Every yarn needs to comply to certain specifications depending on your application and your specifications. At Senbis we are specialized in measuring the properties of yarns for both quality control and R&D. We do so according to renowned standards like ISO, ASTM and BISFA. A large amount of properties can be measured in our dedicated polymer laboratory. We have to categorize due to the broad topic: Mechanical analyses, structural analyses, and other general analyses.
For yarns and fibers mechanical properties are of the the most important quality parameters. This can be tested at the polymer laboratory of Senbis. We often measure both synthetic and biobased yarns for third parties and report linear density in dTex (g/10.000 meter yarn), tensile strength (N), elongation at break (%) and breaking tenacity (mN/Tex). The breaking tenacity is the tensile strength normalized for the linear density. This means that this parameter allows comparison between yarns of different thicknesses.
Also time to failure measurements are commonly conducted here. Yarns are, in some applications, subjected to a constant load for long periods of time. Important to know is how long can a yarn be subjected to a specific load before it breaks. This load is often lower than the tensile strength. So it is smart to measure.
Mechanical properties of yarns can be enhanced by applying techniques such as drawing and twining. Drawing often used in combination with the spinning process. Such processes are called spin-draw-winding (SDW) processes. Crystallinity is induced by drawing increasing the strength of a yarn. During drawing the linear density decreases. Filaments are spun with a higher linear density to compensate for this effect. Twining is a post processing step and is only beneficial when multifilament yarns or multiple yarns are used. Twining ensures a better load distribution over all the filaments, therefore increasing strength. There is an optimum in the amount of twines per meter, which can be determined at our laboratory. For weaving it is very important to use S and Z twines (clockwise and anti-clockwise) for the integrity of a fabric.
The above mentioned drawing process is more complicated than one might think, because the draw ratio can be varied. High draw ratio’s, mean high crystallinity, but it is impossible to infinitely draw a yarn. This means that there is an optimum in the draw ratio. When a draw ratio higher than the optimum is applied, microscopic tears in the surface of filaments occur. Which, diminishes the strength of the entire yarn. This phenomena is called 'over drawing'. When lower draw ratios are used, certain properties can be achieved. Think of chain alignment in the amorphous phase without inducing crystallinity. This increases strength to some extend too.
In order to obtain the maximum strength in a yarn by drawing, the filaments core must be drawn as much as the surface. In some cases we receive yarns which exhibit a clear core-shell effect. Meaning that the shell of the yarn is fully drawn, but the core is not yet drawn to its maximum. This effect diminishes the full potential of a yarn. We can solve these problems and increase strength in your yarns. This might possibly allow yarns with lower linear density to be applied in the same application saving material and costs.
A general rule of thumb is that orientation of polymeric chains increases strength in a yarn. The amount of orientation can be determined in detail by a combination of 5 analysis; density, ubbelohde viscometry, birefringence, pulse propagation method and x-ray diffraction.
- The overall density of a yarn is determined with a davenport gradient column. This information is later used in calculations.
- Ubbelohde viscometry is used to determine the molecular weight of the polymer of interest and is also used in later calculations.
- Birefringence, determined by means of a microscope using polarized light, gives information about the average amount of orientation originating from both the crystalline and amorphous phases.
- The pulse propagation method (PPM) determines the maximum amount of orientation in the yarn, composed of crystalline domains and the amorphous phase. As a soundwave is send through a yarn the propagation speed is measured. The first and fastest wave is most important, because sound travels fastest through orientated materials. The maximum orientation can be mathematically determined.
- X-ray diffraction is a method to measure the size, orientation and density of crystalline domains in the yarn using x-rays. The density of the crystalline fraction is most important in order to determine the total amount of crystalline material in a yarn. By subtracting the x‑ray diffraction results from the pulse propagation method results, the maximum orientation in the amorphous phase is determined.
This is a perfect example of how multiple analysis can be combined to determine the structure of a yarn. The molecular weight, viscosity, density, percentage of crystalline domains, density of crystalline domains, amount of orientation in the amorphous phase of a yarn is known after doing 5 analysis and mathematics. By doing these analysis we can understand the polymer and relate its physical structure to process settings and predict properties. Process optimization can be done and steered into a specific direction suiting your needs.
Besides this structural analysis, more common analysis such as cross section analysis, entanglement tests, hot air shrinkage and yarn evennesyarn evenness measurements can be done here. Many of these analysis are usually performed in combination with each other to construct a complete general image of the properties and specifications of yarns.