A novel phosphorus-containing lignin-based flame retardant and its application in polyurethane
Introduction
Polyurethane (PU), a polymer comprised of urethane-repeating structural units, is usually obtained by the reaction of hydroxyl groups with isocyanate groups (–NCO) [1]. PU has been widely applied as foams, coatings, paint films, and adhesives because of their low density, good ductility, and excellent stability [2]. Among them, PU films are characterized by excellent adhesion, flexibility, impact resistance and abrasion resistance, corrosion and so on [[3], [4], [5]]. However, polyol (the raw materials of PU films) is costly and non-renewable, as this substance is dependent on petrochemical resources [6]. Therefore, the use of non-petroleum-based polyol to prepare PUs has become the focus of current research. PU is also a kind of flammable polymer, thus greatly limiting its wide application. Improving the fire resistance and lowering the cost of PUs are thus highly meaningful [7].
Lignin is presently the second largest biopolymer in nature after cellulose, and it has the advantages of non-toxicity, degradability, and low price [8,9]. Researchers have made great efforts to prepare lignin-based polymers, including phenolic resins, polyesters, and epoxy resins [10]. As an aromatic macromolecular polyol, lignin can replace petroleum-based polyols and react with isocyanate groups to provide a new choice for PU. He et al. [11]provided an approach to prepare PU elastomers by reacting lignin with tolylene 2,4-diisocyanate to consequently obtain excellent mechanical properties and toughness. The aromatic structure of lignin also has high char-forming ability that can slow down to a certain extent the burning speed of polymers, enabling the product to be used as a charring agent for flame retardants [12]. The preparation of lignin-based flame retardant (LMD) not only can save resources but also broaden the application of PUs. LMD is a topic with extremely high economic benefits and far-reaching social significance [13].
As a kind of macromolecular polyol, lignin usually follows two methods of PU synthesis. The first method is the chemical modification of lignin by means of etherification, esterification, and amination to enable the hydroxyl groups to become highly susceptible to isocyanate reaction [14]. Chung et al. [15] used Lewis acid to modify lignin. Their findings showed that the hydroxyl content in lignin increased by 28%, and the reactivity was greatly improved. Although this method can improve the reactivity of –OH groups, the chemical modification of lignin will increase production cost and worsen the environmental impact [16]. Therefore, the direct application of non-chemically modified lignin is more preferred than the direct reaction of polyol with isocyanate. At the same time, considering the limited hydroxyl groups in lignin, other suitable polyols are usually added for copolymerization [17]. The addition of a polyol can also serve as soft segments in the PU architecture, and the resultant PU can achieve good flexibility [18].
The introduction of flame retardants by physical blending is a common method in improving the fire resistance of polymer materials [19,20]. However, this method usually damages the mechanical properties of materials [6]. Thus, improving the compatibility between the flame retardant and the matrix is preferred in enhancing comprehensive performance [21]. In our previous work [22], hexamethylene diisocyanate was applied as a bridge between LMD and the lignin-based polyurethane (LPU) prepolymer matrix to synthesize a kind of flame-retardant LPU, and excellent flame retardancy and coating properties were achieved. Nonetheless, the mechanical properties of PU can still be improved and its cost need to be further lowered. The use of diphenylmethane diisocyanate (MDI) with two symmetrical benzene rings in its molecular structure seems to be a comparatively good choice [23]. In addition, the phosphorus-containing intermediate 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) have attracted great attention owing to its highly effective flame-retardant effect [24].
Here, a novel phosphorus-containing LMD was synthesized via the reactions of lignin, MDI, and DOPO, and excellent flame retardancy was achieved. Moreover, the LPU prepolymer was prepared via the reactions of MDI, lignin, and polyols. Polyethylene glycol (PEG) 2000 was added to adjust the flexibility of PU. LMD was inserted into the LPU prepolymer to prepare a flame-retardant polyurethane (FLPU). Given the same lignin and urethane structures and the co-curing between LMD and the LPU prepolymer, an interface compatibility between the flame retardant and the PU prepolymer was well resolved. The FLPU, with its excellent flame-retardant and mechanical properties, could be prepared successfully and subsequently characterized.
Section snippets
Preparation of the flame-retarding PU composites (FLPU films)
The synthesis route of LPU is presented in Scheme S2. First, the synthesis of lignin-based flame retardant (LMDs) could be obtained in the supporting information. Polyurethanes was successfully synthesized through condensation reactions between –NCO in 4,4′-diphenylmethane diissocyanate (MDI, 99.6%) and the hydroxyl groups of lignin, polyol 204 and PEG 2000. Meanwhile, PEG 2000 was used to adjust the flexibility of PU segments. L15-80PU (content of lignin was 15%, polyol 204 accounted to 80% of
Flame-retardant properties of FLPU
Flame-retardant properties were investigated on the basis of the limiting oxygen index (LOI). The results of LOI tests are shown in Table 1. The LOI value of pure PU was only about 17.1% [22], and the LOI of L15-80PU (lignin in LPU was 15%) was 21.5% without dripping. The result means that the substantial aromatic structures of lignin, together with abundant polyphenol biopolymeric structures, can accelerate the formation of carbon layers in the PU matrix and reduce burning rate. When the
Conclusions
In this study, LMDs were prepared via the in-situ reaction of lignin with MDI and DOPO. The LMDs achieved excellent thermal stability, and its residual char increased greatly with the lignin content, and reached as high as 34.9% at 700 °C. Furthermore, MDI, lignin, and polyols were applied with PEG 2000 for LPU preparation to adjust the flexibility of PUs. Then, LMDs were added to the LPU prepolymer and co-cured for FLPU preparation. The LOI value of FLPU increased with the addition of LMD and
CRediT authorship contribution statement
Yuliang Wang: Data curation, Writing - original draft. Yumei Zhang: Visualization, Investigation. Biying Liu: Investigation. Qi Zhao: Formal analysis. Yunxia Qi: Resources. Yanmiao Wang: Resources. Zhaoyan Sun: Writing - review & editing. Baijun Liu: Validation. Niaona Zhang: Data curation. Wei Hu: Supervision. Haiming Xie: Funding acquisition.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China [Nos. 21404013, 5187306], the Science and Technology Development Plan of Jilin Province, China [Nos. 20160101323JC, 20170101110JC, 20180201075GX, 20180201076GX, 20180519014JH, 20200401036GX], the Jilin Provincial Development and Reform Commission, China [2018C041-1], “13th Five Year Plan” Science and Technology Project of Jilin Provincial Department of Education, China [JJKH20191297KJ], the Open Research
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