Study On Novel Heat-resistant Aqueous Dispersions Based On Resins Containing Phthalazinone Moieties

In recent years, with the environmental pollution becoming serious, water-borne coating has been a topical subject for low volatile organic compounds (VOC) coating. However, most of them have low heat-resistant temperature that means their applications are limited in certain areas. In order to obtain heat-resistant waterborne coatings, one of the important ways is to use some heat-resistant resins. But it is difficult to obtain a finely dispersed system due to their rigid structures. Therefore, few papers have been reported on aqueous dispersions based on these resins. In this paper, several novel methods are designed and applied in preparing heat-resistant waterborne coatings, which are theoretical foundations in their industrialization and application in the future.Poly(phthalazinone ether nitrile ketone)s (PPENK) containing phthalazinone moieties developed by our group is one of the scientific achievements of Hi-tech Research and Development Program of China. PPENK has much better heat resistance and solubility than similar high performance polymers. For better properties, the polymers have a strong competitive capability in international market. PPENK is modified in a hydrolysis reaction in order to prepare heat-resistant waterborne coatings. The CN groups of PPENK are successfully converted into hydrophilic groups, COOH or CONH₂, which give macromolecule hydrophilicity. The structures of hydrolyzates (HPPENK) are confirmed by FT-IR and ¹H-NMR. The properties of hydrolyzates in different reaction time, HPPENKa (0.5 h), HPPENKb (1.5 h) and HPPENKc (3.5 h), are measured. The results indicate that the nitrile groups conversion ratio increases, glass transition temperature (T_g) of hydrolyzates increases slightly whereas the weight loss ratio of HPPENK decreases as hydrolysis time is prolonged. As expected, the hydrophilicity of HPPENK is improved greatly, e.g. when CN conversion ratio is 93.82 %, the water contact angles are decreased from 75.3°of PPENK to 54.4°of HPPENK. The kinetic of hydrolysis reaction is studied. When the reaction temperature are 100℃,110℃,120℃and 130℃, the apparent first-order rate constant K_ap1 for hydrolysis of PPENK are 7.2×10^-3 min^-1,1.0×10^-2 min^-1,1.5×10^-2 min^-1å’Œ2.2×10^-2 min^-1, respectively. The activation energy for the hydrolysis reaction is determined as 47.8 kJ/mol (CN)。By studying the effect of the different reaction co-solvents, reaction temperature and NaOH concentration on hydrolysis, the optimal synthetic technique could be concluded that it is of 6 mol/L NaOH solution and DMAc as co-solvent at 120℃. Aqueous dispersions based on three modified resins were prepared and their stability was in the following order: HPPENKc＞HPPENKb＞HPPENKa. Especially, the HPPENKc aqueous dispersion played good stability and good film-forming property. Curing agents and emulsifiers are screened and applied in HPPENKc aqueous dispersion. Cured film properties of HPPENKc were as follows: pencil rigidity (6 H), heat-resistance (300℃, 24 h), impact-resistance (50 cm) and adhesion (grade 1).From the molecular design, a series of novel aqueous polyurethane dispersions (PUDs) were synthesized by cation self-emulsification. The main raw materials were TDI, PEG, TEA and DHPZ. The FT-IR spectra confirmed that the phthalazinone segments were introduced into the main chain of polyurethane. Under the same conditions, stable PUDs are obtained when molecular weight of PEG were 600, 1000 and 2000, and the tiptop content of DHPZ (Feeding weght ratio) were 18.03 %, 22.97 % and 20.03 %, respectively. The emulsion could not be dispersed if the tiptop content of DHPZ was exceeded. The solid content of PUDs was between 22.3～31.2 %. The particle size and viscosity of PUDs increased with DHPZ content, the same conclusion as PEG molecular weight.The rigidity of polyurethane chain is enhanced and heat-resistance of cured films was improved due to the aromatic structure of the phthalazinone moiety. As to PU-A4 with 18.06 % DHPZ content, the T_g and 5 % weight loss temperature of hard segments were 263℃and 252℃, respectively, outclass the PUDs without DHPZ. With the DHPZ content increased, the water contact angle, hardness and tensile strength of cured films increased, in contrast, water swell, impact resistance and elongation at break decreased significantly. With the PEG molecular weight increased, the water contact angle, hardness and tensile strength of cured films decreased, in contrast, water swell, impact resistance and elongation at break increased. According to different applications, the novel PUDs with different heat resistant class and perfect combination properties could be exploited by controlling the molecule weight of PEG and content of DHPZ and DMPA.Phase inversion emulsification is applied in preparing aqueous dispersion based on PPENK (η=0.51 g/dL, in CHCl₃) without any modification. The orthogonal layout of four factors and three levels (L₉(3⁴)) was designed to study the effect of concentration of PVA/water, concentration of PPENK/CHCl₃, W_PVA/W_PPENK, and stirring velocity on properties of dispersions. Among the formulations, PD-4 played perfect combination properties. The optimal technology of PD-4 was as follows: 2.5 % PVA/water, 3.0 % PPENK/CHCl₃, W_PVA/W_PPENK=1.25, and 3000 r/min rotational speed of dispersion machine. PD-4 emulsion played good stability. The particle size of PPENK micro-particle was 365～666 nm. The micro-particles of PPENK could be observed fractal characteristic by TEM and stably dispersed in water phase. PD-4 aqueous dispersion played perfect film-forming property and its cured film baked in 180℃showed adhesion(1 grade), pencil hardness(6H) and impact resistance (50cm). The cured film of PD-4 could be used at 250℃for a long time.Supercritical anti-solvent (SAS) process was applied to preparation of PPENK micro-particle of mean particle size 170～400 nm when chloroform and supercritical CO₂ were used as organic solvent and anti-solvent, respectively. This confirmed that it was possible to prepare aqueous dispersion based on the micro-particle. Upper limit value of PPENK/CHCl₃ concentration was 10.0%. When PPENK/CHCl₃ concentration was 3.0 %, particle size of PPENK micro-particle was less than that of 1.0 % PPENK/CHCl₃ at the follow operation conditions: 10 MPa-12 Mpa, 35℃, CO₂ velocity of flow 1.0 m³/h, and PPENK/CHCl₃ solution velocity of flow 4 mL/min. The experimental data indicated that it was a complex relationship between pressure variation and particle size. Keeping the other operation conditions, particle size of PPENK micro-particle was shown the variation trend of from increase to decrease when operation pressure was 9 MPa-17 MPa. When PPENK/CHCl₃ concentration was 3.0% and operation pressure was 9 MPa-12 MPa, particle size of PPENK micro-particle was similar as that of 1.0 % PPENK/CHCl₃. The polymer micro-particles possessed fractal characteristic.