Absorbent Core Design
I am a fan of mathematical modeling; however, the tripod is the best analogy that I have found to simply explain how diaper core performance really works. Others may claim there are many more properties to consider. I agree; however, the tripod incorporates the most important properties. The absorbent core performance in a modern diaper is based on a tripod; it is supported by three key diaper design properties. The three key properties (each one represents a leg of the tripod) are:
• Retentive Capacity,
• Strike Through Time
In the same way that a camera or a telescope will fall if the legs are not firmly adjusted, diaper performance will also suffer if these three properties are not correctly balanced; a defective or less than perfect product will be the result. A typical error is to exceed one leg (even at the expense of cost) without adjusting the other two legs accordingly. Balancing is an art learned after many years of diaper testing experience, not just the learning you may acquire in a simple laboratory environment but also the learning you get from the analysis of the recovered diapers from the field and the feedback from infant test centers. Having experienced the unique opportunity to test many diaper designs from all over the world has given me a chance to better understand how it all works.
The Diaper Core Tripod
First Leg Of The Tripod: Retentive Capacity
Total absorbent capacity, which is obtained by measuring the total amount of urine that can be absorbed by a diaper after immersing it in urine for a fixed time and then allowing it to drip (also referred to as free swell capacity), is no longer an important diaper performance variable; at least not as much as it was in the past. In fact, in my opinion, total absorbent capacity today is meaningless! Why? It can mislead you. The amount of urine a diaper can absorb is of little value to us if it is not capable of retaining the urine under pressure, which is why total absorbent capacity is not one of the legs of the tripod.
Instead, absorbency under load (AUL) has assumed this key role. AUL represents the maximum amount of urine that a diaper will hold under the normal pressures of the users such as the every day use of a diaper by a baby. It is a function not only of how well the liquid is retained under pressure but also of how well the diaper is utilized so there are no wasted resources. Some of the best diapers that I have tested have a retentive capacity of around 270 ml for a medium size baby diaper (around 5 KPa of equivalent pressure). Exceeding this value and many brands do, brings little advantage to consumers.
The Second Leg: The Rewet
The second leg of the tripod is the rewet. It represents how wet the surface of the diaper is when it is subjected to pressure. Ideally, it is supposed to be totally dry. Several tests have shown that the actual sensitivity of the skin of an average adult human to detect humidity depends on the temperature of the fluid and the individual; however, it’s been generally accepted that it starts at around 0.11 to 0.13 ml as measured by the typical rewet test at 2.5 Kpa when the fluid is applied at 37°C; any rewet below this amount will not even be detected by the user, but higher rewet numbers will increase discomfort to the baby or the adult incontinence user.
Oftentimes, babies wake up at night not because of leakage but because of discomfort from wet skin; having a mass of cold urine near the body is quite uncomfortable. Imagine that you have two diapers with the same identical retentive capacity but one of them has a much drier surface area than the other. Even when both have the same exact retention, the drier diaper will be regarded as the better product and users will perceive it as being more absorbent. This is the same impression the mother gets when she removes the morning diaper from the baby and notices humidity on the skin, even when both diapers did not leak.
There are a lot of false myths in the industry about rewets. The first one is to incorrectly extrapolate the rewet you get from the laboratory to the rewet from an actual user. Lots of care is taken to avoid handling the diaper before it gets to the lab to measure its rewet when, in fact, the diaper will be subject to deformation and the loss of density in the very first minutes of use by the baby or the adult. This is also quite misleading. If you have ever tried testing a diaper after a baby has worn it for a few minutes, you will understand what I mean.
Another error is taking rewet numbers directly without considering the effect they have on human perception. The most important factors affecting rewet are: the content of superabsorbent polymer (SAP) in the mix; the properties of the SAP, including absorbency under load (AUL); the relative position of the SAP in the mix; the grams per square meter and quality of the acquisition distribution layer (ADL) used to separate the skin from the wet core; and the three dimensionality of the core design (the ratio between pad density at the target zone as compared to the back, which will be explained in this article). I have proposed a new way to measure rewets based on a maximum value of 50 points using a Logarithmic equation that better correlates the relationship between skin sensitivity and the amount of liquid measured at the surface (the old rewet).
The Third Leg: The Strike Through Time
The strike through time represents how quickly the core can absorb the liquids. If it is too slow, there is an increased risk for leakage even when the diaper has not been fully utilized. The better the containment in the diaper, such as in well designed gathers and liquid impervious leg cuffs, the less important this leg of the tripod may be, and you may be able to afford longer times without a decrease in performance.
More than half of the brands in developing markets have leg cuffs that do not really work; thus, in this case, it is extremely important to reduce the time of liquid acquisition. The higher the concentration of SAP in the mix, the slower it is for the liquids to get in. Typically, the thickness of the ADL resulting in higher GSM’s has to be increased in response to higher concentrations in SAP. If you increase the amount of SAP without adjusting the ADL you end up with an unbalanced tripod.
Many core designers tried to clone the old Pampers design, which was made with a special synthetic curly fiber in combination with ADL. Pampers had excellent acquisition times, while many of the clones did not. For this reason Pampers was able to get away with open channels at the end of the leg cuffs, while competitors, imitating the same open channels, suffered from night leakage. The trick is not to make the most expensive diaper, but to make your diaper well balanced according to its particular performance and market segmentation to optimize its performance.
Putting It All Together
A good way to look at diaper core performance is to compare the accumulative values of the three legs of the tripod in a normalized way. I propose using a system of points, where the retentive capacity has a value of 100 points; each normalized rewet for the first two insults has a value of 50 points for a total of 100 points; and each one of the first two strike throughs has a value of 25 points, for a total of 50 points. The reason why the strike through has a lower value than the other two legs of the tripod is because individually it is less important when a diaper with a good containment (good leg cuff barriers) system is being used. Here is an example of core performance based on U.K. diapers purchased last summer using my methodology.
Three-Dimensional Ratios For The Absorbent Core
During the 1990s and early 2000s, most multinational brand owners were designing their absorbent cores with an ever-increased three dimensional pad ratio, which made sure the target zone was not only thicker but also heavier than the back of the diaper to optimize retention at the target zone.
But the whole purpose of a 3D diaper is to make sure the core is better utilized when the diaper is discarded. In a typical flat diaper (with a flat core), you will find the probability of leakage is much higher near the stomach than in the back. This is the reason why cores have been redesigned. In the past, 3D ratios of ranging from 75-150%, were not uncommon. In my opinion, many exceeded the need. Today, most modern diapers do not exceed more than 40-50%. The key design issue for 3D core geometry has to be the probability of leakage; you want a well-balanced diaper where the probability of leakage for the front and back is the same. If you find out that you end up with a higher probability of leakage in the back than in the front, it probably means you exceeded the 3D requirement in your diaper core. The same will happen if you use a much higher weight ADL or its use will help reduce the need of a very high 3D ratio. Some diapers today use as much as 170 gsm of ADL, like in the Huggies Pure & Natural. Just 10 years ago it was hard to find anyone using more than 40 gsm.
New Trends In Diaper Bag Packaging
Not too long ago, purchasing a diaper bag from the multinationals meant obtaining a bag so tight that removing a diaper resulted in the total destruction of the bag. The reason is quite simple: packaging and freight costs may represent as much as 9-11% of the total cost of the product when cardboard is used and as much as 7-8% when not. To save money, the easy solution was to increase the compression force during packaging. This is no longer the case. After 2003, most diaper manufacturers have reversed the trend. Instead of increasing the bag compression with hydraulic pistons at high pressures, today they prefer to reduce the thickness of the diaper altogether.
Compression index measures the height of the stack of diapers in the bag and compares it against the height of the same stack of diapers under a weight of 1 Kg. A compression of 0% meant it was the same height. Not long ago, we were able to check for bags with compression indexes as high as 45% or more where it was impossible to remove the diaper from the bag. Today’s trend is to limit bag compression to no more than 15%. Viewed from an historical point of view, it was ridiculous to force the user to destroy the bag just to get the diaper out, but it happened, believe it or not! New products today are much thinner because they do not use any pulp; this way the bag does not need to be under high compression in order to reduce its volume.
Thinner and Narrower Is Better
Not long ago, most people thought that diapers in the U.S. were already narrow enough. But with the launch of the new Pampers Dry Max in the U.S. and in Europe in March 2010, we have seen it is still not the case. The new Pampers received plenty of criticism initially , but after eight months on the market they seem to have gained acceptance. Recently, Tesco’s private label in the U.K. has followed the same trend as well as many other private label products like in Spain and other developed markets. Thinner products are not as well accepted in developing markets because of the natural cultural tendency to disbelieve that they will hold the same liquid as thicker products. Typically this is dealt with through education and marketing, but it requires a large investment and some time to show its benefits.
Diapers are not only getting thinner, they are also getting narrower. Of all the diaper trends that have originated in developed markets, the narrow diaper is the one facing most resistance from consumers in developing markets. In some areas, it will take several years for this trend to arrive and be fully accepted. On the other hand, all markets—rich and poor—are quickly adopting the use of cloth-like back sheets and printed back sheet designs.
More Product Trends
In addition to diaper core performance, there are a few other areas to consider. Craftsmanship has to do with how well process control is being implemented during the manufacturing of the products. The trend is to reduce standard deviation as much as possible in order to reduce costs.
The most important factor to control is the standard deviation for key properties, such as those in the tripod. You will not survive for long if your process is out of control. Some ways to measure craftsmanship is to look at the core symmetry index; the standard deviation for the retentive capacity (this is a perfect correlation of the SAP variation between diaper to diaper); the standard deviation for the rewets and the strike through times. If you want to learn more about the diaper industry you can join my Diaper Industry Network at LinkedIn using the following link: http://www.disposablediaper.net/e/gis/136568.
Diaper industry consultant, Carlos Richer can be reached at: firstname.lastname@example.org. He maintains an comprehensive diaper industry website, www.disposablediaper.net.