Study On Relationships Between Molecular Structure And Properties Of Dynamic Rheology And Membranes For Enviromental Friendly Waterborne Polyurethane

Driven by environmental regulation, waterborne polyurethane (WPU) has attracted more and more attention. However, in the preparation process of WPU, certain amount of organic solvent was added to adjust the viscosity of PU prepolymer. Although drying rate of the emulsion can be enhanced by adding low boiling point solvent, and film-forming properties can be improved by adding high boiling point solvent, organic solvent addtion not only brings pollution to the environment but also leads to high product cost. Therefore, completely replacement of organic solvent by water is of growing importance, which can make WPU a real sense of environmental friendly material. In addition, much emphasis has been placed on the application of this system. There are relatively little systematic reports on fundamental insights into the relationships among structure, rheological properties and application properties, which would restrict the deep research and exploitation of WPU to a certain extent. In this research, different crosslinkers were introduced to change the size and distribution of particles, thereby to study how controlled changes in particle interaction alter the phase behavior, micromorphology, rheology behavior, thermal properties and mechanical properties, which could provide fundamental information for film-forming performance and performance that the systems are likely to encounter during use. Simultaneously, the study can guide the synthesis, processing and modeling of these materials. Furthermore, in-situ polymerization was used to synthesize polyurethane micro-emulsion without any organic solvent. In this research, two series of polyurethane/polyacrylate composite emulsions were succesfuly prepared by seed emulsion polymerization and in-situ emulsion polymerization, named PUA₁ and PUA₂, respectively. At the same time, self-crosslinking monomer (trimethylolpropane, TMP), functional monomer (2-hydroxyethyl acrylate, HEA), (n-methylolacrylamide, HAM) and epoxy monomer (E-44) was introduced into the system.With the action of polyurethane stabilizer, effect of polymerization temperature, monomer concentration and initiator concentration on copolymerization kinetics were investigated. Model function of cumulative conversion versus reaction time--Boltzmann Fnction was built up, and was employed to fit the samples of cumulative conversion versus reaction time. The maximum polymerization rate(MV), average polymerization rate(AV) in the steady stage, average nucleation rate(NV) and the corresponding cumulative conversion at moment of nucleation stage ceasing and at the steady stage completing were estimated by calculating extremums and inflexions of its fitted Boltzmann Function. Whereafter, the polymerization rate versus each single polymerization parameter was analyzed with non-linear fitting and the related equation was obtained, that is, Rp∝[M]^2.38[I]^0.883. Finally, least squares linear regression was performed on the model, the result show that this model own excellent forecasting ability to this copolymerization.The factors influencing the stability of PUA₂ emulsion were studied. It was found that aggregate took place among particles with the addition of electrolyte , while the stability of PUA₂ emulsion can be improved by the addition of HEA, HAM, acrylamide (AM) and waterborne initiator (ZAM). The system prepared with the mixture of MMA and BA as the continuous phase of prepolymer has comparative stability with the system using organic solvent (such as N-methyl pyrrolidone, NMP), and superior to the system using methyl methacrylate (MMA) or butyl acrylate (BA), providing accurate basis for the preparation of polyurethane/polyacrylate composite emulsion without any organic solvent. The stability of PUA₂ system reach optimum under the following conditions: the dosage of MMA and BA ranges from 10% to 30%, R value ranges from 1.2 to 1.4, copolymerization temperature ranges from 55℃to 70℃, HAM dosage ranges from 10% to 20%, and epoxy monomer dosage is lower than 20%.Effects of crosslinker on the micromorphology, particle size and distribution of emulsion were systematically investigated. It was found that particles of pure polyurethane adhere to each other, similar to wormlike micelle. The particle size of pure polyurethane was very small, showing unimodal distribution. It was also found that core-shell structure didn't form in PUA₁ system prepared by seed emulsion polymerization, and an embedded structure was detected instead. Moreover, individual PU and PA particle was detected in this system, and part PA was grafted into PU, which can also be demonstrated by nuclear magnetic resonance (NMR). As the increase of TMP dosage, the particle size increases from 90.9nm to 103.9nm, with bimodal distribution. And the particle size decreases from 90.9nm to 72.3nm with the increase of HAM dosage. However, the PUA₂ emulsion displays sandwich morphology with relative dispersed particles, and hydrophobic polyacrylate/polyurethane interpenetrating network was formed. Aggregation tends to take place among particles as the increase of HAM dosage, resulting in the decrease of emulsion stability. But particles in the emulsion modified with epoxy monomer are highly dispersed with core/shell spherical morphology, and it is difficult to discern the boundary between core and shell. The particle size of PUA₂ emulsion was 48.0nm, smaller than that of PUA₁ emulsion, exhibiting bimodal distribution with distribution coefficient of 0.111. The particle size of EPUA₂ emulsion modified with epoxy and acrylate is about 53.4nm with bimodal distribution which is higher than EPU and similar to PUA₂. But the PUA₂ system shows broader distribution. In addition, the particle size increases from 44.1nm to 48.0nm, and distribution coefficient increased greatly from 0.035 to 0.111 through post polymerization of vinyl monomer. The particle size increases from 48.0nm to 67.9nm as the dosage of MMA and BA increases. The particle size decreases from 70.2nm to 48.0nm when the HAM dosage increases from 10% to 15%, with distribution coefficient decreasing from 0.127 to 0.111.Steady rheology study shows that the viscosity of polyurethane is about 10²Pa·s and kept invariable with shear rate, exhibiting Newtonian liquid behavior before the phase inversion. While reaching the phase inversion point, the viscosity increased to 10⁵Pa·s, accompanied by the decrease of viscosity after phase inversion. Moreover, an oil-in-water system was built instead of water-in-oil system; and the viscosity at lower shear rate increases owing to the interaction between waterborne particles, exhibiting shear thinning behavior.A Newtonian response is obtained for PU emulsion, as well as PUA₁ emulsion. And no thixotropy behavior was detected in this system. The zero shear viscosity of PUA₁ emulsion was 0.016Pa.s, decreasing with the TMP dosage. While an increase in HEA dosage caused the increase of PUA₁ viscosity. Furthermore, shear thinning behavior and thixotropy behavior can be observed in this sytem with TMP addition, which can't be obtained by HEA addition. If TMP and HEA were simultaneously added into the system, its viscosity increased with HEA dosage, and complex thixotropy behavior was observed due to the crosslinking structure between core and shell.For the PUA₂ system, the viscosity of the PUA₂B emulsion prepared with BA as the continuous phase of prepolymer reaches 10²Pa.s, presenting strong pseudoplastic behavior. The viscosity of PUA₂M emulsion prepared with MMA as the continuous phase was reduced by two orders of magnitude, showing weaker pseudoplastic behavior. While steady rheologival cure of PUA₂MB emulsions prepared with mixture of MMA and BA as continuous phase is similar to that of PUA₂N emulsion prepared with NMP as the continuous phase, their viscosity were about 0.1 Pa.s, presenting Newtonain flow behavior. In a word, the viscosity of PUA₂ was higher than that of PUA₁, and PUA₂ system was endowed with certain thixotropy. The zero shear viscosity increased with the increase of the CAPA molecular weight and HAM dosage, and decreased with the increase of TMP dosage. Furthermore, yielding stressσ_y and plasticity existed in the PUA₂MB emulsion modified by epoxy resin, presenting non-Newtonian flow behavior. Moreover, the viscosity at low shear rate was relative higher, beneficial to the improvement of leveling. And pseudoplastic flow was observed with the increase of shear rate, which was good for brushing.Dynamic rheological investigation was also performed on the system. As to pure PUA₁ system, it was found that storage modulus(G') kept invariable with time, while the storage modulus increased with TMP and HEA addition, but internal friction tanδwas opposite, indicating a transformation from pure viscous behavior to viscoelastic behavior, and enhanced interaction between particles. It was also found that the rising amplitude increased as TMP and HEA increased, which can be ascribed to the occurrence of crosslinking reaction in the emulsion.For the PUA₂ system, an obvious relaxation behavior was detected. tanδof PUA₂M was about 0.65, and a platform area was observed in the low frequency region for G'. Moreover, the intersection of G' and G" was found at 0.35Hz, implying higher emulsive elastic. Under such circumstances, it is easy for particles to aggregate or accumulate, resulting in the decrease of emulsion stability. At the same time, strong frequency dependence was found in both PUA₂MB and PUA₂B system, and PUA₂B system own higher randomness. Furthermore, compared with PUA₂M, PUA₂MB and PUA₂B were endowed with better emulsion stability, tanδof PUA₂MB was about 1.1, tanδof PUA₂B was about 1.3.With respect to PUA₂MB system, the viscosity decreases with the increase of TMP dosage, and the thixotropy was weakened. The fluidity of this system was improved with relatively disordered structure. Furthermore, tanδof the emulsion increases from 1.1 to 1.5, while the storage modulus showed contrary tendency, and a transition from viscoelastic behavior to pure viscous behavior took place in this system, leading to the decrease of emulsion stability. When the epoxy resin(E-44) dosage was 8%, a strong frequency dependence and good fluidity were observed in the emulsion; but when E-44 dosage increase to 20%, a turning point was detected in both G'～ωand tanδ～ωcurves, which indicated that phase separation and relaxation behavior occur in the emulsion.Simultaneously, thermal stability, mechanical properties and solvent resistance of the system were studied. The results showed that thermal stability, mechanical properties and solvent resistance of the system can be enhanced to a extent with proper amount of TMP,HEA,HAM and epoxy resin, as well as CAPA with higher molecular weight. At the same time, thermal degradation kinetics was investigated. It was found that the correlation coefficients fitted by Kissinger and Flynn-Wall-Ozawa method all exceed 0.97, indicating that these two methods were suitable for PUA₁ system. It was also found that the correlation coefficients fitted by Flynn-Wall-Ozawa and Friedman method for PUA₂ system were also over 0.97. The increase of the degradation activation energy in some stage manifests the enhancement of thermal stability.In addition, the initial decomposition temperature increased from 236.31℃to 271.36℃by the introduction of E-44, the tensile strength increased from 12.7MPa to 20.8MPa, T-peel strength increased from 2.1N.mm^-1 to 8.1 N.mm^-1, while elongation at break decreased from 456% to 191%; water absorption decreased from 12.7% to 2.14%, toluene absorption reduced from 112% to 70%, the contact angle increased from 90.00°to 106.49°. However, the comprehensive properties of the system will be weakened with excess crosslinking agent.Finally, compatibility and phase behavior of the system were investigated by Dynamic Mechanical Analysis. Two peaks were observed in the the tanδ～T curve of film prepared from blend of polyurethane and epoxy resin (PU/ER) .They were -56.2℃and 130℃, respectively, indicating higher degree of phase separation. Two internal friction peaks were also found in the PUA₂ system, -29.6℃and 50.5℃, respectively. But the compatibility was improved to an extent. However the compatibility can be improved greatly with E-44 addition, and only one internal friction peak was observed at 26.6℃.