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Wood Sci Technol (2004) 38: 93–107
DOI 10.1007/s00226-003-0207-3
ORIGINAL
G. J. Goroyias ? M. D. Hale
The mechanical and physical properties of strand boards treated
with preservatives at different stages of manufacture
Received: 5 May 2001 / Published online: 25 March 2004
Springer-Verlag 2004
Abstract The physical and mechanical properties of boards treated with a
preservative at di?erent points during the manufacture process were evaluated
to determine the best stage for the application of preservative. A copper boron
tebuconazole amine water-based preservative was used in 3% PF-bonded
strand boards to achieve ?ve di?erent retentions. Preservative addition was
examined at di?erent stages of the manufacture cycle, namely, green strand
di?usion, dry strand vacuum treatment, glue-line spray addition, heat and cold
quench of manufactured board, and by post-manufacture vacuum treatment.
The treatment methods had marked e?ects on the mechanical properties of
some of the boards when the boards with the highest preservative retention
were compared with their respective untreated controls. The best results were
achieved where the preservative was applied by vacuum treatment of dry
strands or by di?usion of green strands before board manufacture. Increasing
preservative retention had minimal e?ects on board properties with these two
methods but signi?cant deterioration was noted when the preservative was
applied by spraying dry strands or by post-board-manufacture heat and cold
quench. An increase of pressing temperature resulted in signi?cant improve-
ments to the mechanical properties of the spray-treated boards. Post-manu-
facture vacuum treatment of boards caused excessively high losses in internal
bond strength.
Introduction
Oriented strand board (OSB) is a structural board widely used in building
construction and other applications mainly for interior use or for short-term
exterior out-of-ground contact exposure. The use of OSB in exterior conditions
is restricted due to its low durability against decay fungi (Chung et al. 1999;
Goroyias and Hale 2000a), swelling under high moisture conditions, and the
resultant reduction in internal bond strength (Goroyias and Hale 2000a).
Preservative treatment is readily applied to solid wood and may also be
applied to improve the properties of panel products. In this context it is applied
G. J. Goroyias ? M. D. Hale (&)
School of Agricultural and Forest Sciences,
University of Wales, LL57 2UW Bangor, U.K.
E-mail: m.d.hale@bangor.ac.uk
94
for commercial use to plywood after manufacture. A product of this type,
treated with a suitable preservative, shows su?cient decay resistance for ground
contact use (AWPA C9 1999). However, the lower cost of production of OSB
compared to plywood and solid wood gives a cost-competitive incentive to
develop a system to improve decay resistance and the dimensional stability of
OSB. Fluid preservative treatment of OSB after manufacture results in high
swelling so alternative application stages in the manufacture cycle need to be
investigated. For this, a series of factors, including time of treatment, stage of
preservative addition, physical and mechanical properties of the treated board,
resistance of the preservative to leaching, and decay resistance (with and
without leaching) need to be considered. The combination of preservative and
adhesive type and the stage of addition play an important role in the successful
production of a preservative-treated board.
Goroyias and Hale (1999, 2000a, 2000b) have recently reviewed the literature
related to the e?ect of point of preservative application on board properties.
Preservative addition to wood-based panel products has been shown to result in
signi?cant losses to the mechanical and physical properties (Boggio and Ger-
tjejansen 1982; Laks et al. 1988; Vick 1990; Vick et al. 1990; van Acker and
Stevens 1993; Jeihooni et al. 1994; Barnes et al. 1996).
Many preservatives were applied by several researchers to prevent wood-
based panel products from decay. These include pentachlorophenol, didecyl
dimethyl ammonium chloride (DDAC), DDAC with copper, DDAC with
carbamate, sodium ?uoride, ammonium hydrogen bi?uoride, ammoniacal
copper zinc arsenate (ACZA), borate preservatives, copper naphthenate (CuN),
copper octoate, chlorpyrifos, zinc naphthenate. However, various problems
may occur: substantial vapour losses of toxic preservatives may occur during
pressing, preservatives may interfere with the adhesive, and the resultant sig-
ni?cant loss of board mechanical properties result in restricted widespread
commercial application (Huber 1958; Vintila et al. 1967; Deppe and Petrowitz
1969; Becker 1972; Laks et al. 1988; Vick 1990; Vick et al. 1990; Fushiki et al.
1993; Jeihooni et al. 1994). However, non-acidic preservatives (e.g. DDAC-
based) and emulsi?ed CuN have shown fewer negative e?ects on board
mechanical properties (Vick et al. 1990; Schmidt 1991). Azaconazole, applied as
a powder to aspen wafers after the application of resin, had no signi?cant e?ect
on the board properties tested (Schmidt and Gertjejansen 1988).
Composite boards can be preserved at a variety of points during and after
manufacture. Di?usion treatment using several preservative formulations [i.e.
ammoniacal copper arsenate (ACA), copper-chromium-arsenic (CCA)] before
blending has been shown to produce a panel with good properties, above
standard speci?cations (Hall et al. 1982), but with poorer properties when
compared to the untreated board (Boggio and Gertjejansen 1982; Jeihooni et al.
1994).
Surface addition of preservative as a liquid (i.e. ACA, CCA) by spray or as a
powder during, before, or after resin blending has been shown to result in a
signi?cant decrease of board properties (Hall and Gertjejansen 1979; Jeihooni
et al. 1994; Schmidt and Gertjejansen 1988). The extent of in?uence depends
on the degree of compatibility of the preservative with the adhesive. It has been
shown that several preservative formulations (chlorpyrifos-CP, dichlorophen-
thion-ECP, sila?uofen-HOE, propetanphos-PP, IF-IF-100, IPBC) do not
interfere with UF resins. However, UF is not suitable for exterior speci?cation
95
boards as UF-bonded boards fail dramatically when wetted (Becker and Deppe
1969; Deppe 1987; Subiyanto et al. 1994). Several types of powder preservative
formulations have been applied with the resin before board manufacture
(Schmidt and Gertjejansen 1988). This method gives good mechanical
properties only when the preservative is heat-stable and compatible with the
adhesive.
Liquids may be applied post-manufacture by dipping, spraying, brushing, or
vacuum or vacuum-pressure, but due to drying, conditioning, and mechanical
strength limitations of such methods (Deppe 1967; van Acker and Stevens 1993;
Barnes et al. 1996), approaches with vapour treatments (Turner et al. 1990) and
supercritical ?uids (Acda et al. 1997) have been investigated. These novel
approaches have given some promising results with no changes in board
mechanical or physical properties. Brushing, dipping, and spraying frequently
yields unsatisfactory depths of penetration and products treated with these
methods are only suitable for out-of-ground use, because preservative pene-
tration is limited only to the surface regions of the board.
It can be concluded from the literature that a variety of isolated studies have
looked at the preservation of di?erent types of panel products. These studies
involve various preservatives, various formulation types (e.g. powder, oil, or
waterborne), application of preservatives at di?erent stages in the manufacture
process, and di?erent wood species, resins, and pressing conditions. These
studies cannot be directly compared with each other to determine the best
method for the application of preservation because they involve too many
unrelated variables. However, they do give indications that some ground-con-
tact preservatives are not suitable. This paper is a comprehensive study on
numerous physical and mechanical properties of strand boards, preserved with
a new generation ground-contact wood preservative formulation (Tanalith
3485) at di?erent stages of the manufacture process. Where di?erent pressing
conditions have been used to achieve successful board production comparable
controls have been included. The leach and decay resistance of these boards will
be reported in a later publication.
Experimental
PF-bonded Scots pine unoriented strand boards treated with Tanalith 3485
(copper azole borate) were made under laboratory conditions to achieve ?ve
)3
di?erent preservative retentions (0, 1.5, 3, 6, and 12 kg m ). The internal bond
strength (IB), modulus of elasticity (MOE), modulus of rupture (MOR),
thickness swelling (TSw), water absorption (Wabs), shear strength, and density
of the boards were then evaluated.
Board treatment
Five di?erent treatment methods were evaluated: di?usion treatment of
‘‘green’’ strands, vacuum treatment of dried strands, spray treatment of dried
strands in the resin blender, heat and cold quench (HCQ) of manufactured
boards, and post-manufacture vacuum treatment of boards. Goroyias and Hale
(1999, 2000a, 2000b) have described in detail the methods of preservative
addition used in this study.
96
Board manufacture
Boards were produced to a target thickness of 15 mm and a target density of
)3
650 kg m , using commercial Scots pine OSB strands with an average density
)3
of 400 kg m . Board manufacture variables are presented in Table 1. Four
di?erent types of control (untreated) boards were made (Table 1).
Board testing
Sampling and cutting of the large-sized boards (Table 1) was based on
standard EN 326–1:1993. When this was not possible, due to the smaller size
of the boards, the sampling and cutting was based on a speci?c pattern which
was determined from the results of an analysis of variance (one way ANOVA
and Tukey’s pairwise comparisons) of properties (IB, MOR, MOE, TSw,
Wabs) between replicate boards. For this purpose small untreated
(400·400 mm) boards were produced and tested. The sample variance of each
property tested was then analysed using ANOVA (p=0.05) and Tukey’s
pairwise comparisons between replicate boards. The results showed that there
was no signi?cant di?erence in properties tested between the replicate small-
sized boards.
Samples were then tested for various mechanical and physical properties and
the variances for each property between the boards were analysed (ANOVA,
p=0.05, Tukey’s pairwise comparisons).
The density pro?le of the boards was measured by using an ATR density
pro?le machine (software v2.09) and average densities were calculated. MOE,
MOR, IB, TSw, and Wabs were tested according to EN 310:1993, EN
319:1993, EN 317:1993. In addition, a modi?ed, non-standard shear strength
test was performed on blocks with 50·25·15-mm thickness (Fig. 1). Load
was applied parallel to the strand direction along the sample length in
specially designed steel jigs. The load versus displacement curves are shown
in Fig. 2.
3
Table 1 Process variables for the manufacture of the test boards
Treatment method
Resin
content content time
Wax
Blending Temp. Press Sizeb
Number
of boards
cyclea
(s) (m)
(%)
(%)
(min)
(C)
Di?usion
3
3
3
3
3
3
3
3
3,4,6,10
1.2
30
30
30
30
30
30
30
30
30
210
210
210
210
210
210
230
210
210
360
360
360
1·1
5
5
2
32
8
5
2
2
2 4
·
Heat and cold quench (HCQ)
Control (di?usion and HCQ)
Vacuum-pressure
Control (vacuum-pressure)
Spray 1
Spray 2
Spray Control
Post-manufacture vacuum
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1·1
1·1
360 0.4·0.4
360 0.4·0.4
660
540 0.4·0.4
660
1·1
360 0.2 0.2
1·1
3
·
c
a
Pressing cycle includes 90 s press close time and 30 s decompression
Board untrimmed size is displayed
For post-manufacture vacuum treatment the controls used were the same boards but were
untreated. The replicates in this case were two boards for each resin content.
b
c
97
Fig. 1 Block shear test
Fig. 2 Load versus
displacement curves for eight
specimens using the block
shear test
The assessment of results was based on comparisons of the properties of the
)3
highest retention preservative-treated boards (12 kg m ) with their appropri-
ate untreated controls (ANOVA, p=0.05, Tukey’s pairwise comparisons using
Minitab version 11). Fifteen replicates were tested for each property, except for
the boards produced with vacuum-pressure-treated strands where 30 replicates
were tested.
To examine for the e?ect of increasing preservative retention on board
properties, a correlation test (Pearson correlation, Minitab version 11) between
preservative retention and each property was performed. The MOR and MOE
)3
tests for the boards treated to the intermediate retentions (1.5, 3, 6 kg m )
included four replicates from the core, and the density pro?le, IB, shear, TSw,
and Wabs tests included six core replicates.
98
Results
Results for the e?ect of resin concentration on IB and TSw of the post-man-
ufacture vacuum-treated boards are presented in Table 2. Results for the dif-
ferent treatment methods on the physical and mechanical properties at each
preservative level and their correlations are shown in Table 3.
Discussion
)3
The null hypothesis that the 12 kg m -treated boards do not di?er signi?cantly
from their respective controls was tested using ANOVA and Tukey’s pairwise
comparisons. When no signi?cant di?erence is shown, preservative addition did
not result in a change of board properties, but where there is a signi?cant
di?erence, an improvement or a reduction of board properties occurred.
Post-manufacture vacuum treatment
Post-manufacture treatment was not a successful treatment method in a
board containing only 3% resin as high IB loss (42%) and TSw (34%) oc-
curred and sub-standard boards resulted. Higher resin content boards (4–
10%) were made and tested for IB and TSw after treatment. Increasing the
resin content from 3 to 4% resulted in boards of su?cient IB (Table 2) to
)2
pass standards (EN 300, 0.30 N mm ) but even these had high IB losses (50–
56% as compared to their control values, Table 2) and showed no
improvement in TSw after treatment. Further increases in resin content above
4% did not result in proportionately higher IB values after treatment. Irre-
versible TSw was observed even at high resin contents (Table 2). This dete-
rioration is in accordance with work by Hall et al. (1982) who examined
aspen waferboards treated under vacuum.
Table 2 Mean IB (mean of 4 samples) before and after post-manufacture vacuum treatment,
IB loss (%), and percentage thickness swelling (TSw) immediately after post-manufacture
vacuum treatment and after the drying (102C) of boards made with di?erent resin contents.
Standard deviations are shown in brackets
Resin content
(%)
IB (EN 319)
TSw
Untreated
)2
Preservative-treated
)2
Loss
(%)
Wet
(%)
Dry
(%)
(N mm
)
(N mm
)
3
0.38
(0.0)
0.57
(0.01)
0.72
(0.02)
0.69
(0.04)
0.16
(0.04)
0.32
(0.02)
0.36
(0.03)
0.38
(0.01)
42
56
50
55
34.2
25.19
(9.70)
28.45
(5.5)
27.76
(1.85)
31.90
(2.32)
(8.48)
35.79
(4.50)
39.85
(0.07)
33.97
(5.25)
4
6
10
99
100
101
‘‘Green’’ strand di?usion
Green strand di?usion proved to be a successful method for board manufac-
ture, producing a board with good properties, although slightly inferior to
boards made from vacuum-treated strands. There were no signi?cant di?er-
–3
ences between the control and the 12 kg m -treated boards for the IB, MOE,
Wabs (2 h), TSw (2 h and 24 h), and shear, but the intermediate preservative
loading boards were marginally worse (Table 3) for the MOR and Wabs (24 h).
)3
However the 3 kg m loading appears to have reduced the Wabs (2 h) (Ta-
)3
ble 3). The lower MOR value with the 12 kg m loading board is an edge e?ect
of the test samples.
Similar works (Hall and Gertjejansen 1979; Boggio and Gertjejansen 1982)
using di?usion into wet strands with di?erent kinds of copper-based preser-
vatives showed pronounced negative e?ects on a variety of mechanical and
physical properties of the boards. This is in contrast with the results of our
study in terms of severity. No correlations were found between preservative
retention and board properties (Table 3). The lack of correlation shows that the
preservative addition did not a?ect the board properties and poor results (e.g.
)3
12 kg m ,MOR) can be explained by other factors, e.g. edge e?ects.
Vacuum pressure treatment of dry wood strands
Vacuum-pressure-treated strands produced boards with better properties than
all of the other treatment methods. The mechanical and physical testing results
are no di?erent from their respective controls although signi?cantly less Wabs
(2 h) and consequently less TSw (2 h) occurred (Table 3). However, with pro-
longed soaking (24 h) these di?erences were no longer signi?cant. No other
correlations were found between preservative retention and board properties
with this treatment method (Table 3). This shows that this method of pre-
servative application does not have any signi?cantly negative e?ects on board
properties.
Vick et al. (1990) found that vacuum preservative treatment of boards with
ACA and CuN did not interfere with PF resin bonding as assessed by a lap
shear test. Similar results are seen here as both IB and shear test data show no
e?ect of preservative. The vacuum treatment method gives a good distribution
of preservative within the wood strands and the preservative is encouraged to
?x with the wood, which may reduce preservative interference.
Spray application
Boards produced by spraying the preservative at the glue-line stage showed
poor properties when compared to their respective controls; these were signif-
icant for every property tested except MOE and MOR (Table 3). Boards
treated to intermediate levels showed better values for MOE.
High correlations between preservative retention and board properties are
noted for spray treatment (Table 3). However, the correlations for the MOE
and MOR are much lower than those for the other properties, especially IB and
shear. This shows that in strand board the MOE and MOR are not directly
related to the quality of wood–resin bond. The orientation and the large size of
strands proved a key factor for the board bending strength.
102
Table 4 Mechanical and physical properties of spray-treated (12 kg m)3) boards pressed at
230C. Standard deviations are shown in brackets
Property
Spray
(230C)
IB (N mm)2
)
0.50 (0.08)
9575 (991)
)2
MOE (N mm
MOR (N mm
)
)
)2
46.62 (7.37)
24.42 (3.69)
29.66 (4.07)
60.55 (13.59)
74.23 (12.61)
2.51 (0.24)
a
TSw (2 h) %
a
TSw (24 h) %
b
Wabs (2 h) %
b
Wabs (24 h) %
)2
Shear (N mm
)
a
Thickness swelling after 2- and 24-h immersion.
Water absorption after 2- and 24-h immersion.
b
As the spray-treated boards pressed at 210C were so inferior, another series
of boards were pressed at 230C. Preserved boards pressed at 230C (Table 4)
had very good mechanical properties but these boards initially absorbed faster
(Wabs, 2 h) than the 210C untreated controls and also showed increased initial
swelling (TSw, 2 h). Rapid ?uid uptake during the immersion test and rapid
subsequent swelling may have been caused by cell wall damage, i.e. crack for-
mation, although other possibilities, such as increased resin–wood bond sti?-
ness (low ?exibility resin unable to accommodate swelling), cannot be ruled out.
Preservative addition by spraying resulted in an increase in mattress moisture
content from 5 to 9%. This alone had a signi?cant negative e?ect on board
prop