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Utility of the Poly(alkylene oxide)s
Published in F. E. Bailey, Joseph V. Koleske, Alkylene Oxides and Their Polymers, 2020
F. E. Bailey, Joseph V. Koleske
In the early 1950s, flexible foams were made by the prepolymer process. In this process, a prepolymer is prepared by reaction of a diisocyanate with a diol, usually a linear polyether diol similar to those described in Section IV.A for elastomers. The prepolymer polyether diisocyanate, usually a poly (propylene oxide) diisocyanate, has a molecular weight in the range of 1000 to 2000. Water is used as a chain extender in making the prepolymer and also to provide branch sites to increase the functionality of the prepolymer through formation of biuret linkages.
Shrinkage in Conventional Monomers During Polymerization
Published in Rajender K. Sadhir, Russell M. Luck, Expanding Monomers, 2020
Russell M. Luck, Rajender K. Sadhir
Prepolymers have also been used to reduce shrinkage. A prepolymer is a viscous, medium molecular weight version of its solid, high molecular weight counterpart. A prepolymer is made by polymerizing the monomers to a viscous, but still liquid, stage prior to gellation and then terminating the polymerization process. This prepolymer is added to the base monomer mix forming a low to moderate viscosity solution which can be poured into a mold, etc. and then polymerized to completion and cured. Since the prepolymer has already been partially polymerized, it will exhibit less shrinkage in the molding resin cure process.33
Materials Engineering
Published in Quamrul H. Mazumder, Introduction to Engineering, 2018
When thermosetting polymers are to be processed into usable form, there are normally two steps, similar to the preparation of a two-part epoxy. In the first step a resin is made. This resin is called the prepolymer, and it normally has a low molecular weight. In the second stage, the prepolymer is formed into the final shape in a mold. Polymerization, or curing, takes place when heat and/or pressure and/or catalysts are added to the prepolymer.
On the possibility of using combined polymerization and drying in synthesis of polyacrylamide
Published in Chemical Engineering Communications, 2019
A. A. Lipin, A. G. Lipin, R. Wójtowicz
However, synthesis of polyacrylamide in concentrated aqueous solutions has some difficulties resulting from the fact that the final stage of polymerization takes place in a highly viscous medium. In this case, the reaction mixture changes its state from liquid to solid and rubber like. Under these conditions, the role of diffusion processes increases substantially. Diffusion limits not only the reaction of chain termination and initiation, but also all elementary reactions. The polymerization rate decreases so much that in some cases it is not possible to obtain the required degrees of monomer conversion. The use of equipment in which transport of highly viscous reaction mixture is constructively solved does not solve these problems (Lipin et al., 2002a, 2002b). This paper’s authors think that one way to overcome these difficulties is to combine polymerization to high conversion degrees with solvent elimination from the prepolymer. Heat treatment of the prepolymer leads to reduction of diffusional limitations on reaction initiation and polymer chain growth, which promotes the reaction of polymerization. This method allows to reduce the technological cycle. In addition, energy costs for drying are reduced, since some of the moisture evaporates due to the heat of the polymerization reaction. Analysis of the literature showed a gap of research in the area of combined process of polymerization and drying of polyacrylamide. Papers often discuss and try to tackle a single aspect of the problem either the synthesis of polymer in concentrated aqueous solutions (Wang et al., 2011; Wu et al., 2010) or drying of the aqueous emulsion (Hemingway et al., 2010). However, in combination these processes have not been studied to date.