Publicação
PURscCO2: Supercritical carbon dioxide-assisted consolidation of polyester-based polyurethane foams with an alkoxysilane mixture
| Resumo: | Polyurethane (PUR) foams are among the most unstable plastics in museum collections. Ideally, consolidation treatments should allow deep and uniform penetration of the consolidant while maintaining or improving the foam’s flexibility. This is especially important for polyester-based polyurethane (PUR-ES) foams, which degrade primarily by hydrolysis, causing drastic losses of cohesion throughout the entire foam network. Current alkoxysilane-based consolidation treatments, such as the combination of 3-aminopropyl(diethoxy)methylsilane (APDEMS) and n-octyltrimethoxysilane (OTMS), have shown promising results in reinforcing PUR-ES foams while enhancing their hydrophobicity. However, traditionally used application techniques present significant limitations, particularly immersion, which is not adequate for large-scale objects as it requires large quantities of solvent. Supercritical carbon dioxide (scCO2) emerges as a potential alternative for the consolidation of PUR-ES foams due to its unique properties combining the relatively high density of a liquid with the low viscosity and high diffusivity of a gas. The premise of this technology is that it may carry the consolidant deeper, acting as both a solvent and carrier, promoting uniform diffusion and deposition, without the use of hazardous solvents or direct handling of fragile objects during treatment. Therefore, with the main goal of finding a more sustainable and effective consolidation treatment for PUR-ES foams, the current study investigates the suitability of scCO2 as a solvent and carrier to diffuse a binary mixture of alkoxysilanes within PUR-ES foams. A key aspect of this research involved assessing the solubility of alkoxysilanes in scCO2 and the foam’s sorption capacity for CO2, as temperature and pressure variations can significantly influence the dissolution and diffusion properties of the consolidant. Understanding these parameters was crucial for designing experimental conditions. Based on a theoretical discussion and validation of the experimental process, a proof-of-concept study was conducted comparing scCO2-assisted consolidation with traditional spray application. Results showed that both methods preserved the visual appearance of treated samples. However, chemical analyses revealed that while spray application primarily led to superficial consolidation, scCO2 treatment promoted a more uniform and in-depth consolidant penetration. Refinements were made to the consolidation process based on the findings from the proof of concept. The impact of PUR-ES foam ageing conditions on the study's representativeness and treatment efficacy, the polymerisation behaviour of alkoxysilane monomers and mixture miscibility in scCO2, were assessed. ATR-FTIR analysis confirmed that the degraded foams used were representative for cross-study comparisons and that there were slight differences between the polymerisation of the monomers in room conditions and in scCO2. Experimental modifications, such as monomer positioning within the reactor, improved mixture homogeneity in scCO2. Other factors influencing polymerisation, such as pH and residual water, were also explored. The study further investigated the impact of varying APDEMS and OTMS ratios, treatment duration, and water addition on consolidant uptake, penetration depth, hydrophobicity, and mechanical reinforcement. Chemical analysis confirmed the presence of the consolidant in all treated samples, although with a heterogeneous distribution (more consolidant on the external surfaces than in the inner layers), likely due to degradation pathways. Water-containing treatments exhibited non-uniform consolidant distribution and lower uptakes, whereas treatments without water yielded higher uptakes (especially with extended exposure to scCO2) and more uniform consolidant distribution within the cell walls. Hydrophobicity assessment revealed inconsistent results, yet higher APDEMS content mainly increased water absorption. Compression-force-deflection tests showed that treated samples behaved like controls and the results were not reproducible, though high data variability limited statistical confirmation. Nonetheless, qualitative observations indicated improved resistance to compression and surface cohesion in consolidated samples. Ultimately, this investigation identifies limitations faced in laboratory-scale testing and provides a framework for further optimising treatment conditions. Future research should focus on expanding the sample dataset, assessing re-treatment and long-term performance, and scaling up to real objects, a crucial step in validating scCO2-assisted consolidation for collections. Overall, this research demonstrates the potential of scCO2 as a viable and sustainable method for consolidating aged PUR-ES foams. |
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| Autores principais: | Soares, Inês |
| Assunto: | polyurethane foam supercritical carbon dioxide treatment contemporary heritage sustainability |
| Ano: | 2025 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade Nova de Lisboa |
| Idioma: | inglês |
| Origem: | Repositório Institucional da UNL |
| Resumo: | Polyurethane (PUR) foams are among the most unstable plastics in museum collections. Ideally, consolidation treatments should allow deep and uniform penetration of the consolidant while maintaining or improving the foam’s flexibility. This is especially important for polyester-based polyurethane (PUR-ES) foams, which degrade primarily by hydrolysis, causing drastic losses of cohesion throughout the entire foam network. Current alkoxysilane-based consolidation treatments, such as the combination of 3-aminopropyl(diethoxy)methylsilane (APDEMS) and n-octyltrimethoxysilane (OTMS), have shown promising results in reinforcing PUR-ES foams while enhancing their hydrophobicity. However, traditionally used application techniques present significant limitations, particularly immersion, which is not adequate for large-scale objects as it requires large quantities of solvent. Supercritical carbon dioxide (scCO2) emerges as a potential alternative for the consolidation of PUR-ES foams due to its unique properties combining the relatively high density of a liquid with the low viscosity and high diffusivity of a gas. The premise of this technology is that it may carry the consolidant deeper, acting as both a solvent and carrier, promoting uniform diffusion and deposition, without the use of hazardous solvents or direct handling of fragile objects during treatment. Therefore, with the main goal of finding a more sustainable and effective consolidation treatment for PUR-ES foams, the current study investigates the suitability of scCO2 as a solvent and carrier to diffuse a binary mixture of alkoxysilanes within PUR-ES foams. A key aspect of this research involved assessing the solubility of alkoxysilanes in scCO2 and the foam’s sorption capacity for CO2, as temperature and pressure variations can significantly influence the dissolution and diffusion properties of the consolidant. Understanding these parameters was crucial for designing experimental conditions. Based on a theoretical discussion and validation of the experimental process, a proof-of-concept study was conducted comparing scCO2-assisted consolidation with traditional spray application. Results showed that both methods preserved the visual appearance of treated samples. However, chemical analyses revealed that while spray application primarily led to superficial consolidation, scCO2 treatment promoted a more uniform and in-depth consolidant penetration. Refinements were made to the consolidation process based on the findings from the proof of concept. The impact of PUR-ES foam ageing conditions on the study's representativeness and treatment efficacy, the polymerisation behaviour of alkoxysilane monomers and mixture miscibility in scCO2, were assessed. ATR-FTIR analysis confirmed that the degraded foams used were representative for cross-study comparisons and that there were slight differences between the polymerisation of the monomers in room conditions and in scCO2. Experimental modifications, such as monomer positioning within the reactor, improved mixture homogeneity in scCO2. Other factors influencing polymerisation, such as pH and residual water, were also explored. The study further investigated the impact of varying APDEMS and OTMS ratios, treatment duration, and water addition on consolidant uptake, penetration depth, hydrophobicity, and mechanical reinforcement. Chemical analysis confirmed the presence of the consolidant in all treated samples, although with a heterogeneous distribution (more consolidant on the external surfaces than in the inner layers), likely due to degradation pathways. Water-containing treatments exhibited non-uniform consolidant distribution and lower uptakes, whereas treatments without water yielded higher uptakes (especially with extended exposure to scCO2) and more uniform consolidant distribution within the cell walls. Hydrophobicity assessment revealed inconsistent results, yet higher APDEMS content mainly increased water absorption. Compression-force-deflection tests showed that treated samples behaved like controls and the results were not reproducible, though high data variability limited statistical confirmation. Nonetheless, qualitative observations indicated improved resistance to compression and surface cohesion in consolidated samples. Ultimately, this investigation identifies limitations faced in laboratory-scale testing and provides a framework for further optimising treatment conditions. Future research should focus on expanding the sample dataset, assessing re-treatment and long-term performance, and scaling up to real objects, a crucial step in validating scCO2-assisted consolidation for collections. Overall, this research demonstrates the potential of scCO2 as a viable and sustainable method for consolidating aged PUR-ES foams. |
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Atividades financiadas
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