United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL 474

Effectiveness of

Fire-Retardant

Treatments for

Shingles After

10 Years of

Outdoor

Weathering

Susan L. LeVan

Carlton A. Holmes
Abstract

Some building codes require wood shingles to be

fire-retardant treated. Because exterior fire-retardant

treatments are subjected to weathering, treatment durability

and leach resistance are critical for insuring adequate fire

protection. We examined the effectiveness of various

fire-retardant treatments on wood after 0, 2, 5, and 10 years

of outdoor exposure.

We used a Class C burning-brand test (ASTM E 108) and a

Schlyter flamespread test to evaluate effectiveness. Most

shingle treatments evaluated were either pressure

impregnated or coated at the Forest Products Laboratory;

however, a commercial treatment was used as a control.

After 10 years of exposure, most treatments passed the

Class C burning-brand test, but lost considerable

effectiveness in the Schlyter test method. The commercial

treatment was the most effective after 10 years of

weathering.

Keywords: Exterior fire-retardant treatments, wood shingles,

weathering, leach resistance, Class C burning-brand,

Schlyter test.

May 1986

LeVan, Susan L.; Holmes, Carlton A. Effectiveness of fire-retardant

treatments for shingles after 10 years of outdoor weathering. Res. Pap.

FPL 474. Madison, WI: U.S. Department of Agriculture, Forest Service,

Forest Products Laboratory; 1986. 15 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 over 1,000 libraries in the

United States and elsewhere.

The Laboratory is maintained in cooperation with the University of Wisconsin.
Effectiveness of

Fire-Retardant

Treatments for

Shingles After

10 Years of

Outdoor Weathering

Susan L. LeVan, Chemical Engineer

Carlton A. Holmes, Research Forest Products Technologist1

Forest Products Laboratory, Madison, WI

Introduction

Wood shingles and shakes are esthetically desirable and

durable, and they are being used in both commercial and

residential construction in a variety of ways. Because

building codes in some localities require wood shingles to

be fire-retardant treated, the Forest Products Laboratory

(FPL) undertook a study both to examine the effectiveness

of potential treatments and to evaluate the durability of

these treatments after outdoor weathering. This paper

discusses the effectiveness of various treatments after

10 years of outdoor weathering.

In prior research at FPL we evaluated the performance of

various fire-retardant treatments using three methods: the

8-foot tunnel furnace (American Society for Testing and

Materials Designation E 288-89) (ASTM 1984); the modified

Schlyter test (Forest Service 1959); and a Class C

burning-brand test (ASTM Designation E 108-83) (ASTM

1984). Treatment systems that displayed fire-retardant

effectiveness were then evaluated for durability.

In the earlier work we used two accelerated weathering

procedures to evaluate durability. In the first procedure,

panels were placed outdoors for 28 days and sprayed daily

with water. The daily water spray, plus any additional

rainfall, totaled 30 inches of rain, the average annual

precipitation for Madison, Wisconsin. In the second

procedure, panels were given a 1,000-hour ultraviolet (UV)

light-plus-water spray exposure in an accelerated

weathering apparatus developed to simulate outdoor

weathering, ASTM D 2898-81 (ASTM 1982, Holmes 1973).

After both weathering procedures, specimens were

conditioned to constant weight at 80°F and 30 percent

relative humidity (RH) and then tested. Treatments that

demonstrated the most promise in fire tests after

accelerated weathering (table 1) were then evaluated for

durability after outdoor exposure. A previous paper (Holmes

and Knispel 1981) reported the results after 2 and 5 years

of exposure. This paper is the final report on the same

treatments after 10 years of outdoor exposure.

1Now retired.
Table 1 .—Description of treating solution

Panel

number Treatment1 Loading Method Treating solution Percent Drying schedule

1 A THPC-1 4.1 Ib/ft3 Pressure impregnation

1 B THPC-1 6.2 lb/ft3

2A THPC-2 8.4 Ib/ft3

2B THPC-2 11.9 Ib/ft3

3A DPF-1 7.3 lb/ft3

3B DPF-1 8.2 lb/ft3

4A DPF-2 9.5 lb/ft3

4B DPF-2 8.4 Ib/ft3

5A DP-1 6.2 lb/ft3

do. do. Do.

do. Tetrakis (hydroxymethyl)

phosphonium chloride

(80 pct in water)

sodium hydroxide

(50 pct in water)

Urea

A liquid melamine

Water

Do.

do. do. Do.

do. Dicyandiamide

Phosphoric acid

(85 pct)

Formaldehyde

(37 pct)

Water

Solution prereacted.

8.64

11.83

0.83

78.7

Kiln dried using kiln

schedule not

exceeding 130 °F dry

bulb to 6 pct moisture

content. Temperature

slowly raised to

185 °F. Shingles cured

at 185 °F for 24 hours.

do.

do.

do. Do.

do.

5B DP-1 6.3 lb/ft3

do.

6A DP-2 7.7 Ib/ft3

do.

6B DP-2 7.2 lb/ft3

do.

7A Pyresote-1 7.1 lb/ft3

do.

Tetrakis (hydroxymethyl)

phosphonium chloride

(80 pct in water)

Sodium hydroxide

(50 pct in water)

Urea

A liquid melamine

Water

do.

Dicyandiamide

Phosphoric acid

(85 pct)

Formaldehyde

(37 pct)

Water

Solution prereacted.

do.

Dicyandiamide

Phosphoric acid

(85 pct)

Water

do. Do.

Dicyandiamide

Phosphoric acid

(85 pct)

Water

9.3 Do.

12.6

78.1

do. Do.

Zinc chloride 7.92

Ammonium sulfate 7.92

Boric acid 5.65

Sodium dichromate 1.13

Water 77.38

Same as THPC-1 and

THPC-2.

12.55

2.16

2.05

4.35

78.89

18.66

3.24

2.98

6.47

68.65

10.90

14.95

1.05

73.10

6.98

9.45

83.57

Kiln dried using kiln

schedule not

exceeding 130 °F dry

bulb to 6 pct moisture

content

Do.

Do.

Same as DPF-1 and

DPF-2 except cured

for 5 hours.

2
Table 1.—Description of treating solution-continued

Panel

number Treatment1

Loading Method Treating solution Percent Drying schedule

7B Pyresote-2 4.5 lb/ft3 Pressure impregnation

and brush coating

8B Pyresote-3 5.0 Ib/ft3 Pressure impregnation

8A Pyresote-4 4.7 lb/ft3 Pressure impregnation

and brush coating

9A Pyresote-5 3.8 lb/ft3do.

9B Pyresote-5

1OA UDFP-1

4.0 lb/ft3

7.1 lb/ft3

do.

Pressure impregnation

1OB

11A

UDFP-1

UDFP-2

7.4 Ib/ft3do.

10.3 lb/ft3do.

Same as pyresote-1

Two applications of a

solution consisting of

80 pct sealer A and

20 pct tricresyl

phosphate. Sealer A

is a mineral spirits

solution containing a

water repellent,

pentachlorophenol,

and other

chlorophenols; it

meets Federal

Specification

TT-W-572 Type II.

Additional coating

reapplied after

5 years.

Zinc chloride

Ammonium sulfate

Boric acid

Sodium dichromate

Water

Same as pyresote-3.

Two applications of a

solution consisting of

80 pct sealer A and

20 pct tricresyl

phosphate. Additional

coating reapplied after

5 years.

Same as pyresote-2.

Four applications of a

solution consisting of

80 pct sealer A and

20 pct tricresyl

phosphate.

do.

Urea

Dicyandiamide

Formaldehyde

(37 pct)

Phosphoric acid

Water

do.

Urea

Dicyandiamide

Formaldehyde

(37 pct)

Phosphoric acid

Water

do.

5.95

5.95

4.25

0.85

83.0

1.08 Same as THPC-1 and

4.54 THPC-2.

4.32

7.06

83.0

1.44

6.03

5.75

9.38

77.4

After pressure

impregnation, shingles

kiln-dried using kiln

schedule not

exceeding 130 °F, dry

to 6 pct moisture

content. Coating

allowed to air-dry

between coats.

Same as THPC-1 and

THPC-2.

Same as pyresote-2.

Do.

Do.

Do.

Do.

11B UDFP-2 7.7 lb/ft3do. Do.

3
Table 1 .—Description of treating solution-continued

Panel

number Treatment1

Loading Method Treating solution Percent Drying schedule

12A Epoxy paint 77.6 ft2

/gal Brush coating

12B

13A

13B

14A

Epoxy paint

Untreated

Untreated

NCX

75.8 ft2/gal do.

– –

– –

Pressure impregnation

14B

16A

NCX

FR coating

––

– –

– –

 – –

66.0 ft2/gal

do.

Brush coating

16B FR coating 77.8 ft2/gal do.

Do.

– –

– –

– –

‘See text for explanation of treatment abbreviations.

Manufactured

commercially in

accordance with

Military Specification

MIL-C-46081.

Allowed to air-dry

between two coats.

do.

– –

– –

Commercial treatment,

class C labeled by

Underwriters’

Laboratories, Inc.

do.

Clear fire-retardant

coating.

do.

– –

Allowed to air-dry.

Do.

4
Procedures

Materials

Shingles used in this study were western redcedar (Thuja

plicata Donn) conforming to the grading rules of commercial

standard CS-31-52 (U.S. Department of Commerce 1952)

as No. 1 Grade: 16 inches long and 2 inches across the

butts of five shingles. These random-width sawn shingles

were edge grain and 100 percent clear heartwood. Shingles

were purchased from a local lumber dealer and treated at

FPL (except for the commercially treated ones). We

obtained commercial shingles labeled as Underwriters

Laboratory (UL) Class C to provide some data by which to

judge the severity of the test procedures.

Six chemical formulations were used in the treatments:

1. Tetrakis (hydroxymethyl) phosphorium chloride (THPC).

2. Dicyandiamide-phosphoric acid-formaldehyde (DPF).

3. Dicyandiamide-phosphoric acid (DP).

4. Pyresote.

5. Urea-dicyandiamide-formaldehyde-phosphoric acid

(UDFP).

6. Kopper’s NCX2 (NCX).

These formulations and their application are described in

table 1.

The pyresote formulation, a leachable interior inorganic

soluble salt, was included for two reasons: First, to serve as

a reference for leach-resistance treatments; second, to

determine if a water-repellent sealer coating could be used

effectively with this type of treatment to provide leach

resistance.

Preparation of Specimens

The impregnation treatments were made by the full-cell,

vacuum-pressure process. The sealed treatment cylinder

was first evacuated to 27-1/2 inches of mercury, and this

vacuum was held for 15 to 30 minutes. The treating solution

was then drawn into the cylinder, and a pressure of 75 to

90 pounds per square inch (Ib/in2) was maintained for

1-1/2 to 2 hours. Solution temperature depended on type

and solubility of chemical. The dry chemical retention in

pounds per cubic foot (lb/ft3) of wood was calculated for

each bundle using the solution concentration (table 1). The

treated shingles were either air dried or kiln dried. Special

drying conditions for some of the treatments are included in

the description (table 1). See Holmes (1971) for further

description of the treating process.

2

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

information and convenience of the reader. Such use does not constitute an

official endorsement or approval by the U.S. Department of Agriculture of any

product or service to the exclusion of others that may be suitable.

For coating treatments we followed the manufacturer’s

recommended procedure for application technique and

loading levels. Coating treatments by brush or spray were

applied to the shingles only after they had been conditioned

to constant weight at 80 °F and 30 percent RH. After

treatment, shingles were reconditioned for at least 30 days

to insure evaporation of the solvent.

For each treatment system and retention level, eight

Schlyter specimen panels and eight burning brand-specimen

panels were made up. All specimen panels were

conditioned to constant weight at 80 °F and 30 percent RH.

Two Schlyter panels and two burning-brand panels were fire

tested without any outdoor exposure (0 yr); the remaining

panels were placed outside in the Madison, Wisconsin, area

on racks facing south at a slope of 22.6° from horizontal

(fig. 1) for 2-, 5-, and 10-year exposure periods. At the end

of each period, two panels for Schlyter tests and two panels

for burning-brand tests of each treatment and retention level

were removed from the outside weathering rack, conditioned

at 80 °F and 30 percent RH, weighed, and fire tested.

Construction of Test Panels

Each test panel consisted of the treated shingles on a

decking or backing used to provide support. Shingles were

applied to the decks following the recommendations of the

Red Cedar Shingle and Handsplit Shake Bureau (Grondal

1963). Holmes (1971) describes the construction of the

panels in more detail. Four panels of each treatment at each

treatment level were constructed. For the burning-brand test,

the decking was made of western white pine boards,

3-1/2 inches wide by 1-inch nominal thickness, laid across

the shorter dimension of the deck and spaced 1-1/2 inches

apart. For the modified Schlyter test, the decking of each

panel was 3/8-inch-thick Douglas-fir plywood, 11-7/8 inches

wide by 31 inches long.

Fire Test Methods

Class C burning-brand test. —We followed ASTM E 108-83

standard methods of fire tests of roof coverings for the

Class C burning-brand test. The ASTM E 108 standard

includes five separate tests to evaluate the fire-retardant

characteristics of roof coverings as follows:

1. Intermittent flame exposure test

2. Spread of flame test

3. Burning-brand test

4. Flying-brand test

5. Rain test.

However, evaluations in this study were limited to the

Class C burning-brand test, which measures the resistance

of the treated shingles to fire penetration. In the Class C

burning-brand method, the roof-section assembly consisted

of a 40- by 52-inch section of a shingled roof, with a

simulated eave and cornice (fig. 2). The roof sloped 22.6°

from the horizontal. A previous publication details the

construction of the test structure (Holmes 1971).

5
Figure 1.—Outside weathering test fence located in Madison, WI. Panels placed outside in January

1974. (M141 902-13)

Figure 2.—Class C burning-brand test. (M127 768)

6
A large fan placed 60 inches from the front edge of the deck

was positioned so that it generated a 12-mile-per-hour (mi/h)

wind over the surface of the structure when air velocity was

measured midway up the slope of the specimen panel at its

center and edges. A Class C brand was ignited and

centered over the joint between two shingles in the same

course and just below the butt edge of a shingle in the

course above. The number of brands that could be used on

a specimen panel was limited by the number and location of

these joints. All our panels had eight such joints. The test

was continued for each brand until it was consumed and all

evidence of flame, glowing, and smoke disappeared, or until

failure (as shown by sustained flaming on the underside of

the deck).

Modified Schlyler test.—We used the Schlyter test to

measure the vertical flamespread property of the material

(fig. 3). Two matched specimen panels were held parallel in

vertical position, with the test surfaces facing each other

2 inches apart. The bottom of one panel was supported

4 inches higher than the bottom of the other. Behind the

testing rack was a ruler for recording flame height. The

arrangement of the panels gives a chimney effect to

promote combustion, since each panel radiates heat to the

other.

In this test we used 6 cubic feet per hour (ft3/h) of natural

gas flowing through a wing-top Bunsen burner as the

ignition source. At the start of the test, the burner was

placed between the panels and the gas ignited. The initial

height of the gas flame was recorded immediately and at

every 15 seconds thereafter. At the end of 3 minutes of

exposure, the gas flame was shut off. The times until all

flaming stops and afterglowing is no longer visible are

recorded. If flaming or glowing does not self-extinguish in

5 minutes, the operator extinguishes it with water.

Figure 3.—Modified Schlyter test. (M85 0412)

7
Results and Discussion

Weather Conditions

The average rainfall in Madison for the 10-year period was

32.3 inches water equivalents, and the average amount of

possible sunshine was 52.8 percent (table 2). These

observations are typical of interior North America; however,

different results would be expected in the West, South, and

Southeast.

Visual Observations

Visual changes in the shingles between 0 and 2 years and

between 2 and 5 years have been described (Holmes and

Knispel 1981). The shingles showed little visual change

between 5 years and 10 years (table 3). All specimens

exhibited some checks, splits, and cracks. There was no

apparent decay in any of the specimens, including the

untreated. Significant quantities of mold were apparent on

the untreated shingles (fig. 4) and on the THPC shingles

(fig. 5). Since the other shingles showed no significant

amounts of surface mold, the fire-retardant treatment must

have imparted some degree of mold resistance. For

specimens with mold, it was more pronounced on

plywood-backed panels than on decking-backed panels,

probably because the plywood backing retained more

moisture.

Table 2.—Weather conditions during 10-year period

Year Water equivalents Possible sunshine

1973 35.53

1974 36.06

1975 34.53

1976 21.08

1977 32.53

1978 36.44

1979 28.12

1980 34.38

1981 32.10

1982 31.58

1983 31.67

1984 33.72

Mean 32.31 52.75

Median 33.12 52.00

SD 4.24 4.29

SE mean 1.22 1.24

Maximum 36.44 63.00

Minimum 21.08 46.00

In Pct

49

51

5 1

63

56

56

52

52

53

50

46

54

The THPC and DPF specimens exhibited some cupping,

more so with the plywood-backed specimens than with the

decking-backed specimens.

The untreated specimens were dark gray after 10 years

(fig. 4), while most of the other shingles were medium gray.

The graying is due to accumulation of fungal spores and

mycelium on the surface (Black and Mraz 1974, Feist and

Mraz 1978). The commercial NCX treatment specimens

(fig. 6) were medium gray, with a slight reddish hue. The

commercial treatment must have imparted a small fungicidal

effect to the shingles. The pyresote specimens (fig. 7)

showed only a moderate amount of graying. The epoxy

paint did not hold up well after 3 years of weathering, as

indicated in figure 8.

Burning-Brand Test

Specimens from all except the pyresote treatments retained

the level of flame retardancy (less than 20 pct failures)

needed to pass the burning-brand test (table 4). The

pyresote treatment in which two coats of sealer were

reapplied after 5 years also retained effectiveness in the

burning-brand test. Because pyresote is water soluble,

degradation of the sealer coating is the primary reason for

loss of flame retardants.

The epoxy paint retained some of its flame retardancy, even

though 80 percent of it had flaked off. This was probably

because paint remained in the cracks between the butt

joints (fig. 8).

Before weathering, the transparent coating (panel No. 16)

passed the burning-brand test but did not withstand a 3-year

outdoor exposure. Buchanan3 has found this coating to pass

the burning-brand test after accelerated weathering in the

laboratory. The lack of durability under actual outdoor

weathering conditions suggests that the coating gives limited

protection against UV light and biological degradation.

3Buchanan, Brian. Personal communication. Texas Forest Products

Laboratory, Lufkin, TX. 1984.

8
Table 3.—Visual observation after 1, 2, 5, and 10 years of weathering

Treatment1

1 year 2 years 5 years 10 years

THPC Medium gray in color, Light gray in color, Medium gray in color, Dark gray in color

(1 and 2) shingles cupped slight cupping. with slight cupping. with black specks.

slightly. Lots of moss which

gives green color

to shingles. Lots of

checking and

splitting. Plywood

backed panels

support more

mildew growth.

Lots of cupping.

Surface mildew

can be scraped off.

No apparent

decay.

DPF

(3 and 4)

Light tan to light gray in

color.

Light gray in color. Medium gray. Cupping

on two of the panels

but other panels

were flat.

DP Light tan to light gray in

(5 and 6) color.

Light gray in color. Medium gray, slight

cupping on panels.

Pyresote-2, 3, 5

with sealer

(7B,8A, 9A and B)

Greenish tan in color with

some spots of gray.

Light gray in color with

some small tan

areas.

Medium gray with some

small tan areas.

Pyresote-1, 4

without sealer

(7A 8B)

UDFP

(10 and 11)

Medium gray in color. Light gray in color. Medium gray in color.

Lots of checking;

more checking on

plywood-backed

panels. Some

discoloration

around bottom of

shingles (yellow

green in color)

probably due to

chemicals. No

apparent decay.

Lots of discoloration.

Yellow green in

color. Cupping on

these panels with

lots of checks.

Mildew and moss

present.

Plywood-backed

panels have more

mildew. No

apparent decay.

Specimen looks

good, 9 looks

better than 8, 10,

11, or 15. Very

little checking even

on plywood-backed

panels. No

apparent decay.

Dark gray in color

with lots of mildew.

Lots of checks and

splits which were

very deep. No

apparent decay.

Light tan to almost gray

in color, 6 checks

average per shingle.

Light tan to light gray

in color, 6 checks

average per shingle.

Light reddish brown,

6 checks average per

shingle.

Dark gray color. Lots

of checks and

splits, mostly

emanating from the

edges. No

apparent decay.

9
Untreated

(13)

Dark gray. Medium gray in color. Deep gray in color.

Table 3.—Visual observations after 1, 2, 5, and 10 years of weathering-continued

Treatment11 year

Epoxy paint Blueish white color, no

(12) checking.

2 years

Lot of paint worn off

especially near

bottom of panel, no

checking.

5 years

Approximately 50

percent of paint worn

off, no checking.

10 years

Eighty percent of

paint peeled off,

lots of mold,

especially where

the paint was

peeling. No

apparent decay.

Lots of mold, several

splits, lots of

checking. Mold and

mildew growth

especially on

grooves between

shingles. No

apparent decay.

NCX

(14)

Light tan color, slight

checking.

Light tan to almost

gray color, light

checking.

Medium gray color with

a slight red tone,

slight checking.

Some discoloration

light and dark gray

mixture. Little

streaking of brown.

Looks pretty good

compared to others

but not as good as

8 and 9. No

apparent decay.

1See text for explanation of abbreviations.

Figure 4.—Appearance of panel 13B8 before

exposure (left) and after 10 years (right) of exposure.

(M141 757, M84 0195)

Figure 5.—Appearance of panel 1A8 before

exposure (left) and after 10 years (right) of exposure.

(M141 753, M84 0213)
Figure 6.—Appearance of panel 14A8 before

exposure (left) and after 10 years (right) of exposure.

(M142 250, M84 0192)

Figure 8.—Appearance of panel 12A8 before

exposure (left) and after 10 years (right) of exposure.

(M141 754, M84 0190)

Figure 7. —Appearance of panel 9A8 before

exposure (left) and after 10 years (right) of exposure.

(M141 748, M84 0197)

11
Schlyter Test Results

The Schlyter test presents a very severe fire exposure

because of the chimney effect and because reradiation

between the two panels promotes the vertical spread of

flame. In this test, the ability of a treated shingle to

self-extinguish after removal of the igniting flame is an

important indication of fire retardancy.

Although most of the treatments passed the burning-brand

test after 10 years of outdoor weathering, the Schlyter

results indicate considerable loss of fire retardancy (table 4).

The biggest changes in fire retardancy occurred between

0 and 2 years; retardancy leveled off between 5 and

10 years, with most of the treatments exhibiting similar

flamespread values after 10 years. The commercially

treated shingles retained greater fire retardancy than the

other specimens.

Correlation Between Accelerated

Weathering and Outdoor Exposure

To estimate how well accelerated weathering simulates

natural outdoor weathering, we compared Schlyter results

for 1,000 hours in the accelerated weathering chamber with

outdoor results (table 5). The levels of treatment were not all

the same between accelerated weathering and outdoor

weathering so all treatments are not represented. For the

DPF and DP formulations, the epoxy paint and the

commercial treatment, the 1,000 hours of accelerated

weathering simulates about 2 years of actual outdoor

weathering. These results are similar to those reported by

Feist and Mraz (1978) and Black and Mraz (1974) which

show that 24 weeks (4,632 h) in the Weatherometer

corresponds to between 5 and 6 years of outdoor exposure.

Although 1,000 hours of accelerated weathering simulates

the equivalent of 34 years of average rainfall, the

accelerated weathering may not properly simulate the UV

light degradation and biodegradation which occurs in

outdoor weathering. These factors may be just as important

in the permanency of fire-retardant treatments as leach

resistance. To improve exterior fire-retardant formulations,

UV light inhibitors and preservatives will have to be

incorporated into the treatment.

Chemical Treatments

Tetrakis (hydroxymethyl) phosphorium chloride.—

THPC had no burning-brand failures after 2, 5, and 10 years

of outdoor exposure. However, the Schlyter test showed

high flamespread values, comparable to those in untreated

panels. Postweathering weight loss was only slightly greater

in THPC panels than in untreated panels, indicating some

leach resistance. Our THPC treatment had lower amounts of

nitrogen-containing compounds than those of commercial

manufacturers. Commercial manufacturers use an excess of

nitrogen-containing components in the formulation to

produce a flame-retardant finish that is completely

crosslinked and durable to water leaching or washing with

detergent at elevated temperatures (Stephenson4

). The use

of insufficient nitrogen-containing components may lead to

the formation of mainly linear polymers of lower molecular

weight. These may be more susceptible to hydrolytic and

photolytic decomposition than crosslinked polymers of a

higher molecular weight.

Dicyandiamide-phosphoric acid-formaldehyde.—The

DPF formulation had 6 burning-brand failures out of 32 after

10 years of weathering. No burning-brand failures occurred

after 2 or 5 years of weathering. The Schlyter test results for

the DPF formulation after 10 years of weathering were poor,

with flamespread values just slightly lower than untreated.

The postweathering weight loss for DPF-treated shingles

after 10 years was about twice as great as for untreated

shingles.

Dicyandiamide-phosphoric acid.— Like the DPF

formulation, the DP formulation had four burning-brand

failures after 10 years of weathering. No burning-brand

failures occurred after 2 or 5 years. The Schlyter test results

were poor, with values comparable to the untreated. The

weight loss for DP shingles after 10 years’ weathering was

twice as great as the weight loss for the untreated

specimens. The durability of this formulation is questionable.

Pyresote.—The results of burning brand for 7A and the

Schlyter test for 8B (no sealer coatings) indicate very little

fire retardancy after 2 years of weathering, giving values

comparable to the untreated specimens. The pyresote-5

formulation with four coats of sealer applied before

weathering retained considerable fire retardancy after 2 and

5 years of weathering. After 10 years, however, the

fire-retardant effectiveness was reduced considerably,

although it still provided some protection. The pyresote-2

and pyresote-4 formulation (in which a second coat of sealer

was applied after 5 yr) did not retain fire retardancy as well

as pyresote-5, in which four coats of sealer were initially

applied to the specimens. Again, some degree of

effectiveness remained after 10 years’ weathering, although

retardancy was reduced.

4Stephenson, J. E. Personal communication. Albright and Wilson LTD, West

Midlands, Great Britain. 1984.

12
Table 4.—Burning-brand and modified Schlyter results after 0, 2, 5, and 10 years’ weathering

Burning brand Schlyter

Panel

number Treatment1

 Loading Weight loss Number of failures per 16 brands Flamespread in inches

5 years 10 years 0 year 2 years 5 years 10 years 0 year 2 years 5 years 10 years

Lb/ft3

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

1 A

1 B

2A

2B

3A

3B

4A

4B

5A

5B

6A

6B

7A

7B

8A

8B

9A

9B

10A

1OB

11A

11B

12A

12B

13A

13B

14A

14B

16A

16B

THPC

THPC

THPC

THPC

DPF

DPF

DPF

DPF

DP

DP

DP

DP

Pyresote-1

Pyresote-2

Pyresote-4

Pyresote-3

Pyresote-5

Pyresote-5

UDFP

UDFP

UDFP

UDFP

Epoxy paint

Epoxy paint

Untreated

Untreated

NCX

NCX

Clear coating

Clear coating

4.1

6.2

8.4

11.9

7.3

8.2

9.5

8.4

6.2

6.3

7.7

7.2

7.1

4.5

4.7

5.0

3.8

4.0

7.1

7.4

10.3

7.7

––

––

––

––

––

––

––

––

––

––

2.9

4.0

4.5

5.5

6.9

6.9

10.1

9.8

9.8

8.7

10.2

10.7

12.8

6.2

6.4

10.5

5.5

4.7

6.9

7.1

7.0

6.5

3.5

2.8

3.7

3.2

3.2

3.6

8.0

7.0

8.6

9.8

10.6

8.5

14.0

14.4

14.5

13.6

14.1

16.6

15.7

11.4

9.2

16.3

11.8

9.7

11.9

11.6

10.8

11.7

3.9

2.1

6.4

7.2

6.9

7.2

––

––

––

0

––

0

––

0

0

––

––

0

0

––

0

––

0

––

––

0

––

0

0

––

––

0

15

––

0

––

––

2

––

0

––

0

––

0

0

––

––

0

0

––

1 5

––

0

––

––

1

––

0

0

––

––

0

14

––

0

––

––

4

10

––

0

––

0

––

0

0

––

––

0

0

––

16

––

4

––

––

3

––

0

3

––

––

3

1 6

––

0

––

––

––

––

0

––

0

––

3

3

––

––

2

2

––

16

––

3

––

––

8

––

0

0

––

––

3

1 6

––

0

––

––

––

42 41

–– ––

36 29

–– ––

1 0 18

–– ––

–– ––

10 22

10 24

–– ––

–– ––

1 0 23

–– ––

7 39

–– ––

28 41

26 34

–– ––

1 0 20

–– ––

–– ––

11 19

27 38

–– ––

–– ––

48 47

–– ––

11 1 2

45 446

–– ––

39

––

36

––

29

––

––

32

35

––

––

36

––

45

––

43

39

––

29

––

––

32

44

––

––

44

––

25

––

2

44

––

2

39

––

37

––

––

40

44

––

––

44

––

44

––

45

47

––

38

––

––

36

44

––

––

44

––

3

29

––

1See text for explanation of abbreviations.

2Heavy smoke, considerable glowing.

3Self-extinguished.

4Evaluated after 3 years.

Table 5.—Comparison of natural outdoor exposure with accelerated results for Schlyter

test

Exposure

Natural Accelerated

Treatment1Loading

1,000

0 year 2 years 5 years 10 years 28 days hours in

chamber

DPF 7.3 10 1 8 29 37 –– 21

DP 7.7 10 36 44 10

Epoxy paint –– 27

2 3

38 44 44 ––

25

36

Untreated –– 48 47 44 4 4 46 46

NCX –– 1 1 12 25 29 12 1 3

1See text for explanation of abbreviations.

13
Heavy applications of water-repellent sealers appear to aid

in retaining fire retardancy for treated wood, but periodic

reapplication of the sealer is necessary. Resistance to

biodegradation and improved appearance of the shingles

are additional advantages. However, more work is

necessary to determine the best interior treatment, the initial

application level to insure best retention, the reapplication

schedule to insure continued fire retardancy, and the effect

on exterior fire-retardant durability.

Urea-dicyandiamide-formaldehyde-phosphoric

acid.—The UDFP formulation had no burning-brand failures

after 2, 5, or 10 years of weathering. The Schlyter test

indicates considerable loss of fire-retardant effectiveness

after 10 years, although the performance was better than for

untreated shingles. The shingles used in this study were

treated at FPL and may not have had the same treatment

level as those produced commercially. Also, the treated

shingles were not heat cured as is done commercially; this

influences the extent of leach resistance.

Kopper’s NCX.—After 10 years of weathering the two

commercially treated specimens had no burning-brand

failures, had a flamespread of 29 inches, and

self-extinguished. of all treatments evaluated after 10 years,

only the NCX-treated specimens self-extinguished in the

Schlyter test, indicating a high degree of leach resistance.

The NCX specimens had a postweathering weight loss of

only 7 percent, equivalent to the weight loss of the untreated

specimens after 10 years of weathering. The increase in

flamespread from 11 inches at the outset (no weathering) to

29 inches after 10 years’ weathering could result from

photodegradation and defiberization of the surface wood

cells by UV light; fire-retardant chemicals are probably lost

along with the degraded wood fibers.

14
Summary Literature Cited

Of all the treatments evaluated, the commercial treatment

NCX performed best in fire tests after 10 years of outdoor

weathering. Other treatments, such as the UDPF and the

DP, performed well in the burning-brand test but

unacceptably in the modified Schlyter test for flamespread.

The accelerated weathering test of 1,000 hours light coupled

with water spray corresponds to about 2 years of outdoor

exposure. Although the equivalent of 34 years of average

rainfall is used in the 1,000-hour accelerated weathering

test, there is probably not enough UV light exposure.

Photodegradation by UV light and biological degradation is

just as important in maintaining retardancy in treated

shingles as is leach resistance.

The use of copious amounts of water-repellent sealer

coatings over interior-type fire retardant showed some

promise in providing a degree of leach resistance. However,

several sealer coatings must be applied initially and then

reapplied periodically. More research is necessary to

determine the proper reapplication schedule to insure

continued fire protection. Such treatments should also be

useful in extending the life of exterior fire-retardant

treatments.

2•5-5186

15