Synergism-HALS

Synergism can also arise from cooperative effects between mechanistically different classes of antioxidants, e.g., the chain breaking antioxidants and peroxide decomposers (heterosynergism) [42]. For example, the synergism between hindered phenols (CB—D) and phosphites or sulphides (PD) is particularly important in thermal oxidation (Table 2). Similarly, effective synergism is achieved between metal dithiolates (PD) and UV-ab-sorbers (e.g., UV 531), as well as between HALS and UV-absorbers, (Table 3). 

HALS exhibit a complex behavior when present in combination with other antioxidants and stabilizers. Effective synergism in both melt and thermal stabilization has been achieved when secondary and tertiary HALS were used in combination with both aromatic and aliphatic phosphites the synergistic optimum depends on the structure of the phosphite. HALS also synergize the action of UV-absorbers, e.g., benzo-triazoles, in different polymers such as polypropylene, polystyrene, and ABS. " ... 

In many stabilizer formulations, different types of stabilizers are used. As processing stabilizer phenolic antioxidants and phosphites are applied, for long-term heat stability phenolic antioxidants and thioethers are used and for UV stability, combinations of HALS with other types of UV stabilizers can be applied. HALS stabilizers show interactions with these types of stabilizers that can lead to synergisms as well as antagonisms. 

The interaction between UVAs and HALS is one of the most investigated interactions. In some cases, a synergism between these stabilizers is reported [16,122,124—127], although antagonisms are reported too [16,122,127]. 

FIGURE 17.5 Half-life time (time to 50% retained elongation) of different HDPE plaques (2 mm thick) showing synergism of HALS (LMW-HALS-1) and UVA (UVA-BZT-1) (total amount UVA + HALS — 0.2 wt%) [124]. 

In HDPE combination of HALS with UVA-HTZ-1 (at an overall level of 0.1 % w/w) exposed in a Microscal Unit at about 50°C led to weak antagonism for LMW-HALS-3, LMW-HALS-4, and LMW-HALS-9. However, for LM-HALS-26 and HMW-HALS-4 a slight synergism was observed [127]. 

Low and High Molecular Weight HALS It was shown that combinations of low and high molecular weight HALS led to an unexpected synergism in PP tapes (Table 17.4), although the effect of the combination on the UV stability is not... 

Two High Molecular Weight HALSs The synergistic effect of a LMW-HALS and a HMW-HALS can be ascribed to the difference in diffusion and evaporation rate. However, this is not the case for the combination of two HMW-HALSs, nevertheless synergisms were shown, which must be a result of very specific interactions or specific protection mechanisms of the components [122]. 

As mentioned above for PP synergisms between low and high molecular weight HALS as well as between two high molecular weight HALSs were reported. 

In contrast to polyamides in polystyrene phenolic antioxidants are not able to reduce the decrease in tensile strength during UV irradiation [141]. UVAs as well as HALS stabilizers are effective in polystyrene (PS). The best protection can be reached by using combinations of a UVA and a HALS. According to Gugumus [133,134] the synergism between UVA and HALS is distinct when the time until a AYI = 5 is taken as failure criterion (see Table 17.12). However, when as criterion the yellowness index after 1600 h in a Weather-Ometer is taken, the synergism between a UVA and a HALS is much smaller (see Fig. 17.14). 

F. Gugumus, Possibilities and limits of synergism with light stabilizers in polyolefins 1. HALS in polyolefins, Polym. Degrad. Stab. 2002, 75, 295-308. 

Fig. 8. Effect of molar composition of mixture of HALS and Irgafos 168 (total [AO] = 7 X 10 mol/kg in each case) on (a) extent of synergism and antagonism during photooxidation of PP (193), and (b) thermoxidative stability of PP in air oven (195).

Light stabilization is mandatory for SB and ABS because of their butadiene content. Combinations of UVA with HALS lead to a notable improvement of mechanical properties during service of the components. The good light-protective effect of the UVA/HALS combination is attributed to the observed synergism, where the UV absorber protects the interior layers and HALS provides the necessary surface protection. This goes hand in hand with significant improvement in color behavior [86]. 

The HALS compounds are largely used for polyolefin photostabiUsation due to their competitive costs and high performances, which permit the utilization of lower amounts than UV absorbers such as benzophenone and benzotriazole compounds. The mixtures of HALS compounds with benzophenones present some synergic photostabUization effects [11], being reported also an antirad efficiency of them [44,45]. 

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