Most webs have insufficient strength in the unbonded form. Individual fibers or filaments must be tied together by gluing, thermally bonding or mechanically entangling.

LATEX RESIN BONDED
Latex bonding is a common technique. A web, supported on a moving belt or screen, has an adhesive resin called a binder applied to it by dipping the web into the binder and removing the excess, or by spraying, foaming or printing the latex onto the web. These methods of application are simple and can also be used to color the webs by adding pigments to the binder solutions. However, the process requires large amounts of heat to remove water and dry and set the binder into the fabric. More heat is needed for dipping or printing compared to that needed for spraying or foaming. The largest end-uses for resin bonded staple nonwovens are cover stock, wipers, fabric softener substrate and interlinings. This class of nonwovens has lost share in these markets over the last decade as thermal bonded, spunbonded and spunlaced nonwovens replaced resin bonded versions in numerous applications.

THERMAL BONDED
The use of thermal bonding techniques has grown significantly. In these methods, fiber surfaces are fused to each other either by softening the fiber surface, if it melts at low temperatures, or by melting fusible additives in the form of powders or fibers. Bonding powders and fibers can be blended in with the web fibers before the web is formed or they can be sprayed on and into the web with a spray gun. The spray gun for nonwovens is predominantly the non-electrostatic type. Electrostatic guns can be used for webs that are capable of being charged. Two common thermal bonding methods are through-air heating and calendaring. The through-air method uses hot air to fuse fibers within the web and on the surface of the web to make high loft, low density fabrics. Hot air is either blown through the web in a conveyorized oven or sucked through the web as it passes over a porous drum within which a vacuum is developed. In calender point bonding the web is drawn between heated cylinders that have an embossed pattern so that only part of the web is exposed to extreme heat and pressure. This type of calendering produces strong, low loft fabrics. Ultrasound in the form of ultrahigh frequency energy, applied to small areas, can also be used to cause localized fusion and bonding of fibers, thereby creating a pattern for web stabilization, laminating or quilting. Thermal bonding fibers and powders are made from fusible polymers such as polyethylene, polypropylene and polyester. When calender bonding is employed, the binder fibers used are often monocomponent. The fusing process destroys their shape at the bond point, but retains it in the unbonded regions, as the polymer softens and flows to form the bond between fibers. In bicomponents fibers, one polymer, the low melting component, either covers all of the surface of the higher melting component (sheath/core structure), or is extruded alongside the higher melting component (side by side structure). During fusion and subsequent bonding, the low melting component softens and flows to form the bond while the high melting component maintains its fiber shape and thereby its structural integrity. Approximately half of North American consumption of thermal bonded carded fabrics is in cover stock. Thermal bonded carded fabrics have also gained share at the expense of resin bonded nonwovens in the interlining market.

SOLVENT BONDING
Solvent bonding can be used, for a few solvent susceptible fibers, to partly dissolve their surfaces and thereby create an adhesive of themselves. Removing the solvent causes resolidification of the fiber surface and bonding at the fiber crossover points.

MECHANICAL BONDING
Mechanical bonding, the oldest technique for consolidating a web, is used to enmesh or entangle fibers to give strength to what are usually dry-laid webs. The most common methods are needlepunching and hydroentangling (also called spunlacing). Mechanical bonding methods typically are slower than binder and thermal bonding. However, they yield advantages in strength and aesthetics. Needlepunching is preferred to heavy fabrics, such as those used in geotextiles and for heat and sound insulation, while spunlacing produces soft, lighter weight fabrics used in disposable hospital goods, wipes and home furnishings.

ULTRASONIC BONDING
Ultrasonic bonding is similar to thermal bonding. This process can bond a single nonwoven web or laminate several webs together, including film. In this process, the nonwoven material or materials are drawn between a 'horn', which produces high frequency sound waves, and a rotary calendar, referred to as the 'anvil'. The sound energy generates localized heat through mechanical vibration at the anvil's embossing points to fuse the material. The process is cool, energy efficient and often used to bond or laminate fabrics which would be affected by the other more heat intensive thermal bonding processes. The technology continues to evolve and ultrasonic bonding speeds are rivalling thermal calendar bonding capabilities.

NEEDLEPUNCHED
In needlepunching, barbed needles are punched through the web, hooking tufts of fibers through it and bonding it in the needlepunched areas. The needles enter and leave the web while it is trapped between two plates called a bed plate and stripper plate. The web is pulled through the needle loom by draw rolls. Sometimes needle looms with less closely spaced needles, called tackers, are used to give the web dimensional stability before it enters the main needle loom. The production of needlepunched fabrics starts with carded, air laid or spunbonded webs that are characteristically bulky. Looms are made to needle webs from the top, the bottom, and from the top and bottom. The largest needle looms are for making papermakers felts. These felts make up the highspeed belts that squeeze water out of pulp as it is being formed into paper in the papermaking process. The design of a needle for every application reflects the balance between its fiber tuft carrying capacity, the damage it may do in breaking and abrading the fibers and the function and aesthetics desired in the product. Exciting new loom and needle designs have permitted the production of ribbed and velour surfaces for carpeting, wall coverings and other textured surfaces. For example, a rib pattern is obtained when forked, instead of barbed, needles are used to carry the tufts into channels instead of the customary perforated bed plate. When the needles have a crown profile, and the bed plate is replaced with a bristle brush bed, a random pile velour results. The two largest markets for needlepunched nonwovens are automotive trim and geotextiles. Other major end-uses, in order of importance, are coated/laminated fabric backings, bedding and home furnishing materials, filters, interlinings, roofing and landscape fabrics. There is an extraordinarily large array of specialized end-uses for custom-designed needlepunched materials. A few examples include draft and decorative felts, desiccant materials, insulation, apparel linings, diaper soaker pads, lubricating pads, marine materials, noise absorbent insulation, office partition fabrics and polishing pads.

SPUNLACED
The Spunlaced (hydroentangled) process uses fine, high velocity jets of water to impact a fibrous web and cause the fibers to curl and entangle about each other. The water jets perforate the web and entangle the fibers, producing fabrics that reflect the pattern of the forming belt which carries the web under the water jets. This produces a fabric with a conventional textile fabric appearance and excellent drapability. Binder is not required. However, a small amount of binder is added to some spunlace fabrics to increase their strength and dimensional stability or to make them liquid repellent. The process is used predominantly on dry laid webs. More recently it has been successfully used on wet laid webs. A lower energy version of spunlacing, using lower velocity water jets, gives products that require a significant amount of binder to improve their strength since the fibers are less entangled. Medical packs and gowns are the largest spunlaced application by far. Wipers are the second largest spunlaced end-use followed by medical sponges. A very large number of applications make up the remainder. These include protective industrial apparel, interlinings, mattress pad fabrics, absorbent product components, coated fabric substrates, reinforced plastic components, window treatments and other home furnishings, other medical supplies, automotive components, fire block fabrics, roofing materials, wall coverings, scrub apparel, filter fabrics, geotextile materials and other advanced composites.

STITCHBONDED
A mechanical bonding process called stitchbonded uses a continuous filament or staple yarn to lock a web of unbonded fibers into a fabric with a stitch pattern. This method is used in applications such as shoe components, mattress ticking and coating substrates.


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