Question

The light fastness of azoic dyed material decreases mainly due to \(\underline{\hspace{2cm}}\)

• A. Depth of color
• B. Type of fabric
• C. Humidity
• D. Type of substrate

Hint:

General Rule for Light Fastness: For most dyes, light fastness is lowest in pale shades and highest in deep shades. Think of it like a painted wall: a single scratch is very visible on a thinly painted surface but less noticeable on a surface with many thick coats of paint.

Answer 1

The Correct Option is A

Step 1: Understand Azoic dyeing. Azoic dyes (or naphthol dyes) are formed directly inside the fiber by reacting two components: a Naphthol component and a Diazonium salt (base). 
 

Step 2: Understand light fastness. Light fastness refers to a dye's ability to resist fading when exposed to light. Fading is a photochemical reaction that destroys dye molecules. 
 

Step 3: Analyze how the options affect light fastness. 
- (A) Depth of color: For most dye classes, including azoics, light fastness is highly dependent on concentration. In pale shades (low depth of color), there are fewer dye molecules on the surface. When a certain number of molecules are destroyed by light, the color change is very noticeable. In deep shades, the same number of destroyed molecules represents a much smaller fraction of the total, so the color change is less apparent. Therefore, light fastness is generally much poorer in pale shades and improves with the depth of color. The question asks what causes it to decrease, so a low depth of color is a major reason. 
- (B) Type of fabric / (D) Type of substrate: While the substrate can have some effect (e.g., the chemical environment), it's a less dominant factor than the concentration of the dye itself. 
- (C) Humidity: Humidity can influence the rate of photochemical degradation for some dyes, but again, the depth of shade is a more universal and significant factor. 

Conclusion: The most significant factor influencing the decrease in light fastness for a given azoic dye is a lower depth of color (i.e., dyeing in pale shades).