Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

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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1002 - 12-10120 September 8, 1961 7339

EFFECT OF FIBER COMPOSITION ON TEXTILE FLAMMABILITY

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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 FLAMMABILITY

REPORT OF PRELIMINARY WORK

by

Marjorie W. Sandholzer

ABSTRACT

A group of ten fabrics., similar in

construction and weight but each knit from

a different textile fiber, were subjected

to three established textile flame tests

and to a test designed to measure the heat

transferred from a burning fabric to an adjacent surface. Because of the pronounced

difference in response to heat and in burning behavior between the fabrics of natural

fibers and those of synthetic fibers, valid

comparisons between fabrics of the two types

were difficult. The fabrics of natural fibers in the group were placed in the order

of wool, cotton, and viscose rayon on the

basis of increasing burning rate, as indicated by the three methods used. Two synthetic fabrics, Dynel and Dacron, did not

support combustion in any of the tests; the

others showed burning rates in a range comparable to that for the fabrics of natural

fibers. The heat transferred to an adjacent

surface varied with the fiber and construction of the material and, as registered by

the method employed, appeared to range from

approximately 20-50 percent of the heat produced by the burning fabric.

1. INTRODUCTION

The ease of ignition and the burning rate of a fabric are

greatly influenced by the physical characteristics of the material, such as the weight, type of weave, and surface texture.

Because normally available fabrics differ widely in these physical characteristics, it has been difficult to arrive at more

than a very general assessment of the effect of fiber composition on the burning behavior of a material. In order to substantially reduce the physical variables, a group of ten fabrics,
2

as similar as feasible in weight and construction but each

knit from spun staple stock of a different fiber composition,

was obtained on special order from the North Carolina State

College. It is believed that such a group of fabrics should

prove helpful in studying the fire hazards connected with

various textile applications.

2. Flammability Tests and Results

The first work with the group of fabrics has consisted

in determining their behavior in several standard textile

flame tests. Two of the tests used are described in Federal

Specification CCC-T-191b as Method 5902 and Method 5906. The

third is described in the N.F.P.A. Tentative Standard for

Classification of the Flammability of Wearing Apparel, No. 702-T.

The three methods have several similarities. They require test

specimens of similar size (11-12 inches long and 2-3 inches wide),

and for each, the specimen is placed in a metal holder which

clamps each long edge of the strip leaving a center width of

1-1/2 or 2 inches exposed to flame progress. All use a flame

applied to an edge of the fabric as an ignition source, although

the igniting flame varies in size. The methods differ primarily

in the position in which the specimen is tested and, in

consequence, in the means adopted for gauging flame progress.

Method 5902 was designed to indicate the comparative flame

resistance of nonflammable or difficultly flammable textiles

and is not readily adaptable to a comparison of easily flammable

materials. 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 flaming

are used as criteria of performance. With a readily flammable

material the flame front is so poorly defined that visual determination of the burning rate is not feasible, although, for

fabrics which burn in a similar manner, a rough comparison may

be based on the time required for the entire specimen to burn

and all flaming to cease.

In Method 5906 the fabric specimen is supported in a horizontal position, ignited at one end and, after burning of the

fabric has become established, the time required for the flame

front to progress a 10-inch distance is recorded manually with

a stopwatch. The method provides little differentiation among

difficultly flammable fabrics, but permits calculation of a burning rate for readily flammable materials.
- 3

-

The N.F.P.A. Tentative Standard uses a specimen position

inclined 15 degrees from the vertical. The specimen is ignited

at the lower edge by a defined flame, and the time interval between application of the igniting flame and the burning of a

taut crossj^thread 10- inches from the lower end of the specimen,

is automatically registered on a stopwatch. This method also

permits calculation of a burning rate, although the flame progress is defined by factors which differ from those of the horizontal method.

In all of the tests the basic difference in burning behavior between the natural fibers and most of the synthetic fibers

was obvious. In general the natural fibers are not thermoplastic,

and the specimens burned or charred completely while remaining

in position. The wool intumesced and showed a somewhat plastic

condition but commonly produced a brittle, foamed char which remained in place. Most of the synthetic fibers, on the other hand,

are highly thermoplastic and soften ahead of the flame. The fabric

structure flowed into melted, flaming globules which either dropped

away or collected along the clamped edge of the specimen, and

flame progress was irregular and difficult to gauge. Smoke evolution was notably much heavier from all of the synthetic fabrics

than from those of natural fibers.

The results obtained with the ten fabrics in the three test

methods are summarized in Table 1. Considered individually,

each method showed decided variations in flammability and rate

of burning among the different materials. In addition, although

the criteria of flame progress differ among the methods, the data

suggest some general effects of specimen position on burning behavior and burning rate. In the vertical test, five of the fabrics burned the full specimen length of 11 inches, and of these,

the wool, cotton, and viscose rayon burned in a reasonably similar

and consistent manner. For these three the duration of afterflaming (flaming of the fabric after withdrawal of the igniting

flame) can provide a rough comparison of burning rate, and would

place them in the same relative order as the other two methods.

For the two synthetic fabrics, Acrilan and Orion, which were completely destroyed in the vertical test, the duration of afterflaming had little significance since it represented primarily

the haphazard flaming of globules clinging to the clamped sides

of the specimen. With the other five synthetic fabrics the length

burned indicated essentially the distance directly affected by

the igniting flame, which generally appeared as an open, flameshaped area from which the fabric had melted and drawn away; the

afterflaming consisted of continuted flaming along the edges of

this area, usually without further fabric destruction.
- b -

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As would be expected, the burning rate appeared to increase

as the specimen position approached the vertical, and to become

most rapid in that position, as long as the flame could maintain

contact with the fuel. The vertical position, however, favored

the melting and withdrawal from the flame to which the synthetic

fabrics were prone, and several which burned in the horizontal

position failed to burn in the vertical position. When the specimen position was inclined slightly from the vertical they burned

more rapidly than in the horizontal position, although on some individual specimens of two of the fabrics the flaming portions still

dripped away to the extent that flame spread was stopped before

the specimen was consumed.

With regard to the burning rates of the various fabrics relative to each other, the natural fibers were placed in the order

of wool, cotton, and viscose rayon, on the basis of increasing

rate, by each of the three test methods. For the synthetic fabrics, only general conclusions appear justified, inasmuch as their

burning was commonly uneven and the differences noted in rate

might well have resulted primarily from differences in melting

temperature and consequent dripping tendency. It is significant,

however, that the Dacron and Dynel failed to support combustion

in any of the test methods used, and the other synthetic fabrics

appeared to burn at rates in approximately the same range as

those 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 surface

from a burning fabric is also of interest in studies of the hazard

presented by various textile applications. Following the design

of a British method for measuring such heat transfer, an apparatus

was assembled and used in testing a number of fabrics, including

the group of similarly constructed knit materials.

For the tests, a specimen 30 inches long and 4 inches wide

was suspended vertically 1/2 inch from an asbestos millboard

back panel. In order to prevent severe rolling of the edges of

the knit fabrics, however, those specimens were cut 4 1/2 inches

wide and a narrow frame (about 5/16 inch wide) of heavy paper

was stapled to the outside edges of the specimen. The amount

of paper introduced was so small and well removed from the center

of the specimen that it could not materially affect the heat produced by the burning fabric at the points of measurement. In
- 6 -

test position the specimen was held at the top by a spring clamp

and, to prevent fluttering or curling during burning, it was

laced under and over six appropriately spaced cross wires, held

at a 1/2 inch distance from the back panel by two metal spacing

strips running lengthwise of the panel. These spacing strips

were set 6 inches apart, and the suspended specimen was centered

between them. The specimen was ignited across the full width of

the lower end and allowed to burn freely. To provide a measurment of the heat supplied to the asbestos board backing at various heights, four cylindrical copper plugs of 1/2 inch diameter

were set into the back panel, flush with the panel surface and

spaced h inches apart, the uppermost being 5 1/2 inches below the

top edge of the specimen. The plugs were surface coated with

India ink to improve heat absorption, and a thermocouple attached

to the back of each plug permitted a continuous recording of the

temperature of the plug. From the recorded temperature rise (disregarding the various small heat losses from the plug and wires)

and the characteristics of the plug, the heat dosage was calculated

from the relation:

Q = cptQ cal cm

1 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 cm

9 = temperature rise in °C.

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

The results obtained on the five knit fabrics which supported combustion in the vertical position are presented in Table 2.

Data for three woven fabrics tested under comparable conditions

are also shown. The difference in heat dosage values obtained in

the two plug positions used in the tests did not appear to be

significant, and the results at only one plug position (5 1/2 inches

below the upper end of the specimen) are included in the table.
Table 2.

Heat

Transfer

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- 8 -

Presented also in Table 2 are modified "potential heat"

values which provide an approximate measurement of the actual

amount of heat released by the various fabric strips in burning.

The values were obtained by a differential bomb calorimetric

method* and were calculated as the difference between the heat

of combustion of the original fabric and that of the residue

remaining after the fabric was allowed to burn freely in air.

For the cellulosic fabrics, no significant residue remained after

burning in air, but for the wool and synthetic materials, air

burning left charred residues amounting to 30-^0 percent of the

weight of the original sample. By taking into account the fabric

weights, values for the heat produced by the burning strips may

be calculated, permitting a rough indication of the proportions

of that heat which were registered by the copper plug indicators.

The values given in Table 2 are, for the most part, based upon

one determination.

From observations during the tests it was evident that the

manner in which the fabric burned had a considerable effect on

the temperature rise obtained. If the charred or nearly ashed . fabric structure remained hanging in place after flaming had

ceased, the temperature continued to rise for some time, usually

until the char fell away terminating radiant heating of the plug.

In general the cotton and rayon fabrics exhibited this type of

behavior, although there were noticeable differences among them

in the tenacity of the char. The char of the woven fabrics tended

to- fall away more quickly than that of the knit fabrics, and the

knit rayon remained in position until the ash had become almost

a powder. With the restriction in air flow introduced by the

backing panel, the wool burned poorly and unevenly, and two of

the five specimens tested burned only part way. On the two synthetic fabrics, Acrilan and Orion, the flames spread violently

and rapidly with the characteristic melting and dripping of the

material, which continued to burn in a mass at the base of the

panel. For the most part, the plugs were subjected to a vigorous

but relatively transient burst of flame, though at times parts of

the molten, flaming material caught on the lacing wires or stuck

to the asbestos panel near a plug and provided some additional

exposure.

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

Loftus ,

, Gross , and Robertson, was used except that free burning

in air was substituted for firing in a muffle furnace.
- 9 -

The numerical results as well as the test observations

suggest that the heat dosage determined by this method is a function of fabric construction and burning behavior, in addition to

fabric weight and perhaps fiber content. The wool values are

questionable because of erratic burning and are therefore inconclusive. The group of cellulosic fabrics showed the most uniform burning and provided probably the most reliable results among

the materials tested. With the exception of the muslin, the cellulosic fabrics gave similar potential heat values, with an indication of slightly lower values for rayon than for cotton. The

higher potential heat obtained for the muslin may have been due

to the considerable dressing which the material carried and which

the other fabrics did not have. The heat dosage values for the

cellulosic materials, however, showed pronounced differences which

appeared to bear a considerable relation to the type of construction. Thus, the two knit cellulosic fabrics gave closely similar

heat dosage values, which amounted to approximately half the heat

exposure calculated on the basis of fabric weight. The sateen

and bengaline, both woven fabrics but quite different in type of

weave, also gave almost identical heat dosage values. The value

for the bengaline was again nearly half the calculated heat exposure, but that for the sateen was not more than one third the

calculated exposure. The very light weight of the muslin was,

of course, reflected in a very low heat dosage value, which, however, was only about one fourth of the calculated heat exposure.

The heat dosages determined for the two knit acrylic fabrics were

decidedly lower than those for the knit cellulosic materials and

represented relatively small proportions of the calculated heat

exposures, an effect which appeared consistent with the type of

burning observed.

*+. Summary

Due to a basic difference in their response to heat, the

fabrics of natural fibers and those of synthetic fibers showed

a pronounced difference in burning behavior. In general the

natural fibers are not thermoplastic and the fabric structure

remained in position during burning, while most of the synthetic

fibers are highly thermoplastic and the fabric structure quickly

melted into viscous flaming masses which dripped away from the

test position. This basic difference in burning behavior makes

it difficult to arrive at valid comparisons between fabrics of

the two types as to burning rate, heat production, and other characteristics by which the relative hazards may be gauged.
10 -

The fabrics of natural fibers which were studied may be

listed in the order of wool, cotton, and viscose rayon, on the

basis of increasing burning rate as indicated by the three test

methods employed. In the measurement of heat transferred from

a burning fabric to an adjacent surface, the determinations on

wool were not sufficiently reliable to justify a comparison with

the other fiber So The cotton and rayon knit fabrics supplied

heat to an adjacent surface in essentially the same amount, which

appeared to be approximately one-half the heat produced by their

burning. A woven cotton sateen of comparable weight, however,

supplied to the adjacent surface only about 60% as much heat as

the knit cellulosic fabrics, or about one-third of the heat produced by its burning.

Among the synthetic fabrics studied, the Dynel and Dacron

did not support combustion in any of the tests applied. The two

acrylic fabrics, Acrilan and Orion, burned readily in either the

horizontal or vertical position, but the other three synthetic

fabrics (cellulose acetate, Arnel, and nylon) failed to burn in

the vertical position because of melting and drawing away from

the flame. The burning rates of the synthetic fabrics, as determined in the horizontal and 15° angle positions, were in a range

which appeared reasonably comparable to that for the fabrics of

natural fibers. The two acrylic fabrics (the only ones of the

synthetic group which burned in the required vertical test position) supplied about 50-60% as much heat to an adjacent surface

as the knit cellulosic fabrics, amounts which appeared to be

around 20% of the heat produced by their burning.

US-C0MM-NBS-DC
U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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front cover.

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