Type: Master/ Innovative Technical Textiles
Thesis advisor: Assoc. Prof. Dr. İkilem GÖCEK
Student's name/surname: Gözdenur ULU
Nonwoven textiles have found numerous applications in various fields today. They have the potential for use in automotive industry as upholstery materials, in the healthcare sector for surgical masks and sterile garments, and in the packaging industry for packaging materials, among many other areas. Nonwoven textiles are textile materials produced without the need for weaving or knitting, using different bonding or entangling methods. They are typically obtained by combining fibers through thermal, mechanical, or chemical processes. Nonwoven textiles offer a faster production process compared to traditional textile manufacturing methods such as weaving or knitting. This provides a significant advantage in mass production and meeting urgent demands. Additionally, the production of nonwoven textiles generally requires less labor and energy, reducing costs and offering a more economical option.
The automotive sector constitutes a significant area where nonwoven surfaces are utilized. One of the key requirements expected from textiles used in the automotive industry is acoustic performance. Various sources such as the engine, powertrain, suspension system, tire and road noise, aerodynamic sources, and other noise-vibration sources are known as acoustic sources that affect passengers and drivers in a vehicle during driving. Materials developed for the automotive sector are expected to deliver acoustic performance. Acoustic performance encompasses sound absorption and sound insulation. Acoustics is the branch of science that studies sound. Accordingly, sound is a form of energy that generates sound waves through vibrations. When sound waves emitted from a sound source encounter a surface, they can interact in different ways. One of these interactions is sound absorption by the surface. Sound absorption refers to the partial loss of sound energy within the surface, in contrast to sound reflection. On the other hand, sound insulation is related to sound transmission loss, indicating how much energy of a sound wave passing through a surface is transferred to the second medium. Sound insulation aims to minimize the transmission of sound from one environment to another, while sound absorption aims to maximize the absorption of sound energy by the material within the environment.
The experimental study focuses on improving the acoustic performance of polymer-coated nonwoven textiles that can be specifically used as floor mats or trunk mats in the automotive sector. In line with this purpose, the study has been completed by following the roadmap outlined below:
Firstly, nonwoven textiles with different weights were tested to determine the optimum weight. The term "weight" refers to the mass per unit area and is related to the thickness of the nonwoven textile.
In the next step, two different types of polymer coatings with two different weights were applied to the selected weight (thickness) of the nonwoven textile, and the acoustic performance behavior depending on the type and thickness of the polymer coating was observed.
In the polymer-coated samples, in order to observe which side of the sample was efficient to employ for acoustic performance, the samples were tested twice by flipping the front and back sides.
Based on the selected polymer coating and weight (thickness), different types of fillers were used in the coating to achieve superior acoustic performance. Finally, the selected types of fillers were added to the coating in different proportions to evaluate the effect of filler dosage on acoustic performance.
In the first step, the tested samples for determining the optimum weight of the nonwoven textile were made of PET (polyethylene terephthalate) fibers and had weights of 120, 220, 300, and 400 gsm. These surfaces were produced in the facilities of Hassan Tekstil Sanayi ve Ticaret A.Ş. using the needle-punching method. PET fibers were chosen because they are commonly used fibers in automotive textiles.
In the second step, experiments were conducted with different types of polymer coatings, and the preferred polymers were polypropylene and polyethylene. Both polymers were tested at weights of 400 and 800 gsm. The application of the polymers onto the nonwoven textiles was done using the extrusion coating technique. Only one side of the samples was coated with the polymer using the extrusion technique. The production of the polymer-coated samples was done in the facilities of Hassan Tekstil Sanayi ve Ticaret A.Ş.
In the third step, samples were prepared using different types of fillers in the polymer coating. The fillers used were pumice, barite, dolomite, calcite, and talc minerals. In this step, these fillers were added to the polymer coating at a constant ratio of 30%.
In the final stage, selected types of fillers were added at ratios of 25.5% and 18%, and they were also tested separately to examine the effect of filler amount and determine the optimum quantity.
The acoustic performance analysis was conducted through the tests on sound absorption coefficient and sound transmission loss. The impedance tube method was used as the measurement technique. The sound absorption coefficient test was performed using a two-microphone setup. Samples were prepared and tested separately for the large tube and small tube configurations. The measurements from the large tube and small tube were combined using software to obtain a general distribution. For the sound absorption coefficient measurement, the sample was placed in the apparatus in contact with a rigid support, ensuring there are no gaps. The sound transmission loss test was conducted using a four-microphone setup. The test was performed for two conditions: with and without sound reflection. The samples used for the sound absorption coefficient test were also used for the sound transmission loss test since both of the tests are non-destructive tests, and no additional samples were prepared.
At the end of the thesis study, it was observed that increasing the weight or thickness of nonwoven textiles without coating improved both the sound absorption coefficient and sound transmission loss. As expected, the frequency-dependent variation of the sound absorption coefficient in nonwoven textiles increased. It was observed that the sound absorption coefficient values of nonwoven textiles increased as the frequency increased, indicating better sound absorption performance at higher frequencies. In terms of sound transmission loss, there was no significant frequency-dependent variation. However, in general, the sound transmission loss values of nonwoven textiles were quite low. In determining the appropriate weight, not only achieving high results was considered but also taking the production process and costs into account. As a result, the 220 gsm sample was considered optimal.
When deciding on the type and weight of the polymer coating, both the sound absorption and sound transmission loss properties were evaluated together. The 800 gsm polypropylene-coated nonwoven textile was selected as the optimal choice.
When the samples were tested on both sides, it was observed that the sound transmission loss results remained almost the same, but there were changes in the sound absorption coefficient. This is due to the fact that in the measurement of sound transmission loss, the sound wave passes through the entire material. In other words, the order of the layers does not significantly affect the sound transmission loss. What matters is the type of layers. However, in terms of sound absorption performance, the surface characteristics with which the sound wave interacts are important. Therefore, the absorbed energy will not be the same when the sound wave hits the polymer coating or the nonwoven surface.
In order to differentiate the effect of filler type on the acoustic performance of the samples coated with a polymer containing 30% pumice, dolomite, barite, calcite, and talc fillers, behaviors obtained at low frequencies were specifically examined. Therefore, experiments were conducted using pumice, calcite, and talc in the final stage.
In addition to the experiment conducted with a 30% ratio of talc, another experiment was conducted with an 18% ratio, considering the negative effects caused by talc on the mechanical properties of the final composite nonwoven structure. It was decided that despite meeting the acoustic expectations, talc was not a suitable option due to the undesirable mechanical effects.
Increasing the filler amount had different effects on the samples with pumice and calcite fillers. While increasing the filler amount improved sound transmission loss in pumice-filled samples, it had a negative effect on sound absorption performance. In calcite-filled samples, on the other hand, increasing the filler percentage had a negative effect on sound transmission loss but provided good results in terms of sound absorption. When looking at the overall distribution, fluctuations were observed, especially in the sound absorption graphs, indicating frequency-dependent variations for both fillers. The sound transmission loss results were more stable in this regard.