Choosing the Right Nonwoven
Step 1: Selecting the Right Fiber
Nonwoven fabrics begin with either synthetic or natural fibers in a staple fiber form or as continuous filaments. Staple fibers are short, chopped fibers that can be blended with other fiber types. Both staple and continuous filament fibers can be made with varying diameters. Typically a fiber’s diameter is measured in D.P.F. (denier per filament).
Fibers can be made from a single synthetic polymer, but they can also be combined to form a bi-component (sheath-core) or bi-constituent (side-by-side) fiber. Even though round is the most common fibrous shape, fibers can be extruded in different cross-section shapes depending on desired functionality.
Step 2: Determining Web Forming Technique
All nonwoven fabrics start out in the same form: loosely connected masses of fibers. To convert this loose mass into a reliable end-product, the fibers are put through one of four web forming processes.
Spunbond (or spunlaid)
The initial stages of the spunbond process are similar to the principles of a spaghetti press or meat grinder where a lump of dough or meat is pressed through a grate. The resulting resin is then melted and fed through a spinneret to create string-like filaments.
These strings are drawn down through an attenuation process to alter the denier (diameter) and jetted onto a moving conveyor belt to form the web. There are various methods of attenuation and jetting which dictate the type of spunlaid fabric manufactured. Meltblown is another type of spunlaid with extremely fine denier fibers.
- Spunbond End Products: Medical drapes & gowns, dryer sheets, interlining, landscape fabric
In this process, fibers are arranged so they lay parallel, then cylinders with wire “teeth” roll over the fibers creating a webbed structure. Carded nonwovens tend to be stronger in the direction that the machine’s “teeth” roll, but can be reinforced to increase the strength or absorbency of the fabric by layering.
- Carded End Products: Cover stock, fabric softener, wipes, interlinings
Unlike carding, which relies on strict structure, air laying is a bit more erratic. Specially engineered machines shoot short fibers in a jet-like air stream that collides with a moving screen. The impacts caused by the moving screen meshes the short fibers into webbed formation.
- Airlaid End Products: Medical and industrial wipes, premoistened wipes, absorbent hygiene products
This process takes staple fibers and puts them in aqueous slurry. The slurry is then laid down on a mesh table and dried, leaving a loose mechanically-bonded web. The most common wetlaid process is paper manufacturing.
- Wetlaid End Products: paper, filtration media
Step 3: Bonding
Typically the web that is created through one of the forming processes above is held together by weak mechanical bonds. There are several bonding technologies available to strengthen the fabric.
Typically synthetic fibers are thermoplastic, which means they soften and re-melt when heat is applied, providing an opportunity for adjacent fibers to bond together. The main types of thermal bonds are:
- Flat Bond - By applying heat and consistent pressure in the form of a flat calender across the web, flat bonding creates a smooth surface where fibers are bonded to each other.
- Point Bond - Also known as pattern bond, point bonding is the process of applying a heated roll with a pattern embossed in the roll. Fibers are bonded together only at the points of the roll.
- Thru-Air Bonding – This bonding process draws the web through a heated drum, creating bonds throughout the fabric without applying specific pressure.
This bonding process uses fine high-pressure jets of water sprayed down on top of the fabric. The pressure from the jet causes fibers to become mechanically entwined through the depth of the fabric.
This bonding process coats the web with a specified type of resin, creating chemical bonds among the resin and fibers.
Step 4: Finishing Process
By the time the fabric undergoes the bonding stage, it is in finished form. The only step left is to apply any desired finishing processes for additional functionality.
- Coating – coatings can be in the form of aqueous dip, or knife coating or extruding a film on top of the fabric.
- Laminating – combining more than one layer of a nonwoven fabric together, usually by a calendaring process, then applying heat and pressure to bond fibers from each layer.
- Ultrasonic Welding – another process by which layers of fabric or substrates can be combined. This technology applies ultrasonic vibrations and pressure to a point of the fabric. The fabric absorbs the vibrations and melts, creating a weld.
- Slitting – One of the real benefits of nonwovens is that they can be slit to custom widths without concern for fraying or unraveling.
Fiberweb® Technical Nonwovens will engineer your material based on the necessary performance characteristics. For more information on Fiberweb Technical Nonwoven fabrics or to discuss the engineering of a custom fabric, visit our contact page.