Flame retardant additives for dispersions used in coatings

Flame retardant (FR) additives for polymer dispersions like PVAC (Polyvinyl Acetate), styrene-butadiene (SBR), styrene acrylics (SAE), pure acrylics, and VAE (Vinyl Acetate Ethylene) typically focus on halogen-free, non-toxic, and water-compatible systems since these binders are primarily used in water-based coatings and adhesives.

Flame Retardant Additives for Polymer Dispersions

The goal of these additives is to interrupt the combustion cycle, usually by forming a protective char barrier or releasing non-combustible gases. The most common types of additives for these waterborne systems are inorganic and intumescent types:

Additive Class	Examples	Mechanism	Compatibility	Applicable Dispersions
Inorganic Hydroxides	Aluminum Trihydrate (ATH), Magnesium Hydroxide (Mg(OH)2​ or MH)	Endothermic decomposition, releases water vapor, which cools the flame and dilutes combustible gases. Acts as a filler and smoke suppressant.	Excellent compatibility, non-toxic, eco-friendly.	PVAC, VAE, Styrene Acrylics, Pure Acrylics, SBR. VAE is noted as compatible with ATH and MH.
Intumescent Systems	Ammonium Polyphosphate (APP), Pentaerythritol, Melamine. Often used in synergy with other agents like Expandable Graphite (EG).	Forms a swollen, insulating char layer (intumescence) when heated, which acts as a physical barrier to heat and oxygen transfer.	Requires careful formulation for dispersion stability and final film integrity. APP/EG systems show excellent synergy.	Especially effective for Styrene Acrylics (SAE) to achieve high FR ratings (e.g., UL-94 V-0). Used across all types.
Phosphorus/Nitrogen Based	Various halogen-free water-based fire retardants (e.g., specific liquid phosphates, Synthro-Nyl type additives).	Acts in the condensed phase (promotes char) and/or gas phase (radical quenching).	Good compatibility is achieved with specialized water-based formulations.	All types, often marketed as halogen-free solutions.
Halogenated FR	E.g., Brominated compounds.	Releases halogen radicals into the gas phase, quenching the flame.	Less common now due to environmental and toxicity concerns.	Used when high performance is critical, but generally avoided in modern waterborne coatings.

Flame retardant (FR) additives for polymer dispersions like PVAC (Polyvinyl Acetate), styrene-butadiene (SBR), styrene acrylics (SAE), pure acrylics, and VAE (Vinyl Acetate Ethylene) typically focus on halogen-free, non-toxic, and water-compatible systems since these binders are primarily used in water-based coatings and adhesives.

Flame Retardant Additives for Polymer Dispersions

The goal of these additives is to interrupt the combustion cycle, usually by forming a protective char barrier or releasing non-combustible gases. The most common types of additives for these waterborne systems are inorganic and intumescent types:

Additive Class	Examples	Mechanism	Compatibility	Applicable Dispersions
Inorganic Hydroxides	Aluminum Trihydrate (ATH), Magnesium Hydroxide (Mg(OH)2​ or MH)	Endothermic decomposition, releases water vapor, which cools the flame and dilutes combustible gases. Acts as a filler and smoke suppressant.	Excellent compatibility, non-toxic, eco-friendly.	PVAC, VAE, Styrene Acrylics, Pure Acrylics, SBR. VAE is noted as compatible with ATH and MH.
Intumescent Systems	Ammonium Polyphosphate (APP), Pentaerythritol, Melamine. Often used in synergy with other agents like Expandable Graphite (EG).	Forms a swollen, insulating char layer (intumescence) when heated, which acts as a physical barrier to heat and oxygen transfer.	Requires careful formulation for dispersion stability and final film integrity. APP/EG systems show excellent synergy.	Especially effective for Styrene Acrylics (SAE) to achieve high FR ratings (e.g., UL-94 V-0). Used across all types.
Phosphorus/Nitrogen Based	Various halogen-free water-based fire retardants (e.g., specific liquid phosphates, Synthro-Nyl type additives).	Acts in the condensed phase (promotes char) and/or gas phase (radical quenching).	Good compatibility is achieved with specialized water-based formulations.	All types, often marketed as halogen-free solutions.
Halogenated FR	E.g., Brominated compounds.	Releases halogen radicals into the gas phase, quenching the flame.	Less common now due to environmental and toxicity concerns.	Used when high performance is critical, but generally avoided in modern waterborne coatings.

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Challenges in Developing Flame Retardant Coatings

Developing effective flame-retardant coatings presents several formulation and performance hurdles:

1. Maintaining Physical and Mechanical Properties: The high loading of FR additives (especially mineral fillers like ATH/MH or intumescent components) required for effective flame retardancy can negatively impact the coating’s essential properties, such as:
   • Mechanical Strength/Flexibility: Decreased elasticity, increased brittleness.
   • Adhesion: Reduced bond strength to the substrate.
   • Water/Weather Resistance: Porosity can increase, compromising durability.
2. Compatibility and Stability in Dispersions: The FR additive (often a solid powder) must be easily dispersed and remain stable within the polymer emulsion (dispersion) without causing:
   • Coagulation or Flocculation of the polymer particles.
   • Settling or poor storage stability.
   • Foaming during processing or application.
3. Required High Loading: For sufficient fire protection, FR additives must often be added at a high concentration (high load), which raises cost and exacerbates the property compromises listed above.
4. Environmental and Health Concerns: There is a strong industry drive towards halogen-free, low-toxicity, and low-smoke FR systems. Replacing effective but problematic halogenated flame retardants with environmentally benign alternatives that offer comparable performance is a significant challenge.
5. Achieving Multifunctionality: Modern coatings often require multifunctionality (e.g., flame retardancy, corrosion resistance, self-healing, hydrophobicity). Incorporating multiple functional additives can lead to conflicts where one property is compromised to achieve another.

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Differences in Flame Retardant Properties within Polymers

The inherent flammability of the base polymer itself affects the type and amount of FR additive needed.

• VAE (Vinyl Acetate Ethylene):
  • Inherent Property: VAE generally has a lower flammability index than Styrene Acrylic and Styrene Butadiene. This makes it comparatively easier and more economic to formulate with FR additives to meet fire standards.
  • Reason: The presence of the vinyl acetate component, which typically combusts less vigorously than aromatic structures, contributes to its lower inherent flammability.
• Styrene-Butadiene (SBR) and Styrene Acrylics (SAE):
  • Inherent Property: These polymers, particularly SBR and SAE, contain the styrene component, an aromatic hydrocarbon that is highly combustible and produces significant smoke and soot when burned. They are considered to have a very high flammability and a low Limiting Oxygen Index (LOI).
  • FR Requirement: This high flammability means they require more potent and/or higher loadings of FR additives. Intumescent systems like APP/EG are particularly vital for SAE to achieve high fire ratings (e.g., UL-94 V-0), as they effectively generate a char to isolate the highly flammable base material.
• PVAC (Polyvinyl Acetate) and Pure Acrylics:
  • Inherent Property: Both are combustible, but pure acrylics and PVAC tend to have an inherent flammability somewhere between VAE and Styrene-based polymers. Pure acrylics are fully acrylic and highly carbonaceous, and while they may exhibit better UV stability than styrene-containing polymers, they still require FR treatment for fire-rated applications. PVAC is chemically similar to the vinyl acetate part of VAE, but without the ethylene component.
  • FR Requirement: They readily respond to the common FR additives (ATH, MH, APP), and the specific formulation depends heavily on the final application’s fire requirements.

Flammability index

The flammability index is a numerical measure that indicates the potential fire hazard of a material. It quantifies how easily a material can ignite and its ability to sustain combustion.

A higher flammability index value typically corresponds to a material that is more easily ignited and poses a greater fire risk.

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Key Characteristics

The flammability index is a result of standardized fire testing and combines multiple factors related to a material’s burning behavior:

• Ignition: How quickly the material ignites when exposed to a flame or heat.
• Flame Spread: How rapidly the flame spreads across the material’s surface.
• Heat Factor/Release: The amount of heat generated during burning.
• Burning Time: The duration for which the material continues to burn after the ignition source is removed.

It’s important to note that the term “flammability index” can refer to different specific standards and scales, depending on the country or application (e.g., building codes, transportation, etc.).

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How the Flammability Index is Determined

The flammability index for a material is determined through standardized fire testing protocols. One common method, such as the one specified in the Australian Standard AS 1530.2, uses a Vertical Burn Test and is typically designed for thin, pliable materials like textiles or sarking (thin sheets used in construction).

The AS 1530.2 Test Method

1. Preparation: A specimen of the material is mounted vertically on a frame.
2. Ignition: A flame, often from a pure alcohol heat source, is applied to the base of the material for a set period.
3. Measurement: Several criteria are observed and measured:
   • Height of flame: The maximum height reached by the flame.
   • Time of flame: The duration of the flame on the material.
   • Heat factor: A measurement related to the heat produced.
4. Calculation: The index number is a calculated result combining these criteria, often resulting in a score that ranges from 0 to 100, though specific regulations may require a score to be much lower for a material to be considered compliant (e.g., a score of 5 or 6).

This index is used to assess the potential fire hazard of a material during the early growth of a fire, providing a quantitative way to compare the flammability of different materials.

Typical VAE applications

Vinyl Acetate-Ethylene (VAE) copolymer dispersions are exceptionally useful as binders in a wide variety of coatings and applications, particularly where flexibility, strong adhesion to polar substrates, and environmental friendliness (low VOC) are key requirements.

Their versatility stems from the incorporation of ethylene, which lowers the polymer’s glass transition temperature (Tg​), resulting in a soft, flexible film without the need for external plasticizers.

Here is a breakdown of the coatings and applications where VAE is most useful:

1. Architectural Coatings (Paints)

VAE is a leading choice for water-based paints, especially for interior applications, due to its excellent combination of performance and environmental profile.

Application/Product	Key VAE Benefit
Interior Wall Paints (Flat and Semi-Gloss)	Low Odor & Low VOC: VAE is a major component in eco-friendly and emission-free paints.
High-Performance Interior Paints	Excellent Scrub Resistance: Provides a durable film that can withstand cleaning and washing.
Film Formation at Low Temperatures	VAE can form a continuous, durable film at temperatures near 0∘C without the need for added coalescing solvents.
Good Hiding Power (Opacity)	Contributes to the paint’s ability to cover the underlying surface effectively, often allowing for a reduction in titanium dioxide.
Textured Coatings	Provides durability and flexibility for thicker, textured finishes.
Exterior Wall Coatings	Offers good water resistance and adhesion, though some high-end exterior applications may prefer other polymers for superior UV resistance.

2. Construction and Building Materials

VAE is widely used in its liquid emulsion form and also as a Redispersible Polymer Powder (RDP), which is mixed with dry-mix mortars.

Application/Product	Key VAE Benefit
Cementitious Membranes	Flexibility and Crack Resistance: Improves the elasticity of cement-based products, allowing them to better handle movement and prevent cracking.
Tile Adhesives and Grouts	Superior Adhesion: Enhances the bond to various substrates and improves strength and workability.
Repair Mortars and Troweling Compounds	Flexural Strength and Workability: Improves mechanical strength and makes the material easier to apply.
Concrete Sealers & Sealants	Provides tough, flexible, and water-resistant films.

3. Adhesives

VAE’s natural polarity makes it an exceptional binder for materials like wood and paper.

Application/Product	Key VAE Benefit
Woodworking Adhesives	Strong Adhesion to Polar Substrates: Excellent bonding to wood, paper, and textiles.
Packaging and Paper Adhesives	Fast Setting Speed and good wet tack.
Flooring Adhesives	Provides flexibility and moisture resistance.

4. Textiles and Nonwovens

VAE dispersions are used as binders and finishes for fabrics.

Application/Product	Key VAE Benefit
Nonwoven Fabric Processing (Medical, Hygiene)	Functions as a binder to consolidate loose fibers, improving mechanical strength and flexibility.
Textile Printing	Acts as a pigment binder for color fastness and wash resistance.
Carpet Backing	Used to bind the carpet fibers and provide durability.

Key Performance Advantages of VAE dispersions

The properties of VAE that make it so useful in these areas include:

• Flexibility & Toughness: Due to the internal plasticizing effect of the ethylene monomer, films are highly flexible and tough without the migration of external plasticizers.
• Adhesion: Exhibits excellent bonding to a wide range of substrates, particularly polar materials like wood, paper, and concrete.
• Environmental Profile: Water-based, low odor, and helps formulators meet low-VOC (Volatile Organic Compound) standards, often eliminating the need for coalescing agents.
• Water Resistance: Forms a film with good resistance to water and alkali.

Reduce or replace ATH content with VAE dispersions

Aluminum Trihydrate (ATH) is a widely used, non-halogenated flame retardant, but there are several alternatives, especially when higher processing temperatures are required or when seeking different flame retardant mechanisms.

The primary alternatives can be grouped into other metal hydroxides, other inorganic minerals, and non-halogenated organic compounds.

I. Metal Hydroxides (Same Mechanism, Different Temperature)

These compounds function similarly to ATH—they decompose endothermically to absorb heat and release water vapor, which cools the material and dilutes flammable gases.

Alternative	Chemical Formula	Decomposition Temperature	Key Advantage over ATH	Primary Applications
Magnesium Hydroxide (MDH)	Mg(OH)2​	≈300–330∘C	Higher Thermal Stability: Can be used in polymers that require higher processing temperatures, such as polypropylene and polyamides.	Polypropylene, polyamides, wire & cable, engineering thermoplastics.
Aluminum Oxide Hydrate (Boehmite)	AlOOH	≈320∘C	Higher Thermal Stability: Suitable for engineering thermoplastics and acts as a synergist with metal phosphinates.	Engineering thermoplastics, printed circuit boards.

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Note on Metal Hydroxides: While highly effective, both ATH and MDH often require high loading levels in the polymer to achieve sufficient flame retardancy, which can sometimes negatively affect the material’s mechanical properties.

II. Other Inorganic and Mineral Flame Retardants

These options often function as smoke suppressants, char formers, or synergists.

• Zinc Borate (ZnB)
  • Mechanism: Multifunctional, acting in both the solid and gas phases. It promotes the formation of a stable, glass-like char layer and acts as a smoke suppressant and afterglow suppressant. It can also release its own water of hydration above 290∘C.
  • Use: Often used in combination with other flame retardants (including ATH and MDH) for a synergistic effect, or as a replacement for antimony trioxide (a halogenated synergist).
• Expandable Graphite (EG)
  • Mechanism: It is an intumescent material. When exposed to heat, it expands significantly to form an insulating layer of char, which shields the underlying polymer from heat and oxygen.
  • Use: Effective at reducing fire hazards, often used synergistically with other non-halogenated flame retardants like ATH or Red Phosphorus.

III. Non-Halogenated Organic and Chemical Flame Retardants

These systems often work by forming an insulating char layer (intumescence) or interfering with the combustion chemistry.

• Phosphorus-Based Compounds
  • Examples: Red Phosphorus (RP), Ammonium Polyphosphate (APP), and various organo-phosphates.
  • Mechanism: They are primarily active in the solid phase by promoting the formation of a char layer on the polymer surface, which acts as a barrier to heat and oxygen. Some forms may also release volatile phosphorus compounds to scavenge free radicals in the flame (gas phase).
• Nitrogen-Based Compounds
  • Examples: Melamine, Melamine Cyanurate (MCA), Dicyandiamide (DICY).
  • Mechanism: They often release inert nitrogen gases when heated, which dilutes the oxygen concentration in the flame. They can also work in conjunction with phosphorus compounds in intumescent systems to help form a char.
• Intumescent Systems
  • Mechanism: These are complex formulations that typically include an acid source (e.g., APP), a carbon source, and a blowing agent. When heated, they swell to form a thick, protective, foamed-char layer.

The best alternative depends entirely on the specific application, the polymer being used, the required processing temperature, and the desired fire performance and cost.