Baby diapers and training pants: a market overview.

a look at the latest trends and developments

The importance of the absorbent products sector to the nonwovens business as a whole cannot be too strongly stressed, amounting to an estimated end product sales value of US $33-34 billion on a worldwide basis. Of the absorbent product applications, the baby diaper area is the largest volume user, but applications in adult incontinence currently show the highest growth.

Consideration of the production volume of 'coverstock' type nonwovens in relation to total nonwovens production in major producer areas is illustrated in Figure 1.

Product Group Profile

The three sub-groups - baby diapers, feminine hygiene products and adult incontinence materials - although they target significantly different applications, are linked by a common objective: to produce an absorbent structure that can provide a workable means of managing waste products excreted from the body and, because we are considering only the single use products, to allow subsequent disposal of the used product in a safe and environmentally compatible manner.

In terms of disposable baby diapers, training pants and pant diapers, the principal requirement of this subgroup is that they provide an effective absorbent structure to receive, absorb and contain urine and fecal waste from babies over the early period of their life when they have the problem of double incontinence.

The initial phase of handling the baby's incontinence is covered by the baby diaper, while the more recent introduction of the training pant/pant diaper (which normally differs in its absorbent capacity) as a concept has extended into the "toilet training" phase of the child's development.

There are still two forms of disposable baby diaper available in the market, the one piece "all-in-one" type, which currently accounts for over 90% of the market, and the two piece pant and pad "insert" type, which is still found in some markets and preferred by some mothers.

Disposable baby diapers should be capable of providing the functionality referred to above in such a way as to not encourage irritation of the baby's skin and to prevent contamination of the baby's clothing and/or surroundings.

They should also be capable of disposal in a safe and efficient manner, be cost effective in use and of a design and construction appropriate to the geographic market served.

Training pants and pant diapers represent an overlap and/or an extension of the diaper use for toddlers by providing a confidence building product at the toilet training stage to provide a backup system of protection in case of "accidents."

The distinction between training pants and pant diapers was originally related to the absorbent capacity of the product, with training pants having more limited absorption capacity. This distinction is now much less, with both types of product available with sufficient capacity for nighttime use.

Evolution And Development

"Those who do not learn from the past are condemned to repeat it." Consistent with this quotation, it is worth asking, "How did we get from reusable cotton toweling diapers to the highly complex structure of a disposable diaper of today?" If we find an answer, then we need to ask "Why?"

Development of disposable diapers as we know them today have their origins in Sweden in 1936, where, using the newly available unbleached, highly creped tissue paper known as cellulose waddings, the first attempts were made to provide a disposable absorbent product for use as an underpad in hospitals.

Although these pads, which still exist today in some markets, could hardly be described as "high performance," they were rather significant as they introduced the concept of single use disposable absorbent products.

By 1942, faced with the fact that cotton had been classified as a strategic material because of the war, the idea of using a pad of cellulose wadding inside a pair of reusable rubber pants with an elasticated waist band was introduced to the Swedish market [ILLUSTRATION FOR FIGURE 2 OMITTED].

By 1950 this simple concept had been developed into a product based on a bleached cellulose wadding strip (the first absorbent core) with a knitted mesh cover (the first coverstock-see Figure 3), which could be inserted into pockets at either end of the rubber pant. This was the first example of a two piece or "insert" diaper system.

These products had a very limited performance specification, which was solely based on the cellulose wadding's ability to absorb liquids. However, because it was by nature a disposable product, this represented the first extension of the concept of disposable hygiene products and at the same time introduced the related concept of a product cost that was consistent with a single use product. It is worth noting that it took eight years for this relatively simple development to take place!

The main problems associated with production of these early products were mainly related to the discontinuous nature of the process. It should be remembered that we did not have hot melt adhesives and the type of fabrication expertise available today and consequently relatively little market penetration or development took place.

Also in 1950, Johnson & Johnson, New Brunswick, NJ, introduced a rectangular underpad product based on cellulose wadding and with a plastic film backing and using a wet strength tissue as a coverstock.

By 1957, Molnlycke, Molnlycke, Sweden, had introduced a shaped insert type diaper, which was constructed using defibered pulp held in place with layers of wet strength tissue and with a layer of cotton wool fiber on the top side. The whole of this "absorbent core" was held in a knitted mesh cover, which had crimped edges and other areas across the structure to help stabilize the pad. The product was used with rubber pants.

From 1960 onwards, the pace of developments accelerated and some of the more significant steps are listed in Tables 1 and 2.

From this initial overview of the development of the modern all-in-one disposable diaper we can see that the product has evolved from a simple creped cellulose wadding structure that, despite its limitations, worked, to a complex multicomponent structure as illustrated in Figure 4. Despite its complexity it is still basically destined to be a single use product. We should pose the question therefore, why has it apparently become necessary to have so many different components in such a complex form to fulfill essentially the same tasks?

Consistent with our introductory quotation, let us consider those initial simple products, that despite their simplicity, had a remarkably long product life compared with current products.

The earliest products, based on cellulose wadding, had an absorption capacity determined by the quantity and characteristics of the cellulose wadding used. The more absorbent the wadding, the more absorbent the diaper.

Early developments were centered around "two piece" diaper systems, where the absorbent pad was kept basically simple (the original was simply a rectangle) and this was used with reusable rubber pants with pockets at the front and back to take the pad. This provided a simple but workable absorption and containment system.

The "improved" version used a cotton mesh coverstock that prevented the wet cellulose wadding from adhering [TABULAR DATA FOR TABLE 1 OMITTED] to the baby's skin and provided a degree of separation of the core material from the baby.

It was not really possible to talk about "fit" other than in general terms. Also, without any form of leg seal in the early products, leakage was a potential problem, as the fluid - initially absorbed into the cellulose wadding - was easily forced out again by changes in the baby's position.

The performance of all interrelated functions (absorption, transport, wicking etc.) were required to be delivered by the one structure.

Even when there was a move towards the all-in-one configuration, it did not provide much more except that the coverstock material was changed to a carded web latex bonded nonwoven. The basic simple fluid mechanics remained the same.

Moving rapidly through the intervening developments to the current all-in-one product designs, we can see how these are now produced from a whole range of different components. This has been brought about by recognition of the fact that it is basically better to incorporate specific components designed to fulfill specific tasks, rather than try to accommodate all the differing requirements with one material, i.e. fluff pulp.

In addition, the modern diaper design is the subject of the continuing development of thinner products, which in turn affects the choice of synergistically complementary materials in an attempt to use the same assembly line mentality to produce the product. The product itself has to be capable of being fitted to a multitude of different size babies, exhibit zero leakage and be the ultimate in thinness, unobtrusive and cost effective!

Major Structural Components Of Current Diapers

We have shown how the evolution of the modern baby diaper has resulted in a product that is built up, in an assembly line fashion, of increasingly complex and numerous component parts.

To prepare for what the future may hold, it is prudent to review the major components and their functionality in a typical modern diaper.

The major structural components are:

* Topsheet (coverstock).

* Acquisition and/or transport or distribution layer.

* Absorbent core.

* Backsheet.

Secondary component materials are:

* Barrier leg cuffs.

* Elastomeric materials.

* Hot melt glues.

Topsheet / Facing Fabric / Coverstock. The topsheet is that part of the diaper in direct contact with the baby's skin. It is therefore necessary that the tactile characteristics of the material, both when wet and dry, be such that is has a soft feel and does not cause irritation of the baby's skin.

Secondly, in combination with the acquisition/transport layer immediately underneath it, the topsheet produces [TABULAR DATA FOR TABLE 2 OMITTED] a high degree of surface dryness, the "dry feel" effect.

Current nonwoven materials used in topsheet applications include spunbonded polypropylene (usually produced on a multibeam system), SMS (spunbond/melt blown/spunbond) polypropylene composites and carded polypropylene thermal bonds. In some applications, thermally bonded bicomponent structures are being used.

An essential requirement of this layer is that it will transport urine in the Z direction of the web in a rapid and effective manner to prevent surface puddling, whatever the insult rate of flow.

It should also be inherently soft and once it has transferred the initial insult through to the acquisition layer, should not have lost its effectiveness or softness while handling subsequent insults.

Polypropylene spunbonded webs, treated to render them hydrophilic (or

partially treated to make them hydrophilic in zones) have proved popular materials for coverstock applications, but they have not always been rated as soft as other materials, i.e. staple fiber-based thermally bonded polypropylene.

However, recent developments in polymer technology with the availability of metallocene polypropylene would appear to provide a route to improve the resultant web softness. Alternative web forming technologies, (such as those of Ason Engineering, Ft. Lauderdale, FL and Kobe, Tokyo, Japan) with the capability to produce bicomponent and microdenier webs, can now provide materials with better web formation, better softness and improved strength, allowing a reduction in web weight and consequently the possibility of a reduction in cost.

Acquisition and transport layers. One of the most important components in the latest ultra-thin diaper is the acquisition and distribution or transport layer(s). These layers (in some diapers there are two layers of different material, in others only one) are a receiving layer for the transferred miction through the topsheet. They are designed to take up brine and transport it across the top of the core structure below; in so doing they assist in handling the potential problem of gel block due to the presence of very high levels of superabsorbent polymer in the core.

Thus the acquisition/transport layers receive the fluid transmitted through the cover sheet and temporarily hold that fluid - hence the terminology "acquisition layer." It also transports the fluid in a lateral direction across the surface of the absorbent core and transfers it at multiple positions, making more efficient use of the superabsorbent polymer.

The essential requirements of this layer are that it must provide sufficient void volume to capture the miction volume. It must also generate a capillary force gradient between the acquisition/transport layer and the absorbent core to ensure rapid transfer of the fluid to the core, leaving the pore structure of the acquisition layer empty and ready to receive the next miction.

There are a number of different approaches to the production of a satisfactory acquisition/transport layer:

* To use a fluff pulp/thermoplastic fiber structure that has been thermally bonded to stabilize the web.

* To use a totally synthetic fiber web such as a bicomponent fiber structure that has been thermally bonded.

* To use a modified and crosslinked cellulose fiber layer, etc.

Some manufacturers prefer to use one layer to provide both functions, but designs also exist that use two adjacent layers to enhance the transport function. For high superabsorbent levels in diapers, the acquisition/transport function is critical to the effective use of the absorbent core.

Absorbent Core. The absorbent core structure is the key functional layer within the diaper. The core has evolved from a structure formed from pre-formed wet laid cellulose wadding to a structure comprising fluff pulp at ever decreasing levels and increasingly large quantities of granular superabsorbent polymer.

Quite obviously the structure and function of the absorbent core of today's diaper is rather different than the structures of previous designs.

Herrmann(2) recently published the information in Figure 5 on the way in which the level of SAP has increased over time.

Although there are other ways of producing an absorbent core structure, current manufacturing systems are designed to use fluff pulp and granular superabsorbent polymer.

It is obvious that once the composition of the absorbent core has reached the point where the SAP content is over 50% of the total, then the SAP becomes the determinant component.

Because the mechanism of liquid storage in SAP's is for them to swell to a multiple of their own weight (and size), the liquid absorption often has to take place through a pre-swollen gel structure.

This requirement of fluid transmission through the gel adds another function to those required of the SAP, which is already required to provide for the fluid retention and absorption under pressure. This has led to the development of permeable SAP's, which improve the liquid transport in homogeneous SAP/fluff pulp mixtures with a high SAP content.

However this still leaves complex problems associated with the retention of such high levels of granular SAP's and the overall stability of the core structure.

Fluff pulp. As we have noted, the other component in the absorbent core is the fluff pulp. There is a tendency to talk as if "fluff pulp" was a clearly defined single material, but it should be realized that because it is a natural product it will be subject to some variations in physical (and chemical) properties. The characteristics and performance of the material classified as fluff pulp will very much depend on the species of tree from which the pulp is extracted, the geographical location of the tree, the pulping technology use to extract the pulp and whether or not chemical additives are added to the pulp to assist in the dry defibration.

Backsheet Materials Technology

Conventional film. Early developments used conventional plastic films as backsheet materials. The primary function was containment, to keep the rest of the diaper in place and prevent transfer of urinary/fecal waste from within the diaper structure.

Early films were not entirely satisfactory and developments centered around the production and use of multilayer film structures to provide greater softness (which improves noise) combined with adequate strength.

The problem was not simple to resolve. A simple blend of polyethylene and polypropylene was found to be unsuitable because the crystallization characteristics of the two polymers are different. The problem could be summarized as follows:

* Polypropylene gives good strength but has poor softness.

* LD polyethylene has good softness but inadequate strength.

* LLD polyethylene promises a better balance of properties.

In the event, it was concluded that a blend of LDPE and LLDPE in one extrusion might provide a satisfactory answer.

With improving technology, the film producers became able to produce multilayer films. These films are produced by the use of a special extrusion head [ILLUSTRATION FOR FIGURE 6 OMITTED].

This co-extrusion allowed the production of five layered films that exhibited a very acceptable balance of properties and economics. Illustrative structure of such films are shown in Figures 7 and 8.

Breathable films. In Japan and other countries in the Far East, there was an early adoption (1983) by manufacturers of breathable films as backsheet materials and over 70% of the diapers now produced use this type of film. Application of this type of backsheet material has only recently spread to North America and Europe.

There are two types of breathable film structure available, one based on a microporous structure and the other on a monolithic film. It is the former structure that is used in diaper backing sheet application because it has a much higher water vapor transport rate.

The concept embodied in the use of a breathable film, which allows water vapor, but not liquid water, to pass through the film, is that by creating a means of water vapor transport from within the diaper core to the outside, the structure can achieve a higher level of perceived dryness at the skin contact layer, which enhances protection of the baby's skin.

This type of microporous film is produced by the incorporation of specially selected and graded calcium carbonate powders into the extruded web. When the film is subsequently oriented, the polymer substrate stretches but the calcium carbonate particle stays as it is, creating a void on the front side of the particle.

It is the controlled creation of these voids that gives the film its breathable properties.

Clothlike backsheets. Nonwoven/film composites (the so-called clothlike backsheets) were initially used as barrier backsheets on training pants where the additional cost could be more easily accommodated. This feature has now spread to many types of diapers as well.

The rationale for adding a nonwoven fabric to a conventional diaper film backsheet for use in training pants was purely aesthetic, enabling the training pant to appear and feel more like a child's cloth panty rather than a diaper. As training pants were an extension of the diaper concept and a premium priced product, a clothlike backsheet appealed to mothers and was valued by them.

However, there is a much less convincing argument for their use in diapers and initially their use was confined to premium range products.

Recent developments have centered around reducing the cost of these materials and there are now a number of different technologies used to produce these materials.

Combination of a breathable film with a nonwoven would appear to offer some technical advantages as well as aesthetic ones, in that the greater surface area created on the outside of the laminate should enhance water vapor transport within the system.

Future Directions: "Don't Imitate - Innovate!"

With all of the developments that have taken place and all of the changes that have been incorporated in the baby diaper of today, where do we go from here?

Success in the promotion of the concept of "thin is beautiful" has resulted in a latent need in the developed markets for still thinner diapers, with maintained or improved performance, improved fit, no leakage and at an acceptable cost. We have seen how, in a number of key areas, present technologies could be up against some fundamental barriers if expected to achieve these objectives.

Franck Courtray of Courtray Consulting, Douai, France(3) has published a number of studies on the evaluation of baby diapers. From a study to measure customer needs, he produced Table 3.

Within the most important category "Keep dry," Mr. Courtray also notes that there are actually two criteria:

Rank1: The diaper keeps the baby's environment dry = no leakage.

Rank 2: The diaper keeps the baby dry = skin dryness/wetness.

In the same series of studies, Mr. Courtray has evaluated the reproducibility of consecutive diapers from the same bulk pack and has measured and observed (by video) the variability in the absorption and flow patterns within the diapers fitted to a mannequin.

The variations are surprisingly large considering that, in theory, the diapers are made in exactly the same way, from the same raw materials and almost at the same time.

All of this indicates that we may have systems that deliver a multitude of "bells and whistles" but we still have a long way to go to meet the customers' perceived needs and priorities and to produce uniformity in our products.

It would appear that the current complex baby diaper with which we are now familiar is a result more of what the producer can make and how each producer achieves product differentiation, than a "real world" reflection of what the consumer actually wants!

[TABULAR DATA FOR TABLE 3 OMITTED]

So if the present assembly line processes leave something to be desired and we have seen how current trends are pushing this process technology beyond its limits and it isn't producing what the consumer really wants in any case, then where do we go from here? What alternatives are there?

Pre-formed absorbent core structures are definitely the subject of the most speculation in the business today. The technology that we prefer to call short fiber air laid to distinguish it from the staple fiber air laid webs made by the Rando Weber process, is based on the technology originally developed by Karl Kroyer in Denmark. It was originally conceived as a way to make paper without the use of water!

In more recent times, the earlier work by M&J Fibertech and DanWebforming, both also located in Denmark, has resulted in "turn-key" machine systems that allow the production of multilayer webs based on cellulose fluff pulp, synthetic fibers or mixtures.

The main limitation is that the fiber length must usually be shorter than 12 mm, but this is not vitally significant for the production of absorbent products. The formed webs may be thermally bonded, latex emulsion bonded or both systems may be used.

The layers of the formed web may be interleaved with pre-formed webs from different technology bases, i.e. spunbonded or SMS and granular superabsorbents or fibrous superabsorbents may be laid down as a layer or throughout the fiber layer. (For more information on air laid nonwovens, see "Technology in Search of Markets" on page 64).

It would appear that using this technology, with the ability to make use of the whole range of cellulose fibers available as fluff pulp and combined with pre-formed insert webs to provide, for example, a specially formed bicomponent acquisition layer or to incorporate a precisely located, thin layer of superabsorbent fibers, will enable the complex webs required for our future diaper products to be made in a controlled and reproducible way.

Adoption of such systems would necessitate large expenditures of the equipment required for the conversion of these composite rolls into finished products and the currently installed capacity would require some fairly large scale additions, but it would seem that we basically have a simple choice.

Either we continue to push the existing technology that has evolved over the past 50+ years to and beyond its limits and still end up making a variable performance product or we invest in the next 50 years and bring innovative new forms of not only the baby diaper, but also feminine hygiene and adult incontinence products produced using the vast spectrum of raw materials now available to us. It does not seem to me to be that difficult a choice!

References.

1. EDANA Absorbent Hygiene Products Program Notes, September 1998/June 1999 Edition Module 1.

2. E. Herrmann (Stockhausen GmbH), Permeable Superabsorbents-Function and Importance for the Application in Diapers. EDANA 1998 International Nonwovens Symposium, 4-5 June 1998 Disneyland, Paris, France.

3. Franck Courtray (Courtray Consulting), "An Expert's View on Laboratory Testing of Baby Diapers," EDANA Absorbent Hygiene Products Program Notes, September 1998/June 1999 Edition Module 4.