Hybrids, sandwich structures and composites: Winning combinations at INDEX™17
Different types of nonwovens fabrics are often much more effective working together with others than on their own.
The obvious example is the combination in layers of spunbond and meltblown nonwovens fabrics that is widely employed in the absorbent hygiene products industry.
Spunbond
The one-step spunbonding process involves a series of integrated operations including melt preparation, filament extrusion and drawing, web forming and bonding.
Polymers suitable for this process, include polypropylene (PP), polyethylene terephthalate (PET), and polyethylene (PE).
Polypropylene is the most widely used for spunbond nonwoven production, providing the highest yield and the ability to form low-density webs. To reduce costs, scrap and PP fibres of inferior quality can also be re-pelletized and then blended with fresh polymer to produce first-grade spunbond fabrics.
Proprietary
The first spunbonding systems originated from the proprietary technology of companies such as DuPont in the USA, Rhône-Poulenc in France and Freudenberg in Germany.
The first commercial spunbonding system to be offered to the market was Docan technology, developed in the 1960s by the German Lurgi engineering group and licensed to various companies who are now part of the Avintiv group in Europe, and to Kimberly-Clark in the USA.
The next major step towards the global commercialisation of the spunbond process was with the introduction of the Reicofil system in 1984 by Reifenhäuser in Germany. The Reicofil system has been constantly improved over the years and is now one of the leading suppliers of technology for the hygiene market.
Meltblown
Meltblowing produces fibrous webs directly from polymeric resins using high-velocity air to form the filaments.
The process is unique because it is used almost exclusively to produce microfibres, which generally have diameters in the range of 2-4 microns. Fibres in this size range have a positive effect on the softness, cover, opacity and porosity of the materials obtained.
US Navy
The basic technology for producing microfibres was first developed in the 1950s by the Naval Research Laboratory. The commercial significance of the work was recognised by Exxon, which subsequently developed the technology further. Researchers at Exxon extended the basic design and first demonstrated the production of meltblown microfibres on a commercial scale by modifying sheet die technology.
Many companies today use the technology to produce commercial nonwoven products for a range of end-uses.
Sandwich constructions
To supplement the limited physical strength of normal meltblown webs and also to provide enhancement of other properties, sandwich constructions are now widely used and SMS –spunbond/meltblown/spunbond – structures are the most dominant. Integrated SMS systems, which are now extremely common for the production of material for absorbent hygiene products, are made from three separate spinning beams which extrude and lay the polymers simultaneously.
SMS materials have an enhanced structure in which the fine fibre properties of the meltblown are complemented by the strength and toughness of the spunbond surface fabrics.
Reifenhäuser has expanded Reicofil technology to the use of up to six beams for specific applications, for combinations of materials such as SSMMSS, and much work has taken place on integrating other fibre webs into the fabric systems for specific applications. Similar technology is also manufactured by another INDEX exhibitor, Oerlikon Neumag.
Coform
Kimberly-Clark’s Coform structures are another notable example of meltblown fibres. Coform nonwovens involve a blend of meltblown fibres and less expensive short wood pulp fibres that have been especially successful for use as a substrate for wipes. They are often combined with a spunbond fabric on one or both surfaces to provide additional product versatility.
The Italian machine builder Teknoweb Converting has recently secured a number of patents for its Arvell process which employs a similar combination of fibres and pulp.
Blends with other fibre types have also been produced, such as 3M’s Thinsulate, combining polypropylene or polyester staple fibre with meltblown webs. The injection of solid materials, especially superabsorbent resins, has provided another range of interesting and useful products based on meltblown technology.
Nanofibres
Meltblown nonwovens have also provided the performance required in filter media, but now nanofibres are taking this a stage further to meet new market demands.
The use of nanofibres as coatings and delicate webs already have a range of applications in industrial fields such as filtration, medical materials, battery separators and acoustic insulation and a number of technologies are commercially available for their production. There is still a feeling, however, that there are significant limitations to the existing technology and that much more should be possible.
This was recently summed up by Bryan Haynes, Director of Global Enterprise, Nonwovens Research and Engineering for Kimberly-Clark.
“A sugar cube of polypropylene with sides of 1.58cm weighing 3.5 grams suffices for the production of 15 micron spunbond fibres that would stretch 14 miles,” he observed. “The same amount of polypropylene employed to make 3 micron meltblown fibres, meanwhile, would result in 350 miles of fibre. But that same sugar lump employed to make 300 nanometre nanofibres, would result in enough to stretch 35,000 miles. The questions is, how can we produce these fibres cost effectively on a mass scale?”
The key nonwoven fabric filter media manufacturers with nanofibre technologies include Avintiv, DuPont, Ahlstrom, Johns Manville, Hollingsworth & Vose, Neenah Gessner and Fibertex. In addition, a number of their key customers also have nanofibre web and coating technologies.
One end-use market seeing particular demand for finer filters is that of commercial vehicles and trucks, as a result of increasingly stringent standards for exhaust emissions, particularly in Europe. Nanofibre-containing nonwovens are frequently providing the solutions.
Wetlaid and polyester
Another key trend that is emerging in filter media is the creation of hybrid wetlaid glass and spunbonded polyester nonwovens.
Ahlstrom’s latest Pleat2Save filter, for example, has a patent-pending depth structure that is not just based on moving from a thicker to a thinner web in its construction, but has a specifically shaped mat glass core, enveloped within spunbond polyester nonwovens.
Such materials are combined due to their robustness, with a tensile strength around 2.5 times higher than microglass alternatives, allowing for excellent processing performance, especially during the filter pleating operation.
INDEX™17, the leading nonwovens show which next takes place at Palexpo from April 4th-7th 2017, will be the place to be to discover more about today’s hybrids, sandwich structures and composites, and the uses to which they are being employed.
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