Thermoplastic sheet and method for producing the same

Abstract

To provide a thermoplastic sheet that has sufficient concealability and suppresses reduction in the mechanical strength of the surface layer portion of the sheet, by means of a simplified method.SOLUTION: A thermoplastic sheet comprises a resin composition that includes a thermoplastic resin and an inorganic powder in a mass ratio of 50:50 to 10:90, the resin composition being shaped into a sheet. The thermoplastic sheet includes an inner layer portion having voids, and two surface layer portions disposed sandwiching the inner layer portion, the two surface layer portions having a lower porosity than that of the inner layer portion.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a thermoplastic sheet and a method for making the same. [Background technology]

[0002] 2. Description of the Related Art Conventionally, molded articles of resin compositions containing a thermoplastic resin and an inorganic substance powder have been used for various applications such as optical members, functional members, and packaging materials.

[0003] For example, Patent Document 1 discloses a sheet for a solar cell module having an A layer containing a polyester resin and a B layer containing a urethane resin and an inorganic filler. The sheet is said to have good reflective performance by increasing the porosity of the A layer and decreasing the porosity of the B layer.

[0004] Patent Document 2 discloses a moisture-permeable waterproof sheet having a urethane resin film including a porous portion and a waterproof portion. The sheet is said to be able to achieve both moisture permeability and waterproofness by increasing the porosity of the porous portion and decreasing the porosity of the waterproof portion.

[0005] Patent Document 3 discloses a stretched sheet of a resin composition containing a thermoplastic resin and a large amount of calcium carbonate powder. The sheet not only reduces the amount of resin used, thereby reducing the environmental impact, but also has good hiding power (opacity) due to the voids formed by stretching. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2018-32753 A [Patent Document 2] International Publication No. 2008 / 090877 [Patent Document 3] Patent Publication No. 2021-161350 Summary of the Invention [Problem to be solved by the invention]

[0007] However, as shown in Patent Document 3, a stretched sheet of a resin composition containing a large amount of inorganic powder has voids formed by stretching. Such voids exist not only inside the sheet but also in large numbers on the surface layer of the sheet. Therefore, it has been newly discovered that the mechanical strength (especially tape peel strength) of the surface layer of the sheet is more likely to decrease in the stretched sheet than in a resin sheet that does not contain inorganic powder.

[0008] In response to this, as a method for suppressing the decrease in the mechanical strength of the surface layer of the sheet, as shown in Patent Document 1, it is possible to make the surface layer a multi-layer structure (e.g., a two-type three-layer structure) composed of layers made only of resin, or to reduce the content of inorganic substance powder to suppress the occurrence of voids. However, when a multi-layer structure is used, the manufacturing process becomes complicated, and the manufacturing cost tends to increase. In addition, when the content of inorganic substance powder is reduced, not only is it impossible to reduce the amount of resin used, but it is also difficult to obtain a sheet with sufficient concealment.

[0009] The present invention has been made in consideration of the above circumstances, and aims to provide a thermoplastic sheet that has sufficient hiding power and can suppress a decrease in the mechanical strength of the surface layer of the sheet in a simple manner, and a method for producing the same. [Means for solving the problem]

[0010] The above problem can be solved by the following configuration.

[0011] [1] A thermoplastic sheet obtained by molding a resin composition containing a thermoplastic resin and an inorganic substance powder in a mass ratio of 50:50 to 10:90 into a sheet, the thermoplastic sheet comprising: an inner layer portion having voids; and two surface layer portions arranged on either side of the inner layer portion and having a porosity lower than the porosity of the inner layer portion. [2] A thermoplastic sheet formed into a sheet from a resin composition containing a thermoplastic resin and an inorganic powder in a mass ratio of 50:50 to 10:90, wherein, in the thickness direction of the thermoplastic sheet, one surface is 0% and the other surface is 100%, and a first region is a region of 0% or more and less than 20%, a second region is a region of 20% or more and less than 40%, a third region is a region of 40% or more and less than 60%, a fourth region is a region of 60% or more and less than 80%, and a fifth region is a region of 80% or more and 100% or less, the porosity of the first region and the fifth region is less than 10%, the porosity of the second region and the fourth region is 10% or more and less than 25%, and the porosity of the third region is 25% or more and 50% or less. [3] The thermoplastic sheet according to [1] or [2], which has, when the thermoplastic sheet is subjected to differential scanning calorimetry, a first endothermic peak corresponding to the melting point of the thermoplastic resin and a second endothermic peak at a higher temperature side that is 8°C or more higher than the first endothermic peak. [4] The thermoplastic sheet according to any one of [1] to [3], wherein the resin composition contains the thermoplastic resin and the inorganic substance powder in a mass ratio of 50:50 to 30:70. [5] The thermoplastic sheet according to any one of [1] to [4], wherein the thermoplastic resin contains a polyolefin-based resin. [6] The thermoplastic sheet according to any one of [1] to [5], wherein the inorganic substance powder contains ground calcium carbonate powder. [7] The thermoplastic sheet according to [6], wherein the average particle size of the ground calcium carbonate powder is 0.7 μm or more and 6.0 μm or less. [8] A method for producing a thermoplastic sheet, comprising the steps of melt-kneading a resin composition containing a thermoplastic resin and an inorganic substance powder in a mass ratio of 50:50 to 10:90, and then molding the composition to obtain a sheet; cooling the sheet; contacting a surface of the cooled sheet with a heating roll to preheat the sheet so that the temperature inside the sheet is maintained lower than the temperature of the sheet surface; stretching the preheated sheet in a longitudinal direction; preheating the stretched sheet in a heating furnace; and stretching the preheated sheet in a direction perpendicular to the longitudinal direction. [9] The method for producing a thermoplastic sheet described in [8], in which, when the melting point of the thermoplastic resin is Tm (°C), the sheet is cooled so that the surface temperature of the sheet is equal to or higher than (Tm-100)°C but lower than (Tm-80)°C.

[10] The method for producing a thermoplastic sheet according to [9], wherein in the preheating step with the heating roll, the sheet is heated so that the surface temperature of the sheet is (Tm-15)°C or higher and (Tm-5)°C or lower.

[11] The method for producing a thermoplastic sheet according to [9], wherein in the preheating step in the heating furnace, the sheet is heated so that the surface temperature of the sheet is higher than (Tm-5)°C and lower than (Tm+5)°C.

[12] The method for producing a thermoplastic sheet according to any one of [8] to

[11] , wherein the step of stretching in the longitudinal direction and the step of stretching in a direction perpendicular to the longitudinal direction are performed so that a total stretching ratio is 8 times or more. Effect of the Invention

[0012] According to the present invention, it is possible to provide a thermoplastic sheet that has sufficient hiding power and is capable of suppressing a decrease in the mechanical strength of a surface layer portion of the sheet, and a method for producing the same, by a simple method. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1A is a schematic cross-sectional view showing a thermoplastic sheet according to one embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view showing a thermoplastic sheet according to a modified example. [Diagram 2] FIG. 2A is an SEM image of a cross section perpendicular to the longitudinal direction of the thermoplastic sheet of Example 1, and FIG. 2B is an SEM image of a cross section perpendicular to the longitudinal direction of the thermoplastic sheet of Comparative Example 1. [Diagram 3] FIG. 3 is a curve obtained by measuring the thermoplastic sheet of Example 1 by DSC. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] As described above, a stretched sheet of a resin composition containing a large amount of inorganic substance powder has many voids even in the vicinity of the surface layer, and therefore the surface strength of the sheet is likely to be weakened.

[0015] As a result of intensive research, the present inventors have found that even when produced by single-layer extrusion, by adjusting the temperature conditions and stretching conditions, the number of voids generated near the surface of the sheet can be drastically reduced and voids can be generated only inside the sheet. As a result, it has been found that even by single-layer extrusion, a thermoplastic sheet having a structure similar to that formed by multi-layer extrusion, i.e., a thermoplastic sheet having a porosity of the surface layer lower than that of the inner layer, can be obtained. Such a thermoplastic sheet can suppress the decrease in mechanical strength of the surface layer of the sheet while having sufficient concealment properties.

[0016] Hereinafter, a thermoplastic sheet according to one embodiment of the present invention and a method for producing the same will be described.

[0017] 1. Thermoplastic sheet [Thermoplastic sheet composition] Fig. 1A is a schematic cross-sectional view showing a thermoplastic sheet 10 according to one embodiment of the present invention, and Fig. 1B is a schematic cross-sectional view showing a thermoplastic sheet 10 according to a modified example.

[0018] As shown in Fig. 1A, a thermoplastic sheet 10 according to an embodiment of the present invention is formed into a sheet from a resin composition containing a thermoplastic resin and an inorganic powder. The thermoplastic sheet 10 has an inner layer portion 12 having a void 11, and two surface layer portions 13A and 13B disposed on both sides of the inner layer portion 12. The porosity of the two surface layer portions 13A and 13B is lower than the porosity of the inner layer portion 12. The inner layer portion 12 and the two surface layer portions 13A and 13B are regions of a single layer sheet having different porosities.

[0019] Specifically, in the thickness direction of the thermoplastic sheet 10, when one surface is 0% and the other surface is 100%, and the region of 0% or more and less than 20% is the first region a1, the region of 20% or more and less than 40% is the second region a2, the region of 40% or more and less than 60% is the third region a3, the region of 60% or more and less than 80% is the fourth region a4, and the region of 80% or more and less than 100% is the fifth region a5, the porosity of the first region a1 and the fifth region a5 is lower than the porosity of the third region a3. Also, the porosity of the second region a2 (or the fourth region a4) is the same as or lower than the porosity of the third region a3 and is the same as or higher than the porosity of the first region a1 (or the fifth region a5).

[0020] More specifically, the porosity of the first region a1 and the fifth region a5 is preferably less than 10%. When the porosity of the first region a1 and the fifth region a5 is less than 10%, the mechanical strength of the surface layer of the thermoplastic sheet is increased, and the tape peel strength can be increased. From the same viewpoint, it is more preferable that the porosity of the first region a1 and the fifth region a5 is 8% or less.

[0021] The porosity of the second region a2 and the fourth region a4 is preferably 10% or more and less than 25%, and more preferably 10% or more and 20% or less. When the porosity of the second region a2 and the fourth region a4 is within the above range, the porosity changes continuously between the first region a1 and the third region a3, or between the third region a3 and the fifth region a5, so that peeling between the regions can be further suppressed.

[0022] The porosity of the third region a3 is preferably 25% or more and 50% or less. When the porosity of the third region a3 is 25% or more, the opacity of the thermoplastic sheet is increased, and the hiding power can be sufficiently improved. When the porosity of the third region a3 is 50% or less, the mechanical strength of the thermoplastic sheet can be better maintained. From the same viewpoint, the porosity of the third region a3 is more preferably 25% or more and 40% or less.

[0023] The inner layer portion 12 and the surface layers 13A and 13B are relative to each other. 1A, since the first region a1 and the second region a2 have a relatively lower porosity than the third region, (first region a1 + second region a2) can be called surface layer portion 13A, and the third region a3 can be called inner layer portion 12. Similarly, since the fourth region a4 and the fifth region a5 have a relatively lower porosity than the third region, (fourth region a4 + fifth region a5) can be called surface layer portion 13B, and the third region a3 can be called inner layer portion 12.

[0024] In addition, in FIG. 1B, since the first region a1 and the fifth region a5 have a relatively lower porosity than the second region a2, the third region a3, and the fourth region a4, the first region a1 can be referred to as the surface layer portion 13A, (the second region a2 + the third region a3 + the fourth region a4) as the inner layer portion 12, and the fifth region a5 as the surface layer portion 13B.

[0025] The porosity of each region can be calculated from a cross-sectional image of the sheet observed with a scanning electron microscope (SEM).

[0026] Such a distribution state of porosity in the thickness direction can be adjusted by the temperature conditions and the stretching ratio in each step in the manufacturing method of a thermoplastic sheet described later. For example, when a sheet is preheated and stretched so as to maintain a lower temperature inside the sheet than near the surface, the porosity of the surface layer portions 13A and 13B of the obtained thermoplastic sheet tends to be low and the porosity of the inner layer portion 12 tends to be high. Furthermore, when the total stretching ratio is increased, the porosity of the inner layer portion 12 tends to be high.

[0027] [Thermoplastic sheet composition] As described above, the resin composition constituting the thermoplastic sheet contains a thermoplastic resin and an inorganic substance powder in a mass ratio of 50:50 to 10:90.

[0028] (thermoplastic resin) The thermoplastic resin is not particularly limited, and any thermoplastic resin can be used. Examples of the thermoplastic resin include: Polyolefin resins such as polyethylene resins, polypropylene resins, polymethyl-1-pentene, and ethylene-cyclic olefin copolymers; Functional group-containing polyolefin resins such as ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, metal salts of ethylene-methacrylic acid copolymers (ionomers), ethylene-acrylic acid alkyl ester copolymers, ethylene-methacrylic acid alkyl ester copolymers, maleic acid modified polyethylene, and maleic acid modified polypropylene; Polyamide resins such as nylon-6, nylon-6,6, nylon-6,10, and nylon-6,12; polyester resins such as polyethylene terephthalate and its copolymers, aromatic polyester resins such as polyethylene naphthalate and polybutylene terephthalate, and aliphatic polyester resins such as polybutylene succinate and polylactic acid; Polycarbonate resins such as aromatic polycarbonates and aliphatic polycarbonates; Polystyrene resin; Polyvinyl chloride resins such as polyvinyl chloride and polyvinylidene chloride; Polyphenylene sulfide; Examples of the polyether resin include polyether-based resins such as polyether sulfone, polyether ketone, and polyether ether ketone. The thermoplastic resin may be one type alone or two or more types may be used. Among these, polyolefin-based resins are preferred from the viewpoint of ease of molding, and polypropylene-based resins and polyethylene-based resins are more preferred.

[0029] The polypropylene resin may be a polymer containing 50% by mass or more of a structural unit derived from propylene. Examples of the polymer include a propylene homopolymer and a copolymer of propylene and other α-olefins copolymerizable with propylene. Examples of the other α-olefins copolymerizable with propylene include α-olefins having 4 to 10 carbon atoms, such as ethylene, 1-butene, isobutylene, and 1-pentene. Examples of the copolymer include an ethylene-propylene copolymer, a butene-1-propylene copolymer, and an ethylene-butene-1-propylene terpolymer.

[0030] The polyethylene resin may be a polymer containing 50% by mass or more of a structural unit derived from ethylene. Examples of the polymer include high density polyethylene (HDPE), low density polyethylene (LDPE), medium density polyethylene (MDPE), linear low density polyethylene (LLDPE), ethylene-vinyl acetate copolymer, ethylene-propylene copolymer, ethylene-propylene-1-butene copolymer, ethylene-1-butene copolymer, ethylene-4-methyl-1-pentene copolymer, etc.

[0031] (Inorganic substance powder) The inorganic powder may be, but is not limited to, a carbonate, sulfate, silicate, phosphate, or borate of a metal such as calcium, magnesium, aluminum, titanium, iron, or zinc; an oxide of the above metal; or a hydrate of the above salt or oxide. Examples of inorganic powder include calcium carbonate, magnesium carbonate, zinc oxide, titanium oxide, silica, alumina, clay, talc, kaolin, aluminum hydroxide, magnesium hydroxide, aluminum silicate, magnesium silicate, calcium silicate, aluminum sulfate, magnesium sulfate, calcium sulfate, magnesium phosphate, barium sulfate, silica sand, carbon black, zeolite, molybdenum, diatomaceous earth, sericite, shirasu, calcium sulfite, sodium sulfate, potassium titanate, bentonite, graphite, and the like. The inorganic powder may be one type or a combination of two or more types. Among them, calcium carbonate powder is preferable. Calcium carbonate powder can not only increase the mechanical strength of the sheet, but also reduce the manufacturing cost.

[0032] The calcium carbonate powder may be heavy calcium carbonate powder or light calcium carbonate powder. From the viewpoint of further increasing compatibility with thermoplastic resins, heavy calcium carbonate powder is preferred.

[0033] The shape of the inorganic powder is not particularly limited, and may be any of particles, flakes, granules, and fibers.

[0034] The average particle size of the inorganic powder is preferably 0.7 μm or more and 6.0 μm or less, and more preferably 1.0 μm or more and 5.0 μm or less. When the average particle size of the inorganic powder is 0.7 μm or more, the dispersibility of the inorganic powder in the thermoplastic resin can be further improved. When the average particle size of the inorganic powder is 6.0 μm or less, the deterioration of the appearance due to the unevenness of the surface of the obtained sheet can be further reduced.

[0035] The average particle size of the inorganic powder can be calculated from the results of measuring the specific surface area by the air permeability method in accordance with JIS M-8511. As a measuring device, for example, a specific surface area measuring device SS-100 manufactured by Shimadzu Corporation can be used.

[0036] The mass ratio of the thermoplastic resin to the inorganic powder (thermoplastic resin:inorganic powder) is 50:50 to 10:90, preferably 50:50 to 30:70. The higher the mass ratio of the inorganic powder, the more the amount of resin can be reduced, and therefore the more the environmental load can be reduced. The lower the mass ratio of the inorganic powder, the more likely it is that moldability will be improved.

[0037] The total amount of the thermoplastic resin and the inorganic powder in the resin composition is preferably 97% by mass or more, and can be 98% by mass or more and 99% by mass or less. When the total amount is 98% by mass or more, the properties of the thermoplastic resin and the inorganic powder are more easily obtained. When the total amount is 99% by mass or less, the functions of the other components are more easily obtained.

[0038] (Other ingredients) The resin composition may further contain other components in addition to the above-mentioned thermoplastic resin and inorganic fine powder. Examples of the other components include coloring agents, lubricants, coupling agents, flow improvers, dispersants, antioxidants, UV absorbers, stabilizers, antistatic agents, foaming agents, etc. These can be used alone or in combination of two or more. Among them, lubricants and antioxidants are preferred.

[0039] Examples of lubricants include higher aliphatic alcohols such as stearyl alcohol and cetyl alcohol; higher fatty acids such as lauric acid, stearic acid, oleic acid, and behenic acid; fatty acid amides such as stearic acid amide and oleic acid amide; fatty acid esters such as butyl stearate, stearyl stearate, and polyhydric alcohol fatty acid esters; and metal salts of higher fatty acids such as calcium stearate, magnesium stearate, zinc stearate, and calcium hydroxystearate.

[0040] Examples of the antioxidant include hindered phenol-based, hindered amine-based and phosphite-based antioxidants.

[0041] [Physical properties of thermoplastic sheet] When the thermoplastic sheet is measured by differential scanning calorimetry (DSC), it may have a first endothermic peak on the low temperature side and a second endothermic peak on the high temperature side that is 8°C or more higher than the temperature of the first endothermic peak (see FIG. 3 described later).

[0042] That is, the second endothermic peak on the high temperature side is a peak derived from large lamellar crystals remaining after the thermoplastic resin is not completely melted, while the first endothermic peak on the low temperature side is a peak that does not originate from such crystals and corresponds to the inherent melting point of the thermoplastic resin. In other words, the fact that a thermoplastic sheet has these two endothermic peaks indicates that the sheet was produced through a state in which part of the resin was completely melted and other parts were not completely melted (for example, the sheet was only melted near the surface and the interior was not melted).

[0043] The DSC measurement of the thermoplastic sheet can be carried out according to the following procedure. Approximately 5 mg of the thermoplastic sheet is cut out and placed in an aluminum pan for measurement. This is then set in a differential scanning calorimeter (TA Instruments DSC25) and heated to 200°C at 10°C / min in a nitrogen atmosphere, held for 5 minutes, and then cooled to 30°C at a rate of 10°C / min. The endothermic peak of the first heating process when the temperature is raised to 200°C at the above 10°C / min is observed.

[0044] The thickness of the thermoplastic sheet can be set arbitrarily depending on the application. The thickness of the thermoplastic sheet is, for example, preferably 60 μm or more and 200 μm or less, more preferably 80 μm or more and 180 μm or less. When the thickness of the thermoplastic sheet is 60 μm or more, preferably 80 μm or more, the mechanical strength and concealment of the sheet can be further improved. When the thickness of the thermoplastic sheet is 100 μm or less, the use of petroleum-derived resin can be further reduced.

[0045] The thermoplastic sheet can be used for any purpose, such as a printing sheet, a packaging sheet, or an optical sheet (such as a light diffusing sheet or a reflective sheet).

[0046] 2. Manufacturing method of thermoplastic sheet The method for producing the thermoplastic sheet includes the steps of 1) melt-kneading the resin composition containing a thermoplastic resin and an inorganic powder in a mass ratio of 50:50 to 10:90, and then molding the composition to obtain a sheet, 2) cooling the obtained sheet, 3) preheating the surface of the cooled sheet with a heating roll, 4) stretching the preheated sheet in the longitudinal direction (longitudinal stretching), 5) preheating the stretched sheet in a heating furnace, and 6) stretching the sheet preheated in the heating furnace in a direction perpendicular to the longitudinal direction (transverse stretching).

[0047] Step 1) (Process for obtaining sheets) First, the resin composition is melt-kneaded, and then extruded through a die to form a sheet.

[0048] The resin composition may be prepared by kneading the above-mentioned thermoplastic resin, inorganic fine powder and any other components in a predetermined ratio and then pelletizing the mixture.

[0049] The melt kneading of the resin composition can be carried out, for example, using a single-screw or multi-screw extruder. The melting temperature may be any temperature at which the thermoplastic resin is sufficiently melted. For example, when the melting point of the thermoplastic resin is Tm (°C), the melting temperature can be Tm°C or higher and (Tm+100)°C or lower, preferably (Tm+10)°C or higher and (Tm+70)°C or lower. The melting point of the thermoplastic resin can be specified as the temperature of the first endothermic peak in the above-mentioned DSC measurement.

[0050] Next, the molten resin composition extruded from the die is sandwiched between, for example, a pair of cooling rolls to form it into a sheet. The temperature of the cooling roll is preferably, for example, (Tm-80)°C or higher and (Tm-65)°C or lower. When the temperature of the cooling roll is (Tm-65)°C or lower, the molten resin composition is more likely to be cooled and solidified. On the other hand, when the temperature of the cooling roll is (Tm-80)°C or higher, the cooling time can be further shortened. For example, when the thermoplastic resin is a polypropylene-based resin (melting point 160°C), it can be cooled with a cooling roll of 80°C or higher and 95°C or lower.

[0051] Step 2) (cooling the sheet) The resulting sheet is then further cooled.

[0052] In the present embodiment, the sheet is cooled to a temperature lower than the surface temperature of the sheet when it is formed in the above step 1), for example, to a temperature lower by 5° C. or more, preferably 10° C. or more, and more preferably 15° C. or more. This makes it easier to maintain the temperature inside the sheet low in the preheating and stretching steps described below.

[0053] Specifically, it is preferable to cool the sheet until the surface temperature is (Tm-100)°C or more and less than (Tm-80)°C, and more preferable to cool the sheet until the surface temperature is (Tm-100)°C or more and less than (Tm-90)°C. By making the surface temperature of the sheet less than (Tm-80)°C, the sheet is cooled further to the inside. Therefore, as described later, the sheet can be stretched in a state where only the surface of the sheet is heated while maintaining the temperature inside the sheet low. By making the surface temperature of the sheet at (Tm-100)°C or more, the entire sheet does not cool too much, so that the vicinity of the surface of the sheet can be easily adjusted to a predetermined temperature or higher in the subsequent preheating process. For example, when the thermoplastic resin is a polypropylene-based resin (melting point 160°C), it is preferable to cool the sheet until the surface temperature is less than 80°C, preferably 65°C or less, more preferably 55°C. The surface temperature of the sheet can be measured by a non-contact infrared thermometer or the like.

[0054] The cooling method is not particularly limited, and may be, for example, a method of applying cold air, a method of cooling with a cooling roll, a method of conveying in a low-temperature atmosphere (including air cooling for a long period of time), a method of water cooling, or the like.

[0055] Step 3) (Preheating with a heating roll) Next, the surface of the cooled sheet is preheated by contacting it with a heating roll. In this embodiment, the sheet is heated while being supported and conveyed by heating rolls arranged in multiple stages.

[0056] Then, the surface of the cooled sheet is brought into contact with a heating roll, and preheated so that the temperature inside the sheet is maintained lower than the temperature of the sheet surface. Specifically, it is preferable to heat the sheet at a temperature lower than the melting point, more preferably to heat the sheet so that the surface temperature is (Tm-15)°C or higher (Tm-5)°C or lower, and even more preferably to heat the sheet so that the surface temperature is (Tm-10)°C or higher (Tm-5)°C or lower. If the surface temperature of the sheet is (Tm-5)°C or lower, it is difficult to heat the inside of the sheet, so that only the surface of the sheet can be heated while keeping the temperature inside the sheet lower. If the surface temperature of the sheet is (Tm-15)°C or higher, the sheet can be adjusted to a temperature more suitable for stretching. For example, when the thermoplastic resin is a polypropylene-based resin (melting point 160°C), the surface temperature of the sheet can be, for example, 145°C or higher, preferably 150°C or higher and 155°C or lower. The surface temperature of the sheet can be adjusted by the set temperature of the heating roll.

[0057] The temperature difference in the thickness direction of the sheet can be adjusted not only by the temperature of the heating roll but also by the sheet conveying speed. For example, by increasing the sheet conveying speed, the inside of the sheet is less likely to be heated, and only the surface vicinity of the sheet can be heated while the inside of the sheet is kept at a lower temperature.

[0058] Step 4) (longitudinal stretching step) The preheated sheet is then stretched in the longitudinal direction.

[0059] The sheet surface temperature during stretching can be set to the same as the sheet surface temperature during preheating.

[0060] The preheated sheet is then stretched longitudinally while the temperature inside the sheet is kept low and only the surface of the sheet is heated. At this time, since the temperature near the surface of the sheet is high, the thermoplastic resin is molten and voids are unlikely to form between the thermoplastic resin and the inorganic powder. Meanwhile, since the temperature inside the sheet is low, the thermoplastic resin is not completely melted and voids are likely to form between the resin and the inorganic powder by stretching. As a result, a sheet with few voids near the surface and many voids inside is obtained.

[0061] The stretching ratio in the longitudinal direction is not particularly limited and may be appropriately set according to the required characteristics and the content of the inorganic powder. In this embodiment, the total stretching ratio of the longitudinal stretching and the transverse stretching is preferably adjusted to 8 times or more, and more preferably adjusted to 12 times or more and 20 times or less. Here, the total stretching ratio means the product of the stretching ratio of the longitudinal stretching and the stretching ratio in the transverse direction.

[0062] Among these, the stretching ratio in the longitudinal direction is preferably 2.5 times or more and 5 times or less, and more preferably 3 times or more and 4.5 times or less. When the stretching ratio in the longitudinal direction is 2.5 times or more, the voids tend to grow larger, and the concealing properties of the obtained sheet become higher. When the stretching ratio in the longitudinal direction is 5 times or less, the mechanical strength of the entire sheet is less likely to be impaired.

[0063] The longitudinal stretching can be carried out, for example, by a method in which stretching is carried out using a plurality of rolls with different peripheral speeds (roll method).

[0064] Step 5) (Preheating in a heating furnace) Next, the longitudinally stretched sheet is further preheated in a heating furnace.

[0065] Specifically, the sheet is preferably heated so that the surface temperature is greater than (Tm-5)°C and less than (Tm+10)°C, and may be heated to greater than or equal to Tm°C and less than or equal to (Tm+7)°C. When the surface temperature of the sheet is greater than (Tm-5)°C, the vicinity of the surface of the sheet is less likely to cool, and voids are less likely to form. For example, when the thermoplastic resin is a polypropylene-based resin (melting point 160°C), the sheet surface temperature can be, for example, greater than 155°C, greater than or equal to 160°C, or greater than or equal to 165°C and less than or equal to 170°C.

[0066] Step 6) (transverse stretching step) The preheated sheet is then stretched in a direction perpendicular to the longitudinal direction.

[0067] The sheet surface temperature during stretching can be the same as the sheet surface temperature in the above step 5).

[0068] The stretching ratio in the direction perpendicular to the longitudinal direction may be adjusted so that the total stretching ratio of the longitudinal stretching and the transverse stretching is within the above range. In this embodiment, the stretching ratio in the direction perpendicular to the longitudinal direction is preferably 3.5 times or more and 7 times or less, and more preferably 4 times or more and 6 times or less. When the stretching ratio in the direction perpendicular to the longitudinal direction is 3.5 times or more, the voids tend to grow and become larger, and the concealing property of the obtained sheet tends to be higher. When the stretching ratio in the direction perpendicular to the longitudinal direction is 7 times or less, the mechanical strength of the entire sheet is less likely to be impaired.

[0069] The transverse stretching can be performed by fixing both widthwise ends of the sheet with clips or pins and widening the interval between the clips or pins in the widthwise direction of the sheet (tenter method).

[0070] Even at this stage, near the surface of the sheet, since the resin is melted, voids are less likely to be generated by stretching. On the other hand, since the temperature inside the sheet is low, the resin is not melted, and starting from the voids formed by longitudinal stretching, the voids further grow. As a result, as shown in FIGS. 1A and 1B and FIG. 2A to be described later, a thermoplastic sheet in which there are extremely few voids or almost no voids in the surface layer portion and voids exist only in the inner layer portion can be obtained by single-layer extrusion.

[0071] In addition, in the above-described embodiment, in the method for manufacturing a thermoplastic sheet, the step (step of stretching in a heating furnace) of 4) and the step (step of transverse stretching) of 5) are performed, but these steps can also be omitted according to the required characteristics of the sheet.

Example

[0072] Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited thereto.

[0073] 1. Preparation of materials 1-1. Thermoplastic resin · Polypropylene homopolymer PP1 (E111G manufactured by Prime Polymer Co., Ltd., melting point Tm: 160 ° C)

[0074] <Melting point of PP1> The melting point of PP1 is a value measured as the temperature of the first endothermic peak in the DSC measurement described later.

[0075] 1-2. Inorganic substance powder · Heavy calcium carbonate powder GCC1 (YBB1C manufactured by Imerys Minerals Co., Ltd., average particle diameter 2.3 μm)

[0076] <Average particle diameter of inorganic substance powder> Calculated from the measurement results of the specific surface area by the air permeability method according to JIS M-8511 using the specific surface area measuring device "SS-100 type" manufactured by Shimadzu Corporation.

[0077] 1-3. Other components · Antioxidant Antistatic agent Lubricants

[0078] 2. Thermoplastic Sheet Fabrication <Example 1> (Pellet preparation) The mixture was kneaded for 15 minutes in a mass ratio of PP1 / GCC1 / other components (total) = 40 / 60 / 1 in a 75-liter 3D Banbury mixer manufactured by Kobe Steel, Ltd. The temperature of the discharged resin was 180°C. The mixture was then passed through a 150 mm L / D = 10 strainer manufactured by Katsu Seisakusho Co., Ltd., which was maintained at 180°C, and a strand was extruded from a die at 190°C, cooled with water, and cut to obtain a pellet-shaped resin composition.

[0079] (Preparation of Thermoplastic Sheet) The pellet-like resin composition prepared above was melt-kneaded and extruded at 220°C using a Toyo Seiki Seisakusho Labopla single-screw T-die extrusion molding device (φ20 mm, L / D=25). Next, the molten resin composition extruded from the T-die was brought into contact with a metal roll with a surface temperature set at 80 to 95°C, cooled, and molded into a sheet (1) Step of Obtaining a Sheet). The obtained sheet was cooled to a surface temperature of 60° C. by conveying it for a predetermined time with a cast roll set at 80° C. or less (2) Step of cooling the sheet). Next, the cooled sheet was preheated until the sheet surface temperature reached 150°C while being transported by heating rolls (3) Preheating with heating rolls), and in the longitudinal stretching section, the stretching roll temperature was set to 155°C, and the sheet was stretched 3.5 times in the MD direction (4) Longitudinal stretching step). The longitudinally stretched sheet was then preheated in an oven set at 157°C (5) Preheating in a heating furnace), and in the transverse stretching section, the sheet was stretched 5 times in the oven in the TD direction (6) Transverse stretching), thereby obtaining a thermoplastic sheet with a thickness of 80 μm.

[0080] <Comparative Example 1> In producing the thermoplastic sheet of Example 1, a thermoplastic sheet was obtained in the same manner as in Example 1, except that 2) the step of cooling the sheet was not performed, and 3) in the step of preheating with a heating roll, the sheet was heated so that the surface temperature of the sheet reached 145°C.

[0081] <Evaluation> The cross section of the obtained thermoplastic sheet was observed by SEM, and DSC measurement and porosity measurement were performed. In addition, the hiding power and tape peel strength were measured by the following methods.

[0082] (1) Cross-sectional observation using SEM The obtained thermoplastic sheet was cut, and the cut surface was observed by SEM (TM4000Plus manufactured by Hitachi High-Technologies Corporation) at a magnification of 1000 times and an accelerating voltage of 15 kV.

[0083] (2)DSC measurement Approximately 5 mg of the thermoplastic sheet was cut out and placed in an aluminum pan for measurement. This was set in a differential scanning calorimeter (TA Instruments DSC25) and heated to 200°C at 10°C / min under a nitrogen atmosphere, held for 5 minutes, and then cooled to 30°C at a rate of 10°C / min. The endothermic peak of the first heating process when the temperature was raised to 200°C at the above 10°C / min was observed.

[0084] (3) Calculation of void ratio The porosity of each region was calculated from cross-sectional images of the sheet observed with a scanning electron microscope (SEM). First, the sheet was cut in the thickness direction using a microtome. A 15 keV backscattered electron image of the cross section of the cut sheet was observed. The brightness of the obtained reflected electron image was normalized to 0 to 255, and a portion with a brightness of less than 80 was evaluated as a void. When one surface was 0% and the other surface was 100%, the area between 0% and 20% was defined as the first region a1, the area between 20% and 40% was defined as the second region a2, the area between 40% and 60% was defined as the third region a3, the area between 60% and 80% was defined as the fourth region a4, and the area between 80% and 100% was defined as the fifth region a5. Images of each region were extracted. For each extracted region image, the porosity was calculated as the area with brightness less than 80 divided by the area of ​​the extracted image.

[0085] (4) Opacity (hiding ability) The opacity of the thermoplastic sheet was measured according to JIS P8149 and evaluated based on the following criteria. ○: Opacity is 90% or more △: Opacity is between 85% and 90% ×: Opacity is less than 85%

[0086] (5) Tape peel strength The thermoplastic sheet was cut into a size of 10 cm length x 10 cm width to prepare a test piece. The test piece was attached and fixed on a substrate with a double-sided adhesive tape having a strong adhesive force. A tape peeling test was performed in which "Scotch Tape #600" (protective tape) manufactured by 3M was attached to the surface of the test piece fixed on the substrate by applying a roller load of 2 kg, and the protective tape was then vigorously peeled off immediately thereafter to observe the state of the surface layer. This test was repeated 15 times, and the tape peeling strength was evaluated based on the following criteria. ○: No surface damage or peeling was observed even after 15 repeated tape peel tests. △: No surface damage or peeling was observed even after 10 repeated tape peel tests, but surface damage or peeling was observed on the 11th, 12th, 13th, or 14th test. ×: Surface destruction or peeling of the surface was observed within 10 repetitions of the tape peeling test.

[0087] The evaluation results of the thermoplastic sheets of Example 1 and Comparative Example 1 are shown in Table 1. FIG. 2A shows an SEM image of a cross section in a direction perpendicular to the longitudinal direction of the thermoplastic sheet of Example 1, and FIG. 2B shows an SEM image of a cross section in a direction perpendicular to the longitudinal direction of the thermoplastic sheet of Comparative Example 1. FIG. 3 shows a graph obtained by DSC measurement of the thermoplastic sheet of Example 1.

[0088] [Table 1]

[0089] 2B and Table 1, the thermoplastic sheet of Comparative Example 1 had a uniform distribution of voids in the thickness direction, and the porosity was also uniform in the thickness direction. In addition, it was found that the thermoplastic sheet had good opacity, but poor tape peel strength.

[0090] In contrast, as shown in Fig. 2A and Table 1, the thermoplastic sheet of Example 1 has a high porosity in the inner layer portion (third region) and a low porosity in the surface layer portions (first and fifth regions). It is also found that the sheet has good opacity and high tape peel strength.

[0091] In addition, as shown in Table 1, in the sheet of Comparative Example 1, only an endothermic peak near 170°C (the second endothermic peak on the high temperature side) was confirmed; whereas in the sheet of Example 1, in addition to an endothermic peak near 170°C (the second endothermic peak on the high temperature side), an endothermic peak near 160°C (the first endothermic peak on the low temperature side) was also confirmed (see FIG. 3).

[0092] Usually, when a thermoplastic resin is heated, the lamellar crystals grow and become larger in size, but when the resin is completely melted, the lamellar crystals disappear. In other words, the sheet of Comparative Example 1 was produced under conditions in which the entire resin was not melted, so many large lamellar crystals remained, and only the endothermic peaks derived from the crystals were confirmed. In contrast, the sheet of Example 1 was produced under conditions in which the resin in the interior was not melted, but the resin near the surface was melted, so two endothermic peaks were confirmed. [Industrial Applicability]

[0093] According to the present invention, it is possible to provide a thermoplastic sheet that has sufficient hiding power and is capable of suppressing a decrease in the mechanical strength of a surface layer portion of the sheet, and a method for producing the same, by a simple method. [Explanation of symbols]

[0094] 10 Thermoplastic sheet 11 void 12 Inner Layer 13A, 13B Surface part

Claims

1. A thermoplastic sheet obtained by molding a resin composition containing a thermoplastic resin and an inorganic substance powder in a mass ratio of 50:50 to 10:90 into a sheet shape, An inner layer portion having voids; Two surface layer portions arranged on either side of the inner layer portion and having a porosity lower than the porosity of the inner layer portion; Including, Thermoplastic sheet.

2. A thermoplastic sheet obtained by molding a resin composition containing a thermoplastic resin and an inorganic substance powder in a mass ratio of 50:50 to 10:90 into a sheet shape, In the thickness direction of the thermoplastic sheet, when one surface is 0% and the other surface is 100%, a region of 0% or more and less than 20% is defined as a first region, a region of 20% or more and less than 40% is defined as a second region, a region of 40% or more and less than 60% is defined as a third region, a region of 60% or more and less than 80% is defined as a fourth region, and a region of 80% or more and 100% is defined as a fifth region, The porosity of the first region and the fifth region is less than 10%; The porosity of the second region and the fourth region is 10% or more and less than 25%, The porosity of the third region is 25% or more and 50% or less. Thermoplastic sheet.

3. When the thermoplastic sheet is subjected to differential scanning calorimetry, it has a first endothermic peak corresponding to the melting point of the thermoplastic resin and a second endothermic peak at a higher temperature side than the first endothermic peak by 8° C. or more. The thermoplastic sheet according to claim 1 or 2.

4. The resin composition contains the thermoplastic resin and the inorganic substance powder in a mass ratio of 50:50 to 30:

70. The thermoplastic sheet according to claim 1 or 2.

5. The thermoplastic resin includes a polyolefin resin. The thermoplastic sheet according to claim 1 or 2.

6. The inorganic powder includes ground calcium carbonate powder. The thermoplastic sheet according to claim 1 or 2.

7. The average particle size of the ground calcium carbonate powder is 0.7 μm or more and 6.0 μm or less. The thermoplastic sheet of claim 6.

8. A step of melting and kneading a resin composition containing a thermoplastic resin and an inorganic substance powder in a mass ratio of 50:50 to 10:90, and then molding the composition to obtain a sheet; cooling the sheet; contacting the surface of the cooled sheet with a heating roll to preheat the sheet so that the temperature inside the sheet is kept lower than the temperature of the sheet surface; stretching the preheated sheet in a longitudinal direction; preheating the stretched sheet in a heating oven; stretching the sheet preheated in the heating furnace in a direction perpendicular to the longitudinal direction; Including, A method for producing a thermoplastic sheet.

9. When the melting point of the thermoplastic resin is Tm (°C), In the step of cooling the sheet, the sheet is cooled so that the surface temperature of the sheet is equal to or higher than (Tm-100) ° C. and lower than (Tm-80) ° C. A method for producing the thermoplastic sheet according to claim 8.

10. In the preheating step using the heating roll, the sheet is heated so that the surface temperature of the sheet is equal to or higher than (Tm-15) ° C. and equal to or lower than (Tm-5) ° C. A method for producing the thermoplastic sheet according to claim 9.

11. In the preheating step in the heating furnace, the sheet is heated so that the surface temperature of the sheet is higher than (Tm-5) ° C. and lower than (Tm+5) ° C. A method for producing the thermoplastic sheet according to claim 9.

12. The step of stretching in the longitudinal direction and the step of stretching in the direction perpendicular to the longitudinal direction are performed so that a total stretching ratio is 8 times or more. A method for producing a thermoplastic sheet according to any one of claims 8 to 11.

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