[en] The thermal degradation behavior of a commercial epoxy resin, EpoFix (Struers), has been investigated by thermogravimetry (TG), differential thermal gravimetry (DTG), and differential thermal analysis (DTA) under nonisothermal conditions in an argon atmosphere. Different methods (Kissinger, Flynn-Wall-Ozawa (FWO), Friedman iso-conversion methods, and nonlinear least-squares (NLSQ) estimation method) have been used to analyze the thermal degradation process and determine the apparent kinetic parameters. The methods produce similar results in terms of activation energy estimations. Nevertheless, the NLSQ method has several advantages over the other methods in terms of both characterizing the activation energy and modeling the thermal degradation - i.e., including this model in a resin degradation process simulation. However, it is interesting to combine the NLSQ method with other iso-conversion methods: they can reflect the dependence and variability of the activation energies during pyrolysis processes, while providing a good starting point for a nonlinear procedure, especially with respect to the activation energy E. This work is the first step (apparent kinetic reaction) of complete simulation of experimental oven of degradation of epoxy resin coating of impregnate nuclear fuel sample. (authors)

[en] The dielectric loss behavior of crosslinked polyethylene has been determined over the frequency range 10-5 khz and over the temperature range -150 to 100⁰C. Major loss effects occur at the lower temperatures and are caused by liberation of the organic byproducts of the crosslinking agent, dicumyl peroxide. Analog materials prepared from linear low density polyethylene by blending in of either acetophenone or 2-phenyl-2-propanol confirm the hypothesis. The effects of concentration of byproducts and of orientation of the polymer have been studied. Activation energy analyses suggest that room temperature losses will occur in the microwave region. The behavior is significantly affected by curing with steam, typical loss peaks being an order of magnitude lower than for dry cured specimens. A new loss region appears close to but below, 0⁰C

[en] Degradation mechanisms have been investigated for graphite/polysulfone and graphite/epoxy laminates exposed to ultraviolet and high-energy electron radiations in vacuum up to 960 equivalent sun hours and 10⁹ rads, respectively. Based on GC and combined GC/MS analysis of volatile by-products evolved during irradiation, several free radical mechanisms of composite degradation have been identified. All the composite materials evaluated have shown high electron radiation stability and relatively low ultraviolet stability as indicated by low G values and high quantum yields for gas formation. Mechanical property measurements of irradiated samples did not reveal significant changes, with the possible exception of UV exposed polysulfone laminates. Hydrogen and methane have been identified as the main byproducts of irradiation, along with unexpectedly high levels of CO and CO₂. Initial G values for methane relative to hydrogen formation are higher in the presence of isopropylidene linkages, which occur in bisphenol-A resins

[en] The radiation crosslinking of poly(vinyl chloride), PVC, with trimethylolpropane trimethacrylate, TMPTMA, has been examined. The polyfunctional TMPTMA undergoes rapid polymerization incorporating the PVC into a three-dimensional network. The kinetics and mechanism of these crosslinking reactions were studied with particular reference to dose dependence and thermal treatment. The gel was rapidly formed with a TMPTMA polymerization rate greater than that of the PVC grafting reaction. Only 30 to 40% of the available bonds were used in the initial polymerization. The remaining 60 to 70% of the double bonds predominantly react in the final stages of crosslinking (80 to 100% gelation). The macroscopic properties (e.g., solubility, glass transition temperatures, mechanical characteristics, etc.) of the PVC-TMPTMA blend are discussed in terms of the molecular crosslinking mechanisms. The effect of thermal treatment, during and after irradiation, on the reaction rates and mechanism is examined

[en] The water vapor sorption--desorption isotherms of graft copolymers of methyl methacrylate with natural and chemically modified wool (i.e., oxidized, reduced, methylated, and crosslinked) have been investigated. As a result of graft copolymerization a reduction in water absorption was observed. The results have also been analyzed in the light of Hailwood--Horrobin equations

[en] Properties of electron beam-cured film from 100% reactive mixtures consisting of polyester diacrylourethane oligomer, copolymerizing monomer, and chain transfer agents were investigated. The inclusion of chain transfer agent in such polymerization reaction was aimed to reducing the degree of polymerization of the double at each end of the oligomer, which is believed to be responsible for the low extensibility of these films. In general, these agents were found to result in some retardation in the polymerization reaction, but none of them influenced the mechanical properties of the cured films except mercapto-functional agents, which were found to drastically affect these properties. By the addition of a mere 2 to 3% of these agents it was possile to toughen the EB-cured films by increasing both the extensibility and breaking strength. This confirms the above speculation and brings the properties of these films to a level almost comparable to those of linear polyurethanes. 17 figures, 2 tables

[en] Poly(methyl methacrylate)--lead silicate composites were prepared with different amounts of crystalline or amorphous PbO--SiO₂ binary composition (70 mole % PbO). The polymerization reaction was carried out in aqueous medium and in methanol--water mixture at 40⁰C using sodium bisulfite as initiator. The presence of lead silicate was found to increase the molecular weight of the poly(methyl methacrylate). The prepared composites were characterized by studying their shielding properties to γ radiation, mechanical hardness, dielectric constant, and thermal stability. It was found that lead silicate increases the absorbing power of the composites to γ radiation. This behavior was found to be dependent on the amount and the type of lead silicate (amorphous or crystalline). Gamma irradiation of the composites was found to cause chemical degradation of the poly(methyl methacrylate) leading to a decrease in mechanical hardness. Molecular weights of the poly(methyl methacrylate) for some of the composites were determined before and after irradiation viscosimetrically. Dielectric constants for some of the composites were determined at two different temperatures. The thermal stability of the composites was studied by means of an automatic thermogravimetric analyzer. 2 figures, 5 tables

[en] Various benzoxazoles were synthesized and investigated by mass spectrometry. The relative stabilities of their molecular ions were used in the search for radiostable polybenzoxazoles (PBO). The three polybenzoxazoles corresponding to the three bibenzoxazoles previously investigated were irradiated by 900 keV electrons, and their aging was followed in situ by optical measurements. The most radiostable polymer corresponds to the model compound (diphenylbibenzoxazole), which is characterized by the highest degree of conjugation and by the lowest number of fragmentations in mass spectrometry. The resistance of this new polymer to electron irradiation is better than that of Teflon FEP or Kapton H

[en] X-ray diffraction patterns of the α- and β-crystalline phases of natural, chemically modified, and MMA-grafted wool fibers were investigated. In the grafted fibers variation of the equatorial and meridional scattering intensity during the α-β transformation was inhibited. This indicated that the grafting had stabilized the structure to the α-β transformation. The average crystallite size was found to increase with the extent of grafting, and the degree of orientation appeared to decrease with percent graft-on. SEM studies revealed little damage to the surface topography of wool fibers after chemical treatments. Graft copolymerization with MMA made the fiber surface regular and smooth, mainly because of deposition of polymer in the escarpment, thus reducing the sharpness of the scaly structure. Scanning electron micrographs of the peeled-off surface and cross sections of the grafted fibers indicated that a significant amount of polymer was deposited in the medullae of the fibers

[en] Cracks were found on the surface of drawn nylon 6 filaments irradiated with ultraviolet (UV) light from a mercury lamp under various humidities at room temperature. The cracks were formed perpendicular to the fiber axis and were of varying sizes. No cracks were observed on undrawn filaments or drawn filaments exposed to UV light in a dry atmosphere. Considerable shrinkage was found in drawn samples by thermomechanical analysis indicating the presence of residual stress in the material. The cracking is explained in terms of the residual stress and plasticization by moisture. The presence of water appears to play an important role in the formation of cracks on nylon fibers which have been subjected either to UV light or to ν-ray irradiation