THERMAL BEHAVIOR OF FABRICS A T FLAMING TEMPERA TURES SYDNEY COPPICK’, JAMES M. CHURCH, AND ROBERT W. LITTLE2 Columbia University, New Yorlr, N. Y.
&
T h e protective quality of a fabric may be defined, from a thermodynamic viewpoint, as a function of the quantity and intensity of the heat supplied and generated, the area over which it is effective, and the coefficient of heat transfer. Damage or injury to personnel results from the
calorific transfer through and propagation along the fabric. Tests show that because their thermal conductivity is similar, cellulose fabrics which are adequately flameproofed and glow-retarded have thermal protection qualities equal to fabrics made from the most inert fibers.
1
Flaming source 4AH1
T IS well known that normal cellulosic material will burn quite vigorously when subjected to an instigating source of high intensity. Propagation of the flame along the burning cellulose or the continued glowing of the residual charred product constitutes a potential hazard to material or personnel in the vicinity. This hazard becomes critical if the cellulose is in the form of fabrics where the state of division of the combustible material and the available air supply are ideal for rapid combustion. The maximum danger occurs when such fabrics are used as apparel (10, l l ) , and since the majority of textiles are cellulosic in nature, consideration of the burning phenomenon and its retardation is deemed advisable.
Volatile decomposition products
i
(2)
+ 02 +
+ HzO + AHs Solid decomposition products + +CO + ( 2 0 2 + AH, 0 2
(3)
(4)
4
AH( 2’)
IC (thermal conductivity) d
-= dAH5(Ta)
dt
(5)
1
dt
Here, the fabric may be considwed to undergo an initial pyrolysis with the production of both volatile and nonvolatile products. The decomposition products are then oxidized and the sum total of the heats evolved in the reactions together with that supplied by the source are available for transfer and constitute the hazard. The final danger is, of course, the quantity, intensity, and rate of this thermal transfer to the body, This is dependent not only on the thermal conditions at the fabric surface but also on the heat transfer coefficient of the solid fabric residue, if any, remaining after contact with the flame.
iilthough no mechanism or process is known whereby cellulose might be made inert to flaming temperatures, there are numerous means for reducing the hazard caused by burning fabrics ( I , 7). In the final analysis, the most desirable property is the protective one. Here it is assumed that when a garment is brought into proximity with a high intensity heat source, it should at least limit the amount of heat to that supplied by the source, if any measure of protection is to be attributed to that garment. That is, if the apparel itself by burning augments appreciably the heat to which the body is subjected, its protective property cannot be considered to be significant. Secondly, the area over which the heat is applied should be restricted to that area defined by the source. This means that there should be no appreciable spreading of the combustion area. Thirdly, to be fully protective, the fabric should resist the transfer of heat to the body or to flammable material in the vicinity (4). Thus the protective quality of a fabric may be fully defined from a thermodynamic viewpoint as a function of the quantity and intensity of the heat supplied and generated, the area over which it is effective, and the coefficient of heat transfer.
DECOMPOSITION REACTION
When a cellulose fabric is heated in a nonoxidizing atmosphere, decomposition ensues with the distillation of a tarry product, and there remains a residue of carbonlike material whioh retains the physical form of the original fabric (8). The main effect of fire-retardant treatments is to change the proportions of these products as shown in Table I. If the heating is carried out in an oxidizing atmosphere and provisions are made for the rapid cooling of the distillate, the same course of the reaction is followed. The liquid and solid products are obtained in amounts equal approximately to those obtained in nonoxidizing atmospheres. Continued heating, of course, causes further oxidation of the solid residue.
FABRIC COMBUSTION
During the normal uncontrolled and unpredictable ignition of a fabric via accidental means, the cellulose is usually subjected to a quantity of heat of high intensity. The amount of thermal energy supplied is quite variable but is normally dependent primarily on the time of contact with the heat source. The duration of the thermal subjection is usually of the order of a few seconds and general experience with average flames places their intensity a t about 500’ C. Under these approximate conditions a series of complicated reactions ensue (W), which for convenience may be summarized as follows:
Table I.
Decomposition Products from Fabrics
Fabric Untreated N H I H ~ P O8~. ,2 % N H ~ H ~ P O18.3% I, Borax:boric acid::7:3 6.5% 13 6’7’ Phbspkated fabric Maximum for zero afterfiaining
1 Present 3
+ AH2
COz
PROTECTIVE FUNCTIONS
rr
(1)
Cellulose +decomposition products
address, American Viscose Corporation, Marcus Hook, Pa. Present address, Hercules Powder Company, Wilmington, Del.
415
Product, Mg./Sq. Cm. Tar Char Tar/Char 12.7 0.2 63.5 2.0 8.3 0.24 1.8 14.4 0.12
1.4 0.9 1.8 2.0
7.3 11.2
0.19
.. .
...
5.1
0.0s 0.17
INDUSTRIAL AND ENGINEERING CHEMISTRY
416
BOO nnn
L
I
Vol. 42, No. 3
A.
0' 400 300 0
g400
ii
200
I-
100
O l 0
I
5
I
I
I
10 15 Time, Minutes
20
I 25
-f
/
I I
I
I
/
I
300 - 1 200
yI
I
100
L
I
I
I
Figure 1. Temperature of Fabric
I
0
1 = No fabric (180 CC. N Zor air/minute) 2 = Untreated fabric (180 CC. airlminute) 3 = 30q0 borax:boric acid::l:l o n fabric (180 cc. air/minute) 4 E Untreated fabric (180 CC. Nllrninute)
,
:
I
I
Time, llinutcr
Figure 2. Temperature of Fabric
5 = 30yo borax:boric acid::l:l on fabric (180 C C . N? air/rninute)
A = Untreated fabric; B = 20% N H I H ~ P Oo ~ n fabric
Thc thermal behavior of taliisreaction is shown in Figurc 1 and it may be concluded from curve 4 that the pure deconiposition reaction is not appreciably exothermic. In fact,, there are some indications that it may be endothermic. I n any rase AH2may be considered negligible: Cellulose --+ decomposition products
+ AH2 (negligible)
(2)
OXIDATION OF VOLATILE DECOMPOSITION PRODUCTS
During the normal combustioii of a fabric the deconiposition and oxidat,ion reactions occur simultaneously, and it is only when inert atmospheres or molecular distillation principles are employed to remove the products rapidly that the intermediatc reactions are discernible. In the presence of air (curves 2 and 3 of Figure 11, despite precautions taken to remove the t>arsas rapidly as they are released by the decomposition reaction, some oxidation occurs during their distillat,ion. This highly exothermic: reaction is manifested by a sharp peak in the curve. The isolated tars are highly flammable, and when impregnated on asbestos or glass fabric they cause flame propagation phenomena at rates and intensities comparable to those observed in normal cellulose fabrics (3). There appears to be a definite relation between the amount of tarry distillation products and the tendency of a fabric to flamc, and it is only when these volatile products are reduced beluw 2 mg. per square em. for an 8.5-ounce fabric that zero afterflaming results in the well known vertical strip burning test, as shown in Table I. From a wide variety of such tests it has been concluded that the oxidation of the volatile tarry decoinpositiori products is highly exothermic and is responsible for the flaming reaction. The main function of flame retardants is to reducc the amount arid concentration of these products:
+
Volatile decomposition products 01+ C02 HzO
+
+ AH3 (large)
arc imparted by the ammonium phosp1i:rtes and t o a IC by boric acid. When the chars are heated in a n oxidizing atmosphcre, they are readily oxidized irrespective of whether or not they contain adequate amounts of gloFv rctarditni. There is, however, considerable difference in the rates at which oxidation proceeds, as well as in the composition of thc resultant products of combustion. 9 s shown in Figure 2, fabrics were first subjected to pyrolyzing conditions to eliminate the Haniing reaction, and secondlj. the resultant chars were oxidized in air until no organic reaiduc rcmained. The thermal behavior shows that. t,he fabric treated iyith the glov- retardant exhibits its iiiusimum temperaturc laiei in time t>lianthe untreated fabric. Analysis of the combustion products of the charred fabrics of Figure 2 showed that the carbon in the char from the untreated fabric was mostly accountable as carbon dioxide in the eombustion gases. However, v i t h char from fabric treated wit,h glow retardants, a large proportion of the carbon was oxidized to carbon monoxide. Examples of this behavior are shown in Table 11.
Table 11.
Combustion of Charred Fabrics
Retardsnt Untreated Borax
% Retardant Added
COZ,G./Q. Char
0 0.7 1.2
2.4 2.6 2.5
2.4 2.0 1.2 1.3 1.2 1.2 0.8 0.7 0.8 0.8
3.4 11.6
Boric mid
4.0 4.6
il .9 11.6
Diammonium phosphate
1.0 2.9 3.9 9.4
(3)
OXIDATION OF SOLID DECOMPOSITION PRODUCTS
Tlie charred residues from the initial decomposition reaction are composed mainly of carbon, together with the remnants of the fire retardant, Their principal thermal character is that although t,hry exhibit no flaming combustion, they are more or less readily oxidized by the nonflaniing or glowing reaction. The second maxima of curves 2 R J 3, Figure 1, illustrates this reaction, which appears to be fully as intense as the flaming reaction. The normal flame retardants such as borax or metallic oxides liave no inhibit,ing effect on the glowing reaction, and in man:cases act,ually enhance it. The best examples of glow rctardancy
Froin a variety of similar data it is concluded that the oxidation of the solid residues is highly exotherniic and it is the coniplete oxidation of the char that accounts for the glowing rcaction: Solid decomposition products
T
CO
0,
---+
+ Cot +
AH4 (large)
(4)
The functions of the gloiv retardant appear to be the changing of ihc course of the oxidation (9):
C C
+ 02 +COz + AH = 9 4 . 4 KC + ' / a +CO + AH = 26.4 KC 0 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1950
417
Both reactions proceed to complete gasification of the carbon if su5cient heat is continuously applied. However, the second reaction is not sufficiently exothermic to be self-sustaining, and although it is the preferred course in the presence of glow retardants, it will not continue after the source flame has been removed. THERMAL BARRIER
The heats developed in the above reactions, together with that from the source, constitute a potential hazard which becomes real if the heats are transferred to the body (6). A fabric itself is an excellent thermal insulator, for such is its function. The cellulose itself has a low coefficient of heat transfer and the fibrous nature as well as the woven construction contribute greatly to its quality of poor thermal conductivity. However, if the cellulose is consumed by either flaming or glowing, the full intensity of the source flame or of any surrounding combustion reaction will fall directly on the body. In the case of a fabric, adequately treated with flame and glow retardant, conditions are approximately the same a t normal and at high temperatures. At flame temperatures there remains substantially unchanged the original form of the fabric with all its excellent thermal barrier structure of entrapped air space. Although the cellulose with its low thermal conductivity has changed, in its place there remains an equally good insulator in the form of carbon. 1000
ri
I
1
f
q 400
k
a
0
0.5
0
I
Figure 3. A
= No
I
I
1.0 1.5 Inches Behind Fabric
I
I
2.0
I
0
10
Figure 4.
I
I
20 30 Time, Seconds
I
I
40
50
Temperature 0.25 Inch Behind Fabric 3 = SbiOs-Vinylite fabric 4 = Phosphated fabric
1 = No fabric 2 = Untreated fabric
only in cases of fully flame- and glow-retarded fabrics that a maximum temperature in the vicinity of 100" C. is maintained for any appreciable length of time. Similar comparisons have been made with fabrics prepared from yarns composed of spun Fiberglas and asbestos, as well as with fabrics prepared from Fiberglas and impregnated with neoprene. With an external temperature of 800" C., the time of thermal subjection to attain a temperature of 200" C. in dead air space 0.25 inch behind the fabric was measured. These fabrics were compared singly with the flame- and glow-retarded cellulose fabric and as 2- and 3-ply combinations. The results, shown in Table 111, indicate that the cellulose fabric is a t least as good as and probably is slightly better than those prepared from the thermally inert materials. Fire retardant-treated cellulose, however, must be considered expendable, since once having contact with a flame its service qualities deteriorate and the garment becomes useless. Nevertheless, the protection is available a t the time i t is needed, and this after all is the main consideration. Furthermore, the cellulose fabrics are wearable, and differ only slightly in hand and appearance from the untreated materials. The h a 1 conclusion is that adequately flame- and glow-retarded cellulose fabrics have thermal protection qualities equal to those made from the most inert fibers available, since their thermal conductivity is similar :
Temperatures after 20 Seconds
fabric or untreated fabric; B = phosphated fabric or asbestos sheet
The magnitude of this thermal barrier is illustrated in Figure 3, where a phosphated fabric is compared with asbestos insulation paper of approximately the same weight per unit area. A flame \vas allowed to impinge upon the outer surfaces so that they attained a temperature of 800" C., and temperature measurements Fere taken a t various locations behind the fabric, in dead air space, after 20 seconds' subjection to the flame. Because of the complete combustion of the untreated fabric, thermal equilibria is soon attained and the resultant condition8 are the same as though no fabric had been present. However, conditions behind the treated fabric were found to be identical with those which resulted when the fabric was replaced by the absestos sheet. For this protection, equivalent to that afforded by asbestos, it is essential that the cellulose fabric be fully glowproofed (6). This is demonstrated in the rate of thermal transfer curves of Figure 4. The 10-second protection afforded by the untreated fabric is increased somewhat by the simple flameproofing donated by the antimony oxide-vinylite treatment. However, the glowing continues from the 10- to the 30-second interval with appreciable temperature rise, and it is
(5)
Table 111.
Insulation Value of Fabrics
Code: 0 = no fabric; A = phosphated fabric; B asbestos-glass; C = glass-neoprene Time t o Attain 2000 c Fabric Layers Code Second's' 0 Single fabric 2 A 12 B 10 C 9 26 18
13 18
16
3-Ply fabric
AAA BBB
ccc
AAB AAC BBC ABB ACC BCC ABC
15 35 33 33 32 29 25 28 23 21
28
418
INDUSTRIAL AND ENGINEERING CHEMISTRY LITERATURE CITED
(1) Akin, E. W., Spencer, L. H., and hIacormac, A. R., Am. Dyestvfl Rept., 29, 418, 455 (1940). ( 2 ) Leatherman, M., U. S. Dept. Agr., Circ. 466 (March 1938). (,3,) Little. R. W.. “Flamemoofina Textile Fibers,” New T o t k , Reinhold Pu.blishing Corp., 1947. (4) Natl. Fire Protection Assoc., Boston, Mass., “Reconimendcd
Requirements for Flameproofing of Textiles,” 1941: (5) Katl. Research Council, Div. 9 and 11, Symposium on the Flame-Thrower, Dunbarton Oaks, Washington, D. C. (January 1945). ( 0 ) Satl. Research Council, Project Q.>l.C. 27, Spec. Rept. Subproject 27-R8-11, Columbia University, New York (Junc 12, 1945).
Vol. 42, No. 3
(7) Ramsbottom, J. E., Brit. Dept. Scientific and Industrial Research, London, Royal Aircraft Establishment (1947). (8) Ramsbottom, J. E., and Snoad, A. W., Brit. Dept. Scientific and Industrial Research, London, seoond rept., Fabrics Co-
ordinating Research Committee (1930).
(9) Sisson, W. A., presented a t Office of Quartermaeter General,
conference on Katl. Research Council Project 27 Q.M.C., Washington, D. C. (Dee. 1.5, 1944). (10) State of California, Assembly Bill 726 (passed April 18, I945j. (11) U. S. House of Representatives, H.R. 2496 (introduced RIarch 6 , 1945). RECEIVED January 26, 1950. Contribution from Q.M.C. 27 War Research Project, Department of Chcmioal Engineeiing, Columbia University, Xew York, E.Y .
UATZON OF A M E - R E S I S T A N T FA BRZCS JAMES R‘I. CHURCH, ROBERT W. IJITTLE1,AND SYDKEY COPPIcK2 Columbia University, New Yorlt, N. Y. T h e acceptabiIity of any flameproofed fabric for a given purpose is dependent not only upon its flame resistance, but also upon other textile characteristics which may be altered b y the flameproofing treatment employed. The type of flame-retardant chemical and the conditions used for its incorporation within the fabric not only affect the characteristics of the finished cloth, but also restrict its use for certain applications. The flammability of a fabric is a factor of the combustibility of fiber, type and weight of weave, and effectivenessof any added flame-resistant treatment. The determination of the flame resistance of fabrics has recently been investigated by many laboratories. During the war a n intensive study of this problem was made, the results of which have led to a standardization of flame test methods for a wider acceptance and better agreement between laboratories. Uniform and
consistent results have been obtained by the establishment of rigid procedures for flame tests, w-hich now make i t possible to measure the comparative effectiveness of various flame-resistant treatments and determine the relative value of flame-resistant fabrics for any given purpose. Consideration has been given to not only the extent of flaming produced in the combustion of the fabric but also the rate of burning and the duration of the afterglow, employing the vertical, horizontal, and angle tests. Other methods for testing the acceptability of flame-resistan t fabrics include durability to leaching by tap water, sea water, perspiration, and detergent solutions; deterioration of the fabric or its flame-resistant characteristics i n storage or actual use; measurement of fabric strength and porosity; physiological effeets of flame-resistant treatments, such as toxicitj , skin abrasion, and heat loiid.
ARLY during World War I1 it became evident that flameresistant textiles would be required in order to afford effective protection against the rapidly developingfke typeof warfare, with the use of incendiary bombs, flame throwers, and highly combustible fuels in tanks, planes, and ships. As a protective measure, temporary methods were adopted for the treat’ment of textiles wherever possible in an effort to reduce the losses in personnel, equipment, and supplies due to fire. Unfortunately, the best treatments then available Ivere unsuited for use on clot’hing, mainly because they lacked permanence. An intensive search was immediately undertaken in an effort, to discover more suitable flame-retardant agents and practical methods for their ready applicat,ion with existing facilities, in order to perfect a flame-rmistant fabric capable of meeting the stringent military requirements. Such a project was established as part of the war research program in the Department of Cheniical Engineering a t Columbia Universit,y in the fall of 1942, under the sponsorship of the National Research Council Committee on Quartermaster Problems. It had a threefold objective: (1)
iniprovement of existing flame-resistant t.reatments for temporary adoption; (2) investigation of the fundamental principles of flame retardancy, which included the study of t.hc niechanisms by which flame-retardant chemicals function t>odecrease the combustibility of textiles; and (3) search for newer and better flame-retardant agents and iniproved methods t,o provide a more satisfact,orv treatnient of fabrics for military use. Incidental to this, but equally important in many respects, was the development of adequate test methods for a correct evaluation of flame-resistant agents and treatments, as a means of determining their acceptnbility for the uses intended. This not only involved the utilisation of standard test methods already used by the textile industry for other types of fabrics, but in many cases required that a new test method be devised, with rigid standardization of the test procedures, in order to obtain satisfactory quantitative result8 for a reliable evaluation and comparison of flame-resistant treat,ments. The acceptability of a flame-resistant textile cannot be judged by its flame resistance alone, for any flameproofing treatment mill alter the properties of the original textile fiber to some cxt-ent, anti may change its characteristics sufficiently to render the fabric unsuited for the purpose intended. This is particularly true vhere the treatment requires an excessive amount of the flame-
E
1
Present address, Experiment Station, Hercules Powder Co., Wilmington
90, Dei. 9
Present address, Research Department, American Viscose Corp., 1RIar-
cus-Hook. Pa.
