In-mold molded article, method for producing same, and crystalline resin sheet

Abstract

To provide an in-mold molded article having dramatically superior mechanical strength of the in-mold molded body, and to provide a polypropylene sheet having excellent rigidity and strength despite being a thick film. Provided is an in-mold molded article obtained by integrally forming a resin sheet and an injection molding resin, wherein a base material sheet layer (X) of the molded article derived from the resin sheet is a crystalline resin having a thickness of 60 μm or more, a crystallinity by x-ray diffraction when the molded article is cut and x-rays irradiated from the cut surface of 70% or more, and a ratio of the maximum value or the minimum value to the average thickness (μm) of the sheet layer (X) in the cut surface of within ±10%.

Description

In-mold molded product, its manufacturing method, and crystalline resin sheet

[0001] The present invention relates to an in-mold molded product, a method for producing the same, and a crystalline resin sheet.

[0002] In recent years, development of paint-free decorative films has progressed, and decorative films using polypropylene as a base material and having a decorative layer printed on the surface have attracted attention because polypropylene is inexpensive, moldable, lightweight, and easily recyclable. Decorative films using such polypropylene base materials are usually used as so-called in-mold molded products, in which the base layer is placed on a mold surface and a resin is injected into the decorative layer side by an injection method. For example, a technology for stabilizing the printed surface of a polypropylene film used as an in-mold label is known (see Patent Document 1).

[0003] However, although the technology described in Patent Document 1 is excellent in terms of imparting design to molded products, the in-mold label film itself is flexible and can be shaped, and it is not possible to give the film itself strength.

[0004] Also known is a technology that improves rigidity and scratch resistance by in-mold molding using a highly rigid multilayer biaxially stretched film (Patent Document 2).

[0005] However, although the technology described in Patent Document 2 discloses examples (Examples 8 and 9) of forming a thick sheet by simultaneous biaxial stretching, the film is formed using only a small stretching machine that simultaneously biaxially stretches all four sides using a tenter, and as a result, the film is not highly accurate in thickness overall, resulting in wavy areas near the edges, and there are limitations on the size and shape that can be formed.

[0006] That is, in the current situation, there is no technology in in-mold molding that can provide strength to the sheet itself and is applicable to large molded products such as automobile bumpers and other injection molded automobile parts.

[0007] Patent No. 6779095 International Publication No. 2023 / 204293

[0008] Therefore, an object of the present invention is to provide an in-mold molded article having dramatically improved mechanical strength, particularly an in-mold molded article that can be used for large molded articles such as automobile bumpers and other automobile parts, a method for producing the same, and a polypropylene sheet that is suitable for such in-mold molding and that is thick yet has excellent rigidity, strength, and film thickness accuracy.

[0009] As a result of intensive research to solve the above problems, the present inventors have found that by using a polypropylene sheet as an in-mold label used in producing in-mold molded products, which has a high film thickness and a high degree of crystallinity and which exhibits excellent film thickness precision when formed on an industrial scale, the mechanical strength of the in-mold molded product after injection molding is dramatically improved, and further, when a decorative film is combined with the in-mold label, excellent design properties are exhibited, which has led to the completion of the present invention.

[0010] That is, the present invention relates to an in-mold molded product obtained by integrally forming a resin sheet and an injection-molded resin, wherein the base sheet layer (X) of the molded product resulting from the resin sheet has a thickness of 60 μm or more, and when the molded product is cut and X-rays are irradiated from the cut surface, the crystallinity determined by X-ray diffraction is 70% or more, and the ratio of the maximum value to the average thickness (μ) of the sheet layer (X) at the cut surface is a crystalline resin within ±10%.

[0011] The present invention further relates to a method for producing an in-mold molded product, in which a resin sheet (x) or a molded product thereof is placed in the cabinet of a molding machine, and then the mold is closed. Then, an injection molding resin material is injected from the gate of the molding machine to integrally mold the resin sheet and the injected resin, wherein the resin sheet (x) has a thickness of 60 μm or more, a crystallinity of 70% or more by X-ray diffraction when irradiated with X-rays from the cut surface, and a thickness unevenness (2σtd) of 10% or less over a 560 mm width in the TD direction.

[0012] The present invention further relates to a crystalline resin sheet that is made of a crystalline resin, has a thickness of 60 μm or more, has a crystallinity of 70% or more by X-ray diffraction when irradiated with X-rays from a cut surface, and has a thickness unevenness (2σtd) of 10% or less over a width of 560 mm in the TD direction.

[0013] According to the present invention, it is possible to provide an in-mold molded article having dramatically excellent mechanical strength, particularly an in-mold molded article that can be used for large molded articles such as automobile bumpers and other automobile parts, a method for manufacturing the same, and a polypropylene sheet that is suitable for such in-mold molding and has excellent rigidity, strength, and thickness accuracy despite being a thick film.

[0014] FIG. 1 is a cross-sectional view of a resin sheet (x) of the present invention. FIG. 2 is a diagram showing one embodiment of combining a resin sheet (x) of the present invention with an acceleration film (y). FIG. 3 is a cross-sectional view of an in-mold molded product using a resin sheet (x) of the present invention. FIG. 4 is a cross-sectional view of an in-mold molded product combining a resin sheet (x) of the present invention with an acceleration film (y). FIG. 5 is a conceptual diagram showing a cross-section of a mold of an in-mold molding machine and a state in which a resin sheet (x) is placed in a cabinet. FIG. 6 is a conceptual diagram showing the state of a molded product after in-mold molding and subsequent trimming. FIG. 7 is a diagram conceptually showing the structure of a simultaneous biaxial stretching machine in a plan view.

[0015] The in-mold molded article of the present invention is an in-mold molded article formed by integrally molding a resin sheet and an injection-molded resin, wherein the base sheet layer (X) of the molded article resulting from the resin sheet has a thickness of 60 μm or more, a crystallinity of 70% or more as determined by X-ray diffraction when the molded article is cut and irradiated with X-rays from the cut surface, and the ratio of the maximum to minimum values ​​to the average thickness (μ) of the sheet layer (X) at the cut surface is within ±10%. Because the base sheet layer (X) has such high crystallinity and small thickness unevenness, the base sheet layer (X) exhibits excellent mechanical strength, resulting in good strength of the molded article and excellent strength uniformity of the molded article. In particular, in the in-mold molded article of the present invention, the strength of the sheet layer (X) itself is ensured, and if the sheet has thickness unevenness, strength reduction due to partial thinning occurs. However, this invention does not result in such strength unevenness. The problem of uneven strength is particularly pronounced in the case of large molded products such as automobile bumpers, and the molded product of the present invention is characterized in that it can provide particularly good results in the case of such large molded products.

[0016] Here, the thickness of the base sheet layer (X) is 60 μm or more, and is particularly preferably 80 μm or more, 90 μm or more, or 150 μm or more from the viewpoint of sheet rigidity. On the other hand, the upper limit is preferably 400 μm or less, particularly preferably 350 μm or less, from the viewpoint of excellent formability when the raw material sheet is subjected to in-mold molding.

[0017] Furthermore, the base sheet layer (X) has a crystallinity of 70% or more as determined by X-ray diffraction when the cut surface of the molded article is irradiated with X-rays. Here, the crystallinity determined by X-ray diffraction may be determined by a conventional method, and the crystallinity can be calculated from the ratio of the peak area of ​​the crystalline component to the total peak area (peak area of ​​the crystalline component + halo pattern area of ​​the amorphous component) in X-ray diffraction. For example, it can be measured according to the Hermans-Weiding method described in Weidinger A. Hermans PH., Macromol Chem Phys [1961:50; 98-115]. As mentioned above, the crystallinity must be 70% or more, but a crystallinity of 75% or more, or even 80% or more, is preferred in terms of improving the rigidity of the molded article. Such a degree of crystallinity can be adjusted by adjusting the stretching conditions when the raw material sheet used in in-mold molding is biaxially stretched.

[0018] Furthermore, the sheet layer (X) is characterized in that the ratio of the maximum value to the minimum value to the average thickness (μx) of the sheet layer (X) on the cut surface of the molded article is within ±10%. The ratio of the maximum value to the minimum value to the average thickness (μx) means that on the cut surface of the same plane or curved surface of the molded article, the difference [μx - Tmax] between the average thickness (μx) and the maximum thickness (Tmax) and the difference [μx - Tmin] between the average thickness (μ) and the minimum thickness (Tmin) are both within 10% of the average thickness (μx).

[0019] Such a cross section of the molded article may be a cross section obtained by cutting the molded article so as to obtain the widest cross section area, and the thickness of the sheet layer (X) may be a value measured at 25 mm intervals on a cut surface corresponding to the same plane or the same curved surface of the molded article, for example.

[0020] In order to reduce the thickness unevenness of the sheet layer (X), it is preferable that the resin sheet used as the raw material for the molded product has a thickness unevenness (2σtd) of 10% or less over a 560 mm width in the TD direction. Furthermore, it is also preferable that the resin sheet has a thickness unevenness (2σmd) of 10% or less over a 5 m width in the MD direction, in order to improve the thickness uniformity of the resin sheet.

[0021] Here, the thickness unevenness (2σtd) at a width of 560 mm in the TD direction is expressed as a percentage obtained by dividing 2σ by the average value of the standard deviation (σ) measured at 15 points at 40 mm intervals in the TD direction using an optical sensor with a width of 560 mm for the resin sheet (x). On the other hand, the thickness unevenness (2σmd) at 5 m in the MD direction is expressed as a percentage obtained by dividing 2σ by the average value of the standard deviation (σ) measured at 201 points at 25 mm intervals over 5 m in the MD direction. Therefore, in the present invention, the thickness of a sheet with a width of 560 mm in the TD direction and a length of 5 m in the MD direction is measured at 15 points in the TD direction x 201 points in the MD direction, and the standard deviation σ and average value (μ) in each TD direction are calculated. The thickness unevenness (2σtd) in the TD direction can be calculated as the percentage obtained by dividing 2σ by the average value (μ) as shown in the following formula (1). 2σtd = [2σ / μ] × 100 Formula (1) Furthermore, the standard deviation σ and average value (μ) for each MD direction are calculated, and the thickness variation in the MD direction (2σmd) can be determined as a percentage when 2σ is divided by the average value (μ) as shown in the following formula (2): 2σmd = [2σ / μ] × 100 Formula (2)

[0022] Thus, in the present invention, it is preferable that the thickness unevenness (2σtd) over a width of 560 mm in the TD direction is 10% or less, and that the thickness unevenness (2σmd) over 5 m in the MD direction is 10% or less, but it is particularly preferable that the thickness unevenness (2σtd) over 5 m in the MD direction is 9% or less, and of these, 8.2% or less, and it is also particularly preferable that the thickness unevenness (2σmd) over 5 m in the MD direction is 9% or less, and of these, 8.2% or less, from the viewpoint of good formability.

[0023] Furthermore, since the base sheet layer (X) has a crystallinity of 70% or more despite having a thickness of 60 μm or more, the mechanical strength of the entire molded product is dramatically improved, which in turn makes it possible to reduce the thickness of the injected resin.

[0024] Examples of the resin sheet (x) constituting the base sheet layer (X) that can be used here include crystalline sheets such as polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer resin, polyamide, polyester, and polypropylene, but in the present invention, polypropylene (a) is particularly preferred from the viewpoint of strength. Therefore, that is, since the base sheet layer (X) is composed of a polypropylene sheet, and the polypropylene sheet is a thick film and highly crystallized, and further, the thickness unevenness, particularly the thickness unevenness in the TD and MD directions of the resin sheet (x) that is the raw material, is small, the base sheet layer (X) can exhibit excellent rigidity and impact strength.

[0025] The polypropylene sheet constituting the base sheet layer (X) is preferably used as a biaxially oriented polypropylene sheet, since this can further increase the crystallinity of the polypropylene layer. In particular, a polypropylene sheet produced at the same stretch ratio by the simultaneous biaxial stretching method described below is preferred, since this reduces the anisotropy of orientation due to stretching, suppresses distortion in the molded product obtained by in-mold molding, and provides good shape stability.

[0026] Here, the stretching ratio of the biaxially oriented polypropylene sheet is preferably 3 to 10 times in the TD direction and 3 to 6 times in the MD direction, and particularly preferably 3.5 to 8 times in the TD direction and 3.5 to 8 times in the MD direction. However, as mentioned above, a simultaneous biaxial stretching method in which the stretching ratio is substantially equal in the TD and MD directions, in the range of 4 to 7 times, is preferred from the viewpoint of achieving both physical strength, moldability, and shape stability.

[0027] [Polypropylene Layer (A)] Specific examples of the polypropylene (a) constituting the polypropylene sheet used herein include propylene homopolymers, propylene random copolymers obtained by polymerizing a monomer component containing 7% by weight or less of at least one selected from C2 to C10 α-olefins (excluding C3 α-olefins), and mixtures thereof. Here, the layer composed of such polypropylene (a) may be referred to as the "polypropylene layer (A)."

[0028] Among these, a propylene copolymer obtained by polymerizing a monomer component containing 3% by weight or less of ethylene is particularly preferred, as it can impart excellent stretchability to the sheet while maintaining high rigidity and toughness, and has excellent in-mold formability.

[0029] Here, it is preferable that the molecular weight distribution (Mw / Mn) of the polypropylene (a) constituting the polypropylene sheet is in a relatively wide range of 6 to 20, since this not only improves the accuracy of the thickness of the film, but also allows for both high rigidity and high extensibility, and makes film formation easier.

[0030] Furthermore, the amount of xylene-insoluble matter in the polypropylene (a) is preferably more than 96.5% by mass and not more than 99.5% by mass. The xylene-insoluble matter in polypropylene corresponds to a crystalline isotactic component. In contrast, the xylene-soluble component contained in small amounts in polypropylene corresponds to a non-crystalline atactic component and has a lower molecular weight than the xylene-insoluble component. Furthermore, when the amount of xylene-insoluble matter in the polypropylene-based polymer is more than 96.5% by mass and not more than 99.5% by mass, the rigidity and heat resistance, particularly the rigidity, of the in-mold molded article are improved.

[0031] The crystalline component of the polypropylene (a) preferably has a stereoregularity (mmmm) of 97.5 to 99.5% from the viewpoint that the rigidity, heat resistance, and heat resistance of a molded product obtained by in-mold molding a sheet made of the polypropylene (a) are good.

[0032] When the propylene copolymer containing a small amount of ethylene as a copolymerization component is used as the polypropylene (a) constituting the polypropylene layer (A), the ethylene content in the raw material monomer components is preferably 0.1% by mass or more and less than 1% by mass, more preferably 0.1% by mass or more and less than 0.6% by mass, and particularly preferably 0.1% by mass or more and less than 0.3% by mass, in order to enhance the stretchability while maintaining the toughness and rigidity of the sheet.

[0033] The propylene copolymer has the advantage of being improved in transparency by randomly copolymerizing propylene with ethylene. Furthermore, the ethylene content in the raw monomer component is less than 1% by mass, resulting in excellent rigidity. The lower limit of the ethylene content is not particularly limited and is greater than 0% by mass, but is preferably 0.1% by mass or more, since this facilitates sufficient improvement in transparency.

[0034] The MFR of the polypropylene (a) is 1 to 15 g / 10 min, preferably 2 to 6 g / 10 min. When the MFR is within the above range, the polypropylene (a) has excellent formability when formed into a sheet.

[0035] It is preferable that the polypropylene (a) constituting the resin sheet (x) contains a nucleating agent from the viewpoint of transparency. In the present invention, by using a smaller amount of this nucleating agent than usual, it is possible to reduce haze and further improve transparency.

[0036] Here, the content of the nucleating agent per 100 parts by mass of polypropylene (a) is preferably less than 0.18 parts by mass, particularly preferably 0.15 parts by mass or less. If it is less than the upper limit, excellent thickness precision is easily obtained, and the polypropylene composition has excellent film-forming properties while maintaining toughness and rigidity. Here, the lower limit of the content of the nucleating agent is not particularly limited, but is preferably 0.01 parts by mass or more in terms of the transparency improvement effect.

[0037] The polypropylene (a) constituting the resin sheet (x) preferably has a crystallization rate parameter (t1 / 2) of more than 1 second, more preferably 2 seconds or more. Decreasing the amount of nucleating agent added tends to decrease the crystallization rate and increase (t1 / 2). When (t1 / 2) is greater than the lower limit, excellent thickness accuracy is likely to be achieved. The upper limit of (t1 / 2) is not particularly limited, but is preferably about 5 seconds or less.

[0038] [Nucleating Agent] The amount of the nucleating agent is more than 0 parts by weight and not more than 1.0 parts by weight, preferably 0.05 to 0.5 parts by weight, per 100 parts by weight of polypropylene (a). A nucleating agent is an additive (transparent nucleating agent) used to reduce the size of crystalline components in a resin and thereby enhance transparency. The nucleating agent is not particularly limited, and those commonly used in the field may be used. However, it is preferable to select from nonitol-based nucleating agents, sorbitol-based nucleating agents, phosphate ester-based nucleating agents, triaminobenzene derivative nucleating agents, metal carboxylate nucleating agents, and xylitol-based nucleating agents. Examples of nonitol-based nucleating agents include 1,2,3-trideoxy-4,6:5,7-bis-[(4-propylphenyl)methylene]-nonitol. Examples of sorbitol-based nucleating agents include 1,3:2,4-bis-o-(3,4-dimethylbenzylidene)-D-sorbitol. Examples of phosphate ester-based crystal nucleating agents include lithium 2,2'-methylenebis(4,6-di-tert-butylphenyl)phosphate-based crystal nucleating agents.

[0039] [Petroleum Resin] In the present invention, the inclusion of a petroleum resin in the polypropylene (a) used in the base sheet layer (X), and further in the polypropylene resin (b) or modified olefin resin (c) described below, is preferred in terms of further reducing thickness variations in the resin sheet and improving thickness uniformity. Here, petroleum resin refers to a resin obtained by polymerizing a C4-C5 fraction (mainly the C5 fraction) or a C5-C9 fraction (mainly the C9 fraction) produced as a by-product by thermal decomposition of petroleum naphtha in a mixed state. Examples include aliphatic petroleum resins (C5 petroleum resins), aromatic petroleum resins (C9 petroleum resins), aliphatic / aromatic petroleum resins (C5 / C9 petroleum resins), and hydrogenated versions of these (hydrogenated petroleum resins). Of these, hydrogenated petroleum resins are preferred in terms of sheet thickness uniformity. The amount of petroleum resin blended is preferably 1 to 20% by mass of the total mass of the polypropylene (a), polypropylene resin (b), or modified olefin resin (c).

[0040] [Other additives] The polypropylene (a) used in the base sheet layer (X) can contain other additives other than the crystal nucleating agent, as long as they do not impair the effects of the present invention. Examples of other additives include antioxidants, neutralizing agents, chlorine absorbers, heat stabilizers, light stabilizers, UV absorbers, internal lubricants, external lubricants, antiblocking agents, antistatic agents, antifogging agents, flame retardants, dispersants, copper inhibitors, plasticizers, crosslinking agents, peroxides, oil extenders, and other organic and inorganic pigments, and other conventional additives commonly used in polyolefins. The amount of each additive may be a known amount.

[0041] [Preparation of Polypropylene (a)] The polypropylene (a) constituting the resin sheet (x) described above in detail can be prepared, for example, according to Production Examples 2 to 7 of Japanese Patent No. 6845001.

[0042] When the polypropylene sheet described above is used as the resin sheet (x), it is preferable to have a low-melting-point polypropylene-based resin layer (B) or a modified olefin-based resin layer (C) having a melting point 10°C or more lower than that of the polypropylene layer (A) on at least one side of the polypropylene layer (A), particularly the side opposite to the surface layer, i.e., the side in contact with the decorative layer or the side in contact with the injected resin, in order to achieve excellent adhesion to the decorative layer or the injected resin layer and improve the rigidity and durability of the in-mold molded product.

[0043] <Low-melting-point polypropylene layer (B)> The low-melting-point polypropylene layer (B) is preferably a polypropylene-based resin layer having a melting point of 110 to 158°C, and the (B) layer can be formed by co-extrusion and then stretching the polypropylene sheet when forming the film.

[0044] The polypropylene resin (b) constituting the low-melting-point polypropylene layer (B) has a melting point of 110 to 158°C. The melting point is measured using DSC at a heating rate of 10°C / min from 30°C to 230°C. This melting point range is sufficiently lower than that of the polypropylene constituting layer A, resulting in good fusion properties during hot pressing. Specifically, such polypropylene resins are preferably formed from propylene homopolymer (HOMO); propylene random copolymer (RACO) containing 5% by weight or less of at least one comonomer selected from C2 to C10 alpha olefins (excluding C3 alpha olefins); or a resin composition containing HOMO or RACO. An excessively low comonomer content may result in insufficient fusion with the first layer, while an excessively high comonomer content may result in reduced rigidity of the multilayer sheet. From this perspective, the comonomer content is preferably greater than 0% by weight and less than 4.5% by weight. Ethylene (a C2-alpha olefin) is preferred as the comonomer. The MFR (230°C, load 2.16 kg) of the polymer or resin composition constituting the second layer is not limited, but is preferably 1 to 15 g / 10 min, more preferably 2 to 10 g / 10 min, and even more preferably 3 to 8 g / 10 min.

[0045] The polypropylene (b) may contain a nucleating agent, or may be composed of a resin composition or polymer that does not contain a nucleating agent. When a nucleating agent is contained, from an economical viewpoint, the amount of the nucleating agent is preferably 1 part by weight or less per 100 parts by weight of the polymer that forms the second layer. Therefore, the low-melting-point polypropylene layer (B) is preferably composed of a resin composition that contains HOMO and a nucleating agent, or a resin composition that contains RACO and a nucleating agent.

[0046] The thickness of the biaxially stretched olefin resin film is preferably in the range of 1 to 20 μm, particularly 2 to 10 μm per layer.

[0047] <Modified olefin resin layer (C)> Next, the modified olefin resin layer (C) is composed of a modified olefin resin or a modified olefin resin (c) containing the modified olefin resin and an additive, and can be formed by melt-kneading, film-forming, and then biaxially stretching.For example, the modified olefin resin (c) can be extruded to obtain an unstretched sheet, and the sheet can be biaxially stretched to form the modified olefin resin layer (C).As mentioned above, in the present invention, it is preferable to form a film and stretch it together with other layers by co-extrusion.In the present invention, by providing such a modified olefin resin layer (C), the adhesion to the printed surface of the decorative layer is particularly good.

[0048] The modified olefin resin constituting the modified olefin resin film layer (C) may be, for example, a polyolefin having various functional groups in its molecular structure, such as a carboxyl group, an acid anhydride group, a sulfonic acid group, a phosphoric acid group, a phosphoric acid ester group, an imino group, or an amino group. In particular, polypropylene having a carboxyl group, an acid anhydride group, or an imino group is preferred because of its excellent adhesion to the printing layer or to other members.

[0049] Examples of polyolefins having such functional groups in their molecular structure include acid-modified olefins such as those manufactured by Mitsui Chemicals, Inc., such as "Admer Film QB515," "Admer Film QB550," "Admer Film QB515," "Admer Film QF500," "Admer Film QF551," "Admer Film QF580," "Admer Film QE840," and "Admer Film QE060."

[0050] The base sheet layer (X) in which the polypropylene layer (A) and the low-melting-point polypropylene layer (B) or the modified olefin resin layer (C) described above are laminated may have an (A) layer alone, a two-layer structure of (A) layer / (B) layer, a three-layer structure of (B) layer / (A) layer / (B) layer, a two-layer structure of (A) layer / (C) layer, or a three-layer structure of (B) layer / (A) layer / (C) layer, as described above. While a structure in which the (A) layer is positioned as the outermost layer is preferred in terms of providing a good surface hardness to the in-mold molded article, the base sheet layer (X) may have an (A) layer alone, a two-layer structure of (A) layer / (B) layer, a three-layer structure of (B) layer / (A) layer / (C) layer, and the structure may be appropriately selected depending on the purpose.

[0051] Regarding the thickness ratio of each layer in the above-mentioned multilayer base sheet layer (X), it is desirable to ensure that the thickness of the (A) layer is as thick as possible in order to be able to exhibit the toughness and mechanical strength of the film itself. In the case of a two-layer structure (ab type) of (A) layer / (B) layer, (A) / (B)=98 to 80 / 2 to 20, and in the case of an ac type film, it is desirable that the thickness of the two-layer structure (ac type) of (A) layer / (C) layer is 98 to 80 / 2 to 20. In the case of a three-layer structure (bab type) of (B) layer / (A) layer / (B) layer, it is preferable that the ratio of (B) layer / (A) layer / (B) layer is 2-15 / 96-70 / 2-15, and in the case of a three-layer structure (bac type) of (B) layer / (A) layer / (C) layer, it is preferable that the ratio of (B) layer / (A) layer / (C) layer is 2-15 / 96-70 / 2-15.

[0052] <Production of Substrate Sheet> The resin sheet (x) constituting the above-mentioned single-layer or multi-layer resin sheet layer is (co)extruded from an extruder using a T-die, the sheet is drawn through a cast roll, and film-formed to obtain a sheet raw material having a thickness of 0.5 to 5 mm, which can then be obtained through a sequential biaxial stretching or simultaneous biaxial stretching process. In this case, sequential biaxial stretching is prone to surface scratches due to roll stretching, and the sheet is cooled once after MD stretching, which is prone to cause anisotropy in the orientation and leads to cracking of the molded product. In addition, haze is likely to occur due to crystallization caused by cooling after MD. Therefore, simultaneous biaxial stretching is preferred because it is less likely to cause these problems. Furthermore, for example, the TD direction and MD direction can be simultaneously stretched using a continuous simultaneous biaxial stretching machine such as that shown in FIG. 7. The simultaneous biaxial stretching machine will be described in detail with reference to FIG. 7. The sheet extruded from the T-die after melt mixing in the extruder is, if necessary, enhanced in homogeneity by guide rolls or chill rolls, and if necessary, preheated before being introduced into the simultaneous biaxial stretching machine shown in FIG. 7. In FIG. 7, the open, upward arrow indicates the extrusion direction of the sheet, with the sheet traveling direction being the MD direction and the horizontal direction in the drawing being the TD direction. The simultaneous biaxial stretching machine has two guide rails (61) and, along the guide rails (61), multiple clips (62) movable in the sheet traveling direction. Stretching in the MD direction can be achieved by adjusting the spacing between these clips. That is, MD stretching progresses as the spacing between the clips gradually widens in the sheet traveling direction. On the other hand, as shown in zone II of FIG. 7, the guide rails widen in the traveling direction, and the spacing between the clips increases depending on the angle of this expansion, allowing for TD stretching. That is, the simultaneous biaxial stretching machine has a mechanism in which the sheet is gripped at the entrance thereof and then stretched in both the MD and TD directions in conjunction with the movement of the clip (C).

[0053] In the present invention, it is preferable that the clips are driven by a linear motor. By driving the clips by a linear motor, the distance between the clips can be precisely controlled, and it becomes possible to adjust the stretching ratio to a predetermined value. Here, as shown in Figure 7, the guide rails (61) are arranged such that the clips (62) run clockwise on the right guide rail, and the clips (62) run counterclockwise on the left guide rail.

[0054] The simultaneous biaxial stretching machine is divided into multiple zones in the sheet travel direction. For example, as shown in FIG. 7, the sheet is preheated in Zone I, and biaxially stretched simultaneously in both the TD and MD directions in Zone II. Zone III is a buffer zone, preventing distortion when stretching is relaxed in Zone IV and facilitating smooth sheet travel. Zone IV is an annealing zone, and stretching relaxation is also performed in Zone IV. Zones I to IV can be appropriately set to temperatures ranging from 140°C to 180°C. Zone V is a cooling zone, where the sheet is cooled to room temperature without heat treatment, and then a biaxially stretched substrate sheet is obtained in the form of a roll on a winder (not shown).

[0055] Here, when the resin sheet (x) is composed of polypropylene (a), examples of the resin sheet (x) include a two-kind two-layer, two-kind three-layer, or three-kind three-layer co-extruded sheet such as a base sheet of a monolayer sheet of polypropylene (a), an ab-type base sheet having polypropylene-based resin (b) on one side of polypropylene (a), an ac-type base sheet having modified olefin-based resin (c) on one side of polypropylene (a), a bab-type base sheet having polypropylene-based resin (b) on both sides of polypropylene (a), and a bac-type base sheet having polypropylene-based resin (b) on one side of polypropylene (a) and modified olefin-based resin (c) on the opposite side of polypropylene (a).

[0056] Here, when layer B and / or layer C are laminated in addition to layer A as the base sheet layer (X), the resin sheet (x) can be obtained as a two-kind two-layer, two-kind three-layer, or three-kind three-layer co-extruded sheet using a multi-layer T-die and a feed block method or a multi-manifold method.

[0057] Here, examples of the two-kind, two-layer ab-type or ac-type resin sheet (x) include those shown in FIG. 1, in which the upper layer is an (A) layer made of polypropylene (a), and the lower layer is a (B) layer or a (C) layer made of polypropylene-based resin (b) or modified olefin-based resin (c).

[0058] Although the resin sheet (x) may be used as a final sheet at the same stretch ratio as when it is simultaneously stretched in the stretching zones, it is preferable to relax the stretching by inclining the guide rails inward and shortening the distance between the clips in zone IV as shown in Fig. 7, since this reduces thickness unevenness in the final sheet, improves the appearance, and also provides excellent formability. This stretch relaxation is preferably performed by relaxing the stretching by 5 to 40% from the maximum stretch ratio in zone II.

[0059] The resin sheet (x) thus obtained is a crystalline resin sheet having a crystallinity of 70% or more by X-ray diffraction and a thickness unevenness (2σtd) of 10% or less in a 560 mm width in the TD direction, and more preferably a thickness unevenness (2σmd) of 10% or less in a 5 m width in the MD direction. Because of this crystallinity and thickness unevenness, the sheet itself has excellent rigidity and impact resistance.

[0060] The resin sheet (x) described above is the polyolefin sheet of the present invention, and is used in combination with a decorative film (y) as needed for in-mold molding, as described below. However, the resin sheet (x) can also be used alone to form a sheet product. For example, when the resin sheet (x), i.e., the polyolefin sheet of the present invention, is made into a food tray by vacuum forming or vacuum pressure forming, it is noteworthy that it becomes an unprecedented food tray that exhibits excellent cold resistance, heat resistance, and mechanical strength.

[0061] Furthermore, when the resin sheet (x) is the bab-type co-extruded polypropylene sheet, a plurality of such sheets can be laminated and heat-sealed to form a laminated sheet having a thickness of 600 μm to 3 mm. The laminated sheet thus obtained is a transparent sheet that has excellent rigidity, toughness, and impact resistance while maintaining excellent moldability and shaping properties.

[0062] A specific structure of such a laminate sheet can be obtained, for example, by stacking multiple bab-type coextruded polypropylene sheets and laminating aab-type sheets on both surface layers with the a layer positioned on the surface, i.e., aab / bab / bab / ...bab / bab / baa, and then heat-sealing each sheet, i.e., the b layer and the b layer together. The transparent laminate sheet obtained in this manner is useful as a building material such as a balcony waist wall, a handrail, a window material, or a solar cell mount. Furthermore, by combining it with a decorative film or by printing on the resin sheet (x) itself, it becomes an ultra-high-rigidity, moldable decorative sheet. Such a decorative sheet can be used as an automotive exterior panel material or an automotive interior material.

[0063] <Decorative film layer (Y)> In the present invention, the resin sheet (x) can be set directly in a molding machine and injection molding can be performed, but the base sheet layer (X) can also be used as a surface layer, and an in-mold molded product can have a decorative sheet layer (Y) on the injected resin side. When providing the decorative sheet layer (Y), the resin sheet (x) constituting the base sheet layer (X) and the decorative film (y) constituting the decorative film layer (Y) are previously bonded by fusion or adhesion, etc., and then subjected to in-mold molding (Method 1), or the resin sheet (x) and the decorative film (y) are set in a mold cabinet in a superimposed state without being bonded, and then injection molded (Method 2). Here, the decorative film (y) can be obtained by forming a decorative layer on the surface of the resin film.

[0064] Here, to form the decorative layer, the decorative ink can be printed on the resin film by a standard method such as gravure printing or screen printing, or by various printing methods such as silk screen printing, gravure printing, offset printing, letterpress printing, and flexographic printing; or various coating methods such as flow coating, drop casting, spin coating, bar coating, spray coating, curtain coating, dip coating, and die coating.

[0065] In this case, it is preferable to subject the surface of the resin film to treatment such as 1) corona treatment, 2) provision of an anchor coat layer, or 3) provision of an anchor coat layer after corona treatment before printing or coating, in order to improve the adhesion strength of the printed or coated surface.

[0066] The decorative ink is preferably one that has excellent adhesion to polypropylene, and examples thereof include urethane resins such as polyether-type urethane resin, polyether polyester-type urethane resin, polyether polyester-type polyurethane polyurea, acid group-containing polyester polyurethane polyurea, and styrene-acrylic resin-modified polyether-type urethane resin; modified polypropylenes such as chlorinated polypropylene, acrylic acid-modified polypropylene, and acrylic acid-maleic acid-graft-modified polypropylene; and mixed resins of the above urethane resins and modified polypropylene. Among these, modified polypropylene is particularly preferred because of its excellent adhesion and adhesion to the injection resin.

[0067] These ink resins may be solvent-based or water-soluble or water-dispersible, but from the standpoint of reducing the environmental load and safety, alcohol-based monosolvent, water-based or water-dispersible resins are preferred.

[0068] Next, the resin film has a thickness of 30 μm or more and less than 400 μm, and various film substrates can be used, such as polyethylene film, PET film, OPET film, polycarbonate film, polypropylene film, etc. Among these, in the present invention, polypropylene film having a thickness of 30 μm or more and less than 400 μm is preferred because it has good adhesion to the injection molding resin.

[0069] Examples of the polypropylene film include unstretched polypropylene film and biaxially stretched polypropylene film. The unstretched polypropylene film is preferred because it has excellent adhesion to the resin sheet (x) and shape stability after insert molding, while the biaxially stretched polypropylene film is preferred because it has a more significant effect of improving the mechanical strength of the in-mold molded article.

[0070] The decorative sheet (y) described above may be combined with only one resin sheet (x), but by stacking multiple decorative films (y) of different hues, a deeper color can be expressed.

[0071] When using a decorative sheet (y), it is preferable to use an ab-type sheet or an ac-type sheet as the resin sheet (x) and laminate the decorative film (y) so that the printed layer is in contact with the (B) layer or the (C) layer, as this provides excellent adhesion between the resin sheet (x) and the printed layer of the decorative film (y).

[0072] To manufacture an in-mold molded product using the resin sheet (x) described above in detail, for example, the resin sheet (x) is placed in an injection molding machine cabinet as shown in FIG. 3, or the resin sheet (x) is previously shaped to fit the mold of the injection molding machine and placed therein, and then the mold is closed. After that, a resin material for injection molding is injected from a molding machine gate portion 43 shown in FIG. 5, whereby an in-mold molded product in which the resin sheet and injected resin are integrally molded can be manufactured.

[0073] In this case, one or more decorative films (y) are introduced into the molding machine cavity together with the resin sheet (x) so that they are positioned on the injected resin side, or a sheet molded product in which the decorative film (y) has been pre-shaped to fit the shape of the injection molding machine mold may be introduced into the molding machine cavity together with the resin sheet (x).

[0074] In particular, when one or more decorative films (y) are used together with the resin sheet (x) so as to be positioned on the injected resin side, it is preferable to introduce the resin sheet (x) and the decorative film (y) into the cavity of a molding machine, close the mold, and then heat the mold to 30 to 120°C, preferably 45 to 120°C, and more preferably 60 to 120°C, and then inject the injection molding resin material, since this ensures good adhesion between the resin sheet (x) and the decorative film (y).

[0075] Here, when the resin sheet (x) is of ab type, ac type, bab type, or bac type, it is preferable to arrange the (b) layer or (c) layer in the resin sheet (x) so that it is in contact with the printed or coated layer in the decorative film (y) in order to improve the adhesion of the printed or coated layer. For example, an example of an arrangement in which the ab-type or ac-type resin sheet (x) and the printed or coated layer in the decorative film (y) are in contact is shown in Figure 2. In addition, it is preferable to place the resin sheet (x) and two decorative sheets (y) in advance by heat fusion so that the printed or coated surfaces of the decorative sheets (y) face each other, and then place the laminated sheet in the molding machine cabinet and perform in-mold molding, as this improves the adhesion of each layer.

[0076] The resin temperature when being injected into the mold is preferably 180 to 280°C from the viewpoints of fluidity and moldability. After injection, the resin is cooled as is, and then the mold is opened to obtain the desired in-mold molded product.

[0077] FIG. 3 shows an example of a conceptual diagram of a cross section of an in-mold molded product when a decorative film is not used, and FIG. 4 shows an example of a conceptual diagram of a cross section of an in-mold molded product when a decorative film is used.

[0078] The injection resin used here may be any resin that can be injection-molded, including, for example, vinyl polymers such as polyvinyl chloride and polyvinylidene chloride; styrene resins such as polystyrene, acrylonitrile-styrene copolymer, and acrylonitrile-butadiene-styrene copolymer resin (ABS resin); acrylic resins such as polymethyl (meth)acrylate and polyacrylonitrile; polyolefins such as polyethylene and polypropylene; polyester resins such as polyethylene terephthalate, ethylene glycol-terephthalic acid isophthalic acid copolymer, and polybutylene terephthalate; and polycarbonate resins. Among these, polyolefins are preferred, with polypropylene being particularly preferred, due to their excellent adhesion to the resin sheet (x), molded product strength, and moldability. Furthermore, various additives such as antioxidants, heat stabilizers, UV absorbers, light stabilizers, flame retardants, plasticizers, fillers, lubricants, mold release agents, antistatic agents, and colorants can be added to the injection resin as needed.

[0079] The in-mold molded article described above in detail can have its scratch resistance further enhanced by providing a hard coat on its outermost surface, and various functions can be imparted to the hard coat layer by adding a weathering agent and a flame retardant.

[0080] Examples of in-mold molded articles obtained in this manner include automobile exterior components such as automobile bumpers, front pillars (A-pillars), center pillars (B-pillars), rear pillars (C-pillars), front door panels, rear door panels, back door panels, roof materials, hoods, and fenders; automobile interior components such as dashboards, center clusters, and door linings; home appliance housings such as refrigerator housings and freezer housings; electronic device housings such as smartphone housings and PC housings; silicone solar cell backsheets; and perovskite solar cell stands.

[0081] Examples 1 to 5 <Production of base sheets x1 to x5> The following resin materials (a1, a2) as A layer raw materials and resin materials (b1, b2) as B layer raw materials were melt-kneaded in an extruder, and then biaxially stretched using a two-kind, three-layer (A / A / B) simultaneous biaxial stretching machine schematically shown in FIG. 7 so as to have the layer structure and thickness ratio shown in Table 1, thereby obtaining sheets x1 to x5 (the A / A layer was assumed to be a single layer). a1: Polypropylene (melting point: 165°C, ethylene content: 0.2 mass%, Mw / Mn = 9, xylene insoluble content: 98.2 mass%, mmmm = 98.3, nucleating agent content: 0.05 mass%, t1 / 2: 2.3 seconds) a2: Polypropylene (melting point: 152°C, ethylene content: 2 mass%, Mw / Mn = 6, xylene insoluble content: 97.5 mass%, mmmm = 96.2, nucleating agent content: 0.1 mass%, t1 / 2: 4.5 seconds) b1: Ethylene-propylene random copolymer (melting point: 150°C) b2: Ethylene-propylene random copolymer (melting point: 137°C)

[0082] The resin components (a1, a2, b1, b2) were prepared by melt-kneading a hydrogenated petroleum resin (Imarve P140 manufactured by Idemitsu Kosan Co., Ltd.) in advance to a concentration of 10 mass %.

[0083] (Film-forming conditions) T-die: lip opening width 3 mm The temperature of each zone was appropriately adjusted in the range of 140°C to 180°C, and the stretch ratio was relaxed by 29% from the maximum stretch ratio in zone IV to obtain an isotropic biaxially stretched film of 5 times in the MD direction and 5 times in the TD direction.

[0084] [Measurement of Thickness Unevenness (2σtd, 2σmd)] For each of the obtained substrate sheets x1 to x5, the thickness was measured at 15 points at 40 mm intervals in the TD direction and at 25 mm intervals in the MD direction at a width of 560 mm using an optical sensor while film formation was performed using a simultaneous twin-screw extruder. 201 points over 5 m were evaluated in the MD direction. The standard deviation σ and average value (μ) of the measured values ​​in the TD direction were calculated, and the thickness unevenness (2σtd) was calculated using the following formula: 2σtd = [2σ / μ] x 100. The average of this 5 m was calculated as 2σtd·avg, and the results are listed in the table. The standard deviation σ and average value (μ) in the MD direction were calculated, and the thickness unevenness (2σmd) was calculated using the following formula: 2σmd = [2σ / μ] x 100. The average of 2σmd at 15 points in the width direction was calculated as 2σmd·avg, and the results are listed in the table.

[0085] [Degree of Crystallinity of Sheet] Each of the obtained substrate sheets x1 to x5 was cut in the TD direction, and the sample was set so that the substrate sheet on the cut surface was exposed to X-rays, and measurement was performed according to the Hermans-Weidinger method.

[0086] <In-mold molding> Each biaxially oriented polypropylene sheet obtained by the above method was placed in a flat cabinet mold. After the mold was closed, the following injection molding polypropylenes were injected through the molding machine gate 43 to obtain in-mold molded articles measuring 350 x 350 mm with the thicknesses listed in the tables. Injection resin 1: SunAllomer "CMB70M" (MFR: 70 g / min) Injection resin 2: SunAllomer "PM970A" (MFR: 30 g / min) The resulting in-mold molded articles were cut near the center, and the cut surface was irradiated with X-rays to measure crystallinity. Other physical property evaluations were also performed for the following items. The results are shown in Table 1.

[0087] [Crystallization Degree] A sample was cut out from the molded article obtained in each of the Examples and Comparative Examples, and the sample was set so that the substrate sheet on the cut surface was exposed to X-rays, and measurement was performed in accordance with the Hermans-Weidinger method.

[0088] [Thickness Unevenness (Maximum / Minimum Values)] An in-mold molded product (350 × 350 mm) was cut in one direction (350 mm length), and the thickness was measured at 25 mm intervals. The difference between the average thickness (μx) and the maximum value (Tmax) [μx - Tmax] and the difference between the average thickness (μx) and the minimum value (Tmin) [μx - Tmin] were measured. The larger value is shown in the table.

[0089] [Gloss] Measured in accordance with JIS K5600 "4-7 Specular gloss." [Scratch resistance (pencil hardness)] Measured in accordance with JIS K5600 "5-4 Scratch hardness (pencil method)."

[0090] [Appearance Evaluation] When visual inspection revealed any partial appearance defect due to melting, the result was marked with "x", and only when no appearance defect was observed, the result was marked with "o". The results are shown in Table 1.

[0091] [Flexural Modulus] The tensile modulus was measured in accordance with JIS K7171 under the following conditions: Equipment used: AUTOGRAPH AG-X plus (Shimadzu Corporation) Test piece: 25 mm x 150 mm strip test piece Support distance: 16 times the thickness of the test piece Test speed: 1 mm / min Stroke: 1 mm Elastic modulus calculation method: Slope of two points at strokes of 0.05 mm and 0.25 mm Starting point of plot collection: 3 N

[0092] [Dental impact strength (23°C, -30°C)] Tests were conducted in accordance with JIS K7211-2, with a striker diameter of 20 mm, a receiving diameter of 40 mm, and a test speed of 4.4 m / s. The test was conducted after the test specimen was set at the set temperatures (23°C and -30°C) and allowed to stand for a sufficient time for the temperature of the specimen to stabilize. Analysis was then conducted within a range of half the maximum impact force point to calculate the puncture energy (unit: J (joules)).

[0093] [Linear expansion coefficient] Tests were conducted in tension mode using Hitachi High-Tech Science's "TMA7100C." Test conditions were a temperature range of -60°C to 120°C in a nitrogen atmosphere, with a heating rate of 5°C / min.

[0094] Comparative Examples 1 and 2 In Comparative Examples 1 and 2, injection molding was performed using Injection Resin 1 or Injection Resin 2 without using a substrate sheet to obtain molded articles. In Comparative Example 3, an in-mold molded article was obtained in the same manner as in Example 1, except that an unstretched polypropylene film (100 μm thick) was used. In Comparative Example 4, an in-mold molded article was obtained in the same manner as in Example 1, except that a BAA-type co-extruded film manufactured by Santox (layer structure: B / A / A, thickness: 50 μm, thickness ratio of B / A / A: 5 / 90 / 5, stretch ratio: 5×8 times) was used. A: Polypropylene, B: Ethylene-propylene random copolymer (melting point 151°C)

[0095] Example 6 In-mold molding was carried out in the same manner as in Example 9, except that injection molding was carried out using Injection Resin 1 so that the molded product had a thickness of 2 mm. From this result, it was found that good strength could be obtained even when the amount of Injection Resin 1 used in Comparative Example 2 was reduced to one-third.

[0096] Evaluation results

[0097]

[0098]

[0099] REFERENCE SIGNS LIST 1 Base sheet 2 Decorative film 3 Injected resin 4 Molding mold 41 Female mold 42 Male mold 43 Gate portion 5 In-mold molded product X Base sheet layer Y Decorative film layer 6 Simultaneous biaxial stretching machine 61 Guide rail 62 Clip I Preheating zone II Stretching zone III Buffer zone IV Annealing zone V Cooling zone x Base sheet y Decorative film X Base sheet layer Y Decorative film layer

Claims

1. An in-mold molded product in which a resin sheet and an injection-molded resin are integrally formed, wherein the base sheet layer (X) of the molded product resulting from the resin sheet has a thickness of 60 μm or more, and when the molded product is cut and X-rays are irradiated from the cut surface, the crystallinity determined by X-ray diffraction is 70% or more, and the ratio of the maximum or minimum value to the average thickness (μ) of the sheet layer (X) at the cut surface is within ±10%.

2. An in-mold molded product according to claim 1, wherein the resin sheet has a thickness unevenness (2σtd) of 10% or less over a width of 560 mm in the TD direction.

3. An in-mold molded product according to claim 2, wherein the resin sheet has a thickness variation (2σmd) of 10% or less over a 5 m width in the MD direction.

4. The in-mold molded product according to claim 1, 2 or 3, wherein the base sheet layer (X) contains a petroleum resin.

5. The in-mold molded product according to claim 4, wherein the resin sheet is a polyolefin sheet.

6. An in-mold molded product according to claim 5, wherein the base sheet layer (X) has a polypropylene layer (A) made of copolymerized polypropylene having an ethylene content of 0.1% by mass or more but less than 1% by mass, and a low-melting-point polypropylene layer (B) or a modified olefin resin layer (C) on the injected resin side, the low-melting-point polypropylene layer having a melting point 10°C or more lower than that of the polypropylene layer.

7. The in-mold molded product according to claim 5, wherein the surface of said base sheet layer (X) has a hardness of HB or more as determined by a pencil hardness test.

8. The in-mold molded product according to claim 7, further comprising a decorative layer (Y) on the injected resin side of the base sheet layer (X).

9. The in-mold molded product according to claim 8, wherein the decorative layer (Y) comprises a printed or coated layer on a polypropylene film having a thickness of 10 to 350 μm.

10. The in-mold molded product according to claim 9, wherein the base sheet layer (X) and the decorative layer (Y) are positioned so that the (B) layer or (C) layer in the base sheet layer (X) contacts the printed or coated layer in the decorative layer (Y).

11. A method for producing an in-mold molded product in which a resin sheet (x) or a molded product thereof is placed in the cabinet of a molding machine, and then the mold is closed, and an injection molding resin material is injected from the gate of the molding machine to integrally mold the resin sheet and injected resin, wherein the resin sheet (x) is a crystalline resin sheet having a thickness of 60 μm or more, a crystallinity of 70% or more by X-ray diffraction when X-rays are irradiated from the cut surface, and a thickness unevenness (2σtd) of 10% or less over a 560 mm width in the TD direction.

12. The manufacturing method according to claim 11, wherein the resin sheet has a thickness variation (2σmd) of 10% or less over a width of 5 m in the MD direction.

13. The manufacturing method according to claim 12, wherein the resin sheet (x) is a co-extruded laminated sheet of polypropylene (a) and, on the injected resin side, low-melting point polypropylene (b) or modified olefin resin (c) having a melting point 10°C or more lower than that of the polypropylene.

14. A manufacturing method according to claim 13, in which a decorative film (y) having a printed or coated layer is placed on the injected resin side of the resin sheet (x) and in such a way that the printed or coated layer is in contact with the layer of the resin sheet (x) composed of (b) or (c), and molding is then carried out.

15. A crystalline resin sheet that is made of crystalline resin, has a thickness of 60 μm or more, has a crystallinity of 70% or more by X-ray diffraction when irradiated with X-rays from the cut surface, and has a thickness unevenness (2σtd) of 10% or less over a width of 560 mm in the TD direction.

16. A crystalline resin sheet according to claim 15, wherein the resin sheet has a thickness variation (2σmd) of 10% or less over a width of 5 m in the MD direction.

17. A crystalline resin sheet according to claim 15, wherein the resin sheet (x) is a co-extruded laminate sheet of polypropylene (a) and, on the injected resin side, a low-melting point polypropylene (b) or a modified olefin resin (c) having a melting point 10°C or more lower than that of the polypropylene.

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