CA1149593A

CA1149593A - Absorbent structure

Info

Publication number: CA1149593A
Application number: CA000360543A
Authority: CA
Inventors: Dennis C. Holtman
Original assignee: Johnson and Johnson Baby Products Co
Current assignee: Johnson and Johnson Professional Inc
Priority date: 1979-09-19
Filing date: 1980-09-17
Publication date: 1983-07-12
Also published as: ES8200218A1; ES495166A0; JPS5653745A; AU530540B2; AU6028880A; NZ194821A; PH16726A; DE3034718A1; ZA805785B; BR8005964A; FR2465833A1

Abstract

ABSTRACT OF THE DISCLOSURE

An absorbent structure is made from non-delignified wood pulp fibers by commingling from 1% to 20% by weight of a hydroxyl-bearing polymer component with the fibers. The resulting absor-bent structure has improved wicking rates and structure integrity.
A process is also disclosed whereby the absorbent structure is prepared by forming an aqueous slurry of non-delignified wood pulp fibers with at least about 1% by weight of the fibers of a hydroxy-bearing polymer component, forming feed material from the slurry, grinding the feed material and air-laying the resulting fibers to form a web.

Description

Absorbent Structure -Background of the rnvention The present invention relates to an absorbent structure containing non-delignified wood pulp fibers which struc-ture is particularly suitable for use in absorbent productsrequiring a su~stantial liquid holding capacity~

Non-delignified wood pulp fi~ers, such as the thermo-mechanically produced ~ood pulp fibers, refiner produced wood pulp fi~ers, or the like, have become quite important in the last few years. These wood pulp fibers also refer-red to as "high yield" w-ood pulp fibers, have become increasingly important for several reasons. The processes used to produce the fibers utilize more of the raw material.
For instance, these types of processes use 90% or more of the tree as compared to approximately 50% use of the tree in the instance of typical chemical processing.
Furthermore, the non-delignified wood pulp processes reduce the environmental problems caused by chemical processing. Specifically, the "high yield" processes cause considerably less air pollution and water pollution than do the counterpart chemical processes. These various factors and the concomitant economic considerations make the high yield processes, such as the thermomechanical pulp process, very attractive.

Non-delignified wood pulp processes have been known for some time and are usually developed primarily for paper grade wood pulps, newsprint, and the like. These wood pulps have not been well accepted in absorbent type products, such as sanitary napkins, disposable diapers, and the like, primarily because of their relatively poor per~ormance as the absorbent core for such products.

Conventional chemically processed wood pulp fibers have a degree of cohesive strength when placed in an air-~aid web structure. Typically chemically processed wood pulp fibers are somewhat collapsed and appear in ribbon-like form. This form permits fiber entanglement during the air-laid web processing and hence results in a web having a degree of cohesiveness and fibrous web integrity.

In contrast, the non-delignified wood pulp fibers are non-collapsed, stiffer and more resilient. Webs formed of these fibers, although possessing a greater potential liquid holding capacity, have poor integrity and hence tend to break apart.

Furthermore, absorbent structures made from non-delignified wood pulp fibers are substantially hydrophobic and not readily wettable. For any a~sorbent structure to be satisfactory, it is highly desirable for the structure to ~1) readily accept liquid, ~2~ easily transport the liquid from one portion of the structure to another and ~3) hold ~5 the liquid accepted.

Various techniques have been developed or suggested for improving the a~sor~ent characteristics of non-delignified wood pulp, such as removing the fines from the wood pulp 30 product or providing various solvent or other chemical treatments to the wood pulp product to both bleach the pulp and improve its absor~ency However, these techniques increase the econo~ics or cost of the wood pulp and, in some instances, increase the pollution problem and, hence, do not take full advantage of the non-delignified wood pulp process.

Other techniques or developing absorbent products util-izing non-delignified wood pulps have been suggested.
One technique is disclosed in British Patent 1,500,Q53 and uses fibers of specific measurement; that is, length and diameter, The surface hydrophilicity of the fibers is increased by bleac~ing and the hydrophilic fibers are air-laid in we~ form and compressed to a specific density.
Bleaching followed by compression substantially increases the wetta~ility of the otherwise hydrophobic structure, lS but at the same time, reduces the liquid holding capacity of an absorbent structure made from non-delignified wood pulp fibers.

As mentioned above, for any absorbent structure to be satisfactory, it is not only necessary for the structure to hold liquid but also t,o readily accept liquid and transport it. The liquid holding capacity of the absorbent structure relates to the pore size of the fibrous bed and the wet bending modulus of the fibers. If the pore si~e (i.e. the spaces surrounding the fibers) is large and the wet bending modulus ~,i.e. stiffness) of the fibers is high, then the structure will have a relatively high liqùid holding capacity but generally does not accept and trans-port (wick2 liquid readily. On the other hand if the pore 3Q size is smaller and the bending modulus relatively low, the structure readily accepts and wicks liquid but will have a lower liquid holding capacity.

.

The fibers from the non-delignified wood pulp process can provide an a~sorbent structure having a large pore size and a high wet bending modulus of the fibers, however, such absorbent structures do not readily accept liquid, nor will the structure be readily densified or embossed to promote wicking~

Summary_of t`he Present Invention It has been discovered that an a~sor~ent structure can be made from non-delignified ~ood pulp fibers which will readily accept and wick liquid w~ile retaining a desirable substantial liquid holding capacity Furthermore, the new absorbent structure of the present invention takes advan-tage of the improved economics available when using non-delignified wood pulp fibers~

In its broadest aspect, the present invention provides an absorbent structure comprising a fibrous bed of non-2Q delignified wood pulp fibers and at least a~out 1% byweight of a hydroxyl-bearing polymer component commingled therewith.

The new absorbent structure is made by defibrating non-delignified wood pulp feed material and air-laying the fibers. The feed material is in the form of a board, crumbs (clumps~, or bale material, or the like, with which the hydroxyl-bearing polymer has been commingled.

Detailed Description of the Invention The non-delignified wood pulp processes do not remove the ~5-lignin from the wood fibers as t~e chemical processes do. The presence of the lignin in or on the fiber provides the fi~er with resilience and resistance to wet compression.
When providing an a~sor~en~ structure made from a fibrous batt w~ich is to be used in products such as disposable diapers, sanitary napkins, and the like, it is desirable to provide the batt ~ith the ability to wick the liquid from t~e point of entry of the liquid to other areas o~
the absor~ent structure.
The "wicking" o~ liquid is the transporting of liquid from one point to another in the absorbent structure.
~Yicking has heretofore ~een accomplished by densifying a selected area to create a smaller capillary structure resulting in an increased capillary pressure. T~is concept is discussed in further detail hereinafter. One method of densifying an area is disclosed in Burgeni U. S. Patent 3,017,304 wherein a "paper-like" skin is formed on one surface of an absor~ent batt used in a diaper. Another method of densifying a selected area of fibers is found in Rep~e, U.S. Patent 3,938,522 wherein densified regions are provided.

In these absor~ent structures wherein a portion of the fibrous batt is densified to decrease the capillary radius between adjacent fibers resulting in an ability ~o wick more liquid in a given period of time, the densified area does not hold as much liquid resulting in a reduction of the liquid holding capacity of the absorbent structure.
The present invention improves the wickability and inte-grity, i.e., stability of the absorbent structure without this loss of liquid holding capacity.

.A~ 35~3 ~6 -The absorbent structure of the present invention is a fibrous batt formed of loosely compacted non-delignified wood pulp fibers which are primarily held together by interfiber ~onds requiring no added adhesive. In order for the absorbent structure to perform, the batt should be substantially wettable. It has been discovered that the addition of at least about 1% of a hydroxyl-bearing polymer component commingled with the fibers of the fibrous batt permits rapid wetting of the fibrous batt.
The fibers used to form the absorbent structure of the present invention may be produced from any of the various soft woods, such as spruce, balsam, western hemlock, douglas fir, white fir, and the various pines, especially southern pine, such as loblolly and slash pines. For ex-ample, thermomechanical pulp is formed when ~he wood is initially cut into chips and the chips are pre-steamed in a pressurized steaming vessel. The pre-steaming softens the wood and allows defibration by a disc refining technique. The forming of non-delignified wood pulp fibers is generally well known in the art.

The hydroxyl-bearing polymer is starch, polyvinyl alcohol, guar gum or a hydroxyl-bearing polymer which is dispersible in water. The starch may be any starch material whether it be cereal starch, tuber starch, waxy, or a high amylose starch. The starch may ~e cross linked or otherwise modi-fied to form a cationic starch, an anionic starch or non-ionic starch. Derivatives such as starch phosphates, hydro-xyethyl starch, starch acetate, and the like, may be used.In addition, polyvinyl alcohol has been found to be suit-able, as is guar gum The hydroxyl-bearing polymer compon-. : .

ent is present in an amount at least a~out 1% by weight, preferably from a~out 1% to a~out 20% and most preferably from about 1% to about 5~

In a preferred embodiment of the present invention, the hydroxyl-bearing polymer in the form of starch, ~in an amount from about 1% to about 2Q% ~y weight of the thermo-mechanical pulp fibersl is slurried in water to form a slurry less than about 30% by weig~t solids. The slurry lQ is then heated to ~5C and the starch component is "cooked"
for about 20 minutes~ rn the meantime, non-delignified thermomechanical wood pulp fibers from the refinery are provided in dilute aqueous sl~rry form. The two water slurries are combined and mixed. The resulting mixture lS is then formed into board After formation of the board, the board is ground and the absorbent structure is air-laid as a we~.

The absorbent structure in the form of a batt may be 2Q laminated with a moisture-impermeable film, such as a backing on one side and a moisture~permeable facing on the other side, to form a diaper. On the other hand, the batt may be wrapped in a suitable wrapper to produce a sanitary napkin or it may 6e processed in various ways so as to be incorporated in tampons, underpads, dressings or other absorbent products.

In the instance wherein the absorbent structure is to be used in a disposable diaper, it is particularly advanta-3Q geous to treat the absor~ent batt so as to provide wicking.When an infant voids into the diaper structure, all of the urine enters the diaper in one limited area. Thus it is highly desirable to provide a wicking or transporting system to the absorbent structure to rapidly wick the urine away from the point of entry. As heretofore men-tioned there are two well-kno~n systems for rendering absorbent structures effective for wicking liquid.

The Burgeni ~or paper-like skin) generally formed on the side of the batt adjacent the backing of the diaper, assists in drawing the urine from ~he entry point to the back side of the diaper a~ay from the infant. Densified regions generally applied as longitudinal lines on the absorbent batt assist in wicking the liquid to the ends of the absorbent batt.

Non-delignified wood pulp fi~ers air-laid as a web are not readily densified nor can a Burgeni skin be formed effec-tively. However, surprisingly,an absorbent batt formed in accordance with the present invention readily accepts densified regions and, if desired, a Burgeni skin. In other words, the incorporation of the hydroxyl-bearing 2Q polymer into the absorbent batt structure greatly enhances the ability of the fibrous structure to receive densified regions. Furthermore, a paper-like (Burgeni) skin is readily formed by conventional methods. Either the densi-fied regions or the Burgeni skin or a combination thereof greatly enhance the integrity of the absorbent batt.
Heretofore it has not been possible to improve both the ~icking properties and the integrity of an absorbent batt consisting of non-delignified wood pulp fibers. At this time it is theorized that in the situation of densified 3a regions and Burgeni skin as applied to an absorbent batt of chemical pulp, the presence of moisture causes a ~iber collapse thus creating the densified region. In the instance of an absorbent batt formed from non-delignified wood pulp fibers 5 the fibers do not collapse in the presence of moisture. The addition of ~he hydroxyl-bearing polymer for some reason permits densifying of the batt fibers in S the presence of pressure and/or moisture.

It is another embodiment of the present invention to provide an absorbent structure comprising non-delignified wood pulp fibers in com~ination with up to about 50% by 10 weight of chemically processed wood pulp fibers, cotton linters, mixtures thereof or mixtures containing small amounts of rayon or the like.

These absorbent structures containing fibers o~her than 15 non-delignified wood pulp fibers may be formed by pre-mixing the fibers, forming a feed material and then grinding and forming the air-laid web, or the absorbent structure may be formed of a layer of non-delignified wood pulp fibers in close association with or laminated 20 to another layer of chemically processed wood pulp fibers or the like.

The following example illustrates a preferred embodiment for forming an absorbent structure in accordance with the 25 present invention~

Example Seventy grams of non-delignified thermomechanical wood 3Q p~llp fibers are soaked in water and made into a twelve by twelve inch board in a sheet former. This sample is used as a control sample.

-.5~33 Another seventy grams of the pulp are soaked in water.
A five hundred milliliter slurry is prepared containing seven grams of a slightly acid-modified starch. The starch mi~ture is cooked at 95C for twenty minutes and added to the thermomechanical pulp slurry and mixed.
The mixture is formed into a board of the same size and type as the control board. The control board weighs 66.2 grams and the board containing the starch weighs 70.1 grams. The boards are ground in a hammer mill and the fibers air-laid to provide a fibrous ~at~. The batt is divided into samples and tested for wicking, tensile strength, embossed lines, etc. in accordance with the following test procedure. A test to determine the wicking properties of a fi~rous web supported in a vertical position is as follows.

The test apparatus consists of two matching clear-plastic plates measuring 5" x 11". The gap between the plates is regulated with adjusting screws so as to adjust the thickness of the batt to obtain the desired density.
Fibrous batt strips measuring 3" ~ 8" (the batts generally weighing about 8 o2/yd2~ are placed between the plates with the bottom edge of the batt strip matching the bottom edge of the plates. The plate gap is adjusted to give whatever dry density is desired. The apparatus is then hung in a vertical position. A large Petri dish, contain-ing 1.59~ saline solution,is placed on an adjustable stand and is raised to contact the bottom surface of the batt strip. The distance wicked by the liquid and the weight of the solution wicked is determined after a prescribed period of time.

The results of the tests are in Table 1 below.

INCHES LIQUID WICKED
DENSITY WICKED GmLiq/
5 SAMPLE GM WEIGHT gm/cc IN 8 MIN. Total Grams GmSample Control A 5.48 0.1 4.50 20.1 3.7 B 4.72 0.1 4.25 16.8 3.6 C 5.25 0.1 5.00 18.3 3.5 D 5.16 0.1 4.75 19.6 3.8 With Starch . . . _ E 3.83 0.1 4.50 16.0 4.2 F 4.70 0,1 4.50 20.7 4.4 G 4.75 0.1 4.75 19.1 4.0 15 H 5.18 0.1 4.50 22.4 4.3 When comparing the control samples with the samples containing starch, it is noted that the distance the liquid wicked in 8 minutes is about the same. However, 2Q the amount of liquid wicked by the samples containing starch is greater by at least 13% in each instance as shown in the last column of the table above.

Embossed lines were placed on the panels. Each line was one-eighth inch in width and was applied by an embossing machine at a pressure of 20 psi. The embossed line was then cut out of the panel and subjected to a tensile strength test on an Instron instrument wherein the full scale load was 100 grams. The embossed lines of the control sample exhibited a tensile strength of 38.6 grams whereas the lines of the sample with starch had a tensile strength of 60.2 grams.

5~;3 It is readily seen from the data above that the starch improves the amount of liquid wicked by at least 10%. Thus more liquid is wicked just as far in the same time period. Furthermore the integrity of the fibrous structure is improved by more than 50%.

The foregoing description and example are illustrative but are not to be taken as limiting. Other variations and modifications are possible without departing from the spirit and scope of the present invention~

Claims

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

1. An absorbent structure comprising a fibrous batt of non-delignified wood pulp fibers and at least about 1% by weight of a hydroxyl-bearing polymer component selected from the group consisting of starch, modified starch, derivatized starch, poly vinyl alcohol and guar gum commingled therewith.

2. The absorbent structure of claim 1 wherein the hydroxyl-bearing polymer component is present in an amount from about 1%
to about 20% by weight.

3. The absorbent structure of claim 2 wherein the hydroxyl-bearing polymer component is present in an amount from about 1%
to about 5% by weight.

4. The absorbent structure of claim 1 wherein the hydroxyl-bearing polymer component is commingled with the fibers in substantially uniform distribution.

5. The absorbent structure of claim 1 wherein the non-delignified wood pulp fibers are soft wood fibers.

6. The absorbent structure of claim 5 wherein the soft wood fibers are selected from the group consisting of spruce, balsam, western hemlock, douglas fir, white fir, southern pine, loblolly and slash pine wood fibers.

7. The absorbent structure of claim 1 wherein the fibrous batt contains up to about 50% by weight of chemically-produced wood pulp fibers or cotton linters or a mixture thereof.

8. The absorbent structure of claim 1 wherein the fibrous batt contains densified regions.

9. The absorbent structure of claim 1 wherein the fibrous batt is placed between a moisture-impermeable backing and a moisture-permeable facing to form a disposable diaper.

10. A process for preparing an absorbent structure which comprises forming an aqueous slurry of non-delignified wood pulp fibers with at least about 1% by weight of the fibers of a hydroxyl-bearing polymer component, forming feed material from the slurry, grinding the feed material and air-laying the resulting fibers to form a web.

11. The process of claim 10 wherein the hydroxyl-bearing polymer component is slightly acid-modified starch and is present in an amount from about 1% to about 5% by weight.

12. The process of claim 11 wherein the starch is formed into a slurry of about 30% by weight solids and is cooked for about 20 minutes at about 95°C and then added to the aqueous slurry of non-delignified wood pulp fibers.