Summary of work

A gas tight set-up was designed and made to allow the entrapment of v/VOCs. Sources of formaldehyde, toluene, limonene and dodecane were created to result in known low concentration flows within the set-up. These four v/VOCs were selected for testing as they represent a wide range of polarity and basic chemical diversity. The v/VOC flow was made to pass through a sample holder, where sheep wool and mineral wool fibres were allowed to sorb v/VOCs. The unsorbed v/VOCs were analysed using thermal or solvent desorption connected to a GC-FID or HPLC, and the amounts of v/VOCs that were sorbed were subsequently calculated. This set-up allowed the determination of v/VOC sorption profiles by fibres exposed to gaseous concentrations close to what would be encountered in real life scenarios.

To determine the total sorption capacity of fibres, a DVS set-up was used; this was only done for formaldehyde. It allowed small fibre samples to be exposed to several cycles of very high formaldehyde concentrations whilst recording the fibres’ weight change.

8.2.1 Sorption profiles

Consistently throughout all sorption experiments with any of the v/VOCs used, mineral wool sorbed less than any of the sheep’s wools, and unscoured wools sorbed more than their scoured counterparts. This shows that the keratinous surface of wool fibres offers more interactive platforms for surrounding molecules than non-organic fibres do. Also, although scouring of wool is a requirement for industrial processing, the material removed obviously

142 enhances sorption profiles. It is probable that this material consists mainly of lanolin, but also contains a host of biological and inorganic contaminants.

Variations in the sorption profiles of different scoured wool types were clearly evident. Although not significant in the case of low levels of formaldehyde, the total sorption capacity clearly differed. More so, the higher the sorption capacity for polar formaldehyde for a fibre type was, the lower its sorption of low concentration non-polar VOCs proved to be, and vice versa. This shows that surface polarity has an effect on the sorption profile of a specific v/VOC. It was concluded that, for Light Herdwick wool, sorption of non-polar VOCs was preferred over the polar vVOC. This preference was highlighted by the fact that it sorbs most of the dodecane (most non-polar VOC used) it is exposed to even at low concentrations; the percentage of sorbed limonene (non-polar VOC) on the other hand increases at higher concentrations, but that of formaldehyde (polar v/VOC) decreased at higher concentrations. This preference is also in agreement with Van der Wal et al. (1998), who showed that “adsorption increases as the boiling point of the compound increases and the vapour pressure decreases.” However, these relationships may vary to a slight or considerable degree, or perhaps even reverse for other wool types having different surface polarities.

Maxwell and Huson’s explanation of the dynamic outer lipid layer fits well with the fact that several cycles of formaldehyde and no-formaldehyde exposure are required to achieve full sorption capacity. The movement of the surface lipids in response to the nature of the surrounding gases logically have to do with the sorption process. If their movement does not assist with furthering sorption and the locking-in of formaldehyde, then there would be no need for fibre to undergo several cycles of exposure; in that case, upon the first exposure formaldehyde would have passively diffused through the fibre surface, increasing the mass of the fibre in one cycle only. The dynamic nature of the lipid layer also fits in with the fact that when constant low concentration flows of formaldehyde were used, no differences are notable between unscoured wool types. It is reasonable to expect the degree of movement of the lipids to be dependent on surrounding environment; if more lipids are present, they move inwards more and if less are present, they move inwards less. This means that the percentage sorbed at this level would be the same regardless of the formaldehyde concentration, in accord with experimental results. Adding to that, modifications that cause an alteration of the lipid layer and/or its neighbouring fibre surfaces is clearly seen to disrupt this process – chemical modifications that result in the addition of polar functionalities lead to a reduction in the total sorption capacity of formaldehyde.

143 FTIR spectra reinforces the conclusion that fibre surface polarity effects sorption, with the C=O peaks at 1720cm-1 _{giving an indication of relatively how much 18-MEA is bound as a lipid layer}

via thioester linkages.

The higher distribution and cumulative volume of pores greater than 30nm for Blackface is likely a result of patches within the surface lipid layer. This coincides with enhanced formaldehyde sorption capacity.

Satisfactory fits for moisture sorption isotherms and sorption hysteresis were possible, and can be used as a basis for kinetics studies of different v/VOC sorption. Using the Vrentas and Vrentas model, fits were possible only by modifying some of the input parameters - especially the glass transition temperature of water. The Zimm-Lundberg model showed the onset of water clustering in the fibres at moisture contents in excess of 15-18% equilibrium moisture content. Parallel exponential kinetics model was found to provide extremely good fits to the experimental data, and clarified the effect the fast and slow sorption processes have on hysteresis; hysteresis is accompanied with the differences in moisture content associated with the fast process only below an RH of 70%, after which the slow process contributes negatively. The calculated fibres moduli were much larger than experimental values at the lower end of the hygroscopic range, but were in reasonable agreement at the upper end of the hygroscopic range.

8.2.2 Modifications and additives

Although tests regarding mechanical modification with tensile stretching were inconclusive, ball milling the fibres certainly enhances low-concentration formaldehyde, toluene, limonene and dodecane sorption. Such an enhanced sorption over the whole range of v/VOCs polarities tested shows that the cause of it is the simple increase of surface area. Although FTIR analysis shows an increase in CH2 and loss of C-O-C groups on the surface, there does not seem to

be a preferential exposure of chemical functionalities of a certain polarity. Keratinous debris, evident from SEM imaging, pore size distribution and cumulative pore volumes, contribute to the aforementioned observations.

Chemical modifications, regardless of the polarity of the introduced function, led to a decrease in the total sorption capacity of formaldehyde. This is likely to the disruptive effect of new polar functions on the dynamic surface movement, and the repelling effect of new non-polar functions on formaldehyde gas. Of the two, the disruptive effect of polar functionalities seems to have a larger effect, as they result in a lower sorption capacity. When examining low concentration levels, where no cycles of polar/non-polar environments are being employed, chemical modifications (control and with DDSA) seem to have no diminishing effect on

144 formaldehyde sorption, but alter toluene’s, limonene’s and dodecane’s sorption. Modification with DDSA enhances the sorption of all three of these non-polar VOCs, but the other anhydrides diminish the sorption of limonene and dodecane. The latter effect can be explained by the steric hindrance the new functionalities are subjected to by the naturally occurring 18- MEA molecules, and/or by the effect of the carboxylic group the new functionalities possess. In short, modification with DDSA enhances wool fibres’ sorption of low levels of non-polar v/VOCs and does not reduce that of polar v/VOCs, although it does reduce the total sorption capacity of polar v/VOCs. Pore size distribution and cumulative pore volumes imply that newly introduced chemical functionalities occur in larger mesopores. The modification leads to a decrease in specific heat capacity, possibly due to the reduction in vibrational space on the molecular level, and not due to a change in moisture uptake behaviour. It also may lead to a very slight decrease in thermal performance as an insulator; a more detailed and comprehensive experimental procedure would clarify the matter.

Carbon as an additive is compatible in its fibrous form with wool fibres and is extremely efficient at sorbing low levels of non-polar v/VOCs. On the other side of the spectrum, chitosan can be used to enhance the total sorption capacity of polar v/VOCs without hindering the sorption of v/VOCs at low concentrations. That is because it does not mask the wool fibre’s surface functionalities; although C=O and C-O-C groups are not detected by FTIR post coating, sorption at the molecular level does not seem to be affected. It is also compatible with chemical modification of wool fibres for the same reason, with the newly introduced chemical functionalities being still visible on the FTIR spectrum post coating. However, the enhancement in sorption capacity of polar v/VOCs is not as large as that of chitosan-sprayed unmodified wool.