Colloidal Stability Analysis of a Core-Shell Emulsion Polymerization

Chapter 2

Downloaded by UNIV OF NEW SOUTH WALES on September 14, 2015 | http://pubs.acs.org Publication Date: November 6, 2001 | doi: 10.1021/bk-2002-0801.ch002

Colloidal Stability Analysis of a Core-Shell Emulsion Polymerization E. C. Kostansek Rohm and Haas Company, 727 Norristown Road, P.O. Box 904, Spring House, PA 19477

During the course of emulsion polymerization reactions many parameters which affect colloidal stability are varying significantly. This is especially true of core/shell systems such as the one discussed in this paper. In order to identify potential colloidal stability problem areas in the process, we took periodic samples from full-scale production emulsion polymerization reactions and measured/analyzed both latex and process parameters. Latex parameters included particle size, zeta potential, surface acid, surface hydrophobicity, and surfactant coverage. Process parameters included viscosity, agitation (shear rate), solids, and pH. Shear stability was measured and also calculated from D L V O theory with, and without, hydrophobic attraction energies. The results indicated a problem area in the process in terms of shear stability and possible gel formation and coagulation in the hydrophobic second stage of the process. Hydrophobic interactions appeared to be very important during that stage of the reaction based on the shear stability calculations and surfactant coverage data. This is the first time that such a detailed colloidal analysis of a commercial emulsion polymerization process has been reported in the literature.

© 2002 American Chemical Society

In Polymer Colloids; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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14 Emulsion polymerizations are dynamic, colloidally complicated processes where colloidal parameters are changing continuously during the entire course of the reaction. This is especially true for core/shell latexes where the surface and bulk compositions change dramatically from stage to stage. Successive stages can have quite different hydrophobicities, glass transition temperatures, and morphologies. Because of these changes, colloidal stability of the latex particles can vary significantly from stage to stage. This can be manifested by poor mechanical stability, coagulation, and gel formation. Often it is difficult to solve stability problems because end-use performance limits the options for stabilizing the latex. For example, increased surfactant levels can sometimes improve colloidal stability, but might lead to increased foaming or decreased adhesion and water resistance in the final application. Often a combination of chemical and process changes is needed to solve the problem. Because latex systems are colloidally complex, it can be difficult to ascertain the root causes of instability during the reaction. This makes it very challenging to do troubleshooting. Given this situation, we decided to thoroughly analyze all stages of a core/shell emulsion polymerization process which has demonstrated instability problems. The magnitude and practical importance of the problem is evident when one has to remove 10,000 gal. of coagulated latex from a kettle and start over! In order to accomplish our study, we took actual production samples of latex from all of the important points in the process and analyzed them for the most critical colloidal parameters. To our knowledge, this is first time in the literature that this has been attempted. The methods and results have been used to identify the causes of instability and should be applicable to other latex systems in general.

Experimental Latex System The latex system we studied was a seeded core/shell three-stage emulsion polymerization reaction containing the monomers methyl methacrylate ( M M A ) , n-butyl acrylate (BA), and styrene (Sty). The seed, Stage I, and Stage III monomers were primarily M M A . Stage II was a crosslinked ΒΑ/Sty copolymer. The surfactant was sodium dodecylbenzenesulfonate (SDBS). Three 10,000 gal. batches were sampled at strategic points in the reaction and analyzed as described in the following sections. In this paper we will concentrate mainly on the results at the ends of each of the three stages. Results were very consistent from batch to batch and we report here the average values for the three batches.


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Latex Variables The latex variables studied were particle size, zeta potential, surfactant coverage, surfactant surface area at saturation, surface acid, and shear stability. Particle size was measured using a Brookhaven BI-90 instrument and electophoretic mobility was measured using a Malvern ZetaSizer III. Zeta potentials were calculated from the mobilities using the method of O'Brien and White ( i ) . Surfactant coverage and surface area at saturation were determined by SDBS surfactant titration using a Wilhelmy plate tensiometer. Shear stability was measured using a Rheometrics D M A instrument with a cone and plate configuration.

Process Variables Process variables studied were pH, temperature, polymer solids, viscosity, residual monomer, and shear rate of mixing. Polymer solids were determined gravimetrically and viscosity was measured using a Brookfield viscometer. Temperature and residual monomer were measured, but will not be discussed in this paper. The maximum shear rate the latex particles were subjected to was estimated based on agitator design and tip speed.

Results and Discussion Particle Size, Solids, and Viscosity The polymer solids, particle size, pH, and viscosity of the latex at various points in the polymerization reaction are listed in Table I. Solids ranged from 7.3% to 50%, particle size grew from the 105 nm seed to the 289 nm final product, and p H stayed in a narrow range between 5.7 and 6.8 during the reaction. Viscosity increased from the initial value of 3 cps to a maximum of 73 cps in Stage III to 42 cps for the final product. As mentioned previously, most of the remaining discussion will concentrate on the parameters associated with the three major stages of the polymerization.

Zeta Potential Zeta potentials for the three reaction stages are shown in Figure 1. Measurement conditions were chosen to match actual reaction conditions as closely as possible. Zeta potential changed very little during the course of the


16 reaction. This would suggest that colloidal stability should be similar at all stages of the reaction. Unfortunately, our experience with this process tells us that this is not the case, so other parameters such as surface acid, surfactant coverage, and surface hydrophobicity needed to be explored to search for the causes of instability.


Table I. Selected Latex Parameters at Different Reaction Stages Reaction Stage

% Solids

Seed ( M M A ) I (MMA) II (BA/Sty) III ( M M A ) Final

7.3 33.3 45.6 50.7 50.0

Particle Diameter 105 206 294 296 289

pH

Viscosity

6.8 6.5 5.7 6.2 6.0

3 6 39 73 42

Note: Solids in weight %, particle size in nm, and viscosity in cps.

Figure 1. Latex zeta potential as a function of reaction stage.

Surface A c i d It is well known that surface acid can help stabilize latex particles (2-4). Even though we did not purposely add acidic monomer to this reaction, we measured surface acid in case some had formed through hydrolysis. Figure 2 shows the surface acid, expressed as % M A A based on polymer, as a function of reaction stage. There is indeed measurable surface acid which peaks at Stage II, but it is a small amount which probably contributes minimally to the stability.


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Colloidal Stability Analysis of a Core-Shell Emulsion Polymerization

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