Rapid and low-cost carbon/carbon composites by using graphite slurry impregnated prepregs

A rapid and low-cost carbon/carbon (C/C) composites preparation method is proposed: graphite prepreg-coated carbon fiber fabric (CFF) is formed by hot pressing, followed by hot isostatic pressing and high temperature graphitization, to prepare C/C composite with low porosity and high crystallinity. In this method, the carbon fiber (CF) mass fraction can be precisely regulated in the range of 40–95% by the impregnation process conditions of CFF in graphite prepreg. The graphite particles in the preform were graphitized and bonded with CFF by high temperature graphitization. Finally, a ZrO₂ anti-ablative layer was applied using sol-gel method. The results show that when the CF mass fraction is 50%, the C/C composite with a crystallinity of 92.21 and a porosity of 3.47% can be obtained, with mass ablation rate of 0.23 mg/s and density of 1.62 g/m³. The method can prepare C/C composites with uniform density and high ablation resistance.     

Growth and characterization of Ag–Al2O3 composites thin films for thermoelectric power generation applications

Thin films of Ag–Al₂O₃ composites were successfully grown on Si substrate by thermal evaporation method and their thermoelectric performance was modulated using post growth annealing technique. Pellet of Ag and Al mixture having 1:4 ratio was evaporated on Si substrate using the vacuum tube furnace. As grown sample was cut into pieces and post-growth annealing was performed at different temperatures using muffle furnace. XRD results suggested that as-deposited sample has amorphous nature, but crystallinity of the samples increase as an annealing temperature increase from 600 to 900^oC. This structural behavior of annealed samples was further verified by Raman spectroscopy measurements. We have reported an optimal annealing temperature (800 ⁰C) for the best thermoelectric performance of investigated composites. At this specific annealing temperature, charge carriers are highly mobile which resulted in the enhancement of thermoelectric power generation performance of Ag–Al₂O₃ composite. The value of power factor (1.38x10^?2 W/m-K^?2) reported in the current study is the highest value for Ag–Al₂O₃ composites so for reported in the literature according to the best of our knowledge.     

Annealing effects on the crystallinity of polyetheretherketone (PEEK) and its carbon fiber composite

The degree of crystallinity of polyetheretherketone (PEEK) has been measured using both the density gradient technique (DGT) and differential scanning calorimetry (DSC). The difference in results between the methods was shown to depend on crystallization taking place during the heating scan in the DSC. By freezing the sample at different stages of the DSC thermogram and measuring its crystallinity in the density gradient column, the existence of induced crystallization for PEEK was established. Though this induced crystallization is not visible in the DSC thermogram, it must be taken into account when comparing the degree of crystallinity measured by the two methods. The induced crystallization was in turn interpreted as a result of an increase in crystal perfection that is also commonly observed during the initial stages of the annealing process. Accordingly, the effect of annealing on the crystallinity was also investigated. DSC scans on annealed samples exhibited a small endothermic peak at approximately 10°C above the annealing temperature. This peak was observed in both neat PEEK and its carbon fiber-reinforced composite. Annealed PEEK shows, therefore, two melting transitions, a low one which depends on the annealing temperature and a high one which is independent of annealing temperature conditions. Collectively, the results of this study demonstrate that processing conditions and morphological features must be considered in characterizing semicrystalline-based matrix polymers for high performance composites.     

Dielectric cure determination of a thermosetting epoxy composite prepreg

Evaluation of degree of cure (DoC) of a glass reinforced epoxy composite prepreg used for manufacturing of printed circuit board (PCB) is an intensive issue because of its practical importance and cost reduction in industry. Typical techniques such as differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR) are destructive and require curing a material during a chosen time, quenching the sample to stop cure before performing analysis. Thus, it is necessary to remove the temperature influence on the determination of DoC. In this study, the feasibility of nondestructive dielectric sensing method as an in situ DoC measuring technique through cure monitoring of prepreg is presented, where a vacuum packing configuration has been established so as for the prepreg to evaluate accurately the DoC in a quenched state at an ambient temperature. The optimal curing condition to get the fully cured state of a prepreg material is determined by the dielectric cure monitoring based on the behavior of ion viscosity. The temperature effect compensated DoC of prepreg is correlated and compared with that evaluated by DSC and FTIR. The correlated DoC with ion viscosity has identified the curing behavior of prepreg by determining cure kinetic parameters. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44707.     

Effect of crystallinity on enthalpy recovery peaks and cold‐crystallization peaks in PET via TMDSC and DMA studies

Poly(ethylene terephthalate) (PET) sheets of different crystallinity were obtained by annealing the amorphous PET (aPET) sheets at 110°C for various times. The peaks of enthalpy recovery and double cold‐crystallization in the annealed aPET samples with different crystallinity were investigated by a temperature‐modulated differential scanning calorimeter (TMDSC) and a dynamic mechanical analyzer (DMA). The enthalpy recovery peak around the glass transition temperature was pronounced in TMDSC nonreversing heat flow curves and was found to shift to higher temperatures with higher degrees of crystallinity. The magnitudes of the enthalpy recovery peaks were found to increase with annealing times for samples annealed ≤30 min but to decrease with annealing times for samples annealed ≥40 min. The nonreversing curves also found that the samples annealed short times (≤40 min) having low crystallinity exhibited double cold‐crystallization peaks (or a major peak with a shoulder) in the region of 108–130°C. For samples annealed long times (≥50 min), the cold‐crystallization peaks were reduced to one small peak or disappeared because of high crystallinity in these samples. The double cold‐crystallization exotherms in samples of low crystallinity could be attributed to the superposition of the melting of crystals, formed by the annealing pretreatments, and the cold‐crystallizations occurring during TMDSC heating. The ongoing crystallization after the cold crystallization was clearly seen in the TMDSC nonreversing heat flow curves. DMA data agreed with TMDSC data on the origin of the double cold‐crystallization peaks. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Structural analysis of poly(ethylene terephthalate) reinforced with glass fiber: Thermal behavior and correlation between PA-FTIR and DSC measurements

In this article, the correlation between PA-FTIR and DSC techniques was carried out to obtain a more comprehensive interpretation for the fiber–matrix interface of glass fiberreinforced PET and the dependence on annealing temperature. In contrast to the thermal behavior of an unannealed matrix or an annealed one at low temperatures (below 110°C), glass transition is barely perceptible and no crystallization peak can be found in the composite. Therefore, the lack of these thermal characteristics indicates the absence of the primary isomerization in the matrix. Similarly to the isolated matrix, low-melting and high-melting peaks appear in the thermograms of the annealed composite beyond 135°C, but the size and perfection of crystallites seems to be less in the composite. Conversely to the spectroscopic results, the crystallinity values of the composite tend only to coincide with those of the annealed isolated matrix at temperatures beyond 150°C, and below this temperature, they are always lower, until the primary isomerization of the matrix. A comparison between the results obtained with both techniques seems to indicate an improvement on the fiber–matrix interface in which an interfacial structure with low degree of perfection changes to more perfect crystals due to the annealing treatment. © 1996 John Wiley & Sons, Inc.     

Preparation of high-modulus and high-strength poly(ethylene terephthalate) fiber by zone annealing

To prepare high-modulus and high-strength PET fiber, a new method using zone drawing and zone annealing has been studied. The apparatus used for this method is the usual tensile tester equipped with a band heater 2 mm wide and a sample holder which can apply a high tension to the fiber. The experimental procedure consists of two stages: zone drawing and zone annealing. The zone drawing was done on the original as-spun fiber in order to produce a fiber with as high an orientation and as low a crystallinity as possible. The zone-drawn fiber was subsequently zone annealed under high tension by moving the band heater from one end to the other of the fiber at a temperature above the crystallization temperature at a considerably low moving speed. In spite of the simple apparatus and procedure, Young's modulus of the fiber obtained was 19.4 × 10¹⁰ dyn/cm², which is comparable to the maximum value of the high-tenacity PET filament commercially available. In order to elucidate the change in the superstructure with zone drawing or zone annealing, optical, x-ray, IR, DSC, and dynamic mechanical measurements were performed. It is suggested that the zone-annealed fiber consists of almost perfectly oriented crystallites and fully extended amorphous chains.     

Gas sorption in poly(butylene terephthalate). II. Influence of crystallinity and molecular orientation

Sorption measurements of CO₂, Ne, and Ar in poly(butylene terephthalate) (PBTP) films were studied by the gravimetric method with a recording microbalance at 298 K in the pressure range of 1–22 bar. The semicrystalline samples were annealed and oriented at 373 K. The sorption isotherms for CO₂ in PBTP films in the glassy state can be well described by the dual-sorption theory. The nonlinear sorption behavior of Ne and Ar can be satisfactorily analyzed using the sorption model developed for noble gases in PBTP. For the undrawn annealed films, it has been found that the increment of crystallinity leads to the reduction of the equilibrium gas concentration. For the oriented films, the gas concentration rises with increasing draw ratio. It appears that the sorption behavior for all tested gases in the oriented PBTP films does not depend on the changes of crystallinity and crystalline morphology under extension. The difference of the critical pressure p^* indicates the change in the size of the frozen microvoids existing in the noncrystalline phase, which was altered by annealing and drawing. © 1993 John Wiley & Sons, Inc.     

The crystallization behavior and mechanical properties of polylactic acid in the presence of a crystal nucleating agent

The crystallization behavior of polylactic acid (PLA) was studied in the presence of a crystal nucleating agent, ethylenebishydroxystearamide (EBH). The crystallization rate and crystallinity were significantly increased with addition of EBH. The isothermal crystallization half-time at 105°C was decreased from 18.8 minutes for neat PLA to 2.8 minutes for PLA with 1.0 wt % of EBH. The crystallinity of PLA with 1.0 wt % EBH was about 35% after 5-minute annealing at 105°C. Like neat PLA, the double melting peaks were also observed for nucleated PLA. The changes of the double melt peaks were investigated with various crystallization temperatures, heating rates, and annealing times. The heat deflection temperature (HDT) of nucleated PLA was up to 93°C after annealing. The correlation between crystallinity and HDT was demonstrated. A percolation threshold of crystallinity was found corresponding to HDT. The crystal size of nucleated PLA was significantly decreased with addition of EBH. The mechanical properties of annealed PLA blends simultaneously; showed improved modulus and impact strength. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Crystallinity and dicumyl peroxide diffusivity in low density polyethylene with different thermal histories

The absorption diffusivity of dicumyl peroxide (DICUP) in low density polyethlene pellets that have been annealed and either cooled slowly or quenched has been measured and correlated with the corresponding changes in crystallinity as measured by DSC. Annealing at 125°C resulted in an increased crystallinity and decreased DICUP diffusivity (3.2 × 10^?7 cm²/s for as supplied pellet to 5.2 × 10^?8 cm²/s for annealed and slowly cooled pellets). Quenching the annealed pellet reduced the increase in crystallinity and corresponding decrease in diffusivity. The time course of annealing and secondary crystallization processes was defined through further DSC measurements. The relationship between crystallinity and pressure for both hot press films and 4 mm diameter extrudate was found to be identical, Indicating the importance of pressure on bulk crystallization phenomena regardless of the source of the pressure.     

Mechanical characterization of carbon fiber reinforced phenolic resol and epoxy condensate plastic

A matrix resin for carbon fiber reinforced composite was developed that consisted of resol type phenolic and difunctional epoxy resin (PR-EP) condensate or adduct. Carbon fiber reinforced composite with fiber volume fraction of 0.6 was prepared with PR-EP matrix containing 0, 50, 100, 150, and 175 parts of epoxy resin per hundred parts of phenolic resin (php), especially a synthesized resol type. One-shot and prepreg techniques have been adopted and the study of loss of volatiles has indicated the superiority in terms of favorable processability of prepreg technique over the other. FTIR spectroscopic analysis confirmed the PR-EP adduct formation at the prepreg preparation stage. The improvement in properties such as tensile strength and elongation at break was observed in resin matrices with epoxy and phenolic resin; however, the flexural strength and modulus remained more or less unaltered. The prepreg technique of composite preparation has resulted in substantial improvement in mechanical properties and the same was attributed to the formation of PR-EP adduct and low volatiles during cure. Composites of carbon fiber reinforced PR-EP matrix developed here are likely to meet the requirement of aerospace structures in view of the realization of a wide spectrum of properties.     

Crystallinity and crystallite size measurement in polyamide and polyester fibres

X-ray diffraction methods for evaluating crystallinity and crystallite size in fibres generally neglect the possibility of peak overlap, and separate peaks and background by arbitrary procedures. The method described here is based on the resolution of normalized diffraction peaks in terms of combined Gaussian-Cauchy profiles for each peak, together with a polynomial background. Peak area crystallinity is then measured as the total area under the resolved peaks over a defined range. Measurements of apparent crystallite size are obtained from the widths of the resolved peaks. Application of this method to high tenacity nylon-6, nylon-6,6, and PET fibres after annealing, indicates that peak area crystallinity is about 100% for the polyamides and 85% for the polyester. The increase in apparent crystallite size is considered to be due to both an increase in the true mean size and to improvements in local lattice order.     

Structural and free-hole volume characterization of high-density polyethylene-chitosan composites plasticized with palm oil

A series of high-density polyethylene (HDPE) – chitosan (CS) composites with varying concentrations of chitosan have been prepared using a peroxide-initiated melt mixing process with maleic anhydride as compatibilizer. Palm oil has been added as a plasticizer to the optimized HDPE/CS5 composite. The variation in the free hole volume, microstructure, and crystallinity of the prepared composites has been studied using Positron annihilation lifetime spectroscopy (PALS), dynamic mechanical analysis (DMA), and X-ray diffractometry (XRD) respectively. The interaction between HDPE and chitosan is evidenced from the FTIR spectroscopic studies. The addition of palm oil to optimized HDPE-CS composite increased the crystallinity upto 41.3%. A shift as well as a decrease in loss modulus was obtained for 5 wt% chitosan added HDPE sample which can be ascribed to the better crosslinking observed in the composite. The increased amorphization of HDPE-chitosan composites with a higher concentration of chitosan beyond the optimized concentration is observed from the tan delta values. Free-hole value is reduced by 20 ų with the addition of 5 wt% palm oil to HDPE/CS5 composite system. The microstructural analysis shows evidence of efficient interaction of HDPE-chitosan system that can be used for further applications in medical field and food packaging industry.     

Tensile properties of random copolymers of propylene with ethylene and 1-butene: effect of crystallinity and crystal habit

The tensile modulus of elasticity and yield strength of semicrystalline random copolymers of propylene with different amount on ethylene or 1-butene co-units were analyzed as a function of the crystallinity and the crystal habit/shape. Samples were prepared by cooling the melt to ambient temperature, and subsequent annealing at elevated temperature. Variation of the cooling rate between 10⁻¹ and 10³ K s⁻¹ and of the temperature of annealing allowed preparation of semicrystalline specimens with either lamellar or non-lamellar crystals of different size, and with different crystallinity between about 30 and 70%. Young’s modulus and yield strength increase with increasing crystallinity and consistently are lower for samples containing nodular, that is, almost isometric, non-lamellar crystals of low aspect ratio. For samples of identical crystallinity and crystal habit, an only minor effect of presence of co-units in the crystalline and amorphous phases is observed.     

Effects of thermal history on isotactic polypropylene

The effect of past thermal history on the melting behavior of isotactic polypropylene is investigated in some detail. It is shown that a series of stepwise annealing treatments at steadily increasing temperatures will raise the final melting point and will result in a double endothermic peak if the final anneal temperature is at or close to 160°C. It is also shown that a series of stepwise annealing treatments at steadily decreasing temperature will lead to multiple DSC peaks. The number of such separate peaks is equal to or greater than the number of annealing steps. Even low-temperature anneals (100–130°C) affect the melting endotherm, while high-temperature anneals have a marked effect on both the degree of crystallinity of the sample and the final melting temperature. For a 3-min anneal, the highest degree of crystallinity is produced by an anneal temperature of 155°C. The highest melting temperature (～182°C) is produced by a 30-min, or longer, anneal at about 160°C. The implications of these results in terms of crystal thickening and perfection are discussed.     

预浸料技术的新进展

本文根据复合材料低成本制造技术，介绍了预浸料技术的新进展，包括可常温储存的预浸料；低温固化高温使用的预浸料；大丝束碳纤维预浸料和快速固化预浸料。     

Experimental investigation and optimization of printing parameters of 3D printed polyphenylene sulfide through response surface methodology

Today fused filament fabrication is one of the most widely used additive manufacturing techniques to manufacture high performance materials. This method entails a complexity associated with the selection of their appropriate manufacturing parameters. Due to the potential to replace poly-ether-ether-ketone in many engineering components, polyphenylene sulfide (PPS) was selected in this study as a base material for 3D printing. Using central composite design and response surface methodology (RSM), nozzle temperature (T), printing speed (S), and layer thickness (L) were systematically studied to optimize the output responses namely Young's modulus, tensile strength, and degree of crystallinity. The results showed that the layer thickness was the most influential printing parameter on Young's modulus and degree of crystallinity. According to RSM, the optimum factor levels were achieved at 338°C nozzle temperature, 30 mm/s printing speed, and 0.17 mm layer thickness. The optimized post printed PPS parts were then annealed at various temperatures to erase thermal residual stress generated during the printing process and to improve the degree of crystallinity of printed PPS's parts. Results showed that annealing parts at 200°C for 1 hr improved significantly the thermal, structural, and tensile properties of printed PPS's parts.     

T700/3234预浸料自动铺带工艺研究

通过研究自动铺带设定温度对预浸料粘性的影响以及自动铺带橡胶辊压力和带间间隙对层压板性能的影响,讨论了T700/3234单向环氧预浸料自动铺带工艺特性,确定了T700/3234的自动铺带工艺参数,满足了大型复合材料壁板类产品的制造要求。     

Cure monitoring of aerospace epoxy resins and prepregs by Fourier transform infrared emission spectroscopy

Spectral analysis of the infrared radiation emitted from thin films of resin transferred from the surface of high performance aerospace carbon fibreepoxy composite prepregs and heated to the cure temperature allows the cure chemistry and kinetics to be monitored in real time. Quantitative spectra with excellent signal-to-noise ratio are obtained by heating a thin resin film on a platinum hotplate fitted to the external optics of a Fourier transform infrared (FTIR) spectrometer and referencing the resulting emission (with the platinum emission subtracted) to a graphite black body at the same temperature. The resulting spectra are identical to absorption spectra and the quantitative features of the analysis are demonstrated by the appearance of isosbestic points during the curing reactions, so indicating that concentration profiles of the reacting species may be obtained. From the initial rate of amine and epoxy consumption, activation energies of 75kJ mol⁻¹ were obtained for both functional groups in the uncatalysed resin 4,4′-tetraglycidyl diamino diphenyl methane (TGDDM) with 27% 4,4′-diaminodiphenylsulfone (DDS), while values of 74 and 89kJ mol⁻¹ were obtained for amine and epoxy consumption from the TGDDM/DDS prepreg catalysed with boron trifluoride monoethylamine (Hercules 3501–6), consistent with homopolymerization occurring in the prepreg as well as amine–epoxy addition. Analysis of the FTIR emission at 177°C of resin from prepreg aged up to 90h at 23°C and 55% relative humidity shows a lowering of epoxy and amine concentration and a higher rate of cure, consistent with the formation of catalytic species. This technique may be used to monitor changes in surface properties such as tack and resin transfer, in addition to changes in the cure profile of the aged epoxy propreg.     

Correlation crystallinity and physical properties of heat-treated cellulose triacetate fibres

Previous measurements of crystallinity in fibres of cellulose triacetate have been qualitative or semi-quantitative; in this work quantitative measurements of relative crystallinity have been made on cellulose triacetate yarn heat treated in the range 20–300°C following the x-ray diffraction method of correlation crystallinity index. The onset of crystallization is clearly marked by a transition in the crystallinity index at 172°C, and beyond this annealing temperature tenacity and crystallinity are inversely correlated. Orientation also improves with annealing temperature with a less well defined transition around 120–180°C. Initial Young's modulus increases with temperature whilst other physical properties have optimum values in the range 180–220°C. Cutting and grinding are found to have an adverse effect on the correlation crystallinity index which is in fact a measure of lattice perfection.