Polyesters, such as poly(alkylene terephthalates), for example, polyethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), are subject to thermal degradation, especially at elevated temperatures. It is known, generally, to modify such polyester compositions containing 0 to 60 weight percent of fillers with a polymeric modifier such as polycaprolactone (PCL) but the degree of thermal stabilization provided in this manner is not sufficient for many uses.

U. S. Patent 3, 835, 089, to Fox and Wambach, recognizes that the physical properties of blends of PBT and PCL are improved over those obtainable with the individual components. This patent does not, however, recognize that PBT PCL blends can be still further improved with respect to thermal aging properties.

European patent application EP 57, 415 A2 of Ogawa and Akagi discloses compositions of polyester and a modified polycaprolactone. The compositions can further contain various additives such as inorganic fillers, e.g., glass fibers, crystal nucleating agents, phosphorus compounds, epoxy compounds, and other additives, such as flame retardants, antioxidants and heat stabilizers. The addition of phosphorus compounds is said to enhance the heat resistance of the composition. Examples of phosphorus compounds given in this European application include phosphates, phosphites, phosphonic and phosphinic compounds.

Trimethyl phosphate and triphenyl phosphite are listed as examples of preferred phosphorus compounds. We have found that phosphates and aryl phosphites are not effective in enhancing the thermal stability of blends of poly(butylene terephthalate) and poly(caprolactone).

The compositions described in the above documents do not possess the degree of thermal stabilization sufficient for many uses.

It has now been discovered that polyester compositions comprising a polyCalkylene terephthalate) and a polylactone can be provided with exceptional thermal stability by the addition of a carbodumide and an aliphatic phosphite.

One aspect of this invention comprises a composition comprising at least about 30% of a polyCalkylene terephthalate), at least about 3% of a polylactone, about 0.05% to about 10% of a carbodumide and at least about 0.05% to about 10% of an aliphatic phosphite, all percentages being by weight based on the weight of the composition.

Another aspect of this invention comprises a method of improving the thermal stability of a composition comprising a poly(alkylene terephthalate) and a polylactone which comprises admixing therewith a carbodumide and an aliphatic phosphite in amounts effective to thermally stabilize the composition.

A further aspect of this invention comprises an insulated wire comprising an electrical conductor coated with a composition comprising at least about 30% of a polyCalkylene terephthalate), at least about 3% of a polylactone, about 0.05% to about 10% of a carbodumide and at least about 0.05% to about 10% of an aliphatic phosphite, all percentages being by weight based on the weight of the composition.

Another aspect of this invention comprises a cable jacketed with a composition comprising at least about 30% of a polyCalkylene terephthalate), at least about 3% of a polylactone, about 0.05% to about 10% of a carbodumide and at least about 0.05% to about 10% of an aliphatic phosphite, all percentages being by weight based on the weight of the composition.

Yet another aspect of this invention comprises a heat recoverable article comprising a composition comprising at least about 30% of a polyCalkylene terephthalate), at least about 3% of a polycaprolactone, about 0.05% to about 10% of a polycarbodiimide and at least about 0.05% to about 10% of an aliphatic phosphite, all percentages being by weight based on the weight of the composition.

The improved thermal stability of the composition of this invention is both dramatic and unexpected. With the incorporation of one weight percent of distearyl pentaerythritol diphosphite and two weight percent polycarbodϋmide, the time to failure (TTF) at 200°C for a flame retarded PBT PCL composition increased markedly from 46 up to about 206 hours.

The composition of this invention comprised a polyCalkylene terephthalate), such as polyCethylene terephthalate) and polyCbutylene terephthalate), in an amount of at least about 30% by weight Call percents given herein are by weight based on the weight of the composition unless otherwise stated). Preferably, the polyCalkylene terephthalate) is present in the amount from about 30% to about 90%, more preferably from about 40% to about 65%. PolyCalkylene terephthalates) are known polymers, some of which are commercially available. The polymers are prepared by polymerizing terephthalic acid or an ester thereof, which may be substituted, for example by halogen, such as bromine, with a glycol, for example ethylene or butylene glycol. The glycol may also be substituted, for example, by halogen, such as bromine. The use of polyCbutylene terephthalate) is particularly preferred. These polymers typically have a molecular weight greater than about 10, 000 weight average molecular weight.

The composition also contains at least about 3% polylactone. Preferably, the composition contains about 3 to about 40%, more preferably from about 5 to about 30% polylactone. Preferred polylactones have repeating units of the general formula:

- --O— R— CO→-

wherein R is divalent alkylene of, e.g., from 2 to 30 carbon atoms, straight chain and branched, and the number of repeating units is such that the average molecular weight is up to about 100, 000.

More particularly, the polylactone has the general formula:

-4-0— --C -R4- ² — r- CO→ _Ϊ -

wherein Rl and R2 are hydrogen or alkyl, e.g., methyl or ethyl, m is, for example, 2-5 and n is from about 25 to about 1500. Especially preferred compounds within this family will comprise those in which R and R2 are each hydrogen, or are methyl or ethyl on the carbon adjacent to the linking oxygen atom. The most preferred such polylactones are polyCbeta- propiolactone), poly(gamma-butyrolactone), polyCdelta-valerolactone), polyCepsilon-caprolactone) or mixtures of at least two of them. The use of polycaprolactone is especially preferred.

The polylactone can be made by various methods. For example, by polymerizing the corresponding lactone. Further details on preparative procedures for polylactones may be obtained by reference to The Encyclopedia of Polymer Science and Technology, Vol. 11, John Wiley and Sons, Inc, New York, 1969, p 98-101; H. Cherdron et al., Makromol Chem. 56, 179-186 and 187-194 (1962); U. S. 2, 933, 477 and U. S. 2, 933, 478.

The composition also contains about 0.05% to about 10% of a carbodumide. Preferably the composition contains about 0.1% to about 5% carbodumide. The carbodumide is represented by the formula:

Xi— R ₁ - N=C=N-R ₂ → _s -N=C=N~ R ₃ -X ₂

where Ri, R2 and R3 are C1-C12 aliphatic, C6-C13 cycloaliphatic, or C6-C13 aromatic divalent hydrocarbon radicals, and combinations thereof, Xi and X2 are H,

— N- C- N-R ₄ H O

II I II

H O R ₅ ^or — N- C- 0-R ₆

where R4, R5 and R& are C1-C12 aliphatic, C6-C13 cycloaliphatic and C6-C13 aromatic monovalent hydrocarbon radicals and combinations thereof and additionally R4 or R5 can be hydrogen; and n is 0 to 100. The useful carbodiimides have an average of at least one carbodumide groups (i. e., one -N=C=N-group) per molecule and an average molecular weight of less than about 500 carbodumide groups. These carbodiimides can be aliphatic, cycloaliphatic or aromatic carbodiimides. The terms aliphatic, cycloaliphatic and aromatic as used herein indicate that the carbodumide

group is attached directly to an aliphatic group, a cycloaliphatic group or an aromatic nucleus respectively. For example, these carbodiimides can be illustrated by formula (I): wherein Ri, R2 and R3 are independently aliphatic, cycloaliphatic or aromatic divalent hydrocarbon radicals and n is at least 0 and preferably 0 to 100. Xi and X2 are defined as hereinbefore. Carbodiimides useful for the compositions of this invention have one or more carbodumide groups and thus one or more of the three divalent hydrocarbon groups (i.e., Ri, R2 and R3) and each of these hydrocarbon groups can be the same or different from the others so that the diimides can have aliphatic, cycloaliphatic and aromatic hydrocarbon groups in one carbodiimide molecule. The use of a polycar bodiimide is preferred.

Carbodiimides can be prepared to use in this invention by well known procedures. Typical procedures are described in U. S. Patent Nos. 3, 450, 562 to Hoeschele; 2, 941, 983 to Smeltz; 3, 193, 522 to Neumann et al, and 2, 941, 966 to Campbell.

The composition also contains about 0.05% to about 10% of an aliphatic phosphite. The phosphite compound preferably is present in an amount of about 0.1% to about 5%.

The term "aliphatic phosphite" is used herein to include hydrogen phosphites, monophosphites, diphosphites, triphosphites, polyphosphites and the like, having at least one aliphatic carbon atom bonded to at least one of the oxygen atoms of the phosphite moiety. The phosphites may be mono- di or tri- esters of phosphorous acid or phosphonic acid. The aliphatic carbon may be part of a straight or branched chain aliphatic group, such as an alkyl group, and may be saturated or unsaturated and may be substituted by one or more substituents, such as chlorine or hydroxyl or carboxyl groups, which do not interfere with its action in enhancing the thermal stability of the composition. The aliphatic group may be substituted by aromatic moieties as long as the carbon atom bonded to the oxygen atom of the phosphite moiety is an aliphatic carbon atom. The aliphatic group is preferably an alkyl group containing at least 1 carbon atom, more preferably the aliphatic group is an alkyl group containing about 6 to about 30 carbon atoms. Other aliphatic groups are also suitable and are exemplified in the following list of aliphatic phosphites.

Illustrative aliphatic phosphites include, for example, distearyl pentaerythritol diphosphite, diisodecyl pentaerythritol diphosphite, bis(2, 4- di-t-butylphenyl) pentaerythritol diphosphite, diphenyl isodecyl phosphite, diphenyl isooctyl phosphite, phenyl diisodecyl phosphite, diisooctyl phosphite, triisooctyl phosphite, dilauryl phosphite, trilauryl phosphite, tristearyl phosphite, di-tridecyl phosphite, ethylhexyl diphenyl phosphite, diisooctyl octylphenyl phosphite, diphenyl didecyl (2, 2, 4-trimethyl-l, 3- pentanediol) diphosphite, tris(2-chloroethyl) phosphite, tris(dipropyleneglycol) phosphite, heptakis(dipropyleneglycol) triphosphite, tetraphenyl dipropyleneglycol diphosphite, poly(dipropyleneglycol) phosphite, trilauryl trithiophosphite, bis(tridecyl) hydrogen phosphite, dioleyl hydrogen phosphite, and the like.

As will be noted from the examples below only those compounds which have an aliphatic or substituted aliphatic group attached to an oxygen of the phosphite compound are effective in increasing thermal stability of the PBT/PCL composition.

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; 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, 1, 2-bis(tetrabromophthalimido) ethylene; pigments such as titanium dioxide and carbon black, and the like. Mixtures of such additives can be used.

One or more additives may be present in an amount of up to 67% of the composition. It is to be understood that for any specified proportions of the other components, additives are present in amounts to provide 100%.

The composition can be prepared by mixing together the components in any appropriate mixer such as, an internal mixer, for example, a Brabender or Banbury, or a twin screw extruder, for example, a ZSK extruder, or the like.

The composition of this invention is melt processable and can be readily formed into any desired shape. The composition can be used in many applications. A preferred use of the composition is as wire insulation, i.e., as a coating on an electric conductor. The composition is also useful as cable jacketing on electrical or fiber optic cable. The composition can also be used in the preparation of heat recoverable articles, such as heat shrinkable tubing and molded parts.

Heat recoverable articles are well known. 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 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-stable 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 invention will be better understood by reference to the illustrative examples which follow.

Examples 1 - 10

Formulations of this invention containing about 57% of poly(butylene terephthalate), about 14% of polycaprolactone, 2.0% polycarbodiimide (PCD) and about 24% of a flame retardant mixture containing 1, 2- bis(terephthalimido) ethylene, antimony trioxide and magnesium hydroxide, 2.0% of a hindered phenolic antioxidant and 1.0% of an aliphatic phosphite, as listed in Table I, are prepared in a twin screw extruder at temperatures between 175-290°C.

The formulations are extruded using a 1 inch extruder at about 220- 280°C onto tin coated copper 20AWG conductor as 10 mil wall insulations.

The samples are evaluated for thermal aging stability using the mandrel wrap method. In general, for each sample four specimens are prepared and heat aged at 200°C in an air-flow oven. At specified intervals, typically 20-24 hours apart, the wire specimens are removed from the oven and cooled before they are wrapped around a 1/2 inch diameter mandrel for visual inspection. Any cracks or defects caused by thermal degradation are recorded accordingly. A sample failure is recognized when 50% or more of the specimens (two in this case) showed one or more cracks. The hours to failure are taken to be the average between the time at which a sample failure was noted and the time at which the prior inspection is made. The results are shown in Table I.

TABLE I

Examples 11-14 (comparative)

The procedures of examples 1-10 are repeated using aromatic phosphites as listed in Table II in place of the aliphatic phosphites used in Examples 1-10. The results are shown in Table II and can be compared to the results in Table I. As can be seen, while the use of aliphatic phosphites dramatically improves the performance of the compositions as shown in Table I, the use of aromatic phosphites, i. e., a phosphite having aromatic carbon atoms bonded to the oxygen atoms of the phosphite moiety, does not.

Examples 15 - 17 Ccomparative)

The procedures of Examples 1-10 are repeated using 1.0% of four commonly used antioxidants as listed in Table III in place of the aliphatic phosphite used in Examples 1-10. The results are shown in Table III. These results indicate that the addition of commonly used antioxidants do not significantly improve the performance of the compositions. On -he other hand, a composition of this invention containing 1.0% of distearyl pertaerythritol diphosphite does not fail when heat aged up to 190 hours.

Examples 18 - 22

The procedures of Examples 1 - 10 are repeated varying only the amounts of distearyl pentaerythritol diphosphite, as listed in Table IV. The results are shown in Table IV.

TABLE TV

The procedures of Examples 1 - 10 are repeated varying the amounts of aliphatic phosphite, polycarbodiimide (PCD) and hindered phenolic antioxidant as listed in Table V. The results are shown in Table V.

Table V

These results show the dramatic improvement in heat aging performance when the composition contains polycarbodiimide and an aliphatic phosphite.