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doi: 10.1002/masy.19971150103pmid: N/A
The influence of manufacturing and processing parameters on the photooxidative stability of aqueous acrylic based latices is examined by FTIR and hydroperoxide analysis. These are based on emulsion polymerised formulations of methyl methacrylate and butyl acrylate. Here acrylic latices based on different formulations are chosen as homo‐polymers and copolymers. The nature of impurities and oxidation products generated during their manufacture are characterised and inter‐related to their influence on subsequent photooxidative degradation. This involves the use of reflectance FT‐IR spectroscopy to show functional group changes together with colourmetric U.V. analysis to determine the photochemical generation of hydroperoxides. The early chemical changes and their subsequent influence on the physical and chemical properties of the latices during the later stages of photooxidation are found to exhibit a close inter‐relationship. To date the nature of the surfactant and the composition of the latex in terms of end group modification are important parameters. De‐esterification and hydroperoxide formation are found to be important processes during latex photooxidation while the addition of low levels of co‐monomer to the emulsion give latices with improved photostability. While co‐reactive hindered piperidine stabilisers are found to be effective the incorporation of simple terminal dialkyl acrylamide/methacrylamide groups are equally as effective. Oxygen scavenging via the formation of alkylamino radicals is disussed.
doi: 10.1002/masy.19971150104pmid: N/A
Volatile products present in poly(vinyl chloride), PVC, after outdoor weathering have been compared with volatiles from laboratory, UV exposed PVC surfaces. Compounds both present in the photo‐degraded polymer surface layer and directly emitted from the polymer were measured in the laboratory experiments. The hydrocarbons, oxygenated compounds and chloro‐organic species were measured and identified by capillary gas chromatography with a mass spectrometer detector.
doi: 10.1002/masy.19971150105pmid: N/A
Parabolic model of the transition state was used for analysis of activation energies of reduction of benzoquinone (Q), aminyl (Am*), and nitroxyl (AmO*) radicals by peroxyl radicals of different structure. It was proved that radicals HO2* and R2C(OH)O2* reduce Am*, and AmO* with zero activation energy due to low triplet repulsion in the transition state. On the contrary alkylperoxyl radicals reduce Am* and AmO* slowly due to high triplet repulsion and sufficiently high activation energy. That is why AmH and AmOH perform cyclic chain termination in oxidizing substances where peroxyls with OH‐group are generated.
Gardette, Jean‐Luc; Posada, Fabrice; Philippart, Jean‐Louis; Kappler, Patrick
doi: 10.1002/masy.19971150106pmid: N/A
This article reports the results on the photooxidation mechanisms of two copolymers of fluorinated olefins and allyl or vinyl ethers. It is shown that the presence of the fluorine atoms influences strongly both the orientation of the reaction and the photooxidation kinetics. Due to the neighbouring fluorine atoms, the methylene groups in α‐position of the oxygen of the ether groups are not equivalent regarding oxidation and the secondary carbon becomes more oxidable than the tertiary one. Because these unexpected results were obtained, the study has been extended to non fluorinated polyethers. On the basis of the results obtained, a general mechanism of the primary oxidation of polyethers is given, and the role played by the fluorine atoms on the orientation of the reactions is discussed.
George, Graeme A.; Celina, Mathias; Lerf, Claude; Cash, Gregory; Weddell, David
doi: 10.1002/masy.19971150107pmid: N/A
A model for the heterogeneous oxidation of polypropylene (PP) is proposed in which it is considered that there is a small initial fraction, po, of oxidizing centres which have a high local rate of oxidation. Within these zones there is a free radical chain reaction producing secondary oxidation products, volatiles and chemiluminescence (CL) from peroxy radical termination reactions. These zones progressively spread (rate coefficient b/s−1) and the free radical reactions die away within the volume of the original zones, producing a measurable concentration of oxidation products (rate coefficient α/s−1). Analysis of the CL‐time curve as representing the instantaneous infectious, fraction, pi, in the spreading model enables the parameters po, b and α to be determined and profiles of the remaining fraction (pr) and dead or oxidized fraction (pd) constructed. Analysis of CL curves from 120°C to 150°C gives an activation energy for spreading in PP particles of 96kJ/mol. Both single particles and groups of particles of different types of PP have been examined and evidence is presented of rapid surface spreading of oxidation from particle to particle.
Habicher, Wolf D.; Bauer, Ingmar; Scheim, Kerstin; Rautenberg, Claudia; Loßack, Annett; Yamaguchi, Kazuo
doi: 10.1002/masy.19971150108pmid: N/A
The antioxidative action of mixtures of phenols, phosphites, HALS, a) and some of their transformation products in various compositions has been studied in the thermo‐ and photo‐oxidation of hydrocarbons and polypropylene under different conditions. In the AIBN‐initiated oxidation of hydrocarbons at low temperatures (< 80°C), hindered phenols, hindered aryl phosphites and the nitroxyl derivatives of HALS act antioxidatively when used individually in appropriate concentrations. Secondary HALS do not show any induction period, but a certain retardation of the oxidation process after some reaction time. The inhibiting efficiency of nitroxyls observed cannot be explained completely by the currently accepted action mechanisms of HALS, but is also related to the reaction of the nitroxyls with alkylperoxyl radicals. In mixtures with hindered phenols, HALS have almost no influence on the rate of thermooxidation at low temperatures. Their nitroxyl derivatives, however, always exhibit synergism, most pronounced when both stabilizers are used in equimolar ratios. During the photooxidation phenols lower the efficiency of HALS. The influence of mixtures of stabilizers on the oxidative stability of polypropylene is rather different and depends on the oxidation conditions, the structure, the concentration and the ratio of the stabilizers. Synergistic as well as antagonistic effects are observed. Both aliphatic and aromatic phosphites studied act synergistically when used together and with phenols. This demonstrates that for acting as synergist for phenols, the hydrogen peroxide decomposing capability of the phosphites, but not their chain breaking activity is important. HALS‐phosphites and phosphonites, containing amine and phosphorus units in one molecule, are highly effective inhibitors of photo‐ and thermooxidation and exhibit lower critical antioxidant concentrations and longer induction periods than phosphites alone. They even exceed the efficiency of phenols in many cases. Transformation products of phenolic antioxidants investigated act differently and in many cases contrarily under photo‐ and thermooxidative conditions. Therefore, they influence the efficiency of stabilizer mixtures also in a different way.
doi: 10.1002/masy.19971150109pmid: N/A
Ultraviolet absorbers (UVA's) undergo photodegradation with quantum yields on the order of 10−6 even in glassy, unreactive matrices such as poly(methyl methacrylate) (PMMA). The kinetics of UVA loss in a film or coating can be described by the equation: A = log10((1‐To)10 (A o‐kt) + 1) where A is the absorbance at time t, To is the initial transmission, Ao is the initial absorbance, and k is an “infinite absorption” zero order rate constant. The derivation and implications of this equation and a general review of UVA degradation chemistry are discussed.
Pospíšil, Jan; Nešpůrek, Stanislav
doi: 10.1002/masy.19971150110pmid: N/A
Deactivation of free radicals, hydroperoxides and harmful solar radiation are the principal mechanisms increasing stability of commodity and engineering polymers. The stabilization process is accompanied by transformations of individual additives. New compounds influence specifically the integral protecting mechanism. Products contributing to the stability are most beneficial. Mechanistic data are used for optimisation of the inherent chemical efficiency and physical persistence and for explanation of deactivation of stabilizers.
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