N2 plasma-assisted grafting of fluorinated chains onto partially cured epoxy resins

Two routes for the grafting of fluorinated molecules to an epoxy resin were studied. The first one deals with the grafting of the liquid-state resin whereas the second one is focused on the grafting onto the solid-state resin. These grafting reactions were shown to be similar as studied through FTIR and XPS spectroscopies. However, it appears that the grafting onto the solid-state resin is limited by the curing advancement. N₂ plasma-activation was used to solve this drawback and enhanced the grafting yield. This grafting improvement was mainly explained in terms of the surface wetting improvement and the attachment of nitrogen containing groups at the surface of the treated resin.     

Simulating tape resin infiltration during thermoset pultrusion process

The thermoset tape pultrusion is a widely adopted manufacturing process to produce long, constant cross-section composite structural parts. For high volume production, low cost can be achieved by maximizing the production rate which is a function of the material and process parameters, more specifically the rate of resin infiltration and resin cure. During resin infiltration, the resin saturates the dry reinforcement either under positive pressure in the pressure chamber, or, by the action of capillary and surface forces, within the resin bath. In either case, the saturation must be completed as the tape is squeezed into the final cross-sectional form at the entrance of the heated mold where the resin will be cured to form the composite part.This paper models the resin infiltration process during pultrusion, by modifying the pre-existing simulation tool for liquid molding processes. The formulated capability can be used not only to optimize the impregnation dynamics within the pressure chamber, but can also be used to predict the required forces for the selected pulling rate. The proposed model does allow one to handle a variety of tape cross-sections, not just rectangular prisms.     

Fluorinated epoxy resin as a low adhesive mould for composite material

The aim of this work is to decrease the adhesion between a cured modified epoxy-based substrate and an in situ cured virgin epoxy-based piece. The effect of perfluorinated additives on the non-adhesion output is investigated through an adapted pull-off test. It appears that additive migration initiates the surface fluorination. Longer the fluorinated chain is, higher the surface fluorination is and weaker the adhesion strength is. The weak chemical affinity between these two epoxy resins is shown to be mainly responsible for these results leading to an adhesive rupture.     

Surface-Fluorinated Coatings

Fluoropolymers have many properties that are desirable in coatings. However, because of their poor solubility, formulating coatings containing fluoropolymers can be very difficult. Fluorine-based polymers that have been modified to increase their solubility have been used in a number of resin systems to bring the properties of fluoropolymers to the final coating. Typically these systems have fluorine throughout the entire resin and consequently have a high total fluorine level. We have found that by designing fluorinated molecules that stratify to the surface in the wet coating, we can deliver systems with the excellent surface properties of fluoropolymers but at very low total fluorine levels.     

Beam-type modeling for the free vibration of a long thick laminated conical tube

The free vibration of a long thick laminated conical tube is studied in this paper with a beam-type model. Taken as a beam, the tube is divided into elements by cross-sections. Each cross-section is a node. In the beam-type model, the nodal forces are assumed to be uniformly distributed along the middle circle, which is the intersection of the cross-section with the middle surface of the tube. The nodal torsional moment is assumed to be produced by a uniformly distributed tangential force along the middle circle. The nodal bending moments are assumed to be produced by sinusoidally distributed axial forces along the middle circle. Three nodal displacements and three nodal rotations corresponding to these nodal forces and moments, respectively, are determined by the energy principle and the assumed deformation pattern. Thus, in the beam-type model, there are only six degrees of freedom at each node. The stiffness matrix in terms of these nodal degrees of freedom is derived from a thick-shell theory together with a semi-analytical method. Based on the stiffness matrix, the frequencies of free vibration are calculated. The numerical results are checked by experimental measurements with good agreement.     

Effect of fluid–particle-interactions on dispersing nano-particles in epoxy resins using stirred-media-mills and three-roll-mills

Fibre reinforced composites are indispensable in the field of modern lightweight structures, such as used in aerospace, automotive industry or in wind power plants. Those materials provide high weight savings and increase the efficiency of a structure significantly. Therefore, various efforts are made to continuously improve the quality of the matrix and the fibres. By embedding nano-particles into the epoxy matrix, the mechanical properties as well as the electrical and thermal characteristics can significantly be improved [1]. In most cases these nano-sized particles are produced as dry powders not as single primary particles but rather as particle collectives consisting out of several primary particles. For the application in reinforced composites the particles must be suspended in epoxy resin as separately dispersed primary particles or in a certain aggregate size. Generally, the influencing parameters to break up the aggregates in a dispersion process can be divided into the stress mechanism, the intensity and the frequency of the dispersing machine itself, the properties of the dispersed particles (e.g. the particle–particle interactions) the properties of the homogenous phase and the particle–resin-interactions. Besides the effect of the chosen dispersing machine the optimization of the dispersing process was investigated by applying modified particle surfaces and varying the fluid properties. The results show that the surface properties of the particles must fit to the epoxy resin properties and the attractive forces between the primary particles must be reduced or the stabilization improved, respectively. An indication for an improved stabilization and adjustment of the particles surface properties to the fluid properties can be obtained by measurements of the contact angle and the rheological properties. Generally, an increase of viscosity and mass fraction of the product leads to a higher energetic efficiency of the dispersion process in the stirred media mill and three-roll-mill.     

Moisture absorption by cyanate ester modified epoxy resin matrices. Part III. Effect of blend composition

The moisture absorption of thermally cured cyanate ester modified epoxy resin matrices has been studied under constant hygrothermal conditions and with a series of thermal spikes. Various resin blends of differing composition were studied, so that the effect of resin structure on moisture absorption could be assessed. It was found that intermittently applied thermal spikes can enhance the moisture absorption by cyanate esters and their blends with epoxy resin. The equilibrium moisture concentration was found to increase with the fraction of cyanate ester in the blend. The results of the desorption study on both control and spiked specimens showed that some of the water molecules remain entrained in the resin, probably as a result of hydrolysis of the polymer matrix. Up to a thermal spiking temperature of 160 °C, the crosslink density remained constant but at higher temperatures tended to decrease, as observed by examination of the modulus above the glass transition temperature.     

In-plane permeability characterization of a unidirectional flax/paper reinforcement for liquid composite molding processes

Principal in-plane permeabilities of a unidirectional flax/paper reinforcement are characterized in terms of reinforcement material and manufacturing parameters at a constant fiber volume fraction (V_f). ANOVA result shows that surface density of the unidirectional flax layer is the most important parameter on the mean and variance of the K₁ permeability. On the other hand all four studied parameters are concluded to affect the K₂ permeability. The K₁ permeability is found close to that of a twill weave flax fiber fabric reported in the literature and only one order of magnitude lower than a plain weave glass fiber fabric. Impregnation of the reinforcement with epoxy resin shows that a large area of the molded plaques was dominated by capillary forces during resin injection. This means capillary number and subsequently the resin injection velocity should be optimized for reducing void content in the final composite.     

Surface Electrical Properties of Fluorinated Epoxy Resin and Influence of Ambient Humidity

As-prepared epoxy resin samples were surface fluorinated using a F₂/N₂ mixture. The fluorination resulted in a remarkable increase in surface conductivity of the epoxy sample by over three orders of magnitude. Corona charges deposited on the fluorinated surface could not be stored even at room temperature. Obvious dependence of surface conductivity and steady state surface current on ambient humidity was found for the fluorinated sample. ATR-IR analysis and SEM imaging of surface and cross-section demonstrated substantial changes in surface layer chemical composition and morphology due to the fluorination, which are responsible for the changes in surface electrical properties.     

Low viscosity cyanate ester resin for the injection repair of hole-edge delaminations in bismaleimide/carbon fiber composites

The repair efficiency of bisphenol E cyanate ester (BECy) resin was investigated for the injection repair of high temperature polymer–matrix composites by ultrasonic C-scan mapping, fluorescent dye penetration, optical microscopy, hole plate shear (HPS), and post delamination compression tests. Bismaleimide/carbon fiber (BMI–cf) composites were chosen as a model substrate. A vacuum-based resin injection repair method was used for repairing the pre-damaged composite specimens. The effect of surface wettability on the repair efficiency of BECy on BMI–cf composite substrate was studied by temperature dependent contact angle measurements. C-scan, fluorescent dye penetration, and optical microscopy images of pristine, delaminated, and repaired specimens reveal efficient infiltration of resin in specimens repaired at elevated temperatures. The repair efficiency calculated from HPS and post delamination compression tests was observed to be 155% and 100%, respectively, illustrating the capability of BECy for repairing high temperature structural composites.     

Modification of polyester resin based composites induced by seawater absorption

This research is a study on seawater absorption ability and on mechanical performance (before and after immersion in seawater) of two composites that basically differ for what concerns the polyester resin (isophthalic or orthophthalic) employed in boats manufacture. Experimental tests, carried out on the two resins, evidenced that they differ for what concerns their structural organization, water diffusion coefficient, thermal stability, wet ability toward glass fibre beside commercial cost. Isophthalic resin resulted well bonded to glass fibres so that seawater absorption resistance was higher compared with that of orthophthalic one. As a consequence, the composite containing isophthalic resin showed higher flexural stiffness and strength, as well as higher shear strength.     

Molecular dynamics study of the interfacial strength between carbon fiber and phenolic resin

The interfacial strength between carbon fiber and phenolic resin is studied using molecular dynamics simulations to demonstrate that carbon fiber-reinforced carbon matrix composites (C/C composites) have improved tensile strength. Simulations are performed using two carbon fiber models, one of which has only carbon atoms and the other has carbon atoms and some fluorinated carbon groups. The carbon fiber models are regarded as two-layer graphite, and the phenolic resin model is treated as cross-linked structures. All force field parameters are based on the Dreiding force field. The tensile stress and interfacial fracture energy are calculated for the estimation of the interfacial strength. The results show that the model including the fluorinated carbon groups has lower interfacial strength than the model having only carbon atoms, up to a certain coating ratio of fluorinated carbon groups. Similarly, within the limits of the coating ratio, the interfacial fracture energy of the fluorinated carbon fiber model is lower than that of carbon fiber model having only carbon atoms.     

功能化纳米TiO2/环氧树脂超疏水防腐复合涂层的制备与性能

超疏水材料在金属防腐领域具备巨大的潜在应用前景。为得到疏水性能及防腐性能俱优的纳米TiO₂/环氧树脂复合涂层材料,首先以三甲氧基十七氟癸基硅烷和γ-氨丙基三乙氧基硅烷(KH550)对纳米TiO₂表面功能化;以全氟辛基甲基丙烯酸酯对固化剂二乙烯三氨(DETA)进行氟化;最后通过一步共混法和两步喷涂法分别制备出两种复合涂层。利用FTIR、XPS、1HNMR分析氟化固化剂(F-DETA)和氟化纳米TiO₂(f-TiO₂)的物相组成和组织结构。接触角测试仪和静置实验表明,当三甲氧基十七氟癸基硅烷和γ-氨丙基三乙氧基硅烷的摩尔比为1∶15时f-TiO₂的性能最佳,所制备的复合涂层接触角达到164.9°。SEM表征结果显示通过两步法制备的f-TiO₂/环氧树脂复合涂层具备更均匀的粗糙表面、涂层内部孔隙率较低且环氧树脂层与f-TiO₂层具备梯度结构。摩擦实验证明两步法制备的f-TiO₂/环氧树脂复合涂层的超疏水性具备较好的机械稳定性。Tafel极化曲线和电化学阻抗谱(EIS)研究表明,通过两步法制备的f-TiO₂/环氧树脂复合涂层具有优异的防腐性能,其腐蚀抑制效率高达99.99%。      

Effect of alumina,silk and ceria short fibers in reinforcement of Bis-GMA/TEGDMA dental resin

The present study is focused to investigate influence of short fibers such as Alumina Microfibers (AMFs), Silk Microfibers (SMFs) and Ceria Nanofibers (CNFs) as reinforcements in Bis-GMA/TEGDMA resin towards development of composite dental filler. Morphologies of AMFs, SMFs, CNFs and their representative fracture surfaces of the reinforced dental resins/composites were examined by SEM. X-ray Diffraction Analysis was done to analyse the phase of the fibers used in this study and degree-of-conversion of the fiber incorporated base resin was studied by FTIR. Viscosity study of fiber resin mixture, depth of cure and mass change behaviour of the fibers resin composites in artificial saliva were done to analyse the flow ability and physical properties of the fiber resin composites. Mechanical properties of the composites were tested by a universal testing machine. This study demonstrated that incorporation of 10% AMFs, 5% SMFs, and 3.33% CNFs individually in Bis-GMA/TEGDMA dental resin resulted in similar degree of conversion compared to the control. Also the fiber reinforced composites (10% AMFs, 5% SMFs, and 3.33% CNFs) demonstrated significant improvement in mechanical properties compared to Bis-GMA/TEGDMA resin (Control). However, depth of cure was significantly reduced due to incorporation of fibers in the resin. The reinforcement effect of AMFs, SMFs in dental resin was superior due to their uniform distribution and good interfacial bonding between fibers and resin matrix. In case of CNFs, rapid increase in viscosity during mixing of fibers with resin and inhomogeneous mixing were the major problem encountered during formulation, which was mainly associated with high surface to volume ration of the nanofibers. The resultant composite containing CNFs had less improvement in mechanical properties which may be due to less fiber content, formation of agglomerates and improper distribution of the fibers in the composite which subsequently resulted in reduction of adhesive strength.     

SWNT composite coatings as a strain sensor on glass fibres in model epoxy composites

The preparation of a model glass-fibre/epoxy composite with single-walled carbon nanotubes (SWNTs) incorporated as a strain sensor on the fibre surface is described. A micromechanical study of stress transfer at the fibre–matrix interface followed using Raman spectroscopy properties is reported. The SWNTs were distributed along the fibre surface either by dispersing them in an amino-silane coupling agent or coating with an epoxy resin solution containing the SWNTs. The point-by-point mapping of the fibre strain in single fibre fragmentation tests has been undertaken for the first time using SWNTs on the fibres and the interfacial shear stress distribution along the fibre length was determined using the embedded SWNTs. The behaviour was found to be consistent with the classical shear-lag model. The effects of SWNT type and preparation procedure on the sensitivity of the technique were evaluated and optimized from single fibre deformation tests.     

Anomalous radial migration of single DNA molecules in capillary electrophoresis

We report the unexpected radial migration of DNA molecules in capillary electrophoresis (CE) with applied Poiseuille flow. Such movement can contribute to anomalous migration times, peak dispersion, and size and shape selectivity in CE. When Poiseuille flow is applied from the cathode to the anode, DNA molecules move toward the center of the capillary, forming a narrow, highly concentrated zone. Conversely, when the flow is applied from the anode to the cathode, DNA molecules move toward the walls, leaving a DNA-depleted zone around the axis. We showed that the deformation and orientation of DNA molecules under Poiseuille flow was responsible for the radial migration. By analyzing the forces acting on the deformed and oriented DNA molecules, we derived an expression for the radial lift force, which explained our results very well under different conditions with Poiseuille flow only, electrophoresis only, and the combination of Poiseuille flow and electrophoresis. Factors governing the direction and velocity of radial migration were elucidated. Potential applications of this phenomenon include an alternative to sheath flow in flow cytometry, improving precision and reliability of single-molecule detection, reduction of wall adsorption, and size separation with a mechanism akin to field-flow fractionation. On the negative side, nonuniform electroosmotic flow along the capillary or microfluidic channel is common in CE, and radial migration of certain analytes cannot be neglected.     

Characterization of epoxy functionalized graphite nanoparticles and the physical properties of epoxy matrix nanocomposites

This work presents a novel approach to the functionalization of graphite nanoparticles. The technique provides a mechanism for covalent bonding between the filler and matrix, with minimal disruption to the sp² hybridization of the pristine graphene sheet. Functionalization proceeded by covalently bonding an epoxy monomer to the surface of expanded graphite, via a coupling agent, such that the epoxy concentration was measured as approximately 4 wt.%. The impact of dispersing this material into an epoxy resin was evaluated with respect to the mechanical properties and electrical conductivity of the graphite–epoxy nanocomposite. At a loading as low as 0.5 wt.%, the electrical conductivity was increased by five orders of magnitude relative to the base resin. The material yield strength was increased by 30% and Young’s modulus by 50%. These results were realized without compromise to the resin toughness.     

Direct fluorination of Twaron fiber and the mechanical, thermal and crystallization behaviour of short Twaron fiber reinforced polypropylene composites

An experimental study of the incorporation of non-fluorinated and fluorinated Twaron fibers in polypropylene (PP) is presented. Surface modifications were made to Twaron fiber by direct fluorination technique using elemental fluorine in order to improve the interfacial adhesion between the fiber and matrix. Composites of PP/Twaron fiber (both Fluorinated and non-fluorinated) with 0.6%, 1.25%, 5% and 10% of Twaron fibers (w/w) were prepared by a solution method. Mechanical behaviour was estimated by the measurement of the tensile strength. The mechanical properties of PP improve significantly with the incorporation of Twaron fibers and fluorinated fiber composites show superior mechanical properties compared to the non-fluorinated system. The morphology was determined by scanning electron microscopy (SEM), showing good dispersion of the fibers. The thermal and crystallization behaviour of PP/Twaron fiber composites were studied by thermogravimetry (TG) and differential scanning calorimetry (DSC). The effect of fiber content and fiber surface treatments on the thermal properties was evaluated. DSC analysis exhibited an increase in the crystallization temperature and crystallinity, melting temperature upon the addition of fluorinated fibers to the PP matrix. This is attributed to the nucleating effects of the fiber surfaces. Also the thermal stability (from TG) and surface energy (determined from contact angle measurement) increased for fluorinated fiber composites. Surface modification of Twaron fibers leads to improved adhesion with the PP matrix and hence an improvement in properties of the Twaron fiber-PP composites.     

Effects of treatment with low molecular weight phenol formaldehyde resin on the surface characteristics of oil palm (Elaeis quineensis) stem veneer

Even though oil palm (Elaeis quineensis) stem (OPS) is highly potential as an alternative raw material in wood industry, it possesses some inferior characteristics. One of the critical weaknesses is a high degree of veneer surface roughness that resulted in high resin consumption during plywood manufacture. The objective of this study was to investigate effects of treatment with low molecular weight phenol formaldehyde (LMWPF) resin on the wettability and surface roughness of OPS veneer. OPS veneers were segregated into two categories namely outer and inner layer veneer, prior to soaking in LMWPF resin solution to obtain weight percent gain of 16–20%. The wettability of OPS veneers was assessed with contact angle measurement according to the sessile drop method. The veneer surface roughness was evaluated by determining the average roughness (R_a), mean peak-to-valley height (R_z), and maximum roughness (R_max) using a stylus profilometer in accordance with DIN standard 4768. The results show that the effect of LMWPF resin treatment on the surface roughness of the veneers is statistically significant. The technique used in the study was able to enhance the surface properties as well as improved the physical and mechanical properties of OPS plywood.     

A hybrid numerical and imaging approach for characterizing defects in composite structures

In this study, a hybrid approach coupling hyperspectral near infrared imaging with a progressive finite element method is proposed for characterization of the elastic and failure response of composites with non-uniform variations of the wrinkles profile through the thickness and across the structure dimensions. In this approach, hyperspectral near infrared spectroscopy is used to create a 3D profile of the surface resin pockets with the capability of measuring resin thickness from approximately 125 to 2500 μm. These resin pockets are directly correlated to underlying ply level wrinkling as confirmed by optical microscopy. The 3D mapped resin plane obtained from the hyperspectral imaging is used to morph a ply-by-ply finite element model of a carbon-fiber/epoxy resin laminated plate using a progressive damage failure methodology. The results show the capability of the hybrid method to predict the structural response in laminated composites containing spatially distributed and non-uniform ply-level wrinkling.