Aging types, aging tests and anti-aging methods of polymer materials
01 Current status of development of polymer materials
Polymer materials, because of their excellent properties such as light weight, high strength, temperature resistance, and corrosion resistance, are now widely used in many fields such as high-end manufacturing, electronic information, transportation, building energy conservation, aerospace, and national defense and military industries. played a huge role.
It is not only an important basic industry of the national economy, but also a leading industry of the country;
It is not only a strategic emerging industry in the petrochemical industry, but also an important supporting material for strategic emerging industries such as electronic information, aerospace, national defense, and new energy;
Not only does it have high technological content and high added value, it is also an important direction for the transformation and upgrading of the petrochemical industry.
Therefore, polymer materials have always been a development area that developed countries and multinational companies attach great importance to. This not only provides a broad market space for the new polymer materials industry, but also puts forward higher requirements for its quality performance, reliability level, and support capabilities.
Therefore, how to maximize the functions of polymer material products based on the principles of energy conservation, low carbon and ecological development has attracted more and more attention. Aging is an important factor affecting the reliability and durability of polymer materials.
02 Aging types of polymer materials
During the processing, storage and use of polymer materials, due to the combined effects of internal and external factors, their properties gradually deteriorate and eventually lose their use value. This phenomenon belongs to the aging of polymer materials.
This not only causes a waste of resources, but may even lead to larger accidents due to functional failure, and the decomposition of materials caused by aging may also pollute the environment.
Due to different types of polymers and different usage conditions, they have different aging phenomena and characteristics. In general, the aging of polymer materials can be classified into the following four types of changes:
1. Changes in appearance
The appearance of stains, spots, streaks, cracks, bloom, chalking, stickiness, warping, fish eyes, wrinkling, shrinkage, scorching, optical distortion, and changes in optical color.
2. Changes in physical properties
Including changes in solubility, swelling, rheological properties, cold resistance, heat resistance, water permeability, air permeability and other properties.
3. Changes in mechanical properties
Changes in tensile strength, flexural strength, shear strength, impact strength, relative elongation, stress relaxation and other properties.
4. Changes in electrical properties
Such as changes in surface resistance, volume resistance, dielectric constant, electrical breakdown strength, etc.
03 Factors causing the aging of polymer materials
1. Macro analysis
Because during the processing and use of polymers, they will be subject to the combined effects of environmental factors such as heat, oxygen, water, light, microorganisms, and chemical media. Their chemical composition and structure will undergo a series of changes, and their physical properties will also change accordingly. Deterioration, such as hardness, stickiness, brittleness, discoloration, loss of strength, etc. These changes and phenomena are called aging.
2. Microscopic analysis
High molecular polymers will form excited state molecules under the action of heat or light. When the energy is high enough, the molecular chains will break to form free radicals. The free radicals can form chain reactions inside the polymer, continue to cause degradation, and may Cause cross-linking.
If oxygen or ozone is present in the environment, a series of oxidation reactions will be induced to form hydroperoxides (ROOH), which will further decompose into carbonyl groups.
If there are residual catalyst metal ions in the polymer, or metal ions such as copper, iron, manganese, cobalt, etc. are introduced during processing and use, the oxidative degradation reaction of the polymer will be accelerated.
04 Aging test
In the development or improvement of new materials, in order to verify their service life or anti-aging effect, aging testing is required. Common aging tests include natural aging and laboratory accelerated aging.
1.Natural aging
Natural aging is to expose the material sample directly to the natural environment. Usually the sample is installed on the exposure rack at a certain angle. Common exposure angles are 5°, 45°, and 90°. Relevant testing standards include ISO 877 Plastics - Methods of exposure to solarradiation; ISO2810 Paints and varnishes - Natural weathering of coatings - Exposure and assessment; ASTMG7 Standard Practice for Atmospheric Environmental Exposure Testing ofNonmetallic Materials, etc.
The natural aging test method is simple and low-cost, but its test cycle is too long, which affects the optimization progress of product design. Moreover, since it is a natural environment and the climate conditions cannot be controlled, in order to ensure the reproducibility of the test results, the selection of the test site is particularly important. The United States established a natural climate field in South Florida in 1931, which is a standard hot and humid climate exposure field in the United States. The test site established in central Arizona is a standard dry heat exposure site. The Turpan Exposure Test Site of my country's National Motor Vehicle Product Quality Supervision and Inspection Center is also a typical dry and hot climate exposure site. The maximum temperature in Turpan area from May to August is above 40 ℃, the extreme maximum temperature is 49.6 ℃, and the average annual precipitation is only 8 mm. The exposure field in Qionghai, Hainan has typical hot and humid climate conditions. The annual average temperature is 27.4 ℃, and the average annual precipitation is as high as 2134 mm.
2. Accelerated aging in the laboratory
In order to speed up the test cycle and obtain aging data faster, the laboratory usually uses artificial light sources to simulate solar radiation, match different temperature, humidity and rain conditions, etc., and can simulate various natural climates.
1) Selection of light source
Commonly used artificial light sources include xenon arc lamps, metal halide lamps and ultraviolet fluorescent lamps. UV fluorescent lamps can simulate sunlight very well in the medium-wave UV and short-wave UV ranges. Xenon arc lamps and metal halide lamps can simulate sunlight very well in the full spectrum. Therefore, test chambers that use xenon lamps and metal halide lamps as light sources can well simulate sunlight radiation, while aging chambers that use fluorescent ultraviolet lamps are not intended to imitate sunlight, but only simulate the aging effect of sunlight. In addition, there are aging boxes on the market that use carbon arc lamps as the light source. However, the carbon arc spectrum does not have a good correlation with the sunlight spectrum, and the use of carbon arc lamp testing is for historical reasons.
2) Relevance of accelerated aging
Correlation refers to the degree of consistency between the accelerated aging results in the laboratory and the aging results of the material in the actual use environment. Only when the accelerated aging test is relevant can it truly reflect the weather resistance of the material and truly predict the service life of the material. Unreasonable accelerated testing will reduce the relevance of the test and even lose its meaning.
3) The development trend of accelerated aging in laboratories
As mentioned at the beginning, the influencing factors of material aging include solar radiation, temperature, water and other factors. The aging of materials is the result of the joint action of these factors, but it is not a simple superposition of the effects of various factors. The synergy between them also needs to be considered. Therefore, a more comprehensive simulation of the actual use environment of the material can lead to better relevant results. For example, according to the ISO 20340 standard, the test is based on a cycle of 7 days. On the 1st to 3rd day, a UV test with a light and dark cycle is performed according to ISO 11507. On the 4th to 6th day, a salt spray test is performed according to ISO 9227. On the 7th day (-20 Low temperature test of ±2)℃. Compared with the traditional weather resistance test, it integrates more aging influencing factors and is more in line with the actual use conditions of the material, so it can better reflect the actual aging of the material. We know that mold, ozone concentration, etc. all have an important impact on the aging of plastic products. How to integrate more aging factors in testing will be one of the development directions of accelerated aging in laboratories.
05 Methods for anti-aging of polymer materials
At present, the main methods to improve and enhance the anti-aging properties of polymer materials include the following:
1. Physical protection (such as thickening, painting, outer layer compounding, etc.)
The aging of polymer materials, especially photooxygen aging, first starts from the surface of the material or product, manifesting as discoloration, powdering, cracking, gloss loss, etc., and then gradually penetrates deeper into the interior. Thin products are more likely to fail prematurely than thick products, so the service life of the product can be extended by thickening the product. For products that are prone to aging, you can apply a layer of coating with good weather resistance on the surface, or compound a layer of material with good weather resistance on the outer layer of the product to attach a protective layer to the surface of the product. Slow down the aging process.
2. Improve processing technology
Many materials also have aging problems during the synthesis or preparation process. For example, the influence of heat during polymerization, thermal oxygen aging during processing, etc. Correspondingly, the impact of oxygen can be mitigated by adding an oxygen removal device or a vacuum device during polymerization or processing. However, this method can only guarantee the performance of the material when it leaves the factory, and this method can only be implemented from the source of material preparation, and cannot solve the aging problem during its reprocessing and use.
3. Structural design or modification of polymer materials
Many polymer materials contain groups that are very susceptible to aging in their molecular structure. Therefore, through the molecular structure design of the material, replacing the groups that are prone to aging with groups that are not prone to aging can often achieve good results. Or, functional groups or structures with anti-aging effects can be introduced into the polymer chain through grafting or copolymerization, giving the material itself excellent anti-aging functions. This is also a method often used by researchers, but the cost is relatively high. High, large-scale production and application cannot be achieved for the time being.
4. Add anti-aging additives
At present, an effective and common way to improve the aging resistance of polymer materials is to add anti-aging additives, which are widely used because of their low cost and the absence of changes to existing production processes. There are two main ways to add these anti-aging additives:
Direct addition method of additives: that is, the anti-aging additive (powder or liquid) is directly mixed and stirred with raw materials such as resin, and then extruded for granulation or injection molding, etc. Because this addition method is simple and easy to implement, it is widely used by the majority of pelletizing and injection molding factories.
Anti-aging masterbatch addition method: Manufacturers with higher requirements for product quality and quality stability more often use the method of adding anti-aging masterbatch during production. Anti-aging masterbatch is obtained by using a suitable resin as a carrier, mixed with a variety of efficient anti-aging additives, and then co-extruded and granulated by a twin-screw extruder. Its application advantage lies in the use of anti-aging additives in the masterbatch preparation process. Firstly, pre-dispersion is achieved, and then in the later material processing process, the anti-aging additives are secondary dispersed, achieving the purpose of uniform dispersion of the additives in the polymer material matrix, which not only ensures the quality stability of the product, but also It avoids dust pollution during production, making production more green and environmentally friendly.