Polymer stabilization transformations

Tetraorganotins Used in manufacture of R nX compounds from SnCI4 catalysts for olefin polymers stabilizers for transformer oils corrosion inhibitor in lubricating oils (CEC 1978 WHO 1980 Davies and Smith 1982)... 

Once the precursor (i.e., pitch, polymer) is transformed into a fiber shape by a suitable spinning process and is carbonized after a proper stabilization stage, the activation of the resulting CF is needed to increase its adsorption capacity. 

Several reactions may be involved in providing protection to polymers against photodecomposition destruction. The contribution of each of the processes depends on the nature of the polymer and the oxidation conditions. The time during which a hindered amine is transformed into nitroxyl in polypropylene is short compared to the induction period. Therefore, piperidines and corresponding nitroxyl radicals are almost equally effective for polymer stabilization. In rubber, which is more rapidly oxidized than polypropylene, nitroxyl radicals are fairly more effective than hindered amines. Subsequent transformations of nitroxyl radicals, formed through Reactions 6-8 are shown in Scheme 6 (53). 

Shlyapintokh, V. Ya. "Photochemical Transformations and Polymer Stabilization," Khimia, Moscow, 1979. 

The radical mechanism of thermal degradation of pristine PE has been widely discussed in a framework of random scission type reactions [14-22], It is known that PE decomposition products comprise a wide range of alkanes, alkenes and dienes. Branching of PE chains causes enhanced intermolecular hydrogen transfer and results in lowering thermal stability. The polymer matrix transformations, usually observed at lower temperatures and involving... 

Chemical reactivity of QI is reflected in the complexity of chemical processes taking part in aging of unsaturated vulcanized rubbers [3,5,24,58], QI are oxidants and are transformed in high yields into the respective reduced formes, e.g. 11 or 83. From the point of view of the polymer stabilization, the process should be understood as regeneration of the AO/AF species. This was observed in the reaction with 2-mercaptobenzimidazole and other thiols during the vulcanization process [4,77], The majority of ArSH was bound to the arylene ring of the reduction product, e.g. in 88. 

Scavenging mechanism has traditionally been considered as the principal contribution of HAS to polymer stabilization. It is more important in polymers where the photoinitiation is mainly due to ROOH photolysis, e.g. in PP [134]. Scavenging of C- and O-centred radicals participating in the initiation and propagation steps of the chain oxidation of polymers has been envisaged for the original structures of HAS and their active transformation products (nitroxides, O-alkylhydroxylamines, hydroxylamines). 

Hydroperoxides formed in the chain propagation step of oxidation of polymers initiate new chains after thermal, photolytic or catalytic homolysis. Heterolytic deactivation of ROOH represents one of the principal modes of polymer stabilization [25]. HAS in their original form ( NH, NR) reduce hydroperoxides effectively. The ROOH decomposing mechanism works in very early stages of PO oxidation, before radical scavenging HAS transformation products are formed [15,26,134,136],... 

Polymer stabilizers are very reactive systems. This is expressed not only in scavenging chain-carrying radicals or deactivation of ROOH but also in interactions between various stabilizers or their transformation products [250], This chemical cooperation may have a positive effect accounting for additivity or even synergism. Unfortunatelly, some interactions are negative, i.e. antagonistic, and should be avoided. 

Under the conditions of polymer stabilization the typical reactions of phenoxyls are those (Scheme 2) in which only I or II take part, L e. coupling via C—O or C—C, formal rearrangement connected with formation of benzyl radical (Chap. II A), or disproportionation (see the Chap. II B), on the one hand, and the reactions with radicals R0 2, RO, or with oxygen (see the Chap. IIC) on the other. The transformation of phenoxyl and the final composition of the transformation products are affected very markedly by conditions of inhibited oxidation. 

Xiang, X., Dahlgren, J., Enlow, W R, and Marshall, A. G., "Analysis of Phosphite Polymer Stabilizers by Laser Desorption/Electron Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry," Anal. Chem., 64, 2862-2865, 1992. 

Identification of nitroxides in the oxidized hydroearbon substrate doped with HAS is considered as a proof of the photoantioxidant activity of HAS. The latter involves unique mechanistic features in polymer stabilization differing thus from the activity of conventional chain-braking antioxidants. The developed nitroxides are able to scavenge polymeric alkyls formed not only in the surface layers but also in areas of oxygen insufficiency in deeper polymer layers where they may be formed due to presence of residual oxygen. Participation of the polymeric alkyls in oxidative transformation into alkylpeioxyls POO" (2) is thus disabled. Resulting 0-alkylhydroxylamines >NOP are formed not only from >NH but also from various A-substituted HAS (>NX, X=alkyl, aryl, alkoxyl) and are considered as active reservoirs of HAS efficiency (6). 

In spite of some imeertainties in the individual steps of the HAS meehanism in polymer stabilization due to the speerfie effeets of the polymer matrix and the environmental stress, the HAS-based nitroxides are eonsidered the key intermediate in the HAS reaetivity meehanism. Detection and quantification of the formed nitroxides using ESRI spectroscopic technique has been exploited for confirmation of the primary transformation step in HAS mechanism [15, 16, 20], as a consequence of interactions of HAS with oxygenated radical and molecular products of polyolefins (5). Monitoring of the nitroxide development enables tracing of the oxidation process within the polymer matrix. Consequently it is also a tool for marking the heterogeneity of the oxidative transformation of semicrystalline carbon chain polymers [polypropylene (PP), polyethylenes (PE)] or amorphous polymers [copolymers of ethylene with norbomene, polystyrene (PS), high impart polystyrene (HIPS), acrylonitrile-butadiene-styrene polymer (ABS)]. 

Other types of antagonistic chemistry often involve relatively strong acids or bases (either Bronstead or Lewis) that can interact with the antioxidants in such a way as to divert them into transformation chemistries that have nothing to do with polymer stabilization. 

Discoloration is typically caused by the formation of colored conjugated quinonoid-type transformation products with visible absorption wavelengths, typically >400 nm (125). Schemes 15 and 16 show some key reactions of hindered phenols used for polymer stabilization to illustrate the effects of different para-substituents ... 

Fig. 27 Top SEM images of CaCOs particles grown on a glass slip in the early reaction stage, PEO-6-PMAA, [CaCb] = 10 mM, lgL , 5h. a CaCOs particles with either spherical or hollow structures, b Zoom showing the calcite rhombohedral subunits grown on the surface of the hollow structure and the inner part consisting of tiny primary nanocrystals with a grain size of about 320 nm as indicated by the arrow (sacrificial vaterite template). Bottom Proposed formation mechanism of the calcite hollow spheres a polymer-stabilized amorphous nanoparticles b Formation of spherical vaterite precursors c aggregation of the vaterite nanoparticles d vaterite-calcite transformation starting on the outer sphere of the particles e formation of calcite hollow spheres under consumption of the sacrificial vaterite precursors. Reproduced in part from [61] with permission of the American Chemical Society...

Plastics usually contain stabilizers that must protect the polymer during melt processing. They also contribute to the service life time of the finished article upon exposure (especially for out-door applications) to light and heat. The stabilizers are partially consumed during the processing and exposure of the plastic parts, thus they must be replaced. The amount of required stabilizer depends on the reprocessing conditions as well as on the envisaged use of recycled polymers. It must be noted that the stabilizers transformation products remain in the plastics. 

Figure 12.34. Schematic illustration of a polymer-stabilized cholesteric texture (PSCT) in (a) the planar texture, (b) the focal conic texture, and (c) the homeotropic texture. Transforming the homeotropic ahgned state (c) back to the planar of focal conic state depends on the history of the voltage.

Thermoanalytical techniques are a quick way for assessing the relative performance of AOs in polymers, rubbers, lubricants, etc. and have been widely used. DSC-OIT is used to study base polymer stability, optimum additive level, the degree of material deterioration during processing and the effect of multiple shear histories while reprocessing. DSC is also useful to determine the effective AO concentrations among all the transformation products present in a polyolefin formulation. 

---