Elsevier

Polymer

Volume 52, Issue 2, 21 January 2011, Pages 233-238
Polymer

Polymer Communication
A novel phosphorus-containing poly(lactic acid) toward its flame retardation

https://doi.org/10.1016/j.polymer.2010.11.023Get rights and content

Abstract

An inherently flame-retardant poly(lactic acid) (PLA) was synthesized via the chain-extending reactions of dihydroxyl terminated pre-poly(lactic acid) (pre-PLA), which was synthesized by direct polycondensation of l-lactic acid using 1,4-butanediol as initiator and stannous chloride (SnCl2) as catalyst, using ethyl phosphorodichloridate as chain extender. The resulting phosphorus-containing poly(lactic acid) (PPLA) was characterized by gel permeation chromatography (GPC), 1H and 31P nuclear magnetic resonance (1H, 31P NMR) and homonuclear correlation spectroscopy (COSY) and inductively coupled plasma-mass (ICP). A comprehensive flame retardant property of PPLA was evaluated by microscale combustion calorimetry (MCC), limiting oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter test (CCT). PPLA has excellent flame retardancy and also can be used as a flame retardant for commercial PLA. Only 5 wt.% of PPLA added into PLA can obtain good flame retardant properties. As the content of PPLA is further increased to 10 wt.%, PLA can have much better flame retardancy (LOI = 35 and UL-94 V-0 rating), lower peak heat release rate (pHRR) and longer ignition time (TTI) than neat PLA. All those results mean that this novel approach to impart flame retardancy to PLA is very effective.

Introduction

Poly(lactic acid) (PLA) is one of the most promising candidates in the field of biobased polymers because it is biodegradable and can be produced from renewable resources (sugar beets, corn starch, etc.). Recently PLA has been considered as alternative in replacing petrochemical polymers due to its excellent mechanical properties, high degree of transparency, and ease of fabrication [1]. Usually, PLA can be synthesized by direct polycondensation of lactic acid, or chain extension after polycondensation and by ring-opening polymerization of lactide [2], [3]. Currently, although the main use of PLA is still in biomedical application, its other applications are increasing because of its excellent potential properties. Actually, PLA has been used in electronic industries now, such as the housings of portable word processors [4]. Unfortunately, PLA is still as flammable as common synthetic thermoplastics, such as polyethylene, polyester, and so on owing to its own intrinsic chemical composition and molecular structures. Predictably, the flammability will limit its application and development, especially its potential wide application in the electronic field. Therefore, the improvement of flame retardant performance of PLA has been an important and urgent task.

Unfortunately, so far few research reports have focused on flame retarded PLA in literatures. Several report employed ammonium phosphate, [5], [6] melamine phosphate, [7] aluminium hydroxide, [8] silica gel [9] and compounds containing halogen and talc [10] as additive flame retardants for PLA matrix. Lately, Casetta and Bourbigot et al. evaluated the efficiency of intumescent formulations to flame-retardant PLA; those are composed of ammonium polyphosphate (APP), pentaerythritol (PER), lignin and starch [11]. The results show the flame retardancy of PLA has been improved greatly as the flame retardant reached a loading level of 40wt%. In our latest studies, a series of flame retardant toughened PLA composites have been prepared using poly(ethylene glycol) (PEG6000) and ammonium polyphosphate (APP) [12].

In the present study, we developed a novel approach to the preparation of flame-retardant PLA. We used a reactive flame retardant, ethyl phosphorodichloridate, as a chain extender to synthesize poly(lactic acid) containing phosphorus in the backbone (PPLA), shown in Scheme 1. The inherently flame-retardant PPLA has excellent flame retardancy, and can also be used as a flame retardant for PLA. The flame retardancy of PPLA and its blends with PLA has been investigated via microscale combustion calorimetry (MCC), limiting oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter test (CCT).

Section snippets

Materials

l-Lactic acid was supplied from Guangshui Chemical Reagent Corp (Hubei, China). 1,4-Butanediol was provided by Bodi Chemical Reagent Corp. (Tianjin, China). Stannous chloride (SnCl2·2H2O) was provided by Jinshan Chemical Reagent Corp. (Chengdu, China), ethanol and phosphoryl trichloride (POCl3) were provided by Kelong Chemical Reagent Corp. (Chengdu, China). Trichloromethane (CHCl3) was provided by Changlian Chemical Corp. (Chengdu, China). PLA resin (Mw = 105 g/mol) was provided by Nature

Synthesis and structural characterization of phosphorus-containing PLA (PPLA)

1,4-Butanediol was used as an initiator for polymerization of lactic acid in the presence of stannous chloride, as shown in Scheme 1, so as to obtain dihydroxyl terminated pre-poly(lactic acid) (pre-PLA) with Mn of 0.8 × 104 g/mol (Mw = 1.1 × 104 g/mol, PDI = 1.4), which would benefit the conduction of the following chain-extending reaction. The molecular weight data were measured by GPC. To determine the optimal conditions for preparing phosphorus-containing PLA (PPLA) via chain-extending

Conclusions

A novel approach to the preparation of flame-retardant PLA is pioneered in this study. An inherently flame-retardant poly(lactic acid) has been synthesized successfully via the chain-extending reactions of dihydroxyl terminated pre-poly(lactic acid) using ethyl phosphorodichloridate as chain extender. The flame-retardant element phosphorus is incorporated into the backbone of PLA macromolecules. The resulting phosphorus-containing PLA (PPLA) exhibits excellent flame retardancy compared with

Acknowledgements

This research was supported by the National Science Foundation of China (50703026), the Key Project of the National Science Foundation of China (50933005), China Postdoctoral Science Foundation funded project (20080440182, 200902615) and the International Foundation for Science (IFS, F/4285-2).

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