Synthesis and properties of poly(bisphenol A acryloxyethyl phosphate) as a UV curable flame retardant oligomer

Abstract

Poly(bisphenol A acryloxyethyl phosphate) (BPAAEP) being used for UV curable flame retardant coatings and adhesives, was synthesized from phosphorus oxychloride, hydroxylethyl acrylate and bisphenol A as raw materials, and characterized using ¹³C NMR, ³¹P NMR, FTIR, MS and GPC measurements. A series of formulations with different ratios of BPAAEP to urethane acrylate, EB220, were prepared to obtain flame retardant resins. The flame retardancy of the UV cured films was investigated by the limiting oxygen index (LOI). A synergistic effect between phosphorus and nitrogen was observed when 1.5 wt% phosphorus was presented in the resin. Their maximum photopolymerization rates $(R_{\mathrm{p}}^{\max })$ and final unsaturation conversion (P^f) in the cured films at the presence of a 3 wt% photofragmenting initiator were investigated. The results showed that the P^f increased with increasing EB220 content photo-DSC analysis. The data from dynamic mechanical thermal analysis showed that BPAAEP has good miscibility with EB220. Moreover, the crosslink density and T_g of the cured film decreased along with the content of BPAAEP in the blend.

Introduction

For about 20 years, photoinitiated reactions have been used in several applications in thin layers (microlithography, microelectronics, lacquers and varnishes for surface treatment, protective coatings, printing inks, and antiadhesive support), as well as in thick layers (dental stratification, stereolithography) [1]. The usage of this technology and the progress in the development of resins and curing systems have contributed towards its high and fast development. The rate of the polymerization is one of the main advantages of UV curing which leads to highly crosslinked materials. Important types of oligomers/monomers commonly used in UV curable coatings are acrylated epoxies, acrylated urethanes, unsaturated polyesters and acrylated polyesters (or polyethers). Urethane acrylates are widely used in various UV coating industries because of their favorable properties with their cured films offering excellent adhesion to most substrates and high flexibility. However, they do not exhibit sufficient flame retardancy and are easily burned. Therefore, the studies emphasizing the development of technologies to promote flame retardancy and creating flame retardant materials have increased over several years [2], [3], [4]. It has been found that phosphorus-containing compounds are most interesting since they are environmental-friendly and offer numerous advantages [5], [6]. The flame retardants containing phosphorus can avoid the concomitant undesirable changes in physical and mechanical properties observed when high concentrations are used, but without emission of toxic gases. When burning, they usually form a protective layer of char and poly(phosphoric acid) which inhibits heat and oxygen transfer into the polymer bulk, decreasing the diffusion of combustible gases into the zone of pyrolysis [7].

Organophosphorous compounds act according to several mechanisms. Numbers of articles had reported that they can form an intumescent flame retardant (IFR) system with nitrogen-containing compounds [2], [8]. The action of IFRs is mainly through a condensed-phase mechanism. While burning, it gives a swollen multicellular char. The typical formulation of IFR is made of three constituents: an essentially phosphorus-containing additive, whose function is to form, during the combustion, an impermeable semisolid and vitreous layer essentially composed of polyphosphoric acid, and to activate the formation process of intumescence; a second additive, containing nitrogen, which serves as foaming agent; and a third, carbon-containing additive, which acts as a carbon donor to allow an insulating cellular carbonaceous layer (char) to be formed between the polymer and flame [9], [10], [11], [12].

The comparison of reactive-type flame retardants with additive-type has been extensively performed. It was found that additive-type flame retardants could migrate from the polymers during their uses, leading to weakening the fire-resistant properties over time. Thus, in order to avoid problems with migration and the diminution of flame retardancy, it is more advantageous for the organophosphorous reactant to be chemically bonded to the polymer backbone [13]. The main advantage of using reactive-type flame retardants is the ability to bestow the permanent flame retardancy and, simultaneously, to maintain the original physical and mechanical properties of polyurethanes in a better way. Therefore, the study on incorporating phosphorus-containing groups onto the polymer backbone has attracted much attention recently [14], [15], [16], [17], [18], [19], [20], [21]. However, little work has been reported on halogen-free flame retardant oligomers and monomers used as compositions in UV curable systems.

In this work, we mainly aimed to design and synthesize a flame retardant containing phosphorus, poly(bisphenol A acryloxyethyl phosphate) (BPAAEP), which can be UV cured. The structure of this oligomer was characterized by FTIR, ¹³C NMR, ³¹P NMR and mass spectroscopy (MS). The molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC). The density of double bond was determined by titration method. The flame retardancy of its UV cured film was characterized by the limiting oxygen index (LOI). The viscosity and photopolymerization kinetics of the flame retardant systems and dynamic mechanical thermal properties of the cured films were also investigated.