Biomacromolecules as novel green flame retardant systems for textiles: an overview

Abstract

The field of flame retardancy for polymeric materials (i.e. plastics, foams and in particular textiles) is currently facing several changes and challenges because some of the current halogenated or phosphorus-based flame retardants (FRs) have proven to be persistent, bioaccumulative, carcinogenic and/or toxic for animals and humans. Thus, the search for highly efficient green flame retardant products, which are exploitable by using simple and environmentally-friendly techniques (i.e. impregnation/exhaustion, layer-by-layer), is driving the researchers towards the development of worthy alternatives. In this context, very recently, biomacromolecules (in particular proteins and deoxyribonucleic acid) have been thoroughly investigated because they exhibit significant potentials as novel green FRs for selected fabrics (cotton, polyester and their blends), as well as for bulk polymers (ethylene vinyl-acetate copolymers) and foamed polyurethane substrates. This work aims to review our recent results related to the “unconventional” use of these biomacromolecules as FRs with low-environmental impact for fabric substrates, as well as the challenges and the perspectives that these products may offer in the forthcoming years in the field of flame retardancy for textiles. To provide the basic knowledge necessary for understanding the role of biomacromolecules as FRs for textiles to the readers, first of all the description of the structure, main properties and conventional applications of proteins and deoxyribonucleic acid is provided; the thermal and thermo-oxidative stability, the reaction to a flame exposure or to an irradiative heat source of the selected fabrics – cotton, polyester and their blends – will be discussed, as well.

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Giulio Malucelli

Giulio Malucelli got his Master Degree in Chemical Engineering in 1992. He received his Ph.D. in Chemistry in 1996. From 2003 he is Associate Professor of Materials Science and Technology at the Department of Applied Science and Technology of Politecnico di Torino, Italy. He has co-authored more than 160 peer-reviewed papers; his main current research interests include the synthesis and characterization of nanocomposite structures (hybrid organic–inorganic coatings and polymeric nanocomposites), as well as the modification of plastics and textiles for improving their flame retardant features.

Francesca Bosco

Francesca Bosco got her Master Degree in Biological Science in 1987. She received her Ph.D. in Biology and biotechnology of fungi in 1995. She is Assistant Professor at the Department of Applied Science and Technology of Politecnico di Torino, Italy. She has co-authored more than 30 peer-reviewed papers. Her main research interests concern the production of microorganisms and biomolecules of industrial interest, valorisation processes of wastes and industrial by-products in the production of biomolecules.

Jenny Alongi

Jenny Alongi got her Master Degree in Chemistry from the University of Genoa, Italy in 2002 and received her Ph.D. in Chemical Sciences in 2006. At present she has a post-doc fellowship at the Department of Applied Science and Technology of Politecnico di Torino, Italy. She has co-authored more than 70 peer-reviewed papers. Her main research interests concern polymers and nanostructures, as well as smart fibres and textiles, with particular attention to their flame retardancy properties.

Federico Carosio

Federico Carosio got his Master Degree in Materials Science and Engineering from Politecnico di Torino in 2008 and received his Ph.D. in Materials Science and Technology in 2012. At present he has a post-doc fellowship at the Department of Applied Science and Technology of Politecnico di Torino, Italy. He has co-authored more than 30 peer-reviewed papers. His main research interests concern polymers and nanostructures, as well as smart fibres, textiles and polyurethane foams with particular attention to their flame retardancy properties achieved through the layer by layer deposition technique.

Alessandro Di Blasio

Alessandro Di Blasio got his Bachelor Degree in Materials Science at the University of L'Aquila, Italy in 2004. At present he is the Laboratory Manager of Consorzio Prometeo (Italy), a research consortium founded by Politecnico di Torino. He has co-authored more than 20 peer-reviewed papers. His main research interests concern nanostructured polymers, with particular attention to their photo-oxidation resistance and flame retardancy.

Chiara Mollea

Chiara Mollea got her Master Degree in Biological Science in 1998. She received her Ph.D. in Biology and Biotecnology of Fungi in 2002. She is Laboratory Technician at the Department of Applied Science and Technology of Politecnico di Torino, Italy. Her main research interests concern extraction of biomacromolecules from food industry wastes for their valorisation as well as the extraction of aromatic molecules for the improvement of food products.

1. Introduction

In the last decade, several studies involving the researchers from academics and industry have been specifically devoted to the search of environmentally-friendly flame retardants that exhibit a remarkable “green” character and ensure fire performances comparable to those achieved by the conventional FR compounds, which have been usually designed based on halogens or halogen-derivatives.¹ Although the halogenated compounds are the most efficient and widely used FRs, some of them (like pentabromodiphenyl ether, decabromodiphenyl ether and polychlorinated biphenyls) have been clearly shown to be persistent, bioaccumulative, and/or possess environmentally toxic features for both animals and humans.²

Among the currently commercially-available flame retardants, phosphorus-based compounds may represent a suitable alternative to their halogen-based counterparts. In particular referring to cellulosic fabrics, the current attention is focused either on the production of effective halogen-free substituents for coatings and back-coated textiles or on the use of N-methylol phosphonopropionamide derivatives (Pyrovatex®) or hydroxymethylphosphonium salts (Proban®). The latter shows some drawbacks referring to the formaldehyde release during the application of the FR and service life.²

At a lab scale, some considerably successful examples also concern the combination of phosphorus-based FRs with sol–gel derived oxidic phases, exploiting the joint or synergistic effects.^3–6

Very recently, Horrocks published a historical and comprehensive review of the progress achieved during the second half of the 20^th century, clearly describing the pros and cons of the use of the different FRs developed during this period.⁷ Furthermore, the USA and EU directives regarding the “chemistry” of FRs are becoming increasingly rigid and severe, thus limiting or even banning some of the currently used products; as a consequence, both the academic and industrial communities are continuously investing time and funds in order to find worthy alternatives to traditional FRs.

In this context, we have recently started to assess the effectiveness of different biomacromolecules (whey proteins, caseins, hydrophobins, and deoxyribonucleic acid – DNA) to be a valuable unusual green alternative to the conventional flame retardants for textiles.⁸ Indeed, it was a successful attempt to demonstrate that such compounds show significant enhancements as far as the resistance to an irradiative heat flux or to flame propagation are considered, when applied onto the surface of the selected substrate (like cotton, polyester or their blends).

The application of biomacromolecules is considerably easy and could exploit the methods that are already designed and optimized for textile finishing (like impregnation/exhaustion) or even layer-by-layer depositions, with significant enhancement.

One of the major advantages related to the use of biomacromolecules in flame retardancy refers to their low environmental impact and toxicity because these products are usually dissolved or suspended at low concentrations in aqueous media such that no VOC (volatile organic carbon) species are produced. Furthermore, their effectiveness in conferring flame retardant features is quite significant and, in some cases, comparable to the conventional phosphorus-based flame retardants.

In addition, some of these biomacromolecules (i.e. caseins, whey proteins) are considered to be by-products or even waste from the agro-food industry and their recovery and subsequent use as flame retardants may comply with the current needs of the valorisation of agro-food crops, avoiding their landfill confinement. Moreover, despite the considerably high cost of deoxyribonucleic acid as compared to traditional chemical FRs (differences are around three orders of magnitude), the availability of this biomacromolecule has become competitive with those of other chemicals because of the large scale method that was recently developed by Wang and co-workers; they proposed the extraction and purification of DNA from salmon milt and roe sacs.⁹

In this context, the present work aims to review the recent results obtained by our research group regarding the use of these biomacromolecules to be novel green flame retardant additives for cotton, polyester and their blends, showing the up-to-date achievements that can be provided to the treated fabrics in terms of resistance to an irradiative heat flux and to flame propagation, and discuss the challenging issues and the future perspectives that these biomacromolecules could present in the coming years.

2. Biomacromolecules: structure, main properties and conventional applications

2.1 Whey proteins

Whey proteins are globular molecules with a substantial content of α-helix motifs, in which the acidic/basic and hydrophobic/hydrophilic amino acids are distributed in a fairly balanced way along the polypeptide chains.¹⁰ The major components of whey proteins are β-lactoglobulin (β-LG), α-lactalbumin (α-LA), bovine serum albumin (BSA) and immunoglobulin (IG). Whey proteins constitute about 20% of the total proteins in milk (about 80% is constituted by caseins) and they have a high nutritional value because of their high essential amino acid content, in particular the sulfur-containing ones (methionine, cysteine).^11,12 In particular, β-LG and α-LA contain 5 and 8 cysteine residues, respectively. Furthermore, whey proteins show functional properties (e.g. high solubility, water absorption, gelatinization and emulsifying capacities) which are essential in food applications.¹³ Whey protein products are available in three major forms: whey protein concentrate (WPC), whey protein isolate (WPI), and whey protein hydrolysate (WPH). Unlike whey protein concentrates, individual whey proteins have their own unique nutritional, functional and biological characteristics.¹⁴

Whey proteins and peptides are exploited as important emulsifiers in food due to their amphiphilic properties because they possess both hydrophobic and hydrophilic residues.¹⁵ The potential of whey protein isolate (WPI) hydrolysates as emulsifiers in nanoemulsions has been reported in the literature.^16,17 WPC micro-, submicro- and nanocapsules have been exploited for encapsulating bioactive substances that are suitable for the development of novel functional foods (e.g. the antioxidant, β-carotene).¹⁸

In addition, blends of milk proteins (either whey protein isolate, WPI or sodium caseinate, SC) were investigated for the encapsulation of astaxanthin to enhance its stability and application in food systems.¹⁹

Recently, Wang and co-workers reported that whey proteins could be used to produce a new paper glue by the addition of polyvinylpyrrolidone; the globular structure of the WP can be unfolded and modified by chemical or physical means, which could make their structures closer to typical adhesives.²⁰

Edible films based on whey proteins appear to be a very promising route for the delivery of probiotics in food systems, where they cannot be incorporated by direct inoculation. The application of probiotic WP edible films on a staple bakery system (i.e. pan bread) reduced L. rhamnosus GG viability losses throughout drying and storage.²¹

Referring to whey protein isolate (WPI) systems and whey protein fractions (i.e. α-LA and β-LG), Gounga et al. emphasized their potential to form edible and biodegradable films for use in food packaging. These films show high mechanical properties and outstanding barrier features for oxygen; thus, they are suitable for the production of transparent, colorless, odorless and flexible films by the addition of very low amounts of forming agents and plasticizers, such as glycerol and polyethylenoxide.²²

More recently, whey protein isolated edible films with encapsulated antimicrobial materials have also been investigated for different applications in the active packaging of food. In addition to antimicrobial treatments, phage therapy is an alternative approach that can be used to control both human and food borne pathogens. The ability of WPI films to encapsulate, stabilize, and release bacteriophages T4 to the surrounding environment has been very recently demonstrated by Vonasek and co-workers.²³

2.2 Caseins

Caseins are the major fraction of milk proteins and, perhaps, the most widely studied food proteins, which are obtained as co-products during the production of skim milk. They are phosphoproteins, a family of proteins synthesised in the mammary gland composed of α_S1, α_S2, β, and κ-caseins, according to the nomenclature developed for the bovine caseins.

α_S1-Casein is the major protein fraction of bovine milk; it is a highly phosphorylated compound containing 8 or 9 bound phosphate groups/mol. α_S2-Casein is also highly phosphorylated; bovine milk comprises four differentially phosphorylated isoforms containing 10–13 phosphate groups per mol.

β-Caseins are, in particular, rich in glutamines that bear –NH₂ groups and they are characterised by a single major phosphorylation site near the N-terminus; the number of phosphorylation sites and the level of phosphorylation are less than that observed for α_S1- and α_S2-caseins. Bovine β-casein exists in a single fully phosphorylated form containing 5 phosphate groups/mol; κ-casein has the smallest phosphate component as compared to any other casein. The phosphorylation sites are confined to the C-terminal region of the molecule and they are present as singles sites.²⁴

Apart from cheese farming, caseins have been mainly used as a food ingredient for enhancing physical properties such as foaming and whipping, thickening and water binding, emulsification and texture, and also for improving the nutritional values.²⁵

Furthermore, caseins show specific coating properties, which widen their application range to papermaking, printing, leather finishing and synthetic fibers manufacturing.²⁶

Recently, casein micelles were used in the preparation of gold nanoparticles (GNPs) for catalytic applications; on the basis of their excellent colloidal stability and biocompatible surface, the casein micelle-stabilized GNPs exhibited good potentials in nanoscience and biomedical applications.²⁷

Caseins are also exploited for nanoencapsulating the medicinal drugs. In particular, β-casein and its complexes with chitosan were used as nanovehicles for the delivery of a platinum anticancer drug.²⁸

Casein-based micelles loaded with flutamide, a hydrophobic anti-cancer drug, were successfully developed in a powdered form via spray-drying technique.²⁹

Finally, the ability of the gold nanoparticles to act as carriers/electrocatalytic labels was also combined with the properties of κ-casein derived peptides; the binding of the specific K88 bacteria fimbriae was successfully achieved.³⁰

2.3 Hydrophobins

Hydrophobins are produced by filamentous fungi, and they consist of a large family of small amphipathic proteins with low molecular masses (7–9 kDa).³¹

Because of its cysteine distribution and the clustering of hydrophobic and hydrophilic amino acid residues, two main hydrophobin classes have been identified: class I (HFBI) proteins, which form aggregates that are highly insoluble in aqueous solution and have low wettability and class II (HFBII) proteins, which form aggregates that easily dissolve in aqueous media.³² Despite the low sequence similarity, the two classes have a characteristic pattern of eight cysteine residues forming four non-sequential disulphide bonds, which stabilize the protein tertiary structure.³³ Hydrophobins are present in large amounts in fungal cell walls, where they are present in the outermost layer (the so-called “rodlet layer”), they form a water-repellent monolayer on the surfaces of spores and fruiting bodies and they reverse the wettability of the surface, on which they are formed. Some hydrophobins secreted in the environment show the ability to convert hydrophobic surfaces to hydrophilic ones and vice versa by self-assembling amphipathic monolayers at the hydrophobic–hydrophilic interfaces;^34,35 indeed, hydrophobins are among the most surface-active molecules known.³⁶

In nature, hydrophobins provide an important way for fungi to deal with surface phenomena in the environment; for example, they play a role in cell wall maturation and in facilitating spore-dispersion in the air.³⁵ In addition, individual proteins show features that are suitable for specific roles (e.g. the reduction of surface tension of the medium or of the substrate in/on which the fungi grows, the involvement in fungus–host interactions).³⁷

From an application point of view, hydrophobins play a role as a coating/protective agent, in adhesion, surface modification, or other functions that require surfactant-like properties.³⁸ Recently, they have become attractive as surfactants and foaming agents for protein immobilization in food industry, biosensor field, pharmaceuticals and tissue engineering.³⁴

The application of hydrophobins to stabilize the dispersed air bubbles in the manufacturing of aerated foods (e.g. ice cream) and as stabilizing agent in emulsions has been reported in the literature.^39,40 The class II hydrophobin (namely HFBII) is an air structuring protein that confers exceptional stability to foams.⁴¹ Furthermore, the foam and air-/oil-filled emulsion-forming capacity of hydrophobins has been exploited to protect nanoparticles and drug formulations.³⁸

Because of their amphiphilic nature and self-assembly properties, hydrophobins show some potentials to be highly effective surfactants, emulsifiers and nanoencapsulating agents in food systems and drugs;⁴² thus, they were found to be promising nanovehicles in hydrophobic nutraceuticals for food and clear beverage enrichment.³⁶

Further possible applications of hydrophobins include their use in membranes to immobilize cells or proteins to surfaces (e.g. in biosensors), tuning of surface hydrophobicity to increase biocompatibility in tissue engineering.^36,43

2.4 Deoxyribonucleic acid

DNA consists of two long chain polymers of nitrogen-containing bases, adenine (A), guanine (G), cytosine (C) and thymine (T), with backbones made of five-carbon sugars (deoxyribose units) and phosphate groups connected through ester bonds. The chains are rolled-up around the same axis and bonded together in a double helix by hydrogen bonds between the bases that are placed side by side and coupled in a specific way (i.e. adenine and cytosine bases paired with thymine and guanine, respectively). In this spatial organization, the phosphoric residues and deoxyribose units are placed towards the outside, whereas the coupled bases are in the inner portion of the polymer and are stabilized with hydrophobic interactions. In nature, during different stages of the cell development, DNA usually exists as both a well-organized double helix structure and also as a single chain polymer; double helix DNA can be also artificially divided when subjected to thermal treatments above 80 °C, or to extreme pH values; this feature can be exploited in different applications.⁴⁴

The field of DNA technology is rapidly growing with the recognition that DNA can be useful as a “generic” material with the capability to incorporate and bind other compounds. The ability of DNA to form double-stranded structures has been used to obtain various DNA-based nano-scale materials, ranging from life science to computing. A variety of DNA-based nanostructures such as DNA-linked metal nanoparticles, semiconductor particles, DNA-directed nanowires and DNA functionalized carbon nanotubes have been successfully synthesized.⁴⁵ In a very recent review, the advances in the development of self-assembled DNA nanostructures for the efficient delivery of bioactive compounds were described.⁴⁶ Furthermore, the chitosan–DNA complex provides an opportunity for the development of safe and effective non-viral vectors for the in vivo delivery of biologically active macromolecules.⁴⁷

DNA-directed immobilization (DDI) is a highly efficient method for embedding components on surfaces. The DDI method has proven its robustness and versatility in numerous applications, ranging from biosensing and biomedical diagnostics to biomimetic assembly schemes in materials science. The use of nucleic acid hybridization can be applied to protein, small-molecules (peptides, glycan, and steroids) and to colloidal compounds, such as metal or semiconductor nanoparticles.⁴⁸

The DNA application in biosensing was developed in the early 90's; optical and voltammetric DNA sensors were designed with detection limits of nano or subnanograms of the complementary DNA. In addition, in the same period, bulk acoustic wave (BAW) biosensors with single stranded DNA immobilized on the sensing area of the device, which were usually made with Au- or Ag-plated quartz crystals, received a significant attention because they were simple, miniaturizable and highly sensible to the target DNA probe.⁴⁹

The structural properties of DNA also make it a very interesting biomaterial coating. The surface treatment of titanium with multilayered DNA coatings via electrostatic self-assembly (ESA) technique was recently described.^50,51

Hydrogels bearing chains functionalized with carboxyl groups were used in DNA immobilization, achieving a uniform distribution of DNA strands in the layer, with a possibility of controlling the thickness of the polymer matrix.⁵²

3. Thermal and fire behaviour of textiles

Thermal stability represents one of the most important issues in the design of a new flame retardant for fabrics. In particular, before ignition, any polymeric material undergoes thermo-oxidation and subsequently starts to burn. Thus, in order to develop a new FR, it is necessary to thoroughly investigate its thermal stability in air (namely, its behaviour towards thermo-oxidation).

Usually, this aspect is studied by thermogravimetry, which is considered to be the most widely employed tool for studying the thermal and thermo-oxidative degradation of polymers. It supplies quantitative results regarding the weight loss of a sample (TG) and corresponding rate (dTG), as functions of temperature or heating time (in isothermal conditions), provided that thermal effects and temperature gradients across the sample are taken into consideration and properly corrected. These measurements are often carried out at lower heating rates (namely 2–20 °C min⁻¹) as compared with those occurring during the thermo-oxidation provoked in combustion tests.

Recently, more sophisticated instrumentations (like fast TGA and flash pyrolysis) are becoming available for mimicking the occurrences during combustion. However, a good agreement between the standard TGA performed at low heating rates and combustion tests can be found, which is demonstrated in the scientific literature.⁵³

In regards to cotton, its thermo-oxidation is a very well-known process, which is reported in several studies until now.^54–58

In detail, the thermal degradation of cellulose takes place within 300–400 °C through two competitive processes, namely depolymerisation (1) and dehydration (2), which is schematized in Fig. 1.

Fig. 1 Scheme of the thermal and thermo-oxidative degradation of cotton.

Depolymerisation is initiated by the scission of acetal bonds between the chains of glycosidic units, followed by the successive splitting of volatile levoglucosan, the cyclic monomer of cellulose, from the ensuing chain ends.⁵⁶

The competitive dehydration reactions lead to the formation of thermally stable aliphatic structures (carbonaceous structure called char I), which are subsequently converted to aromatic structures (char II) with water, methane, carbon mono and dioxide evolutions (400–600 °C). Char II (ca. 18%) turns out to be thermally stable up to 800 °C.

These processes and the resulting equilibrium between the volatile release and char formation have proven to strongly depend on the heating rate of the adopted process. In particular, on increasing the heating rate, the volatile production becomes favoured as compared to the char formation (Fig. 2, unpublished results).

Fig. 2 TG and dTG curves of cotton, heated at the rate 2, 5 and 10 °C min⁻¹.

It can be observed that by increasing the heating rate from 2 to 10 °C min⁻¹, it is possible to observe a significant reduction of the char amount (from 25 to 19%). The same trend has been demonstrated also at very high heating rates (100–300 °C min⁻¹).⁵⁹

These results suggest that cotton possesses intrinsic features for protecting itself from thermo-oxidation by inhibiting the production and release of volatile species that can further favour its degradation, and thus its combustion. Moreover, the tendency of cotton to form a thermally stable residue (char) represents the most promising way to make it a flame retardant. Indeed, the most valuable commercially-available FRs (ammonium salts, Proban® and Pyrovatex®) exhibit a condensed phase, favouring the char formation. This is also a useful approach for the polymers that are not intrinsically char formers, like polyolefins, apart from the use of vapor phase and depolymerisation agents (like NOR-HALS), which cause the polyolefin to drip away from the flame, thus acting as effective flame retardants.

As far as polyester (PET) is concerned, this polymer undergoes ionic⁶⁰ or radical⁶¹ scission of its ester bonds upon heating (Fig. 3). As a consequence, carboxylic and vinyl terminated chain fragments are formed.⁶² These reactive chain end groups can either produce volatile cyclic oligomers via a back-biting depolymerisation mechanism, or being unstable at the scission temperature (400–500 °C), they can further decompose and form volatile species, including carbon mono and dioxide, methane, ethylene, benzene, benzaldehyde, formaldehyde and acetaldehyde.⁶³

Fig. 3 Scheme of the thermal and thermo-oxidative degradation of polyester.

Progressive statistical chain fragmentation may produce chain fragments having sufficiently low molecular weight, which are volatile at the degradation temperature. The probability of the volatilization of chain fragments increases with time due to the progressive molecular weight decrease of PET. The interconnection of the PET aromatic rings, promoted by reactive species produced either by the decomposition of chain fragment end groups or by the scission of ester–phthalic bonds present in the ring was proposed for the thermal stabilisation of chain fragments issued by chain scission, with the production of about 10% charred residue, which is stable at least up to 800 °C.⁵⁹

For cotton–polyester blends, their thermal stability is proven to be dependent on composition; their behaviour can be considered a perfect overlap of the percentage contribution of the two components, as efficiently depicted in Fig. 4, where the TG and dTG curves of two blends containing 15 and 35 wt% of cotton, respectively, are plotted.⁶⁴ Similarly, the combustion behaviour of a cotton–polyester blend also mimics cotton or polyester on the basis of the main constituent.

Fig. 4 TG and dTG curves of cotton, polyester and two blends heated at 10 °C min⁻¹.

Specifically referring to polyester and polyester–cotton blends, the most desirable FRs are expected to favour char formation instead of the production of volatile species because a coherent and consistent char is able to stop the dripping of incandescent polyester drops, which is the main drawback to overcome. Once again, the thermal protection of polyester or polyester–cotton blends strongly affects their resistance to combustion generated by a flame or an irradiative heat flux.

To assess the real efficiency of these FRs, different approaches can be carried out. Usually, two types of tests are exploited, i.e. the resistance of the flame retarded fabrics to a flame application (in a horizontal or vertical configuration) or to an irradiative heat flux, generated by an electrical resistance like a cone calorimeter^65,66 or a radiating panel.

In the former case, rectangular specimens of different sizes are ignited in a flame, the characteristics of which (length and gas type) as well as its application times are chosen on the basis of the employed standard, which depends on the application field. Two types of tests can be carried out, in particular, the specimen can be placed in horizontal or vertical configuration; usually, a methane flame (25 mm length) is applied on the short site of the specimen for at least 3 s. The significant parameters such as total burning time and rate, as well as the final residue are measured. An efficient flame retardant must be able to suppress the fabric combustion, leading to the self-extinguishment of the material; if the flame retardant is capable of significantly reducing the burning time and rate as well as significantly increases the final residue, it can be considered as a valuable and suitable choice for several applications.

An alternate method for qualitatively assessing the resistance of a treated fabric to a flame application is represented by the LOI (Limiting Oxygen Index) tests. In detail, the LOI of a polymeric material is considered to be the minimum concentration of oxygen (expressed as a volumetric percentage) that can support the combustion of the chosen polymer. It is measured by fluxing a mixture of oxygen and nitrogen over a burning specimen, and reducing the oxygen concentration until a critical level is reached. Although the LOI is useful as a quality control test and is often usually reported in the literature with reference to the performances of FRs (i.e. the higher the LOI value, the better are the performances of the FR), it is not clear that its results correlate with those of any other fire test, or comparatively with those of real fires.⁶⁷ In addition, materials that drip and flow can have a high LOI; they may burn readily even when prevented from dripping away from the flame source.

When a flame retarded fabric is subjected to an irradiative heat flux (e.g. generated by a cone calorimeter or a radiating panel), it is expected that an efficient FR must reduce its total heat release, accordingly modifying all the related parameters. For this aim, different tests can be used; recently, our research group had set up an optimised procedure, employing a cone calorimeter for studying the resistance of a flame retarded fabric following the standard ISO 5660 designed for the plastic materials.⁶⁵ The measurements are usually carried out under a 35 kW m⁻² irradiative heat flux (that can be related to the early stages of a developing fire) in a horizontal configuration. The parameters such as time to ignition (TTI, s), total heat release (THR, MJ m⁻²), peak of heat release rate (pkHRR, kW m⁻²) are evaluated. Other important parameters concerning the smoke, such as total smoke release (TSR, m² m⁻²), smoke factor (SF, calculated as pkHRR × TSR, MW m⁻²) and CO and CO₂ release (ppm and % or g s⁻¹) are assessed, as well. Overall, an ideal FR should increase the TTI of the fabric, reduce THR and pkHRR, and significantly decrease the amount of smoke released during the combustion (TSR, SF, CO and CO₂) and its optical density; obviously, these statements are considered valid regardless of the substrate type.

Overall, within its interactions with the selected fabric, an efficient FR should promote:

• An increased residue at high temperatures, both in inert and oxidative atmospheres (TG analyses).

• A rapid self-extinguishment of the fabric or, at least, a significant reduction of total burning rate and time (flame spread tests).

• An increased LOI value.

• A decreased THR and combustion rate (pkHRR) (cone calorimetry).

• A decreased smoke production (TSR, CO and CO₂ yields) (cone calorimetry).

In regards to TTI, although an increase of this parameter is ideally required, it is important to note that many FRs (among which some biomacromolecules, as discussed in ref. 4) react by char formation mechanisms and often ignite sooner (i.e. reducing TTI) than the non-flame retarded polymer because they are activated at low temperatures and form char in response to flame damage and ignition. As a consequence, despite of the reduction of TTI, when the FRs address all the other fire criteria, or, only address some of these, they can still provide an acceptable fire safety performance when the fabrics are exposed to a fire risk scenario.

Finally, referring to an FR that needs activation for conferring flame retardant properties to a fabric substrate, although it may appear counterintuitive, its protection mechanism will need quick degradation of the additive, so that its interactions may occur with the fabric substrate, leading to the char formation.

4. Application of biomacromolecules to selected textiles: effects on their thermal and fire behaviour

This portion aims to thoroughly investigate the thermal stability and fire behaviour of the selected fabrics (mainly cotton, although some examples concerning polyester and cotton–polyester blends will be presented as well) as a consequence of the treatments with the biomacromolecules. To enhance their thermal stability and flame retardant features, the fabrics have been previously subjected either to impregnation/exhaustion processes, using the aqueous solutions/suspensions of the biomacromolecules or to layer-by-layer treatments, exploiting very diluted aqueous solutions/dispersions that have opposite electrical charges. More details concerning the materials, the coating and characterization techniques are reported in the ESI,† along with some structures of the employed biomacromolecules (Fig. S1†).

4.1 Applications of biomacromolecule-based coatings to cotton fabrics

The excellent mechanical and barrier properties of WPI coatings, described in the ref. 2, can be employed for preventing, delaying, or partially inhibiting the thermal degradation of a thin polymer substrate, such as a cellulosic fabric, both in inert and oxidative atmosphere. In addition, their moisture adsorption features could be exploited for partially dissipating the heat evolved during the combustion of the fabric, and diluting the produced combustible volatile species. In this context, whey protein isolate coatings (consisting of folded or unfolded/denatured chains) have been successfully applied to cotton via an impregnation process, reaching final dry add-ons on the fabrics around 25 (folded proteins) and 20 wt% (unfolded proteins).⁶⁸

It was assessed by thermogravimetric analyses that the WPI coatings, regardless of the protein type, were responsible for a strong sensitization of the cellulose decomposition, which is indicated by the T_onset10% values, listed in Table 1; this result was ascribed to the thermal decomposition of the proteins, which starts at ca. 100 °C, giving rise to the formation of oligopeptides and amino acids bearing carboxylic groups, which are able to catalyse the cellulose decomposition.⁶⁹ However, the presence of the protein coating yielded a very high amount of residue at 750 °C, as compared to the untreated fabric.

Table 1 Thermogravimetric data of untreated and WPI-treated cotton fabrics in nitrogen⁶⁸

	Tonset10% [°C]	Tmax^a [°C]	Residue at Tmax [%]	Residue at 750 °C [%]
a From derivative curve.
Cotton	329	362	45.0	8.0
Cotton treated with folded WPI	276	355	45.0	18.0
Cotton treated with unfolded WPI	294	366	45.5	17.0

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In a similar way, the presence of the coating was found to sensitize the cellulose decomposition in an oxidative atmosphere (see T_onset10% values, Table 2); however, it simultaneously favoured the formation of a thermally stable product after the first degradation step (see residue at T_max1) that evolved at high temperatures by other two steps (T_max2 and T_max3) and left a final residue, which was slightly higher as compared to the untreated cotton. Because only the protein-treated fabrics showed the third and last weight losses, this phenomenon was ascribed to some chemical or physical interactions taking place among the species produced by the degradation of cellulose and the proteins at high temperatures.

Table 2 Thermogravimetric data of untreated and WPI-treated cotton fabrics in air⁶⁸

	Cotton	Cotton treated with folded WPI	Cotton treated with unfolded WPI
a From derivative curve.
Tonset10% [°C]	323	283	292
Tmax1^a [°C]	343	341	345
Tmax2^a [°C]	489	487	496
Tmax3^a [°C]	—	580	575
Residue at Tmax1 [%]	48.0	57.0	56.0
Residue at Tmax2 [%]	2.0	14.0	13.0
Residue at Tmax3 [%]	—	2.5	3.0
Residue at 600 °C [%]	<1.0	1.5	2.5

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In regards to the flame retardant properties of the WPI-treated cotton fabrics, Table 3 lists the data collected after the flammability tests were performed in the horizontal configuration. It is important to note that the WPI coatings promoted a significant decrease of the burning rate, thus showing their ability to partially protect the fabric from the flame (the original texture of the fabrics was preserved after these tests), as depicted in Fig. 5.

Table 3 Results from flammability tests performed on untreated and WPI-treated cotton fabrics⁶⁸

	Total burning time [s]	Burning rate [mm s^−1]	Residue [%]
Cotton	78	1.5	—
Cotton treated with folded WPI	126	1.0	30
Cotton treated with unfolded WPI	133	1.1	5

---

Fig. 5 SEM images of the residues of the cotton treated with folded proteins, after flammability tests.

Although self-extinguishment was not achieved in the presence of the protein coatings, the combustion process took place for a longer time, but with a small flame.

It appears very reasonable to assume that they exploit their barrier properties to hinder the oxygen diffusion; simultaneously, they are able to partially absorb the heat evolved during the combustion. In addition, despite the efficiency of both the folded and unfolded protein coatings, only the former was able to promote the formation of a very high amount of residue after the flammability tests; the final residue found in the unfolded WPI-treated fabrics was extremely brittle and not completely coherent.

Pursuing this investigation, cotton fabrics were treated with caseins to exploit the flame retardant features that could be derived from the phosphate groups present in their structure.⁷⁰ In particular, cotton fabrics were treated with an aqueous suspension of caseins, which was spread on the substrates with a spatula; then, the excess was removed by gently pressing it with a rotary drum and the fabrics were dried until a constant weight was reached. The final dry add-on was around 20 wt%.

The thermal and thermo-oxidative stability of both the plain and treated cotton was assessed through TG measurements. The obtained data are collected in Table 4. In presence of nitrogen, as already described for the WPI coatings, the presence of the caseins strongly sensitized the cellulose decomposition, which is clearly shown by the T_onset10% values listed in Table 4 that shift toward the lower temperatures after the treatment with biomacromolecules. Furthermore, it is noteworthy that the T_max1 values were almost unchanged, whereas the residues found at 600 °C significantly increased after the treatment; this result was ascribed to the release of acidic species (like phosphoric acid), which catalyse the dehydration of cellulose for the formation of a thermally-stable char. Therefore, the decomposition of caseins can be considered considerably similar to that of ammonium polyphosphate salts, which exhibit thermal degradation profiles analogous to caseins. The only difference is the temperature at which phosphoric acid is released, in particular, in the case of caseins this occurs at lower temperatures as compared with ammonium polyphosphate salts because of the stronger covalent bonds of the phosphate groups in the main chain of these salts.⁷¹

Table 4 Results from thermogravimetric analyses performed on untreated and caseins-treated cotton fabrics⁷⁰

	Tonset10% [°C]	Tmax1^a [°C]	Tmax2^a [°C]	Tmax3^a [°C]	Residue at Tmax1 [%]	Residue at Tmax2 [%]	Residue at Tmax3 [%]	Residue at 600 °C [%]
a From derivative curve.
Atmosphere: Nitrogen
Cotton	329	362	—	—	48.0	—	—	8.0
Cotton treated with caseins	285	354	—	—	45.0	—	—	18.0
Atmosphere: air
Cotton	324	347	492	—	48.0	4.0	—	<1.0
Cotton treated with caseins	275	338	485	602	55.0	16.0	2.0	2.0

---

In air, the degradation profile of the fabrics treated with the caseins showed the same sensitization effect as that in the presence of nitrogen; the only differences were for the occurrence of two further degradation steps (T_max2 and T_max3, Table 4), which can be attributed to specific coating–substrate interactions, during which the evolution of the thermally stable product that was formed during the first step takes place, leading to the formation of a final residue (at high temperatures, i.e. 600 °C) that is slightly more than that of pure cotton.

In regards to the flammability tests in the horizontal configuration, Table 5 presents the obtained data. Unlike the untreated cotton that burns rapidly and vigorously upon the application of the methane flame for 3 s, the treated counterparts showed a significant increase in the total burning time (+40% as compared to untreated cotton), as well as a remarkable decrease of the total burning rate (−35%), simultaneously promoting the formation of a consistent final residue with the texture of the fabric was still preserved.

Table 5 Results from flammability tests performed on untreated and caseins-treated cotton fabrics⁷⁰

	Total burning time [s]	Burning rate [mm s^−1]	Residue [%]
Cotton	78	1.5	—
Cotton treated with caseins	100	1.0	34

---

In addition, it is noteworthy that the morphology of the residues of the treated fabrics, which was assessed by SEM after the flammability tests, indicates the formation of globular micrometric structures, which increase in size during the combustion (Fig. 6).

Fig. 6 SEM images of the residues of cotton treated with caseins, after flammability tests.

The resistance to an irradiative heat flux for cotton fabrics treated with caseins has been also investigated. For this investigation, cone calorimetry tests (heat flux: 35 kW m⁻²) were performed on square fabric samples (50 × 50 × 0.5 mm³), before and after the treatment with the casein suspensions. As a result, a significant reduction of the time to ignition (28 vs. 10 s, for the plain and caseins-treated cotton, respectively) was found. Furthermore, a significant decrease of the heat release rate peak was achieved (−27% as compared with the untreated cotton); this is beneficial for the fabric substrate, which burns considerably slowly.

Pursuing this investigation, cotton fabrics were treated with an aqueous solution of mixed class I and class II hydrophobins (protein concentration 5%). The fabrics were dipped into the protein solution for 1 min, then gently pressed with a rotary drum and finally dried until a constant weight was obtained. The final achieved dry add-on was about 20 wt%.⁷⁰ As previously mentioned, hydrophobins are small cysteine-rich proteins bearing non-sequential disulphide bonds. Thermogravimetric analyses in nitrogen indicated that these bonds start to degrade at about 200 °C, releasing sulphidric acid that promotes the char formation of the cellulosic fabric and the crosslinking of the amide groups.^72,73

Similar to caseins, hydrophobins also sensitized the cotton degradation, which is shown by the T_onset10% values collected in Table 6, which are significantly lower as compared to the untreated fabric.

Table 6 Results from thermogravimetric tests performed on untreated and hydrophobins-treated cotton fabrics⁷⁰

	Tonset10% [°C]	Tmax1^a [°C]	Tmax2^a [°C]	Tmax3^a [°C]	Residue at Tmax1 [%]	Residue at Tmax2 [%]	Residue at Tmax3 [%]	Residue at 600 °C [%]
a From derivative curve.
Atmosphere: Nitrogen
Cotton	329	362	—	—	48.0	—	—	8.0
Cotton treated with hydrophobins	295	362	—	—	45.0	—	—	19.0
Atmosphere: air
Cotton	324	347	492	—	48.0	4.0	—	<1.0
Cotton treated with hydrophobins	292	336	499	620	61.0	14.0	3.0	4.0

---

Table 6 also collects the TG data in air; the effect of the presence of the hydrophobin coating on the thermo-oxidative stability of cotton is substantially comparable to that of the caseins-treated counterpart. In fact, degradation involves three main steps, among which the first is anticipated because of the presence of the protein coating.

As a result of the application of a methane flame to the treated fabrics, an increased total burning time as well as a decreased total burning rate were achieved (Table 7). Furthermore, the residue was found to increase, although in a less pronounced way as compared to the caseins-treated cotton. This result can be ascribed to the different morphologies of the fabrics treated with hydrophobins after the flammability tests. Unlike caseins, in hydrophobins-treated fibre the cleavage of the disulphide bonds and the subsequent crosslinking of the amide groups is less efficient and gives rise to the formation of several small pearl-like bubbles (Fig. 7).

Table 7 Results from flammability tests performed on untreated and hydrophobins-treated cotton fabrics⁷⁰

	Total burning time [s]	Burning rate [mm s^−1]	Residue [%]
Cotton	78	1.5	—
Cotton treated with hydrophobins	104	1.1	19

---

Fig. 7 SEM images of the residues of cotton treated with hydrophobins, after flammability tests.

In regards to the resistance to an irradiative heat flux, similar to the caseins but more efficiently, the fabrics treated with hydrophobins showed a decreased TTI with respect to cotton (10 and 28, respectively); however, simultaneously, a considerably higher decrease of pkHRR (−45% as compared with the untreated cotton) was observed.

The most effective fire-resistant effect with biomacromolecules on cotton has been achieved using deoxyribonucleic acid.^74,75

This biomacromolecule can be considered as an all-in-one intumescent material. Its three main constituents can be referred to as the proper intumescent flame retardant formulation because an intumescent material typically consists of three chemical components:⁷⁶ (i) an acid source, acting as a char promoter (e.g. ammonium phosphates or polyphosphates), (ii) a char source (e.g. pentaerythritol, arabitol, sorbitol, inositol, saccharides, and polysaccharides), and (iii) a blowing agent (e.g. urea, guanidine and melamine), which upon heating releases high amounts of gases (e.g. ammonia and/or carbon dioxide). Upon exposure to a heat flux, an intumescent material develops a multicellular foamed carbonaceous char that acts as a physical barrier, which is able to limit the heat, fuel and oxygen transfer between flame and polymer, thus leading to flame extinguishment.

DNA aqueous suspensions (2.5 wt%), obtained by slowly dissolving the biomacromolecule powder in acidified water (pH = 5.5), were applied to cotton by impregnation, achieving final dry add-ons equal to 5, 10 and 19 wt%.

Table 8 presents the thermogravimetric data of the plain and DNA-treated cotton fabrics in nitrogen and air. As already observed for the proteins, DNA was responsible for the strong sensitization of the cellulose decomposition in both atmospheres; furthermore, this phenomenon was found to strictly depend on the biomacromolecule dry add-on on the fabric, as clearly shown by the T_onset10% values listed in Table 8: the higher the DNA uptake, the greater is the sensitization effect. In addition, similarly to ammonium polyphosphate, DNA started to decompose at about 200 °C, releasing phosphoric acid and thus promoting the formation of a char that was thermally stable up to 600 °C. In air, the biomacromolecule coatings anticipated both T_onset10% and T_max1, giving rise to a residue that was stable beyond 500 °C (see T_max2 values, Table 8).⁷¹

Table 8 Results from thermogravimetric tests performed on pure DNA, untreated and DNA-treated cotton fabrics⁷⁵

	Tonset10% [°C]	Tmax1^a [°C]	Tmax2^a [°C]	Tmax3^a [°C]	Residue at Tmax1 [%]	Residue at Tmax2 [%]	Residue at Tmax3 [%]	Residue at 600 °C [%]
a From derivative curve.
Atmosphere: Nitrogen
Cotton	335	366	—	—	46.0	—	—	8.0
Pure DNA	195	180	230	270	91.0	85.0	75.0	50.0
Cotton + 5%DNA	285	318	—	—	63.0	—	—	30.0
Cotton + 10%DNA	265	314	—	—	64.0	—	—	34.0
Cotton + 19%DNA	243	309	—	—	67.0	—	—	35.0
Atmosphere: air
Cotton	324	347	492	—	45.0	4.0	—	<1.0
Pure DNA	171	180	226	333	90.0	85.0	69.0	49.0
Cotton + 5%DNA	282	313	506	—	65.0	19.0	—	8.0
Cotton + 10%DNA	263	302	511	—	69.0	24.0	—	13.0
Cotton + 19%DNA	238	299	515	—	68.0	29.0	—	19.0

---

Flammability tests carried out in the horizontal configuration (Table 9) clearly showed that the DNA coatings containing at least 10% of biomacromolecules are capable of self-extinguishing the flame in few seconds after its application; therefore, the higher the DNA uptake, the shorter is the time required for self-extinguishment. Taking also into account the significant increase of the LOI values in the presence of increasing DNA add-ons on the fabrics, it can be concluded that this biomacromolecule is an efficient fire-resistant additive for cotton, when applied beyond the minimum add-on.

Table 9 Results from flammability tests performed on untreated and DNA-treated cotton fabrics⁷⁵

	Total burning time [s]	Char length [mm]	Burning rate [mm s^−1]	Residue [%]	LOI [%]
a Flame out for all the three specimens tested.
Cotton	66	—	1.5	—	18
Cotton + 5%DNA	64	100	1.6	12.5	23
Cotton + 10%DNA^a	18	35	1.9	67.0	25
Cotton + 19%DNA^a	2	6	3.0	98.0	28

---

Cone calorimetry tests that were performed at two different heat fluxes (i.e. 35 and 50 kW m⁻²) combined with the SEM observations on the corresponding residues have allowed for the further investigation of the intumescent character of DNA. The obtained combustion data are presented in Table 10; Fig. 8 displays the typical micrographs of the residues after the combustion tests for the cotton treated with 19% DNA (heat flux: 50 kW m⁻²); the texture of the fabric is preserved after the irradiation under the cone.

Table 10 Combustion data for pure and DNA-treated cotton fabrics⁷⁵

Sample	TTI [s]	Flame out [s]	Combustion time [s]	pkHRR [kW m^−2] (reduction, %)	Residue [%]
a 2 out of 5 specimens do not ignite.
Heat flux: 50 kW m^−2
Cotton	16	40	24	128	<3.0
Cotton + 19%DNA	10	19	9	51 (−60)	17
Heat flux: 35 kW m^−2
Cotton	45	75	30	125	<3.0
Cotton + 19%DNA	No ignition				24
Cotton + 10%DNA	19	29	10	62 (−50)	15^a
Cotton + 5%DNA	24	39	15	68 (−56)	15

---

Fig. 8 SEM images of the residues of 19% DNA-treated cotton after, cone calorimetry tests (heat flux: 50 kW m⁻²).

The formation of a surface topography rich in spherical structures that are randomly dispersed within the burnt area indicates that the biomacromolecule, when activated by the heat flux, promotes the formation of bubbles, which increase in size and may break, releasing inert gases. The latter can dilute the volatile species; thus, preventing it to reach the flammability limit. These phenomena easily occur at 35 kW m⁻², whereas at 50 kW m⁻² the heating rate is significantly fast, and although the DNA coating is still able to form the bubbles, they cannot increase in size as quickly as it occurs; as a consequence, the flame retardant properties provided by the biomacromolecules are less marked as compared to those achieved at a low heat flux.

At 35 kW m⁻², when the final dry DNA add-on on cotton was 19%, no ignition of the tested samples occurred; however, for all the other samples, only three specimens out of five ignited, when the DNA add-on was 10%. The combustion took place in a very short and slow way, and was accompanied by the propagation of a small flame on the tested specimens; furthermore, it is worthy to note that a significant decrease of the pkHRR values (about −50% for both the DNA add-ons) was achieved.

Another approach that has been very recently exploited for treating cotton with biomacromolecules, in particular DNA, and achieving remarkable results in the fire-proofing of the fabrics is the layer-by-layer (LbL) deposition. Fig. 9 shows a schematic of the LbL process. This self-assembly technique, firstly presented by Iler⁷⁷ in 1966 and then rediscovered by Decher⁷⁸ in the 90's, consists of an alternate adsorption of chemical species on a selected substrate (plastic film, fabric and foam), exploiting different interactions (e.g. covalent bonds, hydrogen bonds, etc.), in addition to the electrostatic attraction that is the most often utilized. The latter usually requires the alternate dipping of the substrate into oppositely charged polyelectrolyte solutions or nanoparticle colloidal suspensions. As a consequence, an assembly of positively and negatively charged layers piled up on the substrate is built, exploiting a total surface charge reversal after each immersion step.⁷⁹ The main advantages shown by this approach are as follows: (i) the use of very diluted aqueous polyelectrolyte solutions or nanoparticle colloidal suspensions (with remarkable environmental benefits), (ii) the opportunity for the automation of the entire LbL deposition line, exploiting the roll-to-roll processes, (iii) the possibility of using spray-assisted LbL deposition methods, in addition to dip-coating techniques and (iv) the fine tuning of the LbL coating composition.

Fig. 9 General scheme of the LbL process.

The first example on the use of the LbL approach involving biomacromolecules was proposed by Grunlan and co-workers,⁸⁰ they combined cationic chitosan (which represents an efficient carbon source when used in intumescent LbL assemblies^81,82) and anionic phytic acid (the major storage form of phosphorus in cereal grains, beans and oil seeds⁸³) on cotton fabrics, aiming to reduce their flammability. It was assessed by vertical flame tests that the fabrics coated with up to 37 bilayers (BLs) completely extinguished the flame, while the uncoated cotton was completely consumed. Furthermore, microcombustion calorimetry tests confirmed that all the LbL-coated fabrics were able to lower the peak heat release rate by at least 50% as compared to the untreated counterpart. These findings were attributed to the high phosphorus content of the phytic acid, which was found to enhance the intumescent behavior of the obtained nanocoatings.

Therefore, we decided to couple chitosan with DNA. For this investigation, cotton fabrics were treated with 5, 10 or 20 DNA–chitosan BLs (corresponding mass gains: 2.5, 7 and 14%, respectively), thus exploiting the hydrophilic nature of the fabrics. Then, their flame retardant features were tested through flammability (in the horizontal configuration) and combustion (using a cone calorimeter) tests.⁸⁴

FT-IR ATR spectroscopy suggested the occurrence of an overall exponential growth of the DNA/chitosan bilayers, which can be attributed to the initial island-growth of the coating during the building up of the first BLs, followed by a diffusional growth, according to which at least one of the coating components diffuses in or out of the coating itself.

The morphology of the fibres was modified after the LbL deposition; it was observed that the appearance of a smooth coating on the fibre surface replaced the rough surface, which is typical for cotton and other natural fibres.

The results of flammability tests in the horizontal configuration are presented in Table 11. It is worthy to note that unlike 5 BLs (which did not affect either the burning time or the burning rate), 10 and even 20 BLs promoted a significant increase in the final residue, reducing the burning rate, increasing the burning time and involving a progressively smaller portion of the fabric, until self-extinguishment was achieved. The latter, in the case of 20 BL coatings allowed the preservation of almost all the sample from combustion (final residue: 88%). In addition, the increase of the LOI values of the treated fabrics further confirmed the significant FR properties observed in the flammability tests in the horizontal configuration.

Table 11 Results from flammability tests performed on untreated and DNA-treated cotton fabrics⁸⁴

	Total burning time^a [s]	Burning rate^a [mm s^−1]	Residue [%]	LOI [%]
a Flammability tests performed on specimens 150 mm length.
Cotton	104	2.2	—	18
Cotton + 5 BLs	106	2.2	8	21
Cotton + 10 BLs	154	1.9	48	23
Cotton + 20 BLs	56	1.5	88	24

---

The intumescent features of the LbL assemblies promoted by the presence of DNA were confirmed by SEM measurements performed on the fabric residues after the flammability tests (Fig. 10). First of all, it is noteworthy that the burnt portions of the fabrics still showed the original texture and shape; in addition, intumescent-like bubbles (rich of carbon, oxygen and phosphorus, as assessed by elemental analysis) appeared on and within the LbL treated fabrics, as a consequence of the coating reaction to the flame application.

Fig. 10 SEM micrographs of the residues of LbL-treated cotton (20 BLs), after flammability tests.

Finally, cone calorimetry tests performed at 35 kW m⁻² (the corresponding data are listed in Table 12) clearly highlighted the fire-proof features of the LbL coatings, as revealed by the decrease of both the pkHRR and THR values, as well as by the increase of the final residue. It is also interesting to note that, unlike the specimens after the flammability tests, the corresponding counterparts tested under the cone did not exhibit the bubble-rich morphology; this result was ascribed to the higher heating rates adopted during the cone calorimetry analyses compared to the flammability tests.

Table 12 Combustion data for pure and LbL-treated cotton fabrics⁸⁴

Sample	TTI [s]	pkHRR [kW m^−2]	THR [MJ m^−2]	Residue [%]
Cotton	39	97	1.9	2
Cotton + 5 BLs	17	73	1.7	11
Cotton + 10 BLs	20	60	1.5	12
Cotton + 20 BLs	23	57	1.3	13

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4.2 Application of biomacromolecule-based coatings to polyester fabrics and to cotton–polyester fabric blends

In regards to the synthetic fabrics like polyester, one of the main aim is still to develop new halogen-free flame retardants, which would be able to stimulate char formation and to suppress the dripping phenomena because the formation of incandescent melt droplets can easily spread the fire to other surrounding ignitable materials.⁸⁵ To assess the feasibility of using biomacromolecules for conferring flame retardant features to polyester (PET) and polyester-rich/cotton blends (PET–COT), very recently our group started to apply casein suspensions to these fabrics, reaching a final 20% dry add-on. The resulting thermal and flame retardant features were investigated by means of thermogravimetric analyses, flammability tests in the horizontal configuration, LOI measurements and cone calorimetry tests.⁸⁶

The thermal and thermo-oxidative behaviour of plain and caseins-treated PET and PET–COT is summarized in Table 13.

Table 13 Results from thermogravimetric tests performed on untreated and caseins-treated PET or PET–COT fabrics⁸⁶

	Tonset10% [°C]	Tmax1^a [°C]	Tmax2^a [°C]	Tmax3^a [°C]	Residue at Tmax1 [%]	Residue at Tmax2 [%]	Residue at Tmax3 [%]	Residue at 600 °C [%]
a From derivative curve.
Atmosphere: Nitrogen
PET	400	426	—	—	51.0	—	—	14.0
PET + 20% caseins	315	397	—	—	53.0	—	—	22.0
PET–COT	332	351	423	—	73.0	37.0	—	15.0
PET–COT + 20% caseins	304	334	405	—	75.0	42.0	—	22.0
Atmosphere: air
PET	392	422	547	—	47.5	1.5	—	0
PET + 20% caseins	310	404	538	—	50.5	13.0	—	2.0
PET–COT	323	339	419	508	79.0	37.0	7.0	1.0
PET–COT + 20% caseins	311	335	416	525	82.0	43.0	9.5	2.0

---

As already found for cotton, the casein coatings were responsible for a strong sensitization of polyester and PET–COT decomposition both in an inert and oxidative atmosphere, as confirmed by the decrease of the corresponding T_onset10% values (Table 13). Even for these fabrics, the phosphate groups present on the shell of the casein micelles, on heating, evolve into phosphoric acid that catalysis the degradation reactions of the fabrics, leading to the formation of a considerably stable char. This phenomenon was favoured in the case of polyester–cotton blends, as shown by comparing the residues at T_max1 and T_max2, collected in Table 13.

In air, the presence of caseins again remarkably sensitized the decomposition of the two fabrics under study, and simultaneously favoured the formation of a thermally stable product that evolved at high temperatures (T_max2). PET–COT, unlike PET, showed a third weight loss that shifted from 508 °C to 525 °C in the presence of the protein coating, thus clearly confirming the char promoting character of the protein.

The results from flammability tests are presented in Table 14.

Table 14 Results from flammability tests performed on untreated and caseins-treated PET or PET–COT fabrics⁸⁶

	Total burning time [s]	Burning rate [mm s^−1]	Residue [%]	Dripping	LOI [%]
PET	57	1.8	43	Yes	21
PET + 20% caseins	54	0.6	77	Yes	26
PET–COT	104	1.1	34	No	19
PET–COT + 20% caseins	171	0.7	55	No	21

---

As far as PET is concerned, the caseins coating were found to significantly reduce its burning rate (−67%) and block the flame propagation within 30 mm, leading to a strong increase of the final residue (77%); the only drawback observed was dripping, which was inhibited, but not completely suppressed (indeed, small incandescent drops were still formed). Caseins-treated fabric blends burnt slower than their untreated counterparts and showed higher total burning times. At the end of the test, a consistent and coherent residue was obtained. In order to complete the information provided by the flammability tests in the horizontal configuration, the LOI values were evaluated. As shown in Table 14, the protein coatings were responsible for a remarkable increase of the polyester LOI, unlike the blend, which exhibited a slight increase of this parameter.

Referring to the resistance to a heat flux (cone calorimetry tests), Table 15 presents the typical values of the different parameters. It is worth noting that regardless of the type of fabric substrate, the protein coatings were found to reduce the TTI values, but at the same time decrease the pkHRR and increase the final residue, thus proving their char-former effect on polyester fabrics.

Table 15 Combustion data for pure and protein-treated fabrics⁸⁶

Sample	TTI [s]	pkHRR [kW m^−2]	Residue [%]
PET	112	72	2
PET + 20% caseins	62	70	11
PET–COT	30	60	3
PET–COT + 20% caseins	12	51	5

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5. Conclusions and future perspectives

Some years ago, idea that biomacromolecules, like proteins (whey proteins, caseins, and hydrophobins) and deoxyribonucleic acid can act in fire proofing would not have been considered. The recent results achieved by our group and described in the present review have clearly demonstrated that it is possible, at least at a lab scale, to address the chemical features of these biomacromolecules toward the development of novel, environmentally-friendly and at the same time efficient flame retardants for different fabrics. In addition, some of these green additives show another added value because they can be considered as by-products or even waste from the agro-food industry and their recoveries and subsequent use as flame retardants may comply with the current needs of valorisation of agro-food crops, avoiding their landfill confinement.

Their potentiality in the flame retardant field is quite high, although all the discussed approaches are still currently under investigation. At present, there are some limitations related to the development of efficient FRs based on these biomacromolecules; however, the possibility of adjusting this green technology to a large scale (semi-pilot, pilot or industrial) is still under evaluation, depending on the cost-effectiveness of the proposed biomacromolecules. Undeniably, some of these biomacromolecules, like DNA, are currently very expensive, although a reduction of their cost may be foreseen in the next years because the production capacity of the industrial extraction plants for these biomacromolecules should increase. In addition, the possibility of exploiting the industrial apparatus already utilized in the textile finishing has still to be assessed.

Finally, the durability to the laundering of the proposed treatments deserves particular attention because, at present, the biomacromolecules-based coatings are not resistant to washing treatments. Because of their waterborne character, these coatings come off from the textile when subjected to washing according to the specific standards (and even in “mild” conditions, i.e. at a low temperature e.g., 30 °C and without using surfactants). This certainly represents a significant current limitation in the use of the treated fabrics because the durability of the FR treatments is mandatory for several textile applications. Therefore, further work will also be devoted to explore possible solutions for this limitation, keeping in mind that an acceptable balance between the green features of the biomacromolecules and the use of chemical products for permanently fixing the biomacromolecules to fabrics should be pursued. Some chemicals that are being already used for textile finishing can be very effective for this purpose; however, they could contribute to the loss of the “green” character of the proposed treatments. This does not exclude the near future exploitation of biologically-derived chemical treatments, or at least, of chemicals with a low environmental impact, which could make the proposed biomacromolecules more durable than they are today, simultaneously maintaining their effective flame retardancy.

Thus, the high fire proofing features provided by these bio-treatments along with their safety and environmental features appear to represent a robust starting point for the development of FR alternatives to the current phosphorus-based synthetic products.

Acknowledgements

The authors thank the European COST Action FLARETEX (MP1105) “Sustainable flame retardancy for textiles and related materials based on nanoparticles substituting conventional chemicals”.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06771a

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