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.
The morphology, rheology, and mechanical properties of blends of polysulfone (PSF) with up to 65% of a wholly aromatic liquid crystalline polymer (LCP) were investigated. In injection molded specimens a skin-core morphology was observed with the LCP minor phase oriented in the skin and globular in the core. Scanning electron microscopy of fractured surfaces showed sharp phase boundaries, suggesting low interfacial adhesion. The neat PSF and blends with low amounts of LCP exhibited a low shear Newtonian plateau not observed in the blends with high LCP levels. The addition of LCP to PSF resulted in an increase in stiffness, a small increase in tensile strength, and a significant improvement in processability.
The hydrolytic stability of a new commercial polycarbonate (Calibre 300, Dow Chemical USA) was investigated and compared with that of other commercial polycarbonates. The tests were conducted between 56% and 95% relative humidity (R. H.) at 100°C. Also performed were water immersion tests at 80 and 100°C. The water diffusivity was found to be 8.7 × 10−7 cm2/s at 100°C with an activation energy of 7.9 kcal/mole. These values are similar to other glassy polymers. The equilibrium water sorption, C∞, was found to increase with temperature and R.H. The isotherm at 100°C was determined to be: C∞ = 0.005945 [R.H.]. When samples immersed in a water bath at 100°C were transferred into room‐temperature water, visible aqueous microcavities were formed due to the condition of super‐saturation, and under stress may become crack initiation sites. For the polycarbonate investigated here, it was found that the decrease in weight‐average molecular weight (M̄)w was a first‐order process under a constant R.H. and temperature, and that hydrolytic embrittlement, i. e., (M̄)w <34,000, was reached after ca. 188, 143, 99, and 66 days under 56%, 73%, 87%, and 95% R.H., respectively, at 100°C. A comparison with reported hydrolytic stability data for other polycarbonates showed large differences in their stability which are believed to be due to the extent of end‐group capping (over 95% in Calibre 300) and resin purity: both phenolic end‐groups and some additives (i.e., fire retardants, thermal stabilizers) are known to accelerate hydrolytic degradation.
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.
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