Questions on Basic Knowledge of Acrylic Fiber and Cationic Dyes

Q:1. Why choose 80 ℃ oxygen bleaching for fabrics containing acrylic fiber?

The glass transition temperature Tg of acrylic fiber is between 70-85 ℃ (the temperature at which the high elastic state transitions to the glass state). Beyond this temperature, the fiber will exhibit brittleness and begin to turn yellow; Dyeing with a glass transition temperature not exceeding that of acrylic fiber can avoid damage to the fiber and achieve optimal whiteness.

The whiteness of acrylic fiber is most ideal at 75 ℃. Some factories consider the efficiency of hydrogen peroxide use and the cooking effect of other blended fibers, and use 80 ℃ oxygen bleaching. If the best oxygen bleaching effect is required, the highest temperature is also 85 ℃. It is not ruled out that some peers may use oxygen bleaching above 85 ℃, but in terms of long-term product quality, theoretically, oxygen bleaching above 85 ℃ should be avoided.

Q:2. Why does acrylic fiber turn yellow when exposed to alkali?

Under alkaline conditions, polyacrylonitrile in acrylic fiber hydrolyzes and releases ammonia gas. The unhydrolyzed cyanide group reacts to form an amidine group, which is a chromophore and produces yellow color. This is the reason why acrylic fiber turns yellow and darker under certain alkaline conditions (even becoming more yellow as it is boiled).

Q:3. Why can't acrylic fiber be made into special white?

According to the comparison of three oxidants, the whiteness effect of pretreatment of acrylic fiber under appropriate conditions is as follows: sodium chlorite (i.e. bleaching agent)>sodium hypochlorite (i.e. bleaching agent)>hydrogen peroxide (i.e. bleaching agent).

In actual production, Yapiao has high requirements for mechanical equipment and labor protection, which is not something that ordinary dyeing factories have the conditions to do; Although chlorine bleaching has a simple process and low cost, it is not suitable for fabrics containing spandex (sodium hypochlorite can cause significant damage to spandex), and there are also labor protection issues (high concentrations of chlorine gas are harmful to the human body, while low concentrations can be used for disinfection); So considering the equipment conditions of most dyeing factories, only using oxygen bleaching is more suitable, although the whiteness effect is not good.

The fluorescent whitening agent for acrylic fiber should use a dedicated cationic whitening agent (although the improvement in whiteness is indeed limited). When applying cationic whitening agents, it is also necessary to consider adding an appropriate amount of slow-release agents such as sodium silicate powder and leveling agents; If it is used in the same bath as other types of whitening agents, such as direct cotton whitening agents, anti precipitation dispersants can be considered.

Q:4. Why add sodium silicate powder to acrylic dyeing?

Glauber’s salt powder is an inorganic salt that plays a role in slow dyeing, but its slow dyeing effect is not as good as cationic leveling agents. The two are usually used in combination.

Q:5. Why can't we add more acrylic leveling agent?

There are two main types of cationic dye leveling agents:

(1) Anionic leveling agent, it first forms a complex with cationic dye, and when the dyeing temperature reaches the glass transition temperature of the fiber, it begins to ionize, and then the dye slowly attaches to the dye seat of the fiber; It is prone to precipitation and is currently less commonly used.

(2) Cationic leveling agent can be regarded as a colorless dye. It first binds to the fiber and is gradually replaced by the dye when the dyeing reaches a certain temperature, thus achieving the purpose of slow dyeing; It has good uniformity, but it can occupy the dye seat and affect the dyeing of other dyes.

Regardless of the type used, excessive use can affect the depth of staining; If it is a cationic type, using it too much may even cause the depth of repair to not be restored, so it is important to pay attention to the calculation of saturation value during use.

Q:6. What is the temperature for dyeing acrylic cationic dyes?

The dyeing temperature of acrylic cationic dyes is 97-105 ℃, generally using 98-100 ℃ (not exceeding the boiling point, easy to discharge in the dyeing tank). For ultra dark colors, it can be increased to 105 ℃, but beyond 105 ℃, the fabric begins to become harder.

Q:7. How to post-treatment acrylic cationic dye dyeing?

Use cationic cleaning agents or dispersants (such as DN powder) at a dosage of 2-3 grams per liter and treat at 70-80 degrees Celsius for 15-20 minutes. If necessary, increase the number of cleaning cycles; It is also possible to wash both cationic cleaning agents and cotton with soaping agents in the same bath, while treating both types of fibers (simultaneously cleaning cationic and soaping active materials) to save time.

Q:8. Why can modified polyester be dyed with cationic dyes?

Cationic dyes

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