Aging Of Surface Properties Of High Performance Fibers Treated With Atmospheric Pressure Plasma

Plasma processing of materials has been developed fast in recent years for it advantages that it only affects the chemical and physical properties of the substrate's outmost layer without altering the bulk properties. As an environmental-friendly process, it has been widely used in many applications such as improving the wettability, printability or dyeability of polymeric materials, enhancing adhesion between fibers and matrices, and promoting biocompatibility of implanting parts. Recently more attention has been paid to atmospheric plasma treatment due to advantages such as no need for a vacuum system, online process capabilities, high efficiency, and scalability to larger area.One of the problems that hinder the application of plasma treatment technology is that the polymer surfaces show a gradual hydrophobic recovery over the time and the surface properties simultaneously decreases to the original value after plasma treatments and it is named aging effect. The aging effect of plasma treatment is very complicated from a theoretical point of view, which is mainly related to the dynamic behavior of polymer surface. After the plasma treatment, a great quantity of polar groups is introduced to the polymer surface and surface free energy increases strongly. The mobility of polar groups or polymer chains minimizes the surface free energy by rotational and translational motions in order to tune the interfacial energy differences between the plasma treated polymer surfaces and its environment. The ageing behavior is influenced by many factors, summarizing, it is influenced by the plasma process gas and condition, the treatment time, the chemical composition, molecular and crystallinity structure of the polymer and the temperature and relative humidity of storage environment. This research was aimed to employ the atmospheric pressure plasma treatment on the high performance fibers such as ultra high modulus polyethylene (UHMPE) and poly(p-phenylene terephthalamide) (PPTA) fibers to improve their surface properties including the enhancement of the wettability, surface energy and the adhesion to resins. The change of the surface properties after atmospheric pressure plasma treatment and the aging effect of the treatment were studied and the mechanisms were discussed systematically based on the surface analysis method such as contact angle measurement, scanning electron microscope (SEM), the Atomic force microscopy (AFM), X-ray diffractometry (XPS), electron spinning resonance (ESR), and interfacial shear strength (IFSS) of fiber/epoxy resin interface.To investigate the relationship between aging of the treatment effect and the gas composition of atmospheric pressure plasma treatment, UHMPE fibers were selected as a model fiber to study the aging behavior of atmospheric pressure plasma jet (APPJ) treated fiber surfaces for 0, 7, 14, 21 and 40 days after the initial plasma treatment. Pure helium, helium + 1% oxygen, and helium + 2% oxygen were used as the working gas, treatment nozzle temperature was 100℃, output power was 10 W, and sample treatment or stationary time was 1 s. It was found that the IFSS for the pure helium and the helium + 1% O₂ groups were more than three times as much as that of the control and the highest IFSS was obtained for the helium + 1% O₂ group, while that for the helium + 2% O₂ group only increased 80%. The improvement of adhesion between fibers and matrices was due to increase of surface roughness, as shown in AFM images, as well as the change of the surface chemical composition detected with XPS. During the 40 days' storage, it was found that adding 1% of O₂ to helium increased effectiveness of the plasma in polymer surface modification and suppressed aging after the treatment, while adding 2% of O₂ had a negative effect on the APPJ modification results and accelerated aging. In addition, no significant difference in single fiber tensile strength was observed between the control and the plasma treated fibers.The effect of the treatment duration on wettability improvement and aging behavior of the UHMPE fibers with dielectric barrier discharge (DBD) was investigated. The UHMPE fibers were treated for 30, 60, 90 and 120 s with Ar/O₂ plasma by DBD. It was found that plasma treatment time has a significant influence on the treatment effect and the aging behavior of the treated fiber surfaces. The etching effect was more intensive and the water contact angles decreased with the increase of the plasma processing time through SEM observation and contact angle measurement. Nevertheless, there was no significant decrease in water contact angle if the treatment time was longer than 60 s. The aging effect of the plasma treatment could be suppressed by increasing the length of the plasma treatment duration. Thirty days after the initial DBD plasma treatment, the loss in the value of IFSS for the 90 and 120 s treatment groups was not as significantly as the 30 and 60s treatment groups, as well as the decrease of the water contact angles and the surface oxygen concentration. The results can be explained by the different thickness of the oxidized layer on the polymer surface. Increasing the plasma treatment will increase thickness of the oxidized layer on the polymer surfaces and the saturation degree of the oxidation of the treated surfaces, which might hinder the migration of the hydrophobic polar functional groups from the surface to the bulk of the polymer.The UHMPE fiber was chosen as a model system to investigate the influence of the storage conditions on the aging process of APPJ treated fiber surfaces for 0, 7, 15 and 30 days after the initial plasma treatment. The fiber was first plasma treated and then stored at temperatures of-80, 20 and 80℃at the same 0% relative humidity (RH) and at RH levels of 0%, 65%, and 100% at the same temperature of 20℃. Immediately after the plasma treatment, SEM showed a roughened fiber surface. XPS analysis showed changes in surface chemical composition. Contact angle measurements showed increased surface wettability and microbond test showed an increase in IFSS. The aging behavior was strongly affected by the storage temperature as well as relative humidity. With increasing relative humidity or decreasing temperature, the IFSS value decreased and the contact angle increased more slowly. The reasons for the observed aging behavior could be that decreasing temperature or increasing relative humidity hindered the reorientation of the polar groups and rearrangement of polymer chains on the fiber surface after the plasma treatment.One of the main differences between a low-pressure plasma treatment and an atmospheric pressure plasma treatment is that in atmosphere, the substrate material may absorb significant amount of water which may potentially influence the plasma treatment effects, which does not need to be taken into account in low pressure plasma treatment because all the moisture will have to be removed before the treatment chamber can reach the required degree of vacuum.It was investigated how the moisture absorbed by aramid fibers during the atmospheric pressure plasma treatment influenced the aging behavior of the modified surfaces. Kevlar 49 fibers with different moisture regains (0.5%, 3.5% and 5.5% respectively) were treated with APPJ with helium as the carrier gas and oxygen as the treatment gas. Immediately after the plasma treatment, SEM photographs, ESR and XPS presented the increased surface roughness, free radicals and oxygen concentration, respectively. Furthermore, it was shown that the surface rougliness, the amount of surface free radicals and oxygen concentration were higher, while the water contact angles were smaller for the groups with higher moisture regain than those of the lower moisture regain group. The results indicated that the moisture regain of Kevlar fibers played a positive role in surface etching and chemical modification of the fiber during the atmospheric pressure plasma treatment.The aging effects of atmospheric plasma treatment on Kevlar fibers were studied over a period of 30 days. At the end of 30 day aging period, the fibers treated with 5.5% moisture regain had a lower water contact angle and more polar groups on the fiber surface, leading to 75% improvement of IFSS over the control fibers, while those for the 0.5 and 3.5% moisture regain groups were only 30%. It was found that the moisture regain of the fibers during the atmospheric plasma treatment has a strong effect on hydrophobic recovery after treatment. For the bigger moisture regain group which absorbed more water to make the microvoids and larger, more polar side groups can be created in the microvoids and more fraction of aramid fibers was oxidized during APPJ treatment to restrict the aging behavior of plasma treatment. Finally, the UHMPE fiber was selected as a model fiber to investigate the effect of the organic solvents pretreatment on the aging effect of atmospheric pressure plasma treatment. The pretreatment of the three selected organic solvent such as ethanol, decane and dimethyl sulfoxide (DMSO) was employed to improve the wettability and reduce the aging effect of APPJ treatment of UHMPE fibers. Higher fluorescent dye diffusion and surface oxygen concentration were found for the decane and DMSO pretreated groups after the atmospheric pressure plasma jet treatment with laser scanning confocal microscope (LSCM) analysis. On the other hand, the hydrophobic recovery could be retard through the pretreatment of decane and DMSO. Thirty days after the initial plasma treatment, the smaller water contact angle, higher surface oxygen composition and IFSS to epoxy resin were found for the decane and DMSO pretreated groups. The improvement of the plasma treatment effectiveness and the reducing of the aging effect through the pretreatment of the solvents were attributed to the swelling effect of the solvents to the UHMPE fiber surfaces. The crystallinities of the polymer and intermolecular forces were lowered as well as a certain number of pores and voids in the fiber structure were formed in the structure, which facilitated the oxidation process and increased the depth of the oxidized layer on the fiber surfaces.In conclusion, the hydrophobic recovery of the atmospheric pressure plasma treated high performance fibers have been studied through surface analysis methods. In addition to the temperature and relative humidity of the storage conditions, the difference of the hydrophobic recovery of the plasma treated polymer surface was determined by several factors, including the size of surface amorphous region, intermolecular forces, surface cross-linking degree, and surface oxidized depth. The treatment effect can be optimized and the aging effect of the atmospheric pressure plasma treatment can be retarded through most suitable process parameters to facilitate the polymer surface oxidization and increase the depth of the treated layer on the surfaces as well as carrying out the proper storage conditions.