JP2018002982A - Polyacetal resin, and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyacetal resin that has low volatility, and also has reduced occurrences of MDs (mold deposits) and bleeding.SOLUTION: In a polyacetal resin, in a crystal structure measured by pulse NMR, the percentage of a crystal phase is 45 wt%-50 wt%, the percentage of an intermediate phase is 45 wt%-50 wt%, and a melting point is 160°C-170°C.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a polyacetal resin and a method for producing the same.

Polyacetal resin is a resin excellent in mechanical strength such as rigidity and toughness, slidability, and creep properties, and is widely used mainly in automobile parts, electrical / electronic devices and various mechanical parts.
In recent years, it has become important to reduce volatile substances generated from resins, mainly in the field of automobile parts and the like. Volatile substances are organic compounds that easily volatilize in the atmosphere at normal temperature and pressure, and formaldehyde is a problem in polyacetal resins. Therefore, a technique for reducing volatilization of formaldehyde from the polyacetal resin is required.
As a conventional technique for reducing volatilization of formaldehyde from a polyacetal resin, for example, a technique of adding a hydrazide compound, an imide compound, and a sterically hindered phenol as additives is known (Patent Document 1). Moreover, the technique which prescribes | regulates the terminal of a polyacetal to a specific structure, and adds a guanamine compound and an ester compound is disclosed (patent document 2).

JP-A-2005-336304 JP 2007-051205 A

  However, the technique for reducing volatilization of formaldehyde is based on the addition of a nitrogen compound that reacts with formaldehyde, and there are many problems arising from the additive. For example, generation of deposits (mold deposit, which may be abbreviated as MD hereinafter) on the mold during injection molding, generation of additive bleeding from the molded piece, and the like can be mentioned. As described above, it is difficult to suppress the occurrence of MD while reducing the volatilization of formaldehyde (hereinafter referred to as “MD property”) and to reduce the occurrence of bleed (hereinafter referred to as “bleed property”). Met.

  As a result of intensive investigations to solve the above problems, the present inventors have found that the volatilization of formaldehyde can be reduced by controlling the ratio of the crystal phase and the intermediate phase in the crystal structure in the polyacetal resin to a specific range, The present invention has been reached.

That is, the present invention is as follows.
[1] A polyacetal resin having a crystal phase ratio of 45 wt% to 50 wt%, an intermediate phase ratio of 45% to 50%, and a melting point of 160 ° C. to 170 ° C. in the crystal structure measured by pulse NMR.
[2] The polyacetal resin according to [1], wherein a crystal phase ratio in a crystal structure measured by pulse NMR is 48 wt% to 50 wt% and an intermediate phase ratio is 48 wt% to 50 wt%.
[3] The polyacetal resin according to [1] or [2], wherein the ratio of the intermediate phase / (the ratio of the crystal phase + the ratio of the intermediate phase) in the crystal structure measured by pulse NMR is 0.49 or more.
[4] The polyacetal resin according to any one of [1] to [3], wherein the amount of fluorine in the polyacetal resin is 10 ppm or less.
[5] A molded article comprising the polyacetal resin according to any one of [1] to [4].
[6] A step of premixing a cyclic ether and / or a cyclic formal and a polymerization catalyst, and a step of polymerizing the mixture obtained in the premixing step and other monomers, [1] to [4] ] The manufacturing method of the polyacetal resin in any one of.
[7] The method for producing a polyacetal resin according to [6], wherein in the premixing step, an instrument whose inner wall has an arithmetic average roughness of 3 μm or less is used.

  According to the present invention, it is possible to provide a polyacetal resin with less formaldehyde volatilization and excellent MD and bleed properties.

FIG. 5 is an attenuation curve obtained by fitting using Equation (1) to the attenuation curve obtained by pulse NMR measurement for the polyacetal resin of Example 1. FIG.

Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “the present embodiment”) will be described in detail.
In addition, this invention is not limited to the following description, In the range of the summary, various deformation | transformation can be implemented.

[Polyacetal resin]
In the present embodiment, the polyacetal resin refers to a polymer compound having an oxymethylene structure as a unit structure, and among them, a copolymer is preferable.

  Typical examples of the polyacetal copolymer include a polyacetal copolymer obtained by copolymerizing trioxane and a cyclic ether and / or cyclic formal in the presence of a polymerization catalyst. Trioxane is a cyclic trimer of formaldehyde and is generally obtained by reacting an aqueous formaldehyde solution in the presence of an acidic catalyst. Since this trioxane may contain impurities that cause chain transfer such as water, methanol, formic acid, and methyl formate, it is preferable to remove and purify these impurities by a method such as distillation. In that case, the total amount of impurities acting as a chain transfer agent is preferably 1 × 10 −3 mol or less, more preferably 5 × 10 −4 mol or less, relative to 1 mol of trioxane. By reducing the amount of impurities as indicated above, the polymer produced has excellent thermal stability (less formaldehyde volatilization). Moreover, MD property and bleed property when mixed with additives and the like are excellent.

Cyclic ether and / or cyclic formal is a component copolymerizable with the trioxane, such as ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, epibromohydrin, styrene oxide, oxatan, 1,3-dioxolane, ethylene. Examples include glycol formal, propylene glycol formal, diethylene glycol formal, triethylene glycol formal, 1,4-butanediol formal, 1,5-pentanediol formal, and 1,6-hexanediol formal.
Among these, 1,3-dioxolane and 1,4-butanediol formal are particularly preferable.
These may be used alone or in combination of two or more.
The addition amount of cyclic ether and / or cyclic formal is preferably 1.0 × 10 −2 mol or more and 7.0 × 10 −2 mol or less, more preferably 3.0 × 10 −2 mol or more and 6.5 mol or less with respect to 1 mol of the trioxane. 0 × 10 −2 mol or less.
When the copolymerization ratio of the cyclic ether and / or the cyclic formal is within this range, it becomes easy to adjust the ratio of the crystal phase and the intermediate phase to the range described later.

Examples of the polymerization catalyst include boric acid typified by Lewis acid, tin, titanium, phosphorus, arsenic and antimonide, especially boron trifluoride, boron trifluoride hydrate, and oxygen atom or sulfur atom. A coordination complex compound of an organic compound containing and boron trifluoride is preferable. For example, boron trifluoride, boron trifluoride diethyl etherate, and boron trifluoride-di-n-butyl etherate are preferable examples.
These may be used alone or in combination of two or more.
The addition amount of the polymerization catalyst is preferably in the range of 1 × 10 −6 to 1 × 10 −4, more preferably in the range of 3 × 10 −6 to 5 × 10 −5 mol, and even more preferably with respect to 1 mol of the trioxane. Is in the range of 5 × 10 −6 to 4 × 10 −5 mol.
When the addition amount of the polymerization catalyst is within the above range, a long-time polymerization reaction can be carried out stably.

The polyacetal resin of the present embodiment has a crystal phase ratio of 45 wt% to 50 wt%, an intermediate phase ratio of 45 wt% to 50 wt%, and a melting point of 160 ° C. to 170 ° C. in the crystal structure measured by pulse NMR. It is. The proportion of the crystal phase is preferably 48 wt% to 50 wt%. The proportion of the intermediate phase is preferably 48 wr% to 50 wt%.
Here, pulse NMR is one of solid-state NMR, and is a technique for detecting a 1H nuclear magnetic relaxation time (an index representing molecular mobility) of a sample by detecting a response signal to the pulse. In response to the pulse, a free induction decay (FID signal) is obtained. When a sample having a plurality of components having different molecular motility such as a crystalline resin is measured, the obtained FID is the sum of the FIDs of the plurality of components having different relaxation times, and this is separated using the least square method. Thus, the amount of each component can be detected.
In general, the crystalline resin is often separated into three components: a component having a relaxation time of 0.01 to 0.02 ms, a component having 0.03 to 0.1 ms, and a component having 0.1 ms or more, A component having a relaxation time of 0.01 to 0.02 ms corresponds to a crystalline phase, a component having a relaxation time of 0.1 ms or more corresponds to an amorphous phase, and a component having a relaxation time of 0.03 to 0.1 ms is an intermediate phase. (However, the amounts of these components are different from the crystallinity obtained by analysis by other measurement means such as WAXS and DSC).
The above-mentioned FID (sum of FID of each component) is represented by the following formula (1), where the proportion of the crystal phase is represented by Cr and the proportion of the intermediate phase is represented by Ca.

Here, M (t) represents the magnetization strength at time t (ms), and Cr, Ca, and Cm represent the proportions (wt%) of the crystal phase, intermediate phase, and amorphous phase contained in the polyacetal resin, respectively. Tc, Ta, and Tm represent relaxation times (ms) of the crystal phase, the intermediate phase, and the amorphous phase, respectively.
Wa is a fitting parameter for representing the attenuation curve of the intermediate phase. b is a parameter reflecting the order of proton arrangement. The values of Wa and b vary depending on the object to be measured. In this embodiment (polyacetal resin), Wa = 1.2 and b = 170.

  When the ratios Cr and Ca of the crystal phase and the intermediate phase of the polyacetal resin are in the above ranges, the volatilization of formaldehyde can be reduced, and the MD property and the bleed property when mixed with an additive or the like are excellent. The reason why the volatilization of formaldehyde can be reduced when the ratio of the crystal phase and the intermediate phase is in the above range is not clear, but since such polyacetal resin has an appropriate gap in the crystal structure, formaldehyde is scattered during production. This is presumably because the amount of formaldehyde remaining in the resin is reduced. However, the mechanism does not depend on this.

  Further, the value of the ratio of the intermediate phase / (the ratio of the crystal phase + the ratio of the intermediate phase) is preferably 0.49 or more. If the ratio of the intermediate phase to the total ratio of the crystal phase and the intermediate phase is high to some extent, it is estimated that the above-mentioned effect (the crystal structure has an appropriate gap and formaldehyde is likely to be scattered during production) is more exhibited. .

The method for obtaining the polyacetal resin of this embodiment is not limited. For example, at least a part of the cyclic ether and / or cyclic formal is premixed with the polymerization catalyst in advance, and then another monomer (for example, trioxane). ). At this time, the cyclic ether and / or cyclic formal may be premixed in the whole amount, or a part thereof may be premixed and the remaining amount may be mixed in another monomer (trioxane).
The material used for the premixing (for example, pipes and mixing containers) is preferably a material having acid resistance and pitting corrosion resistance of SUS304 or higher, or a fluororesin, glass or the like. The surface roughness of the inner wall of the device that contacts the cyclic ether and / or cyclic formal and the polymerization catalyst is preferably 3 μm or less, and more preferably 2.5 μm or less. Alternatively, it is also preferable that the surface is processed with a foot print resin. Here, the surface roughness is an arithmetic average roughness (Ra) defined in JIS B0601-2013. Specifically, from the roughness curve obtained for the inner wall, a reference length (l) is extracted in the direction of the average line, and the absolute value of the deviation from the average line of the extracted portion to the roughness curve is summed and averaged. When the roughness curve is represented by y = f (x), it is a value represented by the following formula.
When premixing is performed using an instrument having a small inner wall surface roughness, a polyacetal resin in which the ratio of the crystal phase and the intermediate phase is within the above-mentioned range is likely to be generated.
Thus, by mixing at least a part of the cyclic ether and / or cyclic formal with the polymerization catalyst in advance, and further reducing the surface roughness of the inner wall of the instrument used at that time, The polyacetal resin of this embodiment can be obtained.

  In addition, after the pre-mixing step, a conventionally known polymerization step and steps such as deactivation, drying, terminal stabilization, and stabilizer blending may be performed as necessary.

Further, when trioxane is used as a monomer in the polymerization, the ratio of the crystal phase and the intermediate phase can be easily set within the above range by reducing the amount of impurities.
Trioxane generally contains impurities that act as chain transfer agents such as water, methanol, formic acid, and methyl formate. Therefore, it is preferable to remove and purify these impurities by a method such as distillation. In that case, the total amount of impurities to be chain-transferred is preferably 1 × 10 −3 mol or less, more preferably 5 × 10 −4 mol or less with respect to 1 mol of trioxane.

The melt index (MI) (ASTM-D-1238-57T, 190 ° C., 2.16 kg) of the polyacetal resin of the present embodiment is preferably 0.5 g / 10 min or more and less than 120.0 g / 10 min, more preferably 1.0 g. / 10 min or more and less than 80.0 g / 10 min, more preferably 2.0 g / 10 min or more and less than 60.0 g / 10 min.
When MI is in the above range, the characteristics of polyacetal resin, such as mechanical strength and abrasion resistance, are sufficiently developed.

The melting point of the polyacetal resin of this embodiment is 160 ° C to 170 ° C. By setting the melting point to 160 ° C. to 170 ° C., volatile components can be further reduced.
Furthermore, the polyacetal resin of this embodiment preferably has a fluorine content of 10 ppm or less. Fluorine is generally due to the residue of the polymerization catalyst, and the amount of fluorine can be reduced by sufficiently performing catalyst deactivation and kneading after polymerization. A smaller amount of fluorine is better and there is no lower limit. For example, if the amount of fluorine is 5 ppm or more, there is little problem. By setting it as such a range, the quantity of a crystal phase and an intermediate phase can be controlled moderately.

The polyacetal resin of the present embodiment is an additive used in combination with a conventional polyacetal resin within a range not impairing the effects of the present invention, for example, an antioxidant, a heat stabilizer, a light stabilizer, a release agent, You may add and use a coloring agent etc.
These may be used alone or in combination.

  Although it does not specifically limit as a molded object of the polyacetal resin of this embodiment, The molded object shape | molded by injection molding, extrusion molding etc. is mentioned, For example, a motor vehicle field, household appliances OA field | area, a medical field, industrial It can be used in the material field, and can be suitably used particularly in the automobile field, home appliance OA field, and medical field.

Hereinafter, the present invention will be described in detail with specific examples and comparative examples, but the present invention is not limited to the following examples.
In addition, the measurement method of the term and characteristic in an Example and a comparative example was as follows.

<Measurement of ratio of crystal phase and intermediate phase by pulse NMR>
Apparatus: Minispec MQ20 (Bruker Biospin Co., Ltd.)
Nuclide: 1H
Measurement: T2
Measurement method: Solid echo method Integration count: 256 times Repeat time: 1.0 sec
Measurement temperature: 30 ° C
Using the above apparatus and conditions, about 900 mg of sample pellets were filled in a 10 mmφ sample tube, and pulse NMR was measured to obtain an attenuation curve.
The obtained attenuation curve was analyzed by fitting using the following formula (1), and the ratios of crystal phase, intermediate phase and amorphous phase in the polyacetal resin, Cr, Ca and Cm (wt%), and relaxation Times Tc, Ta and Tm (ms) were obtained. For fitting and analysis, software attached to the measurement apparatus was used.
Cr, Ca, and Cm represent the proportions (wt%) of the crystalline phase, the intermediate phase, and the amorphous phase, respectively.
Tc, Ta, and Tm represent relaxation times (ms) of the crystal phase, the intermediate phase, and the amorphous phase, respectively.
Wa is a fitting parameter for representing the attenuation curve of the intermediate phase, and Wa = 1.2. b is a parameter reflecting the order of proton arrangement, and b = 170.

<Measurement of volatilization amount of formaldehyde (mg / kg)>
Using a 100GN injection molding machine manufactured by Toshiba Machine Co., Ltd., a test piece of 40 mm × 100 mm and a thickness of 3 mm was molded from the polyacetal resin pellets at a molding temperature of 220 ° C. and a mold temperature of 80 ° C. The obtained test piece was left for 1 day in an environment of a temperature of 23 ° C. and a humidity of 50%. Thereafter, the amount of formaldehyde generated at 60 ° C. × 3 hr was measured according to the VDA275 test of the German Industrial Association (VDA).

<Evaluation of MD>
Using a 100 GN injection molding machine manufactured by Toshiba Machine Co., Ltd., 2000 shots of 40 mm × 100 mm test pieces having a thickness of 3 mm were continuously molded from polyacetal resin pellets at a molding temperature of 200 ° C. and a mold temperature of 30 ° C. The state of the mold surface after 2000 shot molding was observed, and the results were evaluated in the following five stages.
Here, “mold” refers to a molding frame on the mold surface.
5. No deposit was observed inside or outside the cabinet.
4. A small amount of deposit was observed outside the cabinet. No deposits were observed in the cabinet.
3. A large amount of deposits were observed outside the cabinet. No deposits were observed in the cabinet.
2. A small amount of deposit was observed in the cabinet.
1. A large amount of deposits were observed in the cabinet.

<Evaluation of bleeding>
Using a 100GN injection molding machine manufactured by Toshiba Machine Co., Ltd., a test piece of 40 mm × 100 mm and a thickness of 3 mm was molded from the polyacetal resin pellets at a molding temperature of 220 ° C. and a mold temperature of 80 ° C. The test piece was allowed to stand for 300 hours in an environment of a temperature of 80 ° C. and a humidity of 90%, the state of the surface of the test piece was observed, and the results were evaluated in the following five stages.
Here, “bleed” means that an additive or the like is raised on the surface of the test piece.
5. No bleed occurred.
4. Bleed occurred on less than 10% of the test piece surface.
3. Bleeding occurred on 10% or more and less than 30% of the test piece surface.
2. Bleed occurred on 30% or more and less than 60% of the test piece surface.
1. Bleed occurred in 60% or more of the test pieces.

<Measurement of polymerization yield (wt%)>
The mass of the obtained polyacetal resin was determined by dividing by the total mass of trioxane, cyclic ether and cyclic formal used in the polymerization.
<Measurement of melting point (° C.)>
5 mg of resin was collected from the polyacetal resin pellets, and the melting point was measured under the following conditions.
・ Device: DSC-7 manufactured by PerkinElmer Co., Ltd.
Conditions: The temperature was raised to 200 ° C. at 80 ° C./min, held for 2 min, then lowered to 50 ° C. at 80 ° C./min, raised to 200 ° C. at 2.5 ° C./min, and the melting point was measured.
<Measurement of MI (g / 10 min)>
According to ASTM D1238, measurement was performed at 190 ° C. and a load of 2.16 kg using a MELT INDEXER manufactured by Toyo Seiki Seisakusho.

<Measurement of fluorine content>
The measurement was performed with a fluorine ion meter.
Ion meter: HORIBA's ion meter Fluorine electrode: HORIBA's fluorine ion electrode a) Preparation of measurement sample Polyacetal resin pellets 3g, 1N HCl 15g were charged in a pressure-resistant bottle, thermally decomposed at 130 ° C for 3 hours, cooled and then decomposed 8g A sample solution was prepared by adding 32 g of pure water and 60 g of Buffer solution.
b) Preparation of Buffer liquid To 500 ml of pure water, 121 g of tris (hydroxymethyl) aminomethane, 158 g of tris (hydroxymethyl) aminomethane hydrochloride, and 230 g of sodium tartrate dihydrate were added and dissolved. It adjusted so that it might become 1L.
c) Preparation of standard solutions Three types of commercially available fluorine standard solutions with known concentrations are prepared, and 3 g of standard solutions are prepared by adding 60 g of the above buffer solution and 38 g of pure water to 2 g each, and using these standards for calibration Created a line.

[Example 1]
A jacketed biaxial paddle type continuous polymerization reactor (made by Kurimoto Steel Works, diameter 2B, L / D = 14.8) through which a heat medium can pass was adjusted to 80 ° C.
As premixing, the polymerization catalyst and cyclic ether and / or cyclic formal were mixed using a T-shaped pipe (arithmetic average roughness of the inner wall of the pipe 2.8 μm) so that both collided from the front. Specifically, boron trifluoride-di-n-butyl etherate 0.18 g / hr and cyclohexane 6.5 g / hr continuously mixed as a polymerization catalyst, cyclic ether and / or cyclic formal 1 , 3-Dioxolane 120.9 g / hr was continuously introduced from each sleeve of the above-mentioned T-shaped pipe, and both were collided in front to obtain a pre-mixed solution. The temperature of the polymerization catalyst and cyclic ether and / or cyclic formal during premixing was 20 ° C.
From the separate pipes, 127.58 g / hr of the pre-mixed liquid obtained in this way and a mixed liquid in which methylal 2.1 g / hr as a molecular weight regulator was continuously mixed with 3500 g / hr of trioxane in a pipe. The polymerization was continuously supplied to the polymerization reactor, and polymerization was carried out under the conditions of a polymerization reactor set temperature of 80 ° C., a polymerization peak temperature of 110 ° C., and a polymerization time (residence time) of 5 minutes. The amount of impurities in trioxane was 7 × 10 −4 mol / mol-trioxane.
What was discharged from the polymerization reactor was put into a triethylamine aqueous solution (0.5% by mass) to completely deactivate the polymerization catalyst, followed by filtration, washing and drying to obtain a crude polyacetal copolymer. . Triethyl (2-hydroxyethyl) ammonium formate as a quaternary ammonium compound was added to the obtained crude polyacetal copolymer so as to be 10 ppm in terms of the amount of nitrogen. After uniformly mixing, it was dried at 120 ° C.
The resulting polyacetal resin (polyacetal copolymer) had a polymerization yield of 80% and a melting point of 164 ° C.
Next, 0.35 parts by mass of triethylene glycol-bis- [3- (3-tert-butyl-5-methyl-4-hydroxyphenyl) propionate] is added to and mixed with 100 parts by mass of the dried polyacetal resin, and the mixture is heated to 200 ° C. It is supplied to a set twin-screw extruder with a vent (L / D = 40) together with 2 parts by weight of 0.8% by weight triethylamine aqueous solution, decomposed and stabilized while degassing under reduced pressure at 21 KPa, and pelletized by a pelletizer did. Thereafter, drying was performed at 100 ° C. for 2 hours to obtain stable polyacetal resin pellets. The results are shown in Table 1.

[Example 2]
The same operation as in Example 1 was performed except that the amount of impurities in trioxane was 4 × 10 −4 mol / mol-trioxane and the amount of methylal was 2.9 g / hr. At this time, the polymerization yield was 81%. The results are shown in Table 1.
Example 3
The same operation as in Example 1 was performed, except that the arithmetic average roughness of the inner wall surface of the T-shaped pipe used for premixing was 2.1 μm. At this time, the polymerization yield was 81%. The results are shown in Table 1.
Example 4
The same operation as in Example 1 was performed except that the supply amount of 1,3-dioxolane was changed to 43.2 g / hr. At this time, the polymerization yield was 81%. The results are shown in Table 1.

[Comparative Example 1]
The operation was performed in the same manner as in Example 1 except that the arithmetic average roughness of the inner wall surface of the T-shaped pipe used for premixing was changed to 5 μm. The polymerization yield was 79%. The results are shown in Table 1.
[Comparative Example 2]
The same operation as in Example 1 was performed, except that 1,3-dioxolane was continuously mixed and supplied to trioxane without performing premixing, and the amount of the polymerization catalyst was 0.18 g / hr. At this time, the polymerization yield was 79%. The results are shown in Table 1.
[Comparative Example 3]
The same operation as in Example 1 was performed except that the supply amount of 1,3-dioxolane was changed to 14.4 g / hr. At this time, the polymerization yield was 78%. The results are shown in Table 1.

  The polyacetal resins of Examples 1 to 4 had less formaldehyde volatilization and excellent MD and bleed properties than those of the comparative examples.

  Since the polyacetal resin of the present invention has little volatilization of formaldehyde and is excellent in MD property and bleeding property, it can be used for various applications. In particular, the polyacetal resin of the present invention can be suitably used for automobile parts, electrical / electronic devices and the like.

Claims (7)

  A polyacetal resin having a crystal phase ratio of 45 wt% to 50 wt%, an intermediate phase ratio of 45% to 50%, and a melting point of 160 ° C to 170 ° C in a crystal structure measured by pulse NMR.   2. The polyacetal resin according to claim 1, wherein the proportion of the crystal phase in the crystal structure measured by pulse NMR is 48 wt% to 50 wt% and the proportion of the intermediate phase is 48 wt% to 50 wt%.   The polyacetal resin according to claim 1 or 2, wherein the ratio of the intermediate phase / (the ratio of the crystal phase + the ratio of the intermediate phase) in the crystal structure measured by pulse NMR is 0.49 or more.   The polyacetal resin in any one of Claims 1-3 whose fluorine amount in a polyacetal resin is 10 ppm or less.   The molded object containing the polyacetal resin in any one of Claims 1-4. A step of premixing a cyclic ether and / or a cyclic formal and a polymerization catalyst, and a step of polymerizing the mixture obtained in the premixing step and other monomers.
The manufacturing method of the polyacetal resin in any one of Claims 1-4 containing this.   The manufacturing method of the polyacetal resin of Claim 6 using the instrument whose arithmetic mean roughness of an inner wall is 3 micrometers or less in the said pre-mixing process.