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Additives are used in a wide range of applications in the world of plastic industry. lndeed, modern technologies and everyday life would not be the same without the many excellent properties of plastics and that can only be achieved by using additives.

Synergies between Antioxidants and Light Stabilizers

The desirable mechanical, optical and processing properties of polymers are achieved through the proper design of molecular weight and molecular weight distribution in the polymerization process. The reactor product undergoes various high temperature extrusion and molding processes to be shaped into a final use article. During all these steps, the molecular design of polymer will be modified due to the combined action of heat, shear and oxygen. Antioxidant and processing stabilizers are necessary to prevent this degradation, maintaining the properties of the polymer material.

Besides protecting the polymer design, using adequate processing stabilizers positively influences the stability of the plastic against the detrimental action of UV radiation. Preventing the formation of UV degradation-sensitive oxidation groups in the polymer backbone improves the UV stability of a final article. A synergy exists in the combination of properly selected processing stabilizers and UV stabilizers, implying the combination of both results in higher stability than the one obtained with the individual systems.

Reduce melt flow and increase lifetime

Change in melt flow after multiple extrusion of PP – Influence of processing stabilizer

Figure 1: Change in melt flow after multiple extrusion of PP – Influence of processing stabilizer

Processing stabilizers are necessary to protect the polypropylene from thermo-mechanical oxidation during processing. The melt flow always increases with increasing thermal stress, suggesting a scission of the molecular chains. Stabilizers system combining a primary antioxidant such as Irganox® 1010 with a phosphite such as Irgafos® 168 are most commonly used.


The effect of stabilization can be shown in a multiple pass experiment, where the change in melt flow is plotted against the number of extrusion pass (Figure 1). Adding a phosphite significantly reduces the melt flow after the fifth extrusion step.

Change in melt flow after multiple extrusion of PP – Influence of processing stabilizer

Figure 2: UV stability of PP tapes - influence of processing stabilizer systems

The contribution of the improved processing stability provided by antioxidants and phosphite on the UV stability is illustrated in Figure 2. The polypropylene has been processed into oriented 50 µm tapes and exposed to artificial UV radiation. A good protection during processing will contribute to enhanced UV stability and therefore to a greater useful lifetime of the final article.

The advantages of combining antioxidants with UV stabilizers

The effect can also be observed in the presence of an effective light stabilizer for PP tapes (30 µm) such as Tinuvin® 770 (figure 3) on samples exposed in Florida.

Combined effect of processing stabilizer and HALS on the UV stability of PP tapes exposed in Florida (140kly/y)

Figure 3: Combined effect of processing stabilizer and HALS on the UV stability of PP tapes
exposed in Florida (140kly/y)

Polyethylene usually shows a molecular weight increase when processed in an extruder at elevated temperatures. This is illustrated in Figure 4. The melt flow after different extrusion steps decreases and a good processing stabilization system is characterized by a rather flat curve. It is achieved in the example shown below with a blend of 0.05% Irganox® 1076 plus 0.10% Irgafos® 168.

Change in melt flow [190°C;10kg] of HDPE after multiple extrusion at 260°C

Figure 4: Change in melt flow [190°C;10kg] of HDPE after multiple extrusion at 260°C

The effect of the good processing stability on the UV stability of the polymer is as well shown for HDPE in Figure 5. A 10 to 15% increase in lifetime is possible for a material adequately stabilized during processing.

Influence of processing stabilizer on UV stability of Polyamide 66 plaques

Figure 5: UV-Exposure of HDPE - influence of processing stabilizer

Selecting the right antioxidant to enable optimum light protection

In some cases, the effect of insufficient processing stability results in a significant reduction in UV-stability. This is illustrated in Figure 6 for a rotomolded polyethylene article, stabilized with a processing stabilization system composed of 0.05% Irganox® 1076 and 0.1% Irgafos® 168 whereas 0.3% Chimassorb® 944 is additionally used as light stabilizer:

The used processing stabilization is definitely not sufficient for a curing time of 25 minutes at the set temperature. However, considering that this longer curing time might be necessary to achieve the desired results in the molding process, the processing stabilization package will be optimized in a next step.

Influence of processing on the UV stability of rotomolded PE parts.

Figure 6: Influence of processing on the UV stability of rotomolded PE parts.

The synergy between good processing stability and UV stabilizers is not unique to polyolefins but also valid for engineering plastics. An example is given in Figure 7 for polyamide 66.

Influence of processing stabilizer on UV stability of Polyamide 66 plaques

Figure 7: Influence of processing stabilizer on UV stability of Polyamide 66 plaques

Protecting the polymer during the high temperature processing has a significant influence on the durability of polymers exposed to UV light. Type and concentration of processing stabilizer has to be selected together with the UV stabilizer system for optimal UV stability.

Find out more about UV stabilization


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