Crystallization of poly(etheretherketone) (PEEK) in carbon fiber composites

The tendency of carbon fiber to nucleate the zation of poly(etherettterlcetone) (PEEK) has been evaluated by DSC and other techniques. As the carbon fiber content was increased, the supercooling necessary for PEEK crystallization decreased. The repeated melting (at 396°C) of the same PEEK sample results in a decrease of the number of nuclei for crystallization. At equivalent thermal histories, PEEK with carbon fiber was found to have a higher nucleation density than PEEK itself. The surface of carbon fibers and nuclei in the PEEK matrix compete for crystallization growth. As the holding time in melt was increased, the number of matrix spherulites formed on cooling decreased, hence a more pronounced transcrystalline region was developed. Correspondingly, the composites preheated in the melt for 100 min showed about two times the transverse tensile strength and strain-to-failure of those preheated for only 30 min. Corresponding fracture surface produced in tension showed that the former samples had a greater matrix adhesion to the carbon fiber than the latter. A strong interfacial bond is thus developed by crystallization on carbon fiber surface. Destroying nuclei in the PEEK matrix by long preheating enhances crystallization on the carbon fiber.     

Crystallization kinetics of poly(phenylene sulfide) (PPS) and PPS/carbon fiber composites

The evolution of crystallinity of neat PPS and of the carbon fiber reinforced polymer under different processing conditions is studied. Crystallization from the amorphous state at low temperatures (cold crystallization), crystallization from the melt during cooling, and crystal melting processes are analyzed using calorimetric techniques under both isothermal and nonisothermal conditions. Cold and melt crystallization kinetics are described using an Avrami equation and an Arrhenius expression for the temperature dependence of the kinetic constant. Also, the melting kinetics of the, reinforced and of the unreinforced polymer are studied in this work. The effect of carbon fibers on the crystallization kinetics of PPS is analyzed, and a comparison of the crystallization behavior of PPS and other semicrystalline thermoplastic matrices, such as poly(etheretherketone) (PEEK), is presented.     

Processing effects and damage tolerance in poly(etheretherketone) composites

Thermoplastic composites in general have been found to display improved resistance to impact damage over their thermosetting counterparts. However, in earlier work, we presented data that suggested that in poly(etheretherketone) (PEEK) based composites, damage tolerance may be adversely affected if cooling rates are too slow in the processing of these parts. In this work, ultrasonic C-scan and compression-after-impact testing were implemented on PEEK APC-2 plaques cooled at three different cooling rates, in addition to toughened epoxy and bismaleimide plaques processed conventionally as a comparison. It was found that the damaged areas as measured ultrasonically varied significantly between the PEEK samples cooled at different rates, and that under the slowest cooling conditions, the damaged areas approached those of the thermosetting systems tested. However, because compressive strengths before impact were found to be greater on slow cooling, the differences in compression-after-impact were less dramatic than one would have expected from the C-scan results alone. Variations in crystallinity as well as spherulite size were the source for the differences in damaged areas as well as in compressive strength.     

Fiber orientation and its effect upon thermoelastic properties of short carbon fiber reinforced poly(etheretherketone) (PEEK)

Experimental and analytical techniques are employed in the present study to investigate the influence of microstructure on thermoelastic properties of short carbon fiber reinforced poly(etheretherketone). The test specimen geometry is an edge gated, injection molded dogbone tensile bar. Typical of injection molded structures, three distinct layers of fiber orientation were discernable through the sample thickness. The thermoelastic properties of the surface layer (machined from the specimen) are measured for direct correlation with a micromechanics model. In addition to measuring the volume fractions and constituent properties, microstructural features such as fiber aspect ratio and the process-induced fiber orientation distribution are quantified. Correlation of experimental data with micromechanics model predictions is found to be quite good.     

Crystallization and thermal behavior of multiwalled carbon nanotube/poly(butylenes terephthalate) composites

Multiwalled carbon nanotube/poly(butylene terephthalate) composites (PCTs) were prepared by melt mixing. The nonisothermal crystallization and thermal behavior of PCTs were respectively investigated by X‐ray diffractometer, polarized optical microscope, differential scanning calorimeter, dynamic mechanical thermal analyzer, and thermogravimetric analyzer. The presence of nanotubes has two disparate effects on the crystallization of PBT: the nucleation effect promotes kinetics, while the impeding effect reduces the chain mobility and retards crystallization. The kinetics was then analyzed using Ozawa, Mo, Kissinger, Lauritzen‐Hoffman, and Ziabicki model, and the results reveal that the nucleation effect is always the dominant role on the crystallization of PBT matrix. Thus the crystallizability increases with increase of nanotube loadings. In addition, the presence of nanotubes nearly has no remarkable contribution to thermal stability because nanotubes also play two disparate roles on the degradation of PBT matrix: the Lewis acid sites to facilitate decomposition and the physical hindrance to retard decomposition. Hence the nanotubes act merely as inert‐like filler to thermal stability. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

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     

Solubility characteristics of poly(etheretherketone) and poly(phenylene sulfide)

The crystalline high-performance polymers poly(etheretherketone) (PEEK) and poly(phenylene sulfide) (PPS) are generally considered to be highly resistant to dissolution in most common solvents. This article reveals numerous compounds that dissolve in excess of 25 wt % of each polymer at elevated temperatures. Typical binary-phase diagrams depicting the minimum solubility temperature vs. concentration are also presented for several representative polymer–solvent mixtures for both PEEK and PPS. The phenomenon of the solvent-induced crystallization of PEEK is presented. The importance of these polymer–solvent systems for the preparation of permselective membranes is discussed.     

Evaluation of interlaminar-toughened poly(etherlmide)-modified epoxy/carbon fiber composites

Amorphous poly(ether imide) has been used as interlaminar toughening particulate agent in laminated carbon fiber/epoxy composites. Mode I and Mode II delamination fracture toughness was characterized using the double cantilever beam (DCB) and end-notched flexure (ENF) specimens. The delamination surface was examined using a scanning electron microscopy (SEM) to investigate relationships between the morphology and properties. The results revealed that the PEI-modified composites exhibited a significantly increased fracture toughness, which increased with the PEI content. G_IC was improved from 165 to 540 J/m² (at 1 mm/min crosshead speed). G_IIC was improved more significantly from 290 to 1300 J/m². It is believed that these values could be further improved if the processing cycle were to be optimized.     

Interfacial adsorption and crystallization of polycarbonate in carbon fiber composites

A polycarbonate (PC)/carbon fiber (CF) composite system has been examined with regard to interfacial adsorption and crystallization by altering times and temperatures of annealing. Times up to 180 min and temperatures of 245, 275, and 300°C have been investigated. Tranverse tensile, tranverse toughness, and scanning electron microscopy results on unidirectional, continuous-fiber composites indicate improved fiber/matrix adhesion at longer times and higher temperatures of annealing. Improvements in transverse toughness and transverse tensile strength of a factor of two is achieved. The data indicate that primarily adsorption rather than secondary interfacial crystallization is the likely mechanism for increased adhension. Isothermal transverse toughness values have been found to fit well to a Langmuir-type expression. The temperature dependence of adsorption as measured by transverse toughness is described well by an Arrhenius equation. The dependence of transverse toughness on PC molecular weights from M_w = 26,600 to 39,800 was found to be large, with higher molecular weights adsorbing more effectively.     

Crystallization behavior of poly(ethylene terephthalate)/multiwalled carbon nanotubes composites

Crystallization behavior of poly(ethylene terephthalate)/multiwalled carbon nanotubes (PET/MWNTs) composites have been investigated under isothermal conditions and in comparison with the conventional nucleating agents, sodium benzoate, and micrometric carbon/glass fibers. In the PET/MWNTs composites, MWNTs promote the crystallization of PET as a heterogeneous nucleating agent, and the nucleation efficiency is greatly enhanced when MWNTs was homogeneously dispersed in PET matrix. In comparison with pure PET, spherulites size of PET/MWNTs composites is significantly reduced, and the shape becomes quite irregular. TEM images indicate that MWNTs bundles locate in the center of spherulites of PET and act as nuclei. Fold surface free energy during nucleation process for MWNTs nucleated PET is just half of pure PET, suggesting that MWNTs are efficient nucleating agents for PET. The sequence of nucleating ability of is given as follows: sodium benzoate>MWNTs>talc>carbon fibers≈glass fibers. The nucleation in the presence of sodium benzoate is a chemical nucleation process that may cause severe degradation of PET, but MWNTs nucleate PET through “particle effect,” which does not affect the molecular weight of PET. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Residual stress development in neat poly(etheretherketone)

Material property models for poly(etheretherketone) (PEEK) have been combined with a residual stress model to provide a means for investigating the effect of crystallization process on the residual stress development in semicrystalline materials. The analysis shows that crystallization causes an increase in the residual stress levels. This increase is affected through an increase in the resin modulus values and through the resin modulus build-up at higher temperatures. The shrinkage due to crystallization was found to have no effect on the residual stress development in neat PEEK.     

Nonisothermal crystallization kinetics of poly(etheretherketone) (PEEK)

Analysis of the crystallization kinetics of poly(etheretherketone) (PEEK) was achieved with dynamic differential scanning calorimetry results. A new kinetic model for the nonisothermal crystallization was derived and the possibility of its application was investigated. By evaluating the parameters in the model, the crystallization behavior of PEEK was analyzed. The experimental and predicted crystallinity change showed good agreement, which indicated that the model equation was appropriate to describe the nonisothermal crystallization kinetics of PEEK. As the melt temperature was increased the number of heterogeneous nuclei decreased, hence the crystallization was delayed.     

Physical aging of amorphous poly(etheretherketone) (PEEK)

Thermal treatment of amorphous poly(aryl-ether-ether ketone) below the glass transition temperature has been studied. The extent of aging was measured by differential scanning calorimetry. X-ray and Fourier transform infrared spectra were used to study both melt-cast and annealed samples. The effect of the thermal treatment of viscoelastic response was evaluated using creep tests. Aging has been shown to be accompanied by a marked change in the transport properties of the material.     

Preparation and characterization of poly(vinyl chloride)–continuous carbon fiber composites

In this article, we report on the preparation and characterization of novel poly(vinyl chloride) (PVC)–carbon fiber (CF) composites. We achieved the reinforcement of PVC matrices with different plasticizer contents using unidirectional continuous CFs by applying a warm press and a cylinder press for the preparation of the PVC–CF composites. We achieved considerable reinforcement of PVC even at a relatively low CF content; for example, the maximum stress (σ_max) of the PVC–CF composite at a 3% CF content was found to be 1.5–2 times higher than that of the PVC matrix. There were great differences among the Young's modulus values of the pure PVC and PVC–CF composites matrices. The absolute Young's modulus values were in the range 1100–1300 MPa at a 3% CF content; these values were almost independent of the plasticizer content. In addition, we found a linear relationship between σ_max and the CF content and also recognized a linear variation of the Young's modulus with the CF content. The adhesion of CF to the PVC matrix was strong in each case, as concluded from the strain–stress curves and the light microscopy and scanning electron microscopy investigations. The mechanical properties of the PVC–CF composites with randomly oriented short (10 mm) fibers were also investigated. At low deformations, the stiffness of the composites improved with increasing CF content. Dynamic mechanical analysis (DMA) was used to determine the glass‐transition temperature (T_g) of the PVC–CF composites. The high increase in the Young's modulus entailed only a mild T_g increase. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Crystallization behavior of polyphenylene sulfide (PPS) and PPS/carbon fiber composites: Effect of cure

Summary The effect of cure on the crystallization rates of polyphenylene sulfide (PPS) and PPS/carbon fiber composites has been studied by differential scanning calorimetry (DSC). The crystallization rate of PPS increased with increasing degree of cure. The carbon fiber acted as a nucleating agent to enhance the crystallization rate of PPS. The fully cured PPS seemed to be saturated with nucleating sites formed through the cure reaction. Therefore, the presence of carbon fibers gave little effect on the crystallization rate of the fully cured PPS.     

Victrex® poly(ethersulfone) (PES) and Victrex® poly(etheretherketone) (PEEK)

Properties of two high performance engineering thermoplastics, amorphous polyethersulfone (PES) and semicrystalline polyetheretherketone (PEEK), are discussed. Both resins can be processed by conventional techniques, compounded with high performance fibers, and have high service temperature (up to 300°C). Due to the amorphous character PES can be dissolved and spray coated into metals.     

Crystallization kinetics of poly(butylene terephthalate) and its talc composites

Crystallization kinetics of poly (butylene terephthalate) (non‐talc‐PBT) and its 0.1 wt % talc composites (talc‐PBT) was determined for a wide range of cooling rates and isothermal temperatures. The critical cooling rate to suppress crystallization is 2000 K s^?1 for non‐talc‐PBT and 7000 K s^?1 for talc‐PBT. The cooling rate dependence of the total enthalpy change and heating rate dependence of enthalpy of cold crystallization are quantitatively discussed on the basis of the Ozawa's method. For isothermal crystallization, the annealing‐temperature (T _iso) dependence of crystallization half‐time (t _1/2) shows a bimodal curve with two minima. Talc shortens the t _1/2 at T _iso above 340 K and acts as a heterogeneous nucleation agent. Tammann's approach revealed that the t _1/2 is shortened by pre‐nucleation for non‐talc‐PBT but not for talc‐PBT. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44739.     

Strength properties of discontinuous fiber composites

The longitudinal strengths of unidirectional discontinuous fiber composites have been calculated based on the concept of perturbation effect and the distortional energy criterion, utilizing the finite-element method. The theoretical results thus obtained are compared with the experimental data as well as the results of shear lag analysis for tungsten-copper, boron-aluminum, glass-epoxy and boron-epoxy composites. Based on the Jackson-Cratchley equations, modified for discontinuous fibers, a formula is also proposed for calculating the strengths of randomly oriented discontinuous fiber composites. The results calculated from this formula are compared with the experimental data for aluminum oxide-aluminum-silicon and glass-epoxy composites.     

Mechanical,morphology, and thermal properties of carbon fiber reinforced poly(butylene succinate) composites

Biodegradable poly(butylene succinate) (PBS)/carbon fiber (CF) composites were prepared by melt blending method using twin‐screw extruder followed by injection molding. Mechanical properties, crystallization behavior, morphology, crystal structure, and thermal stability of PBS/CF composites were investigated with different CF contents (0, 5, 10, 15, and 20 wt%). It was found that the tensile and impact properties of the composites were improved markedly with the addition of CF; while too much CF would lead to agglomeration and thus weaken the improvement. Scanning electron microscopic photographs on the fracture surfaces showed superior interfacial adhesion between fibers and PBS matrix. Crystallization peak temperature of PBS in its composites was increased due to the heterogeneous effect of CF. The spherulite size of PBS/CF composites decreased and the nucleation density increased drastically. The crystal structure was not affected by the incorporation of CF, as confirmed from the wide‐angle X‐ray diffraction analysis. thermogravimetric analysis showed that the thermal stability of PBS/CF composites was also enhanced. POLYM. COMPOS., 36:1335–1345, 2015. © 2014 Society of Plastics Engineers