JPS60216B2 - Crosslinked polyethylene molded product and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a plastic molded product formed from a sheet-like plastic by a method such as vacuum molding or Shoku molding, and a method for manufacturing the same.
This invention relates to a crosslinked polyethylene molded product with improved heat resistance and heat sealability, and a method for producing the same.

In addition, plastic molded products made from sheet plastic by vacuum forming or pressure forming are used in electrical products, such as inner boxes and door casings for electric refrigerators, lighting equipment, etc.; in packaging containers, they are used in frozen dessert containers, simple cups, etc. , fruit and meat packaging trays, etc.; in advertising, shoulder boards, stands, name plates, etc.;

Plastic materials include polystyrene, acrylonitrile-butadiene-styrene copolymer, acrylonitrile-sterene copolymer, polyvinyl chloride, methacrylic resin, cellulose resin, polycarbonate, polyethylene, polypropylene, polyvinylidene chloride, polyacetal, polyamide, polyurethane. Most of the commonly known resins, such as fluororesins and the like, are used for various purposes depending on their properties. Among them, polystyrene and polyvinyl chloride are used in large quantities because of their excellent processability and low cost. In particular, polystyrene packaging containers are used in combination with plastic films, sheets, paper, etc. in packaging systems such as stretch wrapping, shrink wrapping, and sealed bag packaging to prevent contamination, prevent wear, and prevent spoilage by shielding the packaged items from the outside world. It is used in large quantities for prevention purposes. In stretch packaging and shrink packaging, after placing the item in a container, the entire container is wrapped in plastic film and the film is stretched.
This method is used to obtain tight (adhered) packages by sealing, but usually the films are sealed together at the bottom of the container using a hot plate, hot wire, or hot roll. A method of fixation is being taken. In addition, in the seal-put method, after putting the item to be packaged into the container, a lid material such as a film or sheet is placed on the top of the container, and then the top edge of the container and the lid material are heat-sealed and glued together. A method is used in which a hot melt adhesive is applied to at least one of the upper edge or the lid material and the adhesive is bonded by heat treatment. but,
In conventional plastic containers, when heated above the softening point or crystalline melting point of the resin, the strength decreases dramatically;
If the temperature is below the softening point or crystal melting point, heat sealing will not work properly, so the range of suitable sealing temperatures will be extremely narrow.If the container is heated too much, it will melt, create holes or become deformed, and conversely, if the temperature is too low, There was a drawback that complete heat sealing could not be achieved if there were temperature irregularities.

In addition, hot melt adhesion also has drawbacks such as difficulty in temperature control and the adhesion of foreign matter such as dust and moisture to the adhesive-applied area, resulting in uneven adhesion.

In addition, various properties are required in fields that require boiling water sterilization, such as heat resistance, cold resistance in the inner box and interior fields of electric refrigerators, and film resistance in advertising, and it is necessary to use different resins depending on the application. was there. The present inventors focused on the above-mentioned drawbacks of conventional plastic containers, conducted extensive studies, and as a result arrived at the present invention.

Therefore, an object of the present invention is to provide a crosslinked polyethylene molded product having improved moldability, heat resistance, and heat sealability and suitable for vacuum forming and pressure forming, and a method for producing the same.
This objective is achieved by the present invention.

The present invention relates to a cross-linked polyethylene molded product and a method for producing the same, which are characterized in that after ethylene resin is molded into a sheet, it is crosslinked by irradiation with ionizing radiation, and then hot molded by a method such as vacuum molding or Shoko molding. It is in.

The crosslinked polyethylene molded product obtained by the present invention is
Although there are differences depending on the degree of crosslinking, the molding temperature is 10 to 50C0 wider than that of uncrosslinked polyethylene, so it not only has extremely good moldability, but also has heat resistance comparable to that of polypropylene. has been improved to.
Furthermore, in the case of food packaging trays, when cross-linked polyethylene film is used as the lid material, the proper sealing temperature during heat sealing is polyethylene tray - polystyrene film, polypropylene tray - polypropylene film, polyethylene tray - polyester. Compared to the proper sealing temperature of tyrene film, polyvinyl chloride tray, polyvinyl chloride film, etc., it is possible to obtain a wider range of 10 to 40 qC or more. In addition, the properties of cross-linked polyethylene include cold resistance, impact resistance, stress cracking resistance,
Needless to say, it has various properties such as fecal resistance.
In addition, talc,
It is known that uncrosslinked polyethylene molded products can be improved as well as polystyrene, polyvinyl chloride and polypropylene by adding alumina, silica and various other inorganic fillers. A characteristic of ethylene resins is that they can be freely molded from soft to hard molded products by varying the proportion of inorganic filler added. But it goes without saying that it will also be a feature. In the present invention, the ethylene resin refers to polyethylene of various densities or ethylene with a polyethylene content of 5% by weight or more, propylene, one or more Q-olefins such as 1-butene or 1-pentene, vinyl acetate, acrylic acid, etc. , an acrylic acid ester, or a copolymer with one or more vinyl monomers such as vinyl chloride, and one or a mixture of two or more of such polyethylene resins can also be used.

In addition, in order to obtain molded products with various moduli of elasticity, talc, alumina silica, and various other inorganic fillers were added.
It is also possible to use other polymers such as polypropylene, polystyrene, polyvinyl chloride, polybutene, polyptadiene, polyacrylic acid, polyacrylic acid ester, etc., in an amount of 5% by weight or less. Sheet molding of ethylene resin can be done by any of the commonly used extrusion molding methods, hot press molding methods, and hot roll molding methods, but it is also possible to mold the ethylene resin into a tube shape and then cut it into a sheet shape. It's okay. In this case, the thickness of the sheet is sufficient as long as it can be uniformly irradiated with ionizing radiation, and is determined by the shape and thickness of the molded product, but it is usually 50 to 1,500 µm to ensure uniform irradiation crosslinking for handling purposes. It is also appropriate from a practical standpoint. The sheet forming temperature is
It may be selected appropriately depending on the type and melt index of the resin, and the type and amount of additives. The degree of compliance due to irradiation with ionizing radiation is expressed by the gel fraction, but in order to exhibit the effects of the present invention, a gel fraction range of 10 to 70% is appropriate, preferably 10 to 60%. is good.
The gel fraction was determined by extracting the sample with boiling p-xylene.
The percentage of non-scaly portion of the polymer portion is expressed by the following formula. (Sample weight of polymer part before p-xylene extraction) - (sample weight extracted into p-xylene) Gel fraction (%) 2 (sample weight of polymer part before p-xylene extraction) x 100 Gel fraction is At 10% fullness, no significant crosslinking effect is expressed.

On the other hand, when the gel fraction exceeds 80%, the molding temperature becomes extremely high and the resin does not stretch, which limits the shape of the molded product. Therefore, in practical terms, it is appropriate for the molded product to have a gel fraction in the range of 10 to 70%, preferably 10 to 60%. The molding temperature of the molded product varies depending on the type of polymer, gel fraction, type and amount of additives, and shape of the molded product, but in the vacuum molding method, the temperature ranges from the melting point of the resin to 100% above the melting point.
In the case of pressure molding, a range of 2000° C. below the melting point of the resin to 100° C. above the melting point is suitable.
Small amounts of other additives and waste agents commonly used in plastic processing, such as antioxidants, heat stabilizers, slip agents, pigment colorants, antistatic agents, etc., can be used in the production of the molded articles of the present invention. .

Next, the present invention will be explained by examples. Example 1 High density polyethylene (MI=1.0d=0.950) was processed using a 45 rib extruder equipped with a T-tar at a die temperature of 220.
After extruding it into a sheet using an oo, it was cooled using a cooling roll to form a sheet with a thickness of 300 mm.

This sheet was irradiated twice on the back and front sides using an electron beam irradiation device under conditions of 500 KV and 25 mA so that the gel fraction was 50%. Under these conditions, the irradiation dose was 15 megarads. This cross-linked sheet is 190 willow (vertical) x 95 ribs (
An aluminum mold for a meat tray with dimensions of 20 (width) x 20 (depth) was attached. Vacuum molding was carried out using an infrared heating type vacuum molding machine under conditions of a mold temperature of 8030 ℃ and a molding temperature of 150 pm. This molded product had strong strength, uniform wall thickness, and heat resistance over a wide range of heat sealing temperatures with crosslinked polyethylene film. Various properties are shown in Table 1. Table 1 *1 Thickness unevenness (%) = (Minimum thickness of molded product) ÷ (Maximum thickness of molded product) x 100 *2 For tensile strength measurement, samples were taken from the bottom of the molded product in the vertical direction.

*3 Heat resistance was determined by hanging a sample similar to the one used for tensile strength measurement vertically in a temperature-variable temperature dryer, and placing one at the bottom.
The load was lowered and the temperature was raised in 5°C increments, and after being left for 30 minutes, the temperature was set to be 5°C lower than the temperature at which 10% or more deformation occurred.

*4 Heat-seal a cross-linked polyethylene film with an appropriate heat sheet temperature and thickness of 15 and a gel fraction of 50% and the edge of the molded product using an in/f Luss heat sealer to obtain a sealing strength of ~. The temperature range was set to be 5k9' or above in the skin and in which no pitting occurred at the edge of the molded product.

Comparative Examples 1 to 6 The sheets of Example 1 before irradiation were vacuum formed using the same vacuum forming machine at different forming temperatures.

The appropriate molding temperature was 140 to 16,000. The thickness unevenness was worse than that of the molded product of Example 1, and the tensile strength, heat resistance, and heat sealability were also inferior. The various properties are shown in Table 2. Table 2 Examples 2 to 10 Comparative Examples 7 to 8 Vacuum molding was performed in the same manner as in Example 1, changing the molding temperature and gel fraction.

The various properties are shown in Table 3. Table 3 Example 11 Low density polyethylene (MI=0.4 Density=0.920
) and 5% by weight of Yuruku were extruded using the same extruder as in Example 1 at a temperature of 220 q○ to form a sheet of 300 square meters.

This sheet was irradiated with an electron beam so that the gel fraction was 40%, and vacuum molded under the same conditions as in Example 1. This molded product was strong, had a uniform wall thickness, and had good strength, heat resistance, and heat sealability. The various properties were as shown in Table 4. 4 Example 12 An ethylene monovinyl acetate copolymer (MI=1.0, density=0.927) with a vinyl acetate content of 1% by weight was molded into a sheet of 500 mounds.

After irradiating this sheet with 15 megarads, a cup mold with upper part x depth x lower part = low height x 10 molars x 7 molars was used in the same vacuum forming machine as in Example 1, and the mold temperature was 60 m.
Vacuum forming was carried out under the conditions of ℃ and sheet temperature of 140 oo. A good molded product with a thickness unevenness of 75% was obtained. In addition, when molded from an unirradiated sheet, the thickness unevenness was 5 under the same conditions with this equipment.
It was 4%. Examples 13-14 A 500 μm sheet was prepared from a composition in which 1 and 3% by weight of polypropylene (MI=8) were added to the same ethylene-vinyl acetate copolymer as used in Example 12, respectively. After molding and 15 megarad irradiation, 170℃
The same molded product as in Example 12 was vacuum molded under the molding temperature conditions.

Thickness unevenness was 72% for the molded product with a weight percent addition of 1, and 70% for the molded product with a weight percent addition of 3.The non-irradiated items were 48% and 32%, respectively. The sheet that was crosslinked by irradiation at the same time as in Example 1 was molded using an air pressure molding machine equipped with the mold used in Example 1 at an air pressure of 4k9'.
Pressure molding was carried out at a mold temperature of 70 qo and a sheet temperature of 12 pi.

Claims (1)

[Scope of Claims] 1. A crosslinked polyethylene molded product, characterized in that it is a hot vacuum or pressure molded product made of an ethylene resin sheet with a gel fraction of 10 to 70% by irradiation with ionizing radiation. 2. The crosslinked polyethylene molded product according to claim 1, wherein the ethylene resin is polyethylene. 3. The crosslinked polyethylene molded product according to claim 1, wherein the ethylene resin is a copolymer of 50% by weight or more of ethylene and a vinyl monomer. 4. The crosslinked polyethylene molded product according to claim 1, wherein the ethylene resin is a copolymer of 50% by weight or more of ethylene and another α-olefin monomer. 5. The crosslinked polyethylene molded product according to claim 1, wherein the crosslinked polyethylene molded product is a tray or a cup-shaped packaging container. 6. Molding ethylene resin into a sheet shape, irradiating the resulting molded product with ionizing radiation so that the gel fraction becomes 10 to 70%, and reducing the temperature from 20°C below the melting point of the resin to 100°C above the melting point.
A method for producing a crosslinked polyethylene molded product, characterized by vacuum or pressure molding at a temperature in the range of °C.