Study On The Effect Of Transcrystallinity Structure Induced By Nano-TiO₂ On The Interface Structure And Interface Mechanical Properties Of Bamboo Plastic Composite

Thermoplastic polymer material is a widely used packaging material with the characteristics of light weight,high transparency,good barrier and high toughness.However,as a load-bearing material to prepare plastic pallets or turnover boxes,its strength and stiffness are not enough,so that it is difficult to recycle for many times.Due to the low price,high specific modulus and strength,renewable and biodegradable,natural fibers have been used as mechanical reinforcing fillers to prepare natural fiber/thermoplastic polymer composites(NFTPC)which has broad application prospects in the production of packaging boxes,packaging pallets,turnover boxes,and even military equipment packaging boxes.However,natural fiber as reinforcement is usually underestimated due to the poor compatibility and the lower load transfer ability of the interface between hydrophilic plant fiber and hydrophobic polymer.Physical or chemical treatment of natural fiber or plastic matrix,and adding coupling agents such as maleated polyolefin and silane are common methods to improve the interfacial compatibility of NFTPC.However,the above methods may significantly increase the cost or bring environmental pollution problems.In recent years,a“green interfacial reinforcement”technology has attracted the attention of researchers,i.e.inducing semi crystalline polymers(such as polypropylene,PP)to form a special interfacial crystal structure namely transcrystallity(TC)on the surface of natural fiber.The growth control and interface interaction mechanism of TC are quite complex which is closely related to the specific composite system.In this study,the TC of bamboo fiber/polypropylene(PP)system namely bamboo plastic composite was used as the object.Nano-TiO₂particles modified by silane coupling agent KH-570 were deposited on the surface of bamboo fiber to induce the TC at the composite interface.And the effect of TC on the interface structure and interface mechanical properties of composite was explored.Firstly,by the single factor experiment,the factors influencing the growth of TC were optimized to achieve the purpose of efficient and controllable growth of TC.Secondly,the gradient samples of bamboo plastic composite from bamboo fiber to interface phase to PP matrix were studied to explore the effects of TC on the crystal structure and thermal performance of interface phase and matrix phase of composite.Finally,the effect of TC on the load transfer efficiency of bamboo plastic composite interface was explored quantitatively from the interface mechanical scale.Gradient changes of local mechanical properties of composite from bamboo fiber to interface phase to PP matrix were explored from the nano-mechanical scale.The main results are as follows:(1)The highly dispersed nano-TiO₂particles modified by silane coupling agent KH-570 were deposited on the surface of bamboo fiber,which changed the surface of bamboo fiber bundle from smooth to rough.There was interaction between the hanging bonds on the surface of the nano-TiO₂and the molecular chains of bamboo fiber,and the water absorbed by nano-TiO₂from air could combined with the hydroxyl groups of bamboo fiber to form hydrogen bonds.Natural bamboo fiber had low heterogeneous nucleation ability and thus could not induce the TC formation at the composite interface.Nevertheless,after nano-TiO₂particles deposited on its surface,the heterogeneous ability could be significantly improved,and the complete TC structure could be induced finally.Based on the average growth rate of TC and the final width of TC,the optimal growth conditions of TC are as follows:the crystallization temperature was 135^oC,the concentration of nano-TiO₂modified solution was 0.035 mol/L and the isothermal crystallization time was 19 min.(2)The formation of interfacial TC made the structure and properties of bamboo plastic composites vary from interfacial phase to PP matrix.Due to the strong adsorption of bamboo fiber bundles of the 0.035 mol/L nano-TiO₂modified bamboo fiber/PP composite(0.035-BF/PP)on PP molecular chains,the spherulite density near fiber was lower than that of matrix.The crystallinity increased from near the fiber bundle side to the PP matrix,but the crystal form was consistent,which was?monoclinic.The addition of bamboo fiber bundle and the formation of TC at the interface made the symmetrical expansion vibration peak of-CH₂move to the direction of low wave number,the bending vibration peak of-CH₂be divided into two peaks and the-CH₃bending vibration peak move to the high wave number direction.The vibration peak in matrix phase was consistent with that of pure PP.The addition of bamboo fiber bundle and the formation of TC made the crystal melting temperature(T_m)from near the fiber bundle to PP matrix gradually increase,while the non-isothermal recrystallization temperature(T_c)decrease,and the thermal stability decrease.(3)The formation of interfacial TC improved the load transfer efficiency of bamboo plastic composite.Under the condition of isothermal crystallization for 19 min,The IFSS increased by 48.9%caused by the formation of TC.And the TC width was positively correlated with IFSS.The formation of interfacial TC was beneficial to improve the bonding between fiber reinforced phase and PP phase and the local mechanical properties of interface phase.The results of static nonoindentation showed that the elastic modulus and hardness of 0.035-BF/PP forming interfacial TC both were the bamboo fiber bundle>PP matrix phase?interface phase,the properties change rules of bamboo fiber/PP composite(BF/PP)without interfacial TC were bamboo fiber bundle>PP matrix>interface phase.Dynamic nanoindentation(nano-DMA)modulus imaging analysis showed that there was a slow transition region in the composite modulus and storage stiffness curves of the 0.035-BF/PP,while there was phase separation in the mechanical curves of BF/PP.Moreover,the composite modulus and storage stiffness of the former were uniformly distributed in the interface phase,and were significantly higher than those of the latter.In conclusion,the in-situ deposition of nano-TiO₂on the surface of bamboo fiber bundle can significantly improve its heterogeneous nucleation ability,induce the formation of TC structure,thus improving the bonding between fiber reinforced phase and PP phase and the local mechanical properties of interface phase.The improvement of interfacial properties is expected to make natural fibers play a full role in mechanical reinforcement.And this study provides theoretical support and technical guidance for the preparation of bamboo plastic pallets and turnover boxes with light weight and high strength.