Mechanical modeling of the transcrystalline interphase behavior in commingled PBT/glass fiber composites

In a previous work, a mechanical model was proposed to predict the reinforcement of amorphous polymers by particulates as well as by unidirectional fibers over wide ranges of volume fractions of fillers and temperatures (or frequencies). This model is based on both the definition of a representative morphological pattern (RMP), accounting for the presence of fiber clusters, and a quantitative morphology analysis, based on the percolation concept. In this work, such an approach is extended to describe the viscoelastic properties of a semicrystalline polymer, poly(butylene terephthalate), commingled with 30 and 50 vol % of unidirectional glass fibers. It is found that aggregates constituted by both fiber clusters and a transcrystalline region (TCR) can act as the continuous phase. Based on the use of a mechanical model in a reverse mode, the actual viscoelastic behavior of this TCR is extracted and compared to that displayed by the unfilled polymer. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2513–2524, 2000     

An investigation on the rheology,morphology, thermal and mechanical properties of recycled poly (ethylene terephthalate) reinforced with modified short glass fibers

This work was done with the aim to solve an important environmental issue regarding poly (ethylene terephthalate), (PET) wastes. Samples of recycled PET (r-PET) were reinforced with 10 to 30 wt% modified short glass fibers (SGF) through a melt mixing process in an internal mixer and their performance were assessed and compared with those of commercial glass reinforced PET through investigation of their rheology, morphology, thermal, and mechanical properties. It was found that the mechanical properties of the glass reinforced r-PET composites in most cases were comparable or even higher than those of the commercial grades. The impact strength of the 30 wt% SGF filled r-PET composite was about 30% higher than the commercial grades. This led to a conclusion that the PET wastes can be successfully converted to easily moldable thermoplastic materials by incorporation of 30 wt% SGF having a good balance of properties. Through investigation of rheological and morphological properties the optimum conditions for the best reinforcement performance were determined. The r-PET with 30 wt% glass fiber content showed the highest level of orientation and improved interaction with the r-PET matrix while having an acceptable flow behavior and processability. In spite of significant fiber breakage during the melt mixing process, leading to about 20 times reduction in the fiber aspect ratio, the composites maintained their good mechanical properties and showed a shear thinning behavior at high shear rates. The incorporated glass fibers acted as nucleating agents and improved the crystallization rate of r-PET leading to an overall increase in the crystallinity. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Weld-line characteristics in short fiber reinforced thermoplastics

Fibers can greatly improve the mechanical properties of polymers but may also severely weaken molded parts at their weld-line compared to their bulk strength. The tensile properties and fiber orientation of injection and compression molded fiber reinforced Noryl and polypropylene samples with and without a weld-zone were studied. Distinct differences in structure and mechanical properties of weld-containing and weld-free samples were identified. In unfilled Noryl and unfilled polypropylene, the presence of a weld-line was found to only have a small effect on the tensile strength and modulus, while in the corresponding fiber reinforced systems, orientation of the fibrous reinforcement parallel to the weld-line caused a significant reduction of the tensile strength compared to the weld-free products. The strength ratio of welded and unwelded specimens was found to decrease with increasing fiber concentration. Quantitative determination of the glass fiber orientation distribution within the weld-line region and in the bulk was carried out by analyzing photomicrographs of polished sections at desired locations.     

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.     

Welding of thermoplastics reinforced with random glass mat

Thermoplastics reinforced with random glass mat have high strength and stiffness; the fibers dominate the mechanical behavior of these composites. The results of this investigation have shown that fibers are ineffective for reinforcing hot-tool and vibration welded butt welds. The maximum weld strengths attained with GMT are comparable to the strengths of good welds of the unfilled material. The optimum hot-tool welding parameters for the reinforced materials are different from those for the unfilled material. Unfilled polypropylene is easier to weld than unfilled polyamide. This characteristic is also true of the reinforced materials. In vibration welding, high welding pressures and high amplitudes result in lower mechanical properties. The optimum penetration depends on the fiber content of the bulk material. This penetration dependence is different from that for unfilled thermoplastic, for which the mechanical properties are independent of the penetration once a steady state has been attained.     

Crystallization of poly(hydroxybutrate-co-hydroxyvalerate) in wood fiber-reinforced composites

Wood fiber-reinforced composites were prepared from poly(hydroxybutyrate) (PHB) and poly(hydroxybutyrate-co-hydroxyvalerate) (PHB/HV) copolymers containing 9 and 24% valerate. The effects of fibers on crystallization were investigated. Thermomechanical pulp, bleached Kraft fibers, and microcrystalline cellulose filler were used as the reinforcing phase. The crystallization of PHB/HV in composite materials was examined using Modulated Differential Scanning Calorimetry (MDSC) and hot-stage microscopy. Hot-stage microscopy showed that polymer crystallites are nucleated on the fiber surface and that the density of nuclei was greater in fiber-reinforced composites than in unfilled material. Dynamic crystallization experiments showed that bleached Kraft, thermomechanical pulp, and microcrystalline cellulose increased the crystallization rate of PHB and PHB/HV both from the glass and melt. However, ultimate crystallinity determined from the heat of crystallization was the same in unreinforced and reinforced materials. The kinetics of PHB/HV crystallization were examined using nonisothermal Avrami-type analysis. Unreinforced and Kraft-reinforced PHB were characterized and compared with unreinforced PHB/9%HV. The Avrami exponent of crystallization, related to nucleation mechanism and growth morphology, is 2.0 for unreinforced PHB, 2.8 for kraft-reinforced PHB, and 3.0 for unreinforced PHB/9%HV. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1785–1796, 1997     

长玻纤增强丙烯腈-苯乙烯复合材料的研制

采用熔体浸渍工艺制备了长玻纤增强丙烯腈-苯乙烯共聚物复合材料(LGF/AS),研究了不同玻纤含量对LGF/AS复合材料力学、动态力学性能和形态的影响。结果表明:随着玻璃纤维含量的提高,LGF/AS复合材料的力学和动态力学性能逐渐增加;通过SEM证明了玻璃纤维在基体树脂中的具有良好的分散性。     

Fracture behavior of glass bead-filled poly(oxymethylene) injection moldings

The mechanical properties of glass bead filled poly(oxymethylene) were investigated as a function of glass bead content and glass bead diameter using injection molded test pieces. Fracture toughness measurements were made using single edge-notched tension and single edge-notched bend specimens. The effect of notch orientation with respect to the mold fill direction on fracture toughness was studied using single gate and double gate moldings. Tensile strength and flexural modulus were measured using standard test pieces. It was found that; (i) fracture toughness of the filled and unfilled polymer was relatively independent of notch orientation, (ii) the presence of weldlines in the molded test pieces did not affect the fracture toughness of unfilled polymer or its composites, (iii) fracture toughness of filled polymer was always considerably lower than that of the unfilled polymer; fracture toughness decreased sharply with increasing bead concentration, (iv) fracture toughness was not a sensitive function of glass bead diameter; it decreased slightly as bead diameter increased, (v) strain energy release rate as measured under impact decreased with increasing bead concentration, (vi) tensile strength decreased linearly with increasing glass bead concentration and was inversely proportional to the square root of the bead diameter, (vii) weldlines did not affect the tensile strength of the polymer or its composities, (viii) flexural modulus increased linearly with increasing glass bead concentration according to the Einstein equation.     

PE-HD-g-MAH共混改性PA612性能研究

通过马来酸酐接枝高密度聚乙烯(PE-HD-g-MAH)熔融共混改性尼龙612(PA612),研究了玻纤增强改性后的PA612复合材料。结果发现,随着PE-HD-g-MAH含量增加,PA612的结晶温度逐渐下降。通过肉眼和光泽仪研究PE-HD-g-MAH对玻纤增强PA612表观的影响,结果发现随着接枝物的引入,相同条件下复合材料注塑后的样件浮纤更少,光泽度更高。尤其在低模温的条件下,这种差异更加明显。加入PE-HD-g-MAH共混改性后的玻纤增强PA612的复合材料对模温的要求更低,表观更好,改善了其成型性。同时,PE-HD-g-MAH的引入使拉伸强度有小幅度的下降(6%),同时可以提高其缺口冲击强度(18%),对PA612有一定的增韧效果。最后,研究了共混改性后对复合材料的吸水性及尺寸稳定性的影响。PE-HD-g-MAH可以有效降低PA612的吸水性,提高PA612复合材料制件的尺寸稳定性。     

The influence of knit-lines on the tensile properties of injection molded parts

When injection molding complex parts, often knit-lines are formed during the mold filling stage. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. Using three test materials, polystyrene (PS), polycarbonate (PC) and polyoxymethylene (POM), the relationship between a design and a processing parameter and the effects of the knit-lines on the tensile properties of injection molded plates with holes is quantified by the variation of a socalled knit-line-factor a_k1. Finally, for PS a comparison of the influence of knit-line-formation by a splitted melt stream and by multi-gating on the mechanical level is given.     

熔融浸渍法制备PE-HD/黄麻纤维复合材料工艺及性能研究

采用特殊设计的天然纤维熔融浸渍模具制备黄麻长纤维颗粒,通过注塑工艺,制备了长黄麻纤维增强高密度聚乙烯（PE-HD）复合材料。研究了纤维含量、浸渍模具温度对PE-HD/黄麻纤维复合材料力学性能、微观断面形貌的影响。结果表明,利用熔融浸渍工艺制备PE-HD/黄麻纤维复合材料,有效地保障了黄麻纤维的长度,可显著提高复合材料的力学性能;当黄麻纤维含量为45 %,浸渍模具温度为210 ℃时,PE-HD/黄麻纤维复合材料的拉伸强度和弯曲强度最优,相对纯PE-HD分别提高了49.1 %和137 %。     

The Influence of Sepiolite Orientation and Concentration,on the Morphological,Thermal and Mechanical Properties of Bio-Polyamide 4.10 Nanocomposites

Nowadays, trends in automotive sector are toward high-performance materials, but also the concern about the environment has become an important driver for car manufacturers. In this sense, reinforced polymers are lightweight materials that can replace metals in some structural applications with an outstanding contribution to reduce the carbon dioxide emissions. In short fiber-reinforced polymers, processed by injection molding, the fibers are oriented in multiple and arbitrary directions. Due to the arrangement of the fibers, these materials present different thermomechanical behavior. In this study, bio-polyamide 4.10/sepiolite (0–15 wt%) nanocomposites obtained by melt compounding were injected using a square plate mold. Specimens were mechanized in different directions (0°, 45°, and 90°) from this square plate and morphologically and thermomechanically tested. The sepiolite reinforcement results showed improvement in the thermomechanical properties. Moreover, despite the nanometer size of the reinforcement, the mechanical properties were also dependent on the fiber orientation during the injection molding of the nanocomposites. POLYM. ENG. SCI., 60:1035–1043, 2020. © 2020 Society of Plastics Engineers     

Enhanced glass fiber-reinforced phenolphthalein poly(ether ketone) composites by blending poly(phenylene sulfide)

The mechanical properties of glass fiber-reinforced phenolphthalein poly(ether ketone)/ poly(phenylene sulfide) (PEK-C/PPS) composites have been studied. The morphologies of fracture surfaces were observed by scanning electron microscope. Blending a semicrystalline component, PPS, can improve markedly the mechanical properties of glass fiberreinforced PEK-C composites. These results can be attributed to the improvement of fiber/matrix interfacial adhesion and higher fiber aspect ratio. © 1996 John Wiley & Sons, Inc.     

Effect of packing pressure on fiber orientation in injection molding of fiber‐reinforced thermoplastics

Injection molding of fiber‐reinforced polymeric composites is increasing with demands of geometrically complex products possessing superior mechanical properties of high specific strength, high specific stiffness, and high impact resistance. Complex state of fiber orientation exists in injection molding of short fiber reinforced polymers. The orientation of fibers vary significantly across the thickness of injection‐molded part and can become a key feature of the finished product. Improving the mechanical properties of molded parts by managing the orientation of fibers during the process of injection molding is the basic motivation of this study. As a first step in this direction, the present results reveal the importance of packing pressure in orienting the fibers. In this study, the effects of pressure distribution and viscosity of a compressible polymeric composite melt on the state of fiber orientation after complete filling of a cavity is considered experimentally and compared with the simulation results of Moldflow analysis. POLYM. COMPOS. 28:214–223, 2007. © 2007 Society of Plastics Engineers     

Effect of Fiber Length on Hysteretic Heating of Discontinuous Fiber-Reinforced Polypropylene

Fatigue application of a polymeric material is limited due to its hysteretic heating behavior. In this work, effort has been made to understand the effect of fiber reinforcement and length of the reinforcing fibers on hysteretic heating behavior and its mechanisms. Unreinforced, 20% short glass fiber-reinforced and 20% long glass fiber-reinforced polypropylene materials have been injection-molded and subjected to a finite number of fatigue cycles. The load required for constant deflection and the surface temperature of materials during testing was measured and correlated with the hysteretic heating mechanism. During material deformation, the presence of the reinforced fibers and fiber-matrix interface in the reinforced material contributed more internal friction, and resulted in higher heat generation than unreinforced material. Higher fiber density and the inferior fiber-matrix bonds existing in the short fiber-reinforced material generated higher heat than that of long fiber-reinforced material during testing.     

Recycling of poly(ethylene terephthalate) as polymer‐polymer composites

Microfibrillar reinforced composites (MFC) comprising an isotropic matrix from a lower melting polymer reinforced by microfibrils of a higher melting polymer were manufactured under industrially relevant conditions and processed via injection molding. Low density polyethylene (LDPE) (matrix) and recycled poly(ethylene terephthalate) (PET) (reinforcing material) from bottles were melt blended (in 30/70 and 50/50 PET/LDPE wt ratio) and extruded, followed by continuous drawing, pelletizing and injection molding of dogbone samples. Samples of each stage of MFC manufacturing and processing were characterized by means of scanning electron microscopy (SEM), wide‐angle X‐ray scattering (WAXS), dynamic mechanical thermal analysis (DMTA), and mechanical testing. SEM and WAXS showed that the extruded blend is isotropic but becomes highly oriented after drawing, being converted into a polymer‐polymer composite upon injection molding at temperatures below the melting temperature of PET. This MFC is characterized by an isotropic LDPE matrix reinforced by randomly distributed PET microfibrils, as concluded from the WAXS patterns and SEM observations. The MFC dogbone samples show impressive mechanical properties—the elastic modulus is about 10 times higher than that of LDPE and about three times higher than reinforced LDPE with glass spheres, approaching the modulus of LDPE reinforced with 30 wt% short‐glass fibers (GF). The tensile strength is at least two times higher than that of LDPE or of reinforced LDPE with glass spheres, approaching that of reinforced LDPE with 30 wt% GF. The impact strength of LDPE increases by 50% after reinforcement with PET. It is concluded that: (i) the MFC approach can be applied in industrially relevant conditions using various blend partners, and (ii) the MFC concept represents an attractive alternative for recycling of PET as well as other polymers.     

Effect of compounding on the properties of short fiber reinforced injection moldable thermoplastic composites

The mechanical properties of short-fiber-reinforced thermoplastic composites depend on the degree of interfacial bond strength between the fibers and polymer matrix. This interfacial bond strength can be increased by appropriate coupling agents. This study shows, for example, that an amino silane coupling agent improves the bond strength of nylon-aluminum fiber composites, but not polycarbonate-aluminum fiber composites. For cases where appropriate coupling agents are not available it is important to maintain as high a fiber aspect ratio as possible in a molded part. This study shows that a single screw compounder does less damage to glass or carbon fibers than a twin screw compounder under similar processing conditions when the polymer is in the form of pellets. When the polymer is supplied as a powder, satisfactory dry blends can be produced and the twin screw compounder does less damage to the fibers. In both cases, however, fibers initially 6 mm long are reduced to an average length less than 0.5 mm. The greatest degree of fiber size retention was observed when extrusion coated fiber pellets were used in the injection molding machine. The relationship between a fiber's tensile strength and the interfacial shear strength between a fiber and matrix yields a critical fiber aspect ratio below which the maximum reinforcing capability of the fibers are not being utilized. For the polymers investigated in this program, the critical aspect ratio for carbon fibers was found to be between 16 and 25 to 1. The polymers investigated include flame-retardant grades of acrylonitrile-butadiene-styrene (ABS) and poly(phenylene oxide)/polystyrene blend, nylon 6/6 and poly(phenylene sulfide).     

Effect of cure cycle on mechanical properties of thick section fiber-reinforced poly/thermoset moldings

One of the major factors of concern in compression molding of fiber-reinforced thermosets is the mold cycle time which directly affects the processing cost. An ideal system would be the one which cures in a relatively short time resulting in excellent mechanical and physical properties. However, in practice, a compromise has to be made between the mold cycle time and ultimate property requirements. The effects of cure cycle time, temperature, preheating and post-cooling on mechanical properties of continuous as well as chopped glass fiber reinforced polyester and vinyl ester systems involving 1/4 to 1 in thick sections have been studied. Mold cycle time is strongly influenced by the part thickness and mold temperature. Internal heat generation due to curing reaction causes high thermal gradients across the thickness. Preheating offers advantages of reducing both the mold cycle time and the thermal gradient. The flexural and interlaminar shear strengths are strongly dependent or, the mold cycle time. Maximum strengths are obtained when the mold is opened at the instant when there is no thermal gradient across the thickness.     

Formation mechanism of high-pressure water penetration induced fiber orientation in overflow water-assisted injection molded short glass fiber-reinforced polypropylene

Recently, there has been growing interest in water-assisted injection molding (WAIM) not only for its advantages over gas-assisted molding (GAIM) and conventional injection molding (CIM), but also for its great potential advantages in industrial applications. To understand the formation mechanism of water penetration induced fiber orientation in overflow water-assisted injection molding (OWAIM) parts of short glass fiber-reinforced polypropylene (SGF/PP), in this work, the external fields and water penetration process within the mold cavity were investigated by experiments and numerical simulations. The results showed that the difference of fiber orientation distribution in thickness direction between WAIM moldings and CIM moldings was mainly ascribed to the great external fields generated by water penetration. Besides, fiber orientation depended on the position both across the part thickness and along the flow direction. Especially in the radial direction, fiber orientation varied considerably. The results also showed that the melt temperature is the principal parameter affecting the fiber orientation along the flow direction, and a higher melt temperature significantly facilitated more fibers to be oriented along the flow direction, which is quite different from the results as previously reported in short-shot water-assisted injection molding (SSWAIM). A higher water pressure, shorter water injection delay time, and higher melt temperature significantly induced more fibers to be orderly oriented in OWAIM moldings, which may improve their mechanical performances and broaden their application scope.     

Modification to weld lines in extruded thermoplastic pipe using a rotating die system

When a polymer melt flows around the cross head or past the fins of a torpedo in a pipe die, a weld line is produced where the separated melt streams rejoin. This weld line may result in an area of weakness in the extruded pipe, although in the case of unfilled materials the effect can be reduced by using a compression section in the die. However, With fiber reinforced polymers it is unlikely that fibers will cross the weld line, and a reduction in strength is inevitable. It is shown theoretically that by rotating the core of the extrusion die the weld line can be modified to form a spiral around the pipe wall, with a large-surface area for improved strength. Experimental investigations, which employ photographs of thin sections of pipe containing carbon black particles, support these conclusions.