Improving fire performance of wood

This section contains following topics:

1. Mechanisms of flame retardants for wood

The flame retardant techniques for wood aim at delaying the ignition of wood and reducing the heat released during combustion [25]. These goals can be pursued for example the following ways:

Many practical fire retardant systems combine different mechanisms. For instance, systems based on protecting the surface with an isolating intumescent coating often include components that modify the pyrolysis reaction.

Combinations of fire retardants with different mechanisms are often used in order to make the treatment more efficient and to create synergisms.

Several overviews have been presented [26, 27, 28, 29, 30]. Some new ideas have been published [31].

1.1. Changing the pyrolysis of wood

The most common and best known fire retardance methods for wood are based on changing the pathway of pyrolysis. In this simple and inexpensive method, wood is treated with a substance that enhances the pyrolysis reaction of cellulose through the pathway leading mainly to char formation (the lower pathway in Figure 2a). Ideally, the reactions would proceed so that cellulose decomposes to char and water: (C6H10O5)n → n(6 C + 5 H2O). In practice, fire retardants based on this principle reduce the amount of burning pyrolysis products and thus decrease the heat released by the product. Substances used for changing the pyrolysis of wood are introduced in Table 4.

Substances influencing on pyrolysis often react with the hydroxyl group attached to the sixth carbon atom of cellulose molecule, leading eventually to the stabilization of the structure through the formation of a double bond between the fifth and sixth carbon atom. Reactions take place through either dehydration or esterification as shown in Table 5. The fire retardant acts as a catalyst in the reactions. Substances are usually added as, for example, ammonia salts decomposing in heat and producing phosphoric or boric acid.

A fire retardant may also slow down pyrolysis reactions and stabilize the chemical structures of wood against decomposition. For instance, aluminium sulphate added to wood creates bonds between cellulose molecules in increased temperatures, thus preventing thermal decomposition.

Some of the fire retardants changing the pyrolysis are also active against after-glowing, e.g. several phosphorous products and boric acid. Others do not prevent after-glowing, or may even increase it, e.g. boric salts. Table 4. Examples of substances used for changing the pyrolysis of wood.

Table 5. Chemical mechanisms of fire retardants [32].

1.2. Protecting the surface of wood with isolating layers

The surface of a material can be protected with a layer that delays the temperature rise and reduces the evaporation of pyrolysis gases and the access of oxygen on the surface. These effects can be accomplished using intumescent coatings, i.e. substances that expand strongly when temperature increases. A porous, carbon-rich layer is formed on the surface of wood. This layer is a good thermal insulator and does not burn. Intumescent coatings are usually very effective in inhibiting combustion. Their drawbacks, however, are costs and a tendency to cover the appearance of wood. Most of them are also lacking mechanical properties both initially and especially after fire exposure.

The chemicals used in intumescent fire retardants can be divided to three groups on the basis of their way of action: substances 1) forming char, 2) enhancing intumescence, and 3) enhancing dehydration and esterification. The last-mentioned substances are usually the same as those affecting pyrolysis, that is, phosphates and boron compounds (see Table 4 and Table 5). Substances enhancing intumescence include dicyandiamide, melamine, guanidine and urea. In addition to their intumescence property, it is required that these substances produce incombustible combustion products (e.g. CO2, H2O and NH3). Substances forming char are typically carbon hydrates (e.g. sucrose or starch) or polyhydric alcohols. The application of isolating layers is limited to indoor end uses.

1.3. Changing the thermal properties of wood

The thermal properties of a product, such as density, specific heat and thermal conductivity, have an effect on the ignitability and flame spread.

The easiest way to make wood poorly burnable is to wet it. This means has two physical effects. Firstly, water changes the effective specific heat of wood. Water has a higher specific heat than dry wood, and heating up and evaporating water consumes heat. Secondly, water evaporating from a surface reduces the combustibility of the mixture of air and pyrolysis gases.

Technical solutions in fire retardance of wood are based on adding components with a high thermal inertia and diffusivity to a product. The warming-up of the product is thus delayed: the rate of temperature increase is slower and heat is conducted away from the surface. The most commonly used components are metal layers. Their main drawback is the large amount of metal needed for sufficient effects. As a result, the machineability of the product is deteriorated, and its weight and price are increased.

A Japanese research suggests that the warming-up of a wooden specimen can be slowed down even by combining wood layers of different kinds [33]. The conclusion is based on a computational study for wooden specimens made of springwood and summerwood with varying grain orientation. The thermal conductivity k of summerwood (k = 1.0 W/(mK)) was assumed to be tenfold compared to springwood (k = 0.11 W/(mK)), whereas their density and specific heat were assumed to be the same (540 kg/m3 and 1370 J/(kgK), respectively). In practice, such a high thermal conductivity requires very high moisture of wood. The radiative heat exposure was assumed to be 5 kW/m2. Under this condition, wood does not ignite and different thermal properties cause clear differences in the warming-up of the surface. Due to the assumptions made on the material properties and exposure conditions, these results should be considered very critically. Examples of computational surface temperatures are presented in Figure 3. Even if the analysis is not relevant for the conditions at fire exposures, the same principles may be applied.

Figure 3. Results from a Japanese study of the thermal properties of wood on warming-up of its surface: a) the studied composites, and b) calculated surface temperatures for composites B and C, and a fir specimen with the surface across the layers [33].

1.4. Reducing combustion by diluting pyrolysis gases

The combustion gases evolved during pyrolysis may be diluted by gases released from fire retardants. One example is a fire retardant as e g aluminium hydroxides releasing water vapour at temperatures just below the thermal degradation temperature. Another example is a fire retardant releasing carbon dioxide or another non-combustible gas.

1.5. Reducing combustion by inhibiting the chain reactions of burning

Some fire retardants are active by inhibiting reactions in the gas phase as radical scavengers. Halogenes are the best-known example of such chemicals and used quite a lot in the plastics industry. They may retard also the gas phase combustion of wood products, but are not active in the solid phase and not preventing after-glowing. However, they should be avoided for wood products, mainly due to environmental aspects.

2. Practical techniques for fire retardance of wood

Fire retardant treatments of wood can be divided to three classes: 1) impregnation of wood with a fire retardant using vacuum and overpressure, 2) addition of a fire retardant as a surface treatment, and 3) addition of a fire retardant to a product during its manufacturing process. In addition, some novel methods outside the above-mentioned classes are covered in this chapter.

2.1. Pressure impregnation

For pressure impregnation of wood with fire retardants, pressure treatment equipment enduring both overpressure and vacuum are needed. The size of industrial equipment is very variable, ranging from a few to several dozen of cubic metres.

Pressure impregnation is most often used for fire retardant treatments of timber, but it can be adapted also to wooden boards. In the fire retardant treatment of plywood, for example, pressure impregnation has been applied in two different ways: by impregnating the veneers (especially the surface veneer) separately before gluing or by impregnating the pressed plywood product in one piece.

The impregnation process can be divided to the following phases:

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Vacuum for removing air from the cells of wood.
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Insertion of the fire retardant to the impregnating chamber (in low pressure).
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Overpressure phase during which the fire retardant is forced into the wood.
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Removal of the fire retardant from the impregnating chamber (after removing the overpressure).
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End vacuum reducing the egress of the fire retardant from wood.

Fire retarded wood is usually desiccated after impregnation because many fire retardants are hygroscopic and delay the drying of wood. Drying must be controlled to avoid deformation.

Wood species that are difficult to impregnate can be pre-treated to improve the penetration of the fire retardant. Possible pre-treatments include the mechanical incision or perforation, and for some species also pre-steaming.

The durability of a pressure impregnation treatment is mostly dependent on the properties of the fire retardant used. Also the details of the impregnation process have an influence on the durability. On the basis of the chemicals used, pressure impregnated fire retarded wood products can be divided to three types. The division is based on the end-use conditions of the products, see New service classes for different end use applications

Pressure impregnation is considered as the most reliable way of treating wood products. The challenge for fire retardant pressure impregnation is to find suitable chemicals to use with good durability and minimum negative influence on other wood properties. New innovations may include new chemicals or combinations. Pressure impregnation is a general technology used also for wood modification, see Modification of wood .

2.2. Surface treatments

The main practical difference between pressure impregnation and surface treatment in fire retardant treatments of wood is the depth of penetration of the fire retardant. In pressure impregnated pine, for example, the whole sapwood is usually thoroughly impregnated. The penetration depths in surface treatments are usually of the order of 1 mm or less.

Since ignition and burning are surface processes, surface treatments may prevent ignition and burning just as well as treatments penetrating deeper into the wood. In terms of long-term durability, pressure impregnation is usually a better option than a surface treatment. In certain applications, however, fire retardant treatment with pressure impregnation is impractical, expensive or impossible. Examples of such applications are ready-made or previously constructed objects (walls, doors etc.) and temporary construction. In these cases, fire retardant treatment as a surface treatment can be an appropriate solution.

Fire retardants for surface treatments can be divided to two groups according to their operation: intumescent and non-intumescent coatings. Intumescent coatings are usually varnishes or paints. They form a clearly visible surface on the product. Non-intumescent coatings are substances similar to those used in pressure impregnation. They do not form a film or distinctly change the appearance of a wooden surface.

2.2.1. Intumescent varnishes and paints

Intumescent varnishes and paints are used both to improve the reaction-to-fire performance of surface linings and increase the fire resistance of constructions, especially for steel constructions.

When exposed to high temperatures, intumescent coatings swell and form a heat resistant and insulating cover on the surface of a product. The cover protects wood from flames and heat, and prevents the access of oxygen on the surface.

In principle, intumescent coatings are used in the same way as ordinary varnishes and paints. To obtain adequate fire performance, however, a relatively thick surface layer is usually needed. A typical coating consumption is of the order of 500 g/m2, corresponding to a thickness of a few hundreds of micrometers.

Intumescent coatings, both varnishes and paints, are strongly hygroscopic. This feature makes the coated surfaces very sensitive to humidity. A special top coat has to be used, but the coated product is still to be used only indoors.

Increasing the fire resistance time of wooden constructions is one of the most common applications of intumescent coatings. The advantage of this protection method compared to e.g. gypsum plasterboards is that the grain of wood remains visible which is often desirable for architectural reasons. Intumescent coatings in fire resistance applications have also some drawbacks, for example their sensitivity to humidity and expensiveness compared to gypsum plasterboard. Furthermore, the cover formed as a result of swelling is often fragile. Therefore, it can easily break and drop away, leaving the wooden surface unprotected. However, for fire resistance purposes, the benefits and costs for coatings should be compared to slightly thicker dimensions of the original wood product. They may be worthwhile to use if high reaction-to-fire performance is required.

2.2.2. Non-intumescent coatings

Non-intumescent surface coatings have an effect on pyrolysis mainly through chemical means. Due to their slight swelling ability, however, these substances work partly through the physical phenomena described above.

In order to perform an efficient fire retardant surface treatment for wood, it is important to use chemicals specifically designed for surface treatments. A surface treatment using chemicals intended for pressure impregnation is usually not successful. In the worst case, a surface treatment with a fire retardant of a wrong type can even increase the ignitability and heat release of a wood product.

Transparent surface treatments of wood using fire retardants are not very common. The main reason for this is that achieving a sufficient improvement of fire properties is not straightforward using only a surface treatment. However, efficient fire retardants for surface treatment of wood are available on the market.

2.3. Addition of a fire retardant during manufacturing process

The fire retardant treatment of wood products manufactured by pressing is easy to implement by adding the fire retardant to raw materials before the pressing phase. The most common example of such products is fire retarded particle board.

The amount of fire retardant added to raw materials is easy to control in order to achieve desired fire properties, and the treatment is homogeneous. Thus, the fire performance of e.g. fire retarded particle board can be made high enough for demanding applications. When the fire retardant treatment is integrated to the manufacture of a product, it is relatively easy to tailor the properties of the fire retardant suitable for a specific product.

The drawback of this kind of fire retardant treatments of e.g. particle board is that the properties other than fire performance usually decline when the amount of fire retardant increases. The mechanical strength and surface properties of wooden boards with improved fire performance can therefore be inferior to those of non-fire retarded boards.

2.4. Other fire retardant treatment methods

2.4.1. Nanocomposite systems

Fire performance of plastics can be improved by using nanocomposites made of layered silicates and organic polymers. The most often used silicate is the clay montmorillonite, but other clays as well as natural and manmade micas are also used. The mechanism of fire retardancy of nanocomposites is generally considered to be due to the structure of the char formed during combustion, which enables the char to thermally insulate the polymer and inhibit the formation and escape of volatiles [34].

Nanocomposite fire retardant treatments might be adapted also to wood products. However, few results have been published so far. A major problem in the application of nanocomposite technique on the fire retardance of wood is related to the general principle of nanocomposite fire retardants. In the case of plastics, the most efficient nanocomposite fire retardants have intercalated structure; that is, the nanocomposite consists of either a single monomer or extended polymers sandwiched between the host silicate layers. This results in a well ordered multi-layered structure comprising alternating silicate and polymer layers [34]. This kind of structure can easily be created when plastics and clay are combined in an appropriate process. In the case of wood, however, generating intercalated structure for cellulose molecules and clay in nanometre scale is more difficult.

2.4.2. Boron gas treatment

Boron compounds are known as effective wood protective liquids and fire retardants. The most common way of treatment is the pressure impregnation of timber using water-based boron compounds. An adequate amount of boric acid against decay fungi is typically ca. 2–4 kg/m3, but for an effective fire retardant treatment at least 30–40 kg/m3 of the active component is needed.

An alternative method to pressure impregnation is a treatment based on gaseous boron compounds. The source compound is trimethylborate (TMB) that is in liquid state at room temperature. Due to its low boiling point (68.7 °C) TMB is easily evaporated in an elevated temperature and low pressure. The compound reacts with the water molecules of wood forming boric acid and methanol as a side product. One of the benefits of this patented method is the good penetration of boron gas also to wood species that are difficult to impregnate.

The suitability of the method for fire retardant treatments has not been studied. The treatments have aimed to obtain an adequate amount of boric acid and good penetration to wood for rot-prevention. This goal is best achieved when the moisture content of wood is less than 10 %. With larger moisture contents the penetration of boron gas is crucially deteriorated.

2.4.3. Modification of wood

Wood can be modified either chemically of physically. An example of physical modification is compression of wood. It increases the specific weight and surface hardness of wood, but does not usually change significantly the fire properties of wood. An exception is very high surface density that delays the time to ignition. Thermal modification changes the chemical composition of wood to some degree, resulting in reduction of moisture deformation, decrease of equilibrium moisture content, and improved rot resistance. Heat treatments do not improve the fire performance of wood.

In chemical modification of wood, functional groups can be covalently bonded to the OH groups of hemicellulose and lignin. The resulting changes of properties include reduction of moisture deformation, decrease of equilibrium moisture content, and improved rot resistance. The drawback is the weakening of the mechanical strength or at least the embrittlement of wood. Since the functional groups bonded to OH groups are mainly organic, these treatments usually have no importance in fire retardance of wood. A possible exception might be ureamelamine resin that forms bonds to the cell wall of wood. Melamine compounds are known to have fire retarding properties.

2.4.3.1. A chemoenzymatic method for modifying cellulose materials

A novel method for the modification of cellulose-based materials based upon the high natural affinity of the plant polysaccharide xyloglucan for crystalline cellulose [35] has been developed at the Laboratory of Wood Biotechnology at KTH Biotechnology. The method is generally applicable to a wide range of cellulose materials from regenerated cellulose to cotton fibers and chemical and mechanical wood pulps. This implies that the scope of the method may be further broadened to include the attachment of functional groups, including those with fire retardant properties, to wood materials such as lumber, chips, and sawdust.

Xyloglucan forms part of the dynamic network which comprises the cell wall of a wide range of plants. In this structure, xyloglucan coats and crosslinks cellulose microfibers through numerous hydrogen-bonding interactions as shown in Figure 4 [36]. In essence, xyloglucan has evolved to tightly bind cellulose (thus giving rise to a strong, yet flexible composite structure). This interaction is exploited for cellulose modification.

Figure 5 outlines the general chemo-enzymatic method for tailoring fiber surface chemistry. A modified xyloglucan oligosaccharide bearing a desired functional group (in this case, XGO-FITC) is incorporated into high mass (Mr) xyloglucan polysaccharide (XG) through the catalytic action of the enzyme xyloglucan endotransglycosylase (XET) (Figure 5A). The average length of the modified xyloglucan (XG-FITC) is conveniently controlled by adjusting the parameters of the enzyme reaction (Figure 5B), which can be used to alter the surface density of the functional group. The yellow color of the chromophore fluorescein (from XG-FITC) provides clear evidence of adsorption (Figure 5C). The gentle binding conditions employed (aqueous solution, room temperature, pH < 12) preserve cellulose-cellulose chain interactions, fiber-fiber interactions and, ultimately, material strength properties.

Figure 4. Representation of the cellulose-xyloglucan network in the primary plant cell wall [36].
Figure 5. Xyloglucan-based method for cellulose modification. A. The general method, using xyloglucan (XG), derivatised xyloglucan oligosaccharides (XGO-FITC), and XET enzyme. XGOs are convenient, well-defined starting materials for chemical modification. B. Size exclusion chromatogram demonstrating the enzyme-catalyzed incorporation of XGO-FITC into XG and time-dependent decrease in XG chain length. C. XG-FITC adsorbed on filter paper; the control sample shows that XGO-FITC is too short to bind to cellulose, and it is therefore removed by washing with water.

The method has a broad scope and has been used to introduce a range of functional groups to cellulose [35]. Extension to solid wood materials (e.g., lumber, chips and sawdust) could bring about novel prospects for fire-retarded wood products. In particular, it has recently been shown that this method can be used to anchor polymers directly to cellulose surfaces using a “grafting from” technique, which may allow radical alteration of wood surface properties, e.g. by generating nanocomposites [37].

The extension of this method to include solid wood materials (e.g., lumber, chips and sawdust) could bring about novel prospects to fire retarded wood products.

3. Conclusions on fire retardants for wood

It is relatively easy to obtain an improved fire performance of wood products. Most existing fire retardants are effective in reducing different reaction-to-fire parameters of wood such as ignitability, heat release and flame spread. The highest European and national fire classifications for combustible products can be reached. However, high retention levels are needed compared to ordinary preservation treatments used to protect wood against biological decay. However, fire retardants cannot make wood non-combustible.

Fire retardant treatments for wood can be categorised in several different ways

  1. Mechanisms of action to reduce combustion
  2. Types of active chemicals
  3. Ways to add to wood products
  4. End use applications and requirements
  5. Choice of fire retardants in relation to product and process demands

Mechanisms of action to reduce combustion include

Types of active chemicals include

The chemicals are often based on phosphorus, nitrogen, boron and silica. Combinations may be synergistic. Traditional examples of active chemicals are ammonium phosphates, ammonium sulphate, borax/boric acid and melamine phosphate. New more permanent treatments are needed.

Ways to add fire retardants to wood products include

End use applications and requirements are mainly for

New systems for documentation of the durability of the improved fire performance in different end uses are underway.

Choice of fire retardants in relation to product and process demands depend on several factors. Potential service life problems have to be eliminated. Important factors to consider are

Fire retardants, if correctly applied, provide added value to wood products and extend the market potential of the world's most natural building material.

4. Structural properties

It has been observed in the USA that FR wood (mainly but not exclusively plywood) used as roof sheathing has lost its mechanical strength during service conditions. Several incidents have occurred. Extensive studies have been performed and the main phenomena seem to have been explained [13, 14]. High temperatures in roof structures have initiated a decay process in the wood caused by some types of fire retardants. New ASTM standards to predict the behaviour have been developed [15, 16]. However, the mechanical strength is important only for few applications of FR wood products. In most cases other properties, e.g. durability against weathering, are far more essential.

5. Suggestions for further research

5.1. Chemical modification

Different chemicals should be utilised and chosen among those with a superior performance together with other types of products, e.g. natural and synthetic polymers. Some completely new ideas should also be explored. Some examples are:

Different low-molecular weight compounds may be selected and analysed for possible reaction with functional groups of cellulose and possibility to form copolymers. The most promising ones should be incorporated in depth in wood and subjected to conditions of increased temperature or catalysis to react with the functional groups of cellulose. The chemicals should mainly be applied to solid wood by vacuum pressure impregnation cycles. In a first phase, fire impregnated products should be manufactured in laboratories and different retention levels and time-pressure cycles studied. Later, successful products should be checked by pilot manufacture trials in industrial scale.

5.2. Physical modification

The physical modifications to be studied in the project can include, for instance, combinations of different wood species, methods for higher surface densities, and composites.

If the top layer of a wood product consists of a specific species of wood with a relatively low heat release, the heat release peak is smaller, offering possibilities to improve the reaction-to-fire class of the product. Alternatively, FR treated lamellas can be included in wood products as surface layers. Using this method, the consumption of fire retardant is reduced compared to wood products that are FR treated as a whole.

Ignition can be delayed by introducing a high-density surface layer on a wood product. High pressure laminate, for example, might be used for this purpose.

Composite structures offer a wide variety of different solutions for high fire performance wood products. A thin layer of wood on the surface of a composite product can be used to give a wood-like appearance to a product consisting of other materials [38]. If a product made mainly of wood is desired, a protective layer of non-combustible material can be introduced between a thin wooden surface and a thick solid wood substrate.

A limited study on the effects of different melamine resin coatings on plywood have been performed at VTT. The resin was impregnated either directly into the plywood surface or into a separate ply that was glued on the surface of the plywood. The treatments delayed the ignition of plywood, but had a minor effect on its heat release. However, the method might be worth further studies with e.g. different surface layers.

5.3. Nanocomposites

Fire performance of plastics can be improved by using nanocomposites made of layered silicates and organic polymers. The mechanism of fire retardancy of nanocomposites is generally considered to be due to the structure of the char formed during combustion, which enables the char to thermally insulate the polymer and inhibit the formation and escape of volatiles. Nanocomposite FR treatments might be adapted also to wood products.

5.4. Chemoenzymatic modification

Broadening the chemoenzymatic modification method of cellulose materials described above in A chemoenzymatic method for modifying cellulose materials to include the attachment of functional groups to solid wood materials offers new possibilities for fire retardant treatments of wood products.

The possible extension of the method to include solid wood materials (e.g. lumber, chips and sawdust) can be studied in the following phases:

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selection of suitable reporter groups useful for light and/or electron microscopy analysis, such as FITC and biotin,
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production of the corresponding modified xyloglucans (XGO-FITC, XGO-biotin),
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preliminary surface binding studies,
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development of suitable conditions for the pressure treatment of wood materials with modified xyloglucans,
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microscopy analysis of pressure treated samples to determine the distribution of modified xyloglucans throughout the material,
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development of strategies to incorporate fire retardant treated materials.