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Why does plastic material turn yellow after injection molding? The main causes and improvement of yellowing

October 20, 2023

Yellowing refers to the phenomenon of an item turning yellow when exposed to natural light, ultraviolet radiation, heat, oxygen, stress, chemical etching, moisture, and other conditions, as well as improper production processes. This is the basic definition of yellowing.


The main reasons for yellowing are:


1. The influence of polymer structure itself is that there is bond energy between polymer macromolecular chains. When the provided energy is greater than the bond energy, the molecular chains are prone to activity concentration, leading to gradual degradation and yellowing of the polymer during use and storage.


2. The influence of light, when the applied material absorbs light energy, the molecular chains on the absorption site will produce the cleavage of carbon carbon bonds or hydrocarbon bonds. Produce yellowing phenomenon.


3. Under the influence of heat and oxygen molecules, applied materials will undergo oxidation reactions over time, and heat will accelerate the oxidation process of the material. After the formation of a peroxidized structure, it is easy to form free radicals, leading to the discoloration of transparent, light colored, and white materials. Oxygen has the most significant destructive effect on unsaturated diene materials. The effect of heat not only activates oxidation, but also leads to the fracture of the - C-C - bond and the rupture of the double bond, leading to material yellowing.


4. The yellowing of other factors is also related to the additives added to the applied materials, the presence of moisture, impurities, and processing and production processes. Raw materials mixed with various chemical or mechanical impurities can reduce the stability of the polymer. The reasons are quite complex and require targeted analysis and investigation.


Conventional solutions:


1. Based on the various characteristics of the polymer, consider using additives such as ultraviolet absorbers, antioxidants, stabilizers, etc, in the product formula.


2. Strictly control the quality of raw materials, and all technical indicators must meet the established requirements.


3. Polymer materials should undergo strict drying treatment before processing.


4. The production environment should be controlled within a reasonable temperature and humidity range.