Short Fibers and The Paradox of Pad Integrity, Myth or Reality?
I have been involved in the diaper industry for the last 34 years. Back in 1984, when I asked about the absorbent core in diapers, I was told that the core was made with cellulose fibers. The longer these fibers were, the better they would be for the diaper. We had to send samples of the fluff to our pulp suppliers every few months, just to make sure the hammer mills were still doing a good job. If the average fiber length was starting to get shorter, for example under 1.8 mm or less in length instead of the optimal 2.3 to 2.6 mm (depending on the specific type of wood pulp we used), we would need to replace the breaker bar of the mill with a new sharp edge and later retest the mill for compliance. We basically preferred wood pulp suppliers who could deliver longer fiber lengths if the price was the same.
A few years later, after the mid 80’s, diapers started to use super absorbents. The amount of fluff started to decrease with the need to increase the retentive capacity while reducing the shelf space at the same time. Increasing the ratio of SAP in the core created a brand-new problem. It took longer for the liquid to penetrate into the absorbent core and this resulted in premature diaper leakage. In order to reduce acquisition times, we had to use a new diaper component. It was an Acquisition and Distribution Layer, or ADL for short. Soon we noted that the higher the SAP ratio, a higher GSM for the ADL was needed to avoid any increase in times.
For years we kept struggling trying to make thinner products that many times did not have the best core integrity; once the diaper was fully saturated it could break apart. Fiber length and pad density were always blamed, it was the first line of defense when we had to answer to customer feedback related to pad integrity. We were also unable to increase pad density much more, at the risk of selling “cardboard” diapers. In addition, pad density had little meaning once a user had a chance to use the diaper for a few minutes.
As the machine speeds increased, so was the need to replace the tissue core wrap with light weight nonwoven to avoid the material breaking during the automatic splices. This change helped reduce surface rewets but increased the problem of pad integrity even more, until it became critical.
Hot melt suppliers soon developed a specialty hot melt that could be used to fix the problem, they named it “pad integrity” hot melt. Home user tests demonstrated that adding this specialty adhesive did a very good job at keeping the core intact and reduce the complaints even when the diaper was fully saturated.
As SAP ratios continued to go well above 70%, new kinds of SAPs and ADLs needed to be developed. Exceeding the GSM of an ADL, for example above 80 or 100 GSM, created a new kind of challenge. Micro droplets could be easily trapped on a very thick ADL resulting in diapers with excellent acquisition times but poor performance in rewets, in particular during the very first insult. Those ADLs that worked the best, took advantage of the change of denier between the topsheet and the ADL and also the change of deniers within the ADL itself. These high-tech ADLs used multi denier layers and they did a great job.
It became clear that a density gradient between the topsheet and the ADL, and a denier gradient within the ADL itself, helped to move liquids faster, creating a check valve-type effect and reducing the possibility of trapping micro droplets, all these resulted in dryer rewets and making the product more comfortable to the user. They also had one key advantage, they were able to do a better job at lower GSMs.
As we have seen, modern diapers today have taken advantage of density gradients for the topsheet and the ADL, they have also taken advantage of the use of SAPs of different properties by placing specific SAPs at different depths of the core; on the other hand, modern diapers have fallen short regarding the use of different wood pulps made with different deniers. One thing we know today is that not all fibers behave the same way. The hydrogen bonding forces between different types of fibers are not the same, a short fiber from another type of tree can have higher bonding forces at the same fiber length, than the fluff from pine trees.
A typical adult diaper line uses two drum formers. Most adult diapers and pants use a smaller pad placed on top of a larger pad. We have almost 50 years of history telling us the core has to use longer fibers, even when the issue of pad integrity has already been solved, and we have solid evidence that a density gradient helps to move liquids away from the surface, like a check valve. Maybe it is time to review our history and reconsider our paradigms.
Clearly short fibers increase the capillarity of the core, this is obvious, cores are also softer and capable of supporting increased densities, so we should expect that diapers using a higher mix of short fibers will result in increased retentive capacities while reducing diaper thickness, when compared with diapers made with long fibers like pine.
We should also expect that diapers with short fibers will have drier rewets, due to the better wicking of the core. This will not show on a typical lab test, due to the test protocol, but I believe it can be seen from the diaper usage when a user is interacting adding dynamic pressures to the core. At the same time, on the negative side, a higher density core made with short fibers will probably need better ADLs to avoid the expected slower acquisition times.
A core made from two double drum formers can bring all of the benefits of the short fiber, without the need to sacrifice anything, by simply using different pulps in each drum. It is easy to imagine many situations where the use of the short fibers will improve the overall performance of the diaper. It is also easy to imagine products that will not benefit from the use of the short fiber, for example adult diapers made with a single layer with no leg cuffs, where the increased acquisition times of the short fiber could result in increased leakages, especially if the diaper was not balanced using a better ADL.
In conclusion, we can generate new core configurations to improve diaper performance without increasing the cost, we can make diapers softer and more compressed, increase capacity and enjoy better wicking, if we are open to consider the use of short fibers in the cores. Those with the right diaper core design, will gain interesting performance gains.