Durability of polypropylene filament geotextile

Geotextile is commonly known as geotextile, is the use of manufacturing, non-woven or compilation methods, the fiber or yarn system into a continuous plane-like material. Geotextiles are widely used in railroads, highways, water conservancy, mining and metallurgy and environmental protection and other major projects related to national livelihood, with reinforcement, filtration, drainage, protection, separation and other important role. Usually, geotextiles in accordance with the different materials, can be divided into polypropylene geotextiles, polyester geotextiles, polyethylene geotextiles, polyvinyl acetal fiber geotextiles and polyamide geotextiles, etc., of which, the more common on the market is polypropylene geotextiles and polyester geotextiles. Geotextiles can also be divided into filament geotextiles and staple geotextiles according to the length of their fibers, which are obtained by filament spinning and bonding and staple fiber pack carding respectively. The geotextile prepared by filament spinning and bonding has uniform texture, high tensile strength, and high production efficiency compared with short fiber geotextile. For polyester filament geotextile, whether it is equipment, technology or quality, our country has been ranked among the ranks of the world's advanced level, but the development of polypropylene filament geotextile is very slow, especially the equipment manufacturing is significantly behind foreign countries. Although through the introduction of three foreign production lines of polypropylene filament geotextiles, gradually get rid of the long-standing polypropylene filament geotextiles rely on imports of the situation, but there are still more problems in the use of the process, especially the lack of a systematic assessment of the durability of polypropylene filament geotextiles.

Geotextiles are usually used in permanent projects, and studies have shown that no significant degradation of laid geotextiles has been detected after more than 21 or even 35 years of use, but this does not ensure that they will reach their design service life, so researchers usually use accelerated aging tests to study the long-term performance and service life of geotextiles. Polypropylene filament geotextiles still lack data records of actual engineering applications, and there is no systematic research on accelerated aging test. Therefore, it is necessary to systematically sort out the development and current status of polypropylene filament geotextiles to fully understand their aging degradation mechanism under harsh environments, which is conducive to the understanding of their performance and service life in long-term applications.

The article summarizes the domestic and international research on the durability of polypropylene filament geotextiles, mainly including heat and oxygen aging, light and oxygen aging, chemical corrosion resistance, freeze-thaw resistance, etc., to comprehensively elaborate the durability characteristics and limitations of polypropylene filament geotextiles, and to provide a reference for the research and application of polypropylene filament geotextiles in China.

1 Domestic and foreign polypropylene filament geotextile durability performance research

1.1 Heat and oxygen aging resistance of polypropylene filament geotextiles

The thermo-oxygen resistance of polypropylene filament geotextiles is mainly characterized by the thermal stability of single polypropylene filaments and the changes in the mechanical properties of the geotextiles before and after thermo-oxygen aging treatment.Karademir et al. analyzed the tensile properties of single polypropylene filaments as a function of temperature. The results showed that the yield strength of the filament decreased with increasing temperature, this is due to the fact that, the hardness of the polypropylene material decreases with increasing temperature, the decrease in the hardness of the material decreases its ability to withstand elastic deformation under tension. In addition, the addition of titanium dioxide to polypropylene can increase the crystallinity of polypropylene, its thermal stability has been significantly improved, and, the higher the amount of titanium dioxide added, the better the thermal stability.

Fang Yuan et al. put the polypropylene filament geotextile into the drying oven at 110 ℃, aging 14d, testing its mechanical properties after thermo-oxidative aging, and observing its surface morphology. The results showed that, after the aging experiment, the polypropylene filament geotextile did not have tensile strength, and the surface structure of the fiber showed the characteristics of fracture along the axial direction and smooth and flat section, which indicated that the polypropylene filament geotextile had poor resistance to degradation by thermo-oxidative aging under the condition, which was due to the fact that, the tertiary hydrogen atoms contained in the molecular chain were less stable, and were prone to form carbonyl groups in the thermo-oxidative environment, and, decompose rapidly.

Deng Zongcai et al. analyzed in detail the thermo-oxidative aging performance of polypropylene filament geotextile after treatment at 110℃ for 25d, and the results showed that the thermo-oxidative aging process of polypropylene filament geotextile is mainly divided into three stages: the first stage is the early stage of aging, at high temperature, polypropylene molecules are further crystallized, and there is a small increase in the fracture strength of the geotextile; the second stage is the middle stage of aging, and the appearance of peroxides and other Active intermediates, thermo-oxidative aging mainly occurs on the surface of the geotextile, and its fracture strength is a slow decline in the trend; the third stage is the late stage of aging, the geotextile in the active intermediates increase, thermo-oxidative aging from the surface of the material to the internal expansion of the material, resulting in the breakage of polypropylene molecular chain, accelerated decomposition of the material, and a significant decline in fracture strength.

1.2 Light and oxygen aging resistance of polypropylene filament geotextiles

Polypropylene molecular structure, tertiary carbon atoms of the carbon-hydrogen bonding energy is small, and, more unstable, sensitive to light, in the ultraviolet irradiation is prone to degradation, resulting in polypropylene filament geotextiles mechanical properties reduced or even lost. When the polypropylene filament geotextile is applied in uncovered and exposed sites, it is inevitably exposed to sunlight, of which, in the summer or tropical areas where strong light is exposed, the service life of the geotextile is seriously affected. In addition, during transportation and storage, when there are no effective protective measures (e.g., covering, addition of light stabilizers, etc.), photo-oxidative aging degradation also occurs, leading to a reduction in the mechanical strength of the material. Based on this, it is very important to study the photo-oxidative aging resistance of polypropylene filament geotextiles.

The photo-oxidative degradation of polypropylene filament geotextiles can be carried out in the laboratory by accelerated tests or by directly exposing the samples to the outdoor environment, both of which provide quantitative and qualitative information.Carneiro et al. exposed polypropylene geotextiles to UV irradiation in the laboratory (accelerated aging for 84 d) and to the outdoor environment (natural conditions for 2 months) to assess the resistance to photo-oxidative degradation by monitoring the changes in tensile properties. The tensile properties of the materials were quantitatively evaluated by monitoring the changes of their tensile properties. The results showed that both laboratory and outdoor aging caused damage (transverse cracks) to the polypropylene fibers of the geotextiles, and their tensile properties were reduced by 73.9% and 27.9%, respectively.Yang et al. investigated the effects of laboratory accelerated aging test and outdoor exposure test on the performance of polypropylene filament yarns under the UV irradiation strengths of 81.58, 162.58, and 325.25 W/m2, respectively, to find out the effects of two types of UV radiation on the performance of polypropylene filament yarns, and thus to find out the effects of two types of UV radiation test on the performance of polypropylene filament yarns. properties of polypropylene filament yarns at 81.58, 162.58, and 325.25 W/m2, respectively, in order to find the relationship between the two aging test methods. The results showed that at relatively low UV irradiation intensities (81.58 and 162.58 W/m2) and outdoor exposure, carboxylic acid carbonyls were formed on the surface of the specimens, which proved that hydroperoxide decomposition reaction occurred, and the correlation between the two test methods was strong; whereas, ester carbonyls and low molecular weight acids appeared in the specimens under high UV irradiation intensities (325.25 W/m2), which indicated that, under high UV irradiation intensities, the materials' UV irradiation intensity, the decomposition mechanism of the material is different from the outdoor exposure test, and the correlation between the two is poor.

In order to improve the photo-oxidative aging resistance of polypropylene filament geotextiles, it is necessary to add light stabilizers.Carneiro et al. added hindered amines (light stabilizers) and carbon black to polypropylene geotextiles, respectively, and, subjected them to natural outdoor aging. When the aging time was 36 months, the geotextile without light stabilizer decomposed into small fragments, while the surface of the geotextile with the addition of hindered amine (light stabilizer) and carbon black did not change significantly, corresponding to the retention rate of tensile strength of 18.4% and 60.9%, respectively. The photo-oxidative aging resistance of geotextiles was significantly improved by the addition of carbon black, which was due to the fact that, carbon black slowed down the UV radiation penetration.

Hua-rui Bit et al. studied the effects of gray masterbatch (light shielding agent) and UV88 anti-aging masterbatch (hindered amine light stabilizer) on the enhancement of photo-oxidative aging resistance of polypropylene filament geotextiles. The results showed that, compared with the unadded anti-aging masterbatch, the photo-oxidative aging resistance of polypropylene filament geotextiles with the addition of gray masterbatch and UV88 anti-aging masterbatch was greatly improved after 48h of ultraviolet irradiation, and its tensile strength retention rate reached 91.8%. In addition, compared with the fabrics with gray masterbatch, the fabrics with UV88 anti-aging masterbatch have better photo-oxidative aging resistance, which is due to the fact that the gray masterbatch can only reflect ultraviolet light, while the UV88 anti-aging masterbatch can reduce the rate of the chain reaction by capturing free radicals and decomposing peroxides, thus effectively slowing down the decomposition of the materials. Electron microscope to observe the changes in fiber structure of polypropylene materials added with light stabilizers after exposure to UV illumination, the results show that there is no obvious fiber damage on the surface of polypropylene after 1,000 h of UV irradiation, which suggests that the addition of light stabilizers can effectively prevent fiber degradation.

Although the addition of light stabilizers in the preparation process can obtain polypropylene filament materials with better UV resistance, the addition of light stabilizers caused the fibers to break during the spinning process, which reduced the overall mechanical properties of the materials. In addition, the price of the light stabilizer is high, which increases the preparation cost. TiO2 is widely used in the preparation of high-performance composites due to its better UV shielding ability, lower toxicity and production cost.Zhao et al. directly infiltrated the polypropylene filament geotextile in aqueous polyurethane and TiO2 solution, removed it, and then dried and treated it to study its aging performance under UV illumination. The results showed that the infiltrated filament geotextile had better UV resistance, and its fracture strength retention rate (52.44% longitudinally and 55.12% transversely) was significantly improved compared with that of the untreated geotextile (0.07% longitudinally and 0.08% transversely), which provided a new idea for the development and design of polypropylene filament geotextiles with high UV resistance.

1.3 Chemical resistance of polypropylene filament geotextiles

When polypropylene filament geotextile is applied in landfill, storage pool liquid, mining and other projects, it is easy to be eroded by acid, alkali, salt and other corrosive fluids, so the material needs to have good chemical corrosion resistance.Mathur et al. immersed the polypropylene geotextile in brine (pH ≈ 8), strong alkali (pH ≈ 10) and acidic (pH ≈ 3) media, and, at 95 ℃ temperature, accelerated aging 6 months, the effects of pH and saline environments on polypropylene geotextiles were systematically investigated. The test results showed that the polypropylene geotextiles did not undergo significant degradation under the above conditions, and the physical and mechanical properties were relatively stable. Nie Songlin et al. also studied the acid and alkali resistance of polypropylene filament geotextile, and compared it with polyester filament geotextile, the results show that under the condition of the same surface density, polypropylene filament geotextile is more resistant to acid and alkali corrosion compared with polyester filament geotextile, this is due to the fact that, polypropylene mainly consists of a large number of nonpolar alkane chains, with lower polarity, better resistance to swelling, and higher stability in acid and alkaline solutions. This is due to the fact that polypropylene is mainly composed of a large number of non-polar alkane chains, low polarity, good resistance to swelling, and high stability in acid and alkali solutions, while polyester, the main component of which is polyethylene terephthalate, has a large number of polar groups, and some of them are easily destroyed in acid and alkali solutions, resulting in degradation of the molecular chain and a decline in performance.

1.4 Freeze-thaw resistance of polypropylene filament geotextiles

Most of the geomaterials used in northern and alpine areas, dams or coastal projects need to withstand the effects of freeze-thaw cycles, which requires high resistance to freeze-thaw, so it is very important to study the freeze-thaw resistance of polypropylene filament geotextiles applied in the above fields. Nie Songlin et al. conducted freeze-thaw resistance experiments on polypropylene filament geotextile and polyester filament geotextile respectively, and the results showed that after 20 cycles of freeze-thaw, polypropylene filament geotextile did not have obvious mechanical property degradation, while polyester filament geotextile's fracture strength retention was only 60%, which showed that polypropylene filament geotextile had better freeze-thaw resistance compared with polyester filament geotextile. Allen et al. not only carried out freeze-thaw cycles on polypropylene filament geotextiles in cold and dry environments, but also put them into water environments and salt water environments and carried out freeze-thaw cycle tests. The test results showed that the load-strain strength of polypropylene filament geotextile did not change significantly under the above test conditions, and it still has good freeze-thaw resistance, which is mainly due to the fact that polypropylene has good hydrophobicity and resistance to swelling.

1.5 Others

Polypropylene filament geotextiles have high chemical stability, strength and density, making them widely used in many geotextiles. When polypropylene filament geotextiles are applied below ground, photodegradation no longer affects their service life and performance. However, in the process of use, under load or pressure, polypropylene filament geotextiles are prone to deformation and creep phenomenon, resulting in material damage and reduced service life. Moreover, the creep properties are very sensitive to pressure, when the load pressure is doubled, the creep increases by 3 to 5 times, and the mechanical properties are significantly reduced. In addition, with the increase of pressure, polypropylene filament geotextile is more sensitive to oxygen, which further enhances its oxidative degradation rate, seriously affecting the long-term performance and life of the material.

2 Conclusion

Polypropylene has extremely weak polarity, resistance to swelling and other characteristics, therefore, polypropylene filament geotextiles have better acid and alkali resistance and freeze-thaw resistance, so that it has an irreplaceable role in the acid and alkali environment and the engineering applications in the alpine region. However, the tertiary hydrogen atoms contained in the polypropylene molecular chain are less stable, resulting in the existence of polypropylene geotextiles with poor thermal stability, low UV resistance and other defects, greatly limiting its use in high temperature or tropical regions. Although the addition of antioxidants or light stabilizers significantly improves the performance of polypropylene filament geotextiles in terms of heat and oxygen aging resistance and light and oxygen aging resistance, the study of the mechanism of action is relatively small, and the addition of antioxidants or light stabilizers increases the cost of preparation and processing difficulty, which seriously limits the application and development of polypropylene filament geotextiles. Therefore, in-depth study of the mechanism of aging resistance of polypropylene filament geotextiles, the development of durable special materials, the preparation of high-performance durable polypropylene filament geotextiles is a key research direction in the future.