Effects Of Surface Modified Bamboo Powder Cellulose Microcrystals On The Flammability Of Epoxy Resin And Polylactic Acid

Epoxy resin(EP),as a typical thermosetting material,is used in carbon fiber composites,adhesives,structural laminates and electrical equipment,due to its excellent mechanical properties,chemical resistance and strong surface adhesion.Polylactic acid(PLA)is a green polymer obtained from lactic acid,which possess excellent mechanical properties,good processability and high transparency,PLA demonstrates great potential for industrial production.Although EP and PLA have quite good application prospects,the inherently flammable properties of polymer materials give them safety hazard during use.Therefore,flame retardant modification of EP and PLA is a necessary means for their wide application.Ammonium polyphosphate(APP)is the most widely used and highly efficient intumescent flame retardant.It is usually used as an acid source for intumescent flame retardant systems.Intumescent flame retardant systems often need to be used in combination with carbon sources,such as pentaerythritol.Pentaerythritol has the disadvantage of poor compatibility.In this paper,cellulose microcrystals are used as carbon sources.As a green renewable resource,it can not only increase the expanded carbon layer,but also reduce the influence of flame retardants on mechanical properties.Cellulose microcrystals(CMC)were extracted from bamboo powder by acid hydrolysis method,and their sizes were observed by scanning electron microscopy.Flame-retardant EP and flame-retardant PLA were prepared by introducing CMC and APP into the resin matrix by additive processing.The flame retardant properties and mechanical properties of the flame retardant EP and flame retardant PLA were studied.The flame retardant mechanism and mechanical strengthening mechanism of composites were characterized.The main results of this work are as follows:(1)Cellulose crystallite is a linear polysaccharide with glycosidic bond,which is surface-modified by 3-aminopropyltriethoxysilane coupling agent(KH550).The micro structure of KH550-CMC was characterized by scanning electron microscopy and infrared spectroscopy.CMC and APP were introduced into the epoxy resin by casting to obtain a flame-retardant EP composite.The combustion test results show that 7.3 wt%ammonium polyphosphate(APP)and 3.7 wt%KH550-CMC in EP achieves a maximum limited oxygen index of 28.9%,a vertical burning grade of V-0,and a significant reduction in peak heat release reta.The peak heat release rate was reduced from 1055 kW/m2 to 286 kW/m2.The introduction of KH550-CMC can also improve the mechanical properties of EP composites.Its tensile strength is higher than other EP composites reaching 57.36 MPa,and its impact strength is increased to 3.25 KJ/m2,which is higher than that of pure EP.Electron microscopy results of the sample section showed that KH550-CMC has better compatibility with rasin than CMC and bamboo powder,and the interface bonding force is stronger.The addition of CMC allows the material to have a better char formation effect,promotes the formation of a continuous dense carbon layer,and can be esterified and crosslinked with APP to function in the condensed phase.(2)The CMC was modified with an aluminate coupling agent,and then ACA-CMC and APP were compounded into an epoxy resin matrix to prepare a flame-retardant EP composite.Through combustion test,mechanical properties,electron microscopy characterization,etc.,the following results were obtained:the combination of flame retardant APP and CMC can synergistically improve the flame retardancy of epoxy resin.The pHRR of the composite containing 5g ACA-CMC is 290 kW/m2,a 72%decrease compared to that of pure EP.The APP/ACA-CMC combination is more effective than APP or APP/CMC in improving flame retardancy and mechanical properties.The impact strength of the sample containing ACA-CMC was higher than that of the sample containing KH550-CMC,reaching 3.70 KJ/m2.The cross-sectional microstructure of the sample also indicates that the modified CMC has better interfacial bonding with the matrix.The SEM characterization of the carbon residue of the sample shows that the introduction of ACA-CMC makes the epoxy resin composite have good char forming effect.The APP/ACA-CMC/EP sample carbon residue effectively promotes the continuous dense carbon layer,which is more stable than other samples,thus effectively protecting the matrix.(3)The methacrylic acid modified cellulose microcrystals(MA-CMC)and ammonium polyphosphate were introduced into the PLA by melt blending,and the flame retardancy and mechanical properties of the polylactic acid were improved.The flame retardancy of the material was characterized by Cone Calorimeter Test(CONE),Vertical UL-94 Test and Limiting Oxygen Index(LOI).The results show that the LOI of the PLA composite containing 3%MA-CMC and 7%APP can reach 26.8%,and the V-0 rating can be passed in the UL-94 test.It also reduces the pHRR from 556 kW/m2 to 456 kW/m2 and forms a continuous,dense,uniform carbon residue during the combustion process,forming a barrier to prevent further combustion.Raman spectroscopy analysis shows that 3%of MA-CMC can make the residual carbon of PLA composites have higher degree of graphitization,and the carbon residue structure is more stable.At the same time,the tensile strength and impact strength of the PLA composite material were significantly improved,and the unnotched impact strength was 8.16 KJ/m2.