Polypropylene nonwovens are divided into durable nonwovens (such as apparel, civil construction) and environmentally friendly degradable nonwovens (such as environmentally friendly shopping bags, agricultural nonwovens, etc.) according to their durability [1,2 ]. The aging of polypropylene nonwoven fabrics during use is mainly caused by ultraviolet radiation in sunlight. The macroscopic evaluation methods for the photoaging properties of polypropylene nonwoven fabrics include natural aging test methods and artificial aging test methods. Microscopic analysis Methods include intrinsic viscosity method, differential scanning calorimetry, infrared spectroscopy, etc. This article will discuss the reaction mechanism of polypropylene nonwovens photoaging and its macroscopic and microscopic evaluation methods.
1 Photoaging reaction mechanism of polypropylene
Since the 1980s, many scholars have conducted extensive and in-depth research on the aging resistance of polypropylene, and found that the aging of polypropylene is mainly related to the large number of tertiary carbon atoms in the macromolecular chain [3,4]. Carbon atoms have a strong ability to lose electrons. In the presence of oxygen, only a small amount of energy can break the C—H bond to form a lively tertiary carbon free radical, which corresponds to the chemical bond energy in polypropylene. After the action of ultraviolet light energy [5], various reactions of molecular chains occur, such as chain growth, chain breakage, etc. [4], and finally manifest as aging phenomena such as discoloration, strength decline, and surface cracking of polypropylene materials.
Partial wavelengths of ultraviolet energy in sunlight are very close to the energy of certain chemical bonds in polypropylene molecules, so the corresponding chemical bonds in polypropylene can absorb ultraviolet energy, resulting in the breakage of chemical bonds, thereby triggering photoaging [3,6]. The corresponding relationship between partial sunlight ultraviolet energy and typical chemical bond energy in polypropylene is shown in the table below.
Gardette et al. summarized the reaction mechanism of polypropylene photoaging [4], the reaction mechanism is: after the active tertiary carbon atom absorbs the ultraviolet light energy, it oxidizes with O2 in the air to form a peroxide, and then continues Under the action of ultraviolet energy, reactions such as chain growth, chain breaking, chain termination, and branched chain formation further occur, and finally manifest as macroscopic aging behavior; no matter in which direction the photoaging of polypropylene proceeds, there are carbonyl groups in the final product Therefore, many researchers use the carbonyl index to characterize the photoaging degree of polypropylene [2,7,8]. These research results provide a new idea for how to evaluate and measure the photoaging performance of polypropylene nonwovens, that is, analyze the microscopic changes in the photoaging process by various means to evaluate its photoaging performance. Modern analytical instruments can provide More stable and reliable data makes the evaluation results more accurate and reliable.
2 Macroscopic evaluation method of light aging performance of polypropylene nonwoven fabric
The existing macroscopic test and evaluation methods and standards for the photoaging properties of polypropylene nonwovens are mainly divided into two methods: natural aging and artificial accelerated aging.
2.1 Natural aging
The natural aging evaluation method is to expose the sample to outdoor natural conditions for a specified time, and use natural environmental conditions, including sunlight, day and night temperature difference, rain and air, to conduct aging tests on it to evaluate its optical properties, mechanical properties and other related properties. Variety. The natural aging test is close to the actual use of the material, and the obtained weather resistance is relatively reliable.
In the last century, the United States, the Soviet Union, Japan, and European developed countries successively established exposure fields for polymer materials. my country also established exposure fields in Guangzhou and Hainan Island in the early 1960s to study the weather resistance of polymer materials. Test [6,9]. At present, the natural aging performance test standard of polypropylene nonwoven fabrics in my country mainly adopts GB/T 3681-2011 "Exposure test methods for natural sunlight weathering of plastics, sunlight weathering after glass filtration and Fresnel mirror accelerated sunlight weathering"[ 10], there are three environmental conditions in the standard, and the experimental conditions can be selected according to the actual use situation, which has certain pertinence, but there are too many unstable factors, such as climate change, geographical location difference, etc., and the time is long.
Since the natural environmental conditions in the natural aging test are uncontrollable, it is difficult to guarantee the reproducibility and consistency of the test, so this method is mostly used on specific products for special purposes, and is usually carried out near the corresponding actual use site Testing, such as durable geotextiles used in large-scale engineering projects, degradable nonwovens used in specific environments, etc.
2.2 Artificial accelerated aging
The artificial accelerated aging evaluation method is to use laboratory light sources (xenon arc lamps, fluorescent ultraviolet lamps or open carbon arc lamps to simulate the use conditions of materials, and to evaluate the optical properties and mechanical properties of the samples after exposing them for a specified time under controlled temperature and humidity conditions. Changes in performance and other related performance.
At present, the artificial accelerated aging performance test standards of polypropylene nonwovens in my country mainly include GB/T 16422 "Plastic Laboratory Light Exposure Test Methods" series of standards [11-14], which are equivalent to ISO 4892 "Plastics--Methods of exposure to laboratory light sources "[15] series of standards. The standard is characterized by "acceleration" and "strengthening". It is clearly stated in the general provisions of the standard that the test results can only be used to compare the relative durability of exposed materials in a certain environment, and cannot be used to determine the relative durability of the same material in different environments. , so when testing its light aging performance, the experimental conditions and test cycle are usually set according to the requirements of product users, and the performance of the product is measured by comparing with the performance of the control sample. The GB/T 16422 series of standards includes three test methods for accelerated aging of xenon arc lamps, fluorescent ultraviolet lamps and open carbon arc lamps, which can simulate lighting conditions under various conditions of use, and the conditions are controllable, the time is shorter, and the heavy Reproducibility and consistency are more reliable and suitable for most products.
The natural aging and artificial accelerated aging evaluation methods have their own advantages and disadvantages: the natural aging test is closer to the actual use situation, and the photoaging performance evaluation of products for specific purposes is more reliable; the artificial accelerated aging test conditions are controllable, and it can be realized at different times. The photoaging properties of the products are compared, and the results have good reproducibility and reproducibility. Due to the different test conditions, the results of natural aging and artificial accelerated aging of the same sample are not comparable.
3 Microscopic evaluation method of light aging performance of polypropylene nonwoven fabric
The macroscopic changes in the photoaging properties of nonwovens are closely related to the changes in the microstructure of the material, such as changes in the relative molecular mass and the generation of new functional groups, which will eventually manifest as changes in its physical properties. The microstructural changes in photoaging can be measured through intrinsic viscosity , differential scanning calorimetry (DSC) curve, infrared spectrum and other methods to evaluate.
3.1 Intrinsic viscosity
There is a quantitative relationship between the intrinsic viscosity and the relative molecular mass of the polymer, which can be used as a measure of the relative molecular mass, and is linearly related to the physical properties such as the strength of the nonwoven fabric, and can be used to characterize the photoaging degree of the polypropylene nonwoven fabric. The research of Yang Xudong [16] shows that the breaking strength and intrinsic viscosity of polypropylene nonwovens have the same variation law no matter under natural aging or artificial accelerated aging conditions. Yang Xudong and others also used the intrinsic viscosity retention rate as an indicator of the degree of photoaging, and established a life prediction equation based on accumulated ultraviolet radiation energy.
Using intrinsic viscosity to characterize the light aging performance of polypropylene materials can reduce the random errors caused by testing the macroscopic properties of polypropylene nonwoven materials.
3.2 Differential scanning calorimetry
Differential scanning calorimetry (DSC) can be used to analyze the melting point and crystallization of polymers. The melting point reflects the difficulty of melting the material and is related to the relative molecular mass of the material. The change of the crystallization curve can reflect the change of the crystallization situation, which can reflect the degree of photoaging of polypropylene nonwovens to a certain extent. Xie Hao [17] used DSC to study the changes of melting temperature and crystallization of polypropylene nonwovens during natural aging and artificial accelerated aging. Gradually increase; while the melting temperature decreases continuously during the artificial aging process, and the crystallinity temperature also decreases continuously.
Differential scanning calorimetry can be used to analyze the changes in microscopic properties such as crystallization and relative molecular weight of materials during the photoaging process, and it is very important for analyzing the photoaging mechanism of polypropylene nonwovens.
3.3 Infrared spectroscopy
Polypropylene polymer materials will inevitably form new groups during the photoaging process, and infrared spectroscopy can analyze the newly generated groups and molecular structure changes. Since carbonyl groups are generated regardless of the direction of polypropylene photoaging, The degree of photoaging of polypropylene can be characterized by calculating the carbonyl index. At the same time, the photoaging mechanism of polypropylene can also be studied according to the different carbonyl absorption peaks. Now infrared spectroscopy has become the most important method for studying the photoaging process and mechanism of polypropylene materials. One of the means.
Zhang Xiaodong [18] studied the weather resistance of several weather-resistant polypropylenes through infrared spectroscopy, and found that the method of carbonyl group generation can compare the weather resistance of different products; Li Ning et al. [2] used infrared spectroscopy to compare The difference in the photoaging process of two different PP materials. These research results provide us with a new idea, that is, to use a more direct carbonyl index to characterize the degree of photoaging of polypropylene. Wang Huaquan et al.[19] studied the photoaging induction period of polypropylene materials by using infrared spectroscopy, and found that in the first stage after the photoaging started, the material itself had already aged to generate new groups, but the macroscopic properties did not change. It shows that the macro performance has a certain hysteresis, which is not suitable for mechanism research. Yang Xudong et al[7,16] found through infrared spectrum analysis that under the conditions of different irradiation intensities and the same amount of irradiation, the products of polypropylene photoaging are different. The bigger the better, the irradiation intensity should be controlled within a certain range, so that the photoaging process is closer to natural aging, and the test results obtained are closer to reality.
4 Research on Life Prediction of Polypropylene Nonwovens
Due to the long period of the natural aging test, people hope to predict the life of polypropylene nonwovens under natural use conditions by establishing the correspondence between artificial accelerated aging and natural aging. Many researchers have conducted a lot of research over the past few decades. But there is no predictive model that can be widely applied.
Although the current standard clearly states that the use of "acceleration factors" is not recommended, research in this area has not stopped. The initial research used time as the characterization index [9], that is, the ratio of the time required for a certain property of natural aging and artificial accelerated aging to reach a certain preset value; on this basis, the method of sunshine time conversion coefficient appeared, namely When a certain performance reaches a certain preset value, the ratio of sunshine hours (excluding nighttime) to artificial accelerated aging illumination hours; with the deepening of research, it is found that the 290~400nm band in sunlight causes the photoaging of polypropylene materials Therefore, the principle of "energy equivalence" appeared, that is, when the absorbed ultraviolet radiation energy is equal, the degree of photoaging of polypropylene is consistent [20]. These methods are all based on an idea: the breakage and production of chemical bonds in molecules are related to the absorbed energy, and when the absorbed energy is the same, the changes that occur should also be consistent. This kind of thinking occupied the mainstream before the 1990s, but it ignored an important issue, that is, the aging response may change when the energy intensity is too large.
With the deepening of research, the principle of "energy equivalence" began to be questioned: Philippart [21] found that strong ultraviolet radiation would have an impact on the photoaging process of polypropylene, Yang Xudong et al [7,16] used UVA-351F40 ultraviolet fluorescent lamps to conduct experiments , found that when the irradiation intensity does not exceed a certain value (162.58W/m2), the positions of the carbonyl characteristic peaks of polypropylene nonwovens artificially accelerated aging and natural aging products are almost the same, but there is a significant difference after exceeding; Xie Hao [17 ] studies have shown that a large number of hydroxyl groups (-OH) are produced in the natural aging process, but artificial aging does not, and it is believed that this is the effect of water. These studies all show that artificial accelerated aging should not be considered simply from "acceleration" and "strengthening", nor should it only consider whether the changes in the macroscopic properties of polypropylene nonwovens are the same. We should try to choose appropriate experimental conditions to make artificial accelerated aging The microscopic process of aging is closer to natural aging, so that the principle of "energy equivalence" can really be applied.
5 Conclusion
With the emergence of degradable polypropylene nonwovens, how to comprehensively evaluate its photoaging properties has become very important, but the current standards have been difficult to meet market demand, so it is more important to start with the mechanism of polypropylene photoaging Comprehensively simulating the natural aging process, making the changes in microscopic properties during the aging process closer to natural aging, and developing testing methods and evaluation standards that can quickly and effectively predict product life will become the future direction.
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