Research On SRIM Process And Properties Of The Parts Prepared By SRIM Process

Reaction injection molding (RIM) is a new process for polymer processing which was developed during the 1970s. RIM technology has high production efficiency, low energy consuming and low cost of the equipment. However, RIM parts have large linear shrinkage ratio and high coefficient of linear thermal expansion (CLTE), which limits the applications. Structural reaction injection molding (SRIM) technology retains the advantages of RIM technology, and markedly reduces the shrinkage ratio and CLTE, and thus increases the dimension stability of the parts. Moreover, SRIM technology significantly increases the mechanical properties of the parts, which can be used as structural parts. Research on SRIM technology have been maturely developed, the corresponding applications have achieved industrialization at a certain extent. However, few studies on SRIM process and its effects on the morphology and properties of SRIM parts have been reported. Funded by National 863 plan, the materials system, important process ing parameters and the morphology and properties of the parts have been systemically investigated through DMTA, DSC, TG, SEM, TEM, Confocal-LSM and adiabatic temperature rise(ATR) measurement and other methods in this paper. The results of these studies have therefore established correlations between processing, morphology and properties for a systematic series of PU composites by SRIM technology.(1) Effects of the processing parameters (the weight percent of the fiber and the injection pressure) and the test methods on the permeability of CSM were discussed. The materials system for SRIM technology were determined both from SRIM processing and the properties of the materials, which acts as the foundation for the following studies.(2) The reaction kinetics model and viscosity model during PU polymerization were established through ATR measurement and Brookfield rheometer, in which the numerical solutions of the parameters were obtained by means of non linear optimizing. These models can exactly predict the curves of the temperature of the PU system, the conversion of isocyanate and the viscosity of the PU system, the inlet pressure during filling and the mold cavity temperature during filling and curing. (3) Three processing parameters (the mold temperature, the initial material temperature and the reinforcement) and their effects on the rheology of the system, the morphology and mechanical properties of SRIM parts were thoroughly investigated. The results of these research show good mixing, well microphase-separated morphology and high mechanical properties can be obtained when the mold temperature is 80℃, the initial materials temperature is 50℃. The TG data confirm the presence of water on the surface of the reinforcement. Therefore, the reinforcement should be treated before use, which would reduce the percent of water, and thus reduce the amount of air bubble from the reaction of isocyanate and water, and consequently improve the mechanical properties.(4) The surface morphology of SRIM parts was investigate through Confocal-LSM, and presented advanced technologies for improving the surface performance by using appropriate nonwoven fabrics as additional surface layers or holding appropriate pressure after fiber impregnation. The results show the effects of combination of two different nonwoven fabrics are superior to a single nonwoven fabric, combination of nonwoven fabrics B and C can markedly improve the surface performance. When holding pressure 8.0 MPa, combining with non-woven fabrics B and C as surface layers, good surface performance is obtained according as the results of the surface morphology through Confocal-LSM.(5) Effects of the presence of the reinforcement on the morphology of the matrix in SRIM parts were firstly characterized through DMTA. When the weight percent of CSM is 38.9%, the soft and hard segment glass transition temperature were changed from -27℃to -19℃, from 146℃to 154℃, respectively. The presence of the reinforcement improves the dimension stability of SRIM parts. When the weight percent of CSM is 38.9%, the linear shrinkage ratio of SRIM parts is 8.0×10^-4 cm/cm, the thermal droop is 0.1mm, the CLTE is 13×10^-6/℃(close to that of the steel parts(12×10^-6/℃)) and could be matched with the steel parts.(6) Effects of thermal treatment and the hard segment structure on the morphology of the SRIM parts were thoroughly investigated through DMTA, DSC, SEM, TEM and the mechanical properties test. The results of these studies show, thermal treatment can make the reaction complete and improve the degree of microphase separation, which increases the mechanical properties of the parts. At a given temperature (120℃), the treatment time is increased to 4 h, the degree of the microphase separation is improved and thus increases the mechanical properties of the parts, which would be reduced when thermal treatment continues. At a given treatment time (4 h), the increase of treatment temperature could improve the degree of the microphase separation and thus increase the mechanical properties of the parts. After treatment at 120℃, well microphase-separated morphology is obtained. When treatment temperature is increased to 140℃, the microcrack and a certain microphase mixing occur, which reduces the mechanical properties of the parts. MDIPA reacts with isocyanate at a rate comparable to the trimerization of isocyanate, therefore a random array of black copolymer spherical inclusions occurred on the TEM micrographs; In contrast, DETDA reacts faster than MDIPA, the TEM micrograph shows only the microphase-separated morphology without large inclusions.(7) Mode I and II interlaminar fracuture properties were carried out using double cantilever beam and end notch flexure specimens. The results show the values of G_{II c} are two or three times higher than that of G_{I c}. The values of G_{I c} and G_{II c} of SRIM parts are both an order of magnitude greater than that for unidirectional carbon fiber reinforced epoxy composites. The toughness of the resin matrix in SRIM parts is different from that of the epoxy resin. Values of G_{I c} for SRIM parts are higher than that for the pure resin and show significant variation due to fiber bridging. Effects of initial crack lengths on the the degree of dispersing of G_{I c} were discussed. The fracture areas in different regions were analyzed by SEM to identify mode I fracture characteristics.