Synthesis and processing of PMR-15/LaRC-TPI semi-IPN systems

To improve the fracture toughness of PMR-15 polyimide and to alleviate its high susceptibility to microcracking induced by thermal cycling, a thermoplastic polyimide, LARC-TPI, was incorporated to form a sequential semi-interpenetrating polymer network (semi-2 IPN). The imidization kinetics of LARC-TPI in the semi-IPNs were studied using a thermal gravimetric analyzer. Both the solvent and the glass transition temperature of the semi-IPN were found to have significant effects on the imidization kinetics. The kinetics could be modeled by a two-step reaction: the first step being a second-order reaction followed by a second step, which is a first-order diffusion-controlled reaction. Differential scanning calorimetry was chosen to investigate the curing of PMR-15 and PMR-15/LARC-TPI semi-IPNs. The curing process was well correlated by a first-order reaction kinetics, which suggested that the reverse Diels-Alder reaction of the Norbornene end group was the rate controlling step. The glass transition temperatures of these semi-IPNs were again found to play important an important role in dictating the curing kinetics. A higher proportion of LARC-TPI or a higher glass transition temperature of the semi-IPN prepolymer tended to result in a slower curing reaction. The optimum molding cycle of PMR-15 and PMR-15/LARC-TPI semi-IPNs were then determined from the obtained kinetics. © 1995 John Wiley & Sons, Inc.     

R-curve behavior and flexural strength of zirconia-toughened alumina and partially stabilized zirconia composite laminates

A composite-laminate formed by thick layers (~ 320?µm) of zirconia-toughened alumina (ZTA) with thin (~ 50?µm) interlayers of zirconia partially stabilized (Y-PSZ) has been fabricated by tape casting and pressureless sintering. Fracture behavior and strength has been investigated and compared to a “monolithic” reference, e.g. a stack of zirconia-toughened alumina (ZTA) without interlayers. The fracture behavior has been analysed using stable crack growth in V-notched specimens loaded in 3-point bending. The ZTA+Y-PSZ composite laminate presented a rising crack resistance with maximum values between 6 and 14?MPa?m^1/2. In contrast, the “monolithic” ZTA laminate shows a plateau R-curve behavior at 2.7?MPa?m^1/2. Several toughening mechanisms were identified in the ZTA+Y-PSZ composite laminate, such as, crack arrest/slow down, micro cracking and bifurcation. These toughening mechanisms are most likely caused by high tensile residual stresses that were estimated theoretically.     

Prediction of thermal and mechanical properties of glass-epoxy composite laminates

This paper provides an experimental test of the validity of laminate theory used for calculating thermoelastic properties of a unidirectional composite laminate. Theoretically calculated values of elastic moduli and coefficients of thermal expansion for unidirectional glass/epoxy laminates were compared with the measured values. Laminates (8″ × 8″ × 1/4″) as a function of ply orientation were made and their elastic moduli, thermal expansion coefficients, fiber volume, and void contents were measured.     

Effect of degree of cure and fiber content on the mechanical and dynamic mechanical properties of carbon fiber reinforced PMR-15 polyimide composites

A unidirectional continuous carbon fiber reinforced polyimide composite is fabricated using the PMR-15 polyimide as a matrix. Mechanical and dynamic mechanical properties of the composite are studied. The effect of variation of the fiber content ranging from 55% to 70% by volume on the properties of the composite is determined. The effects of the laminate thickness and degree of curing of the matrix on the Izod impact strength are examined. The effect of the degree of curing of the matrix on the dynamic mechanical spectrum is studied. Finally, variables such as humidity, environment temperature, vacuum, and pressure during laminate processing, and variation of monomer composition are discussed in terms of change in matrix structure, which is reflected in a variation of Tg between batches.     

Effect of thermal shock cycling on the quasi-static and dynamic flexural properties of flax fabric-epoxy matrix laminates

The aim of the present work is to investigate the influence of thermal shock cycling on the quasi-static and dynamic flexural properties of epoxy matrix composites reinforced with natural flax fibers fabric. Polymer composite laminates reinforced with four plies of natural flax fiber fabric have been manufactured. The samples have been exposed to different number of thermal shock cycles (0, 50, 100, 200, 300, 400), at a temperature range from −40 °C to +28 °C. Dynamic mechanical analysis (DMA) tests were performed on both pristine and thermally shocked specimens in order to determine their viscoelastic response. Due to the thermal shock cycling and after 100 thermal shock cycles, a maximum decrease in storage and loss modulus on the order of 50% was observed. After 100 thermal shock cycles, no further degradation of dynamic properties was observed. On the contrary, damping factor and glass transition temperature values showed a minor variation as number of thermal shock cycles increased. In addition, the time–temperature superposition principle (TTSP) was successfully applied, confirming the fact that the flax fiber fabric-epoxy laminate is a thermo-rheologically simple material. Likewise, quasi-static three-point bending tests were executed and a maximum decrease of 20% in flexural strength was observed after 400 thermal shock cycles. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48529.     

Effect of processing parameters on compression molded PMR-15/C3K composites

The consolidation and curing history during the processing of composite materials affects the final properties of a part in various inter-related ways. In order to improve the quality of composites, these process-property relations must be understood in detail. Using a computer-controlled compression molding and data acquisition system, the processing of PMR-15/C3k composites has been investigated. The process parameters considered were the pressure, the time at which it was applied and the crosslinking temperature. Parts were tested for inter-laminar shear strength and flexural modulus, and measurements were made for void content and thickness. Full compaction strength, optimum compaction strength and void sensitivity factor have been defined.     

Effect of water and isooctane absorption on the flexural fatigue strength of a sheet molding compound

The effect of water and isooctane absorption on the flexural fatigue strength of a sheet molding compound (SMC-R30) is investigated. Absorption of water reduces the fatigue life of specimens of SMC-R30 as compared to the fatigue life of specimens tested in air. Absorption of isooctane reduces the fatigue life much more than absorption of water.     

The synergistic effects on enhancing thermal conductivity and mechanical strength of hBN/CF/PE composite

In this work, a simple and novel method was applied to prepare polymer composites by taking the advantage of melt flow shear force driving orientation of the fillers. By using this method, hexagonal boron nitride/polyethylene (hBN/PE) and hexagonal boron nitride/carbon fibers/polyethylene (hBN/CF/PE) composites were fabricated to be possessed of high thermal conductivity and mechanical properties. A high thermal conductivity of 3.11 W/mK was realized in the composite containing 35 wt% hBN and 5 wt% CF, which was over 1,200% higher than that of unfilled PE matrix. Under this component, the compressive strength and modulus of hBN/CF/PE composite were determined to be 30.1 and 870.9 MPa, respectively, which were far higher than that of unfilled PE accordingly. The bending performance was also somewhat enhanced. Meanwhile, the bulk resistivity of the composite material reached 2.55 × 10¹¹ Ω·cm, which was basically the same as that of pure PE. The novel composites with high thermal conductivity, excellent mechanical properties, and controllable electrical insulation could be a potential thermal management material for electrical and electronics industries.     

Miscibility,crystallization, and morphology studies of thermosetting polyimide PMR-15/PEK-C blends

Miscibility, crystallization, and mechanical properties of blends of thermosetting polyimide PMR-15 and phenolphthalein poly(ether ketone) (PEK-C) were examined. With the exception of the 90/10 blend, which has two glass transition peaks, all the blends with PMR-15 less than 90 wt % are miscible in the amorphous state according to DMA results. Addition of PEK-C hindered significantly the crystallization of PMR-15, indicating that there must exist some kind of interaction between molecular chains of PMR-15 and those of PEK-C. The semi-IPN system of PMR-15/PEK-C blends exhibits good toughness. Two distinct microphases, interweaving at the phase boundaries, were found in the PMR-15/PEK-C 60/40 blend. The toughness effect of the blends is discussed in terms of the interface adhesion between the two distinct phases and the domain sizes of the phases. The relation between miscibility and toughness of the blends was investigated. © 1996 John Wiley & Sons, Inc.     

预应力对PC/CF层合板力学性能的影响

采用预应力拉伸技术与薄膜堆叠法制备了聚碳酸酯/碳纤维(PC/CF)复合材料.对不同预应力水平的实验样品进行了拉伸性能的测试;用扫描电子显微镜观察了材料拉伸断面的微观形貌;用光学显微镜与相机观察了CF在PC基体中的排列情况.结果表明,随着预应力的提高,CF在PC中的排列情况逐渐改善;PC/CF层合板的拉伸强度随着预应力的...     

Effects of plasticizer/cross-linker on the mechanical and thermal properties of starch/PVA blends

Biodegradable blends of potato starch and polyvinyl alcohol were prepared by solution casting method. Citric acid was employed to introduce the plasticizing effect into the starch materials. Glutaraldehyde as cross-linker was used to enhance the properties of the blend films. Cross-linking is a common method to improve the strength and stability of starch products. The effects of citric acid and glutaraldehyde on the mechanical properties, thermal properties and swelling degree were investigated. The prepared films were measured for their antibacterial activities and biodegradability. The blend samples were characterized by the thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and FTIR analysis techniques. From the mechanical properties study, it was analyzed that the blend films showed improvement in their tensile strength after cross-linking with glutaraldehyde. The SEM micrographs indicated that the blend films were smooth without any cracks, pores and were well cross-linked. The TGA curves showed that there was an increase in the thermal stability of the blend films after cross-linking as compared to uncross-linked blend films. The prepared films showed good antibacterial properties against Gam-positive and Gram-negative bacteria. The biodegradability of the blends was determined by placing the samples in compost soil for different time intervals and were found to be biodegradable in nature.     

改性纳米结晶纤维素/酚醛树脂复合材料的弯曲强度研究

以MPS(3-甲基丙烯酰氧基丙基三甲氧基硅烷)作为NCC(纳米结晶纤维素)颗粒的表面处理剂、MPS改性NCC作为增强填料和PF(酚醛树脂)作为基体,制备了改性NCC/PF复合材料,并对其结构和弯曲强度进行了表征。研究结果表明:当φ(MPS)=8.0%(相对于MPS-乙醇溶液的体积而言)时,PF基体的左、右侧接触角分别降低了19.2%、19.0%;当w(MPS改性NCC)=0.5%~1.0%(相对于PF基体质量而言)时,改性NCC在PF基体中分散良好,改性NCC/PF复合材料的结晶结构中位于22.1°、32.5°处的特征衍射峰显著增强;与对照样相比,1.5%MPS改性NCC可使PF复合材料的弯曲强度(达到98.6 MPa)提高15.7%。     

Improving the interlaminar shear strength and thermal conductivity of carbon fiber/epoxy laminates by utilizing the graphene-coated carbon fiber

The poor interlaminar properties restrict the application of carbon fiber reinforced polymer (CFRP) composites. In this work, a novel method for fabricating a graded interface structure is developed to improve the through-thickness thermal conductivity of CFRP composites. High-strength graphene nano-plates (GnP) and phenolic resin (PF) were selected to deposit on the surface of carbon fiber to design a novel CF/Epoxy laminates, where a simultaneous improvement of interlaminar shear strength (ILSS) and through-thickness thermal conductivity was observed. With addition of 1 wt % of GnP-PF in CF, 37.04% increase of the ILSS, and 16.67% enhancement of thermal conductivity compared to the original CFRP. The mechanism for improvement of both ILSS and thermal conductivity was studied by scanning electron microscopy and nano-indentation, where a better interface formed by GnP-PF has been clearly observed. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47061.     

Low dielectric loss LNT/PEEK composites with high mechanical strength and dielectric thermal stability

(3-Aminopropyl)triethoxysilane treated La_(2−x)/3Na_0.06TiO₃ (x = 0.06) (LNT) microparticles filled polyetheretherketone (PEEK) composites were prepared using hot pressing process. The effects of variation of LNT ceramic filling fraction on dielectric properties, water absorption, thermal stability and mechanical strength were investigated. All composites demonstrate low water absorption (less than 0.4%) when the ceramic filling fraction is lower than 0.6V_f. The obtained composites exhibited dielectric permittivities varying from ~4 to ~22 as the ceramic fillers increased from 0.1 to 0.8V_f and low losses (~10⁻⁴ @1 MHz, 3~5 × 10⁻³ at the frequencies of microwave (10 GHz) and millimeter wave (29-50 GHz), respectively). The mechanical strength, dimensional and dielectric thermal stability of the composite are remarkably improved by the addition of LNT ceramic fillers. A composite with near zero temperature coefficients of dielectric permittivity or resonant frequency and flexural strength of ~140 MPa could be obtained. The out-of-plane coefficient of thermal expansion (CTE) could be reduced to ~20 ppm/°C as the ceramic filler loading reached 0.7V_f.     

Effects of crosslinking on thermal and mechanical properties of polyurethanes

The effects of chemical crosslinking on the thermal and dynamic mechanical properties of a polyurethane system were examined. The polyurethanes were prepared from poly(propylene glycol), a diol; trimethylolpropane propoxylate, a triol; and poly(propylene glycol), tolylene 2,4‐diisocyanate terminated, a diisocyanate monomer. The crosslink density was controlled by varying the triol concentration from 10 to 70 mol % and the isocyanate‐to‐hydroxyl (NCO/OH) ratio from 1.0 to 1.3. All the samples had one glass‐transition temperature and no crystalline regions. In addition, there were larger increases in glass‐transition temperature over the range of triol concentrations studied than over the range of NCO/OH ratios studied. For all samples, the Dibenedetto equation relating glass‐transition temperature to extent of crosslinking fit the data very well. Also, samples with higher crosslink densities had much larger elastic moduli for temperatures above the glass‐transition temperature. By assuming the system was a phantom network, approximate crosslink densities for stoichiometric samples were obtained from the dynamic mechanical data and these agreed fairly well with theoretical predictions. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 212–223, 2002     

Barrier and mechanical properties of pulp fiber/polymer laminates and blends

Paperboard laminates coated with two grades of poly(ε‐caprolactone) (PCL), poly(hydroxy butyrate‐co‐valerate) (PHBV) or a liquid crystalline copolyester (LCP) were prepared by compression molding, and the influence of the processing conditions and polymer content of the laminate on the laminate properties was studied. Ligno‐cellulose fiber/polymer blends were prepared from wet pulps and PCL and PHBV. The morphology, water vapor transmission rates, creasability, curl and twist and mechanical properties of the laminates and blends were studied. LCP and slowly cooled high molar mass PCL laminated paperboards showed the best creasing properties and the paperboards that were penetrated by the polymer showed the smallest degree of curl and twist. Extensive penetration occurred during compression molding of the paperboard with the low molar mass PCL at all temperatures and with PHBV and LCP at the higher molding temperatures. The water vapor transmission rates ranged from 1 to 300 times that of polyethylene depending on the polymer used and on the thermal history. In the case of blends, competitive properties were obtained only in those with a high polymer content. The laminate stiffness decreased and the strength increased in two polymer concentration regions, at ～20 wt% due to fiber‐fiber separation and at ～60 wt% due to phase inversion.     

Microstructure and flexural strength of the Y:TZP/BG composite

The addition of bioactive glasses to a Y:TZP matrix represents a feasible alternative to provide bioactivity to this material and optimize osseointegration. This work evaluated the effect of the BG concentration (0 and 10 wt%) and the sintering temperature (1200°C and 1300°C) on the microstructure, relative density, and flexural strength of the composite Y:TZP/BG. The Y:TZP and Y:TZP/BG powders were uniaxially pressed and sintered at 1200°C or 1300°C for 1 h. The microstructure was characterized by X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive X-ray Spectroscopy. Relative density was calculated from density values obtained using the Archimedes’ principle. For the flexural strength, specimens (n = 6) were fractured in a biaxial flexural setup using a piston-on-three-balls fixture in a universal testing machine. Bioactivity test was performed in simulated body fluid solution. The results suggested that BG addition decreased the grain size of the composite, increased porosity and caused a significant decrease in the relative density and flexural strength. Crystalline phases of calcium stabilized cubic zirconia and sodium zirconium silicate were formed after the addition of BG. Finally, it was concluded that composite specimens sintered at 1300°C showed the highest density values and larger grains compared to those sintered at 1200°C.     

Effect of surface roughness on interlaminar peel and shear strength of CFRP/Mg laminates

In this study, grit blasting with different abrasive particle sizes was carried out on magnesium alloy sheets, then the carbon fiber reinforced polymer (CFRP)/magnesium alloys laminates were prepared using a hot-press process. The surface characteristics of magnesium alloy, and the interlaminar strength of CFRP/Mg laminates were examined, in order to investigate comprehensively the effect of surface roughness on interlaminar strength of laminates under peel and shear loading conditions. The results show that the rougher surface significantly improves the peel strength of laminates, while the shear strength of laminates increases only slightly with increasing surface roughness. Hence, the rougher surface exhibits a good overall interlaminar strength under peel and shear loading when compared to the smoother surfaces.     

Effects of thermal and thermomechanical induced mechanical changes of C/C composites

Carbon fiber reinforced carbon composites are produced by modified chemical vapor infiltration (CVI) of a textile fiber preform. The modified process called rapid CVI (r-CVI) allows a fast production of the matrix as well as the adjustment of graphene orientation of the matrix. At a processing temperature of 1200 °C graphene stacks are produced which are oriented roughly in fiber orientation. Mechanical testing at different temperatures as well as after temperature treatment and optional surface siliconization was carried out on C/C. Thermal treatment leads to growth and realignment of the graphene stacks in the matrix resulting in an intensified anisotropy of the composite. High temperature tests beyond processing temperature on C/C lead to mechanically induced straightening of graphene stacks in loading direction. The improved orientation of the graphene structure which is proven by XRD measurements leads to higher loading capacity and therefore to better mechanical properties in 0/90° fiber direction and to reduced mechanical performance in ±70° fiber orientation. This effect is also shown in transverse tensile tests when the graphene layers in the matrix are loaded perpendicular to their orientation. Furthermore a new method to microstructural assessment is presented.