Why does the surface of transparent TPU material turn yellow?
TPU is a type of elastomer that can be plasticized by heating and dissolved by solvents. It boasts excellent comprehensive properties, including high strength, high toughness, wear resistance, and oil resistance. It also features good processability and is widely used in industries such as national defense, medical care, and food processing.
TPU is a type of elastomer that can be plasticized when heated and dissolved in solvents. It boasts excellent comprehensive properties, including high strength, high toughness, wear resistance, and oil resistance. It also exhibits good processability and is widely used in industries such as defense, medical care, and food processing. Next, let’s explore together the main reasons behind the yellowing phenomenon on the surface of transparent TPU materials:
Transparent TPU material
I. Factors Related to the Material Itself
Raw material quality
Purity Issues: TPU is synthesized through the polymerization of raw materials such as polyols, isocyanates, and chain extenders. If the purity of these raw materials is insufficient and they contain impurities—for example, polyols with trace amounts of unsaturated double bonds or impurity metal ions—these impurities may trigger oxidation reactions during processing and use, leading to yellowing of the material.
Additive Effects: To enhance certain properties of TPU, various additives such as antioxidants, light stabilizers, and lubricants are often incorporated. If these additives are of poor quality or improperly selected, they may cause yellowing. For instance, certain low-quality antioxidants, when exposed to prolonged heat or sunlight, can themselves undergo decomposition and discoloration, thereby leading to yellowing of the TPU material.
Polymerization process
Incomplete Reaction: During TPU polymerization, if reaction conditions—such as temperature, reaction time, and catalyst dosage—are not properly controlled, the polymerization reaction may remain incomplete. Unreacted monomers or oligomers will remain in the material, and over time or under the influence of external factors, these residues may undergo chemical reactions, leading to yellowing of the material.
Side reactions produce impurities: During the polymerization process, some side reactions may occur, leading to the formation of colored impurities. For example, isocyanate groups may react with certain impurities to form colored compounds, which can cause changes in the surface color of TPU materials.
II. The Impact of the Processing Procedure
The processing temperature is too high.
During processing of TPU materials—such as injection molding, extrusion, or blow molding—if the processing temperature is too high, it can lead to thermal degradation of the material. At elevated temperatures, the chemical bonds in the TPU molecular chains may break, generating free radicals. These free radicals can trigger oxidative reactions, causing the surface of the material to turn yellow. For example, when TPU is injection-molded at a temperature significantly higher than its recommended processing range, yellowing is likely to occur easily.
Processing time is too long.
Prolonged processing, such as extended exposure to high-temperature environments, increases the contact time between the material and air, thereby intensifying oxidation reactions. Moreover, prolonged mechanical shearing—such as that occurring in screw extruders—can also cause the TPU molecular chains to break and reorganize, leading to changes in the molecular chain structure and, consequently, alterations in the material’s color.
Mold or equipment contamination
If the surface of the mold or equipment used for processing is contaminated with dirt, rust, or other impurities, these contaminants may transfer onto the surface of the TPU material during the processing. For example, residues from release agents on the mold surface or metallic rust can react chemically when they come into contact with the TPU material, causing the material to yellow.
III. External Environmental Factors
Lighting
Ultraviolet Radiation: TPU material is relatively sensitive to ultraviolet (UV) radiation. Prolonged exposure to sunlight or environments with intense UV radiation can cause the energy of UV rays to break chemical bonds in the TPU molecular chains, generating free radicals. These free radicals trigger a series of photooxidation reactions, leading to yellowing of the material’s surface. This phenomenon is particularly noticeable in transparent TPU products used outdoors—such as transparent TPU phone cases and transparent TPU protective films for cars—which are prone to yellowing due to UV radiation.
Visible Light Effects: Although visible light carries less energy than ultraviolet light, prolonged exposure to intense visible light can still have certain effects on TPU materials. In display environments exposed to strong sunlight, transparent TPU materials may gradually undergo color changes.
Oxygen and ozone
Oxidation Reaction: TPU materials undergo an oxidation reaction when exposed to oxygen in the air. This oxidation reaction is particularly accelerated under high-temperature and high-humidity conditions. Oxygen molecules attack the molecular chains of TPU, triggering oxidative degradation and causing discoloration on the material’s surface. Moreover, ozone is also a strong oxidizing agent; even at relatively low concentrations, it can induce oxidation in TPU materials, leading to yellowing.
Chemical substance contact
When TPU material comes into contact with certain chemicals, it may undergo a chemical reaction and turn yellow. For example, exposure to certain organic solvents (such as acetone and toluene) or acidic and alkaline solutions can cause the TPU surface to dissolve or erode, disrupting its molecular structure and leading to yellowing. Additionally, contact with chemicals containing elements such as sulfur and nitrogen can also trigger discoloration reactions.
Contact Information
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Email: wendy.wang@yanchuangpu.com
Email: ella.duan@yanchuangpu.com
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