SynopsisThe kinetics of simultaneous thermal and hydrolytic degradation of poly( 1,P-cyclohexylenedimethylene terephthalate) (PCHDT) were evaluated by using a 1.5-in.-diam. melt extruder (=20/1 length/diameter ratio) aa a reactor. The effects of extrusion temperature (295°-3300C), residence time (2.6-17.5 min), and moisture content (<0.001% to 0.2%) of the supply polymer on degradation were determined. The rate of degradation was measured in terms of the rate a t which inherent viscosity (I.V.) decreased and the rate at which carboxyl endgroup concentration increased. The contributions of both thermal and hydrolytic degradation to the total degradation of PCHDT could be separated because the hydrolysis was rapid enough that it could be considered as occurring prior to thermal degradation. Thus, the hydrolysis merely adjusted the initial properties of the supply polymer, which was then subjected to thermal degradation. Equations were developed from an analysis of the kinetic data based on a random chain scission mechanism. The activation energies for decrease in I.V. and increase in carboxyl endgroup concentration of PCHDT from thermal degradation were determined as 33.5 and 41 kcal/mole, respectively. INTRODUCTIONPoly( 1,4-cyolohexylenedimethylene terephthalate) (PCHDT) is a synthetic polyester which was discovered by Tennessee Eastman Company1 and became the basis for Kodel polyester fiber. It currently finds extensive use as a carpet fiber.During the melt extrusion of PCHDT to produce fibers, the extrusion temperature and the moisture in the supply polymer cause degradation. In this paper, we discuss the thermal and hydrolytic degradation of this polyester during passage through a 1.Bin.-diam. melt extruder.In earlier work, Gregory and Watson used a laboratory extruder to determine the effects of oxygen and temperature on the degradation of PCHDT.2 They found that the oxygen content of the gas used to purge the polymer during drying prior to extrusion had no measurable effect on degradation when I.V. and carboxyl endgroup concentration were used as responses. Gregory and Watson2 also showed that the overdl kinetio equations describing thermal degradation of PCHDT were in agreement with a random chain scission mechanism. 3254 WAMPLER AND GREGORYSeveral workers have investigated the thermal -degradation of poly-(ethylene terephthalate) (PET) 3--6 and other p~lyesters.~ Investigations by Ritchie3 on the mechanism of thermal degradation of PET, using related model compounds, showed that the initial breakdown in the decomposition is a primary alkyl-oxygen scission of the P-hydrogen type. Poh14 interpreted his chemical and infrared examinations of the products of degradation of PET as typical of random chain scission to produce new endgroups and shorter chains. Marshall and Todd6 studied the kinetics of degradation of PET in an oxygen-free atmosphere by measuring the change of melt viscosity as a function of time. They calculated the initial rate of degradation from the decrease in melt viscosity and found it to be ...
Shear sensitivities of molten polymers may be determined from measurements made on individual samples subjected in one test to a series of either increasing or decreasing shear rates. Many polymers, including polyesters, degrade when molten. If the effect of degradation is significant, and is ignored during data analysis, a significant error could result. The magnitude of the error depends directly on the magnitude of the reaction rate constant for degradation, which differs from polymer to polymer and which increases as temperature increases. In rheological characterization of molten polymers, one should know or determine the degradation behavior of the polymer being investigated and then account for it in the data analysis. Experimental results for poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) are presented and discussed. Ways to account for degradation in shear sensitivity measurements are also presented.
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