Charles R. Hegedus

Cationic polymerization of emulsified epoxy resins

Abstract Whereas free-radical emulsion polymerization has been studied extensively, published reports of cationic (i.e., acid catalyzed) polymerizations of emulsified monomers of any type are rare, and we have found no reported examples of non-reversible emulsion polymerizations catalyzed by Bronsted acids. We recently discovered that treatment of an emulsion of liquid epoxy resin with select superacid catalysts yields polymeric polyols. Catalysis with 1% perchloric acid at room temperature yields a product with a number average molecular weight of 1650, and a polydispersity of 5.0 as measured by gel permeation chromatogram. The polyol’s structure differs from that of conventional high molecular weight epoxy resins prepared by the advancement process in several ways, including the incorporation of two glycidyl units in the repeat unit. The molecular weight of the product depends on the superacid catalyst employed. The product was shown to be much lower in levels of residual bisphenol-A diglycidyl ether (BADGE) and bisphenol-A than conventional epoxy resins, which is an issue of significant importance to producers of can linings for foods and beverages. Polyols prepared by this process were cross-linked with melamine–formaldehyde resins to produce water-borne coatings free of added cosolvent that develop excellent solvent resistance at lower bake temperatures than traditional epoxy resins.

Waterborne acrylic-epoxy coatings

Waterborne two-component acrylic-epoxy coatings are gaining popularity as topcoats in moderate duty industrial and high performance architectural (HIPAC) applications. This increased popularity is due to their attractive handling, application, and performance properties, along with their low solvent content and odor. The objectives of this work were to characterize the cure and property development of these coatings, evaluate performance properties of cured films, and investigate a new epoxy resin dispersion in existing acrylic-epoxy formulations. These evaluations confirmed that existing acrylic-epoxy coatings have long pot life and short dry times while displaying a range of chemical resistance and physical properties. IR spectroscopy and differential scanning calorimetry (DSC) results indicated that the extent of cure at ambient conditions over a 21-day period was minimal; however, dynamic mechanical analysis (DMA) and solvent swell results did illustrate noticeable crosslink density development under these conditions. DSC results demonstrated more complete reaction and cure after heating. Direct substitution of a novel epoxy resin dispersion into these formulas resulted in lower required solvent content, shorter dry time, higher gloss, higher crosslink density, and improved water and scrub resistance.

New applications in studies of waterborne coatings by atomic force microscopy

Abstract Atomic force microscopy (AFM) is rapidly emerging as an important tool for coatings characterization. We report several new applications of AFM of particular value to the development of improved waterborne coatings systems. First, an AFM method was developed to quantitatively assess the extent of coalescence and film formation for latex films by measurement of particle number density of protruding (uncoalesced) particles in dried coatings. Second, the use of topographic imaging to evaluate environmental (temperature) effects on film formation was investigated for a waterborne latex system. Finally, specular gloss of waterborne epoxy coatings was studied by AFM and optical measurements, and topographic features analyzed using power spectral density calculations were found to correlate with optical gloss measurements. Mechanisms for gloss reduction over time (particularly in early pot life coatings) were elucidated in the studies. Further applications in coatings studies will be driven by the development of new modes of AFM (friction force, force modulation, and phase contrast) that can be used to map mechanical properties (friction, stiffness, and adhesion) while simultaneously imaging topography. Examples of the use of the phase contrast mode to identify chemically different domains in early pot life waterborne epoxy coatings are presented.

New polymeric polyol for thermoset coatings: Superacid-catalyzed copolymerization of water and epoxy resins

Though free-radical emulsion polymerization has been studied extensively, published reports of cationic (i.e., acid-catalyzed) polymerizations of emulsified monomers are rare. It was recently discovered that treatment of an emulsion of liquid epoxy resin with select superacid catalysts yields a polymeric polyol. Catalysis with one percent perchloric acid at room temperature yields a product with a number average molecular weight of 1650, and a polydispersity of 5.0 as measured by GPC. The polyol’s structure differs from that of conventional high molecular weight epoxy resins prepared by the advancement process in several ways, including the incorporation of two glycidyl units in the repeat unit. In essence, the product is a copolymer of the epoxy resin and water, in which water is incorporated in the repeat unit structure by reaction with two epoxide groups. A similar product can be prepared by solution polymerization, where the molecular weight is controlled by the ratio of water to epoxy resin. The product was shown to have lower levels of residual bisphenol-A diglycidyl ether (BADGE) and bisphenol-A than conventional advanced epoxy resins. Polyols prepared by these new processes were crosslinked with melamine-formaldehyde resins in waterborne coating formulations which were free of added cosolvent, as well as solventborne coating formulations. The coatings developed excellent solvent resistance at lower bake temperatures than traditional epoxy resins.

Submicron pyrogenic silica and its use in polymer and coating systems

We describe briefly the manufacturing process and commercial grades of pyrogenic silica. We discuss their unique chemical and physical properties. We illustrate the physical and mechanical properties which they impart as fillers in polymer systems and to suggest explanations for these properties in relation to silica surface-polymer interactions