How To Choose The Right Flame Retardant? Here Are The Factors To Consider...

How To Choose The Right Flame Retardant? Here Are The Factors To Consider...

In the previous two installments, we systematically explored the core characteristics, main classifications, and typical application scenarios of flame retardants in modified plastics.
Flame Retardant Solutions for Modified Plastics (Part 1): Product Characteristics + Application Scenarios, Explained in One Article
Flame Retardant Solutions for Modified Plastics (Part 2): Mainstream Halogen-Free Categories + Emerging Nanotechnology

After understanding these fundamentals, selecting the most suitable flame retardant for a specific product and process becomes a critical decision for engineers and R&D personnel. This installment will focus on the selection logic for flame retardants, providing a clear and practical screening guide from multiple dimensions including application requirements, performance balance, cost control, and processing adaptability.

The choice of a flame retardant depends on:

• The type of application scenario.

• The specific flame retardancy standards that must be met.

• The relevant regulations that need to be complied with.

Many other issues must also be considered when selecting the optimal flame retardant system for a particular use.

I. Selection Criteria for Brominated Flame Retardants
Factors Influencing the Selection of Brominated Flame Retardants

1. Bromine Type and Content
For a selected brominated flame retardant to be effective, it needs to decompose when the polymer burns while remaining stable during polymer processing. This requirement dictates the type of bromine in the flame retardant. Furthermore, the retardant must contain sufficient bromine content to achieve the required flame retardant performance, and the addition level should not adversely affect the material's physical properties or system cost.

2. Thermal Stability
The selected brominated flame retardant must remain stable during compounding and injection molding. Decomposition during these processes can lead to material discoloration, polymer degradation, and equipment corrosion. Therefore, selecting the appropriate retardant and pairing it with necessary heat stabilizers and synergists is crucial.

3. Aging Characteristics
The resin system may need to withstand various factors causing premature degradation and discoloration. When selecting the most suitable flame retardant and required stabilizers for a specific system, factors such as UV resistance, thermal stability, and migration must be comprehensively considered.

4. Processing Characteristics
Depending on the processing temperature, some flame retardants are melt-blendable, while others are used as fillers. This difference impacts the processing and the final product's physical properties.

5. Standards to be Met
The selection of a flame retardant largely depends on the chosen resin system and the standards that must be complied with.

6. Cost of Use
The total cost of the entire flame retardant system must be considered. This depends not only on the price of the brominated flame retardant itself and its required loading but also on the cost of other additives needed alongside it to build a functional system.

7. Environmental Considerations
The use of brominated flame retardants comes with specific environmental constraints. A core issue is reducing toxic hazards throughout the entire lifecycle, from manufacturing and end-use to disposal.

8. Blooming Resistance
Blooming refers to the phenomenon where the flame retardant slowly migrates to the plastic surface, causing a hazy appearance, often with a bronzed-like color. This is particularly undesirable for components that also serve aesthetic functions, such as casings and housings. Therefore, blooming resistance is a key consideration in many applications.
Generally, the severity of blooming depends on the compatibility of the flame retardant with the polymer matrix and the molecular weight of the retardant: better compatibility and higher molecular weight result in less blooming.

9. UV Stability
In many applications, flame-retardant resins may need to withstand various environmental conditions that can easily lead to premature degradation and discoloration. Therefore, selecting an appropriate brominated flame retardant is crucial for applications requiring UV resistance, such as outdoor uses.

II. Aluminum Hydroxide (ATH) and Magnesium Hydroxide (MDH)
Selection Criteria
When selecting Aluminum Hydroxide (ATH) or Magnesium Hydroxide (MDH) products for a flame retardant formulation, the following key material parameters need to be emphasized:

• Median Particle Size.

• Particle Size Distribution.

• Specific Surface Area.

• Particle Morphology.

• Surface Chemistry.

• Color.

These product characteristics (of the base materials) will directly influence the compounding process and the final properties of the composite.

1. Comparison of Physical Properties

2. Comparison of Thermal Stability
The figure below compares the thermal decomposition characteristics of Aluminum Hydroxide (ATH) and Magnesium Hydroxide (MDH):

• Aluminum Hydroxide begins to decompose at approximately 220°C (428°F).

• Magnesium Hydroxide decomposes at approximately 330°C (626°F).

Therefore, Magnesium Hydroxide has higher thermal stability, providing a wider temperature window for compounding. Aluminum Hydroxide is suitable for thermosets, which are typically processed below 200°C, as well as some PVC and polyolefin-based plastic compounds.

Magnesium Hydroxide is preferred when formulating plastic composites that need to be processed at or near temperatures above 220°C (428°F), for example, polypropylene and engineering thermoplastics. Using Magnesium Hydroxide for low-melting-point thermoplastics or elastomers can also enable higher processing temperatures and higher compounding output.
When heated and decomposed, both Aluminum Hydroxide and Magnesium Hydroxide release water of crystallization: on one hand, cooling the polymer matrix, and on the other, diluting the smoke produced by combustion (ATH releases about 35% of its weight as water, MDH about 31%). Simultaneously, their endothermic decomposition processes remove a significant amount of heat, further suppressing the combustion reaction. On a per-unit-weight basis, Magnesium Hydroxide absorbs more heat (328 cal/g) than Aluminum Hydroxide (280 cal/g), making it more efficient in high-temperature scenarios.

3. Filler Loading
To achieve the expected flame retardant effect, the loading of Aluminum Hydroxide (ATH) and Magnesium Hydroxide (MDH) typically needs to reach around 150 phr (60 wt%). However, high loading levels can easily lead to reduced processability and compromised mechanical properties (such as tensile strength, impact strength) and physical properties (such as toughness, weatherability).
Although hydroxide fillers are less expensive per unit than most other types of flame retardants, the comprehensive cost advantage is not significant due to the high loading required. To address this, the industry typically adopts the following solutions:

• Use of Synergistic Additives: Adding synergistic additives to hydrated metal hydroxide fillers can reduce the filler loading without compromising flame retardant performance, mitigating the decline in processability and mechanical properties.

• Surface Modification Treatment: Surface treating hydroxide fillers with organosilanes, zirconates, or titanates can improve their compatibility with the polymer matrix, thereby increasing their efficiency of use.

• Combined Use: There is a synergistic effect between Aluminum Hydroxide and Magnesium Hydroxide. When the total loading remains unchanged, combining them can enhance flame retardant performance, or when the flame retardant performance remains unchanged, it can reduce the total loading. Furthermore, combining complex metal hydroxides with ATH/MDH can also further improve the flame retardant effect.

Application Advantages of Complex Metal Hydroxides
In extrusion trials, replacing pure MDH with MDH/ATH mixtures in different ratios achieved the following processing optimizations:

• Die pressure reduced by 15% to 20%.

• Torque reduced by 16% to 21%, lowering energy consumption.

Using Aluminum Hydroxide/Magnesium Hydroxide mixtures and synergistic mineral flame retardants can reduce costs. The price ranking of various related additives from high to low is: Polypropylene > MDH > ATH > Common white fillers.
Leveraging this price difference and the synergistic effect of combination allows for a good balance between flame retardant performance, physical/mechanical properties, and cost, typically achieving 25%-30% cost savings.
As specifically shown in the figure below: The legend is MDH control group, MDH/ATH mixture, MDH/ATH/Kaolin mixture, MDH/ATH/Calcium Carbonate mixture. The horizontal axis is the cost index, ranging from 0-150.

At this point, you can preliminarily determine the type of flame retardant based on the application scenario. However, further verification of the compatibility between this flame retardant and the plastic matrix in your formulation is still needed. The final selection must simultaneously match the polymer's processing characteristics, performance in use, and flame retardancy requirements.