The nonwoven textile fabric industry had an exploratory beginning in the late 1940s which entered into a development phase in the 1950s followed by commercial expansion in the 1960s. During the next 35 years, the nonwoven industry matured and established markets for nonwoven fabrics by either providing cost-effective performance as alternatives to conventional textiles or providing products specifically developed for targeted end-uses. The industry has survived recessions better than conventional textiles and has grown at a faster rate. Its health and safety problems are similar to those of the rest of the textile industry (i.e., noise, airborne fibres, chemicals used in bonding fibres, safe working surfaces, pinch points, burns from thermal exposures, back injuries and so on).
The industry generally has a good safety record, and the number of injuries per standard work unit is low. The industry has responded to challenges associated with clean water and clean air acts. In the United States, the Occupational Safety and Health Administration (OSHA) has promulgated a number of worker protection rules which require safety training and manufacturing practices that have improved worker protection significantly. Responsible companies throughout the world are adopting similar practices.
The raw materials used by the industry are generally similar to those used in conventional textiles. The industry has been estimated to use almost 1 billion kg of a mix of raw materials annually. The natural fibres used are predominately cotton and wood pulp. The manufactured fibres include rayon, polyolefins (both polyethylene and polypropylene), polyesters and, to a smaller degree, nylons, acrylics, aramids and others.
There was an early growth in the number of nonwoven processes to approximately ten. These include; spunbond, melt blown, air-laid pulp and blends, wet laid, dry laid (bonded by either needlepunching, thermal bonding or chemical bonding) and stitch bonding processes. In the United States, the industry has saturated many of its end-use markets and is currently searching for new ones. A major growth area for nonwovens is developing in the area of composites. Laminates of nonwovens with films and other coatings are broadening markets for nonwoven materials. The storage of nonwoven roll goods has recently come under scrutiny because of the flammability of some products that have very low densities and high surface areas. Rolls whose volume-to-weight ratio is greater than a certain roll loft factor are considered to pose storage problems.
The volume of bleached cotton used in nonwoven fabrics has been steadily increasing, and cotton-polyester and rayon-polyester blends in nonwoven fabrics, bonded by hydroentangling, have become attractive combinations for medical and feminine hygiene applications. There has been an interest in using unbleached cotton in nonwoven processes, and some attractive experimental fabrics have been produced through the use of the hydroentangling process.
Rayon has encountered some pressure from environmentalists who are concerned about the impact that by-products of the process have on the environment. Some rayon-producing companies in the United States abandoned the industry rather than face the cost of complying with regulatory requirements imposed by the clean water and air acts. Those companies that chose to meet the requirements now appear to be comfortable with their modified processes.
Wood pulp fibres are a major component of disposable diapers, incontinence products and other absorbent products. Fibres from hardwood and kraft fibres are employed. In the United States alone, use of pulp fibres totals more than 1 billion kg annually. A small percentage is used in air-laid nonwoven processes. The products are popular as towels in applications which range from the kitchen to sports.
The two most popular polyolefin fibres are polyethylene and polypropylene. These polymers are either converted into staple-length fibres which are subsequently converted into nonwoven fabrics, or else converted into spunbonded nonwoven fabrics by extruding the polymers to form filaments which are formed into webs and bonded by thermal processes. Some of the fabrics produced are converted into protective apparel, and by 1995, more than 400,000,000 coveralls had been made using a popular spunbonded polyethylene fabric.
The largest single use for a nonwoven fabric in the United States (approximately 10 billion square metres) is as the cover sheet in disposable diapers. This is the fabric which contacts the baby’s skin and separates the baby from the other diaper components. Fabrics from these fibres are also used in durable products and in some geotextile applications where they are expected to last indefinitely. The fabrics will degrade in ultraviolet light or some other types of radiation.
Thermoplastic fibres from polyester polymers and copolymers are widely used in nonwovens in both staple fibre and spunbonded processes. The combined volume of polyester and polyolefin polymers used in the United States in nonwoven fabrics has been estimated to be more than 250 million kg annually. Blends of polyester fibres with wood pulp which are wet laid and then bonded by hydroentangling and subsequently treated with a repellent coating are widely used in disposable surgical gowns and drapes. By 1995, the use of disposable medical nonwovens in the United States alone exceeded 2 billion square metres annually.
Nylon fibres are used only sparingly in the form of staple fibres and in a limited volume in spunbonded nonwovens. One of the largest uses for spundbonded nylon nonwovens is in the reinforcement of carpet pads and in fibreglass filters. The fabrics provide a low friction surface to carpet pads that facilitates the installation of carpets. In fibreglass filters, the fabric helps retain the fibreglass in the filter and prevents glass fibres from entering the filtered air stream. Other specialty nonwovens, such as aramids, are used in niche markets where their properties, such as low flammability, recommend their use. Some of these nonwovens are used in the furniture industry as flame blockers, to reduce the flammability of sofas and chairs.
Spunbonded and meltblown
In the spunbonded and meltblown processes, suitable synthetic polymers are melted, filtered, extruded, drawn, charged electrostatically, laid down in web form, bonded and taken up as rolls. The process requires good safety practices common to working with hot extruders, filters, spinnerets and heated rolls used for bonding.
Workers should wear proper eye protection and avoid wearing loose clothing, neckties, rings or other jewellery that may be caught in moving equipment. Also, these processes almost always involve the use of large volumes of air, and special precautions must be taken to avoid designs that might lead to fires, such as placing light ballasts in an air duct. Extinguishing a fire in an air duct is difficult. It is important to maintain safe working-floor surfaces, and the floors around any nonwoven equipment should be free of contamination that can lead to unsafe footing.
Spunbonded and meltblown processes call for cleaning some of the process equipment by burning away any accumulated polymer residue. This usually involves the use of very hot ovens for both cleaning and storing the cleaned parts. Obviously, these operations require proper gloves and other thermal protection, as well as appropriate ventilation to reduce heat and exhaust fumes.
Spunbonded processes owe their economic advantages in part to the fact that they are relatively fast and the take-up rolls can be changed while the process continues to run. The design of the roll-changing equipment and the training of the operators should provide for an adequate margin of safety to handle these changeovers.
Processes that involve opening of bales of fibres, blending the fibres to provide a uniform feed to a carding machine, carding to form webs, cross-lapping the webs to provide optimum strength in all directions and then forwarding the web to some bonding process are similar in their safety requirements to conventional textile processes. All exposed points that could trap a worker’s hands in roll interfaces need protection. Some dry-laid processes involve the generation of small amounts of airborne fibres. The worker should be provided with adequate respiratory PPE in order to avoid inhalation of any respirable part of these fibres.
If the webs formed are to be bonded thermally, there will normally be a small amount (on the order of 10% by weight) of a lower-melting fibre or powder that has been blended into the web. This material is melted by exposure to a hot air oven or to heated rollers and then cooled to form the fabric’s bonds. Protection against exposure to the heated environments should be provided. In the United States, approximately 100 million kg of thermally bonded nonwovens are produced annually.
If the webs are bonded by needle punching, a needle loom is used. An array of needles is mounted in needle boards, and the needles are driven through the web. Needles capture surface fibres, carry them from the top to the bottom of the fabric and then release the fibres on the return stroke. The number of penetrations per unit area can range from a small number (in the case of high-loft fabrics) to a large number (in the case of needled felts). A loom may be used for needling from both the top and bottom sides of the web and for use with multiple boards. Broken needles must be replaced. Safety-locking the looms is required in order to prevent accidents during such maintenance. As in the case of carding, some small fibres may be generated by these processes, and ventilation and respirators are recommended. In addition, eye protection is advised to protect against flying debris from broken needles. In the United States, approximately 100 million kg of needlepunched nonwovens are manufactured annually.
If the webs are bonded by chemical adhesive, the process normally calls for spraying the adhesive on one side of the web and passing it through a curing area, normally a through-air oven. The web direction is then reversed, another application of the adhesive is made and the web is sent back through the oven. A third pass through the oven is sometimes used if needed to complete the curing process. Obviously, the area must exhaust the oven gases and it is necessary to capture and remove any toxic effluents (in the United States, this is required by various state and federal clean air acts). In the case of adhesive bonding, there has been worldwide pressure to reduce the release of formaldehyde into the environment. In the United States, the EPA has recently tightened limits on the release of formaldehyde to one tenth of the previously acceptable limits. There are concerns that the new limits challenge the precision of currently available laboratory methods. The adhesive industry has responded by offering new binders which are formaldehyde free.
There is some nomenclature confusion in regard to air-laid nonwovens. One of the variations of carding processes includes a card that includes a section that randomizes the fibres being processed in an air stream. This process is often referred to as an “air-laid nonwoven process”. Another, very different, process, also called air laid, involves the dispersion of fibres in an air stream, usually using a hammer mill, and directing the airborne fibre dispersion to a device that deposits the fibres on a moving belt. The web formed is then spray bonded and cured. The laydown process may be repeated in line with different types of fibres to produce nonwoven fabrics from layers with different fibre compositions. The fibres used in this case can be very short, and protection to prevent exposure to such airborne fibres must be taken.
The wet laid nonwoven process borrows technology developed for making paper and calls for the formation of webs from dispersions of fibres in water. This process is assisted by the use of dispersion aids that help avoid non-uniform clumps of fibres. The fibre dispersion is filtered through moving belts and dewatered by pressing between felts. At some point in the process a binder is often added which bonds the web during the heat of drying. Alternatively, in a newer method, the web is bonded by hydroentangling using high-pressure jets of water. The final step involves drying and may include steps to soften the fabric by microcreping or some other similar technique. There are no known major hazards associated with this process, and the safety programmes normally are based on common good manufacturing practices.
This process is often excluded from some definitions of nonwovens because it can involve the use of yarns to stitch webs into fabrics. Some definitions of nonwovens exclude any fabrics which contain “yarn”. In this process a web is presented to conventional stitchbonding machines to produce knit-like structures that offer a wide variety of combinations including the use of elastic yarns to produce fabrics with attractive stretch and recovery properties. Again, no exceptional hazards are associated with this process.
Finishes for nonwoven fabrics include flame retardant, fluid repellent, antistatic, softeners, anti-bacterial, fusible, lubricants and other surface treatments. Finishes for nonwovens are applied either on-line or as off-line, post-manufacturing treatments, depending on the process and the type of finish. Frequently, antistatic finishes are added on-line, and surface treatment such as corona etching is normally an on-line process. Flame-retardant and -repellent finishes are often applied off-line. Some specialized fabric treatments include exposing the web to a high-energy plasma treatment to influence the polarity of fabrics and improve their performance in filtration applications. The safety of these chemical and physical processes varies with each application and must be considered separately.