Nonwovens

Module 1: Overview of Nonwovens

• Nonwovens: What are they?

Introduction to nonwovens

Definitions of nonwovens
Nonwovens are defined in many ways by different standards and different associations. They are stated below.
As per ISO 9092, nonwoven is defined as “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 or be formed in situ.
Note: To distinguish wet-laid nonwovens from wet-laid papers, a material shall be regarded as a nonwoven if

more than 30 % by mass of its fibrous content is made up of fibers (excluding chemically digested vegetable fibers) with a length to diameter ratio greater than 300 and its density is less than 0.40 g/cm3.”
The aforesaid definition of nonwoven, given by ISO 9092, has been adapted by CEN (EN 29092), and consequently by DIN, AFNOR, and all standardization offices in the EU.
But, ASTM prefers to define nonwoven as “a textile structure produced by bonding or interlocking of fibers, or both, accomplished by mechanical, chemical, thermal, or solvent means, and combinations thereof.
Note: the term does not include paper, or fabrics which are woven, knitted, tufted, or those made by wool or other felting processes.”
This definition is available in many ASTM standards including D 123, D 1117, D 2646, D 3786, D 3787, D 5684, D 5732, D 5733, D 5734, D 5735, and D 5736.
Today, there are two leading associations of nonwovens in the world, namely EDANA (The European Disposables and Nonwovens Association) and INDA (The North America’s Association of the Nonwoven Fabrics Industry). They have been the voice of the nonwoven industries in Europe and North America, respectively. Nonwovens are defined by them as well. Though EDANA has adopted the ISO 9092 or CEN 29092 definition of nonwoven. but, INDA prefers to define nonwovens in another way. According to INDA, “Nonwoven fabrics are broadly defined as sheet or web structures bonded together by entangling fiber or filaments (and by perforating films) mechanically, thermally or chemically. They are flat, porous sheets that are made directly from separate fibers or from molten plastic or plastic film. They are not made by weaving or knitting and do not require converting the fibers to yarn.”
It is quite interesting to note that these definitions include raw materials, structures, and manufacturing processes employed to prepare nonwovens. However, there is no unique definition of nonwovens found to exist in the world.

It is often asked “Nonwovens: How do they look like?” This refers to the structure of nonwovens. It is necessary to have a sense of structure of nonwovens also to make the study of nonwoven technology more meaningful. As in case of woven and knit fabrics, the structure of the fabrics in conjunction with the properties of the constituent fibres and yarns determine the properties of the fabrics, so is the case in case of nonwovens. While the structure of the woven fabrics can be described in terms of the weave geometry and the density of warp and weft, similarly the knitted fabrics are described in terms of the intermeshing structure and the density of courses and wales [1], the structure of nonwovens is described primarily in terms of fibre packing arrangement and fibre directional arrangement. Sometimes, the geometry of pores in nonwovens is also studied. Before developing a fundamental understanding of nonwoven structure, let us first study the characteristics of the constituent materials, that is, fibres – the building block of fibre-based nonwovens.
Fibre characteristics
Fibres are the building block of fibre based nonwovens. One such fibre is displayed in Figure 1.3. Let us denote fiber mass by m and fibre length by l. The fibre fineness is defined by mass per unit length of a fibre. Then, the fibre fineness t can be written as . Let fiber cross-sectional area is s, then fiber volume V can be expressed as . As we known that the mass per unit volume of a fibre defines the density of the fibre, then, r denotes fiber density. If fiber perimeter is denoted by p, then fibresurface area A can be expressed as , provided the area occupied by the top and bottom surfaces of a fibre, being very very small, can be neglected.
Let us write the expression of fibre density in another manner. This is shown below.

It can be written as . What it follows is that the larger fibre (higher value of s) may come out to be finer (smaller value of t) than the smaller fibre (lower value of s). This is because of the influence of fibre density on fibre fineness, the coarser fibre need not to be heavier always. Let us illustrate. A 3 denier polyester fibre possesses cross-sectional area of 2.49´10-4 mm2, but a 2.5 denier polypropylene fibre possesses cross-sectional area of 3.05´10-4 mm2. According to the definition of fineness, the polyester fibre is coarser than the polypropylene fibre, but according to the cross-sectional area, the former is smaller than the latter. It indicates that the fibre fineness is not a purely geometrical characteristic and the traditional definition of fiber fineness does not truly express the fiber size. The size of fibres can be better characterized by the cross-sectional area or  ratio of the fibres. But, as the measurement of fibre volume is difficult in practice, the textile technologists prefer to use fibre fineness in order to characterize the size of fibres.
When the cross-section of the fibres is circular, we can write  where  represents fibre diameter. Then the diameter of the fibres can be calculated from the following expression
.

But, when the cross-section of fibres is not circular, then we assume that the cross-sectional area s of non-circular fibres is equal to the cross-sectional area of a circle of diameter d., then  is called as equivalent fibre diameter.
It is well-known that the perimeter of a fiber having non-circular cross-section is always higher than the perimeter of a circle. Then, it is logical to introduce a quantity called fiber shape factor q such as
.
Clearly, the shape factor of a circular fibre is equal to zero. The fibre perimeter can be written as , consequently, the fibre surface area can be expressed as .
Often, the surface area of fibres in divided by the mass of the fibres and the resulting ratio is termed as specific surface area. If the specific surface area of a fibre is denoted by a then it is possible to write that
.
This terminology is very commonly used in case of nano fibres. But, one can see that the specific surface area is not a purely geometrical characteristic. In order to obtain a purely geometrical characteristic, the surface area of fibres is often divided by the volume of the fibres and this ratio is termed as fibre surface area per unit volume. This is expressed as follows
.
The aforesaid fibre characteristics are often used to study the effect of fibre properties on physical and transport properties of fibre based nonwovens.
Let us now solve some numerical problems
Problem 1.1: Calculate the equivalent diameter of a non-circular polyester fiber of 3 denier fineness and the equivalent diameter of polypropylene fiber of 2 denier fineness. Comment on the results obtained.
Solution 1.1: We take polyester fiber density as , then the equivalent fiber diameter d is calculated as follows

Then we take polypropylene fiber density as  , then we calculate its equivalent fiber diameter d as follows

Thus, we see two different fibers having two different fineness show almost the same diameter. The traditional definition of fiber fineness does not truly reflect the fiber geometry

Problem 1.2: Consider a nonwoven wipe of 100 g weight which is made up of non-cylindrical cotton fibers of 25.4 mm length, 3 dtex fineness, and shape factor of 0.05. Calculate the total surface area occupied by the fibers in the wipe.
Solution 1.2: We take cotton fibre density as , then, the cross-sectional area s of a cotton fiber is calculated as follows
.
Then we obtain the volume V  of a cotton fiber as follows
.
Now the mass m of a cotton fiber is obtained as follows
.
The equivalent diameter d of cotton fiber is calculated as follows
.
Then the surface area of a cotton fiber is calculated as follows
.
Now the number n of cotton fibers present in the wipe having mass M of 100 g is calculated as follows
.
Then the total surface area Y occupied by all cotton fibers present in this wipe is
.
Fibre packing arrangement
The fiber packing arrangement is usually characterized by fiber packing density. Note that fiber packing density is known as fiber volume fraction or solid volume fraction. Figure 1.4 displays the image of a nonwoven fabric. Let the volume of this fabric is  and the volume of all fibers in the fabric is V. As the fibre packing density is defined by the ratio of the volume occupied by the fibres to the volume of the fabric, we can write , where µ denotes fiber packing density. Evidently, the value of fibre packing density lies from zero to one. Sometimes fiber packing density is interpreted in terms of density. Let the mass of a nonwoven fabric be M, then the density of the fabric is . Similarly, the density of fiber is . As we know , then . Also, fiber packing density is interpreted in terms of sectional area. Figure 1.5 shows a thin section of a nonwoven fabric and a thin section of a fibre. The total volume of this infinitesimally thin section of a nonwoven fabric is , where is the sectional area of the thin section of the fabric. The volume of all fibers in the thin section of the fabric is , where  is the sectional area of the fibres in the thin section of the fabric. Then the fibre packing density can be expressed as follows
.
Let us now solve some numerical problems.
Problem 1.3: Consider a laminated nonwoven material which is made up of polypropylene fiber and has the following dimensions: 10 cm length, 10 cm width, and 0.7 mm thickness. Its weight is 2.82 g. Calculate fiber packing density in this material.
Solution 1.3: The weight (M) of the nonwoven material is 2.82 ´10-3 Kg and its volume (VF) is 7´10-6 m3. So, its (mass) density (r*) is calculated as follows:

Let us consider the polypropylene fiber density (r) be 910 Kg×m-3. Therefore, fiber packing density (m) is calculated as follows
.
Problem 1.4: Take a needlebonded fiber web, made up of polyester fiber, has 2 mm thickness and 100 g×m-2 basis weight. Calculate fiber packing density in this material.
Solution 1.4: The basis weight (W) of the needlebonded fiber web is 100´10-3 Kg×m-2  and its thickness is (T) is 2´10-3 m. So, its (mass) density (r*) is calculated as follows
.
Let us consider the polyester fiber density (r) be 1380 Kg×m-3. Therefore, fiber packing density (m) is calculated as follows
.
            Often the packing arrangement of real nonwoven structure is compared with the idealized fibrous structures. In an idealized fibrous structure, the circular fibers are parallelly and uniformly distributed along the axis of the structure [2]. In such a structure, the fibers are arranged in a configuration around a single core fiber as shown in Figure 1.6(a). The repeat of the unit structure gives an equilateral triangle as shown in Figure 1.6(b). The packing density of such a structure can be determined as follows

Let us further consider four variants of this idealized structure. In the first variant, the fibre are touching each other, that is, h=0. Then the packing density can be obtained as m=0.91. Let us call this structure as the tightest structure. It shows the highest resistance to mechanical deformation, but it offers least softness. In the second variant, , then the packing density takes on . Let us call this structure as tight structure. It shows the high resistance to mechanical deformation, but it offers low softness. In the third variant, , then the packing density takes on . Let us call this structure as intermediate structure. It shows the moderate resistance to mechanical deformation, and at the same time it offers moderate softness. In the forth variant, , then the packing density takes on . Let us call this structure as loose structure. It shows the least resistance to mechanical deformation, but it offers highest softness. Figure 1.7 gives a better visualization of these idealized structures.
Fibre directional arrangement
The arrangement of fibers in nonwovens is described not only by the fiber packing arrangement, but also by the fiber directional arrangement (fiber orientation). Let us now discuss about fibre orientation [2]. We consider one such fiber, shown by thin green line in Figure 1.8. As shown, the curvature of the fibres varies widely. Simply, it has many directions. Then how can we describe this fiber in terms of its direction? To answer this question, let us divide this fiber into small segments. Thus, we get fiber segments. Consider a fiber segment of finite length (thick green line), which can be assigned by a suitable orientation vector i. This vector has a direction of joining the end points (A and B) of the fiber segment and it is unitary. When  its direction is given by its tangent.
It is generally known that the orientation of fibres in the thickness direction (z-direction) is not significant and we will discuss here about the orientation of fibres in two-dimensional plane of nonwovens. Such an orientation of fibres is known as planar orientation of fibres. We will first consider planar isotropic orientation of fibres, that is, purely random orientation of fibres in plane. Let us think about fiber segments. The direction of these fiber segments can be described by their orientation angles , as shown in Figure 1.9. In this case, all fiber segments are equally distributed in all directions. Therefore, the probability density function of their orientation is . Then
(Think that the area under the probability curve is equal to one.) then, . The behavior of this function is shown in Figure 1.10.
Often, fiber segments, during their processing, take a direction near to a preferential direction because they are mechanically affected by 1) other fiber segments of the same or other fiber and 2) technological (machine) elements like pins, cylinders, rollers, etc.  This can be analogously described by a flexible felt equipped with spikes being extended in a particular direction (preferential direction), as shown in Figure 1.11 [2]. Here,
. Then,
 . Then, . Further, differentiation of  with respect to  yields
        Here,  denotes the probability density function of orientation before extension. Then, the relative frequency of fiber segments in an elementary class before extension is . Let us consider that  represents the probability density function of orientation after extension and  refers to the relative frequency of fiber segments in an elementary class after extension. Both of these relative frequencies must be the same. Therefore,  Substituting  and  into this, we obtain . Clearly, the maximum value of  is  and the minimum value of  is . The value of extension C is known as a measure of preference, that is, measure of anisotropy. Here, . The behavior of the probability density function for different values of the anisotropy parameter C is shown in Figure 1.12. The corresponding distribution function is

            Sometimes, the preferential direction of fibre orientation may be something other than the direction of y-axis. Let us assume that the preferential direction (/) is described by the preferential angle with y-axis, shown in Figure 1.13. Here, x is the angle of fiber segment from preferential direction and y is the angle of fiber segment from y-axis. Then
. Since  and , we obtain .
Let us see if the orientation of fibres in a real nonwoven fabric can be explained in terms of the aforesaid theory of fibre orientation. A nonwoven fabric was developed by using polyester fibres of 51 mm length and 1.1 dtex fineness and employing opening, carding, cross-lapping, needle-punch technology. This fabric is shown in Figure 1.14. A sample of this fleece was placed on a mirror, and the light was allowed to pass through the fleece and reflect off the mirror surface vertically back to the camera. The fibers, regardless of their position within the fleece, could merely block the light, appeared dark, and were in focus. The result was an image with excellent contrast and uniformity. The image was “thresholded” to separate the fibers from the black and white background to obtain a binary image. This binary image was then analyzed to determine the orientation of 1500 fixed 24-pixel fiber segments. Thirty such images were taken randomly from different parts of the fleece, thus the orientation of 45000 fiber segments was determined. This large set of orientation data was summarized by frequency distribution in ten classes each of 18 degree width. The resulting histogram is displayed in Figure 1.15. The continuous line corresponds to the probability density function of fiber orientation in nonwoven for C=1.1678. This was obtained by using the standard non-linear regression technique. Evidently, the theoretical result corresponded well to the experimental one. Note that this nonwoven material shows lowest value of probability density function of fibre orientation in the machine direction . Why? The answer lies in understanding of the cross-lay process. The readers are encouraged to first study the cross-lay process and then provide a meaningful answer to this question.
The images of some major types of nonwovens are shown in Figure 1.16. The manufacturing processes of these fabrics will be discussed in detail later on.

 

 

 

 

 

 

 

 

 

 

 

 

Pore structure in nonwovens
A nonwoven fabric generally consists of a large number of fibers and a large (volume) portion of this material is occupied by air. If we project this fabric onto a two-dimensional plane, we obtain an “elementary plane” of this fabric. This is shown in Figure 1.17. Evidently, this plane consists of numerous fibers and many air spaces are surrounded by fibers. We call these air spaces as pores. One such pore is shown in Figure 1.17. We will now study the structure of these pores present in such an elementary plane of a nonwoven fabric.
Let the fibers be distributed randomly in an elementary plane of unit area and the probability P(n) that any given point is covered by n fibers present per unit area of the plane is given by Poisson distribution of the following form
,
where c is the total projected area of fibers per unit area of the plane. (c is termed as coverage.)
Evidently, , where l is fiber length, and d is fiber width (diameter). According to the geometrical definition of probability, P(n) is the area covered by n fibers per unit area of the plane, that is, P(n) is the fraction of unit area covered by n fibers. If so then P(0) is the fraction of unit area not covered by fibers, then P(0) represents the open area fraction of the plane. Then, . Clearly, 1-x is the fraction of the unit area covered by fibers. Similarly, P(1) is the fraction of area covered by one fiber, P(2) is the fraction of area covered by two fibers, and so on. Let us assume that a point covered by n fibers is covered by n-1 crossings. This is illustrated in Figure 1.18.
Therefore, the total area Ac occupied by all fiber crossings per unit area of the plane is . Thus, the total area of fiber crossings Ac is the difference between the total coverage cand the fraction of the unit area covered by fibers (1-x). The area ac occupied by one fiber crossing (Figure 1.19) is given by . Then the number nc of fiber crossings per unit area of the plane is

Fiber crossings form the vertices of polygons in the plane. Each polygon simulates a pore. According to Kallmes and Corte [4], the expression for number of such pores np is . The mean cross-sectional area ap of pore is given by
.  Generally, , then, .
Thus, the mean pore cross-sectional area ap is inversely proportional to the square of the total length (nl) of fiber deposited. We consider the mean cross-sectional area ap of pore is equal to the area of a circle of diameter . Then we write
We call  as equivalent pore diameter or simply, pore diameter. Here the pore diameter (dp) is inversely proportional to the total length (nl) of fiber deposited. Note that we assumed all fibers were randomly deposited in an elementary plane, therefore, all these relations are valid for such plane only.
So far we have studied about the pore structure in an elementary plane. We imagine that many such identical elementary planes are stacked one on the other in order to make the ultimate nonwoven fabric. Let us now assume that (1) The thickness T of such a plane is equal to the thickness of two fibers, that is, T=2d (Figure 1.20) and (2) The center of fibers lie on or within this plane, that means, in principle, as many as three fibers can intersect within this plane. Hence, this model holds whenever P(4) <<1. Apparently, the pore structure of an elementary plane is decided by the pore structure of other elementary planes. Then, how many elementary planes must be considered to determine the effective pore structure of a given plane? This depends upon how densely the elementary planes are packed together. This is, of course, decided by the fiber material used and the process technology employed for making the nonwovens. Whatsoever, at least minimum of two elementary planes always interact to determine the effective pore structure of a given plane. Let  be the number of elementary planes that must be taken into account together to describe the pore structure  in nonwoven and we assume  Then,

Here V is total volume of all fibers, VF is total volume of fabric, m is packing density, y is porosity. 
            Let us see if the pore structure of a real nonwoven fabric can be explained in terms of the aforesaid theory of pore structure. A series of spunbond nonwovens was prepared with varying basis weight (weight per unit area), thickness, and porosity. The fibre diameter was measured as 3.82 micron. The pore diameter was measured by using the standard image analysis technique. Table 1.1 shows the results of experiments. The pore diameter was also calculated from the above theory. The theoretical results are also shown in Table 1.1. Evidently, the theoretical results correspond very well with the experimental results.

Basic nonwoven processes
Let us take that the nonwoven manufacturing technology that produces fibre-based nonwovens generally consists of four basic nonwoven processes namely, fibre preparation process, web formation process, web bonding process and finishing process. A brief summary of the nonwoven processes is illustrated in Figure 1.21.

Figure 1.21
The fibre preparation process prepares fibres for web formation process. It includes fibre opening and mixing and fibre feeding to carding machine or air-lay machine. The web formation process, as the name implies, indicates those processes that disperses the fibres or filaments to form a sheet or web and also stacks the webs to form multi-layered webs, sometimes called as batts. The web bonding processes imparts integrity to the web and the resulting material is often called as fabrics. Often, the fabrics undergo mechanical or chemical finishing or both in order to achieve enhanced properties and appearance.
As expected, these processes along with the choice of fibres determine the structures and properties of the nonwoven fabrics. It is often said that the nonwoven fabrics are engineered in a unique way by the nonwoven technology employed and the fibre materials used so as to offer a set of targeted properties. Here, the word technology implies a system of sequence of basic processes. As a nonwoven technologist, it is imperative to learn how these basic processes can be arranged to engineer a wide spectrum of nonwoven fabrics for a wide variety of applications.
Process augmentations
Often the technology of nonwovens is implied by the name of the nonwoven materials which are prepared by utilizing a particular sequence of basic nonwoven processes [1]. For example, carded nonwovens refer to a technology in which the staple fibres undergo a sequence of opening and mixing process, followed by web formation by carding processes, and then followed by adhesive bonding or thermal bonding. Similarly, perpendicular-laid nonwovens refer to a technology in which the staple fibres undergo a sequence of opening and mixing process, followed by web formation by carding processes, and then web stacking by perpendicular-lay process and finally followed by through-air thermal bonding process. In a similar manner, air-laid nonwovens refer to a technology in which the staple fibres undergo a sequence of opening and mixing process, followed by web formation by air-lay processes, and then followed by thermal bonding or adhesive bonding. In the same vein, wet-laid nonwovens refer to a technology in which the staple fibres undergo a sequence of opening and mixing process, followed by web formation by wet-lay processes, and then followed by adhesive bonding or thermal bonding. In these cases, the web formation processes tend to give the name to the technology.
In contrast, the needle-punch technology often refers to systems in which the staple fibres, taken from bales, are opened, carded by using roller top card or flat top card, cross-lapped to achieve the desired basis weight of the final web (batt), passed through a needle loom for bonding and then given a finishing treatment. Similarly, hydroentanglement technology refers to a system in which the staple fibres are opened and formed into a web by using air-lay technology, and finally bonded by hydroentanglement machine. To enhance the properties of fabrics for specific end-uses, an additional step of resin bonding, drying, and curing may follow; one may think of this as a finishing process. Note that the hydroentanglement technology is also called as spunlace technology. In these cases, the web bonding processes tend to give the name to the technology.
The next set of technologies was developed as a combination of at least polymer preparation process, web formation process, and web bonding process. We refer to spunbond technology and meltblown technology. In these technologies, the characteristics of the polymers used and the principles of the processes followed are highly different; as a result, the structures and the properties of the resulting nonwovens are very much different, which demands for different applications of these nonwovens.
In this course, we will first focus on the nonwoven technologies that are relevant to staple fibre nonwovens. Then we will focus on polymer-extrusion based nonwoven technologies namely, spunbond technology and meltblown technology.
Note that in recent years many attempts are made to develop new nonwoven technologies by combing these processes in a unique way, for example, Evolon technology, Coform technology, SMS technology, etc. These technologies are often described as composite nonwoven technologies, which are, however, beyond the scope of this course.
References