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Flame Retardants for Fire Proof Plastics

Flame retardant (FR) plastics are essential to devices we use every day, providing a valuable tool in fire prevention, but their technology is complex. While some resins are inherently flame resistant, others need special additives to minimize the propagation of smoke and flames.

How to overcome this daunting task of right flame retardant selection for a specific application?

Learn here, the detailed knowledge on flame retardants to ease your process while exploring:


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What are Flame Retardants?


TAGS:  Innovation in Flame Retardants    

Flame Retardants for Plastics
Flame retardants (FR)are chemical compounds added with an objective to inhibit/retard the ignition/burning of the plastic. To prevent combustion, it becomes necessary to design a thermally stable polymer that has a lesser probability of decomposing into combustible gases under heat stress.

However, thermally stable polymers may exhibit performance limitations and are often too expensive and difficult to process. Therefore, manufacturers add various flame retardants to impart flame retardancy to a plastic.

Additive vs Reactive Flame Retardants


There are several chemical classes of flame retardants used with polymers including Brominates FR's, Organo Phosphorus FR's, Melamine-based FR's, Metal Hydroxide FR's, etc. Apart from these chemical classes, there are other flame retardants that can be incorporated into a polymer. They may act as additive and reactive flame retardants.

Both categories may largely influence similar properties of different polymers like viscosity, flexibility, density, etc. Some characteristics of reactive and additive flame retardants are mentioned in the table below for a better understanding of their individual properties.

Additive Flame Retardants Reactive Flame Retardants
Added to the polymer through physical mixing Are added to the polymer via chemical reactions
Do not bind to the polymer chemically (do not  undergo any chemical reactions) Once incorporated become a permanent part of  the polymeric structure (bind chemically)
Can be incorporated into the polymeric mixture at  any stage of its manufacturing and hence have an  added advantage over the Reactive FR's Must be incorporated only during the early stages  of manufacturing


Why use Flame Retardants?


Flame Retardants Contribute Directly to the Saving of Lives

In most cases polymers initiate or propagate fires because, being organic compounds, they decompose to volatile combustible products when they are exposed to heat.

However, in many fields such as electrical, electronic, transport, building, etc., the use of polymers is restricted by their flammability, whatever the importance of the advantages their use may bring.

The present diffusion of synthetic polymers has greatly increased the "fire risk" and the "fire hazard" that is respectively the probability of fire occurrence and its consequence either on humans or on structures.

To fulfil these legal requirements flame retardants need to be added into the polymer. In order to increase the escape time of persons, the role of these additives is to:

  • Slow down polymer combustion and degradation (fire extinction)
  • Reduce smoke emission
  • Avoid dripping
Escape Time of Persons with and without Flame Retardants


The severity of the regulations will depend on the time needed to escape an environment!

 » View All the Commercially Available Flame Retardants Here!

This polymer additives database is available to all, free of charge. You can filter down your options by suitable pigment, system or application (plastisols, compoundings...), supplier and regional availability.

Let's understand the uses of flame retardants in fire protection in detail...



Fire Protection Using Flame Retardants


The goals for fire retardant are universal and can be simply stated in the following items:

1. Prevent the fire or retard its growth and spread i.e. the flash over:
  • Control fire properties of combustible items
  • Provide for suppression of the fire
Fire Dynamics
Flash Over Time vs Fire Retardant Use

Under the conditions of fire the use of the flame retardant gives a significant increase in the escape time available.

2. Protect Occupant from the Fire Effects
  • Provide timely notification of the emergency,
  • Protect escape routes,
  • Provide areas of refuge where necessary and possible.
Fire Dynamics
Smoke Release vs Fire Spread


The use of fire retardant reduces the flame spread and so the rate at witch the smoke develops. Less smoke production gives an increase in the escape time available.


3. Minimize the Impact of Fire
  • Provide separation by tenant, occupancy, or maximum area.
  • Maintain the structural integrity of property,
  • Provide for continued operation of shared properties.

Fire Dynamics
Example of Functionalities to be Maintained During First Steps of Fire


4. Support Fire Service Operations
  • Provide for identification of fire location,
  • Provide reliable communication with areas of refuge,
  • Provide for fire department access, control, communication, and selection.

To prevent the fire or retard its growth and spread, material and product performance testing is used to set limits on the fire properties of items which represent the major fuels in the system. The majority of fire safety requirements consist of material fire performance test criteria to retard its growth and spread. Based on test methods that evaluate fire properties of individual materials, the test methods are generally based on the measurement of the flame-spread speed. 

Table below shows the brief overview of the fire retardant and fire resistant characteristics:

  FR Fire Retardant FRT Fire Resistant
WHY To save lives
HOW Delaying the fire growth Limiting the physic progression of fire from one to another area
MEANS Decreasing the fire kinetic Using fireproof barriers to compartment the fire areas
WHEN At the early stage of fire delaying the flash over phenomenon During fire from the early to the post flashover periods
What is assessed The reaction to fire in term of contribution to fire:
  • nil
  • low
  • medium
  • high
The resistance to fire in term of maintaining certain functionalities:
  • Smoke and heat Insulation
  • Integrity
  • Load bearing
 Test scenario

  • To submit a sample to a heat flux
  • To ignite the gaseous decomposition products
  • To follow the fire development

  • To submit the sample to an increasing heat flux
  • To follow the functionality evolution during the exposure time
 Key parameters
  • Heat release
  • Dripping
  • Flame spread
  • Smoke opacity
  • Smoke Toxicity
Time failure of functionality studied:
  • Smoke Insulation
  • Heat Insulation
  • Integrity
  • Load bearing


Learn more about fire scenario & fire parameters by watching this video tutorial:

What fire test for E&E, building, mass transport, automotive



Flame Retardants Mechanism of Action


Flame Retardants Mechanism of ActionFire is the result of three factors:
  • Heat
  • Fuel
  • Oxygen
Heat produces flammable gases from the pyrolysis of polymer. Then, an adequate ratio between these gases and oxygen leads to ignition of the polymer. The combustion leads to a production of heat that is spread out (delta H1) and fed back (delta H2). This heat feedback pyrolyses the polymer and keeps the combustion going.

To limit the establishment of this combustion circle, one (or several) ingredient has to be removed. Several techniques are available in order to break down this combustion circle.

Flame retardants have to inhibit or even suppress the combustion process. Depending on the polymer and the fire safety test, flame retardants interfere into one or several stages of the combustion process: heating, decomposition, ignition, flame spread, smoke process.

Flame retardants can act:
  • Chemically in the condensed/gas phase, and/or
  • Physically

However, we have to remember that both of them occur during a complex process with many simultaneous reactions.

Let's understand their mechanism of action deeply:

1. a. Chemical Effect (Condensed Phase)


In condensed phase two types of reactions can take place:

  1. Breakdown of the polymer can be accelerated by flame retardants. It leads to pronounced flow of the polymer which decreases the impact of the flame which breaks away.
  2. Flame retardants can cause a layer of carbon (charring) on the polymer's surface. This occurs, for example, through the dehydrating action of the flame retardant generating double bonds in the polymer. These processes form a carbonaceous layer via cyclizing and cross-linking processes cycle.
Char and intumescence formation
Char and Intumescence Formation


Intumescence


Flame retarding polymers by intumescence is essentially a special case of a condensed phase mechanism. The activity in this case occurs in the condensed phase and radical trap mechanism in the gaseous phase appears to not be involved.

In intumescence, the amount of fuel produced is also greatly diminished and char rather than combustible gases is formed. The intumescent char, however, has a special active role in the process. It constitutes a two-way barrier, both for the hindering of the passage of the combustible gases and molten polymer to the flame as well as the shielding of the polymer from the heat of the flame.

In spite of the considerable number of intumescent systems developed in the last 15 years, they all seem to be based on the application of 3 basic ingredients:
  • A "catalyst" (acid source),
  • A charring agent and
  • A blowing agent (Spumific).

Additives combining the last three ingredients leading to intumescent effect are commercially available. However, intumescent formulations can simply be developed and are more suitable than some commercial grades for some specific applications. Table 1 below summarize usual catalyst, charring and blowing agents.

Catalyst
(Acid source)
Charring agents Blowing agents
(Spumific)
Ammonium salts Phosphates, polyphosphates Polyhydric compounds Amines/amides
Sulfates
Halides
Starch
Dextrin
Sorbitol Pentaerythritol, monomer, dimer, trimer
Phenol-formaldehyde resins
Methylol melamine
Urea
Urea-formaldehyde resins
Dicyandiamide
Melamine
Polyamides
Phosphates of amine or amide Others Charring  
Products of reaction of urea or Guanidyl urea with phosphoric acids
Melamine phosphate
Product of reaction of ammonia with P2O5
Polymers (PUR, PA, …)  
Organophosphorus compounds
Tricresyl phosphate
   
Alkyl phosphates
Haloalkyl phosphates
   


1. b. Chemical Effect (Gas Phase)


The flame retardant or their degradation products stop the radical mechanism of the combustion process that takes place in the gas phase. The exothermic processes, which occur in the flame, are thus stopped, the system cools down, the supply of flammable gases is reduced and eventually completely suppressed.

The high-reactive radicals HO· and can react in the gas phase with other radicals, such as halogenated radicals resulted from flame retardant degradation. Less reactive radicals which decrease the kinetics of the combustion are created.(see figure below)



Flame inhibition studies have shown that the effectiveness decreases as follow: HI>HBr>HCl>HF

Mechanism of Action of Halogenated Flame Retardants
Mechanism of Action of Halogenated Flame Retardants


Brominated compounds and chlorinated organic compounds are generally used because iodides are thermally unstable at processing temperature and effectiveness of fluorides is too low. The choice depends on polymer type. The behavior of the halogenated fire retardant in processing conditions (stability, melting, distribution, etc…) and/or effect on properties and long-term stability of the resulting material are among the criteria that have to be considered.

Moreover it is particularly recommended to use an additive that produces halide to the flame at the same range of temperature of polymer degradation into combustible volatile products. Then, fuel and inhibitor would both reach the gas phase according to the "right place at the right time" principle.

The most effective fire retardant (FR) polymeric materials are halogen-based polymer (PVC, CPVC, FEP, PVDF...) and additives (CP, TBBA, DECA, BEOs...). However the improvement of fire- performance depends on the type of fire tests i.e. the application.

They perfectly illustrate the previously described chemical modes of action. Severe perturbations of the kinetic mechanism of the combustion lead to incomplete combustion.

Synergism with Antimony trioxide (Sb2O3)


To be efficient the trapping free radicals needs to reach the flame in gaz phase. Addition of antimony trioxide allows formation of volatile antimony species (antimony halides or antimonyoxyhalide) capable to interrupt the combustion process by inhibiting H* radicals via a series of reactions proposed bellow. This phenomenon explains the synergistic effect between halogenated compounds and Sb2O3.

For most applications, these two ingredients are present in the formulations.




2. Physical Effect


Formation of a Protective Layer


The additives can form a shield with low thermal conductivity, through an external heat flux, that can reduce the heat transfer deltaH2 (from the heat source to the material). It then reduces the degradation rate of the polymer and decreases the "fuel flow" (pyrolysis gases from the degradation of the material) that feeds the flame.

Phosphorus additives may act the same way. Their pyrolysis leads to thermally stable pyro- or polyphosphoric compounds which form a protective vitreous barrier. The same mechanism can be observed using boric acid based additives, zinc borates or low melting glasses.

Formation of protective layer inhibiting, combustion and volatiles

Figure 1: Formation of protective layer inhibiting, combustion and volatiles


Cooling Effect


The degradation reactions of the additive can influence the energy balance of combustion. The additive can degrade endothermally which cools the substrate to a temperature which is below the one required for sustaining the combustion process. Different metal hydroxides follow this principle and its efficiency depends on the amount incorporated in the polymer.

Dilution


The incorporation of inert substances (e.g. fillers such as talc or chalk) and additives (which evolve as inert gases on decomposition) dilutes the fuel in the solid and gaseous phases so that the lower ignition limit of the gas mixture is not reached. In recent work, the isolating effect of a high amount of ash (resulting from certain silica-based fillers) has been shown in fire-retarded systems. Moreover, it highlights also an opposite effect as thermal degradation of the polymer in the bulk is increased by heat conductivity of the filled material.


Flame Retardant Chemistries


There are several chemical classes of flame retardants used with polymers, such as:

  1. Halogenated Flame Retardants
  2. Phosphorus-based flame Retardant
  3. Melamine Flame Retardants
  4. Metal Hydroxide Flame Retardants
  5. Silicon-based Flame Retardants
  6. Phosphate Flame Retardants

Let's discuss each class in detail:


1. a. Brominated Flame Retardants


Brominated flame retardants (brominated FR) are by far the most commonly used class of FR's used today. This family of flame retardants is very versatile and provides the best balance between: flame retardant performance, mechanical properties, process ability and cost in use.

Brominated flame retardants for industrial uses are produced by the bromination of Bisphenol-A with bromine in presence of a solvent, such as:
  • Methanol or a halocarbon
  • 50% hydro-bromic acid or aqueous alkyl monoethers

BFR's when combined with minerals help in improving the mechanical properties and reduce the opacity and corrosivity of the fumes generated. This helps to diminish the environmental hazards arising from the incineration of fumes.

These flame retardants can provide you with efficient solutions to meet your regulation requirements as well as giving outstanding performances to your product.

Continue reading to learn more about brominated flame retardants and explore:


Brominated Flame Retardants Selection

Flame Retardants Selection depends on your application and the specific flame retardant standards and regulations that you have to meet. There are a number of others issues which must be considered when selecting the best FR system for a given use.

Following are the factors which may impact brominated flame retardants selection:

1. Bromine Type and Content

In order to be effective the selected brominated flame retardant must decompose when the polymer burns but remain stable during polymer processing; this in turn dictates the type of bromine in the FR. It must also have sufficient bromine content to allow you to obtain the FR performance you require while not adversely effecting physical properties and overall system cost due to high loadings.

2. Thermal Stability

The selected brominated flame retardant must remain stable during compounding and injection molding. Decomposition during these steps can lead to color formation, degradation of the polymer and equipment corrosion. Hence selection of the correct FR along with any heat stabilizers and synergist that may be required is extremely important.

3. Aging characteristics

Your resin system may have to withstand various factors that can cause premature degradation of properties and color formation. Factors like UV stability, thermal stability and migration will dictate the best flame retardant to use in your system along with any stabilizers required.

4. Processing Characteristics

Depending on your processing temperature certain FR's are melt blendable while others act as fillers. This can affect your processing and final physical properties.

5. Standard to be met

Flame retardants' selection will depend heavily on your resin system you have chosen and the standards to be met.

6. Cost in use

The overall cost of the entire package needs to be taken into account which is not just a function of the cost of the brominated flame retardant but its required loading and what other additives are required to be used with it, in order to obtain a viable system.

7. Environmental

The use of brominated flame retardant induces specific environmental constraints. One of the key topics is to reduce toxic hazards at each step of your production process (from manufacturing to end-use and disposal).

8. Non-Blooming

Blooming is very slow process where the flame retardant migrates to the surface of the plastic resulting in a hazy surface, which often has a bronze like appearance.
Brominated Flame Retardants Selection

This effect is particularly undesirable for parts that also have an aesthetic function such as enclosures and housings. For this reason, Blooming is an important criteria to consider for some applications.

Generally, Blooming depends on the compatibility of the FR with the polymer additive as well as the FR's molecular weight. The higher the compatibility and the molecular weight, the lower the blooming.

9. UV Stability

In many applications, the flame retardant resin may have to withstand various conditions that can cause premature degradation of properties and discoloration.
For this reason selecting the right brominated flame retardant is critical for UV stable applications and in particular for outdoor applications.


How can Plastics & Printed Circuit Boards Containing Brominated Flame Retardants be Managed


Plastics containing BFR's have proven to be fully compatible with all methods of waste management, especially recycling and recovery.

For example: Certain plastics/BFR combinations are actually already being specified by leading manufacturers of photocopiers, in part because of their excellent stability in the recycling process.

Recycling is already taking place with 30% of some new copiers containing recycled plastic with brominated flame retardants. A recent study concluded that ABS plastic containing a BFR was superior to other plastics in terms of recyclability and could be recycled five times in full compliance with the strictest environmental and fire safety requirements.

The Swedish company, Boliden, has developed a recycling process for electrical and electronic equipment waste, in compliance with Swedish regulation, whereby the metals are recycled. The plastics provide some of the energy in the smelting process. Brominated flame retardant containing plastics have been tested in this process and fully meet the smelter's requirements.

In short, the presence of plastics containing brominated flame retardants in the waste stream provides producers of many products with a wide variety of environmentally sound and economically feasible options for waste recovery and recycling.


Brominated Flame Retardants Applications


Brominated flame retardants are used in numerous applications. Some major applications of brominated flame retardants include:

Application Description
Printed Wiring Boards

Brominated Flame Retardants Applications

  • Printed Wiring boards (PWB) are used in many applications such as computing, telecommunications, and industrial controls.
  • Most rigid PWB's are made of epoxy resins or phenolic resins (thermosets), that require flame retardants to meet the required flammability standards.
Connectors

Brominated Flame Retardants Applications

  • Large connectors
    For FR large connectors the use of a flame retardant possessing an excellent dispersibility and molding performance is recommended.

  • Thin Walled Connector
    Flame retardants are typically added to the formulation because of safety and regulation concerns.
Wire & Cable

Brominated Flame Retardants in wires and cables

  • FR's prevent any arcing igniting the compound, and subsequently
  • Prevent the spread of fire throughout a structure along the wiring
Electronic Enclosures

Brominated Flame Retardants in Electrical Applications

  • Flame retardant resin system for enclosures is heavily driven by fire safety standard, cost, performance, and health and environment requirements.
  • Enclosures should meet high fire safety standards such as the UL-94 V0 or similar flame retardant specification.
Construction

Brominated Flame Retardants in Construction

  • Brominated flame retardants are added in end-products used for flooring, roofing, insulation foam, plastic wood composites...
  • Keeping health issues in mind, brominated compounds are under intense research to allow efficient flame retardancy as well as increased environmental friendliness.
Furnishing

Flame Retardant Furniture

  • Flexible polyurethane foams are commonly used as padding in many types of furniture.
  • Brominated FR's can be used to flame retard flexible polyurethane foam.
Textiles

Flame Retardant Textile

  • Flammabilty of fabrics is a key concern within the textile industry.
  • The use of Flame Retardants for Textiles is increasing due to the increased severity of the latest safety regulations.


Also, brominated flame retardants that were used for textile treatment are compiled in the table below:

Chemical name Main applications
Pentabromodiphenyl ether (PeBDE) Textiles, PUR
Disodium salt of tetrabromophthalate Textiles, Coatings
Pentabromoethylbenzene (5BEB) Unsaturated Polyesters, SBR, Textiles
(source: OECD, 1997)



1. b. Chlorinated Flame Retardants


Chlorinated compounds are molecules containing high concentration of chlorine and act chemically in gas phase. They are oftenly used in combination with antimony trioxide as synergist. The parameters to consider for the selection of a chlorinated compound is the Chlorine content, thermal stability, the volatility and physical form. We can distinguish two main families of chlorinated compounds:

  • Chlorinated paraffins
  • Chlorinated alkyl phosphate

Chlorinated Paraffins Flame Retardants


The general structure of chlorinated resin is:

chlorinated Resin Chemical Structure


There are various products available depending on the length of the paraffinic chain. Liquid grades are produced from short chain paraffins while solid grades,containing 70-72% of chlorine, are produced from higher molecular waxes.

Chlorinated Resin Applications
The main application of chlorinated resins is as plasticizer for flexible PVC in combination with DOP or DINP. This resin improves flame retardant properties in applications like flooring and cables.

Solid grades with high chlorine content as used in thermoplastics like LDPE in CTI cable jacketing in combination with antimony trioxide.

Chlorinated Alkyl Phosphate


The most common molecules are:

TCEP Tris(2-Chloroethyl)phosphate Chemical Structure TCPP Tris (2-Chloro-1-methylethyl)phosphate Chemical Structure
TCEP Tris(2-Chloroethyl)phosphate TCPP Tris (2-Chloro-1-methylethyl)phosphate
 
TDPP Tris (2-Chloro-1-(chloromethyl)ethyl)phosphate Chemical Structure
TDPP Tris (2-Chloro-1-(chloromethyl)ethyl)phosphate


The main application of these products is on rigid and flexible polyurethane foam generally introduced at concentration between 5 and 15% depending on foam density and test severity.

Examples of flame retardant standard achievable with Chlorinated phosphorus products are:

  1. Flexible foam BS4735
  2. Rigid foam BS 476, NFP92-501, DIN 4102

Chlorinated Cycloaliphatic


Chlorinated Cycloaliphatic Chemical Structure Dodecachlorodimethanodibenzocyclo-octane is a molecule commercially available.

This product can be used in numerous polymers including polyamide, polyolefins, polypropylene. They can be combined with various synergists like Antimony trioxide and Zinc Borate.


Key benefits are:

  • High temperature resistance up to 320°C
  • Good resistance to UV ageing
  • Non plasticizing product
  • Unsoluble filler and non-blooming
  • CTI values greater than 400°C in FR nylon
  • Low smoke generation
  • Low density and cost effective


2. a. Organophosphorus Flame Retardants


One of the major classes of flame retardants for thermoplastics and polyurethane foams is that of organic phosphorus compounds (typically phosphates and phosphonates). These may also include phosphorus-halogen compounds and blends of phosphorous with halogenated flame retardants (typically brominated FR's).

Thermoplastic alloys such as PC/ABS and PPO/HIPS are often required to meet stringent FR standards such as UL94 V0. Phosphate based FR's work efficiently in these resins and give good physical properties and good UV stability.

In many applications, rigid and flexible polyurethane foams are required to exhibit a degree of flammability resistance in order to pass specific flammability tests in any given country. Phosphorus based flame retardants, both chlorinated (chlorophosphates) and non-halogenated are extensively used in these applications and are considered an ideal choice, giving a good balance of:

  • Processability
  • Flame retardancy, and
  • Physical properties

In some instances Phosphorous bromine blends are used particularly where low scorch is required.

Depending on the final application, its key requirements and the flammability standards they must meet, PUR foam producers have the flexibility to choose among reactive additive, halogenated and non-halogenated phosphorus based flame retardants. These options provide a versatile selection for addressing the market needs of:

  • Performance
  • Compatibility
  • Efficiency
  • Physical properties
  • Process ability
  • Cost


Applications of Organophosphorous Flame Retardant Additives


Applications of Organophosphorous FR


Selection Criteria for Organophosphorous Flame Retardants


Viscosity

Influence of Phosphorus FR on Viscosity The addition of phosphorus flame retardant into a PU foam formulation often has an influence on its viscosity.

The viscosity of the phosphorus FR influences:
  • The process ability: In most cases, low viscosity is needed
  • The foaming process: Foaming is a complex process and the rheology of the material will influence the cells' size distribution and foam density
  • The foam performance

Fogging & VOC

Fogging is the condensation of volatile substances from various materials used in car interiors on colder surfaces. This particularly happens on the windscreen and leads to a "clouding" on the glass surface.


It is well know that evaporation of plasticizers from dashboard materials contribute to the fogging but Phosphorus FR used in flexible PU foams also can have a contribution particularly when FR is more volatile or has volatile impurities.


As major automotive producers are putting lots of effort to minimise this undesired effect, some solutions are available today to reduce the Fogging contribution from FR’s while maintaining excellent FR performance.


Migration

As traditional phosphorus based FR exhibit low molecular weight, they tend to migrate out of the material with time. This can result in undesired effects such as:

  • Reduction of the FR performances after a few months. (No compliancy)
  • Changes on surface properties (lower adhesion, printability, "greasy" touch...)

To solve these issues, higher high molecular weight phosphorus based FR's have been developed.


Scorching

During the manufacturing of PU foam, heat generation and the presence of oxygen can lead to discoloration and even degradation (particularly in the core) which makes it unacceptable for many end uses. This phenomena is called "SCORCH."

Most of the time, Scorch can be minimized by addition of specific antioxidants. However, the addition of phosphorus based FR (such as chlorophosphates) might have an influence on scorching based on the concentration and nature of the FR used.


Foam Density

As opposed to rigid PUR foams, flexible PUR foams are based on open cells allowing for an easy circulation of air.

As the surface of contact between air and the material increases when density decreases, the density of the PU foam will have a strong influence on the concentration of phosphorus FR needed to pass a specific FR standard.


 Influence of PU Foam on Concentration of Phosphorus FR Needed


For densities higher than 40 kg/m3, 0 to 10 phr of phosphorus are generally needed. For densities between 18 and 25 kg/m3, 10 to 35 phr of phosphorus FR are needed. Of course, the severity of the test to pass will also influence the concentration fo FR needed.

For very demanding applications, Melamine is often used in combination with the phosphorus FR.


Flame Retardant Regulations

One key criteria to consider for the development of a FR foam composition is the standard the material must pass.

The majority of fire safety requirements consist of material fire performance test criteria to measure how well the FR retards the growth and spread of fire. Based on test methods that evaluate fire properties of individual materials, the test methods are generally based on the measurement of the flame-spread speed.


Measurement of Flame Speed

The severity of the test depends strongly on the specific environment in which the material is used. The regulation depends strongly on the region/country, the ignition source, as well as the final application.

In general, the higher the severity of the test the higher the concentration of phosphorus FR required to pass the test.

 » Explore All Phosphorous-based Flame Retardant Grades Here! 


2. b. Red Phosphorus Flame Retardants


The term Red-phosphorus (P-red) is used for describing one of the allotropic forms of Phosphorus. It is obtained by heating White Phosphorus (P-w) at a temperature close to 300°C in absence of oxygen. The color ranges from the orange to the dark violet depending on:
  • Molecular weight
  • Particle size
  • Impurities.

P-red is largely amorphous inorganic polymer, although X-rays have established the existence of several crystalline forms, normally present in a limited extend (< 10%w). It is well known that P-red is active as single additive in nitrogen and/or oxygen containing polymers such as:

Thermoplastics Thermosets Natural fibers
Polyamides Polyurethanes Cellulose
Polyesters Epoxies Cotton
Polycarbonates Melamine formaldehyde
Ethylene-vinyl acetate Polyisocyanates


Red phosphorus flame retardants While it has to be applied with spumific and carbonific agents and/or with inorganic hydroxides in polyolefins, styrenics, rubbers, a.s.o. P-red is the most concentrate source of phosphorus. Therefore it is an effective flame retardant additive at a concentration ranging from 2% to 10%w based on polymer.

Red phosphorus flame retardants are generally applied for meeting high demanding flammability requirements. They do not form toxic smokes. Red phosphorus flame retardants show good electrical ( i.e: high CTI value) and mechanical characteristics. Today its application seems only excluded, for color reasons, from white or very light colored final articles, but is widely applied from black to medium gray.

The high thermal stability of Red phosphorus based flame retardants allow the product to overcome drastic extrusion temperature (up to 320°C) without:
  1. Decomposing
  2. Releasing dangerous substances
  3. Producing carbonaceous residues
  4. Causing corrosion to the extrusion equipments

Red Phosphorus Flame Retardants - Mode of action


The mechanism of red phosphorus flame retardants is still under discussion however the most accepted one is based on the activity of the product in intumescent systems. Following this mechanism P-red is regarded as an acid source which:

  • Is mainly active in solid phase;
  • Extracts oxygen and/or water from the polymers producing phosphorus acid derivatives which undergo the dehydration at high temperature;
  • Catalyses char formation.

This mechanism rises by the following facts:

  • P-red is especially active as sole additive in oxygen and/or nitrogen containing polymers,
  • It needs co-agents in all oxygen lacking polymers
  • No massive content of phosphorus moieties are generally detected in the smokes during pyrolysis,
  • The LOI index of polymer articles is not very much affected by the presence of P-red.


However it has been also suggested the formation of P radicals occurring during the pyrolysis and combustion of P-red containing polymer articles and it has been proven, by EPR measurements, in nylons.

These radicals are assumed to react either with oxygen, by producing phosphoric structures, or with polymers, by acting as prodegradant, so promoting the dripping.

In addition to the above mentioned mechanisms, showing the product is active in solid phase, it has been also suggested that P-red can operate in gas phase as flame poisoning likely to volatile phosphorus compounds. According to this mechanism, P-red could generate volatile phosphorus moieties (P2, PO, PO2, HPO) which are in position to scavenge H radicals.


3. Melamine Compounds as Flame Retardants


Melamine based flame retardants represent a small but fast growing segment in the flame retardant market. These products offer particular advantages overMelamine Flame Retardants existing flame retardants:

  • Cost effectiveness
  • Low smoke density and toxicity
  • Low corrosion
  • Safe handling
  • Environmental friendliness

In this family of non-halogenated flame retardants, three chemical groups can be distinguished:
  • Pure melamine
  • Melamine derivatives, i.e. salts with organic or inorganic acids such as boric acid, cyanuric acid, phosphoric acid or pyro/poly-phosphoric acid, and
  • Melamine homologues such as melam, melem and melon

Melamine based flame retardants show excellent flame retardant properties and versatility in use because of their ability to employ various modes of flame retardant action

FR Mechanism Melamine derivatives Halogen/
Antimony
Organo
phosphorus
Metalhydroxides
Chemical interference  
Heat sink    
Char formation    
Intumescence    
Inert gas  
Heat transfer (dripping)      

Currently, main areas of application for melamine based flame retardants are flexible polyurethane foams, intumescent coatings, polyamides and thermoplastic polyurethanes. Through continued research and application development work, the market for melamine based flame retardants will further expand in the near future, f.i. in the direction of polyolefins and thermoplastic polyesters.


Melamine-based Flame Retardants Mechanism of Action


Flame retardants function by interference with one of the three components that initiate and/or support combustion: heat, fuel and oxygen. Melamine shows excellent flame retardant properties because of its ability to interfere with the combustion process in all stages and in many different ways.

Stages of Combustion 


In the initial stage melamine can retard ignition by causing a heat sink through endothermic dissociation in case of a melamine salt followed by endothermic sublimation of the melamine itself at roughly 350°C. Another, even larger, heat sink effect is generated by the subsequent decomposition of the melamine vapours.

Melamine can be regarded as a "poor fuel" having a heat of combustion of only 40% of that of hydrocarbons. Furthermore, the nitrogen produced by combustion will act as inert dilluent. Another source of inert dilluent is the ammonia which is released during breakdown of the melamine or self-condensation of the melamine fraction which does not sublimate.

Melamine can also show considerable contribution to the formation of a char layer in the intumescent process. The char layer acts as a barrier between oxygen and polymeric decomposition gases. Char stability is enhanced by multi-ring structures like melem and melon, formed during self-condensation of melamine. In combination with phosphorous synergists melamine can further increase char stability through formation nitrogen-phosphorous substances. Last but not least melamine can act as blowing agent for the char, enhancing the heat barrier functionality of the char layer.


4. Metal Hydroxide Flame Retardants


Metal hydroxides are the most commonly used family of Halogen Free Flame Retardants. These mineral compounds are used in polyolefins, TPE, PVC, rubbers, thermosets and can also be used in some engineering polymers (such as polyamide). Aluminium trihydroxide (ATH) is selected when processing temperature is under 200°C. When processing temperature exceeds 200°C, Magnesium dihydroxide (MDH) is then required.

Metal Hydroxide Flame Retardants Benefits



5. Silicon-based Flame Retardants


Silicone Flame Retardants
Silicon based flame retardants have a lot of potential as they can produce protective surface coatings during a fire, caused by a low rate of heat released. Low levels of silicon in certain organic polymer systems have been reported to improve their LOI and UL-94 performance.

Some compounded silicon (polydimethylsiloxane-type) contains dry powders with a variety of organic plastics. Particularly in PS, they showed that an additive level, as low as 1 to 3 %, reduces the rate of heat released by 30 to 50%. They reported similar improvements in HIPS, PP, PS-blends, PP and EVA.

By studying silicon-modified polyurethane, a significant decrease of the rate of release of these materials in comparison with unmodified polyurethanes has been observed. The proposed mechanism is the following one: while burning, formation on material surface of a silicon dioxide layer which can act as a thermal insulator and prevents the feedback of energy to the substrate by re-radiating the external heat flux.

New silicon based flame retardants for polycarbonate (PC) and PC/ABS resins offer both good mechanical properties (strength, moulding) and high flame retardancy performance (UL-94, 1/16 inch V-0 at 10 phr). Linear and branched chain-type silicon with (hydroxy or methoxy) or without (saturated hydrocarbons) functional reactive groups have been evaluated. The silicon, which has a branch chain structure and which contains aromatic groups in the chain and non-reactive terminal group is very effective. In this case, the silicon is finely dispersed in the PC resin and it may move to the surface during combustion to form a highly flame-retarding barrier on it.


6. Phosphate Flame Retardants


There are numerous phosphate based molecules available in the market for flame retardancy and we will not disclose all of them.

Some common products based on phosphate molecules are:

TPP, Diphenylphosphate Chemical Structure TCP, Tricresylphosphate Chemical Structure
TPP, Triphenylphosphate
TCP, Tricresylphosphate
CDP, Cresyldiphenyl phosphate Chemical Structure TIPP, Tri(isopropylphenyl)phosphate Chemical Structure
CDP, Cresyldiphenyl phosphate TIPP, Tri(isopropylphenyl)phosphate

Triphenyl phosphate which can be used is ABS/PC blends, in other engineering plastics like PPO and eventually in phenolic resins.

Tricresylphosphate is mainly used in PVC as flame retardant plasticizer, in styrenic compositions. Commercially available products is a mixture of ortho, meta and para isomers. However, the ortho is very toxic and excluded as much as possible.

Bisarylphosphates


Commercial Bisarylphosphates are :

Resorcinol bis diphenylphosphate (RDP) Resorcinol bis diphenylphosphate (RDP) Chemical Structure
Bisphenol A bis-diphenylphosphate (BDP) Bisphenol A bis-diphenylphosphate (BDP) Chemical Structure


RDP is colorless liquid generally used in ABS/PC, PBT, PPO. These products exibit lower volatility, high thermal resistance, lower plasticizing effect compared to arylphosphates or alkylphosphates. 10-15 phr are generally needed to pass traditional FR test. At lower levels RDP can improve the processability in thin wall injection molding of ABS and styrenics.

BDP is very similar to RDP and it is used in the same applications at around 20phr.

Compared to RDP, BDP provides better melt stability to polymers and lower volatility.

The product also have good hydrolitical stability beneficial for polymers like polycarbonate.

Alkyl phosphonates - The general structure of a phosphonate is:

phosphonate general structure

Dimethyl methyl phosphonate is a very effective flame retardant due to its high phosphorus content. However, its high volatility limits its use in rigid PU and highly filled polyester.

Dimeric or Oligomeric cyclic phosphates are also commercially available. They are generally highly viscous liquids and consquently rather difficult to handle. Some producers are proposing masterbatch solutions.

Dimeric cyclic phosphonate can be introduced in the PET at around 6 wt% for FR PET fibers. It can be used in rigid polyurethane without the volatility drawback.



Flame Retardants Compatibility with Polymers


Compatible Polymer
High Impact Polystyrene

brominated flame retardant compatibility 

  • HIPS is used in many applications because of its excellent balance of properties and low cost. Electrical/Electronics and appliances are the two most important segments requiring flame retardancy in applications where temperature does not exceed 80°C.
  • Brominated FR's are the most cost efficient materials used for imparting flame retardancy to HIPS.
  • However, for PPO-HIPS Blends - Phosphorous FR's are best suited.
Polyolefins

brominated flame retardant compatibility 

  • Polyethylene, EVA - Wiring and cables (low and medium voltage) Cable jacketing
  • Polypropylene - Power cables, Connectors, Public facilities seats (Stadium) Fitting Enclosures PP fibers (carpets, seats)
  • TPO - Roofing membranes, Interior automotive applications Automotive applications, Flexible cables, Shrinkable films

Brominated FR are the most cost efficient materials used for imparting flame retardancy to following polyolefins
Polyamide

brominated flame retardant compatibility 

  • Polyamide applications requiring flame retardancy are mainly components and enclosures for electrical and electronic applications.
  • Selecting the right brominated FR for connectors is important taking all the specifications in account.
  • MDH flame retardant allows processors to produce flame retarded polyamide without halogen- or phosphorous-containing compounds.
Polybutylene Terephthalate

brominated flame retardant compatibility 

  • Selecting the right brominated FR for connectors is important to meet material specifications and processability (thin wall) with the lowest cost as possible.
Acrylonitrile Butadiene Styrene

brominated flame retardant compatibility

  • Acrylonitrile contributes chemical resistance and heat stability; butadiene delivers toughness and impact strength; and the styrene component provides ABS with rigidity and processability.
  • Best Brominated FR for ABS depends strongly on the requirements of your final application.
  • Phosphorus FR's are best suited for PC-ABS blends.
Polyurethane Foams

brominated flame retardant compatibility

  • Rigid PU foams requiring flame retardancy are essentially used for insulation in building and construction (Roofing, wall Sheeting) and refrigeration.
  • To achieve this level of performance, reactive brominated FR's are recommended.
  • Right phosphorus FR has a major impact on the final performances.
  • Selection of the right phosphorus FR has a major impact on the final performance
Polyethylene

Polyethylene 

  • PE main markets include: Wiring and Cables, Automotive, Building and Construction
  • ATH and MDH appears to be the solution of choice to develop demanding fire resistant applications.
Polypropylene

Polypropylene

  • Polypropylene stands out at the edge between commodity and engineering plastics.
  • MDH appears to be the solution of choice to develop demanding applications.
Polyvinylchloride

PVC 

  • PVC belongs to the group of less flammable plastics, however, plasticizer addition leads to a dramatic increase in flammability and smoke density.
  • MDH or ATH can be added as FR's to achieve desired properties
Natural and Synthetic Rubbers

Rubbers 

  • Typical applications include: seals, gaskets, conveyor belts, cables, profiles, foams or protective coverings...
  • Flame retardancy of cross-linked elastomers using MDH or ATH has been state-of-the-art for many years.


Commercially Available Flame Retardants






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