Download Dyeing of Wool, Silk and Acrylic

Dyeing of Wool, Silk, Nylon and
Acrylic
Faiza Anwar
Acid Dyes
• So called because these are usually applied under
acidic conditions.
• Dyes are normally very large aromatic molecules
consisting of many linked rings.
• Acid dyes usually have a carboxyl or amino group on
the molecule.
• General formula
sulphonic acid
DSO3-Na+ i-e sodium salt of
Acid Dyes
• Attached to fiber by ionic or salt linkage at “dye
site”.
• Dye site in wool is amino group (NH2);
• Applied under acidic conditions, their types are:
– Good leveling characteristics
– Average leveling characteristics
– Poor leveling characteristics
Types of Acid Dyes
• Good leveling
– They have poor substantivity that’s why they have good levelling
property.
– applied in 3.5-4.5 ph.
• Average leveling
– They have average substantivity that’s why they have average levelling
property.
– applied in 5-6 pH,
• Poor leveling characteristics
• They have very good substantivity that’s why they have poor levelling property.
– applied in 6-7 pH.
Acid Dyes
• Dark shades achievable on wool b/c of its amorphous
nature and plenty of amino groups.
• Same is the mechanism for silk and synthetics like
polyamide
• Synthetic fibers may form an ionic or salt linkage.
• They are highly substantive dyes so uneven dyeing may
result
• Evenness achieved by use of retarders such as Na2SO4.
Dyeing with Acid Dyes
• Applied under acidic conditions maintained by
the addition of
– sulphuric acid in dye solution for good leveling dyes
– Acetic Acid in dye solution for average and poor
leveling dyes
• Good substantivity (so chances of unlevelness)
• Sodium Sulphate is used as a Retarder for level
dyeing.
Addition of Acid
• Addition of acid acts as an exhausting agent,
because strongly acidic conditions makes
more cationic sites available and thus
available dye anions got combined with these.
Mechanism of Acid Dyes
• Fixation mechanism
• Wool-NH2
+ H+
Wool-NH3 +
– Chromophore is -ively charged
– Amino group of Wool is made +ively charged so
dyes attaches to it by the addition of acid which
liberates H+
– DSO3- + 3HN-wool
Addition of Salt
• Electrolyte in the acid dye bath act as a retarding agent
because of chlorides ions attracted by the positive sites
at the fiber.
• Anions of the retarders (chloride or sulphate radicals)
are negatively charged and smaller than the dye anion
and can move more rapidly in the dye liquor.
• Dye sites of the fiber polymer are rapidly occupied by
the chloride/sulphate radical and in effect compete
with the acid dye anion for the dye sites.
Effect of Heat
• Dye molecules have the greater affinity for the
dye sites but the sulphate radicals retards the
rate at which the dye molecules occupies the dye
sites results in uniform dyeing.
• The application of heat assists the dyeing process
by increasing the kinetic energy of the dye
molecules which are slowly overcoming the
retarding effect of the sulphate radicals.
• Thus, the dye anion will gradually replace the
sulphate radical that has been attached to the
dye sites.
Properties of Acid Dyes
• Light fastness is good being rating of about 5.
– Since the chromophore of acid dyes are stable and
can bear the UV exposure present in sunlight.
–
• Wash fastness is poor to good depending
upon substantivity of the type of dye used.
– W.F is 2-3 with good leveling characterstics
– 3-4 for those whose average leveling
– 4-5 for poor leveling characterstics.
Reasons for Low wash fastness
– Firstly, This is because the ionic and hydrogen
bonds are hydrolyzed in water.
– Secondly these dyes are acidic in nature so can
not bear alkaline treatments during washing and
laundering.
Basic Dyes
• Can be used for wool, silk and acrylic and
modacrylic
• But poor properties for fibers other than acrylic
• Also called cationic dyes b/c they acquire +ive
charge in solution
• Are MOST BRILLIANT dyes
Dyeing with Basic Dyes
• Applied under slight acidic solution normally
6-7.
• Good substantivity (so chances of unlevelness)
• Retarder used for level dyeing
Fixation mechanism
– Chromophore is +ively charged
– Acrylic has –ively charged dyesites normally
sulphonate groups so dyes attaches to it
– D+ + AcSO3-
AcSO3-+ D
Properties of Basic Dyes
• Light fastness is excellent 6-7
• Wash fastness is very good 4-5
– Due to high substantivity and hydrophobicity of
acrylic
• Brightness
– Excellent brilliance and color intensity
References
• Textile Science, by: E.P.G Gohl