Reaction Injection Molding Systems Research Articles

LABORATORY-SCALE REACTION INJECTION MOLDING OF POLY(CAPROLACTONE) ELASTOMERS FOR RAPID PROTOTYPING OF STIMULI-RESPONSIVE THERMOSETS

ABSTRACT Semicrystalline polymer networks offer valuable shape-memory and shape-shifting properties; however, material processing and scale-up remain major challenges. A laboratory-scale reaction injection molding (RIM) apparatus and methodology are described to mold and cure urethane linked poly(caprolactone) networks from commercially available polyols and polyisocyanate. The highly customized RIM system is capable of making small shot sizes (∼39 g) at precise stoichiometric ratios and forming elastomers with low run-to-run variation of mechanical properties. Experiments conducted with a dye in one feed stream enable evaluation of mixing efficacy, and component mixing is substantially improved by recirculating reagents at higher pressures. Networks prepared with excess isocyanate exhibited higher modulus, which is most likely due to the formation of allophanates.

Microphase separation in RIM polyureas as studied by solid-state NMR

The microphase separation (MPS) in polyureas based on methylene diphenyl diisocyanate (MDI) hard segment, diethyltoluenediamine chain extender, and amino-terminated polypropylene glycol soft segment prepared by reaction injection molding (RIM) was studied by advanced solid-state NMR spectroscopy. Incomplete microphase separation leads to the presence of mobilized hard segments dispersed in the soft segment domains as well as immobilized soft segments residing in the hard domains. This is detected by 1H-NMR spectra recorded under spinning at the magic angle (MAS) as well as two-dimensional wide-line separation (WISE) NMR spectra. The sizes of the various domains as well as the interfaces between them are quantified by spin diffusion measurements. In this way the impact of annealing, method of polymerization, and hard segment content on MPS is studied. Whereas annealing at temperatures up to 170°C results in improving the MPS, major changes are observed after annealing at higher temperatures (190°C), where the system changes from “soft-in-hard” to “hard-in-soft” behavior. The MPS decreases with increasing hard segment content. The highest MPS is observed for solution polymerized samples. The various NMR experiments clearly reveal the nonequilibrium nature of RIM systems. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 693–703, 1998

Three‐dimensional modeling of reaction injection molding. I

AbstractIn this research a model to simulate both the filling the curing stages of a reaction injection molding (RIM) process in complex three‐dimensional molds is developed. This model can be used to predict not only the temperature and conversion changes with time but also the front position during filling. Using given physical and chemical properties of the RIM system, moldability can be determined in advance. The numerical techniques used in this research include adaptation of the SIMPLE algorithm developed by Patankar for a moving‐front, two‐phase system with non‐negligible inertial effects, and exothermic chemical reaction. The model predictions of temperature and conversion compare favorably with available data on simple two‐dimensional molds. The ability of the model to predict the dynamics of filling in more complicated molds was verified by comparison to mold filling experiments with water and a polyurethane foam.

Three‐dimensional modeling of reaction injection molding. II: Application

AbstractThe objective of this research is to demonstrate the application of a general model of reaction injection molding (RIM) to a complex shaped mold part. Using the fundamental physical and chemical properties of the RIM system, prediction of RIM moldability can be made. The numerical techniques used in this research are presented in a previous paper (1). The rigorous modeling of the transport processes and the dynamics of the filling front for a particular application are described in this work.

Rheokinetic studies for the reaction injection molding of polydicyclopentadiene

AbstractDicyclopentadiene (DCPD) based liquid engineered resins are emerging among the newest commercial reaction injection molding (RIM) systems. Since the filling stage is coupled with chemical reaction, an understanding of the chemorheological changes is critical for establishing moldability criteria. An adiabatic reactive viscometry technique which involves the simultaneous measurement of viscosity and temperature changes was used to investigate the chemorheological changes during the adiabatic copolymerization of a DCPD based RIM system. The influence of impingement mixing conditions and initial material temperature on the reaction kinetics, rheology, as well as the resulting final physical properties of the system were investigated. The “apparent gel time” under adiabatic conditions is estimated from the intersection of the tangents to the initial and final portions of the viscosity curve. An autocatalytic kinetic model with an Arrhenius rate constant was used to model the reaction kinetics.

Rotary injection reaction injection molding (RI‐RIM). Part I: Basic design features

AbstractThis paper describes the basic design features of a new reaction injection molding (RIM) processing device, Rotary Injection RIM (RI‐RIM). The new design includes a novel mixing concept which furnishes high intermaterial contact area upon shear imposed rotary injection of the RIM components for effective in situ polymerization. This system operates in low pressure and laminar flow conditions, as opposed to the high pressure and turbulent flow, found in conventional RIM systems. The mixing process is described and quantified in terms of the various forces which govern the injection process. A progressive diminution in the average size of the dispersions generated is found with increasing rate of shear, continuous‐phase viscosity, and injection rate. These results are compared with those expected from traditional shear mixing (bulk convective shearing) under comparable conditions and the current system found to be more efficacious. Reaction molding experiments with RI‐RIM using a model polyurethane system are described and the influence of operating conditions on the mechanical properties of the finished moldings are elucidated. A detectable change in the morphology of the system is observed following increase in the total shear strain imparted to the initial mix of the multiphase reactants. It is suggested that the observed change is affected by a segregation between the components of the segmented polymer.

Microdispersive interfacial mixing in fast polymerizations

AbstractInterfacial activity between liquid reaction injection molding (RIM) reactants was observed using light microscopy. Diisocyanates were brought into contact with various diols and diamines in a thin (250 μm) gap. A dynamic phenomenon, rapidly producing a well‐mixed, intermaterial phase, was discovered. It was revealed that the rate of growth of this newly formed region was dependent on both the rate of reaction and the physical properties of the initially formed product species at the reaction interface. Spontaneous interfacial mixing also occurred without chemical reaction when a reaction product was dissolved in “capped” contacting reactant liquids. It is believed that strong interfacial intermolecular forces, inducing flow through the polymerization product layer formed immediately upon reactant contact, are responsible for the initial explosiveness of this microscopic process. The range of eventual thicknesses of the mixed, interfacial region (100–800 μm) for various typical RIM systems indicates that this microscopic subprocess may be significant to the fate of the overall process.

Reaction kinetics of a polyurea reaction injection molding system

AbstractPolyureas have the potential for Improving reaction Injection molding (RIM) productivity through shorter demo Id times and elimination of external mold release agents and post‐cure. Kinetics of urea formation are necessary for process modeling of polyurea RIM systems. Adiabatic temperature rise measurements were used to monitor the extent of reaction of an aliphatic diamine and an aromatic diamine reacting competitively with a single diisocyanate. The aliphatic diamine reacted much faster; essentially complete polymerization occurred in the RIM mixhead. A similar three‐component system was studied in dimethylacetamide and diethylene glycol dimethyl ether, and the sequential nature of the two competing reactions was confirmed. The RIM polymerization of the aromatic diamine, 3,5 diethyltoluene (2,4 and 2,6) diamine and;4,4′‐diphenylmethane diisocyanate was studied using an unreactive poly ether as the solvent. The exotherms could be fit with a third order model. Adiabatic temperature rises for all RIM experiments were in good agreement with the values predicted by heats of reaction measured in solution.

Adiabatic reactive viscometry for polyurethane reaction injection molding

AbstractAdiabatic reactive rheometry involves the simultaneous measurement of viscosity and temperature changes during adiabatic polymerization. Using the adiabatic reactor method to relate temperature to fractional conversion results in a useful rheokinetic tool ideally suited for fastreacting reaction injection molding (RIM) systems for which the mold‐filling step is nearly adiabatic. In this work, a small laboratory RIM machine is used to mix the reactants and deliver them to a constant stress rheometer retrofitted with a wide‐gap Couette geometry and two thermocouples. Measurements on two polyurethane systems are reported. A simple cross‐linking system is used to verify the adiabatic rheokinetic method through comparison to a known gel conversion. Subsequent measurements on a phase‐separating RIM system show that increases in catalyst level, hard segment content, and initial reactant temperature result in a decreased gel time and an increased gel conversion. The viscosity rise profiles aid our understanding of the onset and development of phase separation. They are also essential for mold‐filling models and establishing moldability criteria for these RIM systems.

Processing of polyurethane/polyester interpenetrating polymer networks by reaction injection molding (RIM). Part I: Design of a high pressure RIM system

AbstractThis is the first of a three‐part paper investigating the feasibility of processing polyurethane/polyester Interpenetrating Polymer Networks (IPN) by Reaction Injection Molding (RIM). The fundamental mixing process is discussed. Various approaches reported to analyze mixing are also critically reviewed. Good part quality in RIM mixing activated systems suggests the need for the creation of a rapid and intense turbulent state within the small impingement chamber. In order to generate the conditions of reactive components intermingling down to submicron dimensions, a high pressure RIM unit was built and tested. The device can inject reactants at high stream Reynolds numbers (around 10,000) and impingement pressures (up to 100,000 psi, 6,895 bar.).

Kinetics and conversion monitoring in a RIM themoplastic polyurethane system

AbstractKinetic data are reported for the fast polymerization of a thermoplastic polyurethane under conditions similar to that of commercial reaction injection molding (RIM). The components were a 2000 molecular weight polyester polyol, butanediol and 4,4′‐diphenylmethane diisocyanate. Three catalysts–dibutyltin dilaurate, phenyl mercuric propionate, and triethylenediamine–as well as uncatalyzed formulations were studied. Kinetic parameters were obtained by numerically fitting adiabatic temperature rise data with both second‐order and hyperbolic models. The hyperbolic model gave consistently better fits and is supported by mechanistic studies in the literature. Activation energies compare well to literature values. The uncatalyzed rate was found to be significant. The kinetic parameters obtained by this method are useful measures of catalyst performance in the RIM systems. Moreover, the models provide a conveient way to predict the extent of reaction during the production of parts by the RIM process. The shape of the reaction pathway (extent of reaction time) may be important in the development of physical properties of polymers produced by the RIM process. Physical properties for these samples compare favorably to those for a conventionally produced (batch) polyurethane of the same formulation.

Viscosity changes during isothermal and adiabatic urethane network polymerization

AbstractViscosity was measured as a function of time for the polymerization of a reaction injection molding (RIM) type urethane system: 4,4′‐diphenyl methane diisocyanate (MDI) with a polyester triol and dibutyl tin dilaurate catalyst. Viscosity was found to be independent of shear rate. Isothermal viscosity rise was related to extent of reaction and to Mw, by branching theory. Isothermal results at several temperatures were able to predict viscosity rise under adiabatic conditions. The gel point test commonly used to evaluate RIM systems is shown to be essentially an adiabatic test.