Short Fibers and the Absorbent Integrity Paradox: Myth or Reality?
I have been working in the diaper industry for 34 years. In 1984, when I asked about the absorbent core of diapers, I was told it was made of cellulose fibers — and that the longer those fibers were, the better the performance. Periodically, we had to send fluff samples to our suppliers just to ensure that the hammer mills were still operating properly. If the average fiber length started to fall below 1.8 mm (when the ideal range was between 2.3 and 2.6 mm, depending on the pulp used), it was necessary to replace the mill discs with sharper blades and test it again. In practice, if the price was the same, we preferred suppliers that delivered longer fibers.
A few years later, in the second half of the 1980s, diapers began incorporating superabsorbent polymers (SAP). As a result, the amount of fluff in the core was reduced to increase retention capacity and reduce product volume on store shelves. However, increasing the SAP proportion introduced a new problem: liquid took longer to penetrate the core, causing premature leakage. To address this, a new component was introduced — the acquisition and distribution layer (ADL). We soon realized that the higher the SAP proportion, the higher the basis weight of the ADL needed to be in order to prevent an increase in absorption time.
For many years, we sought to manufacture increasingly thinner products, but this often compromised core integrity. When the diaper became fully saturated, the core could fall apart. Fiber length and core density were always pointed out as the culprits, serving as standard justifications in response to consumer complaints. It was also difficult to increase core density without risk, as overly dense products became hard, like “cardboard.” In addition, density measured before use did not represent real behavior after a few minutes in contact with the body.
As machine speeds increased, new challenges emerged. It became necessary to replace the core wrap (previously made of tissue) with lightweight nonwoven materials to prevent rupture during automatic cutting. This change helped reduce rewet to the surface but worsened the structural integrity issue. The situation became so critical that suppliers developed specific hot-melt adhesives for “core integrity.” User testing showed that applying this special adhesive was highly effective in keeping the core intact, even when fully saturated.
As the SAP proportion exceeded 70%, new types of SAP and new ADLs were developed. However, very thick ADLs — above 80 or 100 GSM — created new problems. Microdroplets could become trapped within them, resulting in excellent absorption times but worse rewet, especially during the first insult. The most efficient ADLs turned out to be those using combinations of different deniers between the topsheet and the ADL itself, creating multiple functional layers. This design improved liquid transport, reduced entrapment, and delivered drier products, while also working better with lower basis weights.
Modern diapers take advantage of these density gradients in the topsheet and ADL, as well as distributing SAPs with different characteristics in separate layers of the core. However, there has been little progress in the use of different types of cellulose, particularly regarding fiber deniers. Today, we know that hydrogen bonding forces vary among different fiber types. A short fiber from a given tree species may have stronger bonds than a long pine fiber.
In adult diapers, a typical scenario involves two forming drums: a smaller core placed over a larger one. For nearly 50 years, it was assumed that the core should use long fibers, even after structural integrity issues had already been solved. There is also strong evidence that density gradients help move liquid away from the surface, acting like a retention valve. Perhaps it is time to revisit this tradition and reassess these paradigms.
It is clear that short fibers significantly increase core capillarity. Cores made with short fibers tend to be softer and can support higher densities. Therefore, we can expect diapers with a higher proportion of short fibers to deliver greater absorption capacity and reduced thickness when compared to those made with long fibers such as pine. We can also expect less fluid rewet and drier products due to improved core drainage. Although this may not always appear in traditional laboratory tests, the difference becomes evident in real-use testing, where users apply dynamic pressure to the core. On the other hand, denser cores made with short fibers will likely require higher-performance ADLs to prevent increased absorption time.
A core produced with two forming drums can combine the best of both worlds by using different pulps in each layer and extracting maximum benefit from short fibers without compromising integrity. There are many applications in which the use of short fibers improves overall performance. On the other hand, it is also easy to imagine products where short fibers are not advantageous, such as adult diapers with a single core and no side panels, where slower absorption may lead to leakage — especially if not properly balanced with a higher-efficiency ADL.
In conclusion, it is entirely possible to create new core configurations that improve diaper performance without increasing cost. We can develop softer and thinner diapers with higher capacity and capillarity, provided we are open to considering the use of short fibers in our formulations. The company that arrives at the ideal absorbent core design will secure significant competitive advantages.