Note: Descriptions are shown in the official language in which they were submitted.
Process for densifyinq low density wood_
Background of the Invention
This invention relates to an improved process for
producing a densified solid wood product.
It has been known for many years that wood can be
plasticized for forming by treatment with an hydrous
ammonia. For instance, U.S. Patent 3,282,313 teaches that
wood plasticized with an hydrous ammonia can be compressed
by cold compression in a press to convert the wood to a
much harder r mar-resistant, glossy-surfaced product.
While such processes have been known for many years,
practical and inexpensive commercial systems have not been
developed.
It is, therefore the object ox the present invention
to provide an improved process whereby low quality, low
density woods can be inexpensively densified or compressed
to form wood products having the characteristics of high
quality natural hardwoods.
Summary of the Invention
In the process of the present invention, a low quality,
low density solid wood of high moisture content is firstly
impregnated with an hydrous ammonia whereby the wood member
is plasticized to a sponge-like form saturated with water
and ammonia. This plasticized wood member is placed
62~
between the press plates of a cyclic press and, while
maintaining the temperature of the plasticized wood member
below 100C, it is subjected to a plurality of compression
cycles with the wood member being compressed to a predator-
mined thickness, held at that thickness for a short period
of time and released during each cycle. In this manner,
water and ammonia are squeezed out of the wood member down
to a moisture content of less than 30~ to obtain a damp
wood member of predetermined reduced thickness. Tilts damp
compressed wood member is then dried to obtain a dry,
permanently densified solid wood product having the
characteristics of high quality natural hardwood.
Description of the Preferred Embodiments
The wood which is used in the present invention is a
low quality, low density wood preferably of the hardwood
type, typically having a density of about 300 - ~00 Kg/m3.
Coniferous woods of high gum content, e.g. pine, spruce,
balsam, etch, are generally not suitable. Among
particularly useful woods for the process of this
invention, there can be mentioned poplar, alder, cotton-
wood, rubber tree and soft maple. With some woods, such
as poplar, it is preferable to use swooped only,
substantially free of any hearted.
It is particularly advantageous with the process of
this invention to utilize freshly cut, green wood of high
moisture content. Such wood will typically have a moisture
of at least 50% and usually at least 80~.
For densifying in accordance with this invention, the
wood can be cut into planks having thicknesses of up to 2
inches or it can be in a form of a thin veneer, e.g. having
a thickness of about 1/8 inch, or it can be in the form of
a reglued wood laminate.
Ammonia Treatment
An hydrous ammonia is a strong, hydrogen bonding, low
molecular weight solvent which penetrates not only into
amorphous areas of the fiber cell wall but also into the
~36;~55~
lignin binding material of a wood member. Some of the
hydrogen bonds responsible for the rigidity of wood are
broken by the ammonia. This results in a softening or
plasticizing of the giber structure so that the wood
assumes a sponge-like characteristic and can be compressed
at moderate pressure. As the ammonia is removed from the
wood, hydrogen bonds are again formed between the polymer
chains, although not necessarily at the same locations on
the polymer chains or between the same microfibrils,
resulting in the wood member once again becoming rigid.
It is necessary that an intimate contact be achieved
between the ammonia and the fiber structure. To achieve
this, the wood member being impregnated with ammonia
generally contains about 20 -30~ moisture, although
moisture levels both above and below this range may be
used. However, if the moisture content is too low, a
proper plasticization will not be achieved.
For treatment with ammonia, the solid wood members are
placed in a vacuum chamber and a negative pressure is
applied. Typically a negative pressure of about one
atmosphere (30" Hug may be used for about 15 to 20
minutes. Ammonia gas may be added to the chamber while
still under vacuum. At the end of the vacuum stage,
ammonia gas is pumped into the chamber, conveniently at
coo temperature, and at a pressure of preferably about
100 to 150 psi. This ammonia pressure is maintained for
usually about to 8 hours depending on the thickness of
the wood, the species, etc. At the end of the ammonia
pressure stage, the chamber is vented and evacuated at a
negative pressure for about 15 minutes.
This ammonia treatment stage may also conveniently be
used for crying the wood. Thus, at the beginning of the
ammonia treatment, the wood may be first impregnated with
a dye which uniformly dyes the wood member throughout its
thickness. Dying in this manner has the advantage that
-- 4
when the densified wood members are cut or planed, the
newly exposed surfaces are of uniform color.
cyclic Pressing_ _
The wood members removed from the vacuum chamber are
thoroughly plasticized and are in a sponge-like form
saturated with water and ammonia. The plasticized wood
members are then placed between the press plates of a
cyclic press and, while maintaining the temperature of the
plasticized wood members below 100C, they are subjected
to a plurality of compression cycles with the wood members
being compressed to a predetermined thickness, held at
that thickness for a short period of time and released
during each cycle. In this manner, water and ammonia are
squeezed out of the wood members down to a moisture
content of less than about 30~ to obtain a damp wood
member of predetermined reduced thickness. In a typical
procedure for densifying planks having a thickness of 1 to
2 inches, the cyclic pressing is conducted at a press
pressure of-about 175 psi for a total of about 2-5
minutes. Each press cycle has a duration of about 1/2 to
1 minute. In this manner, the thickness of the wood can
be reduced by an amount of up to 50~ and the density
increased from about 300 - 400 Kg/m3 to as much as 1000
Kg/m -
It is to be understood that the above pressing limes
and pressures can be varied quite widely depending on the
species and thickness of the wood member being densified
and the densification required. For instance, a wood
member plasticized with ammonia will undergo a densifi-
cation of about I without any external compressing.
Roth the top and bottom press plates are preferably
perforated so that during the cyclic pressing, the water
and ammonia can emerge not only from the side edges of the
wood member but also from the top and bottom faces.
us During this stage, the water and ammonia are squeezed out
of the wood much in the manner of squeezing a sponge.
~23~Z~ -
Large quantities of water and ammonia (both dissolved and
gaseous) can be removed from the wood member very
inexpensively and in a very short period of time using
this simple mechanical squeezing technique. For instance,
it can reduce the moisture content from as high as 80-90%
down to as low as 20-30%. This makes it practical
according to this invention to start work with a freshly
cut, green wood member containing ~30-90% moisture.
This is an important economic advantage of the present
invention. It is a very expensive procedure to prudery
wood down to the optimum moisture range of 20 -30~ for `
impregnating with ammonia. By starting with green wood of
very high moisture, impregnating this with ammonia and
squeezing ammonia and water out of this quickly at quite
low pressures and temperatures, great savings are realized.
When applying cyclic pressing to a thick wood member,
it is preferable to use temperatures below 100C, thereby
avoiding formation of steam and resultant damage to the
wood. It has also been found to be particularly
advantageous to subject the wood member to high frequency
vibration during cyclic pressing, since this helps the
removal of water and ammonia from the wood.
At the end of the pressing stage, the wood member
remains in a compressed state of reduced thickness while
still containing a considerable quantity of moisture.
The above cyclic pressing technique is particularly
useful for densifyin~ thick wood members, such as planks
and reglued laminates. However, when densifying thin
veneers, e.g. 1/8 inch thick, steam formation is not a
serious problem and so they can be densified by simpler
pressing methods and at higher temperatures. For example,
thin veneers can be densified at temperatures as high as
200C and the densification can be carried out between
rollers.
~3~25~
By using several pairs of rollers in series, the
complete densification and final drying of thin veneers
can be carried out in one stage. The first pair of rollers
squeeze out water and ammonia, intermediate rollers fix
the veneer at a predetermined reduced thickness and final
heated rollers fully dry the veneer.
Dry
The damp compressed wood member from the cyclic
pressing stage is subjected to kiln drying at an elevated
temperature, preferably below 100C, whereby the moisture
content is reduced down to approximately 8%. During
drying, it is not necessary to continue pressing the wood
member but it is necessary to firmly retain it between
support plates to prevent any warping during the drying
stage.
The product emerging from this drying stage is a
permanently densified, high quality hardwood having
substantially the same length and width as the original
stock, but having a substantially reduced thickness. The
densification is consistent throughout the thickness of
the wood member and it can be sanded, planed, ruptured,
drilled, nailed, screwed and sawn similar to other
hardwoods. The product can be stained, polished and
finished at least as well as regular hardwoods and has a
very dense surface with a minimum of texture and porosity,
requiring very little preparation prior to finishing.
pertain preferred embodiments of the present invention
are illustrated by the following examples:
Example 1
Roughly sawn planks of 1 and 2 inch thicknesses were
cut from green poplar and alder. The test planks had a
width of inches, a length of 1 1/2 - 2 feet and a
moisture content of I
The samples thus prepared were placed in a gas retort
with spacers between the planks. The chamber was evacuated
~23 foe
with a negative pressure of approximately one atmosphere
for 15 to 20 minutes, with ammonia gas being introduced
into the chamber during that time. At the end of the 15 to
20 minutes, the vacuum was discontinued and ammonia gas was
pumped into the chamber at room temperature and a pressure
of approximately 100 - 150 psi. This pressure was main-
twined for approximately 6 hours. Following this 6 hour
period, the chamber was vented and evacuated at a negative
pressure (vacuum) of one atmosphere for 15 minutes.
Thereafter, the pressure was released, the door opened and
overhead venting continued.
The plasticized wood samples thus obtained were placed
in a cyclic press between perforated plates. While
maintaining an elevated temperature below 100C, a
pressure of about 175 psi was applied with a cyclic
pressing procedure consisting of 1 minute press cycles for
a total period of 5 minutes. During each down cycle the
perforated pressure plates reduced the thickness of the
wood member-by about 50%, were held in this position for
approximately 45 seconds and then released. In this
manner, the level of water in the wood was reduced from
80% down to about 20 - 30~, at the same time driving out
dissolved ammonia, gaseous ammonia and water, thereby
reducing the plasticization effect on the wood fibers. At
the end of the cyclic pressing stage, wood samples were
obtained having their thickness reduced to about 50~ of
their original thickness.
These samples having a moisture content of about 20 -
30% were placed in a kiln and dried while being firmly
restrained between perforated plates. In this manner they
were dried down to a moisture content of about 8%.
The high quality hardwood boards thus obtained were
subjected to a series of qualitative and quantitative
tests and these were compared with the characteristics of
regular oak, maple and birch boards. The results are
shown in Table I below:
1~36~
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The poplar wood stock produced a walnut-looking hard-
wood, while the alder produced an ebony-looking hardwood.
The densified woods were approximately 1/3 denser than the
3 natural hardwoods, with the densified poplar having an
hardness near equal to the high quality natural hardwoods,
while the densified alder had an hardness twice that of the
high quality natural hardwoods. The natural hardwoods have
a class-III fire rating, while the densified poplar has a
class-II fire rating and the densified alder has a
Claus fire rating
Another important characteristic of the densified wood
products is that they absorbed 40 - 50% less water during a
2 hour soak period than did the high quality natural
hardwoods.
The strength properties of static bending and
compressive strength for the densified wood products were
approximately equal in most cases and in some cases were
superior to the high quality natural hardwood Abrasion
tests indicated that the densified poplar wore down 3 times
less than oak, while the densified alder wore down 1 1/2
times less than oak, during the same period of time.
The densified wood products also showed excellent
glue ability and very good resistance to fungus.
