Fracture surface morphology in thermosets modified with hollow microspheres

Fracture surface morphology in relation with toughening of thermosets modified with hollow microspheres was studied. Two different toughening methods were employed—one was with (MEH) and the other without (ME) compressive residual stresses around microspheres, respectively. The compressive residual stresses were to increase effective stress intensity factor. Various conditions arising from toughening method, properties of constituent materials, bonding between matrix and microspheres, relativity between bonding and microsphere strengths, and plane stress/strain were derived for part of generalization of fracture surface morphology. Mohr circle representations were employed for relative stress components analysis. New deformation mechanisms contributing to toughening were proposed. A major difference in toughening mechanism between ME and MEH methods was found to be in the location of plastic deformation under plane strain. The plastic deformation of ME was dominantly in matrix and appeared in the form of matrix cavitation. In the case of MEH, it was dominantly in microspheres. It was suggested that compressive residual stress promotes plastic deformation of microspheres caused by “extrusion” effect. The microsphere deformation in MEH was found also under plane stress although it was not as much as under plane strain. Matrix cavitation in ME under plane stress, however, was not found. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Fracture properties of thermosets

Fracture surface energy (FSE) measurements using a cleavage technique gave values of 42J/m² for the polyester, 80J/m² for the epoxide and 310J/m² for a rubber-modified epoxide with standard errors of 6J/m², 5J/m² and 10J/m² respectively. A modified method of calculation was introduced to accommodate the considerable within-sample variability of FSE of some samples. The effect of water on unmodified thermosets tested was examined and found to be relatively small.     

Curing characteristics of particulate-filled thermosets

A model to describe the effect of participate fillers on the heat transfer and curing characteristics of a thermosetting polymer is presented. It is assumed that the reaction kinetics are not altered by the addition of the filler and that the thermal characteristics of the composite are adequately described by the “spherical inclusion model.” The model is used to examine the effect of fillers on several important molding characteristics: time-temperature profiles, peak temperatures, extent of reaction, and gel time. Results are presented for the curing of a typical polyester resin with varying filler content and filler type (glass, calcium carbonate, and aluminum). These results indicate that:

• mold cycle time can be reduced at higher filler content by increasing the mold temperature without inducing thermal degradation, and
• the progression of cure fronts through the thickness of the mold is significantly altered with increasing filler content; at low filler content the cure front progresses from the centerline toward the wall; at higher filler content, the cure front progresses from the wall toward the centerline.

     

Fracture surface characteristics and impact properties of poly(butylene terephthalate)

In this article, the relationship between fracture surface feature and impact properties of poly(butylene terephthalate) (PBT) was investigated. The results indicated that the fracture surface morphology of notched impact specimens tested in the temperature range from 196 to 180 °C could be differentiated into brittle (T ≤ 20 °C) and ductile appearances (T > 20 °C). The fracture surface roughness was characterized by surface roughness ratio (R _s) and fractal dimension (D _b). The fracture mode significantly influenced the relationship between impact strength and fracture surface roughness. When PBT fractured in a brittle mode, both the measured values of R _s and D _b could correspond to impact strength appropriately. On the contrary, when PBT fractured in a ductile mode, their relationship became not statistically significant because the area of the plastic deformation zone instead of fracture surface roughness might be the major factor influencing impact strength.     

热固性塑料的回收利用

对于热固性复合材料废弃物的回收与利用问题有不同的看法。有人认为只有热塑性复合材料才能直接进行回收与利用而热固性复合材料则不能,其理由是热塑性复合材料中的聚合物不发生化学交联,它们可以重新熔化和加工成新产品。而热固性复合材料中的聚合物产生化学交联生成体型网状结构,这种     

Fracture toughness of highly ordered liquid crystalline epoxy thermosets achieved by optimized composition of curing agents

Liquid crystalline (LC) epoxy resin was prepared by using different compositions of aromatic amine as curing agents, in order to control curing rates and chemical compositions. The progress of the curing reaction was investigated based on the gel fraction and epoxy groups of conversion determined by Fourier-transform infrared spectroscopy. The ordered networked polymer structure was analyzed by polarized optical microscopy and X-ray diffraction. Highly oriented network chains in the obtained epoxy thermosets were promoted by the incorporation of flexible chains in the network and the provision of sufficient time for vitrification. Furthermore, it was clarified that a curing temperature higher than T_g is required to promote the transition to the smectic LC phase in order to prepare highly ordered epoxy thermosets. The increase in the formed smectic LC phase in the network chains resulted in significant higher fracture toughness and achieved up to 2.7 times higher value.     

热固性塑料的气辅注射新工艺

流体辅助注射工艺正逐步应用于注射加工热固性塑料制件,一种新式的“排气”技术对气体辅助注射热固性塑料非常有效,应用该技术加工的首批商业化产品是冰箱门的中空拉手。     

Property analysis of triglyceride-based thermosets

Triglycerides with acrylate functionality were prepared from various oils and model triglycerides. The triglyceride-acrylates were homopolymerized and copolymerized with styrene. The cross-link densities of the resulting polymer networks were predicted utilizing the Flory-Stockmayer theory. Although the model predictions overestimated the cross-link density, the trends in the cross-link density predictions matched the experimental results. In both cases, the cross-link density was found to increase gradually at low levels of acrylation and then linearly at higher levels of acrylation. The deviation in the experimental results and model predictions were the result of intramolecular cross-linking. Approximately 0.5 and 0.8 acrylates per triglyceride were lost to intramolecular cyclization for homopolymerized triglyceride-acrylates and triglycerides copolymerized with styrene, respectively. The glass transition temperature (T_g) increased approximately linearly with the cross-link density from as low as −50 °C to as high as 92 °C. Simple models accurately predicted the effect of cross-link density on T_g. The tensile strength and modulus of triglyceride-based polymers increased exponentially at low levels of acrylate functionality, but increased linearly at higher levels of acrylate functionality, as predicted by vector percolation theory.     

Fracture surface features of polyethylene pipe samples

Slow crack growth behavior of medium-density polyethylene pipe samples has been investigated by applying standard and nonstandard fracture mechanics test methods. Slow crack growth rate varied by several orders of magnitude among the PE2306 materials from different manufacturers. Slow crack generated surfaces exhibit fibrous textures of varying degrees of coarseness when examined under the scanning electron microscope. When the measured fiber heights, δ/2, of these samples were plotted against the applied stress intensity factor, K_I, they showed a simple power law relationship (of exponent 2). Assuming that the pulled-out fibers transmit almost uniform tensile stresses across the crazed zone at the crack front, the Dugdale-Barenblatt model was introduced to predict the fiber height. Although the model needs further modification, it provides a general power law relationship between the fiber height and the applied stress intensity factor.     

The injection molding of thermosets

Current industry trends toward increasing automation, as well as increased flexibility in material selection have focused a strong spot light on thermoset injection molding. The growth of thermoset injection molding is reviewed comparatively with the technical history and growth of thermoplastic injection. The progress we see to date and the promise for the future in material characteristics and properties, mold design features, equipment features, changing concepts and new thoughts are discussed.     

Continuous ultrasonic devulcanization of unfilled butyl rubber

The devulcanization of resin‐cured unfilled butyl rubber with a grooved‐barrel ultrasonic reactor under various processing conditions was carried out. The experiments indicated that, because of the lower unsaturation and good thermal stability of butyl rubber, its devulcanization could be successfully accomplished only under severe ultrasonic‐treatment conditions. Gel permeation chromatography measurements were carried out for the virgin gum and sol part of devulcanized samples to study the changes in the rubber network during the devulcanization process. The obtained data showed a significant molecular weight reduction and a broadening of the molecular weight distribution upon devulcanization, which indicated that the devulcanization and degradation of butyl rubber occurred simultaneously. The rheological properties showed that devulcanized butyl rubber was more elastic than the virgin gum. The vulcanizates of the devulcanized butyl rubber showed mechanical properties comparable to those of the virgin vulcanizate. The thermal behaviors of the virgin and devulcanized butyl rubber were different and were correlated to the double‐bond content. The structural characteristics of the devulcanized butyl rubber were simulated with the Dobson–Gordon theory of rubber network statistics. A fairly good agreement between the experimental data and theoretical prediction was achieved. The simulation of devulcanized butyl rubber indicated that the rate of crosslink rupture was much higher than that of the main chain. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1316–1325, 2004     

Chemorheology of unfilled and filled epoxy resins

Filled thermosetting resins used for adhesives, sealants, and coatings are two-phase systems whose rheological behavior is determined by the properties of both phases as well as by the interactions between the matrix and the filler. The chemorheological changes during the crosslinking of an epoxy system (EPON 828 with curing agent Y from Shell Chemical Co.) were investigated. The influence of an untreated silica filler on the reaction kinetics and the Rheological behavior of the system were also analyzed. The concentration of the filler was varied between 0 and 17.25 vol%. The influence of the filler on the gel point was explained in terms of the accelerating effect of the filler on the reaction kinetics.     

Continuous ultrasonic devulcanization of unfilled NR vulcanizates

Sulfur-cured unfilled natural rubber (NR) is successfully devulcanized in a continuous extrusion process under the application of high-power ultrasonic energy. The die characteristics and ultrasonic power consumption are measured. A unique correlation is found between the crosslink density and gel fraction of the devulcanized NR. This correlation is independent of the processing parameters, such as barrel temperature, die gap, flow rate, and amplitude of ultrasound. However, these parameters do influence the degree of devulcanization. In most cases, the degree of devulcanization is found to pass through a maximum at an intermediate level of ultrasonic energy. It is hypothesized that simultaneous breakup and reformation of crosslinks occur during the devulcanization of NR, with the relative contribution of each being determined by the process parameters. The cure curves and mechanical properties of the revulcanized NR are studied. The mechanical properties are found to depend on the revulcanization recipe. On optimizing it, tensile strength as high as 14.2 MPa is achieved, which is about 70% of that of the virgin NR vulcanizate. Ultimate elongation as high as 670% is obtained, which is the same as that of the virgin NR vulcanizate. Such stress–strain behavior is an indication that the devulcanized NR maintains the strain-induced crystallization characteristics inherent to the virgin NR vulcanizates. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2007–2019, 1998     

Flame retardant novolac-bisphthalonitrile structural thermosets

Void-free, thermoset networks prepared from phenolic novolacs cured with bisphthalonitrile (BPh) reagents display attractive mechanical, thermal, and fire characteristics. The network properties depend strongly on the novolac-BPh composition. Networks containing 15 wt[percnt] BPh or greater had T_gs above 180 °C and good fracture toughness (K_1c∼0.8 MPa m^1/2). Cone calorimetry results for an 80:20 (wt/wt) novolac/BPh network, at an incident heat flux of 50 kW/m², demonstrated a peak heat release rate of 137 kW/m² and char yields [gt ]50[percnt]. These excellent flame properties are similar to those of phenolic resole networks, which are brittle but known to be flame retardant.The cure reaction between the novolac and BPh was monitored by the disappearance of the nitrile peak at 2200 cm⁻¹ in the FTIR. The nitrile was ≈90[percnt] consumed after 1 h at 200 °C and ∼20 min at 220 °C.The molecular structure of a novolac-BPh crosslink was studied in model reactions using monofunctional phenols. 2-Hydroxydiphenylmethane was reacted with BPh in the melt at 200 °C for 3 h using a 25:1 molar ratio (6.1:1 eq. ratio) of 2-hydroxydiphenylmethane/BPh. FTIR and ¹H NMR suggested that the product generated from the model melt reaction contained a diiminoisoindoline structure.     

应该给热固性塑料一个公平的地位

新辟栏目《观点》,并不代表本刊的观点。《观点》乃一家之言,对错否？有无益？相信读者的分析能力和辨别能力。原汗原味,不妨看看。     

Continuous ultrasonic devulcanization of unfilled butadiene rubber

The devulcanization of sulfur‐cured unfilled butadiene rubber (BR) with a grooved‐barrel ultrasonic reactor under various processing conditions was carried out. The experiments indicated that BR had a narrow devulcanization window. Outside this window, significant degradation or no devulcanization occurred. Gel permeation chromatography (GPC) measurements were carried out with the sol part of virgin and devulcanized samples to study the breakdown of the polymeric chains. The GPC data showed a significant molecular weight reduction and a broadening of the molecular weight distribution upon devulcanization, indicating that the devulcanization and degradation of BR occurred simultaneously. The rheological properties showed that devulcanized BR was more elastic than the virgin gum. The vulcanizates of the blends of virgin and devulcanized BR showed a considerable enhancement of the mechanical properties. The thermal behaviors of the virgin and devulcanized BR were found to be different. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1166–1174, 2004     

Tailoring the viscoelasticity of epoxy thermosets

This research work is aimed to understand the effect of epoxy resin chemistry on the viscoelasticity of the cured epoxy thermosets. In this article, two model systems are selected based on epoxy‐amine reactants. In the model Systems I, the functionality of the epoxy is varied. In model System II, the pendant (side) chain length in the amine is varied. It is found that by varying the initial properties (e.g., functionality, pendant chain length, mixing ratio (r), aromaticity, etc.) the network properties (crosslink density and flexibility) of the cured (or hardened) epoxy changed. The changes (or shift) in the viscoelastic properties (or viscoelasticity) of the cured epoxy is mainly due the changes in the network properties. Further, to study the time and temperature effects the viscoelastic master curves for the two model systems are generated using Time‐Temperature‐Superposition (TTS) principle. The shift in the viscoelastic master curves is modeled with a simple Kohlrausch‐Williams‐Watts (KWW) fit function. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013