Heat Fixation: A Key Process in Polyester Dyeing

Disperse dyes are highly hydrophobic and have very low water solubility. Heat fixation is the key step in ensuring they are firmly dyed onto similarly hydrophobic polyester fibers. Heat fixation is the core process for imparting vibrant color and excellent fastness to polyester fabrics.

I. What is Heat Fixation?

Heat fixation involves the use of high temperatures to diffuse and penetrate disperse dyes adsorbed on the surface of polyester fibers into the fiber interior, achieving a durable dyeing process. Without this step, the dye would simply float on the surface, exhibiting extremely poor color fastness and having no practical value.

II. Main Heat Fixation Methods

High-Temperature and High-Pressure Method

Principle: In a sealed, high-pressure dyeing machine (such as a high-temperature and high-pressure overflow dyeing machine), a dyeing environment with temperatures exceeding 125-135°C and elevated pressure is created. This high temperature intensifies the molecular chain motion of the polyester fiber, instantly expanding the amorphous regions within the fiber and creating more and larger "voids." At the same time, the dye molecules gain significant energy, increasing their kinetic energy, allowing them to diffuse through these gaps from the fiber surface and become fixed within the fiber.

Features: Vibrant color, excellent dyeing levelness, soft feel, and high permeability. This method is the most commonly used and established dyeing method for knitted and woven fabrics.

Hotmelt Method

Principle: This is a continuous pad dyeing process. The fabric is first padded with the dye solution and then baked at a high temperature of 180-220°C for a short time (usually 60-90 seconds). This instantaneous ultra-high temperature rapidly melts the fiber surface, allowing the dye to "melt" or "diffuse" directly into the fiber, completing the color fixation.

Features: High production efficiency, suitable for large-scale woven fabrics (such as shirts and curtain fabrics). However, this method has a slightly stiffer feel and its permeability is not as good as that of the high-temperature, high-pressure method.

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II. Key Factors Affecting Thermal Fixation:

Temperature: The most important factor. Insufficient temperature prevents effective dye diffusion; excessive temperature may cause dye decomposition or fiber damage.

Time: Fixation requires sufficient time for the dye to diffuse completely. Time and temperature are interrelated and require precise control.

PH: The dye liquor should be kept slightly acidic (pH 4.5-5.5) to prevent hydrolysis of the dye at high temperatures, which can cause lightening and discoloration.

Auxiliary agents: Using appropriate dispersants and leveling agents will help ensure stable dye dispersion and uniform dyeing..