CN112521732B - Physical aging resistant bidirectional stretching polylactic acid film material, preparation method and application thereof - Google Patents

Physical aging resistant bidirectional stretching polylactic acid film material, preparation method and application thereof Download PDF

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CN112521732B
CN112521732B CN201910876385.0A CN201910876385A CN112521732B CN 112521732 B CN112521732 B CN 112521732B CN 201910876385 A CN201910876385 A CN 201910876385A CN 112521732 B CN112521732 B CN 112521732B
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polylactic acid
acid film
temperature
stretching
quenching
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CN112521732A (en
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陈鹏
黄威
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Ningbo Institute of Material Technology and Engineering of CAS
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Ningbo Institute of Material Technology and Engineering of CAS
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/18—Manufacture of films or sheets
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C69/00—Combinations of shaping techniques not provided for in a single one of main groups B29C39/00 - B29C67/00, e.g. associations of moulding and joining techniques; Apparatus therefore
    • B29C69/02—Combinations of shaping techniques not provided for in a single one of main groups B29C39/00 - B29C67/00, e.g. associations of moulding and joining techniques; Apparatus therefore of moulding techniques only
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2067/00—Use of polyesters or derivatives thereof, as moulding material
    • B29K2067/04—Polyesters derived from hydroxycarboxylic acids
    • B29K2067/046—PLA, i.e. polylactic acid or polylactide
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29L—INDEXING SCHEME ASSOCIATED WITH SUBCLASS B29C, RELATING TO PARTICULAR ARTICLES
    • B29L2007/00—Flat articles, e.g. films or sheets
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2367/00—Characterised by the use of polyesters obtained by reactions forming a carboxylic ester link in the main chain; Derivatives of such polymers
    • C08J2367/04—Polyesters derived from hydroxy carboxylic acids, e.g. lactones

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  • Chemical & Material Sciences (AREA)
  • Manufacturing & Machinery (AREA)
  • Materials Engineering (AREA)
  • Health & Medical Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Medicinal Chemistry (AREA)
  • Polymers & Plastics (AREA)
  • Organic Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Shaping By String And By Release Of Stress In Plastics And The Like (AREA)

Abstract

The invention discloses a physical aging resistant biaxially oriented polylactic acid film material and a preparation method and application thereof. The polylactic acid film comprises 35wt% -85 wt% of crystalline phase and 15wt% -65 wt% of amorphous phase; the amorphous phase comprises 5wt% to 55wt% of metastable phase, and the characteristic peak of the metastable phase in an infrared spectrogram appears at 918cm‑1To (3). The biaxially oriented polylactic acid film provided by the invention has the characteristic of physical aging resistance, the change rate of tensile strength and bending strength before and after storage is small, the change of elongation at break is small, the crystallinity and orientation degree are high, products such as polylactic acid packaging materials and the like prepared from the biaxially oriented polylactic acid film not only have the strength equivalent to or superior to that of the existing polylactic acid packaging materials, but also have excellent physical aging resistance, the stability of size and performance can be kept in the storage logistics stage, and meanwhile, the biaxially oriented polylactic acid film is simple in preparation process, beneficial to large-scale production and wide in application prospect.

Description

Physical aging resistant bidirectional stretching polylactic acid film material, preparation method and application thereof
Technical Field
The invention relates to a polylactic acid material, in particular to a physical aging resistant two-way stretching polylactic acid film material, a preparation method and application thereof (packaging material, envelope window, disposable cup, dinner plate and the like), belonging to the technical field of film materials.
Background
With the vigorous development of the petroleum industry and the demand of the production industry, a large amount of petroleum-based polyesters, polyolefins, polystyrenes and other resins are made into films every year, and the films are used in the fields of packaging and film-covering materials. In addition, the share of plastic packaging products in the market is increasing year by year, and especially composite plastic flexible packages are widely applied to the fields of food, medicine, chemical industry and the like. The proportion of food packages is the largest, such as beverage packages, quick-frozen food packages, steamed and cooked food packages, fast food packages and the like, and the products bring great convenience to the lives of people. However, the uncertainty of food safety due to precipitation of the petroleum-based resin and related additives, and the "white" pollution caused by the non-degradability of the petroleum-based resin to the environment cover the shadow of the wide application of petroleum-based films in the field of food packaging. Therefore, it is necessary to develop a degradable resin film for use in the field of food packaging.
Polylactic acid has good biocompatibility and biodegradability, can be degraded into lactic acid in nature, and finally forms carbon dioxide and water through microbial decomposition, and is one of biodegradable polymers certified by the U.S. food and drug administration. Has wide application prospect in various fields of tissue engineering, food packaging, non-woven fabrics, drug sustained release and the like. In particular, polylactic acid films are required to have a sufficiently high toughness and a sufficiently long retention time of mechanical properties to various degrees in the field of polylactic acid packaging materials. Polylactic acid is reported as a semi-crystalline polymer, and the amorphous phase is highly susceptible to physical aging below the glass transition temperature, which makes the material brittle rapidly (Macromolecules 2007,40, 9664-9671). Wherein physical aging is a process in which a molecular segment spontaneously transitions from a non-equilibrium state to an equilibrium state; and, a network structure of coherent entanglements is formed (Macrornol. Symp.1997,124, 15-26). This makes polylactic acid articles very susceptible to physical degradation during the warehouse logistics stage, which can lead to severe embrittlement of the articles. However, at present, no report is found on the development of a polylactic acid film resistant to physical aging through process optimization.
The preparation of polylactic acid films is reported in many publications, and the main purpose is to improve the stretching ratio, toughness and plasticity of the films. CN103788603A reports a polylactic acid biaxially oriented frosted film and a preparation method thereof, wherein a modification auxiliary agent and polylactic acid are subjected to melt blending granulation, tape casting tabletting and biaxial orientation to prepare a frosted tough film. CN102453319A reports a biaxially oriented polylactic acid film prepared by adding inorganic functional filler to have excellent processability and balanced rigidity and toughness. CN104105745A reports a polylactic acid based film or sheet obtained by a melt film forming process; preferably, after the melt film forming process and before the cooling process, a residual stress relaxation process is provided, and the temperature is set to be 20-70 ℃ below the melting point of the polylactic acid. CN106147160A reports a microporous oriented film, which is prepared by obtaining an unoriented film through various film forming processes, and then biaxially stretching the unoriented film through steam heating or liquid heating. On the basis of the method, the microporous polylactic acid oriented film is prepared by a general film-making process such as additional heating and shaping.
Although the above prior arts can prepare the polylactic acid film, there are the following limitations: (1) under the current technical conditions, the heat-set biaxially oriented polylactic acid film is easy to be physically aged in the storage logistics stage to cause rapid embrittlement; (2) the heat-set two-way stretching polylactic acid film needs to be subjected to high-temperature hot rolling for preparing food packaging materials, and the brittleness caused by further accelerated physical aging is caused; (3) the biaxially oriented polylactic acid film which is not subjected to heat setting is subjected to high-temperature hot rolling, and is easy to physically age at the stage of storage and logistics to cause brittleness; (4) the biaxially oriented polylactic acid film subjected to heat setting or hot rolling and the packaging material thereof are very easy to be physically aged in the use stage to cause rapid brittleness; therefore, it is necessary to develop a new polylactic acid film resistant to physical aging and a method for preparing the same to satisfy the high requirements of various applications for the overall properties thereof.
Disclosure of Invention
The invention mainly aims to provide a physical aging resistant biaxially oriented polylactic acid film material, a preparation method and application thereof, thereby overcoming the defects in the prior art.
In order to achieve the purpose, the technical scheme adopted by the invention comprises the following steps:
the embodiment of the invention provides a physical aging resistant biaxially oriented polylactic acid film material, which comprises 35wt% -85 wt% of crystalline phase and 15wt% -65 wt% of amorphous phase; the amorphous phase comprises 5wt% to 55wt% of a metastable phase which emerges from a characteristic peak in an infrared spectrumNow at 918cm-1To (3).
Further, the polylactic acid film has a glass transition temperature (T)g) After a sufficient storage time, the DSC curve shows a value after the glass transition temperature (post-T)g) An endothermic peak having an enthalpy value (Δ H) sufficiently large not to vary with the temperature increase rate in the DSC test, and having an infrared spectrum within 918cm-1The band shows a characteristic peak whose intensity increases with the storage time, and whose rate of change in tensile strength is less than 15%, rate of change in flexural strength is less than 15%, and rate of increase in elongation at break is less than 20%. Preferably, the sufficient time is more than or equal to 1 hour. Preferably, the delta H is more than or equal to 1J/g.
The embodiment of the invention also provides a method for preparing the physical aging resistant biaxially oriented polylactic acid film material, which comprises the following steps:
(1) providing a dry polylactic acid or a dry mixture of polylactic acid;
(2) extruding the dried polylactic acid or the dry polylactic acid mixture through a casting device to form a casting sheet;
(3) fully preheating the casting sheet, then carrying out biaxial stretching, firstly carrying out longitudinal stretching on the film, then carrying out transverse stretching on the film, and then rapidly quenching to room temperature at a quenching rate of 2.5-140 ℃/s for 1-59 seconds to obtain a biaxially stretched polylactic acid film;
further, in the step (1), the water content of the polylactic acid or the dry mixture of the polylactic acid is less than 60 ppm.
The embodiment of the invention also provides application of the physical aging resistant biaxially oriented polylactic acid film in preparing a packaging material.
Further, rolling the biaxially oriented polylactic acid film at a certain hot rolling temperature for a certain hot rolling time; and rapidly quenching to room temperature at a quenching rate of 3-200 ℃/s, wherein the quenching time is 1-59 seconds, and thus the packaging material is prepared.
Further, the embodiment of the present invention also provides a method for sterilizing, packaging and storing polylactic acid packaging material, which comprises: providing the foregoingAny of the embodiments of the polylactic acid packaging material with resistance to physical aging, and packaging the polylactic acid packaging material in its TgThe packaging and storage is sterilized as follows.
Compared with the prior art, the polylactic acid film provided by the embodiment of the invention has the characteristic of physical aging resistance, the change rate of tensile strength and bending strength before and after storage is small, the change of elongation at break is small, and the crystallinity and orientation degree are high.
Detailed Description
The present invention will be more fully understood from the following detailed description. Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed embodiment.
In one aspect of the embodiments of the present invention, a physical aging resistant polylactic acid film has a crystalline phase content of 35% to 85% (by mass, unless otherwise specified), an amorphous phase content of 15% to 65%, and a metastable phase in an amorphous phase (infrared spectrum 918 cm)-1Appearance of characteristic peak) is 5% -55%.
In some more preferred embodiments, the polylactic acid film has a crystallinity of 55wt% to 75wt%, and the metastable phase in the amorphous phase is contained in an amount of 10wt% to 30 wt%.
Further, the polylactic acid film material has a glass transition temperature (T) of polylactic acidg) After storage (shelf life) for a sufficient time, the DSC curve shows T as measured by Differential Scanning Calorimetry (DSC)gAn endothermic peak having an enthalpy value (Δ H) sufficiently large is present nearby, and the enthalpy value (Δ H) of the endothermic peak does not change with the temperature increase rate in the DSC test. Wherein, the storage time is not limited; however, in general, the time is 1 hour or more. The enthalpy (Δ H) is also not limited; however, in general, the enthalpy (. DELTA.H) is 1J/g or more.
Further, the polylactic acid film is represented by formula TgAfter storage (shelf life) for a sufficient time, 918cm of an infrared spectrum is obtained by detection by means of a Micro-infrared spectrometer (Micro-FTIR)-1Characteristic peaks appear in the bands and the intensity of the characteristic peaks rises with prolonged storage time. Wherein, the storage time is not limited; however, in general, the time is 1 hour or more.
Further, the polylactic acid film is represented by formula TgThe rate of change of tensile strength before and after storage is generally less than 15% after a sufficient period of storage (shelf life); the change rate of the bending strength before and after storage is lower than 15%; the rate of change before and after elongation at break is generally less than 50%, and there is no transition from toughness to brittleness.
Further, any one or a combination of more of a polymer blend (for example, polyhydroxyalkanoate, polyglycolic acid, polycaprolactone, etc.), a plasticizer, a compatibilizer, a capping agent, a flame retardant, an antioxidant, a lubricant, an antistatic agent, an antifogging agent, a light stabilizer, an ultraviolet absorber, a pigment, a mildewproofing agent, an antibacterial agent, a foaming agent may be added to the polylactic acid film within a range not to hinder the achievement of the process object of the present invention, and is not limited thereto.
The physical aging resistant polylactic acid film provided by the embodiment of the invention can be obtained by the processes of melt extrusion, casting and tabletting, biaxial stretching, quenching and the like of casting equipment.
In some embodiments, the method of making specifically comprises:
(1) providing a dry polylactic acid or a dry mixture of polylactic acid;
(2) extruding the dried polylactic acid or the dry polylactic acid mixture through casting equipment to form a casting sheet, wherein the extrusion temperature is 160-220 ℃, and the casting roller temperature is 20-50 ℃;
(3) and fully preheating the cast sheet, then carrying out biaxial stretching, firstly carrying out film longitudinal stretching (MD) at the stretching temperature of 85-140 ℃ and the stretching multiple of 2-5 times, then carrying out film transverse stretching (TD) at the stretching temperature of 90-145 ℃ and the stretching multiple of 2-5 times, and then rapidly quenching to room temperature at the quenching rate of 2.5-140 ℃/second for 1-59 seconds to obtain the biaxially stretched polylactic acid film.
In some embodiments, step (1) comprises: drying the polylactic acid or the dry mixture of the polylactic acid by hot air until the water content is lower than 60 ppm.
In some embodiments, step (1) comprises: and (3) carrying out melt blending, extrusion granulation and hot air drying on the dried polylactic acid dry mixture until the water content is lower than 60 ppm.
Wherein the dry blend of polylactic acid may comprise one or more of the additives described above.
In some embodiments, the weight average molecular weight of the polylactic acid in step (1) is 8 to 80 ten thousand, wherein the molar content of the L optical isomer is 85 to 99%; in a preferred embodiment, in order to retard the disorientation of the polylactic acid molecular chain and increase the relaxation time of the polylactic acid molecular chain, the weight average molecular weight of the polylactic acid is preferably 20 to 50 ten thousand, and the molar content of the L optical isomer is preferably 88 to 98%.
In some embodiments, the extrusion temperature used in step (2) is 180 to 210 ℃ and the casting roll temperature is 20 to 40 ℃.
In some embodiments, the longitudinal stretching temperature used in step (3) is 90 to 140 ℃ and the stretching ratio is 3 to 5 times, the transverse stretching temperature is 95 to 145 ℃ and the stretching ratio is 3 to 5 times, and the quenching time is 1 to 30 seconds. Wherein:
when the weight average molecular weight of the polylactic acid is 8-20 ten thousand, the longitudinal stretching temperature is 85-115 ℃, the stretching multiple is 3-5 times, the transverse stretching temperature is 90-120 ℃, the stretching multiple is 3-5 times, the quenching rate is 20-100 ℃/s, and the quenching time is 1-5 s;
when the weight average molecular weight of the polylactic acid is 20-40 ten thousand, the longitudinal stretching temperature is 95-125 ℃, the stretching multiple is 3-5 times, the transverse stretching temperature is 100-130 ℃, the stretching multiple is 3-5 times, the quenching rate is 7-20 ℃/s, and the quenching time is 5-15 s;
when the weight average molecular weight of the polylactic acid is 40-60 ten thousand, the longitudinal stretching temperature is 105-145 ℃, the stretching multiple is 3-5 times, the transverse stretching temperature is 110-145 ℃, the stretching multiple is 3-5 times, the quenching rate is 4-7 ℃/s, and the quenching time is 15-30 s.
In some embodiments, the thickness of the biaxially oriented polylactic acid film in step (3) is 20 to 200 μm.
The preparation method provided by the foregoing embodiment of the present invention prepares the polylactic acid film, i.e. the physical aging resistant polylactic acid film, at a proper casting temperature and quenching rate and time by optimizing the molecular weight and optical isomer content of polylactic acid. The polylactic acid film has good physical aging resistance, does not form a tight kinking point (a physical aging phenomenon occurs) in shelf life, but forms a polylactic acid metastable phase, so that the change rate of the tensile strength and the bending strength before and after storage is small, the change rate of the elongation at break is small, and the crystallinity and the orientation degree are high.
Another aspect of the embodiments of the present invention provides a use of the physical aging resistant biaxially oriented polylactic acid film for preparing a packaging material.
For example, an embodiment of the present invention provides a method for preparing a packaging material, including:
preparing a physical aging resistant biaxially oriented polylactic acid film material by any one of the methods; and
hot rolling the biaxially oriented polylactic acid film to synthesize a packaging material, wherein the adopted hot rolling forming temperature is 140-190 ℃, preferably 150-180 ℃, and the hot rolling forming time is 1-10 seconds, preferably 1-8 seconds; the quenching rate is 5-150 ℃/s, and the quenching time is 1-30 s. Wherein:
when the weight average molecular weight of the polylactic acid is 8-20 ten thousand, the hot rolling forming temperature is 140-160 ℃, and the hot rolling forming time is 1-3 seconds; the quenching speed is 24-140 ℃/s, and the quenching time is 1-5 s;
when the weight average molecular weight of the polylactic acid is 20-40 ten thousand, the hot rolling forming temperature is 150-180 ℃, and the hot rolling forming time is 3-5 seconds; the quenching speed is 7-30 ℃/s, and the quenching time is 5-15 s;
when the weight average molecular weight of the polylactic acid is 40-60 ten thousand, the hot rolling forming temperature is 160-190 ℃, and the hot rolling forming time is 5-8 seconds; the quenching speed is 4-10 ℃/s, and the quenching time is 15-30 s.
Further, another aspect of the embodiments of the present invention also provides a method for storing a polylactic acid film, including: providing a polylactic acid film resistant to physical aging according to any of the preceding embodiments, and subjecting the polylactic acid film to the treatmentgThe following is stored.
Further, in the foregoing embodiment, the preparation of the packaging material, i.e. the sterilization and packaging, may be completed before entering the warehouse logistics stage; the set temperature of the sterilization packaging and the storage logistics stage (shelf life) is lower than Tg。
Furthermore, the packaging material prepared in the foregoing embodiment of the present invention is obtained by hot-rolling and molding a polylactic acid film (i.e., the foregoing polylactic acid film resistant to physical aging), and has high crystallinity, stable orientation and no relaxation, and mechanical properties equivalent to those of the existing polylactic acid packaging material. In addition, the polylactic acid metastable phase formed in the storage logistics stage can obviously reduce the internal stress of the polylactic acid film material in the embodiment of the invention and improve the stability of the product. Therefore, the polylactic acid packaging material provided by the embodiment of the invention has excellent physical aging resistance, can keep the stability of size and performance in a warehouse logistics stage, and breaks through the performance bottleneck of the conventional polylactic acid packaging material, so that the use requirement is met, and the application field is expanded.
The technical scheme provided by the embodiment of the invention has the advantages that:
(1) the relaxation time of polylactic acid molecular chains in the product cooling process is improved by selecting the polylactic acid with high molecular weight;
(2) molecular chain disentanglement and orientation of polylactic acid occur under the action of higher shearing (casting extrusion temperature and extrusion pressure), and the generation of polylactic acid melt with a certain orientation degree is promoted;
(3) molecular chain disentanglement of the polylactic acid melt is further carried out under the pressure action of the casting roller, and a polylactic acid casting sheet with a certain orientation degree is generated;
(4) the polylactic acid casting sheet is subjected to further orientation and disentanglement of molecular chains under the action of longitudinal stretching, so that a polylactic acid longitudinal stretching film with a regular longitudinal structure and a certain crystallinity is promoted to be generated;
(5) the polylactic acid longitudinal stretching film is subjected to further orientation and disentanglement of molecular chains under the action of transverse stretching, so that the polylactic acid with the same and regular transverse structure and higher crystallinity is promoted to be generated; quenching immediately to ensure that the polylactic acid molecular chain in the further oriented amorphous region is not subjected to de-orientation and entanglement;
(6) the bidirectional stretching polylactic acid film material is quickly hot-rolled to synthesize a packaging material at a certain hot rolling temperature and hot rolling time; quenching immediately to ensure that the molecular chain of the polylactic acid in the amorphous region is not subjected to disorientation and entanglement;
(7) the polylactic acid film and the packaging material only form a polylactic acid metastable phase in the storage logistics stage, and the physical aging phenomenon cannot occur;
(8) melt extrusion and tape casting molding provide a high shear flow field, and induce polylactic acid molecular chains to be disentangled and oriented; rapid quenching hinders relaxation of polylactic acid molecular chains, so that disentanglement structures of amorphous divided sub chains are maintained, and physical aging resistance of polylactic acid products is improved;
(9) in the storage logistics stage, the temperature is lower than the polylactic acid TgUnder the condition, the polylactic acid forms a polylactic acid metastable phase under the driving of dipole-dipole interaction, and the long-term performance stability of the polylactic acid product is improved.
In summary, according to the technical scheme provided by the embodiment of the invention, a high shear flow field is provided by tape casting extrusion to induce disentanglement and orientation of polylactic acid molecular chains, then a high longitudinal and transverse stretching flow field is provided by bidirectional stretching to respectively induce disentanglement and orientation of the longitudinal and transverse molecular chains, and then the relaxation of the polylactic acid molecular chains is hindered by rapid quenching, so that the disentanglement structure of the amorphous molecular chains is maintained, and the physical aging resistance of the polylactic acid product is improved. The film has orientation states in the longitudinal direction and the transverse direction by the aid of the biaxial stretching treatment, longitudinal and transverse molecular chain relaxation (physical aging) in a shelf life (storage logistics stage) and a use stage is effectively reduced, and the biaxial stretching polylactic acid is accompanied with dipole-dipole interaction by providing proper temperature and time in a storage process, so that a polylactic acid metastable phase is formed, internal stress of a polylactic acid film is reduced, and performance stability of a polylactic acid product in the shelf life (storage logistics stage) and the use stage is improved.
In conclusion, the biaxially oriented polylactic acid film material of the embodiment of the invention has high crystallinity, stable and non-relaxed longitudinal and transverse orientations, mechanical properties equivalent to those of the currently known polylactic acid film material, and particularly has excellent physical aging resistance, so that the biaxially oriented polylactic acid film material, a packaging material formed by the biaxially oriented polylactic acid film material and the like can keep the stability of size and performance in shelf life (storage logistics stage) and use stage, and the performance bottleneck of the conventional polylactic acid film material is broken through, thereby fully meeting the use requirements and greatly expanding the application field of the biaxially oriented polylactic acid film material.
The technical solution and effects of the present invention will be further described with reference to the following examples. Wherein the melting point is measured by Differential Scanning Calorimetry (DSC); the crystallinity is measured by X-ray diffraction (XRD) method. The method does not adopt a DSC method to calculate the crystallinity, and the measured crystallinity is higher than a true value because the secondary crystallization is caused by heating a sample in the DSC test process as well known. In the following examples of the invention, the formation of the metastable phase of polylactic acid was identified by microscopic infrared (Micro-FTIR).
Comparative example 1:
taking polylactic acid with the weight-average molecular weight of 8 ten thousand and the molar content of L optical isomer of 99 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 40 ppm; extruding the dried polylactic acid granules by using casting extrusion equipment to form a casting sheet, wherein the extrusion temperature is 180 ℃, and the casting roll temperature is 40 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 85 ℃ and the stretching multiple of 3 times; secondly, performing Transverse Direction (TD) stretching on the film, wherein the stretching temperature is 90 ℃, and the stretching multiple is 3 times; the thickness of the biaxially oriented polylactic acid film is 20 micrometers. Then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material, the hot-rolling forming temperature is 150 ℃, and the hot-rolling forming time is 3 seconds. Through detection: the glass transition temperature of the biaxially stretched polylactic acid film before storage is 57 ℃; the biaxially oriented polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, while the biaxially oriented polylactic acid film after storage at 30 + -5 ℃ for half a year has an obvious endothermic peak near the glass transition temperature. The enthalpy value of the endothermic peak near the glass transition temperature of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is increased along with the increase of the temperature rising rate of DSC, which shows that the endothermic peak is the enthalpy relaxation phenomenon which is specific to physical aging. After half a year of storage at 30. + -. 5 ℃ WAXD determined the crystallinity of the biaxially stretched polylactic acid film to be 80%. In addition, the transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film before storage are 24MPa and 28MPa respectively; elongation at break in the transverse direction was 115% and elongation at break in the longitudinal direction was 125%; the transverse tensile strength of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is 29MPa, and the longitudinal tensile strength is 35 MPa; the transverse elongation at break is 65% and the longitudinal elongation at break is 60%; the results show that the biaxially oriented polylactic acid film prepared by the comparative example does not have physical aging resistance.
Comparative example 2:
taking polylactic acid with the weight-average molecular weight of 80 ten thousand and the molar content of the L optical isomer of 95 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 42 ppm; extruding the dried polylactic acid granules by using casting extrusion equipment to form a casting sheet, wherein the extrusion temperature is 220 ℃, and the casting roller temperature is 50 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 140 ℃ and the stretching multiple of 4 times; secondly, performing Transverse Direction (TD) stretching on the film, wherein the stretching temperature is 145 ℃, and the stretching multiple is 3 times; the thickness of the biaxially oriented polylactic acid film is 80 microns. Then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material, the hot-rolling forming temperature is 180 ℃, and the hot-rolling forming time is 8 seconds. Through detection: the glass transition temperature of the biaxially stretched polylactic acid film before storage is 53 ℃; the biaxially oriented polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, while the biaxially oriented polylactic acid film after storage at 30 + -5 ℃ for half a year has an obvious endothermic peak near the glass transition temperature. The enthalpy value of the endothermic peak near the glass transition temperature of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is increased along with the increase of the temperature rising rate of DSC, which shows that the endothermic peak is the enthalpy relaxation phenomenon which is specific to physical aging. After half a year of storage at 30. + -. 5 ℃ WAXD determined the crystallinity of the biaxially stretched polylactic acid film to be 45%. In addition, the transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film before storage are 42MPa and 45MPa respectively; elongation at break in the transverse direction is 105%, and elongation at break in the longitudinal direction is 105%; the transverse tensile strength of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is 49MPa, and the longitudinal tensile strength is 56 MPa; the transverse elongation at break is 40%, and the longitudinal elongation at break is 40%; the results show that the biaxially oriented polylactic acid film prepared by the comparative example does not have physical aging resistance.
Comparative example 3:
taking polylactic acid with the weight-average molecular weight of 40 ten thousand and the molar content of the L optical isomer of 90 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 38 ppm; extruding the dried polylactic acid granules by using casting extrusion equipment to form a casting sheet, wherein the extrusion temperature is 190 ℃, and the casting roller temperature is 40 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 125 ℃ and the stretching multiple of 4 times; secondly, performing Transverse Direction (TD) stretching on the film, wherein the stretching temperature is 130 ℃, and the stretching multiple is 3 times; the thickness of the bidirectional stretching polylactic acid film is 50 microns; then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material, the hot-rolling forming temperature is 180 ℃, and the hot-rolling forming time is 5 seconds. Through detection: the glass transition temperature of the biaxially stretched polylactic acid film before storage is 55 ℃; the biaxially oriented polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, while the biaxially oriented polylactic acid film after storage at 30 + -5 ℃ for half a year has an obvious endothermic peak near the glass transition temperature. The enthalpy value of the endothermic peak near the glass transition temperature of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is increased along with the increase of the temperature rising rate of DSC, which shows that the endothermic peak is the enthalpy relaxation phenomenon which is specific to physical aging. After half a year of storage at 30. + -. 5 ℃ WAXD determined the crystallinity of the biaxially stretched polylactic acid film to be 60%. In addition, the transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film before storage are 32MPa and 34MPa respectively; elongation at break in the transverse direction is 130% and elongation at break in the longitudinal direction is 132%; the transverse tensile strength of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is 39MPa, and the longitudinal tensile strength is 42 MPa; elongation at break in the transverse direction was 52%, and elongation at break in the longitudinal direction was 58%; the results show that the biaxially oriented polylactic acid film prepared by the comparative example does not have physical aging resistance.
Example 1:
taking polylactic acid with the weight-average molecular weight of 15 ten thousand and the molar content of the L optical isomer of 95 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 50 ppm; extruding the dried polylactic acid granules by using casting extrusion equipment to form a casting sheet, wherein the extrusion temperature is 190 ℃, and the casting roller temperature is 35 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 90 ℃ and the stretching multiple of 5 times; secondly, performing Transverse Direction (TD) stretching on the film, wherein the stretching temperature is 100 ℃, and the stretching multiple is 3 times; the two-way stretched polylactic acid film is obtained by rapidly entering a rapid cooling device, the quenching speed is 30 ℃/s, and the quenching time is 3 s; the thickness of the bidirectional stretching polylactic acid film is 30 microns; then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material,the hot rolling forming temperature is 160 ℃, and the hot rolling forming time is 2 seconds; the quenching rate was 45 ℃/sec and the quenching time was 3 seconds. Through detection: the glass transition temperature of the biaxially stretched polylactic acid film before storage is 55 ℃; the biaxially oriented polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, while the biaxially oriented polylactic acid film after storage at 30 + -5 ℃ for half a year has an obvious endothermic peak near the glass transition temperature. The enthalpy value of the endothermic peak of the biaxially oriented polylactic acid film stored for half a year at the temperature of 30 +/-5 ℃ does not depend on the temperature rising rate of DSC, which shows that the endothermic peak is the structural transformation of the metastable phase of the polylactic acid and is not the enthalpy relaxation phenomenon which is specific to physical aging; it is further shown that the low temperature heat treatment process does not have physical aging, but forms a metastable phase of polylactic acid. The crystallinity of the biaxially stretched polylactic acid film was 70% as measured by WAXD after storage at 30. + -. 5 ℃ for half a year. The biaxially stretched polylactic acid film was stored at a temperature of 30. + -. 5 ℃ for half a year at 918cm-1Characteristic peaks appear, further indicating the formation of metastable phase of polylactic acid, the content of metastable phase is 15%. In addition, the transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film before storage are respectively 25MPa and 30 MPa; the transverse elongation at break is 120%, and the longitudinal elongation at break is 130%; the transverse tensile strength of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is 23MPa, and the longitudinal tensile strength is 28 MPa; elongation at break in the transverse direction was 125%, and elongation at break in the longitudinal direction was 133%; the two-way stretching polylactic acid film prepared by the embodiment has obvious physical aging resistance.
Example 2:
taking polylactic acid with the weight-average molecular weight of 8 ten thousand and the molar content of L optical isomer of 99 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 40 ppm; extruding the dried polylactic acid granules by using casting extrusion equipment to form a casting sheet, wherein the extrusion temperature is 180 ℃, and the casting roll temperature is 40 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 85 ℃ and the stretching multiple of 3 times; then, Transverse Direction (TD) stretching is carried out on the film, the stretching temperature is 90 ℃, and the stretching ratio is 3Doubling; the two-way stretched polylactic acid film is obtained by rapidly entering a rapid cooling device, the quenching speed is 70 ℃/s, and the quenching time is 1 s; the thickness of the bidirectional stretching polylactic acid film is 20 microns; then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material, the hot-rolling forming temperature is 150 ℃, and the hot-rolling forming time is 3 seconds; the quenching rate was 40 ℃/sec and the quenching time was 3 seconds. Through detection: the glass transition temperature of the biaxially stretched polylactic acid film before storage is 57 ℃; the biaxially oriented polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, while the biaxially oriented polylactic acid film after storage at 30 + -5 ℃ for half a year has an obvious endothermic peak near the glass transition temperature. The enthalpy value of the endothermic peak near the glass transition temperature of the biaxially oriented polylactic acid film after being stored for half a year at the temperature of 30 +/-5 ℃ does not depend on the temperature rising rate of DSC, which shows that the endothermic peak is the structural transition of the metastable phase of the polylactic acid and is not the enthalpy relaxation phenomenon which is specific to physical aging; it is further shown that the low temperature heat treatment process does not have physical aging, but forms a metastable phase of polylactic acid. After half a year of storage at 30. + -. 5 ℃ WAXD determined the crystallinity of the biaxially stretched polylactic acid film to be 85%. The biaxially stretched polylactic acid film was stored at a temperature of 30. + -. 5 ℃ for half a year at 918cm-1Characteristic peaks appear, further indicating the formation of metastable phase of polylactic acid, the content of metastable phase is 5%. In addition, the transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film before storage are 24MPa and 28MPa respectively; elongation at break in the transverse direction was 115% and elongation at break in the longitudinal direction was 125%; the transverse tensile strength of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is 23MPa, and the longitudinal tensile strength is 29 MPa; the transverse elongation at break is 120%, and the longitudinal elongation at break is 130%; the two-way stretching polylactic acid film prepared by the embodiment has obvious physical aging resistance.
Example 3:
taking polylactic acid with the weight-average molecular weight of 8 ten thousand and the molar content of the L optical isomer of 85 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 40 ppm; extruding the dried polylactic acid granules by a casting extrusion deviceForming a casting sheet, wherein the extrusion temperature is 180 ℃, and the casting roller temperature is 40 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 85 ℃ and the stretching multiple of 3 times; secondly, performing Transverse Direction (TD) stretching on the film, wherein the stretching temperature is 90 ℃, and the stretching multiple is 3 times; the two-way stretched polylactic acid film is obtained by rapidly entering a rapid cooling device, the quenching speed is 70 ℃/s, and the quenching time is 1 s; the thickness of the bidirectional stretching polylactic acid film is 20 microns; then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material, the hot-rolling forming temperature is 150 ℃, and the hot-rolling forming time is 3 seconds; the quenching rate was 40 ℃/sec and the quenching time was 3 seconds. Through detection: the glass transition temperature of the biaxially stretched polylactic acid film before storage is 57 ℃; the biaxially oriented polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, while the biaxially oriented polylactic acid film after storage at 30 + -5 ℃ for half a year has an obvious endothermic peak near the glass transition temperature. The enthalpy value of the endothermic peak near the glass transition temperature of the biaxially oriented polylactic acid film after being stored for half a year at the temperature of 30 +/-5 ℃ does not depend on the temperature rising rate of DSC, which shows that the endothermic peak is the structural transition of the metastable phase of the polylactic acid and is not the enthalpy relaxation phenomenon which is specific to physical aging; it is further shown that the low temperature heat treatment process does not have physical aging, but forms a metastable phase of polylactic acid. After half a year of storage at 30. + -. 5 ℃ WAXD determined the crystallinity of the biaxially stretched polylactic acid film to be 55%. The biaxially stretched polylactic acid film was stored at a temperature of 30. + -. 5 ℃ for half a year at 918cm-1Characteristic peaks appear, further indicating the formation of metastable phase of polylactic acid, with metastable phase content of 22%. In addition, the transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film before storage are 22MPa and 25MPa respectively; elongation at break in the transverse direction was 145%, and elongation at break in the longitudinal direction was 155%; the transverse tensile strength of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is 23MPa, and the longitudinal tensile strength is 26 MPa; the transverse elongation at break is 150%, and the longitudinal elongation at break is 150%; the two-way stretching polylactic acid film prepared by the embodiment has obvious physical aging resistance.
Example 4:
taking polylactic acid with the weight-average molecular weight of 80 ten thousand and the molar content of the L optical isomer of 95 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 42 ppm; extruding the dried polylactic acid granules by using casting extrusion equipment to form a casting sheet, wherein the extrusion temperature is 220 ℃, and the casting roller temperature is 50 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 140 ℃ and the stretching multiple of 4 times; secondly, performing Transverse Direction (TD) stretching on the film, wherein the stretching temperature is 145 ℃, and the stretching multiple is 3 times; the two-way stretched polylactic acid film is obtained by rapidly entering a rapid cooling device, the quenching speed is 4 ℃/s, and the quenching time is 30 s; the thickness of the bidirectional stretching polylactic acid film is 80 microns; then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material, the hot-rolling forming temperature is 180 ℃, and the hot-rolling forming time is 8 seconds; the quenching rate was 5 ℃/sec and the quenching time was 30 seconds. Through detection: the glass transition temperature of the biaxially stretched polylactic acid film before storage is 53 ℃; the biaxially oriented polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, while the biaxially oriented polylactic acid film after storage at 30 + -5 ℃ for half a year has an obvious endothermic peak near the glass transition temperature. The enthalpy value of the endothermic peak near the glass transition temperature of the biaxially oriented polylactic acid film after being stored for half a year at the temperature of 30 +/-5 ℃ does not depend on the temperature rising rate of DSC, which shows that the endothermic peak is the structural transition of the metastable phase of the polylactic acid and is not the enthalpy relaxation phenomenon which is specific to physical aging; it is further shown that the low temperature heat treatment process does not have physical aging, but forms a metastable phase of polylactic acid. After half a year of storage at 30. + -. 5 ℃ WAXD determined the crystallinity of the biaxially stretched polylactic acid film to be 35%. The biaxially stretched polylactic acid film was stored at a temperature of 30. + -. 5 ℃ for half a year at 918cm-1Characteristic peaks appear, further indicating the formation of metastable phase of polylactic acid, with metastable phase content of 55%. In addition, the transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film before storage are 42MPa and 45MPa respectively; elongation at break in the transverse direction is 105%, and elongation at break in the longitudinal direction is 105%; at 30 +/-5 DEG CThe transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film after half a year of storage are respectively 43MPa and 46 MPa; the transverse elongation at break is 100%, and the longitudinal elongation at break is 100%; the two-way stretching polylactic acid film prepared by the embodiment has obvious physical aging resistance.
Example 5:
taking polylactic acid with the weight-average molecular weight of 80 ten thousand and the molar content of the L optical isomer of 88 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 38 ppm; extruding the dried polylactic acid granules through casting extrusion equipment to form a casting sheet, wherein the extrusion temperature is 210 ℃, and the casting roller temperature is 40 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 140 ℃ and the stretching multiple of 5 times; secondly, performing Transverse Direction (TD) stretching on the film, wherein the stretching temperature is 145 ℃, and the stretching multiple is 3 times; the two-way stretched polylactic acid film is obtained by rapidly entering a rapid cooling device, the quenching speed is 5 ℃/s, and the quenching time is 25 s; the thickness of the bidirectional stretching polylactic acid film is 100 microns; then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material, the hot-rolling forming temperature is 170 ℃, and the hot-rolling forming time is 6 seconds; the quenching rate was 5 ℃/sec and the quenching time was 25 seconds. Through detection: the glass transition temperature of the biaxially stretched polylactic acid film before storage is 53 ℃; the biaxially oriented polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, while the biaxially oriented polylactic acid film after storage at 30 + -5 ℃ for half a year has an obvious endothermic peak near the glass transition temperature. The enthalpy value of the endothermic peak near the glass transition temperature of the biaxially oriented polylactic acid film after being stored for half a year at the temperature of 30 +/-5 ℃ does not depend on the temperature rising rate of DSC, which shows that the endothermic peak is the structural transition of the metastable phase of the polylactic acid and is not the enthalpy relaxation phenomenon which is specific to physical aging; it is further shown that the low temperature heat treatment process does not have physical aging, but forms a metastable phase of polylactic acid. After half a year of storage at 30. + -. 5 ℃ WAXD determined the crystallinity of the biaxially stretched polylactic acid film to be 50%. Biaxially stretched polylactic acid film after storage at 30. + -. 5 ℃ for half a year at 918cm-1Characteristic peaks appear, further indicating the formation of a metastable phase of polylactic acid, with a metastable phase content of 30%. In addition, the transverse tensile strength of the biaxially oriented polylactic acid film before storage is 40MPa, and the longitudinal tensile strength is 42 MPa; elongation at break in the transverse direction was 117% and elongation at break in the longitudinal direction was 115%; the transverse tensile strength of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is 43MPa, and the longitudinal tensile strength is 40 MPa; elongation at break in the transverse direction is 112%, and elongation at break in the longitudinal direction is 110%; the two-way stretching polylactic acid film prepared by the embodiment has obvious physical aging resistance.
Example 6:
taking polylactic acid with the weight-average molecular weight of 60 ten thousand and the molar content of the L optical isomer of 92 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 50 ppm; extruding the dried polylactic acid granules by using casting extrusion equipment to form a casting sheet, wherein the extrusion temperature is 200 ℃, and the casting roller temperature is 45 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 135 ℃ and the stretching multiple of 4 times; secondly, performing Transverse Direction (TD) stretching on the film, wherein the stretching temperature is 140 ℃, and the stretching multiple is 3 times; the two-way stretched polylactic acid film is obtained by rapidly entering a rapid cooling device, the quenching speed is 6 ℃/s, and the quenching time is 20 s; the thickness of the bidirectional stretching polylactic acid film is 60 micrometers; then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material, the hot-rolling forming temperature is 180 ℃, and the hot-rolling forming time is 6 seconds; the quenching rate was 8 ℃/s and the quenching time was 20 seconds. Through detection: the glass transition temperature of the biaxially oriented polylactic acid film before storage is 54 ℃; the biaxially oriented polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, while the biaxially oriented polylactic acid film after storage at 30 + -5 ℃ for half a year has an obvious endothermic peak near the glass transition temperature. The enthalpy values of endothermic peaks near the glass transition temperature of the biaxially oriented polylactic acid film stored at 30 +/-5 ℃ for half a year do not depend on the DSC heating rate, which indicates that the endothermic peaks are structural transition of polylactic acid metastable phase and are not substancesThe enthalpy relaxation phenomenon peculiar to physical aging; it is further shown that the low temperature heat treatment process does not have physical aging, but forms a metastable phase of polylactic acid. After half a year of storage at 30. + -. 5 ℃ WAXD determined the crystallinity of the biaxially stretched polylactic acid film to be 55%. The biaxially stretched polylactic acid film was stored at a temperature of 30. + -. 5 ℃ for half a year at 918cm-1Characteristic peaks appear, further indicating the formation of metastable phase of polylactic acid, with metastable phase content of 22%. In addition, the transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film before storage are 36MPa and 36MPa respectively; elongation at break in the transverse direction of 127% and elongation at break in the longitudinal direction of 120%; the transverse tensile strength of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is 38MPa, and the longitudinal tensile strength is 35 MPa; elongation at break in the transverse direction is 122%, and elongation at break in the longitudinal direction is 122%; the two-way stretching polylactic acid film prepared by the embodiment has obvious physical aging resistance.
Example 7:
taking polylactic acid with the weight-average molecular weight of 40 ten thousand and the molar content of the L optical isomer of 90 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 38 ppm; extruding the dried polylactic acid granules by using casting extrusion equipment to form a casting sheet, wherein the extrusion temperature is 190 ℃, and the casting roller temperature is 40 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 125 ℃ and the stretching multiple of 4 times; secondly, performing Transverse Direction (TD) stretching on the film, wherein the stretching temperature is 130 ℃, and the stretching multiple is 3 times; the two-way stretched polylactic acid film is obtained by rapidly entering a rapid cooling device, the quenching speed is 5 ℃/s, and the quenching time is 20 s; the thickness of the bidirectional stretching polylactic acid film is 50 microns; then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material, the hot-rolling forming temperature is 180 ℃, and the hot-rolling forming time is 5 seconds; the quenching rate was 10 ℃/sec and the quenching time was 15 seconds. Through detection: the glass transition temperature of the biaxially stretched polylactic acid film before storage is 55 ℃; the biaxially stretched polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, and can be stored at 30 + -5 deg.C for half a yearThe stretched polylactic acid film has obvious endothermic peak near the glass transition temperature. The enthalpy value of the endothermic peak near the glass transition temperature of the biaxially oriented polylactic acid film after being stored for half a year at the temperature of 30 +/-5 ℃ does not depend on the temperature rising rate of DSC, which shows that the endothermic peak is the structural transition of the metastable phase of the polylactic acid and is not the enthalpy relaxation phenomenon which is specific to physical aging; it is further shown that the low temperature heat treatment process does not have physical aging, but forms a metastable phase of polylactic acid. After half a year of storage at 30. + -. 5 ℃ WAXD determined the crystallinity of the biaxially stretched polylactic acid film to be 60%. The biaxially stretched polylactic acid film was stored at a temperature of 30. + -. 5 ℃ for half a year at 918cm-1Characteristic peaks appear, further indicating the formation of metastable phase of polylactic acid, with a metastable phase content of 20%. In addition, the transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film before storage are 32MPa and 34MPa respectively; elongation at break in the transverse direction is 130% and elongation at break in the longitudinal direction is 132%; the transverse tensile strength of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is 33MPa, and the longitudinal tensile strength is 32 MPa; elongation at break in the transverse direction is 132% and elongation at break in the longitudinal direction is 128%; the two-way stretching polylactic acid film prepared by the embodiment has obvious physical aging resistance.
Example 8:
taking polylactic acid with the weight-average molecular weight of 20 ten thousand and the molar content of L optical isomer of 96 percent for hot air drying, wherein the drying temperature is 95 +/-2 ℃, the drying time is 8 hours, and the water content is 38 ppm; extruding the dried polylactic acid granules by using casting extrusion equipment to form a casting sheet, wherein the extrusion temperature is 180 ℃, and the casting roll temperature is 40 ℃; fully preheating the cast sheet, and then performing biaxial stretching, namely performing longitudinal (MD) stretching on the film at the stretching temperature of 115 ℃ and the stretching multiple of 5 times; secondly, performing Transverse Direction (TD) stretching on the film, wherein the stretching temperature is 120 ℃, and the stretching multiple is 3 times; the two-way stretched polylactic acid film is obtained by rapidly entering a rapid cooling device, the quenching speed is 20 ℃/s, and the quenching time is 5 s; the thickness of the bidirectional stretching polylactic acid film is 60 micrometers; then putting the polylactic acid film into a warehouse for standby. When in use, the bidirectional stretched polylactic acid film is hot-rolled to synthesize the packaging material, the hot-rolling forming temperature is 160 ℃, and the hot-rolling forming is carried outThe time is 3 seconds; the quenching rate was 25 ℃/sec and the quenching time was 5 seconds. Through detection: the glass transition temperature of the biaxially stretched polylactic acid film before storage is 56 ℃; the biaxially oriented polylactic acid film before storage has a weak endothermic peak near the glass transition temperature, while the biaxially oriented polylactic acid film after storage at 30 + -5 ℃ for half a year has an obvious endothermic peak near the glass transition temperature. The enthalpy value of the endothermic peak near the glass transition temperature of the biaxially oriented polylactic acid film after being stored for half a year at the temperature of 30 +/-5 ℃ does not depend on the temperature rising rate of DSC, which shows that the endothermic peak is the structural transition of the metastable phase of the polylactic acid and is not the enthalpy relaxation phenomenon which is specific to physical aging; it is further shown that the low temperature heat treatment process does not have physical aging, but forms a metastable phase of polylactic acid. After half a year of storage at 30. + -. 5 ℃ WAXD determined the crystallinity of the biaxially stretched polylactic acid film to be 65%. The biaxially stretched polylactic acid film was stored at a temperature of 30. + -. 5 ℃ for half a year at 918cm-1Characteristic peaks appear, further indicating the formation of metastable phase of polylactic acid, with a metastable phase content of 25%. In addition, the transverse tensile strength and the longitudinal tensile strength of the biaxially oriented polylactic acid film before storage are respectively 30MPa and 32 MPa; elongation at break in the transverse direction is 132% and elongation at break in the longitudinal direction is 135%; the transverse tensile strength of the biaxially oriented polylactic acid film after storage for half a year at 30 +/-5 ℃ is 33MPa, and the longitudinal tensile strength is 30 MPa; elongation at break in the transverse direction is 132%, and elongation at break in the longitudinal direction is 130%; the two-way stretching polylactic acid film prepared by the embodiment has obvious physical aging resistance.
In addition, the inventors of the present invention have also made experiments with other materials, process operations, and process conditions described in the present specification with reference to the above examples, and have obtained preferable results.
It is to be understood that the above-described embodiments are part of the present invention, and not all embodiments. The detailed description of the embodiments of the present invention is not intended to limit the scope of the invention as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention. In addition, the polylactic acid material system provided by the invention is added with one or more additives such as biodegradable polymers (polyhydroxyalkanoate, polyglycolic acid, chitosan, chitin, polycaprolactone and the like), metal alloy materials (such as magnesium alloy, wherein the magnesium alloy is composed of one of magnesium-aluminum alloy, magnesium-manganese alloy, magnesium-zinc alloy, magnesium-zirconium alloy, magnesium-rare earth alloy, magnesium-lithium alloy, magnesium-calcium alloy or magnesium-silver alloy or ternary or multi-element magnesium alloy formed by combining the systems), antibacterial agents (silver, copper, acyclouridine and the like), essential elements for tissue growth (magnesium phosphate, calcium phosphate, sodium alginate and the like), vitamin K3 and the like, and is also protected by the invention.

Claims (14)

1. The preparation method of the physical aging resistant biaxially oriented polylactic acid film material is characterized by comprising the following steps:
(1) providing dry polylactic acid or a dry polylactic acid mixture, wherein the weight average molecular weight of the polylactic acid is 8-80 ten thousand, and the molar content of an L optical isomer is 85-99%;
(2) extruding the dried polylactic acid or the dry polylactic acid mixture through casting equipment to form a casting sheet, wherein the extrusion temperature is 160-220%oC. The temperature of the casting roller is 20-50 DEGoC；
(3) Fully preheating the casting sheet, and then carrying out biaxial stretching, namely firstly carrying out longitudinal stretching on the film, then carrying out transverse stretching on the film, and then carrying out biaxial stretching on the film by 2.5-140 timesoRapidly quenching to room temperature at the quenching rate of C/s for 1-59 seconds to obtain a biaxially oriented polylactic acid film;
after the polylactic acid film is stored for a sufficient time below the glass transition temperature, an enthalpy value appears after the glass transition temperature in a DSC curve of the polylactic acid filmΔH is a sufficiently large endothermic peak, the enthalpy of the endothermic peak does not change with the temperature rise rate of DSC test, and the infrared spectrogram of the polylactic acid film is within 918cm-1A band appears with a characteristic peak corresponding to a metastable phase, the intensity of which increases with the storage time, and of the polylactic acid filmThe change rate of tensile strength is lower than 15%, the change rate of bending strength is lower than 15%, the increase rate of elongation at break is lower than 20%, the sufficient time is more than or equal to 1 hour, the tensile strength and the bending strength are different, the increase rate of elongation at break is lower than 20%, the sufficient time is more than or equal to 1 hour, and the tensile strength and the bending strength are differentΔH ≥1 J/g；
And the polylactic acid film material stored below the glass transition temperature for a sufficient time comprises 35wt% to 85wt% of a crystalline phase and 15wt% to 65wt% of an amorphous phase, the amorphous phase comprising 5wt% to 55wt% of a metastable phase.
2. The biaxially stretched polylactic acid film according to claim 1, wherein: the polylactic acid film has a crystallinity of 55 to 75wt% and a metastable phase content of 10 to 30wt% in the amorphous phase after being stored for a sufficient time below the glass transition temperature.
3. The biaxially stretched polylactic acid film according to claim 1, wherein: the polylactic acid film further comprises any one or more of a polymer blend, a plasticizer, a compatibilizer, an end-capping agent, a flame retardant, an antioxidant, a lubricant, an antistatic agent, an antifogging agent, a light stabilizer, a pigment, an antimildew agent, an antibacterial agent and a foaming agent.
4. The biaxially stretched polylactic acid film according to claim 1, wherein the step (1) comprises: drying the polylactic acid or the dry mixture of the polylactic acid by hot air until the water content is lower than 60 ppm.
5. The biaxially stretched polylactic acid film according to claim 1, wherein the step (1) comprises: and (3) carrying out melt blending, extrusion granulation and hot air drying on the dried polylactic acid dry mixture until the water content is lower than 60 ppm.
6. The biaxially stretched polylactic acid film according to claim 1, wherein the weight average molecular weight of the polylactic acid in step (1) is 20 to 50 ten thousand, and the molar content of the L optical isomer is 88 to 98%.
7. The biaxially stretched polylactic acid film according to claim 1, wherein the extrusion temperature used in the step (2) is 180 to 210%oC. The temperature of the casting roller is 20-40 DEG CoC。
8. The biaxially stretched polylactic acid film according to claim 1, wherein: the longitudinal stretching temperature adopted in the step (3) is 90-140 DEGoC. The stretching ratio is 3-5 times, and the transverse stretching temperature is 95-145oC. The stretching multiple is 3-5 times, and the quenching time is 1-30 seconds.
9. The biaxially stretched polylactic acid film according to claim 1, wherein the step (3) specifically comprises:
when the weight average molecular weight of the polylactic acid is 8-20 ten thousand, the longitudinal stretching temperature is 85-115 DEGoC. The stretching multiple is 3-5 times, and the transverse stretching temperature is 90-120 timesoC. The stretching ratio is 3 to 5 times, and the quenching rate is 20 to 100oC/s, and quenching time is 1-5 seconds;
when the weight average molecular weight of the polylactic acid is 20-40 ten thousand, the longitudinal stretching temperature is 95-125 DEG CoC. The stretching ratio is 3-5 times, and the transverse stretching temperature is 100-130 timesoC. The stretching ratio is 3 to 5 times, and the quenching rate is 7 to 20oC/s, and the quenching time is 5-15 seconds;
when the weight average molecular weight of the polylactic acid is 40-60 ten thousand, the longitudinal stretching temperature is 105-145%oC. The stretching ratio is 3-5 times, and the transverse stretching temperature is 110-145oC. The stretching ratio is 3 to 5 times, and the quenching rate is 4 to 7oC/s, and the quenching time is 15-30 seconds.
10. The biaxially stretched polylactic acid film according to claim 1, wherein: the thickness of the two-way stretching polylactic acid film material is 20-200 microns.
11. Use of a physically ageing resistant biaxially oriented polylactic acid film according to any one of claims 1 to 10 for the preparation of packaging materials.
12. A method of making a packaging material, comprising:
providing a physically aging resistant biaxially stretched polylactic acid film according to any one of claims 1 to 10; and
the biaxially oriented polylactic acid film is hot-rolled to synthesize a packaging material, wherein the hot-rolling forming temperature is 140-190%oC, hot rolling and forming time is 1-10 seconds, and quenching rate is 5-150oC/s, and the quenching time is 1-30 seconds.
13. The method of manufacturing according to claim 12, wherein: wherein the hot rolling forming temperature is 150-180 DEG CoAnd C, hot rolling and forming for 1-8 seconds.
14. The method of manufacturing according to claim 12, wherein:
when the weight average molecular weight of the polylactic acid is 8-20 ten thousand, the adopted hot rolling forming temperature is 140-160 DEGoC, hot rolling forming time is 1-3 seconds, and quenching rate is 24-140oC/s, and quenching time is 1-5 seconds;
when the weight average molecular weight of the polylactic acid is 20-40 ten thousand, the adopted hot rolling forming temperature is 150-180 DEGoC, hot rolling and forming time is 3-5 seconds, and quenching rate is 7-30oC/s, and the quenching time is 5-15 seconds;
when the weight average molecular weight of the polylactic acid is 40-60 ten thousand, the adopted hot rolling forming temperature is 160-190%oC, hot rolling and forming time is 5-8 seconds, and quenching rate is 5-10oC/s, and the quenching time is 15-30 seconds.
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