United States

Department of

Agriculture Effect of Fire-Retardant

Forest Service

Forest Treatment and Redrying

Products

Laboratory on the Mechanical

Research

Paper

FPL-RP-485 Properties of Douglas-Fir

and Aspen Plywood

J. E. Winandy

S. L. LeVan

E. L. Schaffer

P. W. Lee
Exterior grades of Douglas-fir and aspen plywood were

treated with interior-use fire-retardant (FR) chemicals

and redried after treatment using several kiln-drying or

press-drying temperatures. FR treatments included

borax-boric acid, chromated zinc chloride, minalith,

pyresote, and a commercial proprietary formulation.

Bending strength (MOR), load-carrying capacity (RZ),

modulus of elasticity (MOE), stiffness (El), work to

maximum load (WML), and horizontal (rolling) shear

strength (TAU) were evaluated. With the exception of

chromated zinc chloride, all of the FR treatments

appeared to have similar “relative” effects on the

mechanical properties tested. Chromated zinc chloride

treatment reduced strength and stiffness more than the

other FR treatments tested. While MOE and El were not

affected by most combinations of treatment and

redrying, all five FR treatments followed by

post-treatment kiln-drying temperatures in excess of

160 °F considerably reduced MOR, RZ, and WML for

both species and TAU for Douglas-fir. Press drying at

temperatures of 250 °F appeared to have a comparable

effect to kiln-drying after treatment temperatures of 180

to 200 °F. Except for the differential effect on TAU, FR

treatment and redrying had a similar effect on the two

species of plywood tested.

Keywords: Fire retardants, structural plywood, bending

strength, shear strength, Douglas-fir, aspen,

borax-boric acid, chromated zinc chloride, minalith,

pyresote, kiln-drying, press-drying, plywood, panel

products.
United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL-RP-485 Errata

June 1988

Errata

Effect of Fire-Retardant Treatment

and Redrying on the Mechanical Properties

of Douglas-Fir and Aspen Plywood

Page 6, Figure 2

The MOE values on the vertical axes of Figure 2 should be divided by 2;

that is, MOE should range from 0 to 2.5 million psi.

Winandy, J. E.; LeVan, S. L.;

Schaffer. E. L.; Lee, P. W. 1988.

Effect of fire-retardant

treatment and redrying on the

mechanical properties of

Douglas-fir and aspen plywood.

Res. Pap. FPL-RP-485. Madison WI:

U.S. Department of Agriculture,

Forest Service, Forest Products

Laboratory. 20 p.
Effect of Fire-Retardant

Treatment and Redrying

on the Mechanical

Properties of Douglas-Fir

and Aspen Plywood

J. E. Winandy, Research Forest Products Technologist

S. L. LeVan, Chemical Engineer

E. L. Schaffer, Research Engineer

Forest Products Laboratory, Madison, WI

P. W. Lee, Professor

Seoul National University, Suwon, Korea

Introduction

For many types of multifamily residential and

nonresidential constructions, building codes require

that softwood lumber and plywood be treated with

fire-retardant (FR) chemicals. Although such treatment

effectively retards combustion, it also reduces

strength. Strength reduction can be magnified when

the lumber or plywood is improperly treated and dried.

To complicate matters, little is known about the effects

of FR treatment and subsequent redrying on

low-density hardwood species, now commonly used as

core stock in commercial softwood-faced plywood.

FR treatments yield a wood product with a high

moisture content. The wood must be dried to achieve

dimensional stabilization and reduce shipping weight.

Kiln-drying after treatment (KDAT) can reduce the

strength of the treated wood unless the redrying is

accomplished under relatively mild drying conditions

(Adams et al. 1979; Gerhards 1970; Graham 1964;

Jessome 1962; Johnson 1967, 1979; King and Matteson

1961). To minimize strength loss, current standards of

the American Wood-Preservers’ Association (AWPA

1986a) for FR-treated plywood require that the dry-bulb

kiln temperature shall not exceed 160 °F until the

average moisture content of the wood has dropped to

25 percent or less.

The primary objective of our study was to assess the

effect of various FR treatments and redrying

techniques on the drying time and degrade (e.g., warp,

checking) of FR-treated plywood. This aspect of the

study has been previously reported (Lee and Schaffer

1982). The secondary objective of this study, and the

specific objective of this report, was to determine the

effects of various FR treatments and redrying regimes

on the bending and horizontal rolling shear properties

of two species of exterior-grade, structural plywood. We

also investigated the effect of press drying on the

strength properties of FR-treated plywood. The

statistical design of this study emphasized our primary

objective and gave less importance to our secondary

objective. Consequently, the statistical analysis of data

reported here was limited, as will be discussed in

Results and Discussion.
Most FR chemicals alter the treated wood’s

hygroscopicity and mechanical properties; equilibrium

moisture content (EMC) and volume are usually

increased, and strength is usually reduced. However,

strength loss should not always be construed as an

equivalent loss in load-carrying capacity. For example,

a FR-treated panel could ostensibly be reduced in

bending strength but have increased load-carrying

capacity because swelling results in an increased

section modulus (Lehmann and Schaffer 1980).

Because FR-treated plywood commonly serves as a

structural panel, its load-carrying capacity (RZ) and

stiffness (El) are often emphasized. Reductions in

bending strength (MOR) and modulus of elasticity

(MOE) are usually of secondary importance, although

they are of interest in assessing treatment effects. This

important distinction between bending strength and

load-carrying capacity will be addressed later in this

report.

Historically, an increase in EMC at high relative

humidity (RH) has been a major problem in the use of

FR-treated lumber and plywood. For example, in one

study red pine lumber treated with various FR

formulations and held at 77 °F (25 °C) and 60 percent

RH reached an EMC of 9 to 13 percent (depending on

FR treatment and adsorption/desorption condition)

while untreated controls reached an EMC of 8 to

11 percent: at 77 °F and 80 percent RH, FR-treated red

pine reached an EMC of 19 to 27 percent and controls

an EMC of 13 to 14 percent (McKnight 1962). Although

the slightly higher EMC of most FR-treated wood does

not cause problems at RH below 70 to 75 percent,

moisture-related problems may occur at RH above

these levels. The higher EMC can cause fasteners to

corrode and loosen from excessive joint swelling and

shrinkage under changing conditions. Recently, a

series of newer “second-generation” FR treatments

have been developed that claim to have overcome these

problems. These second-generation proprietary

formulations were not included in our study because

they were not commercially available at that time.

The effect of FR treatments and subsequent redrying

on wood strength can be categorized by the FR used

and the maximum dry-bulb temperature in the kiln

(table 1). If FR-treated wood is kiln-dried, the effect on

MOR, MOE, and especially work to maximum load

(WML) can be significant (Gerhards 1970). The literature

(table 1) suggests reductions in engineering design

stresses for FR treatment and kiln drying consistent

with the modification factors recommended by the

National Forest Products Association NDS (1986) for

FR-treated lumber (table 2).

Current AWPA (1986a, 1986b) specifications for redrying

FR-treated wood dictate that dry-bulb temperatures

during kiln drying should not exceed 160 °F (71 °C)

until the mean treated-wood moisture content is

25 percent or less. This requirement recognizes that

elevated temperatures interact with the excess

moisture in freshly treated wood to accelerate thermal

degradation. The presence of water promotes

hydrolysis in untreated wood at temperatures

exceeding 212 °F (100 °C). For example in untreated

wood, Stamm (1964) estimated that a kiln load of

softwood heated for 2 days at 160 °F (71 °C) and an

additional 5 days at 200 °F (93 °C) would result in a

reduction in MOR of 3.0 percent. In the first 2 days of

drying at 180 °F, while the wood still contained free

moisture, Stamm estimated the incremental loss at

2.5 percent, with an additional loss of only 0.5 percent

in the next 5.day period at 200 °F because moisture

content would be far lower. In addition, the presence of

some FR can cause wood hydrolysis and thermal

degradation at even lower temperature levels (Eickner

1986). Unfortunately there are no reported fundamental

studies that quantify the effect of both the presence of

moisture and heat for FR-treated wood.

Steam- or oil-heated platens at temperatures ranging

from 250 °F (121 °C) to as high as 500 °F (260 °C) are

commonly used to press dry the panels. Because

contact pressure between platens and the material

influences wood compaction and drying rates, a platen

pressure of 50 pounds per square inch (Ib/in2) is

commonly suggested as an optimum tradeoff. Still,

with green wood the loss in thickness by compaction

at 50 lb/in2 may be twice that observed after air or kiln

drying. This shrinkage could significantly influence

load-carrying capacity and stiffness in bending; this

problem was more thoroughly discussed in a previous

report (Lee and Schaffer 1982).

The elevated temperatures employed in press drying

might also be expected to affect strength. Yet, limited

evidence exists that press drying softwood

laminated-veneer lumber at platen temperatures of

375 °F (190 °C) and contact pressures of 50 lb/in2

 does

not affect shear strength compared to other kiln-drying

methods (Forest Products Laboratory 1977). Toughness,

a sensitive strength property to treatment effects, was

shown to be comparable in press-dried and kiln-dried

hardwoods (Hittmeier et al. 1968). Information on the

effects of press drying on the strength and stiffness of

FR-treated wood is not available.

2
Methods

Design Drying

Two species (Douglas-fir and aspen) of 5/8-inch-thick,

five-ply, A-C exterior-grade plywood were evaluated. The

test panels were cut from commercially manufactured

4- by 4-foot aspen plywood and 4- by 8-foot Douglas-fir

plywood. For each species, we selected 280 12- by

24-inch panels with no visible defects. These panels

were randomly assigned to individual treatment-drying

groups (sample sizes of four panels per group except

for control and water-treated/air-dried groups) to

distribute within-panel and between-panel variability

resulting from differences in the original plywood

sheets. The panels were then equilibrated at 74 °F and

65 percent RH for several weeks, treated, and redried

as described below:

For every species-treatment combination, four plywood

panels were kiln-dried after treatment at each of seven

levels: 120°/115° (dry bulb/wet bulb), 140°/135°,

160°/155°, 180°/175°, 200°/190°, 230°/190°, or

260°/190°F. Similarly, for every species-treatment

combination, four plywood panels were press-dried at

each of three platen temperatures: 250, 300, or 350 °F.

Platen pressures of 50 lb/in2 were employed at each

press-dry temperature. Twenty water-treated specimens

for each species were air-dried at 80 °F. A more

detailed account of the drying procedures and their

impacts on drying degrade has been previously

reported (Lee and Schaffer 1982).

Following drying, all specimens were equilibrated at

74 °F and 65 percent RH prior to mechanical testing.

These conditions produced about a 12 percent EMC in

the untreated wood.

Drying1

Group Treatment Control Air Kiln2 Press3

BRX Borax-boric acid

CZC Chromated zinc

chloride

MIN Minalith

PYR Pyresote

NCF Commercial

proprietary

formulation

H2O Water

CTL Control

28 12

28 12

28 12

28 12

28 12

20 28 12

20

1Number of panels treated for each species-treatment.

drying combination.

2Seven levels (120, 140, 160, 180, 200, 230, and 260 °F);

4 panels per level.

3

Three levels (250, 300, and 350 °F); 4 panels per level.

Treatment

The H2O group of panels was treated full-cell to a water

retention of 32.9 pounds per cubic foot (lb/ft3). Based

on this retention, panels for the BRX, CZC, MIN, and

PYR formulations were treated full-cell to target

retention of 5 Ib/ft3. The NCF panels were treated in

accordance with the manufacturer’s recommendation.

Except for the proprietary NCF, the specific FR

formulations used are given in table 3.

Mechanical Property Tests

Bending Test

Each 12- by 24-inch panel was subjected to third-point

bending on a 22-inch span loaded across the 12-inch

dimension at a load rate of 0.15 inch per minute (fig. 1).

Load and resulting center-span deflection were

recorded until the maximum load was obtained. MOR,

MOE, WML, RZ, and El were calculated for each panel.

Horizontal (Rolling) Shear Test

After the bending test, two 3- by 6-inch bending

specimens were cut from undamaged portions of each

12- by 24-inch panel and subjected to center-point load

on a 4-inch span to maximum load. The 6.4

span-to-depth ratio produced a horizontal (rolling) shear

failure in the center ply rather than a bending failure in

the outer ply of each specimen. The maximum

horizontal shear stress (TAU) was calculated employing

a standard homogeneous strength-of materials

shear-strength formula. The two observations of TAU

from each panel bending test were averaged because

(1) there was far less within-panel variability than

between-panel variability, and (2) averaging allowed

direct comparison of shear strength to other

mechanical properties.

3
Results and Discussion

Figure 1—Bending test setup for evaluating bending strength

and stiffness of 12- by 24-inch plywood panels. (M 150 381-6)

The sensitivity of statistical analysis of the data to

treatment-drying effects was restricted by the small

sample sizes employed. This resulted from the

secondary emphasis placed on assessing mechanical

property effects in the experimental design. Adding to

the problem was our inability to differentiate

within-panel variability from between-panel variability.

(Recall that specimens were randomly rather than

systematically assigned to groups to evenly distribute

number of specimens from individual panels among the

groups.) These factors reduced our ability to discern

treatment and/or drying effects from natural variability.

Still, the overall results merit discussion because of the

general lack of information on the effects of FR

treatment and redrying on mechanical properties and

because of the potential impact of the results on safe

engineering design.

While the literature suggests that FR treatment has a

different effect on MOR and RZ and on MOE and El

because of chemical bulking, no differences of

practical importance were noted in our study.

Apparently the increase in section modulus from

bulking was not sufficient to offset the effects of FR

treatment on strength and MOE. Thus, the following

discussions of the effects of FR treatments upon MOR

and MOE also apply to RZ and El, respectively.

For each species-treatment combination, average test

results (MOE, MOR, WML, and TAU) versus redrying

temperature are shown in figures 2 to 5. Several

significant treatment-drying interactions occurred

(table 4). For example, note that interactions occurred

between CZC and NCF treatments for MOE of

press-dried Douglas-fir (fig. 2) between CZC and H2O

treatments for MOR of kiln-dried aspen (fig. 3), and

between CZC and PYR treatments for TAU of

press-dried aspen (fig. 5). With closer inspection,

additional interactions can also be noted.

In an attempt to enhance statistical analysis, an

analysis of covariance (COV) was considered involving

two traditional mechanical property covariates, specific

gravity (SG) and moisture content (MC). But the use of

COV was rejected because both covariates are

themselves significantly influenced by FR treatment.

We also considered grouping redrying conditions that

exhibited statistically similar mechanical property

responses. We thought that the grouping process

might effectively reduce treatment-drying interaction

and increase replication; however, it did not entirely

eliminate the interactions. Thus, the use of grouping

was rejected.

4
Keeping in mind that interactions in the data limit

interpretation (table 4), we will highlight the important

general trends in the data while also showing the

statistical limits of these data. The results of Tukey

tests for mean redrying effect on MOR, WML, and TAU

of treated aspen and Douglas-fir plywood are shown in

tables 5-7. These results highlight the insensitivity of

the analyses to historically significant trends (table 1).

The magnitude and consistently negative trend of the

FR treatment-redrying effect in our data (tables 8, 9)

leads us to suspect that the limited number of samples

used was the primary cause of the statistical

insensitivity (tables 5-7). Still, practical and conclusive

results can be distinguished by prudently considering

the effect of each treatment and drying factor on

individual species-property combinations (table 10).

Using table 10 as a guide to what we perceive as being

practically important, we can finally discuss overall

trends.

Species Effects

Both species of plywood were affected by FR and

redrying to about the same relative degree except for a

differential effect on TAU; TAU was reduced in

Douglas-fir but not in aspen (table 10). Overall, the

properties of treated aspen plywood were slightly

higher than those of Douglas-fir plywood (figs. 2-5).

These differences probably reflect the initial veneer

quality or phenolic-adhesive contents because the

specific gravity of the Douglas-fir controls was actually

higher than that of aspen (0.52 versus 0.49).

Treatment Effects

In general, FR treatment did not affect the MOE of both

species and the TAU of aspen, whereas it reduced the

other mechanical properties of both species (tables

8-10). CZC treatment had a far greater negative effect

on almost every species-property combination studied

(compared to controls) than did the other FR

treatments or water treatment (table 10). It is probable

that the high level of chloride in CZC treatment

promoted hydrolysis via an intermediate formation of

hydrochloric acid.

With few exceptions, the minalith (MIN), pyresote

(PYR), borax-boric acid (BRX), and the nondisclosed

commercial proprietary formulation (NCF) all

apparently had a comparable “relative” effect upon

each of the properties studied (table 10). Water

treatment had less effect than FR treatments, but a

greater effect than no treatment at all (figs. 2.5).

Drying Effects

MOR, WML, and TAU were generally reduced as

redrying temperatures increased (tables 8 and 9). At

redrying temperatures of 160 °F or below, the effects of

FR treatment on both plywood species studied were

generally comparable to those cited in previous reports

(table 1). However, when FR treatment was followed by

kiln-drying temperatures in excess of 160 °F, each of

the five FR treatments studied seemed to further

reduce MOR, WML, and TAU (tables 8-10).

Press drying after FR treatment was apparently less

degrading towards mechanical properties than kiln

drying at nearly equivalent temperature. Still, press

drying was most often more degrading to mechanical

properties than KDAT temperatures <160 °F. At 250 °F,

press drying apparently had about the same relative

effect on strength as KDAT temperatures of 180 to

200 °F (tables 8, 9).

These findings, while directly applicable to only

FR-treated plywood, appear to reaffirm support for the

160 °F AWPA redrying temperature limitation during the

high MC periods of the redrying process.

5
Figure 2—Effects of various fire-retardant treatments and redrying conditions on

average values of modulus of elasticity (MOE) for 5/8-inch aspen and Douglas-fir plywood.

B = borax-boric acid, H = H2O, M = minalith, N = NCF (commercial proprietary

formulation), P = pyresote, x = control, Z = chromated zinc chloride. (ML87 5474)

6
average values of modulus of rupture (MOR) for 5/8-inch aspen and Douglas-fir plywood.

(See figure 2 for letter designations.) (ML87 5475)

Figure 3—Effects of various fire-retardant treatments and redrying conditions on
Figure 4—Effects of various fire-retardant treatments and redrying conditions on average

values of work to maximum load (WML) for 5/8-inch aspen and Douglas-fir plywood.

(See figure 2 for letter designations.) (ML87 5476)

8
Figure 5—Effects of various fire-retardant treatments and redrying conditions on average

values of horizontal rolling shear (TAU) for 5/8-inch aspen and Douglas-fir plywood.

(See figure 2 for letter designations.) (ML87 5477)

9
Conclusions Literature Cited

Aspen and Douglas-fir 5/8-inch plywood were treated

with various FR chemicals and then either kiln-dried

(seven temperature levels) or press-dried (three

temperature levels) after treatment. Each specimen was

tested in bending and in rolling shear; several material

properties were analyzed.

Overall, no practical differences were found between

the effects of FR treatment and redrying on the two

species of plywood studied. Mechanical properties

were apparently most degraded by CZC treatment and

least degraded by water treatment. The effect of the

other treatments—BRX, MIN, NCF, and PYR—lay in

between that of CZC and water. Except for TAU, both

species were comparably affected by the five FR

treatments when subsequently redried at temperatures

<160 °F. For both species, FR treatment did not affect

MOE and reduced MOR and WML. TAU was unaffected

in aspen specimens and reduced in Douglas-fir

specimens. However, when FR-treated plywood was

redried at >160 °F, each of the five FR treatments

studied further reduced MOR, WML, and TAU

(tables 8, 9). Press-drying at 250 °F appeared to have a

comparable effect to KDAT temperatures of 180 to

200 °F.

The American Plywood Association (APA) recently

removed its long-recommended design modification

factors for FR treatment of - 17 percent for design

stresses and - 10 percent for MOE from its Plywood

Design Specification (APA 1985); they now specify that

the design engineer should obtain design stress

modification factors from the FR formulators. The

National Forest Products Association (NFPA) recently

considered the same approach in its new National

Design Specification for Wood Construction (NFPA

1986), but instead decided on the modification factors

shown in table 2 and recommended a quality control

procedure for assuring those FR-treated lumber

properties (Appendix Q, NFPA 1986). Our data indicate

that in the absence of published modification factors

for plywood, it seems prudent to support reductions in

allowable bending and shear stresses for FR-treated

plywood similar to those currently required for

FR-treated lumber.

Finally, because it appears that post-treatment redrying

at >160 °F significantly reduces the strength of

FR-treated products and because temperatures >150 °F

permanently reduce the strength of untreated wood

(MacLean 1945, 1951, 1953), we recommend that

extended exposure of FR-treated wood to temperatures

>150 °F should especially be avoided.

10
Koppers Co. 1985. Strength properties of DRICON

treated wood. Product Data Sheet WOL-232.

Pittsburgh, PA: Koppers Co.

Lee, P. W.; Schaffer, E.L. 1982. Redrying fire-retardanttreated structural plywood. Wood and Fiber. 14(3):

178-199.

Lehmann, W.F.; Schaffer, E.L. 1980. Determining

optimum thickness-to-weight ratio for structural

flakeboard panels In: Proceedings, Washington State

State University, Pullman, WA: 127-140.

University Particleboard symposium, Washington

MacLean, J.D. 1945. Effect of heat on the properties

and serviceability of wood. Forest Prod. Lab. Rep.

R1471. Madison, WI: U.S. Department of Agriculture,

Forest Service, Forest Products Laboratory. 12 p.

MacLean, J.D. 1951. Rate of disintegration of wood

under different heating conditions. American

Wood-Preservers' Association Proceedings. 47:

155-168

MacLean, J.D. 1953. Effect of steaming on the strength

of wood. American Wood-Preservers' Association

Proceedings. 49: 88-112.

McKnight, T.S. 1962. The hygroscopicity of wood

treated with fire-retarding compounds. Forest Prod.

Res. Rep. 190. Ottawa, Canada: Department of

Forestry

National Forest Products Association. 1986. National

design specification for wood construction.

Washington, DC: National Forest Products

Association.

Osmose Company. 1984. Personal communication with

J.C. Gauntt, Director of Codes and Product

Acceptance, Griffin, GA.

Stamm, A.J. 1964. Wood and cellulose science. New

York: Ronald Press: 307-311.

11
Table 1—Published information on the effects of fire-retardant treatments on mechanical properties expressed as percent change from their

respective untreated controls

– – – –

– – – 30

– – – 65

+ 26

– 20

– 48

– 8 – 5

– 19 – 19

– 17 – 3

– 16 – 3

– 13 – 5

– 11 – 8

– 17

– 10

– 13

– 8

– 16 – 5

– 14 – 8

– 12 – 2

– 10 – 8

– 17 – 5

– 29 – 12

+ 20 + 19

– 3 + 4

+ 2 + 9

+ 6

– 2

+ 9

+ 4

– 4

– 5

– 1

– 1

– 29 – 5

– 23 – 5

– 3 – 1

– 13 – 1

– 15 – 9

– 16 – 9

– 10 + 2

– 2 + 2

– 13 – 10

– 11 – 5

– 5 0

– 15 – 3

– 14 – 1

– 6 – 6

– 11 – 24

– 10

– 57

– 21

– 44

– 45

– 34

– 26

– 33

– 40

– 32

– 24

– –

– 28

– 32

– 37

– 62

– 55

– 24

– 29

– 6

– 2

– 42

– 12

– 8

– 55

– –

– –

– –

– 1

– 10 0

– 9

0

– 2

0

– 10

– 26

– 11

– 2

– 5 – 12

– 10 0

0

0

0

0

0

– – – – – 25

– 31

– 21 – 5 – 9

– 7

0

0

– 3

– 5

0

Author/manufacturer Year Chemical1 Species2 Redry

temperature MOR MOE Energy3 C-par4

T-par5

----------------- Pct -----------------

Adams, Moore, Brazier 1979

Countryman 1957

Gerhards 1970

Noncom X

6,7 Sects pine

FRT

MIN D. fir ply

AP

APAS

S. pine

FRT Combo.

S. pine

Noncom X

6

 D. fir

S. pine

Glulam

PYR S. pine

D. fir

Graham 1964 Borax

AS

Type B

Type C

Type D

FRT

FRT

Jessome 1962 APAS

Johnson 1967 FRT

King and Matteson 1961

HOOVER

Universal

TWP, Inc.

Koppers Cc.

1984

1985

OSMOSE Inc., W. Pres. Div. 1984 FI.Pf.LH6

ZAB

FRT

Pro-Tex6

Dricon6

D. fir

Red pine

D. fir

D. fir ply

D. fir

Red pine

D. fir

D. fir ply

D. fir ply

150

Kiln

Kiln

Kiln

Kiln

Kiln

150

Kiln

Kiln

Air

Air

140

158

75

Combo. 160

S. pine

D. fir

Spruce

S. pine ply

D. fir ply

Combo.

°F

221

140

198

– –

140

Kiln

– –

Kiln

Kiln

Air

Air

160

180

1

 AP = Ammonium phosphate; AS = ammonium sulfate; APAS

= AP + AS; Borax = boric acid + sodium tetraborate;

4Compression parallel-to-grain.

FRT = unidentified; ZAB = zinc ammonium borate.

5

Tension parallel-to-grain.

2Combo. = various species; D. fir = Douglas-fir; S. pine =

southern pine.

3

 Energy-related properties (include work, toughness, impact

bending, etc.).

6Proprietary commercial formulation.

7

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.

12
Table 2—Fire-retardant-treatment adjustment factors for modification of allowable design stresses from the National Design Specification for Wood Construction1

Property Adjustment factor

Extreme fiber in bending 0.85

Tension parallel-to-grain .80

Horizontal shear .90

Compression perpendicular-to-grain .90

Compression parallel-to-grain .90

Modulus of elasticity .90

Fastener design loads .90

1

 National Forest Products Association, 1986.

Table 3—Individual flre-retardant chemical compositions

Chemicals Percent

Borax-boric acid (BRX)

Borax (Na2B4O7) 60

Boric acid (H3BO3) 40

Chromated zinc chloride (CZC)

Chromated zinc chloride 80

Ammonium sulfate ((NH4)2SO4) 10

Boric acid (H3BO3) 10

Minalith (MIN)

Diammonium phosphate ((NH4)2HPO4) 10

Ammonium sulfate ((NH4)2SO4) 60

Sodium tetraborate anhydrous (Na2B4O7) 10

Boric acid (H3BO3) 20

Pyresote (PYR)

Zinc chloride (ZnCl2) 35

Ammonium sulfate ((NH4)2SO4) 35

Boric acid (H3BO3) 25

Sodium dichromate (Na2Cr2O7H2O) 5

Table 4—Results of an analysis of variance on four mechanical

properties tested for aspen and Douglas-fir plywood1

Source Degrees of

freedom MOE MOR WML TAU

ASPEN AND DOUGLAS-FIR

Species (S) 1

Chemical (C) 5

Drying (D) 9

S*C 5

S*D 9

C*D 45

S*C*D 45

R-square

ASPEN ONLY

C 5

D 9

C*D 45

R-square

DOUGLAS-FIR ONLY

C 5

D 9

C*D 45

R-square

0.0006

.0000

.0001

.0001

.0962

.0001

.0108

.7448

.0000

.0001

.0001

.8155

.0001

.0001

.0001

.6003

0.0001

.0000

.0001

.0001

.1357

.0001

.0369

.6906

.0000

.0001

.0001

.7940

.0001

.0001

.0208

.4958

0.0007

.0000

.0001

.0001

.3040

.0024

.0713

.6328

.0001

.0001

.0765

.6551

.0001

.0001

.0012

.5621

0.0000

.0001

.0001

.1555

.1425

.0001

.0025

.7268

.0001

.0001

.0001

.6563

.0001

.0001

.0001

.6406

1

Untreated control and air-dried water-treated control not

included.

13
14
Table 6—Results of Tukey’s test of means for work to maximum load (WML) of aspen and Douglas-fir plywood after treatment1,2

15
Table 7—Results of Tukey’s test of means for horizontal (rolling) shear strength (TAU) of aspen and Douglas-fir plywood after treatment1,2
Table 8—Percent change in three mechanicaI properties of aspen plywood caused by several treatment and redrying regimes compared to

untreated controls

17
Table 9—Percent change in three mechanical properties of Douglas-fir plywood caused by several treatment and redrying regimes compared

to untreated controls
Table 10—Trends exhibited by the three main variables for each species-property combination as interpreted from figures 2-5. For

treatment and drying factors the trends are: slightly increasing (+), no difference (=), slightly decreasing (–), and strongly decreasing

(–); for species-to- species differences the trends are: no difference (=) and different (*)

2.5-1/88

19

U.S. GOVERNMENT PRINTING OFFICE: 1988/542-039/60010