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. Thanks to its improved tenacity and elasticity, the yarn can stand without problems the tensions and the rubbing caused by weaving.
The functions of the sizing operation are:-
1. To lay in the protruding fibers in the body of the yarn and to cover weak places by encapsulating the yarn by a protective coating of the size film. The thickness of the size film coating should be optimized. Too thick a coating will be susceptible to easy size shed-off on the loom.
2. To increase the strength of the spun warp yarn without affecting its extensibility. This is achieved by allowing the penetration of the size into the yarn. The size in the yarn matrix will tend to bind all the fibers together. The increase in strength due to sizing is normally expected to be about 10 to 15% with respect to the strength of the unsized yarn. Excessive penetration of the size liquid into the core of the yarn is not desirable because it affects the flexibility of the yarn.
3. To make a weaver’s beam with the exact number of warp threads ready for weaving.
Sizing machine:
A sizing machine is used to apply the size material to the yarns. The major parts of the sizing are the creel, size box, drying units, beaming and various control devices.
The size box is probably the most important section of the sizing machine. During the sizing process, the sheet of yarns is passed through the size box which contains the hot water solution or mixtures of sizing agents. The yarns pick up the required quantity of size solution in the size box, any excess size is squeezed oft as the yarns pass through squeeze rolls. Depending on the size material, warp quality and density, single and double immersion rolls and single squeeze and double squeeze configurations are used. Multiple size boxes can also he employed. In general, single box sizing machines have two squeezing rollers and two box machines have a single roller in each box. It is important that the rollers provide uniform squeezing pressure. The squeezing system determines the degree of size pick up to a large extent. While providing size consistency, the roll pressure should be adjusted to get around 125—130% wet pick up for cotton yarns, 110—115% for poly/cotton and 95—105% for polyester. On average, open-end yarns pick up around 10—15% more wet size than a comparable ring spun yarn. Therefore, about 10% more water should be added to get the same add-on. The bottom rollers are usually made of steel and the top rollers are rubber coated.
The critical parameters to watch in the sizing process are size homogeneity, constant speed of the sizing machine, constant size concentrations and viscosity. Flooding or dry zones should be prevented in the size box. Temperature of the size box is important for proper size pick up. For 100% polyvinyl alcohol (PVA) sizing, a temperature of 70 – 75 °C is recommended. Constant size temperature can be obtained in two ways:
1. Direct heating in which steam is injected into the size.
2. Indirect heating in which steam flows in pipes around the double walled size box.
A cooker is used to prepare the size; powdered size is metered into weight stations and then transferred to the cooker. Size mix is also called size liquor. Liquor concentration and the amount of water absorbed by the warp after the squeezing rollers are measured using a microwave measuring head, which enables the degree of sizing to be calculated. The degree of sizing can be calculated as the absolute degree of sizing or as a relative value. If the measured degree of sizing is different than the set data, then the squeeze pressure in the box is changed with a controller. The squeeze pressure is also adjusted according to speed of the system.
Rotor cooker is normally used for size preparation, in a rotor cooker; the size is prepared without pressure. The ingredients are added cold and mixed with a rotor and raking agitator. Additional breaking elements can be used to obtain high shearing effect. The size ingredients are automatically fed-in to the size box.
After the size box, the yarns go through the dryer section. The wet yarns are dried by using hot air, infrared radiation or cylinder drying. Cylinder drying is done using steam heated hot rolls which are called drying cylinders. Sometimes, a combination of drying methods is used on the same machine.
The drying temperature is critical; excess temperature increases the penetration of size into the yam, which can lead to excessive hairiness and even yarn breakage. The typical temperature range is 80 – 105°C. In cylinder machines, the evaporation rate can be calculated as the mass of water evaporated per unit contact area between warp and drying cylinder per unit time. A typical evaporation rate in a modern slasher is around 13 kg/hr/m2. The maximum recommended machine speed is 120 m/min.
Splitting the warps after sizing and separate drying reduces the risk of adjacent yarns sticking together which reduces the number of yarn breaks. For air-jet weaving, a minimum of 75% open space on the dry cans is recommended for any type of yarn.
Due to the nature of sizing, the yarns in the sheet may be stuck together at the exit of the dryer section. Therefore, they are separated into individual ends by using leasing rods. First, the individual sheets of yarns from each warper’s beam are separated followed by pins in the expansion comb to separate the yarns within each sheet. Then the yarns are wound onto a loom beam for weaving (weaver’s beam).
Beam arrangements in the creel are usually two types:
1. Groups of 2, 4, 6 or 8, one to four tiers
2. Staggered, two-tier arrangement
Some sizing machines can have up to 24 beam positions. The beams can be controlled in groups or individually. The let-off can be individual let- off, single group let-off or wrap-round let-off.
Sizing machines are classified based on the method of drying (cylinder, hot air or infrared) or according to the method of yarn supply (single end, direct and indirect). In single – end slashers, yarns are fed to the size box directly from the supply packages. This type of creel is generally used for a small number of yarns and textured yarns. In direct sizing, yarns are fed to the size box from a single creel beam or warper’s beam. In indirect sizing, several warper’s beams (“section beams”) are combined sheet to sheet, forming a final beam for weaving (weaver’s beam). It is also possible to draw the yarns from a combination of package creels and warp beams simultaneously.
Sizing variables:
A good sizing depends on various factors. During sizing the following variables should be checked and controlled where necessary:
· Viscosity of the size solution.
· Sizing machine speed.
· Size add-on levels.
· Concentration of the size mixture.
· Volume of the size box.
· Threading arrangements.
· Condition of squeeze rolls.
· Squeezing pressure.
· Hardness of squeeze rolls.
· Diameter of squeeze rolls.
· Number of size boxes.
· Yarn count and size box warp density per unit space.
Sizing Chemicals & their importance:
Film forming materials: Starch is the oldest film forming material used in sizing of cellulosic fibers. It is also the most widely used in the world due to its low cost and ease of availability. In Pakistan, mostly maize starch is used, whereas, potato starch is more popular in Europe.
PVA is the second largest film former used in sizing. It is mostly used on synthetic yarns such as polyester and poly/cotton blends. PVA coating is strong, abrasion resistant and can easily be desized in hot water. Its strength is greater than starch and also more flexible than most standard starches. PVA is less prone to setup in the size box compared to starches. PVA can form foam in the size box which is controlled with a deformer. PVA may be too strong for some sizing applications. In this case, some weaker film forming polymers such as starch are added to modify the mixture, which also reduces the cost, since PVA is more expensive than starch.
The most widely used size materials are starch and PVA. However, other size materials have been developed and used for specific purposes. Carboxymethyl cellulose (CMC) is produced from wood pulp and cotton lint and has good adhesion to cotton.
Polyacrylic acid based sizes (polyacrylates and polyacrylamides) are used to size hydrophobic fibers and their blends such as nylon, acrylics, polyester, etc because of their good bonding.
Properties of a good sizing material:
· Environmentally safe.
· Good film former.
· Reasonable use economics.
· Penetration of yarn bundle.
· Elasticity.
· Good film flexibility.
· Good specific adhesion.
· Good frictional properties.
· Transparency.
· Bacterial resistance.
· Reasonable strength.
· Controllable viscosity.
· Water soluble or water dispersible.
· Good hygroscopicity characteristics.
· Uniformity.
· Clean split at bust rods.
· Improves weaving efficiencies.
· No effect on drying.
· Reasonable extensibility.
· Recoverable and reusable.
· Low static propensity.
· No skimming tendency.
· Easily removed.
· Easily prepared.
· Lack of odor.
· No beam blocking.
· Compatible with other ingredients.
· Good abrasion resistance.
· Neutral pH.
· High fold endurance.
· Insensitive to high heat.
· Low BOD.
· No build up on dry cans.
· Reduced shedding.
· Rapid drying.
· No re-deposition of size.
· Insensitive to changes in relative humidity.
Add-on materials: In addition to a film former polymer various additives are added to a size mix. Additives may be included in the size mix depending on the particular weaving machine requirements or if a particular type finishing is required after the fabric is woven. A huge variety of additives are used. Most common of them are as follows:
Binders: There are a number of polymeric materials that can be used in sizing as binder materials. Binders are true film formers but are generally not used alone for sizing; some sizes are useful as both a primary size and as a binder. Binders are typically used to increase weave ability by promoting the adhesion of the primary film forming size to a specific fiber substrate while reducing the cohesion between sized yarns. By judicious selection of a binder, additional sizing advantages can be realized.
Most commonly there are three types of binders used;
· Acrylics:
Ø Pearl corn starch.
Ø Modified starch.
Ø Starch derivatives.
Ø Polyvinyl alcohol.
· CMC.
· Polyester resin binders.
· Vinyl acetate resins.
Lubricants: Lubricant is almost always added to increase abrasion resistance of the yarn which is especially useful for rapiers and projectile machines. Lubricants with anti-sticking agents (lecithin) also prevent sticking of PVA to dry cans. Emulsifiers are added to the wax to improve desizablity. Paraffin or marine glycerides are added to harden the wax and better lubricate the yarns; however, if not removed properly during desizing some lubricants can cause problems in later operations.
Various other additives include humectants, anti-static and anti-foam agents, removable tints (for warp or style identification), preservatives (if the warp or the fabric is to be stored for long periods of time), penetrating agents (to allow the size to penetrate into tightly constructed styles), weighting agents (to make cloth heavier), anti-mildew.
Softeners such as lubricants, soaps and waxes are used to make the yarn sufficiently extensible, they also prevent cracking of size during weaving.
Humectants, such as urea, sugar and glycerin are used to retain moisture in the size product. Moisture makes the size film more flexible and less brittle.
