Heat-resistant polylactic acid composite material and preparation method thereof

Abstract

The present invention discloses a heat-resistant polylactic acid composite material and a preparation method thereof, belonging to the technical field of preparation of high-performance polymer materials. The heat-resistant polylactic acid composite material is prepared by melt blending polylactic acid as a raw material, halloysite nanotube / nanoalumina composite as a nucleating agent, and coconut shell fiber grafted maleic anhydride as a reinforcing agent, filler and compatibilizer. The polylactic acid composite material prepared by the present invention has high tensile strength, impact strength and heat deformation temperature, low viscosity and complete biodegradability, and is mainly used in food packaging fields such as packaging boxes and packaging bags, and has significant economic value and social benefits.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparation of high-performance polymer materials, and in particular relates to a heat-resistant polylactic acid composite material and a preparation method thereof. Background Art

[0002] With the development of the economy and society and the improvement of people's living standards, the earth's resources have been overexploited, and problems such as global warming, oil resource depletion and energy shortages are becoming increasingly serious. Therefore, eco-friendly and widely used biodegradable polymer materials have attracted widespread attention from scientific researchers. Polylactic acid is currently the most widely studied and applied biodegradable polymer material. It has good mechanical properties and processability and is widely used in disposable products and biomedicine. However, the helical molecular chain structure of polylactic acid and the chiral space of the lactic acid structural unit make it easy for the polylactic acid crystal structure to form a large number of defects. In addition, the weak mobility of the polylactic acid molecular chain leads to a slow crystallization rate and low crystallinity during the rapid cooling and shaping process. Due to the above reasons, the heat deformation temperature of polylactic acid is relatively low, which limits its application in food packaging fields such as disposable takeout boxes and degradable food packaging bags. Summary of the Invention

[0003] This invention addresses the low heat deformation temperature caused by numerous crystal defects and slow crystallization rate in polylactic acid (PLA). It provides a heat-resistant polylactic acid composite material and its preparation method. The resulting polylactic acid composite material exhibits high tensile strength, impact strength, and heat deformation temperature, low viscosity, and is fully biodegradable. It is primarily used in food packaging applications such as packaging boxes and bags, offering significant economic and social benefits.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] A heat-resistant polylactic acid composite material is prepared by melt blending polylactic acid as a raw material, halloysite nanotube / nanoalumina composite as a nucleating agent, and coconut shell fiber grafted maleic anhydride as a reinforcing agent, filler and compatibilizer.

[0006] The preparation method of the heat-resistant polylactic acid composite material comprises the following steps:

[0007] (1) 0.6-1 g initiator and 0.8-1.2 g maleic anhydride were added to 80-120 mL organic solvent in sequence, ultrasonicated at room temperature for 4-8 min, and then mechanically stirred for 10-20 min. Then, 15-30 g coconut shell fiber was added and mechanically stirred at 80-100 °C for 4-6 h. The mixture was filtered, washed, and dried to obtain coconut shell fiber grafted with maleic anhydride.

[0008] (2) 20 g of halloysite nanotubes and 20 g of nano-alumina were added to 40 mL of 1 mol / L hydrochloric acid, respectively, and allowed to stand at room temperature for 12 to 24 h. Activated halloysite nanotubes and activated nano-alumina were obtained by filtration, washing, and drying.

[0009] (3) 1-3 g of triethylamine and 4-6 g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane were added to 30-50 mL of toluene in sequence, and ultrasonicated at room temperature for 4-10 min. Then, 2-4 g of activated halloysite nanotubes were added and mechanically stirred at 80-100 °C for 16-24 h. The modified halloysite nanotubes were obtained by filtration, washing, and drying.

[0010] (4) Add 2-4 g of γ-(2,3-epoxypropoxy)propyltrimethoxysilane to 80-100 mL of anhydrous ethanol, ultrasonicate at room temperature for 4-10 min, then add 2-3 g of activated nano-alumina, mechanically stir at 80-100 °C for 4-6 h, filter, wash, and dry to obtain modified nano-alumina.

[0011] (5) Add 1-2 g of triethylamine to 80-100 mL of anhydrous ethanol, ultrasonicate at room temperature for 4-10 min, then add 0.5-2 g of modified halloysite nanotubes and 1-2 g of modified nano-alumina in sequence, mechanically stir at 100-120 °C for 8-12 h, filter, wash, and dry to obtain a halloysite nanotube / nano-alumina composite.

[0012] (6) 30-40 g of polylactic acid, 5-10 g of coconut shell fiber grafted maleic anhydride and 1-3 g of halloysite nanotube / nanoalumina composite were sequentially added into an internal mixer at a mixing temperature of 170-210 °C, a mixing pressure of 0.2-0.4 MPa, a rotor speed of 40-60 r / min and a mixing time of 8-12 min. The mixed material was cooled and crushed to obtain the heat-resistant polylactic acid composite material.

[0013] The initiator is any one of azobisisobutyronitrile, benzoyl peroxide or dicumyl peroxide.

[0014] The organic solvent is any one of toluene, chloroform, N,N-dimethylformamide or acetone.

[0015] The halloysite nanotubes have an inner diameter of 10-30 nm, an outer diameter of 40-70 nm, and a length of 0.5-1.5 μm.

[0016] The diameter of the nano-aluminum oxide is 30 nm.

[0017] The beneficial effects of the present invention are:

[0018] (1) Halloysite nanotubes and nanoalumina, as nucleating agents, can both increase the heat deformation temperature of polylactic acid. However, halloysite nanotubes and nanoalumina are difficult to disperse in polylactic acid and tend to agglomerate. This is not only detrimental to increasing the heat deformation temperature of polylactic acid, but also creates structural defects within the polylactic acid, reducing the mechanical properties of the polylactic acid. The present invention compounds halloysite nanotubes and nanoalumina through chemical bonds, which not only avoids the problem of easy agglomeration when they are dispersed separately, but also improves the compatibility between polylactic acid and the composite material through the organic chain between them.

[0019] (2) Coconut shell fiber is a natural polymer material with advantages such as wide availability, low price, and biodegradability. The present invention uses coconut shell fiber to reduce the cost of polylactic acid composite materials and improve the mechanical properties of polylactic acid composite materials. However, the compatibility between coconut shell fiber and polylactic acid is poor. Therefore, the present invention uses a solution grafting method to prepare coconut shell fiber grafted with maleic anhydride to improve the compatibility between coconut shell fiber and polylactic acid. At the same time, the coconut shell fiber grafted with polylactic acid prepared by the present invention can also be used as a compatibilizer between polylactic acid and the composite.

[0020] (3) The polylactic acid, coconut shell fiber, halloysite nanotubes and nano-alumina used in the present invention are all green and environmentally friendly materials such as biodegradable polymer materials or inorganic powder materials. Therefore, the heat-resistant polylactic acid composite material prepared by the present invention is an environmentally friendly and completely biodegradable polymer material, which conforms to the national environmental protection concept and meets the requirements of sustainable development.

[0021] (4) The polylactic acid composite material prepared by the present invention has high tensile strength, impact strength and heat deformation temperature, low viscosity and is completely biodegradable. The tensile strength is 70.1~73.9 MPa and the impact strength is 10.8~13.5 kJ / m 2 The heat deformation temperature is 73.5~76.3 ℃, and the melt index is 15.8~17.7 g / 10min. It is mainly used in food packaging fields such as packaging boxes and packaging bags, and has significant economic value and social benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the infrared absorption spectrum of the coconut shell fiber grafted with maleic anhydride prepared in Example 1.

[0023] Figure 2 This is the infrared absorption spectrum of the halloysite nanotube / nanoalumina composite prepared in Example 1. DETAILED DESCRIPTION

[0024] The advantages and effects of the preparation method of the heat-resistant polylactic acid composite material in this embodiment are further described below through several groups of examples and comparative examples. It should be understood that the specific embodiments described herein are only used to illustrate the present invention and are not intended to limit the present invention.

[0025] Example 1

[0026] (1) 0.8 g of dicumyl peroxide and 1 g of maleic anhydride were added to 100 mL of toluene in sequence. The mixture was ultrasonicated at room temperature for 6 min and then mechanically stirred for 15 min. 20 g of coconut shell fiber was then added and mechanically stirred at 90 °C for 5 h. The mixture was filtered, washed, and dried to obtain coconut shell fiber grafted with maleic anhydride.

[0027] (2) 20 g of halloysite nanotubes and 20 g of nano-alumina were added to 40 mL of 1 mol / L hydrochloric acid, respectively, and allowed to stand at room temperature for 18 h. Activated halloysite nanotubes and activated nano-alumina were obtained by filtration, washing, and drying.

[0028] (3) 2 g of triethylamine and 5 g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane were added to 40 mL of toluene in sequence, and ultrasonicated at room temperature for 7 min. Then, 3 g of activated halloysite nanotubes were added and mechanically stirred at 90 °C for 20 h. The modified halloysite nanotubes were obtained by filtration, washing, and drying.

[0029] (4) 3 g of γ-(2,3-epoxypropoxy)propyltrimethoxysilane was added to 90 mL of anhydrous ethanol, ultrasonicated at room temperature for 7 min, and then 2.5 g of activated nano-alumina was added. The mixture was mechanically stirred at 90 °C for 5 h, filtered, washed, and dried to obtain modified nano-alumina.

[0030] (5) 1.5 g of triethylamine was added to 90 mL of anhydrous ethanol, and ultrasonicated at room temperature for 7 min. Then, 1 g of modified halloysite nanotubes and 1.5 g of modified nanoalumina were added in sequence. The mixture was mechanically stirred at 110 °C for 10 h. After filtration, washing, and drying, a halloysite nanotube / nanoalumina composite was obtained.

[0031] (6) 35 g of polylactic acid, 8 g of coconut shell fiber grafted maleic anhydride, and 2 g of halloysite nanotube / nanoalumina composite were added into an internal mixer in sequence. The internal mixing temperature was 190 °C, the internal mixing pressure was 0.3 MPa, the rotor speed was 50 r / min, and the internal mixing time was 10 min. The internal mixing material was cooled and crushed to obtain the heat-resistant polylactic acid composite material.

[0032] The halloysite nanotubes have an inner diameter of 10-30 nm, an outer diameter of 40-70 nm, and a length of 0.5-1.5 μm.

[0033] The diameter of the nano-aluminum oxide is 30 nm.

[0034] Figure 1 This is the infrared absorption spectrum of the coconut shell fiber grafted with maleic anhydride prepared in this example. The figure shows that compared with the ungrafted coconut shell fiber, the infrared absorption spectrum of the coconut shell fiber grafted with maleic anhydride has a higher value at 1750 cm -1 An obvious C=O absorption peak appears at , which indicates that maleic anhydride has been successfully grafted onto coconut shell fiber.

[0035] Figure 2 This is the infrared absorption spectrum of the halloysite nanotube / nanoalumina composite prepared in this example. As shown in the figure, compared with the unmodified halloysite nanotube, the halloysite nanotube modified with N-(2-aminoethyl)-3-aminopropyltriethoxysilane has a higher absorption spectrum at 2900 cm -1 The CH stretching vibration absorption peak appeared near 2900 cm, indicating that N-(2-aminoethyl)-3-aminopropyltriethoxysilane has been successfully grafted onto the surface of halloysite nanotubes. Compared with the unmodified nano-alumina, the nano-alumina modified with γ-(2,3-epoxypropyloxy)propyltrimethoxysilane has a stronger absorption peak at 2900 cm -1 The CH stretching vibration absorption peak appeared near the surface of the nanoalumina, indicating that γ-(2,3-epoxypropyloxy)propyltrimethoxysilane has been successfully grafted onto the surface of the nanoalumina. After the modified halloysite nanotubes were composited with the modified nanoalumina, the absorption peak at 1230 cm -1 The CN stretching vibration absorption peak appears nearby, indicating that the modified halloysite nanotubes and modified nano-alumina are successfully composited.

[0036] Example 2

[0037] (1) 0.6 g of azobisisobutyronitrile and 0.8 g of maleic anhydride were added to 80 mL of chloroform in sequence, ultrasonicated at room temperature for 4 min, then mechanically stirred for 10 min, and then 15 g of coconut shell fiber was added. Mechanical stirring was continued at 80 °C for 6 h. After filtration, washing, and drying, coconut shell fiber grafted with maleic anhydride was obtained.

[0038] (2) 20 g of halloysite nanotubes and 20 g of nano-alumina were added to 40 mL of 1 mol / L hydrochloric acid, respectively, and allowed to stand at room temperature for 12 h. Activated halloysite nanotubes and activated nano-alumina were obtained by filtration, washing, and drying.

[0039] (3) 1 g of triethylamine and 4 g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane were added to 30 mL of toluene in sequence, and ultrasonicated at room temperature for 4 min. Then, 2 g of activated halloysite nanotubes were added and mechanically stirred at 80 °C for 24 h. The modified halloysite nanotubes were obtained by filtration, washing, and drying.

[0040] (4) Add 2 g of γ-(2,3-epoxypropoxy)propyltrimethoxysilane to 80 mL of anhydrous ethanol, ultrasonicate at room temperature for 4 min, then add 2 g of activated nano-alumina, mechanically stir at 80 °C for 6 h, filter, wash, and dry to obtain modified nano-alumina.

[0041] (5) 1 g of triethylamine was added to 80 mL of anhydrous ethanol, and ultrasonicated at room temperature for 4 min. Then, 0.5 g of modified halloysite nanotubes and 1 g of modified nanoalumina were added in sequence. The mixture was mechanically stirred at 100 °C for 12 h. After filtration, washing, and drying, a halloysite nanotube / nanoalumina composite was obtained.

[0042] (6) 30 g of polylactic acid, 5 g of coconut shell fiber grafted maleic anhydride and 1 g of halloysite nanotube / nanoalumina composite were added into an internal mixer in sequence. The internal mixing temperature was 170 °C, the internal mixing pressure was 0.2 MPa, the rotor speed was 40 r / min, and the internal mixing time was 12 min. The internal mixing material was cooled and crushed to obtain the heat-resistant polylactic acid composite material.

[0043] The halloysite nanotubes have an inner diameter of 10-30 nm, an outer diameter of 40-70 nm, and a length of 0.5-1.5 μm.

[0044] The diameter of the nano-aluminum oxide is 30 nm.

[0045] Example 3

[0046] (1) 1 g of benzoyl peroxide and 1.2 g of maleic anhydride were added to 120 mL of N,N-dimethylformamide in sequence. The mixture was ultrasonicated at room temperature for 8 min and then mechanically stirred for 20 min. 30 g of coconut shell fiber was then added and mechanically stirred at 100 °C for 4 h. The mixture was filtered, washed, and dried to obtain coconut shell fiber grafted with maleic anhydride.

[0047] (2) 20 g of halloysite nanotubes and 20 g of nano-alumina were added to 40 mL of 1 mol / L hydrochloric acid, respectively, and allowed to stand at room temperature for 24 h. Activated halloysite nanotubes and activated nano-alumina were obtained by filtration, washing, and drying.

[0048] (3) 3 g of triethylamine and 6 g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane were added to 50 mL of toluene in sequence, and ultrasonicated at room temperature for 10 min. Then, 4 g of activated halloysite nanotubes were added and mechanically stirred at 100 °C for 16 h. The modified halloysite nanotubes were obtained by filtration, washing, and drying.

[0049] (4) Add 4 g of γ-(2,3-epoxypropoxy)propyltrimethoxysilane to 100 mL of anhydrous ethanol, ultrasonicate at room temperature for 10 min, then add 3 g of activated nano-alumina, mechanically stir at 100 °C for 4 h, filter, wash, and dry to obtain modified nano-alumina.

[0050] (5) Add 2 g of triethylamine to 100 mL of anhydrous ethanol, ultrasonicate at room temperature for 10 min, then add 2 g of modified halloysite nanotubes and 2 g of modified nano-alumina in sequence, mechanically stir at 120 °C for 8 h, filter, wash, and dry to obtain a halloysite nanotube / nano-alumina composite.

[0051] (6) 40 g of polylactic acid, 10 g of coconut shell fiber grafted maleic anhydride and 3 g of halloysite nanotube / nanoalumina composite were added into an internal mixer in sequence. The internal mixing temperature was 210 °C, the internal mixing pressure was 0.4 MPa, the rotor speed was 60 r / min, and the internal mixing time was 8 min. The internal mixing material was cooled and crushed to obtain the heat-resistant polylactic acid composite material.

[0052] The halloysite nanotubes have an inner diameter of 10-30 nm, an outer diameter of 40-70 nm, and a length of 0.5-1.5 μm.

[0053] The diameter of the nano-aluminum oxide is 30 nm.

[0054] Comparative Example 1

[0055] (1) 20 g of halloysite nanotubes and 20 g of nano-alumina were added to 40 mL of 1 mol / L hydrochloric acid, respectively, and allowed to stand at room temperature for 18 h. Activated halloysite nanotubes and activated nano-alumina were obtained by filtration, washing, and drying.

[0056] (2) 2 g of triethylamine and 5 g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane were added to 40 mL of toluene in sequence, and ultrasonicated at room temperature for 7 min. Then, 3 g of activated halloysite nanotubes were added and mechanically stirred at 90 °C for 20 h. The modified halloysite nanotubes were obtained by filtration, washing, and drying.

[0057] (3) Add 3 g of γ-(2,3-epoxypropoxy)propyltrimethoxysilane to 90 mL of anhydrous ethanol, ultrasonicate at room temperature for 7 min, then add 2.5 g of activated nano-alumina, mechanically stir at 90 °C for 5 h, filter, wash, and dry to obtain modified nano-alumina.

[0058] (4) 1.5 g of triethylamine was added to 90 mL of anhydrous ethanol, and ultrasonicated at room temperature for 7 min. Then, 1 g of modified halloysite nanotubes and 1.5 g of modified nanoalumina were added in sequence. The mixture was mechanically stirred at 110 °C for 10 h. After filtration, washing, and drying, a halloysite nanotube / nanoalumina composite was obtained.

[0059] (5) 35 g of polylactic acid and 2 g of halloysite nanotube / nanoalumina composite were added into an internal mixer in sequence. The internal mixing temperature was 190 °C, the internal mixing pressure was 0.3 MPa, the rotor speed was 50 r / min, and the internal mixing time was 10 min. The internal mixing material was cooled and crushed to obtain the finished product.

[0060] The halloysite nanotubes have an inner diameter of 10-30 nm, an outer diameter of 40-70 nm, and a length of 0.5-1.5 μm.

[0061] The diameter of the nano-aluminum oxide is 30 nm.

[0062] Comparative Example 2

[0063] (1) 20 g of halloysite nanotubes and 20 g of nano-alumina were added to 40 mL of 1 mol / L hydrochloric acid, respectively, and allowed to stand at room temperature for 18 h. Activated halloysite nanotubes and activated nano-alumina were obtained by filtration, washing, and drying.

[0064] (2) 2 g of triethylamine and 5 g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane were added to 40 mL of toluene in sequence, and ultrasonicated at room temperature for 7 min. Then, 3 g of activated halloysite nanotubes were added and mechanically stirred at 90 °C for 20 h. The modified halloysite nanotubes were obtained by filtration, washing, and drying.

[0065] (3) Add 3 g of γ-(2,3-epoxypropoxy)propyltrimethoxysilane to 90 mL of anhydrous ethanol, ultrasonicate at room temperature for 7 min, then add 2.5 g of activated nano-alumina, mechanically stir at 90 °C for 5 h, filter, wash, and dry to obtain modified nano-alumina.

[0066] (4) 1.5 g of triethylamine was added to 90 mL of anhydrous ethanol, and ultrasonicated at room temperature for 7 min. Then, 1 g of modified halloysite nanotubes and 1.5 g of modified nanoalumina were added in sequence. The mixture was mechanically stirred at 110 °C for 10 h. After filtration, washing, and drying, a halloysite nanotube / nanoalumina composite was obtained.

[0067] (5) 35 g of polylactic acid, 8 g of coconut shell fiber and 2 g of halloysite nanotube / nanoalumina composite were added into an internal mixer in sequence. The internal mixing temperature was 190 °C, the internal mixing pressure was 0.3 MPa, the rotor speed was 50 r / min, and the internal mixing time was 10 min. The internal mixing material was cooled and crushed to obtain the finished product.

[0068] The halloysite nanotubes have an inner diameter of 10-30 nm, an outer diameter of 40-70 nm, and a length of 0.5-1.5 μm.

[0069] The diameter of the nano-aluminum oxide is 30 nm.

[0070] Comparative Example 3

[0071] (1) 0.8 g of dicumyl peroxide and 1 g of maleic anhydride were added to 100 mL of toluene in sequence. The mixture was ultrasonicated at room temperature for 6 min and then mechanically stirred for 15 min. 20 g of coconut shell fiber was then added and mechanically stirred at 90 °C for 5 h. The mixture was filtered, washed, and dried to obtain coconut shell fiber grafted with maleic anhydride.

[0072] (2) 35 g of polylactic acid and 8 g of coconut shell fiber grafted maleic anhydride were added into an internal mixer in sequence. The internal mixing temperature was 190 °C, the internal mixing pressure was 0.3 MPa, the rotor speed was 50 r / min, and the internal mixing time was 10 min. The internal mixing material was cooled and crushed to obtain the finished product.

[0073] Comparative Example 4

[0074] (1) 0.8 g of dicumyl peroxide and 1 g of maleic anhydride were added to 100 mL of toluene in sequence. The mixture was ultrasonicated at room temperature for 6 min and then mechanically stirred for 15 min. 20 g of coconut shell fiber was then added and mechanically stirred at 90 °C for 5 h. The mixture was filtered, washed, and dried to obtain coconut shell fiber grafted with maleic anhydride.

[0075] (2) 20 g of halloysite nanotubes and 20 g of nano-alumina were added to 40 mL of 1 mol / L hydrochloric acid, respectively, and allowed to stand at room temperature for 18 h. Activated halloysite nanotubes and activated nano-alumina were obtained by filtration, washing, and drying.

[0076] (3) 2 g of triethylamine and 5 g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane were added to 40 mL of toluene in sequence, and ultrasonicated at room temperature for 7 min. Then, 3 g of activated halloysite nanotubes were added and mechanically stirred at 90 °C for 20 h. The modified halloysite nanotubes were obtained by filtration, washing, and drying.

[0077] (4) 3 g of γ-(2,3-epoxypropoxy)propyltrimethoxysilane was added to 90 mL of anhydrous ethanol, ultrasonicated at room temperature for 7 min, and then 2.5 g of activated nano-alumina was added. The mixture was mechanically stirred at 90 °C for 5 h, filtered, washed, and dried to obtain modified nano-alumina.

[0078] (5) 35 g of polylactic acid, 8 g of coconut shell fiber grafted with maleic anhydride, 0.8 g of modified halloysite nanotubes and 1.2 g of modified nano-alumina were added into an internal mixer in sequence. The internal mixing temperature was 190 °C, the internal mixing pressure was 0.3 MPa, the rotor speed was 50 r / min, and the internal mixing time was 10 min. The internal mixing material was cooled and crushed to obtain the finished product.

[0079] The halloysite nanotubes have an inner diameter of 10-30 nm, an outer diameter of 40-70 nm, and a length of 0.5-1.5 μm.

[0080] The diameter of the nano-aluminum oxide is 30 nm.

[0081] Comparative Example 5

[0082] (1) 0.8 g of dicumyl peroxide and 1 g of maleic anhydride were added to 100 mL of toluene in sequence. The mixture was ultrasonicated at room temperature for 6 min and then mechanically stirred for 15 min. 20 g of coconut shell fiber was then added and mechanically stirred at 90 °C for 5 h. The mixture was filtered, washed, and dried to obtain coconut shell fiber grafted with maleic anhydride.

[0083] (2) 20 g of halloysite nanotubes were added to 40 mL of 1 mol / L hydrochloric acid and allowed to stand at room temperature for 18 h. The activated halloysite nanotubes were obtained by filtration, washing, and drying.

[0084] (3) 2 g of triethylamine and 5 g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane were added to 40 mL of toluene in sequence, and ultrasonicated at room temperature for 7 min. Then, 3 g of activated halloysite nanotubes were added and mechanically stirred at 90 °C for 20 h. The modified halloysite nanotubes were obtained by filtration, washing, and drying.

[0085] (4) 35 g of polylactic acid, 8 g of coconut shell fiber grafted maleic anhydride, and 2 g of modified halloysite nanotubes were added into an internal mixer in sequence. The internal mixing temperature was 190 °C, the internal mixing pressure was 0.3 MPa, the rotor speed was 50 r / min, and the internal mixing time was 10 min. The internal mixing material was cooled and crushed to obtain the finished product.

[0086] The halloysite nanotubes have an inner diameter of 10-30 nm, an outer diameter of 40-70 nm, and a length of 0.5-1.5 μm.

[0087] Comparative Example 6

[0088] (1) 0.8 g of dicumyl peroxide and 1 g of maleic anhydride were added to 100 mL of toluene in sequence. The mixture was ultrasonicated at room temperature for 6 min and then mechanically stirred for 15 min. 20 g of coconut shell fiber was then added and mechanically stirred at 90 °C for 5 h. The mixture was filtered, washed, and dried to obtain coconut shell fiber grafted with maleic anhydride.

[0089] (2) Add 20 g of nano-alumina to 40 mL of 1 mol / L hydrochloric acid, let it stand at room temperature for 18 h, filter, wash, and dry to obtain activated nano-alumina;

[0090] (3) Add 3 g of γ-(2,3-epoxypropoxy)propyltrimethoxysilane to 90 mL of anhydrous ethanol, ultrasonicate at room temperature for 7 min, then add 2.5 g of activated nano-alumina, mechanically stir at 90 °C for 5 h, filter, wash, and dry to obtain modified nano-alumina.

[0091] (4) 35 g of polylactic acid, 8 g of coconut shell fiber grafted maleic anhydride and 2 g of modified nano-alumina were added into an internal mixer in sequence. The internal mixing temperature was 190 °C, the internal mixing pressure was 0.3 MPa, the rotor speed was 50 r / min, and the internal mixing time was 10 min. The internal mixing material was cooled and crushed to obtain the finished product.

[0092] The diameter of the nano-aluminum oxide is 30 nm.

[0093] The products obtained from the three groups of embodiments and six groups of comparative examples were subjected to a tensile strength test according to GB / T1040.2-2022, an impact strength test according to GB / T1043.2-2018, a heat deformation temperature test according to GB / T1634.2-2019, with a heating rate of 120°C / h, and a melt index test according to GB / T3692-2000, with a test temperature of 190°C and a test pressure of 2.16 kg. The test results are shown in the following table.

[0094]

[0095] From the test results of the embodiments and comparative examples, it can be seen that a polylactic acid composite material with high tensile strength, impact strength and heat deformation temperature, low viscosity and complete biodegradability can be prepared by melt blending polylactic acid as a raw material, halloysite nanotube / nanoalumina composite as a nucleating agent, and coconut shell fiber grafted maleic anhydride as a reinforcing agent, filler and compatibilizer.

[0096] The foregoing description shows and describes preferred embodiments of the present invention. As previously mentioned, it should be understood that the present invention is not limited to the form disclosed herein and should not be construed as excluding other embodiments. Instead, the present invention can be used in various other combinations, modifications, and environments and can be modified within the scope of the inventive concept described herein by the teachings above or by techniques or knowledge in the relevant art. Modifications and variations made by those skilled in the art without departing from the spirit and scope of the present invention are intended to be within the scope of the appended claims.

Claims

1. A method for preparing a heat-resistant polylactic acid composite material, characterized in that: The heat-resistant polylactic acid composite material is prepared by melt blending polylactic acid as a raw material, halloysite nanotube / nanoalumina composite as a nucleating agent, and coconut shell fiber grafted maleic anhydride as a reinforcing agent, filler and compatibilizer; The following steps are involved: (1) 0.6-1 g initiator and 0.8-1.2 g maleic anhydride were added to 80-120 mL organic solvent in sequence, ultrasonicated for 4-8 min, stirred for 10-20 min, and then 15-30 g coconut shell fiber was added. The mixture was stirred at 80-100 °C for 4-6 h. The coconut shell fiber grafted with maleic anhydride was obtained by filtration, washing, and drying. (2) 20 g of halloysite nanotubes and 20 g of nano-alumina were added to 40 mL of 1 mol / L hydrochloric acid, respectively, and allowed to stand for 12 to 24 h. Activated halloysite nanotubes and activated nano-alumina were obtained by filtration, washing, and drying. (3) 1-3 g of triethylamine and 4-6 g of N-(2-aminoethyl)-3-aminopropyltriethoxysilane were added to 30-50 mL of toluene in sequence, and ultrasonicated for 4-10 min. Then, 2-4 g of activated halloysite nanotubes were added and stirred at 80-100 °C for 16-24 h. The modified halloysite nanotubes were obtained by filtration, washing, and drying. (4) Add 2-4 g of γ-(2,3-epoxypropoxy)propyltrimethoxysilane to 80-100 mL of anhydrous ethanol, ultrasonicate for 4-10 min, then add 2-3 g of activated nano-alumina, stir at 80-100 °C for 4-6 h, filter, wash, and dry to obtain modified nano-alumina. (5) Add 1-2 g of triethylamine to 80-100 mL of anhydrous ethanol, ultrasonicate for 4-10 min, then add 0.5-2 g of modified halloysite nanotubes and 1-2 g of modified nanoalumina in sequence, stir at 100-120 °C for 8-12 h, filter, wash, and dry to obtain a halloysite nanotube / nanoalumina composite. (6) 30-40 g of polylactic acid, 5-10 g of coconut shell fiber grafted maleic anhydride and 1-3 g of halloysite nanotube / nanoalumina composite were sequentially added into an internal mixer at a mixing temperature of 170-210 °C, a mixing pressure of 0.2-0.4 MPa, a rotor speed of 40-60 r / min and a mixing time of 8-12 min. The mixed material was cooled and crushed to obtain the heat-resistant polylactic acid composite material.

2. The method according to claim 1, wherein: The initiator is any one of azobisisobutyronitrile, benzoyl peroxide and dicumyl peroxide.

3. The method according to claim 1, wherein: The organic solvent is any one of toluene, chloroform, N,N-dimethylformamide and acetone.

4. The method according to claim 1, wherein: The halloysite nanotubes have an inner diameter of 10-30 nm, an outer diameter of 40-70 nm, and a length of 0.5-1.5 μm.

5. The method according to claim 1, wherein: The diameter of the nano-aluminum oxide is 30 nm.

6. A heat-resistant polylactic acid composite material prepared by the method according to any one of claims 1 to 5.

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