CA1090060A - Vapor modified cellulosic fibers - Google Patents

Abstract

VAPOR MODIFIED CELLULOSIC FIBERS

ABSTRACT

An improved wet-lay papermaking process is provided which employs substantially modified cellulosic fibers, said fibers being characterized by improved properties, such as reduced swellability and reduced natural fiber-to-fiber bonding. The fibers are produced by a process which comprises treating dry fibers with formaldehyde vapor to react with and internally cross-link the fibers by reacting with the formaldehyde itself and with the fibers. The fibers, when employed as a component of a papermaking furnish, produce improved cellulosic sheet materials which exhibit increased bulk and reduced swellability and dry tensile strength.

Description

-3l.f~q3o~36t) : BACKGROUND OF THE INVENTION
Field of the I nvention . ~ The present invention relates, generally, to modified cellulosic fibers, to a process for preparing sheet materials with said fibers and to said 5 sheet materials.
Description of the Prior Art -~ In a conventional papermaking operation, cellulosic fibers are dispersed in water, drained on a wire screen, pressed into close physical contact and dried. The result is a sheet in which the individual fibers are inti-10 mately associated with one another through what is commonly referred to as papermaking bonds, hydrogen bonding or fiber-to-fiber bonding. It is ~i ~ generally believed that this bonding between fibers is what gives strength to the dry sheet. When the dry sheet is re-wet, this bonding between .' fibers is reduced, and the sheet loses most of its strength. To prevent this strength loss, various chemical treatments have been employed.
Among the most successful treatments is the use of synthetic resins with the cellulosic fibers, either before or after a sheet is formed, followed by curing or polymeri~ing the resin which can significantly increase the wet 'i l strength of the sheet. Most commonly used are the urea-formaldehyde and ~-~ 20 melamine-formaldehyde type resins. These resins are referred to as being substantive with respect to cellulosic fibers, because they are cationic and . are, therefore, easily deposited on and retained by the anionic papermaking fibers.
Attempts have been made to produce paper sheets having improved bulk, or thickness, without significantly increasing the weight or cost of ~ the product or adversely affecting the other properties thereof. These ',,........ attempts have included the reduction or elimination of wet pressing which ` increases the bulk but requires modified equipment, more extensive drying and cannot be run on conventional papermaking equipment. Non-fibrous 30 chemical additives, such as gas filled spheres of thermoplastic resins as ` described in U. S. Patent No. 3,556,934, have also been suggested but ~- these often interfere with normal papermaking operations and adversely affect some of the other properties (e . g ., absorbency, etc. ) of the product.
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Cross-linking of cellulosic fibers or fabrics is well known in the :
textile and papermaking fields. See, for example, U.S. Patent No. 3~069~311 (Harpham et al), U.S. Patent No. 2~010~635 (Kantorowicz), U.S. Patent No. 3~224~926 ~Bernardin), U.S. Patent No. 3,434,918 (Bernardin), U.S.
~atent No. 3~440~135 (Chung) and U.S. Patent No. 3~455~778 (Bernardin).
U.S. Patents 3~765~913 (E. A. Wodka) entitled MODIFIED CELLU-LOSIC FIBERS AND PRODUCTS CONTAINING SAID FIBERS and 3~819~470 - . -. (D. L. Shaw et al) entitled MODIFIED CELLULOSIC FIBERS AND METHOD
. FOR PREPARATION THEREOF, both produce modified fibers similar to the -' 10 present invention, but both begin the process by treating an aqueous slurry of cellulosic fibers with a substantive polymeric compound. This ; aqueous treatment step has inherent limitations and the use of the substan-:~ ~ tive polymeric compound in the aqueous system results in some loss of the compound during subsequent de-watering steps. This reduces the effi-ciency of the process. A somewhat similar process for producing modified cellulosic fibers that also begins by treating an aqueous slurry of the ;~ fibers with a wet strength resin (substantive polymeric compound) is disclosed in Belgium Patent Nos. 826~360 and 826~361 both assigned to Buckeye Cellulosic Corp. (P. ~ G.). All of these processes require a refiberi~ing method fo resuspend the fibers in a papermaking furnish.
. Treating cellulosic textile fabrics, especially cotton fabrics, with ,~ formaldehyde vapor to impart permanent press and crease-resistant proper-ties to the fabric has been disclosed in U.S. Patent Nos. 3~663~158 and ~; 3,653/805. However, these patents do not suggest the treating of paper-; 25 making cellulosic fibers for subsequent use in a wet-lay papermaking ~ furnish, nor do they suggest that improved papermaking properties are `. obtainable by substantially modifying the papermaking cellulosic fibers with vapor-phase formaldehyde.
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SUMMARY OF THE INVENTION
In accordance with one aspect of this invention there is provided a method of producing modified cellulosic, wet-lay papermaking fibers which comprises:
reactin!3 dry papermaking, cellulosic wood pulp fibers with a sufficient quantity of formaldehyde vapor to substantially impede the ,.~ 35 hydrogen bonding capability of the cellulosic fiber;
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~; -3 -said dry cellulosic fibers being in substantially individual fiber . form during reaction with the formaldehyde vapor.
In accordance with another aspect of this invention there is provided an improved method of making wet-layed paper comprising:
forming an aqueous furnish containing formaldehyde vapor-modified cellulosic fibers usually with conventional papermaking cellulosic fibers, draining said furnish through a foraminous surface to form a paper web of the fibers contained in said furnish.
In accordance with another aspect of this invention, there is provided ' 10 a process of making paper comprising:
; reacting papermaking cellulosic fiber pulp in substantially indi-vidual fiber form with formaldehyde vapor in sufficient quantity to modify the pulp fibers by substantially impeding the hydrogen bonding capability , of the pulp fibers;
making a papermaking furnish by combining the modified fibers .; with water;
~;~ draining the papermaking furnish through a foraminous surface to wet-lay the modified fibers as a paper sheet on the foraminous surface;
removing the paper sheet from the foraminous surface; and : 20 drying the paper sheet.
~i In accordance with another aspect of this invention there is provided ' a process of making modified papermaking cellulosic fiber pulp comprising reacting papermaking cellulosic fibers in substantially individual fiber form . with sufficient formaldehyde vapor to substantially impede the hydrogen bonding capability of the fiber.

~ DETAILED DESCRIPTION OF THE INVENTION
:~ Dry cellulosic fibers in substantially individual fiber form are treated with formaldehyde vapor in sufficient quantity and under treatment condi-tions to react with the cellulosic fibers and substantially impede their ability to form hydro~en bonds.
Dry cellulosic fibers, the type suitable for use in the present inven-tion, are papermaking wood pulp cellulosic fibers normally produced by the ; Kraft or sulfite pulping processes, usually bleached by conventional bleaching techniques and dryed to a standard dryness level usually referred .~
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to in the papermaking industry as "air-dryed" fibers. Such fibers usually contain some moisture depending upon ambient conditions which vary from day to clay and usually result in about 5% to 10% moisture in the fibers (90% to 95% oven dry). The fibers are of short papermaking length in 5 order to be suitable for dispersion in an aqueous furnish required for conventional wet-lay papermaking. Such conventional fibers usually average about 4 millimeters in length.
A commercially available source of suitable fibers is available under the commodity designation "lap pulp" which is the product of 3 pulp mill 10 that has been de-watered for shipping convenience. Lap pulp can be ,either hardwood or softwood pulp produced by the sulfite, Kraft, polysul-fide or other well-known pulping techniques followed by bleaching according to any of the standing bleaching techniques.
;Formaldehyde vapor (CH20) is reacted with the cellulosic fibers under 15 appropriate reaction conditions. Preferably, the formaldehyde vapor is obtained by the decomposition of paraformaldehyde. Commercially available, anhydrous formaldehyde gas is also suitable. Because the reaction between -formaldehyde and wood pulp cellulosic fibers proceeds slowly under ambient conditions, elevated temperature is preferred to promote the reaction 20 between formaldehyde and cellulose. The reaction of the formaldehyde vapor with the fiber is preferably catalized with an acid such as sulfuric or hydrochloric acid or with ammonium chloride. The pulp can be treated with the catalyst prior to being exposed to formaldehyde vapor.
The formaldehyde vapor is also capable of reacting with itself to 25 result in an oxymethylene polymer deposited within and upon the cellulosic fiber. Formaldehyde or the polymer is capable of internally cross-linking the cellulosic fibers. This intra-cross-linking is substantiated by the reduction in swellability. However, no significant cross-linking between fibers occurs since there is no wet strength enhancement imparted to a 30 paper sheet by modified fibers of the present invention.
The dry cellulosic fibers should be in substantially individual fiber form during reaction with formaldehyde vapor. A pulp fluff such as that produced by dry fiberizing pulp is done in apparatus such as Joa Fiberizer (manufacturer by Curt E. Joa Inc.). Such dry fiberized pulp is a typical 35 example of wood pulp in substantially individual fiberized form. Other , . , 1~19~

means of dry fiberizing lap pulp or other sources of suitable cellulosic wood pulp papermaking fibers can be used in the present invention.
The amount of formaldehyde reacted with the cellulosic, wood pulp, ; papermaking fibers should be sufficient to substantially reduce the hydrogen bonding capability of the fibers. A simple test to determine whether the hydrogen bonding capability of the fibers has been substantially reduced is by comparing the dry tensile strength of two comparable hand sheets, one made with formaldehyde-treated fibers and another made with compar-able fibers but not treated with formaldehyde. If the handsheet made with formaldehyde-treated fibers has a dry tensile strength of less than half of the dry tensile strength of the hand sheet made from the untreated fibers, then the hydrogen bonding capabilities of the fibers have been substan-tially reduced since such bonds are the major contribution to the dry tensile strength of the hand sheet. Preferably, the quantity of formalde-hyde reacted with the fibers results in a handsheet produced with 100%
treated fibers having essentially no dry tensile strength.
In addition to a tensile strength reduction, the improved tissue paper-making qualities of fibers reacted with the formaldehyde according to the present invention is manifested in other physical properties of a sheet produced on the fibers. A substantial increase in handsheet caliper can be detected between handsheets produced with fibers treated according to the present invention and handsheets produced with comparable fibers but not treated with formaldehyde according to the present invention. The substantial increase in handsheet caliper reflects the increase in bulk , 25 obtainable in paper webs due to the presence of the formaldehyde-treatedfibers. The bulk enhancement is believed to be attributable to the reduced tendency of the treated fibers to form hydrogen bonds and accordingly fibers form a less dense web when the web is produced according to conventional wet-layed papermaking techniques employing a wet-pressing step.
Wood pulp fibers reacting with formaldehyde according to the present invention also exhibit a reduced tendency towards swelling in the presence of water. This reduced swellability is believed attributable to the formalde-hyde penetrating into the fiber, reacting with the fiber and cross-linking within the fiber which reduces the fiber affinity for water and lowers the tendency of the fibers to form papermaking bonds (hydrogen bonds).

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Commercially available gas reaction chambers are suitable for treating cellulosic wood pulp fibers in a dry fluff state ~substantial individual fiber form) with formaldehyde vapor at ambient or elevated temperature.
A significant advantage of the present process for producing modified .` 5 cellulosic fibers over other processes for forming low-bulk, twisted non-swellable fibers as disclosed in U.S. Patent Nos. 3,819,470 and 3,756,913 is that the present process is essertially a single-step process of reacting fibers with the formaldehyde vapor while the fibers are substantially . individual and dry. Accordingly, the fibers produced by the process of the present invention can be added directly to a papermaking furnish for use in making wet-layed paper according to conventional papermaking techniques. The incorporation of the modified fibers of the present inven-tion into a conventional papermaking furnish significantly increases the ~ bulk, sGftness and drape of the paper and reduces the dry tensile of the ~ 15 paper. Such a combination of property modifications of paper is highly desirable in light-weight tissue-type paper products. Preferably, the modified fibers of the present invention comprise from about 10% to about 90% of the fibers in the furnish.
The following example substantiates the significantly improved fiber properties possessed by fibers of the present invention and the improved sheet properties of paper made with a furnish containing fibers of the present invention. All proportions used herein are by weight unless otherwise indicated.
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EXAMPLE
Lap pulp consisting of northwestern softwood pulp produced by the sulfite pulping process was used for making four handsheets designated A, I B, I and ll. The handsheet of Sample A was made by dry-fiberizing the - lap pulp and then making a water furnish for use in making a handsheet according to the standard procedure with a Noble and Wood handsheet apparatus employing a 100 mesh screen. Sample B used the same proce-dure for making a handsheet as Sample A except that the dry-fiberi~ed lap pulp was screened to remove knots and similar larger particles prior to making the handsheet. For Sample 1, dry-fiberized lap pulp was first reacted with formaldehyde according to a process described hereinafter and . .
~; '; '" ' '~ ' f~9 then a furnish was made with the formaldehyde-reacted fibers for use in the Noble and Wood apparatus employin~ a 100 mesh screen. Sample l l was made employing the same procedure as Sample I except that the dry-fiberized pulp was screened prior to reacting with formaldehyde to remove 5 knots and other similar large impurities.
The dry fibers used in Samples I and l l were reacted with formalde-hyde by a process employing a 4-liter resin kettle enclosed in a heating mantle which was used as a reactor. The reactor was vented near the top to remove excess vapor. A charge of dry-fiberized pulp (approximately 49.2 grams containing about 10% moisture) was added to the reactor on a metal screen near the bottom of the reactor. Below the metal screen was an evaporating dish to which was added 50 grams of paraformaldehyde, a magnetic stirrer and a small beaker containing 25 grams of a catalyst (18.5% hydrochloric acid solution). The reaction vessel was heated to 176 for seven minutes, the catalyst combined with the paraformaldehyde by activating the magnetic stirrer and the reactor was then maintained at 176~F for fifteen minutes. During this time, formaldehyde vapor exited the reactor through the vent line at the top, thus indicating that formalde-hyde vapor was passing up through the fiberized pulp suspended above the paraformaldehyde. After fifteen minutes, the reactor was cooled, the fibers removed and made into an aqueous furnish for use in a standard Noble and Wood handsheet apparatus.
In addition to Samples A, B, I and ll, two additional hand sheets were tested ~C and D). Handsheet C was made with modified fibers pro-duced according to the process disclosed in U. S. Patent No. 3,819,470 while handsheet D was made with such fibers produced according to the process described in U. S. Patent No. 3,756,913.
The handsheets were made to have a bisis weight of about 28 pounds per ream and were tested for caliper and dry tensile. The results of the tests are reported in the following table. In addition to testing the hand-sheets, samples of the pulp used to produce each handsheet were tested for Canadian Standard freeness (ml) and for swellability (ml/lOOg).
The caliper test for the handsheets of Samples I and l l had to be approximated, they had no dry tensile and could not be removed from the , .'' ' ' '' ' ~ ' ' '' ' .::
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wire without falling apart. The caliper of the handsheet and wire com-bination was tested, and the caliper of the wire alone was determined.
The caliper of the handsheet was estimated by the difference between the two calipers.
TABLE
SAMPLE A B C D I l l Handsheet caliper (.0001/in.) 6.8 6.9 12.1 10.810 10 Dry Tensile (oz./
in.) 16.9 18.6 6.0 2 0 0 Pulp C. S . Freeness (ml) 743 695 764 765 771 752 Swellability (ml/lOOg) 64 64 50 49.5 19.313.0 As can be seen from the results reported in the Table, the modified fibers produced according to the process of the present invention have significantly altered pulp properties for making wet-lay paper. In addition, the properties are significantly improved over the sirmilar pulp fibers 20 produced according to the two referenced U . S . Patents (Samples C and D) .

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Claims (7)

WHAT IS CLAIMED IS:

1. A process of making paper comprising:
reacting papermaking cellulosic: fiber pulp in substantially individual fiber form with formaldehyde vapor in sufficient quantity to modify the pulp fibers by substantially impeding the hydrogen bonding capability of the pulp fibers;
making a papermaking furnish by combining the modified fibers with water;
draining the papermaking furnish through a foraminous surface to wet-lay the modified fibers as a paper sheet on the foraminous surface;
removing the paper sheet from the foraminous surface; and drying the paper sheet.

2. The process of claim 1 wherein the reacting of the cellulosic pulp fibers with formaldehyde vapor is performed at an elevated temperature.

3. The process of claim 2 wherein the elevated temperature is at least 150°F and the quantity of formaldehyde reacted with the pulp fibersresults in a sufficient reduction in hydrogen bonding capability of the fibers so that the dry paper sheet has a dry tensile strength of less than half the dry tensile strength of a comparable dry paper sheet made without reacting the fibers with formaldehyde vapor.

4. The process of claim 2 wherein conventional wood pulp fibers are combined with the modified fibers in the papermaking furnish with the modified fibers comprising at least 10% of the fibers in the furnish.

5. A process of making modified papermaking cellulosic fiber pulp comprising reacting papermaking cellulosic fibers in substantially individual fiber form with sufficient formaldehyde vapor to substantially impede the hydrogen bonding capability of the fiber.

6. The process of claim 5 wherein the reaction with formaldehyde vapor is performed at an elevated temperature.

7. The process of claim 6 wherein the elevated temperature is at least about 150°F and the quantity of formaldehyde vapor reacted with thefibers is sufficient to reduce the hydrogen bonding capabilities of the fibers by at least 50%.