Material advances: the next phase
by Seshadri Ramkumar, Ph.D.
The COVID-19 pandemic, with its mutating variants, continues to rage worldwide, but it’s not the only pressing global issue. Climate change, too, has impacted nature and contributed to catastrophic fires and floods, and impacted our natural world in other concerning ways.
According to the U.S. Environmental Protection Agency (EPA), heat waves are the primary weather-related cause of casualties in the U.S. Infectious disease and climate change episodes have highlighted the need for sustainable materials, which can counter the ill effects of natural and man-made calamities. These recent events are revitalizing research and manufacturing of advanced materials such as phase change materials, biomimetic products, lightweight fibrous materials and self-sensing materials.
President Biden’s one trillion-dollar infrastructure plan, which has passed the U.S. Senate, has many opportunities for the materials sector to develop advanced and sustainable products. The infrastructure bill invests about $110 billion for roads and bridges, where opportunities are plenty for new fibrous products like phase change materials (PCMs), geotextiles and geogrids, and there is funding allocated for countering air, soil and water pollution, as well—about $75 billion, in fact, for such initiatives. Improvements in filtering products and other advanced materials that counter pollution will be needed.
Recent catastrophic events will force stakeholders, including government, industry, academia and research laboratories to work on mission-linked projects that will drive the advanced manufacturing sector to the next phase.
Phase change materials (PCM) are advanced materials used most often in energy, medical and protective clothing sectors. These materials can play a critical role in fighting climate change, pandemics and other natural calamities. According to Fortune Business Insights™, this field is expected to show growth in double digits, offering opportunities in this sector that could be impactful in addressing future challenges. The PCM field is not mature yet, but it is well recognized by the research community and stakeholders in the advanced materials industry.
PCM-based fibrous materials have been successfully used and commercialized in thermal regulation clothing. Polyethylene glycol is a common PCM used in thermal products. Recently, a collaboration between China and Switzerland-based scientist successfully spun polyethylene glycol-incorporated hollow polypropylene filaments with good reversibility of latent heat storage and discharge. PCM are micro-encapsulated so that they are held intact when they phase change their states to maintain and regulate temperature.
Many research projects are endeavoring to use biomimicry to create thermal regulating products. Research in this field has been inspired by bird feathers as well as hairy structures that are capable of regulating temperature.
Prof. R.S. Rengasamy’s group at the New Delhi-based Indian Institute of Technology is investigating the natural processes in goose down to develop thermal comfort clothing. The feathers of these birds are fine fibers and are in x-y-z planes (in all directions). These fibers are hollow in nature which help to trap air, Rengasamy explained recently in an international virtual conference. Also, the high surface area helps in reducing radiative heat loss. While the structure of goose down fiber offers fascination and motivation for biomimetic research, so far, we have not been successful in mimicking it in the laboratory, Rengasamy added.
As found in nature, microencapsulated PCMs, such as polyethylene glycol particles, change their state depending on the outside ambience and temperature. Switzerland-based Schoeller Textil AG has been at the forefront in developing PCM-based clothing of this kind.
The basic principle behind a PCM is that when outside temperature rises, a PCM encapsulated within the coated structure melts down holding heat, thereby providing comfort. When the outside temperature cools down, PCMs release heat maintaining a thermal balance. According to Schoeller, microencapsulated PCM have a thin layer of coating as a protective sheath which helps to avoid leaking when they change their state, also enabling them to be applied to various textile materials.
It is appropriate to consider types of PCMs as disruptive technologies, capable of providing better thermal balance compared to high-loft nonwoven waddings for insulation. One of the recent advancements in spunmelt nonwoven fabrics is the segmented filament structures. Developments in spinnerets have enabled the development of sheath/core, segmented pie, and hollow filament structures. Such structures are common in some bird feathers (and goose down), which have inspired the hollow structures with a sheath that enables air to be trapped for thermal control.
While PCMs are extremely useful for thermal regulation, surface features like wettability, hydrophilicity and hydrophobicity also influence their application and usage in clothing, particularly sportswear, performance wear and active wear. The textile industry has ably adopted concepts from high energy physics, such as plasma finishing, to provide surface effects to textiles.
Plasma finishing has evolved from being a batch-based vacuum type of finishing, to the continuous atmospheric pressure-based technology, which has made higher productivity possible. Selective surface treatments are possible using plasma technologies. These may include high wicking, next-to-skin fabrics for comfort in sportwear applications. Plasma processes can enhance functionalities in fabrics, and adding nanoparticles using plasma as a carrier helps to enhance the surface area and can alter wettability characteristics.
Utilizing structural aspects
Innate material aspects in phase change materials have been exploited to develop high performance clothing, in particular. New methods are being used to alter surface characteristics, which have conventionally been altered using coating and padding applications, In this way, other properties, such as moisture vapor transport, permeability, smoothness and stiffness, are not affected while surface characteristics change.
Super critical gas and liquid extraction techniques, and atmospheric pressure plasma processes are some of the promising technologies that can be employed by the advanced textiles industry. Characteristics of fibers at the structural level, such as crystallinity, surface area and surface features, play important roles in developing high performance textiles.
Oleo and hydrophilic or hydrophobic characteristics depend on the molecular and structural characteristics at fiber level. Recent research at Texas Tech University has shown that the fineness of cotton plays a very important role in influencing the oil absorbing properties of nonwoven mats.
As cotton is a natural material, environment plays a critical role in its maturity and growth, and affects the physical and structural characteristics of cotton, as well. Therefore, it is useful to exploit the structural aspects of natural fibers in developing value-added products, as is the case with the oil-absorbing nonwoven mats made from less mature fibers. The less mature fibers are finer, which provides higher surface area, making them ideal for higher sorption applications.
A collaborative future
The textile sector has been moving towards being interdisciplinary in nature, which will support development in new materials, including PCMs. The advanced textile sector can creatively utilize this information about natural materials and existing technologies to manufacture layered hybrids from knits, wovens and nonwovens to alter sorption, comfort and filtration properties.
Collaborations with basic science and engineering disciplines will lead to new manufacturing methods, which in due course will lead to new products. Such collaborations are already making waves in fields such as wearable textiles, fabrics with microfluidic devices and active fabric systems. A similar approach in the development of improved PCMs can fuel growth in this sector, as well.
Dr. Seshadri Ramkumar is a professor in the Nonwovens and Advanced Materials Laboratory, Texas Tech University, and a frequent contributor to Advanced Textiles Source.