Rigid polyurethane foams with improved reaction to fire and low emission properties

Abstract

In this work, different combustion modifiers were evaluated in rigid polyurethane/polyisocyanurate foam with regard to their reaction to fire and emission performance. Terminology of combustion modifiers cover both flame retardants and smoke suppressants. The difference between flame retardants and smoke suppressants derive from the action they exhibit during the combustion. Flame retardants delay the combustion action whereas smoke suppressants aid lowering the smoke and harmful compounds generated during the burning of the substance. Having said that it in the research or in the application area, it is possible to see that combustion modifiers and flame retardants definitions can be used interchangeably. Initially, a literature screening was completed to choose the right flame retardants that are commercially available for rigid polyurethane foams. While most of the found candidates are phosphorous based, few examples such as Hexion TL 91-805D polyol that is nitrogen based, was also in scope. In the flow of the study, flame retardants were then classified according to being reactive or not towards isocyanates. This classification is particularly important when evaluating emission performance of the said substances. On the smoke suppressants side, Zinc borate and ferrocene represent non-phosphorous substances that are suitable to incorporate in this particular application. Performance examination of each combustion modifier was completed using 2 methodology. First methodology was determined as the incorporation of one combustion modifier each time at the same weight in the selected formulation. It was then followed by the incorporation of a combination of one flame retardant and one smoke suppressant in rigid Polyurethane foams. DIN 4102 small scale flame device and NBS smoke chamber instruments were chosen to perform the analysis. This method was useful to reveal synergy between different candidates in the foams. Results showed that among the candidates, interaction of Triethyl phosphate and Zinc Borate as well as Triethyl phosphate and Ferrocene created a synergic impact and greatly improved combustion properties in Polyisocyanurate foams. For the investigation of the found results, thermogravimetric and scanning electron microscope characterizations were carried out. It was revealed that Zinc borate creates a thermal barrier and prevents the cell from a complete destruction once the foam is exposed to ignition. While reaction to fire performance was improved, it was detected that addition of Zinc borate has an impact on the reactivity and free rise density of the foams. Gel time occurred to be longer and density of the foams were measured to be higher with respect to reference. This might be explained by Zinc borate acting as an inert filler. Triethyl phosphate-Ferrocene study also put forward interesting results. While addition of a little amount of Ferrocene provided the best fire performance in rigid Polyisocyanurate foams, more than certain amount of Ferrocene incorporation has led the complete burning of the foam. Therefore, for an enhanced smoke and fire performance, Ferrocene amount should be optimized in the formulations. In the latter step, three compound combustion modifier combinations were examined. Loading of oligomeric Triethyl phosphate into the Triethyl phosphate and Zinc borate combination aided to provide superior performance in fire properties with respect to Triethyl phosphate and Zinc borate containing foam. To confirm the results with the same amount of combustion modifiers loading, the second methodology was used: Analyses were successively repeated by adjusting the foams to the same molded density and P% content in the final material. A cone calorimeter was selected to perform the ultimate combustion test and displayed additional parameters such as Total Heat Release, Peak Heat Release Rate, and Total Smoke Production in 11 formulations. The outcome of the cone calorimeter study was evaluated using Triethyl phosphate (mod 1) containing formulation as the reference. In this way, it was possible to confirm the synergism in other formulas. Formulations that surpassed the performance of reference foam were found to be the same as those completed in the laboratory: Triethyl phosphate-zinc borate (mod 3), Triethyl phosphate-ferrocene (mod 12), and Triethyl phosphate-oligomeric Triethyl phosphate-zinc borate (mod 9) combinations. These 3 combinations displayed either a lowered Total Heat Release or Total Smoke Production Rate or both than the reference. In the final stage, further analysis was completed to check the emission properties of these 3 foams and reference using the Headspace gas chromatography-mass spectrometry characterization method. While Triethyl phosphate showed an elevated pique especially in the reference foam due to the high addition amount, Ferrocene also confirmed to migrate because of the sublimation at high temperatures. Cone calorimeter and headspace analysis confirmed that the Triethyl phosphate-oligomeric Triethyl phosphate-zinc borate combination proved to be the most efficient combination with regards to both reactions to fire and emission properties in rigid Polyisocyanurate foams. This result is also proof of how oligomeric substances can enhance the emission properties of end material. As a final word, this study showed that the fire and emission properties of rigid Polyurethane Polyisocyanurate foams can be enhanced through the addition of the right combustion modifiers at the right amount. Said properties are not only governed by the P% content but also synergism and molecular structure might play an important role in improving the properties in polyurethane formulations.