Chemistry for textile

L06-07: viscosity and viscosity measurementDiffusion and laws of diffusion

viscosity

It is the resistance against flow of any fluid.

Viscosity of a solution shows the degree of entanglement of molecules as they move past each other.

viscosity

The viscosity of heavy lubricating oil is greater than gasoline because of their greater molecular length.

viscosity

Honey is much more viscous than water but if we dissolve a gum in water, its viscosity increases too.

viscosity

Why the viscosity of water increases when a gum is dissolved in it?

viscosity

Why some gum solutions are thicker than the others?

Viscometery

Ostwald viscometer

Viscometery

Obbelohde viscometer

Viscometery

Falling ball viscometer

Viscometery

Cup viscometer

Viscometery

Rotary viscometer

Textile printing

In textile printing viscosity of printing paste is very important factor which controls the quality of pirinting.

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Diffusion

Diffusion is defined as the random movement of particles, in a solution or gas, from a region of higher concentration to a region of lower concentration.

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Diffusion Flux Flux: the rate of migration of

matter is measured by it’s flux, which is the amount of matter passing through unit area per unit time. Hence:

Flux = dQ/dtWhere dQ is a very small amount of matter that passes through a unit area in a very small time period.

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Diffusion Gradient Concentration gradient:

Change in concentration of solute while travelling a unit length from the region of higher concentration to the region lower concentration within a system. Hence: concentration gradient = dC/dxwhere dC is a very small change in concentration over a very small length in the direction of decreasing concentration.

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Diffusion

The proportionality of the flux of matter to the concentration gradient is called Fick’s law of diffusion.Fick’s first law is used in steady state

diffusion.Fick’s second law is used in non-steady or

continually changing state diffusion.

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Ficks laws of diffusion Fick’s equations describe the diffusion of a dye within a fiber. Fick’s

second law sates that the rate at which the dye diffuses across a unit area in the fiber is proportional to the concentration gradient across that area.

dQ/dt is proportional to dC/dxor dQ/dt = -D dC/dxwhere:Rate of diffusion = dQ/dt mol/m2/sConcentration gradient = dC/dx mol/m3mproportionality constant = -D (m2 s)

The negative sign gives a positive value of the diffusion coefficient because the concentration gradient is negative as the concentration decreases in the direction of diffusion (i.e. along the x axis)

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Diffusion – Importance in dyeing The transfer of a dye molecule from the dye solution into a fiber is

usually considered to involve the initial mass-transfer from bulk solution to the fiber surface, that is adsorption of dye on the fiber surface, followed by diffusion of the dye into the fiber. The diffusion of dye within the fiber is thought to be the rate controlling step. Diffusion in a polymer is much more difficult than in solution because of dye fiber interaction and obstruction by the fiber molecules in the pores. For example the rate of diffusion of direct dyes in cotton is 10000 times slower than in water.

The concentration of adsorbed dye at the fiber surface quickly reaches a steady state equilibrium value. Any net transfer of dye from the solution to the interface then occurs only when the dye diffuses into the fiber.

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Diffusion of dye within solution

The diffusion of dye within the solution is a simpler process as there is less obstruction in the process of diffusion.

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Diffusion of dye within the fiber The process of

diffusion into the fiber is a slow step in dyeing. The complicated polymer structure of fiber and obstructions in the pores control the diffusion of dye molecules within the fiber.

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Diffusion of dye in dyeing system