Polyester blends may undergo transesterification during processing, resulting in molecular rearrangements, transient properties, and eventually, degradation. To suppress transesterifcation, the use of organophosphites has been suggested in the patent and technical literature. The effectiveness of organophosphites, however, is variable and sometimes inconsistent. Our recent studies suggest a clue to the inconsistent behavior and provide a simple way to enhance the effectiveness of these stabilizers. Using solid state 31P NMR it was shown that for bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite a conversion of the phosphite group to a phosphonate, via hydrolysis, is a prerequisite for a n effective inhibition of transesterification. This conversion occurs readily during melt compounding if the polymers are not completely dry. However, if rigorous drying is employed and phosphite conversion does not occur, then transesterification is not arrested. It was also found that over a period of time the conversion of the phosphite to a phosphonate may take place at room temperature as well. After aging for about a year in the laboratory, the originally ineffective compound, has become a very effective inhibitor of transesterification in blends containing poly(ethy1ene terephthalate), poly(buty1ene terephthalate), polycarbonate, and polyarylate. Thus, a simple way to enhance the phosphite effectiveness is to expose it to a humid environment prior to blending.
synopsisA two-stage emulsion polymerization procedure has been developed and used to prepare relatively uniform populations of heterogeneous acrylic latex particles (HLP). One class of particles (HLPl) can be described as composite materials comprising a glassy continuous phase and a rubbery discrete phase. Another class (HLPP) can be described (at high rubber content) as composite materials comprising a rubbery continuous phase and a glassy discrete phase. The phase structure of the HLPl is sufficiently stable to allow fabrication of composites having a uniform spatial distribution of inclusions by direct compression molding. Although the observed particle structure of the HLPP does not depend markedly on crosslinking, the phase structure and mechanical properties of compression moldings do. Crosslinking of the glassy stage appears to stabilize HLPS phase structure during molding, while crosslinking of the rubbery stage favors phase inversion. The observed HLPP particle structures and the morphology of molded HLPl specimens are consistent with a shell-core model. It is found that the modulus and thermal expansion coefficient of many of these materials can be adequately described in terms of a simple theoretical model for the elastic and thermoelastic properties of particulate composites, provided that an interaction parameter interpreted as a maximum packing fraction is introduced.
SynopsisThe physical properties of heterogeneous polymer-polymer composites such as rubbermodified plastics depend not only on the properties of the constituent polymers, but also on structural characteristics such as phase geometry and morphology. The dependence of the viscoelastic properties of particulate composites on phase morphology and composition has been treated in a previous paper. This analysis is further modified and applied to dynamic Young's modulus ( E * ) data on several types of heterogeneous acrylic polyblends prepared in this laboratory. By taking into account interparticle interactions, copolymerization effects, and phase inversion, a nearly quantitative representation of E* is obtained. Deviations from calculated behavior can be explained by assuming more complicated models of phase structure and composition and by postulating higher-order and temperature-dependent interaction effects.
The phase behavior and mechanical properties of a series of polyarylate/polycarbonate blends were studied. The polymers are known to transesterify, the extent of which depends upon the thermal and shear history and affects phase behavior and properties. Single screw extrusion, twin screw extrusion, and solution casting were employed for blend preparation. Two transition temperatures, corresponding to a polycarbonate‐rich phase and to a polyarylate‐rich phase, were seen in blends that were solution cast or compounded in a single screw extruder at 285°C. But after injection molding a single Tg was observed, When annealed at 180°C for several hours the molded blend was found to phase separate. Blends that were compounded in a twin screw extruder exhibited a single Tg and could not be phase separated. The flexural and tensile properties of blends that were prepared in a twin screw extruder show a small positive synergism. But the impact properties were substantially below the rule of mixtures values, probably the result of advanced exchange reaction and thermal degradation.
SynopsisEster-exchange reactions during melt extrusion of polyarylate, polycarbonate, and polyethylene terephthalate ternary blends result in a decrease of the melting point and the heat of fusion of the poly (ethylene terephthalate) . When an organophosphite is used to suppress the ester-exchange reactions, it was found that whenever the phosphite is converted into a phosphonate, there is little change in both the melting point and the heat of fusion of the polyethylene terephthalate during prolonged exposure to high temperature in the blends. 31P nuclear magnetic resonance is employed to investigate the chemical changes and differential scanning calorimetry to evaluate the thermal stability of the blends. The organophosphite appears to be transformed through side-chain hydrolysis into other phosphorous species, such as diphosphonate in order to be an effective suppressant of the ester-exchange reactions.
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