U.S. patent number 3,891,738 [Application Number 05/305,531] was granted by the patent office on 1975-06-24 for method and apparatus for pressing particleboard.
This patent grant is currently assigned to Canadian Patents & Development Limited. Invention is credited to Kuo-Cheng Shen.
United States Patent |
3,891,738 |
Shen |
June 24, 1975 |
Method and apparatus for pressing particleboard
Abstract
The invention relates to a method for pressing particleboard
whereby steam under pressure is introduced into the particleboard
mat during pressing. The invention provides for reduced press time,
improved particleboard quality and makes practical greater board
thickness. The apparatus comprises a pair of platens having
apertures and associated conduits for steam and exhaust, and means
for enclosing the mat while steam is being injected. Steam is
introduced into one platen and exhausted through the other.
Inventors: |
Shen; Kuo-Cheng (Ottawa,
CA) |
Assignee: |
Canadian Patents & Development
Limited (Ottawa, CA)
|
Family
ID: |
23181178 |
Appl.
No.: |
05/305,531 |
Filed: |
November 10, 1972 |
Current U.S.
Class: |
264/101; 264/120;
264/122 |
Current CPC
Class: |
B27N
3/086 (20130101) |
Current International
Class: |
B27N
3/08 (20060101); B29J 005/00 () |
Field of
Search: |
;264/109,120,82,122,101 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: White; Robert F.
Assistant Examiner: Hall; J. R.
Attorney, Agent or Firm: Bitner; Ronald G.
Claims
What is claimed is:
1. A process for pressing particleboard in a press including a pair
of platens wherein each platen has apertures opening to one surface
thereof which is adjacent to the other platen and chamber defining
means enclosing the region between said platens, comprising the
steps of:
a. compressing a mat comprising wood particles and a thermosetting
adhesive binder between said platens to the desired particleboard
thickness;
b. introducing steam under pressure into the compressed mat through
the apertures of one platen with sufficient pressure to pass
through the mat from one surface thereof to the other surface
thereof, and exhausting the steam through the apertures of the
other platen, restricting the exhaust rate to provide an elevated
and substantially uniform pressure and temperature condition within
the chamber while steam is passed through the mat, for a length of
time sufficient to cure the adhesive binder; and
c. releasing the steam pressure from the chamber and separating the
platens for removal of the particleboard.
2. The process of claim 1 wherein the press platens are maintained
at a temperature at least equal to that of the steam.
3. The process of claim 1 wherein exhausting of the steam is
delayed until the pressure within said chamber defining means
reaches a predetermined magnitude.
4. The process of claim 1 wherein the direction of steam flow is
reversed.
5. The process of claim 1 wherein said mat is prepressed prior to
being pressed between said platens.
Description
This invention relates to a method for pressing particleboard, and
more particularly to a method and apparatus comprising the
introduction of steam under pressure into the particleboard mat
during the pressing thereof.
In the production of particleboard, a thermosetting adhesive binder
is mixed with wood particles, a mat is formed and pressed. In
conventional pressing the heat required to cure the adhesive binder
is transferred predominately by conduction from the surfaces of the
hot press platens and some time is required to raise the
temperature at the center of the mat. With increasing mat
thickness, press time does not vary linearly with board thickness
but increases more rapidly.
In the classic "steam shock" or "steam jet" technique the surface
layers of the mat are given a high moisture content, for example,
by spraying with water. As the hot platens contact the surface
layers of the mat the water vaporizes and moves towards the center
of the mat. In this way the temperature of the core can be raised
more quickly. This technique has the limitation that the amount of
moisture must be optimumly balanced because the higher the moisture
content, the faster the temperature rise, but also the longer the
press time to eliminate the excess moisture which interferes with
the curing of the adhesive binder and causes blisters. With this
method there is also difficulty in applying water at the bottom
surface of the mat because of the hot caul plate on which the mat
is formed.
It has been proposed to reduce press time by passing low pressure
steam through the mat from one edge to the other during
conventional hot pressing. However, with this method it was found
that the maximum temperature which could be reached in the center
of the mat at the discharge side was 100.degree.C. The main
drawbacks of this method are that temperatures and moisture
gradients develop along the steam flow direction across the mat
which would probably result in warping of a full size (4 .times. 8
foot) board, and that larger mats would require a longer time for
the steam to pass through.
In the commercial production of particleboard, press time is the
"bottleneck" on the production line and is the major factor in
determining production efficiency. With increased board thickness
production efficiency is decreased further. Another disadvantage of
conventional pressed particleboard is that it is susceptible to
thickness swelling when exposed to moisture unless treated after
pressing.
It has been found that since a particleboard mat is normally porous
in construction, it is possible to force steam under pressure
through the mat by means of apertured press platens. The injected
steam provides rapid heat transfer to the mat for rapid curing of
the adhesive binder. Steam is injected through an apertured platen
on one side of the mat and exhausted through another apertured
platen on the opposite side of the mat. By confining the mat in a
suitable chamber and restricting the exhaust, elevated temperatures
and pressures can be maintained within the chamber to obtain both
rapid cure of the adhesive binder and a relaxation of the
compressive stresses placed on the wood particles when the mat is
originally compressed to the desired board density. The steam can
be rapidly released from the mat at the end of the cycle with the
release of pressure from the chamber through the apertures in the
platens.
In accordance with the present invention particle-board is pressed
using a press including a pair of platens wherein each platen has
apertures opening to one surface thereof which is adjacent to the
other platen and chamber defining means enclosing the region
between the platens. The process comprises pressing a mat
comprising wood particles and a thermosetting adhesive binder
between the platens, introducing steam under pressure into the mat
through the apertures of one platen and exhausting the steam
through the apertures of the other platen, restricting the exhaust
rate to maintain an elevated pressure and temperature within the
chamber for a time sufficient to cure the adhesive binder, and
releasing the steam pressure from the chamber and separating the
platens for removal of the particleboard.
Valves associated with the chamber and steam source provide means
for pressurizing the chamber, controlling the steam flow through
the mat and releasing the pressure from the chamber.
An object of this invention is to provide a method for pressing
particleboard with shorter press time.
Another object is to provide for the making of particleboard having
a thickness greater than has previously been practical or
economical in a flat press process.
Another object is to provide particleboard which has improved
characteristics particularly with respect to water absorption and
thickness expansion.
Yet another object of this invention is to provide dimensional
stabilization of the particleboard with the pressing operation.
The invention will be further described with reference to the
drawings in which:
FIG. 1 is a schematic sectional view of the apparatus;
FIG. 2 is a sectional view taken at 2--2 of FIG. 1;
FIG. 3 shows an alternate embodiment of a portion of the
apparatus;
FIG. 4 shows an example of the typical relationship between press
time and board thickness for a conventional process and the
steam-press process of the present invention.
Referring to FIG. 1 the apparatus comprises a pair of press platens
1 and 2 which are adapted to be moved relative to one another by
suitable means (not shown). Between the platens 1 and 2 is an
annular sealing frame 3 of the desired thickness, which rests on
the lower platen 2. When the platens are pressed together against
the sealing frame member 3, a sealed chamber 4 for the mat 5 is
defined. The inner face of the platens have a plurality of
apertures 6 and 7 interconnected with passageways 8 and 9,
respectively, which communicate with a pressurized steam source
through conduits 10 and 11. Inlet valves 12, 13, and 14 control the
steam input. The passageways 8 and 9 also connect with conduits 15
and 16. Exhausting and release of steam pressure are controlled by
valves 17 and 18.
Each platen includes passageway means 21 connected with conduits 22
to a source of a heating fluid, preferably also steam, which keeps
the platens at the desired temperature.
In operation, the particleboard mat 5, preferably prepressed and
contained by two screens 25 for ease of handling, is placed on the
lower platen 2 within the sealing frame 3. The platens 1 and 2 are
brought together relative to one another in sealing engagement with
the frame member 3, defining a chamber 4. In order to raise the
pressure within the chamber quickly steam inlet valves 12 and 13
are opened, while the outlet valves 17 and 18 remain closed. After
the desired pressure is reached, the steam inlet valve 13 is closed
and the outlet valve 18 is opened partially to allow steam to flow
through the mat 5 while maintaining the desired pressure in the
chamber 4. After the desired steaming time the inlet valve 12 is
closed and all exhaust valves are opened to release the steam
pressure before the press is opened. During the above operation the
temperature of the platens is controlled by the temperature of the
heating fluid through the passageways 21. The temperature of the
platens is preferably 5.degree. to 20.degree.F higher than that of
the steam injected.
The apparatus of FIG. 1 also provides for changing the direction of
steam flow. By correctly manipulating the inlet valves 12 and 13
and the outlet valves 17 and 18 the steam can be injected into the
mat through the top or bottom platen 1 and 2 respectively. It was
found that for thick boards, reversing the steam flow direction at
some point during the steam injection operation, provides for more
uniform and rapid heating of the mat.
In the alternate embodiment of FIG. 3, radial passageways 39 are
used to facilitate varying the spacing of the apertures 37 radially
from the center of the platen 32 to provide uniform steam flow to
all portions of the mat. The density at the edges of a laterally
unconfined mat will be lower hence the porosity of the mat in this
region will be higher. Having more widely spaced apertures at the
edges provides more uniform flow through all regions of the
mat.
In the following examples the apparatus used was generally of the
type shown in FIGS. 1 and 2. All particleboard mats were prepressed
for ease of handling. The platens 1 and 2 each had dimensions of 24
.times. 24 .times. 2 5/8 inches. Passageways 8 and 9 were formed 3
inches apart, 1/2 inch from each platen's inner surface. The
apertures 6 and 7 were spaced 2 1/2 inches along the passageways 8
and 9 respectively. Asbestos gaskets 23 and 24 were placed between
the platens 1 and 2 and the sealing frame member 3 to enable the
chamber 4 to maintain steam pressures of at least 300 psi with a
press force of approximately 150 tons. When the desired steam
pressure was reached within the chamber 4 the outlet valve 18 was
opened to allow steam flow of about 0.4 to 0.5 lb. per minute while
maintaining the pressure within 2 percent. After the desired steam
time had elapsed the inlet valve was shut off and the exhaust
valves opened to release the pressure in the chamber and the mat.
The release time was normally less than 30 seconds. The indicated
"press time" refers to the time for the whole press cycle from the
time the press was loaded until opened. "Steam-time" indicates the
duration of steam flow through the mat. Platen temperature was
maintained above the steam temperature to prevent condensation.
Steam-pressing refers to the pressing method according to the
present invention while conventional pressing refers to pressing
without the use of steam injection.
EXAMPLE 1
Wood of two species, poplar (Populus grandidentata Michx) and sugar
maple (Acer saccharum Marsh) was cut into flakes or made into
splinters by hammer-milling veneer. All the flakes and splinters
had a moisture content between 6 and 9 percent before resin
application.
A commercially available, liquid phenol-formaldehyde resin
containing 43 percent solids at pH 11.6 to 11.9 was used for all
boards. It was sprayed on in a laboratory rotating drum
blender.
No paraffin was used in any of the boards.
Three types of board were made: a homogeneous board (splinters-1/8
to 1/4 inches screened, with 4 percent resin content); flakeboard
(flakes-0.035 .times. 1 1/2 .times. 1/8 to 2 inches, with 5 percent
resin content); and a three-layer board made with fine splinters
(1/16 inches screened, with 8 percent resin content) on face layers
and coarse splinters (1/8 to 1/4 inches screened with 6 percent
resin content in the core).
The pressed board was weighed immediately and then cut in half.
Each half was weighed again and its thickness measured. One half
was immediately placed in an oven for a 16-hour post-cure at
220.degree.F and oven-dry weight determination. The other half was
conditioned to equilibrium at 70.degree.F and 65 percent relative
humidity. The same conditioning was given to the post-cured halves
before testing. Density was calculated on the basis of oven-dry
weight and conditioned volume.
Torsion-shear strength was measured on 1-inch square specimens,
systematically sampled from each half board. The normal
torsion-shear strength was obtained from ten specimens which were
tested dry. The wet torsion-shear strength was measured on another
ten specimens which had been subjected to an accelerated aging
treatment (2-hour immersion in boiling water) and were tested
wet.
All the torsion-shear values reported are averages of ten
measurements made at the centre plane. The internal bond strength
in psi is about 11.3 times the torsion-shear value in ft-lb for a
1-inch square specimen.
Thickness expansion and water absorption tests
PRESS MOISTURE THICK- STEAM TIME BOARD CONTENT FAILING NESS WATER
PRES- STEAM PLATEN [STEAM THICK- DEN- (HOT TORQUE EXPAN- ABSOR-
BOARD TYPE SURE TEMP. TEMP. TIME] NESS SITY BOARD) (ft.-lb.) SION
PTION & THICKNESS (psig) (.degree.F) (.degree.F) (min.) (in.)
(pcf) (%) DRY WET (%) (%)
__________________________________________________________________________
MAPLE SPLINTERS 200 388 400 2 [1] 0.949 44.7 12.4 12.9 4.4 10.0
60.3 1-inch 200 388 400 3 [2] 0.929 45.7 13.4 13.5 4.6 10.8 59.0
200 388 400 4 [3] 0.903 45.8 11.6 13.0 5.3 9.2 56.6 200 388 400 5
[4] 0.894 45.9 13.0 13.5 5.1 12.0 53.9 200 388 400 6 [5] 0.900 45.9
13.5 12.6 4.9 9.1 57.9 200 388 400 8 [7] 0.891 46.4 12.1 13.6 5.7
9.0 55.3 200 388 400 10 [9] 0.889 44.6 11.6 11.5 4.7 8.4 57.3 260
410 415 3 [2] 0.924 56.8 13.1 21.0 8.7 9.6 38.4 (CONVENTIONAL 400
20 0.984 40.9 2.1 10.8 4.5 23.4 89.2 PRESS) 400 25 0.981 41.0 1.5
13.6 5.2 23.5 90.5 400 30 0.983 43.2 1.2 11.5 4.3 24.4 89.5 400 20
0.984 47.0 3.2 13.0 2.3 34.4 83.5 400 25 0.986 45.2 2.4 12.6 2.6
34.0 89.0 400 30 0.985 44.2 1.9 14.0 2.9 32.0 88.8 400 20 0.986
53.1 4.2 24.8 4.1 42.9 79.6 2-inch 200 388 400 2 [1/2] 2.014 47.3
9.6 15.1 3.2 11.1 54.4 225 396 405 5 [3] 1.953 42.7 10.1 11.2 4.9
10.8 55.6 225 396 405 5 [3] 1.950 42.5 10.6 11.2 4.7 11.0 60.0 225
396 405 5 [3] 1.952 42.5 10.5 11.4 4.9 10.1 58.3 250 405 415 2 [1]
1.986 47.3 9.7 16.8 5.3 8.9 51.3 (CONVENTIONAL 400 50 2.013 47.6
6.2 16.9 0.0 56.6 94.6 PRESS) 5-inch 200 388 400 11 [4] 4.894 45.6
15.8 14.2 5.2 9.7 57.6 MAPLE FLAKES 200 388 400 6 [3] 1.967 43.0
10.6 15.8 4.7 9.2 57.6 2-inch 225 396 400 6 [3] 1.962 42.1 10.0
12.7 4.9 9.0 54.2 POPLAR FLAKES 150 367 380 5 [3] 1.945 41.0 14.0
7.5 3.6 11.7 58.0 2-inch 150 367 380 5 [3] 1.875 37.1 15.1 6.2 2.0
11.3 68.7 150 367 380 5 [3] 1.856 35.7 14.7 5.4 2.9 10.5 72.6 200
388 400 7 [4] 1.843 45.2 14.2 8.6 3.9 9.4 57.2 200 388 400 7 [4]
1.845 45.3 14.0 8.6 3.7 9.6 56.7 3-inch 225 396 405 7 [5] 2.895
35.5 15.2 4.6 2.7 7.6 67.1 225 405 405 7 [5] 2.875 37.5 14.7 4.9
2.9 9.8 58.5 225 396 405 5 [3] 2.965 35.9 14.2 4.5 2.5 15.2 80.0
120 350 380 7 [5] 2.902 38.5 14.6 6.0 1.3 13.1 82.5 POPLAR
SPLINTERS (THREE-LAYER) 2-inch 200 388 400 5 [3] 1.897 37.5 13.6
4.3 2.1 11.7 57.3 3-inch 225 396 405 7 [5] 2.886 36.6 15.0 4.3 1.9
10.2 57.8
__________________________________________________________________________
were carried out on 1-inch square specimens from both the
post-cured and control half of each board. For 1-inch thick board,
ten specimens were placed face to face with their upper edge 1 inch
below water level. The water temperature was kept within 2.degree.
of 70.degree.F during the 24-hour soaking period. Thickness and
weight increase were recorded on a percent of original basis at 1,
2, 4, 6, and 24 hours.
The table indicates that the dry torsion-shear strengths were
generally the same for steam-pressed and conventionally pressed
board, but the wet torsion-shear strengths were higher for
steam-pressed board after the accelerated aging treatment referred
to above. The steam-pressed board showed substantial improvement of
dimensional stability. Both thickness expansion and water
absorption were reduced.
The series of 7 1-inch maple boards show the effect of steaming
time at 200 psi. In general board quality was not significiantly
improved by prolonging the steam time.
EXAMPLE 2
About 28 lb. of hammer milled hard maple particles (>1/16,
<1/4 mesh size) were sprayed with a commercial liquid
phenol-formaldehyde resin (4 percent solid resin based on oven dry
particle weight), and measured into 6 lb. batches, and each batch
was formed and prepressed into a 1 1/2 inch mat. A series of 1-inch
boards (43-48 pcf density) was made from these mats at saturated
steam pressures varying from 50 to 300 psi with two different
steaming times (1/2 and 3 min.).
It is evident that the highest steam pressure
__________________________________________________________________________
STEAM-PRESS CONDITIONS STEAM STEAM PLATEN STEAM FAILING TORQUE
THICKNESS WATER PRESSURE TEMP. TEMP. TIME THICKNESS DENSITY
(ft.-lb.) EXPANSION ABSORPTION (psig) (.degree.F) (.degree.F.)
(min.) (in.) (pcf) DRY WET (%) (%)
__________________________________________________________________________
1 50 297 350 1/2 0.977 45.1 8.0 0 23.2 91.6 2 100 338 350 1/2 0.970
46.0 9.8 1.1 18.4 68.3 3 150 367 380 1/2 0.959 44.2 9.2 1.6 14.1
71.8 4 200 388 400 1/2 0.958 44.3 13.2 2.4 12.1 65.4 5 250 405 415
1/2 0.942 45.6 14.6 2.2 13.6 63.3 6 300 422 430 1/2 0.942 46.7 16.9
5.6 10.1 54.3 7 50 297 350 3 0.975 43.0 11.5 1.9 18.2 72.7 8 100
338 350 3 0.963 43.0 12.5 2.4 12.9 74.6 9 150 367 380 3 0.946 43.9
12.9 4.8 9.8 60.5 10 200 388 400 3 0.900 45.8 13.0 5.3 10.6 56.6 11
250 405 415 3 0.899 45.9 12.0 5.1 7.0 52.6 12 300 422 430 3 0.879
48.3 11.6 4.9 8.7 50.0 13 300 422 430 9 0.828 44.2 10.9 2.7 5.2
56.3
__________________________________________________________________________
(300 psi, 423.degree.F) combined with a 3-min. steam time, produced
the board having the least thickness expansion. The same steam
pressure combined with a 1/2-min. steam time resulted in the board
having the highest torsion-shear strength. It appeared that a
higher steam pressure or a longer steam time would further reduce
the thickness expansion but at the expense of torsion-shear
strength. This was confirmed by one board, steam-pressed at 300 psi
steam pressure (423.degree.F) for 9 min. which showed a lower
torsion-shear strength (10.9 ft-lb) and a lower thickness expansion
(5.2 percent), than the boards steam-pressed at 300 psi for 1/2 and
3 min.
EXAMPLE 3
Hammer milled spruce particles (1/4-1/20 mesh size) were sprayed
with a commercial liquid urea-formaldehyde resin (10 percent solid
resin on oven dry particle weight). Several batches of this
material, each 5.7 lb (10 percent moisture content) were formed
into 15 .times. 15-inch mats. The mats were prepressed at 150 psi
pressure to consolidate the mat to 1 1/2 inch thick for easy
handling. The prepressed mats were steam-pressed into 1 inch thick
boards at various steam pressures with different press and steaming
time as shown in the following table. However the press close time
(1/2 min.), steam release time (1 min.), and platen temperature
(350.degree.F) were constant. Torsion-shear strength and thickness
expansion for 5 steamed-pressed boards and one
conventionally-pressed board are also shown.
__________________________________________________________________________
Thick- Steam Press ness Temp. Time Torsion- Expan- Steam [Platen
[Steam Pressure Temp.] Time] Density Shear sion (psi) .degree.F
(min.) (pcf) (ft-lb) (%)
__________________________________________________________________________
Steam Pressed 1 30 269 3 [1 1/2] 40.3 12.6 8.2 2 50 297 3 [1 1/2]
42.0 13.0 7.0 3 100 338 2 [1/2] 40.3 11.2 7.9 4 100 338 3 [1 1/2]
42.1 12.6 5.8 5 100 338 5 [3 1/2] 42.6 1.8 (delaminated)
Conventionally Pressed 6 [350.degree.] 15 43.3 12.7 16.1
__________________________________________________________________________
It was further found that there was a significant difference in
springback (permanent swelling after the specimen had been boiled
in water for 2 hours and reconditioned to reach equilibrium at
70.degree.F and 65 percent relative humidity) between steam-pressed
and conventionally-pressed boards. The springback for steam-pressed
poplar flake-boards was found to be 2 to 5 percent while for
conventionally pressed boards it was 21 to 33 percent.
Ths moisture content of steam-pressed boards was found to be
reduced by 1 to 2 percent from an initial moisture content ranging
from 11 to 14 percent. Lower steam pressure or a longer steam time
tended to produce boards with lower moisture content than did
higher steam pressure or a shorter steam time. It was also found
that thick boards had higher moisture contents than thin boards and
that flakeboards had higher moisture contents than homogeneous
boards after steam pressing.
The optimum steam time for proper curing is determined partly by
the porosity of the particleboard and also the nature of the
thermosetting adhesive binder. It was found that a less porous mat
required longer steam-time and/or higher steam-pressure. Porosity
is dependent on the wood particle configuration and board density.
For example, flakeboard is less porous than a homogeneous splinter
board of the same wood species and density.
Press time is shortened by increasing steam temperature, pressure
and steam flow rate through the mat.
For the laboratory press used, it was found that for a 1 inch
phenolic bonded flakeboard the steam consumption for the
steam-press process was in the order of one third that of the
convention process in which steam is used to heat the press
platens.
It will be understood that other means for maintaining platen
temperature such as electrical heating means may be used in place
of steam.
It will be evident that particleboard thickness will be determined
by the spacing of the platens due to the sealing frame with
allowance for springback which is dependant on the physical
properties of the mat and the pressing condition.
The present invention is not to be limited to thickness of 5 inches
or less but appears to be equally applicable to thickness of
greater than 5 inches.
It will be understood that the present process is not limited to
the use of the apparatus described herein.
* * * * *