DNA coatings on cotton fabrics: Effect of molecular size and pH on flame retardancy

Deoxyribonucleic acid (DNA) has been mostly considered as a carrier of genetic information and gene expression. Recently, we have demonstrated that this biomacromolecule is very effective in providing cellulosic fabrics with flame retardant features, thanks to its intrinsic intumescent-like behaviour. Here, we have investigated the role played by different parameters on the overall flame retardant features: DNA molecular size, pH of the DNA aqueous solutions, and number of impregnations. For this purpose, herring DNA, at three different molecular sizes, low (100–200 base pairs, bp), medium (400–800 bp) and high (2000–10,000 bp), has been selected as model biomacromolecule; its effect as fire protective coating on cotton fabrics has been assessed in terms of resistance to a flame application (through horizontal flame spread tests) and to an irradiative heat flux of 35 kW/m2 (by cone calorimetry). Furthermore, thermogravimetric analyses have been exploited for evaluating thermal and thermo-oxidative stability of the treated fabrics. The results clearly indicate that, despite a similar composition, the coatings containing low molecular size DNA exert a superior fire protection on the fabric substrate, as clearly indicated by horizontal flame spread and cone calorimetry tests. Multiple impregnation treatment turned out to be more performing than the single one, being equal the final add-on achieved. More specifically, the use of multiple impregnations at pH = 4 and 8 allowed achieving self-extinction for 86 and 74% of the tested samples; in addition, 45 and 25% reductions of total heat release and heat release rate peak under 35 kW/m2 have been found with respect to untreated cotton. These findings may be attributed to themorphology of the low molecular size DNA coating, which shows a greater penetration into the microfibrillar surfaces of the component fibres within the fabric and to its higher thermal stability in air.