JPH09300391A - Gas injected hollow molded object - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas injected hollow molded object good in gas flowability, having large sink preventing effect and excellent in strength or the like and heat resistance. SOLUTION: A gas injected hollow molded object is composed of a styrenic (co)polymer characterized by that a wt. average mol.wt. is 300000-600000 and a methanol soluble component is 5wt.% or less and a ratio (Mw/Mn) of a wt. average mol.wt. (Mw) and a number average mol.wt. (Mn) is 2.4 or a styrenic resin compsn. containing the styrenic (co)polymer. By injecting gas when the molded object is produced, a cavity part is provided to the interior of the molded object.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a gas injection hollow molded article. More specifically, the present invention relates to a gas-injected hollow molded article which has good gas fluidity and therefore has a large effect of weighing and preventing sink marks of the molded article, and which is excellent in mechanical strength such as impact strength and heat resistance. It is a thing.

[0002]

2. Description of the Related Art Styrene resins are widely used for molding because of their rigidity, excellent dimensional stability, and low cost. In recent years, in the field of application for injection molding, the size of molded products has been increasing, and therefore there is a demand for weight reduction. In order to meet the demand, a gas injection hollow structure in which a gas is injected into the molded product to provide a cavity. It is widely used as a molded body. Further, in a molded body having a thick portion, sink marks at the thick portion become a problem, and gas is injected into the thick portion in order to prevent sink marks.

A method of injecting a gas at the time of injection molding to form a gas injection hollow molded article is called a gas assist injection molding method,
It is being used actively in recent years. For details of the gas-assisted injection molding method, see, for example, Furuhashi Akira, Plastics, Vol. 45, no. 8 (22-32, 1994)
Year) etc.

When a gas-injected hollow molded body is obtained by gas-assisted injection molding, particularly in the case of a large-sized molded body, there is a demand to increase the hollow ratio as much as possible. In the case of a gas-injected hollow molded article having a large hollow rate, the gas needs to be easily flown into the molded article.

As an attempt to meet such a demand, for example, a method of lowering the molecular weight of the resin and increasing the fluidity of gas is proposed. However, this method has a problem that the strength of the resin is lowered and the resin is damaged during the extrusion process of the molded product or during use of the molded product. Further, as a method of increasing the fluidity of gas, for example, a method of adding a plasticizer such as mineral oil to a resin is proposed, but in this method, the plasticizer reduces heat resistance and impact strength of the resin. There was a problem.

As a method of maintaining the heat resistance and impact strength of the resin at a satisfactory level and improving the fluidity of gas, for example, there is a method of raising the resin temperature or the mold temperature during molding. However, this method increases the cooling time of the molded body and thus the molding cycle,
There is a problem that the production cost becomes high. In addition, in the method of increasing the gas injection pressure to increase the gas fluidity, there is a problem that the molding machine and the mold are large and the manufacturing cost is high. There is a problem in that the gas exudes even into the thin-walled portion that should not be injected, and the strength and appearance of that portion deteriorates.

[0007]

In such a situation,
The problem to be solved by the present invention is to provide a gas-injected hollow molded article that has good gas flowability and thus has a large effect of preventing sink marks, and that is also excellent in mechanical strength such as impact strength and heat resistance. Exist.

[0008]

That is, according to the present invention, the weight average molecular weight is 300,000 to 600,000, the methanol-soluble component is 5% by weight or less, and the weight average molecular weight Mw and the number average molecular weight Mn are A molded product comprising a styrene-based (co) polymer having a ratio (Mw / Mn) of 2.4 or more or a styrene-based resin composition containing the styrene-based (co) polymer, and producing a molded product It is intended to provide a gas-injected hollow molded article characterized by having a cavity inside the molded article by injecting gas at times. Hereinafter, the present invention will be described in detail.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Styrene series (co) used in the present invention
Examples of the styrene compound which is a monomer constituting the polymer include styrene, α-substituted alkylstyrene such as α-methylstyrene, and nuclear-substituted alkylstyrene such as p-methylstyrene.

In the styrene-based (co) polymer used in the present invention, a compound copolymerizable with the above-mentioned compound, together with the styrene-based compound, such as acrylonitrile, methacrylonitrile, methacrylic acid, Vinyl monomers such as ester derivatives such as methyl methacrylate, maleic anhydride, maleimide, and nucleus-substituted maleimide may be used in combination.

The styrene (co) polymer used in the present invention has a weight average molecular weight of 300,000 to 600,000, preferably 30.
10,000 to 400,000, and the amount of methanol-soluble matter is 5% by weight or less,
It is preferably 3% by weight or less, and has a weight average molecular weight M.
Styrene-based (co) having a ratio of w and number average molecular weight Mn (Mw / Mn) of 2.4 or more, preferably 2.4 to 3.4.
It is a polymer.

If the molecular weight is too small, the impact strength is poor,
If it is too large, the fluidity of the gas will be poor. If the soluble content is excessive, the heat resistance is poor. Here, the methanol-soluble component is measured by the following method. That is, about 1 g of a styrene-based (co) polymer is dissolved in 10 ml of methyl ethyl ketone at room temperature, 300 ml of methanol is added to cause reprecipitation, solids are collected by filtration, dried and precisely weighed. The weight ratio of the styrene-based (co) polymer reduced by such an operation to the styrene-based (co) polymer initially used is defined as the methanol-soluble component.

If the Mw / Mn value is too small, the fluidity of the gas is poor. Here, Mw / Mn is generally used as an index showing the breadth of the molecular weight distribution.

As a method for obtaining a styrene (co) polymer having a wide molecular weight distribution, for example, a high molecular weight polystyrene and a low molecular weight polystyrene are blended, or a polystyrene having a wide molecular weight distribution is prepared by using a multistage polymerization method. There is a method of doing so, which is disclosed in, for example, Japanese Patent Publication No. 57-30843 and Japanese Patent Publication No. 62-61231.

The styrene-based (co) polymer having the characteristics of the present invention has a number of branch points in the Z-average molecular weight of 1 to
A styrene-based copolymer having a ratio of 20, preferably 2 to 10 is preferable. As a method for obtaining such a styrene-based copolymer, for example, a styrene-based compound and a compound containing two or more vinyl groups are added to
There is a method of polymerizing by mixing 0.00 to 1000 ppm by weight, which is disclosed in JP-A-7-166013. Here, the number of branch points in the Z-average molecular weight is a concept corresponding to the number of branch points contained in a molecular chain that represents a high-molecular weight component of a styrene-based copolymer having a certain molecular weight distribution. Required by the method. That is, it can be determined by a viscosity-GPC method using gel permeation chromatography (GPC) equipped with a differential refractometer and a viscometer as a detector. For details, see Japan Rubber Association Journal, No. 45. Volume, Issue 2, 10
5 to 118 (1972). The number of branch points (Bn (M)) at the molecular weight M is determined by the following equation.

[IV (M) / IV _L (M)] ^2/3 =
[(1 + Bn (M) / 7) ^1/2 +4/9 ・ Bn
(M)] ^-1/2 where IV (M) and IV _L (M) are viscosity-
It is the intrinsic viscosity at the molecular weight M of the linear polystyrene as a sample and a standard sample measured by the GPC method.

The styrenic resin composition of the present invention is a lubricant for the above styrenic (co) polymer, if necessary.
Antistatic agents, antioxidants, heat stabilizers, UV absorbers, pigments, dyes, elastomers such as styrene-butadiene block copolymers may be added. Further, a plasticizer such as mineral oil may be added within a range that does not impair the effects of the present invention.

There is no particular limitation on the method for producing a gas injection hollow molded article by using the styrene-based (co) polymer of the present invention or the styrene-based resin composition containing the styrene-based (co) polymer. Known techniques such as a gas-assisted injection molding method can be used.

[0019]

Hereinafter, the present invention will be described based on examples.
The present invention is not limited to these examples.

Among the evaluation items, the items other than the above-mentioned items were evaluated as follows. (1) Gas fluidity (gas flow length) 10 mm thick elliptical spiral mold and BM-T2400 / 2 manufactured by Buttenfeld Co.
Using an X630 molding machine, maximum cylinder temperature 230 ° C,
Gas-assisted injection molding was performed under the conditions of an injection speed of 70 mm / sec, a gas pressure of 50 bar, and a measured value of 70 mm. As the injection gas, N ₂ gas was used. A value obtained by subtracting 80 mm, which is the length of the spool portion, from the length from the gas inlet of the obtained molded product to the tip of the hollow portion was obtained as the gas flow length,
Individual molded bodies were sampled and the average value was obtained. The longer the gas flow length, the better the gas flowability.

(2) Impact strength (falling ball impact strength) According to JIS K7211, except that the sample was press-molded at 200 ° C. to a size of 50 × 50 × 2 mm and the weight of the ball was 28.8 g. The value of 50% breaking height was measured. Here, in low-pressure molding such as gas-assisted injection molding, one of the features is that the residual stress (strain) of the molded body is reduced as compared with the conventional injection molding method. Since the impact strength of the molded body obtained by the molding method is less likely to decrease due to residual stress,
The impact strength of the gas-injected hollow molded article is considered to largely depend on the impact strength of the resin forming the hollow molded article. Therefore, it can be said that the higher the impact strength of the press molded body, the higher the impact strength of the gas injection hollow molded body.

(3) Heat resistance (Vicat softening point) Measured under a load of 5 kg in accordance with JIS K6871.
Here, the heat resistance of the gas-injected hollow molded article is considered to largely depend on the heat resistance of the resin forming the gas-injected hollow molded article. Therefore, it can be said that the higher the value, the higher the heat resistance of the gas-injected hollow molded article. . (4) Prevention of sink mark The degree of sink mark of the gas-injected hollow molded article obtained in (1) above was visually evaluated as follows. ○: No sink mark has occurred. △: Some sink marks have occurred. X: A sink mark is generated as a whole.

Examples 1, 22 Using divinylbenzene (purity 55% by weight, manufactured by Tokyo Kasei Kogyo Co., Ltd.), which is a compound containing at least two vinyl groups,
A solution prepared to have a monomer composition shown in Table 1 was continuously supplied to a continuous bulk polymerization reaction layer, polymerization was carried out at a polymerization temperature and a final conversion rate shown in Table 1, and the polymerization mixture was vacuum degassed at 240 ° C. The unreacted monomer was recovered by passing through, and a resin pellet was used to produce a molded body. The results are shown in Table 1.

Comparative Examples 1 to 4 Molded articles were produced in the same manner as in Example 1 except that divinylbenzene was not used and the polymerization was carried out under the conditions shown in Table 1. The results are shown in Table 1.

Comparative Example 5 A molded body was manufactured in the same manner as in Example 2 except that gas injection was not performed. The results are shown in Table 1.

[0026]

[Table 1] ─────────────────────────────────── Example Comparative Example 1 2 1 2 3 4 5 ─────────────────────────────────── Polymerization monomer Styrene wt% 95 95 95 95 95 91 95 Ethylbenzene wt% 5 5 5 5 5 5 5 Divinylbenzene wtppm ^{* 1} 150 300 0 0 0 0 300 Mineral oil wt% 0 0 0 0 0 0 4 0 Polymerization temperature ℃ 140 145 155 145 120 130 145 Final conversion wt% ^{* 2} 60 60 70 60 60 60 60 Weight average molecular weight 10 ⁴ 37 38 21 29 38 35 38 Mw / Mn ^{* 3} 2.5 3.0 2.0 2.0 2.0 2.0 3.0 Methanol soluble content wt% 1.2 1.2 1.2 1.2 1.2 5.2 1.2 Number of branch points ^{* 4} 2.1 4.0 0 0 0 0 4.0 Gas injection Yes Yes Yes Yes Yes Yes Yes No [Evaluation result] Gas flow length cm 20 26 42 20 9 30 0 Falling ball impact strength cm 42 45 30 35 45 40 45 Vicat softening point ℃ 102 102 102 102 102 88 102 Sink prevention ○ ○ ○ ○ △ ○ × ──────── ───────────────────────────

* 1 Divinylbenzene: The value is ppm by weight based on the styrene compound. * 2 Final conversion: About 0.1 g of the polymerization solution extracted from the final polymerization reaction layer from the middle of the pipe to the preheater was precisely weighed, and 8
It was vacuum dried at 0 ° C. for 1 hour and at 150 ° C. for 2 hours, and the weight after drying was divided by the weight before drying. The value expressed as a percentage was taken as the final conversion rate. * 3 Mw / Mn: Ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) * 4 Number of branch points: number of branch points in Z average molecular weight [Bn (Mz)]

The results show the following. Examples 1 and 2 satisfying the conditions of the present invention show excellent results in all evaluation items. On the other hand, the weight average molecular weight is 30
Comparative Examples 1 and 2, which are less than 10,000, are inferior in impact strength.
Ratio of weight average molecular weight Mw and number average molecular weight Mn, Mw / M
Comparative Example 3 in which n is 2.4 or less is inferior in gas fluidity and sink prevention. Comparative Example 4 having a methanol-soluble content of 5% by weight or more is inferior in heat resistance. In Comparative Example 5 in which gas injection was not performed, sink marks were generated.

[0029]

As described above, according to the present invention,
It is possible to provide a gas-injected hollow molded article that has good gas fluidity and thus has a large effect of preventing sink marks, and that is excellent in mechanical strength such as impact strength and heat resistance.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display area C08L 25:04

Claims (2)

[Claims] 1. A weight average molecular weight of 300,000 to 600,000,
A styrene-based (co) polymer or a styrene-based (co) polymer having a methanol-soluble content of 5% by weight or less and a ratio (Mw / Mn) of the weight average molecular weight Mw to the number average molecular weight Mn of 2.4 or more. A hollow body for gas injection, which is a molded body made of a styrene resin composition containing a polymer and has a hollow portion inside the molded body by injecting a gas during the production of the molded body. 2. The gas-injected hollow molded article according to claim 1, wherein the styrene-based (co) polymer is a styrene-based copolymer in which the number of branch points in the Z average molecular weight is 1 to 20.