Bureau of Standarda

.NOV i93^*

U. S. DEPARTMENT OF COMMERCE

BUILDING

MATERIALS

AND

STRUCTURES

REPORT BMS4

Accelerated Aging

of

Fiber Building Boards

DANIEL A. JESSUP

SAMUEL G. WEISSBERG

and CHARLES G. WEBER
The program of research on building materials and structures imdertaken by the

National Bureau of Standards is planned with the assistance of the following advisory

committee designated by the Subcommittee on Design and Construction of the Central

Housing Committee.

TECHNICAL RESEARCH GROUP

Harold D. Hynds, Chairman

Walter Junge, Federal Housing Administration, Vice Chairman

Pierre Blouke,

Home Owners' Loan Corporation.

C. W. Chamberlain,

Procurement Division.

Charles E. Mayette,

United States Housing Authority.

Vincent B. Phelan,

National Bureau of Standards.

A. C. Shire,

United States Housing Authority.

George W. Trayer,

United States Forest Service.

Elsmere J. Walters,

Quartermaster Corps, War Department.

Sterling R. March,

Secretary.

The program is administered and coordinated by the following staff committee of

the Bureau:

Hugh L. Dryden, Chairman

P. H. Bates W. E. Emley A. S. McAllister

H. C. Dickinson G. E. F. Lundell H. S. Rawdon

The Forest Products Laboratory of the United States Department of Agriculture

is cooperating with the National Bureau of Standards in studies of wood constructions.

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UNITED STATES DEPARTMENT OF COMMERCE • Daniel C. Roper, Secretary

NATIONAL BUREAU OF STANDARDS • Lyman J. Briggs, Director

BUILDING MATERIALS

and STRUCTURES

REPORT BMS4

Accelerated Aging of Fiber Building Boards

by DANIEL A. JESSUP

SAMUEL G. WEISSBERG

and CHARLES G.WEBER

ISSUED OCTOBER 11, 1938

The National Bureau of Standards is a fact-finding organization;

it does not "approve" any particular material or method of construction. The technical findings in this series of reports are to be construed accordingly.

UNITED STATES GOVERNMENT PRINTING OFFICE • WASHINGTON

FOR SALE BY THE SUPERINTENDENT OF DOCUMENTS. WASHINGTON, D. C. • PRICE lO CENTS
Foreword

Vegetable Fiber Boards, a relatively new class of products, have some very desirable properties, but there is little published information as to how well the original

properties are retained when in use. It is obviously not practicable to wait for the

results of normal aging over a long period of years, yet some information as to the

durabihty of these materials is needed before it can be determined whether it is good

economy to substitute them for more costly materials in low-cost house construction.

This is a progress report, describing accelerated aging tests based on the results of the Bureau's experience in the use of similar tests on paper and other cellulosic materials. Some typical test data are included. The results obtained so far show that some of the boards have excellent stability, while a few lose certain desirable properties

rather rapidly under the accelerated test. It is expected that this work will finally result

in a performance specification and test method wliich will make possible the selection

of material that is most likely to prove satisfactory.

Lyman J. Briggs, Director.
Accelerated Aging of Fiber Building Boards

by DANIEL A. JESSUP, SAMUEL G. WEISSBERG, and CHARLES G. WEBER

CONTENTS

Page

Foreword ii L Introduction 1 IL Testing for stability 2

1. Accelerated aging treatment . . 2

2. Measurement of aging effects. . 3

Page

II. Testing for stability—Continued.

3. Miscellaneous exposures ... 5

III. Results of accelerated aging .... 5 IV. Summary and conclusions 7

ABSTRACT

Fiber Building Boards are being tested to determine

whether they have satisfactory permanence to justify their wide use in low-cost house construction. Accelerated aging is being used because this information

is urgently needed, inasmuch as it would take years to obtain the effects of normal aging. The aging treat- ments include cycles of wetting, drying, freezing, and

heating in dry air, and the stability is judged by changes

in physical and chemical properties. The properties most affected by aging are water absorption, nail-holding

pwDwer, flexural strength, and permeability to air. Results thus far obtained are favorable. After being

subjected to the aging cycles for a total of 300 hours,

only two of the boards tested showed serious loss of strength and other desirable properties, despite the

drastic nature of the treatments.

I. INTRODUCTION

For a Number op Years the National Bureau

of Standards has made many studies of buildingmaterials to determine their physical properties

as related to their performance in service.

Supplementing its regular studies the Bureau is now engaged in a program of research to determine the properties and suitability of buildingmaterials with particular reference to their use

in low-cost housing.^ The durability of materials is one of the problems included in this work,

and this report describes the accelerated aging

procedure and tests developed by the Paper

> The purpose of the housing investigation and scope of the problems

assigned to the National Bureau of Standards are described in the first report of this series, Hugh L. Dryden, Research on Building Materials and Structures for use in Low-Cost Housing; Building Materials and

Structures (1938) NBS Rep BMSl. Price 10 cents, stamps not accepted.

Section of the Bureau to determine the durability

of fiber building boards.

Fiber building boards are sheet materials

designed particularly for use in house construction, and are made in forms and sizes adapted for that purpose. They are made in

sizes that can be rapidly and economically

applied with a minimum of skilled labor. The

boards are made resistant to moisture by the use of rosin, asphalt, waxes, or other sizings

and coatings. There are three general uses for them in house construction: as heat-insulating

components in walls and roofs; as sheatliing

under brick, stone veneer, or clapboards; and

as finish for walls and ceilings.

The ordinary properties of the boards are

well known. Data on wall boards and insulating boards are contained in two previous

publications ^ ^ by members of the staff of the

National Bureau of Standards. So far as the

original values of the strength, insulation,

water resistance, density, and rigidity are concerned, the boards have been found to be satis- factory. However, httle is known as to the

retention of these suitable properties under

conditions of use. The boards are made of

ground wood, or crudely refined fibers from

wood or other vegetable growths. The pulping and refining of the fibers are not carried

far enough to remove the lignins and gums

naturally present in the plant, nor are the

fibers bleached. Papers made of fibers in this

2 Misc. Pub. BS 132 (1931).

3 Ind. Eng. Chem. 27, 896 (1935).

81544°—38 [1]
unpurified condition have frequently been

found to be unstable and to deteriorate rapidly.

Therefore, even though the materials when

new may have desirable properties which make

them suitable for use in low-cost house construction, the question of how well these

desirable properties are retained under conditions of use must be answered before the econ- omy of substituting them for more costly

materials of known stability can be justified.

Obviously, the most conclusive data on the

permanence of the materials in service could

be obtained from field and laboratory observations after years of service. However, the

impracticability of waiting years for the results

of such a procedure is apparent. Hence, use must be made of the alternative method of

judging the performance on the basis of data

obtained by accelerated aging. The materials

are subjected to conditions Mhich produce in a

short time in the laboratory effects similar to those arising from long periods of natiu'al

aging. These conditions must be, of necessity,

much more drastic than the deteriorating conditions encountered in use, in order to achieve

results in a comparatively short time. However, they are qualitatively similar, and experience with paper and some other materials has

Figure 1.iSleaiii cabiiui used to /Jiudacc llic high humidity -phase of the accelerated aging cycle for fiber building boards.

shown that a high order of stability to accelerated aging means satisfactory permanence,

wliile low stability to accelerated aging means

unsatisfactory permanence.

II. TESTING FOR STABILITY

1. Accelerated Aging Treatment

Dry Heat was tried first as a means of accelerating the changes occurring during natural aging.

Whereas heating 72 hours at 100° C (212° F) is often sufficient to make some papers and certain

other fibrous materials brittle, practically no

change in this respect was noted for the fiber

building boards when heated for 150 hours at 100° C (212° F).

A high-humidity phase was added to find the

effects of alternate wetting and drying. A

cabinet was attached to a low-pressure steam

line, and condensed steam at 90° to 95° C (194°

F) allowed to fall on the test specimens for 6 hours daily, after which they were kept in a drying oven at 100° C for the remaining 18

hours. This cycle was repeated for a total of

300 hours. Some boards showed considerable

loss in flexural strength. Low temperatures en- countered in certain localities sometimes cause moisture to condense and freeze within walls. The freezing of water within fiber board may

damage the board through the fibers being

forced apart. Consequently, a freezing phase was added to the aging procedure. All further

references to the effects of "accelerated aging" on the properties of the boards refer to the fol- lowing aging cycle.

The test specimens are: Immersed in water at 50° C (122° F) for 1 hour; sprayed with con- densed steam at 90° to 95° C (194° to 203° F)

m the cabinet shown in figure 1 ; stored 20 hours

at -12° C (10.4° F); heated 3 hours at 100° C

(212° F) in dry air; again treated for 3 hours

with condensed steam, then heated 18 hours at 100° C (212° F) in the oven shown in figure 2. This cycle of operations is repeated for a total of 300 hours during which time the specimens

are exposed 40 hours to condensed steam, 130

hours to dry heat and 130 hours to freezing

temperature. This procedure produces suffic- iently large changes in the properties of the

boards to permit classifying them into distinct

groups.

[2]
Although the fiber boards are not, in general,

designed for use as exterior finish, some are recommended for such use and the Bureau re- ceives a large number of inquiries regarding

their suitability for this piupose; hence, tests are

in progress relative to this problem. The tests consist essentially of outdoor exposures, with

water spraying at regular intervals to accelerate

the normal aging. Painted and unpainted specimens, with edges sealed against moisture, are exposed in racks on the roof of the Industrial

Building, as shown in figure 3. The exposure is for the duration of 1 year with periodical ex- amination for changes in appearance, warping,

and deterioration of surface finish.

2. Measurement of Aging Effects

The effects of aging are detected by determining the changes in strength, permeability

to water and air, and chemical composition.

The tests made are for flexural or transverse

strength, nail-holding strength, water absorption, water penetration, permeability to air and water vapor, and cellulosic purity.

The flexural strength tests are made in

accordance with the standard method described

in a previous publication ^ and in a Federal

Misc. Pub. BS 132 (1931),

Figure 2.Drying oven used as a phase of the accelerated aging cycle for fiber building boards.

Specification ^ for fiber building boards. This

test is made by applying a load at midspan of a specimen 3 inches wide, laid flatwise on two

parallel supports spaced 12 inches apart. The

load is applied by means of a tensile testing

macliine, as shown in figure 4. The machine

records the rupturing load and the deflec- tion of the specimen at rupture. The flex- ural strength is an important property inci- dent to the economical handling of the material,

and the amount of loss may be considered

as indicative of the extent of deterioration.

Any structural strength and rigidity imparted to a buflding by

a fiber board is dependent, to some extent at

least, on the nail-holding

strength of the board.

If during aging the board

loses an appreciable part

of this strength, its usefulness as a component

part of the structure may

be materially reduced.

For measurmg nafl-holding strength, a method

simflar to one suggested

by a manufacturer ^ was

foUowed. This test measures the force required to move a 6 -penny common

wire nail to the edge of

Figure 3.

Roof exposures of painted and unpainted panels of fiber building boards.

' Fed. Spec. LLI^r-321a, Fiber Board; Insulating.

6 This method was proposed by M.

S. Wimderlich, of the Insulite Co.

[3]
Figure 4.

FLcxurul strength test.

a board from a position % inch from the edge.

The test is illustrated in figure 5. The existing method for determination of

water absorption was found to be suitable for use in this investigation. Specimens approximately 3 by 5 inches are weighed and immersed in water for 2 hours, removed and al- lowed to drain for 10 minutes, then reweighed.

The increase in weight represents the amomit

of water absorbed and is expressed as percentage of the original volume.

The rate of water penetration through the

boards is determined by the dry indicator ^

^ method. An indicator consisting of a mixture

of eosin dye and powdered sugar is sprinlded on

one surface of the board and a glass cover is sealed over it to prevent moisture reaching the

dye except through the board. The specimen

with edges sealed by a wax coating is then

floated on water. The time of transudation of

water in a sufficient amount to develop color

in the indicator is a measure of the relative

water resistance. The test was developed for paper and paper products in general by F. T. Carson of the Bureau after he had found that numerous methods in use were unsound in one

respect or another. The test is illustrated in

figure 6. Air permeability of fiber building boards is important in its relationship to the insulating

' Fed. Spec. LLI^F-321a, Fiber Board; Insulating.

8 Paper Trade J. 98, No. 21 (May 24, 1934).

• Tappi Std. T433m, Tech. Assn. Pulp and Paper Ind. 122 E. 42d

Street, New York, New York.

value of the boards. The thermal insulation can be made less effective if heat is lost through

the boards by infiltration of air. The measure- ment of air permeability serves also as a means

of detecting changes in the structure of the

material resulting from aging. The apparatus

developed by F. T. Carson ^° is used for the

determinations. This apparatus is illustrated

in figure 7. Investigations of the stability of writing

papers have shown a close relationship between

cellulosic purity and stability, papers with high

alpha-cellulose content generally being more

stable. It has been shown also that the alphacellulose content of paper is lowered by natural

aging and by accelerated aging treatments,

the amount of change being an indication of

stability. The extent of chemical deteriorai» BS J. Research 12, 567 (1934) RP681.

Figure 5.

Nail-holding strength test.

[4]
tion in boards, as indicated by

decrease in alpha-cellulose content, is measured by use of tlic modified volumetric method "

suggested by H. F. Launer of this Bureau.

3. Miscellaneous Exposures

Investigations have shown that

light exerts a destructive influence

upon textiles, paper, and other

similar substances. Light from

the carbon arc, as in the WeatherOmeter, has been used extensively

in methods of accelerated aging.

0. G. Stricter recently found

this type of exposure very satis- factory for determining the weathering quality of roofing materials,

and it was investigated as an aging test for the

fiber boards. Specimens were irradiated with

light from the carbon arc and intermittently

sprayed with water, the spray being shut off for 8 hours out of each 24 to allow the specimens

to become thoroughly dry. The results of

exposures up to 500 hours appeared to be principally surface effects. In view of this and

since the boards are seldom, if ever, exposed

unpainted to direct sunlight, the test was

discontinued.

11 J. Research NBS SO, 87 (1938) RP1068.

12 J. Research NBS 30, 159 (193S) RPinrs.

Figure 7. —^1 ir pcnncability test.

Figure 6. —Water penetration test. Inasmuch as the presence of moisture within

a waU during summer weather might be conducive to the growth of rot-producing fungi, a test

is in progress to obtain data on any susceptibility of the boards to rotting. The method consists in inoculatmg speciniens with three of the

typical rot-producing fungi commonly found

around wood construction, storing the speci- mens under warm, moist conditions favorable

to the growth of the fungi, and observing the

spread of the fungi and their efi^ects on the

boards. The selection of fungi was based on a recommendation of the Division of Forest

Pathology, United States Department of Agriculture, and the cul- tures used were prepared in the

Forest Pathology Laboratory. Inoculation was accomplished by

placmg pieces of culture approximately ji inch square on these

specimens. Each specimen was in- oculated with cultures from all three fungi.

III. RESULTS OF ACCELERATED AGING

While the Test Data for the extent of deterioration imder the

accelerated aging treatment are not yet complete, some typical

results are sho%\Ti in the accompanying charts. Losses in flexural

[5]
UNA6ED

til 1

1

i

B U SAMPL E FiGUEE 8.

Effects of accelerated aging on the flexural strength of 1 1 fiber building boards.

strength produced by accelerated aging are shown in figure 8. The scale to the left represents the strength in pounds. The shaded

blocks are the values for unaged samples, and

the unshaded blocks are values for samples

which had been carried tlurough the accelerated

aging cycles for a total of 300 hours. Only two

of these boards showed a serious loss in flexural strength, despite the severity of the aging

treatment, the losses ranging from approximately 10 percent to about 60 percent.

90

80

70 -

;60

50

4 0

3 0

D

UNAGEO

AGED

i i

1

I i

I

I

I

I

143

I

I

B A 0 J SAMPLE

P'iGURB 9.

Effects of accelerated aging

holding strength of nine fiber building

I H

on the nail- boards.

>20

/I D

IV >^ V

A L SAMPL E Figure 10.

Effects of accelerated aging on the water

absorption of 11 fiber building boards. The losses in nail-holding strength corre- sponded to the losses in flexural strength.

Some results of the test are shown in figure 9. Here, serious losses in strength occurred in the

two boards which were the weakest originally.

For new boards, the force required averaged 57

pounds for the weakest board and 143 pounds

for the strongest. After aging, the values were

31 and 115 pounds, respectively. In the test of the force required to pull the head of a common 6-penny wire nail through the board.

< \

D

I c SAMPLE Figure 11.

Effects of accelerated aging on the air 'per- meability of 11 fiber building boards.

[6]
the values ranged from 44 to 146 pounds.

The values for the aged boards ranged from 19

to 127 pounds.

The effect of the aging procedure on absorption of water was quite marked for some boards,

while others showed little change. Some results are shown graphically in figure 10 which presents

values for water absorption before and after accelerated aging. Some boards showed in- creases in water absorption of several hundred

percent. Corresponding changes in water penetration resulted from the accelerated aging.

Some aged boards allowed water to penetrate in

1/200 the time required for the unaged board,

while others showed no change on aging.

The changes in air permeabilities of the

boards are a reflection of the internal structural

changes produced by aging. Some results of

this test are shown in figure 11. The vertical

scale values show permeabilities expressed in

liters of air passing through 1 m^ of material a

second, with a pressure difference of 1 g/cm^.

Chemical analyses showed the alpha-cellulose content in aged boards to be from 2

to 6 percent less than for new boards. These

losses are in line with those reported for the

natural aging of paper, and suggest that the

accelerated aging treatment produces changes

comparable with those produced by natural

aging.

IV. SUMMARY AND CONCLUSIONS

Accelerated Aging offers a convenient means

of determining the stability of fiber building

boards without the necessity of waiting years

to observe the eft'ects of natural aging. Data

thus far obtained indicate that some of the

boards have excellent stability, while others

lose certain important properties rather rapidly

under the accelerated test. The water resist- ance is the property that changes to the greatest extent in the boards which do not resist aging well, and tliis property may prove to be the most sensitive measure of the effects of aging.

Washington, June 14, 1938.

[7]

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