Effect Of Water Absorption On The Long-Term Properties Of Flax Fiber Reinforced Composites

In recent years,with the popularization of environmental protection concepts and the requirements of low-carbon life,plant fiber as reinforcement of composite materials has become a new research direction in the field of new materials.Compared with traditional inorganic reinforcements,such as glass fiber and carbon fiber,plant fiber has many advantages: being affordable,abundant,environment-friendly,renewable,readily biodegradable,high specific properties and human-agreeable.Therefore,the composite materials with plant fiber as reinforcement have aroused strong interest and extensive attention in academia and industry.But plant fiber is mainly composed of easily combustible substances such as cellulose,lignin and pectin,leading to their flammable properties.A lot of research work has shown that the flame retardant treatment of plant fiber and their composites can make their flame retardant properties comform the flame retardant requirements of industrial applications.On the other hand,the existence of hydroxyl groups and other polar groups in plant fiber make it has inherent hydrophilicity,and the composites reinforced by plant fiber are easy to absorb water,resulting in degradation of mechanical properties.The flame retardant treatment may further deteriorate the long-term performance of the plant fiber reinforced composites.When the flame retardant plant fiber reinforced composites are exposed to humid outdoor conditions,the mechanical properties and flame retardant properties of the flame retardant plant fiber reinforced composites may further deteriorate due to water absorption,resulting in potential safety hazards in the long-term use of the flame retardant plant fiber reinforced composites.Therefore,it is very important to study the water aging of flame retardant plant fiber reinforced composites.In this paper,the water absorption aging of flame retardant plant fiber reinforced composites was studied.With unidirectional flax fiber reinforced phenolic resin matrix composites as the research object,three flame retardant modification methods were used to modify them,namely,dimethyl methylphosphonate(DMMP)or 9,10 dihydro9 oxa-10 phenanthroline-10 oxide(DOPO)physical impregnation method,DMMP chemical grafting method,nano montmorillonite(MMT)and DMMP or DOPO synergistic modification method.Firstly,the flame retardancy of the composite was studied.The flammability of the composite was evaluated by vertical combustion test and limiting oxygen index(LOI)test.The thermal stability,heat release and smoke release of the composite were analyzed by thermogravimetric analysis(TGA)and cone calorimetry(CONE)to reveal the flame retardancy mechanism;Moreover,the research on the water absorption performance of flame retardant flax fiber reinforced phenolic resin composite was carried out.The water absorption capacity of the composite was measured by weighing method,and the influence of different flame retardant modification methods on the water absorption performance of the composite was studied.Then the influence of water absorption on the flame retardancy and mechanical properties of flame retardant flax fiber reinforced phenolic resin composite was studied.The cross-sectional morphology observation,element analysis,vertical combustion,LOI,TGA,CONE,tensile and interlaminar shear properties test and other characterization and testing methods were used to analyze the changes of surface morphology,cross section elements,flammability,thermal stability,heat and smoke release,and mechanical properties of the composites before and after water absorption.Finally,based on the water absorption aging mechanism of flame retardant flax fiber reinforced composites,the residual strength correction model was proposed,and the correlation between the flame retardant properties and long-term mechanical properties of flax fiber reinforced phenolic composites was established.It lay the foundation for the study of long-term performance prediction methods of flame retardant plant fiber reinforced composites.The results show that the physical impregnation of flax fiber with hydrophilic flame retardant DMMP or hydrophobic flame retardant DOPO can greatly improve the vertical combustion performance,LOI,thermal stability and carbon residue rate of flax fiber reinforced phenolic composites,reduce the heat release and smoke release,improve the flame retardancy of the composites.However,during the water absorption process of the composite treated by this method,the flame retardant is easy to precipitate,which causes the decline of the flame retardancy of the composite.At the same time,the release of flame retardant provides more storage space for water molecules to accelerate the water absorption of the composite,further reducing the mechanical properties of the composite.Compared with DOPO,DMMP has better flame retardancy due to its higher phosphorus content,but its hydrophilicity will cause the composite to absorb more water,and its flame retardancy and mechanical properties will decline more after water absorption.The modified flax fiber reinforced phenolic composite by grafting DMMP can not only improve the flame retardant efficiency of DMMP,but also effectively reduce the hydrophilic hydroxyl groups on flax fiber,thereby reducing the water absorption of the composite.Because DMMP is chemically grafted on plant fibers,it is not easy to precipitate during the water absorption process of the composite,thus slowing down the decline of the flame retardant and mechanical properties of the composite after water absorption.Because MMT forms a mixed structure of peeling and intercalation in phenolic resin,it not only has synergistic flame retardant effect with DMMP or DOPO,but also can reduce the water absorption of the composite by reducing the water absorption of the resin.Which further slows down the decline of the flame retardancy and mechanical properties of the flame retardant composite after water absorption.The residual strength model considering damage can effectively predict the tensile strength of flame retardant flax fiber reinforced phenolic composites by introducing the flame retardant property degradation rate parameter.The validity of the model is proved by comparing with the experimental data.In conclusion,the water aging of flame retardant plant fiber reinforced composites is studied experimentally and theoretically in this paper,the degradation mechanism of flame retardancy and mechanical properties of flame retardant plant fiber reinforced composites after water absorption is clarified.The water absorption mechanism of flame retardant plant fiber reinforced composites is determined.The research results in this paper provide a basis for improving the flame retardancy and durability of plant fiber reinforced composites,and lay the foundation for their large-scale applications in aerospace,automotive and civil infrastructure.