The effect of inorganic salts on the flame-spread and smoke-producing characteristics of wood

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The effect of inorganic salts on the flame-spread and smoke-producing

characteristics of wood

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T H E EFFECT O F INORGANIC SALTS ON THE F L A M E SPREAD

AND SMOKE PRODUCING CHARACTERISTICS

OF

by

K. Surni2F, Y. Tsuchiya* and D . P . C. Funge*

The Division of Building Research, National Research Council, in cooperation with the Eastern F o r e s t P r o d u c t s Laboratory, has

undertaken studies an the effect of inorganic s a l t s on the combustion

and decomposition of wood and cellulose. The effect of additives on the pyralysis products a n d flammability of cellulose has been r e p o r t e d in previous papers. (1, 23

The purpose of the study r e p o r t e d in this paper w a s to s t u d y the effect of r e t a r d a n t s on the flame-spread and s m oke-producing

characteristics af Douglas fir ~ l y w o o d and W e s t e r n r e d c e d a r

particleboard. The ASTM E84 t e s t method (31 w a s used to determine the surface s p r e a d of f l a m e and the smoke d e n s i t y classifications.

The effectiveness of flame r e t a r d a n t s w a s evaluated by

conducting flammability t e s t s on cellulose samples t r e a t e d with eleven

different inorganic compounds. The Oxygen Index Test, developed

by Fenimore and Martin (41, w a s used f o r this purpose.

In

test, the flow of oxygen and nitrogen w a s metered, and the mixed gas

was allowed to flow up a _chamber in which a thin specimen measuring 2 in. _{by 6 in.} w a s positioned vertically. The top of the specimen w a s

then ignited by a g a s flame. The minimum oxygen concentration in

a slowly r i s i n g gaseous a t m o s p h e r e that w i l l s u s t a i n the downward

burning of the specimen w a s determined. The t e s t results a r e expressed

a s "oxygen indext1, where

LO,

J

In

of oxygen index.

*

**

The results of these experiments are given

in

I.

one per cent concentration l e v e l of additives, the m o s t effective

treatments were phosphoric acid and pot a s s ium carbonate, followed

by monoammonium phosphate, zinc chloride and borax. The relative

effectiveness of the a d d i t i v e s on the flammability of cellulose depends on the concentration level. F o r example, subs equent experiments

showed that monaamrntlniurn phosphate is more effective than potassium carbonate at higher concentration levels.

Flame-Spread Rating

O n e of the main uses of flame retardants in the construction i n d u s t r y is L o d e c r e a s e the surface burning rate of wood p r o d u c t s , The importance of this p r o p e r t y is w i d e l y recognized by building

authorities and restrictions in the- u s e of m a t e r i a l s baaed on t e s t s to evaluate this characteristic a r e included in m o s t building codes. F o r example, the National Building Code of Canada ( 5 ) has require

-

ments f o r materials based on ASTM Standard E84.

In

25-ft tunnel, using asbestos-ceYnent board and red oak as r e f e r e n c e s t a n d a r d s . The d e n s i t y of smoke developed from the specimen c a n

a l s o be evaluated by this t e s t .

Two of the flame r e t a r d a n t s that. w e r e effective by t h e oxygen i n d e x t e s t w e r e examined on a l a r g e r scale using the E84 t e s t . The effect af monoammonium phosphate and phosphoric acid treatments

on cornrnerciab Western r e d cedar particleboard and Douglas fir plywood w a s investigated

(6).

E84

A

$

at a loading of 20 Ib per 1000 square feet gave a flame-spread r a t i n g

of 7 0 ; untreated particleboard gave a rating of 169. A s i m i l a r particle- board t r e a t e d with equal amounts of monoammonium phosphate and

phosphoric acid at a loading of 40 1b per

I O Q O

phosphate and phosphoric acid w a s applied t o *-in. Douglas f i r

plywood, the flame-spread index w a s r e d u c e d from 102 t s 59. Thus,

t w o of the retardants t h a t w e r e effective in the oxygen index t e s t had

a significant effect on the flame-spread index as obtained by the

large -scale tunnel test. The particleboard results show that the

the flame-spread i n d e x already obtained by the use of monoammonium phosphate, A plausible explanation for this is that the monoammonium phosphate loading was either near the optimum or in excess of t h e

optimum required for lowering the surface flame-spread index of particleboard, and further i n c r e a s e in f l a m e retardants had very l i t t l e

effect.

The potential use of surface finish materials can be i n c r e a s e d

by lowering the flame-spread rating. According t o the National Building Code of Canada 1970, the maximum allowable flame-spr ead

rating for w a l l and ceiling finishing for combustible construction is

150. For '%assembly" occupancies ( e . g. theatres, schools, churches) and ''institutionaltT occupancies [ e . g. hospitals, prisons, old peoplet s

homes) the r equirernent i s m o r e stringent, Some ass ernbly buildings reqnire a flame-spread rating of 25 or less, and still others require a rating of 75 or less. Institutional occupancies of combustible

construction a l s o require a flam e-spread r a t i n g of 7 5 or l e s s . For noncombustible construction, surface finish materials with a flame-

s p r e a d rating of 2 5 or less a r e required.

The untreated particleboard that w a s tested would not have been acceptable according to the National Building Code because its flame-spread rating w a s g r e a t e r than 150. The t r e a t e d p a r t i c l e b o a r d would have been acceptable, with c e r t a i n exceptions. The potential u s e of Douglas fir plywood has a l s o been increased by the! use of a

r e t a r d a n t because the flame-spread index of the treated board w a s

d e c r e a s e d to a value below 75.

The presence of flame r e t a r d a n t s on wood products m a y

result in increased smoke production when the t r e a t e d m a t e r i a l s are involved in fire. Since wood produces m o r e smoke under s m o u l d e r i n g condition than under flaming condition, the ts eatrnent with inorganic flame retardants m a y increase smoke f r o m wood p r o d u c t s by altering

t h e mechanism of combustion, especially in the development s t a g e of

a fire, f r o m f l a m i n g to smouldering.

In

by the ASTM E84 t e s t ( p r e s e n t e d in Table 11) w e r e examined. The smoke density classification of untreated particleboard was 147; for those treated w i t h inorganic flame retardants, it w a s

221

253. The classification of u n t r e a t e d plywood was 58 and that of the t r e a t e d plywood w a s 290.

h

The potential hazard of s m o k e at building fires is receiving increasing attention in building regulations. F o r example, new regulations concerning m a t e r i a l s that are l i k e l y to produce a great

d e a l of smoke in a f i r e have been introduced in the 1970 edition of

t h e National Building Gode of Canada. The n e w requirements on smoke-producing potential apply to materials us ed in _{tall buildings.}

The maximum allowable smoke density rating as obtained by ASTM

E84 t e s t is 25 for ceiling surface and 300 for wall and floor surfaces.

The requirement f o r exit and other c ~ i t i c a l areas i s m o r e restrictive. Although the treatment of Douglas f i r plywood and W e s t e r n r e d cedar p a r t i c l e b o a r d with inorganic flame r e t a r d a n t s i n c r e a s e d

the smoke density classification of these materials, aU the ratings obtained w e r e l e s s than 300. The new smoke density regulations of the National Building Code, therefore, have relatively little

effect in limiting the use of these materials in buildings. Conclusion

Monoammonium phosphate, which w a s found to be effective

in a s c r e e n i n g test f o r flame r e t a r d a n t s , w a s also effective in

lowering the surface flame-spread index of wood pr aducts. Although

flame-retardant treatments for wood can reduce the rate of flame

s p r e a d , they may i n c r e a s e smoke production f r o m the burning of these

materials. U n t i l recently, the re w e r e no limitations on the p e r m i s s i b l e

u s e of materials b a s e d o n s m o k e - ~ r o d u c i n g p r o p e r t y but regulations

b a s e d on this property a r e increasingly used in building codes.

The results of ASTM E84 t e s t s indicate that the presence of inorganic f l a m e retardants increases smoke production f r o m wood

products during the development s t a g e of a f i r e by altering t h e m e c h a n i s m of combustion f r o m that which i s p r e d o m i n a n t l y f l a m i n g to one that is pr edorninantly smouldering.

Refer

enc

1.

Y.

-

2. D, P,

C.

To

3 , Standard Method of T e s t f o r Surface Burning Characteristics

of Building M a t e r i a l s , ASTM E84-68.

4.

C.

J.

-

1966).

5. National Building C o d e of Canada 1970. National Research

Council of Canada, 463 pp.

Table 1

Plarnm ability ( I t Oxygen Index'') of Cellulose

Treated with

17,

1

Treatment Oxygen Index

Untreated c e l l u l o s e

Calcium c h l o r i d e P o t a s s i u m c h l o r i d e

P a t a s sium phosphate, monobasic

P o t a s s i u m phosphate, dibasic P o t a s s i u m bicarbonate P o t a s s i u m b r o m i d e Borax 8 Zinc chloride Manoammonium phosphate Potassium carbonate Phosphoric acid 0.173 0. 178 0. 782 0 . 1 8 5

0.186

Table 2 R e s u l t s of A.S. T . M . Tunnel T e s t Material P a r t i c l e b o a r d [untreated) P a r t i c l e b o a r d C monoammonium phosphate P a r t i c l e b o a r d

+

phosphate t phosphoric acid

Plywood [untreated) P l y w o o d -t- monoammonium phosphate

+