Special finishing process

During gassing or scorching, protruding fibre hairs are burned off. This finishing process smooths the surface of sewing threads made from spun threads. This gives the thread a higher sheen. A gas flame (hence the term ‘gas treatment’) or an electrically heated burner is used for burning. Due to the high winding speed, it is not possible for the material being processed to burn. Only protruding fibre ends are burned, which are then immediately removed by an suction system.

Cotton threads are mercerised before dyeing. During mercerisation, the cotton is treated with caustic soda under tension, causing the fibre to swell and change its cross-section. Mercerisation is usually carried out on the hank material. This additional production step is necessary to give the cotton yarns a silky sheen, increase their tear resistance and improve their dye absorption capacity.

Polyester/cotton core spun threads and pure cotton threads are sometimes polished or dressed to give them a smoother, more uniform surface and at the same time increase their abrasion resistance. In this finishing process, the dyed yarns are dipped in starch or a synthetic wax preparation and then brushed. This brushes any protruding fibres in one direction close to the yarn surface. This finish also has advantages in terms of sewing, as the more closed yarn surface provides greater resistance to untwisting.

In this special treatment, an additional synthetic preparation for film formation (known as a bonding substance) is applied after the dyeing process. The sewing threads are immersed in a bath containing the bonding substance and then passed through a squeezing mechanism that removes any excess bonding material. A directly connected heating channel then ensures that the bonding is condensed or dried.

The bonding substance in the immersion bath is applied as an aqueous dispersion or as a solvent-based product. Depending on the choice of bonding substance, the sewing threads must then be additionally lubricated to ensure the usual sewing performance.

In addition to this conventional and most commonly used type of bonding, there are other methods: for polyamide filament yarns, for example, bonding can be based on treatment with acid. The bonding effect is achieved by dissolving the polyamide filaments under the influence of acid.

Bonding, colloquially referred to as ‘gluing’, ensures better cohesion between the single yarn elements of a thread. The bonding substance creates a special bond between the threads on the surface of the sewing thread, resulting in a better thread bond. Bonded sewing threads are therefore easy to recognise: on the one hand, they are stiffer than normal sewing threads due to the additional preparation (this is immediately apparent when pulling the product off the spool), and on the other hand, these sewing threads are difficult to untwist. You can clearly feel a resistance, which is due to the effect of the bonding.

This special treatment is generally only considered for synthetic filament sewing threads in the coarser ticket range from ticket 60 onwards. For filament yarns, which have a less effective twist due to the smooth surface of the individual threads, this preparation is necessary and useful for special applications. Classic areas of application are seams that cause the sewing thread to twist or the so-called twist shift during sewing, such as multidirectional seams on airbags or zigzag seams on sails (twisting and turning effect of the sewing thread depending on the direction of the zigzag stitch).

For some applications, e.g. shoes or outdoor textiles, an additional hydrophobic finish of the sewing thread is useful. Hydrophobically finished sewing threads can significantly delay water penetration at the seams. However, waterproof seams cannot be achieved by using hydrophobic sewing threads. The hydrophobic finish is usually applied on separate finishing machines together with the standard finish.
The AMANN range includes a wide variety of products with a water-repellent finish.

For seams that are exposed to intense and prolonged sunlight, e.g. sails, awnings, conservatory shading, additional UV protection may be required for the polyester sewing threads used. One option is UV protection via the finish, although the potential of this effect is often overestimated. For UV protection, the standard preparation is supplemented with a UV blocker. This finish is applied as described above.

Sewing thread finishes usually contain silicones. If these cause problems in the final application, e.g. in filter production for paint shops or in clean room production, sewing threads can be finished with a silicone-free finish by changing the finish recipe. There are various options for this, depending on the field of application. Some silicone-free finishes contain paraffins. If these are also undesirable, it is possible to finish sewing threads with aliphatic esters that are both silicone-free and paraffin-free.