Consolidation of degraded polyurethane foams by means of polysiloxane mixtures: Polycondensation study and application treatment

Abstract

Polyurethane (PUR) ester foams are found in 20th century museum collections in a variety of artworks and objects. Unfortunately PUR ester foams are prone to degradation, and present – among other plastic materials – important conservation issues. They are very sensitive to moisture, and their hydrolysis leads to several degradation processes such as yellowing and embrittlement, drastically changing the artwork aspect and materiality. Few researches were conducted on the consolidation of PUR ester foams. In this paper, a curative treatment that should limit the hydrolysis of the degraded PUR foams, and that mechanically consolidate the foams’ structure is presented. This work focuses on the innovative use of alkoxysilanes mixtures and on their application modes on foam samples. Different binary mixtures of OTMS (n-octyltriethoxysilane, hydrophobic and trifunctional) and AMDES (3-aminopropylmethyldiethoxysilane, hydrophilic and bifunctional) in various proportions were tested. The kinetic evolution of their polycondensation using spectral decomposition of FTIR spectra revealed that OTMS does not react with atmospheric water vapor and that the presence of an amine (AMDES) is required to initiate its polycondensation. This result justifies the use of a mixture of both siloxanes. The different treatments were then applied on artificially degraded PUR ester foams using concentration of 2.5% and 10% of alkoxysilanes in HMDS (hexamethyldisiloxane, solvent). The treatment was applied by pouring the solution over the samples and resulted in a homogeneous impregnation of the foams, whereas vaporization modes were unsuccessful regarding the penetration of the treatment in the foam cells. The hydrophilicity of the treatments was characterized using water contact angle measurements and it appears that at least 25% of hydrophobic OTMS in the mixture is needed to decrease the wettability of the samples, in order to avoid further hydrolysis. The reinforcement properties were studied using mechanical tests, and resulted in a need of a solution of 10% (in HMDS) for an acceptable reinforcement. Finally, colorimetric measurements showed that the visual aspect of the degraded samples was improved with the treatment.