JP2024112092A - Polybutylene Terephthalate and Polyester Films - Google Patents

Abstract

To provide: polybutylene terephthalate having a sufficiently low volume resistivity value at a temperature suitable for film formation and being capable of high-speed film formation; and a polyester film using the polybutylene terephthalate.SOLUTION: Provided is polybutylene terephthalate having a volume resistivity value at 250°C(ρV(250°C)) of 13×107 Ω cm or less, preferably polybutylene terephthalate having a volume resistivity value at 285°C (ρV(285°C)) of 10×107 Ω cm or less and a ratio, (ρV(250°C)/ρV(285°C)), of the volume resistivity value at 250°C (ρV(250°C)) to the volume resistivity value at 285°C(ρV(285°C)) is 3.0 or less. Also provided is a polyester film obtained by film formation of a film forming raw material comprising the polybutylene terephthalate in an amount of 50 mass%.SELECTED DRAWING: None

Description

The present invention relates to polybutylene terephthalate (hereinafter sometimes abbreviated as "PBT"), which allows for high-speed film production, and to polyester films using this polybutylene terephthalate.

PBT, which uses terephthalic acid (hereinafter sometimes abbreviated as "TPA") as the main dicarboxylic acid component and 1,4-butanediol (hereinafter sometimes abbreviated as "BDO") as the main diol component, has excellent mechanical properties, heat resistance, moldability and recyclability, and has high mechanical strength and excellent chemical resistance, so it is widely used as a material for industrial molded products such as connectors, relays and switches for automobiles and electrical and electronic devices. BPT is also widely used as a molding material for films, sheets, fibers (filaments), etc., and as a result, high-quality PBT is in demand.

PBT film in particular has excellent resistance to bending fatigue, abrasion, and pinholes, and, like polyethylene terephthalate (hereinafter sometimes abbreviated as "PET"), has excellent chemical resistance. Another desirable feature is that it does not absorb moisture like nylon, and does not experience dimensional changes due to moisture absorption. For these reasons, PBT film, and in particular biaxially oriented PBT film, has been attracting attention in recent years.

In film applications, problems have arisen in recent years with the increasing speed of film production, such as deterioration of flatness during film melt extrusion and reduced productivity due to film breakage. In the case of a film production method in which an unstretched sheet is obtained by a method of extruding the film through a T-die and casting it onto a cooling roll, and then stretching the unstretched sheet, it is necessary to increase the adhesion between the extruded sheet-like molten material and the surface of the cooling roll in order to eliminate surface defects in the obtained film and increase the thickness uniformity.
As a method for enhancing the adhesion between the sheet-like molten material extruded from a T-die and the surface of a cooling roll, a method (electrostatic adhesion casting method) in which a wire-shaped electrode is provided between the extrusion die and the cooling roll, a high voltage is applied to the wire electrode, static electricity is generated on the surface of the unsolidified sheet-like molten material, and the sheet-like molten material is rapidly cooled while being in close contact with the surface of the cooling roll is effective.
In order to effectively carry out this electrostatic contact casting method, PBT is required that has a low volume resistivity (hereinafter sometimes abbreviated as "ρV") when molten and has high adhesion to the cooling roll during film melt extrusion.

Patent Document 1 discloses polyester resin pellets having a volume resistivity of 8× ¹⁰ Ω·cm or less when melted at 285° C., but the only polyester resin specifically described in Patent Document 1 is PET, and there is no specific disclosure about PBT, much less any description or suggestion about the volume resistivity of PBT at 250° C. PET does not melt at 250° C., so the volume resistivity at 250° C. cannot be measured.

Patent Document 2 discloses a biaxially stretched polyester film for protecting the rear surface of a solar cell that satisfies the volume resistivity ρν≦10× ¹⁰ Ω·cm at 285° C., but even in this Patent Document 2, the only polyester resin specifically described is PET, and there is no specific disclosure of PBT, much less any description or suggestion of the volume resistivity of PBT at 250° C. As described above, PET does not melt at 250° C., so the volume resistivity at 250° C. cannot be measured.

Patent Document 3 discloses a biaxially stretched polybutylene terephthalate film having a melt resistivity of 0.10×10 ⁸ Ω·cm or more and 1.0×10 ⁸ Ω·cm or less, and describes the polyester resin composition used therefor as having a melt resistivity of 1.0×10 ⁸ Ω·cm or less, preferably 0.5×10 ⁸ Ω·cm or less, and more preferably 0.25×10 ⁸ Ω·cm or less at 265°C. However, in the examples of Patent Document 3, the volume resistivity of the PBT (Novaduran 5020 made by Mitsubishi Engineering Plastics, melting point 220°C) used in the production of the PBT film at 250°C exceeds 13×10 ⁷ Ω·cm, as shown in Comparative Example 8 below.
Furthermore, Patent Document 3 describes that it is preferable to contain the alkaline earth metal compound and the phosphorus compound in the film in such a manner that the mass ratio (M2/P) of the alkaline earth metal atom (M2) to the phosphorus atom (P) is 1.2 or more and 5.0 or less, and describes that if the M2/P value exceeds 5.0, it is undesirable because it will cause more adverse effects, such as the promotion of foreign matter generation and coloring of the film, rather than the effect of reducing the melt resistivity.

JP 2016-132733 A JP 2014-80609 A International Publication No. 2015/072163

As described above, in order to effectively perform the electrostatic contact casting method, polyester resins with low volume resistivity have been investigated. However, PBT with a sufficiently low volume resistivity at temperatures suitable for film formation has not yet been provided.
An object of the present invention is to provide a polybutylene terephthalate having a sufficiently low volume resistivity at a temperature suitable for film formation, which allows for high-speed film formation, and a polyester film using this polybutylene terephthalate.

The present inventors have discovered a PBT that can solve the above problems, and have completed the present invention.
That is, the present invention relates to the following.

[1] Polybutylene terephthalate having a volume resistivity at 250° C. (ρV(250° C.)) of 13×10 ⁷ Ω·cm or less.

[2] The polybutylene terephthalate according to [1], having a volume resistivity at 285° C. (ρV(285° C.)) of 10×10 ⁷ Ω·cm or less.

[3] Polybutylene terephthalate according to [1] or [2], in which the ratio (ρV(250°C)/ρV(285°C)) of the volume resistivity at 250°C (ρV(250°C)) to the volume resistivity at 285°C (ρV(285°C)) is 3.0 or less.

[4] The polybutylene terephthalate according to any one of [1] to [3], having a solution haze of 40% or less as measured by the following method.
<Method of measuring solution haze>
For a solution in which 2.70 g of polybutylene terephthalate is dissolved in 20 mL of a mixed solvent of phenol/tetrachloroethane = 3/2 (mass ratio), the solution haze is measured using a cell length of 10 mm.

[5] The titanium atom (Ti) content is 37 to 100 ppm, the alkaline earth metal atom (M2) content is 15 ppm or less, and the phosphorus atom (P) content is 9 ppm or less,
The polybutylene terephthalate according to any one of [1] to [4], wherein the mass ratio (M2/P) of the alkaline earth metal atom (M2) to the phosphorus atom (P) is 5.1 or more.

[6] A polyester film produced by forming a film from a film-forming raw material containing 50% by mass or more of the polybutylene terephthalate described in any one of [1] to [5].

The polybutylene terephthalate of the present invention has a low volume resistivity at a temperature suitable for film formation, can be formed at high speed, and has excellent film productivity. Furthermore, by adjusting the titanium atom content and solution haze of the polybutylene terephthalate to a predetermined value, a higher quality film with fewer fish eyes, which are defects in the film, can be obtained.
In addition, the polybutylene terephthalate of the present invention has a low volume resistivity at a temperature suitable for forming a blend film of PET (pellets) and PBT (pellets), enabling high-speed film formation, and is also excellent in productivity for a blend film of PET and PBT, making it possible to obtain a high-quality PET/PBT blend film with few fisheyes, which are a defect in the film.
Therefore, by using the polybutylene terephthalate of the present invention, polyester films such as PBT films and PBT/PET blend films can be produced efficiently with high productivity and yield.

The present invention will now be described in detail. However, the following description of the components is a representative example of the embodiment of the present invention, and the present invention is not limited to these contents.

<PBT>
(Dicarboxylic acid component, diol component, copolymer component)
In the present invention, PBT refers to a polymer having a structure in which a terephthalic acid (TPA) component and 1,4-butanediol (BDO) are ester-bonded, in which 50 mol % or more of the dicarboxylic acid component is composed of the TPA component, and 50 mol % or more of the diol component is composed of BDO. The proportion of the TPA component in all dicarboxylic acid components is preferably 70 mol % or more, more preferably 80 mol % or more, and particularly preferably 95 mol % or more, and the proportion of BDO in all diol components is preferably 70 mol % or more, more preferably 80 mol % or more, and even more preferably 95 mol % or more. If the TPA component or BDO is less than 50 mol %, the crystallization rate of PBT decreases, and bending fatigue resistance, abrasion resistance, pinhole resistance, and chemical resistance deteriorate.

In the present invention, the dicarboxylic acid component other than terephthalic acid constituting PBT is not particularly limited, and examples thereof include aromatic dicarboxylic acids such as phthalic acid, isophthalic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 4,4'-benzophenonedicarboxylic acid, 4,4'-diphenoxyethanedicarboxylic acid, 4,4'-diphenylsulfonedicarboxylic acid, and 2,6-naphthalenedicarboxylic acid; alicyclic dicarboxylic acids such as 1,2-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid; aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid; and derivatives thereof. These dicarboxylic acid components other than terephthalic acid may be used alone or in combination of two or more.
As a method for producing PBT, there are a direct polymerization method using terephthalic acid (TPA) as a dicarboxylic acid component and a DMT method using a terephthalic acid ester such as dimethyl terephthalate. From the viewpoint of production costs, it is preferable to use TPA as a raw material for the dicarboxylic acid component of the PBT of the present invention and produce PBT by the direct polymerization method.

In the present invention, the diol component other than BDO constituting the PBT is not particularly limited, and examples thereof include aliphatic diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 1,8-octanediol, and dibutylene glycol; alicyclic diols such as 1,2-cyclohexanediol, 1,4-cyclohexanediol, 1,1-cyclohexanedimethylol, and 1,4-cyclohexanedimethylol; and aromatic diols such as xylylene glycol, 4,4'-dihydroxybiphenyl, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone.
As for these diol components other than BDO, one type may be used alone, or two or more types may be used in combination.

The PBT of the present invention can further contain one or more copolymerization components such as monofunctional components such as hydroxycarboxylic acids, alkoxycarboxylic acids, stearyl alcohol, benzyl alcohol, stearic acid, benzoic acid, t-butylbenzoic acid, and benzoylbenzoic acid, including lactic acid, glycolic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, 6-hydroxy-2-naphthalenecarboxylic acid, and p-β-hydroxyethoxybenzoic acid; and polyfunctional components having three or more functional groups, including tricarballylic acid, trimellitic acid, trimesic acid, pyromellitic acid, gallic acid, trimethylolethane, trimethylolpropane, glycerol, and pentaerythritol.

(Terminal carboxyl group concentration, terminal hydroxyl group concentration, terminal vinyl group concentration)
The terminal carboxyl group concentration of the PBT of the present invention is usually 0.1 to 50 equivalents/ton, preferably 1 to 40 equivalents/ton, more preferably 5 to 30 equivalents/ton, particularly preferably 7 to 25 equivalents/ton, and most preferably 10 to 19 equivalents/ton. If the terminal carboxyl group concentration is too high, the hydrolysis resistance may deteriorate. In order to make this value less than 0.1 equivalents/ton, for example, economically disadvantageous conditions such as a very small production scale must be adopted, which is not realistic.

The terminal hydroxyl group concentration of the PBT of the present invention is usually 23 to 60 equivalents/ton, preferably 25 to 50 equivalents/ton, more preferably 27 to 45 equivalents/ton, and particularly preferably 30 to 40 equivalents/ton. If the terminal hydroxyl group concentration is too high, the amount of tetrahydrofuran (hereinafter sometimes abbreviated as "THF") generated due to decomposition of the terminal hydroxyl groups during melting such as molding will increase. For example, THF will be generated during film formation, causing poor appearance due to THF, which is undesirable. If the terminal hydroxyl group concentration is equal to or higher than the above lower limit, PBT can be easily manufactured.

The terminal vinyl group concentration of the PBT of the present invention is usually 0.1 to 13 equivalents/ton, preferably 0.5 to 12 equivalents/ton, and more preferably 1 to 11 equivalents/ton. If the terminal vinyl group concentration is too high, it will cause deterioration in color tone. The terminal vinyl group concentration tends to increase further due to the thermal history during molding, so if the molding temperature is high or if the manufacturing method includes a recycling process, the color tone will deteriorate even more significantly. If the terminal vinyl group concentration is equal to or higher than the above lower limit, PBT can be easily manufactured.

The terminal carboxyl group concentration, terminal hydroxyl group concentration, and terminal vinyl group concentration of PBT can be determined by the method described in the Examples section below.

<Method of manufacturing PBT>
The PBT of the present invention is produced through a process in which a dicarboxylic acid component mainly composed of TPA and a diol component mainly composed of BDO are mixed in a predetermined ratio with stirring to obtain a raw material slurry, then the raw material slurry is heated under normal or reduced pressure to cause an esterification reaction to obtain a PBT oligomer, and then the obtained oligomer is gradually reduced in pressure and heated to cause a melt polycondensation reaction to obtain PBT in a melt polycondensation process.

An example of the process for producing oligomers is a method in which an esterification reaction is carried out using a single esterification reaction tank or a multi-stage reaction apparatus in which multiple esterification reaction tanks are connected in series, with or without a catalyst, under normal or reduced pressure, while removing water produced in the reaction and excess diol components from the system, until the esterification reaction rate (the proportion of all carboxyl groups in the raw dicarboxylic acid component that have reacted with the diol component and been esterified) usually reaches 90% or more.

In general, the temperature of the esterification reaction is preferably 212 to 233° C., more preferably 220 to 230° C., and even more preferably 223 to 228° C. If the esterification reaction temperature is low, the esterification reaction tends to be delayed, whereas if the esterification reaction temperature is high, the amount of THF produced as a by-product tends to increase, and the molar consumption ratio of the raw material tends to increase.
The pressure for the esterification reaction is preferably about 10 to 133 kPa, and the residence time in the esterification reaction tank is preferably about 1 to 4 hours.

An example of the melt polycondensation process is a method in which a single melt polycondensation tank or multiple melt polycondensation tanks are connected in series, and a multi-stage reaction apparatus is used, for example, with the first stage being a complete mixing type reactor equipped with an agitator, and the second and third stages being horizontal plug-flow type reactors equipped with agitators, in which the diol produced is distilled out of the system while being heated under reduced pressure in the presence of a catalyst.

The polycondensation reaction is usually carried out at a temperature of 210 to 280°C, preferably about 220 to 250°C, under reduced pressure of 27 kPa or less, preferably 13 kPa or less. The reaction tank may be a single tank or multiple tanks, but in order to prevent discoloration and deterioration and to suppress an increase in terminal groups such as vinyl groups, it is recommended that at least one reaction tank be carried out under a high vacuum of usually 1.3 kPa or less, preferably 0.3 kPa or less. To increase the reaction rate, it is recommended to adopt conditions such as increasing the degree of reduced pressure, accelerating the rate of temperature rise, and increasing the rate of renewal of the reaction liquid surface.

The PBT obtained by the polycondensation reaction is usually extracted in the form of a strand or sheet from an outlet provided at the bottom of the polycondensation reaction tank, and then cut with a cutter while being cooled with water or after being cooled with water to form pellets, chips, or other granular material (e.g., about 3 to 10 mm in length).

<Solid-state polymerization of PBT>
If necessary, the PBT obtained by the above melt polycondensation reaction can be subjected to solid-phase polymerization to obtain the PBT pellets of the present invention.
In order to prevent oxidation deterioration of PBT, the solid-state polymerization is usually carried out under reduced pressure or in an inert gas stream. The temperature is not particularly limited, but if it is too high, coloring tends to increase, and if it is too low, the molecular weight does not increase at a practical rate, so it is usually 150 to 220 ° C, preferably 160 to 215 ° C, more preferably 180 to 213 ° C, and even more preferably 190 to 210 ° C. The time of solid-state polymerization is not particularly limited, but in the case of a copolymerization system with a low melting point, a low temperature and long time condition is selected, and in the case of increasing production efficiency, a high temperature and short time condition is selected. In general, the solid-state polymerization time after reaching a predetermined temperature is usually 2 to 20 hours, preferably 3 to 15 hours, and more preferably 4 to 12 hours.

<Polycondensation catalyst>
When the oligomer obtained by the esterification reaction between the diol component and the dicarboxylic acid component is polycondensed, a titanium compound and preferably a compound of a metal of Group 2A of the Periodic Table (alkaline earth metal) are usually used as a catalyst.
These catalyst components may be used in the esterification reaction and then used in the polycondensation reaction, or may not be used in the esterification reaction, or may be used only as a titanium compound catalyst, with the remaining catalyst components added at the polycondensation stage. Furthermore, a part of the catalyst amount to be finally used may be used in the esterification reaction, and then appropriately added as the polycondensation reaction proceeds. In any case, in the present invention, titanium and preferably an alkaline earth metal, which is a metal of Group 2A of the periodic table, are inevitably contained in the PBT finally obtained, and the amount thereof will be described later.

Specific examples of titanium compounds include inorganic titanium compounds such as titanium oxide and titanium tetrachloride, tetraalkyl titanates such as tetramethyl titanate, tetraisopropyl titanate and tetrabutyl titanate, and tetraaryl titanates such as tetraphenyl titanate, etc. These may be used alone or in combination of two or more.
Of these, tetraalkyl titanates are preferred, and of these, tetrabutyl titanate is preferred.

The amount of titanium (Ti) catalyst used is preferably 37 to 100 ppm by mass as the titanium atom (Ti) content of the resulting PBT, more preferably 38 to 95 ppm, even more preferably 39 to 90 ppm, and particularly preferably 40 to 85 ppm. If the Ti atom content is higher than the above range, the color tone, hydrolysis resistance, and solution haze of the resulting PBT tend to deteriorate (as the amount of Ti precipitated inevitably increases), and the number of fish eyes in the resulting film tends to increase. On the other hand, if the Ti atom content is lower than the above range, the amount of BDO converted to THF increases, polymerization reactivity deteriorates, and it tends to take a long time to obtain PBT with the desired degree of polymerization, or the desired degree of polymerization does not tend to be reached.

In the process in which Ti functions as a catalyst during the production of PBT, the Ti catalyst reacts with the raw material TPA, raw material BDO, or by-products generated in the reaction of the esterification step, and precipitates as a component that is not dissolved in the reaction system, and no longer functions as a catalyst. In this case, the amount of Ti catalyst decreases, and the volume resistivity of the resulting PBT increases. In other words, PBT that is not suitable for high-speed film formation is produced. This is thought to be due to the fact that the Ti compound becomes Ti metal and no longer contributes to the effect of lowering the volume resistivity.
Furthermore, if the Ti catalyst reacts with the raw material TPA, the raw material BDO, or by-products generated in the reaction of the esterification step and precipitates as a component that is insoluble in the reaction system, this is undesirable because it causes an increase in solution haze.

One way to suppress the deposition of Ti catalyst is to adjust the BDO/TPA consumption molar ratio, as described below.

Specific examples of the periodic table Group 2A metal (alkaline earth metal) compound in the present invention include various compounds of beryllium, magnesium, calcium, strontium, and barium, but from the viewpoints of ease of handling and availability, and catalytic effect, magnesium compounds and/or calcium compounds are preferred, and magnesium compounds with excellent catalytic effect are particularly preferred. Specific examples of magnesium compounds include magnesium acetate, magnesium hydroxide, magnesium carbonate, magnesium oxide, magnesium alkoxide, magnesium hydrogen phosphate, etc. Specific examples of calcium compounds include calcium acetate, calcium hydroxide, calcium carbonate, calcium oxide, calcium alkoxide, calcium hydrogen phosphate, etc. These periodic table Group 2A metal compounds may be used alone or in combination of two or more.
Of these, magnesium acetate is preferred.

The content of the Group 2A metal of the Periodic Table, i.e., the alkaline earth metal, in the PBT of the present invention is not particularly limited, but is preferably 15 ppm or less in terms of the mass ratio of alkaline earth metal atoms (M2) to the PBT. This amount is more preferably 5 ppm or more, and even more preferably 10 ppm or more. If the content of the Group 2A metal of the Periodic Table is too high, the color tone, hydrolysis resistance, etc. will deteriorate, and if it is too low, the polymerizability will deteriorate.

The molar ratio (M2/Ti) of alkaline earth metal (M2) atoms to titanium atoms contained in the PBT of the present invention is usually 0.01 to 100, preferably 0.05 to 10, more preferably 0.07 to 3, and even more preferably 0.10 to 1.5.

When producing the PBT of the present invention, reaction aids such as antimony compounds such as antimony trioxide, germanium compounds such as germanium dioxide and germanium tetroxide, manganese compounds, zinc compounds, zirconium compounds, cobalt compounds, sodium hydroxide, and sodium benzoate may be used in addition to the titanium compounds and periodic table Group 2A metal compounds.

In addition, in the production of polyester resins, phosphorus compounds may be used as thermal stabilizing aids. However, while phosphorus compounds contribute to improving thermal stability, they also inactivate Ti catalysts. Therefore, when phosphorus compounds are added during esterification or polymerization reactions, the reactivity is deteriorated, the desired reaction activity is not obtained, and the esterification or polymerization reaction is not completed or is delayed. In addition, phosphorus compounds tend to deteriorate the hydrolysis resistance of the resulting PBT. For this reason, when phosphorus compounds are used, they are preferably added so that the phosphorus atom (P) content is 9 ppm or less relative to the resulting PBT, more preferably 7 ppm or less, and even more preferably 5 ppm or less. It is particularly preferable to add phosphorus compounds below the detection limit, i.e., not to add phosphorus compounds.

The content ratio (mass ratio) (M2/P) of alkaline earth metal atoms (M2) to phosphorus atoms (P) in the PBT of the present invention is preferably 5.1 or more, more preferably 7.0 or more, and even more preferably 8.0 or more. If this M2/P mass ratio is 5.1 or more, the volume resistivity value described below can be satisfied. When no phosphorus compound is added, this M2/P mass ratio is 10.0 or more, taking into account the lower detection limit of P.

The metal content of titanium atoms (Ti), alkaline earth metal atoms (M2), and other metals in PBT, as well as the phosphorus atom (P) content, can be measured using methods such as atomic emission, atomic absorption, and inductively coupled plasma (ICP) after recovering the metals in the PBT using a method such as wet ashing.

<BDO/TPA Consumption Molar Ratio>
In order to adjust the volume resistivity value of the obtained PBT within the specified range of the present invention, in the PBT production process, the molar ratio of consumed BDO and TPA components ([consumed BDO (moles)]/[consumed TPA (moles)], hereinafter sometimes referred to as the "BDO/TPA consumed molar ratio") is preferably 1.2 or more and less than 2.0, and this BDO/TPA consumed molar ratio is more preferably 1.4 or more and 1.9 or less, and even more preferably 1.5 or more and 1.8 or less.
If the BDO/TPA consumption molar ratio is less than 1.2, it tends to cause a decrease in esterification reactivity and deactivation of the Ti catalyst, which significantly affects the volume resistivity of the resulting PBT. For example, even if the Ti content is the same, if the Ti compound is deactivated as a catalyst, for example, in the form of a complex compound with TPA or BOD, the volume resistivity of the resulting PBT will be high, which is considered to be undesirable.
On the other hand, when the BDO/TPA consumption molar ratio is 2.0 or more, the thermal efficiency decreases and the amount of by-products such as THF tends to increase.
The BDO/TPA consumption molar ratio can be controlled by controlling the BDO/TPA molar ratio used in the esterification reaction, the amount of Ti catalyst, and the esterification reaction temperature. These BDO/TPA molar ratio used in the esterification reaction, the amount of Ti catalyst, and the esterification reaction temperature act in a complex manner on the BDO/TPA consumption molar ratio.

The above "BDO/TPA molar ratio used in the esterification reaction" refers to the sum of the BDO that enters the reaction vessel from outside the reaction vessel, including the BDO supplied together with the TPA component as the raw material slurry, the BDO supplied independently of these, and the BDO used as a catalyst solvent. If the BDO/TPA consumption molar ratio is smaller than the BDO/TPA molar ratio added to the esterification reaction vessel, the excess BDO will be recycled.

<Physical properties of PBT>
The physical properties and preferred physical properties of the PBT of the present invention are as follows:

(Intrinsic Viscosity)
When the PBT pellets of the present invention are used for extrusion of films, sheets or filaments, the intrinsic viscosity of PBT is usually 0.85 to 1.60 dL/g, preferably 1.03 to 1.50 dL/g, more preferably 1.05 to 1.55 dL/g, even more preferably 1.10 to 1.50 dL/g, and particularly preferably 1.15 to 1.35 dL/g. When the intrinsic viscosity is less than 0.85 dL/g, the extrusion moldability is deteriorated, which leads to drawdown of the resin and molding defects, and the mechanical strength of the extrusion molded product such as a film becomes insufficient, or the melt viscosity becomes low and the fluidity becomes too high, resulting in deterioration of the extrusion moldability. On the other hand, when the intrinsic viscosity exceeds 1.60 dL/g, the melt viscosity becomes high, the fluidity becomes poor, and the extrusion moldability tends to deteriorate.

The intrinsic viscosity of PBT can be determined by the method described in the Examples section below.

(Solution Haze)
The solution haze of the PBT of the present invention measured by the following method is not particularly limited, but is usually 40% or less, preferably 10% or less, more preferably 5% or less, even more preferably 3% or less, and particularly preferably 1% or less. PBT with high solution haze is considered to have a large amount of foreign matter mixed in due to the deactivation of the titanium catalyst. This phenomenon significantly reduces the commercial value.
<Method of measuring solution haze>
For a solution in which 2.70 g of polybutylene terephthalate is dissolved in 20 mL of a mixed solvent of phenol/tetrachloroethane = 3/2 (mass ratio), the solution haze is measured using a cell length of 10 mm.

More specifically, the solution haze of PBT can be determined by the method described in the Examples section below.

(Volume resistivity)
The PBT of the present invention has a volume resistivity at 250°C (ρV(250°C)) of 13 x ¹⁰ Ω·cm or less, preferably 12 x ¹⁰ Ω·cm or less, and more preferably 11 x ¹⁰ Ω·cm or less. If ρV(250°C) is higher than the upper limit, the high-speed film-forming property of the film tends to decrease.

When forming a blend film of PBT with polyethylene terephthalate (PET), polyethylene naphthalate (hereinafter sometimes abbreviated as "PEN"), or polybutylene naphthalate (hereinafter sometimes abbreviated as "PBN"), it is necessary to form the film at a higher temperature of 285° C. In this case, the volume resistivity value (ρV(285° C.)) of the PBT of the present invention at 285° C. is preferably 10×10 ⁷ Ω·cm or less, more preferably 9×10 ⁷ Ω·cm or less, and even more preferably 7×10 ⁷ Ω·cm or less.

Furthermore, the PBT of the present invention has a ratio (ρV(250°C)/ρV(285°C)) of the volume resistivity at 250°C (ρV(250°C)) to the volume resistivity at 285°C (ρV(285°C)) of preferably 3.0 or less, more preferably 2.8 or less, even more preferably 2.5 or less, and particularly preferably 2.0 or less. On the other hand, this ratio (ρV(250°C)/ρV(285°C)) is preferably 0.5 or more, more preferably 0.7 or more, even more preferably 0.8 or more, and particularly preferably 0.9 or more.
If ρV(250° C.)/ρV(285° C.) is within the above range, the film is excellent in high-speed film formation property in both the case of forming a film of PBT alone or a film of a blend of PBT and other polyester resin such as PET.

The volume resistivity of PBT can be determined specifically by the method described in the Examples section below.

<PBT Composition>
The PBT of the present invention may be mixed with various particles, additives, and resins other than PBT as described below to form a PBT composition.

(particle)
When the PBT of the present invention is molded into a film, inorganic particles and/or organic particles can be added to prevent blocking on the film surface. However, when the film requires transparency for optical applications, it is preferable to add as little particles as possible or to add no particles at all.
Examples of inorganic particles include calcium carbonate, barium sulfate, alumina, silica, talc, titania, kaolin, mica, zeolite, and the like, as well as surface-treated particles of these particles with a silane coupling agent, a titanate coupling agent, or the like.
Examples of organic particles include particles of acrylic resins, styrene resins, crosslinked resins, and the like.
The average particle size of these particles is preferably in the range of 0.05 to 5.0 μm.
The amount of these particles added, in terms of the content in the PBT composition, is usually 0.001% by mass, preferably 0.05% by mass, with the upper limit being usually 2.0% by mass, preferably 1.0% by mass, more preferably 0.5% by mass.
These particles can be added during or after the polycondensation reaction step of PBT.

(Additives)
The PBT of the present invention may contain other conventional additives as necessary. Examples of other additives that may be added to the PBT of the present invention include antioxidants, ultraviolet absorbers, light stabilizers, antistatic agents, lubricants, antiblocking agents, antifogging agents, nucleating agents, plasticizers, and colorants. These additives may be used alone or in combination of two or more. These additives may be added during or after the polycondensation reaction step of PBT.
The amount of each of the other additives added is usually 5% by mass or less, preferably 0.05 to 2% by mass, in terms of the content in the PBT composition.

(Mixing Method)
The method of blending the various particles and additives is not particularly limited, but a method using a single-screw or twin-screw extruder having a vent port for volatilization as a kneader is preferred. Each component, including the additional component, can be fed to the kneader all at once. Alternatively, they can be fed sequentially. Also, two or more components selected from each component, including the additional component, can be mixed in advance.

<Polyester film>
The PBT of the present invention has a low volume resistivity at a temperature suitable for film formation and is excellent in high-speed film formation property, and is therefore suitably used as a film-forming material for polyester films.

In the present invention, film includes sheets. Generally, a film is a thin, flat product whose thickness is extremely small compared to its length and width, and whose maximum thickness is arbitrarily limited, and which is usually supplied in the form of a roll (Japanese Industrial Standard JIS K6900:1994), and a sheet is generally a thin, flat product, as defined in JIS, whose thickness is generally small compared to its length and width. However, since the boundary between a sheet and a film is unclear, in the present invention, film includes sheets. Therefore, "polyester film" may also be "polyester sheet".

In the polyester film using the PBT of the present invention as a film-forming raw material, the content of the PBT of the present invention in the film-forming raw material is preferably 50% by mass or more. If the content of the PBT of the present invention in the film-forming raw material is 50% by mass or more, the effect of high-speed film-forming property due to the use of the PBT of the present invention having a low volume resistivity can be effectively obtained.
The content of the PBT of the present invention in the film-forming raw material is preferably 55% by mass or more, more preferably 60% by mass or more, and even more preferably 62% by mass or more. The content of the PBT of the present invention in the film-forming raw material may be 100% by mass.

The film-forming raw material containing the PBT of the present invention may be a PBT composition containing the PBT of the present invention described above, and this PBT composition may contain, in addition to the above-mentioned particles and other additives, polyester resins other than PBT, such as PET, PEN, and PBN, or resins other than polyester resins.
Due to its low volume resistivity, the PBT of the present invention can exhibit excellent high-speed film-forming properties not only in films made of PBT alone, but also in blend films of PBT and polyester resins other than PBT, such as PET, PEN, and PBN.

The method for forming the polyester film of the present invention using the PBT of the present invention is not particularly limited, and any known method can be used.
In particular, as a method for forming a biaxially stretched film, a conventionally known method is used, such as a sequential biaxial stretching method in which the PBT pellets of the present invention or a film-forming raw material containing the PBT pellets are melt-extruded into a film or sheet, and then quenched by a cooling drum to form an unstretched film or sheet, and then the unstretched film or sheet is preheated and stretched in the longitudinal direction and subsequently in the transverse direction, or a simultaneous biaxial stretching method in which biaxial stretching is performed simultaneously in the longitudinal and transverse directions. The stretching ratio in this case is usually in the range of 2 to 6 times in both the longitudinal and transverse directions, and if necessary, heat setting and/or heat relaxation is performed after biaxial stretching. The thickness of the biaxially stretched film is usually about 1 to 300 μm.

The lower limit of the temperature for melting the PBT pellets of the present invention is preferably 200°C, more preferably 210°C. If the melting temperature is less than 200°C, unmelted material may be generated and extrusion may become unstable. The upper limit of the melting temperature is preferably 290°C, more preferably 270°C, and even more preferably 260°C or less. If the melting temperature exceeds 290°C, deterioration of the resin may occur.

The method for forming a film when blending the PBT pellets of the present invention with other polyester resin pellets having a higher melting point than PBT, such as PET pellets, to produce a film is not particularly limited, and the same known methods as those described above can be used.
The lower limit of the melting temperature of the blend pellets is preferably 200° C., more preferably 210° C. If the melting temperature is less than 200° C., unmelted material may be generated or extrusion may become unstable. The upper limit of the melting temperature is preferably 300° C., more preferably 290° C., and even more preferably 285° C. If the melting temperature exceeds 300° C., deterioration of the resin may occur.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as they do not exceed the gist of the invention.

[Measurement and evaluation method]
The methods for measuring and evaluating PBT in the following examples and comparative examples are as follows.

(1) Volume resistivity (ρV)
15 g of the resin sample was placed in a branched test tube with an inner diameter of 20 mm and a length of 180 mm, and the inside of the tube was thoroughly replaced with nitrogen, and then the tube was immersed in an oil bath at 250°C, and the inside of the tube was vacuum-dried for 20 minutes by reducing the pressure in the tube to 1 Torr or less using a vacuum pump. The oil bath temperature was then raised to 285°C to melt the sample, and the nitrogen pressure was restored and reduced repeatedly to remove any air bubbles that were mixed in. Two stainless steel electrodes with an area of ¹ cm2 were inserted in parallel at an interval of 5 mm (the backs of the electrodes not facing each other were covered with an insulator) into the melt, and after the temperature was stabilized, a direct current voltage of 100 V was applied using a resistance meter (Hewlett-Packard's "MODEL HP4339B") to measure the resistance at 250°C or 285°C, and the volume resistivity values ρV(250°C) and ρV(285°C) (Ω·cm) were calculated.

(2) Intrinsic viscosity (IV)
The viscosity was measured using an Ubbelohde viscometer in the following manner: A mixed solvent of phenol/tetrachloroethane (mass ratio 1/1) was used, and a PBT solution with a concentration of 1.0 g/dL and the solvent alone were measured at 30° C. The number of seconds it took to fall was measured and calculated using the following formula.
IV=((1+4K _H η _sp ) ^0.5 -1)/(2K _H・C)
(Here, η _sp =η ₀ -1, η is the number of seconds it takes for the PBT solution to fall, η ₀ is the number of seconds it takes for the solvent to fall, C is the concentration of the PBT solution (g/dL), and K _H is Huggins' constant. , 0.33 was adopted.)

(3) Titanium Atom (Ti) Content, Alkaline Earth Metal Atom (M2) Content, and Phosphorus Atom (P) Content PBT was wet decomposed with high-purity sulfuric acid and nitric acid for the electronics industry, and the contents were measured using a high-resolution ICP (Inductively Coupled Plasma)-MS (Mass Spectrometer) (manufactured by Thermoquest).

(4) Terminal hydroxyl group concentration/terminal vinyl group concentration PBT was dissolved in a mixed solvent of deuterated chloroform/deuterated hexafluoroisopropanol/deuterated pyridine (volume ratio 21/9/1) containing a trace amount of tetramethylsilane, and ¹ H NMR spectrum was measured using an AVANCE NEO spectrometer (manufactured by Bruker). The chemical shift reference was set to the signal of tetramethylsilane at 0.00 ppm. In Tables 1 and 2, the terminal hydroxyl group concentration is indicated as "terminal OH group concentration."

(5) Terminal Carboxyl Group Concentration This was determined by dissolving 0.5 g of PBT or an oligomer in 25 mL of benzyl alcohol and titrating the solution with a 0.01 mol/L solution of sodium hydroxide in benzyl alcohol.
In Tables 1 and 2, the terminal carboxyl group concentration is indicated as "terminal COOH group concentration."

(6) Color Tone: Using a color difference meter "ZE6000" manufactured by Nippon Denshoku Co., Ltd., evaluation was performed in the L, a, b color system. The lower the b value, the less yellowish the color is, which is preferable. However, when the b value is lower than -2.2, the color tone is unfavorable because the color tone is less yellowish but more blueish.
The b value is preferably in the range of -2.2 to 1.5, more preferably -2.1 to 1.3, and further preferably -2.0 to 1.2.

(7) Solution Haze 2.70 g of PBT was dissolved in 20 mL of a mixed solvent of phenol/tetrachloroethane = 3/2 (mass ratio) at 110 ° C for 30 minutes, and then cooled in a thermostatic water bath at 30 ° C for 15 minutes, and the haze was measured using a turbidity meter "NDH-300A" manufactured by Nippon Denshoku Co., Ltd. with a cell length of 10 mm. The lower this value, the better the transparency.

(8) Electrostatic Adhesion PBT or a blend of PBT and PET was dried at 120° C., melt-extruded at 250° C., and rapidly solidified on a rotating cooling drum whose surface temperature was kept at 40° C. to prepare an unstretched film. When this unstretched film was wound up, electrostatic pinning method was used to evaluate electrostatic adhesion based on the following criteria, based on the speed at which the film could be wound up stably and with small unevenness in film thickness. If it was rated as ◯ or ⊚, it was determined that the productivity was good.
○: Winding speed 30 m/min or more ×: Winding speed less than 30 m/min

(9) Measurement of the number of fisheyes PBT or a blend of PBT and PET was dried at 140°C for 4 hours under a nitrogen atmosphere, and a film was formed using a continuous extrusion film forming apparatus ("ME-20/26V2&CR-7&FS-5" manufactured by OCS) under the following conditions, and the number of fisheyes (number/ ^m2 ) was measured. The number of fisheyes was measured by automatically counting the number of fisheyes with a major axis of 16 μm or more present in an area of ¹ m2 using a CCD camera attached to the apparatus while the film was being formed. The smaller this value, the more excellent the molded appearance. It is preferable that the number of fisheyes is 1,000 pieces/ ^m2 or less.
・Cylinder temperature (temperature at four points between the nozzle and the bottom of the hopper):
PBT only = 250°C - 250°C - 250°C - 250°C
PBT + PET = 285°C - 285°C - 285°C - 285°C
Screw rotation speed: 100 rpm
Resin pressure: 75 MPa
Chill roll temperature: 40°C
Film thickness: 50 μm

[Example 1]
The PBT was prepared as follows.
A slurry in which 1.80 moles of BDO were mixed with 1.00 moles of TPA was continuously fed to an esterification reaction tank (abbreviated as "Es" in Tables 1 and 2) having a screw-type agitator filled with PBT oligomer with an esterification rate of 99%, and an esterification reaction was carried out. A BDO solution containing a tetrabutyl titanate catalyst in an amount such that titanium was 40 ppm relative to PBT was fed to the esterification reaction tank. Additional BDO was fed to the esterification tank so that the molar ratio of BDO to TPA (Es-charged BDO/TPA molar ratio) was 2.6. The temperature of the reaction tank was 226°C, the pressure was 60 kPa, and the average residence time was 180 minutes.
The PBT oligomer with an esterification rate of 96.5% was then continuously transferred to the first polycondensation reaction tank. In the first polycondensation reaction tank, polycondensation reaction was continuously carried out in the presence of magnesium acetate tetrahydrate catalyst in an amount such that magnesium was 10 ppm relative to PBT. The reaction temperature was 230°C, the pressure was 3.9 kPa, and the average residence time was 120 minutes. The product was then transferred to the second polycondensation reaction tank, where polycondensation reaction was continuously carried out. The reaction temperature was 240°C, the pressure was 130 Pa, and the average residence time was 80 minutes. The product was then continuously supplied to a third polymerization reaction tank adjusted to a temperature of 238°C and a pressure of 130 Pa, and the polymerization reaction was further carried out under stirring for a residence time of 80 minutes to obtain a polymer.

The obtained polymer was passed through a discharge line by a discharge gear pump and a filter, and was continuously discharged in the form of strands from a die head and cut with a rotary cutter to obtain PBT pellets (major axis about 3 mm, minor axis about 2 mm, length about 4 mm).
The intrinsic viscosity (IV) of the resulting PBT was 1.20 dL/g.
The molar ratio of BDO to TPA consumed was 1.67. The evaluation results of the obtained PBT pellets are summarized in Table 1.

[Examples 2 to 4, Comparative Examples 1 to 3]
PBT pellets were obtained in the same manner as in Example 1 except that the Ti content, Mg content, and BDO/TPA consumption molar ratio were set as shown in Tables 1 and 2, and evaluations were performed in the same manner. The evaluation results are summarized in Tables 1 and 2.
In Example 4 and Comparative Example 3, the mixture was blended with PET pellets having the physical properties shown below in the ratios shown in Tables 1 and 2 during film formation, and the films were evaluated.

[PET pellets]
Intrinsic viscosity (IV): 0.82dL/g
Ti content: 4ppm
Mg content: 6ppm
P content: 6 ppm
Mg / P mass ratio: 1
Volume resistivity at 285°C (ρV(285°C)): 210 x ¹⁰⁷ Ω·cm

[Comparative Examples 4 to 7]
In an ester exchange reaction tank equipped with a stirrer, a nitrogen inlet, a heater, a thermometer, and a distillation tube, 71.8 parts by mass of dimethyl terephthalate, 39.3 parts by mass of BDO, and tetrabutyl titanate as a catalyst were added as a BDO solution so that the titanium atom equivalent was 33 ppm relative to the polymer produced. Next, the liquid temperature in the tank was kept at 150°C for 60 minutes, and then the temperature was raised to 210°C over 90 minutes and held at 210°C for 30 minutes. During this time, the ester exchange reaction was carried out for a total of 180 minutes while distilling off the produced methanol.
15 minutes before the end of the transesterification reaction, magnesium acetate tetrahydrate was dissolved in BDO and added so that the magnesium atom was 48 ppm relative to the polymer produced, and further a hindered phenol-based antioxidant (manufactured by BASF Ltd., Irganox 1010: tetrakis (methylene-3 (3,5-di-t-butyl-4-hydroxy-phenyl) propionate) was added as a BDO slurry in an amount of 0.15 parts by mass relative to the polymer produced. Subsequently, tetrabutyl titanate was added as a BOD solution so that the total content as titanium atoms relative to the polymer produced was the amount shown in Table 2. The mixture was then transferred to a polycondensation reaction tank equipped with a stirrer, a nitrogen inlet, a heater, a thermometer, a distillation tube, and a pressure-reducing exhaust port, and the polycondensation reaction was carried out by applying pressure reduction.

The pressure in the tank for the polycondensation reaction was gradually reduced from normal pressure to 0.4 KPa over 85 minutes, and was maintained at 0.4 KPa or less. The reaction temperature was kept at 210°C for 15 minutes from the start of pressure reduction, and then increased to 240°C over 45 minutes and maintained at this temperature. The reaction was terminated when a predetermined stirring torque was reached. The time required for the polycondensation reaction (the time from the start of pressure reduction to the time of pressure recovery with nitrogen) was 150 minutes.
Next, the inside of the tank was restored from the reduced pressure state with nitrogen, and then pressurized for polymer discharge. The temperature of the heat medium in the die during discharge was set to 235° C., and the polymer was extruded from the die in the form of a strand. The strand was then cooled in a cooling water tank and cut with a strand cutter to obtain PBT pellets.

The obtained PBT pellets were evaluated in the same manner as in Example 1, and the evaluation results are summarized in Table 2.
In Comparative Example 7, the above-mentioned PET pellets were blended in the ratio shown in Table 2 during film formation, and the film was then evaluated.

[Comparative Example 8]
A similar evaluation was carried out on commercially available PBT pellets ("Novaduran 5020" manufactured by Mitsubishi Engineering Plastics), and the evaluation results are summarized in Table 2.

From the above results, it is clear that the PBT pellets of the Examples are PBT pellets that have excellent electrostatic adhesion when forming a film and produce a film with fewer fisheyes, that is, PBT pellets that are excellent in high-speed film forming ability and film quality.
In contrast, the PBTs of the comparative examples all had large ρV (250° C.) and ρV (285° C.), poor electrostatic adhesion, and some had many fish eyes and poor molded appearance.

The PBT of the present invention has a low volume resistivity at a specified temperature, and when forming a PBT film alone or a blend film of PBT and other polyester resins such as PET, the film can be formed at high speed. Using the PBT of the present invention, it is possible to produce high-quality polyester films with high productivity.

Claims (6)

Polybutylene terephthalate having a volume resistivity at 250° C. (ρV(250° C.)) of 13×10 ⁷ Ω·cm or less. 2. The polybutylene terephthalate according to claim 1, which has a volume resistivity at 285° C. (ρV(285° C.)) of 10×10 ⁷ Ω·cm or less. The polybutylene terephthalate according to claim 1, in which the ratio (ρV(250°C)/ρV(285°C)) of the volume resistivity at 250°C (ρV(250°C)) to the volume resistivity at 285°C (ρV(285°C)) is 3.0 or less. 2. The polybutylene terephthalate according to claim 1, having a solution haze of 40% or less as measured by the following method.
<Method of measuring solution haze>
For a solution in which 2.70 g of polybutylene terephthalate is dissolved in 20 mL of a mixed solvent of phenol/tetrachloroethane = 3/2 (mass ratio), the solution haze is measured using a cell length of 10 mm. The titanium atom (Ti) content is 37 to 100 ppm, the alkaline earth metal atom (M2) content is 15 ppm or less, and the phosphorus atom (P) content is 9 ppm or less,
2. The polybutylene terephthalate according to claim 1, wherein the mass ratio (M2/P) of the alkaline earth metal atom (M2) to the phosphorus atom (P) is 5.1 or more. A polyester film obtained by forming a film-forming raw material containing 50% by mass or more of the polybutylene terephthalate according to any one of claims 1 to 5.