Disposable Nappies (Diapers)
A disposable nappy consists of an absorbent pad sandwiched between two sheets of nonwoven fabric. These nappies are made by a multi-step process in which the absorbent pad is first vacuum-formed, then attached to a permeable top sheet and impermeable bottom sheet. The components are sealed together by application of heat or ultrasonic vibrations. Elastic fibres are attached to the sheets to gather the edges of the diaper into the proper shape.
The earliest disposables used wood pulp fluff, cellulose wadding, fluff cellulose, or cotton fibres as the absorbent material. While these early models met a need, unfortunately, they did not absorb very much moisture for their weight. Consequently, nappies made from these materials were extremely bulky, and became quite wet. More efficient absorbent polymers were developed to address this issue.
Almost a 1,000 patents related to nappy design and construction have been issued in the last 25 years. Today’s nappies are not only highly functional, they include advanced features such as special sizing and colouring for specific gender and age, colour change indicators to show when the child is wet, and reattachable VelcroTM-type closures (some of which are by products of petroleum). These innovations have enabled disposables to capture a large share of the nappy market. For example, in the mid nineties, disposable nappy sales exceeded US$4 billion in the United States alone.
The single most important property of a nappy, cloth or disposable, is its ability to absorb and retain moisture. Cotton material used in cloth nappies is reasonably absorbent, but synthetic polymers far exceed the capacity of natural fibres. Today’s disposable nappy will absorb 15 times its weight in water. This phenomenal absorption capacityis due to the absorbent pad found in the core of the diaper. This pad is composed of two essential elements, a hydrophilic, or water-loving, polymerand a fibrous material such as wood pulp. The polymer is made of fine particles of an acrylic acid derivative, such as sodium acrylate, potassiumacrylate, or an alkylacrylate. These polymeric particles act as tiny sponges that retain many times their weight in water. Manufacturers have optimized the combinations of polymer and fibrous material to yield the most efficient absorbency possible.
The absorbent pad is at the core of the diaper. It is held in place by nonwoven fabric sheets that form the body of the diaper. Nonwoven fabrics are different from traditional fabrics because of the way they are made. Traditional fabrics are made by weaving together fibres of silk, cotton, polyester, wool, etc. to create an interlocking network of fibre loops. Nonwovens are typically made from plastic resins, such as nylon, polyester, polyethylene, or polypropylene, (by products of petroleum) and are assembled by mechanically, chemically, or thermally interlocking the plastic fibres. Polypropylene is typically the material used for the permeable top sheet, while polyethylene is the resin of choice for the non-permeable back sheet, by products of petroleum.
There are a variety of other ancillary components, such as elastic threads, hot melt adhesives, strips of tape or other closures, and inks used for printing decorations, which are also by products of petroleum.
Nappy production does not produce significant by-products; in fact the nappy industry uses the by-products of other industries. The absorbent polymers used in nappy production are often left over from production lines of other chemical industries. The polymer particles are too small for other applications, but they are well suited for use in nappies.
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