POLYMER PROCESSING USING SUPERCRITICAL FLUIDS

Autor: Aionicesei, Elena
Přispěvatelé: Knez, Željko
Jazyk: slovinština
Rok vydání: 2009
Předmět:
Zdroj: Maribor
Popis: Tradicionalne metode za procesiranje polimerov uporabljajo nevarna hlapna organska topila in kloro-floro-ogljikovodike. Zaradi povečanih izpustov nevarnih topil se pojavlja potreba po uporabi čistejših metod za procesiranje polimerov. Eno možnost predstavlja superkritični ogljikov dioksid (scCO2) kot mehčalo pri procesiranju polimerov. Velika uporabnost superkritičnih fluidov se kaže pri procesiranju polimerov za potrebe biomedicinskih pripomočkov (kot so mikrodelci, mikrokapsule, pene, membrane, kompoziti). Prednosti metode so predvsem v odsotnosti nevarnih organskih topil, učinkoviti ekstrakciji topil in nečistoč, procesnih pogojih, nižji temperaturi, nadzorovanemu oblikovanju delcev in pen z enostavnim reguliranjem tlaka in temperature. Navkljub velikemu potencialu scCO2 kot “zelenemu” topilu za procesiranje biokompatibilnih in biorazgradljivih polimerov, je podaktov o faznih ravnotežjih, ki so potrebni za načrtovanje postopka, dokaj malo. Nadaljnje raziskave so potrebne za optimiranje procesnih tehnik in parametrov (tlak, temperatura). Podatkov o uporabi scCO2 za procesiranje kompozitov polimer/keramika za biomedicinske aplikacije je še posebej malo na razpologo. Cilj te disertacije je uporaba scCO2 kot “zelenega” topila za procesiranje biorazgradljivih polimerov in kompozitov, ki se uporabljajo kot biomateriali. V raziskavah smo uporabili dva biorazgradljiva polimera, poli(L-laktid) (PLLA) in poli(D,L-laktid-ko-glikolid) (PLGA). Raziskali smo tudi kompozite polimerov z bioaktivnim keramičnim prahom, hidroksiapatitom (HA). Glavni cilj raziskav je bil pridobiti porozen polimer ali kompozit, primeren za tkivni inženiring, pri nizki temperaturi in brez uporabe dodatnih organskih topil. Študirali in razložili smo obnašanje obeh polimerov v zmesi s CO2. Z določitvijo topnosti in difuzijskega koeficienta CO2 v polimerih pri določeni temperaturi in tlaku, smo pridobili več podatkov o faznem ravnotežju polimer-plin, ki so pomembni za razumevanje vpliva in optimiranje procesnih parametrov. Topnost CO2 v polimerih smo izmerili pri treh različnih temperaturah (308, 313 in 323 K) in v območju tlaka 10 – 30 MPa. Izbrane temperature so bile višje od kritične temperature za CO2, vendar še vedno dovolj nizke, da ne bi vplivale na bioaktivnost spojin ali proteinov dodanih v sistem med procesiranjem. Pri testiranju poimerov in kompozitnih materialov smo uporabili enako temperaturno in tlačno območje. Raziskali smo učinkovitost mešanja v prisotnosti scCO2 za pridobivanje kompozitnega materiala iz PLLA in HA ter PLGA in HA in postopek primerjali s postopkom koprecipitacijie. Nadalje smo določili topnost in difuzijski koeficient CO2 v kompozitnih materialih ter jih primerjali z rezultati pridobljenimi za čiste polimere. Tako smo lahko določili vpliv keramičnega polnila na absorpcijo plina. Ocenili smo možnosti pridobivanja poroznih struktur z uporabo visokotlačne tehnike s CO2 kot vpihovalnim medijem brez oziroma z dodanim porogenom. Raziskali smo vpliv tlaka, temperature, ekspanzijske hitrosti in prisotnost porogena na končno porozno strukturo. Eksperimentalne rezultate smo primerjali s podatki iz literature in z rezultati dobljenimi z matematičnim modeliranjem. Rezultati kažejo, da postopek plinskega penjenja biorazgradljivih polimerov predstavlja obetavno tehniko pridobivanja opornih tkiv z željeno strukturo. V prihodnjih raziskovah bodo potrebne nadaljnje študije in optimiranje procesnih parametrov glede na naravo substrata in željen končni produkt. The traditional methods for polymer processing use environmentally hazardous volatile organic solvents and chlorofluorocarbons. Due to the increase of hazardous solvent emission and generation of aqueous waste streams, there is an obvious need of finding new and cleaner methods for the processing of polymers. Supercritical carbon dioxide (scCO2) has attracted particular attention for these applications due to its tremendous potential as a plasticizer in polymer processing. A particular interest is shown to the use of supercritical fluids for processing polymers destined for biomedical applications (as microspheres, microcapsules, foams, membranes, polymer/drug composites). The method offers important advantages related to the absence of harmful organic solvents or, when necessary, the efficient extraction of solvents and impurities, the mild processing conditions and the control of particle and foams morphology by simple variation of pressure and temperature. Despite the huge potential of scCO2 as a “green” solvent for processing biocompatible and biodegradable polymers, the phase equilibrium data, essential for process design, are quite scarce. Optimum processing techniques and parameters (pressure, temperature) still need consideration and study. The data are especially scarce regarding the scCO2 processing of polymer/ceramic composites for biomedical applications. On this basis, this thesis is aimed to open new perspectives over the use of scCO2 as a “green” solvent for the processing of biodegradable polymers and composites used as biomaterials. Two biodegradable polymers were chosen for this study, poly(L-lactide) (PLLA) and poly(D,L-lactide-co-glycolide) (PLGA). Their composite with a bioactive ceramic powder, hydroxyapatite (HA), was also studied. The main idea followed by this thesis was the obtaining of porous polymeric or composite material scaffolds suitable for tissue engineering under mild temperature conditions and without the use of additional organic solvents. The behavior of the two polymers under dense CO2 had been studied and explained. More data about the polymer-gas phase equilibrium, necessary for understanding and optimizing the effect of processing parameters, were obtained by determining the solubility and diffusion coefficients of CO2 in the polymers for certain values of temperature and pressure. The solubility of CO2 was measured for each polymer for three different temperatures (308, 313 and 323 K) in the pressure range 10 – 30 MPa. The temperatures were chosen higher than the critical temperature for CO2, but still low enough so as not to affect the bioactivity of any drugs or proteins that could be introduced in the system during processing. The same range of temperature and pressure was employed for all tests involving the studied polymers or their composite materials. The efficiency of mixing in the presence of scCO2 for obtaining composite materials from PLLA and HA and respectively PLGA and HA was studied by comparison with coprecipitation. The solubility and diffusion coefficient of CO2 in the composite materials were afterward determined, and the results were compared with the ones obtained for the polymer alone in order to determine the effect of the ceramic filler on the gas uptake. The possibility of obtaining porous scaffolds was assessed by using a pressure quench technique using dense CO2 as blowing agent, with and without the presence of a porogen. The effect of pressure, temperature, depressurization rate and porogen on the final porous structure was investigated. The experimental results were compared with literature data and with data obtained by mathematical modeling, employing equations of state commonly used for polymers or polymer/solvent systems. The results indicate that gas foaming of biodegradable polymers represents a promising technique for obtaining tissue engineering scaffolds with the desired structure. Still the processing parameters need to be studied and optimized, according to the nature of the substrate and of the aimed final product.
Databáze: OpenAIRE
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