JP7630126B2 - Regenerated polyester fiber and method for producing the same - Google Patents

Description

The present invention relates to a new flame-retardant regenerated polyester fiber and a method for producing the regenerated polyester fiber, which is made from recycled polyester raw materials derived from used polyester products and recycled polyester raw materials derived from unused polyester generated in the process of manufacturing polyester products.

Polyethylene terephthalate (hereinafter sometimes abbreviated as PET) has a high melting point, is resistant to chemicals, and is relatively low cost, and therefore is widely used in fibers, films, and molded products such as PET bottles.
These polyester products inevitably generate waste during the manufacturing and processing stages, and are often disposed of after use, but if they are incinerated, the high heat generated will cause significant damage to the incinerator and shorten its lifespan, while if they are not incinerated, they will remain semi-permanently as they do not decompose.
In recent years, it has become a problem that once-used polyester products, such as plastic containers discarded as garbage, end up in rivers and into the ocean, where microplastics are broken down into small pieces by the action of waves and tides, accumulate in the bodies of marine organisms, and are concentrated up the food chain, adversely affecting the marine ecosystem. Plastic is a major cause of marine pollution, and there is a global movement to reduce the use of plastics and switch to biodegradable plastics.

In order to reduce the amount of plastic products used and to address environmental issues, various methods of recycling are being used to reuse resources. Regarding polyester products, methods are being considered for recycling polyester waste generated during the manufacturing process, as well as recovering products that were once on the market and discarded, and reusing them as raw materials. In particular, in recent years, textile products that have been awarded the Eco Mark, which is certified as having achieved a certain recycling rate, have become widespread.

Furthermore, in recent years, there has been an increasing demand for flame-retardant synthetic fibers and various plastic products from the perspective of fire prevention.
Various attempts have been made to impart flame retardancy to polyester, but from the viewpoints of performance and productivity, a method of copolymerizing a flame retardant-imparting substance during polyester production is common. Phosphorus compounds are considered to be advantageous as flame retardant-imparting substances used for this purpose from the viewpoints of flame retardant performance, cost, environmental pollution, and safety. It has been disclosed that flame retardancy is expressed by blending a specific phosphorus compound with polyester fibers (for example, Patent Document 1). Polyester fibers that have flame retardancy due to the inclusion of such phosphorus compounds and that have the same performance as those made from virgin polyester while using recycled polyester as the raw material have not yet been proposed.

In other words, unlike virgin polyester raw materials, recycled polyester raw materials themselves contain a large amount of foreign matter and are poor in uniformity, but the addition of a phosphorus compound further reduces the uniformity of the resin, so that when a polyester resin composition containing a phosphorus compound in recycled polyester raw materials is melt spun, thread breakage occurs easily during spinning, and operability is poor. In addition, there is a problem that the obtained fibers have a large fluctuation rate of fiber diameter and poor thermal stability. Products using these fibers are of poor quality, and in particular, when the obtained fibers are short fibers, the obtained nonwoven fabric has poor texture and poor uniformity.

Patent Publication 2016-108706

The present invention aims to solve the above problems and provide a recycled polyester fiber made of a resin composition in which a phosphorus compound is added to a polyester resin made from recycled polyester raw materials derived from used polyester products or recycled polyester raw materials derived from scraps generated in the process of manufacturing polyester resins and products, and which has the same performance (no unevenness in fineness and excellent thermal stability) as polyester fibers obtained by melt spinning using virgin polyester resins. The present invention also aims to provide a manufacturing method by which such recycled polyester fiber of the present invention can be obtained.

As a result of extensive research into the problems of the prior art, the inventors discovered that by using recycled polyester raw materials and manufacturing them through a specific manufacturing process, it is possible to obtain recycled polyester fibers that have the same performance and flame retardancy as those made from virgin polyester resin, and thus completed the present invention.

That is, the present invention relates to the following (i) to (iii).
(i) A regenerated polyester fiber containing 40% by mass or more of at least one type of recycled polyester raw material, which is a) a used polyester product and b) an unused polyester generated in the process of manufacturing a polyester product, and which satisfies all of the following (1) to (3):
(1) The regenerated polyester fiber according to (1), which contains an organic phosphorus compound having two or more ester-forming functional groups, and the content of phosphorus atoms in the polyester resin is 1,000 to 10,000 ppm by mass. (2) In differential scanning calorimetry, when the temperature is lowered from 310° C. to 25° C. at a rate of 20° C./min, no crystallization temperature is detected. (3) The rate of variation of the fiber diameter is 10% or less. (b) The single fiber fineness is 0.35 to 1.0 dtex, and the fiber length is 20 to 44 mm.
(c) A method for producing recycled polyester fibers, which comprises melt-kneading at least one type of recycled polyester raw material, which is a) a used polyester product and b) an unused polyester generated in a process for producing a polyester product, and an organophosphorus compound having two or more ester-forming functional groups in an extruder, passing the mixture through a metering pump and melt-spinning the mixture through a melt spinning nozzle equipped with a spinneret, cooling the spun fiber in a cooling tube, and taking it up with a take-up roller, wherein the temperature during melt-kneading in the extruder is a, the temperature of the metering pump is b, the temperature of the melt spinning nozzle is c, and the temperature of the cooling tube is d, and the temperatures a to d satisfy the following conditions:
a is 280 to 290° C.
b is a+5 to a+10°C
c is 310 to 320° C.
d is c-150 to c-50°C

The recycled polyester fiber of the present invention is made from at least one recycled polyester raw material, which is a) used polyester products and b) unused polyester generated in the process of manufacturing polyester products, and further contains a phosphorus compound, and can be melt spun with good spinning operability, so there is little unevenness in the length direction and it has excellent thermal stability. Therefore, the nonwoven fabric obtained using the recycled polyester fiber of the present invention has good texture and excellent quality.

In addition, the method for producing the recycled polyester fiber of the present invention makes it possible to obtain the recycled polyester fiber of the present invention with good operability, which is a great practical advantage.

The present invention will be described in detail below.
The regenerated polyester fiber of the present invention (the fiber of the present invention) comprises at least one recycled polyester raw material selected from a) used polyester products and b) unused polyester generated in the process of manufacturing polyester products.

The above-mentioned used polyester products (a) include, for example, polyester molded products (including fibers) that were once distributed on the market and then collected after use, etc. Representative examples thereof include containers such as PET bottles and packaging materials.
The unadopted polyester generated in the process of manufacturing the polyester product of b) above is polyester that has not been commercialized. Examples of such unadopted polyester include resin pellets that do not meet the specifications, materials that are no longer needed during molding, pieces cut during molding, scraps generated during molding or processing, cut-up pieces of transitional products generated when the brand is changed, cut-up pieces of prototypes or defective products, etc.

The above a) and b) are not limited in their form, and may be pelletized by further processing such as crushing and cutting as necessary, or may be melted and pelletized. The above a) and b) may be used alone, or a mixture of the two may be used.

The recycled polyester raw materials a) and b) above may be either crystalline or amorphous. For example, pellets of amorphous polyester waste that have not been heat-treated, crystalline pellets that have been heat-treated, or a mixture of crystalline and amorphous pellets can be used. In the present invention, it is preferable to use crystalline recycled polyester raw materials, particularly for the purpose of preventing the pellets from fusing together when being fed into the device. Therefore, materials a) or b) above that have been crystallized by heat treatment (crystallized pellets, etc.) can be preferably used.

The properties of the recycled polyester raw materials in a) and b) above are not limited, and may be in the form of a) and b) above, or may be in the form of cut pieces, crushed material (powders), etc. obtained by further processing such as cutting and crushing, as well as in the form of solids such as molded bodies (pellets, etc.) obtained by molding these. More specifically, examples include pellets obtained by cooling and cutting melted polyester waste, and cut pieces obtained by finely cutting polyester molded products such as PET bottles. In addition, the above-mentioned cut pieces, crushed material (powders), etc. may be in the form of a liquid obtained by dispersing or dissolving them in a solvent. When manufacturing polyester products using these raw materials, they can be melted at a temperature above their melting point and introduced into the device as a melt, if necessary.

The regenerated polyester fiber of the present invention preferably contains 40% by mass or more, and more preferably 50% by mass or more, of the recycled polyester raw material which is at least one of the above a) and b).
If the content of the recycled polyester raw material is less than 40% by mass, the objective of considering environmental issues cannot be achieved. Although there is no particular upper limit to the content of the recycled polyester raw material, according to the production method of the present invention described later, it is possible to easily obtain a recycled polyester resin having a recycled polyester raw material content of 40 to 100% by mass.

ただし、Ｒ１は炭素数１～１２のアルキル基又はアリール基を示し、Ｒ２は炭素数１～１８のアルキル基、アリール基、モノヒドロキシアルキル基あるいはＲ１を介した環状体又は水素原子を示し、Ｒ３は炭素数１～１８のアルキル基、アリール基、モノヒドロキシアルキル基又は水素原子を示し、Ａは２価以上の炭化水素基を表す。また、ｎは、Ａの価数から１を引いた数を表す。 The regenerated polyester fiber of the present invention contains an organic phosphorus compound having two or more ester-forming functional groups (hereinafter, may be referred to as phosphorus compound (Y)), and has a phosphorus atom content of 1000 to 10000 ppm. As the phosphorus compound (Y), a compound represented by the following formula (1) is preferable in terms of the residual rate of the organic phosphorus compound, etc.

In the formula, R1 represents an alkyl group or aryl group having 1 to 12 carbon atoms, R2 represents an alkyl group, aryl group, or monohydroxyalkyl group having 1 to 18 carbon atoms, or a cyclic structure via R1, or a hydrogen atom, R3 represents an alkyl group, aryl group, or monohydroxyalkyl group having 1 to 18 carbon atoms, or a hydrogen atom, A represents a divalent or higher hydrocarbon group, and n represents the valence of A minus 1.

リン化合物が、ポリエステル繊維中に、リン原子の含有量が１０００～１００００ｐｐｍとなるように含まれていることにより、ポリエステル繊維に難燃性を付与することができ、難燃性を有する各種繊維製品を得ることが可能となる。繊維中のリン原子の含有量は、１０００～１００００ｐｐｍであるが、中でも２０００～７０００ｐｐｍであることが好ましい。 Specific preferred examples of the organophosphorus compound represented by the above formula (1) include those represented by the following structural formulas (a) to (d). Among them, the compound represented by structural formula (a) is particularly preferred in terms of flame retardancy. These may be used as they are or may be esterified products.

By containing the phosphorus compound in the polyester fiber so that the phosphorus atom content is 1000 to 10000 ppm, it is possible to impart flame retardancy to the polyester fiber, and it becomes possible to obtain various flame-retardant fiber products. The phosphorus atom content in the fiber is 1000 to 10000 ppm, and preferably 2000 to 7000 ppm.

If the phosphorus atom content in the recycled polyester fiber is less than 1,000 ppm, the flame retardant performance will be poor. On the other hand, if the phosphorus atom content exceeds 10,000 ppm, the melting point of the polymer will decrease, causing thread breakage during spinning and reducing operability. In addition, the resulting recycled polyester fiber will have low strength and a high rate of variation in fiber diameter.

The content of phosphorus atoms in the polyester fiber can be adjusted to fall within the above range by adjusting the amount of the phosphorus compound (Y) contained in the polyester fiber.
In the present invention, as a method for incorporating the phosphorus compound (Y) into the polyester fiber, a method of blending the phosphorus compound (Y) with the recycled polyester resin containing the above-mentioned recycled polyester raw material can be mentioned. More specific methods will be described in detail in the explanation of the production method.

The polyester resin constituting the recycled polyester fiber of the present invention preferably has an acid component mainly composed of terephthalic acid, and more specifically, when the total acid component is taken as 100 mol %, it is preferably 90 mol % or more, and more preferably 92 to 98 mol %.

In the polyester fiber of the present invention, examples of dicarboxylic acid components other than terephthalic acid include isophthalic acid, phthalic acid, 5-sodium sulfoisophthalic acid, phthalic anhydride, naphthalenedicarboxylic acid, adipic acid, sebacic acid, dimer acid, etc., and two or more of these may be used in combination, or ester-forming derivatives of these acids may be used.

The polyester resin constituting the recycled polyester fiber of the present invention preferably has a glycol component mainly composed of ethylene glycol, and more specifically, when the total glycol component is taken as 100 mol %, it is preferably 90 mol % or more, and more preferably 95 to 98 mol %.

In addition, examples of the diol components other than ethylene glycol that can be used in the polyester resin of the present invention include neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, diethylene glycol, dimer diol, an ethylene oxide adduct of bisphenol S, and an ethylene oxide adduct of bisphenol A.

The polyester fiber of the present invention has the above-mentioned composition and also has the following characteristic values (2) to (3).
(2) In differential scanning calorimetry, when the temperature is lowered from 310° C. to 25° C. at a rate of 20° C./min, no crystallization temperature is detected. (3) The rate of variation of the fiber diameter is 10% or less. The polyester fiber of the present invention having these characteristic values can be obtained by the production method of the present invention described later.

The fiber of the present invention has the characteristic value (2) of not detecting a crystallization temperature in differential scanning calorimetry when cooled from 310°C to 25°C at a rate of 20°C/min. If a crystallization temperature is detected under the above conditions, the fiber has poor thermal stability because crystallization occurs due to various foreign matter acting as crystal nuclei. If the fiber has poor thermal stability, the resulting nonwoven fabric will have poor texture, strength, elongation, and other properties in both dry and wet nonwoven fabrics.

Furthermore, if the crystallization temperature is detected under the above conditions, it is highly likely that crystals are generated in the fibers during the spinning process, and that these crystals become foreign matter and cause yarn breakage. It is also highly likely that the rate of variation in the fiber diameter of the resulting fibers is also large.

The fiber of the present invention has a fiber diameter fluctuation rate of 10% or less as a characteristic value (3). The fiber diameter fluctuation rate refers to the degree of variation in fiber diameter. The fiber diameter fluctuation rate is preferably 8% or less, more preferably 5% or less. If the fiber diameter fluctuation rate exceeds 10%, the resulting short fibers will have poor uniformity, and the resulting nonwoven fabric will have poor texture and poor quality. In addition, there is a high possibility that broken yarns will increase during spinning, deteriorating operability.
The rate of variation in fiber diameter in the present invention is calculated by measuring the fiber diameters of 50 fibers (single fibers) when observing the fiber cross section with an optical microscope or the like, calculating the standard deviation and average value, and then calculating according to the following formula.
Fluctuation rate of fiber diameter (%) = (standard deviation of fiber diameter / average value of fiber diameter) x 100
In addition, the fiber diameter refers to the major axis in the case of an ellipse, and refers to the maximum length of the diagonal in the case of a polygonal shape.

The fiber of the present invention preferably has a single yarn fineness of 0.35 to 1.0 dtex, more preferably 0.4 to 0.8 dtex, and a fiber length of 20 to 44 mm, more preferably 25 to 32 mm.
Although the fiber of the present invention is made from recycled polyester as a raw material, it can satisfy the above characteristic values (1) to (3) by the production method described below, even if the single yarn fineness is small.
Therefore, when the fiber of the present invention is used to prepare a dry or wet nonwoven fabric, the processability is improved and a high-quality nonwoven fabric having a uniform texture can be obtained.

Whether dry or wet nonwoven fabrics are produced using the fiber of the present invention, they can be made with uniform texture and high quality, but it is preferable that the fiber has a strength of 1.5 cN or more and an elongation of 100% or less. Of these, it is preferable that the strength is 2.0 to 5.0 cN and the elongation is 50 to 80%. Outside these ranges, the performance of the resulting dry or wet nonwoven fabrics tends to be insufficient.

Furthermore, the regenerated polyester fiber of the present invention preferably has the above-mentioned thermal stability and strength and elongation, and also has a melt viscosity of 50 to 100 Pa·s at a temperature of 310° C. and a shear rate of 1000 s ⁻¹ .

The fiber of the present invention can be obtained by employing the production method described below, which satisfies the above-mentioned characteristic values.
The method for producing the fiber of the present invention will be described. In the production method of the present invention, at least one recycled polyester raw material (hereinafter, sometimes simply referred to as recycled polyester raw material) consisting of a) used polyester products and b) unused polyesters generated in the process of producing polyester products is melt-kneaded in an extruder with an organic phosphorus compound having two or more ester-forming functional groups. First, the following methods (a) and (b) can be adopted as a method for blending the recycled polyester raw material with the phosphorus compound (Y).

(a) A method of directly blending a phosphorus compound with recycled polyester raw materials; and (b) A method of blending a polyester resin (master resin) containing a high concentration of a phosphorus compound with recycled polyester raw materials. For both of the production methods (a) and (b), for example, a batch blending method in which a screw-type extruder is used to simultaneously add the recycled polyester raw materials and the phosphorus compound or a master resin containing a phosphorus compound and melt and knead them, or a separate blending method in which the recycled polyester raw materials are melted and kneaded, and then the phosphorus compound or a master resin containing a phosphorus compound is supplied from another supply port of the extruder and melted and kneaded.

The manufacturing method of the present invention involves blending the recycled polyester raw material with the phosphorus compound (Y) as described above, melt-kneading the mixture in an extruder, passing the mixture through a metering pump, melt-spinning the mixture from a melt-spinning nozzle equipped with a spinneret, cooling the spun fiber in a cooling tube, and taking it up with a take-up roller.
The temperature during melt kneading in the extruder is designated as a, the temperature of the metering pump as b, the temperature of the melt spinning nozzle as c, and the temperature of the cooling cylinder as d, and it is necessary that the temperatures a to d satisfy the following condition.
a is 280 to 290° C.
b is a+5 to a+10°C
c is 310 to 320° C.
d is c-150 to c-50°C

By satisfying the above conditions, the temperatures of a and b are set to a relatively low temperature of 300°C or less for the extrusion device and metering pump. On the other hand, by setting the temperature of the melt spinning nozzle, which is temperature c, at 310-320°C, it is possible to suppress thermal decomposition of the resin while melting foreign matter in the polyester resin and to keep the melt viscosity low, so that fine fibers can be obtained stably. Furthermore, by setting the temperature of the cooling tube to 50-150°C, especially 80-130°C lower than the temperature of the melt spinning nozzle, uniform cooling can be performed, and the obtained fibers have small unevenness in fineness and are of sufficient yarn quality.

If the temperatures a to d do not satisfy the above conditions, foreign matter in the resin will not melt and the melt viscosity cannot be reduced, making it difficult to stably obtain fine fibers and resulting fibers will have many uneven finenesses and poor yarn quality.
In the case of a fiber obtained by melt spinning without melting the foreign matter in the resin, the crystallization temperature is detected when the temperature is lowered from 310° C. to 25° C. at a rate of 20° C./min in differential scanning calorimetry, which means that the fiber does not satisfy the characteristic value (2) of the fiber of the present invention.

After the melt-spun fiber is taken up by a take-up roller according to the above-mentioned production method, the undrawn yarn is converged to form a tow of 30 to 80 ktex, which is then drawn, heat-treated, and crimped, and then cut to a length of 20 to 44 mm to obtain the fiber of the present invention.
According to the production method of the present invention, polyester fibers with little unevenness in fineness can be obtained with good productivity without causing troubles such as yarn breakage in any of the steps such as melt spinning, drawing, heat treatment, and crimping.

The manufacturing method of the present invention can produce fibers of the present invention having the following characteristics: single filament fineness of 0.5 to 25.0 dtex, strength of 0.1 to 6.0 cN/dtex, and elongation of 20 to 600%. As mentioned above, the manufacturing method of the present invention can easily produce fibers of the present invention having a single filament fineness of 0.35 to 1.0 dtex, which is difficult to produce.

The present invention will now be described in detail with reference to examples, in which the measurement and evaluation methods for various property values and the like are as follows.
(a) Melt Viscosity The obtained recycled polyester fibers were measured using a flow tester "CFT-500" (manufactured by Shimadzu Corporation) under the conditions of a polyester resin residence time of 1 minute, a measurement temperature of 310° C., a shear rate of 1000 sec ⁻¹ , and a die diameter of 0.5 mm×2 mm.
(b) Content of phosphorus atoms in polyester fiber The obtained regenerated polyester fiber was dissolved in a mixed solvent of deuterated trifluoroacetic acid and deuterated chloroform in a volume ratio of 1/11, and 1H-NMR was measured using a JNM-ECZ400R/S1 NMR device manufactured by JEOL Ltd., and the content was calculated from the integrated intensity of the proton peak of each component in the obtained chart.
(c) Crystallization Temperature by Cooling The obtained regenerated polyester fiber was measured using a PerkinElmer differential scanning calorimeter DSC-7 in a nitrogen stream at a temperature range of 25 to 310° C. and a cooling rate of 20° C./min.
(d) Fluctuation Rate of Fiber Diameter The fluctuation rate of fiber diameter was measured and calculated by the above-mentioned method using the obtained regenerated polyester fiber.
(e) Fiber manufacturing runnability (yarn cuts)
Regarding the number of times of yarn breakage in the melt spinning process, less than three times per ton of spinning amount was rated as pass (◯), and three or more times was rated as fail (×).
(f) Fiber Fineness The single fiber fineness of the obtained regenerated polyester fiber was measured according to JIS L1015 8.5.1 B method.
(g) Fiber strength and elongation The obtained recycled polyester fibers were used to measure the strength at which the yarn broke using a tensile tester AG-100G manufactured by Shimadzu Corporation in accordance with JIS L1013, with a gripping distance of 500 mm and a pulling speed of 500 mm/min.
(h) Texture of Nonwoven Fabric The texture of the resulting nonwoven fabric was visually evaluated on the following two-level scale.
○: The distribution of the constituent fibers is uniform, with very few irregularities
×: The distribution of the constituent fibers is very uneven, and spots are noticeable.
(i) Flame Retardancy The flame retardancy of the obtained nonwoven fabric was evaluated in terms of limiting oxygen index (LOI) according to JIS K 7201 A2. An LOI value of 26% or more was judged to be flame retardant.

Example 1
(Short fiber production)
PET bottle scraps were used as the recycled polyester raw material, and a polyethylene terephthalate resin containing 30% by mass of a phosphorus compound represented by the structural formula (a) was used as the master resin.
20 parts by mass of the master resin was used per 100 parts by mass of PET bottle scraps, and these were fed into an extruder at a temperature of 285° C. After passing through a metering pump at a temperature of 295° C., melt spinning was carried out using a spinneret having a temperature of 315° C., 2160 holes, and a hole diameter of 0.16 mm, with a discharge rate of 312 g/min, a spinning temperature of 315° C., a spinning speed of 1176 m/min, and a cooling tower temperature of 215° C.
That is, the temperature a during melt kneading in the extruder was 285° C., the temperature b of the metering pump was 295° C., the temperature c of the melt spinning nozzle was 315° C., and the temperature d of the cooling cylinder was 215° C. In addition, the polyester resin obtained by melt kneading in the extruder had a recycled raw material content of 80% by mass.
Next, after melt spinning, the undrawn yarn taken up by the take-up roller was converged to form a tow of 50 ktex, and was drawn under the conditions of a drawing temperature of 70°C and a draw ratio of 3.2 times. After drawing, the tow was heat-set with a heat drum at a temperature of 199°C, and then an oil agent mainly composed of potassium lauryl phosphate was applied to the tow by a spray method. Next, the tow was heated with a heating roller at 170°C, and then heated with steam, and introduced into a push-in crimper to give it crimping. Then, the tow was cooled on a cooling conveyor without being heat-treated with a dryer, and cut to 32 mm to obtain short fibers.
(Production of nonwoven fabric)
The resulting staple fibers and Unitika's core-sheath type heat-fused fibers with a melting point of 110°C (staple fibers with a core of polyethylene terephthalate and a sheath of a copolymer polyester with a melting point of 110°C, 2.2 dtex x 51 mm) were mixed in a mass ratio of 80/20 to produce a carded web (basis weight 100 g/ ^m2 ). The carded web was heat-treated in a hot air dryer at a temperature of 150°C, an air volume of 38 ^m2 /min, and a treatment time of 1 minute to produce a dry nonwoven fabric with a basis weight of 100 g/ ^m2 .

Examples 2 and 3, Comparative Examples 1 and 2
Short fibers were obtained in the same manner as in Example 1, except that the amount of master resin per 100 parts by mass of PET bottle scraps was changed to that shown in Table 1.
Using the resulting short fibers, a drylaid nonwoven fabric was produced in the same manner as in Example 1.

Examples 4 to 7, Comparative Examples 3 to 9
Short fibers were obtained in the same manner as in Example 1, except that the temperature a during melt kneading in the extruder, the temperature b of the metering pump, the temperature c of the melt spinning nozzle, and the temperature d of the cooling cylinder were changed to those shown in Table 1.
Using the resulting short fibers, a drylaid nonwoven fabric was produced in the same manner as in Example 1.

Example 8
PET bottle scraps were used as the recycled polyester raw material, and the polyester raw material and the phosphorus compound were fed into an extruder at a temperature of 285° C. so that the phosphorus compound represented by structural formula (a) was 6 parts by mass per 100 parts by mass of the recycled polyester raw material. After passing through a metering pump at a temperature of 293° C., melt spinning was performed using a spinneret at a temperature of 315° C., with a hole number of 2160 H and a hole diameter of 0.16 mm, at a discharge rate of 312 g/min, a spinning temperature of 315° C., a spinning speed of 1176 m/min, and a cooling tower temperature of 215° C.
That is, the temperature a during melt kneading in the extruder was 285° C., the temperature b of the metering pump was 293° C., the temperature c of the melt spinning nozzle was 315° C., and the temperature d of the cooling cylinder was 215° C. In addition, the polyester resin obtained by melt kneading in the extruder had a recycled raw material content of 100% by mass.
Next, after melt spinning, the undrawn yarn taken up by a take-up roller was converged to form a tow of 50 ktex, which was then drawn, heat-treated, etc. in the same manner as in Example 1 to obtain staple fibers.
Using the resulting short fibers, a drylaid nonwoven fabric was produced in the same manner as in Example 1.

Examples 9 and 10, Comparative Examples 10 and 11
Short fibers were obtained in the same manner as in Example 8, except that the amount of the phosphorus compound represented by the structural formula (a) added per 100 parts by mass of PET bottle scraps was changed to that shown in Table 1.
Using the resulting short fibers, a drylaid nonwoven fabric was produced in the same manner as in Example 1.

Table 1 shows the property values of the regenerated polyester fibers obtained in Examples 1 to 10 and Comparative Examples 1 to 11, evaluation of spinning operability, and evaluation of the texture and flame retardancy of the obtained dry nonwoven fabrics.

As is clear from Table 1, in Examples 1 to 10, recycled polyester fibers were obtained with good spinning operability, and the obtained recycled polyester fibers satisfied all of the above conditions (1) to (3). Therefore, the obtained dry nonwoven fabric was flame retardant and had a uniform texture without any spots.

On the other hand, the recycled polyester fiber obtained in Comparative Example 1 had a low phosphorus atom content of 630 ppm by mass, and therefore the flame retardancy of the obtained nonwoven fabric was low.
The recycled polyester fiber obtained in Comparative Example 2 had a high phosphorus atom content of 11,040 ppm by mass, and therefore had a low melting point, which resulted in yarn breakage during spinning and poor operability. As a result, the recycled polyester fiber obtained had low strength, a high rate of variation in fiber diameter, and the texture of the obtained nonwoven fabric was also poor.
In Comparative Example 3, the temperature of the extruder was as low as 275°C, so that the foreign matter in the resin did not melt, and the obtained recycled polyester fiber had a crystallization temperature that was detected by differential scanning calorimetry, and the variation rate of the fiber diameter was large. Therefore, the texture of the obtained nonwoven fabric was also poor.
In Comparative Example 4, the temperature of the extruder was as high as 295° C., which caused thermal decomposition, leading to yarn breakage during spinning and poor operability. As a result, the resulting recycled polyester fiber had low strength and a high rate of variation in fiber diameter, and the resulting nonwoven fabric also had poor texture.
In Comparative Example 5, the temperature of the metering pump was as low as 285°C, so that the foreign matter in the resin did not melt, and the obtained recycled polyester fiber had a crystallization temperature that was detected by differential scanning calorimetry, and the variation rate of the fiber diameter was large. Therefore, the texture of the obtained nonwoven fabric was also poor.
In Comparative Example 6, the temperature of the metering pump was as high as 305° C., which caused thermal decomposition, leading to yarn breakage during spinning and poor operability. As a result, the resulting recycled polyester fiber had low strength and a high rate of variation in fiber diameter, and the resulting nonwoven fabric also had poor texture.
In Comparative Example 7, the nozzle temperature was as low as 295°C, so that the foreign matter in the resin did not melt, and the obtained recycled polyester fiber had a crystallization temperature that was detected by differential scanning calorimetry, and the variation rate of the fiber diameter was large. Therefore, the texture of the obtained nonwoven fabric was also poor.
In Comparative Example 8, the nozzle temperature was as high as 325° C., which caused thermal decomposition, leading to yarn breakage during spinning and poor operability. As a result, the resulting recycled polyester fiber had low strength and a high rate of variation in fiber diameter, and the resulting nonwoven fabric also had poor texture.
In Comparative Example 9, the temperature of the cooling cylinder was as low as 215°C, so that the foreign matter in the resin did not melt, and the obtained recycled polyester fiber had a crystallization temperature that was detected by differential scanning calorimetry, and the variation rate of the fiber diameter was large. Therefore, the texture of the obtained nonwoven fabric was also poor.
The regenerated polyester fiber obtained in Comparative Example 10 had a phosphorus atom content of only 600 ppm by mass, and therefore the flame retardancy of the obtained nonwoven fabric was low.
The recycled polyester fiber obtained in Comparative Example 11 had a high phosphorus atom content of 11,300 mass ppm, and therefore had a low melting point, which resulted in yarn breakage during spinning and poor operability. As a result, the resulting recycled polyester fiber had low strength, a high rate of variation in fiber diameter, and the resulting nonwoven fabric had poor texture.

Claims (3)

A regenerated polyester fiber containing 40% by mass or more of at least one type of recycled polyester raw material, which is a) a used polyester product and b) an unused polyester generated in a process for producing a polyester product, and which satisfies all of the following (1) to (3):
(1) It contains an organic phosphorus compound having two or more ester-forming functional groups, and the phosphorus atom content is 1,000 to 10,000 ppm by mass. (2) In differential scanning calorimetry, when the temperature is lowered from 310° C. to 25° C. at a rate of 20° C./min, no crystallization temperature is detected. (3) The rate of variation of the fiber diameter is 10% or less. The recycled polyester fiber according to claim 1, having a single fiber fineness of 0.35 to 1.0 dtex and a fiber length of 20 to 44 mm. 3. A method for producing a recycled polyester fiber, comprising the steps of melt-kneading at least one recycled polyester raw material, which is a) a used polyester product and b) an unused polyester generated in a process for producing a polyester product, and an organic phosphorus compound having two or more ester-forming functional groups in an extruder, passing the material through a metering pump, melt-spinning the material through a melt spinning nozzle equipped with a spinneret, cooling the spun fiber in a cooling tube, and taking it up with a take-up roller, wherein the temperature during melt-kneading in the extruder is a, the temperature of the metering pump is b, the temperature of the melt spinning nozzle is c, and the temperature of the cooling tube is d, and the temperatures a to d satisfy the following conditions:
a is 280 to 290° C.
b is a+5 to a+10°C
c is 310 to 320° C.
d is c-150 to c-50°C