LESSON 9 NON-WOVENS AND BRAIDS STRUCTURE 9.0 OBJECTIVES 9.1 INTRODUCTION 9.2 PRODUCTION PROCESS 9.3 WEB FORMATION 9.

Transcription

1 LESSON 9 NON-WOVENS AND BRAIDS STRUCTURE 9.0 OBJECTIVES 9.1 INTRODUCTION 9.2 PRODUCTION PROCESS 9.3 WEB FORMATION 9.4 BONDING OF WEBS 9.5 CHARACTERISTICS OF NON-WOVENS 9.6 USES OF NON-WOVEN FABRICS 9.7 FIBRES USED AS NON-WOVENS 9.8 FELTS FELTING PROCESS PROPERTIES OF FELT USES OF FELT 9.9 FLOCKING 9.10 BRAIDS 9.11 ASSIGNMENTS CLASS ASSIGNMENTS HOME ASSIGNMENTS 9.12 SUMMING UP 9.13 POSSIBLE ANSWERS TO SELF-CHECK QUESTIONS 9.14 TERMINAL QUESTIONS 9.15 REFERENCES AND SUGGESTED FURTHER READING 9.16 GLOSSARY 1

2 9. NON-WOVENS AND BRAIDS In the previous two lessons, you were introduced to the weaving and knitting processes respectively, which are the commonly used methods of fabric construction. In this lesson you will learn about Non-wovens. As the term implies, the fabrics in this category are not woven; they are constructed through various techniques of bonding fibres or yarns through mechanical, chemical or thermal means. Such fabrics are gaining importance in India and their basics will be considered in this lesson. 9.0 Objectives After going through this lesson, you will be able to: Understand how non-woven fabrics are produced Know about web formation, and their bonding. Know which fibres are suitable for making fabrics. Gain an understanding of the characteristic features and varied applications of non woven fabrics. 9.1 Introduction The non-woven fabrics are produced by techniques which can form a fabric from fibres or even directly from yarns. Such fabrics are used in varied applications like consumer and healthcare goods and in automobiles, engineering, furnishings etc. apart from their textile uses. Non-wovens are thus exclusive fabrics which are porous, soft, bulky according to the type, but are less durable than woven or knitted goods. Due to the practical and functional uses of non wovens, they are much in demand for varied end uses. The Textile Institute defines a fabric as a manufactured assembly of fibres and/or yarns, which has substantial area in relation to its thickness and sufficient mechanical strength to give the assembly inherent cohesion. Fabrics are most commonly woven or knitted, but the term includes assemblies produced by lacemaking, felting, net-making and the so-called non-woven processes. We have already seen that woven and knitted fabrics are made by interlacing and interlooping of linear assemblies of filaments and fibres (i.e. yarns). Non-wovens are made by bonding of web-like arrays of fibres or filaments. The webs may be made from fibres of discrete lengths (ranging from a few millimeters to a few metres) by the carding or wet-laying processes, or they may be produced by laying or blowing filaments as they are being melt-extruded. The bonding of the loose assembly of fibres in non-wovens is achieved through mechanical, chemical or thermal means. 2

3 A whole variety of fibres and filaments are used for producing of nonwovens. Both natural and man-made fibres are used but the use of man-made fibres is growing at a rapid rate. An outstanding advantage of non-wovens is that they can be produced very fast. Though different methods of production work at different speeds, it may be interesting to note that the per hour rate of production of a one-meter wide fabric is 0.5 to 10 meter for weaving, 0.20 to 80 meter for knitting, and to 10,000 meter for non-wovens. Non-woven fabrics can be used for making both disposable and durable products. Some of the disposable products are baby diapers, medical textiles and wipes. Durable products include automotive fabrics, interlinings, home furnishing and as filtration media. Geotextiles represent a very important and large market for non-wovens. 9.2 Production Process Though a variety of processes may be used to produce non-wovens, a typical production process consists of the following steps: Selecting the fibre: A wide range of fibres can be used, based on the property desired in the end-product. Laying the fiber to make a web. Laying the web to make a fleece, and Bonding the web or fleece to make a fabric. 9.3 Web Formation There are different ways of laying the fiber to make a web. Some of them are described below: Parallel-laid web: The fibres are passed through a carding machine which puts them in parallel alignment. Thin webs of parallel fibres are laid over each other to form multi-layer webs. This produces fabric that has high lengthwise strength but low crosswise strength. In majority of cases, the woollen or worsted card is used on account of its greater flexibility and productivity. Cross-laid web: The webs are criss-crossed successively over each other. The fabric has greater crosswise strength than the parallel-laid web type. Random-laid web: Loose fibres are blown onto a rotating drum on which the fibers form a uniform web of randomly placed fibres. These fabrics have uniform weight & strength in all directions. High velocity sprayed web: Thermoplastic fibers are sprayed on a belt in the form of a random web. Heat & pressure are then applied causing the fibers to fuse. 3

4 Air-laid web: In this process, the fibres are suspended in air within a forming system and deposited on a moving forming screen or rotating perforated cylinder. The randomly oriented air-formed batt is bonded together with heat, glued with a coating of adhesive or held together by mechanical entanglement, usually involving needles that grab the fibres and pull them through the web. Air laid non-wovens are highly absorbent fabrics. Wet and dry wipes and absorbent cores for hygiene products are two uses for air laid non-wovens. Wet-laid web: Fibers are mixed with chemicals and are processed through beaters & pulpers to pass through a wire screen. The wet-laid non-woven process is a fiber forming process, and wet-laying takes place between the head box and the forming wire. In this area, the fibers are suspended in water slurry and deposited on a moving screen. The water drains out and a fiber web is formed. The wet-laid web is further dewatered, consolidated, by pressing between rollers, and dried. Wet-laid web forming allows a wide range of fibre orientations. Wet-laid non-wovens are developed and manufactured for products such as filters, tea bags, wallpaper, hygiene products, medical wrap and drapes, wipes, Fig. 9.1 Typical Wet laid Wipes food packaging, etc. Some typical wet-laid wipes are shown in Fig Bonding of Webs The fibres in the webs are bonded together to form non-woven fabrics using the following methods: Resin bonding: A suitable bonding agent is applied to the fiber with high melting point. The web is heated to a temperature, which fuses the fibres and bonds them with one another. Spun bonding: The production of spunbonded non-wovens is a continuous operation, starting from fluid polymer and ending with the fabric. Filaments are drawn from the spinneret by air & sprayed on a belt in a random manner. The web is passed between rollers that Fig. 9.2 Spun-bonding Technique apply heat & pressure, which in turn bind the thermoplastic fibres together. In Fig. 9.2, a production unit is shown schematically. 4 Thermoplastic bonding: Thermoplastic fibres with low melting points are blended with either non-thermoplastic fibres or thermoplastic fibres with

5 high melting point. The web is heated to a temperature, which bonds them to one another. Spun-laced bonding: The production of spun-laced non-wovens involves the use of water jets to consolidate the web. Any web forming process can be used to create the web, typically made from pulp, staple or filament fibers and/or other raw materials. The web is then consolidated by applying water through fine, high pressure jets which cause the fibers to curl and entangle about each other. The spun-laced non-woven is strong, soft and pliable. The end products are medical packs and gowns and wipes. Needling: The needling technique binds the fibre web using barbed needles. This generally consists of sandwiching two cross-laid webs and a reinforcing scrim (fabric) or warp threads by means of barbed needles (Fig. 9.3). The mechanical bonding occurs by transporting the fibre from the horizontal to the vertical plane of the web. Fig. 9.3 The Needling Technique Stitch-through bonding: Thermoplastic threads are used to stitch laminated webs together. Heat is applied that causes thread to soften and bond the web. Self-check Questions 1. What is a non-woven fabric? 2. Name the various types of webs from which non-wovens are made. 5

6 3. Name the methods of bonding of webs. Activity 1. Make a chart for your future reference which shows salient points regarding web formation and web binding. 9.5 Characteristics of Non-wovens Non-woven fabrics look like paper or felt. The texture may vary from soft to hard. The fibres can be as thin as tissue paper or can be very thick. Porosity may range from free-flow to impermeable. 9.6 Uses of Non-woven Fabrics Non-woven fabrics may be classified as durables or disposables. The following items for domestic and industrial uses are made from non-woven fabrics Durables: Caps, interlinings, interfacing, home furnishing mattress padding, carpet backing. Industrial uses include filters, insulation, and geotextiles for road-bed stabilization, etc. Disposables: Dusting cloth, diapers, napkins, surgical and industrial mask, bandages & towels. 9.7 Fibres Used as Non-wovens Almost all fibres have found some use in non-wovens. Taking spunbonded non-wovens as an example, polypropylene fibre has been used for making non-wovens for applications as diverse as carpet backing, upholstering furniture, replacing paper, jute and cotton packaging materials and construction engineering. A growing application for them is as geotextile mainly in situations where there is contact with water, acids, or alkalis and where good abrasion resistance is required. Spun-bonded non-wovens based on polyethylene have found uses as labels, for making flags and for bookbinding. Polyester fibre-based spun-bonded non-wovens have found wide use in the geotextile area because of their high chemical resistance, low creep behaviour, high melting point (265 C), which permits their use in contact with bitumen in road construction, good resistance to ultra-violet light, high tear and penetration resistance, etc. They also find extensive use as disposable protective clothing, carpet backing, filters for liquids and gases, interlining for clothing industry, table cloth and other household textiles, wall coverings, etc. Polyamide fibres have also found considerable use particularly as primary backing in tufted carpets. 6

7 9.8 Felts Felts and non-woven fabrics are similar in that they are made directly from fiber. Felt usually refers to fabric made from wool, whereas the term non-woven applies to non-woven fabrics made from other fibres Felting process The crimp and scales on the surface of woollen fibres cause them to cling and intermesh and allow them to be pressed into a compact fabric in the presence of pressure, moisture and heat. When warm water is sprinkled on the webs and they are passed through a pair of rollers, felts are formed. In the presence of pressure, heat and moisture, further felting occurs. The assembly is allowed to cool for 24 hours, and a lubricant is applied. After this pounding with wooden hammers stabilizes the structure and makes it firm Properties of felt Felts have no warp, filling or selvedge. They do not fray or ravel. Seam edge of felt does not need finishing. Felt has little tensile strength. It tears easily. It has hardly any elasticity and therefore is not suitable for draping. Felt can be shaped with the help of blocks into any shape, for example hats from it can retain the shape it has been moulded into. Felt absorbs sound and shocks. Felts cannot be washed and need dry-cleaning Uses of felt Some of the uses of felt are: Industrial - Padding, Sound Proofing, Insulation, Filtering, Polishing, Wicking, shoe insoles, earmuffs, Decorations, etc. They are extensively used for removing moisture from paper during paper production. 9.9 Flocking Flocking is the technique of binding very small fibres to form designs on fabric. A suitable adhesive in a design is roller printed on to a fabric. Then flocks of cotton, wool, viscose, nylon or acrylic fibre are applied to the fabric in a manner that causes it to stick in an upright position & produce a velvet-textured design Braids Braiding also falls under the category of unconventional weave, in which the yarns are twisted and locked together in varieties of forms to produce various fancy braids. 7 Fig. 9.4 A Typical Braided Structure

8 To braid is to interweave or twine three or more separate strands of one or more materials in a diagonally overlapping pattern. Simple braids with more than three strands can be flat or tubular and generally contain an odd number of strands. Complex braids have been used to create hanging fiber artworks. Braiding of fiber yarn creates a strand or rope that is thicker and stronger than the strands would have been separately. Braided rope is preferred by arborists and rock climbers because it does not twist under load, as does an ordinary twisted-strand rope. These ropes consist of one or more concentric tubular braided jackets surrounding a single untwisted yarn of straight fibers. A typical braided structure is shown in Fig Self-check Questions 4. What is a felt? 5. How is a braid made? 6. Can we have a knitted braid structure also? 7. What is flocking? 8. Fill in the blanks: i) Per hour rate of production of a one-meter wide fabric is for woven, for knitted and for non wovens. ii) Fibres, when passed through carding machines, become. iii) In process the fibres are suspended in the air. iv) threads are used in the process of stitch through bonding. v) Needling technique binds the fibre web using needles Assignments Class assignments i) Collect samples of non-woven fabrics, try to recognize the web laying type and label them. 8

9 Home assignments i) Obtain samples of braids, felts and flocking, paste them on small square cards and label them Summing Up The information provided in this lesson can be summed up as follows: Non-woven fabrics are broadly defined as sheet or web structures bonded together by entangling fiber or filaments mechanically, thermally or chemically. They are flat, porous sheets that are made directly from separate fibers or from thermoplastic filaments. They are not made by weaving or knitting and do not require converting the fibers to yarn. Non-woven fabrics are engineered fabrics that may have a limited life as a single-use fabric or may be more durable. Non-woven fabrics provide specific functions such as absorbency, liquid repellency, resilience, stretch, softness, strength, flame retardancy, washability, cushioning, filtering, bacterial barrier and sterility. These properties are often combined to create fabrics suited for specific jobs, while achieving a good balance between product use-life and cost. In combination with other materials they provide a spectrum of products with diverse properties, and are used alone or as components of apparel, home furnishings, health care, engineering, industrial and consumer goods Possible Answers to Self check Questions 1. A non-woven fabric is one, which is neither woven nor knitted. It is made by bonding web like arrays of fibres or filaments. 2. Parallel laid web, cross laid web, random-laid web, high velocity sprayed web, air-laid web, wet-laid web. 3. Resin-bonding, spun-bonding, thermoplastic bonding, spun-laced-bonding, needling, stitch through-bonding. 4. Felts and non-woven fabrics are similar in that they are made directly from fiber. The term felt is generally used for a non-woven fabric based on wool fibres. 9

10 5. Yarns are twisted and locked together in a variety of forms to produce various fancy braids. 6. No 7. Flocking is the technique of binding very small fibres to form designs on fabric. 8. Fill in the blanks: i) 0.5 to 10 mts., 0.20 to 80 mts and 0.19 to 10,000 mts ii) Parallel iii) Air-laid web iv) Thermoplastic v) barbed 9.14 Terminal Questions 1. List some durable non-woven items and some disposable non-woven fabrics. 2. What are the main characteristics of non-woven fabrics? 3. State the various applications of non-woven fabrics References and Suggested Further Reading 1. Corbmann, P Textile: Fibre to Fabric. McGraw-Hill Book Company. 2. Non-wovens, (Ed.). Gulrajani, M.L., 1992, The Textile Institute (North India Section) and Porritts & Spencer Ltd., New Delhi. 3. Internet Websites: i) ii) Glossary 1. Thermoplastic A plastic material which softens as temperature is raised and melts at high temperature. It solidifies on cooling. 2. Bonding held together 10

11 3. Porosity having pores 4. Adhesion to stick 11