THEORY EXAMINATION (SEM–IV) 2016-17 PHYSICAL CHEMISTRY OF DYEING

Course: B.Tech (Textile Chemistry / Textile Engineering)
Subject Code: EC404
Subject Title: Physical Chemistry of Dyeing
Exam Type: Theory
Duration: 3 Hours
Maximum Marks: 100

SECTION – A (10 × 2 = 20 Marks)

Short and conceptual questions testing theoretical definitions

SECTION – B (5 × 10 = 50 Marks)

Analytical and descriptive questions combining concepts of thermodynamics and kinetics

(a) Absorption & Light Interaction

Absorption: Process where dye molecules absorb specific wavelengths of light.

Laws: Governed by Lambert–Beer’s law.

Effect of Wavelength: Higher wavelength → lower energy absorption; affects shade and depth of color.

(b) Significance of Beer’s Law

Deviations caused by instrumental errors, chemical interactions, or solution changes (pH, aggregation).

Accurate spectrophotometric measurements depend on maintaining ideal conditions.

(c) Fiber Structure Effect

Cellulosic Fibers (Cotton): Hydroxyl groups form hydrogen bonds with reactive and direct dyes.

Protein Fibers (Wool, Silk): Amine and carboxyl groups interact ionically with acid or basic dyes.

Fiber crystallinity, porosity, and orientation directly influence dye uptake and uniformity.

(d) Equilibrium Dyeing Measurement

Expressed as concentration of dye in fiber vs solution at equilibrium.

Governing equation:

• CfCs=K\frac{C_f}{C_s} = KCs​Cf​​=K

where CfC_fCf​ = concentration in fiber, CsC_sCs​ = in solution, KKK = equilibrium constant.

(e) Electrical Effects in Dyeing

Electrostatic interactions between charged fiber and dye molecules determine adsorption rate.

For example, cationic dyes have strong attraction to anionic fibers like acrylic.

(f) Chemical Potential in Dyeing

Dyeing occurs when chemical potential of dye in solution > that in fiber.

The process continues until potentials equalize → equilibrium dyeing achieved.

(g) Diffusion Coefficient Measurement

Techniques include Time–lag method, Fiber Sectioning, and Desorption methods.

Diffusion rate depends on fiber porosity, dye size, temperature, and swelling.

(h) Temperature Effect on Dyeing Rate

Temperature increases kinetic energy and diffusion rate.

At higher temperatures, fibers swell more, reducing diffusion resistance.

SECTION – C (2 × 15 = 30 Marks)

In-depth, applied, and theoretical questions

Q3. Thermodynamics of Dyeing

Involves enthalpy (ΔH), entropy (ΔS), and free energy (ΔG) changes.

• ΔG=ΔH−TΔS\Delta G = \Delta H - T\Delta SΔG=ΔH−TΔS

Negative ΔG → spontaneous dyeing.

Entropy of dyeing: reflects randomness when dye molecules distribute within fiber.

Q4. Dyeing Rate and Its Limitations

Rate equation:

• dCfdt=k(Cs−Cf)\frac{dC_f}{dt} = k(C_s - C_f)dtdCf​​=k(Cs​−Cf​)

Controlled by diffusion and adsorption.

Under equal affinity conditions, the dye rate depends only on diffusion and temperature.

Limitations: Surface saturation, non-uniform fiber structure, aggregation.

Q5. Theories of Dyeing

Pore Model: Diffusion of dye through interconnected pores within fiber structure.

Free Volume Model: Dye molecules migrate through free spaces in polymer matrix; thermal motion creates transient paths.

Both explain dye penetration mechanisms in amorphous polymeric fibers.

Summary

This Physical Chemistry of Dyeing (EC404) paper comprehensively tests: