US3504073A - Method of preparing fiber boards - Google Patents

Description

United States Patent METHOD OF PREPARING FIBER BOARDS John F. Rakszawski, Somerset, N.J., assignor to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed Oct. 23, 1967, Ser. No. 677,019 Int. Cl. D04h 1/16; B29j 5/00 US. Cl. 264-113 9 Claims ABSTRACT OF THE DISCLOSURE Low to medium density fiber boards with smooth, hard finished surfaces ready for printing are prepared by cold prepressing a fiber board mat with an outer pith layer at low pressures and then hot pressing the mat at higher pressures.

BACKGROUND OF THE INVENTION This invention relates to the manufacture of fiber board, especially board made from milled bagasse. More particularly it relates to improved boards which have smoothhard surfaces and low to medium density interiors.

It is known to make fiber board from wood chips, vegetable fibers, or other fibrous materials. Generally the materials are milled or refined to a convenient fiber size range and then coated with a thermosetting resin. The coated fibers are then formed into a loose mat and pressed in a high temperature press at conditions under which the thermosetting resin cures and hardens to rigidly bind the mat in a final desired board structure. The pressing operation can be carried out in a single stage hot press, but often it is more desirable to first press the fibrous mat in a cold press to form a structure as nearly like the final board structure as is feasible and then finish the operation by a final press and cure in a hot press. In the manufacture of many types of fiber board it is desirable to provide surfaces which are dense, hard, strong, smooth, and which have excellent features of printability and uniformity of texture and dimension. Providing a high density surface is relatively easy in the manufacture of boards which have high density interiors, i.e., hardboards. However, in the manufacture of boards in which low density or medium density interiors are desired, e.g., for insulating or acoustical purposes or where a low board weight is desirable, it is very difiicult to produce hard surfaces.

One material which has been found to be very useful in the manufacture of high quality fiber boards of low to medium density is bagasse, the milled fibrous byproduct of a sugar cane mill.

Bagasse, a by-product of sugar manufacture, is the dried crushed residue of the sugar cane stalk. It consists principally of fibrous components and parenchyma, generally (and hereinafter) referred to as pith. The fibrous components include the rind fiber and the fibrovascular fiber bundles or cells from which the juice has been extracted. The fibrous components, especially the fibre-vascular fiber bundles or shives, are united or bonded together with the pith, though some of the pith is found in the bagasse in loose powder form.

The pith is a nonorganized, nonfibrous, pulpy substance. Its chemical nature, light cellular structure and high surface area, relative to its mass, make it a highly reactive and absorbent material of limited commercial utility. It is, in fact, most often a waste product.

In prior art processes, the pith, which constitutes a major proportion of the bagasse, is separated via chemical or mechanical means from the fibrous materials as completely as possible, and then discarded. This has been considered necessary to provide a raw material suit- 3,504,073 Patented Mar. 31, 1970 able for the formation of structural boards or finished products of optimum structure and quality, viz, optimum for machine operation, product quality and low chemical cost per ton of pulp produced. In the separation, large quantities of the fibro-vascular fiber are also unavoidably removed along with the pith, and generally discarded therewith. This represents additional waste and, though bagasse is variable in composition, the waste can range above about 50 percent based on the initial weight of the bagasse.

The surface qualities which can be developed from bagasse fiber in conventional processes can be varied to some extent depending upon the processing or extent of preparation of the fibrous materials remaining after separation of the pith, i.e., the conventional reject fraction of the bagasse. Thus, after removal of the pith, relatively coarse fibers are left behind. On the one hand, the coarse fibers can be formed into a mat and processed into a board of homogeneous structure, the surface quality of which is rather coarse. On the other hand, a portion of the pith can be reapplied upon the coarse fiber and pressed to form a hard surface layer.

Although the pith surface on a bagasse fiber board has outstanding properties when properly pressed to high densities, it is very difficult to achieve these properties in the manufacture of low to medium density boards, i.e., densities from about 10 to about 40 lbs./ft. When the boards are pressed only to such low densities the surfaces which result tend to be too soft and powdery and therefore have poor printability characteristics and a very low surface strength.

SUMMARY OF THE INVENTION It is an object of the present invention to provide the art with improved fiber boards having very hard surfaces yet having only low to medium density interiors. It is a more specific object to manufacture a low to medium density fiber board from bagasse fibers and provide the board with high density surfaces composed of pith and bagasse fines. A particular objective is to provide a process by means of which bagasse can be used to make low density boards with high density pith surfaces suitable for printing, inking, and other surface treatments.

These and other objectives can be achieved in accordance with the present invention. This invention contemplates a process for making a low to medium density fiber board in which a loose mat is formed of thermoplastic resin-coated ligno-cellulosic fibers covered by a layer of particulate surface-forming material and the material is first cold pressed at low pressures and then hot pressed at high pressures.

While any ligno-cellulosic fibers can be used, an especially high quality board is produced by using depithed milled bagasse in preparing the mat. In combination with the fibers, the pith and fines which are obtained by milling and depithing bagasse are outstandingly effective for use as the particulate surface-forming material of this invention. The pith and fines fraction is obtained by milling green bagasse, e.g., in a conventional Bauer mill, until the bagasse fibers are separated and broken to suitable mat-making sizes, generally ranging from about /2 to 2 inches, or even longer. In so milling t he bagasse the pith and fines fraction is recovered by screening or air classification as a product generally less than about 10 mesh (Tyler) and preferably less than about 30 mesh.

Conventional binder rasins can be used although especially preferred resins include phenol-formaldehyde and ureaformaldehyde resins. The resin is added generally as an aqueous solution and can be added to or coated on, the fiber and pith fractions either before or after the pith fraction is separated. When the resin is added to the milled bagasse before the pith and fines fraction is separated, about 7 to wt. percent based on the total solids, may be used. .When the resin is added separately to the fibers and to the separated pith and fines fraction the amount used varies from about 7 to 14 Wt. percent on the fibers and about 13 to 30 wt. percent on the pith and fines fraction.

In preparing the loose mat in accordance with this invention conventional techniques can be employed. Preferably air felting. is used, and the mat is formed having at least a layer of fibrous material and a layer of surface forming particulate material such as the pith and fines fraction of milled bagasse. Preferably, in making bagasse fiber boards both the upper and lower sides of the fiber layer are covered by a pith and fines layer in the mat.

The loose mat is first cold pressed at low pressures. Preferably only sufficient pressure is used to compress the matinto a structure to hold together to transport the cold press to the hot press. It is essential that the cold press be operated at sufiiciently low pressure to avoid compressing the. mat to densities greater than about three-fourths the final desired board density. In any event, the initial press or prepress should be carried out at pressures less than about 400 p.s.i.g. and generally will range from about 50 to about 300 p.s.i.g. When the final desired board density is in the low density range, i.e., about 19 to about 25 lbs./ ft. the cold prepress pressure is preferably about 50 to about 150 p.s.i.g. When the final board density is in the medium density range, i.e., about 25 to about 40 lbs/ft. the cold prepress is preferably carried out at about 150 to about 300 p.s.i.g. In general, the lower the prepress pressure the denser the board surfaces will be in the final board structure. It is thus essential only to press the mats in the cold press at sufficient pressure to give them a structural rigidity sufficient to hold the mat in one piece and avoid breakage when it is conveyed or transported from the cold press to the hot press. i

The hard surface for the'fiber board is prepared in the hot press by closing the latter as rapidly as possible to build up sufficient backpressure in the mat to compact the particulate surface into a dense smooth form. The surface density'reaches asuificiently highvalue to form a compact and smooth surface even before the center of the mat, i.e., the fibrous interior, is substantially compressed. The hot pressclosure rate is then slowed to conventional rates andlclosed to a final desired board density and held in position until the center of the board is cured and the resin is hardened.

- Inthe hot press cycle, it is essential that the initial closure of the platens or pressingsurfaces be at high rates sufficient to compact the particulate surface-forming material. In the caseof 'pith coated bagasse fiber board it is foundthat initial closure rates must be at least about 2.5 inches per minute and preferably at least about 10 inches per minute. In any event, the faster the closure rate the harder and more densely compacted will be the pith surface of the finished board.

It is not clear why a 'low pressure cold prepress is superior to a high pressure cold prepress in making a hard surface board. It is believed, however, that itis essential to avoidover-compression in the cold prepress cycle in order to obtainsufiicient back pressurein the final hot pressing cycle to harden thesurface. Thus, if the fiber board mat is over-compacted in the cold prepress step, then everrwith very rapid closure of the press in hot press cycle, insufiicient back pressure will build up before the overall board density reaches a maximum desirable density and thus the surfaces will not have reached a sufficiently high density to provide the desired characteristics.

In the following examples green bagasse is first milled in a conventional disc mill, i.e., Bauer mill, and the trash and part of the pith is discarded. The remaining portion contains principally fibers with about 20 wt. percent pith and fines.

The bagasse is then mixed with a solution of phenolformaldehyde resin in the amount of about 14 wt. percent, based on total fiber fines and pith. The application of the resin is carried out in a second milling step in which the fibers are broken down generally to lengths less than about 2 inches and the pith is essentially completely milled loose from the fibers. After milling is complete, the pith is separated from the fibrous material by conventional air classification. A substantial quantity of very finely divided particulate fibrous materialis removed along with the pith and no attempt is made to separate the fines from the pith. The pith and fines fraction is remilled to degrade any extraneous long fibers and a minus 10 mesh fraction is recovered.

Mats are prepared from the resin treated fibers and pith and fines fractions by air felting. The pith and fines are fed from a hopper and blown onto a moving conveyor belt to deposit a smooth layer thereon. Fibers are then deposited from a second hopper onto a layer of pith and fines, and finally a top layer of pith and fines is felted onto the fibers. Although ratios of pith and fines to fibers canrange as low as about 12% to about in these examples the total pith and fines to fibers is about 17%. The pith and fines fraction is divided approximately equally on the lower and upper sides of the fibrous interior fraction of the mat.

Loose felted mats of various thicknesses are prepared and prepressed in a cold press at ambient temperatures under various pressures as summarized in the Table I. The prepressing is carried out until the density of the mat is increased to a density corresponding to the final desired board density in the ratio indicated in the table.

The cold prepressed mats have sufiicient strength to be transferred without breakage from the cold press to the hot press by conventional means, i.e., by conveyor belts or manually. The most critical variable is the strength of the lower pith and fines surface since it is subjected to various shocks and scrapes in transporting the mat and may tend to be loosened from the fibrous material unless prepress pressures are above at least about p.s.i.g.

The hot press platens are maintained at temperatures between about 200 F. and 400 F., about 300 F. in these examples. The temperature is sufiicient to cure the phenol-formaldehyde resin. The press is closed at about 12 inches per minute until the pressure builds up to an amount sufficient to provide good compaction and high density of the surface pith and fines fraction. This pressure is generally found to be in the range of about 50 to 150 p.s.i.g. In these examples the pressure is built up to about p.s.i.g. and then the closure rate of the press is decreased or completely stopped. -The press is held in stopped position only for a very short period, if at all, suflicient to cure the surface resin and to allow any vaporized moisture to escape. Thus the time for holding the press in stopped position varies depending upon the thickness and density of the board being pressed. After the holding time in stopped position is complete, the press is closed slowly at about 0.2 inch per minute until final board thickness is achieved. It is then held in this final position long enough to provide a total holding time in the hot press suflicient to cure the resin in the interior of the board. The total hot press cycle holding times are also indicated in the table.

Examination of the finished boards indicates excellent characteristics have been obtained. Thus the printability of the surface is excellent, and printed letters have excellent clarity and no detrimental absorption or diffusion into the pith and fines surface is noted.

TABLE I Hot press holidng time Product density Product thiek- Prepress pres- Density Initial 2 Total 3 (lbs/ftfi) ness (inches) sure (p.s.i.g.) ratio 1 (seconds) (minutes) Low (1 1% 50 0. 67 Low-medium (23) 1% 75 0. 59 0 15 Low-medium (28)- 1% 230 0. 44 2 18 Medium (36) 230 0. 44 3 6 1 Ratio of final board thickness to mat thickness after prepress.

2 Initial hot press holding time is the time after initial rapid partial closure of the press that it is held without .iurther closure to allow the mat to breathe.

3 Totalhot press cycle holding time is the total time the mat is pressed in the hot press until pressure is released to remove the product board.

In sharp contrast to the foregoing when the col-d prepress step is carried out at pressures in range of about 700 p.s.i.g. the final board products are found to have loose powdery surfaces which tend to diffuse and absorb ink and fail in the standard printability test. The surfaces are also soft and can be easily scratched with a fingernail.

Comparisons of the modulus of rupture (MOR) and the modulus of elasticity (MOE) of boards prepared using high pressure cold prepresses with those of boards prepared with low pressure cold prepresses as shown below in' Table II illustrate the outstanding improvement obtained by employing the present invention.

What is claimed is: 1. A process for making a low to medium density fiber board product having a ready to print hard, finished surface consisting essentially of coating lignocellulosic fibers with thermosetting resin, forming a loose mat containing a layer of the resincoated fibers and at least one other layer of particulate surface-forming lignocellulosic material capable of being pressed and cured into a rigid surface, prepressing the loose mat in a press at about ambient temperatures insuflicient to cure and harden said mat and at pressures less than about 400 p.s.i.g. and which are too low to compress the mat to a density greater than about three-fourths the final desired fi-- ranging from about 10 to about 40 lbs/ft at temperatures sufiicient to cure and harden the mat to form a fiber board having a hard, finished surface ready for printing without further surface treatment.

2. The process of claim 1 wherein said lignocellulosic fibers comprise depithed bagasse fibers,

3. The process of claim 2 wherein said particulate surface-forming material comprises a pith fraction of milled and depithed bagasse.

4. The process of claim 3 wherein said mat comprises layers of said pith fraction on both sides of said layer of fibers.

5. The process of claim 4 wherein said pith fraction includes a thermosetting resin binder.

6. The process of claim 1 wherein the prepress pressure ranges from about 50 to about 300 p.s.i.g.

7. The process of claim 6 wherein the prepress pressure is about 50 to about 150 p.s.i.g. and the fiber board product has a density ranging from about 10 to about 25 lbs./ft.

8. The process of claim 6 wherein the prepress pressure ranges from above about 150 to about 300 p.s.i.g, and the fiber board product has a density ranging from above about 25 to about 40 lbs./ft.

9. The process of claim 6 wherein the thermosetting resin in said pith fraction and in said fiber coating comprises phenol-formaldehyde resin.

References Cited UNITED STATES PATENTS 3,021,244 2/1962 Meiler 264- 3,413,395 11/1968 Schnitzler et al 264l20 FOREIGN PATENTS 791,256 2/1958 Great Britain.

ROBERT F. WHITE, Primary Examiner J. R. HALL, Assistant Examiner US. Cl. X.R. 264-120