LU100184B1

LU100184B1 - Polyolefin copolymer and compositions thereof

Info

Publication number: LU100184B1
Application number: LU100184A
Authority: LU
Inventors: Linnea Petersson; Henrik Hillborg; Gang Zhang; T C Mike Chung
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 polyolefin copolymer comprising pendant phenol antioxidant groups extending from a polyolefin backbone, wherein the pendant phenol antioxidant groups have a structure according to formula I: (<fH2)n P (CH2)m wherein n is less than or equal to 8, m is greater than or equal to 0, and the phenol is optionally substituted with one or more groups selected from alkyl, alkoxy, hydroxy, amine, monoalkylamine and dialkylamine. The present invention also discloses a method of manufacturing such a polyolefin copolymer. The present invention further discloses film capacitors and electric power system components comprising such polyolefin copolymers

Description

Polyolefin copolymer and compositions thereof TECHNICAL FIELD

The present invention relates to 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 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). Although polypropylene has an intrinsically high dielectric strength and low dielectric loss, its dielectric constant is low (ε = 2.2), resulting in that BOPP-based film capacitors typically provide energy density only in the range of 1-2 J/cm3.

Another limitation of polypropylene is its propensity to degrade through oxidation due to exposure to heat and UV radiation. In order to limit or avoid such degradation, polypropylene is therefore 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 benign non-radical products. In this manner the propagation of the oxidation radical chain reaction is broken.

In order to address the intrinsically low dielectric constant of polypropylene and reduce its susceptibility towards oxidative degradation, modified polypropylene copolymers have been investigated. Zhang et al (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) have disclosed copolymer stabilizers containing a polypropylene backbone and several pendant hindered phenol groups. Some of these copolymers show significantly higher thermal-oxidative stability than the pristine polypropylene and commercial polypropylene products stabilized with conventional antioxidants. A film formed from the one of the copolymers (A-PP-HP-2) also showed a significantly higher dielectric constant (ε = 3) as compared to conventional polypropylene (ε = 2.2).

There remains a need for polymers with improved properties for use in film capacitors. SUMMARY OF THE INVENTION

The inventors of the present invention have identified deficiencies in prior art polymeric dielectrics. Although it is possible to increase the oxidative stability and dielectric constant of the polymeric materials by incorporating phenol antioxidants into the polymer, this leads to a corresponding increase in dielectric loss. Some copolymers with pendant phenols show dielectric losses (tan δ) of about 0.01 to about 0.03, which is significantly higher than the dielectric losses for conventionally stabilized polypropylene. High dielectric loss leads to heat being generated during operation, which deforms the polymer thin films and results in early breakdown. Increased losses will also result in more electricity being lost in the system, resulting in lower efficiency and higher operating costs.

It is thus an object of the present invention to provide polyolefin copolymers that have reduced dielectric loss whilst maintaining excellent resilience against oxidative degradation and high dielectric constant.

These objects are achieved by a polyolefin copolymer according to the appended claims.

The polyolefin copolymer comprises pendant phenol antioxidant groups extending from a polyolefin backbone, wherein the pendant phenol antioxidant groups have a structure according to formula I:

wherein n is less than or equal to 8, m is greater than or equal to 0, and the phenol is optionally substituted with one or more groups selected from alkyl, alkoxy, hydroxy, amine, monoalkylamine and dialkylamine.

By limiting the length of the linker moiety of the pendant phenol antioxidant group which connects the phenol antioxidant group to the backbone of the copolymer, the dielectric loss of the copolymer can be significantly reduced without significantly reducing the dielectric constant of the copolymer.

The phenol of the pendant phenol antioxidant group may be ortho-disubstituted (i.e. disubstituted ortho to the phenolic hydroxyl group) with a C1-C4 alkyl group, preferably a t-butyl group. This improves the antioxidant properties of the pendant phenol antioxidant groups and hence the oxidative stability of the polyolefin copolymer.

The alkyl spacer chain -(CFh)™- may have a value of m greater than or equal to 2, preferably equal to 2. Such a value of m again improves the antioxidant properties of the pendant phenol antioxidant groups and hence the oxidative stability of the polyolefin copolymer. Furthermore, this moiety can also engage in a facile crosslinking reaction to form a PP network during the oxidation reaction.

The linker moiety -(Chkjn- may have a value of n from 2 to 6, preferably 4. This is sufficiently short to reduce the dielectric loss of the polyolefin copolymer without negatively impacting the dielectric constant.

The polyolefin copolymer may comprise monomer units selected from ethylene, propylene, 1-butene, isobutylene, methylpentene, dicydopentadiene, ethylidene norbornene and vinyl norbornene. The polyolefin copolymer may preferably comprise polypropylene monomer units. Thus, the invention is readily applicable to a wide range of polyolefin copolymers.

The polyolefin copolymer may comprise from 0.2 to 10 mol % of pendant phenol antioxidant groups, preferably from 0.5 to 5 mol%. By ensuring a sufficient amount of pendant phenol antioxidant groups, the oxidative integrity of the polyolefin copolymer may be ensured, even for prolonged use in challenging environments.

The polyolefin copolymer may further comprise a crosslinking moiety derived from dimerization of the pendant phenol antioxidant groups, wherein each crosslinking moiety constitutes an intra- or intermolecular link within the polyolefin copolymer. By crosslinking in this manner, a polyolefin copolymer with enhanced mechanical integrity may be obtained.

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 polyolefin copolymer as described above. By utilizing the polyolefin copolymer in a composition, the dielectric and oxidative stability 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 polyolefin copolymer, calculated with respect to the total weight of the polyolefin composition. This provides compositions with improved resistance to oxidative degradation, high dielectric constant and low dielectric loss.

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 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 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 and a higher dielectric constant 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 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 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 polyolefin copolymer according to the appended claims. The method comprises the steps: a) providing a hydroxy-functional polyolefin copolymer comprising pendant hydroxyalkyl groups extending from a polyolefin backbone, wherein the pendant hydroxyalkyl groups have a structure -(GH2)n-OH, wherein n is less than or equal to 8; and b) coupling the hydroxy-functional polyolefin copolymer with a phenol antioxidant having a structure according to formula II, or an ester, salt, amide or acid halide thereof:

wherein m is greater than or equal to 0, and the phenol is optionally substituted with one or more groups selected from alkyl, alkoxy, hydroxy, amine, monoalkylamine and dialkylamine.

Such a method uses readily available reagents and is compatible with known polymerisation catalysts such as homogenous and heterogeneous Ziegler-Natta or metallocene catalysts. It also allows for the manufacture of copolymers having a tapered microstructure or copolymers having a random microstructure.

The hydroxy-functional polyolefin copolymer and the phenol antioxidant may be coupled by a Steglich esterification using an organic carbodiimide and Ν,Ν'-dimethylaminopyridine. This allows for a convenient, robust, quantitative coupling of the hydroxy-functional polyolefin copolymer with the phenol antioxidant.

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 polyolefin copolymer have a high dielectric constant, lower dielectric loss and excellent endurance at elevated temperatures. This means that film capacitors utilizing such films may have reduced dielectric loss combined with high maximum operating temperature, and therefore a lesser need for cooling during operation. The dielectric film may preferably comprise a 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 polyolefin copolymer as described herein. Insulator materials of the polyolefin copolymer have reduced dielectric loss as well as excellent resistance to oxidative degradation. 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 1 schematically illustrates a polypropylene copolymer comprising pendant 3,5-di-t- butyl-4-hydroxyphenyl propionate moieties bound to the polypropylene backbone by linker moieties -(CH2)n-

DETAILED DESCRIPTION

The present invention addresses the problem of high dielectric loss in polyolefins having incorporated pendant phenolic antioxidant groups.

Optimal dielectric films in thin film capacitors should be capable of having high energy densities whilst still exhibiting low dielectric loss. The linear energy density equation states that energy density (J/cm3) = ΛΑ εE2, where ε is the dielectric constant of the dielectric film and E is the applied electric field of the film. The applied electric field is in turn limited by the breakdown strength of the polymeric film. A contributing factor in breakdown strength, besides quality factors such as polymer molecular weight, morphology, impurities and defects, is the dielectric loss of the polymeric film, since the energy loss is manifested as heat which can deform the film and result in accelerated breakdown.

Losses during the charge-discharge cycle of the dielectric material are manifested as hysteresis in the D-E loop (electric displacement field D vs. electric field E). Dielectrics with little loss exhibit almost no hysteresis. This means that the polarization-depolarization cycle is almost fully reversible.

The present invention represents a step towards finding polymers that exhibit high dielectric constant whilst at the same time exhibiting low dielectric loss. The inventors of the present invention have discovered that by linking the incorporated phenol antioxidants to the copolymer backbone using alkyl chains that are shorter than those known in the art, the dielectric loss (tan S) of the copolymer may be reduced without significantly affecting the dielectric constant (ε).

The dielectric constant (ε) of a polymer is due to a combination of induced electronic polarization (o and π electrons) and segmental motion (including dipole orientation) of the polymer macromolecules. The induced electronic polarization is fast and highly reversible, whereas the segmental motion is slow and usually not fully reversible in the time scale (milliseconds) relevant for capacitor applications. Therefore, the slow randomization of the poled polar groups in the polymer can cause significant hysteresis in the D-E loop, i.e. dielectric loss.

Without wishing to be bound by theory, it is thought that copolymers having phenol antioxidants linked to the copolymer backbone using alkyl chains that are shorter than those known in the art exhibit a high dielectric constant due to good electronic polarization, whilst they at the same time exhibit a lower dielectric loss due to limited segmental motion.

The polyolefin copolymer comprises pendant phenol antioxidant groups extending from a polyolefin backbone. The polyolefin copolymer may comprise from 0.2 to 10 mol% of pendant phenol antioxidant groups, or may preferably comprise from 0.5 to 5 mol% of such groups. The mol% of pendant phenol antioxidant groups in the polyolefin copolymer is defined as the mol% of monomer units in the copolymer having pendant phenol antioxidant groups, 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 groups 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 groups 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 groups L extending from the backbone of the polypropylene). 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.

The pendant phenol antioxidant groups have the structure:

wherein L is an n-alkyl linker moiety extending from the backbone of the polyolefin copolymer, Sp is an optional n-alkyl spacer moiety, PhOH is a phenol moiety optionally substituted with one or more groups selected from alkyl, alkoxy, hydroxy, amine, monoalkylamine and dialkylamine. The linker moiety L comprises at most eight methylene moieties (n < 8), and may preferably comprise no more than 6 methylene moieties (n < 6). The linker moiety L may preferably comprise at least two methylene moieties (n > 2). The spacer moiety Sp is optional, i.e. the carboxyl group may be directly bound to the phenol as - 02C-Ph0H (m=0). However, if the spacer moiety is present it may comprise any number of methylene moieties (m > 1), such as from one to six methylene moieties (1 < m < 6). The spacer Sp may preferably be present and comprise two methylene moieties (m=2), i.e. together with the carboxyl group -CO2- and phenol moiety -PhOH form a 4-hydroxyphenylpropionate group. The phenol moiety may preferably be mono- or di-substituted ortho to the phenolic hydroxy group with a linear or branched C1-C4 alkyl group. The phenol moiety may most preferably be disubstituted ortho to the phenolic hydroxy group with t-butyl groups. Thus, the most preferred pendant antioxidant phenol group may comprise a linear C2-C6 alkyl linker extending from the polymer backbone and terminating in a 3,5-di-t-butyl-4-hydroxyphenylpropionate group (2 < n < 6, m=2).

The polyolefin copolymer may comprise further pendant groups resulting from the use of an olefin in the co-polymerisation reaction forming the polyolefin copolymer. For example, if propylene is used as a co-monomer in the polymerization then methyl pendant groups will be formed.

The polyolefin copolymer may be produced by coupling a hydroxy-functional polyolefin with a phenol antioxidant, as described below. However, other methods of producing the polyolefin copolymer are also conceivable, such as one-pot polymerization of one or more olefins together with an alpha-olefin monomer functionalized with a phenol pendant antioxidant group.

The hydroxy-functional polyolefin comprises pendant hydroxyalkyl groups extending from a polyolefin backbone, wherein the pendant hydroxyalkyl groups have a structure -(CH2)n-OH, wherein n is less than or equal to 8 . It is produced by co-polymerization of one or more olefins together with a hydroxy-functionalized olefin, or a masked or protected derivative thereof. Suitable olefins include, but are not limited to, ethylene, propylene, 1-butene, isobutylene, methylpentene, dicyclopentadiene, ethylidene norbornene and vinyl norbornene. Propylene is preferred since BOPP is the most commonly used material in film capacitors. The hydroxy-functionalized olefin is a straight-chain alpha C3-C10 olefin having a single hydroxy group on the carbon atom most remote from the double bond. Suitable hydroxy-functionalized olefins include 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 alcohol functionality may be masked as for example a borane group, or protected using for example a silane group prior to polymerization. In such a case, a de-masking/de-protection step will be necessary after the polymerization and prior to coupling with the phenol antioxidant.

Suitable polymerization conditions and catalysts may be determined by the person skilled in the art. Heterogeneous Ziegler-Natta catalysts such as TiCI3 may be used, as may homogeneous metallocene catalysts such as zirconium metallocenes, such as the rac-Me2Si[2-Me-4-Ph(lnd)]2ZrCl2/MAO catalyst system. Depending on the catalyst used, copolymers with a random microstructure (i.e. co-monomer randomly distributed in polyolefin chain), or copolymers with a tapered microstructure (i.e. co-monomers frequency increasing towards one or more ends of the chain) may be obtained.

The one or more olefins and hydroxy-functionalized olefin are used in a ratio sufficient to obtain from 0.2 to 10 mol% of hydroxy functionalities, preferably from 0.5 to 5 mol%.

After production of the hydroxy-functional polyolefin, it should be coupled with the phenol antioxidant. The phenol antioxidant comprises a carboxylic acid functionality (or derivative thereof, such as a salt, ester, amide or acid halide), which forms an ester bond with a hydroxy group of the hydroxy-functional polyolefin. The phenol antioxidant may preferably be 3,5-di-t-butyl-4-hydroxyphenylpropionic acid, or a salt, ester, amide or acid halide thereof.

The hydroxy-functional polyolefin and phenol antioxidant may be coupled using any known ester-coupling chemistry. One such known method is the Steglich esterification which uses an alkyl carbodiimide such as dicyclohexylcarbodiimide (DCC) or l-Ethyl-3-(3-dimethylaminopropyQcarbodiimide (EDC) as a coupling reagent, together with 4-dimethylaminopyridine (DMAP) as an esterification catalyst.

The polyolefin copolymer once produced may optionally be crosslinked by exposure to oxidative conditions, such as by heating under air. This causes dimerization of some pendant phenol antioxidant groups which thereby constitute intra- or intermolecular link within the crosslinked polyolefin copolymer. The degree of crosslinking may be controlled by regulating the duration and temperature of heating.

The polyolefin copolymer may be used in polyolefin compositions to provide compositions having superior dielectric 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 polyolefin copolymer of the invention may be added to polyolefin homopolymer compositions comprising commercial mono- or oligo-functional phenol antioxidants. In such a case, the 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 and/or improved dielectric properties.

The dielectric properties, mechanical properties and resistance to oxidative degradation make the 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 having lower levels of dielectric loss. Such applications include insulation materials for power cables, cable joints, bushings and cable terminations.

Examples

Atomistic molecular dynamics (MD) simulations were performed in order to determine the dielectric constant (ε) and dielectric loss (tan δ), as described below.

The systems studied were polypropylene comprising 1-2 wt% of Irganox 1010; and polypropylene copolymers (PP-HP) comprising 1-2 wt% of pendant phenol antioxidant groups having a structure according to Figure 1 and formula (III) below. Percent by weight (wt%) is defined relative to the total weight of the system studied. The polypropylene copolymers studied had n=4 and n=8.

MD simulation of dielectric properties

Classical molecular dynamics (MD) simulations were performed in order to determine the dielectric constant (ε) and dielectric loss (tan δ). In MD simulations a molecular system of desired resolution, in this cases atomistic, is simulated in real-time by integrating Newton's equations of motion. The polymeric system was modelled by using 10 polymer chains with 500 repeating units. For the PP-HP simulations the active groups was placed at random positions along the polymeric chains with the given concentrations. The OPLS-AA force field [1] was used to throughout the simulations. The Particle Mesh Ewald (PME) method [2] was used to describe the electrostatic interactions with a real-space cut-off of 1.0 nm and 0.12 nm Fourier spacing. Van der Waals interactions were treated with a Lennard-Jones potential with a cut-off of 1.0 nm with the corrections for the potential and pressure added in order to eliminate the cut-off dependency. Periodic boundary conditions were used in every dimension. All covalent bonds were constrained using the LINCS algorithm [3], enabling the use of larger time steps in the integration of the equations of motion. A Leap-Frog integrator [4] was used to integrate the equations of motion with a time step of 2 fs. All MD simulations were performed using Gromacs 4.5.5 [5].

All systems were prepared using the following procedure: a large simulation box (100 x 100 x 100 nm) was filled with the comprising molecules, after which an energy minimization was performed. In order to obtain the correct box size MD simulations in the NPT-ensemble were performed. These compression simulations lasted for 50 ns and were performed at a temperature of 298 K, maintained by the Berendsen thermostat [6] with a coupling constant of 1.0 ps and the Berendsen barostat [6] with a pressure of 1000 bar and a coupling constant of 10 ps. Once the systems had been compressed to a reasonable size the simulation boxes were rescaled to correspond to the experimental densities, followed by 500 ns long simulations in the NVT-ensemble using the Berendsen thermostat with the same settings as mentioned before. Once the systems were equilibrated production simulations were performed in the NVT-ensemble, employing the Bussi's velocity scaling thermostat [7] with a coupling constant of 2.0 ps. Production simulations were run for 1000 ns each and the total dipole moment of the system was collected every 0.5 ps. From the total dipole moment, M, the dielectric constant was determined according to

Where kB is the Boltzmann constant and T is the absolute temperature. The factor 2.24 is the experimental dielectric constant of PP and is added to compensate for the electronic contribution to ε(0) from the polymer (which is not explicitly included). The dielectric losses can be computed indirectly from the autocorrelation function, Φ(ί), of the total dipole moment

Then ¢(¢) is fitted to a so-called double stretched exponential function

From this fitted function the complex dielectric constant

can be computed by Fourier transforming the negative derivative of Φfit [8]

From this equation it is clear, after applying Euler's rule, that the frequency dependent dielectric constants, real and imaginary are [8]

and

where ε(0) is the dielectric constant at a frequency of zero and is the limiting higher frequency dielectric constant.

Results of MD Simulations

The results of the MD simulations are shown in the table below. PP-HP denotes a polypropylene-hindered phenol copolymer in accordance with the present invention.

It can be seen that both polypropylene copolymers having pendant phenol antioxidant groups have a higher dielectric constant and higher dielectric loss than conventionally stabilized polypropylene (PP + Irganox 1010). What is of greater interest however is the difference between the polypropylene copolymers (PP-HP) having n=4 and n=8. It can be seen that at both 1 wt% and 2 wt% of pendant phenol groups the dielectric constant of PP-HP with n=4 is more or less the same as for n=8. However, PP-HP having n=4 has a much lower dielectric loss than PP-HP having n=8 for any given amount of phenol antioxidant groups. The effect is especially pronounced for PP-HP having 2wt% pendant phenol antioxidant groups. On shortening the linker chain from n=8 to n=4, the dielectric constant is reduced by less than one percent (3.17 vs. 3.14), whereas the dielectric loss is decreased by a substantial 29% (24.1 vs 17.0).

Cited references [1] W. L. Jorgensen, D. S. Maxwell and J. Tirado-Rives, "Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids," J. Am. Chem. Soc., vol. 118, no. 45, pp. 11225-11236,1996.

[2] "Essmann, Ulrich; Perera, Lalith; Berkowitz, Max L.; Darden, Tom; Lee, Hsing; Pedersen, Lee G.," J. Chem. Phys., vol. 103, no. 19, pp. 8577-8593,1995.

[3] B. Hess, "P-LINCS: A Parallel Linear Constraint Solver for Molecular Simulation," J. Chem. Theory Comput., vol. 4, no. 1, pp. 116-122, 2007.

[4] D. Frenkel and B. Smit, Understanding Molecular Simulation, San Diego: Academic Press, 2002.

[5] B. Hess, C. Kutzner, D. van der Spoel and E. Lindahl, "GROMACS 4: Algorithms for Highly Efficient, Load-Balanced, and Scalable Molecular Simulation," J. Chem. Theory Comput., vol. 4, no. 3, pp. 435-447, 2008.

[6] H. J. C. Berendsen, J. P. M. Postma, W. F. van Gunsteren, A. DiNola and J. R. Haak, "Molecular dynamics with coupling to an external bath," J. Chem. Phys, vol. 81, p. 3684,1984.

[7] G. Bussi, D. Donadio, Parrinello and Michele, "Canonical sampling through velocity rescaling," J. Chem. Phys., vol. 126, p. 014101, 2007.

[8] G. Williams, "The use if the dipole correlation function in dielectric relaxation," Chemical Review, vol. 72, pp. 55-69,1972.

Claims

A polyolefin copolymer comprising supernatant phenolic antioxidant groups extending from a polyolefin backbone, the supernatant phenolic antioxidant groups having a structure according to the following Formula I:

wherein n is less than or equal to 8, m is greater than or equal to 0, and the phenol is optionally substituted with one or more Grappen selected from alkyl, alkoxy, hydroxy, amine, monoalkylamine and dialkylamine.

2. Polyolefin copolymer according to spoke 1, wherein the phenol is disubstituted in the ortho position with a Ci-C4-Alkylgrappe, preferably a t-Butylgrappe.

3. Polyolefin copolymer according to one of the preceding claims, wherein m is greater than or equal to 2, preferably equal to 2.

4. A polyolefin copolymer according to any one of the preceding claims, wherein n is from 2 to 6, preferably 4.

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

A polyolefin copolymer according to any one of the preceding claims, wherein the polyolefin copolymer comprises 0.2 to 10% by mole, preferably 0.5 to 5% by mole of phenol antioxidant supernatants.

A polyolefin copolymer according to any one of the preceding claims, further comprising a crosslinking residue derived from a dimerization of the phenolic antioxidant groups, each crosslinking residue forming an intra- or intermolecular bond within the polyolefin copolymer.

A polyolefin composition comprising a polyolefin copolymer according to any one of claims 1 to 7.

The polyolefin composition of claim 8, wherein the polyolefin composition comprises from 10% to 99.9% by weight of polyolefin copolymer, calculated relative to the total weight of the polyolefin composition.

The polyolefin composition of any one of claims 8 to 9, further comprising a mono- or oligofunctional phenolic antioxidant, and / or further comprising a second polyolefin polymer.

The polyolefin composition of claim 10, wherein the mono- or oligo-functional 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 the preparation of a polyolefin copolymer, comprising the steps of: a) providing a hydroxy-functional polyolefin copolymer comprising supernatant hydroxyalkyl groups extending from a polyolefin backbone, wherein the supernatant hydroxyalkyl groups have a structure - (CH 2) n- OH, where n is less than or equal to 8; and b) coupling the hydroxy-functional polyolefin copolymer with a phenol antioxidant having a structure according to formula II or an ester, salt, amide or acid halide thereof:

wherein m is greater than or equal to 0 and the phenol is optionally substituted with one oddf of a plurality of groups selected from alkyl, alkoxy, hydroxy, amine, monoalkylamine and dialkylamine.

The process of claim 12, wherein the hydroxy-functional polyolefin copolymer and the phenolic antioxidant are coupled by a Stlichlich esterification using an organic carbodiimide and N, N'-dimethylaminopyridine.

14. A film capacitor comprising a dielectric film, wherein the dielectric film comprises a polyolefin copolymer according to any one of claims 1 to 7.

A component of a power supply system comprising an insulator, the insulator comprising a polyolefin copolymer according to any one of claims 1 to 7.