WET-LAID ABSORBENT PULP SHEET SUITABLE FOR IMMEDIATE CONVERSION INTO AN ABSORBENT PRODUCT

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to absorbent pulps; more specifically this invention relates to wet-laid pulp sheets which are convertible to an end product without the added steps of disintegrating a pulp sheet to make a fluff pulp and then air-laying the pulp to make a sheet that is then cut up into appropriate sizes to obtain the desired product. Still further, this invention relates to the method of preparing a wet laid pulp sheet from cold caustic extracted pulp. Description of the Related Art

In the production of absorbent devices such as baby diapers, incontinence and catamenial devices and wound dressings, (i.e., absorption intensive devices), conventionally these products are made by first making a pulp sheet of a dense, board- like nature. Typically, these sheets are made by wet-laying the pulp and with the aid of vacuum suction, and drying, making a rolled up bale of pulp. Such rolled-up bales of pulp are then sent to the customer who disintegrates the stiff, board-like sheet with a special device to make a "fluff" pulp. To reduce the power required for the disintegration, (i.e., fluffing) manufacturers of pulp will sometimes include additives. These additives do, however, affect the cost and pulp properties as used in the ultimate product.

After the disintegration, the fluff pulp is then air-laid either directly on an appropriately sized wire screen; or, more typically, the pulp is air-laid on a tissue sheet on a wire and then an air-laid sheet is formed. Subsequently, the air-laid sheet is cut up into the desired product shape such as a baby diaper and like. As it is evident from the above description, not only are a number of additional steps required to form the air-

laid sheet, but considerable capital investment is needed to reconstitute, in the necessary form, and then to cut up such sheet.

Hence, it has been a desideratum to form immediately, i.e., wet-lay such sheet on a paper machine. However, the inherent properties of pulp have made it unacceptable, if not impossible to form an acceptable product of sufficient caliper (and other properties) for the desired density or of sufficient lightness for the caliper obtained. To obtain the desired absorption, insult (or re-wetting) , and liquid retention properties, softness, pad integrity, etc., a number of additives are employed directly during the pulp making stage, or more typically the product is made of layers of pulp of different properties such as a core layer including super absorbent polymer (SAP) and an acquisition layer which has the desirable properties for acquisition and re¬ wetting.

To eliminate the multiple steps in product formation, a number of approaches have been proposed and implemented. For example, machines have been designed to form "pads" or other devices from pulp products in a female mold or cavity. Such machines are typically large diameter devices of a number of cavities around the circumference of a large drum or a belt. Such operations are very capital intensive, and somewhat cumbersome and slower than typical pulp forming machines employing wire screen technology.

BRIEF DESCRIPTION OF THE INVENTION It has now been found that by employing a properly prepared pulp of characteristics which are desirable in the ultimate consumer product, a wet-laid sheet may be formed which not only has desirable properties but also may be rolled up, sent to the converter and then immediately cut up without the costly and capital intensive re-manufacture of a fluff pulp and an air-laid sheet. Such wet-laid sheets are now made

possible by the novel combination of properties derived from the pulp disclosed in U.S. Application Serial No. 08/370,571, filed January 18, 1995 for COLD CAUSTIC EXTRACTION OF PULPS FOR ABSORBENT PRODUCTS, METHOD OF THEIR PREPARATION, AND PRODUCTS IMPROVED THEREBY by Phyllis Leithem et al. The disclosure of the above application is incorporated by reference herein. Processing of such sheet, either entirely made of the pulp disclosed in the above application or by combining such pulp in a wet-laid state with other pulps in appropriate proportions to obtain the desired end uses eliminates a number of steps and makes absorbent pulps of the cold caustic extracted type more competitive with other types of pulp. Not only are kraft pulps useful, but also sulfite pulps. Sheets of appropriate caliper (bulk) and density are obtainable by appropriate combinations of pulps of different characteristics, in wet laid sheets which heretofore was not possible. Such added benefits are to be considered especially because the disintegration and air-laying steps have been entirely eliminated.

DETAILED DESCRIPTION OF THE INVENTION, EXAMPLES AND EMBODIMENTS THEREOF

Example I — Kraft Pulp

To demonstrate that cold alkali extracted pulps would be soft enough and absorbent enough to be used in the form of a wet laid pulp sheet (i.e., no pressure applied to the formed fibrous sheet to decrease moisture content) directly in an absorbent product, eliminating the fluffing step entirely, a bleached Southern pine conventional kraft pulp was cold caustic extracted (CCE) with 15 and 18% NaOH, in accordance with the disclosure in serial No. 08/370,571 and then wet laid to form sheets at 500 and 1000 g/m ² basis weight (BW) . Some of the sheets were air-dried and others were dried under heated conditions in a tunnel dryer (47°C) . Normal, 650 g/m ²

standard BW sheets that were pressed at 850 psi prior to drying were also prepared for comparative purposes. Caliper and density measurements were made, and these results are summarized in Tables 1-1,2,3.

TABLE I-l Cold Caustic Extraction conditions

Condition Sam.ple A-l Sample B-l

Alkali used NaOH NaOH

Solution strength, % 15 18 Temperature, °C 25 25 Time, H:M 0:10 0:10

Consistency, % 3.0 3.0 in 100 ml. of water

TABLE 1-2 Caliper and Density of Pulp Sheets Prepared from A-l

Sample C-l D-l E-l F-l No;

Standard, Wet Laid, Wet Laid, Wet Laid, 650 g/m ² - 1000 g/m ² - 1000 g/m ² - 500 g/m ² - BW, Tunnel BW, Tunnel BW, Air BW, Tunnel Dried Dried Dried Dried

Caliper, 1.9 14.8 14.5 7.4 mm:

Density, 0.34 0.07 0.07 0.07 g/cm ² :

TABLE 1-3

Caliper and Density of Pulp Sheets Prepared from B-l

Sample G-l H-l I-l J-1 K-l NO.

Standard, Wet Laid, Wet Laid, Wet Laid, Wet 650 g/m ² -BW 1000 g/πr- 1000 g/m ² - 500 g/m ² -BW Laid, Tunnel BW Tunnel BW Air Tunnel 500 g/m ² Dried Dried Dried Dried Air Dried

Caliper 1.4 14.2 14.3 6.9 6.8 , mm:

Density 0.46 0.07 0.07 0.07 0.07 , g/cm ² :

It is noted that density was not affected by the type of drying or by changes in the basis weight. It appears that the density is primarily a function of the 5 pressing. The density of the wet-laid sheets is about two-thirds that of the new "ultrathin" diaper fluff cores now produced by Kimberly Clark and about one-half that of typical diaper fluff core test pads (at equivalent basis weights) . The type of drying did not affect the caliper

10 or density of the sheet.

The various sheets prepared (C-l to K-l) from the cold caustic extracted pulps (A-l and B-l) were evaluated "as-is" (by cutting them up into appropriate sizes) along with normal fluff Rayfloc-J® (a commercial

15 fluff grade bleached kraft south pine pulp produced at Jesup, Georgia by Rayonier Inc.) in a variety of absorption tests. A number of comparisons were also made with three types of a product made in a conventional

manner, improved by one layer and also by chemical cross¬ linking and layering of cross-linked material with an acquisition layer and including a material with a superabsorbent polymer core. The various tests carried out and their results are presented and discussed below.

Evaluation and Results Pad Integrity is a measure of the strength of an absorbent core. The pad integrity value is measured as the maximum force required to push a 1 3/4" piston through the pad. Normal pad integrity fluff pads have a basis weight of 500 g/m ² and a density near 0.05 g/cm". Samples F-1, J-1 and K-l had a basis weight comparable to routine Rayfloc-J fluff sheet previously identified above. The novel samples showed an increase in pad integrity of approximately 40%. All the novel samples had higher pad integrity values than routinely measured for Rayfloc-J® fluff. Pad integrity test results are listed in Table 1-4.

GATS (Gravimetric Absorbency Testing Svstem) testing measures the absorbency characteristics of an absorbent core. The GATS test was performed with M/K Mark II GATS test equipment. The test measures the quantity of deionized water absorbed into the test sample through a 2 mm diameter hole. The test is controlled so that there is no pressure head on the liquid. When the pad is saturated, it is adjusted so that there is a one inch height differential between the test piece (higher) and the liquid reservoir (lower) . This allows loosely retained liquid to drain back into the test reservoir. The retention value is the amount of liquid absorbed after draining is completed. The dry pad height is measured as absorption begins and the wet pad height after draining is completed.

GATS test results are shown in Tables 1-5 and 1-6. Each sample was cut to a square weighing 1 gram

(fluff samples are prepared in a 2 inch diameter cylinder and weigh 1 gram) . The steady state absorption rates of

the unpressed wet-laid sheets were greater than what was observed for fluffed Rayfloc-J®. All the wet-laid sheets reached their absorption capacity in about 20 seconds. This is approximately twice as fast as a conventionally made fluffed pad. The retention for the unpressed sheets (all samples except C-l and G-1) was approximately 40% of that of a fluffed pad. The pressed sheets (samples C-l and G-1) had even lower liquid retention. The GATS test also measures the change in pad height as liquid is being absorbed (Table 1-6) . The novel unpressed sheets maintained approximately 70% of their height, compared to about 45% for conventional fluff pads. The pressed sheets swell with absorption and increase about 130% in caliper. The acguisition laver test was done with the unfluffed wet laid pulp sheets. This test is also done on absorbent products to evaluate their performance. This test simulates the performance of the sheet as an absorbent core. The sample is cut to 7 cm x 17 cm and placed on a stack of 7 blotter sheets. For this example, the blotter sheets would simulate a layer of a superabsorbent polymer (SAP) under the pad. A piece of non-woven cover stock is placed on top of the sample. A 50 ml "insult" of synthetic urine is introduced to the pad via 2" diameter tube centered over the sample. The time for the synthetic urine to completely penetrate the cover stock is recorded. A stack of five blotter sheets is placed on the sample 90 seconds after the synthetic urine is absorbed. A weight applying 1 psi of pressure to the sample is placed on the blotter stock. After 120 seconds the weight and blotter sheets are removed. The amount of synthetic urine absorbed by the blotter sheets is the rewet value.

Product test results for three designs of Proctor & Gamble diapers are included with the test results in Table 1-7. The Pampers Uni® (made in Thailand) is a simple one-piece fluff core with 13%

superabsorbent polymer (SAP) . The Pampers® (made in Japan) is a two-piece fluff core with 21% SAP in the lower pad. The Pampers UltraThin® (made in the United States) has an upper layer of curly acquisition fiber and a lower layer of fluff and 35% superabsorbent. Lower absorption times are preferable as are lower rewet times. The unpressed cores (all samples except C-l and G-1) had absorption times comparable to the Japanese Pampers® but worse than the Pampers Ultrathin®. The rewet of the lower basis weight (samples F-1, J-1 and K-l) wet laid pulp sheets was equivalent to the Japanese Pampers® and the Pampers Ultrathin®. All the unpressed cores outperformed the Pampers Uni® diaper. The pressed pulp sheets (samples C-l and G-1) had very low absorption times.

TABLE 1-4 PAD INTEGRITY RESULTS Sample Number Pad Density Basis

Integrity (g/cm ³ ) Weight Pressing sm. (q/m ³ )

C-l 31.8 0.34 650 850psi

D-l 22.6 0.07 1000 None

E-l 20.1 0.07 1000 None

F-1 11.1 0.07 500 None

G-1 40.0 0.46 650 850 psi

H-l 26.3 0.07 1000 None

I-l 23.9 0.07 1000 None

J-1 10.6 0.07 500 None

K-l 10.9 0.07 500 None

Rayfloc-J® 7.7 0.05 500 N/A

TABLE '1-5 GATS ABSORBENCY TEST RESULTS

Sample No. Absorption Absorption Licruid

Rate Time(sec) Retention

(ml/sec) (g/q)

C-l 0.19 21.4 4.44

D-l 0.63 22.8 9.27

E-l 0.66 21.7 9.77

F-1 0.55 20.1 9.69

G-1 0.14 20.0 3.52

H-l 0.79 16.4 8.48

I-l 0.67 17.6 8.07

J-1 0.71 20.6 9.23

K-l 0.56 21.3 8.85

Rayfloc-J® 0.40 45.2 14.4

TABLE 1-6 GATS PAD HEIGHT TEST RESULTS

Sample No. Drv Pat Wet Pad Wet:Drv (mm) (mm) 1*1

C-l 2.09 2.72 130

D-l 10.96 7.46 68

E-l 11.36 7.36 67

F-1 6.20 4.68 75

G-1 1.79 2.36 132

H-l 10.68 7.44 70

I-l 10.70 7.46 70

J-1 6.11 4.65 76

K-l 5.83 4.40 75

Rayfloc-J® 14.87 6.67 45

TABLE 1-7. ACQUISITION LAYER/PERFORMANCE TEST RESULTS

Time(sec) Rewet(g)

SAMPLE 1st 2nd 3rd 1st 2nd 3rd

C-l 27.4 103.0 153.2 4.16 4.45 11.30

D-l 6.4 25.6 37.8 9.08 16.60 18.68

E-l 0.8 32.8 93.8 8.48 16.48 17.26

F-1 3.2 41.3 69.4 2.52 8.98 21.43

G-1 42.1 128.2 178.0 3.36 8.39 8.32

H-l 1.2 12.4 24.0 7.31 15.00 16.56

I-l 0.9 28.6 37.8 6.96 18.91 13.72

J-1 3.7 40.2 83.6 2.14 7.15 20.22

K-l 3.4 34.4 74.1 8.12 6.84 21.50

Pampers Uni® 4.6 46.4 66.5 22.01 30.11 38.48

Pampers® (Japan) 8.5 46.6 63.1 8.79 13.43 21.29

Pampers 8.0 23.6 30.9 1.94 15.11 21.71 Ultrathin® (USA)

The overall absorbency results presented above are outstanding as they indicate that the cold caustic extracted, wet-laid, pulp sheet is an excellent candidate material for absorbent cores. This material can

outperform multi-layer, SAP containing devices. This material exhibits excellent absorption and pad integrity. Performance of the novel sheets is equivalent to diapers without a special acquisition layer fiber. It does not appear that the solution strength of the caustic used in the CCE stage (15% or 18% studied here) or the type of drying (air or tunnel dried) has much effect on the absorbent or strength properties. Moreover, less wet pad collapse is observed in the wet-laid pulp sheet compared to fluff pads.

Example II — Sulfite Pulp

Two series of cold caustic extracted (CCE) Ethenier-F-UHV® (Ultrahigh Viscosity Sulfite Southern Pine pulp produced by Rayonier Inc.) wet laid pulp sheets were tested in a manner similar to Example I for use as an absorbent core without fluffing. The first series was treated with an 18% cold caustic extraction (CCE) and the second was treated with a 15% CCE. Each series contained samples at 650 g/m ² (basis weight) that were conventionally pressed, and 1000 g/m ² and 500 g/m ² sheets that were not pressed. Some of the unpressed sheets were air dried and others were dried in a tunnel drier.

Table II-l summarizes the properties of the wet laid sheets. Density and caliper results are listed in Table II-2. The density was not affected by the type of drying or by changes in the basis weight. It appears that the density is primarily a function of the pressing. These results are similar to those observed with Porosanier-J-HP® (mercerized kraft Southern Pine pulp produced by Rayonier Inc.).

Evaluation and Results Pad integrity test results are listed in Table II-3. Pad integrity is a measure of the strength of an absorbent core. Normal pad integrity fluff pads have a basis weight of 500 g/m ² and a density near 0.05 g/cm ³ . CCE samples at a basis weight comparable to Rayfloc-J® fluff showed an increase in pad integrity of

approximately 6%. The CCE samples were slightly higher in density than Rayfloc-J®. Pad integrity for the CCE wet laid sheets appears to be proportional to the basis weight, as the higher basis weight sheets were higher in pad integrity. Pressed sheets had greater pad integrity values.

GATS (Gravimetric Absorbency Testing System) testing measures the absorbency characteristics of an absorbent core. GATS test results are shown in Tables II-4 and II-5. Each sample was cut to a square weighing 1 gram (fluff samples are prepared in a 2-inch diameter cylinder and weigh 1 gram) . The steady state absorption rates of the unpressed wet-laid sheets were greater than what was observed for Porosanier-J-HP® and Rayfloc-J®. All the wet-laid sheets reached their absorption capacity in about 20 seconds. This is approximately twice as fast as fluffed pads. The retention for the unpressed sheets was approximately 60% of that of fluffed pads. The pressed sheets had even lower liquid retention. The GATS test also measures the change in pad height as liquid is being absorbed. The results are shown in Table II-5. The unpressed sheets maintained approximately 70% of their height, compared to about 45% for fluff pads. Because the unpressed sheets have less wet collapse, their absorption rate is higher than regular fluff pads. The pressed sheets swell with absorption and increase in caliper about 170%.

The acquisition laver test was done with the unfluffed wet laid pulp sheets. This test is also done on absorbent products to evaluate their performance.

This test stimulates the performance of the sheet as an absorbent core. Product test results for three designs of Proctor & Gamble diapers are included with the test results in Table II-6. The Pampers Uni®, Japanese Pampers®, and Pampers UltraThin® were again used for comparison (as in Example I) .

Lower absorption times are better as are lower rewet times. For an absorbent core, rewet is as important as fast liquid absorption. The unpressed cores had absorption times comparable to the Japanese Pampers® and the Pampers Ultrathin®. The rewet of the lower basis weight wet laid pulp sheets were slightly worse than the Japanese Pampers® and the Pampers Ultrathin®, however, the thinner pads transferred some urine to the under layer of blotter stock, which may account for the lower rewets observed in these sheets. All the unpressed cores outperformed the Pampers Uni® diaper.

Unpressed CCE Ethenier-F-HV® (a sulfite chemical Southern Pine pulp produced by Rayonier Inc.) pulpsheets according to the invention exhibit excellent absorption and pad integrity. Their performance is equivalent to diapers without a special acquisition layer fiber. There also appears to be less wet pad collapse in the wet-laid pulp sheets compared to fluff pads.

Table II-l WET LAID PULPSHEET PROPERTIES

SAMPLE CAUSTIC BASIS PRESSING DRYING NO. TREATMEN WEIGHT TYPE

T (%) (g/m ² )

A-2* 18 650 850 psi Tunnel

B-2 18 1000 None Tunnel

C-2 18 1000 None Air

D-2 18 500 None Tunnel

E-2 18 500 None Air

F-2* 15 650 850 psi Tunnel

G-2 15 1000 None Air

H-2 15 1000 None Tunnel

1-2 15 500 None Air

J-2 15 500 None Tunnel

* — Standard sheet.

Table II-2

CALIPER AND DENSITY OF PULP SHEETS

SAMPLE NO. BASIS CALIPER DENSITY WEIGHT (mm) (g/cm ³ )

(g/m ² )

A-2* 650 2.3 0.28

B-2 1000 13.3 0.08

C-2 1000 14.0 0.07

D-2 500 7.4 0.07

E-2 500 7.0 0.07

F-2* 650 2.4 0.27

G-2 1000 13.6 0.07

H-2 1000 15.1 0.07

1-2 500 8.4 0.06

J-2 500 7.6 0.07

Test Fluff Pad Typical 500 14.2 0.03 Properties to 0.04

Pampers Uni® - 13% SAP 490 10.7 0.06

Pampers® (Japan) - 21% SAP 840 9.3 0.07

Pampers UltraThin® (USA) ** 670 8.1 0.10 - 35% SAP

* — Standard sheet.

** — Includes curly fiber acquisition layer.

Table II - 3

PAD INTEGRITY RESULTS

SAMPLE NO. PAD DENSITY BASIS PRESSING

INTEGRITY (g/cm ³ ) WEIGHT

(N) (g/m ² )

A-2* 37.9 0.28 650 850 psi

B-2 19.5 0.08 1000 None

C-2 24.1 0.07 1000 None

D-2 7.9 0.07 500 None

E-2 9.0 0.07 500 None

F-2* 27.1 0.27 650 850 psi

G-2 20.8 0.07 1000 None

H-2 21.0 0.07 1000 None

1-2 8.1 0.06 500 None

J-2 7.5 0.07 500 None

Rayfloc-J® 7.7 0.05 500 N/A

* — Standard sheet.

Table II - 4

GATS ABSORBENCY TEST RESULTS

SAMPLE NO. ABSORPTION ABSORPTION LIQUID

RATE TIME (sec) RETENTION (g/g)

( L/sec)

A-2* 0.20 17.9 3.51

B-2 0.57 21.2 8.41

C-2 0.61 19.0 7.89

D-2 0.72 19.8 9.86

E-2 0.60 22.9 10.0

F-2* 0.09 20.3 2.98

G-2 0.60 18.5 8.74

H-2 0.53 22.7 8.93

1-2 0.58 25.1 10.4

J-2 0.55 23.9 10.1

Rayfloc-J® 0.40 45.4 14.4

* — Standard sheet.

Table II - 5

GATS PAD HEIGHT TEST RESULTS

SAMPLE NO. DRY PAD WET PAD WET:DRY (%) (mm) (mm)

A-2* 1.30 2.08 166

B-2 9.93 6.98 70

C-2 11.38 7.75 68

D-2 5.85 4.45 76

E-2 6.00 4.55 76

F-2* 0.86 1.71 189

G-2 10.11 7.19 71

H-2 8.10 5.86 72

1-2 4.70 3.45 73

J-2 5.18 4.00 76

Rayfloc-J® 14.87 6.67 45

* — Standard sheet.

Table II - 6

ACQUISITION LAYE /PERFORMANCE TEST RESULTS

TIME(sec) REWET(g)

SAMPLE NO. 1st 2nd 3rd 1st 2nd 3rd

A-2* 20.3 78.8 125.4 4.31 5.93 15.08

B-2 0.4 7.4 21.4 8.50 13.11 19.91

C-2 0.5 7.1 35.4 9.11 14.91 16.29

D-2 1.7 26.2 44.5 3.86 13.32 19.84

E-2 1.8 27.7 49.4 5.02 9.11 21.53

F-2* 21.6 87.4 132.9 5.03 5.44 13.53

G-2 0.6 11.5 18.5 8.04 15.30 22.07

H-2 0.4 9.7 17.9 10.77 16.27 19.00

1-2 1.3 14.7 25.1 7.64 12.49 13.48

J-2 1.5 19.5 37.2 8.47 12.71 24.07

Pampers Uni® 4.6 46.4 66.5 22.01 30.11 38.48

Pampers® 8.5 46.6 63.1 3.79 13.43 21.29 (Japan)

Pampers 8.0 23.6 30.9 1.94 15.11 21.71 UltraThin® (USA) **

* — Standard sheet.

** — Includes curly fiber acquisition layer.

Example III

A composite of CCE pulp and conventional fluff pulp is formed by mixing the pulps in a 50/50 CCE to conventional pulp ratio. The resultant composite would exhibit properties approximate to the average of the properties for the individual CCE and conventional pulps. For example, the values for absorption rate, absorption time and liquid retention of the composite pulp would be expected to be between the corresponding values for the individual pulp materials. Intermediate results would be achieved with a mixture of CCE pulp with amount of CCE pulp being as low as 10% by weight. Such composites would be useful for designing materials with specific properties for use in adsorptive devices.

From this it follows that property-wise, the method as described herein and the products obtained accordingly have substantial advantages over the conventionally produced products, i.e., products obtained by disintegration (fluffing) and air-laying the fluffed pulp. The strength of the material, measured in terms of pad integrity, is at least equal to and often exceeds that of conventionally produced products of the same basis weight. As the pad integrity is often improved, the present product is stronger despite its softness. Based on the extensive data concerning absorption rates and absorption capacity, the disclosed method produces pulps for the disclosed devices of much improved properties with absorption rates that have been improved from about 37% to 98% based on comparable conventionally produced product. Although the above discussion has been with respect to pulp made of cold caustic extracted kraft or sulfite pulps, the equivalent property definition provides a ready vehicle for evaluating properties for absorption intensive applications. By combining lesser quality pulps with the novel pulps to achieve results achieved by the prior art more expensive pulps, one is

provided with an economically viable alternative to fashion outstanding products for intense absorption purposes.