Characterising wood fibre mats as reinforcements for liquid composite moulding processes

Liquid composite moulding (LCM) processes are commonly used techniques for the manufacture of advanced composite structures. This study explores the potential of wood fibres as reinforcement for LCM preforms, considering discontinuous fibre mats produced using four different methods. Modified paper manufacturing techniques were employed to produce two types of wet formed mats, the other two being manufactured using dry methods. The dry compaction response of these mats has been investigated, required compression loads being measured up to a fibre volume fraction of 0.4. A complex non-elastic compression response was observed which has significant influence on forces generated within moulds. Saturated compaction tests were also carried out, the samples infiltrated with two different test fluids. A significant reduction in compaction load was observed due to wood softening when using a water based fluid. On the other hand, a non-water-based solution had little less influence on the compaction of the wood fibre mats. In addition, permeability of all four types of mats was measured as a function of fibre volume fraction. Reinforcement permeability and compaction response data are required to simulate LCM processes.     

Fiber-reinforced composites based on epoxy resins modified with elastomers and surface-modified silica nanoparticles

The properties of fiber-reinforced composites made using epoxy resin formulations can be improved using modified epoxy resins. As epoxies are inherently brittle, they are toughened with reactive liquid rubbers or core–shell elastomers. Surface-modified silica nanoparticles, 20 nm in diameter and with a very narrow particle size distribution, are available as concentrates in epoxy resins in industrial quantities for the past 10 years. Some of the drawbacks of toughening like lower modulus or a loss in strength can be compensated when using nanosilica together with these tougheners. Apparently, there exists a synergy as toughness and fatigue performance are increased significantly. Some of these improvements in bulk resin properties can be found for fiber-reinforced composites as well. In this article, the literature published in the last decade is studied with a focus on mechanical properties. Results are compared, and the mechanisms responsible for the property improvements are discussed. A relationship between the improvements of the fracture energy of the cured bulk epoxy resins and the fracture energy of the fiber-reinforced composites could be established.     

Permeability of natural fiber reinforcement for liquid composite molding processes

We investigate the influence of liquid type on the saturated permeability of natural fabrics in liquid composite molding processes. The permeability of flax woven fabric was characterized with two different liquids which have different viscosity, wettability, and sorption characteristics with flax fiber. From the experimental data, it was observed that the saturated permeability values were different for the liquid type. The fiber swell during the mold filling process and the corresponding change of fabric microstructure were assumed to be the main reason for this dependency of saturated permeability on the liquid type. The fiber swell due to the liquid sorption was characterized as a function of time, and the corresponding change of fiber diameter was investigated. The effective fiber volume fraction of wet natural fabric was defined in terms of fiber swelling ratio. The predictions by the classical Kozeny–Carman model and by the modified Kozeny–Carman model with two model constants were compared with the experimental data. It was shown that the modified Kozeny–Carman equation considering fiber swell could predict very well the saturated permeability of natural fabrics regardless of liquid type.     

Effective permeabilities of multilayer fabric preforms in liquid composite moulding

A homogenisation method was developed to predict effective permeabilities of multilayer fabric preforms by considering interlayer continuity and coupling between in-layer flow and trans-layer flow. A simplified approach was addressed first, from which the common approaches of arithmetic mean formation and harmonic mean formation can be deduced to define the effective permeability by neglecting coupling between in-layer flow and trans-layer flow. For multilayer preforms in liquid composite moulding, the permeability predicted by such an approach, however, is generally different from the actual one because of neglecting effects of micro-structure between layers and synchronisation between in-layer flow and interlayer flow. A conceptual model for interlayer flow, based on the hydraulic radius theory, was then proposed to quantitatively characterise the effect of interlayer micro-structure on the effective permeability of multilayer fabric preforms. The predictions show good agreement with experimental results available in literature.     

Influence of surface treatment on hybrid wollastonite-polyethylene composite resins for rotational moulding

The purpose of this research was to develop a reinforcing material for polyethylene-based composite manufacture by rotational moulding. Wollastonite, sisal fibres and PE are premixed by blending and compounding with a single screw extruder and then granulated to particles with diameter about 0.5 mm prior to rotational moulding, for which the mixture is placed in a mould that is heated from the outside to 250 °C for a period of about 10 min. Aminosilane was used as a surface treatment for wollastonite. It was found that incorporating wollastonite microfibres improved the tensile properties of the system. When wollastonite fibres were coated with aminosilane, the impact strength and processability were enhanced greatly. Sisal fibres were added to improve the impact properties. Scanning electron microscopy revealed good adhesion between the coated fibre reinforcement and the polyethylene matrix at the fracture surface. The mechanism of this phenomenon is discussed.     

Numerical prediction and experimental characterisation of meso-scale-voids in liquid composite moulding

Pressure gradients that drive the resin flow during liquid composite moulding (LCM) processes can be very low while manufacturing large composite parts. Capillary pressure becomes the predominant force for tow impregnation and thus meso-scale-voids can be generated, reducing the part quality. In contrast, micro-voids are created at high resin pressure gradients. In this work, a numerical method is presented to predict the creation of meso-scale-voids and their evolution. Experimental validation is conducted by measuring void content of produced composite parts with micro-computed tomography (μ-CT). Additionally, the void content as a function of the modified capillary number Ca^* is determined and the influence of the fibre volume content in the bundles on the meso-scale- and micro-void content is studied.     

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.     

Modelling flow and filtration in liquid composite moulding of nanoparticle loaded thermosets

This paper presents analytical and numerical models of liquid moulding of hybrid composites. An 1-D analytical solution of Darcy’s problem, accompanied by nanoparticle filtration kinetics and conservation, has been developed. A non-linear finite difference model incorporating variations in permeability, porosity and viscosity as a function of local nanoparticle loading was formulated. Comparison of the two models allowed verification of their validity, whilst a mesh sensitivity study demonstrated the convergence of the numerical scheme. The limits of validity of the analytical solution were established over a range of infiltration lengths and filtration rates for different nanoparticle loadings. The analytical model provides an accurate and efficient approximation of through thickness infusion of hybrid composites, whereas use of the numerical scheme is necessary for accurate simulation of in-plane filling processes. The models developed here can serve as the basis of process design/optimisation for the production of hybrid composites with controlled distribution of nano-reinforcement.     

Effect of processing variables on the durability of epoxy resins for composite systems

The Flory approach has been used to described the molecular characteristics of the non-linear polymerization of tetraglycidyl diamino diphenyl methane (TGDDM) cured with diphenyl sulfone (DDS).The cure behavior of commercial grade TGDDM-DDS mixtures was analyzed by means of differential scanning calorimetry (DSC), high-pressure liquid chromatography (HPLC), and steady and dynamic viscosity measurements. The gelation limits and molecular distribution measured in isothermal tests were in good agreement with experimental determinations. The gelation theory was further combined with the WLF treatment of the temperature dependency of the viscoelastic behavior for the reacting system in order to describe the viscosity profiles during complex cure cycles.Furthermore, this work relates moisture sorption and plasticization of two epoxy systems to hydrothermal and processing variables. A low cross-linked diglycidyl ether of bisphenol-A (DGEBA) cured with linear amines, such as ethylene diamine (EDA), diethylene triamine (DETA) and triethylene tetramine (TETA), and the highly cross-linked TGDDM-DDS system, have been studied. An increase in the equilibrium moisture content was observed as a result of thermal cycling in liquid environments. The hydrothermal interactions are shown to produce changes in the epoxy network structures and in the moisture sorption behavior.     

Effect of silica nanoparticles on compressive properties of an epoxy polymer

The effect of nanosilica on compressive properties of an Epikote 828 epoxy at room temperature was studied. A 40 wt% nanosilica/epoxy masterbatch (nanopox F400) was used to prepare a series of epoxy based nanocomposites with 5–25 wt% nanosilica content. Static uniaxial compression tests were conducted on cubic and cylindrical specimens to study the compressive stress–strain response, failure mechanisms and damage characteristics of the pure and nanomodified epoxy. It was found that the compressive stiffness and strength were improved with increasing nanosilica content without significant reduction in failure strain. The presence of nanosilica improved ductility and promoted higher plastic hardening behaviour after yielding in comparison with the unmodified resin system. This result suggested that nanoparticles introduced additional mechanisms of energy absorption to enhance the compressive properties without reducing the deformation to failure.     

Thermal expansion and swelling of cured epoxy resin used in graphite/epoxy composite materials

This paper presents results of experiments in which the thermal expansion and swelling behaviour of an epoxy resin system and two graphite/epoxy composite systems exposed to water were measured. It was found that the cured epoxy resin swells by an amount slightly less than the volume of the absorbed water and that the swelling efficiency of the water varies with the moisture content of the polymer. Additionally, the thermal expansion of cured epoxy resin that is saturated with water is observed to be more than twice that of dry resin. Results also indicate that cured resin that is saturated with 7.1% water at 95° C will rapidly increase in moisture content to 8.5% when placed in 1° C water. The mechanism for this phenomenon, termed reverse thermal effect, is described in terms of a slightly modified free-volume theory in conjunction with the theory of polar molecule interaction. Nearly identical behaviour was observed in two graphite/epoxy composite systems, thus establishing that this behaviour may be common to all cured epoxy resins.     

Strength of unidirectional glass/epoxy composite with silica nanoparticle-enhanced matrix

A technique was developed to improve the strength of unidirectional composites by enhancing the matrix properties through nanoparticles infusion. A commercially available standard DGEBA epoxy with silica nanoparticles (Nanopox F 400) was used as the matrix to make fiber composites. The silica nanoparticles in Nanopox were grown in situ via a sol–gel process resulting in a concentration of 40 wt% which was later diluted to 15 wt% particle loading. TEM images showed very uniform dispersion of silica nanoparticles with a size distribution of about 20 nm. Compression test revealed a substantial improvement (40%) in elastic modulus of the modified epoxy. A modified vacuum assisted resin transfer molding process was used to fabricate unidirectional E-glass fiber reinforced silica/epoxy nanocomposites. Inclusion of silica nanoparticles dramatically increased the longitudinal compressive strength and moderately increased the longitudinal and transverse tensile strengths. A microbuckling model was used to verify the compression testing results.     

Fatigue crack growth behavior of silica particulate reinforced epoxy resin composite

In the present work, fatigue crack growth tests of epoxy resin composite reinforced with silica particle under various R-ratios were carried out to investigate the effect of R-ratio on crack growth behavior and to discuss fatigue crack growth mechanism. Crack growth curves arranged by ΔK showed clear R-ratio dependence even under no crack closure, where the values of ΔK_th were 0.82 and 0.33 MPa √m for R = 0.1 and 0.7 respectively. However, crack growth curves arranged by K_max merged into almost one curve regardless of R-ratio, which indicated that crack growth behavior of the present composite was time-dependent. The value of K_max,th were in the range from 0.78 to 1.12 MPa √m. In situ crack growth observation revealed the crack growth mechanism: micro-cracking near the interface between silica particle and resin matrix occurs ahead of a main crack and then micro-cracks coalesce with a main crack to grow. The crack path was in the epoxy matrix, which was consistent with the time-dependent crack growth.     

Effect of reinforcement and solvent content on moisture absorption in epoxy composite materials

The effect of fibrous reinforcement and solvent content on moisture uptake in composite laminate was investigated. Two materials using identical epoxy resin systems but different reinforcements—glass vs. carbon fibers—and of different solvent content—low vs. normal—were examined. Samples were characterized in terms of water absorption and desorption. Mechanical and thermal properties including flexural modulus, flexural strength, and glass transition temperature were measured. Results clearly show the contribution of the fiber reinforcement as well as solvent content on the water absorption rate and mechanical property changes.     

Measurements of normal stress distributions experienced by rigid liquid composite moulding tools

Mould tools used for LCM processes such as Resin Transfer Moulding (RTM) and Injection/Compression Moulding (I/CM) must withstand local forces due to compaction of the fibre reinforcement, and due to resin pressure generated within the laminate. A series of RTM and I/CM experiments have been carried out, with the focus placed on measurement of normal stress distributions exerted on the mould surface. In addition, total mould clamping force and injection gate pressure histories have been recorded. I/CM experiments using force-controlled secondary compaction were also undertaken, and compared to the velocity-controlled cases. Observed fluid pressure fields showed good agreement with theory, namely a logarithmic distribution during fluid injection and a quadratic distribution during the compression driven filling phase of I/CM. Significant spatial variation in normal stress due to reinforcement compaction was observed. The influence of the fluid pressure on the total stress experienced by the mould was observed to be a function of both the fibre volume fraction of the part and the applied injection pressure, the latter being more pronounced at lower part volume fractions.     

Promising commercial reinforcement to the nanodiamond/epoxy composite by grafting ammonium ions

A simple, economic, efficient and eco-friendly nanodiamond (ND) modifying method to reinforce the ND/epoxy composite for the industrialization of the high-performance ND/epoxy composite is always desired. In the present work, the ND was successfully modified only using aqueous ammonia through an easy-to-operate method by replacing the hydrogen atoms in the carboxyl group with ammonium ions. Ammonia, which is the only pollutant in the process, could be recycled. The modified ND/epoxy composite showed an overwhelming advantage over the neat epoxy or the ND/epoxy composite in storage modulus in their glassy state without any degradation of tensile strength, hardness and fracture toughness.