Novel eco-friendly maleopimaric acid based polysiloxane flame retardant and application in rigid polyurethane foam

Highlights

• A novel maleopimaric acid-modified polysiloxane flame retardant was synthesised.

• The synthesised flame retardant was integrated into a rigid polyurethane foam.

• The rigid MPA and imide groups inhibit the “back-bite” of polysiloxane.

• They also promote the formation of a silica-rich hybrid char layer.

Abstract

Polysiloxane flame retardants have been widely studied because of their high heat resistance and nontoxicity. The flame retardant efficiency may be further improved by inhibiting the release of cyclic siloxanes during combustion. In this study, a novel flame retardant was synthesised by imidisation of an amino-polysiloxane using biorenewable resource-maleopimaric acid (MPA). The MPA inhibits the conversion of polysiloxane to cyclic siloxane, thereby enhancing thermal stability. The synthesised flame retardant was employed as a part of the soft segment to produce rigid polyurethane foam (RPUF) by a “one-pot” process. The rigid MPA and imide groups inhibit the “back-bite” of polysiloxane and promote the formation of a silica-rich hybrid char layer on the backbone of RPUF to reduce flammability. Compared with pure RPUF, the minimum oxygen concentration necessary to support the combustion of modified foam increased by 39.7%, and the peak heat release rate (PHRR) of modified RPUF decreased by more than 50%. Simultaneously, the imide groups and the rigid hydrogenated phenanthrene ring of MPA improved the compressive strength of polysiloxane-modified RPUF. The technique to synthesise a highly effective polysiloxane flame retardant for RPUF is given herein.

Introduction

Polyurethane (PU) is ranked 6th in worldwide polymer production. The main uses of PU include foams (65%), coatings (13%), adhesive sealants (7%) and elastomers (12%) [1]. RPUF is a type of thermoset polymer which can be formulated to meet specific requirements with applications such as thermal insulation, packing, and transportation [2]. Unfortunately, RPUF is highly flammable due to its porous structure and high specific surface area [3,4]. Therefore, it would be valuable to endow RPUF with excellent flame retardancy.

Compounds containing halogens such as bromine or chlorine may be used to improve flame-retardance of RPUF. The compounds act in the vapour phase by a radical mechanism to disturb the exothermic processes and thereby inhibit combustion [5]. However, they also have significant disadvantages: the possibility of corroding metal components and, more pressingly, the formation of toxic halogen gas during combustion [6]. Alternatively, phosphorus-containing compounds impede combustion in the condensed phase by facilitating char generation on the surface of the condensed material, which acts as a shield to suppress pyrolysis products from spreading into the flame and isolates the material surface from oxygen and heat [7]. Unfortunately, phosphorous compounds usually degrade the mechanical properties of materials, and the poly (phosphoric acid) formed during combustion can also corrode metals in the fire vicinity [8,9]. Some industries are therefore wary of the application of phosphorus-containing flame-retarding materials. Consequently, halogen-free, eco-friendly, and non-toxic have gradually become the main directives for the development of novel flame retardants [10]. Under such requirements, silicon-based flame retardants such as natural silicates, siloxane, polyhedral oligomeric silsesquioxane (POSS) and organic polysiloxane have been widely studied in recent years [[11], [12], [13], [14], [15]].

Polysiloxane flame retardants exhibit good processability, high flexibility [16], water resistance [17], high thermal stability [18], minimal sensitivity to external heat flux [19], low heat release rate, and do not generate toxic gases during combustion [20,21]. The silicones of polysiloxane compounds tend to migrate and concentrate on the surface of the material during combustion, at which point an inorganic silica-rich hybrid residue is formed and serves as an ‘‘insulating blanket’’ and reduces the rate of heat release [22]. Moreover, the inorganic silica-rich hybrid residue acts as a protective layer and isolates the underlying polymer from incoming external heat flux, protecting internal polymer against degradation [19,23,24]. However, polysiloxane can “back-bite”, promoting intramolecular redistribution reactions during thermal oxidation, which then produce flammable cyclic siloxanes with low molecular weight [25,26]. In the work by Hshieh et al. the generation of cyclic siloxanes can seriously weaken the integrity of the silica-rich hybrid char layer, which leads to the low efficiency of the silicon-based flame retardant [27]. Reducing the release of cyclic siloxanes to ensure the integrity of the silica-rich hybrid char layer is, therefore, an urgent research target of polysiloxane flame retardant to be resolved.

Aromatic functional groups have been demonstrated to improve the thermal stability of polysiloxane by sterically hampering the siloxane bond rearrangement during thermal oxidation and thereby reducing the formation of cyclic siloxanes [28,29]. The thermal stability of aromatic-containing polysiloxane is also conferred to its modified materials, such as epoxy thermosets, polycarbonate and polyurethane [[30], [31], [32], [33]]. Recently, many biological resources can be used in place of traditional aromatic compounds. Rosin, a material produced from some tree resins, has a bulky hydrogenated phenanthrene ring analogous to some aromatic compounds. In our previous work, the maximum weight loss temperature and char yield of polysiloxane were improved by the incorporation of rosin into the crosslinked network, owing to the hydrogenated phenanthrene ring being of higher steric hindrance and thereby limiting Si–O bond scission [34,35]. To the best of our knowledge, the effect of rosin on the flame retardancy of polysiloxane has not yet been investigated in other polymer materials.

In the present work a kind of rosin derivative, maleopimaric acid (MPA), was synthesised by linking rosin and maleic anhydride via a Diels–Alder reaction. The MPA is then linked via imidisation to the amine groups of methyl terminated amino-polysiloxane (MTASO) to generate a polysiloxane derivative with enhanced flame-retardance and thermal stability, referred to as maleopimaric acid grafted polydimethylsiloxane (MGSO). The MGSO was then employed as part of the soft segment to prepare more flame-retarding RPUF by a “one-pot” process. The effects of modified polysiloxane on flame retardancy, thermal stability, and the combustion process of RPUF were examined by limiting oxygen index (LOI), thermogravimetric analysis infrared (TG-IR), and the cone calorimeter test. To further investigate the flame retardancy of modified RPUF, scanning electron microscope (SEM) images, as well as X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectra were obtained to analyse the char residue. This novel flame retardant not only improves the flame retardancy of RPUF, but also endow it with better mechanical properties.