ISBN 952-10-2817-3 (pdf)
ISSN 1235-4449
5.4. Biodegradable substances in wood protection
Anna Hyvönen, Petteri Piltonen, Jouko Niinimäki
University of Oulu, Fibre and Particle Engineering Laboratory, Department of Process
and Environmental Engineering, P.O. BOX 4300, 90014 University of Oulu, E-mail:
[email protected] ()
INTRODUCTION
Timber is a sustainable, economical, completely renewable, CO2-neutral organic
material, which requires less energy to process than other construction materials. Wood
is also a strong, aesthetically pleasing, and very durable material when properly
maintained and used. However, under certain conditions of exposure or use —
particularly when it becomes wet — wood may be rapidly decomposed due to organic
decay or natural weathering agents. Decay is caused by the action of fungal microorganisms under adequate moisture and oxygen conditions for an extended period of
time (Sell and Leukens 1971). The various effects of combinations of light, water,
mechanical forces and heat, are covered in the blanket term weathering (Wood
finishing… 1975).
The deterioration of wood is thus caused by a combination of biological,
chemical, and physical processes, with water playing an important role in every case.
The decay and discoloration caused by fungi, and to a lesser extent by bacteria, are
major sources of quality loss in both timber production and the various uses of wood. In
order to ensure a long, useful, and safe life, timber needs protection from the hazards of
fungal decay and weathering.
WOOD/WATER INTERACTION
The wood cell wall is mainly composed of polymers with hydroxyl and other oxygencontaining groups which attract moisture through hydrogen bonding. Hemicelluloses,
the non-crystalline portion of the cellulose, the surfaces of the cellulose crystallites, and
to a small degree lignin are all responsible for moisture uptake in wood. Three forms of
water exist in wood: free water, bound water, and water vapour. Free water or
capillarity water is the liquid water that fills the lumen; the driving force is absorption
by capillary action, which is comparable to the way a sponge soaks up water. These
water molecules are known as free water because they are not bound to the wood.
Bound water is found in the cell wall, where it is adsorbed owing to the attraction of
water molecules to the hydroxyl groups. Water is bound to the hydroxyl groups either
through monomolecular adsorption, or polymolecular adsorption, meaning that one or
more water molecules is bound to a single hydroxyl group (Fig. 1). Bonding is stronger
in monomolecular adsorption than in polymolecular adsorption; thus, it takes more
Anneli Jalkanen & Pekka Nygren (eds.) 2005. Sustainable use of renewable natural
resources — from principles to practices. University of Helsinki Department of Forest
Ecology Publications 34. http://www.honeybee.helsinki.fi/mmtdk/mmeko/sunare
Hyvönen et al.
Fig. 1. Polymolecular and monomolecular bonding of water to the cellulose chain, and
two cellulose chains bound together.
effort to remove the last water molecule from the hydroxyl group. Water vapour is
water in its gaseous phase, which is located in the cell lumina and cavities within the
cell wall. (Siau 1971, Banks 1973)
Because water is adsorbed into the cell wall, the volume of the cell wall increases
nearly proportionally to the volume of water added (Stamm 1964). Swelling increases
until the cell wall is saturated with water (FSP, fibre saturation point; Fig. 2). Any water
available beyond this point remains as free water in the void structure, and does not
cause further swelling. This process is reversible because below the fibre saturation
point wood shrinks as it loses moisture. Wood is anisotropic material, which means that
it shrinks and swells to a different extent in three anatomical directions. Wood shrinks
most in the direction of the annual growth rings (tangentially), about one-half as much
across the rings (radially), and only slightly along the grain (longitudinally) (Wood
Handbook… 1999). Stresses arise in the wood due to moisture gradients between the
surface and the interior. Unbalanced stresses result in surface warping, twisting, and
checking.
Fungi can only cause serious damage to wood when the moisture content is above
the fibre saturation level (26% to 32% of the dry wood weight, depending on the
species). This amount of moisture cannot be acquired as water vapour absorbed from
humid air, however, as it can only exist in green wood or as a result of exposure to
liquid water. The moisture content of wood exposed to atmospheric water vapour only
normally ranges from 8% to 15% (Levi 1973). Wood is protected very effectively from
the deterioration as long as its moisture content is maintained below its fibre saturation
point (FSP).
According to the general rule set out by Cartwright and Findley (1958),
microbiological degradation can only occur if the wood has a moisture content
exceeding 20% of its oven dry weight. Although this is substantially below the
approximate 30% minimum required for fungal decay, a lower moisture content is still
advisable, because this provides a margin of safety in the event that the material does
not dry uniformly (Scheffer 1973). The most widely used and effective means of
protecting wood is to dry it soon after it comes out of the tree, and thereafter to handle it
and put it into service with appropriate precautions to keep it dry. When safe moisture
content cannot be reasonably assured, protection through the treatment with
preservatives, surface coatings or water repellents is usually the logical alternative.
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Biodegradable substances in wood protection
Fig. 2. Different moisture contents of wood (modified from Puunsuojaus 1988).
PREVENTION OF WOOD DETERIORATION
The classical wood preservation is based on the principle of toxicity. Wood
impregnation with biocides (containing e.g. creosote, arsenic, zinc, copper, and
chromium) prevents biological degradation. The European directive on the use of
biocides place restrictions on the number of active substances, above all arsenic, that
can be used in wood preservation, and their field applications (The European
Commission Directive 2003/2/EC). Furthermore, wood treated with common
preservatives is classified as hazardous waste (Ministry of the Environment 1129/2001).
Apart from the risks involved in using such materials for treatments, there is increasing
concern over the problems appearing in the disposal of the timbers after the end of their
commercial lifetime. Systems enhancing the durability of wood should be sustainable
both in production and use. In addition to this, the treated wood products should, at the
end of their life, be suitable for energy production by combustion, composting or for use
as a secondary fibre source by related industries, without presenting any problems of
residual chemicals arising from the treatment.
In addition to toxicity, preventing wood degradation can also be based upon the
idea to interfere in the basic physiological requirements for the growth and development
of micro-organisms, including: favourable temperatures, an adequate supply of oxygen,
at least a certain amount of moisture, suitable food material, and certain essential
growth factors (Stamm 1964). If steps can be taken to create unfavourable conditions
for microbial growth in wood, the risk of attack by wood degrading organisms will be
significantly reduced. For practical purposes, controlling the temperature or removing
the oxygen supply are generally not a good practical means of preventing the
deterioration of wood. More practical and effective way to protect timber is to control
the moisture content of the wood.
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Hyvönen et al.
Fig. 3. Hypothetical swelling over time for water repellency and dimensional stability
treatments (modified from Rowell and Banks 1985).
TREATMENTS TO REDUCE WOOD/WATER INTERACTIONS
Treatment methods
In order to stabilize wood as far as possible, it must be protected from moisture
variations. Various structural and chemical methods can be used to accomplish this, at
least in theory. One widely used structural method involves surface coatings, although
the most successful methods involve the application of water repellents and dimensional
stabilizers.
Rowell and Banks (1985) divided treatments to reduce the tendency of wood to
take up moisture and change its dimensions into two categories: water repellents and
dimensional stabilizers. The effectiveness of water repellency treatment can be
determined as the ability of a treatment to prevent or control the rate of liquid water
uptake, both in the cell wall and the capillaries. In contrast, the effectiveness of
dimensional stabilizing treatment can be determined as its ability to reduce the swelling
and shrinking of wood due to moisture pickup. Since only water that has penetrated into
the cell wall can cause swelling, free liquid water in the capillaries is permissible.
Dimensional stability depends more on the amount than the rate of water uptake since it
defines the equilibrium. Some treatments can reduce both the swelling and the water
uptake rate of the wood. Figure 3 shows typical trends of swelling over time for
untreated and hypothetically treated wood specimens (Rowell and Banks 1985). The
upper curve shows how an untreated specimen quickly takes up water through capillary
action and swells to its maximum extent. The lower curve shows the theoretical impacts
on a treated wood specimen. The reduced water uptake rate indicates increased water
repellency, while the reduced extent of swelling indicates greater stability.
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Biodegradable substances in wood protection
Fig. 4. Different ways of intervention in the wood water relations (modified from Van
Eckeveld 2001).
Figure 4 illustrates the principles of intervention in wood-water interactions. In an
untreated cell wall, the water molecules absorbed into the wood settle between the wood
polymers, forming hydrogen bonds between the hydroxyl groups and between the
individual water molecules.
Surface coatings
The use of surface coatings is based on the formation of a surface film of polymeric
materials, such as varnish, lacquer, or paint. As long as the coating film is perfect and
has no cracks, openings, or other imperfections, water does not penetrate into the wood
or, depending upon the permeability of the film to liquid water or water vapour, at least
any penetration is very slow. However, in practice there is no such thing as a perfect
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Hyvönen et al.
film. Microscopic cracks develop more or less rapidly depending upon the severity of
exposure to weathering. Once a crack has developed, nothing can stop the penetration of
water, which in most cases lifts the surrounding film, widens the cracks, and accelerates
the destruction of the film (Borgin 1961).
The main reason for this destructive process is the phenomenon of preferential
wetting: the affinity of water for wood is much higher than the affinity between wood
and the coating film. As a result of this preferential wetting, the stronger wood-water
hydrogen bonding displaces weakly bonded film deposits from the wood surface. This
type of process depends to some extent on the mechanical strength of the film, but it
cannot be prevented if the wood is exposed to water for a sufficiently long time (Borgin
1961).
Water repellents
The water uptake rate can be considerably reduced either by providing a water barrier,
or by rendering the wood hydrophobic, although very few materials are truly
hydrophobic (Borgin and Corbett 1970). Depending on the amounts used, water
repellents applied to wood fill in the cell lumina or are deposited on the external and to
some extent on the internal pore surfaces (Fig. 4) and impart the hydrophobic properties
to the surface. Hence, the water cannot spontaneously penetrate the wood pores through
capillary action and the water absorption rate is thus limited (Banks and Voulgaridis
1980). However, it is not possible to make the cell wall inert to water without wood
modification. Over time, wood treated with water repellent and exposed to water swells
to the same extent as untreated wood. Although water repellents do not stop all water
absorption, they are an excellent treatment for wood used outdoors, because they inhibit
the absorption of liquid water during rain, yet allow the wood to dry after rain (Wood
Handbook… 1999). The reduction in average moisture content and the period of time
when the wood is wet enough to allow attack by micro-organisms slow down the rate of
fungal attack (Stamm 1964, Feist and Mraz 1978). Bacterial attack is favoured by the
presence of free water, and a certain amount of free water is necessary for optimal
fungal growth. (Rowell and Banks 1985)
An idealized model of water-repellent-treated wood is an envelope of cells evenly
coated with a hydrophobic layer, surrounding an untreated core. Because the surfaces of
cells within the treated zone are hydrophobic, liquid water is unable to penetrate unless
an external pressure greater than the capillary pressure is applied (Rowell and Banks
1985). Theoretically, a coherent monomolecular layer should be sufficient to induce this
effect (Razzaque 1982). Provided that the hydrophobic layer remains intact, deep
penetration of wood with the hydrophobic deposit is unnecessary. In practice, it has
been noticed that deeper or complete impregnation leads to a more durable effect
(Borgin 1965). This is probably due to the fact that deposits in immediate contact with
water are gradually degraded. If deep impregnation is achieved, fresh undegraded
deposits are continually exposed offering extended protection (Razzaque 1982).
Water repellents are usually non-chemically bonded, complex blends of various
materials, such as waxes, oils, natural or synthetic resins, fungicides or insecticides, and
solvents (Borgin and Corbett 1970, Feist and Mraz 1978, Razzaque 1982). They are
applied to wood, usually by immersion or vacuum impregnation, in solutions of organic
solvents (Rowell and Banks 1985). These water repellent substances seem to be bonded
to the cell wall only by relatively weak Van der Wall forces (Razzaque 1982). Many of
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Biodegradable substances in wood protection
the developed water repellents, like the classical wood preservatives, have the drawback
of being detrimental to the environment.
Several researchers (e.g. Feist and Mraz 1978, Banks and Voulgaridis 1980) have
shown that treatment of wood with these kinds of non-chemically bonded water
repellents gives significant control of water uptake for a reasonable period of time.
Eventual loss of water repellent effectiveness may be associated to the failure of the
bond between the cell wall and the deposit, resulting largely from the degradation of the
wood surface (Banks and Voulgaridis 1980). The phenomenon of preferential wetting,
as described above, may also occur in wood treated with water repellent. When the
water repellent material is exposed to water for a sufficiently long time, the hydrophobic
substances are displaced by the water, thus diminishing the performance of the
treatment. The weak Van der Waals wood-deposit bond is then replaced by a stronger
wood-water hydrogen bond (Banks 1973, Razzaque 1982).
Dimensional stabilizers
External and internal coatings, while effective in some cases as water repellent
treatments, give only low dimensional stability. There are many ways to control
dimensional changes in wood, ranging from species and geometric selections, through
cross lamination and reducing hygroscopicity, to various chemical methods (Rowell and
Banks 1985).
A treatment that reduces the tendency of wood to take up water may result in a
reduction in the extent of water uptake, and thus in a reduction in shrinking and
swelling. Optimal treatments of this type would remove the hydroxyl groups on the cell
wall polymers and, thus, remove the sites for hydrogen bonding to water (Fig. 4).
Removal of hydroxyl groups is theoretically possible through reductive reactions;
however, the wood would be destroyed (Rowell and Banks 1985).
In the chemical modification process, timber is treated with a chemical that
produces changes within its cellular structure (The modification of… 2003). These
changes may be due to new chemical bonding within the cellular structure (the direct
method), or the blocking of chemical and physical processes by a non-bonded material
(the indirect method). The hydroxyl groups play the leading role in many chemical
modification reactions. The chemical added into the wood undergoes a chemical
reaction with a hydroxyl group within the wood structure. In this case wood reacts as an
alcohol. The result is a strong covalent bond (similar to the bonds holding the atoms in
the wood together), which is fairly stable. This produces a new form within the wood
structure, and alters the properties of the original material. Modification may lead to
reduction of OH-groups, cross-linking and/or undesired cleavage of the chains. The
decrease in the number of accessible OH-groups leads to a limited interaction with
water, and improved dimensional stability.
Many chemicals have been used to modify wood. The main reaction types are:
chemical cross-linking, where the structural units of the wood cell wall are chemically
bound together; polymerizing addition, where chemicals react with a hydroxyl group
and then polymerize; and single-site addition, where chemicals react with a single
hydroxyl group (Rowell and Banks 1985, The modification of… 2003). Acetylation is
one of the most thoroughly examined chemical wood modification methods. It results in
the substitution of the wood hydroxyl groups by acetyl groups. As a result, the wood is
less hygroscopic, and the enzyme systems of fungi do not identify the wood as a food
source.
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Hyvönen et al.
Instead of using a chemical to produce a change in the wood structure, it is also
possible to use heat to affect changes. Heat treatment reduces the hygroscopicity of
wood. These changes are dependent on the temperature and length of the treatment. The
mechanism imparting a reduction in hygroscopicity is the thermal degradation of the
hemicellulose components of the wood cell wall (Stamm 1964, Viitaniemi and Jämsä
1996).
ENVIRONMENTALLY
WOOD PROTECTION
FRIENDLY,
BIODEGRADABLE
SUBSTANCES
IN
Biodegradable substances are often associated with environmentally friendly products,
which can be broken down by natural processes into more basic, innocuous
components. Such products are usually broken down by bacteria, fungi, or other simple
organisms. If substances are very toxic to those organisms, the biodegradation is not
possible. By this definition, most of the chemicals are ultimately biodegradable, but the
rate of breakdown varies considerably. However, the rate of biodegradation is not as
important as what the product breaks down into. The ideal final products of any
complex would be carbon dioxide (CO2) and water (H2O), but where many more
complex chemicals are concerned this is difficult. For example, the banned, hazardous,
toxic pesticide DDT is biodegradable, although only rather slowly. A more serious
problem is that its breakdown products DDD and DDE are even more toxic than the
original DDT. So, even though a product may be biodegradable, it may not be
environmentally friendly. (Toxicological profile… 2002, Environmental Handbook…
2003) In this context, the term thoroughly biodegradable substance means a substance
which breaks down completely, and whose breakdown products are not toxic.
As mentioned above, many water repellents as well as the classical wood
preservatives have the drawback of being detrimental to the environment. Increased
environmental awareness in recent years, and the consequent spread of policies
favouring the use of renewable resources and environmentally friendly chemicals have
led to increased interest in “non-biocidal”, environmentally friendlier methods in wood
protection. The term “non-biocidal” includes natural oils, waxes, silicones, resins,
polymers, chemical modification, and heat treatments (Militz 2001).
Environmentally friendlier water repellents like extractives from trees and natural
resins have been successfully tested in the laboratory (Voulgaridis 1993, 2001, Passialis
and Voulgaridis 1999, Var and Öktem 1999, Voulgaridis and Passialis 1999). Water
repellency and dimensional stability of wood treated with natural oils has also been
studied (Borgin 1961, Borgin and Corbett 1970, Natural resin report 1999, Paajanen et
al. 1999, Van Eckeveld 2001, Van Eckeveld et al. 2001a, 2001b), as well as the
biological efficacy of these substances (Jermer et al. 1993, Sailer et al. 1998, Natural
resin report 1999, Paajanen et al. 1999, Ritschkoff et al. 1999, Van Acker et al. 1999,
Sailer and Rapp 2001). Natural oils appear to be capable of preventing water uptake by
wood and their chemical and physical composition is promising (Sailer et al. 2000).
Additionally, unsaturated oils (such as drying oils) can oxidize when exposed to
atmospheric oxygen (Porter et al. 1981, Porter and Wujek 1984), which results in a
more protective layer on the wood surface.
One of these natural oils is tall oil, which is a by-product of the pulp and paper
industry. The dark-coloured crude tall oil (CTO) is not composed of pure triglycerides,
like other vegetable oils, but is rather a mixture of fatty acids, rosin acids, and
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Biodegradable substances in wood protection
unsaponifiable substances like sterols, waxes, hydrocarbons. The possible use of tall oil
as a wood protection agent has already been studied (Jermer et al. 1993, Paajanen et al.
1999, Van Eckeveld 2001, Van Eckeveld et al. 2001a, 2001b, Paajanen and Ritschkoff
2002, Hyvönen et al. 2005). Treatments with environmentally friendly, thoroughly
biodegradable tall oil are known to greatly reduce the capillary water uptake of pine
sapwood (Van Eckeveld 2001, Hyvönen et al. 2005). Jermer et al. (1993) tested the
effect of tall oil derivatives against biological degradation in comparison to
preservatives currently in use. In the field test, two tall oil derivatives were almost as
effective against decay as CCA and creosote. However, the retentions applied to the
wood were extremely high. This increases mass of treated wood, and wood application
may consequently become impractical and uneconomical, e.g. because of higher
transport costs. These high retention levels also affect the oil stability within the wood.
Due to high retention, oxidation slows down, and oil tends to be exuded from the wood.
Problems related to high oil retention levels in the wood can be solved with the
process technique. Retention level can be controlled by changing the treatment process
parameters (duration, pressure, temperature) or liquid properties (viscosity,
concentration). Hyvönen et al. (2005) chose the dilution of tall oil as the retention
control method. Instead of the most commonly used organic solvents, water was used as
a thinner, due to environmental concerns, cost and safety. High water repellency
efficiency was reached with considerably lower retention levels. The total amount of oil
needed could be even halved. Unsaturated natural oils can oxidize when exposed to
atmospheric oxygen, which results in a more protecting layer at the surface. Because of
the lack of oxygen the polymerisation does not occur inside the wood. The
unpolymerized oil exudes from the wood over time forming pitch-like surface. Oil
exudation can be prevented by oil modification, in which oil oxidation and
polymerisation are improved especially under predominant conditions within the wood.
The addition of metal catalysts has proven to be promising method to prevent oil
exudation (Piltonen et al., unpubl. data). The effectiveness of a water repellent treatment
is likely to depend on both the amount of deposit and its precise location within the
treated wood (Razzaque 1982). Therefore, the effectiveness of the treatment with
smaller amounts of oil may be based more on the location of the oil within the wood,
and not the amounts of oil and deposit (see water repellent treatments in Fig. 4).
The main routes for liquid penetration into wood are provided by capillaries;
namely the lumina of the cells and the cell wall openings (pits) interconnecting them. In
the heartwood, moisture absorption is very low, since the pits are permanently aspirated
(closed), the extractives are deposited on the pit membranes, and the pore size is
smaller. Sapwood pits also aspirate during the drying of wood, but not permanently. If
the tall oil deposit could be positioned on the membranes of aspirated sapwood pits
(Fig. 4), they would remain in aspirated state due to a “glueing” effect and the water
uptake would be considerably reduced. A notable factor is that tall oil consists of the
same components as heartwood extractives, which are the principal source of decay
resistance in the heartwood. Due to this high natural durability and the low moisture
absorption rates that make heartwood difficult to impregnate, wood protection usually
aims to enhance the durability of sapwood. Using this method (Hyvönen et al 2005,
Piltonen et al., unpubl. data) increases the durability of sapwood, and also makes the
wood more homogenous, while also crucially meaning that the wood can be burned
after the end of its useful life. In simple terms, the aim of this method is to artificially
turn sapwood into heartwood.
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Hyvönen et al.
CONCLUDING REMARKS
Wood preservation is a way to increase the biological durability of wood by adding
chemicals with a biocidal effect. The effects of such treatments are clearly dependent on
concentrations. At specific biocide levels, fungi are either killed, or their ability to
obtain nutrients through their enzymatic functioning is inhibited. Non-biocidal
treatments act quite differently. Mechanisms such as moisture exclusion or
hydrophobation retard or stop colonisation by fungi. Because these treatments are only
effective above a certain level or when a certain degree of substrate modification is
reached (heat treatment), it is seldom possible to establish a dose response curve (Van
Acker and Stevens 2000). Thus, it is clearly more important to estimate the service life
in different hazard classes than to look for total efficacy. Most non-biocidal treatments
are linked to biological hazard class 3 (Van Acker and Stevens 2000), where the general
service situation is above ground, and exposed to outdoor conditions, and the general
moisture level is frequently above 20% (EN 335-1/2/3).
Environmentally friendly, thoroughly biodegradable, natural oily substances are
highly suitable for wood protection when the aim is to prevent moisture uptake by the
wood. With these substances wood moisture content can be kept below 20%, and the
requirements for wood degradation by fungi are not met. However, these
environmentally-friendly oily substances cannot compete with biocides in terms of the
biological durability of wood that is in contact with the ground or water (hazard class
4), where moisture levels in the wood are permanently over 20%.
The preservatives commonly used today can achieve a 50-year service life.
However, the importance of a long service life should be questioned. Does the life
expectancy of garden furniture have to be so long, for instance? Wood will eventually
become grey in any case. Could a 10-year service life be long enough? Even if the life
expectancy of impregnated railway sleepers is 50 years, mechanical wear breaks them
down much earlier than this. Poles become hollow during ageing, since the biocideimpregnated sapwood is undamaged, but the naturally durable, hard to impregnate
heartwood is totally destroyed. An additional consideration is that wood treated with
common preservatives is classified as hazardous waste. Timber could therefore be
protected with environmentally friendly, thoroughly biodegradable substances, in order
to meet the requirements of various end use situations, so that the treated wood is “fit
for the purpose”, and to ensure that it has a useful and safe life cycle, including
production, use and eventual disposal.
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