Airflow Permeability in Tea Bag Non-Woven Materials

How Non-Woven Fabric Porosity Affects Tea Extraction: What the Numbers Mean for Your Tea Bag Line

When a consumer drops a triangle tea bag into a cup of hot water, the rate at which the water passes through the filter fabric - and how completely it extracts flavor from the leaves - depends on a property called airflow permeability. For non-woven tea bag materials, this property is determined by fiber diameter, fabric weight (GSM), and the density of the fiber web created during the carding and bonding process. Understanding these relationships helps tea bag manufacturers choose the right filter material for their specific product.

What airflow permeability measures

Airflow permeability is expressed as the volume of air that passes through a square centimeter of fabric in one second, under standard pressure conditions. It is typically reported in millimeters per second (mm/s) or cubic centimeters per square centimeter per second (cm3/cm2/s). For tea bag applications, this number is a proxy for how quickly water will flow through the filter during brewing.

A higher permeability value means a more open fabric structure. Water moves through faster, extraction happens more quickly, and the brew tends to be lighter in body. A lower permeability value means a denser fabric. Water moves through more slowly, contact time between leaves and water increases, and the brew tends to be stronger and more tannic.

The relationship between GSM and permeability

In non-woven tea bag fabrics, fabric weight (GSM) and permeability are inversely related within a given fiber type. Heavier fabrics have more fiber mass packed into the same area, which creates a denser web and reduces airflow. Lighter fabrics have less fiber mass, leaving more open space and increasing airflow.

For example, in our PLA non-woven range, the 21 GSM fabric has a more open structure than the 25 GSM variant. In practical terms, this means that a 21 GSM bag will produce a faster, lighter extraction, while a 25 GSM bag will produce a slower, fuller-bodied extraction. The difference is not dramatic - usually 10 to 20 percent variation in flow time for a standard 200-milliliter cup - but it is consistent and measurable.

Fiber diameter matters too

Beyond GSM, the individual fiber diameter plays a role. Finer fibers (lower denier) can pack more densely at the same weight, creating a fabric that is heavier in filtration effect than a coarse-fiber fabric at the same GSM. PLA fibers used in tea bag non-woven typically range from 1.5 to 3.0 denier, depending on the supplier and the intended application. We do not manufacture the fibers ourselves, but we check incoming material for denier consistency because variations can change how the fabric behaves on the sealing station.

The role of fiber bonding method

Non-woven fabrics are bonded through either thermal, chemical, or mechanical means. Thermal bonding, which is the standard for PLA non-woven, uses heat to fuse fiber intersection points. The degree of bonding affects both the structural integrity of the fabric and its permeability. Over-bonded fabric becomes stiff and brittle, which can cause tearing during high-speed forming on tea bag machines. Under-bonded fabric is too soft and may not hold its shape in the cup. The bonding parameters are set by the fiber supplier during production, and we verify bonding quality through tensile strength testing on incoming rolls.

Practical implications for tea bag manufacturers

If you are formulating a new tea blend and you want to control extraction time, adjusting the filter material is one lever you have alongside leaf cut size and dose weight. A finer-cut tea in a 21 GSM bag will extract quickly and may taste thin if the consumer steeps for too long. The same tea in a 25 GSM bag will extract more slowly and may taste fuller, but if the cut is too fine and the fabric too dense, you risk clogging the filter surface and creating channeling - where water finds paths of least resistance and bypasses portions of the leaf bed.

For herbal blends with larger, bulkier particles, the 25 GSM fabric is usually the better starting point. The denser structure contains the particles without slowing flow to the point of under-extraction. For traditional black or green teas with smaller particle sizes, the 21 GSM fabric is often preferred because the faster flow prevents over-extraction and astringency.

The best approach, in our experience, is to test both weights with your actual tea on your actual machine. The numbers on a spec sheet give you a starting point, but the real answer comes from brewing cups and tasting them. We provide sample rolls for exactly this purpose.