Thermal and oxidative stability

Secondary antioxidants or peroxide decomposers such as phosphites and thioesters inhibit oxidation. They are usually combined with primary antioxidants to produce synergistic effects. Careful selection of suitable stabilisers is necessary for food contact applications. Interaction with light stabilisers and other additives must be considered during selection (see Chapter 4). 

Dan and co-workers [21] used DTA and TGA amongst other techniques in their study of the thermal and thermo-oxidative stability of chlorinated natural rubber and showed that the carbonyl group and tertiary C-Cl groups were factors causing poorer stability in the rubber prepared by the latex route. 

Pielichowski and Flasik [22] studied the thermal properties of a new family of catalysts on the bulk doped and surface doped heteropolyanion-doped polyaniline by TGA and DTA. It was found that all systems exhibited good thermal stability at the temperature of their catalytic application (up to 250 C) in both oxidative and inert reaction atmospheres. Thermal stability varied significantly with the dopant used, being highest for 12-tungstosilicic acid and the lowest for 12-molybdophosphoric acid. 

Polybutene and crosslinked polyethylene oxidation induction periods have been determined by DTA [23]. 

As with other organic polymers, HIPS is susceptible to degradation when heated in the presence of oxygen. Degradation is a broad term that takes on different meanings to the end user. While it is not easily seen at the molecular level, degradation is recognizable when a finished part is put into its intended use. Typical macroscopic evidence is seen in 

The last two are the result of a reduction in the polymer s molecular weight. It is also interesting that degradation occurs to some degree immediately after the commercial polymer is produced. It can occur during various phases of the polymer s life cycle  

In cases where no additional oxygen is present, polystyrene can undergo nearly pure thermal degradation. The two prevalent mechanisms are sequential elimination of monomer units, which is called unzipping or depolymerization. In this case, styrene monomer is formed. Random chain scission can also occur. It is sometimes combined with unzipping at the reactive broken chain ends. At temperatures approaching 300 °C, up to 40 % of a polystyrene molecule can be converted to styrene monomer. 

While the primary antioxidant serves a critical role, it cannot stop all polymer peroxy radicals from propagating. This is where a second class of antioxidants, called peroxide decomposers, comes in. These molecules catalyze the decomposition of the peroxides to nonradical species, thus breaking the repetitive cycle of radical formation. Phosphites and thioesters commonly serve as secondary antioxidants. Phosphites are commonly used in HIPS resins, but care must be taken to use hydrolysis-resistant molecules to avoid the degradation of these species into black specks that render the final product unacceptable. Phosphites are usually found at levels between 500 and 2000 ppm. 

The first patent on HIPS, a blend of synthetic rubber and transparent polystyrene, was granted in Great Britain as early as 1912. The first graft copolymerization of styrene in the presence of rubber was carried out by Ostromislensky [5]. The decline in the demand for styrene monomer and styrene-butadiene rubber and the simultaneous availability of natural rubber on the world market in the late 1940s drove the development of styrene copolymer processes. 

---

HalogenatedFluids. Chlorocarbons, fluorocarbons, or combinations of the two are used to form lubricating fluids (see Chlorocarbons and CHLOROHYDROCARBONS Fluorine COMPOUNDS, ORGANIC). Generally, these fluids are chemically inert, essentially nonflammable, and often show excellent resistance to solvents. Some have outstanding thermal and oxidation stability, because they are completely unreactive even in Hquid oxygen, and extremely low volatility. 

The most commonly used siloxane modifiers are those having phenyl, trifluoro-propyl and cyanopropyl substituents. Introduction of phenyl units into the polydimethylsiloxane backbone either in the form of methylphenylsiloxane or diphenyl-siloxane increases the thermal and oxidative stability, glass transition temperature and the organic solubility characteristics of the resulting copolymers. At low levels (5-10 percent by weight) of incorporation, bulky phenyl groups also break up the regularity of polydimethylsiloxane chains and inhibit the crystallization (Tc... 

Good Thermal and Oxidative Stability High Permeability to Gases Excellent Atomic Oxygen (Plasma) Resistance Physiological Inertness (Biocompatibility)... 

Poly(organosiloxane)s have a number of outstanding properties such as thermal and oxidative stability, water and chemical resistance, electric insulating capacity, selective permeability to gases, and biocompatibility. 

Classical characterization methods (gas sorption, TEM, SEM, FTIR, XPS and elemental analysis) were used to describe the resulting porous carbon structures. Temperature-dependent experiments have shown that all the various materials kept the nitrogen content almost unchanged up to 950 °C, while the thermal and oxidation stability was found to be significantly increased with N-doping as compared to all pure carbons. Last but not least, it should be emphasized that the whole material synthesis occurs in a remarkably energy and atom-efficient fashion from cheap and sustainable resources. 

Although Parylene-N possesses an outstanding combination of physical, electrical, and chemical properties, the benzylic C—H bonds present are potential sites for thermal and oxidative degradation. It is well known that replacing a C— bond with a C—F bond not only enhances the thermal stability of the resulting polymer, but also reduces the dielectric constant. Because incorporation of fluorine is known to impart thermal and oxidative stability, it became of interest to prepare poly(a,a,a, a -tetrafluoro- p -xylylene), Parylene-F Joesten reported that the decomposition temperature of poly(tetrafluoro-j9-xylylene) is ca. 530°C. Thus, it seemed that the fluorinated analog would satisfy many of the exacting requirements for utility as an on-chip dielectric medium. 

A very clear, transparent, strong plastic Good mechanical properties High impact strength Good thermal and oxidative stability... 

Crystalline with good mechanical properties, high impact strength, good thermal and oxidative stability, transparent, selfextinguishing, low moisture absorption Good heat resistance, dimensional stability, resistance to cold flow, solvent, dielectric properties... 

Polysiloxanes, also called silicones, are characterized by combinations of chemical, mechanical, and electrical properties which taken together are not common to any other commercially available class of polymers. They exhibit relatively high thermal and oxidative stability, low power loss, high dielectric strength, and unique rheological properties, and are relatively inert to most of the ionic reagents. Almost all of the commercially utilized siloxanes are based on polydimethylsiloxane with trimethylsiloxy end groups. They have the widest use... 

MW 27,500) with no cofactors or metal ions reqnirement for its function, it displays Michaelis-Menten kinetics and it is secreted in large amounts by a wide variety of Bacillus species. Subtilisin is also among the most important industrial enzymes due to its use in laundry detergents. Protein engineering strategies for subtilisin have focused on a number of aspects, namely catalysis, substrate specificity, thermal and oxidative stability and pH profile. We will describe briefly each of these aspects. 

Polyesters usually have good thermal and oxidative stability (up to 200°C) but have poor hydrolytic stability at elevated temperatures. 

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. 

These materials also have high thermal and oxidative stability. Flexible segments such as amide siloxane can be incorporated into the imide-based structure for hot melt or injection applications. General Electric (GE) and Hoechst-Celanese are suppliers of these high performance plastics. 

---