Textiles are subject to growing requirements for fire resistance. It’s a very broad field: the risks of exposure to fire are very different for a firefighter, a bath towel, a bed sheet or a curtain. Similarly, the consequences of a fire on a boat, on a train or a building are very different.
It is therefore not surprising that a wide variety of solutions exists to make a textile flame-retardant, nor that a large number of standards have emerged to respond to this wide variety of situations.
In order to obtain a fabric meeting one or more of these standards, following options are possible:
- To use an intrinsically fireproof material: asbestos, fiberglass or, more recently, basalt.
- To add a flame-retardant product to the mass of the fiber. This obviously only works with artificial fibers (viscose type) or synthetic fibers (such as polyester or polyamide).
- To perform a treatment on the fabric itself.
The first solution has the advantage of being radical but not necessarily comfortable nor cheap. Furthermore, the resources used are often non-renewable.
The second solution has the advantage to give the certainty of acquisition of the permanent character of the property. However, this involves greater logistic constraints because it is necessary to have at the desired time the necessary wire, in good fineness and good twist. Beside that the fibers tend to be more expensive, the synthetic fibers are inclined to melt in case of fire. The resultant droplets can cause deep and painful burns. This feature makes the pure synthetic fibers poorly suited for protective clothing.
The rest of this article will be devoted to flame-retardant treatments on fabrics, which offer the overall advantage of being able to be applied late in the manufacturing process, which eases logistic constraints. In addition, the treatment can often be modulated by varying the amount of product. This enables to optimize the result, taking into account multiple constraints: fire resistance, mechanical and coloristic characteristics, handle, price, …
Multiple axes can serve as a classification here: the nature of the fibers (cellulose, polyester, wool,…), the wash resistance of the treatment (from not washable to permanent), the application process (padding or coating, mainly) and finally fire/smoke requirements characterized by national or international standards, the latter particularly for maritime, air or rail transport.
In the framework of the FARBioTY project, we are focusing on flax fibers, composed mainly of cellulose, for use in the railway sector. In this context, the only interesting point is the durability of the treatment.
From non-permanent to permanent
Non-permanent and semi-permanent treatment
In this category, the principle consists of dissolving a salt in water, impregnating the fabric with this solution and drying the fabric. The salt remains on the fabric and helps to delay the appearance or spread of fire. Impregnation is often performed by padding; in the textile field, this operation refers to dipping the fabric in a bath, followed by a passage between two superimposed rollers on which applies a high pressure. This device called ‘mangle’ makes it possible to standardize the impregnation, to get the product deeply into the fabric and to recover the excess, thus facilitating the drying that follows.
This is a very simple technique to implement, relatively cheap and applicable to many fibers. However, there is no question of putting this cloth in machine to wash it because the salt would disappear immediately. It is therefore suitable for applications where the fabric is not exposed to water. Note also that some products in this class are based on halogens (especially bromine). Environmental and / or health issues have led to the ban of some of them such as decaBDE.
Semi-permanent treatments are used for substrates that may be exposed to water without being soaked. This concerns, for example, furniture fabrics, which can be rubbed and moistened to remove a stain. By cons, we rarely put a chair or an armchair in a washing machine! The biggest difference lies in the type of chemical used.
In this case, two types of treatment exist for cellulose: the first is to chemically bind a molecule to the cellulose, using one of its OH groups. The other is based on a polymerization of a product with itself, forming a network in and around the fiber, without creating any chemical bonding. In both cases, the product used is a phosphorus derivative.
In the first case, it is necessary to work in an acid conditions, which causes a loss of fiber strength, typically 15 to 30%. This solution was obviously not chosen for reinforcing fabric.
The other solution makes it possible to maintain almost unchanged mechanical strengths, it can also be used on cellulose / synthetic fiber mixtures and it gives a better resistance to fire. On the other hand, the fabric contains remnants of residual formaldehyde and the handle is more rigid.
It is applied as follows:
- Padding in a bath containing the flame-retardant product. It is a pre-condensate of THPC (Tetrakis Hydroxymethyl phosphonium chloride).
- Drying according to well-defined criteria.
Passage into a machine filled with gaseous ammonia allowing a polycondensation chemical reaction to take place. The ammonia group serves as a link between the different molecules.
- Washing and oxidation to remove unbound products and to stabilize phosphorus at a five-fold bond
- Finally drying
This technique allows obtaining excellent fireproof results, up to M1 and B1classifications, while maintaining the mechanical characteristics of the products. For now, it is mainly used for the production of protective clothing for trades exposed to fire risks: firefighters, military, metallurgists, electricians, …
This article proposes a brief overview of techniques for obtaining fire retardant fabrics. All the points discussed deserve further development, in particular the question of the physicochemical mechanisms used. We hope, however, that this little text has clarified some ideas.