JPH0355502B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing pre-expanded polypropylene resin particles, and more particularly to a method for producing pre-expanded polypropylene resin particles having good moldability. Conventionally, foamed polystyrene and foamed polyethylene have been mainly used for various purposes such as cushioning materials and packaging materials. Along with these foams, polypropylene resin foam moldings have been used, and the applicant has already filed an application for a method for obtaining pre-expanded particles for use in a polypropylene resin bead molding method (Japanese Patent Publication No. 56-1344). issue). This method is revolutionary in that it is possible to easily obtain relatively highly foamed pre-expanded particles from polypropylene resin particles, which have been considered extremely difficult to foam. This method has some problems and still leaves room for improvement. In other words, (1) Although relatively high foaming can be obtained, 25
The limit is about twice as much, and even if you try to obtain a product with a magnification higher than this, the closed cell ratio will decrease and it will not be possible to use it for molding. (2) The cells in the obtained pre-expanded particles tend to be fine, and even if a molded article is attempted to be obtained using such pre-expanded particles, it is difficult to obtain a strong foam with good dimensional accuracy. The present invention aims to overcome the problems of the prior art described above. That is, one of the objects of the present invention is to provide a method for easily producing pre-expanded polypropylene resin particles that have a high closed cell ratio and can be molded even at a high expansion ratio, for example, an expansion ratio of about 50 to 150 times. It is to be. Another object of the present invention is to easily produce pre-expanded polypropylene resin particles that have good moldability, are strong, and can produce molded bodies with good dimensional accuracy, regardless of whether they are low-foamed or highly foamed. The object of the present invention is to provide a method for manufacturing the same. As a result of intensive research aimed at achieving the above object, the present inventors have found that by heating pre-expanded polypropylene resin particles with steam, which have a specific relationship between the expansion ratio and the number of cells, the expansion ratio can be improved, and the cell diameter can be improved. Pre-foamed particles with large (small number of bubbles) can be obtained, such pre-foamed particles have excellent moldability, and the foam molded product obtained using the same has good dimensional accuracy and is strong. Headline,
The present invention has now been completed. That is, in the present invention, when the expansion ratio is E and the number of bubbles per 1 mm ² of cross section is n, the following formula: 2<E ^1/3 ×
A polypropylene resin characterized in that pre-expanded polypropylene resin particles satisfying the relationship n ^1/2 <45 are heated with steam and foamed to obtain pre-expanded particles having a larger expansion ratio than the original expansion ratio E. The gist of this paper is a method for producing pre-expanded resin particles. Examples of the polypropylene resin used in the present invention include propylene homopolymer, ethylene-
Examples include propylene random copolymers, ethylene-propylene block copolymers, mixtures thereof, and ethylene-propylene random copolymers are particularly preferred. The pre-expanded polypropylene resin particles used for foaming in the present invention are expressed by the following formula, where E is the expansion ratio and n is the number of cells per 1 mm ² of cross section.
These are pre-expanded particles that satisfy E ^1/3 ×n ^1/2 <45.
If E ^1/3 ×n ^1/2 is less than 2, the bubbles in the pre-expanded particles obtained by heating and foaming will become too coarse, making it difficult to obtain a foamed molded product with good physical properties. E ^1/3 ×
When n ^1/2 is 45 or more (in such a case, either the expansion ratio E is extremely high or the number of cells n is extremely large, or both cases), such pre-foaming is necessary. Even if you try to foam the particles by heating them, they tend to shrink, resulting in extremely low foaming efficiency, and if you try to foam them forcibly, it will lead to a decrease in the closed cell ratio. Even if foam molding is carried out using this method, the resulting foam molded product has poor dimensional accuracy and does not have satisfactory physical properties. In the present invention, the first pre-expanded particles as a starting material to be subjected to heat foaming can be obtained, for example, by the following method. That is, polypropylene resin particles, a volatile blowing agent, and a dispersion medium are placed in a closed container, heated to a temperature higher than the temperature at which the resin particles soften to incorporate the volatile blowing agent into the resin particles, and then one end of the container is closed. This can be obtained by opening the container and simultaneously releasing the resin particles and the dispersion medium into an atmosphere having a lower pressure than the inside of the container. In the present invention, it is not particularly necessary to actively impart foaming ability to the pre-expanded particles. but,
If the original pre-expanded particles are expanded to 1.4 times or more, or if the original expanded particles are 10 times or less, highly expanded particles cannot be obtained with a single foaming operation, so it is preferable to impart foaming ability. The foaming ability is imparted by allowing the pre-expanded particles to contain an inorganic gas, a volatile foaming agent, or a mixed gas of an inorganic gas and a volatile foaming agent, and varies depending on the desired expansion ratio. , usually 1.5
An internal pressure of ~10Kg/cm ² ·ab is applied. Examples of the inorganic gas include air, nitrogen, argon, helium, etc., but air is usually used.
Examples of volatile blowing agents include aliphatic hydrocarbons such as propane, butane, pentane, and hexane, cycloaliphatic hydrocarbons such as cyclobutane and cyclopentane, trichlorofluoromethane, and dichlorodichloromethane. Halogenated hydrocarbons such as fluoromethane, dichlorotetrafluoroethane, methyl chloride, ethyl chloride, methylene chloride, etc. can be used. In the present invention, the polypropylene resin pre-expanded particles are heated with steam. This heating method is carried out, for example, by placing the pre-expanded particles in a container and introducing steam directly into the container while stirring. The above steam may be, for example, 0.5 to 3 kg/
cm ² (G) of steam is usually used, and the heating time is usually within 1 minute, preferably within 30 seconds. By using steam in this manner, heating can be performed in a shorter time than in the case of hot air, and highly expanded particles can be obtained, resulting in improved productivity. In the present invention, the following formula: 2<E ^1/3 ×n ^1/2 <45
Polypropylene resin particles having the following relationship are heated with steam and foamed, but this operation is not limited to one operation, and the pre-expanded particles that have been heated and foamed are further heated with steam to foam them. May be repeated. Such operations may be repeated any number of times as long as the polypropylene resin pre-expanded particles to be subjected to heat foaming satisfy the above relationship. In the present invention, the expansion ratio E means the true expansion ratio. The true expansion ratio is the reciprocal of the density of the pre-expanded particles divided by the resin density of the base resin. Here, the density of the pre-expanded particles can be measured, for example, as follows. First, a predetermined amount of pre-expanded particles with a known weight is placed in a predetermined amount of water previously placed in a graduated cylinder, and the total volume is then measured. The volume of the pre-expanded particles is then determined by subtracting the original water volume from the total volume, and can be determined by dividing the weight by the volume. The pre-expanded particles obtained by the present invention usually have an expansion ratio of 1.15 times or more than that of the original pre-expanded particles, for example, 3 to 100 times, preferably 10 to 60 times, and have an expansion ratio of 0.1 to 200 times. /mm ² bubble count. The pre-expanded polypropylene resin particles obtained according to the present invention are used for producing foam molded articles. First, the pre-expanded particles are aged for a predetermined time at room temperature and pressure, and then aged for a predetermined time at a predetermined pressure using an inorganic gas such as nitrogen or air or a mixed gas with a volatile blowing agent, if necessary. Next, the pre-expanded particles are filled into a mold having small holes on the mold surface through which a heating medium such as water vapor can pass, and foamed according to the mold by heating and foaming with water vapor of, for example, 2 to 5 kg/cm ² (G). A molded body can be obtained. The polypropylene resin foam molded product described above can be used, for example, in packaging materials, cushioning materials, heat insulating materials, heat insulating materials, construction materials, vehicle components, flotation materials, food containers, and the like. As explained above, according to the present invention, it is possible to obtain polypropylene resin pre-expanded particles with even higher foaming, and the foam molded product obtained using the pre-expanded particles has good moldability and stiffness. is strong,
It also has excellent properties such as good dimensional accuracy. Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples. Examples 1 to 16 and Comparative Examples 1 to 15 Pre-expanded particles of ethylene-propylene random copolymer (melting point 145°C) having the internal pressure shown in 1 of Table 1 and 2 of Table 1 obtained by a conventional method were placed in a container. While stirring, the mixture was heated directly with steam having the pressures shown in Table 1, 1 and 2. 2.5Kg/cm ² (absolute pressure) with air to the obtained pre-expanded particles
An internal pressure of . Next, the pre-expanded particles were filled into a mold and heated and foamed with 3.2 Kg/cm ² (G) of steam to obtain a foamed molded product. The dimensional accuracy and fusion properties of the obtained foam molded product were measured. The results are shown in Table 1, 1 and 2. Among the pre-expanded particles used in these Examples and Comparative Examples, the pre-expanded particles with an expansion ratio exceeding 25 times before treatment were subjected to two-stage foaming according to the method of the present invention. I used the one that I had made there. The obtained data were plotted with the horizontal axis as logE and the vertical axis as logn (Examples are indicated by ○, comparative examples are indicated by ●). The results are shown in FIG. In Figure 1, straight line 1 shows logn = -2/3logE + 2log45, and straight line 2 shows logn = -2/3logE + 2log2, and straight line 1 corresponds to the formula: E ^1/3 × n ^1/2 = 45. , straight line 2 corresponds to the equation: E ^1/3 ×n ^1/2 = 2.

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【table】 [Brief explanation of drawings]

FIG. 1 is a diagram plotted with the horizontal axis as logE and the vertical axis as log n, based on data on the expansion ratio E and the number of bubbles n of pre-expanded particles obtained in Examples and Comparative Examples.

Claims (1)

[Claims] 1 When the expansion ratio is E and the number of bubbles per 1 mm ² of cross section is n, pre-expanded polypropylene resin particles having the following formula: 2 < E ^1/3 × n ^1/2 < 45 are heated with steam. A method for producing pre-expanded polypropylene resin particles, which comprises foaming the particles to obtain pre-expanded particles having a larger expansion ratio than the original expansion ratio E.