Method for producing polyoxymethylene resin composition

Abstract

A polyoxymethylene resin composition from which formaldehyde is generated in a reduced amount upon molding, without carrying out a stabilization treatment for stabilizing an unstable terminal group in a crude oxymethylene copolymer. A hindered phenol-type antioxidant agent and an ethylene-(methacrylic acid) copolymer resin or an ethylene-(acrylic acid) copolymer resin or a salt thereof are melted and kneaded together to produce a polyoxymethylene resin composition without carrying out a stabilization treatment for stabilizing an unstable terminal group in a crude oxymethylene copolymer for which a polymerization catalyst is deactivated after the completion of copolymerization, and in which the unstable terminal group is not yet stabilized. When the polyoxymethylene resin composition is extrusion-molded, the discoloration of a molded product or the deterioration in properties of the molded product can be prevented for a long period of time. The resulting polyoxymethylene resin composition is suitable for forming an extrusion-molded article.

Claims

1. A method of producing a polyoxymethylene resin composition, comprising supplying and melt-kneading the following: a crude oxymethylene copolymer for which a polymerization catalyst has been deactivated after completing the copolymerization but for which unstable terminal groups have not been stabilized, a hindered phenol-type antioxidant, an ethylene-methacrylic acid copolymer resin, and an alkaline earth metal compound, without carrying out a stabilization treatment to stabilize the unstable terminal groups, and in the absence of a salt of said ethylene-methacrylic acid copolymer resin, wherein an extruder having a degassing vent opening at one or more places is used, and wherein at least one selected from water and a low-boiling alcohol is further supplied within a range of 0.1 parts by mass to 10 parts by mass with respect to 100 parts by mass of the crude oxymethylene copolymer at a discretionary location from the main feed to the degassing vent opening.

2. A method of producing a polyoxymethylene resin composition according to claim 1, wherein the alkaline earth metal compound is an aliphatic carboxylic acid metal salt or an oxide.

3. A method of producing a polyoxymethylene resin composition according to claim 1, wherein the alkaline earth metal compound is calcium stearate, and further, the calcium stearate is used in a range of 0.003 mass % to 0.020 mass % with respect to the crude oxymethylene copolymer.

4. A method of producing a polyoxymethylene resin composition according to claim 1, wherein the polyoxymethylene resin composition is used for forming an extrusion molded article.

Description

EXAMPLES

(1) Below, the present invention is specifically explained with examples, but the present invention is not limited to these examples.

(2) <Preparation of the Crude Oxymethylene Copolymer>

(3) Using a continuous mixing reactor having a cross sectional form of two circles which partially overlap, and a barrel provided with a jacket though which a heating (cooling) medium flows on the outer side, and in the lengthwise direction of the inner portion of this barrel, two rotating shafts each provided with respective paddles for mixing and driving, the following polymerization reaction was carried out.

(4) Hot water at a temperature of 80 C. was passed through the jacket, and the two rotating shafts were rotated at a rate of 100 rpm, and trioxane comprising 0.05 mass % of triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] as an antioxidant, 3.3 mass % of 1,3-dioxolane as a comonomer, 700 ppm (mass based) of methylal as a chain transfer agent, was continuously supplied to the reactor, and along with this, in parallel, a solution (1 mass % concentration) of boron trifluoridedibutyl etherate dissolved in cyclohexane was continuously added at a concentration of 10 ppm (mass based) of boron trifluoride to the total monomers (the total amount of trioxane and 1,3-dioxolane), and copolymerization was carried out. Next, to the crude polyoxymethylene copolymer discharged from the exhaust opening of the reactor, an aqueous solution comprising 0.1 mass % of triethylamine was added, and the catalyst was deactivated. This mixture was subjected to centrifugation treatment, and further dried, to obtain the crude oxymethylene copolymer.

(5) In the crude oxymethylene copolymer, the hemiacetal terminal group amount was 2.2 mmol/kg, and the formyl terminal group amount was 1.7 mmol/kg, and the unstable terminal group amount (the amount of the terminal unstable portions) was 1.18 mass %.

(6) Further, in the present invention, the hemiacetal terminal group amount and the formyl terminal group amount were evaluated by the following technique.

(7) The crude oxymethylene copolymer was dissolved in hexafluoroisopropyl alcohol, and N,O-bis(trimethylsilyl)trifluoroacetamide and pyridine were added and after reacting and air drying, vacuum drying was performed at 400 C. to remove the remaining solvent and unreacted substances. The obtained reaction product was dissolved in deuterated hexafluoroisopropyl alcohol with a concentration of 5 mass %, the solution was filled into a sample vial for NMR, and the NMR spectrum was measured at room temperature (refer to Japanese Unexamined Patent Application, First Publication No. 2001-11143).

(8) The hemiacetal terminal group amount (mmol/kg) and the formyl terminal group amount (mmol/kg) were each calculated based on the corresponding NMR absorption peaks.

(9) NMR Device: AVANCE 400 type FT-NMR manufactured by Brucker Corporation

(10) Measurement Conditions: Pulse Flip Angle 30, integration repetition time 10 sec, number of integrations 128 times

(11) Further, in the present invention, the unstable terminal amount (the amount of the unstable portions of the terminals) was evaluated by the following technique.

(12) About 1 g of the crude oxymethylene copolymer was precisely weighed, and was introduced into a pressure resistant closed reaction vessel along with 15 mg of calcium hydroxide and 100 ml of a 60 vol % methanol aqueous solution comprising 0.5 vol % ammonium hydroxide, and after heat treating at 170 C. for 60 min, the vessel was cooled, opened, and the solution inside was removed. The amount of formaldehyde dissolved in the solution, arising from the decomposition of the unstable terminal portions was measured according to the JIS K0102, item 29.1, acetylacetone absorption photometry, and the proportion with respect to the crude oxymethylene copolymer was calculated as mass %.

(13) <Preparation of the Stabilized Oxymethylene Copolymer>

(14) With respect to 100 parts by mass of the above crude polyoxymethylene copolymer, 0.01 parts by mass of triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] and 2 parts by mass of triethylammonia aqueous solution (0.72 mass % concentration: the triethylamine was 1.4 mmol calculated for a teritary amine nitrogen per 1 kg of the crude polyoxymethylene copolymer) were added and uniformly mixed.

(15) Next, this mixture was supplied to the above mentioned twin extruder having a degassing opening, with a vent vacuum of 20 mmHg (2.7 kPa), a cylinder temperature of 200 C., and an average residence time of 300 sec, the volatile matter was removed from the degassing opening (vent) while melt-kneading, and a stabilized oxymethylene copolymer was obtained in pellet form.

(16) In the stabilized oxymethylene copolymer, the hemiacetal terminal group amount was 1.2 mmol/kg, the formyl terminal group amount was 1.3 mmol/kg, and the unstable terminal amount (the amount of the terminal unstable portions) was 0.56 mass %.

Example, Reference Example and Comparative Example

(17) TABLE-US-00001 TABLE 1 Reference Comparative Example Example Example 1 2 3 4 5 6 1 1 2 Crude POM 99.38 99.38 99.38 99.38 99.38 99.38 99.4 99.38 copolymer Stabilized POM 99.4 copolymer Hindered phenol- 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 type antioxidant Ethylene- 0.02 0.02 0.02 0.02 0.02 methacrylic acid copolymer resin Ethylene-acrylic 0.02 acid copolymer resin Ethylene- 0.02 methacrylic acid copolymer resin zinc salt Calcium stearate 0.005 0.02 0.1 0.02 0.02 0.1 0.1 Magnesium oxide 0.02 Sliding property 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 enhancing agent (units are mass %)

(18) In Table 1, the various materials are as follows.

(19) 1. Crude POM copolymer

(20) The crude oxymethylene copolymer obtained by the above mentioned <Preparation of the Crude Oxymethylene Copolymer>

(21) 2. Stabilized POM copolymer

(22) The stabilized oxymethylene copolymer obtained by the above mentioned <Preparation of the Stabilized Oxymethylene Copolymer>

(23) 3. Hindered phenol-type antioxidant

(24) pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] (product mane: Irganox 1010, manufactured by BASF Japan Ltd.)
4. Ethylene-methacrylic acid copolymer resin ethylene-methacrylic acid copolymer resin (product name: Nucrel N1525, manufactured by Du Pont-Mitsui Polychemicals Co., Ltd.)
5. Ethylene-acrylic acid copolymer resin (product name: Primacor 3460, manufactured by The Dow Chemical Company)
6. Zinc salt of ethylene-methacrylic acid copolymer resin
7. Alkaline earth metal compound calcium stearate magnesium oxide
8. Sliding property enhancing agent low molecular weight polyethylene (product name: Sanwax 161-P, Sanyo Chemical Industries, Ltd.)

(25) The materials shown in Table 1, in the proportions shown in Table 1 (the units are parts by mass) were input into a TEX 30 extruder (The Japan Steel Works, Ltd.). Next, the above mentioned raw materials which are input into the extruder are kneaded under the extrusion conditions below. Next, the kneaded raw materials are extruded in a strand form, and after cooling, cut into pellets, and dried at 140 C. for 4 hrs. In this way, the polyoxymethylene resin composition pellets (diameter: 1 to 2 mm, length 2 to 3 mm) according to the Examples and Comparative Examples were produced.

(26) (Extrusion Conditions)

(27) Cylinder temperature: 170 to 200 C.

(28) Extrusion amount: 18 kg/hr (using fixed quantity feeder)

(29) Rotation rate: 120 rpm

(30) <Evaluation>

(31) [Amount of Formaldehyde Generated from Melt]

(32) 5 g of the pellets were precisely weighed, and after holding in a metal vessel at 200 C. for 5 min, the atmosphere inside the vessel was absorbed into distilled water. The amount of formaldehyde of this aqueous solution was quantified according to JISK0102, item 29. (formaldehyde item), and the amount of formaldehyde gas generated by the pellets (ppm) was calculated. If the formaldehyde gas amount was less than 70 ppm, it was judged as , and if it was 70 ppm or more, it was judged as x. The results are shown in Table 2.

(33) [Long Term Color Stability (E)]

(34) Using the above mentioned composition pellets, and using a 30 mm uniaxial solidification extrusion molding device, with an extrusion rate of 5 mm/min, round bars with 140 were solidification extruded. Then, the obtained round bar was cut into test pieces according to the ISO 527-11B model.

(35) After this, the above mentioned test pieces were held in an oven at 140 C. for a long period, and the color was evaluated using a colorimeter. The color change degree (E) was calculated as below.
E=[(L.sub.1L.sub.0).sup.2+(a.sub.1a.sub.0).sup.2+(b.sub.1b.sub.0).sup.2].sup.1/2

(36) In the formula, L, a, and b are respectively values of color measured by a color difference meter, and the subscript 1 for L, a, and b indicates the colors after holding in a oven at 140 C. for 20 days, and the subscript 0 indicates the colors measured immediately before putting in the oven.

(37) Then, when the calculated results were less than 10, it was deemed , and if from 10 to 15, it was deemed as . The results are shown in Table 2.

(38) The test pieces prepared for the above mentioned [Long Term Color Stability (E)] were held in an oven at 140 C. for 20 days, and the tensile strength was evaluated. The evaluations were according to ISO 527. If the tensile strength retention rate after long term holding was 100% or more with respect to the tensile strength before long term holding, they were evaluated as , and if the tensile strength rate was less than 100%, the tensile strength retention rate was evaluated as x. The results are shown in FIG. 2.

(39) TABLE-US-00002 TABLE 2 Reference Comparative Example Example Example 1 2 3 4 5 6 1 1 2 Generated 52 55 60 57 55 58 62 55 105 amount of X formaldehyde (Unit: ppm) Long term color 8.3 7.1 11 7.5 8.2 7.6 10.5 7.5 11.3 stability (E) Long term 100 100 100 100 100 100 100 85 100 properties X (Tensile strength retention rate)

(40) On comparing Examples 1 to 6 and Reference Example 1, it can be said that in the case of using a polyoxymethylene resin composition obtained by melt kneading a crude oxymethylene copolymer with a hindered phenol-type antioxidant and an ethylene-methacrylic acid copolymer resin or an ethylene-acrylic acid copolymer resin or their salts as a material for forming a resin molded article, even though no stabilizing treatment for stabilizing the unstable terminal groups has been carried out, it was possible to suppress the generated amount of formaldehyde to approximately the same degree as for the case of using a polyoxymethylene resin composition obtained from a stabilized oxymethylene copolymer for which the stabilization treatment has been carried out. From this, it can be said that according to the present invention, it is possible to provide a polyoxymethylene resin composition where the generated amount of formaldehyde when molding is sufficiently suppressed, even without carrying out the above mentioned stabilization treatment.

(41) Further, even when the polyoxymethylene resin composition was extrusion molded, long term color changes and long term property deterioration was suppressed, whereby the polyoxymethylene resin composition of the present invention was suitable for forming extrusion molded articles (Examples 1 to 6).

(42) On the other hand, in the case of using an ethylene-methacrylic acid copolymer resin singly as an additive, and further extrusion molding the polyoxymethylene resin composition, it was not possible to suppress degradation of the long term property retention rate (Comparative Example 1). Also, the case of using an alkaline earth metal compound was unsuitable because it was not possible to appropriately suppress the generated amount of formaldehyde (Comparative Example 2).