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 and chain adsorption of poly(etheretherketone) in discontinuous pitch-derived carbon fiber composites

The crystallization kinetics of poly(etherether ketone) (PEEK) in chopped mesophase pitch-derived carbon fiber/PEEK composites have been studied. Various processing techniques are used in order to obtain controlled fiber length in the composites. Scanning electron microscopy performed on properly etched long fiber composite samples reveals that the nucleating sites density is low at the carbon fiber surface: transcrystalline layers are rarely observed. This is confirmed by differential scanning calorimetry. However, samples processed by mixing carbon fibers and molten polymer in a high temperature mixer have a widely different behavior: the nucleation density and the crystallization rate increase, the glass transition of these samples is displaced towards higher temperatures, and the solubility is dramatically lowered. We ascribe these phenomena to the adsorption of the polymer chains on carbon particles created by attrition during the mixting.     

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.     

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.     

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.     

Processing and characterisation of three-layer alumina-based composites with enhanced damage tolerance

Three-layer alumina-based composites reinforced with iron in the inner layer and with chromium in the outer layers were fabricated by first uniaxially pressing the three-layer assembly, followed by cold isostatic pressing at 300 MPa and sintering in a graphite vacuum furnace at 1500 °C for 1 h.

The residual compressive stresses in the outer Cr–Al/Al₂O₃ layers and the residual tensile stresses in the Fe–Al/Al₂O₃ inner layer were predicted as a function of composition and the thickness ratio of the outer and inner layers. Theoretical calculations showed that the compressive stresses in the outer layers increased while the tensile stresses in the inner layer decreased with decreasing outer layer thickness. The existence of compressive stresses was verified by microscopic evidence, which showed that propagation of cracks perpendicular to the interface is suppressed in the outer layer, but promoted in the inner layer.

Indentation and subsequent strength testing showed that these layered composites exhibited improved damage tolerance. Three-layer composites showed four-point bend strengths exceeding the bend strength of unindented monolithic Al₂O₃ even after indentation at 300 N.     

Thermal behavior of transparent poly(etheretherketone)(PEEK) film

The thermal behavior of poly(etheretherketone)(PEEK) film heated in an open differential scanning calorimetry (DSC) pan at 20°C/min is distorted by relaxation of the strained film. PEEK film in a closed pan or quenched PEEK in open or closed pans shows a glass-transition temperature (T_g) around 144°C, cold crystallization (～22 J/g) at 177°C, melt-temperature (T_m) peaking at 335–340°C, with an enthalpy of fusion of 32–34 J/g, and recrystallization on cooling at 285°C, with a crystallization exotherm of about 40 J/g. The enthalpy of fusion decreases with increasing heating rate from 2–100°C/min and approaches the enthalpy of cold crystallization. With increasing heating rate, further crystallization of PEEK during the DSC scan is suppressed. With increasing cooling rate, PEEK melt crystallizes at larger supercoolings to a lesser extent. Crystallization on cooling the melt was more complete than cold crystallization and annealing on heating.     

Role of interface on dynamic modulus of high‐performance poly(etheretherketone)/ceramic composites

High‐performance printed circuit board or electronic packaging substrate with low warping particularly at high frequency is the key demand of manufacturers. In the present work, poly(etheretherketone) (PEEK) matrix composites reinforced with untreated micron size aluminum nitride (AlN) and alumina (Al₂O₃) particles have been studied for dynamic modulus in the temperature range varying from 30 to 250°C. At 48 vol % particles, the room temperature modulus of the PEEK/AlN composites increased by approximately fivefold (～ 23 GPa), whereas it increased by twofold for PEEK/Al₂O₃ composite. The reinforcing efficiency is more pronounced at higher temperatures. The significant improvement in modulus was attributed to the better adhesion between the matrix and the AlN particles. Scanning electron microscope (SEM) and Kubat parameter showed that the poor adhesion between the matrix and the Al₂O₃ particles resulted in comparatively smaller increase in modulus of PEEK/Al₂O₃, despite higher intrinsic modulus of Al₂O₃ than that of AlN. SEM showed almost uniform distribution of particles in the matrix. The experimental data were correlated with several theoretical models. The Halpin–Tsai model with ξ (xi) is equal to four correlates well up to 48 vol % AlN composites while ξ is equal to two correlates only up to 18 vol % Al₂O₃ composites. Guth–Smallwood model also correlates well up to 28 vol % AlN and 18 vol % Al₂O₃‐filled composites. Thereafter, data deviated from it due to the particles tendency to aggregate formation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Processing of poly(amide imides) as matrices for high-performance composites

Poly(amide imide) (PAI) has proven to be an excellent engineering thermoplastic. Injection and compression molding processes are currently used to produce poly(amide imide) parts when superior toughness, solvent resistance, and high-temperature heat performance are required. The objective of this work was to determine the essential features necessary to use these systems as matrices for carbon-fiber-reinforced composites. The principal steps utilized were: impregnation of the carbon fibers with a PAI solution, drying the solvent from the individual plies, stacking the plies for final consolidation, and a postcure. A processing cycle was developed applicable to resins with different solvent concentration and molecular weight, giving special attention to the solvent evaporation step. A model capable of predicting the rate of evaporation as a function of temperature was developed using the principle of time–temperature superposition. A viscosity model similar to that for a curing thermoset was developed by combining Arrhenius expressions for the temperature and concentration effects. Collectively, this work then provided insight for matrix requirements in the formulation of PAI composites by solvent impregnation.     

Application of the parallel Avrami model to crystallization of poly(etheretherketone)

We have reexamined the parallel Avrami model recently proposed by Velisaris and Seferis (1) to describe the non-Isothermal crystallization of poly(etheretherketone), PEEK. We show that, based on considerations of morphology development, the crystallization process with the larger Avrami exponent has the higher melting point, whereas the process with the smaller Avrami exponent has the lower melting point. This assignment differs from that of Velisaris and Seferis. In addition, we have used the infinite crystal melting point, as required by crystallization theory, to determine the Avrami rate parameters for the two processes. With this revision of the parallel Avrami model, we have applied the model to non-isothermal crystallization of APC-2 PEEK composite. Under the assumption that the linear growth rate determines the Avrami rate parameter, both the transport activation energy, U, and the kinetic parameter, K_g, are found to compare favorably with the values previously determined from isothermal crystallization of neat resin PEEK.     

A study of CO2 and excimer laser treatments of poly(etheretherketone)

CO₂ and excimer laser (193, 248, 350 nm) treatments were performed on poly(etheretherketone) (PEEK). High fluence excimer laser irradiation induced efficient etching, mainly due to thermal effects. The irradiation with CO₂ lasers and high fluence excimer lasers introduced limited changes in surface chemistry and morphology. Low fluence excimer laser irradiation, particularly at 193 nm and 248 nm, favored the occurence of photochemical phenomena. Surface chemical modifications leading to oxygen depletion and preferential elimination of the ketonic bridge were assessed. At the same time surface morphological alterations were found, diffraction effects led to cone-like structures while the redeposition of heavy oligomers caused the formation of debris on the surface.     

Processing,microstructure, and failure behavior in short-fiber-reinforced poly(ether ether ketone) composites

Short-fiber-reinforced poly(ether ether ketone) (PEEK) composites were prepared by a specially designed mold. Both compression- and extrusion-molded plaques were obtained under the same thermal history. The fiber length distribution, fiber volume fraction, and fiber orientation are characterized. The fibers show an in-plane random orientation in compression-molded plaques, but they exhibit a 3-layer fiber orientation well-known for injection moldings in extrusion-molded composites. It is the final aim to simulate the rheological and morphologic behavior in injection moldings by using the laboratory designed extrusion/compression mold. Static compact tension (CT) specimens and electron microscopy (EM) were used to investigate the failure behavior. Results showed that crack initiation is the dominant failure energy absorption process in a brittle fracture, whereas crack propagation is dominant in a ductile failure. The extruded composites were mechanically characterized in two orthogonal directions (T- and L-type). The anisotropy factor is reported as 1.2.     

Processing effects on poly(ethylene terephthalate) from bottle scraps

Processing of virgin and recycled poly(ethylene terephthalate) (PET) in a twin screw extruder evidences the degradative effect caused by thermal decomposition of poly(vinyl chloride) (PVC) and other impurities, e.g. adhesives, at the processing temperature. Lower melt viscosity and molecular weight, along with higher carboxylic end group concentration, were observed for recycled PET, the extent depending on PET purity. In an attempt to investigate the correlation between the kinetics of degradation phenomena and the level of thermomechanical stress, a novel dynamic method of evaluating thermal stability in processing conditions was developed. Such a method allows the achievement of long equivalent residence times while using lab-scale extruders. As a result of these experiments, PVC-rich recycled PET was shown to reach very low melt viscosity after less than 10 min in processing conditions, while virgin PET retained high viscosity even after 30 min.     

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.     

The effect of acid-base interaction on the thermal and transport properties of poly(etheretherketone) based composite membranes

Binary composite membranes were prepared by the solution casting method from sulfonated poly(etheretherketone) (SPEEK) and organic additives such as hydroxyquinolinesulfonicacid (HQS), 4-tertiary butylpyridine (TBP), imidazole and succinimide. Ternary composite membranes were prepared from SPEEK, inorganic phosphotungstic acid (PWA) and the same organic additives. The acid base interaction characteristics of the composite membranes were not observed by ATR-FTIR analysis. TGA results showed that the thermal stability of the composite membranes was enhanced in the temperature range up to about 400 °C by the addition of the organic additives. The acid-base interaction between SPEEK and the organic additives of HQS, TBP and imidazole decreased the water uptake, methanol permeability and proton conductivity of the binary and ternary composite membranes. However, the addition of succinimide did not decrease the water uptake, proton conductivity and methanol permeability of the composite membranes. The composite membranes containing succinimide made little acid-base interaction but made hydrogen bonding with SPEEK. The hydrogen bonding proved to be weaker than the acid-base interaction. The selectivity of the composite membranes increased by the addition of PWA, and the selectivities of the composite membranes containing succinimide were higher than those of the other composite membranes.     

Processing and structural properties of in situ polymerized poly(tetrafluoroethylene)/graphite fluoride composites

Novel poly(tetrafluorethylene)‐based composites were polymerized in situ on the fluorinated petroleum coke as graphite fluoride microparticles. This allows providing high adhesion between poly(tetrafluorethylene) and filler particles which is of much important to improve sufficiently mechanical properties of the polymer. The present work aimed at investigating the relationships between mechanisms of polymerization kinetics and structural properties of the composites. In the presence of fluorinated coke, the polymerization kinetics of tetrafluorethylene, started at the surface of the filler, is characterized by a long induction period followed by the explosive growth rate. This effect was supposed to be originated by the particles exfoliation due to the anchoring of growing polymer chains to the accessible interface of the filler. Scanning electron microscope confirmed this hypothesis showing fragmentation of graphite fluoride microparticles. The resulting fragments are show to be well distributed in the polymer matrix. Wide‐angle X‐ray diffraction experiments demonstrates that graphite fluoride crystals are still present after polymerization of poly(tetrafluorethylene) without modification of the crystalline peak (001) position. This asserts that no overall exfoliation mechanism of graphite fluoride into (CF)_n monolayers is active during the in situ polymerization. POLYM. ENG. SCI., 53:2549–2555, 2013. © 2013 Society of Plastics Engineers     

Off-axis damage tolerance of fiber-reinforced composites for aerospace systems

Off-axis strength retention of continuous carbon fiber-reinforced dense ZrB₂-based ceramics (C_f/ZrB₂) after thermal or indentation damage was evaluated. Thermal damage was in-situ induced and characterized by cyclic dilatometric analysis. Indentation damage was induced through Vickers indentation and then characterized by digital microscopy. The investigation of Vickers imprints suggested that residual stresses promoted the material pileup onto the fibers’ plane and the appearance of out-of-plane freed fibers (OFF). On the other hand, thermal damage reduced the residual stresses and left inner freed fibers (IFF) that enhanced the elastic response. Finally, the flexural tests on damaged specimens unexpectedly revealed that C_f/ZrB₂ kept its load bearing capability either after thermal or indentation damage (in both cases) and showed damage insensitivity although tested in fully matrix-dominated loading configuration (off-axis configuration).