Oxidative stability of polymers

Measuring the relative oxidative stability of polymers is important. Measurements can be used to determine dependencies on structural and molecular weight/weight distribution or the effectiveness of an antioxidant, or to perhaps assess the amount present in a polymer sample, etc. The preferred and commonest method consists in raising the sample temperature to a predetermined level, while in an inert atmosphere, then switching the atmosphere to air or oxygen. The time to the onset of exothermic reaction is measured. 

LPD deposition of ceramic thin films is a potentially general approach to improving the abrasion resistance and thermo-oxidative stability of polymers. Dutschke et al.14,15 have deposited titania on variously treated polystyrene (PS). Continuous, adherent anatase films form on PS either after hydroxylation in... 

The effect of transition metals on the thermo-oxidative stability of polymers is ascribed to reactions such as initiation, in which electron or ligand transfer between organic substrate and transition metal ion takes place. In these reactions the transition metal ions may act either as electron donors or as electron acceptors. 

Mechanical and Optical. Clear 15-mil-thick films of EVA and plPVB, 4-mil PVF and 3-mil BAgMMA were exposed in a circulating air oven at about 150°C for periods of 0 to 26 days. This type of test is used extensively throughout the polymer industry as a screening tool for comparing the oxidative stabilities of polymers and compound formulas. 

Figure 5.5. Example of a third-order measurement of combinatorial materials. Oxidative stability of polymers from measurements of UV-VIS reflection spectra from each polymeric composition in a materials array as a function of reaction temperature and time. (A) General view of the materials array on a gradient temperature heater (B) representative UV-VIS spectra from a single material in the array as a function of reaction time and temperature. Reaction temperatures Ti> T2> T. Reaction progress is shown as spectra changes from spectrum 1 to spectrum 7.

Adding two more independent variable parameters to the response of a first-order system obviously makes it a third-order measurement approach. An example of such system is spectroscopic monitoring (wavelength is the variable parameter 7) of the reaction progress of combinatorial materials (time is the variable parameter J) at different process temperatures (temperature is the variable parameter K). Alternatively, a second-order system can be implemented for the measurements as a function of one or more independent parameters. Examples of second-order systems are excitation-emission luminescence measurement systems, GC-MS, and HPLC-diode array UV systems, among others. An example of one of our third-order measurement approaches for combinatorial screening is illustrated in Figure 5.5. It was implemented for the determination of oxidative stability of polymers under different process conditions (temperature / and time J). 

Another relatively recent development in thermal technique for polymers is equipment for reliable measurement of chemiluminescence. The luminescence is thought to be from the drop of an excited carbonyl group back to the neutral state after excitation from the termination of two peroxy radicals. Its use is primarily in determination of oxidation stability of polymers to determine an oxidative induction time (OIT) (194,195). It has also been used to monitor oxidation of polymers subjected to stress (196) and simulated weathering (197). 

OIT is a widely used screening parameter for the oxidative stability of polymers, edible oils, and lubricants, which is typically used as a quality control tool to rank the effectiveness of various oxidation inhibitors. It is a kinetic parameter (i.e. dependent on both time and temperature) and not a thermodynamic property. As a parameter dependent on test time and temperature, the OIT value appears to be decreasing with time but in a well-behaved and predictable manner. OIT is either a measure of the amount of antioxidant present in the polymer or the effectiveness of the particular AO used. If the amount of AO in the polymer is known, then OITime or lOTemperature allow monitoring residual AO contents and calculation of the linear rate of AO consumption. A major limitation of DSC-OIT is that if the isothermal test temperature is lowered below the standard 200° C temperature to reveal small differences in AO concentration at low levels, the polymer s exothermic oxidation rate may decrease below the limits of DSC detectability. Lugao et al. [120] have recently introduced a temperature dependent oxidative induction time (TOIT) in order to cope with some limitations of the traditional OIT method. 

The effective method for increasing thermal oxidation stability of polymers is control of the physical structure [7], The additive effect on the physical structure of PI film was studied with the help of X-ray structural analysis. The film possesses mesomorphous regularity, of which the presence of an intrachain order in the absence of interchain packing regulation is typical. As shown on the diffraction pattern, such structure manifests itself by a single narrow peak of the intrachain order (5-6 deg) and wide amorphous halo (Figure 19). Diffraction patterns show high intrachain orderliness of the stabilized sample. This difference is... 

In terms of processing there is no need for pre-drying PCHE granules, a standard extruder screw as used for polycarbonate may be used and discs are said to release well from the mould. Question marks remain on the oxidative stability of the polymer and on the quality of adhesion of the reflective layer but Dow claim that metallising is possible. 

Physical properties of carbon black-filled EPR and EPDM elastomers have been found to be comparable with the suUur-cured analogues [372]. Aromatic oils increase the optimum dose requirement for these compounds due to the reaction of the transient intermediates formed during radiolysis of the polymer with the oil as well as energy transfer which is particularly effective when the oil contains aromatic groups. The performance and oxidative stability of unfilled EPDM as well as its blend with PE [373], and the thermal stabdity and radiation-initiated oxidation of EPR compounds are reported by a number of workers [374,375]. 

Ranby, B. and Rabek, J., Photo Degradation, Photo Oxidation and Photo Stabilization of Polymers, Wiley, New York, 1975, 75. 

Such hybrid molecules and supramolecular solids offer the promise of systems with the flexibility, strength, toughness, and ease of fabrication of polymers, with the high temperature oxidative stability of ceramics, and the electrical or catalytic properties of metals. Polyphosphazene chemistry provides an illustration of what is possible in one representative hybrid system. 

Figure 3.7. Thermal/oxidative stability of the PFCB thermoset polymer.

Oxidation kinetics of oriented PP was measured under conditions of external priming. The parameter specifying the oxidability of a polymer is slightly dependent on deformation. For instance, at 200°C it only decreases by 1.5 times with X changing from 0 to 10. This unambiguously clarifies that the main reason for increase in thermal-oxidative stability of deformed PP is a sharp drop in the escape of a branching agent (hydroperoxide), i.e. a decrease in hydroperoxide escape. 

Other high performance polymer backbones have been explored as PEM materials in addition to poly-(arylene ether)s and polyimides. Ductile copolymers with high modulus and glass transition values are desirable PEM candidates. The hydrolytic and oxidative stability of many of these materials remains to be determined. Nevertheless, interesting synthetic methodologies have been employed to investigate these materials, which have been instructive in the search for new PEM candidates. 

The oxidative stability of the exposed surface of most organic polymers is less than that in inert conditions and within solids. A description of the many known variations of thermal degradation of polymers is beyond the scope of this book. 

Nadimide- and V-CAP type PMR resins use aliphatic endcaps (multiple carbon-carbon double bonds) to affect crosslinking. Such aliphatic moieties adversely effect the thermal oxidative stability of the cured system. Effort is therefore directed towards PMR resins which use reactive aromatic endgroups to obtain cured polymers free of aliphatic chain- or crosslinking segments. 

Stabilization of polymers against such photo-oxidation can be... 

Thermal and Oxidative Stability. In general, polyolefins undergo thermal transitions at much lower temperatures than condensation polymers thus, the thermal and oxidative stability of polyolefin fibers are comparatively poor. Preferred stabilizers are highly substituted phenols such as Cyanox 1790 and lrganox 1010, or phosphites such as Ultranox 626 and Irgafos 168. 

Much effort has been expended toward the improvement of the properties of polyacetylenes made by the direct polymerization of acetylene. Variation of the type of initiator systems (17—19), annealing or aging of the catalyst (20,21), and stretch orientation of the films (22,23) has resulted in increases in conductivity and improvement in the oxidative stability of the material. The improvement in properties is likely the result of a polymer with fewer defects. 

---