REVIEWS

SYNTHETIC FIBERS WHICH ARE FLAMMABLE TO A LIMITED EXTENT A. V. Volokhina

UDC 677.494:536.468

The:«raditional synthetic fibers which are produced on a huge industrial scale and are based on polymers of organic nature -- polyamides, polyesters, polyacrylonitrile, and polypropylene-- are combustible materials, as is well known. Among the fibers which are limitedly flammable, we may list the following: i) chlorine- and fluorine-containing fibers of the type of polyvinyl chloride, polytetrafluoroethylene (Teflon, Polyfen), and so on; 2) polyamide, polyester, and polyacrylonitrile fibers of the type of nylon 6 (Kapron), nylon 66 (Anid), Lavsan (Dacron, Terylene), Orlon (Nitron), and so forth, which have been modified by various methods and which have fire-protective properties; 3) new synthetic heat-resistant fibers, which, in distinction from those considered above, do not melt and do not undergo combust~on in air without additional modification. They are graphically distinguished4 from fibers of the first group by their infusibility and good textile properties. After appropriate surface treatments, the heat-resistant fibers of several types do not burn eren in an atmosphere which has been enriched in oxygen up to 4075% by wt. Fibers of the third group are most effective for making very important non-burning materials of the textile assortment (decorative finishing articles of the type of furniture coverings, curtains, screens, airplane lounges, ships' lounges, railway lounges, special pro" tective clothing for metallurgists and welders, flight clothing for pilots and cosmonauts, military equipment, etc.). It is these which are examined in the present review. In their resistance to combustion, the heat-resistant synthetic fibers, as materials of organic nature, are considerably inferior to mineral fibers (asbestos, ceramic, glass, metal, or carbon fibers and the like); however, they excel them in textile properties, particularly in resistance to flexing and abrasion resistance. It is weil known that inorganic fibers are heavy materials. As a rule, the density of the heat-resistant fibers is 1400-1450 kg/m 3" Heat-resistant fibers a r e a new, important group of synthetic fibers, prepared by treatment of aromatic heterochain or heterocyclic high-molecular compounds, which are distinguished from the aliphatic fiber-forming polymers used to prepare such fibers as Kapron, Lavsan, Nitron, etc., by a high thermal stability. While the maximum service temperature of traditional synthetic fibers does not exceed 150°C, the thermally resistant fibers can be used for a long time in air at 200-250°C, and sometimes even 300°C. Among the thermally resistant fibers, a group of high-strength, high-modulus materials presents special interest (Kevlar and Kevlar 49, Arenka, Vniivlon and SVM, Terlon, etc.), whose strength is 2 to 3 times as great, and whose modulus is 10-20 times as great as those of the usual synthetic fibers. Although these fibers also do not undergo combustion in air, they are most often used to reinforce constructional plastics, tires, and other technical rubber articles. Thereforè, they will be almost eliminated from consideration in the present communication. Most of the fibers indicated in Table i are used not only as heat-resistant fibers for the filtration of hof industrial gases, high-temperature electrical insulation, making ironing cloth, etc., hut also as materials having a reduced susceptibility to combustion. Kynol and Enkatherm, and also Nomex, Fenilon, PVU, and other fibers modified by thermochemical treatment are inherently "fire-resistant." The physicochemical properties of the traditional and the new heat-resistant fibers are practically identical. As concern~ their ability to burn -- by the oxygen index (O1) -- the heat-resistant fibers considerably excel the widely used textile materials (the Ol values were found experimentally by lighting the material in its upper part): Translated fromKhimicheskie Volokna, No. 4, pp. 6-9, July-August, 1983. article submitted December 24, 1982.

0015-0541/83/1504-0237507.50

Original

© 1984 Plenum Publishing Corporation

237

Fibre Cotton Cotton, fi~eproofed Wool Viscose Acetate Nylon Polyeste~ Polya«ylonitrile Polyplopylene Polyvinyl chloride.,

0I, % 16--19 27--52 24--25 19--19,7 17--18,5 20--23

Literattue sou~ce [4; 6--9; 13; 14; 17] [4; 17] [3; ö; 7; 9; 17] [6; 9; 14; 17] [3; ö; 9; 14; 17]. [3; 6; 7; 9; 15; 17]

20--21

[3;6;7;9; 17]

17--20 17--20 35--45

[3; 6; 7; 9; 14; 17] [3; ö; 7; 14; 17] [3; 6; 7; 9; 14; 17]

It has become customary to consider [7] that polymeric materials having an Ol equal to or less than 26% are flammable, although, as is well known, there is about 21% by wt. oxygen in the air and a material should have an Ol equal to or less than 21% to be combustlble (thereupon, obviously, the lowering of this index when the sample is llghted from beneath is taken into account [18]). Moreover» for some materials which burn unstably, the accuracy in determining the Ol is about 4% overall [17]. Consldering this, it may be stated that the heat-resistant fibers being examined do not burn in air. Conversely, almost all textile fibers areflammable in an air medium. The fireproofed materials are exceptions» for example, cotton which has been subjected to speeial treatment, and materials of the type of PVC or the so-called modacrylic fibers, whieh are prepared from acrylonitrile, vinyl chlorlde, and vinylidene chloride, and which contain a large amount of chlorlne. However, for varlous reasons (change in color, increased stlffness, inadequate resistant to washing, etc.), the first of these still have not received wide circulation, and the second have comparatively low melting or decomposition temperatures. Among the heat-resistant fibers» Nomex has obtained the greatest development; it is produced on an industrial scale at a rather high price (about $14 per kg). The remaining fibers are, as yet, produced on a low-tonnage, experimental, or pilot-plant basis and are apparently still more expenslve. Thus, the initial cost of PVU flber was $200 per ib. [i0], hut even at present this flber is priced at a level of $66 per kg [2]. The only heat-reslstant fiber which approaches the ordinary synthetic fibers in price is a polyoxadiazole fiber of the Oksalon type [i, 2]; however, it cannot be assigned to the fibers which burn to a limited extent. As i s evident from Table 1, i n the case of Nomex, PVU, e t c . , i t i s p o s s i b l e to r a i s e their 0£ values considerably (up to 40-75%) by modification. This modification, as a rule, is effected by a surface treatment of the fibers with halogen- or phosphorus-containing reagents. Thus, heat-resistant fibers which have not been subJected to additional treatment are not inferior to fireproofed textile materials of traditional natural or man-made flbers in their susceptibility to com5ustion, and even excel them. Flameproofed heat-resistant fibers have no equals among materials oforganic nature. Only polytetrafluoroethylene fibers of the type of Teflon, Polifen, or Ftorin have higher Ol values, reaching 95% [7, 17]; however, these fibers melt in a flame. Moreover, in textile properties and comfort they do not satisfy the requirements of customers.

Attempts of some workers have been described in the scientific literature to find correlations between the ability of a polymeric material, including heat-reslstant fibers, to. burn and its other characteristics whlch are readily determinable experlmentally or by calculation. Among these attempts is the work of ran Krevelen [7], who proposed to use the "coked residue" of a material as an index of its degree of nonflammability: 01 x i00 = 17.5 + 0.4 x C, where C is the "coked residue" or the yield of coke formed on heating the material to 850°C. As it turned out» its value can be calculated from the number of carbon atoms in a basic unit of the polymer. For i00 investlgated polymers a rather good agreement wlth experiment was found, since the deviation In all was ±3.5%. However, there are many exceptions from ran Krevelen's emplrical rule. In particular» for polyoxadiazole fiber, which forms a coked residue in the amount of 35-40% by wt., the Ol should be 32-34%~ However~ this fiber burns well in airp and only after the introduction of 3 to 5% by wt. of bromine» chemically bonded to the benzene ring of the polymer» does the polyoxadiazole fiber 0ksalon S acquire the property of not maintaining combustion in air (01 of 25-30% [2» 4]). Asatisfactory correlation has been obtained between the ability of various polymerie materials to burn and their hydrogen contents [18]. However, eren in this case polyphenylene1,3,4-oxadiazole departs from the general rule. At a hydrogen content of 2.8% by wt.» which is comparable with the analog0us figures for aromatic polyimides, Its susceptibility to combustion is far higher. 238

TABLE i. Fibre Nomex Conex

Properties of Heat-Resistant Fibers Company (country) Dupont (USA) Teijin (Japan).

Fenilon USSR Brominated Fenilon • .USSR Durette . Durette NT -4

Monsanto (USA) Dupont (USA)

Starting polymer Polyqn-phenyleneisophthalamide Same

Production volume, Strength at Elongation at metric tons/yeur ..~ " break,%

Literature source

OI,, %

10 000

51--54

15--22

26,7----30

18O--360

44--54

15---50

26---30

[i; 2]

44--45 18,5---31,2

15--25

15,1--19,4

25--30 37--42,5 40--46

[2] [S]

19,6--44,1 33,4

17--20 6

85--38 41

[5]

15-53,9

20--26

42--52 29---35

[5; 8; 10] [2; 3; 6; 7; 9--15]

17,7--29,5

22--26

34--52

[2; 6; 9; 12; 13; 15]

18--54

8--20

28---32

60 45--50 45--50 45

13 6---8 6---8 23

36,5 35 48 38--43

[l; 9; 16; 18] [2; 31 [1; 18] [4] [1; 3; 5--7; 11; 19]

Bromine-containing poly-m-phenyl eneisophthalamideI Prepared by surface thermochomical treat-

m

[I;2; 3; 6--9; I I - --18] [1; 2; 4; 8]

[2; 5; 8; 10--13]

ment of Nomex,

Nomex-T Kynol

Dupont (USA) Carbonmdum

Enkatherm

Enka (Netherlands)

Quermel .

RhonePoulenc f~mnce) Celanese (USA)

PRD-14 Admid Afiraid T RVU~

(USA)

USSR USSR

Celanese (USA)

type fibres Phenol-formalde- Capacity, 150 dahyde polymer metric/tons/day Chelated polyProduction ceased torephthaloyloxbis-amidrazone Polyamidoimide Polyimide Same

u

Polybonzimidazole Announcedconstruction of plant having capacity of 450 metric tom/

m

year in 1982 and .

RVU RVU-T Lola Vniivlon Vniivlon M Tulen

• Prepared by thermochemical treatment of RVU USSR USSR USSR USSR

13,000-15,000 .ton= in 1985.

m

Polyheteroerylene Same

35---40 147--167

2--6 3--7

42---49

[5; I0]

65--75 48--54 43 66 48

[5; I0] [4; 18; 20]

[2; 4] [4l [41

In [3], thecombustibility of polymers is compared with their thermal stability indices, as evaluated by the thermogravimetric analysis method under special conditions. In this work a clear dependence between the OI of a polymer fiber, if lighting is carried out from below, and its decomposition temperature was established, if this decomposition takes place at a rate of 3.3% per min and the atmosphere consists of 1% oxygen by wt. and 99% nitrogen by wt. (heat rate, 15°C per min). According to the results obtained, under severe test conditions (thin materials, lighted from below) only the VVV ladder-structure polymer, from which a fiber with the same name was prepared on a laboratory scale [i, 2], RVU, Kapton polyimide film, and NT-4 fail to burn in an air medium-- the polymers in which the OI exceeds 20.9% by test data (the content of oxygen in air). Even in this case, pDlyphenylene-l,3,4-oxadiazole may be noted as an exception. The thermal stability of the fiber based on it is at the thermal stability level of other fibers from heterocyclic polyarylenes, but its ability to burn is far higher. Apparently the reason for this is a specific mechanism for the decomposition of the polymer in a flame. In [12], the combustibility index (CI) of a polymeric material is calculated from its specific heat capacity, Cp, the ignition temperature, t~, a n d the specific heat of combustion, AH, using the formula CI ffi Cpti/AH , thereupon OI ii CI + 0.009. Obviously the correlations examined should be taken as very simplified and approximate. In all cases the OI figure was used as the only combustibility characteristic of the polymeric material; this has not always been estimated sufficiently accurately. At an equal fiber OI, in fabrics based on them which have an identical weave and weight, the rate of flame propagation may be different [4]. Other, although less universal than the OI, criteria for evaluating the fire risk of materials are known (flame height, length of burned section, dummy tests, etc.) are also known. It is only necessary to stress that most of the thermally resistant fibers whose properties have been briefly examined in the present communication excel other synthetic fibers, even with respect to all these criteria.

239

The problem of devising an incombus=~ble fibrous polymeric material for broad utilization cannot be considered solved, even in this case. The fact is that such a material should be cheap, and on heating in a flame should not evolve toxic gases or a large amount of smoke. It is necessary to carry out a wide search in thls direction in the future. This problem has first place amonE other problems in promising investigations in the synthetic fiber field [22]. Apparently it is easier to solve it along the path of devising thermally resistant fibers, although the problems are posed differently thereupon. A high thermal stability of a fiber presupposes preservations of its mechanical properties at elevated temperatures for a long time thereupon; and the ability of a polymer to resist burning is connected with the behavior of the material under critical situations upon ignition. But the two problems are largely combined. The basic demands on heat-resistant fibers are these: infusibility of the polymer, high thermo-oxidative decomposition temperature, low hydrogen content in the basic polymer unit; these are common for thermally resistant and for incombustible materials. Therafore, as a rule, thermally resistant fibers are also synthetic fibars which have limited flammability, CONCLUSlONS As a result of examining the literature data, one can draw the conclusion that the new synthetic fibers which belong to the class of thermally resistant fibers are very resistant with respect to a flame. Certain forms of heat-resistant fibers do not maintain comhustion eren in a oxygenenriched atmosphere, up to 40-75% by wt. LITERATURE CITED i. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

240

A. V. Volokhina and V. D. Kalmykova, Itogi Nauki i Tekhniki» Ser. "Khim. Tekhnol. Vysokomol. Soadin.," VINITI, Moscow, 15, 3-71 (1981). G . A» Budnitskli, G. I. Kudr~avtsev» G. G. Frenkel', T. I. Shein, and A. M. Shchetinin (Review Information)., NIITEKhlM, Moscow (1978). D. E. Stuetz, A. H. Di Edwardo, et al., J. Poly, Sci., Poly. Cham. Ed., 18, 667-985, 987-1009 (1980). A. F. Zhevlakov, Yu. M. Groshev, A. V. Volokhina, I. S. Ermakova» M. A. Zharkova» and A. M. Shchetinln, Khim. Voloknaj No. i, 41-42 (1982). N. J. Abbott, Ekspress. Inform. "Termostoikie Plastiki»" No. 38» 11-15 (1978). D. W. ran Krevelen, Angew. Makromol. Chem.» 22, 133-157 (1972). D. W. ran Krevelen» Polymer, 16, No. 8, 615-620 (1975). O. I. FetisovD A. S. Chegolya, E. P. Krasnov, L. N. Zuhovj and I. N. Kullkova» Second International Symposlum on Man-Made Fibers, Kallnin (1977)» Preprlnts» Vol. 4, pp. 69-79. H. Görlach» Chemiefasern Text. Ind.» No. 7» 611 (1972). E. R. Kaswe11, Lenzinger Ber., 33, 12-22 (1972). C. E. Hathaway and C. L. Early, J. Appl. Polym. Symp., No. 21, 101 (1973). J. H. Ross, Ekspress. Inform. "Termostolkie Plastiki," No. 22, 13 (1975). Can. Text. J., 90, No. 6, 107-110 (1973). J. Soc. Fiber Sci. Technol, Jpn., 28» No. 9, 359 (1972). F. C. A. ran Berkel, J. Appl. Polym. Symp., No. 21, 67-80 (1973). R. Pigeon and P. Allard, Angew. Makromol.~ Chem., 40-41, 139-158 (1974). J. L. Ysaacs, Ékspress. Inform. "Pozharnaya Okhrana»" No. 44, 6-10 (1975) [sic]. A. F. Zhevlakov, I. A. Bolod'yan, A. S. Melekhov, and V. A. Tret'yakov, Khim. Volokna, No. 5, 28-30 (1976). Text. World, No. 10, 30 (1980). Khim. Volokna, No. 3, 36-37 (1975). K. Kisbore and Das K. Moban, Colloid Polym. Sci., 258, No. 1, 95-98 (1980). P. W. Morgan, J. Makromol. Sci., Chem., A15 (6), 1113-1131 (1981).