Preheat Prepolymer

The polyurethane lining systems in use today are normally of the 100% solids type without solvents. The individual components, prepolymer and isocyanate are preheated and pumped to the mix/dispenser for metering and mixing. After mixing, the material is pumped through an orifice where it is... 

Molds must be checked for damage prior to use and preheated to a temperature equal to the maximum that the exotherm will reach when the prepolymer and curative are reacted together. If required, a thin coating of mold release is applied to the mold to assist in the demolding process. It is often found that more that one release can be obtained from the mold that has previously been prepared. 

The compositions of these adhesives are given In tables I and II. Further Information on preparative procedures for the prepolymers, curing agents, and final adhesives are described In depth elsewhere ( ). Prepolymer and curing agents were mixed In the ratios given in the tables and then degassed to remove entrapped air as rapidly as possible (3-8 minutes). In most cases, freshly prepared adhesive was Immediately transferred to preheated fixtures. 

Mixtures of isocyanates are commonly used for convenience in commercial production of the diisocyanate, since the pure toluene 2,4-diisocyanate is more expensive to produce. The resulting prepolymer is then mixed with either a glycol, such as 1,6-hexanediol, or a deactivated (sterically hindered) diamine plus pigment if required, and then promptly poured into a preheated mold of the desired shape. In about half an hour the mixture sets to a pliable shape with stiffness and elasticity controlled by the components and processing details used [29]. Similar procedures produce high-strength polyurethane fiber (e.g., Perlon U) or elastomeric fibers (e.g., Spandex and Lycra). 

When the temperature of the prepolymer has dropped to 80 + 2 C, It Is stirred together with 1R)CA, which Is at 122-124 C. After agitating for about two minutes, the resultant solution at a tenqierature of 87-88 C Is poured Into the mold which was preheated to the same temperature and then cured.at that temperature for 6 hours. 

Styrene, 300 ml. (2.61 mol), 408 ml. of methanol and 3.545 g. (13 mnol) of initiator obtained from diethylene glycol and AIBN were mixed and heated to 64°C. in a preheated bath. After a reaction time of 359 min. (30% decomposition of the initiator) the reaction mixture was quenched and the product was removed by filtration, dissolved in benzene, precipitated with methanol and dried at room temp, in vacuvm. Prepolymer, 52.2 g., was obtained. 

Polymers of this nature can be polymerized either in solution or in bulk in the latter case they are normally reacted at high temperatures, e.g., 100-150 C. Since our goal was a casting resin, the formulations were reacted in bulk and at lower temperatures to protect heat sensitive electronic components furthermore, low reaction temperatures minimize side reactions that can lead to crosslinking and polymer insolubility. In this process the polyols and diisocyanates were mixed and allowed to react for about 25 minutes at 71 C to form the prepolymer formation while longer times resulted in material too viscous to cast or deaerate. After the indicated time, 1,4-butanediol was added followed by deaeration and subsequent encapsulation of a preheated (71 C) electronic device. A second deaeration of the encapsulated part is usually necessary. Pot life for such a system is about 15 minutes. Final reaction or "cure" was 24 hours at 71 C. 

Amine stoichiometry was kept at 95% based on prepolymer content. Except where noted, prepolymers were preheated at 93°C and mixed with curatives at desired temperature. The prepolymer, curative and prepolymer-curative mixture were each vacuum degassed prior to pouring the mixture into a preheated mold. Degassing before and after mixing is especially important to achieve optimum cures. The mold was closed when gelation started. Demolding time was one hour or less. The sample was then post-cured for 16 hours at 120°C. The elastomer was conditioned for one week at 50% relative humidity at room temperature prior to physical properties measurement. 

The polyimlde molding compound prepared by any of the above techniques Is processed In a preheated mold at 550-600 F, followed by post cure to 625 F for 2-8 hours for development of maximum physical properties. Absence of volatile by-products permits facile production of sound moldings having thick sections, while the modest (1300) molecular weight of the prepolymer permits flow at reasonable temperatures. 

Before making the castings, pot life determinations were performed to determine the processing window of the prepolymer and chain extender materials. As a standard practice, both the prepolymer and chain extender were preheated to the desired temperatures, degassed thoroughly, mixed at the desired stoichiometry (90 or 95%) and poured into a thermostatted vial. The viscosity was measnred, nsing a Brookfield viscometer, as a function of time. The measurements were terminated when either the time of measurement reached 20 minutes or the viscosity of the mix reached 0.1 Pa-s. 

Paste adhesives. Paste adhesive formulations are produced by mixing the base resin with low-molecular weight diluents or solvents, if necessary, to reduce viscosity. Other prepolymer resins may be added and agitated until dissolved. Viscosity and temperature are continuously monitored and more diluents or resin added as required. Finally, one or more types of fillers are added and mixed and the viscosity of the mixture is adjusted. In incorporating the filler into the resin, best results are obtained when the filler is preheated... 

Vernonia oil-sebacic acid prepolymer was charged into a three necked flask, followed by the styrene mixture. The temperature was then raised to 80 C to initiate the styrene polymerization. During this stage, the reaction vessel was stirred under a nitrogen atmosphere. Just before the styrene-DVB component gelled, the mixture was poured into preheated molds and the reaction continued at 80 C for 24 hours. After this time, the temperature was raised to 140 C for 30 hrs. to complete the VOSA network formation. 

Recently, an in-line pyrolysis unit for the MS study of the thermal stability of light oils or volatile liquid prepolymers has been presented [a.l3]. A sealed glass ampoule is dropped into the furnace, which is preheated and controlled at the desired temperature. After the chosen pyrolysis time has elapsed, a piston is screwed down to break the ampoule, and the pyrolysis products are thereby released and impelled by helium carrier gas via a heated capillary into the mass spectrometric source - for GC-MS analysis the products pass directly through the GC column before entering the MS source. This new pyrolysis unit has small dead volume, is precisely temperature-controlled and contains no cold regions where products would condense. 

A series of TPU-CNF and TPU-CNFOX composites were synthesized following a two-step polymerization procedure. Stoichiometric amounts of MDI and PCL were reacted at 80°C under nitrogen gas atmosphere for 150 min to produce a prepolymer. In a second step, 80 g of molten prepolymer were hand-mixed with a proper amount of CNF or CNFOX, 6.5 g of BDO, and the catalyst. Mole ratio of 6 1 5 of MDI PCL BDO was used producing 33 wt. % of hard segments. The mixtme was pomed in a chaotic mixer preheated at 110 °C, and mixed for 5 minutes. TPU composite materials were collected, and compression molded at 220 °C. Neat TPU and composite systems were then kept at room temperature for further analysis. 

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