•flits report Iras -imtoL

•for informatioD and record

pt^eraraHr^

NATIONAL BUREAU OF STANDARDS REPORT

8308

HEATING CHARACTERISTICS OP CORDAGE FIBERS

by

M„ W. Sandholzer,

Jo J. Loftus

and

D 0 Gross

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1002-12-10120 March 10, 1964 8308

HEATING CHARACTERISTICS OF CORDAGE FIBERS

by

M. Wo Sandholzer

J. J. Loftus

and

D. Gross

for

General Services Administration

Agreement No. GS-OOP-22529 (SCM)

IMPORTANT NOTICE NATIONAL BUREAU OF ST,

for use within the Government, and review. For this reason, the whole or in part, is not author Bureau of Standards, Washingtc

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

>s accounting documents intended

subjected to additional evaluation

listing of this Report, either in

! Office of the Director, National

the Government agency for which pies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

HEATING CHARACTERISTICS OF CORDAGE FIBERS

by

M. Wo Sandholzer, J. J. Loftus and D„ Gross

1, Introduction

Fires in various types of fibers during shipment and storage have

not been uncommon over the years. The major difficulties have been ex- perienced in the handling of cotton, wool, and jute, and the heating

characteristics of those fibers have been the subject of considerable

study. In general the so-called "hard fibers" such as hemp and sisal,

used in the manufacture of cordage, have not been considered particularly liable to self-heating and have not been as thoroughly

investigated. Hemp is known to heat to the point of fiber damage very

quickly after wetting, however, and there have been fires in baled

sisal storage which suggest that "spontaneous" or self-heating may be- come a problem in large quantity stockpiling.

One such fire occurred in 1963 in cordage fibers, stockpiled by

the General Services Administration near Cincinnati, Ohio, which had

become rain-soaked when the. building roof was lost in hurricane winds.

The fiber stock was covered with tarpaulins to protect it from further

weather exposure and about fen days later fire developed in the storage.

To help determine and understand the cause of the fire, the National

Bureau of Standards was asked to investigate some of the heating characteristics of the types of fibers included in the storage.

2. Materials and Characteristics Studied

The General Services Administration provided samples of four

different cordage fibers for study. Three were labelled abaca fibers

and identified respectively as Davao, non-Davao, and C. A. Light. The

fourth was identified as Haitian dry sisal.

Information on the tendency of the fibers to heat spontaneously,

and on the heat which might be produced upon wetting them, appeared

to have an important bearing on the problem. Furthermore, from the

data developed in determining the self -heating tendency of a material

it is possible to estimate, for a given surface (or ambient) temperature, the critical size of a storage pile beyond which there may be a hazard of self-ignition. Accordingly, determinations were made of the

heats of wetting and of the self-heating tendencies of the four fibers

submitted, and a few additional points were explored briefly.
- 2 -

3„ Heat of Wetting Determinations

The absorption of moisture by cellulosic fibers results from the

affinity between water molecules and the free hydroxyl groups in the

cellulose structure and is directly related to the proportion of amorphous material in the cellulose chain. It is an exothermic process arid the amount of heat evolved will vary with the particular fiber considered;, and also with the amount of moisture already present in the

fiber and the amount of moisture absorbed. The heat of wetting may be

defined as the heat evolved when the material is completely wetted with

a large excess of water, and it will be greatest when the material is

wetted from a thoroughly dry state. It is expressed in calories per

gram of the dry weight of the material. Determinations were made on

the four test fibers in an oven dry condition and also after conditioning in atmospheres of higher relative humidity.

The calorimeter used for determination of the heats of wetting

was developed at the National Bureau of Standards for measuring the heat

of solution of cement and is described in detail in a paper by Newman

and Wells [l]. Briefly, the calorimeter was filled with water at 25°C

and the weighed specimen, in a water-tight container, was immersed in

the water. A resistance thermometer was inserted to indicate the temperature of the water, which was agitated by a motor-driven stirrer.

After assembly, the calorimeter was placed in a constant temperature

bath controlled automatically at 25°€. When the. rate of temperature

rise of the water, caused by stirring and thermal adjustment, had

reached a constant value, the specimen was expelled quickly into the

water (by suitable means) and the resulting temperat-ure rise noted.

The heat evolved was calculated from the rise in temperature and the

heat capacity of the calorimeter contents.

For the comparative measurements with which this study was con- cerned, the heat capacity of the calorimeter contents was calculated

rather than precisely measured. The water was weighed and the heat

capacity obtained from the weight and the known specific heat, while

a constant, estimated value was assigned for the other contents of the

calorimeter (specimen container, supports, stirrer, etc.).

In all cases the fibers were cut into short lengths (mostly about

1/8- 1/4 inch) for these determinations and the cut material was dried

in a circulating-air oven at 1G0-105°C for at least four hours. For

the determinations of heat of wetting from the dry state, the sample

container was filled directly from the oven and cooled in a desiccator

before weighing. Inasmuch as most fibers exhibit hysteresis, the material conditioned to higher moisture contents was also oven-dried first

so that equilibrium would be approached from the same direction in all
- 3 -

instances. Two conditioning rooms were available, one maintained at

73 ± 2 °.F and 50 ± 5% relative humidity, the other at 72 ± 2 °F and 65

± 2 % relative humidity, and the fiber specimens were conditioned three

days or longer before test. The moisture regain (weight percent of

moisture present, based on the dry weight of the material) was calculated from the weight increase during conditioning. At least two

determinations were made at each condition and the average values obtained are given in Table 1. Published values for the heats of wetting

from the dry state of cotton, wool, and jute are 11,0, 26.9, and 18.2

cal/g, respectively.

Table 1. Heats of Wetting of Cordage Fibers

Conditioning

Fiber type

oven dry 50% RH 65% RH

ht. of wetting

moist

.

regain

ht. of

wetting

moist

.

regain

ht . of

wetting

cal/S % cal/g % cal/g

Abaca

,

C. A. Light 12.1 8.0 4 .

6

10.7 3.3

Abaca, Davao 12.2 8.2 4.4 10.4 3.3

Abaca

,

non-Davao 13.0 8.3 5.0 10.2 3.3

Sisal

,

Haitian dry 16.4 8.4 5.6 10.6 4.2

4. Self-Healing Measurements

The apparatus and method used for the self-heating studies and

analysis of the data obtained have been previously described [2,3].

Thermocouples were placed within a cylindrical specimen 2 in. in

diameter by 2 in. high. The specimen was prepared by spiral winding

of fiber bundles of appropriate length and approximately 1/2 in. diameter into pancake discs 2 in. in diameter. Several discs were stacked

to make an overall height of 2 inches. The assembled specimen was then

placed in an open wire basket or a stainless steel beaker and mounted

within the furnace designed for the self-heating measurements. Two

tests for each material were performed in order to establish satisfactory reproducibility between duplicate tests.

From an initial temperature setting of 150°C, all of the cordage

fibers tested indicated self-heating properties which led to eventual

ignition. The time required for the samples to reach a pre-set furnace
- 4 -

cut-off temperature of 280°C are listed as follows:

Abaca, C. A. Light -- 12 hours

Abaca, Davao -- 9 hours

Abaca, non-Davao -- 11 hours

Dry Haitian sisal -- 17 hours

From the above it is seen that of the four materials Davao required

the shortest time to ignition.

The slope of the time-temperature curve at several temperatures

was plotted against reciprocal absolute temperature and results are shown for one test on each material in Figure 1, A straight line has

been drawn through these points and the activation energy "E" determined from its slope. Comparative rates of self-heating at any temperature may be read directly from the graph; for example, at 190°C

(1/T = ,00216°K"^) the dry Haitian sisal exhibited self-heating at a rate of 0.175 deg C/min. The values of the kinetic constants determined from the lines in Figure 1 are listed in Table 2 for the applicable temperature ranges. Although all specimens were conditioned in an atmosphere maintained at 73 ± 2 °F and 50 ± 5% relative humidity

prior to test, the kinetic properties are considered typical of dry

materials because of the elevated temperatures of test.

Table 2, Physical and Kinetic Properties

Material

Temp

Range Density*

P

Specific*

Heat

c

Thermal*

Conductivity

k

Activation

Energy

E

Heat

Generation

Coef f

A

°C g/ cmJ cal/ g °C ca 1/sec cm°C kcal/mole cal/sec cm

J

Abaca, C. A. Light 150-250 0 . 64 0.32 .00041 26,7 2.48x10^

Abaca, Davao 150-250 0.64 0.32 ,00041 27.2 6.55x10“

Abaca, non-Davao 150-250 0.64 0.32 ,00041 25.8 1.02xl0 ii

Haitian Dry sisal 150-250 0 . 64 0.32 .00041 25.7 4.89X1010

* Values assumed to be typical for stored bales.
- 5

-

Thermal conductivity and specific heat measurements were not made

for each of the materials. However,, if the assumption is made that a typical bale of cordage fiber material would have a density of 40 pcf

(.64 gm/cm^) the values for thermal conductivity and specific heat

would approximate those of wood which at this density would have values

of 0.1 Btu hr"l ft"l °F -

1 (4.13 x 10”^ cal sec~^- cm"^ °C"1) and .32 cal

g“l °C“1, respectively.

5. Calculation of Critical Size

In order to estimate the critical size for ignition of a mass of

self-heating material, the analysis presented by Enig, Shanks and

Southworth [4] was used. This related the half-thickness of a material of given thermal and kinetic properties with the temperatures at

the center and the surface, under critical steady-state conditions.

The assumption was made that the kinetic properties measured over higher temperature ranges may be applied to the temperature range of

practical interest for ordinary storage (20-100°C) and it was further

assumed that the thermal properties would apply over the whole temperature range. Determinations of the critical diameter for a sphere,

and the critical thickness for a slab with lateral dimensions large

compared to thickness, have been made for each material and are listed

in Table 3 and shown graphically in Figure 2.

6. Additional Exploratory Tests

Again assuming the specific heat of 0.32 eal/g °C to be typical

for stored bales of the fibers, the values determined for the heats

of wetting of the fibers would correspond to temperature increases

ranging from 10-50 degrees C. In actual storage the most serious

situation, that of completely dry fibers becoming completely wetted,

would not be likely to occur, and hence the greater increases within

that range could not be expected to result from wetting alone. The

stored fibers might well have a moisture content similar to that re- sulting from the conditioning at 50 percent relative humidity, however,

and temperature increases of around 15 degrees C would appear quite

possible from a heavy wetting of the storage.

Several tests were performed in an effort to examine briefly the

combined effects of the heat of wetting and chemical kinetic selfheating. Because of the drastic cooling effect of liquid water on a self-heating specimen, the adiabatic furnace test was arranged so that
Table

3.

Critical

Size

Calculations 6 *

33 4-c

CJ) fa 4-1 3 O o o 4-1 rH O CM

co CU r-c inm

m o

)-i <-l CNI « o e m 3 —/ o o o o o 43

cd CO J-! CO cu

cu3 i"

h - & cu O 0 4-4 y H o CM

•H

O CM vO av

m rH

-3 cu LO • o • H O w vO vO 00

cd r-4 4-1 3 J-l O 3

°H CO

O fa 4-1 43 0 0 4-4

°r4 J-l

O 00

M o CM vO 00 00 CO co U 4-1 v^j in in m 3

co

cu3

CU 33

J4 4-1

a fa 4-1

y

cu

O 0 0 4-4 •H JS O CM -3 a. CU -4 o av ov CM 4-1 co J-4 i—1 CM o « o « 3 s-/ 1—

o o rH 0

4-1 4-1 43 o cd

J-4 33

J4 cuy

(U Q- 4-1 j-i •u 6 fa 4-1 3

cu cu

O o a e H o CM 6 3 O CM ® ® 0 o

cum .—i cn CM LO

y

a

y

w .—i *--i rH r-H

cd

y

r-4 4-4 00

cd J-l J4 y 3 43 3

CO CJ fa 4-1 rH

43 o o •H J-4 O 00 CM CM rH 33

J-i o CM v£> r—

r—

00 rH

y

O 4-1 r—

i—

r—

B3

co33

cod

4-1 o

-3

u °H

00 o no CO •H cd 3

hJ > d

y

rH cd Cd

B

cd * o Q •H

•r4 < cd i 4H 33

1-4 > 3 -H y • cd O cd r—

j-j

u

Q 3 D5 3

cd >3 S - *\ C\ y

cd cd 3 r-H 43 a a y cd 3

cd cd 3 CO 4

rQ rO ®Q •H

<3 <3 <3 to *
- 7

-

steam could be introduced into the test specimen through a perforated

teflon tube embedded about midway between the center and outside of

the specimen. When steam amounting to approximately 25 percent of

the weight of the specimen was supplied, with the specimen temperature at about 15Q°C, temperature increases ranging up to 17 degrees C

resulted. When the steam was introduced at an initial sample temperature of 96 °C, the temperature rose to 126 °C„ These increases are in

accord with those suggested by the heat of wetting data, and it would

appear that, under some conditions, they might be sufficient to raise

the temperature of a large storage pile to the critical point from

which self ^heating to ignition could proceed.

In addition to the above tests, differential thermal analyses

were made on two of the fibers, the Davao abaca and the Haitian sisal.

In this analysis, which compares the heating behavior of a test material with that of an inert material, specimens of the two materials

are placed in the same furnace, which is then heated at a given rate.

Automatic recording equipment provides a continuous record of the

difference in temperature between the two specimens, as indicated by

a differential thermocouple, and of the actual temperature of the test

material, as indicated by a separate thermocouple. It is assumed that

the effect of the furnace heating alone is fairly represented by the

temperature of the inert material, and that pronounced differences in

the temperatures of the two specimens indicate some change, exothermic

or endothermic, in the test material. In Figure 3, the temperature

difference between the inert and the test material is plotted against

the actual temperature of the test material.

The two fibers showed a generally similar behavior, but the

changes differed notably in degree. Thus, the endothermic reaction,

(likely the production of gaseous degradation products) which became

predominant at around 300°C in both cases, was decidedly more pronounced for the sisal than for the Davao abaca. This difference in

heat absorption is probably related to the longer time required for

the sisal to heat to ignition, which was noted in the self -heating

determinations. The differential thermal analyses indicate further

that ignition resulted in a more rapid evolution of heat above 400°C

from the sisal than from the Dayao abaca.

7. Conclusions

Measurements of the heats of wetting and the self -heating properties of four cordage fiber materials have been made. All materials

showed self -heating over the temperature range of 150-250°C with slight
- 8 -

differences among the fibers. Activation energy values ranged from

25.7 to 27.2 kcal/mole, values typical of fibrous wood products.

From the kinetic data, critical size calculations have been made

for surface temperatures ranging from 20-100°C. The critical thick- ness values calculated for normal storage temperatures of 20-30°C

(68-86°F) are sufficiently high that any usual type of storage of the

dry fibers at those temperatures would appear to present no hazard of

self -ignition. Should the storage temperature exceed that range significantly, however, and approach perhaps 50°C (122°F), the data

clearly suggest that the specification of a permissible pile size

would be advisable.

Prolonged exposure to elevated ambient temperatures will, of

course, raise the temperature of stored fibers to some extent. The

temperature may also be increased if the fibers become wetted, the

extent of the increase depending on the original condition of the

fibers, the degree of wetting, and the various conditions of storage.

The data obtained indicate that, under some conditions, an increase

of 15 or more degrees C might result from wetting the fibers. Such

an increase could produce a temperature from which, under favorable

conditions, self-heating to ignition would appear possible in large

storage piles.

These experiments and the calculations which have been made indicate that definite precautions would be advisable for fiber storage

which may be subject to elevated ambient temperatures or to wetting.

It should be noted, however, that the calculations are based on assumed thermal properties which may be considerably different from

the actual properties of the baled fibers. The density was assumed

to be 40 pcf, based upon the reported bale size of approximately

2 by 2 by 4 ft and a weight of approximately 650-700 lb. A thermal

conductivity value of 0.1 Btu/hr ft °F, representative of wood of

similar density, was arbitrarily assigned. Although critical pile size

is a (square-root) function of the ratio of thermal conductivity to

density (which are roughly proportional to each other) , it may be

noted that a 507. increase in thermal conductivity at the same density

(or a corresponding 50% decrease in density at the same thermal con- ductivity) would result in a 25% Increase in calculated critical size

at all temperatures. Finally, it must be emphasized that calculation

of critical size for ignition is based upon the very broad assumption

that kinetic properties measured at elevated temperatures apply at the

lower temperatures. In some cases, the simple theory for thermal ignition assuming a single reaction with no loss of reactant may be

inadequate to permit extrapolation to normal storage temperatures.
9

REFERENCES

1. Newman, E„, and Wells, L„, J. Research NBS K), 825 (1938),

RP 1107.

2. Raskin, W. H., and Robertson, A. F„, "An Adiabatic Apparatus

for the Study of Self Heating of Poorly Conducting Materials,"

Rev. Sci. Inst. 25, 541-544, June 1954.

3. Gross, D., and Robertson, A. F., "Self-Ignition Temperatures

of Materials from Kinetic Reaction Data," J. Research NBS 61 , 413-417, November 1958.

4. Enig, J. W. , Shanks, D„, and Southworth, R. W., "The Numerical

Solution of the Heat Conduction Equation Occurring in the Theory

of Thermal Explosions," NAVQRB Rep. 4377 (1956), Astia Document

No. AD-116 873.

USCOMM-NBS-DC

RATE

OF

TEMPERATURE

RISE,

°C/MIN.

FIG.I-ADIABATIC SELF-HEATING DATA

ON FOUR CORDAGE FIBERS
CRITICAL

THICKNESS

OR

DIAMETER

,

FEET

SURFACE TEMPERATURE, °F

FIG. 2 — COMPUTED CRITICAL DIMENSION

AND SURFACE TEMPERATURE FOR

FOUR CORDAGE FIBERS
+

JLUVHD U3QaOD3U NO S30VdS ‘30N3d33dia 3UniVH3dW3±

u

ouT

CL<

O)

lL

O

UJ

OCZj

h-

<

ccUJ

CL2

UJ

hCO

crUJ

CD

Ll

UJ

CD

<

Q

oro

CJ

o

hO

CO

UJ

CO

<

<

<1

cr

l J

Kc £

f-

* /

LJ

cr: UJ

Ll

Ll

Q

rO

O

Ll