Case Study: Why Does Flame-Retardant Modified Material Turn Yellow?

Case Study: Why Does Flame-Retardant Modified Material Turn Yellow?

Yesterday, a client mentioned to me that their newly developed flame-retardant nylon fiberglass material often turns yellow during production, and they weren’t sure why.
Before they finished speaking, three possible reasons immediately came to my mind:

1. The flame retardant used has poor heat resistance.

2. Nylon degradation caused by high temperatures during the extrusion process (this may be related to the extruder’s temperature settings, screw shear strength, etc.).

3. Other additives have poor heat resistance, or reactions occurred between additives or between additives and nylon.

After further discussion, I learned that the client had already conducted cooling experiments regarding temperature factors, but the yellowing issue persisted. They then asked if I could help identify the exact cause.

In the sample formulation provided by the client, we identified a specific grade of PA6, a flame retardant (FR), and three additives (A, B, C) (the exact formulation is confidential and cannot be disclosed here). We performed thermogravimetric analysis (TGA) on these four components, including the flame retardant and additives. The TGA curve for the flame retardant is shown in the figure below.

Clearly, the initial decomposition temperature of flame retardant FR does not meet the processing requirements for PA6/30%GF, while the other three additives are suitable for nylon processing temperatures. Therefore, I recommended replacing the flame retardant or switching to a different batch.

A few days later, the client called back to confirm that the current batch of flame retardant indeed had issues and could not meet the processing requirements for PA6/GF. After replacing it, the yellowing problem was resolved.

In fact, yellowing or blackening issues during plastic processing are frequently discussed in the modified materials industry, and the causes are varied. We know that color results from the interaction between light and matter, involving many physical and chemical changes. For plastics, color changes are often caused by the oxidation of certain components. Of course, in some cases, color changes may also result from inadequate cleaning of processing machinery.

Based on past experience, I believe the main reasons for discoloration of flame-retardant modified materials during processing or storage include the following:

1. Poor stability of the raw materials
Some resins on the market are produced without antioxidants or stabilizers. Plastic products made from them may discolor immediately after extrusion or during storage. Switching to resins from other manufacturers or grades often resolves the issue. This indicates varying quality in market resins, and it is essential to find and use materials with good stability.

2. Poor heat resistance of the flame retardant
If the flame retardant decomposes at temperatures below the material’s processing temperature, it generates small molecules that cause discoloration of the entire system. For example, PP is processed at 170–230°C, PA6 typically at 230–290°C, and PA66 at 240–310°C. The flame retardants used for PP, PA6, and PA66 may differ accordingly.

3. Excessive metal impurities in fillers
Excessive impurities such as iron, nickel, or chromium can undergo incomplete oxidation under certain conditions, forming corresponding oxides that cause plastic discoloration. For instance, iron oxidizes in air to form red iron oxide, making plastic products appear darker. Common fillers include titanium dioxide and calcium carbonate.

4. Other additive factors
Common processing and modification additives include antioxidants, plasticizers, reinforcing agents, coupling agents, lubricants, colorants, cross-linking agents, flame retardants, antistatic agents, antibacterial agents, impact modifiers, and degradants. During processing (especially under high temperatures), these additives must not react with each other and must maintain their own stability to prevent oxidative decomposition.

5. Environmental factors
Various substances in the air can adsorb or react with modified materials. For example, some NOx compounds are inherently colored, such as nitrogen dioxide (NO₂), a reddish-brown, highly reactive gaseous substance. When NOx heavily adheres to or is adsorbed onto the exposed edges of plastic products, it inevitably causes discoloration. External factors like light and heat can also trigger chemical reactions in certain plastic components, leading to degradation and discoloration.

The above five points are common causes of plastic discoloration during production and storage. However, specific cases still require specific analysis. Only by considering all possible influencing factors can modification engineers ensure that product color quality meets market and client demands, ultimately achieving mutual benefit and win-win outcomes.