Thermoformed glass fiber reinforced polypropylene: Microstructure,mechanical properties and residual stresses

The objective of this work was to characterize the microstructure, mechanical properties and residual stresses in glass fiber reinforced polypropylene (PP) composites with respect to the thermoforming parameters and as a function of the fiber-matrix interface quality. First, differential scanning calorimetry (DSC) was used to investigate the crystallization behavior of the PP matrix. Second, short beam shear tests and tensile tests in the ±45° directions have been conducted to characterize respectively the interfacial strength and the matrix properties in the composites. Finally, residual stresses were measured via the curvatures of unsymmetric cross-plied laminates. The cooling rate was found to be a critical parameter of the molding process since the matrix crystallization temperature, the interfacial strength as well as the residual stresses showed large variations with various cooling rates. At slow cooling, the crystallization process initiates at higher temperatures and covers longer time periods resulting in more spherulitical matrix structures. In this case, the composites becomes stiffer but also fragile indicating a decrease in the stress transfer efficiency at the interface level. This effect was also observed in the improved interface system, suggesting that the fiber-matrix interaction operates through the amorphous phase surrounding the fibers. The fiber-matrix interface improvement was accompanied by an increase in residual stresses, possibly due to the inhibition of some stress relief mechanism.     

Some observations on stress graphitization in carbon-carbon composites

The in situ stress graphitization behavior of hard carbons in unidirectionally aligned carbon-carbon (C-C) composites was studied for three carbon fibers (PAN-based T-50, pitch-based PX7 and rayon-based WCA) and two carbon precursor resins [phenol-formaldehyde (SC1008) and polyarylacetylene (PAA), a high char-yielding, low shrinkage resin]. Graphitization was followed by measurements of density, transverse thermal expansion, d-spacing by X-ray diffraction (XRD) and by scanningelectron microscopy (SEM). In conjunction with xenon-ion etching, the SEM technique was found to be particularly effective in identifying localized regions of graphitized matrix. Results reveal that the graphitization of the composite is significantly greater than graphitization of fiber or matrix alone to the same temperatures. SEM observations indicate that graphitization is confined to the matrix, usually as a sheath-like structure adjacent to the fiber and 1–3 μm thick. Such localized graphitization, usually termed stress graphitization, is believed to be the result of thermally induced tensile or compressive stresses acting at the fiber-matrix interface. Debonded regions, which are believed to either initiate at heatup or grow from pre-existing cracks in the resin-matrix composite, show less stress graphitization than well-bonded regions, presumably because the debond gaps impede stress buildup at the fibermatrix interface.Studies with three different fibers and one matrix (PAA) in matrix-rich composites showed variable degrees of localized stress graphitization, suggesting that the thermal expansion stresses responsible for stress graphitization vary with different fiber-matrix combinations. One consequence of a well-oriented stress-graphitized sheath was found to be debonding of fiber and matrix. Possible reasons for such debonding are discussed briefly.     

Dynamic mechanical analysis of FRP composites based on different fiber reinforcements and epoxy resin as the matrix material

Three-ply composite laminates prepared from E-glass or N-glass chopped strand mats (CSMs) and jute (J) fabrics as reinforcing agents and amine-cured epoxy resin as the matrix material were subjected to dynamic mechanical thermal analysis at a fixed frequency of 1 Hz over a temperature range of 30–180°C. The volume fraction of fibers ranged between 0.21 and 0.25. The reinforcing effect for the three fibers is in the order E-glass > N-glass ≫ jute. Glass-reinforced composites show a higher storage modulus (E′) than that of jute-reinforced composites. The E′ values of glass-jute hybrid composites lie between those of glass-reinforced and jute-reinforced composites. Odd trends in temperature variability of the loss modulus (E′) and the damping parameter, tan δ, and in the glass transition temperature (T_g) for the three different unitary and four different hybrid composites are interpreted and understood on the basis of odd differences in (1) the chemical nature and physical properties of the three different fibers (E-glass, N-glass, and jute), (2) the void content and distribution, (3) the thermal expansion coefficients of the main phases in the composites, (4) the degree of matrix stiffening at or near the fiber-matrix interface, and (5) the extents of matrix softening in the zone next to the interface. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2467–2472, 1997     

Effect of fiber treatment on the mechanical properties of LDPE-henequen cellulosic fiber composites

The degree of mechanical reinforcement that could be obtained by the introduction of henequen cellulosic fibers in a low-density polyethylene, LDPE, matrix was assessed experimentally. Composite materials of LDPE-henequen cellulosic fibers were prepared by mechanical mixing. The concentration of randomly oriented fibers in the composite ranged between 0 and 30% by volume. The tensile strength of these composite materials increased up to 50% compared to that of LDPE. There is also a noticeable increase in Young's modulus for the composite materials that compares favorably with that of LDPE. As expected, the addition of the fibers decreases the ultimate strain values for the composite materials. The thermal behavior of the LDPE-henequen cellulosic fibers materials, studied by differential scanning calorimetry, DSC, showed that the presence of the fibers does not affect the thermal behavior of the LDPE matrix; thus, the interaction between fiber and matrix is probably not very intimate. Preimpregnation of the cellulosic fibers in a LDPE-xylene solution and the use of a silane coupling agent results in a small increment in the mechanical properties of the composites, which is attributed to an improvement in the interface between the fibers and the matrix. The shear properties of the composites also increased with increasing fiber content and fiber surface treatment. It was also noted that the fiber surface treatment improves fiber dispersion in the matrix. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 197–207, 1997     

Structural analysis of poly(ethylene terephthalate) reinforced with glass fiber: Thermal behavior and correlation between PA-FTIR and DSC measurements

In this article, the correlation between PA-FTIR and DSC techniques was carried out to obtain a more comprehensive interpretation for the fiber–matrix interface of glass fiberreinforced PET and the dependence on annealing temperature. In contrast to the thermal behavior of an unannealed matrix or an annealed one at low temperatures (below 110°C), glass transition is barely perceptible and no crystallization peak can be found in the composite. Therefore, the lack of these thermal characteristics indicates the absence of the primary isomerization in the matrix. Similarly to the isolated matrix, low-melting and high-melting peaks appear in the thermograms of the annealed composite beyond 135°C, but the size and perfection of crystallites seems to be less in the composite. Conversely to the spectroscopic results, the crystallinity values of the composite tend only to coincide with those of the annealed isolated matrix at temperatures beyond 150°C, and below this temperature, they are always lower, until the primary isomerization of the matrix. A comparison between the results obtained with both techniques seems to indicate an improvement on the fiber–matrix interface in which an interfacial structure with low degree of perfection changes to more perfect crystals due to the annealing treatment. © 1996 John Wiley & Sons, Inc.     

Treatment of ramie fiber with different techniques: the influence of diammonium phosphate on interfacial adhesion properties of ramie fiber-reinforced polylactic acid composite

Ramie fiber-reinforced polylactic acid (PLA) composites were successfully prepared by hot compression molding. Different treatment techniques were used to modify the surface of ramie fiber. The influence of diammonium phosphate (DAP) on the interfacial adhesion between ramie fiber and PLA composites was investigated by the contact angle measurements, FTIR and SEM analyses. The contact angle measurement results showed that alkali treatment combined with DAP was very efficient in decreasing the hydrophilicity of fibers. After treatment, the hydrophilicity of untreated ramie fiber from 5.9 ± 1.3 decreased to 2.0 ± 0.8 mJ/m². The wettability of alkali/silane/DAP-treated ramie fiber/PLA composite was higher (95.4° ± 1.3°) than that of pure ramie fiber/PLA composite (87.3° ± 1.9°). The FTIR results were consistent with the wetting measurements as the increment of hydrophilicity. Thermal analysis indicated that DAP-modified ramie fiber/PLA composites exhibited a lower thermal decomposition temperature, unique decomposition behavior and more residual char formation at decomposition temperature. The tensile, flexural and impact properties of DAP-modified ramie fiber composites were comparable to those of untreated ramie fiber composite. Moreover, proper alignment and uniform distribution of ramie fibers within the PLA matrix were found to be excellent. The morphological structures observed by SEM showed that well-modified ramie fibers enhanced the failure of the PLA composites in tensile, flexural and impact tests.     

Debond behavior of copper fibers in thermoset matrices and their effect on fracture toughening

Single fiber pullout experiments were conducted to determine the adhesion quality, debond behavior and subsequent matrix fracture behavior for a variety of end-modified copper fibers. The matrices were: two different epoxy resins, polyester and polyurethane; the end-modified copper fibers were: straight, flat end-impacted, flat end-impacted with release agent applied and straight end-oxidized. The goal was to determine how the bonding and debonding behavior as well as the pullout behavior of the various fiber-matrix combinations affected the composite fracture toughness increment (ΔG). Results indicate that the greatest improvement in the calculated ΔG occurred with a fiber-matrix combination that had a moderate interface bond strength with an interfacial bond failure, minor matrix damage during fiber pullout and moderate post-debond interface friction. Selective oxidation of the fiber end was performed to determine if chemical anchoring of the fiber end could be as effective as mechanical (end-shaping) anchoring of the fiber into the matrix. Improvement in the adhesion bond strength as a result of the chemical anchoring resulted in a significantly lower ΔG compared to the end-impacted fibers because interfacial failure was not possible. This indicates that for the materials tested, mechanical anchoring of the fiber was better than chemical anchoring in improving ΔG. To decrease the adhesion bond strength and allow the fibers to debond, a release agent was applied to the flat end-impacted fiber prior to embedment into the matrix. This resulted in a significantly lower ΔG compared to straight and flat end-impacted fibers for all matrices tested, because the resulting debonding force and friction were significantly reduced. Pullout curves showed that with release agent applied, the end-shape did not effectively anchor the fiber into the matrix. The reduction in the pullout work indicates that the friction at the fiber-matrix interface plays a crucial role in actively anchoring the end-shaped fiber into the matrix after debonding.     

Effect of fiber-matrix adhesion on the properties of short fiber reinforced ABS

The tensile strength, fracture energy, and impact strength of ABS reinforced with discontinuous crystalline fibers such as Fybex

1 Du Pont trademark.

can be controlled by manipulating fiber-matrix interfacial adhesion. In “good bonding” situations composite tensile strength, thermal expansion coefficient, and elastic moduli are significantly improved over the unfilled resin. The excellent impact strength of unreinforced ABS can be retained by lowering the fiber-matrix interfacial adhesion. This results in a corresponding reduction in the improvements in tensile and flexural strength. However, the elastic moduli and thermal expansion coefficients are relatively insensitive to changes in adhesion. Consequently, a material with high modulus (>500,000 psi), high Izod impact resistance (7.0 ft-lb/in.), and low expansion coefficient (3.0 × 10^?5 in./in./°F) can be obtained. A material with this unique combination of properties should find use in large parts such as camper tops, truck grilles, and snowmobile bodies. Fiber-matrix adhesion was measured directly by an x-ray analysis technique which could be employed because of the fiber's crystallinity and unique growth habit. This independent measurement allowed a correlation between bonding and composite properties. The interfacial bond strength was manipulated by a variety of fiber coatings and resin additions.     

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.     

Flexural modulus of SiC/SiC composites sintered by microwave and conventional heating

To deeply study the variation mechanisms of mechanical properties, flexural modulus of SiC fibers reinforced SiC matrix (SiC/SiC) composites prepared by conventional and microwave heating at 800?°C–1100?°C was discussed in detail. The elastic modulus of fibers and matrix, interface bonding strength and porosity of SiC/SiC composites were considered together to analyze the changing tendencies and differences in their flexural modulus. The elastic modulus of fiber and matrix was determined by nanoindentation technique and interface characteristics applying fiber push-out test. The porosity and microstructure examinations were characterized by mercury intrusion method, X-ray Diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Moreover, two conflicts between the changing trends of elastic modulus and chemical compositions of composite components were focused and explained. Results indicate that three factors played different roles in the flexural modulus of SiC/SiC composites and residual tensile stress in composite components led to the conflicts between their elastic modulus and chemical compositions.     

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.     

Effect of heating on the structure of carbon fiber polyphenylenesulfide-matrix composite,as studied by electrical resistance measurement

Heating and cooling were found to affect the structure of carbon fiber polyphenylenesulfide-matrix composites, as shown by their effects on the DC electrical resistance of the composite in the fiber direction. Matrix flow during first heating above T_g and thermal stress buildup during cooling in any thermal cycle caused the resistance to increase, whereas isothermal crystallization at 180°C caused the resistance to decrease. Thermal stress buildup was reversible upon thermal cycling. Prior annealing at 180°C diminished the matrix flow because of improved fiber-matrix bond, and it increased the matrix crystallinity. Annealing at 180°C for 15 h or more was sufficient to eliminate the matrix flow.     

Ionic interphase of glass fiber/polyamide 6,6 composites

Interfacial polymerization to polyamide 6, 6 followed by introduction of ionic groups was performed on the surface of short glass fibers. The ionic interphase-modified fibers were used with poly(ethylene-co-methacrylic acid) (DuPont Surlyn) to prepare composites with specific fiber-matrix interactions. Fiber treatment increased composite tensile and bending properties. An increase in the average fiber length was observed, which was attributed to a decrease in the fiber attrition during mixing. The effect of increasing temperature on the composite mechanical properties was studied. Different behavior was observed before and after the glass transition temperature, T_g, of the matrix. The dynamic mechanical measurements showed an increase in the T_g of the matrix after the treatments, which is attributed to a decrease in chain mobility at the interface resulting from increased interactions of the treated fiber surface with the polymer. Scanning electron microscopy of fractured composites after tensile tests revealed a smooth fiber surface with no polymer at the surface for the untreated composites. Adhered polymer was clearly observed on the surface of treated fibers, indicating better fiber wetting by the matrix. This improved adhesion was attributed to the grafted nylon molecules at the glass fiber surface.     

Interfacial adhesion between ionic copolymers and high-modulus surface-treated carbon fibers

High-modulus carbon-fiber-reinforced thermoplastic composites typically fail at the interface due to poor adhesion between fiber and matrix. To increase interfacial strength, the research described herein focuses on modifying the fiber surface (via high-temperature acid treatment or zinc electrolysis) to facilitate chemical functional groups on the fiber that might increase fiber-matrix inter-actions. The thermoplastic matrix materials used in this study were random copolymers of ethylene and methacrylic acid in which the carboxyl groups in the methacrylic acid segments were neutralized with either sodium or zinc counterions. Mechanical tests were performed to determine the macroscopic effects of fiber pretreatment on the ultimate mechanical properties of the composites. Fabrication was designed such that fiber-matrix separation provides the dominant contribution to mechanical gracture. Composites containing fibers treated with nitric acid, or a mixture of nitric and sulfuric acids exhibit a 20 to 25 percent increase in transverse (tensile) fracture stress relative to composites fabricated with as-received fibers. Scanning electron microscopy of the fiber-matrix interface at fracture allows one to “zoom-in” and obtain qualitative details related to adhesion. Fracture surface micrographs of the above-mentioned acid-treated fiber-reinforced composites reveal an increase in the amount of matrix material that adhered to the fiber surface relative to the appearance of the fracture surface of composites fabricated with as-received fibers. The presence of acid functionality in the matrix, rather than the divalent nature of the zinc counterions, produces the largest relative enhancement of transverse (tensile) fracture stress in the above-mentioned composites containing surface-treated carbon fibers.     

Glass fiber 'sizings' and their role in fiber-matrix adhesion

Silane coupling agents are but one of the many ingredients in commercial sizings that are applied to glass fibers. The action of epoxy-compatible silane coupling agents alone is to increase the fiber-matrix adhesion; however, the action of a silane coupling agent-containing sizing system is not well understood. Research has been conducted in order to determine to what degree an epoxy-compatible glass fiber sizing alters the adhesion between fiber and matrix, as well as to what degree it changes the mechanical properties of the resulting composite. By using blends of epoxy-compatible sizing with bulk matrix, it has been possible to model the properties of the fiber-matrix interphase formed when the sizing interacts with the matrix during composite processing and fabrication. It has been shown in this case that the sizing's interaction with the matrix produces a material with a higher modulus, a greater tensile strength, but a lower toughness. The level of fiber-matrix adhesion increases along with a change in failure mode of the composite caused by the presence of the lower toughness interphase. The results from this study show that a chemical interaction theory of adhesion is not sufficient to explain the effect of fiber-matrix adhesion on composite properties. An interphase-based theory in which the mechanical properties of the interphase are considered along with the chemical interactions between the fiber surface and the sizing offers the best approach for developing these relationships.     

The effects of thermal history on the structure/property relationship in polyphenylenesulfide/carbon fiber composites

The purpose of this investigation was to examine the effects of thermal history during cooling from the melt on the degree of crystallinity, morphology, and mechanical properties of polyphenylenesulfide (PPS)/carbon fiber composites. Three thermal treatments were employed in this tudy: isothermal crystallization from the melt at 140, 160, 180, 200, and 22O°C; quenching from 315°C and then annealing at 160 and 200°C; and nonisothermal crystallization from the melt at rates varying from 0.4°C/min to 38°C/s. The effect of varying the thermal history of the sample on the degree of crystallinity developed in the matrix polymer was determined using differential scanning calorimetry (DSC). The effect of thermal history on and the resulting matrix morphology was examined by scanning electron microscopy (SEM). The subsequent effects of the degree of crystallinity and the morphology on the mechanical behavior of the samples were monitored by transverse tensile tests and flexural tests. In all cases, the transverse tensile and flexural moduli increased as the amount of crystallinity in the samples increased. However, samples with greater amounts of crystallinity did not always yield higher transverse tensile or flexural strengths. Upon examination of the composite samples by electron miscroscopy, it was observed that large increases in the values of the transverse tensile and flexural strengths could be correlated with structural changes in the matrix.     

Structure and dyeability of Bombyx mori silk fibers with different filament sizes

This study deals with the analysis of structure, physical properties, and dyeing behavior of silk fibers with different filament sizes. Fine and coarse silk fibers were obtained from Akebono and Ariake cocoon varieties, respectively. Both samples exhibited a fairly similar x-ray crystallinity, while the degree of molecular orientation increased with decreasing the fiber size. Tensile strength and energy of fine silk fibers were significantly higher, while elongation at break did not change in relation to the fiber size. Fine silk fibers exhibited a slightly higher thermal stability, as shown by the upward shift of both the DSC decomposition temperature and the TMA final extension step at above 300°C. The TGA and DMA (E″) patterns remained unchanged regardless of fiber size. The amino acid analysis confirmed the absence of any difference of chemical structure between fine and coarse silk fibers, the content of acidic, basic, and other characteristic amino acid residues being exactly the same. Accordingly, both samples adsorbed the same amount of hydrochloric acid. Ex-haustion dyeing tests with various direct, acid, and reactive dyes showed the occurrence of appreciable differences in the perceived color between fine and coarse silk fibers. The size of color difference was quantitatively evaluated by reflectance measurements and discussed in relation to silk fiber morphology, structure, as well as dyeing conditions. The kinetics of diffusion of two model dyes was not significantly affected by the different fiber size and structure. © 1996 John Wiley & Sons, Inc.     

Mechanical and thermal shock properties of Cf/SiBCN composite: Effect of sintering densification and fiber coating

In this work, resin-derived carbon coating was prepared on carbon fibers by polymer impregnation pyrolysis method, then silicoboron carbonitride powder was prepared by mechanical alloying, and finally carbon fiber-reinforced silicoboron carbonitride composites were prepared by hot-pressing process. The effects of sintering densification and fiber coating on microstructure, mechanical properties, thermal shock resistance, and failure mechanisms of the composites were studied. Fiber bridging hinders the sintering densification, causing more defects in fiber-dense area and lower strength. However, higher sintering temperature (1800–2000°C) can improve mechanical properties significantly, including bending strength, vickers hardness, and elastic module, because further sintering densification enhances matrix strength and fiber/matrix bonding strength, while the change of fracture toughness is not obvious (2.24–2.38 MPa·m^1/2) due to counteraction of higher debonding resistance and less pull-out length. However, fiber coating improves fracture toughness greatly via protecting carbon fibers from chemical corrosion and damage of thermal stress and external stress. Due to lower coefficient of thermal expansion, lower fiber loading ratio, less stress concentration at the fiber/matrix interface, and better defect healing effect, lower sintering temperature favors thermal shock resistance of composites, and thermal shock recession mechanisms are the damage of interface.     

Studies on thermal,mechanical, morphological,and viscoelastic properties of polybenzimidazole fiber reinforced high density polyethylene composites

High density polyethylene (HDPE) and polybenzimidazole fiber (PBI) composites were prepared by melt blending in a twin screw extruder. The thermomechanical properties of PBI fiber reinforced HDPE composite samples (1%, 4%, and 8%) of fiber lengths 3 mm and 6 mm were investigated using differential scanning calorimeter (DSC), universal testing machine, rheometer, and scanning electron microscopy (SEM). The effects of fiber content and fiber lengths on the thermomechanical properties of the HDPE‐PBI composites were studied. The DSC analysis showed a decrease in crystallinity of HDPE‐PBI composites with an increase of fiber loading. SEM images revealed homogeneous distribution of the fibers in the polymer matrix. The thermal behavior of the composites was evaluated from thermogravimetric analysis and the thermal stability was found to increase with the addition of fibers. The evidence of homogeneous distribution was verified by the considerably high values of tensile strength and flexural strength. In the rheology study, the complex viscosities of HDPE‐PBI composites were higher than the HDPE matrix and increased with the increasing of PBI fiber loading. POLYM. COMPOS., 5–13, 2016. © 2014 Society of Plastics Engineers     

Green Composites from Natural Rubber and Oil Palm Fiber: Physical and Mechanical Properties

Natural rubber (NR) composites were prepared by incorporating short oil palm fibers of different lengths (viz., 2, 6, 10, and 14 mm) into natural rubber matrix in a mixing mill according to a base formulation. The curing characteristics of the mixes were studied and the samples were vulcanized at 150°C. The vulcanization parameters, processability characteristics, and tensile properties of these composites were analyzed. The effects of fiber length, orientation, loading, and fiber-matrix interaction on the mechanical properties of the green composites were studied. The reinforcement property of the alkali-treated fiber was compared with that of the untreated one. The extent of fiber orientation was studied from green strength measurements. From anisotropic swelling studies, the extent of fiber alignment and the strength of fiber–rubber interface adhesion were analyzed. Scanning electron microscopic (SEM) studies were carried out to analyze the fiber surface morphology, fiber pullout, and fiber–rubber interface.