1 Experimental section
1) Experimental materials and equipment
Polypropylene spun bond filament non-woven geotextile; short-cut polypropylene geotextile; 1800 W heating rod; sulphuric acid; calcium hydroxide; KH-55 A electric thermostat; H10 K-S mechanical testing machine.
2) Preparation of geotextile samples
Geotextile samples were taken by GB /T 13760-2009 “Sampling and sample preparation of geosynthetic materials”. The geotextile specimens are referred to as HPP geotextile and PP geotextile for short-cut polypropylene geotextile, both of which have an external dimension of 300 mm × 200 mm and are accurately measured by vernier calipers with an accuracy of 0. 02 mm. The surface density of HPP geotextile is 200 g/m2 and that of PP geotextile is 400 g/m2.
3) Thermal and oxygen aging test method and performance test
The temperature of the thermal oxidation test was set at 110, 120, and 130 ℃. The HPP and PP geotextiles were suspended vertically on the specimen rack in the oven by GB/T 17631-1998 “Test method for oxidation resistance of geotextiles and related products”, avoiding contact with metal to prevent the geotextiles from accelerated degradation and affecting the test results. Throughout the experiment, the temperature in the oven fluctuates by no more than ± 1 °C, the temperature distribution deviation is no more than ± 1. 5 °C and the volume fraction of oxygen in the air is 21%. The geotextiles were aged in the oven for 25 d. During this period, five geotextile samples were taken out every 5 d. The change in mass was tested using a balance with an accuracy of 0.5 g. The geotextile breaking strength was tested according to GB/T 15788-2005 “Tensile test of geotextiles and related products in wide strips”, and the decay pattern of mass and breaking strength was analyzed. The decay law of mass and breaking strength is analyzed.
4) Chemical durability test methods and performance tests
Three groups of chemical durability tests for acid and alkali corrosion and pure water immersion were carried out by GB/T 17632-1998 “Test methods for the resistance of geotextiles and related products to acid and alkali liquids”. The HPP geotextile specimens are placed in a container without any stress and at least 10 mm between the specimens, between the specimens and the container, and between the specimens and the liquid surface. The sulphuric acid solution and the calcium hydroxide suspension are stirred at least once a day. The liquids and specimens were kept out of the light and the temperature of the solution was maintained at (60 ± 1) °C using a thermostat device.
The corrosion period of each geotextile in each of the three durability experiments was 20 d.
The specimens were washed in water, then in 0.01 mol/L ammonium carbonate solution, and finally in water to ensure that the corrosion solution was cleaned, and then dried at room temperature before measuring the geotextile mass and fracture strength. The chemical solution is prepared as follows.
1) Acid corrosion test formulation: 0.025 mol/L sulfur (p H = 1.3) and water of class 3 (a classification for evaluating the quality of surface water in China). (a classification for evaluating the quality of surface water in China).
2) Alkali corrosion test formulation: saturated suspension of calcium hydroxide at a mass concentration of 2. 5 g/L. 2) Alkali corrosion test formulation: saturated suspension of calcium hydroxide at a mass concentration of 2. 5 g/L (pH = 11. 65) in Class 3 water.
3) Pure water immersion formulation: water of class 3.
2 Results and discussion
1) Analysis of thermal oxygen aging test results
a. Mass loss and analysis of geotextile specimens
The thermal oxidative degradation of polypropylene is mainly due to the oxygenation of the chemical bonds of the molecular chain. The thermal oxidative degradation of polypropylene is mainly the oxygenation of the chemical bonds of the molecular chains. As the temperature rises, it eventually leads to the breakage of the chemical bonds and the reduction of the physical functions, which is reflected in the macroscopic decrease of the mass and strength. Figure 1 shows the measured mass retention rate of geotextiles after thermal oxidation at different temperatures versus time. Figure 1 shows the measured mass retention rate versus time after thermal oxidation of geotextiles at different temperatures.
As can be seen from Fig. 1, at the three temperatures, with the increase of time, the mass retention rate of the geotextile is reduced.
ig． 1 Curves of mass retention rate versus time in Thermo-oxidative aging of HPP geotextiles ( a)and PP geotextiles ( b)
The quality retention rate of HPP geotextiles gradually decreases, and the higher the thermal and oxygen aging temperature, the faster and more quality is lost. The difference in the mass retention rate of HPP geotextiles gradually increased as the thermal oxidation aging temperature increased. 110 °C showed that the mass of HPP geotextiles decreased more slowly in the first 15 d, and the rate of mass loss increased after 15 d. This was because the thermal oxidation reaction of HPP geotextiles mainly occurred on the surface of the fabric in the first 15 d, and the rate of mass loss increased after 15 d. This is because the thermal oxidation reaction of HPP geotextiles mainly occurred on the surface of the fabric in the first 15 d. With the extension of time, the thermal oxidation reaction gradually developed to the inner part of the fabric, and the rate of thermal oxidation aging reaction accelerated. At 130 °C, the slope of the curve increased significantly, indicating a faster rate of mass reduction of the HPP geotextile. 99. 06%, 98. 85% and 98. 10% of the mass of the geotextile was retained at the three temperatures at 25 d, respectively. The mass change pattern was similar to that of the HPP geotextile: at 110 and 120 ℃, the mass of PP geotextile decreased slowly in the first 10 d, and the slope of the curve increased after 10 d. At 130 ℃, the mass reduction rate increased after 5 d.
At 130 ℃, the mass reduction rate increased significantly after 5 d, and the mass retention rate at 25 d was 97. 78%.
The mass retention rate at 25 d was 97. 78%.
The mass changes of the two geotextiles were similar at the three temperatures, but at 110 °C, the mass reduction rates of HPP and PP geotextiles started to increase at 15 and 10 d, respectively, and the final mass retention rates of HPP and PP geotextiles at 25 d were 99. 06% and 98. 89%, respectively. The slope of the mass retention curve of HPP geotextile at 130 °C was significantly higher than that of PP geotextile, as the thermal oxidation reaction of HPP geotextile started from the fiber surface to its interior later than that of PP geotextile. At 130 ℃, the slope of the mass retention curve of HPP geotextile was significantly smaller than that of PP geotextile. The slope of the mass retention curve of HPP geotextile was significantly lower than that of PP geotextile at 130 ℃, and the mass retention rates at 25 d were 98. 10% and 97. 78% respectively. This indicates that the slope of the quality retention curve of HPP and PP geotextiles was significantly lower than that of PP geotextiles at 130 ℃. The higher the temperature, the greater the difference between the thermal oxidation performance of HPP and PP geotextiles. The higher the temperature, the greater the difference in heat oxidation performance between HPP and PP geotextiles.
b. Mechanical properties of geotextile specimens
The relationship between the fracture strength and thermal oxidation aging time of the two types of geotextiles at different temperatures is shown in Fig. 2. The relationship between the fracture strength and thermal oxidation aging time of the two geotextiles at different temperatures is shown in Fig. 2. The experimental data of HPP geotextile at 130 ℃ were more discrete, and the relationship between the fracture strength and the thermal oxidation aging time of the two geotextiles was shown in Fig. 2. The coefficient of variation of the five groups of experimental data at five thermal oxidation aging times was 2 The coefficients of variation for the five groups of data were 2. 5%, 5. 2%, 6. 1%, 6. 5%, and 8. 5% respectively. The data of PP geotextiles were more discrete at 120 ℃, and the coefficients of variation of the five groups of data were 2. 5%, 5. 2%, 6. 1%, and 8. 5% respectively. The coefficients of variation for the five data sets were 1. 5%, 3. 6%, 5. 8%, 6. 5%, and 9. 5% respectively. The fracture strength retention rates of the two geotextiles are shown in Figure 3. The fracture strength retention rates of the two geotextiles are shown in Figure 3.
Fig． 2 Curve of longitudinal breaking strength and thermo-oxidative aging time of HPP geotextiles ( a) and PP geotextiles ( b) at different temperatures
The longitudinal fracture strength of HPP geotextiles decreases rapidly at 130 °C. At 130 °C, the longitudinal fracture strength of HPP geotextiles does not increase but keeps decreasing significantly, and the decrease in longitudinal fracture strength accelerates between 15 and 20 d. The decrease in longitudinal fracture strength begins to slow down between 20 and 25 d. The longitudinal fracture strengths of HPP geotextiles at the three temperatures are 13. 41, 12. 62 and 11. 22 kN/m, respectively. The changes in the longitudinal fracture strength of PP geotextiles were similar to those of HPP geotextiles at 110 and 120 ℃, with the rate of decrease of PP geotextiles being significantly faster than that of HPP geotextiles in the first 10 days. At 130 ℃, the longitudinal fracture strength of PP geotextile kept decreasing rapidly.
The final longitudinal fracture strengths of PP geotextiles at the three temperatures were 21. 72, 18. 39 and 16. 96 kN/m. The fracture strengths of HPP and PP geotextiles varied in four stages: slight increase, gentle decrease, drastic decrease, and gentle decrease. The mechanism is as follows: polypropylene is a crystalline polymer, in the first 0-10 d, the fracture strength increases slightly as crystallization continues to develop within the molecule under the thermal-oxidative aging environment, and the chemical properties of the geotextile are relatively stable; in the middle 10-15 d, the effect of thermal-oxidative damage on the material gradually appears, and the aging begins to expand into the material, and the fracture strength decreases slowly; in the later 15-20 d, the fracture strength decreases slowly as the material accumulates in the material. ～The thermal oxygen aging reaction expands to the interior of the material, causing damage to the polypropylene macromolecules, resulting in a rapid decrease in fracture strength; after 20 to 25 d, the rate of thermal oxygen aging reaction gradually slows down.
Figure 3 shows the longitudinal and transverse fracture strength retention rates of the two geotextiles. It can be seen that the strong retention rate of both geotextiles decreases to different degrees as the thermal oxygen aging temperature increases. At 110 ℃, the longitudinal and transverse strength retention rates of HPP geotextile were higher than those of PP geotextile by 2. 85% and 2. 16% respectively; at 120 ℃, the longitudinal and transverse strength retention rates of HPP geotextile were higher than those of PP geotextile by 1. 69%. At 120 ℃, the longitudinal and transverse strength retention rates of HPP geotextile were higher than those of PP geotextile by 1. 69% and 8.95% respectively.
As the temperature increases, the difference in the thermal and oxygen aging properties of the two geotextiles gradually emerges, with the retention rates of the transverse and longitudinal strengths of HPP geotextiles being significantly higher than those of PP geotextiles. In other words, HPP geotextiles are significantly better than PP geotextiles in terms of resistance to thermal and oxygen aging as the temperature increases.
c. HPP geotextile life projections
The chemical reaction equation for the life of an HPP geotextile is
a A + b B + c C → e E + f F
The rate of chemical reaction γ can be defined as
γ = k [A] a [B] b [C] c
where: [A], [B] and [C] are the concentrations of the reactants; a, b, c, e, and f are the coefficients of the equilibrium stoichiometry, which are related to the concentrations of the reactants and products; k is the chemical reaction rate constant, which is related to the temperature and pressure of the chemical reaction.
Fig． 3 Longitudinal ( a) and lateral ( b) breaking strength retention rate of two geotextiles
The series of chemical reaction rates n is
n = a + b + c
Most chemical reactions have a rate step of 0, 1 or 2. Table 1: [A]t is the concentration of reactant A at time t; [A]0 is the initial concentration; x = [A A] t is the concentration of reactant A at time t; [A] 0 is the initial concentration; x = [A t / A] 0 is the initial concentration; x = [A] t / [A] 0, expressed by the equation x = f( t), and the reaction rate is expressed as dx/dt.
Since the number of reaction steps in polymer aging is usually 0, 1, or 2, it is not possible to determine the number of reaction steps in advance, so the experimental data can only be fitted to each of the functions in Table 1 separately, and the equation with the smallest deviation (with the largest correlation coefficient) is the equation for the number of reaction steps to determine the reaction rate constant for the material at different temperatures. A linear fit of link to a graph of Arrhenius data at the corresponding temperature of 1/T was carried out to determine the equation lnk = a( 1/T) + b, which gives the reaction rate constant at a certain temperature T0: k( T0) = exp[a( 1/T0) + b]. After obtaining the reaction rate constant k( T0), the retention rate of the strength of the geotextile at a given time t can be calculated as follows: the thermal oxygen aging temperatures are 110, 120, and 130 °C, and the oxygen concentration is the standard atmospheric oxygen concentration, respectively, using the three chemical reaction level functions in Table 1. The best correlation was found for the 1-stage reaction function. The results are shown in Figure 4 and the number of reaction steps for this thermal oxygen aging was determined to be 1.
The reaction rate constants of HPP geotextiles at three temperatures are 0. 011 82, 0. 018 29 and 0. 024 82. The Arrhenius equation is applied to make a straight line between the dependent variable lnk and the independent variable 1 /T to obtain the reaction rate function equation lnk = – 5 677. 44 / T + 10. 355 92 The average annual temperature in Beijing is 12. 2 °C and the volume fraction of oxygen is 21%, the reaction rate constant of thermal oxygen aging of HPP geotextiles is calculated as lnk = – 5 677. 44 / ( Assuming a design life of 50 a, the strength retention rate of the HPP geotextile at 50 a in the thermal oxygen environment is The strength retention rate of HPP geotextile at 50 an under thermal oxygen environment is x(50) = e-kt= e-7. 113 × 10-5 × 365 × 50= 27. 30%.
d. Mass loss and analysis of HPP geotextile specimens
The literature shows that common polypropylene geotextiles (PP) have good chemical resistance, so this paper focuses on the resistance of HPP geotextiles to acid, alkali, and water immersion. the mass retention curves of HPP geotextiles at different temperatures and solutions versus time are shown in Figure 5.
Fig． 5 Mass retention rates of HPP geotextiles in different liquid environments
The final mass retention rate of the HPP geotextile was 99. 41% at a concentration of 0. 025 mol/L sulphuric acid (pH = 1. 3); the greatest mass loss occurred at an alkaline concentration of 2. 5 g/L calcium hydroxide suspension (p H = 11. 65) with a final mass retention rate of 99. 41%. The final mass retention rate was 99. 37%; however, the mass of the HPP geotextile increased in the pure water environment. This is due to the fact that water molecules are smaller in size compared to other ions and their penetration This is because water molecules are smaller in size compared to other ions and penetrate HPP geotextiles at a higher rate, making it easier for water molecules to penetrate along The water molecules are more likely to penetrate along the macromolecular chains of polypropylene or in the gaps between the macromolecular chain segments of polypropylene. The water molecules continue to penetrate into the geotextile, resulting in an increase in the mass of the HPP geotextile in a pure water environment. Geotextiles have an increased mass in pure water. In an acidic or alkaline environment, the sulfate ions and calcium ions of the solute will be mixed with the water molecules. ions and calcium ions combine with water molecules to form hydrated ions, increasing the molecular half The rate of diffusion into the interior of the polypropylene material is reduced by the increase in molecular half diameter. It can be seen that The mass of HPP geotextiles increases in water, decreases less in acidic environments, and decreases in alkaline environments. The mass of the HPP geotextile increases in water decreases less in an acidic environment and decreases more in an alkaline environment. However, in the two corrosive liquids, However, the two corrosive liquids, the mass-loss rate did not exceed 1. 0%, indicating that the HPP geotextile is well resistant to acids and alkalis.HPP geotextiles have good resistance to acids and alkalis
e. Loss of strength and analysis of HPP geotextile specimens
The strength loss of HPP geotextiles in longitudinal and transverse directions under different liquid conditions and different times was analyzed. The curves of the retention of strength at break in the longitudinal and transverse directions versus time are shown in Fig. 6. The strength of HPP geotextiles in sulphuric acid solution is more discrete. The coefficients of variation of the five data sets were 2. 1%, 1. 5%, 5. 6%, 4. 8%, and 5. 0% respectively. The coefficients of variation for the five data sets were 2. 1%, 1. 5%, 5. 6%, 4. 8%, and 5. 0% respectively.
Fig． 6 Curves of longitudinal ( a) and lateral ( b) breaking strength retention rate and time of HPP geotextiles
As can be seen from Fig. 6, the longitudinal and transverse fracture strength retention rates of HPP geotextiles varied in a similar pattern with a relatively gentle curve, indicating that the strength of HPP geotextiles decayed very little under different liquid environments. 2. 5 g/L of calcium hydroxide In the alkaline environment of 2.5 g/L calcium hydroxide suspension, the longitudinal strength retention was 95. 80% and the transverse strength retention was 97. 60%. The longitudinal strength retention was 95.80% and the transverse strength retention was 97.60%. The smallest retention rate was 95.00% in pure water and 95.75% in transverse strength. It can be seen that: at 20 d, HPP geotextiles lost the most strength in pure water, followed by alkaline and acid. The loss of strength of HPP geotextiles at 20 d was greatest in pure water, followed by an alkaline environment and least in an acid environment. The reason for this is that polypropylene The reason is that the HPP geotextile is composed of a large number of non-polar polymer equivalent alkane chains. When the polypropylene is immersed in non-oxidizing acid and alkali solutions, the ions in the acid and alkali form hydrated ions with When polypropylene is immersed in non-oxidizing acid and alkali solutions, ions from the acids and bases form hydrated ions with the water molecules, increasing the radius of the molecule and reducing the rate of diffusion of hydrated ions into the polypropylene molecule. The rate of diffusion of the hydrated ions into the polypropylene molecule is reduced, even when the hydrated ions penetrate into the polypropylene molecules, they do not chemically This means that the infiltration of these media does not change the molecular structure. This means that the infiltration of these media This means that the penetration of these media does not remove the van der Waals forces between the macromolecules, or that the effect is very weak. The small size of the water molecules means that the rate of penetration into the polypropylene material is high. After the water molecules have penetrated into the polypropylene molecules, the polypropylene molecular system The penetration of water molecules into polypropylene molecules results in the dissolution, migration, or extraction of relevant additives and other soluble substances within the polypropylene molecular system. The water molecules penetrate into the polypropylene molecules and the dissolution, migration, or extraction of other soluble substances within the polypropylene molecular system leads to a greater reduction in the fracture strength of HPP geotextiles. The fracture strength of HPP geotextiles decreases more. Overall, the geotextiles did not show significant strength degradation in the three liquids. This indicates that HPP geotextiles have good resistance to acids, alkalis, and water immersion. The geotextile is resistant to acid, alkali, and water immersion.
1) The quality and transverse and longitudinal fracture strength of HPP and PP geotextiles decrease to different degrees as the thermal and oxygen aging time increases; the thermal and oxygen aging performance of HPP geotextiles is better than that of PP geotextiles at the same temperature. As the temperature increases, HPP geotextiles exhibit better thermal and oxygen aging performance.
2) The mass retention rates of HPP geotextiles were 99. 06%, 98. 85% and 98. 10% when thermally and oxygen-aged at 110, 120, and 130 ℃ for 25 d, respectively, while the mass retention rates of PP geotextiles were 98. 89%, 98. 78% and 97. 78%, respectively. The longitudinal fracture strength retention of HPP geotextiles was 75. 00%, 67. 25% and 64. 71% respectively, while the longitudinal fracture strength retention of PP geotextiles was 72. 15%, 65. 56% and 54. 00% respectively. This shows that the heat and oxygen aging resistance of HPP is significantly better than that of PP geotextile and has better heat and oxygen aging resistance.
3) An expression for the thermal oxygen reaction rate of HPP geotextiles was fitted according to Arrhenius’ theorem. Based on the environmental conditions in Beijing, the strength retention rate of HPP geotextiles was predicted to be 27. 30% after 50 an under thermal-oxidative aging.
4) The mass of the HPP geotextile increases in water decreases less in acidic environments and decreases more in alkaline environments; and the mass-loss rate does not exceed 1. 00% in either of the two corrosive liquids, with the HPP geotextile showing good resistance to acids and alkalis.
5) The retention of transverse and longitudinal fracture strength of HPP geotextiles was 98. 20% in 0. 025 mol/L acid environment; 95. 80% in 2. 5 g/L calcium hydroxide suspension in alkali environment; 95. 00% in pure water. The strength loss of the HPP geotextile did not exceed 5.00% after 20 d of acid and alkali corrosion and pure water immersion, indicating that the HPP geotextile has good resistance to acid, alkali, and water immersion.
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