Influence of the Processes Parameters on the Properties of The ...
Influence of the Processes Parameters on the Properties of The ...
Influence of the Processes Parameters on the Properties of The ...
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Chapter 2.<br />
<str<strong>on</strong>g>Processes</str<strong>on</strong>g> to Manufacture Foams and to Functi<strong>on</strong>alize <str<strong>on</strong>g>the</str<strong>on</strong>g> Surface<br />
(t sat ). An increase <str<strong>on</strong>g>of</str<strong>on</strong>g> temperature (T sat ) above T c enhances <str<strong>on</strong>g>the</str<strong>on</strong>g> free volume and <str<strong>on</strong>g>the</str<strong>on</strong>g> increase <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure<br />
(P sat ) above P c causes <str<strong>on</strong>g>the</str<strong>on</strong>g> sorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> scCO 2 . <strong>The</strong> decrease <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> temperature is at <str<strong>on</strong>g>the</str<strong>on</strong>g> origin <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
departure <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 molecules and <str<strong>on</strong>g>the</str<strong>on</strong>g> depressurizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> chamber causes <str<strong>on</strong>g>the</str<strong>on</strong>g> pore formati<strong>on</strong> inside <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
polymer.Evoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> this phenomen<strong>on</strong> can be fur<str<strong>on</strong>g>the</str<strong>on</strong>g>r explained as under.<br />
<br />
<br />
Period I: <strong>The</strong> CO 2 is compressed to a pressure vessel where a polymer sample had already been<br />
placed (Part I <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> Figure 2.20). Generally, <str<strong>on</strong>g>the</str<strong>on</strong>g> process is carried out until a value above <str<strong>on</strong>g>the</str<strong>on</strong>g> critical<br />
pressure (P c ). Pressure and temperature increase with <str<strong>on</strong>g>the</str<strong>on</strong>g> compressi<strong>on</strong> until desired values called<br />
saturati<strong>on</strong> pressure (P sat ) and saturati<strong>on</strong> temperature (T sat ). <strong>The</strong> sorpti<strong>on</strong>-diffusi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 begins but<br />
since <str<strong>on</strong>g>the</str<strong>on</strong>g> time <str<strong>on</strong>g>of</str<strong>on</strong>g> this period takes maximum 2 minutes, it can be neglected.<br />
Period II: <strong>The</strong> sorpti<strong>on</strong>-diffusi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 takes places. <strong>The</strong> solubility (phase equilibrium) is <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
limiting factor <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> sorpti<strong>on</strong>-diffusi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 into <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer since <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer is not soluble in<br />
CO 2 . Moreover, under high pressure, <str<strong>on</strong>g>the</str<strong>on</strong>g> sorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 into <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer brings <str<strong>on</strong>g>the</str<strong>on</strong>g> structural<br />
phase transiti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> polymer. Polymer swells as <str<strong>on</strong>g>the</str<strong>on</strong>g> CO 2 sorbed and <str<strong>on</strong>g>the</str<strong>on</strong>g> transiti<strong>on</strong> occurs from glassy<br />
to plasticized (rubbery) state, by lowering <str<strong>on</strong>g>the</str<strong>on</strong>g> glass transiti<strong>on</strong> temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer. This<br />
period is called <str<strong>on</strong>g>the</str<strong>on</strong>g> saturati<strong>on</strong> time (t sat ) (Part II <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> Figure 2.20).<br />
Figure 2.19: Schematic presentati<strong>on</strong> for scaffold generati<strong>on</strong> during scCO 2 foaming.<br />
Modified from [Cooper, 2003]<br />
<br />
Period III: Period II is followed by <str<strong>on</strong>g>the</str<strong>on</strong>g> depressurizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure chamber (III). <strong>The</strong> rate <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> depressurizati<strong>on</strong> (dP/dt) can be c<strong>on</strong>trolled. With <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure drop <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure chamber<br />
comes <str<strong>on</strong>g>the</str<strong>on</strong>g> temperature drop (dT/dt). <strong>The</strong> temperature drop is proporti<strong>on</strong>al to <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure drop.<br />
When <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure is reduced from <str<strong>on</strong>g>the</str<strong>on</strong>g> equilibrium state, <str<strong>on</strong>g>the</str<strong>on</strong>g> formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> nuclei occurs as a result <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
supersaturati<strong>on</strong>. <strong>The</strong>se nuclei grow by <str<strong>on</strong>g>the</str<strong>on</strong>g> desorpti<strong>on</strong>-diffusi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> gas from <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer matrix.<br />
We can state that <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure difference between <str<strong>on</strong>g>the</str<strong>on</strong>g> two sides <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> pore interface is <str<strong>on</strong>g>the</str<strong>on</strong>g> driving<br />
force <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> pore growth. One must remember that, during <str<strong>on</strong>g>the</str<strong>on</strong>g> depressurizati<strong>on</strong>, <str<strong>on</strong>g>the</str<strong>on</strong>g> CO 2 is not<br />
supercritical anymore, and its molecular volume is greater than that <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> supercritical state.<br />
Actually, <str<strong>on</strong>g>the</str<strong>on</strong>g> pore growth is provided by <str<strong>on</strong>g>the</str<strong>on</strong>g> expansi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 (Part III <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> Figure 2.20).<br />
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