Influence of Stabilizers
Stabilizers are common additives used in plastics, sealants and adhesives to improve durability and weatherability . Several classes of stabilizers are commercially available, such as UV absorbers (UVAs), antioxidants, hindered amine light stabilizers (HALS) and others. Often combinations of stabilizers are used to increase the sealant performance and to utilize synergistic effects which sometimes exist between stabilizers.
In Table 5, a PU-1 formula without stabilizers (reference) is compared with a sealant formula stabilized with an antioxidant and HALS (formula A), and with a formula which had, in addition to antioxidant and HALS, an UV absorber added (formula B). The nonstabilized reference gave rather poor results, especially with respect to cracking, chalking, and yellowing. The performance could be significantly improved by using stabilizers (formulas A and B). The additional use of an UV absorber in formula В did not result in a further improvement of the sealant performance.
The series given in Table 5 illustrates that in many cases stabilizers or suitable combinations of stabilizers will improve sealant durability compared to a nonstabilized sample. However, there are instances where stabilizers do not improve the overall durability or even have a negative influence. Examples for such a behavior are given in Tables 6 and 7.
In Table 6, a PU-1 formula without stabilization (reference) is compared with two formulas which were stabilized with 0.2 % of a UVA each. In the total durability rating, no significant difference between the nonstabilized sealant and the formulas with added UVA could be seen. However, when comparing chalking performance, the formula stabilized with an oxalanilide gave a worse result than the nonstabilized reference; in addition, with respect to heat stability, the formula containing a benzotriazole derivate showed a performance slightly inferior to the nonstabilized sealant.
Another example for negative effects that stabilizers can give on occasion is shown in Table 7 for two antioxidants; Addition of antioxidants to the unstabilized reference resulted in an increase of the yellowing.
“Formula A: 0.2 % antioxidant (trifunctional sterically hindered phenol) and 0.2 % HALS (dipiperidinyl seba – cate derivative); formula B: 0.2 °k UVA (benzotriazole derivative). 0.2 % antioxidant (same as formula A) and 0.2 % HALS (same as formula A); Film thickness: 2 mm; samples exposed in a xenon arc weathermeter6, exposure time: 500 h.
bCracking 1: no cracks; 2: very small cracks; 3: small cracks; 4: large cracks; 5: entire surface covered with large cracks.
cChalking: 1: no chalking; 2: onset of chalking; 3: some chalking; 4: chalking; 5: severe chalking.
‘‘Yellowing: relative rating based on visual comparison of the exposed with an unexposed sample; 1: no color change; 2: beginning of yellowing: 3: slight yellowing; 4: yellowing; 5: severe yellowing. eHeat stability: change of Shore A hardness at 80°C after 2 months relative to the value before heat exposure; 1: 0-5 %; 2: 5-33 %; 3: 34-67 %; 4: 67-95 %; 5: 95-100 %.
In the case of the second antioxidant (2,4-bis(dodecylthiomethyl)-6-methylphenol) the chalking was also worse than for the reference. In this example, none of the properties studied were actually improved by addition of an antioxidant only.
In conclusion, it can be said that stabilizers often improve the weathering performance and durability of sealants (Table 5), However, there are also cases in which a stabilizer can have a negative impact on the total weatherability of a sealant, or on selected weathering properties (Tables 6 and 7). Thus, each stabi-
TABLE 7—Influence of antioxidants on the performance of a PU-1 sealant".
TABLLi 8—Influence of the catalyst amount of a MS-1 sealant on the accelerated weathering performance (cracking)“&.
‘“Film thickness: 3mm; samples exposed in an open flume carbon arc wcathermeler.
bCracking 1: no cracks: 2: very small cracks; 3: small cracks; 4: large cracks; 3: entire surface covered with large cracks.
cphr: Pans of catalyst (dibutyltin diacetatc) per hundred parts of resin. dChalking was observed.
lizer or mixture of stabilizers needs to be carefully tested and optimized: The use of a standard stabilizer pack in all formulas might give good results in many cases, but may also result in a reduced weathering performance for others.