JP6749640B2 - Method for manufacturing resin molded body - Google Patents

Description

The present invention relates to a method for manufacturing a resin molded body.

When blending inorganic fine powder with a thermoplastic resin to produce a resin molded body, it is important to balance optical properties such as whiteness and opacity with mechanical properties such as elastic modulus and bending strength. ..

In order to obtain a sheet of a resin molded product having a high content of inorganic fine powder, a method has been proposed in which inorganic fine powder is highly mixed with a resin to form a sheet, and the sheet is further stretched. That is, in Patent Document 1, after kneading a resin composition containing 60% by weight to 82% by weight of an inorganic powder and 18% by weight to 40% by weight of a thermoplastic resin, the resin composition is formed into a sheet through a die, and then vertically or/and Disclosed is a method for producing a thin film sheet having a high content of an inorganic substance powder, which is produced by stretching in the transverse direction to improve whiteness and opacity.

JP, 2013-10931, A

A plastic sheet containing a thermoplastic resin as a main component is generally transparent or nearly transparent. When this sheet is filled with the inorganic fine powder, the opacity of the sheet is improved, but the whiteness remains at a low level.

Even in the above case, when the stretch ratio is increased, the whiteness is remarkably improved and the opacity is slightly improved, but the bending strength is decreased. However, after that, practically, a problem began to occur in the stretched sheet, particularly in bending strength.

On the other hand, when a resin molded sheet containing a thermoplastic resin as a main component is subjected to a stretching treatment, a heat treatment (so-called annealing treatment) is usually performed after stretching at a glass transition point or a softening point of the raw material thermoplastic resin or below the softening point. There are many. The purpose is to prevent dimensional changes and reduction in strength that occur after the product is finished due to residual stress in the sheet that has occurred up to the preceding process.

However, as the heat treatment called the above-mentioned annealing treatment, there are usually many methods of maintaining the high temperature for a long time or longer.

The present invention has been made in view of the above circumstances, and provides a method for producing a resin molded article having excellent elastic modulus and bending strength while maintaining optical properties such as whiteness and opacity as high as possible. The purpose is to

The inventors of the present invention first conducted an experiment to find out how the balance between the optical properties of whiteness and opacity and the bending strength of the sheet was changed by stretching, and the results shown in Table 1 below were obtained. That is, when the inorganic fine powder is filled and the stretching is strengthened, the whiteness is remarkably improved, and the opacity is slightly increased, and the measured value reaches 100% in some cases. However, the bending strength was remarkably reduced, and the tensile strength also tended to decrease.

In order to improve the above-mentioned problems, the inventors of the present invention perform a heat treatment temperature after stretching for two types of thermoplastic resins having different melting points at a temperature higher than one melting point and equal to or lower than the other melting point to obtain a whiteness degree. Further, they have found that a resin molded product having an excellent elastic modulus and bending strength can be obtained while maintaining the opacity as high as possible, and completed the present invention. More specifically, the present invention aims to provide the following.

(1) A method for manufacturing a resin molded body, comprising:
A step of obtaining a resin composition by mixing a first thermoplastic resin, a second thermoplastic resin having a higher melting point than the first thermoplastic resin, and an inorganic fine powder,
Stretching the resin composition below the melting point of the first thermoplastic resin;
And a step of heat-treating the stretched resin composition at a temperature not lower than the melting point of the first thermoplastic resin and lower than the melting point of the second thermoplastic resin.

(2) The method according to (1), wherein in the step of obtaining a resin composition, the inorganic fine powder is mixed in an amount of 50% by mass or more based on the mass of the entire resin composition.

(3) The method according to (1) or (2), wherein the first thermoplastic resin is polyethylene and the second thermoplastic resin is polypropylene.

(4) The method according to any one of (1) to (3), wherein the inorganic fine powder contains calcium carbonate.

According to the present invention, it is possible to obtain a resin molded article having an excellent balance of whiteness and opacity and bending strength and elastic modulus.

Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments. It can be implemented with appropriate modifications within the scope of the object of the present invention.

In the method for producing a resin molded product of the present invention, a resin composition is obtained by mixing a first thermoplastic resin, a second thermoplastic resin having a melting point higher than that of the first thermoplastic resin, and an inorganic fine powder. A step of stretching the resin composition at a temperature below the melting point of the first thermoplastic resin, and a step of stretching the resin composition at a temperature equal to or higher than the melting point of the first thermoplastic resin and lower than the melting point of the second resin. And a step of performing heat treatment at a temperature.

Since the present invention includes a step of stretching the resin composition containing the first thermoplastic resin, the second thermoplastic resin and the inorganic fine powder at a temperature lower than the melting point of the first thermoplastic resin, A large number of holes are formed in the resin composition. Further, in the subsequent step, by heat-treating the resin composition at a temperature equal to or higher than the melting point of the first thermoplastic resin and lower than the melting point of the second resin, the elastic modulus and the bending strength are improved, While the internal strain is removed, most of the pores derived from the first thermoplastic resin are closed, and most of the pores derived from the second thermoplastic resin remain, so the whiteness and opacity are relatively high. Maintained. As a result, it is possible to obtain a resin molded product having an excellent balance of optical properties such as whiteness and opacity and mechanical properties such as elastic modulus and bending strength.

Hereinafter, each step in the method for producing a resin molded product of the present invention will be described.

[Step of obtaining resin composition]
In the step of obtaining the resin composition in the present invention, the first thermoplastic resin, the second thermoplastic resin and the inorganic fine powder are mixed to obtain a resin composition. Specifically, the first thermoplastic resin, the second thermoplastic resin, and the inorganic fine powder are melted at a melting point of the second thermoplastic resin or higher and kneaded to obtain a resin composition. The resin composition may be obtained by kneading in advance with a kneader and once producing mixed pellets, and then forming into a sheet with an extruder, or by direct kneading by biaxial extrusion molding or the like. The sheet may be formed into a sheet by performing molding with a single machine. From the viewpoint of uniformly dispersing the inorganic fine powder in the thermoplastic resin, it is preferable to apply a high shearing stress to knead, for example, a biaxial kneader is preferable. Further, since the resin composition containing the inorganic fine powder has a high viscosity when melted, it is designed such that the kneading portion of the extruder is lengthened.

[Step of stretching the resin composition]
In the step of stretching the resin composition in the present invention, the resin composition is stretched at a temperature lower than the melting point of the first thermoplastic resin. By stretching the resin composition at a temperature lower than the melting point of the first thermoplastic resin, voids are easily generated in the resin composition, and a resin molded product having high opacity and whiteness is easily obtained.

The stretching conditions need to be appropriately set according to the desired specific gravity (density) according to the resin molded product, and the stretching treatment may be uniaxial in the longitudinal or transverse direction, or sequential or simultaneous biaxial stretching in the longitudinal and transverse directions. Adopt one of.

The necessary draw ratio can also be calculated. The weight per square meter (also referred to as the basis weight) W (g/m ² ) of the resin composition before being stretched is measured, and the apparent specific gravity D and aspect ratio (longitudinal direction) of the product determined in the production plan are measured. The ratio of the draw ratio in the transverse direction) R and the target value T (cm) of the thickness of the product after the transverse drawing are used to determine the draw ratio (longitudinal direction X times, transverse direction Y times) by the following formula Can be performed, and can be easily estimated from the operating experience of each device.
(Formula 1)
X ² =W×10 ⁻⁴ /(D×Z×R×T)
X=RY
In the formula, D: apparent specific gravity of the product specified in the production plan
R: Aspect ratio defined in the production plan (ratio of stretching ratio in the machine direction and the machine direction)
W: Weight per square meter of thin film material before longitudinal stretching (g)
X: draw ratio in the machine direction
Y: draw ratio in the transverse direction
Z: Shrinkage ratio or expansion ratio of the transverse length of the sheet by longitudinal stretching

[Process of heat treatment]
In the heat treatment step of the present invention, the stretched resin composition is heat treated at a temperature which is equal to or higher than the melting point of the first thermoplastic resin and lower than the melting point of the second thermoplastic resin. As a result, in the heat-treated resin molded body, the internal strain is removed, at least some of the pores derived from the first thermoplastic resin are closed, and the pores derived from the second thermoplastic resin remain. .. As a result, it is possible to obtain a resin molded product having excellent bending strength and elastic modulus while maintaining whiteness and opacity at a considerable level.

The numerical values of whiteness and opacity greatly change by changing the condition of the heat treatment process, especially the temperature of the heat treatment up and down. When the stretched resin composition is heat-treated at a temperature lower than the melting point of the first thermoplastic resin, the bending strength and the elastic modulus are reduced although the opacity and whiteness are secured. When the stretched resin composition is heat-treated at a temperature above the melting point of the second heat-curable resin, the opacity and whiteness are reduced even though the bending strength and elastic modulus are secured.

The temperature for heat treatment is not particularly limited as long as it is a temperature equal to or higher than the melting point of the first thermoplastic resin and lower than the melting point of the second thermoplastic resin, and may be appropriately set according to both melting points. From the viewpoint of ensuring bending strength and elastic modulus, it is preferable that the melting point of the second thermoplastic resin is closer to that of the second thermoplastic resin than the melting point of the first thermoplastic resin, for example, the melting point of the second thermoplastic resin −40° C. To the melting point of the second thermoplastic resin, preferably the melting point of the second thermoplastic resin −30° C. to the melting point of the second thermoplastic resin, and more preferably the melting point of the second thermoplastic resin. It is more preferably −20° C. to the melting point of the second thermoplastic resin, and even more preferably −10° C. to the melting point of the second thermoplastic resin. On the other hand, from the viewpoint of high whiteness and opacity, the temperature of the heat treatment is preferably closer to the melting point of the first thermoplastic resin than the melting point of the second thermoplastic resin, for example, the first thermoplastic resin. From the melting point of the first thermoplastic resin to the melting point of the first thermoplastic resin+40° C., more preferably from the melting point of the first thermoplastic resin to the melting point of the first thermoplastic resin+30° C., The melting point of the thermoplastic resin to the melting point of the first thermoplastic resin +20°C is even more preferable, and the melting point of the first thermoplastic resin to the melting point of the first thermoplastic resin +10°C is even more preferable.

[Thermoplastic resin]
The selection of the first thermoplastic resin and the second thermoplastic resin is not particularly limited as long as the second thermoplastic resin has a higher melting point than the first thermoplastic resin. For example, polyethylene (high density polyethylene/low density Polyethylene, ultra-high molecular weight polyethylene), ultra-high molecular weight polyethylene, polypropylene, polystyrene, polyethylene terephthalate and other resins, the first thermoplastic resin and the second thermoplastic resin can be selected in consideration of the melting point. it can. Among them, polyethylene resin (especially high-density polyethylene or ultra-high molecular weight polyethylene) is used as the first thermoplastic resin and polypropylene resin (especially isotactic resin) is used as the second thermoplastic resin because of its superiority in elastic modulus and bending strength. Chick homopolypropylene) is preferably used.

The blending ratio of the first thermoplastic resin and the second thermoplastic resin may be appropriately set according to the desired resin molded product, and is not particularly limited, but the stretching temperature, whiteness and opacity, and bending From the viewpoint of easily balancing the strength and the elastic modulus, it is preferably 1:0.2 to 5, more preferably 1:0.3 to 3.3, and 1:0.5 to 2 Is particularly preferable.

The molecular weight distribution Mw/Mn, which is the ratio of the weight average molecular weight Mw and the number average molecular weight Mn of the second thermoplastic resin, is preferably 1 or more and 20 or less, more preferably 5 or more and 15 or less. The molecular weight distribution Mz/Mn, which is the ratio of the average molecular weight Mz and the number average molecular weight Mn of the second thermoplastic resin, is preferably 10 or more and 100 or less, and more preferably 20 or more and 50 or less. When the molecular weight distributions Mw/Mn and Mz/Mn of the second thermoplastic resin are in the above ranges, it is easy to obtain a resin molded product having an excellent balance between whiteness and opacity, bending strength and elastic modulus. If the molecular weight distribution is too narrow, the kneadability of the two thermoplastic resins deteriorates, whiteness and opacity tend to become non-uniform, and the physical properties of the sheet also become non-uniform. If the molecular weight distribution is too wide, the spot of the melting point becomes wide, so that it becomes difficult to control the stretching temperature and the heat treatment temperature, and it becomes difficult to obtain desired whiteness and opacity, and bending stiffness and elastic modulus.

The molecular weight distribution Mw/Mn, which is the ratio of the weight average molecular weight Mw and the number average molecular weight Mn of the first thermoplastic resin, is preferably 1 or more and 100 or less, more preferably 10 or more and 40 or less. The molecular weight distribution Mz/Mn, which is the ratio of the average molecular weight Mz and the number average molecular weight Mn of the first thermoplastic resin, is preferably 50 or more and 200 or less, and more preferably 80 or more and 150 or less. When the molecular weight distribution Mw/Mn·Mz/Mn of the first thermoplastic resin is within the above range, it is easy to obtain a resin molded product having an excellent balance of whiteness and opacity, bending strength and elastic modulus. If the molecular weight distribution is too narrow, the kneadability of the two thermoplastic resins is deteriorated, whiteness and opacity are likely to be nonuniform, and the physical properties of the sheet are also nonuniform. If the molecular weight distribution is too wide, the spot of the melting point becomes wide, so that it becomes difficult to control the stretching temperature and the heat treatment temperature, and it becomes difficult to obtain desired whiteness and opacity, and bending stiffness and elastic modulus.

In the present invention, the weight average molecular weight Mw, the number average molecular weight Mn, and the average molecular weight Mz are measured by a gel permeation (GPC) method.

The melt mass flow rate (MFR) of the first thermoplastic resin is preferably 0.02 g/10 or more and 2.0 g/10 minutes or less, and 0.1 g/10 or more and 1.0 g/10 minutes or less. It is more preferable that the first thermoplastic resin has a melt mass flow rate in the above range, and thus it is easy to obtain a resin molded product having an excellent balance of whiteness and opacity, bending strength and elastic modulus.

The melt mass flow rate (MFR) of the second thermoplastic resin is preferably 0.02 g/10 or more and 2.0 g/10 minutes or less, and 0.1 g/10 or more and 1.0 g/10 minutes or less. Preferably. When the melt mass flow rate of the second thermoplastic resin is within the above range, it is easy to obtain a resin molded product having an excellent balance of whiteness and opacity, bending strength and elastic modulus.

The melt mass flow rate is an index showing fluidity at the time of melting, and means a value measured according to JIS K 7210. As a method for measuring the melt flow rate, specifically, according to JIS K 7210, a melt indexer is used to measure a load of 21.18 N, a temperature of 230° C. for polypropylene resin, and a temperature of 190° C. for polyethylene resin. There is a way to measure.

[Inorganic fine powder]
The compounding ratio of the inorganic fine powder is not particularly limited, but as the amount increases, voids are more likely to occur during stretching and the opacity becomes good, so 50% by mass or more based on the mass of the entire resin composition is preferable. It is preferably 60% by mass or more, and even more preferably 60% by mass or more. The upper limit of the amount of the inorganic fine powder contained in the entire composition is not particularly limited, but 80% by mass or less is more preferable from the viewpoint of kneading property and bending strength.

The average particle diameter of the inorganic fine powder is preferably 0.1 μm or more and 50 μm or less, more preferably 1.0 μm or more and 15 μm or less. When the average particle size of the inorganic fine powder is within the above range, it is easy to obtain a resin molded product having an excellent balance of whiteness and opacity, bending strength and elastic modulus. If the average particle size of the inorganic fine powder is too large, the inorganic fine powder easily separates from the surface of the resin molded body. If the average particle size of the inorganic fine powder is too small, the viscosity increases when kneaded with the thermoplastic resin, and the kneadability tends to deteriorate. The average particle size of the inorganic fine powder in the present invention is a 50% particle size (d50) obtained from a distribution curve of integrated% measured by a laser diffraction type particle size distribution measuring device.

Examples of the inorganic fine powder include calcium carbonate, titanium oxide, silica, clay, talc, kaolin, aluminum hydroxide and the like, and calcium carbonate is particularly preferable. These may be used alone or in combination of two or more. Further, in order to enhance the dispersibility or reactivity of the inorganic fine powder, the surface of the inorganic fine powder may be modified in advance by a conventional method.

The resin composition of the present invention is selected from lubricants, antioxidants, ultraviolet absorbers, coloring pigments, dispersants, antistatic agents, flame retardants, etc. in addition to the above-mentioned thermoplastic resin and inorganic fine powder. One or more auxiliaries can be added within a range not deviating from the purpose.

The application of the produced resin molded product is not particularly limited, but since it is particularly suitable for the balance of whiteness and opacity and bending strength and elastic modulus, for example, printing paper, information paper, packaging, card ( For example, it is suitable for business cards), processed cardboard, etc.

Hereinafter, the present invention will be described based on examples, but the present invention is not limited to these examples.

[Reference test]
Samples A and B of high-density polyethylene resin sheets filled with calcium carbonate fine powder were prepared, and changes in optical properties and mechanical properties were evaluated.

The sheet of sample A was obtained by directly kneading calcium carbonate fine powder and polyethylene resin at a mass ratio of 60:40 using a twin-screw coaxial extruder and extruding into a sheet form from a Т die. It was produced by stretching under the conditions shown. Sample B was produced by changing the mass ratio of the fine powder of calcium carbonate and the polyethylene resin to 70:30, extruding it into a sheet like Sample A, and then stretching it under the conditions shown in Table 1 below.

As shown in Table 1, it was found that the high-density polyethylene resin sheet contains a large amount of calcium carbonate, and thus high opacity can be obtained. Moreover, when the stretching ratio was increased, the whiteness was remarkably improved and the opacity was also slightly improved. On the other hand, Taber stiffness (bending strength) decreased.

[Example 1]
As the first thermoplastic resin, a high-density polyethylene homopolymer having a molecular weight distribution Mw/Mn=20.8, a molecular weight distribution Mz/Mn=121, a melt mass flow rate MFR=0.3, and a melting point of 133° C. (Keiyo Polyethylene Corporation). Manufacturing: B5803) (hereinafter referred to as PE) was prepared. As the second thermoplastic resin, a polypropylene homopolymer having a molecular weight distribution of Mw/Mn=10.3, Mz/Mn=29.1, melt mass flow rate MFR=0.5, and melting point=165° C. (Prime Polymer ):E111G) (hereinafter referred to as PP) was prepared. As the inorganic fine powder, calcium carbonate (manufactured by Shiraishi Calcium Co., Ltd.: Ryton S4) (hereinafter referred to as CC) was prepared. These were directly kneaded with a twin-screw coaxial extruder at a charge ratio of PE/PP/CC=2/2/6, extruded into a sheet form from a T die at 250°C, and cooled with a cast roll at 70°C. It solidified and the resin composition was obtained. This resin composition was preheated at 125° C. and stretched twice in the machine direction. Then, the stretched resin composition was heat-treated at 150° C., cooled to 30° C., and wound by a winder. Table 2 shows the physical properties of the obtained sheet-shaped resin molded product.

[Example 2]
In the same manner as in Example 1, the first thermoplastic resin, the second thermoplastic resin, and the inorganic fine powder were used at a charging ratio of PE/PP/CC=2/2/6 using a twin-screw coaxial extruder. Was directly kneaded, extruded into a sheet form from a T die at 250° C., and cooled and solidified by a cast roll at 70° C. to obtain a resin composition. This resin composition was preheated at 125° C. and stretched twice in the machine direction. Then, the stretched resin composition was heat-treated at 140° C., cooled to 30° C. and wound with a winder. Table 2 shows the physical properties of the obtained sheet-shaped resin molded product.

[Example 3]
In the same manner as in Example 1, the first thermoplastic resin, the second thermoplastic resin, and the inorganic fine powder were used at a charging ratio of PE/PP/CC=2/2/6 using a twin-screw coaxial extruder. Was directly kneaded, extruded into a sheet form from a T die at 250° C., and cooled and solidified by a cast roll at 70° C. to obtain a resin composition. This resin composition was preheated at 125° C. and stretched twice in the machine direction. Then, the stretched resin composition was heat-treated at 135° C., cooled to 30° C., and wound by a winder. Table 2 shows the physical properties of the obtained sheet-shaped resin molded product.

[Comparative Example 1]
In the same manner as in Example 1, the first thermoplastic resin, the second thermoplastic resin, and the inorganic fine powder were used at a charging ratio of PE/PP/CC=2/2/6 using a twin-screw coaxial extruder. Was directly kneaded, extruded into a sheet form from a T die at 250° C., cooled and solidified by a 70° C. cast roll, preheated at 125° C., and stretched twice in the longitudinal direction. Then, the stretched resin composition was heat-treated at 125° C., cooled to 30° C., and wound by a winder. Table 2 shows the physical properties of the obtained sheet-shaped resin molded product.

The physical properties of the sheet-shaped resin molded product were measured according to the methods described below.
Density: JIS K 7112
Basis weight: JIS P 8124
Strength at break and elongation at break: JIS K 7162
Tear strength: JIS K 7128-3
Bending stiffness: JIS K 7106
Whiteness: JIS P 8148
Opacity measurement: JIS P 8149
Smoothness: JIS P 8155
Elastic modulus: JIS K7161

[Evaluation]
From the results of Table 2, the sheet resin moldings of Example 1, Example 2 and Example 3 have moderate strengths such as strength at break, elongation at break, and tear strength as in Comparative Example 1. It can be seen that the bending stiffness is improved as compared with Comparative Example 1 while ensuring the opacity and whiteness. In particular, it can be seen that the sheet resin molded body of Example 1 has greatly improved bending strength and elastic modulus as compared with Comparative Example 1.

Claims (3)

A method of manufacturing a resin molded body,
A step of mixing a first thermoplastic resin, a second thermoplastic resin having a melting point higher than that of the first thermoplastic resin, and an inorganic fine powder to obtain a resin composition;
Stretching the resin composition below the melting point of the first thermoplastic resin;
The resin composition after stretching, have a, a step of heat treatment at the first and at least the melting point of the thermoplastic resin and said temperature below the melting point of the second thermoplastic resin,
The first thermoplastic resin is polyethylene,
The second thermoplastic resin is polypropylene,
The compounding ratio of the first thermoplastic resin to the second thermoplastic resin is 1:0.2 to 1:5,
Method. The method according to claim 1, wherein in the step of obtaining a resin composition, the inorganic fine powder is mixed in an amount of 50% by mass or more based on the mass of the entire resin composition. The method according to claim 1 or 2 , wherein the inorganic fine powder comprises calcium carbonate.