IE43658B1 - Absorbent articles - Google Patents

Abstract

The insert to be worn on the body is capable of absorbing body fluids. For this purpose, it comprises an inner layer (24), an outer layer (22) and an absorbent basic body (26) of wood cellulose fibres in between. This wood cellulose contains lignin, so that the wettability is reduced and the elasticity of fibres wetted by use is improved. The essential feature is that the average length of the fibres is 1.0 to 1.9 mm. This insert can be used as diaper or sanitary towel.

Description

The present invention relates to absorbent articles.

Although for convenience the absorbent articles of the present invention will be described primarily . as disposable diapers, it will be understood that the invention is applicable to absorbent articles of other types,such as sanitary towels and maternity napkins.

Various absorbent articles, such as disposable . diapers, have been proposed for capturing and retaining body fluids when placed against a wearer. For example, disposable diapers are normally made with a fluid-pervious cover sheet, a fluid-impervious backing sheet, and an absorbent pad positioned between the . cover sheet and the backing sheet. Many of such pads are made from wood fluff which is normally formed by fiberizing or comminuting pulp board. The pulp board itself is normally formed from trees through a pulping process. The pulping processes may be categorized as . chemical, semimechanical, mechanical and thermomechanical, and are described in detail below.

All of the wood fluff used in absorbent articles sold in the United States of America known to the applicant has been exclusively formed from a chemically produced pulp. For a given species of wood, the . chemical pulping process produces a pulp having fibres with a longer length than the fibres produced by the other pulping processes, particularly the mechanical pulping process. Accordingly, the industry has sought chemically produced pulp for use in disposable pads . since the long fibres enhance the structural integrity - 2 43653 and loft of the pad.

Although the chemical pulp results in a pad v/ith desirable character!sties, a number of disadvantages are inherent in tho use of such pulp. First, the . chemical pulping process is relatively inefficient in that the yield of pulp to the amount of wood used in pulping is in the range of 40 to 55%, whereas the yield of the mechanical and thermomechanical pulping processes is as high as 90-95%. The disparity in . yields between the processes is due to the removal of lignin, cellulose and hemicellulose from the wood during digestion in the chemical process. Accordingly, chemically produced pulps are significantly higher in cost than mechanical and thermomechanical pulps, . necessarily resulting in a more costly item to the consumer. Also, tho low yield is wasteful of natural resources.

Second, environmental considerations favour the use of pulps which are not produced by the chemical . process. In the chemical sulphite process, it is relatively difficult to reclaim the cooking chemicals used during pulping, and disposal of the chemicals involves water contamination problems. Although it is less difficult to reclaim the chemicals used in chemical kraft . pulping,the latter process is characterized by the emission of gases containing malodorous substances, such as mercaptans and organic sulphides.

Third, the energy required to fiberize pulp board solely of the chemical type is greater than that . necessary for a pulp board containing mechanical or - 3 43658 thermomechanical pulp. This is because lignin of the fibres is removed during chemical pulping, thus increasing hydrogen bonding between dry fibres of the chemical pulp.

The invention provides an absorbent article having an absorbent pad which is less subject to the said disadvantages and yet which may have a fluid holding capacity under loads which is superior to pads of similar size made solely of chemical fluff.

According to the present invention an absorbent article for placement against a wearer for capturing body fluids comprises an absorbent pad having a front surface, a back surface, and a mass of fibres, at least some of the fibres comprising wood fluff formed from a thermomechanically produced pulp (as hereinafter defined) and having an average fibre length in the range from 1.0 mm to 1.9 mm, and a sheet or sheets covering the front and back surfaces of the pad.

Such a pad can be significantly cheaper than an otherwise comparable pad of wood fluff formed from a chemically produced pulp.

The fibres of the thermomechanically produced pulp contain a substantial portion of their natural lignins. The lignins of the fibres are relatively hydrophobic and reduce the wetting of the hydrophilic fibres during use of the article and thus increase the wet resiliency of the pad. Also, the resilient fibres retain the interfibre spaces of the pad open so that the fluid holding capacity of the pad is retained under loads.

The invention may be performed in various ways and a specific embodiment will now be described by--------- 4 43658 way of example with reference to the accompanying drawings, in which:Figure 1 is a fragmentary plan view of an absorbent article embodying the invention, namely . a disposable diaper; and Figure 2 is a fragmentary sectional view taken along the line 2-2 of Figure 1.

Referring to Figures 1 and 2, there is shown a disposable diaper 20 having a fluid-impervious backing . sheet 22, a fluid-pervious cover sheet 24 and an absorbent pad 26 between the backing sheet 22 and the cover sheet 24, such that the backing sheet 22 covers the back surface 28 of the absorbent pad 26 while the cover sheet 24 covers at least a portion . of the front surface 30 of the pad. As shown, the backing sheet 22 may have side margins 32 which are folded over and secured to the sides of the cover sheet 24. The diaper may also have a top wadding sheet 34 between the front surface 30 of the pad 26 . and the cover sheet 24, and a back wadding sheet 36 between -the back surface 28 of the pad 26 and the backing sheet 22. The wadding sheets 34 and 36 serve to maintain the structural integrity and prevent balling of the absorbent pad 26 when the pad becomes . wet during use.

The diaper 20 may also include a pair of tape fasteners 38. The fasteners 38 may have a backing 40 with adhesive 42 coated on one surface of the backing. The adhesive 42 on one end 44 of the fasteners . 38 is attached to the backing sheet 22 of the diaper, - 5 43658 while the adhesive on the other end 46 of the fasteners may he releasably covered hy a release sheet AB.

During placement of the diaper, the release sheets 48 are removed from the other ends 46 of the fasteners . 38, and the fasteners are utilized to secure the diaper about the infant. The diaper 20 may be utilized in its flat condition as shown, or may be folded into a box-pleat configuration, or other suitable configuration, as desired.

. . The absorbent pad 26 comprises a mass of loosely formed fibres, such as wood fluff. As described of the wood fluff below, at least some of the fibres/are formed from a thermomechanically produced pulp. If desired, 100% of the fibrous mass may be formed from the thermomechanically . produced pulp, or the mass may comprise a mixture or blend of fibres from thermomechanically produced pulp and other pulps Thus, as shown, the pad 26 may have fibres 50 which have been formed from a pulp other than thermomechanically produced pulp, which are mixed with fibres 52 formed from a . thermomechanically produced pulp.

As -a background to facilitate understanding of the invention, the various pulping processes are discussed below. Pulping itself may be defined for the present purposes as a procedure for rupturing the . fibres of wood. The resulting pulp may be used for making paper or in this case absorbent pads. The separated fibres of the pulp are normally formed into pulp board which may be wound into rolls for convenience of handling during shipment and by the user. The rolls . are fiberized or comminuted by the user to form a loosely - 6 43658 formed fibrous mass which is cut into lengths for use as absorbent pads for the disposable diapers.

Wood itself is primarily composed of cellulose, hemicellulose and lignin. Lignin is an amorphous . polymer of high molecular weight, that serves to hold the fibres of wood together. Cellulose is highly hydrophilic, while lignin has a significantly smaller affinity for liquid than cellulose and is relatively hydrophobic. Since pulping is concerned with ruptur10. ing the bonds between the wood fibres, the middle lamella between the fibres, which is composed mostly of lignin, must be ruptured during the procedure.

Wood logs are transported to the processor, after which bark from the logs may be removed. Generally, . the logs are ground into chips and the chips are used in the pulping procedure to separate fibres in the chips. The fibres are then washed to produce the unbleached pulp, after which the pulp may be bleached to a lighter colour pulp. The process differs primarily . in the manner whereby the wood is pulped.

The. pulping procedures may be categorized as mechanical, chemical, semichemical and thermomechanical. In mechanical pulping, the logs themselves may be ground by a roughened stone to grind fibres out of the . wood. Alternatively, wood chip may he shredded or ground between metal shearing discs in a machine called a refiner. The mechanical pulps produced in this manner are characterized by relatively short fibres due to damage of the fibres during the procedure. Such . groundwood processes are relatively efficient in that -7.43058 approximately 95% of the dry weight of the wood is converted into pulp, since materials, such as lignin, are not specially removed from the pulp.

In chemical pulping, the wood chips are cooked . in a vessel or digester with chemical reagents to separate the fibres, termed a digesting procedure. During digesting, the pulping reagents degrade and dissolve the lignin to break the bond between the fibres in order that they may be separated. However, . the reagents also degrade some of the cellulose and hemicellulose, and the loss of these materials, including lignin, accounts for the relative inefficiency of chemical pulping. Thus, the yield from chemical pulping usually may range from 40 to 50% of . the weight of the wood, with a maximum yield of 55%. Accordingly, chemically produced pulps are significantly higher in cost than mechanical and thermomechanical pulps (the yield of the latter being said to approach 95%) not to mention the loss of valuable . materials during .chemical pulping.

Tha chemically produced pulp is characterised by relatively long fibres which are mostly completely separated. As noted above, the lignin is removed, and the hydrophilic fibres thus produced are susceptible . to increased wetting.

The two most common chemical procedures are the sulphite and kraft processes. In the sulphite procedure, an acidic mixture is used as the reagent which is relatively difficult to reclaim, thus posing a risk . to the environment when disposed of by the processor. •χ. - 8 4 3 6 5s In the kraft or sulphate process, the chips are cooked in a solution of sodium hydroxide, sodium carbonate and sodium sulpnide. This process results in the emission of gases containing malodorous substances, and is also repugnant to the environment.

In the semichemical process, such as tne neutral-sulphite process, the wood chips or logs are softened with a chemical, after which the wood is fiberized mechanically, frequently in disc refiners.

In the thermomechanical procedure, wood chips are steamed at an elevated temperature and pressure to soften the lignin. Thus, the binding force between the fibres is greatly lessened through application of heat to permit separation of the fibres. The fibres may be separated by a refiner under pressure or pressure changes. Pulp made by the thermomechanical procedure is herein termed thermomechanically produced pulp.

The thermomechanical procedure produces a large number of substantially undamaged fibres, in contrast to the mechanical process, although some of the fibres remain together, as will be seen below. The average fibre length of a thermomechanically produced pulp is dependent upon the particular species of wood, as in other pulps. For example, a softwood, i.e. the wood of a coniferous, needle-bearing tree, normally results in a longer fibre than a hardwood, i.e. the wood of a broad-leaf tree, for a given pulping process.

In general, the fibre length of a thermomechanically produced pulp is greater than that of a mechanical pulp and less than that of a chemical pulp for a given species of wood. The greater fibre length of chemical pulps imparts structural integrity and loft to wood fluff, and thus accounts for its widespread use. For the absorbent pads of the articles of the present invention a thermomechanically produced pulp is selected having an average fibre length in the range from 1.0 mm to 1.9 mm.

Since digestion is not used during the thermomechanical procedure, lignin remains on the fibres after being separated. Thus, the hydrophilic fibres have relatively hydrophobic or non-hydrophilic surface portions of lignin which reduce wetting of the fibres, as will be described below. Also, since materials are not removed from the wood by digestion, the yield of the thermomechanical process is substantially higher than the chemical procedure, and - 9 may approach or exceed 95%. As discussed above, the cost of thermomechanically produced pulp is thus substantially less than chemical pulp, reducing the cost of an article produced from the thermochemically produced pulp to the consumer.

When thermomechanically produced pulp, or a mixture thereof with other kinds of pulp, is formed into pulp board, the hydrogen bonding between the dry fibres is significantly reduced by lignin on the thermomechanically produced pulp fibres, in contrast to fibres of chemical pulp board containing thermomechanically produced pulp is less than that required to perform the same operation on a pulp board of 100% chemical pulp. Again, the reduced power requirement is reflected in a saving of cost.

After the pulp has been formed into wood fluff, the absorbent pads and articles are constructed, as described in connection with Figures 1 and 2.

The pads may include fibres which are formed entirely from thermomechanically produced pulp, or fibres from other pulps may be mixed with fibres from the thermomechanically produced pulp. For example, fibres formed from a chemically produced pulp may be mixed with fibres formed from a thermomechanically produced pulp to add structural integrity and loft to the pad. Also, fibres from a mechanical pulp may be mixed with fibres from a thermomechanically produced pulp to serve as a filler for the pad. It is also contemplated that fibres from a thermomechanical pulp may be mixed with fibres from a semi chemical pulp, and that the pad may comprise a mixture of fibres from thermomechanically produced pulp with fibres from two or more of the various pulps described above.

The fibres from the thermomechanically produced pulp enhance the fluid holding capacity of the pad under loads for a given fibre length as follows. It is known that when wood fluff is wetted, the fluid holding capacity of the cellulosic fibres themselves contribute a relatively small amount to the total fluid holding capacity of the pad. Rather, the total fluid holding capacity of the pad is more directly related to the interfibre spaces in the pad. For example, in chemically produced fibres, approximately 13% of the liquid in a wetted pad may be retained by the fibres, while the remaining 87% of the liquid would be retained in the interfibre spaces of the pad. Thus, measure of the total fluid holding capacity of the pad is to a great extent a measure of interfibre spaces in the pad, and normally the larger the spacing between fibres, - 10 4 3 6 5 3 the greater the total fluid holding capacity of the pad.

In use of the diapers, the pads are wetted and are placed under loads by the infants. As noted above, the cellulosic fibres formed from a chemically produced pulp are highly hydrophilic. When such fibres are wetted, the fibres become soft, and become readily compressible. When the wetted pad is placed under loads, fibres in the pad collapse, thus reducing the interfibre spaces and comprising the fluid holding capacity in that region of the pad.

In contrast, the relatively hydrophobic lignin on fibres formed from a thermomechanically produced pulp reduces wetting and prevents full saturation of the fibres. Accordingly, such fibres have a greater degree of resiliency than chemically produced fibres, particularly when the fibres are wetted and placed under loads. This feature is enhanced by the lesser extent of fibre separation during thermomechanical pulping relative to chemical pulping, resulting in thermomechanically produced fibre structures of an average greater thickness than chemically produced fibres, which also increases resiliency Of the thermomechanically produced fibrous mass. The resilient fibres serve to maintain the interfibre spaces of the pad open, and prevents collapsing of the pad when the pad is wetted and placed under loads. Thus although the length of the fibres from thermomechanically produced pulp is on an average shorter than that of chemically produced fibres for a given species of wood, the fluid holding capacity of a pad formed from a thermomechanically produced pulp when wetted and placed under loads during use exceeds the fluid holding capacity of a pad solely formed from a chemically produced pulp.

Claims (8)

1. An absorbent article for placement against a wearer for capturing body fluids, comprising: an absorbent pad having a front surface, a back surface and a mass of fibres, at least some of the fibres comprising wood fluff formed from a thermomechanically produced pulp (as hereinbefore defined) and having an average fibre length in the range from 1.0 mm to 1.9 mm, and a sheet or sheets covering the front and back surfaces of the pad.

2. An article as claimed in Claim 1 wherein the mass of fibres consists wholly of a wood fluff formed from a thermomechanically produced pulp. - 11 4365

3. An article as claimed in Claim 1 wherein the mass of fibres includes fibres formed from a chemically produced pulp.

4. An article as claimed in Claim 1 wherein the mass of fibres includes fibres formed from a mechanically produced pulp.

5. An article as claimed in Claim 1 wherein the mass of fibres includes fibres formed from a semichemically produced pulp.

6. An article as claimed in any of the preceding claims which is a disposable diaper,

7. An article as claimed in Claim 6 which includes a fluid-pervious cover sheet covering at least a portion of the front surface of the pad, and a fluid-impervious backing sheet covering at least a portion of the back surface of the pad.

8. An absorbent article substantially as described with reference to the accompanying drawings.