High Absorbent Nonwovens | Production Process of High Absorbent Nonwovens

ABSTRACT:

NONWOVEN fabric is a fabric-like material made from long fibres, 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 fibres 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-fibre 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.

1. INTRODUCTION: 
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.

2. DEFINATION 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 fibres that is consolidated in different ways. NONWOVEN fabric is made out of fibres, without any restriction, but not necessarily from fibres. These can be very short fibres of a few millimetres length as in the wetlaid process; these can be “ordinary” fibres, 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 fibre it is ultimately made of. These fibres can be natural or man-made, organic or inorganic; the characteristic of a fibre being that it is longer than its thickness, or diameter. Such fibres 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 fibres, they essentially differ from paper because there aren’t any hydrogen bonds linking such fibres 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. Fibres 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 fibres, 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 fibres 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 fibre 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

3. 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 fulfil 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 absorbant 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.

4. RAW MATERIALS:
 Virtually all kinds of fibres can be used to produce NONWOVEN bonded fabrics. To produce NONWOVEN bonded fabrics chemical fibres of both cellulosic and synthetic origin as well as natural and inorganic fibres are mainly used.

ABSORBENT POLYMERS: 
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 napkins)

Even though Superabsorbent 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.

ABSORPTION MECHANISM:
 In contrast to other liquid-binding raw materials (cellulose fibres, 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.

MANUFACTURING 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.

SUSPENSION POLYMERIZATION: 
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.

SOLUTION POLYMERIZATION:
 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-SITU POLYMERIZATION: 
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 fibre 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 

↓ 

Polymerization
↓
Chopping of the Gel
↓
Wood Fluff + SAP Mixing
↓
SAP Powder formation
↓
Milling & Stifling
↓
Drying 
↓
PAD formation

PAD FORMATION FROM IN-SITU PROCESS:

CELLILOSE FIBERS:
 Vegetable fibres serve as raw material for producing high ABSORBENT NONWOVENS as the most important constituent of vegetable fibres 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 confort 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 artile has to adapt shape to the wearer. The specific adjustment of the flexibility of the inventive material can be achieeved by changing the structure of the surface e.g. by means of perforation.

HYGIENE PRODUCTS USE ONE OF THE FOLLOWING AS FLUID ABSORBENT:

CELLULOSE BASED: I
ndividualized cellulosic fibres 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 fibres (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 hyygiene, 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.
 

AIR LAID: 
Airlaid materials, which are essentially made from fluff wood pulp fibres. The airlaid material can comprise superABSORBENT 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 fibres 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 fibres atleast 50% of fibres being cellulosic fibres and consisting of a core and atleast one perforated surface layer having a perforation.

In fibrous Air laid structures, the individual cellulose fibres 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 fibres or binder particles. If a punctual pressure is applied to such a Airlaid structure, the fibres below the pressure point will be compressed but the fibres around the pressure point will atleast partially re-orient responding to the applied pressure. These reoriented fibres 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 fibres 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 sturcture creating a network of connected fibres. 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 fibres exposed to local pressure.

Figure 2 illustrates the behavior of an Airlaid material with a surface layer comprising interconnected fibres 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 perfora- tion 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 fibres. At least 50% of the fibres are cellulosic fibres. The core is produced by an Airlaid process and thus the fibres are only partly interconnected to each other. The core may comprise further components such as superABSORBENT polymers, e.g. superABSORBENT fibres. The surface layer comprises bonded fibres so that the fibres form a contiguous layer made of interconnected fibres. 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 fibres "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
• Reusable
• 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 fibres used in medical NONWOVENS can be classified in natural and synthetic categories.

a.) The natural fibres 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 fibres 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
• Biodegradable
• Excellent drape and conformability
• Good heat resistance
• Excellent water retaining capacity
• Nonallergenic and nonirritant fibres

b.) The synthetic fibres mostly used in this application are: polypropylene for its excellent properties
The properties of synthetic fibres 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.

LIMITATIONS: 
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.

CONCLUSION:
 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. 

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Shout Out at Felt | Manufacturing Process of Felt

Introduction:

Most fabrics are either woven or knitted these days; with woven fabrics taking almost 60% of the total textiles and apparel. In reality, textile goes beyond woven and knitted fabrics. It actually starts from Felt. Felt is a material that requires neither the weaving technology, nor the sophisticated knitting technology.

HISTORY:
 Felt may be the oldest fabric known to man; it predates both weaving and knitting. Since felt is not woven and does not require a loom for its production, ancient man made it rather easily. Some of the ancient felt remain were found in the frozen tomb of nomadic horsemen in the Siberian Tiai mountains and date to around 700BC. These tribes made clothing, saddles and tents from felt. Legends have it that during the Middle Ages, St. Clement who was to become the fourth Bishop of Rome, discovered the process of felt making by accident. It is said stuffed his sandals with tow (short flax or linen fibres) in order to make them more comfortable. St. Clement discovered that the combination of moisture from perspiration and ground dampness coupled with pressure from his feet matted these tow fibres together and produced a cloth. After becoming a Bishop, he set-up a group of workers to develop felting operations, this made him the patron saint for hat makers, who extensively utilize felt to this day. 

Melted fabric

MANUFACTURING METHODS OF FELT: 
Basically, there are two methods of manufacturing felt fabrics: 

• Wet felting (traditional felting)
• Needle-felting (Dry felting)

WET FELTING:
This process uses the inherent nature of wool and other animal hairs, because the fibres have scale on them which are directional and the hairs have kinks in them. It is the combination of these properties that reacts to the stimulation of friction and causes the phenomenon known as felting. It tends to work well with wool fibres as their scales, when aggravated, curl and bond together to form cloth.

RAW MATERIALS: 
Wet fitted fabrics are produced from wool, which grips and mats easily, and a synthetic fibre that gives the felt some resilience and longerity. Synthetics cannot be turned into felt by themselves (using wet felting) but can be felted if they combine with wool. Typical fibre combination include; wool and polyester or wool and nylon.

Cheaper felt (artificial felt) if produced using the wet method, has a minimum of 30% of wool fibres combine with other artificial fibres, this is the minimum required to hold a fabric together with the fibres alone.

Other raw materials used include; steam, sulphuric acid (used in the thickening process) and soda ash (sodium chloride) which is used to neutralizes the sulphuric acid. 

CLEANING, OPENING AND BLENDING PROCESS:

1. The fibres (animal wool) contain a lot of fats and are thoroughly cleaned and made free of fats and dirt by scouring process. 

2. Since some felts uses more than one type of fibre, the cleaned fibres must be mixed and blended together before any processing begins. Usually, bale openers are used to accomplish this purpose.

3. These blended fibres are now passed into a carding machine using hopper feeders. This makes the fibres parallel to one another and delivers them in web form. At least two carding machines (carding cycles) are employed by passing the first web (through a transporter) to a second machine; this produces a new web which is thicker and fully carded.

The stuff has to be distributed as evenly and uniformly as possible both horizontally and longitudinally on the actual web makers.

The basic rule is: 
The quality of nonwoven bounded fabrics such as felt can only be as good as that of the fibre web or fleece the web has been made into. Since the carders are not equipped with any storage facilities, any mass fluctuation in the feed will reappear unchanged in the card web, and once the fleece has been formed it cannot in the course of further processing be made more even and the irregularities present often have a decidedly negative effect on the mechanical and physical properties such as strength fiter effect e.t.c.

4. Several different web are combined (sandwich laid) to create one thick web. Four layers of web considered a standard single roll, sometimes referred to as a batt. Batts are layered in order to create different thicknesses of felt. 

THE MANUFACTURING PROCESS:

The batts for making felted material must be hardened or matted together in order to create thick, densely felted material; this is achieved through the process out line below:

1. The batts are subjected to heat and moisture by passing them through a steam table.

2. The wetted batts are fed into a plate-hardener that shrinks the width of the fabric. The plate-hardener consist of a large square flat bed with a large plate that drops down over the wetted hot batts, exerting pressure on the material and compressing it. At the same time, the plate-hardener oscillates from edge to edge further matting the fibre to a specific width. Alternatively, a roller hardening machine can be used; here the batts are pressed by rollers rather than by plates. From the hardening machine, the fabric (except cushioning and padding felt) is sent to a fulling mill, where it is shrunk up to 50% in both length and width.

3. The above batts are fed into a fuller or fulling machine, which shrinks the length to specific measurement. As it shrinks, the felt becomes denser. The batts are fed through a set of upper and lower steel rollers that are covered with hard rubber or plastic and are molded with treads much like a car tire, enabling them to move across the felt. The felt is continuously wetted with hot water and sulphuric acid solution. The upper rollers remain stationary as the lower rollers are moved upwards to put pressure on the fabric and push it against the upper rollers. All the rollers, (both upper and lower) move together forward and backward. The pressure, the acid, the hot water and the movement causes the batts to shrink in length, making the felt even denser. Example, a piece of felt that is 38 yards long may come out of the fuller at only 30 yards in length.

4. The wet felt has sulphuric acid residue and must be neutralize by running it into neutralizing tanks filled a sodium chloride (soda ash) and warm water solution. The speed of passage at this stage is carefully timed.

5. The neutralize felt is then run through a refulling machine in which heavy rollers run over the surface of the fabric one last time to smooth out any irregularities.

6. If felts are to be dyed, the wet pieces are taken to a dye bath and if otherwise, the washed felts are finished in several ways. Finishes include those designed to make the fabric water proof or moth proof. Then the product is finally passed to a calendering or tendering machine. The felt is roll up and sent to a centrifugal dryer that spins out the water, while other companies have huge dryers in which the felt is pinned in place on a dryer bed.

7. Once dry, some companies press or iron the felt to ensure consistent thickness. Some manufacturers use this ironing to make dense felt even denser as ironing can shrink it slightly.

8. The finishing step includes placing the felt on a gauging table in which the edges of the felt are neatly trimmed. The fabric is now ready for packing, labeling and selling.

DRY FELTING (NEEDLE-FELTING): 
A needle felted fabric is a non-woven fabric made from webs or batts of fibres in which special barbed felting needles on an industrial felting machine are used. The barbs catch the scales on the fibre and push them through the layers of web, tangling them and binding them together. This needling action interlocks the fibres and hold the structure together much likes the wet felting process, and it’s popular for two-dimensional and three-dimensional felted work.

With needle felting, any fibre will work even man-made fibre and hair. All other wholly artificial felts are actually needle-felts.

MANUFACTURING PROCESS:
 A web or batts of fibre are transported by a feeding device between upper and lower hole-plates. The bearded needles periodically penetrate through the holes in the plates and through the batts. In every stroke, the barbs of the needle seized fibres and pull the fibres through the web creating fibre bundle. As the needle withdraws, the batt is released and moves a small step towards take-off rolls.

The level of web densification is among others a function of the number of punches per unit area of the web, the number of needles in the needle board. Attainable frequency of the needle board determines the performance of the machine.

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Posted in: Nonwovens

An Overview of Flame Retardant Nonwoven Fabrics

Introduction Nonwovens: 
Are finding more and more applications requiring flame retardancy in areas that were once the sole domain of woven textiles, Nonwoven products are mainly manufactured using synthetic fibers such as polyolefin, polyester or nylon that represent highly flammable products. Polypropylene, in particular, burns very rapidly with a relatively low amount of smoke and without leaving a char residue because of its wholly aliphatic hydrocarbon structure . Its self ignition temperature is around 570 °C and it presents a rapid decomposition rate compared with wood and other cellulosic materials.

The use of nonwovens manufactured with synthetic fibers can thus lead to an increased fire risk in many cases. This has to be taken into account even more nowadays since there is a trend to replace high cost materials by lower cost materials, for example polypropylene. 

Flame Retardant Nonwoven

Flame retardancy of nonwovens can be achieved in two ways: additive (mechanically blending the FR chemistry with the polymer prior to extrusion) and topical (coating the fiber or fabric with the FR chemistry). Additive types are useful with thermoplastics, while topical treatments can be used with thermoplastics, thermosets and natural fibers.

What are Flame Retardants and How do They Work? 
There are several stages in the combustion process: heating, decomposition, ignition, flame spread and smoke generation. The combustion leads to the production of heat that is fed back and pyrolyzes the polymer, produces more fuel, and keeps the combustion process going. Flame retardants are chemicals that interfere in one or several of the steps in the combustion process. This is done in four distinct modes of action: 

1. Reaction in the gas phase,
2. Reaction in the condensed phase,
3. Cooling effect and
4. Dilution effect.

A flame-retardant additive for nonwovens must meet the following requirements: 

• The product must not adversely affect the natural color or coloration of the fiber.
• It must be non-smoking during fiber production.
• It must have no adverse effects on short- and long-term fiber properties.
• It must have no adverse effect on ultraviolet (UV) durability.
• It must pass the newest standards in a global market.

Basics of Flame Retardancy:
 The process of ignition and burning can be described in short as a gas phase reaction. Thus, a substance must become a gas for burning. As with any solid, a textile fabric exposed to a heat source undergoes a temperature rise. If the temperature of the source (either radiative or a gas flame) is high enough and the net rate of heat transfer to the fabric is high, pyrolytic decomposition of the fiber substrate will occur. The products of this decomposition include combustible gases, non-combustible gases and carbonaceous char. The combustible gases mix with the ambient air and oxygen. The mixture ignites, yielding a flame, when its composition and temperature are favorable.

Part of the heat generated within the flame is transferred to the fabric to sustain the burning process and part is lost to the surroundings. The considerable fire hazards posed by textiles both in historical times and to the present day are a consequence of the large surface area of the fibers and the ease of access to atmospheric oxygen. The goal of flame retardancy is then to inhibit or even suppress the combustion process acting chemically and/or physically in the solid, liquid or gas phases. It can interfere with combustion during a particular stage of this process, e.g. during heating, decomposition, ignition or flame spread. 

Various methods can be used to protect materials more effectively from fire. The first method is to use inherently flame retarded polymers or high performance polymers but it implies the use of specific materials that might not have the required properties. The second method is to chemically modify the existing polymer to synthesize the FR polymer. The third method is to use flame retardants and/or particles (micro- or nanodispersed) directly incorporated in the materials (e.g. thermoplastics, thermosets or synthetic fibers) or in a coating covering their surface (e.g. structural steel or textiles). In this section we only focus on the mechanism of action of FR materials. Our intention is to provide the reader with the general principles of flame retardancy. 

The various ways in which a flame retardant can act do not occur singly but should be considered as complex processes in which many individual stages occur simultaneously, with one dominating (e.g. using hydroxides causes an endothermic decomposition, cooling down the substrate and diluting the ignitable gas mixture due to the formation of inert gases associated with the formation of the oxide protective barrier).

Physical Action:
 There are several ways in which the combustion process can be retarded by physical action: 

1. By formation of a protective layer. Under an external heat flux the additives can form a shield with a low thermal conductivity that can reduce the heat transfer from the heat source to the material.
2. By cooling. The degradation reactions of the additive can play a part in the energy balance of combustion. The additive can degrade endothermally, which cools down the substrate to a temperature below that required for sustaining the combustion process.
3. By dilution. The incorporation of inert substances (e.g. fillers such as talc or chalk) and additives that produce inert gases on decomposition dilutes the fuel in the solid and gaseous phases so that the lower ignition limit of the gas mixture is not exceeded.

Chemical Action The most significant chemical reactions interfering with the combustion process take place in the condensed and gas phases: 

1. Reaction in condensed phase. Here two types of reaction can take place. Firstly, breakdown of the polymer can be accelerated by the flame retardant causing a pronounced flow of the polymer and, hence, its withdrawal from the sphere of influence of the flame, which breaks away. Secondly, the flame retardant can cause a layer of carbon (charring), a ceramic-like structure and/or a glass to be formed on the polymer surface.
2. Reaction in gas phase. The radical mechanism of the combustion process that takes place in the gas phase is interrupted by the flame retardant or its degradation products.

The fire retardant additive systems may be used alone or in association with other systems in polymeric materials to obtain a synergistic effect, i.e. the protective effect is higher than is assumed from the addition of the separate effects of each system. 

Different Approaches for Flame Retardant Nonwovens:
 Most fibers are highly combustible (except high performance fibers) and the flammability of derived fabrics largely depends on the construction and density of the fabric. Several approaches can be used to enhance the fire behavior of fiberbased fabrics used either alone or in blends with other fibers: 

1. Coatings and/or finishing treatments may be applied to shield fabrics from heat sources and prevent volatilization of flammable materials. These may take the form of simple protective coatings or, more commonly, the treatment of fabrics with inorganic salts that melt and form a glassy coating when exposed to ignition sources.
2. Thermally unstable chemicals, usually inorganic carbonates or hydrates, are incorporated in the material, often as a back-coating so as to preserve the surface characteristics of the carpet or fabric.
3. Materials that are capable of dissipating significant amounts of heat are layered with the fabric or otherwise incorporated in a composite structure. These may be as simple as metal foils or other heat conductors or as complicated as a variety of phase-change materials that absorb large quantities of heat as they decompose or volatilize. If sufficient heat is removed from the point of exposure, the conditions for ignition are not reached.
4. Char-promoting chemical treatments that may be fiber-reactive or unreactive to yield launderable or non-durable flame retardancy, respectively.
5. Chemicals capable of releasing free radical trapping agents, frequently organobromine or organochlorine compounds, may be incorporated into the fabric.
6. In the particular case of synthetic fibers (approaches listed above are valid for both natural and synthetic fibers), the direct incorporation of additives (microfillers and/or nanoparticles) or the chemical grafting/copolymerization of specific groups.

Flame Retardant Fibers for Nonwovens:
To make flame retarded fibers, several approaches can be considered: 

1. the incorporation of FR additive(s) in the polymer melt or in the solution prior to extrusion, 
2. the copolymerization or the grafting of FR molecules to the main polymeric chain, and 
3. the use of semi-durable or durable finishing. The only candidates for applying the first two approaches are synthetic fibers .

Synthetic fibers: 
Are numerous and all of them require flame retardancy, appropriate to their chemical formulation. We will then focus on the most established and used fibers. They include polylactic acid (PLA), polyester, polyamide and polypropylene. All these fibers are used for making nonwovens. 

The multifilaments were knitted and the flammability studied using cone calorimetry at an external heat flux of 35 kW/m². Depending on the clay loading, the peak value of RHR is decreased by up to 38 % demonstrating the improved fire performance of these PLA fibers. Formation of char is observed in the case of the nanocomposites suggesting a mechanism of condensed-phase. 

Polyester fibers are the main synthetic fibers used in the industrial manufacturing sector and can be found in several areas of application. As polyester fibers are easily flammable, flame retardancy is a significant issue.

Recent work by Horrocks has been the investigation of the effect of adding selected flame retardants based on ammonium polyphosphate, melamine phosphate, pentaerythritol phosphate, cyclic phosphonate and similar formulations into nylon 6 and 6.6 in the presence and absence of nanoclay. They found that in nylon 6.6 all of the effective systems comprising the nanoclay demonstrated significant synergistic behavior except for melamine phosphate because of the agglomeration of the clay. They then report in the case of nylon 6 that the presence of nanoclay acts in an antagonistic manner (in terms of LOI). To explain why the nanoclay lowered the LOI value of the FR-free nylon 6 film but not that of the nylon 6.6, they proposed that the nanoclay reinforces the fiber structure both in solid and molten phases, thereby reducing its dripping capacity. 

Polypropylene (PP) is presently one of the fastest growing fibers for technical end-uses where high tensile strength coupled with low cost are essential features. 

An acceptable flame retardant for polypropylene, especially fiber-forming grades, should have 

1. a thermal stability up to the normal PP processing temperature (< 260 °C),
2. a good compatibility with PP and no migration of the additives,
3. flame retardancy properties when present in the fiber and
4. efficiency at a relatively low level (typically less than 10 wt.%) to minimize its effect on fiber/textile properties as well as cost.

Applications of Flame Retardant Nonwovens:
In order to ensure the safety of the public with regard to fire, standards, regulations and requirements in this field are continually discussed and modified. It is not easy to navigate the maze of testing methods and standards. In Europe, harmonization was initiated in the 1990s and is still progressing. The new regulations present new hallenges to the flame retardancy industry. Examples of applications of FR nonwovens are reported below. 

Protective Garments:
 The field of protective garments is relatively large with differing requirements since it incorporates the protection of men at work and military applications, as well as clothing for firefighters. This problem is also relatively complex since a number of properties are required for a material to be used in this field of application. Indeed, heat protective performance is needed, but also heat-moisture transfer properties and comfort performance including lightness, for example, have to be taken into account, and usually a balance between the heat and moisture barrier has to be found. Usually, protective fabrics are multilayer clothing containing up to five or six layers. Fire protective clothing for firefighters consists of at least four layers: outer and inner shell, moisture barrier, and thermal liner. These layers are expected to provide adequate heat, flame, liquid, chemical and mechanical protection. Nonwovens composed of high performance fibers are typically used as thermal liners in protective garments.

Fire-blockers for Seat and Upholstery:
 Fire-blockers are usually highly fire resistant materials that are placed beneath the exterior cover fabric of furnishing and the first layer of cushioning materials in seats, mattresses and upholsteries. The bulky cushioning materials represent the major fuel source and therefore the greatest hazard potential. The fire-blocker acts as a barrier between the heat source (flame, cigarette, etc.) and the cushioning materials limiting fire growth and development. Fabric-like fire-blockers include woven and needle punched fabrics made from highly resistant textile fibers such as glass, Nomex, Kevlar, PBI, etc. Some of the fabric-like fire-blockers available are engineered textile products that use a combination of different fibers and fabric treatments. The first modern generation of fire-blockers was introduced by Dupont under the trade name VonarTM during the 1970s.

Other Applications:
 Another application of FR nonwovens is flexible insulation panels for building construction. Although traditional thermal insulation materials such as mineral wool or polystyrene are widely used, the return to ecology and nature noticed in several application fields is also observed in the building industry. Natural wool, coconut or even duck feathers are used to design thermal insulation panels. However, all these materials burn easily and thus FR treatments are required. The development of needle punched nonwovens and air-laid nonwovens based on fire retardant modified natural fibers has been reported and has been demonstrated that such materials meet the requirements for use in building applications. Finally, it is noteworthy that there are applications where FR properties are required in disposable nonwovens, for example for surgical drapes used in operating rooms as well as air filters used in the automotive industry.

Conclusion:
 After many years of research and development and extensive analytical testing of the active additives and master batches produced, the polyester fiber manufacturer's final product. Demand of application for flame retardant nonwovens is increasing annually. So nonwovens manufacturers are entering new markets where they must meet established FR requirements. 

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