**2.1. Producing a nonwoven fabric of polylactide (PLA) using melt blowing and carbon nanotubes**

The most suitable materials to produce nonwoven fabrics via melt blowing are those whose melt flow index is high. Most thermoplastic polymers can be used in this technique. However, as a result of very good processing properties and low price the most commonly used is polypropylene. Polycarbonate, polyester, polyamide and polystyrene are also frequently used to make nonwoven fabrics. When selecting an appropriate polymer for the construction of a chemical vapour sensor both the Hildebrandt and the Flory–Huggins theories should be followed [7, 26], especially when a compound is influenced by a solvent (swelling, dissolution). This happens when the solubility parameters of the polymer and solvent used are similar. The Flory–Huggins solubility parameter has been presented in previous work [7].

The polymeric sensor produced by melt blowing was assumed to have been manufactured from PLA to which multi-walled carbon nanotubes (MWCNT) were then introduced. Pre‐ liminary experiments made it possible to establish a percolation threshold for nonwovens produced by melt blowing. A conductive nonwoven containing 2% MWCNT was found to be adequate to produce a 4060D PLA polymer (98% PLA 4060D/2% MWCNT) [7]. Increasing the content of MWCNT to 4% was found to cause a linear increase in the conductivity of nonwo‐ vens, but a decline in the strength of the product.

Nonwoven fabrics made of PLA and MWCNT were produced in two stages. First, the polymer PLA 4060D (NatureWorks) had 4% MWCNT (Nanocyl® 7000, Nanocyl SA, Belgium) added to it. A ready-made nanocomposite (PLA/4% MWCNT) was then mixed with pure PLA 4060D to obtain the final composition PLA/2% MWCNT. The two-step process was designed to ensure carbon nanotubes were uniformly distributed in the structure of the product and that the formation of undesirable agglomerates was reduced. The nanotubes were 90% pure, 9.5 nm in diameter and 1.5 µm in length. The polymer selected had a molar mass of up to 87 000 Da and a D-isomer content of 12% [7].
