Selecting Flame Retardants for Adhesives and Sealants

Need for Flame Retardant Adhesives

Great attention is being paid to how materials perform in combating the effects of fire. This is partly due to several terrifying incidents associated with the outbreak of fire in public facilities that have led to significant damage and personal injury.

In addition to a greater level of awareness regarding fire safety, several paths have been identified to mitigate the destruction caused when fire erupts. These include:

Improved design and
The use of fire resisting materials

The use of adhesives is intimately connected with both.

In certain applications the adhesive may require fire resistant properties through the addition of a flame retardant. These properties may be directed at the bulk adhesive itself or at the entire bonded joint.

An example of the latter case is flexible circuitry used in electronic and transportation industries. Here the laminating adhesive may be asked to provide sufficient flame retardance to effect the final product (polyester film bonded to copper foil), and in this case the entire joint is tested, not only the adhesive.

A flame resistant adhesive formulation may be required to meet the following criteria:

Resist burning
Retain joint integrity at high temperatures
Produce minimal smoke, and
Release negligible toxic material when heated beyond its decomposition temperature

In addition, a flame retardant must not affect processing methods or the normal service performance of the adhesive (adhesion, durability, chemical resistance, etc.)

Let's explore how to prevent the four stages of fire by studying about the functions provided by flame retardants in detail...

Functioning of Flame Retardants

Flame retardants inhibit or delay the spread of fire by suppressing the chemical reactions in the flame or by the formation of a protective layer on the surface of a material.

They may be mixed with the base material (additive flame retardants) or chemically bonded to it (reactive flame retardants). Mineral flame retardants are typically additive while organic compounds can be either reactive or additive.

Designing Fire-Retardant Adhesive

A fire effectively has four stages:

Initiation
Growth
Steady State, and
Decay

Comparison of the Degradation Temperatures of a Typical Thermoset Adhesive
With Those Reached in Various Stages of a Fire

Each state has a corresponding degradation temperature as shown in Figure. In designing a fire-retardant adhesive, formulators must put their efforts on delivering temperature resistance on the right fire stage for the application:

In electronic manufacturing, for example, an adhesive must suppress any tendency of the electronic component to catch fire - or initiate - if there is a fault-induced rise in temperature.

For bonding tiles or panels, adhesives need to resist detachment in the growth and steady state stages, even when in direct contact with the flame.

They must also minimize toxic gases and smoke emitted. Load-bearing structures are likely to experience all four stages of the fire.

Limiting Combustion Cycle

To limit the combustion cycle, one or several of the processes contributing to fire must be removed by either:

Elimination of the volatile fuel, as by cooling
Production of a thermal barrier, as by charring, thus eliminating fuel by reducing heat transfer, or
Quenching the chain reactions in the flame, as by adding suitable radical scavengers

Flame retardant additives do this by acting chemically and/or physically in the condensed (solid) phase or in the gas phase by providing one of the following functions:

Char formers : Usually phosphorus compounds, which remove the carbon fuel source and provide an insulation layer against the fire’s heat. There are two char forming mechanisms:
- Redirection of the chemical reactions involved in decomposition in favor of reactions yielding carbon rather than CO or CO₂ and
- Formation of a surface layer of protective char

Heat absorbers : Usually metal hydrates, such as aluminum trihydrate or magnesium hydroxide, which remove heat by the evaporation of water from the flame retardant’s structure.

Flame quenchers : Usually bromine- or chlorine-based halogen systems which interfere with the reactions in a flame.

Synergists : Usually antimony compounds, which enhance performance of the flame quencher.

Significance of Flame Retardants in Fire Protection

Flame retardants are an important part of fire protection as they not only reduce the risk of a fire starting, but also that of its propagation. This increases escape time and, thus, protects humans, property, and the environment.

There are many ways to establish an adhesive as a fire retardant. Let's understand the classification of flame retardants in detail.

Classification of Fire Retardants

Flame or fire retardants are classified on two basis:

#1. On the Basis of Reactivity

Flame retardants for polymers can be classified as:

Reactive - the flame retarding agent reacts chemically with the polymer to become an integral part of the molecule
Additive - nonreactive agents that are simply blended or mixed into the compound

Additive flame retardants are more common in practice as the choice of a reactive flame retardant is a bit complex.

The development of systems based on reactive flame retardants is more expensive for the manufacturer, who in effect has to develop novel co-polymers with the desired chemical, physical and mechanical properties, as well as the appropriate degree of flame retardance.

Reactive Flame Retardants

They become a part of the polymer either by becoming a part of the backbone or by grafting onto the backbone. Therefore, they have little influence on mechanical properties. These include tetra-bromo-biphenyl A, di-bormo-neophenyl glycol, brominated styrene, etc.

Non-reactive Flame Retardants

These are not chemically reacted onto the base polymer backbone and, therefore, may migrate out of the system during processing or service life reducing the effectiveness over time. Typical non-reactive flame retardants include chlorinated paraffins, brominated organics, phosphate ester, aluminum trihydrate, magnesium hydroxide, borates, and antimony trioxide.

#2. On the Basis of Chemical Group

Flame retardants can also be classified by the major chemical group which provides the flame retardant properties.

a. Classification on the Basis of Family

With this classification system, there are four main families of flame-retardant chemicals as shown in below.

Family	Characteristics
Inorganic	The main inorganic flame retardants are aluminum trihydroxide, magnesium hydroxide, ammonium polyphosphate, and red phosphorus. This group represents about 50% by volume of the worldwide flame retardant production. Some of these chemicals are also used as flame retardant synergists, of which antimony trioxide is the most important.
Halogenated	Halogenated products are based primarily on chlorine and bromine. This group represents about 25% by volume of the worldwide production. Synergists such as antimony oxides are frequently used with halogenated flame retardants.
Organophosphorous	Organophosphorus products are primarily phosphate esters and represent about 20% by volume of the worldwide production. Phosphate esters, with or without halogen, are the predominant phosphorus-based flame retardant in use. Phosphoric acid esters provide plasticization in addition to flame retardancy.
Nitrogen	Nitrogen-based flame retardants are used for a limited number of polymers. Nitrogen-based compounds can be employed in flame-retardant systems or form part of intumescent flame-retardant products. Melamine, melamine cyanurate, other melamine salts, and guanidine compounds are currently the most predominant group of nitrogen-containing flame retardants.

b. Classification on the Basis of Chemical Type

The major flame retardant products that are being used today in adhesive and sealant formulations and their primary applications are shown in below.

Types	Typical Products	Typical Applications
Alumina Trihydrate	Available in various particle size grades and in surface treated grades	Polyesters Epoxies Polypropylene and ethylene / rubber compounds
Antimony Oxide	Dusting and non-dusting grades are available of various particle sizes	Works synergistically with reactive or additive halogenated compounds, also: Polyethylene Polypropylene Thermoplastic and Unsaturated polyesters
Borate-based	Zinc borate, Barium metaborates Ammonium fluoroborate Boric acid	Flexible PVC Polyolefins Thermoplastic and unsaturated polyesters Epoxies Polyamides Urethanes
Bromine-based	Aromatic bromines (e.g. decarbromophenyl oxide ether, tetrabromo-bisphenol A, pentabromodiphenyl oxide) Aliphatic bromines Ionic bromines Benzene-ethyltribromo derivatives Polypentabromobenzyl acrylate	Work synergistically, also: Polyolefins Polyesters: thermosets and thermoplastics Polyurethanes Epoxies Polyamides
Chlorine-based	Chlorinated paraffins, Tris (dichloropropyl) phosphate, methyl pentachloro stearate, and other chlorinated phosphates Cycloaliphatic chlorine Chlorendic anhydride	Reactive intermediate in making polyester and epoxy flame retardant resins, also: LDPE PVC Polyurethane Polypropylene Polyamide
Magnesium-based	Magnesium hydroxide, Magnesium carbonate	Synergist with alumina trihydrate for smoke reduction, also: Polyesters Epoxies Polypropylene and Ethylene / rubber compounds
Melamine-based	Pure melamine, Melamine derivatives (salts and inorganic acids), melamine homologues	Polyurethane Polyamide Polyolefins Thermoplastic Polyesters
Phosphorous-based	Phosphate esters and others (halogenated and non-halogenated)	Polyurethane Polyesters PVC Celullosics Polyethylene Polypropylene Ethylene / propylene copolymers
Silicone	Polydimethylsiloxane	Works by producing a char surface with: Polyolefin and polyolefin blends EVA Polyurethane

Selection Criteria for Flame Retardants

The criteria in selecting and using the right flame retardant system for a specific adhesive formulation can be broad and somewhat complex.

Care must also be taken with regard to choosing the proper test method and determining the parameters of the test. The materials chosen to perform the function of flame retardation must not interfere with the final product’s performance.

The major problem with incorporating flame retardants in adhesives is that very often a significant amount is required, and they interfere with the other properties of the adhesive and contribute to the cost. Consultation with the flame retardant suppliers is generally necessary to optimize the selection process.

The key criteria used to select a particular flame retardant are:

Fire retardancy
Efficiency / cost
Smoke and combustion products
Ease of compounding
Compatibility
Adequate processing temperature stability
Effect on physical, chemical, electrical and other properties
Environmental regulations

Fire Retardancy

Identification of the flammability test requirements is the initial criterion for selection of the appropriate flame retardant. Flame-retardant selection is affected by the test method to be used to assess flame retardancy.

Some tests can be passed with relatively low levels of flame retardants, while high levels of very powerful flame retardants are needed to pass other tests. Test requirements and standards are usually unique to an industry such as electrical / electronic or building and construction.

Most test requirements apply to the completely assembled and manufactured product and not simply to the adhesive material.

Several fire retardancy characteristics for major chemical families of flame retardant additives are provided. These characteristics are:

Acting site (gas or solid phase)
Mode of action (chemical or physical)
Relative efficiency

Characteristic	Chemical Family of Flame Retardant
Characteristic	Inorganic	Hydrogenated	Organo- phosphorus	Nitrogen
Acting site	Condensed (solid) / gas phase	Gas phase	Condensed (solid) / gas phase	Condensed (solid) / gas phase
Mode of action	Physical	Chemical	Chemical / physical	Chemical / physical
Flame retarding efficiency	Poor	Good	Good	Good

Characteristics of Major Chemical Families of Flame Retardant Additives

Efficiency/Cost

The relationship between fire retardant efficiency and cost is an essential consideration in the selection of a flame retardant.

The greater the flame retardant efficiency of a product for a particular resin, the less will be needed. The amount to be used also depends on the application and specification.

The performance / cost ratio of several common flame retardants are shown below.

Flame Retardancy Performance and Relative Cost of Common Flame Retardants
(Dover Chemical Corporation, January 2016)

It can be concluded from the figure that:

Aluminum trihydrate - It is the lowest cost additive and is used mostly in polymers processed at low temperatures:
- Epoxy
- Unsaturated polyesters
- Polyethylene
- PVC
However, high loadings are required and this can affect the physical properties of the polymer.

Chlorinated paraffins - These paraffins are low cost and can be applied in all polymers processed under 240°C.

Bromine based - These flame retardants are more effective on a weight basis. However based on cost performance, chlorinated paraffins are more effective than aromatic bromine flame retardants.

Smoke and Combustion Products

When fire occurs in a confined space, such as a home or airplane passenger compartment, often the gases generated due to the combustion process provides the most serious threat. As a result, considerable attention has been given to the generation of smoke and toxic combustion products.

Given free access to oxygen, polymers like polyethylene that are based entirely on carbon and hydrogen will burn to give a mixture of water vapor and CO₂, but form toxic CO if without enough oxygen. Polymeric materials which contain the elements chlorine, nitrogen and sulfur will always burn to produce noxious and toxic materials whether there is an adequate oxygen supply or not.

The table below provides characteristics for major chemical families of flame retardant additives. These characteristics are:

Acting site (gas or solid phase)
Mode of action (chemical or physical)
Side effects (smoke and decomposition products)

Characteristic	Chemical Family of Flame Retardant
Characteristic	Inorganic	Hydrogenated	Organo- phosphorus	Nitrogen
Acting site	Condensed (solid) / gas phase	Gas phase	Condensed (solid) / gas phase	Condensed (solid) / gas phase
Mode of action	Physical	Chemical	Chemical / physical	Chemical / physical
Minimization of smoke and toxic combustion products	Good	Poor	Good	Good

Characteristics of Major Chemical Families of Flame Retardant Additives

Ease of Compounding

Flame retardant additives may adversely affect the processing characteristics of polymers. Changes occurring in the viscosity of liquid systems or in the flow of polymers that are melted during processing can cause problems.

Non-melting flame retardants can make the base polymer more difficult to process. However, melt bendable and plasticizing additives such as phosphoric acid esters may be desirable. These can act as processing aids, reduce internal stress in the bond line, and generally provide the adhesive with higher flexibility and impact strength.

Flame retardants can be supplied in various forms to aid in processing of the final adhesive. The table below shows the available forms of some common flame retardant additives:

Type	Typical Form
Type	Dispersion	Emulsion	Flakes	Granules	Liquid	Paste	Powder
Alumina Trihydrate							√
Antimony Oxide	√	√					√
Borate-based					√
Bromine-based	√			√	√		√
Chlorine-based	√	√	√		√		√
Magnesium-based							√
Melamine-based							√
Phosphorous-based				√	√	√	√
Silicone					√

Various Forms of Common Flame Retardants

Compatibility

Polymer compatibility indicates how well the flame retardants interact with the base polymer with which it is mixed. Issues related to solubility, chemical resistance, or reactivity with other formulation components may prevent the use of an otherwise desirable flame retardant.

Significant alteration of the rate of reaction of thermoset polymers or the speed and degree of crystallization of thermoplastic polymers may also result from the use of some flame retardants.

The flame retardant should be chemically compatible with the base polymer as well as mix and disperse well and even dissolve in the polymer if possible.

Typical Base Polymers	Type of Flame Retardants
Typical Base Polymers	Alumina Trihydrate	Antimony Oxide	Borate-based	Bromine-based	Chlorine-based	Magnesium-based	Melamine-based	Phosphorous-based	Silicone
Acrylic & Acrylic Co-polymers							√	√
Epoxies	√	√	√	√	√	√	√	√
Ethylene Coter-polymer Solids (EVA, EMA)					√		√	√	√
Natural Rubber							√
Polyamides			√	√	√		√	√
Poly-chloro-vinyls (PVC, PVDC)		√	√	√				√
Polyesters	√	√	√	√	√	√	√	√
Polyolefins		√	√	√	√		√	√	√
Polyurethanes		√	√	√			√	√	√
Silicones							√		√
Synthetic Rubbers			√			√	√	√

Flame Retardants Suggested for Use with Various Base Polymers

Incompatible flame retardants are more likely to migrate out of the formulation during processing or service aging. Higher molecular weight polymeric additives are a response to this problem and many are available. Reactive flame retardants become a permanent part of the polymer chain and cannot migrate or volatilize.

Adequate Processing Temperature Stability

For the formulator, the flame retardant must be sufficiently stable to survive the processing temperatures. The temperatures routinely used to process many polymers severely restrict the number of flame retardants suitable for incorporation.

Yet it must not be too stable to function as an effective flame retardant when in service. Typically, thermal gravimetric analysis (TGA) is used to evaluate the decomposition or volatilization temperatures of flame retardants and to aid in the selection of the most cost effective materials.

For example: The figure below compares the decomposition characteristics of alumina trihydrate and magnesium hydroxide:

Alumina trihydrate decomposes at about 220°C
While magnesium hydroxide decomposes at about 330°C, thus having a higher thermal stability allowing for a wider window for compounding processing.

Fire Retardant Adhesives for Sealants/Fire-Retardant-Adequate-Processing

Decomposition Temperatures of Alumina Trihydrate (ATH) and Magnesium Hydroxide (MDH)
Measured by Thermal Gravimetric Analysis
(Hubber Fire Retardant Additive Solutions, J.M. Hubber Corporation, 2011)

Effect on Properties

A flame retardant must be chosen for a specific polymer to minimize the degradative effect that it may have on mechanical and thermal properties. Since flame retardants are frequently used at high levels, they often have a dramatic effect on the basic mechanical properties of adhesive systems in which they are used.

Some common problems include reduction of:

Strength
Rigidity
Toughness
Heat resistance

When flame retardants are added to polymers, following changes occur:

Physical properties (density, hardness, melting point, glass transition temperatures, and thermal expansion) and appearance (color, transparency) often change significantly.

Electrical properties (resistance, dielectric strength, loss factor) are frequently altered.

Adhesive’s durability due to factors such as oxidation, UV radiation, and high temperatures may be reduced. The chemical properties of a flame retardant are also often of great importance in the selection process. Resistance to exposure to water, solvents, acids, bases, oils or other substances may be a requirement for use.

Environmental Regulations

The environmental demands placed on today’s flame retardants have changed considerably over the past and play a decisive role in selection. There are some environmental concerns that have developed over recent years regarding flame retardants. The following could influence their selection for a specific application:

Findings of certain brominated flame retardants in the environment
Animal and plant life, and in humans
Some concern about certain phosphate esters in indoor air
Flame retardant persistence in the environment, bio-accumulation, toxicity, etc.

Although bromine and chlorine-containing flame retardants are still used in some products, the need for new alternatives is being driven by a convergence of:

Policy
Standards and
Pressure from environmental groups

Environment regulations tend to be industry and region-specific

A large group of the studied flame retardants has been found to have a good environmental and health profile. These are:

Ammonium polyphosphate
Aluminum diethylphosphinate
Aluminum hydroxide
Magnesium hydroxide
Melamine polyphosphate
Dihydrooxaphosphaphenanthrene
Zinc stannate
Zinc hydroxstannate

Overall, they were found to have a much lower tendency to bioaccumulate than for studied brominated flame retardants.

Standards and Test Methods for Flame Retardants

Standards related to fire testing are aimed at determining the performance of a material with respect to flame, smoke, and toxicity (FST). Several tests have been widely used to determine the resistance of materials to these conditions.

Selected Tests for Flame Retardants

Resistance to Burning

ASTM D635: “Rate of Burning of Plastics”
ASTM E162:“Flammability of Plastic Materials”
UL 94: “Flammability of Plastic Materials”
ISO 5657: “Ignitability of Building Products”
BS 6853: “Flame Propagation”
FAR 25.853: “Airworthiness Standard – Compartment Interiors”
NF T 51-071: “Oxygen Index”
NF C 20-455: “Glow Wire Test”
DIN 53438: “Flame Propagation”

Resistance to High Temperatures

BS 476 Part No. 7: “Surface Spread of Flame – Building Materials”
DIN 4172: “Fire Behaviors of Building Materials”
ASTM E648: “Floor Coverings – Radiant Panel”

Toxicity

SMP 800C: “Toxicity Testing”
BS 6853: “Smoke Emission”
NF X 70-100: “Toxicity Testing”
ATS 1000.01: “Smoke Density”

Smoke Generation

BS 6401: “Specific Optical Density of Smoke”
BS 6853: “Smoke Emission”
NES 711: “Smoke Index of Products of Combustion”
ASTM D2843: “Smoke Density from Burning Plastics”
ISO CD5659: “Specific Optical Density – Smoke Generation”
ATS 1000.01: “Smoke Density”
DIN 54837: “Smoke Generation”

Testing Resistance to Burning

In most tests that measure the resistance to burning, suitable adhesives are those that do not continue to burn for any significant period after removal of the source of ignition. In these tests the cured adhesive sample may be subjected to ignition independent of any adherend (the adhesive is tested as a free film).

Although this approach does not simulate practical reality, it does provide useful data on the relative resistance of the adhesive to burning.

Sample structures with both adhesive and adherend can also be tested. These results may be more representative of the performance of the adhesive in an actual fire since the contribution provided by the adherend could be either positive or negative.

UL-94 Vertical Burning Test

It provides a preliminary assessment of relative flammability and dripping for polymers used in electrical equipment, electronic devices, appliances, and other applications. It addresses such end-use characteristics of ignition, burn rate, flame spread, fuel contribution, intensity of burning, and products of combustion.

Working and Set Up - In this test a film or coated substrate sample is mounted vertically in a draft free enclosure. A burner is placed beneath the sample for 10 seconds and the duration of flaming is timed. Any dripping that ignites surgical cotton placed 12 inches below the sample is noted.

The test has several classifications:

94 V-0: No specimen has flaming combustion for more than 10 seconds after ignition. Specimens do not burn up to the holding clamp, drip and ignite the cotton, or have glowing combustion persisting for 30 seconds after removal of the test flame.
94 V-1: No specimen shall have flaming combustion for over 30 seconds after each ignition. Specimens do not burn up to the holding clamp, drip and ignite the cotton, or have an afterglow of more than 60 seconds.
94 V-2: This involves the same criteria as V-1, except that the specimens are allowed to drip and ignite the cotton below the specimen.

Other Strategies for Measuring Burning Resistance

Another method for measuring the burning resistance of a material is to measure the limiting oxygen index (LOI). The LOI is the minimum concentration of oxygen expressed as a volume percent of mixture of oxygen and nitrogen that just supports flaming combustion of a material initially at room temperature.

The resistance of an adhesive to high temperatures in the case of a fire needs special consideration aside from the flame, smoke, and toxicity effects. Often the substrate will protect the adhesive from a fire. However, if the adhesive loosens or degrades due to the temperature of the fire, the joint can fail causing separation of the substrate and the adhesive. If this happens, the adhesive itself becomes exposed together with the secondary substrate. These fresh surfaces then can contribute further to the fire.

The NIST smoke density chamber (ASTM D2843, BS 6401) is used widely in all industrial sectors for the determination of smoke generated by solid materials and assemblies mounted in the vertical position within a closed chamber. Smoke density is measured optically.

When an adhesive is sandwiched between two substrates, the fire resistance and thermal conductivity of the substrates control the decomposition and smoke emission of the adhesive

In smoke density tests, adhesives can be tested alone as a free coating to impose a worst case condition.

Find Suitable Flame Retardant Grade

View a wide range of flame retardant grades available in the market today, analyze technical data of each product, get technical assistance or request samples.

Suppliers

Brands