Through a period of investigation and collection, many users reflect that the phone case they use soon turns yellow, and the transparent color of the phone case inexplicably turns yellow. We are very that accept this phenomenon because it looks dirty and old. So why is this happening? We consulted Dr. Juye, a materials engineer, and here’s what he had to say about this phenomenon:
After repeated research, in fact, this is related to the material used in the phone case, the shell is a kind of elastomer material called polyurethane TPU, and yellowing is its most typical feature.
5 Points Help You Figure It Out
1. TPU phone case yellowing analysis
TPU is a polymer material synthesized by the reaction of diisocyanate molecules, macromolecular polyols and low molecular polyols (chain extender). Diisocyanate can be divided into aliphatic and aromatic according to its connecting group, aromatic for the unsaturated molecular structure. TPU raw materials containing aromatic groups are prone to react with negative ions in the air when processed at high temperatures, which leads to product aging and discoloration.
In fact, not only the cell phone shell made of TPU will turn yellow in a long time, but also many bags made of TPU will turn yellow. With the technological innovations in the raw material industry, many experts in the field of elastomers have made efforts. Some of them can successfully prevent the yellowing phenomenon, some can delay the yellowing time.
From my laboratory research: TPU in the air will absorb moisture in the air and become white, or this is the migration process of adding additives, the main reason is that the smoothing agent is opaque, yellowing has become a characteristic of TPU. Polyurethane is a yellowing resin. the MDI in ISO will turn yellow under UV radiation. So how to prevent polyurethane from yellowing? Avoid direct sunlight storage areas should be cool and ventilated. At the same time, it can be packed in plastic bags and put in a place without sunlight.
2. Spray yellow paint (the common method)
When spraying in-mold or out-of-mold, the anti-yellowing paint will form a protective layer on the surface of polyurethane products in order to avoid yellowing contamination caused by the contact of polyurethane skin with the atmosphere.
3. Adding an anti-yellowing agent
In order to improve the anti-yellowing ability of polyurethane products, it is usually necessary to add a special anti-yellowing agent to the raw material. However, anti-yellowing agents are expensive, and their economic benefits should be weighed when using anti-yellowing agents. If our black color is not sensitive to yellowing, we can use cheaper non-yellowing materials instead of adding an anti-yellowing agent. Since an anti-oxidant is a raw material additive, when adding an anti-yellowing agent in the production, we are required to mix it evenly to prevent discoloration, otherwise, partial yellowing (oxidized color) will occur.
4. Mixing test
Adoption of a good mixing ratio and processing technology appropriate ratio of I and P can achieve the fullest mixing reaction without incomplete reflection. When these phenomena occur, the finished product’s resistance to hydrolysis and anti-yellowing ability will be greatly reduced, especially when the ISO ratio is too large, if the processing temperature is too high, the high temperature lasts too long, it will accelerate the yellowing process of the finished product to aliphatic isocyanate to get a permanent yellow color.
5. Protect the product from contact with organic matter
Avoid man-made pollution and the use of recycled materials in the finishing or salvage process, more polyurethane products are contaminated, such as human sweat, organic solvents, etc. Therefore, special attention should be paid to the cleanliness of the polyurethane products in contact with the body, and minimize the finishing process. Additives contained in recycled materials can indirectly contaminate the product, thus affecting the effectiveness of anti-yellowing additives.
TPU phone case is one of the most popular phone case categories on the market, both buyers and sellers should have a certain understanding and feeling of the advantages and disadvantages of such phone cases and the use of the TPU phone case will turn yellow after a long time, which is not a simple oxidation reaction. I believe that we have a certain understanding, we can also explore how to clean TPU or Silicone these phone cases, we may have a lot of phone cases, but a short period of yellowing we can also clean ourselves, which is very simple.