Novel barrier coatings based on nanoclay-polymer composites

Murima, Douglas (2015-04)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The investigation of the barrier properties of highly filled polymer-clay hybrid latex films is described. Montmorillonite (MMT) clay contents ranging from 10–30 wt.% were effectively incorporated into polystyrene-butyl acrylate (PSBA) random copolymers, via miniemulsion polymerization. The optical properties of the films were evaluated using UV-Vis spectroscopy. Compared to the neat films, the PSBA nanocomposites retained remarkable visual properties. The light transmittance for PSBA films with styrene/n-butyl acrylate (S/BA) comonomer contents of 40:60 and 50:50 (mol.%) only decreased from 70% in the neat films to 50% in the nanocomposite films containing 30 wt.% clay. The best optical properties were observed in the films with S/BA comonomer contents of 30:70 (mol.%), the light transmittance only decreased from 85% (neat film) to 60% in the nanocomposite films containing 30 wt.% clay. The improved optical properties for the PSBA-30:70 films (compared to the PSBA-40:60 and PSBA-50:50 counterparts) were attributed to an increase in the low UV-absorbing butyl acrylate component of the copolymer, which at the same time has a low Tg that probably facilitated dispersion of the rigid MMT platelets in the matrix. In this study, the overall water vapour transport behaviour was governed by the MMT clay presence and less affected by the copolymer composition variation. The lower diffusion coefficients in the polymer clay nanocomposites (PCNs) were a result of the impermeable clay platelets which forced the water vapour molecules to follow longer and more tortuous paths to diffuse through the nanocomposite films. The irregular shape in the PSBA-40:60 and PSBA-30:70 neat latex particles was lost in the hybrid particles and well defined, dumb-bell shaped particles were observed. This was because of the faceting effect of the rigid MMT clay platelets. The MMT clay platelets were predominantly adhered to the surface of the PSBA latex particles because MMT clay particles have a larger size than the effective size of the copolymer particles. The stable overall transport coefficients in the PSBA-30:70-MMT films were attributed to the morphological organization of clay platelets in the matrix. The storage modulus of the materials decreased with an increase in clay content. This was attributed to the dual role played by the organoclay, firstly as nanofiller and reinforcing agent leading to the increase in storage modulus, and secondly as a plasticizer leading to a decrease of storage modulus.

AFRIKAANSE OPSOMMING: Die versperringseienskappe van hoogsgevulde polimeer-klei saamgestelde latekslae is beskryf. „n 10–30 wt % Montmorilloniet (MMT) klei inhoud is inkorporeer in polistireenbutielakrilaat (PSBA) onreëlmatige kopolimere, via miniemulsie polimerisasie. Die optiese eienskappe van die lae is bepaal m.b.v. UV-Vis spektroskopie. In vergelyking met die lae sonder klei (sogenaamde „neat films‟), het die PSBA nanosamestellings interressante visuele eienskappe getoon. Die ligtransmissie van die PSBA lae met „n stireeen/n-butielakrilaat (S/BA) komonomeerinhoud van 40:60 en 50:50 (mol %) het slegs afgeneem vanaf 70% in die „neat films‟ tot 50% in the nanosaamgestelde lae wat 30% klei bevat het. Die beste optiese eienskappe is waargeneem vir die lae wat „n 30:70 (mol %) S/BA komonomeerinhoud bevat het; die transmissie het slegs afgeneem vanaf 85% in die „neat films‟ to 60% in the nanosaamgestelde lae wat 30% klei bevat het. Die verbeterde optiese eienskappe van die PSBA-30:70 films (in vergelyking met die -40:60 and -50:50 films) is toegeskryf aan „n toename in die lae UV-absorberende butielakrilaat komponent van die kopolimeer. Terselfdetyd het laasgenoemde „n lae Tg-waarde, wat dispersie van die onbuigbare MMT kleiplaatjies in die matriks gefasiliteer het. In hierdie studie is die algehele waterdampvervoer deur die teenwoordigheid van die MMT klei beheer; dit is minder geaffekteer deur variasie in die samestelling van die kopolimeer. Die lae diffusiekoëffisiënte in die polimeer-klei nanosamestellings is as gevolg van die ondeurdringbare kleiplaatjies, wat die waterdampmolekules dwing om langs langer en meer gekronkelde paaie te diffundeer deur die nanosaamgestelde lae. Die onreëlmatige vorm wat gesien is in die PSBA-40:60 and PSBA-30:70 latekspartikels (sonder klei) het geleidelik verdwyn in die saamgestelde partikels, en goed-gedefineerde partikels met die vorm van handgewigte is waargeneem (in TEM beelde). Die rede hiervoor is die sogenaamde „faceting‟ effek, wat deur die onbuigbare MMT kleiplaatjies veroorsaak is. Die MMT kleiplaatjies sit hoofsaaklik aan die oppervlaktes van die PSBA latekspartikels. Die rede hiervoor is dat die MMT kleipartikels groter is as die effektiewe grootte van die kopolimeerpartikels. Die stabiele vervoerkoëffisiënte in die PSBA-30:70-MMT films is aan die unieke morfologiese eienskappe toegeskryf. Die bergingsmodulus van die materiale het monotonies afgeneem met „n toename in klei-inhoud. Dit is toegeskryf aan die tweedelige rol wat die organoklei speel – eerstens as 'n nanovuller en versterkingsmiddel, wat „n toename in bergingsmodulus tot gevolg het, en tweedens as „n plastiseerder, wat „n afname in bergingsmodulus tot gevolg het.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96994
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