Effect of fiber composition on textile flammability - Govinfo.gov

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NATIONAL BUREAU OF STANDARDS REPORT

7339

EFFECT OF FIBER COMPOSITION ON TEXTILE FLAMMABILITY

REPORT OF PRELIMINARY WORK

By

Marjorie W. Sandholzer

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

THE NATIONAL BUREAU OF STANDARDS

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NBS PROJECT MBS REPORT

1002 -12-10120 September 8, 1961 7339

EFFECT OF FIBER COMPOSITION ON TEXTILE FLAMMABILITY

NATIONAL BUREAU OF STANDARDS REPORTS are usually preliminary or progress accounting documentsIntended for use within the Government. Before material In the reports Is formally published It Is subjected

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REPORT OF PRELIMINARY WORK

By

Marjorie W. Sandholzer

IMPORTANT. NOTICE

It. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

EFFECT OF FIBER COMPOSITION ON TEXTILE FLAMMABILITYREPORT OF PRELIMINARY WORK

by

Marjorie W. Sandholzer

ABSTRACT

A group of ten fabrics., similar inconstruction and weight but each knit froma different textile fiber, were subjectedto three established textile flame testsand to a test designed to measure the heattransferred from a burning fabric to an ad-jacent surface. Because of the pronounceddifference in response to heat and in burn-ing behavior between the fabrics of naturalfibers and those of synthetic fibers, validcomparisons between fabrics of the two typeswere difficult. The fabrics of natural fi-bers in the group were placed in the orderof wool, cotton, and viscose rayon on thebasis of increasing burning rate, as indi-cated by the three methods used. Two syn-thetic fabrics, Dynel and Dacron, did notsupport combustion in any of the tests; theothers showed burning rates in a range com-parable to that for the fabrics of naturalfibers. The heat transferred to an adjacentsurface varied with the fiber and construc-tion of the material and, as registered bythe method employed, appeared to range fromapproximately 20-50 percent of the heat pro-duced by the burning fabric.

1. INTRODUCTION

The ease of ignition and the burning rate of a fabric aregreatly influenced by the physical characteristics of the mater-ial, such as the weight, type of weave, and surface texture.Because normally available fabrics differ widely in these phy-sical characteristics, it has been difficult to arrive at morethan a very general assessment of the effect of fiber composi-tion on the burning behavior of a material. In order to sub-stantially reduce the physical variables, a group of ten fabrics,

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as similar as feasible in weight and construction but eachknit from spun staple stock of a different fiber composition,was obtained on special order from the North Carolina StateCollege. It is believed that such a group of fabrics shouldprove helpful in studying the fire hazards connected withvarious textile applications.

2. Flammability Tests and Results

The first work with the group of fabrics has consistedin determining their behavior in several standard textileflame tests. Two of the tests used are described in FederalSpecification CCC-T-191b as Method 5902 and Method 5906. Thethird is described in the N.F.P.A. Tentative Standard forClassification of the Flammability of Wearing Apparel, No. 702-T.The three methods have several similarities. They require testspecimens of similar size (11-12 inches long and 2-3 inches wide),and for each, the specimen is placed in a metal holder whichclamps each long edge of the strip leaving a center width of1-1/2 or 2 inches exposed to flame progress. All use a flameapplied to an edge of the fabric as an ignition source, althoughthe igniting flame varies in size. The methods differ primarilyin the position in which the specimen is tested and, inconsequence, in the means adopted for gauging flame progress.

Method 5902 was designed to indicate the comparative flameresistance of nonflammable or difficultly flammable textilesand is not readily adaptable to a comparison of easily flammablematerials. The specimen is supported in a vertical position,a defined flame exposure is applied to the lower end, and t>he

resulting length of fabric char and duration of fabric flamingare used as criteria of performance. With a readily flammablematerial the flame front is so poorly defined that visual deter-mination of the burning rate is not feasible, although, forfabrics which burn in a similar manner, a rough comparison maybe based on the time required for the entire specimen to burnand all flaming to cease.

In Method 5906 the fabric specimen is supported in a hori-zontal position, ignited at one end and, after burning of thefabric has become established, the time required for the flamefront to progress a 10-inch distance is recorded manually witha stopwatch. The method provides little differentiation amongdifficultly flammable fabrics, but permits calculation of aburning rate for readily flammable materials.

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The N.F.P.A. Tentative Standard uses a specimen positioninclined 15 degrees from the vertical. The specimen is ignitedat the lower edge by a defined flame, and the time interval be-tween application of the igniting flame and the burning of ataut crossj^thread 10- inches from the lower end of the specimen,is automatically registered on a stopwatch. This method alsopermits calculation of a burning rate, although the flame pro-gress is defined by factors which differ from those of the hor-izontal method.

In all of the tests the basic difference in burning behav-ior between the natural fibers and most of the synthetic fiberswas obvious. In general the natural fibers are not thermoplastic,and the specimens burned or charred completely while remainingin position. The wool intumesced and showed a somewhat plasticcondition but commonly produced a brittle, foamed char which re-mained in place. Most of the synthetic fibers, on the other hand,are highly thermoplastic and soften ahead of the flame. The fabricstructure flowed into melted, flaming globules which either droppedaway or collected along the clamped edge of the specimen, andflame progress was irregular and difficult to gauge. Smoke evol-ution was notably much heavier from all of the synthetic fabricsthan from those of natural fibers.

The results obtained with the ten fabrics in the three testmethods are summarized in Table 1. Considered individually,each method showed decided variations in flammability and rateof burning among the different materials. In addition, althoughthe criteria of flame progress differ among the methods, the datasuggest some general effects of specimen position on burning be-havior and burning rate. In the vertical test, five of the fab-rics burned the full specimen length of 11 inches, and of these,the wool, cotton, and viscose rayon burned in a reasonably similarand consistent manner. For these three the duration of after-flaming (flaming of the fabric after withdrawal of the ignitingflame) can provide a rough comparison of burning rate, and wouldplace them in the same relative order as the other two methods.For the two synthetic fabrics, Acrilan and Orion, which were com-pletely destroyed in the vertical test, the duration of after-flaming had little significance since it represented primarilythe haphazard flaming of globules clinging to the clamped sidesof the specimen. With the other five synthetic fabrics the lengthburned indicated essentially the distance directly affected bythe igniting flame, which generally appeared as an open, flame-shaped area from which the fabric had melted and drawn away; theafterflaming consisted of continuted flaming along the edges ofthis area, usually without further fabric destruction.

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As would be expected, the burning rate appeared to increaseas the specimen position approached the vertical, and to becomemost rapid in that position, as long as the flame could maintaincontact with the fuel. The vertical position, however, favoredthe melting and withdrawal from the flame to which the syntheticfabrics were prone, and several which burned in the horizontalposition failed to burn in the vertical position. When the spe-cimen position was inclined slightly from the vertical they burnedmore rapidly than in the horizontal position, although on some in-dividual specimens of two of the fabrics the flaming portions stilldripped away to the extent that flame spread was stopped beforethe specimen was consumed.

With regard to the burning rates of the various fabrics rel-ative to each other, the natural fibers were placed in the orderof wool, cotton, and viscose rayon, on the basis of increasingrate, by each of the three test methods. For the synthetic fab-rics, only general conclusions appear justified, inasmuch as theirburning was commonly uneven and the differences noted in ratemight well have resulted primarily from differences in meltingtemperature and consequent dripping tendency. It is significant,however, that the Dacron and Dynel failed to support combustionin any of the test methods used, and the other synthetic fabricsappeared to burn at rates in approximately the same range asthose for the fabrics of natural fibers.

3. Heat Transfer Test

In addition to burning characteristics and rate of burning,the amount of heat evolved and supplied to an adjacent surfacefrom a burning fabric is also of interest in studies of the hazardpresented by various textile applications. Following the designof a British method for measuring such heat transfer, an apparatuswas assembled and used in testing a number of fabrics, includingthe group of similarly constructed knit materials.

For the tests, a specimen 30 inches long and 4 inches widewas suspended vertically 1/2 inch from an asbestos millboardback panel. In order to prevent severe rolling of the edges ofthe knit fabrics, however, those specimens were cut 4 1/2 incheswide and a narrow frame (about 5/16 inch wide) of heavy paperwas stapled to the outside edges of the specimen. The amountof paper introduced was so small and well removed from the centerof the specimen that it could not materially affect the heat pro-duced by the burning fabric at the points of measurement. In

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test position the specimen was held at the top by a spring clampand, to prevent fluttering or curling during burning, it waslaced under and over six appropriately spaced cross wires, heldat a 1/2 inch distance from the back panel by two metal spacingstrips running lengthwise of the panel. These spacing stripswere set 6 inches apart, and the suspended specimen was centeredbetween them. The specimen was ignited across the full width ofthe lower end and allowed to burn freely. To provide a measur-ment of the heat supplied to the asbestos board backing at var-ious heights, four cylindrical copper plugs of 1/2 inch diameterwere set into the back panel, flush with the panel surface andspaced h inches apart, the uppermost being 5 1/2 inches below thetop edge of the specimen. The plugs were surface coated withIndia ink to improve heat absorption, and a thermocouple attachedto the back of each plug permitted a continuous recording of thetemperature of the plug. From the recorded temperature rise (dis-regarding the various small heat losses from the plug and wires)and the characteristics of the plug, the heat dosage was calculatedfrom the relation:

Q = cptQ cal cm1

where c = specific heat of the plug in cal gm x°C

p= density of the plug in gm cm 3

t - thickness of the plug in cm9 = temperature rise in °C.

Use of a dual-channel recorder permitted measurement of the tem-perature rise of two of the plugs during the same test.

The results obtained on the five knit fabrics which suppor-ted combustion in the vertical position are presented in Table 2.Data for three woven fabrics tested under comparable conditionsare also shown. The difference in heat dosage values obtained inthe two plug positions used in the tests did not appear to besignificant, and the results at only one plug position (5 1/2 inchesbelow the upper end of the specimen) are included in the table.

Table

2.

Heat

Transfer

Results

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Presented also in Table 2 are modified "potential heat"values which provide an approximate measurement of the actualamount of heat released by the various fabric strips in burning.The values were obtained by a differential bomb calorimetricmethod* and were calculated as the difference between the heatof combustion of the original fabric and that of the residueremaining after the fabric was allowed to burn freely in air.For the cellulosic fabrics, no significant residue remained afterburning in air, but for the wool and synthetic materials, airburning left charred residues amounting to 30-^0 percent of theweight of the original sample. By taking into account the fabricweights, values for the heat produced by the burning strips maybe calculated, permitting a rough indication of the proportionsof that heat which were registered by the copper plug indicators.The values given in Table 2 are, for the most part, based uponone determination.

From observations during the tests it was evident that themanner in which the fabric burned had a considerable effect onthe temperature rise obtained. If the charred or nearly ashed

.

fabric structure remained hanging in place after flaming hadceased, the temperature continued to rise for some time, usuallyuntil the char fell away terminating radiant heating of the plug.In general the cotton and rayon fabrics exhibited this type ofbehavior, although there were noticeable differences among themin the tenacity of the char. The char of the woven fabrics tendedto- fall away more quickly than that of the knit fabrics, and theknit rayon remained in position until the ash had become almosta powder. With the restriction in air flow introduced by thebacking panel, the wool burned poorly and unevenly, and two ofthe five specimens tested burned only part way. On the two syn-thetic fabrics, Acrilan and Orion, the flames spread violentlyand rapidly with the characteristic melting and dripping of thematerial, which continued to burn in a mass at the base of thepanel. For the most part, the plugs were subjected to a vigorousbut relatively transient burst of flame, though at times parts ofthe molten, flaming material caught on the lacing wires or stuckto the asbestos panel near a plug and provided some additionalexposure.

* The method described in ASTM preprint "Potential Heat" byLoftus

,, Gross

,and Robertson, was used except that free burning

in air was substituted for firing in a muffle furnace.

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The numerical results as well as the test observationssuggest that the heat dosage determined by this method is a func-tion of fabric construction and burning behavior, in addition tofabric weight and perhaps fiber content. The wool values arequestionable because of erratic burning and are therefore incon-clusive. The group of cellulosic fabrics showed the most uni-form burning and provided probably the most reliable results amongthe materials tested. With the exception of the muslin, the cell-ulosic fabrics gave similar potential heat values, with an indi-cation of slightly lower values for rayon than for cotton. Thehigher potential heat obtained for the muslin may have been dueto the considerable dressing which the material carried and whichthe other fabrics did not have. The heat dosage values for thecellulosic materials, however, showed pronounced differences whichappeared to bear a considerable relation to the type of construc-tion. Thus, the two knit cellulosic fabrics gave closely similarheat dosage values, which amounted to approximately half the heatexposure calculated on the basis of fabric weight. The sateenand bengaline, both woven fabrics but quite different in type ofweave, also gave almost identical heat dosage values. The valuefor the bengaline was again nearly half the calculated heat ex-posure, but that for the sateen was not more than one third thecalculated exposure. The very light weight of the muslin was,of course, reflected in a very low heat dosage value, which, how-ever, was only about one fourth of the calculated heat exposure.The heat dosages determined for the two knit acrylic fabrics weredecidedly lower than those for the knit cellulosic materials andrepresented relatively small proportions of the calculated heatexposures, an effect which appeared consistent with the type ofburning observed.

*+. Summary

Due to a basic difference in their response to heat, thefabrics of natural fibers and those of synthetic fibers showeda pronounced difference in burning behavior. In general thenatural fibers are not thermoplastic and the fabric structureremained in position during burning, while most of the syntheticfibers are highly thermoplastic and the fabric structure quicklymelted into viscous flaming masses which dripped away from thetest position. This basic difference in burning behavior makesit difficult to arrive at valid comparisons between fabrics ofthe two types as to burning rate, heat production, and other char-acteristics by which the relative hazards may be gauged.

10 -

The fabrics of natural fibers which were studied may belisted in the order of wool, cotton, and viscose rayon, on thebasis of increasing burning rate as indicated by the three testmethods employed. In the measurement of heat transferred froma burning fabric to an adjacent surface, the determinations onwool were not sufficiently reliable to justify a comparison withthe other fiber So The cotton and rayon knit fabrics suppliedheat to an adjacent surface in essentially the same amount, whichappeared to be approximately one-half the heat produced by theirburning. A woven cotton sateen of comparable weight, however,supplied to the adjacent surface only about 60% as much heat asthe knit cellulosic fabrics, or about one-third of the heat pro-duced by its burning.

Among the synthetic fabrics studied, the Dynel and Dacrondid not support combustion in any of the tests applied. The twoacrylic fabrics, Acrilan and Orion, burned readily in either thehorizontal or vertical position, but the other three syntheticfabrics (cellulose acetate, Arnel, and nylon) failed to burn inthe vertical position because of melting and drawing away fromthe flame. The burning rates of the synthetic fabrics, as deter-mined in the horizontal and 15° angle positions, were in a rangewhich appeared reasonably comparable to that for the fabrics ofnatural fibers. The two acrylic fabrics (the only ones of thesynthetic group which burned in the required vertical test posi-tion) supplied about 50-60% as much heat to an adjacent surfaceas the knit cellulosic fabrics, amounts which appeared to bearound 20% of the heat produced by their burning.

US-C0MM-NBS-DC

U. S. DEPARTMENT OF COMMERCELuther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDSA. V. Astin, Director

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