Plasma surface modification of advanced organic fibres

Aramid and extended-chain polyethylene fibres have been treated in ammonia and oxygen plasmas in order to enhance adhesion to vinylester resins and thereby improve fibre/resin interfacial properties in composites made from these materials. For both aramid/vinylester and extended-chain polyethylene/vinylester composites, the plasma treatments result in significant improvements in interlaminar shear strength and flexural strength. Extended-chain polyethylene/vinylester composites also exhibit increased flexural modulus. Scanning electron and optical microscopic observations have been used to examine the microscopic basis for these results, which are compared with results previously obtained for aramid/epoxy and extended-chain polyethylene/epoxy composites. It is concluded that the increased interlaminar shear and flexural properties of vinylester matrix composites are due to improved wetting of the surface-treated fibres by the vinylester resin, rather than covalent chemical bonding.     

Plasma surface modification of advanced organic fibres: Part V Effects on the mechanical properties of aramid/phenolic composites

Aramid fibres have been treated in ammonia and oxygen plasma to enhance adhesion to resole phenolic resins. The plasma treatments resulted in significant improvements in interlaminar shear strength (ILSS) and flexural strength of composites made from these materials. Composites containing aramid fibres with epoxide groups reacted on to the ammonia plasma-treated fibre surface also showed further improvements in ILSS and flexural strength. Scanning electron and optical microscopic observations were used to examine the microscopic basis for these results, which have been compared with those obtained previously for aramid/epoxy and aramid/vinyl ester composites. For composites containing oxygen and ammonia plasma-treated fibres, the enhanced ILSS and flexural strength are attributed to improved wetting of the surface-treated aramid fibres by the phenolic resin. However, for those containing fibres with reacted epoxide groups on the ammonia plasma-treated fibre surfaces, the enhanced composite properties may be due to covalent chemical interfacial bonding between the epoxide groups and the phenolic resin. Effects of catalyst levels and cure cycle on the ILSS of composites laminated with untreated fabric has also been examined and optimum values have been determined. The catalyst concentration has an influence on the phase-separated water domain density in the matrix which in turn, affects the available fibre/matrix bonding area and hence the composite ILSS and flexural strength. This revised version was published online in November 2006 with corrections to the Cover Date.     

The interfacial properties of aramid/epoxy model composites

Many attempts have been made to measure, evaluate and improve the level of interfacial adhesion in aramid/epoxy composites. Different surface treatments have been developed in order to promote chemical bonding between the fibre and the matrix but it is found that most of the surface treatments developed have shown little or no improvement in the level of interfacial adhesion. The interfacial properties of a model composite are often determined by measuring the interfacial shear strength using micromechanical test methods that employ different loading configurations. However, the values of interfacial shear strength determined using different test methods are found to be dependent upon the variation of localized stress in the samples due to the different loading configurations and often give different results. Using Raman spectroscopy it is shown that the strain-dependent shift of the 1610 cm^–1 aramid Raman band can be used to determine the point-to-point variation of axial fibre strain along aramid fibres embedded in epoxy resin matrices from which the interfacial properties can be derived. The interfacial properties of aramid/epoxy model composites have been determined using Raman spectroscopy where the properties of the fibre, including different surface treatments, and the matrix have been changed systematically. The results are reviewed here and compared to those obtained using conventional micromechanical test methods. It is also demonstrated that the Raman technique can be used to characterize the interfacial properties of aramid/epoxy model composites deformed using different micromechanical test methods. In this way the interfacial properties can be determined at different loading levels enabling the progressive failure of the fibre/matrix interface to be monitored and defined accurately.     

Plasma surface modification of advanced organic fibres

The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.     

Plasma surface modification of advanced organic fibres

A marked improvement in the interlaminar shear strength and flexural strength of aramid/ epoxy composites is observed when the fibres are pretreated in an ammonia or ammonia/ nitrogen gaseous discharge (plasma) to introduce amine groups on to the fibre surface. Scanning electron and optical microscopic observations are used to examine the microscopic basis for these results. Scanning electron micrographs of shear fracture surfaces show clean fibre/matrix separation in composites made from untreated fibres, indicative of weak interfacial bonding. In contrast, shear fracture surfaces of composites containing plasma-treated fibres exhibit clear evidence of fibre fibrillation and matrix cracking, suggesting stronger interfacial bonding. Optical microscopic examination of flexure specimens shows that enhanced strength results mainly from reduced compressive fibre buckling and debonding, due to an increase in fibre/matrix interfacial bond strength. This increase is not accompanied by any significant change in the interlaminar fracture energy or flexural modulus of the composites, but there is an appreciable loss in transverse ballistic impact properties. These results are also examined in terms of the observed increase in fibre/matrix interfacial strength.     

Composite materials based on wood and nylon fibre

Nylon-wood fibre and polypropylene-wood fibre composite materials were manufactured without any additives to determine the effects of wood fibre on the mechanical properties of the different composites. The raw materials used were eucalypt hardwood fibre, Nylon fibre obtained from stockings, and polypropylene (PP) pellets. A hot press technique was used to manufacture the composite materials, and improvements in the manufacturing methods are suggested. Tests were carried out on the manufactured boards to determine tensile strength and modulus of elasticity. Fracture surfaces were examined using scanning electron microscopy to investigate failure mechanisms. An increase in tensile strength and modulus of elasticity was observed in wood fibre/Nylon matrix composites, indicating that interfacial bonding occurred between these two phases. Bundles of wood fibres with internal voids prevent achieving maximum mechanical properties. The tensile strength of the PP based composites decreased significantly with increasing wood fibre content.     

Shear strength and fracture toughness of carbon fibre/epoxy interface: effect of surface treatment

Textile-reinforced composites have become increasingly attractive as protection materials for various applications, including sports. In such applications it is crucial to maintain both strong adhesion at fibre–matrix interface and high interfacial fracture toughness, which influence mechanical performance of composites as well as their energy-absorption capacity. Surface treatment of reinforcing fibres has been widely used to achieve satisfactory fibre–matrix adhesion. However, most studies till date focused on the overall composite performance rather than on the interface properties of a single fibre/epoxy system. In this study, carbon fibres were treated by mixed acids for different durations, and resulting adhesion strength at the interface between them and epoxy resin as well as their tensile strength were measured in a microbond and microtensile tests, respectively. The interfacial fracture toughness was also analysed. The results show that after an optimum 15–30 min surface treatment, both interfacial shear strength and fracture toughness of the interface were improved alongside with an increased tensile strength of single fibre. However, a prolonged surface treatment resulted in a reduction of both fibre tensile strength and fracture toughness of the interface due to induced surface damage.     

Effect of acid and TETA modification on mechanical properties of MWCNTs/epoxy composites

Acid treatment and triethylene-tetramine (TETA) modification of multi-walled carbon nanotubes (MWCNTs) purposing to attain better dispersibility and stronger interfacial bonding between MWCNTs and epoxy matrix have been carried out in this paper. The epoxy and MWCNTs/epoxy composites were produced by cast molding method. Stress–strain curves show that TETA-MWCNTs/epoxy hold the greatest toughness of all samples with 0.5 wt.% nanoparticles. The Young’s modulus of TETA-MWCNTs/epoxy has a significant increase about 38% compared to the neat epoxy, while the Young’s modulus of unmodified MWCNTs/epoxy or acid-modified MWCNTs/epoxy has a bit of decrease. Tensile and impact strength tests reflect that TETA-MWCNTs reinforced epoxy composites have an obvious improvement of tensile strength about 30% and an enhancement of impact strength over 34% compared to the pure epoxy composites with only 0.5 wt.% loading of TETA-MWCNTs. Scanning electron microscopy images of fractured surface of MWCNTs/epoxy indicate homogeneous dispersibility of TETA-MWCNTs and strong interfacial adhesion between the TETA-MWCNTs and the epoxy in the MWCNTs/epoxy composite.     

Effects of fibre/matrix adhesion on carbon-fibre-reinforced metal laminates—I.: Residual strength

The role of interfacial adhesion between fibre and matrix on the residual strength behaviour of carbon-fibre-reinforced metal laminates (FRMLs) has been investigated. Differences in fibre/matrix adhesion were achieved by using treated and untreated carbon fibres in an epoxy resin system. Mechanical characterisation tests were conducted on bulk composite specimens to determine various properties such as interlaminar shear strength (ILSS) and transverse tension strength which clearly illustrate the difference in fibre/matrix interfacial adhesion. Scanning electron microscopy confirmed the difference in fracture surfaces, the untreated fibre composites showing interfacial failure while the treated fibre composites showed matrix failure. No clear differences were found for the mechanical properties such as tensile strength and Young's modulus of the FRMLs despite the differences in the bulk composite properties. A reduction of 7·5% in the apparent value of the ILSS was identified for the untreated fibre laminates by both three-point and five-point bend tests. Residual strength and blunt notch tests showed remarkable increases in strength for the untreated fibre specimens over the treated ones. Increases of up to 20% and 14% were found for specimens with a circular hole and saw cut, respectively. The increase in strength is attributed to the promotion of fibre/matrix splitting and large delamination zones in the untreated fibre specimens owing to the weak fibre/matrix interface.     

高强高模聚酰亚胺纤维/环氧树脂复合材料力学性能与破坏机制

以高强高模聚酰亚胺（PI）纤维为增强体,以航空级环氧树脂（EP）为基体,通过热熔法制备预浸料并采用热压罐成型技术制备了PI/EP复合材料层合板,对其力学性能和破坏形貌进行了分析。结果表明:高强高模PI纤维与EP具有良好的界面结合力,PI/EP复合材料的层间剪切强度为65.2 MPa,面内剪切强度为68.6 MPa;良好的界面结合状态能充分发挥PI纤维优异的力学性能,PI/EP复合材料的纵向拉伸强度达1 835 MPa,弯曲强度为834 MPa;PI/EP复合材料纵向拉伸破坏模式为散丝爆炸破坏,同时由于高强高模PI纤维还具有优异的韧性和较高的断裂伸长率,PI/EP复合材料从受力到失效断裂的时间较长;PI/EP复合材料纵向压缩破坏模式为45°折曲带破坏。高强高模PI/EP复合材料为航空航天先进复合材料增加了一个全新的选材方案。      

Effect of UV and water spraying on the mechanical properties of flax fabric reinforced polymer composites used for civil engineering applications

The lack of data related to durability is one major challenge that needed to be addressed prior to the widespread acceptance of natural fibre reinforced polymer composites for engineering applications. In this work, the combined effect of ultraviolet (UV) radiation and water spraying on the mechanical properties of flax fabric reinforced epoxy composite was investigated to assess the durability performance of this composite used for civil engineering applications. Specimens fabricated by hand lay-up process were exposed in an accelerated weathering chamber for 1500 h. Tensile and three-point bending tests were performed to evaluate the mechanical properties. Scanning electron microscope (SEM) was used to analyse the microstructures of the composites. In addition, the durability performance of flax/epoxy composite was compared with synthetic (glass and carbon) and hybrid fibre reinforced composites. The test results show that the tensile strength/modulus of the weathered composites decreased 29.9% and 34.9%, respectively. The flexural strength/modulus reduced 10.0% and 10.2%, respectively. SEM study confirmed the degradation in fibre/matrix interfacial bonding after exposure. Comparisons with other composites implies that flax fabric/epoxy composite has potential to be used for civil engineering applications when taking its structural and durability performance into account. Proper treatments to enhance its durability performance will make it more comparable to synthetic fibre reinforced composites when considering as construction building materials.     

Mechanical Properties of Thermoplastic and Thermoset Composites Reinforced with 3D Biaxial Warp-knitted Fabrics

In this study, two types of thermoplastic matrices (low melting point polyethylene terephthalate (LPET) fiber and polypropylene (PP) fiber) and glass fiber/epoxy resin/multi-walled carbon nanotubes (MWCNTs) were used to fabricate the thermoplastic and thermoset composite materials with 3D biaxial warp-knitted fabrics. Thermoplastic and thermoset composites were fabricated using hot-press and resin transfer molding (RTM) methods. The fabricated samples were tested with tensile and three-point flexural tests. In thermoplastic composites, samples in the 90° direction and LPET matrix showed the best tensile and flexural properties with an improvement of 39 and 21% tensile modulus and strength, 16 and 8% flexural modulus and strength compared to the PP samples in the same direction. In thermoset composites, samples in the 90° direction and MWCNTs showed the best improvement of the flexural modulus and strength with 97 and 58% compared to the samples without MWCNTs. This improvement can most likely be attributed to an increase in interfacial adhesion due to the presence of the carbon nanotubes.     

Assessment of interfacial bonding between polymer threads and epoxy resin by transverse fibre bundle (TFB) tests

Two experimental approaches were employed to assess the fibre/matrix adhesion between polymer threads and epoxy resin by transverse fibre bundle (TFB) tests. The first approach was to measure interfacial bonding strength of the fibre/matrix interface in dog-bone-shaped tensile specimens by applying normal stress until failure, simulating the Mode I failure mode. The second approach was to determine the fibre/epoxy interfacial bonding strength in shear (simulating the Mode II failure mode) by means of a V-notched beam shear testing method, i.e. a modified Iosipescu test. In both methods, polymer threads were transversely incorporated in the middle section of the specimens. It was found that both methods were simple, reliable, and sensitive to changes in the fibre/matrix adhesion conditions, though interpretation of the test results was somewhat complex. The two experimental approaches were able to produce consistent results and can thus be adopted as alternative methods for determining the interfacial bonding properties between fibres and matrix in composite systems where conventional micro-mechanical or macro-mechanical testing methods cannot be used.     

The bonding mechanism of aramid fibres to epoxy matrices

Despite considerable attempts to increase aramid-epoxy adhesion, to date, the adhesion levels achieved between aramid fibres and epoxy matrices are less than optimum for some applications. A combination of the aramid fibres' morphology, physical and chemical properties, and the interfacial mechanical stresses is responsible for the lack of success in increasing aramid-epoxy adhesion level. A key to improving the aramid-epoxy adhesion is a basic understanding of the interfacial mechanisms by which fibre and matrix interact. There is a considerable number of publications on aramid fibres and their composites. This paper reviews some of the literature relevant to aramid-epoxy bonding mechanisms.     

Tensile,impact and dielectric properties of three dimensional orthogonal aramid/glass fiber hybrid composites

Aramid/glass hybrid composites with three different stacking sequences and their corresponding single fiber type composites have been fabricated and their tensile, impact and dielectric properties were investigated. The trend of tensile strength and modulus of the composites followed the rule of mixture (ROM) closely and a small but positive hybrid effect for tensile strength of the hybrid composites was observed. The hybrid composites in general had a higher impact resistance than the single fiber type composites and the hybrid composite in which fiber volume fractions for glass and aramid fiber were the most balanced showed the highest impact ductility. The aramid fiber composite showed a lower dielectric constant and a higher dielectric loss than the glass fiber composites. However, the dielectric constant of the hybrid composites decreased first and then increased as the volume fraction of aramid fiber increased, which did not follow the mixing rule for dielectric constants of compounds. The dielectric loss of the composites increased monotonically as the volume fraction of aramid fiber increased which agreed well with the mixing rule.     

Mechanical properties of poly(vinyl alcohol) fibres and composites

This paper describes the performance of poly(vinyl alcohol) (PVOH) fibres and their composites. PVOH fibres have been studied, with the emphasis on long-term properties. Results indicated that the long-term properties of PVOH fibres are superior to those of high-performance polyethylene (HP-PE) fibres. Mechanical tests on PVOH/epoxy composites showed that, in general, their structural performance is between those of plasma-treated HP-PE and aramid fibre-reinforced composites. Structural mechanical properties of PVOH composites are better than those of HP-PE based composites due to the combination of a strong interfacial bond strength and a less pronounced anisotropic fibre character. However, the strong bonding between PVOH and epoxy results in a rather brittle failure mode of the composite and consequently relatively poor impact properties.     

Preparation and properties of polypropylene composites reinforced with wheat and flax straw fibres: Part II Analysis of composite microstructure and mechanical properties

The microstructure and mechanical properties of polypropylene composites containing flax and wheat straw fibres are discussed. Particular emphasis has been given to determining the nature and consequences of fibre damage induced during melt-processing operations, fibre orientation occurring in mouldings, and possible interfacial adhesion between the matrix and fibres. Compared to unfilled polypropylene, addition of flax and wheat straw caused a significant increase in tensile modulus, particularly, in the case of flax fibres, which also gave higher tensile yield strength and Charpy toughness, despite a lack of interfacial bonding. Tensile strength was increased further through inclusion of 5% by weight of maleic anhydride-modified polypropylene, which was shown to promote adhesion between fibres and matrix. This revised version was published online in November 2006 with corrections to the Cover Date.     

Experimental push out testing and analysis of fibre reinforced composites: applicability and test considerations

Abstract

The applicability of the push out technique to fibre composite systems with both excellent and poor interfacial bonding has been considered through experimental push out studies on model polymer matrix and metal matrix composite systems, respectively. Some factors which affect the reliability and reproducibility of data from the push out test have also been established. The findings are applicable to ceramic matrix composites. The interfacial properties of the model steel fibre/epoxy matrix, copper fibre/epoxy matrix, steel fibre/aluminium matrix, and SiC fibre/aluminium matrix composite systems were studied. Factors which influence the reproducibility of interfacial properties such as the interfacial bond strength and the matrix shrinkage pressure as determined from the push out test have been identified. These include the thickness/fibre diameter ratio, the relative diameter of the fibre and the specimen, and the aspect ratio of the fibre in the push out specimen. It was also important to establish the nature offailure at the interface.     

三维正交机织复合材料的拉伸力学性能及介电性能研究

三维纺织复合材料的发展越来越广泛,其轻质,抗分层等优点是层合板式复合材料无法比拟的.本工作对自行设计并制作的五种玻璃纤维芳纶纤维混合增强的环氧树脂复合材料的拉伸力学性能和介电性能进行了研究.结果表明纯芳纶结构的三维复合材料有着最高的比强度和比模量;而在玻璃纤维含量较多的结构材料里介电性能呈现集中且稳定的趋势.在不同的应用中,可以将不同纤维混合作为增强体以发挥各种纤维的优势和特点,满足不同的设计和实际需要.     

Effect of coupling agents on reinforcing potential of recycled carbon fibre for polypropylene composite

Polypropylene (PP) composites reinforced with recycled carbon fibre have been prepared through extrusion compounding and injection moulding. The reinforcing potential of the recycled fibre was increased by improving the interfacial adhesion between the fibre and PP matrix and this was done by the addition of maleic anhydride grafted polypropylene (MAPP) coupling agents. Three MAPP couplers with different molecular weights and maleic anhydride contents were considered. The effects on the mechanical properties of the composite were studied, and scanning electron microscopy (SEM) was used to study the fracture morphology of the tensile specimens. It was observed that with the addition of MAPP the interfacial adhesion was improved as fewer fibres were pulled-out and less debonding was seen. A microbond test was performed and a significant improvement in interfacial shear strength was measured. This resulted in composites with higher tensile and flexural strengths. The maximum strength was achieved from MAPP with the highest molecular weight. Increased modulus was also achieved with certain grades of MAPP. It was also found that the composite impact strength was improved significantly by MAPP, due to a higher compatibility between the fibre and matrix, which reduced crack initiation and propagation.