Comfort-Related Properties of Double-Layered Woven Car Seat Fabrics

Today, the average car has approximately 30 kg of textiles and this is a growing trend, as high-performance and intelligent fibers, yarns, and compound materials are assuming an ever increasing number of comfort, safety, acoustic, and fuel-saving functions with the development of lightweight constructions. Automotive textile materials are used for various applications such as fabrics for passenger safety, airbags, and safety belts, fabrics for passenger comfort of car seat covers and car interior design, as well as for different filters built in motor vehicles [1]. Textiles provide a means of decoration and a warm soft touch to the seats and the interior of the car, but they are also used in more functional applications. Carpets and textile headliners not only contribute to the overall comfort and decor of the interior, but they also play an important role in damping of sound and vibration [2].

The three main fabric-forming structures that have a place in automotive manufacturing are woven, knitted, and non-woven, and among these structures, interior automotive fabrics produced by weaving technology dominate this sector with 51%. Each technology has advantages and limitations, and car manufacturers use consumer's criteria or priorities for different fabrics. Fabrics produced by weaving are mainly dobby weaves having 32% of the share, followed by jacquard weaves with 18% which shows an increasing trend nowadays. Jacquard and dobby-designed fabrics are mainly used in the seat area whereas the dobby woven plain fabric are used in the seat wings, bolsters, and rear covers of the front seats [3].

Some of the most stringent performance requirements in the textile industry are enforced by vehicle manufacturers on their fabric suppliers. The main criteria involved in the development of textiles and components in automotive are: tensile strength, abrasion resistance, pill resistance, air permeability, compression resistance, elasticity, light fastness at high temperatures, stiffness, ease of cleaning, separation force, dimensional stability, flame resistance, certain friction factor, antistatic property, anti-fogging resistant, and resistance to adverse climatic conditions. The others processing requirements are mold ability, susceptibility to compression, sew ability, weld ability, adhesive properties, vulcanizing properties [4,5,6,7]. In car production, the development of components, parts, pieces and materials is orientated by comfort, functionality, safety, and economy and ecology criteria [8]. Comfort in a car is a complex phenomenon and comprises such different aspects as, for example, noise, driving behavior, or ease of handling. The comfort is the first criterion that values the customer, specifically psychological comfort—makes reference to the esthetic aspects—and physiological comfort captured by the view and touch. Thermo-physiological comfort is not just a pleasant feeling while sitting as it improves the performance and concentration of drivers [9].

Automotive car seats are constructed by mainly three elements, such as metal structure, filling material that includes molded polyurethane foam and seat cover, and exterior fabric, foam and support material as reinforcement part. Fabric, which is mostly used for the car seats, is polyester with high abrasion resistance, UV light resistance and cleaning facility combined together with a reasonable price, but the disadvantage is that it neither absorbs nor transports humidity. The inner layer of foam varies from few millimeters to 12 mm, and its cushioning effect absorbs the seat surface irregularities, improves the comfort (compressibility, resilience), and indicates the stitches of the sewing lines with an adequate depth. The reinforcement material has the task to give the dimensional stability to sandwich structure and facilitates the sewing and seam resistance. It can be a polyamide mesh or polyester and a non-woven as well. These three layers, fabric-foam-reinforcement, are fixed by flame laminate [2, 3, 5, 10].

In the literature, there are a few studies examining the handle-and comfort-related properties of the textiles used exteriorly for automobile seat covers. These studies generally focus on the properties of the fabrics that are commonly used in the market. Simiona et.al analyzed the factors that influencing thermal comfort inside the vehicles. In the study, the factors were classified in two classes: measurable factors, including air temperature, air velocity, radiant temperature and relative humidity and personal factors including activity level and clothing insulation. Based on these factors predicted mean vote (PMV) representing the average thermal sensation felt by a group of people sitting in the vehicle was determined [11]. El Homassani et.al investigated the impact of automotive fabrics made of chenille yarns containing PCMs on the properties relating to comfort and concluded that increasing PCMs percentage in the produced fabrics lead to an increase in thermal comfort of car seat fabrics and suggested using PCMs for improving the comfort properties with little energy cost, resulting in reduced air conditioning and fuel consumption [10]. Havelka et.al investigated the performance of car seat fabrics in terms of physiological comfort of sitting, specifically the water vapor resistance and air permeability properties were determined and presented an alternative approach to increase the effectiveness of car seat fabrics through a combination of newly designed middle layer with forced convection achieved by a supplementary suction ventilation device. The results showed that water vapor transport was increased by approximately 20% compared to the standard way of measurement by means of SGHP because of forced air flow in the plane of ribbed-channeled structure in the middle layer of car seat [9]. In a study, the relationship between abrasion and tensile strength properties of automobile seat fabrics was examined. It was found that the increase in the number of cycles reduced the strength of the fabric in a systematic manner [12]. Pamuk and Çeken examined the changes in thickness and weight values of automobile seat cover fabrics after 10,000 rubs of abrasion [13]. Ujevic et.al examined chemical, physical, and mechanical properties of artificial leather lined with a textile fabric on the backside. Samples of artificial leather with woven fabrics on the backside had a greater breaking force than samples of artificial leather with knitted fabric on the backside, in the warp, longitudinal, and weft directions, whereas in other diagonal directions artificial leather with knitted fabric on the backside have higher breaking forces [14]. Cengiz and Babalık compared the ramie blended fabrics with the polyester fabrics in terms of their thermal comfort properties. It was found that cars with ramie blended seat covers were more comfortable based on the subjective evaluation collected from a questionnaire of five questions and these results were consistent with the objective measurements [15]. Jerkovic et.al examined Martindale, Schopper, and Transverse abrasion resistance testers and the methods were compared with each other using P320, P400, P600 abrasive papers [16]. Mecit et.al investigated automotive industry applications for recyclable products. Two different fabrics with different yarn linear densities and constructions were produced with recycled and virgin fibers and their properties were tested in accordance with the automotive requirements. As a result of the study, it was found that the values obtained by using recycled fibers were in the acceptable range in terms of customer satisfaction despite some differences in yarn and fabric test results [17]. Koc et.al investigated the effect of filament cross section on automotive upholstery fabric performance in detail [18]. Avcu and Gürkan investigated the mechanical performances of double-layered woven fabrics used in car seat upholstery by measuring tear strength, breaking strength, and abrasion properties [19].

This study is distinguished from previous studies in terms of the development of double-layered woven fabrics with high mechanical properties aimed to be used use in automotive seat upholstery and investigated in terms of comfort related properties.

Circular bending rigidity values of the fabrics and main effects plot for each process variable were presented in Figure 3. When the results were examined in detail, weaving process variables such as bottom layer, number of interlacing warps in a unit report, and number of interlacing picks per warp had a statistically significant effect on the circular bending rigidity of the fabrics (Table 3). When the importance level of each factor on circular bending rigidity was considered (Table 3), the most effective parameter was found to be the number of interlacing warps in a unit report (Δ=1.228). As the number of interlacing warps in a unit report increased, the circular bending rigidity of the double-layered fabrics also increased due to tight structure obtained. The second important parameter was the number of interlacing picks per warp yarn (Δ=0.694). Increasing both the number of interlacing picks per warp yarn and number of weft skips resulted in longer floats which produced a loose structure in the double-layered woven fabric structure. Thus, increasing the number of interlacing picks per warp yarn caused a decrease in the circular bending rigidity of the double-layered woven fabrics. Based on the bottom layer patterns, the lowest circular bending rigidity values were obtained with panama and the highest values were with ribs 2/2. Since these two patterns are derivatives of plain weave, it is thought that long floats with more than one warp yarn made the fabrics softer which caused low circular bending rigidity values.

Figure 3

Average roughness (R_a), the sum of the maximum profile peak height and the valley depth of the fabrics (R_z) values and main effects plot for each process variable were presented in Figure 4. Based on the bottom layer pattern, the highest roughness values were obtained with panama fabrics. In the production of double-layered woven fabrics, the ones having panama pattern in bottom layer was produced with lower pick density (26 picks/cm) compared to the others, therefore this weft density made the fabrics looser compared to those produced with 28 picks/cm. Thus, surface roughness of the panama fabrics was higher than twill and ribs constructed fabrics. There was no statistically significant effect in the process variables such as bottom layer, number of interlacing warps in a unit report, number of interlacing picks per warp, and number weft skips (Table 4). Increasing the number of interlacing warps in a unit report was expected to reduce the surface roughness (R_z) due to tighter or stiffer structure. However, increasing this process variable increased the maximum peak height and maximum valley depths due to the connection between the top layer warp and the bottom layer weft. As the number of interlacing picks increased, surface roughness values of the fabrics decreased. This rougher fabric surface may be due to the result of the longer floats, however increasing the number of interlacing picks makes the fabric smoother. Increasing the number of weft skips caused longer floats which resulted in higher R_z values as expected.

Figure 4

Average air permeability results of the fabrics and main effects plot for each process variables were presented in Figure 5, and variance analysis and importance level of each factor were given in Table 5. According to the statistical analysis, all weaving parameters had a significant effect on the air permeability properties of the fabrics. Bottom layer had a higher importance level of four and the most air permeable fabrics were composed of panama patterns (Δ=19.23). The reason of supplying more permeable structure with the panama fabrics in bottom layer was due to the low pick density. As it was mentioned, these fabrics had low pick densities that caused more porous structure. As expected, increasing the number of interlacing warps in a unit report affected the air permeability of the fabrics negatively. This parameter had also an important influence on the air permeability of the fabrics (Δ=39.76). Increasing the number of warps in a unit report made a tighter structure which decreased the air permeability of the fabrics. The most important parameter was number of interlacing picks per warp (D=42.35). As the number of interlacing picks per warp yarn was increased, it was expected that the air permeability of the fabrics would be increased due to the increased number of intersections. However, this parameter has to be considered along with the number of weft skips. Increasing both the number of interlacing picks per warp and number of weft skips made longer floats which caused an increase in the air permeability of the fabrics.

Figure 5

Thermal resistance values of the fabrics and main effects plot for each process variable were presented in Figure 6, and variance analysis and importance level of each factor were given in Table 6.

Figure 6

As given in Table 6, thermal resistance rates of the fabrics measured by Hot plate instrument were affected by all parameters investigated here significantly. Bottom layer pattern, number of interlacing picks per warp, number of interlacing warps in a unit report, and number of weft skips were respectively important parameters according to calculated ranking (Δ) values. However, the differences that were caused based on the investigated parameters were very low, and this difference can be neglected at all in practice. The maximum Δvalue was 0.0054 which was approximately 6% of the thermal resistance value, caused by the differences in bottom layer pattern. Generally, thermal resistance of the fabrics is influenced mainly by the fabric thickness and thermal conductivity. As the materials used to produce fabrics were the same, the only parameters that changed the thermal resistances of the fabrics were the weaving parameters.

However, the thickness values of the fabrics were almost the same after stentering. Therefore, the only parameter affecting the thermal resistance of the fabrics was thermal conductivity. As the fabric thickness values and the temperature difference were the same, the only parameter that influenced the thermal conductivity was heat flux (q). Heat flux can be defined as the energy flow per unit area per unit time. Since the material used to produce the all fabrics were the same, the only difference that would make change in the heat flux values would depend on the weaving parameters.

Liquid transport properties of the fabrics measured by MMT were given in Table 7.

When the OMMC values of the fabrics compared with the grading table given by the manufacturing company (0–0.2: very poor, 0.2–0.4: poor, 0.4–0.6: good, 0.6–0.8: very good, >0.8: excellent), all the fabrics were in the “poor” category in terms of moisture management capacity except F1 which was categorized as moisture management fabric according to MMT Manual. All the other fabrics were evaluated as slow absorbing and slow drying or water proof fabrics. Based on the statistical analysis, main effects of the weaving parameters and their importance were presented in Figure 7 and Table 8, respectively.

Figure 7

In this study, double-layered woven fabrics were produced from 450 denier polyester yarns, of which upper layer was produced as Twill 2/2 Z which is commonly used in automotive seat upholstery. Four different parameters were used in the production of fabrics, which were bottom layer pattern, number of interlacing warps in a unit report, number of interlacing picks per warp, and number of weft skips. After weaving stage, the fabrics were fixed on a stenter. Based on the performed measurements, the following results were obtained;

All the investigated parameters except number of weft skips have statistically significant effect on the circular bending rigidity of the fabrics. Among these parameters, the most effective one was found to be number of interlacing warps in a unit report. Since bending rigidity is related to the handle characteristics of the fabrics, increased results indicate stiffer structure. Therefore as the number of interlacing warps in a unit report increases, the number of interlacing picks per warp yarn decreases whereas the circular bending rigidity of the fabrics increases, meaning that the handle of the fabrics gets tighter. Due to the long floats, the lowest circular bending rigidity values were obtained with panama construction while the highest values were supplied by ribs 2/2 structure.

The effects of the investigated parameters on the average roughness (R_a) and the sum of the maximum peak height and the valley depth of the fabrics (R_z) were not statistically significant. However, based on the bottom layer pattern, the highest roughness values were obtained with panama fabrics due to the lower weft density when compared to the twill and ribs pattern fabrics. As the number of interlacing picks increases and the number of weft skips decreases, surface roughness of the fabrics decreases due to the longer floats, which means that the fabrics is becoming smoother.

Air permeability of all fabrics was affected by the investigated parameters significantly. Panama structure as bottom layer weaving construction resulted in a more permeable fabric. Increased number of interlacing warps in a unit report and higher number of intersections of picks per warp caused lower air permeability, however when the number of interlacing picks per warp and number of weft skips are considered together, the effect changes to increased air permeability due to the longer floats.

All the weaving parameters influenced had a statistically significant effect on the thermal resistance of the fabrics. However, this statistical significant effect was very low in practical usage. Thus, the effects of the weaving parameters were suppressed after stentering process.

In terms of liquid transport properties measured by MMT instrument, all the fabrics were in the “poor” category except fabric coded as F1.