Acrylic fibre & its dyeing
Acrylic Fibers
The term acrylic derive from the Latin word Acryl, which means bitter, irreritting or pungent.
The acrylic fibres are man-made, synthetic polymer based, polyacrylonitrile filaments or staple fibres.
Acrylic Fibers
• Common trade names for acrylic fibers include
• Acrilan
• Creslan
• Orlon
• Sayelle
• Zefran.
Acrylic Fibers
They are divided into two types:
Polyacylonitrile fibre generally referred to as the acrylic fibre.
Modified Polyacylonitrile fibre generally referred to as mod acrylic fibres.
ACRYLIC
Acrylic fibers are formed from wet or dry spinning of copolymers containing at least 85% acrylonitrile units.
On the other hand, the mod acrylic fibre must be composed of at least 35% but not more than 85% by weight of acrylonitrile units.
After texturizing, acrylic fibers have a light bulky wool-like handle and over all wool-like aesthetics.
Fibre morphology
The acrylic fibre appear as
regular
translucent
slightly wavy filaments or staple.
The polymer system
The main component of the acrylic polymer is the acrylonitrile monomer.
CH2—CH--CH2—CH--CH2—CH--CH2—CH--CH2—CH--CH2—CH--CH2—CH--CH2—CH--CH2
CN CN CN CN CN CN CN CN
n
Acrylonitrile
group
Nitrile group
Methylene
group
The polymer system
It is a linear polymer, with a degree of polymerisation of about 2000.
This make it about 500nm long, with a thickness ranging from 0.3 nm at methylene group to about 0.53 nm at the nitrile groups.
It is also one of the longest man-made fibre polymers extruded to form a textile filament.
Manufacturing
The copolymer is formed through free radical emulsion polymerization.
After precipitation the copolymer is dried and dissolved in an appropriate organic solvent and wet or dry spun.
The degree of polymerization of the copolymers used for fiber formation varies from 150 to 200 units.
Pure polyacrylonitrile will form satisfactory fibers.
Manufacturing
Owing to the extensive tight packing of adjacent molecular chains and the high crystallinity of the fiber, comonomers must be introduced to lower the regularity and Crystallinity of the polymer chains to make the fiber more dye able.
Manufacturing
Extensive hydrogen bonding occurs between a hydrogens and the electronegative nitrile groups on adjacent polymer chains, and strong van der Waals interactions further contribute to the packing of the acrylic chains.
The periodic comonomer units interfere with this packing and therefore decrease the overall Crystallinity of acrylic fibers.
Manufacturing
Acrylic fibers are usually smooth with round or dog-bone cross sections as in figure .
Many bicomponent acrylic fibers are produced in order to provide a bulky texturized structure on drawing.
Physical properties
Tenacity
The fair to strong tenacity of the acrylic fibres is attributed to the very crystalline nature of their polymer systems, as well as to their very long polymers.
The loss in tenacity that occurs when acrylic fibre become wet indicates that the fibres are slightly amorphous, enabling water molecules to enter and reduce the van der waal’s forces between polymer.
Tenacity
The tenacity of acrylic fibers varies from 18-36 g/tex.
On wetting, the tenacity drops to 13-27 g/tex.
Elastic-plastic nature
Acrylic have soft handle.
This mean that, although the polymer system is very crystalline acrylic polymer must be able to give or slide over each other when the acrylic filament or staple fibre is bent or crushed.
Elastic-plastic nature
The displacement of polymers in the acrylic polymer system is evidenced by the wrinkle and distortion in response to bending, stretching and crushing.
The elongation at break varies from 20% to 50% for the various acrylic fibers.
Elastic-plastic nature
At 2% elongation the recovery of the fiber is 99%; however, at 5% elongation the recovery is only 50%-95%.
2.5% under standard temperature and humidity conditions. The fiber is soluble in polar aprotic solvents such as dimethylformamide. The fiber exhibits good heat and electrical insulation properties. Acrylic fibers do build up moderate static charge and soften at 190°-250°C.
Hydrophobic nature
Acrylic fibre fibre are hydrophobic because the polymer system is highly crystalline.
Very few water molecule are absorbed because of the very slightly amorphous nature of the polymer system and the slight polarity of the nitrile groups in the acrylic polymer.
Thermal properties
Acrylic are most heat sensitive of the synthetic fibre commonly used for apparel purposes.
Acrylic fibre are the most flammable fibre in common use.
Chemical Properties
Effect of acids
The acrylic fibre are resistant to acid because their polymer do not contain any chemical groups which will attract or react with weak acid radicals.
The fibers are only attacked by concentrated acids.
Effect of alkalis
The vary crystalline nature of the acrylic polymer system prevents the ready the entry of alkaline substances.
However, surface alkaline hydrolysis or surface saponification will occur.
This means that any nitrile groups and anionic or basic groups on the surface react with sodium or cation of the alkali.
Effect of bleaches
Acrylic fibres are not usually bleach in practice.
As a result, little is known about the effect of bleach on acrylic polymers.
Acrylics are unaffected by oxidizing and reducing agents except for hypochlorite solutions at elevated temperatures.
Effect of sunlight & weather
Acrylic fibres are the most sunlight and weather resistant fibre in common use.
Acrylic fibre, when exposed to sunlight will initially suffer a small loss in tenacity.
Characteristics of Acrylic Fabric
• It is lightweight, soft, and warm.
• It dyes to bright colors with excellent fastness.
• It absorbs and releases moisture quickly, thus allowing the fabric to "breathe".
• It is resilient, retains its shape, and resists shrinkage and wrinkles.
Characteristics of Acrylic Fabric
• It has flexible aesthetics for wool like, cotton like or blended appearance.
• It is resistant to moths, oil and chemicals, and sunlight degradation.
• It is static.
• It suffers from piling and abrasion problems.
Dyeing with basic dyes
Basic dyes
When these dyes were originally developed for acrylic fibres, they were referred to as modified basic dyes.
Since the original basic dyes, which were in the past used on cellulosic fibres, are no longer used, this new range of modified basic dyes is now commonly referred to be basic dyes.
Basic dyes
Basic dyes are also known as cationic dyes.
Because, the colored portion of basic dye is cation or positively charged radical of the dye molecule.
This cationic radical is attracted to the anionic (acidic) radical on the acrylic polymer.
Basic dye
Basic Brown 1 is an example of a cationic dye that is readily protonated under the pH 2 to 5 conditions of dyeing.
Dyeing with basic dyes
Basic dyes are applied to acrylic fibre from a slightly acidic dye liquor.
Basic dyes have good substantivity for acrylic fibre and exhaust well within narrow limits of temperature.
Because of their poor levelling properties, care must be exercised when applying basic dyes to acrylic fibre to avoid unlevelldyeing.
Dyeing with basic dyes
This is achieve through the use of a retarder and carefully regulating the temperature during dyeing.
The color component of basic dye is the cation. The dye cation is absorbed on the fibre surface which is negatively charged.
The negative potential of the fibre is thus neutralized.
Dyeing with basic dyes
Increasing the temperature of the dye liquor provide the dye sufficient energy to enter the fibre polymer system.
The dyeing process can be represented as follows:
• AcSO-3 +D+ AcSO3
-+D (Acrylic fibre ( dye cation) (dye attached to dye
site on acrylic fibre polymer)
Polymer with
dye site)
Dyeing with basic dyes
The cationic retarder competes with the cationic dye molecules for the dye site on the acrylic fibre polymers.
This prevents the dye molecule from rushing onto the fibre.
The presence of the cationic retarder on the dye site means that the basic dye which has greater substantivity for the acrylic fibre compare with the retarder will only be able to replace the cationic retarder slowly ensuring a more level dyeing.
85 o C
50 o C
100 o C
5 min
Dye Solution
60-70 min
1 o C /min
30 min
60 o C
Dyeing Profile
Form of basic dye
Basic dyes are used in
powder
liquid form
Properties of basic dye
Light fastness
Dye and printed acrylic fibre using basic dye have excellent light fastness.
The light fastness rating of basic dyes about 6-7.
Washing fastness
Acrylic fibre dyed with basic dyes have good washing fastness.
The washing fastness rating of basic dye is 4-5.
Bright colors
Basic dyes are characterised by their brilliance and intense hues.
The bright colors achieved from basic dyes do not usually occur with other dye classes.
Non-ionic Disperse dyes
for Acrylic
Disperse dye
Acrylic fibre which are hydrophobic are readily dyed with the non-ionic disperse dyes.
There other fastness properties also good due dye molecules are non-ionic and insoluble in water and hydrophobic nature of acrylic polymer fibres.
Disperse dyes are rarely used now a days.
End-Use
Mostly use for Apparel
Sweaters
Socks
Fleece
End-Use
circular knit apparel
sportswear
End-Use
children wear
Home Furnishing
• Blankets
• area rugs
• Upholstery
Industrial Uses
• Asbestos replacement
concrete and stucco reinforcement
Other Uses
• Craft yarns
• sail cover cloth
• wipe cloths
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