Influence of thickness on the final mechanical properties

The final mechanical properties determined on different completely postcured PU sheets after more than two months revealed the interesting fact that even if these samples were made and stored simultaneously under the same conditions, the final mechanical properties were not totally similar (Table 4.30). 

As observed, the values of tensile stress and tensile strength were somewhat higher in thinner films probably because in this case the more polar urea group formation was favored to the prejudice of the appearance of allophanate groups. The 100% tensile stress value was similar in aU the cases due to the fact that, as 

Another aspect of possible postcure reactions, which however, is not yet studied enough, is the role of the atmospheric oxygen on the superficial processes also leading to possible NCO groups consumption both in the thermal cure treatment at 110°C, as well as in the postcure processes. 

Although, it is known that the atmospheric oxygen leads to the degradation of PUs in time, it should be possible that oxygen may improve some consolidation of polymer by crosslinking in the first step of the process. 

Recently (year 2010), our studies have been continued by Pichon et al. [347], who also investigated the evolution during maturation and the consequences on the PUs elastic properties. The material used in their work was based on the diisocyanate MDI, and on the polyester PCL. Hydroquinone bis(j0-hydroxyethyl)ether (HQEE), was preferred to the chain extender EDO, because of its higher incompatibility with the polyester SS [9,347]. The polymers were cast with a small excess of isocyanate (6%) and kept in time under atmospheric humidity at ambient temperature. DMA 

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