JP2013044536A - Porosity measuring method for porous resin sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure a state of submicron-size holes of a porous resin sheet for a battery separator in a noncontact manner, and contribute to quality improvement by performing in-line measurement on porosity and air permeability resistance related to the porosity in a film forming step and a slit or processing step.SOLUTION: A characteristic value correlated with porosity is obtained by using the multiple scattering of minute holes and the change of sonic velocity for each frequency, so that the porosity of a porous resin sheet is measured. In addition, a manufacturing method for a porous resin film having the measuring method is provided.

Description

  The present invention relates to a method for measuring the porosity of a porous resin sheet and a method for producing a porous resin sheet.

  Porous films in which polyolefin films such as polypropylene films and polyethylene films are microporous have been developed for separators of lithium ion secondary batteries and electrolytic capacitors because of their air permeability and low specific gravity.

  The separator for a lithium ion secondary battery has a function of ensuring insulation between the positive electrode and the negative electrode of the battery and permeating the electrolyte impregnated in the separator from the positive electrode to the negative electrode. That is, since the formation and density of the vacancies that allow lithium ions to permeate affect the characteristics of the battery, the porosity, which is the ratio of the vacancies in the resin sheet, is an important quality item. The measured value resulting from the structure of the porous resin sheet such as the porosity has air permeability resistance. Air permeation resistance is an index of the ease of permeation of air or ions that permeate the air holes.

  Here, the following method is known as a quantification method of the porosity that is an index of porosity, that is, the ratio of the void volume of the resin sheet divided by the volume of the resin sheet. The total volume is obtained from the weight and bulk density of the resin sheet, the pore volume is obtained from the difference between this value and the actual volume, or the resin sheet is impregnated with a solvent such as ethanol or water, and the weight of the impregnated ethanol. Is converted into a volume, and this is used as a void volume. However, both methods measure and measure a sample off-line after manufacturing the sheet. The porous resin sheet for the separator is flexible, and the pores inside the sheet are apt to be crushed, and depending on the type of solvent, impregnation into the porous resin sheet causes the resin to shrink and the size of the pores changes. There was a problem.

Air permeation resistance, which is one of important characteristics of the separator, is attributed to the structure of the porous resin sheet such as porosity. There is a Gurley tester method as a method for measuring air resistance (JISP 8117 “Paper and paperboard (air permeability test method) Gurley tester method”). This test method is an in-line method in which measurement is performed while a sheet is conveyed in accordance with JISP 8117, and a probe is pressed onto a traveling porous resin sheet to suck air. Similar to the Gurley test, this method measures the time required for a certain volume of air, for example, 0.1 × 10 ⁻³ m ³ to pass through the porous sheet. However, it is necessary to prevent the leakage of air by bringing the probe into close contact, and the contacted probe may be rubbed to generate a scratch.

  On the other hand, as a method for measuring air voids in a resin in a non-contact manner, there is a conventional measurement method using ultrasonic waves in a resin molded product or the like. In general, ultrasonic measurement is often performed in water. This is because the acoustic impedance of air and resin is greatly different in the atmosphere, so that it is difficult for ultrasonic waves to propagate from the air to the sheet. Since the production of the porous resin sheet is performed in the air, the ultrasonic measurement is performed in the air. In other words, the measurement must be performed with a very small amount of ultrasonic transmission.

  As shown in Patent Document 1, an ultrasonic element is installed on one side of a resin molded product, and an ultrasonic wave oscillated from the element and reflected by the opposite surface of the resin molded product is received by the same ultrasonic element. It is. That is, Patent Document 1 discloses a technique for measuring a propagation time until an ultrasonic wave is incident on a resin molded product and is reflected on the back surface of the molded product by using a reflected wave on the back surface of the resin molded product. ing. The propagation speed of ultrasonic waves is related to the Young's modulus and density of a resin molded product, but a resin molded product having a large void volume has a low density and the void volume is measured using the principle that the propagation speed is slow. Using the correlation between the void amount and the propagation velocity obtained in advance, ultrasonic waves are incident on the sample and the void amount is converted from the propagation velocity. The technique of Patent Document 1 is used when the object is a thick object such as a resin molded product and the difference in propagation time can be sufficiently measured. According to the knowledge of the present inventors, in the method of measuring a reflected wave with the above configuration, an incident wave and a reflected wave are overlapped in a sample having a small thickness such as a sheet. That is, the thickness of the porous resin sheet is several tens of μm, and the thickness is smaller than the wavelength and it is difficult to obtain a difference in propagation time. That is, the technique of Patent Document 1 is limited to measuring an air void of about 1 mm in an object having a thickness of several tens of mm. That is, it was not possible to measure the pores in the porous resin sheet as thin as 30 μm or less.

  Furthermore, as a method for measuring the porosity of a solid material using ultrasonic waves, a method for determining the porosity from the propagation delay is known by paying attention to the point that the ultrasonic waves hit the pores and become a detour wave. (Patent Document 2). The technique disclosed in Patent Document 2 obtains a pore function by deconvolution integration processing from a comparison between a response waveform of an incident ultrasonic wave and a response waveform obtained from the same material in which no bubbles are present. Taking the time on the horizontal axis and the pore function on the vertical axis, the propagation delay time, the half value width of the pore function or the peak value of the pore function is obtained. The porosity is calculated using the correlation between the porosity result obtained by the volume substitution method, the propagation delay time, and the peak value. With this method, it is possible to detect bubbles up to several μm order.

  According to the knowledge of the present inventors, the resolution of the response waveform depends on the wavelength of the ultrasonic wave. The wavelength, which is the reciprocal of the frequency, is 50 μm in the case of 20 kHz that is easily used for airborne ultrasonic waves. In the measurement of Patent Document 2, the thickness of the sample is 15 mm, the thickness of the sheet is sufficiently larger than the wavelength, and the incident waveform and the detour wave are observed separately. However, in the sheet having a thickness smaller than the wavelength, the incident waveform and the detour wave are observed to overlap each other, and it is difficult to obtain the propagation delay time and the peak value of the pore function. That is, it was not possible to measure the proportion of fine pores of 0.3 μm or less in the porous resin sheet used for the separator.

  That is, according to the knowledge of the present inventors, the conventional technology cannot continuously measure the pore state and air permeability resistance of the porous resin sheet in a non-contact manner. Furthermore, the conventional method using the attenuation amount of the transmission intensity of ultrasonic waves, the delay of propagation time, and the peak function in the wavelength cannot measure the wavelength below the ultrasonic wave, and is 1 μm or less, particularly 0. Measurement of pores of 3 μm or less was not possible. For this reason, in the manufacturing method of the porous resin sheet, there was no means for continuously evaluating the pore state and the air permeation resistance in-line.

JP-A-8-210965 Japanese Patent Laid-Open No. 6-18403

  The present invention solves the above-described problems of the prior art. That is, in the measurement method for measuring the pore state of the porous resin sheet in a non-contact manner using ultrasonic waves, and obtaining the pore function from the difference in ultrasonic propagation velocity and the response waveform, the sheet thickness below the wavelength of the ultrasonic wave It was not possible to measure the submicron sized holes.

  The present invention solves the above-mentioned conventional problem, and measures the porosity of a porous resin sheet using a characteristic value correlated with the porosity using multiple scattering of fine pores. Objective. Moreover, it aims at providing the manufacturing method of the porous resin film which has this method.

  The measuring method according to the present invention is as follows. The inventors provide a method for non-destructively evaluating the pore characteristics in a solid material using ultrasound. Using ultrasonic waves with a wavelength longer than the sheet thickness, and using the propagation waveform scattered by the holes when passing through the sheet, obtain the porosity of the solid material from the change in the phase velocity for each frequency or the decrease in the amount of transmission. Suggest a method. In addition, a relational expression between the porosity and the air resistance is obtained in advance, and a method for calculating the air resistance from the porosity is provided.

  That is, according to the present invention, in a porous resin sheet having a large number of pores inside the sheet, 10 kHz to 2 MHz ultrasonic waves transmitted in the air are transmitted through the porous resin sheet, and scattering in the pores is caused. There is provided a method for measuring a porosity of a porous resin sheet by receiving a transmitted sound wave containing the sound wave and measuring the porosity of the porous resin sheet.

  According to a preferred embodiment of the present invention, ultrasonic waves having different frequencies are transmitted to the porous resin sheet, and the received ultrasonic waveform has a wavelength that is greater than the thickness of the porous resin sheet in the sound velocity measurement result for each frequency. There is provided a method for measuring a porosity of a porous resin sheet, in which the porosity of the porous resin sheet is measured from a change in phase velocity for each frequency using a long ultrasonic wave.

  Further, according to a preferred embodiment of the present invention, ultrasonic waves having different frequencies are transmitted to the porous resin sheet, and the attenuation rate measurement result is used for each frequency of the received ultrasonic waveform, and the porosity of the porous resin sheet is determined. A method for measuring the porosity of a porous resin sheet is provided.

  Moreover, according to the preferable form of this invention, the porosity measuring method of the said porous resin sheet which measures the said porosity for 40 to 80% with respect to the weight of resin amount is provided.

  According to another aspect of the present invention, the porosity measurement result of the porous resin sheet and the relationship between the porosity and the air resistance obtained in advance are used to determine the air resistance of the porous resin sheet. A method for measuring the air permeability of a porous resin sheet to be measured is provided.

  Moreover, according to the preferable form of this invention, the air permeability measurement method of the porous resin sheet which measures air resistance 80 second or more and 400 second or less is provided.

  According to another aspect of the present invention, there is provided a method for producing a porous resin sheet in which a resin is molded into a sheet shape and continuously formed into a film, wherein the porous resin is stretched after being molded into a sheet. There is provided a method for producing a porous resin sheet comprising a method for measuring the porosity of a sheet.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to measure the porosity by the distribution condition of the 1 micrometer or less pore which exists in the inside of a porous resin sheet in air | atmosphere, without destroying a porous resin sheet.

  Further, the air resistance value can be obtained by utilizing the correlation between the porosity of the porous resin sheet and the air resistance. As a result, in the method for producing a porous resin sheet, since the pore state and air permeability resistance can be continuously evaluated in-line, not only the quality is stabilized but also the variation in the porosity is large. Production loss can be minimized.

It is the schematic which shows the ultrasonic measurement method of this invention. It is a conceptual diagram which shows the ultrasonic wave which permeate | transmits the porous resin sheet of this invention. It is a conceptual diagram which shows the ultrasonic scattering condition in the void | hole of this invention. It is the schematic of the film forming process which has the ultrasonic measuring device of this invention.

  Next, embodiments of the present invention will be described with reference to the drawings.

  In addition, as a porous resin sheet which is a measuring object concerning this embodiment, the separator film for lithium ion secondary batteries is applied preferably. As a material for the separator film, an olefin film such as polyethylene or polypropylene is preferably used, but a resin such as polytetrafluoroethylene, polyamide, or polyvinylidene fluoride is also preferably used as a measurement target. The porous resin sheet contains an antioxidant, a heat stabilizer, an antistatic agent, a lubricant composed of inorganic or organic particles, and a porous material containing various additives such as an antiblocking agent, a filler, and an incompatible polymer. A resin sheet is also included in the measurement object.

  In FIG. 1, an ultrasonic wave oscillating means 2 having a radiation part 21 that emits ultrasonic waves in the air with a porous resin sheet interposed therebetween, and an ultrasonic wave having a wave receiving part 31 that receives the ultrasonic waves and converts them into electric signals. The sound wave receiving means 3 is arranged to face each other. The ultrasonic oscillating means 2 includes an ultrasonic vibrator, and includes a radiation unit 21 that radiates an ultrasonic wave US, and a drive circuit 22 that drives the ultrasonic vibrator at a predetermined frequency. The ultrasonic wave US is converged and radiated by giving the ultrasonic vibrator a curvature. The ultrasonic wave receiving means 3 receives the ultrasonic wave US and receives a wave receiving unit 31 having an ultrasonic vibrator that generates an electric signal having an amplitude corresponding to the intensity of the ultrasonic wave US, and an output (electric signal) of the ultrasonic vibrator. ) Is converted into a DC voltage signal having a voltage value corresponding to the magnitude thereof. The signal received by the ultrasonic wave receiving means 3 is a signal that has passed through the measurement object disposed between the ultrasonic wave oscillating means 2 and the ultrasonic wave receiving means 3.

  The ultrasonic oscillating means 2 radiates ultrasonic waves having different frequencies from the radiating unit 21. Two or more standing waves may be emitted in multiple times for ultrasonic waves having different frequencies. However, it is preferable to radiate a chirp wave that continuously changes the frequency and emit it from the radiation unit 21. Used. The control unit 4 includes a known microcomputer (not shown) including a CPU, a ROM, and a RAM, and controls the ultrasonic oscillating means 2 and the ultrasonic wave receiving means 3 according to a stored program. Specifically, the control is connected to the ultrasonic wave oscillating means 2 via the drive circuit 22 and is controlled so as to continuously emit pulse waves. The chirp wave described above radiates a component having a frequency of 10 kHz to 2 MHz as a pulse wave, and the number of pulse radiations can be continuously radiated 1000 to 10000 times. Then, the ultrasonic wave receiving means 3 sequentially captures the transmission waveform of the pulse wave repeatedly emitted. The acquired waveform is recorded and calculated by the recording unit 5. The calculation has a function of Fourier-transforming the waveform and displaying the power for each frequency.

  Next, the measurement method will be described with reference to FIG. The present inventors have found that the porous resin sheet of the separator for a lithium ion secondary battery propagates ultrasonic waves in the air rather than a general resin sheet or resin molded product. The ultrasonic wave US0 irradiated into the air from the ultrasonic wave transmitting means 2 is incident on the porous resin sheet S, propagates through minute air holes, passes through the porous sheet, and becomes a transmitted ultrasonic wave US1. 2A and 2B, the ultrasonic wave US0 is schematically represented, and the ultrasonic wave US0 indicates a chirp wave having a frequency of 10 kHz to 2 MHz. Conventionally, it was thought that the ultrasonic wave US0 was not easily transmitted through the resin sheet in the air. In the porous resin sheet S, a network structure 18 called minute fibrils of 0.1 μm or less is formed discontinuously, and air holes are not connected and are located discontinuously. In each of the holes, the ultrasonic wave US is repeatedly attenuated and incident on the air / resin interface, so that the attenuation is large, and it is considered that the ultrasonic wave is more difficult to transmit in the porous resin sheet S. The rate measurement was considered difficult.

  However, as shown in FIG. 2B, it is found that the ultrasonic wave US0 can propagate through this complicated and discontinuous air hole, and further, it collides with the resin fibril forming each air hole and repeats incidence and transmission. The present inventors have found that measurement of the porosity of vacancies smaller than the wavelength of the ultrasonic wave US can be performed by paying attention to the waveform of the ultrasonic wave US1 including the multi-scattered sound wave 19 in which the ultrasonic waves are scattered multiple times.

  That is, since the thickness of the sheet and the size of the pores are smaller than the wavelength, the sound wave 19 that is multiply scattered by the pores is superimposed on the transmission response waveform that is transmitted through the porous resin sheet S. The multiple scattered sound wave 19 has a smaller intensity than the pulse waveform oscillated from the ultrasonic wave oscillating means 2, and the propagation speed is delayed. Therefore, the waveform received by the ultrasonic wave receiving means 3 of FIG. 2A is Fourier transformed, the intensity for each frequency is analyzed, and the change of 19 minutes of multiple scattered sound waves is obtained. Here, the size and shape of the pores of the porous resin sheet S and the scattered sound wave have wavelength dependency. When the scattering state is observed with two or more sound waves having different frequencies, the frequency response changes depending on the size and shape of the holes, so that information on the holes can be obtained from the frequency analysis result. That is, correlate with the vacancy state, that is, the porosity, by analyzing the frequency of the received waveform using a chirp wave or sweep wave whose frequency changes continuously and paying attention to the characteristic value of a specific frequency band I found. Information on transmission attenuation, sound velocity, and phase velocity is effective as the characteristic value. The change can be measured with high accuracy when represented by the ratio or difference between the waveform acquired with the porous resin sheet S and the waveform acquired with the resin sheet without pores and the waveform measured with the porous resin sheet S. .

The multiple scattering phenomenon of the porous resin sheet S having a sheet thickness smaller than the ultrasonic wave wavelength found by the present inventors will be described below.
(1) When the porosity is the same for the same sheet thickness, the attenuation of ultrasonic waves for each frequency is small.
(2) When each hole diameter is large, scattering tends to increase and the attenuation of the transmission waveform is large.
(3) The attenuation of the ultrasonic wave is larger as the sheet thickness is thicker.
(4) The hole size and attenuation characteristics are frequency dependent. Depending on the size of the holes, the frequency band that is likely to be scattered changes. As a tendency, when the hole is large, the attenuation is large in a wide frequency band. This is because it is easy to attenuate even a sound wave having a high frequency, that is, a short wavelength. Therefore, when the frequency characteristic is taken, the attenuation is large in a wide frequency band.
(5) The phase velocity representing the speed of sound travel is frequency dependent. As the size and porosity of the holes increase, the phase velocity at which specific positions of the sonic peaks and valleys move increases.

  Note that the sound speed includes a group speed and a phase speed. The phase velocity is the phase, that is, the velocity at which a specific position of a wave peak or valley moves. The speed is the speed of moving along a straight line and represents the distance traveled per unit time, while the phase speed represents how fast one point on the outer circumference of the circle moves. For example, if you focus only on the height of one point on the outer circumference of a circle that rotates at a fixed position, the point will move up and down. When the vertical position of the point is the vertical axis and the horizontal axis is the time axis, it is represented by a sine wave, and one point on the circumference is a peak or valley of one sine wave. The phase velocity represents the moving velocity at the outer periphery of that one point. An example of a phase velocity is the movement of a worm. Humps appear to wave when you look at the insects walking from the side. The undulation of the hump is the phase velocity, and the movement of the insects is the group velocity. When waves propagate through the medium, the phase velocities of components with different frequencies are different. That is, even when the wavelength of the ultrasonic wave propagating is longer than the thickness of the sheet, the change in the ultrasonic wave propagation state can be known from the phase velocity. In the present invention, the phase velocity at the time of propagation of the ultrasonic wave scattered by the vacancies varies depending on the size and ratio of the vacancies, and the phase velocity may be easily delayed or increased with respect to the sound wave frequency. That is, it is possible to evaluate the porosity of the porous resin sheet by using the frequency characteristics.

  One embodiment of a porosity measurement method using the ultrasonic transmission phenomenon of a porous resin sheet having a sheet thickness smaller than the above wavelength will be described below.

  First, using a porous resin sheet whose porosity has been measured in advance, the transmission waveform is subjected to frequency analysis, and a correlation with the porosity is derived. A calibration curve was created by measuring the thickness and porosity, and analyzing the frequency of the transmission waveform, focusing on a specific frequency. Specifically, as a sample with a known porosity, a frequency analysis was performed on the result of transmitting the ultrasonic wave US through a sample with a porosity of 30, 40, 50, or 60%. At this time, since the frequency characteristics of the scattering are different when the hole size is 0.1 μm, 0.05 μm, 0.02 μm, 0.01 μm, for example, separate calibration curves are prepared for each hole size. Keep it. In the in-line measurement of the film formation, the size of the pores is substantially constant depending on the film formation conditions such as stretching, and it is possible to determine which size of the calibration curve to use. As the frequency, a frequency such as 10 kHz, 100 kHz, 1 MHz or the like where the hole information can be clearly discriminated is selected, but it is preferable to graph the frequency on the horizontal axis and the characteristic value on the vertical axis. As the characteristic value, a change in velocity of the transmission amount or propagation time (for example, phase velocity) compared with the measured waveform when there is no porous resin sheet is preferably used.

  Next, the measurement sample is measured, and the porosity is calculated from the similarity to the above-described calibration curve or frequency analysis graph. At this time, when the thickness of the sheet is required, it is preferable to measure in advance using an existing thickness meter.

  As described above, the porosity measurement of a porous resin sheet using ultrasonic waves has an advantage that ultrasonic waves can be propagated and continuously measured without contact without contacting the sheets.

  In addition, the air resistance has a correlation with the value of the porosity, but is influenced by the size of the pores and the number per unit volume. This is because, even if the porosity is the same, the air resistance tends to be small when the size of the holes is large and the number is small, and the air resistance tends to be large when the size of the holes is small and the number is large. According to the frequency analysis result of the multiple scattering by the transmission waveform of the ultrasonic wave according to the present invention, information on the size of the holes can be obtained, so that there is an advantage that the correlation with the air permeation resistance can be obtained more. Also in the case of measuring air resistance, it is necessary to prepare a calibration curve, a graph of the amount of transmission for each frequency, a propagation delay, and a change in phase velocity in advance. The ultrasonic measurement of the present invention can be evaluated with better correlation than the air resistance is evaluated from the X-ray or infrared absorption amount, which is proportional to the product of the thickness and the density.

  Next, a method for making a polyolefin film porous to which the porosity measurement method of the present invention can be suitably applied will be described. There are a wet method and a dry method as a method for producing the porous resin sheet. In this method, the organic liquid material is extracted and removed from the extruded sheet by adding an ultra-high molecular weight polyethylene resin and an organic liquid material such as paraffin as an extraction resin, and the film is made porous and then stretched. The advantage of the post-extraction stretching method is that the pores are easy to spread and the air permeability is reduced even without stretching at a high magnification. As a result, since the air permeability is lowered by low magnification stretching in the transverse stretching step, it is possible to produce a microporous film having a low air permeability and a low shrinkage stress and a low shrinkage rate. On the other hand, as a dry method, for example, by adopting low temperature extrusion and high draft ratio at the time of melt extrusion, the lamella structure in the film before stretching formed into a sheet is controlled, and this is uniaxially stretched in the longitudinal direction, thereby lamellar interface There is known a method of generating a void by generating a cleavage at a point. As a dry method, a method is proposed in which a large amount of incompatible resin is added as inorganic particles or polypropylene, which is a matrix resin, and a sheet is formed and stretched to cleave at the interface between the particles and the polypropylene resin to form voids. Has been. For example, in the case of a polypropylene film, a method of forming voids in a sheet by utilizing the crystal density difference and crystal transition between α-type crystal (α crystal) and β-type crystal (β crystal), which are polymorphs of polypropylene. It has been known. The porosity measurement of the present invention can be applied to any method for producing a porous resin sheet.

  As for the wet method, in both the wet method and the dry method, one of the problems in the formation of a porous film is the uniformization of pore size, porosity, and air resistance. In order to achieve uniformity, in the manufacturing method, the stretching unevenness due to temperature unevenness during stretching in the width direction has been reduced to make the porosity uniform.

  Embodiment which applied the porosity measuring method of this invention to the porous resin sheet manufacturing process by a dry method is described using FIG. FIG. 3 is a schematic process diagram showing an example of a method for producing a porous resin sheet by a dry method. In FIG. 3, the sheet moves in the direction of the arrow to produce a porous resin sheet. A raw material in which a high molecular weight polypropylene resin and a low density polyethylene resin are mixed is heated at a melting temperature or higher by the kneading extrusion means 10 and the resin is extruded from the die 12 onto the cast drum 13 to obtain an unstretched resin sheet. At this time, in order to control the β crystal fraction, the surface temperature of the cast drum 13 is set to 110 ° C. to 130 ° C., and the β crystal fraction of the unstretched resin sheet is controlled to be high. The obtained unstretched sheet is biaxially stretched. The sheet is stretched in the sheet longitudinal direction by the stretching means 14 in the sheet longitudinal direction, and subsequently stretched in the sheet width direction by the stretching means 15 in the sheet width direction. The draw ratio is 3 to 8 times in the longitudinal direction and 3 to 8 times in the sheet width direction. Thereafter, after passing through a transition 17 such as cutting an edge, the winding means 16 winds the porous resin sheet around the core.

The ultrasonic measuring instrument 1 of the present invention is installed between the winding means 16 after being stretched in the longitudinal direction and the sheet width direction. In FIG. 3, it is installed downstream of the sheet width direction extending means 15 with the resin sheet to be conveyed interposed therebetween. The distance between the porous resin sheet and the outermost surface of the oscillating portion 21 of the ultrasonic oscillating means 2 is set to 5 mm to 20 mm. If it is smaller than 10 mm, the oscillating portion 21 may come into contact with the porous resin sheet. Moreover, since the attenuation | damping in the air of an ultrasonic wave will be large and sensitivity will fall when it exceeds 20 mm, 5-20 mm is used preferably. A distance of 10 mm to 15 mm is more preferable.
As a result, the porosity is continuously measured in the sheet conveying direction so that the porosity is kept uniform downstream of the stretching means 15 for forming the holes. The ultrasonic measuring device 1 for measuring the porosity may be scanned in the sheet width direction, or a plurality of ultrasonic measuring devices 1 may be arranged. When scanning in the sheet width direction, it is performed at a speed of about 10 mm / second.

  The film forming speed of the porous resin sheet is 10 m / min to 80 m / min. Since the porosity measuring apparatus of the present invention is installed so that in-line measurement can be performed in the film forming process, it is possible to continuously measure the porosity over the entire length of the sheet, and to achieve uniform quality and quality control. . Conventionally, the porosity and air permeation resistance have been measured off-line from a product by unwinding the surface layer after winding the sheet on a roll. For this reason, a product loss may occur, but the present invention has an effect that an abnormality can be detected early as an on-line process monitor in the longitudinal direction, and the loss of a product can be reduced.

  The porosity to which the porosity measurement of the present invention is applied is 20% to 80%. If it exceeds 80%, the mechanical strength of the film becomes too low, and the film is compressed by the film's own weight at the roll core when stored in the state of being wound on the roll, and the strength when used as a separator cannot be obtained. There is a case. More preferably, it is still 65 to 75%.

  The porous polypropylene film in the present invention preferably has a pore diameter of 20 to 300 nm from the viewpoint of achieving both excellent battery characteristics and safety when used as a separator. This is because if the diameter of the holes is less than 20 nm, the air resistance increases, and the energy loss during ion transmission may increase. On the other hand, if the diameter of the pore exceeds 300 nm, ions may permeate excessively. Therefore, the hole diameter is more preferably 20 to 300 nm.

  The porous polypropylene film suitably used in the present invention preferably has a total film thickness of 5 to 50 μm. The film thickness of the porous resin sheet is preferably 5 to 35 μm, more preferably 5 to 25 μm, from the viewpoint of battery energy density and safety. When the film thickness is less than 5 μm, insulation is not sufficient, and it is difficult to ensure safety, which is not preferable. Moreover, since the energy density of a battery will become low when a film thickness is larger than 50 micrometers, it is unpreferable.

  The porous polypropylene film in the present invention may be a laminated film formed by laminating a plurality of layers having different compositions or the same composition. A laminated film is preferable because the film surface characteristics and the overall film characteristics may be individually controlled within a preferable range. In addition, a lamination | stacking thickness ratio is not restrict | limited in the range which does not impair the effect of this invention. The puncture strength is 2.5 N / 20 μm or more, preferably 2.8 N / 20 μm or more, and more preferably 3.0 N / 20 μm or more. When the puncture strength is lower than 2.5 N / 20 μm, there is a problem in safety and handling during handling.

  After the product measured in this manner is wound up once, the width of the final product is preferably a secondary slit with an appropriate width according to the size of the intended use and power storage device. 0.005 to 1 m width, more preferably 0.01 to 0.5 m width.

  The porous polypropylene film roll of the present invention is not only excellent in air permeability and porosity, but also has extremely small variations in air permeability and porosity, so there is no variation in characteristics between electricity storage devices when used for a separator for electricity storage devices. Small and uniform quality can be obtained. Therefore, it can be particularly preferably used as a separator for large-sized lithium ion secondary batteries used in electric vehicles and the like.

Hereinafter, an example of an embodiment of the present invention will be described using examples. Various measurements were performed as follows.
(1) Measurement of air permeability resistance A porous resin sheet after film formation is cut out, and a Gurley tester and its measurement method in accordance with JIS P 8117 (paper and paperboard-air permeability test method-Gurley test method) are used. used. At that time, the inner diameter of the gasket of the Gurley tester was 28.6 mm, and 100 mL of air was allowed to pass through. The time required for 100 mL to pass through was measured, and the air resistance was determined as [sec / 100 mL].
(2) Measurement of porosity The porous resin sheet was cut into a 100 mm square, and a sheet free of bubbles was obtained by pressurizing 5 MPa at a temperature 150 ° C. higher than the melting temperature. The weight and volume of this resin sheet were determined, and the density d was calculated. As a result, the density of the sheet made of polypropylene resin was 0.92 g / cm ² . The specific gravity ρ of the porous resin sheet cut out to 100 mm square was measured using an electronic hydrometer. As the electronic hydrometer, SD-120L manufactured by Mirage Trading Co., Ltd. was used. From the specific gravity ρ of the porous resin sheet and the resin density d, the porosity was calculated by the following formula.

Porosity (%) = [(d−ρ) / d] × 100
The thickness of the porous resin sheet was measured at an ambient temperature of 23 ± 2 ° C. using a micro thickness gauge (type KBN, terminal diameter Φ5 mm, measurement pressure 62.47 kPa) manufactured by Toyo Seiki.
(3) Size of pores The porous resin sheet sample was processed so that the pores were not crushed, and the size and distribution of the pores were calculated by image processing from the result of observation with an electron microscope.
(4) Measurement of ultrasonic porosity The calibration curve was created off-line using a sample whose porosity obtained in (2) and the size of the pore obtained in (3) are known. Samples having porosity of 40%, 50%, 60%, and 70% were used for the calibration curve and the frequency characteristic graph for calibration. As the diameter of the holes, a porosity of 40%, a center value of 50 nm, and a porosity of 70% and 75 nm were used. Ultrasonic measurement for creating a calibration curve was performed as follows. The calibration curve was carried out by fixing the sheet cut off offline. As an aerial ultrasonic element, the model number is 3K12.7NR10 (manufactured by Japan Probe Co., Ltd.), and the transmitting and receiving devices face each other. In the ultrasonic wave, the center of the oscillation frequency was 300 kHz, and the frequency of the chirped wave that pulsated was 10 kHz to 2 MHz. The oscillating part 21 of the ultrasonic oscillating means 21 and the receiving part 31 of the ultrasonic receiving means 3 were installed with an interval of 16 mm, and the porous resin sheet was arranged almost at the center. A pulsed chirp wave was transmitted to the porous resin sheet at intervals of 20 μsec, and the received pulse waveform was integrated 1000 times to obtain a transmission waveform A of the sample. Furthermore, the state without a porous resin sheet, that is, the porous resin sheet was removed, and the same measurement was performed to obtain a reference transmission waveform B. The A / B value was subjected to frequency analysis, and the frequency from 10 kHz to 1 MHz was taken on the horizontal axis. Measurement was performed at an ambient temperature of 23 ± 2 ° C.

  Moreover, the phase velocity of the frequency component of 10 kHz, 100 kHz, and 1 MHz was obtained. For the phase velocity, the angular velocity rotating with a sine wave for each frequency is ω [rad / sec], the distance traveled in one second on the outer circumference, that is, the degree of phase progression at the length is the wave number κ [rad / μm], and the phase velocity The velocity Vp was determined from Vp = ω / κ [μm / sec]. As described above, a calibration curve (frequency analysis graph) representing the correlation between the porosity, the ultrasonic transmission intensity, and the change in the propagation velocity was created.

  As shown in FIG. 3, the ultrasonic porosity measuring device has an interval of 16 mm between the oscillating unit 21 of the ultrasonic oscillating means 21 and the receiving unit 31 of the ultrasonic receiving means 3 downstream of the transverse stretching process 15 of the film forming process. Opened. It was installed in an environment with a temperature of 23 ° C. and a humidity of 50%. At this time, since the ultrasonic waves are affected by the temperature, the temperature management was performed by placing them in an acrylic box so as to cover the transmitter and the receiver.

  As a polypropylene resin, 94 parts by mass of a commercially available homopolypropylene resin with an MFR of 8 g / 10 minutes, 1 part by mass of a commercially available MFR of 2.5 g / 10 minutes with a high melt tension polypropylene resin, and an ultra low density polyethylene resin 5 with a melt index of 18 g / 10 minutes. A raw material is prepared by mixing 0.2 parts by mass of N, N′-dicyclohexyl-2,6-naphthalenedicarboxamide and 2 parts by mass in advance using a twin screw extruder 10. At this time, the melting temperature is preferably 270 to 300 ° C. Next, the mixed raw material was melt-extruded at 220 ° C. It is discharged from the die 12 onto the cast drum 13 to obtain an unstretched sheet. At this time, the surface temperature of the cast drum 13 was 120 ° C., and the β crystal fraction of the cast film was controlled to be high. In order to bring the sheet into close contact with the drum, an air knife was used to blow air. The unstretched sheet thus obtained is biaxially stretched to form pores in the film. As a biaxial stretching method, the film is stretched in the longitudinal direction of the film and then stretched in the width direction, or the sequential biaxial stretching method of stretching in the longitudinal direction after stretching in the width direction, or the longitudinal direction and the width direction of the film are stretched almost simultaneously. A simultaneous biaxial stretching method or the like can be used, but a sequential biaxial stretching method was adopted, and the film was stretched 5.2 times in the longitudinal direction and then 7 times in the width direction. The film forming speed was 35 m / min.

  With respect to the porous polypropylene film which was formed and stretched under the above conditions, the film end held by the clip was trimmed, and only the central part of the film was wound around the core to obtain a sheet roll body. The winding length was 4000 m.

  The ultrasonic measuring instrument 1 was installed at the center in the sheet width direction. The conveyed porous resin sheet was measured continuously. The configuration of the ultrasonic element used was the same as that in (4) Ultrasonic porosity measurement. The chirp wave used was also equivalent. The frequency of the ultrasonic measurement result was analyzed, and the porosity was estimated to be 55% from the approximation of the calibration curve obtained in advance and the frequency analysis graph of the transmitted ultrasonic waveform. The variation in measurement was +/− 5%.

  The present invention relates to a method for measuring porosity or pore diameter using ultrasonic waves. More specifically, the present invention relates to the porosity of the pores existing in the material without destroying the material by attaching an input / output sensor to the solid material and transmitting ultrasonic waves and analyzing the response waveform. The present invention relates to a method for measuring an average pore diameter.

DESCRIPTION OF SYMBOLS 1 Ultrasonic measuring device 2 Ultrasonic oscillation means 21 Radiation part 22 Oscillation drive circuit 3 Ultrasonic reception means 31 Reception part 32 Reception circuit 4 Control part 5 Recording part 10 Kneading extrusion means 11 Extrusion means 12 Base 13 Cast drum 14 Sheet Moving direction extending means 15 Sheet width direction extending means 16 Sheet winding part (winder)
17 Sheet Thickness Gauge US Aerial Ultrasonic S Porous Resin Sheet

Claims (7)

In the porous resin sheet having a large number of pores inside the sheet, 10 kHz to 2 MHz ultrasonic waves transmitted in the air are transmitted through the porous resin sheet, and the transmitted acoustic waves including scattering in the pores are received, A method for measuring a porosity of a porous resin sheet, comprising measuring a porosity of the porous resin sheet. Transmitting ultrasonic waves having different frequencies to the porous resin sheet, and using the ultrasonic wave having a wavelength longer than the thickness of the porous resin sheet in the sound velocity measurement result for each frequency of the received ultrasonic waveform, the phase velocity for each frequency The porosity measuring method of the porous resin sheet according to claim 1, wherein the porosity of the porous resin sheet is measured from the change of the above. The ultrasonic wave having a different frequency is transmitted to the porous resin sheet, and the porosity of the porous resin sheet is measured using an attenuation measurement result for each frequency of the received ultrasonic waveform. 2. A method for measuring the porosity of a porous resin sheet according to 1. The porosity measurement method for a porous resin sheet according to claim 1 or 2, wherein the porosity is 40% or more and 80% or less with respect to the weight of the resin amount. The air permeability resistance of the porous resin sheet is measured using the porosity measurement result of the porous resin sheet according to any one of claims 1 to 4 and the relationship between the porosity and the air resistance obtained in advance. A method for measuring the air permeability of a porous resin sheet. The air permeability measurement method for a porous resin sheet according to claim 5, wherein the air resistance is measured from 80 seconds to 400 seconds. A porous resin sheet according to any one of claims 1 to 4, wherein the method is a method for producing a porous resin sheet in which a resin is molded into a sheet shape and continuously formed into a film, wherein the sheet is molded and then stretched. A method for producing a porous resin sheet, comprising:

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