Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~935Z
BACKGROUND OF T~l~ INVENTION
Folii~erous wood (i.e., hardwood) is generally
easy to impregnate. Solutions containing preservatives can
be rapidly forced by means of pressure into the wood. As a
rule the entire mass of the foliiferous wood is penetrated
by the preservative solution, in contrast with pine wherein
only the sap wood is penetrated. It had been earlier
though that this caused few problems since good results had
been obtained by the pressure treatment of sleepers of beech
with creosote oil.
This opinion has recently been modified. It is
clear that foliiferous wood treated with preserving salt
solutions exhibit prolonged periods of use but the preser-
ving treatment has been much less effective than expected.
Since the salts are well fixed in the wood, the lessened
effect of the treatment has not been considered to depend on
the leaching effect of the preservative. The preserving
salt has, in all probability, been effective against the
wood destroying organisms which appear in and destroy folii-
ferous wood. The low effect of the treatment has been observ-
ed in several different types of preservatives of different
origin. An explanation for the observed long-term ineffec-
tiveness of the preservatives is believed to have been
found by electron microscopic studies of the wood. It appears
from these studies that the preservatives have not penetrated
the cell walls. The inner part of the cell walls have thus
J
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remained untrcated. The wood destroying organisms have thus
been able to grow within the inner part of the cell walls
and destroy the wood.
SUMMARY OF TI~E INVENTION
~ he present invention discloses an improved method
of satisfactorily preserving foliiferous wood. The method
is based upon the use of specific types of preservatives in
combination with specific methods of penetrating and distri-
buting these preservatives within the wood.
10The preservatives used according to the present
invention must be such that they remain in dissolved form
(i.e., in an unfixed state) in the wood for a certain period
of time after the preserving treatment. Only then are they
"fixed" within the wood. It is thus possible to ensure that
these preservatives pass through the cell walls. The preser-
vation methods of this invention provide for the introduction
of the preservative into the wood with the wood thereafter
being kept in an undried condition for a period of time ,
during which no fixation of the preservative occurs. During
this time the preservative can diffuse through the cell
wall. Thereafter the conditions for the storing of the
wood are changed (i.e., the wood is dried) so that the
preservative is fixed within the wood.
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DETI\II,E:D DESCRIPTION OF Tl-1~3 INVENTION
.
Ordinary water soluble preservatlves are not suit-
able for use in the method of the present invention. For
example, such well known preservatives now in use are
described in U.S. Patent 2,565,175 (containing copper,
chromium and arsenic compounds) which is a development of the
preservative disclosed in U.S. Patents 2,139,747; 2,149,284
(based upon copper compounds and arsenic trioxide dissolved
in ammonia); 1,984,256 (containing zinc compounds and arsenic
trioxide dissolved in acetic acid); and 2,749,256 (containing
copper formate).
These preservatives all contain either copper or
zinc as the active cation and arsenic as the active anion.
The copper or zinc together with arsenic compounds provide
good protection against wood destroyers, with the arsenic
being of special value for protection against termites,
which usually are present in the areas where treatment of
foliiferous wood is particularly of interest.
These specific preservatives exhibit different
properties. However, in order to adapt them to the penetra-
tion treat~ent of the present invention, their properties
have to be changed.
The first described preservative may consist of
copper oxide, chromic acid and arsenic acid (arsenic pent-
oxide). The presence of the acid ingredients, i.e., the
chromic acid and to a certain extent the arsenic acid, ensures
that the preservative can be kept in solution. When the
solution has been introduced into the wood the chromium ion
in the chromic acid is converted (i.e., reduced) from a
,
dm~ ~ 3 ~
(
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negative valcnce ~i.e., -6~ to a posltive valence ti-e-,
+3). A5 a result of this ch~nge the components of the pre-
servative precipitate and become fixed within the wood.
Arsenates and basic salts are formed which solubilize with
difficulty.
The preservative solution must necessarily be
acidic in nature in order to initially dissolve the preserv-
ative. The use of a pH as low as 2 is common. The higher
the acidity the faster is the fixation of the preservative
in the wood. The use of ordinary pressure treatments under
these circumstances is generally no drawback since the
penetration takes place so fast that the preservative is
distributed within the wood before the fixation occurs. A
slower introduction of the solution results in considerable
fixation during the penetration and causes poor distribution
of the preservative in the wood. As soon as the preservative
becomes fixed it will not be further distributed within the
wood, and cannot therefore penetrate the cell walls in
foliiferous wood.
The first preservative can be changed so that it will
not be fixed as described above so that it can be used accord-
ing to the present invention. This is obtained if the
preservative is made alkaline by adding ammonia in a suitable
manner. A small amount of ammonia causes precipitation but
further ammonia additions ensure dissolution. By making the
preservative solution alkaline in nature the reduction of the
chromic acid is delayed or avoided after the introduction of
the solution into the wood. The reduction begins to occur
durin~ drying when the ammonia evaporates and the solution
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becomes less alkaline and more concentrated within the wood.
At that time a slow fixation of the preservative takes
place.
As copper has a strong affinity for wood it is
desirab].e to use an excess of ammonia therewith to avoid an
early fixation of the copper. Furthermore, carbon dioxide ;~
can preferably be added in order to delay the evaporation of
the ammonia and to increase the stability of the solution.
A typical preservative solution containing copper,
chromium and arsenic as disclosed within U.S. Patent 2,565,175
may contain preservative components in the following
exemplary amounts:
In Solid Form In Water Solution
.
CuO 19.6 percent by CuO 0.27 percent by
weight weight
CrO3 35.3 CrO3 0.~8
As2Os 45.1 AszOs 0.61
In order to neutralize the chromic and arsenic
acids in said solution and to dissolve the copper-oxide an
ammonia content of at least about 0.7 percent by weight is
required. , -
Thus the preservative water solution may have the
following exemplary composition if adapted for use in the ~ '
present invention.
CuO 0.27 percent by weight
CrO3 0.~8
~s2Os 0.61
~H3 0.91
CO2 1.40
. .
dm~ ,, ~ 5 ~
,- .: ,, . . ,,, , : :
~1~93SZ
The preservative solution of U.S. Patent 2,1~9,284
is bascd on a coppcr compound and arsenic trioxide dissolved
by means of ammonia. ~fter the wood preservation treatment
the ammonia evaporates and the soluble preserving ingre-
dients are precipitated within the wood. The fixation
proceeds quickly but slower than the fixation of a preserv-
ative containing copper oxide, chromic acid, and arsenic
acid. In order to provide a preservative suitable for use
in the present invention the amount of solvent (i.e., the
ammonia) has to be increased since the affinity of the
copper to wood is high, and the solution should preferably
also be stabilized with carbon dioxide. This may be simply
carried out by increasing the content of ammonia by about 20
percent by weight based on the original ammonia content with
the addition of a major portion of it in the form of ammonium
bicarbonate. It is further advantageous to replace the
arsenic trioxide in the preservative by arsenic acid to
provide a more stable solution.
In the ammoniacal preservatives several possi-
bilities exist whereby the amount of the active ingredientsmay be varied to obtain the desired effect. This is more
difficult in the acid preservatives which contain chromic
acid. These preservatives must, in order to be dissolvable,
have a high content of chromic acid (and arsenic acid) even
if these amounts are not needed for the fixation or to
provide the preservative effect.
In the ammoniacal preservatives the chromic and
arsenic acids provide no dissolving function. Therefore the
arsenic acld is intro-uced only in an amount required to
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produce the desired effect. The arsenic acld is fixed by
the copper compound. If the arsenic acid is present in a
high amount or a higher fixation of it is desired chromic
acid may be introduced only in the amount needed to provide
for the fixation of the arsenic acid.
Such a preservative adapted for the present invention
may contain preservative components in the following exem-
plary amounts:
In Solid Form In Water Solution
-
CuO6.5 percent by CuO 0.30 percent by
~ weight weight
- CrO33.5 CrO3 0.16
As20s 3~5 As2Os 0.16
NH320.7 NH3 0.95
CO239.0 CO2 1.80
Water 26.8
As this preservative is of special interest it
should be noted that the preservative may be used in differ-
ent concentrations in the different treating methods. The
exemplary solutions may thus be used for full cell pressure
treatment or open-tank treatment. If the preservative is
introduced by the Lowry Method when about half the amount of
solution is introduced per unit of wood as compared to a full
cell treatment it is desirable to use a double strength
solution in order to provide the desired amount of preserv-
ative in the wood. Such a doubled strength may also be used
when treating incompletely dried wood by a full cell method.
For soaking diffusion treatment of green wood the concen-
tration of the solution may, for instance, be two to four
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,..
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935~
tlme~s higher than the exemplified solutlons. ~lowever, this
method consumes large quantities of raw materials.
If zinc compounds are included in the preservative
solution instead of copper compounds the ammonia and carbon
dioxide amounts should be increased around 50 per cent. The
zinc preservatives are somewhat more difficult to dissolve
and they also exhibit a lower effect. On the other hand,
they do not color the wood nearly as much such that this
method is generally preferred.
The preservative solution containing zinc compounds
and arsenic trioxide dissolved in acetic acid may also be
introduced into the wood by a pressure treatment. It
differs from the two other preservative solutions in that
it is not so as quickly fixed since the acetic acid evapo-
rates rather slowly from the wood. This property would be
an advantage in the present invention except that the acetic
acid evaporates incompletely. The result is a poor fixation
and the preservative exhibits a lowered effect due to
leaching.
In order to improve the fixation of preservatives
of this type special steps must be taken. In U.S. Patent
2,749,256, a heating of the wood is recommended during which
the rate of fixation is increased. It is also disclosed,
however, that the wood is treated with a copper formate
solution.
In order for this preservative to be most suitable
for use in the present invention the amount of acetic and/or
formic acid should be increased 10 to 25 per cent. Further-
more/ steps have to be taken to improve the fixation after
the preservative has penetrated the cell wall.
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Besides such water preserving solutions, preserv-
atives dissolved in organic solvents can be used even if
they are not to be preferred, they are also rather expensive.
The wood may be treated with a preservative solution
as follows. For example, the wood may be initially pressured
treated according to the full cell method. Treatment of
the wood using a method according to Lowry or Ruping may
also be used.
After the pressure treatment the wood is stored
for some time under conditions such that it cannot dry and
such that the preservation composition solvents cannot
evaporate. This can be done, for example, in a closed space
or the wood can be covered. Of course, the wood can also be
stored or kept in contact with a preservative solution, but
as a rule such arrangements are not very practical. The
storing time may last as long as two to four weeks but can
be even longer. During this time the preservative diffuses
through the cell walls. The diffusion process occurs
slowly but with great effectiveness.
After the diffusion step the wood is dried whereby
the solvents evaporate and the preservatives are
precipitated in fixed form within the wood. ~ -
In addition to this treatment it is advantageous
to warm the wood. The wood can be warmed either before,
during or after the introduction of the preservative
solution.
,
A warming prior to the introduction of the preserv-
ative will facilitate the penetration of the preservative
into the wood. It is most conveniently done with steam or
water at, for example, 80 to 110C. The enhancement of the
wood treatment is a function of the temperature of the wood
and also of the dimensions of the wood as a longer time is
dm:~ ~ ~ 9 ~
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requircd for w~rming wood of lncreased dimensions. Thc wood
may be warmed in any conventional manner For example,
boards with a thickness of 25mm can be steamed or warmed in
water for one or two hours, while poles have to be treated
for at least two to four hours. Longer warming periods are
rarely employed.
The warming of the wood may also be carried out
during the diffusion period after the preservative treatment.
As a general rule the same high temperature cannot be kept.
The temperature has to be limited to a maximum of 40 or 50C
on cases wherein ammonia and carbon dioxide (e.g., in the
form of ammonium bicarbonate) are used as dissolving agents
in view of their tendency to readily evaporate. If, for
example, acetic acid is used as a dissolving agent, higher
temperature may be employed. Lower temperature warming can
be used for an extended time, for example, during the entire
diffusion period, with the warming increasing the rate of
the diffusion.
The wood may also be warmed at the end of the
diffusion period in order to increase the drying rate of the
wood and the fixing rate of the preservatives.
The preservatives may be introduced into the wood
by several methods. If the wood is incompletely dried, a
pressure preservation treatment with a strong solution, for
example, a double strength solution, may be applied. Such a
pressure treatment does, of course, not provide for full
penetration of the wood. During the following diffusion
period, however, the wood treatment is improved because the
preservative penetrates and is distributed by diffusion into
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i~l9352
the wood. It also slowly penetrates the cell wal]s. Even
undried (green) wood can thus be preserved. This is accom-
plished by soaking the wood in a strong solution from which
the preservative diffuses into the wood. The soaking is
then followed by a diffusion period as earlier described.
Such treatments are, however, time consuming.
Diffusion by means of soaking in strong conditions
is a known treatment. Soluble salts have been used in this
treatment and the salts have remained soluble ~i.e., unfixed)
even after the treatment. The salts may have a high preserv-
ing effect but they have not been able to completely protect
the wood since they are later leached out from the wood.
The use of chromic acid solutions, for example, should
generally provide very poor penetration and distribution of
the preservative in the wood due to early fixation. One
criteria, among others, which aids in the successful perserv-
ation of wood is thus the use of solutions according to the
present invention in order to provide greater diffusion of
the preservatives throughout the wood. `
A preservation method of special interest is open-
tank preservation. This treatment is carried out by first
warming wood by suitable means such as by steam, water or
a hot preservation solution, and then soaking the wood in the
preservative solution. During the soaking step the wood
absorbs the solution and is penetrated by the preservative.
The desirable warming of the wood may thus be included as a
part of the preservation step itself. If the preservation ;
solutions of this invention are used and if the wood there-
after is diffusion stored as desribed, foliiferous wood can
be successfully preserved. The method is simple both in
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vicw of apparatus and performance, which isa great advan-
ta~e in less industri~lly developed countries where folli-
ferous wood often exists. In such areas the simple but
tlme consuming method of soaking in solutions according to
this invention may be acceptable.
It should be noted that there is a difference
between diffusion that provides introduction and distribu-
tion of the preservative in the wood and diffusion that
gives penetration through the cell wall.
The diffusion that provides for introduction and
distribution of the preservative in the wood occurs by the
time consuming diffusion methods of soaking in strong solu-
tions. The problem is to introduce the preservative into
the wood. The penetration of the wood occurs in one direc-
tion from the surface to the inner parts of the wood. For
thicker wood a longer diffusion time is needed because the
preservative must diffuse through a larger mass of wood.
Diffusion through the cell walls is distinct from
diffusion into the wood. In this case the preservative
surrounds the cell and the preservative must pass through
the cell wall from all sides and different directions. It
is a very short and comparatively constant length of travel
for the preservative and is independent of the passing
direction and the dimension of the wood.
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S~PPLEMENTARY DISCI.OSUR~
It has now been found that, with an ammoniacal pre-
servative solution of the type taught on Pages 5 and 7 of
the principal disclosure, the ammonia content of the solution
can generally range from about 0.6 or 0.7 percent to about 5
percent by weight. The amount of ammonia may exceed about 5
percent by weight but such would not increase the effectiveness
of the preservative treatment as the solution would already
be rendered sufficiently alkaline to achieve the aims of the
invention by using amounts of ammonia within the stated
range. Enough ammonia need only be added to neutralize the
arsenic and chromic acids as well as providing an excess
thereof. The pH of the solution is advantageously increased
to at least about 8.2 in order to provide the desired
alkalinity, and preferably ranges from about 8.2 to about
10. Advantageously, such a solution will contain from about
0.16 or 0.2 percent to about 0.8 percent by weight of copper.
Preferred aqueous ammoniacal preservative solutions for
use in the present invention have been found to have about
the following composition:
CuO 0.2 to about 1.0 percent by weight
CrO~ 0.1 to ab~out 0.5
As205 0.1 to about 0.7
NHa 0.6 to about 2.8
CO2 0.8 to about 2.8
In a preservative solution for use in the present
invention which contains acetic acid as the dissolving agent
for the copper and zinc compounds, a preferred formulation
comprises an aqueous solution containing from about 2 to
nbout 6 percent by weight of a preservative having about the
following composition:
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copper ~Icetate 20 pcrcent by wei~ht
zinc acetate 70
acetic acid lO.
With ammoniacal preservative solutions, the increased
effectiveness of the preservatio~, treatment of the present
invention is believed to be due to the fact that a correlation
have been found to exist between the preservative content of
the wood (e.g., copper) and the loss in dry mass of the wood
when ammoniacal preservatives are employed together with
maintaining the treated wood in an undried state for a period
of time sufficient for the preservative to diffuse through
the cell walls (i.e., delayed drying). No such correlation
has been found to exist with such preservative solutions
when conventional drying procedures are employed. In fact,
the loss in dry mass has been found to be significantly higher
for wood preservatives with conventional (i.e., normal) drying
as opposed to delayed drying. Accordingly, the wood must be
kept in an undried condition for a sufficient period of time
to provide the required penetration of the wood fibers by the
preservative.
Table 1 below sets forth the results of field tests
which confirm the previous statements. Several types of
preservative solutions were used in the field tests for
the sake of comparison. The wood samples used were initially
treated with the preservative solutions and subjected to either
delayed drying (DD) or normal drying (ND) conditions. The cubes
were then exposed to the elements for a period of 18 months,
with the average loss in dry mass of the cubes being measured
after 12 and 18 months.
The following preservative solutions and procedures were
used to determine the effectiveness of the preservative
~ 14-
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trcatment of the ~resent invention:
A) KP-N special N.: 800 grams of a commercial
ammoniacal prescrvative (tradename Cuprinel Tryck) having a
copper content of 9.7 percent is used as the base solution
and iS described in U.S. Patent 4,001,400. The solution
is ad~nixed with an additional 400 grams of dissolving
ammonium bicarbonate to increase the stability of the
preservative and render it more alkaline. The solution is
admixed with 25 liters of water to provide a solution
containing 3.2 percent of preservative.
B) KP-N special 2N.: The ingredients employed above
to produce KP-N special N are doubled and dissolved in 25
liters of water to provide a preservative concentration of
6.4 percent.
C) Amline special: Amline is a tradename of a
preservative containing 6 percent Cu, 4 percent As20s~ and
4 percent CrO3. Ammonium bicarbonate is used as a dissolving
agent to provide stability. A solution is made up which
contains 4.25 percent of the preservative.
D) KP-N: 800 grams of the preservative Cuprinel
Tryck were dissolved in 25 liters of water to provide a
concentration of 3.2 percent.
E) CCA: The preservative contains As20,, CrO3 and Cu
corresponding to 11.8 percent Cu, 13.8 percent chromium,
and 22.2 percent arsenic and was used in a concentration of
1.8 percent.
The wood samples were impregnated in various ways
using the above solutions. Those samples which were treated
with one of solutions A, B or D were contacted with the
preservative at a vacuum of -0.75 bars for 45 minutes followed
- by a pressure treatment at 11 bars for 1 hour. Those samples
J~ 15-
.
~193S~
trented with solutions C and E were contacted wlth the
preservatlve at a vacuum of -0.95 b~rs for 30 minutes and
a pressure of 10 bars for 1 hour.
Wood samples treated with solutions A, B and C were
separately tightly wrapped ln plastic and stored for three
weeks outdoors under cover (i.e. ? delayed drying (DD)
conditions) at an average temperature of 15C. Samples
treated with solutions D and E were allowed to dry outdoors
under cover (i-.e., normal drying (ND) conditions).
At the end of three weeks the wrapped wood samples
were exposed to air and allowed to dry in the same manner
as the non-wrapped wood. After a 1~ month drying period
the samples were placed in contact with the ground to
determine their resistance to rot. Samples were buried in
soil at about a depth of 30 millimeters in a greenhouse at
a temperature ranging from 10 to 20C depending upon the
season. Samples of the buried wood were extracted from the
soil after exposure periods of 12 and 18 months to determine
their loss in dry mass.
The degradation of the wood was examined using light
microscopy and scanning electron microscopy (Cambridge S 150).
Thin transverse and longitudinal sections of the wood were
stained with safranin and viewed under the light microscope.
Polarized light was used for the longitudinal sections. The
results of the tests are set forth below in Table 1.
~ 16-
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~9~52
TABLE 1
Retention of Preservative Elements and Loss in
ry Mass oE Test Wood Samples
Retention (mg/~ Loss in Dry Mass
Treatment Cu Cr As 12 months 18 months
4.25% Amline
sp DD
Beech 3.14 1.01 1.40 5.0 5.5
Birch 3.40 1.07 1.44 4.4 5.0
3.2% KP-N
sp DD
Beech 2.09 -- -- 3.6 9.0
- Birch 2.51. -- -- 2.3 7.1
6.4% KP-N
sp DD .
Beech 5.05 -- -- 1.5 1.6
Birch 3.87 -- -- 2.7 4.3
3.2% KP-N
ND
Beech 2.69 -- -- 11.0 10.1
Birch 2.61 -- -- 7.6 15.3
1.8% CCA
ND
Beech 1.83 1.91 2.54 7.7 15.8
Birch 2.71 2.78 4.86 8.4 20.9
Untreated
Control
Beech -- -- -- 46.9 60.7
Birch -- -- -- 37.1 47.3
jl/:,' -17-
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Soft rot wns the only typc of nttnck observed in the
wood. Both cavity formation (Type l) and erosion of the
cell walls (Type 2) was observed.
In the wood treated with solution C with delayed drying,
Roft rot attack in the beech wood was confined mainly to the
outermost parts of the blocks. Type 1 and 2 attack occurred
in patches between areas of non-decayed fibers. The birch
wood treated with solution C exhibited only a minor attack
of Type 2 after 12 months and no cavities were found. After
18 months both types of attack were observed in the outermost
parts of the block.
The main type of attack observed in the wood treated
with solution A with delayed drying after 12 months was
Type 1. Strong patches of erosion were observed after 18
months. Both Type 1 and 2 attack was found in the birch wood.
Only a minor amount of attack was observed in the beech
and birch wood treated with solution B with delayed drying.
Both Type 1 and 2 attack occurred in the outermost parts of
the beech wood. Large areas were not attacked at all. Only
Type 2 was observed in the birch wood after 12 months and the
attack was restricted to the outermost portions of the wood.
No attack was observed in the birch wood after 18 months.
Solution D treated birch and beech wood exhibited
strong soft rot attack of both types.
The wood treated with solution E exhibited homogeneous
soft rot attack. Type 1 appeared to be the dominate type of
attack in the beech wood, while the birch wood was degraded
- by both types of soft rot.
It has also been found that while the actual period
during which the treated wood must be kept in an undried
condition will vary with the preservative employed and the
Jl~ 18-
3~
type of wood beln~ treated, it can be stated thnt the wood
should be kept in an undried condition for a sufficient
period of time to provide the necessary retention of the
preServatlve (e.g., copper) in the individual wood fibers
(i.e., sufficient diffusion through the cell wall has
occurred). An energy dispersible X-ray spectometer may be
used to determine the preservative content of the wood fiber.
While the invention has been described in connection
with several preferred embodiments, it is to be understood
that the teachings are illustrative rather than restrictive
and further modification may be resorted to without departing
from the spirit of the invention or the scope of the claims.
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