An engineering analysis on the unsaturated polyester (UPE)-polyurethane(PU) interpenetrating polymer networks(IPN's) was carried out for the UPE rich compositions and reaction injection molding(RIM) formulations. Fundamental studies of reaction kinetics, viscosity behavior, phase separation behavior mechanical properties, and computer simulations were conducted.
The unsaturated polyester resin composed of phthalic anhydride, maleic anhydride, and propylene glycol was crosslinked with styrene monomer in the presence of methyl ethyl ketone peroxide(MEKPO) and cobalt naphthenate(Co. Na.). The polyurethane network was prepared by reacting poly(tetramethylene ether) glycol (PTMG), trimethylol propane(TMP), ad 4, 4'-diphenyl methane diisocyanate(MDI) using Co. NA. as a catalyst.
The reaction kinetic model was established reflecting the interaction of constituent components through measuring the reaction exotherms of IPN's by the differential scanning calorimetry(DSC). The results in reaction kinetics of IPN formation showed that there were complicated interactions between the two reactions in spite of different reaction mechanisms. It was revealed that the MDI and Co. Na. complex had a significant catalytic effect on the UPE reaction. However, the final conversion of UPE reaction decreased in the IPN by the cage effect of the PU network which restricted the diffusion of the styrene monomer.
The intermediate reaction products having a certain conversion were prepared in order to obtain conversion dependence of viscosity. The proper rheological model describing the viscosity change during the reaction was established as a function of temperature and conversion. The composition dependence of blend viscosity showed that the Takayanagi's model was adequate in heterogeneous blend region, but the logarithmic mixing rule was preferred in homogeneous region.
The phase separation behavior during IPN formation was observed by light scattering and turbidity experiment. An obvious feature in the light scattering experiment was that the dominant phase separation process was spinodal decomposition due to fast polymerization. The high reaction rate at elevated reaction temperature or high catalyst concentration led to earlier gelation which restricted the phase separation. From the measured $d_m$, the spinodal temperature at the onset of phase separation was calculated, and the proper relationship between $T_s$ and conversion was obtained. The transmission electron micrographs(TEM) for various process conditions showed similar results as the light scattering experiment. IPN's had complex morphology which showed the dispersed sub-domains of polyester phase within the large PU domains. Some co-continuous morphology which indicated the development of spinodal decomposition was also observed.
The mechanical properties of IPN's were measured with various processing conditions. In the case when the crosslink density of PU was high and the isocyanate level was equivalent, the possibility of impact modification of UPE by adding rubbery PU without lowering the modulus and hardness seriously could be obtained. Moreover, the synergistic effect showing enhanced tensile strength than that of pure UPE appeared in some IPN systems due to the higher interpenetration between UPE and PU.
In order to analyze the RIM process the curing step was simulated using proper mathematical models. Form the results of temperature and conversion profiles at the local position in the mold, the domain size distribution in the RIM mold was also estimated using the relationship between spinodal temperature and conversion. The result of computer simulation showed that it was better to maintain the wall temperature as low as possible and adjust the catalyst concentration to higher value in order to obtain the higher reaction rate or short eject time without too much rise of maximum temperature. Results in the domain size distributions in the mold showed that the size was minimum at the center position, while it was maximum at the wall side due to the difference of the reaction rate. The moldability diagram showed that the possible operating region was relatively small in the case of 80/20(UPE/PU) IPN because the exotherm of polyester was high. In the case when the PU content was 30%, the moldability window was enlarged because the total exotherm was smaller and the UPE reaction rate decreased due to the cage effect of polyurethane.
