The preparation of phosphorus and nitrogen-containing structure towards the enhancement of flame retardancy for thermoplastic polyurethane elastomer

Abstract

Thermoplastic polyurethane elastomer (TPU) is an easily flammable materials with serious droplets and smoke toxicity, which critically restrict its practical application. In order to improve the flame retardancy of TPU, the present work designed a halogen free flame retardant (AL-APP) containing phosphorus and nitrogen elements via the modification of ammonium polyphosphate (APP) with sodium alendronate (Al). It was found that the limiting oxygen index of TPU/AL-APP₅ composites increased from 20.7 % to 26.5 % and got past the V-0 rating without any droplets in the UL-94 vertical measurement. Meanwhile, the peak heat release rate (PHRR) and total heat release (THR) of TPU/AL-APP₅ system reduced from 1021 to 255 kW/m² and 87.5–61.2 MJ/m² in cone calorimeter (CC) measurement when comparing to that of purchased TPU. According the thermogravimetric-infrared and Raman spectroscopy results, the production of the inert gas and formation of continuous carbon layer of AL-APP mutually improved the fire safety of TPU composites. Furthermore, the elongation at break of TPU/AL-APP₅ was significantly increased by 19.9 % than that of neat TPU, showing a better softness and elasticity than neat TPU. According the above results, it was concluded that this work advancing in the high flame-retarding efficient and good mechanical property for TPU composites via a simple method.

Introduction

Thermoplastic polyurethane (TPU) is widely applicated in many fields including cables, automobile and aerospace due to its excellent wear resistance, high mechanical properties, ozone resistance, radiation resistance, remarkable low temperature flexibility and great biocompatibility, etc. [1], [2] However, the neat TPU is easily flammable with serious droplets, which is attributing to the high content of hydrocarbon elements in the molecular chain of TPU [3]. Meanwhile, it will cause serious heat hazard and smoke toxicity (especially the carbon monoxide and nitric oxide), which severely threatening the safety of people's lives and greatly limiting its wide application [4], [5], [6], [7]. Therefore, it is of extremely importance to develop highly efficient flame-retarding TPU based composites [8].

For a long time, halogen-containing flame retardants have been mostly used [9], [10], [11], [12]. Although good flame-retarding effect has been achieved, the produced irritating and corrosive hydrogen halide gas during the burning process, which will seriously endanger the environment and human health [13]. Therefore, the development of high-efficiency, halogen-free, non-toxic and low-smoke flame retardants has become a hot research direction in recent years [14], [15]. At present, there are two ways to improve the flame retardancy of TPU in halogen-free flame retardant field: "additive" flame retardant and "reactive" flame retardant, especially the former are commonly used attributing to wide source, simple synthesis process, high flame-retarding efficiency, etc [16], [17]. Commonly used additive flame retardants include layered double hydroxide (LDH) [18], melamine and its salts[19], ammonium polyphosphate (APP) [20] and its derivatives [21], etc. [22], [23], [24], [25], [26], [27], [28]. It should be noted that the APP and derivatives are commonly researched attributing to the excellent flame-retarding performance, low smoke, and no corrosive gas (ammonia, water and so on) generation [29], [30]. Meanwhile, an expanded and continuous carbon structure will be formed on the surface of the material via the catalyzation of generated phosphoric acid, which is helpful to heat insulation, oxygen insulation, smoke suppression and drip prevention [31], [32], [33]. Zhang and the coworkers modified APP with PA-Co to incorporated into the polyurethane foam, the results showed that the smoke suppression of polyurethane foam was obviously improved due to the formation of a dense carbon layer [34]. Therefore, it is challenging to modify APP with high flame-retarding effect.

Alendronate sodium (AL) is one of the third generation bisphosphonates and the most widely used anti-osteoporosis drug in the world, which could increase the density of bone and reduce the risk of fracture [35], [36]. Generally, AL is used as an effective inhibitor for bone resorption without the limits of the gender for patients [37]. He et al. modified collagen with alendronate sodium and mainly used for bone regeneration [38]. In addition, Bernardo Carvalho et al. also studied the application of AL in the treatment of intrabuccal bone defects, especially the defects caused by periodontitis [39]. Meanwhile, it contains high proportion of P and N elements, which is potential to applied in the enhancement of fire safety of polymer materials.

In this work, a halogen-free flame retardant (AL-APP) containing high phosphorus and nitrogen elements synthesized by ion exchange between AL and APP. The detailed structure was characterized by SEM, FTIR, ¹H NMR demonstrated the successful preparation of AL-APP. The thermal stability and mechanical property of TPU and its composites were measured by TGA and stretching machine. The flame retardancy and combustible behavior of the composites were performed by UL-94, LOI and CC tests and the relative flame-retarding mechanism was discussed by TG-FTIR and Raman spectrum.