Study On Preparation And Properties Of Crop Straw/PLA Degradable Biomass Composites

With the oil resources scared gradually and the aggravation of"white pollution",the new biomass materials was sought to replace the plastic products which was essential in daily life.PLA（polylactic acid）is the second highest consumption biomass plastic in the world,which has been widely used in medicine,packaging and other fields.PLA which is an emerging green and environmentally friendly material has excellent properties such as renewability,degradability,environmental friendliness,high strength and well antibacterial property.However,PLA is expensive.In order to reduce the preparation cost of PLA-based composites and expand application fields,sorghum straw,soybean straw,rice straw and corn straw was used to fill PLA respectively in this paper,extrusion injection molding process was adopted to prepare composites,so as to explore the best filling straw and the best ratio;based on Box-Bhenken experimental design method,the process of composite materials was discussed and the best preparation process parameters was determined;the changes of physical,mechanical and thermal properties of composites during hydrothermal aging and thermal aging were studied separately.;degradation experiments in different types of soil were carried out to study the degradation performance of composites by different soils.The main conclusions are as follows:（1）The optimal filling straw and ratio of PLA-based composites were determined.Sorghum straw was selected as the filler fiber in the research of composite ratio.When the ratio of PLA to sorghum straw is 7:3,the composite has the best comprehensive properties,and the tensile strength,flexural strength,impact strength,tensile strain at break and contact angle are 34.05 MPa,56.29 MPa,7.35KJ/m²,4.4955%and 88.8°respectively.When the mass ratio of straw/PLA composites was 7:3,compared with PLA composites filled with sorghum straw,soybean straw,rice straw and corn straw,soybean straw/PLA biomass composites have the best comprehensive properties,whose tensile strength,flexural strength,impact strength,tensile strain at break,contact angle,24-hour water absorption and hardness of 36.15 MPa,57.01 MPa,9.13 KJ/m^2,5.45%,3.83%and 110.33 HR,respectively;（2）With Box-Bhenken experimental design method,the temperature of extruder cavity,barrel temperature and circulation time in cavity were taken as three factors,and the tensile strength,flexural strength,impact strength,hardness and contact angle of composites were taken as optimization parameters.The quadratic regression equations of five optimization parameters were obtained,and the best factors of preparation technology has been determined as follows:the extruder cavity temperature was 185℃,the barrel temperature was 175℃,and the cycle time was0min.Comprehensive analysis of physical,mechanical,thermal properties and the interfacial bonding quality of composites showed that the cycle time in the cavity has the most significant influence on the properties of composite materials.Under the optimum conditions,the tensile strength,flexural strength,impact strength,water absorption and contact angle of soybean straw/PLA biomass composites were 41.86MPa,74.24 MPa,9.53 KJ/m2,3.16%and 88.85°respectively.（3）The soybean straw/PLA composites prepared under the optimum technological parameters were subjected to three-period hydrothermal aging test at three temperatures（23℃,40℃,60℃）,with one period every three days.The hardness of composites decreased with the increase of hydrothermal temperature,while the water absorption and expansion rate increased with the increase of hydrothermal aging temperature.The water absorption and expansion rate of composites were the highest at 60℃,which were 13.31%and 15.30%,respectively.The tensile strength and flexural strength of the composites decreased continuously with the increase of hydrothermal aging time,while the impact properties increased in one period under hydrothermal aging at 23℃and 40℃,and showed a downward trend on the whole.The higher the hydrothermal aging temperature was,the more obvious the mechanical properties of the composites decreased.By observing the tensile cross section and micro-morphology of surface of the composites,the phenomena of fiber pull-out,delamination and cracks appeared gradually with the increase of hydrothermal temperature and aging time.Through the infrared spectrum,it was found that the hydrolysis of PLA resulted in the decrease of aldehyde groups and ester groups,and the infrared absorption peaks of soybean straw/PLA composites did not shift obviously after hydrothermal aging,but sharpened.After hydrothermal aging,the temperature required for soybean straw/PLA composites to reach the main pyrolysis weight loss stage increased,and hydrothermal aging at 40℃had little effect on the thermal stability of the composites;（4）The thermo-oxidative aging experiments of soybean straw/PLA composites at 40℃,60℃,80℃and 100℃were studied,and the invalid condition was that the tensile strength of the composites decreased to 50%.The results showed that the toughness of the composites after thermo-oxidative aging generally showed a downward trend.The tensile strength,flexural strength and hardness of the composites continued to increase at 40℃,and the two properties increased by 30.53%,14.28%and 2.14%respectively after three periods（42 d）.After thermo-oxidative aging at other temperatures,the hardness of the composites decreased to varying degrees.After thermo-oxidative aging at 40℃and 60℃,the infrared spectrum of the composites did not have any obvious change.At 80℃and 100℃,the infrared absorption peak was sharp and the absorption intensity increased.This phenomenon was similar to the infrared spectrum of the composites after hydrothermal aging,which showed that soybean straw could be pyrolyzed to a certain extent under the condition of thermo-oxidative above 80℃.In the thermal stability test,TG curves almost coincide,and thermo-oxidative aging did not have any significant effect on the thermal stability of composites;（5）The degradation of soybean straw/PLA composites buried in loam,clay and sand was studied.The degradation performance of composites in loam and clay was the best,with the mass loss rate of 7.52%and 7.44%respectively.The tensile strength retention rate of composites degraded by clay was the lowest,which decreased by57.87%,the flexural strength and impact strength of the composites degraded by loam decreased most obviously by 55.15%and 60.34%;the moisture retention rate of sand is poor,which made it difficult for microorganisms to survive and had the weakest degradation ability.The mass loss rate,tensile properties,flexural properties and impact strength decreased by 0.57%,41.6%,26.91%and 51.90%respectively in three periods（63 d）.In the infrared spectrum,the characteristic absorption peak of cellulose disappeared,indicating that the degradation degree of exposed soybean straw in the composite material was higher,and the characteristic absorption peaks such as carboxyl group were enhanced due to hydrolysis and chain breakage of PLA ester group.Because of the high degradation degree and the lower pyrolysis temperature of soybean straw,the main pyrolysis weight loss temperature region of composites shifted to higher temperature after degradation.By observing the micro-morphology of the degraded composites,the surface morphology of the composites in loam and clay was seriously damaged.After three periods of degradation in clay,the surface of the composites could hardly form a continuous state,while the surface morphology of the composites in sand maintained better,and the change of micro-morphology was consistent with the change rule of degradation quality loss.