Research on Photodegradation of Flame-Retardant ABS: Which Flame-Retardant and Anti-Aging System is More Effective?

Research on Photodegradation of Flame-Retardant ABS: Which Flame-Retardant and Anti-Aging System is More Effective?

I. Introduction

Acrylonitrile-butadiene-styrene (ABS) resin has a relatively low limiting oxygen index (LOI) of only 18.3-20, making it a highly flammable polymer. When ignited, it produces a large amount of black smoke and continues to burn even after the flame source is removed. The material softens, chars, and drips as it melts. Electrical and electronic components made from ABS are at risk of ignition due to short circuits, which limits its application in these fields. Therefore, flame-retardant ABS has been developed to meet these needs.

This study investigates the effects of flame retardants, UV stabilizers, and titanium dioxide on the photodegradation resistance of flame-retardant ABS by examining the color difference changes before and after aging, providing guidance and support for the application of the material.

II. Flame Retardancy and Weatherability of ABS

There are three main approaches to flame-retardant modification of ABS:

1. Blending with flame-retardant polymers to form alloys.

2. Chemical modification through the addition of a fourth monomer.

3. Incorporating flame retardants.

Among these, the third method achieves a balance between cost and performance and is the most widely used. Halogenated flame retardants, especially brominated flame retardants, offer the highest efficiency. When combined with synergists such as antimony trioxide (Sb₂O₃) and flame retardant additives like polytetrafluoroethylene (PTFE), the LOI can reach above 27, and the vertical burning performance can achieve UL94 V-0 rating.

The polybutadiene rubber in ABS resin contains unsaturated carbon-carbon double bonds, which are susceptible to reactions with light, heat, oxygen, and moisture in the atmosphere. This leads to the formation of C=O chromophoric groups, resulting in discoloration, powdering, cracking, and degradation of mechanical properties. Additionally, the presence of brominated flame retardants in flame-retardant ABS generates acidic substances such as HBr and free radicals (R•, Br•) during processing. These substances further initiate and promote reactions in the ABS resin, worsening its weatherability.

III. Comparison of Photodegradation between Flame-Retardant and Ordinary ABS

Commonly used brominated flame retardants in styrene-based materials include decabromodiphenylethane (DBDPE), tetrabromobisphenol A (TBBA), brominated epoxy oligomer (BER), tris(tribromophenoxy)s-triazine (TBM), and brominated polystyrene (BPS). Among these, the flame-retardant ABS (FRABS) systems based on TBM, BER, and TBBA exhibit the most balanced performance and are the most widely applied. This study compared the color change due to photodegradation between these widely used FRABS and ordinary ABS (ABS) under xenon lamp aging, with the results shown in Figure 1.

As seen in Figure 1, the addition of brominated flame retardants significantly impacts the color difference change during xenon lamp aging. The color difference increased dramatically from 3.5 for ordinary ABS to over 40 for FRABS after 336 hours of aging. Moreover, the color chart of FRABS showed noticeable cracking around 500 hours. This confirms the previous analysis that the acidic substances and free radicals generated during the processing of brominated flame retardants can accelerate the reaction and discoloration of ABS under light exposure. Among them, TBBA has the poorest thermal stability (with a 2% decomposition temperature of only 285°C in air), making it more prone to degradation during processing and resulting in the worst weatherability.

Considering the comprehensive factors of performance and cost, FRABS based on the TBM system offers greater market value. Therefore, subsequent research on flame-retardant ABS will use this TBM-based FRABS as the reference material for comparative studies, without further explanation.

IV. The Impact of Weathering Agents on the Photodegradation of Flame-Retardant ABS

To enhance the weatherability of ABS, light stabilizers are commonly added to inhibit or slow down the photodegradation rate of the material. The main types of substances used to improve the photostability of materials include:

Light Screening Agents, such as carbon black, titanium dioxide, and zinc oxide. Their stabilization mechanism involves absorbing or reflecting ultraviolet (UV) light, thereby reducing the likelihood of polymer materials absorbing UV radiation.

UV Absorbers, including salicylate esters, benzophenones, and benzotriazoles. These UV absorbers have a much stronger UV absorption capacity than the chromophores in polymers. They can suppress the early initiation stage of polymer degradation by absorbing UV energy and immediately converting it into harmless forms, such as heat-dissipated infrared energy, or phosphorescence and fluorescence, thereby releasing the absorbed UV energy in a manner that is non-damaging to the polymer.

Quenchers, primarily nickel complexes in divalent form. Their stabilization mechanism involves electron transfer with the excited-state molecules in the polymer material. The excited-state molecules lose their activity and return to the ground state, thereby preventing photochemical reactions from proceeding.

Hindered Amine Light Stabilizers (HALS), which work by capturing free radicals in the material and decomposing hydroperoxides. This keeps the concentration of hydroperoxides in the material low, thereby slowing down the reaction rates of chain initiation, chain propagation, and chain branching. As a result, the photodegradation rate of the material is reduced.

In this study, the most widely used benzotriazole UV absorber (Weathering Agent A) and hindered amine light stabilizer (Weathering Agent B) were selected to investigate their individual and combined effects on the photodegradation of flame-retardant ABS. The results are shown in Figures 2 to 4.

As shown in Figure 2, the addition of UV absorbers can significantly enhance the photostability of flame-retardant ABS. Adding 3‰–5‰ UV absorber can reduce the color difference value by approximately 50% after 300 hours of exposure. Additionally, while the color chart without weathering agents began to crack after 300 hours, the one with UV absorbers showed no significant cracking even after 672 hours.

The results shown in Figure 3 indicate that the addition of HALS (hindered amine light stabilizer) has little impact on the photodegradation of the flame-retardant ABS. Figure 4 presents the photodegradation results of flame-retardant ABS when benzotriazole and HALS are used in combination. The results are essentially similar to those obtained when benzotriazole is used alone, also indicating that HALS is ineffective in this case.

HALS typically features N-H structures and N-methyl-substituted derivatives. Its photostabilization mechanism is quite complex. It is generally believed that the hindered amine nitroxyl radicals are the active species directly responsible for the photostabilization of polymers. HALS itself acts as a precursor to the active photostabilizing structures. Under photo-oxidative conditions, the polymer inevitably contains or generates reactive oxidative species such as ozone, singlet oxygen, hydrogen peroxide, peroxy radicals, and alkyl peroxides. The N-H and N-methyl structures in the piperidine framework of HALS are readily oxidized by these reactive species into nitroxyl radical structures. The nitroxyl radicals of HALS can capture free radicals generated during photodegradation, thereby interrupting further detrimental reactions, as shown in Figure 5.

However, due to its amine characteristics, HALS exhibits a certain degree of alkalinity. When encountering acidic substances, it becomes protonated, and the activity of the resulting nitroxyl radicals is reduced. This may also be the primary reason for the ineffectiveness of Weathering Agent B in flame-retardant ABS.

N-alkylated hindered amines (N-CH₃) are slightly less basic than hindered amines with secondary amine structures (N-H). Hydroxylamine structures, O-alkylated hydroxylamine structures, and acetylated hindered amines even exhibit a certain degree of weak acidity. From a practical standpoint, hindered amine light stabilizers with weaker alkalinity, such as O-alkylated hydroxylamine structures and acetylated hindered amine derivatives, are generally preferred. These types of HALS may be more suitable for flame-retardant ABS systems that are slightly acidic, and further exploration can be conducted in this direction.

V. The Impact of Titanium Dioxide and Its Combination with Weathering Agents on the Photodegradation of Flame-Retardant ABS

Rutile titanium dioxide (TiO₂) is known for its stable performance and strong light reflection, making it an efficient light-screening agent. However, rutile TiO₂ particles also have some photocatalytic defects. When used as a UV light shield, they generally need to be coated with an inorganic film such as SiO₂ or Al₂O₃ to shield the photocatalytic active sites in their crystal structure. This study investigated the effect of TiO₂ content on the photodegradation of flame-retardant ABS, with the results shown in Figures 6 and 7.

Figure 6 gives the results of the effect of different titanium dioxide additions on the light aging performance of flame retardant ABS. The results show that the addition of titanium dioxide can significantly reduce the color difference change of flame retardant ABS after light aging, the more titanium dioxide is added, the smaller the color difference change of flame retardant ABS after light aging, when 2% of titanium dioxide is added, after 336h xenon lamp aging, the color difference of flame retardant ABS is reduced from the original more than 40 to 17.1, and when the amount of titanium dioxide added is further increased to 4%, the color difference is reduced to 11.8. Due to the fact that the addition of titanium dioxide will lead to the increase of density of material, the toughness of material will decrease. The addition of titanium dioxide will lead to a rise in the density of the material, the toughness of the material decreases, the overall consideration, for the higher weathering requirements of flame retardant ABS, the addition of titanium dioxide about 2% can achieve a balance between weathering and performance.

In the flame retardant ABS system with compound weathering agent, the addition of titanium dioxide can also effectively improve the light aging weathering resistance of the material. The color difference after aging under xenon lamp for about 300h was also examined, and the color difference of the compound weathering agent flame-retardant ABS without adding titanium dioxide reached 17.3, and the values of the color difference were reduced to 13.2, 11.0, and 8.0 by adding titanium dioxide at 1%, 2%, and 4% respectively, which is comparable to that of ordinary ABS.

Conclusion

1. The bromine flame retardant system has a greater effect on the light aging of ABS materials, the color difference becomes obviously larger, and cracking occurs after 500h.

2. UV absorber can effectively improve the light aging resistance of flame retardant ABS, but the effect of hindered amine is not obvious.

3. The addition of titanium dioxide can also effectively improve the light aging resistance of flame retardant ABS, by compounding with weathering agent, the light aging resistance of flame retardant ABS can reach comparable with ordinary ABS.