Plastic in the use, processing, and storage process, due to external factors, such as light, oxygen, heat, radiation, odor, rain, mold, bacteria, etc., destroy the chemical structure, so that the original good performance is missing, this phenomenon is commonly known as aging. Plastic aging is mainly manifested in the appearance of discoloration, physical property changes, mechanical property changes, and electrical property changes.
Undoubtedly, the relationship between plastic and our human life is closely related. It has the advantages of being lightweight, durable, and easy to process, so it is widely used in various fields. However, with time, plastic will also face a very serious problem, that is, aging. Aging plastic products will lose their original performance and appearance, and even cracks, deformation, and other problems.
1、The scope of the plastic aging resistance test:
Plastic particles, plastic wire, plastic rope, plastic tape, hose, hard pipe, bellows, plastic sheet, plastic film, plastic switch, plastic doors and windows, plastic rods, artificial leather, cables, foam materials, plastic profiles, and car accessories.
2、Plastic aging performance
Plastic aging and how to extend the service life of plastic products, has been an important topic that the plastics industry practitioners always have to face. So what kind of standard is there for plastic aging? At present, the industry generally believes that, including plastics, rubber, and fiber, the three major types of polymer materials, their aging can be attributed to the following four types of changes.
① Changes in appearance
Stains, spots, silvering, cracks, frosting, chalking, sticking, warping, fish-eye, wrinkles, shrinkage, optical aberrations, and changes in optical color.
② Changes in physical properties
Including solubility, swelling, and rheological properties, as well as cold, heat, water permeability, air permeability, and other performance changes.
③ Changes in mechanical properties
Tensile strength, bending strength, shear strength, impact strength, relative elongation, stress relaxation, and other performance changes.
④ Changes in electrical properties
Such as surface resistance, volume resistance, dielectric constant, electric breakdown strength, and other changes.
3、The causes of plastic aging
We already know the main characteristics of aging, then what factors lead to these aging phenomena? Based on scientific research data, the industry has reached a general understanding of the causes of aging.
On the one hand, it is determined by the molecular nature of the plastic itself. Unsaturated double bonds, branched chains, carbonyl groups, hydroxyl groups on the end, etc. contained in the molecular chain of plastics are easy to cause the composition of aging, and a certain degree of degradation will occur with time. In the process of degradation, the products of degradation form free radicals, making the molecular chain chain reaction, and accelerating the rate of degradation.
On the other hand, under the influence of environmental and external factors, such as sunlight, oxygen, heat, water, radiation, industrial gases, molds, bacteria, insects, etc., the internal molecular weight of the plastic changes, resulting in the phenomenon of aging.
4、 Leading to plastic aging of the four factors
①Light
Plastic in the sun will lead to aging, this is because the ultraviolet rays will destroy the plastic molecular chain, making it fragile. Especially in a high-temperature environment, the destructive effect of ultraviolet rays is more obvious. Therefore, plastic products exposed to sunlight for a long time are prone to aging.
②Thermo-oxidative aging
Plastic in a high-temperature environment is prone to thermo-oxidative aging, this is because oxygen will react with the plastic molecules, so that its molecular chain breakage, thus leading to plastic aging. In addition, high temperatures will make the plastic molecules intensify the movement of the molecular chain is easy to break, thus accelerating the plastic aging.
Only UV has been irradiated products, after a long time the surface of the products will form a protective layer that will no longer age. A humid environment will cause thermal expansion and contraction, the products will age faster. If there is a spray, the impact on the surface of the products, damages the protective layer, resulting in the more pronounced aging phenomenon.
③Chemical aging
Plastic in contact with chemical substances is prone to chemical aging, this is because the chemical substances will react with the plastic molecules, so that its molecular chain breakage, thus leading to plastic aging. For example, plastic products are susceptible to aging when exposed to acid, alkali, and other chemical substances for a long period.
④Mechanical stress
Plastic is prone to aging when subjected to mechanical stress, which is because mechanical stress will cause the plastic molecular chain to break, thus leading to plastic aging. For example, plastic products are susceptible to aging when subjected to mechanical stresses such as extrusion and stretching for a long period.
The causes of plastic aging mainly include light, thermal, and oxygen aging, chemical aging, and mechanical stress. To extend the service life of plastic products, we should try to avoid the influence of these factors. For example, avoid direct sunlight when storing, avoid prolonged exposure to chemical substances, etc. At the same time, we should also choose good quality, aging-resistant plastic products to ensure their service life and safety.
5、Plastic aging test method
The most common aging factors for plastics in the natural environment are heat and ultraviolet light. Because plastic materials are most exposed to the environment are heat and sunlight. The study of plastic aging caused by these two types of environment for the actual use of the environment has a particularly important significance. Aging tests can be broadly divided into two categories: outdoor exposure and laboratory-accelerated aging tests.
5.1 Outdoor Exposure Test
Direct outdoor exposure refers to direct exposure to sunlight and other climatic conditions and is the most direct method of assessing the weathering performance of plastic materials.
Advantages.
Low absolute cost, very good coincidence, simple, easy to operate.
Disadvantages:
Usually very long cycle times, global climate diversity, and inconsistent sensitivity of different samples in different climates.
5.2 Laboratory accelerated aging test methods
5.2.1 Xenon arc lamps simulate the complete full solar spectrum, which encompasses the ultraviolet, visible, and infrared spectra. Filtered xenon arc lamps are the best light source for testing the light stability of products such as masterbatches and pigments, which are sensitive to long wavelengths of sunlight and visible light. The xenon arc lamp can be precisely adjusted for its spectral energy distribution. It can simulate natural light under a variety of conditions as well as everything from sunlight to daylight through glass windows. In addition, by varying the irradiation intensity, temperature, humidity, and other parameters of the xenon lamp, it is possible to simulate the environment in which different materials are used, such as automotive interiors and exteriors. Currently, artificial accelerated aging tests using xenon lamps have become a preferred and universal light aging test method. There are many corresponding xenon lamp aging test methods, such as ISO, ASTM, SAE J, GM, and so on.
5.2.2 UV fluorescent lamp light aging test method
The UV aging test mainly simulates the deterioration effect of UV light from sunlight on products. It also reproduces the damage produced by rain and dew. It is conducted by exposing the material to be tested to a controlled interactive cycle of sunlight and moisture while raising the temperature. UV fluorescent lamps are used to simulate sunlight, while the effects of moisture can also be simulated by condensation or spraying.
Fluorescent UV lamps are low-pressure mercury lamps with a wavelength of 254 nm, which are converted to longer wavelengths by the addition of phosphorus coexistants. The energy distribution of fluorescent UV lamps depends on the emission spectrum produced by the phosphorus coexistence and the diffusion of the glass tube. Fluorescent lamps are usually classified as UVA versus UVB. The material exposure application determines which type of UV lamp should be used.
5.2.3 Carbon arc light aging test methods
Carbon arc lamps are an older technology and carbon arc instruments were originally used to assess the lightfastness of dyed textiles. Carbon arc lamps are divided into closed and open carbon arc lamps. Regardless of the type of carbon arc lamp, the difference between its spectrum and that of sunlight is relatively large. Due to the long history of the technology in this project, the original artificial simulated light aging techniques used this equipment. Therefore, the method can still be found in earlier standards, especially in Japan, where the carbon arc lamp technology is often used as the artificial light aging test method in earlier standards.
5.2.4 Ozone Aging Test
Ozone is an extremely rare gas in the atmosphere, but it is extremely destructive to plastic materials. Ozone can have an irreversible chemical reaction with the unsaturated bonds and reducing groups in the chemical structure of plastic materials. This leads to oxidative degradation of the plastic material, which can cause it to lose its usefulness. Ozone aging tests on plastic materials are usually carried out with an ozone aging tester. The ozone required for the test is supplied by an ozone generator, the concentration of which can be adjusted by mixing it with air through a mixer. The concentration of ozone is generally determined by the environmental conditions under which the material is used. In addition, the temperature, humidity, and other factors inside the ozone aging chamber can also be adjusted to achieve the purpose of the test. In turn, the ozone aging resistance of the material as well as the behavior and law of ozone aging are obtained.
5.2.5 Hot air aging test
Heat is one of the main factors leading to the aging of plastic materials, heat can accelerate the movement of polymer chains, leading to polymer chain breakage, generating reactive free radicals, causing free radical chain reaction, leading to degradation or cross-linking of polymers. The hot air aging test is one of the main test methods for evaluating plastic materials and researching the aging resistance of plastic materials, which is usually carried out in the constant temperature blast drying test box. The temperature in the drying box can be set according to the test requirements, plastic materials are exposed to the dryness of the regular sampling, and testing, to obtain the aging behavior of plastic materials and the law, to target the plastic material modification, improve its performance;
5.2.6 Temperature alternating aging test
Temperature is another important factor leading to the aging of plastic materials, high and low temperature alternating aging tests to evaluate the aging test method of temperature resistance of plastic materials, usually in the temperature alternating aging test chamber, from a certain temperature T1 (generally room temperature) to a constant rate of heating to a certain temperature T2, maintain the T2 temperature for a certain period, and then a constant rate of cooling down, cooling down to a certain temperature T3, and then a certain temperature T3. Maintain the T3 temperature for a certain period, and then in the temperature to T1, this is a temperature cycle. Cycle length, according to the requirements of the specific test to determine;
5.2.7 Damp heat aging test
The moisture and heat aging test is an effective method to evaluate the aging resistance of plastic materials under high humidity and high-temperature environments. Under a high-humidity environment, moisture can penetrate the interior of plastic materials, leading to the dissolution of plastic materials and hydrolysis of some hydrophilic groups, resulting in the aging degradation of plastic materials. In addition, the penetration of water into the plastic material will also lead to the dissolution and migration of additives (such as plasticizers, and compatibilizers) and other substances inside the plastic material, affecting the mechanical properties of the plastic material. Heat can promote the polymer chain movement intensified, so that the intermolecular force is reduced, promote the penetration of water, and accelerate the degradation of plastic materials;
5.2.8 Medium aging test
Certain plastic materials in the use of the process of long-term immersion in a certain medium, such as offshore or submarine operation of the equipment above the plastic material to be long-term immersion in seawater, aviation aircraft certain parts to long-term contact with aviation fuel, etc., which require plastic materials have a strong resistance to media aging performance. Media aging test is a common test method to assess the aging resistance of plastic materials and predict their life in certain media. Plastic testing of the media used, according to the specific use of plastic materials in the environment for the preparation, can be man-made seawater, salt water, rain, acid, and alkali solutions, fuel oil, and other organic solvents, etc..
6、Anti-aging method of polymer materials
Sunlight consists of ultraviolet, infrared, and visible light, of which visible light has seven colors red, orange, yellow, green, blue, indigo, and violet. Sunlight, broadly defined, is electromagnetic radiation from all spectrums of the sun. On Earth, sunlight is visible when the sun is above the horizon, and solar radiation filtered through the Earth’s atmosphere to the Earth’s surface is called daylight.
For plastics, most of the sensitive wavelengths of ultraviolet light contained in terrestrial sunlight are between 280 and 400nm, which is the sensitive wavelength at which plastics are most susceptible to damage. The energy of irradiation can break the chemical bond between the molecules of the material itself, making the material appear gloss reduction, chalking, fracture, and other aging phenomena, what methods can be used to better combat the problem of aging, to extend the service life of plastic products?
6.1 Physical protection (such as thickening, coating, outer layer composite, etc.)
Polymer material aging, especially light oxygen aging, first from the surface of the material or product, manifested as discoloration, chalking, cracking, gloss reduction, etc., and then gradually to the internal depth. Thin products are more likely to fail earlier than thick products, so the service life of the products can be extended by thickening the products. For products that are easy to age, a layer of weather-resistant coating can be coated on the surface, or a layer of weather-resistant material can be compounded on the outer layer of the products so that a protective layer can be attached to the surface of the products to slow down the aging process.
6.2 Improve the processing technology
Many materials in the synthesis or preparation process, there is also the problem of aging. For example, the influence of heat in the polymerization process, thermal and oxygen aging in the processing, etc. Then, accordingly, it can be added to the polymerization or processing of oxygen removal devices or vacuum devices to slow down the impact of oxygen. However, this method can only guarantee the performance of the material at the time of leaving the factory, and this method can only be implemented from the source of the preparation of the material, and can not solve the aging problem in its reprocessing and use.
6.3 Structural design or modification of polymer materials
Many polymer materials’ molecular structures of the existence of extremely easily aging groups, then through the molecular structure of the material design, not easy to aging group instead of easy to aging group, often can play a good effect. In the polymer molecular chain through grafting or copolymerization method to introduce functional groups or structures with an anti-aging effect, to give the material itself excellent anti-aging function, is also often used by research workers, but the cost is high, and temporarily can not achieve large-scale production and application.
6.4 Add anti-aging additives
At present, the effective way to improve the aging resistance of polymer materials and the commonly used method is to add anti-aging additives, which are widely used due to the low cost, and no need to change the existing production process. There are two main ways to add these anti-aging additives:
① Direct addition of additives: anti-aging additives (powder or liquid) and resin and other raw materials are directly mixed and stirred, and then extruded and pelletized or injected into the mold, etc. This method of addition is simple and easy to implement. This addition method is simple and easy to implement, so it is widely used by the majority of granulation and injection molding factories.
②Anti-aging masterbatch addition method: in the product quality and quality stability of higher requirements of manufacturers, more is used in the production of anti-aging masterbatch added way.
Anti-aging masterbatch is a suitable resin as a carrier, mixed with a variety of highly effective anti-aging additives, and then co-extruded by the twin-screw extruder granulation obtained, the advantage of its application is that the anti-aging additives in the preparation of the masterbatch in the process of the first pre-dispersion, and then in the process of the later material processing, the anti-aging additives to get the second dispersion, to achieve the purpose of the additives uniformly dispersed in the polymer matrix, not only to ensure the quality and stability of products, but also to avoid production of the product. This not only ensures the quality stability of the product but also avoids dust pollution during production, which makes the production more green and environmentally friendly.
7、Summary
The anti-aging test for plastics is designed to achieve a satisfactory pass rate. Almost all plastic products must be aged before they leave the factory. One reason for aging is to improve the reliability of the product. So far, there are no alternatives. For new products, where the performance of new components or complete machines needs to be evaluated, the aging index is higher. Expose the defects of aging products before leaving the factory, such as the reliability of welding joints and various defects in product design, material, and process; make the performance of aging products enter the stable range before leaving the factory, and lower the rate of return for repair.
Plastic aging resistance testing mainly simulated plastic products in the real environment during the use of a variety of harsh conditions of high-intensity testing, according to the actual requirements, a reasonable prediction of the service life of plastic products. Considering the complex aging mechanism, it is necessary to carry out a comprehensive assessment after a basic understanding of the role of various major factors on polymers. Therefore, to explain the complexity of polymer stability under the influence of various parameters, it is necessary to accumulate more basic information and endeavor to establish practical accelerated aging test methods.
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