Study on interfacial properties of unidirectional flax/vinyl ester composites: Resin manipulation on vinyl ester system

Flax fibers are widely used as reinforcements in bio‐based polymer matrix composites. This study investigated the hydrophilic nature and surface purity of flax fiber that affects fiber/matrix adhesion in combination with hydrophobic structural polymers via matrix modification and the utilization of fiber treatment, specifically in a flax/vinyl ester (VE) composite. A new method to manipulate the vinyl ester system with acrylic resin (AR) was developed to produce flax reinforced. On the other hand, different types of chemical and physical treatments were applied on the flax fiber. FTIR was applied to evaluate the effects of surface treatments. Dynamic mechanical analysis (DMA) was used to analyze the unmodified and modified VE resin system. The surface of untreated and treated flax fibers and their composites were analyzed by scanning electronic microscopy (SEM). Sodium ethoxide‐treated flax/VE with 1% (wt) AR caused the best mechanical performance among all the flax/VE composites evaluated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Flax fiber surface modifications: Effects on fiber physico mechanical and flax/polypropylene interface properties

This study arises on the opportunities of using flax fibers as reinforcement for polypropylene (PP) matrix composites. For this purpose, untreated flax fiber bundles obtained by a retting process have been used. For improving compatibility between flax fiber bundles and PP matrix, fiber surface treatments such as maleic anhydride, maleic anhydride polypropylene copolymer, and vinyltrimethoxy silane have been carried out. On the other hand, alkali treatment has also been carried out for fiber modification. The effect of surface modification on tensile properties of single fibers and also on fiber‐matrix interfacial shear strength (IFSS) has been analyzed. Finally, both optical microscopy and atomic force microscopy have been used for characterizing flax fiber microstructural features. The current study completes previous results to elucidate the influence of treatments on fiber surface and flax fiber‐PP interface. POLYM. COMPOS. 26:324–332, 2005. © 2005 Society of Plastics Engineers.     

Unidirectional hemp and flax EP‐ and PP‐composites: Influence of defined fiber treatments

In some technical areas, mainly in the automotive industry, glass fiber reinforced polymers are intended to be replaced by natural fiber reinforced polymer systems. Therefore, higher requirements will be imposed to the physical fiber properties, fiber‐matrix adhesion, and the quality assurance. To improve the properties of epoxy resins (EP) and polypropylene (PP) composites, flax and hemp fibers were modified by mercerization and MAH‐PP coupling agent was used for preparing the PP composites. The effects of different mercerization parameters such as concentration of alkali (NaOH), temperature, and duration time along with tensile stress applied to the fibers on the structure and properties of hemp fibers were studied and judged via the cellulose I–II lattice conversion. It was observed that the mechanical properties of the fibers can be controlled in a broad range by using appropriate mercerization parameters. Unidirectional EP composites were manufactured by the filament winding technique; at the PP matrix material, a combination with a film‐stacking technique was used. The influence of mercerization parameters on the properties of EP composites was studied with hemp yarn as an example. Different macromechanical effects are shown at hemp‐ and flax‐PP model composites with mercerized, MAH‐PP‐treated, or MAH‐PP‐treated mercerized yarns. The composites' properties were verified by tensile and flexural tests. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2150–2156, 2004     

Mechanical properties of glass/flax hybrid composites based on a novel modified soybean oil matrix material

In the Affordable Composites from Renewable Resources (ACRES) program at the University of Delaware, soybean oil and other plant triglycerides have been made amenable to polymerization using a broad range of chemical routes. The resultant polymers range in properties from soft rubbers to hard thermosets for composite applications. In this paper we present an investigation of the mechanical properties of glass/flax hybrid composites based on these thermoset matrix materials. Composites with different glass/flax ratios and different fiber arrangements were made using a modified soybean oil matrix material. The fiber arrangement was varied to make symmetric and unsymmetric composites. The latter were tested in different modes in flexural tests and drop weight impact tests. The mechanical properties of the composites were found to depend upon the glass/flax ratio and the arrangement of fibers in the composite. On proper selection of the arrangement of fibers in the composite, the glass fibers and flax fibers were found to act synergistically resulting in an improved flexural and impact performance. POLYM. COMPOS., 26:407–416, 2005. © 2005 Society of Plastics Engineers     

Composites of poly(lactic acid) with flax fibers modified by interstitial polymerization

Natural fiber composites were designed and optimized to achieve good mechanical properties and resistance to growth of living organisms. Composite materials were prepared from poly(lactic acid) (PLA) with flax fibers, where the flax fibers had been subjected to interstitial polymerization to replace the water in the cellulose fibers. Before polymerization, the flax fibers were extracted with sodium hydroxide and acetone to remove lignin, pectin, and waxes from the cellulose. Differential scanning calorimetry was used to study the crystallization and melting of the composites as compared with pure PLA. The surface wetting of the fibers and morphology of the composites were studied by scanning electron microscopy and optical microscopy. Mechanical properties were studied using dynamic mechanical analysis. The influence of the interstitial polymerization on the dynamic storage modulus was found to be significant. The composites of polymerization treated flax, with acetone washed fibers, had higher storage moduli than the unwashed fiber composites, which suggested adhesion between flax fibers, and the matrix was improved by the treatments. The composites were subjected to moist environmental conditions to test for development of mold and fungi, and the acetone washed polymerization treated flax composites were resistant to these growths. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Composites of poly(lactic acid) with flax fibers modified by interstitial polymerization

Natural fiber composites were designed and optimized to achieve good mechanical properties and resistance to growth of living organisms. Composite materials were prepared from poly(lactic acid) (PLA) with flax fibers, which had been subjected to interstitial polymerization to replace the water in the cellulose fibers. Prior to the polymerization, the flax fibers were extracted with sodium hydroxide and acetone to remove lignin, pectin, and waxes from the cellulose. Differential scanning calorimetry was used to study the crystallization and melting of the composites compared to pure PLA. The surface wetting of the fibers and morphology of the composites were studied by scanning electron microscopy and optical microscopy. Mechanical properties were studied using dynamic mechanical analysis. The influence of the interstitial polymerization on the dynamic storage modulus was found to be significant. The composites of polymerization treated flax with acetone washed fibers had higher storage moduli than the unwashed fiber composites, which suggested that the adhesion between the flax fibers and the matrix was improved by the treatments. The composites were subjected to moist environmental conditions in order to test for development of mold and fungi, and the acetone washed polymerization treated flax composites were resistant to these growths. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3620–3629, 2006     

Surface Characterization of Differently Pretreated Flax Fibers and Their Application in Fiber-Reinforced Composites

Strong natural bast fibers, especially flax fibers, can be used to replace glass fibers in reinforced composites. The properties of natural fibers depend largely on maturity, retting and processing. Two chemical treatments were applied to retted and semiretted flax fibers to create better fiber to resin bonding and to show the effect of retting degree and successive purification processes on the mechanical properties of natural composite materials. Retted and semiretted flax fibers have been scoured and bleached with the objective of removing surface impurities and developing finer structure. To investigate the effect of adhesion promoter on the mechanical properties of natural fiber composite, a composite sample was prepared from bleached retted flax pretreated with adhesion promoter Isostearoyltitanate (ISTT).

After treatments the fibers got cleaner and the measurements showed that the fiber fineness as well as the surface free energy increased. The treatments were accompanied by decrease in the fiber tenacity but it has been found not to be reflected to the final mechanical properties of the composite. No improvement was remarked by using Isostearoyltitanate for surface modification.     

Ternary composites based on PP‐EPDM blends reinforced with flax fibers. Part I: Processing and thermal behavior

The simultaneous effect of the incorporation of flax fibers and terpolymer ethylene‐propylene diene rubber (EPDM) on the properties of ternary composites with a PP matrix is investigated in this study. In order to evaluate the effects of the matrix composition and fiber percentage on the properties of the composite, an experimental design based on a Doehlert Uniform Net was employed. First, the processing conditions of these ternary composites were analyzed through a rheological study, where a noticeable increase of the viscosity upon the incorporation of the fiber and elastomer was observed. Then, the thermal behavior of the composites was studied with dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The results indicate a strong nucleation ability of the elastomer, showing a considerable increase of the rate of crystallization. These results concur with those obtained with an optical microscope, where it was observed that a higher number of active nuclei were formed in the presence of the elastomer. A previous characterization of the flax fibers used in this study has been also carried out by measuring the length and diameter distribution as well as the mechanical and thermal properties.     

Influence of two carbon plies on adhesion of unidirectional flax‐fibers reinforced epoxy composites

In this work, we undertook a comparative study of the dynamic dielectric analysis of two unidirectional epoxy composites: flax‐fiber‐reinforced epoxy and flax/carbon‐fiber‐reinforced epoxy (FCFRE). In both composites, three relaxation processes were identified. The first one is the water dipoles polarization imputed to the presence of polar water molecules in flax fiber. The second relaxation process associated with conductivity occurs as a result of the carriers charges diffusion noted for high temperature above glass transition and low frequencies. As for the third dielectric relaxation associated with the interfacial polarization effect is attributable to the accumulation of charges at the fibers/matrix interface. The presence of two carbon plies in the reinforcement gives rise to two interfacial polarization effects in the FCFRE composite. The analysis of the Maxwell–Wagner–Sillars and the water dipoles polarizations using the Havriliak–Negami model revealed that the presence of two plies of carbon can locally decrease the adhesion of flax fibers in the matrix. This analysis was supported by the thermal properties using a differential scanning calorimety and the mechanical properties using a short beam shear test. POLYM. COMPOS., 241–253, 2016. © 2014 Society of Plastics Engineers     

A comparative study on the influence of epoxy sizings on the mechanical performance of woven carbon fiber-epoxy composites

In the present work the effect of epoxy sizings on the fracture behavior of woven carbon fiber tetrafunctional epoxy composites has been investigated. Three-point flexural, short beam shear (SBS) and Mode-II interlaminar fracture toughness (ENF) tests have been carried out. Wettability and Atomic Force Microscopy (AFM) studies have been performed on commercial sized, desized and 0.7 wt% TGDDM and 0.7 wt% DGEBA sized carbon fibers. Dynamic mechanical thermal analysis and Scanning Electron Microscopy (SEM) studies were also carried out on the different carbon fiber/epoxy composites. The used sizing treatments provided composites with improved mechanical properties due to the enhancement achieved in the fiber-matrix adhesion. Polym. Compos. 25:319–330, 2004. © 2004 Society of Plastics Engineers.     

Studies of the effect of fiber surface and matrix rheological properties on nonwoven reinforced elastomer composites

The influence of fiber type and fiber-surface properties on matrix flow behavior was investigated using structural reaction injection-molding (SRIM). The influence of fiber type, fiber-surface properties, and matrix type on strength properties in elastomeric composites reinforced with nonwoven fibrous structures was investigated using tensile tests on elastomer composite samples from SRIM and latex coagulation (LC) fabrication methods and the microbond strength method on individual fibers. The fibers used were PET, LLDPE, and p-aramid. Fibers were treated with epoxy, styrene, and isocyanate derivatives, which make the surface chemically reactive. Treatments were also made with NaOH and a copolymer of polyester and polyol ether, causing a change in the fiber surface energy. The matrix types were polyurethane elastomer and natural rubber. The results show that the surface treatments which produced a change in the surface energy influenced the flow rate of the matrix polymer during the composite fabrication process. The treatments resulted in chemically reactive fiber surfaces which improved the fiber-matrix bond strength without affecting the Young's modulus of the composite material. Good correlation was found between bond strength and surface energy including the dispersive component of surface energy in the case of polyurethane elastomer and surface-modified PET fibers. The age of the polyurethane matrix has a marked influence on the bond strength. The fiber volume fraction in composites has a strong influence on the Young's modulus of the elastomer composite. © 1995 John Wiley & Sons, Inc.     

Surface modification of sisal fibers: Effects on the mechanical and thermal properties of their epoxy composites

The aim of this paper is to evaluate the mechanical and thermal properties of sisal fiber reinforced epoxy matrix composites as a function of modification of sisal fiber by using mercerization and silane treatments. The changes introduced by the treatments on the chemical structure of sisal fibers have been analyzed by infrared spectroscopy (FTIR). Thermal behavior of both sisal fibers and composites has been studied by thermogravimetric analysis (TGA). Both treatments clearly enhanced thermal performance and also mechanical properties of fibers, being other physical properties also modified. Mercerization, above all when combined with silanization, led to significant enhancement on mechanical properties of composites as a consequence of increasing mechanical properties of fibers and improving fiber/matrix adhesion. POLYM. COMPOS., 26:121–127, 2005. © 2005 Society of Plastics Engineers     

Mercerization of Flax Fiber Improves the Mechanical Properties of Fiber-Reinforced Composites

In this article, modification of mercerized flax (MFx) through graft co-polymerization with methylmethacrylate (MMA) using ferrous ammonium sulphate–potassium per sulphate (FAS–KPS) redox initiator has been reported. Water uptake and moisture absorbance properties of methylmethacrylate grafted mercerized flax (MFx-g-MMA) and mechanical behavior of raw flax, mercerized flax, and MFx-g-MMA fibers reinforced—polystyrene matrix–based composites also have been evaluated. Four reaction parameters, reaction temperature, reaction time, initiator molar ratio, and monomer concentration, have been optimized to get maximum graft yield. Maximum graft yield of 138.35% has been obtained at optimum reaction conditions. The graft co-polymers thus formed were characterized by FTIR, TGA, and SEM techniques. Mercerized flax fiber reinforced showed better results than raw flax and MFx-g-MMA fibers reinforced composites.     

Concentration effect of γ‐glycidoxypropyltrimethoxysilane on the mechanical properties of glass fiber–epoxy composites

In this study, glass fibers were modified using γ‐glycidoxypropyltrimethoxysilane of different concentrations to improve the interfacial adhesion at interfaces between fibers and matrix. Effects of γ‐glycidoxypropyltrimethoxysilane on mechanical properties and fracture behavior of glass fiber/epoxy composites were investigated experimentally. Mechanical properties of the composites have been investigated by tensile tests, short beam tests, and flexural tests. The short‐beam method was used to measure the interlaminar shear strength (ILSS) of laminates. The tensile and flexural properties of composites were characterized by tensile and three‐point bending tests, respectively. The fracture surfaces of the composites were observed with a scanning electron microscope. On comparing the results obtained for the different concentrations of silane solution, it was found that the 0.5% GPS silane treatment provided the best mechanical properties. The ILSS value of heat‐cleaned glass fiber reinforced composite is enhanced by ∼59% as a result of the glass fiber treatment with 0.5% γ‐GPS. Also, an improvement of about 37% in tensile strength, about 78% in flexural strength of the composite with the 0.5% γ‐GPS treatment of glass fibers was observed. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Short sisal fiber‐reinforced tire rubber composites: Dynamic and mechanical properties

The world tendency toward using recycled materials demands new products from vegetable resources and waste polymers. In this work, composites made from powdered tire rubber (average particle size: 320 μm) and sisal fiber were prepared by hot‐press molding and investigated by means of dynamic mechanical thermal analysis and tensile properties. The effects of fiber length and content, chemical treatments, and temperature on dynamic mechanical and tensile properties of such composites were studied. The results showed that mercerization/acetylation treatment of the fibers improves composite performance. Under the conditions investigated the optimum fiber length obtained for the tire rubber matrix was 10 mm. Storage and loss moduli both increased with increasing fiber content. The results of this study are encouraging, demonstrating that the use of tire rubber and sisal fiber in composites offers promising potential for nonstructural applications. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 670–677, 2004     

Influence of textile treatment on mechanical and sorption properties of flax/epoxy composites

The aim of the present work is to study the effect of conventional textile treatments of woven flax on the mechanical properties and the water sorption of flax/epoxy composites. The flax fabrics are standard 2/2 twills. Various treatments are carried out on fabric such as mercerization, bleaching, and leaching for long fibers or on yarn such as leaching for short fibers. A model, based on a modified rule of mixture applied to composite reinforced with woven fabric, is developed to include the effect of fiber and porosity volume fractions on composite stiffness and strength. Most treatments improve tensile stiffness and strength of flax/epoxy composite and reduce composite water sorption. We prove by X‐ray fluorescence analysis, thermogravimetric analysis, and tensile tests of dry fabric that it is due to an improvement in the interfacial bonding between fibers and matrix. The best performances are achieved with bleaching and mercerization treatment. The weakest performances are obtained with the composites made with leached yarns. POLYM. COMPOS., 34:1761–1773, 2013. © 2013 Society of Plastics Engineers     

The moisture and temperature effect on mechanical performance of flax/starch composites in quasi‐static tension

The effect of temperature and moisture on mechanical behavior of flax fiber/starch based composites was investigated experimentally. Elastic modulus, the nonlinear tensile loading curves, and failure strain were analyzed. Neat matrix and composites with 20 and 40% weight content of fibers were tested. It was found, performing tests with different amplitudes, that microdamage development with stress is rather limited and the related elastic modulus reduction in this type of composites is not significant. It was shown that the composite elastic modulus and failure stress are linearly related to the maximum tensile stress in resin. The sensitivity of the maximum stress of the resin with respect to temperature and moisture is the source of composites sensitivity to these parameters. Constant interface stress shear lag model for stress transfer assuming matrix yielding at the fiber/matrix interface has been successfully used to explain the tensile test data. It indicates that the sensitivity of the used composite with respect to the matrix properties change could be significantly reduced by increasing the average fiber length from 0.9 mm to 1.5 mm. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Physical and mechanical properties of kenaf/flax hybrid composites

This research investigates the physical and mechanical properties of hybrid composites made of epoxy reinforced by kenaf and flax natural fibers to investigate the hybridization influences of the composites. Pure and hybrid composites were fabricated using bi-directional kenaf and flax fabrics at different stacking sequences utilizing the vacuum-assisted resin infusion method. The pure and hybrid composites' physical properties, such as density, fiber volume fraction (FVF), water absorption capacity, and dimensional stability, were measured. The tests of tensile, flexural, interlaminar shear and fracture toughness (Mode II) were examined to determine the mechanical properties. The results revealed that density remained unchanged for the hybrid compared to pure kenaf/epoxy composites. The tensile, flexural, and interlaminar shear performance of flax/epoxy composite is improved by an increment of kenaf FVF in hybrid composites. The stacking sequence significantly affected the mechanical properties of hybrid composites. The highest tensile strength (59.8 MPa) was obtained for FK2 (alternative sequence of flax and kenaf fibers). However, FK3 (flax fiber located on the outer surfaces) had the highest interlaminar shear strength (12.5 MPa) and fracture toughness (3302.3 J/m²) among all tested hybrid composites. The highest water resistance was achieved for FK5 with the lowest thickness swelling.     

Effect of stacking sequence on mechanical properties of hybrid flax/jute fibers reinforced thermoplastic composites

In this study, the hybrid composites were prepared by stacking jute/PP nonwoven and flax/MAPP woven fabrics in defined sequences. Polypropylene (PP) and maleic anhydride grafted polypropylene (MAPP) were used as matrix materials. Jute and flax fibers were treated with alkali solution in order to improve the interface properties of the resultant composites. The mechanical properties of these hybrid composites were analyzed by means of tensile, flexural, and drop‐weight impact tests. The effect of fabric stacking sequence on the mechanical properties of the composites was investigated. The stacking of nonwovens at the top and in alternate layers has resulted in maximum flexural strength, flexural stiffness, and impact force. It was also shown that hybrid composites have improved tensile, flexural, and impact properties in comparison to neat PP matrix. POLYM. COMPOS., 36:2167–2173, 2015. © 2014 Society of Plastics Engineers     

Influence of fiber surface treatment on the properties of sisal-polyester composites

The effect of several chemical treatments, viz. organotitanate, zirconate, silane, and N-substituted methacrylamide, on the properties of sisal fibers used as reinforcement in unsaturated polyester resin (∼50 vol%) was investigated. An improvement in the properties was observed when sisal fibers were modified with surface treatments. Under humid conditions, a decrease of 30 to 44% in tensile and 50 to 70% in flexural strength has been noted. The strength retention of surface-treated composites (except silane) is high compared with untreated composites. It is observed that N-substituted methacrylamide-treated sisal composites exhibited better properties under dry as well as wet conditions. Fractographic evidence such as fiber breakage/splitting and matrix adherence on the pulled-out fiber surface explains such behavior.