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Sunday, January 18, 2015

Drying Methods

Definition of drying

The warp sheet after leaving the size box gets impregnated with wet size paste to the extent of about 90% - 140% of its own weight. The process of drying involves complete removal of the moisture from the size paste and at the same time retaining the moisture in the yarn. This is particularly so in the case of cellulose or any other natural fibre yarns. It is to be noted here that the degree of drying largely influences the yarn properties such as strength, flexibility, brittleness, and plasticity and so on. Also the rate of drying determines speed and production of the sizing machine. Hence it is to be effected in such a way so as to retain the desirable properties of the yarn and economy maintained with respect to production and energy consumption. The time and temperature of drying play an important role, and proper care and caution is to be exercised in this aspect as otherwise it would result in over or under drying of yarns that could create problems in subsequent process. There are different methods of drying sized yarns.

Methods of drying

There are three important methods of drying in sizing. These are:
-         Conduction method
-         Convection method, and
-         Radiation method

Conduction method

In this method the drying of the warp sheet is effected by conducting or transmitting heat from metallic surface of drying cylinder which is steam heated. This is also known as contact type of drying, and is most popularly used in most of the slasher sizing machines nowadays. It is also very economical with respect to energy consumption. The only drawback with this method is at a certain degree of flattening takes place due to contact of yarn with cylinders.

Convection method

In this method the drying of the warp sheet is effected circulation of hot air around in the drying zone. The air may be heated either electrically or by steam. The hot air is circulated by fans and ducts. The main advantage with this method is that the yarn is not flattened as in the previous case.

Radiation method

In this method the drying of warp sheet is effected by heat transferred to it by direct radiation. Electrical heaters are used for this purpose. Energy is transmitted to the yarn in the form of heat waves, by means of electromagnetic radiations, and this removes the water present in the yarn. In this method, also, as in the previous one, no flattening of yarn takes place, since the yarn has no contact surface. It is considered very poor with respect to economy in energy consumption and hence is seldom used. It is however suitable for laboratory trials as well as large scale operations that need quick drying.

Multi cylinder drying

This method is most commonly adopted in the industries. In this case, the number of drying cylinders used ranges from 5 - 13. The size of the cylinders is much smaller compared to those used in the convectional twin-cylinder machine. In some special cases, where dyeing and sizing are done simultaneously, such as in the dye-sizing operation for denim, the number of drying cylinders are much more, and may go up to 32. The cylinder diameter is about 762 mm. Steam pressures are normally higher and vary with different places. In some places, the steam pressures range between 2 – 2.5 kg/cm2, where as in other places it can range as high as 5 – 7 kg/cm2, higher steam pressures permit greater running speeds. The cylinders of a multi cylinder slasher machine are arranged in two rows.

Drying method

One of the advantages of the multi cylinder is that the cylinders are positively driven, and this prevents any undue tension on the yarn. The cylinders rotate smoothly since they are mounted on ball bearings. Each cylinder is provided with independent steam supply valve, so that the temperature of each drying cylinder can be individually controlled to suit the type of yarn and the machine. A number of parameters, such as type of yarn, speed of machine, number of cylinders etc., govern the temperature of each cylinder. The first cylinder is at higher temperature, and the temperature falls at successive cylinders. The last cylinder is kept cool so the dried yarn leaves the drying zone in a cool condition. The first one or two cylinders are coated with Teflon so as to prevent the problem of ‘smudging’ (size stain). The cylinder temperatures vary with the type of yarn sized. It ranges from a maximum of about 1500C for cotton yarns and minimum of 800C for acetate rayon filament. Polyester and polyamide yarns are dried at 85 – 900C.

Friday, January 16, 2015

Slasher Sizing machine

This method is universally adopted and is suitable for large scale operations. In this method a number of warp yarns as per requirement, is assembled in the form of a continuous sheet, sized and then wound onto a weaver’s beam. These operations are supported by auxiliary operations such as separation of ends, measuring warp length, marking into cut lengths and winding the warp of equal length to form the warp of the fabric. All these operations are performed by the slasher or tape sizing machine.
The slasher sizing machines are classified according to the method of drying. Accordingly there are three types, namely –
-         Cylinder drying: Twin and multi-cylinder type.
-         Hot air drying.
-         Electrical drying.
The first method is based on the principle of conduction and is widely used in industry. Twin cylinders were used for drying of sized warp in the conventional machines. Later multi cylinders have been used and are commonly prevalent nowadays. The second method is based on the principle of convection, and the third method is based on the principle of radiation.

Passage of material through a slasher machine

The general passage of warp through a two cylinder slasher sizing machine is shown in the following fig:
Slasher sizing machine

The yarn from the warper’s beams usually containing about 500-800 warp ends, are made to pass into a size box through a guide roller. The warper’s beams also known as the back beams are placed in a stand called creel. The number of warper beams depends upon the total number of ends required in the cloth or weaver’s beam. The warp from rear beams pass over and under the successive back beams. The warp sheet emerging from the back beams enters a size box that is kept heated by constant supply of steam through pipes. The warp sheet is made to go under a partially immersed immersion roller, and then passes between the nips of the size and squeeze roller. The squeeze roller impregnates the size into the yarn structure and also removes external excess size at the same time drags the warp sheet through the paste.
The wet size yarn then enters the drying zone comprising of either two or multi-cylinders or hot air chamber. The sized yarn is dried here. The residence time of the sized warp in the drying zone is regulated in such a way so as to avoid over drying. All the cylinders are steam heated except the last one, which is kept cool by supply of cold water. This is done so as to cool the warp sheet when it leaves the drying zone.
The sheet of warp yarns after leaving the drying zone is then split into as many sections or layers as there are beams in the creel. This is done by means of lease rods or split rods. The purpose is to eliminate stickiness of neighboring warp threads. The split warp threads are then recombined and then made to pass through an ‘expanding or zig zag’ reed or comb. This comb regulates the width of the warp sheet to the required dimensions. The warp is then made to pass between the nips of a ‘drag roller’, and is finally wound on a beam called ‘weaver or sizers’ beam. In short, the entire slasher machine can be divided into three important zones, namely –
-         Creel and size box forming the rear or back side of the machine.
-         Drying arrangement forming the middle zone of the machine and
-         Front zone or head stock consisting of weaver’s beam, and various controls.

Sizing and its Importance

Definition of Textile Sizing

Sizing is the intermediate process of weaving preparation. The yarn after being warped is sized or coated with a thin film of adhesive. The purpose of sizing the yarn is to improve its weave-ability. A single unsized yarn is incapable of withstanding the strains of weaving and also affects the loom production and efficiency. Hence it becomes crucial to size the yarn, particularly the single yarn. In fact, it is the most important in the weaving preparatory process. It would not be an exaggeration to say that to size well is to weave well, just as the saying goes that to card well is to spin well. The size materials used in sizing consists of a number of ingredients mixed together in suitable proportions to form a viscous paste. The type of sizing material differs for different types of textile materials.

Objects of Sizing

The objectives are manifold and are as follows –
-         Add strength to the yarn so as to withstand the stresses and strains of the weaving process.
-         Improve the abrasion resistance of the yarn.
-         Makes the yarn smoother.
-         Add more weight.
-         Cover the protruding fibres on the yarn surface.

Important Technical Considerations of Sizing

The following are the basic considerations to be made –
-         Type of the weaving machinery on which the sized yarn is to be used. The extent of tension that the yarn is subjected to depends on the loom type.
-         The type of textile material being sized. Different materials (cotton, polyester etc.) respond differently to the strains encountered in weaving. The sizing material used depends on the type of textile material being used.
-         The stresses and strains that the yarn is subjected to during the process.
-         Energy consumption and cost.
-         Characteristics of the ingredients used, methods of size paste preparation and their concentration of the ingredients, viscosity of the size paste etc.
-         Desizability of the sized material after weaving and the cost involved.
-         The sophistication of the machine.
-         Economy of the process to get the best quality of warp at the lowest cost.

Suitability of size for different Textile materials

Each type of textile material responds differently to the stresses and strains to which it is subjected in the weaving process. For example cotton material responds differently as compared to polyester material. Continuous filament yarns have good resistance to the weaving tensions due to their better cohesion and continuity of filament length. The purpose of sizing in such cases is only to improve the abrasion resistance rather than strength. The toughness of the textile material is even more important than its strength. Synthetic fibres are advantageous in this aspect. Each type of textile material has its own merits and demerits and the type of size ingredients have to be used accordingly to compensate for their demerits.

Sizing Ingredients

Definition of sizing ingredients

The fundamental objectives of the sizing can be achieved only by impregnating the yarn with the appropriate size mixture consisting of various ingredients in suitable proportion. Each ingredient should impart a particular property to the yarn. Accordingly there are two types of ingredients, namely the primary and secondary ingredients. The primary ingredients are those that are essentially required in the size paste where as the secondary ingredients are those that may or may not be added to the size mixture according to the requirements. The primary ingredient is the main ingredient and it helps to give additional strength to the yarn and also improves its abrasion resistance. The other ingredients known as the secondary ingredients give additional properties to the yarn such as feel, weight, appearance etc.

Sizing Ingredients

Types and list of sizing ingredients with their functions

The following are the types of ingredients used in sizing –
Primary:
-         Adhesives
-         Softeners/Lubricants
-         Antiseptics
Secondary:
-         Humectants
-         Antistatic
-         Weighting agents
-         Brightening/Bluing agents
-         Wetting agents
-         Antifoaming agents
 
Adhesives
Adhesives are those materials that can adhere to the warp yarns throughout the weaving process by increasing the yarn strength and also making its surface smooth so as to withstand abrasion between neighboring warp threads. The adhesives improve the yarn strength by binding the fibres in the yarn together thus making the yarn strong enough to withstand the weaving stresses and strains. In other words the adhesive is that which binds the fibres together and brings about better cohesion between them. Additionally the adhesive should have a certain level of viscosity so that it can form a coating or thin uniform film around the surface of the yarn thereby reducing its frictional resistance and thus reduce abrasion between neighboring warp threads. Also the adhesive should bind the protruding fibres to the surface of the yarn.
A number of adhesives are available and these are listed below:
-         Starch and starch products - Ex: Maize starch, Tapioca starch, Potato starch, Sago starch, Wheat starch etc.
-         Thin boiling starch or soluble starch - Ex: British gum, Dextrin
-         Natural adhesives other than starch - Ex: Tamarind Kernel powder, gum Arabic, gum tragacanth, gum Karaya, beam gums.
-         Protein substances - Ex: Glue and gelatin, Casein, Soya bean protein.
-         Pectin materials - Ex: Alginic acid
-         Starch, gum and cellulose derivatives - Ex: Starch ethers and esters, gum guar, Carboxymethyl cellulose.
-         Synthetic adhesives - Ex: Polyvinyl alcohol, Acrylic polymers, Vinyl polymers, Polyethylene glycol, Polystyrene etc.
 
Softeners/Lubricants
These are next in importance to the adhesives. The adhesives produce a film on the yarn which is rather stiff and thus renders the yarn inflexible. Hence in order to make the adhesive film flexible or pliable, the softener is added. Another problem is that the adhesive film on the yarn is not smooth i.e. has rough or uneven surface. This tends to generate frictional forces between neighboring warp threads in the loom during shed formation and also between yarn and certain loom parts such as reed, heald eyes etc. The effect would be that the rough coating of size would come off the yarn and subsequently lead to yarn breakage in the loom. Therefore it is necessary to make the yarn surface smooth. The softener thus performs the dual function making the yarn flexible and smooth. In other words it also acts as a lubricant. Softeners/lubricants used are the following –
Ex: Oils and fats, waxes, mutton tallow, oils and emulsions, stearic acid emulsions, vegetable tallow, soaps, sulphated oils and fats, mineral oils, paraffin wax, plasticizers etc.
Of the many softeners available, mutton tallow is considered to be the best since it acts well as a softener and lubricant.
 
Antiseptics
These are also known as preservatives. Cellulosic or protein materials which are sized are particularly susceptible to attack by microbiological organisms, moth, insects and mildew. This is due to the hygroscopicity of the textile material and the adhesive with which they are coated. The following antiseptic agents are used in sizing –
-         Metallic salts - Ex: Zinc chloride, copper sulphate, zinc sulphate, sodium silicofluoride.
-         Organic compounds - Ex: Sailcyclic acid, Phenol, Cresol, Benzoic acid etc.
Among the various antiseptics used, zinc chloride is the best since it acts both as an antiseptic as well as humectants.
 
Deliquescents
These are basically hygroscopic agents and therefore are also called humectants. They help the sized yarn to regain its moisture and flexibility. In other words the deliquescent prevent the yarn from over drying and becoming brittle, by retaining the moisture in the cotton yarn in the required level of 8-10% and thereby maintain the flexibility of the yarn. These are, however, not required in modern sizing machines that are equipped with moisture controllers. The various types of deliquescent that have been used in sizing are Zinc chloride, magnesium chloride, Glycerin etc.
 
Antistatics
Though the antistatic agents are considered to be under the category of secondary ingredients, they are of importance in the sizing of synthetic hydrophobic that are prone static electricity. Fibres such as polyester, nylon, cellulose acetate are highly prone to static electricity. The static charges generated create problems in the subsequent processes, difficulty in unwinding of the weaver’s beams, cloth rolls, and in extreme cases can even cause fire hazards. Hence it becomes necessary to control the static charge generation. The antistatic agents used are glycerol, polyethylene glycol, lissapol NX. Out of these Lissapol NX is the best. Besides, there are also several types of antistatic agents that may be of anionic, cationic, and non ionic or amphoteric in nature.
 
Weighting agents
These are also variously known as weightening agents, filling agents, fillers etc. They help to add weight to the yarn so as to make the fabric get a feel or fullness. This is of importance when the cloth is to be sold in the grey state on a weight basis. Some of the well known weighting agents are China clay, Gypsum, Talc or French chalk or soapstone, Barium sulphate, Epsom salt etc.
 
Brightening/bluing agents
These are coloring/whitening agents. Here too, just as in the case of the weightening agents, these agents are required when the cloth is sold in a grey state. Cotton materials have natural coloring matter which makes them dull or creamish in appearance. Hence in order to improve the aesthetic look of the material, they are generally made whiter or given a bluish tinge. Some of the well known brightening/bluing agents used are Ultramarine blue, acid dyes, fluorescent brightening agents etc.
 
Wetting agents
In the case of synthetic textile materials such as polyester, nylon etc. the natural adhesives such as starch cannot wet the fibres, since they are hydrophobic in nature. Therefore synthetic adhesives will have to be applied with some suitable wetting medium. The wetting medium which is ideally suited for the purpose is Turkey red oil. Other oils such as P4 oil can also be used.
 
Antifoaming agents
During the sizing there is always the possibility of foam generation in the size or sow box. The foam is generated by surface active agents such present in the size paste such as soaps, Turkey red oil etc. Alkali foam is formed due to the churning action of the squeezing rollers. The foam that is carried by the warp sheets and causes smudging of the drying cylinders. Hence in order to reduce the foaming tendency of the size paste, anti-foaming agents are to be added. Some of the well known anti-foaming agents used are amyl alcohol, turpentine, pine oil etc.
 

Saturday, January 18, 2014

Definition, Objects and Techniques of Sizing

Definition of sizing | Objects of sizing | Different techniques of sizing | Sizing machines | General specification of sizing machines

Sizing:
Sizing is a complementary operation which is carried out on warps formed by   spun yarns with insufficient tenacity or by continuous filament yarns with zero twist. In general, when sizing is necessary, the yarn is beam warped, therefore all beams corresponding to the beams are fed, as soon as warping is completed, to the sizing machine where they are assembled. Sizing consists of impregnating the yarn with particular substances which form on the yarn surface a film with the aim of improving yarn smoothness and tenacity during the subsequent weaving stage.
The main parts of a sizing machine are: the creel; the framework and drive; the sizing unit consisting of a size box and an immersion and squeezing rollers.
We can say that, the process of applying a protective adhesive coating upon the yarn surface is called sizing.
Objects of sizing:
          • To improve the weave ability of warp yarn.
          • To maintain good fabric quality by reducing hairiness, weakness and by increasing smoothness, strength of yarn.
          • To increase the tensile or breaking strength for cellulose yarn.
          • To increase the elasticity.
          • To remove the projecting fibre.
          • To reduce electrostatic formation for synthetic or blended yarn.
Different techniques of sizing:
          • Hot melt sizing.
          • Solvent sizing.
          • Foam sizing.
          • High pressure sizing.
          • Electrostatic sizing.
          • Emulsion sizing.
          • Combined sizing.
          • Conventional sizing.
Sizing machines:
The sizing process consists in impregnating with size the warp unwound from the warping beams, drying separating the ends and beaming. Moreover, during the sizing process, the warp is marked for separating it into pieces of predetermined length.
The main parts of a sizing machine are: the creel for the accommodation of the warping beams; the framework and drive; the sizing unit consisting of a size box and an immersion and squeezing rollers; the drying section where the excess moisture is removed from the sized and squeezed warp ends; the headstock. A special device is also provided therein for additional treatment, i.e. emulsification and waxing of sized warp; and automatic instruments for checking the sizing process.
Depending upon the arrangement of the drying section, all sizing machines maybe classified as follows:
          • Cylinder sizing machine – in which the warp is dried by contact with the hot surface of the cylinder;
          • Hot air sizing machine – in which drying is effected in chambers with hot air;
          • Combined sizing machine – in which the warp is dried by contact with the hot surface of the cylinders and by passing through hot air chambers;
          • Special sizing machine - in which the warp is dried by electric heating, high frequency currents, by infra-red rays etc.
General specification of sizing machines:
          • Wrap speed at slow run – 3 to 7 meter/min.
          • Warp speed at work – 100 to 120 meter/min.
          • Maximum length in the drying part – 15 to 25 meter.
          • Maximum temperature of the drying cylinder surface – 1200 C.
          • Warping beam diameter – 600 to 800 mm.
          • Size beam diameter – 600 to 700 mm.