Revealing the impact of ageing on a flame retarded PLA
Introduction
The use of synthetic polymers has grown during the past decades [1] and is forecasted to grow to 1 million kt/year by 2100. Therefore, a move from fossil-based polymers to new polymers exhibiting further properties, especially bio-based polymers, would be appropriate. The worldwide capacity of bio-based plastics, according to company announcements, will increase from 0.36 Mt in 2007 to 3.45 Mt in 2020 [2]. It is projected that the most important representatives by 2020 will be starch based plastics (1.3 Mt), polylactic acid (PLA) (0.8 Mt) and bio-based polyethylene (PE) (0.6 Mt) [2]. Of the above mentioned bio-based plastics, PLA has drawn attention these last years as bio-sourced polymeric material [2], [3]. This bio-based polymer appears as an interesting material in several application areas, particularly in packaging, E&E and automotive. PLA is very easy to process and its production requires 20–50% less fossil fuel resources than the production of petroleum-based polymers [2], [3].
However, to be used in certain sectors (e.g. E&E), PLA has to be flame retarded and its flame retardant (FR) properties as well as its properties of use must be kept for the product lifetime. That is why the interest for flame retarding PLA kept growing these past few years (Fig. 1); more than 110 papers are dedicated to FR-PLA in 2014.
Flame retarded PLA can be obtained by incorporating conventional flame retardants (phosphates or phosphinates) at relatively low loading (1–10 wt.-%) [4], [5], [6] which can be combined with nanoparticles (i.e. organomodified montmorillonite) [7], [8].
Focusing on ageing, numerous studies can be found in the literature concerning the effect of temperature and/or relative humidity on the degradation of neat PLA [9], [10], [11], [12], [13]. The degradation of a PLA matrix containing fillers (sepiolite, fluorohectorite …) has also been investigated [9], [14], [15], [16]. It was demonstrated that PLA is extremely sensitive to hydrolysis phenomenon, which is accelerated when temperature increases (from 30 to 60 °C) [12]. Hydrolysis of PLA involves the scission of ester bonds leading to the depolymerization and releases of hydrolyzed monomers from the polymers.
Most of the time, this phenomenon leads to a decrease (i) of the molecular mass of the material, (ii) of its glass transition temperature and (iii) of the mechanical properties of the polymer [12], [16]. The incorporation of fillers such as organomodified montmorillonite could increase the degradation of the PLA matrix [14]. It was reported the relative hydrophilicity of the clay might play a crucial role in the hydrolytic degradation by accelerating the process [14]. Moreover, organomodified montmorillonite is sensitive to the melting process when incorporated into a polymer matrix (degradation of the surfactant). Conventional surfactants of montmorillonite are alkylammonium and they can undergo Hofmann decomposition [17] around 200 °C which can catalyze the polymer decomposition [18], [19], [20], [21], [22], [23].
Only few papers deal with the ageing (thermal, water/moisture, UV irradiation/weathering) of flame retarded materials [24], [25], [26], [27], [28], [29], [30], [31]. Zuo et al. [30] reported the thermal-oxidative ageing of tris(tribromophenyl) cyanurate Polyamide 6 (PA6)/long glass fiber (LGF) with different exposure times at 160 °C. Authors demonstrated the existence after ageing of a surface migration effect of the flame retardant. Due to this FR migration, FR/PA6/LGF composites exhibited a more protective char layer structure leading to better Limiting Oxygen Index (LOI) values and excellent UL-94 ratings. Other researchers such as Oztekin et al. [25] recently highlighted the influence of water/moisture on the flame retardancy of poly(ether ether ketone) (PEEK) studied in cone calorimeter. When a small amount of moisture is present in PEEK, the water is released just after the polymer melts (343 °C). This release leads to vigorous bubbling and results in shorter time to ignition (from 207 to 110 s at an irradiance of 50 kW/m2) but lower pHRR (from 355 to 280 kW/m2). Levchik et al. [31] also studied the impact of hydrolytic degradation induced by artificial ageing on the fire performance of flame retarded polycarbonate (PC)/acrylonitrile butadiene styrene (ABS) blends. Three aryl phosphates, triphenyl phosphate (TPP), resorcinol bis(diphenyl phosphate) (RDP) and bisphenol A bis(diphenyl phosphate) (BDP) were tested as flame retardants. They demonstrated that these three additives show comparable FR efficiency at the same phosphorus level. However, under artificial ageing at high temperature and in the presence of water, authors established that hydrolysis of aryl phosphates leads to the release of acidic species, which attack the polycarbonate. It results in a decrease of the fire retardant properties of the PC/ABS blend compared to those before artificial ageing.
In all the works reported above, the effect of accelerated ageing on the FR properties was mainly reported, but the authors do not establish clear relationships between these properties and changes of the thermo-physical properties of the polymers during ageing process. This relationship is however a necessary criterion to elucidate the ageing mechanism and thus to design long-term durable flame retardant formulations. A recent study of our group reported the effectiveness of the combination of melamine, ammonium polyphosphate (APP) and organomodified montmorillonite (C30B) on FR properties of PLA. In this paper we investigated the effect of accelerated ageing on such FR-PLA.
A pristine PLA, a PLA fire retarded with ammonium polyphosphate and melamine (FR-PLA), and a PLA fire retarded with ammonium polyphosphate, melamine and organoclay Cloisite 30B (FR-PLA-C30B) were subjected to accelerated ageing at 50 °C and 75% relative humidity for 2 months. Fire performances of the materials were assessed by Limiting Oxygen Index (LOI) and Mass Loss Calorimeter (MLC). The effects of ageing on (i) the samples morphology, (ii) the glass transition and melting temperatures, (iii) the molecular mass and (iv) the dynamic viscosity of the three formulations were evaluated using different techniques, including Electron Probe Micro Analysis (EPMA), Solid-state Nuclear Magnetic Resonance (NMR), Differential Scanning Calorimetry (DSC), Gel Permeation Chromatography (GPC) or Melt Flow Index (MFI), respectively.
Section snippets
Materials
PLA (residual monomer content = 0.2%, d-isomer content = 1.5%, density = 1.24 g/cm3, melting point = 155–170 °C, number average molecular mass (Mn) = 82,550 g/mol determined by gel permeation chromatography) was supplied by NatureWorks LLC (Ravago distribution center, Arendonk, Belgium), conserved in sealed bags to avoid pre-degradation and dried overnight at 70 °C before use.
The flame retardants (FRs) used were: Melamine (99%) obtained from Sigma–Aldrich (St. Louis, MO). APP (Exolit AP 422,
Flame retardant properties
The impact of ageing on the neat PLA and the two flame retarded was firstly visually evaluated. The numerical pictures of PLA and FR-PLA undergoing 0 day, 30 days and 60 days of ageing at 50 °C and 75% relative humidity are shown in Fig. 2. No real difference is noticeable between PLA and FR-PLA as function of ageing.
FR-PLA-C30B depicted in Fig. 3 shows a different behavior. Indeed, in presence of Cloisite 30B, white spots appear at the surface of the samples from 46 days (Fig. 3 (c)) until
Conclusion
This paper reported novel studies on the ageing of flame retarded PLAs. It was shown that a 2 month exposure at 50 °C and 75% relative humidity (T/RH conditions) had a dramatic impact on various properties of flame retarded PLA formulations. Indeed, results of analyses indicated a significant decrease of the molecular mass of the materials. According to literature, chain scission is the driving phenomenon in the hydrolysis of polyesters.
The addition of FR additives with or without Cloisite
Acknowledgments
The authors would like to acknowledge the University of Lille 1 for the funding of the PhD thesis and the FEDER (European Funds for Regional Development) for the funding of the EPMA instrument. They are also indebted to Ms. Aurélie Malfait, Dr. Valérie Miri and Dr. Gregory Stoclet from the UMET for skillful assistance and discussion, as well as to Dr. Pierre-Olivier Bussière, Dr. Sandrine Therias and Prof. Jean-Luc Gardette from ICCF (Clermont Ferrand) for helpful discussion.
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