The role of intumescent flame retardants on the fire behavior of poly(vinyl acetate) and poly(ethylene-co-vinyl acetate): a mechanistic and kinetic study.

The fire behavior of polymer model systems poly(vinyl acetate) and poly(ethylene-co-vinyl acetate) with 73 and 60 weight% vinyl acetate, PVAc and EVA 73 and EVA 60 respectively, are studied in combination with three different solid-state flame retardants: ammonium polyphosphate (APP), melamine isocyanurate (MIC) and silicate nanocomposites. The fire performance of these flame retardants (FRs) is tested with cone calorimetry. The link with lab-scale thermal analysis is made by means of difference thermograms calculated out of the oxidative degradation as measured with thermogravimetric analysis (TGA). Thermogravimetry combined with Differential Thermal Analysis (TGA-DTA) is used to measure the production or absorption of heat during the degradation of the composites. A mechanistic degradation model of both the inert and oxidative degradation of the pure polymer and flame retardant composites is developed using the combination of thermogravimetric analysis coupled with mass spectrometry (TGA-MS) and solid-state NMR measurements performed on partially degraded samples. This degradation mechanism is used for modeled kinetic analysis. Experimental thermogravimetric data originating from isothermal, constant heating rate and Hi-ResTM measurements were used for this purpose. An optimisation routine is used to obtain a best fit between calculated and measured weight profiles by the software program FITME. The resulting findings are then compared with the fire behavior of the composites. In an inert environment PVAc and EVA copolymers first undergo a deacetylation reaction: acetic acid is eliminated from the polymer backbone, leaving a double bond. For PVAc, EVA 73 and EVA 60 this process is autocatalytic. In the mechanistic model, this is described by three different reactions: a non-catalytic deacetylation and a catalytic deacetylation reaction and a deactivation step. By the study of the kinetics of the inert deacetylation, it is observed that the importance of this last process depends on the (co)polymer composition: the more ethylene entities are incorporated into the EVA copolymer, the higher the rate of the deactivation step. After deacetylation, the formed polyene-like structure degrades upon further heating with chain scission reactions producing both aliphatic and aromatic volatiles. Ethylene entities degrade into aliphatic volatiles, unsaturated entities into aromatics. The rate parameters of the chain scission reactions were optimized simultaneously for a wide range of copolymer compositions. The ethylene entities in EVA 60 have a lower thermostability and EVA 60 is therefore not incorporated in the kinetic study. In an oxidative environment, the rate of the catalytic deacetylation increases, influenced by the presence of oxygen. After deacetylation, the polyene is reformed into a fully aromatic structure or char. Ethylene entities are not incorporated into this charred structure and degrade similar to inert conditions. The formed char is degraded into CO2 upon further heating. The rate parameters of the charring step and oxidation into CO2 are optimized simultaneously for EVA 73 and PVAc. Both have identical thermostability. By mixing APP into a PVAc matrix, the plates ignite earlier when measured with cone calorimetry. The burning intensity, indicated by the rate of heat release (RHR), is on the other hand lowered, even if small amount APP are added. The EVA 73 and EVA 60 / APP composites posses excellent fire behavior; with a mixing ratio of 20 parts APP per one hundred matrix (phm) the plates do not ignite. The earlier ignition of the plates is explained by an acceleration of the de