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Yarn & Spinning

Yarn conditioning – a technology of achievement

Abstract:

It is a matter of question for spinners that how much money they would allow to evaporate? For example, during purchase the moisture content in raw cotton is about 8 to 9% but at the end of yarn spinning process it get reduced to 5% .In this respect spinners are buying cotton with water & selling yarn with less quantity water. Obviously the matter is loosing money . Yarn conditioning is a process to regain the moisture of yarn at the end before selling to customers. Yarn conditioning simultaneously work as heat setting that provides flexibility, better performance in onward process for all types of yarn.

Introduction:

Heat setting is a term used in the textile industry to describe a thermal process taking place mostly in either a steam atmosphere or a dry heat environment. The effect of the process gives fibers, yarns or fabric dimensional stability and, very often, other desirable attributes like higher volume, wrinkle resistance or temperature resistance also used to improve attributes for subsequent processes. Yarns tend to have increased torque just after spinning, cabling or twisting. Heat setting can influence or even eliminate this tendency of undesirable torquing. At the winding, twisting, weaving, tufting and knitting processes, an increased tendency to torquing can cause difficulties in processing of the yarn. In practice, the moisture-absorbing properties of a fiber are described by a term known as the “Moisture Regain”. This is the weight of moisture present in a textile material expressed as a percentage of its oven-dry weight.

Why conditioning is needed:

1) High speed spinning machines generate more friction thus giving additional heat to the yarn and as a result of such heat transfer the yarn moisture content is vaporized. Rising speeds in spinning result in decreased yarn quality for other processes and it is well known that dry yarns have worse properties. Below graph shows the moisture reduction scenario in cotton process (Fig.:a)

2) Moisture in atmosphere has a great impact on the physical properties of textile fibers and yarns. A high degree of moisture improves the physical properties of yarn and it helps the yarn to attain the standard moisture regain value of the fiber. Yarns sold with lower moisture content than the standard value will result in monetary loss. Therefore the aim of yarn conditioning is to provide an economical device for supplying the necessary moisture in a short time, in order to achieve a lasting improvement in quality.

3) Cotton fiber is hygroscopic material and has the ability to absorb water in the form of steam. It is quite evident that the hygroscopic property of cotton fibers depends on the relative humidity. The higher the humidity is more the moisture absorption. The increase in the relative atmospheric humidity causes a rise in the moisture content of the cotton fiber.

1Fig.(a): Cotton process & moisture content

4) The fiber strength and elasticity increase proportionately with the increase in humidity. If the water content of the cotton fiber is increased, the fiber is able to swell, resulting in increased fiber to fiber friction in the twisted yarn structure. This positive alteration in the properties of the fiber will again have a positive effect on the strength and elasticity of the yarn.

5) The yarn curls, meaning that the yarn tries to reach a state of equilibrium in which twists in the opposite direction from the original twist direction balance the yarn’s torque. These twists are also called negative twists. In this state of equilibrium, the inner torsional tensions cancel each other out.

6) Although unevenness of yarn contains thick & thin place. The thicker yarns are less twisted than fine ones, the inner tension rises opposite to the yarn size. Further positive aspects of steaming are the reduction of curling and, at the same time, the setting of the physical properties of closeness and extension imparted to the yarn by twisting.

7) The effect of the process gives fibers, yarns or fabric dimensional stability and very often other desirable attributes like higher volume, wrinkle resistance & temperature resistance.

8) Weight gaining is a very useful utilization of yarn conditioning. Increment of weight depends on the conditioning process especially in cotton. Temperatures, vacuum time, steaming time, types of steam are the main variables of yarn conditioning.

9) Another crucial achievement of conditioning is the development of quality level. The hairiness, unevenness reduces for conditioning.

10) Again the strength & elongation of cotton is more in wet condition, as a result of conditioning the strength also increased. Below figure shows the development.

2Fig.(b): Improvement of strength & elongation for moisture.

Now we will discuss the moisture response of some

Traditional fibres:

Moisture in cotton:

As living plant cells, cotton fibers within a growing boll are literally full of water. Water is critical for fiber growth and the availability of water during early boll growth is related to the final length achieved by the fiber. When the boll matures and opens, fibers lose water and equilibrate with the ambient humidity. Both constituents of seed-cotton– fiber and seed – are hygroscopic, for example sensitive to moisture, but to different degrees.

In its mature dried form, nearly 90 percent by weight of the cotton fiber is cellulose. In fact the cellulose found in cotton fibers is the purest form of cellulose found in all plants. The cellulose in cotton fibers is mostly (88–96.5 per cent) cellulose1. The non-cellulose components (4–12 percent) are located either on the outer layers of the cotton fiber in the cuticle and primary cell wall or inside the residual protoplasm called the lumen. The secondary wall of mature fibers is primarily cellulose in its most highly crystalline and oriented form ( Figure c).

3Fig(c).: Representation of the structure of a cotton fiber.

Figure (d) shows the structure of the cellulose molecules in cotton. From a physical view point the molecule is a ribbon-like structure of linked six-membered rings each with three hydroxyl groups (OH) on the C2, C3 and C6 atoms projecting out of the plane of the ribbon. As well as providing structural stability the hydroxyl groups allow extensive intermolecular hydrogen bonding with many molecules, including water. The accessibility of water to these hydroxyl groups depends on the spacing between crystal lattice planes. From completely dry state water molecules will form hydrogen bonds with hydroxyl groups that are not already linked within crystalline regions.

4Fig.(d): Assembly of cellulose molecules in a sheet. Hydrogen bonds are shown by dotted lines. Circled carbon atoms; C2,C3 and C6, show location of hydroxyl (-OH) groups.

As humidity is increased, water will be attracted to these accessible hydroxyl groups. In this respect, available hydroxyl groups will ordinarily be located on the surface of crystalline fibrils, a unit of the cellulose crystal structure. The first water molecules to be adsorbed will be directly attached to hydroxyl groups. Later adsorption may occur on the remaining available hydroxyl groups or form secondary layers attached to already adsorbed water molecules.

Again below figure (e) is interesting because it shows the change in density of cotton with changes in moisture content. From the dry state the density of cotton increases as empty space (in proximity of available hydroxyl groups) within the cellulose structure fills with water. Direct adsorption of water molecules onto these hydroxyl groups results in more efficient molecular packing and gives an initial increase in density. The density then decreases as moisture content increases past four per cent, and layers of water molecules in effect dilute the density of the cotton cellulose structure.

5Figure(e): Change in cotton cellulose density (g/cm3) with moisture regain (per cent)

Moisture has important effects on the physical properties of cotton – particularly tensile properties and other property descriptions normalized for weight. The increase in strength with increased moisture content is attributed to the release of internal stresses as hydrogen-bonding is weakened and to the ability of the structure to be pulled into a more oriented form.

In one study, at 55 percent relative humidity, tenacity was 25.8 g/tex and this increased to 29.1g/tex after conditioning at 75 percent relative humidity. Fiber crimp, compress-ability and torsional rotation properties are also affected by high humidity.

Wool:

Wool is an excellent regulator of humidity it can absorb up to 30% of its weight in humidity without feeling damp or breaking. This hydrophilic property of wool allows it to breathe.

Wool is composed of about 97% protein, 2% lipids, and 1% minerals (Onar and Sariisik, 2004). As shown in below figure(f), wool fiber has a so called skin-core structure. In this structure, the inner cortex is hydrophilic, due to the existence of large number of the polar groups it contains. The outer surface is the hydrophobic cuticle. This cuticle cell is about 0.3-0.6 μm thick and approximately 30 μm long.

Exocuticle (rich in disulphide crosslinks – exocuticle A = 35% cystine; exocuticle B = 15% cystine) and the softer endocuticle (3% cystine). The intercellular cement contains only 1% cystine. The cuticle is 10% of the weight of wool, and the cortex is 90%.

6Fig.(f): Structure of wool

Cortical Cells have a complex interior structure that includes:

Twisted Molecular Chain and Helical Coil

These cells are protein chains that are coiled in a helical shape like a spring. The chains are stiffened by hydrogen and disulfide bonds, linking each coil of the helix, helping to prevent it stretching. Though the helical coil is the smallest part of the fiber this little spring gives wool its flexibility, elasticity and resilience; helping wool fabric keep its shape and remain wrinkle free.

Microfibril

These cells make up the units, lying inside the Matrix. The microfibrils are like the steel that is embedded in concrete to provide the strength and flexibility. The microfibrils contain three right-handed helices wrapped around each other in a left-handed coil where they are held together by more H-bonds and sulfur bridges (protofibril). Nine of these protofibril coils cluster around two more so that the microfibril contains a total of eleven coils each consisting of three α-helices.

Matrix

The matrix consists of high sulfur proteins. This makes the wool absorbent because they attract water molecules. Wool can absorb up to 30% of its weight in water and can also absorb and retain large amounts of dye. The Matrix region is responsible for wool’s fire resistance and antistatic properties.

Macrofibril

Inside the cortical cells are the macrofibrils that are made up of bundles of hundreds of the even finer filaments (the microfibrils). These are surrounded by the matrix region.

During steaming the yarn or doubled yarn twist is set. Of course, the morphological structure of the fibers must be considered when equalizing the tensions by steaming. Since the woolen fiber very quickly gets the temperature for breaking up the hydrogen bridges and the steam for hydrolysing the cystine bridges, a relatively quick twist modification is possible which roughly corresponds to the values of an autoclave moderated yarn; however, the steaming quality of the Steamatic steaming process is much better with reference to the evenness of moisture absorption.

Nylon:

Nylons, or polyamides (PA), are high performance semi-crystalline thermoplastics with attractive physical and mechanical properties that provide a wide range of end-use performances important in many industrial applications. It is a polyamide fiber, derived from a diamine and a dicarboxylic acid, because a variety of diamines and dicarboxylic acids can be produced, there are a very large number of polyamide materials available to produce nylon fibers. The two most common versions are nylon 6:6 (polyhexamethylene adiamide) and nylon 6 (Polycaprolactam, a cyclic nylon intermediate). Raw materials for these are variable and sources used commercially are benzene (from coke production or oil refining), furfural (from oat hulls or corn cobs) or 1,4-butadiene (from oil refining). The chemical reactions are as follows.

7Fig.(g): Structure of Nylon

All nylons are hygroscopic (moisture sensitive), which is an important factor to be considered during material pre-selection, parts design, mechanical performance prediction and optimization.  In general, the moisture content in nylon is a key variable affecting processing (polymerization, compounding, molding, welding, etc.) and end-use performances (mechanical, dimensional, surface appearance, etc.). The absorbed water in polymer behaves like plasticizer, which affects material properties such as strength, stiffness, and ductility. Water also results in deterioration of electrical properties.

8Fig.(h): Moisture response of Nylon.

Polyester:

Polyester is currently defined as: “Long-chain polymers chemically composed of at least 85 percent by weight of an ester and a dihydric alcohol and a terephthalic acid.Polyester from polyethylene terephthalate is an extremely strong fiber with a tenacity of 3–9 g/d (27–81 g/tex). The elongation at break of the fiber varies from 15% to 50% depending on the degree of orientation and nature of crystalline structure within the fiber. The fiber shows moderate (80%–95%) recovery from low elongations (2%–10%). The fiber is relatively stiff and possesses excellent resiliency and recovery from bending deformation. The fiber has a specific gravity of 1.38. The fiber is quite hydrophobic, with a moisture regain of 0.1%–0.4% under standard conditions and 1.0% at 21°C and 100% RH. It is swollen or dissolved by henols, chloroacetic acid, or certain chlorinated hydrocarbons at elevated temperatures. Polyethylene terephthalate polyester is highly resistant to chemical attack by acid, bases, oxidizing and reducing agents, and is only attacked by hot concentrated acids and bases. Biological agents do not attack the fiber.

The name “polyester” refers to the linkage of several monomers (esters) within the fiber. Esters are formed when alcohol reacts with a carboxylic acid:

9Fig.(i): Structure of polyester

There are, therefore, many possible variations of the generic polyester fiber. Two that are currently produced commercially are polyethylene terephthalate (PET) and poly-1,4, cyclohexylene dimethylene (PCDT).

Polyester is a smooth fiber with an even diameter. The fiber diameter usually ranges from 12-25 micrometers (10-15 denier). The undyed fiber is slightly off-white and partially transparent. The fibers are approximately 35% crystaline and 65% amorphous.

10Fig.(j): Close up of a polyester fiber

Below graph shows the comparative moisture performance of above four fibers discussed:

11Fig(k):Comparative moisture response

For conditioning steam is required in specific status. Let us discuss about different types of steam.

Types of steam:

If water is heated beyond the boiling point, it vaporizes into steam, or water in the gaseous state. However, not all steam is the same. The properties of steam vary greatly depending on the pressure and temperature to which it is subject.

Saturated (dry) steam results when water is heated to the boiling point (sensible heating) and then vaporized with additional heat (latent heating). If this steam is then further heated above the saturation point, it becomes superheated steam (sensible heating).

Saturated Steam (Dry)

As indicated by the black line in the above graph, saturated steam occurs at temperatures and pressures where steam (gas) and water (liquid) can coexist. In other words, it occurs when the rate of water vaporization is equal to the rate of condensation.

Saturated steam has many properties that make it an excellent heat source, particularly at temperatures of 100 °C (212°F) and higher.

12Fig.(l): Pressure-Temperature Relationship of Water & Steam

Unsaturated Steam (Wet)

This is the most common form of steam actually experienced by most plants. When steam is generated using a boiler, it usually contains wetness from non-vaporized water molecules that are carried over into the distributed steam. Even the best boilers may discharge steam containing 3% to 5% wetness. As the water approaches the saturation state and begins to vaporize, some water, usually in the form of mist or droplets, is entrained in the rising steam and distributed downstream. This is one of the key reasons why separation is used to dis-entrain condensate from distributed steam.

Superheated Steam

Superheated steam is created by further heating wet or saturated steam beyond the saturated steam point. This yields steam that has a higher temperature and lower density than saturated steam at the same pressure. Superheated steam is mainly used in propulsion/drive applications such as turbines, and is not typically used for heat transfer applications.

It is advantageous to both supply and discharge the steam while in the superheated state because condensate will be generated inside steam-driven equipment during normal operation, minimizing the risk of damage from erosion or carbonic acid corrosion.

Supercritical Water

Supercritical water is water in a state that exceeds its critical point: 22.1MPa, 374 °C (3208 psia, 705°F). At the critical point, the latent heat of steam is zero, and its specific volume is exactly the same whether considered liquid or gaseous. In other words, water that is at a higher pressure and temperature than the critical point is in an indistinguishable state that is neither liquid nor gas.

Supercritical water is used to drive turbines in power plants which demand higher efficiency. Research on supercritical water is being performed with an emphasis on its use as a fluid that has the properties of both a liquid and a gas, and in particular on its suitability as a solvent for chemical reactions

A practical Implementation:

The process of heat setting involves exposing yarns in the form of saturated steam sprays. One of the older methods of heat setting fabrics is the autoclave which, although still in use, is being replaced by more efficient methods with higher turnover rates. An autoclave is an enclosed vessel that heats its contents under high pressure or deep vacuuming. The yarn are loaded into the autoclave in skeins, on bobbins, or in a separate container and heated to stabilize the fibers.

13Fig.(m): Steaming process flow diagram (two cycle with heated up accumulator).

The thermal conditioning uses low-temperature saturated steam in vacuum. With the vacuum principle and indirect steam, the yarn is treated very gently in an absolutely saturated steam atmosphere. The vacuum first removes the air pockets from the yarn package to ensure accelerated steam penetration and also removes the atmospheric oxygen in order to prevent oxidation. The conditioning process makes use of the physical properties of saturated steam. The yarn is uniformly moistened by the gas. The great advantage of this process is that the moisture in the form of gas is very finely distributed throughout the yarn package and does not cling to the yarn in the form of drops. This is achieved in any cross-wound bobbins, whether the yarn packages are packed on open pallets or in cardboard boxes. According to above two cycle with heated up accumulator cotton yarn conditioned & changes are observed in count(Ne), CSP(Count Strength Product) ,weight that are shown below.

14Fig(n): Change of yarn count for conditioning

Conclusion:

Although effective, the autoclave method is slow, labor intensive, and does not lend itself to integration into a seamless production process. Textile materials produced from natural fibers with high moisture regain (eg. wool 13.6% – 18.0% and cotton 8.5%) are more likely whereas nylon with low regain (4.0%) and polyester with only slight moisture regain (0.4%). At conditioning the individual fibers start to move in one direction, however cannot return to their original position. As a result of the mechanical action, the fibers become entangled and locked together, causing the dimensional stability of yarn as well as the fabric. According to the theory of DFE (Directional Frictional Effect) it provides benefit.

References:

1. “The Classification of Cotton”, USDA Agricultural Marketing Service, cotton Division, Agricultural Handbook 566.
2. Duckett, K.E.: “Surface Properties of Cotton Fibers“, Surface Characteristics of Fibers and Textiles, edited by M.J.Schick. “Fiber Science Series”, Marcel Dekker.
3. Matthew’s “Textile FibersTheir Physical, Microscopic and Chemical Properties”, edited by Herbert R. Mauersberger, 6th edition, John Wiley & Sons, Inc.
4. A. B. Thompson Fiber structure Edited by J. W. Hearle and R. H. Peters, Butterworth & Co. Ltd. and the Textile Institute.
5. Operational  Manual of  XORELLA , Xorella Machinery Co., 5430 Wettingen , Switzerland.
6. “Steam Classification & Operation”, TLV steam system, Kakogawa City, Japan
8. U.S. cotton Market, Monthly Economic Letter, Cotton Incorporated, Market Research.
9. Stuart Gordon, Susan Horne and Marinus van der Sluijs,“Moisture in cotton – the fundamentals”.
10. SIRO Materials Science and Engineering,Henry Street, Belmont Vic 3216, Australia

About Author:
Md.Mominul motin(Tusher)
B.Sc. in Textile Tech.(D.U.)
Commonwealth M.B.A.(O.U.)
Servicing as General Manager of a reputed Spinning Mills

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