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The Rupp Report: Can Textile Additives Save Lives?

Jürg Rupp, Executive Editor

Yes, they can. But let's start from the beginning — and with a question: Do you know SAP? Yes, you may say, this is computer software. Yes, but not this time. SAP here means superabsorbent polymers, which are mainly used for disposable nonwovens such as baby diapers. SAPs are probably one of the least known components in the entire textile industry — if the nonwovens industry is considered to be part of that sector.

According to its website, Germany-based BASF SE produces annually some 400,000 metric tons of superabsorbers, claiming to be one of the world's biggest manufacturers of the products. But what are superabsorbers? BASF gives this description: "Superabsorbers are polymers of acrylic acid and one of its salts, usually a sodium acrylate. In a polymerization reaction, these two basic components form long chains that are crosslinked by additionally incorporated chemical bridges to produce a loose yet water insoluble bundle of molecules. The high water absorption is mainly due to the high osmotic pressure generated by the sodium acrylate, which in this case is actually osmotic suction: positively charged sodium ions which together with negatively charged chloride ions also form our familiar common salt, draw up long water molecules into the superabsorber until it can expand no further." So much for the chemical aspects of SAPs.

Baby Diapers
But what are superabsorbers made for? BASF says that more than 90 percent of its superabsorber output is used in specialty products for the hygiene industry, mainly for baby diapers and sanitary napkins as well as incontinence products. And indeed, baby diapers are the main application sector for superabsorbents; however, they are also applied in wound dressings, bandages, office chairs and plastic pads for raw meat.

The most important property of baby diapers or similar products, mentioned above, is their extreme moisture absorbency and retention capability. Natural fibers applied in traditional fabric diapers are reasonably absorbent; however, synthetic polymers are much more absorbent than natural fibers. And today, a disposable baby diaper is a high-tech product with outstanding properties.

Today's disposable baby diapers will absorb 15 times or more of their weight in water, thanks to the specially designed absorbent SAP pad in the diaper's core. Originally, disposable baby diapers were made with some 50 grams of wood pulp. Today, typically, there are only about 10 to 13 grams of wood pulp. The reason for the lower amount today is that the SAPs are applied in the absorbent pad between the top sheet and the back sheet.

The nonwoven top and bottom sheets are shaped to form the diaper and help prevent leakage. The components are sealed together using heat or ultrasonic methods. Elastic fibers attached to the sheets gather the diaper edges into the required shape to fit the body of the baby and to prevent any leakage.

Nonwovens Play A Key Role
As mentioned, for the textile professional outside the nonwovens industry, it is interesting to know that nonwovens play a key role in baby diapers production: Nonwovens are typically made from polymers such as polyamide, polyester, polyethylene, or polypropylene. They are mainly produced through a wet or dry laid process. A dry laid process, such as meltblown, is usually applied to produce nonwovens for baby diapers. In the meltblown process, molten polymers are extruded through very small holes, the spinning beams. The microfilaments are laid on a conveyor belt and form the nonwoven, and then go through a set of rollers to bond the fibers into a solid sheet. Usually, polypropylene is used for the permeable top sheet, and polyethylene is used for the non-permeable back sheet.

More Water For Mankind
In baby diapers, the SAPs provide a valuable and well-appreciated service for babies - and their parents as well. However, SAPs can do a lot more: One application is water storage for dry soils; they are already in use to prevent soil from drying out.

One big problem of humankind is providing enough drinking water for everyone. Based on SAPs' well-appreciated inherent properties — and babies know exactly about the comfort of a diaper stuffed with SAPs — German researchers want to use them to desalinate seawater. The SAP white powder in pure water takes on up to 500 times its own weight in liquid and swells to form a gel. In saline fluids, it is less 30 to 50 times its own weight in liquid. The swelling capacity depends on the salinity, because it is based on osmosis — the same effect that will burst sausages in hot water.

These plans of the German researchers are another example of work being done in the cross-linking sciences. What an achievement if it would be possible to solve one of the great problems of humanity: to provide enough water for everyone. The Rupp Report will inform you of further developments in this story. If you know, dear reader, another interesting story about another sector of the textile industry that is applying cross-linking sciences, write to jrupp@textileworld.com.

October 2, 2012