What Is Heat Setting?

Heat setting ^[1] refers to the use of heat to eliminate the internal stress generated by the fabric fibers during the stretching process, to cause macromolecules to relax to a certain degree, and to fix the shape of the knitted fiber.

1. Styling: refers to the elimination of the stress and strain accumulated in the textile through some physical or chemical treatment, so that it can obtain a certain form in the state, size or structure, and achieve a certain stability.

2, the problem to be solved

Remove internal stress

Improve the ability to recover from deformation and maintain the state when it is set

Refers to the process of heat treatment of synthetic fiber fabric under a certain tension to make its size and shape stable. Its significance is: sizing: thermal dimensional stability is improved, shrinkage rate is reduced flat sizing: eliminate wrinkles and improve wrinkle resistance improve taking performance: improve the elasticity, feel and fluffing phenomenon

The concept of fibre stereotypes

Shaping: The process of stress relaxation of fibers and reaching a stable form.

The process of shaping is divided into two steps:

The forces between some macromolecular chains are quickly destroyed;

Macromolecules quickly rebuild new intermolecular forces at new locations and fix them.

Heat setting mechanism

1. Causes of poor thermal stability of synthetic fabrics.

The internal stress is hidden during the fibrillation process, and the arrangement of macromolecules has not reached the most stable state. After heating, the thermal vibration of the molecules intensifies and bends, causing the fiber to retract and reduce the size.

Constant stress during the textile process, and uneven force, exacerbated the tendency of shrinkage and uneven shrinkage when heated, causing the fabric to wrinkle and difficult to remove.

2. Heat setting method

That is, the fabric is supplied with thermal energy under appropriate tensile tension, so that the large fractions are rearranged, and then quickly cooled, so that the fiber is in a stable state with lower energy.

The process is divided into three phases:

Macromolecular relaxation phase: When heated above Tg, the force between the molecular chains , the internal rotation of the macromolecules , the compliance , the degree of orientation , the internal stress .

Segment reorganization stage: With the thermal vibration of the macromolecular chain , the opportunity for the active groups to meet , it is easy to establish a new intermolecular force. If tension is applied, the macromolecules rearrange according to the direction of tension to establish a new balance.

(3) The finalization stage: before the external force is removed, the temperature is lowered, and the fiber is fixed in the new form.

In summary, the process of heat setting can be expressed as:

That is, the thermoplastic fiber is heated to be flexible the chain segment is deformed under external force deformed cooled under external force, and the new form is fixed

3. The fiber structure changes during heat setting

The heat-set T is between Tg and Tm (melting point refers to the larger and complete crystal melting T in fiber). When it is at Tg, the atoms and groups in the molecular chain vibrate at the equilibrium position, and the displacement movement of the segment begins, and with T and , until the Tm is reached, the entire macromolecular chain displacement begins to occur. The fibers contain various crystals of different sizes and integrity, with different melting points.

When heat setting under T1 (& lt; Tm), part of the crystals with small size and poor integrity will melt, and the more uniform and large crystals will increase or more complete, so that the crystallinity, grain size and integrity distribution reach A new state. At this time, if it is further subjected to relaxation heat treatment on T1, the number of crystals that fiber can melt thermal stability of the size, and the dyeing performance, stiffness, elasticity, etc. also change.

The setting mechanism of acrylic is different from polyester

The heat setting causes the macromolecules in the amorphous area of acrylic fiber to rearrange due to the intensified thermal movement, eliminates internal stresses, and rebuilds some stronger new junctions, which improves the integrity of the crystalline region and thus improves the thermal stability of the fiber.

First, eliminate the internal stress generated in the fiber during the stretching process, make the macromolecules relax to a certain degree, and improve the shape stability of the fiber.

Second, improve the physical and mechanical properties of the fiber, such as increasing the fiber's crystallinity, elasticity, knot strength, abrasion resistance, etc., and fixed curl or fixed twist

Third, remove the water brought in by the fiber during the drawing and oiling process, so that the fiber reaches the required moisture content of the finished product, and at the same time, the possibility of yellowing due to the long-term storage of the fiber due to the dryness of the oil agent can be avoided.

Fourth, improve the fiber dyeing performance

Heat setting processing method

1. Moist heat setting: The fabric is in a wet state, and it is mostly used for synthetic fibers such as nylon and acrylic with high hydrophilicity. Soft and plump after setting, divided into two types: water bath and steam setting

2. Dry heat setting: The fabric is dry and suitable for polyester and blended fabrics, including hot air setting, infrared radiation setting, and contact (hot metal roller) heat setting.

Heat setting equipment and process

1. Equipment: needle chain heat setting machine (domestic M-751, M-751A)

Dual-use needle plate cloth

Needle tape and cloth tape can be used for both shape setting and stentering.

2. Fabric heat setting process

Feeding (2 ~ 4% overfeeding) Needle stenter (width of super finished product 2 ~ 3cm) into heating zone (180 ~ 210 ) cooling drop (T & lt; 50 )

Heat setting wet heat setting

1. Water bath setting

Treatment with boiling water for 0.5 2h, poor setting effect

In autoclave, T can reach 125 135 , 20 30 has better effect

2. Steam setting:

Saturated steam: similar to water bath 130 140 , 20 30

Superheated steam: close to dry heat setting

Heat setting method

Dry and hot air shaping;

Contact heat setting;

Damp heat setting of water vapor;

Shaping (water, glycerin, etc.)

Evaluation of heat setting effect

1.Critical dissolution time (CDT)

The polyester ring sample is placed in phenol, and the time (seconds) required to contact the phenol at a certain temperature until the fiber dissolves is called CDT.

CDT is related to fiber crystallinity, crystal size and crystal integrity, and can reflect the thermal history of polyester theory. , This method is more sensitive to determine the heat setting effect of polyester, and it is also suitable for nylon.

2. Thermal shrinkage

Treat the sample in a relaxed state under certain conditions, and then measure its change in length and width, and express it as a percentage of shrinkage. There are five types: dry heat shrinkage, boiling water shrinkage, hot water dissolution, steam shrinkage and ironing shrinkage.

The test is simple and can reflect the shrinkage performance of the fabric.