Search Results
Abstract
A short description of the thermal analysis methods used for the investigation of polymer degradation processes is given. The fields of application of TG, DTA and DSC for polymers are presented. A kinetic approach to the thermal degradation processes is also given.
The usual method of the evaluation of the thermogravimetry of a polymer is shown to be of a questionable value in the case of the random scission degradation. Several physically founded approximations of this type of degradation are given for the homopolymer and for the co-polymer or the co-polycondensate of two differently evaporating monomers. The accuracy of some of these approximations is tested by the mathematical simulation of the degradation process.
On the kinetics of polymer degradation in solution
XIII. Radiolysis of poly(2,2,2-trichloroethyl methacrylate) in dichloromethane solution
Abstract
Poly(2,2,2-trichloroethyl methacrylate), PTCMA, was irradiated in dilute dichloromethane solution with 16 MeV electron pulses of6 0Co 
-rays. Both in the presence and absence of O2, G(S)=9–10 was found at [PTCMA]<9 g/dm3. Indirect and direct radiation effects contributed to main-chain scission at a ratio of 7
3. In contrast to earlier findings with dioxane solutions, at concentrations of up to 20 g/dm3 indications of crosslinking were not obtained. With CH2Cl2 solutions two modes of light scattering intensity (LSI) decrease were detected indicating the existence of two different macroradicals, one generated by the direct and the other by the indirect action of radiation. These radicals decayed with rate constants of 1.0·103 s–1 and 1.4·101 s–1 and also reacted with C2H5SH with rate constants of 8·104M–1·s–1 and 1·103M–1·s–1, respectively. In O2-saturated solutions both radicals formed peroxyl radicals, PO2, wich combined generating products whose decomposition involved main-chain scission. The combination of PO2-radicals was rate-determining in the consecutive series of reactions as inferred from the 2nd order decrease of the LSI.
Abstract
Vinylidene chloride copolymers containing a predominance of vinylidene chloride (85-90%) have long been important barrier polymers widely used in the plastics packaging industry. These materials display excellent barrier to the ingress of oxygen and other small molecules (to prevent food spoilage) and to the loss of food flavor and aroma constituents (to prevent flavor scalping on the supermarket shelf). While these polymers have many outstanding characteristics, which have made them commercial successes, they tend to undergo thermally-induced degradative dehydrohalogenation at process temperatures. The dehydrochlorination occurs at moderate temperatures (120-200C) and is a typical chain process involving initiation, propagation and termination phases. Defect structures, namely internal unsaturation (allylic dichloromethylene groups), serve as initiation sites for the degradation. These may be introduced during polymerization or during subsequent isolation and drying procedures. If uncontrolled, sequential dehydrohalogenation can lead to the formation of conjugated polyene sequences along the polymer mainchain. If sufficiently large, these polyenes absorb in the visible portion of the electromagnetic spectrum, and give rise to discoloration of the polymer. The dehydrochlorination process may be conveniently monitored by thermogravimetric techniques. Both initiation and propagation rate constants may be readily obtained.
Abstract
Several dynamic methods for estimating activation energies have been developed. This development has arisen largely as a matter of convenience and the desire to minimize analysis time. While these methods generally afford values which are somewhat similar, the agreement among values from various methods is never outstanding. Further, the values obtained are often, at best, only approximations of the values obtained by the traditional isothermal approach. To better ascertain the utility of dynamic methods for the determination of activation energies, the activation energy for the thermal degradation of a standard vinylidene chloride/methyl acrylate (five-mole percent) copolymer has been generated by a variety of methods. The degradation of this polymer is an ideal reaction for evaluation of the various methods. At modest temperatures (<200C), the only reaction that contributes to mass loss is the first order evolution of hydrogen chloride, i.e., there is only one significant reaction occurring and it is not impacted by competing processes. The best values (most reproducible; best correspondence to values obtained by titrimetry and other methods) are those obtained by plotting the natural logarithm of rate constants obtained at various temperatures vs. the reciprocal of the Kelvin temperature. Various dynamic methods yield values which are less reproducible and which approximate these values to a greater or lesser degree. In no case is the agreement good.
Abstract
ABS is a well-known and widely-used rigid engineering polymer. The mechanical properties of ABS are critical to its proper functioning in a given application, such as a medical device. It is therefore important to retain those properties during processing, fabrication, and use. To that end, thermal analysis and mechanical testing were employed to understand the origin of observed mechanical property losses. Oxidation onset temperature (OOT) testing and differential scanning calorimetry (DSC) analysis indicated a slightly lower onset temperature and a higher glass transition temperature, respectively, for parts which demonstrated a reduction in mechanical properties. It is also demonstrated that degradation of the butadiene-phase of the ABS is responsible for the mechanical property reduction, and that this degradation only proceeds at an appreciable rate at elevated temperatures in the presence of oxygen.
Abstract
Attempts are made to use kinetics parameters from thermal decomposition experiments at high temperatures to predict service lifetimes of polymeric materials at lower temperatures. However, besides the obvious measurement and extrapolation errors (which can be considerable), there are two fundamental reasons why quantitative long range extrapolations can not be made for complex condensed phase systems. They are: 1) Arrhenius kinetics parameters can not be extrapolated through phase transitions or softening temperatures; 2) Arrhenius kinetics parameters can not be extrapolated through the ceiling temperature region. Satisfactory lifetime prediction methods can be developed only after a thorough analysis of the causes of service failure. A real method has been taken from literature to illustrate the correct procedures.
) It is observed that the sample prepared with Zr, T120-Zr1, is more stable with the temperature increase until the beginning of polymer degradation that occurs approx. at 320 °C than the one prepared without Zr, T120-Zr0, that begins to degrade around
