Brominated Flame Retardants

8. Alternative Flame Retardants

8.1 Organophosphorus
8.1.1 Triphenyl Phosphate
8.1.2 Tricresyl Phosphate
8.1.3 Resorcinol bis(diphenylphosphate)
8.1.4 Phosphonic acid, (2-((hydroxymethyl)carbamyl)ethyl)-, dimethyl ester
8.1.5 Phosphorus and Nitrogen Containing Thermosets
8.2 Inorganic
8.2.1 Aluminium Trihydroxide
8.2.2 Magnesium Hydroxide
8.2.3 Ammonium Polyphosphate
8.2.4 Red Phosphorus
8.2.5 Zinc Borate
8.3 Nitrogen Containing
8.3.1 Melamine

A large number of non-halogen compounds that can be used as substitutes for brominated flame retardants exist. In the following section representatives of the main groups of alternatives will be described.

To the extent reviews of environmental and health hazards of the compounds are available, a short summary will be given in the section. It has been out of the scope of this study to perform a thorough review of the literature on exposure and environmental & health hazards of the compound.

Chlorinated flame retardants

The scope of the present study is brominated flame retardants, but in the endeavour at developing alternatives, the focus more generally is on halogenated versus halogen-free flame retardants. For this reason chlorine containing alternatives to BFRs will not be included.

Seven chloro-organic phosphates used as flame retardants in polyurethanes and unsaturated polyesters have in 1995 been evaluated under the Nordic Council of Ministers /119/. In the aim of evaluating the need for risk reduction measures, human toxicity and ecotoxicity data were gathered for these seven flame retardants among other chloro-organic compounds. Although data on some of the compounds were scarce, it was in the report concluded that human toxicity and ecotoxicity data indicate that the assessed substances are dangerous to human health and the environment, and that the substances are not ready biodegradable and may accumulate in the environment.

Included compounds

The section will include the following compounds:

Organophosphorus

Inorganic

Nitrogen containing

Risk assessments

It has not been possible to identify a comprehensive risk assessment of any of the alternatives in question.

Environmental Health Criteria

Reports in the Environmental Health Criteria series have been prepared for two of the compounds, triphenyl phosphate /120/ and tricresyl phosphate /121/.

Environmental Health Criteria reports have moreover been prepared for the following flame retardants: hexachlorocyclopentadiene (EHC 120), tris(2,3-dibromopropyl) phosphate and bis(2,3-dibromopropyl) phosphate (EHC 173), chlorendic acid and anhydride (EHC 185), and chlorinated paraffins (EHC 181).

Swedish flame retardants project

Within the framework of the Swedish flame retardants project, human health hazard assessments for a number of flame retardants have been carried out (1995) /122 /. The summaries for phosphonic acid, (2-((hydroxymethyl)carbamyl)ethyl)-, dimethyl ester (Pyrovatex*) and melamine in the following will be based on the these assessments.

Beside these compounds the Swedish report includes assessments for:

Risk and benefits in the use of flame retardants

More recently an analysis of risk and benefits in the use of flame retardants in consumer products has been carried out by the Polymer Research Centre, University of Surrey for the UK Department of Trade and Industry, Consumer Safety Unit. The analysis includes assessments of human toxicity of the flame retardants aluminium trihydroxide, antimony trioxide, DeBDE (and other PBDEs), TBBPA, melamine, and tris-(chloropropyl)-phosphate (TCPP). The summaries of aluminium trihydroxide and melamine, in the following will be based on these assessments.

Trade names

Trade names in the section refer, if nothing else is mentioned, to ‘The index of flame retardants’ from 1997 /13/. The list of trade names is not be complete; only examples are given.

8.1    Organophosphorus

A wide range of organophosphorus compounds may be used as flame retardants in plastics and textiles. Many of the compounds contain halogens.

Of the halogen-free organophosphorus flame retardants in particular triaryl phosphates have been used as alternatives for brominated flame retardants (triaryl indicates the presence of three benzene rings).

The actually used compounds are often considered confidential. In the following two representatives of the triaryl group, triphenyl phosphate and tricresyl phosphate will be described. Information on the use pattern, however, indicates that there is a tendency towards using resorcinol bis(diphenylphosphate) in thermoplastics.

The organophosphorus flame retardants have come under intense environmental scrutiny. Results on acute toxicity to algae, invertebrates and fish indicate substantial differences between the various aryl phosphates /123/.

Other halogen-free phosphorus flame retardants are dimethylmethyl phosphonates used as additive flame retardants in rigid polyurethane foams and polyester resins, and diethylethyl phosphonate.

Organophosphorus compounds are widely used for textile applications. Phosphonic acid, (2-((hydroxymethyl)carbamyl)ethyl)-, dimethyl ester (Pyrovatex®) is included as representative of the group.

Phosphorus compounds - often in combination with nitrogen compounds -incorporated into the polymer structure are some of the main candidates for substituting brominated flame retardants for thermosets. The actually used compounds will vary with the application. As an example, compounds for epoxy based laminates are described.

8.1.1    Triphenyl Phosphate

Triphenyl phosphate (TPP) is used as flame retardant PC/ABS blends, in other engineering thermoplastics, and in phenolics. In the following chemical/physical properties and risk identification summary are epitomised from ‘Triphenyl phosphate’, International Programme on Chemical Safety, 1991 /120/.

In Denmark the limit value of triphenyl phosphate in workplace air is 3 mg/m3.

Human health risk

Animal data indicate that TPP has low toxicity. It produces no irritant effects on animal skin. Despite an early report to the contrary, TPP is not considered neurotoxic in animals or man. TPP is not mutagenic. The available data indicate no hazards to man.

No evidence that TPP causes delayed neurotoxicity has been found in animal experiments. No adequate data on the effects of TPP on reproduction are available. Contact dermatitis due to TPP has been described.

Exposure

Exposure of the general population to TPP through various environmental media is likely. TPP has often been detected in urban air, although the levels are low. There are insufficient data to evaluate the significance of the general population exposure to TPP (1991).

The presence of TPP and other organophosphorus compounds in the indoor environment has recently been reported (e.g. /124 , /21/).

Environmental fate and levels

Triaryl phosphates (including TPP) enter into the aquatic environment mainly via hydraulic fluid leakage as well as by leaching and volatilisation from plastics, and to a minor extent, from manufacturing processes. Triphenyl phosphate is rapidly adsorbed on sediments. Its biodegradation is rapid. The bioconcentration factors measured for several species of fish range from 6-18,900 and the depuration half-life ranges from 1.2 to 49.6 hours.

Maximum environmental levels reported are 23.2 ng/m3 in air, 7,900 ng/l in river water, 4,000 ng/g in sediment, and 600 ng/g in fish.

Effects on organisms in the environment

The growth of algae is completely inhibited at TPP concentrations of 1 mg/l or more, but is stimulated at lower concentrations.

TPP is the most acute toxic of the various triaryl phosphates to fish, shrimps and daphnids. The acute toxicity index of TPP for fish (96 h LC50) ranges from 0.36 mg/l in rainbow trout to 290 mg/l in bluegills. Sublethal effects on fish include morphological abnormalities such as congestion, degeneration, and haemorrhage from the smaller blood vessels and behavioural abnormalities. The immobility of fish exposed to 0.21-0.29 mg per litre completely disappeared within 7 days, when the fish were transferred to clean water.

IPCS concludes that as water concentrations of TPP in the environment are low and toxic effects on aquatic organisms are unlikely. Since TPP is removed rapidly from the tissues of fish when exposure ends and bioconcentration factors are moderate, bioaccumulation is not considered to be a hazard. However, disposal of TPP-treated vinyl fabric upholstery into a pond would result in a sufficiently high concentration of TPP to kill fish.

Effects on aquatic communities are possible near production plants.

Recommendations of IPCS 

There is a need for further research (1991). There are no recommendations of the IPCS on the use of TPP.

8.1.2    Tricresyl Phosphate

Tricresyl phosphate (TCP) is used as PVC plasticiser, flame retardant flame retardants in polystyrene and other thermoplastics. The use of tricresyl as flame retardant seems, however, not to be widespread.

In the following chemical/physical properties and risk identification summary are epitomised from ‘Tricresyl phosphate’, International Programme on Chemical Safety, 1990 /121/.

The commercial product is a mixture of the isomers tri-o-cresyl phosphate (CAS no. 78-30-8), tri-m-cresyl phosphate (CAS no. 563-04-2), and tri-p-cresyl phosphate (CAS no. 78-32-0). There is a significant difference in toxicity between the isomers. The o-isomer is very toxic and is usually excluded as much as possible from the commercial products.

In Denmark the limit value of tri-o-cresyl phosphate in workplace air is 0.1 mg/m3.

Human health risk

Human poisoning involving accidental ingestion of tri-o-cresyl phosphate (TOCP) or occupational exposure of workers has frequently been observed. These examples are usually in relation to the use of the compound as hydraulic fluid and not the use as flame retardant. Both the o-isomer and isomeric mixtures containing TOCP are considered major hazards to human health.

Accidental human exposure to a single large dose results in gastrointestinal disturbance. In the case of exposure to small cumulative doses "delayed neurotoxicity" gradually proceeds after a latent period of 3-28 days. Some neurophysiological studies indicate widespread neurotoxic patterns and prolongation of terminal latencies with relatively small decreases of motor nerve conduction velocities. TCP produced from synthetic cresol, which contains less than 0.1% of o-cresol, is not neurotoxic.

No adequate data on mutagenicity and carcinogenicity (1990). TCP is not toxic to chick embryos are available.

Exposure of the general population to tricresyl phosphate (TCP) through environmental media, including drinking-water, can be regarded as minimal. TCP has been measured in air at concentrations up to 70 ng/m3 in Japan but reached a maximum of only 2 ng/m3 on a production site in the USA.

The release of TCP to the environment derives mainly from end-point use.

Environmental effects

The measurements of environmental concentrations of TCP in water have shown only low levels of contamination. As a consequence of the physico-chemical properties of TCP, there is a high potential for bioaccumulation. However, this does not occur in practice, owing to low concentrations of TOCP in the environment and living organisms and to its rapid degradation. Residues in fish and shellfish of up to 40/ ng/g have been reported, but the majority of sampled animals contained no detectable residues.

TCP bound to sediment accumulates in the environment and sediment concentrations with up to 1,300 ng/g in river sediments and 2,160 ng/g in marine sediments have been reported. Since there is no information either on the bioavailability of these residues on burrowing or bottom-living organisms or on their hazards, the possibility of effects on such species cannot be discounted (1990).

TCP spillage leads to hazard to the local environment.

Freshwater algae are relatively sensitive to TCP, the 50% growth inhibitory concentration ranging from 1.5 to 5.0 mg/l. Among fish species, the rainbow trout is adversely affected by TCP concentrations below 1 mg/l, with sign of chronic poisoning, but the tidewater silverside is more resistant (LC50 is 8,700 mg/l). TCP does not inhibit cholinesterase activity in fish and frogs, but it has a synergistic effect on organophosphorus insecticide activity.

Recommendations of IPCS

When tri-substituted creosols are used in the synthesis and manufacture of other compounds, the purified m- and p-isomers should be used in order to avoid the accidental synthesis of o-substituted products.

There are no recommendations of IPCS on the use of TCP.

8.1.3    Resorcinol bis(diphenylphosphate)

Resorcinol bis(diphenylphosphate) is used in engineering thermoplastics. Compared to the triaryl phosphates, resorcinol bis(diphenylphosphate) is less volatile at the high temperatures required for processing.

No summaries of the human and environmental toxicity and fate of resorcinol bis(diphenylphosphate) have been identified.

8.1.4    Phosphonic acid, (2-((hydroxymethyl)carbamyl)ethyl)-, dimethyl ester

Phosphonic acid, (2-((hydroxymethyl)carbamyl)ethyl)-, dimethyl ester is widely used as flame retardant in textiles under trade names as Pyrovatex® and Spolapret®.

Human health hazard of the compound has been summarised within the Swedish flame retardants project on which the following is based /122/.

Human toxicity

Very little is known about the toxicology of Pyrovatex CP. Biochemical studies have shown that Pyrovatex CP is a weak inhibitor of the enzyme acetyl choline esterase (AChE) activity and the microsomal enzyme system. Acute toxicity studies in rats have shown Pyrovatex CP to be practically non-toxic (LD50 13 g/kg). Further, generic tests showed that the compound may induce chromosome aberrations and reverse mutations, but at high concentrations only (20% of the metaphase cells at a dose of 10 mg/ml).

The assessment concludes that too little is known about the toxicology of the compound for a health risk assessment to be made.

Environmental toxicity and fate of the compound

No summaries of the environmental toxicity and fate have been identified.

8.1.5    Phosphorus and Nitrogen Containing Thermosets

Thermoset resins can be made flame retardants by the use of nitrogen and phosphorus containing constituents that form flame retardant structures during processing and curing of the material.

The thermosets may be made from different precursors and have very different environmental and health properties.

For a halogen-free thermoset developed for use in printed circuit boards and electronic components encapsulates toxicity test of combustion products have been reported in /103,104/. The thermoset was made from tailored amine hardener, polyarylamino isocyanate, and epoxy resins containing structures of cyclic phosphonate esters.

The focus on the toxicity of the halogen-free thermosets has been the formation of toxic fumes during combustion.

No summaries on environmental and human toxicity of halogen-free thermosets have been identified.

8.2    Inorganic

The most used inorganic flame retardants are the metal hydroxides: aluminium trihydroxide, antimony trioxide and magnesium hydroxide. Both aluminium trihydroxide and magnesium hydroxide are used in halogen-free formulations substituting for brominated flame retardants.

Inorganic phosphor compounds, red phosphor and ammonium polyphosphate, are widely used as substitutes for bromine.

Of the zinc compounds zinc borate is used in some halogen-free formulations, but other zinc compounds like zinc stannate (ZN) and zinc hydrostannate (ZHN) may as well be used as synergists together with other flame retardants. There is a growing market for zinc stannate and zinc hydrostannate substituting for antimony trioxide.

The inorganic flame retardants are incorporated into the plastics as a filler, and they are usually considered immobile in the plastics, contrary to the organic additives. Emission during use is thus considered negligible.

8.2.1    Aluminium Trihydroxide

Aluminium trihydroxide is the most used flame retardant. Aluminium trihydroxide functions as a flame retardant in both the condensed and vapour phase. When heated it decomposes and releases water that forms an envelope around the flame, which tends to exclude air and dilute the flammable gases. In addition the decomposition is endothermic, lowering the ambient temperature.

Aluminium hydroxide has a tendency of suppressing smoke evolution. The disadvantage of the compound is that very high loading is necessary (up to 50%) affecting the physical properties of the plastic.

The toxicity of aluminium trihydrate has recently been assessed by Stevens et. al 1998 /100/.

The appraisal of the assessment is that alumina, aluminium hydroxide and aluminium compounds in general have very low levels of toxicity except when there are high exposure levels or unusual routes of exposure.

In view of the lack of reported adverse effects from the very extensive environmental exposure to aluminium compounds, including alumina, it is estimated to be extremely unlikely that any adverse effects would ensue from the levels of exposure to aluminium trihydroxide in consumer products.

Environmental toxicity

Environmental toxicity is not included in the assessment.

The environmental effects of release of aluminium trihydroxide from flame retarded plastics are presumably insignificant, but this has to be proved by an assessment.

8.2.2    Magnesium Hydroxide

The mechanism of magnesium hydroxide is basically the same as that of aluminium trihydroxide. Magnesium decomposes at 330*C; about 100*C higher than aluminium trihydroxide, and can be used in polymers that are processed at higher temperatures. Magnesium hydroxide forms char and produces less smoke than aluminium trihydroxide.

Environmental toxicity and fate of the compound

No summaries of the environmental toxicity and fate of magnesium hydroxide have been identified. The environmental effects of release of magnesium hydroxide from flame retarded plastics are presumably insignificant, but this has to proved by an assessment.

8.2.3    Ammonium Polyphosphate

The relatively water-insoluble ammonium polyphosphate is produced from ammonium phosphates. There are several crystal forms, and the commercial products differ in molecular weight, particle size, solubility, and surface coating /123/. Insoluble ammonium polyphosphate consists of long chains of repeating OP(O)(ONH₄) units.

Ammonium polyphosphate is used in intumescent and conventional paints and sealants. In thermoplastics it is used in combination with other flame retardants, e.g. aluminium trihydroxide or melamine.

No summaries of the human and environmental toxicity and fate of ammonium polyphosphate have been identified.

8.2.4    Red Phosphorus

This allotropic form of elemental phosphorus is unlike white phosphorus not spontaneously flammable but is, however, easily ignited. It is commonly used in polyamides, but may be used in a range of plastics.

The mechanism of red phosphorus is basically the same as that of the other phosphorus compounds. The phosphorus has a high affinity for oxygen that results in initial deoxidation of the polymer in a subsequent dehydration leading to a formation of a protective char.

Red phosphorus is classified F;R11;R16

In Denmark the limit value of red phosphorus (designated yellow phosphorus) in workplace air is 0.1 mg/m3.

No risk evaluation summary for red phosphorus is available.

Human health risk

There are risk factors working with red phosphorus including flammability and autoignition, and disproportionation will give toxic phosphine.

The use of red phosphor and handling hazards have been discussed in several articles from producers of flame retardants or compounds /125 ,126 /.

To overcome the handling problems the phosphorus is used in a microincapsulated and stabilised form. In masterbatches the red phosphorus grains are typically encapsulated in a polymer.

Red phosphorus is widely used, but due to the handling problems there is a tendency that especially smaller producers of plastic products avoid the use of red phosphorous.

Environmental effects

The environmental effects of release of elemental phosphor from flame retarded plastics are assumed to be insignificant, but this has to be proved.

8.2.5    Zinc Borate

Zinc borate is the most widely used of the zinc flame retardants and is particularly used as substitute for antimony trioxide. Compounds having varying amounts of zinc, boron and water of hydration are available /123/.

Zinc borate has a synergistic effect with aluminium trihydroxide and is often used in combination with other flame retardants.

In its mode of action zinc borate behaves like aluminium trihydroxide by endothermic release of water.

No summaries of the human and environmental toxicity and fate of ammonium polyphosphate have been identified.

8.3    Nitrogen Containing

8.3.1    Melamine

Melamine and melamine derivatives, e.g. melamine cyanurate and melamine polyphosphate, are used as flame retardants in a range of plastics, sealants, and intumescent paints. The melamine is used as blowing agent in the intumescent (swelling) systems.

Human health hazard of melamine has been summarised within the Swedish flame retardants project (1995) /122/ and recently by Stevens et. al (1998) /100/.

Human health effects

The acute oral LD50 of melamine in rodents is between 3.2 and 7.0 mg/kg body weight. According to Swedish criteria, melamine would be classified as a moderately acute toxic substance /122/

It has been demonstrated that melamine is readily absorbed in the small intestine in rats. The compound is rapidly excreted via the urine. There is no data on the melamine absorption rate through skin /122/).

Melamine was non-sensitising in test on guinea-pigs. Patch test on humans produced no evidence of primary irritation or sensitisation /100/.

The major subchronic effects of melamine on rats (1.6-1.9% in food) were a decrease in the urinary concentrations of sodium, potassium, ammonium calcium and chloride and pH. Melamine also caused a chronic injury to the bladder caused by stones formed during exposure. The formation of stones was not seen in female rats. There was a strong correlation between the presence of bladder stones and carcinoma. Melamine-induced carcinoma is species as well as sex dependent /122/.

Genotoxic studies in bacteria, rat hepatocytes, fly and mice showed no mutagenic effects /122/.

Melamine caused no malformed litter when female rats were exposed on the 4th and 5th day of gestation. Although melamine increased the number of recorptions, it should not be classified as a teratonic chemical /122/.

The toxic effects of melamine are resorption of litter (not significant), changed electrolyte composition of urine, formation of bladder stones and bladder stone induced chronic inflammation, and cancer in the bladder was observed in a single species or in one sex and must be further investigated /122/.

None of the assessments include data on emission from products in use.Steven et al. appraise /100/ that since the available information indicates that melamine has low acute and chronic toxicity no adverse effects are envisaged from the level of exposure expected from the use of melamine as a flame retardant. At the level of exposure precipitation in the renal tubulus and in the bladder should not be a significant risk.

Environmental toxicity

No summaries of the environmental toxicity or fate of melamine have been identified.