The concentration of colorant in pu color masterbatch and the TiO2 concentration added to dilute color affect the heat resistance of HDPE. The graph shows that two colorants (C. L. PO 73 and C. I. PR48:2) impact the heat resistance of HDPE, but with no regular pattern—both benefits and drawbacks. Adding 1% TiO2 affected the heat resistance of the original colored HDPE, and its synergistic effect was also irregular. The C. I. PO pigment, whether in its colored or diluted form with TiO2, significantly improved the heat resistance of HDPE. This irregular impact may be due to the complex combined effects between HDPE, colorants, and TiO2.
The fading process of organic substances is actually a function of temperature and dwell time. In standard experimental methods, the dwell time is fixed at 5 minutes, which is longer than the normal processing time for plastics. Therefore, experience shows that as long as the dwell time is short, organic colorants can be processed at temperatures slightly above the thermal stability provided by the data samples.
On this point, it is crucial that the size of the injection molding machine matches the dimensions and quality of the TPU color masterbatch plastic products being processed. This requirement is generally achievable, but it can be challenging when the products are extremely small, due to technical limitations that prevent plastic processing equipment from being arbitrarily reduced in size. When processing extremely small products, it is inevitable to use molding equipment that is much larger than the product size. In such cases, the residence time of the TPU color masterbatch colored plastic melt in the injection molding machine may exceed 5 minutes, leading to fading and degradation of the plastic material and colorants starting at temperatures below those provided in the data samples.
Another reason for color fading in colored plastic made with TPU masterbatch is the use of hot runners during injection molding. The dwell time of the colored plastic in the hot runners should be added to the total dwell time within the injection molding machine. Typically, the total dwell time should be short enough to prevent any color fading, so this parameter should be carefully considered. Poor design in the hot runner structure is a major cause of color fading, such as when the nozzle or other parts of the hot runner are very small, leading to frictional heat that is difficult to calculate and control. Excessive temperature can cause thermal damage to both the organic colorant and the plastic, which can occur not only in the manufacturing of plastic products but also in the production of granular colorant, known as masterbatch.
