JP7076301B2 - Polyacetal powder, its usage, and additional manufacturing method - Google Patents

Description

The present invention relates to polyacetal powder, a method for using the same, and an additional production method.

In recent years, additional manufacturing techniques have become widespread as one of the techniques for producing shaped products having a complicated structure.

Since this additional manufacturing technique is a method that does not use a mold, there is a merit that it can be prototyped in a short period of time, and it is often used for trial production for function confirmation, for example. In addition, there is an increasing need for application not only to prototypes but also to direct production of small-lot, high-mix products. Against this background, among the additional manufacturing techniques, 3D printers that use powdered resin as a modeling material, such as the selective laser sintering method, are attracting particular attention.

In the selective laser sintering method, a powder material is laid on a modeling table with a roller or a blade, the powder material is selectively irradiated with a laser, heated and sintered, and these are repeated in sequence to form a laminated product. It is a method of making (Patent Document 1). Since the selective laser sintering method is a method that can use the thermoplastic resin currently used for industrial parts made of resin, it can be used for other additional manufacturing techniques (liquid tank photopolymerization method, material injection method, etc.). In comparison, it is superior in terms of strength, reliability, and dimensional stability of the modeled product. Another feature of the selective laser sintering method is that it enables the production of a modeled product having a considerably complicated shape and contour, which cannot be achieved by a conventional method 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 a selective laser sintering method.

US Pat. No. 4,863,538 International Publication No. 1996/006881 International Publication No. 2011/051250

In addition molding using polyacetal powder in the selective laser sintering method, the stackability of the powder at high temperature, the powder placement on the molding site, and the mechanical properties of the modeled product are very important.
However, in Patent Documents 1 to 3, these have not been sufficiently studied, and there is still room for improvement in terms of improving the stackability of polyacetal powder at high temperature, the powder being placed on the modeled portion, and the mechanical properties of the modeled product. was there.

Therefore, the problem to be solved by the present invention is excellent in stackability at high temperature and powder placement on the modeled portion, which can be used in the additive manufacturing technique and can obtain a modeled product having high mechanical properties. It is also to provide polyacetal powder. Further, an object to be solved by the present invention is to provide a method of using the above-mentioned polyacetal powder and an additional manufacturing method capable of obtaining a modeled product having high mechanical properties.

As a result of diligent research to solve the above problems, the present inventors have excellent polyacetal powder having specific powder characteristics and containing hydrophobic silica, which is excellent in stackability at high temperature and powder placement on a molded part. , And have found that it is possible to manufacture a modeled product having high mechanical properties, which can be used in an additional manufacturing technique, and completed the present invention.

That is, the present invention is as follows.
[1]
The average particle size (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 that.
[2]
The polyacetal powder according to [1], wherein the hydrophobic silica is drywall silica.
[3]
The polyacetal powder according to [1] or [2], wherein the average primary particle size of the hydrophobic silica is 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 hydrophobicized 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], wherein the weight loss rate when the polyacetal powder is held at a temperature 10 ° C. lower than the melting point for 1 hour is 5% or less.
[7]
The polyacetal powder according to any one of [1] to [6], wherein the value obtained by dividing the solidified bulk density by the loosened bulk density (solidified bulk density / loosened 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 twice or more that of 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 the 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 the D50 is 3 vol% or less.
[11]
Contains a polyacetal copolymer having 0.1 mol or more and 1.3 mol or less of comonomer units with respect to 100 mol of oximethylene units.
The melting point is 167 ° C or higher and 178 ° C or lower.
The crystallization start temperature is 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 the differential scanning calorimetry (DSC) measurement is 0 ° C. or higher and 5 ° C. or lower.
[13]
The polyacetal powder according to any one of [1] to [12], wherein the crystallization rate is 30 seconds or more and 50 seconds or less.
[14]
The polyacetal powder according to any one of [1] to [13], wherein the average minor axis / major axis ratio of the particles constituting the polyacetal powder is 0.50 or more.
[15]
The polyacetal powder according to any one of [1] to [14], wherein the average surface smoothing shape coefficient of the particles constituting the polyacetal powder is 0.80 or more.
[16]
The polyacetal powder according to any one of [1] to [15], which is used for additive production.

[17]
The method for using a polyacetal powder according to any one of [1] to [16], which is characterized by being used for additive production.

[18]
An additional manufacturing method, which comprises using the polyacetal powder according to any one of [1] to [16] as a modeling material.

According to the present invention, there is provided a polyacetal powder which can be used in an additive manufacturing technique, can obtain a modeled product having high mechanical properties, and has excellent stackability at high temperature and excellent powder loading on a modeled portion. be able to. Further, according to the present invention, it is possible to provide a method of using the above-mentioned polyacetal powder and an additional manufacturing method capable of obtaining a modeled product having high mechanical properties.

It is a secondary electron image which observed the surface of the polyacetal powder obtained in Example 1 with a scanning electron microscope. It is a secondary electron image which observed the surface of the polyacetal powder obtained in Comparative Example 1 with a scanning electron microscope. It is a secondary electron image which observed the surface of the polyacetal powder obtained in the comparative example 2 with a scanning electron microscope.

Hereinafter, embodiments for carrying out the present invention (hereinafter referred to as the present embodiment) will be described in detail.
The present invention is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist thereof.
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 size (D50) of 30 μm or more and 70 μm or less, and containing hydrophobic silica in an amount of 0.05% by mass or more and 1.0% by mass or less.
The polyacetal powder of the present embodiment has excellent stackability at high temperature and powder placement on a molding site, and can be suitably used as a modeling material for additive manufacturing, and a modeled product having high mechanical properties can be obtained.
Hereinafter, the powder characteristics, resin characteristics, particle characteristics, and the like of the polyacetal powder of the present embodiment will be described.

<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. When the average particle size (D50) of the polyacetal powder is within the above range, workability can be sufficiently ensured, for example, it can be spread evenly to a thickness of about 100 μm, and minute voids in the obtained modeled product can be obtained. The generation of (voids) can be sufficiently suppressed.
If the D50 of the polyacetal powder is less than 30 μm, the workability deteriorates, while if it exceeds 70 μm, it becomes difficult to evenly spread the polyacetal powder to a thickness of, for example, about 100 μm, and minute voids (voids) are formed in the obtained modeled product. May occur in large numbers.
From the same viewpoint, the D50 of the polyacetal powder is preferably 33 μm or more and 67 μm or less, and more preferably 35 μm or more and 65 μm or less.

The D50 of the polyacetal powder of the present embodiment has a 50% particle diameter (median diameter) accumulated from the fine particle size side, which is obtained from a volume-based particle size distribution measured by a wet method (solvent: water) by a laser diffraction / scattering method. This means, more specifically, it can be measured by the method described in Examples described later.
Further, the average particle diameter (D50) in the above range is a value measured for the polyacetal powder containing hydrophobic silica, that is, for the polyacetal powder in which the hydrophobic silica adheres to the particle surface and exists 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 fluidity of the polyacetal powder is very important. At the molding site, the polyacetal powder melts and dents due to heating with a laser, but if powder with poor fluidity is used, the surface roughness of the dent will increase, and the powder will not be easily placed on the modeling site, resulting in voids. As a result, the mechanical properties of the modeled product may be impaired or the modeling may be stopped. In this regard, the present inventors have found that the polyacetal powder contains hydrophobic silica in an amount of 0.05% by mass or more and 1.0% by mass or less, and that the hydrophobic silica is added to the surface of the particles constituting the polyacetal powder. It has been found that the adhesion can improve the fluidity of the powder, improve the stackability at high temperature and the powder loading on the molded part, and enhance the mechanical properties of the modeled product.
Further, as the polyacetal powder, a powder having a narrow particle size distribution curve (hereinafter, simply referred to as “particle size distribution curve”) displayed as a frequency distribution based on the number is generally preferred, but the particle size distribution curve is wide. It has been found that the same effect as described above can be obtained by using the powder. 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. Further, when the content of hydrophobic silica exceeds 1.0% by mass, the laser absorption of the polyacetal powder is hindered by the hydrophobic silica, so that the amount of heat required for heating-sintering cannot be obtained, and the voids ( Voids) may occur and the mechanical properties and modeling accuracy of the modeled product may be impaired. Further, 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 viewpoint, 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, 0.2. It is more preferably mass% or more and 0.5 mass% or less.

In the polyacetal powder of the present embodiment, hydrophobic silica is attached to the surface of the particles constituting the polyacetal powder, that is, at least a part of one hydrophobic silica particle is exposed on the surface of the particles constituting the polyacetal powder. This includes the case where the remaining portion is buried in the particles. On the other hand, if the hydrophobic silica particles are not exposed on the surface of the particles constituting the polyacetal powder due to the polyacetal composition being kneaded with hydrophobic silica to produce the polyacetal powder, the present invention does not include it. ..
Whether or not 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 the secondary electron image obtained by observing the surface of the polyacetal powder according to the present embodiment by SEM, a large number of fine hydrophobic silica particles are attached to the surface of the polyacetal powder (for example, FIG. 1). On the other hand, when manufactured without silica and when hydrophilic silica is used instead of hydrophobic silica, fine silica particles are not observed on the surface of the polyacetal powder (for example, FIGS. 2 and 3). ).
As will be described later, the above-mentioned polyacetal powder can be obtained by adding a predetermined amount of hydrophobic silica to the polyacetal powder obtained after the classification step and mixing them (hydrophobic silica mixing step).

In the present embodiment, the hydrophobic silica is not particularly limited, and those usually used in industrial applications can be used. For example, either dry silica or wet silica can be used, among others. It is preferable to use dry method silica. Wet method silica may contain impurities that decompose polyacetal in the manufacturing process, which may impair the resin properties of polyacetal powder. On the other hand, dry-type silica does not have the possibility of containing such impurities in the manufacturing process, and does not impair the resin properties 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. When the average primary particle size of the hydrophobic silica is within the above range, the fluidity of the polyacetal powder can be further enhanced, and the stackability at high temperature, the powder placement on the molding site, and the mechanical properties of the molded product can be further improved. Can be improved.
From the same viewpoint, the average primary particle size of the hydrophobic silica is more preferably 5 nm or more and 15 nm or less, and further preferably 7 nm or more and 13 nm or less.
The average primary particle size of hydrophobic silica can be measured wet (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. When the BET specific surface area of the hydrophobic silica is within the above range, the fluidity of the polyacetal powder can be further enhanced, and the stackability at high temperature, the powder placement on the molding site, and the mechanical properties of the molded product can be further improved. can.
From the same viewpoint, 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 further 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 silica is not particularly limited, and one usually used for industrial purposes can be used, but the surface hydroxyl group (silanol group) of the hydrophobic silica is made hydrophobic with a surface modifier. It is preferable that the product is made of silica. Hydrophobic silica hydrophobicized with a surface modifier reduces the surface hydroxyl groups, enhances the adhesion of the particles constituting the polyacetal powder to the surface, and can further enhance the fluidity of the polyacetal powder.
The surface modifier is not particularly limited as long as it is usually used for surface modification treatment of silica, and examples thereof include dimethyldichlorosilane, trimethylchlorosilane, silicone oil, octylsilane, and hexamethylsilazane. .. Above all, it is preferable that the surface modifier is hydrophobized with at least one surface modifier selected from the group consisting of dimethyldichlorosilane, trimethylchlorosilane, and silicone oil.
The surface modifier may be used alone or in combination of two or more.
As the hydrophobic silica hydrophobized with the surface modifier, a commercially available product may be used, or hydrophobic silica obtained by hydrophobizing with 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 a comonomer unit of 0.1 mol or more and 1.3 mol or less with respect to 100 mol of the oximethylene unit. When the content of the comonomer unit of the polyacetal copolymer contained as a resin component is within the above range, a polyacetal powder having good thermal stability, suppression of gas generation during laser irradiation, and a molded product having few microvoids can be obtained. be able to. In addition, polyacetal powder that does not deteriorate even if it stays in the molding machine for a long time and can be recycled can be obtained.
From the same viewpoint, the content of the comonomer unit of the polyacetal copolymer is more preferably 0.3 mol or more and 1.1 mol or less, and further preferably 0.5 mol or more and 0.9 mol or less with respect to 100 mol of the oximethylene 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.

The comonomer unit contained in the polyacetal copolymer in the present embodiment is not particularly limited as long as it can be copolymerized with the oximethylene unit, but is preferably an oxyalkylene unit having 2 or more carbon atoms. Both ends or one end of the polyacetal copolymer may be sealed with an ester group and / or an ether group.
Then, the polyacetal copolymer having a specific comonomer unit content as described above can be produced, for example, by the production method described later.

The content of comonomer units in the polyacetal copolymer can be determined by the following procedure using the ¹ H-NMR method.
The following procedure 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 with hexafluoroisopropanol to a concentration of 1.5% by mass over 24 hours, and ¹ H-NMR analysis was performed using this solution to obtain an oximethylene unit and 2 or more carbon atoms. The content of the oxyalkylene unit (bmol) having 2 or more carbon atoms (b / a: mol / 100 mol) with respect to the oxymethylene unit (a = 100 mol) from the ratio of the integrated value of the assigned peak to the oxyalkylene unit of Can be asked.

<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 modeling area at the time of modeling is widened (). For example, even if the temperature of the modeling area is lower than the set temperature due to heat dissipation to the surroundings, it is possible to suppress the occurrence of warpage during modeling), and the modeling accuracy of the obtained modeled product can be improved. Can be enhanced. Further, when a large amount of modeled products are mass-produced at one time, the tolerance of the polyacetal powder for the temperature difference in the modeling device can be widened, and the variation in the modeling accuracy between the modeled products can be suppressed.
From the same viewpoint, the melting point of the polyacetal powder is more preferably 168 ° C. or higher and 176 ° C. or lower, and further preferably 169 ° C. or higher and 174 ° C. or lower.

Here, the melting point of the polyacetal powder means the inherent melting point of the polyacetal powder, and can be measured as the melting point of the second scan by differential scanning calorimetry (DSC) measurement. 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 inherent melting point of the polyacetal powder, such as when it is freeze-ground 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 is once melted and solidified at the time of the first scan measurement and then melted again. It can be a unique melting point.

The polyacetal powder having a melting point in the above range can be obtained by adjusting the content of the comonomer unit within the above range to control the crystallinity of the polyacetal copolymer.
In general, increasing the content of comonomer in the polyacetal copolymer lowers the crystallinity and lowers the melting point of the polyacetal powder. On the contrary, if the content of comonomer in the polyacetal copolymer is reduced, the crystallinity can be increased and the melting point of the polyacetal powder can be increased.

<Crystallization start temperature>
The polyacetal powder of the present embodiment preferably has a crystallization start temperature of 140 ° C. or higher and 152 ° C. or lower.
The temperature in the modeling area and the modeling apparatus during the additional manufacturing is always kept higher than the crystallization start temperature of the resin powder. However, when a slight temperature difference (temperature unevenness) occurs in the modeling area or the modeling device, the polyacetal in the portion cooled to the crystallization start temperature in the heated-sintered modeling layer crystallizes, especially in the stacking direction. Warped, and the molding accuracy was lowered in the obtained molded product. In this regard, the present inventors can widen the tolerance for the temperature difference (temperature unevenness) in the modeling area and the modeling apparatus by increasing the temperature difference between the melting point of the polyacetal powder and the crystallization start temperature. It was found that it was possible to suppress warpage during modeling and improve the modeling accuracy of the obtained modeled product. Then, they have found that when a large amount of modeled products are mass-produced at one time, it is possible to suppress variations in modeling accuracy among the modeled products.
From the same viewpoint, the crystallization start temperature of the polyacetal powder is more preferably 144 ° C. or higher and 152 ° C. or lower, and further preferably 147 ° C. or higher and 151 ° C. or lower.
The crystallization start temperature of the polyacetal powder can be measured by differential scanning calorimetry (DSC) measurement, and more specifically, by the method described in Examples described later.
The polyacetal powder having a crystallization temperature in the above range can be obtained by adjusting the content in the main chain of the comonomer unit of the polyacetal copolymer and / or the amount of the crystal nucleating agent.

<Difference between the melting point of the first scan and the melting point of the second scan>
The polyacetal powder of the present embodiment preferably has a difference between the melting point of the first scan and the melting point of the second scan in the differential scanning calorimetry (DSC) measurement of 0 ° C. or higher and 5 ° C. or lower.
The melting point of the polyacetal powder, which determines the size of the process window, which is important in the addition manufacturing using the resin powder as the modeling material, corresponds to the melting point of the first scan in the DSC measurement. The melting point of the first scan reflects the heat history received before the polyacetal powder was produced.
On the other hand, the melting point of the second scan is the melting point of the resin itself as an actual ability, and a higher melting point is preferable. If the melting point of the second scan is high even if the melting point of the first scan is low, it is possible to increase the melting point of the first scan by performing post-treatment such as annealing described later in advance.

If the difference between the melting point of the first scan and the melting point of the second scan is 0 ° C. or higher and 5 ° C. or lower, a wide process window close to the ability of the resin can be achieved.
From the same viewpoint, the difference between the melting point of the first scan and the melting point of the second scan in the DSC measurement in the polyacetal powder is more preferably 0 ° C. or higher and 4 ° C. or lower, and further preferably 0 ° C. or higher and 3 ° C. or lower. preferable.
The difference between the melting point of the first scan and the melting point of the second scan in the DSC measurement is the melting point of the first scan and the melting point of the second scan measured by the DSC measurement, which is the melting point of the higher temperature to the melting point of the lower temperature. Is subtracted from the value.
Polyacetal powders having a difference between the melting point of the first scan and the melting point of the second scan within the above range can be annealed (lower than the melting point and at a lower temperature than the melting point) to control the conditions of freeze crushing in the manufacturing process, and after freeze crushing before or after the classification step. It can be obtained by holding the crystal at a high temperature at which crystal growth is possible for a certain period of time, for example, by growing the crystal at 120 ° C. for 1 hour.

<Crystalization rate>
The polyacetal powder of the present embodiment preferably has a crystallization rate of 30 seconds or more and 50 seconds or less. When the crystallization rate is within the above range, temperature unevenness in the modeling area during modeling and between layers can be tolerated, warpage during modeling can be further suppressed, and the modeling accuracy of the obtained modeled product can be improved. On the other hand, if the crystallization rate is less than 30 seconds, the resin crystallizes too quickly, and temperature unevenness in the modeling area or between layers during modeling cannot be tolerated, causing warpage during modeling, resulting in modeling. There is a risk that the modeling accuracy of the product will decrease. Further, if the crystallization rate is more than 50 seconds, the crystallization of the resin is too slow, the precision of the boundary line between the modeled portion and the non-modeled portion is impaired, and the modeling accuracy of the contour of the modeled product is lowered. There is a fear.
From the same viewpoint, the crystallization rate of the polyacetal powder is more preferably 30 seconds or more and 47 seconds or less, and further preferably 31 seconds or more and 45 seconds or less.
Here, in the DSC measurement, the crystallization rate is maintained at the predetermined temperature by rapidly lowering the temperature of the once melted polyacetal powder and holding it at a predetermined temperature near the crystallization start temperature and observing the exothermic peak due to crystallization. It is the time from the start to the maximum temperature of the exothermic peak (peak top temperature). More specifically, it can be measured by the method described in Examples described later.

Examples of the method for controlling the crystallization rate of the polyacetal powder include adjusting the comonomer content of the polyacetal copolymer and the amount of the crystal nucleating agent added. Specifically, if the content of the comonomer unit is increased, the crystallization degree is lowered, so that the crystallization rate tends to be slowed down. Further, the crystallization rate can be slowed down by not including an additive that becomes a crystal nucleating agent.
As described above, the 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 of the crystal nucleating agent, the type, the addition amount, and the like. ..

<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 a high molecular weight substance such that the MFR is less than 70 g / 10 minutes, a modeled product having excellent creep characteristics can be obtained.
Further, it is also preferable that the polyacetal powder of the present embodiment has a melt flow rate (MFR) of 2.5 g / 10 minutes or more. Since the MFR of the polyacetal powder is 2.5 g / 10 minutes or more, when it is melted by a laser, the polyacetal flows to fill the gaps between the particles more efficiently, which makes it possible to fill the gaps between the particles more efficiently. Voids are reduced, and a modeled product having excellent mechanical properties such as tensile strength can be obtained.
From the same viewpoint, the MFR of the polyacetal powder is more preferably 5 g / 10 minutes or more and 55 g / 10 minutes or less, and further preferably 20 g / 10 minutes or more and 40 g / 10 minutes or less.
Here, the MFR of the polyacetal powder is measured in accordance with ISO1133 (condition D, temperature 190 ° C.).
The 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 step 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 at a temperature 10 ° C. lower than the melting point of the polyacetal powder for 1 hour. When the weight reduction rate of the polyacetal powder is 5% or less, high thermal stability can be maintained. For example, when a polyacetal powder made of polyacetal is used, a volatile organic substance (VOC) such as formaldehyde due to heating is used. ) Can be sufficiently suppressed.
From the same viewpoint, the weight loss rate of the polyacetal powder is more preferably 3% or less, further preferably 1% or less.
The weight loss rate of the polyacetal powder can be measured by a thermogravimetric measuring device, and more specifically, it can be measured by the method described in Examples described later.
The 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 below, and various additives. Can be done.

<Hardened bulk density / Loose bulk density>
In the polyacetal powder of the present embodiment, the value obtained by dividing the solidified bulk density by the loosened bulk density (solidified bulk density / loosened bulk density) is preferably 1.45 or less. If the value obtained by dividing the solidified bulk density by the loosened bulk density is within the above range, the polyacetal powder has excellent fluidity and stackability, can further suppress the generation of voids, and makes the modeled product dense and mechanical. The physical properties can be further enhanced.
From the same viewpoint, the value obtained by dividing the solidified bulk density by the loosened bulk density is more preferably 1.40 or less, and further preferably 1.36 or less.
The loose bulk density is a density measured in a state where powder is sparsely filled (not tapped) in a container having a predetermined volume. The solidified bulk density is a density measured in a state where a container of a predetermined volume filled with powder is repeatedly dropped (tapped) from a constant height at a constant speed and densely packed until the bulk density of the powder in the container becomes almost constant. Is. Specifically, the solidified bulk density and the loosened bulk density can be measured by the methods described in Examples described later.

Polyacetal powder having a value obtained by dividing the compaction bulk density by the loose bulk density in the above range has a particle size distribution (for example, the ratio of particles having a diameter of 2 times or more of D50, which will be described later, and a diameter of 0.2 times or less of D50). It can be obtained by controlling the particle shape (for example, the average minor axis major axis ratio, the average surface smooth shape coefficient, etc., which will be described later) and the like.

<Particle size distribution>
In the polyacetal powder of the present embodiment, the proportion of particles having a diameter of twice or more the average particle diameter (D50) (for example, particles having a diameter of 100 μm or more when D50 is 50 μm) is 10 vol% or less. Is preferable. When the above ratio is 10 vol% or less, the fluidity of the polyacetal powder can be improved, the density of the modeled product can be increased, the mechanical properties can be improved, and the surface of the modeled product can be made smoother. Since the molding accuracy is improved, it becomes easy to manufacture precise parts.
From the same viewpoint, the proportion of particles having a diameter of 2 times or more the diameter 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 the polyacetal powder of the present embodiment, the proportion of particles having a diameter of 0.2 times or less the average particle diameter (D50) (for example, particles having a diameter of 10 μm or less when D50 is 50 μm) is 10 vol. % Or less is preferable.
Since the laser beam used for modeling can make the irradiation spot size very small, the resolution of the contour of the modeled product can be made very clear. However, due to heat conduction from the location fused by heating with the laser, the powder outside the location where the laser was scanned fused directly to the part to be sintered and crossed the laser scanning region, thus reducing the design dimensions. There is a risk that the powder will fuse beyond that. Further, if the heat from the sintering remains, even if the powder is newly laid on the layer, the powder is sintered on the sintered portion of the previous layer, and so-called interlayer growth occurs. There is a fear. When such interlayer growth occurs, the molding accuracy is lowered and it becomes difficult to remove the unsintered powder from the finished product. In this regard, the present inventors have determined that particles having a diameter of 0.2 times or less of D50 are particularly easy to fuse and affect the molding accuracy, and that the ratio is 10 vol% or less. , It has been found that the molding accuracy is improved and it becomes easier to manufacture precision parts. Further, when the ratio is 10 vol% or less, it is possible to improve the fluidity of the polyacetal powder, increase the density of the modeled product, and enhance the mechanical properties.
From the same viewpoint, 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.

Further, in the polyacetal powder of the present embodiment, it is preferable that 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. When the proportions of the particles having a diameter of 0.2 times or less the D50 and the diameter are within the above ranges, the stackability of the polyacetal powder can be further improved, and the mechanical properties of the obtained modeled product can be further improved. ..
From the same viewpoint, it is more preferable that D50 is 63 μm or less and the proportion of particles having a diameter of 0.2 times or less of D50 is 2 vol% or less, D50 is 62 μm or less, and the diameter is 0. It is more preferable that the proportion of particles having a value of 2 times or less is 1 vol% or less.

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

<Particle shape>
The particles constituting the polyacetal powder of the present embodiment are preferably spherical from the viewpoint of fluidity. In general, it is difficult to directly form fine particles of a resin such as polyacetal as a particle material by polymerization, and in order to produce a powder using such a resin, an operation such as crushing pellets obtained by an arbitrary method is performed. You will need it. Therefore, at that time, the crushing method and crushing conditions are appropriately adjusted to directly obtain particles close to a sphere, or the crushed powder is stirred while being heated by post-processing, and the corners of the particles are rounded off. Therefore, spherical particles can be obtained.
Hereinafter, the shapes of the particles constituting the polyacetal powder of the present embodiment will be described more specifically.

<< Average minor axis major axis ratio of particles >>
The particles constituting the polyacetal powder of the present embodiment preferably have an average minor axis major axis ratio of 0.50 or more. When the average minor axis major axis ratio of the particles is 0.50 or more, it is possible to avoid irregularities, increase the fluidity, and spread the powders without gaps.
From the same viewpoint, the average minor axis major axis ratio of the particles is more preferably 0.65 or more, and further preferably 0.70 or more.
Here, the average minor axis major axis ratio means the ratio of the minor axis to the major axis of the particles (minor axis / major axis), and the average minor axis major axis ratio is the minor axis major axis ratio of 100 arbitrarily selected particles. The average value. The minor axis and the major axis refer to the short side and the long side of the circumscribed rectangle that minimizes the area circumscribed on the boundary pixel of the particle, respectively.
The average minor axis major axis ratio of the particles constituting the polyacetal powder can be measured by the method described in Examples described later.
The polyacetal powder in which the average minor axis major axis ratio of the particles is 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 coefficient of particles >>
The particles constituting the polyacetal powder of the present embodiment preferably have an average surface smoothing shape coefficient of 0.80 or more. When the average surface smoothing shape coefficient 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 viewpoint, the average surface smoothing shape coefficient of the particles is more preferably 0.85 or more, and further preferably 0.90 or more.
Here, the surface smooth shape coefficient is an index representing the circularity or ellipticality of the particles, and is expressed by the following formula using the measured values of the minor axis, the major axis, and the peripheral length of the particles.
Surface smooth shape coefficient = [π × (major axis of particle + minor axis of particle)] / [2 × perimeter of particle]

The surface smoothness coefficient is 1 when the particles are perfect circles and approximately 1 when the particles are perfect ellipses. The irregular shape rather than a smooth circle or ellipse increases the perimeter of the particles, so that the surface smooth shape coefficient is smaller than 1.
The average surface smoothing shape coefficient means the average value of the surface smoothing shape coefficient of 100 arbitrarily selected particles.
The average surface smoothing shape coefficient of the particles constituting the polyacetal powder can be measured by the method described in Examples described later.
The polyacetal powder in which the average surface smoothing shape coefficient of the particles is within the above range can be obtained by adjusting various conditions in the crushing step and / or the classification step of the method for producing the polyacetal powder.

[Component composition of polyacetal powder]
Hereinafter, the component composition of the polyacetal powder of the present embodiment will be specifically described.

<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 with respect to 100 mol of oximethylene 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 is preferable that the polyacetal copolymer is contained in an amount of 85% by mass or more, more preferably 90% by mass or more, based on 100% by mass of the resin component. , 95% by mass or more is more preferable, and 97% by mass or more is further preferable.
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.

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

(1) Polymerization step In the polymerization step, known polymerization methods (for example, US Pat. No. 3,027,352, US Pat. No. 3,803,094, German Patent Invention No. 1161421, German Patent Invention No. 1495228. , German Patented Invention No. 1720358, German Patented Invention No. 3018898, JP-A-58-98322 and JP-A-7-70267) in a reactor, A crude polyacetal copolymer with unstabilized ends can be obtained by copolymerizing the main monomer and the comonomer using a polymerization catalyst.
Although the crude polyacetal copolymer itself can be used as the material for the resin powder, it is preferable to use one in which the ends of the crude polyacetal copolymer are stabilized by a terminal stabilization step described later.

1) Main Monomer Examples of the main monomer used in the production of the polyacetal copolymer include formaldehyde or a cyclic oligomer such as trioxane which is a trimer thereof or tetraoxane which is a tetramer.
In the present embodiment, the "main monomer" means a monomer unit contained in an amount of 50% by mass or more based on the total amount of the monomer.

2) Comonomer The comonomer used for producing the polyacetal copolymer is not particularly limited, and examples thereof include a cyclic ether compound having an oxyalkylene unit having 2 or more carbon atoms in the molecule.

The cyclic ether compound is not particularly limited, and is, for example, ethylene oxide, propylene oxide, 1,3-dioxolane, 1,3-propanediolformal, 1,4-butanediolformal, 1,5-pentanediolformal, 1, Examples thereof include 6-hexanediol formal, diethylene glycol formal, 1,3,5-trioxysepan, 1,3,6-trioxyocan, and mono- or di-glycidyl compounds capable of forming a branched or crosslinked structure in the molecule.
The cyclic ether compound may be used alone or in combination of two or more.

The main monomer and comonomer preferably contain as little as possible impurities having a polymerization termination action and a chain transfer action during the polymerization reaction, such as water, methanol and formic acid. By using a main monomer and a comonomer having a small amount of these impurities, an unexpected chain transfer reaction can be avoided and a polyacetal copolymer having a desired molecular weight can be obtained. Above all, the content of the impurity that induces a hydroxyl group in the polymer terminal group 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 the monomers. Is even more preferable.

As a method for obtaining a main monomer and a comonomer having a small amount of impurities, a known method (for example, the method described in JP-A-3-123777 and JP-A-7-33761 for the main monomer, and the comonomer is specified. The methods described in Kaisho 49-62469 and JP-A-5-271217) can be mentioned.

3) Chain transfer agent In the production of the polyacetal copolymer, it is preferable to use a chain transfer agent. The chain transfer agent is not particularly limited, and examples thereof include 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.

In order to obtain a long-chain branched polyacetal copolymer, it is preferable to use a polyether polyol and an alkylene oxide adduct of the polyether polyol as a chain transfer agent.

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

As the chain transfer agent, it is preferable to use one containing as little as possible impurities having a polymerization termination action and a chain transfer action during the polymerization reaction, such as water, methanol, formic acid, and acetic acid. As a method for obtaining a chain transfer agent having a small amount of impurities, for example, a chain transfer agent that is widely used and has an available water content exceeding a specified amount is bubbled with dry nitrogen, and impurities are removed by an adsorbent such as activated carbon or zeolite. , Purification method and the like.
The chain transfer agent may be used alone or in combination of two or more. In any case, the obtained polyacetal copolymer preferably has a small number of unstable terminals.

4) Polymerization catalyst The polymerization catalyst used for producing the polyacetal copolymer is not particularly limited, but a cationically active catalyst such as Lewis acid, protonic acid, and ester or anhydride of protonic acid is preferable.

The Lewis acid is not particularly limited, and examples thereof include halides of boric acid, tin, titanium, phosphorus, arsenic, and antimony. Specific examples thereof include boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentafluoride, phosphorus pentachloride and antimony pentafluoride, and complex compounds or salts thereof.

The ester or anhydride of the protonic acid and the protonic acid is not particularly limited, and examples thereof include perchloric acid, trifluoromethanesulfonic acid, perchlorolic acid-quaternary butyl ester, acetylparklorate, and trimethyloxonium hexafluorophosphate. Can be mentioned.

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

As the polymerization catalyst, for example, a catalyst that reduces the formation of the terminal formating group described in JP-A No. 05-05017 may be used in combination, if necessary.

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, based on 1 mol of the total amount of all the monomers. When the amount of the polymerization catalyst used is within the above range, the reaction stability at the time of polymerization and the thermal stability of the obtained polyacetal powder can be further improved.

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

The catalytic neutralizing deactivating agent is not particularly limited, and for example, amines such as ammonia, triethylamine and tri-n-butylamine; hydroxides of alkali metals and alkaline earth metals; inorganic acid salts; organic acid salts and the like. Can be mentioned.
The catalyst neutralization deactivating agent may be used alone or in combination of two or more.

A method of contacting a vapor such as ammonia and triethylamine with a polyacetal copolymer to inactivate the polymerization catalyst, or contacting with at least one of hindered amines, triphenylphosphine and calcium hydroxide with a mixer deactivates the catalyst. A method of causing it to be used can also be used.

5) Reactor The reactor used in the production of the polyacetal copolymer is not particularly limited, but for example, a reaction tank with a batch type stirrer, a continuous conider, a twin-screw screw type continuous extrusion kneader, and a twin-screw paddle type continuous reactor. Examples include a mixer.
The reactor preferably has a structure on the outer periphery of the body capable of heating or cooling the reaction mixture such as a jacket.

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

The method for decomposing and removing the unstable end portion contained in the crude polyacetal copolymer is not particularly limited, but for example, a known decomposition using a single-screw screw extruder with a vent, a twin-screw extruder with a vent, or the like is used. A method of melting a crude polyacetal copolymer in the presence of a removing agent to decompose and remove an unstable terminal portion can be mentioned.

When melt-kneading for terminal stabilization, it is preferable to replace the atmosphere with an inert gas or degas by a one-stage or multi-stage vent in order to maintain the quality and working environment. The temperature at the time of melt-kneading is preferably 250 ° C. or higher, which is equal to or higher than the melting point of the polyacetal copolymer.

1) Decomposition remover As a decomposition remover used for decomposing and removing unstable terminal portions contained in crude polyoxymethylene copolymer, for example, ammonia, aliphatic amines; alkali metal such as calcium hydroxide or alkaline earth metal water. Examples include basic substances such as oxides; inorganic weak salts; and organic weak salts.
The decomposition removing agent may be used alone or in combination of two or more.

<Other resins>
The polyacetal powder of the present embodiment may contain a small amount of a resin other than the above-mentioned polyacetal copolymer having a specific comonomer unit content as the resin.
Specific examples of the other resin that can be contained as the resin component are not particularly limited and may be appropriately selected depending on the intended purpose. For example, polyamide 6, polyamide 11, polyamide 12, polyamide 66, polypropylene, polyether. Examples thereof include crystalline resins such as ether ketone (PEEK), polyethylene terephthalate (PET), and polybutylene terefurate (PBT). These may be used alone or in combination of two or more.

<Additives>
Further, the polyacetal powder in the present embodiment is, for example, an antioxidant, a stabilizer (heat stabilizer, etc.), a weather resistant (light) agent, a lubricant (mold release agent, etc.), a crystal nucleating agent, an inorganic / organic filler. , Conductive agent / antistatic agent, appearance improving agent (pigment, dye, etc.) and other additives may be contained. In particular, the polyacetal powder in the present embodiment preferably contains an antioxidant and a heat stabilizer as additives. The antioxidant is preferably a hindered phenolic antioxidant, and the heat stabilizer is preferably one containing formaldehyde-reactive nitrogen.
The additive may be used alone or in combination of two or more.

<< Antioxidant >>
As the antioxidant, a hindered phenolic antioxidant is preferably used.
Examples of the hindered phenolic antioxidant include n-octadecyl-3- (3', 5'-di-t-butyl-4'-hydroxyphenyl) -propionate and n-octadecyl-3- (3'-. Methyl-5'-t-butyl-4'-hydroxyphenyl) -propionate, n-tetradecyl-3- (3', 5'-di-t-butyl-4'-hydroxyphenyl) -propionate, 1,6- Hexadiol-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.
The antioxidant may be used alone or in combination of two or more.

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

Specific examples of the amino-substituted triazine compound include 2,4-diamino-sym-triazine, 2,4,6-triamino-sim-triazine, N-butyl melamine, N-phenyl melamine, N, N-diphenyl melamine, and N. , N-diallyl melamine, benzoguanamine (2,4-diamino-6-phenyl-sym-triazine), acetoguanamine (2,4-diamino-6-methyl-sym-triazine), 2,4-diamino-6-butyl -Sym-triazine and the like can be mentioned.

Specific examples of the amino-substituted triazine compound and the adduct of formaldehyde include N-methylol melamine, N, N'-dimethylol melamine, N, N', N "-trimethylol melamine and the like.

Specific examples of the condensate of the amino-substituted triazine compound and formaldehyde include a melamine-formaldehyde condensate.

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

Examples of hydrazine derivatives include hydrazide compounds. Specific examples of the hydrazide compound include dicarboxylic acid dihydrazide, and more specifically, malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, pimelic acid dihydrazide, speric acid dihydrazide, and azelaic acid dihydrazide. Examples thereof include sebatic acid dihydrazide, dodecanedioic acid dihydrazide, isophthalic acid dihydrazide, phthalic acid dihydrazide, and 2,6-naphthalenedicarbodihydrazide.

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

Specific examples of the imide compound include succinimide, glutarimide, and phthalimide.

Examples of other stabilizers include hydroxides, inorganic acid salts, carboxylates and alkoxides of alkali metals or alkaline earth metals, and specifically, sodium, potassium, magnesium, calcium or the like. Examples thereof include hydroxides such as barium, carbonates of the above metals, phosphates, silicates, borates, carboxylates, and layered compound hydroxides.

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 a hydroxyl group. Specific examples of saturated or unsaturated aliphatic carboxylates include calcium dimyristate, calcium dipalmitate, calcium distearate, calcium (myristic acid-palmitic acid), calcium (myristic acid-stearic acid), and (palmitic acid). -Stearic acid) Calcium and the like are mentioned, and calcium dipalmitate and calcium distearate are preferable.

Examples of the layered double hydroxide include hydrotalcites represented by the following formulas.
[(M ²⁺ ) _1-X (M ³⁺ ) _X (OH) ₂ ] ^{X +} [(An-) _{X / n} ^・mH ₂ O] ^X-
(In the above formula, M ²⁺ is a divalent metal, M ³⁺ is a trivalent metal, An- indicates an anion having an n valence ( ⁿ is an integer of 1 or more), and X is 0 <X ≦ 0.33. In the range of, 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 examples of M ³⁺ . Can be Al ³⁺ , Fe ³⁺ , Cr ³⁺ , Co ³⁺ , In ³⁺ , etc. Examples of ^An- are OH-, F- ^{, Cl- ,} Br- ^, NO ^3- , Examples thereof include CO ₃ ^2- , SO ₄ ^2- , Fe (CN) ₆ ^3- , CH ₃ COO- ^, oxalate ion, and salicylate ion. As examples of ^An- , OH- ^and CO ₃ ^2- are preferable.

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. Examples thereof include synthetic hydrotalcite represented by 5H ₂ O, Mg _4.3 Al ₂ (OH) _12.6 CO ₃ and the like.
The stabilizer may be used alone or in combination of two or more.

<< Weatherproof (light) agent >>
Examples of the weather resistant (light) agent include benzotriazole-based ultraviolet absorbers, oxalic acid anilide-based ultraviolet absorbers, and hindered amine-based light stabilizers.

Specific examples of the benzotriazole-based ultraviolet absorber include 2- (2'-hydroxy-5'-methyl-phenyl) benzotriazole and 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'- Examples thereof include bis- (α, α-dimethylbenzyl) phenyl] -2H-benzotriazole, 2- (2'-hydroxy-4'-octoxyphenyl) benzotriazole and the like.

Specific examples of the oxalic acid anilides-based UV absorber include 2-ethoxy-2'-ethyloxalic acid bisanilide, 2-ethoxy-5-t-butyl-2'-ethyloxalic acid bisanilide, and 2-. Ethoxy-3'-dodecyl oxalic acid bisanilide and the like can be mentioned. 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 can be mentioned.

Specific examples of the hindered amine-based light stabilizer include N, N', N'', N'''-tetrakis- (4,6-bis- (butyl- (N-methyl-2,2,6,6-). Tetramethylpiperidin-4-yl) amino) -triazine-2-yl) -4,7-diazadecan-1,10-diamine, dibutylamine 1,3,5-triazine N, N'-bis (2, Polycondensate 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}], a condensate of dimethyl succinate and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinethanol, decane 2 Reaction product of acid bis (2,2,6,6-tetramethyl-1- (octyloxy) -4-piperidinyl) ester with 1,1-dimethylethylhydroperoxide 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- Condensations of pentamethyl-4-piperidinol and β, β, β', β'-tetramethyl-3,9- [2,4,8,10-tetraoxaspiro (5,5) undecane] diethanol, etc. Preferably bis (2,2,6,6-tetramethyl-4-piperidyl) -sevacate, 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] A condensate with diethanol.
The weatherproof (light) agent may be used alone or in combination of two or more.

<< Lubricant >>
Examples of the lubricant include alcohols, fatty acids and fatty acid esters thereof, polyoxyalkylene glycols, olefin compounds having an average degree of polymerization of 10 to 500, silicones and the like.
The lubricant may be used alone 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, leaf wax, clay, diatomaceous earth, silicates such as wollastonite; iron oxide, titanium oxide, alumina and the like. Metal oxides; Metallic sulfates such as calcium sulfate and barium sulfate; Carbonated salts such as calcium carbonate, magnesium carbonate and dolomite; Others Commonly known in resins such as polyacetal such as silicon carbide, silicon nitride, boron nitride and various metal powders. It suffices as long as it is a subdivided solid of the crystalline nucleating inorganic substance. Boron nitride and talc are preferable as the crystal nucleating agent.

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

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

<< Inorganic / organic filler >>
Examples of the inorganic filler include metal powder (aluminum, stainless steel, nickel, silver, etc.), oxides (silicon oxide, iron oxide, alumina, titanium oxide, zinc oxide, etc.), and 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.

Examples of the organic filler include natural products (linter, wood, rice husks, silk, leather, etc.) and synthetic materials (aramid, Teflon (registered trademark), viscose, etc.).

Above all, it is preferable to select from inorganic and organic fillers that can be added to resins such as conventional polyacetal resins. That is, if a filler having strong acidity and alkalinity is added to a resin such as a polyacetal resin, the stability may be lowered. It is preferable to be selected from among them.

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

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

The inorganic / organic filler may be used alone or in combination of two or more.

<< Conductive agent / antistatic agent >>
Examples of the conductive agent include conductive carbon black, carbon nanotubes or nanoparticles or nanoparticles, metal powders or fibers.

Examples of the antistatic agent include aliphatic polyethers (excluding compounds having a fatty acid ester terminal), aliphatic polyethers having a fatty acid ester terminal, and free hydroxyl groups obtained from fatty acids and polyhydric alcohols. Polyalkylene polyols or alkali metal salt-dissolved polyalkylenes based on a fatty acid ester of a polyhydric alcohol, a borate ester of a glycerin monofatty acid ester, an ethylene oxide adduct of an amine compound, a basic carbonate or an anion exchanger thereof. Examples thereof include an antistatic agent in which polyols are encapsulated.
The conductive agent / antistatic agent may be used alone or in combination of two or more.

<< Appearance improver >>
Examples of the appearance improving agent include inorganic pigments and organic pigments, metallic pigments, fluorescent pigments, and various dyes.

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

Examples of organic pigments include condensed zo-based, inon-based, phthalocyanine-based, monoazo-based, diazo-based, polyazo-based, anthracinone-based, heterocyclic, pennon-based, quinacridone-based, thioindico-based, berylene-based, and dioxazine-based pigments. Examples include phthalocyanine-based pigments.
The appearance improving agent may be used alone or in combination of two or more.

[Manufacturing method of polyacetal powder]
The polyacetal powder in the present embodiment is, for example, a step of granulating a resin material such as polyacetal (granulation step), a step of crushing the granulated resin material (grinding step), and sieving the crushed 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 in which hydrophobic silica adheres to the surface of particles (hydrophobic silica mixing step) are carried out. Thereby, it can be manufactured.
Hereinafter, each step will be specifically described.

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

Here, when the resin material is melt-kneaded, it is preferable to replace the atmosphere with an inert gas or degas by a one-stage or multi-stage vent in order to maintain the quality and working environment. The temperature at the time of melt-kneading is preferably 250 ° C. or higher, which is equal to or higher than the melting point of the resin material, and may be 20 to 50 ° C. higher than the melting point of the resin material.

Further, in the granulation step, the above-mentioned additives may be added to the resin material and mixed, and the obtained resin composition may be melt-kneaded. At this time, the additive may be mixed with the resin material in several portions.
Alternatively, in the granulation step, in order to enhance the dispersibility of the additive with respect to the resin material, a part or all of the resin material is arbitrarily crushed before mixing, and a Henshell mixer, a tumbler, a V-shaped blender, or the like is used. The amide compound and the above-mentioned additives may be added and mixed, and the obtained resin composition may be melt-kneaded. In this case, a spreading agent may be used to further enhance the dispersibility.

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

As the amide compound, an amide compound having a melting point of 300 ° C. or higher is preferable. Since the amide compound having a melting point of 300 ° C. or higher does not melt during molding, the generation of foreign substances can be suppressed.

Examples of the amide compound 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 the amide compound 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 anthranyl amides.

The amount of the amide compound added is preferably 0.1 part by mass or more, more preferably 0.2 part by mass or more, and 0.5 part by mass with respect to 100 parts by mass of the resin material such as polyacetal. The following is preferable. When the amount of the amide compound added is within the above range, it is possible to obtain a modeled product having a good appearance, a small amount of foreign matter generated, 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 are suppressed even if the additive stays at a high temperature for a long time in the molding machine, recycling is possible, and the mechanical strength of the obtained modeled product is also good. There is a tendency. From the same viewpoint, the content of the additive in the resin composition is more preferably 50 parts by mass or less, and further preferably 25 parts by mass or less with respect to 100 parts by mass of the resin material.

-Spreading agent-
The spreading agent is not particularly limited, and examples thereof include aliphatic hydrocarbons and aromatic hydrocarbons, modified products thereof, and fatty acid esters of polyols.
The spreading agent may be used alone or in combination of two or more, such as an aliphatic hydrocarbon, an aromatic hydrocarbon and a mixture of these modified products.

[[Crushing process]]
In the pulverization step, the pellets obtained in the granulation step can be pulverized to obtain a powder. The pulverization method is not particularly limited, and examples thereof include room temperature pulverization, freeze pulverization, and the like. Further, there is also a method of dissolving the pellets obtained in the granulation step in a solvent and then precipitating them.

[[Classification process]]
In the classification step, the powder obtained in the pulverization step is sieved to have an average particle diameter (D50), a proportion of particles having a diameter of 2 times or more of D50, and particles having a diameter of 0.5 times or less of D50. The polyacetal powder of the present embodiment can be finally obtained by appropriately adjusting the ratio and the like.

Further, in the method for producing polyacetal powder of the present embodiment, the crystallinity may be adjusted by heat treatment before or after the classification step, if desired.

[[Hydrophobic silica mixing step]]
In the method for producing polyacetal powder of the present embodiment, a hydrophobic silica mixing step is carried out after the classification step.
In the hydrophobic silica mixing step, by mixing the polyacetal powder obtained in the classification step with the hydrophobic silica, the hydrophobic silica adheres to the surface of the particles constituting the polyacetal powder, and the fluidity of the polyacetal powder is improved. Can be made to.
The mixing method is not particularly limited, and a normal powder mixer, for example, a Nauter mixer, a Henschel mixer, a tumbler mixer, or the like may be used for mixing. Alternatively, the mixture may be mixed by manual stirring or the like.

[Use of polyacetal powder]
The polyacetal powder of the present embodiment is suitably used for addition manufacturing techniques, and is particularly preferably used for a selective laser sintering method. Since the polyacetal powder in the present embodiment has been optimized for powder characteristics and particle characteristics, it can be evenly spread to a thickness of, for example, about 100 μm. Therefore, by performing additional production using this resin powder in the selective laser sintering method, it is possible to obtain a modeled product having high mechanical properties in which minute voids are sufficiently suppressed. Such shaped products can be widely applied to mechanical parts such as automobile parts, electric / electronic parts, industrial parts, medical parts, and the like.

[How to use polyacetal powder]
The method of using the polyacetal powder of the present embodiment includes using the polyacetal powder of the present embodiment described above for addition production.
The additional manufacturing technique used for additional manufacturing is not particularly limited, and a selective laser sintering method or a heat absorber is applied to a part to be formed in a modeling area covered with resin powder, and heat is applied from the upper surface with a heater. Examples include the heat melting method for modeling.
In addition, in the method of using the polyacetal powder of this embodiment, specific usage conditions and the like are not particularly limited, and the polyacetal powder is appropriately selected according to the additional manufacturing technique to be used, the use and purpose of the modeled product, and the like. Can be used.

[Additional manufacturing method]
The addition manufacturing method of the present embodiment includes using the polyacetal powder of the present embodiment described above as a modeling material. According to the additional manufacturing method of the present embodiment, since the polyacetal powder of the present embodiment described above is used, it is possible to obtain a modeled product having high modeling accuracy and mechanical properties.
The addition manufacturing technique used in the addition manufacturing method of the present embodiment is not particularly limited as long as the resin powder can be used as a molding material, and examples thereof include a selective laser sintering method and a heat melting method. Be done.
The specific modeling conditions and the like in the additional manufacturing method of the present embodiment are not particularly limited, and the additional manufacturing technique to be used, the intended use of the modeled product, and the like can be appropriately selected and modeled.

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

[Measuring method]
For the polyacetal powders obtained in Examples and Comparative Examples, the average particle diameter (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, and the comonomer unit. Content rate, melting point, crystallization start temperature, melting point difference between first scan and second scan in DSC measurement, crystallization rate, weight loss rate, melt flow rate, compacted bulk density / loose bulk density value, average minor axis major axis of particles The ratio and the average surface smoothness coefficient of the particles were measured. Each measurement method is as follows.

[[Particle diameter, etc.]]
Using a master sizer manufactured by Malvern, the average particle diameter (D50), the proportion of particles having a diameter of 2 times or more of D50, and the proportion of particles having a diameter of 0.2 times or less of D50 were measured.

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

[[Melting point, crystallization start temperature, crystallization rate]]
Based on the DSC curve obtained by measuring according to the following procedures 1 to 8 using a differential scanning calorimeter (Parkin Elmer: DSC-2C), the first scan melting point, the second scan melting point, and the crystallization start temperature, And the crystallization rate was 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 the heating furnace of the differential scanning calorimeter.
The temperature is raised until it reaches 220 ° C. at a heating rate of 2:10 ° C./min, and the highest point of the endothermic peak (melting point peak) at that time is defined as the "first scan melting point".
3: Maintain the temperature for 2 minutes after reaching 220 ° C.
4: Next, the temperature is lowered to reach 149 ° C. at a temperature lowering rate of 80 ° C./min, the temperature is maintained for 10 minutes, and the exothermic peak due to crystallization is measured.
The time from the start of holding at 5: 149 ° C. to the arrival of the highest exothermic peak is referred to as the "crystallization rate". The longer this time is, the slower the crystallization rate is.
6: Next, the temperature is raised to 210 ° C. at a heating rate of 20 ° C./min, and the temperature is maintained for 5 minutes.
7: Next, the temperature is cooled from 210 ° C. to 100 ° C. at a temperature lowering rate of 20 ° C./min. The temperature (high temperature side) at the start point of the exothermic peak (crystallization peak) at this time is defined as the “crystallization start temperature”.
8: Next, the temperature is raised from 100 ° C. to 210 ° C. at a heating rate of 20 ° C./min, and the highest endothermic peak (melting point peak) at that time is defined 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 obtained by DSC measurement as described above, the melting point difference between the first scan and the second scan (referred to as "1st 2nd melting point difference" in Table 1) was obtained.

[[Weight loss rate]]
Using a thermal weight measuring device (manufactured by Perkin Elmer, trade name "Pyris1 TGA"), sample weight: 5 mg, air flow rate: 10 mL / min, temperature rise 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 after holding at that temperature for 1 hour was measured.

[[Melt Flow Rate (MFR)]]
Using "P-111" manufactured by Toyo Seiki Seisakusho, MFR was measured in accordance with ISO1133 (Condition D) under the conditions of 190 ° C. and a load of 2.16 kg.

[[Hardened bulk density / loosened bulk density]]
Using a powder tester PT-X type (manufactured by Hosokawa Micron), the solidified bulk density and the loosened bulk density were measured.
Specifically, in a 100 cm ³ cylindrical container made of stainless steel, the sample feeder is vibrated until the powder is piled up in the container, the powder flows down, and the excess powder on the container is scraped off with a blade for measurement. The loosened bulk density was defined as the loosened bulk density (g / cm ³ ).
In addition, cover a stainless steel 100 cm ³ cylindrical container with a cap, vibrate the sample supply device to let the powder flow down, and tap with a stroke length (tapping height) of 18 mm, a tapping speed of 60 times / minute, and a tapping number of 180 times. gone. Then, the density measured by scraping off the excess powder on the container with a blade was defined as the solidification bulk density (g / cm ³ ).
The measured value of the compacted bulk density measured as described above was divided by the measured value of the loosened bulk density to obtain the value of the compacted bulk density / the loosened bulk density.

[[Average minor axis major axis ratio of particles]]
Images taken with a digital microscope (using "VH-7000" and "VH-501 lens" manufactured by Keyence Co., Ltd.) are saved in 1360 x 1024 pixels, TIFF file format, and image processing analysis software (stock). Using "Image HyperII" manufactured by the company Digimo, the average value of the minor axis / major axis of 100 particles was calculated.

[[Average surface smoothness coefficient of particles]]
Images taken with the above digital microscope are saved in 1360 x 1024 pixels, TIFF file format, and the major axis, minor axis, and peripheral length of 100 particles are measured using the above image processing analysis software. Then, each surface smooth shape coefficient was obtained according to the following formula, and the average value was obtained.
Surface smooth shape coefficient = [π × (major axis of particle + minor axis of particle)] / [2 × perimeter of particle]

[Powder productivity]
For Examples and Comparative Examples, the yield (%) after the pulverization / classification step was determined by the following formula based on the mass of the pellets subjected to the pulverization / classification step and the mass of the powder obtained in the pulverization / classification step. rice field.
Yield = (mass of powder obtained in crushing / classification step / mass of pellets used in crushing / classification step) × 100
The obtained yield (%) was evaluated according to the following evaluation criteria. The higher the obtained yield (%), the better the productivity.
(Evaluation criteria)
◯: Yield of 50% or more ×: Yield of less than 50%

[Evaluation of powder stackability and modeled products]
Using the polyacetal powders obtained in Examples and Comparative Examples, they were molded into strips having a length of 5 cm, a width of 1 cm, and a thickness of 4 mm in the stacking direction by a commercially available selective laser sintering method. Then, the stackability of the polyacetal powder, the warp at the time of modeling, and the mechanical properties of the modeled product were evaluated according to the following evaluation methods.

[[Polyacetal powder stackability at high temperature]]
The powder supply unit is heated to 155 ° C and the modeling area is heated to 160 ° C to perform modeling, and the surface condition of the modeling area during modeling is visually observed. Was evaluated.
⊚: There is no agglomerate when laminating the powder, and the powder is uniformly laminated on the modeled portion during modeling.
◯: There is no agglomerate when the powder is laminated, and the powder is partially unevenly laminated on the modeling portion during modeling.
Δ: About 1 to 2 agglomerates.
X: There is an agglomerate on the entire surface and it cannot be modeled.
The more uniform the powder is laminated on the modeled portion without agglomerates, the better the stackability at high temperature.

[[Powder on the modeled area]]
The strip-shaped shaped parts were visually observed during the molding, and the powder loading property of the polyacetal powder on the shaped parts was evaluated according to the following criteria.
⊚: The powder is evenly laminated on the molten portion irradiated with the laser.
◯: In the initial stage of modeling, the powder is not sufficiently placed on the periphery of the strip, but the powder is gradually and evenly stacked.
Δ: The powder is unevenly placed on the periphery of the strip.
X: The powder is unevenly placed on the entire molten portion irradiated with the laser.
The more the powder is uniformly laminated on the entire molten portion irradiated with the laser, the better the powder can be placed on the shaped part.

[[Mechanical properties of modeled products]]
The molded test piece was subjected to a tensile test at a tensile speed of 5 mm / min in accordance with ISO527-1, and the tensile fracture stress was measured. The obtained measured values were evaluated according to the following criteria.
⊚: 40 MPa or more ◯: 20 MPa or more and less than 40 MPa Δ: 10 MPa or more and less than 20 MPa ×: less than 10 MPa The larger the value of the tensile fracture stress, the better the mechanical properties.

(Example 1)
<Preparation of polyacetal copolymer>
(1) Polymerization step A self-cleaning type twin-screw paddle type continuous mixing reactor with a jacket that allows a heat medium to pass through (screw diameter 3 inches, length ratio to diameter (L / D) = 10) at 80 ° C. Adjusted to. The flow rate of trioxane as the main monomer is adjusted in the range of 3750 g / hr, 1,3-dioxolane as the comonomer is adjusted in the range of 5 to 150 g / hr, and methylal as the chain transfer agent is adjusted in the range of 2.0 to 8.0 g / hr. , Continuously fed into 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 catalyst was 2.0 × 10 ^-5 mol with respect to 1 mol of trioxane. Then, polymerization was carried out to obtain polymerized flakes. The amount of the chain transfer agent added was adjusted so that the melt flow rate of the obtained polyacetal powder was desired.
After pulverizing the obtained polymerization flakes, the pulverized product was put into a 1% by mass aqueous solution of triethylamine as a catalyst neutralization deactivating agent and stirred to deactivate the polymerization catalyst. Then, the triethylamine 1% by mass aqueous solution containing the polymerization flakes was sequentially filtered, washed and dried to obtain a crude polyacetal copolymer.

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

<Granulation process>
The polyacetal copolymer collected by filtration was dried in a gear oven (set at 80 ° C.) under a nitrogen atmosphere after confirming a product temperature of 80 ° C. for 3 hours.
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 Chivas Specialty Chemicals Co., Ltd.)). 0.2 parts by mass was mixed with a Henchel mixer for 1 minute. Then, the obtained polyacetal composition was put into a screw type twin-screw extruder with a vacuum vent set at 200 ° C. (manufactured by Rastic Engineering Laboratory Co., Ltd., BT-30, L / D = 44, L: twin-screw extruder). The screw rotation speed was 100 rpm at the distance (m) from the raw material supply port to the discharge port, D: the inner diameter (m) of the twin-screw extruder, and melt-kneading was performed 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. The results are shown in Table 1.

<Crushing / classification process>
The obtained pellets were frozen and pulverized with a Linlex Mill (registered trademark) (manufactured by Hosokawa Micron Co., Ltd.) while freezing in liquid nitrogen, and sieved to appropriately adjust the powder characteristics to obtain a powder.

<Hydrophobic silica mixing process>
To 10 kg of the obtained powder, 30 g of dry silica (Aerosil (registered trademark) R974 manufactured by Aerosil Japan Co., Ltd.) hydrophobicized with dimethyldichlorosilane as hydrophobic silica was added, mixed with a tumbler mixer, and the surface of the particles was mixed. A polyacetal powder having hydrophobic silica attached to it was obtained.
The obtained polyacetal powder was measured and evaluated as described above. The results are shown in Table 1.

(Examples / Reference Examples 2 to 11, Comparative Examples 1 to 8)
Presence / absence of hydrophobic silica addition, type of hydrophobic silica, addition amount, average primary particle size, BET specific surface area, type of surface modifier, comonomer addition flow rate in preparation of polyacetal copolymer, classification condition in preparation of polyacetal powder, Polyacetal powders of Examples / Reference 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, the same as in Example 1 in Examples / Reference Examples 2, 4 to 6 and 9 to 11 and Comparative Examples 3 to 7, and in Reference Example 3, a dry method hydrophobized with dimethyldichlorosilane. Silica (Leoloseal manufactured by Tokuyama Corporation (registered trademark) DM20-S), dry silica (HM-20L manufactured by Tokuyama Corporation) hydrophobicized with trimethylchlorosilane in Example 7, and wet silica (Tosoh silica) in Reference Example 8. The average particle size of the secondary aggregate of SS-20 manufactured by the same company (2 μm) was used. Silica was not used in Comparative Example 1, and hydrophilic silica (QS-102 manufactured by Tokuyama Corporation) was used in Comparative Example 2. In Comparative Example 8, the pellets obtained by melt-kneading hydrophobic silica similar to those in Example 1 were crushed and classified in the granulation step without performing the hydrophobic silica mixing step to obtain a polyacetal powder.
Each of the obtained polyacetal powders was measured and evaluated as described above. The results are shown in Table 1.

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

As shown in Table 1, the polyacetal powder according to the example is excellent in stackability at high temperature and powder placement on the molding site, and the molded product formed by the selective laser sintering method using the polyacetal powder according to the example is It was found that it has high mechanical properties and high molding accuracy.

According to the present invention, it is possible to provide a polyacetal powder which can be used in an additive manufacturing technique, can obtain a modeled product having high mechanical properties, and has excellent stackability at high temperature and excellent powder loading on a modeled portion. Can be done. Further, according to the present invention, it is possible to provide a method of using the above-mentioned laminated powder and an additional manufacturing method capable of obtaining a modeled product having high mechanical characteristics.

Claims (17)

It contains a polyacetal copolymer having 0.1 mol or more and 1.3 mol or less of a comonomer unit with respect to 100 mol of an oximethylene unit, and 0.05% by mass or more and 1.0% by mass or less of hydrophobic silica .
The average particle size (D50) is 30 μm or more and 70 μm or less.
The melting point is 167 ° C or higher and 178 ° C or lower.
The crystallization start temperature is 140 ° C or higher and 152 ° C or lower.
A polyacetal powder characterized by that. The polyacetal powder according to claim 1, wherein the hydrophobic silica is drywall silica. The polyacetal powder according to claim 1 or 2, wherein the average primary particle size of the hydrophobic silica is 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 hydrophobicized 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, wherein the weight loss rate when the polyacetal powder is held at a temperature 10 ° C. lower than the melting point for 1 hour is 5% or less. The polyacetal powder according to any one of claims 1 to 6, wherein the value obtained by dividing the solidified bulk density by the loosened bulk density (solidified bulk density / loosened bulk density) is 1.45 or less. The polyacetal powder according to any one of claims 1 to 7, wherein the proportion of particles having a diameter of twice or more that of D50 is 10 vol% or less. The polyacetal powder according to any one of claims 1 to 8, wherein the proportion of particles having a diameter of 0.2 times or less the D50 is 10 vol% or less. The polyacetal powder according to any one of claims 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 the D50 is 3 vol% or less. 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 the differential scanning calorimetry (DSC) measurement is 0 ° C. or higher and 5 ° C. or lower. The polyacetal powder according to any one of claims 1 to 11 , wherein the crystallization rate is 30 seconds or more and 50 seconds or less. The polyacetal powder according to any one of claims 1 to 12 , wherein the average minor axis / major axis ratio of the particles constituting the polyacetal powder is 0.50 or more. The polyacetal powder according to any one of claims 1 to 13 , wherein the particles constituting the polyacetal powder have an average surface smoothing shape coefficient of 0.80 or more. The polyacetal powder according to any one of claims 1 to 14 , which is used for additive production. The method for using a polyacetal powder according to any one of claims 1 to 15 , wherein the polyacetal powder is used for additive production. An additional manufacturing method, which comprises using the polyacetal powder according to any one of claims 1 to 16 as a modeling material.

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