Coatings and treatments at the fiber level are primarily utilized as a process aid. These ingredients can be used to heat stabilize the fiber while it is spun and stretched. They can also make the fibers easier to convert into the final fabric, and they can even be washed away during the process.
Meanwhile, coatings, finishes and treatments for fabric at the final material stage are primarily utilized to add functionality to the final product. Functionality is additive to the material. These chemistries are generally not temporary, but rather are expected to be long lasting or even permanent. Fiber-to-fiber friction is the most important attribute in holding a fabric together in most applications, and thus this a major application for coatings, finishes and treatments at this stage.
Sizing
Some common examples of coatings and treatments utilized in the fiber stage that aid in the processing of fibers into a fabric or material include chemistries that reduce friction. The terminology utilized for coating a fiber is called “sizing.” The sizing is a mixture of various chemicals, typically, but not exclusively, in an aqueous solution, which is ultimately safer to handle and better for the environment. This is a complex blend of ingredients, which can include polymeric components, coupling agents, lubricants and a range of additives. Additives can include surfactants, plasticizers, rheology modifiers, anti-static agents and adhesion promoters, to name just a few. The assorted blend is specific to the final fiber and requires different sizing chemistries. However, what is typical and relatively consistent is that the solids content is between 5-15%.
The sizing chemistry can also contribute to mechanical properties such as tensile strength, impact and fatigue resistance, as well as others. Chemical property attributes are also affected by sizing chemistry in the forms of oil resistance, water resistance and even corrosion resistance on metallic fibers, as well as many other factors.
Lastly, and somewhat indirectly, these coatings can affect regulatory and safety compliance, which is very important for medical and protective apparel applications. The specifics of these chemistries are often trade secrets and are viewed as key differentiators, especially with regard to high-performance fibers.
Sizing machines are usually defined by drying method. Two techniques for drying include hot air and cylinder drying. In hot-air drying, the fiber is dried in an enclosed chamber and the air is heated either by electricity or steam. Cylinder drying passes the fiber around a drying cylinder. The fiber is pressed against the cylinder under pressure or tension in an open environment. The fiber is only dried on one side in this process, so it has to be wound around multiple cylinders to accommodate both sides. The only thing that is constant between both methods is that each setup is engineered and configured specifically to the filament or spun yarn’s unique requirements.
Application examples
Some common functions of coatings and treatments at the fabric and material stage include bodily fluids repellency, fire retardancy, stretch and durability. These treatments are applied to finished textiles and are typically very long lasting.
Chemistries that offer functionality to fabrics often involve a complex blend of ingredients with a significant amount of customizability in both their material and final structure. Treatments can be produced in micro or nano structures and are very versatile. While some coatings are permanent, others are engineered to have controllable biodegradability, and advances in filaments and yarns can accommodate rapid degradation in the correct conditions. This trend is mostly utilized in single-use products and is becoming an end-of-life requirement when permissible.
The protective efficacy of facemasks, for example, can be a combination of filament chemistry and fabric treatment in the form of corona or plasma treatments. Polypropylene is the most frequently used fiber, because it is both hydrophobic in nature and has a capacity for wicking, ensuring a dry and comfortable microclimate between the mask and face.
The COVID-19 effect
With the advent of COVID-19 there has been a significant amount of development in the area of coatings and treatments that have antiviral effects. Surface protectors are coatings that make use of either metals, like copper and silver, or biomolecules.
One popular strategy is incorporating antiviral nanoparticles into the structure of a mask or on the surface of a respirator. Other techniques include making the surface of the mask superhydrophobic, so viruses within aerosols would not be able to remain on the mask because of the ultra-waterproof coating.
There are many different coatings and treatments that have been developed for hygienic and antiviral applications over the past 18 months with varying degrees of effectiveness. Surface protectors are coatings that make use of either metals like copper and silver or biomolecules. These compounds can be formulated into long-lasting protective coatings that can last from one week to 90 days, depending on the material and environments.
Antiviral coatings are also a known subset of antimicrobial coatings. Some of these antimicrobial coatings possess the capability to kill viruses, bacteria and other microorganisms like fungi and mold, whereas others do not destroy viruses. Both powder and liquid forms are available today for coating surfaces to prevent the spreading of several diseases. These coatings and treatments can be used in a variety of applications:
- Single-use caps, gowns, facemasks, scrub suits and shoe cover.
- Contamination control gowns
- Transdermal drug delivery patches
- Heat packs
- Drapes, wraps and packs
- Lab coats
- Underpads, dressings and wipes
- Procedure packs
- Isolation gowns
- Bed linens
The wildfire effect
When considering the wildfires currently impacting various parts of the world, antiviral and flame-resistant coatings and treatments are being employed in a number of ways in protective apparel and even some construction materials. Antiviral coatings provide that extra protection for first responders and firefighters on the inside of the garments, while flame-resistant polymer coatings add an additional level of protection to high-temperature-resistant fabrics. In construction materials, anti-microbial coatings are utilized to manage mold and mildew in house wraps and other vapor barriers.
Future trends
One of the most significant trends in coating and treatments for protective apparel is the use of bio-based and biodegradable compounds on polymeric materials. An example of this application would be the move from halogen-containing flame retardants to those that are halogen-free.
Until recently few would have imagined that bio-macromolecules such as carbohydrates (cellulose, starch, chitosan, etc.), phenolic compounds (lignin, tannins), and others could be used in flame-retardant applications. There are many studies in the marketplace that are showing that these alternative approaches to flame retardance are possible, at least at the lab and pilot scale. The industrial scale-up of such bio-macromolecules is still under evaluation due to certain limitations. As an example, the development of bio-based FR additives at such a large scale is still not practically feasible, and more work needs to be done in testing the thermal processing of such additives. The other major factor that limits the scaling of bio-macromolecules at this point is cost – they are not cheap. These materials are just at the limits of current technologies, but with increasing legislative pressures and environmental regulations, they could soon be commonplace.
Training
For a bonus “Tech Talk” video on plasma treatment, visit: bit.ly/filter-treatment
* International Fiber Journal is owned by INDA, Association of the Nonwoven Fabrics Industry (inda.org).
