Improvement of surface wettability and interfacial adhesion of poly-(p-phenylene terephthalamide) by incorporation of the polyamide benzimidazole segment

In order to investigate the effect of the polyamide benzimidazole group on the surface wettability and interfacial adhesion of fiber/matrix composites, surface features of two kinds of aramid fibers, poly (p-phenylene terephthalamide) fiber (Kevlar-49) and poly-(polyamide benzimidazole-co-p-phenylene terephthalamide) (DAFIII), have been analyzed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and contact angle analysis (CAA) system, respectively.The results show that with the incorporation of the polyamide benzimidazole segment, more polar functional groups exist on DAFIII surface. The contact angles of water and diiodomethane on DAFIII surface get smaller. The surface free energy of DAFIII increases to 36.5 mJ/m², which is 2.3% higher than that of Kevlar-49. In addition, DAFIII has a larger rough surface compared with that of Kevlar-49 due to different spinning processes. The interfacial shear strength (IFSS) of DAFIII/matrix composite is 25.7% higher than that of Kevlar-49/matrix composite, in agreement with the observed results from surface feature tests. SEM micrographs of failed micro-droplet specimens reveal a strong correlation between the fracture features and the observed test data.     

Preparation and Characterization of Para-Aramid Fibers with the Main Chain Containing Heterocyclic Units

Abstract

A para-aramid fiber whose main chain contained heterocyclic units was prepared by low temperature copolycondensation, wet-spinning, and high temperature thermal treatment. The prepared fibers (named F-368) and two commercial aramid fibers, K49 (Kevlar 49, Dupont de Nemours Co., USA) and APMOC (Kamenskvolokno and Tver’khimvolokno, Russia), were characterized and analyzed in detail. Infrared spectroscopy (IR) and wide-angle X-ray diffraction (WAXD) were employed to characterize their chemical and aggregation structures, respectively. The results showed the introduction of heterocyclic units into the wholly para-aromatic polyamide backbone of K49 in the F-368 and APMOC reduced the crystallinity significantly. The tenacity of F-368 and APMOC were 32.2 and 30.5cN/dtex, which were about 68% and 59% higher than that of K49, respectively. Thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) were used to investigate their thermal properties; the results indicated that these aramid fibers showed exceptional thermal properties with glass transition temperatures of 240–260?°C, and decomposition temperatures at 510–560?°C, both in nitrogen and air. The TGA results also showed the decomposition mechanism of K49 and the heterocyclic aramid fibers in nitrogen and air were different. The decomposition temperature of K49 was higher than that of the heterocyclic copolyaramid fibers both in nitrogen and air. On the contrary, the char yields of the heterocyclic copolyaramid fibers at 800?°C were higher than that of K49 in both nitrogen and air.     

Enhanced surface free energy of polyimide fibers by alkali treatment and its interfacial adhesion behavior to epoxy resins

In this article, polyimide (PI) fibers were modified by alkali treatment, and PI fiber-reinforced epoxy composites were fabricated. The effects of different alkali treatment times on the surface properties of the PI fibers and the adhesion behaviors of PI fiber/epoxy composites were studied. The surface morphologies, chemical compositions, mechanical properties, and surface free energy of the PI fibers were characterized by atomic force microscopy, X-ray photoelectron spectroscopy, single-fiber tensile strength analysis, and dynamic contact angle analysis, respectively. The results show that alkali treatment plays an important role in the improvement of the surface free energy and the wettability of PI fibers. We also found that, after the 3 min, 30 °C, 20 wt% NaOH solution treatment, the fibers possessed good mechanical properties and surface activities, and the interlaminar shear strength of the composites increased to 64.52 MPa, indicating good interfacial adhesion properties.     

Influence of aramid fiber moisture regain during atmospheric plasma treatment on aging of treatment effects on surface wettability and bonding strength to epoxy

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. This paper investigates how the moisture absorbed by aramid fibers during the atmospheric pressure plasma treatment influences the aging behavior of the modified surfaces. Kevlar 49 fibers with different moisture regains (MR) (0.5, 3.5 and 5.5%, respectively) are treated with atmospheric pressure plasma jet (APPJ) with helium as the carrier gas and oxygen as the treatment gas. Surface wettability and chemical compositions, and interfacial shear strengths (IFSS) to epoxy for the aramid fibers in all groups are determined using water contact angle measurements, X-ray photoelectron spectroscopy (XPS), and micro-bond pull out tests, respectively. Immediately after the plasma treatment, the treated fibers have substantially lower water contact angles, higher surface oxygen and nitrogen contents, and larger IFSS to epoxy than those of the control group. 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%.     

Surface Modification of Ultrahigh-molecular-weight Polyethylene Fibers with Coupling Agent during Extraction Process

Ultrahigh molecular weight polyethylene (UHMWPE) fibers were treated with a coupling agent following the extraction of gel fibers, resulting in modified fibers after subsequent ultra-drawing. The structure and morphology of the modified UHMWPE fibers were characterized and their surface wetting, interfacial adhesion, and mechanical properties were investigated. It was found that the coupling agent was absorbed into the UHMWPE fiber and trapped on the fiber surface. Compared with unmodified UHMWPE fibers, the modified fibers had smaller contact angle, higher crystallinity, and smaller crystal size. The interfacial adhesion and mechanical properties of UHMWPE fibers were significantly improved with increasing coupling agent concentration and gradually reached a plateau value. After treatment with 1.5 wt% solution of a silane coupling agent (γ -aminopropyl triethoxysilane, SCA-KH-550), the interfacial shear strength of the UHMWPE-fiber/epoxy composites was increased by 108% and the tensile strength and modulus of modified UHMWPE fibers were increased by 11% and 37% respectively.     

Surface analysis of oxygen plasma treated poly(p-phenylene benzobisoxazole) fibers

The influence of oxygen plasma treatment on surface properties of poly(p-phenylene benzobisoxazole) (PBO) fibers and aging effect of the oxygen plasma modified PBO fiber surfaces were investigated by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and dynamic contact angle analysis (DCAA), respectively. The results indicated that the oxygen plasma treatment introduced some polar groups to PBO fiber surfaces, enhanced surface roughness and changed surface morphologies of PBO fibers by plasma etching and oxidative reactions. Surface wettability of PBO fibers may be significantly improved by increasing surface free energy of the fibers via oxygen plasma treatment. Aging effect of the oxygen plasma treated PBO fibers showed that the fiber surface wettability degraded in the first several days after the plasma treatment, and it was found to be changeless as the aging time continued as long as 30 days.     

Preparation of vertically aligned carbon nanotube arrays grown onto carbon fiber fabric and evaluating its wettability on effect of composite

Vertically aligned carbon nanotube (CNT) arrays have been grown onto the carbon fiber fabric using a catalytic chemical vapor deposition (CCVD) method. The as-synthesized CNT arrays are about 20 μm in height, and the nanotube has a mean inner and outer diameter of 2.6 nm, 5.5 nm, respectively. The CNT-grafted carbon fabric shows a hydrophobic property with a contact angle over 145°, and the single CNT-grafted carbon fiber shows a sharp increase of dynamic contact angle in de-ionized water from original 71.70° to about 103°, but a little increase does in diiodomethane or E-51 epoxy resin. However, the total surface energy of carbon nanotube-grafted carbon fiber is almost as same as that of as-received carbon fiber. After CNTs growth, single fiber tensile tests indicated a slight tensile strength degradation within 10% for all different lengths of fibers, while the fiber modulus has not been significantly damaged. Compared with the as-received carbon fibers, a nearly 110% increase of interfacial shear strength (IFSS) from 65 to 135 MPa has been identified by single fiber pull-out tests for the micro-droplet composite, which is reinforced by as-received carbon fiber or CNT-grafted carbon fiber.     

Ballistic behavior of Heracron®-based composites: effect of fiber density and fabrication method

In this study, a Heracron^® aramid fiber-based helmet was made, and its ballistic properties were investigated. The effect of fiber density was studied in depth. For the same weight and number of plies, a helmet manufactured from the HT-2820-based composite showed improved ballistic properties compared with one made from the HT-15000-based composite. This result suggested that fiber density may strongly affect the ballistic properties of armor. HT-2820, containing more multifilament fibers, provides more efficient energy absorption and dissipation. The influence of fabrication method on the ballistic behavior of a helmet was studied. The V₅₀ of a helmet made by the film laminating method was 10% better than that for a helmet made by the resin dipping approach. Based on these findings, the film laminating method, which forms the composite by directly attaching aramid fabric to the matrix film, may be a good candidate for improving the ballistic behavior. The required composite interfacial strength will be application-dependent. Greater fiber-matrix adhesion may be advantageous in certain cases.     

A Fluoropolymer Coating on Carbon Fibers Improves Their Adhesive Interaction with PTFE Matrix

Carbon fibers were treated in a HF glow discharge in tetrafluoroethylene and octafluorocyclobutane in order to improve their adhesion to poly(tetrafluoroethylene) matrix. As the result of the plasma treatment, a thin (20–140 nm) fluoropolymer coating was deposited onto the fiber surface. The structure of this coating was studied by means of IR spectroscopy, XPS, AFM and SEM techniques. The coating material appeared to be similar to PTFE in its chemical composition but distinguished by branched, partially crosslinked, amorphous structure and included unsaturated chemical bonds. The coating thickness of 70 nm was sufficient to effectively screen the field of molecular forces of the initial substrate, thus, decreasing the surface energy of the fibers and improving their compatibility with the PTFE matrix. The adhesive strength in the PTFE–carbon fiber systems, measured by means of the microbond test, more than doubled upon the plasma treatment (the local interfacial shear strength increased from 10.7 to 29.7 MPa, apparent IFSS from 4.3 to 7.8 MPa), and the interfacial frictional stress increased by 70%. The new composite material consisting of 20% short coated carbon fibers in the PTFE matrix showed better mechanical, thermal and tribological characteristics as compared with the composite reinforced with untreated fibers.     

Surface treatment of aramid fiber by air dielectric barrier discharge plasma at atmospheric pressure

Aramid fiber samples are treated by air dielectric barrier discharge (DBD) plasma at atmospheric pressure; the plasma treatment time is investigated as the major parameter. The effects of this treatment on the fiber surface physical and chemical properties are studied by using surface characterization techniques. Scanning electron microscopy (SEM) is performed to determine the surface morphology changes, X-ray photoelectron spectroscopy (XPS) is analyzed to reveal the surface chemical composition variations and dynamic contact angle analysis (DCAA) is used to examine the changes of the fiber surface wettability. In addition, the wetting behavior of a kind of thermoplastic resin, poly(phthalazinone ether sulfone ketone) (PPESK), on aramid fiber surface is also observed by SEM photos. The study shows that there seems to be an optimum treatment condition for surface modification of aramid fiber by the air DBD plasma. In this paper, after the 12 s, 27.6 W/cm³ plasma treatment the aramid fiber surface roughness is significantly improved, some new oxygen-containing groups such as C-O, CO and OC-O are generated on the fiber surface and the fiber surface wettability is greatly enhanced, which results in the better wetting behavior of PPESK resin on the plasma-treated aramid fiber.     

Roles of interfaces between carbon fibers and epoxy matrix on interlaminar fracture toughness of composites

The effect of atmospheric-pressure plasma treatment on high strength PAN-based carbon fibers had been studied in terms of fiber surface energetics and mode I and II interlaminar fracture toughness of unidirectional carbon fibers/epoxy matrix composites. The surface characterization of plasma treated carbon fibers was investigated by X-ray photoelectron spectroscopy (XPS) and contact angles. As a result, the plasma treatment changed the surface properties of the carbon fibers, mainly through formation of oxygen functional groups like hydroxyl, carbonyl, and carboxyl groups. According to contact angle measurements, it was observed that plasma treatment led to an increase in surface free energy of the fibers, mainly due to the increase of its specific component. Fracture toughness test results employing double-cantilever beam (DCB) and end notched flexure (ENF) specimens also showed that the increase in specific components or hydrogen bonding between the –OH groups on carbon fibers and the =O ring in epoxy matrix resins played an important role in improving the degree of adhesion at interfaces, resulting in an increase in the interfacial fracture toughness of the composites studied.     

Surface wettability of atmospheric dielectric barrier discharge processed Armos fibers

Wettability of Armos fibers has been investigated after exposed to dielectric barrier discharge (DBD) plasma, which was performed at atmospheric pressure in air while varying the sample treatment time between 9 and 27 s. Contact angles and surface free energy of the original and plasma-treated fibers were measured with dynamic contact angle analysis (DCAA) to reveal the correlation between the fiber wettability and the surface treatment, including surface composition and topography modifications, which were evaluated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. It was found by XPS analysis that the O/C atomic ratio on Armos fiber surface can be increased from 0.134 to 0.248 after the 18 s exposure to the plasma and many polar functional groups were proved to be incorporated into the surface, which aided good wetting. In addition, AFM observations revealed the changes of fiber surface microstructure, showing significant enhancement of the surface roughness after the treatment, which could also bring the fiber better wettability. What's more, the impacts of fiber surface treatment on its tensile properties were characterized by single fiber tensile strength (SFTS) testing. Results showed that Armos fibers exhibited only slight reductions in their tensile strengths with the great enhancement in fiber surface free energy.     

Enhancing the interfacial strength of glass/epoxy composites using ZnO nanowires

This paper investigated the application of ZnO nanowires (ZnO NW) to enhance the interfacial strength of glass/epoxy composites. ZnO NW were grown on glass fibers by hydrothermal method, tensile properties of bare and ZnO NW coated fibers were measured by single fiber tensile testing, wettability of fiber with resin was studied by contact angle measurements and finally the interfacial strength and mechanisms were determined by single fiber fragmentation testing of glass/epoxy composites. The surface coverage of ZnO NW on glass fibers was fairly uniform without formation of major clusters. The coating of ZnO NW slightly reduced the tensile strength and improved the tensile modulus of fibers. Wettability tests showed reduction in contact angles for ZnO NW coated fibers because of enhanced wetting and infiltration of epoxy resin into nanowires. In fragmentation testing of microcomposites, smaller and concentrated interfacial debonding zones for ZnO NW coated fibers indicated good stress transfer and strong interfacial adhesion. A new form of crossed and closely spaced stress patterns were observed for nanowires of high aspect ratios. The interfacial strength of ZnO NW coated fibers increased by at least 109% and by 430% on average, which was attributed to the increased surface area and mechanical interlocking provided by ZnO NW.     

Comparison of F-12 aramid fiber with domestic armid fiber III on surface feature

A comparison of F-12 aramid fiber with domestic armid fiber III (DAF III) on surface feature was carried out by scanning electron microscope (SEM), atomic force microscopy (AFM), elements analysis and X-ray Photoelectron Spectroscopy (XPS) analysis. It is found that the two aramid fibers are of “skin-core” structure and fibrillar structure. The microfibrils orient along the fiber axis and rather poorly bond in transverse direction. Many defects exist on the surface of two fibers. Carbon, hydrogen, nitrogen and oxygen are the major elements of two aramid fiber. The element content of the same aramid fiber from surface to interior is different. The surface carbon contents of F-12 aramid fiber and DAF III are increased by 10.75% and 9.95% than those in fiber interior respectively, the surface nitrogen content decreased by 9.72% and 27.02% respectively, and the surface oxygen content increased by 13.99% and 37.95% respectively.     

Effects of Plasma Etching on Surface Modification and Gas Permeability of Bisphenol‐A Polycarbonate Films

The surface fluorination of polycarbonate bisphenol‐A thin films was carried out by treatment with plasma of sulfur hexafluoride/argon (SF₆/Ar) using a radio frequency (RF) discharges. Effects of treatment time and SF₆ concentration have been studied. Surface characterization was performed using X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy with attenuated total reflectance (FT‐IR/ATR), atomic force microscopy (AFM) and contact angle measurements. The effects of the surface modification on the carbon dioxide (CO₂) gas permeation of the fluorinated films were evaluated. The results showed effective fluorination of all samples, with C‐F, C‐F₂ and even C‐F₃ groups attached to the polycarbonate, depending on the treatment conditions. The presence of incorporated fluorine molecules absorbed into the polymer structure during treatment was also shown. The polymer films showed an increase of their surface roughness and a decrease in their surface tension due to the plasma treatment. Plasma fluorination also decreases polycarbonate CO₂ permeability considerably. The films treated with the lower concentration of SF₆ showed the highest barrier values; however, significant differences between the permeability values for the samples treated for different times were not observed.     

Improvement of aramid fiber III reinforced bismaleimide composite interfacial adhesion by oxygen plasma treatment

Domestic Aramid Fiber III (DAF III) was modified by oxygen plasma treatment. The effects of oxygen plasma treatment power on fiber surface and DAF III reinforced bismaleimides (BMI) composite interfacial properties were investigated, respectively. The fiber surface characteristics were analyzed by X-ray photoelectron spectroscopy, Scanning Electron Microscopy, Atomic Force Microscopy and Dynamic Contact Angles Analysis, respectively. The results showed that oxygen plasma treatment introduced new oxygen containing groups such as C=O and –COO on to the fiber surfaces, changed the fiber surface morphologies and enhanced surface roughness by oxidative reactions and plasma etching. Finally, the fiber surface wettability was effectively improved. The total free energy increased from 49.8 to 71.7 mJ/m2 at maximum with 300 W oxygen plasma treatment. The composite interlaminar shear strength (ILSS) was evaluated by short beam shear measurement. The ILSS value increased from 49.3 to 59.8 MPa (by 21.3%) within 300 W plasma treatment.     

The effect of surface modification on the properties of sisal fiber and improvement of interfacial adhesion in sisal/starch composites induced by starch nanocrystals

A facile approach was utilized to introduce starch nanocrystals (SNCs) onto sisal fiber (SF) to improve the interfacial adhesion between SF and starch. For this, fibers were treated with alkali and then subjected to cold plasma treatment to increase the accessibility with SNCs, which was confirmed through X-ray photoelectron spectroscopy (XPS). It was found that due to the influence of cold plasma treatment, new functional groups were introduced onto SF. The surface characteristics of SF were examined by Fourier transform infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM). The observed results suggested that SNCs were successfully distributed onto SF. Tensile strength and interfacial shear strength of fibers treated under different conditions were calculated and compared through a two-parameter Weibull model. The highest interfacial shear strength of 3.05 MPa was obtained by Alkali-300 W-SNCs, which indicated an increase of 80.6% than untreated SF. It has also been proved that the starch nanocrystals produced hydrogen bonding and physical interlocking between sisal fiber and starch. Notably, the outcome of this investigation indicates that SNCs may be applied for the fabrication of high performance, environmentally friendly sisal/starch composites for a range of technological applications.     

Fabrication of superhydrophobic wood surface by a sol-gel process

The superhydrophobic wood surface was fabricated via a sol-gel process followed by a fluorination treatment of 1H, 1H, 2H, 2H- perfluoroalkyltriethoxysilanes (POTS) reagent. The crystallization type of silica nanoparticles on wood surface was characterized using X-ray diffraction (XRD), the microstructure and chemical composition of the superhydrophobic wood surface were described by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), the bonding force between the silica nanoparticles and POTS reagent was analyzed by Fourier transform infrared spectroscopy (FT-IR) and the superhydrophobic property of the treated sample was measured by contact angle (CA) measurements. An analytical characterization revealed that nanoscale silica spheres stacked uniformly over the wood surface, and with the combination of the high surface roughness of silica nanoparticles and the low surface free energy film of POTS on wood surface, the wood surface has turned its wetting property from hydrophilic into superhydrophobic with a water contact angle of 164° and sliding angle less than 3°.     

Influence of γ-ray radiation grafting on interfacial properties of aramid fibers and epoxy resin composites

This research used Co⁶⁰ γ-ray radiation to modify Armos fibers in 1,2-epoxy-3-chloropropane. After the treatment, the interlaminar shear strength (ILSS) values of aramid/epoxy composites were improved by about 20%. Surface elements of Armos fibers were determined by XPS analysis, which indicated that the oxygen/carbon ratio was increased. The surface of the fibers treated was rougher than that of the untreated fibers when examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Fourier transform infrared (FT-IR) spectra confirmed that the epoxy group was grafted onto the fibers. The wettability of the fibers' surface was also enhanced by the treatment. Nanoindentation technique analysis showed that the nanohardnesses of the various phases (the fiber, the interface and the matrix) in the composite, whose fibers were treated, were correspondingly higher than those in the composite, whose fibers were untreated. The results indicate that γ-ray irradiation grafting technique, which is a suitable batch process for industrialization, can modify the physicochemical properties of Armos fibers and improve the interfacial adhesion of its composite.     

Optimization of mechanical strength of titania fibers fabricated by direct drawing

Nanostructured polycrystalline titania (TiO₂) microfibers were produced by direct drawing from visco-elastic alkoxide precursors. The fiber crystallinity and grain size were shown to depend on post-treatment calcination temperature. Tensile tests with individual fibers showed strong sensitivity of the elastic modulus and the tensile strength to microstructural details of the fibers. The elastic modulus of as-fabricated fibers increased about 10 times after calcination at 700 ^°C, while the strain at failure remained almost the same at ～1.4 %. The highest tensile strength of more than 800 MPa was exhibited by nanoscale grained fibers with a bimodal grain size distribution consisting of rutile grains embedded into an anatase matrix. This structure is believed to have reduced the critical defect size, and thus increased the tensile strength. The resultant fibers showed properties that were appropriate for reinforcement of different matrixes.