The effect of alkaline and silane treatments on mechanical properties and breakage of sisal fibers and poly(lactic acid)/sisal fiber composites

The main goals of this work were to study the effect of different chemical treatments on sisal fiber bundles tensile properties as well as on tensile properties of composites based on poly(lactic acid) (PLA) matrix and sisal fibers. For this purpose, sisal fibers were treated with different chemical treatments. After treating sisal fibers the tensile strength values decreased respect to untreated fiber ones, especially when the combination of NaOH + silane treatment was used. Taking into account fiber tensile properties and fiber/PLA adhesion values, composites based on silane treated fibers would show the highest tensile strength value. However, composites based on alkali treated and NaOH + silane treated fibers showed the highest tensile strength values. Finally, experimental tensile strength values of composites were compared with those values obtained using micromechanical models.     

化学处理和吸湿水对剑麻纤维及其与树脂界面性能的影响

使用KMnO₄、NaOH、阻燃剂、硅烷对剑麻纤维进行表面处理。采用单丝拉伸和微脱粘方法分别测试了剑麻纤维的拉伸性能及其与改性丙烯酸酯、环氧树脂的界面性能,考察了吸湿水对剑麻纤维表面形貌、拉伸性能及其与树脂界面粘结的影响,分析了相应的破坏模式。结果表明,经过表面化学处理后剑麻纤维的拉伸强度和模量均有不同程度的下降,其中经KMnO₄和硅烷处理后,纤维拉伸强度下降了44%,经NaOH处理后其拉伸强度降低了27%,阻燃剂处理对剑麻性能的影响不明显。表面化学处理还会降低剑麻纤维与改性丙烯酸酯的界面粘结强度,其下降的幅度与纤维拉伸强度下降程度不一致,阻燃剂处理的剑麻/改性丙烯酸酯的界面强度最低,仅为2.0 MPa,较未处理剑麻纤维复合体系下降了80%。经硅烷处理后,剑麻纤维的吸水率下降,吸水后其拉伸性能保留率高于未处理剑麻纤维。湿态条件下未处理剑麻纤维与环氧树脂的界面强度为6.6 MPa,高于硅烷处理剑麻/环氧树脂的界面强度,其断口形貌表明硅烷处理可导致微纤之间的弱粘结,从而降低了剑麻纤维自身及其与树脂的界面性能。     

A study of the mechanical properties of short natural-fiber reinforced composites

The degree of fiber–matrix adhesion and its effect on the mechanical reinforcement of short henequen fibers and a polyethylene matrix was studied. The surface treatments were: an alkali treatment, a silane coupling agent and the pre-impregnation process of the HDPE/xylene solution. The presence of Si–O–cellulose and Si–O–Si bonds on the lignocellulosic surface confirmed that the silane coupling agent was efficiently held on the fibres surface through both condensation with cellulose hydroxyl groups and self-condensation between silanol groups.

The fiber–matrix interface shear strength (IFSS) was used as an indicator of the fiber–matrix adhesion improvement, and also to determine a suitable value of fiber length in order to process the composite with relative ease. It was noticed that the IFSS observed for the different fiber surface treatments increased and such interface strength almost doubled only by changing the mechanical interaction and the chemical interactions between fiber and matrix.

HDPE-henequen fiber composite materials were prepared with a 20% v/v fiber content and the tensile, flexural and shear properties were studied. The comparison of tensile properties of the composites showed that the silane treatment and the matrix-resin pre-impregnation process of the fiber produced a significant increase in tensile strength, while the tensile modulus remained relatively unaffected. The increase in tensile strength was only possible when the henequen fibers were treated first with an alkaline solution. It was also shown that the silane treatment produced a significant increase in flexural strength while the flexural modulus also remained relatively unaffected. The shear properties of the composites also increased significantly, but, only when the henequen fibers were treated with the silane coupling agent. Scanning electron microscopy (SEM) studies of the composites failure surfaces also indicated that there is an improved adhesion between fiber and matrix. Examination of the failure surfaces also indicated differences in the interfacial failure mode. With increasing fiber–matrix adhesion the failure mode changed from interfacial failure and considerable fiber pull-out from the matrix for the untreated fiber to matrix yielding and fiber and matrix tearing for the alkaline, matrix-resin pre-impregnation and silane treated fibers.     

Effect of fiber surface treatments on mechanical and abrasive wear performance of polylactide/jute composites

The main focus of this work is to improve the adhesion of jute fiber with polylactide (PLA). For this purpose, surface of the jute fiber was modified by alkali, permanganate, peroxide and silane treatments. The surface modified fibers were characterized by FTIR spectroscopy. Unidirectional composites were prepared with treated jute fibers and PLA matrix by hot pressing of solvent impregnated prepregs. Surface treatments resulted in enhancement of tensile and flexural properties and reduction in Izod impact strength. Dynamic mechanical analysis (DMA) results showed that, treated composites have higher storage modulus and lower tangent delta with respect to untreated composite. The degree of interfacial adhesion between the jute fiber and PLA was estimated using adhesion parameter obtained through DMA data. The results of thermogravimetric analysis (TGA) showed a higher thermal stability for silane treated composites. Experimental results on abrasive wear tests revealed that the wear resistance of composite is sensitive to fiber/matrix adhesion.     

Chemical treatment of sisal fiber using alkali and clay method

In this study the chemical treatment of sisal fiber using the combined alkali (NaOH) and clay is discussed. The purpose of this fiber treatment is to improve the fiber–matrix compatibility, interface strength, mechanical, thermal and water barrier properties. The phase change due to chemical treatment of raw sisal fiber was examined by Fourier Transform Infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) methods. The result shows the presence of about 20 wt.% clays in NaOH–clay treated sisal fiber with 2.6× reduced water uptake and also with improved mechanical and thermal properties. Subsequently the treated and untreated fibers were reinforced in polypropylene (PP) matrix and the mechanical and thermal properties were examined. The result indicates that the fiber–matrix interface strength, adhesion, glass transition temperature and tensile properties of composites were improved in NaOH–clay treated fiber composites.     

Glass fiber reinforced ROMP-based bio-renewable polymers: Enhancement of the interface with silane coupling agents

In this study, the influence of silane coupling agents on interfacial adhesion in glass fiber reinforced polymers from the ring-opening metathesis polymerization (ROMP) of a linseed oil-based monomer and dicyclopentadiene is investigated experimentally. Two types of silane coupling agents, norbornenylethyldimethylchlorosilane (MCS) and norbornenylethyltrichlorosilane (TCS), are examined. Interfacial shear strength (IFSS) is evaluated by the microbond technique. The IFSS increases by about 150% for the MCS-treated fibers and by about 50% for the TCS-treated fibers compared to untreated fibers. Dynamic mechanical analysis of composite panels made with untreated and silane-treated fibers reveals that MCS-treated fiber composites have the highest storage modulus and glass transition temperature, indicating strong interfacial interactions at the glass/matrix interface. Short beam shear tests and scanning electron microscopy of fracture surfaces also confirm that MCS is more effective than TCS at improving interfacial adhesion.     

Preparation and properties of sisal microfibril/gelatin biomass composites

In this work, the natural sisal fibers were fibrillated by enzyme hydrolysis or mechanical disintegration into microfibrils with a width of 5-10 μm and different aspect ratios. The sisal microfibrils or microfibril mats were added into the gelatin to prepare biomass composites, by solvent-casting or solution impregnation techniques, respectively. The morphology, mechanical properties, biodegradation property, and water adsorption behaviors of the composites were investigated. It was found that the tensile strength of the composites was dramatically increased with the addition of sisal microfibrils. The degradation ratio of the composites decreased continuously with increasing the sisal fibril content. The addition of sisal microfibrils decreased the water uptake at equilibrium and the water diffusion coefficient. Scanning electron microscopy characterization showed that the sisal microfibrils were very well embedded in the gelatin matrix, showing a good interfacial adhesion.     

Mercerization of sisal fibers: Effect of tension on mechanical properties of sisal fiber and fiber-reinforced composites

Sisal fibers were mercerized, under tension and no tension, to improve their tensile properties and interfacial adhesion with soy protein resin. Mercerization of fibers under tension is known to minimize fiber shrinkage and to lower the microfibrillar angle by aligning them along the fiber axis. Mercerization improved the fracture stress and Young’s modulus of the sisal fibers while their fracture strain and toughness decreased. Mercerized sisal fiber-reinforced composites with soy protein resin showed improvement in both fracture stress and stiffness by 12.2% and 36.2%, respectively, compared to the unmercerized fiber-reinforced composites. Scanning electron microscope (SEM) photomicrographs of the composite fracture surfaces showed shorter fibrils protruding in the mercerized fiber-reinforced composites resulting in better sisal fiber/soy adhesion. Changed fiber surface properties were also responsible for better adhesion.     

Mechanical property improvement of unsaturated polyester composite reinforced with admicellar-treated sisal fibers

Sisal fiber was treated by admicellar polymerization with a poly(methyl methacrylate) film coating in order to enhance the interfacial adhesion of the fiber/polymer composite for mechanical property improvement. Properties of the admicellar-treated sisal fiber were investigated by measuring its moisture absorption and electrostatic charge. Thermal stability study by thermogravimetric analysis and film identification by FTIR was also carried out. The treatment was shown to improve the tensile and flexural properties, impact strength, and hardness of the composite. SEM micrographs of the tensile fracture surface of sisal/unsaturated polyester composites also show interfacial adhesion improvement of the composite prepared with admicellar-treated sisal.     

Mechanical properties and flammability of sisal/PP composites: Effect of flame retardant type and content

In this research, magnesium hydroxide (Mg(OH)₂) and zinc borate, as flame retardants, were incorporated into sisal/PP composites. Maleic anhydride grafted polypropylene was also used as a compatibilizer. Adding flame retardants into sisal/PP composites reduced burning rate and increased thermal stability of the composites. No synergistic effect was observed when both magnesium hydroxide and zinc borate were incorporated in the sisal/PP composites. In addition, the sisal/PP composites exhibited insignificant difference of shear viscosity at high shear rate indicating that types of flame retardants used in this study had no impact on the processability of the composites. Good distribution of flame retardants and sisal fiber in PP matrix was also observed. All PP composites had lower impact strength than the neat PP. However, the sisal/PP composites with the addition of Mg(OH)₂ and zinc borate exhibited comparable tensile and flexural properties to the sisal/PP composites without adding those flame retardants. Therefore, the addition of Mg(OH)₂ and zinc borate enhanced flame retardancy of sisal/PP composites without sacrificing their mechanical properties.     

Effect of surface modification of bamboo cellulose fibers on mechanical properties of cellulose/epoxy composites

Bamboo cellulose fibers were treated with NaOH aqueous solution and silane coupling agent, respectively, before they were applied into epoxy composites. The effect of surface modification on mechanical properties was evaluated by tensile and impact tests under controlled conditions. Compared with the untreated cellulose filled epoxy composites, the NaOH solution treatment increased the tensile strength by 34% and elongation at break by 31%. While silane coupling agent treatment produced 71% enhancement in tensile strength and 53% increase in elongation at break. The scanning electron microscopy (SEM) was used to observe the surface feature of the cellulose fibers and the tensile fractures as well as cryo-fractures of the composites. The Fourier transform infrared (FTIR) was employed to analyze the chemical structure of the cellulose fibers before and after modifications. The results indicated different mechanisms for the two modifications of cellulose. The NaOH solution partly dissolved the lignin and amorphous cellulose, which resulting in splitting the fibers into smaller size. This led to easier permeating into the gaps of the fibers for epoxy resin (EP) oligmer and forming effective interfacial adhesion. Based on the emergence of Si–O–C and Si–O–Si on the cellulose surface, it was concluded that the enhancement of mechanical properties after coupling agent modification could be ascribed to the formation of chemical bonds between the cellulose and the epoxy coupled with the coupling agent.     

硅溶胶改性碳纤维对碳纤维/环氧树脂复合材料界面性能影响

以硅烷偶联剂和正硅酸乙酯(TEOS)为前躯体, 以固体酸-对甲苯磺酸为催化剂制备硅溶胶, 利用硅溶胶对碳纤维进行表面改性后, 以环氧树脂为基体, 制备碳纤维增强环氧树脂复合材料。利用SEM、 TEM、 万能试验机、 偏光显微镜等对表面改性前后的碳纤维形态、 力学性能及碳纤维/环氧树脂复合材料的界面性能进行表征, 研究了硅溶胶改性碳纤维对其复合材料界面性能影响。结果表明, 硅溶胶处理碳纤维后, 在碳纤维表面原位生成具有膜-粒结构的表面层, 改性后碳纤维的强度由2.41 GPa提高到3.00 GPa, 界面性能也得到了明显改善, 界面剪切强度(IFSS)提高了51.41%。     

Effect of glass fiber hybridization on properties of sisal fiber–polypropylene composites

Natural fiber reinforced polymer composites became more attractive due to their light weight, high specific strength, and environmental concern. However, some limitations such as low modulus, poor moisture resistance were reported. This study aimed to investigate the effect of glass fiber hybridization on the physical properties of sisal–polypropylene composites. Polypropylene grafted with maleic anhydride (PP-g-MA) was used as a compatibilizer to enhance the compatibility between the fibers and polypropylene. Incorporating glass fiber into the sisal–polypropylene composites enhanced tensile, flexural, and impact strength without having significant effect on tensile and flexural moduli. In addition, adding glass fiber improved thermal properties and water resistance of the composites.     

Development of slate fiber reinforced high density polyethylene composites for injection molding

During the last decade the use of fiber reinforced composite materials has consolidated as an attracting alternative to traditional materials due to an excellent balance between mechanical properties and lightweight. One drawback related to the use of inorganic fibers such as those derived from siliceous materials is the relative low compatibility with conventional organic polymer matrices. Surface treatments with coupling agents and the use of copolymers allow increasing fiber–matrix interactions which has a positive effect on overall properties of composites. In this research work we report the use of slate fiber treated with different coupling agents as reinforcement for high density polyethylene from sugarcane. A silane (propyltrimethoxy silane; PTMS) and a graft copolymer (polyethylene-graft-maleic anhydride; PE-g-MA) were used to improve fiber–matrix interactions on HDPE-slate fiber. The effect of the different compatibilizing systems and slate fiber content were evaluated by scanning electron microscopy (SEM), dynamic thermomechanical analysis (DTMA) as well as mechanical properties (tensile, flexural and impact). The results show that the use of silane coupling agents leads to higher fiber–matrix interactions which has a positive effect on overall mechanical properties. Interesting results are obtained for composites containing 30 wt.% slate fiber previously treated with propyltrimethoxy silane (PTMS) with an increase in tensile and flexural strength of about 16% and 18% respectively.     

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.     

Long-term moisture effects on the interfacial shear strength between surface treated carbon fiber and epoxy matrix

In recent years, carbon fiber reinforced polymer (CFRP) composites have found increasing applications in marine and offshore area, where the CFRP components are subjected to a persistent attack of moisture. The performance degradation of composites under those critical service conditions becomes a key issue. In this work, silane coating and multiwalled carbon nanotubes were applied on carbon fibers to enhance the fiber/matrix interfacial bonding strength. The long-term effects of moisture on the interfacial shear strength (IFSS) of the composites in underwater environments, such as de-ionized water and simulated seawater, have been studied using single fiber microbond method. The silane coating and carbon nanotube-modified silane coating are found to contribute 14.5% and 26.3% increase in IFSS of the CFRP in dry air, and well maintain this improvement during a 120-day immersion test in de-ionized water and simulated seawater.     

Evaluation of mechanical properties of banana and sisal fiber reinforced epoxy composites: Influence of glass fiber hybridization

In this work, the effect of glass fiber hybridization with the randomly oriented natural fibers has been analyzed. The banana (B), sisal (S) fibers were chopped and woven E-glass (G) synthetic fibers were reinforced with epoxy matrix. Nine different kinds of laminates were prepared in the following stacking sequence of B, S, BS, G/B/G, G/S/G, G/BS/G, G/B/G/B/G, G/S/G/S/G and G/BS/G/BS/G. Mechanical properties like tensile strength, flexural strength and impact strength were evaluated and compared. Interfacial analysis was also carried out with the help of Scanning Electron Microscope (SEM) to study the micro structural behavior of the tested specimen. It was observed that the addition of two and three layer of glass fiber can improve the tensile strength by a factor of 2.34 and 4.13 respectively. The flexural properties were enhanced on banana–sisal fiber with two layers of glass fibers rather than three layers and the laminate with sisal and three glass ply offers better impact strength.     

Processing of PLA/sisal fiber biocomposites using direct- and extrusion-injection molding

The processing strategy adopted to develop biocomposites plays a significant role in determining their characteristics. The present experimental investigation explores the feasibility of using direct-injection molding (D-IM) process for processing of sisal fiber (3?mm and 8?mm) reinforced poly-lactic acid biocomposites with a fiber weight fraction of 30%. For a comparative analysis, mechanical and morphological behavior of biocomposites developed using D-IM process is compared with biocomposites developed using extrusion-injection molding (E-IM) process. The mechanical behavior in terms of tensile, flexural and impact properties is compared and discussed in relation to extracted fiber morphology and fiber orientation as well as dispersion within the developed biocomposites. Morphological investigation of extracted fibers revealed severe fiber attrition and fiber length variation during E-IM process as compared with D-IM process. However, short sisal fiber (3?mm) reinforced biocomposites developed using both the processes exhibit uniform fiber dispersion and orientation, resulting in comparable mechanical properties. The tensile and flexural strength of D-IM-SF biocomposites increased remarkably by 34.7% and 15.9%, respectively, as compared with D-IM-LF biocomposites. Similarly, the tensile and flexural modulus of D-IM-SF biocomposites increased significantly by 92.5% and 56.7%, respectively, as compared with D-IM-LF biocomposites. However, D-IM process incorporating long fibers exhibit better impact properties.     

Mechanical properties of basalt fibers and their adhesion to polypropylene matrices

This work is aimed to study the mechanical properties of basalt fibers, and their adhesion to polypropylene (PP) matrices. Single filament tensile tests were used to calculate the strength of different types of fibers, characterized by different providers and surface treatment. Single fiber fragmentation tests (SFFT) were used to calculate the critical length of the fibers, in a homopolymer PP matrix and in a maleic anhydride modified PP matrix. It was shown that the tensile strength of the fibers is not significantly influenced by the origin or the surface treatment. Only fibers without any sizing show very reduced mechanical properties. On the other hand, the tensile strength was shown to be severely dependent on the filament length. Weibull theory was used in order to calculate the fitting parameters σ₀ and β, which were necessary in order to extrapolate the tensile strength to the critical length determined by SFFT. This allowed calculating the adhesion properties of the basalt fibers. It was shown that fiber–matrix adhesion is dependent on both the presence of sizing on the fiber surface, as well as on the modification of the matrix.     

Investigation of the electrical and mechanical properties of short sisal fiber-reinforced epoxy composite in correlation with structural parameters of the reinforced fiber

Chemical modification of the sisal fiber is done through dewaxing. Increment in the degree of crystallinity, crystallite size, and bulk density are observed in case of the dewaxed sisal fiber. The partial removal of wax, hemicellulose, and lignin content is confirmed from the FTIR spectra. Better flexural strength and tensile strength are observed in case of dewaxed sisal fiber-reinforced epoxy composite (DSFREC) in comparison to the raw sisal fiber-reinforced epoxy composite (RSFREC). This may be due to the improvement in the adhesion between the fiber and matrix. Lower values of dielectric constant (ε_r) and dielectric loss (tanδ) are also observed in case of DSFREC. The shifting of M″_max toward higher frequency side with rise in temperature ascribing a correlation between motions of mobile ions and suggests a spread of relaxation times. Moreover, the structural parameters of the fiber are correlated with the mechanical and electrical properties of the composite.