Approved for public release by the

Director of the National Institute of Qgfj

Standards and Technology (NIST)

NATIONAL BUREAU OF STANDARDS REPORT o.

)

7325

SURFACE FLAMMABILITY OF CELLULAR PLASTIC FOAMS

By

D. Gross

And

J. J. Loftus

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1002 - 12-10120 August 22, 1961 7325

SURFACE FLAMMABILITY OF CELLULAR PLASTIC FOAMS

by

D. Gross

and

J. J. Loftus

IMPORTANT NOTICE

NATIONAL BUREAll Of STANDARDS REPORTS are usually preliminary or progress accounting documents

Intended for use within the Government. Before material In the reports Is formally published It Is subjected

to additional evaluation and review. For this reason, the publication, reprinting, reproduction, or open-literature

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the Office of the Director, National Bureau of Standards, Washington 25, D. C. Such permission Is not needed,

however, by ?he Government agency for which the Report has been specifically prepared If that agency wishes

to reproduce additional copies for Sts own use,

U, S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

SURFACE FLAMMABILITY OF CELLULAR PLASTIC FOAMS

by

D. Gross

and

J. J. Loftus

ABSTRACT

Flame spread indices for twelve cellular

plastic foams, including fire-retardant treated

and untreated polyurethane and polystyrene foams,

are reported. There appear to be no serious

drawbacks to the use of the radiant panel test

method for evaluating surface flammability of

cellular plastic foams. Discussion is included

on the use of slightly modified data techniques

in particular applications.

1. INTRODUCTION

The "radiant panel" flame spread test method has been

used to measure comparative surface flammability of a wide

variety of building finish materials (1, 2, 3)* Requiring

a relatively small specimen and only a modest amount of

equipment and space, it has been found possible to obtain

meaningful flame spread indices ranging from about 1 to

over 1000.

An investigation of the applicability of the method for

measuring the flammability of cellular plastic foams was

undertaken in conjunction with a round robin test series

sponsored by the Flammability Committee of the Cellular

Plastics Division of the Society of the Plastics Industry,

Inc. For these tests, it was felt that no major changes

should be effected in the standard test requirements (4)

,

such as specimen exposure to different irradiation levels,

but that modifications might be made in the techniques for

obtaining more meaningful data and in their interpretation.

A particular goal of this study was to examine the extent

to which the test method could be used to differentiate

between conventional and fire-retardant treated foams.
2

2. TEST METHOD

The flame spread index as originally devised provides

a means for classifying materials according to their

rerlative surface flammability properties. This numerical

classification appears to be generally consistent with

information available on the behavior of such materials

during fires. The index is based upon measurements of

(a) the times at which surface flaming reaches

equally-spaced distances along the specimen

length, and

(b) the maximum temperature rise in the stack

which receives the hot combustion products.

In some instances, the standard method of calculating

flame spread index must necessarily be based upon limited

test information and interpretation of the data and the

test observations may be difficult. Upon careful analysis

and interpretation of specific applications, therefore,

the computations are sometimes modified to make the flame

spread index more meaningful.

For cellular plastics as well as other materials,

particularly those with flame-retardant treated surfaces,

a significant delay may occur in the start of surface flaming,

followed by a very rapid flame progression which may envelop

one or more markings before a sustained flame establishes

itself. In many cases, the progression of the sustained

flame front after this has occurred, will be regular and

times for the flame front to pass succeeding 3~i-n * marks may

be readily measured. However, in some cases, the sustained

flame front progression will be limited, providing no

additional time-distance data. If the rapid flaming envelops two or more markings, the time interval(s) involved,

even if measurable, would result in a disproportionately

high number being used in summing up the components of the

flame spread factor. In order to obtain more representative

factors in such situations, the following procedures have been

adopted
(1) Measure or estimate the distance D, in inches,

and the corresponding time t, in minutes,

when the flaming establishes very rapidly past

the 3-in. mark or past the 3

- and 6-in. marks

or past the 3~> 6- and 9-in. marks.

(2a) In the case where succeeding time-distance

data are available, plot D on rectangular

coordinates as a function of t on logarithmic

coordinates

.

or (2b) In the case where no succeeding time-distance

data are available, determine the distance D 0

corresponding to a time of 1.0 minutes from

the following equation, D = D 0 + 6.5 l°ge t.

(3a) Extrapolate the curve (or line) back to obtain

estimated times for the 3 “in* or 3“ and 6-in.

markings

.

or (3^) Using the value of D 0 and the same equation

from (2b), calculate the corresponding times

for distances of 3“> 3“ and 6-, or 3“? 6- and

9-inches

.

(4) Use these extrapolated or calculated times and

corresponding distances to calculate the flame

spread index.

For some materials (including some low-density cellular

plastics), the flaming is rapid and limited to the very

early part of the test exposure. Due to the response time

of the stack thermocouples, a slight temperature rise may

go undetected. In such cases, the value of A6 for the test

specimen should be determined with respect to the stack

thermocouple temperature for the asbestos-cement board specimen

at a time corresponding to that at which the maximum stack

thermocouple . temperature was observed for the test specimen.

This response time correction is intended to be applied only

in those cases where the temperature rise is not large.

Therefore, its use is arbitrarily restricted to instances

where the maximum stack thermocouple temperature for the

specimen does not exceed the mean stack thermocouple temperature during test of an incombustible asbestos-cement board

specimen by more than '10 degrees F. Copti-nwou's recording of

the stack thermocouple temperature is required in such cases.
- 4 -

3. MATERIALS and TEST PROCEDURE

A total of twelve cellular plastic foam materials were

made available by manufacturers and are listed in Table 1.

Included was a series of four foams of the same composition

in which the blowing agent was varied in order to vary the

bulk density. With the exception of the latter group, the

materials were designated as 2 lb/ft^ foams; the measured

densities as received are also listed in Table 1.

The standard test method (4) with the modifications

described, was used for these tests, except that pre-drying

of the specimens was omitted. With one exception, four

replicate tests were performed on each material. A continuous record of the stack thermocouple temperature was obtained by use of a pen-type recording potentiometer. The

chart speed was 3 in. /min and, using a 5 millivolt "bucking”

signal, the temperature scale could be read to less than

1 degree C per mm of chart. An event marker pen on the same

recorder was actuated by a hand-operated microswitch to

measure the time for flames to advance past the 3“in* markings.

Measurements were also made of the deposit of smoke on a glass

fiber filter paper placed in the stack above the burning

specimens

.

4. RESULTS and DISCUSSION

Typical distance-time data for the 2 lb/ft^ foams

are shown in Figure 1 which also includes data for balsa

wood for comparison. Cellulosic materials previously tested

(1) generally plot as straight lines on semi-logarithmic

coordinates and a similar pattern is observed with many cellular plastics. However, a distinctive upward curvature is

observed for materials 1 and 5 corresponding to an unusual

type of flame propagation. The shape of the curve for

material 9 appeared to be due to rapid enhancement of flaming

following the separation of a protective char coat during the

test.

The temperature rise modification in computation of the

flame spread index suggested in Section 2' was necessary in

only a few instances and is noted in Table 2 which summarizes

the test results. The extrapolation technique was used only

to fill in an occasional unrecorded distance-time point and

did not materially affect the computations. From Table 2, it

is seen that the flame spread index for tfie 2 lb/ft3 fireretardant treated polyurethane foams ranged from 10 to 1440
- 5 -

whereas, the two untreated foams (Nos. 2 and 5 ) were

rated at 1490 and 2220, respectively. Two direct comparisons showing the marked improvement of fire-retardant over

untreated polyurethane and polystyrene foams are available,

i.e., 2 versus 3, and 7 versus 8, respectively.

The opportunity for exploring the effect on the flame

spread index of an appreciable range of material bulk

density appeared to be more convenient with plastic foams

than with natural or processed cellulosic materials.

Accordingly, one cellular plastic foam was made available

in four densities corresponding to an eight-fold density

change. Unfortunately, this foam was a flame-retardant type,

and, upon separation of its protective char coat during

test, a rapid enhancement of flaming for all specimen densities resulted, severly hampering measurement of flammability

differences due to density. Although density appears to

have an appreciable effect, it is evident that an untreated

foam, or a flame-retardant treated foam with normal surface

flammability behavior would be more suited for evaluating

density effects.

With few exceptions, the quantity of smoke deposited

by the cellular plastic foams tested was significantly greater

than that from typical cellulosic materials.

Limited comparisons (not shown) were made with preliminary

data from other test methods used in the round robin series.

For those tests which provide a numerical indication of the

burning rate (ASTM D1692. ASTM D 757 , and Nat. Aniline CF-TM-11),

the range of values for eight materials was considerably less

than the corresponding range in the flame spread indices. In

some instances, the burning rate values for fire-retardant and

untreated foams were not significantly different or were in

reverse of the expected order.
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Table 2. Surface Flammability Test Results

Mean values based on four replicate tests

Flame Spread Index Heat

Is Evolution

Coeff .of Factor Smoke

Symbol Mean Var

.

Q Denosit Remarks

~T~ mg.

1 1000 9.7 15.4 1.0 No melting or dripping

2 1490 13.4 15.3 1.0 Rapid melting and

dripping

3 10 51.0 4.2 3-1 Rapid melting and

dripping. Some

flaming to 3 in * 4 1440 28.9 26.3 7.4 No melting or dripping

5 2220 13-0 29.1 3-5 No melting or dripping

6 880 39.4 15.5 6.9 Intumescence. No

melting or dripping

7 114* 14.

5

3-1 1.7 Melting and flaming

drips

8 13* 100.0 1.4 0.5 Some flaming to 3

and 6 in.

9 1090 12.8 11.9 11. 3^

Separation of pro10 3200 14.8 34.4 12.8 1 20.4

tective char coat

f during test results

11 2590 83 .O

24.6

36.0 in very rapid

flaming

12 1310 38.3 33-1 J

*Base'd upon stack thermocouple temperature for asbestos -element

board at time of maximum. See discussion, Section 2.
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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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