Polymer materials in its synthesis, storage and processing and final application of various stages may occur metamorphism, that is, the performance of the material deteriorates, such as yellowing, molecular weight decline, products surface crack, luster loss, more serious is the impact strength, flexion strength, tensile strength and elongation and other mechanical properties decreased significantly, thus affecting the normal use of polymer materials products To use. This phenomenon is called plastic aging. From the perspective of chemistry, plastic materials, whether natural or synthetic, have certain molecular structure, some parts of which have some weak bonds, these weak bonds naturally become the breakthrough of chemical reactions. The essence of plastic aging is nothing more than a chemical reaction, that is, starting from the chemical reaction of weak bonds (such as oxidation reaction) and triggering a series of chemical reactions. It can be caused by many causes, such as heat, ultraviolet light, mechanical stress, high-energy radiation, electric fields, and so on. It can be a single factor or a combination of factors. The result is that the molecular structure of the polymer material changes and molecular weight decreases or produces crosslinking, thus the material performance deteriorates and can not be used.
The most common causes of aging are heat and ultraviolet light, because from the production, storage, processing and use of plastic products are most exposed to the environment of heat and sunlight (ultraviolet light). The study of plastic aging caused by these two environments is of particular importance to the practical operator. Therefore, this book focuses on the discussion of thermal aging and UV aging of plastics and their anti-aging techniques. If you are interested in other aging and anti-aging technologies, please refer to the relevant specialized books.
The so-called anti-aging, also known as stabilization, is to take certain measures to prevent or delay the chemical reaction to aging. Strict approach, can not completely prevent aging, can only delay the aging process. At present, the more suitable anti-aging measures are as follows:
① Improve the chemical structure of copolymer, introduce the structure containing stable groups, such as the use of ethylene monomer containing antioxidant copolymerization modification .
② The active terminal group was deactivated and stabilized, which was mainly used for polyacetal polymers.
③ Physical stabilization, such as tensile orientation.
④ Add additives, such as antioxidants and light stabilizers.
Among them, method 4 is the most common method of plastic aging prevention, its advantages are simple, effective and flexible. Usually, the amount of additives in the range of 0.1%~1%, add the additive time, from the synthesis of the polymer to the final application of the various stages can be, but in principle to as early as possible to add people. In application, the core problem is how to choose the right additive system, which is not only a technical problem. The right choice is the result of a combination of many factors, including technical, economic, social and legislative factors. For example, in addition to the type of resin and the use of its products as well as the cost, user acceptance, food application licensing, environmental requirements, legal restrictions and all technological developments related to the application must also be considered.
Excerpt from "Plastic Aging and Anti-Aging Technology"