Shape stabilisation of cellulose-poly(ethylene glycol) phase change composites for building construction

Abstract

In this thesis a series of unique formulations were developed and investigated for the stabilisation of PEG using cellulose fibre (CF) as the supporting material by implementing the highly feasible tricarboxylic acid (TCA) crosslinking reaction using deionised water as solvent. This resulted in producing a series of shape stabilised TCA crosslinked composite PCMs of different chemical compositions. The TCA crosslinked composite PCMs were characterised by FTIR, XRD, SEM, DSC, TGA and POM analysis. FTIR results confirmed the formation of a crosslinked structure, and the presence of weak intermolecular forces of attraction between the unreacted PEG particles and CF. Hence, leakage problem of solid-liquid PEG PCM was successfully overcome in the current study; and therefore, PEG was able to retain its original solid state during the phase transition process. Microwave (MW) irradiation had no major effects on the final structure of TCA composite PCM, but it appeared to have reduced the moisture content significantly since the intensity of peak appearing at 3450 cm-1 corresponding to the presence of OH groups had reduced significantly which may indicate moisture loss or changes in the polarity strength of the O-H group. XRD results indicated that the crystallinity of pristine PEG had changed drastically after TCA crosslinking reaction. The major two crystalline peaks appearing at 2θ = 19.10 and 23.23° disappeared after chemical crosslinking reaction; thus, broad humps existed in the composite PCMs indicating the presence of amorphous structures which was attributed to the incorporation of amorphous polymer (e.g., CF). The crystallinity index (CI) value of PEG8000 was 52.94% while TCA composite PCMs had CI value in the range 8.92 to 45.3%. The crystallinity of the resulting composite PCMs were dictated by the PEG content, PEG/CF ratio and PEG/CA ratio. Ten minutes MW radiation exposure was observed to enhance the crystallinity of TCA composite PCM with blend ratio of 4.0:1:7.5 from 35.65 to 47.96% (e.g., 34.53% increase) but twenty minutes MW radiation exposure reduced it from 35.65 to 32.32% (e.g., 9.34% decrease) which was attributed to destruction of PEG crystal lattice structure by the thermal effect of MW radiation. DSC results showed that pristine PEG8000 had a very high latent heat capacity of 181.5 J/g which was attributed to the fact that PEG was comprised of simple linear polymer chains. This indicated that pristine PEG8000 are susceptible to crystallisation; thus, could easily form crystals. After TCA crosslinking reaction the latent heat capacity of PEG had reduced significantly. Thus, TCA crosslinked composite PCMs had latent heat capacity in the range 18.19 to 58.08 J/g which was attributed to the reduced flexibility of the PEG chain during the crystallisation process. The phase transition temperature of PEG changed after TCA crosslinking reaction which was due to the changes in the thickness of the crystal lamellae. TGA results showed that the thermal stability of PEG had increased after TCA crosslinking reaction attributed to the formation of the crosslinked structure. POM images indicated major changes in the spherulitic crystal structure of PEG after chemical crosslinking reaction due to the formation of the TCA crosslinked structure. Hence, the crystal structure became less prominent, and its size was observed to have reduced significantly attributed to the formation of crystal defects during chemical crosslinking reaction. SEM images indicated the possibility of enhancing the structural integrity; thus, the homogeneity of the resulting composite PCMs using MW radiation exposure for ten minutes MW radiation exposure was observed to produce composite PCMs with compact structure and even topography; however, results suggested that twenty minutes MW radiation exposure was undesirable since cracks were observed to have formed due to severe moisture loss by the thermal effect of MW radiation.