CA1068144A - Machine made light weight glass fiber web material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A machine-made light weight glass fiber web material of exceptionally uniform fiber distribution is comprised of micron diameter glass fibers having a fiber length of about 1/4 inch or more and a basis weight of about 5-30 grams/square meter. The web material exhibits an isolated fiber bundle de-fect count of less than 10 per 100 square feet and a visually perceptible overall uniform fiber distribution essentially de-void of "cloud effect" fiber density variations.

Description

The present invention relates generally to wet-laid inorganic fibrous sheet material and is more particularly con-cerned with a new and improved inorganic fibrous web of light weight produced on production size papermaking machines.
Inorganic fibrous web materials, such as glass fiber papers, have been manufactured for a considerable period of time but have constantly presented the papermaker with special uniform fiber distribution problems. In this connection the art has recognized that uniformity of fiber dispersion pTior to sheet formation is ine~orably tied to uniform fiber forma-tion ~ithin the resultant web material. Due to the difficul-ties associated with achieving the necessary uniform fibersuspension, the resultant inorganic webs of fine diameter fi-bers were of a heavy basis weight, i.e., about 50 gra~.s/square meter and heavier, since the heavier weight materials were sufficiently thick to mask the non-uniform characteristics of the resultant fiber array. In the typical wet-laid paper-making process, the fibers are micron diameter glass fibers and are supplied to the dispersing medium in the form of bun-dles chopped from continuous multiple strand glass rovings.
The dispersing medium is usually an acidic aqueous solution and may be slightly viscous in order to promote and maintain ''' - : : . . . :. : : . .. ..

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i819~4 the dispersion and isolation of the individual fibers within the multiple strand bundles. The fibers within the dispers-ing medium are agitated within a beater to affect bundle separation and then the stock is conveyed to holding tanks -containing con~entional mixing units to maintain the fibers within their desired suspended condition. As can be appre-ciated, failure to provide sufficient agitation during the initial dispersion sf the fibers causes incomplete separa-tion of the glass fibers and fiber bundles are visible within the resultant continuous sheet material.
In recent years glass fibers longer than conven-tional papermaking length, namely, fibers having a length of between a~out 1/4 inch to one inch and more have been used.
However, when these fibers have been dispersed in accordance with the prior known technique, it was found that the indi-~idual fibers tended to snag within the beater and holding tanks and cou~d not easily be redispersed, resulting in clumps or other irregularities within the sheet product. It ; was also found that the long glass fibers reaccumulated in such a manner as to form fiber bundles exhibiting the con-figuration of a haystack or spider. Although these "hay-, .
stacks" can be tolerated in the heavy weight materials and for certain applications where the aesthetic appearance of the sheet material is not of concern, they are considered major defects in light weight materials and for those appli-cations where the glass sheet provides a surface veil or is intended to provide a smooth surface of a reinforced plastic structure.
The thicker, heavy weight sheets have been used in vinyl flooring tile and the like to provide dimensional ~06~

stability. However, the heavy weight glass material has ; poor resin penetration characteristics and therefore poor lamination resulting in a tendency of the tiles to delami-nate. Thin, light weight, hand sheets having good fiber distribution can be individually formed when appropriate care is taken. However, the uniform fiber distribution ne-cessary to provide for elimination of the visually percepti-ble, overall density variation referred to as the "cloud ef-fect" coupled with substantial minimization of isolated fi-ber bundles or "haystacks" has not been achieved on continu-ous papermaking machines when producing light weight glass fiber web material.
In a continuous papermaking operation on a produc-tion basis, long fiber sheet material is typically produced from very dilute fiber suspensions using an inclined wire or ; similar type of papermaking machine. In such machinery there is used a conventional open type headbox of sufficient vol-ume to establish a calm and relatively placid fluid approach to the web forming zone. The advantage of such a headbox is that sufficient time is provided in the headbox for the re-lease of air bubbles from the fiber suspension prior to web formation. However, the desired calm and placid fluid ap-proach has a distinct disadvantage for long glass fiber sus-pensions. It has been found that as the air bubbles are re-.
leased at the headbox they tend to permit and even encourage the formation of fiber "haystacks". The bubbles carry these fiber bundles to the surface causing them to be deposited on the surface of the web material as it is being formed. This provides not only an un~cceptable sheet materi~l from a vis-ual appearance standpoint, but also produces an irregular or .,' ~
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roughened surface feel that is readily detected by simply passing a hand across the surface of the sheet material.
Accordingly it is a primary object of the present invention to provide a new and improved long fiber glass web material of extremely light weight yet of uniform fiber for-mation that is produced on production size papermaking ma-chinery.
Another object of the present invention is to pro-vide a new and improved glass fiber web material of the type described that exhibits a visually perceptible, overall uni-form fiber distribution and a minimum of isolated fiber bun-dle defects. Included in this object iscthe provision for a light weight glass sheet material of continuous length that is essentially devoid of visible "cloud effect" fiber den-k sity vaTiations.
Still another object of the present invention is to provide a light weight glass fiber material that exhibits improved aesthetic and physical properties and renders the material well sui~ed for use in reinforced plastic films, tiles and the like.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects will be achieved in ac-cordance with the present invention by providing a continu-ous machine-made light weight inorganic fiber web material comprised o~ micron diameter inorganic fibers having a fiber length of about 1/4 inch or more and a minor amount of a binder for the inorganic fibers. The web material has a basis weight of about 5-30 grams/square meter and exhibits an isolated fiber bundle defect count of less than 10 per ~ -4-.' ' :

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100 square feet. Further, the web exhibits a visually per- -ceptible overall uniform fiber distribution essentially de-void of "cloud effect" fiber density variations.
A better understanding of this invention will be obtai~ed from the following detailed description and the ac-companying drawin~ wherein the article of manufacture pos-;~ sesses the features, properties and relation of elements de- -scribed and exempli~ied herein.
The single sheet of drawing shows a block diagram : 10 of a preferred technique used in forming the light weight ;~ web material of the present invention.
As mentioned hereinbefore, a major factor in ob-taining the desired uniform fiber distribution within the re-sultant sheet product is the achievement of a complete and uniform fiber suspension of the glass fibers within the dis-persing medium and the conveyance of that dispersion intact to the forming area. Thus, for clarity of description and ease of understanding, the glass web material of the present invention will be described in connection with the preferred technique OT method used for its manufacture.
Numerous factors affect the quality of an aqueous fiber dispersion and its ability to be fed to the forming area of a papermaking machine. Among these are the type of fiber, including the fiber finish and the condition of the strand rovings used to supply the fibers, the chopping or cutting performance, the composition and characteristics of the dispersing medium, the performance of the mixing or dis-persing apparatus and the treatment of the fiber stock mate-rial after it lea~es the disperser. Although each of these -factors is important, it has been found in accordance with . .
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the present invention that a substantial and significant factor is the dwell time of the fibers within the system be-tween the point at which they enter the disperser and the point at which they are removed from the dispersion at the web forming zone of the papermaking machine. Thus, in ac-cordance with the present invention, it has been determined that best results are achieved by completely eliminating the holding tanks utilized heretofore and by using an in-line disperser rather than the batch mixers utilized in the past.
,~, In conjunction with the elimination of the holding tanks is the immediate conveyance of the dispersed fibers to a dilu-tion station and the utilization of a smooth, low volume headbox characterized by high turbulence and high stock ve-locity. In such a system the flow of the fiber suspension from the disperser to the forming area of the papermaking machine occurs within a matter of a few seconds and the dwell time within the disperser is a major time-controlling factor for the passage of the glass fibers through the sys-tem~ Such time control is important since it has been found that optimum dispersion of long glass fibers is reached rel-atively quickly, that is, within about one to two minutes, and is maintained in its most uniformly dispersed condition for a period of only four to five minutes. Thereafter, the glass fibers tend to accumulate, cling to each o~her or form the undesirable "haystacks" or multi-fiber bunches mentioned hereinbefore. It will of course be appreciated that the wet papermaking process is a dynamic system which is affected by numerous other conditions or factors within the system, such as the viscosity of the dispersing mediumJ the fiber ~068 '~
consistency, the rate at which the fibers are metered into the disperser and numerous other process variables. ~onse-quently, the exact dwell time will vary depending on these various conditions or factors. However, best results have been achieved with controlled dwell times within the disperser of less than ten minutes and generally from about one to seven minutes. An acceptable operating range falls between approximately two to six minutes while the preferred dwell time is about two and one half to five minutes.
Although the inorganic fibers that may be used in the present invention includes substantially all of the con-ventional inorganic materials commercially available in fi-ber form, such as aesbestos, mineral wool and the like, glass fibers are generally preferred. The fibers will vary substantially in thickness although in the preferred embodi-ment the fiber diameters are within the coarser fiber range such as between about 5 microns to 15 microns. It will of course be appreciated that somewhat finer or coarser diame-ter fibers may be used for particular applications. The glass fibers constitute the major portion of the fiber con-tent and preferably account for as much of the fiber content as possible. Thus, about 85-90 percent or more of the fi-bers within the sheet structure are inorganic, and prefera-bly glass fibers. As exemplified herein mixtures of differ-ent types and sizes of glass fibers may be employed or the sheet can be forme~ from only a single type and size of glass fiber.
Due to the type of preferred glass fibers utilized, ; it is generally desirable to provide a binder in the inor-ganic sheet material. Although a binder can be applied as a : . ~' ' : . . ' ' :
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- dilute solution after the web is formed or can be incorpo-rated within the fiber furnish as a portion of the dispers-- ing medium, it is generally preferred to provide binder fi-bers which cons~itute up to about 10-15 percent of the total fiber content and preferably about 5 to 10 percent thereof.
Various binder fibers can be used with good results, among these, polyvinyl alcohol fibers have been found to produce superior results relative to post formation spraying with adhesives and the like. The binder fibers also enhance the handling characteristics of the web through the papermaking machine. Preferably the fibers are activated or at least softened in the dTier section of the machine to provide the sheet material with its desired structural integrity.
The binder fibers are preferably added to the fi-; ber suspension during or after dilution of the fiber consist-ency and prior to the flow of the suspension to the headbox of the papermaking machine. Thus the polyvinyl alcohol fi-bers which act as the binder component of the fiber web can be added conveniently at an adjustable speed fan pump down-stream of the dilution operation without interfering with the dispersion of the glass fibers within the uniformly dis-persed fiber stock material. If desired subsequent size press treatment or other binder treatments can be utilized depending upon the particular end use for which the sheet material is intended.
Referring now specifically to the drawing, it has been found desirable in the preferred technique to provide a controlled or metered feed of long glass fibers in order to achieve the best fiber dispersion characteristics. The fi-bers are preferably metered at a selected rate into a ..

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continuous in-line disperser and from the disperser are fed ,j~' directly to the dilution and forming area of the conventional papermaking machine. This arrangement obviates the need for retaining the dispersed fibers within a stock chest or other holding tank and the resultant deterioratiGn of the quality of the dispersion. Additionally, it is an advantage of the ~
present invention that the continuous dispersing equipment is ', of relatively simple construction and inexpensive compared to conventional stock preparation equipment. If desired the fi-bers can be precut and fed by a dry fiber meter or can be fed ''~
' as continuous strands and cut or chopped as they are delivered to the in-line disperser.
, In the preferred embodiment it has been found advan-tageous to provide a cutter at the inlet to the disperser so that continuous lengths of glass rovings can be fed from , spools and cut for immediate delivery to the disperser. This ' delivery of the continuous filaments provides excellent con-trol over both the fiber length and the rate at which the fi-bers are fed to the disperser. Additionally, it provides flexibility by permitting the utilization of different fiber lengths and adjustable control over the fiber lengths.
, Where prechopped or precut fibers are employed it is possible to provide control over the fiber feed rate to the disperser by employing a weigh belt or the like between the - dry fiber meter and fiber dîsperser, in which event the dry fiber meter functions as a pre-feeder with its speed modulated and controlled by a signal from the weigh belt, in order to ' achieve the desired feed rate for the fibers.
, The fluid used as the dispersing medium is also fed to the inlet of the disperser to provide the desired fiber ~.
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consistency therein. This fluid is an acidic aqueous solution that may contain a suitable agent for controlling the viscosity of the dispersing medium. Thus, in accordance with the pre-ferred embodiment an aqueous solution of dilute sulphuric acid having a pH of between 2 and 4 and containing a sufficient amount of a viscosity forming agent is employed. Typically the solution exhibits a viscosity between about 5 and 20 centi-poise. The viscosity producing agent may be a natural or syn-thetic material or blends or combinations thereof. The agents are preferably water soluble materials such as resins or natu-ral gums which can be used along or in combination with other materials to provide the desired viscosity. Examples of natu-ral gum materials are locust bean gum and guar gum derivitives.
Among these, the guar gum derivatives are preferred and excel-; lent results have been obtained with an aqueous solution of a guar gum derivative sold by General Mills Company under the A tradename "Gendriv"~ In addition to the natural viscosity pro-ducing agents it is also possible to utilize synthetic materi-als such as high molecular weight resins, dispersants, surfac-tants and the like to control the properties of the dispersing medium. These synthetic materials are preferably water soluble and are stable within the acidic environment utilized for the - glass fibers. Among the synthetic viscosity producing materi-als, the preferred resins are polyacrylamide polymers which can be used in dilute aqueous solutions at low concentration (e.g., ; 0.025-0.2 percent) to provide the desired control over the vis-cosity. Typical of such materials is the polyacrylamide resin ; sold by Dow Chemical Company under the tradename "Separan ~P-30" and by American Cyanamide Company under the tradename "Cytame 5".

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'' The viscous dispersing medium is utilized since it prevents fiber entanglement during the dispersing operation and assists to maintain the fibers in their dispersed state during passage of the suspe~sion through the disperser. As will be appreciated the viscosity of the solution will affect the dwell time required and must be adjusted for the particu-lar fiber and fiber consistency utilized. A high viscosity medium and a short dwcll ~ime might lead to an under-dispersed fiber stock while a low viscosity and a long dwell time could lead to over-dispersion and the formation of "haystacks" and other major defects. A viscosity in the range of about 5-10 centipoises and a dwell time of about 2.5-5.0 min. has been ;~ found to produce good dispersion results. As will be appreci-ated other additives, such as dispersing aids, e.g., surfac-tants such as sodium hexametaphosphate sold under the trade-A name "Calgon",~ may be added to the dispersing medium i~ order to achieve the desired control oveT the dispersed fibers and to assist in preventing the recombination of fibers into the undesirable haystack configurations.
As mentioned, it has been found that the fibers are dispersed quite rapidly within the dispersing medium and reach a peak of percent fibers dispersed within a relatively short time following which the fibers tend to cling or bind together ~ slightly to form the undesirable "haystacks". Thus upon reach-; ing optimum dispersion, it is desirable to maintain the agita-tion for a limited period of time and control the dwell time of the fibers within the disperser so that prolonged agitation is avoided. In this connection it has also been found that even ater the optimum dispersion has been reached at the desired . . .
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dwell time, the agitators within the disperser cannot be shut off without damage to the quality of the dispersion. Of course as will be appreciated, surface treatment of the fibers will substan*ially affect the ability of the fibers to toler-ate a prolonged dwell time. However, for most glass fibers presently available on a commercial basis, it has been found that the optimum dwell time is between 2 1/2 and 5 minutes when operating with a dispersing medium having a viscosity of about 5-10 centipoises and a pH of about 2-3 at a solution temperature of approximately 80-100F and a fiber consistency of about 0.3-1.0 percent.
Preferably the disperser should be of the type that exhibits a relatively smooth interior surface and is free of any edges or surfaces on which the long glass fibers can snag or drape. However the disperser may consist of a plurality of mixing or dispersing stations or compartments with continu-ous flow directly from station to station in order to provide the desired dwell time characteristics.
As will be appreciated the specific design of the disperser can vary so long as it achieves the desired function of separating the individual fibers from the fiber bundles fed ~` to the disperser and produces a uniform dispersion of the in-dividual fibers while conveying the fiber dispersion through ;
the disperser within the required dwell time. As will be ap-preciated the fibers are metered into the dispersing medium flowing through the disperser to provide the desired fiber consistency. Usually the consistency is substantially higher than the fiber consistency within the headbox by a factor of from as much as 10-100 times. In accordance with the pre-; 30 ferred embodiment the fiber consistency is less than two '',;

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percent and generally is in the range of about 0.3-1.3 per-cent with a preferred range of about 0.5-0.9 percent.
As mentioned hereinbefore the fiber dispersion moves rapidly from the disperser to the forming portion of the papermaking machine and in fact reaches the forming wire within a few seconds after leaving the disperser. However, during that period the fiber consistency of the dispersion is adjusted so as to more fully dilute the fiber stock. This can be achieved by feeding the dispersion to a separate flow-through mix tank where it is mixed with the main white water flow from the web forming operation~ The fiber c4nsistency is diluted from a value of 0.3-1.2 percent to a value of about 0.005-O.OS percent. Thus, as can be seen, the dilution is greater than 10 to 1 and usually 15-25 to 1 in order to provide the highly dilute fiber suspension fed to the headbox of the papermaking machine.
As indicated in the drawing, the headbox utilized in accordance with the present invention is unlike the open headbox of the conventional inclined-wire papermaking ma-" 20 chines and is provided with a smooth contour and a reduced -volume so that the highly dilute fiber suspension flows rap-; idly through the headbox toward the web forming area. The ;
reduced volume headbox with its smooth contour not only in-creases the velocity of the fiber suspension traYeling there-through but also increases the level of random turbulence im mediately over the forming zone. The increased level of tur-bulence prohibits the accumulation of foam and fiber masses that would otherwise float to the surface and form "hay- -stacks" or other fibeT defects. As will be appreciated flow control of the dilute fiber dispersion can be achieved by a .

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suitable flow control mechanism such as a variable speed fan pump, provided however that the pump is of smooth configura-tion and free of elements that would produce eddies in the flow or otherwise cause fiber entanglement. Thus the head-box utilized in accordance with the present invention pre-vents holding of the fiber dispersion for a prolonged period of time, thereby preventing the dispersed fibers from recom-bining to form defects in the sheet structure.
The following examples are given in order that the - 10 effectiveness of the present invention may be more fully un-deTstood. These examples are set forth for the purpose of illustration only and are not intended to in any way limit the practice of the invention. Unless otherwise specified all parts are given by weight.
EXAMPLE I:
A light weight glass fiber web material was pro-; duced using production size, papermaking machinery. Glass fibers having a fiber diameter of 9 microns were cut to a length of 1/2 inch from strands of glass rovings fed from bobbins. The cut fibers were delivered directly to an in-line disperser at a rate of one pound per minute. The in-line disperser has a capacity of 100 gallons and was operated at a throughrate of 30 gallons per minute, thus providing a dwell time of slightly more than 3 minutes. The dispersing media used was a dilute sulphuric acid solution containing a ::~ A guar gum derivative (Gendriv ~ 92 SR) in amounts sufficient to provide a solution viscosity of about 5 cps at a pH of 2.3 and a temperature of 88F. The fiber dispersion at a fiber consistency of 0.4 percent was fed from the disperser to a mix tank where the fiber consistency was diluted at a ratio -~ ~e9is~e/~e~ ~a,~k 14-10~

of approximately 24:1. Polyvinyl alcohol fibers were added to the dilute suspension in amounts sufficient to provide a polyvinyl alcohol fiber concentration of 8 percent based upon the weight of the glass fibers. The fiber dispersion was then fed to a low volume high velocity h~adbox at a con-sistency of 0.017 percent and a glass fiber web was formed at a medium speed production rate.
The resultant web material had a basis weight of 13.6 grams/square meter, a thickness of 84 mîcrons and an air porosity of 8263 liters per minute per 100 cm2 at 12.7 mm H2O pressure. The light weight web had a dry tensile strength of 507 gm/25 mm. in the machine direction and 333 gm/25 mm. in the cross direction. It exhibited tongue tear of 34 gms in the machine direction and 44 gms in the cross direction.
Samples taken from various portions of the sheet material exhibited a major defect count of 0-2 and a minor defect count of 0-5 per 100 square feet corrected to a basis weight of 17 grams/square meter. A maior defect is catego-; 20 rized as a fiber bundle either of an undispersed or partially dispersed nature or of a haystack configuration while a minor defect is categorized as two or three fibers which have re-.,. ;
, mained undispersed or been drawn together. Commercially ac-- ceptable light weight materials are considered those which have about 10 or less and preferably 5 or less major defects , :- .
per 100 square feet of web material. The minor defects are not considered significant. The sheet material also exhib-ited a uniform fiber distribution substantially free of any -density variation upon a visual examination.
EXAMPLES II - VI

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The procedure of Example I was repeated on the same papermaking machine except for variations in the process op-erating conditions, the fiber furnish and the basis weight ofthe material produced. The results are tabulated below:
TABLE
Ex. II Bx. IIIEx. IV Ex. VEx. VI
Fiber 9 micron (%) 70 46 90 70 22 13 micron (%) 22 46 -- 22 70 Binder ~%) 8 8 10 8 8 Basis weight19~8 18.3 22.0 22.4 23.1 ~, (gmlm2) Thickness 123 115 133 138 115 , (microns) Air porosity5648 6552 4742 5512 6149 (l/min.) Dry tensile (gm/25 mm) CD gl5 7~5 1034 1362 1037 Ton~ tear (gms) ` MD 51 60 40 62 89 Defect Coun~
per 100 ft.
Major 0-3 0-4 0-3 0-1 0 Minor 3-4 0-5 7-13 1-4 2-4 ********
EX~LES VII - IX
The procedure of the preceding examples was repeated on a small size production machine using finer diameter glass 30 fibers and no binder fiber. In each instance, the glass -'' fibers constituted lOO percent of the fiber component and were 1/2 inch in length and 6 microns in diameter. The basis weight and defect count per 100 square feet are given below.
The high minor defect count reflects the very fine fiber di-ameter and the subjective determination of the analyst but in each instance is considered a perfect sheet material from a commercial standpoint.
Defects Basis WRight Ex. #tgm/mG) Major Minor VII 15.8 1 222 VIII 16.6 0 356 IX 17.6 0 198 . ********
As will be apparent to persons skilled in the art, various modifications, rariations, and adaptations can be made from the foregoing specific disclosure without departing `. from the teachings of the present invention.
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Claims (10)

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

1. A continuous, wet-laid, machine-made, light weight, inorganic fibrous web of uniform fiber formation comprising inorganic fibers having a fiber length of about 1/4 inch or more and up to about 15 percent by weight of a binder for the inorganic fibers; said web having a basis weight of about 5-30 grams/square meter, an isolated fiber bundle defect count of less than 10 per 100 square feet, and a visually perceptible uniform fiber distribution essentially devoid of "cloud effect" fiber density variations.

2. The fibrous web of claim 1 wherein the inorganic fibers are glass fibers having a micron diameter thickness.

3. The fibrous web of claim 1 wherein the inorganic fiber content is about 85 percent by weight or more.

4. The fibrous web of claim 1 wherein the web has a basis weight of about 10-25 grams/square meter.

5. The fibrous web of claim 1 wherein the inorganic fibers are glass fibers having a diameter in the range of 5-15 microns and a length in the range of 1/4 - 1 inch.

6. The fibrous web of claim 1 wherein the inorganic fibers include a mixture of glass fibers of different micron diameter size.

7. The fibrous web of claim 1 wherein the inorganic fibers constitute about 90 percent by weight of the web and are glass fibers having a fiber diameter in the range of 5-15 microns, said web exhibiting a major defect count of less than 10 per 100 square feet.

8. The fibrous web of claim 1 having a major defect count of about 5 or less per 100 square feet.

9. The fibrous web of claim 1 wherein the binder is initially incorporated into the web in fiber form.

10. The fibrous web of claim 1 wherein the inorganic fibers are glass fibers having a diameter of less than 15 microns and a length of about one inch or less, said glass fibers constituting at least about 90 percent by weight of the web, the binder being of a thermoplastic material initially incorporated into the web in fiber form, said web having a basis weight of about 25 grams per square meter or less and exhibiting a major defect count of about 5 or less per 100 square feet.