Preparation method of high-rate foamed polylactic acid (PLA) sheet
A preparation method of a high-rate foamed polylactic acid (PLA) sheet includes first-stage extrusion, second-stage extrusion, and foamed sheet extrusion. The method requires the following raw materials in parts by mass: 88 to 94 parts of PLA, 1 to 2 parts of a nucleating agent, 2 to 5 parts of a foaming agent, and 2 to 5 parts of an additive. The new method effectively solves the problems of low foaming rate, low strength, and the like in the industrial production using carbon dioxide, and a prepared PLA sheet with high foaming rate and excellent surface performance can be used in the fields of food packaging, disposable fully-degradable lunch boxes, and the like.
1. A preparation method of a high-rate foamed polylactic acid (PLA) sheet, comprising first-stage extrusion, second-stage extrusion, and foamed sheet extrusion, wherein the first-stage extrusion is conducted with the following raw materials in parts by mass: 88 to 95 parts of PLA, 1 to 2 parts of a nucleating agent, 2 to 5 parts of a foaming agent, and 2 to 5 parts of an additive; and the foaming agent is a fluorine-containing compound, wherein the foaming agent is any one from the group consisting of difluoromonochloromethane, trifluorotrichloroethane, and tetrafluorodichloroethane, and wherein the high-rate foamed PLA sheet has a foaming rate of 13 to 16, a tensile strength of 80 Mpa to 88 Mpa, and a bending strength of 120 Mpa to 128 Mpa.
2. The preparation method of the high-rate foamed PLA sheet according to claim 1 , wherein the additive is nitrogen; and the nucleating agent is talcum powder.
3. The preparation method of the high-rate foamed PLA sheet according to claim 1 , wherein the foamed sheet extrusion comprises: after a melt at an outlet of the first-stage extrusion has a pressure of 12 Mpa to 15 Mpa and a temperature of 190° C. to 200° C. and the melt at an outlet of the second-stage extrusion has a pressure of 3 MPa to 6 MPa and a temperature of 150° C. to 160° C., setting a screw speed of a first-stage screw extruder to 20 r/min to 50 r/min and setting a screw speed of a second-stage screw extruder to 10 r/min to 20 r/min to extrude a foamed sheet through a sheet die.
4. The preparation method of the high-rate foamed PLA sheet according to claim 3 , wherein in the first-stage extrusion, a screw of the first-stage screw extruder has a diameter of 135 mm, a length-to-diameter ratio of 36:1, and an initial speed of 5 r/min to 15 r/min; and eleven temperature zones are set in sequence from a feeder to an extruder neck of the first-stage screw extruder, with a temperature range of 180° ° C. to 200° ° C.
5. The preparation method of the high-rate foamed PLA sheet according to claim 3 , wherein in the second-stage extrusion, a screw of the second-stage screw extruder has a diameter of 150 mm, a length-to-diameter ratio of 36:1, and an initial speed of 5 r/min to 7 r/min; eight temperature control units are set in sequence from a melt entrance of the second-stage screw extruder first temperature control unit to a die head, with a temperature range of 150° C. to 170° ° C.; and a melt has a pressure of 4 MPa to 6 MPa.
6. The preparation method of the high-rate foamed PLA sheet according to claim 1 , wherein the PLA is a modified foam-grade PLA raw material with a molecular weight of 120,000 to 150,000 and a PLA content of ≥97%.
7. The preparation method of the high-rate foamed PLA sheet according to claim 2 , wherein the nitrogen is injected at a pressure of 10 MPa to 15 MPa and a purity of ≥99.9%; and the foaming agent is injected at a pressure of 16 MPa to 20 MPa.
8. The preparation method of the high-rate foamed PLA sheet according to claim 1 , wherein an extrusion product is cooled and formed by a cool molding device, the extrusion product is split and flattened, the extrusion product is cooled and drawn by a drawing machine at a set drawing speed, and the extrusion product is shaped and wound by a winding machine to finally obtain the high-rate foamed PLA sheet.
9. The preparation method of the high-rate foamed PLA sheet according to claim 8 , wherein the cool molding device is an anti-static cool molding sleeve, and a formed sheet is subjected to annular cooling using two cooling fans and to water-cooling in the anti-static cool molding sleeve; and the drawing machine is a roller drawing machine, with a drawing speed of 8 m/min to 12 m/min.