Effect of hot-air temperature on the properties of high-strength and high-elongation geotextiles

Abstract

Short-fiber needle punched nonwoven geotextiles are widely used in civil, hydraulic, and transportation engineering, owing to their simple production process and low cost. However, limitations in raw fiber materials and manufacturing processes lead to defects, such as loose structure and low strength. Currently, it is an urgent issue to develop a short-fiber needle punched geotextile with a tight structure, high strength, and elongation. In this study, polypropylene/ethylene-propylene side-by-side (PP/ES) fiber webs were prepared by needle punching and thermal bonding, offering a convenient, cost-effective, and scalable method of producing high-strength nonwoven geotextiles. In the work presented here, PP/ES fiber webs were prepared by needle punching reinforcement and thermal bonding, offering a convenient, cost-effective, and scalable method for producing high-strength nonwoven geotextiles. First, studying the fiber properties at different temperatures serves as a basis for understanding the performance changes of the geotextiles. Investigation of the resulting PP/ES geotextiles revealed that, under thermal bonding, PP fibers and ES fibers bonded with each other, forming a closely bonded fibrous network structure. Tensile strength testing revealed a tensile strength of PP/ES short-fiber nonwoven geotextiles of up to 1057 N, confirming excellent mechanical properties. Additionally, with an increase in thermal air bonding temperature and the addition of thermal press bonding, the effective aperture of the geotextiles decreased slightly, while the air permeability and vertical permeability coefficients remained at a minimum of 760 L/(m²/s) and 3.11 mm/s, indicating favorable air and water permeability. This study provides an effective strategy for the preparation and application of high-strength nonwoven geotextiles.

Keywords

Nonwoven geotextile needle punching reinforcement thermal bonding mechanical properties tight structure

With increasing soil erosion and infrastructure development, the market demand for geomaterials is increasing by the day. At present, the global geomaterials market has reached a considerable scale.^1,2 As a vital engineering material, nonwoven geotextiles play a significant role in construction projects.^3–5 Nonwoven geotextile is a product manufactured through fiber web formation and consolidation processes, and is characterized by low cost,^6,7 simple processing,^8–10 and excellent water permeability^11,12 compared with traditional geotextiles. In recent years, the development of high-strength nonwoven geotextiles for engineering applications has been widely researched.^13–15 Smith and Brown noted that the lack of load-bearing capacity and durability of traditional geotextiles is driving the need for high-strength materials.¹⁶ Zhang and Wang noted that the improvement in load-bearing capacity and ductility of nonwoven geotextiles, especially their potential in high-strength applications, supports the current demand for high-performance materials.¹⁷

At present, nonwoven geotextiles are mainly prepared by filament spun-bond needle punching and short-fiber needle punching. Filament spun-bond needle punching involves spinning polymers into a web by spunbonding, followed by reinforcement through needle punching. In short-fiber needle punching, a fiber web is formed by carding after opening and mixing the short fibers, which is then strengthened through needle punching and other consolidation methods.^18,19 The manufacturing process of nonwoven geotextile is an important influence on its mechanical properties. Currently, the commonly used nonwoven geotextile is made of filament, which has high strength and excellent water filtration performance, but it has the problems of high production cost, single species, and difficulty in adjusting for width.^20,21 Traditional short-fiber nonwoven geotextiles effectively address these issues, but their strength is inferior to that of the filament geotextiles; this imposes many restrictions on their practical production applications.^22,23 Peng and Zhu discussed the aging mechanism of polypropylene nonwoven geotextiles in high-temperature acidic conditions and in the presence of heavy metals, especially the loose structure but low strength. Koerner and Koerner discussed the filtration role of nonwoven geotextiles in geotechnical engineering and pointed out the strength and stability problems of these geotextiles in practical applications, owing to their loose structure.²⁵

At present, geotextiles are mainly divided into two kinds: polyethylene terephthalate (PET) and polypropylene (PP). For technical reasons, China mainly produces PET geotextile. PET geotextile has the advantages of high mechanical strength and good ultraviolet (UV) and aging resistance, but its alkali resistance is poor; it is difficult to satisfy increasingly serious geological pollution environment requirements using PET geotextile. PP fiber, because of its high strength, abrasion resistance, low-temperature resistance, and excellent resistance to acid and alkali, has become an important alternative material in acid and alkali environments and cold conditions. However, traditional PP staple-fiber needle punched geotextile is limited in its application, owing to its large thickness, fluffy structure, and low strength. In contrast, PP filament geotextile has high strength and good water filtration, but the production technology is not mature and the cost is high. To enhance the mechanical properties and permeability, two-component PP/ethylene-propylene side-by-side (ES) fibers were used to prepare nonwoven materials by needling and thermal bonding to meet the performance standards of PP spun-bond filament geotextiles.

The question of how to significantly improve the mechanical properties of short-fiber nonwoven geotextiles, drawing on the maturity and simplicity of the production process, has become urgent, because of the need to solve the problem. Several researchers have improved the mechanical properties of short fibers through modification methods, thereby endowing geotextiles with excellent mechanical performance. Joshi et al.²⁶ modified polypropylene–clay nanocomposites and prepared composite filaments using hexadecyl trimethyl ammonium bromide by ion exchange reaction and characterized the composite filaments with the addition of modified clay to improve the tensile strength and modulus.

Wu and Li²⁷ prepared high-density polyethylene (HDPE) and modified graphene-reinforced PP composite fibers through a melt extrusion process. Their study showed that, by controlling the content of HDPE and graphene, the tensile strength of PP fibers reached 2.46 × 10² MPa. Fiber modification enables high fiber strength to be achieved to meet engineering-specific demands. However, modification requires sophisticated experimental equipment and chemicals, leading to a significant increase in production costs. Moreover, the use of chemical agents for modification usually has negative environmental effects,^28,29 making it difficult to achieve green and sustainable product development.

Nowadays, improving the technology of reinforcing fiber webs has become a research topic to realize high strength in nonwoven geotextiles. A few researchers have developed high-strength nonwovens with excellent performance by adding thermal bonding on top of the needle punching reinforcement; this makes the fiber web structure more compact. For example, Lin et al.³⁰ used a thermal press bonding process to combine polyester fabrics, fiberglass fabrics, and PP woven edges into a geotextile. They found that the thermal press bonding process not only combines the fabrics but also enhances the tensile strength and water penetration resistance of the geotextile. Li et al.³¹ observed that the mechanical properties of needle punched Kevlar–PET–low-melting-point PET nonwoven geotextiles were improved after thermal treatment, and pointed out that the proportion of fiber blend significantly affects the performance of geotextiles. Lin et al.³² combined PET filaments on geotextiles by thermal press bonding, indicating that, in addition to the needle punching process parameters, the thermal press bonding temperature is, likewise, a key factor, affecting the mechanical properties of geotextiles. However, the discussion of most studies on thermal bonding techniques is limited to the effect of process parameters, without an in-depth discussion of fiber properties and structural changes in the geotextile. In addition, there are relatively few reports on the use of thermal bonding technology to enhance the mechanical properties of short-fiber nonwoven geotextiles.

The aim of this study was to develop a short-fiber nonwoven geotextile with high strength and good ductility to address the shortcomings of conventional short-fiber geotextiles in terms of strength and structural stability. A combination of needling and thermal bonding was employed to reinforce the PP/ES fiber network. By adjusting the hot-air bonding temperature and controlling the incorporation of thermocompression bonding, we successfully prepared nonwoven geotextiles with tight structure and excellent mechanical properties. In addition, we investigated the properties of PP fibers at different heat treatment temperatures, as well as the thickness, apparent density, effective pore size, and vertical permeability of the nonwoven geotextile, to further optimize the comprehensive performance of the geotextile.

Materials and methods

Materials

The following raw materials were used in the experiments. PP fiber was purchased from Jinan Jiaxin Fiber Co., Ltd., with a thread density of 8 dtex, a length of 78 mm, and a melting point of 172°C. ES fiber was also purchased from Jinan Jiaxin Fiber Co., Ltd., with a thread density of 6 dtex, a length of 53 mm, and a skin–core structure, with a skin layer of polyethylene (PE), and a melting point of 134°C. The core of the ES fiber was PP, and the melting point was 171°C. Hot-air nonwoven fabric with a surface density of 28 g/m² is offered by Beijing Jinglan Nonwovens Co.

Preparation of PP/ES geotextile

The preparation process is illustrated in Figure 1. Initially, PP and ES fibers are mixed at a fiber ratio of 9:1 using a carding machine to open and blend the fibers. Subsequently, the fibers are formed into a web through carding and lapping, followed by the use of a model MNP-3000 needling machine, and passed through a needling fiber mesh with a needling density of 35 needles/cm², a needling depth of 7 mm, and a frequency of 800 needles/min, to obtain a short-fiber needled geotextile with a surface density of 200 g/m². The needled geotextile is then reinforced by thermal air bonding (TAB) using hot-air rollers (model, HAF-2000) at temperatures of 130°C, 135°C, 140°C, 145°C, and 150°C, at a hot-air roller speed of 8 m/min. Finally, the geotextiles undergo thermal press bonding (TPB) through a thermal roller at 100°C to achieve nonwoven geotextiles with varying properties. Additionally, a control sample of PP short-fiber needle punched geotextile without ES fibers was prepared under the same needle punching conditions, for comparison.

Thermal treatment of fibers

The thermal treatment of the fibers begins by setting the oven temperature to the desired thermal treatment temperature. Several fibers, both in a crossed state and in a free state, are then placed in a petri dish, which is subsequently placed in the oven for thermal treatment for 5 min. Once the thermal treatment is completed, the petri dish is removed and allowed to cool naturally to room temperature.

Characterization

The melting temperature (T_m) of the fibers was tested utilizing a Netzsch DSC200 F3 differential scanning calorimeter. The fiber sample (5–10 mg) was placed evenly in the sample tray to avoid spillage or uneven distribution. Then the lid was pressed gently, to ensure good thermal contact, and sample was placed in a nitrogen atmosphere and heated from room temperature to 300°C at a rate of 10°C/min.

The strength of thermal bonding between fibers (Figure 2(d)) and the strength of pulling out fibers (Figure 2(e)) were tested using a YM-06A electronic single-fiber strength tester.

Measurements were made using a Bruker D8 Discover X-ray diffractometer with a set voltage of 40 kV and a current of 40 mA, scanning the fibers over a 2θ angle range of 5°–40°, to investigate changes in the crystallinity of PP fibers after heat treatment under different temperature conditions. The formula is

X_{c} = \frac{I_{c}}{I_{a} + I_{c}} \times 100 %

(1)

where X_c is the crystallinity of the fiber; I_c is the crystalline sharp diffraction peak integral intensity; and I_a is the amorphous diffuse scattering peak integral intensity.

Wide-angle X-ray diffraction measurements of the fibers were made using a Bruker D8 Discover X-ray diffractometer at a wavelength of 0.154 nm. One-dimensional diffractograms were obtained by integrating the crystal surfaces according to the measured two-dimensional diffractograms and were fitted to the peaks for peak-splitting treatment. The degree of orientation is obtained as

F_{c} = \frac{180 - H}{180} \times 100 %

(2)

where F_c is the orientation of the fiber; and H is the one-dimensional curve half-height width.

The breaking strength and elongation at breakage of the fibers, as well as the thermal bonding strength between fibers, were tested using a YM-06A electronic single-fiber strength tester, according to the method given in standard GB/T 14337-2008.³³

Several PP and ES fibers were placed in an oven that had reached the specified temperature for heat treatment; after a while, they were removed and cooled at room temperature for 10 min for length measurement.

A scanning electron microscope (Phenom XL) was used to observe the surface morphology of the nonwoven geotextile: first, the surface of the sample was sprayed with gold, then the sample was placed in a specific position, the hatch was closed, and the sample was allowed to have a certain vacuum. Then the sample was placed underneath the electron gun and it was switched on, to observe the effect on the fiber structure of different processing conditions.

The thickness of the nonwoven geotextile was tested using an FY080 thickness tester with a presser foot area of 25 cm² and a pressure of 2 kPa.

The areal density of geotextile is measured by using relevant instruments, under the provisions of GB/T 13762-2009,³⁴ using the formulas

G = \frac{m}{A} \times 10^{4}

(3)

G_{1} = \frac{G_{2} - G_{3}}{G_{2}} \times 100 %

(4)

where G is the face density of the geotextile (g/m²); m is the mass of the geotextile (g); A is the area of the specimen (cm²); G₁ is the rate of increase of the face density of the fibers (%); and G₂ and G₃ are the face densities of the geotextile without the thermal reinforcement process and after the thermal reinforcement process (g/m²), respectively.

A YT030 type geotextile effective aperture tester was adopted, using the standard GB/T 17634-1998 method for testing.³⁵

The air permeability of the nonwoven geotextile was tested using a YG461 air permeability tester, according to GB/T5453-1997,³⁶ with a test area of 20 cm² and a test differential pressure of 100 Pa.

Vertical permeability is an important basis for reflecting the drainage effect of geotextile. Adopting the standard GB/T 15789-2005,³⁷ the relevant data were obtained using an FY020-type geotextile vertical permeability tester. The formula is

k = \frac{V R_{T}}{A t} \times \frac{δ}{H}

(5)

where k is the vertical permeability coefficient of the geotextile (mm/s); R_T is the correction factor for water temperature at 20°C; V is the volume of water (mm³); A is the area of the test over water (mm²); t is the time to reach the volume of water V (s);

δ

is the thickness of the geotextile specimen (mm); and H is the difference in the head of water on both sides of the geotextile specimen (mm).

The tensile strength and elongation at breakage of the geotextile fabrics were tested using a universal strength tester (Instron 5969, Instron, USA) in accordance with the method of standard GB/T 3923.1-2013,³⁸ by taking a specimen of size 5 cm × 25 cm and testing it using a tensile tester (with a clamping distance of 200 mm and a tensile speed of 100 mm/min).The tensile breaking strength and elongation of the geotextile, as well as the slope change of the breaking curve, were studied under different temperature conditions. The formula is

K = \frac{F}{L} \times 100 %

(6)

where K is the slope of the tensile rupture curve; F is the tensile rupture strength of the geotextile (N); and L is the rupture elongation of the geotextile (mm).

The tearing strength of the geotextile was measured according to the standard GB/T 13763-2010,³⁹ using a YT010-1000 electronic geotextile comprehensive strength machine.

The breaking strength of the geotextile was measured by using a YT010-1000 electronic geotextile comprehensive strength machine according to standard GB/T 14800-2010.⁴⁰

Results and discussion

Properties of fibers after thermal treatment

The thermal properties of PP and ES fibers have a significant effect on the interfiber thermal bonding effect and the structure of nonwoven geotextiles. Therefore, we investigated the melting behavior of PP and ES fibers. Figure 2(a) shows the thermal profiles obtained from the differential scanning calorimetry analysis of PP and ES fibers. The figure indicates that the melting temperature range of PP fibers is 150.4°C to 180°C, with a peak at 172°C. ES fibers, which are composed of PE and PP components in a skin–core structure, exhibit two melting peaks. From the figure, it can be seen that the melting temperature range of the PE component of the skin layer in the ES fiber is 127°C to 139°C, with a peak at 135°C, while the melting temperature range of the PP in the core layer is 155°C to 176°C, with a peak at 169°C. In the process of TAB, thermal bonding is affected by the melting of the PE component. Therefore, the heating temperature should be higher than the melting temperature of the PE component, but not exceed the melting temperature of PP, to prevent destruction of the fiber body in the PP/ES geotextiles.

After thermal treatment at high temperatures, bonding was observed between PP and ES fibers as well as between ES fibers themselves. The thermal bond strength between fibers at different thermal treatment temperatures is shown in Figure 2(b), which visually reflects the variation of thermal bond strength with thermal reinforcement temperature in PP/ES nonwoven geotextiles. PE forms the skin layer in ES fibers, and is the main component of thermal bonding between fibers. Therefore, the melting effect of PE in the thermal treatment process plays a decisive role in the thermal bonding strength between fibers. When the thermal treatment temperature is on the low side, the melting effect of the PE component is insufficient and the bonding effect is weak where the crystallization is not perfect. With the increase in thermal treatment temperature, the PE part of the ES fiber skin layer melts sufficiently and the bonding point between the fibers is firmly established; this increases both the bonding strength between the PP/ES fibers and that between the ES fibers. When the thermal treatment temperature reaches 140°C, the PE component is fully melted, since the temperature has exceeded the melting point of PE; at this time, the bonding strength between the fibers is unchanged. It is worth mentioning that the thermal bonding strength between PP/ES fibers was again significantly increased, to 0.66 cN. The thermal properties of PP and ES fibers have a significant effect on the interfiber thermal bonding effect and the structure of nonwoven geotextiles at a treatment temperature of 150°C. This may be because 150°C is approaching the melting temperature of PP fibers, which leads to partial melting of the PP fibers and consequently enhances the bonding strength between the PP/ES fibers once again.

The role of the fiber thermal bonding is limited, as the ES fibers in the nonwoven geotextile account for 10% of the nonwoven geotextile. Excluding excessive pullout strength, the pullout fiber strength reflects, to a certain extent, the needle entanglement force of the geotextile. The effect of thermal treatment temperature on the pullout fiber strength in nonwoven geotextiles is illustrated in Figure 2(c). The pullout fiber strength before the thermal treatment temperature of 150°C is about 12.5 cN. No significant difference was detected in the distribution of pullout fiber strength for the samples after thermal treatment at different temperatures. This proves that the thermal treatment temperature does not affect the pullout fiber strength in nonwoven geotextiles within a certain temperature range. However, the pullout fiber strength increased from 13 cN to 21.06 cN with the increase in thermal treatment temperature from 145°C to 150°C. This may be because the PP fibers are partially melted at 150°C, which leads to a significant increase in pullout fiber strength by overcoming not only the needle punching entanglement strength but also the fiber melt bonding strength during the fiber pullout process.

PP fiber accounts for 90% of PP/ES nonwoven geotextile; as such, its performance after high-temperature thermal treatment reflects, to a certain extent, the change in performance of the geotextile after thermal reinforcement. With a view to investigating the optimum TAB temperature for the preparation of PP/ES nonwoven geotextiles, the crystallinity, orientation, thermal shrinkage, breaking strength, and elongation of PP fibers after thermal treatment were investigated. Figure 3(a) demonstrates the X-ray diffraction (XRD) spectra of PP fibers after different thermal treatment temperatures. All the XRD diffraction curves show characteristic peaks at the angles 2θ = 14.1°, 16.8°, and 18.6°, which correspond to the (110), (040), and (130) crystal planes of the PP crystal.⁴¹ The intensity of these peaks grows gradually with the increase in the thermal treatment temperature, indicating that the crystallinity of PP fibers is significantly increased after thermal treatment. This is caused by thermal treatment: the original crystalline molecular chain segments with defects gain a certain amount of energy and enter the crystal lattice to form secondary crystallization, resulting in an increase in crystallinity.⁴² The orientation of PP fibers at different thermal treatment temperatures is shown in Figure 3(b). With the increase in the thermal treatment temperature, the crystallinity of PP fibers increases significantly up to 62.76%, owing to the further enhancement of the molecular chain mobility and the increase of the perfection of the crystalline structure.⁴³ However, the degree of orientation of PP fibers showed a significant negative correlation with the thermal treatment temperature. Compared with the fibers without thermal treatment, the degree of orientation of PP fibers after thermal treatment at 150°C was reduced by 26.67%. This may be a result of the PP fibers being in a free state during thermal treatment, with the original crystalline and amorphous zones partially unoriented.

Figure 3(c) demonstrates the fracture strength and elongation of PP fibers at different thermal treatment temperatures. The breaking strength of PP fibers gradually increased, from 3.88 cN/dtex to 5.38 cN/dtex, after thermal treatment at high temperatures, and the elongation at breakage also increased, from 22.01% to 34.07%. The increase in fracture strength and elongation with increasing thermal treatment temperature primarily results from changes in fiber crystallinity and orientation. The crystallinity of PP fibers rises with thermal treatment temperature, resulting in a larger zone of aligned regularity in the fiber and a perfect crystalline structure. The overall macromolecular chain is less prone to movement; this endows the fibers with better tensile resistance. Moreover, the reduction in the orientation of PP fibers indicates an increase in the randomness of the molecular chain segments in the fibers and a significant increase in flexibility; this makes it easier for the fibers to deform without breaking. This observation suggests that thermal treatment of PP fibers at high temperatures contributes to the increase in crystallinity and decrease in orientation of PP fibers, thereby increasing the fracture strength and elongation of PP fibers.

The effect of thermal treatment temperature on PP fiber length is shown in Figure 3(d). The length of the fibers decreased significantly as the thermal treatment temperature increased from room temperature to 150°C. This may be due to the disorientation of a large number of highly oriented amorphous regions in the fibers after thermal treatment, along with a gradual shift in the crystalline morphology to a stable crystalline form. Thermal shrinkage behavior of the fibers occurs under the effect of internal stresses from constrained molecular chains and a few crystalline defects. The crystallinity and grain size of the fibers increased gradually with the increase in thermal treatment temperature, resulting in an increase in the degree of shrinkage of the fibers.

Properties of nonwoven geotextiles

The fibers on the surface of the nonwoven geotextile without thermal reinforcement are distributed haphazardly and are cross-tangled with each other; these are the typical characteristics of a needle punching reinforcement nonwoven fiber structure, as shown in Figure 1. Figure 4 clearly illustrates the surface micromorphology of the nonwoven geotextile after thermal bonding.^44,45 It can be clearly observed that, on the basis of the cross-entanglement of the fibers, thermal fusion bonding points appear between some fibers.⁴⁶ The emergence of thermal bonding points confirms that the addition of a thermal bonding process to the needle punching reinforcement when producing the nonwoven geotextile provides a compact fiber structure; this provides an effective guarantee for the development of high strength in the nonwoven geotextile. In addition, by comparing Figure 4(a) to (e), it can be found that the thermal bonding points in the surface of the nonwoven geotextile increase continuously with the increase of the TAB temperature from 130°C to 150°C. Consequently, the combination between the fibers becomes more compact, and the mechanical properties of the nonwoven geotextiles are improved by the increase in interfiber compactness. Additionally, a comparison of the scanning electron micrographs shown in Figure 4(f) to (j) of the surface of the nonwoven geotextile reinforced by hot air and the nonwoven geotextile reinforced by hot-air hot rolling shows that the pores of the nonwoven geotextile become smaller after the hot-rolling reinforcement; this may be because the nonwoven geotextile is subjected to temperature and pressure after the hot rolling treatment, and the adhesion between the fibers is further strengthened, so that the structure of the nonwoven geotextile becomes more compact. In summary, the fact that the PP/ES nonwoven geotextile structure becomes more compact is attributed to the TAB and TPB processes.

The variation of thickness with temperature of the nonwoven geotextile after thermal reinforcement is shown in Figure 5(a). The thickness of the nonwoven geotextile increased with the elevation of temperature after TAB but showed a decrease at 150°C. This is because the rising temperature of the TAB causes the fibers to gradually shrink and come closer to each other, thus increasing the thickness of the nonwoven geotextile. However, fiber shrinkage or excessive melting occurs at overly high temperatures, reducing the thickness of the nonwoven geotextile instead. Figure 5(b) demonstrates the trend of areal density of nonwoven geotextile with the TAB temperature. The nonwoven geotextile’s areal density presents a tendency to increase with the addition of TAB temperature from room temperature to 150°C; this mainly originates from the change in the area of the nonwoven geotextile. After the thermal bonding process, the fibers in the nonwoven geotextile show different degrees of contraction, and the fiber shrinkage rate gradually declines with the increase in temperature (Figure 3(d)). As the temperature increases, the area of the thermally bonded nonwoven geotextile decreases, without a significant change in mass, so that the areal density appears to markedly increase. Furthermore, it is worth mentioning that the addition of a TPB process after the nonwoven geotextile is subjected to mechanical pressure and temperature from the rolls of the press, resulting in a closer bond between the fibers. As a result, the thickness of the nonwoven geotextile was significantly reduced after TPB, while the surface densities appeared to be elevated.

Figure 6(a) shows the variation of the effective aperture of nonwoven geotextile with TAB temperature. The effective aperture decreases from 0.147 mm to 0.115 mm with the increase in the TAB temperature from room temperature to 150°C; this arises from the change in the structural tightness of the fiber web.⁴⁷ In addition, TPB reduces the thickness of the geotextile and tightens the fiber web so that the effective aperture is further reduced.

Geotextiles with excellent air permeability and water permeability effectively intercept sand and soil loss in engineering applications; this plays a role in reinforcing the soil and protecting the ecology.^48,49 The effect of TAB temperature on permeability and vertical permeability is illustrated in Figure 6(b). As expected, as in Figure 6(c), the change in air permeability with the increase in TAB temperature is consistent with the change in effective aperture, which gradually decreases with the increase in TAB temperature for nonwoven geotextiles.⁵⁰ Conversely, the vertical permeability coefficient of the nonwoven geotextile showed the same decreasing trend with the increase in TAB temperature. This may be because PP/ES nonwoven geotextiles prepared at higher TAB temperatures have a compact structure and smaller effective aperture, resulting in poorer air permeability and vertical permeability. There was also a decrease in the effective aperture, air permeability, and vertical permeability coefficient of the nonwoven geotextile with the addition of TPB, compared with the nonwoven geotextile that underwent the TAB process. This implies that TPB causes changes in the effective aperture and fiber web structure of the nonwoven geotextile, leading to a decrease in permeability and vertical permeability.

The mechanical properties of geotextiles are of great significance for their application in practical engineering. Geotextiles with high tensile strength, small porosity, and a regular overall structure can easily withstand the pressure of soil and water.⁵¹ Geotextiles with high tensile strength, small porosity, and a regular overall structure can easily withstand the pressure of soil and water. Therefore, the tensile strength and elongation at break of PP/ES nonwoven geotextiles were evaluated and the tensile fracture mechanism was analyzed. Figure 7(a), (a1), and (a2) are the strength–displacement curves derived from the tensile strength testing of PP/ES nonwoven geotextile, ES thermal nonwoven geotextile, and PP/ES needle punched geotextile, respectively. It was found, by comparison, that the tensile fracture curves of PP/ES nonwoven geotextiles could be roughly divided into two parts, in which part 1 was similar to the tensile fracture curves of ES thermal nonwoven geotextiles, while the tensile fracture curves of PP/ES needle punched geotextiles were close to part 2. For further validation, ES thermal nonwoven geotextiles and PP/ES needle punched nonwoven geotextiles were compared by using mathematical analysis to derive the tensile rupture curves, as shown in Figure 7(b), (b1), and (b2). It can be further observed that part 1 is in high conformity with the slope–displacement curves of ES thermal nonwovens; similarly, part 2 has a high degree of similarity with the slope–displacement curves of PP/ES needle punched nonwovens. This proves that the fracture mechanism of PP/ES nonwoven geotextiles during tensile fracture is close to that of ES thermal nonwoven geotextiles and PP/ES needle punched nonwoven geotextiles. Combined with the tensile fracture mechanism of thermal nonwoven and short-fiber needle punched nonwoven geotextile, the following can be observed. During the stretching process of PP/ES high-strength geotextile, the tensile strength acts first on the thermal bonding point between the fibers. The geotextile structure becomes more compact under the action of the tensile strength, and the tensile strength appears to increase at a faster rate (the slope increases rapidly). Subsequently, the fiber bonding point is destroyed: at this time the fiber entanglement point caused by needle punching is subjected to tensile strength, and the increase in tensile strength is slowed down (the slope rises slowly). The fibers are subjected to tensile stresses when the fiber entanglement point is broken, and the tensile strength increases again at a faster rate (the slope increases with increasing speed). Finally, mass fracture of fibers occurs when it is difficult for the fiber strength to withstand more tensile strength. At this time, the structure of the geotextile is destroyed, and the tensile strength and its slope decrease sharply.^52,53

The transverse tensile breaking strength and elongation of part 1, part 2, and the total curve in PP/ES nonwoven geotextile after TAB are shown in Figure 7(c). The results indicate that the tensile strength and elongation at breakage of part 1 was gradually enhanced with the increase in TAB temperature. This is because the interfiber thermal bonding strengths for both parts show an upward trend along with the increase in the TAB temperature (Figure 2(b)), and the tensile strength is provided by the interfiber thermal bonding points during the tensile fracture process in part 1. The tensile strength and elongation at breakage for part 2 increased from 645 N and 85% to 997 N and 95% as the TAB temperature increased from 130°C to 150°C. The tensile strength is borne by both the needle punching entanglement points and the fibers themselves during the tensile breaking process (part 2). Both breaking strength and elongation at breakage of PP fibers improved with increasing temperature (Figure 3(c)), while the needle punching entanglement strength was unaffected by the temperature change (Figure 2(c)). It was found that an increase in the TAB temperature leads to an improvement in the tensile strength and elongation at breakage of the PP/ES nonwoven geotextile, by combining the trends of tensile strength and elongation at breakage with temperature changes in part 1 and part 2.

Figure 7(d) and (e) demonstrates the tensile strength and elongation at breakage of PP/ES nonwoven geotextile in different parts of the transverse direction after blank conditions, the addition of TAB (145°C), and the addition of TPB (100°C). The tensile strength of nonwoven geotextile increased from 730 N to 1004 N after TAB, and the elongation at breakage increased from 93% to 100.75%, compared with the blank sample. This is because some of the fibers are thermally bonded in the nonwoven geotextile after TAB, and the fiber web structure becomes more tightly packed. The tensile strength and elongation at breakage of PP fibers in nonwoven geotextiles are greatly increased, resulting in the elevation of tensile strength and elongation of nonwoven geotextiles. In addition, it can be seen that the tensile breaking strength and elongation at breakage of the nonwoven geotextile increase again after increasing the amount of hot-rolled reinforcement. The breaking strength reached 1057 N, and the elongation at breakage increased by 11%. The tensile strength and elongation at breakage of part 1 did not exhibit large variations after adding TPB, indicating that it did not result in a large increase in the interfiber bonding strength. The improvement of tensile strength and elongation at breakage in part 2 is attributed to the fact that the mechanical pressure and temperature during the TPB process make the fiber web structure more compact, leading to a tighter fiber web structure, which again enhances the mechanical properties of the nonwoven geotextile. The addition of the TPB process causes an increase in the tensile strength and elongation at breakage of PP/ES nonwoven geotextiles by combining the trends of tensile strength and elongation at breakage with temperature, in parts 1 and 2.

We added significance analysis to enhance the scientific validity and reliability of our data. Specifically, we used the t test to compare the tensile fracture strength, effective pore size, and permeability coefficient of the materials before and after heat treatment. The combination is given in Table 1.

Table 1.

Significance test of heat treatment process on strength, penetration, and effective pore size

	Average value		t	p
	Before thermal processing	After thermal processing	t	p
Tensile strength, N	740	1027.5	−9.45	0.003
Effective aperture, mm	0.154	0.105	20.247	0.002
Vertical permeability coefficient, mm/s	4.98	4.02	35.796	0.001

As shown in Table 1, the tensile breaking strength, after testing gives p = 0.003, which is less than 0.01; the results are very significant, indicating that the thermal bonding process significantly increased the nonwoven geotextile tensile breaking strength. The effective pore size after testing gave p = 0.002, which is less than 0.01; the results are very significant, indicating that, after the thermal bonding process, the effective pore size of the nonwoven geotextile is significantly reduced. For the coefficient of permeability, in the test, p = 0.001, which is less than 0.01, and the result is very significant, indicating that the effective pore size of the nonwoven geotextile is significantly reduced in the thermal bonding process. This clearly demonstrates the superiority of PP/ES geotextile performance after the heat treatment process.

The burst strength test and tear resistance test are both important methods for evaluating the performance of geotextiles, and are commonly used to assess the performance of geotextiles under different stress conditions.^54,55 The burst strength and tear strength of thermally bonded nonwoven geotextiles are given in Table 2. The fracture of the fiber web and the loosening of the fiber web are the main events that occur in nonwovens during the breakage process. The burst strength of the nonwoven geotextile after TAB increased from 1954 N to 2491 N along with elevation of the TAB temperature from 130°C to 150°C. This is closely related to the increase in the fiber strength and the densification of the fiber web structure after TAB. Fiber strength and fiber web structure are also crucial factors for tear strength. Therefore, the tear strength of the nonwoven geotextile also shows an increasing trend after TAB. Furthermore, it was found that the addition of TPB resulted in an improvement in the burst strength and tear strength of the nonwoven geotextile. This is because TPB leads to a tighter fiber web structure, which results in the nonwoven geotextile being less susceptible to damage through bursting and tearing.

Table 2.

Burst and tear strength of geotextile after thermal bonding

Samples with different parameters	Burst strength, N	Tear strength, N
Samples with different parameters	Burst strength, N	Machine direction	Transverse direction
Blank	1760	298	308
Thermal air bonding
130	1954	311	338
135	2137	347	354
140	2213	361	411
145	2326	397	445
150	2491	410	462
Thermal air bonding plus thermal press bonding
130	1987	323	356
135	2181	355	373
140	2252	394	430
145	2386	422	461
150	2524	456	497

Through these tests, in this study, we synthesized the market, production, and cost. The best processing parameters are: PP/ES fiber ratio, 90/10; needling depth, 7 mm; frequency, 450 needles/min; hot-air temperature, 145°C; hot-air time, 5 min; hot rolling temperature, 100°C; hot rolling time, 4 min; and hot rolling pressure, 10 kg. The PP/ES nonwoven geotextile was prepared with a sample size of 200 mm × 50 mm, and the relevant properties of the PP/ES nonwoven prepared by the best process were comparable with those of GB/T 17639-2008 Geosynthetics: Synthetic filament spun-bond and needle punched nonwoven geotextiles.⁵⁶ The relevant properties of the PP/ES nonwoven fabric prepared by the best process were compared with those of GB/T 17639-2008 Geosynthetics: Synthetic filament spun-bond and needle punched nonwoven geotextiles⁵⁶ and GB/T 17638-2017 Geosynthetics: Synthetic staple fibers needle punched nonwoven geotextiles,⁵⁷ as well as the PP spunbonded filament geotextile of a certain company. The results are given in Table 3. These tests demonstrate the excellent mechanical properties of PP/ES nonwoven geotextile. Three factors may be used to explain this:

Table 3.

PP/ES fiber nonwoven geotextile properties and national standard

Evaluation indicator	Subject preparation	Polypropylene spun-bond filament geotextile	Staple-fiber needle punched geotextile, national standard	Spun-bond filament geotextile, national standard, surface density
Surface density, g/m²	230	200	200	200
Thickness, mm	2.2	1.6	1.6	1.6
Rupture strength, kN/m	16.8	13	8	10
Elongation at break, %	100.75	65	20–100	40–80
California bearing ratio top break strength, kN	2.386	2.0	1.4	1.9
Tear strength, N	461	500	200	280
Vertical permeability coefficient, mm/s	4.03	2.0	K × (10⁻¹ to 10⁻³)K = 1.0–9.9	K × (10⁻¹ to 10⁻³)K = 1.0–9.9
Effective aperture O90, mm	0.11	0.20	0.07–0.2	0.05–0.2

The fiber web after needle punching reinforcement forms a disorganized and tangled structure.

Thermal bonding points are formed after TAB, building a compact fiber web structure.

Mechanical pressures and temperatures in TPB tighten the fiber web structure again.

This shows that the TAB and TPB processes effectively improve the mechanical properties of PP/ES nonwoven geotextiles, which can be increased by raising the temperature of TAB to improve the mechanical properties of the nonwoven geotextiles.

Conclusions

In summary, PP/ES short-fiber nonwoven geotextile with tight structure, high strength, and elongation was successfully prepared using a combination of needle punching reinforcement and thermal bonding. The elevation of TAB temperature and the addition of the TPB process produce a significant improvement in the properties of the nonwoven geotextile, toward densification of the fiber web structure and improvement of the mechanical properties of the geotextile. The conclusions reached are as follows.

By adjusting the process parameters, the nonwoven geotextile achieved a tensile strength of 1057 N, an increase in elongation at breakage of 11%, and a vertical permeability coefficient of 4.03 mm/s. These key results highlight the material’s significant advantages in terms of increased tensile strength and good permeability.

The thermal adhesion between PP and ES fibers, as well as between ES fibers, tends to increase as the heat treatment temperature increases. When the heat treatment temperature was increased to 150°C, the strength and elongation of PP fibers increased to 5.38 cN/dtex and 34.07%, respectively, compared with that of non-heat-treated fibers; similarly, the length of PP fibers decreased to 69.62 mm³. Scanning electron micrographs showed that bonding points appeared in the fiber web after thermal bonding; the thermal bonding part gradually became larger with the increase in TAB temperature and the addition of the TPB process, leading to a more compact fiber web structure. The areal density of nonwoven geotextile reached 277 g/m², and accordingly, the effective aperture was reduced to 0.11 mm. Meanwhile, the permeability and vertical permeability coefficient of nonwoven geotextile were reduced to 760 L/(m²/s) and 3.11 mm/s.

In this study, we not only explore the effects of thermal bonding on fiber properties and geotextile structure at different temperatures, but also verify, through systematic experiments, how these factors can enhance the overall mechanical properties of geotextiles; in this paper, we discuss how these processes can alter the fiber properties and the microstructure of the observed fibers. Overall, PP/ES staple-fiber nonwoven geotextiles have excellent ductility and mechanical properties, in addition to simple processes and low production costs. Owing to its excellent tensile strength and durability, the geotextile is suitable for use in highway foundations and pavement reinforcement, providing good load-bearing capacity and long-term stability. The material can also be used in tunnel linings to resist soil erosion and water intrusion and ensure structural safety; in dams, canals, and flow control projects, the composite’s high permeability and strength help to improve water flow management and reduce erosion. In the future, we intend to explore the effects of different fiber combinations and processing methods on performance to further enhance the material’s mechanical properties and durability, and carry out real-world application tests to assess the material’s performance under extreme environmental and loading conditions, to confirm its long-term stability and reliability.

Footnotes

Declaration of conflicting interests

The author(s) declares no conflict of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Key Research and Development Program of Shandong Province, China (grant number 2021CXGC011001).

ORCID iDs

Haiyang Lei

Yaming Duan

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