JP2007131868A - Heat-shrinking film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat-shrinking film having excellent heat shrinkage, heat-shrinking stress and adhesion to a container, and also having alkali-solubility. <P>SOLUTION: The heat-shrinking film is an oriented film comprising a polymer consisting essentially of a polylactic acid containing 0.01-1 wt.% particles, and has ≥20% heat shrinkage at least in one direction of the film in hot air at 120°C, and ≤5.0 MPa maximum heat shrinkage stress of the film. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a heat-shrinkable film that is soluble in an alkaline aqueous solution and not only excellent in heat shrinkage properties but also excellent in mechanical properties and heat resistance, and used for shrink labels, food packaging, and the like.

  Conventionally, heat shrink films have been used as shrink labels used in bottles, polyethylene terephthalate (hereinafter abbreviated as PET) bottles, and the like.

  In particular, as a shrink label for food packaging, a stretched film having a large shrinkage rate due to heat, for example, an ethylene-vinyl acetate copolymer, an ethylene-ethyl acrylate copolymer, polyvinylyl chloride, or polystyrene has been mainly used. It was.

  However, in recent years, polyester-based heat-shrinkable films having excellent safety and hygiene, chemical resistance, and transparency have been demanded, and stretched films made of polyester obtained by copolymerizing neopentyl glycol or isophthalic acid with PET have been developed. It has been proposed (Patent Document 1, Patent Document 2, Patent Document 3).

  However, it cannot be said that these films have sufficiently reduced the heat shrinkage stress, and there is a problem that the heat shrinkage rate is insufficient when the film is deformed or contracted without deformation of the container.

In recent years, it has been requested to collect bottles and PET bottles. In order to collect these containers, it is necessary to remove the heat-shrinkable film attached to these containers. In the case of a label using a water-soluble paste, the label can be removed from the container by immersing it in an aqueous solution. However, in the case of a heat shrinkable film, it is brought into close contact with the container by heat shrinkage. It cannot be removed from the container by immersion. In order to recover the container, there was a problem that the heat shrink film had to be removed from each container.
JP-A 63-156833 JP-A-63-202429 Japanese Examined Patent Publication No. 63-7573

  Accordingly, an object of the present invention is to provide a heat-shrinkable film that is excellent in heat-shrinkage characteristics and container adhesion, and can be dissolved in an alkaline aqueous solution and peeled from the container.

  The object of the present invention is a stretched film composed of a polymer mainly composed of polylactic acid containing 0.01 to 1% by weight of particles, wherein the film has a heat shrinkage rate in at least one direction in 120 ° C. hot air. It is 20% or more, and is achieved by a heat shrinkable film characterized in that the maximum heat shrinkage stress of the film is 5.0 MPa or less.

  The heat-shrinkable film obtained in the present invention is a stretched film composed of a polymer mainly composed of polylactic acid containing 0.01 to 1% by weight of particles, and the film is at least one direction in hot air at 120 ° C. By providing a heat shrinkage rate of 20% or more and a maximum heat shrinkage stress of the film of 5.0 MPa or less, a heat shrinkable film having excellent heat shrinkability, heat shrinkage stress properties, container adhesion, and alkali solubility is provided. can do.

  Hereinafter, preferred embodiments of the present invention will be described.

  The particles used in the heat-shrinkable film of the present invention must be contained in an amount of 0.01 to 1% by weight in a polymer mainly composed of polylactic acid. More preferably, it is 0.01 to 0.5% by weight. The particles to be contained are not particularly limited as long as they are inactive to the polylactic acid polymer, but examples of the particles include inorganic particles, organic particles, crosslinked polymer particles, and internal particles generated in the polymerization system. it can. Two or more kinds of these particles may be contained.

  The inorganic particles are not particularly limited, but calcium carbonate, kaolin, talc, magnesium carbonate, barium carbonate, calcium sulfate, barium sulfate, lithium phosphate, calcium phosphate, magnesium phosphate, aluminum oxide, silicon oxide, titanium oxide, zirconium oxide. And lithium fluoride. Examples of the organic particles include calcium oxalate, terephthalate such as calcium, barium, zinc, manganese, and magnesium.

  Examples of the crosslinked polymer particles include homopolymers or copolymers of vinyl monomers such as divinylbenzene, styrene, acrylic acid, and methacrylic acid. In addition, organic fine particles such as polytetrafluoroethylene, benzoguanamine resin, thermosetting epoxy resin, unsaturated polyester resin, thermosetting urea resin, and thermosetting phenol resin are also preferably used. As the internal particles generated in the polymerization system, those generated by a known method in which an alkali metal compound, an alkaline earth metal compound or the like is added to the reaction system, and a phosphorus compound is further added can be used. If the particle content is less than 0.01% by weight, it is difficult to impart the slipperiness of the film. On the other hand, when the amount exceeds 1% by weight, when the heat-shrinkable film is dissolved with an alkaline aqueous solution when the PET bottle is collected, the particles are likely to adhere to the PET bottle as a dissolution residue, and when the particles are contained in the collected PET, the moisture attached to the particles. And lactic acid is not preferable because it tends to have an adverse effect. The average particle diameter to be contained is preferably 0.001 to 10 μm, and more preferably 0.01 to 2 μm. If the average particle size is less than 0.01 μm, it is difficult to impart easy lubricity, and conversely if it exceeds 10 μm, defects in the film tend to occur, which is not preferable.

  Moreover, the polymer used for the heat shrinkable film of the present invention needs to be mainly composed of polylactic acid. Examples of the polymer mainly composed of polylactic acid include D-lactic acid homopolymer, L-lactic acid homopolymer, D-lactic acid / L-lactic acid copolymer, D-lactic acid / hydroxycarboxylic acid copolymer, and mixtures thereof. The polymer mainly composed of polylactic acid is mainly composed of an L-lactic acid component and preferably has a D-lactic acid component content of 0 to 30% by weight from the viewpoint of heat resistance and heat shrinkage characteristics. The content of the D-lactic acid component is preferably 0 to 20% by weight.

  The polymer mainly composed of polylactic acid in the present invention can be obtained by the following method. As raw materials, hydroxycarboxylic acids such as glycolic acid, hydroxybutyric acid, hydroxyvaleric acid, and hydroxycaproic acid can be used in combination, mainly D-lactic acid and / or L-lactic acid. Furthermore, dicarboxylic acids and glycols can also be used. A polymer mainly composed of polylactic acid can be obtained by a method of directly dehydrating polycondensation of the raw materials or a method of ring-opening polymerization of the cyclic ester intermediate. For example, when producing by direct dehydration polycondensation, lactic acid or lactic acid and hydroxycarboxylic acid are preferably subjected to azeotropic dehydration condensation in the presence of an organic solvent, particularly phenyl ether solvent, and particularly preferably polymerized by azeotropy. As a result, a high molecular weight polymer suitable for the present invention can be obtained. The molecular weight of the polymer is preferably in the range of a weight average molecular weight of 10,000 to 1,000,000 from the viewpoint of moldability as a film.

  In addition, the polymer mainly composed of polylactic acid of the film of the present invention includes polyglycolic acid, polybutyric acid polyhydroxybutyrate, etc. having a hydroxycarboxylic acid component as constituents, and dicarboxylic acid components and glycol components as constituent components. Polyethylene terephthalate (PET), Polypropylene terephthalate (PPT), Polyethylene-2,6-naphthalate (PEN), Polybutylene terephthalate (PBT), Polyhexamethylene terephthalate (PHT), Polyethylene isophthalate (PEI), Polycyclohexanedimethylene terephthalate (PCT), polyesters such as polybutylene succinate, or a blend with a copolymer mainly composed of these polyesters may be used. In the case of a copolymer, it may be a random copolymer or a block copolymer. Moreover, you may add another alkaline water-soluble polymer in the range which does not impair the effect of this invention.

  Examples of other alkali biodegradable polymers include cellulose acetate, cellulose, starch, and polyvinyl alcohol. By making the polymer used for the heat-shrinkable film a polymer mainly composed of polylactic acid, it can have alkali solubility. If necessary, the polymer mainly composed of polylactic acid may contain various additives other than particles as long as heat shrinkage, heat shrinkage stress, and alkali solubility are not impaired. Also good. For example, an organic lubricant such as a flame retardant, an antioxidant, an antifungal agent, a heat stabilizer, a lubricant, a crystal nucleating agent, an ultraviolet absorber, a colorant, a terminal blocking agent, a fatty acid ester, a wax, or a polysiloxane may be blended. it can.

  In addition, the heat shrink film in the present invention needs to be a stretched film. The stretching method is biaxially stretched by any of the inflation simultaneous biaxial stretching method, the stenter simultaneous biaxial stretching method, and the stenter sequential biaxial stretching method. In that respect, a film formed by a stenter sequential biaxial stretching method is preferred.

  Further, the heat shrinkable film in the present invention is required to have a heat shrinkage rate of at least 20% in at least one direction at 120 ° C. Preferably it is 40% or more. When the heat shrinkage rate is less than 20%, the heat shrinkability becomes insufficient, and this is not preferable in that the film does not sufficiently adhere to the container.

  The film in the present invention is required to have a maximum heat shrinkage stress of 5.0 MPa or less. Preferably, the maximum heat shrinkage stress in the longitudinal direction is preferably 5.0 MPa or less. If it exceeds 5.0 MPa, the container is easily deformed, which is not preferable.

  The heat-shrinkable film in the present invention is preferably dissolved by 50% or more in an aqueous solution exceeding pH 7. Preferably, it is 80% or more, more preferably 95% or more. If it is less than 50%, it is difficult to remove the film adhered during container recovery, which is not preferable. The aqueous solution exceeding pH 7 is an alkaline aqueous solution, and examples thereof include alkaline aqueous solutions such as sodium hydroxide, calcium hydroxide, and ammonium hydroxide. A strong alkaline aqueous solution is easy to dissolve the film, but too strong is not preferable because the working environment is deteriorated. Preferably it is pH 7.1-9. A neutral solution of pH 7 and an acidic aqueous solution of less than pH 7 are not preferable because the film is difficult to dissolve. More preferably, heating an aqueous solution exceeding pH 7 to 50 ° C. or more is preferable because the solubility of the film easily proceeds.

  In the heat shrinkable film of the present invention, a polymer mainly composed of polylactic acid can be laminated on a base material mainly composed of polyester or polyolefin resin.

  Here, the polyester is a polymer having a dicarboxylic acid component and a glycol component as constituent components. For example, polyethylene terephthalate (PET), polypropylene terephthalate (PPT), polyethylene-2,6-naphthalate (PEN), polybutylene terephthalate ( Polyesters such as PBT), polyhexamethylene terephthalate (PHT), polyethylene isophthalate (PEI), polycyclohexanedimethylene terephthalate (PCT), polybutylene succinate, or blends of these polyesters as a main component It may be a body. In the case of a copolymer, it may be a random copolymer or a block copolymer. Polyolefin resins are polyethylene and polypropylene resins, and copolymers of propylene, butene-1, hexene-1, 4-methylpentene-1, and octene-1 with an ethylene content of 0 to 6% by weight. It is. The copolymer of propylene, butene-1, and octene-1 may be a simple substance, a mixture, or a terpolymer such as propylene and butene-1. In particular, the base material is preferably a polyester resin such as polyethylene terephthalate, or a polyolefin resin such as polyethylene or polypropylene, from the viewpoint of cost and heat resistance. The substrate may be any of a non-stretched film, a uniaxially stretched film, and a biaxially stretched film, but a biaxially stretched film is preferable for obtaining uniform heat shrinkability and heat shrinkage stress properties.

  In the heat shrinkable film of the present invention, the base film (A) and the polymer (B) mainly composed of polylactic acid can be laminated in the order of B / A / B.

  Although the thickness of the film of the present invention is not particularly limited, it is effectively used at 1 to 300 μm, preferably 5 to 100 μm.

  Although the heat shrink film of this invention is not specifically limited, For example, it can manufacture using the following method. For example, in the case of a biaxially stretched film, the polylactic acid polymer or polylactic acid copolymer having the composition of the present invention described above is sufficiently dried using a normal hopper dryer, paddle dryer, vacuum dryer, etc., and then an extruder. And melted at 150 to 230 ° C., extruded onto a casting drum by a T-die extrusion method, and rapidly cooled to obtain an unstretched film. As an adhesion method to the casting drum, any of an electrostatic application method, an adhesion method using surface tension such as water, an air knife method, a press roll method, etc. may be used, but the flatness is good. As a method for obtaining a film having a small smooth surface defect, it is particularly preferable to use a contact casting method or an electrostatic application method utilizing surface tension of water or the like because a film having a uniform thickness can be obtained. At this time, an unstretched film having a desired thickness can be obtained by adjusting the slit width of the die, the discharge amount of the polymer used for the film, and the number of rotations of the casting drum.

  Subsequently, a biaxially stretched film can be obtained by biaxially stretching the unstretched film simultaneously or sequentially. In the case of sequential biaxial stretching, the stretching order of the film may be the longitudinal direction and the width direction, or vice versa. Furthermore, in the sequential biaxial stretching, the stretching in the longitudinal direction or the width direction can be performed twice or more. The draw ratio in the longitudinal direction and width direction of the film can be arbitrarily set according to the degree of orientation, strength, elastic modulus and the like of the target film heat shrinkability, heat shrink stress property, and adhesion to the container. Preferably it is 2.0 to 7.0 times. Either the stretching ratio in the longitudinal direction or the width direction may be increased or the stretching ratio may be the same. There is no restriction | limiting in particular about the extending | stretching method, Methods, such as roll extending | stretching and a tenter extending | stretching, are applied, and what kind of things, such as a flat form and a tube form, may be sufficient in a shape surface. Further, the stretching temperature can be set to a temperature in the range of not less than the glass transition temperature of the polymer mainly composed of polylactic acid and not more than the crystallization temperature. Usually, 30-120 degreeC is preferable. Furthermore, after the film is biaxially stretched, heat treatment may be performed for the purpose of improving strength, stability over time, and shrinkage characteristics. This heat treatment can be performed by any method such as in an oven or on a heat-treated roll. The heat treatment temperature can be any temperature not lower than the stretching temperature and not higher than the melting point of the polymer mainly composed of polylactic acid, but is preferably 50 to 150 ° C. or lower. Moreover, although the heat processing time can be made arbitrary, it is preferable to carry out normally for 1 to 60 seconds. The heat treatment may be performed while relaxing the film in the longitudinal direction and / or the width direction. The heat-treated film may be rapidly cooled to the glass transition temperature or lower after the heat treatment, or may be cooled stepwise.

Next, the effects of the present invention will be described with reference to examples, but the present invention is not limited to these examples. First, the characteristic value measurement method and evaluation method will be described below.
[Method for Measuring / Evaluating Characteristic Value] The characteristic value of the present invention is determined by the following measuring method.
(1) The coefficient of static friction (μs) was measured in accordance with ASTM-D-1894 for the slipperiness between slippery films. The judgment criteria were evaluated as x for μs of 2.0 or more, Δ for a range of 1.6 to 2.0, and x for less than 1.2.
(2) Heat shrinkage ratio and appearance Samples were cut into strips of 250 mm in the longitudinal direction and 10 mm in the width direction, left in an atmosphere of 23 ° C. and 60% humidity for 30 minutes, and 200 mm in the longitudinal direction in that atmosphere. Two marks are made at intervals of, and the distance between the marks is measured using a linear scale length measuring device, and the value is set to LA. Next, after heating for 5 minutes using hot air at 120 ° C, cooling to a 23 ° C, 60% humidity atmosphere for 1 hour, adjusting the humidity, the distance between the first marks is measured with a linear scale measuring machine, and the value is LB And The heat shrinkage rate is obtained by the following formula.

Heat shrinkage rate = (LA−LB) / LA × 100 Then, the average heat shrinkage rate in the MD and TD directions is 20% or more, ○ is 10-20%, Δ is less than 10% Was evaluated as x. Further, after heat shrinkage, those having appearance defects such as whitening were evaluated as xx, and those having good appearance were evaluated as ◯.
(3) The maximum heat shrinkage stress film was sampled into a strip shape having a width of 5 mm, and the temperature rising rate from room temperature to the melting point of the film was 10 ° C. by using a thermal analysis system MTS9000 type and heat absorption stress measuring machine TM9400 type manufactured by Vacuum Riko Co., Ltd. The shrinkage force generated when heated at 1 min was measured, and the heat shrinkage stress with respect to temperature was determined. The maximum heat shrinkage stress was evaluated as ○ when the stress was less than 5.0 MPa, Δ when the stress was in the range of 5.0 to 6.0 MPa, and x when 6.0 MPa or more.
(4) Adhesion to container After the film was shrunk and packaged in a PET bottle (for 1000 ml) with a dryer, the appearance was evaluated by classification as follows. The film is sufficiently shrunk to wrap the container, and the container has the original shape ○, the film is sufficiently shrunk to wrap the container, but the container has a part that is slightly deformed △ The case where the film was insufficiently shrunk and the container could not be packaged well and the film was sufficiently shrunk to wrap the container but had a part that was clearly deformed were evaluated as x.
(5) The film weight of about 5 g of alkali solubility is accurately measured, and the value is defined as G1. After immersing in 200 ml of 2 wt% aqueous sodium hydroxide solution at a temperature of 60 ° C. for 8 hours, the aqueous solution is filtered with a filter paper, thoroughly washed with pure water, and the residue on the filter paper is dried at 110 ° C. for 1 hour, then 23 ° C. Then, after cooling in a 60% humidity atmosphere for 1 hour and adjusting the humidity, the weight of the residue is measured and the value is defined as G2. The alkali solubility is obtained by the following formula.
Alkali solubility = (G1-G2) / G1 × 100 Then, ◎ if the alkali solubility is 95% or more, ○ if 80% or more but less than 95%, △ if 50% or more but less than 80%, 50% The solubility in alkali was evaluated with x below.
(6) The container is collected in a PET bottle (for 1000 ml), wrapped with a dryer and packaged, and then immersed in 5000 ml of a 2% by weight aqueous sodium hydroxide solution at a temperature of 60 ° C. for 8 hours to dissolve the film. Was removed and washed with water. Collectability was evaluated by visually classifying the adhesion state of particles to the PET bottle as follows. The case where the particles are attached to the container is indicated as x, the case where the particles are not attached is indicated as ○, and the intermediate portion is indicated as Δ.
(7) Comprehensive evaluation Regarding heat shrinkage stress properties, heat shrinkability, adhesion to containers, and alkali solubility as a heat shrinkable film, it is evaluated as ○ for superior, △ for slightly inferior, and × for inferior. did.

  Next, the effects of the present invention will be described with reference to examples.

  Example 1 Mixing polylactic acid A having a weight average molecular weight of 170,000 having a composition ratio of 95/5 with L-lactic acid / D-lactic acid containing 0.15% by weight of wet-process silica particles having an average particle diameter of 1.1 μm It supplied to the back twin-screw extruder and was made into the extrusion pellet at 200 degreeC. The pellets were dried at 120 ° C. under reduced pressure for 3 hours, then led to a T-die die using a separate extruder, extruded at a temperature of 250 ° C., and cast on a cooling drum to prepare a sheet-like unstretched film. Next, the sheet was stretched 2.0 times in the longitudinal direction between 65 ° C. heating rolls and cooled, and then led to a tenter type stretching machine, heated to a temperature of 60 ° C., and 3.0 times at 70 ° C. in the width direction. After stretching, heat treatment was performed at a temperature of 85 ° C., normal temperature air was blown onto the film, and the film was cooled to a temperature of 40 ° C. Next, the film was stretched 1.5 times at 75 ° C. in the machine direction, heat treated at 85 ° C., cooled, then air at room temperature was blown onto the film, cooled to a temperature of 40 ° C., wound into a roll, and heat-shrinked. A film was obtained. The average thickness of the film was 25 μm.

  Example 2 A poly (polysiloxane) having a weight average molecular weight of 150,000 having a composition ratio of L-lactic acid / D-lactic acid and hydroxycaproic acid of 0.15% by weight of colloidal silica particles having an average particle diameter of 1.1 μm is 80/5/15. A heat shrink film was obtained in the same manner as in Example 1 except that lactic acid B was used. The average thickness of the film was 25 μm.

  Example 3 Blend ratio of polylactic acid A / 3-hydroxybutyric acid (92 mol%)-3-hydroxyvaleric acid (8 mol%) copolymer containing 0.15% by weight of calcium carbonate particles having an average particle diameter of 1.1 μm A heat-shrinkable film was obtained in the same manner as in Example 1 except that polylactic acid C having a weight average molecular weight of 170,000 which was 70/30 was used. The average thickness of the film was 25 μm.

  Examples 4-5, Comparative Examples 1-2 No wet method silica particles having an average particle diameter of 1.1 μm were added (Comparative Example 1), 0.02% by weight (Example 4), 0.8% by weight (Examples) 5) A heat-shrinkable film was obtained in the same manner as in Example 1 using polylactic acid A contained so as to be 2% by weight (Comparative Example 2). The average thickness of each film was 25 μm.

  Example 6 Polylactic acid A having a weight average molecular weight of 180,000 having a composition ratio of 95/5 with L-lactic acid / D-lactic acid containing 0.15% by weight of wet-process silica particles having an average particle diameter of 1.1 μm was used. A film having an average thickness of 25 μm was obtained in the same manner as in Example 1 except that the stretching conditions were 65 ° C., 2.0-fold longitudinal stretching, 70 ° C., 3.0-fold transverse stretching.

  Comparative Example 3 Similar to Example 1 except that 17.5 mol% of isophthalic acid copolymerized polyethylene terephthalate having a weight average molecular weight of 32,000 and containing 0.15 wt% of wet process silica particles having an average particle diameter of 1.1 μm was used. Thus, a film having an average thickness of 25 μm was obtained.

  Comparative Example 4 A film having an average thickness of 25 μm was obtained in the same manner as in Example 1 except that the stretching conditions were only 65 ° C., 1.4-fold longitudinal stretching, and 85 ° C. heat treatment.

  Comparative Example 5 In Example 1, the same stretching conditions as in Example 1 except that the stretching conditions were 65 ° C, 1.2-fold longitudinal stretching, 70 ° C, 1.5-fold transverse stretching, and 85 ° C heat treatment only. A film with a thickness of 25 μm was obtained. Table 1 summarizes the quality evaluation results of the above heat shrinkable films.

  As can be seen from the results in Table 1, the heat-shrinkable films obtained in Examples 1 to 6 were excellent in heat-shrinkability, heat-shrinkage stress and container adhesion, and had alkali solubility.

  That is, in order to obtain the above film properties from Table 1, it is a stretched film composed of a polymer mainly composed of polylactic acid containing 0.01 to 1% by weight of particles, and is at least one-way heat shrinkage in hot air at 120 ° C. It was found that the rate was 20% or more and the maximum heat shrinkage stress of the film was 5.0 MPa or less.

  On the other hand, although the film obtained in Comparative Example 1 had good heat shrinkability and heat shrinkage stress properties, the slipperiness was poor. Although the film obtained in Comparative Example 2 has good heat shrinkability, heat shrinkage stress properties, and easy slipperiness, when dissolved in an alkaline aqueous solution during container recovery, particles adhere to the container as a dissolution residue, The particles were mixed in the recovered PET, and the recoverability of the container was poor. The film obtained in Comparative Example 3 had a large heat shrinkage stress, and a portion where the container deformed was observed. The films obtained in Comparative Examples 4 to 5 did not have sufficient adhesion to the container. Any film was not preferable as a heat shrink film.

Claims (8)

A stretched film composed of a polymer mainly composed of polylactic acid containing 0.01 to 1% by weight of particles, wherein the film has a thermal shrinkage rate of 20% or more in at least one direction in 120 ° C. hot air, A heat shrink film having a maximum heat shrink stress of 5.0 MPa or less. 2. The heat shrinkable film according to claim 1, wherein the film has a heat shrinkage rate of 40% or more in at least one direction in hot air at 120 ° C. 3. The heat-shrinkable film according to claim 1, wherein the heat-shrinkable film is a stretched film made of a polymer mainly composed of polylactic acid containing 0.01 to 0.5% by weight of particles. The heat-shrinkable film according to any one of claims 1 to 3, wherein the heat-shrinkable film is dissolved in an aqueous solution exceeding pH 7 by 50% or more. The heat-shrinkable film according to claim 1, wherein the heat-shrinkable film is dissolved in an aqueous solution exceeding pH 7 by 80% or more. The heat-shrinkable film according to any one of claims 1 to 5, which dissolves 95% or more in an aqueous solution exceeding pH 7. The heat-shrinkable film according to any one of claims 1 to 6, which is dissolved by 80% or more in a 2% aqueous sodium hydroxide solution. The heat-shrinkable film according to any one of claims 1 to 7, wherein the heat-shrinkable film is dissolved by 95% or more in a 2% aqueous sodium hydroxide solution.