Novel Hydroxyl Polyacrylate For Two Component Waterborne Polyurethane Coatings

Polyurethane (PU) coatings possess excellent properties and have attracted great interesting in the coatings field due to their micro-phase separated domains from immiscibility of soft and hard blocks, hydrogen bonding, and chemically induced separation of urethane-rich and urea-rich domains. Among them the two component solvent-borne PU coatings display excellent performance and appearance which satisfy the main market requirements. Due to increased stringent legal requirements and market demands for higher-performance, formaldehyde free, thermosetting water-based coatings, two component waterborne polyurethane coatings (2K-WPU) have become increasingly popular over the past several years. 2K-WPU are composed of hydroxy-functional waterborne polymer (polyol), oligomeric polyisocyanate crosslinker derived from hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI) which may or may not be hydrophilically-modified, optional co-solvent(s), and water. Various types of polyols including polyester, polyacrylate, polyurethane, and their hybrids can be used in 2K-WPU systems. Among them, waterborne acrylic polymers attracted more attention because of their excellent resistance to hydrolysis and low cost. Hydroxy-functional waterborne acrylic polymers can be divided into two parts, namely, acrylic polymer emulsions (PAE) and acrylic polymer dispersions (PAD). 2K-WPU film based on PAE which possess high molecular weight and glass transition temperature exhibited rapid physical drying properties at room temperature before the onset of chemical cross-linking. However, it is difficult to achieve an acceptable film appearance and fulfill most of customers need due to some surfactants and protective colloids remained in the drying films which come from the PAE. On the other hand, 2K-WPU based on PADs normally have excellent film appearance due to absence of surfactants and protective colloids in PAD. However, the high concentration of hydrophilic acid groups in PADs often leads to the decrease of water-/ethanol-resistance properties of the film. In present paper, we describe novel preparation processes of hydroxy functional acrylic polymer dispersions (NPAD) and acrylic emulsion (PAH) to improve the 2K-WPU properties, and illustrate their morphologies, film formations and pot life.The polyacrylate emulsions with high hydroxy value and high solids content (≥45.0%) for 2K-WPU were synthesized by semi-batch seeded emulsion polymerization withβ-hydroxypropyl methacrylate (HPMA) as a hydroxy monomer. The effects of the hydroxy value on the emulsion polymerization stability, the particle sizes and distributions of latexes, and film performances were investigated, and the influence of molar ration of NCO and OH groups on film performace were also discussed. The results indicated that with increasing of hydroxy value, the latex particle sizes and its distributions of latexes increased and the emulsion polymerization stability decreased, while the crosslinked degree and hardness of the cured films were improved. The films with excellent properties including the water-/alcohol-resistance and gloss of the cured film were achieved when the hydroxy value of polyacrylate emulsions were 65.9 mgKOH/g and 82.4 mgKOH/g, respectively. The molar ratio of -NCO and -OH groups of 2K-WPU ranging from 1.5 to 1.8 afford the acceptable film properties. The FT-IR spectra showed that -NCO groups react completely with -OH groups in the 2K-WPU films over 7 days. TGA curves indicated that 2K-WPU cured films show higher thermally stability than the polyacrylate films.The novel acrylic hybrid (PAH) emulsions with 45.0 wt% solid contents were synthesized by semi-batch seeded emulsion polymerization withβ-hydroxyethyl methacrylate (HEMA) as a hydroxyl monomer, while the polar monomers especially carboxylic monomers were multi-stages added and in-situ neutralized during the polymerization process. TEM photos exhibited that PAHs composed of two kinds of polymer emulsion particles, one kind of particles was covered mainly with carboxy groups'polymers (P1), the other carried hydroxy groups'polymers (P2). Nano-granularity analysis indicated that the PAHs exhibited smaller average diameters and broader particle distributions than the common hydroxyl polyacrylate emulsions. The acceptable 2K-WPU film properties from PAH was obtained when the mass ratio of carboxy and hydroxy groups in P1 and P2 were 1:0 and 1:3, respectively. The FT-IR spectra demonstrated that the NCO groups react completely with OH groups in the 2K-WPU films in 3 days. AFM pictures indicated that the surface of 2K-WPU cured films were compact and flat.Novel self-emulsifying acrylic polymer aqueous dispersions (NPAD) for 2K-WPU were prepared by the emulsification of self-emulsifying polymer blends in water, which the polymer blends typically consist of two kinds of acrylic polymers, one is a salt group containing polymer (P3), and the other is a polymer without salt groups (P4). The dynamic light scattering (DLS) analysis showed that the NPAD have a bimodal particle size distribution and the particle diameters can be controlled by the amount of salt group containing polymer and the concentration of salt groups in the polymer. Transmission electron microscopy (TEM) images also testified that the NPADs possess small particles consisted of P3 and large particles with core-shell structure, P3 in the shell and the P4 in the core. The properties of the NPAD, conventional polyacrylic dispersion (CPAD) and poylacrylic emulsion (PAE) for 2K-WPU were measured. The NPADs possess higher solid contents up to 45 wt% and much lower carboxy salt contents than the CPAD, and the NPAD-based 2K-WPU films displayed excellent performance,such as 90% of the gloss, 0.81 of the pendulum hardness, better solvents- and water-resistance than those of the CPAD and PAE. The TGA curves indicated the NPAD-based 2K-WPU film had good thermal stability.2K-WPU coatings were prepared from hydrophilically-modified polyisocyanate and PAE and its dispersion (PAD). The hardness, gloss and chemical resistance of the product were measured to investigate the physicochemical behaviours of 2K-WPU during the pot life. The results indicate that (1) with the increase of storage time, the viscosity of PAD-based 2K-WPU slowly decreases, along with an increase in bubbles and micro-foams in the film, while the water and chemical resistance decreases,the film hardness first decrease and then increase; (2) as for PAE-based 2K-WPU, the viscosity first decrease and then increase with the increase of storage time, while the film hardness and the chemical resistance decreases, and the bubbles in the film even disappear; (3) neither the particle diameter nor the gloss of PAE- and PAD-based 2K-WPU shows obvious change during their pot life; and (4) PAD-based 2K-WPU exhibit better film performance but slightly shorter pot life than the PAE-based one (about 4h). It is thus concluded that it is not the viscosity but the film performance that determines the pot life of 2K-WPU before application.