Impact resistance and secondary transitions

Instrumented Charpy impact measurements were made on polycarbonate from liquid nitrogen temperature to room temperature. The polymer had a transition from brittle fracture to ductile failure at ?130°C. Scanning electron micrographs of the fracture surfaces did not correlate with the secondary transition or test temperature. A Fourier analysis of the impact pulse covers a wide range of frequencies, but the dominant frequencies at room temperature are below 200 Hz. Time–temperature superposition shows that the secondary transition occurs over a broad frequency range centered at 7 MHz at room temperature. Impact strength and the secondary transition (at impact frequency) both have a maximum value around ?75°C.     

The influence of matrix modification on fracture mechanisms in rubber toughened polymethylmethacrylate

The fracture behavior of composite rubber particle-toughened polymethylmethacrylate has been investigated over a wide range of test speeds, encompassing impact conditions. When the entanglement density of the matrix was increased and its glass transition temperature reduced by copolymerization, there were significant increases in the crack initiation and propagation resistance of the particle-toughened materials at low to intermediate speeds. At impact speeds, on the other hand, where crazing became the dominant matrix microdeformation mechanism in all the materials investigated, the fracture response of the copolymer matrix was closer to that of the polymethylmethacrylate homopolymer, and the toughening effect of the rubber particles was no longer effective in either case. This is discussed in terms of the onset of the matrix β transition, associated with the transition from shear to crazing, and the α transition of the rubber domains, both of which occurred in the temperature range immediately below room temperature in low frequency dynamic torsion measurements.     

Investigation of fracture properties of epoxy at low temperatures

The work of fracture of a number of epoxide resins has been measured at room temperature, 77K, and 4.2K, using a three‐point‐bending method with deeply notched specimens. Various types of yield behavior were noted, and the results and observations have been used to classify resins as brittle or semi‐flexible at room temperature and to record the presence of plasticity even at lower temperatures. For semi‐flexible resins at room temperature, the geometry dependent elastic/plastic contribution to the work of fracture is discussed. This work indicates how the work of fracture and the glass transition temperatures of the resin may be used to guide developments of resins for low temperature applications and possibly to derive a numerical term as a ‘Cracking Index.’ These results were used to explain the observed thermal shock resistance of these materials. A scanning electron microscope (SEM) and optical microscopy were used to investigate the fracture surfaces associated with the different types of yield and fracture behavior. The fracture surface microstructure has been used to explain the observed mechanical behavior, including the existence of plastic deformation in some resins at 77K. The results are of help in understanding the process of crack initiation, growth, and the crack tip deformation processes in epoxide resins.     

Off-axis tensile properties and fracture in a unidirectional graphite/polyimide composite (celion 6000/PMR 15)

Tensile properties of unidirectional Celion 6000 graphite/PMR 15 polyimide composites prepared by hot molding and cold molding processes were measured at room temperature and 316°C, the upper use temperature of the polyimide resin, at both 45 and 90° to the fiber axis. The resulting fractures were characterized by scanning electron microscopy and materialographic techniques. Variation in tensile properties with processing history occurred in the elastic modulus and strain to failure for specimens loaded at 90° at 316°C, and in the fracture stress, and hence the in-plane shear stress, for those loaded at 45° at room temperature. Significant plastic deformation was observed in the 45° orientation at 316°C for material produced by both processing methods. In general, fracture occurred by both failure within the matrix and at the fiber-matrix interface; the degree of interfacial failure increased with temperature. Secondary cracking below the primary fracture surface also was observed.     

The Effect of Modification of an Epoxy Resin Adhesive with ATBN on Peel Strength

The effects of rubber content, rate of peel and temperature on peel strength of ATBN modified DGEBA based epoxy resin adhesives have been investigated. The fracture surfaces of peel test specimens and the distribution of rubber particles in cured bulk epoxy resin have been observed with SEM and TEM, respectively. The mechanical properties of bulk rubber modified epoxy resin have been also measured. The peel strengths increased with increasing rubber content, peel rate, and decreasing temperature. The peel strengths were superposed as a function of rate and temperature. Plots of the shift factors against temperature gave two straight lines, which followed an Arrhenius relationship. The region of temperature below the intersection of the two straight lines, temperature somewhat lower than T_g of epoxy adhesive, gave markedly high peel strengths and a stick-slip failure due to plastic deformation of the adhesive, and a number of micro holes produced by the rupture of rubber micro particles on the fracture surface. The region of temperature above the intersection gave lower peel strengths and an apparent interfacial failure with ductile fracture of the adhesive, and larger, shallow holes or no holes. From these results, the marked increase of peel strength was concluded to be mainly attributed to the plastic or viscoelastic deformation of epoxy matrix, the strong bond at the interface between rubber particles and epoxy matrix, and the dilation and rupture of a number of rubber particles.     

Mechanical properties of polyketone terpolymer/rubber blends

Blends of aliphatic polyketone terpolymer and a core-shell rubber (CSR) were melt processed with varying CSR concentration of 0-40 wt%. The obtained morphology was of finely dispersed CSR particles in the polyketone matrix. The thermal properties of the matrix polymer remained unaffected by the addition of the CSR phase. The crystallinity remained constant at 35 wt% and the melting temperature was not changed. The tensile modulus and yield stress were decreased by the addition of the rubber phase to the aliphatic polyketone polymer. The deformation was strongly delocalised with increasing CSR content. The temperature development during fracture was also strongly reduced with increasing rubber concentration. The CSR phase was found to toughen the aliphatic polyketone matrix very effectively, the brittle to ductile transition temperature was lowered from 90 to −40 °C with the highest rubber concentration (40 wt%). Cavitation experiments revealed that the macroscopic cavitation strain remained constant with increasing rubber content. A study of the deformation zone below the fracture surface showed that voids were produced by cavitation of the rubber phase. The voids were strongly deformed by the plastic deformation of the matrix polymer. At high strain rates a relaxation layer was found below the fracture surface, where the voids were no longer present. This relaxation zone was found to be due to the adiabatic temperature rise of the material during fracture at high strain rates.     

Temperature dependence of effective thermal conductivity and thermal diffusivity of treated and untreated polymer composites

Thermal properties, such as thermal conductivity, thermal diffusivity, and specific heat, of treated and untreated oil palm fiber–reinforced PF composites were measured simultaneously at room temperature and normal pressure using the transient plane source (TPS) technique. An increase in thermal conductivity was observed in the fiber‐treated and resin‐treated composites. Surface modifications of fibers by prealkali, potassium permanganate, and peroxide treatments increased the fiber–matrix adhesion by increasing porosity and pore size of the fiber surfaces. The increase in crosslinking enhanced the thermal conductivity of a composite of resin treated with peroxide compared to other composites. Also an attempt was made to explain the temperature dependence of thermal conductivity and thermal diffusivity of amorphous polymer samples using the same technique. It was observed that at the glass‐transition peak of the polymer, thermal conductivity and diffusivity were maximum. Below and above this temperature their values decreased. This has been explained on the basis of predominant scattering processes. An empirical relationship was established for the theoretical prediction of thermal conductivity and diffusivity. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1708–1714, 2003     

Toughening mechanisms in commercial thermoplastic polyolefin blends

Impact fracture mechanisms in a variety of commercially available and experimental thermoplastic polyolefin (TPO) blends were studied using the double‐notch four‐point‐bend Charpy impact test, followed by microscopy observations. It was shown that the failure mechanisms and the sizes of the subcritically formed crack‐tip damage zone before fracture were quite different among the TPO systems investigated. The room temperature Izod impact strengths of the TPOs investigated were found to correlate qualitatively well with the sizes of the damage zone. At room temperature the main fracture mechanisms observed in TPOs include matrix crazing, particle–matrix debonding, rubber particle internal cavitation, and shear banding. At low temperature (−40°C), the operative fracture mechanisms in TPOs are limited only to crazing, particle cavitation, and debonding. A strategy for improving impact strength without sacrificing scratch/mar resistance of TPOs is discussed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 311–319, 2000     

Depolarization current of a novolac p-fluorophenol-formaldehyde resin

A Novolac p-fluorophenol-formaldehyde (NFF) resin was prepared by condensation of p-fluorophenol with formaldehyde. DSC showed the glass transition effect or coinciding endothermal peak depending upon the thermal history of samples. It is supposed that the peaks are caused by breaking of the intermolecular bondings in the resin during the glass transition. The bondings are formed in the resin during the storage at room temperature. Thermally stimulated depolarization current (TSDC) measurements were carried out in the temperature range of 290 to 350 K, with the samples having an average number molecular weight M?_n of 375 and 434. TSDC curves mainly showed the dipolar relaxation α peaks. The influence of poling temperatures, the influence of M?_n, the activation energy E_a, and the physical ageing of the samples were investigated. Physical ageing was determined as the reciprocal polarizability R_p vs. time of ageing. Samples. with higher M?_n showed a higher glass transition temperature, a lower E_a, and a higher increase in R_p than the sample with lower M?_n. The increase in M?_n increased the rigidness of NFF samples. The effects are attributed to the strong hydrogen bonding. The comparison with analogous results in novolac phenol-formaldehyde resin without fluorine is given.     

Crazing,yielding, and fracture of polymers. I. Ductile brittle transition in polycarbonate

Most thermoplastics far below their glass transition give a brittle fracture when de-formed in uniaxial tension. Bisphenol-A polycarbonates are an exception and deform in a ductile manner. However, it has been observed in Izod impact studies of notched samples that the mode of failure changes from a ductile to a brittle fracture on annealing samples below T_g. It has been found that, when notched samples are stressed, a Griffith type flaw is formed under the notch. The criterion for the ductile brittle transition is evaluated in terms of σG (the stress required to propagate the Griffith flaw), and σ_y, the yield stress for the polymer. It has been found that the density and yield stress for the samples annealed at various temperatures are dependent upon previous thermal history and in particular on the molecular weíAght. On the basis of these measurements, it is concluded that many of the so-called anomalous effects observed with polycarbonate can be explained.     

端环氧基硅油改性环氧树脂性能研究

采用端环氧基硅油及其预反应物来改性双酚A型环氧树脂。采用热分析、扫描电镜和力学性能等测试方法系统探讨了改性方法、有机硅含量对环氧树脂性能的影响。采用端环氧基硅油直接物理共混改性的EP,其耐热性几乎不变,但力学性能下降较大。采用5份端环氧基硅油预反应物改性的EP,其玻璃化转变温度由未改性的163.23 ℃提高到165.90 ℃,拉伸强度几乎保持不变,断裂伸长率由7.6 %提高到16.7 %,冲击强度由20.23 kJ/m2提高到27.19 kJ/m2。拉伸断面的SEM照片表明,环氧树脂固化物显示出明显的增韧效果。     

The Effect of Temperature of Test on the Adhesion of Polyethylene Coatings Applied to Metals as a Hot Melt

The adhesion of polyethylene coatings applied as a hot melt to steel, zinc and copper with various surface pretreatments has been studied over a temperature range from ambient to 70 or 80°C. Tensile properties and tear strength of the polymer itself were measured over the same temperature range. Substrates which give high adhesion at room temperature give a fall in adhesion with temperature. This can be understood in terms of a fall in fracture energy of the polymer as indicated by tensile and tear tests. Substrates which give low adhesion at room temperature show first a significant rise and then a fall in adhesion as temperature is raised. Examination of the fracture surfaces by electron microscopy shows a progressive increase in plastic deformation of the polymer as the adhesion rises. The rise in adhesion and change in failure mode are interpreted in the light of the change in mechanical properties of the polymer. The adhesion maxima are not viscoelastic in origin as time-rate equivalence was not observed.     

Electrical properties of a composite comprising epoxy resin and α-hematite nanorods

Electrical properties of pure epoxy and epoxy-hematite nanorod composites have been investigated. The nanorods were synthesized by the forced hydrolysis method and further mixed with epoxy to obtain the nanocomposite. TEM analysis revealed that they have an average diameter of about 8 nm, with an average aspect ratio of 25. DC-conductivity and DC-current relaxation measurements showed a significant influence of Fe₂O₃ nanorods on the DC-electrical properties of the epoxy matrix. However, the observed effects of the filler below and above the glass transition are different. Because of their high specific surfaces, nanorods affected segmental mobility of epoxy molecules to a large extent, which resulted in an increase in the glass transition temperature (T_g) and a decrease in the real part of dielectric permittivity in high frequency/low temperature region. It is further observed that at elevated temperatures (above T_g) and low frequencies the real part of dielectric permittivity of the nanocomposite exceeds that of the pure matrix, i.e. there is a transition towards microcomposite-like dielectric behaviour.     

Study of jute fiber reinforced polyester composites by dynamic mechanical analysis

Cyanoethylation of jute fibers in the form of nonwoven fabric was studied, and these chemically modified fibers were used to make jute–polyester composites. The dynamic mechanical thermal properties of unsaturated polyester resin (cured) and composites of unmodified and chemically modified jute–polyester were studied by using a dynamic mechanical analyzer over a wide temperature range. The data suggest that the storage modulus and thermal transition temperature of the composites increased enormously due to cyanoethylation of fiber. An increase of the storage modulus of composites, prepared from chemically modified fiber, indicates its higher stiffness as compared to a composite prepared from unmodified fiber. It is also observed that incorporation of jute fiber (both unmodified and modified) with the unsaturated resin reduced the tan δ peak height remarkably. Composites prepared from cyanoethylated jute show better creep resistance at comparatively lower temperatures. On the contrary, a reversed phenomenon is observed at higher temperatures (120°C and above). Scanning electron micrographs of tensile fracture surfaces of unmodified and modified jute–polyester composites clearly demonstrate better fiber–matrix bonding in the case of the latter. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1505–1513, 1999     

New aspects of aging in epoxy networks. II. Hydrothermal aging

An amine cured epoxy is prepared in two different network states for hydrothermal aging. The “low‐crosslinked” network has a considerable amount of residual reactive groups and a relatively high‐molecular mobility. The low‐crosslinked matrix contains high‐crosslinked regions. In contrast, the “highly crosslinked” epoxy system has little reactive groups and a lower molecular mobility. Here, low‐crosslinked regions are found in a high‐crosslinked matrix. Hydrothermal loading for both networks is performed in demineralized water at temperatures below their glass transition. The water plasticizes both kinds of networks which remain in the glassy state, however. As a consequence, in the low‐crosslinked epoxy, the increased molecular mobility promotes an ongoing curing reaction leading to the consumption of epoxy groups until an almost complete network has formed. As a new aging process, phase separation occurs in the highly crosslinked epoxy. The new phase is more mobile than the matrix because it has its own glass transition at a lower temperature. In addition, thermooxidative degradation is observed for both network states. Certainly, these chemical and structural changes in the epoxy networks should influence the performance of an adhesive joint, a coating, or a fiber‐reinforced composite. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 369–377, 2004     

Chemically stable polyphenylene ether microcapsules prepared using a facile method for the self‐healing of polymers

Chemically stable polyphenylene ether (PPO) microcapsules (MCs) filled with epoxy resins (PPO‐EP MCs) were prepared using low‐molecular‐weight PPO with vinyl end‐groups as shell wall and epoxy resins as core material using an oil‐in‐water emulsion solvent evaporation method. This method for synthesizing MCs with PPO shell walls is simple, convenient and novel, which can avoid the influence of processing parameters on the chemical stability of the epoxy resin core material. The resulting PPO‐EP MCs exhibit good chemical stability below 255 °C mainly owing to the absence of a polymerization catalyst of the epoxy resins. The initial thermal decomposition temperature of the MCs is about 275 °C. The MCs were embedded in a 4,4′‐bismaleimidodiphenylmethane/O,O′‐diallylbisphenol A (BMI/BA) thermosetting resin system. When processed at high temperature (up to 220 °C), the microencapsulated epoxy resins could be released from the fractured MCs to matrix crack surfaces and bond the crack surfaces. An amount of 8 wt% MCs restored 91 and 112% of the original fracture toughness of the BMI/BA matrix when heated at 220 °C/2 h and 80 °C/1 h + 220 °C/2 h, respectively. The MCs only slightly decreased the thermal property of the matrix. © 2016 Society of Chemical Industry     

The synthesis of a secondary branched epoxy-modified silicone resin and its application at low temperature

To enhance the fracture toughness of epoxy resin at low temperature, a secondary branched epoxy-terminated silicone resin (ESR-6) was synthesized and incorporated into bisphenol A epoxy resin at different contents. The structure of ESR-6 was characterized by Fourier transform infrared spectroscopy and nuclear magnetic resonance (¹H NMR), and the fracture surface of the composites was observed by scanning electron microscope (SEM) and atomic force microscopy. At room temperature and − 70°C, the maximum values of elongation at break were 15.78% and 12.55% with 10 wt% ESR. Compared with those of neat epoxy resin, the values of elongation at break of the composite were increased by 50.86% and 36.12%. The results of dynamic mechanical analysis also showed that the toughness of the modified resin had been improved. The SEM images of the fracture surfaces suggested that the fracture mode of the modified resin changed from brittle one to plastic one because of the addition of ESR-6, which further confirmed the toughening effect of ESR-6. These research results may provide a new strategy for enhancing the low-temperature toughness of epoxy resins.     

Influence of pyrolysis temperature on fracture response in SiOC based composites reinforced by basalt woven fabric

The fracture resistance of ceramic based composites reinforced by various ceramic fibres can be dramatically enhanced when an efficient fracture mechanism takes place during the crack propagation. Presented work shows an effect of the pyrolysis temperature of the composite matrix on the fracture behaviour of the composite. The matrix is formed from the polysiloxane resin precursor and the reinforcement is a basalt woven fabric. The temperature range under investigation was from 600 °C, where the onset of fracture properties were observed up to 800 °C. Above this temperature basalt fibres suffer by rapid degradation of the microstructure. The optimum stage of the polysiloxane resin transformation maximizing the fracture resistance of the composite was identified. The fractographic analysis of the fracture surfaces revealed the differences in the acting fracture mechanism.     

Einfluß des Verstreckens und Temperns auf das Dehnungs- und Bruchverhalten von Polyäthylenterephthalat

The stress-strain diagrams of undrawn and drawn polyethylene terephthalate were measured at room temperature and above the glass transition temperature. Before the stress-strain measurements the undrawn samples had been crystallized at various temperatures, whereas the drawn samples had been crystallized in the undrawn state, then were drawn at various temperatures and finally were crystallized again. The influence of the temperature of crystallization and the temperature of drawing on the Young's modulus, the tensile strength, and the fracture strain were of special interest. The fracture strain as a function of the crystallization temperature shows a minimum at room temperature. This minimum disappears above the glass transition temperature. Young's modulus and tensile strength generally are found the higher, the higher the degree of orientation in the sample. Crystallization of the undrawn samples therefore does not change these values significantly. But a drawing of the samples leads to a significant increase which is still more pronounced if the sample is crystallized after the drawing. Crystallization before drawing of a sample leads to a decrease of Young's modulus and tensile strength because in this case apparently the formation of a sufficient orientation during the drawing cannot take place. An increase of the drawing temperature above the glass transition temperature also leads to a decrease in the mentioned values.     

Epoxy/alumina nanoparticle composites. I. Dynamic mechanical behavior

We studied the influence that alumina nanoparticle addition has on the dynamic mechanical spectra of an amino‐cured epoxy resin. A suppression of the short‐scale cooperative motions related to the β relaxation and an increase in the activation energy for the β relaxation of the epoxy matrix was observed as the alumina content increased. This is explained in terms of an antiplasticization effect of the alumina nanoparticles on the epoxy resin. An estimation of the effective thickness of the nanoparticle–matrix interfacial region was done based on the reduced damping. The dependence of the composites' reduced modulus on the alumina nanoparticles content is very well fitted by the generalized Kerner equation. The best‐fit parameter values suggest the presence of small and strong agglomerates in the composites at room temperature. At temperatures above the T_g, these agglomerates start to behave as weak ones because of the polymer matrix softening and particle–particle and particle–matrix slippage and friction. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3774–3785, 2003