Moisture Absorption Properties Of Interface In Bamboo Strips/Vinyl Ester Composites And Its Interface Characterization By AFM

As the improvement of the consciousness to build an eco-friendly society, natural fibres reinforced composites have been recently attracted more and more attention. Bamboo is a natural plant with the advantages of resource abundance, high mechanical properties, biodegradability, and etc. The development of bamboo reinforced composites can meet the requirements of environment protection as well as improve the utilization rate of bamboo, which has the important significance for economic and social progress in China. However, bamboo is hydrophilic and absorbs moisture when it is exposed to humid enviroment. During the processing of bamboo reinforced composites, the moisture content can lead to poor processability, porous product, dimensional instability and aggravative interfacial incompatibility with hydrophobic polymer matrix. Moreover, when bamboo reinforced composites are subject to moisture environment for outdoor use, the mechanical properties of the composites decrease considerably due to the moisture characteristics of bamboo. Moisture not only interacts with the polymer matrix and bamboo themselves, but also affects the interface, leading to poor stress transfer efficiencies from matrix to reinforcement. Therefore, it is essential to investigate the moisture absorption properties of the interface in bamboo strips/vinyl ester composites, including the effects of environment humidty on interfacial shear strength during processing and the water absorption aging properties of interface during its immersion in the water. In addition, though the interfacial shear strength of composites can be directly measured and calculated from pull-out tests, the nanoscale nature of the interface should be examined, which is important to fully characterize the interfacial properties of the composites. Interfacial characterization of bamboo strips reinforced vinyl ester resin composites by atomic force microscope (AFM) in this study is a tentative method of interfacial characterization technology for composites.The aim of this investigation includes:(1) to quantify the influence of environment humidty during composites manufacture on the interfacial adhesion of bamboo/vinyl ester resion composites, (2) to find out a suitable chemical treatment for bamboo strips to reduce its moisture absorption property and improve the interfacial moisture resistance of bamboo strips reinforced vinyl ester resin composites, and (3) to characterize the nanoscale interfacial properties using AFM by topography and nanoindentation for indicating the interfacial shear strength of composites.For these purposes, the first part of this study is to investigate the moisture absorption behaviors of bamboo strips in different humidity and its effect on the interfacial shear strength of bamboo strips/vinyl ester resin during manufacturing process. Results show that the moisture sorption property of bamboo is similar to other natural fibres, such as cotton and flax, which also experiences an anisotropic dimensional expansion, increases the extensibility and reduces the elastic modulus of the bamboo strips with the absorption of moisture. The humidity during composite manufacture has a severe impact on the interfacial shear strength (IFSS) of the composites. It descends linearly with the increase of RH levels at the initial stage, then, the decreasing rate accelerates until the IFSS value is to 0 MPa. As compared with the composites fabricated at dry condition, the IFSS achieved at standard room conditions (20±2℃,65±2 %RH) is only a half of what is achieved on dry bamboo strips. Composites produced at high relative humidity conditions (80 %RH and 90%RH) have negligible interfacial strength.To improve the interfacial moisture absorption resistance and adhesion between bamboo strips and vinyl ester resin, four different chemical treatments are utilized to modify bamboo strips, i.e., alkaline, acetylation, potassium permanganate and silane treatments. In the second part of this paper, it compares the efficiency of four chemical treatments on the moisture absorption of bamboo strips and their adhesion to vinyl ester resin at relative humidity conditions. The results suggest that acetylation treatment is the most efficient in reducing moisture absorption capability, including equilibrium moisture content and diffusion coefficient. Potassium permanganate and silane treatments slightly decrease the moisture absorption, whereas alkaline treatment enhances the moisture wetting ability. The IFSS of dried bamboo strips and vinyl ester resin can be enhanced by all the chemical treatments, which silane, acetylation and potassium permanganate treatments show the much better effect than alkaline treatment, while there is no obvious difference according to the data statistics.With the increase of fabrication RH levels, the IFSS for both untreated and treated samples reduce sharply. At 80%RH fabrication condition, all of the IFSS values become almost negligible except that obtained by acetylation. The IFSS of the acetylated samples obtained at 80 % RH is approximately equal to that of untreated samples fabricated at 40 % RH, as well as to that of other three chemical treated bamboo strips/vinyl ester resin fabricated at 60 % RH condition. Results indicate that the severe negative impact on the interfacial adhesion caused by fabrication RH levels is greatly minimized if the bamboo strips were subjected to acetylation treatment.As one part of the investigation on the interfacial moisture absorption properties of bamboo strips/vinyl ester resin, this study also focuses on the residual interfacial shear strength after being immersed in the water for different periods of time. It shows that the accumulated loss of interfacial shear strength mainly cause in the beginning of the nine days, which amounts to about 38 %. Thereafter, the interfacial shear strength remains very stable as the immersion time extended up to 100 days. The effects of four chemical treatments on the water absorption aging of bamboo strips reinforced composites make clear that silane treatment has the best effect when the samples are immersed in the water for short period of time, while the effect is highlighted by acetylation treatment as the immersion time prolonged.In this study, the IFSS of bamboo strips/vinyl ester resin composites is directly calculated according to the pull-out test results. However, because of the interface with nanoscale and complexity, its microstructure properties can not be comprehensively characterized by only the IFSS value. Therefore, the last part of this paper involves the nanoscale characterization of interface in bamboo strips reinforced vinyl ester composites using AFM topography and nanoindentation mode. From the topography morphology, it can be clearly seen that there are two different kinds of interfaces between bamboo strips and vinyl ester resin due to the bamboo itself is a natural lignose-cellulosic composite, which are named bamboo cellulose-vinyl ester interface and bamboo lignose-vinyl ester interface. The AFM indentation analyses results exhibit that little interface layer exists in the untreated bamboo strips/vinyl ester resin and the elastic modulus and nanohardness show a non-continuous distribution. However, an obvious transition region can be observed between chemical treated bamboo strips and vinyl ester resin, which can be regarded as the interface. The elastic modulus and nanoindentaion from bamboo strips to resin is a gradient distribution. Meanwhile, a clear trend is revealed of a marginal increase in elastic modulus and nanoindentation of bamboo strips after chemical treatments. This observation is thought to be consisitent with the results obtained from tensile test. In summary, this technique is proven to be a very useful to identify the nanoscale nature of the interface including the information of microstructure morphology and mechanical properties distribution.The conclusions on the moisture absorption properties of the interface in bamboo strips/vinyl ester resin in this study could provide the theoretical basis and industrial guidance for further application of the other natural fibres reinforced compsites. It can stabilize and optimize the properties of natural fibres reinforce composites products. Moreover, the appicaltion of AFM tapping mode and nanoindentation to characterize the nanoscale interface of bamboo strips/vinyl ester resin composites could also afford a feasible method to represent the interfacial nature of the composites.