Light stabilizers polypropylene

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. 

There are four modes of action important in stabilizing polypropylene (1) free radical scavenging (2) decomposition of peroxides (3) regeneration of inhibitors in synergistic antioxidant systems (4) absorption of ultraviolet light and re-emission of absorbed energy in nondegrading forms. 

Bauer I, Habicher W D, Korner S and Al-Malaika S (1997) Antioxidant interaction between organic phosphites and hindered amine light stabilizers effect during photooxidation of polypropylene, Folym Degrad Stab 55>217-224. 

The mode of action of hindered piperidine compounds continues to be rather controversial. A recent study of the effect of different N-substituents on the light stabilizing activity of a particular hindered piperidine molecule in polypropylene showed that the nitroxyl radical —O was the most effective... 

Polyolefins - An Arrhenius model has been developed for lifetime prediction of the light stability of polypropylene . Photooxidation processes in blends of polypropylene with poly(butylene terephthalate) (PBT) are complicated by the thermal sensitivity of the polypropylene and the screening effect of the terephthalate ester . This effect is shown in Scheme 1. 

As shown in Table I, these stable radicals showed strikingly higher light stabilization activity in polypropylene than that of the UV absorber tested. We felt that their activity was related to their radical scavenging ability. This hypothesis is supported by the observation that the coupled products (32) and (33) were obtained by the reaction of the nitroxyl radicals (2) and (27), respectively, with a C-radical derived from AIBN (10). The radical scavenging ability of the stable nitroxyl radicals is now well known to play a major role in the mechanism of light stabilization by hindered amine compounds (13). 

These results suggest that hindered amine compounds could be converted to stable nitroxyl radicals through the corresponding amino radicals in polymers that is, they are the precursors of the stable radicals. In fact a key compound, 2,2,6,6-tetramethyl-4-oxopiperi-dine (42), showed high light-stabilizing activity in polypropylene, comparable to that of the nitroxyl radicals, lending support to this interpretation. 

Synthesis and Stabilizing Activity of Hindered Amines. As mentioned previously, the hindered piperidine compounds showed excellent light-stabilizing activity in polypropylene. In order to find more efficient compounds, various derivatives of 2,2,6,6-tetramethyl-4-oxopi-peridine (42) were synthesized and tested. In this section we describe some typical examples from the great number of derivatives prepared in our laboratory. 

Figure 7. Light stability of polypropylene tapes (Florida) KLY to 50% retention of tensile strength.

Figure 8. Light stability of 2 mm polypropylene plaques (Arizona) KLY to 507, retention of impact strength.

The properties of hindered diazacycloalkanones were evaluated as light stabilizers and antioxidants for polypropylene. A totally hindered piperazinone was found to be excellent light stabilizer with many other desirable properties as a polymer additive. Partially hindered decahydroquinoxalinone derivatives were found to be surprisingly good antioxidants. The relationship between structure parameters and activity was investigated. 

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