What Are Nonwovens and Why Used Heavily In The Healthcare?

The first written definition of nonwoven fabrics came from the American Society for Testing and materials in 1962 which defined them as “textile fabrics made of carded web or fiber web held together by adhesives.” Currently, INDA, the association of the nonwoven fabrics industry, defines a nonwoven as “sheet or web structures bonded together by entangling fiber or filaments (and by perforating films) mechanically, thermally or chemically. These substrates are flat, porous sheets that are made directly from separate fibers or from molten plastic or plastic film. They are not made by weaving or knitting and do not require converting the fibers to yarn (INDA). It is this nature of the fabric that makes it superior to woven fabrics in terms of infection control. 
 
Technical definitions express the fundamental basis for the nonwoven processes, but due to the wide variety of production techniques, a general description of nonwoven fabrics is not enough. As with woven or knitted fabrics, each process possesses unique characteristics. The resulting fabrics do not have much in common aside from being categorized as nonwoven. Nonwoven components such as; fiber selection, web formation, bonding, and finishing techniques can be altered to manipulate fabric properties or reverse engineer fabrics based on functional requirements. Due to its assortment of achievable characteristics nonwoven fabrics penetrate a wide range of markets including medical, apparel, automotive, filtration, construction, geotextiles, and protective.
There are several types of nonwovens processes by which these materials are made.
 
Airlaid Nonwovens
Compared to other nonwovens technologies, airlaid has the unique ability to lay down short fibers, either 100% pulp fibers, or mixtures of pulp and short cut synthetic fibers, to form a homogeneous and continuous web. It is also possible to mix in superabsorbent powders or fibers thereby creating highly absorbent webs.

The airlaid web can be bonded in several ways. In latex bonding (LBAL), a liquid binder is applied to both sides of the web, which is thereafter dried and cured to achieve the dry and wet strength needed. Typical applications are table top products, dry and wet wipes, industrial wipes and household products.  Thermal bonding airlaid (TBAL) includes bonding fibers,  typically bicomponent fibers, in the web formation, and the web is heated to activate the melting components of the synthetic fibers to bond the web. Typically used for absorbent cores, where superabsorbent owder can also be present and locked into the web structure by the synthetic fibers.

Multi bonding (MBAL) is abonding process where latex and thermal bonding are combined, typically where the inner part of the produdt is thermal bonded and the surfaces have a slight layer of binder to eliminate dust and linting. Typically used for absorbent cores, household products, dry and wet wipes, these materials can also contain SAP.

In hydrogen bonding (XBAL), bonding is achieved by a combination of pressure, humidity and temperature to create so called hydrogen bonds, eliminating the need for other bonding methods. Typically used for absorbent cores.


Air Through Bonding (Thermal Bonding)
Through air bonding is a type of thermal bonding that involves the application of heated air to the surface of the nonwoven fabric. During the through air bonding process, heated air flows through holes in a plenum above the nonwoven material. Unlike hot ovens, which push air through the material, the through air process uses negative pressure of suction to pull the air through an open conveyor apron holding nonwoven as it is drawn through the oven. Pulling air through the material allows the rapid and even transmission of heat to minimize distortion of the nonwoven material.

The binding agents used in the through air bonding process include crystalline binder fibers and powders, which melt to form molten droplets throughout the cross-section of the nonwoven. As the material is cooled, bonding occurs at these droplet points. Nonwovens created by the through air process have the characteristics of being soft and bulky.

Meltblown

Meltblown nonwovens are produced by extruding melted polymer fibers through a spin net or die consisting of up to 40 holes per inch to form long thin fibers which are stretched and cooled by passing hot air over the fibers as they fall from the die. The resultant web is collected into rolls and subsequently converted to finished products. The extremely fine fibers (typically polypropylene) differ from other extrusions, particularly spunbond, in that they have low intrinsic strength but much smaller size offering key properties. Meltblown nonwovens can benefit from extremely fine fibers and are often used in respirators, face masks and filtration media. Often meltblown is added to spun bond to form SM or SMS webs, which are used in the disposable diaper and feminine care industries.


Spunlace (Hydrotentanglement)
Spunlace (also known as hydroentanglement) is a bonding process for wet or dry fibrous webs made by either carding, airlaying or wet-laying, the resulting bonded fabric being a nonwoven. This process uses fine, high pressure jets of water which penetrate the web, hit the conveyor belt (or “wire” as in papermaking conveyor) and bounce back causing the fibers to entangle.
Spunlace non woven fabrics used short staple fibers, the most popular is viscoseand polyester staple fibers but polypropylene and cotton are also used. Main applications for spunlace include wipes, facial sheet masks and medical products.
 
 
Spunlaid (Spunbond)
Spunlaid, also called spunbond, nonwovens are made in one continuous process. Fibers are spun and then directly dispersed into a web by deflectors or can be directed with air streams. This technique leads to faster belt speeds, and cheaper costs. Several variants of this concept are available. Polypropylene spunbonds run faster and at lower temperatures than PET spunbonds, mostly due to the difference in melting points. The polypropylene-based spunbond is widely the material of choice in hygiene products like baby diapers and feminine hygiene products as well as in medical garments. Polyester-based spunbond materials are typically used in durable nonwovens applications like roofing and construciton, automotives and geotextiles.


Spunmelt/SMS
Spunbond has been combined with melt-blown nonwovens, conforming them into a layered product called SMS (spun-melt-spun). Melt-blown nonwovens have extremely fine fiber diameters but are not strong fabrics. SMS fabrics, made completely from PP are water-repellent and fine enough to serve as disposable fabrics. Melt-blown is often used as filter media, being able to capture very fine particles. Spunlaid is bonded by either resin or thermally.
 

Wetlaid
In the wetlaid process, staple fibers of up to 12 mm fiber length, very often mixed with viscose or wood pulp, are suspended in water, using large tanks. Afterwards the water-fiber- or the water-pulp-dispersion is pumped and continuously deposited on a forming wire. The water is sucked off, filtered and recycled. Besides synthetic fibers, glas ceramic and carbon fibers can be processed.
To distinguish wetlaid nonwovens from wetlaid papers more than 30% by mass of its fibrous content is made up of fibers with a length to diameter ratio greater than 300, its density is less than 0.40 g/cm3. Wetlaid is commonly used in applications like tea bags and coffee filters and dispersible wipes.

Reources: 

 1. EDANA, a global association and voice of the nonwovens and related industries

2. Nonwovens Industry Publications

3. INDA, Association of the Nonwovens Industry