In order to avoid the photodegradation and aging problems of plastic products and extend their service duration, it is necessary to add UV stabilizers and antioxidants to the formulation. They can help plastics have the ability of uv resistance and oxidation resistance that may occur during processing and in nature. So, let us briefly know about these two additives below.
What is UV Resistant?
UV stabilizer is a kind of substances that can inhibit or weaken the degradation effect of UV light on plastic and improve the light resistance of plastic products. Because plastics are often affected by oxygen and light in nature, they will undergo a series of reactions, causing the compounds in the plastics to break, degrade and crosslink, leading to the destruction of plastic.
UV stabilizers are generally divided into 4 categories:
- Hindered Amine Light Stabilizers (HALS): The most widely used type of UV inhibitor, accounting for about 60% of the total consumption. Their mechanism is to decompose hydroperoxides, quench excited oxygen, remove free radicals by photo-oxidation, and the effective groups can be recycled and regenerated (Denisov cycle). It is a long-acting heat stabilizer that can effectively limit the photodegradation process. Although the HALS products on the market are quite different in structure, they all adopt 2,2,6,6-tetramethylpiperidine ring structure. At present, the most widely used models are 622, 944, 783, Tinuvin326, Tinuvin327, etc.
- Ultraviolet absorbers: the second largest UV stabilizer, accounting for about 25% of total consumption. The role of ultraviolet stabilizers is generally to absorb radiation from ultraviolet light, and then disperse these energy into low-level heat. Ultraviolet absorbers mainly include benzophenones, benzotriazoles, triazines and salicylates. The most commonly used UV absorber is UV326.
- Quenchers: Quenchants are substances that rapidly and effectively quenched excited molecules by means of energy transfer between molecules, which are converted into heat energy, and the radiation dissipates and returns to the ground state. Industrial products are mainly Nickel Quenchers, and those are a commonly used type in agricultural film. However, due to the heavy metal nickel and resulting in color changes of plastics (change to brown or green) , some areas have stopped using it.
- UV Light shielding agent: Carbon black masterbatch, titanium dioxide and zinc oxide are mainly UV light shielding agent. Carbon black is one of the most effective and commonly used ultraviolet stabilizers.
Major foreign manufacturers of UV stabilizers include： BASF, Clariant, Cytec, Eastman; Asahi Kasei, etc.
What is Antioxidant?
Plastic antioxidants are substances that trap active free groups to produce inactive free groups, or that can break down the polymer hydroperoxides produced during oxidation, stopping the chain reaction and delaying the oxidation of the polymer. Thus the polymer can be processed smoothly and the service life is extended.
Antioxidants are divided into primary antioxidants and secondary antioxidants.
- Primary antioxidants： The most commonly used primary antioxidants are hindered phenolic antioxidants. Hindered phenol antioxidants can react with free radicals in the resin to generate lower active groups and stable compounds. Hindered phenolic antioxidants have low molecular weight, high volatility, and migration, and are easy to color plastic products. At present, the most commonly used model of polyphenol antioxidants is 1010 and 1076. It accounts for about 60% of the total consumption of antioxidants at local and abroad.
2. Secondary antioxidants include phosphate esters and thiomalonic acid antioxidants: it can effectively decompose the peroxide (ROOH) formed by the process of primary antioxidants and prevent the thermal-oxidation reaction during polymer processing. The commonly used secondary antioxidants include antioxidant 168, antioxidant DLTP, DSTP and so on.
How to choose anti UV stabilizers and antioxidants
When choosing anti uv stabilized and antioxidants, it is necessary to consider various factors such as the type and model of the plastic, the processing equipment and processing conditions, the selection and usage of other chemical additives, the use environment, and service span of plastic products. The main reference principles are:
- Compatibility: Because both have different degrees of polarity, the compatibility of the two substances should be considered when selecting. Antioxidants, uv stabilized, and polymers are usually combined at high temperatures. Note that when designing the formula, the upper melting point of the solid antioxidant and UV stabilizer should not be lower than the processing temperature of the plastic polymer.
- Migration: The oxidation reaction usually occurs on the surface of plastic products, which requires anti uv stabilizers and antioxidants to continuously migrate from the inside of the plastic products to the surface to play a protective role. However, the speed of migration needs to be controlled. If the migration is too fast, the plastic cannot be protected for a long time. Therefore, when choosing, you can choose models with larger relative molecular weight and higher melting point.
- Stability: Stability also should be considered when applying UV stabilizers and antioxidants. Factors that need to be considered include: low loss of volatilization in various processing and use environments, no discoloration, no decomposition, no reaction with other additives, and no damage to machinery and equipment. For example, hindered amine light stabilizers are generally low-alkaline substances. In order to avoid reactions, acidic additives should not be included in the formulation, and also such plastic products with HALS cannot be used in acidic environments.
- Processability: Adding anti uv stabilizers and antioxidants will affect the melt viscosity of the resin. Therefore, if their melting point is lower than the processing temperature by more than 100 degrees, they should be made into a certain concentration of masterbatch, and then processed into the resin mixture.
- Environmental protection: Another question should be considered is environmental impact. It is also important whether they are toxic, and will pollute humans, animals, soil and water.
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