High Stiffness Natural Fiber‐Reinforced Hybrid Polypropylene Composites

Abstract

Natural fibers are potentially a high‐performance non‐abrasive reinforcing fiber source. In this study, pulp fibers [including bleached Kraft pulp (BKP) and thermomechanical pulp (TMP)], hemp, flax, and wood flour were used for reinforcing in polypropylene (PP) composite. The results show that pulp fibers, in particular, TMP‐reinforced PP has the highest tensile strength, possibly because pulp fibers were subjected to less severe shortening during compounding, compared to hemp and flax fiber bundles. Maleic‐anhydride grafted PP (MAPP) with high maleic anhydride groups and high molecular weight was more effective in improving strength properties of PP composite as a compatiblizer. Coupled with 10% glass fiber, 40% TMP reinforced PP had a tensile strength of 70 MPa and a specific tensile strength comparable to glass fiber reinforced PP. Thermomechanical pulp was more effective in reinforcing than BKP. X‐ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM) were used to aid in the analysis. Polypropylene with high impact strength was also used in compounding to improve the low‐impact strength prevalent in natural fiber‐reinforced PP from injection molding.     

Effect of fiber reinforcement on the tensile, fracture and thermal properties of syntactic foam

This paper examines the effect of the fiber content and fiber length on tensile, fracture and thermal properties of syntactic foam. Results showed that a hybrid structure demonstrates a significant increase in the ultimate tensile strength, σ_uts, and Young's modulus, E, with increasing fiber loading. Interestingly, the fracture toughness, K_Ic, and energy release rate, G_Ic, increased by 95% and 90%, respectively, upon introduction of 3 wt% short carbon fibers in syntactic foam, indicating the potent toughening potential for short carbon fibers in syntactic foam systems. SEM and OM studies identified the presence of several toughening mechanisms. An estimate of the contribution from each toughening mechanism by composite theory and fractography revealed that the specific energy required to create new surfaces was enhanced by the presence of fibers and was the main contributor to the toughness of the short fiber reinforced syntactic foam.     

Effects of thermal history on mechanical behavior of PEEK and its short-fiber composites

The effect of crystallinity differences induced by mold wall temperature and annealing on mechanical behavior is evaluated for poly(etheretherketone) (PEEK) resin and its composites. The systems investigated were neat PEEK, glass fiber (GF) reinforced PEEK, and carbon fiber (CF) reinforced PEEK. Both composite systems were reinforced with 10, 20, and 30 wt% fiber. The degree of crystallinity (X_c) of PEEK was found to increase by processing at higher mold temperatures, by annealing, and by fiber length reductions, which appears to indicate the ability of short fibers to nucleate the crystallization of PEEK under favorable thermal conditions. Improvements in Young's modulus and strength together with ductility reductions are generally obtained as crystallinity increases in both neat PEEK and its composites. The contribution of crystallinity to mechanical behavior is significant only for neat PEEK and PEEK reinforced by 10% fiber. SEM micrographs reveal that this is due to a change in failure mode. When PEEK is reinforced by carbon fibers or by 20–30% glass fibers, a macroscopic brittle mode of failure is observed irrespective of matrix crystallinity, and mechanical behavior is principally determined by the nature and content of the reinforcing fibers.     

Study on depositing SiO2 nanoparticles on the surface of jute fiber via hydrothermal method and its reinforced polypropylene composites

To improve the interfacial compatibility of jute fiber reinforced polypropylene (PP) composites, hydrothermal method was used to deposit SiO₂ nanoparticles on the surface of pretreated jute fibers and the effect of reaction factors (tetraethoxysilane [TEOS] concentration, ammonia concentration, and reaction temperature) on the deposition of SiO₂ nanoparticles were evaluated. The results of FTIR, XRD, SEM, and TEM showed that the amorphous SiO₂ nanoparticles with an average particle size of 65.0 nm were successfully deposited on the surface of jute fibers at the TEOS/H₂O volume ratio of 1:2, ammonia of 0.55 M, reaction temperature of 100 °C (0.15 MPa) for 5 h. Compared with the sol–gel method, SiO₂ nanoparticles obtained by the hydrothermal method possessed smaller particle size and were less agglomerated, which can better fill in the surface defects of the jute fibers and result in a 12.9% increase in the tensile strength. The study on the mechanical properties and interface performance of the jute fiber reinforced PP composites indicated that the interfacial compatibility between jute fibers and PP was obviously improved. The tensile and impact strength of the composites reinforced with nano‐SiO₂ deposited jute fibers were increased by 26.87% and 25.65%, respectively, compared with the untreated jute fibers. J. VINYL ADDIT. TECHNOL., 26:43–54, 2020. © 2019 Society of Plastics Engineers     

Effects of fiber length on the tensile strength of epoxy/glass fiber and polyester/glass fiber composites

In discontinuous fiber-reinforced composites, the shear strength at the fiber–matrix interface plays an important role in determining the reinforcing effect. In this paper, a method was devised to accurately determine this shear strength, taking the strength distribution of glass fiber into consideration. Calculated strength values based on the shear strenght obtained by the method were in better agreement with the experimental observations than those calculated by employing the shear strength obtained on the assumption that the fiber strength was uniform. The tensile strength of composites increases with increasing aspect ratio of the reinforcing fibers. This trend is almost the same regardless of the kind of matrix, the nature of interfacial treatment, and the environmental temperature. When composites are reinforced with random-planar orientation of short glass fibers of 1.5 times the mean critical fiber length, the tensile strength of composite reaches about 90% of the theoretical strength of composites reinforced with continuous glass fiber. Reinforcing with glass fibers 5 times the critical length, the tensile strength reaches about 97% of theoretical. However, from a practical point of view, it is adequate to reinforce with short fibers of 1.5–2.0 times the mean critical fiber lenght.     

Study on long fiber–reinforced thermoplastic composites prepared by in situ solid‐state polycondensation

Long glass fiber–reinforced thermoplastic composites were prepared by a new process, in situ solid‐state polycondensation (INSITU SSP). In this process reinforcing continuous fibers were impregnated by the oligomer of PET melt, and then the impregnated continuous fibers were cut to a desired length (designated prepreg); finally, the prepreg was in situ polymerized in the solid state to form the high molecular weight matrix. SEM, FTIR spectra, short‐beam shear stress test, flexural strength test, impact strength test, and the intrinsic viscosity measurement were used to investigate the wetting and interfacial adhesion, the mechanical properties of the composite, and the molecular weight of matrix resin in the composite. The results showed that the molecular weight of PET in the matrix resin and mechanical properties could be adjusted by controlling the SSP time and that the high level of interfacial adhesion between reinforcing fibers and matrix resin could be achieved by this novel INSITU SSP process, which are attributed to the good wetting of reinforcing fibers with low molecular weight oligomer melt as the impregnation fluid, the in situ formation of chemical grafting of oligomer chains onto the reinforcing fiber surface, and the in situ formation of the high molecular weight PET chains in the interphase regions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91:3959–3965, 2004     

Structure and property of polyurethane/SiO2 composite elastomer prepared via in situ polymerization

A series of polyurethane (PU)/SiO₂ composites were prepared via in situ polymerization through a prepolymer process. The effect of SiO₂ content on mechanical and solvent‐resistant properties of PU/SiO₂ composites was investigated. It was found that with increasing SiO₂ content, the tensile strength and toughness, hardness, and modulus of PU composites increased. PU/SiO₂ composites exhibited only one loss peak corresponding to the glass transition temperature of the soft‐segment of PU, which shifted insignificantly with increasing SiO₂ content. The equilibrium swelling ratio and swelling rate constant in cyclohexanone and xylene were reduced by increasing SiO₂ content, indicating the enhancement of the solvent resistance of the PU elastomer. Morphology observation of PU/SiO₂ composites showed that the acicular SiO₂ dispersed uniformly in PU matrix, and there was no obvious aggregation even at 9 wt % loading of SiO₂. The reinforcing and toughening effects depended largely on the dispersion of SiO₂ in PU matrix and the interfacial layer formed between the two phases. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Investigation of mechanical properties of alumina nanoparticle‐loaded hybrid glass/carbon‐fiber‐reinforced epoxy composites

This research work investigates the tensile strength and elastic modulus of the alumina nanoparticles, glass fiber, and carbon fiber reinforced epoxy composites. The first type composites were made by adding 1–5 wt % (in the interval of 1%) of alumina to the epoxy matrix, whereas the second and third categories of composites were made by adding 1–5 wt % short glass, carbon fibers to the matrix. A fourth type of composite has also been synthesized by incorporating both alumina particles (2 wt %) and fibers to the epoxy. Results showed that the longitudinal modulus has significantly improved because of the filler additions. Both tensile strength and modulus are further better for hybrid composites consisting both alumina particles and glass fibers or carbon fibers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39749.     

Comparison of mechanical performance and fracture behavior of gelatin composites reinforced with carbon fibers of different fiber architectures

Gelatin‐based composites reinforced, respectively, with continuous carbon fibers, short carbon fibers, plain woven carbon fibers, and carbon fiber felt were investigated. Tensile and shear strengths, and their changes with fiber volume fraction (V_f) of these four composites were compared. It was demonstrated that at all fiber levels, the composite containing continuous carbon fibers showed the largest strength, while the composite reinforced with carbon fiber felt exhibited the lowest strength of the four composites. The above results were analyzed by comparing the fracture surfaces of the four composites. SEM confirmed the great differences in fracture surfaces for composites of different fiber architectures. The presence of a large number of pores in the C_F/Gel composite was responsible for its lowest strength, and cracks within fiber tows caused the lower strength of the C_W/Gel composite when compared to its C_L/Gel counterpart. It was suggested that fiber architecture exerted a great effect on composite performance and the effect was dependent on the nature of the matrix material.     

Characterization of Short Glass Fiber–Reinforced Castor Oil–Based Polyurethane Polystyrene Interpenetrating Networks

ABSTRACT

Chopped strand glass fiber–reinforced particle-filled castor oil–based polyurethane polystyrene composites with varying weight fractions of glass fibers were investigated for morphology, tensile strength, and absorption of various chemicals. The short glass fiber fraction was varied from 1% to 16% (by wt.) of the total composite system. The tensile strength of these composites was much higher than that of unfilled IPNs for the same concentration of polystyrene. The tensile strength of the IPN composites increases with the increase in fiber content up to a fiber percentage of 9%. After that, there is a sharp decline in tensile modulus as well as elongation at break. The chemical absorption showed an increase with increasing glass fiber content.     

The properties of glass fibers recycled from the thermal processing of scrap thermoset composites

Glass fiber reinforced composites can be recycled by combustion of the polymer in a fluidized bed and reclamation of the reinforcement. The effect of heat on the reinforcing properties of glass fibers was studied by measuring the strength of a composite incorporating heat cleaned glass cloth. This work was then extended by measuring the tensile strength of single fibers. An initial study of the residual strength of fibers processed in a fluidized bed was also undertaken by measuring the tensile strength of fibers recovered at two temperatures. It is concluded that low bed temperatures and short bed residence times are required to maximize the strength of the recovered fibers.     

Surface treatments of subdenier monofilament polypropylene fibers to optimize their reinforcing efficiency in cementitious composites

The surface properties of polypropylene (PP) fibers have an important effect on their reinforcing efficiency in cementitious composites. Two new methods of modifying the surface of subdenier monofilament polypropylene fibers were introduced, as well as the performances of the fiber‐reinforced mortar. The results show that the surface modification improved the mechanical performance of the fiber‐reinforced mortars, such as compressive strength and flexural strength, and the reinforcing efficiency depends on the adopted method. The enhanced interfacial bonding between treated fibers and the cementitious matrix, compared with that of unmodified fibers, was investigated using scanning electronic microscopy. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2637–2641, 2004     

Adhesive joints between carbon fiber and aluminum foam reinforced by surface‐treated aramid fibers

Short aramid fibers have been successfully used to reinforce the interface adhesive property between carbon fiber/epoxy composites and aluminum foam, and to form aramid‐fiber “composite adhesive joints.” In this study, to further improve the reinforcing effect of the aramid‐fiber‐reinforced adhesive joints, aramid fibers were ultrasonic treated to conduct different surface conditions. Critical energy release rate of the carbon fiber/aluminum foam sandwich beams with as‐received and treated interfacial aramid fibers were measured to study the influence of the surface treatment on aramid fibers. It was found that reinforcements in critical energy release rate were achieved for all samples with treated aramid fiber as measured under double cantilever beam condition. The interfacial characteristics of the short aramid fibers with different surface condition were investigated and discussed based on scanning electron microscopy observations. It is suggested that advanced bonding between aramid fibers and epoxy resin was conducted after surface treatment, and more energy was therefore absorbed through fiber bridging during crack opening and extension process. POLYM. COMPOS., 36:192–197, 2015. © 2014 Society of Plastics Engineers     

Non‐traditional lightweight polypropylene composites reinforced with milkweed floss

Lightweight composites are preferred for automotive applications due to the weight restrictions and also due to the presence of inherent voids that can enhance the sound absorption of these composites. The density of the reinforcing materials plays a crucial role in such lightweight composites. Milkweed is a unique natural cellulose fiber that has a completely hollow center and low density (0.9 g cm^?3) unlike any other natural cellulose fiber. The low density of milkweed fibers will allow the incorporation of higher amounts of fiber per unit weight of a composite, which is expected to lead to lightweight composites with better properties. Polypropylene (PP) composites reinforced with milkweed fibers have much better flexural and tensile properties than similar PP composites reinforced with kenaf fibers. Milkweed fiber‐reinforced composites have much higher strength but are stiffer than kenaf fiber‐reinforced PP composites. Increasing the proportion of milkweed in the composites from 35 to 50% increases the flexural strength but decreases the tensile strength. The low density of milkweed fibers allows the incorporation of higher amounts of fibers per unit weight of the composites and hence provides better properties compared to composites reinforced with common cellulose fibers with relatively high density. This research shows that low‐density reinforcing materials can more efficiently reinforce lightweight composites. Copyright © 2010 Society of Chemical Industry     

Effect of macromolecular coupling agent on the property of PP/GF composites

Silane‐grafted polypropylene manufactured by a reactive grafting process was used as the coupling agent in polypropylene/glass‐fiber composites to improve the interaction of the interfacial regions. Polypropylene reinforced with 30% by weight of short glass fibers was injection‐molded and the mechanical behaviors were investigated. The results indicate that the mechanical properties (tensile strength, tensile modulus, flexural strength, flexural modulus, and Izod impact strength) of the composite increased remarkably as compared with the noncoupled glass fiber/polypropylene. SEM of the fracture surfaces of the coupled composites shows a good adhesion at the fiber/matrix interface: The fibers are coated with matrix polymer, and a matrix transition region exists near the fibers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1537–1542, 1999     

Structure–property relationships in glass reinforced polyamide,part 2: The effects of average fiber diameter and diameter distribution

We present the results of an extensive study of the influence of average fiber diameter and the width of the diameter distribution on the performance of injection‐molded glass‐fiber reinforced polyamide 6,6. In the average fiber diameter range from 9 to 18 μm, dry‐as‐molded (DaM) composite unnotched impact and tensile strength decreased significantly. The composite notched impact performance and tensile modulus showed little dependence on fiber diameter. The influence of broadening the fiber diameter distribution by blending glass fiber samples of different average diameter was found to be particularly negative on the level of composite unnotched impact when compared at equal number average diameter. After hydrolysis treatment, the composite tensile strength and modulus exhibited a large drop compared to the DaM results. In contrast, the unnotched impact results became insensitive to fiber diameter after hydrolysis. The average level of unnotched impact after hydrolysis was sufficiently high to show an increase over DaM when the fiber diameter was above 14 μm. Residual fiber length correlated significantly with fiber diameter with a lower average length for thinner fibers. The interfacial shear strength was found to be in the range of 26–34 MPa for DaM composites. There was a highly significant inverse correlation between the DaM interfacial strength and the average fiber diameter. It is shown that results from both tensile and unnotched impact measurements can be brought back to single trend lines by using a Z average value for the average fiber diameter, which is more heavily weighted to the thicker fibers in the distribution. POLYM. COMPOS., 28:331–343, 2007. © 2007 Society of Plastics Engineers     

The effect of high temperature heat-treatment on the strength of C/C to C/C-SiC joints

Joining of carbon fiber reinforced carbon matrix composite to a carbon fiber reinforced C-SiC dual matrix composite has been realized through a reaction joining process using boron-modified phenolic resin with micro-size B₄C and nano-size SiO₂ powder additives. The effect of the heat-treatment temperature on the retained strength of the joints, calculated by dividing the strength of the heat-treated joints by the strength of the joints before heat-treatment, was studied. The maximum retained strength of the joints is 91.9% after heat-treatment at 1200 °C for 30 min in vacuum, indicating good heat-resistance of the joints. The interlayer with a thickness of about 25 μm is uniform and densified. There are no obvious cracks or pores at the interfaces. The interlayer is composed of B₄C, SiO₂, glassy carbon, B₂O₃ and borosilicate glass. Si diffuses from the interlayer into the substrates and reacts with carbon to form SiC. Both B and O migrate from the interlayer into the substrates, contributing to the interfacial bonding. The B₄C and the SiO₂ powder additives contribute to the densification of the interlayer, the bonding at the interfaces and the heat-resistance of the joints.     

Comparison of foam core sandwich panel and through‐thickness polymer pin–reinforced foam core sandwich panel subject to indentation and flatwise compression loadings

Through‐thickness polymer pin–reinforced foam core sandwich (FCS) panels are new type of composite sandwich structure as the foam core of this structure was reinforced with cylindrical polymer pins, which also rigidly connect the face sheets. These sandwich panels are made of glass fiber–reinforced polyester face sheets and closed‐cell polyurethane foam core with cylindrical polymer pins produced during fabrication process. The indentation and compression behavior of these sandwich panels were compared with common traditional sandwich panel, and it has been found that by reinforcing the foam core with cylindrical polymer pins, the indentation strength, energy absorption, and compression strength of the sandwich panels were improved significantly. The effect of diameter of polymer pins on indentation and compression behavior of both sandwich panels was studied and results showed that the diameter of polymer pins had a large influence on the compression and indentation behavior of through‐thickness polymer pin–reinforced FCS panel, and the effect of adding polymer pins to FCS panel on indentation behavior is similar to the effect of increasing the thickness of face sheet. The effect of strain rate on indentation behavior of FCS panel and through‐thickness polymer pin–reinforced FCS panel were studied, and results showed that both types of composite sandwich panels are strain rate dependent structure as by increasing strain rate, the indentation properties and energy absorption properties of these structures are increased. POLYM. COMPOS., 37:612–619, 2016. © 2014 Society of Plastics Engineers     

Tensile behavior of unidirectional polyethylene fibers PMMA and glass fibers—PMMA composite laminates

Unidirectional (UD) composite laminates based on glass fibers (GF) and high‐performance polythylene fibers (PEF) were prepared with partially polymerized methyl methacrylate (MMA) at room temperature, followed by heating at 55°C (well below the softening point of PEF) for 2 h. The tensile strength, modulus of elasticity, fiber efficiency and strength efficiency of both the composite laminates, loaded parallel to the fibers, at the same volume fraction range, were investigated. All the properties were compared between the two composite laminates. It was observed that the measured tensile strength and modulus of elasticity deviated from the values calculated from the Rule of Mixture (ROM). The deviation was minimal at the lower volume fraction of fibers, and increased with the fiber volume. An interesting feature that was observed was that the efficiencies of PEF‐reinforced composite was higher than that of the GF‐reinforced composite at the same volume fraction of the fibers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1489–1493, 2000     

Influence of interfacial adhesion on the structural and mechanical behavior of PP‐banana/glass hybrid composites

Hybrid composites of polypropylene (PP), reinforced with short banana and glass fibers were fabricated using Haake torque rheocord followed by compression molding with and without the presence maleic anhydride grafted polypropylene (MAPP) as a coupling agent. Incorporation of both fibers into PP matrix resulted in increase of tensile strength, flexural strength, and impact strength upto 30 wt% with an optimum strength observed at 2 wt% MAPP treated 15 wt% banana and 15 wt% glass fiber. The rate of water absorption for the hybrid composites was decreased due to the presence of glass fiber and coupling agent. The effect of fiber loading in presence of coupling agent on the dynamic mechanical properties has been analyzed to investigate the interfacial properties. An increase in storage modulus (E′) of the treated‐composite indicates higher stiffness. The loss tangent (tan δ) spectra confirms a strong influence of fiber loading and coupling agent concentration on the α and β relaxation process of PP. The nature of fiber matrix adhesion was examined through scanning electron microscopy (SEM) of the tensile fractured specimen. Thermal measurements were carried out through differential scanning calorimetry (DSC) and the thermogravimetric analysis (TGA), indicated an increase in the crystallization temperature and thermal stability of PP with the incorporation of MAPP‐treated banana and glass fiber. POLYM. COMPOS., 31:1247–1257, 2010. © 2009 Society of Plastics Engineers