Open Journal of Forestry
2013. Vol.3, No.2, 57-61
Published Online April 2013 in SciRes (http://www.scirp.org/journal/ojf) http://dx.doi.org/10.4236/ojf.2013.32009
A Wood Preservative Based on Commercial Silica
Nanodispersions and Boric Acid against Fungal Decay
through Laboratory and Field Tests
Sabrina Palanti*, Elisabetta Feci
CNR IVALSA, Istituto per la Valorizzazione del Legno e delle Specie Arboree, Sesto Fiorentino, Italy
Received December 18th, 2012; re vi sed Fe bruary 14th, 2013; accepted March 6th, 2013
Copyright © 2013 Sabrina Palanti, Elisabetta Feci. This is an open access article distributed under the Creative
Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
The paper is based on the development of a wood preservative without metal salts to be used in use
classes 3 and 4 (EN 335), eco-friendly and harmless to humans and animals. Boric acid was used as a
biocide, due to its effectiveness against fungi and insects. It is also known to be easily leached from wood
exposed to weather action. Colloidal silica was therefore added in the formulations to guarantee the fixa-
tion of boric acid to wood. The different formulations were tested for the protective efficacy against decay
fungi through laboratory tests (EN 113) and field trials (EN 252). The results were promising, especially
those concerning boron fixation and efficacy against decay fungi through laboratory tests, where some
formulations and retentions gave a durability class 1 (very durable) according to EN 350-1. The fourth
evaluation, after 50 months of field trials showed only a slight difference between the treated samples and
controls.
Keywords: Basidiomycetes; Boric Acid; Colloidal Silica; Wood Durability; Field Test
Introduction
Protectin g wood from biotic decay is a necessity and also an
obligation for producers of wooden artefacts. The wood pre-
servatives have to ensure the resistance of wood to biotic agents
for a long time in service conditions, so it is important that
active ingredients are permanently fixed to the walls of wood
cells.
Currently the most used products for use classes 3 and 4 (EN
335) are formulations based on copper salts and other co-fo-
rmulates as azoles, organic compounds and boron (Forest Prod-
ucts Laboratory, 2010). The disadvantage of the se compounds is
due to utilization of metal compounds and to the facility of
leaching them away when utilized in outdoor conditions.
Boric acid is a substance effective against fungi and insects
xylophages and is also non-toxic to humans and the environ-
ment if used under the limit indicated in Biocides Directive
98/8/EC. Nevertheless, this substance alone cannot be applied
to wood exposed to the atmospheric agents because it is easily
washed away into the ground. The recent classification of boric
acid and borates as toxic to the human reproduction, category
1B (EU Regulation 790/2009) means that they fall within the
scope of the measures of risk reduction provided by the
REACH regulation (licensing procedures and restrictions). The
justification for the use of boric acid is linked to the fact that
the repro-toxic effect occurs at a threshold concentration of
5.5%. Below this threshold this compound is considered safe.
There are some compounds that form complexes with boric
acid and thereby lock it when introduced into the wood. Their
employment in the industry is hampered by some technical
difficulties during the preparation.
In order to develop an environmentally friendly preservative,
this research work has had as its aim the development of wood
preservative formulations based on boric acid and colloidal
silica nanodispersions with different weight percentagesin SiO2
and size part ic les.
Silicic acid in combination with boric acid has been shown to
be effective against wood decay fungi and termites, when ap-
plied to wood with vacuum-pressure impregnation cycle (Ya-
maguchi, 2001, 2003, 2005).
The theorized mechanism for the attachment of boron is due
to the atomic structure of the colloidal silica performed by the
compound SiO4. This compound has tetrahedral structure with
the silicon atom in the centre. Each molecule of silica is com-
bined with others so that each silicon atom has available three
oxygen atoms of the tetrahedron. It is assumed that the boron
atoms, which have a structure similar to Si, may replace one
atom into the tetrahedron (Ueda, 1993). Once heated, the mix-
ture, boric acid and colloidal silica, takes a glassy appearance,
blocking the vessels of wood, with flame-retardant property. In
this case, the attachment of boron is due to the fact that boron
at o ms ar e replaced with those of silicon i n the c o lloida l solution,
once introduced into the wood by impregnation with vacuum-
pressure cycle and hence is permanently blocked by the gelling
of colloidal silica after heating wood.
*Corresponding author. In this research boric acid was added as the active biocidea-

S. PALANTI, E. FECI
gainst fungi.
The evaluations of the experimental formulations were there-
fore concerned with:
 Antifungal efficacy through laboratory tests;
 Fixation of boric acid into the wood;
 Antifungal efficacy through in field tests.
Material and Methods
Treatments Based on Colloidal Silica and Boric Acid
The treatments were based on boric acid and nanodispersions
of colloidal silica.
Some commercial silica dispersions (Eka Chemicals AB and
Nissan Chemical Industrex) of different weight percentage and
different silica nanoparticles size were utilized.
EN 113 wood blocks and EN 252 stakes were treated with a
formulation based on 20.7% of a colloidal dispersion of silica
particles in slightly alkaline water, Bindzil® CC 30 (Eka
Chemicals AB) and 2.3% boric acid, technical grade.
Previous experiments to evaluate the fixation of boric acid
into the wood were performed according to EN 84 on wood
blocks treated with a formulation based on 20.5% colloidal
silica (Snowtex C, Nissan Chemical) and 2.59% boric acid.
The treatment emulsions were impregnated into the wood
according to the following cycle: vacuum 0.7 kPa for 30 min-
utes, pressure 200 kPa for 5 minutes, 400 kPa for 5 minutes,
600 kPa for 5 minutes and 700 kPa for 30 minutes.
Retentions were calculated as amount of dry residual ex-
pressed in kilograms per cubic meter of wood.
Wood Species
The wood samples were derived from Scots pine (Pinus syl-
vestries L.) sapwood .
Laboratory Decay Test (EN 113: 1996)
Laboratory decay test were carried out in accordance with
European standard EN 113, 1996.
Treated wood samples, dimensions: (50 ± 0.5) × (25 ± 0.5) ×
(15 ± 0.5) mm3, were exposed to fungal attack for 16 weeks in
a conditioning room (22 ± 2˚C, 70 ± 5% RH). The test requires
the inclusion of three certified fungi, one obligatory—the brown
rot fungus Coniophora puteana (Schumacher ex Fries) Karsten
(strain BAM Ebw.15) and the two additional brown rot fungi
Poria placenta (Fries) Cooke sensu J. Eriksson (strain FPRL280)
and Gloeophyllum trabeum (Persoon ex Fries) Murrill (strain
BAM ebw.109). For each fungus, ten treated samples were test-
ed. A treated sample was placed with an untreated reference
one in a Kolle flask, containing one of the above described brown
rot fungus grown on 20 ml of 4% malt and 2.5% agar medium.
A modification of the EN 113 (1996), that consists in the divi-
sion into two parts of the mycelium inside the Kolle flask be-
fore the exposure to wood samples (Figure 1), so that the fun-
gal strain virulence (tested on control samples) was prevented
from inhibition by the preservative washed away from the
treated samples. In fact, the inhibition of the fungus by boric
ac id was he d away from treated sample against contro l s pec ime ns ,
as resulted from preliminary tests, did not allow for demonstrat-
ing the effectiveness of the product, according to the criteria of
the standard. This effect due to the leaching of boron from
non-leached treated samples has been observed in preliminary
Figure 1.
Mycelium within the Kolle flask, divid e d into portions.
non-published experiments of the authors.
Furthermore, six treated wood blocks (called in EN 113
(1996) check blocks) were put in contact with culture medium
without fungal strains for the determination of the correction
coefficient, which was used to calculate the possible mass loss
due to factors different from fungal decay. Another set of six
untreated wood specimens for each tested fungus was used to
check for virulence of fungus.
The resistance against fungi was evaluated through the
measurement of the percent mass loss of wood, which was
calculated for each individual block as the difference between
the dry mass before the impregnation process and after the fun-
gal exposure, corrected by correction coefficient. A minimum
of 20% mass loss on control (untreated) samples was required
for the test to be valid.
Leaching Tests (EN 84:1997)
Treated wood samples, dimensions (5 ± 0.1) × (15 ± 0.1) ×
(30 ± 0.5) mm3, were subjected to leaching according to EN 84,
1997. This standard describes an accelerated ageing test of
specimens treated with a preservative for simulating the service
conditions, in particular exposure to rainfall. The cycle con-
sisted of an initial 4 kPa vacuum-atmospheric pressure cycle
with distilled water. Then, wood specimens were maintained in
water (ratio of water to wood 5:1) for 14 days with 9 water
changes, and then conditioned to constant mass.
The leaching waters were subjected to analysis by ICP (In-
ductive Coupled Plasma) for the determination of boron.
The percentage of residual boric acid in the wood was calcu-
lated by the difference with the amount obtained from the
analysis on initial formulation, where its content i s 100%.
Field Test (EN 252: 1989)
In ground test was performed in accordance with EN 252,
1989. This standard describes a method for the determination of
efficacy of preservative applied by impregnation plant in
ground condition. The method is also utilized for the determi-
nation of natural durability of wood in ground condition (EN
350, 1994).
The test sites were respectively situated in Cesa, Arezzo (lat
43.3˚N; long 11.8˚E), and Follonica, Grosseto (lat 42.9˚N; long
58

S. PALANTI, E. FECI
10.8˚E), Italy.
Duration of the test is a minimum 5-years period or until the
stakes fail. The wood samples were placed in situ in October
2007 and in this paper the fourth evaluation was reported, after
respectively 54 and 50 months for Cesa and Follonica.
Stakes, dimensions (500 ± 1) × (50 ± 3) × (25 ± 0.3) mm3,
were cut and conditioned before being impregnated and condi-
tioned to constant mass.
The stakes were placed vertically in the soil leaving half of
length exposed. Untreated stakes were also buried up to half
their length in the soil test site.
Ten replicates for treatment and reference controls were util-
ized.
The fungal decay was evaluated in the area in ground contact.
Annual inspections were carried out by giving a light blow to
the upper part of each stake followed by removal from the
ground. Surface examination was performed with an awl. The
fungal decay was evaluated on the basis of the depth of fungal
softening and extension area according to a specific rating sys-
tem: 0 sound, 1 slight attack, 2 moderate attack, 3 severe attack
and 4 failure (stake breaks in the ground after blowing). The
evaluation was performed in accordance with guideline for EN
252 of Nordic Wood Preservation Council (Borsholt & Hen-
riksen, 1992). After evaluation, each stake was re-installed in
the original position.
As preservative reference, ten stakes treated with Impralith
KDS (Rütger Organics GmbH), with a retention use class 4
(EN 599-1, 2009), were used. As control reference ten untreat-
ed were utilized.
Conferred durability classes were calculated according to EN
350-1, 1994.
Results
Retention
The impregnation of EN 113 wood blocks gave the follow-
ings results: 201.37 ± 11.54 kg/m3 (N = 36). Impregnation of
wood mini-blocks for the leaching test gave as result 194.48 ±
4.03 kg/m3 (N = 10).
The impregnation of EN 252 wood stakesgave the followings
results: 55.6 ± 15.50 kg/m3 (N = 20). The stakes impregnated
with reference preservative Impralith KDS had an average re-
tention of 8.65 ± 1.26 kg/m3 (N = 25).
Decay Tests
Virulence of fungal strains confirmed the validity of the test
(mass loss of untreated samples more than 20%).
Results of antifungal efficacy were expressed as average
mass loss and standard deviation of treated samples with the
formulation tested (Table 1). All reference wood blocks lost
more than 20% mass, the minimum value required for the va-
lidity of the test according to standard EN 113.
The mean mass loss due to the check block was 16.11% ±
3.23%, this results was due to the leaching of boron from wood
blocks into the medium.
Leaching Test
Results of leaching are reported in Figure 2, where the per-
centages of residual boric acid for the formulation impregnated
into the wood are graphed. Number 0 corresponds to the
Figure 2.
Residual percentage of boric acid into the wood with respect
to the initial concentration. The starting point (Water change
number 0) represents the initial formulation and the corre-
sponding boric acid content is 1 00 %.
Table 1.
EN 113, decay test results; sd = standard deviation, N = number of
samples.
Fungus Wood blocks treated % mass loss average (sd)
N = 10
Wood block s refere nc e s
average % mass loss
N = 10
C. puteana6.87 (1.42) 59.16 (3.92)
P. placenta6.65 (0.66) 48.25 (5.16)
G. trabeum6.27 (1.02) 30.87 (5.45)
amount determined by ICP analysis in the impregnation solu-
tion, which was assumed to be entirely and homogenously
penetrated into wood. This assumption can be considered real-
istic because at the concentration of 2.3% the boric acid is well
solved into the water of silica dispersion.
The ICP boron analysis has shown a decreasing presence of
the active ingredient in consecutively obtained leaching waters.
The loss of boric acid was 23.0%. Compared to the amount
present in the initial solution, boric acid decreased from 2.59%
to 1.99%.
Field Test (EN 252)
Annual evaluations on stakes from the two experimental
fields are shown in Table 2.
For each evaluation, the average decay values of the stakes
treated with colloidal silica-boric acid formulation, the stakes
treated with the reference wood preservative and the untreated
stakes are reported.
Discussion
The different average retentions obtained for each test sam-
ples group were probably due to different size of wood speci-
mens. It has been remarked that the different dimensions of
specimens utilized in different standardized tests could have
been influenced the final retention of formulations tested: the
smaller the wood samples, the higher were the retentions, (e.g.
EN 113, EN 84); the lower performance was obtained with the
EN 252 stakes.
The expression of EN 113 decay test results as conferred du-
rability of wood according to EN 350-1 (1994) led to durabil-
ity class 1—very durable, with an expected service life exceed-
ing 25 years (Eaton and Hale, 1993).
In Table 1, the masses losses indicate that the treatment gave

S. PALANTI, E. FECI
60
Table 2.
EN 252 Decay results.
15th month by
installation 29th month by installation 40th m onth by installation 54th m onth by installation
Place: Cesa
Average decay
CC30 colloi dal silica and boric acid 0.33 1.33 2.67 3.27
Impralith KDS 0.15 0.69 1.46 1.46
Untreated control 0.60 2.40 2.70 3.00
Place: Follonica 13th month by
installation 24th month by installation 36th m onth by installation 50th m onth by installation
CC30 colloi dal silica and boric acid 0.30 2.30 3.60 4.00
Impralith KDS 0 0 0.30 0.70
Untreated control 2.5 4 - 3.8 (the 2nd series)
protection against fungi with respect to untreated wood. Never
theless, this preservative was not efficacy in accordance with
the criteria of validity of EN 113 (1996), where is stated that a
wood preservative is efficacy when its mass loss is below than
3%.
The amount of boric acid leached was probably an exceeding
part that did not participate in the complex formation and con-
sequently did not remain fixed in the wood. If it is assumed that
after the impregnation the concentration of boric acid into the
wood was the same as in the initial formulation, the fixation of
boric acid was more than 75%. These results support the theory
on the interaction between silica and boric acid and dem onst rate
the ability of silica to block the active ingredient in the wood.
Evaluation of EN 252 in two sites gave very similar results
with the retention tested 55.6 kg/m3.
In Cesa, an agricultural soil near Arezzo, during the fourth
evaluation, the colloidal silica-boric acid treated stakes had an
higher decay (3.27) than the untreated control stakes (3.00).
During the first two evaluations there was a gap between the
two sets of stakes, infact the decay grade were respectively 1.33
and 2.40 for silica-boric acid and untreated stakes, suggesting
the treatment was better than untreated wood. During the third
year this gap was plugged, the two series scoring the same
grade of decay (2.67 and 2.70).
On the contrary, in Follonica, an agricultural soil close to the
coast, the higher decay grade was reached by control stakes
after 50 months from beginning and another series of control
stakes has been installed. At the fourth evaluation (after 50
months) the decay grade of silica-boric acid was 4.00. In this
case also there was not a substantial difference between treated
and untreated specimens, suggesting the treatment is no better
than untreated controls.
In both sites the copper salt wood preservative gave better
results, reaching respectively 1.46 in Cesa and 0.70 in Follo-
nica.
The fungi, principally white rots, found in Follonica, resulted
more aggressive versus untreated and colloidal silica-boric
acidtreated stakes with respect to those found in Cesa.
Conclusion
The formulations colloidal silica-boric acid tested in this work
have been very successful at retention 201 kg/m3 and 194 kg/m3
respectively in terms of resistance to fungal decay through
laboratory testand with regard to fixation into the timber.
The results obtained in EN 252 field test with the lower re-
tention, 55 kg/m3, support the idea that this retention value
could be used in use class 3, not in contact with the ground,
because this service condition is considered to severe, reaching
a very high decay grade only after 50 - 54 months.
Acknowledgements
The authors thank the Regional Agency for Agriculture De-
veloping and Innovation (ARSIA), TLF srl, Arezzo that sup-
ported this research and Mrs. Anna Maria Torniai from CNR
Trees and Timber Institute who helped with fungi cultures.
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