JPH0957841A - Polypropylene container - Google Patents

Abstract

(57) [PROBLEMS] To provide a polypropylene container which has characteristics such as softness and strength peculiar to polypropylene and which can be smoothly removed from blocks in a stacked state. SOLUTION: This is a shape that can be stacked with a thermoformed body of polypropylene sheets, for example, a lightweight container such as a hood pack, and one molded product is sequentially processed, for example, by suction or the like.
When taken out at a speed of 0 mm / sec, at least 95 pieces can be removed one by one, and the friction coefficient of at least one of the outer surface and the inner surface of the container, which come into contact with the same kind of container at the time of stacking, is 0.01 to 0.7 Is a polypropylene container. The polypropylene container is preferably manufactured by melt-extruding polypropylene into a sheet and cooling it, then applying a lubricant to the surface of the rolled sheet with a pair of rolls, and then thermoforming at a temperature below the melting point.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a novel polypropylene container. Specifically, a soft feeling peculiar to polypropylene, which is preferably used as a lightweight container for packaging foods,
The present invention relates to a polypropylene container that has characteristics such as strength and transparency and that can be smoothly removed from blocks in a stacked state.

[0002]

2. Description of the Related Art A food container for packaging food is a container in which a tray and a lid are integrally formed (hereinafter referred to as a food pack), or the tray and the lid are separately molded and the lid is fitted to the tray when used. Used as a container (hereinafter abbreviated as fitting container)
Etc.

Among these food containers, for the packaging of foods in supermarkets, convenience stores, etc., containers using expanded polystyrene (OPS) sheets having excellent transparency so that the food as the contents can be seen easily ( Hereinafter, an OPS container) is generally used.

The structure of the container used for the above-mentioned application usually has a side wall having a large opening area with respect to the bottom surface 2 of the container 1 and a side wall inclined so as to slightly widen toward the opening 3, as shown in FIG. It is formed into a shape that can be stacked, and as shown in FIG. 1, it is handled by a block in which several hundred pieces are stacked at the maximum. Then, at the time of filling foods or the like, the containers are used by removing them one by one from the block manually or automatically.

[0005]

In the OPS container having the above-mentioned structure, due to the inherent slidability of the raw material sheets, the OPS sheets can be smoothly removed from the stacked piling blocks, but the OPS sheets can be handled easily. It has a problem that it tends to emit a peculiar metallic noise. Further, the above-mentioned container is generally used by fixing the lid to the container with a stopper such as a stapler or a cellophane tape, but it was difficult to open the container smoothly because it was easily torn from the stopper.

On the other hand, as a material having a soft feeling and strength, a polypropylene container made of a thermoformed polypropylene sheet is conceivable. However, the polypropylene container is not only inferior in transparency but is also more likely to be stacked than blocks. It has been found that when the containers are removed, the containers are likely to block each other, which significantly impairs workability in packaging foods and the like in the containers.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to solve the above problems, and as a result, in a container made of the above thermoformed polypropylene sheet, an outer surface and an inner surface which come into contact with other containers during stacking. By making at least one of the friction coefficients smaller than a specific value, the workability at the time of removing such a container can be improved, and the use of such a polypropylene container as a packaging container is enabled, and the present invention has been completed. .

[0008] That is, the present invention is a container having a shape capable of being stacked, which is obtained by thermoforming polypropylene sheets, and a stack having 100 containers stacked along the shape is 1000 mm / sec. When the containers are removed from the stack one by one at a speed of 1, the polypropylene container is characterized by having a coefficient of stacking friction such that the number of removable one by one is at least 95.

In the present invention, examples of the polypropylene constituting the polypropylene sheet include copolymers of propylene homopolymer, block copolymers of propylene and α-olefins other than propylene, random copolymers, graft copolymers and the like. You can list coalescence. Examples of the α-olefin other than propylene include ethylene, butene-1, pentene-1, and hexene-1, and the copolymerization ratio of these α-olefins is 10 mol% or less. It is suitable for exerting rigidity.

Further, as the polypropylene in the present invention, a mixture of the above-mentioned propylene homopolymers or copolymers can be used. Among these,
A propylene homopolymer and a propylene-ethylene random copolymer which are excellent in transparency are particularly preferable. Further, the polypropylene, if necessary, ethylene-propylene copolymer, ethylene-butene copolymer,
Olefin-based elastomers such as ethylene-propylene-butene copolymers, styrene-butene copolymers, styrene-based elastomers such as styrene-ethylene-butene copolymer styrene elastomers and the like in the range that the characteristics of the present invention are not impaired, generally It can be appropriately blended in a ratio of 1 to 10 parts by weight.

The melt index of the polypropylene is 0.3 in order to favorably perform the molding process described later.
It is preferably in the range of 10 g / 10 minutes.

In addition, it is effective to add a crystallization nucleating agent to the above polypropylene, if necessary, in order to improve the transparency of the obtained container. Such crystallization nucleating agents include talc; silica; methylene phosphate (2,4-tert.
-Butylphenyl) sodium or other aromatic phosphoric acid compound metal salt; benzoic acid, toluic acid, p-tert-butylbenzoic acid or other aromatic monocarboxylic acid metal salt; 1, 3, 2,
Examples thereof include sorbitols such as 4- (dimethylbenzylidene) sorbitol; polymeric nucleating agents such as poly (3-methyl-pentene-1). In particular, metal salts of aromatic phosphoric acid compounds, metal salts of aromatic monocarboxylic acids, sorbitols; organic crystallization nucleating agents such as polymer nucleating agents are preferable,
Further, sorbitols are particularly preferable in terms of the transparency improving effect. The addition amount of the crystallization nucleating agent is not particularly limited, but it is 0.000 per 100 parts by weight of polypropylene.
It is preferably in the range of 1 to 1 part by weight.

Further, if necessary, additives such as a stabilizer, an antistatic agent, an antifogging agent, a UV absorber, an antiblocking agent, and a coloring agent can be appropriately added to the polypropylene. As the polypropylene sheet, a sheet obtained by a known method can be used without particular limitation. In general, there is a method in which polypropylene is melt-extruded into a sheet shape, cooled, and then rolled at a temperature lower than the crystal melting point.

The thickness of the polypropylene sheet of the present invention is in the range of 0.1 to 0.8 mm, preferably 0.15 to 0.4 mm in order to obtain practical rigidity, high transparency, flexibility and heat resistance. It is in. If the thickness exceeds 0.8 mm, the effect of improving transparency is not sufficient, and if the thickness is less than 0.1 mm, the rigidity and heat resistance are poor and it is difficult to use as a container.

The polypropylene container of the present invention comprises a thermoformed product of the polypropylene sheet of the above-mentioned material. As a thermoforming method, a known method can be adopted without particular limitation. For example, vacuum forming, pressure forming, and vacuum pressure forming methods may be mentioned. Specifically, a vacuum forming machine, a pressure forming machine, or a vacuum pressure forming machine in which these are combined is used to perform thermoforming by hot plate heating or indirect heating with a far infrared heater. In this case, it is desirable to perform vacuum pressure forming at a relatively high pressure of 3 kg / cm ² or more in order to secure good mold transferability. The sheet surface temperature during thermoforming must be below the melting point of polypropylene in order to obtain a highly transparent food container. Usually 130
It is preferable to select from the range of to 165 ° C from the viewpoint of transparency of the obtained container.

The polypropylene container of the present invention may have any shape as long as it can be stacked. For example, the food pack, the fitting container and the like are common. In addition,
When the lid of the container is also a thermoformed polypropylene sheet, it is included in the container of the present invention.

A typical shape of the polypropylene container described above is shown in FIGS. 1 and 2. That is, FIG. 1 shows an aspect of the food pack. The food pack of FIG. 1 has a mode in which a lid 4 is integrally formed on one side of the container 1. The lid 4 is bendable at the joint with the container 1. After the food is packed in the container, the lid 4 is bent so as to cover the opening of the container, and the lid and the container are fastened with a stopper such as a stapler. To use.

In this case, the container is the bottom surface 2 as described above.
On the other hand, a structure is adopted in which the area of the opening 3 is increased and the side wall 5 has a side wall that is inclined so as to slightly widen toward the opening 3 so that stacking is possible.

FIG. 2 shows a fitting container. The container 1 and the lid 4 are separately molded, and after the food is packed in the container, the lid is fitted into the container for use. In FIG. 2, the container 1 has the partition 6 provided, but even when the partition 6 is provided in this way, the partition 6 is partitioned so that the opening is narrower than the bottom surface of the container partitioned by the partition 6. A structure that allows stacking by adopting an inclined side wall is adopted.

In the present invention, the structure of the container is not particularly limited as long as it satisfies the above requirements. For example, the planar shape of the container is not limited to the rectangular shape shown in FIGS. 1 and 2, but may be any shape such as a square shape, a circular shape, or an oval shape. In addition, the size of the container is determined by the type of food or the like that is the content, and although it is not limited to any general rule, it is usually 100 to 400 (vertical) x 100 to 400 (horizontal) x 1
It is selected from the range of 0 to 50 (depth) mm.

The polypropylene container of the present invention has a structure in which 100 containers are stacked along the shape thereof to obtain 1000 stacks.
It is necessary that at least 95 pieces can be removed one by one at a speed of mm / sec. In order to be able to remove at least 95 pieces at a speed of 1000 mm / sec, the coefficient of friction of the outer surface at the time of stacking is preferably 0.0.
It is in the range of 1 to 0.7, and more preferably 0.01 to 0.5. In this case, the coefficient of friction is a general term for the coefficient of static friction and the coefficient of dynamic static friction, and both must be within the above range in the present invention.

If the number of pieces that can be removed one by one at the speed of 1000 mm / sec is less than 95 and the coefficient of friction exceeds 0.7, the effect of improving the slipperiness of the container is not sufficient.
Containers cannot be removed smoothly from stacked blocks by manual extraction or automatic extraction device,
The object of the present invention cannot be achieved. Also, the friction coefficient is 0.0
It is difficult to economically manufacture containers less than one.

The outer surface and the inner surface of the container, which come into contact with the same kind of container at the time of stacking, refer to either one of the front and back surfaces of the container. Depending on the case, the entire surface of the container, the entire back surface of the container, and the entire front and back surfaces of the container may have such a friction coefficient.

The method of adjusting the friction coefficient of either the outer surface or the inner surface of the polypropylene container of the present invention to the above range is not particularly limited. For example, a method of increasing and adjusting the amount of the lubricant added to the polypropylene, a method of applying the lubricant to the surface of the container, and the like can be mentioned.
However, a method of applying a lubricant to the surface of the container is preferred as a means for improving the slipperiness and not impeding the transparency of the container described later. The method of applying such a lubricant is not particularly limited. Typical examples include a mode in which a lubricant is partially or entirely applied to the surface of a polypropylene sheet before molding in a container, a mode in which it is applied to a container during or after molding. In particular, application in the state of a polypropylene sheet is preferable in terms of workability.

As a method of applying the above-mentioned lubricant, a method of spraying a solution in which the lubricant is dispersed in water or a solvent onto a polypropylene sheet or a polypropylene container surface with a sprayer,
Examples thereof include a method of coating the polypropylene sheet surface with a coater such as a gravure coater, a reverse roll coater, and a kiss coater. Among these, the method using a gravure coater is preferable because a uniform and thin coating film can be formed. The above-mentioned lubricant is generally used, for example, in a form in which it is applied as a solution in which the lubricant is dispersed in water or a solvent and then dried in a drying furnace.

As the lubricant to be applied, it is preferable to select a lubricant which is safe and does not impair the transparency of the sheet, and an aqueous emulsion of silicone oil is particularly preferable. The water-based emulsion of silicone oil is an oil-in-water emulsion in which silicone oil is emulsified with various emulsifiers or a type in which silicone oil is emulsified by a special method, and the active ingredient in the range of 25 to 65% is preferably used. .

Further, for the purpose of improving the adhesion of the lubricant to the surface of the polypropylene sheet, it is preferable to perform a surface treatment such as corona treatment on the surface of the polypropylene sheet before applying the lubricant. The surface treatment is preferably performed so that the critical surface tension of the sheet surface becomes 35 to 55 dyne / cm.

In the present invention, in order to improve the transparency of the obtained polypropylene container to a degree comparable to that of the OPS container, the polypropylene sheet has a haze value of H.
(%) And H / t when the wall thickness is t (mm) are preferably in the range of 0.01 to 20, and more preferably 0.01 to 15. When H / t exceeds 20, the effect of improving transparency is insufficient, and it is difficult to economically manufacture a container having H / t of less than 0.01.

The following method is particularly recommended as a method for producing a polypropylene sheet for realizing such transparency. That is, first, polypropylene is melt extruded into a sheet and cooled. Such a method can be carried out by a known technique. For example, polypropylene is supplied to an extruder, melted at a resin temperature of 200 to 280 ° C., extruded in a sheet form from a die, and cooled by a cooling roll having a surface temperature of preferably 20 to 100 ° C., more preferably 30 to 80 ° C. to form a sheet. To mold.

Next, the following method can be preferably adopted as the method of rolling. A method in which the sheet cooled by the cooling roll is sufficiently preheated before passing it between the rolling rolls and then rolled at a temperature not lower than the preheating temperature and lower than the crystal melting point of the sheet can be suitably adopted. Preheating before rolling is an important step in producing a rolled sheet having excellent transparency. The preheating temperature depends on the thickness of the sheet to be preheated,
It is preferable to preheat so that the sheet temperature before rolling satisfies the following formula. 80 + 35 × t1-15 × t1 / t2 ≦ T ≦ 155 + 15 × t1-27 × t1 / t2 (where T is the sheet temperature before rolling (° C) and t1 is the sheet thickness before rolling (mm ), T2 is the thickness (mm) of the sheet after rolling.)

The method for adjusting the sheet temperature before rolling to the above temperature includes a method of passing the sheet through a preheating roll whose surface temperature is adjusted so that the sheet temperature before rolling becomes the above temperature, and an indirect heating method such as a far infrared heater. Examples thereof include a method of preheating and a method of passing the sheet through a medium such as air or water heated to a predetermined temperature. Among these, the method of using a preheating roll is preferable because the temperature can be easily adjusted. Rolling is performed by passing the above preheated sheet between a pair of rolls. The temperature of the rolling roll is preferably 120 to 160 ° C, more preferably 130 to 1
55 degreeC, More preferably, it is 140-155 degreeC. The rolling is preferably performed so that a rolled sheet having a crystal orientation coefficient of 0.2 to 0.95 and a fibril diameter of 0.01 to 0.4 μm can be obtained. For this purpose, it is preferable that the thickness of the rolled sheet be in the range of 1 / 1.5 to 1 / 3.5 of the thickness of the original sheet. The rolling can be repeated several times if necessary.

For the purpose of improving the thermoformability of the rolled sheet obtained as described above, it is preferable to heat the sheet after rolling to a specific temperature. As a method,
It is recommended to perform heat treatment so that the sheet temperature (Ta (° C)) after the heat treatment satisfies the following formula. Sheet temperature after rolling (° C) -15 ≤ Ta ≤ Sheet temperature after rolling (° C) +15

The method of controlling the sheet temperature after the heat treatment within the above range is to preheat by a method of passing the sheet through a heating roll whose surface temperature is adjusted so that the sheet temperature becomes the above temperature, or an indirect heating method such as a far infrared heater. The method may include, a method of passing the sheet through a medium such as air or water heated to a predetermined temperature, or a method of combining these. Among these, the method using a heating roll is preferable because it is simple. The polypropylene container thus obtained preferably has a crystal orientation coefficient of 0.2 to 0.95 and a fibril diameter of 0.01 to 0.4 μm.

[0034]

EFFECTS OF THE INVENTION The polypropylene container of the present invention has a transparency comparable to that manufactured from an OPS sheet even though it is polypropylene, and has a soft feeling, and a stopper such as a stapler or cellophane tape is used as a lid. It is a polypropylene food container which is not only easy to open even when the tray part is used by fastening, but also has excellent slipperiness. Therefore, the polypropylene container of the present invention, for example, meat, fish, sashimi, sushi, vegetables, fruits, confectionery, fried foods, tempura, side dish, yakisoba, takoyaki, spaghetti, salad, sandwich, bento, rice, omelet rice, fried rice, rice ball, Ice, ice cream, cake, gyoza,
It can be suitably used as various food containers such as Shoumai and Shiruko.

[0035]

EXAMPLES Examples will be shown below for specifically explaining the present invention, but the present invention is not limited to these examples. In the following Examples and Comparative Examples, the crystal orientation coefficient is "Advances in X-ray Analysis", Volume 6, p231.
According to the method described in (1963), the fibril diameter can be determined by "Journal of Polymer Science (Jo
urnal of Polymer Science) "vol. 27, p87 (19
58). The haze was measured according to JIS K7105, and the friction coefficient was measured according to ASTM D1894. The non-following number of food packs is
When one molded product was sequentially taken out at a speed of 1000 mm / sec from a stack of 100 food packs stacked by suction or the like, the number that the lower container did not follow was measured.

Examples 1 to 3 Polypropylene having a melt index of 0.9 g / 10 minutes and a melting point of 161 ° C. was melted by a twin-screw extruder having a screw diameter of 65 mm and extruded into a sheet form from a die having a die width of 1300 mm, and a surface temperature of 70 Table 1
Was obtained. This sheet was preheated by passing it through a preheating roll whose surface temperature was set to the values in Table 1. The sheet temperature before rolling was measured by a contact type thermometer and was as shown in Table 1. Subsequently, the preheated sheet was rolled by a pair of rolls having a surface temperature of 135 ° C. so that the final sheet thickness was 0.3 mm.

Then, subsequently, this rolled sheet was heat-treated by passing it through a heat-treatment roll whose surface temperature was set to 140 ° C. The sheet temperature at the exit of the heat treatment roll was measured by a contact type thermometer and was as shown in Table 1. This rolled sheet was a uniaxially oriented sheet in which the C axis was oriented in the MD direction (extrusion direction), and its crystal orientation coefficient and fibril diameter were as shown in Table 1. next,
An emulsion of dimethyl silicone oil with water as a diluent was applied to the surface of this sheet with a gravure coater so that the thickness of the dried silicone coating film was 0.5 μm, and dried. Haze of this sheet and haze value H (%)
Table 1 shows the ratio of H / t to the thickness t (mm).

Subsequently, this sheet was vacuum-pneumatic molded at a sheet surface temperature of 162 ° C. and a pneumatic pressure of 5 kg / cm ² with a vacuum-pneumatic molding machine, and the tray portion size was 160 mm × 100 m.
m x depth 30 mm, lid size 160 mm x 100 mm x
Depth 3 mm, overall size 160 mm x 200 x maximum depth 3
A 0 mm food pack was molded. The obtained food pack produced a small sound during handling, and even if the lid and tray were fastened with a stapler and then opened, the food pack could be easily opened without tearing. Table 1 shows the thickness of the bottom surface of the food pack, the haze of the bottom surface, H / t, the non-following number, the friction coefficient, the crystal orientation coefficient, and the fibril diameter.

Comparative Example 1 A sheet and a food pack were obtained in exactly the same manner as in Example 3 except that dimethyl silicone oil was not applied. Haze and H / t of the obtained sheet
And bottom thickness of food pack, bottom haze, H /
t, non-following number, friction coefficient, crystal orientation coefficient, and fibril diameter were measured. The results are shown in Table 1.

Comparative Example 2 Example 3 is the same as Example 3 except that rolling and dimethyl silicone oil application are not performed in the sheet manufacturing process.
A sheet and a food pack were obtained in exactly the same manner as. The obtained food pack had a small crystal orientation coefficient and no fibril structure was observed. Haze of sheet, H / t and thickness of bottom of food pack, haze of bottom, H /
Table 1 shows the measurement results of t, non-following number, friction coefficient, crystal orientation coefficient, and fibril diameter.

Examples 4 to 5 Polypropylene compositions in which 100 parts by weight of a propylene homopolymer having a melt index of 0.9 g / 10 min were mixed with 0.3 parts by weight of 1,3,2,4- (dimethylbenzylidene) sorbitol. Was melted by a twin-screw extruder having a screw diameter of 65 mm, extruded into a sheet from a die having a die width of 1300 mm, and cooled by a cooling roll having a surface temperature of 70 ° C. to obtain a sheet having a thickness shown in Table 1. This sheet was preheated by passing it through a preheating roll in which the surface temperature of the roll was set to the value shown in Table 1. The sheet temperature before rolling was measured by a contact type thermometer and was as shown in Table 1. Subsequently, the preheated sheet was rolled by a pair of rolling rolls whose surface temperatures were set to the values shown in Table 1 so that the final sheet thickness would be 0.25 mm.

Then, subsequently, this rolled sheet was heat-treated by passing it through a heat-treatment roll whose surface temperature was set to 140 ° C. When the temperature of the sheet at the exit of the heat treatment roll was measured with a contact thermometer, it was as shown in Table 1. MD direction obtained (extrusion direction)
Table 1 shows the crystal orientation coefficient and fibril diameter of the uniaxially oriented sheet in which the C axis was oriented. Next, an emulsion of dimethyl silicone oil with water as a diluent was applied on the surface of this sheet with a gravure coater so that the thickness of the dried silicone coating film was 1 μm, and dried. Haze of this sheet, haze value H (%) and wall thickness t
The ratio to (mm) and H / t are shown in Table 1.

Subsequently, this sheet was vacuum-pneumatic molded at a sheet surface temperature of 162 ° C. and a pneumatic pressure of 5 kg / cm ² with a vacuum-pneumatic molding machine, and the tray portion size was 160 mm × 100 m.
m x depth 30 mm, lid size 160 mm x 100 mm x
Depth 3 mm, overall size 160 mm x 200 x maximum depth 3
A 0 mm food pack was molded. The obtained food pack produced a small sound during handling, and even if the lid and tray were fastened with a stapler and then opened, the food pack could be easily opened without tearing. Table 1 shows the thickness of the bottom surface of the hood pack, the haze of the bottom surface, H / t, the non-following number, the coefficient of friction, the crystal orientation coefficient, and the fibril diameter.

[0044]

[Table 1]

[Brief description of drawings]

FIG. 1 is an example showing a state in which stackable containers are stacked.

FIG. 2 is an example of another container that can be stacked.

[Explanation of symbols]

 1. Container 2. Bottom 3. Opening 4. Lid 5. Side 6. Partition

Claims (5)

[Claims] 1. A container having a shape capable of stacking, which is obtained by thermoforming a polypropylene sheet, and is 100
For stacks stacked along the shape, 1000
A polypropylene container having a stacking coefficient of friction such that when the containers are removed one by one at a speed of mm / sec from the stack, at least 95 can be removed one by one. 2. At least one of the inner surface and the outer surface of the container, which is in contact with the same kind of container in the stack, is at least one surface.
The polypropylene container according to claim 1, which exhibits a coefficient of friction of 01 to 0.7. 3. The polypropylene container according to claim 1, wherein the surface having a coefficient of friction of 0.01 to 0.7 is a surface coated with a lubricant. 4. The polypropylene container according to claim 1, wherein the polypropylene sheet has a H / t value of 0.01 to 20 when the haze value is H (%) and the wall thickness is t (mm). . 5. A polypropylene sheet having a wall thickness of 0.1 to
The polypropylene container according to claim 1, which has a thickness of 0.8 mm.