Multivariable dependency of thermal shrinkage in highly aligned polypropylene tapes for self-reinforced polymer composites

Self-reinforced composites offer a unique combination of properties such as high specific strength, high impact resistance, and recyclability by incorporating highly aligned fibers within a random matrix of the same polymer. However, high temperatures will shrink the system to recover randomness in the aligned segments, compromising the composite thermal stability during processing as self-reinforced tapes are consolidated into the final composite through heating and pressure. Hence, the dynamic nonlinear multivariable (i.e., time, temperature, stress) shrinkage exhibited by self-reinforced polypropylene (SRPP) tapes was measured and modeled at the maximum shrinkage limit achieved in the proximity of the composite processing temperature [∼140 to160 °C]. At high stress (∼7.5 MPa) the thermal shrinkage of the SRPP tapes was reduced and a parallel creep mechanism was activated. The modeling, and prediction of the main factors governing the thermal shrinkage expand and diversify the dynamic design window for new SRPP composites.     

Hybrid effects on tensile properties of hybrid short-glass-fiber-and short-carbon-fiber-reinforced polypropylene composites

Hybrid composites of polypropylene reinforced with short glass fibers and short carbon fibers were prepared using extrusion compounding and injection molding techniques. The tensile properties of these composites were investigated taking into account the effect of the hybridization by these two types of short fibers. It was noted that the tensile strength and modulus of the hybrid composites increase while the failure strain of the hybrid composites decreases with increasing the relative carbon fiber volume fraction in the mixture. The hybrid effects for the tensile strength and modulus were studied by the rule of hybrid mixtures (RoHM) using the tensile strength and modulus of single-fiber composites, respectively. It was observed that the strength shows a positive deviation from that predicted by the RoHM and hence exhibits a positive hybrid effect. However, the values of the tensile modulus are close to those predicted by the RoHM and thus the modulus shows no existence of a hybrid effect. Moreover, the failure strains of the hybrid composites were found to be higher than the failure strain of the single carbon fiber-reinforced composite, indicating that a positive hybrid effect exists. Explanations for the hybrid effects on the tensile strength and failure strain were finally presented.     

Effects of maleated polypropylene on the morphology,thermal and mechanical properties of short carbon fiber reinforced polypropylene composites

The aim of this study was to determine the effect of the maleic anhydride grafted polypropylene (PP-g-MAH) on the properties of short carbon fiber (CF) reinforced polypropylene (PP) composites. The composites were prepared by melt blending and injection molding techniques at different percentages of CF. Tensile tests, hardness, differential scanning calorimeter (DSC) and scanning electron microscopy (SEM) were performed to characterize the physical and morphological properties of the prepared composites. It was observed from SEM photographs that modification with PP-g-MAH improved the interfacial adhesion between the carbon fibers and PP matrix. The ultimate tensile strength, hardness and modulus values of modified PP composites were higher compared to the values of CF reinforced PP composites. Melting temperature of all composites was not changed significantly with increasing CF content; however degree of crystallinity values were decreased with the increasing CF content level.     

The importance of bonding in intralayer carbon fibre/self-reinforced polypropylene hybrid composites

Polymer composites are usually either stiff or tough, but seldom both. Intralayer hybrids of carbon fibre and self-reinforced polypropylene (PP) do offer the potential to achieve a unique combination of toughness and stiffness. In these hybrids, the bonding between carbon fibre prepregs and PP tapes is a crucial parameter. For a weak bonding, the 20% ultimate tensile failure strain and high penetration impact resistance of self-reinforced PP were maintained. For a strong bonding, the ultimate tensile failure strain was strongly reduced, but the flexural performance was improved. For a homopolymer PP matrix in the prepregs, the weak bonding between fibre and matrix caused the penetration impact resistance to reduce according to a linear rule-of-mixtures. For a maleic anhydride modified PP matrix however, the strong fibre–matrix bonding greatly reduced the penetration impact resistance. These results provide new insights into designing hybrid composites with a unique balance of stiffness and failure strain.     

Unrestrained early age shrinkage of concrete with polypropylene,PVA, and carbon fibers

Plastic shrinkage cracking due to restrained shrinkage is a primary problem that often occurs in concrete structures with a relatively large surface area, such as concrete walls, bridge decks, slabs, and overlays. These applications are susceptible to rapid changes in temperature and humidity resulting in high water evaporation and high potential for shrinkage cracking. Free shrinkage at early age is one of the most influential factors leading to tensile stresses in concrete structures. When the tensile stress is higher than the tensile strength, cracking occurs. The addition of micro-fibers in amounts as small as 0.1% by volume is an effective method to control plastic shrinkage cracking. However, the effect of fibers on the free shrinkage and water evaporation of concrete is not clearly understood. The main objective of the study described in this paper is to evaluate the influence of fibers on the free shrinkage and water evaporation of fiber reinforced cement composites during the first 24,h after mixing, with particular attention to fiber content, fiber bond, and fiber stiffness. Prismatic concrete specimens of 1000,mm in length and 100 × 60,mm in cross section were tested to measure their unrestrained shrinkage strain. Simultaneously, flat concrete specimens of dimensions 327 × 230 × 50,mm were tested to determine loss of water by evaporation. The tests were carried out under adverse environmental conditions known to encourage high shrinkage, namely high temperature, low relative humidity, and exposure to high volume and velocity of air flow. Three types of fibers, polypropylene, PVA, and carbon fibers were investigated at four different fiber volume fractions, namely: 0.1%, 0.2%, 0.3%, and 0.4%. Experimental results are presented and conclusions are drawn.     

Environmental effects on bamboo-glass/polypropylene hybrid composites

The effects of environmental aging and accelerated aging on tensile and flexural behavior of bamboo fiber reinforced polypropylene composite (BFRP) and bamboo-glass fiber reinforced polypropylene hybrid composite (BGRP), all with a 30% (by mass) fiber content, were studied by exposing the samples in water at 25°C for up to 1600 h and at 75°C for up to 600 h. Reduction in tensile strength for BFRP and BGRP was 12.2% and 7.5%, respectively, after aging at 25°C for about 1200 h. Tensile and flexural strength of BFRP and BGRP were reduced by 32%, 11.7%, and 27%, 7.5% respectively, after aging at 75°C for 600 h. While the strengths of the bamboo fiber reinforced composites reduce with sorption time and temperature, the environmental degradation process can be delayed by adding a small amount of glass fiber. Moisture sorption and strength reduction are further suppressed by using maleic anhydride polypropylene (MAPP) as a coupling agent in both types of composite system.     

Correlation of constitutive response of hybrid textile reinforced concrete from tensile and flexural tests

This papers addresses the disparities that exist in measuring the constitutive properties of thin section cement composites using a combination of tensile and flexural tests. It is shown that when the test results are analyzed using a simplified linear analysis, the variability between the results of tensile and flexural strength can be as high as 200–300%. Experimental results of tension and flexural tests of laminated Textile Reinforced Concrete (TRC) composites with alkali resistant (AR) glass, carbon, aramid, polypropylene textile fabrics, and a hybrid reinforcing system with aramid and polypropylene are presented. Correlation of material properties is studied analytically using a parametric model for simulation of flexural behavior using a closed form solution based on tensile stress–strain constitutive relation. The flexural load carrying capacity of TRC composites is computed using a back-calculation approach, and parameters for a strain hardening material model are obtained using the closed form equations. While the parametric model over predicts the simulated tensile response for carbon and polypropylene TRCs, predictions are however consistent with experimental trends for aramid and glass TRCs. Detailed discussion of the differences between backcalculated and experimental tensile properties is presented. Results can be implemented as average moment–curvature relationship in the structural design and analysis of cement composites.     

混杂工艺对椰壳-大麻/聚丙烯复合材料力学性能的影响

采用正交分析法,讨论混杂工艺和复合工艺对椰壳-大麻/聚丙烯（PP）复合材料力学性能的影响。结果表明,混杂处理后的椰壳-大麻/PP复合材料的力学性能均比相同复合工艺条件下的大麻/PP复合材料有较大程度的改善。椰壳纤维与大麻纤维质量比对混杂椰壳-大麻/PP复合材料力学性能影响最大,且混杂椰壳-大麻/PP复合材料的力学性能随椰壳纤维含量的增加而线性增大;混杂针刺毡中PP纤维质量分数对混杂椰壳-大麻/PP复合材料的抗弯强度影响较大,最初混杂椰壳-大麻/PP复合材料的抗弯强度随PP纤维质量分数的增加而减小,随后又随PP纤维质量分数的增加有一定程度的增大,而混杂椰壳-大麻/PP复合材料的抗拉强度则随PP纤维质量分数的增大而线性减小;混杂椰壳-大麻/PP复合材料的力学性能随复合层压温度的升高呈下降趋势。      

Date palm wood flour/glass fibre reinforced hybrid composites of recycled polypropylene: Mechanical and thermal properties

Recycled polypropylene (RPP) based hybrid composites of date palm wood flour/glass fibre were prepared by different weight ratios of the two reinforcements. Mixing process was carried out in an extruder and samples were prepared by injection molding machine. Recycled PP properties were improved by reinforcing it by wood flour. The tensile strength and Young’s modulus of wood flour reinforced RPP increased further by adding glass fibre. Glass fibre reinforced composites showed higher hardness than other composites. Morphological studies indicated that glass fiber has good adhesion with recycled PP supporting the improvement of the mechanical properties of hybrid composites with glass fiber addition. Addition of as little 5 wt% glass fibre to wood flour reinforced RPP increases the tensile strength by about 18% relative to the wood flour reinforcement alone. An increase in wood particle content in the PP resulted in a decrease in the degree of crystallinity of the polymer. The tensile strength of the composites increased with an increase in the percentage of crystallinity when adding the glass fibre. The improvement in the mechanical properties with the increase in crystallinity percentage (and with the decrease of the lamellar thicknesses) can be attributed to the constrained region between the lamellae because the agglomeration is absent in this case.     

Creep behavior and manufacturing parameters of wood flour filled polypropylene composites

The influence of wood flour content, coupling agent and stress loading level on the creep behavior of wood flour–polypropylene composites was investigated. Maleated polypropylene (MAPP; Epolene G-3003™) was used as the coupling agent to treat the wood flour used as reinforcing filler for polypropylene composite. The tensile strength and modulus of various wood flour–polypropylene composites (WPCs), manufactured using the melt blending, extrusion, and palletizing methods, were measured before performing the creep test. The residual tensile strength, creep strain, and fractional deflection of the resultant wood flour–polypropylene composites were measured by means of the creep test. It was shown that the tensile strength decreased with increasing wood flour level in the composites. The creep strain also decreased as the wood flour level increased. The presence of the coupling agent increased the tensile strength of the wood flour–polypropylene composites, compared with the specimens made of pure polypropylene. For those composites containing the coupling agent, the creep deflection was significantly lower than those made without any coupling agent. The creep strains of the WPC specimens observed during the creep test fitted perfectly with the four-element burger creep model. Further investigation is required of the effects of combined mechanical and environmental loading in varying proportions.     

Tensile properties of short-glass-fiber- and short-carbon-fiber-reinforced polypropylene composites

Composites of polypropylene (PP) reinforced with short glass fibers (SGF) and short carbon fibers (SCF) were prepared with extrusion compounding and injection molding techniques. The tensile properties of these composites were investigated. It was noted that an increase in fiber volume fraction led to a decrease in mean fiber length as observed previously. The relationship between mean fiber length and fiber volume fraction was described by a proper exponential function with an offset. The tensile strength and modulus of SGF/PP and SCF/PP composites were studied taking into account the combined effect of fiber volume fraction and mean fiber length. The results about the composite strength and modulus were interpreted using the modified rule of mixtures equations by introducing two fiber efficiency factors, respectively, for the composite strength and modulus. It was found that for both types of composites the fiber efficiency factors decreased with increasing fiber volume fraction and the more brittle fiber namely carbon fiber corresponded to the lower fiber efficiency factors than glass fiber. Meanwhile, it was noted that the fiber efficiency factor for the composite modulus was much higher than that for the composite strength. Moreover, it was observed that the tensile failure strain of the composites decreased with the increase of fiber volume fraction. An empirical but good relationship of the composite failure strain with fiber volume fraction, fiber length and fiber radius was established.     

Isocyanate as a compatibilizing agent on the properties of highly crystalline cellulose/polypropylene composites

Composites of a highly crystalline cellulosic microfibres with polypropylene (PP) as well as with maleic anhydride grafted polypropylene (MAPP) were prepared by using 1,6-diisocyanatohexane (DIC) as a compatibilizing agent, their mechanical properties, morphologies, and thermal properties were investigated. Results show that the tensile strength and young’s modulus of the composites improved intensively by using DIC. The enhancement is proposed to be due to stronger interfacial adhesion caused by the reduction of the polarity and hydrophilicity of cellulose fiber in PP-based composites, while much more chemically bound MAPP chains on cellulose fiber in MAPP-based composites. A maximum on tensile properties of the composite can be obtained by optimizing of the DIC content. Scanning electron microscopy (SEM) indicates that the interfacial adhesion between cellulose fibers and PP or MAPP matrix was improved in DIC coupled composites. Furthermore, DIC yields also some effects on thermal dynamic mechanical properties, as well as melting and crystallization behavior of the composites.     

Mechanical behavior of Kevlar/basalt reinforced polypropylene composites

In this study, mechanical behavior of thermoplastic composites reinforced with two-dimensional plain woven homogeneous and hybrid fabrics of Kevlar/basalt yarns was studied. Five types (two homogeneous and three hybrids) of composite laminates were manufactured using compression molding technique with polypropylene (PP) resin. Static tensile and in-plane compression tests were carried out to evaluate the mechanical properties of the laminates. The tension and in-plane compression tests had shown that the composites with the combination of Kevlar and basalt yarns present better tensile and in-plane compressive behavior as compared to their base composites. Improvement in the properties such as elastic modulus, strength and failure strain in both tension and in-plane compression was observed due to the hybridization. Numerical simulations were performed in ABAQUS/Standard by implementing a user-defined material subroutine (VUMAT) based on Chang-Chang criteria. Good agreement between the experimental and numerical simulations was achieved in terms of damage patterns.     

Jute/polypropylene composites I. Effect of matrix modification

Natural fibre-reinforced polymers can exhibit very different mechanical performances and environmental aging resistances depending on their interphase properties, but most studies have been focused on fibre surface treatment. Here, investigations of the effect of maleic anhydride grafted polypropylene (MAHgPP) coupling agents on the properties of jute fibre/polypropylene (PP) composites have been considered with two kinds of matrices (PP1 and PP2). Both mechanical behaviour of random short fibre composites and micro-mechanical properties of single fibre model composites were examined. Taking into account interfacial properties, a modified rule of mixture (ROM) theory is formulated which fits well to the experimental results. The addition of 2 wt% MAHgPP to polypropylene matrices can significantly improve the adhesion strength with jute fibres and in turn the mechanical properties of composites. We found that the intrinsic tensile properties of jute fibre are proportional to the fibre’s cross-sectional area, which is associated with its perfect circle shape, suggesting the jute fibre’s special statistical tensile properties. We also characterised the hydrophilic character of natural fibres and, moreover, humidity environmental aging effects. The theoretical results are found to coincide fairly well with the experimental data and the major reason of composite tensile strength increase in humidity aging conditions can be attributed to both improved polymer–matrix and interfacial adhesion strength.     

Mechanical properties of polypropylene hybrid fiber-reinforced concrete

This paper investigates the mechanical properties of polypropylene hybrid fiber-reinforced concrete. There are two forms of polypropylene fibers including coarse monofilament, and staple fibers. The content of the former is at 3 kg/m³, 6 kg/m³, and 9 kg/m³, and the content of the latter is at 0.6 kg/m³. The experimental results show that the compressive strength, splitting tensile strength, and flexural properties of the polypropylene hybrid fiber-reinforced concrete are better than the properties of single fiber-reinforced concrete. These two forms of fibers work complementarily. The staple fibers have good fineness and dispersion so they can restrain the cracks in primary stage. The monofilament fibers have high elastic modulus and stiffness. When the monofilament fiber content is high enough, it is similar to the function of steel fiber. Therefore, they can take more stress during destruction. In addition, hybrid fibers disperse throughout concrete, and they are bond with mixture well, so the polypropylene hybrid fiber-reinforced concrete can effectively decrease drying shrinkage strain.     

Fabrication and mechanical properties of glass fibre–carbon fibre polypropylene functionally gradient materials

Hybrid fibre mat reinforced polypropylene (PP) composites with carbon(CF) and glass fibre (GF) were prepared and four kinds of functionally gradient materials (FGM) were fabricated by changing the spatial distribution of GF and CF. To measure the mechanical properties of FGMs and hybrid composites, flexural tests and instrumented impact tests were performed. The flexural strengths and the flexural moduli of hybrid composites increased following the rule of mixture as the relative volume content of CF increased. On the other hand, the total impact absorption energy of hybrid composites decreased with the increment of CF relative volume content. Compared with GF–CF PP isotropic hybrid composite, the composites with compositional gradient showed similar flexural strengths, but characteristic flexural moduli. Especially, sandwich-type FGMs with a CF-rich outer layer and a GF-rich inner layer exhibited higherflexural moduli than others. Total impact absorption energies of four FGMs were also similar, but the ratios of crack initiation energy,Ei, to crack propagation energy, Ep, or ductility index, were quite different.     

Recycled carbon fibre-reinforced polypropylene thermoplastic composites

Comingled carbon fibre (CF)/polypropylene (PP) yarns were produced from chopped recycled carbon fibres (reCF) (20 mm in length, 7-8 μm diameter) blended with matrix polypropylene staple fibres (60 mm in length, 28 μm diameter) using a modified carding and wrap spinning process. Microscopic analysis showed that more than 90% of the reCF were aligned along the yarn axis. Thermoplastic composite test specimens fabricated from the wrap-spun yarns had 15-27.7% reCF volume content. Similar to the yarn, greater than 90% of the reCF comprising each composite sample made, showed a parallel alignment with the axis of the test specimens. The average values obtained for tensile, and flexural strengths were 160 MPa and 154 MPa, respectively for composite specimens containing 27.7% reCF by volume. It was concluded that with such mechanical properties, thermoplastic composites made from recycled CF could be used as low cost materials for many non-structural applications.     

Special manufacturing and characteristics of basalt fiber reinforced hybrid polypropylene composites: Mechanical properties and acoustic emission study

Basalt fiber reinforced, polypropylene matrix hybrid composites were manufactured in the process of carding, needle-punching and pressing. Hemp, glass and carbon fibers were applied besides basalt fiber in these composites. In order to achieve a sufficient interfacial adhesion, the fibers were treated with the reaction mixture of maleic acid anhydride and sunflower oil. The hybrid effect in these composites was examined as a function of fiber content and fiber combination. The strength properties of hybrid composites improved owing to surface treatment and this was proven by mechanical tests and microscopic analysis, as well. Acoustic emission methods revealed that there is a correlation between the physical parameters of sound waves that occurred during failure and the mechanical properties.     

Determination of interfacial shear strength of white rot fungi treated hemp fibre reinforced polypropylene

Short untreated and white rot fungi treated hemp fibre, polypropylene (PP) and maleated polypropylene (MAPP) coupling agent were extruded and injection moulded into composite tensile test specimens. The tensile properties of untreated and treated fibre and their composites were measured. The fibre length distributions in the composite were obtained by dissolving the PP/MAPP matrix in boiling xylene to extract the fibre. Both the Single Fibre Pull-Out test and the Bowyer and Bader model were used to determine the interfacial shear strength (IFSS) of these composites. IFSS was found to be lower for the Single Fibre Pull-Out test, which was considered to be largely due to axial loading of fibre and the resulting Poisson’s contraction occurring during this technique. This suggests that the Bowyer and Bader model provides a more relevant value of IFSS for composites. The results obtained from both methods showed that IFSS of the treated fibre composites was higher than that for untreated fibre composites. This supports that the hemp fibre interfacial bonding with PP was improved by white rot fungi treatment.     

Mechanical behavior and microstructural analysis of sugarcane bagasse fibers reinforced polypropylene composites

The compression and injection molding processes were performed in order to evaluate the better mixer method for fiber (sugarcane bagasse, bagasse cellulose and benzylated bagasse) and matrix (polypropylene). The samples (composites and polypropylene plates) were cut and submitted to mechanical tests in order to measure flexural and tensile properties. The morphological and microstructural analyses of fracture surface and specimens from composites can be easily evaluated by microscopic techniques. The fracture surface was evaluated by SEM and selected specimens from composites were analyzed by reflected light in OM. The better tested method for composites obtainment was the injection molding under vacuum process, by which composites were obtained with homogeneous distribution of fibers and without blisters. The mechanical properties show that the composites did not have good adhesion between fiber and matrix; on the other hand, the fiber insertion improved the flexural modulus and the material rigidity.