New development approaches in the field of needling technology
The concept of stabilizing and hardening earthwork with natural materials can be traced back to 3,000 B. C. According to records, woven fabric and reeds were found in Babylon. Thousands of years later, the basic principle hasn’t changed. But the methods and materials can hardly be compared at all with the technique used back then. While materials like wood, stone, concrete and even cotton were still used into the 20th century in the field of road construction, the focus is now mainly on synthetic polymers generally known as plastics. Nonwovens (including thermally bonded spunbond matts) began being used in the middle of the 1960s. At the end of the 1960s, the first needled nonwovens were used in earthworks by Rhône-Poulenc Textile, France. It took another 20 years for standardization and standards for geosynthetics (ASTM, ISO, BSI) to become a serious focal point.
Today, needled nonwovens are used in a wide range of technical applications, as nonwoven in their own right, as geosynthetic clay sealing strips or combined with a geogrid as composite material.
Megatrends such as urbanization and the economic situation are driving factors for the continued growth of geononwovens. According to the report “worldwide outlook for the nonwovens industry”, 682,000 tons of geo-nonwovens were produced in 2014 and a production quantity of 943,000 tons is expected for 2020. This corresponds to a total annual growth of 38 percent over six years. [inda/edana, worldwide outlook for the nonwovens industry 2014 – 2020, page 166]
Development approaches and trial execution
Groz-Beckert uses the cooperation with customers and partners to create synergies and build up internal process knowledge, as well as to improve the advice and optimize products. Various basic investigations are also carried out as part of this process: For example, the goal of a series of tests was to more precisely analyze the impact of the needle’s working part geometry on a textile’s physical properties depending on the fiber orientation and the needling parameters of nonwovens. Here, a particular focus was placed on the application of the geo-nonwovens. Geotextiles must fulfil several requirements simultaneously. Separation, filtration and draining are among its task, whereby the mechanical stress usually tests the limits of its capacity. The geotextile robustness classes dictate what a geotextile can ultimately withstand. The following tests are required for nonwovens:
- Determining the mass per unit area in accordance with DIN EN ISO 9864
- Measurement of the stamping puncture force based on tests in accordance with DIN EN ISO 12236
The measurement values enable the geotextiles to be allocated to the robustness class. Depending on the type of geotextile, we differentiate between five different robustness classes – from GRC 1 to GRC 5. Nonwovens in geotextile robustness class 5 must have a fabric weight of at least 300 gsm and a stamping puncture force of at least 3.5 kN. In order to take into account the production-related product deviations, the requirements always refer to the 5% minimum quantile.
The most suitable needle to meet the product requirements
In sections of the geotextile spectrum, Groz-Beckert customers work with Tri STAR® and Cross STAR®-felting needles, as the surface quality plays a secondary role, but high tensile strengths are required in the product. Both needle types offer high efficiency. Tri STAR® needles have concave sides and Cross STAR® needles have an additional forth edge. Both working part cross-sections were compared with a standard triangular working part in the test series.
In order to correspond to at least robustness class 5, a nonwoven with a fabric weight of 320 gsm was produced. The web formation took place with 90 mm long polypropylene staple fibers with a fineness of 4.4 dtex. The draft in the web drafter VST19 was set at 40 %.
The pre-needling took place on a DI-LOOM OD-II 20 with GEBECON® needles. The stitch density was set to 75 S/cm² and the penetration depth was set to 10 mm.
The main needling on a DI-LOOM OUG-II SB 15 took place with all three working part cross-sections, always with a stitch density of 250 S/cm², in tandem operation. The penetration depth was set to 8 mm.
The testing on the density showed that the needling with Tri STAR® and Cross STAR® needles, compared with a standard triangular working part, achieves a nonwoven with a 10 % higher density with a similar fabric weight. This is due to the higher needling efficiency through improved fiber holding in the barb area (Tri STAR®), or the distribution of the barbs over four edges (Cross STAR®). Tri STAR® and Cross STAR® needles are mainly offered with a C barb spacing. The standard needle used has a R barb spacing and therefore offers a lower needling performance with the same needling parameters.
Further tests show that the tensile strength is also positively influenced with the use of Tri STAR® and Cross STAR® needles. The MD:CD ratio becomes more even and the tensile strength in cross direction, in accordance with DIN EN ISO 9073-3, increases by 8 % with the Tri STAR® – and as much as 12 % with the Cross STAR® needles. Accordingly, the elongation is decreasing. A test in accordance with DIN EN ISO 10319 could not be carried out due to the lack of wide clamping jaws. Instead, a test based on the standard 9073-3 was carried out.
During the measurement of the stamping puncture force in accordance with DIN EN ISO 12236, positive results were achieved across all three working part cross-sections. All needle types achieved the robustness class 5 requirements but the Cross STAR® felting needle achieved the best result. The adjacent table shows the results in graph form. The variations in the fabric weight were already excluded here so that the results can be compared directly.
A further series of tests where the penetration depth of the main needling was increased to 12 mm showed that the stamping puncture force drops drastically. Compared to the tests with 8 mm, the overall cross section, with a penetration depth of 12 mm, is reduced by 11 % on average. This means that the GRC 5 can no longer be achieved with the increased penetration depth. A drop in power between 8 and 12 mm penetration depth is also visible during the test on the maximum traction.
Standard triangular 8 mm penetration depth (left) and 12 mm penetration depth (right)
The higher the penetration depth, the more barbs enter the fiber mat and grab the fibers. In this case, it can be assumed that with a penetration depth of 12 mm, damage to the fiber already occurs. This assumption was confirmed in subsequent laboratory analyses. The image on the left figure shows a penetration hole with a penetration depth of 8 mm. The figure on the right shows a needle hole with significantly more damaged fibers with a penetration depth of 12 mm.
Summary and outlook
This series of tests shows the impact of the working part geometry of felting needles on the physical textile properties of geo-nonwovens. All nonwovens created during the test with a penetration depth of 8 mm achieved the robustness class 5 (> 300 gsm fabric weight and > 3.5 kN stamping puncture resistance).
Tests with a 50 % increase in the penetration depth (12 mm), show a significant drop in puncture force, meaning that only a lower robustness class could be achieved here.
A direct comparison between the three different working part geometries generally shows that the Tri STAR® and Cross STAR® felting needles, compared with the standard triangular working part:
- achieve a higher product density with the same machine adjustment
- achieve better tensile strength properties and a higher isotropy
- offer a higher stamping puncture force
Based on these results, we can say that using Tri STAR® and Cross STAR® felting needles reduces the amount of raw material required with the same mechanical properties. Using the correct felting needles therefore has a significant impact on the material costs and, as such, directly on the manufacturing costs.
The Groz-Beckert Technology and Development Center (TEZ) provides the optimal prerequisites. With the knitting technology, weaving technology and nonwoven technology competence centers, it combines all modern surface formation methods under one roof. The latter is also home to the new staple fiber needling line, which enables Groz-Beckert to expand its own process knowledge and continuously optimize its products as well as develop innovations for the nonwovens industry. The line is also available in various configurations to customers and partners for tests and joint projects in both needle and textile development. Focus is also on optimization of customer products and processes. The new line also provides customers with the option to produce small runs. Groz-Beckert’s total supply portfolio helps prevent resource bottlenecks and production stops on its own lines.