Patent Description:
BOPP films have been widely used in applications of high voltage insulation in power capacitors. The electrical breakdown of a dielectric film of the capacitor is irreversible, thus it is important that high dielectric performance is guaranteed. The dielectric performance of BOPP film is mainly determined by its crystallite structure and crystallinity. More specifically, a uniform distributed crystallite with high crystallinity generally leads to high dielectric performance. The molecular weight distribution (MWD) of BOPP film plays an important role in the crystallite structure, and a narrower NWD tends to generate uniform crystallite and high crystallinity.

<CIT> describes a polypropylene for a film capacitor or a polypropylene sheet for a film capacitor which exhibits excellent stretchability when stretched into a film and provides a film having high breakdown voltage and small thermal shrinkage ratio.

<CIT> describes a method of manufacturing a polypropylene film for film capacitors including a process for manufacturing a propylene homopolymer, where a melt flow rate (MFR) measured with a load of <NUM> at <NUM> conforming to ASTM D1238 is within a range of <NUM>-<NUM>/<NUM> minutes.

JPS <NUM> A describes a capacitor comprising a capacitor element in which a dielectric layer of a film is laminated between electrode foils and wound.

<CIT> describes that a polypropylene film can be produced with a structured surface by first subjecting a surface of the (thicker) film to electron irradiation and then permanently stretching the film biaxially, so that static irregularities are produced on the irradiated surface.

At present, the raw material of polypropylene (PP) used to produce BOPP film has generally a broad molecular weight distribution. The broad MWD is important for the fabrication of BOPP film, indicating that PP components of both high molecular weight (MW) and low molecular weight are required. The component of low MW endows polymer melt with a good flowing property for the extrusion process, while the component of high MW provides adequate melt strength to ensure the stability of film formation. However, for the well-prepared BOPP film, the broad NWD is unfavorable to generate uniform crystallite and high crystallinity, as it would hinder the molecular chains from regularly arranging into crystal lattice.

Accordingly, there is a need for modifying the molecular weight distribution, and improving the crystallites structure and crystallinity of BOPP film.

Embodiments of the invention are set forth with the appended dependent claims.

In general, example embodiments of the present disclosure provide a biaxially oriented polypropylene film, a power capacitor, and associated methods.

In a first aspect, there is provided a biaxially oriented polypropylene film for use in a power capacitor. The biaxially oriented polypropylene film has a reduced molecular weight distribution in a range of <NUM> to <NUM> as a result of a radiation treatment.

In some embodiments, the power capacitor is of an oil-immersed capacitor.

In some embodiments, the power capacitor is of a dry-type metalized film capacitor; and the power capacitor is further obtained as a result of metalizing treatment before or after the radiation treatment.

In some embodiments, the radiation treatment uses radiation source as of an electron beam source or a gamma ray source.

In some embodiments, the molecular weight distribution of the biaxially oriented polypropylene film is in a range of <NUM> to <NUM>.

In some embodiments, a dosage of the radiation treatment is in a range of 1kGy to 300kGy, preferably 10kGy to 100kGy.

In some embodiments, the biaxially oriented polypropylene film is obtained by biaxially stretching a polypropylene layer before the radiation treatment.

In a second aspect, there is provided a power capacitor. The power capacitor comprises the biaxially oriented polypropylene film of the first aspect.

In a third aspect, there is provided a method for processing a biaxially oriented polypropylene film for use in a power capacitor. The method comprises treating the biaxially oriented polypropylene film by radiation so as to reduce a molecular weight distribution of the biaxially oriented polypropylene film into a range of <NUM> to <NUM>.

In some embodiments, the power capacitor is of a dry-type metalized film capacitor; and the method further comprises: metalizing the biaxially oriented polypropylene film before or after the radiation treatment.

In some embodiments, bi-axially stretching a polypropylene layer before the radiation treatment.

In a fourth aspect, there is provided a manufacturing method. The manufacturing method comprises the method for processing the biaxially oriented polypropylene film of the third aspect and winding up the radiation-treated biaxially oriented polypropylene film into a shape of roll. A radiation source of the radiation treatment is configured to move substantially parallel to an axial direction of the winding.

In a fifth aspect, there is provide a system for processing a biaxially oriented polypropylene film for use in a power capacitor. The system comprises a radiation source configured to treat the biaxially oriented polypropylene film by radiation so as to reduce a molecular weight distribution of the biaxially oriented polypropylene film into a range of <NUM> to <NUM>.

In some embodiments, the power capacitor is of a dry-type metalized film capacitor; and the system further comprises: a metallization device configured to metalize the biaxially oriented polypropylene film before or after the radiation treatment.

In some embodiments, the radiation source is an electron beam source or a gamma ray source.

In some embodiments, the molecular weight distribution of the biaxially oriented polypropylene film is in the range of <NUM> to <NUM>.

In some embodiments, the system further comprises a stretching device configured to bi-axially stretch a polypropylene layer before the radiation treatment.

In a sixth aspect, there is provided a manufacturing system. The manufacturing system comprises the system for processing the biaxially oriented polypropylene film according to the fifth aspect; and at least one roller configured to wind up the radiation-treated biaxially oriented polypropylene film into a shape of roll; wherein the radiation source is configured to move substantially parallel to an axial direction of the at least one roller.

Features of the present invention as set forth with the appended claims will become easily comprehensible through the following description.

Through the more detailed description of some embodiments of the present disclosure in the accompanying drawings, the above and other objects, features and advantages of the present disclosure will become more apparent, wherein:.

It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present invention as set forth with the appended claims. The disclosure described herein can be implemented in various manners other than the ones describe below.

As used herein, the term "includes" and its variants are to be read as open terms that mean "includes, but is not limited to. " The term "based on" is to be read as "based at least in part on. " The term "one embodiment" and "an embodiment" are to be read as "at least one embodiment. " The term "another embodiment" is to be read as "at least one other embodiment. " Other definitions, explicit and implicit, may be included below.

It is well known in the art that radiation process radiation process can induce the crosslinking or degradation of polypropylene, depending on whether effective crosslinking agent is present or not. Without crosslinking agent, the polypropylene may be radiation-treated for trash recycling. However, the inventors discovered that the radiation processing exhibits a significant effect on enhancing the dielectric strength of BOPP film, which may be used for both oil-impregnated capacitor and metalized-film capacitor. The BOPP film has demonstrated an outstanding insulating property used for power capacitor.

The radiation-induced narrower NWD that leads to the dielectric strength improvement. As far as the crystallization kinetics is concerned, a narrower MW distribution facilitates the crystallization process, as uniform and regular polymer chains can arrange into crystal lattice more easily. The electrical breakdown tends to occur in the amorphous regions in BOPP film. Therefore the increase in crystalline region will benefit to the dielectric strength improvement.

<FIG> is a flow chart illustrating a method <NUM> for processing a BOPP film for use in a power capacitor in accordance with embodiments of the present disclosure.

In some embodiments, at block <NUM>, the BOPP film may be obtained by bi-axially stretching a polypropylene layer. The block <NUM> may be implemented by a stretching device. In this way, the method <NUM> may be incorporated to a production line for manufacturing BOPP film. For example, the polypropylene layer may be formed by casting, calendaring, or hot-pressing a raw polypropylene material. It is to be understood that the block <NUM> is optional and can be omitted in some implementations. For example, the BOPP film may be commercially available.

At block <NUM>, the BOPP film is treated by radiation such that a molecular weight distribution of the BOPP film can be reduced to a range of <NUM> to <NUM>, preferably of <NUM> to <NUM>. The radiation processing may result in scission of polymer chains, and then crystallite reorganization by the broken chains occurs. As a result, the crystallite reorganization leads to a crystallinity increase and morphology perfection, which further induces an increased dielectric strength.

At block <NUM>, an electron beam source or a gamma ray source may be used as the radiation source. For example, the electron beam source may include a high power electron generator. The gamma ray source may be a Co-<NUM> source, for example. The dosage of the radiation treatment may be in a range of 1kGy to 300kGy, preferably 10kGy to 100kGy.

The power capacitor may be an oil-immersed capacitor or oil-impregnated capacitor. For example, the BOPP film may be a rough BOPP film. The power capacitor may also be a dry-type metalized film capacitor. For example, the BOPP film may be a smooth BOPP film. The method <NUM> may further comprise metalizing the biaxially oriented polypropylene film before or after the radiation treatment. The metallization may be achieved by any method currently known or to be developed in the further, for example, plating, coating, and/or the like.

The BOPP film may have a shape of roll and the radiation treatment may be implemented on the roll. However, due to the limited penetration depth of radiation source, especially electron beam, the radiation treatment may result in a non-uniform improvement in the dielectric performance of the BOPP film. Accordingly, embodiments of the present disclose provide a manufacturing method and system for improving the uniformity of radiation treatment.

<FIG> is a schematic diagram illustrating a side view of a manufacturing system <NUM> in accordance with embodiments of the present disclosure, and <FIG> is a schematic diagram illustrating another view of the manufacturing system <NUM> seen from left side of <FIG>. The manufacturing system <NUM> includes a radiation source <NUM> that generates and emits a radiation, for example, a high energy radiation such as an electron beam radiation or a gamma ray radiation. <FIG> and <FIG> will now be described with reference to an electron beam radiation and it is to be understood that any other suitable radiation source may be used instead.

The BOPP film <NUM> is transferred from a first roller <NUM>, a second roller <NUM>, and winded up by a third roller <NUM> to a shape of a roll. During this process, a layer of the BOPP film <NUM> is treated by the radiation from the radiation source <NUM>. The radiation treatment may be performed on thin BOPP film (for example a single layer) such that the radiation effect will not be affected by the limited penetrating depth of the electron beam. The radiation source <NUM> may be configured such that the radiation can move substantially parallel to the axial direction of the roller <NUM>, as shown in <FIG>. The movement of the electron beam enables the radiation on a broad width of BOPP film, for example, <NUM>-<NUM> meters. In this way, the manufacturing system <NUM> can be easily incorporated into a commercial production line. For example, the BOPP film <NUM> may be biaxially stretched upstream in the production line.

As shown in <FIG>, the radiation source <NUM> may include a high voltage generator <NUM> and a cathode accelerator tube <NUM>. For example, the cathode accelerator tube <NUM> may generate a fan-shaped electron beam <NUM>. The generation of fan-shaped beam may facilitate a uniform radiation of the radiation. The cathode accelerator tube <NUM> may be configured to move substantially parallel to the axial direction of the roller <NUM>, as shown in <FIG>. For example, the fan-shaped electron beam <NUM> may be moved to a new location as indicated by the fan-shaped electron beam <NUM>'. The high voltage generator <NUM> can generate an electron beam with beam energy up to <NUM> keV to provide a radiation dosage in a range of <NUM> kGy-<NUM> kGy, preferably <NUM> kGy-<NUM> kGy on the BOPP film <NUM>.

<FIG> is a flow chart illustrating a manufacturing method <NUM> in accordance with embodiments of the present disclosure. The method <NUM> may be implemented by the manufacturing system <NUM>. At block <NUM>, the BOPP film may be treated by radiation as described above with reference to <FIG>.

At block <NUM>, the radiation-treated BOPP film is winded up into a shape of roll. The radiation source of the radiation treatment is configured to move substantially parallel to an axial direction of the winding.

Some detailed examples and experimental results will now be described. It is to be understood that the examples are described only for the purpose of illustration without suggesting any limitations as to the scope of the present disclosure.

At present, there are mainly two types of ionizing radiation sources in industrial applications, i.e. electron beam and gamma-ray (Co-<NUM>). Both electron beam and gamma-ray have their benefits and drawbacks for modifying BOPP film, which have been summarized in Table <NUM>.

It is known that electron beam radiation has a very high processing efficiency, due to the high-power electron generator. However, electron beam has a rather limited penetration in the target object. The penetration depth s can be calculated according to Equation (<NUM>): <MAT> where E represents the radiation energy, and ρ represents the density of the product being irradiated. The estimated penetration depth of electron beam is around <NUM>. Hence as far as electron beam radiation processing is concerned, the total thickness of roll film should be no larger than about <NUM>. In an example, rough BOPP film of a small roll was treated with electron beam radiation. In order to obtain a uniform radiation inside roll, a multistep radiation is required. For example, an octagon holder as shown in <FIG> may be used to fix the roll during the radiation.

In each time radiation, a small dosage was given on the roll in one direction of the octagon. Some dosimeters (size <NUM> × <NUM> × <NUM>) were inserted in the roll to measure the received doses at different positions (cylinder core, cylinder middle, and cylinder outside).

Gamma-ray has a much larger penetration depth than electron beam. The estimated penetration depth is over <NUM>, which enables gamma-ray to treat a big roll of commercial film, as shown in <FIG>. As shown in <FIG>, eight rolls are treated simultaneously and the arrows indicate the gamma ray direction. On the other hand, gamma-ray has also a drawback of low radiation processing efficiency, indicating a long radiation time and increased processing cost.

The roll of rough BOPP films for electron beam treatment has a length of <NUM>, an inner diameter of <NUM>, and an outer diameter of <NUM>. The roll of rough BOPP films for gamma-ray treatment has a length of <NUM>, an inner diameter of <NUM>, and an outer diameter of <NUM>.

Breakdown test on irradiated BOPP film was first carried out in air with a voltage ramp of <NUM> V/s. The bottom (GND) electrode was composed of a steel plate; the top electrode was made of aluminum (Al) foil (electrode area: ~<NUM><NUM>). At least <NUM> individual measurements were performed for each sample. Table <NUM> shows the test results. It can be seen that both electron beam and gamma-ray led to a significant increase (nearly +<NUM>%) of dielectric strength. Furthermore, it was observed that the dielectric strength increased with the increase of radiation doses.

It is known that the radiation dosage will be attenuated along penetration direction when the roll film is irradiated. Hence an examination on the different positions of roll is important to identify the radiation uniformity. Table <NUM> shows the breakdown test results at different positions of the roll. A decreased dielectric strength was observed along the dosage attenuation direction.

The stability of the dielectric performance of treated film was examined by an annealing test. The annealing test was firstly performed at <NUM> for <NUM> hr, then at <NUM> for <NUM> hr. No obvious change of dielectric strength was found after the annealing treatment. The significant improvement still observed after the annealing.

As it is more important for rough film to be examined in fluids than in the air, a breakdown test of treated film in capacitor fluid (blend of Monobenzyl toluene and Dibenzyl toluene) was carried out. The rough films were immersed in capacitor fluid for <NUM> hr for fully swelling before breakdown test. Table <NUM> shows the breakdown test results in the capacitor fluid. A significant increase of <NUM>%+ dielectric strength was observed for the modified BOPP film in fluid, indicating a favorable application in oil-impregnated capacitors.

The selected roll of smooth film has a much larger size (~<NUM>) than rough film, and the roll has a length of <NUM>, an inner diameter of <NUM>, and an outer diameter of <NUM>. Table <NUM> shows the breakdown test results of irradiated smooth film. A considerable increase of dielectric strength was observed for the irradiated film. However, due to the large roll size of smooth film, there is wide dosage dispersity inside the roll, and the improvement of dielectric strength was not as large as the rough film accordingly.

A series of characterizations were performed on irradiated film so as to obtain the possible mechanism behind the performance improvement.

The gel content was measured on the irradiated film. An <NUM>-hr reflux of irradiated film in xylene solvent at <NUM> was carried out before the measurement. Table <NUM> shows the measured gel content for BOPP film before and after the radiation. A quite similar gel content was observed for irradiated film compared with reference film, indicating that no crosslinking reaction occurred during the radiation processing.

MWD measurement was performed on irradiated film through a GPC (gel permeation chromatography) methodology. Table <NUM> shows the measured results of parameters Mw, Mn, and molecular weight distribution (Mw/Mn). A broad MWD (Mw/Mn of the first type and the second type is <NUM> and <NUM>, respectively) was found for reference film. Nevertheless, a much lowered MWD was observed for the modified film after radiation. Moreover, the MWD decreased with the increase of radiation doses (the roll outside has a higher dosage than roll middle and roll inside).

DSC measurement was performed to characterize the thermal parameters for the irradiated film. For DSC measurement, BOPP film (<NUM>-<NUM>) was heated from <NUM> to <NUM> at a scanning rate of <NUM>/min under a nitrogen atmosphere to test its melting behavior. Table <NUM> shows the measured results. An increased degree of crystallinity (Xc) was observed for the irradiated film compared to reference film before radiation. On the other hand, a slight decrease of melting temperature (Tm) was also observed.

The mechanical strength (including tensile strength and elongation at break) of irradiated film was performed by using a universal materials testing machine. The measured data were compiled in Table <NUM>. A slightly decreased mechanical strength and elongation rate were observed in the roll outside, while comparable mechanical property observed in the roll middle and roll inside.

FTIR spectra of irradiated film and the reference film show that no obvious difference was observed for BOPP film before and after radiation processing.

It was observed that the radiation processing can greatly enhance the dielectric strength of BOPP film. Furthermore, the dielectric strength has a close relationship with the radiation dosage, i.e. the dielectric strength increases with the increase of the dosage. From the results of gel-content test and MWD measurement, it can be concluded that no crosslinking but a breakdown of long polymer chains has occurred during the radiation. The molecular breakdown indicated a lowered MWD in the treated film. Hence, it is the radiation-induced narrower MWD that leads to the dielectric strength improvement. As far as the crystallization kinetics is concerned, a narrower MWD facilitates the crystallization process, as uniform and regular polymer chains can arrange into crystal lattice more easily. The experimental results have demonstrated that the irradiated film has a higher crystallinity, which indicates a crystallite reorganization and perfection (increased crystalline region and decreased amorphous region) occurred in the irradiated film, which could be the main cause of the dielectric strength improvement. It is believed that the electrical breakdown tends to occur in the amorphous regions in BOPP film. Therefore the increase in crystalline region will benefit to the dielectric strength improvement.

In order to examine the effect of radiation on thin film, an E-beam irradiation was performed on single-layer commercial film. Table <NUM> showed the examined results. For both smooth film and rough film, a significant increase of dielectric strength was observed after radiation, and the dielectric strength increased with the increase of radiation dosage. The results indicate that the irradiated thin film is favorable to be applied in both oil-impregnated capacitor and metallized film capacitor.

Claim 1:
A biaxially oriented polypropylene film (<NUM>) for use in a power capacitor, having a reduced molecular weight distribution Mw/Mn in a range of <NUM> to <NUM> as a result of a radiation treatment of the biaxially oriented polypropylene film (<NUM>) using ionizing radiation sources, whereby
the molecular weight distribution Mw/Mn is defined by the quotient of the weight-average molecular weight Mw to the number-average molecular weight Mn and
Mw and Mn are determined by gel permeation chromatography.