KR102206610B1 - method of manufacturing foamed film - Google Patents

Abstract

The present invention relates to a method for manufacturing a foamed film. A resin composition of low-density polyethylene, polypropylene and ethylene-acrylic rubber in addition to an ethylene-vinyl acetate copolymer is used to form a blown film, and thus it is possible to reduce the density of the foamed film, to make the state of foamed cells uniform, and to achieve a balanced expression of the film′s texture, mechanical properties, and heat resistance. The foamed film according to the present invention has excellent heat resistance, and thus can be applied to gaskets, cushioning materials, and packaging materials for electrical and electronic products.

Description

Method of manufacturing foamed film {method of manufacturing foamed film}

The present invention relates to a method for producing a foamed film in which foaming cells are generated in a film by a foaming agent when forming a film so that the film exhibits buffering properties.

Foam films are generally manufactured by foaming a foaming agent in a synthetic resin added with a foaming agent in the process of forming the film, and are light weight, thermal insulation, insulation, elasticity, and cushioning properties by foaming voids formed compared to films that do not contain foamed cells. , Flexibility, buffering properties, etc. are improved, and by using these properties, it is used for various purposes such as various packaging materials, food containers, insulation materials, slip sheets, and gaskets.

As the base resin constituting the foamed film, thermoplastic resins such as polyolefin, polyurethane, and vinyl chloride resin are used, and foam films are manufactured using the properties of each resin.

As an example of the foam film, there is a foam film using an ethylene vinyl acetate copolymer (EVA) as a base resin.

EVA foam film is applied as a cushioning material due to its excellent elasticity, cushioning, and flexibility, but its application as an internal cushioning material for electronic products is limited because durability, weather resistance, heat resistance, and flammability are deteriorated.

On the other hand, the EVA foam film is generally manufactured by a crosslinking foaming method in order to facilitate film formation and to control foam cells and to improve mechanical strength.

The crosslinked EVA foam film by the crosslinking foaming method can change the viscoelastic behavior while first crosslinking the resin under specific conditions, so that the temperature range having viscoelasticity suitable for manufacturing a low density (high foaming) foam can be widened.

In addition, heat resistance and mechanical properties can be improved by crosslinking.

However, since crosslinking is required for molding, the manufacturing process is complicated, and it is difficult to select an appropriate molding condition for controlling mechanical properties and foam cells.

EVA has a very narrow temperature range in the suitable processing area because its viscoelasticity is rapidly lowered at a temperature above the melting point. Therefore, if crosslinking is not performed, only a high-density (low-foaming) foam cannot be obtained, resulting in a low-density film, resulting in lower buffering properties. And mechanical properties are deteriorated.

In other words, the film forming method by the cross-linked foaming method is not easy to control foaming and to improve mechanical and thermal properties. In particular, it is a blown bubble film forming method that has superior productivity and easily manufactures a thin film compared to the T-die method. There is a problem that is not easier to use when using.

Republic of Korea Patent Publication No. 2003-0084849 (EVA-based film for crosslinking foam)

In order to solve the above-described problems, the present invention provides a method for producing a foamed film in which foamability is improved and mechanical and thermal properties are balanced even when a blown bubble film molding method is used without crosslinking the EVA resin. It aims to provide.

In order to solve the above problems, the present invention, 30 to 50 parts by weight of low-density polyethylene, 30 to 50 parts by weight of polypropylene, 30 to 50 parts by weight of ethylene-acrylic rubber, and 2 to 10 parts by weight of talc, based on 100 parts by weight of the ethylene vinyl acetate copolymer. Mixing a resin composition including 0.05 to 5 parts by weight of a foaming agent and a part by weight; And melting the resin composition using a blown bubble film molding machine and extruding it through a circular die to form a blown bubble film.

According to the present invention, by molding into a blown film using a resin composition of low-density polyethylene, polypropylene, and ethylene-acrylic rubber in addition to the ethylene vinyl acetate copolymer, it is possible to reduce the density of the foamed film, the state of the foaming cell is uniform, and the touch of the film. , It becomes possible to appear in a balance between mechanical properties and heat resistance.

The foam film according to the present invention has excellent heat resistance and can be applied to packaging materials, cushioning materials, and gaskets of electric and electronic products.

1 is a diagram showing an example of a blown bubble film forming machine for manufacturing a foam film according to an embodiment of the present invention.

The present invention uses a blown bubble film forming machine, 30 to 50 parts by weight of low-density polyethylene, 30 to 50 parts by weight of polypropylene, 30 to 50 parts by weight of ethylene-acrylic rubber, talc 2 based on 100 parts by weight of the ethylene vinyl acetate copolymer. This is a method of manufacturing a foamed film in which a resin composition including ~10 parts by weight and 0.05 to 5 parts by weight of a blowing agent is melted and extruded through a circular die to form a blown bubble film.

In the resin composition, ethylene vinyl acetate copolymer (EVA) is a copolymer of ethylene and vinyl acetate obtained by radical polymerization of ethylene and vinyl acetate in the presence of a catalyst in a reactor, and flexibility, rubber due to the presence of vinyl acetate group. The elasticity, transparency, and low temperature characteristics become very excellent.

The vinyl acetate content of the EVA of the present invention is 5 to 25% by weight, and if it is less than 5% by weight, the tensile strength, tear strength, impact strength, elongation, flexibility, texture, appearance, heat seal strength of the blown bubble foam film If is insufficient and exceeds 25% by weight, the MI exceeds 5g/10 minutes, resulting in lower tensile strength, tear strength, impact strength, etc., and strong adhesiveness, resulting in poor film formability.

In addition, the density of the EVA is preferably 0.930 to 0.950 g/cm 3, but if it is less than 0.930 g/cm 3, the tensile strength, tear strength, impact strength, elongation, flexibility, texture, and appearance properties of the blown bubble foam film This decreases, and when it exceeds 0.950 g/cm 3, tensile strength, tear strength, impact strength, and the like decrease.

In addition, the melt index (MI) of the EVA is preferably 0.5 to 5 g / 10 min.If it is less than 0.5 g / 10 min, the moldability of the blown bubble foam film deteriorates, resulting in a decrease in productivity, as well as flexibility and texture. If it falls out and exceeds 5g/10 minutes, the tensile strength, tear strength, impact strength, etc. of the blown bubble foam film will decrease.

The EVA foam film foamed by using the EVA copolymer has excellent elasticity and is therefore used as a cushioning material.

When EVA is not crosslinked in the foam film, EVA has a low melting point (especially, as the content of VA increases, the melting point decreases). Continuous use of 80~105℃ Use of cushioning, packaging, and gaskets in electronic and electrical products that require heat resistance. There is a problem to be applied as a low mechanical strength is also a problem.

In addition, since the foaming agent is not easily foamed, there is a problem in that it is difficult to increase the foaming ratio (lower density) and to make the foam cell size uniform.

In addition, there is a problem in that it is not easy to form a film due to low viscoelasticity.

In the present invention, in order to solve this problem, low-density polyethylene (LDPE), polypropylene (PP), and ethylene-acrylic rubber may be used together with EVA in the resin composition.

In the resin composition, LDPE suppresses the strong adhesiveness of the blown bubble foamed film due to EVA and increases the melt viscosity of the resin composition so that the blown bubble does not sink in during the film forming process and is stably molded into a film shape. do.

At this time, LDPE preferably has a melt index (MI) of 0.5 to 5 g/10 min.If it is less than 0.5 g/10 min, the moldability of the blown bubble foam film deteriorates, and if the MI exceeds 5 g/10 min, blown bubble foam The mechanical properties of the film deteriorate.

LDPE preferably has a density of 0.910~0.940g/cm3. If it is less than 0.910g/cm3, the mechanical properties of the blown bubble foam film decrease, and if it exceeds 0.940g/cm3, elasticity, flexibility, elongation, and impact resistance are lowered. Is done.

If the content of LDPE in the resin composition of the present invention is low, molding of the blown bubble foam film is not easy under the molding conditions of the present invention, and the produced blown bubble foam film becomes sticky, and if the content of LDPE is large, the impact resistance decreases. .

In the resin composition, PP improves the heat resistance of the blown bubble foam film to be produced.

In addition, since the gas permeability is lower than that of polyethylene, the gas generated by the foaming agent is confined to form many foam cells. Accordingly, the foaming ratio is increased and the density is reduced, thereby exhibiting an effect of improving the buffering property.

The PP used at this time can be used without limitation, for the purpose of improving heat resistance, such as homo PP, ethylene/propylene copolymer, and a copolymer of propylene and other α-olefins.

In the resin composition of the present invention, when the amount of PP is low, the heat resistance of the blown bubble foam film decreases, and when the content of PP is high, the elasticity, flexibility, elongation, and impact strength are lowered, and the melt viscosity is lowered, resulting in lower film moldability.

The ethylene-acrylic rubber is a ternary copolymer containing ethylene-methyl acrylate as a main component, and may be, for example, a ternary copolymer containing ethylene-methyl acrylate as a main component and an unsaturated organic acid ester as a third component.

In the resin composition of the present invention, ethylene-acrylic rubber is used to improve heat resistance, mechanical strength, flexibility, elasticity, impact resistance, and flame retardancy.

Ethylene-acrylate rubber as a commercial product can be used DuPont Vamac ^ⓡ.

The resin composition of the present invention has increased melt viscosity and viscoelasticity compared to EVA copolymers, so even if the molding (cooling) temperature changes after melt extrusion, the film does not collapse or shake, and molding is easily performed, and heat resistance, elasticity, and impact resistance This can be improved.

In the resin composition, the foaming agent may be an organic or inorganic pyrolytic foaming agent, water, a hydrocarbon-based and freon-based solvent, a gas such as nitrogen, carbon dioxide, propane, and butane.

Among these, examples of the thermally decomposable foaming agent include inorganic foaming agents such as sodium bicarbonate, sodium carbonate, ammonium hydrogen carbonate, ammonium carbonate and sodium nitrite.

Other blowing agents used in the present invention include azo compounds such as azodicarbonamide, azobisisobutyronitrile, azocyclohexylnitrile, and azodiaminobenzene, and N,N'-dimethyl-N,N Nitroso compounds such as ´-dinitrosoterephthalamide, N,N´-dinitrosopentamethylenetetramine, benzenesulfonyl hydrazide, sulfonyl hydrazide such as p,p´-oxybisbenzenesulfonyl hydrazide Compounds, calcium azide, and azide compounds such as 4,4'-diphenyldisulfonyl azide.

If the content of the foaming agent in the resin composition of the present invention is low, the amount of gas for foaming is small, and if the foaming film cannot be obtained, if it is too large, the foaming ratio becomes too large, so that the surface of the foamed film becomes rough and the foaming cells are not uniform, tensile strength, and tearing steel. Also, the surface smoothness decreases.

The foaming agent may be added as it is to the resin composition of the present invention, but it can also be added as a master batch added to the resin component in advance to improve dispersibility.

As a foaming aid, talc, silica, titanium oxide, stearic acid, phthalic acid, zinc stearate, lead stearate, magnesium stearate, calcium stearate, ethylene glycol, glycerin, ethanolamine, urea, urea derivatives, melamine, 2 alkaline lead phosphite, 3 alkaline lead sulfate , Zinc oxide, etc. can be blended.

The foamed film of the present invention is produced by a blown bubble film forming method.

In the blown bubble film forming method, the molten thermoplastic resin is extruded into a tube shape, and the pressure of the air in the tube is slightly increased to expand while stretching and cooling, and the tube-shaped film is folded between a pair of rolls to block the pressure at the same time. It is a method of manufacturing a film by continuously winding the folded tube.

An example of a blown bubble film forming machine is shown in FIG. 1 below.

A resin component, a foaming agent, and an additive constituting the resin composition of the present invention are mixed, and the mixed resin composition is supplied to an extruder of a blown bubble film molding machine to be melted.

Subsequently, the tube of the resin composition extruded through the circular slit die is expanded and cooled by injection of air to produce a blown film.

At this time, it is preferable that the circular die gap width is 0.4 to 1.2 mm. If it is less than 0.4 mm, the melt pressure rises and melt fracture occurs, so the stability of the bubble is poor and the extrusion processing of high discharge is difficult. If it exceeds mm, it is difficult to control cooling due to excessive discharge, and it is difficult to form bubbles, and since the film is oriented only in the longitudinal direction, it becomes a foamed film that is easily torn only in the longitudinal direction.

It is preferable that the blow-up ratio due to expansion is in the range of 1.5 to 4.0, as it can be stretched uniformly in the longitudinal and transverse directions of the bubble. If it is less than 1.5, the shape of the foaming cell is not uniform, and the foamed film is in the longitudinal direction. It is easy to tear, and if it exceeds 4.0, the bubble becomes unstable, making it difficult to process a stable foamed film.

When forming the blown bubble film of the present invention, it is preferable that the take-up speed is in the range of 4 to 15 m/min.If it is less than 4 m/min, orientation does not occur in the longitudinal direction of the film and productivity decreases, and if it exceeds 15 m/min The film is easily torn in the longitudinal direction due to the occurrence of cutting of the film or excessive orientation of the film in the longitudinal direction.

In general, when forming a film, the molten resin is rapidly cooled to reduce stretching stress, improve transparency, and maintain flatness, but in the case of the present invention, when rapid cooling is performed, the crystallinity of the molten resin is lowered and the stress at the time of stretching is lowered. The generation of foam cells decreases.

In the present invention, the cooling temperature of the blown tube bubble is adjusted to adjust the foaming properties such as the degree of occurrence, size, and uniformity of the foam cells.

The cooling temperature is adjusted by blowing cooling air from a first blower installed near the circular slit die to gradually cool the neck of the tube bubble to 70 to 80°C, and cooling with a second blower installed above the first blower. By blowing air, the expansion part of the tube bubble is gradually cooled to 50 to 60°C, and cooling air is blown from the third blowing device installed above the second blowing device to reduce the frost line area of the tube bubble to 30 to 40°C. It can be cooled slowly with.

As a result, the neck portion and the expansion portion of the tube bubble are gradually cooled while crystallization is delayed, but since the resin composition of the present invention has increased viscoelasticity, the film is easily formed.

By cooling the neck portion and the expansion portion to the above temperature, the foaming agent can be prevented from rapidly foaming at the initial stage of film forming. This makes it possible to maximize the release of foaming gas, so that the foaming ratio is high and the size and distribution of the foaming cells are uniform.

In addition, since the expanded portion is in an amorphous state, the resin composition is not stretched only in the longitudinal direction, which is the longitudinal direction, by the melt tension of the resin composition, but is simultaneously stretched biaxially in both the longitudinal and transverse directions. Due to this, a lot of foaming occurs and the resulting foam cells are uniformly distributed in the film, and the size of the foam cells is small and the shape becomes uniform.

In this way, when the molten resin is slowly cooled, crystallization of the resin continues to proceed, so that the degree of crystallinity increases. Accordingly, since peeling from the foaming nucleating agent easily occurs during stretching due to the increased stretching stress, many foaming cells are generated.

In addition, since the resin is maintained at a high temperature by slow cooling, foaming occurs uniformly, and thus the size of the foaming cell is uniform.

The foamed film of the present invention has a density of 0.60 to 0.80 g/cm 3 and a thickness of 30 to 300 μm prepared by the method of the present invention.

At this time, if the density is less than 0.60 g/cm 3, the buffering property is rather deteriorated, mechanical properties such as impact resistance and durability are deteriorated, the touch is also poor, and the quality of the film is deteriorated, and when it exceeds 0.80 g/cm 3, the buffering property is lowered.

The manufacturing process of the foam film of the present invention will be described with reference to FIG. 1 below.

The tubular film extruded from the circular die mounted on the extruder is supplied with air from the air injection pipe to the inside, so that it is rapidly expanded into a film of a predetermined width, pinched by a nip roll to take up, and a take-up reel 14 ) To wind it up.

The circular die has a circular die head and a weld portion.

In the present invention, there is no particular limitation on the configuration of the air injection means for supplying the air injected into the tubular film, for example, the air injection pipe can be connected to the blower or the air injection pipe can be connected to the compressed air cylinder. have.

The molten resin composition is extruded from the circular orifice of the die to form tube bubbles. In the neck portion formed by the bubbles immediately after extrusion, the bubbles are mainly stretched in the longitudinal direction (MD).

Thereafter, the bubble is rapidly expanded so that the bubble has a predetermined diameter. In this expanded portion, the bubbles are simultaneously stretched in MD and TD. There is a frost line near the upper side of the expansion part, where the resin is cooled and solidified.

Above the frost line, the tube bubble cools more slowly.

When forming into a blown foam film using the resin composition of the present invention, by cooling the bubbles as described above, the crystallization of the resin composition is made slowly, so that the size and distribution of the foam cells formed as foaming continues to increase It becomes possible to become uniform.

Hereinafter, the present invention will be described in more detail based on the following examples and comparative examples.

However, the following examples are only for illustrating the present invention, and the present invention is not limited by the following examples, and can be substituted and equivalent to other examples within the scope not departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which the present invention pertains.

[Example 1]

EVA copolymer (vinyl acetate content 12%, density 0.931g/cm3, MI=0.8g/10min) 100 parts by weight of low density polyethylene (density 0.924g/cm3, MI=2.0g/10min) 35 parts by weight, poly 40 parts by weight of propylene (random copolymer, density 0.90 g/cm 3, MI = 7.5 g/10 min), 50 parts by weight of ethylene-acrylic rubber (Vamac G, DuPont), 3 parts by weight of talc, and 1.5 parts by weight of sodium bicarbonate foaming agent ribbon The resin composition was prepared by mixing in a mixer.

The resin composition was put into an extruder of a blown bubble film molding machine as shown in FIG. 1, and a foamed film having a thickness of 100 μm was prepared under the following conditions.

<Film forming conditions>

Extrusion cylinder temperature: 170℃, die temperature: 170℃, extrusion volume: 50kg/hour,

200mm, 다이 갭 폭 : 0.7mm, Circular die:

200mm, die gap width: 0.7mm,

Blow-up ratio: 2.3, take-up speed: 6m/min.

In addition, in the cooling of the tube bubble, the first blowing device is blown to gradually cool the neck portion of the bubble to 80°C, the second blowing device is blown to gradually cool the bubble expansion to 60°C, and the third blowing device The frost line area was gradually cooled to 40°C by blowing air, and the bubble area was maintained at 40°C.

The cylindrical partition made of acrylic resin surrounding the bubble region had two ventilation outlets formed on its upper side. A rod-shaped heater was installed inside the cylindrical partition, and a rectifying plate with an opening rate of 60% was installed above the inside of the cylindrical partition, so that the blown air was discharged to the outside while rising in one direction along the bubble.

[Example 2]

A foam film was prepared in the same manner as in Example 1, except that the blow-up ratio was set to 3.5 under the film forming conditions in Example 1.

When forming the film, it was confirmed that the bubble did not sink or fluctuate and maintained a stable shape, and that the size and distribution of the foam cells of the foam film were uniform.

Since it becomes possible to increase the blow-up ratio, the productivity of the foamed film is increased.

[Comparative Examples 1-6]

A foam film was prepared in the same manner as in Example 1, except that the temperature of the resin composition and the bubble in Example 1 were changed as shown in Table 1 below.

division Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Suzy
Composition
(Part by weight) EVA 100 100 100 100 100 100 100 LDPE 35 0 35 10 35 35 60 PP 40 0 40 40 10 40 60 Ethylene-
acryl
Rubber 40 0 40 40 40 10 60 Talc 3 3 3 3 3 3 3 blowing agent 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Tube bubble
Temperature(℃) Neck 80 80 60 80 80 80 80 Expansion 60 60 50 60 60 60 60 Frost line 40 40 40 40 40 40 40

The properties of the foam films of the Examples and Comparative Examples were evaluated in the following manner, and the results are shown in Table 2 below.

The safety evaluation of the tube bubble during film forming is good depending on whether the desired state of the bubble is maintained by visually observing the state of the tube bubble (a bubble collapsing, shaking, breaking, etc.) when forming under the above conditions. , Marked as defective.

Foam cell formation and size uniformity evaluation was conducted by randomly cutting 5 foamed films prepared by the methods of Examples and Comparative Examples into a size of 1 cm in width and 1 cm in length, and then placed on an optical microscope to form a foam cell and uniformity in size Was observed with the naked eye. At this time, according to the observed form, it is shown in Table 2 as good, normal, and bad.

As an evaluation of heat resistance, the rate of change in strength after heat treatment is measured. After measuring the tensile strength and elongation of the film sample, it is measured by treating it in an oven at 100°C for 168 hours. The higher the ratio (%) obtained by dividing the measured value after treatment by the measured value before treatment, the better the heat resistance.

The tactile feel of the foamed film was evaluated as good if the film was pressed and touched by hand and there was no sticky sensation, and as bad if any.

division Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Bubble stability Good Instability Good usually Good Good Good Density (g/cm3) 0.77 0.86 0.89 0.82 0.81 0.81 0.85 Foam cell status Good Good Bad Bad usually usually Bad Film touch Good Bad Good Good Good Good Good Tensile strength (㎏/㎠) 289 278 293 295 291 292 302 Elongation(%) 143 152 149 145 141 139 141 Tensile strength change rate after heat treatment (%) 108 96 102 107 86 97 112 Rate of change in elongation after heat treatment (%) 87 83 86 87 78 85 92

From Table 2 above, it was confirmed that the foamed film of Example 1 according to the present invention improved the density of the foamed cells, while the foamed cell structure was very dense and uniform, and the stability of the bubbles was improved when molded into a blown film.

From the results of the comparative example, when the content of PP and ethylene-acrylic rubber in the resin composition decreases, heat resistance decreases, and when the content of PP and ethylene-acrylic rubber increases, it is difficult to reduce density and the state of the foam cell is not uniform. It is confirmed that this appears.

In particular, since the foam film according to the embodiment does not have a large change in mechanical properties even after heat treatment, it has excellent heat resistance and can be applied to packaging materials, cushioning materials, and gaskets of electric and electronic products.

Claims (3)

Resin containing 30 to 50 parts by weight of low-density polyethylene, 30 to 50 parts by weight of polypropylene, 30 to 50 parts by weight of ethylene-acrylic rubber, 2 to 10 parts by weight of talc, and 0.05 to 5 parts by weight of a foaming agent based on 100 parts by weight of ethylene vinyl acetate copolymer Mixing the composition; And
Including; melting the resin composition using a blown bubble film molding machine and extruding through a circular die to form a blown bubble film; includes,
In the step of forming the blown bubble film, after the extrusion, the neck of the tube bubble expanded by injecting air into the tube is blown from the first blowing device installed in the circular die, and the neck of the tube bubble is 70 to 80. The tube bubble is gradually cooled to °C and blown from a second blower installed on the first blower to gradually cool the expansion part of the tube bubble to 50 to 60°C, and blown from a third blower installed on the second blower to The method for producing a foamed film, characterized in that gradually cooling the frost line region of 30 ~ 40 ℃. The method of claim 1,
The ethylene vinyl acetate copolymer is a radical polymerization of ethylene and vinyl acetate, the content of vinyl acetate is 5 to 25 wt%, the density is 0.930 to 0.950 g/cm 3, and the melt index (MI) is 0.5 to 5 g/10 Method for producing a foamed film, characterized in that the powder. delete

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