Preparation And Properties Of Poly (Lactic Acid)/Wood Fiber Foam Composites

In present society,which is experiencing rapid technological development,the unrestrained use of non-renewable resources has led to extreme energy shortages and environmental pollution caused by unsustainable development.Therefore,the development and utilization of renewable and degradable materials has become a hot topic in line with the trend of the times.Polylactic acid(PLA)is known as"green plastic"and is a rare substitute for traditional petroleum-based plastics.The raw materials are fermented from crops such as potatoes and corn,and waste PLA products can be completely degraded into carbon dioxide and water under natural conditions,returning to nature.Wood fiber(WF)is also a pure natural and renewable biomass material that can be obtained from waste wood by crushing,grinding,and screening.Introducing WF as a filler into the PLA matrix to prepare PLA-based wood-plastic composites can reduce the production cost of PLA products and give higher added value to natural wood fibers.At the same time,using the low equipment and operation requirements of the molding foaming technology to introduce a microporous structure into the PLA-based wood-plastic composite material can achieve the toughening,lightweight,and low-cost properties of the material.However,PLA has the disadvantages of being a semi-crystalline polymer with a long chain of fatty groups and poor melt strength,which is not conducive to preparing PLA/WF foamed composite materials with excellent foaming structure and stable mechanical properties.At the same time,the flammability and poor high-temperature resistance of PLA have restricted the further promotion of PLA-based foamed materials.The specific research results are as follows:In this paper,PLA/WF foamed composites were prepared by using PLA and WF as raw materials,azodicarbonamide(AC)as chemical foaming agent,and the flexibility of the composites was modified by adding flexible polymer,poly(adipic acid)/butylene terephthalate(PBAT).The thermal properties,foaming properties and mechanical properties of the material were tested and analyzed by universal testing machine,simply supported beam impact testing machine,differential scanning calorimeter(DSC),thermogravimetric analyzer(TGA),scanning electron microscope(SEM)and melt index instrument.In addition,flame retardant(FR)was added to the foamed material to modify and test the flame retardant performance.(1)PLA was mixed with WF in different proportions as matrix,and AC was used as foaming agent to prepare foamed composites by means of molding and foaming.The test and analysis of PLA/WF foam composites show that when PLA and WF are mixed in the ratio of 8:2,the foam structure is good and the cell size is uniform.The cell density is as high as1.8×105/cm3.The material density reached the lowest value of 0.36 g/cm~3,and the foaming ratio was 3.42 times.The impact strength and compressive strength are 3.16 KJ/m~3 and 4.12MPa,respectively.Its comprehensive mechanical properties are outstanding,and it is suitable for food packaging,cushioning materials,construction and other fields.(2)Based on the previous experimental research,the low melt strength and brittleness of PLA/WF(8:2)composites were further modified by blending with PBAT.The results show that the melt strength of PLA/WF composites modified by PBAT is enhanced,and the cell structure is obviously improved.When the content of PBAT reaches 10%,the foam system shows the most excellent cell morphology,the cell density increases to 3.0x105 cells/cm3,and the average cell size decreases to 61.1μm m..(3)The PLA/WF composites were modified by the expandable graphite(EG)/ammonium polyphosphate(APP)intumescent flame retardant system,and their properties were tested.The results showed that the total amount of flame retardant was 20%,and when EG/APP=3:1,the oxygen index of the composites reached the highest value of 38.1%,and reached V0 level through UL-94 test.The oxygen index of APP and EG added alone is relatively low,indicating that EG and APP have synergistic effect,which is mainly because polyphosphates produced by APP decomposition make the carbon layer formed by EG more compact and block oxygen and heat source.The addition of flame retardant greatly controlled the heat release rate,total heat release and smoke release of PLA/WF foamed composites,effectively improved the defect of poor flame retardancy of PLA-based composites and reduced the risk of fire.