JP2009114456A - Method for purifying lubricant for magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for purifying lubricant which excels in lubricating properties and has high stability for a long-term use and which can be used for a magnetic recording medium. <P>SOLUTION: With the supercritical extraction method, lubricant containing a compound having a perfluoropolyether structure is purified. In the same pressure vessel, an extraction medium made of supercritical carbon dioxide and the lubricant are contacted at a first condition, and a perfluoropolyether compound having a low-polar end group is removed. Then, at a second condition, an extraction medium made of the supercritical carbon dioxide and lubricant from which the perfluoropolyether compound having the low-polar end group is removed are contacted, and perfluoropolyether lubricant is extracted and recovered while removing ionic impurities. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a lubricant for a magnetic recording medium, a purification method thereof, and a magnetic recording medium using the lubricant.

  Recording media such as fixed magnetic recording media are widely used as data recording media for computers and the like. A magnetic recording apparatus including such a fixed magnetic recording medium includes a magnetic recording medium driving mechanism, a magnetic head driving mechanism, a magnetic head stop mechanism, and a data transfer mechanism in addition to one or a plurality of magnetic recording media. In recent years, such magnetic recording apparatuses have made remarkable progress in increasing the density, increasing the capacity, and increasing the data transfer speed.

  Further, the conventional fixed magnetic recording apparatus has a contact where the magnetic recording head floats when the magnetic recording medium rotates, and the magnetic head contacts the surface of the magnetic recording medium when the rotation drive motor for rotating the magnetic recording medium stops. -A start / stop (CSS) system is adopted. In this system, the magnetic head slides on the surface of the magnetic recording medium when the magnetic recording medium starts and stops rotating, and friction is generated between the magnetic recording medium and the magnetic head. A protective layer is provided to protect the magnetic layer from such friction, and a lubricating layer is provided to improve surface lubrication characteristics.

Further, in recent years, the rotation speed of the magnetic recording medium of the magnetic recording apparatus has been increased from 7400 to 15000 min ⁻¹ from 5400 min ⁻¹ (rpm) in the past. As a result, the spin migration (spin-off) phenomenon in which the lubricant on the surface of the magnetic recording medium moves to the outer peripheral portion due to centrifugal force or scatters is becoming apparent. In addition, the problem that the lubricant evaporates due to the heat generated at the time of high-speed rotation and the amount thereof is decreasing is becoming apparent.

  The increase in the thickness of the lubricating layer on the outer periphery of the magnetic recording medium due to the spin migration causes an adsorption failure, and the evaporation of the lubricant tends to cause wear of the protective layer due to the decrease in the thickness of the lubricating layer. To do. In the worst case, it will cause a head crash.

  The lubricant layer of the magnetic recording medium needs to be stably formed with a uniform film thickness on the surface of the protective layer in order to suppress spin migration. In addition, high adhesion and bonding properties are required between the lubricating layer and the protective layer. In order to enhance this adhesion, perfluoropolyether lubricants having terminal groups such as hydroxyl groups and piperonyl groups are used. These are commercially available, for example, as “Fomblin Z DOL”, “Fomblin Z TETRAOL” and “AM3001” from Augmond. Many of the perfluoropolyether lubricants currently used are inferior in heat resistance if the molecular weight is too low, and conversely, if the molecular weight is too high, the adsorptivity increases, so that the number average molecular weight (Mn) is 1000 to 1000. 10,000 lubricants, more specifically, about 2000 to 4000 lubricants are used.

  The lubricant as described above has sufficient characteristics with respect to the current rotational speed of the magnetic recording medium, but remains uneasy about durability against a phenomenon such as spin migration at a higher rotational speed.

In Patent Document 1 , by using a lubricant having a high molecular weight and a narrow molecular weight distribution, the problem of the conventional high molecular weight lubricant is solved, and the adhesion with the protective layer is also improved. It is disclosed to improve durability against spin migration.

In Patent Document 1 , a high-molecular-weight lubricant is used. However, in recent years, the influence of lubricant molecules caused by lowering the magnetic head further becomes a problem. That is, as the distance between the magnetic head and the magnetic recording medium becomes shorter, the size of the lubricant molecules may affect the flying of the magnetic head. Accordingly, if the resistance to spin migration is the same, it is preferable that the lubricant has a small molecular weight within a range in which heat resistance can be ensured.

  As described above, the lubricant needs further improvement from the viewpoints of molecular weight, spin migration resistance, heat resistance, and the like.

Further, in addition to the problems from the above viewpoint, the commercially available lubricant contains impurities, and it is necessary to remove these impurities. For example, commercially available perfluoropolyether lubricants contain impurities having CF ₂ Cl, CF ₃ , CF ₂ H groups, etc. at the ends of the perfluoropolyether as impurities. These end groups are less polar than hydroxyl groups and piperonyl groups (original end groups of perfluoropolyether lubricants), and if this component is large, the adhesion and bonding properties with the protective layer are high even if the molecular weight is large. Not improved. Therefore, a purification method for removing such impurities is required.

As a method for purifying the lubricant, for example, Patent Document 2 discloses a technique for purifying by controlling the molecular weight by the GPC method. Patent Document 3 discloses a technique for purifying by controlling the molecular weight of a lubricant using an organic solvent. In the method of purifying a lubricant with an organic solvent, components having high polarity and low molecular weight are preferentially removed. Therefore, in the lubricant after purification, CF ₂ Cl, CF ₃ , CF ₂ having low solubility in an organic solvent is used. There is a possibility that the ratio of the perfluoropolyether having an H group or the like at the terminal increases. Patent Document 4 discloses a purification method for increasing the ratio of terminal functional groups by utilizing supercritical fluid chromatography (SFC). In this application, it is stated that it is cheaper than using an organic solvent and is more productive than the GPC method. However, in the case of increasing the amount to be processed at one time even in the SFC method, the cost and durability of the packed column, etc. Is a problem.

  Further, as impurities of commercially available lubricants, there are ionic impurities in addition to the above low-polar perfluoropolyether. If ionic impurities contained in the lubricant are present on the recording medium, it causes corrosion. The corrosion induces elution of ions from a metal element such as cobalt used in the magnetic layer, and the cobalt ions or the like act as a catalyst to decompose perfluoropolyether molecules used in the lubricating layer. For this reason, it is preferable to remove ionic impurities as much as possible.

  Thus, it is also necessary to remove ionic impurities that will be included in the synthesis of perfluoropolyether.

JP 2001-229524 A JP-A-5-20673 JP-A-11-199882 JP 2001-164279 A

  An object of the present invention is to provide a method for purifying a lubricant usable for a magnetic recording medium, which has excellent lubrication characteristics and is highly stable for long-term use. Furthermore, an object of the present invention is to provide a lubricant having excellent lubrication characteristics and high stability for long-term use by the purification method. Another object of the present invention is to provide a magnetic recording medium having excellent lubrication characteristics and long-term stability using the lubricant.

The above-mentioned subject is achieved by the present invention shown below.
The first of the present invention relates to a method for purifying a perfluoropolyether lubricant by a supercritical extraction method using carbon dioxide in a supercritical fluid state (referred to herein as supercritical carbon dioxide).

A first aspect (reference aspect) of the present invention is a method for purifying a lubricant containing a compound having a perfluoropolyether structure by a supercritical extraction method, and is supercritical under a predetermined condition in a pressure vessel. Contacting an extraction medium made of carbon dioxide with the lubricant to remove ionic impurities selected from sodium ions, potassium ions, chlorine ions, HCO ₃ ions, HSO ₄ ions or sulfate ions contained in the lubricant. Is a purification method characterized by The purification method of this aspect is characterized in that the predetermined conditions are carried out at a temperature and pressure at a density equal to or lower than the density of supercritical carbon dioxide (about 0.7 g / cm ³ ) obtained at a temperature of 60 ° C. and a pressure of 20 MPa. .

A second aspect (reference aspect) of the present invention is a method for purifying a lubricant containing a compound having a perfluoropolyether structure by a supercritical extraction method, and is supercritical under a predetermined condition in a pressure vessel. A group selected from CF ₃ , CF ₂ H, or CF ₂ Cl as an end group contained in the lubricant from a component of a perfluoropolyether lubricant by bringing the extraction medium made of carbon dioxide into contact with the lubricant; The purification method is characterized by removing a perfluoropolyether compound having a low-polar end group, which is a compound containing. In the purification method of this aspect, the predetermined conditions are a temperature and a pressure at which the density is less than or equal to the density of supercritical carbon dioxide (about 0.6 g / cm ³ ) obtained at a temperature of 60 ° C. and a pressure of 16 MPa. .

A third aspect of the present invention is a method for purifying a lubricant containing a compound having a perfluoropolyether structure by a supercritical extraction method, wherein the lubricant is supercritical under the first condition in the same pressure vessel. A group selected from CF ₃ , CF ₂ H, or CF ₂ Cl as an end group contained in the lubricant is brought into contact with an extraction medium made of carbon dioxide and the lubricant. The perfluoropolyether compound having a low-polar end group, which is a compound containing, is removed, and then the extraction medium composed of supercritical carbon dioxide and the perfluoropolyether compound having a low-polar end group are removed under the second condition. The selected lubricant is contacted and selected from sodium ion, potassium ion, chlorine ion, HCO ₃ ion, HSO ₄ ion or sulfate ion A purification method characterized by extracting a purified perfluoropolyether lubricant while recovering it while removing ionic impurities. In the purification method of this aspect, the first condition is a temperature and a pressure at which the density is less than or equal to the density of supercritical carbon dioxide (about 0.6 g / cm ³ ) obtained at a temperature of 60 ° C. and a pressure of 16 MPa. The second condition is characterized in that the temperature and pressure at which the density is less than or equal to the density of supercritical carbon dioxide (about 0.7 g / cm ³ ) obtained at a temperature of 60 ° C. and a pressure of 20 MPa.

  The second of the present invention relates to a perfluoropolyether lubricant purified by the lubricant purification method of the first invention.

  A third aspect of the present invention is a magnetic recording medium in which at least a nonmagnetic underlayer, a magnetic layer, a protective layer and a lubricating layer are sequentially laminated on a nonmagnetic substrate, wherein the lubricant is the perfluoropolyether lubricant of the present invention. The present invention relates to a magnetic recording medium characterized by being an agent.

According to the present invention, by controlling the state of carbon dioxide used for supercritical carbon dioxide extraction, the removal of ionic impurities and the removal of perfluoropolyether components having low polarity end groups are the same pressure vessel. A highly productive lubricant refining method that can be realized within the company can be provided. Furthermore, according to the present invention, a lubricant having a low molecular weight and high binding property can be provided by the above purification method. Further, by using the lubricant purified by the method of the present invention for a magnetic recording medium, the lubricating characteristics of the magnetic recording medium of the fixed magnetic recording apparatus can be improved, and a magnetic recording medium having excellent long-term stability can be obtained. Can be provided.

The present invention will be described below. First, the first invention will be described.
The first invention is a method for purifying perfluoropolyether by supercritical extraction using supercritical carbon dioxide.
A supercritical fluid continuously changes from an extremely dilute low density state to a high density state close to a liquid by a slight change in temperature and pressure. Since the solute dissolution capacity of the supercritical fluid depends on this density, the solute dissolution capacity can be freely changed by controlling the temperature and pressure.

In the purification method of the present invention, the temperature can be in the range of 31 ° C to 100 ° C. Moreover, although a pressure should just be 7.4 MPa, for example, in the said temperature range, it can implement in the pressure range to about 30 MPa. This is because carbon dioxide can be maintained in a supercritical state within this range. Particularly in the present invention, the temperature range is preferably 40 ° C to 100 ° C. This is because when the temperature is 40 ° C. or lower, the solubility of supercritical carbon dioxide changes greatly due to slight fluctuations in pressure, and control becomes difficult. This is because it takes time to control because the dissolving power of critical carbon dioxide does not change. These temperature and pressure ranges correspond to a density range of about 0.4 g / cm ³ to about 0.9 g / cm ³ of supercritical carbon dioxide. In the present invention, the temperature and pressure can be controlled to freely change the solute dissolving ability according to the notation range. Accordingly, in the present invention, the temperature and pressure are controlled to form a supercritical carbon dioxide state having a desired density, that is, a desired elution capacity, and the supercritical carbon dioxide and the perfluoropolyether lubricant are brought into contact with each other. The desired lubricant component can be purified. By using the method of the present invention, the lubricant can be purified by utilizing a slight difference in properties such as molecular weight of perfluoropolyether, functional group, or polarity of each compound or ion contained in the lubricant. Can do.

  The purification method of the present invention can be divided into a first aspect to a third aspect described below according to the component to be removed and the component to be extracted.

First, the first aspect (reference aspect) of the present invention will be described. The first aspect of the present invention is a purification method for removing ionic impurities from a perfluoropolyether lubricant.

  When an ionic impurity is contained in the lubricant, the ionic impurity decomposes the lubricant, causing a problem in long-term stability. Therefore, by removing ionic impurities, the decomposition of the lubricant is suppressed and the long-term stability of the lubricant is ensured. Removal of ionic impurities from the lubricant is achieved by setting an upper limit on the density of the supercritical carbon dioxide used in the method of the present invention and performing extraction below this density.

The ionic impurities that can be removed in the present invention are metal ions such as sodium ions and potassium ions, and inorganic ions such as chlorine ions, HCO ₃ ions, HSO ₄ ions, and sulfate ions. Furthermore, in the present invention, organic ions such as ammonia ions, oxalate ions, and formate ions can also be removed.

  The first aspect will be specifically described below. In the first aspect, the perfluoropolyether lubricant is brought into contact with an extraction medium made of supercritical carbon dioxide under a predetermined condition in a pressure vessel by a supercritical extraction method. This is a purification method for removing ionic impurities contained in. More specifically, the purification method of this embodiment includes the following steps.

(A) introducing a perfluoropolyether lubricant into the pressure vessel;
(B) a step of bringing supercritical carbon dioxide into a pressure vessel in contact with the lubricant under predetermined conditions to extract a perfluoropolyether compound; and (c) perfluoropolyester obtained by the step (b). Recovering the ether compound;

This aspect will be described below along each step.
The method of the present invention is a supercritical extraction method using supercritical carbon dioxide as an extraction medium. This method utilizes the fact that supercritical carbon dioxide can keep the solubility in polar components low under predetermined conditions.

Step (a)
In step (a), a perfluoropariether lubricant as a raw material is introduced into the pressure vessel. The pressure vessel has at least an introduction part for introducing supercritical carbon dioxide, which is an extraction medium, and an outlet part for taking out the medium and the extract, and can control the pressure and temperature in the container arbitrarily. Have Containers and apparatuses that satisfy such requirements include, for example, a thick stainless steel container having an inlet and an outlet, a supercritical carbon dioxide feed pump (for example, SCF-Get manufactured by JASCO Corporation), a fully automatic pressure Examples thereof include a regulating valve (for example, SCF-Bpg manufactured by JASCO Corporation), a column oven used for a normal chromatogram, and the like. The capacity of the pressure vessel may be variously selected according to the lubricant to be purified, and is not particularly limited in the present invention.

The lubricant that can be used in the purification method of the present invention is a perfluoropolyether lubricant, for example, a polar group such as a piperonyl group, a hydroxyl group, a carboxyl group, an ester group, an amino group, or a cyclophosphazene group is introduced. Can be cited as an example. Specific examples include “Fomblin Z tetraol” and AM3001, which are commercially available from Augmont.
A predetermined amount of such a raw material is introduced into the pressure vessel through the introduction portion of the pressure vessel.

Step (b)
In the step (b), the lubricant introduced into the pressure vessel is brought into contact with supercritical carbon dioxide as an extraction medium, and the perfluoropolyether compound is extracted to remove ionic impurities.

  The extraction medium is supercritical carbon dioxide. The supercritical carbon dioxide is brought into contact with the lubricant raw material in the pressure vessel while flowing into the pressure vessel at a predetermined flow rate.

This step (b) can be carried out in the temperature and pressure ranges described at the beginning, but it is preferable to perform extraction at a predetermined temperature and pressure within this range. It is preferable that this temperature and pressure, that is, the upper limit temperature and pressure of this embodiment, can form a state of about 0.7 g / cm ³ at the density of supercritical carbon dioxide. Specifically, the temperature and pressure are, for example, 60 ° C., 20 MPa, 100 ° C., and 26 MPa. By bringing the lubricant into contact with supercritical carbon dioxide under such conditions, the components of the perfluoropolyether compound are selectively extracted, and ionic impurities are left in the residue in the container.

  The flow rate of the supercritical carbon dioxide in this step is not particularly limited because it varies depending on the extraction conditions and the target compound to be extracted. For example, the flow rate is 1 mL / min to 100 mL / min per 100 g of lubricant, preferably 10 mL / min to 50 mL / min per 100 g of lubricant. If the flow rate is too slow, there will be a problem that the processing speed will be slow.If the flow rate is fast, the extraction efficiency will be reduced and carbon dioxide will be consumed unnecessarily. There arises a problem of being pushed out without being purified.

Step (c)
In step (c), the extracted perfluoropolyether compound is collected together with supercritical carbon dioxide, and the perfluoropolyether compound is recovered. The perfluoropolyether compound and supercritical carbon dioxide are taken out from the outlet of the pressure vessel. Each can be separated by returning to normal temperature and normal pressure, and the purified perfluoropolyether lubricant can be recovered. Carbon dioxide can be recovered and reused as needed.

As described above, ionic impurities can be removed from the perfluoropolyether lubricant and the lubricant can be purified. For example, in the case of the above-mentioned Augmont lubricant, ionic impurities such as sodium ions, potassium ions, chlorine ions, HCO ₃ ions, HSO ₄ ions, and sulfate ions can be removed.

  The purified perfluoropolyether lubricant can be used as a lubricant for the magnetic recording medium of the present invention described later. Since the lubricant refined in this embodiment has reduced ionic impurities, it can provide a magnetic recording medium with excellent long-term stability when used as a material for the lubricating layer of the magnetic recording medium.

Next, the second aspect (reference aspect) of the present invention will be described. The second aspect of the present invention is a method for purifying a perfluoropolyether lubricant by removing a perfluoropolyether compound having a low polarity end group from the perfluoropolyether lubricant. In this second embodiment, the lubricant is purified by selecting the end groups of the lubricant. In the perfluoropolyether lubricant, in order to improve the adhesion and bondability with the protective layer, a lubricant containing a large amount of perfluoropolyether compound having a terminal group having high adhesion and bondability with the protective layer is used. It is effective. For this reason, in the case of a lubricant containing a perfluoropolyether compound having a low-polar group at the terminal group, it is necessary to separate and remove these in order to improve the adhesion and bondability with the protective layer. This embodiment provides a purification method in such a case. Examples of the compound that can be removed in this embodiment include perfluoropolyether compounds having a terminal group such as CF ₃ , CF ₂ Cl, and CF ₂ H.

  The second embodiment will be described in detail. In the second embodiment, the perfluoropolyether lubricant is brought into contact with the extraction medium composed of supercritical carbon dioxide under a predetermined condition in a pressure vessel by a supercritical extraction method. This is a purification method for removing the low-polar perfluoropolyether component contained in this lubricant. More specifically, the purification method of this embodiment includes the following steps.

(A) introducing a perfluoropolyether lubricant into the pressure vessel;
(B) a step of contacting a supercritical carbon dioxide in a pressure vessel with the lubricant under predetermined conditions to extract a perfluoropolyether compound having a low-polar end group;
(C) A step of recovering the perfluoropolyether lubricant component left in the step (b).

This aspect will be described below along each step.
Step (a) is as described in the first aspect. Further, the container, the lubricant to be purified, and the like are as described in the first embodiment.

Step (b)
In step (b), the lubricant introduced into the pressure vessel is brought into contact with supercritical carbon dioxide, which is an extraction medium, to extract a perfluoropolyether compound having a low-polar end group and to remove it. To do.

  The extraction medium is supercritical carbon dioxide. The supercritical carbon dioxide is brought into contact with the lubricant raw material in the pressure vessel while flowing into the pressure vessel at a predetermined flow rate.

This step (b) can be carried out in the temperature and pressure ranges described at the beginning, but it is preferable to perform extraction at a predetermined temperature and pressure within this range. This temperature and pressure, that is, the temperature and pressure at the upper limit of this embodiment are preferably those capable of forming a state of about 0.6 g / cm ³ in terms of the density of supercritical carbon dioxide. Specifically, the temperature and pressure are, for example, 15 to 17 MPa at 60 ° C., preferably 16 MPa at the same temperature, 20 MPa to 22 MPa at 100 ° C., preferably 22 MPa at the same temperature. By bringing the lubricant into contact with supercritical carbon dioxide under such conditions, the components of the perfluoropolyether compound having a low-polar end group are selectively extracted and removed. The purified lubricant is left in the container.

  The flow rate of the supercritical carbon dioxide in this step is not particularly limited because it varies depending on the extraction conditions and the target compound to be extracted. For example, the flow rate is 1 mL / min to 100 mL / min per 100 g of lubricant, preferably 10 mL / min to 50 mL / min per 100 g of lubricant. If the flow rate is too slow, there will be a problem that the processing speed will be slow.If the flow rate is fast, the extraction efficiency will be reduced and carbon dioxide will be consumed unnecessarily. There arises a problem of being pushed out without being purified.

Step (c)
In the step (c), the perfluoropolyether lubricant remaining in the container is recovered. The perfluoropolyether lubricant can be recovered by stopping the supply of supercritical carbon dioxide and returning the temperature and pressure of the pressure vessel to room temperature and normal pressure.

  As described above, the low-polarity perfluoropolyether compound can be removed from the perfluoropolyether lubricant and the lubricant can be purified.

  The perfluoropolyether lubricant refined in this embodiment is a perfluoropolyether compound having a highly polar terminal group such as a piperonyl group or a hydroxyl group when the lubricant raw material described in the first embodiment is used. Including mainly.

The purified perfluoropolyether lubricant can be used as a lubricant for the magnetic recording medium of the present invention described later. The lubricant refined in this embodiment has a reduced amount of perfluoropolyether compounds having low-polar end groups. Therefore, when used as a material for a lubricating layer of a magnetic recording medium, the adhesive and binding properties of the lubricant are reduced. It is possible to provide an excellent magnetic recording medium. In particular, the lubricant purified by the present invention has a perfluoropolyether compound having a low-polarity group at the end group removed, so that when compared with an unpurified lubricant of the same molecular weight, the magnetic recording medium Therefore, it is possible to form a lubricating layer having high adhesion and bonding properties to the protective layer, and thus excellent heat resistance.

  Next, the third aspect of the present invention will be described. A third aspect of the present invention is a method for purifying a perfluoropolyether lubricant by removing both perfluoropolyether compounds having low-polar end groups and ionic impurities from the perfluoropolyether lubricant. It is. This embodiment is a highly productive method that can remove both perfluoropolyether compounds having low-polar end groups and ionic impurities in one purification process.

This embodiment will be described in detail. In this embodiment, the perfluoropolyether lubricant is brought into contact with the extraction medium composed of supercritical carbon dioxide under the first condition in the same pressure vessel by the supercritical extraction method. The perfluoropolyether compound having a low-polar end group contained in the lubricant is extracted from the perfluoropolyether compound and then extracted under the second condition with supercritical carbon dioxide; This is a purification method in which a lubricant from which a perfluoropolyether compound having a terminal group is removed is brought into contact with the lubricant to extract and recover the perfluoropolyether lubricant while removing ionic impurities. More specifically, this embodiment includes the following steps.

(A) introducing a perfluoropolyether lubricant into the pressure vessel;
(B) a step of contacting a supercritical carbon dioxide in a pressure vessel with the lubricant under a first condition to extract a perfluoropolyether compound having a low-polar end group;
(C) Supercritical carbon dioxide is brought into contact with the lubricant under the second condition to the perfluoropolyether lubricant left in the step (b) to extract the perfluoropolyether lubricant, The process of recovering.

This aspect will be described below along each step.
Step (a) is as described in the first aspect. Further, the container, the lubricant to be purified, and the like are as described in the first embodiment.

Step (b)
In the step (b), the lubricant introduced into the pressure vessel and the supercritical carbon dioxide as the extraction medium are brought into contact with each other under the first condition, and a perfluoropolyether compound having a low polarity end group is obtained. Extract to remove it. When the lubricant described in the first invention is used as a raw material, perfluoropolyether compounds having terminal groups such as CF ₃ , CF ₂ Cl, and CF ₂ H are removed.

  The extraction medium is supercritical carbon dioxide. The supercritical carbon dioxide is brought into contact with the lubricant raw material in the pressure vessel while flowing into the pressure vessel at a predetermined flow rate.

In this step (b), extraction is performed under the first condition. The first condition is the temperature and pressure range described at the beginning, and within the range not exceeding the temperature and pressure at which the density of supercritical carbon dioxide can form a state of about 0.6 g / cm ^3. Preferably there is. Specifically, the upper limit is, for example, a temperature and pressure of 15 to 17 MPa at 60 ° C., preferably 16 MPa at the same temperature, 20 MPa to 22 MPa at 100 ° C., preferably 22 MPa at the same temperature. By bringing the lubricant into contact with supercritical carbon dioxide under such conditions, the components of the perfluoropolyether compound having a terminal group with low polarity are selectively extracted.

  The flow rate of the supercritical carbon dioxide in this step is not particularly limited because it varies depending on the extraction conditions and the target compound to be extracted. For example, the flow rate is 1 mL / min to 100 mL / min per 100 g of lubricant, preferably 10 mL / min to 50 mL / min per 100 g of lubricant. If the flow rate is too slow, there will be a problem that the processing speed will be slow.If the flow rate is fast, the extraction efficiency will be reduced and carbon dioxide will be consumed unnecessarily. There arises a problem of being pushed out without being purified.

  By this operation, a perfluoropolyether compound having a highly polar group or the like is left in the container.

Step (c)
In step (c), the lubricant component remaining in the pressure vessel is brought into contact with supercritical carbon dioxide, which is an extraction medium, to extract a perfluoropolyether compound having a more polar end group.

  The extraction medium is supercritical carbon dioxide. The supercritical carbon dioxide is brought into contact with the lubricant raw material in the pressure vessel while flowing into the pressure vessel at a predetermined flow rate.

In this step (c), extraction is performed under the second condition. The second condition is the range of the temperature and pressure described at the beginning, and the range that does not exceed the temperature and pressure that can form a state of about 0.7 g / cm ³ at the density of supercritical carbon dioxide. preferable.
Specifically, the upper limit is, for example, a temperature and a pressure of 60 ° C., 20 MPa, or 100 ° C., 26 MPa. In step (c), since the perfluoropolyether having a terminal group having a polarity higher than that of the compound extracted in the previous step (b) is extracted, in the step (b) together with the second condition. It is also preferable to have conditions of temperature and pressure that are equal to or greater than the upper limit values mentioned. That is, the second condition of step (c) is not less than the temperature and pressure at which the density of supercritical carbon dioxide can form a state of about 0.6 g / cm ³ , and the density of supercritical carbon dioxide is about 0. It is most preferable that the temperature and pressure be less than or equal to 7 g / cm ³ .

  By bringing the lubricant into contact with supercritical carbon dioxide under such conditions, the components of the perfluoropolyether compound are selectively extracted, and ionic impurities and the like are left in the residue in the container.

  The flow rate of the supercritical carbon dioxide in this step is not particularly limited because it varies depending on the extraction conditions and the target compound to be extracted. For example, the flow rate is 1 mL / min to 100 mL / min per 100 g of lubricant, preferably 10 mL / min to 50 mL / min per 100 g of lubricant. If the flow rate is too slow, there will be a problem that the processing speed will be slow.If the flow rate is fast, the extraction efficiency will be reduced and carbon dioxide will be consumed unnecessarily. There arises a problem of being pushed out without being purified.

  In step (c), the extracted perfluoropolyether compound is further collected together with supercritical carbon dioxide, and the perfluoropolyether compound is recovered. The perfluoropolyether compound and the supercritical carbon dioxide can be separated from each other by taking out from the outlet of the pressure vessel and returning to normal temperature and normal pressure. The separated perfluoropolyether compound is recovered, and a purified perfluoropolyether lubricant is obtained. Carbon dioxide can be recovered and reused as needed.

  As described above, the lubricant can be purified by removing the perfluoropolyether compound having ionic impurities and low-polar end groups from the perfluoropolyether lubricant.

The perfluoropolyether lubricant purified in this embodiment is a perfluoropolyether compound having a more polar end group such as a piperonyl group or a hydroxyl group when the lubricant raw material described in the first embodiment is used. In addition, ionic impurities such as sodium ions, potassium ions, chlorine ions, HCO ₃ ions, HSO ₄ ions, and sulfate ions are removed.

  The purified perfluoropolyether lubricant can be used as a lubricant for the magnetic recording medium of the present invention described later. When the lubricant refined in this embodiment is used as a material for the lubricating layer of a magnetic recording medium because the perfluoropolyether compound having a low-polar end group is reduced and ionic impurities are also removed. In addition, it is possible to provide a magnetic recording medium having high adhesion and bonding with the protective layer, and thus excellent heat resistance and excellent long-term stability.

  Further, the present invention is not a purification method for purifying a lubricant by a separation means using a packing material such as a column, so that it is inexpensive, excellent in durability, and excellent in productivity as a result. In particular, the method of the present invention is a general-purpose method capable of purifying a small amount to a large amount of lubricant as described in the examples described later. Further, by using the method of the present invention, the lubricant can be purified by utilizing the slight difference in properties such as the molecular weight of perfluoropolyether, the functional group, or the polarity of each compound or ion contained in the lubricant. It can be carried out. Furthermore, the purification method of the present invention can be applied batchwise and continuously by appropriately selecting containers and the like.

Next, the second invention will be described.
The second invention is a lubricant, particularly a magnetic recording medium lubricant, purified by the first invention. More specifically, the lubricant of the present invention is a lubricant from which ionic impurities of the lubricant have been removed, a lubricant from which perfluoropolyether components having low-polar end groups have been removed, or a ionic impurity and a polar one. It is a lubricant from which a perfluoropolyether component having a low end group is removed.

  The lubricant of the present invention is highly polar by selecting perfluoropolyether compounds based on the difference in polarity of the end groups of the perfluoropolyether molecule, the polarity of ionic impurities and the polarity of the perfluoropolyether compound, etc. It is a lubricant with an increased content of perfluoropolyether compounds having terminal groups. For this reason, adhesiveness and bondability with the protective layer are enhanced, and as a result, spin migration is suppressed, and evaporation of the lubricant due to heat is prevented.

Furthermore, the lubricant of the present invention removes ionic impurities that cause the lubricant to be decomposed, so that the lubricant is prevented from being decomposed, and as a result, the long-term stability of the lubricant is ensured. Examples of the ionic impurities of the lubricant of the present invention include ions such as Na ⁺ , K ⁺ , Cl ⁻ , SO ₄ ²⁻ and HSO ₄ ^{− in} the case of the lubricant described in the first invention.

Next, the third invention will be described.
A third invention is a magnetic recording medium using the lubricant of the present invention.

  Hereinafter, the present invention will be described with reference to the drawings. However, the following description is an embodiment, and the present invention is not limited thereto.

  FIG. 1 is a schematic sectional view of a perpendicular magnetic recording medium of the present invention. As shown in FIG. 1, the perpendicular magnetic recording medium of the present invention has a structure in which at least a nonmagnetic underlayer 3, a magnetic recording layer 4, a protective layer 5, and a liquid lubricant layer 6 are sequentially laminated on a substrate.

  In the present invention, as shown in FIG. 1, the substrate is preferably one in which a plating layer 2 (for example, nickel-phosphorus (Ni-P) plating layer) is formed on the nonmagnetic substrate 1 by electroless plating or the like ( In this specification, the nonmagnetic substrate 1 and the plating layer 2 are collectively referred to as a substrate).

  In the present invention, the nonmagnetic substrate 1 is formed of a material conventionally used for a magnetic recording medium. For example, an aluminum (Al) alloy, tempered glass, crystallized glass, a plastic substrate, or the like can be used as the material of the substrate 1.

  A nonmagnetic underlayer 3, a magnetic layer 4, a protective layer 5, and a liquid lubricant layer 6 are sequentially formed on this substrate to obtain a magnetic recording medium.

  Conventionally used materials can be used for the nonmagnetic underlayer 3, the magnetic layer 4, and the protective layer 5. Specifically, the nonmagnetic underlayer 3 is an underlayer made of, for example, Cr, and the magnetic layer 4 is made of a Co alloy, for example, a ferromagnetic alloy such as Co—Cr—Ta, Co—Cr—Pt, and the like. The layer 5 is, for example, a carbon protective layer.

  The liquid lubricant layer 6 uses a perfluoropolyether lubricant purified by the purification method of the present invention.

  Although the magnetic recording medium of the present invention has been described with reference to FIG. 1, this structure is an example, and various modifications can be made according to the purpose of the magnetic recording medium. Further, the shape can be adapted to the equipment using the magnetic recording medium of the present invention, and is not particularly limited. For example, a disk-shaped magnetic recording medium such as that used in an HDD can be used.

  In the method of manufacturing a magnetic recording medium according to the present invention, the above-described substrate is used, the nonmagnetic underlayer 3 is coated on the substrate, and the magnetic layer 4 and the protective layer 5 are sequentially formed thereon. Thereafter, the lubricant of the present invention is applied to the surface of the protective layer 5.

  In the present invention, the nonmagnetic underlayer 3 is preferably a Cr layer, and the magnetic layer 4 is preferably a Co alloy layer.

  The nonmagnetic underlayer 3, the magnetic layer 4, and the protective film 5 can be formed by DC sputtering when these are, for example, a Cr nonmagnetic underlayer, a Co magnetic alloy layer, and a carbon protective film. Moreover, when the protective layer 5 is a carbon protective layer, it may be a carbon protective layer mainly composed of ordinary graphite or a DLC protective layer. The lubricant layer 6 can be applied by a dip coating method, a spin coating method, or the like. The thicknesses of the nonmagnetic underlayer 3, the magnetic layer 4, the protective layer 5, and the lubricant layer 6 and the conditions for film formation can be the same as those used for ordinary magnetic recording media. In addition, said structure does not limit this invention.

  EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples. In the following examples, “Fomblin Z tetraol” (hereinafter referred to as “Z-tetraol”) and “AM3001” commercially available from Augmont are used as examples of perfluoropolyether lubricants.

Example 1
Using an extraction container (thick stainless steel container with an inlet and an outlet) with an internal volume of 80 mL, 40 g of a lubricant (Z-tetraol) was injected into it, and supercritical carbon dioxide was flowed at a feeding rate of 8 mL / min. . The temperature was fixed at 60 ° C., the pressure was changed with the passage of time, and fractionation was performed at each pressure stage to obtain extraction components ZT60-1 to ZT60-10 shown in Table 1. Table 1 shows the pressure and elapsed time together.

Example 2
Using the same container as in Example 1, 40 g of lubricant (Z-tetraol) was injected with the same amount of liquid feeding. The temperature was fixed at 100 ° C., the pressure was changed with the passage of time, and fractionation was performed at each pressure stage to obtain extraction components ZT100-1 to ZT100-11 shown in Table 2. Table 2 shows the pressure and elapsed time together.

Example 3
Using the same container as in Example 1, 40 g of lubricant (AM3001) was injected with the same amount of liquid feeding. The temperature was fixed at 60 ° C., the pressure was changed over time, and fractionation was carried out at each pressure stage to obtain extraction components AM60-1 to AM60-3 shown in Table 3. Table 3 shows the pressure and elapsed time together.

Example 4
Using the same container as in Example 1, 40 g of lubricant (AM3001) was injected with the same amount of liquid feeding. The temperature was fixed at 65 ° C., the pressure was changed over time, and fractionation was carried out at each pressure stage to obtain the extraction components AM65-1 to AM65-3 shown in Table 4. Table 3 shows the pressure and elapsed time together.

Example 5
An extraction container (thick stainless steel container having an inlet and an outlet) with an internal volume of 450 mL was used, 250 g of lubricant (AM3001) was injected into this, and supercritical carbon dioxide was allowed to flow at a feeding rate of 20 mL / min. The temperature was fixed at 63 ° C. and held at a pressure of 14.5 MPa for 60 minutes, during which 100 g of the extracted fraction was removed. Thereafter, the pressure was increased to 16 MPa, and the pressure was maintained for 40 minutes to obtain 100 g of the target component. As a result of measuring the molecular weight of this target component by the GPC method, a lubricant having a number average molecular weight (Mn) of 3400 and a weight average molecular weight (Mw) of 3900 was obtained.

In addition, when the amount of ionic impurities before purification and after purification (the above target component) was measured by ion chromatography, the amount of sodium ions decreased from 1.4 ppm to 0.3 ppm, and the amount of potassium ions Decreases from 1.0 ppm to 0.05 ppm, the amount of sulfate ions (HSO ₄ ions) decreases from 0.03 ppm to 0.01 ppm or less (not detected), and the amount of chloride ions decreases from 0.07 ppm to 0 ppm. Reduced to .04 ppm. Ammonia ions, nitrate ions, formate ions, and oxalate ions were hardly detected in both the unpurified and purified samples.

Comparative Example 1
In Comparative Example 1, various analyzes shown in the following evaluation were performed using a commercially available AM3001 lubricant as it was without purification.

Comparative Example 2
40 g of AM3001 lubricant was stirred with 40 g of butyl acetate at 60 ° C. (fixed) for 30 minutes, and then allowed to stand for 12 hours, and the lower layer portion was separated with a separatory funnel. The butyl acetate remaining in the lower layer portion was removed by an evaporator to obtain 20 g of a purified lubricant (butyl acetate purified AM).

Evaluation 1. Results of analysis by time-of-flight secondary ion mass spectrometry (TOF-SIMS) The extracted fractions of Examples 1 and 2 and the sample of Comparative Example 1 were applied to the surface of a magnetic recording medium not coated with a lubricant. Several micrograms was dripped and the surface was measured by TOF-SIMS. The results are shown in Table 5. Table 5 shows cation and anion fragments among various fragments detected in each extract fraction or sample. Each numerical value represents the amount of these fragments. In addition, a numerical value is the quantity (count number) of the fragment detected per unit time by TOF-SIMS.

As can be seen from the table, many cationic fragments are detected in the fraction extracted on the high pressure side and the extraction residue. As for anion fragments (HCO ₃ , HSO ₄ and Cl), a large amount of HCO ₃ and HSO ₄ is detected in the extraction residue, and Cl is extracted in the low pressure side extracted fraction ( ZT60-1 to ZT60-3 and ZT100-1 to ZT100-4) were also detected. Here, in ZT60-1 to ZT60-3 and ZT100-1 to ZT100-4, a CF ₂ Cl fragment was also detected at the same time as the Cl fragment, but the high pressure extraction fraction and the extraction residue (ZT60-9, ZT60- 10 and ZT100-11), no CF ₂ Cl fragment was detected. Thus, the Cl fragments of ZT60-9, ZT60-10 and ZT100-11 are due to chloride ions, while the Cl fragments of ZT60-1 to ZT60-3 and ZT100-1 to ZT100-4 are CF It is thought to be a fragment derived from a lubricant compound containing ₂ Cl groups.

From the above results, it can be seen that when extraction is performed at a carbon dioxide pressure of 60 ° C. and 20 MPa or more, sodium, potassium, Cl ions, HSO ₄ ions and the like are extracted together, which is not preferable as a lubricant. Moreover, it turns out that the same result is obtained also when extracting at 100 degreeC and 26 Mpa or more. Both carbon dioxide at these temperatures and pressures have a density of about 0.7 g / cm ³ , and in the present invention, supercritical carbon dioxide with a density above this condition is an ionic impurity in the perfluoropolyether lubricant. Is considered to dissolve. From this, in order to remove ionic impurities in the perfluoropolyether lubricant in the present invention, supercritical carbon dioxide is used, and the density of supercritical carbon dioxide at 60 ° C., 20 MPa or less or 100 ° C., 26 MPa or less ( That is, it is preferable to perform supercritical extraction under conditions equal to or less than those corresponding to a density of about 0.7 g / cm ³ or less.

Further, it can be seen from the behavior of the Cl fragment that the low pressure side extract fraction contains a low-polarity perfluoropolyether compound having CF ₂ Cl (low-polarity group) at the end group of the lubricant. Therefore, the fraction extracted with supercritical carbon dioxide (about 0.6 g / cm ³ ) up to about 60 ° C. and 16 MPa, or up to about 100 ° C. and about 22 MPa contains a lot of low-polar perfluoropolyether compounds. it is conceivable that. From this, in order to remove the low-polarity perfluoropolyether compound in the perfluoropolyether lubricant in the present invention, supercritical carbon dioxide is used, and 15 to 17 MPa at 60 ° C., preferably 16 MPa at the same temperature, 100 MPa. It is particularly preferred to perform supercritical extraction under conditions corresponding to a density of supercritical carbon dioxide (ie, a density of about 0.6 g / cm ³ or less) at 20 ° C. to 22 MPa, preferably 22 MPa at the same temperature.

Furthermore, when the above results are combined, in order to remove ionic impurities, it corresponds to the density of supercritical carbon dioxide at 60 ° C. and 20 MPa or less or 100 ° C. and 26 MPa or less (density of about 0.7 g / cm ³ or less). It can be seen that supercritical extraction should be carried out under the following conditions. When reducing the low-polarity perfluoropolyether compound, for example, 15 to 17 MPa at 60 ° C., preferably 16 MPa at the same temperature, 20 to 22 MPa at 100 ° C., preferably 22 MPa at the same temperature, preferably 22 MPa. It can be seen that the supercritical extraction may be performed under the conditions corresponding to the density (density of about 0.6 g / cm ³ or less). Furthermore, in order to obtain a perfluoropolyether lubricant from which ionic impurities and low-polarity perfluoropolyether compounds have been removed, first supercritical carbon dioxide is used, for example, 15 to 17 MPa at 60 ° C., preferably at the same temperature. Supercritical extraction is performed under conditions corresponding to a density of supercritical carbon dioxide (density of about 0.6 g / cm ³ or less) of 16 MPa at 100 ° C., 20 MPa to 22 MPa at 100 ° C., preferably 22 MPa at the same temperature. Supercritical extraction may be performed under conditions corresponding to the density of supercritical carbon dioxide (density of about 0.7 g / cm ³ or less) at 100 ° C., 20 MPa or less, or 100 ° C., 26 MPa or less.

The reason why the above extraction is possible is that, as a characteristic of supercritical carbon dioxide, it is first a non-polar solvent, and secondly, the density is about 0.4 g / cm by changing the temperature and pressure. ^A large change continuously from ³ to 0.9 g / cm ³ is accompanied by an increase in the dissolving power. Therefore, by utilizing the difference in solubility of the target lubricant component and ions with respect to supercritical carbon dioxide, ionic impurities other than the above ionic impurities, such as metal ions, ammonia ions, oxalate ions, formate ions, etc. It is also possible to separate organic ions.

2. Measurement of lubricant characteristics Molecular weight measurement, supercritical chromatography (SFC) measurement, thermogravimetric analysis (TG) temperature measurement at 10% weight loss of lubricant, and magnetic recording medium spin-off test .

(I) Molecular weight measurement The molecular weight of each sample of AM65-2 of Example 4, AM60-2 of Example 3, unpurified AM of Comparative Example 1, and butyl acetate purified AM of Comparative Example 2 was measured. The molecular weight was measured using a known GPC method, and the number average molecular weight (Mn) and the weight average molecular weight (Mw) were determined. The results are shown in Table 6.

(Ii) Measurement of end groups by supercritical chromatography (SFC) method The lubricant fraction (AM65-2) of Example 4 was subjected to SFC. The conditions were as follows.
Mobile phase: 45 ° C., 20 MPa CO ₂ , flow rate 3 mL / min,
Sample injection amount: 5 μg,
Detector: ELSD
Use a silica gel column.

As a result of the measurement, the chromatogram shown in FIG. 2 was obtained. The fraction corresponding to the first peak of this chromatogram was collected and analyzed by TOF-SIMS. As a result, fragments such as CF ₂ Cl and CF ₂ H appeared stronger and the polarity was higher than the material before fractionation. Fragments based on piperonyl groups, which are terminal groups of perfluoropolyether compounds, appeared extremely weakly. From this, it can be seen that the first peak appearing in the SFC method is based on a lubricant molecule having a weakly polar end group. In the TOF-SIMS method, CF ₃ fragments are also generated from the main chain of the lubricant, so it is unclear whether there is a lubricant component terminated with CF ₃ , but if present, SFC Therefore, it is determined to be included in the first peak. The area ratio of the first peak of AM65-2 to all peaks was calculated.

  For each sample of AM60-2 of Example 4, AM60-2 of Example 3, unpurified AM of Comparative Example 1, and butyl acetate purified AM of Comparative Example 2, in addition to AM65-2 of Example 4, the area ratio of the first peak to all peaks Was calculated. These results are shown in Table 6.

(Iii) Measurement of temperature when lubricant is reduced by 10% by thermogravimetric analysis (TG) method An analysis was performed by the TG method in order to measure the loss of lubricant by heating. For each sample of AM65-2 of Example 4, AM60-2 of Example 3, unpurified AM of Comparative Example 1, and butyl acetate purified AM of Comparative Example 2, each sample was heated and the lubricant was 10 wt. The temperature at the time of% reduction was measured by the TG method and recorded. The results are shown in Table 6.

(Iv) Spin-off test of magnetic recording medium Spin-off test was performed on each sample of AM65-2 of Example 4, AM60-2 of Example 3, unpurified AM of Comparative Example 1, and butyl acetate purified AM of Comparative Example 2. went.

  A non-magnetic substrate (material: Al alloy composed of Al / Mg alloy) was subjected to Ni-P plating by electroless plating to form a plating layer, and then the surface was polished by polishing. Next, a substantially concentric groove was produced by a texture using diamond slurry so that the surface roughness (Ra) was 0.8 nm. After cleaning this substrate, a nonmagnetic underlayer of Cr is formed in a commonly used sputtering apparatus, a magnetic layer containing Co, Cr, Ta, and Pt, and an amorphous carbon (a- A protective layer of C: N) was formed by the CVD method. As a result, a magnetic recording medium substrate having a diameter of 95 mm was obtained. A lubricant was applied to the magnetic recording medium substrate by a dipping method so as to have a film thickness of 1.6 nm, thereby obtaining a magnetic recording medium. The above four samples were used as lubricant samples.

The spin-off test was performed by rotating the magnetic recording medium prepared as described above at 7200 min ⁻¹ for 120 hours in an environment at a temperature of 55 ° C. and a relative humidity of 80%. The film thickness with a radius of 18 mm, which is the CSS region, was measured before and after the start of the test, and the amount of film thickness variation (%) was determined. The results are shown in Table 6.

  As shown in Table 6, AM60-2 and AM65-2 purified by the method of the present invention have an SFC first peak area ratio that is less than half that of unpurified AM. From this, it can be seen that the component having a small polarity of the end group of the lubricant molecule is decreased.

  Further, the molecular weights of AM60-2 and AM65-2 were smaller than those of butyl acetate purified AM, but the results of 10% weight loss temperature of the lubricant and spin-off test were similar to those of butyl acetate purified AM. Therefore, the lubricant purified by the method of the present invention has a characteristic that it has a high binding property while having a low molecular weight. This is a result of the removal of the low polarity components of the lubricant end groups.

  The magnetic recording medium using the lubricant purified by the method of the present invention can reduce the influence on the magnetic head even if the flying height of the magnetic head is low. Therefore, the magnetic recording medium using the lubricant of the present invention can be a magnetic recording medium excellent in long-term stability.

1 is a schematic cross-sectional view of a magnetic recording medium of the present invention. It is a SFC elution graph of lubricant AM65-2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Nonmagnetic board | substrate 2 Plating layer 3 Nonmagnetic base layer 4 Magnetic recording layer 5 Protective layer 6 Liquid lubricant layer

Claims (4)

A method for purifying a lubricant containing a compound having a perfluoropolyether structure by a supercritical extraction method, wherein the extraction medium is made of supercritical carbon dioxide under the first condition in the same pressure vessel and the lubricant. A perfluoropolyether compound having a low-polar end group, which is a perfluoropolyether compound containing a group selected from CF ₃ , CF ₂ H or CF ₂ Cl as the end group. Then, in the second condition, the extraction medium consisting of supercritical carbon dioxide and the lubricant from which the perfluoropolyether compound having a low polarity end group has been removed are brought into contact with each other, and sodium ions, potassium ions, chlorine ions, HCO ₃ Perfluoropolyether while removing ionic impurities selected from ions, HSO ₄ ions or sulfate ions A refining method characterized in that the lubricant is extracted and recovered. 2. The purification method according to claim 1 , wherein the first condition is a temperature and pressure at which the density is equal to or less than a density of supercritical carbon dioxide obtained at a temperature of 60 ° C. and a pressure of 16 MPa, and the second condition. Is a temperature and pressure at a density equal to or lower than that of supercritical carbon dioxide obtained at a temperature of 60 ° C. and a pressure of 20 MPa. A perfluoropolyether lubricant purified by the purification method according to claim 1 or 2 . The perfluoropolyether lubricant according to claim 3 , wherein the lubricant is a magnetic recording medium in which at least a nonmagnetic underlayer, a magnetic layer, a protective layer, and a lubricating layer are sequentially laminated on a nonmagnetic substrate. A magnetic recording medium characterized by the above.

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