1. Oxidation and antioxidant of polymers
Polymer materials in processing, storage and use During the process, contact with oxygen is inevitable. Therefore. Oxidation reaction is a major cause of aging of polymer materials.
The oxidation process of polymer compounds is generally considered to be carried out by a free radical chain reaction. BelowRH represents polymer compounds, and R• represents macromolecular free radicals.
(1) Chain trigger
PolymerRH is exposed to the action of heat and oxygen, and breaks at the “weak points” of the polymer structure (such as branch chains, double bonds, etc.) to generate free radicals.
(2) Chain transmission and growth
The existence of free radicals in oxygenDown, automatically oxidized into peroxide free radicalsROO• and macromolecular hydrogen peroxide. Macromolecule hydrogen peroxide decomposes into chain free radicals.
(3) Termination of the chain
Macromolecule free radicals combine And terminate the chain reaction
The latter two termination methods are unstable due to the generation of peroxides, and are also easily cracked to generate macromolecular free radicals, which in turn cause chain initiation and growth.
Increase in chainLength stage, due to the generation of alkoxy radicals, the macromolecular main chain is easily Breakage, this is the degradation of macromolecules. As a result of degradation, the molecular weight decreases significantly, resulting in a decrease in the physical and mechanical properties of the polymer.
Intersection occurs at the termination stage of the chain connections, often forming uncontrolled network structures. This may greatly increase the molecular weight and lead to embrittlement, hardening, and reduced elasticity of polymer materials.
The ease with which a polymer is oxidized is related to the structure of the polymer. It is also related to the relative stability of the chain free radicals generated during oxidation. Relatedly, free radicals are relatively stable, which means that they are easier to form. That is, polymers with this structure are more likely to undergo free radical oxidation reactions. Generally speaking, polypropylene is easier to oxidize than polyethylene, and branched-chain polyethylene is easier to oxidize than straight-chain polyethylene. Polyethylene is easily oxidized and contains polymers without bubbles and chains.�Natural rubber is also relatively easy to oxidize, that’s why.
The role of antioxidants, because it can prevent the free radical chain reaction from proceeding, that is, cut off automatic oxidation into peroxide free radicals “>ROO• and macromolecular hydrogen peroxide chain growth reaction proceeds, this antioxidant is called the main antioxidantagent, also called chain terminator, represented by AH, chain terminator can interact with active free radicals R,ROO• The combination generates stable compounds or low-active free radicals, thereby preventing chain transmission and growth.
In order to better cut off the chain reaction, it is necessary to cut off the macromolecular hydrogen peroxide in the chain growth stage and decompose it into chain free radicals in two steps. Therefore, it is necessary to use an auxiliary antioxidant. Its main function is to decompose the macromolecule peroxide MROOH and generate stable chemical compounds. , thereby preventing the development of the chain reaction. This type of antioxidant is also called hydrogen peroxide decomposition system. Examples of this type of antioxidant include thiols, thioesters, phosphites, etc.
