Morphologies and droplet sizes of alkyd–acrylic hybrids with high solids content (original) (raw)
Abstract
Alkyd–acrylic hybrids having final solids content of 75–80% were prepared by dropping alkyd resin into an acrylic dispersion. The resulting suspo emulsion structures and the alkyd droplet sizes in the hybrids were dependent on the preparation procedure as well as the interfacial tension between the two liquid phases, _σ_12, the shear rate, γ˙, the latex/alkyd ratio and the viscosity ratio, p, between the viscosity of the dispersed phase, _η_d, and the viscosity of the matrix, _η_m. _σ_12 was varied by changing the surfactant concentration and pH of the hybrids, _η_d was varied by changing the viscosity of the alkyd phase but was also affected by variations in the pH, and finally the shear stress was varied by varying the solids content and the amount of latex in the hybrid systems. Parameters that effected the ratio, p, were also studied separately and in combination, and by varying parameters such as the presence of latex particles, alkyd viscosity and solids content, the effect of p on the alkyd droplet size was determined.
Different liquid structures were obtained depending on how the surfactant was added to the system. When surfactant was added to the latex prior to the addition of the alkyd, an O/W emulsion was formed. However, if the surfactant was added to the alkyd prior to the addition of the alkyd to the latex, a multi-emulsion (W/O/W) of alkyd was formed, which was also observed when the hybrids were prepared at a high pH. The structures obtained were stable over time and remained unchanged even after ageing for 1 year at room temperature.
Introduction
The replacement of solventborne surface coatings by their aqueous-based counterparts, has been an important research topic during the past several decades, motivated by health and environmental concerns. One result of these efforts has been waterborne binders, such as acrylic latex dispersions and alkyd emulsions. Both of these binder types perform well in certain applications but improvements are also sought after. For instance, acrylic latex coatings show short drying time, low odour and easy clean up with water. However, penetration of the substrate for acrylic latexes is inferior to that of alkyd emulsions, and problems with levelling and brush marks occur as well. Alkyd emulsions on the other hand have high gloss and good penetration, but the drying time for alkyd emulsions is long.
A research area investigating hybrid systems [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] consisting of latex dispersions and alkyd emulsions arises from the expectation that combining both binders could diminish the negative characteristics each type displays alone. This type of hybrid system would combine the fast drying and colour retention of acrylic latexes with the high gloss and good penetration properties of alkyd emulsions.
Hybrid systems can be achieved in several ways; for instance, the acrylic phase can be polymerised by emulsion [2], [8] or mini-emulsion [1], [2], [8] polymerisation processes in the presence of colloidal alkyd droplets. Another possibility is the mechanical mixing of an acrylic dispersion and an alkyd emulsion. In the present study, the hybrid system was prepared using a high solids content technology in which the hybrid was created by emulsification of the alkyd in the presence of an acrylic dispersion [12], [13]. Under certain conditions, paints prepared using this acrylic–alkyd hybrid system as a binder were not completely stable when subjected to the shear forces arising during application with a brush or a roller. Therefore, an increased and deeper understanding of the hybrid system's structure and morphology with respect to its stability was necessary in order to be able to use the hybrid system in real applications.
We have previously partially reported on the hybrid structure and parameters that affect the alkyd droplet sizes in some of the hybrid systems, which are also reported in this study [12]. In addition, this paper will present an extended discussion on the morphology of the hybrid system in the liquid state and deal in more detail with the parameters that affect the droplet size of the alkyd phase. Both the liquid morphology and the alkyd droplet size affect properties in the wet and in the dry state of a surface coating. For example, the morphology in the liquid state will not only affect the behaviour of the wet binder such as rheology, but will also have an impact on the drying process and the film morphology after application. The droplet size affects the colloidal stability of the system and the ability to withstand high shear deformation during application. And in the solid state of a coating, the gloss and mechanical strength will be affected by the droplet size. To determine what influences the droplet size of the alkyd emulsion, our study focused on surfactant concentration, pH, alkyd viscosity and finally, alkyd concentration (solids content). The studies on the effects of pH and variations of the surfactant concentration also revealed great differences in the liquid structure of the hybrids.
Section snippets
Materials
Linseed oil alkyd (Akzo Nobel Decorative Coatings AB, Sweden) with a Mw of 7300
g/mol was used. The alkyd had an oil length of 75%, i.e. the alkyd composition was 75
wt.% of fatty acid based on the total alkyd and the acid value was between of 6–10
mg KOH/g[14]. The alkyd was liquid at room temperature with a viscosity of 1.3
Pa
s at 23
°C and shear rate 100
s−1. Butyl ester of tall oil fatty acid (Akzo Nobel Decorative Coatings AB, Sweden) with a Mw of 700
g/mol, an acid value of 7
mg KOH/g and a
Morphology in the liquid phase
The morphology for three hybrid dispersions, HS–L, HS–AL and HNaOH, was studied in the liquid state by means of a light microscope (see Fig. 1, Fig. 2, Fig. 3). However, due to the large differences in size between the alkyd droplets and latex particles, it was difficult to focus the light microscope so that the images were sharp with respect to the alkyd droplets as well as to the latex particles. Fig. 1, Fig. 2, Fig. 3 show the images of the hybrids when the light microscope was focused on
Conclusions
When an alkyd resin was added directly to an acrylic dispersion, it was possible to achieve stable hybrids having either O/W emulsions or W/O/W multi-emulsions with a solids content above 80
wt.%. The O/W emulsion hybrids were formed when alkyd was added to the latex dispersion, either when additional surfactant had been added to the dispersion prior to the alkyd addition, or at low pH of the dispersion. At higher dispersion pH, or when the surfactant mixture was added to the alkyd resin prior
Acknowledgements
We are thankful for the financial support from the Research School for Industry: “The Building and its Indoor Environment” organized by “The Foundation for Knowledge and Competence Development (KK-foundation)” and Celanese Emulsions Norden AB, Perstorp, Sweden, and Akzo Nobel Decorative Coatings AB, Malmö, Sweden. The authors thank Professor Frans H.J. Maurer and Professor Björn Bergenståhl for contributions to the discussion.
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