Apart from PVC both of these mixed phosphates found some use in flexible foam formulations [295] sometimes in combination with bromine-containing additives [296]. Even though these phosphates are not very efficient they have good hydrolytic stability and low volatility which is important for automotive foams. tert-Butylphenyl phenyl phosphate was also shown to be a viable flame retardant in high index PIR foams [297]. It has also been used as a flame retardant in PPE/HIPS pallets to pass the large scale UL 2335 test [298], in PPE/elastomer blends [299] and in PC/ABS where it shows good stress cracking resistance [300]. tert-Butylphenyl phenyl phosphate is often added to PPE-based blends as a processing aid even if flame retardancy is not required. Another large application of alkylphenyl phosphates is a plasticizer for thick intumescent coating for offshore oil rigs [301].
Alkyl diphenyl phosphates are products developed to provide improved low temperature flexibility, a fault of triaryl phosphate plasticizers in PVC [302]. There are three commercial phosphates in this family (Formula 2.22), e.g., 2-ethylhexyl diphenyl phosphate, isodecyl diphenyl phosphate and diphenyl phosphate with a mixture of longer (C12-C14) chains. These phosphates generally provide slightly less flame-retardant efficacy but generally produce less smoke compared to triaryl phosphates when the PVC formulation burns [303]. 2-Ethylhexyl and isodecyl diphenyl phosphate find their use in PVC sheet applications, PVC and TPU based artificial leathers and PVC/nitrile rubber tapes for insulative wrap of conduits. C12-C14-alkyl diphenyl phosphate has lower volatility compared to the other two phosphates and therefore it is used in PVC cable jacketing. Other applications of alkyl diphenyl phosphates are in casted polyurethane-polyurea goods [304] and in combination with intumescent flame retardants in thermoplastic elastomers cable jackets [305].
Aromatic phosphates or aromatic phosphate oligomers (mostly diphosphates) are very widely used in PC/ABS and PPE/HIPS blends. Historically triphenyl phosphate (TPP) was the first phosphorus-based flame retardant used in these blends. Although TPP is soluble in these resins and it doesn’t bloom out at room temperature, it deposits on the mold surfaces during molding. Because of the low melting point of TPP (48°C), it leads to bridging at extrusion feeding ports. The next generation of aromatic phosphate FR in PC/ABS and PPE/HIPS was tert-butylphenyl phenyl phosphate, which is still used nowadays in old formulations. However, now oligomeric aromatic phenyl phosphates (mainly diphosphates) are finding broader application than monophosphates because of better thermal stability and lower volatility.
The first product which became commercial was resorcinol bis(diphenyl phosphate) (RDP, Formula 2.23) which is a mixture of oligomers with two to five phosphorus atoms, but with the distribution heavily shifted towards the diphosphate [306]. In commercial PC/ABS blends where ABS content normally does not exceed 25%, RDP gives a V-0 rating at 8-12 wt. % loading [307]. Poly(tetrafluoroethylene) (PTFE) is a necessary ingredient in the formulation, which is usually added at <0.5 wt.% to retard dripping. Since the glass transition temperature of PTFE is below room temperature, it is soft and difficult to handle. To improve PTFE feeding it can be added during the production of ABS so that it is embedded in the polymer [308], or it can be specially treated to become free flowing [309], or pre-processed as a masterbatch [310]. RDP is somewhat less hydrolytically stable compared to other bisphosphates, which limits its application in humid environments and may cause a problem in recycling. This shortcoming of RDP can be alleviated by adding acid scavengers such as epoxies, oxazolines, or ortho esters [311].
Bisphenol A bis(diphenyl phosphate) (BDP) (Formula 2.24) was introduced to the market in the late 90s as an alternative to RDP [312]. Since bisphenol A is less expensive that resorcinol, BDP is more cost efficient despite a lower phosphorus content (8.9 % P for BDP vs. 10.7 % P for RDP). BDP is significantly more viscous than RDP (12500 cP for BDP vs. 600 cP for RDP at 25°C) and therefore it requires a heated storage tank and heated transfer lines, whereas RDP needs only heated transfer lines. Because of the high viscosity, the oligomers content (n>1, Formula 2.24) in the BDP mixture is usually limited to only 10-15% which creates a problem of potential crystallization of BDP during transportation. Despite the many disadvantages over RDP, BDP became the major product used in PC/ABS and the second largest phosphorus-based flame retardant produced. On the positive side BDP has better hydrolytic stability than RDP [313] and can be used in high humidity applications especially if it is further stabilized by adding epoxy [314] as an acid scavenger. PC/ABS with an ABS content less than 25 wt. % usually needs more than 12 wt. % BDP plus a small co-addition of PTFE in order to assure a V-0 rating [315].
BDP and RDP are also used in PC/PBT and PC/PET but further addition of an impact modifier, for example polyethylene copolymer [316] or core-shell copolymer [317] is needed. Recently, new flame-retardant blends of PC/PMMA [318] (copolymer of methyl methacrylate and phenyl methacrylate) which produce very high gloss and have excellent scratch resistance were introduced to the market. New FR blends using as one component a bio-based polymer PC/PLA [319] (polylactic acid) are also being explored for use in electronic equipment. Further addition of talc improves the heat stability of PC/PLA [320]. The content of bisphosphate in these blends depends mostly on PC content, the higher the PC content, the less bisphosphate required to achieve a V-0 rating.
Although major compounders of PC based blends are likely to be well equipped with liquid feeding systems, small and medium size compounders prefer to use solid bisphosphates even if they cost more than BDP. Very close to RDP, hydroquinone bis(diphenyl phosphate) (HDP, Formula 2.25) when made relatively pure with low TPP content and low oligomers content is a solid with a melting point of 105-108°C. It can be fed into the extruder without extensive cooling of the feeding zone and therefore some large compounders also use this product