1. business and industrial
2. energy
3. oil

Royal-treatment
– scientific background and practical application
Presentation at COST E22 Conference in Reinbek, November 2001
A. Treu*, H. Militz* and S. Breyne**
* University Göttingen, Institute of Wood Biology and Wood Technology
** Dr. Wolman GmbH
I.
Introduction
The presented work was part of a project of the Institute of Wood Biology and Wood
Technology (University Göttingen) in cooperation with Dr. WOLMAN Company
Beginning: April 2001, Duration: 3 years
Durability of our native wood species is limited and especially for most of our important
economic timber it is moderate or bad. Pine wood (Pinus sylvestris) and especially pine sapwood has moderate or low durability. In order to build long lasting garden furniture,
panelling, etc. for hazard-class 3 (and 4) pine wood has to be impregnated with wood
preservatives. Durability of a product is influenced by different factors not only by
effectiveness of biocides or natural durability of wood but furthermore by factors like crack
behaviour, UV-stability and water behaviour of wood.
Today wood preservatives with different compounds are used. A critical discussion about
some preservative compounds is becoming increasingly important. It is known that the
addition of water repellents, like oil, to wood has a positive influence on product performance.
The philosophy of wood preservation industry to produce “fit for purpose”-products, having
good durability, good esthetical properties and not negative environmental impact is growing.
In our project a copper based chromium-free wood-preservative is used in combination with
modified linseed oil. This combined treatment, called ROYAL-treatment, is going to be
optimised and therefore the process parameters are investigated. The treated products are
analysed referring to their amount of preservative- and oil-uptake and different characteristics
like leaching, durability against weathering, stability of dimension and aesthetics of surfaces
are investigated.
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II.
Oil and wood
Oil is well known in use of different applications. In combination with wood it is often used
for surface treatment, dipping, vacuum-pressure-application and drying. Because of its ability
to transport heat oil can be used for drying processes (SIMMONS, 1989 “Preparation of
chemically dried cellulosic fuel”; HÄGER,1982 Royal-process) and wood modifications
(MENZ, 1999 heat treatment of wood).
%- reduction of water uptake compared to reference
60
78,8
76,9
78,6
65,9
67,4
80
82,5
83,1
81,9
71,8
66,1
95,1
95,5
96,2
91,2
89,8
100
PS 542
PS 600
30 min
6,9
0
-20
soya-oil
walnut-oil
16,8
20
orange-oil
rape-oil
40
32,6
reduction %
A student project about the use of
natural oils in wood preservation was
performed at the Institute of Wood
Biology and Wood Technology of the
University Göttingen (DÜSING,
LANGEN, NÖLLENHEIDT, 2001).
Different oils (orange-, soya-, rape-,
walnut-, and two tall-oils) were tested
and their reduction of water uptake in
comparison to a reference described.
The positive water repellent effect can
be seen in figure 1.
24 hours
2 weeks
time
For the period of three years SAILER
Fig. 1: screening of different oils
(2001) investigated hemp-oil- and
linseed-oil-impregnated wood samples
(with a high oil uptake) in an outside weathering test. Referenced untreated pine wood
samples have considerable changing of moisture content. It is quite evident that oil
impregnated wood samples have a small
fluctuation or moisture content during the
outside weathering.
Another process were oil is used in a very
different manner is the Menz-process. In
this oil-heat-treatment wood is completely
covered with hot modified linseed oil. Oiltemperatures are between 180 – 220 oC. In
terms of heat-treatment different versions
are
described.
Therefore
different
statements about biological efficacy were
made.
Fig 2: outside weathering test of hempoil and
linseed oil impregnated wood samples
Oil, waxes, resins and water repellent additives can improve the hydrophobity of surfaces of
wood. Therefore it has a positive effect against leaching of preservative compounds
(COOPER, 1997). Oil, especially linseed oil, is often used as basic component for varnishes
and paint. It has good autoxidativ properties and can harden without modification or addition
of siccatives. Responsible for the curing properties of linseed oil are the linoleic and linolenic
fatty acids, which can also be found in other plant oils like hempoil or rapeoil. A non-drying
oil does not possess the power to take up oxygen from the air and to loose its liquid
characteristics (MORELL, 1925).
Linseed oil cannot act as a cell wall bulking agent. Its base molecule is too large and
hydrophobic to enter the cell wall in a short time impregnation process. However, during the
2
impregnation process linseed oil fids cavities such as the tracheid lumina, the rays and any
cracks in wood structure (OLSSON,1999).
It is known that linseed- or rapeoil-treatments have no effect on protection against fungi
(SAILER et al., 1998, VAN ACKER et al., 1999). Linseed oil is also not light stable. In case
of light exposure constituents of oil can change (JANDER, 1932). From that reason a
combined process (wood preservatives – oil) is researched.
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III.
Process characteristics
The ROYAL-treatment, developed by Bror Olof Häger (GB 2088422A, US 4305978, DE
3008263A1) is a combined process. A copper-salt-impregnation (in our case WOLMANIT
CX-8) is followed by an oil-impregnation (modified Linseed oil). This process is yet used in
Europe since some years, but further details of many process parameters are still not
investigated. In Europe only a few companies use the royal-process as a production-process
(see table 1)
Tab. 1: Application of the Royal-process in Europe
Country
Company
Product
amount
Norway
NYE MÖRE TRÄ AS
paneling, nois-proof
Ca. 8000 m³/year
walls
Norway
MARNA BRUK
paneling, nois-proof
Ca. 4000 m³/year
walls
Denmark
FREJLEV TRAE
windows and doors
Ca.1000 m³/year
Germany:
WERTH-HOLZ
garden furniture,
fences, playgrounds
Ca. 5000 m³/year
Significant for the royal-process are two steps and therefore two different vessels are used.
For testings and investigations of this combined process a new pilot plant was developed in
cooperation with the plant-manufacturer (SHARF-Bassum, GERMANY). Picture 1 shows the
pilot plant at University Göttingen.
1. LOWRY-process
The LOWRY-process (LOWRY, 1906)
is a simplified procedure that is used for
impregnation of waterborne wood
preservatives. Mainly two steps are
significant for the LOWRY-process: a
pressure phase (>9 bar, 1 hour) and a
high vacuum at the end of the process
(20 min).
The uptake of preservative is for pine
wood 4 kg/m³ (at 4 per cent solution)
Fig 3: Pilot-plant of Univerity Göttingen
2. Oil-impregnation
The oil-impregnation is a treatment of wood with hot oil in a vacuum. Aim is to dry the oil
from nearly wet state to a wood moisture content of between 12-18 % and to hydrophobize
the surface in order to prevent leaching of preservatives. Therefore a vacuum is set (about 200
mbar) for several hours and the linseed oil is heated up to 80° C. This is done in order to
reduce the steam pressure of water and to boil water out of wood.
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The steam-pressure of water depends on temperature and pressure. A high vacuum needs only
a low temperature for
boiling of water.
The boiled out water
(steam) is cooled in a
condensate-reservoir and
can be absorbed and its
quantity can be assessed.
The wood samples can be
dried to a certain moisture
content in regard of the
condensate.
Tab. 2: Boiling point subject to temperature and pressure
The process is finished when requested moisture content is reached. Different dimensions of
wood samples and moisture contents need different duration of an oil-impregnation-process
(between 2 and 12 hours).
IV.
Results and Foresight
It can be stated yet that variation of moisture content during an outside exposure of Royalimpregnated wood in comparison to CX-8 impregnated and untreated pine wood is less. In an
outside weathering test of 20 month with linseed-oil impregnated wood samples (Lap-joints,
ENV 12037) wood moisture content was below 20 % (SAILER et al., 1999). EDLUND
(1994) carried out comparative studies with horizontal installed royal-, CCA- and unimpregnated wood samples for the period of 65 month. The measured wood moisture content
can be seen in figure 4.
Weathering test
Royal-treated
100
pine untreated
moisture content [%]
spruce untreated
CCA-treated
80
60
40
TREU, 2001
20
0
0
5
10
13
23
month
51
65
EDLUND, 1994
Fig. 4: Changing of moisture content during an outside weathering test
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Figure 5 shows the moisture content during an accelerated weathering test. An outside
weathering test can be simulated in a accelerated weathering machine (QUV). Three different
steps within a weathering-cycle can be chosen: Condensation water system, UV-light system
and spray water system.
moisture content of wood samples during the weathering test (QUV)
40
Royal
cx-8
moisture content [%]
35
untreated
30
25
20
15
10
Step 1 /24
Step 3/1-26
Step 4/2-27
Step 1 /163
Step 4/1-166
Time
Step 3/2-168
Step 4/2-171
TREU, 2001
Fig. 5: moisture content during an accelerated weathering test (QUV-tester)
The change of colour is also an important factor to evaluate wood samples and products.
Because of its missing UV-protection royal-impregnated wood samples are becoming grey
after some month. Figure 6 shows a wood sample in an outside weathering test. The pictures
were taken after 1, 3 and 6 month.
May 2001
August 2001
October 2001
Fig. 6: changing of colour after several month
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The project deals with the Process-development- and optimizing with regard to end product,
moisture content, efficacy and uptake optimization. Wood properties are investigated
especially water-repellancy, UV- stability, esthetical properties, shrinkage/swelling, crack
behaviour, and leaching of wood preservative and oil.
Further research within the next 3 years will focus on the following items:
♦
♦
♦
♦
♦
♦
♦
Migration of wood preservatives to oil during impregnation process
Emission of oil and wood preservatives during exposure to weathering
Long term water repellency
Optimization of the process
V.
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