Non-woven materials - types and technologies

Depending on the desired result, there are several ways to manufacture nonwovens.

The first written definition of nonwovens was given by the American Society for Testing and Materials in 1962, which defined them as "textile fabrics made of carded or fibrous fabric bonded with adhesives".

Currently, INDA, the association of manufacturers of nonwovens, defines nonwovens as "linen structures connected to each other by entangling fibers or threads (perforation of films) by mechanical, thermal or chemical methods. These substrates are flat porous webs made directly from individual fibers or from molten plastic or plastic film. They are not produced by weaving or knitting and do not require the conversion of fibers into yarn (INDA).

Technical definitions express the fundamental basis of nonwovens production processes, but due to the wide variety of production technologies, a general description of nonwovens is not enough. As with woven or knitted fabrics, each process has unique characteristics. The resulting non-woven fabrics do not have much in common with each other, except that they all belong to the category of non-woven materials. Components of nonwovens, such as; the methods of fiber selection, web formation, gluing and finishing can be changed to control the functional properties of the webs, depending on the requirements of the relevant market segment. Due to a wide range of characteristics, nonwovens penetrate a wide range of markets, including the following:

• medicine,
• hygiene,
• automotive industry,
• filtering,
• construction,
• geotextile,
• PPE and others

There are several technologies for manufacturing nonwovens. This article discusses the most basic of them, since quite often the described technologies are used in combination with each other in order to achieve certain unique properties of nonwovens.

Nonwovens«Airlaid» (airlade - aerodynamic styling)

Compared to other non-woven fabric production technologies, Airlaid has a unique ability to stack short fibers, whether it is 100% cellulose fiber or a mixture of cellulose and short synthetic fibers, to form a homogeneous and continuous web. It is possible to add superabsorbent powders (SAP) or fibers (SAF) between layers of nonwoven fabric or in primary raw materials, thereby creating webs with high absorbent capacity.

The canvas "Airlaid" can be bonded in several ways. When latex bonding (LBAL – latex bonded airlaid) a liquid binder is applied to both sides of the web, which is then dried and cured to achieve the necessary strength in dry and wet conditions. Typical applications are table tablecloths, dry and wet wipes, industrial napkins and household goods. Thermal bonding of canvases «Airlaid» (TBAL– thermally bonded airlaid) usually includes thermal bonding of two-component fibers of the "shell-core" type, in which the shell is made of a polymer having a lower melting point (softening) compared to the core, which is mainly responsible for strength. During the formation of the web, heating occurs in order to activate the easily melting components of synthetic fibers for binding the web. For absorbent layers of various medical, hygienic and household products, superabsorbent powder (SAP) is added to the process, which is fixed in the fabric structure with synthetic fibers.

Bonding in several ways at the same time (MBAL – multi-bonded airlaid) is a process that combines latex and thermal bonding. Usually, the inner part of the product is thermally glued, and a small layer of binder is applied to the surface to eliminate dust and lint. These materials, commonly used for absorbent fillers, household goods, dry and wet wipes, may also contain SAP.

Hydrogen bonding (XBAL) The bonding of the web is achieved by combining pressure, humidity and temperature to create so-called hydrogen bonds, which eliminates the need for other bonding methods. It is usually used for absorbent layers.

Nonwovens Air Through Bonded (air thermal through connection)

The technology of through-air bonding is a type of thermal bonding, in which heated air is supplied to the surface of a non-woven material. During the bonding process, heated air passes through the openings in the chamber above the nonwoven fabric. Unlike hot furnaces, which push air through the material, the through-air bonding process uses negative pressure, in which the heated air is sucked through an open conveyor belt holding the non-woven material as it passes through the furnace. The air supply through the material ensures fast and uniform heat transfer, minimizing the deformation of the non-woven material.

The binders used in the process of through-air bonding include crystalline binding fibers and powders, which, when melted, form molten droplets along the entire cross-section of the non-woven material. As the material cools, binding occurs at the points of these droplets. Non-woven materials created using this technology have characteristics such as softness and volume.

Meltblown (meltblown)

Nonwovens blown from the melt are produced by extrusion of the molten polymer through a die having up to 45 holes per inch to form long thin fibers that are stretched and cooled by passing hot air over the fibers, at the moment when they fall out of the die. The resulting canvas is assembled into rolls and subsequently turns into finished products. Extremely thin fibers (usually polypropylene) differ from other extruded materials, especially spunbond, in that they have low internal strength, but a much smaller size (thickness from 0.5 microns), providing key properties. Nonwovens blown from the melt can have extremely thin fibers and are often used in respirators, face masks and filter media. Often, the "meltblown" technology is used in conjunction with the "spunbond" technology; for the formation of composite cloths of the type SMS, which are used in the production of disposable diapers and feminine hygiene items.

Spunlace (hydroweaving)

The "spanlace" technology (also known as hydroweaving) is a process of fastening fibrous webs made by carding (card), air (airlaid) or wet laying (wetlaid), resulting in a non-woven material. In this process, thin jets of high-pressure water are used, which penetrate the web, hit the conveyor belt (or the so-called "wire").; («wire, inclined wire») and bounce back, causing entanglement of the fibers.

Spunlace nonwovens use short staple fibers. The most popular are viscose and polyester staple fibers, but polypropylene and cotton are also used. The main areas of application of spanlace are napkins, fabric face masks and medical products.

Spunlaid, spunbond (spunbond)

The essence of the technology is as follows: the polymer melt (mainly PP and PET) is extruded through a die in the form of thin continuous fibers, which are then stretched in the air stream and, laying on a moving conveyor, forming a non-woven fabric. The formed canvas is subsequently fastened with one of the following methods:

• needle punching,
• chemical impregnation with binders,
• thermal fastening on the calender,
• water jet bonding,
• hot air thermal bonding,

The most common method of fastening is thermal fastening on a calender. The method of fastening the formed canvas determines the characteristics of the resulting material, and consequently, the scope of application. Polypropylene-based spunbonds are widely used in hygiene products, such as baby diapers and feminine hygiene items, as well as in medical clothing. Polyester-based spunbond materials are commonly used in durable nonwovens for applications such as construction (roofing materials, geotextiles) and automotive.

Spunmelt SMS

Materials Spunmelt SMS (SMMS, SSMMMS, etc.) are a combination of spunbond and meltblown technologies, resulting in a multi-layered product called SMS (spunbond-meltblown-spunbond). A layer of non-woven material "meltblown" is not particularly durable and is usually placed between several layers of spunbond bearing a reinforcing function. Meltblown gives high barrier properties to the material, thanks to the extremely thin diameter of the filaments and their uniform laying. The cloths SMS, completely made of polypropylene, have water-repellent properties, increased tactile properties and sufficient tinting so that they can be used in the production of disposable hygienic and medical products (surgical fields, dressing gowns, armbands, caps, disposable sheets, duvet covers, diapers, pads, etc.).

Wetlaid (wet styling)

During wet laying, an aqueous suspension of staple fibers (viscose, polyester, etc.) up to 12 mm long, very often mixed with crushed wood pulp (or separately), is continuously deposited on the molding mesh (enclosed wire, the principle of a paper machine). The water is sucked out, filtered and returned to the process. The cloth formed on an inclined grid is dried on a drum, air (omega) or flat (table) dryer, depending on the raw materials used. For example, due to the high probability of deformation of the web, glass-ceramic and carbon webs are dried only in flat furnaces. Wetlaid is commonly used for the production of materials such as tea bags, coffee/car filters and water-soluble napkins. Sometimes, to obtain materials with additional functional properties, carding machines for forming the canvas-base are placed at the beginning of the line (or a slightly calendered spunbond is unwound), crushed cellulose is laid on an inclined grid on the canvas, and then there is a stage of additional hydro-entanglement.This technology is called Wetlace Composite.