Preparation And Properties Of Polylactic Acid/wheat Straw Fiber Composites Based On Montmorillonite Intercalation Structure

As an environmentally friendly and efficient material,biomass fibers/thermoplastic composites have been widely used in homes and squares.However,some problems,such as poor thermal stability,poor flame retardancy and low ratio of strength to weight have stopped the development of them.In our work,12-aminododecanoic acid(ALA)and dimercaptodimethylammonium chloride(DDAC)surface modified nano-montmorillonite(MMT)particles were used as intercalation agents.Two-step organically modified nano-MMT/polylactic acid(PLA)/wheat straw fiber composites were obtained via simple mixing and melt blending.We investigated the mechanical properties,physical properties and flame retardant properties of PLA/wheat fiber composites were investigated by using organic modified nano-MMT particles as intercalation agents.On this basis,wheat straw fiber was modified by dopamine coating,and the coating effect and mechanism of wheat straw fiber were discussed.At the same time,nano-MMT particles were organically modified by ALA,DDAC combined with y-aminopropyltriethoxysilane(KH550),respectively,and the organic modified nano-MMT modified PLA/wheat straw fiber composites were prepared.We investigated the mechanism of surface modification of nano-MMT paticles,the intercalation and interfacial compatibility enhancement of organically modified nano-MMT particles.It aims to enhance the interfacial bonding strength between PLA and wheat straw fiber,improve the physical and mechanical properties of biomass composite,impart flame retardant properties,establish and improve biomass fiber modification system,and be a large biomass resource.Theoretical basis and technical support are providec for efficient use of scale and optimization of performance of biomass fiber/thermoplastic composite products.The main parts were detailed as hereunder mentioned:(1)The nano-MMT particles were organically modified by a two-step modification method.Meanwhile,the dopamine was hydrolyzed to form a polydopamine coating to modify the surface of the wheat straw fiber.The particle size of the nano-MMT particles,as results shown,is reduced by 4～6 μm after the combination of organic long-chain carbon quaternary ammonium salt and silane coupling agent,which is 360%smaller than the unmodified MMT particle size.And the size distribution is uniform,and the dispersibility is improved.The organosilicon functional group of aminosilane KH550 reacts with the hydroxyl group of MMT to form a hydrogen bond and condenses into a covalent bond on the surface of SiO-MMT,and the XRD diffraction peak shifts to smaller angle,the layer spacing becomes 1.76 nm,which is 76%larger than that of the unmodified MMT.After the dopamine pretreatment of wheat straw fiber,the appearance of infrared characteristic absorption peak at 1076 cm-1 and 1458 cm-1 indicated that the fiber was successfully coated and modified,the surface roughness of the fiber increased,and the contact area was increased.(2)MMT modified PLA/wheat straw fiber composites were fabricated by mixing solution method.The dispersing of MMT in PLA matrix was investigated.The intercalation mechanism of MMT in composites and the mechanical properties of composite materials,the impact of flame retardant properties.It is found that the organic modifier DDAC two-step modified MMT can disperse better in the PLA matrix than the sodium-based MMT and form the structure of intercalation.After MMT secondary modification,the tensile,flexural and impact strength increased by 267%,189%and 173%compared with the composites without MMT.The mechanical properties have been significantly improved.Modified MMT has a certain influence on the thermal stability of the materials,but the influence is tiny.Through the analysis of the flame retardant properties of the composites,the PHRR of the composites treated by the two-step modified MMT was 247.4 KW/m2,which was 42.1%lower than that of the control group,indicating that the organic modified MMT can effectively suppress smoke and hinder oxygen.Through,the composite material has a good flame retardant.(3)The MMT/PLA/wheat straw intercalation composites were produced via melt intercalation way.The mechanical properties and thermal decomposition of the composites were investigated by the unmodified MMT,ALA one-step modified MMT and ALA，KH550 synergistic two-step modified MMT.The impact of kinetics.The results demonstrated that the treatment of PLA with organic MMT particles could improve the mechanical properties of the composites,compared with the control experiment.The mechanical properties of the composites treated by ALA and KH550 two-step modified nano-MMT particles were the most significant.When the two-step modified MMT treated composites,the flexural,tensile and i mpact strength increased by 72.5%,119.5%and 98.1%,respectively,when not added.The organic treatment of the two-step modified MMT can improve the heat resistance of the composite material.The initial degradation temperature(T0)and the termination degradation temperature(Tf)are also enhanced.In addition,the two-step modified MMT layer has good barrier properties,which can hinder the thermal decomposition of the product,help to delay the thermal degradation rate and improve the heat resistance of the material,which indirectly indicates that PLA and the two-step modified MMT form embedded nanometers composite material.From the thermal decomposition kinetics,the thermal decomposition process includes two steps,namely the decomposition of surfactants and the decomposition of PLA/wheat fiber composites,but the OMMT modification has little effect on the thermal properties of the composites in the second stage.Compared with the direct hybrid intercalation method,the melt intercalation method utilizes the advantages of organic MMT and PLA high-temperature intercalation,which grratly improves the mechanical properties of the prepared composites,but the thermal stability changes are not obvious.