The Complete Guide To Desizing-2

 

Desizing is an important process before dyeing-finishing, the desizing results directly affect the inner and appearance quality of fabric dyeing and printing. The previous articles mentioned the identification of sizes, types of desizing agents, and qualitative methods for measuring desizing rates. In this article, let’s take a look at the process of quantitative determination of desizing rate.

Quantitative method for starch desizing rate

Determination principle

Perchloric acid can dissolve starch, starch on fabric can be impregnated with perchloric acid solution and dissolve in the solution. Starch forms a blue complex with iodine in a certain pH range. The maximum absorption wavelength of the aqueous solution of this complex is about 620nm, and it conforms to beer’s law when the starch concentration is in a certain range. So the starch desizing rate can be calculated by determining the starch content before and after desizing by colorimetric method.

Main instruments and reagent solutions for starch desizing rate

(1)Instrument:

Spectrophotometer; Drying oven.

(2)Reagent solution:

Perchloric acid, 42% solution; Acetic acid, prepared into c=2mol/L solution; Sodium hydroxide, prepared into c=6mol/L solution; Potassium iodide, in a 10% solution; Potassium iodate, prepared into c=0.05mol/L solution; Phenolphthalein indicator, 0.5% 90% ethanol solution.

(3)Draw a standard curve: 

​​Accurately measure 50mg starch (accurate to 0.1mg), dissolve it in 42% perchloric acid solution of about 20mL, drop 1-2 drops of phenolphthalein indicator solution, add sodium hydroxide solution of C =6mol/L until the solution turns pale red, and then add drops of acetic acid of C =2mol/L until the pale red disappears.Transfer to a 500mL volumetric flask and dilute to scale with distilled water. The solution contained starch 0.1mg/mL. Absorb 3mL, 6mL, 10mL, 15mL, 20mL, 25mL and 30mL of the solution to the numbered 50mL volumetric flask, add 25mL of c=2mol/L acetic acid solution, 0.5ml of 10% potassium iodate solution, and then add 2mL of C =0.05mol/L potassium iodate solution. Diluted with distilled water to scale (at this time, the mass concentration of starch is 6mg/L, 12mg/L, 20mg/L, 30mg/L, 40mg/L, 50mg/L, 60mg/L), with blank solution as the reference solution, the absorbance was measured [temperature is (25±1)℃.A standard curve was drawn with absorbance as ordinate and starch mass concentration as abscissa.

(4)Test of sample:

​The cloth sample to be tested is cut into small pieces and placed in the oven at 100-105 ℃ for drying. A certain amount of dry cloth sample is accurately measured (the sampling amount is determined according to the pulp content of the cloth sample, the general grey cloth is 0.5-1g, and the desized fabric is 2-5g), placed in a 250mL beaker, and about 30ml of 42% perchloric acid solution is added to make the sample completely immersed in the solution. Leave at room temperature for 30 minutes, stirring constantly.Then add 100mL distilled water and 1-2 drops of phenolphthalein indicator, add sodium hydroxide solution with C(NaOH)=6mol/L until the solution appears light red, and then add C(HAc)=2mol/L acetic acid until the light red just disappears. Filtrate is collected or transferred to a 250mL volumetric flask and diluted to scale with distilled water. Put 5mL of the above solution into a 50mL volumetric flask, add 25mL of c(HAc)=2mol/L acetic acid solution, 0.5ml of 10% KI solution and 2mL of C =0.05mol/L KIO3 solution, dilute to scale with distilled water. After stable color development, colorimetry was carried out immediately and absorbance was measured. Compare with the standard curve to find the starch mass concentration of the measured solution.

(5)Calculate desizing rate:

​Starch content=C·d·V／W×l0-6×100％
C——Corresponding starch mass concentration on the standard curve，mg／L；
d——Dilution ratio of extraction solution，In the above methods, is 50／5=10；
v——Volume of starch extract (250mL in the above method)，mL；
W——Weight of test cloth，g.
Desizing rate = starch content on the blank cloth – starch content on the desized cloth/starch content on the blank cloth ×100%

(6)Notes:

​①The chromogenic reaction of this test should be completed after adding reagent and sitting for 5min, and then colorimetry is carried out. However, as the starch-iodine complex is stable under light for about 15min, over 15min the solution may become cloudy, so the shelving time should not be too long, and it is best to avoid light.
②Starch will be hydrolyzed in acidic medium, so when using perchloric acid to dissolve starch, the temperature should not be too high and the time should not be too long (the temperature in the test was room temperature, the time was 30min). If 42% of perchloric acid is made by diluting 72% of perchloric acid, concentrated acid diluting is an exothermic reaction, so it should be cooled to room temperature before use.

This article is reprented from https://fyitester.com/.  Come here for the full version.