JPS593484B2 - Method for manufacturing thermoplastic resin foam - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a thermoplastic resin foam having uniform, fine cells.

Conventionally, there are various methods for producing thermoplastic resin foams.

For example, a method for producing a foam (US Pat. No. 3,098,831) has been proposed in which a thermoplastic resin is mixed with a crosslinking agent and a blowing agent with a higher decomposition temperature than the crosslinking agent, and the mixture is first crosslinked and then foamed.

According to this method, a low-density thermoplastic resin foam having uniform microcells can be obtained, but it must be treated under pressure and heat for a long time, so the manufacturing process becomes discontinuous.
The drawback was low manufacturing efficiency.

In addition, a thermoplastic resin is mixed with a crosslinking agent and a foaming agent with a higher decomposition temperature, heated at a faster temperature increase rate, and both agents are decomposed almost simultaneously to cause crosslinking and foaming. A method has been proposed (UK Patent No. 112,685).

According to this method, a low-density thermoplastic resin can be produced efficiently, but since the blowing agent does not completely decompose and remains inside the foam, the surface of the foam becomes colored. It easily discolors when exposed to water and has coarse bubbles. Furthermore, a method for producing a foam has been proposed in which a thermoplastic resin is mixed with a normal foaming agent, crosslinked by irradiation with ionizing radiation, and then heated and foamed. However, this method results in a foam with a non-uniform cell structure in which large and small cells are mixed, and furthermore, the manufacturing cost is high due to the expensive manufacturing equipment. As a result of extensive research, the inventor of the present invention has found that by using a low-temperature decomposition type foaming agent, the effective utilization rate of the foaming agent can be increased. We have discovered a method for efficiently manufacturing plastic resin foams. That is, this invention mixes a pyrolytic foaming agent with a thermoplastic resin,
In a method of producing a foam by heating after molding, a low-temperature decomposition type foaming agent whose initial decomposition temperature until a 1% weight loss is 180°C or lower and whose decomposition temperature is 185°C or higher as determined by thermogravimetric analysis. It is characterized by the use of

Next, this invention will be explained in detail.

This invention is based on the investigation of the foaming mechanism of a foaming agent from the following foaming process, and based on this, the discovery of a low decomposition type foaming agent suitable as a foaming agent for thermoplastic resins.

That is, 100 parts by weight of polyethylene, 10 parts by weight of azodicarbonamide as a foaming agent, and 1.0 parts by weight of dicumyl peroxide as a crosslinking agent were added and kneaded, and the resulting sheet with a thickness of 2.0 m1 was molded into a metal sheet at 200°C. Foaming was carried out in a bath, and the specific volume and number of bubbles of the foam were observed. From this experiment, the horizontal axis shows the specific volume (Cc/9) of the foam, and the vertical axis shows the number of bubbles in the foam (Cc/9). /f!) to create a foaming state line.

This figure shows that the number of cells in the foam increases significantly during the foaming process until the specific volume of the foam reaches 3CC/9.
After that, it was found that the number of cells in the foam hardly increased, and the diameters of the existing cells simply increased, increasing only the specific volume of the foam. In other words, it was found that during the foaming process, there are two periods: a period in which the number of bubbles increases (nuclear bubble generation period) and a period in which only the diameter of the bubbles increases. Furthermore, as a result of investigating the generation behavior of nuclear bubbles during the above-mentioned nuclear bubble generation period, it was found that when the generated gas from the blowing agent is dissolved in the thermoplastic resin and reaches the saturated solubility or higher, it becomes bubbles as nuclear bubbles and enters the foam. If the remaining blowing agent, in other words, slowly decomposes into two and generates gas, the generated gas will be dissolved in the thermoplastic resin and will diffuse out before reaching saturation solubility, resulting in the formation of new nuclear bubbles. instead, only gas diffusion into existing bubbles occurs.
Only a foam containing a small number of coarse cells can be obtained.
Based on this, as a result of further research into the relationship between the nuclear bubble generation stage and the blowing agent, we found that the foaming ratio was 3 from the start of decomposition of the blowing agent.
The so-called nuclear bubble generation period up to a certain degree of expansion is a period caused by the decomposition of the blowing agent, which corresponds to a weight loss of about 3%; It has been found that the average temperature for the period can be determined by the degree of decomposition, and that the total decomposition temperature depends approximately on the melting temperature (foaming temperature) of the thermoplastic resin. Based on this research result, the present invention is a low-temperature decomposition type foaming agent with a large temperature difference between the initial decomposition temperature and the foaming temperature, that is, the initial decomposition temperature until the weight decreases by 1% in thermogravimetric analysis is 180 ° C or less, In addition, by using a blowing agent with a total decomposition temperature of 185°C or higher, the initial decomposition rate of the blowing agent is increased, the thermoplastic resin can easily reach saturated solubility with the generated gas, and a large number of In addition to forming nuclear cells, the effective utilization rate of the blowing agent is increased to prevent the blowing agent from remaining in the foam, and the surface of the resulting foam becomes white without discoloration. Therefore, according to the present invention, it is possible to obtain a thermoplastic resin foam that contains a large number of extremely fine cells and becomes obvious when colored with a pigment or the like. Aspects of the present invention will be described below.

First, the thermoplastic resin in the present invention refers to, for example, high-density or low-density olefin polymers such as polyethylene, polypropylene, and polybutene, or ethylene-propylene copolymers, ethylene-butene copolymers, and ethylene monovinyl acetate copolymers. Olefin copolymers such as ethylene-acrylic acid ester copolymers, polyhypyrene polymers such as polyvinyl chloride monochloride monoacetate copolymers, polyacrylic acid, polymethacrylic acid, etc. Or a mixture of two or more types, or a mixture of various rubbers with polyolefin.

Next, a foamable composition is prepared by blending a low-temperature decomposition type foaming agent with the thermoplastic resin, or adding a dispersant to improve mixing of the foaming agent with the thermoplastic resin.

In addition, the low-temperature decomposition type blowing agents used here include, for example, azodicarbonamide, dinitropentamethylenetetramine, PP'-oxybisbenzenesulfonyl hydrazide, azobisisobutyronitrile, paratoluenesulfonyl semicarbazide. , hydrazodicarbonamide,
In the process of synthesizing benzenesulfonyl hydrazide, etc., by controlling various conditions such as raw material particle size, reaction temperature, reaction time, amount of catalyst, etc., the initial decomposition temperature at which weight is reduced by 1% in thermogravimetric analysis can be set to 180%. 120 below ℃
℃ or higher, preferably 175℃ or lower and 120℃ or higher,
The foaming agent has a total decomposition temperature of 185°C or higher, preferably 190°C or higher, and two or more of these foaming agents may be mixed to form a foaming agent that satisfies the above conditions. In this case, a low temperature decomposition type foaming agent having a low initial decomposition temperature and a high temperature decomposition type foaming agent having a high initial decomposition temperature may be used together, and blowing agents with various decomposition behaviors may be used. Further, in order to lower the initial decomposition temperature of the blowing agent, a blowing aid such as zinc oxide or zinc stearate is added if necessary. In the blowing agent of the present invention, the reason why the initial decomposition temperature is set to 180°C or lower and the total decomposition temperature is limited to 185°C or higher is that the initial decomposition temperature is 180°C or lower.
When the temperature exceeds 80°C, the temperature difference with the foaming temperature becomes small and the generation of nuclear bubbles becomes slow, and the thermoplastic resin that is the base material of the foam does not reach saturated solubility due to the gas generated by the blowing agent, resulting in a large number of Since nuclear bubbles cannot be generated, a foam with many fine bubbles cannot be obtained. On the other hand, if the total decomposition temperature is less than 185°C, the temperature at which the thermoplastic resin sufficiently melts (foaming temperature)
Since a large amount of decomposed gas is generated from the blowing agent before the decomposition temperature is reached, the foaming efficiency is low, and the temperature becomes close to the above-mentioned initial decomposition temperature), making it impossible to obtain a foam having a large number of fine bubbles. Note that the amount of low-temperature decomposition type blowing agent added to the base material of the foam cannot be absolutely limited depending on the type of the base material, etc., but usually, when adding the crosslinking agent described below to the base material, 0.1 to 3 of the above foaming agent per 100 parts by weight of the base material
If the crosslinking agent is not mixed in the base material, the foaming agent is added in an amount of 0.0 parts by weight per 100 parts by weight of the base material of the foam.
It is preferable to mix 1 to 40 parts by weight. Then, a crosslinking agent is added to a foam base material containing the above-mentioned foaming agent to prepare a foamable composition.

The crosslinking agents used here include ditertiary butyl peroxide, 1,3 bis(tertiary butylperoxyisopropyl)benzene, and 2,5 dimethy! Le 2,5ji (
These include organic peroxides such as tert-butylperoxy)hexane and dicumyl oxide, azide compounds such as 1,9 nonane bissulfon azide, and organic chlorides such as octachlorocyclobenzene. In this case, in order to perform crosslinking rapidly or uniformly, and to perform foaming three-dimensionally and in the same direction to prevent the foam from sticking to the support, the foaming agent is added to the base material. It is desirable to add crosslinking aids such as allyl cyanurate, triallyl isocyanurate, and trimethylolpropane methacrylate. Furthermore, various additives such as pigments for coloring, anti-aging agents, light stabilizers, flame retardants, etc. may be added as appropriate. Next, the foamable composition or the foamable composition blended with a crosslinking agent is kneaded using a mixing roll, pressure kneader, Banbury mixer, press machine, etc., and then formed into a plate by pressure molding or press molding. Form into shapes, sheets, etc. Subsequently, the molded product made of the foamable composition is crosslinked by irradiation with ionizing radiation, and then heated and foamed. Further, a molded article prepared by adding a crosslinking agent to a foamable composition is heat-treated to cause crosslinking and foaming. This heat treatment method is carried out in an infrared heater, hot air heating furnace, salt bath or metal bath. In this case, in order to correct distortion of the molded body, form a large number of fine foams in the obtained foam, and prevent adhesion between the foam surface and the support surface, the initial heating temperature is set to 100-140℃. Preferably, the temperature is set to a low temperature of about 0.degree. C., and then the heating rate is increased to perform rapid heat treatment. below,
An embodiment of this invention will be described.

It should be noted that all parts shown in the examples mean parts by weight. Example 1 Low density polyethylene 100 with melt index 1.0
．． 0 parts, 10.0 parts of azodicarbonamide, which has an initial decomposition temperature of 172°C resulting in a 1% weight loss in thermogravimetric analysis as a low-temperature decomposition blowing agent, and a total decomposition temperature of 188°C, and dicumylper as a crosslinking agent. 0.9 parts of oxide and 0.2 parts of triallyl cyanurate as a crosslinking aid were blended, and the mixture was press-molded into a sheet having a thickness of 27 ftm using a press.

Subsequently, this sheet plate was immediately heated and foamed at 240° C. in a heating furnace equipped with a wire mesh conveyor to obtain a foamed sheet. The resulting foam sheet has a white surface and an average cell diameter of 0.4m77! It had uniform fine bubbles. Comparative Example As a blowing agent, the initial decomposition temperature at which the weight decreases by 1% is 183°C.
When the same treatment as in Example 1 was performed except that commercially available azodicarbonamide having a total decomposition temperature of 198° C. was used, only a foamed sheet with a large cell diameter of 0.62 ml was obtained.

Example 2 Low density polyethylene 100 with melt index 1.0
．． 0 parts, 10.0 parts of azodicarbonamide as a high-temperature decomposition blowing agent, and 1.5 parts of PP'oxybisbenzenesulfonyl hydrazide as a low-temperature decomposition blowing agent (1% weight reduction in thermogravimetric analysis). (initial decomposition temperature is 170°C, total decomposition temperature is 190°C), 0.7 part of cicumyl peroxide as a crosslinking agent, and 0.2 part of triallyl cyanurate as a crosslinking aid are mixed and kneaded with a mixing roll. After that, press molding to 235LX1
30WX3N-Mw! It was formed into a sheet.

Next, cut this sheet board into small pieces of 3 cm square, and
A foam sheet was made by heating and foaming in a 000C metal bath. The resulting foamed sheet had a white surface and had uniform, fine cells with an average cell diameter of about 0.33.

As detailed above, the present invention is a method for producing a foam by mixing a thermoplastic resin with a pyrolytic blowing agent, molding and heating the mixture, and is capable of producing a foam until the weight decreases by 1% in thermogravimetric analysis. Using a low-temperature decomposition type foaming agent with an initial decomposition temperature of 180°C or lower and a total decomposition temperature of 185°C or higher, a large number of nuclear bubbles are generated during the nuclear bubble generation period to form uniform fine bubbles, Moreover, it is possible to provide a method for easily producing a thermoplastic foam that can increase the effective utilization rate of a blowing agent and make the surface clear.

[Brief explanation of drawings]

The figure is a diagram showing the change in the number of cells in the foam as the specific volume of the foam increases during foaming.

Claims (1)

[Claims] 1 In a method in which a thermoplastic resin is mixed with a pyrolyzable blowing agent, molded, and then heated to produce a foam, thermogravimetric analysis shows that the initial decomposition temperature at which the weight decreases by 1% is 180°C.
A method for producing a thermoplastic resin foam, characterized in that a low-temperature decomposition type foaming agent having a total decomposition temperature of 185° C. or higher is used.