JP2016180085A - Acrylic resin foam and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an acrylic resin foam with a small variation in flexure strength among areas and a manufacturing method therefor.SOLUTION: There is provided an acrylic resin foam having a planar shape and a constitutional unit derived from an acrylic monomer and a constitutional unit derived from styrene and satisfying a following relation when measured by a predetermined measurement method of density. 0≤[(maximum density of test piece)-(minimum density of test piece)]/(average density of test piece)≤0.25. The measurement method of density: total 9 foam pieces are manufactured by dividing an acrylic resin foam into 9 parts by 3×3 in a face direction. The 9 foam pieces are cut so that each of them is segmented in thickness direction into five equal parts to prepare total 45 test pieces. Each density of the 45 test pieces is measured by a method according to JIS K 7222:1999. Maximum density, minimum density and average density of the 45 test pieces are calculated from measured each density of 45 test pieces.SELECTED DRAWING: None

Description

  The present invention relates to an acrylic resin foam and a method for producing the same.

  Since the acrylic resin foam has high strength and is excellent in lightness and heat insulation, for example, a fiber reinforced plastic (FRP) (carbon fiber reinforced plastic (CFRP) or the like) is bonded to the surface, It is used as a member for constituting an automobile interior material, an automobile exterior material, a wall material for a cold room of a freight vehicle, a blade of wind power generation, a table for an X-ray machine for transmitting X-rays, and the like.

As a manufacturing method of an acrylic resin foam, the method shown below is mentioned, for example.
A polymerizable solution in which an acrylic monomer, urea as a foaming agent, and a radical polymerization initiator are mixed is prepared, the polymerizable solution is poured into a mold, and the entire mold is heated to polymerize the acrylic monomer. A foamable acrylic polymer is obtained. Thereafter, the foamable acrylic polymer obtained is heated to a temperature higher than the temperature at which urea decomposes in a mold, and gas is generated to foam, thereby obtaining an acrylic resin foam (for example, Patent Document 1). ).

JP 2014-198795 A

However, in the foam obtained by the conventional manufacturing method as in Patent Document 1, characteristics such as bending strength vary from region to region, and for example, in applications such as automobile interior materials that cannot be combined with FRP. There is a problem that it may not be used.
Then, this invention is made | formed in view of the said situation, and makes it a subject to obtain the acrylic resin foam with the small dispersion | variation in bending strength for every area | region.

  The inventors of the present invention have clarified that, in the acrylic resin foam having a large difference in bending strength for each region, the density varies depending on the region of the foam. As a result of further studies, it has been found that when the density of the foam has a specific relationship, an acrylic resin foam having a small variation in bending strength for each region can be obtained, and the present invention has been completed.

That is, the acrylic resin foam of the present invention has a flat plate shape, and in the acrylic resin foam having a structural unit derived from an acrylic monomer and a structural unit derived from styrene, bubbles are exposed on the surface, It is characterized by satisfying the relationship of the following formula (1) when measured by the following density measuring method.
<Density measurement method>
Procedure (1a): Acrylic resin foam is divided into 3 × 3 in 9 in the surface direction to produce a total of 9 foam pieces.
Procedure (2a): The nine foam pieces produced in the procedure (1a) are cut so that the thickness direction is divided into five equal parts, and a total of 45 test pieces are produced.
Procedure (3a): Each density of the 45 test pieces prepared in the procedure (2a) is measured by a method in accordance with JIS K 7222: 1999.
Procedure (4a): The maximum density, the minimum density, and the average density of 45 test pieces are obtained from the densities of 45 test pieces measured in the procedure (3a).
0 ≦ [(maximum density of test piece) − (minimum density of test piece)] / (average density of test piece) ≦ 0.25
... (1)

The acrylic resin foam of the present invention has a flat plate-like shape, and the acrylic resin foam having a structural unit derived from an acrylic monomer and a structural unit derived from styrene has a skin layer on the surface and has the following density. It satisfies the relationship of the following formula (1) when measured by the measuring method.
<Density measurement method>
Procedure (1b): Acrylic resin foam with the skin layer removed and air bubbles exposed on the surface is divided into 9 × 3 × 3 in the surface direction to produce a total of 9 foam pieces.
Procedure (2b): Each of the nine foam pieces produced in the procedure (1b) is cut so that the thickness direction is divided into five equal parts to produce a total of 45 test pieces.
Procedure (3b): Each density of the 45 test pieces prepared in the procedure (2b) is measured by a method in accordance with JIS K 7222: 1999.
Procedure (4b): The maximum density, the minimum density, and the average density of 45 test pieces are obtained from the densities of 45 test pieces measured in the procedure (3b).
0 ≦ [(maximum density of test piece) − (minimum density of test piece)] / (average density of test piece) ≦ 0.25
... (1)

  The acrylic resin foam of the present invention is suitably used for automobile interior materials.

  The method for producing an acrylic resin foam according to the present invention is a method for producing the acrylic resin foam according to the present invention, wherein a polymerizable monomer containing an acrylic monomer and styrene, a thermal decomposable foaming agent, and a polymerization start And heating the polymerizable solution containing an agent to polymerize the polymerizable monomer to obtain a foamable acrylic polymer, and foam the foamable acrylic polymer obtained in the polymerization step The foaming step includes foaming the foamable acrylic polymer between the heat plates of a mold having a pair of upper and lower heat plates spaced at an arbitrary distance, and the foam is placed on the upper heat plate. And a pressing force releasing operation for pressing the foam with the upper heat plate and then releasing the force with which the upper heat plate presses the foam.

  The pressing force releasing operation is preferably performed when more than half of the processing time of the foaming process has elapsed.

The polymerizable monomer preferably contains 40 to 70% by mass of methyl (meth) acrylate, 15 to 30% by mass of (meth) acrylic acid, and 10 to 30% by mass of styrene.
In addition, it is preferable that the polymerizable monomer further contains 1 to 10% by mass of maleic anhydride and 1 to 10% by mass of (meth) acrylamide.

The acrylic resin foam of the present invention has a small variation in bending strength for each region.
According to the method for producing an acrylic resin foam of the present invention, an acrylic resin foam having a small variation in bending strength for each region can be easily produced.

(Acrylic resin foam)
The acrylic resin foam of the present invention has a flat plate shape and has a structural unit derived from an acrylic monomer and a structural unit derived from styrene.
Examples of the flat acrylic resin foam include a rectangular parallelepiped shape and a disc shape, and the thickness or size thereof is appropriately determined depending on the application.
The acrylic resin foam of the embodiment is, for example, a foamable acrylic polymer containing a copolymer having a structural unit derived from an acrylic monomer and a structural unit derived from styrene, and a pyrolytic foaming agent. Examples include foamed ones.

<Foaming acrylic polymer>
The form of the foamable acrylic polymer is appropriately determined according to the form of the target acrylic resin foam. In the present invention, since the acrylic resin foam has a flat plate shape, the foamable acrylic polymer is preferably a flat plate-shaped molded body.

The foamable acrylic polymer of the embodiment includes a polymer obtained by polymerizing a polymerizable monomer containing an acrylic monomer as a main component (hereinafter also referred to as “polymer A”), a pyrolytic foaming agent, It contains.
“Mainly containing an acrylic monomer” means that the ratio of the acrylic monomer to the total amount (100% by mass) of the polymerizable monomer is 50% by mass or more.
The foamable acrylic polymer is not particularly limited, and a foamable acrylic polymer usually used in the production of a foamed molded product can be used. The foamable acrylic polymer in the present invention may be one in which at least an acrylic monomer and styrene are used as a polymerizable monomer, and other monomers other than these are further used.

≪Polymer A≫
Examples of acrylic monomers include (meth) acrylic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, itaconic anhydride, crotonic acid, methyl (meth) acrylate, ethyl (meth) acrylate, (meth ) Butyl acrylate, lauryl (meth) acrylate, 2-ethylhexyl (meth) acrylate, cyclohexyl (meth) acrylate, benzyl (meth) acrylate, (meth) acrylamide, maleic acid amide, maleic acid imide, etc. It is done.
In the present specification, “(meth) acrylic acid” means one or both of methacrylic acid and acrylic acid, and the same applies to other compounds.

The acrylic monomer is preferably a water-soluble acrylic monomer, more preferably (meth) acrylic acid, and particularly preferably methacrylic acid in that it exhibits excellent solubility in the pyrolytic foaming agent.
Further, as the acrylic monomer, methyl (meth) acrylate is preferable, and methyl methacrylate is more preferable in that the foamability of the foamable acrylic polymer becomes better.
An acrylic monomer may be used individually by 1 type, and may be used in combination of 2 or more type.

The other monomer may be any monomer that is copolymerizable with an acrylic monomer or styrene, and can be appropriately selected and used for the purpose of modifying the foam.
Another monomer may be used individually by 1 type and may be used in combination of 2 or more type.

  As the polymer A, a structural unit derived from methyl (meth) acrylate (hereinafter referred to as “unit (a1)”) and a structural unit derived from (meth) acrylic acid (hereinafter referred to as “unit (a2)”). And a structural unit derived from styrene (hereinafter referred to as “unit (a3)”). Among them, since the effect of the present invention is further enhanced, the units (a1) to (a3), the structural unit derived from maleic anhydride (hereinafter referred to as “unit (a4)”), and (meth) acrylamide are included. Those having a derived structural unit (hereinafter referred to as “unit (a5)”) are more preferable.

40-70 mass% is preferable and, as for the ratio of the unit (a1) with respect to all the structural units (100 mass%) of the polymer A, 45-65 mass% is more preferable. If the ratio of the unit (a1) is at least the preferred lower limit of the above range, the foamability of the foamable acrylic polymer becomes better. If the proportion of the unit (a1) is not more than the preferred upper limit of the above range, it is easy to achieve a balance with other structural units.
15-30 mass% is preferable and, as for the ratio of the unit (a2) with respect to all the structural units (100 mass%) of the polymer A, 20-25 mass% is more preferable. When the proportion of the unit (a2) is at least the preferred lower limit of the above range, the solubility in the pyrolytic foaming agent is further enhanced. If the ratio of the unit (a2) is not more than the preferable upper limit of the above range, it is easy to achieve a balance with other structural units.
10-30 mass% is preferable and, as for the ratio of the unit (a3) with respect to all the structural units (100 mass%) of the polymer A, 15-25 mass% is more preferable. If the ratio of the unit (a3) is at least the preferred lower limit of the above range, the foamability is further improved. If the proportion of the unit (a3) is not more than the preferable upper limit of the above range, it is easy to achieve a balance with other structural units.

When the polymer A has a unit (a4), the ratio of the unit (a4) to the total structural unit (100% by mass) of the polymer A is preferably 1 to 10% by mass, and more preferably 5 to 10% by mass. When the proportion of the unit (a4) is at least the preferred lower limit of the above range, the effect of the present invention is further enhanced. If the proportion of the unit (a4) is not more than the preferred upper limit of the above range, it is easy to achieve a balance with other structural units.
When the polymer A has a unit (a5), the ratio of the unit (a5) to the total structural unit (100% by mass) of the polymer A is preferably 1 to 10% by mass, and more preferably 2 to 8% by mass. If the ratio of the unit (a5) is at least the preferred lower limit of the above range, the effect of the present invention is further enhanced. If the proportion of the unit (a5) is not more than the preferable upper limit value in the above range, it is easy to achieve a balance with other structural units.

  As the polymer A, the unit (a1) 40 to 70% by mass, the unit (a2) 15 to 30% by mass, and the unit (a3) 10 to 10% with respect to all the structural units (100% by mass) of the polymer A. What has 30 mass% is still more preferable. Among them, as the polymer A, the unit (a1) is 40 to 70% by mass, the unit (a2) is 15 to 30% by mass, and the unit (a3) is based on all the structural units (100% by mass) of the polymer A. What has 10-30 mass%, a unit (a4) 1-10 mass%, and a unit (a5) 1-10 mass% is especially preferable.

≪Pyrolytic foaming agent≫
The pyrolyzable foaming agent is preferably one that decomposes at 65 ° C. or higher to generate gas, and more preferably one that decomposes at 100 to 180 ° C. to generate gas.
Pyrolytic foaming agents include urea, urea derivatives, dinitrosopentamethylenetetramine, amidoguanidine, trimethylenetriamine, paratoluenesulfonhydrazine, azodicarbonamide, thiourea, ammonium chloride, dicyandiamide, dioxane, hexane, chloral hydrate Citric acid and the like. Among these, urea and urea derivatives are more preferable, and urea is particularly preferable.
A thermal decomposition type foaming agent may be used individually by 1 type, and may be used in combination of 2 or more type.

  1-15 mass parts is preferable with respect to 100 mass parts of the polymer A, and, as for content of the thermal decomposition type foaming agent in a foamable acrylic polymer, 2-10 mass parts is more preferable. If the content of the pyrolytic foaming agent is at least the preferred lower limit of the above range, an acrylic resin foam excellent in lightness can be easily obtained. If the content of the pyrolytic foaming agent is not more than the preferable upper limit of the above range, it becomes easy to dissolve the pyrolytic foaming agent in the polymerizable monomer. Further, if the content of the pyrolytic foaming agent is not more than the preferred upper limit of the above range, the pyrolytic foaming agent hardly remains in the foam, and foam breakage hardly occurs during heat foaming.

<Acrylic resin foam>
The acrylic resin foam of the present invention is flat and includes those in which bubbles are exposed on the entire surface and those having a skin layer on at least a part of the surface.
Here, the “skin layer” is defined as a layer on the surface of the foam in which bubbles are not exposed.
The shape of the acrylic resin foam may be appropriately determined according to the application and the like, and examples thereof include a plate shape.

  The acrylic resin foam of the present invention satisfies the relationship of the following formula (1) when the density of an arbitrary region is measured by the following measuring method.

0 ≦ [(maximum density of test piece) − (minimum density of test piece)] / (average density of test piece) ≦ 0.25
... (1)

[Method for measuring density of acrylic resin foam]
Procedure (1): A flat acrylic resin foam having air bubbles exposed on the surface is divided into 9 × 3 × 3 in the surface direction to produce a total of 9 foam pieces.
For example, when the shape of the acrylic resin foam is a rectangular parallelepiped, it is cut so that the longitudinal direction is divided into three equal parts and the short direction is divided into three equal parts to produce a total of nine foam pieces. .
For example, when the shape of the acrylic resin foam is a disk, it is cut so that an arbitrary diameter direction and a direction perpendicular to the diameter direction are divided into three equal parts, for a total of nine pieces A foam piece is prepared.
In the case of having a skin layer, an acrylic resin foam in which the skin layer is removed and air bubbles are exposed on the surface is used. In order to expose bubbles on the surface of the acrylic resin foam, for example, the skin having a thickness of 2 mm is removed from the surface of the skin layer.

Procedure (2): Each of the nine foam pieces produced in the procedure (1) is cut so that the thickness direction is divided into five equal parts, and a total of 45 test pieces are produced.
Procedure (3): The density of each of the 45 test pieces prepared in Procedure (2) is measured by a method based on JIS K 7222: 1999.
Procedure (4): The maximum density, the minimum density, and the average density of 45 test pieces are obtained from the densities of 45 test pieces measured in the procedure (3).

The ratio represented by [(maximum density of test piece) − (minimum density of test piece)] / (average density of test piece) (hereinafter referred to as “(maximum−minimum) / average density”) is 0. ~ 0.25.
When the (maximum-minimum) / average density is within the above range, the density variation in the acrylic resin foam is small and the density is more uniform. Since the variation in density becomes smaller, the value of (maximum−minimum) / average density is preferably as small as possible.

  In the acrylic resin foam of the present invention, it can be controlled to satisfy the relationship of the above formula (1) by selecting a polymerizable monomer and operating in a foaming step in the production method as described later.

In addition, the bending strength of the acrylic resin foam of the present invention is a ratio represented by [(maximum bending strength of the test piece) − (minimum bending strength of the test piece)] / (average bending strength of the test piece) (below). “(Maximum−minimum) / average intensity”) is preferably 0 to 0.30.
If the (maximum-minimum) / average strength is within the above preferred range, the strength of the entire foam becomes more uniform. For example, when integrally molded with CFRP or the like, it can be molded under certain conditions. Since the variation of the bending strength for each region becomes smaller, the value of (maximum−minimum) / average strength is preferably as small as possible.
The bending strength of the acrylic resin foam of the present invention is measured by the method described in Examples described later.

As described above, in the acrylic resin foam of the present invention, when the relationship of the above equation (1) is satisfied, the density variation in the foam is reduced, and the density becomes more uniform. For this reason, the acrylic resin foam of this invention has a small dispersion | variation in the bending strength for every area | region.
Such an acrylic resin foam is, for example, a member for constituting an automobile interior material, an automobile exterior material, a wall material for a cold storage room of a freight vehicle, a blade for wind power generation, and an X-ray machine base for transmitting X-rays. It is suitable for such as. Such an acrylic resin foam is particularly suitable for automobile interior materials having a narrow density management range.

(Method for producing acrylic resin foam)
The manufacturing method of the acrylic resin foam of this invention is a manufacturing method of the said acrylic resin foam of this invention, Comprising: It has a superposition | polymerization process and a foaming process.

<Polymerization process>
In the polymerization step, a polymerizable solution containing an acrylic monomer and a polymerizable monomer containing styrene, a thermally decomposable foaming agent, and a polymerization initiator is heated to polymerize the polymerizable monomer to form a foamable acrylic system. A polymer is obtained.
As a method for obtaining such a foamable acrylic polymer, a production method usually used for the production of a foamable acrylic polymer used for the production of a foamed molded product can be employed. For example, a polymerizable monomer, a pyrolytic foaming agent, a polymerization initiator, and other components, if necessary, used for the production of the above-mentioned polymer A are mixed to form a polymerizable solution, and the polymerizable solution is formed into a mold having a predetermined shape. Examples of the method include a method of curing in a frame and heating to polymerize the polymer.

≪Polymerizable solution≫
The polymerizable solution contains a polymerizable monomer, a thermally decomposable foaming agent, a polymerization initiator, and other components as necessary.

  As the polymerizable monomer, an acrylic monomer, styrene, and, if necessary, other monomers are used. About an acrylic monomer and another monomer, the thing similar to the acrylic monomer mentioned above and another monomer is mentioned.

The content ratio of the acrylic monomer in the polymerizable monomer (100% by mass) is, for example, as follows.
The content ratio of methyl (meth) acrylate in the polymerizable monomer (100% by mass) is preferably 40 to 70% by mass, and more preferably 45 to 65% by mass.
The content ratio of (meth) acrylic acid in the polymerizable monomer (100% by mass) is preferably 15 to 30% by mass, and more preferably 20 to 25% by mass.

The content ratio of maleic anhydride in the polymerizable monomer (100% by mass) is preferably 1 to 10% by mass, and more preferably 5 to 10% by mass.
The content ratio of (meth) acrylamide in the polymerizable monomer (100% by mass) is preferably 1 to 10% by mass, and more preferably 2 to 8% by mass.

  10-30 mass% is preferable and, as for the content rate of the styrene in a polymerizable monomer (100 mass%), 15-25 mass% is more preferable.

As a polymeric solution, what contains 40-70 mass% of (meth) acrylic acid methyl as a polymerizable monomer (100 mass%), 15-30 mass% of (meth) acrylic acid, and 10-30 mass% of styrene Is preferred.
Among these, as a polymerizable solution, as a polymerizable monomer (100 mass%), methyl (meth) acrylate 40-70 mass%, (meth) acrylic acid 15-30 mass%, styrene 10-30 mass%, More preferred are those containing 1 to 10% by mass of maleic anhydride and 1 to 10% by mass of (meth) acrylamide.

The content of the thermally decomposable foaming agent in the polymerizable solution is preferably 0.5 to 30 parts by mass with respect to 100 parts by mass of the polymerizable monomer.
If the content of the pyrolytic foaming agent is less than the preferred lower limit of the above range, the foaming factor of the resulting acrylic resin foam may be reduced, and the lightness may be impaired, and when the preferred upper limit of the above range is exceeded. , It becomes difficult to uniformly dissolve the pyrolytic foaming agent in the polymerizable solution, the pyrolytic foaming agent tends to remain in the resulting acrylic resin foam, and foam breakage tends to occur. There is a risk.
When urea is used as the pyrolytic foaming agent, the content of urea in the polymerizable solution is preferably 1 to 15 parts by mass with respect to 100 parts by mass of the polymerizable monomer.

As the polymerization initiator, a redox polymerization initiator is preferable.
Examples of the redox polymerization initiator include t-butyl hydroperoxide, cumene hydroxy peroxide, diisopropylbenzene hydroperoxide, p-menthane hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, and the like. Is mentioned.
A polymerization initiator may be used individually by 1 type, and may be used in combination of 2 or more type.
The content of the polymerization initiator in the polymerizable solution is preferably 0.1 to 5 parts by mass with respect to 100 parts by mass of the polymerizable monomer.

  The polymerizable solution contains, as other components, a polymerization initiation aid, a substance for adding chloride ions, a polymerization inhibitor, a dehydrating agent, a foaming agent other than a pyrolytic foaming agent, a bubble regulator, a plasticizer, and the like. Also good.

Examples of the polymerization initiation aid include sulfinic acid metal salts and amine compounds.
Examples of the sulfinic acid metal salt include sodium benzenesulfinate, sodium toluenesulfinate, sodium hydroxymethanesulfinate and the like. Among these, as the sulfinic acid metal salt, sodium hydroxymethanesulfinate is preferable.
Examples of the amine compound include N, N-dimethylaniline and triethylamine.
One type of polymerization initiation assistant may be used alone, or two or more types may be used in combination.
The content of the polymerization initiation assistant in the polymerizable solution is preferably 0.1 to 5 parts by mass with respect to 100 parts by mass of the polymerizable monomer. Moreover, the usage-amount of a polymerization start adjuvant is 0.1 to 5 times by mass ratio with respect to a polymerization initiator.

Any substance capable of adding chloride ions to the polymerizable solution may be used as the substance for adding chloride ions. Chloride ions contribute to the acceleration of the polymerization reaction.
Examples of substances for adding chloride ions include sodium chloride, copper chloride, ferric chloride, silver chloride, gold chloride, hydrochloric acid, imidazolium salt surfactants, quaternary ammonium salt surfactants, alkylbetaines. Type amphoteric surfactants and the like.

  Examples of the imidazolium salt type surfactant include 1,3-dimethylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-butyl-2,3-dimethylimidazolium chloride, 1-ethyl-3. -Methylimidazolium chloride, 1-methyl-3-n-octylimidazolium chloride, 1-methyl-1-hydroxyethyl-2-tallow alkyl-imidazolium chloride and the like.

  Examples of the quaternary ammonium salt type surfactant include cetyltrimethylammonium chloride, lauryltrimethylammonium chloride, dodecyltrimethylammonium chloride, stearyltrimethylammonium chloride, coconut alkyltrimethylammonium chloride, beef tallow alkyltrimethylammonium chloride, behenyltrimethylammonium chloride. , Polydiallyldimethylammonium chloride, dialkyldimethylammonium chloride and the like.

  Examples of the alkylbetaine-type amphoteric surfactant include lauryldimethylaminoacetic acid betaine and myristylbetaine.

Among them, as the substance for adding chloride ions, at least one selected from the group consisting of sodium chloride, hydrochloric acid and quaternary ammonium salt type surfactants is preferable, and quaternary ammonium salt type surfactants are more preferable.
Of the quaternary ammonium salt type surfactants, cetyltrimethylammonium chloride and lauryltrimethylammonium chloride are more preferable.

The substance for adding chloride ions may be used alone, or two or more kinds may be used in combination.
The content of the substance for adding chloride ions in the polymerizable solution is preferably 0.005 to 5 parts by mass with respect to 100 parts by mass of the polymerizable monomer.

Examples of the polymerization inhibitor include alkaline earth metal salts such as calcium formate.
Examples of the dehydrating agent include sulfates such as anhydrous sodium sulfate and anhydrous magnesium sulfate; zeolite (molecular sieve) and the like.
Examples of the foaming agent other than the pyrolytic foaming agent include a physical foaming agent having a boiling point of 65 ° C. or higher, and a physical foaming agent having a boiling point of 65 ° C. to 180 ° C. is preferable. Examples of the physical foaming agent include alcohols such as isopropanol, cyclopentanol, ethanol, 1-propanol, 2-methyl-2-propanol, and 2-ethyl-1-hexanol.
Examples of the air conditioner include powdered inorganic substances such as metal oxides and diatomaceous earth.
Examples of the plasticizer include sulfonic acid phenyl ester.

≪Polymerization operation≫
When the polymerizable monomer is polymerized by heating the polymerizable solution, the polymerization is preferably performed in a nitrogen atmosphere. The polymerization temperature is preferably 35 to 70 ° C, and more preferably 40 to 60 ° C.
Such polymerization may be performed at a constant polymerization temperature, or may be performed by changing the polymerization temperature in the range of 35 to 70 ° C.
The polymerization time is preferably 10 to 45 hours, more preferably 10 to 35 hours, and further preferably 15 to 25 hours. If the polymerization time is at least the preferred lower limit of the above range, the polymerization reaction will proceed sufficiently and voids and the like will not easily occur. Productivity will become high if polymerization time is below the preferable upper limit of the said range.

When polymerizing the polymerizable solution by heating in the mold, the mold may be appropriately selected according to the desired shape.
It is preferable to filter the polymerizable solution using a filter member having a mesh having a mesh size of less than 0.300 mm before pouring into the mold. Thereby, the foreign material in a polymeric solution is removed and generation | occurrence | production of the void etc. resulting from this foreign material can be suppressed.
Examples of the filtering member having a mesh with a mesh size of less than 0.300 mm include a 60 mesh strainer (mesh size 0.300 mm), a 200 mesh strainer (mesh size 0.077 mm), and the like.

<Foaming process>
In the foaming step, the foamable acrylic polymer obtained in the polymerization step is foamed to obtain an acrylic resin foam.
The foaming step is performed by foaming the foamable acrylic polymer between the heat plates of a mold having a pair of upper and lower heat plates separated by an arbitrary distance, contacting the foam with the upper heat plate, This includes a pressing force releasing operation of pressing the foam with the heat plate and then releasing the force with which the upper heat plate presses the foam.

  In the production method according to the present invention, when producing a flat acrylic resin foam, a flat foamable acrylic resin is provided between the heat plates of a mold having a pair of upper and lower heat plates separated by an arbitrary distance. A polymer is put, and the foamable acrylic polymer is heated above the thermal decomposition temperature of the pyrolytic foaming agent by the heat plate.

When the foamable acrylic polymer is heated by the heat plate, the foamable acrylic polymer may be heated in a state where a lubricant is present on the surface of the foamable acrylic polymer. By heating in a state where a lubricant is present on the surface of the foamable acrylic polymer, it is possible to better prevent the surface of the acrylic resin foam from being cracked.
The method of applying the lubricant is not particularly limited, and examples thereof include a method of applying manually using a brush, cloth, or the like.
Examples of the lubricant include dimethyl silicone and methylphenyl silicone. One type of lubricant may be used alone, or two or more types may be used in combination. The coating amount of the lubricant is appropriately determined in consideration of the expansion ratio of the foamable acrylic polymer or the type of the lubricant.

  The mold used for heat foaming is not particularly limited as long as it has a pair of upper and lower heat plates, and may be appropriately selected according to the shape of the target foam. A foam having a desired shape can be obtained by heating and foaming the foamable acrylic polymer in a mold having a desired inner method.

The heating temperature at the time of foaming the foamable acrylic polymer is equal to or higher than the temperature at which the foamable acrylic polymer is softened, and the thermal decomposition temperature of the pyrolytic foaming agent contained in the foamable acrylic polymer. That's it.
For example, when the pyrolytic foaming agent is urea (thermal decomposition temperature 135 ° C.), the heating temperature is preferably 135 ° C. or higher, and more preferably 145 ° C. or higher. Foaming progresses more efficiently as the heating temperature is higher. The heating temperature is preferably 200 ° C. or lower, and more preferably 180 ° C. or lower. If the heating temperature is not more than the upper limit value, the foam is unlikely to be thermally deteriorated.

  The foamable acrylic polymer is foamed by being heated to a temperature equal to or higher than the thermal decomposition temperature of the thermal decomposition type foaming agent. While the foamable acrylic polymer is foamed, it continues to expand on the lower heat plate, and the produced foam comes into contact with the upper heat plate. The foam is gradually pressed by the upper and lower heat plates.

Next, for example, the force by which the thermal plate presses the foam is released by increasing the distance between the pair of thermal plates (pressing force releasing operation).
Such pressing force releasing operation is preferably performed when 50% or more of the entire processing time of the foaming process has elapsed, more preferably 50 to 90% of the total processing time, and even more preferably the entire processing time. This is performed when 50 to 80% of the processing time elapses.
By performing this pressing force release operation when 50% or more of the entire processing time has elapsed, the variation in the density of the acrylic resin foam is further suppressed, and the density is likely to be more uniform.
By performing this pressing force release operation when a preferable upper limit value or less of the above range has elapsed, an acrylic resin foam molded in a sufficiently foamed state can be easily obtained.

After the force pressing the foam is released, foaming continues and the foam continues to expand and its volume increases.
The time for releasing the pressing force to the foam by the heat plate is preferably 1 to 5 minutes.
Before the pressing force release operation, the force with which the heat plate presses the foam is preferably 0.1 to 1.5 MPa. Then, after the pressing force releasing operation, the force with which the heat plate presses the foam is preferably reduced to 0 MPa.

The force with which the heat plate presses the foam before the pressing force releasing operation is measured, for example, as follows.
A pressure measuring film is attached to the center of the surface of the foamable acrylic polymer (flat plate) before foaming. For the pressure measurement film, EA729Z-2 manufactured by Esco Corporation can be used.
Next, the foaming process is operated. By pressing force release operation, the mold is opened (the distance between the pair of heat plates is increased), and the pressure applied to the pressure measurement film is measured (this pressure is measured by the The force that presses the foam ”.
Note that when the generated foam and the heat plate on the upper side of the mold are separated by the pressing force releasing operation, the pressing force applied to the foam by the heat plate becomes 0 MPa.

Next, after the pressing force release operation, the foam is sandwiched between a pair of heat plates, and the foam is heated to a temperature equal to or higher than the thermal decomposition temperature of the pyrolytic foaming agent in a state where the foam is pressed by the heat plate. Let
After the pressing force releasing operation, the force for pressing the foam by the heat plate is appropriately determined in consideration of the foaming ratio of the foam.
The expansion ratio in the entire foaming process is appropriately determined according to the use of the acrylic resin foam.

As described above, according to the method for manufacturing an acrylic resin foam according to the present invention, the pressing force applied to the foam by the heat plate is released by the pressing force releasing operation in the foaming process. Thereby, an acrylic resin foam having a more uniform density can be easily produced.
Thus, the residual stress in a foam is relieved by once reducing the pressure added to a foam in the middle of a foaming process. For this reason, the variation in density in the foam is reduced, and the density can be made uniform, whereby an acrylic resin foam having a small variation in bending strength for each region can be obtained.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by the following description.
Example 1
The following raw materials were used.
A polymerizable monomer composed of 47% by mass of methyl methacrylate, 25% by mass of methacrylic acid, 16% by mass of styrene, 8.0% by mass of maleic anhydride, and 4.0% by mass of methacrylamide was used.
Urea was used as a pyrolytic foaming agent.
As the polymerization initiator, t-butyl hydroperoxide (“Perbutyl H-69” manufactured by NOF Corporation) was used.
Cetyltrimethylammonium chloride (“Nissan Cation PB-40R” manufactured by NOF Corporation) was used as a substance for adding chloride ions. Calcium formate was used as a polymerization inhibitor. Sodium sulfate was used as a dehydrating agent. As the plasticizer, sulfonic acid phenyl ester (manufactured by LANXESS, “Mesamol”) was used.
Dimethyl silicone (“TMS650” manufactured by Momentive Performance Materials Japan LLC) was used as a lubricant.

Polymerization process:
100 parts by weight of a polymerizable monomer, 5.0 parts by weight of a pyrolytic foaming agent, 0.5 parts by weight of a polymerization initiator, 0.1 parts by weight of a substance for adding chloride ions, and 0.2 parts by weight of a polymerization inhibitor Part, 2.0 parts by weight of a dehydrating agent and 2.0 parts by weight of a plasticizer were heated and stirred at 35 ° C. and filtered to remove residual inorganic salts, thereby obtaining a filtrate.
Subsequently, copper (II) stearate (made by Mitsuwa Chemicals Co., Ltd.) was added to the obtained filtrate so that it might become 650 ng / g with respect to the said filtrate, and the polymeric solution was prepared.
Next, the prepared polymerizable solution was passed through a strainer (60 mesh strainer, mesh 0.300 mm), and a plate shape (cuboid shape) made of Teflon (registered trademark) having an internal method of 1008 mm × 1662 mm × 25 mm. Was filled into a mold.
Next, the mold filled with the polymerizable solution is heated under a nitrogen atmosphere at 43 ° C. for 16 hours, at 52.5 ° C. for 2 hours, and at 58.5 ° C. for 1 hour to form a rectangular solid foam. An acrylic polymer was obtained. It was confirmed that the obtained foamable acrylic polymer was in a sufficiently hard state.

Foaming process:
The obtained foamable acrylic polymer was cut into a size of 436 mm × 872 mm × 25 mm. 25 g of lubricant was applied to the entire surface of the cut foamable acrylic polymer with a cloth.
Next, the foamable acrylic polymer coated with the lubricant is put into a mold having a pair of upper and lower heat plates having an internal method of 1025 mm × 2045 mm × 65 mm, and 175 ° C. exceeding the thermal decomposition temperature (135 ° C.) of urea. For 75 minutes (the entire processing time of the foaming step) to perform foaming.
Specifically, the foaming process was operated as follows.

The mold containing the foamable acrylic polymer was heated for 60 minutes at 175 ° C. for foaming (the molded product obtained by heating for 60 minutes was used as the first foam).
When the heat treatment at 175 ° C. has passed for 60 minutes, the distance between the pair of heat plates in the mold is increased, and the pressing force applied to the first foam by the heat plates is released for about 1 minute (pressing force). Pressure release operation).
At that time, before the pressing force releasing operation, the force with which the heat plate pressed the first foam was 0.7 MPa, and the pressing force releasing operation was reduced to 0 MPa.
After the pressing force releasing operation, the foam is sandwiched between a pair of heat plates, and the foam is continuously heated at 175 ° C. for 15 minutes in a state where the foam is pressed by the heat plate. A rectangular parallelepiped acrylic resin foam (with a skin layer on the surface) was obtained.

(Comparative Example 1)
Polymerization process:
The same operation as in the polymerization step in Example 1 was performed to obtain a rectangular parallelepiped foamable acrylic polymer.
Foaming process:
In the foaming step in Example 1, the pressing force release operation was not performed, and the mold containing the foamable acrylic polymer was continuously heated at 175 ° C. for 75 minutes to perform foaming. An acrylic resin foam (with a skin layer on the surface) was obtained.

(Example 2)
The following raw materials were used.
A polymerizable monomer comprising 61% by mass of methyl methacrylate, 24% by mass of methacrylic acid and 15% by mass of styrene was used.
Urea was used as a pyrolytic foaming agent.
As the polymerization initiator, t-butyl hydroperoxide (“Perbutyl H-69” manufactured by NOF Corporation) was used.
N, N-dimethylaniline was used as a polymerization initiation aid.
Cetyltrimethylammonium chloride (“Nissan Cation PB-40R” manufactured by NOF Corporation) was used as a substance for adding chloride ions. Calcium formate was used as a polymerization inhibitor. Sodium sulfate was used as a dehydrating agent.
Dimethyl silicone (“TMS650” manufactured by Momentive Performance Materials Japan LLC) was used as a lubricant.

Polymerization process:
100 parts by mass of the polymerizable monomer and 5 parts by mass of the pyrolytic foaming agent were mixed and uniformly dissolved to obtain a monomer solution.
Next, 0.54 parts by mass of a dehydrating agent, 0.14 parts by mass of a polymerization inhibitor, 0.45 parts by mass of a polymerization initiator, and 0.45 parts by mass of a polymerization initiation aid with respect to 100 parts by mass of the polymerizable monomer. Then, 0.04 part by mass of the substance for adding chloride ions was heated and stirred at 35 ° C. and filtered to remove residual inorganic salts, thereby obtaining a filtrate.
Subsequently, copper (II) stearate (made by Mitsuwa Chemicals Co., Ltd.) was added to the obtained filtrate so that it might become 150 ng / g with respect to the said filtrate, and the polymeric solution was prepared.
Next, the prepared polymerizable solution was passed through a strainer (60 mesh strainer, mesh 0.300 mm), and a plate shape (cuboid shape) made of Teflon (registered trademark) having an internal method of 1008 mm × 1662 mm × 25 mm. Was filled into a mold.
Next, the mold filled with the polymerizable solution was heated under a nitrogen atmosphere at 56 ° C. for 4 hours, 43.5 ° C. for 11 hours, 51 ° C. for 1 hour, and 58.5 ° C. for 3 hours. A rectangular parallelepiped foamable acrylic polymer was obtained. It was confirmed that the obtained foamable acrylic polymer was in a sufficiently hard state.

Foaming process:
The obtained foamable acrylic polymer was cut into a size of 555 mm × 992 mm × 25 mm. 25 g of lubricant was applied to the entire surface of the cut foamable acrylic polymer with a cloth.
Next, the foamable acrylic polymer coated with the lubricant is placed in a mold having an internal method of 1025 mm × 2045 mm × 65 mm, and is heated at 170 ° C. exceeding the thermal decomposition temperature of urea (135 ° C.) for 75 minutes. Overall processing time) Foaming was performed by heating.
Specifically, the foaming process was operated as follows.

The mold containing the foamable acrylic polymer was heated for 60 minutes at 170 ° C. to perform foaming (the molded product obtained by heating for 60 minutes was used as the first foam).
When the heat treatment at 170 ° C. has passed for 60 minutes, the distance between the pair of heat plates in the mold is increased, and the pressing force applied to the first foam by the heat plates is released for about 1 minute (pressing). Pressure release operation).
At that time, before the pressing force releasing operation, the force with which the heat plate pressed the first foam was 0.8 MPa was reduced to 0 MPa by the pressing force releasing operation.
After the pressing force releasing operation, the foam is sandwiched between a pair of heat plates, and the foam is continuously heated at 170 ° C. for 15 minutes in a state where the foam is pressed by the heat plate. A rectangular parallelepiped acrylic resin foam (with a skin layer on the surface) was obtained.

(Comparative Example 2)
Polymerization process:
The same operation as in the polymerization step in Example 2 was performed to obtain a rectangular parallelepiped foamable acrylic polymer.
Foaming process:
In the foaming process in Example 2, the pressing force release operation was not performed, and the mold containing the foamable acrylic polymer was continuously heated at 170 ° C. for 75 minutes to perform foaming. An acrylic resin foam (with a skin layer on the surface) was obtained.

[Measurement of density of acrylic resin foam]
Using the acrylic resin foam of each example, the maximum density, the minimum density, and the average density of 45 test pieces were determined by the above-described procedures (1) to (4).
In step (1), in order to expose bubbles on all six surfaces of the rectangular acrylic resin foam, the skin layer having a thickness of 2 mm was removed from the surface, and the skin layer was cut.
A ratio represented by [(maximum density of test piece) − (minimum density of test piece)] / (average density of test piece) (denoted as “(maximum−minimum) / average density” in the table). Was calculated. The results are shown in Table 1.

[Measurement of bending strength of acrylic resin foam]
Using the acrylic resin foam of each example, a total of 45 foam pieces were produced for each acrylic resin foam by the above-described procedures (1) to (4), and these were used as test pieces.
Subsequently, each bending strength of 45 test pieces was measured by a method based on JIS K 7221. And the average bending strength of 45 test pieces was calculated | required.
And, the ratio ([(maximum−minimum) / average strength] in the table) represented by [(maximum bending strength of test piece) − (minimum bending strength of test piece)] / (average bending strength of test piece). Calculated). The results are shown in Table 1.

From the results shown in Table 1, the acrylic resin foams of Example 1 and Example 2 to which the present invention was applied had a (maximum-minimum) / average density in the range of 0 to 0.25. .
The acrylic resin foams of Comparative Example 1 and Comparative Example 2 outside the scope of the present invention had (maximum-minimum) / average density exceeding 0.25.
That is, it can be confirmed that the acrylic resin foam of Example 1 has a smaller density variation and a more uniform density than the acrylic resin foam of Comparative Example 1.
Compared with the acrylic resin foam of Comparative Example 2, the acrylic resin foam of Example 2 can be confirmed to have less density variation and a more uniform density.

Further, from the results shown in Table 1, the acrylic resin foam of Example 1 had a value of (maximum−minimum) / average strength smaller than that of the acrylic resin foam of Comparative Example 1.
Similarly, the acrylic resin foam of Example 2 had a smaller value (maximum−minimum) / average strength than the acrylic resin foam of Comparative Example 2.
That is, it can be confirmed that the acrylic resin foams of Example 1 and Example 2 with small variations in density have small variations in bending strength.

  The acrylic resin foams of Examples 1 and 2 to which the present invention is applied can be suitably used particularly for automobile interior materials.

Claims (7)

In the acrylic resin foam having a plate-like shape and having a structural unit derived from an acrylic monomer and a structural unit derived from styrene,
Bubbles are exposed on the surface,
An acrylic resin foam that satisfies the relationship of the following formula (1) when measured by the following density measurement method.
<Density measurement method>
Procedure (1a): Acrylic resin foam is divided into 3 × 3 in 9 in the surface direction to produce a total of 9 foam pieces.
Procedure (2a): The nine foam pieces produced in the procedure (1a) are cut so that the thickness direction is divided into five equal parts, and a total of 45 test pieces are produced.
Procedure (3a): Each density of the 45 test pieces prepared in the procedure (2a) is measured by a method in accordance with JIS K 7222: 1999.
Procedure (4a): The maximum density, the minimum density, and the average density of 45 test pieces are obtained from the densities of 45 test pieces measured in the procedure (3a).
0 ≦ [(maximum density of test piece) − (minimum density of test piece)] / (average density of test piece) ≦ 0.25
... (1) In the acrylic resin foam having a plate-like shape and having a structural unit derived from an acrylic monomer and a structural unit derived from styrene,
Has a skin layer on the surface,
An acrylic resin foam that satisfies the relationship of the following formula (1) when measured by the following density measurement method.
<Density measurement method>
Procedure (1b): Acrylic resin foam with the skin layer removed and air bubbles exposed on the surface is divided into 9 × 3 × 3 in the surface direction to produce a total of 9 foam pieces.
Procedure (2b): Each of the nine foam pieces produced in the procedure (1b) is cut so that the thickness direction is divided into five equal parts to produce a total of 45 test pieces.
Procedure (3b): Each density of the 45 test pieces prepared in the procedure (2b) is measured by a method in accordance with JIS K 7222: 1999.
Procedure (4b): The maximum density, the minimum density, and the average density of 45 test pieces are obtained from the densities of 45 test pieces measured in the procedure (3b).
0 ≦ [(maximum density of test piece) − (minimum density of test piece)] / (average density of test piece) ≦ 0.25
... (1)   The acrylic resin foam of Claim 1 or 2 used for a motor vehicle interior material. It is a manufacturing method of the acrylic resin foam according to any one of claims 1 to 3,
A polymerizable solution containing an acrylic monomer and a polymerizable monomer containing styrene, a thermally decomposable foaming agent, and a polymerization initiator is heated to polymerize the polymerizable monomer to obtain a foamable acrylic polymer. A polymerization step, and a foaming step of foaming the foamable acrylic polymer obtained in the polymerization step,
In the foaming step, the foamable acrylic polymer is foamed between the heat plates of a mold having a pair of upper and lower heat plates separated by an arbitrary distance, the foam is brought into contact with the upper heat plate, A method for producing an acrylic resin foam, comprising: pressing the foam with the heat plate, and then releasing the pressing force with which the upper heat plate presses the foam.   The said pressing force cancellation | release operation is a manufacturing method of the acrylic resin foam of Claim 4 performed when half or more of the processing time of the said foaming process passes.   The acrylic monomer according to claim 4 or 5, wherein the polymerizable monomer contains 40 to 70% by mass of methyl (meth) acrylate, 15 to 30% by mass of (meth) acrylic acid, and 10 to 30% by mass of styrene. Of producing a resin-based resin foam.   The method for producing an acrylic resin foam according to claim 6, wherein the polymerizable monomer further contains 1 to 10% by mass of maleic anhydride and 1 to 10% by mass of (meth) acrylamide.