Heat Setting

Modifying physical structure of a fiber, hence the construction or textile article.

Thermal treatment/ Heat setting increases dye uptake.

Receptivity of disperse by PET depends on the temperature of dry heat setting before dyeing.

Dry heat setting is preferred over steam heat setting due to partial hydrolysis of ester bonds in PET leading to weakening of the fiber.

Steam heat setting can be done for blend fabrics containing PET, and PET technical textiles. It can also be done on nylon fabrics.

Tg and Tm values of a few synthetic polymers:-PolymerTg (C)Tm (C)

PET69265

Nylon 650250

Nylon 6,650270

PAC97340

Cellulose Triacetate 49 478 306

Increase in initial heat setting temperature = Decreasing dye uptake (for 140C - 180C). Beyond this, dye uptake increases (at 200C) and further beyond 200C dye uptake increases significantly!

Graphically shown:-

Reason for increased dye uptake - Due to liberalization of chains with least potential energy inside. And so Less resistance to penetration and diffusion of dye at very high temperature.

Dimensional stability, dyeability etc. of thermoplastic fibers are affected by repeated heating and cooling otherwise known as its heat history.

Purposes of heat setting:-

Stabilize the material in order to prevent shrinkage, distortion and creasing. Purposely crease, pleat or emboss fabrics.

Increase dye uptake (or improve dyeability of fibers).

Heat relaxes/relieves stresses in the amorphous regions of thermoplastic fibers. Once the fiber is heated above its Tg , molecules in the amorphous regions begin to move or vibrate and the material can be formed into a new shape.

When the temperature is decreased (cooling), the material stays in its new shape. Creases that have been developed (unwanted) in the fabric can be pulled out AND the fabrics width can be changed too somewhat in the heat setting process.

Creases can be permanently set in the fabric by heat setting if desired.

Heat setting is used to permanently set twists in yarns at the end of the spinning process (preventing twists from unraveling).

Heat setting can be done before or after any wet treatments, after scouring and mercerizing or after dyeing the fabric.

Fabric heat setting before dyeing resists undesirable creases and wrinkling in the dye process (winch dyeing) and accepts dyes more uniformly, provided that the heat setting process was controlled well.

Non-uniform dyeing can result due to poor heat setting (different temperatures applied along the length and width of fabric due to non-uniform temperature application). This leads to differential dye uptake in regions, giving leveling problems.

Heat setting of grey goods causes size material stains in the fabric difficult to remove (for woven goods).

Heat setting can remove carrier from PET but undesirable discoloration (yellowing) occurs after heat setting carrier dyed fabrics. Tendency of some softeners to develop a yellowish color when heated. Softeners can also condense in dryers and vents. These drip on fabrics producing spots.

Heat setting of PET after dyeing leads to loss of dye by sublimation and thermomigration of the dye. This depends on sublimation fastness of dyes and size or Mw of dyes.

Higher heat setting temperature (175C to 220C) leads to increased dimensional stability AND resistance to pilling.

[Textile Coloration and Finishing NC]

Heat Setting Dimensional stability, dyeability and other properties of thermoplastic fibers are affected by repeated heated and cooling, or the heat history of the material. Purposes of heat setting include:-

Stabilize the material to shrinkage, distortion and creasing

Crease, pleat or emboss fabrics

Improve dyeability of fabrics

Heat relieves stresses in the amnorphous regions of thermoplastic fibers. Once fibers get heated above their Tg, the molecules in the amorphous region can move, and the material can be formed into a new shape. When the temperature is decreased, the material stays in its new shape. Thus, creases that have developed in the fabric can be pulled out, and the width of the fabric can be changed somewhat in the heat setting process. Creases can be permanently set in the fabric by heat setting if desired. Heat setting is used to permanently set twist in yarns.

Heat setting may be done before any wet treatments, after scouring and mercerizing, or after dyeing of the fabric. Fabric heat set before dyeing resists undesirable creasing and wrinkling in the dyeing process and accepts dye more uniformly is the heat setting process was controlled well.

Heat setting can cause non-uniform dyeing if the setting temperature varies along the length or across the width of the fabric (inadequate temperature, moisture, fabric running speed etc.)

Heat setting of greige goods can make size materials and stains in the fabric more difficult to remove. Heat setting after dyeing helps remove carrier from carrier-dyed PET. But undesirable discoloration (yellowing) of the fabric may occur in the fabric heat set after dyeing.

Dye migration (thermomigration) or loss of dye by sublimation may occur in fabrics heat set after dyeing.

Heat history affects the rate at which fibers are able to absorb dye and the amount of dye the fibers will absorb. Eg. Effect of heat setting of PET at 230C on dyeing rate

Shown above is the rate of dyeing isotherms for a disperse dye on PET which was heat set at 230C compared to a control sample which was not heat set. Differences in rate of dye absorption due to differences in heat history may cause non-uniform dyeing of fabrics.

The heat setting process subjects the fibers to higher temperatures than theyve previously experienced. This minimizes the effect of previous heat history and improves the chances for uniform dyeing of the material. Uniform heating of material during heat setting leads to uniform dyeing.

Heat setting can be done with either heat or steam. PET is usually heat set dry while nylon may be heat set either dry or with steam. Continuous heat setting of flat fabrics is usually done with dry heat by contacting the fabric heated rolls, impinging hot air on the fabric in a stenter frame, a combination of these two methods, or by heating with IR radiation.

Steam heat setting is often done in an autoclave or may be done using continuous steaming equipment. Nylon carpet yarns are often steam set.

Temperature used for dry heat setting of PET is usually 180C 200C although temperatures in the range 175C 220C can be used.

Higher setting temperature gives better dimensional stability and resistance to pilling. But, the fabric becomes stiffer and loses crease recovery characteristics when set at higher temperatures.

Temperature selected for heat setting is often that temperature at which dyeability of the fabric can be best controlled.Eg. Effect of heat setting temperature on saturation value of disperse dyes on PET

From the above, its apparent that the temperature used to heat set PET affect the dye uptake of the fiber. The saturation value of different disperse dyes is affected to different degrees by heat setting, but the general effect shown above occurs for most disperse dyes. The saturation value of the dye on the fabric is more critical in batch dyeing than continuous dyeing. Heat setting temperature effects rate of dye uptake as well as saturation value and can affect continuous as well as batch dyeing.

Heat setting of PET at temperatures higher than 200C is difficult because small changes in setting temperature greatly affects dyeability of the fiber.

Effect of dry heat setting on dyeability of nylon is similar to its effect on dyeability of PET. Dyeing rate of the fiber decreases with higher heat setting temperature up to a point and then increases at even higher heat setting temperatures.Effect of dry heat setting on dyeing of nylon 6,6 with C.I. Acid Black 52 at pH 9

Dry heat setting temperatures of 190C - 200C are typical for nylon. Steam setting increases the dyeing rate of nylon with both acid and disperse dyes. The presence of water promotes molecular chain rearrangements which increase the size of openings through which dyes can diffuse.

Lower temperatures are used for heat setting fabrics containing texturized yarn because the crimp may be lost if the yarn is heated to higher temperatures. Typical dry heat setting temperatures are 150 - 175C for texturized nylon and 160 180C for texturized PET.

Problems that may be caused by heat setting or improper control of the heat setting process:-

1. Permanent set wrinkles

2. Strength loss

3. Stiffer fabric handle

4. Permanently set stains

5. Non-uniform dyeing

6. Improper fabric width or weight.

Dryers Textile materials are usually wetted and dried several times in preparation, dyeing and finishing processes.

Moisture can be removed from textile materials by mechanical methods like squeezing, centrifugal extraction, or vacuum extraction.

Mechanical methods remove only the moisture which is very loosely bound to the textile material. Eg. That located in interstices between the yarns comprising the fabric.

Water droplets trapped within the fibers and yarns and water droplets bound to the fibers by secondary forces like H-bonding are not removable by mechanical means and must be vaporized in order to me removed from the material during the drying process.

Vaporization of water requires a large amount of energy so good removal of loosely bound water using mechanical means is desirable to minimize the high drying cost.

Most drying processes for textile materials use thermal energy. The thermal energy heat water in the textile material. As the temperature of water rises, water molecules start to vaporize and escape into the atmosphere around the material.

Mechanisms for heat transfer to textile materials are:-

Indirect

Conduction

Convection

Irradiation

Direct

Dielectric

Microwave

Radio Frequency

Indirect methods all rely on a heat source external to the material to be dried. Conduction heating involves heat being transferred from a hot surface to the material to be dried by contact between the hot surface and the material being dried.

Convection heating involves hot air being circulated around the material to be dried. Super-heated steam or other hot gases can be substituted for air in convection drying.

Radiant drying, or drying by irradiation, uses a source with high content of IR (infrared) waves. Water molecules absorb infrared energy, causing water to heat and vaporize.

Dielectric heating is basically direct heating because it is electric energy which vibrates the entire water molecule causing molecular friction and generating heat inside the wet textile material.

Both conduction drying and convection drying often use steam to deliver the energy to the point of drying Steam is generated in a boiler, usually by burning fuel. The mechanism of steam generation is as follows:-[BTU : British Thermal Unit. 1 BTU = 1055 joules. 1 BTU is the amount of energy needed to cool or heat one pound of water by 1F]

The temperature of water in the boiler rises 1F for each BTU of energy input per pound of water. The heat in the water Is called sensible heat. When the water reaches its boiling point, further energy input causes vaporization of water molecules.

A pound of water absorbs about 970 BTUs in changing phase from liquid to vapor. The heat absorbed during the phase change from liquid to vapor is called the heat of vaporization or the latent heat amount of energy needed to cause a state change of 1kg liquid to vapor. The latent heat in steam is the energy which is used to evaporate water in a drying process.

Convection Dryers

Air for convection drying can be heated using heat exchangers with steam as the heat transfer medium. Alternatively, air can be heated directly from a gas-fired burner.

Dyed yarn packages are often dried by convection. A common type of dryer for yarn packages is called the rapid dryer. The packages on the same perforated cores that were used for dyeing the yarn are placed in the dryer. Hot air is forced through the packages to vaporize the water, The moisture laden air is then passed through a condenser to lower its humidity. The dry air is then reheated in a heat exchanger before again being recirculated through the wet yarn.

Continuous drying by convection is usually done in a stenter frame. Blowers impinge hot air on both the bottom and top of the fabric as the fabric passes through the stenter frame.

The stenter frame is equipped with devices to control the dimensions of the fabric both lengthwise and widthwise. Fabrics may be stretched in the lengthwise direction by applying tension to the fabric as it enters the dryer. Some fabrics, notably knits, must be allowed to relax and shrink in length in order to achieve the desired bulk and stretch characteristics. Overfeeding fabric onto the frame allows this shrinkage as drying occurs.

Width control is achieved by holding the fabric at the required width during drying. Stenter frames are equipped with an endless chain on each side which grips the fabric by both selvedges as the fabric enters the frame. The distance between the chains can be increased or decreased along the length of the stenter frame to either stretch the fabric or allow it to shrink as drying proceeds. Gripping action of the chain can be either by pins or clips. In a pin stenter the fabric edges are simply pushed onto sharp pins protruding from the chain. The numerous small holes punched in the selvedges by the pins may be undesirable in some cases.

Clip stenters grip the fabric with metal clips closely spaced along the chain.

Non-uniform heating where the clips grip the fabric can be a problem with clip stenters.

Stenter frames can be used for curing purposes, heat setting of fabrics as well as for drying.

Higher temperatures are often used for curing or heat setting than for drying.

A stenter for high temperature applications will require a direct flame heat source rather than a steam-supplied heat exchanger to achieve the desired temperature.

Conduction Dryers

Steam-heated drying cylinders or cans help dry materials in continuous processing ranges.

Mechanism of cylinder drying:-

1. Steam is generated in the boiler via. Burning of a fuel or oil

2. Steam is admitted to the cylinder through a rotating joint.

3. Cylinder wall condenses steam inside the cylinder.

4. Latent heat released by the condensing steam heats the cylinder wall.

5. Heat transfers through the cylinder wall, heating the wet fabric.

6. Water in the fabric heats and vaporizes.

7. Heat transfer from the cylinder to the fabric cools the cylinder so the cylinder condenses more steam. Repeats a lot like a cycle.

8. The hot condensate is siphoned (drained off) from the bottom of the cylinder and returned to the boiler. This same mechanism applies to heating of water in jacketed vessels or with closed heating coils. Drying capacity of a continuous processing machine determines the production rate of the machine. Therefore, maximizing the drying rate is often desirable and economically important.

Factors which affect the drying rate in a cylinder drying apparatus are as follows:-

Cylinder dimensions (or number of cylinders used) Amount of heat transfer and therefore the drying rate depends on the length of time the material is in contact with the heated surface. Steam pressure inside the cylinder Heat transfer is a function of cylinder surface temperature which depends on steam pressure inside the cylinders.

Conductivity of the cylinder wall Rate of heat transfer depends on the thickness of the cylinder wall and the thermal conductivity of the material. Since the walls of drying cylinders are thin, conductivity of the wall has only a small influence on drying rate.

Tension exerted on the material to be dried Higher tension on the material increases contact of the material with the cylinder and increase heat transfer rate and drying rate.

Ventilation of the drying apparatus Vaporization rate depends on the humidity of the surrounding atmosphere. Moisture laden air must be continuously replaced by dry air to keep the drying rate high.

Degree to which the material is dried The degree to which the material is dried dramatically affects the drying rate and processing rate of a machine. When the fabric reaches its natural moisture regain, the remaining moisture is chemically bound, and each additional increment (increase) of moisture removal becomes difficult. Thus, drying materials more than is required can have a large effect on production from the machine. None of the items listed above will affect energy consumption much or at all. The amount of energy required to evaporate water is about the same regardless of the rate at which drying occurs. Production speed and drying rate are affected.

Radiant Dryers

Infrared radiant dryers may be either gas-fired or electric.

Gas-fired infrared dryers use burning gas to heat ceramic emitters which impinge the energy on the material passing between banks of the ceramic emitters

Electric infrared dryers use lamps as the infrared source.

Pre-dryers in continuous dyeing and finishing processes use infrared energy because the drying is uniform, and migration of chemicals is minimized.

Infrared drying during application of coatis is also common.

Since infrared waves travel in a straight line, the material must be in the path of the waves in order to absorb the energy and heat up. Thus, the density of the material being dried affects the efficiency of IR drying. Reflectors are sometimes used to improve the efficiency of use of infrared energy in IR drying processes to re-direct the waves and make full use of them for drying purposes.

Dielectric Dryers

Most dielectric drying in textiles is done with radio frequency (RF) energy. Radio frequency is the portion of the electromagnetic spectrum between 1 and 100 MHz.

The fact that this range is used for radio communications accounts for the name radio frequency or (RF) energy.

A radio frequency drying oven uses high voltage and an oscillating electric field.

The material to be dried is exposed to the oscillating electrical field

Eg. Schematic representation of dielectric heating mechanism

Polarity of the electrodes changes at a rate equal to the frequency of the RF energy.

Polar molecules, such as water, attempt to line up their poles with the electrical field. The oscillating polarity causes rapid movement of water molecules. The friction between molecules caused by this motion generates heat in the material which causes the water to vaporize.

RF drying is especially suitable for bulky materials like yarn packages, skeins, and loose fiber. RF energy heats deep within the material and greatly accelerates drying of bulky which dry slowly in convection drying systems. Flat fabrics, nonwoven webs, and yarn sheets can be dried using RF energy, with proper design of the electrode system.

Steamers

Steamers are used in continuous wet processes to rapidly heat fabric to a temperature of about 100C and maintain this temperature for the required processing time.

Schematic diagram of a Festoon Steamer / Loop Steamer

Steam is admitted from a boiler.

The pressure in the steamer should be slightly higher than the pressure of the atmosphere outside the steamer to prevent entry of air into the steamer.

Boiling water in the bottom of the steamer keeps the steam saturated with moisture.

When cold wet fabric contacts steam, the steam condenses on the fabric transferring its heat to the fabric. If the steam is superheated (not saturated), condensation will not occur, and the steam may actually dry the fabric.

Drying of the fabric in the steamer usually adversely affects quality of the material being processed.

Air must be excluded from a steamer since it decreases the transfer of heat to the wet fabric (air being good insulation).

Air is excluded by maintaining a slight positive pressure in the steamer and by using caution with exhaust fans near the steamer.

[PJ]

Drying wet textiles

Water in a wet textile resides in three different areas. Most loosely bound water being on the fabric surface and interstices. Much of this water can be taken out by mechanical means such as squeezing, centrifuging or vacuum extraction.

The remaining water, the water thats held in the yarn capillaries and the water absorbed internally by the fiber, must be removed through vaporization by thermal means.

There are 3 modes of heat transfer mechanisms used to dry textiles. Conduction methods involve direct contact of the wet with heated surfaces. These are the most efficient heat transfer methods, but do not allow for control of fabric width during drying. Eg. Steam heated cylinders. High pressure steam inside cylinders provides the energy needed to dry the fabric.

Convection methods involve contact of the wet textile with hot air and are the most common method used in textiles since they combine high process speeds with control of fabric dimensions during drying. Examples include stenter frames. Air is heated to the desired temperature by gas or oil-fired burners or steam heat exchangers and passed over the fabric by high velocity blowers. Fabric tensions are adjusted in both the width and length directions, allowing for complete control of final fabric dimensions. Radiation is another heat transfer mechanism. Examples include IR dryers and Radio-frequency dryers. Radiant heaters are often used as predryers, removing much of the moisture from wet fabric prior to entering the actual drying process. Use of predryers minimizes finishing chemical or colorant migration and increases dryer productivity since less water must be removed in the actual dryer.

Curing chemical finishes

The same heating equipment used to dry wet textiles can also be used to heat the fabric and finish to the temperatures desired for optimal curing. For all equipment, it must be remembered that the temperature of the fabric cannot exceed 100C until all the water has been removed. Eg. Look at the Temperature and moisture profiles in the stenter:-

When drying and curing are done separately in two steps, the curing time can be controlled easily. As speed is defined by distance divided by time, the curing time can be calculated by the equation:-

For instance, if the fabric content of the machine is 20 meters and the fabric speed is 40 m/min, then the curing time is 0.5 minutes.

Often drying and curing are combined in one process, for example the so-called shock-condensation or shock-curing processes. As the end of the drying phase is not easy to determine, theres a risk of over-or-under-curing with disadvantages. Eg. Over curing could lead to a loss of strength or added harsh handle. Under curing leading to incomplete reaction hence poor performance of whatever finishing formulation used.

The best available solution to this problem is curing controlled by the temperature of the fabric. As shown in Fig. 2.15, only when all the water has evaporated, will the temperature of the fabric rise from the wet-bulb temperature to the temperature of the surroundings and the curing process can begin.

With radiation pyrometers the surface temperature of the fabric is exactly measured free of contact! Thus the end of the drying step and the time of the curing step can be determined and monitored.

Radiation pyrometers are very expensive and often not all sections of long stenters are completely monitored by pyrometers; they are concentrated in the most important section or region of the stenter where drying ends and curing begins.