1. AGING TYPES OF POLYMER MATERIALS
In the process of processing, storage and use of polymer materials, due to the comprehensive action of internal and external factors, their properties gradually deteriorate, resulting in the final loss of use value, this phenomenon belongs to the aging of polymer materials.
This not only causes a waste of resources, but also leads to more serious accidents because of its functional failure, and the material decomposition caused by its aging may also pollute the environment.
Due to different kinds of polymers and different use conditions, there are different aging phenomena and characteristics. In general, the aging of polymer materials can be classified into the following four types of changes:
01.A change in appearance
There are stains, spots, crazes, cracks, frosting, pulverization, sticking, warping, fisheye, wrinkling, shrinkage, scorch, optical distortion and changes in optical color.
02.Changes in physical properties
It includes the changes of solubility, swelling, rheology, cold resistance, heat resistance, water permeability, air permeability and so on.
03.Changes in mechanical properties
The changes of tensile strength, bending strength, shear strength, impact strength, relative elongation, stress relaxation and so on.
04.Changes in electrical properties
Such as the changes of surface resistance, volume resistance, dielectric constant, electrical breakdown strength and so on.
2. AGING TEST
In the development or improvement of new materials, aging testing is needed to verify its service life or anti-aging effect. Common aging tests include natural aging and accelerated aging in the laboratory.
01.Natural aging
Natural aging is the direct exposure of material samples to the natural environment. Usually, the sample is installed on the exposure rack at a certain angle, and the common exposure angles are 5 °, 45 °and 90 °. The relevant testing standards are ISO 877 Plastics — Methods of exposure to solarradiation；ISO2810 Paints and varnishes – Natural weathering of coatings – Exposure andassessment；ASTMG7 Standard Practice for Atmospheric Environmental Exposure Testing of Nonmetallic Materials and so on。
The natural aging test method is simple and the cost is low, but the test period is too long, which affects the optimization progress of product design.Moreover, because of the natural environment, the climatic conditions can not be controlled, in order to ensure the reproducibility of the test results, the selection of the test site is particularly important.
02.Accelerated aging in laboratory
In order to speed up the test cycle and get the aging data faster, the laboratory usually uses artificial light sources to simulate solar radiation, matching different temperature, humidity and rain conditions, and can simulate a variety of natural climate.
001.Selection of light source
The commonly used artificial light sources are xenon arc lamp, metal halide lamp and ultraviolet fluorescent lamp.Ultraviolet fluorescent lamp can simulate daylight very well in the range of medium wave ultraviolet and short wave ultraviolet.The xenon arc lamp and metal halide lamp can well simulate daylight in the full spectrum.Therefore, the test chamber using xenon lamp and metal halide lamp as light source can well simulate solar radiation, while the purpose of aging box using fluorescent ultraviolet lamp is not to imitate sunlight, but only to simulate the aging effect of sunlight.
002.Correlation of accelerated aging
Correlation refers to the degree of consistency between the results of accelerated aging in the laboratory and the aging results of materials in the actual use environment.Accelerated aging test, only with correlation, can truly reflect the weather resistance of materials and truly predict the service life of materials.Unreasonable accelerated test will reduce the relevance of the test, and even lose its meaning.
003.The development trend of accelerated aging in laboratory
The influencing factors of material aging are sunlight 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 various factors, and the synergistic effect needs to be considered.Therefore, a more comprehensive simulation of the actual use environment of materials can get better correlation results.For example, according to the ISO 20340 standard, the experiment takes 7 days as a cycle, ultraviolet test with light and dark cycle according to ISO 11507 on the 1st to 3rd day, salt spray test on the 4th to 6th day according to ISO 9227, and low temperature test of (- 20 ±2) ℃ on the 7th day.Compared with the traditional weathering test, it integrates more aging factors, which is more in line with the actual working conditions of the material, so it can better reflect the actual aging of the material.We know that mold and ozone concentration have an important impact on the aging of plastic products. How to integrate more aging factors in the test will be one of the development directions of accelerated aging in the laboratory.
3.METHODS OF ANTI-AGING OF POLYMER MATERIALS
At present, the main methods to improve the anti-aging properties of polymer materials are as follows:
01.Physical protection (e. G. thickening, painting, outer layer composite, etc.)
The aging of polymer materials, especially photooxidation aging, starts from the surface of materials or products, such as discoloration, pulverization, cracking, glossiness decline and so on, and then gradually goes deep into the interior. Thin products are more likely to fail earlier than thick products, so the service life of products can be prolonged by thickening products. For easily aging products, they can be coated or coated with a coating with good weather resistance, or a layer of materials with good weather resistance can be compounded on the outer layer of the products, so that a protective layer is attached to the surface of the products, thus delaying the aging process.
02. Improve the processing technology
Many materials also have the problem of aging in the process of synthesis or preparation. For example, the influence of heat in the polymerization process, the thermal oxygen aging in the processing process and so on. Accordingly, the effect of oxygen can be mitigated by adding deaerators or vacuum devices in the polymerization or processing process. However, this method can only ensure the performance of the material when it leaves the factory, and this method can only be implemented from the source of material preparation, and can not solve the aging problem in the process of reprocessing and use.
03.Structural Design or Modification of Polymer Materials
There are very aging groups in the molecular structure of many polymer materials, so through the molecular structure design of materials, replacing aging groups with non-aging groups can often have a good effect. Or the functional groups or structures with anti-aging effect are introduced into the polymer chain by grafting or copolymerization to endow the material itself with excellent anti-aging function, which is also a method often used by researchers, but the cost is high. can not achieve large-scale production and application for the time being.
04.Add anti-aging additives
At present, the effective and common way to improve the aging resistance of polymer materials is to add anti-aging auxiliaries, which is widely used because of its low cost and no need to change the existing production process. There are two main ways to add these anti-aging auxiliaries:
Direct addition of auxiliaries: that is, anti-aging auxiliaries (powder or liquid) are directly mixed with resin and other raw materials after extrusion, granulation or injection molding, and so on. Because this kind of adding method is simple and easy, it is widely used by the vast number of granulation and injection moulding factories.
Anti-aging masterbatch addition method: in the manufacturers with high requirements for product quality and quality stability, it is more common to add anti-aging masterbatch during production. The anti-aging masterbatch is obtained by using a suitable resin as the carrier, mixed with a variety of efficient anti-aging auxiliaries, and then co-extruded and granulated by a twin-screw extruder. Its application advantage is that the anti-aging auxiliaries first achieve pre-dispersion in the preparation process of the masterbatch, so in the later process of material processing, the anti-aging auxiliaries get secondary dispersion, achieving the purpose of uniform dispersion of the auxiliaries in the polymer matrix. It not only ensures the quality stability of the product, but also avoids the dust pollution during production, which makes the production more green and environmentally friendly.
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