Mechanism of enhancement of intumescent fire retardancy by metal acetates in polypropylene

Abstract

The effects of cobalt acetate (CoAc), manganese acetate (MnAc), nickel acetate (NiAc) and zinc acetate (ZnAc) as fire retardant additive in intumescent polypropylene (PP) formulations containing PP/ammonium polyphosphate (APP)/pentaerythritol (PER) are reported. The limiting oxygen index (LOI) and vertical burning (UL94) tests and cone calorimetry were used to quantify the enhancement. Environmental chamber rheometry, thermal gravimetric analysis and the morphology of the residual char were used to investigate the mechanism of enhancement. The incorporation of small quantities of metal acetates had a significant influence on the fire behaviour. As an example, 0.7 wt% MnAc improved the UL 94 rating of PP/APP+PER (mass ratio 100/25, with APP/PER=3/1) sample from V-2 to V-0, while 1 wt% MnAc reduced the peak heat release rate and the total heat release by 18% and 12% in the cone calorimeter. Rheological data, cone calorimetry, and photographs of the residual char showed how the fire retardancy of the systems were affected by the melt viscosity, which depended on the loading of metal acetate. During thermal decomposition, the metal acetates promote the crosslinking of the polymer and the fire retardant, reinforcing the protective intumescent layer. While, the effect is most potent at the optimal metal loadings. At higher MnAc loadings, the benefit of a stronger char is overwhelmed by the adverse effect of crosslinking on the transition char layer. Thus, this paper offers a new insight into the mechanism of the intumescent fire retarded PP system.

Fire toxicity – The elephant in the room?

“…Char formation has been enhanced in polyethylene, using functionalised graphene oxide [87]; in PMMA using graphene oxide and a nickel-aluminium layered double hydroxide [88]; in ABS copolymer, with graphene nanosheets combined with a metal hydroxide [89]; in polyamide 6 using graphene supported cobalt oxide and nickel oxide [90]; and in epoxy resins using silica, attached to cobalt-aluminium layered double hydroxide spheres [91]. Enhancement of the barriers can be achieved in intumescent systems, where gas is released within the molten polymer, causing significant swelling, and so increasing the effectiveness of the thermal barrier [92], for example in polyethylene [93], polypropylene [94] and polystyrene [95]. Carbon nanotubes (CNT) have also been used as potential fire retardant additives, given their similarity to carbonaceous char, with mixed success [96].…”

Studies on intumescent flame retardant polypropylene composites based on biodegradable wheat straw

“…Cone calorimeter test has a good correlation with real fire disaster and provides a wealth of information about the combustion behavior, so it is often used to estimate flame properties of materials. Among indexes provided by CCT, heat release rate (HRR) and the total heat release (THR) are very important to evaluate the developing, spreading, and intensity of fires . Figure shows HRR curves of flame retardant WSP composites.…”

“…Among indexes provided by CCT, heat release rate (HRR) and the total heat release (THR) are very important to evaluate the developing, spreading, and intensity of fires. [23][24][25] Figure 2 shows HRR curves of flame retardant WSP composites. From Figure 2, it can be observed that the HRR curve of pure PP presents a sharp peak during a short time range of 0 to 225 seconds and its peak HRR (pHRR) is as high as 1290.5 kW/m 2 .…”

Phosphorylated sodium alginate/APP/DPER intumescent flame retardant used for polypropylene

“…Intumescent formulations normally consist of three basic ingredients, namely, an acid source as a catalyst, a carbon source, and a gas source or blowing agent . In the case of the acid source, compounds of acids, ammonium salts, and phosphates are often used.…”