The influence of microstructure on the failure behaviour of PEEK

In this study, different molecular weight PEEK materials were used to determine the effect of spherulite size on fracture. Melt processing of the PEEK at different temperatures produced samples of different average spherulite size. A permanganic etching technique was used to reveal the spherulites. It was found that for low molecular weight 150P PEEK, the spherulite size increased with melt processing temperature; but, for the higher molecular weight 450G PEEK, the spherulite size remained approximately constant. Also, the average spherulite size was markedly lower for the material of higher molecular weight. The failure behaviour of these samples was studied using a compact tension test. It was found that the fracture toughness of PEEK varied with processing temperature. Also, the average spherulite size of this material had a profound influence on the fracture mechanism.     

Annealing study of poly(etheretherketone)

Annealing of poly(etheretherketone), PEEK, has been studied for two materials cold crystallized from the rubbery amorphous state. The first material is a low molecular weight PEEK synthesized in our laboratory; the second is commercially available neat resin. Differential scanning calorimetry was used to monitor the melting behaviour of annealed samples. The effect of thermal history on melting behaviour is very complex and depends upon annealing temperature, residence time at the annealing temperature, and subsequent scanning rate. Thermal stability of both materials is improved by annealing, and for an annealing temperature near the melting point, the polymer can be stabilized against reorganization during the scan. Variations of density, degree of crystallinity, and X-ray long period were studied as a function of annealing temperature for the commercial material. Wide angle X-ray scattering was used to study the structure of annealed PEEK. An additional scattering peak was observed at higher d-spacing when annealed samples were cooled quickly.     

Mechanical and thermal expansion properties of glass fibers reinforced PEEK composites at cryogenic temperatures

Polyetheretherketone (PEEK) has been widely used as matrix material for high performance composites. In this work, 30% chopped glass fibers reinforced PEEK composites were prepared by injection molding, and then the tensile, flexural and impact properties were tested at different temperatures. The modulus, strength and specific elongation of glass fibers reinforced PEEK at room temperature, 77 K and 20 K have been compared. And the fracture morphologies of different samples were investigated by scanning electron microscopy (SEM). The results showed a dependence of mechanical properties of glass fibers reinforced PEEK composites on temperature. The coefficient of thermal expansion of unfilled PEEK and glass fibers reinforced PEEK were also investigated from 77 K to room temperature. The results indicated that the thermal expansion coefficient (CTE) of PEEK matrix was nearly a constant in this temperature region, and it can be significantly decreased by adding glass fibers.     

The effect of polymerization conditions and crystallinity on the mechanical properties and fracture of spherulitic nylon 6

The molecular and structural parameters controlling the mechanical properties, deformation and fracture of spherulitic nylon 6 have been investigated. The nylon was prepared by the anionic polymerization of ε-caprolactam and the polymerization conditions were varied to give samples having a range of spherulite diameter, molecular weight and degree of crystallinity. The tensile properties and fracture mode of the nylon varied considerably with degree of crystallinity and polymerization temperature. High crystallinity and low polymerization temperatures below 423 K gave a brittle material. Polymerization above 423 K resulted in a ductile material which showed a yield drop. In this material final fracture was preceded by the formation of inter and trans spherulitic cracks which coalesced to form a large cavity that led to final failure. In nylon having a low degree of crystallinity, fracture was fibrillar in nature and occurred by the ductile drawing of the material to strains greater than 250%.     

The effect of molecular weight on the interfacial properties of GF/PP injection molded composites

It is well established that the molecular weight of recycled PP decreases significantly as compared to the virgin material. Hence this study involved 2 PP grades of different molecular weights in order to simulate the recycling process. The effect of weight–average molecular weight on interfacial adhesion between GF and PP was investigated. Tensile test was done and the fiber length distribution around the fracture zone in both composites was compared with the distributions from similar locations of unstressed composites. The effect of PP-grafted maleic anhydride coupling agent was also studied. It was found that a decrease in weight–average molecular weight of PP improved interfacial adhesive strength between GF/PP. The lower molecular weight matrix has a lower viscosity that enables its molecules to penetrate easily into the silane interphase. In that case, the interfacial area that is available for coupling is higher, leading to a more effective coupling. The higher interfacial shear strength between the glass fiber and the lower molecular weight matrix induced more breakage of the glass fiber during tensile test.     

The effect of processing temperature and time on the structure and fracture characteristics of self-reinforced composite poly(methyl methacrylate)

A novel material, self-reinforced composite poly(methyl methacrylate) (SRC-PMMA) has been previously developed in this laboratory. It consists of high-strength PMMA fibers embedded in a matrix of PMMA derived from the fibers. As a composite material, uniaxial SRC-PMMA has been shown to have greatly improved flexural, tensile, fracture toughness and fatigue properties when compared to unreinforced PMMA. Previous work examined one empirically defined processing condition. This work systematically examines the effect of processing time and temperature on the thermal properties, fracture toughness and fracture morphology of SRC-PMMA produced by a hot compaction method. Differential scanning calorimetry (DSC) shows that composites containing high amounts of retained molecular orientation exhibit both endothermic and exothermic peaks which depend on processing times and temperatures. An exothermic release of energy just above T_g is related to the release of retained molecular orientation in the composites. This release of energy decreases linearly with increasing processing temperature or time for the range investigated. Fracture toughness results show a maximum fracture toughness of 3.18 MPa m1/2 for samples processed for 65 min at 128°C. Optimal structure and fracture toughness are obtained in composites which have maximum interfiber bonding and minimal loss of molecular orientation. Composite fracture mechanisms are highly dependent on processing. Low processing times and temperatures result in more interfiber/matrix fracture, while higher processing times and temperatures result in higher ductility and more transfiber fracture. Excessive processing times result in brittle failure.     

The fracture of poly(hydroxybutyrate) Part III Fracture morphology in thin films and bulk systems

This paper is the final part of a three paper series describing the fracture and ageing behaviour of poly(hydroxybutyrate). In the first two parts conventional fracture mechanics methods were used to monitor changes both during the detrimental room temperature ageing process that occurs and after a subsequent annealing process that had been reported to reverse the ageing process. This paper reports on our studies of the morphology of fracture surfaces and how fracture proceeds in different ways in the original, ductile, fresh, material, the more brittle, aged material and the ductile, annealed material. We have used optical and electron microscopic techniques to examine fracture surfaces of samples which had already been well characterised by mechanical testing. The effect of ageing and high temperature annealing on the resultant fracture morphology is detailed for both thin films and bulk samples. We have found that PHB undergoes crazing before failure regardless of annealing history. We have studied the craze morphology using optical microscopy and scanning electron microscopy. Both aged and un-aged samples are found to deform in approximately the same manner, the primary difference on ageing being the volume of material that is plastically deformed. After high temperature annealing a different craze morphology has been observed. In thin films this is characterised by the formation of a dense zone of micro-crazes over a relatively large area. In bulk samples there is a distinct change in the resultant fracture surface. In both thin films and bulk systems there is an increased occurrence of fracture initiation in the spherulite cores after high temperature annealing which helps to extend the craze zone.     

Spherulitic morphology of the matrix of thermoplastic PEEK/carbon fibre aromatic polymer composites

PEEK/carbon fibre composites (derived from APC-2) have been examined with a permanganic etching technique in order to reveal the crystalline, spherulitic morphology of the thermoplastic PEEK polymer matrix. The locations of the nucleation sites for spherulite growth have been categorized. Nucleation can occur both within the matrix and from the carbon fibres. Crystallization at lower temperatures favours matrix nucleation. Nucleation from fibres is dominated by sites associated with fibre-fibre contact. There is no evidence of “transcrystalline” growth. The study also identifies two types of crystal orientation effect in the polymer matrix. The first is a slight orientation that can occur in standard mouldings and is the result of the fibres constraining the shape of the volume into which spherulite growth can occur. The second effect produces abnormally high crystal orientation and is the result of improper processing at too low a melt temperature. Such conditions cause self-seeding during consolidation of laminates which, coupled with flow-induced orientation, can lead to directionally arranged spherulite precursors in different stages of morphological development.     

The effect of crystalline morphology of poly (butylene terephthalate) phases on toughening of poly(butylene terephthalate)/epoxy blends

In an effort to investigate the effect of the crystalline morphology of a poly(butylene terephthalate) (PBT) phase on the toughening of PBT/epoxy blends, the blends, having different degrees of perfectness of the PBT crystalline phase, were prepared by blending PBT and epoxy at various temperatures ranging from 200 to 240 °C. As the blending temperature decreases, the degree of perfectness of the PBT crystalline phase increases as a result of the increase of crystal growth rate. For PBT/epoxy blends, the change in crystalline morphology induced by processing may be the most important cause for the dependency of the fracture energy on blending temperatures. It has been found that PBT phases with a well-developed Maltese cross are most effective for epoxy toughening. This dependency reveals the occurrence of a phase transformation toughening mechanism. Also, the higher relative enhancement of fracture energy of a higher molecular weight epoxy system is further indirect evidence for a phase transformation toughening mechanism. Some other toughening mechanisms observed from the fracture surfaces, such as crack bifurcation, crack bridging, and ductile fracture of PBT phases, have been found to also be affected by the blending temperatures.     

THERMAL TRANSITIONS AND MORPHOLOGICAL BEHAVIOR OF POLYETHER ETHER KETONE AND LIQUID CRYSTALLINE POLYMER BLENDS

Polyether ether ketone (PEEK) and thermotropic liquid crystalline polymer (TLCP) based on hydroxy benzoic acid and hydroxy naphthoic acid (HBA/HNA) were prepared on a single-screw extruder with rotor speed 45 rpm at 350°C. Thermal analysis data, especially the glass transition temperature (T_g), indicated that the blends were incompatible in the entire range of concentration. Melting temperature (T_m) of the blends was found to be close to melting temperature of pure PEEK. Thermogravimetric analysis data show the poor thermal stability of the blends compared to the parent material. The percent weight loss increased with increasing LCP concentration. Analysis with a scanning electron microscope clearly indicated that a distinctive fibrous morphology was developed in the extruded samples at the low concentration of LCP, but the adhesion of the fiber to the PEEK matrix was poor, with circular voids around the LCP phase at higher concentration.     

Monomer casting nylon-6-b-polyether amine copolymers: Synthesis and properties

MC nylon-6-b-polyether amine copolymers were prepared with macro-initiator based on amino-terminated polyether amine functionalized with isocyanate via in-situ polymerization. It was found that the introduction of polyether amine delayed the polymerization process of caprolactam by increasing apparent activation energy and pre-exponential factor, resulting in the decrease of molecular weight of nylon-6. The motion of molecular chain of the copolymers was easy because of the decreased hydrogen bonds and weakened inter-molecular forces. The physical entanglement of molecular chains of the copolymers was significant and strong which increased the entanglement density. Only the nylon-6 phase crystallized in the copolymers and the crystal grain size, spherulite size and crystallinity of the copolymers decreased. A small amount of γ crystal formed at high polyether amine content. The copolymers presented obvious strain hardening behavior in stress-strain curves and the loss factor dramatically increased while the glass transition temperature and storage module decreased. The fracture surface of the copolymers became rough and presented hairy structure, indicating that the toughening mechanism of the copolymers corresponded to the multi-layer crack extension mechanism.     

Consolidation of unidirectional CF/PEEK composites from commingled yarn prepreg

Relationships between impregnation mechanisms, consolidation quality and resulting mechanical properties of CF/PEEK thermoplastic composites manufactured from a commingled yarn system have been investigated. A small compression mould was used to apply the different processing conditions (i.e. pressure, holding time and processing temperature). The consolidation quality of finished samples was characterized mainly through microscopic studies of the microstructure of the material associated with density measurements and evaluations of mechanical properties using a small transverse flexure testing facility. A model for qualitatively describing the impregnation and consolidation processes in commingled-yarn-based thermoplastic composites was developed, which predicts variations of void content during consolidation as well as the time, temperature and pressure required to reach full consolidation. Good correlations between predictions and the experimental data indicate the success of the approach. For a desired, minimum level of void content (X_v = 0.5%), optimum processing windows for manufacturing of CF/PEEK composite parts from the commingled yarn preform are suggested.     

Effect of hot water immersion on the performance of carbon reinforced unidirectional poly(ether ether ketone) (PEEK) composites: Stress rupture under end-loaded bending

This paper describes the behaviour of AS4 and T700SC reinforced PEEK composites (SUPreM™ and ACP-2) under applied compressive bending strain. The effect of an increased molecular weight of the polymer matrix on the residual time under endloaded compression bending conditions is studied. Generally for a given composite material, the higher the testing temperature and the applied strain the faster the failure occurs. At test temperatures exceeding the glass transition temperature or at high strain ratios the time-to-failure for CF/PEEK composites follows a master curve. The residual times under endloaded compression bending conditions increase with increasing toughness of the PEEK matrix but decrease with increasing tensile strength of the reinforcing fibres. It seems that the better the fibre/matrix adhesion the lower is the time to failure of an endloaded composite, because more load is transferred from the matrix into the fibres.In order to simulate composite applications under ‘harsh’ conditions the CF/PEEK composites have been exposed to boiling water. PEEK is known to be highly resistant to environmental effects, but water uptake significantly influences the overall performance of CF/PEEK composites under endloaded compression bending conditions. The tensile properties of the composites have been measured as function of exposure time in boiling water. The fibre dominated uniaxial tensile strength is not/or only slightly affected by the boiling water conditioning even after extended exposure times but the transverse tensile strength decreases significantly after exposure to boiling water. The performance of SUPreM™ CF/PEEK-150 and 450 composites under endloaded compression bending conditions are positively affected by water conditioning whereas APC-2 fails at shorter residual times. The fracture behaviour under endloaded conditions is also affected by the ingress of water into the composite.The obtained results show clearly that applications of thermoplastic composites leading to large out of plane deformations can only be ‘safe’ if the maximum service temperatures of the finished part will be well below the glass transition temperature of the polymer matrix otherwise even at low bending radii a dramatic failure of the material cannot be excluded.     

Recycling effects on microstructure and mechanical behaviour of PEEK short carbon-fibre composites

The effect of recycling on microstructure and mechanical properties has been evaluated for injection-moulded poly-ether-ether-ketone (PEEK) composites reinforced with 10% and 30% short carbon fibres. Microstructure characterization was carried out by determining fibre length distributions, PEEK molecular weight, and by SEM observations of fracture surfaces before and after processing. These studies reveal degradation of fibres and matrix during recycling. Tensile Youngs modulus and strength, as well as impact strength reductions are presented for recycled composites.     

Mikrofraktographie an CFK mit thermoplastischer Matrix (APC-2)

Microfractography of CFRP with thermoplastic matrix(APC-2) The objective of this investigation was to find out microfractographic features for APC-2 material due to specific loading modes. Interlaminar fractures were produced using static mode. I, II and mixed mode loading. Simultaneously, the fracture thoughness were measured. Furthermore, we determined the influence crack growth velocity and the temperature on the fracture morphology. Due to the three loading modes we got fraction surfaces with local changing ductil and brittel fracture morphologies but respectively specific for each mode. The ductil fracture morphology corresponds to lower and the brittle one to higher crack growth velocity. The area with the ductil fracture morphology increases with decreasing crack growth velocity and increasing temperature. At the same time the fracture thoughness of the material increases. This is an under-standable correlation. At loading modes with a tensil stress component (mode I and mixed mode) we found arest lines (lines in which the local crack growth velocity changes) in the fracture surface regions with a brittle fracture morphology. With the help of these arest lines the crack growth direction can be determined. At mode II loadings only the direction of the shear stress can be detected due to the microfractographic deformation direction. Interlaminar fatigue fractures (in notched bending specimens) show specific fracture morphologies which are influenced from the spherolithic microstructure of the semicrystalline matrix. A breakup of the PEEK -matrix in fibrils starting at the fibres is visible. This can be probably traced back to delamination of the amorphous fibrils and a breakup of the crystalline lamellas. On principle CFRP with a thermoplastic matrix has a completely different fracture morphology as a composite with a thermoset epoxy matrix. This is clear if one considers that the fracture strain of PEEK is one order higher than that of epoxy.     

The influence of morphology and molecular weight on ductile-brittle transitions in linear polyethylene

The tensile behaviour of linear polyethylene was examined over a wide range of temperatures. Samples were prepared from low and medium molecular weight polymer with different morphologies, by varying the initial crystallization conditions. It was found that the temperature of the ductile-brittle transition was markedly different for different samples. In particular, low molecular weight polymer crystallized at a low degree of of supercooling, showed brittle behaviour over most of the temperature range, with a ductile-brittle transition near to room temperature. Rapidly quenched material, where the degree of supercooling is high, showed a very low ductile-brittle transition temperature, especially in high molecular weight polymer. The reasons for these differences in behaviour are discussed both at a phenomenological level and in terms of known structural differences between the different materials examined.     

Material characterization of blended epoxy resins related to fracture toughness

The present study describes the effect of the macromolecular modifications on the fracture toughness of an epoxy system. We synthesized epoxy networks by the reaction of diglycidyl ether of bisphenol A (DGEBA) with methyl-tetrahydro-phthalic anhydride (MTHPA), initiated by a tertiary amine. Several materials were obtained by adding a high molecular weight monomer to one with low molecular weight (both based on DGEBA) at different concentrations. In every case, a stoichiometric amount of MTHPA was employed. The glass transition temperature and the Angell’s fragility index, derived from thermo-viscoelastic properties, were used to characterize the materials. Relationship between these two parameters and the fracture properties, including the fracture toughness and the microscopic roughness of the fracture surfaces observed by atomic force microscope (AFM), was then investigated. We found that there were direct correlations among the glass transition temperature, the fragility, the fracture toughness, and the roughness. This study revealed that both the glass transition temperature and the fragility are effective for characterizing material in relation to the fracture toughness of the blended epoxy resins.     

The synthesis of polyetheretherketones and investigations of their properties

Using hydroxyl-containing olygoketones with different condensation degrees and dichloranhydrides of isophthalic and terephthalic acids, polyetheretherketones (PEEK) were synthesized by means of acceptor-catalytic polycondensation, and their properties were investigated. It was shown that PEEK possesses a better solubility, a higher molecular weight and is characterized by high thermal and mechanical properties. The correlation between some properties of polyethers and the degree of condensation of olygoketones was also investigated.     

Influence of processing conditions on bending property of continuous carbon fiber reinforced PEEK composites

The purpose of this study is to achieve an optimum fabrication condition for the continuous carbon fiber reinforced PEEK matrix composites based on a micro-braiding fabrication method. The composite plates were fabricated at three processing temperatures (380, 410 and 440 °C) and three holding times (20, 40 and 60 min), respectively, with a total number of nine different fabrication conditions, and their bending properties were investigated in terms of thermal and fracture characterizations. As a result, the bending performance of the fabricated composites was significantly affected at the 440 °C temperature. Although no significant change in the bending performance was seen at the 380 and 410 °C with all the holding times, the thermal and fracture characterizations implied a degradation of the PEEK matrix property during the fabrication process. In order to avoid the matrix degradation and the decrease of mechanical properties, a lower fabrication temperature with a shorter holding time should be recommended for the carbon/PEEK composites fabricated by the micro-braiding method.     

PEEK复合材料用碳纤维上浆剂研究进展EI北大核心CSCD

热塑性聚醚醚酮(PEEK)复合材料具有优异的断裂韧性、抗冲击性能、耐疲劳性能,广泛应用于航空航天领域。上浆剂作为碳纤维的核心配套产品,对复合材料界面有重要影响。受分解温度限制,传统热固性碳纤维上浆剂难以满足PEEK复合材料使用,制约高性能PEEK复合材料的研制和应用,因此研制匹配PEEK复合材料的碳纤维上浆剂具有重要意义。本文分析了PEEK复合材料界面特性及上浆剂作用机理;重点介绍了改性PEEK、聚酰亚胺前驱体、聚醚酰亚胺等类型上浆剂的研究进展和成果,并对不同体系上浆剂进行分析总结;最后对PEEK复合材料用碳纤维上浆剂的研制提出建议,对上浆剂绿色环保多功能化趋势进行了展望。