Prepolymer monitoring reaction

Once the prepolymer and catalyst are added together, the chain extension (curing) reactions will commence. The time taken to mix must be carefully monitored. It must be sufficient to allow complete mixing, but there must be enough pot life left to allow pouring into the mold while the material is at the lowest viscosity possible. This is to allow the polyurethane to fill the mold completely and any entrapped bubbles to reach the surface. 

In Chaps. 2 to 6, a case study is developed in order to apply and test the methods developed along the whole book. To this purpose, the reaction between phenol and formaldehyde for the production of a prepolymer of phenolic resins has been chosen for several reasons. In fact, this reactive system is widely used in different forms for the production of different polymers moreover, it is characterized by a noticeable production of heat and by a complex kinetic behavior. Such features represent strong challenges for controlling and monitoring tasks. 

FTIR spectroscopy was used with a common kinetics program to monitor the two separate reactions occurring during thermal curing of acetylene-terminated polyisoimide prepolymer (242). 

The prepared phthalonitrile prepolymers were polymerized under MW irradiation using a 3-step cure cycle. The prepolymer sample (I, II, IIIC, or IVC) (0.5 g) in a small Pyrex reaction vial was placed in MW reactor. The curing was carried out with a preprogrammed three step cycles at 260°C for 30 min, at 280°C for 30 min and at 300°C for 30 min. The temperature of the reaction was monitored using a shielded thermocouple (ATC-300) inserted directly into the corresponding reaction chamber. 

The FT-IR analysis was used to monitor the progress of cure reaction under MW irradiation. Figure 4 shows the FT-IR spectrum of prepolymer (IC) and cured resin. With the first 30 min of polymerization at 260°C under MW irradiation the intensity of... 

A typical procedure followed in the preparation of siloxane-urethane segmented copolymers is as follows Calculated amounts of HMDI and PDMS oligomer are introduced into the reaction flask, stirred and heated up to about 60°C. This mixture is not miscible. However, when 0,5 mL of 1.0% DBTDL solution in toluene is added, the mixture turns clear in about one minute indicating a fairly fast reaction between PDMS and HMDI. There is also a dramatic increase in the reaction temperature from 60 to about 90°C, typical for very exothermic urethane formation reaction. Prepolymer formation is followed by FTIR, monitoring the disappearance of the broad hycfroxyl peak centered around 3300 cm Prepolymer obtained is then diluted with THF to about 50% solids and heated to reflux temperature of 64.5 C. Chain extender, BD, is dissolved in THF and added dropwise into the reaction mixture. As the system became viscous as a result of the chain extension reaction, it is diluted with THF and DMF to prevent the premature precipitation of the copolymer formed, which is indicated by the formation of a cloudy solution. Reaction was continued until the complete disappearance of sharp isocyanate peak around 2250 cm in the FTIR... 

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