JPH0722960B2 - Heat treatment method for polyoxymethylene stretched body - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heat treatment method for a polyoxymethylene stretched product. More specifically, the present invention relates to a heat treatment method for a polyoxymethylene stretched product for obtaining a high strength and high elastic modulus polyoxymethylene stretched product having excellent thermal dimensional stability.

BACKGROUND ART Conventionally, a polyoxymethylene stretched product obtained by super-stretching an unstretched polyoxymethylene has a high strength and a high elastic modulus, and therefore, for example, a tensile strength member such as a wire rope, a control cable or an optical fiber cable. It is used as an industrial material such as and various rubber reinforcing agents.

In order to superstretch a polyoxymethylene unstretched product with a filter, for example, a method of stretching the polyoxymethylene unstretched product while heating under pressure or (Japanese Patent Laid-Open No. 60-183122), dielectric heating And the like (Japanese Patent Application Laid-Open No. 60-239539) are known, and the polyoxymethylene stretched product obtained by these methods has excellent strength and elastic modulus, but has a thermal dimension. The stability is not always fully satisfactory. When a high-strength, high-modulus polyoxymethylene stretched body is used as an industrial material, it is often used at a high temperature of room temperature or higher. In such a use at a high temperature, the stretched body has a thermal dimension. If stability is insufficient and shrinkage may occur, even if it has excellent mechanical properties such as high strength and high elastic modulus, it is unsuitable as an industrial material, limiting its use. I cannot escape. Therefore, when the polyoxymethylene stretched product is used as an industrial material, excellent thermal dimensional stability is required in addition to mechanical properties such as high strength and high elastic modulus.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The present invention overcomes the drawbacks of conventional polyoxymethylene stretched products, has high strength and high elastic modulus, and has good thermal dimensional stability. It is made for the purpose of providing.

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to develop a polyoxymethylene stretched product having such excellent characteristics, and as a result, have drawn a polyoxymethylene stretched product having a specific tensile elastic modulus. It was found that the above object can be achieved by heat-treating the body while shrinking the body at a specific relaxation rate, and the present invention has been completed based on this finding.

That is, according to the present invention, when heat-treating a polyoxymethylene stretched product, a stretched product having a tensile elastic modulus (E) in the range of 8 to 45 GPa is used, and a relaxation rate (%) ≧ 14.2-0.3 E (I The present invention provides a heat treatment method for a stretched polyoxymethylene body, which comprises heat-treating while shrinking under conditions satisfying

Hereinafter, the present invention will be described in detail.

The polyoxymethylene used in the method of the present invention is not particularly limited, and a polyoxymethylene homopolymer or polyoxymethylene copolymer obtained by a conventionally known polymerization method, or a blend thereof may be used. The polyoxymethylene may contain various additives for modification, antistatic agents, plasticizers, weather resistance improvers and the like.

In the method of the present invention, these polyoxymethylenes are formed into a stretched body having a tensile elastic modulus in the range of 8 to 45 GPa,
Next, heat treatment is carried out while shrinking this under conditions satisfying the relation of the above formula (I).

FIG. 1 is a schematic view of an example of an apparatus for heat-treating a stretched polyoxymethylene body in the present invention, which is fed from a bobbin 1 by a feeding machine 3 at a constant speed (V ₁ (m / min)). The polyoxymethylene stretched body 2 is heated in the heat treatment furnace 4 and is not sagged in the take-up machine 5 at a constant speed (V ₂ (m / mi
n)) and then by the winder 6. The relaxation rate is expressed by the following equation by the feeding speed V ₁ and the take-up speed V ₂ described above.

Relaxation rate (%) = 100 (V ₁ −V ₂ ) / V ₂ (II) Hereinafter, FIG. 2 will be described in more detail. Feeding machine 3
The pulling machine 5 may be of any type such as a nip roll type or a belt type, but it can reliably grip the polyoxymethylene stretched product from two or three directions, and has a function of feeding or pulling the stretched product at a constant speed. It is necessary to have The heat treatment furnace 4 has a double pipe structure in which steam is flown to the jacket part, a single pipe or a single rectangular pipe structure in which hot air is flown, a heat transfer heater is used from the outside, or a release container is used for ethylene glycol. There are various structures such as a method of filling a heat medium and a heating method, but all of them must have a function of controlling temperature. Considering economy and heating efficiency, it is preferable to use a double pipe structure in which steam is flown to the jacket and hot air is blown into the jacket. The temperature of the heat treatment varies depending on the length of the heat treatment furnace, the take-up speed, and the cross-sectional area of the polyoxymethylene stretched product, but it is preferably in the range of 150 to 190 ° C, and the temperature inside the furnace may be constant or a temperature gradient may be applied. . The heat treatment may be performed independently of the stretching step as shown in FIG. 1, or the heat treatment may be continuously performed after the stretching. In that case, the heat treatment may be performed by installing the heat treatment furnace 4 and the take-up machine 5 between the final take-up machine and the take-up machine in the drawing process.

In the method of the present invention, it is necessary to perform the heat treatment while shrinking the stretched polyoxymethylene body under the condition that the relaxation rate (E) satisfies the relation of the expression: relaxation rate (%) ≧ 14.2-0.3 E (I). is there.

When the relaxation rate is out of the range given by the above formula (I), thermal dimensional stability cannot be imparted to the stretched polyoxymethylene body even if heat treatment is performed, and only mechanical properties are deteriorated. Inevitably, the excellent properties inherent to polyoxymethylene are impaired. The preferred range of the relaxation rate is (16.5-0.3 E) or more, and (18.2-0.3 E)
E) Below.

The thermal dimensional stability in the present invention is represented by the absolute value of the dimensional change rate when heat-treating a sample for 20 hours at a temperature of 120 ° C. in an oven as a measure, and the absolute value of the dimensional change rate is 0.2%. When it was below, the dimension was thermally stable, and when it was 0.1% or less, it was particularly excellent in thermal dimension stability. If a negative sign is added before the dimensional change rate, it means that the sample contracted when the sample was heat-treated freely at a temperature of 120 ° C. for 20 hours.

The method for measuring the tensile modulus of elasticity of the stretched polyoxymethylene body before heat treatment used for calculating the relaxation rate in the present invention is as follows.

Set the stretched body with a grip length of 15 cm in a tensile tester equipped with a fixed position wedge type grip. A slight amount of tension is applied to remove the slack of the stretched body, and then the stretched body is marked with a mark. Set the gauge length to exactly 5 cm, and firmly hold the gauge with a differential transformer type extension end receiver. After installing the detection end on the receiver, perform the test in the same manner as a normal tensile test. When the elongation reaches about 1%, the elongation is stopped, and the linear region of elongation less than 0.3% of the obtained elongation-load curve is extrapolated to obtain the tensile elastic modulus.

EFFECT OF THE INVENTION According to the present invention, thermal dimensional stability can be imparted by shrinking the stretched body at a relaxation rate according to the tensile modulus of the stretched polyoxymethylene and heat-treating the stretched body. Further, the excellent mechanical properties of the stretched body are not impaired in practical use. That is, it is possible to obtain a polyoxymethylene stretched product having high strength, high elastic modulus and excellent thermal dimensional stability, which is extremely suitable as an industrial material.

Examples Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

Examples and Comparative Examples A hollow unstretched body made of polyoxymethylene [manufactured by Asahi Kasei Kogyo Co., Ltd., Tenac (registered trademark) 3010] and having an outer diameter of 4.6 mm and an inner diameter of 1.45 mm is disclosed in JP-A-60-183122. Stretching was carried out by the stretching method described above to produce a polyoxymethylene stretched product having a tensile elastic modulus of 14, 26, 36 and 44 GPa. These stretched bodies were heat-treated at various relaxation rates using the apparatus shown in FIG. The heat-treated stretched body was heat-treated in an oven at 120 ° C. for 20 hours in a free state, and the dimensional change rate was examined. The results are shown in FIG. FIG. 2 shows the relaxation rate and the dimensional change rate when each stretched body was heat-treated. The white plot in the figure shows the relaxation rate within the range of the present invention, and the black plot shows the relaxation rate according to the present invention. And the tensile modulus of the stretched body before heat treatment is 44
GPa, B shows 36 GPa, C shows 26 GPa, and D shows 14 GPa. From this figure, if the relaxed rate is set within the range of the present invention and the stretched body is heat-treated, the absolute value of the dimensional change rate of the stretched body of any tensile elastic modulus becomes 0.2% or less,
It can be seen that thermal dimensional stability is imparted. The dimensional change rate of the unheated stretched product after heat treatment at 120 ° C. for 20 hours was −several% to −10%. The decrease rate of tensile strength and tensile modulus before and after heat treatment is 10 ~ 20%.
It was within the range of practically no problem. However, the tensile elastic modulus of the stretched product having a low elastic modulus was reduced by about 30 to 40%.

[Brief description of drawings]

FIG. 1 is a schematic view of an example of an apparatus for heat-treating a polyoxymethylene stretched product according to the method of the present invention, and FIG. 2 shows the relationship between the relaxation rate and the dimensional change rate when heat-treating a polyoxymethylene stretched product. It is a graph shown. In the figure, reference numeral 1 is a bobbin, 2 is a polyoxymethylene stretched body, 3 is a feeding machine, 4 is a heat treatment furnace, 5 is a take-up machine, and 6 is a take-up machine.

Claims (1)

[Claims] 1. When heat-treating a polyoxymethylene stretched body, a stretched body having a tensile elastic modulus (E) in the range of 8 to 45 GPa is used, and a relaxation rate (%) ≧ 14.2-0.3 E is satisfied. A heat treatment method for a polyoxymethylene stretched product, which comprises heat-treating while shrinking under conditions.