The system used in Europe, North America and Asia to classify technical textiles, developed in the Techtextil, Messe Frankfurt Exhibition GmbH, specifies twelve main application areas [2-4]:

Figure 1 Source: The Authors.

The most common synthetic fibers used in the textile industry are acetate, acrylic, aramid, carbon, fluorocarbon, glass, lyocell, modacrylic, polyamide (PA), olefins, polybenzimidazoles, polyester, rayon, saran, spandex and sulfar fiber [7]. Despite the efficiency of these polymers, it has been necessary to create new polymers to generate technical textiles with specialized functions, as well as technical strategies to modify conventional fibers. One of the most important modifications is the incorporation of nanoparticles in the textile fibers by means of three main methodologies: a) embedding the nanoparticles in the polymer matrix by mixing in melt or solution before generating the fibers, b) incorporating the nanoparticles superficially in fibers through its in-situ formation, c) by decorating the fibers with nanoparticles, that is, where both the fibers and the nanoparticles are each formed separately, and subsequently the surface of both components is modified to achieve better compatibility between them and thus achieve the greatest possible permanence of the nanoparticles on the surface of the fibers [8].

Most conventional fibers (commodities) are manufactured by spinning polymers. There are two main types of spinning processes: spinning molten polymer and spinning polymer in solution (wet and dry). In melt spinning, the polymer is heated to melt it and be able to extrude it through a spinel to form the fibers, which solidify by cooling the polymer and are stretched to obtain the required thickness and properties.

In the spinning solution, the polymer is dissolved in a solvent and extruded by the spinel, after extrusion two variants of the technique can be used: spinning in dry or wet solution. In dry spinning, the solvent present in the newly extruded fibers is evaporated to solidify the fibers, while, in wet spinning, the spinneret where the fibers leave is submerged in a coagulation bath, which is where the fibers take their form as such, while the solvent is removed from the fibers when diffused in the coagulant bath [7]. An advantage of melt spinning is that it does not use solvents either to dissolve the polymer or to coagulate it, and therefore the toxicity of textiles is avoided by the presence of residual solvent [9]. For the above described, to generate fibers on an industrial scale it is preferred to use the melt spinning technique when possible (with thermoplastic polymers), however with some others it is necessary to use the spinning methodology in solution [7].

There is another technique for spinning fibers, but of nanometric dimension, although the use of solvents is necessary: electrospinning. Through this technique, polymeric nanofibers (their diameters are in the submicron range) can be produced continuously by taking advantage of the effect of electrostatic forces generated by the action of an external electric field imposed on the polymer solution. During the electrospinning process, a high voltage potential is applied between the polymer solution and the collector located near the end of the capillary, so that the jet of the polymer solution flows from the drop of solution at the exit of the capillary to the collector. The stretching and thinning of the fibers occurs in milliseconds over a relatively short path, as a consequence of the electrical forces present in the system [10].

Each of the spinning techniques mentioned has been used to generate fibers of pure polymers and polymeric nanocomposites. The incorporation of nanoparticles in the polymeric matrices or on the textile surface is carried out in order to obtain materials with properties superior to those obtained with pure polymers, for example, greater mechanical resistance [11], greater heat resistance [12], gas permeability [13], performance as flame retardant [14], etc. The improvement of the characteristics of the nanocomposites with respect to those of the pure polymers has a particularity, which is that the properties usually exceed the theoretically expected values for the mixture of its components (polymer matrix and nanoparticles). This unexpected improvement is the result of a synergistic effect of the properties of the generated nanocomposites, possibly the result of the exponential increase in the amount of interfacial interactions that are generated when using nanoparticles, being able to form an additional phase, which directly influences the properties of the nanocomposite, even when using low percentages of nanoadditives (<5%). Additionally, the low percentages of nanoadditives necessary to generate a change and / or improvement in the properties of the generated nanocomposite, allow the nanocomposites to be processed to form textile fibers in much the same way that synthetic fibers would be processed with pure conventional polymers [10].

Currently and as a consequence of the serious pollution problem that we have reached worldwide, there is a tendency towards the search and use of polymers, polymeric compounds and polymeric nanocomposites that are biodegradable. The main feature and advantage of biodegradable plastics is that, as they stop their degradation process, their polymer chains tend to break into simpler molecules and structures compared to the original structure, due to the attack of microorganisms, radiation, oxygen, etc. This deterioration of the polymer chains causes that the articles manufactured with this type of materials are broken and disintegrated into small fragments that cause minor damage to the environment or even, in some cases, depending on the polymer, the degradation products can arrive to be beneficial to the environment where disintegration takes place. An example of this is the biopolymers that, being part of a structure of compost, once the degradation of the polymer occurs, the compounds from the degradation could enrich the environment.

With the use of biodegradable polymers, in addition to avoiding environmental pollution, the cost of labor for the disposal of plastic waste can be reduced, the useful life of landfills can be increased as a result of the decrease in volume of garbage, and even, it might be possible to recycle them by microbial and enzymatic treatment to obtain monomers and oligomers [15].

In general, polymers that can be degraded fall into one of the following 3 categories:

1) Biodegradable natural polymers.

These types of natural polymers are also often called environmentally degradable polymers, and are produced by living organisms, so in addition to being totally biodegradable, they are also renewable. Within this classification of polymers are polysaccharides (such as cellulose and starch), proteins (keratin, gelatin and silk), polyhydroxyalkanoates that are synthesized by some bacteria, chitosan, polylactic acid and polyglycolic acid. The main drawback of some of these polymers is their high hydrophilicity that directly affects their glass transition temperature, which in turn alters the mechanical behavior of these materials, these limiting factors being for the manufacture of objects with this type of polymers [15]. However, biodegradability is an unparalleled advantage, which is why the world is constantly looking for ways to use this type of polymers in different fields, especially in the medical area because some polymers of this type are also biocompatible. Biocompatibility makes them an optimal alternative to use in applications that have direct contact with animal cells and body fluids without rejection of the material by the organism to which they are applied.

As mentioned earlier, biodegradable polymers have little mechanical stability to manufacture objects with them. That is why various researchers have sought ways to reinforce this type of materials by incorporating nanoadditives that give rise to biocompatible and / or environmentally biodegradable polymeric nanocomposites. The most commonly used particles are pure hydroxyapatite, hydroxyapatite doped with some metal [16], calcium and phosphorus nanoparticles [17], tricalcium phosphate [18], copper sulphide [19], zinc oxide [20], oxide magnesium [21], strontium carbonate [22], montmorillonite [23], carbon nanotubes in conjunction with silver and iron nanoparticles [24], where each type of nanoparticle gives the polymer nanocomposite exclusive properties. Below are some examples of these types of compounds that have been used to generate technical textiles.

Zhou et al [25] developed polyamide acid / collagen compounds reinforced with nanometric size hydroxyapatite, which they used to make nanofibrous scaffolds useful for bone regeneration guided by the electro-spinning technique. Zhou found that this polymeric nanocomposite has improved dimensional stability compared to the pure polylactic acid / collagen compound, this improvement attributed to the presence of the nanoparticle. In addition, the nanocomposite showed to have better cell adhesion, better propagation, proliferation, differentiation, mineralization and expression of osteogenic marker genes compared to other scaffolds, so it is considered as a material with potential application in the medical area. Another example is the mixture of polyhydroxybutyrate with titanium dioxide nanoparticles, as well as its subsequent use in obtaining nonwoven fabric by means of electrospinning. This textile presented greater thermal stability when exposed to degradation temperatures, as well as better resisting aging by ultraviolet radiation due to the presence of titanium dioxide compared to pure polyhydroxybutyrate. This biodegradable polymer nanocomposite was even considered to design biocompatible materials useful as support for cell growth, as well as self-sterilizing packaging material useful for medical tools [26].

Gelatin is another example of a biodegradable and biocompatible natural polymer that has been used to form scaffolds by electro-spinning from highly aligned fibers of gelatin-based nanocomposites with cerium oxide nanoparticles, which act as sequestrants of oxygen oxidizing species. In addition to acting as anti-oxidants, these fibers have been shown to promote the outbreak of neurites and inhibit cell senescence, thereby promoting the development and alignment of neurites. Due to these qualities, the scaffolds of the proposed nanocomposites represent a promising approach to nerve tissue engineering and regenerative medicine [27].

2) Biodegradable synthetic polymers.

They are polymers susceptible to decomposition by biological attack of enzymes. Examples of this type of materials are aliphatic polyesters, which can be hydrolyzed by the action of lipases and esterases, such as poly (ε-caprolactone) (PCL) that can be degraded by the action of the Pencillium spp. [28], which is why it is one of the most used polymers to generate biodegradable synthetic polymer nanocomposites.

The most recurrent applications for this type of polymers are biocompatible scaffolds useful in tissue engineering, especially for bone regeneration [16,22,29], however, nanocomposite of biodegradable synthetic polymers have also been prepared to generate hydrophobic materials and hydrophilic for the transport of medicines [30], fibers for water treatment [24], as well as antibacterial materials for application as packaging [31].

3) Biodegradable polymer blends.

This classification is made up of materials that are the result of mixing polymers that are biodegradable with polymers that are not. These materials result in susceptibility to biodegradation, in addition to the fact that they are usually less expensive compared to pure biodegradable natural polymers. Some examples are those obtained by mixing low density polyethylene with starch or with poly (3- hydroxybutyrate) [32], where the presence of the biodegradable polymer makes the mixture suitable for partial biodegradation, making it an alternative to the use of fully biodegradable polymers, however, more expensive. These polymeric mixtures have been used as a matrix to form nanocomposites, such is the case of the mixture formed by PA-6 and PCL, to which silica nanoparticles were incorporated. This polymer nanocomposite has been used to form fibrous scaffolds useful in tissue engineering specifically for tendons, where silica nanoparticles improve the biological activity of scaffolds, in addition to modifying topography, wetting capacity, stiffness and degradation rate [33].

The main consumer of technical textiles is the automotive industry in which polyester and PA dominate, which together account for 85% of the market [80]. The automotive industry for the textile sector is of great relevance, since, of the total polymers used in the components, 30% are textiles such as seat fabrics, interior upholstery, airbags, filters, bands, among others [81]. Technical threads are used in the manufacture of woven fabrics where properties such as high strength and elongation uniformity, dimensional stability and good abrasion resistance are required. The weaving process guarantees a high utilization of the resistance characteristics of the initial fibrous material from which the fabric is manufactured. Therefore, new challenges are posed according to the weaving conditions for new complex threads and fabrics that the use of nanotechnology brings. The use of composite materials in auto parts continues to evolve, as does research in this field due to high costs compared to some metals such as steel. Polymer matrix composites are regularly used in the development of decorative components such as side panels, trays, etc. And the fibers used as reinforcement regularly are glass, aramid and carbon depending on the application component (see Table 2) [82].

Table 2

Source: The Authors.

Components such as tires and belts require fibers with high tensile strength, in seat and floor fabrics with high abrasion resistance, in flexible components such as high thermal stability filters, and in general properties such as low flammability, UV resistance, heat and fire are a common requirement for materials applied in the automotive industry. On the other hand, most of the textile structures applied to this sector requires finishes and/or coatings before being suitable for final use. For example, the use of the transmission belt or the hose reinforcement requires a surface treatment to improve adhesion to the surrounding plastic or rubber matrix (neoprene). Airbags constructed with PA-6.6 wires require a coating that guarantees non-porous surfaces, and non-explosive inflation, due to the high risk that the heat released from the reaction can generate for gas (nitrogen) to occur and friction which generates the rapid expansion of the material [83]. Ceramic fabric structures must be applied around the combustion chambers, and in the most important structural elements there are materials reinforced with carbon fiber, and aramid for structures of walls, floors, and fuselage. Similarly, fiber reinforced composite materials are used that replace metals for acoustic and anti-vibrational insulation for marine and aircraft vessels in applications that include: hulls, screens and superstructures. It is important to mention that all textile materials, without exception, are subject to a variety of rigorous fire testing regimes that depend on the severity of this risk, therefore, are considered technical textiles.

Fabrics that contain aluminized fiberglass are commonly used. Recently, fabrics containing para-aramid are used that are designed to contain small explosions, as well as fires. In aviation, some jets use polyester in decorative coatings, and it is also common to find more exotic fabrics such as silk and animal hair blends [84]. Some passenger blankets made of non-woven fabric include flame retardant fibers, such as polyester or wool, or modacrylic [85]. The curtains comprise flame retardant polyester or wool fabrics. In this sense, different nanocomposites have been studied in the last decade in order to significantly reduce the release of heat while increasing carbon formation; this has been achieved with materials based on silicon, boron, phosphorus or oxides synergistically [86]. These charges incorporated in the polymer matrix produce improvements in thermal stability, the particle size has a great effect because the interaction of the charge is high, when you have large surface areas and this in turn will depend on the size. Thus, microcomposites require high amounts of the load to achieve significant results, which implies an increase in the density of the original polymer with dramatically altered mechanical properties [87]. The dispersion of these nanoscale charges affects the surface area, as well as the percolation threshold. Lu et al [88] studied various combinations of materials, designs and manufacturing techniques of nanostructured piezoelectric fibers with a hollow polycarbonate core, surrounded by a multilayer coating of polyvinylidene-based compounds with nanoparticles of BaTiO₃, Pb (Zr_0.52Ti_0.48)O₃ or CNT and as conductive layers used low density polyethylene impregnated with carbon). The resulting fibers exhibited excellent durability with high piezoelectric voltages (up to 6 V and ~4 nA current) in a cyclic flex and release test greater than 26,000 cycles. The applications they identified are the following: sound detectors, energy collectors, blood vessels, smart upholstery for car seats or portable materials, among others.