This paper introduces a new generation of alkyl phenol-free nonionic surfactants. The new surfactants are environmentally compatible, exhibiting effective performance during polymerization and post-polymerization stabilization. These liquid surfactants meet demanding physicochemical properties, regulatory compliance and performance requirements. They exhibit low freezing temperatures, excellent water miscibility, minimum gel phase formation and low viscosity at high solids content. The new surfactants are clear, high-solid liquids at room temperature and are easy to handle and very effective during polymerization, providing overall latex stability. This paper focuses on their chemical and physical properties as well as on their performance in model acrylic formulations for architectural coatings.
IntroductionEarlier generations of nonionic surfactants (Disponil® As and Disponil AFXs) as well as the new surfactants introduced in this paper, were developed in response to increasing global environmental protection regulations and recommendations to phase out the use of Alkyl Phenol Ethoxylates (APEs). The two new high-HLB nonionic surfactants introduced in this paper, Disponil AFX 4061 and Disponil AFX 4070, have been designed to meet environmental requirements while providing effective performance in emulsion polymerization. Also, they are very effective as post-polymerization latex stabilizers or surfactant additives for coating and adhesive formulations. Similar to previous generations, the introduction of these new emulsifiers responds to the emulsion polymerization and coatings industry demands for ecologically acceptable and effective non-ionic emulsifiers.
Surfactant Composition and PropertiesTable 1 shows the composition and properties of the two new nonionic surfactants: Disponil AFX 4061 and Disponil AFX 4070. Selected Disponil As and AFXs have also been included for comparison purposes. The hydrophobes in all the surfactants listed are aliphatic. This portion of the molecules is based on C12-14 natural fatty or synthetic alcohols and primary and secondary isomeric alcohols.
Some of the most important features of the Disponil AFX 4061 and Disponil AFX 4070 can be listed as follows:
- alkyl phenol free (AP-free);
- easier handling at low temperatures;
- high solid content of 60% and 70%;
- minimum gel phase formation;
- low freezing point range of -5 ºC to -9 ºC at 60% and 70%;
- clear, high-solid liquids at sub zero and room temperatures;
- excellent water miscibility; and
- low viscosity.
Performance in Emulsion PolymerizationThe mechanism of emulsion polymerization, as well as the role of surfactants has been discussed in a number of publications.1-4 Extensive surfactant screening in model formulations has shown that the new emulsifiers are very effective in acrylic formulations. Table 2 shows the physical properties of thermal acrylic latexes made combining one nonionic emulsifier at a time with Disponil FES 32 an anionic alkyl ether sodium sulfate, used for particle size control. The results demonstrate the role of an effective surfactant combination in particle size control, coagulum content and overall stabilization. All latexes exhibited desirable small particle sizes, low coagulum, mechanical stability and freeze thaw stability.
The electrostatic stability was obtained by post adding a 0.25% of either Disponil AFX 4061 or Disponil AFX 4070. The model thermal acrylic latex formulation is shown in Table 3.
Although there is an increasing tendency to replace alkyl phenol-based (AP) with alkyl phenol-free (AP-free) surfactants, octyl phenol-40 (OP-40) still is extensively used in many different coating applications in North America. Results listed in Table 2 show remarkable performance similarities between OP-40 and both Disponil AFX 4061 and Disponil AFX 4070. These results clearly show that both surfactants are suitable replacements for OP-40.
Latex Stability: Post Polymerization StabilizationEmulsion polymer and coating producers are increasing their efforts to replace OP-40 and similar AP-based surfactants in a wide spectrum of latexes and coatings. Some of the most important drivers of these efforts are a new North American wave of environmental awareness and efforts to increase sustainability, Canadian regulatory pressures, older European regulations, and recommendations and trends to globalize formulations.
Surfactant post addition is often necessary to stabilize latex formulations with high inorganic filler content such as for architectural and traffic coatings, as well as paper and adhesive coatings. A fundamental property of the surfactant molecules is their ability to orient and adsorb at the interfaces. In latex systems, most of the surfactant is adsorbed at the interface between the particles and the water. This adsorption is a thermodynamic process favored by a decrease in the overall free energy of the system. The adsorbed layer of surfactant molecules is essential for the stabilization of latexes during polymerization and during post polymerization processes. It is well known that the colloidal stabilization depends on the interactions of attractive and repulsive forces between the particles. It is also accepted that electrostatic repulsion stabilization and steric or spatial stabilization are the two most important barriers to flocculation. The addition of inorganic fillers containing divalent metallic ions such as calcium, magnesium and zinc, produces rapid flocculation by disrupting the electrical double layer surrounding the polymer particles. This reduces the electrostatic repulsions between the adsorbed anionic surfactant molecules typically used during the emulsion polymerization process.
A pragmatic remedy to restore stability is the addition of high-HLB surfactants, specifically polyethoxylated nonionic emulsifiers. The hydrophilic polyethoxylated chain provides steric stabilization by orienting and extending towards the water phase. This prevents the particles from entry into the effective space of mutual attraction. Also, it has been proposed that the effectiveness of this type of stabilization is due to the hydrogen bond interactions and high-localized viscosity of the hydrated polyethoxylated chain, which acts as a protective shield preventing flocculation.5-10
High-HLB nonionic surfactants are characterized by a low amount of surfactant molecules adsorbed at the interfaces and high cross-sectional areas at the interfaces. These properties make them ideal as post polymerization stabilizers. Disponil AFX 4061, Disponil AFX 4070 and OP-40 were evaluated as post polymerization stabilizers. In this discussion, the latex stability to divalent cations and hence the electrostatic stability of the system, is measured by the stability of the latex to calcium chloride titration.
The results of the post polymerization stabilization study are shown in Table 4. The evaluation of the latex stability consisted in measuring the coagulum formed in model acrylic latexes after the addition of 0.25%, 0.50% and 1% nonionic surfactant added to acrylic latexes without electrostatic stability. The post addition of nonionic surfactants was followed by the addition of 5% calcium chloride solution. The model acrylic latexes were made with one primary anionic emulsifier, either SLS or alkyl ether sodium sulfate surfactant. Table 4 shows post stabilization results. No coagulum was formed after the addition of only 0.25 % of non-ionic surfactant to the latexes. The excellent performance of Disponil AFX 4061 and Disponil AFX 4070 compared to OP-40 is a clear indication that they can replace OP-40 in commercial applications.
In previous studies it has been observed that the efficiency and effectiveness of surfactants as post polymerization stabilizers is system dependent. It has been observed that certain latexes made with large quantities of anionic surfactants as the primary emulsifier, may require more than 1% nonionic surfactant addition to restore the electrostatic stability.
SummaryThe experimental results presented in this paper illustrate the efficiency and effectiveness of the new Disponil AFX 4061 and Disponil AFX 4070. They can be used in a variety of applications, either to effectively improve adsorption at diverse surfaces and interfaces or to efficiently stabilize emulsion polymers. This new generation of nonionic surfactants is AP-free, biodegradable, low viscosity at high solid content, clear, and easy to handle at low temperatures. These characteristics make them environmentally acceptable and provide an opportunity for developing new AP-free formulations. The new Disponil emulsifiers have been shown to be effective during polymerization and as post-polymerization stabilizers replacing high EO-bearing APEs and synthetic sec-alcohol ethoxylates.
This paper was presented at The Waterborne Symposium sponsored by The University of Southern Mississippi School of Polymers and High Performance Materials and The Southern Society for Coatings Technology, 2009, New Orleans, LA.