Adhesion characteristics of plasma surface treated carbon/epoxy composite

The load transmission capability of adhesive joints can be improved by increasing the surface free energy of the adherends with surface treatments. In this paper, suitable plasma surface treatment conditions for carbon/epoxy composite adherend were investigated to enhance the strength of carbon/epoxy composite adhesive joints using a capacitively coupled radio-frequency plasma system. Effects of plasma surface treatment parameters on the surface free energy and adhesion strength of carbon/epoxy composite were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time. Quantitative chemical bonding analysis determined with XPS (X-ray photoelectron spectroscopy) was also performed to understand the load transmission capabilities of composite adhesive joints with respect to surface treatment time.     

Adhesion characteristics of plasma-surface-treated carbon fiber-epoxy composite with respect to release films used during demolding

The load capabilities of carbon fiber-epoxy composite adhesive joints are affected by surface characteristics of the composite adherends such as surface free energy and chemical composition, which can be altered by plasma surface treatment and the type of release film for demolding carbon fiber-epoxy composites from metal molds. In this paper, suitable plasma surface treatment conditions for carbon fiber-epoxy composite adherends were investigated to enhance the strength of carbon fiber-epoxy composite adhesive joints using dielectric barrier discharges of atmospheric pressure plasmas. The effects of plasma surface treatment on the surface free energy and adhesion strength of carbon fiber-epoxy composites were experimentally investigated with respect to surface treatment time. Also, the surface and adhesion characteristics of carbon fiber-epoxy composites were investigated with respect to release films such as fluorinated ethylene propylene (FEP), high density polyethylene (PE) and Nylon 6.6. Quantitative chemical bonding analysis with X-ray photoelectron spectroscopy (XPS) was also performed to understand the load capabilities of composite adhesive joints with respect to plasma treatment time and release films. From the experimental results, it was found that plasma treatment of carbon fiber-epoxy composites did enhance its adhesion strength, irrespective of the type of release film. Regarding adhesion strength, Nylon 6.6 was found to be the most suitable release film for these composites when no plasma treatment could be applied. From the XPS measurements on carbon fiber-epoxy composites, it was found that the carbon bond ratio of C=O to C-C and C-H reached a maximum at around 10 s treatment time, which corresponded well with the load transmission capability of the composite adhesive joint.     

Investigation of optimal surface treatments for carbon/epoxy composite adhesive joints

Although an adhesive joint can distribute the load over a larger area than a mechanical joint, requires no holes, adds very little weight to the structure and has superior fatigue resistance, but it not only requires a careful surface preparation of the adherends but also is affected by service environments. In this paper, suitable conditions for surface treatments such as plasma surface treatment, mechanical abrasion, and sandblast treatment were investigated to enhance the mechanical load capabilities of carbon/epoxy composite adhesive joints. A capacitively coupled radiofrequency plasma system was used for the plasma surface treatment of carbon/epoxy composites and suitable surface treatment conditions were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time by measuring the surface free energies of treated specimens. The optimal mechanical abrasion conditions with sandpapers were investigated with respect to the mesh number of sandpaper, and optimal sandblast conditions were investigated with respect to sandblast pressure and particle size by observing geometric shape changes of adherends during sandblast process. Also the failure modes of composite adhesive joints were investigated with respect to surface treatment. From the peel tests on plasma treated composite adhesive joints, it was found that all composite adhesive joints failed cohesively in the adhesive layer when the surface free energy was higher than about 40 mJ/m², because of high adhesion strength between the plasma treated surface and the adhesive. From the peel tests on mechanically abraded composite adhesive joints, it was also found that the optimal surface roughness and adhesive thickness increased as the failure load increased.     

Ultraviolet surface treatment for adhesion strength improvement of carbon/epoxy composite

—As the applications of composite structures have increased, various techniques to join composite parts to the structures have been developed in order to meet the required adhesion strength. In this work, surface modification of carbon/epoxy composites was investigated using ultraviolet (UV) surface treatment to increase the adhesion strength between the carbon/epoxy composite and the epoxy adhesive. After UV surface treatment, X-ray photoelectron spectroscopy (XPS) analysis and contact angle measurements were performed to analyze the surface characteristics of the carbon/epoxy composites. From the results of XPS analyses and adhesion strength tests, it was found that the increase of C O bond density on the surface of carbon/epoxy composite caused the enhancement of adhesion strength. Also it was found that the UV-B (wavelength 280–315 nm) surface treatment resulted in a superior adhesion strength compared to the UV-A (wavelength 315–400 nm) surface treatment.     

Surface energy characterization of modified carbon blacks and its relationship to composites tearing properties

The effects of three types of chemical treatments, i.e. as polar acidic, polar basic, and nonpolar oxidations, on virgin carbon blacks have been studied in terms of pH, acid-base surface values, specific surface area, X-ray diffraction analysis, and surface free energy. The acidic chemical treatment leads to significant changes in surface and adsorption properties, surface free energy, and microstructures. The increased acidic surface functional groups on carbon blacks result from reaction between the basic carbon and the acidic chemical solution. Also, based on the determination of surface free energy from contact angle measurements, a good correlation between the London dispersive component or apolar (γ^d _s) of surface free energy and specific surface area (S_BET) (or crystalline size S along the c-axis, L_C) is shown in this work. Particularly, it is found that the γ^d _s of the carbon blacks studied is highly correlated with the mechanical tearing test results based on hydrocarbon rubber compound composites.     

Surface modifications of EVA copolymers induced by low pressure RF plasmas from different gases and their relation to adhesion properties

Two ethylene vinyl acetate (EVA) copolymers (12 and 20 wt% of vinyl acetate,VA, content) have been treated with low pressure RF plasmas from non-oxidizing gases (Ar, N₂) and oxidizing gases (air, a mixture of 4N₂: 6O₂ (v/v), O₂ and CO₂). The formation of polar moieties on both EVAs was more noticeable by treatment with plasmas from non-oxidizing gases than from oxidizing ones (the higher the reactivity, the lower the difference with respect to untreated EVA surfaces). The surface etching with the non-oxidizing plasmas, giving rise to a high roughness, depends on the wt% of VA in the composition of the copolymer because of the different resistances of VA (low) and PE (high) to the non-oxidizing plasma particles bombardment. The adhesion properties obtained using a polyurethane adhesive (PU) showed high T-peel strength values and adhesion failure in EVAs treated with plasmas from oxidizing gases, due to roughness produced causing mechanical interlocking of the adhesive. Lower T-peel strength values were obtained with non-oxidizing plasmas: the values for EVA12 being, in general, lower than those obtained for EVA20. The durability of the treated EVAs/PU adhesive joints after ageing in humidity and temperature was quite good.     

Improved adhesion of EVAs with different vinyl acetate contents treated with sulphuric acid

Four ethylene vinyl acetate copolymers (EVAs) containing 9, 12, 18 and 20 wt% vinyl acetate (VA) were treated with concentrated sulphuric acid to improve their adhesion to polychloroprene (PCP) adhesive. The tensile strength and Young's modulus of EVAs decreased as the VA content increased, due to the reduction in crystallinity of the polyethylene blocks in the copolymer. The modifications produced in the EVAs by treatment with sulphuric acid were followed using contact angle measurements (water, 25 °C), ATR-IR spectroscopy and scanning electron microscopy (SEM). Adhesive-bond strength was obtained by T-peel tests on treated EVA/polychloroprene adhesive joints. The vinyl acetate content in the EVA affected the extent, but not the nature, of the surface modification produced by treatment with sulphuric acid. The treatment produced both sulfonation and oxidation on the EVA surfaces. The higher the vinyl acetate content in the EVA, the more significant the modifications produced. Increased T-peel strengths of EVA/polychloroprene adhesive + 5 wt% polyisocyanate joints were obtained and a mixed failure (adhesion failure + cohesive failure in the adhesive) was produced. It was found that, to be effective, the treatment of EVAs must be carried out with 96 wt% sulphuric acid.     

The effect of silica (SiO2) nanoparticles and ammonia/ethylene plasma treatment on the interfacial and mechanical properties of carbon-fiber-reinforced epoxy composites

A nanoparticle dispersion is known to enhance the mechanical properties of a variety of polymers and resins. In this work, the effects of silica (SiO₂) nanoparticle loading (0–2 wt%) and ammonia/ethylene plasma-treated fibers on the interfacial and mechanical properties of carbon fiber–epoxy composites were characterized. Single fiber composite (SFC) tests were performed to determine the fiber/resin interfacial shear strength (IFSS). Tensile tests on pure epoxy resin specimens were also performed to quantify mechanical property changes with silica content. The results indicated that up to 2% SiO₂ nanoparticle loading had only a little effect on the mechanical properties. For untreated fibers, the IFSS was comparable for all epoxy resins. With ethylene/ammonia plasma treated fibers, specimens exhibited a substantial increase in IFSS by 2 to 3 times, independent of SiO₂ loading. The highest IFSS value obtained was 146 MPa for plasma-treated fibers. Interaction between the fiber sizing and plasma treatment may be a critical factor in this IFSS increase. The results suggest that the fiber/epoxy interface is not affected by the incorporation of up to 2% SiO₂ nanoparticles. Furthermore, the fiber surface modification through plasma treatment is an effective method to improve and control adhesion between fiber and resin.     

Study on surface- and mechanical fiber characteristics and their effect on epoxy composite properties tuned by continuous anodic carbon fiber oxidation

Continuous anodic oxidation was employed to alter the surface chemical properties of carbon fibers. As expected, the wetting behavior by water improved and that of non-polar liquid diiodomethane deteriorated. The calculated surface tensions mirror the changes in the physicochemical surface properties. The zeta (ζ)-potential measurements performed also reflect changes in the surface chemistry of the investigated carbon fibers. A correlation between the measured ζ-potentials and the wetting behavior of water on anodically oxidized carbon fibers was found. The influence of anodic carbon fiber oxidation on the epoxy composite properties was studied by a modified axial tensile test, which allows additionally the measurement of the so-called 'notching force' as a measure of the interfacial composite properties. Common model-composite samples were used to check the reliability of this test. The determined 'notching force' as a measure of adhesion correlates with the increased polar component of the fiber surface tension.     

Effects of atmospheric pressure helium/air plasma treatment on adhesion and mechanical properties of aramid fibers

In order to investigate the effect of atmospheric pressure plasmas on adhesion between aramid fibers and epoxy, aramid fibers were treated with atmospheric pressure helium/air for 15, 30 and 60 s on a capacitively-coupled device at a frequency of 5.0 kHz and He outlet pressure of 3.43 kPa. SEM analysis at 10 000× magnification showed no significant surface morphological change resulted from the plasma treatments. XPS analysis showed a decrease in carbon content and an increase in oxygen content. Deconvolution analysis of C1s, N1s and O1s peaks showed an increase in surface hydroxyl groups that can interact with epoxy resin. The microbond test showed that the plasma treatment for 60 s increased interfacial shear strength by 109% over that of the control (untreated). The atmospheric pressure plasma increased single fiber tensile strength by 16-26%.     

Surface modification of nylon 6 films treated with an He/O2 atmospheric pressure plasma jet

To investigate the effect of the gas composition of the plasma treatment on the surface modification of an atmospheric pressure plasma jet (APPJ), nylon 6 films were treated with APPJ with pure helium (He), He + 1% oxygen (O₂), and He + 2% O₂, respectively. Atomic force microscopy showed increased surface roughness, whereas X‐ray photoelectron spectroscopy revealed increased oxygen contents after the plasma treatments. The plasma‐treated samples had lower water contact angles and higher T‐peel strengths than the control. The addition of a small amount of O₂ to the He plasma increased the effectiveness of the plasma treatment in the polymer surface modification in terms of surface roughness, surface oxygen content, etching rate, water contact angle, and bonding strength. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011