United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL-RP-545

Fire Performance of Wood

Treated With Combined

Fire-Retardant and

Preservative Systems

Mitchell S. Sweet

Susan L. LeVan

Robert H. White

Hao C. Tran

Rodney De Groot
Abstract

The availability of western redcedar has decreased in recent

years, and other species of wood are being considered as

substitute materials for wood shakes and shingles. However,

the wood of these alternative species is more susceptible to

decay than is western redcedar. The objective of this study

was to evaluate the fire performance of combined fireretardant and preservative treatments using different fire test

methods. Several amino resin fire retardants were used in

combination with several wood preservative compounds that

imparted both fire retardancy and decay resistance to wood.

Treated specimens underwent fire tube tests and based on the

results of these tests, two fire retardants and two quaternary

ammonium preservatives were selected for additional fire

testing. These treated materials were subjected to a modified

Schlyter test and a burning brand test. The heat release rate

was also measured. Both weathered and unweathered specimens were evaluated. The unweathered and weathered treated

material exhibited good fire performance.

Keywords: Preservative, fire retardant, shingles, Pacific

silver fir, western hemlock, western redcedar

March 1996

Sweet, Mitchell S.; LeVan, Susan L.; White, Robert H.; Tran, Hao C.;

De Groot, Rodney. 1996. Fire performance of wood treated with combined

fire-retardant and preservative systems. Res. Pap. FPL–RP– 545. Madison,

WI: U.S. Department of Agriculture, Forest Service, Forest Products

Laboratory. 10 p.

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Contents

Page

Introduction................................................................1

Experimental Methods..................................................2

Materials ................................................................2

Procedures...............................................................2

Results......................................................................5

Fire Tube Tests........................................................5

Modified Schlyter Tests.............................................5

Modified Class C Burning Brand Tests.........................8

Heat Release Rate.....................................................8

Discussion .................................................................8

Conclusions .............................................................10

Acknowledgments......................................................10

Literature Cited .........................................................10
Fire Performance of Wood

Treated With Combined

Fire-Retardant and

Preservative Systems

Mitchell S. Sweet, Chemist

Susan L. LeVan, Assistant Director

Robert H. White, Supervisory Wood Scientist

Forest Products Laboratory, Madison, Wisconsin

Hao C. Tran, Staff Specialist

Forest Products and Harvesting Research

USDA Forest Service, Washington, DC

Rodney De Groot, Research Plant Pathologist

Forest Products Laboratory, Madison, Wisconsin

Introduction

The availability of western redcedar (Thuja plicata) logs has

decreased in recent years and as a result, other species, such as

western hemlock (Tsuga heterophylla) and Pacific silver fir

(Abies amabilis), are being considered as substitute materials

for wood shakes and shingles. The wood of these two alternative species is more susceptible to decay than is western

redcedar. Also, to better fill the need for wood shakes and

shingles, treatment with a fire retardant would enable additional and widespread use. Therefore, these alternative species

require a treatment that imparts both fire retardancy and decay

resistance.

Some conventional fire retardants, which are mainly waterborne inorganic salts, provide a certain degree of decay resistance. These chemicals are not suitable for exterior use because they leach out of the wood. A simple one-step process

that gives both resistance to fire and microbial decay and may

be used for exterior applications is currently not available for

the treatment of wood products. This study evaluated various

wood preservative and fire-retardant mixtures that could be

used as a combined treatment for wood products. Related

investigations on improving the decay resistance alone can be

found in De Groot (1994a,b).

A preliminary investigation to determine which combinations of fire retardants and preservatives are compatible was

conducted by placing a quantity of fire-retardant solution (4 to

10 g) in a 35-mL vial. To each vial was added sufficient

preservative to yield a solution with concentration of 1×, 2×,

and 5× the commercial application levels. The vials were

agitated, and the solutions that did not exhibit phase separation or precipitation were judged to be compatible.

The following fire retardants were included:1

• UDPF (urea, dicyandiamide, phosphoric acid and

formaldehyde)

• MDPF (melamine, dicyandiamide, phosphoric acid

and formaldehyde)

• Irotherm 909–200 (commercial version of UDPF)

• Irotherm 909–300 (commercial version of MDPF)

• Fyrol-6 (diethyl N,N-bis (2-hydroxyethyl) aminomethyl

phosphonate)

• Fyrol-51 (proprietary oligomeric phosphorus esters)

• Dricon (guanylurea phosphate and boric acid)

• NCX (undisclosed proprietary formulation)

• DPF (dicyandiamide, phosphoric acid and formaldehyde)

• DP (dicyandiamide and phosphoric acid)

• Amgard TR (ammonium polyphosphate)

• Retardol S (THPS)

(tetrakis(hydroxymethyl)phosphonium sulfate)

1

The use of trade or firm names in this publication is for

reader information and does not imply endorsement by the

U.S. Department of Agriculture of any product or service.
2

The following commercial-use wood preservatives were also

included:

• CCA (chromated copper arsenate)

• ACQ (ammoniacal copper quaternary)

• DDAC (didecyl dimethyl ammonium chloride)

• NP-1 (DDAC with 3-iodo-2-propynyl butyl carbamate)

• ACA (ammoniacal copper arsenate)

• ACZA (ammoniacal copper zinc arsenate)

• ACB (ammoniacal copper borate)

• CuOct-macro (copper octoate, macro emulsion)

• CuOct-micro (copper octoate, micro emulsion)

• CuNaph-macro (copper naphthenate, macro emulsion)

• CuNaph-micro (copper naphthenate, micro emulsion)

• CuNaph-wr (copper naphthenate, with water repellent)

• CuNaph-wb (copper naphthenate, waterborne emulsion)

• ZnNaph-wr (zinc naphthenate, with water repellent)

• ZnNaph-wb (zinc naphthenate, waterborne emulsion)

• CuNaph-ob (copper naphthenate, oilborne)

• ZnNaph-wb (zinc naphthenate, aqueous solution)

• CuZnNaph-wr (copper zinc naphthenate, with water

repellent)

• CuNapSol (copper naphthenate, with solubilizing agent)

Based on the results of the compatibility tests and because

a leach-resistant exterior use fire retardant was desired, three

fire retardants were chosen for further evaluation: DP, DPF,

and UDPF. Six preservatives were selected for combination

with each of the three fire retardants: CCA, NP-1, ACQ,

ZnNaph-wb, ACA, and DDAC. Initially, some combinations

(in particular, UDPF with DDAC or NP-1) were not compatible at higher fire-retardant concentration levels. However,

it was discovered that the addition of isopropanol to the

UDPF/DDAC or UDPF/NP-1 mixtures would act as a cosolvent and solubilize the preservative. A treating solution

containing 30 percent UDPF required approximately

10-percent isopropanol to bring about a homogeneous

solution.

The UDPF fire retardant was developed by researchers at the

Eastern Forest Products Laboratory (EFPL) in Canada (Juneja

1972). Additional work was performed by EFPL scientists to

better understand the UDPF system (Fung and others 1973,

1976; Juneja and Richardson 1974).

This paper discusses the fire tests conducted on selected

combinations of fire retardants and preservatives.

Experimental Methods

This study consisted of two stages. The first part consisted of

selecting several compatible combinations from the first

stage and testing fire tube specimens. The fire tube test

served to give a rough idea of the fire performance of each

combination. The second stage of the study involved treating

test decks with a limited number of combinations of fire

retardants and preservatives. These decks were evaluated for

fire performance by several fire test methods.

Materials

To ensure a uniform and consistent treatment, southern

yellow pine was used for the fire tube specimens, which were

1.02 m long by 19 mm wide by 9.5 mm thick (40 by 0.75

by 0.37 in.). Western hemlock and Pacific silver fir shakes

were used for the modified Schlyter and the burning brand

tests. The shakes were quartersawn with no knots, slash

grain, or other visible defects. The dimensions of the shakes

were 10 mm (0.375 in.) at butt, 460 mm (18 in.) long, and

100 to 200 mm (4 to 8 in.) wide, with a 140-mm (5.5-in.)

exposure when assembled in a deck. For the Ohio State

University (OSU) heat release rate test, Pacific silver fir

shakes were cut to dimensions of 150 by 150 by 13 mm

(6 by 6 by 0.5 in.).

Procedures

Treatments

All treated wood specimens were pressure impregnated with

mixtures of a fire retardant and a preservative using a full-cell

pressure process. A vacuum of 0.1 MPa (15 lb/in2) was

pulled for 30 min, the treating solution was added, and

pressure of 1.0 MPa (150 lb/in2) was applied for 2 h. The

pressure was then released, the treated specimens were removed from the tank, and the excess treating solution was

blotted from the specimens. The dry chemical retention was

calculated from the weight of treating solution each specimen

had absorbed during the treating process by weighing specimens before and after treatment.

Drying

The southern yellow pine fire tube sticks were dried after

treatment in an oven at 65°C (150°F) for 5 days. The specimens were then equilibrated to a constant moisture content at

23°C (73°F), 50-percent relative humidity.

The shakes were kiln dried after treatment. The kiln schedule

used a dry bulb of 49°C (120°F) and a wet bulb of 45°C

(113°F) for 6 days. This equates to a relative humidity of

80 percent or an equilibrium moisture content of 14 percent.
3

After that, both the dry- and wet-bulb temperatures were

increased by 6°C (10°F) on each of the following days until

the dry-bulb temperature reached 82°C (180°F). The shakes

were allowed to cure at 82°C (180°F) for 48 h and then

equilibrated to a constant moisture content at 27°C (80°F) and

30-percent relative humidity.

The OSU heat release rate specimens were oven dried at 49°C

(120°F) for 2 days, then at 71°C (160°F) for 2 days. These

specimens were then cured at 82°C (180°F) for 2 days.

Fire Tube Test

The ASTM Test Method E 69–80 (ASTM 1980) apparatus

was used to measure percentage weight loss caused by combustion. Specimens were exposed to the flame for 3 min.

Each treated specimen was placed in the fire tube (Fig. 1),

and the flame from a gas burner was placed directly beneath

the vertically oriented specimen. Weight loss was recorded for

an additional 7 min after the burner was removed. The percentage weight loss at the end of the 10 min was reported.

Modified Schlyter Test

The modified Schlyter test was used to measure the vertical

flamespread of the hemlock and fir shakes. Two decks were

constructed out of the treated and untreated material following

the procedure in LeVan and Holmes (1986). Two matched

decks were held parallel in a vertical position, with the test

surfaces facing each other 50-mm (2-in.) apart (Fig. 2). The

bottom of one deck was supported 100 mm (4 in.) higher

than the bottom of the other. Behind the testing rack was a

ruler for recording flame height. The arrangement of the decks

promoted combustion, because each panel radiated heat to the

other. At the start of the test, the burner was placed between

the decks and the gas ignited. The initial height of the flame

was recorded immediately and at every 15 s thereafter for

10 min. At the end of 5 min of exposure, the gas flame was

shut off. The maximum flame height was reported.

Stabilizing

ring

Ch.

Specimen

V

R

S

T

Figure 1—Fire tube test apparatus.

Figure 2—Modified Schylter test apparatus.
4

Modified Class C Burning Brand Test

A modified version of the ASTM Test Method E 108–91a

(ASTM 1991a) for the Class C burning brand test of roof

coverings was used. This test measured the resistance of the

treated shakes to fire penetration and the capability of the fireretardant/preservative system to control glowing combustion.

A roof-section assembly was constructed, measuring 0.30 m

(12 in.) wide by 0.79 m (31 in.) long. After conditioning,

the assembly was positioned in the deck with a large fan

placed 1.5 m (5 ft) from the front edge of the deck in such a

manner as to generate a 5.4 m/s (12 mph) wind directed over

the surface of the specimen (Fig. 3). A Class C brand was

ignited and placed over the joint between two shakes in the

same course and just below the butt end of the shake in the

course above. The test was continued until the brand was

consumed and all evidence of flame, glowing, and smoke

disappeared or failure occurred. A failure was defined as burn

through to the underside of the deck. In cases where the first

brand did not result in a failure, a second brand was placed in

the same location as the first successful test. This procedure

was repeated for seven additional brands placed at other joints

as specified in ASTM E 108. Results were reported as the

number of failures out of the number of first tests, then the

number of failures out of the number of second tests.

Heat Release Rate Test

An Ohio State University (OSU) heat release rate calorimeter

(Fig. 4) was used to determine the fire performance of leached

specimens relative to that of unleached specimens. A large

heat release rate generally means a rapid flame spread. ASTM

E 906–83 (1983) was followed except that the oxygen consumption method, rather than the sensible heat method, was

used to measure the heat release rate (Tran 1990). The vertically mounted test specimens were exposed to a radiant heat

flux of 40 kW/m2

. Results were reported as the 5-min average heat release rate for each specimen.

Leaching

For the modified Schlyter test and the Class C burning brand

tests, the leachability of each treatment was determined by

method B of ASTM D 2898-81 (1981). After drying and

conditioning at 27°C (80°F) and 30-percent relative humidity,

test decks were positioned in the accelerated weathering

chamber as described in the test method. The decks were then

subjected to a 24-h exposure cycle consisting of 4 h each of

wetting, drying, wetting, drying, and 8 h of rest. This cycle

was repeated for 6 weeks for a total of 1,000 h. The water

was applied in a fine spray uniformly distributed over the

exposed specimen surface, with the temperature remaining

below 32°C (90°F) during the wetting cycle. During the

drying cycle, air at 65°C (150°F) was obtained by using

ultraviolet lamps and circulating air at a temperature necessary to maintain a constant temperature of 65°C. After

completing the accelerated weathering, the specimens were

equilibrated at 23°C (73°F) and 50-percent relative humidity.

The OSU heat release rate specimens were leached by placing

half the specimens in containers and covering them with

distilled water for 13 days. The other half of the specimens

were unleached. The water was replaced 6, 30, 78, 126, 174,

222, 270, and 318 h after initiation of the leaching cycle.

Upon completion of the leaching, the specimens were dried at

Figure 3—Burning brand test apparatus.

Filter

(glasswool)

Coldtrap

(ice)

Drier

(drierite)

Pump

O2 analyzer

Air in a

CH4

•

3/4 a•

1/4 a•

Pilot 1

Pilot 2

Sample

plus

holder Injector

Annulus

Thermopile

Probe

Baffle

 Figure 4—OSU heat release rate apparatus.
5

49°C (120°F) for 2 days, then at 60°C (140°F) for 2 days.

Finally, both the leached and unleached specimens were

equilibrated at 23°C (73°F) and 50-percent relative humidity.

Results

Fire Tube Tests

Fifteen of the 18 possible combinations of fire retardants and

preservatives were evaluated for fire performance using the

fire tube test. The mean weight loss for each combination,

number of replicates evaluated in each group, and coefficient

of variation (COV) are found in Table 1. The distributions of

individual and mean values for each treatment are found in

Figure 5. A statistical analysis shows little difference between treatments. One group of treatments (DP/ACA,

DP/ACQ, UDPF/DDAC, and UDPF/ACQ) had slightly less

fire-retardant performance, and a second group, comprising all

the other treatments, had somewhat greater fire-retardant

performance with no statistical difference among those

11 treatments (Table 1). Essentially, no particular treatment

stood out as better than the rest.

When each fire retardant was examined, independent of which

preservative it was combined with, subtle differences in fire

performance emerged. Each of the three groups contained

an equivalent level of fire retardant (235 ± 2 kg/m3 (14.7 ±

0.1 lb/ft3)). DPF-1 gave the best fire performance, indicated

by the least weight loss (Fig. 6). The DP retardant had the

next best fire performance, followed by UDPF. Although the

differences between groups are statistically significant, from a

practical standpoint, the differences are small enough to be

negligible (Table 2).

Taking a similar approach in examining the fire performance

of each preservative treatment, independent of the type of fire

retardant used, subtle differences were also found. The fire

performance of ACQ and DDAC were equivalent. Likewise,

the fire performance of DDAC and ACA was comparable to

each other. The four preservatives that exhibited the best fire

performance and were not significantly different from each

other were ACA, NP-1, CCA, and ZnNaph (Fig. 7 and

Table 3).

Based on the results of the fire tube tests alone, we can conclude that a combination of DPF fire retardant and ACA, or

NP-1, or CCA, or ZnNaph would likely give the slightly

better fire performance per given treatment level. Although

these combinations had somewhat lower weight losses, all

combinations had satisfactory results and for all intents and

purposes were the same. Because of other concerns, such as

decay resistance, UDPF was chosen to be combined with

DDAC (De Groot and others 1992, LeVan and De Groot

1993).

Modified Schlyter Tests

Four test decks of western hemlock and Pacific silver fir were

treated with each combination of fire retardant and preservative level. Two decks were weathered and two were not.

Results of the modified Schlyter test (Table 4) generally

indicated an increasing level of fire performance with increasing level of fire-retardant retention. Chemical loading of

fire retardant greater than105 kg/m3 (6.6 lb/ft3) had little or

no effect on the fire performance of both the weathered and

unweathered specimens. The retention level of preservative

did not appear to affect the fire performance of the decks when

used in combination with the fire retardant.

Table 1—Honest significant difference (HSD) between mean percentage weight loss by treatmenta

Fire retardant DPF DPF DPF DPF DP DP DPF UDPF UDPF DPF DP DP DP UDPF UDPF

Preservative ACA CCA ZnNaph DDAC CCA ZnNaph NP-1 ACA CCA ACQ DDAC ACA ACQ DDAC ACQ

Mean weight

 loss

11.71 11.88 12.05 12.45 12.83 12.83 12.85 13.00 13.03 14.03 14.09 15.44 16.47 16.75 17.26

Samples/

 group

(15) (45) (15) (45) (46) (9) (45) (15) (29) (15) (29) (13) (5) (45) (15)

COV (%) 13.4 16.8 18.2 21.7 20.8 19.3 25.5 8.8 14.2 19.4 18.5 19.1 8.9 8.0 8.9

a

Horizontal lines signify treatment groups that showed no statistical difference.
6

30

20

10

0

Weight loss (%)

DP

ACA

DP

CCA

DP

ACQ

Means with confidence interval

DP

DDAC

DPF

ACA

DP

ZnNaph

DPF

ACQ

DPF

DDAC

DPF

CCA

Mixture

DPF

ZnNaph

DPF

NP-1

UDPF

ACQ

UDPF

CCA

UDPF

DDAC

UDPF

ACA

All pairs

Tukey-Kramer

0.05 confidence

Figure 5—Distribution of percentage weight loss by fire-retardant/preservative mixture.

30

20

10

0

Weight loss (%)

Means with 

confidence 

interval

DPF

Fire retardant

UDPF All pairs

Tukey-Kramer

0.05 confidence

DP

Figure 6—Distribution of percentage weight loss by

fire retardant only.

Table 2—Honest significant difference between mean

percentage weight loss from fire retardant only

Fire retardant DPF DP UDPF

Mean weight loss (%) 12.44 13.70 15.24

Samples/group 180 102 104

COV (%) 21.1 20.5 15.9
7

Table 3—Honest significant difference between mean percentage weight loss

from preservative only

Preservative ZnNaph CCA NP-1 ACA DDAC ACQ

Mean weight loss (%) 12.34 12.52 12.85 13.29 14.48 15.76

Samples/group 24 120 45 43 119 35

COV (%) 18.5 19.9 25.5 18.6 18.9 16.4

Table 4—Results of modified Schlyter tests

Retention level Retention level Hemlock (m (in.)) Pacific silver fir (m(in.))

of UDPF

(kg/m3

 (lb/ft3))

of DDAC

(kg/m3 (lb/ft3))

Unweathered WeatheredUnweathered Weathered

2.25 (88)

 5 (0.3) 2.15 (84) 2.05 (80) 2.15 (84) 1.90 (74)

150 (9.4) 0.65 (26) 1.05 (42) 0.75 (30) 1.20 (48)

 5 (9.4) 0.80 (32) 0.80 (32) 0.85 (34) 0.75 (30)

10 (9.4) 0.80 (32) 0.80 (32) 0.90 (36) 0.80 (32)

105 (6.6) 0.80 (32) 1.15 (46) 0.75 (30) 1.30 (52)

 5 (9.4) 0.95 (38) 0.90 (36) 0.70 (28) 0.80 (32)

10 (9.4) 1.00 (40) 1.20 (48) 0.90 (36) 0.95 (38)

45 (2.8) 1.20 (48) 1.40 (56) 1.00 (40) 1.35 (54)

 5 (0.3) 1.10 (44) 1.70 (66) 0.95 (38) 1.75 (68)

10 (0.6) 1.20 (48) 1.65 (64) 1.10 (44) 1.85 (72)

30

20

10

0

Weight loss (%)

ACA

Means with 

confidence 

interval

CCA DDAC

Preservative

ZnNaph

NP-1 All pairs

Tukey-Kramer

0.05 confidence

ACQ

Figure 7—Distribution of percentage weight loss by

preservative only.
8

In this modified Schlyter test, weathering did not have an

observable effect on fire performance at the two higher fireretardant levels (105 and 150 kg/m3

 (6.6 and 9.4 lb/ft3)), but

a decrease in fire performance was observed for the lowest

level (45 kg/m3 (2.8 lb/ft3)). This effect was more pronounced for the Pacific silver fir than for the western

hemlock.

Modified Class C Burning Brand Tests

Because the results of the modified Schylter test looked

promising with UDPF as the fire retardant, we decided that a

closely related fire retardant, MDPF, would also be used to

treat test decks for the modified Class C burning brand test.

Two test decks of western hemlock and Pacific silver fir were

treated at each combination of fire retardant and preservative

level. One deck was weathered and the other was not. The

results of the Class C burning brand test (Table 5) show that

the untreated test decks and the decks treated only with a

preservative suffered many failures. Again, the decks treated

at the higher fire-retardant levels (105 and 150 kg/m3 (6.6 and

9.4 lb/ft3)) did not exhibit failure, nor did the unweathered

decks treated at the lowest fire-retardant level (45 kg/m3

(2.8 lb/ft3 )). The only failures of treated decks occurred on

the weathered decks with the lowest fire-retardant level. These

results confirm that a loading level of 100 to 150 kg/m3 of

fire retardant would likely lessen the negative effect of leaching on fire performance.

Heat Release Rate

The DDAC treatment had a minimal effect on the results of

the burning brand tests; therefore, a similar preservative,

NP-1, was also used for the heat release rate tests. The heat

release rate of the leached and unleached specimens was

measured, and the results are shown in Figures 8 and 9. The

three zones (I, II, III) represent heat release rate values that

seem to correspond with ASTM E 84 flame spread values

that fall into the Class I, II, or III classification of the building codes (ASTM 1991b; LeVan and Tran 1990). The heat

release rate of the leached specimens, for both the UDPF and

MDPF treated material, was greater than for the unleached

specimens. Unleached specimens, at retentions of 7 lb/ft3 and

greater, exhibited heat release rates less than 40 kW/m2. This

level of fire performance could also be achieved with the

leached specimens, although a fire-retardant retention of

9 lb/ft3 or greater is required.

Discussion

The ASTM E 108 standard is typically cited by building

officials as the test method for evaluating the fire performance of roof coverings, including wood shakes and shingles.

Within this test method, there are actually five separate tests

(intermittent flame exposure test, spread of flame test, burning brand test, flying brand test, and rain test) with three

levels of classification within each test. These classification

levels are A, B, and C, with A being the most severe testing

and C the least severe. The burning brand tests performed at

the USDA Forest Service, Forest Products Laboratory (FPL),

were not meant to exactly follow the methodology of ASTM

E 108, but were done with the equipment available at FPL to

get an approximation of how treated material would perform

in a similar test. It is believed that based on the results of our

testing, the combined fire-retardant and preservative treatments would likely pass a full-scale ASTM E 108 test series.

Similarly, the heat release rate tests performed as part of this

study are not mandated by the building codes, but help provide an understanding of the level of fire retardancy in the

treated shakes. For building materials that have exposed

surfaces, the surface flame spread can be quantified byASTM

E 84. The numeric results of this test method are also broken

down into classifications I, II, and III, which are not in any

way related to the classifications of ASTM E 108. The

Class I rating derived from ASTM E 84 is the strictest

classification and usually only noncombustible material and

wood products that have been well treated with a fire retardant

are able to attain a Class I rating. Researchers have found a

correlation between the results of the ASTM E 84 test and

the rate of heat release. By measuring the heat release rate, we

can approximate the surface flame spread of the treated material. Material that has a heat release rate of less than 40

kW/m2 corresponds to a Class I flame spread rating and

indicates that the material is well treated with a fire retardant.

The fixation of UDPF was explored by EFPL scientists in

the mid-1970s to determine the necessary curing time and

temperature (Juneja and Calve 1977). The UDPF treated

wood was cured for 3 to 6 h at 110°C to 150°C. The researchers found that there was some loss of fire retardant

through leaching and the problem of strength loss was also a

concern.

The weathering of the test decks for burning brand and modified Schlyter tests and the leaching of the heat release rate

specimens show that some fire retardancy is lost due to

leaching. What is unknown is the extent of the leaching and

why the loss of fire retardant occurs. It may be that a fixed

amount or percentage of fire retardant never cures into the

wood structure and can be easily washed back out. If that is

the case, then it would be possible to simply treat the wood

with enough fire retardant to compensate for the loss as a

result of leaching. It is also possible that there will be a

constant loss of fire retardant during the exposure of the

wood, and fire retardancy will continue to diminish. We do

not know which scenario is correct. This area of weathering

of fire-retardant-treated wood, both natural and artificial,

clearly needs further investigation.
9

Table 5—Results of Class C burning brand testsa

Retention Hemlock Pacific silver fir

Fire

retardant

Retention

(kg/m3

 (lb/ft3))

DDAC

(kg/m3 (lb/ft3))

Unweathered WeatheredUnweathered Weathered

8/8 6/8 – 2/2 6/8 – 1/1 7/8 – 1/1

 5 (0.3) 7/8 5/8 – 2/2 4/8 5/7 – 2/2

UDPF 150 (9.4) No failure No failure No failure No failure

 5 (0.3) No failure No failure No failure No failure

10 (0.6) No failure No failure No failure No failure

UDPF 105 (6.6) No failure No failure No failure No failure

 5 (0.3) No failure No failure No failure No failure

10 (0.6) No failure No failure No failure No failure

UDPF 45 (2.8) No failure No failure No failure No failure

 5 (0.3) No failure 0/8 – 2/8 No failure 1/8 – 2/7

10 (0.6) No failure No failure No failure 0/8 – 3/8

MDPF 150 (9.4)) No failure No failure No failure No failure

 5 (0.3) No failure No failure No failure No failure

10 (0.6) No failure No failure No failure No failure

MDPF 150 (6.6) No failure No failure No failure No failure

 5 (0.3) No failure No failure No failure No failure

10 (0.6) No failure No failure No failure No failure

MDPF 45 (2.8) 5 (0.3) No failure No failure No failure No failure

10 (0.6) No failure No failure No failure No failure

a

 Results are listed in the format of failures/trials. Passes on the first test were given a second test,

 which is listed after the first. For example, 1/8 – 2/7 means one failure out of eight brands on the first

 test, and two failures out of seven possible brands on the second test. No failure means no failure

 out of eight brands on the first test, and no failure out of eight possible brands on the second test.

125

100

75

50

25

5 min average heat release rate (kW/m2)

0 2 4 6 8 10 12 14

Average fire retardant retention (lb/ft3)

UDPF/DDAC unleached

UDPF/DDAC leached

UDPF/NP-1 unleached

UDPF/NP-1 leached

III

II

I

Figure 8—Heat release rate of leached and

unleached UDPF-treated southern yellow pine.

1 lb/ft3

 = 16 kg/m3.

125

100

75

50

25

5 min average heat release rate (kW/m2)

0 2 4 6 8 10 12 14

Average fire retardant retention (lb/ft3)

MDPF/DDAC unleached

MDPF/DDAC leached

MDPF/NP-1 unleached

MDPF/NP-1 leached

III

II

I

Figure 9—Heat release rate of leached and

unleached MDPF-treated southern yellow pine.
10

Conclusions

In this study, the fire tube tests showed that the fire performance of all the fire-retardant and preservative combinations

used was fairly similar, allowing us to choose which combinations held the most promise, specifically UDPF and

DDAC. Further testing showed that the closely related

MDPF and NP-1 could also be successfully used to provide

fire retardancy to wood. Material treated with each of these

fire retardant/preservative combinations was subjected to

accelerated weathering procedures, and fire tests indicated that

good fire performance could be achieved with weathered and

unweathered specimens.

Acknowledgments

We are grateful to the following individuals for their assistance in our study: Como Caldwell, Jethro Clay, Anne

Fuller, Dave Gutzmer, Ron Knispel, Tedd Mianowski, Carlos Miro, Nicole Stark, and Bessie Woodward.

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