Properties of Sewing Threads | Essential Properties Required for Sewing Threads

Essential properties required for sewing threads: Industrial sewing techniques make specific and often very exacting demands on the threads involved in the sewing process. The sewability of sewing threads is of major importance6, having a very profound effect on seam quality and production costs. The sewing and the seam performance of a sewing thread are largely influenced by the material to be sewn, the sewing technique and the end-use for which the sewn material is intended. These requirements can be defined as:

* The ability of the sewing thread to meet the functional requirements of producing the desired seam effectively.

* The ability of the sewing thread to provide the desired aesthetics and serviceability in the seam.

* The cost of sewing thread and that associated with producing the desired seam.

The different important properties required by a sewing thread are discussed below:

1. Needle thread must pass freely through the small eye of the needle; consequently they must be uniform, knot-free, non-torque and fault free.

2. Tensile strength/breaking strength is one of the essential properties of the thread. It must be capable of withstanding several kinetic/lateral movements during sewing. The strength of the sewing thread must be higher than that of the fabric so that the thread does not rupture during use. During sewing at high speeds, the needle thread is subjected to repeated tensile stresses at very high rates. The thread also comes under the influence of heat, bending, pressures, torsion and wearing. The value of these stresses depends on the sewing speed, machine settings and the thread used. The stresses created within the thread have a negative effect on the processing and functional characteristics of the thread, and there is significant reduction in the thread strength after sewing.

This is a function of the dynamic and thermal loading of the thread and is influenced by the thread frictional properties, thread tensioning during sewing, needle size, stitch length and number of fabric layers in the seam. The thread should therefore possess adequate strength and elongation in order to perform satisfactorily during sewing and in seam 7.

3. For good performance in a sewing machine moderate to low extension-at-break of the thread is usually preferred. Needle thread with different elongation-at-break has been found to behave quite differently during stitch formation. The determinants of success of sewing a thread with certain elongation per cent without any problem are the machine setting and special properties of the sewing thread itself 6.

4. The elasticity of the sewing thread must be uniform along its length in order to enable equal length stitches to be formed, and it must closely match the elasticity of the fabric being sewn; otherwise either seam thread fracture, or tearing of the adjacent fabric may arise during garment use. Clearly, the requirements of woven and knitted fabrics will be different.

5. The forces that are developed in the sewing thread are mostly due to the friction between the thread and machine parts, the most severe action taking place between:

- The thread and the needle.

- The thread and the fabric being sewn.

A controlled level of both static and dynamic friction is required; this must not be too high, which could cause lack of thread control. High static friction values are necessary to allow the stitches to lock and prevent “run-back” of seams. Spun threads are particularly good in this respect when compared with filament thread. The worst is the monofilament threads. The frictional properties are affected by lubrication. The factors that influence the frictional properties are:

• Uniform application of lubricating agents.
• Adhesion of the finishing agent on the thread.

The quantity and quality of finishes are very important. Special finishes like silicone compounds have been found to exhibit clear advantage over standard paraffin wax.

6. Good abrasion resistance is essential for good sewing performance. The thread is under tension condition, especially when the stitch is being set. The thread must be resilient enough to return to shape after the distortions, and then must maintain its physical properties to provide good performance in the seam after the sewing process is complete. Nylon and polyester offer the best resistance to abrasion.

7. Good resistance to heat is a very important requirement of a sewing thread. The temperature reached by the sewing needle during sewing very much depends on:

* the nature of the fabric to be sewn (density, thickness, finish)

* the speed of the sewing machine

* the type of needle used (size, shape, surface finish)

* size and finish of the sewing thread.

The needle temperature is especially critical for fabrics and sewing threads of thermoplastic fibres, where it may exceed their melting temperature. Needle heating causes sewing thread breakage, cross-thread, skipped stitches, seam damage and physical damage to the needle.

Various studies show that the sewing thread influences the needle temperature significantly. Its movement through the needle reduces the needle temperature by an average of 21- 45%, the amount of reduction depends on the sewing condition and the structure, fineness and composition of sewing thread.

Lubrication of sewing thread with a mixture of wax, emulsions with synthetic resins, and silicon based products may minimise heat generation, and the fibres surface of spun yarns may be an advantage in that a thin layer of the surrounding air will move with the thread and promote needle cooling.

8. The hairiness of sewing thread also affects the appearance of the seam. Sewing threads for decorative seams are singed, squeezed and gloss-brushed.

9. The final direction of twist insertion may be important to enable the stitch forming mechanism of the sewing machine to perform correctly; most sewing machine require Z twist, but there are a few where performance is better with S twist.

10. Colour fastness is a general requirement for sewing thread. It is important that the selected shade retain its colour throughout the life of the garment. Two aspects of fastness are important:

• The thread must not change colour.

• The thread must not stain any material adjacent to the seam.

11. Low shrinkage during washing and ironing is required. Shrinkage due to fibre swelling causes seams to pucker, especially if the fabric exhibits less shrinkage than threads. Synthetic threads suffer less from this problem than cotton threads owing to their much lower moisture absorbency; however they are liable to residual shrinkage problems if unsuitable manufacturing processes are employed. Synthetic threads can suffer from the problem of thermal shrinkage during ironing but this difficulty can be solved by the use of high temperature setting, which stabilises the thread at temperature above those normally encountered during the ironing process.

The sewing threads should possess better evenness and should contain minimal number of knots, faults and neps, etc. Thread should have very low level of imperfections and classimat faults.

12. Good lustre in the thread improves appearance of the seam. 

13. Threads must be uniformly dyed in a good match to the materials being sewn and also the dyed thread should have properties like colourfastness to washing, light, perspiration, and sublimation.

14. The ability of the thread to perform efficiently in the sewing machine is defined sewability. It can be assessed by the number of breaks that occur during the sewing of a certain number of stitches. However, owing to the generation of needle heat in high-speed sewing, the threads could be damaged without breaking. The long knot-free evenner yarns in case of rotor and air-jet can give better sewability.

15. The characteristics of properly constructed seam are strength, elasticity, durability, stability and appearance. The relative importance of these qualities is determined by the end-use of the sewn product. The factors that govern these properties are seam and stitch type, thread strength and elasticity, stitches per unit length of seam, thread tension, seam efficiency of the material. The hairiness of sewing thread is important to decide seam appearance. The shrinkage potential of the thread and hence the seam is also major importance for proper seam appearance. The serviceability of a garment depends not only on the quality of the fabric but also on that of the seam. The seam quality is measured by stitching parameters of the threads and seam parameters such as size, slippage and strength. 

The failure of seam produced by traverse loading can generally be classified as: Type I: the failure due to thread breakage, Type II: the failure due to fabric breakage, Seam breakage: the failure due to the slippage of cloth yarns at right angle to the seam. 

Seam slippage is the most probable cause for seam failure that leads to garment rejection in wear. The durability of a seam depends largely on its strength and its relationship with elasticity of the material. It is measured in terms of seam efficiency, where Seam Efficiency = (Seam tensile strength/fabric tensile strength) x 100, generally ranges between 85 to 90%. The minimum loop strength correlates well with the stitch breaking strength. Further resistance to abrasion and wear of the seam during everyday use, including laundering is also essential for the longer seam.

16. Seam pucker can be defined as a differential shrinkage occurring along the line of a seam and is mainly caused due to seam instability, due to high tension imposed during sewing. Though currently available threads have a certain amount of controlled elasticity and elongation they get over-stretched when the sewing tensions are high. During relaxation the thread recovers its original length, thus gathering up the seam. Threads for use in apparel are also required to have good stability to laundering, ironing and other treatments since differential shrinkage between the sewing thread and the fabric of a garment can cause puckering.

Further, Seam pucker can be determined by measuring the differences in fabric and seam thickness under a constant compressive load. The seam-thickness strain is calculated by using the formula:

Thickness strain (%) = (seam thickness – 2 x fabric thickness) x 100 / 2x fabric thickness {ref}