JP7194851B2 - Polyacetal powder, method of use thereof, and additive manufacturing method - Google Patents

Description

The present invention relates to polyacetal powders and methods of use thereof, as well as additive manufacturing methods.

In recent years, additive manufacturing technology has been widely used as one of the techniques for producing a modeled product having a complicated structure.

Since this additive manufacturing technique is a method that does not use a mold, it has the advantage of being able to produce prototypes in a short period of time. Moreover, there is an increasing need for application not only to trial production but also to direct production of small-lot, high-variety products. Against this background, among the additive manufacturing technologies, 3D printers that use powdered resin as a modeling material, such as selective laser sintering, are attracting attention.

In the selective laser sintering method, a powder material is spread on a modeling table with a roller or blade, and the powder material is selectively irradiated with a laser, heated and sintered, and the process is repeated in sequence to create a laminate. It is a method of making (Patent Document 1). Selective laser sintering is a method that can be used with thermoplastic resins, which are currently used for plastic industrial parts, so it is compatible with other additive manufacturing techniques (liquid bath photopolymerization, material injection, etc.). In comparison, it is superior in that the strength, reliability, and dimensional stability of the molded product are high. Another feature of the selective laser sintering method is that it enables the production of shaped articles with rather complex shapes and contours, which cannot be achieved by conventional methods such as machining.

Polyamide 11, polyamide 12, polypropylene and the like are widely known as resins constituting powder materials that can be used in the selective laser sintering method (Patent Document 2). Further, Patent Document 3 discloses a powder composed of polyacetal that can be used in selective laser sintering.

U.S. Pat. No. 4,863,538 WO 1996/006881 WO2011/051250

In additive manufacturing using polyacetal powder for selective laser sintering, the stackability of the powder at high temperatures, the placement of the powder on the molded part, and the mechanical properties of the molded product are very important.
However, in Patent Documents 1 to 3, sufficient studies have not been made on these matters, and there is still room for improvement in terms of improving the lamination property of polyacetal powder at high temperatures, powder placement on molded parts, and mechanical properties of molded products. was there.

Therefore, the problem to be solved by the present invention is that it can be used for additive manufacturing technology, and it is possible to obtain a molded product with high mechanical properties. Another object is to provide a polyacetal powder. Another problem to be solved by the present invention is to provide a method of using the polyacetal powder described above and an additive manufacturing method capable of obtaining a shaped article having high mechanical properties.

The present inventors have made intensive studies to solve the above problems, and as a result, polyacetal powder that has specific powder characteristics and contains hydrophobic silica has excellent lamination properties at high temperatures and excellent powder placement on the modeling site. , which can be used in additive manufacturing techniques, enabling the production of shaped articles having high mechanical properties, and completed the present invention.

That is, the present invention is as follows.
[1]
The average particle diameter (D50) is 30 μm or more and 70 μm or less,
Contains 0.05% by mass or more and 1.0% by mass or less of hydrophobic silica,
A polyacetal powder characterized by:
[2]
The polyacetal powder according to [1], wherein the hydrophobic silica is dry silica.
[3]
The polyacetal powder according to [1] or [2], wherein the hydrophobic silica has an average primary particle size of 1 nm or more and 20 nm or less.
[4]
The polyacetal powder according to any one of [1] to [3], wherein the hydrophobic silica has a BET specific surface area of 50 m ² /g or more and 300 m ² /g or less.
[5]
The polyacetal powder according to any one of [1] to [4], wherein the hydrophobic silica is hydrophobized with one or more selected from the group consisting of dimethyldichlorosilane, trimethylchlorosilane, and silicone oil.
[6]
The polyacetal powder according to any one of [1] to [5], which has a weight loss rate of 5% or less when held for 1 hour at a temperature 10° C. lower than the melting point of the polyacetal powder.
[7]
The polyacetal powder according to any one of [1] to [6], wherein the value obtained by dividing the solid bulk density by the loose bulk density (solid bulk density/loose bulk density) is 1.45 or less.
[8]
The polyacetal powder according to any one of [1] to [7], wherein the proportion of particles having a diameter of at least twice the D50 is 10 vol % or less.
[9]
The polyacetal powder according to any one of [1] to [8], wherein the proportion of particles having a diameter of 0.2 times or less of D50 is 10 vol % or less.
[10]
The polyacetal powder according to any one of [1] to [9], wherein the D50 is 65 μm or less, and the proportion of particles having a diameter of 0.2 times or less of the D50 is 3 vol % or less.
[11]
A polyacetal copolymer having 0.1 mol or more and 1.3 mol or less of comonomer units per 100 mol of oxymethylene units,
a melting point of 167° C. or higher and 178° C. or lower;
a crystallization start temperature of 140° C. or higher and 152° C. or lower;
The polyacetal powder according to any one of [1] to [10].
[12]
The polyacetal powder according to any one of [1] to [11], wherein the difference between the melting point of the first scan and the melting point of the second scan in differential scanning calorimetry (DSC) measurement is 0°C or more and 5°C or less.
[13]
The polyacetal powder according to any one of [1] to [12], which has a crystallization speed of 30 seconds or more and 50 seconds or less.
[14]
The polyacetal powder according to any one of [1] to [13], wherein the particles constituting the polyacetal powder have an average short axis to long axis ratio of 0.50 or more.
[15]
The polyacetal powder according to any one of [1] to [14], wherein particles constituting the polyacetal powder have an average surface smoothness shape factor of 0.80 or more.
[16]
The polyacetal powder according to any one of [1] to [15], which is used for additive manufacturing.

[17]
A method for using the polyacetal powder according to any one of [1] to [16], which is characterized by using it for additive manufacturing.

[18]
An additive manufacturing method, wherein the polyacetal powder according to any one of [1] to [16] is used as a molding material.

According to the present invention, there is provided a polyacetal powder that can be used in additive manufacturing technology and that can be used to obtain a molded article having high mechanical properties, and that has excellent lamination properties at high temperatures and excellent powder placement on molded parts. be able to. Further, according to the present invention, it is possible to provide a method of using the above-described polyacetal powder and an additive manufacturing method capable of obtaining a shaped article having high mechanical properties.

2 is a secondary electron image of the surface of the polyacetal powder obtained in Example 1, observed with a scanning electron microscope. 2 is a secondary electron image of the surface of the polyacetal powder obtained in Comparative Example 1 observed with a scanning electron microscope. 4 is a secondary electron image of the surface of the polyacetal powder obtained in Comparative Example 2 observed with a scanning electron microscope.

EMBODIMENT OF THE INVENTION Hereinafter, the form (henceforth this embodiment) for implementing this invention is demonstrated in detail.
It should be noted that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the present invention.
Further, in the embodiment of the present invention, A (numerical value) to B (numerical value) mean A or more and B or less.

[Polyacetal powder]
The polyacetal powder of the present embodiment is characterized by having an average particle diameter (D50) of 30 μm or more and 70 μm or less and containing 0.05% by mass or more and 1.0% by mass or less of hydrophobic silica.
The polyacetal powder of the present embodiment is excellent in lamination property at high temperature and powder application to the molding site, and can be suitably used as a molding material for additive manufacturing, and can obtain a molded article having high mechanical properties.
The powder properties, resin properties, particle properties, etc. of the polyacetal powder of the present embodiment will be described below.

<Average particle size (D50)>
The polyacetal powder of the present embodiment is characterized by having an average particle size (D50) of 30 μm or more and 70 μm or less. If the average particle size (D50) of the polyacetal powder is within the above range, sufficient workability can be ensured, for example, it can be sufficiently uniformly spread to a thickness of about 100 μm, and the resulting molded product has minute voids. It is possible to sufficiently suppress the generation of (voids).
When the D50 of the polyacetal powder is less than 30 μm, the workability deteriorates. is likely to occur frequently.
From the same point of view, D50 of the polyacetal powder is preferably 33 μm or more and 67 μm or less, more preferably 35 μm or more and 65 μm or less.

The D50 of the polyacetal powder of the present embodiment is the 50% particle diameter (median diameter) accumulated from the fine particle side, which is obtained from the volume-based particle size distribution measured by the wet method (solvent: water) by the laser diffraction/scattering method. More specifically, it can be measured by the method described in Examples below.
In addition, the average particle size (D50) in the above range is a value measured for polyacetal powder containing hydrophobic silica, that is, for polyacetal powder in which hydrophobic silica is attached to the particle surface as described later.

<Hydrophobic silica>
The polyacetal powder of the present embodiment preferably contains 0.05% by mass or more and 1.0% by mass or less of hydrophobic silica with respect to 100% by mass of the polyacetal powder.
In additive manufacturing the flowability of the polyacetal powder is very important. The polyacetal powder is melted and dented in the molding area by heating with a laser, but if powder with poor fluidity is used, the roughness of the surface of the dent becomes large, and the powder does not adhere to the molding area, resulting in voids. There is a risk that the mechanical properties of the molded product will be impaired or the molding will be stopped. In this regard, the present inventors have found that the polyacetal powder contains hydrophobic silica in a content of 0.05% by mass or more and 1.0% by mass or less, and the hydrophobic silica is on the surface of the particles constituting the polyacetal powder. It was found that the adherence can improve the fluidity of the powder, improve the stackability at high temperatures and the powder placement on the modeled part, and improve the mechanical properties of the modeled product.
Furthermore, as the polyacetal powder, a powder having a narrow particle size distribution curve (hereinafter simply referred to as "particle size distribution curve") expressed as a number-based frequency distribution is usually preferred, but the width of the particle size distribution curve is wide. It has been found that the same effect as described above can be obtained even when powder is used. Thereby, the productivity of the polyacetal powder (for example, the yield after the classification step) can also be increased.

On the other hand, when the content of hydrophobic silica in the polyacetal powder is less than 0.05% by mass, the effect of improving the fluidity of the powder is insufficient. In addition, if the content of hydrophobic silica exceeds 1.0% by mass, the laser absorption of the polyacetal powder is inhibited by the hydrophobic silica, so that the amount of heat required for heating and sintering cannot be obtained, and the voids ( Voids) may occur, which may impair the mechanical properties and molding accuracy of the molded product. Even if hydrophilic silica is used instead of hydrophobic silica, it does not adhere to the surface of the particles constituting the polyacetal powder, and the effect of improving the fluidity of the powder cannot be exhibited.
From the same point of view, the content of hydrophobic silica in the polyacetal powder of the present embodiment is more preferably 0.1% by mass or more and 0.7% by mass or less with respect to 100% by mass of the polyacetal powder. More preferably, the content is not less than 0.5% by mass and not more than 0.5% by mass.

In the polyacetal powder of the present embodiment, the hydrophobic silica adhering to the surface of the particles constituting the polyacetal powder means that at least part of one hydrophobic silica particle is exposed on the surface of the particles constituting the polyacetal powder. It also includes the case where the remaining part is embedded in the particle. On the other hand, when the hydrophobic silica particles are not exposed on the surface of the particles constituting the polyacetal powder, such as when the hydrophobic silica is kneaded with the polyacetal composition to produce the polyacetal powder, it is not included in the present embodiment. .
Whether the hydrophobic silica adheres to the surface of the particles constituting the polyacetal powder can be confirmed by observing the surface of the particles constituting the polyacetal powder with a scanning electron microscope (SEM). For example, in a secondary electron image obtained by observing the surface of the polyacetal powder according to this embodiment with an SEM, a large number of fine hydrophobic silica particles adhere to the surface of the polyacetal powder (eg, FIG. 1). On the other hand, in the case of production without using silica, and in the case of using hydrophilic silica instead of hydrophobic silica, adhesion of fine silica particles to the surface of the polyacetal powder was not observed (e.g., FIGS. 2 and 3). ).
The polyacetal powder described above can be obtained by adding a predetermined amount of hydrophobic silica to the polyacetal powder obtained after the classification step and mixing (hydrophobic silica mixing step), as described later.

In the present embodiment, the hydrophobic silica is not particularly limited, and those commonly used in industrial applications can be used. For example, both dry process silica and wet process silica can be used. Dry process silica is preferably used. Wet-process silica may contain impurities that decompose polyacetal during its manufacturing process, and may impair the resin characteristics of the polyacetal powder. On the other hand, dry-process silica is unlikely to contain such impurities during its manufacturing process, and there is no risk of impairing the resin characteristics of the polyacetal powder.

In the present embodiment, the average primary particle size of the hydrophobic silica is not particularly limited, but is preferably 1 nm or more and 20 nm or less. If the average primary particle size of the hydrophobic silica is within the above range, the fluidity of the polyacetal powder can be further increased, and the layerability at high temperatures, the powder placement on the molded part, and the mechanical properties of the molded product can be improved. can be improved.
From the same point of view, the average primary particle size of the hydrophobic silica is more preferably 5 nm or more and 15 nm or less, and even more preferably 7 nm or more and 13 nm or less.
The average primary particle size of hydrophobic silica can be measured by a wet method (solvent: water) by a laser diffraction/scattering method.

In the present embodiment, the BET specific surface area of the hydrophobic silica is not particularly limited, but is preferably 50 m ² /g or more and 300 m ² /g or less. If the BET specific surface area of the hydrophobic silica is within the above range, the fluidity of the polyacetal powder can be further increased, and the lamination property at high temperatures, powder placement on the molded part, and the mechanical properties of the molded product can be further improved. can.
From the same point of view, the BET specific surface area of the hydrophobic silica is more preferably 100 m ² /g or more and 250 m ² /g or less, and still more preferably 130 m ² /g or more and 200 m ² /g or less.
The BET specific surface area of hydrophobic silica can be determined according to the BET method.

In the present embodiment, the dry-process silica is not particularly limited, and one commonly used in industrial applications can be used. It is preferable that the Hydrophobic silica hydrophobized with a surface modifier has a reduced number of surface hydroxyl groups, which enhances the adhesion to the surface of particles constituting the polyacetal powder, and further enhances the fluidity of the polyacetal powder.
The surface modifier is not particularly limited as long as it is commonly used for surface modification treatment of silica, and examples thereof include dimethyldichlorosilane, trimethylchlorosilane, silicone oil, octylsilane, hexamethylsilazane, and the like. . Above all, it is preferable to hydrophobize with at least one surface modifier selected from the group consisting of dimethyldichlorosilane, trimethylchlorosilane, and silicone oil.
One of the above surface modifiers may be used alone, or two or more thereof may be used in combination.
As the hydrophobic silica hydrophobized with the surface modifier, a commercially available product may be used, or a hydrophobic silica obtained by hydrophobization using a surface modifier may be used.

[Polyacetal Copolymer]
The polyacetal powder of the present embodiment preferably contains, as a resin component, a polyacetal copolymer having 0.1 mol or more and 1.3 mol or less of comonomer units per 100 mol of oxymethylene units. If the comonomer unit content of the polyacetal copolymer contained as the resin component is within the above range, a polyacetal powder is obtained that has good thermal stability, suppresses the generation of gas during laser irradiation, and gives a shaped article with few microvoids. be able to. In addition, it is possible to obtain a polyacetal powder that is less deteriorated even if it stays in the molding machine for a long time and that can be recycled.
From the same point of view, the comonomer unit content of the polyacetal copolymer is more preferably 0.3 mol or more and 1.1 mol or less, more preferably 0.5 mol or more and 0.9 mol or less, relative to 100 mol of the oxymethylene unit. preferable.
The comonomer unit may be contained in the main chain of the polyacetal copolymer, may be contained in the side chain, or may be contained in both the main chain and the side chain.

In the present embodiment, the comonomer unit of the polyacetal copolymer is not particularly limited as long as it can be copolymerized with an oxymethylene unit, but an oxyalkylene unit having 2 or more carbon atoms is preferable. Both ends or one end of the polyacetal copolymer may be blocked with an ester group and/or an ether group.
A polyacetal copolymer having a specific comonomer unit content as described above can be produced, for example, by the production method described below.

The comonomer unit content in the polyacetal copolymer can be determined by the following procedure using ¹ H-NMR method.
The procedure below describes a polyacetal copolymer in which the comonomer unit is an oxyalkylene unit having 2 or more carbon atoms.
That is, the polyacetal copolymer was dissolved in hexafluoroisopropanol over 24 hours to a concentration of 1.5% by mass, and ¹ H-NMR analysis was performed using this solution, and an oxymethylene unit and a carbon number of 2 or more were detected. From the ratio of the integral value of the oxyalkylene unit and the peak attributed to, the content of the oxyalkylene unit having 2 or more carbon atoms (bmol) with respect to the oxymethylene unit (a = 100mol) (b/a: mol/100mol) can be asked for.

<Melting point>
The polyacetal powder of the present embodiment preferably has a melting point of 167° C. or higher and 178° C. or lower. If the melting point is within the above range, the temperature difference (process window) from the crystallization start temperature of the polyacetal powder can be sufficiently widened, and the tolerance of the polyacetal powder to the temperature difference in the molding area during molding is widened ( For example, even if the temperature of the molding area is lower than the set temperature due to heat dissipation to the surroundings, it is possible to suppress the occurrence of warping during molding), and the molding accuracy of the obtained molded product can be improved. can be enhanced. In addition, when mass-producing a large number of shaped products at once, it is possible to widen the tolerance of the polyacetal powder to the temperature difference in the shaping apparatus, and to suppress variations in shaping accuracy between shaped products.
From the same point of view, the melting point of the polyacetal powder is more preferably 168° C. or higher and 176° C. or lower, and still more preferably 169° C. or higher and 174° C. or lower.

Here, the melting point of the polyacetal powder means the intrinsic melting point of the polyacetal powder, which can be measured as the second scan melting point in differential scanning calorimetry (DSC). Since the melting point of the first scan measured by DSC is affected by the thermal history of the polyacetal powder to be measured, it may be lower than the intrinsic melting point of the polyacetal powder, such as when it is freeze-pulverized during the manufacturing process. On the other hand, the melting point of the second scan measured by DSC is measured as the melting point when the polyacetal powder melts and solidifies once during the first scan measurement, and then melts again. can have an intrinsic melting point.

A polyacetal powder having a melting point within the above range can be obtained by adjusting the comonomer unit content within the above range to control the degree of crystallinity of the polyacetal copolymer.
Generally, increasing the comonomer content in the polyacetal copolymer lowers the degree of crystallinity, which can lower the melting point of the polyacetal powder. Conversely, decreasing the comonomer content in the polyacetal copolymer increases the crystallinity and can increase the melting point of the polyacetal powder.

<Crystallization start temperature>
The polyacetal powder of the present embodiment preferably has a crystallization initiation temperature of 140° C. or higher and 152° C. or lower.
The temperature in the modeling area and in the modeling apparatus during additive manufacturing is always kept higher than the crystallization start temperature of the resin powder. However, if there is some temperature difference (temperature unevenness) within the modeling area or within the modeling device, the polyacetal in the portion cooled to the crystallization start temperature in the heated and sintered modeling layer will crystallize, especially in the stacking direction. warpage occurred, and the molding accuracy was lowered in the resulting molded product. In this regard, the present inventors have found that by increasing the temperature difference between the melting point and the crystallization start temperature of the polyacetal powder, it is possible to increase the tolerance for temperature differences (temperature unevenness) within the molding area and within the molding apparatus. The inventors have found that it is possible to suppress warpage during modeling and improve the modeling accuracy of the resulting modeled product. They also found that when mass-producing a large number of shaped products at one time, variations in modeling accuracy between shaped products can be suppressed.
From the same point of view, the crystallization initiation temperature of the polyacetal powder is more preferably 144° C. or higher and 152° C. or lower, and still more preferably 147° C. or higher and 151° C. or lower.
The crystallization initiation temperature of the polyacetal powder can be measured by differential scanning calorimetry (DSC), and more specifically by the method described in Examples below.
A polyacetal powder having a crystallization temperature within the above range can be obtained by adjusting the content of comonomer units in the main chain of the polyacetal copolymer and/or the amount of the crystal nucleating agent.

<Difference between First Scan Melting Point and Second Scan Melting Point>
In the polyacetal powder of the present embodiment, the difference between the melting point of the first scan and the melting point of the second scan in differential scanning calorimetry (DSC) measurement is preferably 0° C. or more and 5° C. or less.
The melting point of polyacetal powder, which determines the width of the process window, which is important in additive manufacturing using resin powder as a modeling material, corresponds to the melting point of the first scan in DSC measurement. The fast scan melting point reflects the thermal history that the polyacetal powder was subjected to before it was manufactured.
On the other hand, the melting point of the second scan is the melting point of the resin itself, and the higher the better. Even if the melting point of the first scan is low, if the melting point of the second scan is high, it is possible to raise the melting point of the first scan by performing a post-treatment such as annealing, which will be described later.

If the difference between the melting point of the first scan and the melting point of the second scan is 0° C. or more and 5° C. or less, a wide process window close to the capabilities of the resin can be achieved.
From the same point of view, the difference between the melting point of the first scan and the melting point of the second scan in the DSC measurement of the polyacetal powder is more preferably 0° C. or higher and 4° C. or lower, and more preferably 0° C. or higher and 3° C. or lower. preferable.
In addition, the difference between the melting point of the first scan and the melting point of the second scan in DSC measurement is the melting point of the higher one to the lower melting point of the melting point of the first scan and the melting point of the second scan measured by DSC measurement. is subtracted from the value.
Polyacetal powder having a difference between the melting point of the first scan and the melting point of the second scan within the above range should be annealed (lower than the melting point and It can be obtained by holding for a certain period of time at a high temperature that allows crystal growth (for example, 120° C. for 1 hour) to allow crystal growth.

<Crystallization rate>
The polyacetal powder of the present embodiment preferably has a crystallization speed of 30 seconds or more and 50 seconds or less. If the crystallization rate is within the above range, it is possible to allow temperature unevenness in the molding area and between layers during molding, further suppress warpage during molding, and improve the molding accuracy of the resulting molded product. On the other hand, if the crystallization speed is less than 30 seconds, the crystallization of the resin is too fast, and temperature unevenness in the molding area or between layers during molding cannot be tolerated, and warping occurs during molding, resulting in molding. There is a risk that the molding accuracy of the product will decrease. If the crystallization speed is more than 50 seconds, the crystallization of the resin is too slow, and the precision of the boundary line between the modeled part and the non-modeled part is impaired, and the molding accuracy of the outline of the modeled product is reduced. There is fear.
From the same point of view, the crystallization speed of the polyacetal powder is more preferably 30 seconds or more and 47 seconds or less, and still more preferably 31 seconds or more and 45 seconds or less.
Here, the crystallization rate is determined by DSC measurement, in which the temperature of the once-melted polyacetal powder is rapidly lowered and held at a predetermined temperature near the crystallization start temperature, and an exothermic peak due to crystallization is observed. It is the time from the start until the maximum exothermic peak temperature (peak top temperature) is reached. More specifically, it can be measured by the method described in Examples below.

Methods for controlling the crystallization speed of the polyacetal powder include adjusting the comonomer content of the polyacetal copolymer, the amount of crystal nucleating agent added, and the like. Specifically, if the comonomer unit content is increased, the degree of crystallinity is lowered, so the crystallization rate tends to be slowed down. The crystallization rate can also be slowed by not including additives that act as crystal nucleating agents.
A polyacetal powder having a crystallization rate within the above range can be obtained by appropriately adjusting the content of the comonomer unit of the polyacetal copolymer, the presence or absence, type, and amount of the crystal nucleating agent, as described above. .

<MFR>
The polyacetal powder of the present embodiment preferably has a melt flow rate (MFR) of less than 70 g/10 minutes. If the polyacetal itself is of such a high molecular weight that the MFR is less than 70 g/10 minutes, shaped articles with excellent creep properties can be obtained.
Moreover, the polyacetal powder of the present embodiment preferably has a melt flow rate (MFR) of 2.5 g/10 minutes or more. When the MFR of the polyacetal powder is 2.5 g/10 minutes or more, the polyacetal flows when melted by a laser, so that the gaps between the particles can be filled more efficiently. A shaped article with fewer voids and excellent mechanical properties such as tensile strength is obtained.
From the same point of view, the MFR of the polyacetal powder is more preferably 5 g/10 min or more and 55 g/10 min or less, and still more preferably 20 g/10 min or more and 40 g/10 min or less.
Here, the MFR of the polyacetal powder is measured according to ISO1133 (condition D, temperature 190°C).
A polyacetal powder having an MFR within the above range can be obtained by adjusting the amount of the chain transfer agent added in the polymerization process of the polyacetal copolymer.

<Weight reduction rate>
The polyacetal powder of the present embodiment preferably has a weight loss rate of 5% or less when held for 1 hour at a temperature 10° C. lower than the melting point of the polyacetal powder. When the weight loss rate of the polyacetal powder is 5% or less, high thermal stability can be maintained. ) can be sufficiently suppressed.
From the same point of view, the weight loss rate of the polyacetal powder is more preferably 3% or less, and even more preferably 1% or less.
The weight loss rate of the polyacetal powder can be measured with a thermogravimetry device, and more specifically, it can be measured by the method described in Examples below.
A polyacetal powder having a weight reduction rate within the above range can be obtained by adjusting the types and/or amounts of components other than the polyacetal copolymer, such as hydrophobic silica, other resins described later, and various additives. can be done.

<Fixed Bulk Density/Loose Bulk Density>
In the polyacetal powder of the present embodiment, the value obtained by dividing the solid bulk density by the loose bulk density (solid bulk density/loose bulk density) is preferably 1.45 or less. If the value obtained by dividing the compacted bulk density by the loosened bulk density is within the above range, the polyacetal powder has excellent fluidity and stackability, and the generation of voids can be further suppressed. Physical properties can be further improved.
From the same point of view, the value obtained by dividing the solid bulk density by the loose bulk density is more preferably 1.40 or less, and even more preferably 1.36 or less.
The loose bulk density is the density measured when the powder is loosely packed (untapped) in a container of a given volume. The compacted bulk density is measured by repeatedly dropping (tapping) a container of a specified volume filled with powder from a constant height at a constant speed, and filling the container tightly until the bulk density of the powder in the container becomes almost constant. is. Specifically, the hardened bulk density and loose bulk density can be measured by the methods described in Examples below.

Polyacetal powder whose value obtained by dividing the solid bulk density by the loose bulk density is within the above range has a particle size distribution (for example, the ratio of particles whose diameter is twice or more than D50, which will be described later, and whose diameter is 0.2 times or less than D50 It can be obtained by controlling the ratio of the particles, etc.), the particle shape (for example, the ratio of the average short diameter to the long diameter, the average surface smoothness shape factor, etc., which will be described later).

<Particle size distribution>
In the polyacetal powder of the present embodiment, the proportion of particles whose diameter is two times or more the average particle diameter (D50) (for example, when D50 is 50 μm, particles whose diameter is 100 μm or more) is 10 vol% or less. is preferably When the above ratio is 10 vol% or less, it is possible to improve the fluidity of the polyacetal powder, increase the density of the molded product, improve the mechanical properties, and make the surface of the molded product smoother. Since the molding accuracy is improved, it becomes easier to manufacture precise parts.
From the same point of view, the proportion of particles having a diameter of at least twice the D50 in the polyacetal powder is more preferably 5 vol% or less, further preferably 3 vol% or less, and substantially 0 vol%. is even more preferable.

In the polyacetal powder of the present embodiment, the proportion of particles whose diameter is 0.2 times or less the average particle diameter (D50) (for example, when D50 is 50 μm, particles whose diameter is 10 μm or less) is 10 vol. % or less.
The laser beam used for shaping can have a very small irradiation spot size, so that the contour resolution of the shaped article can be very sharp. However, due to heat conduction from the point fused by heating with the laser, the powder outside the point scanned by the laser fuses directly to the part to be sintered, exceeding the laser scanning area, and thus exceeding the design dimensions. There is a risk that the powder will fuse beyond the limit. In addition, if the heat from the sintering remains, even if the powder is simply laid on top of the layer, the powder will sinter into the sintered portion of the previous layer, causing so-called inter-layer growth. There is fear. When such inter-layer growth occurs, the molding precision is lowered, and it becomes difficult to remove the unsintered powder from the finished product. In this regard, the present inventors have found that particles with a diameter of 0.2 times or less of D50 are particularly prone to fusion, affecting modeling accuracy, and that the ratio is 10 vol% or less. , It was found that the molding accuracy was improved and it became easy to manufacture precise parts. Furthermore, when the above ratio is 10 vol % or less, it is possible to improve the fluidity of the polyacetal powder, increase the density of the molded article, and improve the mechanical properties.
From the same point of view, the proportion of particles having a diameter of 0.2 times or less of D50 in the polyacetal powder is more preferably 5 vol% or less, further preferably 3 vol% or less, and substantially 0 vol%. is even more preferable.

In addition, the polyacetal powder of the present embodiment preferably has a D50 of 65 μm or less and a proportion of particles having a diameter of 0.2 times or less of the D50 of 3 vol % or less. When the D50 and the ratio of particles having a diameter of 0.2 times or less of the D50 are each within the above ranges, the layering property of the polyacetal powder can be further improved, and the mechanical properties of the resulting shaped article can be further improved. .
From the same point of view, it is more preferable that the proportion of particles having a D50 of 63 μm or less and a diameter of 0.2 times or less of the D50 be 2 vol % or less, more preferably 2 vol % or less of particles having a D50 of 62 μm or less and a diameter of 0.2 times the D50. It is more preferable that the proportion of particles having a ratio of 2 times or less is 1 vol % or less.

The proportion of particles with a diameter of 2 times or more D50 and the proportion of particles with a diameter of 0.2 times or less than D50 can be obtained from the particle size distribution and D50 measured by a laser diffraction/scattering method.
The polyacetal powder in which the ratio of particles having a diameter of 2 times or more D50 and/or the ratio of particles having a diameter of 0.2 times or less than D50 is within the above range is obtained by the pulverization step and/or classification of the method for producing polyacetal powder. It can be obtained by adjusting various conditions in the process.

<Particle shape>
From the viewpoint of fluidity, the particles constituting the polyacetal powder of the present embodiment are preferably spherical. In general, it is difficult to form fine particles directly from a resin such as polyacetal as a particle material, and in order to produce a powder using such a resin, an operation such as pulverizing pellets obtained by an arbitrary method is required. necessary. Therefore, at that time, the pulverization method and pulverization conditions are appropriately adjusted to directly obtain nearly spherical particles, or the pulverized powder is stirred while being heated in post-processing to remove the corners of the particles and round them. to obtain spherical particles.
The shape of the particles constituting the polyacetal powder of the present embodiment will be described in more detail below.

<<Ratio of average short diameter to long diameter of particles>>
It is preferable that the particles constituting the polyacetal powder of the present embodiment have an average minor axis to major axis ratio of 0.50 or more. When the average minor axis to major axis ratio of the particles is 0.50 or more, irregularities can be avoided, fluidity can be improved, and the powder can be laid out without gaps.
From the same point of view, the average minor axis to major axis ratio of the particles is more preferably 0.65 or more, and still more preferably 0.70 or more.
Here, the average short diameter to long diameter ratio refers to the ratio of the short diameter to the long diameter of the particles (short diameter / long diameter), and the average short diameter to long diameter ratio is the ratio of the short diameter to the long diameter of 100 arbitrarily selected particles. mean value. The minor axis and major axis respectively refer to the short side and long side of the circumscribing rectangle having the smallest circumscribing area on the boundary pixel of the particle.
The average minor axis to major axis ratio of the particles constituting the polyacetal powder can be measured by the method described in Examples below.
A polyacetal powder having particles with an average ratio of minor axis to major axis within the above range can be obtained by adjusting various conditions in the pulverization step and/or the classification step of the method for producing the polyacetal powder.

<<Average surface smoothness shape factor of particles>>
The particles constituting the polyacetal powder of the present embodiment preferably have an average surface smoothness shape factor of 0.80 or more. When the average surface smoothness shape factor of the particles is 0.80 or more, the particles have a more regular shape with less unevenness, and as a result, the fluidity of the particles can be further improved. From the same point of view, the average surface smoothness shape factor of the particles is more preferably 0.85 or more, more preferably 0.90 or more.
Here, the surface smoothness shape factor is an index representing the circularity or ellipticalness of particles, and is expressed by the following formula using the measured values of the minor axis, major axis and peripheral length of particles.
Surface smoothness shape factor = [π × (long diameter of particle + short diameter of particle)] / [2 × circumference length of particle]

The surface smoothness shape factor is 1 if the particle is a perfect circle and nearly 1 if the particle is a perfect ellipse. The surface smoothness shape factor becomes smaller than 1 when the shape is not smooth circle or ellipse but irregular shape, because the perimeter of the particle becomes longer.
The average surface smoothness shape factor is the average value of surface smoothness shape factors of 100 arbitrarily selected particles.
The average surface smoothness shape factor of the particles constituting the polyacetal powder can be measured by the method described in the examples below.
A polyacetal powder whose particles have an average surface smoothness shape factor within the above range can be obtained by adjusting various conditions in the pulverization step and/or classification step of the method for producing the polyacetal powder.

[Component Composition of Polyacetal Powder]
The component composition of the polyacetal powder of the present embodiment will be specifically described below.

<Polyacetal Copolymer>
The polyacetal powder of the present embodiment contains, as a resin component, a polyacetal copolymer containing 0.1 mol or more and 1.3 mol or less of comonomer units per 100 mol of oxymethylene units. The polyacetal powder of the present embodiment may contain only the polyacetal copolymer as a resin component, or may contain a small amount of a resin other than the polyacetal copolymer.
When the polyacetal powder of the present embodiment contains a small amount of other resin, it preferably contains 85% by mass or more, more preferably 90% by mass or more of the polyacetal copolymer with respect to 100% by mass of the content of the resin component. , more preferably 95% by mass or more, and still more preferably 97% by mass or more.
The polyacetal copolymer having the predetermined comonomer unit content may be a commercially available product or may be produced by the method for producing a polyacetal copolymer described below.

<<Method for producing polyacetal copolymer>>
A method for producing the polyacetal copolymer having the above-described predetermined comonomer unit content, which is contained as a resin in the polyacetal powder of the present embodiment, will be described in detail below.

(1) Polymerization step In the polymerization step, a known polymerization method (e.g., US Pat. No. 3,027,352, US Pat. No. 3,803,094, German Patent 1161421, German Patent 1495228) , German Patent Invention No. 1720358, German Patent Invention No. 3018898, JP-A-58-98322 and JP-A-7-70267), a chain transfer agent or A polymerization catalyst can be used to copolymerize the main monomer and the comonomer to give a crude polyacetal copolymer that is not end-stabilized.
As a material for the resin powder, the crude polyacetal copolymer itself can be used, but it is preferable to use a crude polyacetal copolymer whose terminals are stabilized by the terminal stabilization step described later.

1) Main Monomer Examples of the main monomer used in the production of the polyacetal copolymer include cyclic oligomers such as formaldehyde, its trimer trioxane, and its tetramer tetraoxane.
In the present embodiment, "main monomer" refers to a monomer unit containing 50% by mass or more of the total amount of monomers.

2) Comonomer The comonomer used in the production of the polyacetal copolymer is not particularly limited, but examples thereof include cyclic ether compounds having an oxyalkylene unit with 2 or more carbon atoms in the molecule.

The cyclic ether compound is not particularly limited, but examples include ethylene oxide, propylene oxide, 1,3-dioxolane, 1,3-propanediol formal, 1,4-butanediol formal, 1,5-pentanediol formal, 1, Examples include 6-hexanediol formal, diethylene glycol formal, 1,3,5-trioxepane, 1,3,6-trioxiocane, and mono- or di-glycidyl compounds capable of forming a branched or crosslinked structure in the molecule.
A cyclic ether compound may be used individually by 1 type, and may use 2 or more types together.

The main monomer and comonomer preferably contain as little impurities as possible, such as water, methanol, and formic acid, which have a polymerization terminating action and a chain transfer action during the polymerization reaction. By using main monomers and comonomers containing less of these impurities, unexpected chain transfer reactions can be avoided and polyacetal copolymers with the desired molecular weight can be obtained. Among them, the content of impurities that induce hydroxyl groups to polymer terminal groups is preferably 30 mass ppm or less, more preferably 10 mass ppm or less, and 3 mass ppm or less with respect to the total amount of monomers. is more preferred.

As a method for obtaining the main monomer and comonomer with a small amount of impurities, known methods (for example, for the main monomer, the methods described in JP-A-3-123777 and JP-A-7-33761; methods described in JP-A-49-62469 and JP-A-5-271217).

3) Chain Transfer Agent It is preferred to use a chain transfer agent in the production of the polyacetal copolymer. Examples of chain transfer agents include, but are not limited to, dialkyl acetals of formaldehyde and oligomers thereof; lower aliphatic alcohols such as methanol, ethanol, propanol, isopropanol and butanol. The alkyl group of the dialkyl acetal of formaldehyde is preferably a lower aliphatic alkyl group such as methyl, ethyl, propyl, isopropyl and butyl.

To obtain long chain branched polyacetal copolymers, it is preferred to use polyether polyols and alkylene oxide adducts of polyether polyols as chain transfer agents.

As the chain transfer agent, a polymer having one or more groups selected from the group consisting of hydroxyl groups, carboxyl groups, amino groups, ester groups and alkoxy groups may be used.

As the chain transfer agent, it is preferable to use one containing as little impurities as possible, such as water, methanol, formic acid, and acetic acid, which have a polymerization terminating action and a chain transfer action during the polymerization reaction. As a method for obtaining a chain transfer agent with few impurities, for example, dry nitrogen is bubbled through a generally available chain transfer agent having a moisture content exceeding a specified amount, and impurities are removed with an adsorbent such as activated carbon or zeolite. , purification methods, and the like.
A chain transfer agent may be used individually by 1 type, and may use 2 or more types together. In any case, the obtained polyacetal copolymer preferably has a small number of unstable terminals.

4) Polymerization Catalyst The polymerization catalyst used to produce the polyacetal copolymer is not particularly limited, but cationic active catalysts such as Lewis acids, protonic acids, and esters or anhydrides of protonic acids are preferred.

Examples of Lewis acids include, but are not limited to, boric acid, tin, titanium, phosphorus, arsenic and antimony halides. Specific examples include boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentafluoride, phosphorus pentachloride, antimony pentafluoride, and complex compounds or salts thereof.

Examples of protonic acids and protonic acid esters or anhydrides include, but are not limited to, perchloric acid, trifluoromethanesulfonic acid, perchloric acid-tertiary butyl ester, acetylperchlorate, and trimethyloxonium hexafluorophosphate. mentioned.

As the polymerization catalyst, boron trifluoride, boron trifluoride hydrate, and a coordination complex compound of an organic compound containing an oxygen atom or a sulfur atom and boron trifluoride are preferable. Boron di-n-butyl ether is more preferred.

As the polymerization catalyst, if necessary, a catalyst that reduces formation of terminal formate groups described in JP-A-05-05017 may be used in combination.

The amount of the polymerization catalyst used is preferably 1×10 ⁻⁶ to 1×10 ⁻³ mol, more preferably 5×10 ⁻⁶ to 1×10 ⁻⁴ mol, per 1 mol of the total amount of all monomers. When the amount of the polymerization catalyst used is within the above range, the reaction stability during polymerization and the thermal stability of the obtained polyacetal powder can be further improved.

After the polymerization process, the polymerization catalyst is deactivated by putting the polymer in an aqueous solution or an organic solvent solution containing a catalyst neutralizing and deactivating agent and stirring the slurry in a slurry state for several minutes to several hours. can be done.

Examples of the catalyst neutralizing and deactivating agent include, but are not limited to, ammonia, amines such as triethylamine and tri-n-butylamine; hydroxides of alkali metals and alkaline earth metals; inorganic acid salts; organic acid salts; is mentioned.
The catalyst neutralizing and deactivating agent may be used singly or in combination of two or more.

A method of deactivating the polymerization catalyst by contacting vapors such as ammonia and triethylamine with the polyacetal copolymer, and deactivating the catalyst by contacting at least one of hindered amines, triphenylphosphine and calcium hydroxide with a mixer. A method of allowing the

5) Reactor The reactor used for the production of the polyacetal copolymer is not particularly limited. A mixer etc. are mentioned.
The reactor preferably has a structure, such as a jacket, that can heat or cool the reaction mixture on the outer circumference of the barrel.

(2) Terminal stabilization step In the terminal stabilization step, a stable polyacetal copolymer can be obtained by decomposing and removing unstable terminal portions contained in the crude polyacetal copolymer obtained in the polymerization step. The terminally stabilized polyacetal is preferably dried prior to the production of the resin powder.

The method for decomposing and removing the unstable terminal portion contained in the crude polyacetal copolymer is not particularly limited. A method of melting the crude polyacetal copolymer in the presence of a removing agent to decompose and remove the unstable terminal portion can be mentioned.

When performing melt-kneading in terminal stabilization, it is preferable to replace the atmosphere with an inert gas or deaerate by single-stage or multi-stage venting in order to maintain quality and work environment. The temperature during melt-kneading is preferably above the melting point of the polyacetal copolymer and below 250°C.

1) Decomposing and removing agent Examples of the decomposing and removing agent used for decomposing and removing the unstable terminal portion contained in the crude polyoxymethylene copolymer include ammonia, aliphatic amines, water of alkali metals such as calcium hydroxide, and alkaline earth metals. Basic substances such as oxides; weak inorganic acid salts; and weak organic acid salts.
The decomposing and removing agent may be used singly or in combination of two or more.

<Other resins>
The polyacetal powder of the present embodiment may contain, as a resin, a small amount of a resin other than the polyacetal copolymer having the above-described specific comonomer unit content.
Specific examples of other resins that can be contained as a resin component are not particularly limited and can be appropriately selected depending on the purpose. Examples include polyamide 6, polyamide 11, polyamide 12, polyamide 66, polypropylene, polyether Crystalline resins such as ether ketone (PEEK), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT) are included. These may be used individually by 1 type, and may use 2 or more types together.

<Additive>
Further, the polyacetal powder in the present embodiment includes, for example, antioxidants, stabilizers (thermal stabilizers, etc.), weathering (light) agents, lubricants (release agents, etc.), crystal nucleating agents, inorganic and organic fillers. , a conductive agent, an antistatic agent, and an appearance modifier (pigment, dye, etc.). In particular, the polyacetal powder in this embodiment preferably contains an antioxidant and a heat stabilizer as additives. The antioxidant is preferably a hindered phenolic antioxidant, and the heat stabilizer preferably contains formaldehyde-reactive nitrogen.
An additive may be used individually by 1 type, and may use 2 or more types together.

<<Antioxidant>>
As the antioxidant, a hindered phenol-based antioxidant is preferably used.
Examples of hindered phenol antioxidants include n-octadecyl-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)-propionate, n-octadecyl-3-(3'- methyl-5'-t-butyl-4'-hydroxyphenyl)-propionate, n-tetradecyl-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)-propionate, 1,6- Hexanediol-bis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate], 1,4-butanediol-bis-[3-(3,5-di-t-butyl -4-hydroxyphenyl)-propionate], triethylene glycol-bis-[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)-propionate], pentaerythritol tetrakis[methylene-3-(3' , 5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane and the like. Preferably, triethylene glycol-bis-[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)-propionate] and pentaerythritol tetrakis[methylene-3-(3',5'-di-t -Butyl-4′-hydroxyphenyl)propionate]methane.
An antioxidant may be used individually by 1 type, and may use 2 or more types together.

<<Stabilizer>>
A heat stabilizer is preferably used as the stabilizer.
Examples of heat stabilizers include amino-substituted triazine compounds, adducts of amino-substituted triazine compounds and formaldehyde, condensates of amino-substituted triazine compounds and formaldehyde, urea, urea derivatives, hydrazine derivatives, imidazole compounds, imide compounds, and the like. .

Specific examples of amino-substituted triazine compounds include 2,4-diamino-sym-triazine, 2,4,6-triamino-sym-triazine, N-butylmelamine, N-phenylmelamine, N,N-diphenylmelamine, N , N-diamino-6-butyl, N-diallylmelamine, benzoguanamine (2,4-diamino-6-phenyl-sym-triazine), acetoguanamine (2,4-diamino-6-methyl-sym-triazine) -sym-triazine and the like.

Specific examples of adducts of amino-substituted triazine compounds and formaldehyde include N-methylolmelamine, N,N'-dimethylolmelamine and N,N',N''-trimethylolmelamine.

Specific examples of condensates of amino-substituted triazine compounds and formaldehyde include melamine/formaldehyde condensates.

Examples of urea derivatives include N-substituted urea, urea condensates, ethylene urea, hydantoin compounds, ureido compounds and the like. Specific examples of N-substituted urea include methyl urea substituted with a substituent such as an alkyl group, alkylenebis urea, and aryl-substituted urea. Specific examples of urea condensates include condensates of urea and formaldehyde. Specific examples of hydantoin compounds include hydantoin, 5,5-dimethylhydantoin, 5,5-diphenylhydantoin and the like. Specific examples of ureido compounds include allantoin.

Examples of hydrazine derivatives include hydrazide compounds. Specific examples of hydrazide compounds include dicarboxylic acid dihydrazide, and more specific examples include malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, pimelic acid dihydrazide, sperate dihydrazide, azelaic acid dihydrazide, Sebacic acid dihydrazide, dodecanedioic acid dihydrazide, isophthalic acid dihydrazide, phthalic acid dihydrazide, 2,6-naphthalenedicarbodihydrazide and the like.

Specific examples of imidazole compounds include imidazole, 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole and the like.

Specific examples of imide compounds include succinimide, glutarimide, and phthalimide.

Further, other stabilizers include, for example, alkali metal or alkaline earth metal hydroxides, inorganic acid salts, carboxylates, alkoxides, etc. Specific examples include sodium, potassium, magnesium, calcium or Hydroxides such as barium, carbonates, phosphates, silicates, borates, carboxylates, and layered double hydroxides of the above metals.

The carboxylic acid of the carboxylic acid salt is preferably a saturated or unsaturated aliphatic carboxylic acid having 10 to 36 carbon atoms, and these carboxylic acids may be substituted with hydroxyl groups. Specific examples of saturated or unsaturated aliphatic carboxylates include calcium dimyristate, calcium dipalmitate, calcium distearate, calcium (myristate-palmitate), calcium (myristate-stearate), (palmitic acid -stearate) calcium and the like, preferably calcium dipalmitate and calcium distearate.

Examples of layered double hydroxides include hydrotalcites represented by the following formula.
[(M ²⁺ ) _1−X (M ³⁺ ) _X (OH) ₂ ] ^X+ [(A ⁿ⁻ ) _X/n ·mH ₂ O] ^X−
(In the above formula, M ²⁺ is a divalent metal, M ³⁺ is a trivalent metal, A ^n- is an n-valent (n is an integer of 1 or more) anion, and X is in the range of 0<X≦0.33. , and m indicates a positive number.)
In the above formula, examples of M ²⁺ include Mg ²⁺ , Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Ni ²⁺ , Cu ²⁺ , Zn ²⁺ and the like, and examples of M ³⁺ include Al ³⁺ , Fe ³⁺ , Cr ³⁺ , Co ³⁺ , In ³⁺ and the like, and examples of A ⁿ⁻ include OH ⁻ , F ⁻ , Cl ⁻ , Br ⁻ , NO ³⁻ , CO ₃ ²⁻ , SO ₄ ²⁻ , Fe(CN) ₆ ³⁻ , CH ₃ COO ⁻ , oxalate ion, salicylate ion and the like. Preferred examples of A ^n- are OH ⁻ and CO ₃ ^2- .

Specific examples of hydrotalcites include natural hydrotalcite represented by Mg _0.75 Al _0.25 (OH) ₂ (CO ₃ ) _0.125 ·0.5H ₂ O, Mg _4.5 Al ₂ ( OH) ₁₃ CO _{3.3.5H 2} O, Mg _4.3 Al ₂ (OH) _12.6 CO ₃ , and synthetic hydrotalcites.
A stabilizer may be used individually by 1 type, and may use 2 or more types together.

<<weather (light) agent>>
Examples of weathering (light) agents include benzotriazole-based UV absorbers, oxalic acid anilide-based UV absorbers, and hindered amine-based light stabilizers.

Specific examples of benzotriazole-based UV absorbers include 2-(2'-hydroxy-5'-methyl-phenyl)benzotriazole, 2-(2'-hydroxy-3',5'-di-t-butyl- phenyl)benzotriazole, 2-[2′-hydroxy-3′,5′-bis(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole, 2-[2′-hydroxy-3′,5′- bis-(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole, 2-(2′-hydroxy-4′-octoxyphenyl)benzotriazole and the like.

Specific examples of oxalic acid arinide-based UV absorbers include 2-ethoxy-2'-ethyloxalic acid bisanilide, 2-ethoxy-5-t-butyl-2'-ethyloxalic acid bisanilide, 2- and ethoxy-3'-dodecyloxalic acid bisanilide. Preferably, 2-[2′-hydroxy-3′,5′-bis-(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole, 2-(2′-hydroxy-3′,5′-di -t-butyl-phenyl)benzotriazole and the like.

Specific examples of hindered amine light stabilizers include N,N',N'',N'''-tetrakis-(4,6-bis-(butyl-(N-methyl-2,2,6,6- Tetramethylpiperidin-4-yl)amino)-triazin-2-yl)-4,7-diazadecane-1,10-diamine, dibutylamine/1,3,5-triazine/N,N'-bis(2, Polycondensation product of 2,6,6-tetramethyl-4-piperidyl-1,6-hexamethylenediamine and N-(2,2,6,6-tetramethyl-4-piperidyl)butylamine, poly[{6- (1,1,3,3-tetramethylbutyl)amino-1,3,5-triazine-2,4-diyl}{(2,2,6,6-tetramethyl-4-piperidyl)imino}hexamethylene {(2,2,6,6-tetramethyl-4-piperidyl)imino}], condensate of dimethyl succinate and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidineethanol, decane 2 reaction product of acid bis(2,2,6,6-tetramethyl-1-(octyloxy)-4-piperidinyl) ester with 1,1-dimethylethyl hydroperoxide and octane, bis(1,2,2, 6,6-pentamethyl-4-piperidyl)[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl]butylmalonate, methyl-1,2,2,6,6-pentamethyl -4-piperidyl sebacate, bis(2,2,6,6-tetramethyl-4-piperidyl)-sebacate, 1,2,3,4-butanetetracarboxylic acid and 1,2,2,6,6- condensates of pentamethyl-4-piperidinol and β,β,β',β'-tetramethyl-3,9-[2,4,8,10-tetraoxaspiro(5,5)undecane]diethanol; Preferably bis(2,2,6,6-tetramethyl-4-piperidyl)-sebacate, bis-(N-methyl-2,2,6,6-tetramethyl-4-piperidinyl)sebacate, 1, 2,3,4-butanetetracarboxylic acid and 1,2,2,6,6-pentamethyl-4-piperidinol and β,β,β',β'-tetramethyl-3,9-[2,4,8 , 10-tetraoxaspiro(5,5)undecane]diethanol.
The weathering (light) resistance agents may be used singly or in combination of two or more.

<<Lubricant>>
Examples of lubricants include alcohols, fatty acids and fatty acid esters thereof, polyoxyalkylene glycols, olefin compounds having an average degree of polymerization of 10 to 500, and silicones.
Lubricants may be used singly or in combination of two or more.

<<Crystal nucleating agent>>
Crystal nucleating agents include, for example, talc, silica, quartz powder, glass powder, calcium silicate, aluminum silicate, kaolin, pyrophyllite, clay, diatomaceous earth, silicates such as wollastonite; Metal oxides; metal sulfates such as calcium sulfate and barium sulfate; carbonates such as calcium carbonate, magnesium carbonate and dolomite; It may be a finely divided solid of crystal nucleating inorganic material. Preferred crystal nucleating agents are boron nitride and talc.

A known surface treatment agent may be used for the crystal nucleating inorganic material in order to improve affinity and dispersibility with a resin component such as polyacetal resin.

Examples of surface treatment agents include silane coupling agents such as aminosilane and epoxysilane, titanate coupling agents, fatty acids (saturated fatty acids, unsaturated fatty acids), alicyclic carboxylic acids, resin acids, and metal soaps. be done. The amount of the surface treatment agent to be added is preferably 3% by mass or less, more preferably 2% by mass or less, relative to the crystal nucleation inorganic material.
The crystal nucleating agent may be used singly or in combination of two or more.

<<Inorganic/organic filler>>
Examples of inorganic fillers include metal powders (aluminum, stainless steel, nickel, silver, etc.), oxides (silicon oxide, iron oxide, alumina, titanium oxide, zinc oxide, etc.), hydroxides (aluminum hydroxide, water magnesium oxide, calcium hydroxide, etc.), silicates (talc, mica, clay, bentonite, etc.), carbonates (calcium carbonate, magnesium carbonate, hydrotalcite, etc.), carbon-based substances (carbon black, graphite, carbon fiber, etc.) , sulfate, boron nitride, silicon nitride, and the like.

Organic fillers include, for example, natural fillers (linters, wood, rice husks, silk, leather, etc.) and synthetic fillers (aramid, Teflon (registered trademark), viscose, etc.).

Among them, it is preferable to select from among inorganic and organic fillers that can be added to conventional resins such as polyacetal resins. In other words, if fillers with strong acidity or alkalinity are added to resins such as polyacetal resins, there is a possibility of lowering the stability. It is preferable to choose from among them.

The shape of the inorganic/organic filler may be powdery, scale-like, plate-like, needle-like, spherical, fibrous, or tetrapod-like, and is not particularly limited.

As the inorganic/organic filler, a known surface treatment agent may be used in order to improve affinity with the resin. Examples of surface treatment agents include silane coupling agents such as aminosilane and epoxysilane, titanate coupling agents, fatty acids (saturated fatty acids, unsaturated fatty acids), alicyclic carboxylic acids and resin acids, metal soaps, and resins. and the like. The amount of the surface treatment agent to be added is preferably 3% by mass or less, more preferably 2% by mass or less, relative to the inorganic/organic filler.

The inorganic/organic fillers may be used singly or in combination of two or more.

<<Conductive Agent/Antistatic Agent>>
Conductive agents include, for example, conductive carbon black, carbon nanotubes or nanofibers or nanoparticles, metal powders or fibers, and the like.

Examples of antistatic agents include aliphatic polyethers (excluding compounds with fatty acid ester ends), aliphatic polyethers with fatty acid ester ends, and free hydroxyl groups obtained from fatty acids and polyhydric alcohols. polyhydric alcohol fatty acid ester, glycerin mono-fatty acid ester boric acid ester, ethylene oxide adduct of amine compound, basic carbonate or its anion exchanger as a base, polyalkylene polyol or alkali metal salt dissolved polyalkylene Examples include antistatic agents containing polyols.
The conductive agent/antistatic agent may be used singly or in combination of two or more.

<<Appearance improver>>
Appearance improvers include, for example, inorganic pigments, organic pigments, metallic pigments, fluorescent pigments, and various dyes.

Inorganic pigments are those commonly used for coloring resins, such as zinc sulfide, titanium oxide, barium sulfate, titanium yellow, cobalt blue, combustion pigments, carbonates, phosphates, and acetic acid. Examples include salt, carbon black, acetylene black, and lamp black.

Examples of organic pigments include condensed ouzo, inone, phlotacyanine, monoazo, diazo, polyazo, anthraquinone, heterocyclic, pennon, quinacridone, thioindico, berylene, dioxazine, Pigments such as phthalocyanine-based pigments can be used.
An appearance improver may be used individually by 1 type, and may use 2 or more types together.

[Method for producing polyacetal powder]
The polyacetal powder in the present embodiment can be produced by, for example, a step of granulating a resin material such as polyacetal (granulation step), a step of pulverizing the granulated resin material (pulverization step), and sieving the pulverized resin material. A step of obtaining a powder having desired powder properties (classification step) and a step of mixing with hydrophobic silica to obtain a polyacetal powder with hydrophobic silica attached to the surface of particles (hydrophobic silica mixing step) are performed. It can be manufactured by
Each step will be specifically described below.

[[Granulation process]]
In the granulation step, for example, the resin material such as polyacetal obtained as described above is melt-kneaded using a kneader, roll mill, single-screw extruder, twin-screw extruder, multi-screw extruder, etc. to form pellets. can be obtained.

Here, when the resin material is melt-kneaded, it is preferable to replace the atmosphere with an inert gas or perform degassing by single-stage or multi-stage venting in order to maintain quality and work environment. The temperature during melt-kneading is preferably above the melting point of the resin material and below 250° C., and may be 20 to 50° C. higher than the melting point of the resin material.

In addition, in the granulation step, the above-described additives may be added to the resin material and mixed, and the resulting resin composition may be melt-kneaded. At this time, the additive may be divided into several times and mixed with the resin material.
Alternatively, in the granulation step, in order to enhance the dispersibility of the additive in the resin material, a part or all of the resin material is optionally pulverized before mixing, and a Henschel mixer, tumbler, V-shaped blender or the like is used to The amide compound and the above-described additives may be added and mixed, and the resulting resin composition may be melt-kneaded. In this case, a spreading agent may be used to further enhance dispersibility.

-Amide compound-
For example, when polyacetal is used as the resin material, the above-mentioned amide compound can react with formaldehyde generated by decomposition or the like from the resin composition, and the formaldehyde is oxidized to form formic acid or the like to promote decomposition. can be suppressed.

As the amide compound, an amide compound having a melting point of 300° C. or higher is preferable. An amide compound having a melting point of 300° C. or higher does not melt during molding, so it is possible to suppress the generation of foreign matter.

Examples of amide compounds having a melting point of 300° C. or higher include polyamide resins such as nylon 4-6, nylon 6, nylon 6-6, nylon 6-10, nylon 6-12 and nylon 12.

Other examples of amide compounds include acrylamide and its derivatives, copolymers of acrylamide and its derivatives with other vinyl monomers, polyvalent carboxylic acid amides such as isophthalic acid diamide, and anthranilamide.

The amount of the amide compound added is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, or 0.5 parts by mass with respect to 100 parts by mass of the resin material such as polyacetal. The following are preferred. When the amount of the amide compound added is within the above range, it is possible to obtain a molded product with a good appearance, a small amount of foreign matter, and high grease resistance.

-Additive-
The content of the additive in the resin composition is preferably 100 parts by mass or less with respect to 100 parts by mass of the resin material such as polyacetal. By setting the content of the additive to 100 parts by mass or less, changes in physical properties can be suppressed even if the additive is retained at high temperature for a long time in the molding machine, recycling is possible, and the mechanical strength of the resulting molded product is improved. There is a tendency. From the same point of view, the content of the additive in the resin composition is more preferably 50 parts by mass or less, still more preferably 25 parts by mass or less with respect to 100 parts by mass of the resin material.

-Spreading agent-
Examples of the spreading agent include, but are not limited to, aliphatic hydrocarbons, aromatic hydrocarbons, modified products thereof, fatty acid esters of polyols, and the like.
The spreading agent may be used singly or in combination of two or more such as aliphatic hydrocarbons, aromatic hydrocarbons and mixtures of modified products thereof.

[[Pulverization process]]
In the pulverization step, the pellets obtained in the granulation step can be pulverized to obtain powder. The pulverization method is not particularly limited, and examples thereof include ordinary temperature pulverization, freeze pulverization, and the like. Moreover, after dissolving the pellet obtained by the granulation process in a solvent, the method of precipitating is also mentioned.

[[Classification process]]
In the classification step, by sieving the powder obtained in the pulverization step, the average particle size (D50), the ratio of particles with a diameter of 2 times or more of D50, and the particles with a diameter of 0.5 times or less than D50 By appropriately adjusting the ratio of and the like, the polyacetal powder of the present embodiment can be finally obtained.

In addition, in the method for producing the polyacetal powder of the present embodiment, the degree of crystallinity may optionally be adjusted by heat treatment before or after the classification step.

[[Hydrophobic silica mixing process]]
In the method for producing the polyacetal powder of the present embodiment, the classifying step is followed by the hydrophobic silica mixing step.
In the hydrophobic silica mixing step, by mixing the polyacetal powder obtained in the classification step with hydrophobic silica, the hydrophobic silica adheres to the surface of the particles that make up the polyacetal powder, improving the fluidity of the polyacetal powder. can be made
The mixing method is not particularly limited, and may be mixed using an ordinary powder mixer such as a Nauta mixer, a Henschel mixer, a tumbler mixer, or the like. Alternatively, the mixture may be mixed by manual stirring or the like.

[Applications of polyacetal powder]
The polyacetal powder of the present embodiment is suitable for use in additive manufacturing techniques, and particularly suitable for selective laser sintering. Since the polyacetal powder in the present embodiment has optimized powder properties and particle properties, it can be uniformly laid out to a thickness of, for example, about 100 μm. Therefore, by performing additive manufacturing using this resin powder in the selective laser sintering method, it is possible to obtain a shaped article having high mechanical properties in which minute voids are sufficiently suppressed. Such molded products are widely applicable to mechanical parts such as automobile parts, electric/electronic parts, industrial parts, and medical parts.

[How to use polyacetal powder]
A method of using the polyacetal powder of the present embodiment includes using the polyacetal powder of the present embodiment described above for additive manufacturing.
There are no particular restrictions on the additive manufacturing technology used for additive manufacturing, such as the selective laser sintering method, or applying a heat-absorbing agent to the desired part of the molding area covered with resin powder and applying heat from the top surface with a heater. Modeling, heat melting method, and the like can be mentioned.
In the method of using the polyacetal powder of the present embodiment, specific usage conditions are not particularly limited, and are appropriately selected according to the additive manufacturing technology to be used, the application and purpose of the molded product, etc., and the polyacetal powder is used. can be used.

[Additive manufacturing method]
The additive manufacturing method of this embodiment includes using the polyacetal powder of this embodiment described above as a modeling material. According to the additive manufacturing method of the present embodiment, since the polyacetal powder of the present embodiment described above is used, it is possible to obtain a molded article having high molding accuracy and mechanical properties.
The additive manufacturing technique used in the additive manufacturing method of the present embodiment is not particularly limited as long as the resin powder can be used as a modeling material. be done.
Specific modeling conditions and the like in the additive manufacturing method of the present embodiment are not particularly limited, and can be appropriately selected and modeled according to the additive manufacturing technique to be used and the application of the modeled product.

EXAMPLES The present invention will be described below with reference to examples, but the present embodiment is not limited to the following examples.

[Measuring method]
Regarding the polyacetal powders obtained in Examples and Comparative Examples, the average particle size (D50), the ratio of particles having a diameter of 2 times or more of D50, the ratio of particles having a diameter of 0.2 times or less of D50, the comonomer unit Content rate, melting point, crystallization start temperature, melting point difference between first scan and second scan in DSC measurement, crystallization speed, weight loss rate, melt flow rate, value of solid bulk density/loose bulk density, average short diameter and long diameter of particles ratio, the mean surface smoothness shape factor of the particles was measured. Each measurement method is as follows.

[[Particle size, etc.]]
Using Mastersizer manufactured by Malvern, the average particle size (D50), the ratio of particles with a diameter of 2 times or more D50, and the ratio of particles with a diameter of 0.2 times or less of D50 were measured.

[[Comonomer unit content]]
The comonomer unit content was determined by the following procedure using the ¹ H-NMR method.
That is, the polyacetal copolymer was dissolved in hexafluoroisopropanol to a concentration of 1.5% by mass over 24 hours, and this solution was used for ¹ H-NMR analysis (magnetic field strength: 400 MHz, measurement solvent: hexafluoroisopropanol , reference material: tetramethylsilane, temperature 55° C., number of times of accumulation 500).
The integral value of the attributed peak of the oxymethylene unit obtained by ¹ H-NMR analysis (δ (ppm): 4.8 to 5.4) and the attributed peak of the oxyalkylene unit having 2 or more carbon atoms which is the comonomer unit ( δ (ppm): For example, when the comonomer unit is an oxyethylene unit, the ratio of the comonomer unit (bmol) to the oxymethylene unit (a = 100 mol) from the ratio with the integrated value of 3.6 to 4.0) A content rate (b/a: mol/100 mol) was determined.

[[Melting point, crystallization start temperature, crystallization rate]]
Using a differential scanning calorimeter (manufactured by Parkin Elmer: DSC-2C), the first scan melting point, second scan melting point, crystallization start temperature, and the crystallization rate were determined.
1: 3 mg of polyacetal powder to be measured is enclosed in an aluminum pan for measurement, and placed at a predetermined position in a heating furnace of a differential scanning calorimeter.
2: The temperature is raised at a heating rate of 10°C/min until it reaches 220°C, and the highest point of the endothermic peak (melting point peak) at that time is defined as the "first scan melting point".
3: Hold the temperature for 2 minutes after reaching 220°C.
4: Then, the temperature is lowered at a rate of 80°C/min until it reaches 149°C, and the temperature is maintained for 10 minutes to measure the exothermic peak due to crystallization.
5: The time from the start of holding at 149° C. to the highest point of the exothermic peak is defined as the “crystallization rate”. It should be noted that the longer the time, the slower the crystallization rate.
6: Next, the temperature is raised at a rate of temperature increase of 20°C/min until it reaches 210°C, and the temperature is maintained for 5 minutes.
7: Then, the temperature is lowered from 210°C to 100°C at a rate of 20°C/min. The temperature (high temperature side) at the starting point of the exothermic peak (crystallization peak) at this time is referred to as the "crystallization start temperature".
8: Next, the temperature was raised from 100°C to 210°C at a heating rate of 20°C/min, and the maximum endothermic peak (melting point peak) at that time was taken as the "second scan melting point".

[Melting point difference between first scan and second scan in DSC measurement]
By subtracting the melting point of the first scan from the melting point of the second scan determined by DSC measurement as described above, the melting point difference between the first scan and the second scan (referred to as "1st2nd melting point difference" in Table 1) was determined.

[[Weight reduction rate]]
Using a thermogravimetry device (manufactured by Perkin Elmer, trade name “Pyris1 TGA”), sample weight: 5 mg, air flow rate: 10 mL / min, temperature increase rate: 10 ° C. / min from room temperature, polyacetal to be measured The temperature was raised to a temperature 10° C. lower than the melting point of the powder, and the weight loss rate was measured after holding at that temperature for 1 hour.

[[Melt Flow Rate (MFR)]]
Using "P-111" manufactured by Toyo Seiki Seisakusho, the MFR was measured in accordance with ISO 1133 (Condition D) under conditions of 190°C and a load of 2.16 kg.

[[Fixed Bulk Density/Loose Bulk Density]]
A powder tester PT-X type (manufactured by Hosokawa Micron) was used to measure the hardened bulk density and loose bulk density.
Specifically, the sample supply device is vibrated until the powder piles up in a stainless steel 100 cm ³ cylindrical container to flow down the powder, and the excess powder on the container is scraped off using a blade for measurement. The resulting density was defined as loose bulk density (g/cm ³ ).
In addition, a stainless steel 100 cm ³ cylindrical container is capped, the sample supply device is vibrated to let the powder flow down, and tapping is performed with a stroke length (tapping height) of 18 mm, a tapping speed of 60 times/minute, and a tapping frequency of 180 times. went. After that, the density measured by scraping off excess powder on the container with a blade was taken as the compacted bulk density (g/cm ³ ).
The hardened bulk density measurement, measured as described above, was divided by the loosed bulk density measurement to obtain the hardened bulk density/loosen bulk density value.

[[Average minor axis major axis ratio of particles]]
Images taken using a digital microscope (manufactured by Keyence Corporation, "VH-7000 type", using "VH-501 lens") are saved in 1360 x 1024 pixel, TIFF file format, and image processing analysis software (stock "Image Hyper II" manufactured by Company Digimo) was used to determine the average value of the short diameter/long diameter of 100 particles.

[[Average surface smoothness shape factor of particles]]
Save the image taken using the above-mentioned digital microscope in 1360 x 1024 pixels in TIFF file format, and use the above-mentioned image processing analysis software to measure the major axis, minor axis, and perimeter of 100 particles. Then, each surface smoothness shape factor was determined according to the following formula, and the average value was determined.
Surface smoothness shape factor = [π × (long diameter of particle + short diameter of particle)] / [2 × circumference length of particle]

[Productivity of powder]
For Examples and Comparative Examples, the yield (%) after the pulverization/classification process was calculated by the following formula based on the mass of the pellets subjected to the pulverization/classification process and the mass of the powder obtained in the pulverization/classification process. rice field.
Yield = (mass of powder obtained in pulverization/classification process/mass of pellets subjected to pulverization/classification process) x 100
The obtained yield (%) was evaluated according to the following evaluation criteria. The higher the calculated yield (%), the more excellent the productivity.
(Evaluation criteria)
○: Yield is 50% or more ×: Yield is less than 50%

[Evaluation of powder stackability and shaped product]
Using the polyacetal powders obtained in Examples and Comparative Examples, strips each having a length of 5 cm, a width of 1 cm, and a thickness of 4 mm in the stacking direction were formed using a commercially available selective laser sintering forming apparatus. Then, the stackability of the polyacetal powder, the warpage during molding, and the mechanical properties of the molded product were evaluated according to the following evaluation methods.

[[Lamination property of polyacetal powder at high temperature]]
The powder supply section is heated to 155°C and the modeling area is heated to 160°C to carry out modeling, and the surface condition of the modeling area during modeling is visually observed, and the lamination property (recoating property) of the polyacetal powder is measured according to the following criteria. evaluated.
⊚: No agglomerates were observed during powder lamination, and the powder was evenly laminated even in the forming portion during forming.
◯: No agglomerates were observed during powder lamination, and the powder was partially unevenly laminated on the part being formed.
Δ: About 1 to 2 aggregates.
x: Agglomerates are present on the entire surface, and modeling is not possible.
The more uniform the layering of the powder on the modeled portion without the presence of agglomerates, the better the stackability at high temperatures.

[[Powder placed on molding part]]
The strip-shaped molded portion was visually observed during molding, and the ability of the polyacetal powder to adhere to the molded portion was evaluated according to the following criteria.
⊚: The powder is evenly layered on the melted portion irradiated with the laser.
◯: In the early stage of molding, the powder was not sufficiently applied to the peripheral portion of the strip, but the powder was gradually and evenly laminated.
Δ: Non-uniform powder loading on the periphery of the strip.
x: Powder loading was uneven on the entire melted portion irradiated with the laser.
The more uniformly the powder is layered over the entire melted portion irradiated with the laser, the more excellent the powder can be placed on the modeled portion.

[[Mechanical properties of modeled products]]
The shaped test piece was subjected to a tensile test at a tensile speed of 5 mm/min according to ISO527-1 to measure the tensile breaking stress. The measured values obtained were evaluated according to the following criteria.
A: 40 MPa or more B: 20 MPa or more and less than 40 MPa B: 10 MPa or more and less than 20 MPa X: less than 10 MPa The higher the value of the tensile breaking stress, the better the mechanical properties.

(Example 1)
<Preparation of polyacetal copolymer>
(1) Polymerization step A jacketed self-cleaning twin-screw paddle type continuous mixing reactor (screw diameter: 3 inches, ratio of length to diameter (L/D) = 10) through which a heat medium can pass was heated at 80°C. adjusted to The flow rate is adjusted within the range of 3750 g/hr for trioxane as the main monomer, 5 to 150 g/hr for 1,3-dioxolane as the comonomer, and 2.0 to 8.0 g/hr for methylal as the chain transfer agent. , was continuously fed to a continuous mixing reactor. As a polymerization catalyst, a 1% by mass cyclohexane solution of boron trifluoride di-n-butyl etherate was added to the continuous mixing reactor so that the amount of the catalyst was 2.0×10 ⁻⁵ mol per 1 mol of trioxane. Polymerization was carried out to obtain polymerized flakes. The addition amount of the chain transfer agent was adjusted so that the polyacetal powder obtained had a desired melt flow rate.
After the obtained polymerized flakes were pulverized, the pulverized product was put into a 1% by mass aqueous solution of triethylamine as a catalyst neutralizing and deactivating agent and stirred to deactivate the polymerization catalyst. After that, the 1% by mass triethylamine aqueous solution containing the polymerized flakes was sequentially filtered, washed and dried to obtain a crude polyacetal copolymer.

(2) Terminal stabilization step Add 0.5 parts by mass of water to 100 parts by mass of the crude polyacetal copolymer obtained, set the extruder conditions to a temperature of 200 ° C. and a residence time of 7 minutes, and the polyacetal copolymer becomes unstable. The terminal portion (-(OCH ₂ ) _n -OH) was decomposed and removed.

<Granulation process>
The filtered polyacetal copolymer was dried for 3 hours in a gear oven (set at 80°C) in a nitrogen atmosphere while confirming a product temperature of 80°C.
100 parts by mass of this polyacetal copolymer (a-1), 0.2 parts by mass of an amide compound (H-3 (manufactured by Asahi Kasei Finechem Co., Ltd.)), and an antioxidant (Irganox 245 (manufactured by Ciba Specialty Chemicals Co., Ltd.)) 0.2 parts by mass were mixed with a Henschel mixer for 1 minute. After that, the obtained polyacetal composition was placed in a screw type twin screw extruder with a vacuum vent set at 200 ° C. (BT-30, L / D = 44, L: Twin screw extruder manufactured by Rastic Engineering Laboratory Co., Ltd. The distance (m) from the raw material supply port to the discharge port (D: inner diameter (m) of the twin-screw extruder) was set at a screw rotation speed of 100 rpm and melt-kneaded at 24 amperes to obtain pellets.
The obtained pellets were dried at 80° C. for 2 hours in a gear oven (GPH-102, manufactured by Espec Co., Ltd.) before measurement. Melt flow rate was measured using dried pellets. Table 1 shows the results.

<Pulverization/classification process>
While freezing the obtained pellets with liquid nitrogen, they were freeze-pulverized with a Rinrex Mill (registered trademark) (manufactured by Hosokawa Micron Corporation), sieved, and powder characteristics were appropriately adjusted to obtain a powder.

<Hydrophobic silica mixing step>
To 10 kg of the obtained powder, 30 g of dry silica hydrophobized with dimethyldichlorosilane (Aerosil (registered trademark) R974 manufactured by Nippon Aerosil Co., Ltd.) was added as hydrophobic silica, and mixed with a tumbler mixer to remove the surface of the particles. A polyacetal powder to which hydrophobic silica was attached was obtained.
The obtained polyacetal powder was measured and evaluated as described above. Table 1 shows the results.

(Examples 2 to 11, Comparative Examples 1 to 8)
Presence or absence of addition of hydrophobic silica, type of hydrophobic silica, amount added, average primary particle size, BET specific surface area, type of surface modifier, comonomer addition flow rate in preparation of polyacetal copolymer, classification conditions in preparation of polyacetal powder, Polyacetal powders of Examples 2 to 11 and Comparative Examples 1 to 8 shown in Table 1 were obtained in the same manner as in Example 1, except that the presence or absence of heat treatment was appropriately adjusted.
As the hydrophobic silica, in Examples 2, 4 to 6 and 9 to 11 and Comparative Examples 3 to 7, the same one as in Example 1 was used. Rheolosil (registered trademark) DM20-S manufactured by Tosoh Silica Co., Ltd.), dry silica hydrophobized with trimethylchlorosilane (HM-20L manufactured by Tokuyama Co., Ltd.) in Example 7, and wet silica (SS manufactured by Tosoh Silica Co., Ltd.) in Example 8. −20, the average particle size of the secondary aggregates was 2 μm). In Comparative Example 1, silica was not used, and in Comparative Example 2, hydrophilic silica (QS-102 manufactured by Tokuyama Co., Ltd.) was used. In Comparative Example 8, a polyacetal powder was obtained by pulverizing and classifying pellets obtained by melt-kneading the same hydrophobic silica as in Example 1 in the granulation step without performing the hydrophobic silica mixing step.
Each polyacetal powder obtained was measured and evaluated as described above. Table 1 shows the results.

Secondary electron images of the surfaces of the polyacetal powders obtained in Example 1, Comparative Example 1, and Comparative Example 2 observed by SEM (accelerating voltage: 0.5 kV, magnification: 4000 times (left) and 8000 times (right) )) are shown as FIGS. 1 to 3, respectively. A large number of fine hydrophobic silica particles were uniformly attached to the surface of the polyacetal powder obtained in Example 1 (Fig. 1). On the other hand, in Comparative Example 1, in which silica was not used, and in Comparative Example 2, in which hydrophilic silica was used instead of hydrophobic silica, adhesion of fine silica particles to the surface of the polyacetal powder was not observed (FIGS. 2 and 3). ).

As shown in Table 1, the polyacetal powders according to the examples are excellent in lamination property at high temperature and powder placement on the molded part, and the molded products formed by the selective laser sintering method using the polyacetal powders according to the examples are , have high mechanical properties and high molding accuracy.

According to the present invention, it is possible to provide a polyacetal powder that can be used for additive manufacturing technology and that can obtain a molded article having high mechanical properties, and that has excellent lamination properties at high temperatures and excellent powder placement on molded parts. can be done. Further, according to the present invention, it is possible to provide a method of using the laminating powder described above and an additive manufacturing method capable of obtaining a shaped article having high mechanical properties.

Claims (17)

The average particle diameter (D50) is 30 μm or more and 70 μm or less,
Contains 0.05% by mass or more and 1.0% by mass or less of hydrophobic silica ,
A polyacetal powder, wherein the value obtained by dividing the solid bulk density by the loose bulk density (solid bulk density/loose bulk density) is 1.45 or less . 2. The polyacetal powder according to claim 1, wherein said hydrophobic silica is dry process silica. 3. The polyacetal powder according to claim 1, wherein the hydrophobic silica has an average primary particle size of 1 nm or more and 20 nm or less. The polyacetal powder according to any one of claims 1 to 3, wherein the hydrophobic silica has a BET specific surface area of 50 m ² /g or more and 300 m ² /g or less. The polyacetal powder according to any one of claims 1 to 4, wherein the hydrophobic silica is hydrophobized with one or more selected from the group consisting of dimethyldichlorosilane, trimethylchlorosilane, and silicone oil. The polyacetal powder according to any one of claims 1 to 5, which has a weight loss rate of 5% or less when held for 1 hour at a temperature 10°C lower than the melting point of the polyacetal powder. 7. The polyacetal powder according to any one of claims 1 to 6 , wherein the proportion of particles whose diameter is twice or more the D50 is 10 vol% or less. The polyacetal powder according to any one of claims 1 to 7 , wherein the proportion of particles having a diameter of 0.2 times or less of D50 is 10 vol% or less. The polyacetal powder according to any one of claims 1 to 8 , wherein the D50 is 65 µm or less, and the proportion of particles having a diameter of 0.2 times or less of the D50 is 3 vol% or less. A polyacetal copolymer having 0.1 mol or more and 1.3 mol or less of comonomer units per 100 mol of oxymethylene units,
a melting point of 167° C. or higher and 178° C. or lower;
a crystallization start temperature of 140° C. or higher and 152° C. or lower;
The polyacetal powder according to any one of claims 1-9 . The polyacetal powder according to any one of claims 1 to 10 , wherein the difference between the melting point of the first scan and the melting point of the second scan in differential scanning calorimetry (DSC) measurement is 0°C or more and 5°C or less. The polyacetal powder according to any one of claims 1 to 11 , which has a crystallization speed of 30 seconds or more and 50 seconds or less. 13. The polyacetal powder according to any one of claims 1 to 12 , wherein the particles constituting the polyacetal powder have an average minor axis to major axis ratio of 0.50 or more. The polyacetal powder according to any one of claims 1 to 13 , wherein the average surface smoothness shape factor of particles constituting said polyacetal powder is 0.80 or more. Polyacetal powder according to any one of claims 1 to 14 , which is used for additive manufacturing. A method of using the polyacetal powder according to any one of claims 1 to 15 , characterized in that it is used for additive manufacturing. An additive manufacturing method, characterized in that the polyacetal powder according to any one of claims 1 to 15 is used as a building material.

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