Organo-phosphorus flame retardants when applied to acrylonitrile-butadiene-styrene copolymer

In order to find an effective halogen-free flame retardant for acrylonitrile-butadienestyrene copolymer (ABS), organo-phosphorus compounds were studied and their flame retarding performances were determined by UL 94 vertical test. It is found that the flame retardancy strongly depends on phosphorus (P) content of organophosphorus compounds. Only the mixture of ABS with 2-(6-oxido-6Hdibenz<c,e><1,2>oxaphosphorin-6-yl) methyl diethyl phosphinate (ODOPM-DE), which has the highest P content, i.e., 17.68 % gives V-0 rating for the 70/30 composition.


INTRODUCTION
Polymers are a large and growing fraction of the fire load in homes, commercial environment, and transportation. However, the flammability of organic polymers limits their applications and fire hazard may be greatly increased when these materials are used unsuitably [1-3]. Halogenbased flame retardants, especially aromatic bromine compounds, are the most widely used in polymeric materials. However, so that the generation of toxic, corrosive, and halogenated gases in processing and the release of polybrominated dibenzo-p-dioxins (PBDDs), polybrominated dibenzofurans (PBDFs) in combustion might be avoided, there is a trend toward using non-halogen flame retardants in polymeric materials [4][5]. For example, EU forbids the use of polybrominated biphenyls (PBBs), polybrominated diphenylethers (PBDEs) in electric and electronic materials. Among the flame retardants containing nitrogen, silicone, phosphorus elements, organo-phosphorus compounds, generating less toxic gases and smoke, are the materials of choice [6-7]. Triphenyl phosphate (TPP, Fig. 1(A): structure of TPP) and its analogues are widely known to be the most effective flame retardants for many polymers. But, because the evaporation temperatures of TPP and its analogues are lower than the processing temperature of various polymers, they are not suitable to avoid a considerable amount of TPP to evaporate during processing. On the other hand, aromatic bisphosphates are more thermally stable than monophosphates, tetra-2,6-dimethyl phenyl resorcinol diphosphate (DMP-RDP, Fig. 1(B): structure of DMP-RDP), is one of the phosphorus flame retardant exhibiting higher evaporation temperature compared to TPP. But, when DMP-RDP was employed to examine the flame retardancy, it didn't exhibit the UL94 V-0 rating in styrenic polymers by itself [8].
Flame retardant can be chemically bonded to the macromolecular chain (reactive type) or dispersed mechanically in the polymer matrix (additive type). So far, there have been several approaches to the use of 9,10-dihydro-9-oxa-10-

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phosphaphenanthrene-10-oxide (DOPO, Fig.  1(C): structure of DOPO) by being incorporated into the backbone of the epoxy resin or the amine hardeners [9][10]. However, the phosphorus content of DOPO is low so that it also didn't exhibit the UL94 V-0 rating in styrenic polymers by itself. In this work, cyclic phosphorus compound of DOPO was used to synthesized 2-(6oxide-6H-dibenz<c,e><1,2>oxaphos-phorin-6yl) methanol (ODOPM) via a simple addition reaction. The obtained compound with reactive methanol group was then used to continue the reaction to obtain flame retardant with high phosphorus content.

Synthesis of diethylphosphinic chloride
Diethylphosphinic acid (122.1 g, 1 mol) obtained from Cheil Industries was placed in a roundbottom flask equipped witha temperature controller and a reflux condenser. Thionyl chloride (237.94 g, 2 mol) was added dropwise. The mixture was heated to 75 o C and refluxed at that temperature for 3 h. The reaction product was concentrated on a rotary evaporator, and diethylphosphinic chloride was obtained with a yield of 94% ( 31 P-NMR (CDCl3): a singlet peak at δ = 76.6 ppm). Characterization FT-IR spectra were recorded with an infrared spectrophotometer (Nicolet 380). 1 H-and 31 P-NMR spectra were taken on a Varian Unity Inova 500NB spectrometer. The chemical shift of 31 P-NMR spectrum is relative to the external standard of 85% H3PO4.
Differential scanning calorimeter (DSC) was carried out on a TA 2910 DSC instrument at a heating rate of 10 o C/min under a flow of N2 gas.
The UL 94V test was performed according to the testing procedure of FMVSS 302/ZSO 3975 with test specimen bar of 127 mm in length, 12.7 mm in width, and about 12.7 mm in thickness. During the test, the specimen was subjected to two 10-s ignitions with butane gas flame. Cotton ignition would be considered if material dripping occurred during the test. After the first ignition, the flame was removed and the time for the sample to self-extinguish (t1) was recorded. A second ignition was applied on the same sample; the selfextinguishing time (t2) and dripping characteristic were recorded. If the sum t1 and t2 was less than 10 s with no dripping, the sample would be classified a V-0 ranking.

UL 94V test
The UL 94V test determines the upwardburning characteristics of a solid and is considered as one of the industrial standards for determining the flame retardancy. To evaluate the efficiency of the flame retardant, 30 wt % of flame retardant was mixed with ABS using a HAAKE Plastic-Corder mixer and undergone UL 94V test, whose results are given in Table 1. It was found that the mixture with ODOPM-DE gave V-0 rating while other mixtures burned completely. This result clearly indicated that the flame retardancy strongly depended on phosphorus content. In other words, more effective retardancy could be achieved by employing the organo-phosphorus with higher P content, which clearly supports that our approach to synthesize this material could be justified.