JPH0622920B2 - Method for manufacturing thermoplastic resin in-mold foam molding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to a method for producing a thermoplastic resin in-mold foamed article.

[Conventional technology]

The in-mold foaming method, in which pre-expanded thermoplastic resin particles are filled in a molding die and heat-foamed to form a molded body, has been widely adopted as a method for producing a foamed body, and conventionally, this type of in-molded foaming has been performed. As a molding method, the pre-expanded particles are pressure-treated with an inorganic gas, the inorganic gas is pressed into the particles to give a foaming ability, and then the particles are filled in a molding die to be heated and foamed (Japanese Patent Publication No. 52-22951). JP-A-49-128065
No.), a method in which pre-expanded particles are compressed and filled in a molding die to be foamed by heating (Japanese Patent Publication No. 53-33996),
A method in which pre-expanded particles having an internal pressure substantially equal to the atmospheric pressure are filled in a molding die for heat-expanding, and the resulting molded body is aged at a predetermined temperature (Japanese Patent Publication No. 55-7816, Japanese Patent Laid-Open No. 60- 166442) and the like are known.

[Low points of problems to be solved by the invention]

However, in this method, a long-time pressure treatment is required to apply an internal pressure to the pre-expanded particles, and the time during which the inorganic gas pressed into the pre-expanded particles is held in the particles is relatively short, After prepressing the pre-expanded particles and applying internal pressure, the pre-expanded particles should be molded in a short time, or unless the method of holding the pre-expanded particles under pressure is applied until molding is performed. As a result of the decrease in the internal pressure, the obtained foamed molded product shrinks, resulting in a large variation in dimensions, and there is a drawback that molding takes a long time to prevent the shrinkage. One of the methods is to give pre-expanded particles a foaming ability by compressing the pre-expanded particles, and a facility for compressing and filling the pre-expanded particles is required. Further, it is necessary to link the maintenance for compression filling with the molding machine, which causes a problem that the apparatus becomes complicated. When the pre-expanded particles are not subjected to a pressure treatment as in the other method, and the pre-expanded particles having an internal pressure substantially equal to the atmospheric pressure are used for molding, the pre-expanded particles have many voids due to the poor secondary foaming ability. Only a molded product can be obtained, and if a superheated method of increasing the heating time or increasing the pressure of steam for heating molding is adopted in order to solve such a defect, a molded product taken out from the molding mold to a room temperature atmosphere Expands once and then rapidly shrinks. In the method for this, after molding,
There is a problem that it is necessary to cure the molded body at a predetermined temperature, complicated temperature control and time management are required at the time of curing, the molding cycle becomes long, and efficient molding cannot be performed.

[Means for solving problems]

The present inventor has conducted extensive studies in view of the above points, and as a result, a foamed molded product obtained by filling pre-expanded particles in a mold and heat-foaming it and pressurizing it with an inorganic gas has a poor foaming ability. The inventors have found that an excellent expanded molded article can be obtained without complicated curing after molding even when sufficient pre-expanded particles are used, and have completed the present invention.

That is, the present invention comprises a step of filling thermoplastic resin pre-expanded particles in a molding die and heat-foaming the mixture, and a foamed molded body obtained by heat-foaming is added with nitrogen or an inorganic gas containing nitrogen as a main component in the mold. The gist is a method for producing a foamed molded product in a thermoplastic resin, which comprises cooling after pressing or cooling while pressurizing with an inorganic gas.

As the base resin of the thermoplastic resin pre-expanded particles used in the present invention, low density polyethylene, linear low density polyethylene, high density polyethylene, polypropylene,
Polyolefin resin such as polybutylene, ethylene-propylene block copolymer, ethylene-propylene random copolymer, or a resin composed of a mixture of two or more thereof; polystyrene, poly-p-methylstyrene, styrene-maleic anhydride copolymer Styrene resins such as polymers, styrene-acrylic acid copolymers, styrene-acrylonitrile copolymers, styrene-butadiene acrylonitrile copolymers; polyvinyl chloride, vinyl chloride-vinyl acetate copolymers, polyvinylidene chloride, etc. However, in order to effectively achieve the object of the present invention, it is preferable to use a polyolefin resin.

Pre-expanded particles used in the present invention is a method of foaming expandable resin particles containing a foaming agent under pressure in a pressure vessel, after melt-kneading a resin and a foaming agent in an extruder,
Extrusion foaming to form a foamed strand, a method of cutting this, dispersing resin particles and a foaming agent in a dispersion medium in a pressure resistant container and heating under pressure to impregnate the resin particle with the foaming agent, and then at atmospheric pressure It can be obtained by a method of discharging the foam to the bottom to foam.

The pre-expanded particles are subjected to pressure treatment with an inorganic gas, preferably nitrogen or an inorganic gas containing nitrogen as a main component such as air in the same manner as in ordinary molding, and the inorganic gas is contained in the particles so that the internal pressure of the particles (usually in absolute pressure). It may be subjected to molding after applying a pressure of 1.2 kg / cm ² · abs), but in the present invention, the internal pressure of the pre-expanded particles to be heat-expanded is not necessarily higher than 1.2 kg / cm ² · abs. After pre-expanded particles with an internal pressure reduced to 1.2 kg / cm ² · abs or less after being pressure-treated with an inorganic gas and left under atmospheric pressure, left at atmospheric pressure after production. As with pre-expanded particles having a particle internal pressure substantially equal to the atmospheric pressure, the internal particle pressure is 1.2 kg / cm ² abs due to the gas mainly composed of the inorganic gas.
The following pre-expanded particles can also be used.

In the production method of the present invention, the above-mentioned pre-expanded particles are filled in a molding die and heated for foam-molding. To heat the pre-expanded particles, a steam having a gauge pressure of 2 to 5 kg / cm ² · G is usually used. To be

Further, in the present invention, the pre-expanded particles are filled in a molding die and heated for foam molding, but in molding, it is not always stable due to factors such as a low expansion ratio, a low particle internal pressure, and a high cell membrane strength. In the case of pre-expanded particles that do not cause secondary expansion, for example, ethylene-propylene random copolymer, the bulk magnification is 25 times or less, the internal pressure of the particles is about atmospheric pressure, and differential scanning calorimetry (hereinafter referred to as "DSC"). Energy of the high temperature peak in the melting curve of is 8 J / g
In the case of the above pre-expanded particles having high film strength, it is preferable to fill the particles in a mold and then heat and foam by reducing the pressure in the mold.

The high temperature peak in the DSC melting curve is the high temperature peak in the DSC curve obtained when 1 to 3 mg of polypropylene resin pre-expanded particles are heated to 220 ° C. at a temperature rising rate of 10 ° C./min by a differential scanning calorimeter. For example, the DSC curve when the temperature of the sample was raised from room temperature to 220 at 10 ° C./min was used as the first DSC curve, and then 22
The DSC curve obtained when the temperature was decreased from 0 ° C. to about 40 ° C. at a temperature decrease rate of 10 ° C./minute, and was again increased to 220 ° C. at a temperature increase rate of 10 ° C./minute was used as the second DSC curve. The characteristic peak and the high temperature peak can be obtained from the DSC curve of.

That is, the unique peak is an endothermic peak peculiar to the polypropylene resin forming the expanded particles, and is considered to be due to the so-called endothermic reaction of the resin during melting. The characteristic peak appears in both the first DSC curve and the second DSC curve, and the temperature at the peak apex may differ slightly between the first and second times, but the difference is usually less than 2 ° C. Is.

On the other hand, the high temperature peak is an endothermic peak that appears on the higher temperature side than the above-mentioned intrinsic peak in the first DSC curve, and the melting energy of the high temperature peak means the energy of this high temperature peak.

In the present invention, the pre-expanded particles are heated and foamed in the molding die to form a foamed molded body as in the mold, and then the molded body is pressurized with an inorganic gas in the molding die. By pressurizing the molded body obtained by heat-foaming pre-expanded particles in the molding die with inorganic gas, escape of gas in the molded body is prevented until the molded body is cooled, and the molded body When the gas is pressed into the mold, shrinkage of the molded body is prevented.

The inorganic gas used for pressurization is an inorganic gas containing nitrogen as a main component, such as nitrogen or air. The pressure applied by the inorganic gas varies depending on the material of the base resin, but is usually 0.01 to 5 kg / cm ² · G, preferably 0.1 to 1.5 k
It is about g / cm ² · G.

The pressurization of the molded body in the mold with the inorganic gas is preferably performed in a state where the temperature of the molded body is high to some extent, and usually the temperature of the foamed molded body is between about 60 to 120 ° C. It is preferable to start the pressurization while having. The pressing time varies depending on the type of base resin and the like, but is generally preferably about 1 to 5 minutes.

The molded body obtained by heating and foaming in the mold in the present invention is cooled until it can be easily released, and then taken out from the mold.
This cooling step may be performed after completion of pressurization with the above-mentioned inorganic gas, or may be performed while pressurizing. Generally, when the molded body is insufficiently cooled, the molded body expands too much when the molded body is taken out of the mold, and the surface is likely to crack. The surface pressure of the foam can be used to know whether the molded body has been cooled to such an extent that it can be removed from the molding die. For example,
When the ethylene-propylene random copolymer is used as the base resin, it is preferable to remove the ethylene-propylene random copolymer from the molding die at a surface pressure of about 1 to 0.5 kg / cm ² · G. As a cooling method, in addition to normal water cooling, cooling by vacuum drying may be adopted, and when cooling by vacuum drying increases the hardness of the surface of the molded body,
Mold release can be performed well, and it is not necessary to dry the obtained molded body.

The foamed molded product taken out from the molding die is usually heated and dried after being taken out. For example, in the case of an ethylene-propylene rendam copolymer, this temperature is generally suitable at 50 to 110 ° C., and is dried in a short time. In particular, the temperature is preferably 60 ° C. or higher, and particularly preferably 70 ° C. or higher.

In the present invention, as described above, an excellent molded product can be obtained even by using pre-expanded particles having a low internal pressure of 1.2 kg / cm ² abs or less due to the gas containing an inorganic gas as a main component, When such pre-expanded particles having low internal pressure are used, even low-pressure steam can be sufficiently molded, and economical molding can be performed. That is, in the case of a normal molding method, it is difficult to perform good foam molding when pre-expanded particles having a small foaming capacity are used, and therefore pre-expanded particles having a large foaming capacity with an internal particle pressure exceeding 1.2 kg / cm ² · abs are used. However, when low pressure steam is used for heating such high internal pressure pre-expanded particles, large secondary expansion occurs before the particles are heated to a temperature at which the particles can be fused to each other due to the large expansion ability of the particles, As a result, the gaps between the particles become small and the vapor does not sufficiently penetrate between the particles, resulting in defective fusion of the particles in the molded body. For this reason, when using pre-expanded particles with high internal pressure, it is necessary to heat at once to a temperature at which fusion of particles with high pressure steam may occur, but in the case of particles with low internal pressure, the foaming ability is low, so low pressure steam There is no possibility of causing large secondary foaming before the particles are heated to a temperature at which fusion of the particles may occur even when heated by the method described above, and in the method of the present invention, it is possible to mold using pre-expanded particles having a low internal pressure. Therefore, when pre-expanded particles having a low internal pressure are used, low-pressure steam can be used for efficient molding. Therefore, in the present invention, it is preferable to use the pre-expanded particles having a low internal pressure for economical molding, but even when the pre-expanded particles having the same high internal pressure as the conventional one are used, compared with the conventional molding method, An excellent molded product with less shrinkage can be obtained.

〔Example〕

 Hereinafter, the present invention will be described in more detail with reference to examples.

Examples 1 to 6 The pre-expanded particles shown in Table 1 (the melting energy of the high temperature peak is shown in Table 1 for the ethylene-propylene random copolymer) are 300 mm x 300 mm.
It was filled in a molding die of × 64 mm (inner dimension), heated by steam having a pressure shown in the table, and foam-molded. Then, pressurize with air at a pressure shown in Table 1 for 90 seconds in the mold,
Then, the molded body was taken out from the mold after water cooling until the surface pressure of the molded body reached the value shown in Table 1.

Table 1 shows the properties of the obtained molded body after being dried at 80 ° C. for 12 hours.

Comparative Examples 1 to 6 Pre-expanded particles shown in Table 1 (for those made of ethylene-propylene random copolymer, the melting energy of the high temperature peak is shown in Table 1) were filled in the same mold as in the Examples. After foaming by heating with steam having a pressure shown in the same table, it was immediately cooled and the molded body was taken out from the mold when the surface pressure of the molded body reached the value shown in Table 1. Table 1 also shows the properties of the formed body after the obtained formed body was dried in the same manner as in Examples.

[Effects of the Invention] As described above, according to the present invention, the internal pressure of particles is 1.2.
An excellent expanded molded article can be obtained even by using pre-expanded particles with a low internal pressure of kg / cm ² · abs or less. Therefore, even pre-expanded particles immediately after production do not have to increase the internal pressure by pressure treatment. It can be molded immediately, and since it is not necessary to perform complicated curing after molding, it can be molded in a short cycle, and an excellent molded product can be efficiently manufactured. Further, when pre-expanded particles having a low internal pressure are used, it is possible to perform heat molding with low-pressure steam, which allows economical molding. Furthermore, even when using the same high internal pressure pre-expanded particles as used in the conventional molding method, compared with the conventional method, it is possible to obtain a molded body with less shrinkage and excellent dimensional accuracy. Have.

Claims (3)

[Claims] 1. A foamed molded body obtained by filling thermoplastic resin pre-expanded particles in a molding die and heat-foaming, pressurizing in the mold with nitrogen or an inorganic gas containing nitrogen as a main component, and then cooling. Or a method for producing a thermoplastic resin in-mold foam molded article, which comprises cooling while pressurizing with an inorganic gas. 2. The method for producing a foamed molded product in a thermoplastic resin mold according to claim 1, wherein the thermoplastic resin is a polyolefin resin. 3. The pre-expanded particles to be filled in the molding die are polyolefin resin pre-expanded particles having an internal particle pressure of 1.2 kg / cm ² · abs or less due to a gas containing an inorganic gas as a main component. A method for producing an in-mold thermoplastic resin molded article according to item 1.