Testing3 Dimensions Fibres Yarns

Introduction to the Principles of Textile Testing - MA Wilding

Topic 3: Dimensions of Fibres & Yarns Important Basic Concepts, Special Quantities, Units & Methods of Determination 1.

Introduction

From the point of view of simplicity, it is easiest to consider fibres and yarns before fabrics. Further, because of the general "shape" of fibres & yarns (ie long and thin!) and convenience of measurement, probably the most important aspect of their end-use performance is their tensile properties - ie how they respond to being pulled or loaded along their length. However, before examining tensile behaviour itself, it is really necessary to consider the significance of fibre & yarn dimensions.

2.

Fibre & Yarn Length

The length of a fibre is almost certainly the simplest dimension to understand in principle. Fibres generally range in length from around 1 -10 cm for natural staple (like cotton & wool), up to many kilometres for silk and synthetic continuous filament (such as nylon & polyester). Yarns, whether staple or not, are manufactured in more-or-less continuous lengths up to several kilometres. 2.1 Importance of Fibre Length The length of the fibres within a yarn is of considerable technological importance. For example, in staple yarns it is the fibre length which largely determines strength and uniformity, with longer fibres usually being preferred. It is important to realise that fibre lengths can be extremely variable, both between and within fibre types. Fibre length also has many other, more complex, roles in relation to the performance of textile materials. 2.2 Length Units Any convenient unit can be used to express fibre or yarn length. The SI (Systeme International) unit is the metre, but smaller units such as mm (and even imperial measures like inches) are often used - particularly for staple length. There is in fact no real standardisation of length units. In fact, it is worth noting in passing that textiles in general is awash with all sorts of weird and wonderful units - the logic of which is not always immediately obvious! (This is an unfortunate fact, but one which you really just have to live with, and be prepared for.)

Testing3-Dimensions-Fibres-Yarns.doc 1

Introduction to the Principles of Textile Testing - MA Wilding

2.3 Methods for Determining Fibre & Yarn Length On the face of it, measuring the length of a "long thin thing" like a fibre or yarn would appear to be a rather trivial and straightforward matter, but this is not necessarily so. There may be numerous ways of doing it, but each will usually be found to have some disadvantages. The most appropriate method in any specific case will depend on a range of factors, especially the physical form that the material is in. Moreover, as for any kind of test, certain precautions may be called for. For example, temperature and humidity can have a very marked effect on the physical characteristics (ie "properties") of textile materials - including their dimensions. We will look at this aspect in a future topic, but it is important to realise now that any specimen intended for measurement will usually need to be left in the laboratory atmosphere for a period of several hours to "condition" before testinga. During this time it may extend or contract due to changes in its temperature and the amount of moisture absorbed. Other factors can also complicate the measurement; such as whether or not the fibres (or yarns) are crimped (ie wavy); if so, the question then arises as to whether or not they should be deliberately straightened before measuring. 2.3.1 Yarns Accepting the qualifications made above, measuring the length of a piece of yarn may not be too problematic. Long lengths (of the order of 10s or 100s of metres, say) might be measured using a creel system, as referred to in the later section on yarn count determination. Shorter lengths can often be measured using a metre rule or other convenient scale. If the yarn is composed of continuous filaments then the task is likely to be easier and less subject to error than if it is a staple-spun yarn. 2.3.2 Staple fibres As was pointed out, staple length is an important factor in determining the strength and other properties of spun yarns. However, a given batch of raw staple (eg cotton or wool) may contain hundreds of thousands of fibres having many different lengths. It would plainly be impossible to measure each one individually. What is often required instead is an assessment of the typical (or average) length along with some indication of the distribution of lengths within a relatively small (but still representative) sample. Not surprisingly then, the methods available for staple fibres tend to be statistical in nature. Most of the commercial instruments available are described as "sorters" because they work by grading the fibres within a sample into increasing length classes. Examples include various “comb”, “sledge” and photoelectric sorters (eg the “Shirley” and the “Fibrograph”) instruments). The competent operation of these testers requires much a

This will certainly be the case where the test is to conform to one of the recognised standards (BS, ISO, ASTM etc)

Testing3-Dimensions-Fibres-Yarns.doc 2

Introduction to the Principles of Textile Testing - MA Wilding

training and experience and must therefore be regarded as highlyspecialised. It is usually also extremely labour-intensive & timeconsuming. For these reasons no further discussion will be given here, although reference should be made to Chapter 5 of Booth, for example.

3.

"Thickness" (of a Fibre or Yarn)

The effective thickness of a fibre or a yarn is of considerable importance. Among other things, thickness is a dominating factor in determining fibre and yarn flexibility. The "fineness" of a fibre also contributes to the strength and uniformity of spun yarn. ….. More on this later. 3.1 Linear Density in Relation to Fibre & Yarn Thickness We tend to think of a fibre or yarn as being like a long cylinder. If this were so, then a simple measurement of the diameter (using, say, a calibrated microscope) would enable the cross-section to be determined. But fibres - particularly natural ones - may be very irregular in crosssection, with all kinds of different and often complex shapes (cotton, for example, is hollow, and almost kidney-bean shaped). The shape may also vary along the fibre’s length. Even "regular" twisted yarns will have thick and thin places, loose fibres protruding from the surface, and other irregularities. Therefore, in practice, it is often exceedingly difficult to determine the actual cross-section of a fibre or yarn. For this reason, linear density is generally used as an alternative measure of the "thickness". 3.2

Fibre Fineness

3.2.1 Direct System(s) of Measurement - the "Tex" System The linear density of a fibre gives an indication of its cross-sectional area. This stems from the fact that the mass of a cylinder (or prism) of given length is proportional to its volume, and hence its cross-section. The diagram below should make this clearer.

Mass = V x density (d) = ALd Testing3-Dimensions-Fibres-Yarns.doc 3

Introduction to the Principles of Textile Testing - MA Wilding

Linear density = Mass/Length (eg in tex) = ALd/L = Ad So the linear density is proportional to the fibre's crosssectional area (for given d) The linear density, expressed in an appropriate unit, is called the fineness. The linear density is actually the weight (more properly the mass) of a given length of fibre. It might be expressed in a unit such as g/cm or, more likely (using the so-called "Tex" system), in tex; or one of the submultiples decitex (dtex) & millitex (mtex). •

The tex of a fibre (or yarn) is the weight in g of a 1km length.

•

The decitex of a fibre (or yarn) is the weight in g of a 10km length.

•

The millitex of a fibre (or yarn) is the weight in g of a 1000km length (or the weight in kg of a 1km length - which is the same thing).

But … what if the fibre is shorter than 1km (or 10km etc) …? Clearly, if the fibre is staple it is going to be very much shorter. Its fineness can still be determined. Example Suppose a single fibre weighs 0.00002g (0.02mg). Its length is say 10cm. First, convert its length to km. There are 100,000 cm in 1km, so: 1cm = 1/100,000 km Therefore the fibre's length is 10/100,000 km = 1/10,000 km = 10-4 km. Then, its fineness will be mass(g)/length(km) = 0.00002/10-4 = 2x105 /10-4 = 0.2 tex. We therefore have a 0.2-tex fibre (it is also a 2-dtex fibre, or a 200mtex fibre). Notice that the finer the fibre, the smaller its tex value will be. This being the case, the Tex system is known as a direct system. The Tex system may be applied equally to fibres and yarns. Note: the equivalent fibre cross-section will depend on the bulk density, so that two fibres of different types, but of equal tex values, would generally have different cross-sectional areas. The use of linear density as a measure of thickness is common in textiles, but is rarely used in other Testing3-Dimensions-Fibres-Yarns.doc 4

Introduction to the Principles of Textile Testing - MA Wilding

fields of engineering and technology, the actual cross-section generally being preferred. Readers might be familiar with a rather older, but still direct, measure of linear density called the 'denier' (shortened to den). This equals the weight in grams of 9000 metres. As such it is very nearly, but not quite, equivalent to the decitex. Although its use is gradually declining, it is still an important unit, particularly in the USA. There are also a number of more obscure direct measures of linear density, which can be found described in the literature, but which are no longer in general use. 3.2.2 Indirect System(s) of Measurement - "Counts" One way to specify the fineness of a fibre or yarn is linear density as described above. The implied question there was: "What is the weight of this or that length of fibre or yarn?" The question could be put the other way round: "What length of fibre/yarn would weigh this or that amount?" Thus, in the earlier example, one could have asked: "How long would be a 1-gm fibre of the type described?". The sample of 0.00002g had a length of 10cm, so how long (in principle) would a 1-g sample be? The answer is (1/0.00002 times 10) cm = 500,000 cm, or 5km. This - or something like it - is the basis for what is done using the indirect 'counts' systems. However, it is necessary to take it a little further, and ask: "How many standard-length units of fibre/yarn weigh this or that amount?" The answer may depend on the fibre type (for natural fibres, at any rate), as each traditionally has its own specific counts system. 3.2.2.1 Cotton Count This is perhaps the most commonly applied indirect system. The "standard length" unit (or "hank") in this case is a length equal to 840 yards, and the reference weight is one pound (yes - Imperial!) Thus, in the cotton system, the count is: •

the number of 840-yard hanks required to weigh one pound.

Clearly, the more hanks that are needed, the thinner must be the fibre (or yarn). So, as the fibre/yarn becomes finer, the count becomes bigger. This is why counts represent an indirect system of measurement.

Testing3-Dimensions-Fibres-Yarns.doc 5

Introduction to the Principles of Textile Testing - MA Wilding

Example Suppose a single standard length hank of a particular cotton yarn is found to weigh 1/30 pound. Then 30 such hanks would be needed to weigh one pound, and the cotton would be described as "30’s" cotton. There are many different counts systems, and hanks of length other than 840 yards may be specified, as well as different reference weights. The differences are both historical and geographical (Yorkshire woollen & worsted people would probably have gone to great lengths to avoid using Lancashire cotton counts!! - who said the Wars of the Roses were over?) Note: notwithstanding the somewhat facetious comment above, it is possible to use the same counts system for yarns of different types, but it must be made clear which is in use. Some examples are the woollen, ramie and silk systems. 3.2.2.2 Folding Number Here’s another complication: the number of individual threads ('singles') that have been plied together to form the final yarn is called the folding number. If there is only one, then the yarn itself is often referred to as a "singles yarn". However, depending on the counts system being applied, the folding number will appear differently in the yarn count. Example Using any of the cotton, woollen or worsted systems, a twofold forties cotton would be written as "2/40" with the first digit signifying the folding number, and the second the count of each single ply. In the spun silk system, a twofold forties cotton would be written as "2/20" because officially, the count of spun silk is the total yarn count, with the folding number placed in front (remember that a coarser yarn gives a smaller number). Tip: whenever you have a choice - avoid counts. The direct system is almost always simpler, and generally to be preferred. In any case, the tex is regarded as a sort of "honorary" SI unit! 3.2.3 Technological Importance of Fibre Fineness There are many reasons why one may wish to know the fineness of a fibre or yarn. The following are just some examples of why it is important: •

stiffness, handle and drape of fabrics bending resistance (flexural rigidity) varies as the square of the fibre cross-section; thus for a yarn of given count (or indeed fabric of given weight) flexibility increases as fibre tex decreases;

Testing3-Dimensions-Fibres-Yarns.doc 6

Introduction to the Principles of Textile Testing - MA Wilding

torsional rigidity resistance to twisting varies as the square of the fibre cross-section; thus it is easier to twist fine fibres than coarse ones in yarnspinning; •

•

light reflection for a given yarn count or fabric weight, finer fibres give greater number of reflecting surfaces; thus fine fibres tend to yield a soft sheen, coarse fibres a harsh glitter; other things being equal, in dyed fabrics, fine fibres give lighter shades than coarse fibres;

•

absorption of liquids fine fibres have relatively more surface than coarse ones, and absorb liquids more quickly; thus dye uptake etc depends on fibre fineness;

•

fibre cohesion in yarns, twist results in friction between fibres and this is greater for fine fibres than for coarse, as the surface in contact is greater; thus, other things being equal, for a given yarn count, less twist is needed for fine fibres than for coarse;

•

yarn uniformity very important to the spinner - for a given yarn count, the more fibres in the cross-section, the better the uniformity; thus, other things being equal, fine fibres give better uniformity than coarse ones.

3.3 Importance of "Thickness" in Comparing Different Fibres or Yarns In the context of textile testing, a knowledge of the effective thickness of the fibre or yarn sample is of considerable valuable in the analysis of results - particularly in making a sensible comparison between different fibre types. Example Suppose a particular cotton fibre can withstand a greater load (tension) than a particular nylon fibre without breaking. Does this necessarily imply that cotton is stronger than nylon per se? The question can’t be answered directly unless we say something about the thicknesses of the two fibres tested, compared to one another. The results should, in other words, be "normalised". All other things being equal, a thick fibre will be stronger than a thin fibre of the same type (sic!). In engineering, it is customary to divide the load by the cross-sectional area, so as to give a quantity that is independent of the thickness - the Testing3-Dimensions-Fibres-Yarns.doc 7

Introduction to the Principles of Textile Testing - MA Wilding

breaking stress. In textiles, the cross-section is seldom determined directly, as already said, and instead, the load is divided by the linear density (fineness) for normalisation purposes. ……more on this later. 3.4 Methods for Determining Fineness and Count There are several ways in which the effective "thickness" of a fibre or yarn might be estimated in terms of its linear density or count. These include both gravimetric (ie by weighing) and non-gravimetric methods. Some of them are direct, in the sense that they measure the effective diameter; some are indirect, in that they are based on some other attribute, such as the way the fibres scatter light, or the depth of shade they display when dyed at a given dye concentration. Unfortunately, many techniques are extremely complicated, labour-intensive and time-consuming. Moreover, as for any physical measurement several precautions may need to be taken: eg preconditioning the sample in the laboratory (discussed later). However, there are several fairly rapid methods that can be routinely applied (in a mill situation, for instance), and which have the additional benefit of being relatively straightforward to carry out. It should be appreciated that the most appropriate choice in any given case will depend on a range of factors. An important one is whether it is a fibre or a yarn that is being tested. 3.4.1 Yarns Provided a plentiful supply is available, there is little doubt that the simplest and most reliable method for determining the fineness (in terms of either tex or count) of a yarn is to weigh a known length of it on a standard laboratory balance. Typically, a 100-metre (or some other convenient length) "skein" of yarn is wound off using a “wrap wheel”. This is a simple winding device having a precise diameter - usually 1 metre - and a revolution counter to make length determination easy and accurate. Most instruments are motorised. Suppose 100 metres (ie 0.1km) of a yarn is found to weigh W grams. Remember that tex is defined as the weight in grams of one kilometre. Hence, the yarn sample must have a linear density of W/0.1 tex, or 10W tex. Not surprisingly, the longer the length of yarn tested, the more accurate will be the actual measurements – of both length and weight. However, it should be realised that the result will be an average value for that length of yarn as a whole. This may not be an issue if the yarn is uniform, as it is likely to be if it is a continuous-filament yarn made from synthetic fibres; but if it is a variable spun yarn, for instance, this fact must be taken into account, and it may be necessary to use an alternative method. Short

Testing3-Dimensions-Fibres-Yarns.doc 8

Introduction to the Principles of Textile Testing - MA Wilding

lengths (perhaps a few centimetres) could be measured, for example, using a metre rule and a sensitive electronic balance, although of course the shorter the length the less accurate the measurement is likely to be. The count equivalent can be calculated using the appropriate conversion formula; for example, for English Cotton Count (often abbreviated to NE): NE =

590 .5 tex

On occasion it may be required to determine the count of the yarn(s) making up a fabric. In this case lengths for measuring will need to be carefully extracted. There may be crimp in the yarn, and this must be removed with as little force as possible to enable the straightened length to be measured. 3.4.2 Fibres The range of methods available for fibres is particularly broad, and it is not possible to cover them all here. (Further information can be found in the literature, including the references to this topic.) Many of them are, in addition, quite complicated, labour-intensive and slow to carry out. However, one or two of the more routine fineness tests are worthy of discussion. It should be realised, to begin with, that the appropriate choice of method will depend on various factors, such as: • •

the physical form the fibre is in: ie is it raw fibre in a bale? Or is it in the form of a sliver, roving or yarn, or even a fabric? Is a single-fibre value required? Or is it necessary to assess the overall fineness of a batch of fibre in bulk?

For the uniform synthetic fibres making up a continuous-filament yarn it may often be possible to determine the single-filament fineness (usually expressed in either dtex or denier) simply by knowing how many fibres there are in the yarn cross-section. This is clearly a matter of dividing the linear density of the yarn as a whole by the number of filaments. In many instances, of course, the yarn supplier will have provided information on yarn and filament count, as well as the number of filaments. On the other hand, it may be necessary to count the filaments in order to calculate the fibre fineness. Given the fact that typical fibres are around 20 µm across, and that yarns may contain many hundreds of filaments, this would generally entail a low-powered microscope plus a generous helping of patience!

Testing3-Dimensions-Fibres-Yarns.doc 9

Introduction to the Principles of Textile Testing - MA Wilding

There will be occasions when the fineness of a single fibre is required, and there may be only a small amount available. Since most fibres are much too fine - and often too short - to be weighed reliably on a balance alternative methods must be found, two of which are described briefly below. 3.4.2.1 Single Fibre Fineness Using Projection Microscopy A projection microscope is similar to a conventional instrument except that instead of (or as well as) appearing in the eyepiece, the magnified image is thrown onto a screen where it can be measured relatively easily. In the case of fibre fineness determination, a cross-sectional view of the fibre(s) is needed. This entails quite careful sample preparation, but is not usually too difficult for a skilled operator. By knowing the overall magnification, the area of the fibre cross-section can be estimated using a calibrated grid of small squares superimposed on the image. The method assumes that the fibre’s fineness is proportional to its cross-section area. In order actually to calculate the fineness we also need the bulk density, of course. An approximate value for this may be obtainable from literature provided the fibre type is known. Otherwise, some method of estimating density will also be needed (which we will not go into here). 3.4.2.2 Single Fibre Fineness Using a Vibrascope A ‘vibrascope’ is a very ingenious machine for measuring single-fibre fineness, based on the principle of a vibrating stretched string – much as is the case in a stringed musical instrument. Basic physics shows that for an elastic (or nearly so) filament, the plucked frequency (“pitch”) depends on the applied tension, the linear density and the vibrating length between fixed points (as for example, the distance between the stopped fret and the “bridge” of a guitar). In the case of a vibrascope, the fibre is hung vertically so that it is resting against two knife-edges which define this length. The lower of these is moveable vertically so that the vibrating length can be adjusted. Below is a schematic representation of a vibroscope.

Testing3-Dimensions-Fibres-Yarns.doc 10

Introduction to the Principles of Textile Testing - MA Wilding

The procedure entails attaching a small known weight to the lower end. If it has a mass of m kg it will exert a tension (T) of mg newtons. The stretched fibre is then caused to vibrate by the application of an electrostatic field oscillating at a fixed frequency (f, say). A light source and associated optics enables an enlarged image of the fibre to be viewed on a ground-glass screen. The lower knife edge is then raised or lowered until the fibre is seen to vibrate with maximum amplitude (known as “resonance”). The linear density of the fibre is then determined from the length at resonance, the frequency of vibration and the tensiona. In fact, the control knob for adjusting the length is usually calibrated directly in decitex (assuming a prescribed tension is applied to the fibre). The method is quite rapid and convenient - once the fibre has been prepared and mounted on the instrument. However, it may not be highly accurate as resonance is not always easy to identify precisely. 3.4.2.3 Bulk Fibre Fineness Using Airflow Techniques Sometimes it is the average fineness of a bulk of fibres that is required, rather than that of an individual. This kind of information is often needed, for example, when assessing the overall quality of a batch of raw staple, such as cotton and wool. Under such circumstances the most convenient and rapid methods are probably those based on air-flow. The underlying principle is that the resistance to air-flow depends on the total surface area of the fibres. For a given volume (or weight) of fibres, the total surface area will generally be greater if the fibres are fine than if they are coarse. Another way of saying this is that fine fibres have a higher 'specific surface' than coarse fibres. By way of analogy, compare sand (very fine particles having a large specific surface) with pebbles (much coarser particles having a lower specific surface). Now imagine packing

a

The linear density, σ, is given by:

σ=

T , where T, f and L are defined as above. 4f 2L2

Note that consistent units must be applied: eg, with T in newtons, f in hertz and L in metres σ will be in kg m-1. Multiplying this by 107 gives the equivalent in dtex.

Testing3-Dimensions-Fibres-Yarns.doc 11

Introduction to the Principles of Textile Testing - MA Wilding

sand into a wide pipe and attempting to blow air through it: quite difficult (if not impossible); but if the same pipe contained pebbles it would be somewhat easier. Because, in the case of fibres, there is a relationship between fineness and specific surface, it is possible to use the phenomenon to measure linear density. There are essentially two approaches: • •

either measure the pressure needed to produce an air-flow of specified rate; or measure the flow rate corresponding to a specified pressure difference across the sample

Whichever type of method is used, the procedure consists of preparing a wad of fibres (of prescribed weight) which is then packed under a standard pressure into a perforated chamber through which the air is forced. The figure below gives a very approximate idea of the set-up that might be used.

Typical airflow system (schematic)

Air

Weighted insert with perforated base

Air From pressure meter

Flow meter & suction pump

Chamber packed with wad of fibres of standard weight

As with any experimental technique, several precautions need to be taken to ensure the results are reliable, and there may be some complications. For example, where the fibres are not uniform and/or not cylindrical the results of air-flow measurements will be not necessarily be the same as for uniform, cylindrical fibres, so that some form of calibration may be necessary. In addition the result, whilst being an indication of the overall fineness, will almost certainly not be the simple arithmetic mean for the batch. A particular problem arises in the case of cotton, in which there is a relationship between fineness and maturity (see following section). This complicates the analysis somewhat, although it does enable the same method to be used to estimate both properties. For further details and practical systems for measuring fibre fineness see, for example, Booth, Chapter 5.

Testing3-Dimensions-Fibres-Yarns.doc 12

Introduction to the Principles of Textile Testing - MA Wilding

4.

Maturity (of Cotton)

According to HH Ramey Jr a., a cotton fibre is "a tubular outgrowth of a single cell on the surface of the seed" . The state of maturity (or immaturity) of such 'outgrowths' can have a profound influence on the physical and chemical properties of cotton, and hence on the performance characteristics of yarns and other cotton products. The subject of maturity is therefore of considerable technological and commercial importance. The concept of maturity is essentially confined to cotton, and is rarely, if ever, applied with respect to other fibre types. In order to appreciate why this should be so, it is necessary to consider not only the appearance of cotton fibres look like, but also their growth and structural development. 4.1 Development of Cotton Fibre Structure The microscope pictures below (reproduced from a) show the appearance of a typical batch of cotton fibres. The first noticeable thing about them is that they are definitely not regular cylindrical rods. In cross-section they are seen to have a roughly kidney-bean shape, and looked at from the side, they are rather ribbon-like, with periodic twists, called 'convolutions'. A very important feature is that the fibres are hollow, the hole down the centre of each being called a 'lumen'

Convolution

Longitudinal View

Cross-section

In the first stage of fibre development in the cotton plant, the fibres emerge as regular thin-walled cylinders which elongate over a period of

a

The Meaning and Assessment of Cotton Fibre Fineness by HH Ramey Jr.

Testing3-Dimensions-Fibres-Yarns.doc 13

Introduction to the Principles of Textile Testing - MA Wilding

15 to 20 days. Ultimately, their lengths may be 3000 times greater than their diameters. The following graph, reproduced from The Meaning and Assessment of Cotton Fibre Fineness by HH Ramey Jr., clearly shows that the fibres' lengths develop before their thickness (represented by what is termed the 'secondary wall').

In the second stage of growth, the secondary wall gets thicker, and the lumen gets narrower, but the outside diameter of the fibre does not change because as the fibre develops, new cellulose is laid down on the inside of the cylinder. In the following diagram, the secondary wall is shown in red. It is the ultimate thickness of this, compared to the overall fibre thickness, which essentially determines the state of 'maturity'.

Hole down middle …lumen

In the final stage of development, the fibres are cut off from the plant's transpiration stream, and dry out. In this process they collapses into the flattened, convoluted ribbons shown earlier. However, this final stage does not change their state of maturity. 4.2 Technological Importance of Maturity The overall thickness of the fibres is controlled by purely biological factors, and is dependent upon the cotton variety. However, the state of

Testing3-Dimensions-Fibres-Yarns.doc 14

Introduction to the Principles of Textile Testing - MA Wilding

maturity reached by the fibres of any individual crop will depend very much on the growth conditions, such as temperature, rainfall, and soil fertility. The maturity of a cotton fibre specifies the state of development it had reached up to the drying stage. In practice, as has been stated, this is assessed in terms of the relative thickness of the secondary layer. There can be considerable variation of maturity within a single cotton crop, and this can have a very significant influence on yarns spun from the cotton fibres. In any given sample of cotton, almost all degrees of maturity are present. There is an optimum degree of maturity for a fibre, above which it is too stiff for effective processing, and below which it is too flabby and lacking in resilience. As far as spinning is concerned, the 'over-mature' fibres present far less of a problem than the 'immature' fibres. During yarn processing, for example, these latter tend to form 'neps' - small, tightly-knotted bundles of fibres. It is now clear that neps are almost always associated with very thin-walled, so-called 'dead', fibres.

• Mature/Over-mature

• Immature • “Dead”

Testing3-Dimensions-Fibres-Yarns.doc 15

Introduction to the Principles of Textile Testing - MA Wilding

A nep When, in particular, fine yarns are being spun from fine cotton, neps can be a very serious problem. Even the mature fibres are delicate, but the dead ones are exceedingly so. Thus, neppiness is not easily avoided. Moreover, the neps may be very noticeable since their size can be comparable with the yarn diameter. Neppiness is a factor in the dyeing and printing of cottons, because the thin-walled fibres tend to dye a lighter shade than the mature fibres. For the same reason, if there is not an even blending of fibres of different maturity, the result can be streaky dyeing and other adverse effects. 4.3 Parameters Specifying Maturity, and Methods of Measurement Maturity can be expressed in terms of a number of parameters.

4.3.1 Degree of Thickening The degree of thickening, θ (the Greek letter "theta"), is defined as the ratio of the solid cross-sectional area of the fibre (ie that of the cellulose present, and depicted as A in the following diagrams), to that of a circle bound by its perimeter (A0). Assuming the cotton fibre initially (ie before drying) had a circular cross-section, then the perimeter, P, is easily derived in terms of the fibre diameter. The area of the fibre cross-section is related to the fibre fineness, and the fibre specific surface, S, is equal to the ratio P/A. Thus, fineness, specific surface and maturity are all interrelated.

Testing3-Dimensions-Fibres-Yarns.doc 16

Introduction to the Principles of Textile Testing - MA Wilding

The optimum value of θ for processing efficiency is generally in the range 0.8-0.9. However, it is not common to determine theta by direct measurement. 4.3.2 Maturity Count and Maturity Ratio The average degree of cell-wall thickness can be estimated for a sample of cotton fibre indirectly, making use of the 'maturity count' (or, very often, the 'immaturity count'), and the 'maturity ratio'. The actual counting is usually performed on the tufts of fibre left over from the Baer diagram after the weight per centimetre has been established. A microscopic method is used, in which the fibres are swollen in a dilute caustic soda solution. The mature fibres quickly regain their cylindrical Testing3-Dimensions-Fibres-Yarns.doc 17

Introduction to the Principles of Textile Testing - MA Wilding

shapes, whereas the more immature ones retain their ribbon-like appearance to some degree. The apparent thickness of the secondary wall is determined, along with the apparent width of the lumen. The 'Normal' fibres are those that have become deconvoluted and rod-like. The 'dead' fibres are those whose wall-thickness is measured to be less than onefifth of the apparent lumen width. A count is made of all the fibres in the sample, and of the normal and dead fibres separately. The same is done for each of five tufts, and the results averaged. The number of normal fibres and the number of dead fibres (N and D, respectively), are expressed as percentages of the total number. If required, the proportion of immature (but not dead) fibres can be obtained by subtraction. This is the British standard maturity count. The American Society for Testing and Materials (ASTM), and the (former) USSR standard use variations of this. The 'maturity ratio', M, is defined as M = 0.7 + (N-D) / 200 The arbitrary constants ensure a value of approximately one for a high grade Egyptian or Sudanese cotton (chosen as standard: this of course means that values greater than one are possible for even more superior grades). A cotton for which M is less than about 0.8 would be regarded as immature as a whole. Values less than 0.7 are exceedingly rare. Some workers prefer to use the inverse of M, called the 'immaturity ratio' (I). The following empirical relationship holds: θ = 0.577M 4.4 Indirect Methods of Determining Maturity The micrometric method described above for maturity measurement (along with its variants) is both labour-intensive and time consuming, so that much effort has gone into developing alternatives. Regretfully, this has not been entirely successful.

Alternative methods include: •

Polarized light microscope - makes use of interference colours, which are different for different levels of maturity; - need to measure ca 1000 fibres, so not particularly timesaving; - doubtful correlation with standard maturity count.

•

Differential-dyeing test - mixture of red and green dyes applied to sample; final colour indicates maturity;

Testing3-Dimensions-Fibres-Yarns.doc 18

Introduction to the Principles of Textile Testing - MA Wilding

- needs precise and rigid procedure for dyeing; - measures specific surface, which may mean maturity only if the perimeter is constant; - long-winded •

Air-flow methods - air is forced through a wad of fibres; air passes more easily for coarse fibres than for fine, and provided sufficient other information is available, the results can be related to maturity as well as fineness; - instruments measure either rate of flow for a given pressure, or else pressure for a given rate of flow; examples 'Micronaire', 'Causticaire', 'Arealometer'; - all are fairly quick; - correlation for causticaire is very suspect - correlation for Arealometer is quite good

A note of caution needs to be sounded with respect to measuring the maturity of cotton using air-flow methods: the result is not entirely straightforward because it is airflow is affected by fibre fineness as well as maturity. However, this does lead to the ability of such techniques to yield both quantities, provided the appropriate precautions are taken. For a fuller account of cotton maturity see, for example, Morton & Hearle, Pp 145-151, or Booth, Pp 190-l95.

5.

Suggested Further Reading

JE Booth, "Principles of Textile Testing", Newnes-Butterworth, London (1983). ISBN: 0408014873. Chapters 5 & 6. WE Morton & JWS Hearle, "The Physical Properties of Textile Fibres", 3rd Ed., Textile Institute, Manchester (1997 reprint). ISBN: 1870812417. BP Saville, “Physical Testing of Textiles”, Woodhead, Cambridge (1999). ISBN: 1855733676. Chapter 3.

Testing3-Dimensions-Fibres-Yarns.doc 19