Ralph D. Priester

Effects of liquid crystalline structure formation on the curing kinetics of an epoxy resin

The curing kinetics of a system containing 4,4′-diglycidyloxy-α-methylstilbene (DOMS) and different functionality amines, N-ethylaniline (NEA), aniline, benzenesulfonamide (BSA), and sulfanilamide (SAA), have been studied by differential scanning calorimetry (DSC) under isothermal conditions. The phase transformations during curing of the systems have been monitored by a crosspolarized optical microscope equipped with a hot-stage and photo detector. It has been found that the growth of a nematic liquid crystal structure does not cause a discrepancy from the autocatalytic model for the reactions between aniline and epoxy. There is no liquid crystalline structure formed for the systems containing NEA or BSA, which follow the autocatalytic kinetic models within the temperature range of 120–150°C. For the curing reactions between DOMS and SAA, there is a big deviation from the autocatalytic model when the liquid crystals transfer from a nematic structure to a smectic structure. Unlike the usual decrease of reaction rate resulting from diffusion in a heterogeneous reaction, the reaction rate is enhanced. A modified kinetic model has been constructed for this reaction system by introducing a pseudoconcentration term caused from the liquid crystalline structure formation.

Characterization of Polyurethane Foams by Mid-Infrared Fiber/FT-IR Spectrometry

The use of a mid-infrared transmitting fiber to monitor the cure process of a polyurethane foam is described. A chalcogenide fiber was situated between the spectrometer and a remote detector, but passed through the center of a polyurethane foam. This fiber was used as an internal reflectance element to observe changes in the polyurethane curing process. The appearance and disappearance of absorbances in the NH-stretching, carbonyl stretching (free urea and urethane, hydrogen-bonded urethane, and monodentate and bidentate hydrogen-bonded urea), isocyanate, and isocyanurate regions were monitored. These changes provided information about the reaction kinetics and morphological development of the foam.

The Use of Ft-Ir and Dynamic Saxs To Provide An Improved Understanding of the Matrix Formation and Viscosity Build of Flexible Polyurethane Foams

Many aspects concerning the formation of the polyurethane flexible foam matrix are still poorly understood. While the chemistry of the isocyanate-water and the isocyanate-polyol reactions is well known, the dynamic evolution of the resultant products to form the final polymer matrix is not. The inherent reagent differences between traditional slabstock and HR foams and their contribution to the viscosity profile are not well documented. Also needed for completeness is an understanding of raw material and product interactions: factors such as reagent incompatibility, hydrogen bonding, phase separation and chemical crosslinking all play a major role in the formation of the flexible foam matrix

Vibrating Rod Viscometer: A Valuable Probe into Polyurethane Chemistry

The vibrating rod viscometer has been used to examine the rheology of reacting slabstock foam formulations. The reaction can be divided into three rheological domains. The first begins during mixing of the reagents and continues up to the point immediately prior to cell opening. This region is completely dominated by a reduction in the systems viscosity caused by the increase in the temperature of the reaction. During this stage of the reaction, the molecular weight increase from the urethane reaction has not proceeded sufficiently to cause a measurable increase in viscosity. Interestingly, the systems viscosity is also unaffected by the weakly interacting monodentate urea segments observed by infrared spectroscopy during the latter part of this stage of the reaction

The use of tetraalkylphosphonium-tetrafluoroborate-tetrafluoroboric acid in the curing of a liquid crystalline epoxy resin

Tetraalkylphosphonium–Tetrafluoroborate–Tetrafluoroboric Acid was used as a catalyst in the curing of a liquid crystalline epoxy. Under some conditions the Tetraalkylphosphonium–Tetrafluoroborate–Tetrafluoroboric Acid actually retarded the reaction. An extensive experimental and kinetic analysis is presented anda mechanism for the reaction retardation is proposed.

High Resiliency Polyurea Foam-An Improved Flexible Foam Matrix

It is likely that the development of urea technology for flexible seating foam applications has been hindered simply by a perceived incompatibility between the rapid isocyanate-amine reaction and the traditional «long» gel time foam processing requirements. We will show that the amine-isocyanate reaction rate can be controlled, thereby making urea technology useful in flexible foam applications. Conventional processability evaluations on low pressure metering equipment have shown that amine terminated polyether resins can be used in place of conventional polyols in typical foam formulations without jeopardizing desirable processing parameters. In addition, nonpolymer polyol reinforced polyurea foam matrixes have hardness properties comparable to or better than conventional polymer polyol filled urethane foams

Volume Rise During Flexible Urethane Foaming

ABSTRACTDuring the formation of flexible urethane foams, isocyanate and water react to form urea dimers and oligomers which eventually segregate as a microdisperse solid phase. The role that urea segregation plays on reaction kinetics and flowability during foaming is examined in this work. Temperature and volume rise profiles were measured for a series of foam systems by varying the amount of water (2-6 pphp), isocyanate (90, 110, 130 index), and amine or tin catalysts in the formulation. Model calculations showed that 84-90% conversion of isocyanate (NCO) groups was required to form a crosslinked polymer network. However, volume rise was found to stop at 40-70% NCO conversion. This indicates that gelation is due to phase separation rather than covalent crosslinking. A sudden acceleration in the rate of reaction (dT/dt), detected before foams reached a constant volume, is thought to indicate microphase separation during foaming. A mechanism is presented to correlate temperature and volume rise profiles w...

An Integrated View of Reactive Urethane Foaming

R. D. Priester, Jr., and R. B. Turner Dow Chemical Co., Texas Division Reactive urethane foaming results from a complex combination of kinetic events: bubble growth, microphase separation and urethane polymerization. An understanding of the fundamental role that urea segregation plays on reaction kinetics and cell opening is the purpose of this work. Temperature and height rise profiles were measured for a series of slabstock formulations differing in the amount of water isocyanate and/or catalyst package. Our crosslinking model predicted that 83-90% of the available isocyanate groups must react before a network of covalent bonds could develop. Instead, as the amount of water and/or TDI was increased in the formulation, the experimental gel points decreased from 75-45% conversion of isocyanate. Still, foams were strong enough to withstand cell opening. The source of that strength appeared to be provided by urea phase separation. Independent infrared and rheological measurements confirmed that urea segregation takes place at about 60% conversion of isocyanate, significantly raising the modulus of the reacting mixture and triggering cell opening during foaming. Furthermore, a consistent trend was observed in our experiments: at 40-60% conversion of isocyanate, a sudden acceleration in the rate of reaction appeared just prior to foams reaching is proposed : as a phase separated morphology develops, isocyanate groups become trapped within rich urea-water domains. This results in a more efficient use of the available water and a sharp rise in the rate of reaction and cell opening during foaming. Addition of triethanolamine to 3.5 and 5.5 pphp water foams disrupted the cell opening mechanism by changing the balance between microphase separation and network polymerization during foaming.