JP2000281829A - Foamed body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an ultra fine porous foamed body having a void density of a specific value or more, fine voids in a high void density, and a good balance of rigidity, heat insulation performance, light-weight property and others by using a plastic material containing a polypropylene resin as the raw material. SOLUTION: A foamed body having a void density of 1011 or more, preferably 1012 or more per 1 cm3 of a raw plastic material and having an average void diameter of 2 μm or less is obtained by using a plastic material containing a polypropylene resin (a propylene homopolymer, a propylene copolymer containing 50 mol% or more of propylene monomer unit, or the like) as the raw plastic material. The content of the polypropylene resin is preferably 50 wt.% or more, further preferably 70 wt.% or more, especially preferably 85 wt.% or more. It is preferable that the plastic material contains further a polyolefin elastomer. A possible example of the manufacturing method is a supercritical foaming method comprising a process in which a supercritical fluid is impregnated in a plastic material, and a subsequent process in which the pressure is quickly released to obtain a foamed body.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a polypropylene resin foam having high cell density and fine cells.

[0002]

2. Description of the Related Art Polypropylene resins have an excellent balance of various properties such as heat resistance, chemical resistance and strength when compared with other general-purpose thermoplastic resins, while maintaining the properties of such polypropylene resins. In order to reduce the weight and material cost, foams obtained by foaming the resin have been widely used in various fields such as automobiles, building materials, and miscellaneous goods. However, foams generally have a problem that as the expansion ratio increases, the cell diameter of the foam increases, and the strength, heat insulation, and the like of the foam decrease.

Recently, clean and low-cost carbon dioxide,
A supercritical foaming method has been developed in which an inert substance such as nitrogen is turned into a supercritical fluid under high pressure, and this supercritical fluid is impregnated into the raw material resin, and then the pressure is rapidly released to obtain a foam having fine bubbles. The application of this technology to polypropylene is discussed, for example, in the Materials & Manufacturing Process.
rocesses, 4 (2), 253-262 (1989)) and U.S. Pat. No. 5,160,674. However, foams obtained according to the teachings of these documents are also required to be further improved in strength, heat insulation properties, and the like.

[0004]

SUMMARY OF THE INVENTION Accordingly, the present inventors
As a result of intensive studies to develop a foam of polypropylene resin having excellent strength and heat insulation properties, an ultrafine porous foam having a cell density of 10 ¹¹ or more per 1 cm ^{3 of} the plastic material constituting the foam Was found to be effective, and the present invention was completed.

[0005]

That is, the present invention provides a foam comprising a plastic material containing a polypropylene resin and having a cell density of at least 10 ¹¹ cells per 1 cm ^{3 of the} plastic material.

BEST MODE FOR CARRYING OUT THE INVENTION

[0006] In this specification, the cell density is a concept that means the number of cells present in a part of a foam made of a plastic material having a unit volume (1 cm ³ ).
It is expressed using a unit of individual cells / cm ³ .

In this specification, the bubble density is defined and determined as follows. First, an arbitrary cross section of the foam is observed with an SEM (scanning electron microscope), and the number of bubbles per unit cross-sectional area (1 cm ² ), n (single cells) is determined from the number of bubbles observed in the visual field. Ask for. The magnification of SEM is
As long as the magnification is such that bubbles can be clearly observed, generally,
The magnification is preferably such that about 20 to 50 bubbles can be observed in the visual field, and usually about several hundred to 10,000 times. The 3/2 power of n obtained above is the unit volume (1c
m ³ ), which corresponds to the number of cells in the foam, N (individual cells). On the other hand, for each bubble observed in the field of view of the SEM used for the measurement of the number of bubbles, the longest length was measured, and the average of the values, 2r (cm), was determined, and this was used as the value in the foam. Defined as the average bubble diameter of the bubbles. Assuming that all the bubbles contained in the foam are spheres having a radius of r (cm), the total volume V ₁ (cm ³ ) of the bubbles present in the foam having a unit volume (1 cm ³ ) is calculated by the following equation. calculate. ^{_{V 1 = (4πr 3/3}} ) × volume of plastics material occupying in the foam of N unit volume V
(Cm ³ ) is given by the following equation: V = 1−V ₁ . Therefore, the bubble density in the present invention (number bubbles / cm ³⁾ is the following formula: as defined by the cell density = 1 / {1 / N- 4πr 3/3}.

[0008] The polypropylene resin foam of the present invention comprises:
10¹¹Individual bubbles / cm^ThreeHigher bubble density
Air-tightness from the viewpoint of balance
Degree is 10 ¹²Individual bubbles / cm^ThreeIt is preferable that it is above.
The average bubble diameter is preferably 2 μm or less.

The foam of the present invention is made of a plastic material containing a polypropylene resin as a main component, and the content of the polypropylene resin in the plastic material is 50% by weight.
From the viewpoint of the strength and heat resistance of the foam, the content is preferably 70% by weight or more, more preferably 85% by weight or more.

Examples of the polypropylene resin used in the present invention include a propylene homopolymer and a propylene copolymer containing at least 50 mol% of a propylene monomer unit. As a propylene copolymer,
Binary or ternary copolymers of propylene and ethylene or an α-olefin other than propylene are preferred. α-
Examples of the olefin include linear or branched α-olefins having 4 or more carbon atoms such as 1-butene, 4-methylpentene-1, 1-octene and 1-hexene.
From the viewpoint of copolymerizability with propylene, a linear or branched α-olefin having 10 or less carbon atoms is preferred. From the viewpoint of strength, a propylene homopolymer is preferable,
From the viewpoint of transparency and the like, a propylene-based copolymer is preferred. When a copolymer is used, the content of monomer units other than propylene in the copolymer is preferably 10% by weight or less for ethylene and 30% by weight or less for other α-olefins. Also excellent in strength, from the viewpoint that a foam having fine cells can be obtained, a mixture of a propylene homopolymer and a copolymer of propylene and ethylene, or a copolymer of propylene and an α-olefin other than propylene. Is also preferred. When using such a mixture,
A composition obtained by separately pre-producing a propylene homopolymer and a copolymer of propylene and ethylene or a copolymer of propylene and an α-olefin other than propylene, and then kneading both with a kneader or the like. You may use things,
After performing propylene homopolymerization, propylene and ethylene, or propylene and α-
A so-called block copolymer obtained by copolymerizing an olefin may be used. Of course, the block copolymer may be mixed with a propylene homopolymer, a copolymer of propylene and ethylene, a copolymer of propylene and an α-olefin other than propylene, or the like. The above polypropylene resins may be used alone or in combination of two or more.

[0011] Further, polypropylene resin is disclosed in
It may be a polypropylene resin into which long-chain branching is introduced by low-level electron beam crosslinking as described in JP-A-2-121704, or a polypropylene resin into which an ultrahigh molecular weight component as described below is introduced. .

Examples of the polypropylene resin into which an ultrahigh molecular weight component is introduced include, for example, a crystalline polypropylene polymer (I) having a limiting viscosity of 5 dl / g or more by polymerizing a monomer containing propylene as a main component in the first stage. ), And polymerizing a propylene-based monomer in the second and subsequent steps to continuously produce a crystalline polypropylene-based polymer (II) having an intrinsic viscosity of less than 3 dl / g. Examples of the polymer include. In the ultrahigh molecular weight-containing polymer, the content of the polymer (I) is 0.05% by weight or more and less than 35% by weight in the ultrahigh molecular weight containing polymer from the viewpoint of the melt viscosity during foaming. An ultrahigh molecular weight-containing polymer having an intrinsic viscosity of less than 3 dl / g and an Mw / Mn of less than 10% by weight is more preferable.

When a composition comprising the polypropylene resin and the olefin elastomer is used as a plastic material and subjected to supercritical foaming as described below, a foam having extremely high cell density and extremely fine cells is obtained. Because it is obtained, it is preferable as the plastic material in the present invention.

The olefin-based elastomer includes ethylene-α-olefin copolymer, ethylene-propylene-diene copolymer, polybutadiene and hydrogenated product thereof, styrene-isoprene-styrene copolymer, styrene-ethylene-butadiene- Examples thereof include a styrene copolymer, a styrene-butadiene copolymer, a styrene-ethylene-propylene-styrene copolymer, and a styrene-butadiene-styrene copolymer. As the α-olefin, those similar to the above can be exemplified.

In particular, by foaming a plastic material having a morphology in which an amorphous phase is finely dispersed in a polypropylene resin, a foam having extremely high cell density and extremely fine cells can be obtained.
An olefin-based elastomer-containing polypropylene-based resin composition obtained by mixing an olefin-based elastomer highly finely dispersed in a polypropylene-based resin with the polypropylene-based resin is particularly preferable. Specific examples of such useful olefin-based elastomers include a styrene-butadiene copolymer or a hydrogenated product thereof, and a hydrogenated styrene-butadiene copolymer containing 10% of a styrene monomer unit is preferable from the viewpoint of the morphology. Particularly preferred.

When the foam is composed of an olefin-based elastomer-containing polypropylene-based resin composition, the amount of the olefin-based elastomer per 100 parts by weight of the polypropylene-based resin is preferably at least 0.01 part by weight.
Less than 100 parts by weight is preferred from the viewpoint of oil resistance. Particularly preferably, it is 1 to 18 parts by weight.

The melt flow rate (MFR) of the plastic material used for producing the foam of the present invention is preferably 0.1 or more from the viewpoint of processability, particularly extrudability.
It is preferably 50 or less from the viewpoint that it is easy to maintain a viscosity that can withstand the gas expansion pressure at the time of foaming, suppress foaming, and obtain an ultrafine porous foam.

Further, the plastic material may contain a polyethylene resin. Examples of the polyethylene resin include low-density polyethylene, high-density polyethylene, and linear low-density polyethylene. When a polyethylene resin is blended, the amount is preferably at least 0.01 part by weight, more preferably at least 3 parts by weight, per 100 parts by weight of the plastic material from the viewpoint of the effect. Further, from the viewpoint of the compatibility of the resin with the polypropylene resin, 3
0 parts by weight or less is preferable, and 20 parts by weight or less is more preferable. Further, the MFR of the polyethylene resin is preferably 0.1 or more from the viewpoint of compatibility with the polypropylene resin, and from the viewpoint of suppressing foam breakage and obtaining an ultrafine porous foam, 10 The following is preferred.

The plastic material used for producing the foam of the present invention may be in the form of powder or pellets, and a conventionally known method can be applied for the preparation. For example, a composition comprising a polypropylene-based resin and an olefin-based elastomer can be prepared by a conventionally known method.For example, using a single-screw or twin-screw extruder generally used for kneading, both materials are melted. A method of kneading and extruding to obtain pellets can be used. For kneading, a Banbury-type kneader can be used. The same method can be used when blending the polyethylene resin.

As a method for producing the foam of the present invention using the above-mentioned plastic material, (1) a step of impregnating a plastic material with a supercritical fluid, and (2)
After the step (1), a so-called supercritical foaming method comprising a step of rapidly releasing pressure to obtain a foam can be exemplified.

The supercritical fluid used in the above step (1) is a fluid having a temperature higher than the critical temperature and a pressure higher than the critical pressure. This is the critical state described above.
The substance which is gaseous at normal temperature and normal pressure includes, for example, organic compounds such as butane and pentane, or inorganic compounds such as carbon dioxide, air, hydrogen, nitrogen, neon, and argon. May be used. Of these, carbon dioxide, air, nitrogen, neon, and inert substances such as argon are preferable from the viewpoint of ease of handling, and carbon dioxide, or a mixture of carbon dioxide and other substances, is preferable from the viewpoint of solubility in a polypropylene resin. More preferred.

The amount of impregnation of the plastic material with the substance as a supercritical fluid is appropriately set according to the kind of the substance, the expansion ratio of the target foam, the cell density, and the like. The lower limit of the amount of impregnation of the substance is usually such that fine bubbles are formed at a sufficient expansion ratio. There is no particular upper limit for the amount of impregnation, but it is usually a saturated dissolution amount of the substance in the plastic material or an amount close thereto. The impregnation amount does not necessarily have to reach the saturation dissolution amount. For example, if the substance to be impregnated is carbon dioxide, the impregnation amount is
0.1 parts by weight or more is preferable from the viewpoint that a foam having a high expansion ratio is easily obtained with respect to 0 parts by weight, and 20 parts by weight or less is preferable from the viewpoint of effects. The range is more preferably from 0.1 to 15 parts by weight based on 100 parts by weight of the plastic material.

The impregnation pressure, impregnation temperature, impregnation time and the like for impregnating the plastic material with the supercritical fluid differ depending on the desired impregnation amount. For example, when the critical pressure is about 7.5 MPa
When a supercritical fluid of carbon dioxide is dissolved in a plastic material, the impregnation pressure may be at least this critical pressure, but the amount of dissolved carbon dioxide is sufficient to obtain a foam having fine cells. From the viewpoint that it can be performed, 10 MPa or more is preferable. The upper limit of the impregnation pressure depends on the performance of the apparatus and the like, but is usually about 50 MPa.

The impregnation temperature when impregnating the plastic material with the supercritical fluid is preferably not lower than the critical temperature of the substance which is the supercritical fluid. The upper limit of the impregnation temperature may be any temperature at which the plastic material used does not decompose, and is usually 300 ° C. or less. When a supercritical fluid of carbon dioxide having a critical temperature of about 31 ° C. is used, the impregnation temperature is preferably at least this critical temperature, and from the viewpoint of carbon dioxide permeation rate and productivity of the plastic material, at least 60 ° C. It is preferably 230 ° C. or lower from the viewpoint of the amount dissolved in the plastic material.

The required impregnation time depends on the rate of penetration of the supercritical fluid into the plastic material, and depends on the impregnation pressure and the impregnation temperature. When the impregnation operation is continued, the impregnation amount usually increases up to the saturated dissolution amount.However, the impregnation time is usually set to a time until the impregnation amount of the supercritical fluid reaches the saturation dissolution amount at most, and usually up to several hours. is there. From the viewpoint of productivity, the shorter the impregnation time is, the more preferable it is. It is not always necessary to impregnate until the saturation amount is reached. For example, the impregnation time in the case of carbon dioxide is usually about several minutes to about 5 hours, and is preferably about several minutes to about 3 hours from the viewpoint of the balance between productivity and impregnation amount.

In the step (2), it is preferable to release the pressure in as short a time as possible. If this operation is performed slowly, the energy required for generating cell nuclei may not be provided, and a foam having a desired cell density may not be obtained. Normally, the pressure is released instantaneously from the impregnation pressure to around normal pressure. The phrase “releasing the pressure instantaneously” means that the pressure is reduced from the impregnation pressure to near normal pressure in as short a time as possible. Although it depends on the capacity of the container used for the impregnation, the thickness of the exhaust gas pipe, and the like, the pressure drop time from the impregnation pressure to near normal pressure is usually less than 10 seconds, and preferably about 3 seconds or less.

In the step (2), the temperature at which the pressure is released is not particularly limited, but is preferably equal to or lower than the melting point of the plastic material in order to suppress the breaking of bubbles as much as possible. To achieve the temperature range from the melting point of the plastic material to about 100 ° C.
The temperature at the time of releasing the pressure does not necessarily have to be constant, and usually, the temperature decreases with the release of the pressure. Although it is not always necessary to control the temperature drop, it is preferable to control the temperature drop from the viewpoint of controlling the bubble density.

The rapid release of the pressure in the step (2) may be performed at a higher temperature than the step (1), at a lower temperature than the step (1), or at the step (1). May be performed at the same temperature. In order to achieve a finer cell diameter and a higher cell density, it is preferable to release the pressure in step (2) at a lower temperature than in step (1).

More specifically, for example, the step (1)
The step is performed at a temperature not lower than the critical temperature of the substance to be a supercritical fluid and not higher than the melting point of the plastic material. In step (2), the pressure is rapidly increased at a temperature higher than the temperature of step (1), for example, at a temperature higher than the melting point of the plastic material. To form and grow cell nuclei, and by appropriately controlling the growth of the cell nuclei by utilizing the temperature drop due to rapid release of pressure, a foam can be obtained. Further, in step (1), the plastic material is heated to a temperature equal to or higher than its melting point. In step (2), the plastic material is once cooled to a temperature lower than that in step (1), for example, to a temperature lower than the melting point of the plastic material. The foam can be obtained by rapidly opening to generate cell nuclei and then growing the cell nuclei appropriately. Further, in step (1), the temperature may be lower than the melting point of the plastic material, and step (2) may be performed at that temperature. In order to obtain a finer cell diameter and a higher cell density, it is preferable to release the pressure in step (2) at a lower temperature than in step (1).

Further, in order to more appropriately control the bubble density, in the above step (2), following the process of forming the bubble nucleus, the process of growing the bubble nucleus generated by the rapid pressure release, and the growth of the bubble It is preferable to further control the temperature and the time of both steps of stopping the process.

In the above-described bubble nucleus growth process, the temperature at which the bubble nuclei are grown is controlled in order to minimize bubble breakage.
It is preferable to control the temperature within the range from the crystallization temperature of the plastic material to the melting point. The time for growing the bubble nuclei is appropriately set according to the desired bubble density, and is usually 20 seconds to 30 seconds.

In order to suppress bubble breakage due to excessive growth of bubbles, it is preferable to control not only the temperature of the bubble nucleus growth process but also the temperature of the bubble growth stop process. The temperature at which the growth of the bubbles is stopped is preferably equal to or lower than the crystallization temperature of the plastic material, and it is preferable to sufficiently cool the entire foam until the temperature becomes equal to or lower than the crystallization temperature.

An ultrafine porous foam can be obtained by the method exemplified above. The obtained foam can be suitably used for, for example, automotive materials, food trays or containers, building materials, cushioning materials, heat insulating materials, and the like.

[0034]

The foam of the present invention is an ultrafine porous foam made of a plastic material containing a polypropylene resin and having a cell density of 10 ¹¹ cells or more per 1 cm ^{3 of} the plastic material. While maintaining the characteristics of the contained polypropylene resin, a foam excellent in balance between lightness, rigidity, heat insulation and the like is obtained. Also, by blending an olefin-based elastomer with a polypropylene-based resin and finely dispersing the elastomer in a polypropylene-based resin matrix, a fine bubble diameter of about 2 μm or less can be achieved, and the bubble density can be further increased. .

[0035]

EXAMPLES The present invention will be further described with reference to the following examples, which should not be construed as limiting the present invention.

(Measurement Method) <Average Cell Diameter> After cooling the foam with liquid nitrogen, the foam was cut with a razor and its cross section was photographed with a scanning electron microscope. The magnification was adjusted so that about 50 bubbles could be seen in the field of view of the electron microscope. The maximum length of the bubbles in the visual field was measured from the photograph of the photographed cross section of the foam, and the average value was further determined to be the average bubble diameter (2r).

<Bubble Density> Electrons used for measuring the average bubble diameter
1cm cross-sectional area of foam using micrograph ^TwoYou
Calculate the number of air bubbles (n) and multiply it by 3/2 to obtain a unit
The number of bubbles per product (N) was calculated. This number of bubbles (N)
From the average cell diameter (2r) obtained above,
Number of bubbles per unit volume of plastic material
Chi cell density (cell / cm)^Three).

Examples 1 to 6 90% by weight of polypropylene (polypropylene noblene W101 MFR8 to 10, Sumitomo Chemical Co., Ltd., propylene homopolymer) and hydrogenated styrene-butadiene copolymer rubber (SBR) (1320P manufactured by Nippon Synthetic Rubber Co., Ltd.) Styrene monomer unit content 10%, MFR 3.5) 1
An olefin-based elastomer-containing polypropylene-based resin composition obtained by melt-mixing 0% by weight with a laboplast mill was used as a raw material resin. The raw material resin was press-molded to prepare six sheets (thickness: 1.5 mm, length: 4 cm × width: 2 cm) (press conditions: preheating for 3 minutes at 230 ° C., pressing for 1 minute, and keeping at 30 ° C.). (Cool for 5 minutes on a press platen).
After placing the sheet in a pressure vessel equipped with a pressure gauge and an exhaust pressure valve, each of which is separately heated in advance to a predetermined temperature, and then closing the lid, carbon dioxide is pumped with a carbon dioxide pressure vessel. The carbon dioxide was brought into a supercritical state by being pressed into the inside, and the sheet was impregnated at the impregnation pressure, the impregnation temperature and the impregnation time shown in Table 1. The impregnation operation was continued while maintaining the impregnation pressure at the moment when the pressure gauge reached the predetermined impregnation pressure. After a predetermined impregnation time has elapsed, carbon dioxide was discharged from the pressure-resistant container via the exhaust pressure valve, and the pressure in the pressure-resistant container was released from the impregnation pressure to normal pressure (about 0.1 MPa) at once (opening time: 2 to 2 MPa). 3 seconds).

After the pressure in the pressure vessel reached normal pressure, the obtained foam was taken out and the average cell diameter and cell density were measured. The results are shown in Table 1.

[0040]

[Table 1] 1) Gauge pressure

[Brief description of the drawings]

FIG. 1 is an electron micrograph of a cross section of a foam obtained in Example 1.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B29K 23:00 105: 04 F term (Reference) 4F074 AA09 AA13 AA24 AA24A AA24B AA25A AA25B AA32A BA31 BA32 BA33 BA37 BA39 CA24 CC22X DA02 DA03 DA32 DA33 DA34 DA35 DA50 DA58 4F212 AA03 AA11 AA12 AA45 AA47 AB02 AG20 AH17 AH47 UA17 UB01 UC05 UC06 UN11 4J002 AC032 AC112 BB052 BB121 BB141 BB151 BB152 BC052 BP031 BP021

Claims (5)

[Claims] 1. A plastic material containing a polypropylene resin, wherein the cell density is 1 cm.
Foam with more than 10 ¹¹ cells per ³ 2. The plastic material has a cell density of 1 cm ^3.
2. The foam according to claim 1, wherein the number of bubbles per foam is 10 ¹² or more. 3. The foam according to claim 1, wherein said plastic material further contains a polyolefin-based elastomer. 4. The foam according to claim 3, wherein the polyolefin-based elastomer is a hydrogenated styrene-butadiene copolymer or a propylene-butene copolymer. 5. The foam according to claim 1, which has an average cell diameter of 2 μm or less.