Antioxidants melt processing

To stabilize PO s against degradation during the melt processing, antioxidants (e.g., hindered phenols, sulfide esters, organic phosphites, secondary aromatic amines, and hindered amines), peroxide decomposers (usually a phosphite) and frequently acid acceptors (e.g., calcium stearate) are used. For special applications metal deactivators are also added. 

Transformation products of stabilizers formed during melt processing may exert either or both anti- and/ or pro-oxidant effects. For example, in the case of BHT, peroxydienones, PxD (reactions 9b, b") lead to pro-oxidant effects, due to the presence of the labile peroxide bonds, whereas quinonoid oxidation products, BQ, SQ, and G- (reaction 9 b, c, d) are antioxidants and are more effective than BHT as melt stabilizers for PP [29], The quinones are effective CB—A antioxidants and those which are stable in their oxidized and reduced forms (e.g., galvinoxyl, G-, and its reduced form, hydrogalvi-noxyl, HG) may deactivate both alkyl (CB—A mecha-... 

Another example where antioxidant performance can be improved dramatically lies in the mechanochemlcally initiated addition of reactive antioxidants on rubbers (5.10) or unsaturated thermoplastics such as ABS (12). For example, using thiol antioxidants 2 and 3 as the reactive antioxidants, Kharasch-type addition of the thiol function to the polymer double bond takes place during melt processing to give bound antioxidant adduct (see Equation 1) the polymer becomes much more substantive under aggressive environments. 

Radiation chemistry in polymer research, 168-169 Reactive macroalkyl radicals, formation, 409 Reactive modifiers addition of reactive antioxidants on rubbers, 417 adhesion, 420,422 demanding applications, 414,416 improving additive performance during melt processing, 412 polymer bound antioxidant, 418-419/ Reduced poly(vinyl chloride),... 

Hoang, E.M., Liauw, C.M., AUen, N.S., Fontan, E., and Lafuente, P, Effect of Metal Stearate Antacid on the Melt StahiUzation Performance of Phenolic/Phosphite Antioxidants in Metallocene LLDPE. Part 1 Melt Processing Stahility /. Vinyl Addit. Technol., 10, 3, 137 (2004)... 

Under normal conditions of melt processing, most antioxidants undergo oxidative transformations during the course of their action [21, 22, 23, 24, 25]. The overall stabilisation imparted to polymers by antioxidants, there-... 

To address the question of whether the biological antioxidant would withstand the high melt processing temperatures, dl-a-tocopherol (synthetic vitamin E) was extruded at different concentrations in PE and PP under different processing temperatures ranging from 210 °C to 285 °C. Similar experi-... 

Al-Malaika, S. Issenhuth, S. Burdick, D. The antioxidant role of vitamin E in polymers. V. Separation of stereoisomers and characterization of other oxidation products of dl-a-tocopherol formed in polyolefins during melt processing. J. Anal. Appl. Pyrolysis 2001, 73, 491-503. 

In the presence of a stream of hot air or high temperature and low pressure (e.g., during polymer melt processing) volatility becomes very important it is governed by the rate of diffusion of antioxidants, which, in turn, determines the rate at which the surface is replenished. " The influence of polymer sample shape and the structure and molar mass of antioxidants on volatility has received much attention. The rate of evaporation of antioxidants from rubber and polyethylene, for example, was found to be inversely proportional to the thickness of the sample and directly proportional to its surface area. Furthermore,... 

Al-Malaika, S. Ashley, H. Issenhuth, S. The antioxidant role of ot-tocopherol in polymers. I The nature of transformation products of a-tocopherol formed during melt processing of LDPE. J. Polym. Sci. Part A, Polym. Chem. 1994, 32, 3099-3113. 

Scheme 1.70. Pro-oxidant activity of alkyl mercaptans (a) and both pro-oxidant and delayed antioxidant activity of diphenyl disulfides (b) during melt processing of polymer PH containing hydroperoxide groups POOH. After Scott (1993b).

Phenolic antioxidants are well known for being melt processing stabilizers as well as long-term thermal stabilizers. In the chemiluminescence measurements on the polyethylene films under oxygen (Fig. 3.3), the antioxidant effect of the phenols is clear when these induction times are compared with those of the free additive polyethylene film (0.73 h) (Table 3.2). The results showed that the structure of the phenolic moiety will be a cmcial factor influencing the stabilization performance. 

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