United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL-RP-498

Effects of Fire Retardant

Chemicals on the Bending

Properties of Wood

at Elevated Temperatures

Susan L. LeVan

Robert J. Ross

Jerrold E. Winandy
Abstract

Fire retardant chemicals are designed to lower the

temperature at which thermal degradation occurs.

When these chemicals are used for roof sheathing,

the combination of chemicals, elevated temperatures,

and moisture can sometimes prematurely activate

the fire retardant mechanism. The objectives of our

study were to determine what kind of fire retardant

chemicals are most susceptible to accelerating thermal

degradation and at what temperature or temperatures

this acceleration occurs.

Small, clear Southern Pine specimens were treated with

six different fire retardant chemicals and exposed to

three environments for up to 160 days. The exposure

conditions were (1) 80°F (27°C), 30 percent relative

humidity, (2) 130°F (54°C), 73 percent relative

humidity, and (3) 180°F (82°C), 50 percent relative

humidity. Static bending tests were performed on the

treated specimens and untreated controls to determine

the effect of treatment and exposure on modulus of

Keywords: Fire retardant treatments, wood degrade,

thermal degradation, reduced strength properties

September 1990

LeVan, Susan L.; Ross, Robert J.; Winandy, Jerrold E.

1990. Effects of fire retardant chemicals on bending

properties of wood at elevated temperatures. Res.

Pap. FPL-RP-498. Madison, WI: U.S. Department of

Agriculture, Forest Service, Forest Products Laboratory.

24 p.

A limited number of free copies of this publication

are available to the public from the Forest Products

Laboratory, One Gifford Pinchot Drive, Madison, WI

53705-2398. Laboratory publications are sent to more than

1,000 libraries in the United States and elsewhere.

rupture, modulus of elasticity, and work to maximum

load.

The phosphoric acid and monoammonium phosphate

treatments had the most severe effect on strength. The

180°F (82°C) exposure caused considerable strength

loss in wood treated with some chemicals but not

others. The 80°F (27°C) exposure had no effect on

strength and the 130°F (54°C) exposure a minimal

effect. The results indicate that once an elevated

temperature has caused a fire retardant chemical to

dissociate into its acidic chemical form (the form that

provides the fire retardancy mechanism), the rate of

wood degradation does not change. Thus, the actual

difference between most fire retardant chemical systems

is the time required for the chemical to dissociate at

some temperature into its acidic chemical form.

Our results show that the effectiveness of acidic fire

retardant chemicals rests on achieving a delicate

balance between reducing the flammability of the wood

and maintaining strength properties.

The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin.
Effects of Fire Retardant Chemicals

on Bending Properties of Wood

at Elevated Temperatures

Susan L. LeVan, Supervisory Chemical Engineer

Robert J. Ross, Supervisory General Engineer

Jerrold E. Winandy, Research Wood Scientist

Forest Products Laboratory, Madison, Wisconsin

Introduction

Some building codes and insurance companies permit

fire-retardant-treated wood to be used as an alternative

to noncombustible materials for certain applications.

Fire retardant chemicals drastically reduce the rate at

which flames travel across the wood surface, thereby

reducing the capacity of the wood to contribute to a

fire. The chemicals lower the temperature at which

thermal degradation occurs (LeVan 1984). They

increase the amount of char and reduce the amount

of flammable volatiles. Fire retardant chemicals can

also reduce the strength of lumber or plywood, an

effect related to the nature of the chemicals and to

the redrying temperatures used in the treating process

(King and Matteson 1961; Jessome 1962; Johnson

1967; Gerhards 1970; Winandy and others 1988). This

research indicated that the strength of wood exposed

to room temperatures after treatment is reduced

between 10 and 20 percent depending on chemical

treatment and redrying temperature (Gerhards 1970,

Winandy 1988). In addition, more than 30 years of

field experience at or near room temperatures have

indicated that the initial reduction in strength of fireretardant-treated wood does not change over time.

However, problems related to reduced strength have

developed in some situations where fire-retardanttreated material is exposed to elevated temperatures.

The problem has occurred most often when fireretardant-treated plywood has been used as roof

sheathing, but fire-retardant-treated lumber may also

be susceptible to strength loss if exposed to elevated

temperatures for a sufficient period of time. In the

worst cases, roofs made with fire-retardant-treated

plywood have required replacement. In these cases,

the wood had darkened, was brittle, crumbled easily,

and exhibited excessive cross-grain checking. For the

severely degraded roofs brought to our attention, time

in service has ranged from 1 to 8 years (APA 1989a).

The magnitude of wood degradation depends on the

temperature levels to which the fire-retardant-treated

plywood is exposed. High temperature levels can be

achieved in processing or when the fire-retardanttreated wood is used in roof systems (Heyer 1963). In

addition, the presence of moisture and the particular

fire retardant formulation used affect the magnitude

of wood degradation. Previous articles by LeVan

and Winandy (1990) and LeVan and Collet (1989)

describe suspected mechanisms that cause wood failure.

We believe that the strength degradation is directly

attributable to chemical changes in the constituents

that comprise wood. Consequently, we designed and

conducted an extensive experimental program to

investigate changes in the chemical and mechanical

properties of wood treated with fire retardant chemicals

under several temperature and relative humidity

conditions.

The objective of this report is to discuss research

results that show accelerated thermal degradation

of strength and chemical composition of wood are

functions of fire retardant chemical and temperature.

Experimental Methods

Materials

Figure 1 shows an overview of the experimental

procedures. Preparation of specimens, selection of

fire retardant chemicals, types of environmental

exposures, static bending tests, and chemical analysis

are described in the following sections.
Figure 1—Overview of experimental procedures.

Wood Specimens

Small, clear 5/8-in.- (15.9-mm-) tangential by 1-3/8-in.-

(35-mm-) radial by 12-in.- (305-mm-) long bending test

specimens were cut from nominal l-in.- (25.4-mm-)

thick, vertical grain Southern Pine lumber. Although

strength loss has been documented for fire-retardanttreated plywood, we chose to use clear Southern Pine

specimens to evaluate the fundamental effects of fire

retardant treatment on the chemical and mechanical

behavior of wood. Southern Pine was chosen because it

represents a dominant portion of the structural treated

plywood market. We used clear wood because the

properties of clear wood are less variable than those

of products such as plywood or structural lumber.

Consequently, the differences among treatments can

be distinguished more readily. We are also conducting

a parallel study to evaluate the effect of several fire

retardant chemicals on plywood. These data will be

reported at a later date.

To obtain nearly identical sample characteristics for

the various treatments and exposures, specimens were

sorted according to density and modulus of elasticity

(MOE). Stress wave nondestructive testing techniques

were used to determine MOE of the specimens. The

sort resulted in 161 MOE- and density-matched groups

of 30 specimens each. The average density value for

each group was about 37 lb/ft3 (593 kg/m3), with a

corresponding coefficient of variation of 8 percent. The

average stress wave MOE value for each of the groups

was 2.12 × 106 lb/in2 (14.6 GPa), with a coefficient

of variation of 14 percent. Stress wave MOE values

ranged from 1.5 to 2.7 × 106 lb/in2 (10.3 to 18.6 GPa).

This report discusses the results of 84 (12 exposures of

six fire retardant chemicals and one untreated control)

of these matched groups.

Fire Retardant Chemicals

The extent of degradation is believed to be due to acidcatalyzed dehydration (LeVan and Winandy 1990),

which is influenced by acidity and temperature. Acidity

is determined by the pH. The chemicals selected for

this research project cover a broad spectrum of acidity

and may provide an indication of a property that

can be used to predict the extent of acid-catalyzed

dehydration. The chemicals selected may or may not

be used in commercial formulations. However, the

selection of these particular chemicals (all of which

provide fire retardancy to wood) indicates which

chemicals are activated by temperatures up to 180°F

(82°C), which may occur at the top surface of the roof

sheathing (Heyer 1963). Table 1 lists the chemicals

used and the reasons for their selection.

The specimens were pressure impregnated with the fire

retardant chemicals using a full-cell pressure process.

A vacuum of 30 inHg (0.102 MPa) of mercury was

pulled for 30 min, chemicals were added, and pressure

of 150 lb/in2 (1.03 MPa) was then applied for 60 min.

Concentration of the chemical solutions was adjusted

to provide retention of approximately 3.5 lb/ft3

(56 kg/m3). Table 2 hs ows average chemical retentions,

including solution concentration in percentage of

weight, for each treatment and pH of the chemical

solutions before and after treatment.

To minimize premature drying after treatment, the

specimens were bundled in polyethylene and stored

in a 36°F (2°C) cold room until all specimens could

be dried at once. The specimens were then dried in a

kiln using the moderate kiln conditions summarized in

Table 3.

2
Table 1—Chemical treatments

Chemical

Chemical

classification Comment

Phosphoric acid (PA) Inorganic

Monoammonium phosphate (MAP)

Borax-boric acid (BBA)

Inorganic

Inorganic

Guanylurea phosphate-boric acid

(GUP-B)

Dicyandiamide phosphoric acid

formaldehyde (DPF)

Organic

Organic

Diethyl-N,N-bis(2-hydroxyethyl)

aminomethyl phosphate (OPE)

Organic

Phosphoric acid is not used as a sole ingredient in commercial formulations. However,

phosphoric acid is a good fire retardant. It

was selected as a worst-case control.

Common chemical used in some commercial formulations.

Neutral pH. Used together with other

chemicals to buffer some commercial fire

retardant systems.

Organic phosphate salt. Used

commercially.

Amino-resin system, sometimes used as

exterior fire retardant system (1:1:1 molar

ratio).

Phosphonate ester, specifically designed

for use as a flame retardant in rigid urethane foams. Selected for this study because of its neutral pH and classification as

an organophosphonate ester.

Table 2—Chemical retention and pH levels

Chemical

Average Concenretention tration pH before pH after

(lb/ft3 (kg/m3)) (percent wt) treatment treatment

Phosphoric acid (PA)

Monoammonium phosphate (MAP)

Borax-boric acid (BBA).

Guanylurea phosphateboric acid (GUP-B)

Dicyandiamide phosphoric acid

formaldehyde (DPF)

Diethyl-N,N-bis(2-hydroxyethyl)

aminomethyl phosphonate (OPE)

3.64 (58.2) 8.38 1.43 1.35

3.47 (55.5) 8.14 4.27 4.21

3.52 (56.3) 8.14 7.96 8.06

3.47 (55.5) 8.19 3.10 3.13

3.55 (56.8) 8.24 3.75 3.75

3.46 (55.4) 8.14 6.58 5.46

3
Table 3—Posttreatment drying schedule for small

clear wood specimens for all treatments

Dry bulb Web bulb

temperature temperature Airspeed

Days (°F (°C)) (OF (°C)) (ft/min (m/min))

9 110 (43) 100 (38) 250-300 (76.2-91.4)

7 120 (49) 112 (44) 250-300 (76.2-91.4)

Table 4—Environmental conditions and

exposure times

Relative Moisture

Temperature humidity content Exposure time

(°F (°C)) (percent) (percent) (days)

80 (27) 30 6 3, 160

130 (54) 73 12 3, 7, 21, 60, 160

180 (82) 50 6 3, 7, 21, 60, 160

Procedures

Environmental Exposures

Three environmental exposures were selected. Temperature, relative humidity (RH), and exposure time

are listed in Table 4. The 80°F (27°C)-30 percent RH

exposure represents a common dry-room-temperature

condition; 130°F (54°C)-73 percent RH, a warm, moist

condition; and 180°F (82°C)-50 percent RH, a hot environment. Both the 80°F (27°C)-30 percent RH and

180°F (82°C)-50 percent RH exposures represent approximately 6 percent equilibrium moisture content

(EMC) in untreated wood. The 130°F (54°C) exposure represents an often attainable temperature for

roof sheathing (Heyer 1963). The 180°F (82°C) exposure represents a seldom encountered, but possible,

maximum-heat exposure (APA 1989b). For the 80°F

(27°C)-30 percent RH and 130°F (54°C)-73 percent

RH exposures, we used commercial environmental conditioning chambers. For the 180°F (82°C)-50 percent

RH exposure, we used a dry kiln.

Specimens were removed from exposure after the

appropriate exposure time. For each treatment

group, moisture content (ovendry basis) values of

five specimens with the highest densities were used

to monitor the moisture content of the remaining

25 specimens before reconditioning. In this way, the

five density-matched specimens removed from each

group maintained the matched characteristics of the

remaining 25 specimens. The remaining specimens of

each group were then weighed and reconditioned to

4

constant weight at 73°F (23°C)-65 percent RH, which

represents a 12-percent EMC condition for untreated

wood. An additional 56 matched groups are being

exposed at the Forest Products Laboratory (FPL)

in the environmental chamber at 150°F (66°C) and

75 percent RH for exposure times of up to 5 years.

Results of tests on these specimens will be the subject

of a future report.

Static Bending Tests

Specimens were tested flatwise, and maximum bending stresses were imposed on the radial faces to minimize variability, attributed to density differences between earlywood and latewood regions. This technique is described by Bendtsen and others (1983)

and Winandy and others (1985). A span of 9 in.

(0.229 m) was utilized with center-point loading and

a loading rate of 0.19 in/min (4.8 mm/min). The

length, width, thickness, and weight of each specimen were measured prior to testing. Load-deflection

data were continuously monitored and recorded.

From this information, static MOE, stiffness (EI),

modulus of rupture (MOR), and work to maximum

load (WML) were calculated. After testing, moisture content and density were measured for each

specimen.

Chemical Analysis

Each treatment group (defined by type of chemical,

exposure condition, and exposure time) consisted

of 25 specimens. After static bending tests, a small

section was cut from near the failure point of each

specimen. A portion of these sections was cut and

ground to 40 mesh in a Wiley mill. For each group,

we took a fraction of each ground specimen and

mixed the fractions together for chemical analysis.

Thus, a mixed representative sample was analyzed

for each treatment group. Sugar residues, acid-soluble

lignin, and klason lignin were analyzed following the

procedures of Pettersen and Schwandt (in preparation),

TAPPI Standards Useful Method 250 (TAPPI 1982),

and Effland (1977), respectively.

Results and Discussion

Moisture Content of Unconditioned

and Reconditioned Specimens

Figure 2 illustrates average moisture content (MC) of

the five specimens from each treatment group that

were ovendried immediately on removal from the

exposure chamber (monitoring specimens; Fig. 1).

The relative difference in MC of treated specimens

and untreated controls and the relative trend of each

treatment were consistent over time. The MC values
Figure 2—Average moisture content of treated

and untreated (control) specimens ovendried

after exposure. Specimens were used to monitor

representative moisture con tent for various

exposure times at (a) 130°F (54°C) and

73 percent relative humidity and (b) 180°F

(82°C) and 50 percent relative humidity. PA

is phosphoric acid, MAP monoammonium

phosphate, BBA borax-boric acid, GUPB guanylurea phosphate-boric acid, DPF

dicyandiamide phosphoric acid formaldehyde,

OPE organophosphonate ester, and CTL control.

for specimens exposed to 180°F (82°C) (Fig. 2b)

were essentially uniform for the various fire retardant

chemicals over the exposure times. The MC of controls

was reduced about 1 percent after 60 days. At 130°F

(54°C), the phosphoric acid (PA) and borax-boric

acid (BBA) treatments showed the largest deviation

in MC over the exposure times. The MC of controls

was reduced about 2 percent after 60 days. For both

the 130°F (54°C) and 180°F (82°C) exposures, MC

apparently did not change between 60 and 160 days of

Figure 3—Average moisture content of treated

and control specimens after reequilibration

and bending tests for various exposure times

at (a) 130°F (54°C) and 73 percent relative

humidity and (b) 18O°F (82°C) and 50 percent

relative humidity. See legend to Figure 1 for

definition of treatment abbreviations.

exposure. The ordered rank of MC values for specimens

treated with the various fire retardant chemicals was

found to be the same as that of MC values of the

specimens after reconditioning to 73°F (23°C) and

65 percent RH (Fig. 3).

Mean, median, and 10th, 25th, 75th, and 90th

percentile MC values were determined for the test

specimens. No distributional trends or differential

skewness of the MC distribution were observed.

Consequently, further discussion will focus on mean MC

values after reequilbration.

5
Mean MC values are summarized in Table 5. These

specimens do not include the five monitoring specimens

of each group. These values were obtained after the

specimens were reequilibrated to 73°F (23°C) and

65 percent RH. Note that for each treatment group at

80°F (27°C), no change in MC was observed between

the 3- and 160-day exposure times. At the higher

temperatures, all MC values were slightly reduced

with time (Fig. 3). Exposure to 130°F (54°C) and

180°F (82°C) was previously shown to result in EMC

values of 12 and 6 percent, respectfully, in untreated,

unexposed Sitka spruce (USDA 1987). In our study,

these respective exposures resulted in 10 and 6 percent

EMC in untreated Southern Pine. The differences

between these results may be related to species

differences or equipment difficulties in maintaining

73 percent RH at 130°F (54°C). Nevertheless, the

relative difference between each treatment group

and untreated controls and the relative trend of each

treatment over the exposure times are consistent.

The relative rank of MC values was found to be

dependent upon chemical treatment. In general,

the descending order of MC values for the various

treatments was as follows: phosphoric acid (PA),

borax-boric acid (BBA), monoammonium phosphate

(MAP), guanylurea phosphate-boric acid (GUP-B),

dicyandiamide phosphoric acid formaldehyde (DPF),

control, and organophosphonate ester (OPE). Inorganic

chemicals tended to increase MC more than did the

organic salts when compared to controls. The OPE

treatment, however, resulted in slightly reduced MC

values at all temperatures and RH conditions compared

to that of controls.

Each fire retardant chemical system alters the EMC of

the wood. The magnitude of this alteration depends

on the attraction of the fire retardant chemical for

water. Bendtsen (1966) found that most inorganic salts

increased the EMC of wood 3 to 5 percent depending

on the chemical treatment. The differences between

untreated controls and specimens treated with PA and

BBA are in agreement with Bendtsen’s results; the

differences between the controls and the specimens

treated with MAP, GUP-B, DPF, and OPE are less

similar to Bendtsen’s results, although generally within

1 percent of the control. Because some fire retardant

chemicals, but not others, permanently altered the

EMC in our study, the effect of the chemicals on EMC

should be considered as part of the treatment. Thus,

strength properties were not adjusted to the same

MC level because strength should be compared at a

constant environment rather than at a constant MC.

Furthermore, equations are not available for adjusting

strength properties of treated wood for MC.

6

The MC results shown in Figure 3 also indicate a

permanent reduction in EMC over the exposure time.

This trend presents another reason for not adjusting

the strength values for MC. Overall, the magnitude

of this permanent reduction in EMC was about 1 to

2 percent. This effect was more pronounced for the

specimens subjected to the higher EMC conditions

(130°F (54°C)-73 percent RH). The MC values of the

monitoring specimens indicated similar negative trends

in EMC at 130°F (54°C)-73 percent RH (Fig. 2a) and

180°F (82°C)-50 percent RH (Fig. 2b).

Two possible explanations for this negative trend are as

follows: (1) RH conditions in the exposure chambers

and reconditioning rooms changed over a period of

several months, or (2) long high-temperature exposures

resulted in a permanent loss in the capacity of the

specimens to absorb water. Although RH conditions

of the conditioning chambers could change over a

long period, we doubt that this occurred because the

trends in MC between unconditioned and reconditioned

specimens are nearly identical (Figs. 2 and 3). We

believe the negative MC trends are due to a permanent

loss in the water-holding capacity (hygroscopic affinity

for water) of the specimens after long high-temperature

exposure. This loss in hygroscopic affinity for water

is primarily due to degradation of the hemicelluloses

(Skaar 1972). We believe the slight deviations between

MC values over time do not affect the general trends in

the strength property data.

Modulus of Elasticity

As with MC values, mean, median, and various

percentile values for MOE and EI were determined.

No distributional trends or differential skewness of the

MOE or EI distributions were observed. The MOE

and EI values showed parallel trends. Accordingly,

our discussion will focus solely on mean MOE values.

Overall MOE data are summarized in Table 6, and

mean values are shown graphically in Figure 4. Table 7

shows the reduction in bending properties of the

treated specimens over the exposure time. The MOE

of all untreated specimens (controls) showed no change

after 160 days for all exposures.

For PA, the 180°F (82°C) exposure resulted in an

initial reduction in MOE of almost 45 percent when

compared to the MOE of controls (Table 7). A

significant rate of reduction in MOE compared to

that of controls became apparent after only 7 days of

exposure. The degradation of the PA-treated specimens

exposed for 160 days at 180°F (82°C) was so severe

that the specimens broke during handling, which

precluded mechanical testing.
Table 5—Effect of temperature and exposure time on moisture content

and specific gravity of treated Southern Pine specimens

Moisture content Specific gravityb

Tempera- Exposure (percent)

ture time Standard

Chemical a

(°F(°C)) (days) Mean Range Mean deviation

Control 80 (27)

130 (54)

180 (82)

PA 80 (27)

130 (54)

180 (82)

MAP 80 (27)

130 (54)

180 (82)

BBA 80 (27)

130 (54)

180 (82)

3

160

3

7

21

60

160

3

7

21

60

160

3

160

3

7

21

60

160

3

7

21

60

160

3

160

3

7

21

60

160

3

7

21

60

160

3

160

3

7

21

60

160

3

7

21

60

160

10.5 9.3 - 11.3 0.55

10.5 9.6 - 11.4 0.56

10.5 9.3 - 11.3 0.56

10.6 9.6 - 11.4 0.54

10.5 9.2 - 12.7 0.54

9.8 9.1 - 10.9 0.55

9.9 9.3 - 10.4 0.52

9.4 9.2 - 9.8 0.54

9.0 8.5 - 9.4 0.52

8.7 8.5 - 9.0 0.55

8.0 7.8 - 8.7 0.57

7.9 7.6 - 8.4 0.55

14.0 12.4 - 15.4 0.54

13.9 12.9 - 15.3 0.54

15.0 13.1 - 16.5 0.54

14.7 13.1 - 16.4 0.53

15.2 13.6 - 19.5 0.54

12.8 11.4 - 14.1 0.55

13.6 12.0 - 15.5 0.56

13.9 11.0 - 16.4 0.54

11.8 9.8 - 13.8 0.55

11.2 9.6 - 13.6 0.52

11.8 9.9 - 14.2 0.54

(c) (c) (c)

11.9 11.5 - 12.4 0.57

11.7 11.3 - 12.0 0.57

12.5 12.0 - 13.0 0.56

12.2 11.6 - 12.9 0.56

12.0 11.2 - 12.9 0.56

11.1 10.9 - 11.4 0.58

10.5 9.8 - 11.3 0.56

9.5 9.1 - 10.0 0.57

9.2 9.0 - 9.9 0.56

8.7 8.2 - 9.3 0.57

8.0 7.2 - 8.5 0.57

8.0 7.7 - 8.5 0.55

13.7 12.5 - 14.2 0.55

13.4 13.1 - 13.9 0.56

13.8 13.6 - 14.2 0.55

13.6 12.2 - 14.1 0.56

13.3 12.5 - 13.6 0.56

12.5 12.2 - 12.8 0.57

12.5 11.9 - 12.9 0.54

10.7 9.7 - 11.2 0.57

10.7 10.2 - 11.0 0.56

10.1 10.0 - 10.3 0.57

9.3 9.1 - 10.0 0.59

9.4 9.1 - 9.8 0.56

0.04

0.04

0.04

0.04

0.04

0.04

0.10

0.04

0.05

0.04

0.05

0.05

0.04

0.05

0.05

0.05

0.06

0.04

0.05

0.04

0.05

0.04

0.05

(c)

0.03

0.04

0.06

0.04

0.04

0.05

0.09

0.04

0.05

0.04

0.05

0.04

0.04

0.05

0.04

0.04

0.03

0.04

0.09

0.05

0.05

0.05

0.04

0.04

7
Table 5—Effect of temperature and exposure time on moisture content

and specific gravity of treated Southern Pine specimens-concluded

Moisture content Specific gravityb

Tempera- Exposure (percent)

ture time Standard

Chemicala(°F (°C)) (days) Mean Range Mean deviation

GUP-B 80 (27)

130 (54)

180 (82)

DPF 80 (27)

OPE 80 (27)

130 (54)

180 (82)

130 (54)

180 (82)

3

160

3

7

21

60

160

3

7

21

60

160

3

160

3

7

21

60

160

3

7

21

60

160

3

160

3

7

21

60

160

3

7

21

60

160

11.6 11.3 - 11.8 0.56 0.04

11.5 11.3 - 11.8 0.56 0.04

12.0 11.7 - 12.3 0.56 0.04

11.7 11.4 - 12.0 0.55 0.04

11.6 11.1 - 11.9 0.57 0.03

10.4 10.3 - 10.6 0.55 0.04

10.4 10.2 - 10.8 0.57 0.03

9.2 9.0 - 9.4 0.57 0.04

9.0 8.9 - 9.2 0.57 0.04

8.6 8.5 - 9.0 0.57 0.04

8.3 8.0 - 8.7 0.59 0.04

7.2 7.1 - 7.4 0.57 0.04

10.7 10.4 - 11.1 0.57 0.05

10.7 10.6 - 10.9 0.59 0.05

10.9 10.6 - 11.2 0.57 0.04

10.8 10.6 - 11.2 0.56 0.03

10.7 10.2 - 11.1 0.58 0.07

9.9 9.6 - 10.2 0.57 0.05

9.7 9.4 - 9.9 0.59 0.04

8.5 8.4 - 8.8 0.58 0.05

8.5 8.2 - 8.8 0.58 0.04

8.2 8.1 - 8.5 0.57 0.04

8.0 7.7 - 8.4 0.58 0.04

8.0 7.8 - 8.2 0.56 0.05

10.2 10.0 - 10.5 0.55 0.04

10.3 10.0 - 10.5 0.56 0.04

10.3 10.1 - 10.7 0.57 0.04

10.1 9.7 - 10.5 0.56 0.04

10.0 9.6 - 12.3 0.57 0.04

9.4 9.2 - 9.8 0.55 0.04

9.3 9.1 - 9.6 0.58 0.05

8.3 8.0 - 8.6 0.56 0.05

8.0 7.8 - 8.3 0.56 0.04

8.0 7.8 - 8.2 0.57 0.04

7.8 7.7 - 8.0 0.58 0.04

7.4 4.4 - 7.9 0.55 0.11

aPA is phosphoric acid; MAP, monoammonium phosphate;

BBA, borax-boric acid; GUP-B, guanylurea phosphate-boric acid;

DPF, dicyandiamide phosphoric acid formaldehyde;

and OPE, diethyl-N, N-bis (2-hydroxyethyl) aminomethyl phosphate.

bBased on ovendry, treated conditions.

cThese specimens were not tested in bending because of excessive

degradation, and moisture content values were thus not determined.
Table 6—Effect of fire retardant treatments on modulus of elasticity (MOE), modulus of rupture (MOR), and work to

maximum load (WML)a

MOE MOR WML

(×106 lb/in2 (GPa)) (lb/in2 (MPa)) (in-lb/in3)

d

Tempera- Exposure MOE MOR WML

ture time Standard COVc Standard COV Standard COV

Chemicalb

(°F (°C)) (days) Mean deviation (%) Mean deviation (%) Mean deviation (%)

Control 80 (27)

130 (54)

180 (82)

PA 80 (27)

130 (54)

180 (82)

MAP 80 (27)

130 (54)

3

160

3

7

21

60

160

3

7

21

60

160

3

160

3

7

21

60

160

3

7

21

60

160

3

160

3

7

21

60

160

2.019 (13.92) 0.201 (1.39)

2.099 (14.47) 0.276 (1.90)

2.011 (13.87) 0.189 (1.30)

1.996 (13.76) 0.271 (1.87)

1.953 (13.47) 0.270 (1.86)

2.003 (13.81) 0.185 (1.28)

2.017 (13.90) 0.261 (1.80)

1.953 (13.47) 0.228 (1.57)

2.027 (13.98) 0.158 (1.09)

2.132 (14.70) 0.235 (1.62)

2.034 (14.02) 0.193 (1.33)

2.000 (13.79) 0.214 (1.48)

1.667 (11.49) 0.255 (1.76)

1.648 (11.36) 0.236 (1.63)

1.716 (11.83) 0.282 (1.94)

1.621 (11.18) 0.236 (1.63)

1.913 (13.19) 0.480 (3.31)

1.558 (10.74) 0.262 (1.81)

1.646 (11.35) 0.296 (2.04)

1.533 (10.57) 0.272 (1.88)

1.509 (10.40) 0.266 (1.83)

1.245 (8.58) 0.241 (1.66)

1.104 (7.61) 0.230 (1.59)

(e) (e)

1.807 (12.46) 0.351 (2.42)

1.841 (12.69) 0.223 (1.54)

1.808 (12.47) 0.257 (1.77)

1.769 (12.20) 0.258 (1.78)

2.266 (15.62) 0.246 (1.70)

1.825 (12.58) 0.259 (1.79)

1.919 (13.23) 0.272 (1.88)

9.9

13.1

9.4

13.6

13.8

9.2

12.9

11.7

7.8

11.0

9.5

10.7

17,024 (117.4) 1,874 (12.9) 11.0 14.68

18,550 (127.9) 1,618 (11.2) 8.7 16.91

18,003 (124.1) 1,898 (13.1) 10.5 15.98

16,964 (117.0) 1,835 (12.7) 10.8 14.29

17,341 (119.6) 2,011 (13.9) 11.6 14.68

17,590 (121.3) 1,375 (9.5) 7.8 14.72

17,422 (120.1) 2,030 (14.0) 11.7 14.40

17,260 (119.0) 1,846 (12.7) 10.7 16.25

18,133 (125.0) 1,489 (10.3) 8.2 15.27

18,011 (124.2) 2,275 (15.7) 12.6 14.71

17,219 (118.7) 1,728 (11.9) 10.0 12.24

15,187 (104.7) 2,907 (20.0) 19.1 8.42

8,460 (58.3) 2,301 (15.9) 21.2 3.27

8,252 (56.9) 1,913 (13.2) 23.2 3.14

8,097 (55.8) 1,771 (12.2) 21.9 2.69

7,665 (52.9) 1,731 (11.9) 22.6 2.57

8,036 (55.4) 2,078 (14.3) 25.9 2.77

7,362 (50.8) 2,156 (14.9) 29.3 2.75

7,603 (52.4) 2,141 (14.8) 28.2 2.57

5,942 (41.0) 2,179 (15.0) 36.7 1.83

6,322 (43.6) 1,519 (10.5) 24.0 1.81

4,478 (30.9) 1,624 (11.2) 36.3 0.95

2,960 (20.4) 999 (6.9) 33.8 0.49

(e) (e) (e) (e)

6.03 41.1

2.94 17.4

3.96 24.8

1.45 10.1

3.98 27.1

1.94 13.2

3.06 21.3

4.38 27.0

3.61 23.6

4.80 32.6

3.87 31.6

3.54 42.0

15.3

14.3

16.4

14.6

25.1

16.8

18.0

17.7

17.6

19.4

20.8

(e)

1.59 48.6

1.30 41.4

1.07 39.8

1.23 47.9

1.24 44.8

1.41 51.3

1.27 49.4

1.24 67.8

0.77 42.5

0.59 62.1

0.24 49.0

(e) (e)

19.4 14,767 (101.8) 1,598 (11.0) 10.9 11.80 2.94 24.9

12.1 15,012 (103.5) 1,451 (10.0) 9.7 11.50 2.33 20.3

14.2 13,935 (96.1) 1,965 (13.5) 14.1 11.11 3.81 34.3

14.6 14,388 (99.2) 1,491 (10.3) 10.4 11.27 2.30 20.4

10.9 14,305 (98.6) 1,751 (12.1) 12.2 9.08 2.58 28.4

14.2 13,910 (95.9) 1,819 (12.5) 13.1 9.44 2.55 27.0

14.2 13,186 (90.9) 2,640 (18.2) 20.0 6.42 2.33 36.3
Table 6—Effect of fire retardant treatment on modulus of elasticity (MOE), modulus of rupture (MOR), and work to

maximum load (WML)a-continued

MOE MOR WML

(× 106 lb/in2 (GPa)) (lb/in2 (MPa)) (in-lb/in3

)

d

Tempera- Exposure MOE MOR WML

ture time Standard COVc Standard COV Standard COV

Chemicalb

(°F (°C)) (days) Mean deviation (%) Mean deviation (%) Mean deviation (%)

180 (82)

BBA 80 (27)

130 (54)

180 (54)

GUP-B 80 (27)

130 (54)

180 (82)

3

7

21

60

160

3

160

3

7

21

60

160

3

7

21

60

160

3

160

3

7

21

60

160

3

7

21

60

160

1.888 (13.02)

1.784 (12.30)

1.930 (13.31)

1.818 (12.54)

1.526 (10.52)

1.797 (12.39)

1.930 (13.31)

1.874 (12.92)

2.009 (13.85)

2.322 (16.00)

1.952 (13.46)

1.986 (13.69)

2.113 (14.57)

2.106 (14.52)

2.096 (14.45)

2.146 (14.80)

2.071 (14.28)

1.764 (12.16)

1.821 (12.56)

1.826 (12.59)

1.944 (13.40)

1.814 (12.50)

1.865 (12.86)

1.892 (13.05)

1.961 (13.52)

1.861 (12.83)

1.935 (13.34)

1.945 (13.41)

1.808 (12.47)

0.288 (1.99)

0.188 (1.30)

0.209 (1.44)

0.243 (1.68)

0.207 (1.43)

0.293 (2.02)

0.294 (2.03)

0.177 (1.22)

0.155 (1.07)

0.354 (2.44)

0.171 (1.18)

0.220 (1.52)

0.194 (1.34)

0.217 (1.50)

0.227 (1.57)

0.264 (1.82)

0.168 (1.16)

0.215 (1.48)

0.191 (1.32)

0.194 (1.34)

0.203 (1.40)

0.179 (1.23)

0.194 (1.34)

0.209 (1.44)

0.179 (1.23)

0.192 (1.32)

0.203 (1.40)

0.200 (1.38)

0.283 (1.95)

15.3

10.5

10.8

13.4

13.6

16.3

15.2

9.4

7.7

15.2

8.8

11.1

9.2

10.3

10.8

12.3

8.1

12.2

10.5

10.6

10.4

9.9

10.4

11.0

9.1

10.3

10.5

10.3

15.7

12,945 (89.3)

11,244 (77.5)

11,020 (76.0)

8,856 (61.1)

6,194 (42.7)

16,777 (115.7)

16,731 (115.4)

15,386 (106.1)

16,736 (115.4)

16,990 (117.1)

18,353 (126.5)

17,801 (122.7)

17,861 (123.2)

19,020 (131.1)

19,244 (132.7)

20,180 (139.1)

17,989 (124.0)

15,685 (108.1)

15,631 (107.8)

15,577 (107.4)

15,732 (108.5)

15,612 (107.6)

15,114 (104.2)

15,166 (104.6)

16,110 (111.1)

15,998 (110.3)

15,224 (105.0)

12,144 (83.7)

9,615 (66.3)

2,037 (14.0)

2,191 (15.1)

2,314 (16.0)

2,838 (19.6)

1,484 (10.2)

2,569 (17.7)

2,895 (20.0)

1,985 (13.7)

2,361 (16.3)

1,665 (11.5)

2,458 (17.0)

2,348 (16.2)

2,456 (17.0)

2,627 (18.1)

2,717 (18.7)

3,228 (22.3)

1,786 (12.3)

1,580 (10.9)

1,705 (11.8)

1,750 (12.1)

1,729 (12.0)

1,566 (10.8)

1,651 (11.4)

1,859 (12.8)

1,710 (11.8)

2,046 (14.1)

2,022 (13.9)

3,153 (21.7)

2,631 (18.1)

15.7 6.63

19.5 4.86

21.0 4.21

32.0 3.06

24.0 1.63

15.3 10.05

17.3 9.49

12.9 8.36

14.1 9.02

9.8 8.67

13.4 12.52

13.2 10.28

13.8 9.70

13.8 11.15

14.1 10.72

16.0 12.92

9.9 10.03

10.1 12.01

10.9 11.70

11.2 12.24

11.0 11.25

10.0 10.96

10.9 10.44

12.3 9.44

10.6 10.17

12.8 10.05

13.3 7.99

26.0 5.40

27.4 3.65

1.67 25.2

1.93 39.7

1.67 39.7

1.85 60.5

0.68 41.7

2.56

3.05

2.33

2.61

2.22

3.45

2.63

2.45

2.38

2.85

4.00

2.07

25.5

32.1

27.9

28.9

25.6

27.6

25.6

25.3

21.3

26.6

31.0

20.6

2.76

2.60

3.31

2.37

2.30

2.64

2.32

2.53

2.42

2.35

2.78

2.09

23.0

22.2

27.0

21.1

21.0

25.3

24.6

24.9

24.0

29.4

51.5

57.3
Table 6—Effect of fire retardant treatment on modulus of elasticity (MOE), modulus of rupture (MOR), and work to

maximum load (WML)a-concluded

MOE MOR WML

(×106 lb/in2 (GPa)) (lb/in2

 (MPa)) (in-lb/in3)

d

Tempera- Exposure MOE MOR WML

ture time Standard COVc

Standard COV Standard COV

Chemical b

(°F (°C)) (days) Mean deviation (%) Mean deviation (%) Mean deviation (%)

DPF 80 (27)

130 (54)

180 (82)

OPE 80 (27)

130 (54)

180 (82)

3

160

3

7

21

60

160

3

7

21

60

160

3

160

3

7

21

60

160

3

7

21

60

160

1.732 (11.94) 0.165 (1.14)

1.802 (12.42) 0.227 (1.57)

1.799 (12.40) 0.180 (1.24)

1.799 (12.40) 0.276 (1.90)

1.870 (12.89) 0.158 (1.09)

1.861 (12.83) 0.234 (1.61)

1.883 (12.98) 0.254 (1.75)

1.917 (13.22) 0.223 (1.54)

1.891 (13.04) 0.196 (1.35)

1.851 (12.76) 0.182 (1.25)

2.599 (17.92) 0.318 (2.19)

1.796 (12.38) 0.255 (1.76)

1.716 (11.83) 0.173 (1.19)

1.730 (11.93) 0.201 (1.39)

1.723 (11.88) 0.184 (1.27)

1.801 (12.42) 0.215 (1.48)

1.696 (11.69) 0.168 (1.16)

1.774 (12.23) 0.190 (1.31)

1.755 (12.10) 0.231 (1.59)

1.768 (12.19) 0.239 (1.65)

1.823 (12.57) 0.207 (1.43)

1.787 (12.32) 0.228 (1.57)

2.162 (14.90) 0.506 (3.49)

1.851 (12.76) 0.236 (1.63)

9.5

12.6

10.0

15.3

8.4

12.6

13.5

11.6

10.4

9.8

12.2

14.2

10.1

11.6

10.7

11.9

9.9

10.7

13.2

13.5

11.4

12.8

23.4

12.7

15,412 (106.3) 1,618 (11.2) 10.5 12.02

15,870 (109.4) 1,790 (12.3) 11.3 12.19

14,960 (103.1) 1,524 (10.5) 10.2 10.81

15,347 (105.8) 1,651 (11.4) 10.8 11.08

15,810 (109.0) 1,815 (12.5) 11.5 11.18

15,832 (109.2) 2,102 (14.5) 13.3 11.82

15,419 (106.3) 2,046 (14.1) 13.3 10.42

16,462 (113.5) 1,818 (12.5) 11.0 11.34

16,252 (112.1) 1,933 (13.3) 11.9 10.67

14,946 (103.1) 2,333 (16.1) 15.6 8.42

13,091 (90.3) 2,847 (19.6) 21.7 6.36

10,217 (70.4) 2,759 (19.0) 27.0 4.14

14,480 (99.8) 1,715 (11.8) 11.8 12.56

14,844 (102.3) 1,646 (11.3) 11.1 13.05

14,336 (98.8) 1,898 (13.1) 13.2 12.70

14,472 (99.8) 1,725 (11.9) 11.9 12.10

14,609 (100.7) 1,589 (11.0) 10.9 12.86

14,771 (101.8) 1,868 (12.9) 12.6 12.65

15,345 (105.8) 2,415 (16.7) 15.7 13.48

15,501 (106.9) 2,524 (17.4) 16.3 12.19

15,825 (109.1) 1,934 (13.3) 12.2 12.89

15,552 (107.2) 2,232 (15.4) 14.4 11.32

14,892 (102.7) 1,956 (13.5) 13.1 9.51

12,727 (87.8) 2,165 (14.9) 17.0 6.11

2.73 22.7

2.47 20.3

2.49 23.0

2.94 26.5

2.88 25.8

3.12 26.4

2.38 22.8

2.53 22.3

2.15 20.1

2.54 30.2

2.71 42.6

2.12 51.2

2.82 22.5

2.50 19.2

3.55 28.0

2.59 21.4

3.51 27.3

2.88 22.8

3.29 24.4

2.93 24.0

2.91 22.6

3.12 27.6

2.51 26.4

2.25 36.8

aEach value represents the mean or standard deviation of 25 specimens.

b

See Table 5 (footnote a) for definitions of treatment abbreviations.

c Coefficient of variation.

d

1 in-lb/in3 = 6.8947 kJ/m3.

e These specimens were not tested because of extreme degradation, and moisture content values were thus not determined
Figure 4—Average modulus of elasticity (MOE) of treated and control specimens exposed for

various times to different temperature and relative humidity conditions. See legend to Figure 1

for definition of treatment abbreviations. (106

 lb/in2 = 6.895 GPa)

The MAP-treated specimens showed an initial reduction in MOE of about 11 percent, which did not change

over the exposure time for the 80°F (27°C) and 130°F

(54°C) exposures (Fig. 4, Table 7). At 180°F (82°C),

MOE was reduced 24 percent over the exposure time

compared to that of controls. The MAP-treated specimens exhibited a consistent reduction in MOE after

21 days of exposure at the high temperature.

The initial reduction in MOE of specimens treated

with BBA, GUP-B, DPF, and OPE apparently did

not change over the exposure time for all exposure

temperatures (Fig. 4, Table 7).

12

The MOE values of the various treated specimens,

except the PA-treated specimens, were generally

unaffected by almost all temperatures and exposure

times. This stability in the effect of treatment on MOE

clearly supports historical adjustment factors (NFPA

1986) that assume that the relative performance

of treated compared to untreated wood over time

at room-temperature conditions is constant after

accounting for the initial treatment effect. Evaluation

of treatment effects based solely on MOE would not

give a realistic evaluation of the impact of chemical

type, exposure conditions, and exposure time.
Table 7—Reduction in bending properties of treated

wood over exposure time

Reduction in property (percent)

Treatment Modulus of Modulus of Work to

and elasticity rupture max. load

exposure

(°F (°C)) Initial Final Initial Final Initial Final

Control

80 (27)

130 (54)

180 (82)

PA

80 (27)

130 (54)

180 (82)

MAP

80 (27)

130 (54)

180 (82)

BBA

80 (27)

130 (54)

180 (82)

GUP-B

80 (27)

130 (54)

180 (82)

DPF

80 (27)

130 (54)

180 (82)

OPE

80 (27)

130 (54)

180 (82)

0

0

0

17

17

17

11

11

11

13

13

13

13

13

13

13

13

13

13

13

13

0

0

0

0

0

0

0

0

11

0

0

0

0

0

43

17

17

45

50

50

50

50

50

(a)

78

78

78

78

78

(a )

11 13 13 20 20

11 13 22 20 56

24 13 64 20 89

13

13

13

13

13

13

13

13

13

13

13

13

0

0

0

0

(b )

(b)

32 32

32 32

32 32

8 8 18 18

8 8 18 36

8 44 18 75

9

9

9

15

15

15

9 16 16

9 16 16

40 16 72

15 16 16

15 16 16

25 16 58

a

Specimens not tested because of extreme degradation.

bValues increased by approximately 5 percent.

Modulus of Rupture

As with the MOE values, no distributional trends or

differential skewness of the MOR distributions were

observed. Accordingly, our discussion will focus on

mean MOR values. The MOR data are summarized

in Table 6, and mean values are shown graphically in

Figures 5 and 6. Table 7 shows the reduction in MOR

over the exposure time.

The controls exhibited no reduction in MOR after

160 days of exposure at 80°F (27°C) (Tables 6 and 7)

or 130°F (54°C) (Tables 6 and 7, Fig. 5a). However,

Figure 5—Effect of various treatments on

average modulus of rupture (MOR) for two

exposure conditions: (a) 130°F (54°C) and

73 percent relative humidity, and (b) 18O°F

(82°C) and 50 percent relative humidity. See

legend to Figure 1 for definition of treatment

abbreviations. (103 lb/in2 = 6.895 MPa)

the MOR of controls was reduced 11 percent after

160 days of exposure at 180°F (82°C) (Fig. 5b).

The PA-treated specimens showed an initial reduction

in MOR of 50 percent compared to that of controls

(Table 7, Fig. 6). At 80°F (27°C) and 130°F (54°C),

this reduction in MOR did not change for the duration

of exposure. However, after 160 days at 180°F (82°C),

PA-treated specimens were degraded to such an extent

they broke on handling. A significant rate of reduction

in MOR for PA-treated specimens was apparent after

only 7 days when compared to the MOR of controls.

This loss in MOR agrees with the results for MOE.

13
Figure 6—Average modulus of rupture (MOR) of treated and control specimens exposed for

various times to different temperature and relative humidity conditions. See legend to Figure 1

for definition of treatment abbreviations. (103 lb/in2 = 6.895 MPa)

14
The initial reduction in MOR of MAP-treated specimens did not change over the exposure time at 80oF

(27°C) (Table 7, Fig. 6). At 130°F (54°C), MOR values apparently decreased slightly over the exposure

time, but this difference was not statistically significant. At 180°F (82°C), MOR values were significantly

reduced compared to that of controls after only 7 days.

The BBA-treated specimens showed no significant

decrease in MOR values compared to control MOR

values for all temperature exposures (Table 7, Fig. 6).

In fact, MOR values at both 130°F (54°C) and 180°F

(82°C) increased by approximately 5 percent. We

suspect these increases were a result of crosslinking of

some carbohydrates. Further research is underway at

FPL to explain this anomaly.

The other three treatments, GUP-B, DPF, and OPE,

resulted in an initial MOR reduction of 8, 9, and

15 percent, respectively, compared to that of controls

after 3 days of exposure at 80°F (27°C) (Table 7, Fig.

6). This reduction was constant over the exposure time

at 80°F (27°C) or 130°F (54°C). At 180°F (82°C),

however, the treated specimens showed a significant

rate of reduction in MOR over the exposure time

(Table 7, Fig. 5b).

Modulus of rupture was more sensitive to treatment

effects than was MOE and shows differences between

most treatment groups. As with MOE, the PA

treatment had the most deleterious initial and thermalinduced effects on MOR, especially at 180°F (82°C).

Although not as severe as the effects of PA treatment,

the MAP treatment also had deleterious initial and

thermal-induced effects on the strength properties

of the wood. All other treatments clearly showed an

initial effect on MOR. While thermal-induced effects

were apparent at 180°F (82°C), these effects were not

as severe as those resulting from PA and MAP. Also

note that a similar thermal-induced effect on MOR

was apparent for the controls after 160 days at 180°F

(82%).

Most importantly, note that after the thermal-induced

reduction in MOR at 180°F (82°C) was initiated

(<60 days) and eventually stabilized (>60 days),

the rate of strength degradation (slopes of the lines)

was similar for treated and untreated material even

though large differences in strength occurred (Fig. 5b).

This suggests that once an elevated temperature has

caused a fire retardant chemical to dissociate into

its acidic chemical form, this form catalyzes strength

deterioration in the wood in a similar manner with all

chemicals. Thus, the actual difference between most

fire retardant chemical systems is the time required for

the chemical to dissociate at some temperature.

Figure 7—Effect of various treatments on

average work to maximum load (WML) for two

exposure conditions: (a) 130°F (54°C) and

73 percent relative humidity, and (b) 180°F

(82°C) and 50 percent relative humidity. See

legend to Figure 1 for definition of treatment

abbreviations. (1 in-lb/in3 = 6.8947 kJ/m3)

Work to Maximum Load

As with the other properties, the distributions of

WML for each treatment group were evaluated. No

distributional trends or differential skewness of the

WML distributions were observed. Accordingly, our

discussion will focus on mean WML values. The WML

data are summarized in Table 6, mean values are shown

graphically in Figures 7 and 8, and the reduction in

WML over the exposure time is shown in Table 7.

The WML of the untreated wood (control) was

relatively unchanged over the 160-day exposure at 80°F

15
Figure 8—Average work to maximum load (WML) of treated and control specimens exposed for

various times to different temperature and relative humidity conditions. See legend to Figure 1

for definition of treatment abbreviations. (1 in-lb/in3 = 6.8947 kJ/m3)

16
(27°C) (Table 7) or 130°F (54°C) (Table 7, Fig. 7a).

However, the WML of controls was significantly

reduced by about 43 percent after 160 days of exposure

at 180°F (82°C) (Table 7, Fig. 7b).

The PA-treated specimens showed an initial reduction

in WML of approximately 78 percent compared to

that of the controls after 3 days of exposure at 80°F

(27°C) (Table 7, Fig. 8). At 80°F (27°C) and 130°F

(54°C), this reduction was constant over the exposure

time. However, after 160 days at 180°F (82°C), the

PA-treated specimens could not be tested because the

specimens were too brittle to handle.

The WML of the MAP-treated specimens was initially

reduced approximately 20 percent compared to that of

controls (Table 7, Fig. 8). At 80°F (27°C), WML did

not change over the exposure period. Both the 130°F

(54°C) and 180°F (82°C) exposures caused a significant

reduction in WML over the exposure time (Table 7).

The BBA-treated specimens showed an initial reduction

in WML of approximately 32 percent compared to

that of controls (Table 7, Fig. 8). This reduction was

essentially constant for all three exposure temperatures

although an anomalous increase in WML occurred at

130°F (54°C) and 180°F (82°C) after 60 days (Fig. 8).

Although the results with BBA look promising on a

quantitative basis, the specimens treated with this

chemical experienced ceramic-like failures during the

bending test-they experienced sudden rupture and

smooth, almost glass-like fractures. This embrittlement

is a matter for concern because wood is assumed to

undergo ductile or plastic-like failure, rather than

brittle failure. Ductile failure is often cited as a

structural advantage for wood.

Specimens treated with GUP-B showed an initial

reduction in WML of approximately 18 percent

compared to that of controls (Table 7, Fig. 8). These

WML values did not change over time at 80°F (27°C),

but they were further reduced at the other exposure

temperatures (Table 7). The reduction in WML for

GUP-B-treated specimens was not as severe as the

reduction for specimens treated with PA or MAP.

The initial reduction in WML for specimens treated

with DPF or OPE was generally constant for both

the 80°F (27°C) and 130°F (54°C) exposures over

the exposure time (Table 7, Fig. 8). For the 180°F

(82°C) exposure, WML was reduced a total of 72

and 58 percent for DPF- and OPE-treated specimens,

respectively, over the exposure time (Table 7). At the

high exposure temperature, the rate of change in WML

for both DPF- and OPE-treated specimens paralleled

the rate of change for controls. The DPF and OPE

performance was very similar to that of GUP-B.

Work to maximum load is sensitive to the influence

of chemical type, exposure conditions, and exposure

time. The WML is an indication of the brittleness of

materials, and treatments tend to cause embrittlement

(Winandy and Rowe11 1984). The PA treatment had

the greatest detrimental effect on WML, followed by

MAP. All other treatments were not as severe as PA

and MAP. However, except for BBA-treated specimens,

all other treated specimens and the untreated controls

had reductions in WML over time at 180°F (82°C).

Chemical Analysis

The chemical analysis data revealed a pattern of

degradation for several treatments (Table 8). Because

we were looking for patterns in the degradation of the

sugar residues, we ran only one chemical analysis per

group. Consequently, the differences between control

and specimen groups should be viewed as relative

estimates only. Further chemical analysis is necessary

to establish the quantitative changes in sugar residue

percentages from the untreated controls. Examination

of the degradation patterns provides some clues as to

the causes of strength degradation and gives a starting

point for further extensive chemical investigation. In

addition, degradation patterns in the wood constituents

appeared primarily for the exposures at 180°F (82°C).

Therefore, the following discussion deals with trends in

the degradation patterns of the wood constituents at

180°F (82°C) only.

Control Group

For the control group, the percentage of the wood

constituents remained constant over the various

exposure temperatures and the exposure times. For

the 180°F (82°C) exposure, the precentage of arabinose

appeared to decrease with exposure time. The total

sum of the percentages of the wood constituents in the

control group ranged between 93 and 103 percent. The

sums in Table 8 differ from 100 percent because of the

differences in the samples, experimental variability, and

components that were not taken into account.

Treatment Groups

Phosphoric Acid—The PA-treated specimens showed

the most changes in the percentages of the various

wood constituents. Klason lignin and acid-soluble

lignin showed an increase, and some sugar residues

decreased. With the exception of glucose, all the sugar

residues showed a pattern of decreasing percentage with

exposure time. The percentage of glucose appeared to

remain constant. The sums of the percentages of wood

constituents in the PA-treated specimens were lower

than that of the controls. The sums ranged from 87 to

93 percent, roughly a lo-percent difference from that of

the controls.

17
Table 8—Effect of temperature and exposure time on chemical composition of treated Southern Pine specimens

Composition (percent)

Tempera- Exposure Acidture time Klason soluble

Chemical a

(°F (°C)) (days) lignin lignin Glucose Xylose Galactose Arabinose Mannose Sum

Control 80 (27)

130 (54)

180 (82)

PA 80 (27)

130 (54)

180 (82)

MAP 80 (27)

130 (54)

180 (82)

BBA 80 (27)

130 (54)

180 (82)

3 29.4 0.6 48.1 8.0 3.0 1.3 12.9 103.3

160 27.6 0.4 44.9 6.1 2.1 1.2 11.2 93.5

3 29.6 0.5 47.0 7.5 3.4 1.2 12.6 101.8

7 28.4 0.6 48.0 7.0 2.4 1.3 13.2 100.9

21 29.6 0.6 47.1 7.1 2.6 1.2 12.4 100.6

60 29.4 0.6 45.9 6.6 2.5 1.0 12.0 98.0

160 27.8 0.4 52.9 7.1 2.2 1.3 13.6 105.3

3 29.2 0.6 46.7 7.3 2.6 1.2 12.4 100.0

7 29.3 0.6 46.5 6.9 2.5 1.2 12.3 99.3

21 29.7 0.6 48.0 6.8 2.4 0.9 12.7 101.1

60 29.1 0.7 46.9 6.8 2.6 0.7 12.7 99.5

160 27.6 0.8 53.1 6.9 2.3 0.6 13.4 104.7

3 26.7 0.6 40.9 5.9 2.6 0.7 10.8

160 24.8 0.5 44.3 5.8 1.8 1.2 11.3

3 27.1 0.6 41.5 5.9 2.4 0.8 11.9

7 27.0 0.6 41.6 5.9 2.7 1.1 10.7

21 26.8 0.6 43.6 6.1 2.7 1.1 12.2

60 27.4 0.7 43.2 5.7 2.8 0.8 10.8

160 26.7 0.9 44.1 6.4 1.5 0.6 10.3

3 28.4 0.9 42.0 5.4 2.5 0.7 10.2

7 29.3 1.0 44.2 4.4 1.8 0.6 10.1

21 34.0 1.3 41.2 2.6 1.7 0.2 6.2

60 37.9 1.4 40.7 2.0 0.8 0.0 4.2

160 (b) (b) (b) (b) (b) (b) (b)

88.2

89.7

90.2

89.6

93.1

91.4

90.5

90.1

91.4

87.2

87.0

3 27.3 0.5 43.8 5.9 2.1 0.7 11.3 91.6

160 23.6 0.4 42.5 6.2 2.1 1.0 11.8 87.6

3 27.5 0.5 43.6 6.6 2.4 1.2 11.7 93.5

7 27.2 0.5 41.8 6.4 2.2 0.9 11.0 90.0

21 26.6 0.5 42.6 6.1 1.7 0.7 11.3 89.5

60 27.0 0.5 41.5 6.2 2.6 0.9 11.1 89.8

160 26.3 0.5 44.2 6.4 2.0 0.6 11.6 91.6

3 27.1 0.5 41.8 6.2 2.1 0.6 10.9 89.2

7 27.3 0.6 41.1 5.7 2.0 0.5 10.6 87.8

21 27.7 0.8 42.7 6.0 1.7 0.3 10.5 89.7

60 30.2 1.2 43.5 4.9 1.2 0.0 9.8 90.8

160 26.3 1.8 44.7 2.6 0.2 0.0 5.6 87.2

3 27.8 0.6 42.2 6.7 2.6 1.1 14.8

160 26.3 0.4 39.3 5.9 2.4 1.2 10.5

3 27.1 0.6 42.8 6.5 2.3 1.0 14.7

7 26.8 0.6 44.4 7.0 2.7 1.4 15.5

21 26.7 0.6 43.9 6.5 1.9 1.0 15.7

60 27.0 0.6 43.5 6.8 1.9 1.2 14.9

160 25.7 0.4 44.9 6.5 2.0 1.0 12.4

3 27.0 0.6 43.1 6.7 2.7 1.2 15.7

7 27.1 0.6 43.5 6.3 1.4 0.6 14.8

21 27.0 0.6 46.0 6.8 2.3 0.9 15.4

60 27.0 0.7 42.3 6.5 2.5 0.9 14.5

160 26.3 0.9 43.8 6.1 2.3 0.7 11.4

95.8

86.0

95.0

98.4

96.3

95.9

92.9

97.0

94.3

99.0

94.4

91.5

18
Table 8—Effect of temperature and exposure time on chemical composition of treated Southern Pine

specimens-concluded

Composition (percent)

Tempera- Exposure Acidture time Klason soluble

Chemical a

(°F (°C)) (days) lignin lignin Glucose Xylose Galactose Arabinose Mannose Sum

GUP-B 80 (27)

130 (54)

180 (82)

DPF 80 (27)

130 (54)

180 (82)

OPE 80 (27)

130 (54)

180 (82)

3 28.3 1.5 41.4 5.7 2.3 1.2 11.2 91.6

160 25.9 2.2 44.1 6.0 2.4 1.1 12.4 94.1

3 27.9 1.5 43.0 6.2 1.9 1.1 11.4 93.0

7 27.5 1.6 43.1 5.9 1.8 1.1 11.4 92.4

21 28.1 1.5 42.6 6.3 2.3 1.2 11.6 93.6

60 27.9 1.5 42.4 5.8 1.9 1.3 11.3 92.1

160 25.6 1.7 46.2 6.2 2.3 0.8 12.0 94.8

3 27.9 1.6 42.2 5.7 2.1 1.5 10.9 91.9

7 28.1 1.4 42.3 6.2 2.3 1.4 10.9 92.6

21 29.5 1.2 44.2 6.6 2.4 1.2 11.1 96.2

60 29.3 1.4 44.2 6.5 2.6 0.9 10.4 95.3

160 26.9 2.1 48.4 5.3 1.6 0.3 9.8 94.4

3 27.4 2.6 42.3 6.6 2.1 0.8 11.0 92.8

160 28.0 3.2 40.3 5.6 2.4 1.4 10.5 91.4

3 28.4 2.4 41.6 6.1 2.4 1.1 11.1 93.1

7 27.9 2.5 42.4 6.8 2.2 1.0 11.0 93.8

21 27.4 2.2 43.6 6.5 2.4 1.0 11.4 94.5

60 27.7 2.5 40.7 6.5 2.4 0.8 10.4 91.0

160 28.0 2.9 42.3 5.4 2.3 0.8 10.0 91.7

3 27.7 2.5 42.1 5.7 1.8 0.7 11.2 91.7

7 28.4 2.3 42.5 6.0 1.9 0.8 10.9 92.8

21 27.9 2.3 43.9 6.7 2.0 0.6 10.6 94.0

60 28.3 2.2 45.5 6.0 2.0 0.4 11.2 95.6

160 27.8 2.2 40.5 4.1 1.4 0.3 8.8 85.1

3 27.2 0.6 42.8 6.2 2.4 0.9 10.9 91.0

160 25.9 0.4 44.6 5.9 1.9 1.2 10.7 90.6

3 26.6 0.6 44.8 6.7 2.1 1.1 11.8 93.7

7 26.4 0.6 45.0 6.3 1.8 0.9 11.9 92.9

21 27.0 0.6 42.0 6.0 2.0 1.4 11.1 90.1

60 27.2 0.6 43.8 6.6 2.5 1.1 11.4 93.2

160 25.7 0.5 44.0 5.9 1.5 1.0 10.7 89.3

3 26.8 0.6 42.5 6.3 2.5 0.9 11.8 91.4

7 26.4 0.6 43.7 6.2 2.2 1.0 11.7 91.8

21 26.9 0.6 43.8 6.8 2.1 0.8 11.5 92.5

60 27.5 0.8 45.0 6.8 2.1 0.5 11.4 94.1

160 27.9 0.9 46.5 5.7 1.5 0.5 11.0 94.0

a

See Table 5 (footnote a) for definitions of treatment abbreviations.

bThese specimens could not be analyzed for chemical composition because of excessive degradation.

19
Monoammonium Phosphate—Like the PA-treated

specimens, the MAP-treated specimens exhibited

changes in the percentages of the various wood

constituents, although not to the same extent. As

with the PA treatment group, the acid-soluble lignin

fractions of the MAP treatment group increased

significantly. However, the Klason lignin fraction did

not appear to increase in the MAP treatment group

as it did in the PA treatment group. The percentages

of xylose, galactose, arabinose, and mannose showed a

decreasing pattern like that of the PA treatment group

but to a lesser extent. The sum of percentages for the

MAP treatment group was roughly the same as that

of the PA treatment group, ranging between 87 and

93 percent.

Borax-Boric Acid-Within the wide variability of the

data, the BBA treatment group showed almost no

change in the percentage of wood constituents. Both

lignin fractions as well as the sugar residues appeared

to be constant with exposure time. The sum of the

percentages of constituents was between 91.5 and 98.4,

which is in close agreement with that of the controls.

Guanylurea Phosphate-Boric Acid—The GUP-B

treatment group showed minor reductions in some

sugar residues and no apparent increases in the lignin

fractions. Only the percentages of galactose and

arabinose appeared to decrease with exposure time.

Mannose may have been slightly decreased, but given

the wide variability of the data, it would be difficult

to quantify this decrease. The sum of the percentages

ranged from 91.6 to 96.2 percent, just slightly below

that of controls and the BBA treatment group.

Dicyandiamide Phosphoric Acid Formaldehyde—The DPF

treatment group showed the same pattern in change

of the percentages of wood constituents as that shown

by the GUP-B treatment group. This is reasonable

because these treatments are very similar chemically.

The sum of the percentages ranged from 85.1 to

95.6 percent.

Diethyl-N, N-bis (2-Hydroxyethyl) Aminomethyl

Phosphate— A degradation pattern in wood constituents

was difficult to discern in the OPE treatment group.

The percentages of galactose and arabinose apparently

decreased, but to a lesser extent than that which

occurred in the GUP-B and DPF systems. The sum

of the percentages ranged from 89.3 to 94 percent.

Overall Patterns—Similarities exist between the wood

constituents affected and the chemical treatment.

For example, only with the strongest acid did we

see apparent increases in the Klason lignin. Given

the effect of PA acidity on the carbohydrates, we

believe this increase resulted solely from the drastic

reductions in the other sugar residues, leaving a larger

percentage of Klason lignin. We also observed an

increase in the acid-soluble lignin fraction and cannot

explain this increase. However, there appeared to be

a direct correlation between the observed changes

in the percentage of arabinose and the acid-soluble

lignin. If the arabinose percentage showed a decreasing

trend for a given treatment group, the acid-soluble

lignin showed an increasing trend. Since acid-soluble

lignin was determined by ultra-violet absorbance at

a wavelength of 205 nm, degradation byproducts of

arabinose were possibly measured. We are cautious

on this speculation, however, because carbohydrate

degradation products from the hydrolytic procedure are

known to not interfere with the 205-nm measurement

(Pearl and Busche 1960). Further research is necessary

to positively identify the products that cause the

increase in acid-soluble lignin.

Other significant similarities are the reductions in

arabinose and galactose. These sugar residues appeared

to be the most sensitive to chemical treatment. The

percentage of arabinose decreased in all treatment

groups and controls. Galactose residues decreased in

all treatment groups, except the BBA treatment, but

not in controls. Mannose and xylose residues primarily

decreased with only the highly acidic treatments,

namely PA and MAP. The other treatments may

have caused some reduction in these sugar residues, as

indicated by the data for the 180°F (82°C) exposure

after 160 days (Table 8). However, quantitative

evaluation is difficult because of the wide variability

in the data and limited number of replicates.

Relationship of Chemical Properties

to Mechanical Properties of

Fire-Retardant-Treated Wood

Effect of Treatment on Strength

There is a direct relationship between the effect of

chemical treatment and the effect of treatment pH

on mechanical properties. The PA treatment has the

lowest pH, and it had the greatest effect on mechanical

properties. It is the strongest acid of the selected

chemicals; the hydrogen ion concentration of PA is

approximately 100 times greater than that of MAP.

This difference in pH at the same loading concentration

accounts for the severe effect of PA on mechanical

properties.

Three fire retardant chemicals-MAP, GUP-B,

and DPF–have the same pH. Monoammonium

phosphate is considered an inorganic salt whereas

GUP-B and DPF are considered organic phosphate

salts. Monoammonium phosphate dissociates more

readily at higher temperatures than does GUP-B or

20
DPF, thereby increasing the acid concentration and

decreasing the strength properties more than does

GUP-B or DPF.

The BBA and OPE treatments have pH levels that

are close to neutrality. These treatments exhibited

the least effect on strength properties. However, BBA

and OPE reduce flame propagation only slightly. To

determine whether these chemicals are acceptable

commercial fire retardant treatments, the relationship

between loading level and reduced flame propagation

needs to be examined.

We believe that mixtures of the fire retardant chemicals

used in this study would have intermediate effects.

For example, if MAP and BBA were mixed, BBA

could reduce the acidity of MAP, thereby reducing

strength loss. Thus, a MAP-BBA mixture might

produce effects similar to those produced by GUPB and DPF. On the other hand, we speculate that if

MAP and PA were mixed, the effects of temperature

over time on mechanical properties of the treated

material would be less than the effects of these factors

on material treated with PA alone but greater than

those exerted on material treated with MAP alone.

These intermediate effects would be generally based on

the relative proportions of the chemicals in a mixture.

Effect of Treatment on Wood Chemical Components

Hemicelluloses are the major wood components affected by chemical type, exposure conditions, and exposure time. However, the side-chain constituents are

more susceptible to chemical degradation than is the

linear backbone chain. As previously indicated, arabinose showed the greatest decrease, depending on

fire retardant chemical and exposure temperature. In

softwoods, arabinose is a constituent of arabinoglucuronoxylan, whose complete structure consists of a

linear xylan backbone with side chains of 4-0-methylα-D-glucuronic acid (2 glucuronic groups per 10 xylose groups) and α-L-arabinofuranose (1.3 arabinose

groups per 10 xylose groups) (Sjostrom 1981). Because

of their furanosidic structure, the arabinose side-chains

are easily hydrolyzed by acids and temperature. The

α-L-arabinofuranose is also believed to bond with lignin

(Sjostrom 1981). It is noteworthy that the only sugar

residue affected in the untreated control group at 180°F

(82°C) was the arabinose residue and that WML was

also considerably reduced. As the percentage of arabinose dropped, so did WML.

Following arabinose, galactose appeared to be the next

sugar residue most sensitive to the effects of chemical

type and exposure temperature. In softwoods, galactose

is associated with the hemicellulose galactoglucomannan, which consists of a glucose-mannose backbone

with side chains of acetyl and galactose. Galactose is

easily cleaved from the main chain by acids. Galactose

is also believed to bond with lignin (Sjostrom 1981).

Mannose and xylose were affected similarly, but not to

the same extent as arabinose and galactose. Mannose

and xylose were primarily degraded by PA and MAP at

180°F (82°C), although they may have been degraded

slightly by some of the other treatments, as indicated

previously. Mannose and xylose are the linear backbone

chains of the hemicellulose. Because these sugars are

present in higher percentages than are arabinose and

galactose, small decreases in these constituents could

have a large impact on strength.

Of note is the fact that the side groups of two major

hemicelluloses were most affected by treatment; these

side groups are both suspected of bonding with lignin

(Sjostrom 1981). We speculate that the cleavage of

these side groups between the lignin and hemicelluloses

releases the linkage by which one microfibril of a

wood fiber shares the load with another microfibril.

Disruption of such load-sharing would result in

increased brittleness. This loss in load sharing in

combination with disruption in the hemicellulose

backbone chains would also cause a gradual reduction

in strength (Winandy and Rowe11 1984), and it would

support the increased attenuation of a stress wave

induced on the wood member (Ross and others 1989).

The effect of fire retardant chemicals on wood constituents can be related to the pH of the treatment

solution. The PA treatment is strongly acidic, and it

had the most deleterious effect on strength properties

and chemical composition. The MAP treatment is also

acidic, but much less so than PA; the relative acidity

of MAP is 100 times lower than that of PA. The organic phosphate salts are also acidic, but they require

higher temperatures to dissociate to the same extent

as does MAP. The BBA and OPE treatments are basically neutral and do not tend to degrade wood chemical

components. The decreasing pattern in the hemicellulose residues followed the same decline in MOR and

WML with chemical treatment and exposure temperature. Therefore, there is a definite relationship between strength properties and hemicellulose content.

This finding supports the wood strength-wood chemical

model suggested by Winandy and Rowe11 (1984).

Except for BBA, all fire retardant chemicals tested

include phosphate. The fire retardancy mechanism

of BBA is not clearly understood, but Lyons (1970)

indicated it may be a barrier mechanism instead of

a chemical mechanism. This is probably the primary

reason why BBA-treated specimens did not exhibit

significant degradation in our study. However, as

previously mentioned, we believe that the BBA

21
complexed with some carbohydrates, possibly resulting

in crosslinking, which enhances strength and stiffness

but greatly increases brittleness. The fire retardancy

mechanism of BBA needs further research.

Effect of Temperature on Strength

Besides the initial reduction on mechanical properties

caused by treating and redrying, all treatments had in

general little effect on MOE or MOR over time at 80°F

(27°C) and 130°F (54°C). This was also true for WML,

except for the MAP treatment at 130°F (54°C).

The lack of a reduction in MOE, MOR, or WML in the

untreated controls at 80°F (27°C) and 130°F (54°C)

suggests that no permanent thermal degradation in

strength occurs in exposures at 130°F (54°C) for up to

6 months. Although the National Design Specification

for Wood Construction (Section 2.2.2 and Appendix C,

NFPA 1986) allows intermittent exposure to up to

150°F (66°C), our findings support the idea that

thermal effects are immediate and recoverable in nature

for exposures at 130°F (54°C) for up to 6 months.

At the higher temperature of 180°F (82°C), we noticed

reductions in strength properties of all specimens over

time, including the untreated controls. The MOR

values and, to a greater extent, the WML values, were

reduced in the controls after 160 days of exposure.

Reduction in WML occurred almost immediately.

Also, once the reduction of MOR and WML was

initiated and stabilized, the rate of degradation was

generally similar for all chemical treatments after 21

to 60 days of exposure at 180°F (82°C), as shown by

the similar slopes of MOR curves in Figure 5b and

WML curves in Figure 7b. This reveals that once

the fire retardant chemical is changed into the acidic

chemical form used to reduce flammability, the rate

of wood degradation remains the same. Therefore,

the fire retardant chemicals tested differ in a practical

sense in the temperature-time combination needed for

each chemical to dissociate into its acidic chemical

form. For example, PA was in the acidic chemical

form initially, and therefore, it began to degrade wood

strength immediately at 180°F (82°C). Conversely,

MAP required approximately 21 days at 180°F (82°C)

to dissociate into the acidic chemical form and to

initiate strength reduction. Most other chemicals

required an even longer time at 180°F (82°C) before

they dissociated into their acidic chemical form. Only

the BBA treatment did not appear to follow this

pattern, indicating another chemical mechanism was

operative.

The WML values were reduced earlier than were

comparable MOR values. The loss in WML was a

direct consequence of the cleavage of bonds between

the constituents. We speculate that bond cleavage

decreases the ability of the material to distribute

strain energy between constituents. As a consequence,

embrittlement occurs.

Embrittlement supports the idea that treatment effects

on WML can be used as a tool to predict future trends

in MOR. We believe that prudent considerations of

treatment effects on WML or other energy-related

mechanical properties, such as toughness and impact

bending strength, can provide some assurance of

future performance of the treated material. This

is especially relevant when long-term performance

data for traditional engineering design properties are

incomplete. Although reduction in WML cannot be

used to precisely predict the relative percentage of

reduction in MOR, the decrease in WML can be used

to indicate future trends in MOR. Hence, we believe

that any material that shows a significant loss in WML

over time will eventually exhibit lower MOR.

The effect of the 180°F (82°C) exposure and targeted

MC of 6 percent compared to that of 130°F (54°C)

and targeted MC of 12 percent clearly demonstrates

the dominance of the role of elevated temperature.

Using the 180°F (82°C) exposure, we were able

to distinguish between the various chemicals and

to induce degradation in the untreated controls.

However, untreated plywood has long been used as roof

sheathing, and, to our knowledge, it has not exhibited

significant degradation in the field. The question that

arises then is whether the static exposure of 180°F

(82°C) is representative of the problem that exists in

the field.

The surprising result was the lack of an effect at 130°F

(54°C). Specimens treated with MAP (a chemical cited

to cause degradation in the field) showed no significant

loss in MOR after 160 days at 130°F (54°C). Based on

the report by Heyer (1963), 130°F (54°C) exposures

on roof sheathing at Athens, Georgia, were achieved

for approximately 127 hours per year. Assuming a

correlation exists between Heyer’s real exposures and

our static exposures, the 160-day exposure at 130°F

(54°C) in our study could be translated to represent a

real 130°F (54°C) exposure for approximately 30 years.

Because MAP-treated material has been reported to

fail in as few as 1 to 2 years, we expected MAP to show

a greater effect at 130°F (54°C).

The discrepancy between our research results at

130°F (54°C) and the field problems raises the

question whether another temperature variable is

also a factor. Either the plywood roof sheathing is

reaching temperatures greater than 130°F (54°C) for

considerably longer times, or another exposure of the

material to elevated temperature, such as redrying

22
temperature, was not accounted for in our study. We

redried all the material at <120°F (49°C); the AWPA

C20 and C27 Standards (AWPA 1989) stipulate that

redrying temperature should not exceed 160°F (71°C)

until the MC is <25 percent. Further research is

necessary to determine the effects of redrying under

the AWPA standard, followed by subsequent exposure

at elevated temperatures. Based on our results, we

believe this difference in redrying temperature may

contribute to the general lack of effect we observed

at 130°F (54°C) for MAP and may also explain the

problems reported in the field.

Conclusions

The effect of fire retardant chemicals on the strength

properties of wood depends on the type of chemical

and the exposure temperature. Of the properties

tested, work to maximum load was the most sensitive

to the influence of chemical type, exposure conditions,

and exposure time. The PA treatment had the most

deleterious effect, followed by MAP. Wood treated

with the other chemicals as well as the untreated

controls did not experience such severe effects,

although temperature-induced reductions in MOR of

approximately 20 to 30 percent were observed after

160 days at 180°F (82°C). We suspect that mixtures

of the test chemicals would have intermediate effects on

wood exposed to elevated temperatures.

Once degradation was initiated and stabilized, the

rate of strength loss caused by high-temperature

exposure was similar for the various fire retardant

chemicals. This indicates that similar chemical

reactions are occurring in the wood. The chemicals

differ in the temperature and time needed to begin the

degradation process. Thus, the influence of treatment

on strength properties is highly dependent on the

thermal stability of the fire retardant formulation. No

effect was observed at 130°F (54°C) other than the

initial treatment effect. We expected more strength

reductions for the MAP treatment based on known field

failures (APA 1989a). Therefore, we also suspect that

some other previous exposure to elevated temperature,

such as redrying temperature, may be important.

Hemicellulose content was significantly reduced

depending on the type of chemical, the exposure

temperature, and the specific hemicellulose residue

examined. Treatment and exposure conditions had the

greatest effect on the percentage of arabinose, followed

by galactose, then mannose, and finally xylose. Trends

in the reduction of hemicellulose residues followed the

same trends as the reduction of MOR and WML. The

ranking of the effect of chemical treatments on strength

properties followed the same ranking as that of the

effect of chemical treatment on wood constituents.

We conclude that the degradation of hemicelluloses

plays an important role in the reduction of strength

properties, especially the increased brittleness of the

treated material.

Acknowledgments

We are grateful to the following individuals for their

assistance in our study: Como Caldwell, Vicki Class,

Jethro Clay, Bill Cottingham (deceased), Diego Cuevas,

Mary Doran, Bob Edlund, James Evans, Les Floeter,

Steve Geier, Earl Geske, Cherilyn Hatfield, Chuck Hillary,

John Hillis, Becky Ibach, Jan Kelly, Rommie Klassy,

Ron Knispel, Vyto Malinauskas, Ted Mianowski,

Bill Nelson, Roger Pettersen, Virgil Schwandt,

Flo Schwartz, Fran Silver, Rita Simonsen, Mitch Sweet,

Hao Tran, Roy Travel, and Anita Willis.

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