Nylon Flammability-Effects Compounds
of Thiourea, Ammonium Sulfamate, and Halogen
Kenneth B. Gilleol Speciality Chemicals Division, Allied Chemical Corporation, Butlalo, New York 74210
The burning mechanism of nylon is described in detail. Primary decomposition involves random C-N bond breaking to form oligomers but not volatiles, while secondary decomposition involves rapid depolymerization to yield large amounts of monomer fuel. Both homolytic and heterolytic cleavage of the amide linkage are proposed. The addition of thiourea to Nylon 6 or 6,6 lowers the melting points by 20-40°C and reduces flammability by promoting dripping and also by altering decomposition. Possible mechanisms are discussed. Ammonium sulfamate showed similar but less pronounced effects. Some halogens were found to increase nylon's flammability by preventing dripping in a flame and by catalyzing thermal decomposition. Thermal data indicate that halogen compounds can lower the decomposition temperature of Nylon 6 by 100 "C. TGA, DSC, and 01 data form the basis for most conclusions.
Many nylons are considered to have low flammability. With the exception of aromatic polyamides, nylons are fusible resins with large differences among their melting points and ignition temperatures. Also, a low melt viscosity, which may be the result of degradation, contributes to the apparent low flammability. Light-weight nylon apparel usually drips away when held in a flame and does not continue to burn if ignition occurs a t all. This is not true of heavier upholstery or drapery fabric unless it is treated with melt-promoting additives. In addition, the application of backing, especially halogen-containing material, can greatly increase flammability. Possible mechanisms for these effects will be discussed. The Burning Mechanism of Nylon The burning process which occurs in nylon is not as well understood as it is for some substrates such as cotton. However, enough work has been done to allow basic conclusions to be drawn. For the purpose of this discussion, the burning process will be divided into these distinct stages: stage I, thermal initiation (heat input from an external source involving a flame); stage 11, transition (physical changes including deformation and melting); stage 111, primary decomposition (early chemical changes involving bond breaking not leading to significant amounts of volatile fuel); stage IV, secondary decomposition (rapid pyrolysis where depolymerization occurs to generate substantial amount of fuel); stage V, ignition (flash-ignition point: condition where volatiles produced in stage I11 are ignited by the thermal source); and stage VI, combustion (self-sustaining exothermic oxidation which provides thermal energy to maintain the earlier stages). It should be mentioned that different authors included additional stages and applied varying definitions. Einhorn (19701, for example, defines polymer burning in nine separate stages. Pearce, et a1 (1973), used the term degradation to denote loss of physical properties. Stage I. Thermal Initiation. In discussion actual burning, the thermal source will be a gas burner prescribed by the Sleepwear Standard (DOCFF3-71) and Upholstery Flammability Regulations such as KYPA Method 5903 (Federal Test Method Standard No. 191) and NFPA KO. 701. Upholstery regulations usually call for the application of a 1M-in. flame to the edge of vertically held fabric for 12 sec. This is usually enough time to ignite heavier weight fabrics although a 3-sec burn often is not. The Present address, 3M . Company, St. Paul, Minn. 55101.
thermal initiation stage must provide enough energy to initiate the next four stages a t which point combustion takes over to supply energy. Stage 11. Transiton. Most polyamides, on heating, soften and melt. The transition stage simply involves these physical changes, with very few chemical changes occurring other than those involving hydrogen bonding. In the case of Nylon 6, which will be t h e example used most often, melting occurs at 220-230°C depending on molecular weight, crystallinity, and other variables. The melting point is easily seen in the DSC curve (Figure I) as a small endotherm, in this case occurring a t 223°C. Referring to the TGA curve, it is seen t h a t no noticeable weight loss has occurred at the melting point. Stage 111. Primary Decomposition. This is the stage where chemical bond breaking begins. It is a very import a n t stage, particularly in the case of thiourea treatment which will be discussed later. Again, referring to the DSC and TGA curves, it is seen t h a t a t about 350°C, energy is being absorbed while no significant loss of weight is occurring. In fact, gas evolution and weight loss are not significant until 370°C and higher. Apparently, random bond-breaking rather than depolymerization is occurring u p to around 370°C. Stepniczka (19731, as well as Straus and Wall (19581, have concluded that early pyrolysis of nylon does not involve a pure freeradical depolymerization process, but rather random bond scission. Probably the long nylon chains are being broken down to low volatility oligomers a t this stage without much monomer (caprolactam) formation. At higher temperatures, rapid weight loss is noted and caprolactam is detected as the major product in air or under Nz. The weakest chemical bonds will likely rupture first as thermal energy is applied. Table I lists various chemical bond energies. The C-N linkage, being the weakest, is probably the only one involved in the degradation stage. Achhammer (1951) concluded t h a t homolytic and heterolytic cleavage occur. Stage IV. Secondary Decomposition. We have defined this stage to mean the more vigorous, fuel-producing pyrolysis, which begins after 370" and becomes pronounced beginning at about 385°C as seen in Figure 2. Whereas the primary decomposition stage involved random C-N bond breaking as the predominant process, secondary decomposition entails depolyinerization to monomer. We have found t h a t the major pyrolysis product in decomposition is caprolactam which concurs with the findings of Barker, et al. (1972). Smaller amounts of caprolactam dimer and Ind. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 2, 1974
139
A 275
200
350
425
TEMPERATURE
'
500
OC
0 0
Figure 1. Nylon 6 (in air); DSC, lO"/min rate.
c-c
N-H C-H
c=o
a
66 . O kcal/mol 8 0 . 5 kcal/mol 9 2 . 2 kcal/mol 9 8 . 2 kcal/mol 173 . O kcal/mol
U. S. Patent 2,854,437 U. S. Patent 2,859,206 U. S. Patent 2,881,152 U. S. Patent 2,923,644
The Burning Process One observes that untreated, lightweight nylon fabric, free of spinning oils and lubricants, often extinguishes after a few seconds when the fabric is ignited with a small flame. When a flame is applied, the low melting polymer shrinks away and recedes from the flame or may even melt and drip without burning. The thermal source proDevelop.,Vol. 13, No. 2,
600
500
700
800
9co
C'
T a b l e 11. Thiourea Patents
Janz (1967).
Ind. Eng. Chem., Prod. Res.
400
XK)
Figure 2. Nylon 6, TGA (in air), lO"/min.
trimer, hydrocarbons, cyclopentanone, COz, "3, and water have been detected by Straus and Wall, Achhammer, and Barker, et al. The major difference between the primary and secondary decomposition stages is that considerable volatile fuel is produced in the latter even though both involve chemical bond breaking. Secondary decomposition probably involves C-C as well as C-N bond breaking. A maximum decomposition rate occurs a t 450-470°C. Pyrolysis studies, notably those of Straus and Wall, are not consistent with a purely homolytic cleavage reaction. They concluded that decomposition at moderately high temperatures was about half homolytic scission and half hydrolytic cleavage. A significant amount of water is presumably tightly bound to the polyamide linkages which become involved in the hydrolysis reaction according to Achhammer. Water also may form during pyrolysis by condensation reactions. Stage V. Ignition. Data from Goldstein (1968) and Rieber (1969) show that nylon begins to smoulder a t about 420°C and ignites a t 421-425°C. The ignition point is referred to as the flash ignition temperature and a flame or spark is the ignition source. The autoignition temperature is that point where reactions become self-sustaining to the point of ignition. Stage VI. Combustion. Combustion of nylon, as with most polymers, is a gas-phase reaction which, therefore, requires volatile reactants. The combustion stage will, therefore, be affected by any factors influencing the secondary decomposition stage. One may view the process in simplified form as a vapor-phase oxidation of caprolactam which generates sufficient thermal energy to maintain earlier burning stages. More than likely, a free-radical oxidation predominates, and as such, should be affected by free-radical inhibitors. We have observed that surface temperatures of more than 500°C in the normal burning of nylon occur. Such a temperature is sufficient to allow rapid decomposition of the polymer to gaseous fuel.
140
200
TEMPERATURE
Table 1.e Bond Dissociation Energies C-N
100
1974
U. S. Patent 2,992,726 U. S. Patent 3,308,098 U. S. Patent 3,376,160 U. S. Patent 3,627,767
Urea-thiourea-formaldehyde-
bisulfite A1kyl-thiourea-ureaformaldehyde Urea-thiourea-condensate Thiourea-formaldehydesulfonate Thiourea-formaldehyde Urea-thiourea-formaldehyde resin Thiourea-APO Thiourea-phosphorus compound
vides energy for the transition stage, but physical removal of the fabric from the thermal source occurs before later stages are reached. In other instances, the fabric may ignite, but the heat generated allows the nylon to drip away, carrying the flame with it. The large difference in nylon's melting point and ignition point, in addition to a low melt viscosity, causes certain fabrics to burn incompletely. If, however, a thread of glass wool is sewn into the fabric, the material will burn the entire length. As heavier weight fabric, such as upholstery. is tested, the material cannot recede from the flame as rapidly and ignition occurs. Drip-away often cannot occur rapidly enough to prevent continued burning. Additives which promote this drip-away phenomenon should reduce nylon's flammability provided the sample is burned vertically and drip-away is not mechanically interfered with. Drip-Promoting Nylon Flame Retardants Thiourea. Thiourea has long been known to flame retard nylon. Table I1 lists some of the systems claimed in patent literature. When nylon fabric. which is normally flammable, is treated with a thiourea system, the fabric often does not ignite when held in a flame. However, the area destroyed (melted away) is often significant. In cases where the sample does catch fire, the flaming portion quickly drips away as in the situation with lighter weight fabrics. These observations suggest that thiourea, a t least in part, flame retards nylon by promoting dripping. Douglas (1957) has proposed that thiourea functions by lowering the melting point of nylon and also by lowering the melt viscosity. Table IIIA shows the effects of thiourea and other sulfur compounds on the melting point of Nylon 6,6. Table IIIB gives similar data on Nylon 6. In both cases, the additives were applied from an aqueous solution and the fabric was run through a wringer (laboratory padder) to remove excess pad bath. Samples were dried a t room temperature to constant weight. Douglas obtained melting point values by heating treated fabric in sealed glass tubes until fusion occurred. Those values for Nylon 6 were obtained from DSC curves and so allowances should be made for the differences in these techniques. Thiourea
Table IILa
A. Effects of Additives on Nylon 6,6* Melting point, O C
Treatment None-control Thiourea Ammonium thiocyanate Ammonium sulfamate
245-250' 200-210 200-210 235-240
(10.4% by weight) ( 7 . 8 % by weight) (11.3% by weight)
Flame Retardant Efficiency Treatment
F R efficiency
Thiourea Ammonium thiocyanate Ammonium sulfamate
Good Intermediate Fair
500
350 TEMPERATURE 'C
Figure 3. Nylon 6 (in air), 10%thiourea; DSC, 10"/min. G L A S S COLUMN ( M i a Dam. 45 cm H. x 7.5~171 I.D.
B. Effects of Additives on Nylon 6b
Melting point, C0
Treatment Control Thiourea Ammonium sulfamate
I
I
200
223d 205 216
(10% by weight) (10% by weight)
Flame Retardant Efficiency-Vertical
Test
Treatment
F R efficiency
Thiourea Ammonium sulfamate
Good Fair
uDouglas (1957). *Applied from aqueous pad bath. Measured by heating treated fabric in sealed glass tube. Measured from DSC curves of nylon fabric. depressed the melting point to the greatest degree and was also the most efficient flame retardant. Ammonium sulfamate caused less of a melting point depression and was inferior as a flame retardant. Figure 3 shows the DSC curve for Nylon 6 treated with thiourea. The melting point is seen as a gradual endotherm beginning a t 190°C and ending at about 220°C. The melting point endotherm is not nearly as sharp as for untreated nylon. This is often the case when additives are present. It is probably also the result of a nonhomogeneous sample resulting from a finish application. Careful examination of thermal data for nylon treated with thiourea indicates that the primary decomposition pattern as well as the secondary decomposition pattern is changed. Therefore, thiourea does not just simply lower the polymer melting point. A t about 270"C, a small endo spike is seen in the DSC curve which is not present in curves of Nylon 6 or thiourea alone. This is indicative of a reaction between molten nylon and thiourea. After the 270°C endotherm, heat absorption continues to about 300°C. The usual primary decomposition endotherms occur around 350-370" and then merge into the secondary decomposition endotherms a t 380-385°C. The decomposition slope is suppressed somewhat and is also much smoother. This suggests that while decomposition is still occurring a t a rapid rate, the complexity (total number of reactions) has been reduced. One also observes a sharp exothermic peak a t 420°C which does not correspond to thiourea decomposition. One can thus conclude that thiourea is not simply a "drip-promoter'' but also modifies the decomposition reactions. Pyrolysis studies could give some insight into what is actually happening. Oxygen Index (01)measurements were used to examine the effects of additives on nylon flammability. Figure 4 gives a diagram of the apparatus. Values for thiourea and other additives are given in Table IV. Although thiourea greatly increases the 01 value for
ox YGEN
Figure 4. Limiting oxygen index apparatus: 1, burning specimen; 2, clamp with rod support; 3, igniter; 4, wire screen; 5, ring stand; 6, glass beads in a bed: 7, brass base: 8, tee; 9, cut-off valve; 10, orifice in holder; 11, pressure gauge; 12, precision pressure regulator; 13, filter.
Table IV. 01 Values for Nylon 6 ,
(1) (2) (3 )
Additive (7% by weight)
01 value
Control Nylon 6 Thiourea 1,4,5,6,7,7-Hexachloro-N,N'-bis(thiocarbamoyl) -5-norborene-2, 3-dicarboxamide
0.245 0.340 0.345
nylon, substantial dripping of the sample occurs carrying away fuel and heat. A rough indication of flame retardancy contribution by the drip-away mechanism is obtained by mechanically preventing the sample from dripping. This can be accomplished by adding an inert fiber to the nylon sample such as asbestos or glass or by sewing a glass thread through the fabric sample. A newer technique is to use the liquid sample apparatus developed by General Electric. In this method, powdered sample is burned in a ceramic cup, thus eliminating dripping without introducing other variables. When dripping is prevented, the 01 value is lowered substantially for the control and treated samples. Table V shows this effect. However, the 01 value for the thiourea sample is still higher than the control. One may conclude Ind. Eng. Chem., Prod. Res. Develop.,Vol. 13, No. 2 , 1974
141
/4
Table VI. 01 Values for Acid-Forming Compoundsa
,y--p
gt W
’\CCNTROL
& !
~
216
.__. I
I
I
200
VIOLENT
- - - - - - - - - _.... --
EXOTHERM
.
.
~
....
‘.--.--
0.245 0.205 0.220
0.205 0 .232b
0.220b
500
~ N y l o n6, 2.5% by weight phosphorus. bBarker, et al. (1972).
OC
Figure 5, Nylon 6 (in air), 10% ammonium sulfamate; DSC, lO”/min. T a b l e V. 01 Values for Nylon 6” Additive (7% by weight) Control-none Thiourea 1,4,5,6,7,7-Hexachloro-N,Nf-bis(thiocarbamoyl)-5-norborene-2, 3-dicarboxamide
01 value
0.205* 0.225
Table VII. 01 Values for Halogen Compounds”
Additive
01 value
Control Tris(trichloropheny1) phosphate Tris(tribromopheny1) phosphate Tetrabromobisphenol A Dechlorane
0.245 0.220
0.240 0.240 0.235
”Nylon 6, 6% additive by weight. 0.240
Dripping is prevented mechanically. bThe lower value (normally 0.235-0.245) for untreated Nylon 6 results when dripping is prevented. A value of 0.201 is reported for glassstitched fabric (Lyons, 1970). 5
that although thiourea predominantly flame retards nylon by promoting dripping (transition stage), the compound also influences both decomposition stages. Hasselstrom (1952) suggested that sulfur-containing nylon FR’s produce acidic sulfur compounds on pyrolysis which decompose intermediate ammonium carbonate, releasing GO2. This does not seem to be a satisfactory explanation. Other acid producing species usually increase the flammability of nylon. Table VI gives 0 1 values for phosphorus compounds which yield phosphorus acids on pyrolysis. It is found that the addition of phosphorus compound to nylon often lowers the 01 value and increases overall flammability unless large loadings are used. However, in the case of phosphorus compounds, char formation occurs which interferes substantially with dripping. Therefore, a more realistic control for comparison is a sample of untreated nylon where dripping is prevented. Even when this is done, only a slight increase is seen for phosphorus derivatives as shown in Table VI. Halogen compounds, capable of forming HX, on pyrolysis, also decrease 01 values. Drip prevention does not appear to play an important role, however. As seen in Table VII, Dechlorane, which increased dripping, was less effective than tris(trihromopheny1) phosphate which inhibited dripping. Halogen compounds will be discussed in greater detail in the next section. Lyons (1970) has speculated that thiourea cross-links nylon a t the burning temperature. This could be possible and such a mechanism is compatible with the data. Possible mechanisms by which thiourea may interact with nylon are: (1) reaction with carboxylic acid end groups present or formed; (2) reaction with amine end groups present or formed; (3) nucleophilic attack by sulfur atom on polyamide linkage; (4)reaction with fragments arising from homolytic cleavage in the condensed phase; and ( 5 ) general free-radical inhibition in condensed and vapor phases. Ammonium Sulfamate Ammonium sulfamate has been described in the patent literature as a flame retardant for nylon and cotton. Douglas concluded that this compound acted on nylon by 142
01 value
____._
350 TEMPERATURE
Additive
Control Control, dripping prevented Tris(trichloropheny1) phosphate Tris(pbromopheny1) phosphate Triphenyl phosphate Triphenylphosphine oxide
Ind. Eng. Chem., Prod. Res. Develop.,Vol. 13, No. 2, 1974
a drip-promoting mechanism and gave melting point data (Table 111) to support this view. Melting points are obtained by heating fabric samples in sealed glass tubes. The fire retardant was applied by a pad-bath technique described earlier. Ammonium sulfamate was not as effective as thiourea in flame retarding nylon and the compound did not depress the polymer’s melting point to as large a degree. It would seem then, that a drip-promoting mechanism accounts for the flame retardancy. As was the case with thiourea, thermal data indicate a more complicated situation than simple melting-point depression. The DSC curve (Figure 5) of Nylon 6 fabric treated with an aqueous solution of ammonium sulfamate to give 10% add-on shows a melting point depression to about 216°C. Beginning at 260”C, a gradual endotherm is seen which becomes more pronounced at 272 reaching a maximum a t 284°C. This is a likely indication of a reaction with nylon, and much more pronounced than in the case of thiourea. At 365”C, a violent exotherm occurs, followed by a somewhat normal decomposition pattern. Thus, ammonium sulfamate clearly affects both the transition and decomposition stages of nylon. These data give one some idea of how complex the flame retardancy mechanism can be. While superficially both thiourea and ammonium sulfamate appear to function by melting point depression, complex reactions during pyrolysis occur which, apparently, are different for each of the additives. Thermal analysis is a valuable means of detecting interactions between substrate and additive. Effects of Halogen Compounds Nylon upholstery is customarily backed with a latex in the finishing stage. Once backed, however, the fabric is very difficult to flame retard and thiourea compositions are ineffective. It is observed that latex backings prevent the dripping away of burning nylon. This has been referred to as “grid” or “scaffolding” effect by Kruse (1969). The same phenomenon occurs when strands of glass wool are laid on the fabric. Polyvinyl chloride latex or vinylidene chloride latex, although regarded as self-extinguishing, can greatly increase the flammability of nylon. It has been assumed by most that this was due to the scaffolding effect entirely. We have observed that the addition of many halogen compounds to nylon increases flammability, especially a t low or medium loadings. Table VI1 gives some 01 values for halogen additives. The most obvious explanation for the decreased 01 values, in view of halogen’s known inhib-
1 0
w z
I
200
260
F i g u r e 6.
,
425
350
TEMPERATURE
500
O C
Nylon 6 (in air), 1% PVC latex; DSC, lO"/min.
0
z W
0 W X
1
zoo
260
350
F i g u r e 7.
500
425
TEMPERATURE
"C
PVC latex (in air); DSC, lO"/min.
Table VIII. Flammability of PVC/Nylon Blendsa (12-sec Vertical Flame Test)
Fabric 100% 75% 50% 25% 0%
Afterflame, sec
Char length, in.
85 90 79 1 1
9 10 8 3 2.4
Nylon Nylon Nylon Nylon Nylon
aNylon 6, PVC fiber blend. iting effect on free-radical oxidation, is that the compounds catalyze decomposition of nylon. Busse (1968) and later Barker, et a / . , have observed that halocarbons often catalyze the thermal decomposition of nylon unless the compound is highly thermally stable as, for example, certain aromatic chlorine compounds. Barker, et al., found that both aliphatic and aromatic bromine compounds lowered the decomposition temperature of nylon by 100°C. The usual flame retardancy offered by halogen compounds is, therefore, offset by their ability t o catalyze fuel-forming reactions. DSC data for nylon treated with halogen containing latex showed a much earlier and more rapid decomposition (Figure 6). The melting point of Nylon 6, coated with a vinylidene chloride latex to the extent of only 1% by weight, was depressed slightly to 221°C. A decomposition endotherm began a t 260"C, or about 100" lower than normal. Rapid decomposition commenced a t 355°C. Overall, a small amount of halogen compound was found to cause earlier and much more severe decomposition. One thus concludes that halogen compounds, in intimate contact
with nylon, can increase flammability by physical as well as chemical means. Figure 7 shows the DSC curve for vinylidene chloride latex film. Nylon-PVC yarn blends of upholstery fabric were tested for flammability using a 12-sec vertical burn (1% in. higher burner flame). Table VIII gives results. Samples were very flammable even a t 50% PVC. PVC catalyzed the decomposition of nylon. Presumably, released HCI causes C-N bond rupture to occur a t lower temperatures resulting in more fuel formation. To offset this effect, large amounts of PVC are required preferably containing antimony oxide. The case appears to be similar for bromine compounds according to Barker. A self-extinguishing blend was not achieved until 75% PVC was used. Summary Nylon can be flame retarded by thiourea in certain situations. A drip-promoting mechanism appears to predominate since the additive is not effective when dripping is mechanically prevented. However, thermal and flammability data suggested that a second mechanism operates which involves altering the decomposition process in nylon. Polyvinyl or polyvinylidene chloride, in contact with nylon as a back coating or fiber, increases its flammability at low and medium levels without additives. DSC data show that PVC and other halogen compounds promote the decomposition of nylon to volatile fuel. Acknowledgment The author thanks P. Koch and T. Largman for 01 values, E. Turi and F. Evans for thermal data, and E. Pearce for review of the manuscript. Literature Cited Achhammer, B. G., J. Res. Nat. Bur. Stand., 46, 391 (1951). Barker, R. H., Bostic. J. E.. Jr., Reardon, T. J.. Strong, R. A,, "Effect of on the Flammability of Polyester and Nylon,'' paper presented at the 164th National Meeting of the American Chemical Society, New York, N Y., Sept 1972. Busse, W . - F , , (to E. I. duPont de Nemours 8 Co.), U. S. Patent 3.418.267 ~ . _i l 9 .B A~I Douglas, D. O., J. Soc. Dyers Colour., 73, 258 (1957). Einhorn, I. N., "Fire Retardance of Polymeric Materials," Polymer Conference Series, University of Utah, June 1970. Goldstein, H. E., preprint, Division of Organic Coatings Plastics Chemistry, American Chemical Society, 1968. Hasselstrom, T., Coles, H. W.. Balmer, C. E., Hannigan, M . , Keeler, M. M., Brown, R. J.. Text. Res. J., 22, 742 (1952). Janz, G. J., "Thermodynamic Properties of Organic Compounds," Academic Press, New York, N. Y., 1967. Kruse, W., Melliand Textilber., 50, 460 (1969) Lyons, J., "Current Trends in Techniques for Obtaining Flame Retardancy," Polymer Conference Series, University of Detroit, 1970. Pearce, E. M., Shalaby, S. W.. Barker, R. H., "Flame Retardance of Polymeric Materials," Gordon and Breach Science Publishers, Inc., New York, N. Y., 1973. Rieber, M., Chemiefasern, 69, 375 (1969). Stepniczka, H . , Ind. Eng. Chem., Prod. Res. Develop., 12, 29 (1973) Straus, S..Wall, L., J. Res. Nat. Bur. Stand., 60, 39 (1958).
Received f o r revielc October 12, 1973 Accepted February 28, 1974
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 2,1974 1 4 3
