Influence of plasma surface treatments on kink band formation in PBO fibers during compression

Effect of non-oxidative plasma treatments on the surface properties of poly(p-phenylene terephthalamide) (PPTA) and poly(p-phenylene benzobisoxazole) (PBO) fibres as measured by inverse gas chromatography

It has been shown in previous works that the interfacial adhesion in PPTA- and PBO-epoxy composites can be improved by modifying the surface properties of these high-performance fibres upon exposure to non-oxidative plasma treatments. In this work, the effects developed on both types of polymer surface were examined as a function of treatment gas nature (He or N₂) and exposure time (one or four minutes) using inverse gas chromatography at infinite dilution (IGC). From the adsorption of n-alkanes, it has been proved that non-oxidative plasma treatments led to energetically heterogeneous surfaces in the case of PPTA, and to low-energy surfaces in the case of PBO. Nevertheless, it was proved with the 1-min plasma treatments (either under helium or under nitrogen) that chemical reactivity was enhanced on the PBO surface. Such a behaviour was ascribed to the presence of low-molecular weight oxidized materials. The mechanisms involved in surface activation of PPTA were not equivalent under He or N₂ exposure. Nitrogen plasma exposure led to a PPTA surface that is chemically reactive as a result of polarity enhancement. Helium plasma-treated PPTA surface was characterized by the presence of branched arrangements that intensified the number of chemical contacts onto reactive sites. Finally, for both fibre sets, if the purpose is to enhance the chemical surface reactivity, it makes no sense to increase the plasma exposure time from 1 to 4 min.

Enhancement of hydrophobicity and UV aging resistance of Poly (p-phenylene benzobisoxazole) fibers modified by fluorosilane and UV absorber

Poly (p-phenylene benzobisoxazole) (PBO) fibers were functionalized by 3-Glycidoxypropyltrimethoxysilane (KH-560) and then coated with 1 H, 1 H, 2 H, 2H-Perfluorodecyltrimethoxysilane (HFTES) and UV absorber (UV-328) to improve hydrophobicity and UV aging resistance. The chemical compositions of PBO fibers before and after modification were analyzed by XPS and surface morphologies were observed by SEM. The hydrophobicity of PBO fibers was evaluated by the measurement of contact angle for water and the UV aging resistance was evaluated by the tensile strength retention ratio of PBO fibers. The results showed that KH-560 was successfully introduced onto the PBO fiber surface. The contact angle for water was increased by 128% from 51.7° to 127°, suggesting a huge improvement of hydrophobicity. The UV aging resistance of PBO fibers was also greatly improved. After 400 h UV exposure, the tensile strength retention ratio was increased to 53%, which was much higher than that of 21.4% for pristine PBO fibers. And the results of variable coating times demonstrated that the optimal UV aging resistance was obtained by coating three times.

Oxidation Reactions in Kink Banded Regions of UHMMPE Fiber-Based Laminates Used in Body Armor: A Mechanistic Study

This work demonstrates the synergy between the thermo-mechanical and humidity induced degradation as well as the oxidation reactions in the kink-banded areas of ultra-high molar mass polyethylene (UHMMPE) fiber-based laminates used in body armor. For aged materials, the energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) results reveal high concentrations of oxygen containing products, and the EPR results demonstrate the presence of the peroxyl radicals (RO₂ ^• ) in the kink-banded areas. After one year of dark ambient storage, very long-lived RO₂ ^• radicals were observed primarily in the samples exposed to ageing conditions of elevated temperatures, humidity, and mechanical stress. The total percentage of crystallinity, as measured by differential scanning calorimetry, of the kinkbanded fibers was unchanged, indicating that the degradation occurs primarily in the amorphous region, and may also involve recrystallization processes of the degraded chains. However, the most abundant orthorhombic crystalline phase decreases from 77 % to 70 %. This decrease in the orthorhombic structure leads to more diffusion of oxygen into the kink-banded region, enhancing the oxidation processes. No changes are observed in the monoclinic phase of the kinked fibers, which remained constant and constituted ~2 % of the total crystallinity.