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Powder bonding technology: nonwoven structures and laminates.

POWDER BONDING TECHNOLOGY Powder bonding technology is perhaps the most advanced technology available that affords a manufacturer substantial opportunities to develop new, creative products, provides for a high level of process versatility and yields value added products unlike that of other processes.

The dry powder bonding process is relatively simple but with proper design can be highly versatile. Figure 1 illustrates the typical sequence of equipment for a powder bonding line to manufacture nonwoven fabrics.

Staple fibers are fed into a blender-opener system and chute fed to the individual cards. The carded webs are then layered onto the card conveyor to form a single web. The web is spread to the desired width and conveyed to the adhesive conveyor, where it is transported underneath a dry powder distribution device such as a spray nozzle or a scatter coating applicator. Dry powder is applied to the web at the desired weight level. The weight percentage adhesive applied to the fabric depends upon the end use requirements for strength, softness and wash performance. Use levels of about 20% of the total product weight are common.

The web containing dry powdered adhesive is then conveyed through the first oven, which is infrared heated. The powdered adhesive absorbs energy and melts to a viscous liquid. At this point the web is still neither bonded sufficiently for handling nor for removal from the conveyor belt. The adhesive-activated web must first be condensed through a nip assembly to force adhesive flow through the thickness of the web, to enhance wetting and to impart strength to the nonwoven. The nip surface is controlled at a temperature below the melt point of the adhesive. At the nip point the adhesive cools and crystallization is initiated. The fabric at this point has a sufficient level of properties for additional handling and for selected end uses.

The bonded web is conveyed to a second oven, which may be identical in construction and design to the first. The second oven may be viewed as a finishing oven. It is used to impart additional strength, to impart bulk or loft to the structure and/or to change fabric hand. The second oven can also be used for laminating.

The line schematic in Figure 1 shows both ovens to be identical, with the conveyor belt passing through the nip assembly. For ease of operation and to eliminate belt wear it is desirable not to have to drive heated belts through a nip assembly. Alternate designs for the second oven have been developed and additional developments are likely. Through-air ovens and free-standing nip rolls are two possible alternatives.

Minor modifications and additions to the basic powder bonding production line give lamination capabilities. The placement of unwind stands, tensioning devices and automatic splicing equipment between the two ovens gives the capability of laminating in-line, various materials to the adhesive-activated nonwoven being produced. Materials such as films, scrims, tissue and foam are fed over the first nip roll assembly to force contact with the adhesive-activated nonwoven web, thereby yielding the laminated product. Temperature and nip pressure control in the second oven are often used to alter the properties of the laminate formed at the first oven.

Raw Materials--Fibers And Adhesives

The powder bonding process is not only versatile because of the possible line configurations and process variables, but also because of the range of raw materials that might be used. To form the basic web one can choose from a range of fiber types, including polyester, rayon, nylon, cotton and acrylics.

The other area of raw material selection that provides for enhanced ability for new product development is that of adhesive choice. Basically, any material that is thermoplastic is a potential candidate for an adhesive in powder bonding. Polymers in powder form have been used on fusible interlinings for garment construction since the mid-1960s. Adhesive development for powder bonding through the past five years is an extension of this earlier technology. Adhesives that are currently available include polyesters, polyamides, polyethylene and polyurethanes. The most commonly used adhesives for powder bonding are crystallizable terephtalate copolyesters containing one or more dibasic acids and one or more glycols.

Within a given generic adhesive type there are a number of physical properties of the adhesive that can give substantial differences in fabric properties. Important properties to consider in selecting an adhesive for the development of specific nonwoven products and laminates are melting point), heat of fusion, glass transition temperature, crystallization half times, melt viscosity, adhesive film tensile strength, modulus and elongation. These properties can be determined by the polymer chemistry and are in many cases interactive.

In addition to forming fabrics used powdered adhesives, binder fibers may also be used. Sometimes it is useful to incorporate both binder fiber and powder within the same structure.

Powder-Based Nonwovens And Laminates

A variety of product concepts can be fully developed through powder bonding technology to meet the needs of various markets for nonwovens, composites and laminated structures. It is the versatility of the process that is important to demonstrate, rather than specific properties of any given commercial product.

Among the products that can be produced by powder bonding are flat nonwovens that compete with fabrics made from spunbonded, chemical bonded (latex), wet laid and thermal calendar bonded technologies. Commercial powder bonded, flat nonwoven fabrics are manufactured over a weight range of about 15-60 grams sq. meter. These fabrics are soft and drapable similar to calendar bonded products, but can be made from a range of fiber types and at heavy weights with improved strength. The fabric hand is substantially improved when compared to that of most spunbondeds, wet laids and latex bonded structures. Powder bonded nonwovens can also withstand multiple cycles of home washing and drying, as well as dry cleaning. Products developed from powder bonding technology serve markets for both durable and disposable end uses.

Powder bonded fabrics can be subsequently bonded to other materials with the application of heat and pressure. The adhesive used to form the fabric is reactivated with the application of heat. If higher bonded strength is required in subsequent lamination, the amount of adhesive can be increased. Some webs are manufactured and marketed to serve as adhesive webs where the greater percentage of product weight is adhesive.

Powder bonded nonwovens may be used in dielectric and sonic bonding applications. Because the adhesive is highly thermoplastic and the web has high elongation, powder bonded nonwovens are easily molded.

Bulked Powder Bonded Nonwovens

The dry powder bonding process can be used to provide a flat product or a bulky product from the same base fabric. Selected, high crimp recovery, polyester fiber (3-15D/F) used in conjunction with low melting polyester powdered adhesives give excellent bulk potential. The nonwoven web containing the adhesive is heated sufficiently to melt the adhesive and is then nipped to form a thin, flat, densified fabric. The densified fabric can be continuously made and bulked on line or it can be shipped as a densified, thin product and bulked by the customer with a subsequent heating process. Bulking can be accomplished using through-air ovens, infrared head or steam. The increase in thickness upon bulking of the densified fabric is normally 300-500%.

Laminates And Composite Structures

Through the past several years, laminates and composite structures have been developed using powder bonding technology. Some of these products are made using the lamination capability only, while others combine various materials to the nonwovens being produced on-line. Some examples of laminates and composite structures made on production equipment are listed in Table 1.

As can be seen, the combinations of materials are practically unlimited. In some of the examples, the products were developed for specific identified markets, in others the work was done to develop knowledge and to pursue curiosity. Commercial products have evolved from both endeavors. Often excellent new products come from combinations of two or more competitive products. Powder bonding technology provides unique capabilities to explore these possibilities.

For example, in certain markets powder bonded products compete with selected spunbonded products, while in other markets a combination of the two fabrics best serve the customer when neither product by itself is sufficient. A combination of a stiff, high strength spunbonded fabric and a soft powder bonded fabric can result in a product that has the advantageous properties of each. This concept has been used to achieve fabrics with improved aesthetics and high strength for some applications and improved functional properties in others.

The combination of spunbondeds with powder bonded fabrics will find substantial applications in areas such as filtration, where high strength may be required along with improved filtration that can be accomplished using fine denier staple fibers in the powder bonded structure. Additionally, in formation of combination structure with spunbondeds and powder bonded fabrics it has been found that the resulting strength is often substantially higher than the sums of the strengths of the individual fabrics.

Finally, the versatility of powder bonding might be best demonstrated if one could simultaneously take advantage of all the process capabilities as well as the raw material variables. Imagine a product that has a layer of fibers on one side that is flat, condensed and provides a smooth surface. On the opposite side is a bulky structure several times the thickness of the flat fabric. In the center between these two structures is an open scrim (eg. four ends X four picks/inch). Either or both layers of nonwoven may contain one or more fibers in an intimate blend. The product might be offered in densified form with subsequent rebulking or it might be offered as a bulked structure. This type product doesn't have to be imagined; it, as well as other similar products, has been made on production powder bonding equipment.

Powder bonding is expected to continue to develop and grow as the market demands more sophisticated nonwoven, laminate and composite structure. The capability and versatility afforded by the technology should allow it to play an important role in the future of nonwovens.
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Author:Harris, Frank
Publication:Nonwovens Industry
Date:Oct 1, 1990
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