JP2779882B2 - Method for producing polypropylene resin foam sheet with beautiful appearance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a foamed polypropylene resin sheet, and more particularly to a method capable of performing secondary processing such as thermoforming used for various packaging materials and various product members including food packaging. The present invention relates to a method for producing a foamed polypropylene resin sheet which is hard and semi-hard and has excellent mechanical properties with a low open cell ratio and a beautiful surface appearance.

[0002]

2. Description of the Related Art Polystyrene foam sheets are widely used as foam sheets which can be formed in a secondary form such as food packaging. Since polystyrene resin is an amorphous resin, it does not have a definite melting point. If it contains a foaming agent and is heated and melted, it becomes a molten resin having melt viscoelasticity suitable for foaming. Is obtained. However, expanded polystyrene sheets are not always satisfactory in properties such as heat resistance.

[0003] Since polypropylene resin is a crystalline resin,
It is superior to polystyrene resin in terms of heat resistance and the like. However, the crystalline polypropylene resin has a very low melt viscosity above the melting point of the crystalline melting point, cannot retain the foamed bubbles, and easily breaks. Therefore, the conventional polypropylene foam sheet has a high open cell rate. It was difficult to obtain a foam sheet having good mechanical properties and excellent heat resistance having closed cells. The density of the conventional polypropylene resin foam sheet is 0.5 g / cc.
It was a low foamed product as described above or a highly foamed product having a density of 0.03 g / cc or less.

That is, a low foam sheet having a density of 0.5 g / cc or more is produced by adding a decomposable or reactive foaming agent to a polypropylene resin and supplying it to a sheet molding and extruding apparatus. A highly foamed sheet having a microcellular structure having a density of 0.03 g / cc or less is disclosed, for example, in Japanese Patent Publication No. 46-414.
As shown in No. 74, a crystalline polypropylene resin is dissolved in an activating solution such as methylene chloride, fluorotrichloromethane, perfluorocyclobutane, and the resulting solution is subjected to a pressure higher than the vapor pressure of the solution but 1000 PSi. Extruding foam from an area no higher than to a substantially lower pressure area, thereby evaporating the activating liquid, precipitating solid polymer and freezing the orientation of the polymer. There is.

[0005] An intermediate foam of these two examples has a density of 0.
Although those having around 3 g / cc are also found on the market, they are almost open cells and hardly any closed cells, and therefore cannot be said to be foam sheets having good mechanical properties. Also, after adding the foaming agent to the polyolefin resin, instead of free foaming into the atmosphere immediately from the narrow gap of the mold, foaming is performed under a certain appropriate pressure condition, and this is determined by the dimensions of the molding apparatus. A method for producing a foam sheet having closed cells by cooling and molding to a predetermined thickness and continuously extruding from the molding apparatus at atmospheric pressure is known (Japanese Patent Publication No. 51-14540). reference). However, this method has a drawback that the foaming process is very special and the control is difficult.

[0006] Therefore, in order to extrude and foam a polypropylene resin like a polystyrene resin, it is necessary to widen the extrudable foamable temperature range and to increase the melt viscoelasticity in the temperature range above the melting point. For this reason, attempts have been made to increase the molecular weight of the polypropylene resin and to form a copolymer resin with another olefin. Modification of the resin by these means has given good results in improving the extrudability of the non-foamed sheet, the thermoformability of the non-foamed sheet, and the physical properties of the molded product. It did not lead to elasticity.

[0007] One of the causes that makes the melt viscoelasticity of the polypropylene resin special is that the polypropylene resin is considered to be a very linear polymer. Attempts have been made to solve the problem, and although the foamability has been slightly improved, it has been extremely difficult to obtain the expected effects because crosslinking and decomposition proceed simultaneously as a characteristic of the polypropylene resin.

In the course of various studies to obtain a polypropylene resin suitable for extrusion foaming, characteristics of a resin exhibiting melt viscoelasticity required for foaming can be recognized by a molecular weight distribution value obtained by high-temperature GPC. It has been found that by using a polypropylene resin having a GPC molecular weight distribution value in the range described above, a polypropylene resin sheet having a high closed cell ratio and excellent mechanical properties can be obtained.

The polypropylene resin having a GPC molecular weight distribution value in this specific range has a Z-average molecular weight Mz of at least 2.0 × 10 ⁶ and Mz / Mw of at least 3.0.
And a molecular weight distribution curve determined by gel permeation chromatography, which is a camel-type polypropylene resin having a shape with an overhang of the curve indicating that the polymer region contains a branched polymer.

However, as described above, the foamed sheet obtained by extrusion foaming this polypropylene resin is a foamed sheet having a high closed cell ratio and excellent mechanical properties, but has a large cell diameter. It was hard to say that it was beautiful. In order to remedy this drawback, for example, if the cells in the entire sheet are made finer by using means such as an increase in the amount of a foaming nucleating agent, a phenomenon occurs in which the open cell ratio increases and the expansion ratio decreases.

Further, when a method of suppressing the foaming of the surface by quenching the surface at the time of extrusion to suppress the foaming of the surface and making the bubbles of the surface fine is carried out, as in the method of producing a foamed sheet of a polystyrene resin foam, it is a crystalline resin. In the case of polypropylene, it is solidified in the mold slit and cannot be extruded. Further, even in the case of so-called secondary molding for molding into a container or the like, in the case of a polystyrene resin foam sheet, the sheet foams due to the action of the foaming agent swelling inside, and new microbubbles are generated. The molded article is more beautiful than the sheet because it is produced, whereas the polypropylene resin foam sheet does not produce any new air bubbles,
The bubbles are stretched and conversely coarse.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to provide a polypropylene resin sheet having a low appearance of open cells and excellent mechanical properties. To provide a method for manufacturing the same.

[0013]

The gist of the present invention is that a Z-average molecular weight Mz is at least 2.0 × 10 ⁶ and Mz / Mw
Is at least 3.0 and the molecular weight distribution curve determined by gel permeation chromatography is a camel-type polypropylene having a shape with a protrusion of the curve indicating that the polymer region contains a branched polymer. Resin (A)
A propylene-based copolymer resin (B) composed of 60 to 100% by weight and 80 to 95% by weight of propylene and 5 to 20% by weight of an α-olefin such as ethylene, or a mixed resin of 0 to 40% by weight. This is a process for producing a foamed polypropylene resin sheet having a beautiful appearance, wherein a wax is added, supplied to an extruder, mixed and melted with a foaming agent, and then extruded into the atmosphere and foamed.

That is, in the present invention, when a resin foam sheet is formed from a specific polypropylene resin, polyethylene wax is added to the resin composition, supplied to an extruder, mixed with a foaming agent and melted, and then air-cooled. By extruding and foaming inside, a polypropylene resin foam sheet with a beautiful appearance is obtained. Further, the present invention will be described in detail.

A method for measuring a gel permeation chromatograph (hereinafter referred to as GPC) for specifying the polypropylene resin used in the present invention is as follows. Measurement device: Waters, GPC 150-C type Measurement condition: Column KF-80M 2 pieces (SH
ODEX) Column temperature 145 Inject. Temperature 145 Pump temperature 60 Sensitivity 32 Solvent used O-dichlorobenzene (1.0 ml
/ Min) Run time 50 minutes Inject. The information obtained by this GPC is as follows. (1) Mn number average molecular weight: the most basic average molecular weight obtained by measuring physical properties directly related to the number of molecules of a polymer, and depends on the total number of molecules. (2) Mw weight average molecular weight: the average molecular weight determined when the measured physical property value is directly related to the weight of the polymer, and is the root mean square of the molecular weight, and depends on the higher polymerization degree molecule than Mn. (3) Mz z average molecular weight: the highest average molecular weight,
It is the cube mean of the molecular weight.

The polypropylene resin effective for obtaining the foam of the present invention is a polymer having an Mz of about 2.0 × 10 ⁶ or more and an Mz / Mw ratio of about 3.0 or more. Since the polydispersity index Mw / Mn could not be distinguished between a polypropylene resin that gives good foaming properties and a polypropylene resin that gives only insufficient foaming properties, the numerical value is less important. In addition, the molecular weight distribution curve of the polypropylene resin giving good foaming properties by GPC is always in a shape with an overhang in the polymer region.
It has a shape like the back of a camel. This is called a camel type in the present invention. On the other hand, the molecular weight distribution curve B of the polypropylene resin giving only insufficient foaming property has a simple mountain shape showing a normal distribution. The camel-type molecular weight distribution shows that most of the polypropylene resin is linear, but some components with high molecular weight have many branches. Table 1 shows the properties of commercially available polypropylene resins, such as Mn, Mw, Mz, and GPC curve shape.

[0017]

[Table 1]

The camel type polypropylene resin used in the present invention is considered to be produced by the method of US Pat. No. 4,916,198, and is a branched polymer such as a polypropylene resin sold by Himont Corporation. . As is clear from Table 1, a polypropylene resin having a camel-type overhang shape (referred to as a "camel-type polypropylene resin") alone in a GPC curve can provide a closed-cell foamed sheet, but has further mechanical properties. In order to improve the viscosity, a polypropylene copolymer resin comprising a copolymer of propylene and another α-olefin can be mixed up to 40% by weight.

The polypropylene copolymer resin is a copolymer resin of α-olefin with propylene as a main component, and includes ethylene / propylene random copolymer resin, ethylene / propylene block copolymer resin, ethylene / butene / propylene copolymer resin. It is a resin or the like. The type of the α-olefin of these copolymer resins is not particularly limited, and those having physical properties suitable for the intended use of the obtained foamed sheet may be used. The content of propylene in the copolymer resin is 95% by weight or more, and the effect of improving brittleness is small. When the proportion of propylene in the copolymer resin is 80% by weight or less, brittleness is improved but rigidity is reduced. You will lose. The above-mentioned copolymer resins can be used alone or as a mixture.
In addition, an elastomer such as an α-olefin polymer, an ionomer, and ethylene / propylene rubber (EP rubber) may be further added.

The foamed polypropylene resin sheet according to the present invention may be a camel type polypropylene resin alone or
Although a mixture of the resin and a polypropylene-based copolymer resin is used, when a mixture of a camel-type polypropylene resin and a polypropylene-based copolymer resin is used, the content of the polypropylene-based copolymer resin increases, Since the characteristics of the branched camel-type polypropylene resin having foaming properties are reduced and it is difficult to obtain a good foam sheet having a low open cell ratio, the amount of the polypropylene copolymer resin to be added is up to 40% by weight. Is good.

The density Xg / cm ³ of the foamed sheet of the present invention is from 0.5 to 0.025. The range of the open cell ratio Y% of the good foamed sheet obtained in the research process leading to the present invention satisfies the following relational expression. Y <−50 logX In the formula, the open cell ratio is a value measured by the following method. Measuring device: Air Pycnometer (Toshiba Beckman,
(Model-930) Calculation method: Open cell rate (Y%) = (apparent volume−measured value) × 100 / apparent volume However, the open cell rate of the foamed sheet of the present invention satisfies the above relational expression. did. The apparatus for producing a foam sheet according to the present invention is the same as the conventional apparatus for producing a foam sheet, except that a foaming agent is added to the above-mentioned mixed resin composition and extrusion-molded.

However, in the present invention, polyethylene wax is added to the above raw material resin. The proportion added to the raw resin is 0.01 part to 0.10 part, preferably 0.1 part.
03 parts to 0.08 parts.

The foam sheet of the present invention reduces the shearing force on the mold and the wall surface by the lubricating action of polyethylene wax to prevent excessive heat generation, so that the surface layer becomes fine bubbles and the appearance is beautiful. It becomes a foam sheet. Next, the present invention will be described more specifically with reference to examples.

[0024]

[Examples and Comparative Examples]

Example 1 Hydrocello-a stoichiometric mixture of citric acid and sodium bicarbonate was used as a cell nucleating agent in 100 parts by weight of PF814 (manufactured by Himont), a camel-type polypropylene resin.
HK (manufactured by Behringer) and 0.04 part of polyethylene wax (manufactured by Behringer) are preliminarily mixed with a blender, and the resulting mixture is a tandem type extruder having a diameter of 90 to 115 mm for foam extrusion. No. 1 extruder (9
3.0 parts of butane (n-butane / i-butane = 2/1) was injected at a central zone of 0 mmφ). Then, the extruded tubular extruded foam sheet is cooled and formed by a circular die, and cut at two points on the circumference by a cutter to obtain a width 640.
mm, thickness 1.9 mm, basis weight (total) 300 g / cm
² sheets were obtained.

Comparative Example 1 A stoichiometric mixture of citric acid and sodium bicarbonate hydrocelloline was used as a cell nucleating agent in 100 parts by weight of PF814 (manufactured by Himont), a camel-type polypropylene resin.
HK (manufactured by Behringer) in a blender in advance, and the mixture was supplied to a tandem type extruder for foam extrusion having a diameter of 90 to 115 mm. 1 extruder (9
3.0 parts of butane (n-butane / i-butane = 2/1) was injected at a central zone of 0 mmφ). The extruded tubular extruded sheet is cooled and molded by a circular die, and cut at two points on the circumference by a cutter to obtain a width of 640 m.
m, thickness 1.9 mm, basis weight (total) 300 g / cm ²
Was obtained. The density, open cell ratio and cell size of each of the foam sheets obtained above were measured and the results were as follows.

Cell Size Density Open Cell Ratio Surface Cross Section Example 1 0.14 25.3% 0.77 mm 0.98 mm Comparative Example 1 0.14 24.0 1.02 0.98 As apparent from the results, The cells in the cross section of the foam sheet obtained by the method of the present invention are the same as those in Comparative Example 1, but the surface is finer than that in Comparative Example, and the appearance is beautiful. -

[0027]

As described above, the camel-type polypropylene resin (A) alone or the resin (A) and the propylene copolymer resin (B) composed of propylene and another α-olefin are used. By blending polyethylene wax with the resin raw material composed of the mixed resin composition and extruding and foaming, the bubbles on the surface are fine and the appearance is beautiful while maintaining the excellent heat resistance and mechanical properties of the polypropylene resin. A foamed sheet can be obtained.

[Brief description of the drawings]

FIG. 1 shows the molecular weight distribution curves of a camel-type polypropylene resin used in the present invention and a general polypropylene resin GPC.

Claims (1)

(57) [Claims] 1. The method of claim 1, wherein the Z-average molecular weight Mz is at least 2.0 ×
10 ⁶ , Mz / Mw is at least 3.0, and the molecular weight distribution curve by gel permeation chromatography has a curve overhang indicating that the polymer region contains a branched polymer. 60 to 100% by weight of a camel-type polypropylene resin (A), 80 to 95% by weight of propylene and α-olefin 5 such as ethylene
A polyethylene wax is added to a resin alone or mixed with 0 to 40% by weight of a propylene-based copolymer resin (B) consisting of 0 to 20% by weight and supplied to an extruder. A method for producing a foamed polypropylene resin sheet having a beautiful appearance, characterized by foaming.