High Absorbent Nonwovens: Properties, Manufacturing, Advantages and Limitations
Dept. of Textile Technology
D.K.T.E Society’s Textile Engineering Institute
Ichalkaranji- 416115, MH, India
NONWOVEN fabric is a fabric-like material made from long fibers, bonded together by chemical, mechanical, thermal or solvent treatment. The term is used in the textile manufacturing industry to denote fabrics, such as felt, which are neither woven nor knitted. NONWOVEN materials typically lack strength unless reinforced by a backing. In recent years, NONWOVENS have become an alternative to polyurethane foam. Non-woven fabric is manufactured by putting small fibers together in the form of a sheet and then binding them either with an adhesive or by interlocking them with serrated needles such that the inter-fiber friction results in a strong fabric.
They have high Absorbency because they contain immobilized super ABSORBENT polymer particles. This high absorbency property of the NONWOVENS is used to manufacture many products like baby diaper, medical purpose, geotextiles, vacuumed bags etc. This product shows property like washability, absorbency, softness, filter etc. The method of manufacturing NONWOVENS is divided into several stages like web formation, web bonding etc.
Different methods are used for preparing different products like baby diapers consists of the following methods for its manufacturing like formation of ABSORBENT pad, preparation of NONWOVENS from plastic resins using meltblown process. These products symbolize many advantages for comfortable usage because of its high absorbency power without harming the skin of the body. Many changes have been taken place compared to conventional products and modern ones.
Whilst the first production of a “NONWOVEN fabric” in Europe goes back to the thirties, the existence of a recognizable industry in Europe can be dated to the mid-sixties. NONWOVENS are found in hygiene and health care, in rooting and civil engineering, household an automotive, in cleaning, filtration, clothing, food wrap and packaging, absorbency purpose, to name only a few end-uses.
Definition of Nonwovens in Brief
The term used to designate the products generally known as NONWOVENS, was coined in most languages in opposition to woven fabrics. A NONWOVEN was something that was not woven. Only specialists know that NONWOVENS are unique engineered fabrics which offer cost effective solutions as e.g. in hygiene convenience items, or as battery separators, or filters, or geotextiles, or may be super ABSORBENTS.
As a main characteristic the CEN definition indicates that a NONWOVEN fabric is a fabric made from fibers that is consolidated in different ways. NONWOVEN fabric is made out of fibers, without any restriction, but not necessarily from fibers. These can be very short fibers of a few millimetres length as in the wetlaid process; these can be “ordinary” fibers, as used in the traditional textile industry, or then very long filaments etc. Properties and characteristics of a NONWOVEN fabric depend for a large part from the type of fiber it is ultimately made of. These fibers can be natural or man-made, organic or inorganic; the characteristic of a fiber being that it is longer than its thickness, or diameter. Such fibers can also be produced continuously in connection with the NONWOVEN process itself and then cut to length, or then extruded directly e.g. from polymer granules into a filament and then fibrous structure.
NONWOVEN are not paper and indeed, when made out of a very short, cellulose fibers, they essentially differ from paper because there aren’t any hydrogen bonds linking such fibers together.
NONWOVENS, as indicated by their English or French name, are neither woven fabrics, nor such other textiles as knitted fabric. Behind this statement lies a fundamental characteristic of the NONWOVENS: contrary to woven or knitted fabrics. Fibers that ultimately make up the NONWOVEN fabric need not to go through the preparatory/transitory stage of yarn spinning in order to be transformed into a web of a certain pattern.
A manufactured sheet, web or batt of directionally or randomly oriented fibers, bonded by friction, and/or cohesion and/or adhesion, excluding paper and products which are woven, knitted, tufted, stitch-bonded incorporating binding yarns or filaments, or felted by wet-milling, whether or not additionally needled. The fibers may be of natural or man-made origin. They may be staple or continuous filaments to be formed in situ. (This definition is completed by various notes)
NONWOVEN do not depend on the interlacing of yarn for internal cohesion. Intrinsically they have neither an organized geometrical structure. They are essentially the result of the relationship between one single fiber and another. This provides NONWOVEN fabrics with characteristics of their own, with new or better properties (absorption, filtration) and therefore opens them up to other application
High Absorbent Nonwovens
Most NONWOVENS, disposables or not, are high-tech, functional items, e.g. with ultra-high absorbency or retention for wipes and no wetback properties for those used into hygiene articles, with outstanding barrier because of their pores dimension and distribution, etc. They weren’t manufactured with the aim of disposability but in order to fulfill other requirements. They mainly became disposable because of the sectors they are used in (hygiene, healthcare) and of their cost efficiency. And disposability very often creates an additional benefit to the users. As disposable items have never been used before, there is then a guarantee that they do possess all the properties require as opposed to reused laundered fabrics.
High absorbent nonwovens have various applications due to its property of high absorption of not just liquid but also sound. Due to high absorption of liquids they are used in manufacture of baby diapers, feminine hygiene, adult incontinence products, wet wipes, bandages and wound dressings etc. Also, NONWOVEN fabric has a good sound absorbing property and it is for this reason that they are used in various fields such as NONWOVENS in automotive industry.
Virtually all kinds of fibers can be used to produce NONWOVEN bonded fabrics. To produce NONWOVEN bonded fabrics chemical fibers of both cellulosic and synthetic origin as well as natural and inorganic fibers are mainly used.
ABSORBENT Polymers are water-insoluble, cross-linked polymers which absorb large quantities of aqueous liquids by forming a hydrogel. The gel like mass responds in an elastic manner to external mechanical pressure. The liquid is retained even under pressure. Owing to their characteristic absorption properties, these polymers are used worldwide in the hygiene industry (baby diapers, adult care articles and hospital products, sanitary napkin).
Even though Super absorbent polymers (SAP) do not belong directly to the group of primary raw materials for the production of NONWOVENS, they are inseparably linked to the NONWOVENS industry through a number of technical innovations in hygiene applications. EDANA (European Disposables and NONWOVENS Association) stated for the year 2000 that the main end-use for the Western European NONWOVENS industry is the hygiene market with 341,000 tons.
In contrast to other liquid-binding raw materials (cellulose fibers, foams, etc), cross-linked, partly neutralized polyacrylates do not only absorb large quantities of liquid, but they also store these permanently, even under pressure.
The carboxylate groups in the polymers are strongly solvated in contact with aqueous liquids. There is an accumulation of similarly charged groups along the polymer chains which repel each other electrostatically. This process opens the polymer clusters, resulting in the transformation of the ABSORBENT polymer into a hydrogel. In consequence of the cross-linking, the polymer chains remain firmly connected to one another at some points so that the liquid absorption only results in swelling. The hydrogel does not liquefy even if it consists of 99% water.
Production Process of High Absorbent Nonwovens
The usual grain size of the SAP is within a range of 150-180 ppm. The two most suitable are described below.
In the inverse suspension polymerization, partly neutralized acrylic acid is dispersed. The polymerization is initiated by radical initiators. The crosslinking reaction is carried out through copolymerization of a polyfunctional crosslinker which is added to the monomer solution and by reaction of suitable crosslinking agents with functional groups of the polymer. This leads to the production of small porous droplets. After the polymerization is finished, these droplets are dried.
The more dominant manufacturing process for the production of super ABSORBENT granules is the radical solution polymerization, consisting of the following steps shown in the below figure. In this method monomeric acrylic acid is converted by partial neutralization to sodium acrylate (55-75%) before polymerization. The addition of a crosslinking system then brings about the formation of a three dimensional network. The surface crosslinking system was the most important precondition for the development of ultra thin baby diapers with a low proportion of fluff and high ratio of super ABSORBENT.
Typically, conventional superabsorbent powder polymer production begins with solution polymerization of partially neutralized acrylic acid along with a small amount of a crosslinking agent in water. The polymerization results in a water insoluble, water swellable gel containing approximately 25 to 40% solids, which must then be cut, dried, milled and sifted to produce a powdered SAP product with a typical particle size ranging between 100 to 800 um. The sifting operation typically generates a fines stream that must be recycled back into the production process creating a production bottleneck. The finished SAP product is then shipped to a hygiene industry converter where it is blended with fibrilled wood fluff to form the ABSORBENT core structure of a personal hygiene article such as a diaper.
In an in-situ SAP process, the partially neutralized acrylic acid monomer solution is applied directly to a NONWOVEN substrate and polymerized. The web may be fed to the process either as a pre-manufactured roll good or, preferably, made in-line from bulk stable fiber using a carding operation. The monomer solution may be applied to the web using a variety of application techniques such as brush coating, pressurized liquid spray, air-assisted spray or airless spray.
In-Situ Process Vs. Conventional Process
The in-situ SAP process offers a number of potential advantages for hygiene converters over conventional technology. Since the process produces stable, immobilized SAP structure, it eliminated the need for SAP powder handling and associated dust exposure issues. It also offers the potential to provide more uniform SAP distribution in ABSORBENT core structures and will remain stable during transport. The immobilized SAP particles also remain stable even in the hydrated state. The technology further offers the possibility of reducing production costs through the minimization of SAP processing steps.
Conventional Pad Formation from Solutions Polymerization
Preparation of Monomer Solution
Chopping of the Gel
Wood Fluff + SAP Mixing
SAP Powder formation
Milling & Stifling
Pad Formation From In-Situ Process
Vegetable fibers serve as raw material for producing high ABSORBENT NONWOVENS as the most important constituent of vegetable fibers is cellulose, which is HYDROPHILIC and HYDROSCOPIC.
So far, cellulose has been the most often used raw material to make paper. In the last decades, it has become more and more important with regard to NONWOVENS, too. It has found a wide field of application, preferably, together with super-ABSORBENT powders (SAP), for hygienic purposes which show the property of take-in and transport of moisture, absorbency and low dust emission.
Cellulosic NONWOVEN structures are used in personal care and feminine hygiene products for single use, such as panty liners, sanitary napkins, incontinence articles or diapers as fluid ABSORBENT core. Of high interest especially for this application area is the possibility to integrate super ABSORBENT polymers into this ABSORBENT cellulosic NONWOVEN structure. The present invention is directed to creating a fluid-ABSORBENT structure having a very good resiliency not only perpendicular to the surface but also parallel to the surface providing a certain resistance against getting crumpled. This feature is particularly advantageous for the wear comfort of the hygiene articles, because there is a significantly improved body-fit of the sanitary article. If the hygiene products are in use, they are constantly in movement and the article has to adapt shape to the wearer. The specific adjustment of the flexibility of the inventive material can be achieved by changing the structure of the surface e.g. by means of perforation.
Hygiene Products Use One of the Following as Fluid Absorbent
Individualized cellulosic fibers generated by mechanical opening of wood pulp obtained by means of hammer mills, being deposited as strips in hygiene products (fluff-pulp). Super ABSORBENT powder (SAP) or super ABSORBENT fibers (SAF) are added to this fluff-pulp to increase the liquid absorption capacity, particularly under pressure. The fluff pulp-based ABSORBENT structures are bulky and have especially in the wet condition no mechanical integrity and can not recover to the original shape after mechanical deformation. This implies limited comfort in particular in products for incontinence and feminine hygiene, since the products are easily shaped to a bulky bundle especially after exposure to liquids (menses or urine) which uncomfortable and annoying especially when wearing closely fitting clothes.
Airlaid materials, which are essentially made from fluff wood pulp fibers. The airlaid material can comprise super ABSORBENT materials (SAP or SAF) to increase the absorption capacity together with the fluff pulp. Compared to formerly mentioned fluff-pulp-based products, hygiene products using Airlaid material as ABSORBENT material are much thinner and therefore offer the wearer an increased comfort. Other thermoplastic materials like fibers and powder can be added to the Airlaid material to achieve additional functionality like odour an enhanced absorption, compatibility to ultrsonic welding etc.
The final object is achieved by a fibrous porous fluid ABSORBENT material comprising a NONWOVEN, in particular made by an Airlaid process and comprising of fibers at least 50% of fibers being cellulosic fibers and consisting of a core and at least one perforated surface layer having a perforation.
In fibrous Air laid structures, the individual cellulose fibers are not elastic by themselves and are only partially connected and crosslinked with each other e.g. by addition of a liquid binder, the addition of binder fibers or binder particles. If a punctual pressure is applied to such a Airlaid structure, the fibers below the pressure point will be compressed but the fibers around the pressure point will at least partially re-orient responding to the applied pressure. These reoriented fibers have property of a permanent deformation.
Surprisingly, it turns out that the restoring force against a local deformation of a structure such as fibrous Airlaid significantly increases when the fibers on both surfaces are connected with an additional preferred partially elastic surface. This surface can be formed by applying a binder partially penetrating the surface of the fibrous structure creating a network of connected fibers. Another suitable surface is achieved by adding a welaid tissue paper sprayed with binder to the surface of the fibrous material.
Attempt to Explain Through Figures
The above figure 1 illustrates the behavior of an Airlaid material without a surface layer comprising interconnected fibers exposed to local pressure.
Figure 2 illustrates the behavior of an Airlaid material with a surface layer comprising interconnected fibers exposed to local pressure.
The Airlaid material depicted in FIGURE 3 is similar to the material depicted in figure 2 with an additional perforation of the surface layers. Figure 3 illustrates how the perforation of the surfaces of the Airlaid structure decreases the stiffness of the material. The perforation points form kink or bending points at which the material can be bent without excessively bending the surfaces between these kink or bending points. For a material only perforated in particular zones these zones form areas of enhanced flexibility. This gives the opportunity to create structures with well defined deformation zones which bend into a predetermined direction when a defined force is applied.
This allows the design of hygiene articles with improved wear comfort (diapers, sanitary napkins, incontinence products) wherein the material can adapt to the respective form of the article and can adjust shape in a well defined manner if a force is applied. Compared to the method of embossing or local stretching (ringrolling or other mechanical treatment technologies, see patents above) the surface perforation provides the advantage that the Airlaid structure, which is important for liquid transport, is not altered and desired pore structures (e.g. pore size gradient) are not changed. Manufacturing hygiene products, a local perforation can be integrated into the process e.g. by using needle rollers with a pattern according to the desired perforation structure. Such integration of the perforation step onto the manufacturing line (converter) offers the advantage that the placement of the respective perforated zones is well positioned in the hygiene product. Furthermore, it is avoided that perforated materials with irregular textile properties have to be transported over longer distances on the converting line.
Figure 4 shows an exemplary embodiment of a fibrous porous fluid ABSORBENT material with a core and a surface layer. The core comprises NONWOVEN fibers. At least 50% of the fibers are cellulosic fibers. The core is produced by an Airlaid process and thus the fibers are only partly interconnected to each other. The core may comprise further components such as superABSORBENT polymers, e.g. superABSORBENT fibers. The surface layer comprises bonded fibers so that the fibers form a contiguous layer made of interconnected fibers. The surface layer is perforated by means of perforation holes that created a desired flexural rigidity. The perforation holes can be created by means of a needle roller. The perforation holes preferably have a diameter in the order of 0.2 mm to 0.5 mm. The distance d between the perforation holes in the surface layer preferably is between 1mm and 2mm. The thickness t of the fibrous porous fluid ABSORBENT material preferably is between 1mm and 2mm. Instead of a perforation by means of perforation holes a perforation made of slits can be provided.
Advantages and Properties of High Absorbent Nonwovens
1. Super ABSORBENT fibers “lock in” water, reducing body temperature during heat emergencies. While stabilizing vital body functions, material soothes and comforts victims of heat stroke, heat exhaustion, and other hypothermic conditions. Evaporative cooling method allows cloth to be reused in future traumas. Features and benefits include:
- Super ABSORBENT! – Absorbs 20 times its weight
- Water is “locked in” a stable, comfortable gel for quick, constant reduction of body temperature
- Chemical free
- User friendly – easy to store – easy to use
- Material is soft, comfortable, and comforting
2. Apart from its High level comfort and good Absorbency Power they are also low cost materials. No spinning and weaving is required and the cost of production is comparatively less. Thus, they can be easily used as a disposable product.
3. High ABSORBENT are also involved in the green wall or green roof projects because of their good resistance to roots and high interlocking of water so that vegetation can be grown on them. They have a high scope in India and being used in Germany from last 50 years.
4. They are used in medical products due to following reasons: The fibers used in medical NONWOVENS can be classified in natural and synthetic categories.
a) The natural fibers used are wood-pulp, cotton and rayon. Wood pulp is used for its obvious absorbency, bulk and low cost. Cotton and rayon are good to be used directly on wounds. They have good absorbency and make excellent NONWOVENS.
The reasons natural fibers make excellent medical NONWOVENS are:
- They are highly ABSORBENT of blood
- Excellent breathability
- Good aesthetic characteristics
- Easy launderability and can be sterilized
- Excellent dimensional stability and high operability temperature ~ 175 deg C
- Excellent drape and conformability
- Good heat resistance
- Excellent water retaining capacity
- Nonallergenic and nonirritant fibers
b) The synthetic fibers mostly used in this application are: polypropylene for its excellent properties
The properties of synthetic fibers which are required in many applications:
- Hydrophobicity: to be able to act as a barrier fabric
- Easy to process
- Cost effectiveness
- Better performance due to strength, low density
- Easy to dispose, not hazardous
5. There are some special NONWOVENS like SureSorb® which have special property of absorbing and retain up to 60 times their own weight in fluids and remain more drier and cleaner in the work area. They are highly durable and super ABSORBENT.
Basically NONWOVENS have many advantages but if compared to the woven and knitted fabric they definitely lack in many properties which is shown through the following property map.
It is for this reason that they may be disposable as they are weak compared to spunlaced, knits and weaves. Thus baby diapers, feminine hygiene and other high ABSORBENT products are more often disposable which can be taken as positively as they become a use and throw product.
The main reason why NONWOVENS should be used is their low cost. The company can use the waste material of the spinning and weaving industries and thus maximize their profit and minimize their loss. The number of processes is very less in manufacturing nonwovens which adds to profit.
Nonwovens have a very high absorbency because of the presece of immobilized polymer particles and Celulose pulp. They have high absorbing property which makes nonwovens to serve as baby diapers, sound insulators and water locking systems.
Thus, we conclude that, Nonwoves have a very high scope because of its different properties and the economy of any country can be served positively to a great extent in the field of nonwovens. Also, job oppoutunities are increasing in this field.
- Nonwoven Fabrics by W. Albrecht, H. Fuchs, W. Kittelmann;
- Indian journel of Fiber and textile research(December 2010)
- Processes for high absorbent nonwovens by Vedant Dhandhania in Indian Textile Journal January 2012
- High absorbent Nonwovens in Textile Excellence in 2011
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.