Rheology of coating systems

Moolman, Pieter Lafras (2003-03)

Thesis (MScEng)--Stellenbosch University, 2003.

Thesis

ENGLISH ABSTRACT: Desired behaviour of paint during processing, storage, application and after application is of great importance in the coatings industry. Rheology (the study of flow and deformation behaviour) is used as a method to investigate the behaviour of the flow and deformation properties of the paint during these stages. Some of the more important phenomena that can occur during these stages, which were examined rheologically, are: 1. The rheological behaviour of certain complex raw materials during processing - vesiculated beads suspensions 11. The paint behaviour during storage - in-can stability, e.g. phase separation and sedimentation of particles; Ill. The paint during application - ease of application, spatter, etc.; IV. The paint behaviour after application - sag, layer thickness, levelling (ability to hide brush marks), etc. A rheometer was used to obtain rheological curves from a paint sample (± 1 ml). Correct interpretation of these curves, which display rheological properties of the sample such as the viscosity, shear stress, structural strength and many more, produced information about the properties mentioned in i, ii, iii and iv above. It was found that the rheological data correlated well with empirical tests carried out in the laboratory for spatter, sag, levelling behaviour and in-can stability of the paint. A wide variety of paints were studied ranging from tough/durable outdoor paints to smooth/velvety indoor paints. The rheological behaviour explained the specific end-use properties of the paints. Paints from two different companies were compared on a rheological basis. In some cases large differences in rheological behaviour were observed. Rheology modifiers were tested on a new paint. It was found that specific rheology modifiers could be incorporated into the formulation to give the specific rheological behaviour required. Rheological modelling was performed and it was found that the flow behaviour of paint cou1d be modelled accurately with existing viscosity models (Ostwald/de Waal, Bingham, Casson, Herschel-Bulkley, Cross and Philips-Deutsch). Other rheological properties of paint were also modelled successfully with a simple linear regression model. Rheology was also used as a method to examine the flow and deformation properties of vesiculated beads, a component of paint with very complex rheology. The most important factors that influence the rheological behaviour of the vesiculated beads were determined. It was found that the type of manufacturing process for the beads affected the rheological behaviour. The effect of the raw materials used to manufacture the vesiculated beads were rheologically determined.

AFRIKAANSE OPSOMMING: Dit is baie belangrik dat verf die gewenste gedrag moet toon tydens berging, vervaardiging, tydens aanwending en direk na aanwending. Reologie (die studie van vloei- en vervorming) word gebruik as a metode om die vloei- en vervormingseienskappe van verf gedurende bogenoemde stadia te ondersoek. Sommige van die belangriker verskynsels wat gedurende hierdie stadia kan plaasvind, is d.m.v reologiese toetse ondersoek. Die verskynsels is as volg: i. Die reologiese gedrag van komplekse grondstowwe soos gevesikuleerde partikel suspensies. ll. Verfgedrag tydens berging - stabiliteit van verf in die blik, bv. faseskeiding en sedimentasie van partikels lll. Verfgedrag tydens aanwending - gemak van aanwending, spatsels IV. Verfgedrag direk na aanwending - afsakking, dikte van verflaag, vloei-eienskappe 'n Reometer is gebruik om die reologiese kurwes te bepaal deur gebruik te maak van ± 1 ml monster. Korrekte interpretasie van hierdie kurwes, wat onder meer eienskappe soos viskositeit, spanning en struktuursterkte insluit, lei tot inligting wat die verskynsels in i, ii, iii en iv kan verklaar. Daar is gevind dat inligting wat verkry is i.v.m die verskynsel van spatsels, afsakking, vloei en die stabiliteit van verf in die blik, goed korreleer met empiriese toetse wat in die laboratorium uitgevoer is. 'n Groot verskeidenheid van verwe, wat wissel van duursame/sterk buitemuurse verf, tot fluweelsagte binnenshuise verf, isondersoek. Die reologiese toetse het daarin geslaag om die eienskappe van die eindproduk suksesvol te verduidelik. Verf van twee verskillende maatskappye is ook met mekaar vergelyk op 'n reologiese grondslag. Daar is by sommige van die verwe aansienlike verskille in die reologiese gedrag gevind. 'n Nuwe verf is gebruik om die uitwerking van reologie modifiseerders te ondersoek. Daar is gevind dat daar spesifieke reologie modifiseerders bestaan wat unieke reologiese gedrag verseker. Reologiese modellering is gedoen op verf en daar is gevind dat die vloeigedrag van die verf akkuraat gemodelleer kan word deur van die bestaande viskositeitsmodelle gebruik te maak (Ostwald/de Waal, Bingham, Casson, Herschel- Bulkley, Cross en Philips-Deutsch). Ander reologiese eienskappe is gemodelleer deur gebruik te maak van eenvoudige regressie modelle. Daar is ook van reologiese tegnieke gebruik gemaak om die vloei en vervormingseienskappe van gesuspendeerde gevesikuleerde partikels, wat as 'n grondstof vir verf gebruik word, te ondersoek. Die belangrikste faktore wat die reologiese gedrag beïnvloed het, was onder meer die verskillende prosesse waarmee die gevesikuleerde partikels vervaardig is. Die effek van sekere van die grondstowwe wat gebruik word om die gevesikuleerde partikels te vervaardig, is ook reologies ondersoek.

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