CN115960434A - Wear-resistant polyformaldehyde composition as well as preparation method and application thereof - Google Patents

Abstract

The invention relates to polymer materials, in particular to a wear-resistant polyoxymethylene composition and its preparation method and application. The wear-resistant polyoxymethylene composition comprises the following components in parts by weight: 100 parts of polyoxymethylene polymer, 0.05-5 parts of isocyanate compound, 0.05-10 parts of molybdenum disulfide, 0.05-2 parts by weight of lubricant and antioxidant 0.05‑1 part; the functionality of the isocyanate compound is ≥3. The invention adds the isocyanate compound with a functionality ≥ 3, which can reduce the formaldehyde emission of the wear-resistant polyoxymethylene composition. The invention adds molybdenum disulfide to the preparation process of the wear-resistant polyoxymethylene composition, and improves the friction performance of the wear-resistant polyoxymethylene composition through the sliding between the molybdenum disulfide sheets. And the isocyanate compound with a functionality ≥ 3 can penetrate into the molybdenum disulfide to increase the interlayer delamination of the molybdenum disulfide, thereby reducing the friction coefficient and specific wear of the wear-resistant polyoxymethylene composition filled with molybdenum disulfide.

Description

A wear-resistant polyoxymethylene composition and its preparation method and application

Technical Field

The invention relates to polymer materials, in particular to a wear-resistant polyoxymethylene composition and a preparation method and application thereof.

Background Art

Polyoxymethylene (POM) is a highly crystalline linear thermoplastic polymer with excellent mechanical properties, wear resistance, self-lubrication, oil resistance, chemical resistance, creep resistance, low water absorption, and the ability to maintain its mechanical, chemical and electrical properties over a wide temperature range. It is an engineering plastic with excellent comprehensive properties and is widely used in the automotive, electronics, and household appliances industries.

POM is mainly composed of (-CH ₂ -O-) chain segments, with a small amount of (-CH ₂ CH ₂ O-) or (-CH ₂ CH ₂ CH ₂ CH ₂ O-) chain segments, and a macromolecule with a terminal group of methoxy ether or hydroxyethyl ether structure. This results in POM being easily broken under the action of heat and oxygen during melt processing, and this thermal decomposition is autocatalytic, releasing a large amount of formaldehyde. In particular, in materials that use molybdenum disulfide to improve the wear resistance of POM, molybdenum disulfide will accelerate the decomposition of POM and produce more formaldehyde. Therefore, in the preparation process of polyoxymethylene compositions, the suppression of formaldehyde release is an important aspect.

Many methods for improving the thermal stability of polyoxymethylene have been proposed. In various schemes, additives such as amines, amides and hydrazines that react with degradation gases such as formaldehyde generated by thermal degradation are mainly used. For example, Japanese Patent Publication No. Hei 10-1592 describes the addition of acrylamide and boric acid compounds to polyoxymethylene resins; Japanese Patent Publication No. Sho 59-213752 describes the addition of alanine to polyoxymethylene resins. However, according to these methods, since the additives are thermally unstable, they cause the polymer to turn yellow, and are easily exuded into the polymer to form precipitates in the mold. Therefore, the addition of additives limits the improvement of thermal stability.

DuPont of the United States has published several patents on the thermal stability of polyoxymethylene. The main idea is to absorb the formaldehyde gas generated during the processing by adding amide substances, thereby reducing the release of formaldehyde and stabilizing the processing of polymers. For example, in Chinese patent CN87102759, hydroxyl-containing compounds ethylene-vinyl alcohol copolymer (EVOH), polyvinyl alcohol (PVA) and copolymerized nylon are used to achieve stable polyoxymethylene processing and reduce formaldehyde release. In Chinese patent CN201680008580, formaldehyde release is reduced by introducing polyacrylamide, copolymerized nylon, EVOH and allantoin. In Chinese patent CN201480068593, the amount of formaldehyde release and mold fouling is controlled by controlling the amount of polyacrylamide and copolymerized nylon, while achieving a certain amount of creep. The principle adopted by these patented methods is to react with the generated formaldehyde molecules by adding substances to reduce the formaldehyde content finally present in the product, thereby obtaining lower test results in the formaldehyde release test. Currently, there is no plan to study the intrinsic factors that produce formaldehyde molecules, etc., to fundamentally solve the problem of formaldehyde release generated during the decomposition of POM.

During the production process, molybdenum disulfide products are treated with acidic substances such as hydrochloric acid, which will cause the molybdenum disulfide products to have strong acidity. During high-temperature processing such as extrusion and injection molding, this acidic substance will aggravate the decomposition of the POM molecular chain, thereby affecting the stability of POM.

Summary of the invention

The purpose of the present invention is to overcome the above-mentioned shortcomings of the prior art and provide a wear-resistant polyoxymethylene composition and its preparation method and application. The wear-resistant polyoxymethylene composition of the present invention has the advantages of low formaldehyde emission and high wear resistance.

To achieve the above object, the technical solution adopted by the present invention is:

The first object of the present invention is to provide a wear-resistant polyoxymethylene composition, wherein the wear-resistant polyoxymethylene composition comprises the following components in parts by weight:

100 parts of polyoxymethylene polymer, 0.05-5 parts of isocyanate compound, 0.05-10 parts of molybdenum disulfide, 0.05-2 parts by weight of lubricant and 0.05-1 part of antioxidant;

The functionality of the isocyanate compound is ≥3; preferably, the functionality of the isocyanate compound is 3-4.

In the technical solution of the present invention, due to the effects of heat and shear, the polyformaldehyde molecular chain will break to produce terminal hydroxyl groups, causing the instability of the polyformaldehyde. The present invention adds an isocyanate compound with a functionality ≥ 3, which can react with the terminal hydroxyl groups, thereby inhibiting the decomposition of the polyformaldehyde and playing a stabilizing role.

In the process of preparing the wear-resistant polyoxymethylene composition, the invention adds molybdenum disulfide to improve the friction performance of the wear-resistant polyoxymethylene composition through the slippage between the molybdenum disulfide sheets. In addition, the isocyanate compound with a functionality of ≥3 can penetrate into the molybdenum disulfide, increase the interlayer peeling of the molybdenum disulfide, thereby reducing the friction coefficient and specific wear of the wear-resistant polyoxymethylene composition filled with molybdenum disulfide. The addition of the isocyanate compound with a functionality of ≥3 can also improve the hardness of the wear-resistant polyoxymethylene composition.

As a preferred embodiment of the wear-resistant polyoxymethylene composition of the present invention, the average particle size of the molybdenum disulfide is 1-6 μm.

As a preferred embodiment of the wear-resistant polyoxymethylene composition of the present invention, the average particle size of the molybdenum disulfide is 1.3-3 μm.

When the particle size of molybdenum disulfide is within the above range, the friction performance of the wear-resistant polyoxymethylene composition can be better improved, and the friction coefficient and specific wear loss of the wear-resistant polyoxymethylene composition can be reduced.

As a preferred embodiment of the wear-resistant polyoxymethylene composition of the present invention, the isocyanate compound includes at least one of 4,4'-methylenebisphenyl isocyanate, isophorone diisocyanate, 1,6-hexamethylene diisocyanate, 2,4-toluene diisocyanate or a trimer of 2,6-toluene diisocyanate.

The multifunctional isocyanate compound is introduced into the wear-resistant polyoxymethylene composition. Through the reactivity of the isocyanate compound, the isocyanate compound can react with the terminal hydroxyl group of the wear-resistant polyoxymethylene polymer, thereby inhibiting the decomposition of the wear-resistant polyoxymethylene composition to a certain extent, playing a stabilizing role, and further reducing the formaldehyde release of the wear-resistant polyoxymethylene composition. The coordinated use of molybdenum disulfide and the isocyanate compound can better reduce the friction coefficient and specific wear of the wear-resistant polyoxymethylene composition.

As a preferred embodiment of the wear-resistant polyoxymethylene composition of the present invention, the polyoxymethylene polymer is a homopolymer composed of a formaldehyde monomer represented by formula (I), or a copolymer composed of a formaldehyde monomer represented by formula (I) and a monomer represented by formula (II), or a mixture of the above homopolymer and copolymer;

Formula (I): -(-CH ₂ O-)-; Formula (II): [(CX ₁ X ₂ ) _a O];

_X1 and _X2 are the same or different and are independently selected from the following groups: hydrogen, alkyl or aryl, and a is an integer of 2-6.

As a preferred embodiment of the wear-resistant polyoxymethylene composition of the present invention, the homopolymer is a homopolymer with ester or ether end-capping formed by chemical reaction; the copolymer is a copolymer that is not completely end-capped but has a free hydroxyl end in the comonomer unit, or a copolymer end-capped with an ether group.

Preferably, the polyoxymethylene polymer has a melt index of 2-50 g/10 min under test conditions of 190° C. and 2.16 kg according to ISO 1133-2011 standard.

As a preferred embodiment of the wear-resistant polyoxymethylene composition of the present invention, the antioxidant is a hindered phenol antioxidant; and the lubricant is a stearamide lubricant.

Preferably, the hindered phenol antioxidant is 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, 2,5-di-tert-butyl-4-hydroxybenzyl dimethylamine, diethyl-3,5-di-tert-butyl-4-hydroxybenzyl phosphate, stearyl-3,5-di-tert-butyl-4-hydroxybenzyl phosphate, 3,5-di-tert-butyl-4-hydroxyphenyl-3,5-distearyl-thiotriazolylamine, 2,6-di-tert-butyl-4-hydroxymethylphenol, 2,4-di-(n-octylthio)-6-(4-hydroxy-3,5-di-tert-butyl glycerol allyl ether)-1,3,5-triazine, N,N'-hexamethylenediamine, At least one of (3,5-di-tert-butyl-4-hydroxy-hydrocinnamic acid), N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionyl) hexanediamine, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, pentaerythritol-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], triethylene glycol-bis[3-(3,5-dimethyl-4-hydroxyphenyl) propionate], triethylene glycol bis[β-(3-tert-butyl-4-hydroxy-5-methylphenyl) propionate] or 2,2'-thiodiethyl-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate.

The hindered phenol antioxidant is preferably a compound of triethylene glycol bis[β-(3-tert-butyl-4-hydroxy-5-methylphenyl) propionate] and N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionyl) hexamethylenediamine, for example, the hindered phenol antioxidant is a compound of Irganox@245 and Irganox@1098 (in a ratio of 2:1).

The second object of the present invention is to provide a method for preparing the above-mentioned wear-resistant polyoxymethylene composition, comprising the following steps:

1) adding polyoxymethylene polymer, isocyanate compound, molybdenum disulfide, antioxidant and lubricant into a premixer according to the proportion and mixing to obtain a premix;

2) adding the premix into a twin-screw extruder, melting and extruding at 180° C. to 200° C., cooling, and granulating to obtain the wear-resistant polyoxymethylene composition.

The third purpose of the present invention is to provide the use of the above-mentioned wear-resistant polyoxymethylene composition in gear parts, rearview mirror shafts, and automobile pedal mechanism products.

Compared with the prior art, the present invention has the following beneficial effects:

The present invention provides a wear-resistant polyoxymethylene composition and a preparation method and application thereof. The present invention adds an isocyanate compound with a functionality of ≥3, which can react with a terminal hydroxyl group, thereby inhibiting the decomposition of polyoxymethylene, playing a stabilizing role, and further reducing the formaldehyde release of the wear-resistant polyoxymethylene composition. The present invention adds molybdenum disulfide to the preparation process of the wear-resistant polyoxymethylene composition, and improves the friction performance of the wear-resistant polyoxymethylene composition through the slippage between the molybdenum disulfide layers. In addition, the isocyanate compound with a functionality of ≥3 can penetrate into the molybdenum disulfide, increase the interlayer peeling of the molybdenum disulfide, and thus reduce the friction coefficient and specific wear of the wear-resistant polyoxymethylene composition filled with molybdenum disulfide.

DETAILED DESCRIPTION

In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

In the following examples and comparative examples, the experimental methods used are conventional methods unless otherwise specified, and the materials, reagents, etc. used are all commercially available unless otherwise specified.

The sources of raw materials used in the following examples and comparative examples are as follows:

Polyoxymethylene polymer:

POM resin: brand POM KP20, copolymer POM grade, melt index 9g/10min, produced by Ticona, Germany.

Isocyanate compounds:

Isocyanate compound 1: brand VESTANAT T 1890/100, trimer of isophorone diisocyanate (IPDI trimer), NCO functionality of 3-4, produced by Evonik Specialty Chemicals Co., Ltd.;

Isocyanate compound 2: brand MDI-100, 4,4'-diphenylmethane diisocyanate, NCO functionality is 2, produced by Wanhua Chemical Group Co., Ltd.;

Molybdenum disulfide:

The brand is molybdenum disulfide SF, with an average particle size of 1.3-1.5μm, produced by Luoyang Shenyu Molybdenum Co., Ltd.

The brand is molybdenum disulfide TF, with an average particle size of 3-6μm, produced by Luoyang Shenyu Molybdenum Industry Co., Ltd.

The brand is molybdenum disulfide PR-1, with an average particle size of 1.5-3μm, produced by Luoyang Shenyu Molybdenum Industry Co., Ltd.

The brand is molybdenum disulfide TG, with an average particle size of 12-16μm, produced by Luoyang Shenyu Molybdenum Industry Co., Ltd.

The brand is molybdenum disulfide LG, with an average particle size of 16-30μm, produced by Luoyang Shenyu Molybdenum Industry Co., Ltd.

The lubricant is ethylene bis stearamide, brand EBS B50, produced by Guangzhou Runfeng Chemical Co., Ltd.

The antioxidant is a mixture of triethylene glycol bis[β-(3-tert-butyl-4-hydroxy-5-methylphenyl) propionate] and N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionyl) hexanediamine (in a ratio of 2:1). The trade name of triethylene glycol bis[β-(3-tert-butyl-4-hydroxy-5-methylphenyl) propionate] is Irganox 245, produced by BASF of Germany; the trade name of N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionyl) hexanediamine is Irganox 1098, produced by BASF of Germany.

The performance tests of the products in the embodiments and comparative examples are as follows:

The bending strength and bending modulus tests were conducted according to ISO 178-2019 standard, the sample size was 80mm*10mm*4mm, and the test speed was 2mm/min. The test equipment was an electronic tensile testing machine from Zwick, Germany.

The hardness test standard is ISO 2039.1-2001, the sample size is 60mm*60mm*4mm, and the test equipment is the Aolongxin touch screen digital display plastic Rockwell hardness tester produced by Shanghai Aolong Xingdi Testing Equipment Co., Ltd.

The formaldehyde emission test is carried out according to the VDA 275 test standard. The specific process is as follows:

The polyoxymethylene resin was injection molded at a temperature between 190-200°C, and the size of the product was 100mm*100mm*2mm, and then it was cut into products with a size of 100mm*40mm*2mm. The injection molded sample was fixed in a 1-liter polyethylene bottle containing 50ml of distilled water so that the product would not come into contact with water, and then the bottle was sealed and baked in an oven at 60°C for 3 hours. Then 10ml of the aqueous solution was sampled from the polyethylene bottle, 10ml of acetylacetone and 10ml of ammonium acetate standard solution were added, and after a period of time, the coloration of the water in the bottle was analyzed by a UV spectrophotometer, and then the formaldehyde content was calculated.

The tests of friction coefficient and specific wear (unit: *10 ^-3 mm ³ /kg*km) were carried out according to JIS K7218 standard, with S45C steel as the grinding material, pin-disc mode, linear speed of 0.8m/s and load of 4KG.

The components (in parts by weight) of the wear-resistant polyoxymethylene compositions of Examples 1-13 and Comparative Examples 1-5 are shown in Table 1-2. The preparation method of the wear-resistant polyoxymethylene compositions is as follows:

According to the proportion, polyoxymethylene polymer, isocyanate compound, molybdenum disulfide, antioxidant and lubricant are added to a premixer and mixed for 1-2 minutes to obtain a premix, and then the premix is added to a twin-screw extruder, melt-extruded at 180°C-200°C, the main engine speed is 300-400 rpm, cooled and granulated to obtain the polyoxymethylene composition. Wherein, the temperature of the first zone is controlled at 180-190°C, the temperature of the second zone is 180-190°C, the temperature of the third zone is 180-190°C, the temperature of the fourth zone is 180-190°C, the temperature of the fifth zone is 180-190°C, the temperature of the sixth zone is 180-190°C, the temperature of the seventh zone is 190-200°C, the temperature of the eighth zone is 190-200°C, and the temperature of the ninth zone is 190-200°C.

Table 1 Formulation and performance test results of wear-resistant polyoxymethylene composition (Examples 1-13)

Table 2 Formulation and performance test results of wear-resistant polyoxymethylene composition (Comparative Examples 1-5)

It can be seen from Table 1 and Table 2 that Examples 1-4 adjust the weight fraction of the isocyanate compound. As the weight fraction of the isocyanate compound increases, the formaldehyde release, friction coefficient and specific wear of the prepared wear-resistant polyformaldehyde composition decrease, and the strength of the wear-resistant polyformaldehyde composition also increases.

Compared with Example 3 and Comparative Example 2, adding isocyanate compounds to the preparation system of the wear-resistant polyformaldehyde composition can effectively reduce the release of formaldehyde from the wear-resistant polyformaldehyde composition; and isocyanate compounds with a functionality ≥ 3 can penetrate into molybdenum disulfide, increase the interlayer peeling of molybdenum disulfide, thereby reducing the friction coefficient and wear of the molybdenum disulfide-filled wear-resistant polyformaldehyde composition. Adding isocyanate compounds with a functionality ≥ 3 can also increase the hardness of the wear-resistant polyformaldehyde composition.

In Examples 3 and 5-7, the weight fraction of molybdenum disulfide was adjusted. As the weight fraction of molybdenum disulfide increased, the formaldehyde release of the prepared wear-resistant polyoxymethylene composition increased, and the friction coefficient, specific wear and strength of the prepared wear-resistant polyoxymethylene composition also decreased.

Compared with Example 3, Examples 10-13 change the average particle size of molybdenum disulfide, and the wear resistance of the wear-resistant polyoxymethylene compositions prepared in Examples 3 and 11 is better than that of the wear-resistant polyoxymethylene composition prepared in Example 10; the wear resistance of the wear-resistant polyoxymethylene composition prepared in Example 10 is better than that of the wear-resistant polyoxymethylene compositions prepared in Examples 12-13. When the average particle size of molybdenum disulfide is not within the range of 1-6 μm, the friction coefficient and specific wear loss of the prepared wear-resistant polyoxymethylene compositions are higher, and the wear resistance of the wear-resistant polyoxymethylene compositions of Comparative Examples 12-13 is not as good as that of Example 3.

Compared with Comparative Example 3, when the functional group of the isocyanate compound is 2, only the formaldehyde emission can be reduced; when the functionality of the isocyanate compound is ≥ 3, the formaldehyde emission, friction coefficient and specific wear can be reduced at the same time.

Compared with Comparative Example 4, when the weight percentage of the isocyanate compound is not within the range of 0.05-5 parts, the hardness of the polyoxymethylene composition prepared therefrom will be reduced.

Compared with Example 5, when the weight percentage of molybdenum disulfide in Example 3 is not within the range of 0.05-10 parts, the hardness of the wear-resistant polyoxymethylene composition prepared therefrom is lower and the formaldehyde release is higher. The comprehensive performance of Comparative Example 5 is inferior to that of Example 3.

Compared with Example 1, Example 3 shows that the simultaneous addition of the multifunctional isocyanate compound and molybdenum disulfide in the present invention can synergistically increase the wear resistance of the wear-resistant polyoxymethylene composition, and the formaldehyde emission of the wear-resistant polyoxymethylene composition is also low.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims (9)

1. A wear-resistant polyoxymethylene composition, characterized in that it comprises the following components in parts by weight: 100 parts of polyoxymethylene polymer, 0.05-5 parts of isocyanate compound, 0.05-10 parts of molybdenum disulfide, 0.05-2 parts by weight of lubricant and 0.05-1 part of antioxidant; The functionality of the isocyanate compound is ≥3. 2. The wear-resistant polyoxymethylene composition according to claim 1, characterized in that the average particle size of the molybdenum disulfide is 1-6 μm. 3. The wear-resistant polyoxymethylene composition according to claim 1, characterized in that the average particle size of the molybdenum disulfide is 1.3-3 μm. 4. The wear-resistant polyoxymethylene composition according to claim 1, characterized in that the isocyanate compound comprises at least one of a trimer of 4,4'-methylenebisphenyl isocyanate, isophorone diisocyanate, 1,6-hexamethylene diisocyanate, 2,4-toluene diisocyanate or 2,6-toluene diisocyanate. 5. The wear-resistant polyoxymethylene composition according to claim 1, characterized in that the polyoxymethylene polymer is a homopolymer composed of a formaldehyde monomer represented by formula (I), or a copolymer composed of a formaldehyde monomer represented by formula (I) and a monomer represented by formula (II), or a mixture of the above homopolymers and copolymers; Formula (I): -(-CH ₂ O-)-; Formula (II): [(CX ₁ X ₂ ) _a O]; _X1 and _X2 are the same or different and are independently selected from the following groups: hydrogen, alkyl or aryl, and a is an integer of 2-6. 6. The wear-resistant polyoxymethylene composition according to claim 5, characterized in that the homopolymer is a homopolymer terminated with an ester group or an ether group; the copolymer is a copolymer that is not completely terminated but has a free hydroxyl end in the comonomer unit, or a copolymer terminated with an ether group. 7. The wear-resistant polyoxymethylene composition according to claim 1, characterized in that the antioxidant is a hindered phenol antioxidant; and the lubricant is a stearamide lubricant. 8. A method for preparing the wear-resistant polyoxymethylene composition according to any one of claims 1 to 7, characterized in that it comprises the following steps: 1) adding polyoxymethylene polymer, isocyanate compound, molybdenum disulfide, antioxidant and lubricant into a premixer according to the proportion and mixing to obtain a premix; 2) adding the premix into a twin-screw extruder, melting and extruding at 180° C. to 200° C., cooling, and granulating to obtain the wear-resistant polyoxymethylene composition. 9. Use of the wear-resistant polyoxymethylene composition according to any one of claims 1 to 7 in gear parts, rearview mirror shafts, and automobile pedal mechanism products.