Differential Scanning calorimetry
Differential scanning calorimetry is one most used methods to investigate the interactions of water with cellulose and other hydrophilic polymers. For purpose of comparison, we use this method to characterize microcrystalline cellulose Avicel with approximately 5 to 6 wt% of equilibrium absorbed water and with excess water at around 50wt%. DSC curves of pure water and cellulose Avicel with excess added water of 50wt% and 5 wt% absorbedare shown in Figure 1.
It is possible to see in Figure 1a that the free water in Avicel with 50 wt% shows a crystallization peakat -18 ºC, a melting peak between near 0 °C and vaporization peak after 100 °C. The melting and crystallization peaks are very similar to the peaks observed for puremilli-Q water (Figure 1b) which is in accordance with literature for samples of cellulose with more than 20 % of water. These peaks correspond to free water or freezing water.
For 5 wt% water content (Figure 1c), no peak was observed in DSC because in this case the water absorbed in bounded to cellulose as described by Hatekayama, et.al. (Hatakeyama and Hatakeyama, 1998). According to Park et al 2007 (Park et al., 2007) the non-freezing bound water content of microcrystalline cellulosereported is 3.3 wt% and 25 wt% for wood pulp fiber. This lower value for MCC is attributed to the higher crystalline index of MCC than wood pup fiber and the lower accessibility of water to sites of absorption. It is also reported that for cellulose fiber with water content below 19.6%all water is bounded and DSC did not show any transient due to melting or crystallization (Nakamura et al. 1981). This data shows that DSC is not adequate to characterize water interactions below such values and the need for other tools to characterize bound water in cellulose and other hydrophilic polymers. Since thermogravimetric analysis detect water up to very low concentration its use has been considered, however in dynamic mode a single event is usually observed for a large interval of temperature, making it difficult to characterize different kind of non-freezing bound water in a material.The results reported in Figure 1 shows that DSC shows that free water can be detected in samples with 50 wt% of water and that for samples of microcrystalline cellulose water content up to 5/6 wt%cannot be detected by DSC since it corresponds to non-freezing bound water as reported in literature (Park et al., 2007).
Experimental setup for TGA conditions for cellulose analysis
In order to determine the general conditions for the TGA experiments, cellulose Avicel with 5wt% humidity was analyzed by TGA using dynamic heating program at 3, 10, 20, 30 and 40 ºC.min-1 and by the auto stepwise method with heating rate of 3, 5, 7 and 10 ºC.min-1, from room temperature to 200ºC.
Figure 2a shows the dynamic experiments. It is clear from TGA plotsthere are only one desorption process during the heating, with a curve shift to higher temperatures as the heating rate were increased. The interval of loss was from the start at 25 ºC to 120 – 160 ºC depending on the heating rate.
The auto stepwise results are shown in figure 2b and the total waterdesorption was very close to dynamic method, but without such appreciable temperature shift of the curve as the heating rate was changed.
The results obtained with the auto stepwise program suggests that two process of water loss occur, one up to approximately 50 ºC (loss of about 3.4 %) and a second from 50 up to 120 ºCwith a loss of about 2 %. These loses of absorbed water will be analyzed in detail in the next section
Effect of different water content absorbed in cellulose Avicel on the loss profile
For a more accurate determination of the different kind of absorbed water the auto stepwise program was followed by an isothermal step at room temperature in order to detect the loosely bounded water. Figures 3shows TG curves of conditioned samples in environments with 11.3, 32.8, 43.2, 75.3 and 97.3%ofrelative humidity using dynamic and auto stepwise program at a heating rate of 10 ºC.min-1. The amount of absorbed water in the samples varies from 4 to 13 wt%, depending on the condition of conditioning.
The curves obtained in the dynamic heating (Fig. 3a) showed a single loss of mass being difficult to distinguish different types of water. By other hand, the curves obtained by the auto stepwise program showed clearly three steps of water losses, the first one starting immediately after the sample was placed in the balance at isothermal conditions and two others at higher temperatures.
The first event shows an abrupt loss and begins immediately after the sample was placed in the thermoanalyzer. In this stage, purge gas drag the free water absorbed onto cellulose at constant temperature of approximately 25 ˚C. A second heating starts at 25 ˚C and goes up to 40 ˚C. The third loss step starts after 40 ˚C and proceeds with continuous heating up to 120-150 ˚C. The events observed above 40 ˚C were attributed the non-freezing bound water.
The data for water loss determined by dynamic andauto stepwise methodsare presented in Table 1.
Water losses at first step at isothermal conditions increase with the relative humidity of conditioning, while the losses from bound water that occurabove25 ºC are almost constant and independent on the initial absorbed water content. This behavior indicatesthat the level of saturation of the two non-freezing bound water are achieved for all samples being of around 1.4% for the first loss between 25 ºC and 40 ºC and 2.2 % after 40 ºC. Those results indicate that bound water is limited by the sites of absorption available in cellulose as described in literature (Park et al., 2006; 2007).Non-freezing bound water can be distinguished in two types, one easy to be removed and one hard to be removed. This is a valuable information that can help in understand the structure of absorbed water onto cellulose substrates.
One possible explanation is that the first bound water removed at lower temperature is the second layer of water absorbed and the bound water eliminate at higher temperature is the first layer directed link to cellulose. This result can be used to evaluate the number of sites for the different species of bound water on cellulose and can give an idea of its nature. A sketch showing the three kind of water observed is presented in the Figure 4.
Determination of activation energy of bound water desorption and cellulose degradation
The Ozawa Flynn-Wall non-isothermal method (Ozawa 1965) was used to estimate the activation energy of bound water desorptionand thermal degradation of cellulose. The cellulose degradation energy was also estimated with the purpose of comparison with water desorption. The thermograms showing the water desorption (5.5% at 25-150 ºC) followed by cellulose degradation (88.0% at 270-550 ºC) performed at several heating rates are shown in Figure 5a. The residue from the pyrolysis of cellulose was approximately 6% of the initial weight.
Figure 5b showed the plots of log β vs 1/T for both water desorption and cellulose degradation, where β is the heating rate, according to Equation 1. The data shows a linear behavior with R-square higher than 0.990 for the water desorption and higher than 0.970 for cellulose degradation.
The activation energies for water desorption and for cellulose degradation obtained from the plots of Fig. 5b arepresented in Table 2.
The activation energies of water desorption did not vary appreciable with the degree of conversion and was about 50 kJ.mol-1 and was associated to bound water (Kuo 2018; Hatakeyama and Hatakeyama 1998) The activation energy ofbound water can be considered high since the activation energy for cellulose degradation was in the range f 150 to 170kJ.mol-1when measured for between 12 and 60% conversion.