Water resistance is a fundamental function of any outdoor or sportswear garment to ensure the body stays dry. This performance is usually achieved via durable water repellent (DWR) finishes that can be added to fabrics to make them water-resistant. The pores on the water-repellent fabric should remain open so it can 'breathe', making the garments comfortable to wear. Durability is also repeatedly emphasised as a necessary characteristic of these garments as these kinds of finishes usually wear off over time and retreatment (eg spray-on products for consumers to use) can sometimes be necessary. But, achieving this requirement has often resulted in the use of perfluorinated chemicals (PFCs) due to their superior water, oil and soil repellent abilities. PFASs – polyfluoroalkyl substances – are also used as this group of chemicals is closely related to PFCs, with both containing fluorine and carbon atoms.
The challenge
However, despite their great repellent properties, both PFCs and PFASs are part of the DWR discussion that refers to concerning compounds, and they have been the subjects of severe criticism in recent years. Specifically, longer chain PCS materials, depending on their functional groups, can be very hazardous due to their harmful by-products; it is generally understood that not all PFCs/ PFASs are hazardous.
According to the definition provided by the Organization for Economic Co-operation and Development, long-chain PFASs refer to perfluoroalkyl carboxylic acids (PFCAs) with eight carbons and greater (i.e., with 7 or more perfluorinated carbons) and perfluoroalkane sulfonates (PFSAs) with six carbons and greater (ie with 6 or more perfluorinated carbons). The shorter PFCs compounds (C < 6), have lower oil and water repellency and lower washing durability.
Until a couple of years ago, nearly all stain- and water-repellent finishing materials contained long-chain PFASs like perfluorooctanoic sulfonic acid (PFOS) and/or perfluorooctanoic acid (PFOA) (both having 8 perfluorinated carbons). However, they have been found in wastewater and shown to be bio-accumulative and highly persistent pollutants in the environment. Humans can be exposed to low-levels of such materials through contaminated drinking water or food, which can become polluted through packaging, soil or water used to grow the food. As a result, these materials have found their way into human blood streams and in other living creatures, such as the livers of polar bears in the arctic.
Moreover, people who work at PFCs production sites may be exposed to them, and if humans or other animals ingest traces of PFCs, they will accumulate in the body and stay for a long period of time. Various published studies unambiguously evidence that, if the level of such materials increases, living organisms may start to suffer from their health effects. These materials are also linked with liver and kidney and immunological effects in laboratory animals, as well as increased cholesterol levels, damage to children’s immune systems, lower female fertility and cancer.
Demand for PFC free, regulations
Due to growing concern regarding the adverse effects of fluorinated chemicals, both eight carbon backbone PFOS and PFOA by-products are strictly regulated by environmental laws and regulations at both a regional and national level. The fluorochemical and textile industries have addressed this critical issue primarily by gradually phasing out long-chain perfluorinated compounds and replacing them with short-chain perfluorinated substances, featuring typically 4 to 6 C-atoms.
NGOs contribute to the elimination of hazardous PFCs in outdoor products as well. It was highlighted in a 2012 report by Greenpeace, titled 'Chemistry for any weather', that DWR finishes used on outdoor clothing containing long-chain PFCs are hazardous to human health and the environment, which resulted in demand for finishes free of fluorocarbons. Since then, an increasing number of initiatives have appeared with the aim to replace PFCs and provide support for research into alternatives, to find practical solutions.
Environmental sustainability is an important part of outdoor brands’ strategies as business’ growth strongly depends on conservation of outdoor surroundings. Developing transparency of sustainable activism is of strategic importance for many brands which can be seen through the increasing trend of reporting the environmental impacts of products and production. A relatively large number of sportswear brands and performance textiles companies are therefore upping their efforts to replace PFCs with environmentally-friendly alternatives.
Consumer trends and awareness
To reduce the risks related to these fluorinated substances, a phase-out is necessary, and voluntary avoidance by consumers may be one way to make this happen. Consumer awareness about hazardousness of fluorine based DWR finishes is expected to drive demand for PFC-free alternatives, with millennials and urban consumers are expected to take the lead.
A recent Swedish survey conducted collaboratively by researchers from major technology universities in Sweden, aimed to investigate the extent to which information about hazardous properties of fluorinated substances increases people’s willingness to pay for durable water-repellent outdoor clothes free from hazardous substances. The results show a generally high willingness-to-pay, and that the effects of providing information on non-hazardous options being more expensive improved price increase responses. These results can be useful to brands and manufacturers in attempting to understand the different aspects associated with voluntary substitution of PFASs and their acceptability by the consumer.
Biomimetics: learning from nature
In this matter, during the past few decades, researchers have been interested in learning from nature and identifying how some biological surfaces, such as lotus leaves, rice leaves, butterfly wings, the legs of water strider, cicada wings, or other biomaterials exhibit superhydrophobicity.
In 1974, Professor Wilhelm Barthlott from the Nees-Institute for Biodiversity in Bonn, Germany, discovered that micro-roughness and surface chemistry on the leaf of certain plants can cause water droplets to remain spherical on the surface. This phenomenon was later named the ‘Lotus Effect’. The non-wettable properties of the so-called Lotus Effect, is due to the rough surface structure of the leaves, and the surfaces coated with an ultra-fine layer of wax crystals, which makes them water-repellent.
Ever since this discovery, scientists and engineers have put forward nature-inspired approaches to fabricate artificial non-wettable surfaces by manipulating the surface chemistry or the surface topology of the materials. It has been known that, a good cooperation of surface chemistry that gives low surface energy and a structure that results in intermediate surface roughness usually results in superhydrophobic behaviour.
Alternative sustainable strategies
Presently, the major substitutes for PFCs are shorter fluoroalkyl chain PFCs (C<6), non-fluorinated organic-based chemicals, eg paraffin, dendrimer products and silicones. Meanwhile, some research and development projects are focusing on novel inorganic-based non-PFCs and use of nanoparticles. These substitutes have been shown to have less hazardous environmental and biological impacts compared to PFC water-repellent products.
However, recently published studies clearly indicate that the short-chain perfluorinated replacements also reveal some critical issues and pose questions concerning their impact on the environment and also on human health. Most strikingly, the short-chain perfluorinated compounds were found to be as persistent as their long-chain analogues by virtue of their reluctance to biodegrade. A problematic feature of the short-chain PFASs is, in particular, their higher mobility in aqueous environments compared to the long-chain compounds.
To fabricate super-hydrophobic fabrics, the mentioned materials are commonly applied on textile substrates by methods such as solution immersion, electrospinning, chemical vapour deposition (CVD), layerby-layer (LBL) assembly, sol-gel methods, etc. Among these methods, solution-immersion is one of the simplest and most cost-effective ways to deposit super-hydrophobic coatings on to fabric surfaces.
In terms of achieving required surface roughness, researchers have reported methods such as templating and moulding, lithographic approaches, and plasma treatments. However, these approaches can be used only on certain substrates and generally require complex and/or expensive processing equipment and conditions. A more practical way is to deposit nano/micrometer-sized particles to build the roughness.
Given the importance of substituting the fluorinated water-repellent finishes due to their environmental risks, research in academia and industry has resulted in the development of several alternative approaches. Alongside the commercialised products already in the market, which mostly cannot provide oil repellency, research in academia is still ongoing to develop a durable repellency performance against both water- and oil-based contaminants, to match the technical parameters of PFC materials.
Academic research
There is a lot of research and development being undertaken on the design and fabrication of green water-repellent coatings. These strategies should generally meet several requirements such as: avoiding fluorinated materials to provide a low surface tension, a fast processing method that involves nontoxic materials and solvents, using materials that are commercially available and cheap. In addition, for textile applications processing should occur in a reasonably low temperature environment due to limited thermal resistance of most textile fibres.
Belgian researchers from the University of Louvain have reported on an environmentally friendly method to prepare superhydrophobic fabrics using water-based emulsions and suspensions of nontoxic materials by a simple dip-coating method. The method that they developed, which is published in the Applied Materials and Interfaces journal, volume 10, 2018, can easily be integrated into standard textile industrial processes and has a strong potential for the mass production of environmentally friendly super water-repellent fabrics. They have used water-emulsified polydimethylsiloxane (PDMS) and silica suspensions in water, which are fluorine-free and commercially available, on a polyester fabric. The produced fabrics show good resistance to machine washing and acidic or alkaline treatments. In addition, it is shown that based on the combination of the roughness of the fabric and of the coating formulations, superhydrophobicity can be quantitatively predicted.
In another paper published in the journal of NANO, volume 13, 2018, a researcher from the Myongji University, Korea, has applied a novel method to control the wettability of polymeric substrates, while avoiding any potential toxic chemical treatment. Pre-strained, surface oxidized layers of PDMS were used to form wavy geometries, on which hydrophobic functionalisation was carried out in order to produce irreversible wetting effects. This innovative strategy has demonstrated that the surface wettability of wavy patterned polymer thin films was tuned by the details of the grooved geometries on the surface of the films and the groove geometries were controlled via O2 plasma treatment and mechanical pre-straining. The pre-strained, buckled PDMS films were applied to the fabrication of hydrophobic polystyrene nano-patterns using colloidal self-assembly, where the colloids were arrayed in two-dimensional way. The wavy polystyrene films were found to be more hydrophobic relative to flat polystyrene films. The grooving methodology used in this study could be applied to enhancing the hydrophobicity of other types of polymeric thin films, eliminating the need for chemical treatment.
Another challenge with development of economical water-repellent coatings in commercial and industrial applications is their lack of mechanical wear robustness and long-term durability. Lastly, we report on a study by researchers from Georgia Tech University, USA, that demonstrate a durable water-repellent polyethylene terephthalate (PET) fabric modified with inorganic, methyltrimethoxysilane (MTMS)-based coatings. The results of this study showed that the coating adhesion to the fabric was remarkably high resulting in resistance to various mechanical and chemical wear damage. This developed technology, which is published in the Journal of Colloid and Interface Science, volume 516, 2018, offers opportunities to design low cost, durable and functional textiles for both indoor and outdoor applications.
