Preparatory chemical processing for polyester

Singeing

Singeing is a pretreatment for PET-cellulosic blend fabrics (not 100% PET fabric). The modern gas singeing machines have efficient and computer controlled burners, which can control the width, height, angle and intensity of the flames.

Problems encountered:

• Any uneven heat transfer is liable to cause variations in the setting of PET fibres, resulting in uneven, dyeing of fabric.
• In singeing protruding PET fibres tend to form globules, which are difficult to remove.
• The globules dye deeper shade than the original fibre when dyeing is carried out by exhaust method.

Remedies:

• Singeing as a pretreatment should only be done if the continuous thermosol method is used for dyeing.
• Singeing effect, if one or more passages are given, can reduce the pilling effect on the fabric. Leading to better usage and thus in such cases singeing acts as post treatment machine.

Setting of polyester

In order to achieve good shape retention and prevent running creases from marking during wet processing, PET fibres and their blends must be stabilised. In other words, tension within the individual fibres and in the fabric is leveled out by relaxation. Heat setting of PET is carried out at 180⁰C, to ensure 0% residual shrinkage (natural shrinkage) and completion of primary crystallization.

Problems:

• Measurement of surface temperature of fabric
• Speed uniformity in machine
• Non-uniformity in setting effects for large run of fabrics
• Incomplete removal of residual shrinkage (0% shrinkage never achieved)
• Excessive shrinkage causes problems in subsequent processes

Remedies:

• Accurate and precise measurement of fabric surface temperature using infrared sensing thermometers
• Measuring residual shrinkage in lab (should not be more than 0.5%)
• Achieving precise relaxation of fabric by over feed arrangement
• Using modern hot air stenters for accurate speed adjustment, air humidity measurement and overfeed

Chemical preparation

Processing aids are applied during sizing of microfilament yarns in order to help and protect the delicate polymer fabric during processing. The common feature of all washing processes is the exchange of substances in an aqueous media.

Problems:

• Temperature variations in the batch cause variations in the degree of bleaching
• Poor temperature control can cause fibre tendering
• Variations in reaction time between ends of the cloth on the batch can cause tailing in dyeing
• Efficient and optimal removal of sizes and other processing aids
• Creasing and abrasion marks in the fabric during chemical pretreatments