阅读说明：本技术 一种高强度双向拉伸聚乳酸薄膜及其制备方法 (High-strength bidirectional-stretching polylactic acid film and preparation method thereof ) 是由 施中华 于 2019-10-09 设计创作，主要内容包括：本发明公开了一种高强度双向拉伸聚乳酸薄膜及其制备方法,所述高强度双向拉伸聚乳酸薄膜包括上表层、芯层和下表层,所述芯层的原料包括增韧剂和聚乳酸切片,所述增韧剂的质量占芯层原料总质量的3-5％；所述增韧剂为硅烷偶联剂改性球形二氧化硅颗粒,所述硅烷偶联剂包括苯基三乙氧基硅烷、γ―(2,3-环氧丙氧)丙基三甲氧基硅烷。本发明的双向拉伸聚乳酸薄膜具有拉伸强度大、热封强度高、厚薄均匀性好、可降解等特点,可以用于绿色包装领域。(The invention discloses a high-strength bidirectional stretching polylactic acid film and a preparation method thereof, wherein the high-strength bidirectional stretching polylactic acid film comprises an upper surface layer, a core layer and a lower surface layer, the raw material of the core layer comprises a toughening agent and polylactic acid slices, and the mass of the toughening agent accounts for 3-5% of the total mass of the raw material of the core layer; the toughening agent is silane coupling agent modified spherical silica particles, and the silane coupling agent comprises phenyl triethoxysilane and gamma- (2, 3-epoxypropoxy) propyl trimethoxysilane. The biaxially oriented polylactic acid film has the characteristics of high tensile strength, high heat sealing strength, good thickness uniformity, degradability and the like, and can be used in the field of green packaging.)

1. The high-strength biaxially oriented polylactic acid film is characterized by comprising an upper surface layer, a core layer and a lower surface layer, wherein the core layer comprises a toughening agent and polylactic acid slices as raw materials, and the mass of the toughening agent accounts for 3-5% of the total mass of the core layer raw materials;

the toughening agent is silane coupling agent modified spherical silica particles, and the silane coupling agent comprises phenyl triethoxysilane and gamma- (2, 3-epoxypropoxy) propyl trimethoxysilane.

2. The high-strength biaxially oriented polylactic acid film according to claim 1, wherein the silane coupling agent modified spherical silica particles are prepared by the following method: uniformly dispersing spherical silicon dioxide particles in an organic solvent, adding phenyltriethoxysilane and gamma- (2, 3-epoxypropoxy) propyl trimethoxysilane, stirring for 5-10h at 30-80 ℃, removing the organic solvent by rotary evaporation, washing and drying to obtain the silicon dioxide nano-particles; preferably, the particle size of the spherical silica particles is 1.0-3.0 μm; preferably, the mass ratio of the spherical silica particles to the organic solvent is (5-10): 100, respectively; preferably, the organic solvent is toluene.

3. The high strength biaxially stretched polylactic acid film according to claim 1 or 2, wherein the mass ratio of phenyltriethoxysilane to γ - (2, 3-glycidoxy) propyltrimethoxysilane is (4-5): 1.

4. a high-strength biaxially oriented polylactic acid film according to any one of claims 1 to 3, wherein the mass ratio of said silane coupling agent to said spherical silica particles is (3-5): 100.

5. the high-strength biaxially oriented polylactic acid film according to any one of claims 1 to 4, wherein the raw materials of the upper and lower surface layers comprise polylactic acid chips and an anti-sticking agent; preferably, the anti-sticking agent is silica particles with a particle size distribution of 1.0-4.0 μm; preferably, the mass of the anti-sticking agent respectively accounts for 0.01-5% of the total mass of the raw materials of the upper surface layer and the lower surface layer; preferably, the thickness of the upper surface layer is 0.5-1.5 μm, the thickness of the core layer is 20-25 μm, and the thickness of the lower surface layer is 0.5-1.5 μm.

6. A method for preparing a high-strength biaxially oriented polylactic acid film according to any one of claims 1 to 5, comprising the steps of:

s1, drying the raw materials of the upper surface layer, the core layer and the lower surface layer for 0.5 to 1 hour at the temperature of between 60 and 100 ℃ through a vulcanization bed, and then drying the raw materials for 4 to 6 hours at the temperature of between 60 and 100 ℃ in a drying tower;

s2, adding the dried core layer raw material into a main extruder, respectively adding the upper surface layer raw material and the lower surface layer raw material into two auxiliary extruders, and heating and melting at the temperature of 180-240 ℃ to obtain a core layer melt, an upper surface layer melt and a lower surface layer melt;

s3, converging the core layer melt, the upper surface layer melt and the lower surface layer melt in a three-layer T-shaped die head, and extruding to obtain a mixed melt, wherein the temperature of the three-layer T-shaped die head is 200-240 ℃;

s4, enabling the mixed melt to pass through an electrostatic adsorption wire and then closely attach to the surface of a grounded cold roll to form a cast sheet;

s5, peeling the cast sheet from the cold roll to form a sheet, preheating, and then performing longitudinal stretching, wherein the longitudinal stretching multiple is 2.5-3.5 times, the longitudinal stretching mode is double-point stretching, and then cooling to 15-35 ℃;

s6, preheating the longitudinally stretched sheet, transversely stretching the sheet at the transverse stretching multiple of 2-4 times, and cooling to 40-60 ℃;

s7, heat-setting the transversely stretched sheet into a film at the temperature of 100-180 ℃, and simultaneously adding a catalyst with platinum as an effective component into the front part of the air exchange box in the heat-setting area to improve the cleanliness of the film;

and S8, drawing the heat-set film to a thickness gauge through a drawing device, adding a dust adsorption device at an inlet of the drawing position to perform dust absorption treatment on the film, performing corona or flame treatment on the upper and lower surfaces of the film to enable the surface tension to reach 38-42mN/m, and cooling to obtain the high-strength biaxially-oriented polylactic acid film.

7. The method for preparing a high-strength biaxially oriented polylactic acid film according to claim 6, wherein: in the step 2, the core layer melt is filtered by a disc type filter with the pore size of 10-30 mu m until the moisture content is less than or equal to 150 ppm; preferably, in the step S4, the voltage of the electrostatic adsorption wire is 6 to 20kV, the current is 6 to 20mA, and the operation speed is 5 to 20 mm/min; preferably, the distance between the electrostatic adsorption wire and the lip of the cold roll is 20-100 mm.

8. The method for preparing a high-strength biaxially oriented polylactic acid film according to claim 6 or 7, wherein: in the step S5, the preheating temperature is 40-80 ℃, and the longitudinal stretching temperature is 60-80 ℃; preferably, in the step S6, the preheating temperature is 50 to 120 ℃, and the transverse stretching temperature is 60 to 130 ℃.

9. The method for preparing a high-strength biaxially oriented polylactic acid film according to any one of claims 6 to 8, wherein: in step S6, the cooling step includes cooling at 70-100 deg.C, and cooling at 40-60 deg.C.

10. The method for preparing a high-strength biaxially oriented polylactic acid film according to any one of claims 6 to 9, wherein: in the step S8, the thickness gauge feeds back the detection result to the three-layer T-shaped die head, so that the thickness of the extruded mixed melt is automatically adjusted.

Technical Field

The invention relates to the technical field of films, in particular to a high-strength biaxially oriented polylactic acid film and a preparation method thereof.

Background

Plastics are the most great invention in the twentieth century, films such as BOPP/BOPET/BOPA and the like are deeply applied to various aspects of people's life, but the accompanying plastic garbage disposal problem gradually becomes the biggest problem of environmental protection in the world. With the proposal of the idea of production civilization and construction, the concept of environmental protection is gradually deepened, and the degradability of the plastic is more and more emphasized.

Polylactic acid is chemically synthesized from lactic acid as a monomer, is also called polylactide, is a biodegradable high-molecular polyester material, and has a molecular formula of (C)₃H₄O₂) n is the same as the formula (I). The polylactic acid has the advantages of abundant and renewable raw material sources, no pollution in the production process, natural degradation of products, realization of the circulation in the nature, and ideal green high polymer materials. The polylactic acid film is the only transparent biodegradable polymer film at present, has the characteristics of safety, environmental protection, printing, heat sealing and the like, can be used in the field of degradable packaging products, and is known as one of novel packaging materials with the greatest development prospect in the world. However, the polylactic acid film produced at present in China has the disadvantages of low film production rate, poor thickness uniformity, low tensile strength and the like. Wherein the strength of the produced film of the two-way stretching polylactic acid film is lower, about 100-110Mpa, which can not meet the downstream packaging processing requirements and seriously restricts the market popularization of the polylactic acid film.

Disclosure of Invention

Based on the technical problems in the background art, the invention provides a high-strength biaxially oriented polylactic acid film and a preparation method thereof, and the obtained polylactic acid film has the characteristics of high tensile strength, high heat sealing strength, good thickness uniformity, degradability and the like, and can be used in the field of green packaging.

The invention provides a high-strength biaxially oriented polylactic acid film which comprises an upper surface layer, a core layer and a lower surface layer, wherein the raw material of the core layer comprises a toughening agent and a polylactic acid slice, and the mass of the toughening agent accounts for 3-5% of the total mass of the raw material of the core layer;

the toughening agent is silane coupling agent modified spherical silica particles, and the silane coupling agent comprises phenyl triethoxysilane and gamma- (2, 3-epoxypropoxy) propyl trimethoxysilane.

Preferably, the preparation method of the silane coupling agent modified spherical silica particles is as follows: uniformly dispersing spherical silicon dioxide particles in an organic solvent, adding phenyltriethoxysilane and gamma- (2, 3-epoxypropoxy) propyl trimethoxysilane, stirring for 5-10h at 30-80 ℃, removing the organic solvent by rotary evaporation, washing and drying to obtain the silicon dioxide nano-particles; preferably, the particle size of the spherical silica particles is 1.0-3.0 μm; preferably, the mass ratio of the spherical silica particles to the organic solvent is (5-10): 100, respectively; preferably, the organic solvent is toluene.

Preferably, the mass ratio of the phenyltriethoxysilane to the gamma- (2, 3-epoxypropoxy) propyl trimethoxysilane is (4-5): 1.

preferably, the mass ratio of the silane coupling agent to the spherical silica particles is (3-5): 100.

preferably, the raw materials of the upper surface layer and the lower surface layer comprise polylactic acid slices and an anti-sticking agent; preferably, the anti-sticking agent is silica particles with a particle size distribution of 1.0-4.0 μm; preferably, the mass of the anti-sticking agent accounts for 0.01-5% of the total mass of the raw materials of the upper surface layer and the lower surface layer respectively.

Preferably, the thickness of the upper surface layer is 0.5-1.5 μm, the thickness of the core layer is 20-25 μm, and the thickness of the lower surface layer is 0.5-1.5 μm.

A preparation method of the high-strength biaxially oriented polylactic acid film comprises the following steps:

s1, drying the raw materials of the upper surface layer, the core layer and the lower surface layer for 0.5 to 1 hour at the temperature of between 60 and 100 ℃ through a vulcanization bed, and then drying the raw materials for 4 to 6 hours at the temperature of between 60 and 100 ℃ in a drying tower;

s2, adding the dried core layer raw material into a main extruder, respectively adding the upper surface layer raw material and the lower surface layer raw material into two auxiliary extruders, and heating and melting at the temperature of 180-240 ℃ to obtain a core layer melt, an upper surface layer melt and a lower surface layer melt;

s3, converging the core layer melt, the upper surface layer melt and the lower surface layer melt in a three-layer T-shaped die head, and extruding to obtain a mixed melt, wherein the temperature of the three-layer T-shaped die head is 200-240 ℃;

s4, enabling the mixed melt to pass through an electrostatic adsorption wire and then closely attach to the surface of a grounded cold roll to form a cast sheet;

s5, peeling the cast sheet from the cold roll to form a sheet, preheating, and then performing longitudinal stretching, wherein the longitudinal stretching multiple is 2.5-3.5 times, the longitudinal stretching mode is double-point stretching, and then cooling to 15-35 ℃;

s6, preheating the longitudinally stretched sheet, transversely stretching the sheet at the transverse stretching multiple of 2-4 times, and cooling to 40-60 ℃;

s7, heat-setting the transversely stretched sheet into a film at the temperature of 100-180 ℃, and simultaneously adding a catalyst with platinum as an effective component into the front part of the air exchange box in the heat-setting area to improve the cleanliness of the film;

and S8, drawing the heat-set film to a thickness gauge through a drawing device, adding a dust adsorption device at an inlet of the drawing position to perform dust absorption treatment on the film, performing corona or flame treatment on the upper and lower surfaces of the film to enable the surface tension to reach 38-42mN/m, and cooling to obtain the high-strength biaxially-oriented polylactic acid film.

Preferably, in the step 2, the core layer melt is filtered by a disc filter with a pore size of 10-30 μm until the moisture content is less than or equal to 150 ppm.

Preferably, in the step S4, the voltage of the electrostatic adsorption wire is 6 to 20kV, the current is 6 to 20mA, and the operation speed is 5 to 20 mm/min; preferably, the distance between the electrostatic adsorption wire and the lip of the cold roll is 20-100 mm.

Preferably, in the step S5, the preheating temperature is 40 to 80 ℃, and the longitudinal stretching temperature is 60 to 80 ℃.

Preferably, in the step S6, the preheating temperature is 50 to 120 ℃, and the transverse stretching temperature is 60 to 130 ℃.

Preferably, in step S6, the cooling step includes cooling at a temperature of 70-100 ℃ and then cooling at a temperature of 40-60 ℃.

Preferably, in step S8, the thickness gauge feeds back the detection result to the three-layer T-die, so that the thickness of the extruded mixed melt is automatically adjusted.

The invention has the following beneficial effects:

the high-strength biaxially oriented polylactic acid film prepared by the invention adopts a biaxially oriented process, and silane coupling agent modified spherical silicon dioxide particles are added into a core layerThe granules are used as a toughening agent, wherein the silane coupling agent is the compound of phenyltriethoxysilane and gamma- (2, 3-epoxypropoxy) propyl trimethoxysilane, so that SiO can be improved₂Compatibility with polylactic acid matrix, and enhancement of polylactic acid and SiO₂Interaction between them, thereby increasing tensile strength; because the molecular chain of the phenyltriethoxysilane contains a rigid benzene ring structure, the rigidity of the polylactic acid material is improved, and the tensile strength is improved; the gamma- (2, 3-epoxypropoxy) propyl trimethoxy silane is a flexible long chain containing epoxy groups, and can be intertwined with a benzene ring rigid chain segment so as to form SiO in a polylactic acid matrix₂The three-dimensional network structure plays a role in strengthening, the strengthening effect is better played, and the reaction of epoxy groups with carboxyl and hydroxyl on polylactic acid during melt mixing can further promote SiO₂The three-dimensional network structure and the polylactic acid matrix interact, so that the stiffness and the tensile strength of the polylactic acid film are further improved.

According to the invention, the uniformity of the thickness of the film is excellent by strictly controlling the temperature uniformity of the stretching system and automatically detecting and feeding back to the die head for adjusting the temperature by using the infrared detection system.

The high-strength biaxially oriented polylactic acid film prepared by the invention belongs to degradable green environment-friendly materials, and meanwhile, the high-strength biaxially oriented polylactic acid film has high strength, high heat seal strength and excellent thickness uniformity, so that the film is very convenient and fast in the downstream processing process, part of the performance can reach the level of biaxially oriented polyethylene terephthalate films, and the packaging effect is good.

The high-strength biaxially-oriented polylactic acid film prepared by the invention has the tensile strength of more than or equal to 150MPa, the heat seal strength of more than or equal to 5.0N/15mm, and the 2-delta value (standard deviation) of a thickness gauge scanned for 4 times of less than or equal to 1.0 percent, has the characteristics of high tensile strength, high heat seal strength, good thickness uniformity, degradability and the like, and is mainly used for the field of green packaging.

Detailed Description

The technical solution of the present invention will be described in detail below with reference to specific examples.