Sewing thread quality characteristics include appearance, durability, extensibility and strength. The priority given to each of the characteristics will vary depending on the final product and its uses.
Traditionally, the appearance of stitches and seams has been assessed using the subjective procedure of visual inspection, though instrumental techniques have also been developed. Thread instability can affect the appearance of a seam through the creation of surface undulations along the seam line. For example, threads used for shirts of a polyester/cotton blend need to display a shrinkage of less than 1% in order to avoid an undesirable effect on the appearance of the product. Topstitching is used to define design and construction seams and as such, thick and/or lustrous thread is typically used. Embroidery stitching requiring a smooth, satiny, appearance is achieved by using a fine mercerised cotton, or alternatively, a silk thread.
The durability of a seam may be affected by a variety of factors including ultraviolet (UV) radiation, chemical attack, mould and mildew, abrasion and temperature. Resistance to UV radiation is important, for example, in curtains, where seams are exposed to sunlight for long periods of time. Core-spun polyester or nylon threads may be appropriate for such application, as long as the covering is not cotton. A thread constructed from synthetic resins, acrylic and plasticisers in continuous filament form, plied and bonded, has been specifically developed for products where sensitivity to UV radiation is an issue.
Extensibility is important for threads used to join fabrics which are themselves highly extensible. All sewing threads are extensible to some degree. The variables affecting extension-to-break include the substrate on which the stitching occurs, substrate thickness and the type of thread used. Thread extension-to-break also depends on such factors as yarn structure, twist and fibre type. Silk and a number of synthetic continuous filaments can extend up to 20% whereas a typical spun thread extends only to approximately 10% before breaking. Cotton threads, with low extensibility but high elastic recovery, increase in extensibility as a result of the stitching process, whereas synthetic continuous filament threads with high initial extensibility and low elastic recovery decrease in extensibility as a result of stitching. Increased stitching speed decreases the impact of the stitching process on extensibility.
The strength and elongation of sewing thread must be adequate for good sewing performance as well as good seam strength. Strength and elongation of sewing threads are determined by breaking a single sewing thread on a tensile testing machine and recording the force and elongation at break (ASTM D2256). Sometimes, it is necessary to measure loop strength and loop elongation of a sewing thread, as these properties are also very relevant to the performance. The loop strength and elongation of a sewing thread are a measure of the thread's ability to contribute to seam performance.
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