Polyurethanes degradation mechanisms

TG-FTIR Vulcanisation [32], ageing characterisation [39, 48], sulphur components in rubber [31], polyurethanes [37], polymer degradation mechanisms [30, 40, 41], identification of base polymers [36, 43, 44], thermal stability [46], grafted flame retardants [47], differentiation of EVA rubbers [45] and AN-NBR rubbers [36, 44], degradation of chlorinated natural rubber [42],... 

Lu and Macosko (2004) and Lu et al. (2003) have prepared compatibilized blends of polyurethane with functionalized PP characterizing the blends by rheology, DMA, tensile properties, and morphology. Primary and secondary amine-functionalized PP were more efficient compatibilizers than was PP-g-MA. A degradative mechanism for copolymer formation involving polyurethane chain cleavage was postulated. See also Kobayashi et al. (2011) for related PE/TPU blends. 

Yashitake and Furukawa investigated the thermal degradation mechanism of a.y-diphenyl alkyl allophanates and carbanilates as model compounds for crosslinking sites in polyurethane networks by pyrolysis-high-resolution GC/ FTIR (Py-HR GC/FTIR). Pyrolysis was performed at 250°C, 350°C, 450°C, and 500°C. 

Yashitahe,N. Furufcawa,M. Thermal degradation mechanism of a, y-diphenyl alkyl allophanate as a model polyurethane by pyrolysis-high-resolution gas chromatography/FT-IR. J. Anal. Appl. Pyrol. 1995, 33, 269. 

The response of polyurethanes to thermally activated autoxidation depends upon polymer structure. In general, polyurethane degradation by this mechanism is suppressed by the addition of antioxidant to the polymer. Ultraviolet initiated autoxidation is suppressed by a suitable screen (e.g. carbon black, titanium dioxide) or a combination of antioxidant and ultraviolet absorber. Irganox 1010 and Tinuvin P (Ciba-Geigy) are particularly suitable antioxidant and ultraviolet absorbers, respectively, for polyurethanes. Polyurethane structures with enhanced resistance to ultraviolet initiated autoxidation may be possible. 

A similar degradation mechanism has been proposed for the thermal degradation of polyurethanes which also dissociate into their isocyanate and diol constituents. ... 

Polyurethane degradation is a common issue for polyurethane-based medical implants. The degradation mechanisms vary depending on the type of polyurethane used and their environment. For example, polyester polyurethane undergoes hydrolytic... 

Keywords Polyurethanes, Nanocomposites, Nanoparticles, Thermal stability. Degradation mechanisms, TGA... 

The imphcation from the above work is that polymer present in the galleries of montmorillonite through intercalation and exfoliation has enhanced thermal stability provided by a different thermal degradation mechanism when compared to the pure polymer. Calculations of the activation energy of polyurethane-imide-clay nanocomposites [37] utilizing the Broido [38] and Coats-Redfern equations were consistent with a mechanism that increased the thermal stability of the polymer-clay nanocomposite in relation to the pure polymer. An optimization of melt-blending processing that improved the exfoliation efficiency of polymer-clay nanocomposites resulted in increased thermal stability by TGA when compared to polymer-clay nanocomposites with inferior exfoliation, and pure polymer [39]. 

A two-stage method of waste polyurethane degradation is described in [79]. In the first stage, scission of the polyurethane chain takes place at a temperature of 120°C in the presence of dialkanolamine and metal hydroxide e.g., KOH). Under these conditions, the reaction products include polyols, aromatic amines, short-chain ureas, and urea derivatives. The second stage is based on the alcox-ylation of the hydroxyl groups and the primary and secondary amine groups, e.g., by the use of propylene oxide. In this way, polyols with a hydroxyl number of 156-271 mg KOH/g and viscosity within the range of 1950-57 000 mPa s can be obtained. The flexible foams prepared from recycled polyols revealed favorable mechanical properties. 

Some other degradation mechanisms other than simple hydrolysis are presented for biodegradable polymers. Such mechanisms include oxidative cleavage by a radical mechanism. Oxidative degradation is the main mechanism for non-hydrolyzable polymers, such as polyolefins, natural rubber, lignins, and polyurethanes. For many polymers, hydrolysis and oxidation occur simultaneously in the environment. 

Even though poly(ortho esters) contain hydrolytically labile Linkages, they are highly hydrophobic materiads and for this reason are very stable and can be stored without careful exclusion of moisture. However, the ortho ester linkage in the polymer is inherently thermally unstable and at elevated temperatures is believed to dissociate into an alcohol and a ketene acetal (33). A possible mechanism for the thermal degradation is shown below. This thermal degradation is similar to that observed with polyurethanes (34). 

In an NMR analysis of the effects of /-irradiation induced degradation on a specific polyurethane (PU) elastomer system, Maxwell and co-workers [87] used a combination of both H and 13C NMR techniques, and correlated these with mechanical properties derived from dynamic mechanical analysis (DMA). 1H NMR was used to determine spin-echo decay curves for three samples, which consisted of a control and two samples exposed to different levels of /-irradiation in air. These results were deconvoluted into three T2 components that represented T2 values which could be attributed to an interfacial domain between hard and soft segments of the PU, the PU soft segment, and the sol... 

Degradation of bulk biocompatible polyurethanes occurs mainly by the hydrolytic mechanism and the stage of cellular degradation is initiated only at later stages when cavities appear as a result of polymer surface erosion (Ref. l5), p. 44). 

On the other hand, San Miguel and Duran (Ref 125) showed that the mechanical properties of polyurethane solid proplnt were degraded significantly by gamma irradiation dosages greater than 106 R. The tests used to determine the effects were by swelling, torsion, uniaxial tension and multiaxial tension... 

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