Bum-through time

Figure 26 shows a comparison of bum-through time between the two foams based on polymeric isocyanate (polymeric MDI) and a TDI-prepolymer (65). In the case of polymeric isocyanate-based foams, a remarkable increase in bum-through time resulted after increasing the NCO/OH ratio, which reached a maximum value at 3.0. 

In contrast, TDl-based modified isocyanurate foams did not show any increase in bum-through time, even if the NCO/OH equivalent ratio was increased. This significant difference could be attributed to the difference in flash point of the two isocyanates. The flash points of polymeric isocyanate and liquid-modified MDI oligomers are >200°C, and that of TDl is 135 C. 

The effects of the NCO/OH equivalent ratio and aliphaticity index on bum-through time are shown in Figures 28 and 29. 

In this process the most important factor necessary to provide foams having high flame endurance in terms of bum-through time is the ep-oxy/NCO equivalent ratio. It has been found that the best ratio is in the range of about 0.05 to 0.10. Figure 39 shows this relationship. 

Figure 39. Effect of equivalent ratio on bum-through time (63).

Delay fuzes which detonate shortly after impact giving the projectile time to penetrate the target. These are often black powder charges that may take a few tenths of a second to bum through. 

Calvert et. al. describes a theoretical model to calculate the bum out time as a function of the extrudate thickness without bubble formation [Cal 90]. By increasing early development of porosity within the sample, higher debinding temperatures can be used and consequently faster burnout can be achieved. Furthermore the products of the binder decomposition should have a high vapor pressure and should diffuse rapidly through the extruded body. 

Pipeline wall bum-through events took place on the early stage of the BR-5 reactor operation, since experience in design and operation of sodium systems at that time was poor causing wrong decisions. These events were caused by the short circuit between electric heater and pipeline resulting in the electric arc. In this view, power supply system of electric heaters was modified, namely transformer with insulated neutral conductor. Such approach is used in all domestic fast reactors. No more leaks have occurred for this reason. 

In general, all of the chlorobenzenes are less toxic than benzene. Liquid chlorobenzenes produce mild to moderate irritation upon skin contact. Continued contact may cause roughness or a mild bum. SoHds cause only mild irritation. Absorption through the skin is slow. Consequently, with short-time exposure over a limited area, no significant quantities enter the body. 

Boronic acids (69 and 70) (Fig. 45) with more than one boronic acid functionality are known to form a polymer system on thermolysis through the elimination of water.93 Specifically, they form a boroxine (a boron ring system) glass that could lead to high char formation on burning. Tour and co-workers have reported the synthesis of several aromatic boronic acids and the preparation of their blends with acrylonitrile-butadiene-styrene (ABS) and polycarbonate (PC) resins. When the materials were tested for bum resistance using the UL-94 flame test, the bum times for the ABS samples were found to exceed 5 minutes, thereby showing unusual resistance to consumption by fire.94... 

As the blackpowder core of a safety fuse bums, it produces gases which must escape. At the same time the heat of the combustion melts the bitumen and plastic and thus produces side venting through the textile layers. This results in the production of an increased but constant gas pressure, determined by the equilibrium between gas generation and gas lost sideways. As the rate of burning of blackpowder depends markedly on the pressure, it is this process of equilibration which determines the speed of burning of the fuse. 

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