OK
The material selection platform
Polymer Additives
Car Fire (Automotive Fire Scenario)

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.

Overview

What are Flame Retardants?

What are Flame Retardants?

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.

Types of Flame Retardants Used Today in Polymers


There are several chemical classes of flame retardants used with polymers, such as halogenated compounds, organophosphorus, melamines, metal hydroxides, 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!

Fire Protection Using Flame Retardants

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 which 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


Flame Retardants Mechanism of Action

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 these mechanisms in detail.:

1a. 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


The Process of Intumescence


Flame retardation 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
   


1b. 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)

Mechanism of Action

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 must 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, etc.) and additives (CP, TBBA, DECA, BEOs, etc.). 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.

Watch this free video tutorial to understand different fire scenarios and learn how to deduct the flame retardants that you need based on the thermal stress & fire parameters.

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


Synergism with Antimony trioxide (Sb2O3)


To be efficient the trapping free radicals needs to reach the flame in gas 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.

Ingredients 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

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 also highlights also an opposite effect as thermal degradation of the polymer in the bulk is increased by heat conductivity of the filled material.

Get Inspired: Use cone calorimeter effectively to get valuable insight into the burning behavior & prepare polymer materials for the large variety of industry specific tests »


Halogenated Flame Retardant Types

Halogenated Flame Retardant Types

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.

Brominated Flame Retardants Selection


Flame retardant 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 mportant criterion 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 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
Brominated Flame Retardants ApplicationsPrinted Wiring Boards

  • 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.
Brominated Flame Retardants ApplicationsConnectors

  • 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.
Brominated Flame Retardants in wires and cablesWire & Cable

  • FR's prevent any arcing igniting the compound, and subsequently
  • Prevent the spread of fire throughout a structure along the wiring
Brominated Flame Retardants in Electrical ApplicationsElectronic Enclosures

  • 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.
Brominated Flame Retardants in ConstructionConstruction

  • 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.
Flame Retardant FurnitureFurnishing

  • 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.
Flame Retardant TextileTextiles

  • 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)


Chlorinated Flame Retardants


Chlorinated compounds are molecules containing high concentration of chlorine and act chemically in gas phase. They are often 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                   TDPP Tris (2-Chloro-1-(chloromethyl)ethyl)phosphate Chemical Structure
TCEP Tris(2-Chloroethyl)phosphate (L), TCPP Tris (2-Chloro-1-methylethyl)phosphate (C) & TDPP Tris (2-Chloro-1-(chloromethyl)ethyl)phosphate (R)

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
  • Insoluble filler and non-blooming
  • CTI values greater than 400°C in FR nylon
  • Low smoke generation
  • Low density and cost effective


Phosphorus-based Flame Retardants

Phosphorus-based Flame Retardants

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

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 known 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 minimize 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 criterion 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.

Get Inspired: Gain a better understanding of quantitative analysis methods (LOI, UL 94...) to determine the effect of flame retardants on material properties »

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

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 equipment's


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, and
  • Catalyzes 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 is 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.

A further step forward to innovative HFFR/LSZH/NHFR cable compounds


Melamine Flame Retardants

Melamine Flame Retardants

Melamine-based flame retardants represent a small but fast growing segment in the flame-retardant market. These products offer particular advantages over 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 Organophosphorus 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, 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 diluent. Another source of inert diluent 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. Finally melamine can act as blowing agent for the char, enhancing the heat barrier functionality of the char layer.

Metal Hydroxide Flame Retardants

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).

They provide flame-retardant formulations that meet appropriate standard for many applications. Such formulations produce combustion products for a low opacity, low toxicity, and minimal corrosivity. When properly compounded, inorganic hydroxides offer a cost-effective means to achieve low-smoke flame-retardant formulations.

In addition, inorganic hydroxides are easily handled and relatively non-toxic. Aluminium trihydroxide (ATH), Magnesium dihydroxide (MDH) and a variety of other inorganic hydroxides are replacing halogenated and phosphorus-containing flame-retardants due to their long-term effects on the environment.

Benefits of Metal Hydroxide Flame Retardants
Key Benefits of Metal Hydroxide Flame Retardants


Aluminium Trihydroxide (ATH)


Aluminium trihydroxide (ATH) is the largest selling inorganic hydroxide used as a flame retardant. ATH is processed at temperature below its decomposition point (190-230°C), depending on the particle size. It used as flame retardant in elastomers, thermosetting resins, and thermoplastics that are processed below 200°C.

ATH obtained from the BAYER process is gibbsite whose particle size exceeds 50µm. It can be redissolved and precipitated to produce more highly purified grades of ATH.

Improvements to this process lead to decrease iron, silica or residual solid impurities. Ground hydrates (beige to white, soda silicates iron impurities, 1.5 to 35 µm) and finely precipitated hydrates (white, bright, pure, 0.28 to 3µm) can be distinguished. The major difference between different grades of ATH is essentially the particle size and the surface treatment.

The aim of the surface treatment is to increase one or more specific mechanical properties (e.g., elongation at break, EB).

  • Fatty acids or metal stearates are often used as surface treatment of ATH or MH in order to limit aggregate of additives and to increase the EB property in cable and wire applications.
  • Some silane-based surface treatment are also available with unreactive (alkyl group) and reactive (vinyl, amino, epoxy and methacryl) substituents. The kind of reactive substituent depends on the polymer in which the flame retardant is used.

Due to a higher cost of silane surface treatments compared to fatty acid, they are developed to perfectly suit specific applications.

Other surface-treatment options employ phosphorus, titanium, and zirconium instead of silicon as the central element. A wide variety of titanates are available. Titanates and zirconates have specific applications and are generally more expensive than the silanes.

Applications of ATH

One major use is in styrene-butadiene-rubber latex used in the manufacture and flame retardation of carpets. It is used in the manufacture of flame-retarded rubber-insulated cable, insulating foam, conveyor belting, roofing, and hoses.

It is used to flame retard almost all applications of unsaturated polyester resins, such as those used for laminated countertops and wall covering, sheet molding compounds, bulk molding compounds, and so forth. Such polyesters are used in bathroom ware and enclosures, decorative wall panels, appliance housing, automotive hoods and decks, moulded seating, truck front ends, and so forth.

Electrical applications include stand-offs, insulators, and circuit boards. Pultruded products employ ATH as a flame retardant for producing profiles for construction applications.

Epoxy and phenolic resins use ATH in electrical/electronic and construction applications such as countertops and sinks, bathroom panels, decorative surrounds, and wall panels.

The use of ATH in thermoplastics is widespread and growing, especially in Europe, where the environmental impact of halogenated organic chemical is of considerable concern. ATH is used in flexible and rigid PVC, EPR, EPDM, EVA, EEA, LDPE, HDPE, LLDPE, blends of PE and PP, and the new classes of plastomers and flexomers created by metallocene catalyst technology. General application includes wire and cable, conduit, piping, appliance housing, adhesives, construction laminates, and insulating foams.

Magnesium Hydroxide (MH)


Magnesium hydroxide (MH) is a more thermally stable inorganic flame retardant. It is stable to temperatures above 300°C and finds use in many elastomers and resins, including engineering plastics and other resins that are processed at higher temperatures.

It is produced using different processes from magnesium-containing ores, such as magnesite, dolomite, or serpentinite and from brine and seawater. Some ores such as brucite, huntite, and hydromagnesite can used as flame retardant themselves or converted into MH.

MH used as flame retardant is generally of high purity (>98.5%). It is most often obtained from seawater or brine, though an ore-derived product also can be of high purity. Three different processes are used: Process from seawater and brine, the Aman Process and the Magnifin® process.

Most flame-retardant grades of MH are white powders ranging in median particle size from 0.5 to 5 µm. Surface area range from 7 to 15 m2/g, depending on the particle shape and size. Like ATH, MH is used as high loading levels, usually between 50% and 70%. The small quantity of MH used as flame-retardant comes from the higher price of MH compared to precipitated grades of ATH.

Applications of MDH

Due to their higher decomposition temperature and cost, MH is generally used in thermoplastics and thermosetting resins that are processed above 200-225°C. They find use in PP, PP-blends, ABS, ABS-alloys and blends, fluoropolymers, PPO, PPO-alloys and blends, polyimides and aliphatic polyketone. MH cannot be used in thermoplastics polyester resins, as they catalyse decomposition of the resin.

End-use applications are in wire and cable, appliance housings, construction laminates, piping, and electrical components. As with ATH, a variety of MH is commercially available to meet the various requirements of the application.

ATH vs MDH – Choosing the Right Flame Retardant


Key material parameters considered when selecting an ATH or MDH product for your flame retardant formulation include:

  • Median particle size
  • Particle size distribution
  • Surface area and particle morphology
  • Surface chemistry
  • Color

These product characteristics (of the base material) will have a direct effect on the compounding process and the end properties of your compound.

Property ATH MDH
Formula Al(OH)3 Mg(OH)2
Water Release 35% 31%
δH -280 cal/g -328 cal/g
Decomposition Temperature 230 upto 300°C 330 upto 400°C
Processing Temperature <200°C >200°C
Cost Lower Higher
Physical Property Comparison


Thermal Stability Comparison of ATH and MDH

The figure below compares thermal decomposition characteristics of ATH and MDH. ATH starts to decompose at about 220˚C (428˚F) while MDH decomposes at about 330˚C (626˚F). Therefore MDH has a higher thermal stability offering a wider window for compound processing. ATH is suitable for use in thermosets and in certain PVC- and polyolefin-based plastics compounds in which processing temperatures are generally below 200˚C.

Thermal Stability Comparison of ATH vs MDH
Source: Huber Engineered Materials

MDH is preferred when formulating plastics compounds that need to be processed at temperatures near or above 220˚C (428°F), such as polypropylene and engineering thermoplastics. Using MDH for low-melting thermoplastics or elastomers can also enable higher processing temperatures and increased compounding throughput.

When heated to decomposition, both ATH and MDH release water of hydration that quenches the polymer and dilutes smoke. It is this release water of hydration that quenches the polymer and dilutes smoke. ATH releases about 35% of its weight in water vs 31% for MDH. The process of endothermic decomposition also removes heat thus helping to retard combustion. MDH absorbs more heat (328 cal/g) than ATH (280 cal/g) on the same weight basis. Therefore, higher thermal stability and greater heat removal capacity make MDH a very effective flame retardant.

Filler Loading of ATH and MDH

ATH and MDH are efficient at loading levels in the order of 150phr (60wt%). Such high loading levels can detrimentally reduce processability, mechanical and other physical properties. For the same reason, although hydroxides are cheaper than most other FRs, the end cost is not really favourable.

Using synergistic additives for hydrated fillers offers a means for lowering overall filler levels, hence limiting the drawbacks, without compromising fire performance. Surface treatment of fire retardant additives with organo-silanes, zirconates or titanates can also improve their efficiency.

ATH and MDH have mutual synergistic effects allowing to improve fire retardant properties for a same total loading or to lower the total loading for the same fire behavior. The use of mixed-metal hydroxides as additives in combination with ATH or MH increases FR performances.

Benefits of Using Mixed-Metal Hydroxides

  1. In extrusion trials, the replacement of pure MDH by various MDH/ATH mixtures allows reductions by:
    • 15 up to 20% of the die pressure
    • 16 to 21% of the torque

  2. ATH/MDH mixtures and use of synergistic mineral FR reduces the cost. Prices of additives are, in descending order:
    Polypropylene > MDH > ATH > Common white fillers
    Exploiting this and the synergistic effects, it is possible to converge on a suitable balance of fire performances, fair physical and mechanical properties with a substantial cost saving in the order of 25-30% as illustrated below.

    Cost-effective Benefits Using Mixed-metal Hydroxides
    Cost-effective Benefits Using Mixed-metal Hydroxides


Silicone-based Flame Retardants

Silicone-based 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, molding) 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.


Phosphate Flame Retardants

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

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

Tricresyl phosphate is mainly used in PVC as flame retardant plasticizer, in styrenic compositions. Commercially available products are 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) and Bisphenol A bis-diphenylphosphate (BDP).

Resorcinol bis diphenylphosphate (RDP) Chemical Structure
Resorcinol bis diphenylphosphate (RDP)

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

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


They are generally highly viscous liquids and consequently 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.


Get Better Performance with Flame-retardant Selection Strategies


Talk to T Richard Hull where he will help you get a grip over your flame-retardant selection strategies by learning about halogen-free alternatives (phosphorus, silicates...) their chemistries & problems with currently available commercial technologies.

Halogen-free Flame Retardants

Nanomaterials as Flame Retardants

Nanomaterials as Flame Retardants

  1. Expandable graphite provides good flame retardancy properties at low loading and can be used in thermoplastic resins and thermosetting resins. For natural charring polymer (PA, PU, PVC...), expandable graphite can be successful used alone.

    However, it is often combined with other flame retardants such as phosphates, boron compounds, antimony trioxide or magnesium hydroxide in order to develop a strong enough substrate to support the shield of expandable graphite. The use of graphite can also be used in nanocomposite PP.
    Expandable Graphite is used as a Flame Retardant in Thermosets and Thermoplastics
    Expandable Graphite is used as a Flame Retardant in Thermosets and Thermoplastics

  2. The flame retardant properties of nanoclays, carbon nanotubes, POSS, and others are also a used in halogenated and non-halogenated systems.

    • Nanoclays reduce relative heat release, promote surface char, create an anti-dripping effect, and reduce smoke generation.

      • In non-halogenated formulations, nanoclays allow a lower loading of mineral flame retardant.
      • In halogenated systems, they reduce the amount of brominated or ATO flame retardant needed, providing lower density, low blooming, and better mechanical properties.

    • Multi-walled carbon nanotubes (CNTs) are used commercially for their electrostatic dissipative (ESD), strength properties and flame retardant properties. They are:

      • Effective at forming char,
      • Retard onset of combustion by drawing heat away,
      • Increase viscosity to help prevent dripping, and
      • Do not contribute to depolymerization.

      CNTs are expected to find use in electronics, where they can provide both ESD and flame retardant properties.

      Structure of Carbon Nanotubes
      Structure of Carbon Nanotubes

    • Polyhedral oligosilsesquioxane (POSS)-based hybrid polymers are completely defined molecules of nanoscale dimensions that may be functionalized with reactive groups suitable for the synthesis of new organic-inorganic hybrids. POSS have been successfully incorporated into common polymers via copolymerization, grafting or blending.

      The synthesis of POSS cages, monomers containing POSS cages, POSS-dendrimer cores, POSS-containing polymers (nanobuilding blocks) and POSS nanocomposites lead to specific properties including mechanical, thermal, flame-retardant and viscoelastic properties. They are used commercially as flame-retardant aids in phenolics, as well as in PPE and COC.

      A key advantage of POSS is the action both as intumescent synergist and as a dispersion aid for halogen-free flame retardants (HFFR), which may allow higher levels of HFFR by improving flow. For example, a lithiated POSS aids dispersion, provides thick intumescent char and mitigates loss of mechanical properties compared to using phosphate FRs alone in thermosets such as vinyl esters.

      Silsesquioxane Cage Structure
      Silsesquioxane Cage Structure

  3. Polymer-clay nanocomposites are hybrid organic polymer inorganic layered materials with unique flammability properties when compared to conventional filled polymers. Polyamide-6, polystyrene and polypropylene are some polymers used in combination with clays.


Hyperbranched Polymers

Hyperbranched Polymers

Hyperbranched polymers with their numerous, more or less ordered branches, open the way to hyperfunctionalized additives. They can be prepared from "branched" monomers of the general type AxBy, where: 
  • A and B represent functional groups that can react with each other (i.e., A + B → -A-B-) but not with themselves (i.e., A cannot react with A and B cannot react with B). 
  • x and y are integers, x equal or larger than 1 and y being equal or larger than 2.

For the simplest case of ABmonomers, the obtained polymer is represented as shown in the above figure below. The figure takes into account the benefits of HBP and fire-retardant additives and suggests some possible routes toward new fire-retardant additives.

Fire-retardant Hyperbranched Polymers
Fire-retardant Hyperbranched polymers

Dendrimers, a special form of HBP, starting from a single focal point or core with each branch of the dendrimer dividing into two (or more) other branches and so forth down to the terminal functionalized ends offer additional opportunities. They can host metal atoms to give metallodendrimers. The metal can be situated in the repeat unit, the core or at the end-groups. Metal hosting expands the properties of metallodendrimers.

Thanks to their architecture, dendritic polymers can be active at low levels compared to linear polymers. The following figure compares hyperbranched polymer (HBP) and dendrimer structures with a conventional 'linear' macromolecule having some short branches.

Structure of Dendrimers (L), Hyperbranched Polymers (R) and Linear Macromolecule (Below)
Structure of Dendrimers (L), Hyperbranched Polymers, AB2 Monomer (R) and Linear Macromolecule (Below)

Hyperbranched Polymers
Dendrimers
  • A high degree of branching
  • Multitude of reactive or non-reactive end-groups
  • Numerous end functions
  • A broad molecular size distribution
  • A lower cost than dendrimers

  • A high degree of branching with a better ordered structure leading to highly precise architectures
  • A narrower molecular size distribution
  • A size in the nanoscale domain
  • A compact hydrodynamic volume and a Newtonian flow easing processing
  • A better suitability to host various entities
  • A higher cost only justified for very specific applications
  • Do not contain molecular core
  • Less defined intramolecular cargo space
Comparison of HBP with Dendrimers


Perstorp markets Boltorn™ products produced using polyalcohol cores, hydroxy acids and technology based on captive materials. The dendritic structures are formed by polymerization of the particular core and 2,2-dimethylol propionic acid (Bis-MPA). The obtained base products are hydroxyl-functional dendritic polyesters. Fully aliphatic and consisting only of tertiary ester bonds, they are claimed providing excellent thermal and chemical resistance.

A wide variety of different hyperbranched polymers include – polyamides, polyamidoamines, polyureas, polyurethanes, polyesters, polycarbosilanes, polycarbosiloxanes, polycarbosilazanes, perfluorinated derivatives of many of the previous polymers, etc. These polymers are ideally suited for a variety of specialty coating applications including plastic additives.

Flame Retardants / Smoke Suppressants for Polymers and Elastomers


View a wide range of frame retardants available today, analyze technical data of each product, get technical assistance or request samples.

Key Applications

Suppliers

Brands

Leave a comment


Want to comment?

No Account yet?

Register
Rate this Content
1 Comments on "Flame Retardants for Fire Proof Plastics"
Alireza D Jun 23, 2021
hi dear thanks for nice information i am glad if you could help us during producing of PP-compound consuming in wire and cable insulation usable in automotive industrial , we need to achieve the flame retardancy properties V-2 so please aware me what kind of flame retardant could i use in my compound. please kindly note my compound is include PP+SEBS and some additive like metal deactivator and antioxidant . looking forward to hearing from you

Back to Top