Mechanical bonding, the oldest technique for consolidating fibers in a web, entangles the fibers to give strength to the web. The two most widely used methods are needlepunching and spunlacing, also known as hydroentanglement. Spunlacing uses high-speed jets of water to strike a web to intermingle the fibers. Spunlaced nonwovens made by this method have specific properties such as soft hand and drapability.
The process employs jets of water to entangle fibers and thereby provide web and eventually fabric integrity. Softness, drape, conformability, and relatively high strength are the major characteristics that make spunlaced products unique among all nonwovens.
The technology was officially introduced by Wilmington, Del.-based DuPont in 1973. DuPont obtained five patents on spunlaced nonwovens within the period from 1963 to 1970. Since the 1990s, the technology has been made more efficient and affordable for more manufacturers. There are many specific terms for spunlaced nonwovens: jet-entangled, water-entangled, and hydroentangled or hydraulically needled. However, the term "spunlaced" is the most popular today.
Basically, every nonwoven web-forming process can be applied in the spunlacing process. The general properties of web forming are as follows:
• Isotropic precursor webs can be produced by airlay systems.
• Carding webs can result in final products that have higher machine direction (MD) strength than cross direction (CD) strength. An MD:CD ratio of 1:1 would be the perfect isotropic nonwoven.
• Wet-formed webs especially can be produced with good MD:CD characteristics. The combinations of various types of precursor webs provide numerous options for using in the spunlace process to create various composites.
The formed web is first compacted and pre-wetted to eliminate air pockets, and then water-needled - or hydroentangled. The water pressure generally increases from the first to the last injectors. The jets exhaust most of the kinetic energy primarily in rearranging fibers within the web and, secondly, in rebounding against the substrates, dissipating energy to the fibers. A vacuum within the roll removes used water from the product, preventing flooding of the product and reduction in the effectiveness of the jets to move the fibers and cause entanglement.
Both the fiber and web properties have primary effects on the performance of the finished product. These parameters comprise the web material and area basis-weight, and the way in which the web was manufactured. Spunlace technology demands a high-quality web, especially in its uniformity and isotropic orientation.
Spunlacing can be carried out using drylaid - carded or airlaid - or wetlaid webs made of natural or man-made fibers, or blends. Cellulosic fibers are hydrophilic, chemically stable and relatively colorless. In general, low-micronaire cotton is not recommended for hydroentangled nonwovens because of a higher number of neps and small bundles of entangled fibers, resulting in unsightly-appearing fabric. In spite of this, fabrics made with lower-micronaire fiber show higher strength, probably caused by a higher number of fine fibers and greater surface area. In addition, greige cotton has been used in spunlace technology. It has been shown that the absorbency rate increases with increasing hydroentangling energy. This is the result of oil and wax removal from the fiber surface. These nonwovens subsequently can be bleached, which should raise the strength of the fabric.
All success in the competitive, but highly profit-yielding, market of technical textiles and nonwovens is based on experience and know-how in traditional textile products manufacturing. Some cotton spinners already are working successfully with spunlacing lines. As traditional textile manufacturers, they know how to handle fibers, so there basically is no big difference between spinning and nonwovens, at least at the start of the production process. Fibers are fibers; a card is a card.
High-water-pressure machines mostly are used because when using high pressure, energy can be delivered into a web with fewer water needles and less water. This is economically beneficial. Another basic process parameter having influence on the fabric is the speed of the line. If a constant amount of energy is being delivered to a fabric, the speed of the fabric determines how much energy is going to be absorbed per fabric unit area. Logically, the higher the line speeds, the less the energy that is absorbed by the fabric and the lower the fabric strength that is achieved.
Spunlaced fabrics show high drape, softness and comfortable hand because increased fiber entanglement leads to increased strength without an increase in shear modulus. The softness of the fabric is explained by the fact that the entangled structures are more compressible than bonded ones as well as having mobility and partial alignment of fibers in the thickness direction. The absence of a binder is seen to result in a fabric with more textile-like appearance.
The substrate texture seems to have an important influence on the product. The size of perforations is usually measured in mesh, which is the count of wires per inch of the substrate. Imposing the same energy into two webs with different substrate meshes, the finer substrate yields a stronger product resulting from finer support. A coarser wire support, say, 20 mesh, results in a bulkier product with more permeability, but less strength. Water removal from the fabric has been shown to be dependent on the mesh of the support belt.
For spunlacing, there are two main technology suppliers on the market: Germany-based Fleissner GmbH, a daughter company of Germany-based Trützschler GmbH & Co. KG; and France-based Rieter Perfojet, a subsidiary of Switzerland-based Rieter Textile Systems. Both companies have a lot of experience and are able to supply turnkey installations. It is of utmost importance to install compatible lines without any production problems along the different stages of production. Both companies have their own pilot lines - at Egelsbach, Germany, for Fleissner, and at Montbonnot, France, for Rieter Perfojet. The technical centers enable trials to be performed to ensure the equipment configuration meets the relevant needs.
Fleissner's AquaJet Spunlace System
With its AquaJet Spunlace System, Fleissner supplies machinery for spunlacing, thermal bonding, chemical bonding and general finishing processes including impregnation and drying. Germany-based Erko Trützschler Nonwovens GmbH, its sister company, brings technically advanced equipment for fiber opening, blending, carding, airlaying and needlepunching processes to the table. The AquaJet spunlacing system was put on the market by Fleissner more than 10 years ago and has been used for bonding of all kinds of nonwovens from low weights of 20 grams per square meter (g/m2) to heavy weights of 800 g/m2. Natural and man-made fibers as well as spunbond webs can be processed. More than 80 AquaJet lines have been delivered so far. Standard speed ranges from 5 to 300 meters per minute (m/min). Speeds of up to 600 m/min are possible for spunbond applications.
Rieter's JETlace® Essentiel Configuration
Rieter Perfojet's new JETlace® Essentiel configuration enables the production of most of the lightweight polyester and viscose fabrics in the range of 30-80 g/m2 for hygiene, medical and wipes applications. With two cards, the machine is able to produce any blended products using polyester, viscose and cotton. Four different widths can be selected as well as the machine direction, which can be changed onsite during installation. The equipment is defined, optimized and located in order to reduce maintenance and operating effort. Only four weeks for installation and four weeks for start-up are required, according to Rieter. Production efficiency of up to 92 percent can be expected, and such a line will be able to produce 2 tons per hour.
The market trend is to create fabrics that exhibit patterns, thus creating consumer product differentiation and brand recognition. The ability to create designs on the JETlace 3000 fabric is now possible with the patent-pending patterning sleeve technology. This sleeve design enables special fabrics to be made without affecting the line speed, thus enabling the line to operate at optimal and efficient speeds. This system allows production of a large range of designs and logos for personalizing nonwovens. Patterning is achieved via an embossed cylinder located prior to the final conveyor of the JETlace 3000, following the initial entanglement stages.
Bonding And Softening Spunbonds
The combination of spunbond production and spunlacing is predestined for very high production speeds. The technical design of the Fleissner AquaJet drums is especially favorable for optimal dewatering, which is of decisive importance for high production speeds and the resulting centrifugal forces of the water to be removed.
Fleissner, in cooperation with Germany-based Reifenhäuser Reicofil, has successfully optimized spunlacing of lightweight spunbonds directly after spunbond formation. Much higher strengths have been achieved compared to normally calendered spunbonds with volumes or web thicknesses doubled. These nonwovens are characterized by a very soft touch. Together with other renowned European producers of spunbond lines, Fleissner also has developed spunlaced spunbond products.
An alternative to produce more bulky and absorbent nonwovens is to combine spunbond, pulp and staple fibers. The special advantage here lies in the use of spunbonds for increased web strength while at the same time reducing web weight. Moreover, optimized bonding of the three-layer webs results in a soft touch. Two-layer webs made of staple fibers combined with tissue for surgical gowns or cover sheets are successfully produced using AquaJet lines, and a special filtration system was developed for the circulating water. Fleissner AquaJet lines for multi-layer nonwovens made of fibers combined with pulp are said to have been in use successfully for years.
Key To Spunlace Line Efficiency
Water is the active medium of spunlace technology. With poor filtration quality, the negative impact of clogged strips reduces bonding energy, creates some jet line defects and consequently decreases product quality. The filtration process required to capture the spin finish is based on different filtration steps, such as the use of fine-particle sand filters followed by bag filters. Sand filtration is the main process for capturing spin finish. The fineness of the sand particles used in this type of filter, along with the sand-bed surface area and depth, creates an optimum filtration media.
Additional filtration steps have to be considered when high pollution is anticipated, up to 600 milligrams per liter, examples being the processing of cotton, tissue or pulp. Installation of flotation cells and in some cases large-particle sand filters is required to remove larger-particle elements. These filters will be the preliminary filtration step prior to the fine-sand filter and bag filter processes. The flotation may use some chemicals to help the particle group to be captured by the fine air bubbles. Furthermore, with the recent and unavoidable future implementation of environmental regulations, nonwovens producers will have to pay more attention to their plant waste rejects and filtration systems.
The JETlace 3000 spunlace machine continuously recycles 99 percent of the process water, in order to reduce costs and/or comply with any local restrictions. Actually, the range of fibers employed determines the scope of the filtration plant, with each filtration step incrementally focused on a reduced micron particle capture.
Typical fibers for wipe products are polypropylene, polyester and viscose. Fiber fragments are minor losses because most of the pollution comes from the spin finish. This chemical spin finish enhances man-made fibers carding processability. Most of it is then washed off the fiber into the process water used by the water jets. The spin finish behaves as an emulsion within the process water. Even with its small size in the micron range, the difficulty comes from the ease with which it aggregates into gel-like groups. It is then capable of affecting the performance of the injector's strips and, therefore, product quality.
Water Analysis And Jet-Strip Cleaning
The manufacture of spunlaced products involves forcing water at high pressure through hundreths-of-a-millimeter-wide openings in the jet strip. According to Germany-based Groz-Beckert KG, manufacturer of HyTec® jet strips, the process entails large quantities of process water flowing through the individual jets. Contamination of the process water leaves deposits that clog the jet area. Potential pollutants can include fiber residues, rust, metal particles, limescale particles and chemicals added to the process water. With time, the resulting deposits inside the jets gradually diminish the diameter, thus impeding the flow of water. Contaminants in the entrance area of the jet opening exert a detrimental effect on the formation of the water jet and consequently on the shape of the water curtain. This negatively impacts the physical characteristics of the spunlaced product. Consequently, jet strips require professional cleaning at regular intervals to remove deposits, while care must be taken to protect the sensitive structure of the jet strip. Aspects such as work safety and environmental protection also play an important role. Groz-Beckert provides cleaning instructions for the HyTec jet strips.
Groz-Beckert notes that the quality of process water used is decisive to the manufacture of spunlaced products. The properties of the water influence the service life of various machine components and of the jet strips. The process water also is instrumental to the quality of the end product, and not only when manufacturing hygiene products. Ensuring hygienically sound water quality is therefore essential. Water is a highly versatile medium and can exhibit varying levels of acidity or alkalinity as well as occur in partially or fully deionized form. It can also contain microorganisms, metal particles and other inorganic substances in varying concentrations. Alongside pH value, water hardness and chloride content, there also are a large number of other characteristics that determine water quality. Groz-Beckert recommends its HyTec process water analysis for water-jet hydroentangling systems to precisely analyze the process water in use. The company can perform a comprehensive laboratory analysis at its premises to examine the process water with respect to important hydrochemical parameters.
Spunlaced products have a broad range of applications such as wipes, medical and surgical, cotton pads, coating substrates and other industrial end-uses such as roofing felts and geotextiles; as well as apparel. Fleissner supplies a composite AquaJet spunlace line for an entirely new fabric product suitable for apparel use. The fabric features a combination of an electrospun nanoweb and splittable polyester fibers. Applications include artificial leather and premium sports textiles.