Factors affecting the discoloration of plastic products
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Plastic coloring products can lose their vibrancy over time due to various factors. The fading of these products is influenced by the lightfastness of pigments, dyes, as well as their oxidation resistance, heat resistance, and acid/alkali resistance. Additionally, the type of resin used plays a significant role. Depending on the processing conditions and intended use of the plastic items, the necessary properties of pigments, dyes, surfactants, dispersants, carrier resins, and anti-aging additives can be chosen when producing masterbatches.
Firstly, the lightfastness of colorants directly impacts how quickly a product fades. For outdoor items exposed to intense sunlight, the lightfastness rating of the colorant is crucial. If this rating is low, the product may fade prematurely. For weather-resistant products, the lightfastness should be at least grade six, with grades seven or eight being ideal. Indoor products can typically get away with grades four or five. The carrier resin’s light resistance also matters, as UV exposure can alter its molecular structure and cause fading. Adding UV absorbers or other light stabilizers to the masterbatch can enhance the colorants' lightfastness.
Secondly, the thermal stability of pigments refers to how they hold up under processing temperatures. Inorganic pigments like metal oxides and salts are highly stable and heat-resistant. However, organic pigments can degrade or change color at higher temperatures. This is particularly relevant for plastics like PP, PA, and PET, where processing often exceeds 280°C. When choosing pigments, it's essential to consider both their heat resistance and the duration they need to withstand high temperatures—usually between 4 to 10 minutes. Certain metal ions in pigments can accelerate resin degradation. Pigments containing copper, manganese, iron, and cobalt significantly affect polyolefins negatively, whereas those with potassium, sodium, cadmium, and calcium have less impact. Metal elements tend to have a stronger aging effect in the order of pure metal, metal oxide, and metal salt.
Thirdly, oxidation resistance is another critical factor. Some organic pigments degrade over time due to macromolecule breakdown or oxidation. High-temperature processing and strong oxidants like chromate in chromium yellow can exacerbate this issue. Combining certain pigments like lakes, azo pigments, and chrome yellow can lead to fading, especially in red hues.
Lastly, the fading of acid-alkali stained plastics depends on the chemical resistance of colorants. Molybdenum chromium red, for instance, resists mild acids but is sensitive to alkaline environments, while cadmium yellow lacks acid resistance. These pigments combined with phenolic resins can diminish some colorants, impacting their heat resistance, weather resistance, and fading tendencies.
In summary, achieving durable and vibrant plastic coloring requires careful consideration of multiple factors, including lightfastness, thermal stability, oxidation resistance, and chemical compatibility. Proper selection and formulation can help ensure that colored plastics maintain their appearance over time.
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