Detailed Investigation of Modulus of Elasticity in Linseed
Oil-impregnated Mature Norway Spruce Sapwood
Thomas Ulvcrona 1
ABSTRACT
Various techniques and preservatives are used in wood preservation, some of which are toxic. Hence,
there is a need for further development (and use) of non-toxic alternatives, such as linseed oil
impregnation by the Linotech process, especially for wood such as mature sapwood from Norway
spruce (Picea abies L. Karst.) with anatomical features that make it difficult to impregnate with
preservatives by currently applied industrial processes. In the study reported here we examined the
modulus of elasticity (MOE) of sub-samples of mature Norway spruce sapwood impregnated by this
process to evaluate its effects, caused by possible structural damage, on the wood’s short-term
mechanical properties. Two sets of process settings were applied, “low” and “high” with treatment
times, pressures and temperatures of 2 and 3 h, 0.8 and 1.4 MPa, 60 and 140˚C, respectively. The
treatments resulted in 30-50% mass increases in the specimens. MOE was generally slightly lower in
control (unimpregnated) specimens than in specimens impregnated at low and high settings (9976,
10663 and 10528 MPa, respectively). The results indicate that the Linotech process does not cause
more structural damage than drying without impregnation, and that determination of short-term
mechanical properties followed by basic statistical analysis provides a convenient means for detailed
evaluations of effects of varying process settings.
KEYWORDS
Short-term mechanical properties, Hydrophobic oil, Method development.
1
Swedish University of Agricultural Sciences Unit for Field-based Research Svartberget Fieldstation SE-922 91,Vindeln
SWEDEN, [email protected]
Thomas Ulvcrona
1 INTRODUCTION
Right now there is worldwide a constant evaluation of possible sustainable processes that could
substitute many of the environmentally adverse preservation techniques currently used. As one of
many potential processes, impregnation of wood with hydrophobic oils has already been extensively
evaluated for many years.
Potential sustainable alternatives to many currently used environmentally adverse preservation
techniques are being intensively sought. One of many potential processes, impregnation of wood with
hydrophobic oils, has been known for many years, but further evaluation and development of the
process is still required. Previous studies have shown that it is possible to successfully impregnate
even recalcitrant types of wood, such as mature sapwood from Norway spruce (Picea abies L. Karst),
with the linseed oil product Linogard® using the Linotech process [Ulvcrona et al. 2006, Ulvcrona &
Bergsten 2007]. In this process the linseed oil does not enter the cell wall structure, but the moisture
uptake of the resulting wood-based material is retarded by the formation of a hydrophobic layer
within the wood [Fredriksson et al. in press]. Thus, it probably does not affect the chemical (and
hence structural) contents of the cell walls. However, the temperatures that may be used in the process
are relatively high (60-140 °C) and external pressure may also be applied [Ulvcrona et al. 2006,
Ulvcrona & Bergsten 2007], so it could potentially cause structural changes resulting in alterations of
short-term mechanical properties. Accordingly, Wang [2007] has reported that impregnation with oils
can have structural effects that might restrict use of impregnated materials in constructions. Thus,
clarifying the effects of hydrophobic oil impregnation on wood’s short-term strength properties is an
important first step towards elucidating the potential range of uses for wood treated with hydrophobic
oil.
Thus, in the presented study the Modulus of Elasticity (MOE) of mature Norway spruce sapwood
impregnated by the Linotech process with two sets of process settings was investigated.
2 MATERIALS AND METHODS
2.1 Samples
In total, 15 Norway spruce (Picea abies L. Karst) trees from three stands in a mixed coniferous forest
in northern Sweden (64°10’N, 160-320 m above sea level) were selected, from each of which in all
more than 60 samples (500*25*25 mm) were sawn from the mature sapwood. A third of the samples
(controls) were dried, but not impregnated, a third were impregnated by the Linotech process with
“low” settings (2 h treatment at 0. 8 MPa and 60-140˚C) and the other third by the process with
“high” settings (3 h treatment at 1.4 MPa and 140˚C). The moisture content of the wood (percentage
of wood dry mass) before impregnation was calculated according to standard method EN 384 [1995].
The impregnation resulted in 30-50% increases in the mass of the mature sapwood specimens, and
their linseed oil contents were calculated as a percentage of wood dry mass. For further details of
these procedures, see Ulvcrona et al. [2006].
2.2 Modulus of Elasticity and Macroscopic Cracks
Forty impregnated mature sapwood samples were randomly chosen to test their MOE, as a general
indicator of their mechanical properties, and eight unimpregnated mature sapwood samples were
randomly chosen as controls. The top half of each sample was cut longitudinally with a band saw to
form nine specimens of equal size (219*6*6 mm) with standing annual rings according to Fig. 1, then
specimens representing each treatment, and control specimens, were randomly selected for the MOE
measurements according to Table 1. In addition, material properties of the samples (moisture content
and density) prior to impregnation, and their oil contents post-impregnation, were determined to
assess possible correlations between these variables and the specimen’s MOE values.
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Modulus of Elasticity in Linseed Oil-impregnated Mature Norway Spruce Sapwood
Figure 1. Sketch showing the sawing pattern of specimens from the samples.
Tests were performed using an Instron Universal, 10 kN testing machine with standard equipment for
3-P bending. All specimens were loaded until failure, using a crosshead speed of 4 mm minute-1.
Displacement of the crosshead was recorded and used to calculate the strain. Thus, the absolute
flexural modulus values are not entirely correct, but are still useful for comparisons between reference
and impregnated specimens. Specimens were conditioned and tested at 23°C with 50% humidity.
The elastic flexural strain, ε , on the bottom surface was calculated as:
fl
(1) εfl = 6w0h/l
2
where w0 is the measured deflection, and h is the thickness of the specimen.
Flexural stress, σfl, was calculated as:
2
(2) σfl = 3Pl / 2h b
where P is the applied load, and b is the width.
The flexural modulus, E , was calculated, in MPa, as:
fl
(3) Efl = σfl / εfl
with measurements of applied load taken between 0.01 – 0.05 kN chosen for calculations.
The shear factor was neglected in the calculations of flexural modulus because the l/h ratio (26.7)
made it insignificant [Kollman & Cote, 1984].
The bending strength, ƒm, was calculated, in MPa, as:
(4) ƒm = aFmax / 2W
according to standard method EN 408 [1995].
Macroscopic crack development was measured with the naked eye and a caliper across the whole
(25*25mm2) surface in the middle of samples before further sawing into specimens.
2.3 Statistical Analysis
All statistical calculations were performed using MINITAB software [Anon., 1999]. The data were
tested for normality and heteroscedasticity, and the results indicated that no transformations were
needed. To test for differences in mean values related to the treatments One-way Anova was used.
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Thomas Ulvcrona
3 RESULTS AND DISCUSSION
There were no easily interpretable general trends concerning effects of process settings on the
Modulus of Elasticity (MOE) values of the mature Norway spruce sapwood specimens impregnated
by the Linotech process [Table 1], although they were slightly lower for untreated specimens than for
impregnated specimens [Table 1]. However, MOE values of untreated specimens were not
significantly lower than those of specimens impregnated with high settings [Table 1]. The standard
deviation of the MOE values was also slightly lower for the unimpregnated specimens [Table 1].
Table 1. Mean (and standard deviation), minimum and maximum Modulus of Elasticity (MOE)
values of samples following the applied treatments (numbers of investigated specimens in brackets).
Treatment
Mean
(MPa)
MOE Standard
Deviation
Untreated
Low settings
9976a (48)
10663b (107)
Variance
1330
1556
1556
1568
MOE
Minimum
(MPa)
6643
5593
MOE
Maximum
(MPa)
13666
14255
High settings 10528ab (95)
1568
1330
7983
14480
Note, superscript letters indicate significant differences between treatments according to Tukey’s tests
at the 0.05 probability level.
This tallies to some degree with findings reported by Megnis et al. [2002], who also discuss the
possibility that hydraulic effects of oil present in the cavities may increase the wood’s stiffness in
compression , which in turn affects its MOE and strength. However, the levels of uptake in the
investigated samples seem to have been insufficient to significantly alter the short-term mechanical
properties of the impregnated wood. There appears to have been no more structural damage after
impregnation than after drying without further treatment. Among the examined specimens, there were
no significant differences between impregnation treatments regarding linseed oil uptake; the mean
values of oil uptake were 25.4% of wood dry mass for both impregnation treatments (not further
presented). No significant differences in macroscopic crack development were found either following
the investigated impregnation treatments; mean values of macroscopic crack development were 6.6
and 5.8 mm, in treatments with “low” and “high” settings respectively (not further presented).
Scatterplot of MOE 2 vs Density; MOE 3 vs Density 2
300
MOE 2*Density
15000
325
350
375
400
MOE 3*Density 2
12500
10000
7500
5000
300
330
360
390
420
Figure 2. MOE of specimens impregnated by the Linotech process with “low” (MOE 2) and “high”
(MOE 3) Linotech settings in relation to the samples’ wood density (kg/ m3) before impregnation.
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Modulus of Elasticity in Linseed Oil-impregnated Mature Norway Spruce Sapwood
There were also no significant differences between treatments regarding wood density before
impregnation in the investigated samples [Fig. 2].
Wood density is generally quite strongly correlated with MOE in samples with otherwise similar
variables. There were weak indications of this correlation in the specimens subjected to the Linotech
process with “low” settings, but no such indications in the specimens subjected to the process with
“high” settings [Fig. 2].
There were also no significant differences between treatments regarding wood moisture content
before impregnation in the investigated samples [Fig. 3].
Scatterplot of MOE 2 vs mst ct bef; MOE 3 vs mst ct bef2
50
MOE 2*mst ct bef
15000
100
150
MOE 3*mst ct bef2
12500
10000
7500
5000
50
75
100
125
150
Figure 3. MOE of specimens impregnated by the Linotech process with “low” (MOE 2) and “high”
(MOE 3) settings in relation to their wood moisture content (%) before impregnation.
Previous studies have shown that the wood moisture content (expressed as water-filled porosity) is
positively correlated with uptake of oil in the Linotech process [Ulvcrona et al. 2006, Ulvcrona &
Bergsten 2007]. However, the moisture content before impregnation of the samples examined here did
not have any apparent effect on the MOE of the impregnated wood material [Fig. 3].
Scatterplot of MOE 2 vs linseed oil perc; MOE 3 vs Linseed oil perc
10
MOE 2*linseed oil percentage
15000
20
30
40
MOE 3*Linseed oil percentage 2
12500
10000
7500
5000
0
10
20
30
40
Figure 4. MOE of specimens impregnated by the Linotech process with “low” (MOE 2) and “high”
(MOE 3) settings in relation to the samples’ linseed oil percentage (%).
XII DBMC, Porto, PORTUGAL, 2011
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Thomas Ulvcrona
In addition, there were no significant differences between treatments regarding uptake of oil,
expressed as linseed oil percentage of wood dry mass before impregnation in the investigated samples
[Fig. 4].
Wang [2007] has reported that impregnation with oils can cause structural changes in impregnated
materials that might restrict their use in constructions. However, we found no clear trends regarding
effects of linseed oil uptake on the MOE of the resulting wood material [Fig. 4]. It should be noted the
levels of oil uptake were generally similar following impregnation by the process with both sets of
settings [Fig. 4]. Therefore, it is difficult to draw conclusions regarding possible effects of all levels
of oil uptake. However, this study provides information about effects of impregnation to levels likely
to be applied in commercial practice.
This study shows that the presented methodology can be used to study, in detail, the effects of
variations in process settings on the short-term mechanical properties of impregnated wood, providing
a convenient approach for optimizing parts of the process in screening studies before progressing to
large-scale studies of short-term mechanical properties in full-scale products. Further, use of highly
characterized raw wood materials allowed much of the natural variability present in wood from whole
trees to be excluded, providing opportunities to highlight effects of separate parts of the process.
In addition, the study indicates that impregnation with linseed oil in the Linotech-process does not
have major effects on the MOE of mature Norway spruce sapwood. Possible hydraulic effects in the
wood are not significant when samples are divided into specimens. Neither do any of the investigated
material properties have any obvious effect on MOE in the investigated samples.
Finally, the absence of clear negative effects of the investigated impregnation process on MOE
indicates that further investigations of its effects on both the short-term and long-term mechanical
properties of full-scale products are warranted. However, such investigations should be done in
parallel with studies like this, allowing effects of specific process settings to be identified.
ACKNOWLEDGEMENTS
The author thanks The Swedish University of Agricultural Sciences theme research program “Future
Forests” for financial support and Sees-Editing for professional editing.
REFERENCES
Anon. EN 384. 1995. Structural timber – determination of characteristic values of mechanical
properties and density, European standard 384. European Committee for Standardization. 1-8.
Anon. EN 408. 1995. Timber structures – Structural timber and glued laminated timber –
Determination of some physical and mechanical properties, European standard 408. European
Committee for Standardization, 1-19.
Anon. 1999. Minitab Statistical Software Release 13 for Windows.
Fredriksson, M. Ulvcrona, T. & Wadsö, L. 2010. ´Swelling and moisture sorption of Norway spruce
(Picea abies L. Karst.) impregnated with linseed oil´, Wood Material Science and Engineering. In
press.
Megnis M., Olsson T. Varna J. & Lindberg H. 2002. `Mechanical performance of linseed oil
impregnated pine as correlated to the take-up level`, Wood Science and Technology, 36[1], 1-18.
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Modulus of Elasticity in Linseed Oil-impregnated Mature Norway Spruce Sapwood
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