![]() Mihnea-Alexandru Găman, in Phytochemicals as Lead Compounds for New Drug Discovery, 2020 6.1.1.1 Antioxidant activityĪntioxidants are free radical scavengers which neutralize reactive oxygen species (ROS) produced during aerobic cellular metabolism: superoxide (O 2 −), hydrogen peroxide (H 2O 2) and peroxynitrite (OONO−). It was not absorbed by fibers present in the formulation and did not interact with pigments during fiber spinning. A new polysiloxane-based HALS, with molecular weights appropriate for the application, provided enhanced stabilization in experimental studies and had lower volatility and extractibility. High molecular weight HALS are used in thin sections, such as fibers for upholstery, cover seats, and door panels, where migration to the surface must be minimized. A monomer-oligomer blend, with different migration rates, can reduce the loss of stabilization, but physical properties and molecular weights exhibit a discontinuity. In talc- or calcium carbonate-filled resins, monomers are partially absorbed by the filler, resulting in a loss of stabilization. Monomers, however, can be lost from the surface through evaporation or extracted by fluids such as detergents or rain. Low molecular weight monomers can be used in thick sections such as bumpers, which require HALS migration toward the part surface for effective stabilization. HALS are widely used in stabilizing polypropylene for automotive applications. High molecular weight HALS are used in tapes and fibers, due to the tendency of low molecular weight stabilizers to volatilize or migrate out of the resin during processing, post-extrusion treatment, or long-term exposure. Their resistance to pesticides makes them useful in agricultural film. Because stabilization is through a chemical reaction, they are not limited to thicker sections, such as outdoor seating and trash receptacles, but can be used in applications with high surface-to-volume ratios, such as films, slit tapes, and fibers. ĭue to their effectiveness, hindered amines have been substitued for UV absorbers in applications such as food packaging. The destabilizing effects of halogenated flame retardants can be reduced by using a hindered amine with lower basicity ( Figure 3.9) or by using a UV absorber to provide a screen for the HALS, by reducing the amount of acid generation. Interaction with halogenated flame retardants can deactivate HALS acid generated by the flame retardant reacts with the basic amine structure and interrupts the free radical scavenging reactions. Organic phosphites, which act synergistically with HALS, can be used to reduce the effect of thioesters. ![]() Thioester antioxidants can negatively affect the free radical scavenging activity of HALS. These radicals then couple to form yellow oxidation products. Oxidation products from this reaction can react with BHT in the basic environment provided by hindered amines to produce long-lived free radicals. Discoloration can also occur in the presence of titanium dioxide pigments, which are photoreduced by ultraviolet light at 350–400 nm. Although HALS do not impart color to the resin they do react with phenolic antioxidants to produce deep yellow colors after radiation sterilization. Hindered amines can interact with other additives in the formulation. Use levels vary with substrate thickness, pigmentation, and application requirements levels generally range from 0.1–1.5%. ![]() Although they are more expensive than UV absorbers, they are more cost effective due to lower use levels. They are extraction-resistant and are more compatible with the resin than benzophenones or nickel complexes. They can provide surface protection and can be used alone or in combination with UV absorbers and quenchers. They display low volatility and good stability at high temperatures. HALS are available in a wide variety of molecular weights. The alkoxy amine reacts with peroxy free radicals to regenerate the nitroxyl radical (NO ![]() ) this then reacts with a polymer or impurity free radical (R.The amine is oxidized by reaction with a hydroperoxide to form a nitroxyl radical (NO
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