Improving retention of blowing agent in rigid polyisocyanurate foams Through the introduction of metal-organic frameworks and nanosheets

The building sector accounts for up to 40 % of the global energy consumption and up to 50 % of greenhouse gas emissions, with the energy required to keep buildings warm contributing significantly. The United Nations Environmental Programme has estimated 30 to 80 % of energy consumption for heating can be lowered through effective energy reduction technologies, primarily through the use of insulators to retain building temperatures. Insulating materials have inefficient transport of thermal energy through their structure, with polymeric foam insulators producing some of the most efficient insulating materials available. More than 50 % of heat transfer through polymeric insulators comes from the gaseous blowing agent used within the sealed bubbles, or cells, of the foam, making the blowing agent vital to the performance of the insulator. Many polymeric foam insulators suffer from loss of blowing agent over time as the gas diffuses out of the cells and can be mitigated through introduction of barrier materials in the polymer matrix, such as metal-organic frameworks and nanosheets.

Metal-organic frameworks (MOFs) are three-dimensional co-ordination networks that contain potential voids and consist of regularly ordered structures of metal ions/clusters and organic linkers. MOFs are highly programmable materials that can be tailored by their metals/linkers for specific application and have seen success in separation of gasses when combined in polymer matrixes. Metal-organic nanosheets (MONs) are a unique class of free-standing two-dimensional materials that approach monolayer thicknesses with high aspect ratios. MONs retain the highly tuneable nature of MOFs but add increased surface areas and nanoscopic dimensions which are expected to enhance the formation of tortuous paths for gas separation applications. In this thesis, both MOFs and MONs were added to rigid Polyisocyanurate (PIR) foams to create novel composites to restrict the loss of pentane-based blowing agent from the foam. Additionally, the synthesis of novel MONs and their post-synthetic modification was explored to produce tailored materials for addition to the foams.

In Chapter 2 a robust and reliable accelerated aging method for monitoring the loss of blowing agent from rigid PIR foams was developed. This method was then tested with example layered additives MoS2 and Graphite to determine optimum loadings of additives, and to demonstrate that the influence of gas loss by additives can be measured by the monitoring technique. Copper-based paddle-wheel MOFs and MONs were used as additives in Chapter 3, demonstrating the complex relationship between the surface functionality, size, and stability of MOFs/MONs on the loss of blowing agent from rigid PIR foams. The introduction of a copper aminoterephthalic acid MOF (Cu(ABDC)(DMF)) or a maleic acid functionalized uncentrifuged MON (Cu(MA-ABDC)(DMF) ) demonstrated a reduced loss of blowing agent from the rigid PIR foams. Chapter 4 expands on this work by looking at an alternative MOF; NH2-MIL-53. This MOF was modulated and exfoliated into ultrathin MONs. The use of MOF in this system provided no reduction in blowing agent loss, but uncentrifuged MON did result in a reduction of the loss of blowing agent from the foam. Chapter 5 described the novel synthesis of the MOF Cu(MA-ABDC)(DMF) from Cu(ABDC)(DMF) and the subsequent exfoliation of the nanosheets. The amine functionality of Cu(ABDC)(DMF) was reacted with maleic anhydride to produce alternative carboxylic acid functionality. The 31 % functionalized Cu(MA-ABDC)(DMF) MOF was exfoliated into ethanol to produce predominantly monolayer MONs, that were utilized as an additive in Chapter 3. Finally, Chapter 5 described the attempted addition of polymeric chains to the surface of MONs through ring-opening polymerization techniques to produce polymer enhanced MONs.

This thesis demonstrates the use of MONs as effective barrier materials within rigid PIR foams and provides insights into the synthesis and modification of novel MONs utilized within the work. These rigid PIR foams could be utilized as a basis for the next generation of insulating materials, and the measurement techniques for loss of blowing agents can be utilized for further development of new composite rigid PIR foams.