CO2 desorption kinetics

The capacity of PUs to absorb appreciable quantities of CO2 has been relatively recently mentioned [344, 345]. Previous experiments conducted by us have lead to the conclusion that when our PUs samples are kept for 48 hours under liquid COj pressure at room temperature, they may absorb high quantities of CO2, (up to 14% grav.) [17]. 

In our experiments, PU sheets of variable thickness were introduced in stainless steel pressure tubes and submitted in every case for two days to different CO2 pressures (ranging between 0.5 to 6 MPa). During this time, the PU samples absorbed variable quantities of CO2 depending on the COj pressure to which they were subjected. The samples were withdrawn from the pressure tubes and the COj desorption was followed by the weight decrease in time at atmosphere pressure, 250°C and 50% RH [17]. 

Unusual phenomena were observed in the case of the reversed phenomenon. The desorption curves were quite different depending on PUs sample thickness and apparently unexpected, on the initial absorbed quantity of CO2 (Fig. 4.76). 

We studied the behaviour of two 2 mm thick PUs sheets with two different initial CO2 concentrations Cj = 13.7% grav. and C2 = 3.8% grav., respectively (Fig. 4.76). After a period of time, the concentration of CO2 in the first case became equal to the initial concentration of the second case C2. Surprisingly, the further CO2 desorption rate for the first sample was not equal to the desorption rate observed in the second case, even though at that time, the CO2 content was equal in both cases. It was observed that the lower the initial concentration of CO2, the slower the rate of CO2 desorption (Fig. 4.76). 

This behaviour was explained as owing to a process of supramolecular structure loosening as a result of a polymer plasticization effect determined by CO2, which allowed quicker removal of CO2 [17]. The obvious consequence was that the kinetics equations established for every particular case (PUs sheet thickness or CO2 concentration) depended on the initial CO2 concentration. 

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As observed from the CO2 desorption kinetics, the CO2 diffusion from the polymer was very slow. The quantity of 0.406 g CO2 / 100 g PUs, equivalent to the whole quantity of COj resulted from the reaction, needed at least two weeks to diffuse totally from the 2 mm PUs sheets. The reaction with water proceeded with time and... 

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