A disposable diaper consists of an absorbent pad sandwiched between two sheets of nonwoven fabric. The pad is specially designed to absorb and retain body fluids, and the nonwoven fabric gives the diaper a comfortable shape and helps prevent leakage. These diapers 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 fibers are attached to the sheets to gather the edges of the diaper into the proper shape so 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.
Disposable diapers are a relatively recent invention. In fact, until the early 1970s mothers had no real alternative to classic cloth diapers. Cotton diapers have the advantage of being soft, comfortable, and made of natural materials. Their disadvantages include their relatively poor absorbency and the fact that they have to be laundered. Disposable diapers were developed to overcome these problems. The earliest disposables used wood pulp fluff, cellulose wadding, fluff cellulose, or cotton fibers as the absorbent material. These materials did not absorb very much moisture for their weight, however. Consequently, diapers made from these materials were extremely bulky. More efficient absorbent polymers were developed to address this issue.
Since the 1970s, disposable diaper technology has continued to evolve. In fact, nearly 1,000 patents related to diaper design and construction have been issued in the last 25 years. Today's diapers are not only highly functional, they include advanced features such as special sizing and coloring for specific gender and age, color change indicators to show when the child is wet, and reattachable VelcroTM-type closures. These innovations have enabled disposables to capture a large share of the diaper market. In 1996, disposable diaper sales exceeded $4 billion in the United States alone. Proctor and Gamble and Kimberly Clark are the two largest brand name manufacturers, and their sales account for nearly 80% of the market. Private label manufacturers that produce store brands and generic diapers account for most of the remaining 20%.
The single most important property of a diaper, cloth or disposable, 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 fibers. Today's state-of-the-art disposable diaper will absorb 15 times its weight in water. This phenomenal absorption capacity is 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, 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 alkyl acrylate. These polymeric particles act as tiny sponges that retain many times their weight in water. Microscopically these polymer molecules resemble long chains or ropes. Portions of these chemical "ropes" are designed to interact with water molecules. Other parts of the polymer have the ability to chemically link with different polymer molecules in a process known as cross linking. 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, superabsorbents, or hydrocolloids. Depending on the degree of cross linking, the strength of the gel network can be varied. 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 fibers throughout the polymer matrix. These wood fibers 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.
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 fibers of silk, cotton, polyester, wool, etc. to create an interlocking network of fiber loops. Nonwovens are typically made from plastic resins, such as nylon, polyester, polyethylene, or polypropylene, and are assembled by mechanically, chemically, or thermally interlocking the plastic fibers. There are two primary methods of assembling nonwovens, the wet laid process and the dry laid process. A dry laid process, such as the "meltblown" method, is typically used to make nonwoven diaper fabrics. In this method the plastic resin is melted and extruded, or forced, through tiny holes by air pressure. As the air-blown stream of fibers cools, the fibers condense onto a sheet. Heated rollers are then used to flatten the fibers and bond them together. Polypropylene is typically the material used for the permeable top sheet, while polyethylene is the resin of choice for the non-permeable back sheet.
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.
Formation of the absorbent pad
1 The absorbent pad is formed on a movable conveyer belt that passes through a long "forming chamber." At various points in the chamber, pressurized nozzles spray either polymer particles or fibrous material onto the conveyor surface. The bottom of the conveyor is perforated, and as the pad material is sprayed onto the belt, a vacuum is applied from below so that the fibers are pulled down to form a flat pad.
At least two methods have been employed to incorporate absorbent polymers into the pad. In one method the polymer is injected into the same feed stock that supplies the fibers. This method produces a pad that has absorbent polymer dispersed evenly throughout its entire length, width, and thickness. The problems associated with 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.
A second method of applying polymer and fiber involves 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 center. This blockage holds moisture against the skin and can lead to discomfort for the wearer.
These problems are solved by controlling the mixture polymer and fibrous material. Multiple spray dispensers are used to apply several layers of polymer and fiber. As the fiber is drawn into the chamber and the bottom of the pad is formed, a portion of the polymer is added to the mix to form a layer of combined polymer and fiber. Then more pure fiber is pulled on top to give a sandwich effect. This formation creates a pad with the absorbent polymer confined to its center, surrounded by fibrous material. Gel blockage is not a problem because the polymer is concentrated at core of 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.
- 2 After the pad has received a full dose of fiber and polymer, it proceeds down the conveyor path to a leveling roller near the outlet of the forming chamber. This roller removes a portion of the fiber 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 competed diaper.
Preparation of the nonwoven
3 Sheets of nonwoven fabric are formed from plastic resin using the meltblown process as described above. These sheets are produced as a wide roll known as a "web," which is then cut to the appropriate width for use in diapers. There is a web for the top sheet and another for the bottom sheet. It should be noted that this step does not necessarily occur in sequence after pad formation because the nonwoven fabrics are often made in a separate location. 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.
- 4 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
- 5 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 polyethylene bottom sheet. The polypropylene top sheet is then fed into place, and the compiled sheets are joined by gluing, heating, or ultrasonic welding. The assembled diaper may have other attachments, such as strips of tape or Velcro™, which act as closures.
- 6 The long roll is then cut into individual diapers, folded, and packaged for shipping.
Diaper production does not produce significant byproducts; in fact the diaper industry uses the byproducts of other industries. The absorbent polymers used in diaper 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 diapers. In diaper production, however, considerable amounts of both nonwoven material and polymer particles are wasted. To minimize this waste, the industry tries to optimize the number of diapers obtained from every square yard (meter) of material. Furthermore, every attempt is made to recover the excess fiber and polymer material used in the forming chamber. However, this is not always possible due to clogging of filters and other losses.
There are several methods used to control the quality of disposable diapers, and most of these relate to the product's absorbency. One key is to make sure the polymer/fiber 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 fiber to particle ratio should be about 75:25 to 90:10. Even more critical than this ratio are the size and distribution of these particles. It has been established that particles with mass median particle size greater than or equal to about 400 microns work very well with the fibers to enhance the rate at which the fluid is transported away from the body. If the particles vary 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. 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 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 manufacturers will develop improved elastic bands to hold the waist more tightly without causing chafing 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 to lead to the development of diapers which are less bulky and more biodegradable.
Where to Learn More
"Dueling diapers." The Edell Health Letter, August 1993, p. 6.
McAloney, Regina. "Thin is in." Nonwovens Industry, November 1994 p.52.
Lenzner, Robert, and Carrie Shooc. "The Battle of the Bottoms." Forbes, March 24, 1997, p. 98.
Ohmura, Kin. "Superabsorbent Polymers in Japan." Nonwovens Industry, January 1995, p. 32.
Tissue with Lotion
Tissue with Lotion
Facial tissues belong to a class of paper products used extensively for personal hygiene in modern society. Other products of this type include paper towels, napkins, and sanitary (or toilet) tissue. These products are designed to be highly absorbent, soft, and flexible. These pleasant tactile properties are especially important for facial and bathroom tissues, considering their use. To optimize pleasant skin feel, tissues have been developed with softening agents or lotion-type ingredients to reduce any chafing effect on delicate parts of the body.
Tissues of this type are made by a process in which the nonwoven fabric is made from a solution of cellulose fibers and water, formed into a sheet, then coated with softening agents. Finally, the coated fabric is cut into individual tissues, folded, and packaged for sale.
Tissue softness is a tactile perception characterized by the sheet's physical properties, such as flexibility or stiffness, texture, and frictional properties. Historically it has been difficult to soften the tissue surface without interfering with other properties of the fabric. For example, softness can be increased by adding agents that interfere with the way the fibers within the tissue interact, making them less closely bonded to each other. These are known as debonding agents. However, these materials tend to decrease the tensile strength of the fabric and may irritate skin on contact. Enhanced softness can also be achieved by coating the fabric with oily materials. However, this limits the amount of moisture the tissue can absorb. In fact, the coating can also make the fabric so hydrophobic (water hating) that it can not be processed properly in sewage plants. Another problem is that some coating materials may decrease the strength of the fabric to the point where the tissue is not usable. To overcome this problem, fabric strength may be increased by adding certain resins or by mechanical processes which ensure the fibers bond together better. However, increasing strength tends to make the fabric stiffer and harsher to the touch. Rising to these challenges, tissue manufacturers have designed methods that successfully balance softness with absorbency and strength to create a product that consumers find acceptable.
Nonwoven tissue paper
Tissue paper is a nonwoven fabric made from cellulosic fiber pulp. Common fibers used in tissue paper pulp include wood (from either deciduous or coniferous trees), rayon, bagasse (a type of sugar cane stalk), and recycled paper. These fibers are macerated in a machine known as a hydropulper, which is a cylindrical tank with a rapidly revolving rotor at the bottom that breaks fiber bundles apart. In this process the fibers are mixed in a cooking liquor with water and either calcium, magnesium, ammonia, or sodium bisulfite. This mixture is cooked into a viscous slurry containing about 0.5% solids on the basis of weight. Bleaching agents are added to this mixture to whiten and brighten the pulp. Common bleaching agents include chlorine, peroxides, or hydrosulfites. The pulp is then washed and filtered multiple times until it is the fibers are completely free from contaminants. This mixture of pulp and water, known as a "furnish," is then ready for the papermaking process.
Lotion (Softening additives)
Softening agents are oily or waxy materials that are coated onto the tissue fabric to improve its tactile properties. These materials are too concentrated to coat directly on the paper, so they must be diluted with water first. However, these oils do not dissolve in water, they must be dispersed in water with the aid of chemicals known as surface active agents, or surfactants. A mixture of water, oils, and a surfactant is known as an emulsion. Mayonnaise is an example of a food product emulsion.
The oily materials used in lotions typically include vegetable and mineral oil, plant or animal derived waxes, fatty materials, and silicone-based oils. While theoretically all of these materials would be appropriate tissue paper softening agents, experience has shown that many of them do not function well because they interfere with other desirable properties of the paper, like its absorbency. The tissue industry has had to develop its own patented combinations of lotion materials which, when blended and applied in the correct ratio, provide appropriate softening without negatively affecting the tissue. These materials include polyhydroxy compounds with multiple oxygen-hydrogen groups that allow them to interact with water. Therefore, these compounds are able to soften the paper surface without blocking too much water. Examples of polyhhydroxy compounds include glycerine, propylene glycol, polyoxyethlyne glycol, and polyoxypropylene glycol. They are employed at concentrations between 0.1 and 1% on the basis of the dry tissue weight. Other useful agents include mixtures of petroleum- and silicone-based oils, which are judiciously added to further soften the paper. These oils must be used at low levels to avoid waterproofing the web and robbing it of absorbency. Surfactants are added to disperse the oils in water. A typical surfactant used in paper treatment emulsions is cetyl alcohol, a fatty material whose chemical structure allows it to combine oil and water.
Preparation of the nonwoven
A variety of specialized equipment is used to press the pulp mixture, or furnish, into a nonwoven sheet of fabric-like paper. Nonwoven fabrics are different from traditional fabrics because of the way they are made. Traditional fabrics are made by weaving fibers together to create an interlocking network of fiber loops. Nonwovens are assembled by mechanically, chemically, or thermally interlocking the fibers. There are two primary methods of assembling nonwovens, the wet laid process and the dry laid process. The wet laid process is employed for making the type of nonwoven used in tissue production.
- 1 The slurry flows into a device known as a headbox, which in turn spreads it on a moving wire mesh known as a Fourdrinier. The Fourdrinier is a continuous wire belt, approximately 50 feet (15 m) or more in length, which is stretched out like a table. As the fibers are travel down this belt much of the water drains through the holes in the wire mesh. The wet sheet of fibers is carried by a series of woolen blankets, called felts, between several sets of rolls, which further compress it and remove more water. At this point the sheet is strong enough to be transferred to a drying machine that is especially adapted for making tissue papers.
2 The tissue paper drier is called a Yankee Dryer and consists of a steam-heated, highly polished roller 10-12 feet (3-4 m) in diameter. The wet sheet is carried by a heavy canvas felt, which is threaded over and around the rollers. With each successive pass, the rollers remove more water until the paper is adequately dried. If desired, a pattern may be imprinted in the tissue by juxtaposing the web on an array of supports during the dewatering process. (Alternately, the web can be dewatered and transferred to a separate imprinting line.) The raised supports on the line create bumps and valleys on the fabric. These are regions of varying fiber density and are visible as tiny patterned "pillows" on the final sheet. If necessary, these high bulk areas can be densified even further by applying a vacuum to selected portions of the sheet.
After the fabric been compressed to the desired thickness, it is referred to as a "web." The web is now ready for additional processing. It may be coated or stored on large vertical rollers, known as calendar stacks, to await further operations.
Lotion preparation and application
- 3 The lotion is prepared in steam-heated batch tanks equipped with high speed mixing blades. The oils and water may be preheated and are blended together with high shear to form an emulsion. The completed lotion is ready to be applied to the paper surface and is pumped from the batch tanks to a holding vessel connected to the coating equipment.
- 4 The nonwoven web is fed onto a series of papermaking belts. As it travels over the belts the web comes into contact with an emulsion distributing roller, which pulls lotion out of the holding tank and coats a thin film onto the web. Ideally low amounts of lotion are applied (0.3% or less) to prevent the web from being overcoated. However, higher levels can be used if the coating is designed with additional surfactants that will act as wetting agents to help the tissue absorb moisture through its hydrophobic layer. After passing through the coating rollers, the web continues along the belts to other rollers, which strip excess lotion from the fabric. In between processes the belts are kept clean by belt-cleaning showers that remove paper fibers, adhesives, and other additives.
Forming operations and packaging
- 5 The web passes through a series of rotating knives that cut it to the desired width. The coated tissue is then sliced at tissue-sized intervals, folded, and packaged in boxes or cellophane wrap.
The tissue manufacturing and coating process generates considerable amounts of waste material, but much of this is reclaimable. Waste fibers from the pulping process can be washed and reused. The water used in the slurry and in subsequent operations can be recycled. Unfortunately, there is little or no recovery of the chemicals used in coating and other treatments, and the disposal of the various spent solutions is a problem for the industry.
There are many quality control measures used in the tissue paper industry. The ones related to lotion application include analytical testing and subjective panel evaluations. Since the amount of material deposited on the tissue is critical, the industry has established various tests to measure how much is actually present on the tissue surface. For example, the amount of polyhydroxy compounds present can be determined by stripping the compounds from a tissue sample using a method known as the Webul solvent extraction. The amount of compound is then measured on a spectroscope or chromatograph. The concentration of surfactants can be established in a similar manner.
While these analytical techniques can precisely determine the levels of specific chemicals, they can not evaluate fabric softness. This tactile property is assessed by subjective evaluation by trained panelists. Prior to these evaluations, the tissue fabric is equilibrated to a constant temperature of 72-111°F (22-44°C) and relative humidity of 10-35%. The fabric is then conditioned for another 24 hours at 50% humidity. Panelists are then asked to feel swatches and rate the degree of softness, flexibility, and smoothness. The evaluation is done by paired comparison, as described by the American Society for Testing Materials (ASTM). Subjects are presented with samples on a blind basis and required to choose one on the basis of tactile softness. The results are reported in Panel Score Units which grade the fabric on a scale of "Much Softer," "Slightly Softer," "Equally Soft," "Less Soft," etc.
Absorbency, the ability of the tissue to be wetted with water, is quantified by measuring the period of time required for dry tissue to become completely saturated with water. This measurement is known as wetting time. Once again, the fabric is equilibrated to a specified temperature and humidity. It is then cut into small squares, crumpled into a ball, and placed on surface of a 3-qt (3-1) beaker of water. A timer is started when the ball hits the water and the amount of time for the ball to be completely wetted by the water is measured. Five sets of five balls are tested to obtain an average measurement. Absorbency is measured on fresh tissue samples immediately after manufacture and on samples aged at least two weeks. This is important because absorbency will decrease over time, as the coating agents cure on the surface of the tissue.
The density of the tissue is also measured with a thickness tester to evaluate how thick cloth is, then its mass, volume, and area are calculated. Linting (the amount of loose lint which detaches from the tissue) is measured by abrading a sample against a piece of black wool by a motor driven device known as the Sutherland Rub Tester. Colormetric analysis can then be used to determine the quantity of lint transferred to the wool.
The increased environmental concern about waste chemicals may lead to improved lotion formulations employing biodegradable or recyclable raw materials in the future. The industry is continually researching ways to make the manufacturing process faster and more energy efficient. Finally, methods may be developed to improve the strength of nonwoven fabrics without sacrificing the pleasant tactile characteristics that make lotion-coated tissues so desirable.
Where to Learn More
Ingman, Lars C. "Accepting 'The New.' (new products and techniques in the paper industry)" Pulp & Paper, 62, no. 5 (May 1988): 127(2).
"Nonwovens: making a better product," (International Textile Machinery Association)Textile World 141, no. 12 (Dec 1991):63(4).
"TAPPI brings librarians and paper industry together." (Technical Association for the Pulp and Paper Industry), Library Journal 113, no. 20 (Dec 1988):35(1).
"Pulping conference focus is energy conservation, process optimization." Pulp & Paper 55 (Dec 1981):108(3).