| Recently, cellulose fiber-reinforced thermoplastic composites have attracted increasing attention because of their advantages such as low cost, less molding time, light weight, high specific strength and modulus, impact resistance, chemical resistance and recyclability. The vegetable fiber resources are abundant in China, especially ramie resource, which ranks top one in the world annual production. It is meaningful that the ramie fibers can be developed as high value-added reinforcement in composites. One of major problems limiting this application is that the hydrophilic fiber surfaces are incompatible with relatively hydrophobic polymer matrices, leading to poor mechanical performance of the corresponding composites due to low interfacial adhesion. Although some surface modification methods have been shown effective in improving interfacial adhesion, problems regarding high water and energy consumption, low treatment efficiency, damage to fiber strength and the disposal of pollutant waste still exist, which do not agree with the current idea of sustainable development. Therefore, the eco-friendly plasma technology with high processing capability, high efficiency and less damage to fiber strength is becoming ever more demanding, which would be very promising as a surface modification method for cellulose fibers to improve their interfacial adhesion in composites.However, most plasma technologies are operated under low pressure condition, which requires expensive vacuum system and thus may not process continuously. Besides, the vacuum system can not be used for the samples containing liquids, limiting the further application of plasma technology. Therefore, the atmospheric pressure plasma technologies without vacuum system are more and more popular in the world. Currently, there are mainly two kinds of commercial atmospheric pressure plasma systems:atmospheric pressure plasma jet (APPJ) system and dielectric barrier discharge (DBD) system.The objective of this dissertation is by using APPJ and DBD to modify ramie fibers and ramie fabrics,(1) to investigate the influence of atmospheric pressure plasma treatment on ramie surface properties;(2) to study the joint influence of simultaneous interaction of plasma treatment parameters on the ramie fiber surface modification effect;(3) to examine the aging effect of plasma modification on ramie fiber to check if the treatment effect could be durable;(4) to study the plasma treatment effect on ramie fabric properties as well as the interfacial adhesion and mechanical properties of ramie fabric-reinforced composites. The detailed studies are described as follows:(1) The DBD plasma was applied to modify ramie fiber surface to increase its adhesion with poly(butylene succinate)(PBS) resin. The ramie fiber surface morphology, surface chemical composition, surface wettability and single fiber tensile strength were investigated by scanning electron microscope (SEM), atomic force microscopy (AFM), X-ray photoelectric spectroscopy (XPS), dynamic contact angle analysis (DCAA) and single fiber tensile strength test, respectively. The ramie/PBS interfacial shear strength (IFSS) was determined by single fiber pull-out test. The ramie fiber surfaces became rougher with many visible deposited spots, small notches and large area of protuberances, increasing the mechanical interlocking in ramie/PBS interface. The carbon element and C-C group concentration as well as the contact angle significantly increased. The IFSS value of ramie/PBS interface increased up to46%. Meanwhile, the ramie fiber tensile strength had no significant difference after plasma treatment. Apparently, plasma treatment voltage had significant influence on modification effect of ramie fibers:when the voltage was lower than a threshold value, the plasma treatment almost had no influence on the modified fiber surface properties; but when the voltage was too high, the plasma modification effect could be migitated. Therefore, it is very important to select the optimum plasma treatment parameter.(2) The plasma treatment parameters generally have significant influence on material surface modification effect. Besides, there usually is interaction effect between those treatment parameters. In order to study the simultaneous influence of plasma treatment parameters on ramie fiber surface modification effect, a novel DBD device was designed and improved to modify ramie fiber surface. A central composite design (CCD) response surface methodology (RSM) was used to design these parameters. In this CCD design, the three most influential parameters (treatment current, treatment time and flow rate) based on preliminary experiment were selected to conduct three factorial five level experiments, including15treated groups. The ramie fiber surface morphology, surface chemical composition and surface wettability were investigated by field emission scanning electron microscopy (FESEM), XPS and DCAA, respectively. The IFSS value of ramie/polypropylene (PP) was determined by single fiber pull-out test. The Molecular fragments generated in plasma chamber were identified using a residual gas analysis (RGA) quadrupole mass spectrometry system. It was found interactions between plasma treatment parameters. The IFSS value of ramie/PP increased up to50%under treatment parameters of10mA,5min and8sccm, among which the flow rate was the most influential factor. It was also found contact angle and IFSS values correlated well with RGA analysis, indicating there was strong relationship between the abundance of ethyl groups and plasma modification effect. Besides, the modification made to the traditional plasma system was helpful to control the gas composition, making the treatment process more reproducible and stable.(3) One of the major drawbacks of the plasma technology used in material surface modification is aging behavior which leads to gradual deterioration of surface performance of the modified material. There usually is a long time before plasma-modified material being used in the next steps. Therefore, aging effect is a necessary research topic in plasma research field. In order to study the influence of the hydrophilic recovery of APPJ modified ramie fiber surfaces, ramie fiber surfaces were treated by APPJ and aged for up to150days to perform aging experiment. The ramie fiber surface morphology, surface chemical composition and surface wettability were investigated by SEM, XPS and static water contact angle test, respectively. The ramie/PP IFSS value was determined by single fiber pull-out test. The debonding area morphology of ramie fiber surface after single fiber pull-out test was examined by SEM. The research results showed that the surface composition, surface wettability and ramie/PP IFSS values maintained at a similar level within150days of aging. The debonding area morphology of ramie fiber did not show obvious change. All these results indicated no obvious hydropholic recovery of plasma modified hydrophobic ramie fiber, showing plasma treatment can be used as a permanent surface treatment with negligible aging for at least five months.(4) The DBD plasma was used to modify ramie fabric surface, then the modified fabrics and PP films were stacked together to make composite by hot press method, in order to study the influence of plasma treatment on the modified ramie surface properties and the interfacial and mechanical properties of ramie fabric-reinforced PP composites. The ramie surface morphology, surface chemical composition and fabric surface wettability were investigated by SEM, XPS and contact angle test, respectively. The surface fracture morphology of ramie fabric after interlaminar shear strength (ILSS) test was examined by SEM. The mechanical properties of the composite were tested by ILSS test, flexural test and tensile test. The SEM showed that ramie surfaces became rougher and the interfacial fracture mode of the modified fabric-reinforced composite changed from adhesion failure to cohesive failure:more PP resin was adhered to the fracture surface of the treated ramie fiber, improving the interfacial adhesion between ramie fiber and PP resin. After plasma treatment, the carbon element concentration significantly increased and O/C element ratio dramatically decreased. The contact angle of the treated ramie fabric increased obviously because that the hydrophobic functional groups had been grafted onto the modified ramie fabric surface. The mechanical tests showed significant improvement of mechanical performance:the ILSS, flexural strength and tensile strength of modified composite had up to39%,28%and20%increase, respectively. |
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