1. White Thread - Polyester filament yarn (PFY) Core, Cotton (C) Wrap
i)The parent yarn is strong. Showing an average breaking load of 2235 grams, it is the strongest of the three types of thread in this study. The combination of a strong polyester yarn core and the cohesive inter-fibre grip between the cotton wrapping fibres provides good resistance to tensile loads. This is an important characteristic required of a sewing thread.

ii)Its breaking elongation is the lowest at 9.83 per cent. The presence of cotton, which has a characteristically low extension, lowers the overall extension of the yarn at break. The wrapping fibres can be expected to straighten under the tensile load and break even before the polyester core has completely elongated.

iii)As this yarn has a high breaking load, the energy required to rupture it is also correspondingly high. The energy required to rupture it is 0.38 J.

2. Blue Thread - 100% Polyester (PET) Spun Yarn
i)With a breaking load of 1733 gf, this is the weakest of the three yarns under study. Though of the same linear density as the other two types of thread, the inadequate grip between the constituent polyester fibres lowers their cohesion and hence the overall yarn strength.

ii)The breaking elongation at 15.5 % is intermediate between the other two threads and is due to the polyester fibre present.

iii)The energy to rupture this 100% spun polyester thread is half that required to rupture the polyester core-cotton wrapping thread. The low breaking load and the moderate breaking extension result in the low value of energy of rupture (0.19 J).

3. Grey Thread - Polyester filament yarn PFY Core, Polyester staple fibre (PSF) Wrap
i)This yarn has an intermediate breaking load at 2028 gf. The inter-fibre grip provided by the smooth polyester sheath fibres cannot be expected to be as strong as that of the cotton fibres in the White Thread and thus the breaking strength is lower.

ii)The breaking elongation at 19.59% is higher than the other two yarns mainly because the polyester fibre has an inherently higher extension characteristic.

iii)The energy to rupture this thread is higher than the 100% spun yarn polyester thread. The fairly high breaking load and the high breaking extension contribute to this value (0.25 J).

Besides the above points, other aspects like the linear density of the core continuous polyester filament yarn, the number of filaments it contains, the denier per filament, the level of twist in the wrapping fibres, the number of plies the thread is composed of, etc would also influence the mechanical behaviour of the threads.

Effects of the stitching action and the washing treatment on the threads
Effect on tensile modulus
The results of the tensile moduli, the breaking loads and extensions of the threads unravelled from the seams of the denim samples after the standard industrial wash present some interesting information on the expected behaviour of the threads in the seams.

The general observation is that there is a fall in the tensile modulus at loads of 100 g (MOD 1) and 250 g (MOD 2) for all the three types of thread. Only for the PFY Core-C Wrap White Thread is there an apparent and inexplicable increase in the moduli at 100 g load, in three cases, the 6-spi seam in the warpway and weftway directions and the warpway 8-spi seam. This effect may be due to particular specimen effects and cannot be representative of the sample as a whole.

Ignoring these three cases, the average modulus of the PFY Core-C Wrap White Thread after stitching and washing is 47% of that of the parent yarn at 100-g load. Similarly, the moduli of the 100% PSF Blue Thread and the PFY Core-PSF Wrap Grey Thread are respectively 31% and 27% of the modulus of original parent yarn values at 100-g load. The moduli at the 250-g load are still lower in all the three cases.

There is no discernable effect of the direction of the seam, warpway or weftway, on the modulus values. There is also no particular trend by which the stitch density influences the thread moduli.
It may be thus concluded that the effect of the standard enzyme wash is to lower the moduli of the threads in the seams. Added to the effect of washing, is the effect of the mechanical action of stitching and stitch formation. The speed at which the thread is stitched (2860 stitches /min) and the bending and twisting strains imposed as a result of thread formation cause the threads to be in a state of mechanical stress, whereby it suffers a lowering of its tensile modulus.

The advantage of the combined effect is that the flexibility of the seam and hence its contribution to the drape of the garment will be improved. The disadvantage is that the thread is susceptible to strains in regular garment use to a greater extent. Of course, the extent of such increased strains would depend on the type of denim garment and its tightness of fit on the users. In any case, it is not likely that the 100-g load would be equalled or exceeded in the strains of normal garment use.

Effect on other mechanical properties
Breaking Strength
The PFY Core-C Wrap White Thread shows a drop in breaking load, about 3.5% for the thread from the warpway seam and nearly 13% for that unravelled from the weftway seam. The blue and grey threads, on the other hand, show a fairly uniform increase of about 10% for both the warpway and weftway seams. There is little effect of the stitch density on the strength results.

The change in strength values after washing could reflect a differential change in the linear density of the three types of threads. It is likely that the blue and grey threads have suffered a greater degree of shrinkage than the white thread. The PET sf in the blue thread and the PET yarn and wrapping PET sf in the case of the grey thread are bound to relax and shrink due to the washing treatment. Such shrinkage would cause an increase in the linear density of the threads and this in turn would result in a corresponding increase in the thread strength.

In the case of the white thread, the restraining effect of the relatively rough-surfaced cotton wrapping fibres has probably given rise to a negative effect and hence the strength has dropped. The different behaviour of the warpway and weftway seams is not clear at the moment, but could be ascribed to the 2/1-twill structure of the denim fabric.

Breaking extension and energy of rupture:
The extensions-at-break of the three types of thread are all greater than the breaking extensions of the respective parent threads. The greatest increase is shown by the PFY Core-C Wrap White Thread and this is observed for both the warpway and weftway seams. The other two types of thread show a uniform 9-10% increase in extension.

The PFY Core-C Wrap White Thread shows a fall in the energy to rupture the threads, the weftway thread showing a 20% fall. The other threads show increases in the energy of rupture that correspond with the changes in the breaking load and extension values.
The above changes in the mechanical properties are welcome changes that would contribute to better sewing thread and seam performance in garments.
Effects on Seam Properties on denim fabric
The results of tests on the seams are shown in Table 2 below.

*Sbf - Seam Breaking Force; TPO - Thread Pullout; STB - Sewing Thread Breaks; FTS - Fabric Tear at Seam

Effect of Seam Direction and SPI on Seam Strength

The first observation is that in the case of all of the threads, the warpway seams decrease in strength as the stitch density increases. The strength of the weftway seams, on the other hand, shows an increasing trend with increase in the SPI of the seams.

Warpway Seams: In all of the types of thread investigated, the warpway seam strengths are lower than those of the weftway seams. When a warpway seam is tested for its strength, it is the weft threads in the fabric that are loaded along with the sewing thread composing the seam. The weft yarn in a fabric is also generally not as strong as the warp yarn. Though the weft yarn in the denim fabric under test is coarser than the warp yarn, the picks per inch are much lower than the ends per inch.

As the stitch density increases, fewer weft threads are included inside a single stitch, so there are fewer threads to break as every stitch of the sewing thread takes on the applied load. This is why the seam strength decreases with increase in stitch density. In most of the cases, the effect of sewing thread pullout is more pronounced as there are fewer picks per inch that offer greater space for the sewing thread to slip through.

Weftway Seams: Due to the twill weave structure of the fabric, a greater number of warp ends are available to take the load with every stitch and this results in higher seam strength in this case. This is also the reason for the increase in seam strength with increase in stitch density. Though there are fewer ends to share the load per stitch as the SPI increases, the longer floats of the twill weave offer greater collective resistance to the applied load.

The behaviour of the sewing thread in the majority of the cases here is to break due to the applied tensile forces. The stronger and greater number of warp ends that it has to pass through during the test offers too much of a resistance and hardly any chance of slipping through. The net result is that the sewing thread breaks. Only in the case of the PFY Core-PSF Wrap Blue Thread, at 11 SPI, does the fabric itself tear.

Effect of Sewing Threads Type on Seam Strength
The warpway seam strengths are more or less in conformation with the breaking loads of the respective parent threads. The PFY Core-Cotton Wrap - White Thread and the 100% PET Spun Yarn - Blue Thread show a roughly 10 % difference in strength for all the SPI values. However, the blue thread appears to be the stronger in the seam than the white thread. The PFY Core-PSF Wrap Grey Thread appears to be the weakest of the three types of thread.

When the threads form the weftway seams the behaviour shows a dramatic change. The grey thread is the strongest, next comes the white thread and the weakest is the blue thread! The reason for this turn of events is not clear at the present moment.

Further studies with different kinds of fabric are underway and the findings will be reported in due course. It is hoped that this subsequent work will throw more light on the behaviour of threads and seams.