Flame retardants

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Flame retardants (or fire retardants ) are substances that are intended to limit, slow down or prevent the spread of fires . Flame retardants are used wherever there are potential sources of ignition, such as B. in electronic devices ( electrical short circuit ), upholstered furniture or carpets.

Flame retardants are mainly used in flammable materials and prefabricated parts in order to meet fire protection requirements in construction and transport as well as in the electrical / electronics sector (E&E). The basis for this are regulations on preventive fire protection , which are still largely national in the building industry today, but predominantly international in the field of transport and E&E. However, since 2002, harmonized classification systems and test methods for the fire behavior of construction products have been introduced in the European Union. The aim of preventive fire protection is to minimize the risk of fire and thereby protect human life, health and property as well as the environment.

For 2012, the worldwide annual consumption of flame retardants was estimated at almost two million tons, which corresponds to a sales volume of around five billion US $. It is estimated that the market value would increase to around $ 5.8 billion by 2018. The expected increase, however, depends on the development of the regulations in industrialized and emerging countries, which are supposed to take into account the dangers posed by flame retardants.

Many flame retardants are harmful to health and / or the environment . In house dust , blood serum and breast milk , increasing concentrations of some flame retardants have been found for years. Sometimes they accumulate on the surface of microplastics .

Mode of action of flame retardants

The effect is divided into chemical and physical principles.

A distinction is made between the chemical effects:

In terms of the physical effect, a distinction is made between the following effects:

  • Cooling: The material is cooled through the energy consumption of an endothermic decomposition , for example through the evaporation of (chemically or physically) bound water
  • Formation of a protective layer ( intumescent layer ); the formation of the layer can take place through chemical as well as through physical processes
  • Dilution of flammable gases with inert substances
  • Liquefaction: The heated material melts and flows out of the fire zone so that it is not in the area affected by the flame

Most flame retardants work through one or more chemical and physical processes, each in different proportions.

The process of the radical chain reaction proceeds schematically as follows:

1. Release of halogen radicals (X) from the flame retardant: R-X → R + X
2. Formation of hydrogen halides (HX): R – H + X · → R + H-X
3. Endothermic binding of oxygen via intermediate stages: X + O – O → X – O + O
    X + O → X – O ·
    · O · + H-X → OH + X
    XO · + H-X → 2 X + OH
4th Neutralization of high-energy radicals and recombination: H-X + • OH → H 2 O + X
    R + + OH → R-OH
    R + R → R – R

The reaction of halogen radicals and hydrogen halide with oxygen and its reaction products serves as an endothermic step in order to slow down the strongly exothermic combustion and make it more difficult for the flame to spread . At the same time, the hydrogen halide acts as a diluting gas in the vicinity of the flame and thus reduces the oxygen content in the gas-air mixture. This also has a flame-retardant effect.

The efficiency of halogenated flame retardants can be increased several times over by combining them with antimony oxide (Sb 2 O 3 ). This is called a synergistic effect .

Types of flame retardants

There are basically four types of flame retardants:

  • Additive flame retardants: The fire retardants are incorporated into the flammable substances as additives
  • Reactive flame retardants: The substances themselves are part of the material (see also polymerisation )
  • Inherent flame protection: the material itself is flame retardant
  • Coating : The fire retardant is applied from the outside as a coating

These consist proportionally of the following flame retardant families (production shares worldwide according to a 2012 market study by Townsend Solution Estimates):

  • 40.4% inorganic flame retardants (aluminum hydroxide)
  • 19.7% brominated ( brominated ) flame retardants
  • 11.3% chlorinated ( chlorinated ) flame retardants
  • 14.6% organophosphorus flame retardant (can also contain chlorine or bromine)
  • 8.4% antimony trioxide
  • 5.6% others

classification

Source:

halogenated not halogenated
high performance isobutylated triphenyl phosphate (TBPP) DOPO , phosphinates
medium power Br - polystyrene , Br- epoxy , HBCD , antimony trioxide Ammonium polyphosphate (APP), red phosphorus , zinc borate , triaryl phosphate , melamine polyphosphate (MPP)
for bulk plastics TCPP , TDCP , TBBA , Octa-BDE , Deca-BDE , chlorinated paraffins RDP , BDP , aluminum hydroxide , magnesium hydroxide , trialkyl phosphate

DIN EN ISO 1043-4 classifies flame retardants for plastics and assigns them two-digit code numbers:

  • 1x: halogen compounds
  • 2x: halogen compounds
  • 3x: nitrogen compounds
  • 4x: organic phosphorus compounds
  • 5x: Inorganic phosphorus compounds
  • 6x: metal oxides, metal hydroxides, metal salts
  • 70-74: Boron and Zinc Compounds
  • 75-79: silicon compounds
  • 80: graphite

Flame-retardant plastics contain the addition “ FR (‹code number1› +‹ ›‹code number2› + ..) ” in their abbreviation . For example, " PA6 GF30 FR (52) " stands for a polyamide 6 filled with 30% glass fibers , which is flame-retardant with red phosphorus .

Halogenated flame retardants

The most important representatives are polybrominated diphenyl ethers ( PentaBDE , OctaBDE , DecaBDE ), DBDPE , BTBPE , TBBPA and HBCDD . Until the 1970s, polybrominated biphenyls (PBB) were also used as flame retardants. The chlorinated flame retardants include z. B. Chlorinated Paraffins and Mirex . With the exception of TBBPA, these substances are only used as additive flame retardants. The main areas of application are plastics in electrical and electronic devices (e.g. televisions, computers), textiles (upholstered furniture, mattresses, curtains, sun blinds, carpets), the automotive industry (plastic components and upholstery covers) and construction (insulation materials and assembly foams ).

Halogenated flame retardants are particularly dangerous in the event of a fire. When exposed to heat, they have a fire-retardant effect in that the halogen radicals formed during pyrolysis inhibit the reaction with oxygen . However, there are also high concentrations of polybrominated (PBDD and PBDF) or polychlorinated dibenzodioxins and dibenzofurans (PCDD and PCDF). These are also known under the umbrella term “ dioxins ” for their high toxicity (“ Seveso poison”). In addition, emissions of flame retardants can occur during production, the use phase and disposal.

TBBPA is a special case of brominated flame retardants. It is mainly used as a reactive flame retardant, i. This means that it is chemically integrated into the polymer matrix (e.g. epoxy resins of printed circuit boards ) and is an integral part of the plastic. Other reactive brominated flame retardants are e.g. B. bromostyrene and dibromostyrene, and 2,4,6-tribromophenol . When incorporated into the polymer, the emissions of this flame retardant are very low and usually do not pose any risk. However, the formation of dioxins is not fundamentally lower. However, to a lesser extent, TBBPA is also used as an additive flame retardant. Very little data is available on the breakdown products of TBBPA, which is easily decomposed by light.

After testing the substances within the framework of REACH , the above-mentioned brominated flame retardants were classified as follows: Recordings in Annex XIV (and thus a sales ban):

  • Hexabromocyclododecane , reason: PBT
  • Penta-BDE and Octa-DBE, basic risk to the environment and for the preventive protection of breastfed infants

Not classified as dangerous:

  • TBBPA

The potential hazards of flame retardants such as polybrominated diphenyl ethers (PBDE) and polybrominated biphenyls (PBB) in relation to their formation of PBDD / F have led to a ban by the EU ( WEEE , RoHS , ElektroG ). DecaBDE, which was initially exempt from this ban, was an exception. With the ruling of the European Court of Justice, this is now banned in electrical and electronic equipment from July 1, 2008.

In 2000, 38% of the approximately 5 million tons of bromine worldwide was used for the production of brominated flame retardants.

Content of flame retardants in various plastics:

polymer Salary [%] Flame retardants
Polystyrene foam 0.8-4 HBCD
HIPS 11-15 DecaBDE, brominated polystyrene
Epoxy resin 19-33 TBBPA
Polyamides 13-16 DecaBDE, brominated polystyrene
Polyolefins 5-8 DecaBDE, propylene dibromostyrene
Polyurethane 10-18 PentaBDE, TBBPA ester
Polyethylene terephthalate 8-11 Brominated polystyrene, TBBPA derivative
Unsaturated polyester 13-28 TBBPA
Polycarbonates 4-6 Brominated polystyrene, TBBPA derivative
Styrene copolymers 12-15 OctaBDE , brominated polystyrene

Exclusively composed of halogenated monomers plastics such. B. polyvinyl chloride (PVC) and polytetrafluoroethene (PTFE), but also polydibromostyrene and similar plastics, are non-flammable due to their special chemical properties and are known as inherently flame-retardant. Depending on the flame retardant category , you need little or no additional flame retardant.

Nitrogen-based flame retardants

Examples of nitrogen-based flame retardants are melamine and urea .

Organophosphorus flame retardants

In this class of compounds, aromatic and aliphatic esters of phosphoric acid are typically used, such as:

  • TCEP (tris (chloroethyl) phosphate)
  • TCPP (Tris (chloropropyl) phosphate)
  • TDCPP (tris (dichloroisopropyl) phosphate)
  • TPP (triphenyl phosphate)
  • TEHP (Tris- (2-ethylhexyl) phosphate)
  • TKP (tricresyl phosphate)
  • ITP ("isopropylated triphenyl phosphate") mono-, bis- and tris (isopropylphenyl) phosphates with different degrees of isopropylation
  • RDP (resorcinol bis (diphenyl phosphate))
  • BDP (bisphenol A bis (diphenyl phosphate))
  • Vinyl phosphonic acid

These flame retardants are used, for example, for soft and hard PUR foams in upholstered furniture, vehicle seats or building materials. Recently, however, BDP and RDP have increasingly been used as substitutes for OctaBDE in electrical appliance plastics .

Inorganic flame retardants

Inorganic flame retardants are for example:

  • Aluminum hydroxide [Al (OH) 3 ], the most widely used flame retardant in the world (also called ATH for "aluminum trihydrate"). It has a cooling and gas-diluting effect by splitting off water, but must be added in large proportions (up to 60%).
  • Aluminum sulphate , is used as a substitute for controversial agents containing boron, but it is potentially harmful to health
  • Borax and boric acid , traditionally also used as preservatives in food (e.g. caviar)
  • Magnesium hydroxide [Mg (OH) 2 , MDH, "magnesium dihydrate"] is a mineral flame retardant with a higher temperature resistance than ATH, but with the same mode of action.
  • Expandable graphite / expandable graphite , a mineral flame retardant that works by forming an intumescent layer.
  • Ammonium sulfate [(NH 4 ) 2 SO 4 ], ammonium phosphate and polyphosphate [(NH 4 ) 3 PO 4 ] dilute the gas in the flame by splitting off ammonia (NH 3 ), which burns to form water and various nitrogen oxides , and thereby the flame removes oxygen. At the same time, they cause the formation of a protective layer by the sulfuric (H 2 SO 4 ) or phosphoric acid (H 3 PO 4 ) which, as one of their functions, can interrupt the radical chain reaction. The acids are also non-flammable, highly hygroscopic and have high boiling points. As a result, they condense in the cooler area of ​​the flame and deposit on the material. Phosphoric acid also forms meta- and polyphosphoric acid by splitting off water , which have even higher boiling points.
  • Red phosphorus forms a layer of phosphoric and polyphosphoric acids on the surface and causes it to swell ( intumescence ). This layer has an insulating effect and protects the material from the ingress of oxygen. The phosphates formed here have the same properties as those derived from ammonium phosphate .
  • Antimony trioxide (Sb 2 O 3 ) only works as a synergist in combination with halogenated flame retardants. A disadvantage is its catalytic effect in the formation of dioxins in the event of a fire.
  • Antimony pentoxide (Sb 2 O 5 ) acts like Sb 2 O 3 as a synergist .
  • Zinc borates (see borates ) have a cooling and gas-thinning effect through the release of water from the borate. Zinc compounds can also act synergistically and partially replace the more dangerous antimony trioxide .
  • Slaked lime [Ca (OH) 2 ] was used as a flame retardant for the wood of the roof trusses during the Second World War . It initially binds carbon dioxide from the air by splitting off water and converts to calcium carbonate (CaCO 3 ). As a protective coating , it makes it difficult for oxygen to enter.

literature

Web links

Individual evidence

  1. ↑ Jürgen Troitzsch (Ed.): Plastics Flammability Handbook. Principles, Regulations, Testing and Approval. 3. Edition. Carl Hanser Verlag, Munich 2004.
  2. ↑ Jürgen Troitzsch: Flame retardants. Requirements and innovations. In: plastics . 11/2012, p. 84.
  3. ↑ Market study of flame retardants. on: Ceresana. July 2011.
  4. ↑ Sonya Lunder, Renee Sharp, Amy Ling, Caroline Colesworthy: Study Finds Record High Levels of Toxic Fire Retardants in Breast Milk from American Mothers. 2008.
  5. ↑ Residues of flame retardants in breast milk from Germany with special consideration of polybrominated diphenyl ethers (PBDE). (PDF)
  6. ↑ Andreas Sjödin, Lars Hagmar, Eva Klasson-Wehler, Kerstin Kronholm-Diab, Eva Jakobsson, Åke Bergman: Flame Retardant Exposure: Polybrominated Diphenyl Ethers in Blood from Swedish Workers. In: Environmental Health Perspectives . 107 (8), 1999. PMC 1566483 (free full text) (there are further references in this publication)
  7. ↑ Flammschutz Online: Der Flammenschutzmittelmarkt ( Memento from May 12, 2014 in the Internet Archive ), accessed on June 29, 2013.
  8. ↑ Flame retardants for plastics. Overview of the state of the art and current trends (2010), p. 4.
  9. ↑ Dieter Drohmann: The range of applications of brominated flame retardants: applicability, properties, environmental discussion. ( Memento of September 28, 2007 in the Internet Archive ) 2001.
  10. ↑ R. Gächter, H. Müller: Plastics Additives Handbook. Hanser, Munich 1993.
  11. ↑ S. Kemmlein, O. Hahn, O. Jann: Emissions of flame retardants from building products and consumer goods . project no. (UFOPLAN reference no.) 299 65 321, Environmental Research Program of the Federal Ministry for Environment, Nature Conservation and Nuclear Safety, commissioned by the Federal Environmental Agency (UBA), UBA-FB 000475, Berlin, April 2003.
  12. ↑ Federal Institute for Occupational Safety and Health: Hexabromocyclododecane (HBCDD) and all larger identified diastereoisomeric compounds ( Memento from December 22, 2015 in the Internet Archive ), accessed on December 16, 2015.
  13. ↑ Health and Environmental Hygiene - Flame Retardants ( Memento from December 25, 2008 in the Internet Archive ) on Umweltbundesamt.de, accessed on May 13, 2013.
  14. ↑ Anke Schröter: TBBPA approved for marketing and use. June 18, 2008, accessed May 13, 2013.
  15. ↑ Press release of the Federal Environment Agency from June 30, 2008 (PDF; 44 kB) .
  16. ↑ Linda S. Birnbaum, Daniele F. Staskal: Brominated Flame Retardants: Cause for Concern? In: Environ Health Perspect . 112, 2004, pp. 9-17. doi: 10.1289 / ehp.6559 . PMC 1241790 (free full text).
  17. ↑ Pedro Arias: Brominated flame retardants - an overview. The Second International Workshop on Brominated Flame Retardants, Stockholm 2001.
  18. ↑ SpecialChem4polymers: Flame Retardants Center - Organic Phosphorus Compounds Center