %, based on the weight of the copolymer.

The Group lib metal sulfide is incorporated by mixing the copolymer and the sulfide together by any method. For example, in an internal mixer such a Banbury, a mill, a Brabender, a twin screw extruder, or the like.

The composition may comprise a blend of an ethylene- tetrafluoroethylene copolymer and one or more additional polymers. Particularly advantageous for certain uses are blends of ethylene- tetrafluoroethylene copolymer with a thermoplastic elastomer containing thermoplastic segments containing ethylene- tetrafluoroethylene repeating units and elastomeric segments containing vinylidene fluoride, hexa- or pentafluoropropylene and tetrafluoroethylene repeating units. Such blends are disclosed in published International Patent Application WO88/07063; the entire disclosure of which is incorporated herein by reference.

Various additives can be added to the polymeric composition. Such additives include antioxidants such as alkylated phenols, e.g. those commercially available as Goodrite 3125, Irganox 1010, Irganox 1035, Irganox 1076, Irganox 1093, Vulkanox BKF; organic phosphites or phosphates, e.g. dilauryl phosphite, Mark 1178; alkylidene polyphenols, e.g. Ethanox 330; thio-bis alkylated phenol, e.g. Santonox R; dilauryl thio-dipropionate, e. g . Carstab DLTDP; dimyristyl thiodipropionate, e.g. Carstab DMTDP; distearyl thiodipropionate, e . g . Cyanox STDP; amines, e. g . Wingstay 29, Nauguard 445, etc; UV stabilizers such as [2,2'-thio-bis(4-t-octylphenolato)] n-butylamine nickel, Cyasorb UV 1084, 3,5-ditertiarybutyl-p-hydroxybenzoic acid, UV Chek Am-240; flame retardants such as antimony oxide, decabromodiphenyl ether, perchloropentacyclodecane, l,2-bis(tetrabromophthalimido) ethylene; pigments such as titanium dioxide and carbon black, and the like. Mixtures of such additives can be used. The addition of antimony trioxide has been found to be particularly advantageous. The antimony oxide can be present in an amount of about 0.01 to about 5%, preferably from about 0.1 to about 1%, the percentages being by weight based on the weight of the copolymer. The composition of this invention has numerous uses. In general the composition has good electrical insulating characteristics. The compositions can be formed into shaped articles, coatings, or the like by melt processing or other suitable techniques. A preferred use of the composition of this invention is in wire and cable insulation and/or jacketing. Another preferred use of the composition is the preparation of heat recoverable articles, particularly tubular articles for use as wire and cable harnessing, sleeves, boots, marker sleeves, or molded articles such as transitions, tridents or the like to cover or be installed over straight or branched elongated substrates.

A heat recoverable article is one whose dimensional configuration may be made to change when subjected to an appropriate treatment. Usually these articles recover, on heating, towards an original shape from which they have previously been deformed, but the term "heat- recoverable" as used herein also includes an article which, on heating, adopts a new configuration even if it has not been previously deformed.

In their most common form heat-recoverable articles comprise a heat-shrinkable sleeve made from a polymeric material exhibiting the property of elastic or plastic memory as described, for example, in U.S. Patents 2,027,962, 3,086,242 and 3,597,372. As is made clear in, for example, U.S. Patent 2,027,962, the original dimensionally heat-stable form may be a transient form in a continuous process in which, for example, an extruded tube is expanded immediately after extrusion, while hot, to a dimensionally heat-unstable form. In other embodiments a preformed dimensionally heat-stable article is deformed to a dimensionally heat-unstable form in a separate stage.

In the production of heat recoverable articles, the polymeric material may be crosslinked (as discussed more fully below) at any stage in the production of the article to enhance the desired dimensional recoverability. One manner of producing a heat-recoverable article comprises shaping the polymeric article into the desired heat-unstable form, subsequently crosslinking the polymeric material, heating the article to a temperature above the crystalline melting point of the polymer, deforming the article and then cooling the article while in the deformed state so that the deformed state of the article is retained. In use, since the deformed state of the article is heat-unstable, application of heat will cause the article to assume its original heat-stable shape.

The composition of this invention can be crosslinked, if desired. Crosslinking can be achieved, for example, by use of a suitable crosslinking agent such as a peroxide or amine or by irradiation.

The composition is formed into a shaped article, e.g. by extrusion, prior to irradiation. The article may be annealed by heating it to a temperature of about 100 to about 250°C for about 0.5 to about 4 hours before and/or after irradiating the article to reduce processing stresses in the article and the like.

In a preferred embodiment, the composition is crosslinked by irradiation. The dosage employed in the irradiation step is generally below about 50 Mrads to ensure that the polymer is not degraded by excessive irradiation. The dosage preferably employed depends upon the extent of crosslinking desired balanced against the tendency of the polymer to be degraded by high doses of irradiation. Suitable dosages are generally in the range 0.5 to 40 Mrads, preferably 1 to 20 Mrads, and most preferably 2 to 10 Mrads. The ionizing radiation can, for example, be in the form of accelerated electrons or gamma rays. Irradiation is generally carried out at about room temperature but higher temperatures can also be used.

Prior to irradiation it is preferred to incorporate a crosslinking agent into the composition. Preferred radiation crosslinking agents contain carbon-carbon unsaturated groups in a molar percentage greater than 15, especially greater than 20, particularly greater than 25, based on the total molar amount of (A) and (B). In many cases the crosslinking agent contains at least two ethylenic double bonds, which may be present, for example, in allyl, methallyl, propargyl, or vinyl groups. We have obtained excellent results with crosslinking agents containing at least two allyl groups, especially three or four allyl groups. Particularly preferred crosslinking agents are triallyl cyanurate (TAC) and triallyl isocyanurate (TAIC); other specific crosslinking agents include triallyl trimellitate, triallyl trimesate, tetrallyl pyromellitate, the diallyl ester of l,l,3-trimethyl-5-carboxy-3-carboxyphenyl) indan. Other crosslinking agents which are known for incorporation into fluorocarbon polymers prior to shaping, for ex.ample, those disclosed in U.S. Patents Nos. 3,840,619, 3,894,118, 3,911,192, 3,970,770, 3,985,716, 3,995,091, 4,031,167, 4,155,823 and 4,353,961. Mixtures of crosslinking agents can be used.

The following examples illustrate the invention.

EXAMPLES 1-3

Compositions are prepared by blending an ethylene- tetrafluoroethylene copolymer with zinc sulfide, ZnS, or zinc oxide, ZnO, in the percentages shown in Table I (the percentages being by weight- based on the weight of the copolymer). The compositions also contain a crosslinking promoter (1.5 weight %) and a hindered phenolic antioxidant (0.5 weight %). The components are blended together on a Brabender mixer at 250°C. Slabs of the compositions, 0.75" thick, are prepared by compression molding at 300°C, 40,000 psi for 1 minute. ASTM D 412 die specimens are cut from the slabs and irradiated in a high energy electron accelerator to a dose of about 4 megarads. Tensile measurements (tensile strength and elongation according to ASTM D 412) are made on an Instron with a crosshead speed of 2 inches/minute before and after heat aging at 275°C for 168 hours. S.amples are hung vertically in an oven during aging. Creep is measured as the elongation of the entire specimen during oven aging (at 275°C for 168 hours) and after 4 hours at 350°C ( a heat shock or accelerated aging test).

The results are shown in Table I. As can be seen from these results, the addition of zinc sulfide greatly improves the retention of tensile strength and creep of the composition without adversely affecting the elongation when the composition is subjected to heat aging. Zinc oxide, on the other hand, does not exhibit satisfactory retention of these physical properties of the composition. TABLE I

Compositions of this invention containing zinc sulfide

Example

ZnS (%) Original

Tensile

Strength, psi

Elongation, % After heat aging

Tensile

Strength, psi

Retention, %

Elongation, % Retention, % Creep, %

After heat shock Creep, % 97 13

Comparative compositions containing zinc oxide

Comparative example

ZnO, % Original

Tensile strength, psi

Elongation, % After heat aging

Tensile strength, psi Retention, %

Elongation, % Retention, % Creep, %

After heat shock Creep, % 97 8 8 17 EXAMPLES 4-7

Compositions are prepared by blending an ethylene- tetrafluoroethylene copolymer with zinc sulfide and, optionally antimony trioxide and an amine antioxidant or a hindered phenol antioxidant, in the amounts shown in Table IKamounts are shown by weight percent based on the weight of the copolymer). Test specimen's are prepared and tested as set forth above.

The results are shown in Table II. As demonstrated by these results, the addition of zinc sulfide improves the retention of tensile strength and elongation and reduces creep of the composition.

Creep, % 75 20 1.5 12

EXAMPLES 8-9

Compositions are prepared by blending an ethylene- tetrafluoroethylene copolymer with a thermoplastic elastomer (TPE) having thermoplastic segments containing ethylene-tetrafluoroethylene copolymer repeating units and elastomeric segments containing a vinylidene fluoride, hexa- or pentafluoropropylene and tetrafluoroethylene repeating units, zinc sulfide and , optionally, a hindered phenol antioxidant, antimony trioxide, and titanium dioxide in the amounts shown in Table III (amounts are shown by weight percent based on the weight of the copolymer). Test specimens are prepared and tested as set forth above.

The results are shown in Table III. As shown by these results even with the addition of 30% of a thermoplastic elastomer, the use of zinc sulfide improves the retention of tensile strength and elongation of an ethylene-tetrafluoroethylene-based composition.

^♦After aging 168 hours at 275°C.

Claims

WHAT IS CLAIMED IS:

1. A polymeric composition comprising an ethylene- tetrafluoroethylene copolymer about 30 to about 60 mole percent of ethylene, about 60 to about 35 mole percent of tetrafluoroethylene, and about 0 to about 35 mole percent of one or more other comonomers, and a Group lib metal sulfide in an amount effective to stabilize the composition.

2. A composition according to claim 1, wherein the Group Hb metal sulfide is zinc sulfide.

3. A composition according to claim 1, containing about 0.05 to about 20% by weight of a Group lib metal sulfide, based on the weight of the copolymer.

4. A composition according to claim 1, containing about 0.1 to about 10% by weight of a Group lib metal sulfide based on the weight of the copolymer.

5. A composition according to claim 1 or claim 2, which further comprises antimony oxide.

6. A composition according to claim 1 or claim 2, which has been crosslinked.

7. A composition according to claim 1 or claim 2, which further comprises a thermoplastic elastomer containing thermoplastic segments containing ethylene-tetrafluoroethylene repeating units and elastomeric segments containing vinylidene fluoride, hexa- or pentafluoropropylene and tetrafluoroethylene repeating units.

8. A composition according to claim 7, which further comprises antimony oxide.

9. A composition according to claim 7, which has been crosslinked.

10. A heat recoverable article made from a polymeric composition of any one of the preceding claims.

11. A method of stabilizing a composition comprising an ethylene- tetrafluoroethylene copolymer which comprises incorporating therein in an amount effective to stabilize the composition a sulfide of a Group Hb metal.