Chemical Structure of Cured Films

As intended by the adhesive formulation, no parasitic side reactions such as formation of urea, new uretdione, isocyanurate, biuret, or allophanate are detected for the bulk and for thick films on Al, Cu, and Au (dpu l pm cf Figs. 6.3, 6.7). Even ultrathin PU films on Au correspond to bulk properties to a very good approximation (Fig. 6.8), which is a recommendation for films on Au as references with respect to effects in the interphase on different metals. 

The extent of 1640 cm or 1675 cm band formation on the one hand, and phenylene peak intensity changes on the other, seem to be closely related, as one effect is never observed without the other. Further, there are connections with the availability of moisture during substrate storage, which increases the amount of adsorbed water and hydroxide on the surface that would react with the isocyanate groups. Therefore, urea or urea-like species (showing bands at 1695-1630 cm [30]) are the most reasonable candidates for these chemical structure modifications observed in the interphase on Al and - less intensely - on copper. 

This reaction is associated with the formation of new isocyanate groups, which immediately add to excess hydroxyl groups to form more urethane species. The emergence of carbodiimide is still incomplete after 4 h of post-curing in films on Au and A1 and in the bulk. Only on copper are no carbodiimide bands observed after post-curing, even in thick films (e.g., dpu = 2pm in Fig. 6.7). Instead, a new chemical species develops with an IR band at 1660 cm , which is probably a reaction product of the carbodiimide consumption. 

Uretdione groups (isocyanate dimers traces originate from the as-received Desmodur CD component) are subject to similar effects. In films on copper, uretdion bands (1780 cm ) vanish during post-curing, independently of fihn thickness (Figs. 6.7 and 6.9). 

The specific mechanism of carbodiimide and uretdione conversion on Cu is not sufficiently understood, yet We stiU assume very specific reactions due to the influence of mobile copper ions. These findings can be related to the detection of Cu ions by XPS and TOF-SIMS, even within thick polyurethane layers 

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