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Inherently fire-resistant fibers

There are some commercially available synthetic fibers that do not easily melt or catch fire. These fibers are called inherently fire-resistant [4,258]. The commonly used inherently fire-resistant polymeric fibers are aramid, poly(aramid-imide), polyimide, polybenzimidazole (PBI), polybenzoxazoles, melamine-formaldehyde, phenolic, chlorinated, fluorinated, polyphenylene sulfide, polyetheretherketones, polyether imide, polyacrylate, and semi-carbon. There are also a few inherently fire-resistant inorganic fibers such as glass and ceramic. In order to develop high-performance thermal protective clothing, it is essential to gain a thorough scientific knowledge regarding these inherently fire-resistant fibers.

Aramid: Aramid belongs to the nylon fiber family, and this is an aromatic polyamide-based synthetic fiber. Aramid fiber is mainly produced from various aromatic diamines and diacids or diacids chlorides. This facilitates the generation of aramid with an extensive variation in its molecular structure. The most common aramid fibers are synthesized via a low-temperature polycondensation-based reaction of phe- nylene diamine and terepthaloyl chloride [259-261]. In this aramid fiber, at least 85% of the amide (-CONH) linkages are attached directly to two aromatic rings (Fig. 4.8). As the aromatic groups are all linked and highly oriented in a backbone chain, this fiber does not easily break down into ignitable molecular fragments. The presence of hydrogen bonding in aramid fibers also affects the orientation of polymer chains and provides a basis for improving the orientation and crystallinity of the fibers with heat treatment. This property makes the aramid fiber highly fire-resistant, and it produces little smoke when heated [262,263]. Additionally, the high dissociation energies of C-C and C-N bonds in the main polymer chain (Fig. 4.8) give high thermal stability to aramid fibers.

Aramid fiber

Fig. 4.8 Aramid fiber.

Furthermore, all the aromatic groups can be linked in different positions along the backbone chain. If all the aromatic groups are linked in 1 and 3 positions along the backbone chain, it is called meta-aramid fiber (Fig. 4.9). On the other hand, when all the aromatic groups are linked in 1 and 4 positions along the backbone chain, it is called para-aramid fiber (Fig. 4.10). Both meta-aramid and para-aramid fibers possess no thermoplasticity, and have good tensile as well as other mechanical properties; thus, these fibers are suitable for manufacturing thermal protective clothing. However, meta-aramid fibers become damaged during prolonged exposure to ultraviolet (UV) radiation from both sunlight and artificial light sources, depending upon the UV wavelength, exposure time, radiation intensity, and fiber properties. The double bonds present in meta-aramid fiber absorb the UV rays in the presence of air, causing photodegradation and producing carboxylic acid [264]. It has also been observed that para- aramid fibers do not break down into ignitable molecular fragments as quickly as meta-aramid fibers. This is because para-aramid fibers comprise extreme symmetry (rod-like or straight structure identical to silk protein) within its polymer chains to a greater extent than meta-aramid fibers. This results in greater interchain links, hydrogen bonding, and crystallinity within para-aramid fibers. Additionally, para-aramid fibers can switch between cis and trans confrontation. In the trans confrontation, the fiber is completely stretched out, which allows both amorphous and crystalline regions to come together in a closely packed array and possess a high flexibility. Hence, the tensile strength, rigidity, and moduli of para-aramid fibers are higher than meta-aramid fibers. The increased structural chain crystallinity also raises the glass transition temperature of para-aramid fibers up to 340°C, which is higher than that of meta-aramid fibers (275°C). As a result, the fire-resistivity of para-aramid fibers becomes higher than meta-aramid fibers [258,266]. However, the thermal degradation (>375°C) of para-aramid fibers under radiant heat and/or flame produces carbonaceous char that is similar to the char produced by meta-aramid fibers. Thus, the LOI values of meta-aramid and para-aramid fibers are similar, 28-31%. Despite this, the improved tensile properties of para-aramid fibers, coupled with this charforming ability, indicate they can resist higher temperature and heat fluxes than meta-aramid fibers, and para-aramid fibers are more widely used than meta-aramid fibers, especially where protection to firefighters from intense radiant heat and/or flame is the primary concern. In this context, it’s worth noting that the para-aramid fiber has a poor compression, tendency to fibrillate, poor transverse strength, and less abrasion resistance. These properties can be detrimental to providing proper thermal protection to firefighters.

Meta-aramid fiber

Fig. 4.9 Meta-aramid fiber.

Para-aramid fiber

Fig. 4.10 Para-aramid fiber.

Commercially, the most popular brand of meta-aramid fiber is Nomex, which was developed by DuPont in 1960 and marketed in 1967; other accepted meta-aramid fiber brands are Conex by Teijin and Apyeil by Unitika. Similarly, the most popular para- aramid fiber is Kevlar, which was also developed by DuPont in 1965 and marketed in 1970; other accepted para-aramid fiber brands are Twaron by Acordis and Technora by Teijin. The behaviors of these fibers have been widely investigated through various techniques (X-ray diffraction, scanning electron microscopy, tensile testing, and weight change), and it has been found that the Nomex fibers shrink or break in the presence of very intense temperatures [266]. Due to this limitation, a new fiber with a blend of 95% Nomex and 5% Kevlar fibers was developed by DuPont. This newly blended fiber is highly fire-resistant and commercially marketed under the trade name Nomex-III [267]. Notwithstanding these advantageous features of Nomex-III in comparison to normal Nomex, Song [268] mentioned that Nomex-III can also significantly shrink in a 4 s exposure to a 2 cal/cm2 flash fire. Furthermore, all the blended aramid fibers (Nomex and Kevlar) in Nomex-III are very expensive. Thus, the Nomex and Kevlar fibers are often blended with lower-cost fire-retardant fibers, namely a fire- retardant viscose. One of the most common blended fibers developed by DuPont is Karvin (30% Nomex, 5% Kevlar, and 65% fire-retardant viscose) [269]. This newly developed blended fiber comprises fire retardancy similar to the meta-aramid fibers. However, the formed char by this fiber under thermal environments is weaker in structure. Hence, this fiber does not offer the sustained fire protection at high heat fluxes and temperatures that 100% meta-aramid fiber does. As a consequence, the Nomex and Kevlar fibers are further blended with some other antistatic fibers (eg, P14 by DuPont) to enhance the char tensile strength. For example, 93% Nomex and 5% Kevlar are blended with 2% P14 to produce Nomex-IIIA for firefighters’ protective clothing; 75% Nomex and 23% Kevlar are blended with 2% P14 to produce DeltaT for firefighters’ protective coveralls. Recently, a combination of Conex and Technora fibers called X-Fire has been developed by Teijin. This fiber is capable of resisting temperatures up to 1200°C for 40-60 s and widely used in thermal protective clothing.

Poly(aramid-imide): Poly(aramid-imide) fiber is a type of meta-aramid fiber. The surface of this fiber is very smooth and almost circular in cross section. The shape and modulus of this fiber makes it suitable to use for clothing purposes [270,271]. A commercially available poly(aramid-imide) fiber is Kermel, which was developed in France by a company called Kermel in 1971. The Kermel fiber is produced through polycondensation of toluene diisocyanate and trimellitic anhydride [271]. The chemical structure and fire-resistant property of Kermel resemble the Nomex fiber. The chemical structure of Kermel fiber comprises a high proportion of aromatic groups and combined double bonds (Fig. 4.11). However, the chain segment of this fiber is not very symmetrical; thus, the crystallinity of this fiber is low. Fig. 4.11 also demonstrates that the poly(aramid-imide) fiber comprises imide nitrogen (>N-). This imide nitrogen molecule introduces the rigid and stable heterocyclic rings within the polymer chain; it also lacks the active flammable hydrogen present in the polyamide functional groups. Due to all these structural features, the LOI value of Kermel is 33%; hence, the fire-resistant property of Kermel fiber is almost similar to Nomex fiber. Therefore, the fire resistivity and heat dissipation properties of Kermel fiber are very good [272].

Poly(aramid-imide) fiber

Fig. 4.11 Poly(aramid-imide) fiber.

Although Kermel fiber is good for thermal protective clothing, it is very costly. Therefore, this fiber is frequently blended with fire-retardant viscose fiber to produce underwear; for the outer shell fabrics, Kermel is often blended with Twaron, as a competitor of Nomex/Kevlar. Commonly, 50% Kermel is blended with 50% fire-retardant viscose fiber. The comfort of this blended fiber is very high and LOI value varies within 29-32% [4]. Since 1971, much research has also been conducted to improve the tensile properties and dyeability of Kermel fiber. Through this research, a high performance version has been developed, named Kermel Tech. This newly developed fiber comprises better strength and abrasion resistance than the original Kermel fiber; consequently, this fiber might be more suitable for manufacturing thermal protective clothing. Recently, a high performance composite yarn called KermelHTA was developed by combining a para-aramid fiber core (35%) and a Kermel fiber wrapping (65%). This high modulus yarn can also be used to manufacture thermal protective clothing.

Polyimide: Polyimide is the polymer of imide monomer. This imide monomer is produced through a condensation reaction of an aromatic tetracarboxylic dianhydride and an aromatic diamine. One of the most popular polyimides is marketed by DuPont under the trade name PRD-14. This PRD-14 fiber is produced through the chemical reaction between pyromellitic anhydride and a diamine similar to para-phenylene diamine, 4,4'-diaminodiphenyl, 4,4'-diaminodiphenylether, substituted derivatives, or a mixture thereof. Here, an imide monomer comprises two carbonyl groups that are bound to nitrogen; in Fig. 4.12, a general linear imide functional group is presented, where two acyl groups are attached to -NR. As the chemical structure of PRD-14 fibers comprises nitrogen, this fiber is highly fire-resistant. This chemical structure also contains double bonds, which indicates this fiber does not easily disintegrate in the presence of fire [273,274]. As the number of single bonds per unit length of the PRD-14 chain is lower than the aramid chain, the rigidity and glass transition temperature of the PRD-14 fiber is significantly higher than aramid fibers. The PRD-14 fiber also provides greater resistance to pyrolytically induced chain scission reaction under thermal environments in comparison to aramid fibers. Due to these features, the PRD-14 fiber rarely starts to break down below 500°C under nitrogen and 450°C in air. This shows that the fire-resistant property of the PRD-14 fiber is much higher than any aramid fiber.

Polyimide fiber

Fig. 4.12 Polyimide fiber.

Another very popular, commercially available polyimide fiber is Lenzing P84, developed by an Austrian company of the same name. The LOI value of Lenzing P84 fiber is 36-38% [275]. Therefore, this fiber is suitable for thermal protective clothing. Additionally, this fiber is soft and silky in nature and has acceptable mechanical properties like strength, modulus, etc. These added properties make this fiber more suitable for clothing purposes. This fiber is also compatible to blend with fire-retardant viscose or aramid fibers. Through blending with the viscose (50% Lenzing P84 and 50% viscose), the LOI value of the fiber can reach up to 35%. The resultant fiber also shows a greater moisture absorbency, which is one of the prime requirements for developing thermal protective clothing. The wear and tensile characteristics of the Lenzing P84 fiber can be improved when it is blended with high- tenacity aramid fibers. These newly developed blended fibers are also cost-efficient.

Polybenzimidazole: Polybenzimidazole or PBI is a commercially available, highly fire-resistant fiber. This fiber was developed in the 1960s by Celanese Americas in collaboration with the United States Air Force Materials Laboratory (AFML). PBI is manufactured through chemical reaction of tetra-aminobiphenyl (TAB) and diphe- nylisophthalate (DPIP) (Fig. 4.13) [276].

Fig. 4.13 show that PBI fiber comprises nitrogen, which makes it fire-resistant. Moreover, several aromatic groups are present in a ladder-like manner within its chemical structure. Thus, this fiber does not disintegrate easily in the presence of intense temperature. All these properties make the LOI value of PBI fiber >41%,

Polybenzimidazole (PBI) fiber

Fig. 4.13 Polybenzimidazole (PBI) fiber.

and the fiber can easily withstand temperatures as high as 600°C for a short time, and 300-350°C for longer exposures. Besides the fire-resistant property, PBI fiber has other properties, namely good tensile strength, chemical resistance, moisture regain, softness, and handle, which make this fiber suitable for thermal protective clothing [4,277-279]. In order to increase the flame stability of PBI, it is sometimes treated with sulfuric acid. It has been found that sulfonated PBI shrinks <10% when exposed to direct flame. By substituting sulfuric acid treatment with a phosphoric acid treatment, the flame stability of PBI fiber can be enhanced at a greater rate. This type of phosphonated PBI can be used to develop thermal protective clothing, especially when protection of a firefighter from a very intense thermal environment is the top priority. This fiber could also easily process in conventional textile machinery, which enhances the compatibility of this fiber to blend with other fire-retardant fibers; in many cases, PBI fiber improves the processibility of its partner fibers as well. The products made from PBI blended fiber (eg, PBI Gold, a blend of PBI (40%) and Kevlar (60%)) shows superior flame resistance, soft-hand, and a cotton-like feel. Due to high fire resistivity, mechanical (puncture, tear, and rip) resistivity, and chemical stability along with good textile processibility characteristics, PBI blended fibers have secured a unique position among all the inherently fire-resistant fibers. In the United States and the United Kingdom, PBI Gold fiber is being widely introduced into clothing, underwear, hoods, socks, and gloves for firefighters.

Polybenzoxazoles: This fiber is produced through polymerization of benzoxazoles monomer. Presently, one commercial example of polybenzoxazoles fiber is poly- para-phenylene benzobisoxazole or PBO or Zylon. This Zylon fiber is manufactured by Toyobo [280]. In the chemical structure of this Zylon fiber, it has been found that the bulk aromatic groups of benzoxazoles monomer are attached with its backbone chain (Fig. 4.14). As this Zylon fiber comprises aromatic groups as well as a rigid benzoxazole segment, this fiber is very rigid [281].

Polybenzoxazoles fiber

Fig. 4.14 Polybenzoxazoles fiber.

Fig. 4.14 shows that the chemical structure of Zylon is a linear symmetrical repeating aromatic structure; therefore, it has a very high thermal stability [282,283]. This fiber is inherently fire-resistant due to the absence of any aliphatic -CH group. As a result, the decomposition temperature of this fiber is nearly 650°C, and its LOI value is >68%. It has also been found that a negligible mass loss occurs when Zylon is heated to 500°C in a thermo gravimetric analysis instrument, compared with meta-aramids and para-aramids. The tensile strength and modulus of Zylon are also about twice as high as respective values for meta-aramids and para-aramids [284]. Additionally, although Zylon fiber has hydrolysis problems, this fiber can significantly retain its tenacity when heated at temperatures up to 250°C. These properties make the Zylon the most thermally stable and fire-resistant fiber among all organic polymer-based fibers commercially available on the market. Sometimes, Zylon is also blended with para-aramid fibers. For example, a blend of Zylon and Technora with a Super Shelltite finish is called Millenia, and the cost-efficient blended Millenia fiber can be used in thermal protective clothing.

Melamine-formaldehyde: This fiber is produced through the reaction of melamine and formaldehyde at pH 8.0-9.5 under general acid-base catalysis. This reaction is slightly exothermic, and it primarily forms the methylol compounds. At elevated temperatures, methylol compounds react with each other by oligocondensation. This condensation process eliminates the water and formaldehyde in order to form a threedimensional network of a typical thermosetting resin of methylene ethers. This reaction turns into a high viscosity solution at high temperatures with a sufficient reaction time. This highly viscous solution (300-3000 poise) is extruded through the spinneret to manufacture melamine-formaldehyde filament/fiber. The chemical structure of this fiber is shown in Fig. 4.15. This fiber is marketed under the trade name Basofil by BASF, Germany.

Melamine-formaldehyde fiber

Fig. 4.15 Melamine-formaldehyde fiber.

One of the main characteristics of this fiber is that it is very sensitive to thermal environments. In the presence of radiant heat and/or flame, this fiber continues to polymerize and forms cross-linking between reactive side groups. Due to this cross-linking, the LOI value of this fiber becomes 32% along with a low thermal conductivity and an excellent dimensional stability. Additionally, this fiber has several important properties, like good chemical resistance, high tensile strength, and elasticity, which demonstrate its usefulness to the production of thermal protective clothing. This fiber can also be blended with para-aramid fiber. For example, a combination of 40% Basofil and 60% Kevlar is one of the pioneer blended fibers that can be used in firefighters’ protective clothing.

Phenolic: Phenolic is also known as Novoloid fiber. In the commercial market, the most popular phenolic fiber is available under the trade name Kynol, which is developed by the Kynol Corporation of Japan. Kynol fiber is amorphous and cross-linked and made from phenol-aldehyde (Novolac) [285]. This fiber generally has an elliptical cross section and is light gold in color. This fiber feels soft and is available with or without crimps in different lengths [286]. The chemical structure of this fiber contains ^76% carbon (C), 18% oxygen (O), and 6% hydrogen (H), which is shown in Fig. 4.16. Due to the high carbon content (76%), this fiber can be used as an excellent precursor for carbon or activated carbon fibers and textile materials.

Phenolic or Novoloid (Kynol) fiber

Fig. 4.16 Phenolic or Novoloid (Kynol) fiber.

According to Fig. 4.16, Kynol fiber comprises a lot of bulk aromatic phenolic groups; therefore, the thermal conductivity of this fiber is also very low. Consequently, this fiber has a very high LOI value (30-34%) and ignition temperature (150°C in air and 250°C without air); in other words, this is a highly fire-resistant fiber. Also, this fiber rarely shrinks and generates minimal smoke at high temperatures. The Kynol fiber contains only carbon, hydrogen, and oxygen; therefore, the smoke generated from this fiber comprises water vapor, carbon dioxide, and carbon char. As no toxic gases such as hydrogen cyanide (HCN) and hydrogen chloride (HCl) are formed at the time of smoke generation, this fiber does not have any harmful effect on firefighters [264,287].

Similar to Kynol, another phenolic or novoloid fiber is Philene, developed by the R&D center of the Glass Wool Division of Isovar Saint Gobain, France. Philene is a highly cross-linked phenolic resin and an aromatic glassy polymer with a high carbon content of 72% by weight. This fiber is directly produced by thermosetting and centrifugal spinning, which results in a fine, very long, and naturally crimped fiber, with a circular cross section. Due to all these properties, the LOI value of Philene is 39%. This high LOI value makes this fiber suitable to produce high-performance thermal protective clothing [4].

Chlorinated: Chloro fiber is mainly produced from chloride. One of the main fibers of this family is polyvinyl chloride or PVC. PVC fiber is manufactured by suspension polymerization of vinyl chloride (Fig. 4.17) [288]. This fiber is nonflammable, and it does not burn or emit flames or release molten incandescent drops to spread fire to other combustible materials. Similar behavior is observed in the case of polyvinylidene chloride fiber that is developed through suspension polymerization of vinylidene chloride (Fig. 4.18) [289]. The chemical structures of PVC or polyvinylidene chloride fibers comprise polymeric repeat unit (-CHCl, -CCl2), which creates a high degree of chain order and thermal resistance. However, a high degree of order reduces the textile processibility of these fibers. In order to enhance the processibility, these fibers are frequently copolymers with other vinyl or acrylic co-monomers (eg, vinyl chloride, acrylonitrile, methyl acrylate). The fire-resistant property of PVC, or polyvinylidene chloride fibers, can be increased by incorporating the flame-retardant materials within the fiber structures as additives or finishing agents [290].

Polyvinyl chloride fiber

Fig. 4.17 Polyvinyl chloride fiber.

Polyvinylidene chloride fiber

Fig. 4.18 Polyvinylidene chloride fiber.

In the commercial market, PVC fiber is marketed under the trade name Rhovyl. This fiber is developed by Rhone-Poulenc Group of France. This inherently fire- resistant Rhovyl fiber (LOI value >41%) has properties such as good moisture management, easy care machine washability, quick drying, and high chemical resistance.

One of the disadvantages of this fiber is that it produces hydrogen chloride in contact with radiant heat and/or flame, which is both toxic and corrosive to human beings.

Fluorinated: Fluorinated fiber is produced from fluoride. Although many fluori- nated fibers (polyvinyl fluoride, polyvinylidene fluoride) are available in the market, the most generic example of fluorinated fiber is polytetrafluoroethylene (PTFE). The PTFE fiber is produced through suspension polymerization of tetrafluoroethylene. The chemical structure of PTFE fiber is shown in Fig. 4.19 [4]. Apart from PTFE, the fluorinated fibers such as polyvinyl fluoride (PVF) and polyvinylidene fluoride (PVDF) are also widely used in the commercial market.

Polytetrafluoroethylene fiber

Fig. 4.19 Polytetrafluoroethylene fiber.

According to Fig. 4.19, PTFE fiber is comprised of a carbon-fluorine bond. Because of the aggregate effect of the carbon-fluorine bond, the fire-resistance property of PTFE fiber is very high. It has been observed that the LOI value of PTFE fiber is 98%. Additionally, various features (eg, inherent stability, unreactability of the polymer chain, efficiency of intermolecular forces, symmetrical chain order) make this fluorinated fiber highly chemical-resistant. Because the LOI value and chemical resistance of this fiber are very high, this fiber is mainly used for industrial applications such as cookware, air filtration, etc. Some research has been conducted to develop thermal protective clothing from this fiber [4,291]. It is speculated that its high thermal resistance, along with the chemical stability of fluorinated fiber, can make PTFE fiber suitable to use for thermal protective clothing.

Polyphenylene sulfide: In polyphenylene sulfide (PPS) fiber, the aromatic phenol groups within the polymeric chain bond together with sulfur groups. Due to the presence of aromatic as well as inert sulfur groups, PPS fibers have a good fire resistance. The most easily available PPS fiber is Ryton, developed by the Philips Fiber Corporation. Ryton fiber is produced through polymerization of phenylene sulfide (Fig. 4.20) [4]. Other companies such as Toyobo, Toray, and Celanese have produced PPS fiber under the trade names Procon, Torcon, and Fortron, respectively.

Ryton fiber has a moderately high LOI value of 34-35%, and its melting temperature is 285°C. Additionally, this fiber is highly chemical-resistant (acids, alkali) and mildew-preventive. Thus, Ryton can be used to manufacture thermal protective clothing [4]. However, the application of this fiber for thermal protective clothing is dependent upon the level of applied stress on this clothing during firefighting activities. In this context, a notable point is that the oxidation of PPS fiber can convert it into

Polyphenylene sulfide (Ryton) fiber

Fig. 4.20 Polyphenylene sulfide (Ryton) fiber.

polyphenylene oxide (PPO), which is a highly fire-resistant fiber (LOI = 68%) and can be used in thermal protective clothing.

Polyetheretherketones: Similar to PPS fiber, the aromatic ketone groups within a polymeric chain bond together with oxygen groups in polyetheretherketones fiber. As a consequence, this fiber has a repeat unit of one ketone and two other groups in its chemical structure. This provides a linear, fully aromatic, and highly stable structure containing carbon, hydrogen, and oxygen atoms (Fig. 4.21) [292]. This fiber is marketed by different companies with different names, such as BASF’s PEKEKK, DuPont’s PEKK, and Hoechst’s PEEKK and PEEK. The manufacturing of this fiber faces several challenges such as difficulty in polymerization and the high cost of purifying polyetheretherketones.

Polyetheretherketones fiber

Fig. 4.21 Polyetheretherketones fiber.

As the structure of polyetheretherketones fiber is comprised of bulk aromatic ketone groups, the fire-resistant property of this fiber is relatively high (LOI value = 35%). This fiber emits the lowest levels of smoke and toxic gases. Along with fire resistivity, the chemical and abrasive endurance of this fiber is also trustworthy. Due to these good resistive powers, this fiber can be useful for various purposes in the automotive, aerospace, and related industries [293]. To date, no thermal protective clothing has been developed using the polyetheretherketones fiber. However, the sewing thread produced from this fiber is widely used to manufacture thermal protective clothing. Also, polyetheretherketones are used to produce the hook and loop fasteners for thermal protective clothing.

Polyether imide: Similar to polyetheretherketones fiber, polyether imide (PEI) is also a high performance fiber [294]. Although the temperature-resistant property of PEI fiber is less than polyetheretherketones, PEI fiber is cheaper than polyether- etherketones. The commercially available and widely accepted PEI fibers are Teijin and Acrodis [295,296]. The chemical structures of these two fibers are alike, as shown in Fig. 4.22.

Polyether imide fiber (Teijin or Acrodis)

Fig. 4.22 Polyether imide fiber (Teijin or Acrodis).

According to Fig. 4.22, the Teijin and Acrodis fibers comprise an irregular polymer chain. This makes such fibers amorphous in nature with moderate tenacity (0.25 N/ tex) and elongation (40%). As Teijin and Acrodis fibers comprise imide as well as aromatic groups in their structures, these fibers could resist temperatures up to 190°C. Additionally, the LOI values of these fibers are ^45%, which is higher than polyetheretherketones fibers. This high LOI value represents good stability at high temperatures and resistance to oxidation. Therefore, these fibers might be useful to manufacture thermal protective clothing [273].

Polyacrylate: The polyacrylate fiber is produced through polymerization of acrylate monomers. Acrylate monomers are esters, which contain vinyl groups. In the vinyl groups, two carbon atoms (a, P) are double-bonded to each other and are directly attached to the carbonyl carbon (Fig. 4.23) [4].

Polyacrylate fiber

Fig. 4.23 Polyacrylate fiber.

It is evident from Fig. 4.23 that polyacrylate fiber has a lot of cross-linking. This cross-linking makes the polyacrylate a strong and highly fire-resistant fiber. As the

LOI value of this fiber (43%) is very high, this fiber does not easily melt or burn. It also does not emit harmful gases in the presence of thermal environments [297].

Semi-carbon: This fiber is produced through partial oxidation of polyacrylonitrile polymer using itaconic acid as a co-monomer [298]. Through this oxidation process, partial carbonization of this polymer occurs. Additionally, the co-monomer assists in promoting crystallization of the pendant nitrile groups of polyacrylonitrile (Fig. 4.24) [4]. After crystallization (process shown in Fig. 4.24), density of the polyacrylonitrile fiber significantly changes. This density is directly proportionate with the degree of crystallization, and the crystallized polyacrylonitrile is called semi-carbon fiber. Carbonization and crystallization of viscose fiber can also be used to manufacture semi-carbon fiber; however, this process is cumbersome as well as costly. Thus, this is not a widely used industrial process to manufacture semi-carbon fiber [4].

Crystallization process of polyacrylonitrile

Fig. 4.24 Crystallization process of polyacrylonitrile.

Generally, the highly crystallized structure of semi-carbon fiber shows a greater resistance to temperature. It has been observed that the LOI value of this fiber can reach up to 55% and can resist temperatures up to 1000°C; this fiber also possesses no smoke, toxic gases, or afterglow even during and/or after intensive flame exposure. Additionally, this fiber has other physical properties such as good tensile strength, surprisingly high moisture retainment, etc. However, this fiber fairs poorly in terms of characteristics such as color, abrasion resistance, handling, and wear. Due to these shortcomings, this fiber is frequently blended with other chemically modified fire-retardant fibers. This fiber can be blended with fire-retardant wool fiber in order to manufacture thermal protective clothing [245,299]. Sometimes, this fiber is also aluminized to produce structural firefighters’ clothing.

The commercially available Panotex fiber (developed by Universal Carbon Fibers Ltd.) is one of the pioneer examples of semi-carbon fibers, and this fiber is highly fire- resistant as well as less expensive. These features make the Panotex fiber suitable for thermal protective clothing. Unfortunately, this fiber is black in color, so rarely used alone. Furthermore, this fiber degrades in the presence of oxygen during prolonged exposures at 210°C and is also vulnerable to flexing, crushing, and abrasion. Some other brands of semi-carbon fibers (Celiox by Celion Carbon Fiber, Pyron by Zoltek Carbon Fiber, Panox and Sigrafil by SGL Group, Grafil by Grafil Incorporation) are also commercially available and used in firefighters’ clothing and accessories (gloves, helmet).

Inorganic (glass and ceramic): Glass and ceramic fibers are mainly manufactured from silica-(Si) based compounds such as silicate, silane, carbosilane, silazane, car- bosilazane, borosilazane, siloxane, tremolite, amosite, and chrysotile. Apart from silica, these compounds must also possess sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These compounds are melted, polymerized, cured, and pyrolysed to produce the fibers. There are few popular brands of these fibers, namely, S-1Glass and S-2Glass by AGY, and Nicalon and Hi-Nicalon by Nippon Carbon Corporation Limited. Because these fibers are manufactured from inorganic substances, it is obvious that they possess high strength/tenacity, flame resistivity, and so on. Glass fibers can retain 25% of their initial strength even at 540°C. The properties of these fibers can also be enhanced by chemical treatments. For example, colloidal graphite- and silicone oil-treated glass fiber can sustain a temperature of higher than 400°C for several minutes; the melting temperature of aluminum-treated glass fiber can reach above 1500°C and the radiant heat resistivity of the fiber can also be increased. Basalt, one kind of glass fiber, (composition of mineral, plagioclase, pyroxene, and olivine) can also be used in thermal protective clothing. The melting temperature and LOI of basalt fiber is 1450°C and 70%, respectively. Thus, this fiber can effectively resist temperatures up to 700°C. Additionally, basalt fiber is noncombustible, explosion-proof, and produces no toxic gasses even after long-term exposure in intensive thermal environments. These properties add extra features to developed clothing for firefighters.

 
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