Polymers melts at solid surfaces

In hres, polymers can bnm as a solid with formation and burning of very small bubbles of polymer melt at the surface or with char formation at the... 

Between abont 400 and 900 s in Figure 11.1, PP was burning at the steady state as a solid polymer with formation and burning away of very small polymer melt bubbles at the surface with no accumulation of the polymer melt at the surface. The flames were completely lifted off the surface due to the high release rate of polymer vapors resulting in 0. From Eq. (11.8) and properties of PP, the predicted steady-state 2ch value for the combustion of solid PP is 780 kW/m, which agrees well with the experimental value. 

This closeness of 0 to zero explains the existence of a gas-oversaturated solution area in the polymer melt, when P < Pg, but the entire volume of gas remains in the solution. The degree of oversaturation, particularly upon free foaming (not in flow) can be 2- to 3-fold. In real polymer compositions, there are always solid admixtures, which have poor wetting areas. This reduces the degree of oversaturation at the interface melt-molding tool. Moreover, bubble nuclei can result from fragmentation of gas bubbles in the polymer [16]. Another factor that promotes the formation of bubble nuclei is the presence of localized hot points in the polymer melt they act as nuc-leation centres. Hot points appear either after a chemical reaction in the melt polymer [17], or in overheated areas on the surface of metal equipment [18]. Density of nucleation can be improved via introduction of various agents that reduce tension of the polymer [19]. 

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