Elsevier

Polymer

Volume 42, Issue 18, August 2001, Pages 7617-7625
Polymer

Synthesis of novel flame retardant epoxy hardeners and properties of cured products

https://doi.org/10.1016/S0032-3861(01)00257-9Get rights and content

Abstract

Novel flame-retardant curing agents for epoxy resins, [ODOPM–PN] and [ODOPM–MPN], were prepared from phenol formaldehyde novolac (PN), melamine-phenol formaldehyde novolac (MPN) and a reactive 2-(6-oxid-6H-dibenz〈c,e〉 〈1,2〉oxaphosphorin-6-yl)-methanol (ODOPM) while ODOPM was synthesized through the reaction between 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and formaldehyde. The compounds (ODOPM–PN and ODOPM–MPN) were used as flame-retardant hardener for o-cresol formaldehyde novolac epoxy (CNE) resin in electronic application. The thermal stability and flame retardancy were determined by thermal gravimetric analysis and UL 94 vertical test. The glass transition temperatures were measured by dynamic mechanical analysis. The phosphorus–nitrogen synergistic effect on flame retardancy combined with the rigid structure of ODOPM have resulted in better flame retardancy, higher glass transition temperature and thermal stability for the phosphorus–nitrogen containing epoxy resin system than the regular phosphorus-containing flame retardant epoxy resin system. UL 94-VO rating could be achieved with a lower phosphorus content of as low as 0.81% with 2.36% nitrogen for the ODOPM–MPN cured epoxy resin system and no fume and toxic gas emission were observed.

Introduction

Epoxy resins have the excellent characteristics of moisture, solvent and chemical resistance, toughness, low shrinkage on cure, superior electrical and mechanical resistance properties and good adhesion to many substrate. The versatility in formulation also made epoxy resins widely applied industrially for surface coating, adhesive, painting materials, pottings, composites, laminates, encapsulant for semiconductor and insulating material for electric devices, etc. [1], [2], [3], [4]. The main drawback of epoxy resins which like other organic polymers is their flammability. Traditionally, flame-retardant polymers are achieved by physically blending flame-retardant additive with the polymer. However, a major disadvantage of all flame-retarding additives is that they may be lost in processing and during use of polymer, and this may mean that high loading are initially required. Another way in which to reduce the flammability of polymers is chemically bond the flame retardant to the polymer backbone, i.e. to use a reactive flame retardant. This offers the advantage of permanent attachment of flame-retardant group to polymer and leads to high efficiency in flame retardancy [5], [6], [7], [8] with consequently a much smaller influence upon the physical and mechanical properties of the polymer.

Recently, organophosphorus compounds have demonstrated good ability as flame-retardant for epoxy resins and also being found to generate less toxic gas and smoke than halogen-containing compounds [9], [10], [11], [12], [13], [14], [15], [16], [17]. According to the previous investigations, it was found that introduction of phosphorus into the polymer skeleton can improve the flame retardancy and decrease contamination on pyrolysis. Generally, the phosphorus moiety decomposes at low temperatures relative to the polymer matrix. However, oxidation of the phosphorus char is observed at temperature above 600°C. Thus, a phosphorus-rich char is formed to reduce the production of combustible gases during fire [18], [19], [20], [21].

The main common advantages of nitrogen compounds are their low toxicity, their solid stated and, in case of fire, the absence of dioxin and halogen acid as well as their low evolution of smoke.

Melamine and its salts are widely used as fire retardant additives, particularly of intumescent type [22], [23]. It is known that melamine undergoes progressive condensation on heating with elimination of ammonia and formation of polymeric products named ‘melam’, ‘melem’, and ‘melon’ which are more thermally stable than melamine itself [24], [25], [26], [27]. Therefore, when melamine is incorporated in a polymer, in absence of reactions with the matrix, it should leave the material when thermal degradation occurs, with negligible contribution to charred residue possibly formed.

In this article, the nitrogen–phosphorus synergistic effect on flame retardancy was studied by the incorporation of a rigid cyclophosphoryloxymethyl structure into melamine phenol novolac to form a novel epoxy resin hardener and its effects on flame retardancy and physical properties were investigated.

Section snippets

Materials

All reagents and solvents were reagent grade or were purified by standard methods before use. 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) which was prepared in our laboratory [28]. Paraformaldehyde and melamine from Aldrich were used as received. Xylene, and methyl isobutyl ketone (MIBK) from Acros were used as solvent. The epoxy resin used was o-cresol formaldehyde novolac epoxy (CNE, epoxy equivalent weight, EEW 192) from the Chang Chun Plastic (Taiwan). A phenol formaldehyde

Synthesis of phosphorus-containing compounds

Synthesis of the reactive rigid heterocyclic ring structure containing phosphorus, ODOPM, was performed by starting with DOPO and paraformaldehyde according to Scheme 1. IR spectrum of phosphorus-containing ODOPM exhibited the characteristic aliphatic hydroxyl group absorption at 3308 cm−1. The absorption around 1186 and 1292 cm−1 corresponds to vibration with Pdouble bondO which is characteristic of phosphoric compounds. The ODOPM also showed strong absorption around 962 cm−1 corresponding to P–O–C

Conclusions

Novel flame-retardant curing agents [ODOPM–PN] and [ODOPM–MPN] were successfully synthesized from PN, MPN and ODOPM. The compounds were used as curing agent for CNE resins in semiconductor encapsulation and in electrical laminate applications. The ODOPM–PN and ODOPM–MPN cured system provided not only better mechanical property, flame retardancy and thermal stability but also much less fume in combustion test than the PN cured system. Furthermore, the N–P synergistic effect on flame retardancy

Acknowledgements

Financial support of this work by the National Science Council of the Republic of China is gratefully appreciated (NSC 89-2216-E006-058).

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