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Rigid polyurethane foams (RPUFs) have been fabricated from crude MDI (CMDI) and polypropylene glycols (PPGs) of various functionalities (f) with HFC 365mfc as a blowing agent. Foam density increased, cell size and density distribution decreased with increasing f while the closed cell content was kept constant over 92%. The gel time, tack‐free time, volume change, and the thermal conductivity of the foam showed a minimum with f = 5, and the existence of minimum has been explained in terms of a large mixture viscosity and cell wall resistance.
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Lim et al. (2008) studied this question.
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