Study On The Crosslinking Modification Of Polymer Emulsions For Waterborne Coatings

Waterborne polymers (WPs) for coatings can be divided into polymer emulsions and polymer dispersions according to the preparation methods. They all use functional monomers /surfactants with hydrophilic groups including -COOH, -NHB2B, -SOB3BH, and so on as the (internal) emulsifiers. However, the hydrophilicity groups remained in the film often decreases the film performance.WPs were crosslinking modified with aziridine (AZ), polycarbodiimide (PCDI), diacetone acrylamide (DAAM)/adipic dihydrazide (ADH), and hydrophilic polyisocyanate to eliminate the influence of hydrophilic groups in polymer films. Additionly, in order to decrese the hydrophilic monomer dosages and the volatile organic compound (VOC) contents to improve the properties of polymers, the micro-structure and configuration of polymer particles were designed by optimizing the distribution of functional groups as well as using organic-inorganic hybrid methods, the results were achieved as follows:(1) Anionic aqueous polyurethane dispersion (PUD) was consists of carboxyl groups that served as an internal emulsifier stabilizing aqueous PU dispersions and its film also exhibits a high hydrophilicity. To improve the performance of PUD film, PUDs were crosslinked with aziridine derivatives and polycarbodiimide, respectively. The PUD films were also characterized by FT-IR spectrum and thermogravimetric analysis (TGA). The results indicated that aziridine derivatives and polycarbodiimide were reactive toward the carboxyl groups of PUD at room temperature. Properties of cured PUD film are improved depending on their curing categories. For examples, aziridine derivatives improve greatly water-, solvent- and pollution-resistance properties of cured films attributed to increase its gel content up to 92.6% and its hardness to 0.72 with 5.8 of optimal mass ratio of aziridine derivatives and carboxyl groups. On the other hand, polycarbodiimide improve the adhesion, flexibility and cold crack-resistance of cured films with 10.37 of optimal mass ratio of polycarbodiimide and carboxyl groups. TGA curves show that the thermal stability of cured films increased by polycarbodiimide and decreased by aziridine derivatives.(2) Polyurethane-acrylate hybrid emulsions (PUA) were crosslinked by aziridine derivatives and polycarbodiimides, respectively. The FT-IR spectra show that the carboxyl groups of PUA reacted with aziridine derivatives and polycarbodiimides during the drying process of PUA. The properties of PUA film indicated that aziridine derivatives greatly improve water-, solvent- and pollution- resistance properties due to increase film's gel content and hardness, and the optimal mass ratio of aziridine derivatives and carboxyl group [m(AZ): m(-COOP-P)] is 6.60; while polycarbodiimide improves the PUA film's adhesion, flexibility and cold crack resistance greatly, which the corresponding mass ratio of polycarbodiimide and carboxyl group [m(PCDI): m(-COOP-P) ] is 16.67. The TGA curves determined that both polycarbodiimide and aziridine can improve the thermal stability of PUA films.(3) The ambient self-crosslinkable polyacetylate latexes with sealed function were synthesized by semi-batch seeded emulsion polymerization with diacetone acrylamide (DAAM), methyl acrylate (MMA), methacrylic acid (MAA) and butyl acrylate (BA) as co-monomers. The effects of DAAM and multiple emulsifiers on the emulsion polymerization stability and film performances were investigated. The results indicated that the emulsion polymerization stability decreased and particle size of latex increased with the amount of DAAM increased, while the crosslinked degree of cured film increased and the water- or alcohol-resistance properties of cured film improved, and the optimal DAAM dosage is 3.0% (mass fraction) and the molar ratio of DAAM and adipic dihydrazide (ADH) is 2:1; On the other hand, the emulsion polymerization displayed highly stability (low agglomeration and high monomer conversation), polyacrylate latexes exhibited good electrolyte stability, and its cured films possessed excellent sealed function, which the sodium dodecyl sulphonate(SDS), nonyl phenol ethoxylates (OP-10) and OP-40 were used as multiple emulsifiers with 2.35% (mass fraction) of the optimal dosage, the mass ration of SDS to OP-10 and OP-40 is 1:1.25; and the mass ratio of multiple emulsifies dosages in seeded, core and shell is 1.5:1:2. The FT-IR spectrum shows that keton carbonyl groups of DAAM react with hydrazide groups of ADH to form imines (C=N) during film formation. TGA curves indicated that the thermally stability of cured film was decreased attributing to the presence of C=N groups.(4) Novel polyacrylate hybrids (PAH) emulsion with 45.0% (wt) solids content was synthesized by semi-batch seeded emulsion polymerization withβ-hydroxyethyl methacrylate (HEMA) as a hydroxyl monomer, the polar monomers were multi-stages added and carboxyl groups were neutralized during the polymerization process. TEM photos show that PAHs were composed of two kinds of emulsion particles including one covered with carboxy groups (P1) and the other carrying hydroxy groups (P2). Nano-granularity analysis indicated that average diameters of the PAHs were smaller and its particle distributions were much broader than those of the common hydroxyl polyacrylate emulsion (HPAE). Two component waterborne polyurethane coatings (2K-WPU) were prepared with PAH, and the properties of 2K-WPU films indicated that the optimal mass ratio of carboxy / hydroxy groups in P1 and P2 were 1:0 and 1:3, respectively. The FT-IR spectra show that the reactions between NCO and OH in 2K-WPU films were completely finished in 3 days. AFM pictures indicated that the surface of 2K-WPU cured films were compact and flat.(5) Polyacrylate/SiOB2B composite emulsions (Si/PAE) were prepared by polyacrylate emulsions (PAE) mixing with alkaline silica sol modified by a saline coupling agent and a co-solvent. The effects of kinds and amounts of co-solvents on the sol-gel reaction and the properties of composite films were investigated, the results indicated that isopropanol is the best co-solvent and the optimal dosage is 10 wt% of the silica sol. The nano-granularity analysis and TEM photos showed that co-solvents can make the silica particles distributed more evenly in the composite emulsions and the average diameters of the Si/PAE were smaller than that of the PAE. The FT-IR spectra and AFM pictures indicated that co-solvent could induce the occurrence of sol-gel reaction of silica sol to form Si-based polymers on the surface of films during the film formation. Si/PAE films display excellent properties besides high gloss and compact. TGA curves indicated that the Si/PAE films exhibit higher thermal stability than that of the PAE.