PRIOR ART Polyacetal resins, which are excellent in mechanical properties and have superior resistance to creep, fatigue and wear, are typically applied as engineering plastics mainly for various electric or electronic appliances, automobile parts and general merchandise.

In recent years, application ranges thereof have gradually widened.

Particularly, polyacetal resins are used for bathroom fixtures, sprinklers, conveyer belts and the like, each of which is exposed to tap water or a chlorine-based sterilizing agent. In the case where an article manufactured from polyoxymethylene comes in contact with such tap water or sterilizing agent, the article should be frequently replaced with a new one due to decomposition of polyoxymethylene. For this reason, there has been a need for improving chlorine resistance in polyoxymethylene compounds. That is to say, when any article prepared from polyoxymethylene is in contact with tap water for a long time, the polyoxymethylene is decomposed by the chlorine or chlorides used as tap water sterilizing agents. Further, the chlorine component lowers mechanical properties

of the polyoxymethylene, thereby deteriorating'functions of the polyoxymethylene articles.

In this regard, Japanese Patent Laid-open Nos. Hei. 3-290459 and 7-150005 disclose a method of improving chlorine resistance of polyoxymethylene by adding hydrotalcite and zinc compounds to the polyacetal resin, thus simply chelating (inactivating) chlorine or chlorides. Further, as efforts to increase chlorine resistance of polyoxymethylene, there are reported Korean Patent No. 142504, in which zinc compounds are added to the polyacetal resin, and Japanese Patent Laid-open No. Hei. 8- 100102, in which zinc compounds and organic compounds such as tetrachloroisophthalonitrile are added to the polyacetal resin.

Through the above-mentioned techniques, polyoxymethylene may be improved in chlorine resistance for a short time by inactivating exogenous chlorine or chlorides.

However, the polyoxymethylene is subjected to significant decrease in resistance to chlorine when the chlorine is adsorbed beyond the limited level. In addition, the used hydrotalcite component is in a salt form, and may cause bubbling problems upon processing the hydrotalcite-added polyoxymethylene. Further, such methods are restricted since the polyoxymethylene may be discolored depending on processing methods.

In Japanese Patent Laid-open No. Sho. 57-102943, there is disclosed a stabilized oxymethylene copolymer by use of a stabilizer comprising an initial polycondensed product of amino resin, a hindered phenol and a metal-containing compound such as alkali metal or alkali earth metal hydroxide. However, there is disclosed neither the improvement of chlorine resistance nor the use of the metal oxide.

DISCLOSURE OF THE INVENTION Leading to the present invention, the intensive and thorough research into improvement of chlorine resistance of polyacetal resins, carried out by the present

inventors aiming to solve the problems encountered in the prior arts, resulted in the finding that a metal hydroxide and a metal oxide are added to a polyacetal resin, whereby the decomposition of polyoxymethylene by chlorine can be prevented with inhibition of acidolysis induced by chlorine through inactivation and deacidification of chlorine, and the bubbling due to conventionally used hydrotalcite is avoided, thus increasing chlorine resistance of the polyacetal resin.

Therefore, it is an object of the present invention to provide a polyacetal resin composition, which is advantageous in terms of no bubbling, excellent thermal stability, preparation by various processing methods, and having the desired chlorine resistance even though articles prepared from the above resin composition are in contact with chlorines or chlorine-containing compounds for a long time.

It is another object of the present invention to provide an article prepared from such a polyacetal resin composition.

In accordance with a first aspect of the present invention, there is provided a polyacetal resin composition, comprising 100 parts by weight of a polyoxymethylene homopolymer or copolymer; 0.01-1. 0 parts by weight of an antioxidant; 0.01-1. 0 parts by weight of a thermal stabilizer; 0.01-10. 0 parts by weight of a metal hydroxide selected from the group consisting of magnesium hydroxide, calcium hydroxide, barium hydroxide, cadmium hydroxide, aluminum hydroxide and combinations thereof ; and 0.01-5. 0 parts by weight of a metal oxide selected from the group consisting of synthetic aluminum silicate, calcium oxide, magnesium oxide, aluminum oxide and magnesium aluminate.

In accordance with a second aspect of the present invention, there is provided an article prepared from the polyacetal resin composition, for use in articles coming in direct contact with a chlorine-based sterilizing agent or tap water containing chlorine or chlorides.

BEST MODES FOR CARRYING OUT THE INVENTION In accordance with the present invention, the polyacetal resin (A) used as. a main component is a polyoxymethylene homopolymer having a repeat unit represented by the following Formula 1 or a random copolymer having a monomeric unit of the Formula 1 and a monomeric unit represented by the following Formula 2: Formula 1 - ciizo 4- Formula 2 Wherein Rl and R2, which are the same or different, each represents hydrogen, alkyl groups and aryl groups, and x represents an integer of 2-6.

The polyoxymethylene copolymer is obtained by polymerization performed in the presence of known anionic or cationic catalysts. The polymerization catalyst of trioxane (trimer of formaldehyde) is exemplified by halogens, such as chlorine, bromine or iodine; organic and inorganic acids, such as alkyl or allyl sulfonic acid, HC104, HI04, HC104 derivatives, CPh3C (I04), R3SiHS04, etc.; metal halides, such as BF3, SbF3, SnCLt, - TiCl4, FeCl3, ZrCl4, MoCls, SiF4, etc.; complexes of metal halides, such as BF3 0H2, BF3-OEt2, BF3-CH3COOH, BF3-PFs-HF, BF3-10-hydroxyacetophenol, Ph3CSnCI5, Ph3CBF4, Ph3CSbC16, etc.; metal esters, such as carboxylates of copper, zinc, cadmium, iron, cobalt or nickel; P20s+S02, P205+phosphate ester, etc.; and combinations of organic metals and metal halides. Among them, it is preferred that coordinated compounds of boron trifluoride are used.

An antioxidant (B) used in the present invention includes hindered phenols, for example, 2,2'-methylenebis (4-methyl-6-t-butylphenol), hexamethyleneglycol-bis- (3, 5- di-t-butyl-4-hydroxyhydrocinnamate), tetrakis [methylene (3,5-di-t-butyl-4- <BR> <BR> hydroxyhydrocinnamate) ] methane, triethyleneglycol-bis-3- (3-t-butyl-4-hydroxy-5- methylphenyl) propionate, 1, 3,5-trimethyl-2, 4,6-tris (3,5-di-t-butyl-4- hydroxybenzyl) benzene, n-octadecyl-3- (4'-hydroxy-3', 5'-di-t-butylphenol) propionate, 4,4'-methylenebis (2,6-di-t-butylphenol), 4, 4'-butylidene-bis- (6-t-butyl-3-methyl-phenol), d-stearyl 3,5-di-t-butyl-4-hydroxybenzylphosphate, 2-t-butyl-6- (3-t-butyl-5-methyl-2- hydroxybenzyl) -4-methylphenylacrylate, 3, 9-bis2- [3- (3-t-butyl-4-hydroxy-5- methylphenyl) propionyloxy]-1, 1-dimethylethyl-2, 4,8, 10-tetraoxaspiro [5,5] undecane, etc.; or hindered amines, for example, 4-acetoxy-2, 2,6, 6-tetramethylpiperidine, 4- stearoyloxy-2, 2,6, 6-tetramethylpiperidine, 4-acrylooxy-2,2, 6,6-tetramethylpiperidine, 4- methoxy-2,2, 6,6-tetramethylpiperidine, 4-benzoyloxy-2,2, 6,6-tetramethylpiperidine, 4- cyclohexyloxy-2,2, 6,6-tetramethylpiperidine, 4-phenoxy-2,2, 6,6-tetramethylpiperidine, 4-benzyloxy-2,2, 6,6-tetramethylpiperidine, 4- (phenylcarbamoyloxy)-2, 2,6, 6- tetramethylpiperidine, bis (2,2, 6,6-tetramethyl-4-piperidyl) oxalate, bis (2,2, 6,6- tetramethyl-4-piperidyl) alonate, bis (2,2, 6,6-tetramethyl-4-piperidyl) adipate, bis (2,2, 6,6- tetramethyl-4-piperidyl) sepacate, bis (1, 2,2, 6, 6-pentamethyl-piperidyl) sepacate, bis (2,2, 6,6-tetramethyl-4-piperidyl) terephthalate, 1, 2-bis (2,2, 6, 6-tetramethyl-4- piperidyloxy) ethane, bis (2,2, 6,6-tetramethyl-4-piperidyl) hexamethylene-1, 6-dicarbamate, bis (l-methyl-2, 2,6, 6-tetramethyl-4-piperidyl) adipate, tris (2,2, 6,6-tetramethyl-4- piperidyl) benzene-1, 3, 5-tricarboxylate, etc. Among them, triethyleneglycol-bis-3- (3-t- <BR> <BR> butyl-4-hydroxy-5-methylphenyl) -propionate, 1, 6-hexane-diol-bis-3 (3,5-di-t-butyl-4- hydroxy-phenyl) propionate, or tetrakis [methylene (3,5-di-t-butyl-4- hydroxyhydrocinnamate) ] methane is preferably used. More preferably, triethyleneglycol-bis-3- (3-t-butyl-4-hydroxy-5-methylphenyl)-propionate is used. Such an antioxidant (B) is used in the amount of 0.01-1. 0 parts by weight, and preferably in

the amount of 0.1-0. 5 parts by weight, on the basis of 100 parts by weight of the polyacetal resin (A).

A thermal stabilizer (C) used in the present invention is exemplified by nitrogen- containing compounds which serve to increase thermal stability of polyoxymethylene by reaction with formaldehyde, for instance, 6-phenyl-1, 3,5-triazine-2, 4-triamine (benzoguanamine), 2,4, 6-triamino-1, 3,5-triazine (melamine), carbonyldiamide (urea), dicyandiamide, isophthalic dihydrazide (hydrazine) ; or alcohols, for instance, polyethyleneglycol, ethylene-vinylalcohol copolymer, sorbitol, sorbitan, etc. In particular, 2,4, 6-triamino-1, 3,5-triazine (melamine) is preferably used. The thermal stabilizer (C) of polyoxymethylene is used in the amount of 0.01-1. 0 parts by weight, and preferably in the amount of 0.1-0. 5 parts by weight, on the basis of 100 parts by weight of the polyacetal resin (A).

Further, a metal hydroxide (D) functioning as a chlorine-inactivating agent (chelate compound) in the present invention is selected from the group consisting of magnesium hydroxide, calcium hydroxide,-barium hydroxide, cadmium hydroxide, aluminum hydroxide and combinations thereof. Among them, magnesium hydroxide is preferable in terms of the stability and the suppression of discoloration. The metal hydroxide is used in the amount of 0.01-10. 0 parts by weight, and preferably in the amount of 0.05-1. 0 parts by weight, on the basis of 100 parts by weight of the polyacetal resin (A). In the case that the amount of metal hydroxide is less than 0.01 parts by weight, an addition effect of this component is hardly exhibited. Meanwhile, if the amount exceeds 10.0 parts by weight, the thermal stability of polyacetal has a tendency of being decreased.

According to the present invention, the metal hydroxide is present in polyoxymethylene, and serves to inactivate a chlorine component upon the penetration of the chlorine or chlorides into polyoxymethylene, thus increasing the chlorine r resistance of polyoxymethylene. In particular, magnesium hydroxide is not washed

with water, which stems from the water-insoluble tendency, and is not independently converted to magnesium carbonate, thereby exhibiting its chlorine inactivation effect for a long time.

Furthermore, in addition to the chlorine-inactivating agent, a metal oxide (E) is used for deacidification of acidolysis derivatives formed by chlorine. Examples of the metal oxide are synthetic aluminum silicate, calcium oxide, magnesium oxide, aluminum oxide and magnesium aluminate. Among them, magnesium oxide or magnesium aluminate is preferably used. In particular, magnesium aluminate is more preferable.

The metal oxide (E) is used in the amount of 0. 01-5. 0 parts by weight, and preferably in the amount of 0.05-1. 0 parts by weight, on the basis of 100 parts by weight of the polyacetal resin (A). If the metal oxide is used in the amount less than 0.01 parts by weight, this component does not exhibit functions thereof even though being added.

On the other hand, if the above component is used in the amount exceeding 5.0 parts by weight, the polyacetal resin composition is decreased in its thermal stability.

In the present invention, since the metal oxide is used in a dehydrated form, bubbling problems of polyacetal resin composition may be overcome. Also, the above metal oxide functions to eliminate formic acid generated or present in polyacetal resin composition, thus increasing thermal stability of polyacetal resin composition as well as providing the composition with stability to chlorides.

Optionally, an organic additive (F) may be used for further increasing chlorine resistance. The organic additive is exemplified by alkylene bis-stearamide or hot melt nylon resin, and used in the amount of 0. 01-1. 0 parts by weight, on the basis of 100 parts by weight of the polyacetal resin (A). In particular, ethylene bis-stearamide. is preferably used. According to the prior art references, the amide groups of ethylene bis-stearamide are reported to serve to increase thermal stability of polyacetal compositions. However, the present inventors have noted that the above component may increase the chlorine resistance of the polyacetal composition by reacting with the

residual chlorine to be converted into monochloroamine.

Meanwhile, the stabilized polyacetal resin composition according to the present invention may be prepared by use of any conventional devices employable for heat- melting stabilization. Examples of such a device are a single-screw extruder equipped with single discharge port, a twin-screw extruder equipped with a multi-stage discharge port, and a twin-shaft paddle type continuous mixing extruder. An exemplified embodiment of the stabilization is as follows: Each component of the present composition is mixed with common additives such as a polymerization stabilizing agent. Thereafter, the mixture is processed by the above device under the addition of a chlorine stabilizing agent at the temperature (160- 250 °C), which is higher than a melting point of polyoxymethylene, under reduced pressure of 150-400 torr, followed by being subjected to drying at 80 °C for 2-10 hours for post-treatment. In this regard, if the pressure and drying conditions fall outside of the above ranges, terminal moieties of polyoxymethylene may become increasingly unstable, thus decreasing chlorine resistance of polyoxymethylene.

By using the conventional process, the composition of the present invention may be molded into articles for application involving direct contact with a chlorine-based sterilizing agent or tap water containing chlorine or chlorides, for example, bathroom fixtures, sprinklers, conveyer belts, piping materials, containers, bib cocks, showerheads, swimming pool cleaners, etc.

A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

Hereinafter, chlorine resistance and thermal stability of a resin are measured as follows.

(1) Maintenance ratio of tensile strength (chlorine resistance)

A polyacetal resin composition was injected to a ASTM 1 type dumbbell-shaped test piece for measuring tensile strength, which was then immersed in an aqueous chlorine solution (Cl concentration: 2,000 ppm) obtained by dissolving a predetermined amount of sodium hypochloride used as a chlorine-based sterilizing agent (chlorine sterilization of tap water generally uses chlorine gas, but sodium hypochloride exhibiting the same functions as the chlorine gas is applied in the present invention) in tertiary distilled water, and allowed to stand at room temperature (232 °C) for 3000 hours in a darkroom. A maintenance ratio of tensile strength was calculated from the following Equation 1: Equation 1 Maintenance Ratio (%) = (tensile strength of test piece after immersion/tensile strength of test piece before immersion) x 100 2) Weight decrement (%) (thermal stability) 1 g of a polyacetal resin composition was allowed to stand at 222 °C under an air atmosphere for 1 hour, and its weight decrement (%) was determined.

3) Discoloration and bubbling (chlorine resistance) Discoloration and bubbling of a polyacetal resin composition were observed with the naked eye.

EXAMPLE 1 100 parts by weight of polyoxymethylene was added with 0.1 parts by weight of melamine, 0.1 parts by weight of magnesium hydroxide, 0.05 parts by weight of magnesium aluminate, and 0.3 parts by weight of triethyleneglycol-bis-3- (3-t-butyl-4- hydroxy-5-methylphenyl) -propionate, and extruded using a twin-screw extruder at 230 °C,

to obtain a resin composition, which was then dried at 80 °C for 5 hours, followed by evaluating weight decrement, discoloration and bubbling thereof.

Further, the obtained resin composition was injection-molded at 220 °C using an injection-molding machine, to prepare a ASTM 1 type dumbbell-shaped test piece for measuring tensile strength, which was immersed in an aqueous chlorine solution for 3,000 hours, to measure a maintenance ratio of tensile strength thereof.

EXAMPLE 2 According to the same procedure as in Example 1, a resin composition was prepared by adding 100 parts by weight of polyoxymethylene with 0.1 parts by weight of melamine, 0.1 parts by weight of magnesium hydroxide, 0.1 parts by weight of magnesium aluminate, and 0.3 parts by weight of triethyleneglycol-bis-3- (3-t-butyl-4- <BR> hydroxy-5-methylphenyl) -propionate. Then, properties of the resin composition were measured by the same manner as in Example 1.

EXAMPLE 3 According to the same procedure as in Example 1, a resin composition was prepared by adding 100 parts by weight of polyoxymethylene with 0.1 parts by weight of melamine, 0.1 parts by weight of magnesium hydroxide, 0.1 parts by weight of magnesium aluminate, 0.3 parts by weight of triethyleneglycol-bis-3- (3-t-butyl-4- hydroxy-5-methylphenyl)-propionate, and 0.15 parts by weight of ethylene bis-stearamide.

Then, properties of the resin composition were measured by the same manner as in Example 1.

COMPARATIVE EXAMPLE 1

According to the same procedure as in Example 1, a resin composition was prepared by adding 100 parts by weight of polyoxymethylene with 0.1 parts by weight of melamine, 0.1 parts by weight of magnesium hydroxide, and 0.3 parts by weight of triethyleneglycol-bis-3- (3-t-butyl-4-hydroxy-5-methylphenyl)-propionate. Then, properties of the resin composition were measured by the same manner as in Example 1.

COMPARATIVE EXAMPLE 2 According to the same procedure as in Example 1, a resin composition was prepared by adding 100 parts by weight of polyoxymethylene with 0.1 parts by weight of melamine, 0.1 parts by weight of zinc stearate, and 0.3 parts by weight of triethyleneglycol-bis-3- (3-t-butyl-4-hydroxy-5-methylphenyl)-propionate. Then, properties of the resin composition were measured by the same manner as in Example 1.

COMPARATIVE EXAMPLE 3 According to the same procedure as in Example 1, a resin composition was prepared by adding 100 parts by weight of polyoxymethylene with 0.1 parts by weight of melamine, 0.1 parts by weight of hydrotalcite, and 0.3 parts by weight of triethyleneglycol-bis-3- (3-t-butyl-4-hydroxy-5-methylphenyl)-propionate. Then, properties of the resin composition were measured by the same manner as in Example 1.

The measured results are shown in Table 1, below.

TABLE 1 No. Tensile Strength Weight Decrement (%) Discoloration Bubbling Maintenance (%) Ex. 1 97. 3-23 No No 2 99.5-1. 9 No No 3 100-1.9 No No 1 96.8-2. 7 No No C. Ex. 2 90. 0--6. 0 Yellowing No 3 85. 0-33. 7 Yellowing Yes

INDUSTRIAL APPLICABILITY As described above, the polyacetal resin composition of the present invention is advantageous in terms of excellent chlorine resistance, superior thermal stability, no yellowing and no bubbling.

The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.