Natural Fibers
Natural fibers are strands of long, thin, flexible material that are created in nature by plants, animals, or geological processes (i.e., mineral fibers). Although natural fibers are rarely used in commercial medical masks, they are commonly used in fabric masks to contain droplets to help reduce the spread of COVID-19.
Manufactured Fibers
Manufactured fibers are strands of long, thin, flexible materials that have been significantly altered (in terms of chemical composition, structure, and properties) by industrial processes. These fibers include synthetic fibers (e.g., polyester, polypropylene, and carbon fiber), regenerated fibers (e.g., rayon, viscose, and bamboo), and inorganic fibers (e.g., fiberglass, silver, and copper fibers).
Woven Fabrics
Woven fabrics are created by interlacing fibers or yarns at right angles (90° angles) to each other in a ‘checkered’ pattern. This pattern is created by weaving (or interlacing) a set of horizontal fibers (weft fibers) over and under a set of vertical fibers (warp fibers). The properties of woven fabrics depend on the ways in which the fibers (warp and weft) are interwoven.
Knit Fabrics
Knit fabrics are made by pulling loops of thread or yarn through each other to create a sheet of material. In general, knit fabrics are not recommended for use in masks because they are stretchy. When knit materials are stretched, it increases the size of the holes (pore size) in the materials, which decreases the its ability to filter particles.
Spunbond Nonwovens
Spunbond fabrics are produced by extruding melted polymer through a spinneret to form long thin filaments. These filaments (5 – 20 um diameter) are collected on a moving conveyor belt, where they form a fibrous web. The web of nonwoven fibers is then passed between two heated rollers, which thermally bond the web together to give it better strength and durability. This process is called thermal bonding or calender bonding.
Meltblown Nonwovens
Meltblown nonwovens start off with a spunmelt process similar to that of spunbond fibers. However, the process after filaments are extruded from the spinneret is different. For meltblowns, the extruded fibers are are immediately subjected to high velocity hot air streams coming from both sides of the spinneret. These rapidly moving streams of hot air hit the molten polymer at the edge of the spinneret and blow it into very fine filaments (~1 -5 μm diameter). The rapidly moving air causes these fibers to break into shorter, discontinuous filaments, which are then randomly dispersed and collected into a web on the roll below.
Composite Spunmelt Materials
Spunbond-meltblown-spunbond (SMS) materials are multi-layered fabrics, commonly used in medical gowns, drapes, and sterilization wraps. The spunbond layers give the materials better structural integrity, and the meltblown layers give it better filtration.
Electrospun Nonwovens
The majority of nanofiber materials are manufactured through a process called electrospinning. For electrospun fibers, the precursor materials go through a spinneret, but instead of being blown by hot air as in meltblown processes, electrical fields are used to draw out, and deposit extremely thin (1 nanometer to 500 nanometers in diameter) continuous fibers.