CA1080915A - Medium density, high strength, lignocellulose composition board including exhaustively hydrated cellulosic gel binder - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A medium density, high strength lignocellulose composition board comprises on a dry weight basis from 60 to 95% lignocellu-lose particles and from 5 to 40% exhaustively hydrated cellulosic gel binder, the gel binder being characterized in its gel condition by a TAPPI drain time of at least 350 seconds.

Description

~L~8~5 MEDIUM DENSITY, HISH STRENGTH -LIGNOCELLULOSE COMPOSITION BOARD
INCLUDING EXHAUSTIVELY HYDRATED
CELLULOSIC GEL BINDER
This invention relates to lignocallulosa composition boards.
It pertains particularly to wood composition boards of high strength and medium density, e.g. a density of 20 to 50 pounds -per cubic foot.
In the building trades need exists for a composition board t specifically a wood iber board, of medium density and increased : .. . .
strength which retains the other necessary and desirable iber-board properties of dimensional stability~ thickness swelling~
water absorption, uniformity~ and ~he like. If such a fiber board product were to be available commercially, it could be sub-stituted in building materials speciications for fiber boards of substantially higher density. This would result in significant savings of raw materials, as well as in important economies in building material costs and transportation costs.
Accordingly 9 it is the general purpose of the present inven-tion to provide a composition board product of medium density and gr~atly increased strength which is charact rized also by accep-table properties of dimensional stability, thic]cness swelling, water r0sistancs 9 and uniformity.
It is a urther purpose o the present invention to provide a composition board product of lo~ cost which can be produced in high yield from its original starting materials.
A urther object of the present invention is the provision of a composition board product which does not require the inclu-sion together with its primary Æibrous lignocellulose component o an excessively large quantity of an -xotic binder o high cost.
Still another object of the present invention is the provi-sion of a composition board product including a low cost bind2r which serves the ancillary functions o acting as a dispersant for -th~ lignocellulose fiber compon~nt of the board, as a finas ' ~ ' .

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retention ag~nt/ and also as a board density control a~ent.
A stlll further object of the present invention is the pro-vision of a rapid and economical process for the manufacture o composition boards having the above noted qualities of medium den-sity and high strength.
I now have discovered that the oregoing and other objects of the present invention are achieved by the provision of a medium density composition board comprising broadly~ on a dry weight bas-is, rom 60 to 95% by weight o~ lignocellulose fiber and from 5 to 40% by weight of a unique cellulosic gel binder, As will be described in detail hereinafter, the cellulosic gel binder employ-ed is exhaustively hydra~ed so that it is characteri~ed in its gel condition by a TAPPI drain time o at least 350 seconds, pre-erably at least 900 seconds, and specifically from 900 to 2000 ssconds .
The medium density board product of the invention has a den-sity of from 20 to S0 pounds per cubic oot. Howaver, it has :
strength properties which are characteristic of ~rior art fiber boards having a much higher density. For example, a medium dan-sity fiber board o the invention at a density o 30 pounds per ~`
square inch meets the current product standards of the American Hardboard Association for building siding and furniture cores for wood iber boards having a density o~ 42 pounds per cubic foot.
The standa~ds met include modulus o rupture, tensile strength, thickness swelling and water absorption. The ~avings in raw mat~
erial use, transportation costs, and product cost resulting rom the substitution o the hersind2scribsd medium weight product in building specifications are immediateIy apparent.
The noted im~rovement in strength is achieved, urthermore, at no sacrifice o the other im~ortant properties required of f;b-er board productsO
This desirable result is achieved as a direct consequence of the inclusion in the heraind scrib2d iber board products o~ an

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axhaustivPly hydrated cellulosic gel blndar as a major component of the board-making furnish. In the forming of the board product, ,, the gel component not only acts as an efficient binder in a quick pressing operation to bind the particles of lignocellulose into , , an integral bo~rd, it also acts as a dispersant for the fibers so that a uniform board free from the presence of bunched fiber ag-gregates is obtained~ It also shrinks during drying of the board with the result that it compacts and integrates the other board components. Being water resistant per sa, it augm~nts the water resistant characteristics of the finished board product.
The medium density, high strength, li~nocellulosa composi-tion boards of my invention are illustrated by the following gen-eral and prefarred formulations, wherein percent is given in per-csnt by weight, on a dr~ weight basis:
General Composition(~) Preferred Composition(~

Lignocellulose Particles 60 to 95 70 to 90 ' ' '' Hydrated cellulosic ' ~'' ' gel binder 5 to 40 10 to 30 ' '~ ' The lignocellulose which is the primary structural component , of the hereindescribed medium density composition boards may be derived from a wide variety of souraesO Representative sources includa bagasse and such ~oods as the wood of the sucalyptus, cot~

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tonwood~ willow, alder, Douglas ir and pine, ~"
Whers woody materials are employed the~ may be used in the '' form of chips, planer shavings, flakes or sawdust, T~hat~ver the~
source, the materials first are size reducad, preferably by being ;~
defibratad to the form of lignoceIlulosic fibers and fiber bund This is accomplished in suitable apparatus, such as Bauer or Sprout-Waldron mechanical refiners. A pr~ferred defibrating sys- ' tem includes the conventional Asplund defibrator wherein wood ' pieces are subjected to abrasive reduction in an environm~nt of steam undar pressure of ~rom 40 to 150 pounds per s~uara inch.
, If desirable or necessar,y, the fibrous product of the Asplund ,~ , .

s machine may b~ passed to a secondary refiner such as an Asplund raffinator for ultimate reduct;on to a particle size of 10% maxi-mum plus 12 mesh, U. S Sieve Ssries.
The sPcond primary constituent of the hereindescribed medium density hi~h strength lignocellulose composition boards is an ax-haus-tively hydrated cellulosic gel binder used in an amount of ~rom 5 to 40%, preferably 10 to 30% by weight, dry board basis.
Th~s material distinguishes from the hydrat~d cellulosic gels of the prior art in that it is exhaustively hydrated to a condition in which it has substantially no ~iber struc~ure at aIL
The difference becomes evident when the gels are subJacted to the conventional Schopper-Riegler freeness test, or TAPPI Standard T221-OS-63.
In accordance with TAPPI Standard T221 OS-63~ used to meas-ure the fraeness of various slow pulps such as are under conside~
ation here, a pulp sample is formed into a sheet 6-1/4" in dia- ~ -meter in a Williams sheet mold. The sheet formed in the mold contains 1.2 grams o~ dry pulp or gel. The drainags time requir-ed to orm the shaet is mçasured and becomes an indica~ion of the degree of hydration of the stock.
In using this test, it readily is possible to distinguish the hereindescribed highly hydrated cellulo~ic gel binders from the hydrated cellulosic gel products of the prior ar~, such as those employed in the manufacture of glassines paper, and those employed in the manufacture o prio~ art compos;tion board pro~
ducts such as are disclosed in Roberts U. S. 3~3791608-9 Both o~ these made us~ of hydra~ed cellulosic gels having a drain time, as measured by the above mentioned TAPPI Standard T221-OS-63 of about 300 seconds.

3~ I have discovered~ however, that it is possible to h~drate cellulosic gel to a degree much more highly advanc~d than that characteristic of th~ g~ls employed in either glassines papar or the composition boards o~ the aforesaid patents. Thus, whereas the named prior art gels have a TAPPI standard freenass measured by a drain time of about 300 seconds, the gels employ~d in my in-vention have drain times of at least 350 seconds, preferably above 900 seconds, and in particular from 900-2000 secondsO
Other test methods may be employed to id~ntify and characte~
ize ths hereindescribed exhaustively hydrated cellulosic gel bind-ers.
One such is to determine the shrinkage upon drying of the hand sheet producad by the above described TAPPI test. A suit-ably hydrated gel will form a hand sheet which shrinks upon dry- ~
ing to a diameter which is at least 35% smaller than its original :
diameter.
In a third method~ the hand sheet is dri~d and a small flame applied to its underside. If the cellulose is suficiently hy-drated, the flame instantaneously will produce a blister on the sheet. :
In a fourth test m~thod, 250 mlO o the refinad pul~ slurry is dried into a solid ball. If the gel is sufficiently hydrat~d for the present pur~ose~ the ball will sink when dropped into water and thereater will remain hard without swelling for an in-definite pariod o~ submergence, The use o the thus characterized exhaustively hydrated cellulosic gel binders in the medium density composition boards ~ i of my invention is oritically significant or ssveral reasons.
In the ~irst place~ the gel serYns as a highly e~ficient binder which binds the lignocellulose fibers togsther in an inte-grated product. Upon drying, the gel binder shrinks materially, pulling the particles together and locking them irreversibly in their consolidated condition. This factor is of primary ;mpor-tance in determining the increased strength of the board produat.
Ths gel al80 serves as a binder~ which acts very rapidly toset the thickness and density of the felted sheet when it is in-troduced into the press, This makes possiblz a quick pressing .

operation, i.e. one r~quiring a press time of only 10 seconds to three minutes~ preferably from 10 seconds to 60 seconds. This in turn produces significant economias in requiring only a relatively expensive single opening press, which easily is adaptable to in-clusion in conventional plant production lines.
Several other important advantages stem from the use of the novel cellulosic gel binder described above. The binder imparts water resistance to the board product in which it is contained.
As noted, such produc~s contain from 5 to 40% by weight o~ th~
binder and the binder is so highly hydratad as to be substantial~
insolubls in wat r. This imparts a high degree of binder water resistance to the board product. The gel binder is superior in this respect to conventional fiber board binders such as the urea formaldehyde thPrmosetting resins.
Still further, the gel is a highly ~fficient dispersion aid in the formulation of the fiber slurries rom which the boards are made. In this application, it disperses into individual fib-ers wood fiber clumps which may be oontained in the slurry It also thickens the slurry so that as it is run on the wire at a consistency o~ about 5% ther~ is little tendency or any light partic~aas contained in the slurry to float, or any heavy parti-cles to sink. Still further, it is responsibl~- for the Formation of a very stable slurry which is converted to a finished board product of completely homogeneous cross section, Fourth 3 the gel serves as a fines retention agent. This is of particular importance during vacuum forming o the board from a pulp slurry run onto the screen~ During this procedure, ~inely ~ ;
divided particles present in the slurry tend to be oxtr2cted from the ~ormed sheet by the run-off water. As a retention aid, the gel serv~s the valuable function of r~taining th2se fines within the sheet, thereby conserving raw materials) improving the prop-erties o the boardD and minimizing a waste disposal problem.
The hyd~ated cellulosic gels suitable or the intended ~ur--6~

po5e are produc~s in which water of hydration is added to cellu-lose molecules by the substantially complete beating or refining of cellulose in aqueous medium. The cellulos~Q ther~by is con-verted from a fluffy~ fibrous condition into a gelat;nous condi-tion, the degree of conversion being dependent upon such varia-bles as the duration of the rafining, the naturs of the refining equipment, the presence or absence of extraneous chemiaals, etc.
Conventionally~ the conversion i5 effectuated by mechanically treating cellulose pulp in aqueous medium in disc type refiners equipped with lava tackle or in conical refiners such as the Jor-dan.
The cellulose pulp for the gel manufacture may be derived from any one of several sources, such as the bleached or unbleach ed wood or bagasse pulps manufactured by the convsntional sul-phate or sulphite papermaking processes. If bagasse is employed as the ultimate raw material~ it preferably is depithed before being pulped. The pulps are available on *he large commercial scale in the form of dried pulp sheets~
In the manufacture of ths hereindescribed gel products, the cellulose pulp is rsfined and hydrated exhaustively to a high de-grea at which fiber structure is almost completeIy destroyed. ~--This is accomplished by breakin~ down the cellulose pulp sheets to their component individual fibers or fib~r slumps, preferably by adding the dry sheets and water to a conventional hydrapulper, and hydrapulping at a stock consistency of from 1 to 10~, p~efer- -ably from 6 to 8%, This requires about 30 minutas.
The resulting pulp then is pumped to a storags tank and f~d in con-trolled flo~ to the selected disc type or conical ~ype pri-mary refinerO There preferably are three such refiners arranged in series with a flow restricting valve downstream from the last refiner to insure an adequate refiner dwell time. These abrade the pulp and hydrate it to a high degree.

The resulting partially refined and hydrated pulp is pass~d .
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into a second storage tank which supplies a secondary refiner of the same general class as the primary refiner, but which is effective to complete the hydration and size reduction of the puip to values imparting to the pulp TAPPI standard drain times of at least 350 seconds, preferably above 900 seconds. This is accomplished by a shearing action which almost completely destroys the fiber structure of the pulp and hydrates it ex-haustively. This supplemental and exhaustive refining greatly improves the qualities of the pulp as a binder, dispersing agent, and retention agent when used in the manufacture of the hereindescribed medium density composition boards of high strength.
In particular, it makes of the gel an "irreversible"
binder. This means that a composition board made with the hereindescribed highly refined gel binder can be subjected to the action of boiling water for several hours, with little softening or relaxing of the adhesive bond present between the bonded fibers. If the board is dried after having been sub-jected to such a boil test, it is just as strong as it was before boiling. This i5 the same characteristic exhibited by phenolic resins when they are used as binders. It no doubt is responsible for the improved water resistance of the com-position boards of my invention.
In addition to the lignocellulose fiber and hydrated cellulosic gel binder which are the principal constituents of the hereindescribed composition boards, there may be included ;
in the boards varying amounts of supplemental materials such as sizing material used in the amount required to develop desired properties in the board product. In particular, there may be incorporated in the boards from 0.5 to 3% by weight of conventional wax sizes such as Petrolatum or Hercules* Paracol :'i ': ' ' ' ~ /
*Trademark ~ r - 8 -', ' :' ' . . , ' ' ', '. ': ' ' . . . ',. ',, : ' ' .

wax, in order to improve the water resistance of the boards. .
Other additives which may be added are fire retard-ants such as borax, boric acid and ammonium phosphate; supple-mental fibers such as sisal or fiberglass used in an amount of from 5 to 15%

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to increase toughness and impact resistance; pigments~ and sup-plemental binders such as starch and phenolic rasin binders, used in amount appro~riate to the development of dssired properties.
In the manufacture of the high strenath, medium density ca~
position boards of my invention, there are compounded together ~lows of three separate ~urnish components: Lignocellulose fibsr slurry~ hydrated cellulosic gel, and wax emulsion and/or other additives.
The lignocellulose fiber slu~ry is prepared as indicated above. In a typical instance, pulp logs with or without prelim-inary debarking are reduced to chips in a conventional chipper.
The chips are fed to an Asplund Defibrator followed by a conven-tional Asplund typo Raffinator which reduces the chips to a fib-er slurry having a particle size of 10% maximum plus 12, U. S, Sieve Serias. The slurry has a consistency of about 40~ through the Defibrator and about 30% through the Raffinator.
In a preferred manner of operation the hydrated cellulosic gel component is pr2pared by feeding Kraft pulp to a convention- -al hydrapulper in which it is disintegrated into a fibrous slur~
ry having a consistency of from 1 to 10%~ preferably from 6 to 8%. -The resulting slurry is fed to a battery of thre~- or four -conventional rafiners such as Jones refiners or modified Jordan ref;ners with straight steel taokle, o~ praferably with lava lin-ings. Thæ charge passes from one of these refiners to another and ultimately to a disc type refiner such as a Jonss duo-flow disc refinsr. The flow through the sequence of refin~rs is ,-throttled down by appropriat2 valving to give a dwell or resi-dence time sufficient to develop a h~drated cellulosic gel pro-duct having a drain tima by the above described TAPPI drainage test T221-OS-63 of at least 350 seconds~ preferably at laast 900 seconds~ and specifically from 900 to 2000 seconds~

The additivæ emulsion is pr~pared by emulsîfying in water a .' ': ' ' _g_ ,, , . ~ , :

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conventional industrial wax such as Hercules* Paracol or petrolatum, and mixing in predetermined amounts of any other selected additives.
The three Eoregoing constituents, i.e. the lignocel-lulose fiber slurry, the cellulosic gel slurry, and the addi-tive emulsion, are fed together in metered flows into a mix-ing tank where they are intimately mixed together in the desired proportions.
The mix tank charge then is transferred to the chest of a forming machine which preferably is of the Fourdrinier type providing a high degree of suction for dewatering the stock. In the forming machine, the charge is run into a wet sheet having a pressed thickness of about one and one-half inches, pressed, in a rotary press section, and cut to length.
Thereafter the resulting sheet is quick pressed to a thickness of fr~m 1/4 inch up to one inch on wire in a heated flat press, depending on the desired ultimate board thickness.
It is a particular feature of the invention that use of the hereindescribed novel cellulosic gel binder makes pos-sible carrying out the press operation in a relatively inex-pensive single opening press. This is for the reason that the gel acts to set the thickness and density of the sheet in the press during a very short press time of from 10 seconds to 3 minutes, preferably from 10 seconds to 60 seconds, at a pressure of 50 to 150 psi and a platen surface temperature of from 150 to 300~F. Exemplary conditions for pressing are 30 seconds at 100 psi and 240F.
Other advantages flow from the application of the quick pressing procedure.
~uick pressing dries out the water to a tipple mois-ture content of about 35% by weight, vs. a conventional level *Trademark ~ - 10 -of 55% so that less kiln time is required. This is accomplish-ed without altering the properties of the gel ~inder.
Quick pressing also preheats the sheet so that less drying time is required su~sequently in the kiln. Also, it forms a ~

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smooth top surface on the sheet which rsmains when the sheet ls dried into a board.
Still further, the quick pressing operation permits control of the density of the finishPd board to within the desired 20 to 50 pounds per cubic foot range. Using the gel binder, it may be carried out without splitting of the shee~ when the sheet is re-leased rom the press, This result clearly is attributable to the efective adhesive qualities o~ the gel binder.
It is followed by a kiln drying operation effectuated in three zones at successive zone temperatures of from 500 to 600F
400 to 500F., and 300 to 400F , with the hi~her temperatures at the head end o the kiln. Drying is continued to a final board moisture content of less than 2%, preferably about 1/2% by weight. Drying removes not only the free water from the board, but also most of ths water of hydration from the cellulosic gel.
The resulting boards are cooled, trimmed, finished as need-ed for the contemplated product, and packaged, ready for shipp~
The presently described medium density, high-strength com-~osition boards and their method of manufacturR are illustrated in the following example:
Eucalyptus wood chips were defiberized in an Asplund Deib-rator operated at l50 psi, followed by refining in an Asplund type Raffinator to a fiber product size of less than 10~ plus 12 mesh U. S. Sieve Ser;es, The fibers were mixed with sufficient water to form a ~iber stock having a consistency of 4%, An exhaustively hydrated cellulosic geI was produced by hy~
drapulping unbleached kraft pulp to a consistency of 8%. The resulting pulp was passed through a series of three Jones Fiber-master No. II conical refiners followed by a Jones Duo Flow shearing type disc refiner. In their operation, the refiners were controlled by valving their discharge ports to achieve resi-dence times within the refiners adequate to produce three types .of gel, one havin~ a drain time of 355 seoonds, another of 1200 seconds and another of 1800 seconds, all as measured by TAPPI
test standard T221-OS-63.
The foregoing constituents ~ere mixed in a mix tank with 1~ Hercules* Paracol wax.
The three components of the furnish were mixed to uniformity, transferred to the chest of a Fourdrinier type form-ing machine and run into board. The board was pressed to thick-ness, dried, and tested for modulus of rupture.
Three sets of boards were thus manufactured and test-ed, using the gels having drain times of 355, 1200 and 1800 seconds, respectively. In each set of tests, three boards were prepared having gel contents of 10, 15 and 20~ by weight res-pectively, dry board basis.
When pressing the first two groups of boards, the press pressure was 150 psi, the press temperature was 240F., and the press/duration 40 seconds. In the third group of boards, ~ ;
a press pressure of 100 psi, a press temperature of 240F., and ;~
a press duration of 25 seconds were employed.
The results are given in the table below:

Example Board Board Gel Gel Stock Modulus of Number Thickness Density Drain Usage Drain Rupture (psi) (inches) (psi) Time (~) Time _ (sec) (sec) 1 .517 24.7 355 10 10 870 2 .509 27.2 355 15 14 1085 3 .496 29.6 355 20 30 1470

4 .525 31.8 1200 10 20 1150 .528 30.0 1200 15 55 1900 6 .500 32.6 1200 20 123 2500 7 .505 30.1 1800 10 63 1620 8 .510 29.8 1800 15 97 2550 9 .495 30.0 1800 20 220 3060 It is apparent from the foregoing that the inclusion of the exhaustively hydrated cellulosic gel binder in the stipulated proportions exerts a profound and most remarkably beneficial effect on the strength properties of the composition -boards of my invention. Thus, as shown in Example 9, including 20~ of a gel *Trademark - 12 -, . . , ., . . ~ . . .

having a drain time of 1800 seconds results in the production ofa board having a density of only 30 pounds per cubic foot, but a modulus of rupture of over 3,000 pounds per squarQ inch. This extraordinarily high modulus of rupturs value is characteristic normally of boards not of the medium density 30 pounds per cubic foot class, but of boards of much higher density~ i,e. boards having densities of ths order of 45 pounds per cubic foot, The resulting saving in raw material costs, transportation costs and board manufacturing costs are apparent. These are achieved~ furthermors, while retaining the other properties of light wei~ht, dimensional stability, uniformity~ water resistance and workability which necessarily must characterize structural composition boards to render them suitable ~or their various uses.

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Claims (16)

The embodiments of the invention in which an exclusive pro-perty or privilege is claimed are defined as follows:

1. A felted, pressed and dried lignocellulose composition board of medium density and high strength comprising on a dry weight basis: % by weight Lignocellulose particles 60 to 95 Hydrated cellulosic gel binder 5 to 40 the hydrated cellulosic gel binder in its gel condition being characterized by a TAPPI drain time of at least 350 seconds.

2. The composition board of claim 1 wherein the lignocellulose particles comprise defiberized wood.

3. The composition board of claim 1 wherein the lignocellulose particles comprise defiberized bagasse.

4. The composition board of claim 1 wherein the lignocellulose particles have a particle size determining a TAPPI-CSF freeness of the fiber of less than 750.

5. The composition board of claim 1 wherein the lignocellulose particles are employed in an amount of from 70 to 90% by weight and the hydrated cellulosic gel binder is employed in an amount of from 10 to 30 parts by weight,

6. The composition board of claim 1 wherein the hydrated cellu-losic gel binder is characterized by a TAPPI drain time of at least 900 seconds.

7, The composition board of claim 1 wherein the hydrated cellu-losic gel binder is characterized by a TAPPI drain time of from 900 to 2000 seconds.

8. The composition board of claim 1 wherein the board has a density of from 20 to 50 pounds per cubic foot and a modulus of rupture of at least 2000 pounds per square inch.

9. A felted pressed and dried lignocellulose composition board having a density of from 20 to 50 pounds per cubic foot and a modulus of rupture of at least 2000 pounds per square inch and comprising, on a dry weight basis:

% by weight Lignocellulose fiber 70 to 90 Hydrated cellulosic gel binder 10 to 30 the lignocellulose fiber having a TAPPI-CSF freeness of less than 750 and the hydrated cellulosic binder having a TAPPI drain time of at least 900 seconds.

10. The process of making medium density high strength composi-tion board which comprises:
a) abrading and beating cellulose pulp in aqueous medium to the condition of a hydrated cellulosic gel binder having a TAPPI
drain time of at least 350 seconds, b) forming an aqueous slurry comprising % by weight dry weight basis Lignocellulose particles 60 to 95 Hydrated cellulosic gel binder 5 to 40 and water in amount predetermined to impart a board-forming consistency to the slurry, c) felting the slurry into a wet sheet on a board-forming screen, and d) drying the sheet e) the hydrated cellulosic gel binder in its gel condition be-ing characterized by a TAPPI drain time of at least 350 seconds.

11. The process of claim 10 wherein the lignocellulosic partic-les comprise wood fiber having a TAPPI-CSF freeness of less than 750.

12. The process of claim 10 wherein the gel has a TAPPI drain time of at least 900 seconds.

13. The process of claim 10 wherein the gel has a TAPPI drain time of at least 900-2000 seconds.

14. The process of claim 10 including the step of drying the slurry while pressing it to a density of from 20 to 50 pounds per cubic foot.

15. The process of claim 10 including the step of drying the slurry by quick pressing it at from 50 to 150 psi and 150 to 300°
F. for from 10 seconds to three minutes, followed by kiln drying the resulting sheet to a predetermined moisture content.

16. The process of making medium density composition board which comprises forming an aqueous slurry comprising:
a) % by weight dry weight basis Lignocellulose fiber 60 to 95 Hydrated cellulosic gel binder 5 to 40 and water used in amount sufficient to form an aqueous slurry of board-forming consistency b) the hydrated cellulosic gel in its gel condition being characterized by a TAPPI drain time of at least 350 seconds c) felting the slurry into a sheet on a board-forming screen, and d) drying the sheet by quick-pressing it to a dry density of from 20 to 50 pounds per cubic foot, at 50 to 150 psi and 150 to 300° F. for a time of from 10 seconds to three min-utes, and thereafter kiln drying the pressed sheet to the predetermined moisture content.