**5. Phosphorus-based flame retardants: inorganic compounds**

Inorganic FRs act mainly in the solid phase, with thermal decomposition through the release of phosphoric acid. This leads to substrate carbonization, resembling phosphorus-containing FRs [158].

The German report "Substituting Environmentally Relevant Flame Retardants: Assessment Fundamentals" analyzed some FRs on the basis of several evaluation criteria, including potential to accumulate in environmental media (occurrences in humans and the environment), chronic and acute toxicity, emission trend (production, use, and waste disposal), fire by-products (smoke density, gas toxicity, and corrosivity, and fire extinguishing water charges, etc.), and concluded that red phosphorus and ammonium polyphosphate are the least problematic FRs [158].

### **5.1 Red phosphorus (RP)**

RP is a very effective FR in many polymer applications is. It is a stable form of the element phosphorus, which has an amorphous structure [158].

RP acts as an FR via a solid-phase mechanism. It forms a rigid layer that prevents flammable material replenishment in the gas phase, reducing fire and decreasing fire gas toxicity [159]. For polyphosphoric acid (the main component of the aforementioned rigid layer) to be formed, oxygen must be supplied by the polymer or another material used as matrix. Therefore, RP is a more efficient FR in materials with high oxygen content (such as cellulose or plastics containing oxygen) [158]. The RP concentration varies from 2 to 10% of the total weight. RP reduces the toxic smoke formation and heat release, preventing large fires from occurring [160]. Its use restrictions are based on color—because it is red, it cannot be used in white or light products given that it is difficult to mask even with dyes, [158].

RP is mainly used in condensation polymers (polyamide, polyester, polyurethane, polyisocyanurate, and epoxy resins), but it can also be used in dispersions for textile finishing, polyamide components for electrical and electronic devices, fiberglass reinforced plastics for electrical applications, synthetic glues, and automotive textiles [158].

According to some studies, the use of RP as FR is not problematic because it does not dissolve in water easily, making the risk of environmental contamination with

phosphorus unlikely. Organ intoxication effects are also unlikely, and RP may only cause skin irritation. Therefore, the use of this FR has a low ecological and human health impact, as long as it is not mixed with white or yellow phosphorus [158].

The risks of contaminating the environment with phosphorus as a result of using RP as FR are unlikely. There are no data on RP concentrations in air, soil, or water. Like microencapsulated phosphorus, inert RP does not pose a threat to the environment [158].

The occurrence of phosphorus compounds in environmental samples cannot be analyzed separately from the natural occurrences of phosphorus compounds and cannot be seen as a consequence of the use of RP as FR [158]. Accumulation is hardly a consequence of RP used in plastics because this FR degrades fast, with phosphine and phosphoric acid formation. The effects on aquatic systems are not alarming because phosphorus concentrations are low compared to natural occurrences [158].

RP oral ingestion is unlikely because it is degraded in the environment when it is eliminated in sewage plants through adsorption to sewage sludge. Whether resorption occurs when RP is microencapsulated and ingested orally has not been examined, but resorption is more likely negligible and organ effects are improbable [158].

If inhaled, RP LD50 in rats is 4.3 mg/l, which indicates moderate acute toxicity because metabolism is rapid. Eyes and mucous membranes may become irritated probably due to acid formation. Therefore, affected individuals should not be taken for toxicological evaluation because phosphoric acid may be formed due to the high reactivity of phosphine. Nevertheless, oxides released during a fire should not be ignored because they may irritate the affected persons skin [158].

The RP physicochemical properties prevent it from evaporating at room temperature and from solubilizing in water. Because oxygen is present in the environment, RP forms some phosphates that generate phosphine (PH) through a complex chain of chemical reactions. As toxic as phosphine is (with prescribed exposure limits for humans being TLV/TWA—0.3 ppm or 0.4 mg/m3 ; TLV/STEL—0.75–1 ppm), it is also very reactive and produces non-toxic phosphates [159].

Some techniques for RP production have been developed and improved, so phosphine is no longer a problem. Phosphine arises if RP is treated at high temperatures or if it is exposed to humidity. If during compounding the RP levels are kept below the TLV/TWA limit, phosphine formation may be drastically reduced, and they can be removed in a well-ventilated area [159]. RP should not be stored in closed containers because phosphine may be formed [158].

RP in the powder form is flammable and has therefore been regulated as a potentially hazardous material. The safest and most ecological way to transport RP is to microencapsulate it. But the newly developed RP grade is highly stable, safer to use, and easier to handle in terms of cleaning and service life [159].

During a fire, RP is easily oxidized to phosphorus oxides that do not form phosphine: in fact, phosphorus oxides remain as polymeric phosphoric acid or phosphates that can be removed in the incinerators' flue gas treatment system [159].

#### **5.2 Ammonium polyphosphates (APP)**

APP is a crystalline inorganic salt with the chemical formula NH4PO3. This FR retardant is found in the crystalline or liquor form and contains nitrogen and phosphorus (**Figure 3**). Like most FRs containing phosphorus, APP begins to decompose at temperatures above 225°C and acts by releasing phosphoric acid and carbonizing [158].

*Flame Retardants: New and Old Environmental Contaminants DOI: http://dx.doi.org/10.5772/intechopen.104886*

**Figure 3.** *APP chemical structure.*

The composition of ingredients varies depending on the manufacturer. For example, APP 422 from Clariant GmbH, contains 31.5% phosphorus and 14.5% nitrogen [158]. APP is often used in conjunction with other compounds such as polyalcohol as a carbon dioxide dispenser, melamine as a blowing agent, and aluminum trihydrate (ATH) in resins [158].

In intumescent systems, APP is combined with carbonic compound distributors and blowing agents. This mechanism releases non-flammable gases, preventing oxygen from being supplied to the flammable substrate. APP is used in polyurethane foams (hard and flexible), polypropylene, epoxy and polyester resins, cellulosecontaining systems, and wash-resistant textile backings [158].

Some studies have reported that, from a toxicological viewpoint, APP is not problematic, or that it has a very low ecological and human health impact. Air contamination caused by APP is unlikely due to its physicochemical properties [158].

There are no data available on APP reabsorption by oral ingestion, but a high rate can be predicted from experiments with similar compounds. APP is metabolized to ammonia and phosphate, which integrate with the nitrogen and phosphate cycle. However, there are no considerable concentrations in the body or toxic effects that need to be feared. If APP comes into contact with skin, it causes some irritating or sensitizing effect because hydrolysis occurs in the presence of acid and ammonia salts in an aqueous medium. The LD50 data (>2000 mg/kg) allows no conclusions about chronic toxicity [158].

Rapid APP decomposition into ammonia and phosphate occurs in soil and sewage sludge, so water eutrophication must be taken into account. However, there are no exact data on the relevance of the volume. During a fire from plastics containing APP, nitrogen oxide and ammonia are formed, as well as phosphorus oxide. These gases are aggressive, and the health effects cannot be ignored [158].

**Table 6** summarizes the biological effects caused by organic, halogenated, and inorganic phosphorus-based flame retardants.
