Nanomaterials in nonwovens

Lux Research in the US estimates the market for nanotechnology by 2015 will be worth $3.1 trillion (E24 trillion) spanning all applications. The ‘nano wave’ has caught up quickly in electronics

and medical fields. However, in materials science efforts have been focused mainly on the synthesis of nanomaterials such as nanoparticles and nanotubes. Though these materials have been accepted commercially it is still cost prohibitive to find broad-based and wide applications for them.

In the case of textiles, nanotechnology has found application so

far as a finish enhancer providing select value-added features. Nano-Tex and Schoeller Textil have been successful in bringing nanotechnology products into markets with fabrics having enhanced stain release and moisture management characteristics. Though nano-products have found commercial success in technical textiles, large-scale production and health-related concerns need immediate attention from academia and industry.

Nanoscience is considered to be one of the key technologies of this century and will play an important role in high-tech fields such as material science, electronics and even energy. Big developments deploying nanotechnology in textiles have included the finishing sector, where nano-formulations are used in filtration. In the case of nano-filters, a few companies have been using nanofibre webs in their filtration products over the last decade or so.

A barrier to the widespread commercialisation of nanofibre webs in nonowovens has been lack of productivity in electrospinning. More recently, developments are happening to speed up the commercialisation of nanofibres in filtration, medicine and even energy storage.

Another issue that will bear on what consumer products nanomaterials will be used in are toxicity issues concerning

submicron and nano-sized particles. Theories conflict on the health effects of nanomaterials and governments around the world are investing heavily to understand the health issues surrounding nanomaterials. Thorough knowledge of toxicity and health issues related to submicron fibres and nanomaterials in nonwovens is much needed for the industry.

This article will throw some light on the scalability issues regardingthe manufacturing of submicron fibres and toxicity-related aspects of nanomaterials. More clarity and knowledge of these aspects is essential if the nonwovens industry is to fully exploit the potential

of nanomaterials.

Electrospinning process and continuous nanofibre formation

Electrospinning is recognised as a simple and easy method to produce submicron fibres. The original patent on this technology

is about 100 years old. Since the 1990s, the use of electrospinning

as a processing technique has experienced a resurgence in the production of nanofibres. A nanofibre in the parlance of science generally refers to fibres with diameters ranging somewhere between 100nm and 0.5micron. About 100 laboratories around the world are using electrospinning to develop fibres using polymers, ranging from simple polyethylene oxide to complex protein molecules.

Most recently, researchers from Technion-Israel Institute of Technology reported the successful development of protein nanofibres for wound repairs. The researchers reported that bovine serum albumin has been successfully electrospun for biomedical applications. Such breakthrough developments are possible using nozzle-based electrospinning technology.

If production of nanofibres using simple methods can be translated to the commercial space, nanofibre webs can find myriad applications. The question is what has the nonwovens industry done to tackle commercialisation issues?

Donaldson has been using electrospinning technology to develop value-added products such as filters and synthetic nanofibrillar cell growth surfaces for number of years.

In recent years, a handful of companies have been scaling up nanofibre production processes. Ohio-based NanoStatics Corporation uses the nozzle-based electrospinning process and has developed a high throughput electrohydrodynamic spinning system (EHS). Czech Republic-based Elmarco, which relocated to North Carolina, US, has come-up with a nozzle-free Nanospide continuous technology.

NanoStatics Corporation’s electro-hydrodynamic spinning process produces continuous nanofibres from polymeric fluids by applying high electric fields. This system has an array of electrospinning nozzlesin a matrix of 100 to 400 nozzles per ft2, all producing consistent nanofibres. Each nozzle has a flow rate greater than 0.1ml per minute. This is relatively larger than the low production rate of 0.02ml per nozzle in current low productive systems. EHS results in a five-fold increase in productivity per nozzle. NanoStatics claims that with the increase in the throughput and the number of nozzles per square feet, the productivity gain compared to the current electrospinning systems can be as high as 200 times.

Typically, a 100in-wide EHS unit can contain more than 10,000 nozzles that produce consistent nanofibres at line speeds of 20 to 1,000ft per minute. In addition to the scaling up of the process, NanoStatics is also using environmentally friendly solvents to

produce nanofibres with functional properties. These are trademarked as Green Spinning and Engineered Nanofibres. In 2008 NanoStatics introduced filter media suitable for the HVAC industry with a brand name of NS Enhance.

Elmarco’s Nanospider has been designed to overcome problems caused by the close packing of spinning nozzles. The drum-based system takes care of the problems associated with the Taylor cone, which is very crucial for the development of submicron-size webs. The technology can produce nanofibre webs from 50nm to 1micron for various industrial applications. It also enables scaling up of the nanofibre production process in an energy efficient way.

The modular equipment can be stacked in serial, up to four units, to scale up the throughput and reach a maximum production speed of 15 metres a minute, producing 0.03gsm fabric of 200nm.

With the advent of highly productive systems such as Nanostatics and Nanospider, the industry now has the option to move forward in a high gear in developing nanofibre webs for many applications.

Toxicity of nanomaterials

Nanomaterials that are used to functionalise nanowebs are

prone to have contact with skin and enter into the human body. Therefore, investigating the toxicity of nanomaterials, such as nanometal oxides which are catalytic in nature, is extremely important. For example, nanometal oxides such as iron oxide and zinc oxide, which have catalytic activities on toxic gases, are usedto coat nanofibre webs. But how toxic these particles will be? The health related toxicity of these materials is a determining factor in the acceptance and application of nanofibre composite materials which are used in air and toxic gas filtration. The industry has yet to scratch the surface of this field.

Scientists at The Institute of Environmental and Human Health

at Texas Tech University (TIEHH-TTU) have recently begun dedicated research efforts to understand the toxicity of nanomaterials such

as metal oxides and carbon-based nanomaterials. Shawna Nations and Dr George Cobb at TIEHH-TTU are investigating the acute and chronic toxicity of metal oxide nanoparticles like Fe2O3, ZnO, CuO

and TiO2 to frogs. They have found that acute exposure of nanoparticles were not embryo lethal, but did have dose-dependent effects on the growth of frogs. CuO and ZnO nanoparticles induced malformations resulting in EC15 of 39mg/L and 2mg/L respectively. Both these nanoparticles were found to inhibit metamorphosis upon chronic exposure.

As is evident, long-term exposure of nanoparticles may lead to negative effects. However, the effects in humans have to be determined to have regulations on the use of such nanomaterials for applications in filters and so on. Such a study is warranted and will

be of enormous help to the industry.

The toxicity of carbon nanotubes remains an unresolved issue. As carbon nanotubes are used in a number of fibre-based composites, the health issues surrounding the use of carbon nanotubes are of extreme importance. A recent study by Jonathan Maul at TIEHH-TTU on the interaction between functionalised fullerenes and agriculture chemicals has shown that fullerenes functionalised with carboxylic acid reduced the reproductive toxicity of bifenthrin to aquatic organisms such as Daphnia magna. This result is of particular relevance to the industry as nanofibre-based materials can be functionalised to provide certain properties such as distructive adsorption and filtration.

The toxicity of functionalised nanofibrous materials will be a determining factor in the development and use of chemically

modified nanofibrous materials. A collective effort is needed between the nonwovens industry and toxicologists to gain an overallacceptance of nanofibre materials for those applications where nanowebs will be in contact with human beings.

Recent developments in nanotextiles

The field of nanotechnology in textiles has evolved in the last decade and now focuses mainly on the functionalisation of basic nanowebs. Functionalisation refers to value addition to regular nanofibre webs by incorporating some chemicals or by modifying the physical structure. These functionalised nanofibres provide enhanced applications, including catalysts, sensors, liners for

chemical and biological protective clothing, tissue scaffolds and bioengineered materials.

A recent study in Ramkumar’s group at TIEHH-TTU manipulated the electrospinning collector substrate to develop filter-within-filter honeycomb nanowebs. These nanowebs, in addition to high surface area, due to mesh-in-mesh structure, can act as good filters and can trap fine particles. Such a process of assembling nanofibres to obtain unique patterns for superior performance is commonly referred to as self-assembly. A similar self-assembling project is being carried out by professor Juan Hinestroza at Cornell University which focuses on understanding the self-assembling mechanisms of nanoparticles on the surface of fibres. Dr Hinestroza has found that high surface coverage of fibres with nanoparticles may induce interesting and unique phenomena such as the creation of colour without dyes. This could open up new avenues for the use of nanofibres in camouflage and military applications. His work also focuses on measuring the mechanical properties of bicomponent and tricomponent nanofibres using acoustic force atomic microscopy.

Dr Gajanan Bhat and his coworkers at the University of Tennessee Nonwovens Research Laboratory, in collaboration with eSpin Technologies, has investigated the structure and properties of electrospun nanofibre composites with spunbond and meltblown fabrics in order to produce sustainable biomedical webs. In another collaborative work with ChK Group, Inc. Dallas, TX, Dr. Bhat and his group were able to develop nanophase Mn (VII) oxide (NM7O) incorporatednonwovens to neutralise chemical warfare agents.

This nanoparticle incorporated nonwoven can

also be used as a smart fabric in military and civilian applications.

The cotton research unit at the Southern Regional Research Center, Agricultural Research Services, USDA, New Orleans, has been using nanomaterials to develop value-added cotton products. According to Dr Paul Sawhney of the unit, layer-by-layer deposition of nanoparticles

of appropriate chemicals and their size may be

a more efficient and cost-effective route to

develop value-added textiles. He is of the

opinion that the layer-by-layer deposition will not change the intrinsic properties of textiles as in the case of traditional method of deposition of nanoparticles. As is evident, functionalised nanomaterials will find a number of applications unexplored as yet. More importantly, functionalised nanofibre webs have a number of biomed uses.

Nanotechnology could help solve one of the important issues facing the nonwovens industry – enhancing the comfort aspect of polypropylene.

A potential solution is using environmentally benign technologies. Early indication from a recent research activity at Texas Tech University has shown that plasma treatment may create submicron size indentations on polypropylene nonwovens that may serve as air pockets similar to those in wool. If such results are proven successful, polyolefin-based nonwovens could very well penetrate apparel markets. But there is much work to do.

What’s next in nanotechnology and nonwovens

Barriers to the higher productivity of

nanofibre spinning have been overcome with

multi-nozzle electrospinning and nozzle-free continuous methods. These technologies should create interest in the industry to gear up the production of nanofibres for filtration and biomedical applications. In addition, methods to functionalise nanofibrous materials and the evaluation of health-related issues of nano materials should be seriously undertaken by the technical textile industry.

By Seshadri Ramkumar and Arvind Purushothaman of Texas Tech University