Effect of functional monomer on the stability and film properties of thermosetting core–shell latexes

Abstract

Functionalized core–shell latexes were prepared by copolymerization of butyl acrylate and methyl methacrylate with 2-hydroxyethyl methacrylate (HEMA) or methacrylic acid (MAA), which were added during the first or second stages of polymerization, respectively. The HEMA and MAA concentrations were increased while the equivalent ratio of functional groups remained constant. Colloidal stability, particle size, particle size distribution, film properties and morphology were studied as functions of functional monomer content. The upper limit functionality content was limited by the stability of the system during synthesis. A bimodal particle size distribution was observed for high concentrations of functional monomers. Increase in carboxyl and hydroxyl functionalities improved tensile strength and modulus for un-crosslinked films, and generally higher tensile strength, tensile modulus and storage modulus at high temperature were obtained after the functional latexes were crosslinked with a cycloaliphatic diepoxide.

Introduction

Environmental regulations have been continuously restricting the amount of volatile organic compounds (VOCs) in coatings formulations. As a consequence waterborne systems have developed, and will continue to do so as the logical choice for many applications in the coatings industry [1]. Latex based and water-reducible coatings are two major classes of waterborne coatings replacing solvent-borne applications. Latex dispersions form films at ambient temperatures by coalescence of relatively soft particles containing solid polymer. Cohesive strength development occurs during coalescence when interdiffusion of polymer chains takes place, but also by crosslinking of the polymer prior to or during film formation [2], [3], [5]. Generally, crosslinking is introduced into thermoplastic latex dispersions for improvement of mechanical properties and solvent resistance. Several types of thermosetting latexes have been reported in the literature in which functional monomers have been added during synthesis and then chemically crosslinked [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14].

Previous studies in this group have shown that dispersions of latex particles containing hydroxyl-functional core and carboxylic-functional shell were successfully crosslinked with a cycloaliphatic diepoxide [15], [16]. A schematic representation is shown in Scheme 1. The crosslinking reactions for the system were influenced by several factors including the amount and addition mode of diepoxide, location of functional monomer, type and amount of catalyst, among others. Soucek et al. [17] investigated epoxide addition in terms of morphology and film properties obtained from the latex system. When cycloaliphatic diepoxide was added in the form of solution, increase in hardness and tensile modulus was observed possibly due to more uniform adsorption of the diepoxide on the particles surface, whereas addition of emulsified diepoxide showed lower hardness and tensile modulus. This was postulated to be a result of decreased interaction of emulsified epoxide with the latex particles [17].

In a separate study, the effect of location of hydroxyl functionality was investigated [18]. Phase inversion of the core–shell structure did not occur when hydroxyl functionality was introduced during the first stage of polymerization, indicating concentration of hydroxyl groups within the core. When hydroxyl functionality was located in the core, it promoted hardness and tensile modulus, and when it was present in the shell of the particles, higher crosslink density was observed. Distribution of functionality between both stages did not generally improve film properties [18]. The effect of type and concentration of catalyst was studied previously for latexes with both hydroxyl and carboxyl functional groups as sole functionality (monofunctional systems). Strong acid catalysis was required for crosslinking of hydroxyl functional latexes with cycloaliphatic diepoxide, whereas it did not necessarily improve the coating properties of latexes containing carboxyl groups [19]. It was postulated that the conjugate base of the strong acid catalyst did not undergo re-protonation under the curing conditions.

Purposeful design of these thermosetting latexes relies on the evaluation of composition variables such as addition mode of the crosslinker, type of catalyst and functionality concentration. It is desirable to have 10–20 wt% of total monomer available as functional groups to participate in crosslinking reactions; however, other properties could also be affected by increase in functionality content. Polymerization pathways, particles structure and ultimately film properties might be influenced by such change in composition. In many of previously studied formulations in the literature, functionality varies between 2 and 8-wt% based on total monomer weight. Although such films show improved solvent resistance and enhanced mechanical properties, there is no specific information on the extent of crosslinking [4], [14]. In the present study, optimization of the total amount of functional monomers was investigated, as well as consequent effects on latex and film properties. Two series of latexes were prepared with different glass transition temperatures (T_g=10 °C and T_g=−5 °C) and with increasing content of carboxyl and hydroxyl functional groups. The effects of the reacting system on colloidal stability and on particle size distribution were evaluated with varying functionality. Tensile, thermo-mechanical properties and film morphology were also evaluated.