LU100185B1

LU100185B1 - Crosslinked polyolefin copolymer and compositions thereof

Info

Publication number: LU100185B1
Application number: LU100185A
Authority: LU
Inventors: Linnea Petersson; Henrik Hillborg; Gang Zhang; T C Mike Chung; Joakim Jambeck
Original assignee: Abb Schweiz Ag
Priority date: 2017-05-03
Filing date: 2017-05-03
Publication date: 2018-11-05

Abstract

The present invention discloses a crosslinked polyolefin copolymer comprising pendant phenol antioxidant moieties and further comprising crosslinking moieties derived from dimerization of the phenol antioxidant moieties, wherein each crosslinking moiety constitutes an intra- or intermolecular link within the crosslinked polyolefin copolymer. The present invention also discloses a method of manufacturing such a crosslinked polyolefin copolymer. The present invention further discloses film capacitors and electric power system components comprising such crosslinked polyolefin copolymers

Description

Crosslinked polyolefin copolymer and compositions thereof TECHNICAL FIELD

The present invention relates to crosslinked polyolefin copolymers and compositions thereof. The present invention further relates to methods of manufacturing such copolymers. The present invention also relates to film capacitors and electric power system components comprising such crosslinked polyolefin copolymers.

BACKGROUND ART

Capacitors play an important role in electric power grids by helping to compensate for the reactive power consumption of components connected to the grid. A commonly used type of capacitor is a polymer film capacitor comprising a polymer film as the capacitor dielectric. The most commonly used dielectric material in industrial and power capacitors is polypropylene, and more specifically biaxially-orientated polypropylene (BOPP).

Polypropylene is the second most abundantly produced polymer globally. It used in an extremely wide variety of applications, such as packaging, textiles, construction materials and automotive parts, besides capacitors. However, polypropylene is prone to degradation through oxidation due to exposure to heat and UV radiation, and therefore is often stabilized using antioxidants. The antioxidants most commonly used are of the sterically-hindered phenol type. A common feature of such antioxidants is the presence of an ortho-substituted phenol moiety having a low O-H bond dissociation energy. Such phenols can sacrificially donate a phenolic hydrogen atom to a peroxyl radical, thereby forming a peroxide and a phenolic radical, and preventing the peroxyl radical from abstracting a hydrogen atom from the polymer backbone. The produced phenolic radical reacts further by combinations of disproportionation, reaction with a further peroxyl radical and/or dimerization in order to ultimately provide harmless nonradical products. In this manner the propagation of the oxidation radical chain reaction is broken.

Another problem with polypropylene is its limited mechanical strength at elevated temperatures. Although commercial isotactic polypropylenes commonly exhibit a melting peak at about 165 °C, softening of the polymer occurs at temperatures as low as 70 °C. Therefore, polypropylene is generally limited to applications requiring temperatures not exceeding 75-80 °C. BOPP capacitors for example typically have an operational temperature of at most 80 °C and tend to exhibit problems with aging, especially at elevated temperatures.

Crosslinking is known to provide polymers with improved mechanical strength, temperature stability and resistance to solvents, creep and stress-cracking. Cross-linked polyethylene (XLPE) is for example commonly used in piping and as insulation material for high-voltage electric power cables. Polyethylene is commonly cross-linked by gamma- or electron-beam irradiation, peroxide-induced radical crosslinking or by silane-moisture cure mechanisms. However, none of these approaches are readily applicable to polypropylene, since radical crosslinking would degrade the polypropylene backbone and crosslinkable silane monomers would poison the transition metal polymerisation catalyst.

There have been prior attempts to produce crosslinkable polypropylenes. Lin et al. (Lin, W., Shao, Z., Dong, J. Y., & Chung, T. M. (2009): "Cross-linked polypropylene prepared by PP copolymers containing flexible styrene groups", Macromolecules, 42(11), 3750-3754) discloses a polypropylene butenylstyrene (PP-co-BSt) copolymer. This copolymer can undergo a thermally-induced [4+2] cycloaddition between the pendant styrene groups, thus providing a crosslinked polypropylene copolymer.

There remains a need for polyolefins, and especially polypropylenes, having improved endurance when used at elevated temperatures.

SUMMARY OF THE INVENTION

The inventors of the present invention have identified a number of shortcomings in the known prior art. Commercially available polypropylenes comprising commercial antioxidants are unsuitable for use at elevated temperatures due to the low softening temperature and poor mechanical properties of the polypropylene homopolymer. Moreover, the antioxidants in such commercial polypropylenes are susceptible to migration and leaching due to poor compatibility with the semi-crystalline polypropylene matrix, leaving the polypropylene insufficiently protected against oxidative degradation, especially at elevated temperatures.

Known crosslinkable polypropylene copolymers have good crystallinity, high melting temperature, excellent mechanical strength, and high thin film dimensional stability at elevated temperatures. However, these copolymers have no protection against oxidative degradation and attempting to blend antioxidants with the polymer matrix will lead to similar compatibility issues as for the polypropylene homopolymer, leading to poor antioxidant dispersion in the semi-crystalline polymer matrix, migration or leaching of the antioxidant and ultimately insufficient protection against oxidative degradation, especially at elevated temperatures.

It is therefore an object of the present invention to provide polyolefins having both improved mechanical properties at elevated temperatures as well as excellent resilience against oxidative degradation.

This object is achieved by a crosslinked polyolefin copolymer according to the appended claims.

The crosslinked polyolefin copolymer comprises pendant phenol antioxidant moieties and further comprises crosslinking moieties derived from dimerization of the phenol antioxidant moieties, wherein each crosslinking moiety constitutes an intra- or intermolecular link within the crosslinked polyolefin copolymer.

Such a crosslinked polyolefin copolymer is obtained by oxidative crosslinking of a polyolefin copolymer comprising pendant phenol antioxidant moieties. The pendant phenol antioxidant moieties, being bound to the polymer backbone, are well distributed in the copolymer matrix and cannot migrate or leach from the copolymer. Thus, the copolymer has excellent protection against oxidative degradation. Furthermore, the crosslinks obtained upon the antioxidant moieties fulfilling their protective function provide the crosslinked polyolefin copolymer with improved mechanical properties, such as a greater tensile strength. These properties in combination ensure that the crosslinked polyolefin copolymer is well suited for use at elevated temperatures. The crosslinked polyolefin copolymer also has a higher dielectric constant than typical stabilised polyolefins, as well as a low dielectric loss, meaning that it is well suited as a dielectric film in film capacitors.

The crosslinked polyolefin copolymer may comprise at least 5% crosslinking moieties relative to pendant phenol antioxidant moieties, preferably at least 10%, such as at least 30%, at least 50%, or at least 70%. This may be determined using UV spectroscopy or NMR spectroscopy as described herein. The crosslinked polyolefin copolymer may have a solubility in refluxing xylene of less than 90% by weight, such as less than 70% by weight, less than 50% by weight, less than 30% by weight, or less than 10% by weight. Thus, the extent of crosslinking may be determined by either spectroscopy or quantitative solvent extraction. By ensuring a suitable degree of crosslinking, the suitability of the properties of the crosslinked polyolefin copolymer may be ensured for any given application.

The crosslinked polyolefin copolymer may comprise monomer units selected from ethylene, propylene, 1-butene, isobutylene, methylpentene, dicyclopentadiene, ethylidene norbornene and vinyl norbornene. Thus, any olefin commonly used in the production of polyolefins may be incorporated in the polymer chain. The crosslinked polyolefin copolymer may preferably comprise polypropylene monomer units. Crosslinked polypropylene is difficult to obtain by prior art methods and the semicrystalline polypropylene matrix has poor compatibility with traditional antioxidants, meaning that commercially available stabilized polypropylenes are unsuitable for use at elevated temperatures due to poor oxidative and mechanical stability. These deficiencies are overcome by a polypropylene copolymer according to the present invention.

The pendant phenol antioxidant moieties may each comprise an 3,5-di-t-butyl-4-hydroxytoluene moiety. Such moieties are known to undergo dimerization upon sacrificial oxidation of the antioxidant moiety, thus providing a crosslinking moiety. The pendant phenol antioxidant moieties may preferably each comprise a 3,5-di-t-butyl-4-hydroxyphenyl propionate moiety. Such moieties are known to have excellent antioxidant activity and provide highly conjugated bis-quinonemethide crosslinking moieties upon sacrificial oxidation. The bis-quinonemethide moieties are highly polarizable and contribute to the high dielectric constant of the crosslinked polyolefin copolymer. A linker comprising an ester group may connect each pendant phenol antioxidant moiety to a backbone chain of the crosslinked polyolefin copolymer. Thus, the crosslinked polyolefin copolymer may be synthesised by robust, well-established chemical methods for forming ester bonds, and a variety of commercially available antioxidants and co-monomers may be used in the synthesis of the crosslinked polyolefin copolymer.

The crosslinked polyolefin copolymer may comprise from 0.2 to 10 mol % of pendant phenol antioxidant moieties and crosslinking moieties in total, preferably from 0.5 to 5 mol%. The amount of pendant phenol antioxidant moieties and crosslinking moieties in total is considered to be equivalent to the amount of pendant phenol antioxidant moieties initially, prior to crosslinking; i.e. the amount of pendant phenol antioxidant moieties and crosslinking moieties in total is considered to be independent of the extent of crosslinking of the polymer. By ensuring a sufficient amount of pendant phenol antioxidant moieties and crosslinking moieties, the mechanical and oxidative integrity of the crosslinked polyolefin copolymer may be ensured, even for prolonged use in challenging environments.

According to another aspect of the invention, the object of the present invention is also achieved by polyolefin compositions according to the appended claims. The polyolefin compositions comprise a crosslinked polyolefin copolymer as described above. By utilizing the crosslinked polyolefin copolymer in a composition, the mechanical, dielectric, and degradation properties of the composition may be tailored in order to provide a composition having suitable properties at a reasonable cost.

The polyolefin composition may comprise from 10 weight% to 99.9 weight% of crosslinked polyolefin copolymer, calculated with respect to the total weight of the polyolefin composition. This provides the composition with improved resistance to oxidative degradation and a higher dielectric constant, since the antioxidant moiety is evenly dispersed throughout the composition.

The polyolefin composition may comprise a mono- or oligo-functional phenol antioxidant. The mono- or oligo-functional phenol antioxidant may constitute from 0.1 weight% to 5 weight% of the total weight of the polyolefin composition The crosslinked polyolefin copolymer ensures that the mono- or oligo-functional phenol antioxidant remains dispersed in the polyolefin composition, i.e. it acts as a compatabilizer, even for ternary blends of a crosslinked polyolefin copolymer, a mono- or oligo-functional phenol antioxidant, and a second polyolefin polymer. Thus, higher concentrations of antioxidant may be evenly distributed in the composition with a lesser risk for migration or leaching, meaning that an improved oxidative protection may be obtained at a relatively low cost.

The polyolefin composition may comprise a second polyolefin polymer. The second polyolefin polymer may constitute from 0.1 weight% to 90 weight% of the total weight of the polyolefin composition. In such a case, the crosslinked polyolefin copolymer may co-crystallize with the second polyolefin polymer, thus ensuring a homogenous composition and even distribution of antioxidant moieties throughout the entire composition.

The second polyolefin polymer may be a polyolefin homo- or co-polymer having the same principal monomer as the crosslinked polyolefin copolymer. So, for example, if the crosslinked polyolefin copolymer is polypropylene-based then the second polyolefin polymer may be a polypropylene homo- or co-polymer. The second polyolefin polymer may be a polyolefin homo-or co-polymer selected from polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polybutene-1 (PB-1), polyisobutylene (PIB), ethylene propylene rubber (EPR) and ethylene propylene diene rubber (EPDM).

According to a further aspect of the invention, the object of the invention is further achieved by a method of manufacturing a crosslinked polyolefin copolymer according to the appended claims. The method comprises the steps: a) providing a non-crosslinked polyolefin copolymer comprising pendant phenol antioxidant moieties; and b) subjecting the non-crosslinked polyolefin copolymer to conditions sufficient to induce oxidative dimerization of the pendant phenol antioxidant moieties in the polyolefin copolymer, thereby forming crosslinking moieties constituting intra- or intermolecular links within the crosslinked polyolefin copolymer by dimerization of the phenol antioxidant moieties.

By utilizing the method described, a crosslinked polyolefin is obtained using robust, well-established chemistry, without degrading the polymer backbone, and without poisoning the polymerization catalyst.

The conditions sufficient to induce oxidative dimerization of the pendant phenol antioxidant moieties may comprise heating under an oxidative atmosphere, such as air. The non-crosslinked polyolefin copolymer may be heated to at least 100 °C and may be heated for at least 2 hours. This ensures a degree of crosslinking.

Further suitable conditions sufficient to induce oxidative dimerization of the pendant phenol antioxidant moieties may include treatment of the non-crosslinked polyolefin copolymer with UV or IR.

The polyolefin copolymer comprising pendant phenol antioxidant moieties may be manufactured by post-polymerisation coupling of a functionalized polyolefin copolymer with a phenol antioxidant. Such a method is compatible with known polymerisation catalysts such as homogenous and heterogeneous Ziegler-Natta or metallocene catalysts, and allows for the manufacture of copolymers having a tapered microstructure or copolymers having a random microstructure. In this case, the functionalized polyolefin copolymer produced by polymerization may be a hydroxy-functionalized polyolefin copolymer and the phenol antioxidant may have a carboxylic acid functional group, or ester, amide or acid halide thereof. This allows for a convenient, quantitative post-functionalization of the polyolefin copolymer using well-established coupling chemistry, such as Steglich esterification.

The non-crosslinked polyolefin copolymer comprising pendant phenol antioxidant moieties may be manufactured by copolymerisation of an olefin monomer together with a monomer comprising a phenol antioxidant moiety. This allows for the production of the non-crosslinked polyolefin copolymer comprising pendant phenol antioxidant moieties in a single step.

According to yet another aspect of the present invention, the object of the invention is achieved by a film capacitor according to the appended claims. The film capacitor comprises a dielectric film comprising a crosslinked polyolefin copolymer as described herein. Dielectric films of the crosslinked polyolefin copolymer have a higher dielectric constant, low dielectric loss and improved endurance at elevated temperatures. This means that film capacitors utilizing such films may have an increased capacity, a higher maximum operating temperature and therefore a lesser need for cooling during operation. Such capacitors may also handle higher currents as compared to prior art capacitors. The dielectric film may preferably comprise a crosslinked polypropylene copolymer, such as a biaxially oriented polypropylene copolymer that is optionally metallized on one or both surfaces.

According to yet a further aspect of the present invention, the object of the invention is achieved by an electric power system component according to the appended claims. The electric power system component comprises an insulator comprising a crosslinked polyolefin copolymer as described herein. Insulator materials of the crosslinked polyolefin copolymer have increased resistance to oxidative degradation and improved mechanical stability at elevated temperatures. This means that power system components comprising such insulator materials may be operated at higher temperatures and/or have a longer operative lifespan. The electric power system component may be an electrical cable, a cable joint, a bushing or a cable termination. It may be designed for use in a high voltage DC power system (HVDC) or a high voltage AC power system.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which:

Fig la schematically illustrates the oxidative dimerization of BHT.

Fig lb schematically illustrates the oxidative dimerization of antioxidants comprising a 3,5-di-t-butyl-4-hydroxyphenyl propionate moiety.

Fig. 2a schematically illustrates a polypropylene copolymer comprising pendant 3,5-di-t-butyl-4-hydroxyphenyl propionate moieties.

Fig. 2b schematically illustrates the crosslink formed by oxidative crosslinking of a polypropylene copolymer comprising pendant 3,5-di-t-butyl-4-hydroxyphenyl propionate moieties.

Fig. 3 is a UV-vis spectrogram demonstrating the formation of crosslinking moieties from phenol antioxidant moieties during heating of a polypropylene copolymer sample.

Fig. 4a schematically illustrates the gel content of polypropylene copolymer samples upon heating at various temperatures for 24 hours.

Fig. 4b schematically illustrates the gel content of polypropylene copolymer samples upon heating at 210 °C for various times.

Fig. 5 is a stress-strain diagram illustrating the mechanical properties of polypropylene copolymer samples having various degrees of crosslinking.

Fig. 6 is an isothermal thermogravimetric analysis plot illustrating the oxidative decomposition of a polypropylene copolymer compared to commercial polypropylene samples.

Fig. 7a is a thermogravimetric analysis plot illustrating the oxidative decomposition (conversion) of a polypropylene copolymer at varying heating rates.

Fig. 7b is a plot of log(heat rate) against inverse temperature for a polypropylene copolymer subjected to varying levels of conversion.

Fig. 8 is a plot illustrating the predicted lifetime of polypropylene copolymer and a commercial polypropylene assuming an acceptable conversion of 1 wt%.

DETAILED DESCRIPTION

The present invention is based upon a discovery by the inventors that polyolefin copolymers comprising pendant phenol antioxidant groups may be crosslinked by subjecting them to oxidative conditions. This allows for the provision of a crosslinked polyolefin under relatively mild conditions without degrading the polymer backbone. The resulting crosslinked polyolefin copolymer has excellent mechanical properties, such as a greater tensile strength as compared to the non-crosslinked precursor copolymer. Thus, the crosslinked polyolefin copolymer may have improved dimensional stability and creep resistance at elevated temperatures. Moreover, because the crosslinked polyolefin copolymer has incorporated antioxidant moieties evenly dispersed in the polymer matrix, it has excellent resistance to oxidative degradation, again making it highly suitable for use at elevated temperatures. Due to the polar antioxidant and crosslinking groups being evenly dispersed within the polymer matrix, the crosslinked polyolefin copolymer has a high dielectric constant (ε) whilst still maintaining low dielectric loss (tan 6). The improved mechanical stability and resistance to oxidative degradation at elevated temperatures in combination with increased dielectric constant and low dielectric loss makes the crosslinked polyolefin copolymer highly suitable for use in a dielectric film for film capacitors.

The crosslinked polyolefin copolymer is manufactured by subjecting a polyolefin copolymer having pendant phenol antioxidant moieties to oxidative conditions.

The polyolefin copolymer having pendant phenol antioxidant moieties may be manufactured by any means known in the art. For example, the polyolefin copolymer may be manufactured by post-functionalisation of a suitable polyolefin copolymer with a phenol antioxidant. For example, a polyolefin copolymer having hydroxy functionalities may be manufactured by copolymerization of an olefin with a hydroxy-functionalized olefin, or a masked or protected derivative thereof. The hydroxy-functionalized olefin may for example be a straight-chain alpha C3-C20 olefin having a single hydroxy group on the carbon atom most remote from the double bond. Suitable hydroxy-functionalized olefins include, but are not limited to, 10-undecen-l-ol, 9-decen-l-ol, 8-nonen-l-ol, 7-octen-l-ol, 6-hepten-l-ol, 5-hexen-l-ol, 4-penten-l-ol, 3-buten-l-ol and allyl alcohol. The polyolefin copolymer having hydroxy functionalities may then be coupled with an antioxidant having a carboxylic acid functionality, or equivalent thereof. This coupling may for example be performed using the coupling reagents EDC (l-Ethyl-3-(3-dimethylaminopropyljcarbodiimide) and DMAP (Ν,Ν'-dimethylaminopyridine), as disclosed in Zhang, G., Li, H., Antensteiner, M., & Chung, T. M. (2015); "Synthesis of functional polypropylene containing hindered phenol stabilizers and applications in metallized polymer film capacitors", Macromolecules, 48(9), 2925-2934. However, other means of coupling and coupling reagents are known in the art.

Alternatively, the polyolefin copolymer may be prepared by copolymerisation of an olefin with a monomer incorporating a phenolic antioxidant moiety, such as described in Wilén, C. E., Luttikhedde, H., Hjertberg, T., & Näsman, J. H. (1996): "Copolymerization of Ethylene and 6-tert-Butyl-2-(l, l-dimethylhept-6-enyl)-4-methylphenol over Three Different Metallocene-Alumoxane Catalyst Systems", Macromolecules, 29(27), 8569-8575.

However, other means of preparing the polyolefin copolymer are conceivable. For instance, post-functionalisation of suitable polyolefin copolymers may be performed using any suitable coupling chemistry, such as peptide coupling, transition-metal mediated coupling reactions, and cycloaddition reactions such as the Huisgen copper-catalysed "click" [3+2] cycloaddition.

The phenol antioxidant moieties may be any phenol antioxidant moieties known in the art to undergo dimerization when subjected to oxidative conditions. This includes antioxidants having a 3,5-di-t-butyl-4-hydroxytoluene moiety, which may dimerize to form a stilbenquinone structure as shown in Figure la, or preferably antioxidants having a 3,5-di-t-butyl-4-hydroxyphenyl propionate moiety, which may dimerize to form a bis-quinonemethide structure as shown in Figure lb.

The polyolefin copolymer having pendant phenol antioxidant moieties may be primarily based on monomer units selected from the olefins ethylene, propylene, 1-butene, isobutylene, methylpentene, dicydopentadiene, ethylidene norbornene and vinyl norbornene. Preferably, the polyolefin copolymer is a polypropylene copolymer. The phenol antioxidant moieties may be present in the polyolefin copolymer in amounts of from 0.2 to 10 mol%. The mol% of phenol antioxidant moieties in the polyolefin copolymer is defined as the mol% of monomer units in the copolymer having pendant phenol antioxidant moieties, relative to the total monomer units in the copolymer. This may for example be determined by quantitative 1H NMR spectroscopy of the polyolefin copolymer.

The quantity of pendant phenol antioxidant moieties in the polyolefin copolymer may be expressed either in mol% or in weight%, and these units may be easily interconverted. Taking as an example a polypropylene (PP) copolymer having pendant phenol antioxidant moieties deriving from 3,5-di-t-butyl-4-hydroxyphenylpropionic acid (HP), the PP portion has a unit weight of approximately 42 g/mol (not taking into account the linker moieties extending between the backbone of the polypropylene and the pendant phenol antioxidant moiety). The 3,5-di-t-butyl-4-hydroxyphenylpropionic acid has a molecular weight of approximately 278 g/mol. A polypropylene copolymer comprising 1 mol% of pendant phenol antioxidant groups has therefore approximately 6.3 wt% 3,5-di-t-butyl-4-hydroxyphenylpropionate groups.

Once prepared, the polyolefin copolymer having pendant antioxidant moieties is crosslinked by exposure to oxidative conditions. Such oxidative conditions may for example comprise subjecting the polyolefin to heating under an oxidative atmosphere such as air or oxygen, or may comprise subjecting the polyolefin to IR or UV irradiation under oxidizing conditions. The crosslinking of the polyolefin copolymer may be purposefully accelerated by heating during production of components from the polyolefin copolymer or compositions thereof, or the crosslinking may be obtained over a longer duration by passive crosslinking of the polyolefin copolymer at the operating temperature of a component comprising the polyolefin copolymer. Thus, items produced from the polyolefin copolymer need not necessarily be actively crosslinked, but may be allowed to crosslink over time through use. The rate and degree of crosslinking may be controlled by adjusting the oxidation temperature and reaction time. For example, the polyolefin copolymer may be heated at a temperature in excess of 70 °C, such as 100 °C or 150 °C. When heating, it is preferable to maintain the polyolefin copolymer at a temperature suitably removed from the decomposition temperature of the polyolefin, such as at a temperature less than 300 °C, less than 250 °C or less than 200 °C. The polyolefin copolymer may be heated for a period of time sufficient to obtain the desired degree of crosslinking. For example, the polyolefin copolymer may be heated for at least 30 minutes, such as at least 1 hour, at least 2 hours, or at least about one day. The crosslinking of the polyolefin copolymer may be accelerated by addition of a radical initiating agent such as an organic peroxide. Suitable organic peroxides include but are not limited to dicumyl peroxide (Di-Cup®), bis(t-butylperoxyisopropyl)benzene (Vul-Cup®), t-butyl cumyl peroxide (Luperox® D-16), 2,5-di(t-butylperoxy)-2,5-dimethylhexane (Luperox® 101), n-butyl-4,4'-di(t-butylperoxy)valerate (Luperox® 230), l,l'-di(t-butylperoxy)-3,3,5-trimethylcyclohexane (Luperox® 231), or mixtures thereof. If an organic peroxide is used, this may replace the need for an oxidizing atmosphere, and therefore the polyolefin copolymer may be crosslinked under inert atmosphere using an organic peroxide.

The extent of crosslinking of the crosslinked polyolefin copolymer may be determined by a variety of different methods. Upon dimerization, the phenol antioxidants form a conjugated chromophoric dimer having a different absorption maximum that the parent phenol. For example, the 3,5-di-t-butyl-4-hydroxyphenyl propionate moiety has an absorption maximum at λ = 275 nm, whereas the bis-quinonemethide resulting from dimerization has an absorption maximum at λ = 310 nm. Therefore, the degree of crosslinking of the polyolefin copolymer may be established by means of quantitative UV-vis spectroscopy by comparing the relative intensity of the peaks at 275 and 310 nm.

The extent of crosslinking may also be determined by determining the gel content of the crosslinked polyolefin copolymer. The gel content is defined as the insoluble portion of the crosslinked polyolefin copolymer after refluxing in xylene for 2 hours. Note that since it only requires several crosslinks in each polymer chain in order to form a completely insoluble network, the gel content of the crosslinked polyolefin will typically exceed the degree of crosslinking as determined by UV-vis spectroscopy.

The crosslinked polyolefin copolymer may be used in polyolefin compositions to provide compositions having superior thermo-oxidative and mechanical properties. The crosslinked polyolefin copolymer may constitute from 5 weight% to 99.9 weight% of the total weight of such polyolefin compositions.

Such polyolefin compositions may further comprise a second polyolefin polymer in amounts of from 0.1 weight% to 90 weight%. The second polyolefin polymer may be a polyolefin homo- or co-polymer having the same principal monomer as the crosslinked polyolefin copolymer. So, for example, if the crosslinked polyolefin copolymer is polypropylene-based then the second polyolefin polymer may be a polypropylene homo- or co-polymer. The second polyolefin polymer may be polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polybutene-1 (PB-1), polyisobutylene (PIB), ethylene propylene rubber (EPR) or ethylene propylene diene rubber (EPDM).

The polyolefin composition may further comprise one or more mono- or oligo-functional phenol antioxidants in amounts of from 0.1 weight% to 5 weight% with respect to the total weight of the polyolefin composition. Among the most widely used commercial mono-or oligo-functional phenol antioxidants for polyolefins are the monofunctional 2,6-di-tert-butyl-phenol, 2,6-Di-tert-butyl-4-methylphenol (BHT), 2-tert-butyl-4-methoxyphenol (BHA) and octadecyl-3-(3,5-di-tert.butyl-4-hydroxyphenyl)-propionate (Irganox 1076); the difunctional 2-tert-butyl-6-(2-hydroxy-3-tert-butyl-5-methyl-benzyl)-4-methyl-phenol (Cyanox 2246) and 2-tert-butyl-6-[(3-tert-butyl-5-ethyl-2-hydroxyphenyl)methyl]-4-ethylphenol (Cyanox 425); the trifunctional l,3,5-trimethyl-2,4,6-tris (3,5-Di-tert-butyl-4-hydroxybenzyl)benzene (Ethanox 330) and 1,1,3-tris (2-methyl-4-hydrox-5-tert-butyl phenyl) butane (Topanol CA); and the tetrafunctional pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (Irganox 1010).

The polyolefin composition may comprise further additives known in the art, including but not limited to scorch retardants, pigments, dyes, fillers, UV-absorbers, nucleating agents and flame retardants.

For example, the crosslinked polyolefin copolymer may be added to polyolefin homopolymer compositions comprising commercial mono- or oligo-functional phenol antioxidants. In such a case, the crosslinked polyolefin copolymer may function to improve the compatibility of the homopolymer and the mono- or oligo-functional phenol antioxidant. This may allow a higher concentration of antioxidant to be dispersed in the polymer matrix, thus providing a composition with improved resistance to oxidative degradation.

The dielectric properties, mechanical properties and resistance to oxidative degradation make the crosslinked polyolefin copolymer, and compositions thereof, well suited for use in electrical applications. A film comprising the crosslinked polyolefin copolymer may be used as a dielectric in film capacitors. Such a film may be biaxially oriented, and/or may be metallized. For example, the dielectric film may be a biaxially-oriented polypropylene (BOPP) film. The crosslinked polyolefin copolymer may be used in other electrical applications requiring an insulating material or dielectric. Such applications include insulation materials for power cables, cable joints, bushings and cable terminations.

Examples A polypropylene copolymer having 1 mol% pendant 3,5-bis(tert-butyl)-4-hydroxyphenylpropionate groups was synthesised as described below. The synthesis method is adapted from that described in Zhang, G., Li, H., Antensteiner, M., & Chung, T. M. (2015): "Synthesis of functional polypropylene containing hindered phenol stabilizers and applications in metallized polymer film capacitors", Macromolecules, 48(9), 2925-2934. The resulting polypropylene copolymer is denoted PP-HP (wherein PP-HP stands for polypropylene-hindered phenol copolymer) and its structure is illustrated in Figure 2a.

The polypropylene copolymer was crosslinked under oxidizing conditions to provide the crosslinked polypropylene copolymer x-PP-HP illustrated in Figure 2b. Such a crosslinked polypropylene was subjected to testing in order to determine its oxidative stability and endurance at elevated temperatures, its dielectric properties and its mechanical properties.

Materials

All O2 and moisture sensitive manipulations were carried out inside an argon-filled dry box. rac-Me2Si[2-Me-4-Ph(lnd)]2ZrCl2 catalyst was prepared using a published procedure. Methylaluminoxane (MAO, 10 wt% in toluene), chlorotrimethylsilane, triethylamine, 10-undecen-l-ol (Sigma-Aldrich), 3,5-bis(tert-butyl)-4-hydroxyphenylpropionic acid (Ciba), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, 4-N,IM-dimethylaminopyridine, calcium hydride (VWR Scientific), and propylene (Matheson Gas) were used as received. Toluene (Wiley Organics) and xylene (Alfa Aesar) were distilled over sodium benzophenone under argon. The commercial PP (capacitor grade), includes a mono- or oligo-functional phenol antioxidant (0.5 wt%) and has a Melt Index of 0.5.

Synthesis of PP-OH Copolymers Using rac-Me2Si[2-Me-4-Ph(lnd)l2ZrCl2/MAO Catalyst

The OH group in 10-undecen-l-ol comonomer was protected by the silane group before the polymerization step. In a one-liter flask, equipped with a magnetic stirrer, 34.46g of 10-undecen-l-ol and 20.22g of triethylamine were dissolved into 500ml of THF. 21.72g of chlorotrimethylsilane was slowly introduced at room temperature. White powder appeared immediately. The suspension was stirred at 60 °C for 8 hours, and the white powders were filtered and removed. The resulting yellow solution was distilled under vacuum, and the fraction at 60 eC was redistilled over CaH2 before use.

In a typical polymerization reaction, a dried Parr 450 ml stainless autoclave equipped with a mechanical stirrer was charged with 75 ml toluene, 5ml MAO (5 wt% in toluene), and 2.0g 10-undecen-l-oxytrimethylsilane after purging with propylene gas. About 5 pmol rac-Me2Si[2-Me-4-Ph(lnd)]2ZrCh in the toluene solution was then syringed into the rapidly stirred solution under propylene pressure to initiate the polymerization. After 20 mins of reaction at 40 eC under 120 psi pressure of propylene gas, the polymer solution was quenched with methanol. The resulting product was washed with HCI/methanol (0.5M), methanol and THF 2 times each, then vacuum-dried at 60 °C. About 7.22 g of PP-OH copolymer was obtained with a catalytic activity of 4300 Kg of polymer/mol of Zr h. The comonomer content of the PP-OH copolymer is determined by integration of the peaks of the XH NMR spectrum of the copolymer.

Synthesis of PP-HP Copolymer

The PP-HP copolymer was synthesized by esterification of the PP-OH copolymer with 3,5-bis(tert-butyl)-4-hydroxyphenylpropionic acid in the presence of l-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC) and 4-l\l,N-dimethylaminopyridine (DMAP). In a typical reaction run, under an argon atmosphere, 5 g of PP-OH copolymer with 1 mol% of OH content was mixed with 1.9 g of 3,5-bis(tert-butyl)-4-hydroxyphenyl propionic acid, 0.19 g of DMAP, and 100 ml of toluene in a 500 mL round bottom flask equipped with a stirrer and a condenser. After adding 1.15 g of the EDC reagent, the esterification reaction was carried out at 110 °C for 12 hours. The resulting PP-HP copolymer was precipitated in 600 mL of methanol, then washed with methanol a few times before drying the polymer overnight in a vacuum oven at 70 °C. It could be seen by *H NMR spectroscopy that the esterification of the OH groups in PP-OH was quantitative. For example, a 1:1 peak intensity ratio was observed between the chemical shift at 4.28 ppm for-CH2-0-C(=0)- and a singlet chemical shift at 7 ppm for the two aromatic protons in the 3,5-bis(tert-butyl)-4-hydroxyphenylpropionate moiety.

Instrumentation and characterization

The thermal transition data was obtained with a TA Instruments 02000 differential scanning calorimeter (DSC) at a heating rate of 10 "C/min. Thermogravimetric (TGA) analysis was performed on the TA Instruments Q600. The molecular weights of the polymers were determined by intrinsic viscosity of polymer measured in decahydronaphthalene (Decalin) dilute solution at 135 °C with a Cannon-Ubbelohde viscometer. The viscosity molecular weight was calculated by the Mark-Houwink equation: [η]=ΚΜνα where K=1.05 x 10'4 and a=0.80. UV-visible spectroscopy was investigated by Perkin-Elmer Lambda 950 UV-VIS-NIR Spectrophotometer. The samples were prepared as film form with a thickness around 20-30 μπι. For the gel content test, all the samples were put into an oven for 24h at various temperatures from 150-210 ‘C under air condition. Then the samples were weighted to get Wi. Then, the samples were treated with refluxing xylene for 2h to remove all the soluble part. After that, the samples were put into vacuum oven over night to dry and then weighted to get W2. The gel content was calculated by the difference between the two weights. Gel Content = (Wi-W2)/Wixl00%.

Oxidative Crosslinking Reaction

As illustrated in Figure 2b, under elevated temperature in air, the PP-HP copolymer engages in a thermal/oxidation reaction with oxygen to form a crosslinked x-PP-HP network comprising conjugated bis-quinonemethide (crosslinker) moieties. UV-visible spectroscopy and gel content were used to monitor the thermal/oxidative induced crosslinking reaction in forming the x-PP-HP network. Figure 3 shows the UV-visible spectra of a PP-HP copolymer containing 1 mol% HP group during constant heating at various high temperatures for 24h under air flow condition, as shown in the Table below. The absorption peak (PI) at λ=275 nm, corresponding to the hindered phenol moiety, is gradually reduced with the presence of a new peak (P2) at λ=310 nm, corresponding to the conjugated bis-quinonemethide. Based on the peak intensity ratio, about 90 % of HP groups in PP-HP copolymer are interconverted to x-linkers after the consecutive heating at 150,170,190, and 210 °C for 24h each in air.

Gel content was examined in parallel with the UV-visible spectrum to understand the network structure in each resulting x-PP-HP copolymer. Figures 4a and 4b show the gel content of all heat-treated x-PP-HP samples from the same PP-HP copolymer with 1 mol% HP group content. Figure 4a shows the gel content after heating for 24h at a variety of temperatures. The gel content systematically increases with the increase of heating temperature. The higher the temperature, the greater the crosslinking density in the resulting x-PP-HP copolymer. At 210 eC for 24h in air, the resulting x-PP-HP sample is more-or-less completely insoluble. A small percentage of polymer may be lost during the solvent extraction, filtration, and isolation process. Figure 4b shows the gel content of the same PP-HP copolymer heating at 210 °C for various time periods of from lh to 24h. After lh of heat treatment, the PP-HP copolymer began to crosslink with about 1% insoluble fraction. After 17h of heating treatment, the PP-HP copolymer was nearly fully crosslinked, with 95% insoluble fraction.

The results of the UV-vis crosslinking determinations are quite consistent with the gel content evaluation of all heat-treated samples. It is expected that the experimental result in the gel content value (insoluble polymer fraction) shall be higher than the value of phenol antioxidant conversion to crosslinker as determined by UV-vis spectroscopy. It only requires several crosslinkers in each polymer chains to form a completely insoluble network. The combination of UV-vis and gel content results clearly show the presence of crosslinking reaction in the PP-HP copolymers, with the kinetic rate controlled by the heat treatment condition.

Mechanical Properties

The mechanical properties of the PP-HP crosslinked PP-HP (x-PP-HP) were determined by heat treating the PP-HP polymer at different temperatures, 150,170,190, and 210 °C for 24 hours in order to provide x-PP-HP samples with varying degrees of crosslinking. The PP-HP and x-PP-HP were compressed to form films having a thickness of 50-100 pm. These films were then tested using an Instron 5966 tensile tester. The test conditions used were ambient temperature and a displacement (extension) of 50 mm/min. At least 6 specimens were tested for each PP-HP and x-PP-HP sample. The results of the mechanical testing are shown in Figure 5 as stress/strain curves for each sample, and are summarised in the Table below.

It can be seen that the tensile strength of the samples increase with increasing degree of crosslinking. The sample treated for 24 hours at 210 °C, which according to Figure 4a thereby has a gel content of 98%, has a tensile strength of approximately 20 MPa, which is nearly double that of the non-crosslinked PP-HP. The strain-at-break on the other hand decreases with increasing degree of crosslinking.

Thermal/Qxidative Stability

Isothermal thermogravimetric analysis (TGA) in air was performed at 210 eC on samples of PP-HP. It should be noted that with analogy to the crosslinking results shown in Figure 4b, under the conditions prevailing during the TGA the PP-HP will quickly become crosslinked to an extent due to the oxidative dimerization of the phenol antioxidant moieties. Two commercially available stabilized polypropylenes were also tested as reference compounds in the TGA experiment. The results are shown in Figure 6.

It can be seen that the commercially available polymers (lines 601 and 602) are rapidly degraded, losing a significant proportion of their mass after less than 100 minutes. The PP-HP (line 603) on the other hand is much more stable under the test conditions, and has lost only 3% weight after 1000 minutes, by which time it will have a gel content in the region of 90% by analogy with Figure 4b.

Endurance under Elevated Operation Temperatures

The ASTM E1877 and E1641 standard methods were employed to determine PP-HP endurance time at 110 eC in air. The failure is defined at 1 wt% polymer weight loss. This method involves a series of TGA measurements with heating rates of 2.5 °C/min (line 701), 5 °C/min (line 702), 10 °C/min (line 703) and 20 eC/min (line 704). For comparison, a high quality commercial capacitor-grade PP polypropylene was also examined. The TGA curves for PP-HP are shown in Figure 7a.

Figure 7b shows the plots of log (heating rate) vs. inverse heating temperature (1000/T) under various specific polymer weight loss (conversion) conditions, based on the TGA curves in Figure 6. The slope of each line was used to calculated the activation energy (Ea) of each polymer weight loss (conversion during the polymer chain oxidation and degradation reaction), using the equation from ASTM 1877: Ea=-(R/b)*A log β/Δ(1/Τ); wherein Δ log /?/Δ(1/Τ) = slope of the line obtained in Figure 7b, β= heating rate (K/min), T = temperature (K) at constant conversion, gas constant R= 8.314J/(mol-K), and b= 0.457/K on the first iteration. The Table below summarizes the activation energy (Ea) for both commercial PP (capacitor grade) and PP-HP copolymer with various polymer weight loss conditions.

In all side-by-side weight loss comparisons, PP-HP copolymer shows significantly higher activation energy than commercial PP, indicating that PP-HP is far more resistant to oxygen oxidation and polymer chain degradation. Looking at the activation energy in detail, it is interesting to note that the different trend in activation energy vs. polymer weight loss occurs between the two cases. Commercial PP shows a systematic reduction of activation energy (Ea) with increasing loss of polymer weight (conversion) due to the continuous polymer chain degradation resulting in lower polymer molecular weight and higher chain mobility. On the other hand, for PP-HP the initial reduction of activation energy is somewhat recovered at high conversion levels (15-20% conversion). Without wishing to be bound by theory, this may be due to the in situ formation of crosslinks during oxidation, resulting in an x-PP-HP network that is more durable at elevated temperatures than the original non-crosslinked PP-HP.

With the activation energy (Ea), we can estimate the material endurance time under a specific value of conversion and failure temperature, following the ASTM 1877 equation Tf = Ea/(2.303 R[log tf —log{Ea/(R /?)}+a]); wherein a= approximation integral, tf= estimated life time, and Tf= failure temperature for a given value of conversion. Figure 8 plots the estimated endurance (lifetime) vs. application temperature in air for both commercial PP (capacitor grade) (line 801) and PP-HP (line 802) with 1 wt% polymer weight loss, assuming this weight loss level is acceptable in the application. It is clear that the PP-HP polymer shows much higher endurance than the commercial PP product in the whole elevated temperature range. The vertical line (803) in Figure 8 indicates that the material lifetime for PP is about 100 (10Λ2) days under 110 °C in air. On the other hand, the lifetime of PP-HP, under the same condition, is nearly 10000000 (10Λ7) days. This five-order increase in endurance is astonishing, and, without wishing to be bound by theory, clearly reinforces the idea of PP-bonded HP groups providing a combination chemical and physical protections, by not only preventing the PP chain from thermo-oxidative degradation but also by forming a crosslinked (3-D) x-PP-HP network structure. The experimental results raise the strong possibility that this new x-PP-HP polymer may address the concerns regarding the thermal and oxidative stability (aging issue) of PP polymers operated at elevated temperature conditions.

Dielectric properties

It is very interesting to understand the dielectric properties of the x-PP-HP copolymer. The dielectric constant (ε) and dielectric loss (%) of two x-PP-HP films having different degrees of crosslinking were determined. The films consisted of x-PP-HP copolymers with 1 mol% HP content, which is similar to a PP/Irganox 1010 mixture with 2.5 wt% Irganox 1010 content based on the HP antioxidant moiety content. The first x-PP-HP film was prepared by heating in air at 150 °C for 1 day (5% crosslinking), and the second x-PP-HP film was prepared by heating at the same temperature for 7 days (60% crosslinking). The dielectric measurements were carried out over a temperature range (from 25 to 125 °C) and two frequencies (IK and 10K Hz).

The first (5% crosslinked) x-PP-HP film has a dielectric constant of ε= 3 and the second (60% crosslinked) x-PP-HP film has a dielectric constant of ε=3.3. Therefore, both x-PP-HP films demonstrate a higher dielectric constant than a reference film (commercial PP including a mono- or oligo-functional phenol antioxidant (0.5 wt%)) which has a dielectric constant of ε=2.2. Despite the x-PP-HP copolymer only containing 1 mol% of HP groups, the dielectric effect is significant (~50% increase). The dielectric loss for both x-PP-HP films is less than 1%. Without wishing to be bound by theory, it seems that the x-PP-HP copolymer has a single phase morphology with the well-dispersed 1 mol% HP groups that can provide both ionic and electronic contributions to the dielectric properties, from their polar OH groups and polarizable π-electrons. Furthermore, by utilizing a longer heating time and therefore obtaining a higher degree of cross-linking, the resulting x-PP-HP sample shows a further increase of dielectric constant. This may be interpreted as the extensively π-conjugated bis-quinonemethide crosslinking group providing higher electronic polarization.

Claims

A crosslinked polyolefin copolymer comprising supernatant phenolic antioxidant moieties and characterized in that the crosslinked polyolefin copolymer further comprises crosslinking residues derived from dimerization of the phenolic antioxidant moieties, each crosslinking moiety having an intramolecular or intermolecular bond within the moiety Formed crosslinked polyolefin copolymer.

The crosslinked polyolefin copolymer of claim 1 wherein the crosslinked polyolefin copolymer comprises at least 5%, preferably at least 10% crosslinking, residues with respect to residual phenolic antioxidant moieties.

A crosslinked polyolefin copolymer according to any one of the preceding claims, wherein the crosslinked polyolefin copolymer comprises monomer units selected from ethylene, propylene, 1-butene, isobutylene, methylpentene, dicyclopentadiene, ethylidene norbornene and vinylnorbornene, preferably polypropylene monomer units.

A crosslinked polyolefin copolymer according to any one of the preceding claims, wherein the supernatant phenol antioxidant moieties each comprise a 3,5-di-t-butyl-4-hydroxytoluene residue or, preferably, a 3,5-di-t-butyl-4-hydroxyphenylpropionate residue ,

5. A crosslinked polyolefin copolymer according to any one of the preceding claims, wherein the crosslinked polyolefin copolymer comprises a total of from 0.2 to 10 mol%, preferably from 0.5 to 5 mol% of phenol antioxidant radicals and crosslinking radicals.

A polyolefin composition comprising a crosslinked polyolefin copolymer according to any one of claims 1 to 5.

The polyolefin composition of claim 6, wherein the polyolefin composition comprises 10% to 99.9% by weight crosslinked polyolefin copolymer, calculated relative to the total weight of the polyolefin composition.

A polyolefin composition according to any one of claims 6 to 7, further comprising a mono- or oligo-functional phenolic antioxidant, and / or further comprising a second polyolefin polymer.

The polyolefin composition of claim 8, wherein the mono- or oligofunctional phenolic antioxidant forms from 0.1% to 5% by weight of the total weight of the polyolefin composition, and / or wherein the second polyolefin polymer represents from 0.1% to 90% by weight of the Total weight of the polyolefin composition forms.

A process for producing a crosslinked polyolefin copolymer comprising the steps of: a) providing an un-crosslinked polyolefin copolymer comprising supernatant phenol antioxidant moieties; and b) subjecting the unvulcanized polyolefin copolymer to conditions sufficient to induce oxidative dimerization of the residual phenol-antioxidant moieties in the polyolefin copolymer to thereby form crosslinking moieties which, through dimerization of the phenol-antioxidant moieties, cause intra- or intermolecular bonds within of the crosslinked polyolefin copolymer.

The process of claim 10, wherein the conditions sufficient to induce oxidative dimerization of the residual phenol-antioxidant moieties include heating under an oxidative atmosphere, such as air.

The process of claim 11, wherein in step b) the un-crosslinked polyolefin copolymer is heated to at least 100 ° C for at least 2 hours.

The process of any one of claims 10 to 12, wherein the un-crosslinked polyolefin copolymer comprising residual phenolic antioxidant moieties is prepared by post-polymerization coupling of a functionalized polyolefin copolymer with a phenolic antioxidant.

A process according to any one of claims 10 to 12, wherein the un-crosslinked polyolefin copolymer comprising residual phenol-antioxidant groups is prepared by copolymerizing an olefin monomer together with a monomer aas-1 comprising a phenolic antioxidant moiety.

A film capacitor comprising a dielectric film, wherein the dielectric film comprises a crosslinked polyolefin copolymer according to any one of claims 1 to 5.

16. Component of a power supply system comprising an insulator, wherein the insulator comprises a crosslinked polyolefin copolymer according to any one of claims 1 to 5.