JPS6039541B2 - Method for manufacturing polypropylene containers - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for increasing the heat resistance of polypropylene solid state air-formed containers.

In recent years, a technology has been developed to ripen polypropylene resin (hereinafter abbreviated as PP) sheets in a solid state below the crystalline melting point. Among them, the pressure forming method using plugs as an aid has excellent transparency, gloss, and light weight. It is attracting attention because it produces a sturdy container.

In particular, it is expected to replace polyvinyl chloride for food packaging containers from Tsurumi's hygiene standpoint, and the demand for PP solid phase air-formed products is expected to increase considerably in the future. However, PP solid phase air pressure molded products have the disadvantage of poor heat resistance because they are molded at relatively low temperatures.

This is because the polypropylene crystals are stretched and oriented due to solid-state molding, and the orientation is removed by applying heat and restored to the original state. This becomes clear when comparing a container made by melt-molding the same PP sheet and a container made by solid phase pressure forming. Solid-state air-formed products with a high degree of crystallinity are supposed to have higher heat resistance than melt-formed products with a lower degree of crystallinity, but in reality, the opposite is true, and the heat resistance temperature of PP-phase air-formed products is around 100°C at most. It is. This is a serious drawback as a food container, and food cannot be subjected to retort processing, steam sterilization, etc., and the use of PP aggregate phase air-molded products is severely restricted.

Therefore, in fields that require heat resistance of 100q0 or more, PP melt vacuum-formed products, polycarbonate containers, etc. have been used so far. However, these are opaque or expensive, and a transparent, light, highly rigid, and inexpensive container such as a PP solid-state air pressure molded product has not been found. On the other hand, as a method to increase the heat resistance of molded products, annealing treatment within the mold has been known for a long time.
It is put into practical use in the injection molding field. The present invention has been achieved through repeated studies to apply annealing treatment technology as a technology to improve the heat resistance of PP aggregate phase pressure-molded products, and provides a polypropylene container with excellent heat resistance without impairing transparency. It is something to do.

The method for manufacturing a polypropylene container of the present invention is a method for manufacturing a thin-walled hollow polypropylene container having a heat distortion temperature of 105 qo or higher and 140 qo or lower, and a crystal melting point of 155 qo.
A single-layer or laminated plastic sheet containing the above polypropylene resin as a main component is preheated to a temperature of 15,000 or more and below the crystal melting point, and is preformed into a hollow container shape on the female mold surface by air pressure forming. The preform is fitted tightly into a male mold having the same shape as the molded product and having external dimensions substantially equal to the internal dimensions of the preform, and then the preform is fitted into the male mold. The preformed product in condition 1
It is characterized by heat treatment at a temperature in the range of 150°C to 150°C, followed by cooling.

In the present invention, the heat deformation temperature of the container, which reflects the heat resistance during use of the container, is determined by the heat distortion temperature of the container, which is determined by the heat deformation temperature of the container, which is measured at 2 female per ground length from above along the outer periphery of the flange at the end of the cup-shaped hollow container. The temperature at which the compressive deformation rate in the depth direction of the container reaches a value of 4% of the depth of the container by increasing the temperature of the container in a liquid atmosphere at a rate of 2°C/min while applying a uniform compressive load. Defined by For example, when measuring the heat deformation temperature of a cup-shaped container with an opening diameter of 95 and a depth of 5 m, the circumferential dimension along the flange is approximately 30 mm, so a total of 60 female uniform loads are placed on the flange. Heat the container while pouring
The temperature at which the amount of compressive deformation in the depth direction of the container reaches two bars is defined as the heat deformation temperature. The heat distortion temperature of the thin-walled polypropylene container is 105° C. or higher and 140 qo or lower. When the heat distortion temperature is 10g0, the desired steam sterilization, retort treatment, etc. are impossible. It is difficult to manufacture a container with a heat distortion temperature of more than 140 qo by the method of the present invention, and higher heat resistance is not required for the food use intended by the present invention.

First, the PP sheet is preheated to a temperature of 150° C. or higher and lower than the crystal melting point, and then preformed into a hollow container shape on the surface of a female mold by air pressure forming.

The sheet temperature during preforming is preferably as high as possible without impairing the transparency and gloss of the container. Preheating temperature is 150q0
It is difficult to improve heat resistance below. The preformed product is a so-called solid phase air-formed product, and its heat distortion temperature is approximately 10
0q○, and generally deforms gradually when boiled in hot water at 90°C to 100°C.

In the present invention, the preformed product obtained by molding onto the surface of the female mold is then tightly fitted with iron into a male mold of the same shape and size, heated to 150°C with no 100q0, and heated to 150°C. A container with a high heat deformation temperature is obtained by heat treatment at a temperature higher than that of sand.

The principle of the present invention will be described below.

PP is warmly stretched at a temperature below the crystal melting point by solid phase pressure forming, and both the crystalline phase and the amorphous phase are oriented.

Due to the stretching orientation, the crystalline higher-order structure of the spherical mouse is transformed into an extremely oblate spheroid and then into an east-shaped fiber crystal.
When the cross-section of the side wall of a PP container that has actually been solid-phase pressure-formed is observed under a microscope, flat spherulites and fibrous crystals can be observed. When the aligned PP is heated, the amorphous chain segments move and the orientation is relaxed at relatively low temperatures. Furthermore, at temperatures above the crystal dislocation temperature, folded crystals of intraspherulite lamellae undergo a transition, and some become fibrous crystals. By the way, it is known that when a uniaxially stretched sample is subjected to tension heat treatment at a temperature below the crystal melting point and changes in the orientation coefficients of the crystalline phase and the amorphous phase are observed, the orientation coefficient of the amorphous phase shows a large change. Therefore, the main cause of thermal deformation of a PP interphase air-formed product can be considered to be the phenomenon of relaxation of the orientation of amorphous chain segments. Therefore, since the temperature at which the mobility of amorphous chain segments increases is around 100°C,
This means that even if a component molded product with a heat resistance temperature of 100oC or less is heat treated at a temperature around 100oC, which is considerably lower than its molding temperature or crystal dislocation temperature, a heat distortion temperature higher than the heat treatment temperature can be achieved. To increase the heat resistance of PP solid phase air-formed products,
It is sufficient to relax the amorphous chain segments in the oriented state. The present invention increases the mobility of amorphous chain segments10
It is characterized by heat treatment at 0°C to 15,000°C.

Polyp in the present invention. Pyrene resin has a crystal melting point of 155
00 or more, and is a propylene homopolymer, a propylene-Q-olefin copolymer, or a mixture of a propylene homopolymer and a propylene-Q-olefin copolymer. Furthermore, there is no problem in adding various auxiliary agents to these polypropylenes as required during molding or for the characteristics of the container. When particularly high heat resistance is required, a propylene homopolymer or a mixture of 20% or less propylene-Q olefin copolymer and 80% or more propylene homopolymer may be used, or a nucleating agent such as dibenzylidene sorbitol may be used. It is preferable to use a sheet with high rigidity and good transparency by adding the following: to increase the degree of crystallinity. Various methods are known for the Shoku-forming method during pre-forming, but using a plug as an aid,
An air pressure molding method, or a molding method that uses a vacuum in combination with this method, is preferable for producing transparent and strong preforms. In heat-treating the preform, according to the present invention, a male die is filled with iron and heat-treated at a temperature of 100 OO to 15,000 for at least one inkstone interval. The male mold is preferably made of metal such as aluminum or iron, or plastic such as epoxy resin or fluororesin.

If it is made of metal such as aluminum, it is preferable to install a heater inside to adjust the temperature of the male mold surface. If the male mold is not flooded, a plastic mold with good insulation is more effective. It is necessary that the internal dimensions of the preform and the external dimensions of the male mold be substantially equal.

This can occur with heat treatment. 10
Heat treatment at a temperature below 1000 yen requires a very long treatment time, and the effect of improving heat resistance is small, making it impractical.

Further, at a temperature of 150° C. or higher, partial melting of the crystal layer begins, the orientation is relaxed, and spherulite growth is promoted.

For this reason, the transparency and gloss, which are the characteristics of solid phase air pressure molded products, gradually decrease. In order to further shorten the treatment time, sufficiently increase heat resistance, and prevent deformation of the preform, the heat treatment temperature is preferably 110 qo to 140°C.

For heat treatment, a male mold that tightly fits the preform is used to prevent the preform from being compressed and deformed. Further, in order to prevent the molded product from coming off the male mold during heat treatment and to prevent the flange portion from deforming, it is preferable to use a separate auxiliary mold for fixing the flange portion of the molded product to the male mold. The heat treatment is preferably performed using a heat medium, by mirror heating, or by combining these methods.

The heat medium is heated air, hot water, steam, ethylene glycol, glycerin, etc., and can be used alone or in combination. Heat treatment is carried out by blowing a heating medium onto the preform, or by immersing and contacting the preform with a male mold in the heating medium. A far-infrared heater is preferable as a heat source for specular heating because it has high thermal efficiency and can perform heat treatment in a short time. The preform held in the male mold can be heat treated by passing it through a heating tunnel with far infrared radiation installed inside. Although the polypropylene single-layer sheet has been described above, the method of the present invention can also be applied to any laminated sheet whose main constituent layer is a polypropylene resin. By the method of the present invention, a container having high heat resistance and transparent gas barrier properties is obtained from a laminated sheet of polypropylene resin and saponified ethylene/vinyl acetate copolymer resin, a laminated sheet of polypropylene resin and polyamide resin, etc. be able to.

This will be explained in more detail in Examples below.

In the example, the PP sheet is an extruded sheet of 0.8 rib thickness.

The preformed product was formed by heating the sheet to about 15 psi and using a plug-assisted air pressure forming method. The shape of the preform is a cup with a diameter of 95 mm, a bottom diameter of 85 mm, and a depth of 5 mm, and the area stretching ratio is approximately 3.1 times. The heat resistance of the container or preform was evaluated by heat distortion temperature and heat shape retention.

The heat deformation temperature was determined by applying a compressive load of 60 mm evenly on the flange of the container, immersing the container in silicone oil, and setting it at 2℃/
The temperature was raised in 1 minute and the evaluation was made at a temperature at which the amount of compressive deformation in the depth direction of the container reached 2 ribs. The thermal shape retention of the container was evaluated by the change in appearance after being kept in a heating bath at a constant temperature of 130° C. for 1 minute. The degree of crystallinity was determined from the density measured by the density gradient tube method of JISK=6760. Crystal melting points were measured on samples cut from the sheets using a differential scanning calorimeter. Example 1 A preformed product is formed from a PP sheet made of propylene homopolymer with MI=1, then this preformed product is iron-cast into a male mold of the same shape, and heat treated by blowing hot air of 130 qo for 1 minute. and obtained a container.

Table 1 shows a comparison of the heat resistance of the preform and the container. The material of the male mold is epoxy resin. The containers shown in Table 1 maintained the same transparency and gloss as the preformed products, and were even stronger.

Example 2 Homopolymer 8 base with MI=1 and ethylene polyp with MI=2.

A preformed product is formed from a PP sheet consisting of a pyrene random copolymer base and a base part, and then this preformed product is cast into a male mold, soaked in 13,000 ml of glycerin and 3% sand, and then heated with hot air at 100°C. I sprayed it and did the sand treatment in step 3. Table 2 shows a comparison of the heat resistance of this container and the preform. Table 2 Example 3 A preform formed using the same sheet as in Example 1 was pressed into a male mold and heat-treated for 25 seconds in a heating tunnel equipped with a far-infrared heater.

At this time, the surface temperature of the preform rose to a maximum of 131°C. The heat resistance of this container is shown in Table 3. Table 3

Claims (1)

[Scope of Claims] 1. A method for manufacturing a thin-walled hollow container made of polypropylene with a heat distortion temperature of 105°C or higher and 140°C or lower, which includes a single-layer or laminated plastic whose main component is a polypropylene resin with a crystalline melting point of 155°C or higher. After preheating the sheet to a temperature of 150°C or higher and lower than the crystal melting point and preforming it into a hollow container shape on the female mold surface by air pressure forming, the sheet has an external dimension that is substantially equal to the internal dimension of the preformed product. The male die is tightly fitted into the preform, and then the preform fitted into the male die is heat treated at a temperature in the range of 100°C or higher and 150°C or lower for 10 seconds or more, and then cooled. A method for manufacturing a polypropylene container, characterized by: 2. A pressure forming method in which a plug is assisted by a pressure forming method;
Alternatively, the method for producing a polypropylene container according to claim 1, which is a pressure forming method using a plug as an aid and a vacuum. 3. The method for manufacturing a polypropylene container according to claim 1 or 2, wherein a heat medium is used for the heat treatment. 4. The method for manufacturing a polypropylene container according to claim 3, wherein the heat medium is heated air. 5. The method for manufacturing a polypropylene container according to claim 3, wherein the heat medium is water and/or steam. 6. The method for producing a polypropylene container according to claim 1 or 2, wherein the heat treatment is a radiant heat treatment using a far-infrared heater.