Toward a new approach to synchronously improve the fire performance and toughness of polylactic acid by the incorporation of facilely synthesized ammonium polyphosphate derivatives
Introduction
Nowadays polylactic acid (PLA) is the most vastly researched and applied biopolymer, which derives from fermentation of crops (e.g., sugarcane, wheat, corn, or tapioca) [1], [2]. To tackle the environmental issues from traditional petroleum-based polymers, PLA serves as an alternative in some suitable applications (food packaging, electronic devices, biomedicine and so on) due to its biodegradability, recyclability, easy processability and non-toxicity [3], [4], [5], [6]. However, the easy flammability and heavy molten dripping of PLA burning behavior restrict itself to applying widely in commercial cases. The most efficacious path to enhance the fire resistance of PLA is to blend it with flame retardants [7], [8]. Halogenated flame retardants have been used in a broad range of industrial products, but the intrinsic toxicity brings about lots of environmental issues [9], [10]. Hence, the exploration of halogen-free flame retardants meets the needs of PLA from dual perspectives of the fire safety and the eco-friendly development.
Ammonium polyphosphate (APP) is a kind of widely used non-halogen flame retardants because of its cheap price, low toxicity and good dispersion [11]. The conventional way to improve the fire performance of APP is incorporating charring agents or synergists during the processing of polymeric materials [12], [13], [14], [15], [16]. Unfortunately, the addition of multiple components generally deteriorates the mechanical properties of polymer composites. Therefore, the improvement towards mono-component flame retardants has attracted extensive attentions. In recent years, varieties of amines have been exploited for the surface modifications of APP via ion exchange reaction. For example, piperazine, amino glycerin, ethanolamine, allylamine and vinyltrimethoxysilane are chemically incorporated into APP, respectively, obtaining mono-component intumescent flame retardants for polypropylene [17], [18], [19], [20], [21]. Additionally, APP modified with polyamines (e.g., piperazine, hyperbranched polyethyleneimines, diethylenetriamine and 4, 4-diaminodiphenylmethane) can be used as curing agent for epoxy resin with excellent flame retardancy [22], [23], [24], [25]. In our previous work, tris(hydroxymethyl) aminomethane and p-aminobenzenesulfonic acid have been utilized for the preparation of surface-modified APP through the cation exchange reaction [26], [27]. Nevertheless, the additive amount of these two modified APP in PLA is usually higher than 10 w.t.%, which is expected to be further improved.
The boron-based flame retardant is one of the replacements of halogenated flame retardants in addition to phosphorous-containing and nitrogen-containing compounds [28]. Compared with inorganic boron-containing flame retardants, there is relatively little-known about the application of organic boron-based flame retardants for polymeric materials. Recently, phenylboronic-acid derivatives have been reported that they are effective for inhibiting the flammability of PLA [29], [30], [31]. To the best of our knowledge, there are no reports involving the modification of APP with aromatic boric acid so far.
In this work, a boron-containing ammonium polyphosphate (B-APP) was prepared through facile ion exchange reaction with APP and 3-aminophenyl boronic acid. The B-APP was characterized extensively and then incorporated into PLA using torque rheometer. The flame retardancy, thermal stability and mechanical properties of PLA composites were investigated. Moreover, the corresponding char residue was further studied and the possible flame-retardant mechanism of B-APP was also proposed.
Section snippets
Materials
PLA with a trademark of 3052D (density of 1.24 g/cm3) was supplied by Nature Works Co., Ltd., USA. APP (trademark: AP-422) was offered by Clariant Co., Ltd., Germany, and its polymerization degree was above 1000. 3-Aminophenyl boronic acid was provided from Shanghai Aladdin Reagent Co., Ltd., China. Ethanol (Analytical Reagent) was acquired from Tianjin Damao Chemical Industry Co., Ltd., China.
Preparation of B-APP
The schematic illustration of preparation of B-APP was shown in Fig. 1. Firstly, 3 g 3-aminophenyl
Characterization of B-APP
The FTIR spectra of APP and B-APP are shown in Fig. 2. The characteristic double peaks at 3211 cm−1 and 3066 cm−1 are assigned to NH4+ stretching vibrations [32]. After ion exchange reaction, the absorption peak of –NH3+ stretching vibration is found at the adjacent position of 3388 cm−1 in the spectrum of B-APP. The appearance of peaks in 2895 cm−1 belongs to C–H stretching vibration in benzene ring of 3-aminophenyl boronic acid [33]. Moreover, the peaks in the range of 1630–1500 cm−1 and the
Conclusions
In this work, B-APP, has been successfully synthesized by APP and 3-aminophenyl boronic acid via a facile one-step reaction. At the loading of 5% B-APP, the easy-flammability of PLA is greatly improved. The PLA-5% B-APP sample passes the UL-94 V-0 level and the LOI value is up to 29.4%. By the comprehensive study of TGA analysis, cone calorimeter tests, SEM and Raman spectra, B-APP exhibits superior fire-resistant performance than APP for PLA. Furthermore, B-APP has better compatibility with
CRediT authorship contribution statement
Quan Wu: Conceptualization, Investigation, Writing – original draft. Xinyu Cui: Conceptualization, Investigation, Writing – original draft. Chenzhong Mu: Data curation, Resources. Jun Sun: Conceptualization, Methodology, Writing – review & editing. Xiaoyu Gu: Formal analysis. Hongfei Li: Software, Validation. Sheng Zhang: Conceptualization, Methodology, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The current work was financially supported by National Key R&D Program of China(Grant No. 2018YFD1100403), National Natural Science Foundation of China(Grant No. 51803007 and 21875015)
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These two authors are joint first authors.