Synthesis and properties of flame-retardant epoxy resins based on phosphorus compounds: Fid-Bau Portal

Synthesis and properties of flame-retardant epoxy resins based on phosphorus compounds

Döring, Manfred

Epoxy resins are one of the most versatile classes of thermosets with a great variety of applications. They are generally used as coatings, in fiber-reinforced composite materials, as casting resins and adhesives. Especially in electronic and electrical industries as well as aircraft and vehicle construction, fire resistance of these materials is of particular importance. Recently, phosphorus-modified epoxy resins have received important attention in literature as well as in industry too. Depending on the manufacturing process different phosphorus derivatives should be used. In principle, flame retardancy of epoxies can be achieved by using phosphorus additives, hardeners and epoxy preformulations. Since the phosphacyclic compound, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), was developed in the early 1970's, a large number of applications and modifications of this flame retardant have been published. These systems are commercially applied for printed wiring boards and laminates. The present article will highlight two different approaches to obtaining phosphorus-modified epoxy materials. The first method involves the incorporation of novel non-reactive derivatives of different phosphacycles in an epoxy novolac (DEN 438) and subsequent curing with dicyandiamide (DICY) / fenuron. An alternative process for the manufacture of phosphorus-modified epoxy materials is the reactive introduction (preformulation) of aldehyde adducts of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 2,8-dimethyl-phenoxaphosphine-10-oxide (DPPO) into the backbone of the DEN 438 novolac, followed by curing with 4,4'-diaminodiphenylmethane (DDM). The epoxy materials obtained in this way reached the UL 94 V0 rating at low phosphorus concentrations (0.6-1.4% P) and high T_g values (180 - 190 deg C). Comparison of the results obtained by both methods, however, revealed a slight superiority of the application of non-reactive additives in the epoxy system used. Whereas all formulations containing aldehyde adducts of DOPO and DPPO exhibited an insignificant drop of T_g, this parameter was maintained at the high level of the unmodified material when DOPO-based additives bearing bridging and nitrogen-containing substructures were applied. These novel DOPO derivatives are the first non-reactive additives that do not influence the T_g of an epoxy material. DEN 438 samples containing commercially available and various other already known organophosphorus compounds were manufactured in an analogous manner. All of them showed lower T_g values and poorer flame retardant efficiencies than the novel phosphorus-modified materials. To explain the superior flame inhibition activity of phosphacyclic compounds, the decomposition behaviours of DOPO and DPPO, including their sulfur derivatives, were investigated by means of thermal desorption mass spectroscopy (TDMS) and high-resolution mass spectroscopy (HRMS). PO radicals as well as PS radicals were identified as gas-phase active species. However, such fragments could not be detected when investigating samples containing open-chained phosphorus compounds.