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mechnaics of non-wovens

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producing a non-woven fabric and its mechanical performance

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mechnaics of non-wovens

  1. 1. Non-Woven Fabrics INDA (Association of the Nonwoven Fabrics Industry located in Cary, North Carolina, USA) defines a nonwoven as ‘a sheet, web, or batt of natural and/or man-made fibers or filaments, excluding paper, that is bonded by any of several means.’ EDANA (The European Disposables and Nonwovens Association located in Brussels, Belgium) defines a nonwoven as ‘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 (woven, knitted, tufted, stitch-bonded) Introduction
  2. 2. Non-Woven Fabrics Introduction The term nonwoven refers to web-like assemblages of fibers wherein fiber to fiber bonding replaces twisting and interlacing. We define a nonwoven as an engineered fabric structure that may contain fibrous and non-fibrous elements and that is often manufactured directly from fibers or filaments and may incorporate other types of fabrics. The difference primarily between a nonwoven and its more traditional counterparts (woven, knitted and braided structures) is the structure.
  3. 3. Non-Woven Fabrics Introduction The fibers or filaments in a nonwoven are not interlaced or inter- looped and are somewhat random layered assemblies of fibers held together by a variety of different means. The structure of a nonwoven is defined therefore as its fiber orientation distribution function (ODF). Another important structural aspect to consider is the basis weight (mass per unit area – g/m2 or more commonly referred to as gsm) and its uniformity.
  4. 4. Non-Woven Fabrics Introduction ODF may dictate behavior and weight uniformity dictates failure. The structure-property relationships in a nonwoven depends on the process utilized to form the nonwoven. Therefore, short review of the processes employed in the making of nonwovens is important to describe the mechanical properties of nonwovens.
  5. 5. Non-Woven Fabrics Non-woven production processes .The first question which comes to mind when asked about nonwoven mechanical properties is: what process was used to make it ? Nonwovens production is usually split into two steps: web formation and web consolidation (or bonding).
  6. 6. Non-Woven Fabrics Web formation . Cross Lapper schematic
  7. 7. Non-Woven Fabrics Web formation . Air Laid schematic
  8. 8. Non-Woven Fabrics Web formation . Spun bonding schematic
  9. 9. Non-Woven Fabrics Web formation . Melt blown schematic
  10. 10. Non-Woven Fabrics Bonding . Calendering schematic
  11. 11. Non-Woven Fabrics Bonding . Bond pattern schematic
  12. 12. Non-Woven Fabrics Bonding . Hydroentanglement schematic
  13. 13. Non-Woven Fabrics . Fiber orientation distribution function (ODF) The orientation distribution function [ODF] y is a function of the angle .
  14. 14. Non-Woven Fabrics . The influence of the production method on anisotropy The different web formation processes will impart different initial ODF, which might be further modified by the bonding process. The opening and carding processes have a significant impact on the orientation of the resultant web. The carding process, by nature, imparts a high degree of orientation to the fibers in the machine direction. The main cylinder and the workers in the card align the fibers parallel to the machine direction.
  15. 15. Non-Woven Fabrics . The influence of the production method on anisotropy Inadequate opening of the fibers creates a non-uniform fabric that has a tendency to break easily during processing. A fabric formed from a web with fibers mostly aligned in the machine direction will be expected to have high strength in the machine direction and relatively low strength in the cross direction. Other properties follow the same pattern
  16. 16. Non-Woven Fabrics . The influence of the production method on anisotropy To improve the cross direction strength requires the rearrangement of the fibres so as to have a higher degree of orientation in the cross direction. This can be achieved by several mechanical methods. One method involves stretching the web in the cross direction prior to the consolidation or bonding step. When the web is stretched in the cross direction, fibres are pulled away from the machine direction and realigned in the cross direction.
  17. 17. Non-Woven Fabrics . The influence of the production method on anisotropy Another method commonly employed is a cross-lapper that takes a card feed and cross-laps it into a uniform batt before consolidation or bonding. Most cross-lapped webs have a bimodal fibre orientation distribution. The ODF in the wet lay process also has a machine direction dependency. Here, the ODF can be adjusted by controlling the throughput and the speed of the belt.
  18. 18. Non-Woven Fabrics . The role of ODF on mechanical performance Unit cell
  19. 19. Non-Woven Fabrics . The ODF was measured from a series of such images captured at regular intervals of deformation at each test direction. Images at 0, 25% and 50% strain for 90° direction (cross direction). The role of ODF on mechanical performance
  20. 20. Non-Woven Fabrics . Re-orientation with different testing directions When the samples are tested in the cross direction (90°), the fibers reorient significantly and the dominant orientation angle changes from its initially preferred machine direction towards the loading direction.
  21. 21. Non-Woven Fabrics . Re-orientation with different testing directions In the case of samples tested in the machine direction (0°), where the initially preferred orientation coincides with the loading direction, the deformation-induced effect is, as expected, primarily to increase this preference of fibers
  22. 22. Non-Woven Fabrics . Re-orientation with different testing directions The reorientations due to the test deformations imposed at 34° also show similar changes in the dominant orientation angle but of a much smaller magnitude than that obtained at 90°.
  23. 23. Non-Woven Fabrics . Re-orientation with different testing directions The reorientations due to the test deformations imposed at −34° show similar changes in the dominant orientation angle but of a much smaller magnitude than that obtained at 90°.
  24. 24. Non-Woven Fabrics . The role of ODF on mechanical performance Stress strain behaviour and fracture surfaces
  25. 25. Non-Woven Fabrics . The role of ODF on mechanical performance The samples tested at 34° and −34° directions fall between the two cases of ‘low stress–high strain’ and ‘high stress– low strain’ failure along the cross and machine directions, respectively. Also, the failures are dominated by shear when the fabrics are tested at 34° and −34°. The fracture edges are shown for each case in Fig. As expected, failure tends to propagate along the dominant orientation angle.
  26. 26. Non-Woven Fabrics . The role of ODF on mechanical performance Shear angle as a function of strain.
  27. 27. Non-Woven Fabrics . The role of ODF on mechanical performance The application of a macroscopic tensile strain produces a significant shear deformation along the initially preferred direction in fiber ODF, except when the two directions are either parallel or normal to each other.
  28. 28. Non-Woven Fabrics . The role of ODF on mechanical performance Asymmetry parameter as a function of strain
  29. 29. Non-Woven Fabrics . The role of ODF on mechanical performance The moments are greatest when the test is performed in directions other than the two principal directions (machine and cross). The fabric performance is a function of its structure or the manner in which the fibers are arranged within the structure. The structural changes brought about in the structure and the microscopic deformations are driven by the initial orientation distribution function (ODF) of the fibers and are similar for all structures with the same initial ODF.
  30. 30. Non-Woven Fabrics . The role of ODF on mechanical performance The bonding conditions only dictate the point of failure
  31. 31. Non-Woven Fabrics . Failure Mechanisms The main source of failure in nonwovens is defects. Those defects can be split into two categories:  Non-uniformities, mostly originating from the web formation,  Over/under bonding. Once failure is initiated, its propagation is essentially ruled by the ODF.
  32. 32. Non-Woven Fabrics . Failure Mechanisms The foremost factor in this category is weight uniformity of a nonwoven. This refers to the degree of mass variation in a nonwoven normally measured over a certain scale. Local variation of the mass results in unattractive appearance, but more importantly will lead to, and potentially dictate the failure point of a nonwoven. For example, tensile failure may be initiated and propagated first in areas that are fiber-poor (region with low mass),
  33. 33. Non-Woven Fabrics . Failure Mechanisms Under-bonding occurs when there are an insufficient number of chain ends in the molten state at the interface between the two crossing fibers or there is insufficient time for them to diffuse across the interface to entangle with the free ends of the chains in the other fiber. Thus, only a few tie chains exist and the bonds can be easily pulled out or ruptured under load. The formation of a bond requires partial melting of the crystals to permit chain relaxation and diffusion. In the case of underbonding, insufficient melting has occurred or too little time for diffusion was allowed prior to cooling.
  34. 34. Non-Woven Fabrics . Failure Mechanisms Over-bonding occurs when many chains have diffused across the interface and a solid, strong bond has been formed. The fibers within the bond have also lost some of their molecular orientation (and strength) at the fiber bond interface. They may have also become flat and irregular in shape. The bond site edge becomes a stress concentration point where now the weaker fibers enter.
  35. 35. Non-Woven Fabrics . Failure Mechanisms In a fabric under load, this mechanical mismatch results in the premature failure of the fibers at the bond periphery. Over-bonding occurs when too much melting has occurred. Under-bonded (left) and over-bonded (right) bond spots.

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