Polycarbonate (PC) is a thermoplastic polymer material with advantages of high impact resistance, high light transmission and high heat resistance. However, under long-term sunlight exposure, it will absorb ultraviolet light and cause photodegradation. Using Fourier transform infrared spectroscopy (FTIR) and ultraviolet (UV) light aging tests, Rivaton et al. proposed degradation mechanism of polycarbonate (as shown in figure below). Research of Andrady et al. showed that Photo-Fries rearrangement phenomenon caused by ultraviolet light of polycarbonate leads to production of yellow substances, which affects appearance of polycarbonate products.

 

There are three types of auxiliaries commonly used in PC: one is high temperature processing protection auxiliaries antioxidants, antioxidants are divided into hindered phenolic main antioxidants and phosphite auxiliary antioxidants due to their different protection principles; Second category is lubricants for material processing and molding, and stearic acid lubricants are commonly used; third category is toners for color adjustment, and organic pigments are commonly used.

 

Figure 1 shows color changes before and after heat retention in injection molding machine after adding 168 PCs from different manufacturers and blank PCs.
It can be seen from Figure 1 that after addition of 168 from different manufacturers, yellowness index (YI) value of direct injection molding samples decreased to a certain extent, indicating that 168 has a protective effect on color of material during processing. After 20min heat retention, adding phosphite antioxidant, yellow index of PC is higher than blank PC, possible reason is that phosphite antioxidant is gradually consumed and transformed into pentavalent phosphorus during high temperature process and loses its antioxidant properties. Oxidation ability, small molecules and impurities in antioxidants further affect decomposition of PC at high temperature, resulting in a higher YI value of PC than blank sample after thermal retention at 320℃ for 20 min.

Figure 2 shows color changes before and after heat retention in injection molding machine after adding 1076 PCs from different manufacturers and blank PCs.

 

It can be seen from blue histogram in Figure 2 that YI value of PC samples directly injected with 1076 from different manufacturers is higher than that of blank PC, and yellowness index of material is not reduced like 168. It can be seen from red bar graph that after heat retention for 20min, YI value of 1076 added is higher than that of blank PC, magnitude is increased compared with that before heat retention, indicating that main antioxidant is used as a long-acting antioxidant in high temperature processing. Unlike phosphite antioxidants, it does not effectively protect high temperature processing process, and due to influence of hindered phenol structure, although phenolic oxygen radicals generated after hydrogen loss of hindered phenols are relatively stable, they may produce awakenings in a high temperature environment. Chromophore-like group leads to enlargement of YI and affects optical properties of PC.

Figure 3 shows color changes of PC and blank PC added with PETS from different manufacturers before and after heat retention in injection molding machine.

 

From comparison of blue histogram in Figure 3, it can be seen that there is no significant difference in YI value of blank PC and PC with added lubricant after direct injection molding. YI value increases slightly after thermal retention, but effect is limited. Possible reason is that there is a very small amount of pentaerythritol or stearic acid in lubricant, which will lead to alcoholysis and transesterification of PC under high temperature for a long time, which will accelerate aging of PC, resulting in increased yellowing.

Figure 4 shows color changes of PC and blank PC added with different types of toners before and after thermal retention in injection molding machine.

 

It can be seen from Figure 4 that YI value of PC can be greatly reduced by adding the two toners in same amount, but after 20min thermal retention at 320℃, color difference of toner A rebounds greatly, while color difference of toner B rebounds greatly. Range of change is comparable to that of blank particles, indicating that different types of toners have great differences in application effect under short-time high-temperature processing and long-term high-temperature residence, which may lead to serious discoloration of PC during high-temperature processing.

Toner A and Toner B were tested by TGA respectively, and temperature was raised to 700℃ at a heating rate of 10℃/min in a nitrogen atmosphere. Difference in heat resistance between two is shown in Figure 5. In Figure 5, temperatures corresponding to four points of weight loss 5%, 15%, 25%, and 35% of two toners are shown in Table 1.

 

 

From curves in Figure 5, it can be found that thermal stability of two toners is quite different. Initial decomposition temperature of toner B is about 325 ℃, while initial decomposition temperature of toner A is only 275 ℃. In process of weight loss, toner B is about 50℃ higher than toner A.
At a wavelength of 600 nm, spectrophotometric curves of samples before and after thermal retention of toner A added to PC and blank PC were compared and analyzed, and results obtained are shown in Figure 6. Three curves in figure from top to bottom are blank PC curve, PC curve after adding toner A, and curve before adding toner A.
It can be seen from Figure 6 that Toner A has an obvious absorption peak at 450-700 nm, and absorption peak is significantly weakened after 20min of thermal retention, indicating that some of chromophores of Toner A have changed, resulting in severe discoloration of PC .

 

168, PETS and Toner B were used for further analysis, antioxidants, lubricants, and toners were mixed in pairs or three to carry out heat retention injection molding experiments with PC. Results are shown in Figure 7.

 

From the first two histograms in Figure 7, it can be seen that the YI values of PC with antioxidants and PETS are basically same as that of blank PC, and there is no significant difference. Two have little effect on yellowing ofbmaterial during high-temperature processing. Results of adding two additives alone are same.
Color difference before and after thermal retention of PC with toner and PETS is not much different from that of blank, which indicates that PETS and toner do not affect each other, and have little effect on PC yellowing.
From last two histograms in Figure 7, it can be seen that after combination of antioxidant + toner and combination of the three, yellowing of PC increased, indicating that antioxidant and toner have mutual influence, possible reason is that antioxidant itself or its high temperature decomposition product reacts with toner, which affects chromophore of toner.