CA2035402A1

CA2035402A1 - High bulking resilient fibers through crosslinking of wood pulp fibers with polycarboxylic acids

Info

Publication number: CA2035402A1
Application number: CA002035402A
Authority: CA
Inventors: Bruce J. Kokko
Original assignee: Individual
Current assignee: Fort James Corp
Priority date: 1990-02-01
Filing date: 1991-01-31
Publication date: 1991-08-02
Also published as: ES2075339T3; EP0440472B1; EP0440472A1; FI910467A0; DE69112089D1; DE69112089T2; ATE126556T1; FI910467A

Abstract

ABSTRACT OF THE DISCLOSURE
The invention relates to resilient bulking fibers prepared by crosslinking wood pulp fibers with polycarboxylic acids.

Description

ll BAC~GRO~ND OF TEE INVE~TION Z03540Z
Field of the_Invention ~ he present in~ention relates generally to fiberq exhibitin~ improved re3ilient bulkin~ and absorbent proper-tieq. ~ore particularly, this in~ention relates to an improved method of preparing resilient bulking fibers by cros~linking wood pulp fiber~ with polyc~rboxylic acids.

De~cri~tion of the Related Art ¦ It is known in the art that resilient bulking fibers are 'lluseful for the preparation of bulkier and more absor~ent ilpaper ~tructures. Such paper structures Are useful for the manufactur~ of products such as handsheets, t~ ls, tissues, filters, ',paperboard, diapers, sanitary napkins, ho~pital dressings and ,Ithe like. Onë method for obtaining resilient bulking fibers ¦is by crosslinking cellulose fibers by treatment with a ~,chemical compound. U.S. Patent No. 3,819,470 discloses modified cellulosic fiber~ characterized by reduced ',swellability and a reduce,d capability of natural ,ifiber-to-fiber bonding when compared to unmodified cellulosic 'fib~rs and having a substantive polymeric compound reacted with and attached to the fibers. U.S. Patent No. 4,431,481 di~clo~e~ modified cellulosic fibers produced by treating the fibern with copolymers of male mic acid. Other known techniques include treatment of fibers with cationic urea formaldehyde resins, (U.S. Patent No. 3,756,913), methylol : urea~ and melamines (u.s. Patent No. 3,440,135), formaldehyde . .
'.
.. ,.. ... ., .. .. . .. .. , .. . .... "" . .. , . , ,, ~ , . . .. .... . ... . ... .

.

''. ~:.
.

(U.S. Patent No. 3,224,926), with the condensation product of acrolein ~nd formaldehyde, tU.S. Patent No. 3,183,054), bi~-acrylamides (Eur. Patent No. 0,213,415), and treatment with glyoxal or glutaric dialdehyde (WO 88104704, U.S. Patent No. 4,822,453 and U.S. Patent No. 4,853,086).
The crosslinking method3 of the prior art, however, tend to suffer from the disadvantages of toxicity, high cost, or poor effectiveness. Of these, toxicity is especially disadvantageous in view of the mounting concerns over the environment and safety of the workers. Because of these concerns, mo~t currently available bulXing fibers and the Imethods for making them are not commercially acceptable or ,will be challenged.
Thus, crosslinkers such as epichlorohydrin, divinyl-sulfone, bisaorylamides, formaldehyde, and formaldehyde-based reagents such as 4,5-dihydroxy-1,2-dimethylol-ethylene urea (common textile finish) present serious hazards to workers !and consumers. Formaldehyde-free reagents such a8 4,5-dihydroxy-1, 2-dimethyl-ethylene urea, while ~afer, are very expensive. Other formaldehyde-free reagents such as glyoxal, glutaric dialdehyde, and various resins, while generally considered non-hazardous and reasonably priced, are ¦les3 effective at producing bulking resilient fiber~. For ¦¦example, treatment of cellulosic fibers with male~mic copolymers or other resin~, a~ taught in U.S. Patent No.
4,431,481, results in fibers having equivalent bulk to fibers .. ~ . . - .

-,l ' 21~)35402 without chemical treatment that were heated to the ~ame elevated temperatures as utilized with the resin treatment.
The formation of nits and knots ic a common problem in the preparation of bulking resilient fibers through chemical cro~slinking. Nit formation is particularly prevalent when faster reacting agents, such as aldehydic compound~, or when polymeric agents are used. Practitioner~ of the art usually l¦employ debonding agents, mechanical defibration such as ¦Ihammermilling, and screening to reduce the nit and knot contents of treated fibers. Such measures tend to be costly and can be deleterious to fiber and paper quality.
~, ~he prior art does not disclose the use of ;polycarboxylic acids as crosslinkers or coreactants with other crosslinking systems for the production of bulking absorbent fibers, although the textile industry has demonstrated the use of polycarboxylic acids as crosslinkers or coreactants with other crosslinkers for the enhancement of ,;wrinkle-resistance and durable-pres~ properties in cotton fabrics (U.S. Patents Nosi 3,526,048 and 4,820,307, and Text.
Res. ~. (1967)f 37, 933 and (1972), 42, 274). Also "cellulosic fibers and powders have been crosslinked with ¦Icitric acid to produce ion exchange materials (U.S; ~atent ,~No. 2,759,787).
¦! The present invention overcome~ the problem~ ~nd 'disadvantage~ of the prior art directed to papermaking by providlng high bulking resilient fibers with little or no .... . .

~ ~ ` `J

, . . ;
." '~ ' ' :

" 2035402 ¦nits or knots obtained through crosslinking of wood pulp i fiber~ with polycarboxylic acids ~uch a~ citric acid.
Xt i~ an ob~ect of the present invention to provide ~uch resilient bulking fibers in a manner which will minimize the cost a~d increase the effectivene~s of the fibers produced.
It is an additional ob~ect of the present invention to ¦Iminimize the hazard~ to workers and the environment during ¦preparation of these fiber6.
'¦ Another ob~ect of the present invention iq to increaqe the anionicity of the fibers such that the fibers are more ~receptive to specific additives and are themsèlves more conducive to making acceptable pAper sub~tr~te-q.
, Additional ob~ects and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and advantage~ of the invention may be realized and obtained by means of the instrumentalities and co~binations particularly pointed out ;in the appended claims. ~' SU~NARY OP r~ INV~NTION
To achie~e the foregoing ob~ects r and in accordance with the pUrpo8e~ of the invention as embodied and broadly described herein, there is provided a resilient bulking fiber 1, comprlsing individualized wood pulp cellulosic fibers crosslinked by a i polycarboxylic acid, wherein the degree of crossl1nklng is at least that sufficient to induce twisting and curling and/or resilient bulking tendency in said individualized fibers.
; - 4 -.. , . . I . . . . . .. . , .,, , . ", ", . ~. . .. . .. .. . ..... .. . . . . ..... . ......... ...

, -There is also provided a method ~or preparing resilient bulking fibers comprising individualized cellulosic fibers crosslinked by a polycarboxylic acid comprising the steps of treating wood pulp cellulosic fibers by contact with a polycarboxylic acid; individualizing the cellulosic fibers so treated; and heating individualized cellulosic fibers ! to effect individualized crosslinking of the cellulosic fibers by the polycar~oxylic acid. Individualizing the treated fibers prior to heating them to effect crosslinking ensures that the crosslinking is intra-fiber;
that is, the crosslink bonds are primarily between cellulose molecules of a single fiber. This is in contrast to inter-fiber crosslin~ing where the bonds are formed between cellulose molecules of different fibers. The resulting dry bulking fibers can be incorporated into products through conventional papermaking techniques. These fibers resist relaxation during papermaking, retaining their bulking behaviour throughout the papermaking process.
The invention also includes an improved paper product comprising on a weight basis a majority of wood pulp fiber and sufficient crosslinked wood pulp fiber to impart improved bulk and absorbency properties.
iA,I BRIEF DESCRrPTION OF THE DRAWINGS
Fig. 1 graphically depicts the Attenuated Total Reflectance (ATR) of CAFC fibers (cf Example 4).
Fig. 2 graphically depicts the ATR spectrum of TC fibers (cf Example 2).

Fig. 3 graphically depicts the ATR spectrum of CA fibers (cf Example 6).
Fig. 4 is a microphotograph of fibers that were oven dried and cured without citric acid.

~j '": ~ , .:: .,' ; ~., :
:
.
-~

Il , 203540Z
Fig. 5 is a microphotograph of ~ibers that were oven dried and cured with citric acid.
I D~SCRIPTION OF IqIe P~FE~URED EIDBODIME~TS

! Reference will now be made in detail to the present preferred embodiment of the invention. In accordance with the present invention there is provided resilient bulking l fibers and a method for their preparation by cro~slinking ¦¦individualized wood pulp cellulose fibers with polycarboxylic acids.
¦The termin~logy "individualized crosslinked fibers" as used herein, refers to Icellulosic fibers that ha~e primarily intrafiber chemical i¦crosslinX bonds. That is, the crosslink bonds are primarily .Ibetween cellulose molecules of a ~ingle fiber, rather than lbetween cellulose molecules of separate fibers.
!i The cellulose fibers are treated with an aqueous 501u-'tion comprising a pol~carboxylic acid.and, if desised, an ;,additional agent such as sodium hydroxide or other caustic agent ;or a coreactant/accelerator. It is preferable to select the coreactant~
aGcelerator from the class of inorganic phosphorus compounds. It is more preferable to select the coreactant/
accelerator from the group consisting of phosphates, ii,phosphites, hypophosphites, pyrophosphates and metapho~phates. It is most preferable to use an inorganic . ~pho8phoru ` compound such a9 monosodium phosph~te, 1~ Dry lap or never dried wood pulp fibers can be used, i although it is preferable to use never dried fibers. It is our experience that starting with the never-driet fiber results in maximum bulking levels after crosslinking i . ~ .

.. , .; . . . . ~

~,~ , Z03S40Z
li regardless of the type ef cellulose crosslinker used. Not wishing to be bound by any theory, it i~ believed that ! never-dried fibers allow for homogeneou~ distribution of ! cro~slinking chemical in the cell wall, remain in a more ¦¦in'dividualized ~tate during the crosslinXing proceas, and ¦Imore readily adopt twisted and curled configuration-2 than do predried fiber~.
~¦ Any wood pulp fibers may be used, although it i8 j¦prefer~ble to use chemical thermal mech,nical pulp~, Southern &nd Northern softwood bleached kraft pulps, and secondary fibers.

, .
A~ording to the~present invention, individua~ized wood pulp cellulosic ~fibers are crosslinked Iby a polycarboxylic acid. The degree of cros~linking Lg at ;',least that sufficient to induce twisting and curling and/or resilient bulking tendency in said individualized fibers.
The upper limit would be reached when the degree of ''crosslinking renders the fibers unfit for the intended use.
! Individualized cro~slinked fibers according to this invention thu8 include those crosslinked by from le8~ than 1 mole ~ to more than 25 mole ~, calculated on a cellulosic anhydroglucose molar basis, of a polycarboxylic acid crosslinking agent.

.. ... .. . . .. ..

... :
. ,: ' , ' ;

, ,~, '~

- - 2035402 `

. .
.1 .
¦ Any polycarboxylic acid known to crosslink cellulose may be used to crosslink the fibers ac~ording to the present invention. Preferred polycarboxylic acids include citric acid, propane tricar~oxylic acid, maleic acid, ¦butanetetracarboxylic icid, cyclopentanetetrAcarboxylic ~cid ;and benzene tetracarboxylic acid. It is Also contemplated to i~use polycarboxylic acid precursors and deri~ative~ that will produce the polycarboxylic acid under the ~eaction conditions utilized to crosslink the fibers, as w~ell a~ homopolymers and 'copolymers of polycarboxylic acids. The most preferred ! polycarboxylic acid is citric acid becau~e it is an inexpensive, nontoxic, environmentally safe, readily available, naturally occurring polycarboxylic acid.
The polycarboxylic acid may be present in any concentration in the aqueous solution to allow for a sufficient numker of crosslinks. It iQ advantageous to use ;in the range of a 3-10% aqueous solution of polycarboxylic acid, with about a 5% aqueous solution being most preferred.
A caustic agent may be used, if desired, including sodium hydroxide.
After the fibers are treated with the aqueous ,olution, Ithe fibers may be dewatered by conventional papermaking 3~ ~techniques, for example, through the use of a screw press.
~he dewatering is done to any consistency, although higher consistencies are desirable for economical drying.
Preferably, the fibers are dewatered to a consistency of at least 30%. In order to maximize the bulking and resilient ._ .

~ ; ;

' .;,. .

l,j , Z035402 Ickaracteristics of the crosslinked fibers, it ic important to I minimize compression forces experienced by the fibers prior to cro~81inking and particularly during dewatering.
! The dewatered fibers may be dried by any method that l allows individualization of f ibers ( i . e ., minimizes nits, i ~not9, fisheye~, etc.). For exzmple the fibers may be azeotropically dried in a qo}vent, preferably toluene.
~Alternatively, the filtered fibers may be fluff dried using a ! hot gag 5uCh aS aix or superheated steam.
~! After the fibers have been dried to an individualized state, they are then cured by conventionally Xnown means to ~bring about the crosslinking reaCtion. For example~ the fibers may be cured by heating them at a temperature in the range of from about 150C to about 180C for in the range of about one-half of a minute to about ten minutes.
i Drying and curing can be accomplished either separately or concurrently in either batch or Continuous operations.
In order to maXimize the bulking and resilient characteristics of the fibers prepared according to the preqent invention it is desirable to conduct drying at a lower temperature than that used for curing.
Drying and curing of the treated fibers can be achieved l¦by any meanB th8t allows heating of the fibers to elevated ~¦temperatures~ for example, ovens~ or heating in hot gas strezms such as air, steam, superheated steam, or inert gases such as argon or nitrogen. It is preferred to use reducing atmospheres during drying and curing, such as is achievable .1 ; 203540Z
with systems like superheated steam or inert gases like nitrogen and argon, to minimize charring, darkening, and degradat$on of the fibers.
The cured fibers thus prepared can then be dispersed for use. ~referably, the dispersion step involves contacting the cured fiber~ with water at an elevated temperature.
The~e bulking fibers may then be used -- alone or in blend~ -- to prepare products that exhibit improved bulking ~and absorbent properties. The improvement in absor~ency irelates both to faster rate of absorbency and to increased ',fluid-holding capacity. The amounts of crosslinked fibers used to prepare the products are readily determinable by tho~e skilled in the art. For instance, filtration and absorbent product applications will often be made 100~ from the fibers of the present invention. On the other hand, towel and tissue paper products may be made by blending fibers according to the present invention with a ma~ority of ! ,conventional wood pulp fibers. In such applications, it may be preferable to use crosslinked fibers in an amount of about 25% or less by weight of the paper product.
Additional advantages and modifications will readily occur to those skilled in the art. The invention in its ¦Ibroader ~spects is, therefore, not limited to the specific ¦~details and illustrative examples shown and described.
,~ Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive 'I ' .

. ~ . .. . : . ~

Il Z035~0Z
concept as defined by the appended claims and their equivalents.
Ihe foliowing examples further illu~trate preferred embodiment~ of the present invention. The examples ~hould in I no way be considered limiting, but are merely illustrative of 1, the various features of the presen~ ~nvention.
EsamPle 1:
¦ Never dried Northern bleached softwood kraft fibers ¦(NSWg) were dispersed in a 10~ aqueous solution of citric ¦acid, to which 0.03 equivalents of sodium hydroxide (based on jlequivalents citric acid) had been added. The res~Ltant fibers were filtered to approxim~tely 30% consi~tency, azeotropically dried in toluene, filtered, and heated in an oven at 160C for 10 min. The cured fibers were then jdisintegrated in 100C water for 30 min. (the water ;Itemperature drops to 45C during this time). The resultant fibers are hereafter referred to as "CATC".
Esam~le 2:
Example 1 was repeated without citric acid to produce fibers hereafter referred to as ~TC".

¦EsamDle 3-The fibe~ described in Examples 1 and 2 were made into ¦¦pressed Briti~h handsheets according to standard method3 us-ing the furnish compositions described in Table 1. A~ can be seen from the data provided in Table 1, ~heet~ made with the furnish containing the CATC fibers had the highest bulk after .. . -- 11 --! I 203S402 ,pressing. Thus, for every 1~ incorporation of CATC fibers in , a furnish containing NSWR fibers, a 2.S~ increase in dry ! sheet bulk was seen after pressing.
~ample 4s l Ex~mple 1 was repeated except that no sodium hydroxide i was added to the citric acid ~olution , the fibers were fluff , dried with hot air in lieu of azeotrope dryin~ in toluene, and curing wa~ done at 180C for 2.8 minutes. ~he resultant '¦fibers are hereafter referred to as ~CAECn.
ExamDle 5:
Example 4 was repeated without citric acid to generate ~!fibers hereafter referred,to as "FC".
,E~am~le 6:
Example 4 was repeated without the oven curing step to ,generate fibers hereafter referred to as ~CA".
Exam~le 7.
I Example 4 was repeated without citric a~id and without ',the oven curing step to generate fibers hereafter referred to as ~'FD".
'E~amPle 8:
,' The fibers obtained in Examples 4-7 were used to prepare l British,handsheets as described in Example 3. The pressed bulk data for the resultant sheets ase pro~ided ln Table 2.
¦ The crosslinking pre~umably occurs by the fosmation of diester bonds between cellulose chains. The existence of ester li~kages in the CATC and CAEC fibers is clearly evident from the band at 17.~8 cm ! obtained by IR spectroscopy ( for examp~e see , . Figure 1 ) . Such ester . `
--12-- .

.

:. ~ . : : ., ~' :

I , 2C~35402 ~¦linkages are absent in the untreated or uncured fiber~ (for i examples ~ee Figures 2 and 3). The percent covalently bound citric acid was measured in the CAFC fibers by titration according to the method described in Text. Res. J. (1967), 37:933 and found to be 7 wt~ (based on weight of oven dried fiber). ~his means that 23~ of the available citric acid had !actually reacted with the fiber.
;I The citric acid cros~linking reaction appeared to impart additional kink and curl to the fibers that were o~herwise not achieved by the heat treatments alone. This suggestion 'was supported by comparison of microphotographs o~ fibers ! 'that were oven dried and cured without citric acid (Figure 4) with microphotographs of fibers that were oven dried and cured with citric acid (Figure 5).
The citric acid crosslinking reaction rendered the NS~R
fiber more anionic. This was readily apparent by treating the crosslinked fibers with methylene blue. A deep blue color was retained in the crosslin~ed fibers, whereas little dye was taken up by the untreated NSWR fibers. The total charge of citric acid crosslinked fibers, made according to j~xample 4, was 76 meq/100 g. The total charge of untreated 'fibers wss 4 meq/100 g. This anionicity is a further advantage of the fibers of the present invention over those prepared according to the past art, as the polycarboxylic acid crosslinked fibers should be more receptive to cationic additives important to papermaking. For example, the strength of sheets made from the crosslinked fibers should be _ 13 ~

.
, .

recoverable without compromising the bulk enhancement by incorporation of a cationic ~trength re~in.
~ he polycarboxylic acid cros61inking reaction did not appear to damage the NSW~ fibers. Thus, the average fiber ~length was not c~anged by the crosslinking reaction.
,IFurthermore, the integrity of the fibers was unchanged by the ¦¦crosslinking reaction as evidenced by micro~copic examination jl(compare Figures 4 and 5). There wa~ ~ome brightness ~¦reduction due to the crosslinking reaction ~see Table 1).
E~am~le 9:
¦ The successful achievement of bulking fiber~ is by no ,Imeans limited to crosslinking with citric acid. Any ,polycarboxylic acid known to crosslink cellulose will wor~.
To demonstrate this, NSWR fibers were crosslinXed with butanetetracarboxylic acid according to the method described in Example 1. The resultant fibers, hereRfter referred to as ~BTATC~, were then made into handsheets according to the method described in Example 3. The physical data on these ;sheets are provided in Table 3. The existence of ester bonds ~between cellulose and butanetetracarboxylic acid was ~erified by IR spectroscopy. As can be calculated from the data in Table 3, ~ 25~ incorporation of the BTATC fibers in the NSWK
furni~h re~ults in a 92S increase in pre~sed sheet bulk.
Furthermore, there was no brightness lo~s seen in the preparation of the BTATC fiber~.

~samole 10:

_ 14 ~

-. ... - : ~ . . , . :

.
' ., ~ - ' -:

- : .

Il Partial neutralization of the citric acid prior to fiber ¦¦treatment is not necessary (see Example 4) for the successfu ¦Iprep~ration of high bulking resilient fiber~ as de~cribed abo~e. Thus, ~x~mples 1-3 were repeated without the use of sodium -hydroxide in the preparation of the treatment solu-tion, and the resultant fibers (i.e. CAT fibers) had equivalent performance to that of the CATC fibers (compare ~¦data in Table 4 with that in Table 1). -Since the citric acid treated fibers were at 25~ consistency prior to drying, 39%
of available citric acid (i.e. that acid contained in the dry fiber prior to curing) had reacted with the NSWR fibers to produce the CAT fiber~ described in this example.
Exam~le 11:
Example I0 was repeated except a treatment solution containing only 5 wt% citric acid wa~ u~ed. As can be seen in Table 4, comparable bulking performance is observed with the resultant CAT fibers relative to those prepared with S! " solution~ having twice th,e level of citric acid.
Furthermore, there is a marked improvement in ~rightness accompanying the reduction of citric acid in the treat~ent ; Ibath. It ~hould also be noted that 53% of the available ~citric acid had reacted with the NSW~ flber~ to produce the ~CAT fiber~ described in this example.
'.'P~amele 12s i Example 10 was repeated except a 3 wt% aqueou~ solution of citric acid was used for the treatment. As c~n be seen in ~able 4, there was a slight reduction in the ~ulking ability _ 15 --.
, '' , ' ~

Ii I 2~)35402 realized with the CAT fiber prepared under these conditions.
Neverthele~s, a 2~ bulk enhancement is predicted for every 1 incorporation of these fibers in a NSW~ furnish.
Furthermore, es~entially no reduction in brightness wa~
obser~ed with these fiber~ relative to the control. As was the case with the CAT fibers prepared according to Example 10, about 39~ of the available citric acid had reacted with the NSWR fiber~ to produce the CA~ fibers described in this ! example.
'¦ The percent bound citric acid levels as determined by l titration (7) are consistently lower than those determined by l ion chromatography. The latter method is considered to be ., more reliable as it is not predicated on an assumption of the num~er of active equivalents of carboxyl functionality during .base hydrolysis.
¦ The results of the above examples suggest that the 'bulking resilient fibers can be obtained using dilute ;~solutions of polycarboxylic acids without the in~olvement of other chemical additive~ Such a simple treatment chemi~try jgreatly enhances the attractiveness of the present invention.
,,Neverthele~s, it has been demonstrated by others that certain l additi~s, such as sodium dihydrogen phosphate or sodium ! hypophosphite, can apparently accelerate the reaction of I polycarboxylic acids with cotton fibers. ,~ext. Ch~. Color.
' (1989), 21, 2,13. Such acceleration is useful for the present i.nvention, as shown in Example 13.
Exam~le 13 , - 16 -.~ , , :~ , , . ~

~- .... . .

il i 203S402 NSWR fibers were dispersed in an aqueou5 solution that contain8 5~ citric acid and ~ monosodium phosphate, filtered to about 2~ consistency, fluff dried, and cured at 180 for 90 6econds. A8 Can be seen in Table 5, the resultant fibers (PCAT) are extremely bulXing. The amount of bound citric acid reached in this catalyzed syQtem was 69~ of that avail-able. The effecti~eness of the monosodium phosphate to ac-¦celerate reaction of citric acid with fiber i8 further ~exemplified by the ob~ervation of 1~ bound citric acid afterfluff drying alone (PC~TU). No bound citric acld ha~ been 'lobserved during fluff drying of fibers treated witn only `¦citric acid. Some covalently bound pho~phate was al80 detected by ion chromatographic analy~i~ of hydro8ylat~ of PCAT fibers. Thus, phosphate appears to be coreacting along ,with citric acid, with the cellulose.
' _am~le 14 ¦ The citric acid cro881inking tre8tment is effective at producing bulk and resiliency enhancement in a wide variety iof wood pUlp8. ~ifferen~ wood pulps were treated according to Exa~ple 13, unless otherwise stated, and made into pressed 65 g/m2 hand~heets. The bulk data is pro~ided in Table 6.

~.' _ 17 --.. .

, .: :
.

`-.! , !

N N

~i N N

o '~ ,.
.~ .' ~ ~ O O O ~ I

3 ~ a a .

` ' , Z035402 ll ~ ô -- o e .. ~

C ~ o ~ o - ! ~
C , . s S .

o ,, o, U

c ~ U ~ ~

o , ~o ~, i~ ~ ~ ~

o ~ -- 19 --.~ ~ o ~ ~ .

! ~1 o o ~
o. o ~ ~ l .
I

8 ~ 8 ¦ a ~ 7 !

o ¦ j ~

ii ~ o l 1~

} ;~ ~ ~ 9 a ~ ~ 1;

.

.
: i ~ . . ; - . ~

z03~;40Z
'. ' .
., .
o ~
. ~ ~ 2 ~ ~0 OOO a ~
+l ~ +l +l +l ~ I ~ ~ o In ~
,~ . ~ ~ ~ ~
C ~0~ 0;0 000 0 0 ~ ~ O O

1~ ~ 'Y ~ Q ~ O C~ Ç~ ~ ~
~ . . O- C C

5 ~ 3 Z S E~ Z ~ U ~ Z P ~ Z
o ~ ______ __ `:

ll , ~,l);~5402 . .'~, .
.
j ' ~o-~ooll'.
C ,~ .

2 ~ y .

s~ l o~

. ` ~
i! .

. . - .

: , ' : -' ` . '' ' ::` .
.. .. , . ~ , .. . . .
., . . , -'11 S~bl- 6 cltrlc ~d ~ro~ g o~
.! C~f~r~at vocd pulps E~zni~h' 8ulk_(cm3/q~
100% Husu~ (predried) 1 8 100% HULUr~ ~never dried) 1 6 100~ sswn~ ~ne~er dried) 1 8 100~ Secondary ~ibers~ 1 8 25% Treated Husum ~predried)' 2 3 25S Treated Husu~ (never dried)t 2 6 25S Tre~tet ssWn~l 2 4 1 25% Treated secondary fibersh 2 3 !1 2S~ Treated CT~P1~ 2 8 !i 25~ CTMP (never dried) 100~ NSWX ~nevcr dried) ___________________________________________________________________ a) Mad- into pressed 65 g/m2 ~ritish handsheQts b) Scandinavian bleached spruce kraft pulp ~untreated) c) Southern pin- bleached kraft pulp ~untreated) d) Long fiber fraction of Ponderos3 ~condary tlbcr~ (untreated e) 75~ untrea~ed pr-dried Husun ~) 75$ untr-at-d ~-ver drl-d Husu~
g) 75~ untr-ated SSWX
h) 75~ untreated ~econdary fibers ', i) 75~ NSWK
Il ~) N-var dried Northern softvood bl-ach-d kratt pulp (untreated i k) Son- nlts pre~ent 1) Drl-d and cur-d vlth aup-rh-at-d t-au at l~O'C ~or 30 oconds ~) Startinq CT~P vas n-ver dri-d . . .

... ..

Claims

1. Individualized wood pulp cellulosic fibers crosslinked by a polycarboxylic acid, wherein the degree of crosslinking is at least that sufficient to induce in said individualized fibers at least one of the following, namely twisting, curling and resilient bulking tendency.

2. Individualized wood pulp cellulosic fibers as claimed in claim 1 crosslinked by from about 1 mole % to about 25 mole %, calculated on a cellulosic anhydroglucose molar basis, of a polycarboxylic acid crosslinking agent.

3. Individualized crosslinked wood pulp cellulosic fibers as claimed in claim 1 or claim 2 in which the polycarboxylic acid crosslinking agent is selected from citric acid and butanetetracarboxylic acid.

4. A resilient bulking fiber comprising individualized crosslinked wood pulp cellulosic fibers as claimed in any one of claims 1 to 3.

5. A method for preparing resilient bulking fibers comprising the steps of:
(a) contacting wood pulp cellulosic fibers with a polycarboxylic acid;
(b) individualizing the cellulosic fibers; and (c) curing the individualized cellulosic fibers to effect crosslinking of the cellulosic fibers by the polycarboxylic acid.

6. A method for preparing resilient bulking fibers comprising the steps of:
(a) mixing wood pulp cellulosic fibers with an aqueous solution of polycarboxylic acid;
(b) individualizing said cellulosic fibers of step (a);
(c) curing said cellulosic fibers of step (b) to effect crosslinking of said fibers.

7. A method for preparing resilient bulking fibers comprising the steps of:
(a) mixing wood pulp cellulosic fibers with an aqueous solution of polycarboxylic acid;
(b) dewatering said cellulosic fibers of step (a);
(c) drying said cellulosic fibers of step (b); and (d) curing said cellulosic fibers of step (c) to effect crosslinking of said fibers.

8. The method of claim 7 wherein said dewatering process comprises pressing said cellulosic fibers through a screw press.

9. The method of claim 7 wherein said dewatering process comprises azeotropically drying said fibers in a solvent.

10. The method of any one of claims 7 to 9 wherein said fibers are dewatered to a consistency of at least 30%.

11. The method of any one of claims 7 to 10 wherein said drying step (c) comprises fluff drying the dewatered fibers with hot gases.

12. The method of any one of claims 7 to 11 wherein said drying step (c) is performed at a temperature lower than that used for curing in step (d).

13. The method of any one of claims 6 to 12 wherein said aqueous solution is about 3-10% aqueous solution of a polycarboxylic acid.

14. The method of any one of claims 6 to 13 wherein the aqueous solution includes a caustic agent.

15. The method of any one of claims 6 to 13 wherein said aqueous solution comprises citric acid and a coreactant/accelerator.

16. The method of claim 15 wherein said coreactant/accelerator is selected from phosphates, phosphites, hypophosphites, pyrophosphates and metaphosphates.

17. The method of claim 16 wherein the coreactant/accelerator is monosodium phosphate.

18. The method of any one of claims 5 to 17 wherein said polycarboxylic acid is selected from citric acid and butanetetracarboxylic acid.

19. The method of any one of claims 5 to 18 wherein the curing step comprises heating the fibers at a temperature in the range of from about 150 to about 180°C for a time period in the range of from about 0.5 to about 10 minutes.

20. The method of any one of claims 5 to 19 wherein said wood pulp fibers are selected from chemical thermal mechanical pulps, Southern and Northern softwood bleached kraft pulps, and secondary fibers.

21. The method of any one of claims 5 to 20 wherein said wood pulp fibers are never-dried fibers.

22. An improved resilient bulking and absorbent paper product comprising crosslinked wood pulp cellulose fibers to provide improved bulking and absorbent properties.

23. A paper product as claimed in claim 22 wherein said crosslinked wood pulp cellulose fibers are individualized.

24. A paper product as claimed in claim 22 or claim 23 wherein said crosslinked wood pulp cellulose fibers contain interfiber bonds.

25. A paper product as claimed in any one of claims 22 to 24 wherein the crosslinked wood pulp cellulose fibers are as claimed in any one of claims 1 to 3 or are made by a method as claimed in any one of claims 5 to 21.

26. The paper product of any one of claims 22 to 25 further comprising non-crosslinked fibers.

27. The paper product of claim 26 wherein said non-crosslinked fibers comprise the majority of said product, on a weight basis.

28. The paper product of claim 26 or claim 27 wherein said non-crosslinked fibers are wood fibers.

29. The paper product of any one of claims 22 to 28 wherein said paper product is selected from handsheets, towels, tissues, filters, paperboard, diapers, sanitary napkins, and hospital dressings.