JP2000351114A - Production of transparent thermoplastic resin pellet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain transparent thermoplastic resin pellets containing only a trace quantity of foreign materials stably and efficiently. SOLUTION: In a method for producing pellets in which a transparent thermoplastic resin is melt-extruded by using an extruder into strands, which are cooled and cut by a cutter, a space ranging from the die of the extruder to the inlet of the cutter has a cleanliness of class 7 or below is terms of JIS B9920-1989.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for stably and efficiently producing transparent thermoplastic resin pellets having very little foreign matter.

[0002]

2. Description of the Related Art In recent years, various thermoplastic resins have been developed,
Thermoplastic resins such as polycarbonate resin, polyarylate resin, polymethyl methacrylate resin, polystyrene resin, and amorphous cyclic polyolefin resin having excellent transparency are preferably used as molding materials in the optical field. In these applications, the size and amount of foreign matter in the resin greatly affects the yield and quality of the product. Particularly in the production of optical disc substrates, foreign matter (dust and carbide, etc.) It has a great effect on the reliability of information recording and reproduction. Therefore, it is required that the amount of foreign matter is extremely small for the transparent thermoplastic resin constituting the substrate.

[0003] For this reason, conventionally, foreign substances in resin pellets have been reduced in the purification step, granulation step, and the like. For example, Japanese Patent Publication No. 7-21007 discloses that a thermoplastic resin solution and water are used. A method for obtaining a thermoplastic resin having a reduced amount of foreign substances by centrifuging to separate and collect a resin solution phase is disclosed. Also, Japanese Patent Application Laid-Open No. 5-239334
In Japanese Patent Application Laid-Open Publication No. H11-163, a method of reducing the amount of foreign substances by filtering a polycarbonate resin in a molten state with a polymer filter is disclosed. Japanese Patent Application Laid-Open No. 9-254151 discloses a method for producing polycarbonate resin pellets with a small amount of foreign matter using cooling water having a small electric conductivity and a small amount of foreign matter when cooling a melt-extruded polycarbonate resin. Proposed.

[0004]

However, as described above, the amount of foreign matter in the raw material to be subjected to melt extrusion is reduced, the polymer is filtered with a polymer filter, or the polymer after melt extrusion is cooled with cooling water having a small amount of foreign matter. The cooling method alone was not sufficient, although the amount of foreign substances in the transparent thermoplastic resin pellets was reduced to some extent. The present inventor has found it difficult to extremely reduce the amount of foreign matter in thermoplastic resin pellets after melt extrusion even if the amount of foreign matter in the thermoplastic resin subjected to melt extrusion is extremely small. As a result of the examination, the transparent thermoplastic resin was melt-extruded into a strand using a melt extruder,
After cooling the strand, in a method of obtaining pellets by cutting, in the air space from the die of the melt extruder to the entrance of the cutting machine, paying attention to an increase in the amount of foreign substances, such a specific air space is specified cleanness The inventors have found that a transparent thermoplastic resin pellet having an extremely small amount of foreign matter can be stably and efficiently obtained by setting the class, and the present invention has been achieved.

[0005]

In other words, according to the present invention, a transparent thermoplastic resin is melt-extruded into a strand by using a melt extruder, and the strand is cooled and then cut by a cutter to produce pellets. In the method, the airspace from the die of the melt extruder to the entrance of the cutting machine is JIS B
A method for producing a transparent thermoplastic resin pellet, characterized by having a cleanliness class 7 or less defined in 9920-1989 is provided.

Hereinafter, the present invention will be described in detail. In this specification, the term "foreign matter" refers to a contaminant mixed through various routes in a process of producing a transparent thermoplastic resin from raw materials and obtaining pellets, and includes, for example, raw materials (monomer, solvent, etc.) ) Indicates all components insoluble in methylene chloride, such as impurities and dust contained in, dust adhering to manufacturing equipment or carbide generated during the molding process.

In the present invention, the air space from the die of the melt extruder to the entrance of the cutting machine, preferably the air space from the exit of the cutting machine, is 7 or less in the cleanliness class defined in JIS B 9920-1989. . Further, from the viewpoint of facility easiness and economical efficiency, the cleanliness class is preferably 2 to 7, and 2 to 6 is preferable.
Is particularly preferred. When the cleanliness class is 8 or more, the amount of foreign matters in the obtained transparent thermoplastic resin pellets is undesirably increased.

As a method of setting the air space from the die of the melt extruder to the entrance of the cutting machine to a cleanliness class of 7 or less, at least from the die of the melt extruder to the entrance of the cutting machine is made of glass, wood or plastic. , A partition made of a material such as metal, mortar, or the like, and a method of installing in a space or room with a cleanliness class of 7 or less, which is isolated from a dirty environment. The size of such space or room is
It is preferable that the size is such that the operation of the melt extruder or the like can be sufficiently performed. For example, a width of about 0.5 to 3 m from the machine and a height of about 1.5 to 4 m from the floor are preferably used.

In the present invention, the transparent thermoplastic resin is melt-extruded into strands, and the melt-extruded strands are cooled while the strands are cut by a cutter. As a method for cooling the strands, a method of immersing the strands in water applied to a cooling bath to cool the strands is preferably adopted. In this case, the water used is preferably water filtered with a filter having a filtration accuracy of 5 μm or less, and the filtration efficiency and Filtration accuracy of 0.2 to 5 in terms of reducing the amount of foreign matter in the pellet
μm filter is more preferable, and the filtration accuracy is 0.2 to 3
μm filters are more preferred. The water used for cooling the strand is preferably ion-exchanged water or distilled water, and ion-exchanged water is particularly preferably used. The water temperature in the cooling bath is preferably from 25 to 80C, more preferably from 30 to 70C. A strand cooled at a water temperature in such a range is preferable because air bubbles hardly enter the strand and a large amount of cut debris is reduced when cutting with a cutting machine.

In the present invention, the transparent thermoplastic resin is melt-extruded into a strand by using a melt extruder. As the melt extruder, either a single screw extruder or a twin screw extruder can be used. An extruder with a vent and a filter having a filtration accuracy of 0.5 to 5 μm between a screw and a die is preferably used. As the filter provided between the screw and the die, a disk-shaped, candle-shaped, or leaf-disk-shaped metal filter is preferably used.

The transparent thermoplastic resin which is the object of the present invention is:
A resin capable of transmitting infrared light or visible light, specifically, polycarbonate resin, polyarylate resin, polymethyl methacrylate resin, polystyrene resin, amorphous cyclic polyolefin resin and the like, among which polycarbonate resin is Most preferred. These resins have a common process of manufacturing pellets. Hereinafter, a polycarbonate resin will be described as an example.

[0012] The polycarbonate resin is obtained by reacting a dihydric phenol with a carbonate precursor by an interfacial polycondensation method or a melting method. Representative examples of the dihydric phenol used here include hydroquinone, resorcinol, 4,4'-dihydroxydiphenyl, bis (4-hydroxyphenyl) methane, bis {(4-hydroxy-3,5-dimethyl) Phenyl methane, 1,
1-bis (4-hydroxyphenyl) ethane, 1,1-
Bis (4-hydroxyphenyl) -1-phenylethane, 2,2-bis (4-hydroxyphenyl) propane (commonly known as bisphenol A), 2,2-bis {(4-hydroxy-3-methyl) phenyl} propane, 2,2-bis {(4-hydroxy-3,5-dimethyl) phenyl}
Propane, 2,2-bis {(3,5-dibromo-4-hydroxy) phenyl} propane, 2,2-bis} (3-
Isopropyl-4-hydroxy) phenyl} propane,
2,2-bis {(4-hydroxy-3-phenyl) phenyl} propane, 2,2-bis (4-hydroxyphenyl) butane, 2,2-bis (4-hydroxyphenyl)
-3-methylbutane, 2,2-bis (4-hydroxyphenyl) -3,3-dimethylbutane, 2,4-bis (4
-Hydroxyphenyl) -2-methylbutane, 2,2-
Bis (4-hydroxyphenyl) pentane, 2,2-bis (4-hydroxyphenyl) -4-methylpentane,
1,1-bis (4-hydroxyphenyl) cyclohexane, 1,1-bis (4-hydroxyphenyl) -4-isopropylcyclohexane, 1,1-bis (4-hydroxyphenyl) -3,3,5-trimethylcyclohexane 9,9-bis (4-hydroxyphenyl) fluorene, 9,9-bis {(4-hydroxy-3-methyl) phenyl} fluorene, α, α'-bis (4-hydroxyphenyl) -o-diisopropylbenzene , Α, α'-
Bis (4-hydroxyphenyl) -m-diisopropylbenzene, α, α'-bis (4-hydroxyphenyl)
-P-diisopropylbenzene, 1,3-bis (4-hydroxyphenyl) -5,7-dimethyladamantane,
4,4'-dihydroxydiphenyl ketone, 4,4'-
Examples thereof include dihydroxydiphenyl ether and 4,4'-dihydroxydiphenyl ester, which can be used alone or as a mixture of two or more.

Among them, bisphenol A, 2,2-bis {(4-hydroxy-3-methyl) phenyl} propane, 2,2-bis (4-hydroxyphenyl) butane,
2,2-bis (4-hydroxyphenyl) -3-methylbutane, 2,2-bis (4-hydroxyphenyl)-
3,3-dimethylbutane, 2,2-bis (4-hydroxyphenyl) -4-methylpentane, 1,1-bis (4
A homopolymer obtained from at least one bisphenol selected from the group consisting of -hydroxyphenyl) -3,3,5-trimethylcyclohexane and α, α'-bis (4-hydroxyphenyl) -m-diisopropylbenzene, or Copolymers are preferred. In particular, bisphenol A homopolymer and 1,1-bis (4-hydroxyphenyl) -3,3,5-trimethylcyclohexane and bisphenol A, 2,2-bis {(4-hydroxy- Copolymers with 3-methyl) phenyl} propane or α, α′-bis (4-hydroxyphenyl) -m-diisopropylbenzene are preferably used.

As the carbonate precursor, carbonyl halide, carbonate ester or haloformate is used, and specific examples include phosgene, diphenyl carbonate and dihaloformate of dihydric phenol.

In producing the polycarbonate resin by reacting the dihydric phenol with the carbonate precursor by an interfacial polycondensation method or a melting method, if necessary, a catalyst, a terminal stopper and an antioxidant for the dihydric phenol may be used. Etc. may be used. Further, the polycarbonate resin may be a branched polycarbonate resin obtained by copolymerizing a trifunctional or higher polyfunctional aromatic compound, or a polyester carbonate resin obtained by copolymerizing an aromatic or aliphatic bifunctional carboxylic acid, In addition, 2 of the obtained polycarbonate resin
It may be a mixture of more than one species.

The reaction by the interfacial polycondensation method is usually a reaction between dihydric phenol and phosgene, and is carried out in the presence of an acid binder and an organic solvent. As the acid binder, for example, an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide or an amine compound such as pyridine is used.
As the organic solvent, for example, halogenated hydrocarbons such as methylene chloride and chlorobenzene are used. Further, for promoting the reaction, for example, a catalyst such as a tertiary amine such as triethylamine, tetra-n-butylammonium bromide, or tetra-n-butylphosphonium bromide, a quaternary ammonium compound, or a quaternary phosphonium compound may be used. it can. At that time, the reaction temperature is usually 0 to 40 ° C., the reaction time is preferably about 10 minutes to 5 hours, and the pH during the reaction is preferably maintained at 9 or more.

In such a polymerization reaction, a terminal stopper is usually used. Monofunctional phenols can be used as such a terminal stopper. Monofunctional phenols are commonly used as molecular terminators for molecular weight control, and the resulting polycarbonate resins are capped by groups based on monofunctional phenols, so that Excellent heat stability. Such a monofunctional phenol is generally a phenol or a lower alkyl-substituted phenol, and may be a monofunctional phenol represented by the following general formula (1).

[0018]

Embedded image

[In the formula, A is a hydrogen atom, a linear or branched alkyl group or arylalkyl group having 1 to 9 carbon atoms, and r is an integer of 1 to 5, preferably 1 to 3. ]

Specific examples of the above monofunctional phenols include phenol, p-tert-butylphenol, p-cumylphenol and isooctylphenol.

Other monofunctional phenols include:
Phenols or benzoic acid chlorides having a long-chain alkyl group or an aliphatic polyester group as a substituent, or long-chain alkyl carboxylic acid chlorides can be used. When blocked, they not only function as a terminal terminator or a molecular weight regulator, but also improve the melt flowability of the resin and facilitate not only the molding process, but also the physical properties particularly as an optical disk substrate, especially the resin. It has the effect of lowering the water absorption and the effect of reducing the birefringence of the substrate, and is preferably used. Among them, phenols having a long-chain alkyl group represented by the following general formulas (2) and (3) as a substituent are preferably used.

[0022]

Embedded image

[0023]

Embedded image

Wherein X is -RO-, -R-CO-O
— Or —RO—CO—, wherein R represents a single bond or a divalent aliphatic hydrocarbon group having 1 to 10, preferably 1 to 5 carbon atoms, and n represents an integer of 10 to 50. Show. ]

As the substituted phenols of the formula (2), those wherein n is 10 to 30, especially 10 to 26, are preferred.
Specific examples thereof include decylphenol, dodecylphenol, tetradecylphenol, hexadecylphenol, octadecylphenol, eicosylphenol, docosylphenol, and tricontylphenol.

As the substituted phenols of the formula (3), a compound in which X is -R-CO-O- and R is a single bond is suitable, and n is 10 to 30, especially 10 to 26. Those are preferable, and specific examples thereof include decyl hydroxybenzoate, dodecyl hydroxybenzoate, tetradecyl hydroxybenzoate, hexadecyl hydroxybenzoate, eicosyl hydroxybenzoate, docosyl hydroxybenzoate and tricontyl hydroxybenzoate. .

These terminal stoppers are desirably introduced at least at 5 mol%, preferably at least 10 mol%, based on all terminals of the obtained polycarbonate resin. Or a mixture of two or more.

The reaction by the melting method is usually a transesterification reaction between a dihydric phenol and a carbonate ester,
The method is carried out by a method in which a dihydric phenol and a carbonate ester are mixed while heating in the presence of an inert gas to distill off the produced alcohol or phenol. The reaction temperature varies depending on the boiling point of the produced alcohol or phenol, but is usually in the range of 120 to 350 ° C.
In the latter half of the reaction, the pressure of the system is reduced to about 10 to 0.1 Torr to facilitate the distillation of the alcohol or phenol produced. The reaction time is usually about 1 to 4 hours.

Examples of the carbonate ester include an optionally substituted ester such as an aryl group having 6 to 10 carbon atoms, an aralkyl group or an alkyl group having 1 to 4 carbon atoms. Specific examples include diphenyl carbonate, ditolyl carbonate, bis (chlorophenyl) carbonate, m-cresyl carbonate, dinaphthyl carbonate, bis (diphenyl) carbonate, dimethyl carbonate, diethyl carbonate, dibutyl carbonate, and the like. Is preferred.

A polymerization catalyst may be used to increase the polymerization rate. Examples of the polymerization catalyst include sodium hydroxide, potassium hydroxide, alkali metal compounds such as sodium and potassium salts of dihydric phenol, and water. Alkaline earth metal compounds such as calcium oxide, barium hydroxide and magnesium hydroxide; nitrogen-containing basic compounds such as tetramethylammonium hydroxide, tetraethylammonium hydroxide, trimethylamine and triethylamine; alkoxides of alkali metals and alkaline earth metals Manganese, organic acid salts of alkali metals and alkaline earth metals, zinc compounds, boron compounds, aluminum compounds, silicon compounds, germanium compounds, organic tin compounds, lead compounds, osmium compounds, antimony compounds Compounds, titanium compounds, usually the esterification reaction, such as zirconium compounds, there can be used a catalyst used in the transesterification reaction. The catalyst may be used alone or in combination of two or more. The amount of these polymerization catalysts to be used is preferably 1 × 10 ⁻⁹ to 1 × 10 ^{9 with} respect to 1 mol of the starting dihydric phenol.
^-3 equivalents, more preferably 1 × 10 ^{−8 to} 5 × 10 ⁻⁴ equivalents.

The molecular weight of the polycarbonate resin is preferably from 10,000 to 100,000, more preferably from 11,000 to 45,000, and more preferably from 11,000 to 45,000 in terms of viscosity average molecular weight (M).
000 to 30,000 are particularly preferred. A polycarbonate resin having such a viscosity average molecular weight is preferable because sufficient strength is obtained, the melt fluidity during molding is good, and molding distortion does not occur. The viscosity average molecular weight of 0.7 g of polycarbonate resin in 100 ml of methylene chloride is 2
The specific viscosity (η _sp ) obtained from the solution dissolved at 0 ° C. was obtained by inserting it into the following equation. η _sp /c=[η]+0.45×[η] ² c (where [η] is the intrinsic viscosity) [η] = 1.23 × 10 ^-4 M ^0.83 c = 0.7

In the transparent thermoplastic resin pellets obtained by the production method of the present invention, heat stabilizers (phosphate esters, phosphite esters, etc.), release agents (fatty acid esters, etc.), antistatic agents, ultraviolet absorbers A modifying modifier such as an antioxidant and an antibacterial agent can be appropriately added and used.

The transparent thermoplastic resin pellets obtained by the production method of the present invention have a reduced amount of foreign matter, and the amount of foreign matter is preferably 20,000 or more in the transparent thermoplastic resin pellets. Pcs / g or less, more preferably 15,000 pcs / g or less, and still more preferably 10
000 / g or less, particularly preferably 8000 / g or less.

The transparent thermoplastic resin pellets obtained by the production method of the present invention and having a reduced amount of foreign matter include, for example, magneto-optical disks, various write-once disks, digital audio disks (so-called compact disks), optical video disks ( It can be suitably used as a material for an optical disk substrate such as a so-called laser disk) and a digital versile disk (DVD), or as a material for a container for precision equipment such as a silicon wafer, and particularly suitable as a material for an optical disk substrate. Adopted to.

[0035]

EXAMPLES The present invention will be described below in detail with reference to examples, but the present invention is not limited to these examples. The evaluation was performed by the following method. (1) Viscosity-average molecular weight of polycarbonate resin 0.7 g of polycarbonate resin was added to 100 m of methylene chloride.
1) and determined from the specific viscosity measured at 20 ° C. (2) Foreign material amount in polycarbonate resin
Using the solution dissolved in ml, the amount of foreign substances having a size of 0.5 μm or more was measured by a foreign substance measuring device manufactured by Hiac Leuco.

Example 1 A reactor equipped with a thermometer, a stirrer and a reflux condenser was charged with 219.4 parts of ion-exchanged water and 40.2 parts of a 48% aqueous sodium hydroxide solution.
-Bis (4-hydroxyphenyl) propane 57.5 parts (0.252 mol) and hydrosulfite 0.1
After dissolving 2 parts, 181 parts of methylene chloride were added, and 28.3 parts of the above phosgene was blown in with stirring at 15 to 25 ° C for 40 minutes. After the injection of phosgene was completed, 7.2 parts of a 48% aqueous sodium hydroxide solution and 2.42 parts of p-tert-butylphenol were added, stirring was started, and after emulsification, 0.06 part of triethylamine was added.
After stirring for an hour, the reaction was completed. After completion of the reaction, the product was diluted with methylene chloride, washed with water, acidified with hydrochloric acid, and washed with water.
When the conductivity of the aqueous phase became almost the same as that of the ion-exchanged water, methylene chloride was evaporated at a water liquid temperature of 75 ° C. in a kneader provided with an isolation chamber having a foreign substance take-out port in the bearing portion to remove the polycarbonate resin powder. 145
℃, dried for 6 hours, viscosity average molecular weight 15,000,
A dried powder having a foreign substance amount of 0.5 μm or more and 1500 particles / g was obtained. Tris (2,4-di-tert) is added to this powder.
-Butylphenyl) phosphite 0.004% by weight,
0.06% by weight of stearic acid monoglyceride was added, and 3
The powder was mixed for 0 minutes and subjected to melt extrusion.

Next, a filtration accuracy of 1 between the screw and the die.
After a vent type twin screw extruder [KTX-46 manufactured by Kobe Steel Co., Ltd.] equipped with a SUS304 filter of 0.5 μm, filtration accuracy for cooling the strands is 0.5 μmS.
Using a device equipped with a cooling bath whose water temperature is controlled at 50 ° C. and a cutting machine for cutting the strands into pellets after the cooling bath, apply ion-exchanged water filtered with a US304 filter, Cylinder temperature of the above-mentioned powder type twin screw melt extruder 240 ° C.,
Melting and kneading while degassing at a vent gas suction rate of -5 mmHg, the polycarbonate resin under the condition of cleanliness class 5 defined in JIS standard B 9920-1989 from the die of the twin screw extruder to the outlet of the cutter. A pellet was obtained. Table 1 shows the results.

Example 2 A vent-type single-screw extruder equipped with a SUS304 filter having a filtration accuracy of 1 μm between a screw and a die [Ishinaka Iron Works Ltd. Model VMK
The air space from the die of the twin-screw extruder to the outlet of the cutting machine is JIS B 9920-198 using the vent type 70].
The procedure was performed in the same manner as in Example 1 except that the cleanliness class defined in 9 was performed under the conditions of 6. Table 1 shows the results.

[Embodiment 3] In Embodiment 1, p-te
Except that the amount of rt-butylphenol was changed to 1.55 parts, the same procedure as in Example 1 was carried out.
To obtain a dry powder. Except that this powder was melt-kneaded and extruded at a cylinder temperature of 290 ° C. in a vented twin-screw extruder, polycarbonate resin pellets were obtained in the same manner as in Example 1. Table 1 shows the results.

Example 4 A polycarbonate resin pellet was obtained in the same manner as in Example 1, except that the ion-exchanged water used for cooling the strand was not filtered with a SUS304 filter. Table 1 shows the results.

Comparative Example 1 A vented twin screw extruder [KTX-4 manufactured by Kobe Steel Co., Ltd.] equipped with a SUS304 filter having a filtration accuracy of 1 μm between a screw and a die.
6], filtration accuracy for cooling the strands is 0.
Using ion-exchanged water filtered with a 5 μm SUS304 filter, using a cooling bath whose water temperature is controlled at 50 ° C., and a device equipped with a cutting machine for cutting the strands into pellets after the cooling bath, Example 1
The dried polycarbonate resin powder obtained in the above,
The cylinder temperature of the vented twin-screw extruder is 240 ° C,
Melting and kneading while degassing at a vent gas suction of -5 mmHg, the polycarbonate resin under the condition of cleanliness class 8 defined in JIS B 9920-1989, from the die of the twin screw extruder to the outlet of the cutting machine. A pellet was obtained. Table 1 shows the results.

[Comparative Example 2] A vent-type single-screw extruder equipped with a filter made of SUS304 having a filtration accuracy of 1 µm between a screw and a die [Ishinaka Iron Works Ltd. Model VMK
-Vent method 70], except that Comparative Example 1 was used. Table 1 shows the results.

Comparative Example 3 The viscosity average molecular weight obtained in Example 3 was 23,500 and the amount of foreign substances having a particle size of 0.5 μm or more was 1800.
A polycarbonate resin pellet was obtained in the same manner as in Comparative Example 1, except that the dried polycarbonate resin powder was melt-kneaded and extruded at a cylinder temperature of 290 ° C. in a vented twin-screw extruder. Table 1 shows the results.

Comparative Example 4 A polycarbonate resin pellet was obtained in the same manner as in Comparative Example 1, except that the ion-exchanged water used for cooling the strand was not filtered through a SUS304 filter. Table 1 shows the results.

[0045]

[Table 1]

[0046]

According to the production method of the present invention, transparent thermoplastic resin pellets with extremely small amount of foreign matter can be produced stably and efficiently, and such transparent thermoplastic resin pellets are very susceptible to adverse effects on foreign matter. It is suitably used as a material for a disk substrate, and its industrial effect is outstanding.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yutaka Kurata 1-2-2 Uchisaiwaicho, Chiyoda-ku, Tokyo Teijin Chemicals Co., Ltd. (72) Sumio Okura 1-2-2 Uchisaiwaicho, Chiyoda-ku, Tokyo F-term (reference) in Nippon Kasei Co., Ltd. BB15A BC03A BD02 BD09A BE05A HC01 HC04A KE11

Claims (3)

[Claims] 1. A method for producing a pellet by melt-extruding a transparent thermoplastic resin into strands using a melt extruder, cooling the strands, and cutting the pellets with a cutter. The airspace up to the entrance of
A method for producing transparent thermoplastic resin pellets, which has a cleanliness class of 7 or less defined in JIS B 9920-1989. 2. The method for cooling a strand by immersing the strand in water stretched in a cooling bath to cool the strand, wherein the water is water filtered through a filter having a filtration accuracy of 5 μm or less. A method for producing a transparent thermoplastic resin pellet. 3. The method for producing transparent thermoplastic resin pellets according to claim 1, wherein the transparent thermoplastic resin is a polycarbonate resin.