JPS631183B2 - - Google Patents

Description

[Detailed description of the invention]

[] BACKGROUND TECHNICAL FIELD OF THE INVENTION The present invention relates to a thermoplastic resin film with excellent printability suitable for use as synthetic paper. More specifically, the present invention provides printing, folding,
Trouble caused by falling of compounded filler during secondary processing such as lamination, that is, so-called paper powder trouble.
This invention relates to a filler-containing thermoplastic resin film. By uniaxially or biaxially stretching a thermoplastic resin film containing a fine filler at an appropriate temperature, an opaque thermoplastic resin film can be obtained.
This film is similar to pulp paper in terms of its opacity, whiteness, texture and feel, and can be used as a single-layer structure consisting only of this film or as a multi-layer structure with this film as a surface layer. It is known that it can be used for various paper purposes. Stretched films made of filler-containing thermoplastic resins are opaque and white because peeling occurs at the interface between the resin and filler during film stretching, and as stretching progresses, fine gaps are created between the resin and filler. Therefore, at the end of stretching, fine voids containing filler particles are formed inside the film, and the closed shape of the voids is not maintained near the film surface, resulting in the formation of fine cracks on the surface. This is because light is scattered on the film surface and inside. These surface cracks and internal voids give the film properties similar to pulp paper. However, when performing secondary processing such as printing, folding, bookbinding, and bag making on such opaque film,
As voids near the surface have their closed structure destroyed and become surface cracks, filler particles that have come to exist in a free or semi-free state on the surface adhere to the rolls and blankets of secondary processing machines and cause problems during processing. Failures often occur and the efficiency of secondary processing is significantly reduced. Prior Art Regarding the prevention of such paper dust problems, there are methods of applying a pigment coat on the surface similar to pulp paper to prevent filler particles from falling off, methods of coating the surface with a paste-like substance, and methods of combining filler and resin. A method of blending additives to improve interfacial adhesion,
and so on. However, these methods are not satisfactory. In other words, in the pigment coating method, the film and the coating layer tend to separate easily and the generation of paper dust from the edges of the film is unavoidable, whereas in the method of coating with a paste-like substance, the paste-like substance impairs printability. This is because there are problems in terms of lowering the temperature and the complexity of the process. On the other hand, the method of improving resin/filler particle interfacial adhesion is simple in terms of process, but has the disadvantage that the additives used to improve adhesion are expensive and cannot be used in large quantities. Significant problems arise during stretching in this case. In other words, since the adhesive force at the resin/filler grain interface becomes strong, a large stretching force is required during film stretching, which may make it impossible to produce thick films depending on the stretching equipment, or may cause breakage problems during film stretching. There are problems such as frequent occurrence of problems and the inability to perform stable production. By the way, some of the inventors of the present invention have already made a proposal regarding synthetic paper that is improved in terms of paper dust problems (Japanese Patent Application No. 139674/1982). The synthetic paper according to this prior invention is a synthetic paper having a paper-like layer made of a stretched film of a polyolefin composition containing an inorganic filler, in which a part of the polyolefin is graft-copolymerized with ethylenically unsaturated carboxylic acid, etc. be. One specific example of the synthetic paper according to this prior invention has a multilayer structure consisting of a base layer and this paper-like layer, but in this case, the thickness of the paper-like layer (after stretching) is the sum of the front and back layers. 40-50 for multilayer structures
% (base material layer 50-60 microns, paper-like layer
25 microns x 2). Therefore, the synthetic paper according to this prior invention had the above-mentioned problems regarding stretching. [] Summary of the Invention The present invention aims to solve the above points, particularly the problems recognized in the case of improving interfacial adhesion between resin/filler particles, and provides an additive for improving interfacial adhesion. This objective is achieved by blending only into a thin surface layer to form a laminated structure. Therefore, the thermoplastic resin film with excellent printability according to the present invention has a multilayer structure consisting of a base layer and a printable surface layer made of a stretched film of an inorganic filler-containing polypropylene composition provided on at least one side of the base layer. The film is characterized in that the surface layer is defined as below. (1) The thickness of the surface layer shall be 1/6 or less of the thickness of this multilayer structure film and 3 microns or more. (2) Having minute voids. (3) The inorganic filler-containing polypropylene composition forming the surface layer must contain modified polypropylene in which polypropylene and ethylenically unsaturated carboxylic acid or its anhydride are copolymerized. Effects Modified polypropylene as a resin/filler particle interfacial adhesion enhancer is blended only in the surface layer, and this surface layer is made sufficiently thin, making it possible to create a thermoplastic resin film without the filler falling off during secondary processing. It became possible to produce it stably and economically. The thermoplastic resin film with excellent printability according to the present invention is particularly suitable for use as synthetic paper. [] Detailed Description of the Invention The thermoplastic resin film with excellent printability according to the present invention has a multilayer structure consisting of a base layer and a surface layer. This type of paper is distinguished from film or synthetic paper. 1 Surface Layer The surface layer consists of a stretched film of an inorganic filler-containing polypropylene composition provided on at least one side, preferably both sides, of the base layer. This film has paper-like properties due to the mechanism described above, and also contains modified polypropylene in accordance with one of the features of the present invention. In the present invention, the term "surface layer" refers to a layer containing an inorganic filler and modified polypropylene that is present on the surface of the base layer in relation to the base layer. Therefore, even if the multilayer structure film of the present invention comprising such a base layer and a surface layer has some kind of layer on the surface layer and this surface layer is buried, this layer can be removed from the surface. It's called a layer. 1 Modified polypropylene Modified polypropylene used as a resin/filler particle interfacial adhesion enhancer in the present invention is a combination of polypropylene and ethylenically unsaturated carboxylic acid or its anhydride (hereinafter sometimes referred to as modified monomer). It is a copolymerization of. Specific examples of the polypropylene in this case include a propylene homopolymer, a copolymer of propylene and ethylene or an α-olefin having 4 to 8 carbon atoms, or a mixture thereof. The polypropylene may be blended with a small amount (up to 15% by weight of the polypropylene) of a compatible or compatible thermoplastic resin such as high-density polyethylene, low-density polyethylene, ethylene-vinyl acetate copolymer, etc. . Ethylenically unsaturated carboxylic acids (and their anhydrides) may include maleic acid, itaconic acid, hemicic acid (and their respective acid anhydrides), acrylic acid, and others. Particularly effective in preventing desorption of filler particles according to the present invention are maleic acid and maleic anhydride. Since this reaction is a copolymerization of the polymer polypropylene and the monomer ethylenically unsaturated carboxylic acid or its anhydride, this reaction belongs to the category of so-called graft copolymerization. Graft copolymerization techniques are already known, and the modified polypropylene for the present invention can be obtained by any of these methods, but typical graft copolymerization techniques are classified according to the type of polymerization initiation. For example, it is as follows. (i) A method in which maleic anhydride and polypropylene are subjected to suspension polymerization in an aqueous solution in the presence of a radical initiator. (ii) A method in which a resin composition containing a radical initiator is melt-kneaded using heat and shear force in a kneader. The modified polypropylene produced in this way is a mixture of the graft copolymerized polypropylene, the polypropylene that did not participate in the graft copolymerization, and optionally a polymer of the modifying monomer (the modifying monomer is In the case of a monomer with a large absolute value of the so-called e value, such as maleic acid, almost no polymer of itself will be formed). In such a case,
Since it is virtually impossible and not practical to define the amount of carboxylic acid residues in the modified polypropylene by the weight percentage of the graft copolymer in the modified polypropylene, The amount of carboxylic acid residue is determined by the content of the modified monomer obtained. Further, the content of modified polypropylene in the polypropylene composition for the surface layer is also defined by the amount of the polymerized modifying monomer (details will be described later). The modified polypropylene used in the present invention preferably has a content of such polymerized modifying monomers of 0.01 to 5% by weight, particularly 0.02 to 2% by weight. 2 Inorganic filler Any fine inorganic filler that can be blended into polypropylene can be used as the inorganic filler for the surface layer of the present invention. Specific examples of such inorganic fillers include clay, titanium oxide,
Barium sulfate, calcium carbonate, silica, alumina, silicate, magnesium carbonate, calcium sulfite, diatomaceous earth, gypsum, etc. Among these, calcium carbonate is particularly preferred for use in the present invention because it has a good printing effect, high whiteness, and low abrasion. Since the number of defects and falling off from the stretched film is extremely high, the effect of improving the adhesiveness of the modified polypropylene according to the present invention is particularly noticeable. The inorganic filler is granular and has an average particle size of 0.3~
Preferably it is 20 microns, especially 0.5 to 5 microns. If it is less than 0.3 microns, voids will not be effectively generated, resulting in poor whitening, and secondary aggregation of the filler will occur, resulting in poor dispersion into the resin. When it exceeds 20 microns, the surface of the molded product (film) becomes rough and the void generation rate per unit weight of filler becomes small. The shape of the particles of the inorganic filler is basically arbitrary, and may be, for example, spherical, rod-shaped, conical, pyramidal, or other shapes within the range of granular shapes. However, from the viewpoint of void generation, it is preferable that the material is not flat. The blending amount of the filler is appropriately selected depending on the intended use of the film to be obtained (details will be described later). 3. Polypropylene Composition for Surface Layer The surface layer consists of a polypropylene composition containing an inorganic filler, and this composition contains the modified polypropylene as described above. Here, the fact that the surface layer polypropylene composition contains modified polypropylene does not mean that the composition consists only of this modified polypropylene as a resin component, but also when it is a mixture with "unmodified" polypropylene. It also includes.
Regarding the type of unmodified polypropylene in this case, the above explanation regarding modified polypropylene applies. The polypropylene composition for the surface layer consists of two essential components: modified polypropylene and an inorganic filler (and non-essential or auxiliary components added as necessary). The amount of the inorganic filler to be added is appropriately selected depending on the intended use of the film to be obtained. In the case of films intended for use in printing and writing paper applications where printing or writing is often carried out (sometimes referred to as synthetic paper), the content in the polypropylene composition for the surface layer is between 12 and 12. 70% by weight, especially
It is preferably 30 to 70% by weight. This is because if it is less than 12%, printability or writability will not be sufficient, and if it exceeds 70%, the surface smoothness and surface strength will be low, which is disadvantageous. In addition, when the multilayer structure film of the present invention is used as synthetic paper, there is a preferable range for the filler content of the multilayer structure film as a whole. 8-40% by weight, especially 15-40% by weight. If it is less than 8%, the quality of the resulting multilayer film as synthetic paper will be poor, so in order to satisfy the minimum whiteness, opacity, writability, printability, etc. of synthetic paper, it is necessary to use at least 8%.
%is necessary. If it exceeds 40%, the strength and surface properties of the paper will be insufficient. Since the purpose of blending the modified polypropylene is to prevent the filler from falling off, it is reasonable to specify the amount of the modified polypropylene based on the amount of the filler. The amount of polymerized modifying monomer in the modified polypropylene is 0.01 to 5 parts by weight, particularly 0.03 to 0.15 parts by weight, relative to 100 parts by weight of the inorganic filler in the polypropylene composition for the surface layer. Amounts of modified polypropylene are preferred. If it is less than 0.01 part, the adhesive effect of the filler is small,
Prevention of paper dust problems is insufficient. If the amount exceeds 5 parts, the polypropylene composition undergoes significant hue change and deterioration due to heat, and cannot be used for practical printing purposes. Such a composition consisting of two essential components is a thermoplastic resin composition containing polypropylene as the main resin component, and therefore can contain various non-essential or auxiliary components that thermoplastic resin compositions normally contain. Such aids include, for example, antioxidants, ultraviolet absorbers, dyes and pigments, fluorescent brighteners, and the like. 4 Surface Layer Thickness The thickness of the surface layer should be at least 3 microns in the multilayer structure film after stretching. If it is less than 3 microns, the intended effect of preventing filler from falling off is not sufficient. On the other hand, if the thickness of the surface layer is too large, the forming stability will be poor when carrying out a stretching process to obtain a multilayer structure film having such a surface layer.
Fractures often occur during stretching. Therefore, the thickness of the surface layer should be 1/6 or less of the thickness of the multilayer film after stretching. Note that the thickness of the surface layer here refers to a single layer of the surface layer. Therefore, if surface layers exist on both sides of the base material layer, the total thickness is 1/3 of the thickness of the multilayer structure film. This means the following. When there are two layers, the preferred thickness is 1/5 or less. 5. Stretching In order for surface cracks to be generated as described above, the surface layer should be stretched. The stretching may be uniaxial or biaxial stretching, but in the case of a preferred production method described later, uniaxial stretching is usually used (the stretching ratio will be described later). 6 Void Since it is stretched in this way,
There are cracks on the surface of the surface layer and voids inside the surface layer. From the viewpoint of writability and printability, the void ratio of the surface layer is preferably 10% or more. Note that the void ratio is defined by the following formula. Void ratio (%) = Specific gravity of the film before stretching - Specific gravity of the film after stretching / Specific gravity of the film before stretching 2 Base layer Should have a surface layer made of the polypropylene composition as described above on one or both sides. Various film materials can be used as the base layer. This base material layer may also have a multilayer structure itself (details will be described later). The material for the base layer may be any suitable material, but it is usually one that can be stretched if the preferred manufacturing method described below is followed. Various thermoplastic resins can be used as stretchable materials for the base layer, but polyolefin is preferred among them. Examples of the polyolefin include polypropylene, high-density polyethylene, medium-low density polyethylene, and the like, and either the same kind of polypropylene as the polypropylene of the surface layer main resin or a different kind can be used. The base layer preferably has good stretchability, and therefore the resin used preferably does not substantially contain the above-mentioned modified polypropylene. As mentioned above, it is preferable that the base layer also contains an inorganic filler, but if both the filler and the modified polypropylene are contained, the stretchability will deteriorate. The base material layer is also preferably uniaxially or biaxially stretched. The base layer may also contain auxiliary components as described above for the surface layer. Further, the paper having a large number of fine voids inside has an excellent paper-like feel. As mentioned above, the base material layer itself may have a multilayer structure. An example of such a multilayer structure is one consisting of a biaxially oriented layer and a uniaxially oriented layer bonded preferably on both sides thereof. In this case, the two layers differ in the filler content and/or the type of resin used (differences due to resin type or melting properties), but this difference is useful from the viewpoint of producing the stretched film. It is determined. 3. Manufacturing Some preferred methods for manufacturing the multilayer structure film of the present invention are as follows, for example. (1) Materials for the base layer, such as polypropylene, and inorganic fillers and auxiliary components used as necessary are mixed, kneaded in an extruder, and then formed into a sheet. Place this sheet lengthwise at least
After stretching 1.3 times, on one or both sides,
Materials for the surface layer, that is, modified polypropylene, an inorganic filler, and if necessary, unmodified polypropylene are mixed, kneaded in an extruder, extruded into a sheet, and laminated when melted. The multilayer sheet is stretched at least 3.5 times in the transverse direction. (2) When extrusion laminating a surface layer sheet on one or both sides of the base layer sheet as described above, polyolefin, preferably polypropylene (unmodified), and inorganic filler are added between both sheets. After melt-kneading the mixture with the agent in a separate extruder, it is coextruded from the same die as the sheet for the surface layer to produce a three-layer to five-layer sheet, which is then stretched in the same manner as above. According to the embodiment (2), the base material layer itself has a multilayer structure. In other words, it has a multilayer structure consisting of a biaxially stretched layer and a uniaxially stretched layer (formed at the same time as the surface layer) between this layer and the surface layer (layer containing modified polypropylene). . When the base material layer has such a multilayer structure according to the embodiment (2), there are competitive requirements for the base material layer, that is, it has good biaxial stretchability and has a somewhat high filler content. Being able to skillfully fulfill the following. That is, a biaxially oriented layer is made from a composition that has a relatively low content of a resin and/or filler that is relatively easy to stretch biaxially, and a composition that has a relatively high content of a resin and/or filler that is relatively difficult to stretch biaxially. By forming a uniaxially oriented layer from the composition to form a multilayer structure, it is possible to combine the good mechanical properties and isotropy of a biaxially oriented film with the high brightness or opacity of a oriented film with a high filler content. This is because the base material layer included therein can be easily obtained. 4 Examples In order to make the present invention easier to understand, the following examples will be used to explain the invention. These examples constitute one aspect of the present invention, and can be arbitrarily modified within the scope of the present invention. Example 1 and Comparative Example 1 (1) 5% by weight of calcium carbonate with an average particle size of 1.5 microns was mixed with polypropylene with a melt index (MI) of 0.8, and after kneading in an extruder set at 270°C, it was shaped into a sheet. The product was extruded and cooled using a cooling device to obtain a non-stretched sheet. This sheet was heated to 140°C and then stretched 5 times in the machine direction. (2) 100 parts polypropylene with MI 0.6, 10 parts maleic anhydride, and benzoyl peroxide (BPO)
5 parts were reacted in 600 parts of xylene (all parts by weight) at 120°C for 5 hours, and reprecipitated with a large excess of acetone to obtain powdered polypropylene modified with maleic anhydride. When maleic anhydride in this polymer was quantified by infrared absorption spectrum, it was found that 0.5% by weight of maleic anhydride had been graft copolymerized. (3) Modified polypropylene obtained here 6% by weight
and MI4.0 polypropylene 49% by weight and average particle size
Composition (A) (modified monomer amount 0.07 parts by weight per 100 parts by weight of filler) mixed with 45% by weight of calcium carbonate having a particle size of 1.5 microns was melt-kneaded using an extruder set at 270°C. , a composition (B) containing 55% by weight of polypropylene with an MI of 4.0 and 45% by weight of calcium carbonate with an average particle size of 1.5 microns.
was melt-kneaded in a separate extruder set at 270°C and laminated in a die, and coextruded on both sides of the sheet obtained in (1) above that was stretched 5 times in the longitudinal direction. The layer containing high-quality polypropylene was placed on the outside. Next, this five-layer laminate is 155
After heating to 0.degree. C., the film was stretched 7.5 times in the transverse direction to obtain a five-layer film. Further, as Comparative Example 1, a film obtained in the same manner using the composition (A) without the addition of modified polypropylene was also molded under the same conditions.

[Table] Comparative Example 2 6% by weight of the modified polypropylene obtained in (2) and 49% by weight of polypropylene with MI 4.0 were placed on both sides of the sheet obtained in (1) of Example 1 and stretched 5 times in the longitudinal direction. 45% by weight of calcium carbonate with an average particle size of 1.5 microns
Composition (A) (modified monomer amount: 0.07 parts by weight per 100 parts by weight of filler) was melt-kneaded using an extruder set at 270°C, and then extrusion laminated. A three-layer film shown in Table 2 below was obtained by stretching 7.5 times.

[Table] Example 2 (1) 75% by weight of polypropylene with MI1.0, 10% by weight of high-density polyethylene (HD/PE) with MI1.5, and 15% by weight of calcium carbonate with an average particle size of 1.5 microns.
After kneading with an extruder, the mixture was extruded into a sheet and cooled to obtain a non-stretched sheet. This unstretched sheet was heated to 140°C and then stretched 5 times in the longitudinal direction to obtain a longitudinally stretched film. (2) A mixture of the modified polypropylene obtained in Example 1 (maleic acid content: 0.6% by weight) was prepared in the following proportions.

[Table] As in Example 1, this mixture was coextruded with a composition of 55% by weight of polypropylene with an MI of 4.0 and 45% by weight of calcium carbonate with an average particle size of 1.5μ. The obtained sheet was laminated on both sides and then stretched in the transverse direction at 158° C. by a factor of 7.5 to obtain a printable film. The evaluation results of this film are shown in Table 4.

[Table] * Comparative Example Example 3 Compositions (A) and (B) were applied in the same manner as in Example 1 to one side of a uniaxially stretched sheet obtained in the same manner as in Example 1.
Three-layer films were obtained by coextruding to various thicknesses and then stretching the laminated sheet in the transverse direction. The evaluation results of this film are shown in Table 5.

【table】

[Table] * Comparative Example Example 4 A laminated film having a three-layer structure shown in Table 6 was obtained in the same manner as in Example 3 except that the thickness of each layer was changed as shown in Table 6.

【table】

[Table] * Comparative example

Claims (1)

[Scope of Claims] 1. A multilayer structure film consisting of a base layer and a printable surface layer made of a stretched film of an inorganic filler-containing polypropylene composition provided on at least one side of the base layer, where this surface layer is as follows: A thermoplastic resin film with excellent printability, characterized in that it is defined by: (1) The thickness of the surface layer shall be 1/6 or less of the thickness of this multilayer structure film and 3 microns or more. (2) Having minute voids. (3) The inorganic filler-containing polypropylene composition forming the surface layer must contain modified polypropylene in which polypropylene and ethylenically unsaturated carboxylic acid or its anhydride are copolymerized.