FIRE-RETARDANT TREATMENTS FOR WOOD 1

B y

Forest Products Laboratory, 2 Forest ServiceU. S. Department of Agriculture

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From ancient times, man has associated wood with fire. He has becomeimbued with a wood-fire consciousness acquired by centuries of personalexperience. It is, therefore, not surprising that mention of "fireproofing"treatments for wood should be received with enthusiasm by some andwith skepticism by others. A better understanding of the facts shouldhelp to reconcile these widely divergent attitudes. Possibly the firststep in reaching this understanding would be to avoid the use of the all-inclusive term "fireproof" and select instead one which describes a spe-cific performance resulting from the treatment. Even this is not simpleto do.

The manner in which the spread of flames is retarded and that in whichpenetration of fire is resisted depends upon the nature, form, and arrange-ment of the materials involved in the fire and also on the character of theigniting fire. In a fire test in which the character of the igniting fire iscontrolled, distinction between the two phases of fire performance ispossible and the terms "fire-retarding" or "fire retardant” may be appliedto treatments which limit flaming performance, and the term “fire resist-ance" to the property of the structure which resists penetration by fire.With this brief explanatory background, a description of the fire-retardingtreatments may be more intelligible.

Two general methods are available for reducing the flaming characteristicsof wood by the use of fire -retarding chemicals. One method consists of

1 This report was originally published in the Quarterly of the National FireProtection Association in January 1943 as written by the late G. C.McNaughton, Forest Products Laboratory research engineer. It ispresented here in slightly revised form.

2Maintained at Madison, Wis., in cooperation with the University of Wis-consin.

Rept. NO. 2081 - 1 - Agriculture-Madison

an impregnation treatment which deposits water -borne chemicals withinthe wood. Many chemicals exhibit fire -retarding properties, but becauseof cost or other objectionable characteristics, comparatively few areconsidered generally practical. These are usually combined in variousproportions in treating formulas, and often include mono and dibasicammonium phosphates, ammonium sulphate, borax, boric acid, and zincchloride. Penetration of the treating solution into the wood is usuallyobtained by the vacuum-pressure methods used in the wood preservingindustry, and the operation is controlled to secure a predetermined re-tention of solution. The important considerations are the depth of pene-tration and the amount of chemical deposited in the wood, rather thanthe details of the treating method. Subsequent to treatment, the material

must usually be dried before use.

The other method of controlling flaming characteristics of wood is theapplication of suitable paints to the wood surfaces. The paints may beof the nature of oil-, resin-, or latex-base preparations in which fire-retarding chemicals have been incorporated, or they may be of composi-tions the effectiveness of which depends mainly on their ability to frothand swell at fire temperatures and insulate the wood from the fire. Abenefit that can be derived from these treatments is a reduction in theflammability of the wood, so that it will contribute little fuel to a firealready started. If the spread of flame from an incipient fire can beretarded or prevented, if flaming can be made to decrease and ceaseafter the igniting source has been removed, and if the progress of thechar into the wood can be delayed or held in check, the main purposesof the coating will have been achieved.

Impregnation with fire-retardant chemical has a less pronounced effecton the fire resistance of wood than it has on surface flammability. Never-theless, the treatment can be used to improve fire resistance, probablyby increasing the depth to which the layer of insulating charcoal buildsup, sometimes importantly. The time of failure of wood walls has beenincreased from 20 to 33 percent by chemical treatment.

Comparison of Fire-Retarding Paints and Impregnation Treatments

Effectiveness

Under sustained, severe fire exposure, there are no impregnation treat-ments or fire-retarding paints that will give complete protection. Goodimpregnation treatments are more generally dependable than paint coat-ings, but for controlling the spread of fires of short duration a good fire-retarding paint can be as effective as an impregnation treatment.

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Favorable experience in uncontrolled fires has been built up with impreg-nated material used for interior trim and scaffolding. There may havebeen similar favorable experiences with fire -retardant coatings, with-out available records.

The lack of a satisfactory background of use experience, extravagantclaims by irresponsible promoters, lack of permanence, the uncertaintyof proper application after the paint leaves the manufacturer, and failureof the user to take into account construction features that contribute tofire hazard may be among the reasons why authorities have not always.reacted favorably to fire-retarding paints in the past.

The ultimate fire performance of a specific wood structure is morelikely to be affected by details of design when a fire-retarding paint isused than when the structure is made of impregnated wood. One reasonfor this is that the paint is ordinarily applied to but one surface of thewood. Glowing, once it is established by the fire, is more apt to persistin the untreated wood beneath painted surfaces than in well-impregnatedwood. Even though the spread of fire is prevented on the painted sur-face, ignition of an opposite unpainted side by transmitted heat may per-mit the fire to get out of control. This condition would occur when anincendiary bomb of the magnesium-thermit type comes to rest on spacedboards of an attic floor so that the molten metal can run through thecracks between the boards (fig. 1). If all surfaces have been penetratedby fire -retarding chemicals to an appreciable extent, some of the objec-tions that apply to surface coatings disappear.

The shattering, by a demolition bomb or otherwise, of painted wood ora large partially impregnated timber would expose unprotected surfacesthat would contribute to any fire that might be started. The amount ofuntreated wood exposed in a large impregnated timber would be less,however, than with coated timbers.nated timber

It would be still less with impreg-and lumber of smaller sizes, because the amount of unpene-

trated wood in the interior of impregnated material decreases with sizeand complete penetration may be obtained with thin lumber.

When there is a reasonable expectancy that integrity of surfaces willbe maintained, and the fire can be expected to be of limited duration,coatings may offer worthwhile protection. Good coatings can greatlyretard the spread of fire from small incendiary bombs or other sourcesof fire and thus increase the time for the discovery of the fire and forbringing suppression apparatus into action (compare figs. 2 and 3).Superior coatings or impregnated wood may even permit small fires toburn out unattended (figs. 4 and 5), while the same fire in untreatedwood would spread rapidly (fig. 2).

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The fire -test performance of lumber impregnated with fire -retardingsolutions is related to the amount of dry fire-retarding chemical retainedin the wood. Fire-retardant lumber is usually purchased on the basis of thepounds of chemical retained per cubic foot of wood.

The fire -test performance of wood coated with a good fire -retarding paintis related to the amount or thickness of application. Usually a greaterthickness is required than is customary with ordinary decorative paintcoating 9. There are no generally accepted specifications for the composi-tion of fire -retarding paints ; and, while various ranges in effectivenesshave been reported, there are no generally accepted standards of testperformance.

Application

Pressure impregnation treatments are obviously unsuited for structuresalready erected. For best results in structures to be built, the lumberand timbers should be cut to finished dimensions before treatment andshould not be cut or surfaced after treatment. Easily treated lumber ofsmall cross section, however, may be so completely impregnated thatcutting after treatment is permissible. The retentions and penetrationsobtainable vary considerably with species of wood and between sapwoodand heartwood of the same species. Some species are exceedingly diffi-cult to treat, and considerable skill and careful supervision are requiredto insure an acceptable product.

Fire-retarding paints may be applied to completed structures by brush-ing or spraying, and to all species of wood with equal facility. Theapplication of coatings is simple and requires only moderate equipmentand skill. Inadequate application can be corrected, and the coatingsusually can be renewed as often as required.

Both impregnation and paints may be applied to plywood and other formsof cellulosic material. With plywood, it is possible to impregnate theveneers before gluing, but such procedure requires the use of a gluethat will not be affected adversely by the fire-retarding chemicals in theveneers .

Cost

wood must bear the overhead of a treating plant, technical control andBesides the actual cost of the fire-retardant chemicals, impregnated

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inspection costs, rehandling and redrying costs, and sometimes additionaltransportation charges. The cost of impregnation will vary with the kindof wood, the quantity treated, the thoroughness of impregnation, thechemicals used, and the weight of chemical absorbed per unit of wood.Actual cost figures are likely to be between $70 and $80 per thousandboard feet over the cost of the untreated wood, and sometimes higher.The fact that lumber impregnated with fire-retarding salts is hard onordinary machine tools is another item of increased cost when the woodmust be machined. Special alloy steel tools prove an economy if ma-chine work on fire -retardant -treated wood is extensive.

The retail cost of a gallon of fire-retarding paint can be expected to vary,depending largely upon the paint base. The necessity for thick filmslimits the coverage per gallon and increases the cost per unit area ascompared with ordinary paint. The ultimate cost of a suitable fire-re-tarding coating for a unit of area, however, is usually less than the costof impregnating the wood.

Strength of Wood

Paints are not harmful to wood and do not penetrate it appreciably, hencethere is no reason to expect that they would have an adverse effect uponthe strength of the wood to which they are applied.

Little authentic information is available concerning the effect of thevarious fire -retardant chemicals and treating conditions upon the strengthof the impregnated product. Zinc chloride is known to hydrolyze cellu-losic material when used in high concentrations and at elevated tempera-tures, and the concentration of zinc chloride solutions required to imparthigh fire retardance to treated wood is likely to be harmful to strength.The effect may not be apparent immediately after treatment, but mayincrease with the age of the treated wood, particularly if the wood is usedunder conditions of low humidity and high temperature. Similarly , obser-vations in reworking pine lumber impregnated with borax suggest thatlarge amounts of this salt may make the wood brittle. Other chemicalsmay have a corrosive effect upon metal fastenings used in constructionsand require the addition of a corrosion inhibitor to the treating solution.There is also a probability that, in wood treated with certain chemicals,high kiln-drying temperatures may produce injurious effects, but moreresearch is needed to relate more definitely the extent of the injury withthe particular chemical and the time and temperature of heating.

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There have been few adverse reports concerning strength reduction infire -retardant -treated wood in use, but on the other hand there is noassurance that reduction in strength has not occurred in some cases.For these reasons, the purchaser of impregnated lumber should be cau-tious in using high concentrations of fire-retardant chemicals in timberthat is to be subjected to high working stresses or whose failure wouldhave serious consequences. In timbers whose weakening would createundue hazards, protection may be obtained by leaving the load bearingmembers untreated or lightly treated for decay prevention and thencovering them with 1-inch lumber heavily impregnated with fire-retard-ing chemicals.

General Durability

On account of the water-soluble nature of the fire-retarding chemicals,neither impregnations nor coatings containing the water -soluble chemi-cals are well adapted to damp or wet conditions or to weather exposure, but,under similar exposures to moisture, wood impregnated with fire -retard-ing chemicals should retain its effectiveness somewhat longer than thepainted wood.

The fire -retardant properties of wood impregnated with fire -retardantchemicals should remain permanent if the material is not subjected towater exposure.paints,

On the other hand, fire-retardant paints, like ordinarywould need renewing at intervals, according to the amount of

wear or other damage to which they are exposed in use.

In some instances where impregnated structural wood has been used inoutdoor exposures, outside house paint has been applied to retard theleaching of the chemicals by rain or other water. There is little infor-mation as to how well the paint performs such service. There are indi-cations, however, that once the paint film has become thoroughly dry andfreed of combustible volatiles, it does not detract from the fire-retardingproperties of the treated lumber.

The application of a suitable "waterproof" material over fire -retardingpaints has been suggested as a means of adapting the latter to damp ex-posures , but no evidence has been made available that the scheme wouldbe successful.

In a general way, a comparison of the two methods for improving thefire performance of wood may be summarized as follows:

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Property Coatings

Suitability for application To structures al- To building lumber orready erected timbers before erection

( C o s t Ordinarily less thanimpregnations

Adverse effect on strength Probably noneof wood

Conservation of criticalmaterials

Small amounts used

Durability Not permenent, butrenewable

Limitations Usually not durableoutdoors

Fire -retardant effectiveness Moderately to veryeffective

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Impregnation Treatments

Usually more effectiveand dependable thancoatings

Relatively high

Possible in some cases

Substantial retentionsrequired

More lasting than coat-ings, but not renewable

Fire -retardant leachesaway in rain and groundwater

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