Effect of Flameproofing Agents on Cotton Cellulose H. A. SCHUYTEN, J. W. WEAVER, AND J. DAVID REID Southern Regional Research Laboratory, New Orleans, La.
F
t
ME resistance in cotton cloth is achieved by treatment of the cloth with certain chemicals which so change the course of the normal thermal decomposition and combustion reactions that flaming does not occur. Untreated cellulose when subjected to a flame or other source of high temperature decomposes and produces gases which ignite, propagate the flame, and further decompose the cellulose until only a negligible ash remains. Flameproofed cellulose under the same conditions results in a large carbonaceous residue and correspondingly less gases. Moreover, the gases produced under these conditions do not ignite or burn readily. The term “flameproofed cotton” is understood to mean cotton cloth that will not support combustion after removal of the source of ignition when tested under certain standard conditions. I n the usual vertical flame test, described in Federal Specification CCC-T-19lb ( 1 9 ) , a strip of cloth is exposed to the luminous flame of a Bunsen burner and flameproofness is judged by the length of a tear produced through the charred area by a standard weight. The effects of numerous flameproofing compounds on the thermal decomposition of cotton cellulose have been quite thoroughly studied by a number of investigators, notably by Coppick ( 8 ) ,Church ( 4 )and Tamaru ( 1 7 ) . However, conclusions reached by these workers were based on experiments in which the cellulose was thermally decomposed in closed and in some cases evacuated systems. Ramsbottom ( 1 4 ) has studied the effect of a number of inorganic salts on the burning of cotton cloth. Parks and others (9, 1 2 ) have studied the carbon monoxidecarbon dioxide and “char-tar” ratios obtained by pyrolysis of cellulose under vacuum in apparatus similar to that used by Coppick (8) and have attempted to correlate flameproofing activity with these data. Much effort has been devoted to the development of theories to explain the mechanism by which the burning of cellulose is prevented by the agents applied. Little (8) in his monograph on flameproofing has outlined the various theories proposed to explain flameproofing. I n a previous paper ( 1 6 ) , the authors have discussed the theoretical aspects of the flameproofing of cellulose. I n that paper the existing theories were reviewed and elaborated and a theory of general applicability was presented. The principal points of this theory are: 1. The main reaction involved in the flameproofing of the cellulose is a dehydration brought about by the flameproofing agent acting as a catalyst through a carbonium ion mechanism. 2. The catalytic effect of the flameproofer, in accentuating dehydration, changes the rate of gas production, probably accelerating it, and probably brings this about a t a temperature below the ignition temperature of the gaseous mixture. 3. The flameproofing catalyst is a Lewis acid or base (electron acceptor or donor) and must be present as such or must be produced from the flameproofer a t or near the temperature of the burning cellulose.
This paper presents experimental evidence substantiating the points of this general theory of flameproofing. Experiments under conditions approaching those of cotton cloth burning in air were conducted by placing treated and untreated samples on a hot surface. Rates of thermal decomposition of the treated and untreated cotton textiles are compared a t several tempera-
tures to show that the flameproofing agents accelerate this decomposition and produce the gases a t lower temperatures. Analyses of the residues support the concept of the presence of Lewis acids which increase the efficiency of dehydration of the cellulosic material. The decomposition of some flameproofing agents has been investigated, and the vertical flame test for flameproofness of textiles is discussed in the light of these findings. MATERIALS AND APPARATUS
Four types of treated cloth prepared in this laboratory were used in some of the studies described: 1. Partially acetylated cotton cloth (48 square, 5 ounce), prepared by methods developed in this laboratory (6),having acetyl contents of 8.5, 12.5, 17.5, 22.4, and 27.3% 2. A 6-ounce cotton drill with a pickup of approximately 40% of antimony oxide-chlorinated paraffin-urea-formaldeh yde flameproofing material ( 3 ) 3. An 8-ounce twill treated with tetrakis( hydroxymethyl) phosphonium chloride (THPC)-amine resin (16) 4. An &ounce twill treated with the telomer-polymer of bromoform and triallyl phosphate ( 6 ) . I n addition, two types of untreated cotton cloth were employed for further treatment on these investigations. These were a khaki twill and an undyed duck, both lightly scoured and weighing approximately 8 ounces per square yard. All inorganic salts and acids were reagent grade. The ammonium salts of organic acids were prepared by adding a alight excess of ammonium hydroxide to aqueous solutions of the reagent grade acid. Methylol melamine was commercial grade. Trial1y1 phosphate (TL4P)was furnished in a technical grade by the Victor Chemical Works and tris-( dibromopropyl) phosphate ( 1 8 ) was prepared from this by brominating and washing the product with a dilute solution of sodium carbonate. A partially polymerized triallylphosphate (polyTAP) was obtained by the method of Walter ( 1 1 ) ,modified by the use of 2-ethoxyethyl acetate as the solvent to avoid introduction of chlorine from chlorinated solvents. The polymerized triallyl phosphate was then brominated to approximately 3Oy0 bromine content to yield a brominated compound (brompolyTAP). Treated samples were obtained by applying aqueous solutions of the salts and acids to the cloth using a laboratory padder. Thermally unstable materials were dried a t room temperature; other samples were dried in a forced-draft hot air oven. Polymerized triallyl phosphate was applied from a solution of 2ethoxyethyl acetate and bromopolytriallyl phosphate from ethylene chloride-methanol solution (32% methanol by weight). These samples were cured in a forced draft oven a t 110” C. for 30 minutes. Tris(dibromopropyl) phosphate in benzene was emulsified using triethanolamine oleate as the emulsifier, and added to an aqueous solution of methylol melamine. The emulsion contained approximately l6T0 of the tris ester and 8% methylol melamine. Cloth samples were padded, dried 10 minutes a t 60’ C., and cured a t 150’ C. for 5 minutes. The hot surface used in the experiments was constructed by thermally insulating the heating unit of a conventional 1000-watt laboratory hot plate which was connected to a variable trans-
1433
INDUSTRIAL AND ENGINEERING CHEMISTRY
1434
former capable of furnishing 135 volts a.c. The apparatus was calibrated by measuring temperatures with a surface pyrometer. GENERAL PROCEDURE FOR THERMAL DECOMPOSITION OF COTTON CLOTH
Samples of cloth (approximately 4 grams) were weighed and this weight was corrected for moisture content. Each sample was placed on the hot plate, controlled a t the specified temperature, and a timer started. At the end of the prescribed time the sample was removed with a forceps and momentarily suspended in an atmosphere of carbon dioxide maintained by a lump of solid carbon dioxide in a pail. This treatment instantly extinguished any flaming or glowing. The charred residue, weighed immediately, was assumed to be anhydrous. This weight, divided by the original anhydrous weight, yielded a quotient termed the "per cent residue." No corrections were made for the residue of the reagent applied (except in the case of sodium chloride which is noted in the proper place). TEST METHODS AND ANALYTICAL PROCEDURES
Char lengths of flameproofed samples were determined by the vertical flame test (19). Carbon and hydrogen analyses were carried out on a micro scale. I n some cases, fluxing of the sample was necessary to prevent entrapment of carbon in the residue. Phosphorus was determined by wet digestion, followed by the reduced molybdate colorimetric method (IS). Halogen determinations were made by the Volhard method after decomposition in a Parr peroxide bomb. Sodium and lithium were determined using a Beckman flame spectrophotometer. Conductometric titrations were carried out in the usual manner and the curves were interpreted in accordance with the procedures of Britton ( 2 ) . EXPERIMENTS AND DISCUSSION
Effect of Temperature on Decomposition of Cotton. The thermal decomposition of treated and untreated cotton twill was studied using the hot surface technique. The untreated cotton a t temperatures high enough to cause ignition quickly darkened, smoked, ignited, and burned, leaving only a fluffy ash of negligible weight. The treated samples behaved in a similar manner at first, darkening and liberating large volumes of smoke, but these did not ignite and, instead of a fluffy ash, left a heavy carbonaceous residue. These reactions were characteristic of all the flameproofed samples examined.
Vol. 47, No. 7
The heavy smoke from any treated sample did not ignite even though the heated surface was a t dull red heat (about 500" C.); however, in every case the vapors could be ignited a short distance above the hot plate when a free flame or hot spark was used. The first consideration was t o determine the effect of temperature on the decomposition. The burning temperature of cellulose is about 500" C. (8); this was the temperature obtained when a piece of burning cloth was hung on the junction of the thermocouple. Decompositions of cloth were carried out a t several temperatures to 500" C. and the results are plotted a8 percentage residue against the logarithm of the exposure time. Figures 1 and 2 show these results. Figures 1 and 2 show that untreated cloth burned completely a t temperatures of 400" and 500" C. The flameproofed cloth generally decomposed more rapidly than the untreated cloth but, due to the production of the nonflammable carbonaceous char, approached an equilibrium point. Because of the difference in the nature of the reactions observed, and in order to establish a simple basis for comparison of the rates of thermal decomposition, the time required for the sample to be reduced to 50% of its original weight was taken as a measure of this rate. Figures 1 and 2 show that decomposition of treated and untreated samples at 500" C. takes about the same length of time (8 seconds). At 400" C. the time for untreated cloth is 48 seconds and for treated cloth 22 seconds. At 300" C. by extrapolating the curve, it was estimated that 1400 seconds would be required for 50% decomposition of untreated cloth; treated cloth requires only 300 seconds to reach this point. The flameproofing agent accelerates the rate of decomposition. This same effect may also be studied by considering the relation of the amount of residue left to the temperature a t which the decomposition is conducted. If the reaction is judged complete when no further weight change in the residue is observed, the percentage residue left when equilibrium is reached can be measured. I n the case of untreated cotton, no residue is left when equilibrium is reached a t 400" C. or higher. At 300" C. the experiment was abandoned after 1000 seconds when equilibrium had not yet been reached. With flameproofed samples, the situation was different. At 500' C., 26% residue was left, a t 400° C., 34% residue remained, and a t 300" C., 49% remained. The fact that, a t 500" C., approximately the same time was required to reach 50% decomposition with treated and untreated cloth is somewhat misleading. Closer examination of the data reveals that the decomposition of the treated sample actually proceeds much faster than that of the untreated, and the area
3 00 l o o t
90
ae I 60 W
a 9
W
v)
w
0:
40
40
400'
5000
20
2ol
0 TIME
- SECONDS
Figure 1. Thermal decomposition of untreated 8-ounce twill
0
I
I
2
I
I I 1 1 1 1 1
4
I
6810
TIME
I
I
-
I I1111
100 SECONDS
I
I
I I I I I I I
1000
Figure 2. Thermal decomposition of treated 8-ounce twill
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
/
under the treated curve is smaller than the area under the unCLOTH SAMPLE treated curve. I n order to select the most suitable temperature at which to study the decomposition of flame6" proofed samples, information was obtained on the actual conditions p r e v a i l i n g when flameproofed 5' cloth is tested. The temperatures surrounding the luminous flame used in the standard vertical flame test (19) were estimated, -c by measuring with a. thermocouple the temperatures reached in the space surrounding the -3' flame. The results are shown in Figure 3 . I n each measurement one minute was allowed for the -e" thermocouple to come to equilibrium. This is considerably more time than the 12 seconds' exposure required in the flame test. How.I" ever, the extremely low thermal conductance of cotton textiles makes this only an estimate of t h e c o n d i t i o n s existing in the standard vertical flame test. AlBUNSEN though, the actual temperatures shown in Figure 3 may not be accurate, there is a range in temperatures along the cloth which Figure 3. Temperaapplies in a qualitative way. ture distribution patI n the zone t o 3 inches above the tern in vertical flame flame, the temperature ranges test from 650" to 450' C. This is well above the temperature required for rapid reaction in both treated and untreated samples as shown in Figures 1 and 2. I n this zone, the exposed area of the treated sample is converted very rapidly to char while untreated samples burn completely and rapidly. At points 3 to 6 inches above the flame, the temperature decreases so that a t the 6-inch mark, the temperature is 300" C. Figures 1 and 2 show that a t these lower temperatures the reaction is still taking place rapidly with treated cloth, converting it to a substantially incombustible char while untreated cloth is decomposing very slowly. The significance of this difference in rates is that the treated sample is being decomposed a t a low temperature and the gaseous products are below their ignition temperatures so that if and when a higher temperature is reached by this section of the cloth, there will be nothing left to promote flaming. That is, the gases t h a t would normally propagate the flame have already been liberated and dissipated before their ignition temperature could be reached. I n this manner propagation of the flame is prevented
-
Table I. Flameproofing Agents Studied in Thermal Decomposition of 8-Ounce Twill at 400" C. Flameproofing Agent Lithium chloride Ammonium bromide Diammonium phosphate Rorsx: horic acid. ..~ ...~ ~ ~,6 .: 4 ~
~
Polytriallyl phosphate Brompolyallyl phosphate Tris(dibromopropyl)phosphatemethylol melamine Tetrakis(hydroxymethy1) phosphonium chloride resin Bromoform-triallyl phosphate polymer (6)
8-02. Twill Pickup,
%
21
20 9 7 20
20
20 16 20
1435
and flameproofing is accomplished. I n view of the results illustrated in Figures l, 2, and 3,400' C. was chosen as an operating temperature for further studies on the decomposition of cellulose with various flameproofers. Effect of Various Flameproofkg Agents on Decomposition of Cellulose at 400" C. I n order to study more thoroughly the behavior of flameproofers, a group of known agents, as listed in Table I, was selected for application to samples of cotton cloth. All treatments employed either acids or acid producers. Sodium hydroxide conferred flameproofness, but the chars were too soft to transfer. These treated samples were then decomposed on the hot plate a t 400" C. in accordance with the general procedure. The results obtained in this study are summarized in Figure 4. The cross-hatched area in Figure 4 includes the curves obtained from the 9 flameproofers. No corrections for residues from the flameproofers have been made. Far comparison, the curve for untreated cloth is included. These curves demonstrate that the rapid decomposition"of cellulose noted earlier is a common characteristic of these flameproofing agents. The treated samples reached 50% residue in 7.5 to 22.5 seconds while untreated cellulose required 48 seconds. Moreover, the use of flameproofing agents resulted in residues ranging from 35 to 43% which reached equilibrium in about 40 seconds. I n contrast, untreated cellulose is completely destroyed in 180 seconds.
A = TREATED B = UNTREATED
2ol LB 0
0
,
,
2
I
TIME
3
- MINUTES
Figure 4. Effect of flameproofers on thermal decomposition of 8-ounce twill at 400' C. Three other cases, because of their special interest, are shown separately in Figure 5. Ammonium 2,3-dibromopropionate is apparently a very efficient flameproofer, since the reaction approaches completion in 30 seconds. However, equilibrium is never attained and the amount of residue continues to diminish with time. This slower consumption of the residue is caused entirely by glowing which begins after about 30 seconds, when equilibrium is ordinarily reached. Similar results were obtained with the antimony oxide-chlorinated paraffin-urea-formaldehgde treatment but the reduction of the residue by glowing was more rapid. Glowing is a distinct and separate phenomenon from flaming. Sodium chloride is not a flameproofing agent for cellulose, but the results obtained by treating the cloth with this salt are inter-
Vol. 47, No. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
1436
esting. For better comparison with the untreated sample, the residues derived from this treatment were corrected by subtracting the sodium chloride content. Thus, in this case only, the percentage residue denotes the residue derived from the cellulose alone. It was impossible to correct the other series similarly since the decomposition products of the flameproofing agents were not known.
An Ammonium 2,3-dibromopropionate ,.I I % B = Antimonv oxide- chlorinated paraffin-resin ,40% C = Untreated D * Sodium chloride .24% (corrected$-
100
I I\\ \
Dehydration of Cellulose by Flameproofing Agents. It has been stated in an earlier paper (16)that the essential reaction involved in the flameproofing of cellulose is a dehydration catalyzed by the flameproofing agent or its decomposition products. I n an effort to verify the truth of this statement, the compositions of some of the chars resulting from treated cotton duck at 500' C. were investigated. The examination of these chars involved two studies: 1. Determination of the carbon-hydrogen content of the chars so as to compare them with the products of an ideal dehydration of cellulose. 2. Search for evidence of the presence of a Lewis acid which is believed t o be responsible for bringing about the dehydration. If cellulose could be ideally dehydrated, the products would be carbon and water in accordance with (CeHia0s)n
I
6nC
+ 5nHzO
A progressive theoretical dehydration of cellulose would yield products having the characteristics shown in Table 11.
ac
2
-+
60-
P
u)
w
100
-
a 40
80-
20
60
01 0
,
I
2
I
TIME
Figure 5.
-
be
I
3
- MINUTES
Effect of glowing on residue from 8-ounce twill at 400' C.
Figure 5 shows that sodium chloride causes an increase in the rate of decomposition of the cellulose since soy0residue is reached in about 30 seconds as compared with 48 seconds for untreated cellulose. However, sodium chloride is not a flameproofer and equilibrium is never attained. The relative effectiveness of halogens in certain types of flameproofing agents is demonstrated in Figure 6. Samples with pickups of 10% ammonium acetate, 1570 ammonium chloracetate, and 21 % ammonium bromoacetate were decomposed in the usual manner. These pickups are in approximate proportion to the molecular weights of 77, 112, and 156, respectively, so that the samples contained approximately the same mole per cent of salt. Figure 6 shows that ammonium acetate slightly accelerated the thermal decomposition but did not flameproof cellulose, while ammonium chloracetate, in addition to accelerating the decomposition, produces a residue of about 22y0. Ammonium bromoacetate is the best flameproofer since it caused the most rapid decomposition and resulted in the largest amount of char. Since the thermal decomposition of these halosalts probably involves the formation of ammonium chloride and bromide, the results shown in Figure 6 are compatible with the work of Ramsbottom (1.4)who showed that 22% ammonium chloride was required to flameproof cloth to the same extent as another sample containing 7% ammonium bromide. From this evidence it is concluded that one characteristic which must be possessed by a flameproofing agent is the ability to accelerate the thermal decomposition of cellulose, and t o do so a t a temperature below the ignition temperature of the gases evolved.
I
w a n ij=i
-
e
40-
w
20
-
0
0
20
,
60
40
TIME
Figure 6.
80
100
- SECONDS
Effect of ammonium haloacetates on 8-ounce twill at 400' C.
Investigation of these chars would have been simplified had they consisted only of carbon, hydrogen, and oxygen. This, of course, is not the case, since practically every flameproofing agent results in some sort of residue, and even after the chars have been extracted with water, analyses have shown the existence of small amounts of inorganic materials left in the residue. Carbonaceous residues are undoubtedly deposited by the organic portion of some flameproofers, but these are considered insignificant, and regardless of the nature or amount of these residues, the extent of dehydration of the cellulose is clearly indicated b y the ratio of the amounts of carbon and hydrogen present. Thia t,heoretical dehydration is ilhstrated in Table 11. I n order to compare experimental results with these ideal data, samples of treated cloth were charred on the hot plate, and extracted in Soxhlet extractors with water to remove soluble constituents. The flameproofing agents give rise to Lewis acids which, in these cases, are water soluble inorganic substances.
July 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
I I\ I\ I \
Table 11. Theoretical Dehydration of Cellulose Reaction CsHioOa-Hz0 4CeHaOa CeHs04-Hp0 4 CeHeOs CsHsOs-HzO -+ CaH40z CeHaOz-Hz0 -+ CsHzO CsHz0-Hz0 -+ Ca
- CAnalysis, %H 5.55 4.76 3.70 2.22 0.00
50.0 57.1 66.7 80.0 100.0
Ratio Atoms, C/H 0.75 1.00 1.50 3.00 0.00
Moles Hz0 Lost/ Glucose 1 2 3 4 5
The chars were then analyzed for carbon and hydrogen by the ordinary microcombustion methods, and the extent of dehydration estimated from a curve drawn from theoretical values. The results are given in Table 111.
1437
50
- II I
40
-
x
\
x
ACETYL
NH4Br
A = B
0 17.5
19
c=
0 17.5
D =
21 0 0
A
#
I W
30u)
Table 111. Dehydration of Cellulose by Flameproofing Agents
W K
20
LiCl 9.8 Brompolytriallyl phosphate 19 HsPOa .. Polytriallyl phosphate 29 Borax-boric acid 11 Tetrakis(hvdr0xvmethy1)phospLonium chloride 16 (NHa)zHP04 5.8 (NHa)zHP04 5.5 (NHb)zHPO4 6.5 a Corrected for moisture but dues of flameproofer.
500
64.6
2.08
2.64
3.85
500 500
74.3 75.2
1.79 1.43
3.48 4.41
4.15 4.32
500 500
75.6 76.9
1.65 2.25
3.84 2.75
4.22 3.90
500 70.7 2.13 500 72.3 1.33 400 72.1 1.82 300 62.4 2.88 not for ash, or for carbon
2.78 3.92 4.56 4.35 3.32 4.11 1.82 3.35 and hydrogen resi-
These data clearly indicate an increase in carbon content from the original 44.4% of cellulose to a t least 70y0 in the residue. When this increase is compared to an ideal dehydration reaction, i t may be estimated that about 4 of the possible 5 molecules of water have been removed from 75% of the cellulose, leaving a nonflammable carbonaceous residue, while the remaining 25% of the cellulose has decomposed into volatile materials, presumably carbonaceous gases and water vapor. Further support of this dehydration is obtained by studies on the flameproofing of acetylated cotton. I n this laboratory it has been found that textiles woven from cellulose acetate fibers cannot be flameproofed with any of the known flameproofers for cotton. The partially acetylated cotton cloth produced in this laboratory ( 5 ) which can be obtained with a substitution as high a s 27.3y0 acetyl (almost 1.5 CH3CO-groups per glucose unit) was investigated. The samples available were treated with approximately 20% ammonium bromide and decomposed by the general procedure. Untreated control samples were included for comparison and the results are shown in Table IV.
Table IV. Residues Derived from Acetylated Cotton Cloth
rp
% Acetyl 0 17.5 0 8.5 12.5 17.5 22.4 27.3
NHeBr 0
Residue a t Epuilibrium 0
19 21 21 21 21 18
36 30 22 20 19 17
% 0
0
50% Residue Relative Time, decomposi. seconds tion rate 28.8 1.0 17.5 1.6 7.5 3.8 .. .. 817
..
..
3.3
.. ..
Code in Figure 7
C
D
A
..
B
..
..
To illustrate better the effect of substitution of the acetyl group for the hydrogen of the cellulosic hydroxyl groups, the data from two acetylated and two untreated samples are shown in Figure 7 . Ammonium bromide accelerated the decomposition of both the
-
.B
\
10-
0 0
l
1
1
1
1
I
60
30 TIME
- SECONDS
90
120
Figure 7. Thermal decomposition of acetylated and nonacetylated 48-square cloth at 400' C. acetylated and untreated cloth to about the same extent, but the acetylated sample left only about 20% residue and flaming was observed. Acetylation appears to accelerate the thermal decomposition of cellulose. Since Lewis acids attack esters in much the same manner as alcohols ( I O ) , it is to be expected that flameproofing agents for cellulose would also catalyze the thermal decomposition of acetylated cotton. The inefficiency of the flameproofing treatment of acetylated cotton might be explained by the production of flammable acetic acid in place of water, the normal product in the mechanism postulated. The smaller amounts of char deposited by acetylated cotton can be accounted for partially by correcting the original weight of the sample to the weight of the cellulose fraction of the acetylated cellulose, but such corrected chars are still not equivalent in weight to those from flameproofed cellulose. The presence of the acetyl group in some manner so hinders the dehydration of the cellulose portion that highly acetylated cotton cannot be satisfactorily flameproofed with any of the known flameproofers for cellulose. These data support the contention that the flameproofing of cellulose is brought about through a dehydration reaction leaving a residue, consisting essentially of carbon, which will not produce flammable gases under these conditions. Acid Content of Carbonaceous Chars. It has been postulated ( 1 6 ) that Lewis acids are the catalysts which bring about the dehydration reaction. Although many of the successful flameproofing agents are compounds that cannot be considered as Lewis acids, it is quite possible that these compounds produce the latter a t flaming temperatures. I n the case of organic phosphate resins it would be expected that a t these temperatures decomposition to acids or oxides of phosphorus would take place. To establish this, the aqueous extracts of the chars obtained from cloth samples treated with polytriallyl phosphate, bromopolytriallyl phosphate tetrakis(hydroxymethy1) phosphonium chloride, diammonium hydrogen phosphate, and phosphoric acid were compared. I n addition, phosphoric acid alone was heated
INDUSTRIAL AND ENGINEERING CHEMISTRY
1438
to 400' to 500' C. and the product dissolved in water. These aqueous solutions were titrated conductometrically with standard sodium hydroxide solutions and the extracts were analyzed for phosphorus content. The loss of weight of these chars could not be related in any way to the amount of flameproofer on the cotton. Presumably the char contained water soluble products of the decomposed cellulose, which were extracted together with the soluble residue of the flameproofer. I n the titration of all samples, precipitation was observed that indicated the formation of sodium tri-m-phosphate or sodium tetra-m-phosphate, and the curves obtained were similar. No precipitation took place with ammonium molybdate, but boiling caused the yellow phosphomolybdate to form. These observations point to the presence of metaphosphates and the absence of orthophosphates. Table V gives the results of these investigations. Table V. Analysis of Extracts of Charred Residues from Flameproofed Cloth Analysis of Extract Milliequivalents found H' P 19.17 23.63 33.05 44.06 25.36 36.15 10.03 14.17
Flameproofing Agent Bromopolytriallyl phosphate
H:POa Pol triallyl phosphate (N6a)zHPOa Tetrakis(hydroxymethy1) phosphonium chloride
8.85
10.1
Ratio H +/P 0.81 0.75 0.70 0.71
0.88
The possibility of hydrobromic acid being present in the chars from the bromopolytriallyl phosphate was considered but only negligible amounts of bromide were found, showing that the products formed were volatile. The ratio of hydrogen ion to phosphorus, given in Table V, shows that the solutions contained mixed polyphosphoric acids which are regarded as the Lewis acids which catalyzed the dehydration reaction. These data support the statement that a material which is, or can produce, a strong Lewis acid, can act as a flameproofing agent for cotton cellulose. Thermal Decomposition of Some Flameproofing Agents. As a logical consequence the temperature was determined a t which the catalyst was produced by thermal decomposition of the various organic phosphate resins available. The samples employed were dry powdered resins, approximating the resins deposited on the cotton textiles by the flameproofing treatment. The resins were placed in ordinary melting point capillaries and immersed in oil in a Thiele tube, heated a t a rate approximating 20' C. per minute. All of the resins investigated showed good stability, as indicated by only a slight darkening prior to reaching their decomposition point. All decomposed vigorously over a rather short range of temperature with the evolution of gases and production of a black residue which was liquid in most cases. Since decomposition temperatures near 350" C. were difficult to determine in the Thiele tube, these were confirmed by dropping small samples on the surface of the hot plate employed in the thermal decomposition of treated textiles. For simplicity, the hot plate was heated in 50' C. stages so the decomposition temperature of the samples lies
Table VI. Decomposition Temperatures of Flameproofing Agents -0
Ascent Bromoform-triallyl phosphate polymer Polytriallyl ,phosphate Brompol triallyl phosphate Tetrakis rhydroxymethyl) phosphonium chloride resin
Normal Decomposition Temp., C. Char Length, Hot plate Thiele tube Inches
200-250 200-250 350-400
223,228 235 228 350'(approx.)
5.0-5.6 4.0-4.5 3.5-4.0
400 (approx.)
350 (approx.)
3.0-3.5
Vol. 47, No. 7
somewhere between the extremes recorded. The agreement between results determined by the two methods is good, and is shown in Table VI together with the char length normally obtained with a pickup of 15 to 20% of the flameproofer on 8-oz. twill. The flameproofing resins with the higher decomposition temperatures produce the shorter char lengths, and the char lengths reported correspond in a qualitative manner with the location of the decomposition temperature of that material as shown in Figure 3. Thus, the flameproofing agent must decompose t o furnish a dehydration catalyst and should do so a t a temperature approaching, but below, 500' C., the burning temperature of cellulose. Decomposition a t temperatures much below 500' C. is attended by proportional decomposition of the cellulose and a long tear which is interpreted as a long char length. Some long char lengths of treated samples were apparently due to weakening of the cloth by the heat and action of the flameproofer rather than to actual burning of the sample. Relation of Amount of Char Residue to Flameproofing Efficiency. A nonflameproofer such as sodium chloride can bring about the deposition of a residue which is highly dehydrated. The results of analyses of two series of samples are given in Table VII. The percentage residue derived from the samples treated with sodium chloride has been corrected by subtracting the weight of sodium chloride contained in the treated sample before thermal decomposition, since analyses have shown that all the sodium chloride is retained in the residue. Unfortunately, such corrections cannot be applied to samples treated with lithium chloride. Investigation of one sample with an original pickup of 13% lithium chloride showed that the residue contained only about 15% of the chloride and 30% of the lithium. Thus, correction of the amount of residue by subtraction of the weight of applied salt would give a minimum corrected weight only. Table VII. Analysis of Samples Decomposed with Sodium and Lithium Chlorides % Residue Pickup
Uncorreoted
4.8 7.1 10.7 14.1 21.2 31.1
5.7 8.0 11.0 13.6 18.3 24.5
2.6 4.4 5.8 9.8 15.7 21.4
5.6 10.1 15.9 25.2 31.4 33.2
Correoted
Analysis of Unextraoted
C
NaCl 21.2 17.3 13.7 8.3 5.7 5.1 LiCl 3.1 50.1 62.9 6.1 66.4 11.0 18.0 64.6 20.1 56.6 48.2 19.0
1.2 1.5 1.6 1.2 1.0 0.9
Ratio Atoms, C/H
~~l~~
0.75 0.76 0.52 0.47 0.50
2.07 1.92 1.50 1.33 1.01 0.84
3.6 3.5 3.0 2.8 2.0 1.4
1.70 1.83 1.94 2.05 1.77 1.86
2.89 2.87 2.85 2.62 2.67 2.16
4.0 3.9 3.9 3.8 3.8 3.6
% H
0.86
Hz0
Lost
The data listed in Table VI1 demonstrate that the dehydration efficiencies of both salts decrease with increasing pickup, and although theefficiency of sodium chlorideisinferior to that of lithium chloride, a minimum pickup is equivalent in dehydration efficiency to an excess of lithium chloride. The efficiency of sodium chloride 8s a char producer, however, is negligible. This dehydration effect of nonflameproofers such as sodium chloride, and potassium chloride, has been indicated by the work of earlier investigators ( I , 7 , I I , 17) on the carbonization and pyrolysis of cellulose in closed systems. I n view of the foregoing evidence a flameproofing agent must be considered as one which during combustion dehydrates a major portion of the cellulose, and in doing so deposits a large residue. This deposition of residue is illustrated in Figure 8 which shows the effects of sodium chloride, lithium chloride, and bromopolytriallyl phosphate. All three curves are uncorrected.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
35
/
Brorn-polyTA,E
1439
That the reaction involved in flameproofing is a dehydration of the cellulose is shown by carbon and hydrogen analyses of the char residue. The results compare favorably with those calculated for a n 80% theoretical dehydraticn of the cellulose. Moreover, examination of the charred residues of cloth flameproofed with several organic phosphorus resins has shown the presence of acids of phosphorus. Such agents (Lewis acids) bring about the catalytic dehydration of alcohols and the presence of such an agent suggests a similar mechanism in the decomposition of flameproofed cellulose. These acids attack esters in much the same manner as alcohols but in so doing liberate acids instead of water. Perhaps the production of carbon and acetic acid instead of carbon and water explains why cellulose acetate cannot be flameproofed with the reagents used for flameproofing cotton. ACKNOWLEDGiMENT
,
PICKUP
Figure 8.
-%
16
20
I
24
Residues from treated cotton cloth decomposed at 500’ C.
The authors wish to acknowledge with thanks the helpful interest shown in the project by Stephen J. Kennedy as well as the benefit derived from discussions of the theoretical considerations with Allan J. McQuade, Irvin M. Gottlieb, Ramon Esteve, and George Mixter. The authors are indebted to Ruby K. Worner and to L. E. Brown, H. B. Moore, and R. E. O’Connor for their assistance in carrying out the analyses and tests reported. The Cotton Chemical Processing Section of this Laboratory kindly furnished the treated fabrics used in some of the studies described. LITERATURE CITED
(1) Blayden, H. E., Gibson, J., Riley, H. L., and Taylor, A,, Fuel,
I n an attempt t o relate the amount of char with the ability of an agent to flameproof, aliquots of samples treated with lithium chloride and bromopolytriallyl phosphate were subjected to decomposition on the hot plate and t o the vertical char test (19). The results are shown in Table VIII. Although the evidence is meager i t appears that an agent must produce approximately 20y0 residue in order to be classified as a flameproofer.
Table VIII. Variation of Char Length and Residue with Pickup Agent
Pickup,
%
Residue a t 500’ C.,
%
21 17 13
35 32 29 24 10 7 18 6 16 4 10 Bromopolytriallyl 24 28 phosphate 21 27 16 26 13 23 22 11 20 8 a Burned to end of s a m p l e u s u a l l y about 12 inches LiCl
Vertical Char Length, Inches 3.2 3.3 4.1 5.0 5.4 B. E.5 B. E. 4.5 4.6 5.2 6.0 B. E. B. E.
SUMMARY AND CONCLUSIONS
A flameproofing agent when applied t o cotton cellulose behaves as a catalytic agent when the cloth is ignited, changing the course and speed of the reactions involved. That decomposition of the cellulose is accelerated and gas production is decreased is evidenced by the large amount of carbanaceous material left behind. Furthermore, although the gases produced are flammable, they are evolved at a much lower temperature with treated cloth than with untreated cloth, and are liberated and dissipated at temperatures probably too low for ignition. Similar results were obtained in a study of 9 different flameproofing agents with regard to the speed of decomposition and the amount of char produced.
19, 24 (1940). (2) Britton, H. T. S., “Conductometric Analysis,” Chapman & Hall, London, 1934. (3) Campbell, K. S., and Sands, J. E., Teztile World, 96, No. 4, 118 (1946). (4) Church, J. M., U. S. Quartermaster Corps. Textile Ser. Rept. 38, 308 (1952); released by Off. Tech. Serv. (U. 8. Dept. Comm.) as PB-111007. (5) Cooper, A. S., Voorhies, S. T., Jr., Buras, E. M., Jr., and Goldthwait, C. F., Textile Inds., 116, No. 1, 97 (1952). (6) Frick, J. G., Jr., Weaver, J. W., and Reid, J. David, Textile Research J., 25, 100 (1955). (7) Lessing, R., and Banks, M. A. L., J. Chem. Soc., 125, 2344 (1924). (8) Little, R. W., “Flameproofing Textile Fabrics,” Reinhold, New York, 1947. (9) Little, R. W., Textile Research J . , 21, 901 (1951). (10) Luder, W. F., and Zuffanti, S., “Electronic Theory of Acids and Bases,” Wiley, New York, 1946. (11) Martin, Glenn L. Co., British Patent 688,372 (March 4, 1953). (12) Parks, W. G., Erhardt, J. G., Jr., and Roberts, D. R., Am. Dyestuff Reptr., 39, 294 (1950). (13) Pons, W. A., Jr., Stansbury, M. F., and Hoffpauir, C. L., J. Assoc. Ofic. Agr. Chemists, 36, 492 (1953). (14) Ramsbottom, J. E., “Fireproofing of Fabrics,” H.M. Stationery Office, London, 1947. (15) Reeves, W. A., and Guthrie, J. D., U. S. Dept. of Agriculture, AIC-364, 1953. (16) Schuyten, H. A., Weaver, J. W., and Reid, J. David, Advances in Chem. Sers., No. 9, pp. 7-20, Academic Press, New York, June 1954. (17) Tamaru, K., Bull. Chem. Soc. J a p a n , 24, 164 (1951). (18) U. S. Dept. of Agriculture, Annual Report of the Chief of the Bureau of Agricultural and Industrial Chemistry; abstract Daily News Record, No. 3 3 , l (Feb. 17, 1953),and Chem. Week, 72, No. 8, 12 (Feb. 21, 1953). (19) U. S. Federal Supply Service, Federal Specification CCC-T-19lb, Method 5902 (1951). RECEIVED for review April 24, 1954. ACCEPTED April 21, 1955. This is a report on one phase of a program of research on the flameproofing of cotton textiles being supported a t the Southern Regional Research Laboratory, one of the Laboratories of the Southern Utilization Research Branch, Agricultural Research Service, U. S. Dept. of Agriculture, with funds supplied by the Office of the Quartermaster General, Dept. of the Army, and conducted under the general supervision of the Research & Development Laboratories, Philadelphia Quartermaster Depot, Philadelphia, Pa. The mention of trade products and firms does not imply endorsement b y the Department of Agriculture over similar products or firms not mentioned.
