POLYAMIDES refer to various natural (polypeptides) and synthetic materials containing free amino groups. Examples of polyamides include nylons, wool, and silk. Nylon fibre is commonly dyed with acid dyes which are anionic in character, including premetallized acid dyes, in a batch process referred to as exhaust dyeing. For example, nylon fibre which has been made into fabric may be dyed in a jet-dyeing machine, whereby a continuous loop of the fabric is circulated throughout the dye bath by impinging the dye bath liquor against the fabric in a venturi nozzle. Care must be taken during the dyeing process to obtain a uniform distribution of dye on the fabric, referred to as leveling.
Since acid dyes are negatively charged, the dyes are attracted to positive dye sites appearing in the targeted substrate. In general, acid dyes have a high affinity for protonated polyamide materials, meaning that the dyes have a strong tendency to quickly bind to the polymer. Unfortunately, however, once in contact with the cationic polymer surface, acid dyes have a tendency to poorly diffuse into the polyamide. In such cases leveling agents which works on migration principle works effectively.
Leveling agents or assistants are generally surface-active textile dyeing auxiliaries which have the task of (i) thoroughly wetting the fibre/ fibre blend to be dyed (ii) promoting penetration of the fibres and (iii) preventing too rapid uptake of the dyes, which can lead to unlevelness (spottiness) during the dyeing operation. Suitable leveling assistants include oleyl sulphonates, fatty alcohol sulphonates, fatty acid condensation products, alkyl and alkyl aryl poly glycol ethers and surface-active chemicals in general.
Unlevelness is caused by:
High and varying affinity of the dye on the fibre
High and varying affinity of the fibre for the dye
Uneven distribution of the dye solution on the fabric/fibre
Temperature differences on the fabric or fibre
Insufficient levelness can be prevented by means of suitable dyeing techniques and by means of leveling assistants.
Leveling assistants reduce mainly the rate of dyeing, increase the rate of dye migration within the fabric and improve the compatibility of the dyes.
Leveling assistants can exert two or more of the above mentioned effects at the same time. Leveling assistants can be divided into two groups, those which have an affinity for the fibre and those which have an affinity for the dye. Leveling assistants with an affinity for dyes form an addition compound with the dye whose stability is concentration dependent and normally decreases with increasing temperature. The dye distribution equilibrium between the dye in solution and the dye in the fibre is therefore shifted to the dye in solution. The increased dye concentration in the dye solution makes it possible for regions of the fabric which were dyed in a non-level manner to level out as a result of dye migration.
Effective leveling assistants have an affinity for the dye that is sufficient to reduce the absorption rate or to speed the migration rate. Differences in the absorption behaviour of different dyes can likewise be leveled, so that the dyes in a dye mixture can go on at a uniform rate. Assistants with an affinity for dyes can also be used to level previously dyed materials. Assistants with an affinity for fibres go onto the fibre in competition with the dye. This competition reaction reduces the absorption rate and promotes the migration rate.
Important dye-affinity leveling agents for polyamides are nonionic surfactants, cationic compounds or ethoxylated compounds. Important fibre-affinity leveling agents for polyamides are cationic compounds.
In every dye house, levelness of the dyeing is a major criteria, since unleveled goods are usually not saleable, causing loss to the manufacturer. Migration of applied dyes in a uniform manner throughout the dyed goods is called leveling and it may be a property of the dye or it may require some chemical assistance. Understanding the need, Sarex has developed a product Nylolevel-606(Conc), an excellent leveling agent for polyamide fabrics which works on dye migration principle.
Leveling agent for polyamide fabrics
Excellent migration imparting level dyeing
Corrections of faulty dyeing
Recommended dosage during dyeing of polyamide fabrics with Acid dyes.
Nylolevel-606 (Conc) : 0.5-1 %
The efficiency of the product is evaluated by migration study. Polyamide fabric was initially dyed with 3.5% C.I. Acid Red 1. Dyed (Fabric-1) and Undyed (Fabric-2) polyamide fabrics were treated together with 0.5-1% Nylolevel-606 (Conc) at 98°C for 30 min. and the amount of dye migrated on the undyed fabric was evaluated. More the migration of dye on Fabric-2, better is the leveling property.
The above figure show excellent migration of dye on the fabric resulting into level dyeing.
Dyeing of polyamide was carried out with 2% C.I. Acid Blue 83 at various temperatures i.e. 50°C, 70°C, 90°C, 98°C, 98°C, 15 min and 98°C, 30min. The quantity of dye remaining in the dyebath i.e. retardation property was evaluated by performing second dyeing in the exhausted bath at 130°C for 30-45 min.
From the graph (Retardation curve) it could be seen; In blank dyeing, maximum amount of dye has been rushed onto the fabric as it reaches near to the dyeing temperature i.e. 90°C. This rushing of dye causes unlevel and patchy dyeing. Addition of Nylolevel-606 (Conc) in the dye bath show clear retardation of the dye at this temperature which in turn results into level and uniform dyeing.