OPTICAL
PROPERTIES OF TEXTILE FIBRES
The behaviors shown by a textile
fibre when light falls on it are called as optical properties. The optical
properties of a fibre include birefringence (basis on light refraction or
transmission), dichorism (basis on light absorption) and luster (basis on light
reflection).
Birefringence:
When a beam of light falling on a
textile fibre, it splits up into two refracted beams, one polarized parallel to
the fibre axis and other polarized perpendicular to the fibre axis. The
difference between the refractive index for light, polarized parallel to the
fibre axis and the refractive index for light, polarized perpendicular to the
fibre axis is called as birefringence value of fibre. Birefringence can be
formalized by assigning two different refractive
indices to the fibre for different polarizations. The birefringence
magnitude is thus defined by-
Δ n = n1-n2
Where n1 and n2
are the refractive indices for light, polarized parallel and perpendicular to
the fibre axis respectively. Highest value of birefringence indicates that the
most of the molecules are lined up parallel to the fibre axis and hence the orientation
of fibre is higher. Ideally, oriented fibres have different birefringence value
and the magnitude of birefringence ranges from -0.005 (for triacetate) to 0.188
(for terylene).
Factors
affecting the birefringence of textile fibres:
The birefringence value of textile
fibres depend on-
·
the
degree of orientation and
·
the
degree of asymmetric of the molecular chain (straight/zigzag/with side groups)
Dichorism:
The variation in the absorption of
radiation by a colored fabric with the direction of polarization of light is
called as dichorism, which may result in the difference of depth of shade or
even in the actual color. For dyed fibre exhibiting dichorism, its magnitude is
used as a measure of orientation of the molecules in the fibre. So, we get-
φ = k1/k2
Where, φ = Dichric/dichroitic
constant
K1 = Absorption co-efficient of
light polarized parallel to the fibre axis
K2 = Absorption co-efficient of
light polarized perpendicular to the fibre axis
Lustre:
Lustre is an important property of
textile fibres. When a beam of light falls onto a fibre surface, it may be
reflected along the angle of reflection. The reflection may vary with the angle
of incidence, with the color and polarization of light. Lustre of textile
fibres will be increased with the increase of regular light reflection.
Factors
affecting the lustre of textile fibres:
·
Falling
of light on fibre (across the fibre or along the fibre)
·
Fibre
fineness
·
Regularities
of fibre surface
·
Cross-sectional
shape of fibre
·
Amount
of small particles (TiO2) present in fibre
·
Maturity
of fibre
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