US2925360A

US2925360A - Method of preventing excessive springback of dry felted fiber mats

Info

Publication number: US2925360A
Application number: US681059A
Authority: US
Inventors: James R Roberts; Ernest H Collins
Original assignee: Weyerhaeuser Co
Current assignee: Weyerhaeuser Co
Priority date: 1957-08-29
Filing date: 1957-08-29
Publication date: 1960-02-16
Anticipated expiration: 1977-02-16

Description

Feb. 16, 1960 J R, ROBERTS ETAL 2,925,360 METHOD OF PREVENTING EXCESSIVE SPRING-BACK 4 OFDRY FELTED FIBER MATS Filed Aug. 29, 1957 2 Sheets-Sheet 1- 1 A a: V

Figure 2 Figure 3 V :;\i :72 iii-1 '3 I4 kwmwmammnvmummwfim I Figurei JAMES R. ROBERTS ERNEST H. coLLms mveurons ATTORNEY J. R. ROBERTS ET AL 2,925,360 METHOD OF PREVENTING EXCESSIVE SPRING-BACK OF DRY FELTED FIBER MATS Filed Aug. 29, 1957 2 Sheets-Sheet 2 JAMES R.

ERNEST H.

ROBERTS COLLINS INVENTORS ATTORNEY METHOD OF PREVENTING EXCESSIVE SPRING- BACK OF DRY FELTED FIBER MATS, James RJRobe'rtsQLoingvieW, and Ernest H- Collins, .Kelso, ,Wash fassignors to Weyerhaeuser Company, .;;a corporation of Washington r j invention relatesto manujfacturei of felted" mats from individualized ultimate wood fibers, and has particular reference to a method of preventing excessive spring-back of such'mats' after cold P essing and improvement. ofthe handling properties of such coldpressed mats in the further processing of the matsto consolidate them into densified wood fiber structures. v

- v The manufacture of felted wood fiber'structures may be carried out by two well-known general procedures used ineifecting felted mats-for the production ofv hardboard, .softboard, insulation blankets and 'batts,'and partially tdensified moldable boards. The two general procedures for effecting such mats involve the deposition ,of the wood 7 =fibers from a dilute liquid slurry, or the, deposition of :dry or moist fibers from a gaseous carrier onto a mat- .forming surface. It is tothe ;latter "of thesegeneral procedures, known as/the dryor ainfelting process, that this invention is directed; Generally speaking,:the 'dry' or .air felting method of efiecting the-fibrous mats involves impacting individualized ultimate wood fibers and opened up bundles, or tracheids, of such ultimate 'fibers from "a :felting device onto a traveling foraminous surface by means of applyingsuction underneath the foraminoussur- :face while the mat'is being formed, The fibers are carried to the area of felting inlapneumatic tube under positive :pressure, and passed through a perforated generally:

=cylindricalfsurfa'ce'into'the mat-forming area; After the mat' has been formed, it is carried by the foraminous' belt-into a cold-pressing area'where the mat is consolidated toa predetermined density for further processing. In an alternative method of mat forming, the ultimate desired thickness of the mat may be built up in a series of felting stages wherein successive superimposed layers ofthe inter felted ultimatewood fibers, which may be the same ordiflerent fibers, are formed in integral relation with each other. Ineither case, the ultimate wood fibers supplied to the felter by the air stream under positive pressure are derived from a'defibering procedure such as is efiected in an Asplund defibrator or other similarknown means. Binders, such as thermosetting resins of the phenolaldehyde type, and such other additives .as may be desired areaddedto the fiber mass ahead of the felting area, geiierally'in the defibering area, so that a uniform mixture of the binder and such additives will have been achieved prior to felting.

n. Fur h rt sne fie fie a 'th ll sb efi laid necessary, therefore, ;in order to provide the mat nap ni asfl str h t at a h ra d an used" for such ,purposes as insulation blankets or nioldable o e; r so that it ma e 'handled' fo'r runner pr essin'gfto a densified 'vvoodf fiber structure suchas. further densification iseffected 'by coldw e math i S tab P essin i c a efrolle'rs'or pressure belts traveling around pairs laidffeltsflor mats-'pos'sessgenerally lower ensities than we'tjlaid'felt's, Atypical-air'laidfelt may Base density in the range" of between 1-3 pounds per" cubic;foot? quires a longer press cycle to insure proper dryingand ing the pressing, as in'the case of addition ofsubstantiali Ice of rollers; This densificationwby cold pressing is effective for consolidating the mats for some purposes, particularlyl purposes Where a low density end product is desired;

Where a high density end product is desired, as in the case of densified wood fiber structures, such as hardboard,

certain difficulties have been experienced in'the handling and further processing of .the mat which has been densified by'cold-pressing or pre-pressing. The dry fibrous materialsof which the mat is constituted have the bulk property of res'ponding elasticallyto the release of applied pressure and causing a spring-bac to a thicknessor .densityintermediate the compressed density under the applied pressure, and the density of the mass prior to compression. The mat then having the said intermediate) density resulting from the spring-back tendsto rupture by shearing and blowout when consolidated between platens in a hot press for densification to a thin hard board. These effects are the result of the failure of a low density mat to provide suficient tensile strength to withstand the disrupting stresses caused by the expulsion of the. relativelylarge volumeof air from within the mat upon theclosing ofthe press platens. The rupture due to shearing can be controlled partially and the blowout reduced somewhat by interposing a wire screen'or other rough surface between a press platen-and one surface of the mat to supplement thetnsile strength. Qther means of correcting these faults of low density'interfelted mats have been suggested, such asthe uti-lizationofthe bonding means by apre-setting technique so as fto ,providefa properly compressed density by interfiber adhesion. However, such methodspreclu'deproper use of the resin binders during the final hot-pressing consolidation;

Another suggestion involves subjecting at least one surface of the mat to heat and moisture while'it ispassing through the cold pressing area by surface application of steam carried moisture. This procedure, proposed in US. Pat'- ent No. 2,635,301, issued April 21, 3, to Dal e L, Schubert et al., is subject to the disadvantages that the thus moistened fibers on the surface of the mat tend' to' adhere to the pressure rollers or belts under the pressure rollers and-thus disruptthe surfacestructure of the airlaid mat, andalso, that the non-surface fibers are g to produce an appreciable amount of] spring-bac l Still another'suggestioninvolves adjusting'the moisture content of the mat tnahigh level toelfect'bettrr consolidation on 'prepr ess'ing'. This proposal has the. disadvantage mentioned above of moistened surface fibers adhering to thepress'ure rollers and also of having greater expulsion.

ofwater and loss of water, solubles upon finalconsolida tron. Furthermore, the increased moisture content reconsolidation resulting in increased costs.

It has no vv'been found that dry air laid wood fiber felts may be made intodense, handleable and useful products which do not have the undesirable characteristics. heretofore experienced, as outlined above. Thenovel method.

described below results in a densification of the airlaid! wood fiber felts witha minimum of spring-back 'without the necessity of advancing the thermosetting .of the.

bonding resins appreciably, and without adversely affect;

amounts of moisture.

, Patented Feb. 16,1960:

asaaaeo V I H A cold-pressing area and at such a negative pressure as to provide a quantity of the dry steam which will increase the temperature of the fibers comprising the mat to such a degree that the temperature of the mat at its center of -c 'ross-section', as the mat leaves the cold-pressing area, is at least 90 'F., while' the moisture content of the mat remains substantially the same.

' A typical process sequence for the formation of densified wood fiber mats from ultimate wood fibers, in which this new method of treating the mat with dry steam for effecting an initial consolidation of the fibers prior to cold-pressing and reducing the amount of spring-back after cold-pressing, is that which is carried out in the manufacture of a plane-surfaced whole wood fiber mat. In1such a process whole wood is reduced to chip form and is fed to'a defibering apparatus .such as the Asplund defibra'tor. In the defibrator the chips are subjected to an atmosphere of steam at a pressure in the range of between about 50-200 pounds per square inch and a corresponding. temperature for saturated steam. The product of the defibrator, which consists predominantly of individualized ultimate fibers and opened up bundles or tracheids of such fibers, is discharged from the defibrator as a moist, fluffy mass. A bonding agent, such as a thermosetting resin of the phenol-aldehyde type, is added at the downstream sideof the defibrator, and other additives, such as water repellents and flame-proofing agents, may be added in or ahead of the defibrator. The fibrous material containing the bonding agent and other additives may or may not be separated from the steam in which it is entrained as it emerges from the defibrator V 4 pressure to subatmospheric pressure, are forcibly impacted together as an interfelted mass of fibers or mat 19 of substantially completely randomly oriented particles.

Immediately after the mat of fibers of the desired thickness leaves the felting area, it is carried by the foraminous traveling surface 14 between an overhead steam chamberzll and a suction box 21 located on the underside of the foraminous traveling surface immediately under the steam chamber 20. The steam chamber is a S-sided box or casket with its open side facing the top surface of the mat of fibers. It should be located-as closely as possible to the surface of the mat and provision should be made for lowering and raising the steam chamber in order to accommodate mats of varying thickness. Alternatively, adjustable deckles can be supplied on the sides of the steam chamber so that adjustment to lower levels of the mat surface can be made. Dry

steam is introduced under pressure through steam pipes 22 into the'inner' space of the steam chamber 20, which latter is at atmospheric pressure. I In order to prevent any condensation of the steam on the sides of the steam chamber with resultant dripping onto the 'mat (which marks the finished board), the latter are heated by means of a heat conducting liquid medium which is maintained at a temperature substantially above the boiling pointof water by introduction of steam under pressure to conduits 23. From the steam chamber, the steam, having approxi mately the temperature corresponding to the pressure of and is then dried to a moisture content of not more than 4 0%. It is then processed for the removal of any chips, ribbons sticks, or large fiber bundles as may have been included with the ultimate fibers and small bundles of same upon leaving the defibrator. This fraction may be discarded or, preferably, is reprocessed prior to mixing with the acceptable fraction. The ultimate fiber fraction, which comprises the major portion of the mass, is then adjusted to a moisture content of between about 8 to 18%, and is suspended in superatmospheric pressure air.

At this point in the process reference may be made to the accompanying drawings wherein:

Figure 1 is a side view of a section of the apparatus included in the mat-felting and cold-pressing areas utilizing a single felter unit.

Figure 2 is a top view of the section shown in Figure 1.

Figure 3 is a section of the steam chamber shown in Figures -1 and 2 taken on the line 3--3 of Fig. 2.

Figure 4 is a side view of a section of the apparatus included in the mat-forming and cold-pressing areas utilizing a series of three .felter units arranged in series relationship.

The suspension of the acceptable fiber fraction of the product of the defibrator, processed as above in air under positive pressure, is introduced into the pneumatic conduit 10 leading to the felter head 11, which is cylindrical in shape and provided with a revolving means 12 to aid in'distributing the fibers uniformly, breaking up agglomerations, and forcing the fibers through the perforations in the felter head. At the felter head 11 the fibers are forced through the perforations or foramens principally by the pressure of the air carrying them into a space of atmospheric pressure 13 and onto a foraminous traveling collecting surface 14, which latter is generally a Fourdrinier type wire screen. On the opposite side of the traveling foraminous belt, which serves as the collecting surface for the mat, a predeterminedand independently controlled reduced pressure is maintained through suction box 15 attached to the suction side of blower 16 through

conduits

17, 18. The vehicular air passes through the foraminous collecting surface, whereasjthe fibers, due to the acceleration gained by passing "smas e Pre rad en sm ma n t sphe e the steam supplied by steam pipes 22, is quickly drawn through the mat of fibers into suction box 21,- from which it is evacuated by the negative pressure supplied by blower 16 in the arrangement shown in Fig. 1 where the same blower is used for evacuation of both the steam suction box and the felter suction box, orby-blower 24, as shown in Fig. 4, where an independent blower is utilized for the evacuation of the steam suction box. The duration of the treatment of the mat by the steam being drawn through it is very short and the quantity of steam is adjusted to accommodate dilferent thicknesses of mats. The amount of suction by which the steam is drawn through the mat is also adjusted so that in com bination with the time of travel of the mat under the, steam chamber the quantity of steam passing through the mat will increase the temperature of the mat at its cross section center to approximately F.l30 F. as meas ured at the cold-pressing area and at the same time. will; not appreciably change the adjusted moisture content of the mat. Dependent upon the above conditions and somewhat upon the thickness of the mat, the time of travel under the steamer should be in the range of from A second to 6 seconds. In the arrangement of Fig. l, a balanced damper 25 is interposed between the exit from such suction box 21 and the suction. side of blower 16 to provide control of the amount of reduced pressure applied to the steaming operation. Reduced pressures of from 3" to 15" of water are preferred at the speeds of travel mentioned above.

As the mat 19 passes under the steamer 20, its thickness is reduced by the action of the negative pressure drawing the steam through the mat structure, resulting in the removal of static charges from the fibers, efiecting an initial consolidation of the interfelted wood; fibers of the mat.

Immediately after leaving the steaming area the steamed mat passes through a cold-pressing area where-it is compressed to a predetermined density of from about; 10 to about -25=pounds per cubic foot. This may-be accomplished by a single roller, coactingopposing rollers such as rollers 26 and- 27- of Figure 1, or by moving cone verging pressure'beltsasshown at 28in Fig.4. Th m d of p re i o r rs naa s s fi 1%!- Upon emerging from the cold-pressing .area,- sti ll by the foraminous beltupon which the that was inally felted, the steam-treated and cold-pressed-mat st, h e y. a reeab e sndsnerim rn, s

steaming treatment, due to what is'termed the .fspringe bac ibf the fibers. Any spring-bat; which is experi- 1 enced islimited to a return to .a density of notless than 7 pounds. It' is permanently densifiedto that density at which it has .suflicient structural strength to behan'dled, stored, or transported. "It is, capable of. being 'used for; r

a. number of purposes, or itmaybe placed ,into a platen press having relatively small. openings,.or so-ca lled daylight'provision. .For further consolidation into densified' hardboard the treated 'niat is cut into desired .lengths}andj* transported on cauls', into aheatedlplaten ress; Di .to; the fact that 'the mat .has retained the thickness which it: hadhimmediately greener/n me press area, thus enabling the use of smaller openings in the densifying platen press and a lower total press height, the latter may be closed quickly because of the reduced distance of arrests travel between platens providing faster and 'moreeconomical press cycles. Also, the. use of a pre-densified mat of, theinvention provides for the manufacture of a thicker board. with theIsame density or of aboard of e greater density for. the same thickness from a given ipres's opening than would be possible from theuntreated or' normal cold-pressed felts. Therush of air from the region within themat does not split the 'mat' or tend to blow portions of it out of the press, because: of the greater. density and structural ;st rength.of the mat as it enters the.press. a t f e In view of the high. structural ,strength-.andgreater density of the feltedrn'ats produced in accordance'wi'th this process, 1such"mats,j'gwhenused ain' the production of highly densitied hardboard, maybe placed into the platen hot-presses as self-sustaining units, rather than unitswhich require a caul or screen for carrying a'nd a caul or screen to prevent the air expelled from within the mat from disrupting the mat. Consequently,. the

boards may be pressed using only-a;siugle caul, a single screen, or neither, using in the. Jattercase the press platens for both carrying and covering the mats: The foregoing process may be further illustrated by reference to the following illustrative examples:

Example I White fir Asplund fiber treated in the defibrator with V striieturei it'wzis found that the' felts'which hadnot treated with dry steam were diflicult to handle and tended" to blow out of the hot press and rupture whcnjthe press was closed. The mats which had been steam wereeasilyhandled without damage anddrf rupture or blow out as pressclosed. e

White fir Asplundfiberscontaining 2% Amres resin, j 3%. p um, a a um lp t we e are-2, 'pared-assin Examp I a ;.p 'o s .din o' tte fe ts... in accordance with the gern'e i'al process described. abovc.; Some of the felts produced in accordance withthis-example were; treated with dryQsteam caused to passthrough thestructure .of the felted mat "by suction applied in;a C-.

; cordance .withjthe general process described above gfor approximately 1.3 9 seconds prior .to cold-pressing and raising thetemperature in the range of v120-130" .Others were treatedsimilarly but the quantity of steam:

passing through .thestructure ,ofthefelted mat was jreduced. until. the temperature of the ,mats prior t'o cold g pressing was in the range of 80-90 The felts which were treatedjin accordance with the; process of this-inthickness. of .87",after, cold-pressing and' were easily handledwithout damage and didnot rupture or blo w.

out as the press closed in'further consolidation. It was. found that the mats treated with a reduced. quantity of Estea'mand. havinga lower. mat temperature .assumed a thickness.aftericoldyressing'of 1.257, were diflicult to: handle. and tended to blow.out'.of the hot..press andrupture when the press was'closed.'.--.The moisture con-';

tent ofthe mats made in. accordance with this; example.l :werez-in the range. of fabout .=11-16,% before nndgafter steaming.

7 Example! V White fir (Abies concolor) Asplund fiber individualized ultimate wood fibers 'and'srnallbundles" or tracheids of such fibers and containing 2.5% petrolatum, Vz% aluminum sulphate, and 1% thermosetting phenol-formaldehyde resin solids (Amres 6130, manufactured by American-Marietta Company), based on the suspended in air after adjusting the moisture content to 12%, and made into a flat felt having a thickness'of about 3.25" in accordance with the aboveadescribed general procedure. A continuous mat of thefibers was I made and this was cut into sections, someof which were a merely cold-pressed under an unheated roller, and others submitted to the action of dry steam for -a period of approximately 5 seconds by passing said dry steam from a heated stearncharnber at atmospheric pressure through.

the felt into a suction 'box.- These latter were then cold-pressed with an unheated roller under the same f pressure as the unsteamed mats.

The mats or felts wh'ch werenot treated with the dry steam retained a thickness of about 1 .4 after the cold -pressing'and had a tensilestrength of 8--16 ounces per square inch. The'felts which .were treated with the dry-steam had a thickness of 1.0" after steaming, .retained a thickness of .5 after cold pressing, andhad a tensile strength of 176 ounces per square inch.

It was found that the mat which had notbeen treated with steam contained 12% moisture, while the mats which had been treated with steam contained 11.9% 'moisture.

The temperature of the mats which were treated with dry steam was appreciably higher than that of the un-.

treated mats, and was in the range of from 120-130 F. Upon transporting the felted mats of this example to a hot press for further consolidation into a densified dry weight of the wood fiber and having an adjusted moisture content of about 12% was suspended air at atmospheric pressure and passed continuously through a series of felters as shown in Fig. 4 at 10, following the general procedure described above. The thus suspended fibers having a thickness of 4". The thus prepared mat Was carried on the traveling screen 14 to a cold-pressing area28 during an elapsed time of; approximately 8% seconds, during which travel dry steam introduced at a reduced pressure of 20 pounds into the atmospheric pres-f 7 sure bf the heated steam chamber zowas drawn throughv v themat'for. a period of l /z se'condsby a negative pres- I sure of 10"- of watersupplied through suction box 21 to i V {the underside of the traveling screen. 12. Upon entering thecold-pressing area. the mat passed under a pressure "belt 23 hydraulically loaded to press the mat to a thickness of /z" 3t the'exit side. The mat was-then cut, to desired lineal sizes and passed into a'hot-press where it was pressed to an ultimate thickness of 1s" andadensity ofapproximately 60 pounds per cubic foot..

'Thejfelted mat was reduced from its original jthick-.

'ness of'4 to about 2.25" under the steamer and increased in thickness after passing the cold-pressing area from a thickness of /2" under the exit prepress roller to a mat having a thickness of and a density of 8.6 pounds per cubic'foot. at its cross-section center, after leaving the prepress or cold-pressing -area, was F. The moisture content The temperature of the mat at the mat berets s ami was swing w';' s 1.2- V

LThis xp riment was epeat dwh l omitting the st fdrawing th d y team thr gh the mat w i e pp ying negative pressure under the steamer. When so operating no reduction of the mats, thickness was efiected ahead of the prepress or cold-press area. Upon passing the prepress area the mat increased in thickness from /2'T under the cold-press exit roller'to a mat having a thickness of 2" and a density of 3.2 pounds per cubic foot. The temperature of the mat remained constant at ambient-atmospheric temperature and the moisture content remained constant at 12%.

The mats which were treated by dry steam in accordance with this example were firm and easy to handle for cutting and feeding into the hot press. No blow-on of thefibers or shearing on the caul plate was experienced upon closing the press. The mats whichwere' not treated with dry steam in accordance with this example were unstable and hard to handle for cutting and feeding into th'e'hot press. Considerable blow-out and some shearingon the caul plateswereexperinced upon closing the press Having nowdescribed our invention and illustrated the best means of its operation, we claim as new:

1. In a process for the production of densificd wood fiber structures which comprises dry felting a mixture of ultimate woodfibers and binder having an adjusted moisture content of from 8-18% onto a traveling forarninous surface to form a mat of fibers and binder, consolidating the mat by cold-pressing and further consolidating the cold-pressed mat to its ultimate thickness as a densitied structure, the step of effecting an initial consolidation ofthe mat'whieh comprises drawing high-temperature dry steam-from aspace of atmospheric pressure containing such high-temperature steam in admixture with air through the that after the felting step and before the coldpressing step by application of negative pressure to the underside of the mat through the forarninous surface, said dry steam being drawn through the mat for a' predeterinined length of depending upon the thiclme s of the mat and atl? predetermined negative pressure de pending upon the time'seleeted such that the combination of t e nd, n at r pre s re p o d s a quanti ye do steam to the mat whichrincreases the temperature ofthe mat at its cross-section center after cold-pressing, to at least F. ithout appreciably increa in the moismm content gfi the' I I thickness of A2", the step of eifecting an initial consolidation of the mat which comprises drawing dry steam at 1 the temperature of 135 pounds steam frorn'a space at atmospheric pressure through the mat. after the felting step and before the cold-pressing step by application of negativepressure to the underside of the mat through the foraminous surface, said dry steam being drawn through the mat for a period of 1 /2 seconds at a negative pressure of about 10" of water to produce, a temperature at the cross-section center of the mat after cold-pressing of approximately F. with no appreciable increase in the moisture content of the mat.

3. The process of claim 2 in which the wood fibers are white fir ultimate wood fibers and the binder is a thermosetting phenol-formaldehyde binder.

. References Cited in the file of this patent UNITED STATES PATENTS 2,208,511 Ellis July 16, 1940- 2,441,169 Roman May' 11, 1948 2,480,851 Goss Sept. 6, 1949' 2,464,301 Francis Mar. 15, 1949 2,635,301 Schubert Apr. 21, 1953 2,655,458 Collins Oct. 13, 1953 2,714,081

Burgon July 26, 1955

Claims

1. IN A PROCESS FOR THE PRODUCTION OF DENSIFIED WOOD FIBER STRUCTURES WHICH COMPRISES DRY FELTING A MIXTURE OF ULTIMATE WOOD FIBERS AND BINDER HAVING AN ADJUSTED MOISTURE CONTENT OF FROM 8-18% ONTO A TRAVELING FORAMINOUS SURFACE TO FORM A MAT OF FIBERS AND BINDER, CONSOLIDATING THE MAT BY COLD-PRESSING AND FURTHER CONSOLIDATING THE COLD-PRESSED MAT TO ITS ULTIMATE THICKNESS AS DENSIFIED STRUCTURE, THE STEP OF EFFECTING AN INTIAL CONSOLIDATION OF THE MAT WHICH COMPRISES DRAWING HIGH-TEMPERATURE DRY STEAM FROM A SPACE OF ATMOSPHERIC PRESSURE CONTAINING SUCH HIGH-TEMPERATURE STEAM IN ADMIXTURE WITH AIR THROUGH THE MAT AFTER THE FELTING STEP AND BEFORE THE COLDPRESSING STEP BY APPLICATION OF NEGATIVE PRESSURE TO THE UNDERSIDE OF THE MAT THROUGH THE FORAMINOUS SURFACE, SAID DRY STEAM BEING DRAWN THROUGH THE MAT FOR A PREDETERMINED LENGTH OF TIME DEPENDING UPON THE THICKNESS OF THE MAT AND AT A PREDETERMINED NEGATIVE PRESSURE DEPENDING UPON THE TIME SELECTED SUCH THAT THE COMBINATION OF TIME AND NEGATIVE PRESSURE PROVIDES A QUANTITY OF DRY STEAM TO THE MAT WHICH INCREASES THE TEMPERATURE OF THE MAT AT ITS CROSS-SECTION CENTER AFTER COLD-PRESSING TO AT LEAST 90*F. WITHOUT APPRECIABLY INCREASING THE MOISTURE CONTENT OF THE MAT.