Sewing thread quality characteristics explained: properties and performance
The respective properties of sewing threads and embroidery threads result in different quality characteristics. These determine the sewing performance and the utility value of the finished seam or embroidery. Depending on the selection of raw materials and the specification of manufacturing processes and production parameters, sewing threads and embroidery threads have different processing and usage properties and thus a different level of quality.
Strength
The strength of sewing threads affects sewability and has a significant impact on seam strength. Sewing threads with low strength can break during the sewing process if the stresses, i.e. the forces exerted by the stitch formation, are too high. This can lead to disruptions in the production process or even production downtime, especially when sewing/embroidery machines are used. A distinction is made between the following strength terms and specifications:
The maximum tensile strength is the maximum linear tensile force that can be applied to a thread before it breaks. It is sometimes also referred to as breaking strength, breaking force or linear tensile strength.
It is measured using the simple tensile test in accordance with ISO 2062 and expressed in cN. The simple tensile test is one of the most important yarn or thread tests for compliance with specified quality requirements.
In AMANN's testing laboratories around the world, the maximum tensile strength test is performed more than 5,000 times a day on fully automatic tensile testing machines and evaluated using a stress-strain curve diagram. The curve of the diagram shows the elongation in percent and the effect of the force in newtons until the thread breaks. In the diagram shown, the values of several tests are superimposed. This considerable testing effort is necessary because, as a rule, all the different production stages – yarn, raw thread and finished thread – are tested in order to guarantee optimum quality.
When talking to technicians in the apparel industry, the maximum tensile strength is often given in grams or kilograms rather than centiNewtons or Newtons. Here, 1 g corresponds to ~ 1 cN and 1 kg corresponds to ~ 10 N. For many people, weight designations are still more common and easier to understand. Depending on the raw material, sewing thread type and linear density, AMANN products have very different levels of strength. The finest polyester continuous filament threads, such as Serafil fine 300, have a maximum tensile strength of approx. 640 cN; coarse polyester continuous filament threads can achieve maximum tensile strength values of over 20,000 cN (equivalent to 20 kg).
The maximum tensile strength in relation to linear density is specified in cN/Tex for sewing threads and provides a comparison of the strength of different types of sewing thread.
Example:
Saba 120 has a linear density of 14 Tex × 2 and a maximum tensile force of 1,200 cN. This results in a maximum tensile force relative to linear density of 42.9 cN/Tex. (Calculation: 14 × 2 = 28; 1,200 : 28 ≈ 42.9)
An exemplary comparison of the maximum tensile strength (also known as tenacity) of different types of sewing thread clearly shows the varying strength potential depending on the raw material and thread construction.
| Product | Tenacity (cN/tex) |
|---|---|
| Polyester continuous filament (Serafil) | 50 to 65 |
| Polyester/polyester core spun (Saba) | 40 to 50 |
| Cotton spun (Mercifil GD) | 25 to 35 |
| Polyester bulk yarn (Sabatex) | 30 to 40 |
| Kevlar® continuous filament (Kc-tech) | approx. 160 |
Loop strength is the tensile strength of sewing threads of the same linear density, which are stressed in the form of two loops hooked into each other in the tensile testing machine.
This test method is more closely related to the stresses on the sewing thread in the stitch formation looping of the seam than the linear tensile strength test using a simple tensile test.
The loop strength test subjects the sewing threads to different types of stress at the same time:
- tension in the longitudinal direction
- compression in the transverse direction
- bending around a very small radius
The sewing threads usually break at the point of looping, as the transverse strength of the fibres is lower than their longitudinal strength due to the fibre molecules being oriented in the longitudinal direction of the fibres. The relative loop strength is calculated from the ratio of the maximum loop tensile strength to the maximum tensile strength and is expressed as a percentage. Depending on the sewing thread construction, conventional sewing threads have a relative loop strength of approx. 60 to 75%. Spun threads generally have the highest relative loop strength, while continuous filament threads have the lowest.
Due to their raw material properties, para-aramid yarns have a very low loop strength of only approx. 40%. As a result, a significant portion of the very high linear strength is not effective in the seam. This must be taken into account when calculating seam strength.
Example: Saba 120 has a maximum tensile strength of approx. 1,300 cN and a relative maximum loop tensile strength of approx. 70%.
Elongation and elasticity
The elasticity of sewing threads has a direct influence on sewability and seam elasticity. Therefore, this quality feature is often the focus of attention. It is important to understand the correct use of the different terms used to describe this physical property.
Elasticity is basically divided into an elastic (reversible) and a plastic (permanent) component, with a smooth transition between the two. Elastic elongation is the portion of elongation that completely reverses after the load is removed.
Plastic elongation does not reverse after the load is removed. In common parlance, the former is referred to as elasticity and the latter as elongation.
At AMANN, elongation is determined in accordance with ISO 2062 by testing the maximum tensile force and is specified as maximum tensile elongation in percent on the product data sheet. The standard describes maximum tensile elongation as the change in length up to the breaking point of a sewing thread caused by tensile force in the longitudinal direction (in relation to the original length).
The overall course of the elongation behaviour is represented by the stress-strain curve diagram. Different types of sewing thread have different elongation behaviours and can be compared by superimposing the curves.
The elongation behaviour of sewing threads is determined by the selection of raw materials, the construction and the manufacturing process. Cotton threads such as MercifilGD have very low elongation of approx. 5 to 10%.
Sabaflex, AMANN's highly elastic sewing thread, has an elongation of approx. 60% thanks to its innovative raw material PTT, making it ideal for elastic seams. Standard sewing threads have an elongation of between 10 and 30% and are universally applicable. With the optimal combination of sewing parameters (stitch type, stitch density, thread tension, etc.), they are also suitable for elastic seams.
The two most important raw materials for sewing threads, polyester and polyamide, also differ in their elasticity. Polyamide sewing threads are known for their high elasticity. Polyamide continuous filament threads, such as Onyx, have a maximum tensile elongation of approx. 20 to 25%. In comparison, polyester threads have lower elasticity, depending on the sewing thread construction.
| Product | Maximum tensile elongation ( % ) |
|---|---|
| Polyester spun thread | approx. 12 to 15 |
| Polyester/polyester core spun thread | approx. 15 to 25 |
| Polyester/cotton core spun thread | approx. 15 to 25 |
| Polyester continuous filament | approx. 20 to 30 |
During the sewing process, different tensions (tensile forces) act on the sewing thread. Until the sewing thread is sewn in a double lockstitch, it is periodically exposed to up to 80 times this tensile stress, depending on the stitch density (see diagram below). This principle of operation results in two requirements for the elongation properties of sewing threads. On the one hand, the sewing thread must be able to compensate for the rapidly changing tension stress; on the other hand, the rebound of the sewing thread in the finished seam should be as low as possible to avoid tension puckering.
Abrasion resistance
Abrasion resistance is the resistance of sewing and embroidery threads against abrasion and is assessed on the basis of visible changes in the appearance of the sewing thread, up to and including complete destruction of the thread. The abrasion resistance of sewing threads is measured in abrasion cycles required for destruction.
Resistance to abrasion is one of the most important properties for assessing the performance of sewing threads in seams. This becomes particularly clear when considering, for example, the abrasion stress on decorative seams in upholstery or topstitch seams on shoes.
Embroidery, such as on workwear or children's shoes, can also be exposed to extreme abrasion. At the same time, abrasion resistance is important for the sewing performance of sewing threads. The friction generated during the manufacturing process must not impair the sewing thread or embroidery thread in order to ensure seam strength.
There is no DIN or ISO standard for the abrasion testing of sewing threads and embroidery threads. Internationally recognised abrasion tests only exist for textile surfaces. AMANN has therefore developed its own testing procedure, which optimally reflects the abrasion stresses encountered during subsequent use.
In accordance with DIN EN ISO 12947-2, the Martindale abrasion testing device is used to rub the sewing threads in the seam. The result is visually assessed at intervals using a microscope. The stress intervals are based on the expected number of abrasion cycles until destruction. Unlike testing the strength or elongation of sewing threads, abrasion resistance is not part of standard quality control. Rather, the abrasion tests serve to obtain detailed information about this quality characteristic, which is necessary for competent application advice. Depending on the raw material and construction, sewing threads can have very different abrasion resistance.
A comparison of different types of sewing thread of the same strength using the AMANN test specification shows the following results:
| Product | Number of abrasive rubs before destruction |
|---|---|
| Cotton thread | 4,800 |
| Polyester spun thread | 7,600 |
| Polyester/polyester core spun thread | 17,000 |
| Polyester/cotton core spun thread | 17,000 |
| Polyester continuous filament | 24,000 |
| Polyamide continuous filament | 130,000 |
The abrasion resistance of sewing threads is primarily determined by the raw material and can be classified into different quality grades. The results confirm the superiority of polyamide over polyester and the advantages of synthetic sewing threads over cotton threads. In addition, the sewing thread construction has a significant influence on abrasion resistance. The clear difference between core spun threads and spun threads is interesting here.
A thicker sewing thread does not necessarily result in better abrasion resistance, as – depending on the nature of the material being sewn – it may lie on the surface and therefore be exposed to greater abrasion than a finer sewing thread.
The sewing parameters and the sewing material (the fabric of the abrasion test sample) also have a particular influence on the abrasion resistance of sewing threads in the seam. The sewing parameters – selection of stitch type, stitch density, thread tension – determine the stitch formation and thus the degree of stress. The sewing material used also determines the position of the sewing thread on the sewing material. When processing voluminous, fluffy fabrics, the sewing thread penetrates the fabric and is thus protected against abrasion stress. With hard, dense materials, deep stitch formation is prevented; the sewing thread lies exposed on the fabric and is subjected to high abrasion stress.
Gliding capabilty
Good gliding capability is important for a smooth sewing process (without skipped stitches and thread breaks) and flawless stitch formation. Sewing threads and embroidery threads must be able to be processed under friction conditions that are as constant and low as possible. In addition to the surface properties of a sewing thread, which vary depending on its design, this quality feature is influenced by the lubricant application.
AMANN monitors the gliding properties and thus the lubrication application as a quality feature using a friction measuring device. The aim here is to simulate the stresses caused by the thread tension on the sewing machine. The sewing thread is fed through a tension disc under constant load and at a constant speed; the forces that occur are recorded by the friction measuring device. Trend diagrams of the thread tension show the results.
Minor fluctuations in the diagram indicate optimal lubrication application and ensure good sewability. Large fluctuations, on the other hand, are disadvantageous.
Thread consistency
Thread consistency is understood as deviations in diameter from the average linear density of sewing threads and embroidery threads. The number of defects is recorded in relation to a defined length. Thread consistency directly influences sewing performance. A distinction is made between disruptive defects, i.e. those that hinder the sewing process, and non-disruptive defects. Thread consistencyis particularly important for the qualitative assessment of spun threads and core spun threads, as these, due to their construction, exhibit sporadic irregularities and thick spots even under optimal manufacturing conditions.
For optimum thread consistency, a smooth production process from the spinning mill to the finished thread is essential. AMANN examines both the consistency of the yarns used and the threads themselves. This quality feature is tested and monitored in the laboratory and during production.
An Elcometer is used for laboratory testing, which mechanically scans the sewing threads with a probe and detects defects (diameter deviations of a specified size). Defects are cut out and displayed on defect cards.
The faults are divided into fault classes. These are linked to the various causes that lead to the formation of thick and thin spots. They can be caused in the spinning or twisting process, by nubs, impurities, fibre deposits, loops, knots or poor splice points.
Thread consistency is checked during production using optical sensors on the winding machines. Unlike in laboratory testing, defects are not cut out but simply recorded as part of quality control. This allows goods with an excessive number of defects to be identified and separated.
Shrinkage
Shrinkage refers to the change in dimensions of sewing thread due to thermal or hydrothermal effects. The change in length, which is usually negative, is expressed as a percentage of its original length. This quality characteristic is particularly important for the subsequent product quality. Cleaning and finishing processes such as washing, dry cleaning, steaming and ironing must not cause the sewing threads and embroidery threads in the finished product to shrink.
Factors influencing sewing thread shrinkage:
- The sewing thread itself:
- Raw material
- Construction
Manufacturing process (stretching/drawing, heat setting, dyeing)
- Thermal or hydrothermal exposure
- Treatment temperature
- Exposure time
- Type of medium (water, steam, hot air, etc.)
- Stress state
AMANN conducts shrinkage testing in accordance with DIN 53 866. Length measurements are taken before and after treatment in the respective medium using simple suspension and measuring devices. The test is carried out using the so-called strand measurement method and with a preload force that depends on the linear density.
AMANN tests both thermal and boiling shrinkage using the following parameters:
- Thermal shrinkage: 180°C, 15 min., in a thermal cabinet
- Boil shrinkage: 95°C, 30 min., in boiling water
The thermal shrinkage parameters simulate a finishing treatment, while boiling shrinkage simulates the stress of the washing process. These results help to coordinate the respective thermal and mechanical parameters for sewing thread production. This enables AMANN to ensure compliance with the lowest shrinkage values.
Sewability
Sewability refers to the performance of the sewing thread during the sewing process. Good sewability requires that a sewing thread can be processed without problems not only under favourable conditions, but also under difficult sewing conditions. Good sewing properties ensure flawless stitching and prevent thread breaks and skipped stitches. Sewability is therefore extremely important for both production efficiency and the quality of the seams.
Optimal sewability is achieved through the interaction of the quality characteristics explained here. Due to the lack of DIN or ISO standards, AMANN tests sewability according to specially developed regulations in its own sewing and embroidery laboratories.
Depending on the type and strength of the sewing thread, the following sewability tests are carried out:
- Reverse sewing
- Zigzag sewing
- Multidirectional sewing
- Buttonhole sewing in underwear
- Thermal sewing
- Sewing at high speeds
- Test embroidery
- Water-repellency tests
- Serging seams tests
- Crimping tests
- Fibre abrasion tests
- Abrasion tests
- Tendency to static charge
- Unwinding tests
- Fraying tests
In these sewing positions, the sewing threads are subjected to stress due to their different properties. The production of underwear buttonholes, for example, places high demands on abrasion resistance due to the short stitch length. Reverse and zigzag sewing as well as multidirectional sewing are used to test the compactness of the thread and its tendency to open. This allows the level of twist and any bonding to be assessed. Sewability depends on numerous influencing factors. In addition to the quality characteristics already listed, two further properties are important.
Low-twist sewing threads are open, which means there is a risk that the hook will hit the open thread during the stitch formation process and tear it. Sewing threads that are twisted too tightly tend to curl. Knots and loops can form and lead to sewing problems. At the same time, the ratio of yarn and thread twists to each other is important in order to obtain a compact, balanced sewing thread.
The surface structure is primarily determined by the yarn or thread construction and influences sewability. Due to their textile, fibrous surface, spun threads and core spun threads behave differently on the sewing machine than continuous filament threads, for example. Bulk yarns have different sewing properties than conventional sewing threads and, due to their voluminous nature, cannot be sewn in all sewing positions. Monofilaments are stiff and smooth, making it difficult to secure the loop formation at the ends of the seam.
Lubrication
Lubrication is a combination of silicones, paraffins, waxes and antistatic agents that are applied to the sewing thread for optimum performance. It is important to have both a suitable composition of the lubricant formula and controlled application of the lubricant.
The lubricant fulfils various functions with regard to sewability:
Sewing threads should run as evenly as possible through the thread guide elements and thread tensions of a sewing machine in order to avoid tension fluctuations as far as possible.
Lubrication protects the sewing thread from abrasion during the sewing process. However, the raw material and construction of the sewing thread are decisive factors in terms of abrasion resistance.
High sewing speeds and the processing of dense, hard fabrics cause high needle temperatures, which are problematic for synthetic sewing threads. Lubrication provides a needle cooling effect during the sewing process, which delays the melting of synthetic sewing threads in demanding sewing positions.
The friction of the predominantly synthetic sewing threads on the various thread guide elements of a sewing machine often leads to static charging of the sewing threads, which significantly impairs stitch formation. The lubrication counteracts this behaviour. At the same time, the antistatic components of the lubrication help to prevent uncontrolled thread unwinding (thread ballooning).
Applying too much lubricant causes abrasion on the sewing machine. The thread guide elements become clogged with lubricant residue and impede the sewing process. Applying too little or unevenly applied lubricant causes fluctuations in the gliding behaviour and can also cause sewing problems.
Storage
AMANN uses many different raw materials to manufacture its products. These raw materials are processed and treated in different ways, depending on the end product to be produced. Due to the variety of materials and the different manufacturing and finishing processes, it is not possible to make a general statement about the optimal storage of AMANN sewing threads and embroidery threads.
However, extreme storage conditions (temperature, humidity, sunlight, etc.) should be avoided, as a combination of several of these factors may affect the quality of the sewing threads and embroidery threads. Under normal storage conditions, there are no known adverse effects on the quality of AMANN sewing threads and embroidery threads.
In order to guarantee the high quality of our sewing threads and embroidery threads, we recommend storing them in a cool, dry and dark place:
- in the original sales packaging,
- protected from dirt,
- protected from direct sunlight,
- protected from high humidity,
- protected from chemicals such as oils, gases, etc.,
- at temperatures not exceeding 25°C if possible,
- according to the ‘first in, first out’ principle (to avoid long storage times).
Colour
Colour is a complex quality characteristic for sewing and embroidery threads, which is determined by various colour properties. Colour tone, colour consistency from batch to batch and colour fastness are the primary factors that determine this characteristic. Particularly important, and verifiable by any observer, is the appropriate colour matching of the fabric and the sewing thread or embroidery thread. This is based on a wide range of colours tailored to the requirements of the respective industry and colour consistency of the goods delivered from different batches. Colour deviations are immediately detected and impair the product quality of the finished textile. Colour is defined by three components: shade, brightness and saturation.