Ingeniaritza Kimikoa eta Ingurumenaren Ingeniaritza
Jakintza-arloa:
Kimika
Urtea:
2003
|Poliuretanoak, konposizio eta diseinuaren aldagarritasuna dela eta, aplikazio eremu ugariko polimero familia zabal bat osatzen dute. Zehazki, poliuretano elastomero termoplastikoen, propietate fisiko-kimiko, termiko eta mekanikoen arteko erlazioa dela eta, material garrantzitsuenetakoak kontsidera daitezke.
Poliuretano elastomero termoplastikoak bi segmentuz eratuak daude, bata zurruna, diisozianato eta masa molekular txikiko diol batez eratua, eta bestea malgua, polieter edo poliester motako masa molekular altuko makrodiol batez eratua. Segmentuen arteko bateraezintasun termodinamikoak, hainbat faktoreekiko menpekoa den banaketa mikrofasikoa sortarazten du. Banaketan eragiten dute faktoreak, segmentuen izaera, egitura eta tamaina, erreaktiboen arteko erlazio molarra eta sintesi prozedura eta baldintzak dira, besteak beste.
Ikerketa honetan, poliuretano elastomero termoplastikoak sintetizatu dira eta material hauen propietate makroskopikoengan, sintesi-erreakzioaren zinetikak eta egitura/propietate erlazioak duen eragina ulertu nahian, fisiko-kimikoki, termikoki, mekanikoki eta morfologikoki karakterizatu dira. Makrodiolaren masa molekularraren eta egitura kimikoaren eragina, polikarbonato diol erako lau makrodiol erabiliz aztertu da. Horietako hiruk egitura kimiko berdina, polihexametilenkarbonato diol, baino masa molekular ezberdina dute, eta laugarrenak aurreko baten antzeko masa molekularra, baino egitura kimiko ezberdina dauka, polihexametilen-pentametilenkarbonato diol. Fase banaketarengan erreaktibo arteko erlazio molarraren eragina aztertzeko masan %20 eta %60 arteko segmentu zurruna duten poliuretanoak sintetizatu dira.
Ikerketa zinetikoan, erreakzio zinetikak hirugarren mailako alkoholez eta uretanoz katalizaturiko mekanismo autokatalitiko bat jarraitzen duela egiaztatu da. Alkoholez eta uretanoz katalizaturiko erreakzioen abiadura-konstanteak, hidroxilo eta uretano talde funtzionalen hidrogeno zubiko asoziazio/disoziazio orekak medio, makrodiolaren masa molekularrarekiko eta egitura kimikoarekiko menpekoak direla ikusi da. Aktibazio-energia balioek isozianato taldeek asoziaturiko alkoholekin konplexua eratzeko joera handiagoa dutela erakusten dute.
Segmentu zurrunen artean eta segmentu zurrun/segmentu malgu artean gertatzen diren hidrogeno zubiko elkarrekintza fisikoak, segmentuen egiturarekiko eta tamainarekiko menpekoak dira eta poliuretanoaren mikroegitura baldintzatzen dutela ikusi da. Poliuretano sistema ezberdinen mikrofase malguaren beira trantsizio-tenperaturak makrodiol puruekiko alderatuz, banaketa mikrofasikoa ez dela erabateko erakusten dute. Beira trantsizio-tenperaturaren handitzea, mikrofase malguan disolbaturiko MDI 3-4 unitate baino gutxiagoko segmentu zurrun amorfoei egotzi daiteke. Fase banaketa-maila handitu egiten da segmentu malguaren masa molekularra eta segmentu zurrunaren MDI unitate-kopurua handitzearekin batera. Faseen arteko nahaskortasuna aurresateko Flory-Huggins-en elkarrekintza-parametroan oinarrituriko ikerketa teorikoa egin da eta bat dator esperimentalki lorturiko emaitzekin.
Poliuretano sistema guztientzat segmentu zurrun amorfoen frakzio batek, segmentu zurrunen ordenamendu partzial batekin erlazionaturiko ¿zahartzea¿ edo ¿giro tenperaturako berregostea¿ pairatzen du, T1 tenperaturan endotermia edo trantsizio bat agertuz.
Ekorketazko mikroskopia elektronikoaz poliuretano sistema ezberdinek eratzen dituzten egiturak behatu dira. Segmentu zurrunaren ordenamendu kristalinoak tamainarekiko menpekoak dira, horrela, MDI 8-9 unitatez eraturiko segmentu zurrunek fusio-puntu altuko egitura ordenatuak, esferulitak, eratzen dituzte, aldiz, MDI 4-5 unitatez eraturiko segmentu zurrunak globulutan antolatzen dira. Fusio-puntu altuko egitura kristalino hauek poliuretanoen portaera elastomerikoa tenperatura altuagoetaraino hedatzen dute.
Material hauen portaera mekanikoa fase-banaketaren menpekoa da. Masa molekular baxueneko makrodiolean oinarrPolyurethanes constitute a broad polymer family which due to the versatility in composition and design, can be used for a wide range of applications fields. Specifically, thermoplastic elastomeric polyurethanes are considered one of the most important materials because of their physico-chemical, thermal and mechanical properties relationships.
Thermoplastic elastomeric polyurethanes are formed by two segments, the hard segment formed by a diisocyanate and a low molecular weight diol, and the soft segment composed by a medium molecular weight polyeter or polyester type macrodiol. The thermodynamic incompatibility between segments, which depends on several factors, leads to a microphase separated microstructure. Among the factors affecting the phase separation degree, can be included the nature, structure and size of the segments, the reagents molar ratio and the synthesis procedure, among others.
In this research, thermoplastic elastomeric polyurethanes have been synthesized in order to understand the effect of synthesis reaction kinetics and structure/properties relationships on macroscopic properties of this material. Physico-chemical, thermal, mechanical and morphological characterization has been performed. The effect of molecular weight and chemical structure of macrodiol has been studied using four polycarbonate type macrodiols. Three of them have the same chemical structure, polyhexamethylencarbonate diol, but different molecular weight, while the fourth has similar molecular weight to one of them, but different chemical structure, polyhexamethylene-pentamethylencarbonate diol. The impact of molar ratio into the microphase separation degree has been analyzed synthesizing polyurethanes with hard segment content varying from 20% to 60 % in weight.
The kinetic studies have revealed that the reaction kinetics is dequately described by a third order mechanism autocatalyzed by alcohol and urethane groups. The values obtained for the rate constant of alcohol and urethane catalyzed reaction, suggest that association phenomena by hydrogen bonding, where hydroxyl and urethane groups are implicated, depends on molecular weight and chemical structure of polyol. Activation energy values show that isocyanate groups have greater tendency to form a complex with associated alcohols.
The physical interactions by hydrogen bonding between hard segments and hard/soft segments depend on the structure and size of segments and determine the microstructure of the polyurethane. The glass transition temperature of soft phase in the different polyurethane systems with respect to the glass transition temperature of pure polyol, suggest that microphase separation is not complete. The increase of glass transition temperature is related to the dissolution of hard segments composed with less than 3-4 MDI units into the soft segment. The microphase separation degree increases as soft segment molecular weight and MDI unit number in the hard segment increases. The miscibility between microphases has been theoretically studied by means of Flory-Huggins interactions parameters, and the prediction agrees with experimental results.
In all polyurethane systems, due to the room temperature annealing or physical aging, an endotherm or transition related to some relaxation effects and a partial ordering of amorphous polymer chains in the hard segments is observed at T1 temperature.
The morphologies that the different polyurethane systems show have been observed by atomic force microscopy. Hard segment crystal structures depend on hard segment size, and in this way, hard segments domains with 8-9 MDI units are formed by spherulites, whereas hard segments domains composed of 4-5 MDI units are formed by globules. These hard segment crystals show high melting point and extend the elastomeric behaviour of polyurethanes towards higher temperatures.
The mechanical behaviour of this type of materials depends
