Development of Intumescent Coatings for Fire Protection of Structural Steel

Abstract

Intumescent coatings are used for fire protection of structural steel. These coatings typically contain ammonium polyphosphate (APP), pentaerythritol, melamine, titanium dioxide and a polymeric binder. Six different crystalline phases of APP have been reported in literature. APP-II is commonly used in intumescent coatings due to its superior fire performance and commercial availability.

This work identifies different chemical and physical properties of different samples of APP-II using a range of analytical techniques and develops new methods for analysing APP. APP-I and APP-II are synthesised and characterised. This work assesses the fire performance of intumescent coatings containing different samples of APP-II and different TiO2:APP ratios by comparing furnace test data to a cone calorimetry method developed in this study.

A rapid method for determining the solubility of APP was developed from the relationship between the conductivity of aqueous APP and its solubility. Powder XRD was used to identify phases of APP and to determine the purity of different APP samples. Additional phases of APP-I and APP-V are observed in some samples of commercial APP-II. The presence of APP-I and APP-V results in higher water solubilities and increased viscosities, respectively.

Differences in the FTIR of APP-I and APP-II are determined and used as a quick method for phase identification and purity assessment. XRD and FTIR analysis of dissolved and undissolved fractions of APP after solubility tests show that APP-I is more soluble than APP-II, while XRD suggests that APP-V is less soluble than both APP-I and APP-II.

Thermogravimetric analysis is used to study the thermal decomposition of APP and intumescent coatings. A method for determining the degree of polymerisation of APP using solution state 31P NMR is established. Analysis of different APP-II samples shows that as the chain length decreases, the solubility increases.

A method for assessing the thermal protection of intumescent coatings using cone calorimetry was developed and is in good agreement with the furnace test for determining the crucial, early fire performance of intumescent coatings. Cone calorimetry also provides information regarding the expansion profile of the char and mass loss of the intumescent coatings which cannot be obtained from a furnace test.

The thermal protection of intumescent coatings containing different samples of APP-II is assessed and the results show that there are no significant differences in the thermal protection of the coatings. The thermal protection of intumescent coatings containing various TiO2:APP ratios were then investigated. All formulations containing TiO2 performed better than the formulation without TiO2. Intumescent coatings containing TiO2:APP ratios of 0.2 and 0.4 were found to provide the best thermal protection. This is in good agreement with the theoretical ideal ratio needed to form TiP2O7 which was calculated as 0.353.

The cone calorimeter is shown to be a good model for the furnace test for assessing the thermal protection of intumescent coatings, particularly in the early stages of the intumescent process. Thus, cone calorimetry provides a scientific method for investigating the early fire performance of intumescent coating formulations.