Thermal degradation of DNA-treated cotton fabrics under different heating conditions

Highlights

• The thermo-oxidation of cotton and cotton treated with DNA was investigated.

• At low heating rates, DNA favours cotton dehydration instead of depolymerisation.

• At high heating rates, DNA favours cotton dehydration instead of depolymerisation.

Abstract

Our recent work has demonstrated that deoxyribose nucleic acid (DNA) can act as an effective flame retardant when applied to cotton fabrics as a thin coating. DNA acts as a Lewis acid and promotes the dehydration of cotton cellulose to form char, limiting the production of volatile species. Here, the effect of heating rates on thermal degradation behaviour has been studied in order to understand the thermo-oxidation behaviour under slow heating and fast (flash) pyrolysis. The low heating rate effect has been studied using thermogravimetry coupled with infrared spectroscopy and pyrolysis-combustion flow calorimetry, whereas high heating rate effect was obtained by performing thermogravimetry at 200–500 °C/min and flash-pyrolysis tests coupled with infrared spectroscopy. The information obtained by the latter has been successfully employed in order to (i) better explain the results collected from combustion tests and (ii) demonstrate that the type of the gaseous species and their evolution, as well as the char formation, as a consequence of the fabric thermo-oxidation, are independent of the adopted heating rate.

Introduction

Very recently, the efficiency of bio-macromolecules like proteins [1] and nucleic acids [2], [3] as novel green flame retardant systems for cellulosic substrates has been investigated and demonstrated. It has been possible to achieve the self-extinguishment of cotton, when DNA was coupled with chitosan in a Layer-by-Layer (LbL) assembly [4]. This unexpected behaviour has been ascribed to the chemical structure of DNA; its main components (namely, nitrogen containing bases, deoxyribose units, and phosphate groups) act as an intumescent formulation [5], [6], [7], [8]. On the basis of the results collected by thermogravimetry at very low heating rates (i.e. 10 °C/min), it was possible to assert that the flame retardant feature of DNA mainly consists in its char-forming and char-promoting character [2], [3]; this bio-macromolecule is able to favour the dehydration of cellulose within cotton to form char (instead of allowing the production of volatile species). In doing so, the formed char behaves like a carbonised replica of the original fabric, which continues to act as a thermal barrier [9].

The behaviour of DNA is similar to that of the most commonly used phosphorus- and nitrogen-containing flame retardants, which reduce the formation of volatiles and catalyse the char formation [10], [11]. This finding can be ascribed to their Lewis acid properties; upon heating, they release polyphosphoric acid that phosphorylates the C(6) hydroxyl group in the anhydroglucopyranose moiety, and simultaneously act as an acid catalyst for dehydrating these repeating units [11]. In this scenario, we have recently studied the effect of DNA as a char-former for cotton in the condensed phase [2], [3]; however, an in-depth investigation of the gas phase, and thus on the release of volatile species has not been carried out yet; this latter is the focus of this study. Here, pyrolysis-combustion flow calorimetry and thermogravimetry coupled with infrared spectroscopy have been employed, aiming to monitor the evolution of volatile species at very low heating rates (i.e. 10 °C/min). However, since in a real fire scenario the heating rate is much higher (∼500 °C/min), thermogravimetry and flash-pyrolysis coupled with infrared spectroscopy tests, both performed at 200 and 500 °C/min, have been exploited for better explaining the results previously collected from combustion tests (horizontal flame spread, limiting oxygen index and cone calorimetry tests [3]).

More specifically, our attention has been focused on performing tests that are able to give useful information about the thermal-oxidation of the materials under investigation during pre-ignition or at least the ignition steps, during which oxygen plays a key role. For this reason, all tests have been performed in an oxidative atmosphere, and not in pure nitrogen. Indeed, the aim of the present and of the previous studies was to design a novel flame retardant capable of suppressing cotton combustion by modifying the mechanism through which it degrades in air.

The information obtained has been also used to demonstrate that the type of the gaseous species and their evolution, as well as char formation, as a consequence of the fabric thermo-oxidation, is in general independent of the adopted heating rate.