Thermo-oxidative processes materials

Generally [17] starch is added at fairly low concentrations (6-15%) the overall disintegration of these materials is achieved by the use of transition metal compounds, soluble in the thermoplastic matrix, as pro-oxidant additives that catalyse the photo and thermo-oxidative process [18-21]. 

Crystalline starch beads can be used as a natural filler in traditional plastics [3]. They have been particularly used in polyolefins. When blended with starch beads, polyethylene films biodeteriorate upon exposure to a soil environment. The microbial consumption of the starch component leads to increased porosity, void formation, and loss of integrity of the plastic matrix. Generally, starch is added at fairly low concentrations (6-15 wt%). The total disintegration of these materials is obtained using transition metal compounds, soluble in the thermoplastic matrix, used as pro-oxidant additives to catalyze the photo and thermo-oxidative processes [4]. These products belong to the first generation of degradable polymers that biodeteriorate more than mineralize to CO2 and H2O in a time... 

Results showed the same induction period for all three systems (55-60 min) whereas isothermal heating for 6 h at 150°C caused no changes in the mechanical properties of the material. Therefore, the resistance of all three PP formulations to the thermo-oxidation process is substantially the same. Therefore, the same stablizing effect of these additives during PP processing can be assumed. 

Although there are several compounds (Ascorbic acid (vitamin C) or alpha-tocopherol (vitamin E) which their antioxidant effect is proved to be effective in reducing the oxidation of UHMWPE [60], there are several studies that support the antioxidant capacity of carbon nanotubes (CNTs). For example, in the study of Zeynalov et al. [61] was conducted a simulation of thermo-oxidative processes which take place in the polymer chains, and the results showed an inhibition of oxidation of the polymer when carbon nanotubes are present. Also, P. Castell et al. [62] founded that the incorporation of low concentration of arc-discharged multiwall carbon nanotubes (MWCNTs) can act as inhibitors of the oxidative process on irradiated UHMWPE, proving the radical scavenger effect of this reinforcing material. Also, this study shows that the presence of MWCNTs enhances the chemical stability of the polymer. 

Poly[2,2 -(m-phenylene-5,5 -benzimidazole)] (PBI) is a very high glass transition temperature (Tg 430°C), commercially available material. It possesses excellent mechanical properties, but is difficult to process into large parts and has high moisture regain and poor thermo-oxidative stability at temperatures above approximately 260 °C. Polyimides, especially the thermoplastic polyimides, offer attractive thermo-oxidative stability and processibility, but often lack the thermal and mechanical characteristics necessary to perform in applications such as the matrix for high use-temperature (over 300 °C) structural composites (for example, carbon fiber reinforced) for aerospace use. The attempt to mitigate... 

During the first 100 h of oxidation there were no significant differences between the samples, i.e. they all seemed unaffected by thermo-oxidation and no influence of repeated processing was found [65]. However, after 500 h of thermo-oxidation, all recycled samples showed a marked deterioration in tensile strength whereas the virgin material remained unaffected. The magnitude of the decrease in tensile strength corresponded well to the number of times the material was extruded. After 1200 h of thermo-oxidation the deterioration in mechanical properties was even more pronounced and at this point the polyamide 6.6 recycled three times retained only 20% of its... 

Conductive polyaniline is observed to be quite stable material and its thermal degradation usually involves a three-step process i.e. removal of moisture, then of HCl and finally the breakdown of the polymer backbone. An emeraldine base treated with HBr is reported to be the most stable material against thermal degradation and methyl substitution enhances the thermal stability of polyaniline in inert environments as well as in air. The thermo-oxidative stability of N-substituted polymers is better than the ring-substituted polymers. 

The use of ACECs as modifiers for CECs makes it possible to preserve the desirable processing characteristics of epoxy resins and to improve the mechanical strength, elongation under load and heat resistance. The thermal stability of the crosslinked materials does not depend on the type of modifier. This statement concerns both the thermo oxidative aging in the isothermal and the dynamic conditions. 

However, it is very difficult to propose a mechanistic explanation of the obtained results, but a look to the general features of the accepted thermo-oxidation mechanism of organic materials and polymers can be helpful to have an idea about the processes involved during the degradation. This subject will be explored in the next sections. 

The study of degradation processes of polymers and organic materials is a fundamental topic both from academic and industrial point of view and it can be expected that thermal and thermo-oxidative decomposition mechanisms are reasonably well established. [20,3 8] These complex reactions proceed via a typical free-radical chain as outlined in scheme 2. 

As with all organic polymers, exposure of aromatic polyesters to heat and oxygen can, especially over long time periods, result in degradation of the polymer. This thermo-oxidation manifests as discoloration of materials, loss of physical properties, and complete failure of the substrate. To prevent (or more likely control) such processes it is necessary to incorporate additives which can protect the host polyester against the effects of heat and oxygen antioxidants. 

Bearing in mind that the molecular arrangement possibilities (through synthesis) are almost infinite, one inquires whether a molecular structure can be adjusted for high fracture toughness while maintaining other material requirements such as high —temperature dimensional and thermo-oxidative stability. At present, it does not appear that this can be done with accuracy for very many materials, yet the process of inquiry produces beneficial results, and improves prospects for the future. 

Carbon black (CB) is indisputably the most widely used reinforcing filler in NR formulations. It improves tensile and tear strengths, modulus and hardness, abrasion and thermo-oxidative resistance, etc. of NR-based materials. CB is manufactured by a variety of processes, including the channel process, to produce furnace black, thermal black, lamp black and acetylene black. NR-based composites and nanocomposites with the addition of CB exhibit the monotonous black colour to the finished goods. 

If degradation occurs by a thermo-oxidative mechanism, air should be excluded from the extruder. This can be done by putting a nitrogen bianket on the feed hopper, vent port, or at the die, depending on where the air is introduced. If degradation occurs by hydrolysis, moisture has to be excluded from the process. If degradation occurs by a chemical reaction with the metal surfaces of screw and barrel, a nonreacting material of construction has to be selected for the screw and barrel. 

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