Flame retardants based on ATH and MDH
Flame retardants based on aluminum hydroxide (ATH) and magnesium hydroxide (MDH) release water bound in their crystal structure when exposed to heat, and the temperature at which this process begins distinguishes the two substances. Therefore, the first step in any selection process is to determine the temperature reached during your manufacturing process. If the process reaches temperatures—as is the case with PP—at which ATH would already begin to release water, then MDH should be considered.
Classification
Classify ATH or MDH as a flame retardant
Aluminum hydroxide and magnesium hydroxide in Alpha Calcit's product portfolio
Alpha Calcit’s flame retardants are based on two mineral hydroxides: aluminum hydroxide, with the formula Al(OH)₃, and magnesium hydroxide, with the formula Mg(OH)₂. The abbreviations ATH and MDH are derived from the terms aluminum trihydroxide and magnesium dihydroxide. We market ATH under the ALFRIMAL® brand and MDH under the ALFRIMAL® MG brand. Both active ingredients function through a physical mechanism by releasing water as soon as the material exceeds a certain temperature.
Aluminum hydroxide and magnesium hydroxide from Alpha CALCIT Füllstoff GmbH & Co. KG contain no halogen. In addition to their fire-retardant function, ATH and MDH can serve as fillers to add volume to the formulation and influence stiffness and surface properties. We offer both product lines in several particle sizes and surface finishes.
Aluminum hydroxide and magnesium hydroxide from Alpha CALCIT Füllstoff GmbH & Co. KG contain no halogen. In addition to their fire-retardant function, ATH and MDH can serve as fillers to add volume to the formulation and influence stiffness and surface properties. We offer both product lines in several particle sizes and surface finishes.
Mechanism of Action
At what temperature do ATH and MDH release water when burned?
Three Phases of the Fire Progression at Two Different Decomposition Temperatures
Both hydroxides contain water in their crystal lattice, each accounting for about one-third of their mass. Aluminum hydroxide begins to release this water at approximately 200 °C; magnesium hydroxide retains it until approximately 300 °C and only begins to release it at that temperature. The temperature range within which the material operates determines when the crystal-bound water is released. ALFRIMAL® and ALFRIMAL® MG have a narrow decomposition range, meaning they release their water within a narrow temperature range.
The sequence of events in the event of a fire can be divided into three phases. The physical processes involved occur regardless of the application in which ATH or MDH is used.
Heating to the decomposition temperature
As long as the temperature within the material remains below the decomposition temperature of the respective hydroxide, the water remains bound within the crystal. This same threshold applies even during the manufacturing of your product. In PP processing, for example, ATH would lose water before the product is finished; for MDH, the threshold is higher.
Decomposition with the release of water
If the fire exceeds the decomposition temperature, the hydroxide releases its water. This decomposition consumes energy, thereby removing heat from the fire and slowing down the thermal decomposition. The resulting water vapor lowers the concentration of combustible gases and displaces oxygen from the fire area.
Oxide residue as a barrier
After the water has been released, metal oxides remain. They form a layer on the surface that prevents heat from penetrating the underlying material and blocks the entry of oxygen.
Fabric Comparison
What Distinguishes Aluminum Hydroxide from Magnesium Hydroxide
Documented properties of both active ingredient bases, line by line
| Property | Aluminum hydroxide (ATH) | Magnesium hydroxide (MDH) |
|---|---|---|
| Chemical Formula | Al(OH)₃ | Mg(OH)₂ |
| Brand at Alpha Calcit | ALFRIMAL® | ALFRIMAL® MG |
| Start of Water Distribution | at or above approximately 200 °C | at or above approximately 300 °C |
| Bound Water | about one-third of the mass | about one-third of the mass |
| Thermal Stability | up to about 200 °C | higher than at ATH |
| Surface character | amphoteric | alkaline |
| Viscosity and Flow Behavior | depending on the formulation and application | depending on the formulation and application |
| Fire gases and smoke | halogen-free; no toxic gases are produced | halogen-free; no toxic gases are produced |
| Median particle size d₅₀ | 1.3 to 20 µm, depending on the type | 2 to 8 µm, depending on the type |
| Whiteness Level Ry | 94 to 98, depending on the model | 93 to 95, depending on the model |
| Variants | Grades with different grain sizes and surface treatments, including two precipitated grades | with and without a hydrophobic surface coating and in various grain sizes |
d₅₀ and whiteness are average values and are not legally binding.
When choosing between the two materials, the water release data is usually the starting point, because it indicates whether the flame retardant can withstand the temperatures involved in your processing. In PP processing, for example, this argues in favor of MDH. After that, the surface characteristics can help determine how additives in your formulation behave and how viscosity and flow properties are affected; this can only be determined through testing with your raw materials.
A Comparison of Mill-Finished, Coated, and Uncoated ALFRIMAL® Grades
| Execution | Types | Median particle size d₅₀ | Whiteness Level Ry | Features |
|---|---|---|---|---|
| like | ALFRIMAL® 506 P, 509 P | 1.4 µm or 1.3 µm | 98 and 97, respectively | Very fine-grained; flame-retardant and smoke-reducing, without affecting the product's color, surface, or texture |
| uncoated, very fine | ALFRIMAL® 104, 105, 106 | 4 µm, 3 µm, 2 µm | 97, 97, 98 | An alternative to finely ground ATH; wide range of applications |
| with a hydrophobic surface treatment | ALFRIMAL® 104 HS, 106 HS | 4 µm or 2 µm | white, without a "Ry" designation | Water-repellent surface; for formulations in which hydrophobic interactions with the matrix are required |
All three grades are intended for use in cable compounds, rubber products, and elastomer foams. Among them, the precipitated grades have the smallest average particle size. Surface-treated grades may be suitable when better wetting of the mineral is required in a more nonpolar formulation. In Solid Surface applications, the very fine uncoated grades can also be used in combination with other ATH grades. A joint test with samples will determine which grade is best suited for your compound.
Use
When to Consider Halogen-Free Flame Retardants with ATH or MDH
Application and Target Fire Resistance Rating as a Starting Point
Whether your product requires halogen-free inorganic flame retardants or whether an inorganic filler is sufficient depends, among other things, on the application and the fire resistance class you wish to achieve. Different applications behave differently in the event of a fire and may also be subject to different fire resistance classes; for example, plastic burns more readily than plaster. For higher fire resistance classes, flame retardants such as ATH or MDH should be incorporated into the formulation.
A technical consultation can help determine whether a tailor-made solution would work for your application.
Advantages of ATH and MDH in Fire Protection
One advantage of ATH and MDH is their halogen-free composition. In the event of a fire, no halogenated hydrocarbons are released from the flame retardant itself. No toxic or corrosive gases are produced in the event of a fire.
Both substances can also significantly reduce smoke production. ATH and MDH are environmentally neutral, and the oxide layer that remains after a fire is also harmless.
What the fill level means for your recipe
The effect of both hydroxides is quantity-dependent; therefore, high fill levels are the norm. The required fill level in your formulation depends, among other factors, on the application, the required outcome of the fire safety test, the desired workability, and the surface finish you want your final product to have. We do not specify a general filler content range because the possible filler content depends on your formulation. As the filler content increases, viscosity and density rise, and the mixture becomes more viscous during processing. Therefore, the impact of a higher filler content on your process can only be determined through testing with samples, which we will coordinate with you.
Situations in which ATH is a logical choice
If your processing temperature remains below the temperature range in which aluminum hydroxide releases its water, ATH is available as an active ingredient. It is primarily used in the cable, rubber, and thermoplastic industries. ATH can provide flame retardancy without affecting the color, surface, or tactile properties of the product. Within the ALFRIMAL® line, the very fine grades are primarily intended for cable compounds, while the coarser grades are used, among other things, for resins, paints, coatings, and mineral-based materials. The ALFRIMAL® grades are effective regardless of pH value. We will work with you to determine how ATH interacts with the additives in your formulation through testing.
Situations in which MDH is a logical choice
PP processing is a classic example of MDH, because ATH would already release water at those temperatures, whereas ALFRIMAL® MG does not release its water until approximately 300 °C. ALFRIMAL® MG is intended, among other things, for cable jacket compounds, elastomers, and TPEs, as well as insulating plastics for white goods and roofing membranes. We’ll determine whether a version with or without a hydrophobic coating is suitable based on your specific application.
Selection
Criteria for Choosing Between ATH and MDH
Parameters that narrow down your initial selection
A flame retardant that begins to hydrolyze below the processing temperature loses its effectiveness before the product is finished. This can also lead to scrap or a defective surface during the processing. Therefore, first check what temperature your process reaches in the mixer or extruder; for PP processing, for example, this test yields the MDH. The remaining rows in the table narrow down which type within the selected active ingredient base might be suitable for your formulation.
| Parameter | Technical Significance | Consequences for the selection |
|---|---|---|
| Processing Temperature of the Process | Above the decomposition temperature, water separation begins as early as the mixer or extruder | Determines the preliminary selection between ATH and MDH |
| Active Ingredient Base | Specifies the temperature at which water is released | ATH starting at approx. 200 °C, MDH starting at approx. 300 °C |
| Surface character | ATH: amphoteric; MDH: basic; affects interaction with additives | Include the additives in the formula in the preselection |
| Particle Size Distribution | Affects, among other things, dispersion, surface quality, and the achievable fill level | Finer grades for more uniform surfaces with a greater increase in viscosity; coarser grades for higher loading |
| Surface treatment | Untreated surfaces are hydrophilic; coated surfaces are hydrophobic. | A hydrophobic coating can improve dispersibility and allow for a higher filler content; the effect depends on the specific application |
| Fill Level | The mineral effect depends on the amount used and on the concentration of the specific formulation or application. | The thickness depends, among other things, on the application, test results, workability, and surface finish. |
| Binders and other ingredients in the formulation | Influence when and how the flame retardant works in the final product | Evaluation of the collaboration between your laboratory and the Alpha Calcit laboratory |
This list is not exhaustive; other factors may depend on your formulation and application.
Hydrophobic Coating: The Case of Solid Surface
Solid Surface—that is, mineral-based materials used for countertops, wall cladding, shelves, or window sills—demonstrates the benefits of ATH’s hydrophobic coating. A special coating can reduce the viscosity of the filled resin matrix and help create surfaces that are resistant to chemicals and repel dirt. Because it can also improve dispersibility, the matrix may be filled to a higher degree under certain conditions. There is no general answer as to which type and particle size are suitable, as every formulation is different.
Technical Consulting
Choosing Between ATH and MDH for Technical Consulting
What information we need for the preliminary selection
To provide technical advice, we need, among other things, details about your application, a general description of the processing method with the highest temperature reached, the filler used to date, the required fire safety test, and your specifications regarding color, surface finish, and density. Using this information, we will work with you to determine the active ingredient base and compile a customized selection of suitable ALFRIMAL® types, or assess whether a solution tailored to your application is appropriate. To this end, we will send you samples that you can test in your own formulation. If it makes sense for your application, we’ll prepare the samples so that you can test a coated and an uncoated version side by side.
Contact: Sales · Phone +49 2236 8914-0 · Email sales@alpha-calcit.de
Product Lines
ALFRIMAL® for ATH, ALFRIMAL® MG for MDH
Which product line corresponds to which active ingredient base
ALFRIMAL® encompasses our aluminum hydroxide-based flame retardants. The grades differ, among other things, in particle size and surface treatment. The coarser grades include those with optimized particle size distribution, which can enable high loading levels and improved processability in highly filled systems. You can find an overview under “ALFRIMAL® Types Based on Aluminum Hydroxide.”
ALFRIMAL® MG is the magnesium hydroxide-based product line, available with or without a hydrophobic surface coating. It is a suitable option when your processing—such as with PP—exceeds the temperature range of ATH. Variants and grades are listed under ALFRIMAL® MG on the magnesium hydroxide product page. We process both product lines from controlled sources.
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Frequently Asked Questions
Frequently Asked Questions by Formulation Developers About ATH and MDH
Further Reading
Related Pages on ATH and MDH
- ALFRIMAL® Aluminum Hydroxide-Based Grades · Overview of the ALFRIMAL® Product Line
- ALFRIMAL® MG on the Magnesium Hydroxide product page · Types and variants of the ALFRIMAL® MG line
- Sales Contact · Contact Form, Contact Locations, and Sales Contact
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