POLYOXYMETHYLENE RESIN COMPOSITION AND MOLDED ARTICLE THEREFROM

Abstract: A polyoxymethylene resin composition is provided containing, per 100 parts by weight of a polyoxymethylene resin, 0.02-0.5 parts by weight of inorganic powder having a specific solid solution of magnesium oxide and aluminum oxide. The polyoxymethylene resin composition, even when used in contact with water, water vapor, or the like, or when exposed for long periods of time to high temperature, high moisture environments, will not decrease in its physical properties or durability and will not adversely affect the adjacent metal parts, rubber, plastics, and the like.

Description: TITLE POLYOXYMETHYLENE RESIN COMPOSITION AND MOLDED ARTICLE THEREFROM BACKGROUND OF THE INVENTION Field of The Invention The present invention relates to a polyoxymethylene resin composition which has well balanced mechanical physical properties, excellent durability and fatigue resistance, and molded articles thereof. Description of Related Art Polyoxymethylene polymers, which have well balanced mechanical physical properties, and excellent durability and fatigue resistance, have been used in a broad area of electronic parts and automotive parts. However, polyoxymethylene polymers which are outstanding in durability, if used in an ambient environment, will undergo a gradual deterioration due to the hydrolysis of the polymer itself when used in contact with water, water vapor, or to a high temperature and high moisture environment for a long period of time, which not only decreases the physical properties, but also will present the possibility of releasing highly reactive volatile components, such as formaldehyde monomer components generated by the degradation, or formic acid, or the like, resulting from side reactions, corrosion, or degradation of any adjacent metal parts, rubber, plastic, and the like. For example, when an acetal resin comes in contact with water, the water which diffuses into the resin will end up extracting the formaldehyde and acid components, such as formic acid and acidic acid, in the resin. In some cases, an oxidative degradation occurs furthermore by the formaldehyde being converted to formic acid and methanol as a result of the Cannizzaro reaction. Problems to he Solved bv the Invention It is an objective of this invention to provide a highly reliable polyoxymethylene resin composition which does not lose its physical properties or durability and is free from any adverse effect on adjacent metal parts, rubber, plastic, and the like, even when used in contact with water or water vapor or exposed for a long period of time to a high temperature, high humidity environment, as well as molded articles therefrom. Means Used for Solving the Problems In order to achieve the above objective, the present inventors carried ut an extensive research of materials and discovered that the above objective can be met by a molded article of a resin composition obtained by adding a specific amount of a solid solution of a specific magnesium oxide and aluminum oxide to a polyoxymethylene resin. SUMMARY OF THE INVENTION That is, the invention set forth in Claim (1) comprises a polyoxymethylene resin composition comprising, per 100 parts by weight of a polyoxymethylene resin, 0.02-0.5 parts by weight of an inorganic powder from a solid solution of magnesium oxide and aluminum oxide represented by the formula: (M gl . x Al)0 ] +1/2x (x represents an equality equation, 0 < x < 0.4). The invention set forth in Claim (2) comprises a molded article comprising a molded product using a material containing a polyoxymethylene resin composition as set forth in Claim (1). DETAILED DESCRIPTION OF THE INVENTION The solid solution used in the polyoxymethylene resin composition of this invention is a solid solution of AI2O3 in MgO and can be obtained by calcining a hydrotalcite compound represented by the following formula: Mg,. x Al x (OH)2A-n ^ . m^O (where A" n represents an n-valent anion, x, m, and n are positive numbers which meet the ranges: 0 < x < 0.4; 0 < m < 1; 1 < n < 4), at aoove 400-900°C, preferably at about 500-700°C. The resultant solid solution can be prepared to have a desired average particle size using a suitable pulverizer. The hydrotalcite has a structure in which the solid solution of AI2O3 and MgO is in a layer structure, between which layers water, carbonate ion, or other acidic anions, and the like, are interposed. The solid solution of magnesium oxide and aluminum oxide which is used in the polyoxymethylene resin composition of this invention is a substance under a state in which the anion molecules which have been incorporated between layers of the above hydrotalcite compound have been removed, thereby exhibiting under that state an effect of trapping acidic components, such as formic acid, acetic acid, and the like, so that the above effect cannot be obtained if these molecules are already incorporated between these layers to saturation. It should be mentioned that Japanese Patent Application Publication H2-50948 and Japanese Patent Laid Open H3-290459 disclose compositions in which a hydrotalcite has been incorporated into an acetal resin, but these compositions have anionic molecules already incorporated into the layers, requiring an exchange reaction with anion molecules in order to trap an acidic substance, hence resulting in a poor trapping efficiency for acids. Thus, this fails to improve the antihydrolysis properties. Japanese Patent Laid Open H6-340792 discloses a composition obtained by adding to an acetal resin an ionic adsorbant, mainly comprising an alkali metal oxide and/or alkaline earth metal oxide including a solid solution of AI2O3 and MgO and two or more oxides of trivalent oxides, but this composition is limited to an acetal resin in which the polyoxymethylene polymer generates formaldehyde gas in an amount of 500 ppm or less, with the ionic adsorbant having a limited capability of adsorption; hence, this lacks a technical concept to solve the problems due to hydrolytic degradation, and the like, of the molded polyacetal resin compositions. The Examples below reveal that molded articles of the resin compositions obtained by adding the above solid solution in a range of 0.02-0.5 parts by weight per 100 parts by weight of the polyoxymethylene resin suppress metal corrosion and rubber degradation under high temperature and high moisture conditions, at the same time showing good moldability among excellent effects. Advantageous Effect of the Invention As described above, the present invention can suppress the degradation of the resin under high temperature and high moisture conditions. This makes it possible, for example, to reliably reduce hydrolytic degradation with water, water vapor, or under high temperature, high moisture environments, and any corrosion or degradation of metal, plastic, rubber, and the like, parts adjacent to such resin parts, due to degradation-generated volatile acids, such as acetic acid, formic acid, and the like. Therefore, molded articles from the polyoxymethylene resin composition of this invention can be suitably used, specifically in gears, chassis, cams, rollers, and the like, which are used in audio, video, copiers, printers, computer equipment, and the like, as well as gears, combination switches, and the like, which are used in automotive wipers, door lock devices, etc. EXAMPLES The present invention is further specifically described by the following examples of this invention and comparative examples: First, a description is made of a high temperature, high moisture aging test which was carried out for each sample from the examples and comparative examples described later. In a polystyrene box container were placed 85 g of an oxymethylene test target and water. On this was placed a polystyrene small box holding a piece of steel metal and a roller made of chloroprene which was then covered with a lid. The seams between the lid and box container were sealed off with an adhesive tape, followed by allowing it to stand for one week at 70°C so as to evaluate the extent of metal corrosion and the extent of any deposits (dirtying) by an oily substance on the rubber surface according to a four-step rating. Metal Corrosion: @: No corrosion of the metal piece O: Some rust on the metal surface Δ: Substantial rust on the surface of the metal piece X: Formation of rust on the entire surface of the metal piece Rubber Dirtying: ® : Rubber roller retained original state with no change O: Rubber roller has some cloudiness on the surface Δ: Rubber roller has some partial oily spots X: Rubber roller was entirely covered with oily spots. After the above high temperature, high moisture aging test, 65 g of water was further added and the pH value was checked with a pH meter. The pH value was used to measure the concentration of the acids generated by hydrolytic degradation and was used as a measure to judge any buffering effect upon adding a buffer. Table 1 shows these ratings and measured values. The following is a description of the molding test carried out on each sample from the examples and control examples to be given later. A 2 g sample placed in a glass tube after the introduction of nitrogen gas was kept for 30 minutes at 250°C. This test was carried out to evaluate the heat stability of the sample resin at a hot melt state. A rating was made as to whether the extent of resin weight loss after testing was any of the following four stages and is reported in Table 1. In particular, the samples with observed blistering were judged to be unsuitable for molding and were noted in Table 1 as "Blistering." Moldability: ®: Weight loss less than 0.20% O: Weight loss 0.2-0.4% Δ: Weight loss 0.4-1.0% X: Weight loss of 1% or greater. Example 1 3 kg of an oxymethylene homopolymer, Delrin ® acetal resin 1700P-NC10 (manufactured by E. I. du Pont de Nemours and Company), having a MFR (melt flow rate) of 16.5 g/10 min., measurement temperature was 190°C, and load was 1190g, was mixed with 0.75 g of a solid solution of AI2O3 and MgO (tradename: Kyoward 2200, manufactured by Kyowa Kagaku K.K.) and then melt kneaded in a 35 mm diameter twin screw extruder (manufactured by Toshiba at a temperature set at 200°C). When the mixing conditions stabilized, the resulting resin was cut by a pelletizer to a sample 500 g of a pelletized resin. The sample was dried at room temperature overnight to carry out the above test. The results are given in Table 1. Example 2 A sample was prepared in a manner similar to that of Example 1 except for adding 1.5 g of a solid solution of AI2O3 and MgO followed by the above tests with the results given in Table 1. Example 3 A sample was prepared in a manner similar to that of Example 1 except for adding 6 g of a solid solution of A-2O3 and MgO followed by the above tests with the results given in Table 1. Example 4 A sample was prepared in a manner similar to that of Example 1 except for adding 15 g of a solid solution of AI2O3 and MgO followed by the above tests with the results given in Table 1. Comparative Example 1 The same polymer as that used in Example 1 for the polyoxymethylene homopolymer resin pellets was placed in a box container with no water, followed by being allowed to stand for one week at 70°C. Thereafter evaluations on metal corrosion and rubber dirtying, as well as the measurement on the pH values were carried out. The molding test was also carried out. These results are given in Table 1. Comparative Example 2 85 g of the same oxymethylene homopolymer resin pellets as those used in Example 1 and 15 g of water were placed in the box container, allowed to stand for one week at 70°C, from which metal corrosion and rubber dirtying were evaluated, including pH values. The molding test was also carried out. These results are given in Table 1. Comparative Examples 3-14 3 kg of the same oxymethylene homopolymer resin as that used in Example 1 was mixed with the chemical substances shown in Table 1. Otherwise, it was processed in a manner similar to that of Example 1 to obtain resin pellets. The resultant resin pellets, along with a designated amount of water, were placed in a box container, allowed to stand for one week at 70°C, on which metal corrosion and rubber dirtying were evaluated, as well as the pH values. Molding tests were also carried out. The results are given in Table 1. Comparative Example 15 A sample was prepared in a manner similar to that of Example 1 except for adding 0.3 g of a solid solution of AI2O3 and MgO to carry out the above test. The results are given in Table 1. Comparative Example 16 A sample was prepared in a manner similar to that of Example 1 except for adding 30 g of a solid solution of AI2O3 and MgO to carry out the above test. The results are given in Table 1. Table 1 Additives Conditions Metal Rubber Mold¬ pH Corrosion Dirtying ability Examples: 1 MgO-Al2θ3 Talcite With water O Δ © 5.9 (0.025 parts by weight) 2 MgO-A-203 Talcite With water O O O 6.1 (0.05 parts by weight) 3 MgO-Al2θ3 Talcite With water © O 0 6.4 (0.20 parts by weight) 4 MgO-A-203 Talcite With water ® O Δ 6.4 (0.50 parts by weight) Comparative Examples: 1 None No water ® © © 6.8 2 None With water X X © 4.7 3 Calcium Carbonate With water 0 x Δ 6.3 (0.2 parts by weight) 4 Calcium Carbonate With water 0 x O 5.2 (0.05 parts by weight) 5 Calcium Carbonate With water © x O 5.6 (0.10 parts by weight) 6 Hydrogen sodium carbonate With water © o Blister 7.3 (0.02 parts by weight) 7 Hydrogen sodium carbonate With water © © Blister 7.6 (0.05 parts by weight) 8 Sodium carbonate With water 0 0 Blister 6.9 (0.05 parts by weight) 9 Magnesium carbonate With water Δ Δ Blister 5.6 (0.05 parts by weight) 10 Hydrotalcite With water With water X X O 5.8 (0.05 parts by weight) 1 1 Magnesium silicate With water X X © 5.7 (0.05 parts by weight) 12 Magnesium oxide With water X X Δ 5.6 (0.05 parts by weight) 13 Dihydrogen sodium phos¬ With water o X © 6.7 phate (0.05 parts by wt) 14 Sodium barbiturate With water x X X 5.7 (0.05 parts by weight) 15 MgO-Al2θ3 Talcite With water Δ X © 5.0 (0.01 parts by weight) 16 M O-Al2θ3 Talcite With water © o X 6.5 (1.0 parts by weight) Table 1 clearly shows that Comparative Example 1, with no effective water, did not undergo any hydrolytic degradation, giving good results in metal corrosion, rubber dirtying, and moldability; but the other Comparative Examples 2-16 affected by water were found to be deficient in either metal corrosion, rubber dirtying, or moldability. For example, Comparative Example 2 with no additive showed rust formation on the entire surface of the metal piece, along with oily spots on the entire surface of the rubber roller. Comparative Examples 3-9 compounded with alkali metal or alkaline earth metal carbonate salt additives were confirmed to show an improved effect against metal corrosion and rubber deterioration, but they were found to be troublesome in actual molding operations as they blistered within a short period of time in the melt heat stability test. Comparative Examples 10-14 compounded with a variety of inorganic compounds that are considered to neutralize the acid components failed to show any confirmable improvement effect on metal corrosion or rubber dirtying. Comparative Example 15 was compounded with the same solid solution as that used in Examples 1-3, but failed to provide any significant effect because the amount added was too small. Comparative Example 16 was compounded with too much of the same solid solution as that used in Examples 1-3 which caused a drop in the heat stability of the resin.

Claims: WHAT TS CLAIMED IS: 1. A polyoxymethylene resin composition comprising, per 100 parts by weight of a polyoxymethylene resin, 0.02-0.5 parts by weight of an inorganic powder from a solid solution of magnesium oxide and aluminum oxide represented by the formula: (Mgi- x Al)Oι +1 2x wherein x represents an equality equation, 0 < x < 0.4. 2. A molded article comprising a polyoxymethylene resin composition as set forth in Claim 1.

Inventors: SHINOHARA KENICHI, SHINOHARA, KENICHI
Applicants: DU PONT, SHINOHARA KENICHI, E.I. DU PONT DE NEMOURS AND COMPANY, SHINOHARA, KENICHI
Application year: 1996
Main classification: C08K3
IPC codes: C08K3, C08L59