Synthesis of an intrinsically flame retardant bio-based benzoxazine resin

Highlights

• An intrinsically flame retardant bio-based benzoxazine (DPA-PEPA-boz) was synthesized from renewable diphenolic acid.

• The residual char of P-DPA-PEPA-boz after 400 °C was much higher than those of P-DPA-boz and P-MDP-boz under nitrogen and air atmospheres.

• P-DPA-PEPA-boz had a limiting oxygen index (LOI) of 33.5% and achieved V0 level in UL94 test.

• Thus, this study demonstrates the great potentials of the intrinsically flame retardant bio-based benzoxazine (DPA-PEPA-boz) in the application of high performance matrix resin and composite material.

Abstract

An intrinsically flame retardant bio-based benzoxazine (diphenolic acid pentaerythritol caged phosphate benzoxazine, DPA-PEPA-Boz) monomer was synthesized from bio-based diphenolic acid (DPA) using a four-step process. The monomer of DPA-PEPA-Boz was characterized by FT-IR, ¹H NMR and ¹³C NMR. The curing behavior of DPA-PEPA-boz was studied and compared with those of DPA based benzoxazine (DPA-Boz) and DPA ester derivative (MDP) based benzoxazine (MDP-Boz) without PEPA by means of non-isothermal differential scanning calorimetry. The results indicated that DPA-PEPA-Boz system showed a two-stage curing, assigned to the exothermic opening reactions of oxazine rings and P–O–C ring in PEPA respectively, while the DPA-Boz and MDP-Boz showed a one-stage curing. In addition, the effect of the introduction of PEPA on thermal and inflammable properties of the resin was evaluated. The residual char of the cured DPA-PEPA-Boz (P-DPA-PEPA-Boz) after 400 °C was much higher than those of cured DPA-Boz (P-DPA-Boz) and cured MDP-Boz (P-MDP-Boz) under nitrogen and air atmospheres. Meanwhile, total heat release (THR), peak heat release rate (PHRR) and heat release capacity (HRC) of P-DPA-PEPA-Boz were about half of those of P-DPA-Boz and P-MDP-Boz. P-DPA-PEPA-Boz had a limiting oxygen index (LOI) of 33.5% and achieved V0 rating in UL94 test. P-DPA-PEPA-Boz behaved as a very good intrinsic thermal and flame retardant bio-based benoxazine resin.

Introduction

The development of environmentally friendly polymers from bio-based materials and compounds is one of the current challenges and also opportunities for polymer chemistry. Many new bio-based compounds have been developed and commercialized for the production of bio-based polymers, which have the potential of significant economic and environmental benefits to the society and creating new property profiles that the traditional nonrenewable polymers do not exhibit [1], [2]. Up to now, much effort has been made to synthesize polylactic acid, polycarbonates, polyesters, epoxy resin and benzoxazine resin from renewable feedstock, such as rosin, cyclodextrins, soybean oil, diphenolic acid, itaconic acid, etc [3], [4], [5], [6], [7], [8], [9], [10].

Benzoxazine resins (Bozs) have attracted increasing attention in the past decade due to their excellent high temperature stability, low water absorption, good dielectric properties and structure-design flexibility, in addition to their zero shrinkage or a slight expansion upon curing [11], [12], [13], [14]. These good properties give them a huge potential in industrial applications, such as high-speed printed circuit boards, aerospace structural composites, etc [15], [16], [17], [18], [19]. The bisphenol A (BPA) based Boz (BPA-Boz) stands out from many types of Bozs due to its high structural integrity and superior properties [20]. Diphenolic acid (DPA), which has a structure similar to BPA (Fig. 1), can be prepared from low-grade lignin and cellulose biomass resources [21]. Through plant photosynthesis to synthesize cellulose, it is a renewable material that can alleviate our current large dependence on petroleum for chemicals, preserve the planet nature resources, and obtain good environmental and economic benefits [22]. DPA is commercially available and much cheaper than BPA and has the ability to introduce functional carboxyl group into the polymer structure. Therefore DPA is believed to be a cheap bio-based compound in large scale and can be used as a BPA alternative to synthesize new polymers. In the last decade, DPA has attracted much attention from researchers to replace BPA for synthesizing new polyesters, polycarbonates and Bozs [7], [23], [24], [25], [26], [27].

With the rapid development of aerospace, microelectronics and energy industries, there is an urgent requirement to further improve the properties of Bozs, such as heat resistance, flame retardancy, toughness, low dielectric constant, etc [28]. The flame retardancy of BPA-Boz cannot meet the more demanding requirement. Challenges also exist for the diphenolic acid based Boz (DPA-boz) in this regard. For example, DPA-Boz has been modified using DOPO or DOPO-2Me to prepare Boz foams [27]. But the choice of flame retardant is limited due to the poor compatibility between flame retardant and Boz. Fortunately, with the carboxyl group in the DPA-boz structure, it is possible to introduce flame retardant moieties to improve its flame retardancy. Common flame retardants contain N, P, Si and halogen elements. Researchers pay more attention to P containing flame retardants because of their better flame retardant effect, less smoke, and lower emissions of harmful substances. A representative P containing flame retardant is 4-(hydroxymethyl)-1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane (PEPA), which was synthesized by Verkade for the first time in the 1960s [29]. It has aroused people’s great interest due to the highly symmetrical cage structure in its molecule. PEPA can be used as intumescent type flame retardant because its molecule contains acid source and carbonization agent, which resulting in characteristics of good char-forming ability and excellent thermal stability. Synthesis of cage phosphate ester compounds and their application in flame retardant materials have received wide attention in recent years [30], [31], [32], [33].

In this study, an intrinsically flame retardant bio-based Boz (DPA-PEPA-boz) was synthesized from renewable diphenolic acid. The material was characterized by fourier transform infrared (FTIR) spectrophotometer, ¹H and ¹³C nuclear magnetic resonance (NMR). Its curing behavior, thermal and inflammable properties were evaluated and compared with DPA based Boz (DPA-boz) and DPA ester derivative (MDP) based Boz (MDP-boz).