Thermal antioxidants

Oligomeric hindered amine light stabilizers are effective thermal antioxidants for polypropylene. Thus 0.1% of A[,Af-bis(2,2,6,6-tetramethyl-4-piperadinyl)-l,6-hexanediamine polymer, with 2,4,6-trichloro-l,3,5-triazine and 2,4,4-trimethyl-2-pentaneainine [70624-18-9] (35) (Fig. 5), protects polypropylene multifilaments against oxidation when exposed at 120°C in a forced-air oven (22) for 47 days. 3,5-Di-/ l -butyl-4-hydroxytoluene [128-37-0] (0.1%) affords protection for only 14 days. 

Note 1 Antioxidants acting under specified conditions may be referred to as thermal antioxidants, photo-antioxidants, mechano-antioxidants, etc. 

It might be assumed that, as condensed-phase flame retardants function by modifying the normal thermal degradation processes of polymers, they would also function as thermal stabilizers and that thermal antioxidant stabilizers would show flame-retardant properties. However, these statements are rarely the case, and to understand why, it is necessary to compare the mechanistic aspects of flame retardance as discussed earlier with those of thermal degradation and thermal oxidation as well, briefly alluded earlier, and in the case of the latter, the Bolland and Gee mechanism,17 in Scheme 2.1. 

Photoantioxidants are typified by the class of HAS which although were developed for photostabilization of polyolefins, also possess thermal antioxidant properties. They are generally assumed to function as primary antioxidants in that they scavenge radicals and in particular, peroxy radicals. 

Table I shows the results obtained with various bisphenols which are generally considered quite effective as thermal antioxidants. Most of these additives did not retard the discoloration of the polymer. One compound, 4,4 -methylenebis(2,6-di-terf-butylphenol), showed some promise, but it discolored the polymer initially and, therefore, cannot be considered of practical importance.

Heat-resistant [218] soft foams were prepared from the blends of hdPE with E-P random copolymers. The azodicarbanamide acts as a thermal antioxidant and the crosslinking of the blend was increased by electron beam radiations and foamed at 225 °C with 2320% expansion. A blend of 35 wt.% PE-PP (8 92), 15 wt.% E-P block copolymers, and 50 wt.% EPDM showed accelerated weathering resitance [219] 1000 h probably due to crosslinking between constituents of the block copolymer, polyblend and EPDM. The effect of filler and thermodynamic compatibility on kaolin-filled PE-PP blend was studied by Lipatov and coworkers [220]. The thermodynamic interaction parameter (%) decreased and thermodynamic stability increased by filler addition, the degree of crystallinity decreased with increasing thermodynamic compatibility of the components due to sharp decrease in the phase separation rate during cooling. 

The mechanisms of thermal antioxidant action have been extensively studied over the past thirty years and can be considered to be reasonably well under-stood. Several comprehensive reviews are available (1-3), and antioxidants have been classified into two main types (4,5), namely preventive and chain-breaking. The former act... 

Fully hindered amines are excellent UV stabilizers, but poor thermal antioxidants, for polyolefins. We have found that partially hindered bicyclic amines, such as 3,3-dialkyldecahydroquinoxalin-2-ones, are excellent UV stabilizers and also excellent thermal antioxidants as well. The oxidation of these bicyclic amines with m-chloroperbenzoic acid was studied by electron spin resonance spectroscopy. They form stable, partially hindered nitroxyl radicals (6-line spectra). However, these primary radicals are easily oxidized to a new, fully hindered nitroxyl radical (3-line spectra) which are also very stable. Field desorption mass spectroscopic studies of the oxidation of these amines show that the primary nitroxyl radicals, which form first, lose two hydrogen atoms and add an oxygen. Thus, these bicyclic partially hindered amines are unique in being both stable and at the same time hydrogen atom donors. 

Table 8 shows that over 50% binding of MADA (VIII) can be achieved in both NR and SBR by the mechanochemical procedure (21) and that this increases during vulcanization to about 70%. Other sulfur-containing antioxidants, VII and IX, can be bound to similar levels in both polymers. Figure 5 shows the effect of extraction on MADA-B in NR in comparison with a conventional thermal antioxidant, WSP (II), at 100 C. Similar results for SBR are listed in Table 9 before and after solvent extraction. The hindered phenolic antioxidant, BHBM-B is in general somewhat less effective than MADA-B, but is nevertheless much less affected by extraction than are conventional add it ives. 

The unsaturation and hydroperoxides formed initially in the polymer during the early stages of processing both disappear as a result of the peroxide catalyzed thiol addition to the double bond (41, 42). The resulting BHBM-B is a very powerful thermal antioxidant in an air oven test at 140°C (41), see Figure 9. EBHPT can be similarly bound to PVC to give an effective UV stabilizer (42). 

Thiol antioxidants can also be reacted with polyethylene and polypropylene (40). Figure 10 illustrates the superiority of MADA-B in PP compared with conventional thermal antioxidants when subjected to continuous hot water leaching. It is also an effective processing stabilizer and photoantioxidant. Table 15 shows that the latter effect is resistant to solvent extraction. The loss of activity after extraction corresponds to the amount of unbound MADA present. 

Some of the most powerful uv stabilisers belong to the class of peroxide decomposing preventive antioxidants and it has been suggested that the mechanism of this type of uv stabiliser is not distinguishable from their behaviour as thermal antioxidants although all peroxide decomposers do not behave as uv stabilisers (11). Of paramount importance in the peroxide decomposer -uv stabiliser class are the metal dithiocarbamates (III) (9,22,23,24) the dithiophosphates (IV) (11,24) and the... 

N.S. Allen, A. Chirinos-Padron, and J.H. Appleyard, Photo-stabilizing action of metal chelates in polypropylene. Part III. Thermal antioxidant action and its relationship to photo-stabilization, Polym. Degrad. Stab. 1983, 5, 55-63. 

Best thermal antioxidant performance was seen with a combination of 0.2 wt% phosphite and 0.2 wt% of either of the hindered phenols. 

Russian researchers [18, 19] investigated the efficiency of thermal antioxidants in poly(butylene terephthalate) (PBT). Good results were obtained using combinations Irgafos 168 or Ultranox 626 with Irganox 1010 or stearyI- 3-(3,5-di- butyI-4-hydroxyphenyl) propionate (Irganox 1076 Ciba). 

As well as being excellent UV stabilisers for several polymer applications, there is ample evidence [31] to suggest that hindred amine light stabilisers can also operate as thermal antioxidants. Discussion of these additives will be undertaken in Chapter 8. 

Eew radical scavengers besides the HALS have been suggested as UV stabilisers for aromatic polyesters. Some time ago, it was noted that certain nitro compounds could be used as UV stabilisers for polyesters, but closer reading of the documents showed this to be limited to certain sulfonated copolyesters [202]. Benzofuran-2-ones have been suggested as UV stabilisers with a good chain breaking -acceptor action, i.e., capable of scavenging alkyl radicals [203, 204, 156], but these are more efficacious as thermal antioxidants and process stabilisers, especially when used in conjunction with hindered phenolic antioxidants. 

Table 2.48 Synergism between a UV absorber and a thermal antioxidant (6)...

Carbon black can fnnction as a UV stabilizer, thermal antioxidant, extender in crosslinked polyethylene (XLPE) cable compounds, antistat in vinyl records, modifier of polymerization rate in nnsaturated polyesters, conductive filler, and colorant. Although commonly used in rubbers and thermoplastics, carbon black does not improve the properties of thermosetting resins significantly. However, it is often used as a pigment and for obtaining electrical conductivity. 

THERMAL ANTIOXIDANTS STABILIZATION OF PP AND MECHANISMS OF ANTIOXIDANT ACTION... 

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