Thesis (MScIngwet)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: Paint manufacturing through the dispersion process is an important part of the chemical industry
that relies on limited and expensive supplies of titanium dioxide pigment. The aims of the project
were to: (i) identify and evaluate the factors that affect the opacity and flocculation gradient, (ii)
establish and evaluate the mechanism of the process, and (iii) evaluate and apply the rheology of the
dispersion process.
The mechanism of the dispersion process can be broken down into the following steps: (i) wetting,
(ii) particle break down, and (iii) stabilisation against agglomeration. Wetting can be further broken
down into adhesional, immersional and spreading wetting, which can be evaluated in terms of the
contact angle. Titanium dioxide pigment particles are held together in the agglomerate state through
attractive Van der Waals forces. These forces must be broken down through shear stresses applied
through the Cowles mill or the homogeniser. The sensitivity analysis that was carried out confirms
that Van der Waals forces are effective only for sub-micron-sized particles. Acoustic cavitation also
increases the rate of particle break down in a homogeniser and it depends on turbulence intensity.
The mechanism of dispersion can be evaluated in terms of the Reynolds number.
The opacity and flocculation gradient of paint were found to depend on: (i) the mean pigment
particle size of titanium dioxide and extender, (ii) the particle size distribution of titanium dioxide,
(iii) the pigment volume concentration of titanium dioxide and extender, and (iv) the wavelength of
the incident radiation. Correlations of opacity (contrast ratio) or flocculation gradient could be set
up on the basis of the above explanatory variables in terms of a multiple linear regression. However,
it was found out that the methods used for measuring the contrast ratio and flocculation gradient
were unreliable. In the case of the contrast ratio, there was no standard procedure for preparing
pigment dispersion samples, thus resulting in values that were very high and insensitive to process
changes. Samples whose contrast ratio has to be determined must be diluted with resin.
Measurements of the flocculation gradient were found to be erratic with very low linear association.
Samples of pigment dispersions processed through the dispersion process were found to be shear
thinning. Flow curves obtained showed that the high shear rate and low shear rate behaviour of
dispersions were significantly different. Of the processing variables investigated, dispersant had the
greatest impact on the dispersion process. It significantly affects wetting and the particle break
down rate. Samples of dispersions from the dispersion process are thixotropic and recover their
structure in a reasonable time after the application of shear stress. Overall, the agitator speed and diameter did not impart a significant effect on the dispersion
process. / AFRIKAANSE OPSOMMING: Die vervaardiging van verf met die dispersie proses vorm ‘n belangrike deel van die chemiese
industrie wat staatmaak op die verskaffing van ‘n beperkte voorraad, duur titaniumdioksied
pigment. Die doel van hierdie projek was om (i) die faktore te identifiseer en te evalueer wat die
ondeurskynendheid en flokkulasie gradiënt beïnvloed, (ii) die meganisme van die proses vas te stel
en te evalueer, en (iii) die reologie van die dispersie proses te evalueer.
Die meganisme van die dispersie proses kan verdeel word in die volgende stappe: (i) benatting, (ii)
afbreking van partikels, (iii) stabilisering teen agglomerasie. Benatting kan verder verdeel word in
adhesiebenatting, onderdompelingsbenatting en spreibenatting, wat dan in terme van die hoek van
kontak ge-evalueer kan word. Titaniumdioksied pigment partikels word deur Van der Waals kragte
bymekaar gehou in die agglomeraattoestand en hierdie kragte moet gebreek word met behulp van
skuifspanning wat deur die Cowles Meul en Homogeniseerder aangewend kan word. ‘n
Sensitiwiteits analise het bevestig dat die Van der Waals kragte slegs effektief is vir sub-mikron
grootte partikels. Akoestiese kavitasie verhoog die tempo waarteen partikels breek in die
(homogeniser) en dit hang af van die intensiteit van die turbulensie. Die meganisme van dispersie
kan in terme van die Reynolds getal ge-evalueer word.
Daar is gevind dat die ondeurskynendheid en flokkulasie gradiënt van die volgende afhanklik is: (i)
die gemiddelde pigment partikel grootte van die titanium dioksied en aanvuller, (ii) die
partikelgrootteverspreiding , (iii) die pigment volume konsentrasie van die titanium dioksied en die
aanvuller, en (iv) die golflengte van die invallende bestraling. Korrelasies van die
ondeurskynendheid of flokkulasie gradiënt kan opgestel word op grond van die bogenoemde
verduidelikende veranderlikes in terme van ‘n veelvuldige linêere regressie. Daar is egter gevind
dat die metodes vir die meet van die ondeurskynendheid en flokkulasie gradiënt nie betroubaar is
nie. In die geval van die ondeurskynendheid was daar geen standaard prosedure vir die
voorbereiding van die pigment dispersie monsters nie en die gevolg was waardes wat baie hoog was
en onsensitief was vir proses veranderinge. Monsters waarvan die ondeurskynendheid bepaal moet
word behoort met ‘n hars verdun te word of by relatiewe lae vastestof inhoude te wees. Flokkulasie
gradiënt metings was baie wisselvallig.
Pigment monsters wat deur die dispersie proses geprosesseer is , is pseudoplasties. Die vloeikurwes
dui daarop dat die hoë skuif tempo en die lae skuif tempo gedrag van die dispersies grootliks
verskil. Van die proses veranderlikes wat ondersoek is het die dispersant die grootste invloed op die dispersie proses gehad het. Dit het die benatting en partikel afbreking grootliks beïnvloed.
Dispersie monsters van die dispersie proses is thixotropies en herwin hul struktuur redelik gou na
die aanwending van skuifspanning. Die roerspoed het geen betekenisvolle invloed op die dispersie
proses gehad nie binne die beperkte grense in roerspoed wat ondersoek is.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16609 |
| Date | 10 1900 |
| Creators | Kazembe, Callisto |
| Contributors | Knoetze, J. H., University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xii, 215 leaves : ill. |
| Rights | University of Stellenbosch |
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