WO2019196320A1 - 一种通过预等温冷结晶处理提高聚乳酸发泡倍率的方法 - Google Patents

一种通过预等温冷结晶处理提高聚乳酸发泡倍率的方法 Download PDF

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WO2019196320A1
WO2019196320A1 PCT/CN2018/103673 CN2018103673W WO2019196320A1 WO 2019196320 A1 WO2019196320 A1 WO 2019196320A1 CN 2018103673 W CN2018103673 W CN 2018103673W WO 2019196320 A1 WO2019196320 A1 WO 2019196320A1
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polylactic acid
isothermal
cold crystallization
crystallization treatment
temperature
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French (fr)
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赵国群
李博
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山东大学
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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
    • C08J3/00—Processes of treating or compounding macromolecular substances
    • 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
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
    • C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
    • C08J9/122—Hydrogen, oxygen, CO2, nitrogen or noble gases
    • 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
    • C08J2201/00—Foams characterised by the foaming process
    • C08J2201/02—Foams characterised by the foaming process characterised by mechanical pre- or post-treatments
    • C08J2201/03—Extrusion of the foamable blend
    • 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
    • C08J2203/00—Foams characterized by the expanding agent
    • C08J2203/06—CO2, N2 or noble gases
    • 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
    • C08J2205/00—Foams characterised by their properties
    • C08J2205/04—Foams characterised by their properties characterised by the foam pores
    • C08J2205/044—Micropores, i.e. average diameter being between 0,1 micrometer and 0,1 millimeter
    • 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

Definitions

  • the invention belongs to the field of polymer material processing, and particularly relates to a method for increasing the foaming ratio of polylactic acid by pre-isothermal cold crystallization treatment.
  • the polymer foaming material has excellent properties such as light weight, high material strength, high shock absorption, shock absorption, heat insulation, high efficiency adsorption, etc., and thus has wide applications in the fields of automobiles, home appliances, packaging, construction, medicine, aerospace and the like.
  • the existing polymer foaming materials mostly use petroleum-based plastics such as polystyrene, polypropylene, and polyethylene as the matrix.
  • the large-scale use of these materials not only consumes petroleum resources, but also causes severe depletion of petroleum resources and is difficult to degrade. It is easy to cause ecological problems such as white pollution. Therefore, the development and promotion of bio-based and biodegradable polymer foam materials is extremely urgent.
  • polylactic acid (PLA) materials are mature in technology, low in cost and good in overall performance.
  • polylactic acid is a linear semi-crystalline material, which has a slow crystallization, a poor melt strength, a complicated foaming mechanism, and poor hair expandability.
  • large-magnification foam materials have great application potential in the fields of heat insulation, adsorption and buffering. Therefore, the preparation of large-rate polylactic acid foaming materials is a difficult and hot issue in the current foaming field.
  • Chinese patent CN102911392A discloses a method for preparing a radiation-modified polylactic acid foaming material by using supercritical fluid CO 2 , which first melt-blends polylactic acid and a sensitizer into a sheet, and then carries the above sheet into 60 Co- The gamma ray is irradiated, and finally the irradiated sheet is subjected to supercritical CO 2 foaming.
  • the working principle of the invention is to improve the degree of crosslinking of the molecular chain by radiation, thereby improving the melt strength of the polylactic acid and finally improving the foaming performance of the polylactic acid.
  • the radiation radiation used in the invention is relatively high, and has certain dangers and pollution, and the working environment is demanding.
  • Chinese patent CN106751611A discloses a high melt strength polylactic acid foaming special resin and a preparation method thereof, and the raw materials thereof include polylactic acid, biodegradable copolymer, chain extender, melt enhancer, nucleating agent and lubricant.
  • the core idea of the patent is to improve the melt strength of polylactic acid by chain extension, thereby improving the foaming performance of polylactic acid.
  • the method involves a variety of chemical auxiliaries, which are not only costly, but also easily cause chemical pollution and endanger the health of the operator.
  • Di Y. et al. reported a method for branching and modifying commercial polylactic acid by using a chain extender. After branching, the melt viscoelasticity of polylactic acid was improved, and the final expansion ratio was increased from 10 times to 19 times. However, this method requires the use of a variety of chain extenders, which are costly and highly polluting. [Di Y., et al. Reactively Modified Poly(lactic acid): Properties and Foam Processing [J]. Macromolecular Materials & Engineering, 2010, 290(11): 1083-1090.]
  • the existing methods mostly use chain extender and ray irradiation to modify the chain structure of the linear polylactic acid such as chain extension, branching and cross-linking.
  • the existing methods are complicated in operation, high in cost, and high in pollution.
  • the condensed structure such as crystallization has a significant influence on the polylactic acid foaming, and the existing method ignores the regulation of the condensed structure by the thermal history design method.
  • the present invention provides a method for increasing the expansion ratio of polylactic acid by a pre-isothermal cold crystallization treatment.
  • the present invention increases the crystallization point and crystallinity in the matrix by pre-isothermal cold crystallization treatment, and ensures that more ordered structures are retained in the subsequent saturation process, and physical entanglement is formed.
  • the knot network improves the melt viscoelasticity and further increases the polylactic acid foaming ratio.
  • the technical solution of the present invention is:
  • a method for increasing the expansion ratio of polylactic acid by pre-isothermal cold crystallization treatment the specific steps are:
  • the polylactic acid or a blend thereof is made into an amorphous or low crystalline blank
  • the above-mentioned billet is subjected to pre-isothermal cold crystallization treatment to obtain a pre-isothermal treatment sample having a certain crystallinity and crystal morphology;
  • the polylactic acid or a blend thereof comprises pure polylactic acid, a mixture of polylactic acid and an organic compound, a polylactic acid and an inorganic filler composite.
  • step 2) is:
  • the blank obtained in the step 1) is placed in a heating vessel at room temperature at room temperature, and then the billet is heated to a pre-isothermal cold crystallization treatment temperature, and a certain pre-isothermal cold crystallization treatment time is maintained, and then heating is continued to the saturation temperature of the material;
  • step 2 Or the blank obtained in step 1) is placed in a heating vessel at room temperature at room temperature, and then the billet is heated to a pre-isothermal cold crystallization treatment temperature, and a certain pre-isothermal cold crystallization treatment time is maintained, and then the billet is taken out from the heating vessel. Placed in air or in a liquid to cool;
  • step 3 or first heating the heating vessel to the pre-isothermal treatment temperature, and then placing the blank at room temperature in step 1) in a heating vessel, and maintaining a certain pre-isothermal cold crystallization treatment time, and then taking the billet out of the heating vessel, placing Cooling in air or in liquid;
  • step 4 Or first heat the heating vessel to the pre-isothermal treatment temperature, and then place the blank at room temperature in step 1) in a heating vessel, and maintain a certain pre-isothermal cold crystallization treatment time, and then heat the billet to a saturation temperature.
  • the pre-isothermal cold crystallization treatment is a single-stage pre-isothermal treatment or a multi-stage pre-isothermal treatment, wherein the single-stage pre-isothermal treatment uses a single pre-isothermal temperature for pre-isothermal treatment, and the multi-stage pre-isothermal treatment.
  • the treatment is to perform pre-isothermal treatment step by step using a plurality of pre-isothermal temperatures.
  • the pre-isothermal cold crystallization treatment temperature is between the glass transition temperature T g and the melting point T m of the polylactic acid, that is, 60 to 170 ° C.
  • the pre-isothermal cold crystallization treatment temperature ranges from 70 to 140 °C.
  • the pre-isothermal cold crystallization treatment time is 5 to 120 min, and further preferably, the treatment time is 10 to 60 min.
  • step 3 the specific process of step 3) is:
  • the autoclave is first heated to the saturation temperature of the polylactic acid, and then the pre-isothermally treated sample obtained in the step 2 or 3 is placed.
  • the autoclave is closed; then, for the four conditions of 1, 2, 3 and 4 in step 2), the autoclave is purged with low pressure gas, and the gas is charged to a saturation pressure to maintain a certain saturation time. The pressure was released and the autoclave was placed in ice water for rapid cooling and setting.
  • the saturation pressure in step 3 is 1500-4500 psi; the saturation temperature is 90-160 ° C; and the saturation time is 50-130 min.
  • the gas in step 3) is carbon dioxide, nitrogen, pentane and a mixed gas thereof.
  • the pre-isothermal cold crystallization process is: pre-isothermal cold crystallization treatment, the crystallinity and crystallization point of the matrix are increased, more ordered structures can be retained in the subsequent saturation process, and a physical entanglement network is formed to improve the melt. Viscoelasticity, which in turn increases the expansion ratio of polylactic acid.
  • the invention can significantly improve the foaming performance of the polylactic acid, increase the expansion ratio of the polylactic acid, and broaden the foaming window of the polylactic acid;
  • the present invention uses unmodified conventional commercial linear polylactic acid without destroying the biodegradability and recyclability of polylactic acid, and the whole process The process does not involve any organic solvents and chemical auxiliaries, nor does it require additional chain extenders, radiant radiation and blended fillers. Therefore, not only can the expansion ratio of the polylactic acid be significantly increased, but also the process is simple, the cost is low, and the environment is environmentally friendly.
  • Figure 1 is a temperature profile of a conventional method and a method of the present invention
  • Example 2 is a scanning electron micrograph of a cross section of the sample of Example 1;
  • Figure 3 is a scanning electron micrograph of a cross section of the sample of Example 2.
  • Figure 4 is a scanning electron micrograph of a section of the sample of Example 3.
  • Figure 5 is a scanning electron micrograph of a cross section of the sample of Example 4.
  • Figure 6 is a scanning electron micrograph of a cross section of the sample of Comparative Example 1;
  • Fig. 7 is a scanning electron micrograph of a cross section of a sample of Comparative Example 2.
  • the conventional method is a method which is not subjected to a pre-isothermal cold crystallization treatment step, wherein a is a conventional method, and b is a method of the pre-isothermal cold crystallization treatment step of the present application;
  • a is a conventional method
  • b is a method of the pre-isothermal cold crystallization treatment step of the present application
  • Example 1 Preparation of pure PLA foamed material by pre-isothermal cold crystallization treatment
  • the pure polylactic acid (4032D, Natureworks) pellets were dried in a vacuum oven.
  • the dried polylactic acid particles were extruded by a twin-screw extruder and air-cooled to prepare an extruded sample having a diameter of about 3.5 mm.
  • the autoclave is heated to 80 ° C, and then the above-mentioned extruded sample of a certain length is placed in an autoclave, and isothermally cooled for 15 minutes, followed by pre-isothermal cold crystallization treatment; then the sample is taken out and quickly placed in ice water to cool;
  • the autoclave was further heated to 137 ° C, and the treated sample was placed in an autoclave, the autoclave was purged with low pressure CO 2 , and sealed; charged with a saturated pressure of 2500 psi of CO 2 at a constant temperature and constant pressure for 50 minutes; Quickly relieve pressure and place the autoclave in ice water to cool the setting.
  • the polylactic acid foamed material obtained after foaming had an expansion ratio of about 10.8 times, an average cell size of about 18.8 ⁇ m, and a cell density of about 1.4 ⁇ 10 8 /cm 3 .
  • the cross section of the cell is shown in Figure 2.
  • Example 2 Preparation of pure PLA foamed material by pre-isothermal cold crystallization treatment
  • the pure polylactic acid (4032D, Natureworks) pellets were dried in a vacuum oven.
  • the dried polylactic acid particles were extruded by a twin-screw extruder and air-cooled to prepare an extruded sample having a diameter of about 3.5 mm.
  • a certain length of the above extruded sample was placed in an autoclave, the autoclave was purged with low pressure CO 2 , and sealed; the autoclave was heated to 90 ° C, and isothermally heated for 15 minutes to perform pre-isothermal cold crystallization treatment; To 137 ° C, CO 2 with a saturation pressure of 2500 psi was charged, constant temperature and constant pressure for 50 minutes; rapid pressure relief, and the autoclave was placed in ice water to cool the setting.
  • the polylactic acid foamed material obtained after foaming had an expansion ratio of about 17.7 times, an average cell size of about 18.0 ⁇ m, and a cell density of about 2.2 ⁇ 10 8 /cm 3 .
  • the cross section of the cell is shown in Figure 3.
  • the difference between the second embodiment and the first embodiment is that the embodiment 2 is heated in the manner of the case 1 in the above step 2), and the pre-isothermal temperature is 90 ° C; and the embodiment 1 is heated in the manner of the case 3 in the above step 2).
  • the pre-isothermal temperature is 80 °C.
  • Example 3 Preparation of pure PLA foamed material by pre-isothermal cold crystallization treatment
  • the pure polylactic acid (4032D, Natureworks) pellets were dried in a vacuum oven.
  • the dried polylactic acid particles were extruded by a twin-screw extruder and air-cooled to prepare an extruded sample having a diameter of about 3.5 mm.
  • the autoclave is heated to 100 ° C, and then the above-mentioned extruded sample of a certain length is placed in an autoclave, isothermally cooled for 15 minutes, and pre-isothermal cold crystallization treatment is carried out; then the sample is taken out and quickly placed in ice water to cool;
  • the autoclave was further heated to 137 ° C, and the treated sample was placed in an autoclave, the autoclave was purged with low pressure CO 2 , and sealed; charged with a saturated pressure of 2500 psi of CO 2 at a constant temperature and constant pressure for 50 minutes; Quickly relieve pressure and place the autoclave in ice water to cool the setting.
  • the polylactic acid foamed material obtained after foaming had an expansion ratio of about 8.7 times, an average cell size of about 16.3 ⁇ m, and a cell density of about 2.3 ⁇ 10 8 /cm 3 .
  • the cross-sectional shape of the cross section is shown in Fig. 4. The only difference between Example 3 and Example 1 was that the autoclave preheating temperature of Example 3 was 100 °C.
  • Example 4 Preparation of pure PLA foamed material by pre-isothermal cold crystallization treatment
  • the pure polylactic acid (4032D, Natureworks) pellets were dried in a vacuum oven.
  • the dried polylactic acid particles were extruded by a twin-screw extruder and air-cooled to prepare an extruded sample having a diameter of about 3.5 mm.
  • the autoclave is heated to 90 ° C, and then the above-mentioned extruded sample of a certain length is placed in an autoclave, isothermally cooled for 60 minutes, and subjected to pre-isothermal cold crystallization treatment; then the sample is taken out and quickly placed in ice water to cool;
  • the autoclave was further heated to 137 ° C, and the treated sample was placed in an autoclave, the autoclave was purged with low pressure CO 2 , and sealed; charged with a saturated pressure of 2500 psi of CO 2 at a constant temperature and constant pressure for 50 minutes; Quickly relieve pressure and place the autoclave in ice water to cool the setting.
  • the polylactic acid foamed material obtained after foaming had an expansion ratio of about 9.0 times, an average cell size of about 15.8 ⁇ m, and a cell density of about 2.8 ⁇ 10 8 /cm 3 .
  • the cross-sectional shape of the cross-section is shown in Fig. 5.
  • the difference between the fourth embodiment and the second embodiment is that the fourth embodiment is heated in the manner of the case 3 in the above step 2), and the pre-isothermal time is 60 minutes; and the second embodiment is heated in the manner of the case 1 in the above step 2).
  • the pre-isothermal time is 15 minutes.
  • Comparative Example 1 Preparation of pure PLA foamed material without pre-isothermal cold crystallization treatment
  • the pure polylactic acid (4032D, Natureworks) pellets were dried in a vacuum oven.
  • the dried polylactic acid particles were extruded by a twin-screw extruder and air-cooled to prepare an extruded sample having a diameter of about 3.5 mm.
  • a certain length of the above extruded sample was placed in a room temperature autoclave, the autoclave was purged with low pressure CO 2 , and sealed; the autoclave was heated to 137 ° C, and charged with a saturated pressure of 2500 psi of CO 2 , constant temperature and constant pressure. 50 minutes; rapid pressure relief, and the autoclave was placed in ice water to cool the setting.
  • the polylactic acid foamed material obtained after foaming had an expansion ratio of about 3.2 times, an average cell size of about 8.4 ⁇ m, and a cell density of about 1.6 ⁇ 10 9 /cm 3 .
  • the cross-sectional shape of the cross section is shown in Fig. 6.
  • the pure polylactic acid (4032D, Natureworks) pellets were dried in a vacuum oven.
  • the dried polylactic acid particles were extruded by a twin-screw extruder and air-cooled to prepare an extruded sample having a diameter of about 3.5 mm.
  • the sample was directly placed in an autoclave at room temperature.
  • the autoclave was first heated to 137 ° C, and then the extruded sample was placed in an autoclave and blown with low pressure CO 2 .
  • the autoclave was swept and sealed; charged with 2,500 psi of CO 2 at a constant pressure and constant pressure for 50 minutes; the pressure was quickly released, and the autoclave was placed in ice water to cool the setting.
  • the polylactic acid foamed material obtained after foaming had an expansion ratio of about 2.6 times, an average cell size of about 9.8 ⁇ m, and a cell density of about 1.0 ⁇ 10 9 /cm 3 .
  • the shape of the cross-section cell is shown in Fig. 7.
  • the present invention has the maximum expansion ratio when the pre-isothermal temperature of the polylactic acid foaming material is 90 ° C and the pre-isothermal time is 15 minutes under the conditions of a saturation temperature of 137 ° C and a CO 2 saturation pressure of 2500 psi.

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Abstract

本发明涉及一种通过预等温冷结晶处理提高聚乳酸发泡倍率的方法，包括以下步骤：1）经熔融挤出或热压，将聚乳酸或其共混物制成无定形或低结晶的坯料；2）将上述坯料进行预等温冷结晶处理，获得具有一定结晶度和结晶形态的预等温处理试样；3）将上述预等温处理试样进行高压气体溶胀渗透，通过快速泄压，制得发泡材料。本发明可有效解决半结晶聚乳酸熔体强度低、可发性差、发泡倍率低的技术难题，调控聚乳酸发泡材料的泡孔结构，且工艺简单、过程绿色、成本低廉，并可进一步扩展聚乳酸发泡材料的应用领域。

Description

一种通过预等温冷结晶处理提高聚乳酸发泡倍率的方法 技术领域
本发明属于高分子材料加工领域，具体涉及一种通过预等温冷结晶处理提高聚乳酸发泡倍率的方法。
背景技术
高分子发泡材料具有质轻省材、比强度高、减震缓冲、隔热吸声、高效吸附等优良性能，因而在汽车、家电、包装、建筑、医药、航空航天等领域具有广泛应用。然而，现有高分子发泡材料多使用聚苯乙烯、聚丙烯、聚乙烯等石油基塑料为基体，这些材料的大量使用不仅会消耗石油资源，造成严重的石油资源枯竭，也因其难以降解而易造成白色污染等生态环境问题。因此，开发和推广生物基和生物降解高分子发泡材料迫在眉睫。
在众多生物基和生物降解高分子材料中，聚乳酸(PLA)材料技术成熟、成本低、综合性能好。然而，聚乳酸为线形半结晶材料，其结晶慢、熔体强度差、发泡机理复杂、可发性差。难以制备发泡倍率高、泡孔均匀的聚乳酸泡沫。而发泡倍率是材料发泡性能最直观的体现。同时，大倍率发泡材料在隔热、吸附、缓冲等领域具有极大的应用潜力。因此，制备大倍率聚乳酸发泡材料是当前发泡领域的难点和热点问题。
中国专利CN102911392A公布了利用超临界流体CO 2制备辐射改性聚乳酸发泡材料的方法，该发明先将聚乳酸和敏化剂熔融共混制成片材，再将上述片材进行 60Co-γ射线辐射，最后将辐射后的片材进行超临界CO 2发泡。该发明的工作原理是通过射线辐射提高分子链的交联程度，进而改善聚乳酸的熔体强度，最终提高聚乳酸的发泡性能。但该发明所采用的射线辐射成本较高，且具有一定危险和污染，工作环境要求苛刻。
中国专利CN106751611A公布了一种高熔体强度聚乳酸发泡专用树脂及其制备方法，其原料包括聚乳酸、生物可降解共聚物、扩链剂、熔体增强剂、成核剂和润滑剂。该专利的核心思想是通过扩链提高聚乳酸的熔体强度，进而改善聚乳酸的发泡性能。但该方法涉及多种化学助剂，不仅成本高，而且易造成化学污染，危害操作人员身体健康。
Di Y.等人报道了一种利用扩链剂对商用聚乳酸进行支化改性的方法，支化后改善了聚乳酸的熔体粘弹性，最终发泡倍率可由10倍增加至19倍。但该方法需使用多种扩链剂，成本高且污染大。[Di Y.,et al.Reactively Modified Poly(lactic acid):Properties and Foam Processing[J].Macromolecular Materials&Engineering,2010,290(11):1083-1090.]
综上所述，为改善聚乳酸的可发性，现有方法多采用扩链剂和射线辐射等手段，对线 形聚乳酸进行扩链、支化、交联等链结构层面的改性。然而，现有方法操作复杂、成本高、污染大。同时，结晶等凝聚态结构对聚乳酸发泡同样影响显著，而现有方法忽略了通过热历史设计的方法对凝聚态结构进行调控。通过查阅现有文献资料，也未发现通过预等温冷结晶处理来提高聚乳酸发泡倍率的方法报道。
发明内容
针对上述现有技术中存在的问题，本发明的一个目的是提供一种通过预等温冷结晶处理提高聚乳酸发泡倍率的方法。鉴于聚乳酸结晶慢、熔体强度差的问题，本发明通过预等温冷结晶处理，增加基体中的结晶点和结晶度，保证在后续饱和过程中保留更多的有序结构，并形成物理缠结网络，改善熔体粘弹性，进而提高聚乳酸发泡倍率。
为了解决以上技术问题，本发明的技术方案为：
一种通过预等温冷结晶处理提高聚乳酸发泡倍率的方法，具体步骤为：
1)经熔融挤出或热压，将聚乳酸或其共混物制成无定形或低结晶的坯料；
2)在一定预等温冷结晶处理温度和预等温冷结晶处理时间下，将上述坯料进行预等温冷结晶处理，获得具有一定结晶度和结晶形态的预等温处理试样；
3)在一定饱和压力、饱和温度和饱和时间下，将上述预等温处理试样进行高压气体溶胀渗透，通过快速泄压，制得发泡材料。
优选的，所述聚乳酸或其共混物包括纯聚乳酸、聚乳酸和有机物混合物、聚乳酸和无机填料复合物。
优选的，步骤2)的具体过程为：
①将步骤1)所得坯料在室温下置于室温的加热容器，然后将坯料加热至预等温冷结晶处理温度，并保持一定的预等温冷结晶处理时间，随后继续加热至材料的饱和温度；
②或者将步骤1)所得坯料在室温下置于室温的加热容器，然后将坯料加热至预等温冷结晶处理温度，并保持一定的预等温冷结晶处理时间，随后将坯料从加热容器中取出，置于空气中或液体中冷却；
③或者先将加热容器加热至预等温处理温度，再将步骤1)室温下的坯料置于加热容器中，并保持一定的预等温冷结晶处理时间，随后将坯料从加热容器中取出，置于空气中或液体中冷却；
④或者先将加热容器加热至预等温处理温度，再将步骤1)室温下的坯料置于加热容器中，并保持一定的预等温冷结晶处理时间，随后将坯料加热至饱和温度。
优选的，步骤2)中，所述预等温冷结晶处理为单级预等温处理或多级预等温处理，其中，单级预等温处理采用单个预等温温度进行预等温处理，而多级预等温处理则是采用多 个预等温温度逐级进行预等温处理。
优选的，步骤2)中，所述预等温冷结晶处理温度为介于聚乳酸的玻璃化转变温度T g和熔点T m之间，即60～170℃。
进一步优选的，所述预等温冷结晶处理温度范围为70～140℃。
优选的，步骤2)中，所述预等温冷结晶处理时间为5～120min，进一步优选的，处理时间为10～60min。
优选的，步骤3)的具体过程为：
除步骤2)中上述①和④情况外，对于步骤2)中②和③两种情况，先将高压釜加热至聚乳酸的饱和温度，再将步骤②或③所得预等温处理试样置于高压釜中，封闭高压釜；然后，对于步骤2)中①、②、③和④四种情况，再用低压气体吹扫高压釜，充入气体至饱和压力，保持一定的饱和时间后，快速泄压，并将高压釜置于冰水中快速冷却定型。
优选的，步骤3)中所述饱和压力为1500～4500psi；所述饱和温度为90～160℃；所述饱和时间为50～130min。
优选的，步骤3)中所述气体为二氧化碳、氮气、戊烷及其混合气体。
预等温冷结晶过程的作用为：经预等温冷结晶处理，基体的结晶度和结晶点增加，在后续饱和过程中能够保留更多的有序结构，并形成物理缠结网络，改善熔体的粘弹性，进而提高聚乳酸的发泡倍率。
本发明的有益效果
与通过非等温方式直接加热至饱和温度的常规方法相比，本发明能显著改善聚乳酸的发泡性能，提高聚乳酸的发泡倍率，拓宽聚乳酸的发泡窗口；与通过扩链、支化、交联、共混等方式制备大倍率聚乳酸泡沫的方法相比，本发明使用未经改性的常规商用线形聚乳酸，不破坏聚乳酸的生物降解性和可循环性，且整个工艺过程不涉及任何有机溶剂和化学助剂，也无需额外加入扩链剂、射线辐射和共混填料等。因此，不仅能够显著提高聚乳酸的发泡倍率，而且工艺过程简单、成本低廉、绿色环保。
附图说明
构成本申请的一部分的说明书附图用来提供对本申请的进一步理解，本申请的示意性实施例及其说明用于解释本申请，并不构成对本申请的不当限定。
图1为常规方法和本发明方法的温度曲线；
图2为实施例1的样品断面扫描电镜照片；
图3为实施例2的样品断面扫描电镜照片；
图4为实施例3的样品断面扫描电镜照片；
图5为实施例4的样品断面扫描电镜照片；
图6为对照例1的样品断面扫描电镜照片；
图7为对照例2的样品断面扫描电镜照片。
具体实施方式
应该指出，以下详细说明都是例示性的，旨在对本申请提供进一步的说明。除非另有指明，本文使用的所有技术和科学术语具有与本申请所属技术领域的普通技术人员通常理解的相同含义。
需要注意的是，这里所使用的术语仅是为了描述具体实施方式，而非意图限制根据本申请的示例性实施方式。如在这里所使用的，除非上下文另外明确指出，否则单数形式也意图包括复数形式，此外，还应当理解的是，当在本说明书中使用术语“包含”和/或“包括”时，其指明存在特征、步骤、操作、器件、组件和/或它们的组合。
下面结合实施例对本发明进一步说明。
分析测试方法如下：按照GB/T 6343-2009，测得发泡样品密度ρ f和未发泡样品密度ρ p，发泡倍率R＝ρ p/ρ f。发泡样品在液氮中浸泡脆断，其断面喷金后进行扫描电镜观察，并统计其泡孔结构。
如图1所示，常规方法为未经过预等温冷结晶处理步骤的方法，其中a为常规方法，b为本申请的经过预等温冷结晶处理步骤的方法；从图中可以看出本申请的方法和常规方法的过程中，温度变化不同。
实施例1：经预等温冷结晶处理的纯PLA发泡材料的制备
将纯聚乳酸(4032D，Natureworks公司)颗粒在真空干燥箱中进行干燥处理。利用双螺杆挤出机，将干燥后的聚乳酸颗粒进行挤出并空冷，制备直径约为3.5mm的挤出试样。先将高压釜加热至80℃，再将一定长度的上述挤出试样置于高压釜中，等温15分钟，进行预等温冷结晶处理；随后将试样取出，并快速置于冰水中冷却；将高压釜继续加热至137℃，并将处理后的试样置于高压釜中，利用低压CO 2吹扫高压釜，并密封；充入饱和压力为2500psi的CO 2，恒温恒压50分钟；快速泄压，并将高压釜置于冰水中以冷却定型。发泡后得到的聚乳酸发泡材料的发泡倍率约为10.8倍，泡孔平均尺寸约为18.8μm，泡孔密度约为1.4×10 8个/cm 3。其断面泡孔形貌如图2所示。
实施例2：经预等温冷结晶处理的纯PLA发泡材料的制备
将纯聚乳酸(4032D，Natureworks公司)颗粒在真空干燥箱中进行干燥处理。利用双 螺杆挤出机，将干燥后的聚乳酸颗粒进行挤出并空冷，制备直径约为3.5mm的挤出试样。将一定长度的上述挤出试样置于高压釜中，利用低压CO 2吹扫高压釜，并密封；将高压釜加热至90℃，并等温15分钟，进行预等温冷结晶处理；随后继续加热至137℃，充入饱和压力为2500psi的CO 2，恒温恒压50分钟；快速泄压，并将高压釜置于冰水中以冷却定型。发泡后得到的聚乳酸发泡材料的发泡倍率约为17.7倍，泡孔平均尺寸约为18.0μm，泡孔密度约为2.2×10 8个/cm 3。其断面泡孔形貌如图3所示。实施例2与实施例1的区别在于：实施例2采用上述步骤2)中情况①的方式进行加热，预等温温度为90℃；而实施例1采用上述步骤2)中情况③的方式进行加热，预等温温度为80℃。
实施例3：经预等温冷结晶处理的纯PLA发泡材料的制备
将纯聚乳酸(4032D，Natureworks公司)颗粒在真空干燥箱中进行干燥处理。利用双螺杆挤出机，将干燥后的聚乳酸颗粒进行挤出并空冷，制备直径约为3.5mm的挤出试样。先将高压釜加热至100℃，再将一定长度的上述挤出试样置于高压釜中，等温15分钟，进行预等温冷结晶处理；随后将试样取出，并快速置于冰水中冷却；将高压釜继续加热至137℃，并将处理后的试样置于高压釜中，利用低压CO 2吹扫高压釜，并密封；充入饱和压力为2500psi的CO 2，恒温恒压50分钟；快速泄压，并将高压釜置于冰水中以冷却定型。发泡后得到的聚乳酸发泡材料的发泡倍率约为8.7倍，泡孔平均尺寸约为16.3μm，泡孔密度约为2.3×10 8个/cm 3。其断面泡孔形貌如图4所示。实施例3与实施例1的唯一区别在于实施例3的高压釜预热温度为100℃。
实施例4：经预等温冷结晶处理的纯PLA发泡材料的制备
将纯聚乳酸(4032D，Natureworks公司)颗粒在真空干燥箱中进行干燥处理。利用双螺杆挤出机，将干燥后的聚乳酸颗粒进行挤出并空冷，制备直径约为3.5mm的挤出试样。先将高压釜加热至90℃，再将一定长度的上述挤出试样置于高压釜中，等温60分钟，进行预等温冷结晶处理；随后将试样取出，并快速置于冰水中冷却；将高压釜继续加热至137℃，并将处理后的试样置于高压釜中，利用低压CO 2吹扫高压釜，并密封；充入饱和压力为2500psi的CO 2，恒温恒压50分钟；快速泄压，并将高压釜置于冰水中以冷却定型。发泡后得到的聚乳酸发泡材料的发泡倍率约为9.0倍，泡孔平均尺寸约为15.8μm，泡孔密度约为2.8×10 8个/cm 3。其断面泡孔形貌如图5所示。实施例4与实施例2的区别在于实施例4采用上述步骤2)中情况③的方式进行加热，预等温时间为60分钟；而实施例2采用上述步骤2)中情况①的方式进行加热，预等温时间为15分钟。
对照例1：未经预等温冷结晶处理的纯PLA发泡材料的制备
将纯聚乳酸(4032D，Natureworks公司)颗粒在真空干燥箱中进行干燥处理。利用双 螺杆挤出机，将干燥后的聚乳酸颗粒进行挤出并空冷，制备直径约为3.5mm的挤出试样。将一定长度的上述挤出试样置于室温的高压釜中，利用低压CO 2吹扫高压釜，并密封；将高压釜加热至137℃，充入饱和压力为2500psi的CO 2，恒温恒压50分钟；快速泄压，并将高压釜置于冰水中以冷却定型。发泡后得到的聚乳酸发泡材料的发泡倍率约为3.2倍，泡孔平均尺寸约为8.4μm，泡孔密度约为1.6×10 9个/cm 3。其断面泡孔形貌如图6所示。
对照例2：未经预等温冷结晶处理的纯PLA发泡材料的制备
将纯聚乳酸(4032D，Natureworks公司)颗粒在真空干燥箱中进行干燥处理。利用双螺杆挤出机，将干燥后的聚乳酸颗粒进行挤出并空冷，制备直径约为3.5mm的挤出试样。与对照例1中将试样直接置于室温的高压釜中不同，对照例2中是先将高压釜加热至137℃，然后再将挤出试样置于高压釜中，利用低压CO 2吹扫高压釜，并密封；充入饱和压力为2500psi的CO 2，恒温恒压50分钟；快速泄压，并将高压釜置于冰水中以冷却定型。发泡后得到的聚乳酸发泡材料的发泡倍率约为2.6倍，泡孔平均尺寸约为9.8μm，泡孔密度约为1.0×10 9个/cm 3。其断面泡孔形貌如图7所示。
由上述看出，本发明在饱和温度137℃和CO 2饱和压力为2500psi的条件下，当聚乳酸发泡材料的预等温温度为90℃、预等温时间为15分钟时，其发泡倍率最大，而没有设置预等温冷结晶过程的对比例1和对比例2的发泡倍率则很低。
以上所述仅为本申请的优选实施例而已，并不用于限制本申请，对于本领域的技术人员来说，本申请可以有各种更改和变化。凡在本申请的精神和原则之内，所作的任何修改、等同替换、改进等，均应包含在本申请的保护范围之内。

Claims (10)

  1. 一种通过预等温冷结晶处理提高聚乳酸发泡倍率的方法，其特征在于：具体步骤为：
    1)经熔融挤出或热压，将聚乳酸或其共混物制成无定形或低结晶的坯料；
    2)在一定预等温冷结晶处理温度和预等温冷结晶处理时间下，将上述坯料进行预等温冷结晶处理，获得具有一定结晶度和结晶形态的预等温处理试样；
    3)在一定饱和压力、饱和温度和饱和时间下，将上述预等温处理试样进行高压气体溶胀渗透，通过快速泄压，制得发泡材料。
  2. 根据权利要求1所述的一种提高聚乳酸发泡倍率的方法，其特征在于：所述步骤2)的具体过程为：
    ①将步骤1)所得坯料在室温下置于室温的加热容器，然后将坯料加热至预等温冷结晶处理温度，并保持一定的预等温冷结晶处理时间，随后继续加热至材料的饱和温度；
    ②或者将步骤1)所得坯料在室温下置于室温的加热容器，然后将坯料加热至预等温冷结晶处理温度，并保持一定的预等温冷结晶处理时间，随后将坯料从加热容器中取出，置于空气中或液体中冷却；
    ③或者先将加热容器加热至预等温处理温度，再将步骤1)得到的室温下的坯料置于加热容器中，并保持一定的预等温冷结晶处理时间，随后将坯料从加热容器中取出，置于空气中或液体中冷却；
    ④或者先将加热容器加热至预等温处理温度，再将步骤1)得到的室温下的坯料置于加热容器中，并保持一定的预等温冷结晶处理时间，随后将坯料加热至饱和温度。
  3. 根据权利要求2所述的一种提高聚乳酸发泡倍率的方法，其特征在于：步骤2)中所述预等温冷结晶处理为单级预等温处理或多级预等温处理。
  4. 根据权利要求2所述的一种提高聚乳酸发泡倍率的方法，其特征在于：所述预等温冷结晶处理温度为介于聚乳酸的玻璃化转变温度T g和熔点T m之间，即60～170℃；
    优选的，所述预等温冷结晶处理温度范围为70～140℃。
  5. 根据权利要求2所述的一种提高聚乳酸发泡倍率的方法，其特征在于：所述预等温冷结晶处理时间为5～120min，优选10～60min。
  6. 根据权利要求2所述的一种提高聚乳酸发泡倍率的方法，其特征在于：步骤3)的具体过程为：
    除步骤2)中①和④情况外，对于步骤2)中②和③两种情况，先将高压釜加热至聚乳酸的饱和温度，再将步骤2)所得预等温处理试样置于高压釜中，封闭高压釜；然后，对于步骤2)中①、②、③和④四种情况，再用低压气体吹扫高压釜，充入气体至饱和压 力，保持一定的饱和时间后，快速泄压，并将高压釜置于冰水中快速冷却定型。
  7. 根据权利要求6所述的一种提高聚乳酸发泡倍率的方法，其特征在于：所述步骤3)中所述饱和压力为1500～4500psi。
  8. 根据权利要求6所述的一种提高聚乳酸发泡倍率的方法，其特征在于：所述步骤3)中所述饱和温度为90～160℃。
  9. 根据权利要求6所述的一种提高聚乳酸发泡倍率的方法，其特征在于：所述步骤3)中所述饱和时间为50～130min。
  10. 根据权利要求6所述的一种提高聚乳酸发泡倍率的方法，其特征在于：步骤3)中所述气体为二氧化碳、氮气、戊烷及其混合气体。
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