Electron Spin Resonance Imaging of Degradation and Stabilization Processes:  Behavior of a Hindered Amine Stabilizer in UV-Exposed Poly(acrylonitrile−butadiene−styrene) Polymers

Abstract

Introduction. Polymeric materials exposed to heat, mechanical stress, and ionizing or UV irradiation undergo degradation in the presence of oxygen due to the formation of reactive intermediates such as free radicals R • , RO • , and ROO • and hydroperoxides ROOH. 1-10 The degradation process can be accelerated by chromophores, free radicals, and metallic residues from the polymerization reactions. The structural and morphological changes in the polymer properties due to exposure to environmental factors are often profound and include modification of the chemical structure (for instance, formation of double bonds, chain scission, and cross-linking) and loss of crystallinity, especially at large radiation doses. Often the deleterious effects are not immediately detected but develop over longer periods. The gradual changes in the polymer properties observed in many systems including polyolefins, and the ultimately grave results, are due to trapped radicals that react slowly, to peroxy radicals that decompose in time with formation of reactive radicals and gas molecules, and to trapped gases that lead to local stresses and to cracking. 3,9 While the time scale of these changes may vary, the final results are dramatic: degradation of the structure and collapse of the mechanical properties. The accelerated rate of ozone depletion in the stratosphere due to environmental factors is expected to raise the level of UV-B radiation (λ ) 290-320 nm), thus adding severity to the problem of degradation and urgency to the need for solutions.Hindered amine stabilizers (HAS) rank among the most important recent developments for light and heat stabilization of polymeric materials. Nitroxides and hydroxylamine ethers are the major products of reactions involving HAS. The HAS-derived nitroxides are thermally stable but can react with free radicals (as "scavengers") to yield diamagnetic species; the hydroxylamine ethers can regenerate the original nitroxide, thus resulting in an efficient protective effect. 6,9 The pivotal role of nitroxides (added as such or derived from HAS) has been documented recently in low-molecular-mass model systems. 7 In polymers the situation is more complicated, and the equilibrium concentration of the nitroxides varies markedly with the nature of the polymer matrix, thermal history, humidity, and type of

EPR Imaging Determination of High Molecular Weight Nitroxide Radicals in the UV Degradation of Polycarbonate-Poly(acrylonitrile-butadiene-styrene) Polymers

“…Actually, by recording the spectrum at 330 K no significant differences in the line shape were observed with respect to the room temperature spectrum. This contrasts with what found by Schlick and coworkers [9,10] in the EPR spectra of irradiated ABS samples stabilized with Tinuvin 770. They reported the presence of two superimposed spectra assigned to a ''fast'' component located in domains essentially consisting of polybutadiene sequences (T g % 200 K) and a ''slow'' component located in domains consisting of polystyrene (T g % 370 K) or polyacrylonitrile sequences (T g % 360 K).…”

“…Very recently thermal and UV oxidative degradation of ABS stabilized with Tinuvin 770, has been investigated by means of the EPRI technique by Schlick and co-workers. [9,10] Two spectral components were detected in the EPR spectra: a ''fast'' component located in domains dominated by polybutadiene sequences and a ''slow'' component located in domains dominated by polystyrene or polyacrylonitrile sequences. It was also found that the polybutadienerich domains in ABS are the more vulnerable to UV radiation.…”

Time Evolution of the Concentration Profiles of HALS Stabilizers and of the Corresponding Oxidation Forms across Poly(propylene) Plaques Irradiated with UV-Visible Light

“…The importance of oxygen diffusion in determining the spatial distribution of nitroxides inside irradiated polymers and its role in the degradation mechanism of the polymeric materials may possibly explain the results obtained in different polymeric matrices [21] and requires a deeper investigation.…”

Spectral spatial electron paramagnetic resonance imaging as a tool to study photoactive dimethacrylate-based dental resins

“…They originated from hindered amines used as stabilizers. Lucarini et al carried out detailed studies on PP using 1D EPRI [23][24][25], Marek et al extended the work to polystyrene [26], while Schlick and collaborators used both 1D EPRI and 2D SSI to study the degradation of heterophasic system such as poly(acrylonitrile-butadiene-styrene) and propylene-ethylene copolymers [27,28], or very recently to measure diffusion coefficients of nitroxide radicals in propylene-ethylene copolymers [29] (for review on this topic see Refs. [30][31][32] by Schlick).…”