PA6 nanofibers applied to Reemay® PET scrim
Nan-o-fi-ber: a fiber of material with a thickness or diameter of only a few nanometers
By Eugene Beck, Berry Global, Inc.
Nanofiber composites have been associated with filtration for decades. Multiple niche markets utilize various forms of nanofibers when exacting high levels of filtration efficiency are desired or mandated. Nanofibrous filtering media has been commercialized for use in atmospheric air cleaners, cabin filtration, respiratory and breathing filters, engine oil and fuel, and machinery air intake filters. Numerous polymer systems have deployed using organic, inorganic, and biopolymers depending on the specific challenge.
The primary challenge incorporating nanofibers into HVAC commercial and industrial air filters is balancing the alpha value noting the relationship between resistance and penetration, and the corresponding dust holding capacity and filter operational life expectancy. The benefit of nanofibers as part of a fiberglass, cellulose, or synthetic media composite for industrial cartridge filters is the instant high efficiency, surface loading and releasing characteristics. Heavy amounts of airborne particulate are captured and released via pulsing and the process repeats multiple times a day. Quick capture – quick release.
High performance HVAC filters delivering a total cost of ownership value require the opposite. Low initial resistance and a high capture rate of fine airborne particles are required over an extended period of time. Filter life expectancy can be one year or even more depending on the installation. Previous generations of HVAC grade air filters constructed with nanofiber composites provided low resistance and excellent efficiency, but did not provide sufficient dust holding capacity resulting in short life expectancy. Advances in nanofiber machinery and process controls now provide operating engineers much more control over the application of the nanofiber to support material or scrim. The previous exceptionally flat, film like nanofiber layer is being replaced with a more three-dimensional web matrix to allow more depth loading of airborne particulate. Composite prefilter layers have also advanced to better protect the high efficiency nanofiber layer from loading prematurely, thus extending filter life.
Applying nanofibers to a composite
Nanofibers have traditionally been spun with a needle or nozzle forcing the jets to spin from the exact position of the needle. The latest generation of nanofiber machinery utilizes the science of Free Surface Spinning Electrospinning. Free surface spinning means no needles or nozzles are involved in the process of creating the nanofiber. Fibers are spun from the surface of a wire allowing physics to dictate spacing. Jet spacing is optimized and truly random creating higher uniformity.
Free surface wire electrospinning of nanofibers
Improved nanofiber layer uniformity results in higher performance at lower coating weights while creating opportunity for higher yields and production line speeds, reducing costs. More depth is not required to compensate for inconsistent fiber application. Cross-web uniformity, achieving 3 Sigma is typical creating a highly uniform mechanical nano web matrix or particle barrier, target specific size airborne particulate contaminants.
A major benefit of using nanofibers in HVAC grade air filters is maintaining high efficiency levels after 24 hour IPA vapor exposure as noted and required by the ISO 16890 standard. V-Bank style air filters utilizing nanofibers as part of the media composite have been tested by third party independent laboratories confirming MERV 15/15A performance due to the mechanical web matrix created during the free surface spinning process. High confidence mechanical efficiency in a durable synthetic composite form is possible. Nanofibers can also be applied to traditional fiberglass air filter materials to create special properties.
Nanofibers can now be applied to a wide spectrum of carriers or scrims to achieve specific results. Fibers spun from PA6 can be applied to PET or PP synthetic scrims, fiberglass air filter media as support structure, and even lightweight spunbonds. Historically, the HVAC filtration market has been generalized with limited high efficiency tiers usually ranging from MERV 13 to MERV 15 for mid to high efficiency, and MERV 16 to HEPA for higher purity applications. High efficiency charged synthetic air filter media provide low initial resistance and durability. However, the media can drop up to three MERV values when discharged. Fiberglass paper type air filter media delivers consistent efficiency levels even when discharged but with typically higher resistance. Synthetic nanofibers can be a tool to realize the benefits of each material.
The evolution of nanofiber composites for HVAC now allows for exceptionally high efficiencies targeting a specific particulate size for capture immediately upon filter installation, without efficiency derogation over time. New high value products can be developed for industries such as automotive finishing, turbine, agricultural, healthcare, and pharmaceutical. As an example, if the automotive paint finishing industry has identified an airborne particulate size that must be removed prior to entering the paint environment, air filter manufacturers can utilize nanofibers as the key component of a custom filter media composite targeting the specific unwanted particle size. The advanced uniformity of the nanofiber layer allows a lighter application providing better air flow with reduced resistance and cost.