abstract
-
In this work which consists of two parts, two important aspects in latex production and in the control of latex reactors were studied: the determination of latex particle size using on-line and off-line measurements and the particle nucleation in emulsion polymerization in the absence of micelles.
In the first part, three particle sizing methods (turbidimetry, dynamic light scattering and packed column chromatography) were evaluated for their capability to provide accurate estimates of the particle size distribution (PSD) and for their potential to be applied for on-line measurements to monitor and control the particle size in latex reactors. Dynamic light scattering was successfully applied on-line to follow particle growth during the emulsion polymerization of vinyl acetate in a pilot scale reactor. A layout for the hardware of an on-line sampling device which can be used with packed column chromatography is also suggested. Electron microscopy was used to corroborate results from these methods. Several interesting features were observed during this study and this motivated an in-depth investigation of some aspects related to these methods.
A detailed theoretical and experimental investigation showed for the first time that turbidimetric methods are not expected to provide an estimate of the full PSD in certain cases, and identified these cases. Based on this analysis a controversy and several inconsistencies in the literature were resolved. It was also shown that the wavelength exponent and the turbidity ratio are not different but equivalent methods.
In electron microscopy shrinkage of the latex particle under the electron beam was investigated. It was shown that the diameter decreases in an exponential manner with the time of exposure to the electron beam, and that the largest reduction occurred within the first minutes of exposure.
For the packed column chromatography method, software was developed for the conversion of the observed chromatogram to PSD. This software allows for a retention volume dependent spreading function, mass or turbidity detector and extinction coefficient calculations by taking into account the refractive index of the particles. A chromatographic apparatus operating with two detectors (turbidity and differential refractometer) connected in series was utilized to monitor particle growth during the reaction.
In the second part a systematic study of the emulsion polymerization of vinyl acetate in the absence of soap micelles in a pilot plant reactor was carried out. The effects of several variables on the particle size and the reaction rate were investigated in a series of designed experiments. These parameters were: initiator and monomer concentration, monomer and water soluble impurities and agitation. Both soap free runs and runs with soap concentrations much below the CMC were included in the design. Even though the objective was a systematic study of the soap free polymerization of vinyl acetate, the experiments with soap levels below the CMC were included to provide additional data in an attempt to reconcile the conclusions derived from experiments carried out with no soap and with soap levels above CMC. To test for the reliability and the reproducibility of the results some of the polymerization runs were repeated. Most of the measurements were duplicated or triplicated and the particle size was determined with 3 different methods.
Accurate and reproducible data were obtained. The results were consistent, at all levels of the parameters, both in the soap free polymerizations and in the runs with soap below CMS. These results help to clarify certain mechanisms, and to link phenomena observed in the soap free cases with those in polymerizations with soap above CMC, and to understand the similarities and differences. This set of data can offer valuable information for the modelling of several mechanisms of the homogeneous nucleation.