Abstract
Melt flow and dripping of the pyrolysing polymer melt can be both a benefit and a detriment during a fire. In several small-scale fire tests addressing the ignition of a defined specimen with a small ignition source, well-adjusted melt flow and dripping are usually beneficial to pass the test. The presence of flame retardants often changes the melt viscosity crucially. The influence of certain flame retardants on the dripping behaviour of four commercial polymers, poly(butylene terephthalate) (PBT), polypropylene (PP), polypropylene modified with ethylene-propylene rubber (PP-EP) and polyamide 6 (PA 6), is analysed based on an experimental monitoring of the mass loss due to melt dripping, drop size and drop temperature as a function of the furnace temperature applied to a rod-shaped specimen. Investigating the thermal transition (DSC), thermal and thermo-oxidative decomposition, as well as the viscosity of the polymer and collected drops completes the investigation. Different mechanisms of the flame retardants are associated with their influence on the dripping behaviour in the UL 94 test. Reduction in decomposition temperature and changed viscosity play a major role. A flow limit in flame-retarded PBT, enhanced decomposition of flame-retarded PP and PP-EP and the promotion of dripping in PA 6 are the salient features discussed.
Highlights
Fire behaviour is reflected by the response of defined specimens or components in distinct fire scenarios
The pyrolysis includes the processes of heating the polymer, melting, decomposition, charring, and so on
poly(butylene terephthalate) (PBT) HB decomposed in one decomposition step, with a starting decomposition temperature of T 5 wt % = 369 ̋ C and a maximum mass loss rate (taken from the derivative thermogravimetric analysis (DTG)) at T max = 398 ̋ C
Summary
Fire behaviour is reflected by the response of defined specimens or components in distinct fire scenarios. The pyrolysis of the polymer in the condensed phase, the oxidation of volatile fuel in the gas phase (flame), the mass transport to the flame and the heat transport to the pyrolysis front are the main phenomena controlling burning. Each of these main actors is merely a category comprised of a dozen sub-phenomena. Apart from thermal insulation through intumescence, the role of flame retardants in changing the physical phenomena is often underestimated or neglected. An effective heat reflection (=infrared mirror) increases the time to ignition by an order of magnitude and, reduces the maximum average rate of heat emission (MARHE) below
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