Preparation method of polypropylene film for label

Abstract

The present 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 coated film layer. The base film layer comprises the following raw materials in parts by weight: 60-90 parts of random copolymer polypropylene; 15-30 parts of homopolypropylene; 5-15 parts of polyolefin elastomer; 3-7 parts of polytetrafluoroethylene; 10-25 parts of inorganic filler; 0.5-2 parts of dispersant; 1-3 parts of coupling agent. Among them, the inorganic filler comprises large-particle-size inorganic filler with a particle size of 500-1000 nm and small-particle-size inorganic filler with a particle size < 300 nm. The polypropylene film for labels of the present invention has the advantages of good printing performance, high mechanical strength, and being not easy to generate wrinkles.

Description

Technical Field

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

[0002] Label films are important carriers for product information display. They mainly convey important information such as the brand, name, specifications, production date, and shelf life of products to consumers by printing information such as text, patterns, and barcodes. Common label films include film media made of materials such as PE, PP, PET, and PVC. Among them, polypropylene film is a thermoplastic resin prepared by polymerizing propylene. According to different processing techniques, polypropylene film can be divided into CPP film, BOPP film, IPP film, etc.

[0003] The commonly used biaxially oriented polypropylene film (BOPP) is prepared 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 molecular chain segments and the increase in crystallinity, the tensile strength, tensile elastic modulus, impact strength, tear strength, and flex life 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 resistance, high gloss, good transparency, good gas barrier property, moisture-proof, light weight, non-toxic, odorless, good dimensional stability, good electrical insulation, wide range of applications, excellent quality stability and processability. Its comprehensive performance is superior to transparent films such as moisture-proof glassine paper and polyethylene film, and it is hailed as the "queen of packaging" by the packaging industry.

[0004] However, the existing BOPP films still have the following defects:

[0005] First, poor printing performance: Due to the non-polar surface of the BOPP film, the adhesion of ink and adhesive on its surface is poor, and the printing effect is not ideal. Usually, special inks are required for printing, which not only increases the printing cost, but also the adhesion fastness of the oil film on the printed matter is not high. Especially for labels that require high-resolution printing such as barcodes, the printing quality is difficult to meet the requirements. In addition, the BOPP film has a very small absorption capacity for ink, and the ink is not easy to dry, resulting in the printing speed of the BOPP film being much lower than that of paper products;

[0006] Second, the tensile strength needs to be improved: Thanks to the orderly arrangement of PP molecular chains during the stretching process, the BOPP film can exhibit high tensile strength. However, for label stickers, higher tensile strength means that the label stickers are less likely to break when pulled or torn, which is beneficial to improving their durability and reliability;

[0007] Third, uneven internal stress distribution and easy generation of wrinkles: During the biaxial synchronous stretching process, longitudinal and transverse stretching are carried out simultaneously, and the tensile forces in the two directions compete with each other. The molecular chains are randomly oriented within the film plane, and it is difficult to obtain a highly oriented crystal structure along one direction. This non-uniform distribution of internal stress easily causes the film to wrinkle during subsequent processes. In addition, due to the complexity of the biaxial synchronous stretching process, the film thickness may be uneven, and the thickness unevenness will further cause internal stress in the film during subsequent processes, thereby generating wrinkles. Especially for films used in high-temperature (such as above 80 °C) application environments, the wrinkle phenomenon is particularly serious.

[0008] To solve the problems existing in BOPP films and further improve their performance, those skilled in the art have proposed various improvement methods. Among them, adding a certain amount of inorganic particles, such as calcium carbonate, carbon dioxide, silicon dioxide, barium sulfate, etc., to the raw materials of BOPP films 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, using inorganic particles as the reinforcing phase in BOPP films to improve the mechanical properties of BOPP films is a relatively effective method. For example, Chinese Patent with publication number CN119220018A and title "An Ink-Affinic BOPP Film 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 addition amount is too small, the improvement of the performance of BOPP films, especially the printing performance, is very limited. However, if the addition amount of the inorganic particles is too large and the particle size is too large, it will lead to a decline in the processing performance of BOPP films, such as difficult extrusion and uneven stretching. At the same time, if the addition amount of the inorganic particles is too large and the particle size is too large, it will also lead to poor dispersion of the inorganic particles in the film, easily forming stress concentration points, resulting in a decline in the mechanical properties of the film, such as a decrease in tensile strength and tear strength, as well as a decline in optical properties, such as transparency and gloss.

[0009] In view of this, the present invention is specifically proposed. Summary of the Invention

[0010] The object of the present invention is to provide a preparation method for a polypropylene film for labels in view of the above existing technical problems.

[0011] In view of this, the present invention provides a preparation method for a polypropylene film for labels. The polypropylene film includes a base film layer and a coated film layer. The base film layer comprises the following raw materials in parts by weight:

[0012] Random copolymer polypropylene 60 - 90 parts;

[0013] Homopolypropylene 15 - 30 parts;

[0014] Polyolefin elastomer 5 - 15 parts;

[0015] 3-7 parts of polytetrafluoroethylene;

[0016] 10~25 parts of inorganic filler;

[0017] Dispersant 0.5~2 parts;

[0018] 1~3 parts of coupling agent;

[0019] 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.

[0020] Furthermore, the large-particle inorganic filler is one or more of porous calcium carbonate, porous silica, and porous titanium dioxide particles.

[0021] Furthermore, the base film layer further comprises the following raw materials in parts by weight:

[0022] Nylon 5~10 parts;

[0023] 3~8 parts of compatibilizer.

[0024] Furthermore, the nylon is a modified nylon that has been modified, and the preparation process of the modified nylon is as follows:

[0025] 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.

[0026] 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.

[0027] Furthermore, the coating liquid for coating the film layer includes the following raw materials in parts by weight:

[0028] 8-15 parts of ethylene-acrylic acid copolymer;

[0029] Polyvinyl alcohol 5-10 parts;

[0030] 3-5 parts of xanthan gum;

[0031] Inorganic fiber 10-15 parts;

[0032] 60~90 parts of water;

[0033] Crosslinking agent: 0.5 - 1 part;

[0034] Compatibilizer: 0.5 - 2 parts;

[0035] Dispersant: 1 - 3 parts.

[0036] Furthermore, the inorganic fiber is a modified inorganic fiber, and the preparation process of the modified inorganic fiber is as follows:

[0037] First, mix 5 - 15 parts by weight of dimethyl silicone oil, 3 - 5 parts by weight of emulsifier, and 70 - 100 parts by weight of water, stir and shear to obtain a dimethyl silicone oil emulsion; then disperse 10 - 20 parts by weight of inorganic fiber into the dimethyl silicone oil emulsion, soak it for 10 - 20 min under stirring at room temperature, filter, and dry the inorganic fiber at 30 - 60 °C to obtain the modified inorganic fiber.

[0038] Furthermore, the preparation process of the coating liquid is as follows:

[0039] First, mix the ethylene - acrylic copolymer in the formula amount with water, stir until the ethylene - acrylic copolymer is completely dissolved, add the crosslinking agent and compatibilizer in the formula amount, and heat the temperature of the mixed liquid to 100 - 120 °C and keep it warm for 0.5 - 1 h under stirring. Then add the inorganic fiber and dispersant in the formula amount, stir evenly, add the polyvinyl alcohol and xanthan gum in the formula amount, and continue to keep the mixed liquid at 100 - 120 °C and keep it warm for 0.5 - 1 h under stirring to obtain the coating liquid.

[0040] Furthermore, the preparation method of the polypropylene film for labels includes the steps:

[0041] S1, Mix the raw materials of the base film layer, add them into a twin - screw extruder to extrude and melt, and extrude the molten material through the die head of the twin - screw extruder, and then cool it with a cooling roller to obtain a sheet;

[0042] S2, Stretch the sheet obtained in step S1 by a biaxial synchronous stretching process, and then obtain the base film layer through heat setting, cooling, and winding;

[0043] S3, Preparation of the coating film layer: Dilute the prepared coating liquid and coat it on the base film layer obtained in step S2, and dry it at 40 - 70 °C to obtain the polypropylene film for labels.

[0044] Furthermore, in step S3, dilute the prepared coating liquid to a solid content of 5 - 10 wt% and then coat it on the base film layer, and the wet - based coating amount of the coating film layer is 5 - 20 g / m 2 .

[0045] The beneficial effects of the present invention are:

[0046] The polypropylene film for labels of the present invention has the advantages of good printing performance, high mechanical strength, and being not prone to generating wrinkles, and is particularly suitable for electronic anti-theft soft labels. Specific Embodiments

[0047] The technical solutions in the embodiments of the present application will be clearly described below. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art belong to the scope protected by the present application.

[0048] It should be noted that in the present application, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including a..." does not exclude the presence of additional identical elements in the process, method, article or device including the element. In addition, it should be pointed out that the methods and devices in the embodiments of the present application are not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in a reverse order according to the functions involved. For example, the described methods may be performed in an order different from that described, and various steps may be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0049] A preparation method of a polypropylene film for labels, the polypropylene film includes a base film layer and a coated film layer, and the base film layer includes the following raw materials in parts by weight:

[0050] Random copolymer polypropylene 60 - 90 parts;

[0051] Homopolypropylene 15 - 30 parts;

[0052] Polyolefin elastomer 5 - 15 parts;

[0053] Polytetrafluoroethylene 3 - 7 parts;

[0054] Inorganic filler 10 - 25 parts;

[0055] Dispersant 0.5 - 2 parts;

[0056] Coupling agent 1 - 3 parts;

[0057] Among them, the inorganic filler includes large-particle-size inorganic filler with a particle size of 500 - 1000 nm and small-particle-size inorganic filler with a particle size < 300 nm.

[0058] Among the raw materials of the base film layer, random copolymer polypropylene 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 endow the base film layer with good mechanical properties and high temperature / low temperature resistance, making the polypropylene film for labels prepared by the present invention have 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.

[0059] The addition of the polyolefin elastomer can improve the impact strength of the polypropylene film and its toughness. At the same time, the polyolefin elastomer has good processing performance and fluidity, which can improve the melt fluidity of the polypropylene film, reduce the processing temperature, and improve production efficiency.

[0060] The addition of the polytetrafluoroethylene can improve the corrosion resistance and chemical stability of the polypropylene film. At the same time, the polytetrafluoroethylene has an extremely low surface energy, and its excellent self-lubricity and non-stickiness can improve the processing performance and surface performance of the polypropylene film.

[0061] More importantly, in the present invention, by setting the inorganic filler to include large-particle-size inorganic fillers with a particle size of 500 - 1000 nm and small-particle-size inorganic fillers with a particle size < 300 nm, the good dispersion and compatibility of the small-particle-size inorganic fillers can be used as the 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 using the large-particle-size inorganic fillers to improve the surface roughness of the base film layer, a relatively obvious strengthening effect is further formed on the polypropylene film by the large-particle-size inorganic fillers to improve the mechanical properties of the film. In addition, the addition of the large-particle-size inorganic fillers also helps to improve the dimensional stability of the film, reduce the coefficient of thermal expansion, so that it maintains good dimensional stability during temperature changes and is not prone to shrinkage and wrinkles. Moreover, the large-particle-size inorganic fillers can also form the attachment points of the coating film layer to improve the bonding force between the base film layer and the coating film layer.

[0062] It should be noted that unless otherwise specified, the particle sizes described in the present invention are all median particle sizes.

[0063] Preferably, the inorganic filler includes large-particle-size inorganic fillers with a particle size of 500 - 1000 nm and small-particle-size inorganic fillers with a particle size of 100 nm.

[0064] More preferably, among the inorganic fillers, the weight ratio of the large-particle-size inorganic fillers is 20 - 35 wt%.

[0065] 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, silica, titanium dioxide, hydrotalcite, diatomaceous earth, calcium sulfate, barium sulfate, magnesium sulfate, zeolite, kaolin, etc.

[0066] Preferably, the large-particle-size inorganic filler is one or more of calcium carbonate, silica, and titanium dioxide particles.

[0067] More preferably, the large-particle-size inorganic filler is one or more of porous calcium carbonate, porous silica, and porous titanium dioxide particles.

[0068] As some examples of the present invention, the dispersant is selected from at least one of paraffin wax, polypropylene wax, and pentaerythritol stearate.

[0069] As some examples of the present invention, the coupling agent is selected from one or several of silane coupling agents, titanate coupling agents, aluminate coupling agents, borate coupling agents, etc.

[0070] Furthermore, the base film layer further comprises the following raw materials in parts by weight:

[0071] Nylon: 5 - 10 parts;

[0072] Compatibilizer: 3 - 8 parts.

[0073] 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.

[0074] Preferably, the nylon is modified nylon that has been subjected to a modification treatment.

[0075] Specifically, the preparation process of the modified nylon is as follows:

[0076] Put 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) into an extruder, and after melting, kneading, and homogenizing, extrude and pelletize to obtain the modified nylon. Among them, the length-diameter ratio of the extrusion screw of the extruder is 36 - 48:1, the extrusion temperature is controlled at 170 - 200 °C, and the screw speed is controlled at 200 - 500 r / min.

[0077] As some examples of the present invention, the nylon is selected from one or several of polycaprolactam, polyundecanamide, polydodecanamide, etc.

[0078] Preferably, the modifier is selected from at least one of C5 / C9 copolymerized petroleum resin, C9 hydrogenated petroleum resin, C5 hydrogenated petroleum resin, terpene resin, hydrogenated rosin, disproportionated rosin, and polymerized rosin.

[0079] 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 graft copolymer, ethylene-vinyl alcohol copolymer, ethylene-acrylate-maleic anhydride copolymer, ethylene-acrylate-glycidyl methacrylate copolymer, ethylene-maleic anhydride-glycidyl methacrylate copolymer, etc.

[0080] The modifier has the functions of thickening, tackifying and stiffening. In the present invention, by modifying nylon, a modified nylon material with better adhesiveness and processing fluidity can be obtained. When used for the preparation of the base film layer, nylon can be well compatible with polypropylene, inorganic fillers, etc. in the base film layer, and finally the prepared base film layer has good stiffness, strength and uniformity.

[0081] As a preferred embodiment of the present invention, when the base film layer includes porous large-particle-size inorganic filler particles and modified nylon, during the preparation of the modified nylon, the porous large-particle-size inorganic filler particles and the modified nylon can be premixed in advance to obtain porous large-particle-size inorganic filler particles with the surface coated or filled with modified nylon. The specific process is as follows:

[0082] Put 5-10 parts by weight of the modifier, 30-45 parts by weight of nylon, 1-2 parts by weight of the compatibilizer and 6-10 parts by weight of polyethylene terephthalate (PET) into the extruder from the main feed port, and then add the formulated amount of porous large-particle-size inorganic filler particles from the side feed port according to the raw material ratio, and extrude and pelletize through melting, kneading and homogenization to obtain the modified nylon. Among them, the ratio of the length to the diameter of the extrusion screw of the extruder is 36-48:1, the extrusion temperature is controlled at 170-200 °C, and the screw speed is controlled at 200-500 r / min.

[0083] During the premixing process of the porous large-particle-size inorganic filler particles and the modified nylon, the modified nylon can be fully coated on the surface of the porous large-particle-size inorganic filler particles or filled into the pores of the porous large-particle-size inorganic filler particles. During the subsequent preparation process of the base film layer, the porous large-particle-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, making the processability of the base film layer raw material better. At the same time, the porous large-particle-size inorganic filler particles can form good dispersion in the base film layer.

[0084] Furthermore, the coating liquid for the coating film layer includes the following raw materials in parts by weight:

[0085] Ethylene-acrylic acid copolymer 8-15 parts;

[0086] Polyvinyl alcohol 5-10 parts;

[0087] Xanthan gum 3-5 parts;

[0088] 10 - 15 parts of inorganic fiber;

[0089] 60 - 90 parts of water;

[0090] 0.5 - 1 part of crosslinking agent;

[0091] 0.5 - 2 parts of compatibilizer;

[0092] 1 - 3 parts of dispersant.

[0093] In the present invention, the ethylene - acrylic acid copolymer adopted is a kind of high - performance thermoplastic resin with unique properties. 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 action of hydrogen bonds, the crystallization of the ethylene - acrylic acid copolymer is inhibited, and the linearity of the main chain is damaged. This makes the molecular chain of the ethylene - acrylic acid copolymer more flexible, easier to form a tight bond with other substances, endowing it with excellent adhesiveness, good flexibility and impact resistance, as well as excellent low - temperature resistance. As the main component of the coating solution, it can enable the coating solution to adhere well and stably on the base film layer.

[0094] On this basis, through the addition of polyvinyl alcohol, since a large number of hydroxyl groups and hydrogen bonds are contained in the linear molecular chain of polyvinyl alcohol, polyvinyl alcohol can form a linear interpenetrating network structure with the ethylene - acrylic acid copolymer through cross - linking, hydrogen - bonding and other effects. This network structure can endow the coating film layer with good mechanical strength and the ability to bind to the base film layer.

[0095] Furthermore, through the addition of xanthan gum, on the one hand, the molecular chain of polyvinyl alcohol can stretch, diffuse and interpenetrate into xanthan gum in the high - temperature liquid phase solution. Xanthan gum and polyvinyl alcohol form a second - layer network structure through cross - linking, hydroxyl groups, hydrogen - bonding and other effects, further optimizing the mechanical strength of the coating film layer and the ability to bind to the base film layer.

[0096] In the present invention, through the addition of inorganic fiber, the coating film layer formed by coating the coating solution has a "paper - like structure". This film layer structure containing a large amount of inorganic fiber, on the one hand, can modify the base film layer through the coverage of inorganic fiber, making it easier to combine with ink; on the other hand, it can improve the surface roughness of the polypropylene film for labels, facilitating the adhesion of the oil film on its surface. In addition, inorganic fiber can also fill and adhere to the gaps between inorganic fillers in the base film layer, especially between large - particle - size inorganic fillers, or between inorganic fillers and the matrix material, thereby enhancing the binding force between inorganic fillers, especially large - particle - size inorganic fillers and the matrix material, between inorganic fiber and inorganic fillers, as well as between the base film layer and the coating film layer.

[0097] As some examples of the present invention, the inorganic fibers are selected from one or more of wollastonite fibers, calcium carbonate whisker fibers, calcium sulfate whisker fibers, sepiolite fibers, brucite fibers, etc.

[0098] Preferably, the length of the inorganic fibers is 1 to 100 μm, and the diameter is ≤3 μm.

[0099] Preferably, the inorganic fibers are modified inorganic fibers.

[0100] Specifically, the preparation process of the modified inorganic fibers is as follows:

[0101] 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, stirred and sheared to obtain a dimethyl silicone oil emulsion; then 10 to 20 parts by weight of inorganic fibers are dispersed into the dimethyl silicone oil emulsion, soaked for 10 to 20 min under stirring at room temperature, filtered, and the inorganic fibers are dried at 30 to 60 °C to obtain the modified inorganic fibers.

[0102] 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 dodecylbenzenesulfonate, sodium dodecyl sulfate, etc.

[0103] Preferably, the melting temperature of the ethylene-acrylic acid copolymer is 90 to 100 °C.

[0104] As some examples of the present invention, the crosslinking agent for the coating liquid is selected from one or more of dicumyl peroxide, tert-butylperoxytrimethylsilane, etc.

[0105] 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, etc.

[0106] As some examples of the present invention, the dispersant for the coating liquid is selected from one or more of sodium silicate, sodium tripolyphosphate, sodium hexametaphosphate, sodium pyrophosphate, etc.

[0107] Furthermore, the preparation process of the coating liquid is as follows:

[0108] First, the formulated amount of ethylene-acrylic acid copolymer is mixed with water, stirred until the ethylene-acrylic acid copolymer is completely dissolved, then the formulated amount of crosslinking agent and compatibilizer are added, and the temperature of the mixed solution is heated to 100 to 120 °C and kept warm for 0.5 to 1 h under stirring. Then, the formulated amount of inorganic fibers and dispersant are added, stirred evenly, and the formulated amount of polyvinyl alcohol and xanthan gum are added. Under stirring, the mixed solution is kept warm at 100 to 120 °C for another 0.5 to 1 h to obtain the coating liquid.

[0109] Further, the preparation method of the label made of polypropylene film comprises the steps of:

[0110] S1, Mix the raw materials of the base film layer, add them to a twin-screw extruder for extrusion and melting. After melting, the liquid material is extruded through the die head of the twin-screw extruder and cooled by a cooling roller to obtain a sheet;

[0111] S2, Stretch the sheet obtained in step S1 by a two-way synchronous stretching process, and then perform heat setting, cooling and winding to obtain the base film layer;

[0112] S3, Preparation of the coating film layer: Dilute the prepared coating liquid and coat it on the base film layer obtained in step S2, and dry it at 40-70 °C to obtain the polypropylene film for the label.

[0113] As some examples of the present invention, in step S1, the raw materials of the base film layer are mixed, added to a twin-screw extruder and extruded and melted at 230-250 °C. After melting, the liquid material is extruded through the die head of the twin-screw extruder, the die head temperature is 230-240 °C, and it is cooled by a cooling roller at 20-40 °C to obtain a sheet.

[0114] Further, in step S2, the stretching process of the base film layer includes:

[0115] S21, Keep the sheet obtained in step S1 at 90-120 °C for 1-3 min, and then perform the first two-way synchronous stretching. The stretching temperature of the first two-way synchronous stretching is 140-160 °C, and the stretching ratio is 1.5×1.5-3×3 times;

[0116] S22, Keep the film material obtained in step S21 at 140-160 °C for 1-3 min, and then perform the second two-way synchronous stretching. The stretching temperature of the second two-way synchronous stretching is 160-180 °C, and the stretching ratio is 4×4-6×6 times.

[0117] Preferably, in the first two-way synchronous stretching process in step S21, control the temperature distribution of the film material in the transverse direction to be symmetric and parabolic with a higher middle and lower sides; in the second two-way synchronous stretching process in step S22, control the temperature distribution of the film material in the transverse direction to be symmetric and parabolic with a lower middle and higher sides.

[0118] As some examples of the present invention, in steps S21 and S22, the temperature of the film material in the transverse direction can be determined according to the thickness difference of the film material in the transverse direction. The specific process is as follows:

[0119] In step S21, first detect the thickness distribution of the film material in the transverse direction before the first two-way synchronous stretching, and calculate the average value H of the thickness distribution of the film material in the transverse direction平均1 , the maximum value H max1 and the minimum value H min1 ; then, according to the average value H of the thickness distribution of the film material in the transverse direction 平均1 determine the base temperature T of the film material during the first biaxial synchronous stretching process 基础1 , where the base temperature T 基础1 should be between 140 and 160 °C. Then, according to the maximum value H max1 and the minimum value H min1 and the deviation from the base temperature T 基础1 determine the maximum temperature T and the minimum temperature T of the film material during the first biaxial synchronous stretching process max1 and the lowest 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 an adjustment coefficient, and its value is obtained through experiments or experience; then, take the midpoint of the film material in the transverse direction as the highest point of the temperature value, that is, the temperature at the midpoint of the film material in the transverse direction is T max1 , and the two end points of the film material in the transverse direction are the lowest points of the temperature value, that is, the temperatures at the two ends of the film material in the transverse direction are T min1 ; on this basis, calculate the temperature distribution of the film material in the transverse direction through fitting or the formula of a parabola, and adjust the power of the heating module to control the temperature distribution of the film material in the transverse direction to this set value.

[0120] On the contrary, in the step S22, first detect the thickness distribution of the film material in the transverse direction before the second biaxial synchronous stretching, and calculate the average value H of the thickness distribution of the film material in the transverse direction 平均2 , the maximum value H max2 and the minimum value H min2 ; then, according to the average value H of the thickness distribution of the film material in the transverse direction 平均2 determine the base temperature T of the film material during the second biaxial synchronous stretching process 基础2 , where the base temperature T 基础2 should be between 160 and 180 °C. Then, according to the maximum value H max2 and the minimum value H min2 and the deviation from the base temperature T 基础2 determine the maximum temperature T and the minimum temperature T of the film material during the second biaxial synchronous stretching process max2 and the lowest temperature T min2 , where, Tmax2 = 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 , where k2 is an adjustment coefficient, and its value is obtained through experiments or experience; then, the midpoint of the film material in the transverse direction is taken as the lowest point of the temperature value, that is, the temperature at the midpoint of the film material in the transverse direction is T min2 , and the two end points of the film material in the transverse direction are taken as the highest points of the temperature value, that is, the temperatures at the two ends of the film material in the transverse direction are T max2 ; On this basis, the temperature distribution of the film material in the transverse direction is calculated by fitting or using the formula of a parabola, and the temperature distribution of the film material in the transverse direction is regulated to this set value through the power adjustment of the heating module.

[0121] As some examples of the present invention, in the step S2, the obtained base film layer material can be heat-set at 110 - 130 °C for 10 - 15 min.

[0122] Preferably, in the 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 basis coating amount of the coating film layer is 5 - 20 g / m 2 .

[0123] The following is an example of the polypropylene film for labels and its preparation method of the present invention through specific examples:

[0124] Examples 1 - 2

[0125] Preparation of a polypropylene film for labels:

[0126] S1, Mix the raw materials of the base film layer according to Table 1 below, add them to a twin-screw extruder for extrusion and melting. After melting, the molten material is extruded through the die head of the twin-screw extruder and cooled by a cooling roll to obtain a sheet;

[0127] S2, Stretch the sheet obtained in step S1 by a bi-axial synchronous stretching process, and then perform heat setting, cooling and winding to obtain the base film layer;

[0128] Among them, the stretching process of the base film includes:

[0129] S21, Keep the sheet obtained in step S1 at 90 °C for 3 min, and then perform the first bi-axial synchronous stretching. During the first bi-axial synchronous stretching, control the temperature distribution of the film material in the transverse direction to a fixed value of 145 °C, and the stretching ratio is 2 × 2 times;

[0130] S22. After keeping the film material obtained in step S21 at 140 °C for 3 minutes, perform the second biaxial synchronous stretching. During the second biaxial synchronous stretching, control the temperature distribution of the film material in the transverse direction to a fixed value of 160 °C, and the stretching ratio is 4×4 times;

[0131] S3. Preparation of the coated film layer: Dilute the prepared coating liquid to a solid content of 5 wt% and then coat it on the base film layer obtained in step S2. The wet basis coating amount of the coated film layer is 20 g / m 2 , and obtain the polypropylene film for labels after drying at 40 °C.

[0132] Examples 3 - 4

[0133] Preparation of a polypropylene film for labels:

[0134] S1. Mix the raw materials of the base film layer according to Table 1 below, add them to a twin-screw extruder for extrusion and melting. After the molten material is extruded through the die head of the twin-screw extruder and cooled by a cooling roll, obtain a sheet;

[0135] S2. After stretching the sheet obtained in step S1 by a biaxial synchronous stretching process, perform heat setting, cooling and winding to obtain the base film layer;

[0136] Among them, the stretching process of the base film includes:

[0137] S21. After keeping the sheet obtained in step S1 at 100 °C for 2 minutes, perform the first biaxial synchronous stretching. During the first biaxial synchronous stretching, control the temperature distribution of the film material in the transverse direction to a fixed value of 160 °C, and the stretching ratio is 1.5×1.5 times;

[0138] S22. After keeping the film material obtained in step S21 at 150 °C for 2 minutes, perform the second biaxial synchronous stretching. During the second biaxial synchronous stretching, control the temperature distribution of the film material in the transverse direction to a fixed value of 165 °C, and the stretching ratio is 5×5 times;

[0139] S3. Preparation of the coated film layer: Dilute the prepared coating liquid to a solid content of 8 wt% and then coat it on the base film layer obtained in step S2. The wet basis coating amount of the coated film layer is 15 g / m 2 , and obtain the polypropylene film for labels after drying at 50 °C.

[0140] Examples 5 - 6

[0141] Preparation of a polypropylene film for labels:

[0142] S1. Mix the raw materials of the base film layer according to Table 1 below, add them to a twin-screw extruder for extrusion and melting. After melting, the liquid material is extruded through the die head of the twin-screw extruder and cooled by a cooling roll to obtain a sheet;

[0143] S2. Stretch the sheet obtained in step S1 by a bi-axial synchronous stretching process, and then perform heat setting, cooling and winding to obtain the base film layer;

[0144] Among them, the stretching process of the base film includes:

[0145] S21. After keeping the sheet obtained in step S1 at 120 °C for 1 minute, perform the first bi-axial synchronous stretching. During the first bi-axial synchronous stretching, control the temperature distribution of the film material in the transverse direction to a fixed value of 140 °C, and the stretching ratio is 3×3 times;

[0146] S22. After keeping the film material obtained in step S21 at 160 °C for 1 minute, perform the second bi-axial synchronous stretching. During the second bi-axial synchronous stretching, control the temperature distribution of the film material in the transverse direction to a fixed value of 180 °C, and the stretching ratio is 6×6 times; S3. Preparation of the coating film layer: Dilute the prepared coating liquid to a solid content of 10 wt% and coat it on the base film layer prepared in step S2. The wet basis coating amount of the coating film layer is 5 g / m 2 , and after drying at 70 °C, the polypropylene film for labels is obtained.

[0147] Table 1 Raw material ratio of the base film layer and the coating film layer

[0148]

[0149] Example 7

[0150] The difference between it and Example 1 above is only 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.

[0151] Example 8

[0152] The difference between it and Example 3 above is only 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.

[0153] Example 9

[0154] The difference between it and Example 8 above is only that: the nylon used in the raw materials of the base film layer is the modified nylon described in the present invention, and the modified nylon and the porous calcium carbonate particles are premixed during the preparation of the modified nylon.

[0155] Example 10

[0156] The only difference between this and the above-mentioned Example 5 is that during the first biaxial synchronous stretching process, the temperature distribution of the film material in the transverse direction is controlled to be a symmetric parabola shape with a high middle and low sides. When determining the temperature distribution of the film material in the transverse direction, by adjusting the coefficient k1, the average value of T max1 and T min1 is 140 °C.

[0157] Example 11

[0158] The only difference between this and the above-mentioned Example 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 symmetric parabola shape with a low middle and high sides. When determining the temperature distribution of the film material in the transverse direction, by adjusting the coefficient k1, the average value of T max1 and T min1 is 180 °C.

[0159] Example 12

[0160] The only difference between this and the above-mentioned Example 5 is that during the first biaxial synchronous stretching process, the temperature distribution of the film material in the transverse direction is controlled to be a symmetric parabola shape with a high middle and low sides. When determining the temperature distribution of the film material in the transverse direction, by adjusting the coefficient k1, the average value of T max1 and T min1 is 140 °C; during the second biaxial synchronous stretching process, the temperature distribution of the film material in the transverse direction is controlled to be a symmetric parabola shape with a low middle and high sides. When determining the temperature distribution of the film material in the transverse direction, by adjusting the coefficient k1, the average value of T max1 and T min1 is 180 °C.

[0161] Comparative Example 1

[0162] The only difference between this and the above-mentioned Example 1 is that the inorganic fillers used are all small-particle-size inorganic fillers.

[0163] Comparative Example 2

[0164] The only difference between this and the above-mentioned Example 1 is that the inorganic fillers used are all large-particle-size inorganic fillers.

[0165] Comparative Example 3

[0166] The only difference between this and the above-mentioned Example 3 is that the coating solution used does not contain polyvinyl alcohol.

[0167] Comparative Example 4

[0168] The only difference between this and the above-mentioned Example 3 is that the coating solution used does not contain xanthan gum.

[0169] Comparative Example 5

[0170] The difference between it and the above-mentioned Embodiment 3 is only that: the coating liquid used does not contain inorganic fibers.

[0171] Comparative Example 6

[0172] The difference between it and the above-mentioned Embodiment 3 is only that: the raw materials of the base film layer do not contain nylon and compatibilizer.

[0173] Performance test:

[0174] The following performance tests were respectively carried out on the labels obtained from the above-mentioned Examples 1 to 12 and Comparative Examples 1 to 6 using polypropylene films, where:

[0175] (1) Tensile strength and heat shrinkage rate performance were tested in accordance with GB / T1003-2008;

[0176] (2) Impact strength performance was tested in accordance with GB / T9639;

[0177] (3) The ink adhesion test was carried out using UV inkjet ink. The test method was as follows: Use a grid cutter to draw grids on the surface of the printed matter at an interval of 1 mm up and down, let it stand for 90 seconds, and then test with 810 tape. The test method was as follows: After applying pressure 3 times, pull it up instantly in the 90° direction, and then count the number of peeled paint grids on the surface.

[0178] The test results are shown in Table 2 below:

[0179] Table 2 Performance test results of polypropylene films

[0180]

[0181] In the above description of the embodiments of the present application, without conflict, the embodiments and the features in the embodiments in the present application can be combined with each other. The present application is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present application, those of ordinary skill in the art can also make many forms without departing from the purpose of the present application and the scope protected by the claims, and all belong to the protection scope of the present application.

Claims

1. A polypropylene film for 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; Nylon 5~10 parts; 3~8 parts of compatibilizer; Wherein, the inorganic filler includes 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 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.

2. The 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 polypropylene film for labels according to claim 1, 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.

4. The polypropylene film for labels according to claim 3, 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.

5. The polypropylene film for labels according to claim 1, 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.

6. The polypropylene film for labels according to claim 1, 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.

7. The method for preparing a polypropylene film for labels according to any one of claims 1 to 6, 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.

8. The method for preparing a polypropylene film for labels according to claim 7, 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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