Preparation And Properties Of Weatherable Carboxylic Acid-type Waterborne Polyurethane Dispersions

Polyurethane coatings have excellent mechanical, wear-resistance and adhesive properties, and can be widely applied in the finishing field. Traditional solvent-type polyurethane coatings containing volatile organic solvents pollute the environment. So the development of waterborne polyurethane coatings (WPU) is being prompted. Although WPU possesses many advantages, such as non-pollution, safety, convenient use and excellent mechanical properties, high-quality polyurethane coatings should have weathering resistant characteristic. Therefore, weatherable carboxylic acid-type WPU dispersions were synthesized from aliphatic polyester, alicyclic isocyanate and hydrophilic carboxylic acid-type chain extender as raw materials by addition polymerization in this paper.The reaction kinetics between isophorone diisocyanate (IPDI: as alicyclic diisocyanate) and 2, 2-dimethylolpropionic acid (DMPA: as hydrophilic chain extender) in N-methyl-2-pyrrolidone (NMP: as a solvent) were first studied. The results showed that reaction activity of IPDI was lower, activation energy (Ea) was 68.7kJ/mol and reaction rate constant (k) followed Arrhenius equationFirstly, when polybutylene adipate glycol (PBA3000), IPDI and DMPA were used as main raw materials, the synthesis methods of -NCO terminated polyurethane prepolymers with -COOH groups in the side chain of polymers were classified one-step method and two-step method on the basis of feeding order of DMPA. After, prepolymers were diluted with acetone and cooled to 25~30℃, -COOH groups in the prepolymers were neutralized by triethylamine (TEA). Finally, neutralized prepolymers were dispersed in water and further extended by ethylenediamine (EDA: as auxiliary chain extender). After removing acetone, the carboxylic acid-type waterborne polyurethane dispersions were obtained. The synthetic conditions, dispersion and chain extension of polyurethane prepolymers; the relations between ion group content or R value and conductivity or appearance or critical congregating concentration (Cc.c) of WPU dispersions with the same solid content; microscopic characteristics of particles in WPU dispersions, and mechanical properties, thermal properties and structural characteristics of films made from WPU dispersions, were discussed, respectively. The experiment results showed that mechanical properties of films made from WPU dispersions synthesized by one-step method were similar to those by two-step method, but one-step method was more convenient. When–COOH group content of polyurethane molecule was from 1.09% to 2.37%, WPU dispersions were stable and their conductivities and Cc.c increased with the increasing of–COOH group content. The solid content of stable dispersion could achieve 32.15%, transmission electron microscopic (TEM) showed that the size of particles in dispersions was less than 100nm. With the increasing of -COOH group content of polyurethane molecular, tensile strength of films made from WPU dispersions increased but elongation at break declined. When–COOH group content was 2.37%, tensile strength could achieve 29MPa and elongation at break was 510%. X-ray diffraction (XRD) demonstrated that carboxylic acid-type WPU molecule had higher crystallinity and the crystallinity of soft segment in the polymer was higher than that of hard segmeng. Differential scanning calorimetry (DSC) and thermogravimetry (TG) showed that the thermal stability of carboxylic acid-type WPU was higher and thermal decomposition temperature was more than 330℃.