Burning Behavior of Some Cellulose Ester Film Compositions C. J. MALM, N. G. BAUMER, AND G. D. HIATT Cellulose Acetate Development Division, Kodak P a r k Works, Eastman Kodak Co., Rochester 4,
UCH work has been done in the textile field in treating cotton goods to effect some degree of fireproofing. This work has dealt with the various types of chemical treating agents and with the postulated mechanisms of reaction brought about by these treatments ( 2 , 4 ) . The common explanation for the effect of most chemical treatments is that, in the range of 300" to 500" C., the course of the decomposition is changed so that a large portion of the organic material is converted to carbon or relatively nonvolatile oils. As a result, the portion of volatile and highly flammable gases is reduced, with a corresponding reduction in vigor of burning.
ili.
Y.
Flammability was studied by coating a typical cellulose ester using an e . d y released solvent. Different materials were added to the coating solution, so their antiflame activity could be observed by burning strips of the cast film. TESTING PROCEDURES
A commercial yarn-type cellulose acetate (3!3.2y0 combined acetyl) coated from acetone was chosen as the test material. This ester releases acetone quickly and is easily knife-coated on 8 X 10 inch glass plates t o give test films 0.005 inch thick. Drying a t 50" C. for 2 days was chosen after it was found that a 7-day cure shoved no significant change in burning results. Ten test strips inch wide mere cut in the 10-inch direction for burning Seven holes inch in diameter were punched along the center line of each strip. A wire 0.014 inch in diameter was laced through the holes, pulled tight, and weighted a t the lower end to keep the vertically suspended test strip straight during the test. Without the wire, the strip is apt to curl up on itself during burning. The test m-as run in a covered drum 18 inches in diameter, having a slit in the front for lighting and observation (Figure 1). The strip was lighted with a Bunsen burner and the timer started. If the flame went out or if the fire dropped from the strip, the strip was relighted. The time when the whole length had been consumed was recorded in seconds. The figures for burning time (B.T.) and number of relights (R.L.) represent averages from ten strips. The burning behavior during this test, which approximates the ASTM procedure ( I ) , is a t times confusing. The strip may burn steadily to give easily reproducible results, or may go out because the burning end melts and falls off, carrying the fire with it. The relighted end again will burn for a while, then the heated area will fall, carrying the flame, and go out. This behavior is presumably desirable because, in an actual fire, the flame would not be held aloft to pass on to the mrroundings, and, in going out, the piece is not contributing to the total heat of combustion. PRELIMINARY TESTS
A series of compounds was first tried for its influence on burning, representing a wide range of types (Table I). The unmodified acetate, itself, generally requires 19 to 24 seconds to burn, with little need for relighting. Triethyl phosphate (boiling range 210' to 220' C.), under the test conditions, seems to be too volatile, so that, a t a combustion temperature of 300' to 500" C., the antiflame agent has already volatilized from the thin Table I. Typical Observations o n B u r n i n g Added, Parts per 100 Cellulose Ester' 10 20 J3.T.b R.L.6B.T. R.L. Triethyl phosphate 20 0.1 24 1.8 Triphenyl phosphate 30 7.6 32 9.2 Phosphoric acid, 85% 30 5.9 (1 pt.) 18 2.3 (3 pts.) Triphenyl phosphite 17 18 0.1 34 3.8 Methyl phthalate Triphen'' thiophosph5te 44 18 7"' . 19 .. Octyl phthalitte 19 20 .. 23 Glyptal 2557 0.3 26 1.1 Succinic acid 25 0.2 .. .. 23 0.3 21 0.4 Hexachlorobenzene Titanium dioxide 25 1.6 29 0.4 B.T. of cellulose ester 20 seconds, no R.L. b B.T. Burning time, seconds. R.L. No. of relights. C Modified alkyd resin supplied by General Electric Co.
F i g u r e 1. Test of b u r n i n g behavior
.
Cellulose ester sheeting differs from cotton fabrics in physical construction. Chemically, it is an ester and not an alcohol like cotton. The flame-reducing agent is generally incorporated in the plastic film instead of being applied onto it, as in textile applications. The mechanism of activity, however, is probably th.e same, in that the more effective antiflame agents change the caurse of decomposition as the film is brought to the ignition temperature. 2521
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2522
Table 11. Behavior at Low Plasticizer Levels 10
Tributyl phosphate
10 5
No addition
R.L.
B.T. 52 43 44 32
Parts Triyhenyl phosphate
5 1 0.1
11
9.9 3.1 0.3
1 0.1
30 39 41 28
1 2.8 3.3
..
26
0
0
Table 111. Testing Lower Acid Levels Ester Xormal
Addition None Phosphoric acid pToluenesulfonic acid Sulfuric acid
Batch A
Parts/100 1.0 0.5 0.1
2 1 0.1 0.5 0.1 0.01
None
B.T. 20 27 27 24 22 33 36 26 31 43 40 31
ai
23 30 28
R.L. 716 9.2 9.9 2.5 5.9 13 2.7 7.9 7.6 15 12.3 .. i:7 9.8
Vol. 47, No. 12
and phosphoric acid are incorporated, down to low levels. As the amount of triphenyl phosphate drops below the 5-part level, its effectiveness falls off. One part of triphenyl phosphate should furnish about 0.3 part of phosphoric acid, yet levels of 1% (and preferably less) of phosphoric acid are active. It is possible that most of the triphenyl phosphate vaporizes as such and is not effective. This argument could also be used for the alkyl phosphates which, under certain conditions, are more stable and/or lower boiling and so escape without forming acid. High levels of phosphoric acid, for some reason, appear to allow the “normal” burning pattern. EFFECT OF MINERAL ACIDS OR ACID FORMERS
Low levels of several acids were first dissolved in a little water and then diluted with the acetone solvent. The cellulose ester was dissolved and coated from this mixture. The total water in the finished solution was only about 3%. Very low amounts of the mineral acids and of p-toluenesulfonic acid produce high relight figures, whereas high amounts of acid are less useful (Table 111). Because of the effectiveness of sulfuric acid, some cellulose ester batches (A and B in Table 111) were tested which had been made to retain in a combined form some of the catalyst used in esterification. This amount, 0.04% based on the weight of ester, was left in the unneutralized form by washing the esters in distilled water. The batches show high numbers of relights, as do batches with added sulfuric acid. When portions of batch B were soaked in distilled water containing 0.02% of basic materials, and the ester was centrifuged, dried, and then coated to films, the burning was different, depending on the kind of base used. The ions forming a heatstable cellulose sulfate salt give films which burn like the normal commercial ester check. The ammonium salt, however, apparently decomposes to produce the effect of sulfuric acid. EFFECT OF PLASTICIZER VOLATILITY
film, Triphenyl phosphate (boiling range 407” tlo 410” C.) is sufficiently nonvolatile to be effective in producing slower burning with many relights. Phosphoric acid added a t low levels to the acetone solution prior to coating gives a film with burning properties similar to the film plasticized with triphenyl phosphate. High acid levels actually are less effective. Triphenyl thiophosphate is good, whereas the film treated with triphenyl phosphite is similar to the cellulose ester alone. The three organic acid ester plasticizers ranging from methyl phthalate to the polymeric Glyptal 2557 show only a slight difference, with the methyl phthalate possibly speeding the rate of burning. Succinic acid does not have the same effect as phosphoric acid. Titanium dioxide (TiOz)as an inert filler appears to be only a diluent for the combustible material. Hexachlorobenzene shows some promise, but it or its decomposition products may be too volatile to be effective. These results suggest that one function of triphenyl phosphate as an antiflame agent is to act as a source of phosphoric acid which, in turn, influences the course of decomposition and comhustion. This acid formation is nicely balanced. At no time is too much acid made available so that burning is rapid. Neither I S any released at lower temperatures so as t o rule out employment of this plasticizer under conditions encountered in the use of the product. Low levels of phosphoric acid, though equivalent t o triphenyl phosphate a t high temperature, might be injurious to film compositions under normal conditions.
A salt-stabilized (normal) ester and a sulfate-unstable ester were compared in compositions with different amounts of ethyl phthalate and a polymeric plasticizer (Paraplex G40). Data in Table IV show the loss in relight properties of the unstable ester as the amount of diethyl phthalate is increased. It appears that the combined sulfate has no control over the course of decomposition of the ethyl phthalate, or else the plasticizer is volatile enough to form a fire-encouraging vapor. The less volatile polymeric plasticizer behaves more like the cellulose ester, and slow-burning compositions with many relights are obtained. INSTABILITY AND BURNING RATE
The relation of the heat instability of a composition t o burning time and number of relights was studied by heating film samples in test tubes a t 205’ C. and noting the color after 2 hours of heating or the time for decomposition if less than 2 hours. When
Table IV. Ester Normal
Unstable
Effect of Volatility of Plasticizer Plss ticizer
Amount
Ethyl phthalate
Ethyl phthalate
5 15 30 0 5
15 30 Normal
Paraplex G40’
LOW PLASTICIZER LEVELS
Because only small quantities of acid appear to be necessary to bring about slow burning and many relights, only low quantities of antiflame plasticizers should be necessary. I n Table I1 are data showing burning behavior as various phosphate esters
0
Unstable
Paraplex G40
Polymeric plasticizer, Rohm & Haas C o .
5
15 30 5 15 30
B.T. 24 29 29 30 45 44 39 25 34 31 33 44 40 46
R.L.
.. ..
.. 13.8 6 5 0.7
..
1.2 1 0 0.4
14 1 13.1 12 5
I N D U S T R I A L .AND E N G I N E E R I N G C H E M I S T R Y
December 1955
Heat Stability a t 205' C. Time Color Partial decomp. None, normal ester 1 hr. Black 10 min. None unstable ester Tripdeny1 phosphate 2 hours Black Black TriDhenvl thioohomhate 2 hours Mdnobrbmonaphthklene 15 min. Black Partial decomp. Butyl phthalate 2 hours 2 hours Brown Trioctyl phosphite Trip henyl phosphite 2 hours Light yellow a To normal ester. Additionn
~~~~
The clear extracts were distilled in vacuo to give distillates low in acidity and sirupy residues requiring, on the average, 33 ml. of 0.1N alkali.
Relation of Heat Instability to Burning R a t e
Table V.
~~
B.T. 21 35 30 40 49 20 16 18
R.L.
The neutralized sodium salt solutions, when dried, yielded ash values in the range of 0.61 to 0.67 gram. Attempts a t determining phosphate gave only a scanty magnesium ammonium phosphate precipitate until the sodium salt residues were subjected to an alkali fusion. The percentage of phosphorus in the fused sodium salt was 11.5%. From the amount of acid titration, a disodium monophenyl phosphate would give 0.36 gram of residue and a phosphorus analysis of 14.2%, while monosodium diphenyl phosphate would give 0.89 gram of salt analyzing 11.4% phosphorus. It would appear that the acid responsible for the cellulose degradation is mainly diphenyl monohydrogen phosphate.
..
14 8 7
0 2
~
Table VI. Addition
Effect of Certain Salts and Acids Parts/100 B.T. R.L. 0.1
0.05 0 01
0.1 0.05 0.01
Carbamide phosphoric acid Borophosphoric acid Phosphoric acid
0.1 0.05 0.01 0.05 0.05 0.05 0.01
28 42 38 36 40 36 43 36 38 34 42 36 42 36
0.4 17.7 14.2 5.4 15.2 12.0 9.4 11.3 12.3
BURNING AND ACETYL LEVEL
Because the melting temperature of cellulose acetate varies with acetyl content, i t was thought that the higher acetylated esters would be more resistant to decomposition and, accordingly, burn with fewer relights. Table VI11 shows a hydrolysis series in which samples precipitated and washed in distilled water were divided. One half was dried as LLsalt-free'J;the second half was slurried in distilled water containing 0.02% magnesium carbonate. These samples were centrifuged, dried, and cast from acetone solution. The salt-free samples show burning values which correlate with degree of instability. The stability decreased with decrease in acetyl content. The stabilized samples all have essentially the same burning time.
0.9
14.4 5.8 16 6 a 2
~
~~~~~
Table VII. Acidity Development with Heating Time, Hours
Triphenyl Phosphate 250° C. 300' C.
Filter Paper, 250OC.
2523
Triphenyl Phosphate with Filter Paper, 250' C.
Table VIII. Effect of Acetyl Content on Burning Behavior MgCO;
Salt-Free
111. of 0.10N alkali to neutralize acidity developed.
%
these are compared with burning behavior (Table V), it is seen that materials contributing to instability at high temperatures give longer burning time and increased relights. The addition of phosphites actually improved the stability; the burning of these compositions is rapid.
a
3 hours a t
Treated ~-
Acetyl B.T. R.L. 180' C. B.T. 43 26 .. S1. color 28 41.8 28 0.2 Straw 23 Straw 26 40.9 33 3 Dark straw 26 40.1 33 0.3 40.0 33 2 Decomp. 27 38.8 36 8 Decomp. 28 37.9 35 5 Decomp. 30 All showed only faint yellowing after 6 hours a t 180' C.
R.L. 0:1 ..
..
0.3
..
'0.3
ACIDS AND ACID SALTS
Some acid salts or acid-generating salts were incorporated a t low levels in cellulose ester solutions, a limited amount of water first being used to dissolve the salt (Table VI). These salts are effective, although 0.01 part of aluminum sulfate is apparently low enough to be counteracted by the salts normally found in commercial esters. Earlier work by Robinson (6) employed phosphate salts a t levels equal to the weight of the cellulose acetate. The use of urea-phosphoric acid mixtures as impregnants to render cellulose material flame-resistant has been claimed by Groebe (3). The present Iow levels of salts might be useful in solvent-coated film and sheeting applications, but the higher heat required for molding plastics discolors the compositions.
CONCLUSIONS
'
Addition agents capable of forming an inorganic acid a t high temperature cause cellulose ester test strips to burn slowly and the flame to go out repeatedly. Organic phosphate esters impart different degrees of flameinhibiting action, depending on their instability to heat and/or volatility. Low levels of phosphoric or sulfuric acid have antiflame activity. Neutralizing this acidity with a strong base destroys the flame-inhibiting properties, but acid salts of these acids have much the same effect as the acids. Relatively volatile organic ester plasticizers may counteract the flame-inhibiting action of a strong acid.
ACIDITY DEVELOPMENT
The amount of acidity developed on heating a phosphate plasticizer, such as the triphenyl derivative, is actually small (Table VII). Five-gram samples were heated in a metal block in test tubes with glass-jointed air condensers. At intervals a tube was withdrawn from the heat and cooled, the sample was diluted with 100 ml. of acetone and 50 ml. of water, and the acid was titrated with O . 1 O N alkali. The acidity developed is relatively low; yet, if 0.25 gram of ash-free filter paper is first u t in the plasticizer, the decomposition on heating, as measure% by acidity, is marked. Filter paper heated alone does not produce much acidity. Four additional phosphate-filter paper tubes were heated a t 250' C. for 3 hours. The resulting dark mixtures were extracted with warm distilled water to leave essentially acid-free residues.
LITERATURE CITED (1) Am.
D
SOC.Testing Materials, Philadelphia, ASTAK Test Method 568-43.
(2) Coppick, S., Church, J. A I . , and Little, R. W., IND.ENG.CHEY., 42, 415 (1950).
(3)
Groebe, F. (to General Electric C o . ) . U. S. Patent 2,089,697 (1937).
(4) Little, R. W., "Flameproofing Textile Fibers," Reinhold, New
York, 1947. ( 5 ) Robinson, E. G. (to E. I. du Pont de Temours & Co.), U. Patent 1,310,841 (1919).
S.
RECEIVED f o r review M a y 10, 1955. .ICCEPTED August 11, 1955. Division of Cellulose Chemistry, Symposium on Degradation of Cellulose and Cellulose Derivatives, 127th Meeting, ACS, Cincinnati, Ohio, MarchApril 1955.
