An engineering analysis on the reaction injection molding process of polyurethane-unsaturated polyester IPNs was carried out through model simulations and actual experiments.
The polyurethane network was prepared by reacting 4,4'-diphenyl methane diisocyanate(MDI), poly (propylene glycol) (PPG) and trimethylol propane(TMP) using cobalt naphthenate as a catalyst, and the unsaturated polyester resin composed of phthalic anhydride, propylene glycol and fumaric acid was crosslinked with styrene in the presence of methyl ethyl ketone peroxide and cobalt naphthenate.
For the computer simulation model, the reaction kinetics and the viscosity model were established. Results in the reaction kinetics of IPN formation showed that there are complicated interactions between the two reactions in spite of the different reaction mechanisms. It was revealed that MDI had a significant catalytic effect on the unsaturated polyester reaction. A small amount of MDI raised the rate of unsaturated polyester reaction by accelerating the initiator decomposition. Therefore the reaction rate of unsaturated polyester increased in the polyurethane/unsaturated polyester mixed system. However, the final conversion of unsaturated polyester reaction decreased in the mixed system by the cage effect of the polyurethane network which restricted the diffusion of the styrene monomer. On the other hand, the polyurethane reaction rate in the mixed system was not altered significantly.
For the rheological model, the viscosity changes during the reaction course were described as a function of temperature and conversion. The viscous activation energy for the polyurethane reaction mixture showed a constant value, while that for the unsaturated polyester system revealed linear dependence on conversion.
To simulate the RIM process, balance equations calculating the temperature and conversion change in a disc type mold were established. These equations were solved numerically by a explicit finite difference method with the aid of a computer and this simulation model was verified by comparing the simulated temperature change with the measured temperature rise in actual RIM experiment.
Through the computer simulations, it was found that there existed a range in cobalt naphthenate concentration to obtain high conversion of the unsaturated polyester within a reasonable molding time. This range could be expanded by increasing the amount of MEKPO or by lowering the mold wall temperature down to 100℃.
The mechanical properties of the IPNs which were prepared by the reaction injection molding(RIM) process were measured with variations in composition and crosslink density. From dynamic mechanical analysis(DMA), it was found that the two component polymers had a good compatibility over the whole composition range. The tensile strengths of the blends were greater than those of the pure components and had the maximum value at 50/50 composition. The modulus of elasticity and surface hardness decreased and the impact strength increased as the polyurethane content was increased, but the enhancement of impact strength at low polyurethane content was not as high as expected due to the absence of large scale domain formation of polyurethane rubbery phase. For higher crosslink density, the compatibility was enhanced and the mechanical properties were improved considerably.
폴리우레탄-불포화 폴리에스터 IPN의 반응사출성형공정에 대해 콤퓨터 모사 및 성형실험을 통한 공학적 해석이 행해졌다.
폴리우레탄은 4,4'-diphenyl methane diisocyanete(MDI), poly(propylene glycol)(PPG) 및 trimethylol propane(TMP) 으로 합성되었으며, cobalt naphthenate를 촉매로 사용하였다. 불포화 폴리에스터는 phthalic anhydride, propylene glycol 및 fumaric acid의 축합반응에 의해 중합된 것으로서 styrene monomer로 가교시켰다. 이때 개시제로서 methyl ethyl ketone peroxide(MEKPO)가 사용되었으며, 촉진제로서 cobaltnaphthenate가 첨가되었다.
콤퓨터 모사 모델을 세우기 위한 반응속도식을 구하기 위해 DSC를 이용하여 각조성 고분자와 IPN의 반응열을 측정하여 본 결과, IPN 형성시 각 조성 고분자의 반응이 상호간에 깊은 영향을 미친 다는 사실이 밝혀졌다. 폴리우레탄 반응물인 MDI가 불포화 폴리에스터의 반응속도를 크게 증가시키는 작용을 하는 반면, 폴리우레탄 자체는 불포화폴리에스터 반응물의 확산을 어렵게 만들므로서 불포화 폴리에스터 반응의 전환율을 떨어뜨리는 작용을 한다는 사실이 밝혀졌다.
반응이 일어나는 system의 점도변화를 예측하는 모델을 만들기 위해 반응물의 전환율과 온도를 변화시키면서 점도를 측정하였는데 온도의존성을 나타내는 점성활성화 에너지가 폴리우레탄의 경우에는 전환율에 무관한 일정한 값을나타내었으나 불포화 폴리에스터의 경우에는 반응전환율에 따라 그 값이선형적으로 증가하는 것으로 나타났다.
이와같이 구한 반응속도식과 점도변화식을 이용하여 콤퓨터모사를 위한 수학적모델이 만들어졌으며, 유한요소법을 이용하는 수치해석을 통해 금형안에서의 온도와 전환율분포가 계산되었고, 실제 RIM실험에서 측정된 온도 변화와의 비교를통해서 모사모델의 정확성이 입증되었다.
공정조건의 최적화를 위해 촉매농도와 금형온도를 변화시켜 가며 반응시간과 금형내의 최고온도를 계산하여 본 결과, 적당한 시간내에 충분한 반응전환율, 특히 불포화폴리에스터의 높은 전환율을 얻기 위해서는 cobalt naphthenate 양의 적절한 조절이 매우 중요하다는 사실이 밝혀졌다. 한편 금형내의 최고 온도는주로 불포화 폴리에스터 반응의 최종전환율에 의해 결정되었으며, 반응속도가 빠른경우에 오히려 최고 온도가 낮아지는 현상이 나타났다.
실험에서 얻어진 성형품의 물성을 측정해 본 결과, 두 조성고분자 상용성이 매우 좋음을 DMA 분석을 통해 알 수 있었으며, IPN의 인장강도가 각 조성 고분자의 인장강도 보다 높게 나타나는 바람직한 현상을 보인 반면 폴리우레탄이 소량첨가된 경우에 있어서 충격강도의 향상은 별로 크게 나타나지 않았다.
