JP4014356B2 - How to recycle plastic bottles - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for recycling a plastic bottle. More specifically, the present invention relates to a PET bottle recycling method for producing high-quality PET bottles that can be suitably used for food applications from flakes obtained by pulverizing used food PET bottles.
[0002]
TECHNICAL BACKGROUND OF THE INVENTION
In recent years, the collected used PET bottles have been recycled. Generally, collected used PET bottles are separated from foreign resin bottles such as PVC bottles, foreign materials such as metal, colored PET bottles, etc., crushed, washed with surfactants, rinsed, dehydrated and dried. And then pulverized into flakes, and processed into pellets as necessary to form recycled materials.
[0003]
Such recycled materials in the form of flakes or pellets are mainly used as raw materials for carpets, clothing, textile products such as short fibers, containers for flowerpots, packaging films, etc., and almost used as food containers. It has not been.
[0004]
It is desirable that the PET bottle is originally reused as a PET bottle. However, the flaky or pellet-like recycled raw material often contains foreign matters, and because the degree of polymerization (inherent viscosity) of the resin is reduced by heating during the initial bottle molding, When a bottle is produced using a pellet-shaped recycled raw material, there are problems that the formability is poor and a bottle having sufficient strength and transparency cannot be obtained.
[0005]
In addition, when chemical substances such as chemicals and solvents are filled in used PET bottles, it is possible that used bottles with impurities such as chemical substances mixed into the collected used bottles. . For example, plastic bottles after consumers fill and use chemicals such as thinners, cleaning agents, pesticides, herbicides, pesticides, waxes, motor oils, antifreezes, etc. are mixed into other used bottles. It is thought that it is done. In addition, chemical substances adsorbed on the bottle surface cannot be sufficiently removed by normal cleaning, and thus there has been a problem that it is difficult to ensure food hygiene safety when manufacturing PET bottles using recycled materials. .
[0006]
Under such circumstances, it has been proposed to decompose a PET bottle into dimethyl terephthalate and ethylene glycol and to produce polyester using these as raw materials (see JP-A-11-302443). However, according to this method, the polyester resin molded product is decomposed into monomers and purified, and then polymerized again to produce a polyester, so that many auxiliary materials and energy are required for the decomposition and polymerization, and the process is complicated. As such, it was not economically feasible on an industrial scale.
[0007]
For this reason, the degree of polymerization of the resin of the flakes obtained by pulverizing the PET bottles is recovered, the impurities adhering to or impregnating the flakes and fine solid foreign substances are highly removed, and a PET bottle that can be used safely for food container applications is manufactured. The emergence of PET bottle recycling methods has been strongly desired.
[0008]
OBJECT OF THE INVENTION
An object of this invention is to provide the recycling method of the plastic bottle which manufactures the high quality plastic bottle which can be used suitably also for a food use from the flake which crushed the plastic bottle.
[0009]
SUMMARY OF THE INVENTION
The PET bottle recycling method of the present invention is:
A cleaning process for cleaning flakes obtained by pulverizing PET bottles using an aqueous solution of an alkali metal hydroxide,
A solid phase polymerization step for continuously solid phase polymerization of the washed flakes;
A pelletizing step of melting and granulating the solid phase polymerized flakes using an extruder having a devolatilization means and a filtration means;
It is characterized by comprising a bottle forming step of forming a PET bottle from the obtained pellets.
[0010]
In the PET bottle recycling method of the present invention, the molded PET bottle is also preferably a food PET bottle.
In the PET bottle recycling method of the present invention, the intrinsic viscosity of the solid phase polymerized flakes is preferably 0.8 to 0.95 dl / g. In a continuous solid phase polymerization apparatus, it is also preferable to be a step of contacting with an inert gas at 180 to 230 ° C.
[0011]
Furthermore, it is also preferable to reuse the inert gas used in the solid phase polymerization step in the solid phase polymerization step after removing volatile substances.
Furthermore, in the PET bottle recycling method of the present invention, the extruder used in the pelletizing step has at least one vacuum vent in the melting zone of the resin as a devolatilizing means. It is also preferable to devolatilize volatile substances in the molten resin at a pressure of 70 kPa or less, and it is also preferable to filter and remove particles having a particle size of 25 μm or more in the molten resin in the pelletizing step.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be specifically described.
The flakes obtained by pulverizing PET bottles used in the present invention are pulverized PET bottles having a pulverized shape in the form of flakes (book pieces). Such flakes are collected and used. A flake-like product obtained by separating, pulverizing, and washing with a surfactant, rinsing, dehydrating and drying can be suitably used.
[0013]
Used PET bottles usually have a generally reduced activity of the polymerization catalyst in the resin. Especially in PET bottles filled with the contents at high temperatures, the catalyst in the resin is intentionally deactivated at the time of production. . In general, a resin that does not contain a highly active catalyst does not easily proceed even when subjected to a solid-phase polymerization treatment as described later. However, in the present invention, the raw material to be used is a flake shape of a PET bottle (book piece shape). ), The surface area is large, the thermal conductivity is excellent, the solid phase polymerization can be performed, and the volatile impurities can be removed well.
[0014]
In the PET bottle recycling method of the present invention, by using such flakes obtained by pulverizing PET bottles as raw materials, the recycled PET bottles are manufactured through a washing step, a solid phase polymerization step, a pelletizing step, and a bottle forming step. Recycle used PET bottles into recycled PET bottles.
[0015]
First, the cleaning process will be described.
In the washing step, flakes obtained by pulverizing the PET bottle are washed with an aqueous solution of an alkali metal hydroxide. As the aqueous solution of the alkali metal hydroxide used in this washing step, an aqueous potassium hydroxide solution and an aqueous sodium hydroxide solution are preferred. In such a cleaning step, preliminary cleaning may be performed before cleaning with an alkali metal hydroxide aqueous solution.
[0016]
The concentration of the alkali metal hydroxide aqueous solution used in the washing step is usually 0.5 to 10% by weight, preferably 1 to 5% by weight, more preferably 2 although it depends on conditions such as washing time, temperature and stirring. It is desirable that the amount be about ˜4% by weight. The washing time in the washing step in which the flakes obtained by pulverizing the PET bottle and the aqueous solution of the alkali metal hydroxide are in contact is usually 5 to 120 minutes, preferably 10 to 60 minutes, more preferably 20 to 40 minutes. The degree is desirable. Moreover, it is preferable to perform such a washing | cleaning process under stirring.
[0017]
In such a cleaning process, the surface of the flakes obtained by pulverizing the PET bottle is hydrolyzed at least partially by the aqueous solution of the alkali metal hydroxide. As a result, impurities attached to or impregnated on the surface of the flakes, non-volatile substances such as salts impregnated on the surface, and fine solid foreign substances encroaching on the surface are removed by water washing or surfactant washing. Impurities that cannot be cleaned can be highly washed away. Such a washing step is performed under the condition that 0.1 to 5% by weight, preferably 0.5 to 3% by weight, more preferably 0.5 to 1.5% by weight of the flakes to be washed are hydrolyzed. Is desirable.
[0018]
In the cleaning step, it is desirable to perform rinsing and drying following the cleaning with the aqueous solution of the alkali metal hydroxide.
The flakes thus washed can be subjected to the subsequent solid phase polymerization step.
[0019]
Next, the solid phase polymerization process will be described.
Generally, a resin for producing PET bottles must have an appropriate degree of polymerization (intrinsic viscosity) in order to form a molded article having excellent moldability and desired mechanical strength. is there. On the other hand, the flakes obtained by pulverizing the PET bottle, which is the processing raw material of the present invention, are heated at the time of initial bottle molding, and therefore the degree of polymerization (inherent viscosity) of the resin is usually lower than at the time of initial molding. . In addition, the degree of polymerization of the washed flakes may be further lowered due to hydrolysis in the aforementioned washing step.
[0020]
In the present invention, the washed flakes are subjected to a solid-phase polymerization step to improve the polymerization degree of the resin and adjust the polymerization degree to be appropriate for the production of a molded article, and the volatility contained as impurities in the flakes. Some of the components can be removed.
[0021]
The solid phase polymerization step is carried out by bringing the flakes washed in the washing step into contact with an inert gas at 180 to 230 ° C., preferably 190 to 230 ° C., more preferably 195 to 210 ° C., and performing continuous solid phase polymerization. be able to. As the inert gas, any non-reactive gas can be used under the condition of contact with the washed flakes. Specific examples include nitrogen gas and rare gas. It is particularly preferable to use nitrogen gas.
[0022]
In the solid phase polymerization step, the intrinsic viscosity of the solid phase polymerized flakes is usually 0.8 to 0.95 dl / g, preferably 0.8 to 0.9 dl / g, and particularly preferably 0.85 to 0. It is desirable to carry out under conditions of 9 dl / g.
[0023]
Such a solid phase polymerization step can be performed using a conventionally known polymerization machine or the like used for solid phase polymerization of a resin, but is particularly preferably performed using a moving bed type continuous solid phase polymerization apparatus.
[0024]
The inert gas after being used in such a solid phase polymerization step usually contains volatile substances contained in the washed flakes, but after removing the volatile substances, the inert gas is subjected to the solid phase polymerization step. Can be reused. Examples of a method for removing volatile substances from the inert gas used in the solid phase polymerization step include a method for absorbing or adsorbing volatile substances in the inert gas at the outlet of the apparatus such as a polymerization machine.
[0025]
When the inert gas used in the solid phase polymerization process is reused in the solid phase polymerization process after removing volatile substances, the amount of inert gas used as an auxiliary material can be reduced. It is economical and preferable because the energy related to the heating of the inert gas can be reduced.
[0026]
Next, the solid phase polymerized flakes are subjected to a pelletizing step.
The pelletizing step can be performed by melting and granulating the solid phase polymerized flakes using an extruder having a devolatilizing means and a filtering means.
[0027]
Melting in the pelletizing step can be performed by melt-kneading the solid phase polymerized flakes at a temperature of 260 to 300 ° C., preferably 265 to 280 ° C. in the melting zone of the extruder. The extruder used in the pelletizing step preferably has at least one vacuum vent in the resin melting zone as a devolatilizing means. Volatilization of the volatile substance in the molten resin is preferably carried out under reduced pressure of 70 kPa or less, preferably 65 kPa or less in the molten state by the vacuum vent. Volatile impurities in the resin are also removed in the above-described solid phase polymerization step, but can be more strictly removed by devolatilization in the molten state in such a pelletization step.
[0028]
Further, the extruder preferably has a filter capable of filtering off solid foreign matters having a particle size of 25 μm or more, preferably 15 μm or more, more preferably 10 μm or more in the molten resin as a filtering means. It is desirable that solid foreign matters having a particle size of 25 μm or more, preferably 15 μm or more, more preferably 10 μm or more in the molten resin are filtered out from the resin. Solid foreign matters in the resin are also removed in the above-described washing step, but can be more strictly removed by such filtration removal.
[0029]
The molten resin from which volatile impurities and solid foreign substances have been sufficiently removed in this manner can be pelletized by granulation by extrusion molding into pellets of a desired shape and size by an ordinary method in an extruder. .
[0030]
Next, the pellet obtained in the pelletizing step is subjected to a bottle forming step for forming a PET bottle.
In the bottle molding step, any bottle molding method for producing a PET bottle from a normal polyethylene terephthalate resin can be employed. For example, the pellet obtained in the above pelletizing step is melt-kneaded and extruded or injection molded to form a preform, and the resulting preform is stretch blow molded in a mold to form a bottle. be able to.
[0031]
The resin temperature for molding the preform is not particularly limited, but is usually preferably about 270 to 290 ° C, and the mold temperature for stretch blow molding is preferably about 90 to 110 ° C. Examples of the blowing fluid include air, nitrogen, water vapor, and water. Of these, air is preferably used. If desired, a part or the whole of the stretch blow molded bottle may be heat set.
[0032]
The PET bottle thus obtained is substantially free of volatile impurities and solid foreign matters, has sufficient moldability and mechanical strength, has excellent transparency, and has safety suitable for filling foods. .
[0033]
【The invention's effect】
According to the present invention, a cleaning process involving hydrolysis of at least a part of the raw material flake surface, a solid phase polymerization process for performing solid phase polymerization and removing volatile substances, and a volatile substance for performing melting and granulation and From the flakes obtained by pulverizing PET bottles through the four steps of the pelletization process for further removing the slag and the bottle molding process for molding the PET bottle from the obtained pellets. Regeneration having a degree of polymerization (intrinsic viscosity) suitable for molding Manufactures excellent recycled PET bottles that are substantially free of volatile impurities and solid foreign substances, have sufficient moldability and mechanical strength, have excellent transparency, and have safety suitable for food filling. It is possible to provide a recycling method for plastic bottles.
[0034]
【Example】
EXAMPLES Hereinafter, although this invention is demonstrated further more concretely based on an Example, this invention is not limited to these Examples.
[0035]
[Example 1]
<Preparation of simulated contaminated bottle flakes>
Fill a PET bottle with a mixture of 42.5% by volume of toluene, 42.5% by volume of chloroform, 5.0% by volume of benzophenone, and 5.0% by volume of zinc stearate, and seal it in a 40 ° C. warm bath. For 14 days to prepare a simulated contaminated bottle. Next, the obtained simulated contaminated bottle was made into flakes by a pulverizer to obtain simulated contaminated bottle flakes.
[0036]
When the content of each chemical substance in the simulated contaminated bottle flakes was measured, it was 30,000 ppm of toluene, 43,000 ppm of chloroform, 2,300 ppm of lindane, 1,800 ppm of benzophenone, and 1,100 ppm of zinc stearate.
<Preparation of contaminated samples>
1 part by weight of the above simulated contaminated bottle flake and 1000 parts by weight of commercially available PET bottle flake for recycling were mixed to obtain a contaminated sample.
[0037]
Table 1 shows the calculated values of the contents of each chemical substance in the contaminated sample and the measured intrinsic viscosity values of the contaminated sample.
<Washing process>
The above contaminated sample was washed under stirring with a 3% by weight sodium hydroxide solution at a flake concentration of 10% by weight, 90 ° C. for 20 minutes. Next, rinsing and washing were performed using distilled water and drying to obtain washing flakes. At this time, the hydrolysis rate of the resin was 0.7%.
[0038]
The content of each chemical substance in the obtained washing flakes and the intrinsic viscosity of the washing flakes were measured. The results are shown in Table 1.
<Solid-state polymerization process>
The washing flakes were brought into contact with nitrogen at 200 ° C. for 12 hours in a nitrogen stream, and solid phase polymerization and removal of volatile components contained in the flakes were performed to obtain solid phase polymerization flakes.
[0039]
The content of each chemical substance in the obtained solid phase polymerization flake and the intrinsic viscosity of the solid phase polymerization flake were measured. The results are shown in Table 1. Thereby, it can be seen that the intrinsic viscosity (degree of polymerization) is increased and the amount of the chemical substance remaining in the washing process is further reduced.
<Pelletization process>
Solid phase polymerization flakes are introduced into a single screw extruder having a vent and a filter, and volatile substances are devolatilized in a molten state at a cylinder temperature of 270 ° C. and a vacuum degree of the vent of 60 kPa. The mixture was filtered with a filter having an opening of 10 μm and granulated to obtain pellets.
[0040]
The content of each chemical substance in the obtained pellet and the intrinsic viscosity of the pellet were measured. The results are shown in Table 1. Thereby, it can be seen that the obtained pellets are further reduced in the amount of chemical substances remaining in the washing step and the solid phase polymerization step, and have an appropriate intrinsic viscosity.
[0041]
For example, the threshold for the concentration of contaminating chemicals in polyethylene terephthalate resin for food use as shown by the US Federal Food Sanitation Agency (FDA) is 430 ppb. According to the present invention, as shown in Table 1, Even when a raw material containing a contaminating chemical substance at a high concentration is used, the concentration of the contaminating chemical substance in the obtained pellet is sufficiently reduced. Therefore, it turns out that the PET bottle shape | molded from this pellet can be used conveniently for food filling.
[0042]
In general, a resin having a degree of polymerization having an intrinsic viscosity of 0.75 to 0.85 dl / g is used as a resin for PET bottles. As shown in Table 1, according to the present invention, the obtained pellets It can be seen that has an intrinsic viscosity suitable for molding PET bottles.
[0043]
[Table 1]

Claims (7)

A cleaning process for cleaning flakes obtained by pulverizing PET bottles using an aqueous solution of an alkali metal hydroxide,
A solid phase polymerization step for continuously solid phase polymerization of the washed flakes;
A pelletizing step of melting and granulating the solid phase polymerized flakes using an extruder having a devolatilization means and a filtration means;
A plastic bottle recycling method comprising a bottle molding step of molding a plastic bottle from the obtained pellet. 2. The plastic bottle recycling method according to claim 1, wherein the molded plastic bottle is a food plastic bottle. The method for recycling PET bottles according to claim 1 or 2, wherein the intrinsic viscosity of the solid phase polymerized flakes is 0.8 to 0.95 dl / g. The solid phase polymerization step is a step of bringing the washed flakes into contact with an inert gas at 180 to 230 ° C in a moving bed type continuous solid phase polymerization apparatus. How to recycle plastic bottles. The method for recycling a PET bottle according to claim 4, wherein the inert gas used in the solid phase polymerization step is reused in the solid phase polymerization step after removing volatile substances. The extruder used in the pelletizing step has at least one vacuum vent in the resin melting zone as a devolatilization means,
The method for recycling a PET bottle according to any one of claims 1 to 5, wherein in the pelletizing step, a volatile substance in the molten resin is devolatilized by a vacuum vent at 70 kPa or less. The method for recycling a pet bottle according to any one of claims 1 to 6, wherein in the pelletizing step, particles having a particle size of 25 µm or more in the molten resin are filtered and removed by a filtering means of an extruder.