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Ultrasonic bonded nonwovens.

The first ultrasonic lamination concepts were developed in the 1950-60's. These were the result in part of patent research conducted by Ronald Bull (England, patent no. 1,018,971), Geoffrey Manock, Imperial Chemical Industries (Canada, patent no. 1,092,052), Sonobond (U.S., patent no. 2,946,119) and C.A. Robertson, Cavitron Corporation (U.S., patent no. 3,562,041). The technology that was being developed noted the ability to bond two or more plies of material having a common melting point by the use of high frequency vibrations.

As ultrasonic technology was further explored, it was confirmed that this methodology can be applied metal-to-metal, metal-to-plastic and plastic-to-plastic. Composites can be bonded as well, as long as at least 65% of the composite is comprised of material with a melting point. The ultrasonic bonding process is suited, therefore, for plastics such as acrylic, rayon, nylon, polyester, polyethylene, polypropylene, PVA and PVC and for metals such as copper, aluminum, brass, steel, alloys and others.

The ultrasonic process requires the use of a stationary horn and anvil between which the material is compressed. Multiple horns can be stationed side by side and the anvil factor can be supplied by a rotating cylinder. Given this flexibility, ultrasonics can be used in a "Plunge-Bond" discrete process as well as a multiple horn "Stitch-Bond" continuous process. In this first capacity, ultrasonics have been used to bond metal-to-metal helicopter parts. In this second capacity, ultrasonics have been used to make quilts and other bedding. As a discrete process, the technology has been used to seal, stake, tack, label and welt. As a continuous process, ultrasonic bonding can be used to splice/join, quilt, hem, stitch, emboss and slit/seal.

When used with metals, ultrasonics perform well in bending operations as well as joining in that the metal's memory is reconfigured in the process. Some success has been achieved in bonding dissimilar metals that usually cannot be joined by conventional means such as resistance welding. The ultrasonic energy disperses oxides and surface films during the bonding process exposing the underlying metallurgical elements for a more durable bond. Unnecessary with ultrasonics are some of the consumables required by conventional bonding such as fluxes, fillers and cleaning materials. The process is generally environmentally safe and avoids producing embrittlement, lowered mechanical strength or reduced electrical conductivity at the bond point.

Composites are most suited to ultrasonic welding if they are comprised of layers having similar melt temperatures and similar molecular structures. With similar temperatures and structures, layers will reach the bonding stage at the same time. Different temperatures and structures means that one layer will be ready to bond when another is not. When this second layer of different material reaches the bond-ready stage, the first layer may have moved to a "burn' stage.

Other factors - such as throughput speed, material thickness, material density, uniformity across the material surface, a relatively high coefficient of friction and others - also play a role in achieving quality bonds. Fabrics and films that are too thin, for instance, may not present enough fibrous material at the bond point to generate a seal. Among plastics, polypropylene, polyester and nylon tend to perform very well with ultrasonics, whereas PVC and acrylic tend to perform less well because of these characteristics.

In the welding process itself, the layers of material are held together (usually between the anvil and horn in continuous processes) under relatively low static force. A frequency converter ordinarily draws electricity and converts it into high frequency electrical energy while at the same time transferring the energy to a transducer. This transducer (converter) converts electrical energy into vibratory energy. An amplifier system (booster), next in the process, magnifies this vibratory energy, delivering it to the horn, an acoustic tool immediately in contact with the material to be welded. These supra-auditory frequencies disrupt the molecular bonds in the material, causing fusion of the layers at the bond point interface.

Market Drivers And Major Players

Interest has grown in the use of ultrasonics as a bonding methodology especially with the increased use of plastics. The technology requires that the materials bonded have a meltpoint. In an economy where most products are made of wood, leather, wool, cotton, brick and/or similar nonmelting materials, ultrasonics could not be used. Because of synthetic materials and fabrics, the market opportunities for ultrasonics have a clear and growing basis. The following describes various ultrasonic sealing considerations.

Aseptic Seals. Other bonding methodologies for both fabrics and metals are presented with both micro-and macro-contaminants, both organic and inorganic. Conventional needle and thread, adhesive bonding, heat sealing and the like are often associated with fumes, particles, micro-organisms and other bits and pieces that make a contaminant free environment more difficult to maintain.

Conventional metal. bonding methodologies such as welding, soldering and similar techniques are associated with harmful gases or macrocontaminants such as sparks, burrs and the like. The production of invasive medical devices, ingestible consumer products and electronic storage, discs require aseptic, tamper resistant, dust-free sealing. Ultrasonic bonding methodologies require fewer consumables (needles and thread), less down-time for repair and maintenance and cleaner operations. Ultrasonics represents an opportunity to replace more complicated processes with a simpler, more maintenance free, more automated and cleaner technology.

Hermetic Seals. Current legislation supported by the Occupational Safety and Health Administration (OSHA) and the Food and Drug Administration (FDA) emphasizes the need for the closed seaming of fabrics to ensure that pathogenic contaminants encounter a noncompromised barrier. Fabrics performing filtration functiong down to uniform micron dimensions could fail a variety of standards or tests established by the General Services Administration, American Society for Testing and Measurement or others where penetration or adhesive bonding is used. Conventional needle and thread stitching accomplishes the bonding process only by puncturing the fabric, compromising the material as a uniform barrier.

Still other legislation focuses on the protection of workers who are responding to spills where hazardous liquids and/or fumes might penetrate protection clothing intended to keep the personnel from harm. Needle and thread bonding would require the application of an additional sealant to seam lines to achieve a closed seam, as is sometimes required by tent manufactures to prevent rain from entering inside.

Micro-Controlled Seals. Ultrasonics methodology has introduced a new level of precision in bonding, such as staking in the production of micro-circuitry. The technology gives manufacturers the ability to decide the dimensions, strength, location, uniformity, consistency and other aspects of the bond point with greater exactness. Similar control levels could not be achieved with glues, thread, solder and the other more traditional methods of sealing. With fabrics, ultrasonic bonding can reach new quality levels in controlling drape, fiber integrity, loft and other desirable features than achieved by thermal bonding or embossing. This new level of control often improves cosmetic appearances for enhanced marketability as there are no visible stitches, stakes, screws or rivets in the presentation of the product.

Some of the major ultrasonic bonding equipment suppliers in the U.S. include Sonobond, Branson, Dukane and others. These suppliers tend to specialize in one or more areas of the ultrasonics market - rigid plastics, metal and/or textile welding. What follows is an overview of the various U.S. manufacturers.

Sonobond Ultrasonics, West Chester, PA, was founded in 1948. Sonobond owns and/or controls more than 250 patents. In 1981, Sonobond purchased Cavitron, Inc., which enabled it to broaden its product range into plastic welding, although previously it had specialized in metals. Inductotherm Industries purchased Sonobond in 1988. During the past 10 years Sonobond has become a leader in developing ultrasonic applications for the textile market.

Branson Ultrasonics Corporation, Danbury, CT, was founded in 1946 to supply equipment and processes for the non-destructive testing of metals. During the 1950's, Branson added ultrasonic cleaning capabilities. Holding several hundred patents in ultrasonic technology, Branson is highly experienced in welding woven/nonwoven fabrics. The company introduced the first ultrasonic sewing machine in 1970 and continuous pinsonic web bonding in 1971. Branson has solid experience in rigid plastic and textile welding.

Dukane Corporation, St. Charles, IL, was established in 1922 and has a noted reputation in audio-visual equipment, acoustic transmitters/receivers and ultrasonic bonding equipment. The company has recognized skill in constructing ultrasonic welding systems for automated plastic assemblies.

Forward Technologies, Minneapolis, MN, was established in 1965 and has approximately 11 patents. Forward Technologies specializes in plastic assembly welding and automation. In the nonwovens area, the company offers precision cutting of baby diapers and sanitary napkins, although it is also experienced with a complete range of bonding techniques such as hot plate welding, vibration welding, hot air cold staking and spin welding.

Ultrasonic Seal Company, Aston, PA, was founded in 1959. The general manager of this company, Howard Dean, holds several patents that demonstrate expertise in laminating thin films and fabrics. Ultrasonic Seal Company offers a specialized sealing head for films and is active in the plastic and nonwoven market segments.

Sonics & Materials, Danbury, CT, was founded in 1969 and manufactures equipment for welding injection molded thermoplastics and woven/nonwoven textiles. Sonics & Materials specializes in power ultrasonic equipment used for assembly, slitting and drying thermoplastic materials.

Nonwovens Issues

In the textile market segment in contrast to the rigid plastic and metal segments, ultrasonic technology has a number of challenges that have been more or less successfully met. In the very structure of the bond delivery process, the technology requires drag on the surface of the material. With continuous

web bonding, a stable, non-rotating horn is generally applied from the top to a material surface, which is moving at the same surface speed as a rotating anvil beneath the material. This drag means that the top layer of material is stretched more than the bottom layer. Differential tension can cause warping in some instances or rippling of the top layer in others. Thermal bonding can be accomplished with the use of two anvil rolls, rotating with the same speed as the material without a drag effect. The task of eliminating the drag effect can be costly, even if justified in comparison to other technologies. New horn structures might be developed to minimize or eliminate this effect in the future. Sonobond has introduced a rotary horn within the past four years but the technology is not yet available for full web widths.

The continuous process production in the textile industry also requires uniform coverage of full web widths, some of which extend several feet. Given the relatively narrow width of individual horns (nine inches), a larger web width (72 inches) can be covered only if multiple horns located side by side across the web are made to overlap through diagonal positioning or staggered banks. Individual horns cannot touch each other without damage to the equipment. Unless the manufacturer can accept a pattern gap every nine inches across the web width, not an acceptable outcome in the quilting industry, complicated diagonal or bank placements are required. This response to full coverage requirements results in a fairly sophisticated array of horns, transducers, amplifiers and controls. Future developments may offer more simplified techniques to achieve full coverage.

The market in rigid plastics and metals has remained relatively static during the past decade, while interest in ultrasonics among the textile industry has grown. Some of this interest has been associated with environmental and safety legislation in that ultrasonics processes enable manufacturers to reach higher cleanliness standards, to operate at lower cost and to generate less disposable waste.
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Author:Lutzow, Thomas
Publication:Nonwovens Industry
Article Type:Cover Story
Date:Oct 1, 1993
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