Every technical specialist in general knows that molecular internal structure of plastic determines its tendency to auto-oxidize, as a result of which material turns yellow, hardens, cracks, loses shine, etc., and its physical and mechanical properties deteriorate. also greatly reduced, such as impact strength, bending strength, elongation at break, etc., thereby shortening service life of plastic products. Aside from technical reasons, question may be that I added antioxidants to my product, and is there enough added parts to last until product's expiration date? Therefore, today we will focus on plastic additives - antioxidants, and understand why our products need antioxidants from main mechanism of action.
1. The main cause of plastic aging
There are two main causes of plastic aging, as shown in Table 1-1:
Table 1-1 Main Causes
Second, antioxidants and how they work
To prevent plastic from aging, we need to add what we call antioxidants to it to ensure quality of our products. Antioxidant (AO), as a functional additive to polymer materials, can not only effectively reduce autoxidation reaction rate of plastics, but also slow down aging and degradation of plastics. It is widely used in all plastic products. It is well known as 1010, 1076, 168, BHT and other antioxidants are widely used in plastic products.
Figure 1. Antioxidant 1010
It is generally accepted that process of autoxidation of polymeric materials is carried out according to free radical reaction process, and reaction can be divided into three stages: chain initiation, chain transfer and chain termination. Thermal-oxidative degradation of polymers is mainly caused by a chain-type free radical reaction caused by ROOH hydroperoxide formed in chain initiation step, which generates free radicals R·free radicals when heated.
By way, everyone knows purpose of adding antioxidants to plastics: to scavenge free radicals produced and hydroperoxide ROOH formed during decomposition to prevent RH autoxidation chain reaction process, thereby inhibiting polymerization. substances, thereby ensuring long-term suitability of our products.
Figure 2. Mechanism of action of antioxidants
3. Types of antioxidants
3.1 Phenolic antioxidants
Phenolic antioxidants are compounds with substituents on one or both sides of hydroxyl group of benzene ring, as shown in Fig. 3. It can be seen from figure that phenolic antioxidants contain active hydroxyl groups, which can stop free radical reactions and prevent further polymer oxidation reactions.
Fig. 3. General structure
Among varieties of phenolic antioxidants, high molecular weight phenolic antioxidants, represented by numbers 1010 and 1076, have excellent thermal stability, do not easily volatilize and do not change color, and their consumption is increasing every year.
3.2 Phosphorus antioxidants
The biggest feature of phosphorus antioxidants is that they have a synergistic effect with phenolic antioxidants. Its typical antioxidant is 168. In process of anti-oxidation, phenolic antioxidants capture free radicals of peroxides and then generate hydroperoxides, and then decompose hydroperoxides, thereby inhibiting thermo-oxidative degradation of plastics. In addition, phosphorus antioxidants can also improve color stability of materials and weatherability of products.
3.3 Complex Antioxidant
Composite antioxidant is suitable for plastic processing under severe processing conditions, has high antioxidant activity and low volatility. The most representative compound antioxidant is obtained by mixing BHT and dilauryl thiodipropionate. It not only reduces production cost, but also has better anti-oxidation performance and can prolong service life of plastics.
In addition, antioxidants include sulfur antioxidants, metal deactivators, natural antioxidants, etc. for various purposes.
Different antioxidants have different antioxidant effects in same system, and same antioxidant also has different antioxidant effects in different systems. Therefore, when choosing antioxidants, one should adhere to certain principles: fully consider properties of antioxidants, pay attention to effect of antioxidants in technological environment, combination of primary and secondary antioxidants, and use amount of antioxidants correctly.
Disclaimer: All text/images and other manuscripts marked "Source: XXX" are intended to provide additional information and facilitate industry discussion under this heading and do not constitute agreement with them. The content of article is for reference only. If there is any infringement, please contact us to have it removed. All texts/figures and other manuscripts in which author is indicated at beginning of article are originals with this heading number, if necessary, it is necessary to obtain authorization with this heading number.