What is Absorbency?

Absorbency has always been regarded as one of the most important features desired by the consumers in a disposable diaper. Does anybody know what absorbency is?  When I ask this question, I am not talking only about the simple physical absorbency phenomena, which too is not as simple as we all may think. We have to look for an answer from the point of view of the consumers of disposable diapers.  In the case of baby diapers, the answer has to be provided by the mother of the baby, or whoever takes the decision to buy the diapers for the baby. Similarly, for adult incontinence products, the answer has to be provided by the actual users of the adult products.  We will soon see, there in no simple answer to the question. The objective of this paper is to make you think whether you are answering the question about absorbency the right way. Is your definition of absorbency the same as that of your customers? And last, but not least, is the current design of the core of your diaper really the optimal core for the money, both for the manufacturer and the user? Is it the time to make a change?   We will traverse the absorbency street together in the next hour or so.

Let us start with Webster’s definition of ‘absorbency’; according to the dictionary, absorbency means to suck up, to drink in, to take in and incorporate.  If we accept this definition, then a diaper that is capable of sucking up more urine, or the one that drinks in more liquid, will be regarded as the most absorbent.  Well, as I am going to explain, this is not necessarily true; at least it is not true from the perspective of the adult diaper consumer, or from the point of view of mothers who buy the diapers.

A key issue to understand, before we talk about diaper absorbency, is the development of the absorbent pad from the beginning of the history of diapers to modern times.  This exercise will help us prepare our minds to understand what we need to do, in order to improve diaper absorbency.  It will also help me point out the errors of the past, concentrate on the errors of the present and define a path for the future.

The development of the absorbent diaper core:

The first disposable diapers were made of crepé tissue paper.  This was a very simple process, where multiple layers of paper were pulled out from several unwinders and were folded. The problem with this simple design was the lack of capacity of this core to absorb adequate quantity of urine and the poor wet resistance of the pad. Wicking was excellent but because paper is not good at storing liquids under pressure, its use was good only for the first insult.

The next step in the development of the absorbent core was the use of cellulose fluff, using a hammer or disk mill and a horizontal or vertical flat screen. This was a huge improvement as you were able to ship large quantities of pulp in a highly compressed roll form, which allowed you to make thousands of diapers from every single roll.  The mill opened the fibers, and the fibers were deposited on a moving copper screen, using a chamber under a low vacuum equivalent to around 8 to 12 inches of water. The core needed a tissue wrapping and spray glue in order to transport the pad to the debulker section, as the pad was typically too weak to withstand the pulling forces by itself, without breaking apart.  This pad design used a compression roll at the debulking station, in order to increase the density of the fluff.  It was soon realized that you were able to improve diaper wet strength and integrity by giving it a diamond shape or a similar pattern during compression.

Why was this a good idea and yet, at the same time, it was a bad option?  An easy way to understand that is to watch the effect that high density has on cellulose fibers.  The lower the density of the fluff, the easier (and faster) it is for the fibers to suck up liquid. What we call “the strike trough time” is almost immediate if the density is low. On the other hand, if density is too high, liquids are not absorbed as quickly by the cellulose.  This is mainly because there is less void space between the fibers and the liquid moves fast along the compressed lines before it is absorbed (capillary pressure and movement of liquid increase as the capillarity becomes smaller).  The simple compressing of the fluff helps to spread the liquids into a larger area, before the fibers are fully saturated, but unfortunately it also results in liquids leaking out of the pad quickly, all the way to the edges. This was typical of many core designs of that time which created a compressed pattern all the way to the edges.

The next step in pad technology was the change of the vertical screen because it had too much friction associated to its operation and it was also a fire hazard, the vertical screen was changed to a rotating screen drum.  The addition of the drum helped to increase the vacuum, without creating excessive friction. Typical vacuums of about 15 to 25 inches of water were used.  Even today we can still see many brand new diaper machines made with this simple flat screen drum design.

It was soon realized that you could just as easily have a wider pad, using the same technology, and then cut a longitudinal piece of it with a circular knife, in order to create a double layer of fluff positioned in the center of the pad.   This design reduced the basis weight at the edges of the pad and increased the capacity of the center at the same time, making it more comfortable for the babies while walking or sleeping. We have to remember that before the use of SAP, diapers had to be very thick in order to remain effective for extended times between diaper changes at night. Baby diapers with more than 60 grams of fluff for night usage were not uncommon during the 80’s. Typical density at that time was around 0.11 to 0.13 grams per cubic centimeter. One of the advantages of using fluff was the excellent wicking that ensured almost full utilization of the diapers before they leaked. However, they had to be extremely bulky to achieve anything close to the liquid holding capacity that we have in modern diapers.

In order to reduce the effect of the very thick pad at the crotch, new pad designs using drum pockets with a shaped hourglass design replaced the original flat screen. This also helped to increase diaper pad area without sacrificing diaper fit at the legs.

The revolution that came with the use of SAP forced many changes in pad design.  The first problem was that the super absorbent was not good at moving liquids.  The first generations of SAP provided an excellent way of holding liquids under pressure and to reduce diaper volume but, unfortunately, they were actually very bad at moving or spreading liquids after they were saturated.  The first timers were faced with a brand new problem, which was called “Gel block” and it meant that you could end up with a leaking diaper even when half of the diaper was totally dry or unused.  This meant wasting lots of raw materials.  Something that helped to avoid leaking of liquids was the addition of a leg cuff, which was introduced at that time.

One of the first changes in pad design came in at the debulking station.  The use of a diamond shaped pattern was a terrible choice when using the first generation SAPs. Why was this a problem? The high density lines created by the embossing pattern made the SAP to “gel block” and stop the liquids from moving any farther. When the SAP in the mix is high, because of the abrasive nature of SAP, it made it almost impossible to cut the pad with a circular knife, as it was being done by many vertical screen formers that are still in use.  This process had to be changed.  In order to avoid the problem, the easy way out was to remove the diamond shaped pattern in the compression roll and replace it with a simple flat mirror surface or a one step roller.   

   Another temporary solution to the problem was the use of an intermittent SAP gun inside the drum former. This gun was used in order to control the position of the SAP, using a pneumatic valve and a sophisticated timer.  This method of applying the SAP had several problems. On one hand, the abrasive nature of SAP made this material difficult to handle with a high speed clicking valve. There were several maintenance problems resulting from its use. In addition, it was difficult to actually see the position and the placement of the SAP in the finished product.  In many cases the SAP ended in the wrong position, before it was possible to take corrective actions.  Most diaper producers removed the embossing pattern and replaced it with a flat compression roll and even several years after the SAP problem was corrected with the use of very effective surface cross linkers, most diaper factories have not returned to the original compression rolls with the embossing pattern, though this action may provide a few measurable benefits today.