Phosphorus-based epoxy resins

Shell Oil has a patent covering the use of red phosphorus in a curable epoxy resin, US 5859097 and Hoechst have also disclosed phosphorus-based epoxy resin compositions in US 5854371. 

A phosphorus-containing epoxy resin was prepared from a (2,5-di-alkoxyphenyl-)diphenylphosphine oxide derivative and bis(3-aminophenyl-) methylphosphine oxide (Fig. 4). Other novel phosphine oxide-based polymers and copolymers have also been described. ... 

TBBA, a brominated flame retardant, is used in the epoxy resin laminate in printed circuit boards in most manufacturers products. In 1997, a phosphorus-based alternative to TBBA was developed by the German engineering giant, Siemens,... 

In 2000, NEC developed an epoxy resin with what it describes as a fire-retardant structure that avoids the need for either TBBA or phosphorus-based flame retardants in circuit boards. The new resin contains a metal hydroxide retardant. The company claims the new board is almost totally free of pollutants, and is easy to process and thermally recycle. By also integrating flame retardant properties within the board, use of the metal hydroxide is minimised, while offering good electrical properties, higher heat resistance and improved processing characteristics. ... 

The major developments in reactive phosphorus-based flame retardants for epoxy resins to 2005 have been well reviewed.52 It will suffice here, therefore, to outline just the major developments and to highlight the most recent work. 

Ren, H., Sun, J., Wu, B., and Zhou, Q., Synthesis and properties of a phosphorus-containing flame retardant epoxy resin based on bisphenoxy(3-hydroxy) phenyl phosphine oxide, Polym. Degrad. Stab., 2007, 92, 956-961. 

Varley, R.J. Liu, W. Simon, G.P. (2006). Investigation of the reaction mechanism of different epoxy resins using a phosphorus-based hardener. Journal of Applied Polymer Science, Vol.99, No.6 (March 2006), pp. 3288-3299, ISSN 0021-8995. 

Toldy, A., Toth, N., Anna, P., and Marosi, G. Synthesis of phosphorus-based flame retardant systems and their use in an epoxy resin. Polymer Degradation and Stability, 91, 585-592 (2006). 

By incorporating TS ATH into the epoxy resin system, the level of phosphorus in the backbone of the resin can be reduced. The use of an ATH filler will also reduce the laminate cost. The addition of Martinal TS 601 to the phosphorus-based resin improves the flammability and allows a lower Tg resin system to be used. Other laminate properties are not affected. 

As a result some computer manufacturers, such as Hewlett-Packard, have shifted to phosphorus-based FRs in housings for computer monitors. Siemens has developed a phosphoms-based alternative to the TBBA used in the epoxy resin laminates that are the basis of all circuit boards. 

In general purpose applications competitively priced thermosets are used for the printed circuit board base material which is usually FR4 (Flame Retardant) . One of the main flame retardants used in America is to have tetrabromobisphenol-A reacted into the epoxy resin. Non-halogen systems include additives such as alumina trihydrate, alumina trihydrate/red phosphorus and aromatic phosphates. Flame retardant epoxy coatings are reported to use ammonium polyphosphate with char-forming additives. 

Phosphorus-containing flame retardants can be added to epoxy as additives, or they can be incorporated in an epoxy network by pre-reacting with epoxy resin or by using a phosphorus-based curing agent (Levchik and Weil 2004). 

Levchik, S.V. Camino, G. Costa, L. Luda, M.P. Mechanistic stndy of thermal behaviour and combustion paformance of carbon fibre-epoxy resin composites fire retarded with a phosphorus-based curing syston. Polym. Degrad. Stab. 1996,54, 317-322. 

The flame retardancy of the modified epoxy resins (sul-phone-, silicone- and phosphorus-based) was much better than of the neat epoxy resin. It can be clearly imderstood that the modification of tetrafunctional epoxies with silox-ane and phosphorous moieties play a pivotal role in flame retardancy. 

Synthetic resins, such as phenolic and cresylic resins (see PHENOLIC RESINS), are the most commonly used friction material binders, and are usually modified with drying oils, elastomer, cardanol [37330-39-5], an epoxy, phosphorus- or boron-based compounds, or even combinations of two. They are prepared by the addition of the appropriate phenol and formaldehyde [50-00-0] in the presence of an acidic or basic catalyst. Polymerization takes place at elevated temperatures. Other resin systems are based on elastomers (see Elastomers, synthetic), drying oils, or combinations of the above or other polymers. 

These are mainly relevant to thermosetting resins, such as unsaturated polyesters and epoxies. For polyesters, the main reactive retardants are HET acid (based on chlorine) or dibromoneopentyl glycol (DBNPG). Brominated FRs are said to be 70% more efficient than HET acid (which has also become expensive). With epoxies, the best system (on present evidence) appears to be reactive phosphorus organic compounds, which are toxicologically harmless in fire and are chemically linked to the resin matrix, so that mechanical and chemical properties are not affected. 

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