CN108409985B - A kind of method for improving polylactic acid foaming ratio by pre-isothermal cold crystallization treatment - Google Patents

Abstract

The invention relates to a method for improving the foaming ratio of polylactic acid by pre-isothermal cold crystallization treatment, comprising the following steps: 1) by melt extrusion or hot pressing, polylactic acid or its blend is made into amorphous or low-crystalline billet; 2) subjecting the above-mentioned billet to pre-isothermal cold crystallization treatment to obtain a pre-isothermally treated sample with a certain degree of crystallinity and crystalline morphology; 3) subjecting the above-mentioned pre-isothermally treated sample to high-pressure gas swelling and infiltration, and rapidly releasing the pressure to produce a pre-isothermal treatment sample. Get foam material. The invention can effectively solve the technical problems of low melt strength, poor expandability and low foaming ratio of the semi-crystalline polylactic acid, regulates the cell structure of the polylactic acid foaming material, has simple process, green process, low cost, and can further Expand the application field of polylactic acid foam material.

Description

A kind of method for improving polylactic acid foaming ratio by pre-isothermal cold crystallization treatment

technical field

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.

Background technique

Polymer foam materials have excellent properties such as light weight and material saving, high specific strength, shock absorption and buffering, heat insulation and sound absorption, and efficient adsorption, so they are widely used in automobiles, home appliances, packaging, construction, medicine, aerospace and other fields. However, the existing polymer foam materials mostly use petroleum-based plastics such as polystyrene, polypropylene, polyethylene, etc. as the matrix. The large-scale use of these materials will not only consume petroleum resources, causing serious petroleum resource depletion, but also because it is difficult to degrade. It is easy to cause ecological and environmental problems such as white pollution. Therefore, the development and promotion of bio-based and biodegradable polymer foam materials is imminent.

Among many bio-based and biodegradable polymer materials, polylactic acid (PLA) material has mature technology, low cost and good comprehensive performance. However, polylactic acid is a linear semi-crystalline material with slow crystallization, poor melt strength, complicated foaming mechanism and poor expandability. It is difficult to prepare polylactic acid foam with high foaming ratio and uniform cells. The foaming ratio is the most intuitive manifestation of the foaming performance of the material. At the same time, high-rate foaming materials have great application potential in the fields of heat insulation, adsorption, and buffering. Therefore, the preparation of high-rate polylactic acid foaming materials is a difficult and hot issue in the current foaming field.

Chinese patent ^CN102911392A discloses a method for preparing radiation-modified polylactic acid foam material by using supercritical fluid _CO γ-ray irradiation, and finally the irradiated sheet is subjected to supercritical _CO2 foaming. The working principle of the invention is to increase the degree of cross-linking of molecular chains through radiation, thereby improving the melt strength of polylactic acid, and finally improving the foaming performance of polylactic acid. However, the ray radiation used in this invention has high cost, certain danger and pollution, and has strict working environment requirements.

Chinese patent CN106751611A discloses a high-melt-strength polylactic acid foaming resin and a preparation method thereof. The raw materials include polylactic acid, biodegradable copolymer, chain extender, melt enhancer, nucleating agent and lubricant. The core idea of this patent is to increase the melt strength of polylactic acid through chain extension, thereby improving the foaming performance of polylactic acid. However, this method involves a variety of chemical auxiliaries, which is not only costly, but also easily causes chemical pollution and endangers the health of operators.

Di Y. et al. reported a method of branching and modifying commercial polylactic acid using chain extenders. After branching, the melt viscoelasticity of polylactic acid was improved, and the final foaming ratio could be increased from 10 times to 19 times. However, this method requires the use of various chain extenders, which is costly and polluted. [Di Y.,et al.Reactively Modified Poly(lactic acid):Properties and Foam Processing[J].Macromolecular Materials&Engineering,2010,290(11):1083-1090.]

In summary, in order to improve the expandability of polylactic acid, the existing methods mostly use chain extenders and ray irradiation to modify the chain structure of linear polylactic acid such as chain extension, branching, and cross-linking. However, the existing methods are complicated in operation, high in cost and large in pollution. At the same time, the condensed structure such as crystallization also has a significant impact on the foaming of PLA, while the existing methods ignore the regulation of the condensed structure by the method of thermal history design. By consulting the existing literature, there is no report on the method of improving the foaming ratio of polylactic acid by pre-isothermal cold crystallization treatment.

SUMMARY OF THE INVENTION

In view of the problems existing in the above-mentioned prior art, one object of the present invention is to provide a method for increasing the foaming ratio of polylactic acid by pre-isothermal cold crystallization treatment. In view of the problems of slow polylactic acid crystallization and poor melt strength, the present invention increases the crystallization point and crystallinity in the matrix through pre-isothermal cold crystallization treatment, ensures that more ordered structures are retained in the subsequent saturation process, and forms physical entanglement. The network is formed to improve the melt viscoelasticity, thereby increasing the foaming ratio of polylactic acid.

In order to solve the above technical problems, the technical scheme of the present invention is:

A method for improving the foaming ratio of polylactic acid by pre-isothermal cold crystallization treatment: the specific steps are:

1) By melt extrusion or hot pressing, polylactic acid or its blend is made into an amorphous or low crystalline blank;

2) Under a certain pre-isothermal cold crystallization treatment temperature and pre-isothermal cold crystallization treatment time, the above-mentioned billet is subjected to pre-isothermal cold crystallization treatment to obtain a pre-isothermal treatment sample with a certain degree of crystallinity and crystal form;

3) Under a certain saturation pressure, saturation temperature and saturation time, the above-mentioned pre-isothermally treated sample is swollen and penetrated by high pressure gas, and the foamed material is obtained by rapidly releasing the pressure.

Preferably, the polylactic acid or its blend includes pure polylactic acid, polylactic acid and organic mixture, polylactic acid and inorganic filler composite.

Preferably, the specific process of step 2) is:

① Place the blank obtained in step 1) in a heating container at room temperature at room temperature, then heat the blank to the pre-isothermal cold crystallization treatment temperature, and maintain a certain pre-isothermal cold crystallization treatment time, and then continue to heat to the material's saturation temperature;

②Or place the blank obtained in step 1) in a heating container at room temperature at room temperature, then heat the blank to the pre-isothermal cold crystallization treatment temperature, and maintain a certain pre-isothermal cold crystallization treatment time, and then remove the blank from the heating container. Cool in air or liquid;

③Or first heat the heating container to the pre-isothermal treatment temperature, then place the blank at room temperature in step 1) in the heating container, and keep a certain pre-isothermal cold crystallization treatment time, then take the blank out of the heating container and place it in the heating container. Cooling in air or liquid;

④ Or first heat the heating container to the pre-isothermal treatment temperature, then place the billet at room temperature in step 1) in the heating container, and maintain a certain pre-isothermal cold crystallization treatment time, and then heat the billet to the saturation temperature.

Preferably, in step 2), 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 adopts a single pre-isothermal temperature for pre-isothermal treatment, and the multi-stage pre-isothermal treatment The treatment is to use multiple pre-isothermal temperatures to perform pre-isothermal treatment step by step.

Preferably, in step 2), the pre-isothermal cold crystallization treatment temperature is between the glass transition temperature T _g and melting point T _m of polylactic acid, that is, 60-170°C.

Further preferably, the temperature range of the pre-isothermal cold crystallization treatment is 70-140°C.

Preferably, in step 2), the pre-isothermal cold crystallization treatment time is 5-120 min, and further preferably, the treatment time is 10-60 min.

Preferably, the specific process of step 3) is:

Except for the cases ① and ④ above in step 2), for the two cases of ② and ③ in step 2), first heat the autoclave to the saturation temperature of polylactic acid, and then place the pre-isothermally treated sample obtained in step ② or ③ in the In the autoclave, the autoclave is closed; then, for the four cases of ①, ②, ③ and ④ in step 2), the autoclave is purged with low-pressure gas, filled with gas to the saturation pressure, and after maintaining a certain saturation time, quickly The pressure was released, and the autoclave was placed in ice water for rapid cooling.

Preferably, in step 3), the saturation pressure is 1500-4500 psi; the saturation temperature is 90-160° C.; and the saturation time is 50-130 min.

Preferably, the gas in step 3) is carbon dioxide, nitrogen, pentane and a mixed gas thereof.

The effect of the pre-isothermal cold crystallization process is: after the pre-isothermal cold crystallization treatment, the crystallinity and crystallization point of the matrix increase, and in the subsequent saturation process, more ordered structures can be retained, and a physical entanglement network can be formed to improve the melt. Viscoelasticity, thereby improving the foaming ratio of polylactic acid.

The beneficial effects of the present invention

Compared with the conventional method of directly heating to saturation temperature by non-isothermal method, the present invention can significantly improve the foaming performance of polylactic acid, increase the foaming ratio of polylactic acid, and widen the foaming window of polylactic acid; Compared with the method of preparing high-rate polylactic acid foam by means of chemistry, cross-linking, blending, etc., the present invention uses unmodified conventional commercial linear polylactic acid, does not destroy the biodegradability and recyclability of polylactic acid, and the whole process The process does not involve any organic solvents and chemical additives, and no additional chain extenders, radiation and blending fillers are required. Therefore, not only the foaming ratio of the polylactic acid can be significantly improved, but also the technological process is simple, the cost is low, and it is environmentally friendly.

Description of drawings

The accompanying drawings that form a part of the present application are used to provide further understanding of the present application, and the schematic embodiments and descriptions of the present application are used to explain the present application and do not constitute improper limitations on the present application.

Fig. 1 is the temperature curve of conventional method and the method of the present invention;

Fig. 2 is the SEM photo of the sample section of embodiment 1;

Fig. 3 is the SEM photo of the sample section of embodiment 2;

Fig. 4 is the SEM photo of the sample section of embodiment 3;

Fig. 5 is the SEM photo of the sample section of embodiment 4;

Fig. 6 is the SEM photo of the sample section of Comparative Example 1;

FIG. 7 is a scanning electron microscope photograph of the cross-section of the sample of Comparative Example 2. FIG.

Detailed ways

It should be noted that the following detailed description is exemplary and intended to provide further explanation of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

It should be noted that the terminology used herein is for the purpose of describing specific embodiments only, and is not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly dictates otherwise, the singular is intended to include the plural as well, furthermore, it is to be understood that when the terms "comprising" and/or "including" are used in this specification, it indicates that There are features, steps, operations, devices, components and/or combinations thereof.

The present invention will be further described below in conjunction with the examples.

The analytical test method is as follows: according to GB/T 6343-2009, the density ρ _f of the foamed sample and the density ρ _p of the unfoamed sample are measured, and the foaming ratio R=ρ _p /ρ _f . The foamed samples were immersed in liquid nitrogen and brittle and fractured, and the cross-section was observed by scanning electron microscope after gold spraying, and the cell structure was counted.

As shown in Figure 1, the conventional method is the method without the pre-isothermal cold crystallization treatment step, wherein a is the conventional method, and b is the method of the application through the pre-isothermal cold crystallization treatment step; it can be seen from the figure that the application's During the process of the method and the conventional method, the temperature changes are different.

Example 1: Preparation of pure PLA foamed material treated with pre-isothermal cold crystallization

The pure polylactic acid (4032D, Natureworks) particles were dried in a vacuum drying oven. Using a twin-screw extruder, the dried polylactic acid pellets were extruded and air-cooled to prepare extruded samples with a diameter of about 3.5 mm. First heat the autoclave to 80°C, then place a certain length of the extruded sample in the autoclave, isothermally for 15 minutes, and perform pre-isothermal cold crystallization treatment; then take out the sample and quickly cool it in ice water; Continue heating the autoclave to 137°C, place the treated sample in the autoclave, use low-pressure CO ₂ to purge the autoclave, and seal it; fill with CO ₂ with a saturation pressure of 2500 psi, and keep it at constant temperature and pressure for 50 minutes; Quickly depressurize and place the autoclave in ice water to cool to shape. The foaming ratio of the polylactic acid foamed material obtained after foaming was about 10.8 times, the average cell size was about 18.8 μm, and the cell density was about 1.4×10 ⁸ cells/cm ³ . Its cross-sectional cell morphology is shown in Figure 2.

Example 2: Preparation of pure PLA foamed material treated with pre-isothermal cold crystallization

The pure polylactic acid (4032D, Natureworks) particles were dried in a vacuum drying oven. Using a twin-screw extruder, the dried polylactic acid pellets were extruded and air-cooled to prepare extruded samples with a diameter of about 3.5 mm. A certain length of the above extruded sample was placed in an autoclave, and the autoclave was purged with low-pressure CO ₂ and sealed; the autoclave was heated to 90°C and isothermal for 15 minutes for pre-isothermal cold crystallization treatment; then continued heating to 137° C., filled with CO ₂ with a saturation pressure of 2500 psi, constant temperature and pressure for 50 minutes; depressurized quickly, and the autoclave was placed in ice water to cool and shape. The foaming ratio of the polylactic acid foamed material obtained after foaming was about 17.7 times, the average cell size was about 18.0 μm, and the cell density was about 2.2×10 ⁸ cells/cm ³ . Its cross-sectional cell morphology is shown in Figure 3. The difference between Example 2 and Example 1 is: Example 2 is heated in the manner of case 1 in the above step 2), and the pre-isothermal temperature is 90°C; while Example 1 is heated in the manner of case 3 in the above step 2). , the pre-isothermal temperature is 80 °C.

Example 3: Preparation of pure PLA foamed material treated with pre-isothermal cold crystallization

The pure polylactic acid (4032D, Natureworks) particles were dried in a vacuum drying oven. Using a twin-screw extruder, the dried polylactic acid pellets were extruded and air-cooled to prepare extruded samples with a diameter of about 3.5 mm. First heat the autoclave to 100°C, and then place a certain length of the extruded sample in the autoclave, isothermally for 15 minutes, and perform pre-isothermal cold crystallization treatment; then take out the sample and quickly cool it in ice water; Continue heating the autoclave to 137°C, place the treated sample in the autoclave, use low-pressure CO ₂ to purge the autoclave, and seal it; fill with CO ₂ with a saturation pressure of 2500 psi, and keep it at constant temperature and pressure for 50 minutes; Quickly depressurize and place the autoclave in ice water to cool to shape. The foaming ratio of the polylactic acid foamed material obtained after foaming was about 8.7 times, the average cell size was about 16.3 μm, and the cell density was about 2.3×10 ⁸ cells/cm ³ . Its cross-sectional cell morphology is shown in Figure 4. The only difference between Example 3 and Example 1 is that the preheating temperature of the autoclave in Example 3 is 100°C.

Example 4: Preparation of pure PLA foamed material treated with pre-isothermal cold crystallization

The pure polylactic acid (4032D, Natureworks) particles were dried in a vacuum drying oven. Using a twin-screw extruder, the dried polylactic acid pellets were extruded and air-cooled to prepare extruded samples with a diameter of about 3.5 mm. First heat the autoclave to 90°C, then place a certain length of the extruded sample in the autoclave, isothermally for 60 minutes, and perform pre-isothermal cold crystallization treatment; then take out the sample and quickly cool it in ice water; Continue heating the autoclave to 137°C, place the treated sample in the autoclave, use low-pressure CO ₂ to purge the autoclave, and seal it; fill with CO ₂ with a saturation pressure of 2500 psi, and keep it at constant temperature and pressure for 50 minutes; Quickly depressurize and place the autoclave in ice water to cool to shape. The foaming ratio of the polylactic acid foamed material obtained after foaming was about 9.0 times, the average cell size was about 15.8 μm, and the cell density was about 2.8×10 ⁸ cells/cm ³ . Its cross-sectional cell morphology is shown in Figure 5. The difference between Example 4 and Example 2 is that Example 4 adopts the mode of ③ in the above-mentioned steps 2) to heat, and the pre-isothermal time is 60 minutes; and Example 2 adopts the mode of 1. in the above-mentioned steps 2) to heat, The pre-isothermal time was 15 minutes.

Comparative Example 1: Preparation of pure PLA foam without pre-isothermal cold crystallization treatment

The pure polylactic acid (4032D, Natureworks) particles were dried in a vacuum drying oven. Using a twin-screw extruder, the dried polylactic acid pellets were extruded and air-cooled to prepare extruded samples with a diameter of about 3.5 mm. The above-mentioned extruded sample of a certain length was placed in an autoclave at room temperature, and the autoclave was purged with low-pressure CO ₂ and sealed; the autoclave was heated to 137 ° C, filled with CO ₂ with a saturation pressure of 2500 psi, and the autoclave was kept at constant temperature and pressure. 50 minutes; depressurize quickly and place autoclave in ice water to cool to shape. The foaming ratio of the polylactic acid foamed material obtained after foaming was about 3.2 times, the average cell size was about 8.4 μm, and the cell density was about 1.6×10 ⁹ cells/cm ³ . Its cross-sectional cell morphology is shown in Figure 6.

Comparative Example 2: Preparation of pure PLA foam without pre-isothermal cold crystallization treatment

The pure polylactic acid (4032D, Natureworks) particles were dried in a vacuum drying oven. Using a twin-screw extruder, the dried polylactic acid pellets were extruded and air-cooled to prepare extruded samples with a diameter of about 3.5 mm. Different from the control example 1 where the sample was directly placed in the autoclave at room temperature, in the control example 2, the autoclave was first heated to 137°C, and then the extruded sample was placed in the autoclave, and the low pressure CO ₂ was used to blow the sample. The autoclave was swept and sealed; filled with CO ₂ with a saturation pressure of 2500 psi, constant temperature and pressure for 50 minutes; depressurized quickly, and the autoclave was placed in ice water to cool and shape. The foaming ratio of the polylactic acid foamed material obtained after foaming was about 2.6 times, the average cell size was about 9.8 μm, and the cell density was about 1.0×10 ⁹ cells/cm ³ . Its cross-sectional cell morphology is shown in Figure 7.

It can be seen from the above that the present invention under the condition that the saturation temperature is 137 ° C and the CO ₂ saturation pressure is 2500 psi, when the pre-isothermal temperature of the polylactic acid foam material is 90 ° C and the pre-isothermal time is 15 minutes, its foaming ratio is the largest. , and the foaming ratio of Comparative Example 1 and Comparative Example 2 without pre-isothermal cold crystallization process is very low.

The above descriptions are only preferred embodiments of the present application, and are not intended to limit the present application. For those skilled in the art, the present application may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of this application shall be included within the protection scope of this application.

Claims (5)

1. a method for improving polylactic acid foaming ratio by pre-isothermal cold crystallization treatment, is characterized in that: Concrete steps are: 1) By melt extrusion or hot pressing, polylactic acid or its blend is made into an amorphous or low crystalline blank; 2) Under a certain pre-isothermal cold crystallization treatment temperature and pre-isothermal cold crystallization treatment time, the above-mentioned billet is subjected to pre-isothermal cold crystallization treatment to obtain a pre-isothermal treatment sample with a certain degree of crystallinity and crystal form; 3) Under a certain saturation pressure, saturation temperature and saturation time, the above-mentioned pre-isothermally treated sample is subjected to high-pressure gas swelling and infiltration, and a foamed material is obtained by rapid pressure relief; The concrete process of described step 2) is: ① Place the blank obtained in step 1) in a heating container at room temperature at room temperature, then heat the blank to the pre-isothermal cold crystallization treatment temperature, and maintain a certain pre-isothermal cold crystallization treatment time, and then continue to heat to the material's saturation temperature; ② Or place the blank obtained in step 1) in a heating container at room temperature at room temperature, then heat the blank to the pre-isothermal cold crystallization treatment temperature, and maintain a certain pre-isothermal cold crystallization treatment time, and then remove the blank from the heating container. Cool in air or liquid; 3. Or first heat the heating container to the pre-isothermal treatment temperature, then place the blank at room temperature obtained in step 1) in the heating container, and keep a certain pre-isothermal cold crystallization treatment time, then take out the blank from the heating container, Cool in air or liquid; ④Or firstly heat the heating container to the pre-isothermal treatment temperature, then place the blank at room temperature obtained in step 1) in the heating container, and keep a certain pre-isothermal cold crystallization treatment time, then heat the blank to the saturation temperature; The temperature of the pre-isothermal cold crystallization treatment is between the glass transition temperature Tg and the melting point Tm of the polylactic acid, that is, 70 to 100°C; The specific process of step 3) is: Except for the cases of ① and ④ in step 2), for the two cases of ② and ③ in step 2), first heat the autoclave to the saturation temperature of polylactic acid, and then place the pre-isothermal treatment sample obtained in step 2) in the autoclave Then, for the four cases of ①, ②, ③ and ④ in step 2), the autoclave is purged with low-pressure gas, filled with gas to the saturation pressure, and after maintaining a certain saturation time, the pressure is quickly released , and place the autoclave in ice water for rapid cooling and shaping; In described step 3), saturation temperature is 90～137 ℃; The pre-isothermal cold crystallization treatment time is 10-60 min. 2. a kind of method for improving polylactic acid foaming ratio according to claim 1 is characterized in that: the pre-isothermal cold crystallization treatment described in step 2) is single-stage pre-isothermal treatment or multi-stage pre-isothermal treatment. 3 . The method for improving the foaming ratio of polylactic acid according to claim 1 , wherein the saturation pressure in the step 3) is 1500-4500 psi. 4 . 4 . The method for improving the foaming ratio of polylactic acid according to claim 1 , wherein the saturation time in the step 3) is 50-130 min. 5 . 5. a kind of method that improves polylactic acid foaming ratio according to claim 1, is characterized in that: in described step 3), gas is carbon dioxide, nitrogen, pentane and mixed gas thereof.

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