How to choose the correct breathability in a disposable diaper? -The Paradigm of Diaper Smell

Since mid 90’s when the “breathable” feature was first launched, many brands of baby diapers have been using permeable laminated back sheets. The easiest way to manufacture them is by adding calcium carbonate, a very fine mineral just like talc. The CaCO3 powder is added to the PE pellets and homogeneously mixed before the melting screw. Sometimes layers of different PE polymers are combined for strength and softness, mixing lots of low density with linear low-density poly resins as shown in the Figure below. Once the film is being extruded, it is stretched with a group of rollers while the film is still warm. This stretching results in millions of micro holes that are made into the film. The water vapor can pass the film, but not a drop of water, so it is liquid impervious but gas permeable.

This breathable film can be laminated with a nonwoven sheet using hot melt (also commonly called “hot melt lamination breathable laminate”), or extruded directly into a nonwoven sheet (also called “melt embossed breathable laminate”) to create the breathable textile back sheet that we know so well.

Typical weights for baby diaper Textile Back Sheet (or TBS) range between 20 GSM ( as a minimum for direct melt embossed extrusion) to around 28 GSM (as a minimum for hot melt lamination). The reason why direct extrusion can be done at lower GSMs is that it is possible to handle extruded film at very low GSM`s, as low as 8 GSM, but it is very difficult to unwind a finished roll of PE film at 8 GSM due to limitations in tension control and the impossibility to unwind it. This forces the film, the one that is going to be unwinded for a hot-melt lamination process, to be almost twice the thickness of direct extrusion.

This may seem like a big negative factor on hot melt lamination (due to higher polyethylene costs), however, the feeling and the hand texture resulting from direct extrusion can never compare with the softness and cloth-like feeling of a good hot melt lamination. Most people have to decide between the cost savings or obtaining a better softness. With the arrival of a new generation of PE resins, now it is possible to obtain softer textile back sheets, much softer in comparison to what we had just a few years ago, and this will probably keep improving as a result of current research in polymer technology.

The consumer most times is not able to detect the differences between a breathable TBS and a non-breathable, except maybe for the trained eye and perhaps for the softer feeling that is added by the micro holes. The major difference from the point of view of the consumer will result after the diaper is soiled and you can smell the urine. It is a fact that most users of Clothlike TBS already think that the laminate is breathable, even when it is not. If this is the case, then why are breathable films used in the first place? Well, it has been confirmed by several medical research reports, that there is an improved skin condition when you allow the air to interact with the skin, instead of keeping it totally sealed by a non-previous backsheet. In addition, we can all agree that is critical to change a diaper with feces as soon as possible. Few things are as toxic to the skin as a mix of feces with urine in combination with friction. A breathable diaper makes it easier to detect (even when most times the offensive smell is so strong that even a non-breathable diaper will not be able to hide this smell). If there is a smaller quantity of feces, on the other hand, it may be difficult to detect with a non-breathable TBS.

One of the typical concerns of pediatricians, when they encounter skin irritation in babies, is to suggest to the parents to allow the skin to breathe between diaper changes, in addition to the use of skin barriers such as petrolatum, zinc oxide, or other common medication depending on the specific type of rash. Using a breathable diaper, the air can go into the diaper when the diaper is dry, but it can also get in when the diaper is wet. In the same way, water vapor can get out allowing for some evaporation of the urine. The skin benefit is not so much related to the ability of the diaper to dry itself (even when this may help), as much as the benefit of allowing the air to get close to the skin. The amount of air that can go through the TBS depends on the density and size of the holes, as well as the thickness of the film. The most common way to measure this is by using WVTR, the water vapor transmission rate in a day. There is a simple cup method used to measure the amount of water vapor that is lost through the laminate at standard laboratory conditions, and the result of this weight loss is expressed in WVTR in grams/m2 per day.

Why measure water vapor instead of air transmission rate? Well, one reason may be because of our history. Water vapor was the most important concern with cereal packaging for a long time. Without a good moisture barrier, the cereal lost the “crunchiness” and lost its taste. With time, those same tests were used for everything else, not just cereal. There is a direct relationship between the water vapor and the air transmission rates, so we can say that a higher WVTR also means a higher air transmission rate, and also results in a higher oxygen transmission rate.

What are the typical WVTR used in baby diapers? The table shows the amount of WVTR that are most typically found in disposable baby diapers around the world, as you can see it is a wide range.

Of course, the scale may change depending on the test method you used. What is important to know, is that the higher the WVTR, the higher will be the amount of air getting into the diaper, but also the smell coming out of the diaper. It is equivalent to leakage of smell rather than urine (smile). As we will see, this secondary leakage effect can be fortunate or unfortunate, depending on who is your audience and the local culture.

You can spend good money educating consumers of the benefits of a breathable diaper, but if the resulting smell is not acceptable to parents, they will change brands and will avoid buying your products, independently if it had a benefit to the skin. Several factors determine how much smell consumers accept. Probably the most important of all is the local culture and diaper frequency habits. Culture plays a major role in what is acceptable. In countries like Japan and South Korea, which are famous for having the highest rate of diaper frequency change in the world, consumers not only prefer a high WVTR TBS, they will demand it and would not even consider buying a diaper that is not breathable. For them is important to know when the diaper is wet so they can change the diaper as soon as possible, and to know the skin is at the best. This way they use the sense of smell as an indicator for the need of diaper change, reducing the risk of diaper rash.  In many other regions, in particular in Latin America and the Middle East, as well as basically in all the emerging economies, consumers are more concerned with their economy rather than increasing diaper change frequency. They are unwilling to change the diaper just because it is wet; instead, they continue to use it until it becomes almost fully saturated. In all these cases, the smell can play a major role and it can produce a negative perception of consumer’s preferences. Instead of the welcoming effect of the smell as a signal to change the diaper, for these consumers, it is associated with bad quality.

Contrary to popular belief, the smell of the diaper is not just because of the biodegradation process, which happens only after several hours that the diaper has been wet in a warm environment, but it is also due to fresh urine. Many components in diaper urine, like many of its organic acids, have a bad smell; lucky for parents, the smell gets much worse with age, so it is a more serious problem with the urine from elderly people than with most young kids.  Never the less, the odor is still strong enough in babies to have a negative effect on diaper quality perception.  Of course, if the diaper also had a chance to start biodegradation in order to produce ammonia, this is a more serious skin concern, but in reality, it is not as common as we may think, except for warmer areas and long extended use. This is the reason why odor control using enzyme inhibitors used to delay the biodegradation is not a good solution to remove odor in adult diapers. They do not work at removing the chemical odor of hot melts and other diaper components, and they do not remove the odor of fresh urine. A better solution may be the use of molecular modifiers, making sure each molecule of odor is neutralized.

In summary, what is the best amount of WVTR in a baby diaper? If you decide to use a higher WVTR, you will be limited by surface condensation. Depending on the specific relative humidity and temperature differences between skin temperature and the environment, you may experience condensation and this should be avoided. This may happen at WVTRs as low as 10,000 but more common at above 12,000. You should also be aware that at higher WVTRs you may create added discomfort associated to lower urine temperatures resulting from the evaporation, this in some cases (like during wintertime) could be more negative than the positive effect of having a transfer of air near the skin. Many babies wake up in the middle of the night not because the diaper leaked a few drops of urine, but because they feel cold. I believe there is no sufficient research on the topic of the interaction between skin health and skin comfort for a definite answer.  If you choose to go in the direction of a lower WVTR, you may end up with just a gimmick.  As far as I know, there is no legislation defining what should be the minimum amount of WVTR in order to add a claim of breathability to a bag of baby diapers, but you may be missing the potential benefits. How much is it for real, how much has been exaggerated, and how much can it be a negative due to the colder urine rewets, is still a topic of future discussion and some debate. For sure odor control is much more important with adult users than with babies. As a suggestion, try not to exceed the WVTR of the local market leader.

Thanks to Felix Plastics and to SML for the pictures. This is a recently updated article from the one I originally published at the Nonwovens Industry Magazine a few years ago.