Interfacial Characteristics of Sisal Fiber and Polymeric Matrices

Sisal-fiber-reinforced composites, as a class of eco-composites, have attracted much attention from materials scientists and engineers in recent years. In this article, the effects of fiber surface treatment on fiber tensile strength and fiber-matrix interface characteristics were determined by using tensile and single fiber pullout tests, respectively. The short beam shear test was also employed to evaluate the interlaminar shear strength of the composite laminates. Vinyl ester, epoxy, and high-density polyethylene (HDPE) were chosen as matrix materials. To enhance the interfacial strength, two kinds of fiber surface-treatment methods, namely, chemical bonding and oxidisation, were used. The results obtained showed that different fiber surface-treatment methods produced different effects on the tensile strength of the sisal fiber and fiber-matrix interfacial bonding characteristics. Hence, valuable information on the interface design of sisal fiber–polymer matrix composites can be obtained from this study.     

Interfacial Characteristics of Sisal Fiber and Polymeric Matrices

Sisal-fiber-reinforced composites, as a class of eco-composites, have attracted much attention from materials scientists and engineers in recent years. In this article, the effects of fiber surface treatment on fiber tensile strength and fiber-matrix interface characteristics were determined by using tensile and single fiber pullout tests, respectively. The short beam shear test was also employed to evaluate the interlaminar shear strength of the composite laminates. Vinyl ester, epoxy, and high-density polyethylene (HDPE) were chosen as matrix materials. To enhance the interfacial strength, two kinds of fiber surface-treatment methods, namely, chemical bonding and oxidisation, were used. The results obtained showed that different fiber surface-treatment methods produced different effects on the tensile strength of the sisal fiber and fiber-matrix interfacial bonding characteristics. Hence, valuable information on the interface design of sisal fiber-polymer matrix composites can be obtained from this study.     

Theory and Analysis of Fracture Energy in Fiber Reinforced Composites

A micro-mechanics model is developed to analyze the stress distributions and fracture energies associated with crack propagation and fiber pull-out in reinforced composites. The stress and work mechanisms of interfacial debonding, fiber deformation, and the frictional work of fiber pull-out are considered as semi-independent contributions to fracture toughness. The theoretical expressions of Cottrell for frictional work W_F and Outwater and Murphy for fiber deformational work W_D are obtained as special relations in a general relation for the total work W_T = W_s + W_F + W_D where W_s defines the matrix shear work for interfacial debonding of fiber and matrix. Three dimensional diagrams of fracture energies W_T, W_s, or W_r versus interfacial shear bond strength λ₀ and frictional shear stress λ_f identify regions of optimized fracture energy. The influence of environmental degradation of bond strength upon fracture energy is analyzed in terms of the theory.     

Enhanced interfacial adhesion via interfacial crystallization between sisal fiber and isotactic polypropylene: direct evidence from single‐fiber fragmentation testing

Bioresource natural sisal fiber (SF) was used to prepare single fiber‐reinforced isotactic polypropylene (iPP) composites. Three kinds of interfacial crystalline morphologies, spherulites, medium nuclei density transcrystallinity (MD‐TC) and high nuclei density transcrystallinity (HD‐TC), were obtained in the single fiber‐reinforced composites by implementing quiescent or dynamic shear‐enhanced crystallization and by modulating the compatibility interaction between SF and iPP. The development of interfacial shear strength (IFSS) during the interfacial crystallization process was demonstrated for the first time using a combination of single‐fiber fragmentation testing and optical microscope observation. A close correlation between IFSS and morphological characteristics of interfacial crystallization was well elucidated. The increases in IFSS were very different for spherulitic, MD‐TC and HD‐TC morphologies. The highest IFSS obtained was 28 MPa, after the formation of HD‐TC, which was about 62% of the tensile strength of neat iPP (45 MPa). These results offer powerful and direct evidence that interfacial crystallization could play an important role in the enhancement of interfacial adhesion of real SF/iPP composites. © 2013 Society of Chemical Industry     

Stress Transfer in Single Fiber/ Resin Tensile Tests

Microscale (25 mm gauge length) “dogbone” resin specimens with single carbon fibers embedded through the length of the specimen have been studied as a method for determining the fiber-resin interphase strength. The specimens are pulled in tension until the fiber fragments to a critical length, l_c. Evidence is presented here, based primarily on the relaxation of stress birefringence around the fiber fragment, that this test may not be an unambiguous measure of fiber-resin adhesion. Data obtained for various production lots of AS-4, AS-6, and IM-6 fibers indicate an increase in l_cd with laminate tensile strength. Although there is theoretical justification for this correlation, it requires that the interphase shear strength is relatively constant.

In those instances where interfacial adhesion was expected to be low, i.e., surface contamination or unsurface treated fiber, there was a significant increase in l_c/d and usually a distinct difference in stress birefringence compared to “good” adhesion. However, the distinction in stress birefringence was not always clear cut.     

The Temperature Dependence of Interfacial Shear Strength for Various Polymeric Matrices Reinforced with Carbon Fibers

The influence of temperature on the interfacial shear strength between epoxy thermoset matrices and surface treated, carbon AS4 as well as with surface treated and sized AS4-C carbon fibers was investigated. The thermoset matrices all consisted of DGEBA epoxy resin cured with different amine curing agents resulting in matrices with a range of behavior from brittle, elastic to ductile, plastic. For all systems, the results indicate that the interfacial shear strength (ISS) decreases with increasing temperature and as the T_g of the matrix is approached, a large corresponding decrease in the interfacial shear strength is seen. Moreover, the AS4-C (epoxy sized) system revealed a distinct decrease in interfacial shear strength at temperatures lower than the bulk matrix T_g indicating the formation of an interphase layer of composition different from the bulk matrix. Linear superposition methods were used to generate a master curve for the different matrix materials reinforced with the AS4 fibers. These results allow the prediction of ISS at any temperature.     

Temperature Dependence of Interfacial Shear Strength in SiC-Fiber-Reinforced Reaction-Bonded Silicon Nitride

The interfacial shear strength of AVCO SCS-6 SiC-fiber-reinforced reaction-bonded Si₃N₄ (RBSN) composites was studied as a function of temperature. Fiber "push-through" experiments were conducted with a diamond indenter and a high-temperature microhardness tester. The interfacial shear strength was variable and depended mostly on interfacial bonding at low temperatures (5 to 18 MPa at room temperature) and frictional forces at high temperatures (12 to 32 MPa at 1300°C). The frictional component is attributed to the surface roughness of the fibers. The interfacial shear strength increased with temperature, because of the relief of residual stresses arising from the thermal expansion mismatch between fiber and matrix. Because of the composite nature of these fibers, a number of interfaces were tested in each experiment. The interface which debonded and slid was not always the same. Interfacial fracture took place either between the two outermost carbon layers of the SCS-6 fibers, or between the SiC core and the innermost of the two outer carbon layers. The outermost carbon layer of the fiber always stayed bonded to the Si₃N₄ matrix.     

Utilizing Vacuum Bagging Process to Prepare Carbon Fiber/CNT-Modified-epoxy Composites with Improved Mechanical Properties

In this work, the effects of carbon nanotube-modified epoxy and carbon nanotube-enriched sizing agent on the tensile properties and failure mode of unidirectional carbon fiber/epoxy composites were investigated. Laminates of carbon fiber/epoxy composites at different concentrations of carbon nanotube and sizing agent were fabricated by hand layup vacuum bagging process. Scanning electron microscopy analysis was conducted to unveil the relation between the macroproperties and the composites’ microstructure. Experimental results showed that the carbon nanotube-modified epoxy/carbon fiber composite showed 20% enhancements in the Young’s modulus compared to the pristine epoxy/carbon fiber composite. The scanning electron microscopy analysis of the fracture surfaces revealed that incorporating carbon nanotube into the epoxy matrix with utilizing the vacuum improves the interfacial bonding and minimizes the voids that act as crack initiators. This microstructure enhances the interfacial shear strength and load transfer between the matrix and the fabrics and consequently the tensile characteristics of the formulated composite.     

The relationship between the strength of an adhesive bond and the thermodynamic properties of its components

Maximum stability of any system is achieved when its free energy is minimum, in accordance with the second law of thermodynamics. Considering the adhesive bond as a thermodynamic system, it is proposed that the minimum interfacial energy coincides with (1) the maximum strength, and (2) the maximum durability, understood as bond resistance to degradation under environmental attack. The thermodynamic properties of bond components which play a key role in promoting conditions for maximum strength of adhesion have been identified. The general pattern of the relationship: STRENGTH = function (interfacial energy and related parameters), has been developed based on experimental data covering a variety of adhesives and substrates such as metals (steel and aluminium), plastics, ceramics and glass fibre composites. The influence of adhesion promoters (eg, silanes) has also been considered.

It is shown that conditions for maximum strength coincide with the minimum interfacial energy of the system, acquired when the ratio of the surface energy of the substrate, γ₁, to that of the cured adhesive, γ₂ (ie, a = γ₁/γ₂), has a specific value denoted a_MIN. Systems with energy ratios a a_MIN were found to have engineering utility, because the strength deficiency for a >a_MIN was found to be significantly less than for a MIN.     

芳纶纤维等离子体接枝改性处理研究

采用等离子体接枝对芳纶纤维表面进行改性处理,采用XPS、浸润性、界面剪切强度对等离子体接枝处理前后的表面组成、复合材料界面粘接性能等进行了研究,结果表明:等离子体接枝处理可以有效地提高芳纶纤维表面的极性官能团,增加与基体树脂-环氧树脂的浸润性,进而提高芳纶/环氧复合材料的界面粘接强度.     

Evaluation of Adhesion Property of UHMWPE Fibers/Nano-epoxy by a Pullout Test

Ultrahigh-molecular-weight polyethylene (UHMWPE) fibers have poor wetting and adhesion properties to polymer resins because of the inert surface of the fibers. In our previous study, a reactive nano-epoxy matrix, developed by making a modification on the matrix with reactive graphitic nanofibers (r-GNFs), showed improved wettability to UHMWPE fibers. In this work, fiber bundle pullout tests were conducted to evaluate the adhesion property between the UHMWPE fibers and the nano-epoxy matrices. Analysis of load-displacement curves from pullout tests shows that debonding initiation load and ultimate debonding load increased considerably, because of effective improvement of adhesion between the UHMWPE fibers and nano-epoxy matrix. Stress-controlled and energy-controlled models of interfacial debonding were applied for theoretical analyses. Results from ultimate IFSS, frictional shear stress, and critical energy-release rate are in good agreement with experimental results. Nano-epoxy matrix with 0.3 wt% r-GNFs shows effective improvement in terms of adhesion property between UHMWPE fiber and epoxy.     

Analysis of a pull-out test with real specimen geometry. Part II: the effect of meniscus

This paper continues our study on the platelet model of the pull-out specimen, in which the matrix droplet shape is approximated by a set of thin parallel disks with the diameters varying along the embedded fiber. Using this model, the fiber tensile stress and the interfacial shear stress profiles were calculated for real-shaped matrix droplets, including menisci (wetting cones) on the fibers, taking into account residual thermal stresses and interfacial friction. Then, these profiles were used to numerically simulate the processes of crack initiation and propagation in the pull-out test and to obtain theoretical force-displacement curves for specimens with different embedded lengths and wetting cone angles. Our simulations showed that the interfacial crack in real-shaped droplets initiated at very small (practically zero) force applied to the fiber, in contrast to the popular ‘equivalent cylinder’ approximation. As a result, the equivalent cylinder approach underestimated the interfacial shear strength (IFSS) value determined from the pull-out test and at the same time overestimated the interfacial frictional stress; the smaller was the wetting cone angle, the greater the difference. We also investigated the effects of the embedded fiber length and interfacial frictional stress in debonded areas on the calculated IFSS. The simulated force–displacement curves for the real-shaped droplets showed better agreement with experimental curves than those plotted using the equivalent cylinder approach.     

A new method for study of the fiber-matrix interface in composites: single fiber pull-out from a microcomposite

—A new method, single fiber pull-out from a microcomposite (SFPOM), was developed to study the fiber/matrix interface in composites. By pulling a fiber out of a seven-fiber microcomposite, the SFPOM test provides the real feeling of a fiber pulled out of an environment similar to that in a real composite. Interfacial shear strength decreased as the fiber volume fraction increased in the fiber-matrix system tested in the experiment. Three factors were suggested to be responsible for the phenomenon: (1) poor bonding between fibers when close to each other; (2) shear stress concentration in the matrix between neighboring fibers; and (3) possible change in matrix properties, thus altering the failure mechanism from interfacial debonding to a mixture of interfacial debonding and matrix fracture.     

Oxidation and Temperature Effects on the Interfacial Shear Strength in SCS-6 Fiber-Reinforced Reaction-Bonded Silicon Nitride

The fiber/matrix interfacial shear strength of Textron SCS-6 SiC-fiber-reinforced reaction-bonded Si₃N₄ (RBSN) was studied as a function of temperature after oxidation for 24 h at 600°C. Fiber push-out experiments were conducted using a diamond indenter in a high-temperature micro-hardness tester under vacuum. The interfacial shear strength increased with temperature because of the relief of residual tensile stresses arising from the difference in thermal expansion coefficients between the fiber and the matrix. Most of sublayer 2 of the fiber outer coating, which mainly consisted of carbon in the form of BSU (basic structure unit) aggregates, had disappeared after the heat treatment of the composite. Oxidation resulted in severe changes in the fiber outer coating and caused a lower interfacial shear strength with respect to that of the unoxidized composite.     

Nanoscale toughening of carbon fiber‐reinforced epoxy composites through different surface treatments

Interests in improving poor interfacial adhesion in carbon fiber‐reinforced polymer (CFRP) composites has always been a hotspot. In this work, four physicochemical surface treatments for enhancing fiber/matrix adhesion are conducted on carbon fibers (CFs) including acid oxidation, sizing coating, silane coupling, and graphene oxide (GO) deposition. The surface characteristics of CFs are investigated by Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, interfacial shear strength, and interlaminar shear strength. The results showed that GO deposition can remarkably promote fiber/matrix bonding due to improved surface reactivity and irregularity. In comparison, epoxy sizing and acid oxidation afford enhancement of IFSS owing to effective molecular chemical contact and interlocking forces between the fiber and the matrix. Besides, limited covalent bonds between silane coupling and epoxy matrix cannot make up for the negative effects of excessive smoothness of modified CFs, endowing them inferior mechanical properties. Based on these results, three micro‐strengthening mechanisms are proposed to broadly categorize the interphase micro‐configuration of CFRP composite, namely, “Etching” “Coating”, and “Grafting” modifications, demonstrating that proper treatments should be chosen for combining optimum interfacial properties in CFRP composites. POLYM. ENG. SCI., 59:625–632, 2019. © 2018 Society of Plastics Engineers     

New Coupling Agents as Adhesion Promoters at the Poly(Phenylene Sulfide)/Glass Interface-Studies with Micro and Macro Composites

Carboxy-functionalized poly(phenylene sulfide) having different molar masses and amount of the functional group were prepared in order to be used with γ-aminopropyltriethoxysilane as surface treatment of E-glass fibers. These grafted polymer chains act as connecting chains in order to improve the interfacial shear strength of the PPS-glass interface. According to their chemical nature, which is the same as the PPS matrix, and their ability to crystallize in the same crystalline form as the pure PPS, a continuum of bonding from the fiber surface to the bulk matrix is achieved. A chemical linkage is established at the glass surface by means of hydrolyzed ethoxysilane groups of the γ-APS and from the formation of amide units resulting from the reaction of amine functions of the silane and the carboxylic groups of the modified PPS. A “physical” linkage is expected between the grafted PPS and the PPS chains by means of entanglements and co-crystallization. A large improvement of the interfacial shear strength measured from the microdroplet test is observed when a modified-PPS having a medium molar mass and a low amount of functional groups is used in comparison with untreated or silane-treated glass fibers. This improvement is also observed for short glass fibers/PPS composite materials. In fact, a large improvement is obtained on mechanical properties such as the tensile, flexural, and impact strengths.     

玻璃纤维增强聚丙烯界面处理研究进展

本文综述了提高玻璃纤维增强聚丙烯复合材料界面粘结强度和改善界面层结构的各种有效方法,包括玻璃纤维的偶联剂涂覆、浸润剂浸润、表面接枝等表面处理方法以及在聚丙烯基体中添加功能化聚丙烯对基体进行共混改性等,对玻璃纤维与聚丙烯的粘的结机理进行了讨论,并论述了玻纤／聚丙烯界面横晶对界面粘结强度的影响。     

Acid–base interactions and covalent bonding at a fiber–matrix interface: contribution to the work of adhesion and measured adhesion strength

The work of adhesion, W _A, and the practical adhesion in terms of the interfacial shear strength, τ, in some polymer-fiber systems were determined to establish a correlation between these quantities. An attempt was made to analyze the contributions of various interfacial interactions (van der Waals forces, acid-base interaction, covalent bonding) to the 'fundamental' and 'practical' adhesion. The surface free energies of the fibers were altered using different coupling agents. To characterize the strength of an adhesion contact, the ultimate adhesion strength, τ_ult, was determined for the onset of contact failure. The adhesion of non-polar polymers occurs through van der Waals interaction only; therefore, fiber sizing does not affect the adhesion strength. For polar polymers, such as poly(acrylonitrile butadiene styrene) and polystyrene, adhesion is sensitive to fiber treatments: suppression of the acid-base interaction by using an electron-donor sizing agent γ-aminopropyltriethoxysilane results in a decrease of both 'fundamental' and 'practical' adhesion. In the case of epoxy resins, the main contribution to the work of adhesion is made by covalent bonds. Since the process of their formation is irreversible, the work of adhesion determined from micromechanical tests seems to be more reliable than indirect estimations, such as from wetting and inverse gas chromatography techniques. Fiber treatment by sizing agents results in considerable changes in the intensity of adhesional interaction with the epoxy matrix. A correlation between the work of adhesion, the ultimate interfacial shear strength, and the strength of macro-composites has been found.     

Wettability and Adhesion Characteristics of Plasma-Treated Carbon Fibers

The surface free energy (γ_s) of modified carbon fibers was determined by tensiometry and effects of CF₄-O₂ plasma treatment were evaluated. The treatment with the gas mixture in which oxygen was above 40% accelerated preferentially the oxidation of fiber surfaces and the nondispersive component of the surface free energy, γ^P_S, increased to about three times that of the untreated fiber. On the other hand, the treatment with the gas containing CF₄ above 80% induced fluorination and surface species such as - CF, - CF₂, or - CF₃ were formed. The γ^P_S values decreased to almost zero and the dispersive component became about 18 mJ/m². The calculated work of adhesion between various fibers and the epoxy resin was well correlated with the interfacial shear strength of the composites formed with these materials.     

Effects of Fiber Volume Fraction on Mechanical Properties of SiC-Fiber/Si3N4-Matrix Composites

The effects of fiber volume fraction on composite mechaniacl properties were examined in SiC-fiber-reinforced Si₃N₄ composites fabricated in our laboratories. Fiber volume fraction was found to have significant effects on important composite properties including failure mode, ultimate strength, matrix-cracking stress, fiber–matrix interfacial shear stress, and work-of-fracture. The composite mechanical properties were improved with increasing fiber volume fraction. However, when the fiber volume fraction was sufficiently large, the composite ultimate strength was degraded. This was related to fiber strength loss as a result of fiber damage from contact with surrounding fibers and abrasive matrix particles during hot pressing.