Preparation method of polypropylene film for labels

Abstract

The invention belongs to the technical field of label films, and particularly relates to a preparation method of a polypropylene film for labels, the polypropylene film comprises a base film layer and a coating film layer, the base film layer comprises the following raw materials by weight: 60-90 parts of random co-polypropylene; 15 to 30 parts of homo-polypropylene; 5-15 parts of a polyolefin elastomer; 3-7 parts of polytetrafluoroethylene; 10-25 parts of an inorganic filler; 0.5-2 parts of a dispersant; 1-3 parts of a coupling agent; wherein the inorganic filler comprises a large-particle-size inorganic filler with the particle size of 500-1000nm and a small-particle-size inorganic filler with the particle size of less than 300nm. The polypropylene film for the label has the advantages of good printing performance, high mechanical strength and low possibility of generating wrinkles.

Description

Technical Field

[0001] The invention belongs to the technical field of label films, and in particular relates to a method for preparing a polypropylene film for labels. Background Art

[0002] Label film is an important carrier for product information display. It mainly conveys important information such as the brand, name, specification, production date, shelf life, etc. of the product to consumers by printing text, patterns, barcodes, etc. Common label films include film media made of PE, PP, PET, PVC, etc. Among them, polypropylene film is a thermoplastic resin made by propylene polymerization. Polypropylene film can be divided into CPP film, BOPP film and IPP film according to different processing techniques.

[0003] The commonly used biaxially oriented polypropylene film (BOPP) is made by co-extruding polypropylene into a sheet and then stretching it in both the longitudinal and transverse directions. During the stretching process, due to the orientation of the molecular chain segments and the increase in crystallinity, the tensile strength, tensile elastic modulus, impact strength, tear strength, bending life, etc. of the film medium can be significantly improved, and a strong and tough film with high stiffness can be obtained. In addition, BOPP film has good moisture barrier, high gloss, good transparency, good gas barrier, moisture resistance, light weight, non-toxic, odorless, good dimensional stability, good electrical insulation, wide range of use, excellent quality stability and processability, and its comprehensive performance is better than transparent films such as moisture-proof cellophane and polyethylene film, and is praised as the "Queen of Packaging" by the packaging industry.

[0004] However, the existing BOPP film still has the following defects: First, poor printing performance: Due to the non-polar surface of BOPP film, the adhesion of ink and adhesive on its surface is poor, and the printing effect is not ideal. Special inks are usually required for printing, which not only increases the printing cost, but also the adhesion of the oil film of the printed matter is not high. Especially for labels with high resolution requirements such as barcodes, the printing quality is difficult to meet the requirements; in addition, the absorption of ink by BOPP film is very small, and the ink is not easy to dry, resulting in the printing speed of BOPP film being much lower than that of paper products; Second, the tensile strength needs to be improved: BOPP film can show higher tensile strength due to the orderly arrangement of PP molecular chains during the stretching process. However, for labels, higher tensile strength means that labels are less likely to break when pulled or torn, which is conducive to improving their durability and reliability. Third, the internal stress is unevenly distributed and wrinkles are easy to form: in the process of bidirectional synchronous stretching, longitudinal and transverse stretching are carried out simultaneously, the tension in the two directions competes with each other, the molecular chains are arbitrarily oriented in the plane of the film, and it is difficult to obtain a crystal structure that is highly oriented in one direction. This uneven distribution of internal stress can easily cause wrinkles in the film in the subsequent process. In addition, due to the complexity of the bidirectional synchronous stretching process, the thickness of the film may be uneven, and the uneven thickness will further cause internal stress in the film in the subsequent process, thereby causing wrinkles. Especially for films in high temperature (such as above 80°C) application environments, the wrinkle phenomenon is particularly serious.

[0005] In order to solve the problems existing in BOPP film and further improve its performance, technicians in this field have proposed a variety of improvement methods. Among them, a certain amount of inorganic particles, such as calcium carbonate, carbon dioxide, silicon dioxide, barium sulfate, etc., are added to the raw materials of BOPP film to improve the polarity and roughness of the film surface through the inorganic particles, thereby improving the printing performance of the film; at the same time, inorganic particles can also be used as a reinforcing phase in the BOPP film to improve the mechanical properties of the BOPP film, which is a relatively effective way. For example, the Chinese patent with publication number CN119220018A, entitled A BOPP film with ink affinity and its preparation method and application, discloses a similar method. However, this method still has the following problems: if the particle size of the inorganic particles is too small and the amount added is too little, the improvement on the performance of the BOPP film, especially the improvement on the printing performance, is very limited; but if the amount of inorganic particles added is too much and the particle size is too large, the processing performance of the BOPP film will be reduced, such as extrusion difficulty and uneven stretching; at the same time, if the amount of inorganic particles added is too much and the particle size is too large, the dispersion of the inorganic particles in the film will become poor, and stress concentration points will easily form, resulting in a decrease in the mechanical properties of the film, such as reduced tensile strength and tear strength, as well as a decrease in optical properties, such as transparency and gloss.

[0006] In view of this, the present invention is proposed. Summary of the invention

[0007] The object of the present invention is to provide a method for preparing a polypropylene film for labels in view of the above-mentioned technical problems.

[0008] In view of this, the present invention provides a method for preparing a polypropylene film for a label, wherein the polypropylene film comprises a base film layer and a coating film layer, and the base film layer comprises the following raw materials in parts by weight: Random copolymer 60-90 parts; Homopolymer 15-30 parts; Polyolefin elastomer 5-15 parts; 3-7 parts of polytetrafluoroethylene; 10~25 parts of inorganic filler; Dispersant 0.5~2 parts; 1~3 parts of coupling agent; The inorganic filler includes a large-particle inorganic filler with a particle size of 500-1000 nm and a small-particle inorganic filler with a particle size of less than 300 nm.

[0009] Furthermore, the large-particle inorganic filler is one or more of porous calcium carbonate, porous silica, and porous titanium dioxide particles.

[0010] Furthermore, the base film layer further comprises the following raw materials in parts by weight: Nylon 5~10 parts; 3~8 parts of compatibilizer.

[0011] Furthermore, the nylon is a modified nylon that has been modified, and the preparation process of the modified nylon is as follows: 5 to 10 parts by weight of a modifier, 30 to 45 parts by weight of nylon, 1 to 2 parts by weight of a compatibilizer and 6 to 10 parts by weight of polyethylene terephthalate are put into an extruder, and the modified nylon is obtained by extrusion granulation after melting, mixing and homogenizing, wherein the extrusion screw aspect ratio of the extruder is 36 to 48:1, the extrusion temperature is controlled at 170 to 200° C., and the screw speed is controlled at 200 to 500 r / min.

[0012] Furthermore, when the base membrane layer includes porous large-particle inorganic filler particles and modified nylon, during the preparation process of the modified nylon, the porous large-particle inorganic filler particles and the modified nylon are premixed in advance to prepare porous large-particle inorganic filler particles with surface coating or filling of modified nylon.

[0013] Furthermore, the coating liquid for coating the film layer includes the following raw materials in parts by weight: 8-15 parts of ethylene-acrylic acid copolymer; Polyvinyl alcohol 5-10 parts; 3-5 parts of xanthan gum; Inorganic fiber 10-15 parts; 60~90 parts of water; Cross-linking agent 0.5~1 part; Compatibilizer 0.5~2 parts; Dispersant 1~3 parts.

[0014] Furthermore, the inorganic fiber is a modified inorganic fiber, and the preparation process of the modified inorganic fiber is as follows: First, 5 to 15 parts by weight of dimethyl silicone oil, 3 to 5 parts by weight of emulsifier and 70 to 100 parts by weight of water are mixed, and a dimethyl silicone oil emulsion is obtained after stirring and shearing; then 10 to 20 parts by weight of inorganic fiber is dispersed in the dimethyl silicone oil emulsion, soaked for 10 to 20 minutes under stirring at room temperature, filtered, and the inorganic fiber is dried at 30 to 60°C to obtain modified inorganic fiber.

[0015] Furthermore, the preparation process of the coating solution is as follows: First, a formulated amount of ethylene-acrylic acid copolymer is mixed with water, and stirred until the ethylene-acrylic acid copolymer is completely dissolved. Then, a formulated amount of a cross-linking agent and a compatibilizer are added, and the temperature of the mixed solution is heated to 100-120°C under stirring, and the mixture is kept warm for 0.5-1h. Then, a formulated amount of inorganic fiber and a dispersant are added, and after stirring evenly, a formulated amount of polyvinyl alcohol and xanthan gum are added, and the mixed solution is kept warm at 100-120°C for 0.5-1h under stirring to obtain the coating liquid.

[0016] Furthermore, the method for preparing the polypropylene film for label comprises the steps of: S1, mixing the raw materials of the base film layer, adding them into a twin-screw extruder for extrusion and melting, extruding the molten liquid through the die head of the twin-screw extruder, and cooling them through a cooling roller to obtain a sheet; S2, stretching the sheet obtained in step S1 by a biaxial synchronous stretching process, and then heat-setting, cooling and winding to obtain a base film layer; S3, preparation of coating film layer: dilute the prepared coating liquid and apply it on the base film layer obtained in step S2, and dry it at 40-70° C. to obtain the polypropylene film for label.

[0017] Furthermore, in step S3, the prepared coating liquid is diluted to a solid content of 5-10 wt% and then coated on the base film layer, and the wet base coating amount of the coating film layer is 5-20 g / m 2 .

[0018] The beneficial effects of the present invention are: The polypropylene film for labels of the present invention has the advantages of good printing performance, high mechanical strength and not prone to wrinkles, and is particularly suitable for electronic anti-theft soft labels. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present application will be described clearly below. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments in the present application belong to the scope of protection of the present application.

[0020] It should be noted that, in the present application, the terms "comprise", "include" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises one..." does not exclude the presence of other identical elements in the process, method, article or device including the element. In addition, it should be noted that the scope of the method and device in the embodiment of the present application is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in reverse order according to the functions involved, for example, the described method may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0021] A method for preparing a polypropylene film for a label, wherein the polypropylene film comprises a base film layer and a coating film layer, wherein the base film layer comprises the following raw materials in parts by weight: Random copolymer 60-90 parts; Homopolymer 15-30 parts; Polyolefin elastomer 5-15 parts; 3-7 parts of polytetrafluoroethylene; 10~25 parts of inorganic filler; Dispersant 0.5~2 parts; 1~3 parts of coupling agent; The inorganic filler includes a large-particle inorganic filler with a particle size of 500-1000 nm and a small-particle inorganic filler with a particle size of less than 300 nm.

[0022] Among the raw materials of the base film layer, random copolymer has excellent impact performance, good heat resistance, good low-temperature toughness, and good dimensional stability. Using it as the main raw material of the base film layer can give the base film layer good mechanical properties and high temperature / low temperature resistance, so that the polypropylene film for labels prepared by the present invention has excellent temperature adaptability. On this basis, by adding a certain amount of homopolypropylene, the mechanical strength and corrosion resistance of the prepared polypropylene film for labels can be further improved.

[0023] The addition of the polyolefin elastomer can improve the impact strength and toughness of the polypropylene film. At the same time, the polyolefin elastomer has good processing performance and fluidity, can improve the melt fluidity of the polypropylene film, reduce the processing temperature, and improve production efficiency.

[0024] The addition of polytetrafluoroethylene can improve the corrosion resistance and chemical stability of the polypropylene film. At the same time, polytetrafluoroethylene has extremely low surface energy, excellent self-lubricity and non-stickiness, and can improve the processing performance and surface performance of the polypropylene film.

[0025] More importantly, in the present invention, by setting the inorganic filler to include a large-particle inorganic filler with a particle size of 500-1000nm and a small-particle inorganic filler with a particle size of less than 300nm, the good dispersion and compatibility of the small-particle inorganic filler can be used as a reinforcing phase of the base film layer to improve the strength, toughness, thermal stability and processing performance of the base film layer. At the same time, while improving the surface roughness of the base film layer by using the large-particle inorganic filler, the large-particle inorganic filler further forms a more obvious reinforcing effect on the polypropylene film, thereby improving the mechanical properties of the film. In addition, the addition of the large-particle inorganic filler also helps to improve the dimensional stability of the film, reduce the thermal expansion coefficient, and maintain good dimensional stability when the temperature changes, and is not easy to shrink and wrinkle. Furthermore, the large-particle inorganic filler can also form an attachment point for the coating film layer, thereby improving the bonding force between the base film layer and the coating film layer.

[0026] It should be noted that, unless otherwise specified, the particle sizes described in the present invention are all median particle sizes.

[0027] Preferably, the inorganic filler includes a large-particle inorganic filler with a particle size of 500-1000 nm and a small-particle inorganic filler with a particle size of 100 nm.

[0028] More preferably, in the inorganic filler, the weight proportion of the large-particle inorganic filler is 20-35wt%.

[0029] As some examples of the present invention, the inorganic filler is selected from one or more of calcium carbonate, magnesium carbonate, barium carbonate, talc, mica, silicon dioxide, titanium dioxide, hydrotalcite, diatomaceous earth, calcium sulfate, barium sulfate, magnesium sulfate, zeolite and kaolin.

[0030] Preferably, the large-particle inorganic filler is one or more of calcium carbonate, silicon dioxide, and titanium dioxide particles.

[0031] More preferably, the large-particle inorganic filler is one or more of porous calcium carbonate, porous silica, and porous titanium dioxide particles.

[0032] As some examples of the present invention, the dispersant is selected from at least one of paraffin wax, polypropylene wax, and pentaerythritol stearate.

[0033] As some examples of the present invention, the coupling agent is selected from one or more of a silane coupling agent, a titanate coupling agent, an aluminate coupling agent, a borate coupling agent, and the like.

[0034] Furthermore, the base film layer further comprises the following raw materials in parts by weight: Nylon 5~10 parts; 3~8 parts of compatibilizer.

[0035] In the present invention, by adding a small amount of nylon, the performance of the base film layer can be further improved by utilizing the excellent strength, rigidity and thermal stability of nylon.

[0036] Preferably, the nylon is modified nylon that has been subjected to modification treatment.

[0037] Specifically, the preparation process of the modified nylon is as follows: 5 to 10 parts by weight of a modifier, 30 to 45 parts by weight of nylon, 1 to 2 parts by weight of a compatibilizer and 6 to 10 parts by weight of polyethylene terephthalate (PET) are put into an extruder, and the modified nylon is obtained by extrusion granulation after melting, mixing and homogenizing, wherein the extrusion screw aspect ratio of the extruder is 36 to 48:1, the extrusion temperature is controlled at 170 to 200° C., and the screw speed is controlled at 200 to 500 r / min.

[0038] As some examples of the present invention, the nylon is selected from one or more of polycaprolactam, polyundecamide, polydodecamide and the like.

[0039] Preferably, the modifier is selected from at least one of C5 / C9 copolymer petroleum resin, C9 hydrogenated petroleum resin, C5 hydrogenated petroleum resin, terpene resin, hydrogenated rosin, disproportionated rosin and polymerized rosin.

[0040] As some examples of the present invention, the compatibilizer for the base film layer raw material and the compatibilizer for nylon modification are selected from one or more of PP-maleic anhydride grafts, ethylene-vinyl alcohol copolymers, ethylene-acrylate-maleic anhydride copolymers, ethylene-acrylate-glycidyl methacrylate copolymers, ethylene-maleic anhydride-glycidyl methacrylate copolymers, etc.

[0041] The modifier has the functions of thickening, viscosity increasing and stiffening. In the present invention, by modifying nylon, a modified nylon material with better adhesion and processing fluidity can be obtained. When used for the preparation of the base film layer, the nylon can be well compatible with polypropylene, inorganic fillers and the like in the base film layer, and ultimately the prepared base film layer has good stiffness, strength and uniformity.

[0042] As a preferred embodiment of the present invention, when the base film layer includes porous large-size inorganic filler particles and modified nylon, the porous large-size inorganic filler particles and the modified nylon can be premixed in advance during the preparation of the modified nylon to prepare porous large-size inorganic filler particles with surface coating or filling of modified nylon. The specific process is as follows: 5-10 parts by weight of a modifier, 30-45 parts by weight of nylon, 1-2 parts by weight of a compatibilizer and 6-10 parts by weight of polyethylene terephthalate (PET) are fed into an extruder from a main feed port, and then a formulated amount of porous large-size inorganic filler particles are added from a side feed port according to the raw material ratio, and the modified nylon is obtained by extrusion granulation after melting, mixing and homogenizing, wherein the extrusion screw of the extruder has an aspect ratio of 36-48:1, the extrusion temperature is controlled at 170-200°C, and the screw speed is controlled at 200-500r / min.

[0043] During the premixing process of the porous large-size inorganic filler particles and the modified nylon, the modified nylon can be fully coated on the surface of the porous large-size inorganic filler particles or filled into the pores in the porous large-size inorganic filler particles. In the subsequent preparation process of the base film layer, the porous large-size inorganic filler particles can obtain excellent processing fluidity and dispersibility by virtue of the easy plasticization property of the modified nylon on the surface, thereby making the processability of the base film layer raw materials better. At the same time, the porous large-size inorganic filler particles can be well dispersed in the base film layer.

[0044] Furthermore, the coating liquid for coating the film layer includes the following raw materials in parts by weight: 8-15 parts of ethylene-acrylic acid copolymer; Polyvinyl alcohol 5-10 parts; 3-5 parts of xanthan gum; Inorganic fiber 10-15 parts; 60~90 parts of water; Cross-linking agent 0.5~1 part; Compatibilizer 0.5~2 parts; Dispersant 1~3 parts.

[0045] In the present invention, the ethylene-acrylic acid copolymer used is a type of high-performance thermoplastic resin with unique characteristics. In the molecular structure of the ethylene-acrylic acid copolymer, carboxyl groups are randomly distributed along the main chain and side chains of ethylene. Due to the presence of carboxyl groups and the effect of hydrogen bonds, the crystallization of the ethylene-acrylic acid copolymer is inhibited and the linearity of the main chain is destroyed, which makes the molecular chain of the ethylene-acrylic acid copolymer more flexible and easy to form a close bond with other substances, so that it has excellent adhesion, good flexibility and impact resistance, and excellent low-temperature resistance. When used as the main component of the coating liquid, the coating liquid can be well and stably attached to the base film layer.

[0046] On this basis, through the addition of polyvinyl alcohol, since the linear molecular chain of polyvinyl alcohol contains a large number of hydroxyl groups and hydrogen bonds, polyvinyl alcohol can form a linear interpenetrating network structure with ethylene-acrylic acid copolymer through cross-linking, hydrogen bonding and other effects. This network structure can give the coating film layer good mechanical strength and the ability to combine with the base film layer.

[0047] Furthermore, through the addition of xanthan gum, on the one hand, the molecular chains of polyvinyl alcohol can stretch in the high-temperature liquid solution, diffuse and penetrate into the xanthan gum, and xanthan gum and polyvinyl alcohol form a second network structure through cross-linking, hydroxyl groups, hydrogen bonds, etc., further optimizing the mechanical strength of the coating film layer and its ability to combine with the base film layer.

[0048] In the present invention, the coating film layer formed by coating the coating liquid has a "paper-like structure" by adding inorganic fibers. On the one hand, the film layer structure containing a large amount of inorganic fibers can be used to modify the base film layer by covering the inorganic fibers, so that it is easy to combine with ink; on the other hand, the inorganic fibers can be used to improve the surface roughness of the polypropylene film for labels, so as to facilitate the attachment of the oil film on its surface. In addition, the inorganic fibers can also fill and adhere to the inorganic fillers in the base film layer, especially the gaps between the large-particle inorganic fillers, or between the inorganic fillers and the base material, thereby enhancing the bonding force between the inorganic fillers, especially the large-particle inorganic fillers and the base material, the inorganic fibers and the inorganic fillers, and the base film layer and the coating film layer.

[0049] As some examples of the present invention, the inorganic fiber is selected from one or more of wollastonite fiber, calcium carbonate whisker fiber, calcium sulfate whisker fiber, sepiolite fiber, brucite fiber and the like.

[0050] Preferably, the length of the inorganic fiber is 1-100 um and the diameter is ≤3 um.

[0051] Preferably, the inorganic fiber is a modified inorganic fiber.

[0052] Specifically, the preparation process of the modified inorganic fiber is as follows: First, 5 to 15 parts by weight of dimethyl silicone oil, 3 to 5 parts by weight of emulsifier and 70 to 100 parts by weight of water are mixed, and a dimethyl silicone oil emulsion is obtained after stirring and shearing; then 10 to 20 parts by weight of inorganic fiber is dispersed in the dimethyl silicone oil emulsion, and after soaking for 10 to 20 minutes under stirring at room temperature, it is filtered, and the inorganic fiber is dried at 30 to 60°C to obtain modified inorganic fiber.

[0053] As some examples of the present invention, the emulsifier is selected from one or more of fatty alcohol polyoxyethylene ether, alkylphenol polyoxyethylene ether, Tween, sodium dodecylbenzene sulfonate, sodium dodecyl sulfate and the like.

[0054] Preferably, the melting temperature of the ethylene-acrylic acid copolymer is 90-100°C.

[0055] As some examples of the present invention, the cross-linking agent for the coating liquid is selected from one or more of dicumyl peroxide, tert-butylperoxytrimethylsilane, and the like.

[0056] As some examples of the present invention, the compatibilizer for the coating liquid is selected from one or more of ethylene-vinyl acetate copolymer, polypropylene grafted maleic anhydride, and the like.

[0057] As some examples of the present invention, the coating liquid dispersant is selected from one or more of sodium silicate, sodium tripolyphosphate, sodium hexametaphosphate, sodium pyrophosphate and the like.

[0058] Furthermore, the preparation process of the coating solution is as follows: First, a formulated amount of ethylene-acrylic acid copolymer is mixed with water, and stirred until the ethylene-acrylic acid copolymer is completely dissolved. Then, a formulated amount of a cross-linking agent and a compatibilizer are added, and the temperature of the mixed solution is heated to 100-120°C under stirring, and the mixture is kept warm for 0.5-1h. Then, a formulated amount of inorganic fiber and a dispersant are added, and after stirring evenly, a formulated amount of polyvinyl alcohol and xanthan gum are added, and the mixed solution is kept warm at 100-120°C for 0.5-1h under stirring to obtain the coating liquid.

[0059] Furthermore, the method for preparing the polypropylene film for label comprises the steps of: S1, mixing the raw materials of the base film layer, adding them into a twin-screw extruder for extrusion and melting, extruding the molten liquid through the die head of the twin-screw extruder, and cooling them through a cooling roller to obtain a sheet; S2, stretching the sheet obtained in step S1 by a biaxial synchronous stretching process, and then heat-setting, cooling and winding to obtain a base film layer; S3, preparation of coating film layer: dilute the prepared coating liquid and apply it on the base film layer obtained in step S2, and dry it at 40-70° C. to obtain the polypropylene film for label.

[0060] As some examples of the present invention, in the step S1, the raw materials of the base film layer are mixed, added into a twin-screw extruder and extruded and melted at 230-250°C, the molten liquid is extruded through the die head of the twin-screw extruder, the die head temperature is 230-240°C, and after being cooled by a cooling roller at 20-40°C, a sheet is obtained.

[0061] Furthermore, in step S2, the stretching process of the base film includes: S21, after keeping the sheet obtained in step S1 at 90-120° C. for 1-3 min, perform a first biaxial synchronous stretching, wherein the stretching temperature of the first biaxial synchronous stretching is 140-160° C., and the stretching ratio is 1.5×1.5-3×3 times; S22, after keeping the film material obtained in step S21 at 140-160°C for 1-3 minutes, perform a second biaxial synchronous stretching, the stretching temperature of the second biaxial synchronous stretching is 160-180°C, and the stretching ratio is 4×4-6×6 times.

[0062] Preferably, during the first bidirectional synchronous stretching process in step S21, the temperature distribution of the membrane material in the transverse direction is controlled to be high in the middle, low on both sides, symmetrical, and parabolic; during the second bidirectional synchronous stretching process in step S22, the temperature distribution of the membrane material in the transverse direction is controlled to be low in the middle, high on both sides, symmetrical, and parabolic.

[0063] As some examples of the present invention, in the steps S21 and S22, the temperature of the film material in the transverse direction may be determined according to the thickness difference of the film material in the transverse direction, and the specific process is as follows: In the step S21, the thickness distribution of the film material in the transverse direction before the first biaxial synchronous stretching is detected, and the average value H of the thickness distribution of the film material in the transverse direction is calculated. 平均1 , maximum value H max1 and the minimum value H min1 ; Then according to the average value H of the thickness distribution of the membrane material in the horizontal direction 平均1 Determine the base temperature T of the membrane material during the first biaxial simultaneous stretching process 基础1 , where the base temperature T 基础1 It should be between 140 and 160°C, and then the maximum value H max1 and the minimum value H min1 With the basic temperature T 基础1 The maximum temperature T of the film material during the first biaxial synchronous stretching process is determined by the deviation between max1 and minimum temperature T min1 , where T max1 =k1*T 基础1 *(H max1 -H 平均1 ) / H 平均1 ; T min1 =k1*T 基础1 *(H 平均1 -H min1 ) / H 平均1 , k1 is the adjustment coefficient, and its value is obtained through experiments or experience; then the midpoint of the membrane material in the horizontal direction is taken as the highest point of the temperature value, that is, the temperature of the midpoint of the membrane material in the horizontal direction is T max1The two ends of the membrane material in the horizontal direction are the lowest points of the temperature value, that is, the temperature of the two ends of the membrane material in the horizontal direction is T min1 On this basis, the transverse temperature distribution of the membrane material is calculated by fitting or parabolic formula, and the transverse temperature distribution of the membrane material is regulated to the set value by adjusting the power of the heating module.

[0064] On the contrary, in the step S22, the thickness distribution of the film material in the transverse direction before the second biaxial synchronous stretching is first detected, and the average value H of the thickness distribution of the film material in the transverse direction is calculated. 平均2 , maximum value H max2 and the minimum value H min2 ; Then according to the average value H of the thickness distribution of the membrane material in the horizontal direction 平均2 Determine the base temperature T of the membrane material during the second biaxial synchronous stretching process 基础2 , where the base temperature T 基础2 It should be between 160 and 180°C, and then the maximum value H max2 and the minimum value H min2 With the basic temperature T 基础2 The deviation between the two determines the maximum temperature T of the film material during the second biaxial synchronous stretching process. max2 and minimum temperature T min2 , where T max2 =k2*T 基础2 *(H max2 -H 平均2 ) / H 平均2 ; T min2 =k2*T 基础2 *(H 平均2 -H min2 ) / H 平均2 , k2 is the adjustment coefficient, and its value is obtained through experiments or experience; then the midpoint of the membrane material in the horizontal direction is taken as the lowest point of the temperature value, that is, the temperature of the midpoint of the membrane material in the horizontal direction is T min2 The two ends of the membrane material in the horizontal direction are the highest points of the temperature value, that is, the temperature of the two ends of the membrane material in the horizontal direction is T max2 On this basis, the transverse temperature distribution of the membrane material is calculated by fitting or parabolic formula, and the transverse temperature distribution of the membrane material is regulated to the set value by adjusting the power of the heating module.

[0065] As some examples of the present invention, in step S2, the obtained base film layer material can be subjected to heat setting treatment at 110-130°C for 10-15 minutes.

[0066] Preferably, in step S3, the prepared coating liquid can be diluted to a solid content of 5-10 wt% and then coated on the base film layer, and the wet base coating amount of the coating film layer is 5-20 g / m 2 .

[0067] The polypropylene film for label and the preparation method thereof of the present invention are illustrated by specific embodiments as follows: Embodiment 1-2 Preparation of a polypropylene film for labels: S1, mixing the raw materials of the base film layer according to Table 1 below, adding the raw materials to a twin-screw extruder for extrusion and melting, extruding the molten liquid through the die head of the twin-screw extruder, and cooling it through a cooling roller to obtain a sheet; S2, stretching the sheet obtained in step S1 by a biaxial synchronous stretching process, and then heat-setting, cooling and winding to obtain a base film layer; Among them, the stretching process of the base film includes: S21, after keeping the sheet obtained in step S1 at 90° C. for 3 minutes, perform the first biaxial synchronous stretching, during which the temperature distribution of the film material in the transverse direction is controlled to be a fixed value of 145° C., and the stretching ratio is 2×2 times; S22, after keeping the film material obtained in step S21 at 140° C. for 3 minutes, perform a second biaxial synchronous stretching, during which the temperature distribution of the film material in the transverse direction is controlled to be a fixed value of 160° C., and the stretching ratio is 4×4 times; S3, preparation of coating film layer: dilute the prepared coating liquid to a solid content of 5wt% and then apply it on the base film layer prepared in step S2. The wet base coating amount of the coating film layer is 20g / m 2 , and after drying at 40° C., the polypropylene film for label is obtained.

[0068] Embodiment 3-4 Preparation of a polypropylene film for labels: S1, mixing the raw materials of the base film layer according to Table 1 below, adding the raw materials to a twin-screw extruder for extrusion and melting, extruding the molten liquid through the die head of the twin-screw extruder, and cooling it through a cooling roller to obtain a sheet; S2, stretching the sheet obtained in step S1 by a biaxial synchronous stretching process, and then heat-setting, cooling and winding to obtain a base film layer; Among them, the stretching process of the base film includes: S21, after keeping the sheet obtained in step S1 at 100° C. for 2 minutes, perform the first biaxial synchronous stretching, during which the temperature distribution of the film material in the transverse direction is controlled to be a fixed value of 160° C., and the stretching ratio is 1.5×1.5 times; S22, after keeping the film material obtained in step S21 at 150° C. for 2 minutes, perform a second biaxial synchronous stretching, during which the temperature distribution of the film material in the transverse direction is controlled to be a fixed value of 165° C., and the stretching ratio is 5×5 times; S3, preparation of coating film layer: dilute the prepared coating liquid to a solid content of 8wt% and then apply it on the base film layer prepared in step S2. The wet base coating amount of the coating film layer is 15g / m 2 , and then dried at 50° C. to obtain the polypropylene film for labeling.

[0069] Embodiment 5-6 Preparation of a polypropylene film for labels: S1, mixing the raw materials of the base film layer according to Table 1 below, adding the raw materials to a twin-screw extruder for extrusion and melting, extruding the molten liquid through the die head of the twin-screw extruder, and cooling it through a cooling roller to obtain a sheet; S2, stretching the sheet obtained in step S1 by a biaxial synchronous stretching process, and then heat-setting, cooling and winding to obtain a base film layer; Among them, the stretching process of the base film includes: S21, after keeping the sheet obtained in step S1 at 120° C. for 1 minute, perform the first biaxial synchronous stretching, during which the temperature distribution of the film material in the transverse direction is controlled to be a fixed value of 140° C., and the stretching ratio is 3×3 times; S22, after the film material obtained in step S21 is kept at 160°C for 1 minute, it is subjected to a second biaxial synchronous stretching. During the second biaxial synchronous stretching process, the temperature distribution of the film material in the transverse direction is controlled to be a fixed value of 180°C, and the stretching ratio is 6×6 times; S3, preparation of the coating film layer: the prepared coating liquid is diluted to a solid content of 10wt% and then coated on the base film layer obtained in step S2, and the wet base coating amount of the coating film layer is 5g / m 2 , and drying at 70° C. to obtain the polypropylene film for labeling.

[0070] Table 1 Ratio of raw materials for base film layer and coating film layer Example 7 The only difference between it and the above-mentioned Example 1 is that the large-particle-size inorganic filler therein is porous calcium carbonate particles, while the large-particle-size inorganic filler in Example 1 is spherical non-porous calcium carbonate particles.

[0071] Example 8 The only difference between it and the above-mentioned Example 3 is that the large-particle-size inorganic filler therein is porous calcium carbonate particles, while the large-particle-size inorganic filler in Example 3 is spherical non-porous calcium carbonate particles.

[0072] Example 9 The only difference between it and the above-mentioned Example 8 is that the nylon used in the raw material of the base film layer is the modified nylon described in the present invention, and the modified nylon is premixed with porous calcium carbonate particles during the preparation process of the modified nylon.

[0073] Example 10 The difference between it and the above-mentioned embodiment 5 is that: in the first biaxial synchronous stretching process, the temperature distribution of the film material in the transverse direction is controlled to be a symmetrical parabola shape with high temperature in the middle and low temperature on both sides. When determining the temperature distribution of the film material in the transverse direction, the coefficient k1 is adjusted so that T max1 With T min1 The average value is 140℃.

[0074] Embodiment 11 The only difference between it and the above-mentioned embodiment 5 is that: during the second biaxial synchronous stretching process, the temperature distribution of the film material in the transverse direction is controlled to be a symmetrical parabola with low temperature in the middle and high temperature on both sides. When determining the temperature distribution of the film material in the transverse direction, the coefficient k1 is adjusted so that T max1 With T min1 The average value is 180℃.

[0075] Example 12 The difference between it and the above-mentioned embodiment 5 is that: in the first biaxial synchronous stretching process, the temperature distribution of the film material in the transverse direction is controlled to be a symmetrical parabola shape with high temperature in the middle and low temperature on both sides. When determining the temperature distribution of the film material in the transverse direction, the coefficient k1 is adjusted so that T max1 With T min1 The average value is 140℃; in the second biaxial synchronous stretching process, the temperature distribution of the film material in the transverse direction is controlled to be a symmetrical parabola with low temperature in the middle and high temperature on both sides. When determining the temperature distribution of the film material in the transverse direction, the coefficient k1 is adjusted to make T max1 With T min1 The average value is 180℃.

[0076] Comparative Example 1 The only difference between it and the above-mentioned embodiment 1 is that the inorganic fillers used are all small-particle inorganic fillers.

[0077] Comparative Example 2 The only difference between it and the above-mentioned embodiment 1 is that the inorganic fillers used are all large-particle-size inorganic fillers.

[0078] Comparative Example 3 The only difference between this embodiment and the above-mentioned embodiment 3 is that the coating liquid used does not contain polyvinyl alcohol.

[0079] Comparative Example 4 The only difference between it and the above-mentioned Example 3 is that the coating liquid used does not contain xanthan gum.

[0080] Comparative Example 5 The only difference between it and the above-mentioned embodiment 3 is that the coating liquid used does not contain inorganic fibers.

[0081] Comparative Example 6 The only difference between it and the above-mentioned embodiment 3 is that the base film layer material does not contain nylon and compatibilizer.

[0082] Performance Test: The following performance tests were performed on the polypropylene films for labels obtained in the above-mentioned Examples 1 to 12 and Comparative Examples 1 to 6, respectively, wherein: (1) Tensile strength and thermal shrinkage performance are tested in accordance with GB / T1003-2008; (2) Impact strength performance is tested in accordance with GB / T9639; (3) The ink adhesion test is conducted using UV inkjet ink. The test method is as follows: use a grid knife to draw a hundred grids on the printed surface at 1 mm intervals. After standing for 90 seconds, use 810 tape to test. The test method is as follows: after applying pressure three times, pull it up instantly at 90°, and then count the number of grids where paint has peeled off the surface.

[0083] The test results are shown in Table 2 below: Table 2 Test results of polypropylene film properties The above description of the embodiments of the present application, in the absence of conflict, the embodiments in the present application and the features in the embodiments can be combined with each other, the present application is not limited to the above-mentioned specific implementation methods, the above-mentioned specific implementation methods are merely illustrative and not restrictive, and ordinary technicians in this field, under the guidance of the present application, can also make many forms without departing from the purpose of the present application and the scope of protection of the claims, all of which are within the protection of the present application.

Claims

1. A method for preparing a polypropylene film for a label, characterized in that: The polypropylene film comprises a base film layer and a coating film layer, and the base film layer comprises the following raw materials in parts by weight: Random copolymer 60-90 parts; Homopolymer 15-30 parts; Polyolefin elastomer 5-15 parts; 3-7 parts of polytetrafluoroethylene; 10~25 parts of inorganic filler; Dispersant 0.5~2 parts; 1~3 parts of coupling agent; The inorganic filler includes a large-particle inorganic filler with a particle size of 500-1000 nm and a small-particle inorganic filler with a particle size of less than 300 nm.

2. The method for preparing a polypropylene film for labels according to claim 1, characterized in that: The large-particle inorganic filler is one or more of porous calcium carbonate, porous silicon dioxide, and porous titanium dioxide particles.

3. The method for preparing a polypropylene film for labels according to claim 1 or 2, characterized in that: The base film layer also includes the following raw materials in parts by weight: Nylon 5~10 parts; 3~8 parts of compatibilizer.

4. The method for preparing a polypropylene film for labels according to claim 3, characterized in that: The nylon is modified nylon that has been modified, and the preparation process of the modified nylon is as follows: 5 to 10 parts by weight of a modifier, 30 to 45 parts by weight of nylon, 1 to 2 parts by weight of a compatibilizer and 6 to 10 parts by weight of polyethylene terephthalate are put into an extruder, and the modified nylon is obtained by extrusion granulation after melting, mixing and homogenizing, wherein the extrusion screw aspect ratio of the extruder is 36 to 48:1, the extrusion temperature is controlled at 170 to 200° C., and the screw speed is controlled at 200 to 500 r / min.

5. The method for preparing a polypropylene film for labels according to claim 4, characterized in that: When the base film layer includes porous large-size inorganic filler particles and modified nylon, during the preparation of the modified nylon, the porous large-size inorganic filler particles and the modified nylon are premixed in advance to prepare porous large-size inorganic filler particles with surface coating or filling of modified nylon.

6. The method for preparing a polypropylene film for labels according to claim 1, characterized in that: The coating liquid for coating the film layer comprises the following raw materials in parts by weight: 8-15 parts of ethylene-acrylic acid copolymer; Polyvinyl alcohol 5-10 parts; 3-5 parts of xanthan gum; Inorganic fiber 10-15 parts; 60~90 parts of water; Cross-linking agent 0.5~1 part; Compatibilizer 0.5~2 parts; Dispersant 1~3 parts.

7. The method for preparing a polypropylene film for labels according to claim 6, characterized in that: The inorganic fiber is a modified inorganic fiber, and the preparation process of the modified inorganic fiber is as follows: First, 5 to 15 parts by weight of dimethyl silicone oil, 3 to 5 parts by weight of emulsifier and 70 to 100 parts by weight of water are mixed, and a dimethyl silicone oil emulsion is obtained after stirring and shearing; then 10 to 20 parts by weight of inorganic fiber is dispersed in the dimethyl silicone oil emulsion, soaked for 10 to 20 minutes under stirring at room temperature, filtered, and the inorganic fiber is dried at 30 to 60°C to obtain modified inorganic fiber.

8. The method for preparing a polypropylene film for labels according to claim 6, characterized in that: The preparation process of the coating solution is as follows: First, a formulated amount of ethylene-acrylic acid copolymer is mixed with water, and stirred until the ethylene-acrylic acid copolymer is completely dissolved. Then, a formulated amount of a cross-linking agent and a compatibilizer are added, and the temperature of the mixed solution is heated to 100-120°C under stirring, and the mixture is kept warm for 0.5-1h. Then, a formulated amount of inorganic fiber and a dispersant are added, and after stirring evenly, a formulated amount of polyvinyl alcohol and xanthan gum are added, and the mixed solution is kept warm at 100-120°C for 0.5-1h under stirring to obtain the coating liquid.

9. The method for preparing a polypropylene film for labels according to claim 1, characterized in that: The method for preparing the polypropylene film for label comprises the following steps: S1, mixing the raw materials of the base film layer, adding them into a twin-screw extruder for extrusion and melting, extruding the molten liquid through the die head of the twin-screw extruder, and cooling them through a cooling roller to obtain a sheet; S2, stretching the sheet obtained in step S1 by a biaxial synchronous stretching process, and then heat-setting, cooling and winding to obtain a base film layer; S3, preparation of coating film layer: dilute the prepared coating liquid and apply it on the base film layer obtained in step S2, and dry it at 40-70° C. to obtain the polypropylene film for label.

10. The method for preparing a polypropylene film for labels according to claim 9, characterized in that: In step S3, the prepared coating liquid is diluted to a solid content of 5-10 wt% and then coated on the base film layer. The wet base coating amount of the coating film layer is 5-20 g / m 2 .

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