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Disposable diapers

Properties, components, advantages, and uses of modern disposable diapers

Since their introduction modern single-use diaper products have continuously improved, becoming lighter, more compact, more absorbent and easier to use. Today it is estimated that more than 95 percent of all parents use them – and it is generally recognized that modern disposable diapers are healthier for the infant’s skin. Their softness, lightness and breathable nature provide superior comfort for the baby; they are easy to put on and remove and are more convenient than reusable diapers as they eliminate the need for both constant laundering at very high temperatures to remove germs and drying. The industry is committed to providing baby diapers that give better skin care, a better performance, with reduced resource use and reduced environmental impact, all at an affordable price.

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With the growing demand for more comfortable, healthier and environmentally friendly products, efforts in research and development activities in the textile industry have focused on the utilization of renewable and biodegradable resources as well as environmentally sound manufacturing processes in textiles.

The disposable diaper concept came in the early 1960s in the form of products based on cellulose wadding with plastic backing and nonwoven top sheets. Disposable baby diapers became more popular and widespread during the mid-1980s and by 1990 disposable diapers were used in more than 90% of families in most European and US countries.

Disposable baby diapers are designed to absorb and retain an infant’s urine and faeces while keeping the skin dry and healthy and to be easily and hygienically disposed of in household solid waste. Diapers are available in a wide range of sizes designed to fit all ages of children.

A diaper or nappy is a type of underwear that allows the wearer to defecate or urinate without the use of a toilet, by absorbing or containing waste products to prevent soiling of other clothing or the external environment when diapers become soiled, they require changing, generally by another person.

Wikipedia

Manufacturing of these diapers are done 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. These components are sealed together by application of heat or ultrasonic vibrations. Elastic fibre bands are attached to the sheets to gather in order to form the diaper into the proper shape so that it fits snugly around a baby’s legs and crotch. When properly fitted, the disposable diaper will retain body fluids which pass through the permeable top sheet and are absorbed into the pad.

Benefits to society: Disposable baby diapers have become indispensable for nearly all families across Europe, the USA and developed world.  The convenience created by disposable diapers is a huge benefit in today’s busy lifestyles where time is a precious asset. They lessen the burden of domestic chores, freeing parents to spend more time on other activities.

Advantages of Disposable Baby Diapers

Disposable baby diapers are…

  • Healthy: their usage results in reduced skin rash incidence, skin irritation and infections
  • Comfortable: their softness, lightness and the breathable nature of the materials used provide for superior comfort for the baby
  • Convenient: they are easy to use, faster to put on and remove
  • Hygienic: by preventing leakage of faeces and urine, they reduce risks of transmission of infectious diseases for babies and their carers

How they are made

There are three basic operational processes involved in the manufacture of baby diapers, they are…

  • Ferberization of the fluff pulp, addition of superabsorbent polymer and absorbent pad formation
  • Lamination with films, nonwoven materials and elastic elements
  • Shaping, cutting, folding and packaging.

These 3 basic processes have remained constant over the years. With the introduction of new technologies and more sophisticated designs, major improvements have been made in increasing production line efficiency, product quality and reducing manufacturing waste.

Raw Materials

Wood pulp which would be transformed into the absorbent pad.

  1. Pulp would be mixed with polymer partials to make a fluffy absorbent pad. Polyacrylic acid is a polymer made from the monomer acrylic acid.
  2. The absorbent pad is made of SAP and wood pulp. The hydrocarbons that make up the SAP are both found naturally and made synthetically.  When it is made synthetically, UV light and catalysts are added to the hydrocarbon to remove the salts.
  3. 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. A dry laid process, such as the “melt-blown” method, is typically used to make nonwoven diaper fabrics.
  4. Polypropylene is typically the material used for the permeable top sheet, while polyethylene is the resin of choice for the non-permeable back sheet. 

The most significant important property of a diaper is its ability to absorb and retain moisture.

Cotton material used in cloth diapers is reasonably absorbent, but synthetic polymers far exceed the capacity of natural fibres.

Latest innovations for disposable diaper will absorb 20 times its weight in water. This phenomenal absorption is due to the absorbent pad found in the core of the diaper. This pad is composed of two essential elements, a hydrophilic, polymer and a fibrous material such as wood pulp. The polymer is made of fine particles of an acrylic acid derivative, such as sodium acrylate, potassium acrylate, or an alkylate. These polymeric particles form a gel and act as tiny sponges that retain many times their weight in water.

When checked under a microscope these polymer molecules form long chains. Portions of these long chemical chains are designed to interact with water molecules. During cross-linking, process polymer has the ability to chemically link with different polymer molecules.

When a large number of these polymeric chains are cross-linked, they form a gel network that is not water soluble but that can absorb vast amounts of water. Polymers with this ability are referred to as hydrogels, superabsorbent, or hydrocolloids. The strength of gel can be adjusted by the process of controlling the degree of cross-linking. This is an important property because gel strength is related to the tendency of the polymer to deform or flow under stress.

If the strength is too high the polymer will not retain enough water. If it too low the polymer will deform too easily, and the outermost particles in the pad will absorb water too quickly, forming a gel that blocks water from reaching the inner pad particles. This problem, known as gel blocking, can be overcome by dispersing wood pulp fibres throughout the polymer matrix. These wood fibres act as thousands of tiny straws which suck up water faster and disperse it through the matrix more efficiently to avoid gel blocking. Manufacturers have optimized the combinations of polymer and fibrous material to yield the most efficient absorbency possible.

Other components

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.

Manufacturing and Processing

disposable-diapers

There are five main components that makeup diapers.

  • The first is the absorbent core.  It is made from the combination of Superabsorbent Polymer and wood pulp.
  • The second is 3D leak guard. This prevents to spill outside the diaper.
  • The third is the soft breathable back sheet made from polypropylene/polymer material.
  • The fourth is the magic side tape
  • The fifth is the elastic waistband

Formation of the absorbent pad

The absorbent pad is manufactured on a movable conveyer belt that passes through a long “forming chamber”.  The chamber at various points has pressurized nozzles spray for either polymer particles or fibrous material onto the conveyor surface. The conveyer belt bottom is perforated, and as the core pad material is sprayed onto the belt, with a vacuum pressure applied from below to pull down the fibres in order to form a flat pad.

formation-of-absorbent-pad

At least two methods have been employed to incorporate absorbent polymers into the pad.

Method 1:

In one method the polymer is injected into the same feedstock that supplies the fibres. This method produces a pad that has absorbent polymer dispersed evenly throughout its entire length, width, and thickness.

The problems associated with the method are that loss of absorbent may occur because the fine particles are pulled through the perforations in the conveyor by the vacuum. It is therefore expensive and messy. This method also causes the pad to absorb unevenly since absorbent is lost from only one side and not the other.

Method 2:

A second method of applying polymer and fibre involves the application of the absorbent material onto the top surface of the pad after it has been formed.

This method produces a pad which has absorbent material concentrated on its top side and does not have much absorbency throughout the pad. Another disadvantage is that a pad made in this way may lose some of the polymer applied to its surface. Furthermore, this approach tends to cause gel blocking, since all the absorbent is on the outside of the pad. The moisture gets trapped in this outer layer and does not have a chance to diffuse to the centre. This blockage holds moisture against the skin and can lead to discomfort for the wearer.

To overcome these major problems multiple spray dispensers are used to form several layers of polymer and fibre.

The first step is to draw fibres into the chamber to form a bottom, a portion of the polymer is added in a controlled way to mix and form a layer of combined polymer and fibre.

Then more pure fibre is pulled on top to give a sandwich effect. This formation creates a pad with the absorbent polymer confined to its centre, surrounded by fibrous material. Gel blockage is not a problem because the polymer is concentrated at the core pad. It also solves the problem of particle loss since all the absorbent is surrounded by fibrous material.

Finally, this process is more cost-effective because it distributes the polymer just where it is needed.

After the pad has received a full dose of fibre and polymer through dispensers, it proceeds down the conveyor path to a levelling roller near the outlet of the forming chamber. This roller removes a portion of the fibre at the top of the pad to make it a uniform thickness. The pad then moves by the conveyor through the outlet for subsequent operations to form the completed diaper.

Preparation of the nonwoven

Non-woven fabrics in web form are made at a different location and are brought in roll form then cut to the appropriate width for use in diapers. Sheets of nonwoven fabric are formed from the plastic resin using the melt blown process as described above.

There is a web for the top sheet and another for the bottom sheet. When the manufacturer is ready to initiate diaper production these large bolts of fabric are connected to special roller equipment that feeds fabric to the assembly line.

diapers-bottom-sheet

At some point in the process, stretched elastic bands are attached to the backing sheet with adhesive. After the diaper is assembled, these elastic bands contract and gather the diaper together to ensure a snug fit and limit leakage.

Assembly of the components

At this point in the process, there are still three separate components, the absorbent pad, the top sheet, and the backing sheet. These three components are in long strips and must be joined together and cut into diaper-sized units. This is accomplished by feeding the absorbent pad onto a conveyor with the polyethene bottom sheet.

The polypropylene top sheet is then fed into place, and the compiled sheets are joined by glueing heating, or ultrasonic welding. The assembled diaper may have other attachments, such as strips of tape or Velcro™, which act as closures.

The long roll is then cut into individual diapers, folded, and packaged for shipping.

Quality Control

The main activity of the quality control is to check products absorbency which should be within the set international norms.

One key is to make sure the polymer/ fibre ratio in the absorbent pad is correct. Too much variation will impact the diaper’s ability to soak up moisture. Industry trial and error has shown that for optimal performance and cost, the fibre to particle ratio should be about 75:25 to 90:10. Even more critical than these ratios are the size and distribution of these particles.

It has been established that particles with a mass median particle size greater than or equal to about 400 microns work very well with the fibres to enhance the rate at which the fluid is transported away from the body. If the particles very much outside this range, gel blocking may occur.

There are several standard tests the industry uses to establish diaper absorbency. One is referred to as Demand Wettability or Gravimetric Absorbance. These tests evaluate what is are commonly referred to as Absorbance Under Load (AUL). AUL is defined as the amount of 0.9% saline solution absorbed by the polymers while being subjected to pressure equivalent to 21,000 dynes, or about 0.30 lb/sq in (0.021 kg/sq cm). This test simulates the effect of a baby sitting on a wet diaper. If the diaper has an absorbency of at least 24 ml/g after one hour, the quality is considered acceptable.

Other quality control factors besides absorbency are related to the diaper’s fit and comfort. This is an essential parameter check and Particular attention must be paid to the melt characteristics of the nonwoven fabrics used to form the diaper’s shell. If materials with different melting points are used, the material that melts the quickest may become too soft and stick to the assembly apparatus. When the fabric is pulled off it may be left with a rough surface that is uncomfortable to the user. Finally, the alignment of the components must be carefully checked or leakage may result.

Disposable baby diapers sold in the EU/USA must comply with the European/USA General Product Safety Directive and where they contain lotions and fragrances, must also comply with the requirements of the European/USA Cosmetics Directive.  Our industry does more than simply comply with the legal framework to ensure that our products are

Disposable diaper manufacture is a high technology field which has consistently shown innovation over the last few decades. Nonetheless, there are still a number of areas which require additional improvement. One such area is that of leakage reduction. It is likely that the manufacturers have developed 3D leak guard with elastic bands to hold the waist more tightly without causing chafing or leakage or discomfort. It is also likely that current concern regarding the role of disposable diapers in landfills will impact manufacturing and formulation. This concern may lead to the development of diapers which are less bulky and more biodegradable.

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