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IRG/WP 08-40395
THE INTERNATIONAL RESEARCH GROUP ON WOOD PROTECTION
Section 4 Processes and properties
Feasibility study on three furfurylated non-durable tropical wood
species evaluated for resistance to brown, white and soft rot fungi
Thomas Mark Venås
Danish Technological Institute
Timber & Textile
Gregersensvej
DK-2630 Taastrup, Denmark
Andrew H. H. Wong
Universiti Malaysia Sarawak
Faculty of Resource Science and Technology
94300 Kota Samarahan
Sarawak, Malaysia
Paper prepared for the 39th Annual Meeting
Istanbul, Turkey
25-29 May 2008
IRG SECRETARIAT
Box 5609
SE-114 86 Stockholm
Sweden
www.irg-wp.com
2
Feasibility study on three furfurylated non-durable tropical wood
species evaluated for resistance to brown, white and soft rot fungi
Thomas Mark Venås1 and Andrew H. H. Wong2
1Danish Technological Institute, Timber & Textile, Gregersensvej, DK-2630 Taastrup, Denmark,
thomas.venaas@teknologisk.dk
2Universiti Malaysia Sarawak, FRST, 93400 Kota Samarahan, Sarawak, Malaysia,
awong.unimas@gmail.com
ABSTRACT
Furfurylation can protect non-durable wood species against biological degradation, but the
method used today cannot fully protect the heartwood of Scots pine due to insufficient
penetration. In order to test alternative wood substrates for furfurylation, three Malaysian grown
wood species (Kelempayan, Rubberwood and Sena) were furfurylated and subjected to soil
block decay testing. Their performance was compared to furfurylated Scots pine and furfurylated
Beech modified using the same process. In addition, treatment characteristics were evaluated.
One of the species tested, Kelempayan, seems to be a promising substrate for furfurylation.
Kelempayan is easy to impregnate in both sap- and heartwood, and a 50% higher weight gain
was reached using equivalent amounts of impregnation solution compared to Scots pine. Sena,
Rubberwood and Beech returned weight gains 40-60% lower than Scots pine. Decay protection
was largely comparable at equivalent weight percent gains for all wood species tested, although
differences appeared. Generally, a weight gain of approximately 25% by furfurylation seems to
offer good protection in the chosen soil block test.
Keywords: Kelempayan, Sena, Rubberwood, furfurylation substrate, decay resistance
1. INTRODUCTION
Modification of solid wood with an initiated furfuryl alcohol monomer that is subsequently cured
to yield an inert polymer resin inside the cell wall structure, has for a long time been known to
provide changes in wood properties. Increased durability towards acids and alkali was one of the
first fields of interest (Goldstein 1955), but also improved mechanical properties (Goldstein and
Dreher 1960), dimensional stability (Stamm 1964) and thereby improved biological durability
was found some forty to fifty years ago in several studies. A lot of effort was put into
development of a stable, low viscosity impregnation solution which could penetrate evenly into
the wood cell wall (Goldstein and Dreher 1960). A range of acidic catalysts was tested, but
ZnCl2 was superior concerning storage life of the mixed solution (Goldstein and Dreher 1960)
and because of this - combined with a reasonable polymer yield - it was the preferred choice.
This metal salt however had a negative influence on the long term strength properties of
cellulose (Schneider 1995). Moreover, the distribution of polymeric furfuryl alcohol in the wood
cell wall was uneven in larger samples (Schneider 2002b). In the 1990’s two research groups
simultaneously developed new and better adapted catalytic systems for furfurylation which later
on made it possible to market furfurylated wood (Lande et al. 2004b).
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The main substrate for furfurylation is Scots pine (Pinus sylvestris). This wood species is fairly
easy to impregnate in its sapwood but extremely resistant to impregnation in the heartwood.
Since the sapwood zone generally is quite narrow, most Scots pine wood products will contain
heartwood. When used for modification with a waterborne product like in furfurylation, zones in
the wood will thus remain unmodified and still be susceptible to biological degradation.
Moreover the swelling by treatment will become uneven in products containing both sap- and
heartwood. This challenge has led to research in improved impregnation processes for better
penetration in the heartwood of Scots pine. Larnøy et al. (2007) have reported an improved
penetration of furfuryl alcohol in the outer heartwood using an oscillating pressure method, but
still most heartwood is unmodified.
In the present work another approach was investigated; change of substrate. The use of fast and
uniformly growing plantation wood species for wood modification offers promising prospects.
Many light-weight tropical timbers are easily impregnated in both sap- and heartwood and are
structurally homogenous. In this work we have chosen to work with Malaysian species having
commercial potential, Kelempayan (Anthocephalus spp.), Rubberwood (Hevea braziliensis) and
Sena (Pterocarpus indicus) with comparison to Scots pine and Beech (Fagus sylvatica). The
comparison goes for both attained decay resistance and general treatment feasibility.
2. EXPERIMENTAL METHODS
2.1 Wood
Five wood species were employed: Kelempayan (sapwood and poorly differentiated heartwood),
Rubberwood (sapwood), Sena (heartwood), Beech (without red heartwood) and Scots pine (sap-
wood). Defect free test samples were machined (flat sawn, planed on four sides) from well-
conditioned batches of timber to cubes measuring 20 x 20 x 20 mm3. For density information,
see Tab. 1. Chudnoff (1980) reports that Kelempayan is easy to impregnate, Rubberwood fairly
easy while the information given for Sena is conflicting.
2.2 Chemicals
In order to prepare impregnation solutions that would yield different resin loadings, ethanol (0-
78 wt-% of total solution) was chosen as solvent to furfuryl alcohol (20-90 wt-%) and added in
varying percentages. Ethanol also functions as a stabilizing co-solvent to prevent untimely poly-
condensation during the impregnation step and must be evaporated before curing. Maleic
anhydride (0.4-1.7 wt-%) and citric acid (0.8-3.5 wt-%) served as catalysts to the polyconden-
sation reaction. For details on the chemistry behind the furfurylation process, view Lande et al.
(2004a). Three treatment solutions (I, II and III with increasing furfuryl alcohol content) were
mixed using furfuryl alcohol (98% ACROS Organics®), ethanol (99.9%, HmbG Chemicals®),
citric acid (99%, SYSTEM®), maleic anhydride (99%, Scharlau®) and de-ionized water as
follows: De-ionized water was heated to 40 °C and citric acid added under stirring and then left
to cool to room temperature. Maleic anhydride was added directly to furfuryl alcohol under
stirring and subsequently mixed with the citric acid solution and finally diluted with ethanol
(ethanol only added in solutions I and II).
The wood samples were pre-dried at 103 °C for 16 hours before impregnation and weighed (W1)
after having settled to ambient temperature in a desiccator. Six samples were used for every
combination of wood species, fungus and impregnation solution type (including untreated) - a
total of 360 samples for decay testing.
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2.3 Impregnation modification
The modification treatment consisted of five consecutive steps:
1) Wet vacuum: 0.10 bar for 0.5h
2) Pressure: 12 bar for 2h
3) Evaporation of solvent (solutions I and II): 20-40 °C for 4h (temperature ramp)
4) Curing: 103°C for 16h wrapped in aluminium foil
5) Final drying to evaporate condensation water and unreacted monomer
After modification samples were re-weighed (W2) following the procedure described in section
2.2. Treatment intensity was calculated according to Eq. 1 as weight percent gain (WPG):
WPG = [(W2 – W1) / W1] x 100% (1)
2.4 Decay testing
The basidiomycete decay test was conducted according to the general procedure of the ASTM
soil-block method (ASTM 1975), slightly modified, e.g. filter paper (70 mm circles, qualitative,
ADVANTEC®) soaked in 1% malt extract solution, served as fungal feeder strips. Employed
Malaysian strains of decay fungi were: Pycnoporus sanguineus (white rot) and Gloeophyllum
trabeum (brown rot). Compost medium was adopted at 100% of its water holding capacity
(WHC).
For the soft rot decay test, unsterile compost medium, at 100% WHC, mixed with a Malaysian
isolate of Chaetomium globosum (soft rot), was adopted using the procedure of Wong (2006).
The decay test assembly was incubated in the dark at room temperature (ca. 24 °C) for 28 weeks.
The resistance of the furfurylated wood was evaluated according to Eq. 2 by mass loss percen-
tage (MLP):
MLP = [(W2 – W3) / W2] x 100%, (2)
where W3 is the final dry mass of the specimen after decay testing.
3. RESULTS AND DISCUSSION
3.1 Treatment feasibility
Impregnation solution uptake, polymer yield and WPGs were monitored: Scots pine and Kelem-
payan retained 660-849 and 626-860 kg/m3, respectively with the highest uptake found using the
almost undiluted furfuryl alcohol impregnation solution (III). The resulting polymer yields were
also comparable (not shown). Consequently, the WPG for Kelempayan was much higher than
seen for Scots pine due to the low basic density of Kelempayan (Tab. 1).
Rubberwood, Sena and Beech took up appreciably lower amounts of impregnation solution; 499-
575, 570-692 and 582-779 kg/m3, respectively. The polymer yields in Rubberwood, Sena and
Beech (not shown) were comparable to that of Scots pine and Kelempayan, except for solution II
where the yields were considerably lower. The reason for this has not been determined. The
combined effects of lower retention and higher density yield WPGs for Rubberwood, Sena and
Beech which are 40-60% lower compared to Scots pine when using the same impregnation
solution.
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Table 1 Density and weight percent gain (WPG) data for untreated and furfurylated material.
WPG is based on oven-dry wood mass. I, II and III refer to different impregnation solutions with
increasing furfuryl alcohol content.
Wood species Density
r(0.12) [kg/m3] WPG (I)
[%] WPG (II)
[%] WPG (III)
[%]
Kelempayan 320±44 38±8.4 131±26 183±26
Scots pine 485±31 25±3.2 89±6.9 117±10
Rubberwood 610±37 14±1.9 36±3.9 60±8.6
Sena 666±33 15±2.9 44±5.9 68±10
Beech 701±35 15±2.3 36±2.8 73±6.2
Data is given as mean±standard deviation.
Fig. 1 shows uptake of impregnation solution II as a function of sample mass. The picture was
equivalent for solutions I and III. Rubberwood retains less impregnation solution compared to
the other species of comparable density, while Scots pine has quite good retention. Retention
results are in concordance with work on furfurylation of unspecified pine and beech species by
Schneider (2002a).
2
3
4
5
6
7
234567
Sample mass (g)
Solution uptake (g)
Scots pine
Kelempayan
Rubberwood
Beech
Sena
100 % uptake150 % uptake200 % uptake
75 % uptake
Figure 1: Uptake of impregnation solution II as a function of sample mass for Scots pine, Kelempayan,
Rubberwood, Beech and Sena. Lines indicate uptake relative to sample mass in % (75, 100, 150 and
200%).
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3.2 Decay resistance
In the following, the decay resistance of the furfurylated material is analysed. Performance is
based on mass loss percentage. A number of statistical analyses could be employed, but the size
of the material does not support such. Other general problems with evaluating the efficacy of
modified wood are well covered by van Acker and Stevens (2000). Additionally, the use MLP to
evaluate protection of wood species with very different densities presents problems in itself, but
is at the moment still the most widely used method.
No detailed process development or fine-tuning of impregnation solutions was attempted. This is
however also work of great interest since unpublished results indicate that the polycondensation
of furfuryl alcohol is highly affected by the polarity of the cell wall constituents. For a more
specific comparison of the feasibility of anatomically different wood species for furfurylation,
ajustments of the chemical composition of the impregnation solution might also need to be
considered.
0
10
20
30
40
50
60
70
0 25 50 75 100 125 150 175 200 225
WPG (%)
MLP (%)
Scots pine
Kelempayan
Rubberwood
Beech
Sena
Figure 2: Weight percent gain (WPG) versus mass loss percentage (MLP) of furfurylated Scots pine,
Kelempayan, Rubberwood, Beech and Sena exposed to Gloeophyllum trabeum (brown rot).
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0
10
20
30
40
50
60
70
0 25 50 75 100 125 150 175 200 225
WPG (%)
MLP (%)
Scots pine
Kelempayan
Rubberwood
Beech
Sena
Figure 3: Weight percent gain (WPG) versus mass loss percentage (MLP) of furfurylated Scots pine,
Kelempayan, Rubberwood, Beech and Sena exposed to Pycnoporus sanguineus (white rot).
0
10
20
30
40
50
60
70
0 25 50 75 100 125 150 175 200 225
WPG (%)
MLP (%)
Scots pine
Kelempayan
Rubberwood
Beech
Sena
Figure 4: Weight percent gain (WPG) versus mass loss percentage (MLP) of furfurylated Scots pine,
Kelempayan, Rubberwood, Beech and Sena exposed to Chaetomium globosum (soft rot).
Figs. 2 to 4 indicate that the low and heterogeneous resistance of untreated wood towards all
three fungi was clearly improved and a quite homogenous performance attained for the modified
material. Furfurylation of the chosen substrates in general gave good decay protection when
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WPGs of approximately 20-30% were reached. This is equivalent to what has been reported for
pine, cedar and beech species in comparable studies (Edlund 2007; Lande et al. 2004b; Ryu et al.
1992; Westin 2004). Rubberwood seems to be slightly more difficult to protect from white and
brown rot compared to the other species.
4. CONCLUSIONS
Soil block decay tests have shown that decay protection imparted by furfurylation of Kelem-
payan sap- and heartwood, Sena heartwood and in part Rubberwood sapwood towards three
different fungi was comparable to that of Scots pine sapwood and Beech at equivalent weight
percent gains. The treatment feasibility varied considerably. Scots pine and Kelempayan took up
larger amounts of impregnation solution than Beech, Rubberwood and Sena and this in
combination with comparable polymer yields gave very high weight percent gains for Kelem-
payan due to its comparably lower density.
It is proposed that utilisation of Kelempayan may have promising prospects for furfurylation due
to the fact that decay protection can be attained in both sap- and heartwood and with the use of
much less impregnation solution compared to Scots pine sapwood.
ACKNOWLEDGEMENTS
Technical assistance was provided by Hii Ung Ngo (Faculty of Resource Science and Techno-
logy, Universiti Malaysia Sarawak), Lai Jeow Kok and Willies Chin (both, Sarawak Forestry
Corporation).
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