Seawater ageing of low styrene emission resins for marine composites: Mechanical behaviour and nano-indentation studies

Polyester resins are widely employed for pleasure boat construction. In order to satisfy new environmental legislation on styrene emissions resin suppliers have proposed modified formulations, but these show lower failure strains under tensile loading. This paper examines the influence of wet ageing on mechanical properties of low styrene polyester, together with standard polyester and vinyl ester resins, and their glass reinforced composites. Results are presented from accelerated ageing in natural sea water for nine months, at temperatures of 20, 40 and 60 °C. Nano-indentation was used to study local changes in the resin after ageing. The diffusion kinetics of the low styrene emission resins and their composites are similar to those for standard polyester resins. The low styrene resins appear to lose strength more slowly than the standard resins but after nine months in seawater at 40 °C similar property losses are noted.     

Application of Interlaminar Tests to Marine Composites. Relation between Glass Fibre/Polymer Interfaces and Interlaminar Properties of Marine Composites

The need for improved performance and the development of new composite manufacturing methods require a better understanding of the role of interface phenomena in the mechanical behaviour of these materials. The influence of the cure cycle on the bulk and surface properties of the matrix resin, and of composites based on polyester and epoxy resins reinforced with glass fibres has been studied. While the mechanical properties of the epoxy vary with cure temperature the surface tension is not affected. The increase in interfacial shear strength and interlaminar shear strength with increased cure temperature cannot be simply explained by the wetting of the fibres by the matrix. The importance of thermal stresses, generated at the interface by resin shrinkage and differences in thermal expansion, for the mechanical behaviour of the composite are demonstrated.     

Damage Resistance of Composites Based on Glass Fibre Reinforced Low Styrene Emission Resins for Marine Applications

Composites based on glass fiber reinforced low styrene emission polyester resins have been widely used over the last 10 years, in order to meet increasingly strict safety regulations, particularly in the pleasure boat industry. Previous studies of their mechanical properties suggested that although these resins are generally more brittle than traditional orthophthalic polyester resins this did not adversely affect the properties commonly used for quality control (short beam shear and tensile failure strength of mat reinforced composites). In the present paper results from a more detailed study of damage behaviour are presented. Tests include fracture toughness (K _Ic ) tests on resins, fibre/matrix interface energy, detection of composite damage initiation in tension by acoustic emission, composite delamnation (G _Ic and G _IIc ), and low energy impact. Overall the results indicate that the low failure strain of low styrene emission resins results in significantly lower composite damage resistance.     

The effect of resin failure strain on the tensile properties of glass fibre-reinforced polyester cross-ply laminates

An investigation has been made of the effect of resin properties on the transverse cracking behaviour of glass fibre-reinforced polyester resin three-ply laminates. The polyester resin properties were modified by the addition of a flexibilizing resin to produce five resin systems with failure stresses ranging from 1.75 to 11.1%. The mechanical properties of the resins which were determined, are observed to affect the stress level at which transverse cracking is initiated and the nature of the cracking behaviour. If fibre bunching is taken into account the Kies strain magnification theory can predict the general trend of the results. However, it is concluded that strain-rate effects associated with fibre bunching are worthy of further investigation.     

Novel method for functionalising and patterning textile composites: Liquid resin print

The paper reports a novel method of integrating resin into continuous textile reinforcement. The method presents a print of liquid reactive resin into textile preforms. A series of targeted injections forms a patch which upon consolidation and curing transforms into a stiff region continuously spanning through preform thickness. Enhancing the injected resin with conductive phase allows creating a pattern of patches with controlled dimensions and added functionalities. Patterned composites reveal features which are not typical for conventional composites such as fibre bridged interfaces, regular thickness variation, and gradient matrix properties. The presented study explores the role of these features in (a) the mechanical behaviour of these materials, focusing on their deformation and failure mechanisms in tension, and (b) the feasibility of adding functionality by printing electrically conductive resins containing carbon nano-tubes (CNT). It was shown that resin print is a promising method for local functionalization of structural composites.     

On fibre debonding effects and the mechanism of transverse-ply failure in cross-ply laminates of glass fibre/thermoset composites

The mechanism of transverse-ply failure in cross-ply laminates of glass fibre thermoset composites has been investigated. It is shown that fibre debonding initiates failure, the debonds subsequently joining up to form a transverse crack nucleus. In the epoxy system investigated fibre debonding causes an observable whitening effect and small modulus change; this effect is reversible in that rebonding can be brought about by further heat treatment. It is shown that in the case of the polyester system the larger thermal strains introduced during the curing cycle cause debonding of the composite and therefore the whitening effects are not observed on application of load. Simple models for the prediction of the observed effect of glass fibre volume fraction on transverse failure strain are proposed.     

Could biopolymers reinforced by randomly scattered flax fibre be used in structural applications?

Thermoplastics reinforced by natural fibres are mainly used for fitting-up products in the automotive industry. The aim of this work is to study the tensile properties of natural fibre-biopolymer composites in order to determine whether or not, biocomposites may replace glass fibre reinforced unsaturated polyester resins. The materials used are flax fibre, polylactic acid (PLA), l-polylactide acid (PLLA), poly(3-hydroxylbutyrate) (PHB), polycaprolactone and starch thermoplastic (MaterBi® Z), poly(butylene succianate) (PBS) and poly(butylene adipate-co-terephtalate) (PBAT). The tensile properties of the flax fibres have already been determined [C. Baley, Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase, Comp Part A 2002;33:939–948]. The composites are manufactured using a film stacking technique. After studying the processing parameters, these are then adapted to each thermoplastic composites. Test samples are cut out from the composites to test their mechanical properties under tensile loading conditions. These tensile properties are then compared to those of similar polypropylene flax composites. Preliminary results show that the tensile properties are improved with the fibre volume fraction. The tensile strength and Young’s modulus of PLLA and PLA flax composites are greater than those of similar PP/flax fibre composites. The specific tensile strength and modulus of flax fibre/PLLA composite have proved to be very close to those of glass fibre polyester composites.     

Recycling of shredded composites from wind turbine blades in new thermoset polymer composites

As the energy produced from wind increases every year, a concern has raised on the recycling of wind turbine blades made of glass fibre composites. In this context, the present study aims to characterize and understand the mechanical properties of polyester resin composites reinforced with shredded composites (SC), and to assess the potential of such recycling solution. A special manufacturing setup was developed to produce composites with a controlled content of SC. Results show that the SC in the composites was well distributed and impregnated. The composite stiffness was well predicted using an analytical model, and fibre orientation parameters for strength modelling were established. The stress-strain curves revealed composite failure at unusual low strain values, and micrographs of the fracture surface indicated poor adhesion between SC and matrix. To tackle this problem, chemical treatment of SC or use of an alternative resin, to improve bonding should be investigated.     

Comparison of the mechanical and physical properties of a carbon fibre epoxy composite manufactured by resin transfer moulding using conventional and microwave heating

Microwave processing holds great potential for improving current composite manufacturing techniques, substantially reducing cure cycle times, energy requirements and operational costs. In this paper, microwave heating was incorporated into the resin transfer moulding technique. Through the use of microwave heating, a 50% cure cycle time reduction was achieved. The mechanical and physical properties of the produced carbon fibre/epoxy composites were compared to those manufactured by conventional resin transfer moulding. Mechanical testing showed similar values of flexural moduli and flexural strength for the two types of composites after normalisation of the corresponding data to a common fibre volume fraction. A 9% increase of the interlaminar shear strength (ILSS) was observed for the microwave cured composites. This enhancement in ILSS is attributed to a lowering of resin viscosity in the initial stage of the curing process, which was also confirmed via scanning electron microscopy by means of improved fibre wetting and less fibre pull-out. Furthermore, both types of composites yielded minimal void content (<2%). Dynamic mechanical thermal analysis revealed comparable glass transition temperatures for composites produced by both methods. A 15 °C shift in the position of the β-transition peak was observed between thermally and microwave cured composites, suggesting an alteration in the cross-linking path followed.     

Wettability of glass fibres with different sizings and their adhesion to unsaturated polyester matrices

The wetting characteristics of unsaturated polyester resins on glass fibres with different sizings have been studied by contact angle determination periods. The styrene content of the matrix and the nature of the coatings result to be determinant in the wettability of glass fibres by the polyester matrices. Highest styrene content and styrene-soluble-coatings determine better wetting characteristics. Mechanical properties of the cured specimens and scanning electron microscopy on the fracture surfaces were also performed. Interesting correlations between the nature of the sizing agent and cohesive energy density with mechanical properties and fibre/matrix adhesion were found.[/p]     

Mechanical behaviour of carbon and glass hybrid fibre reinforced polyester composites

Mechanical behaviour of carbon fibre/glass mat/polyester resin hybrid composites of sandwich construction is studied through tension, flexure, impact and post-impact tension tests. Tensile and flexural strength, modulus and failure strain values are compared to the calculated values. Total impact fracture energy and residual (after impact) tensile strength values of hybrid composites are analysed with regard to corresponding values of carbon/polyester composites. Failure of tested coupons was analysed by visual inspection and observation by scanning electron microscopy.     

Dental application of various kinds of fibres in heat curing acrylic resin

The effect of the type of fibres on bending behaviour and impact energy in reinforced acrylic resin was examined. Reinforcing materials such as inorganic glass fibre and cloth and organic polyester and Kevlar fibres were coated with a silane coupling treatment, whereas Co-Cr wire was directly used for the reinforcement. After silane coupling treatment each fibre was added to the heat curing base resin, except polyester fibre. It is concluded that the inclusion of glass linear fibre provides an effective improvement on plain acrylic base resin. An additional way to reinforce the resin matrix was given by the combined use of glass and Kevlar fibres.     

A study of fibre and interface parameters affecting the fatigue behaviour of natural fibre composites

The tension–tension fatigue behaviour of different natural fibre reinforced plastics was investigated. The composites used were made of flax and jute yarns and wovens as reinforcements for epoxy resins, polyester resins and polypropylene.Fibre type, textile architecture, interphase properties, fibre properties and content were found to affect the fatigue behaviour strongly as illustrated with damping versus applied maximum load curves. It was found that natural fibre reinforced plastics with higher fibre strength and modulus, stronger fibre–matrix adhesion or higher fibre fractions possess higher critical loads for damage initiation and higher failure loads. In addition, damage propagation rates were reduced.Furthermore, unidirectional composites were less sensitive to fatigue induced damage than woven reinforced ones.     

Mechanical properties of nano-silica particulate-reinforced epoxy composites considered in terms of crosslinking effect in matrix resins

In this study, the mechanical properties of nano-silica particulate-reinforced epoxy composites with different crosslinking densities were clarified experimentally to consider the interaction effects between nano-particles and the network structure in matrix resin. The matrices were prepared by curing with an excessive mixture of diglycidyl ether of bisphenol A type epoxy resin as the curing agent for the stoichiometric condition. The volume fraction of the silica particles with a median diameter of 240 nm was constantly 0.2 for every composite. The crosslinking densities and glass transition temperatures of the neat epoxy resins were identified from thermo-viscoelastic properties measured by dynamic mechanical analysis. Elastic moduli and strengths of the composites and the neat epoxy resins were measured by three-point bending tests. The glass transition temperatures of the neat epoxy resins decreased linearly as the crosslinking densities decreased from the stoichiometric condition. The glass transition temperatures of the composites were reduced by adding the nano-silica particles. The bending moduli of the composites in the glassy state could be predicted by using a mixture law of the composites regardless of the crosslinking densities and glass transition temperatures. The bending strengths were found to be sensitive to the crosslinking densities: they were both higher (for composites with high crosslinking densities) and lower (for composites with low crosslinking densities) than those of the neat epoxy resin. These results demonstrate that the interaction between nano-particles and network structures reduces the bending strengths, especially for low crosslinking densities.     

Effect of structure on properties of vinyl ester resins

The paper describes the synthesis of vinyl ester resins based on diglycidyl ether of bisphenol-A (epoxy equivalent = 450–465 g/eq) (VR resin) and tetrabromobisphenol-A (epoxy equivalent = 380–420 g/eq) (VR-1 resin). The viscosity of styrenated VR resin was higher than VR-1 resin. The effect of styrene andα-methyl styrene on curing of VR resins was studied. An increase in styrene from 30 to 50 wt% resulted in an increase in gel time and a decrease in exothermic peak. Addition ofα-methyl styrene delayed and depressed the exotherm. The mechanical properties of VR resin sheets and glass fabric reinforced laminates were better than VR-1 resins; whereas LOI of VR-1 was higher. A resin formulation containing 20–30 wt% of VR: VR-1 showed optimum mechanical properties and LOI.     

Wrapped-yarn reinforced composites: Part II—composite properties

The flexural properties of composites fabricated from yarns formed by twisting paper tapes around mono- or multifilament cores and embedded in a polyester resin matrix have been examined in standard four-point bend, notched three-point bend and Charpy impact tests. It was found that the ease of core slippage, and therefore the composite properties, depended on the yarn packing density and the resin curing conditions. The wrapped-yarn composites bent in a ductile manner with a specific bending stiffness comparable to those of steel and aluminum but showed better recovery behaviour and less notch sensitivity than these two metals. The specific impact toughness of the wrapped-yarn composite increased with sample length and compared favourably with that of a commercial pultruded glass/polyester (GRP) composite containing a much higher proportion of glass fibres.     

Strength and adhesion characteristics of elementary flax fibres with different surface treatments

It is known that the best flax fibres can compete in terms of mechanical properties with glass fibres. However, during the manufacturing process flax fibres are often damaged, and hence, the properties can be lowered. Furthermore, these properties change from batch to batch (depending on the time and place of harvest), which means that they are somewhat unpredictable. The most affected fibre property is strength, which can vary in very wide interval due to defects introduced by the manufacturing process. Therefore, there is a need for a simple but reliable testing procedure that allows the estimation of the strength of flax fibres, so called quality control. Regarding the final goal, that is the development of natural fibre composites, another crucial property is the fibre/matrix adhesion. The objective of this study is to investigate the possibility to use the single fibre fragmentation test to characterize strength distribution of flax fibres and to evaluate the adhesion. Untreated flax fibres and fibres coated by a special surface treatment are used. Fragmentation tests are performed on flax fibres embedded in thermoset, vinylester and polyester, resins. Results show that there is a definite improvement in interfacial strength when a fibre surface treatment is applied. Fibre strength distribution is obtained from SFFT and compared with limited results available from single flax fibre tests.     

Effects of alkali treatment and elevated temperature on the mechanical properties of bamboo fibre–polyester composites

Bamboo fibre reinforced composites are not fully utilised due to the limited understanding on their mechanical characteristics. In this paper, the effects of alkali treatment and elevated temperature on the mechanical properties of bamboo fibre reinforced polyester composites were investigated. Laminates were fabricated using untreated and sodium hydroxide (NaOH) treated (4–8% by weight) randomly oriented bamboo fibres and tested at room and elevated temperature (40, 80 and 120 °C). An improvement in the mechanical properties of the composites was achieved with treatment of the bamboo fibres. An NaOH concentration of 6% was found optimum and resulted in the best mechanical properties. The bending, tensile and compressive strength as well as the stiffness of this composite are 7, 10, 81, and 25%, respectively higher than the untreated composites. When tested up to 80 °C, the flexural and tensile strength are enhanced but the bending stiffness and compressive strength decreased as these latter properties are governed by the behaviour of resin. At 40 and 80 °C, the bond between the untreated fibres and polyester is comparable to that of treated fibres and polyester which resulted in almost same mechanical properties. However, a significant decrease in all mechanical properties was observed for composites tested at 120 °C.     

Effect of resin system on the mechanical properties and water absorption of kenaf fibre reinforced laminates

The objective of this study is to compare the mechanical and water absorption properties of kenaf (Hibiscus cannabinus L.) fibre reinforced laminates made of three different resin systems. The use of different resin systems is considered so that potentially complex and expensive fibre treatments are avoided. The resin systems used include a polyester, a vinyl ester and an epoxy. Laminates of 15%, 22.5% and 30% fibre volume fraction were manufactured by resin transfer moulding. The laminates were tested for strength and modulus under tensile and flexural loading. Additionally, tests were carried out on laminates to determine the impact energy, impact strength and water absorption. The results revealed that properties were affected in markedly different ways by the resin system and the fibre volume fraction. Polyester laminates showed good modulus and impact properties, epoxy laminates displayed good strength values and vinyl ester laminates exhibited good water absorption characteristics. Scanning electron microscope studies show that epoxy laminates fail by fibre fracture, polyester laminates by fibre pull-out and vinyl ester laminates by a combination of the two. A comparison between kenaf and glass laminates revealed that the specific tensile and flexural moduli of both laminates are comparable at the volume fraction of 15%. However, glass laminates have much better specific properties than the kenaf laminates at high fibre volume fractions for all three resins used.     

Frequency dependence of gel time in thermosetting resins

The increasing use of thermosetting resins in fibre reinforced composites for structural and insulating applications has necessitated the need to understand the process of crosslinking and systematics of curing time of resins with and without the reinforcing fine fibre filaments. We have employed dynamical measurements of loss tangent and dielectric constant over the frequency range 1 kHz–100kHz for investigating cure characteristics of composites. After adding crosslinking agents to resinous matrices (epoxy and polyester styrenated alkyd resins) the time variation of dielectric parameters was recorded until the completion of curing. The resin flow, the onset of gelation and completion of curing were unambiguously mapped by variations in dielectric parameters. Onset of gelation coincided with precipitous fall of dielectric constant and resonant maximum of dielectric loss. Asymptotic stabilization of dielectric parameters signalled completion of curing. The gel time and curing time were found to be substantially larger for fibre reinforced resins as compared to pure resins. The variation of electrically determined gel and cure time with frequency of ac signal employed and on the libre fraction in the composite are discussed in the paper. The present study shows that if dielectric measurements are to be employed to infer the completion of curing, low frequency measurements would be needed to correctly infer onset of gelation and time for curing. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995