JP2006037056A - Method for continuously crystallizing aliphatic polyester - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an improved method for crystallizing a high molecular weight aliphatic polyester, capable of continuously performing the crystallization of the aliphatic polyester, without having problems such as the fusion and breakage of particles, and also having less discoloration after its solid polymerization. <P>SOLUTION: This method for crystallizing the aliphatic polyester is provided by using a device having a pushing out for flowing property and >5 stages, and bringing the aliphatic polyester in contact with an aqueous alkaline solution of pH≥8 flowing in parallel, continuously for crystallizing the polyester. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an improved crystallization method for aliphatic polyesters, which are biodegradable polymers useful as resins for general and medical applications.

Patent Document 1 discloses a technique relating to a method for producing a high molecular weight aliphatic polyester. That is, an aliphatic polyester having a specific relative viscosity η _rel (1.5) or higher is crystallized and then subjected to solid phase polymerization under specific conditions, thereby having a high molecular weight and a high melting point that can be molded as a fishing line. The aliphatic polyester can be obtained.
However, as for the crystallization technique, it is only described that various conventionally known methods that promote crystallization such as crystallization at room temperature or water cooling can be used, and continuously. There is no suggestion or disclosure regarding crystallization by contact with an alkaline aqueous solution and the definition of the apparatus.

Patent Document 2 discloses a technique related to a method for producing a lactic acid-based copolymer polyester granular material. In other words, the fusion problem can be solved by crystallizing a lactic acid-based copolymer polyester composed of a lactic acid component, a dicarboxylic acid component, and a diol component at a glass transition temperature or higher and lower than a melting temperature. Has been.
As described in Paragraph Nos. [0014] to [0016] of the published patent publication, a lactic acid copolymer polyester is extruded from a die in a molten state to form a strand, and the molten strand is cooled at a constant cooling rate. A technique for crystallizing and cutting the crystallized strands into granules is disclosed. However, there is no suggestion or disclosure regarding crystallization by continuous contact with an alkaline aqueous solution and the definition of the apparatus.

  Patent Document 3 discloses that an aliphatic polyester polymer containing 50% or more of an aliphatic hydroxycarboxylic acid unit having a weight average molecular weight of 2,000 to 100,000 is solid-phase polymerized in the presence of a catalyst to obtain an aliphatic hydroxy having a weight average molecular weight of 50,000 to 1,000,000. A technique relating to a method for producing an aliphatic polyester containing 50% or more of a carboxylic acid unit is disclosed. In this invention, a technique for crystallizing an aliphatic polyester polymer by bringing it into contact with a liquid that does not dissolve the aliphatic polyester polymer, and a description that an organic acid may be added to the liquid used for crystallization There is. However, there is no mention of crystallization by continuous contact with an aqueous alkali solution and the definition of the apparatus.

  Patent Document 4 discloses a technique in which a solid aliphatic polyester is brought into contact with a liquid and crystallized. The liquid of the present invention is an organic solvent in which aliphatic polyesters such as water and alcohol do not dissolve, but there is no mention of crystallization by continuous contact with an alkaline aqueous solution and the definition of the apparatus.

  Patent Document 5 discloses a method of crystallization using a solution in which an organic compound, particularly an organic sulfonic acid as a catalyst component, is added to water. In this method, crystallization is performed under low temperature and stirring to avoid fusion, but crystallization by continuous contact with an alkaline aqueous solution using an apparatus having high extrusion flowability is mentioned. Absent.

JP-A-8-34843 JP-A-8-165339 European Patent No. 953589 specification JP 2000-302855 A JP 2001-192442 A

  The problem to be solved by the present invention is that when aliphatic polyester is crystallized, it is continuously brought into contact with an alkaline aqueous solution having a pH ≧ 8 by using a crystallization apparatus having extrusion flowability of more than 5 stages. In addition, by performing solid-phase polymerization thereafter, it is possible to efficiently produce a high-molecular-weight aliphatic polyester with less coloring, which could not be achieved by the prior art. It is intended to provide a method for crystallizing.

  As a result of intensive studies in view of the above situation, the present inventors have completed the present invention.

  In the present invention, when an aliphatic polyester is crystallized by contacting with an alkaline aqueous solution having a pH ≧ 8, the aliphatic polyester and the liquid are continuously mixed in a continuous flow using an apparatus having extrusion flowability of the number of stages> 5. Provided is a method for crystallizing an aliphatic polyester, which is characterized by contacting and crystallizing.

  A method for crystallizing an aliphatic polyester in which the aliphatic polyester is a polymer of an aliphatic hydroxycarboxylic acid is a preferred embodiment of the present invention.

  A method for crystallizing the aliphatic polyester, in which the aliphatic polyester is a copolymer of an aliphatic hydroxycarboxylic acid, an aliphatic polyhydric alcohol, and an aliphatic polycarboxylic acid, is also a preferred embodiment of the present invention.

  A method for crystallizing the aliphatic polyester in which the aliphatic hydroxycarboxylic acid is lactic acid and / or glycolic acid is also a preferred embodiment of the present invention.

  A preferred embodiment of the present invention is a method for crystallizing an aliphatic polyester comprising at least one alkali component in the alkaline aqueous solution selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, and ammonia. It is.

  According to the method of the present invention, when an aliphatic polyester is crystallized, it can be continuously crystallized without causing problems of fusion and breakage under conditions for suppressing a decrease in molecular weight, and subsequently solid-phase polymerization. To obtain an aliphatic polyester having a high molecular weight and a low degree of coloring. Therefore, by crystallizing the aliphatic polyester by the method of the present invention, a stable operation can be performed continuously without causing problems such as particle fusion, particle breakage, and clogging in the apparatus. The aliphatic polyester can be provided.

The aliphatic polyester of the present invention is a polymer containing an ester bond, and may be any of a homopolymer, a copolymer, or a mixture thereof. In the present invention, the aliphatic polyester preferably includes an aliphatic polyhydroxycarboxylic acid unit, and more preferably those represented by the following two types.
(1) Homopolymer or copolymer of aliphatic polyhydroxycarboxylic acid obtained from aliphatic hydroxycarboxylic acid or a mixture thereof (2) Fat comprising aliphatic hydroxycarboxylic acid, aliphatic polyhydric alcohol and aliphatic polycarboxylic acid Copolymers or mixtures thereof with aliphatic polyesters

  Specific examples of the aliphatic hydroxycarboxylic acid include lactic acid, glycolic acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 4-hydroxyvaleric acid, 5-hydroxyvaleric acid, 6-hydroxycaproic acid and the like. Among these, lactic acid and glycolic acid are preferable. These hydroxycarboxylic acids may be used alone or in combination of two or more. In the case of having an asymmetric carbon atom in the molecule like lactic acid, D-form, L-form, and an equivalent mixture (racemic form) thereof exist, any of which can be used.

  The aliphatic polyhydric alcohol is not particularly limited. Examples of the divalent alcohol include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, 3-methyl-1,5-pentanediol, 1 , 6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, neopentyl glycol, 1,4-cyclohexanedimethanol and the like. These can be used alone or in combination of two or more. Examples of the trihydric or higher aliphatic polyhydric alcohol include glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, dipentaerythritol, xylitol, and inositol. These can be used alone or in combination of two or more.

  The aliphatic polycarboxylic acid is not particularly limited. Examples of the aliphatic dicarboxylic acid include succinic acid, oxalic acid, malonic acid, glutamic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, and 3,3-dimethylpentane. Aliphatic dicarboxylic acids such as diacids and alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid can be mentioned. These can be used alone or in combination of two or more. Examples of trivalent or higher aliphatic polyvalent carboxylic acids include 1,2,3,4,5,6-cyclohexanehexacarboxylic acid, 1,2,3,4-cyclopentanetetracarboxylic acid, tetrahydrofuran 2R, 3T, 4T, 5C-tetracarboxylic acid, 1,2,3,4-cyclobutanetetracarboxylic acid, 4-carboxy-1,1-cyclohexanediacetic acid, 1,3,5-cyclohexanetricarboxylic acid, (1α, 3α, 5β) -1,3,5-trimethyl-1,3,5-cyclohexanetricarboxylic acid, cyclic compounds such as 2,3,4,5-furantetracarboxylic acid and anhydrides thereof, butane-1,2,3,4- Tetracarboxylic acid, meso-butane-1,2,3,4-tetracarboxylic acid, 1,3,5-pentanetricarboxylic acid, 2-methylolpropane tricarboxylic acid, , 2,3-propane tricarboxylic acid, 1,1,2-ethane tricarboxylic acid, compounds and anhydrides thereof, such as 1,2,4-butane tricarboxylic acid. These can be used alone or in combination of two or more.

  The aliphatic polyester used in the crystallization method according to the present invention is preferably an aliphatic hydroxycarboxylic acid polymer, an aliphatic hydroxycarboxylic acid and an aliphatic polyhydric alcohol, and an aliphatic polycarboxylic acid copolymer. Preferably (1) a polymer of lactic acid and / or glycolic acid, (2) a copolymer of lactic acid and / or glycolic acid, an aliphatic polyhydric alcohol and an aliphatic polyvalent carboxylic acid, more preferably polylactic acid, Polyglycolic acid and lactic acid / glycolic acid copolymer are preferred, and polylactic acid is particularly preferred.

There are two methods for producing aliphatic polyesters: dehydrating polycondensation of hydroxycarboxylic acids and ring-opening polymerization of cyclic esters and cyclic dimers as hydroxycarboxylic acid dehydrates, in order to obtain desired resin properties. A known reaction method can be used as appropriate.
Further, during the polyhydric condensation of an aliphatic hydroxycarboxylic acid or in the ring-opening polymerization of a cyclic dimer, which is a dehydration product of a hydroxycarboxylic acid, an aliphatic polyhydric alcohol and an aliphatic polycarboxylic acid are added. In order to obtain a copolymer, a well-known reaction method can be used suitably.

  In addition, as aliphatic hydroxycarboxylic acid, lactic acid alone or glycolic acid alone or a method of dehydration polycondensation of lactic acid and glycolic acid, and lactide dehydration product lactide and glycolic acid dehydration product glycolide are each subjected to ring-opening polymerization. In order to obtain a copolymer, a well-known reaction method can be used suitably.

  The weight average molecular weight of the aliphatic polyester used for the crystallization method of the present invention is not particularly limited as long as it is 2000 or more and 100000 or less, but more preferably 5000 or more and 70000 or less.

  The form of the aliphatic polyester suitable for the crystallization method of the present invention may be solid, molten, or solution. In the solid state, the shape is not particularly limited, and a plate shape, a lump shape, a string shape, a pellet shape, a granular shape, and the like can be used. From the viewpoint of uniform crystallization and ease of handling, pellets or granules are preferred. The method for molding the polymer into pellets or granules is not particularly limited, and a known method is used.

  In the molten state and the solution state, the method of contacting with the liquid is not limited at all. For example, an aliphatic polyester melt or an aliphatic polyester solution in which an aliphatic polyester is dissolved in a solvent is dropped into an alkaline aqueous solution and solidified, and then crystallized in a crystallization apparatus with high extrusion flowability. In this case, spherical pellets can be obtained.

  The pellet shape and grain shape are not particularly limited. The pellet shape and particle shape need not be a specific shape such as a pulverized shape, a chip shape, a spherical shape, a cylindrical shape, a marble shape, or a tablet shape, but in general, a spherical shape, a cylindrical shape, or a marble shape is preferable.

  The pellet manufacturing apparatus is not particularly limited. For example, Sandvik's strip former, funnel former, double roll feeder, Kaiser rotary drop former, piston drop former, Mitsubishi Examples include a drum cooler manufactured by Kasei Engineering, a steel belt cooler manufactured by Nippon Belling, and a hybrid former.

  The melted droplet generator for aliphatic polyester and the solution droplet generator for aliphatic polyester are not particularly limited, but specific examples thereof include a Kastor Pastilator. The size of the pellets and grains is not particularly limited, but is preferably 0.1 mm to 10 mm and more preferably 1 mm to 5 mm in consideration of handling in the manufacturing process and handling in the secondary molding.

  In the present invention, when performing crystallization, the alkaline aqueous solution to be contacted with the aliphatic polyester preferably has pH ≧ 8, more preferably 8 ≦ pH ≦ 13.

  The aqueous alkali solution used for crystallization of the present invention is preferably an aqueous solution of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, ammonia, etc., and these are used alone or in combination of two or more. be able to.

  The crystallization in the present invention is measured by performing a differential scanning calorimeter (DSC) measurement (measurement conditions; sample weight = about 10 mg, temperature conditions = 20 ° C. to 200 ° C., heating rate = 10 ° C./min). This means that the heat of crystallization is 30 J / g or less.

The apparatus having high extrudability in the present invention is an apparatus characterized in that the time during which polymer particles continuously introduced into the apparatus stay in the apparatus is substantially uniform, that is, there is almost no distribution of residence time. is there.
The index representing the extrusion flowability is the number of tank reactors in a tank row model in which the apparatus is virtually divided into tank reactors of equal volume and they are considered to be connected in series. The greater the number of stages, the higher the extrusion flowability. The apparatus in the present invention is an apparatus having a feature that the number of stages is more than 5, preferably 7 or more, more preferably 10 or more.
The method for measuring the number of stages of the crystallization apparatus in the present invention is not particularly limited, but can be measured according to a method generally called a step response method. As a measuring method, from a state in which pellets and water were continuously circulated in advance in the apparatus, pellets having different colors were circulated as tracers from a certain time, and the amount of change at the outlet of the reactor was measured at regular intervals. Taking the weight of the initial pellets at different pellet distribution start times t = 0 as a reference, the change ratio of the weights of the different color pellets at time t is calculated as the dimensionless value by the following equation.

Replacement ratio = (1-weight of pellets circulated in the initial stage) / (weight of discharged pellet at time t)

With respect to this change ratio, a step response curve of the change ratio calculated previously is created for θ = t / τ in which a certain arbitrary time t is made dimensionless in the residence time τ of the entire apparatus.
Next, in the tank row model where the apparatus is virtually divided into tank reactors of equal volume and considered to be connected in series, the number of tanks, which is an indicator of the degree of mixing, that is, extrusion flowability A step response function curve shown in the following equation is created with (number of stages) as N, and compared with an actual step response curve, and the number of stages of the apparatus is measured.

Step response function ^{= 1-e -Nθ [1 +} Nθ + (Nθ) 2/2! + ... + (Nθ) ^N−1 / (N−1)! ]

When the extrusion flowability is low, that is, when the residence time distribution is wide and the number of stages is small, the continuously supplied polymer is dispersed and mixed in the apparatus. Since it is immediately discharged out of the apparatus, crystallization does not proceed sufficiently, fusion between polymer particles in the subsequent drying process occurs, or the hue of the polymer deteriorates in solid phase polymerization, which is not preferable. Since certain polymer particles stay in the apparatus for a long time, not only the water content in the polymer increases and the drying load of the polymer increases, but also the polymer itself becomes brittle and easily crushed.

  As the number of stages of the apparatus having high extrusion flowability used in the present invention, the number of stages calculated by the step response method is preferably more than 5, more preferably 7 stages or more, and more preferably 10 stages or more.

  The weight of the polymer used for crystallization is preferably from 0.01 to 100, more preferably from 0.005 to 50, based on the weight of the aqueous alkali solution. Within this range, crystallization can be performed efficiently. . Crystallization can be suitably performed within the above range. )

  The crystallization temperature of the present invention is preferably not less than the glass transition point (Tg) of the polymer and not more than the boiling point of the aqueous alkali solution used. Crystallization can be suitably performed within the above range.

  Although the crystallization time of the present invention depends on the crystallization temperature, it is preferably 10 to 120 min, more preferably 30 to 100 min. Crystallization can be suitably performed within the above range.

  In this way, it can be dried to remove the water in the polymer crystallized with an alkaline aqueous solution to obtain a final product, or the polymer crystallized with an alkaline aqueous solution can be subjected to solid phase polymerization to increase the molecular weight. it can.

[Solid-state polymerization]
The term “solid phase polymerization” of the present invention means a polymerization method in which the molecular weight is increased by carrying out a polymerization reaction at a temperature lower than the melting point of the polymer present in the reaction system. That is, it means a polymerization method in which the molecular weight is increased by carrying out a polymerization reaction while maintaining the solid state of the polymer present in the reaction system. The reaction temperature of the solid phase polymerization is generally preferably a temperature lower than the melting point of the polymer existing in the reaction system and a temperature equal to or higher than the glass transition temperature Tg, and a temperature lower than the melting point of the polymer existing in the reaction system. More preferably, the temperature is 100 ° C. or higher.

  In addition, solid-phase polymerization methods include, for example, a method in which a polymerization reaction is performed in a circulating gas atmosphere in which an inert gas such as nitrogen gas, helium gas, argon gas, xenon gas, krypton gas, or dry air is circulated, or under reduced pressure or There is a method of performing a polymerization reaction under pressure, but there is no particular limitation as long as the molecular weight of the aliphatic polyester after the completion of the solid phase polymerization is equal to or greater than the molecular weight before the start of the solid phase polymerization. When the polymerization reaction is carried out under reduced pressure or under pressure, the pressure in the reaction system depends on the polymerization rate, the type and amount of catalyst used, and in the case of dehydration polycondensation reaction, the rate at which water produced by the reaction is removed. It is set in consideration of efficiency and molecular weight reached.

  In the present invention, the weight average molecular weight is 80,000 or more, preferably 100,000 or more, more preferably 13 by performing solid phase polymerization after continuous crystallization in an alkaline aqueous solution using an apparatus having high extrusion flowability. Producing a high-molecular weight aliphatic polyester having a low coloration such that the yellowness index (YI) of the index representing the hue of the polymer is not more than 6.5 per 2 mm thickness, more preferably not more than 5.0, more preferably not more than 5.0. Can do.

  The Yellowness Index (YI) shown in the present invention refers to melting an aliphatic polyester at a melting point (Tm) or higher to prepare a 2 mm thick plate. According to JIS K-7103, an SM color computer (model: SM-6-IS) is used. -2-B, measured by Suga Test Instruments Co., Ltd.)

  The aliphatic polyester obtained by contacting with an alkaline aqueous solution with the apparatus with high extrusion flowability according to the present invention and continuously crystallizing it, and then performing the above-mentioned solid-phase polymerization, has sufficient mechanical properties in practice. It has a low coloring degree and can be used for a wide range of applications. Particularly, it can be suitably used for applications requiring low coloration, such as packaging containers, bottles, films, packaging materials and the like.

Hereinafter, the present invention will be described in detail by way of examples. However, the present invention is not limited to the following examples.
In the examples, description will be made focusing on polylactic acid, which is a typical example of aliphatic polyester. The evaluation method used in this example is as follows.
(1) Weight average molecular weight Aliphatic polyester was dissolved in chloroform, and measured by gel permeation chromatography (GPC) at 40 ° C. using chloroform as a solvent. At this time, polystyrene was used as a standard sample.
(2) Yellowness Index (YI)
Aliphatic polyester samples were melted at Tm or more to prepare 2 mm thick plates, and in accordance with JIS K-7103, SM color computer (model: SM-6-IS-2-B, Suga Test Instruments Co., Ltd.) It was measured.
(3) Calculation of the number of stages of the tower-type apparatus Using the above-mentioned step response method, from a state where pellets and water were continuously circulated in advance, pellets having different colors were circulated as tracers from a certain time, and at the reactor outlet The amount of replacement was measured and calculated.

[Production Example 1]
400 kg of L-lactic acid and 2 kg of methanesulfonic acid as a catalyst were charged, and crude dehydration was performed at 140 ° C. and 6700 Pa for 2 hours, and then polycondensation was performed at 160 ° C. and 1300 Pa for 8 hours. Thereafter, pelletized to obtain 250 kg of amorphous polylactic acid having a particle size of 2.8 to 3.4 mm and a weight average molecular weight of 10,000. The heat of crystallization of the obtained pellet was 50 J / g.

[Device Example 1]
Using a tower-type device with an inner diameter of 150 mm and a height of 1000 mm, from a state where pellets and water were continuously circulated in advance, circulate pellets of different colors as tracers from a certain time, and measure the amount of replacement at the tower-type device outlet. The number of steps was calculated by the step response method. As a result, the number of stages was 20.

[Device Example 2]
Using a tower type apparatus having an inner diameter of 250 mm and a height of 1000 mm, the amount of change was measured in the same manner as in Example 1, and the number of stages was calculated by the step response method. As a result, the number of stages was 10.

[Device Example 3]
Using a tower type apparatus having an inner diameter of 500 mm and a height of 1000 mm, the amount of change was measured in the same manner as in Example 1, and the number of stages was calculated by the step response method. As a result, the number of stages was 5.

[Example 1]
The polylactic acid pellets obtained in Production Example 1 were supplied from the top of the tower of the tower type apparatus shown in Apparatus Example 1 at 10 kg / hr, and at the same time, pH 8 sodium hydroxide aqueous solution maintained at 70 ° C. was co-flowed at 50 kg / hr. Made contact. The pellets moved smoothly toward the bottom of the tower without fusing. The pellets obtained from the bottom of the tower were immediately dehydrated with a screw decanter. The heat of crystallization of the obtained pellet was not observed. After the pellet was sufficiently dried, the measured weight average molecular weight was 10,000. Subsequently, solid phase polymerization was carried out at 140 ° C. for 60 hours under a nitrogen stream. As a result, Mw was 130,000, YI = 3.0, and the color of the polymer was good.

[Example 2]
The polylactic acid pellets obtained in Production Example 1 were supplied at 10 kg / hr from the top of the tower-type apparatus shown in Apparatus Example 2 and simultaneously a pH 8 sodium hydroxide aqueous solution maintained at 70 ° C. was flown at 50 kg / hr. Made contact. The pellets moved smoothly toward the bottom of the tower without fusing. The pellets obtained from the bottom of the tower were immediately dehydrated with a screw decanter. The heat of crystallization of the obtained pellet was not observed. After the pellet was sufficiently dried, the measured weight average molecular weight was 10,000. Subsequently, solid phase polymerization was carried out at 140 ° C. for 60 hours under a nitrogen stream. As a result, Mw was 130,000 and YI = 4.5, and the color of the polymer was good.

[Comparative Example 1]
The polylactic acid pellets obtained in Production Example 1 were supplied at 10 kg / hr from the top of the tower-type apparatus shown in Apparatus Example 3, and at the same time, pH 8 sodium hydroxide aqueous solution maintained at 70 ° C. was co-flowed at 50 kg / hr. Made contact. The pellets obtained from the bottom of the tower were immediately dehydrated with a screw decanter. The heat of crystallization of the obtained pellet was 35 J / g. After the pellet was sufficiently dried, the measured weight average molecular weight was 10,000. Subsequently, solid phase polymerization was carried out at 140 ° C. for 60 hours under a nitrogen stream. As a result, Mw was 150,000 and YI = 7.0, and the color of the polymer was poor.

According to the present invention, it is possible to provide a method for continuously crystallizing an aliphatic polyester for obtaining a high molecular weight aliphatic polyester having a low degree of coloring.

Claims (5)

In order to crystallize the aliphatic polyester by bringing it into contact with an alkaline aqueous solution having a pH ≧ 8, the aliphatic polyester and the liquid are continuously brought into contact and crystallized in a cocurrent manner using an apparatus having extrusion flowability of the number of stages> 5. And a method for crystallizing an aliphatic polyester. 2. The method for crystallizing an aliphatic polyester according to claim 1, wherein the aliphatic polyester is a polymer of an aliphatic hydroxycarboxylic acid. 2. The method for crystallizing an aliphatic polyester according to claim 1, wherein the aliphatic polyester is a copolymer of an aliphatic hydroxycarboxylic acid, an aliphatic polyhydric alcohol, and an aliphatic polycarboxylic acid. The method for crystallizing an aliphatic polyester according to claims 2 to 3, wherein the aliphatic hydroxycarboxylic acid is lactic acid and / or glycolic acid. The method for crystallizing an aliphatic polyester according to claim 1, wherein the alkaline component in the alkaline aqueous solution comprises at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide and ammonia.