Mechanical and morphological properties of coextruded wood plastic composites with glass fiber‐filled shell

Coextrusion technology makes various properties of wood plastic composites (WPCs) highly tunable. However, structural and material optimization of core‐shell shaped WPCs is needed to balance manufacturing cost, processing efficiency, and product performance. In this study, various systems of coextruded WPCs were designed and analyzed using short glass fiber (SGF)‐filled shells in combination with three core systems (i.e., weak, moderate, and strong). A comparison of the composite flexural property of the manufactured WPCs (i.e., modulus and strength) shows that SGF reinforcements in the shell layer were optimized at high SGF loading levels regardless of core qualities. Also, SGF alignments in the shell layer played an important role in determining the flexural property of the WPCs. When the shell modulus and strength were lower than these of the core, the increase of shell thickness led to reduced composite properties. On the other hand, when the shell properties were higher than the core properties, the opposite was true. Composite impact strength increased with shell thickness increase for all three core systems. However, at a given shell thickness, the impact strength decreased with the addition of SGFs in the shell. Further increase of SGFs in the shell led to somewhat increased impact strength. The structure–property relationship plots provide a design guide for optimizing performance of coextruded WPCs with various combinations of core‐shell qualities. POLYM. COMPOS., 37:824–834, 2016. © 2014 Society of Plastics Engineers     

Effect of the delignification of wood fibers on the mechanical properties of wood fiber–polypropylene composites

The effect of the delignification of hornbeam fibers on the mechanical properties of wood fiber–polypropylene (PP) composites was studied. Original fibers and delignified fibers at three levels of delignification were mixed with PP at a weight ratio of 40:60 in an internal mixer. Maleic anhydride (0.5 wt %) as the coupling agent and dicumyl peroxide (0.1 wt %) as the initiator were applied. The produced composites were then hot‐pressed, and specimens for physical and mechanical testing were prepared. The results of the properties of the composite materials indicate that delignified fibers showed better performance in the enhancement of tensile strength and tensile modulus, whereas the hardness of the composites was unaffected by delignification. Delignified fibers also exhibited better water absorption resistance. Notched impact strength was higher for delignified fiber composites, but it was reduced at higher delignification levels. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4759–4763, 2006     

Short kevlar fiber–thermoplastic polyurethane composite

Mechanical and dynamic mechanical behavior of short Kevlar fiber-filled thermoplastic polyurethane (TPU) have been studied with respect to fiber loading and orientation. The strength of the composite is improved at higher fiber content with a minimum at 10 phr of fibers. Storage and loss moduli (E′, E″) are increased and tan δ_max is reduced progressively with fiber loading, the effect on moduli being more pronounced at post-T_g temperatures. Anisotropy in strength is evident beyond 15 phr fiber loading. Impact strength is reduced considerably at all fiber loading irrespective of fiber orientation. Study of the fracture furface by scanning electron microscopy (SEM) shows good correlation between the modes of failure and strength of the composites. SEM study of the extracted fibers shows the existence of a kinking stage through which the fiber undergoes severe breakage during processing.     

Influence of thermoplastic elastomers on adhesion in polyethylene–wood flour composites

The mechanical properties of recycled low-density polyethylene/wood flour (LDPE/WF) composites are improved when a maleated triblock copolymer styrene–ethylene/butylene–styrene (SEBS–MA) is added as a compatibilizer. The composites' tensile strength reached a maximum level with 4 wt % SEBS–MA content. The compatibilizer had a positive effect on the impact strength and elongation at break but decreased the composites' stiffness. Dynamic mechanical thermal analysis (DMTA), a lap shear adhesion test, and a scanning electron microscope (SEM) were used to investigate the nature of the interfacial adhesion between the WF/SEBS and between the WF/SEBS–MA. Tan δ peak temperatures for the various combinations showed interaction between the ethylene/butylene (EB) part of the copolymer and the wood flour in the maleated system. The shear lap test showed that adhesion between the wood and SEBS–MA is better than between the wood and SEBS. The electron microscopy study of the fracture surfaces confirmed good adhesion between the wood particles and the LDPE/SEBS–MA matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1845–1855, 1998     

Fiber–matrix interactions in aramid‐short‐fiber‐reinforced thermoplastic polyurethane composites

The mechanical and dynamic mechanical properties of thermoplastic polyurethane (TPU) elastomers reinforced with two types of aramid short fibers, m‐aramid (Teijin‐Conex) and copoly(p‐aramid) (Technora), were investigated in this study with respect to the fiber loading. In general, both types of composites exhibited very similar stress–strain behaviors, except that Technora–TPU was stronger than Conex–TPU. This was primarily due to the intrinsic strength of the reinforcing fibers. Both types of fibers reinforced TPU effectively without any surface treatment. This could be attributed to good fiber–matrix interactions, which were revealed by the broadening of the tan δ peak in dynamic mechanical analysis. Furthermore, the morphologies of cryogenically fractured surfaces of the composites and extracted fibers, investigated with scanning electron microscopy, revealed possible polar–polar interactions between the aramid fibers and TPU matrices. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1059–1067, 2003     

Structurally different short aramid fiber–reinforced thermoplastic polyurethane

The reinforcing effect of two structurally different Aramid short fibers, Technora and Twaron on the mechanical, dynamic mechanical, and thermal properties of an ester‐based thermoplastic polyurethane (TPU) was investigated. A fixed fiber length of 6 mm is used by varying the fiber loading ranging from 3 to 10 phr. The Young's modulus and the low strain modulus of Technora–TPU composite was found three times higher than that of Twaron–TPU composite at all ranges of fiber loading. Optical microscopic analysis revealed that a severe processing‐induced fiber breakage of Twaron is the primary reason behind the inferior properties shown by these fiber‐reinforced TPU composite. A brittle kind of failure has been observed during tensile testing in both the composite at a fiber loading of 10 phr. To solve this problem, an economic pretreatment with maleic anhydride‐grafted polybutadine (PB‐g‐MA) has been applied on the Aramid fiber surface before mixing it with the TPU matrix. A good quality of fiber dispersion with significant improvement in mechanical properties could be achieved with the addition of only 5 phr of PB‐g‐MA. Morphological analyses on the tensile‐fractured and cryogenically fractured surfaces of these composites offer strong evidences for the dispersing and coupling action of PB‐g‐MA with these Aramid fibers and the TPU matrix. POLYM. COMPOS., 35:1767–1778, 2014. © 2013 Society of Plastics Engineers     

Dynamic behavior of short aramid fiber‐filled elastomer composites

Short fiber reinforcement is a suitable way to improve the tribological properties of elastomers. However, rubbers products are often exposed to highly dynamic mechanical loadings. Hereby it is crucial to study the change in dynamic behavior due to the addition of short fibers. Therefore, these properties were investigated in terms of dynamic mechanical thermal analysis, heat build‐up (HBU), and fatigue crack growth resistance under cyclic loadings for two different rubber compounds. A peroxide cured ethylene–propylene–diene rubber (EPDM) and a sulfur cured natural rubber (NR) compound were chosen and reinforced with two types of short aramid fibers. It was found that the short fibers could contribute to the improvement in the crack growth resistance, HBU, and the dynamic mechanical behavior of the composites depending on the testing conditions. POLYM. ENG. SCI., 54:2958–2964, 2014. © 2014 Society of Plastics Engineers     

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     

Glass‐fiber‐reinforced wood/plastic composites

The usage of wood‐plastic composites (WPCs) is rapidly growing because of their many advantages. However, they still suffer from lack of strength and toughness, which can be improved by adding a small amount of glass fiber reinforcement (GFR). Tensile tests of high‐density polyethylene WPC specimens with varying amounts of wood fiber content and 5% of GFR were carried out. Significant improvements in properties were observed. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers.     

Temperature dependence of the tensile strength of glass fiber–epoxy and glass fiber–unsaturated polyester composites

Epoxy and unsaturated polyester resins reinforced with random-planar orientation of short glass fibers were prepared and the temperature dependence of their tensile strength was studied. The tensile strength decreases as the temperature increases, and this tendency can be expressed in terms of critical fiber length l_c and apparent interfacial shear strength τ: where σ_cs is the tensile strength of composite reinforced with random-planar orientation of short fibers, L is the fiber length, d is the fiber diameter, σ_f is the tensile strength of fiber, σ_m is the tensile strength of matrix, u_f is the volume fraction of fiber, v_m is the volume fraction of matrix, and σ′_m is the stress of the matrix at fracture strain of the composite. The experimental strength values at room temperature are considerably smaller than the theoretical values, and this difference can be explained by the thermal stress produced during molding due to the large difference in the thermal expansion coefficient between glass fiber and matrix resin.     

Stamp forming of fabric-reinforced thermoplastic composites

A stamp forming technique has been used to process a fabric woven composite made of glass fibers (GF) and polyetherimide (PEI). A hemispherical mold with a built-in hold-down arrangement was designed and used at room temperature to stamp parts from preheated flat preconsolidated laminates. Tensile properties of the material were measured under similar heating conditions as in the relevant stamp-forming process. Stretch in the fiber direction was found to be smaller than the maximum elastic extension of the glass fibers. Reduction of the angle between the crossing fibers was quite large when the satin woven fabric composite was pulled in the 45° direction. The effect of die geometries and original laminate dimensions on the “shear-buckling” were studied. The results described the correlations between processing parameters and fiber buckling. Finally, the local strain of fiber bundles was investigated in relation to different directions of fiber orientation.     

Characterization of highly filled wood flour–PVC composites: Morphological and thermal studies

Composites with polyvinyl chloride (PVC) as major matrix constituent, ethylene vinyl acetate (EVA) as polymeric plasticizer and wood flour (WF) and fly ash (FA) as filler were extruded. Morphology of the samples was studied using scanning electron microscopy (SEM). Morphological study indicated good dispersion of the constituents. Infrared spectroscopy (IR) indicates interaction between EVA and PVC and also between the polymeric matrix and WF. The effect of various constituents on glass transition temperature (T_g) was evaluated using differential scanning calorimetry. Addition of EVA decreased the T_g, whereas T_g was increased due to addition of WF and FA. Study indicated that reduction in T_g on addition of EVA was compensated by increase in T_g due to addition of WF. The contribution of FA to change in T_g was not significant. Resulting composites have T_g close to that of PVC. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Use of wood fibers in thermoplastic composites: VI. Isocyanate as a bonding agent for polyethylene–wood fiber composites

Linear low density (LLDPE) and high density (HDPE) polyethylenes were reinforced with wood fibers of aspen chemithermomechanical (CTMP) pulp. The different isocyantes: (i) polymethylene (polyphenyl isocyanate), (ii) tolene –2–4-diisocyanate, (iii) 1–6 hexamethylene-diisocyanate, and (iv) ethyl isocyanate used as bonding agents improved the tensile properties of the composites. HDPE performed better in comparison with LLDPE composites. Also, shorter fibers (mesh size 60) produced higher tensile strength and modulus in HDPE. The comparison of HDPE reinforced with aspen, mica, and glass fibers showed the effectiveness of wood fibers in terms of their cost and performance.     

Characterization of electropolymerized graphite fiber–polyacrylamide composites

The synthesis of a series of graphite fiber-polyacrylamide composites was performed electrochemically in dilute sulfuric acid (0.125M)-acrylamide (2M) solution, 1 : 1 sulfuric acid (0.25M) : acetone-acrylamide (2M) solution, and 1 : 1 sulfuric acid (0.25M) : 2-propanol-acrylamide (2M) solution, respectively, using graphite fiber bundles as the working electrode. The graphite fiber-polyacrylamide composites, synthesized in a 1 : 1 2-propanol : sulfuric acid-acrylamide solution, were more easily characterized than those synthesized from the sulfuric acid-acrylamide solution that contained no alcohol. Composites that were synthesized in a dilute sulfuric acid solution were, however, more readily crosslinked. (Fourier transform infrared spectroscopy, FTIR, confirmed the formation of inter-chain and intrachain imide functional groups after the resin was cured at ≈ 200°C.) Polymer weight gain analysis, coupled with surface morphology studies using scanning electron microscopy, showed that the thickness of the coatings, and hence the volume fraction of the resin in the composites, varied linearly with the time of electropolymerization. Scanning electron microscopy revealed an open and folded chain surface structure, which permitted unrestricted permeation of the monomer onto the electrode surface. Differential scanning calorimetry of the electropolymerized resins confirmed a glass transition temperature of between 180 and 207°C. © 1994 John Wiley & Sons, Inc.     

Mechanical and physical characteristics of cellulose‐fiber‐filled polyacetal composites

We investigated the mechanical and physical characteristics of composites composed of polyacetal [alternatively called polyoxymethylene (POM)] and cellulose fiber (CelF) derived from wood pulp [10–52 wt % (9.3–50.1 vol %)] without any fiber surface treatment. The modulus, deflection temperature under load, and thermal conduction coefficient of the POM/CelF composites were effectively enhanced with increasing CelF content, and the composites had an advantage of specific modulus compared to glass fiber (GF)‐filled POM. The flexural modulus of POM/CelF 40 wt % (38.2 vol %) was measured to be about 6 GPa, which was comparable to that of POM/GF 20 wt % (12.1 vol %). In the composites, the CelFs were distributed randomly as monofilaments, and the debonding of the interface between the fibers and POM matrices in the fracture faces was confirmed as less by scanning electron microscopy observation. The POM/CelF composites possessed lower specific wear rates than the POM/GF composites, and they had damping behaviors near that of neat POM. No clear dependence of the melt flow index of the base POM on these characteristics was observed, except on Charpy impact strength. The composites studied here were unique in their performance and ability to be designed in accordance with specific demands, and they could be potential replacements for mineral‐filled and GF‐filled POM composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Structure–properties relations in titanium‐based thermoplastic fiber–metal laminates

This paper investigates the interfacial, tensile, and fatigue properties of a titanium alloy fiber–metal laminate (Ti‐FML) based on woven glass‐fiber‐reinforced polyetherimide (GF/PEI). Initial tests, using the single cantilever beam (SCB) geometry have shown that it is not necessary to surface treat the titanium alloy in order to achieve a high value of metal–composite interfacial fracture toughness. Tensile tests have shown that the mechanical properties of the FML lie between those offered by its constituent materials. Tension–tension fatigue tests have shown that the fatigue lives of these laminates are superior to those offered by the plain titanium alloy. The mechanical properties of this glass fiber/PEI FML have also been compared with those offered by an FML based on a unidirectional carbon‐fiber‐reinforced polyetheretherketone (CF/PEEK) composite. Here, it has been shown that although the fatigue properties of this woven GF/PEI composite are inferior to those of the CF/PEEK FML, they do offer a higher temperature capability due to the higher glass transition temperature of the PEI matrix. Polym. Compos. 27:264–270, 2006. © 2006 Society of Plastics Engineers.     

Preparation and properties of T300 carbon fiber‐reinforced thermoplastic polyimide composites

In this study, a series of T300 carbon fiber‐reinforced polyimide (CFRPI) composites were prepared by laminating premolding polyimide (PI) films with unidirectional carbon fiber (CF) layers. On the basis of PI systems design, the effect of CF volume fraction, processing conditions, and PI molecular structure on the properties of CFRPI composites was studied in detail. In addition, two kinds of nano‐particles, including carbon nano‐tube (CNT) and SiO₂ were filled into the premolding PI films with different concentrations. And the effect of nano‐particles on the properties of CFRPI composites was also investigated. The surface characteristic of T300 CF was measured by X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The properties of premolding PI film and CFRPI composites were measured by dynamic mechanical analysis (DMTA), SANS testing machine, scanning electron microscopy (SEM), and so forth. These experimental results showed that the properties of CFRPI composites were mainly affected by the premolding PI film and molding condition. The change of CF volume fraction from 55% to 65% took little effect on the mechanical properties of CFRPI composites. In addition, the incorporation of nano‐particle SiO₂ could further improve the properties of CFRPI composites, but CNT hardly improved the properties of CFRPI composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 646–654, 2006     

Stress relaxation of woodfiber–thermoplastic composites

Woodfiber–polypropylene and woodfiber–waste polyethylene composites have been produced by injection molding and by hot pressing the thermoplastic between woodfiber mats. The stress relaxation under constant strain in these composites has been studied at 25, 50, and 80°C. The results have been compared with similar experiments performed on neat thermoplastics. It is interesting to note that the presence of woodfibers as reinforcement in the composites restricts the stress relaxation, but their effectiveness decrease with the increase in ambient temperature. Composites made by hot pressing the woodfiber mat and the thermoplastic are found to exhibit a lesser amount of relaxation than those made by injection molding the same combination. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 401–407, 2006     

Production and characterization of innovative carbon fiber–polycarbonate composites

Vapor Grown Carbon Fiber (VGCF)–polycarbonate composites were produced using conventional processing and the performance of the parts thus produced was evaluated. The tensile properties of the composites were only marginally better than those of the nonreinforced polymer, and the impact resistance decreased with the incorporation of the fibers. In an attempt to understand this behavior, the rheological characteristics of the composites were studied, using rotational (steady and oscillatory) and capillary rheometry. A decrease in viscosity with fiber content was observed and explained in terms of poor fiber-matrix adhesion. Evidence of possible weak fiber structures forming above a critical concentration was also found.