The Science of Defoaming

481

9

Control of Foam in

Waterborne Latex

Paints and Varnishes

9.1 INTRODUCTION

Paints make an important contribution to both the preservation and aesthetics of

many different types of artifacts. They are applied to an enormous range of sub-

strates ranging from various metals, woods, and plaster to paper. Traditionally many

types of paints have been formulated to consist of solid pigment particles and “bind-

ers,” dispersed in volatile non-aqueous solvents. After application, the solvent evapo-

rates leaving the binder to coat the substrate with a coherent lm containing the

dispersed pigment. However, such solvents are usually classied as “volatile organic

compounds” (VOCs), which are known to represent environmental hazards. Some

solvents are known carcinogens, the vapors of others are known greenhouse gases,

and most contribute to the formation of excessive tropospheric ozone levels (through

an ultraviolet-induced chain of reactions with oxides of nitrogen). In consequence,

legislation in many countries has been introduced that seeks to minimize the use of

VOCs [1].

The industrial reaction to this situation is one of compliance, with an increasing

use of waterborne paint formulations based on the use of “synthetic latices” as bind-

ers especially in paints for domestic application. Such latices are usually polymeric

colloids, of volume fraction from 0.2 to 0.5, dispersed in aqueous surfactant solution,

prepared by a process of emulsion polymerization. These dispersions have a slightly

turbid appearance, often with a low viscosity of order 1 mPa s. The latices can be

readily prepared as near-monodisperse colloids.

The preparation of waterborne latices by emulsion polymerization usually employs

polymerization in aqueous micellar surfactant solutions. The simplest manifestation

of the process involves the presence in an aqueous medium of emulsied mono-

mer drops, micellar surfactant, and a water-soluble polymerization (free-radical)

initiator. A combination of monomers is often used, exemplied by combinations

of compounds such as methyl methacrylate, butyl acrylate, and styrene. Typically

monomers have only slight water solubility. The classic qualitative picture of this

process was described by Harkins [2] more than 60 years ago. A schematic of that

process is shown in Figure 9.1. The key step is solubilization of the monomers in

the micelles where polymerization is initiated. Both the low solubility of the mono-

mer and the relatively low surface area of the monomer emulsion drops means that

initiation of polymerization is essentially conned to the micelles. Polymerization

now proceeds as more monomer is transported to the swelling micelles from the

482 The Science of Defoaming: Theory, Experiment and Applications

monomer emulsion drops. Eventually the solubilization capacity of the micelles is

exceeded whereupon they are transformed into colloidal polymer–monomer particu-

late nuclei stabilized by adsorbed surfactant. No further nuclei form. However, trans-

port of monomer to the existing nuclei continues from the monomer emulsion drops.

The nuclei continue to grow into particles of colloidal dimensions (~100–500 nm)

until the monomer drops disappear. A scanning electromicrograph of a typical latex

that results from this process is shown in Figure 9.2. At this stage any further poly-

merization concerns only the monomer present in the colloidal particles. Presumably

the low polydispersity of the resulting latex derives at least in part from the low poly-

dispersity of the micellar incubator of the colloid. References [3–6] can be consulted

for more detailed accounts of emulsion polymerization.

Micelle with

solubilized

monomer +

polymer

Swelling colloidal

monomer + polymer

particle

(a)

(b

)

Monomer

Monomer

Monomer

FIGURE 9.1 Schematic illustration of model of emulsion polymerization by Harkins.

(a) Emulsied monomer supplies monomer to micelles where polymerization is initiated.

(b) Process continues as micelles transformed into colloidal particles, which continue to swell

until emulsied monomer entirely consumed. (From Harkins, W.D., J. Am. Chem. Soc., 69,

1428, 1947.)