Abstract
Because phosphorus chemistry is very diverse there are many classes of phosphorus-based flame retardants with specific applications. Red phosphorus is a unique flame retardant which is used in its elemental form. Despite being very flammable in air, red phosphorus is a very efficient flame retardant mostly for thermoplastic polyesters and polyamides. Most inorganic phosphates are water soluble and because of this they are used as non-durable treatment for textiles and wood. Water insoluble ammonium polyphosphate, piperazine polyphosphate and melamine phosphates are very efficient flame retardants especially for polyolefins. Aluminum and calcium hypophosphites and aluminum diethyl phosphinate were introduced to the market about two decades ago but are still actively researched for new applications by industrial labs and for mechanisms of action by academic institutions. Aliphatic phosphates and phosphonates and aromatic phosphates are the oldest classes of organophosphorus flame retardants and plasticizers with well-established applications. Aromatic bisphosphates are broadly used in polycarbonate-based and polyphenylene ether-based blends but their market share grows mostly because these types of thermoplastics grow fast. Fueled by a fast growing sector of high speed and high frequency printed wiring boards, aromatic phosphinates, phosphine oxides and phosphazenes are the most active areas of research both in industry and in academia.
Keywords: Phosphorus flame retardant, intumescent, char, plastic, textile, epoxy resin, polyurethane foam
2.1 Introduction
It is generally accepted that the most efficient flame retardants provide their action both in the condensed and gas phases. Although halogen- and phosphorus-based flame retardants exhibit these two mechanisms of action, the difference is that halogen flame retardants can promote charring of most organic polymers by bromine radicals abstracting hydrogen atoms from polymer chains resulting in formation of double bonds or cross-links [1]. Phosphorus flame retardants are more specific to the polymer chemistry than halogen ones and they are mostly effective in the oxygen- or nitrogen-containing polymers due to the fact they need to react with the polymer e.g., phosphorylate it and thus involve it in the charring. The char impedes the heat flux to the polymer surface and retards diffusion of the volatile pyrolysis products to the flame.
If conditions are right, the phosphorus-based molecules or fragments can volatilize and be oxidized producing active moieties in the flame. Volatile phosphorus compounds can be as effective as halogen radicals in the flame, but even here phosphorus surprisingly works well only in heteroatomic polymers. It has always been challenging to design phosphorus-based flame retardants, which will volatilize into the flame at relatively low temperatures and at the same time will not be lost during polymer processing. Therefore, there are not many commercial phosphorus-based flame retardants that provide mostly gas phase action.
The author of this chapter has published a similar chapter on phosphorus-based flame retardants in the first edition of this handbook [2] and he also co-authored two earlier reviews on phosphorus-based flame retardants [3, 4]. This current chapter is an update and extension of the previous publications. This chapter does not cover the large class of chloroalkyl phosphates since they are not halogen-free, but these products were reviewed previously. Although there is a large body of academic publications and patent literature on new phosphorus flame retardants, this chapter focuses only on flame retardants which, to the best of the author’s knowledge, are in commercial use or in advanced commercial development. A recent review on some commercial phosphorus-based and intumescent flame retardants was published elsewhere [5]. There are also broader non-selective reviews on phosphorus flame retardants [6, 7]. Mechanisms of action of phosphorus flame retardants were recently reviewed by Shartel [8].
2.2 Main Classes of Phosphorus-Based Flame Retardants
The ammonium phosphate treatment of cellulosic materials (canvas, wood, textiles, etc.) has been known for almost three centuries [9]. However, only with commercialization of synthetic polymeric materials in the twentieth century, organophosphorus compounds have become an important class of flame retardants.
All phosphorus-based flame retardants can be separated into three large classes:
• Inorganic represented by red phosphorus, ammonium phosphates and metal phosphites and hypophosphites.
• Semi-organic represented by amine and melamine salts of phosphoric acids, metal salts of organophosphinic acids and phosphonium salts.
• Organic represented by phosphates, phosphonates, phosphinates, phosphine oxides and phosphazenes.
Water-soluble phosphorus flame retardants are mostly used for topical treatment of wood, textile and other cellulosic products. Some water soluble FRs can be further reacted with cross-linkers (cured) which provides durable water resistant treatment. Water-insoluble phosphorus FRs find a very broad range of applications in thermoplastics, thermosetting resins, synthetic foams, coatings, etc.
Phosphorus flame retardants have certain advantages over other flame retardants (mostly halogen based) but also have some disadvantages which are both listed below:
Advantages:
• Low specific gravity which results in light plastic parts
• Achieve flame retardant efficiency at lower phosphorus content compared to the halogen content needed for the same rating
• High comparative tracking index (CTI) test performance
• Better UV stability than most halogen-based FRs
• Less tendency to intensify smoke obscuration
• Less acidic smoke compared to halogen FRs
• Most phosphorus FRs are biodegradable and therefore not persistent or less persistent than halogen FRs
Disadvantages:
• Low efficiency in polyolefins, styrenics and elastomers unless charring agent is added.
• Absence of a good general synergist that works in most polymers.
• Many phosphorus FRs are hydrophilic and possibly cause moisture uptake, limiting use in some applications.
• May hydrolyze to give acids which decrease the molecular weight of acid-sensitive plastics (polycarbonates, polyesters, polyamides, etc.)
• Recycling of acid sensitive polymers is problematic due to the hydrolytic instability of organophosphates.
• Some phosphates are toxic to aquatic organisms. Some phosphates exhibit a certain degree of neurotoxicity.
• Apart from a few selected cases, the cost/efficiency of phosphorus FRs is higher than halogen based FRs.
2.3 Red Phosphorus
Red phosphorus is a polymeric form of elemental phosphorus consisting of non-periodic five- and six-membered rings with some crosslinks. Red phosphorus is made by prolonged heating of white phosphorus at about 250-300°C in anaerobic conditions. At the end of the production red phosphorus comes out in the form of cake like solid chunk which needs to be broken and milled. Being exposed to moist air, non-stabilized red phosphorus slowly reacts with water and oxygen producing phosphine and various phosphorous acids. Oxidation starts on the sharp edges which are considered to be active sites. Smaller dusty particles are the most susceptible to oxidation and because of this red phosphorus is usually sieved out to different fractions with the removal of dusty particles. In the late 80s a process of producing mostly spherical particles of red phosphorus [10] which are considered less prone to oxidation was developed in Japan. Red phosphorus is very thermally stable and environmentally benign [11] because being exposed to weathering conditions it eventually converts to phosphoric acids.
Red