WO2024128083A1 - Method for producing preform and preform - Google Patents

Abstract

A preform is produced more efficiently and at lower cost from resin flakes obtained by crushing recovered polyester resin molded articles into flakes. Resin flakes obtained by crushing recovered polyester resin molded articles into flakes are prepared, and the resin flakes are plasticized after heat treatment under reduced pressure. While removing foreign matter from the molten resin obtained by plasticizing the resin flakes, the molten resin is supplied to an injection molding device, and a preform is injection molded.

Description

Preform manufacturing method and preform

The present invention relates to a method for manufacturing a preform and a preform.

　Traditionally, synthetic resin containers are made by making a bottomed cylindrical preform from a polyester resin such as polyethylene terephthalate, and then molding this preform into a bottle by biaxial stretch blow molding or the like, and these are used in a wide range of fields as containers for various beverages, seasonings, and other contents. This type of container is generally known as a PET bottle, and in recent years, in response to societal demand, a recycling technology known as "bottle-to-bottle" has been considered, in which used PET bottles are collected and reused as recycled material to manufacture PET bottles.

For example, Patent Document 1 discloses a technology in which recovered polyester resin molded products are crushed into flakes to produce resin flakes, which are then decontaminated, melted to produce resin pellets, which are then subjected to solid-state polymerization and then transported to an injection molding device to manufacture preforms.

Japanese Patent Publication No. 2021-98350

In light of the above background technology, the inventors of the present invention have conducted extensive research into how to more efficiently and at lower cost manufacture preforms from resin flakes obtained by crushing recovered polyester resin molded products into flakes, and as a result have completed the present invention.

The method for manufacturing a preform according to the present invention involves preparing resin flakes by crushing recovered polyester resin molded products into flakes, heat-treating the resin flakes under reduced pressure conditions and then plasticizing them, and while removing foreign matter from the molten resin obtained by plasticizing the resin flakes, supplying the molten resin to an injection molding device to injection mold a preform.

The preform according to the present invention is a preform manufactured by the above manufacturing method, and has a residual amount of acetaldehyde of 1 to 15 ppm, a residual amount of bishydroxyethyl terephthalate of 1 to 35 ppm, a residual amount of monohydroxyethyl terephthalate of 1 to 35 ppm, and a residual amount of cyclic trimer of 6000 ppm or less.

According to the present invention, preforms can be produced more efficiently and at lower cost from resin flakes obtained by crushing recovered polyester resin molded products into flakes.

1 is an explanatory diagram conceptually illustrating an entire apparatus in which an embodiment of the present invention is preferably implemented; FIG. 2 is an explanatory diagram showing an example of an apparatus for subjecting resin flakes to a heat treatment in an embodiment of the present invention.

The following describes a preferred embodiment of the present invention.

In this embodiment, first, resin flakes are prepared by crushing the recovered polyester resin molded products into flakes.

When preparing such resin flakes, it is preferable to wash them by any washing method such as alkaline washing or hot water washing in order to remove dirt such as residue of contents remaining on the surface and foreign matter that has been mixed in. "Recovered polyester resin molded products" can include polyester resin molded products such as used PET bottles that have been collected separately as recyclable waste, as well as scrap materials generated during the manufacturing process of polyester resin molded products.

Examples of polyester resins include polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene furanoate, and copolymers thereof. These polyester resins may be polyester resins with an increased biomass content, for example, by using ethylene glycol or its derivatives made from plant-derived bioethanol as a diol component, terephthalic acid or its derivatives made from plant-derived bioparaxylene as a dicarboxylic acid component, or furandicarboxylic acid or its derivatives made from plant-derived fructose as a dicarboxylic acid component.

When polyester resin molded products such as used PET bottles are collected and reused as recycled materials through so-called mechanical recycling to manufacture recycled products, the decrease in intrinsic viscosity due to deterioration of the resin caused by the heat history received during the manufacture of the collected products can cause problems in the manufacturing process of the recycled products. Furthermore, if a large amount of low molecular weight impurities such as acetaldehyde (AA) generated by thermal decomposition of polyester resin, oligomers such as bishydroxyethyl terephthalate (BHET), monohydroxyethyl terephthalate (MHET), and cyclic trimer (CT) generated by depolymerization of polyester resin, and limonene derived from the contents remain in the recycled materials, they not only affect the quality of the recycled products manufactured, but also cause productivity to decrease by fouling the manufacturing equipment.

In this embodiment, to avoid such problems, the prepared resin flakes are subjected to a heat treatment, preferably in an atmosphere with a reduced pressure of 1000 Pa or less and a temperature of preferably 160 to 240°C, to advance the solid-state polymerization reaction of the polyester resin that forms the resin flakes and promote crystallization while keeping the flakes in their flake form, without melting the resin flakes.

By allowing the solid-phase polymerization reaction to proceed, the end groups of the polyester resin, whose degree of polymerization has decreased due to the cutting of the molecular chains caused by deterioration, re-condense, and as the degree of polymerization is restored, the intrinsic viscosity corresponding to the molecular weight can be restored. Furthermore, by subjecting the resin flakes to heat treatment under reduced pressure conditions, particularly by exposing them to a temperature atmosphere of 160 to 240°C under reduced pressure conditions, it is possible to suppress hydrolysis of the polyester resin while effectively promoting crystallization by utilizing the melting point lowering effect. This reduces the free volume between the molecular chains of the polyester resin (reducing the amorphous phase), and selectively bleeds out the low molecular weight impurity components (organic impurity components) as mentioned above that were included in the free volume portion of the resin flakes and the moisture sorbed in the resin flakes to the surface layer of the resin flakes, where they can be volatilized or evaporated, or washed with hot water, steam, organic solvents, etc. as necessary, and efficiently removed.

In this embodiment, the specific method for subjecting the resin flakes to a heat treatment is not particularly limited. For example, it is preferable to perform the heat treatment using a treatment device 100 as shown in FIG. 2, which includes at least one preheating chamber 102 and a treatment chamber 103 capable of adjusting the internal atmosphere to a temperature and pressure suitable for the heat treatment.

2, the processing apparatus 100 is provided with a sealed container 101 constructed to have an internal volume of preferably 7 to 18 ^m3 . A processing chamber 103 is provided inside the sealed container 101, and above the sealed container 101, two preheating chambers 102 are provided which are connected to the sealed container 101 via an opening/closing mechanism such as a sliding shutter (not shown) so as to be able to communicate with the processing chamber 103 while maintaining airtightness from the outside of the system.
In FIG. 2, illustration of incidental equipment such as a heating device, a pressure reducing device, and associated piping for adjusting the atmosphere inside the processing chamber 103 is omitted.

In order to subject resin flakes to a heat treatment using such a treatment device 100, it is preferable to, prior to the heat treatment, put the resin flakes into the preheating chamber 102, seal the preheating chamber 102, heat the resin flakes to a predetermined temperature, and reduce the pressure inside the preheating chamber 102 to a predetermined pressure. At that time, for example, a heated gas (e.g., an inert gas such as _N2 gas) is sprayed into the preheating chamber 102 by providing a spray hole opening on the inner wall surface of the preheating chamber 102, and the spray direction and spray amount are appropriately adjusted to generate an air flow in the preheating chamber 102, and the resin flakes are heated while being stirred, and the preheating chamber 102 is decompressed and sucked, so that at least a part of the impurity components contained in the resin flakes are dispersed and removed outside the system.

In this way, when raising the temperature of the resin flakes, it is preferable to appropriately adjust the final temperature and pressure in the preheating chamber 102 taking into consideration the internal atmosphere of the processing chamber 103. Then, it is preferable to open the opening and closing device and, while maintaining airtightness from the outside of the system, allow the resin flakes in the preheating chamber 102 to fall by their own weight, so that the resin flakes are supplied into the processing chamber 103 without damaging the internal atmosphere of the processing chamber 103, which has been adjusted to a temperature and pressure suitable for heat treatment.

In the processing apparatus 100 shown in FIG. 2, resin flakes are sequentially fed into two preheating chambers 102, and preheating treatments for raising the temperature of the resin flakes are alternately performed. The resin flakes are dropped into the processing chamber 103 in the order of the preheating chamber 102 in which the fed resin flakes have been heated to a predetermined temperature, but this is not limited to this. Depending on the processing capacity of the processing apparatus 100, that is, the total amount of resin flakes that can be subjected to heat treatment per unit time, the number of preheating chambers 102 can be one, or three or more. In any case, it is preferable to repeat the operation of dropping the resin flakes from the preheating chamber 102 into the processing chamber 103, so that the resin flakes are stacked in layers for each group that is dropped. Then, it is preferable to expose the resin flakes to the internal atmosphere of the processing chamber 103 while the resin flakes are piled up in the processing chamber 103, so that the resin flakes can be subjected to heat treatment.

The processing time for heat-treating the resin flakes is usually 30 minutes or more, and is preferably adjusted appropriately within the range of 1 to 8 hours in order to suppress deterioration of the resin's hue while allowing the solid-state polymerization reaction to proceed. It is preferable to manage the processing time by removing the resin flakes accumulated in the processing chamber 103 from the processing chamber 103 in equal amounts to the amount dropped from the preheating chamber 102, starting from the lower layer of the layered deposit as described above, after the processing time has elapsed. In this case, the time required for the preheating process in the preheating chamber 102 depends on the water content and shape of the resin flakes, and there may be variation in the intervals at which the resin flakes are dropped. Therefore, it is preferable to manage the processing time for each set of dropped resin flakes.

In this way, in order to suitably manage the processing time when heat-treating the resin flakes, the processing apparatus 100 shown in FIG. 2 is provided with an opening/closing mechanism 104 on the bottom side of the sealed container 101. The opening/closing mechanism 104 forms the bottom surface of the processing chamber 103 and is configured to serve as a partition between the processing chamber 103 and the removal section 105 provided at the bottom of the sealed container 101. This allows the resin flakes to spill into the removal section 105 when the opening/closing mechanism 104 is opened, and by appropriately adjusting the timing of opening and closing the opening/closing mechanism 104, it is possible to remove an amount of resin flakes equal to the amount dropped from the preheating chamber 102, and although there will be some mixing, it is possible to remove each group of resin flakes dropped from the preheating chamber 102.

As described above, impurity components can be efficiently removed by performing heat treatment under reduced pressure conditions, but it is not preferable for the volatilized or evaporated impurity components to remain in the treatment chamber 103. For this reason, the treatment apparatus 100 shown in FIG. 2 is configured to circulate the gas phase components in the treatment chamber 103, and is provided with a circulation flow path 106 including a catalyst tank 107 in the path for decomposing low molecular weight impurity components derived from polyester resin, and the circulation flow path 106 is connected to the treatment chamber 103. By configuring the treatment apparatus 100 in this way, it is preferable to reduce the impurity components remaining in the treatment chamber 103 by guiding the gas phase components in the treatment chamber 103 to the circulation flow path 106 and circulating them without damaging the internal atmosphere of the treatment chamber 103, for example, by using an _inert gas such as N 2 gas as a carrier gas.

In addition, when cleaning the impurity components selectively bled out on the surface layer of the resin flakes, the resin flakes taken out of the processing chamber 103 can be liquid cleaned using a rotary or convection type liquid cleaner, or steam cleaned using a pressurized steam convection cleaner, although not shown in the figure. When applying liquid cleaning, the resin flakes after cleaning must be dried, and for this drying process, the resin flakes may be subjected to a heat treatment again in an atmosphere equivalent to the internal atmosphere of the processing chamber 103, or may be dried using a hot air circulation dryer. Furthermore, the resin flakes can be dried using two jacketed rolls with spiral grooves on the surface, using the shear heat generated by the roll rotation to semi-weld and press the surface layer of the resin flakes, and further removing organic impurities and moisture from the resin flakes using a rotary forward drying device (open roll type twin screw extruder). In this case, the resin flakes dried in the open-roll twin-screw extruder are easily separated and dispersed into flakes by the pressure of the compression rolls in the cooling section downstream of the rolls.

In this embodiment, the resin flakes are heat-treated under reduced pressure conditions as described above to remove (decontaminate) low molecular weight impurities, and then plasticized. At this time, the heat-treated resin flakes may be fed into the extruder 200 and plasticized while being melted and kneaded by the heat from the heating cylinder and the shear heat from the screw, but it is preferable to use a twin-screw extruder for plasticization. A twin-screw extruder has a high kneading effect, can plasticize the resin flakes more uniformly, and can also shorten the time required for plasticization, making it preferable for suppressing deterioration of the resin, thermal decomposition, and depolymerization.

When plasticizing resin flakes using a twin-screw extruder, it is preferable to configure the twin-screw extruder with a vent port, and to decompress and suck out the gaseous components containing low molecular weight impurities derived from the polyester resin that remain in the twin-screw extruder through a vent port provided through the heating cylinder, for example. This makes it possible to further reduce the impurities remaining in the plasticized polyester resin. In this case, cyclic oligomers, in particular, are generated in an equilibrium reaction during thermal melting, so cyclic oligomers are generated when polyethylene terephthalate is melted. Therefore, it is important to perform the operation of decompression absorption and removal at the vent port of the twin-screw extruder. The optimal conditions (temperature and pressure) for gas-liquid separation of cyclic oligomers are estimated. The melting point of cyclic oligomers is 319°C and they are sublimable. Assuming that the resin melt temperature at the vent port of a polyethylene terephthalate twin-screw extruder is 300°C, the reduced vacuum pressure at which cyclic oligomers can sublimate (transform into gas) is generally 0.1 KPa (100 Pa) or less, so it is necessary to adjust the pressure to a relatively high vacuum. This increases the possibility of the molten resin venting up at the vent port. To avoid venting up, it is preferable to use a twin-screw extruder with counter-rotating (different) screw rotation directions rather than the same (same) screw rotation direction.

In addition, the resin flakes removed from the processing chamber 103 as described above may be directly fed into the extruder 200 for plasticization, or may be stored in the buffer tank 300 and then fed into the extruder 200 for plasticization. When managing the processing time during the heat treatment of the resin flakes as described above, the resin flakes are intermittently removed from the processing chamber 103 according to the interval at which the operation of dropping the resin flakes from the preheating chamber 102 into the processing chamber 103 is repeated, and the amount of resin flakes removed each time also depends on the amount of processing in the preheating chamber 102. In such a case, particularly if there is variation in the interval at which the operation of dropping the resin flakes is repeated, the amount of resin flakes removed per unit time will not be constant. Therefore, if the resin flakes are stored in the buffer tank 300 and can be removed from the buffer tank 300 in fixed amounts at any interval, the amount of resin flakes added can be easily adjusted according to the processing capacity of the extruder 200 so that the amount of resin plasticized in the extruder 200 can always be kept constant.

When installing the buffer tank 300, it is preferable to adjust the internal volume of the buffer tank 300 as appropriate so that more resin flakes can be stored. By doing this, even if a problem occurs in the process following the process of plasticizing the resin flakes, causing the subsequent process to stop, the heating process of the resin flakes can continue without being stopped, and the amount that cannot be sent to the subsequent process can be stored in the buffer tank 300.

Also, in anticipation of such trouble occurring or changes in circumstances such as the processing speed of the subsequent process speeding up or slowing down, a relief tank 400 can be installed midway along the transport path that transports the resin flakes removed from the processing chamber 103 to the buffer tank 300. It is preferable to transport the resin flakes to the buffer tank 300 while allowing them to be evacuated to the relief tank 400 during transport.

By doing this, for example, when the subsequent process is stopped, the resin flakes taken out of the processing chamber 103 can be evacuated to the relief tank 400, and the transport to the buffer tank 300 can be limited. When the processing speed of the subsequent process is increased, the resin flakes evacuated to the relief tank 400 can be transported to the buffer tank 300 together with the resin flakes taken out of the processing chamber 103. When the processing speed of the subsequent process is decreased, a portion of the resin flakes taken out of the processing chamber 103 can be evacuated to the relief tank 400, and the amount of resin flakes transported to the buffer tank 300 can be appropriately adjusted. In this way, when evacuating the resin flakes to the relief tank 400 as necessary, the internal atmosphere of the relief tank 400 is preferably set to 100°C or higher so that the resin flakes do not become hydrated.

In this embodiment, after the above steps, the molten resin obtained by plasticizing the resin flakes is supplied to the injection molding device 500 to injection mold the preform while removing foreign matter from the molten resin obtained by plasticizing the resin flakes. To remove foreign matter from the molten resin obtained by plasticizing the resin flakes, for example, a filter 600 for removing foreign matter may be provided on the discharge port side of the extruder 200. Furthermore, the specific configuration of the injection molding device 500 is not particularly limited, and the preform can be injection molded using, for example, an injection molding device 500 equipped with at least one injection pot and a preform mold configured to measure the supplied molten resin and then inject a fixed amount of molten resin.

Furthermore, when supplying the molten resin obtained by plasticizing the resin flakes to the injection molding device 500, it is undesirable for the resin to deteriorate (especially its hue) during transportation. To achieve this, in order to shorten the time that the molten resin remains in the transportation path, it is preferable to set the transportation distance until it is supplied to the injection molding device 500 after removing foreign matter, more specifically, the distance from the outlet of the gear pump 700 installed downstream of the filter 600 to the inlet of the injection pot equipped in the injection molding device 500, to 1 to 7 m. In this case, it is preferable that the transportation rate of the molten resin is 900 to 1500 kg/h.

According to the present embodiment as described above, preforms can be produced more efficiently and at lower cost from resin flakes obtained by crushing recovered polyester resin molded articles into flakes. In particular, according to this embodiment, although it is produced by recycling recovered polyester resin molded products, the residual amount of acetaldehyde is 1 to 15 ppm, preferably 1 to 10 ppm, more preferably 1 to 5 ppm, the residual amount of bishydroxyethyl terephthalate is 1 to 35 ppm, preferably 1 to 30 ppm, more preferably 1 to 20 ppm, the residual amount of monohydroxyethyl terephthalate is 1 to 35 ppm, preferably 1 to 30 ppm, more preferably 1 to 20 ppm, the residual amount of cyclic trimer is 6000 ppm or less, preferably 4000 ppm or less, more preferably 3000 ppm or less, and preferably, when the mass of the preform is 14 to 50 g, a preform having an L ^* value of 70 to 90 and a b ^* value of 2 to 18 in the L ^* a ^* b ^* color system can be well produced.

Here, the remaining amount and intrinsic viscosity of acetaldehyde, bishydroxyethyl terephthalate (BHET), monohydroxyethyl terephthalate (MHET), and cyclic trimer (CT) can be determined, for example, as follows.

<Residual amount of acetaldehyde>
A piece cut from the preform is used as a sample, and 1.0 g of the sample is weighed into a glass bottle, and 5.0 mL of pure water is added and sealed. This suspension is heated for 60 minutes in an oven adjusted to 120°C, and then cooled in ice water. 1.0 mL of the supernatant of the suspension is taken, to which 0.2 mL of a 0.1% concentration 2,4-dinitrophenylhydrazine-phosphate solution is added, and the mixture is left for 30 minutes and measured by high performance liquid chromatography. At the same time, the standard solution is also measured, and the acetaldehyde content is calculated based on the obtained calibration curve.

<Remaining Amounts of BHET, MHET, and CT>
A piece cut from the preform is used as a sample, and 0.2 g of the sample is weighed out, and 1 mL of a mixed solvent of hexafluoroisopropanol and chloroform (weight ratio 1/1) is added to it to completely dissolve it. After adding 4 mL of chloroform to the solution, 5 mL of acetonitrile is gradually added and left for 3 hours to precipitate the PET polymer. 1 mL is taken from this suspension, filtered through a membrane filter with a pore size of 0.45 μm, and the filtrate is measured by high performance liquid chromatography. At the same time, the standard solution is also measured, and the contents of MHET, BHET and CT in the pellet are calculated based on the obtained calibration curve.

<Intrinsic Viscosity>
A piece cut from the preform is used as a sample, and the sample is vacuum dried at 150°C for 1 hour and weighed out at 0.2 g. A mixed solvent of 1,1,2,2-tetrachloroethane and phenol (weight ratio 1:1) is added to this to adjust the concentration to 1.00 g/dL, and the mixture is stirred at 120°C for 20 minutes to completely dissolve. The dissolved solution is cooled to room temperature, and the relative viscosity is measured using a relative viscometer adjusted to 30°C, to determine the intrinsic viscosity.

The L ^* value and b ^* value in the L ^* a ^* b ^* color system are measured at the body of the preform as the measurement site using a spectrophotometer under the measurement conditions of a D65 light source and a 2° visual field.

The present invention will be explained in more detail below with specific examples.

[Example 1]
PET bottles collected from the market were washed and crushed to prepare resin flakes as a raw material. The intrinsic viscosity and the amount of residual acetaldehyde of the resin flakes were determined in the same manner as described above, and the intrinsic viscosity was 0.803 dl/g and the amount of residual acetaldehyde was 6.0 ppm.

The prepared resin flakes were piled up in a treatment chamber with the internal atmosphere adjusted to a temperature of 202°C and a pressure of 400 Pa, and then subjected to heat treatment. The treatment time was 2 hours. The intrinsic viscosity and amount of remaining acetaldehyde of the heat-treated resin flakes were measured, and the intrinsic viscosity was 0.826 dl/g, and the amount of remaining acetaldehyde was 5.7 ppm.

After the resin flakes were heat-treated, they were fed into a twin-screw extruder and plasticized. During this process, the gaseous components, including low molecular weight impurities derived from the polyester resin that remained in the twin-screw extruder, were sucked out of the system through the vent port by reducing the pressure using the venting process. The degree of vacuum during the venting process was 1 kPa.

While removing foreign matter using a filter installed on the discharge side of the extruder, the plasticized molten resin was fed to an injection molding machine, and 28g preforms were injection molded at a production speed of 750 pieces per minute. At that time, the distance from the outlet of the gear pump installed downstream of the filter to the inlet of the injection pot equipped in the injection molding machine was 6m, the molten resin remained in the transport path for 260 seconds, and the transport rate of the molten resin was 1260kg/h.

For preforms manufactured by controlling the temperature of the molten resin supplied to the injection molding machine to the temperatures shown in Table 1, the reduction rate of intrinsic viscosity relative to the raw material, the amount of remaining acetaldehyde (AA), the L ^* value, and the b ^* value were determined. The results are shown in Table 1.

In this example, the amount of residual acetaldehyde was below 15 ppm at all temperatures, and was particularly below 5 ppm at 280°C. The rate of decrease in intrinsic viscosity was small, at 3.0%, even at a temperature of 300°C.

[Example 2]
A preform was produced in the same manner as in Example 1, except that a single screw extruder was used instead of a twin screw extruder to plasticize the heat-treated resin flakes, and no venting was performed.

For preforms manufactured by controlling the temperature of the molten resin supplied to the injection molding machine to the temperatures shown in Table 2, the reduction rate of intrinsic viscosity relative to the raw material, the amount of remaining acetaldehyde (AA), the L ^* value, and the b ^* value were determined. The results are shown in Table 2.

In this example, the amount of remaining acetaldehyde was below 15 ppm at all temperatures.

[Example 3]
A preform was produced in the same manner as in Example 2, except that the distance from the outlet of the gear pump installed downstream of the filter to the inlet of the injection pot provided in the injection molding device was 8 m.

For preforms manufactured by controlling the temperature of the molten resin supplied to the injection molding machine to the temperatures shown in Table 3, the reduction rate of intrinsic viscosity relative to the raw material, the amount of remaining acetaldehyde (AA), L ^* value, and b ^* value were determined, and the results are shown in Table 3.

In this example, the amount of remaining acetaldehyde was below 15 ppm at 280°C and 285°C.

A comparison between Example 2 and Example 3 confirms that the rate of decrease in intrinsic viscosity and the amount of remaining acetaldehyde are reduced by shortening the transport distance until the plasticized molten resin is supplied to the injection molding device after removing foreign matter. Furthermore, a comparison between Example 1 and Example 2 confirms that the rate of decrease in intrinsic viscosity and the amount of remaining acetaldehyde are further reduced by feeding the heat-treated resin flakes into a twin-screw extruder to plasticize them and then performing a venting process.
Furthermore, the methods described in these examples can be said to be environmentally friendly methods in that they have little thermal history and emit little carbon dioxide.

The present invention has been described above by showing preferred embodiments, but it goes without saying that the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the present invention.

100 Processing device 101 Processing chamber 102 Preheating chamber 107 Catalyst tank 200 Extruder (twin-screw extruder)
300 Buffer tank 400 Relief tank 500 Injection molding device 600 Filter 700 Gear pump

Claims (14)

The collected polyester resin molded products are pulverized into flakes to prepare resin flakes;
The resin flakes are heat-treated under reduced pressure and then plasticized;
A method for producing a preform, comprising the steps of: removing foreign matter from a molten resin obtained by plasticizing the resin flakes; supplying the molten resin to an injection molding device; and injection molding the preform. The method for manufacturing the preform according to claim 1, in which the heat-treated resin flakes are fed into a twin-screw extruder to plasticize them. The method for producing a preform according to claim 2, wherein the twin-screw extruder is provided with a vent port, and gaseous components including low molecular weight impurity components derived from polyester resin that remain in the twin-screw extruder are sucked under reduced pressure through the vent port and discharged outside the system. The method for manufacturing a preform according to any one of claims 1 to 3, in which the transport distance of the plasticized molten resin until it is supplied to the injection molding device after removing foreign matter is 1 to 7 m. The method for manufacturing a preform according to any one of claims 1 to 4, in which the resin flakes are heat-treated in an atmosphere at a temperature of 160 to 240°C, with the pressure reduced to 1000 Pa or less. The method for manufacturing a preform according to any one of claims 1 to 5, comprising the steps of: putting the resin flakes into a preheating chamber, heating the resin flakes to a predetermined temperature, reducing the pressure inside the preheating chamber to a predetermined pressure, and then dropping the resin flakes from the preheating chamber into a processing chamber whose internal atmosphere is adjusted to a predetermined temperature and pressure while maintaining airtightness from the outside of the system, while repeating this operation, and depositing the resin flakes in the processing chamber and subjecting them to a heat treatment. The method for manufacturing a preform according to claim 6, in which the resin flakes that have been subjected to the heat treatment are sequentially removed from the treatment chamber in amounts equal to the amount dropped from the preheating chamber, starting from the bottom layer of the resin flakes that are piled up in the treatment chamber, and plasticized. The method for manufacturing a preform according to claim 6, in which the resin flakes that have been subjected to the heat treatment are sequentially removed from the treatment chamber in amounts equal to the amount dropped from the preheating chamber, starting from the bottom layer of the resin flakes that have been piled up in the treatment chamber, stored in a buffer tank, and then plasticized. The method for manufacturing preforms according to claim 8, in which a relief tank is installed midway along the transport path that transports the resin flakes removed from the processing chamber to the buffer tank, and the resin flakes are transported to the buffer tank while being able to be evacuated to the relief tank during transport. The method for manufacturing a preform according to any one of claims 6 to 9, in which the resin flakes are sequentially fed into a plurality of preheating chambers, and the resin flakes are dropped into the processing chamber in the order of the preheating chamber in which the fed resin flakes have been heated to a predetermined temperature. The method for manufacturing a preform according to any one of claims 6 to 10, in which heated gas is sprayed into the preheating chamber to heat the resin flakes while stirring them, and the preheating chamber is suctioned under reduced pressure. The method for manufacturing a preform according to any one of claims 1 to 11, in which a treatment chamber in which the resin flakes are subjected to a heat treatment is connected to a circulation flow path that allows the gaseous components in the treatment chamber to circulate and includes a catalyst tank in the path that decomposes low-molecular-weight impurity components derived from polyester resin, and the gaseous components in the treatment chamber are introduced into the circulation flow path and circulated, thereby reducing the impurity components remaining in the treatment chamber. A preform manufactured by the method for manufacturing a preform according to any one of claims 1 to 12,
A preform characterized in that the residual amount of acetaldehyde is 1 to 15 ppm, the residual amount of bishydroxyethyl terephthalate is 1 to 35 ppm, the residual amount of monohydroxyethyl terephthalate is 1 to 35 ppm, and the residual amount of cyclic trimer is 6000 ppm or less. The preform according to claim 13, wherein when the mass of the preform is 14 to 50 g, the L ^* value is 70 to 90 and the b ^* value is 2 to 18 in the L ^* a ^* b ^* color system.

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