Nanocomposites combustion peculiarities. A case history: Polylactide-clays
Graphical abstract
Highlights
► The presence of clay in the PLA matrix favours ignition. ► The ignition is attributed to catalysed oxidation of the gases generated by PLA decomposition. ► The catalytic effect avoids the unstable burning stage in LOI nanocomposite testing. ► The presence of clay decreases PLA combustion rate both in the cone calorimeter and in LOI apparatus.
Introduction
A few papers are devoted to flame retardancy of polyesters from renewable resources [1], which mostly concern polylactide (PLA). The reason is that early applications of biosourced polyesters were designed for disposable materials (e.g. packaging) which do not need to be flame retarded. Now the markets turn to durable biobased polymer materials which are expected to substitute fossile sourced commodity plastics in sectors such as transportation, construction and electrical/electronic applications in which fire risk is a relevant issue and flame retardancy is required [2].
The addition of a small amount of well dispersed clay (exfoliated/intercalated) has been shown to reduce the rate of combustion in forced combustion test in the case of many polymers [3], [4], [5] including PLA [6]. A reduction by 40% of peak of heat release rate (pHRR) in the oxygen consumption calorimeter (“cone calorimeter”) test is obtained when about 3–5 wt% of clay platelets are well dispersed inside PLA [7], [8].
The suggested mechanism by which clay nanocomposites act as fire retardants, involves the formation of a char that, together with coalescing clay lamellas, serves as an effective barrier to both mass and heat transfer between flame and polymer. Fukushima et al. [9] reported that charring was observed on burning well-dispersed PLA organomodified montmorillonite nanocomposites, which is absent in pure PLA, confirming the hypothesis concerning the formation of the carbonised inorganic protective surface layer during burning.
On the other hand, performance of polymer materials and nanocomposites in a specific fire test, strongly depends on test scenario, i.e. on the fire model. At present, the fire retardant scientific community takes advantage of basically three fire tests: Limiting Oxygen Index (LOI), Underwriters Laboratories UL 94 vertical test and Cone Calorimeter. LOI and UL94 are generally referred to as “flammability” tests, in which the behaviour of the material exposed to a small flame is addressed, in terms of capability to ignite and to self-sustain a flame, thus representing a scenario in which the material is at the origin of a fire. The cone calorimeter test, referred to as a “combustion test” is representative of a forced combustion, in which the material is burned under controlled external heat flux and supplies time evolution of a full set of combustion parameters, which characterise the material combustion when exposed to a fire started elsewhere.
Taking into account the different fire scenario of the three tests, it is expected that they may deliver different results for the comparison of a given fire retarded formulation with the reference material. With polymer nanocomposites, the differences in performance obtained in flammability and forced combustion test are usually very significant, this having caused an ongoing discussion on the actual effectiveness of nanoparticles as fire retardants [10], [11], [12], [13]. The consequence of these facts is twofold: on the one hand, the need of understanding the relevance of different fire tests to real fire scenarios to which the material may be exposed becomes crucial for the final application of polymer nanocomposites and on the other hand, the scientific significance of standard tests must be carefully evaluated.
In this paper the effects of several factors such as fire retardant additive, clay type, concentration and dispersion degree on the reaction to fire of PLA are studied. These effects, measured by cone calorimeter, UL94V and LOI, are discussed in terms of peculiarity of nanocomposites combustion mechanism. In particular, it will be shown that by extending the use of the Oxygen Index test beyond the simple LOI quotation, very useful information on polymer materials burning process can be acquired.
Section snippets
Materials
Polylactide was a commercial grade supplied by Nature Works (PLA, 3051D). Flamestab NOR 116, an N-alkoxy hindered amine which acts as a flame retardant and flame retardant synergist in polyolefins applications, was supplied by Ciba specialty chemicals.
Commercial organomodified montmorillonite Cloisite 30B (Cl30B) and Cloisite 20A (Cl20A) were supplied by Southern Clay.
The PLA/Talc composite named PLA-MT contains 5.0 wt% of talc, preparation and full characterization was reported elsewhere [14].
Morphology
The XRD curves for PLA and Cl20A or Cl30B nanocomposites are shown respectively in Fig. 1a and b.
PLA is characterised by a broad diffraction peak approximately centred at 2θ = 16°, typical of an amorphous structure. Cl20A is characterised by a diffraction peak at 2θ = 3.5° [15] corresponding to an interlayer distance (d001) related to the presence of the organic modifier molecules between clay layers and accounting for a d001 = 2.5 nm. When Cl20A is blended with PLA, a shift of the main clay
Conclusions
The results of this work show that the reliable and complete assessment of the combustion behaviour of nanocomposites by ranking tests such as UL94 and LOI requires a more detailed evaluation of combustion data supplied by the testing apparatuses than with pure polymers, owing to the fact that polymers and nanocomposites burn in a completely different way. This prevents the evaluation of the relative reaction to fire behaviour by a single parameter such as for example the UL94 ranking or the
References (24)
- et al.
Processing and nanodispersion: a quantitative approach for polylactide nanocomposite
Polymer Test
(2008) - et al.
New trends in polylactide (PLA)-based materials: “Green” PLA–Calcium sulfate (nano)composites tailored with flame retardant properties
Polymer Degrad Stab
(2010) - et al.
Effect of expanded graphite/layered-silicate clay on thermal, mechanical and fire retardant properties of poly(lactic acid)
Polymer Degrad Stab
(2010) - et al.
Catalytic fire retardant nanocomposites
Polymer Degrad Stab
(2008) - et al.
Nanosize and microsize clay effects on the kinetics of the thermal degradation of polylactides
Polymer Degrad Stab
(2009) - et al.
New nanocomposite materials based on plasticized poly(l-lactide) and organo-modified montmorillonites: thermal and morphological study
Polymer
(2003) - et al.
Polyethylene thermal oxidative stabilisation in carbon nanotubes based nanocomposites
Euro Polymer J
(2007) - et al.
Flame retardancy of polylactide: an overview
Polymer Chem
(2010) - Shen L, Haufe J, Patel MK. Product overview and market projection of emerging biobased plastics (PROBIP 2009). In:...
- et al.
Cone calorimeter combustion and gasification studies of polymer layered silicate nanocomposites
Chem Mater
(2002)
Flammability properties of polymer-layered-silicate nanocomposites. Polypropylene and polystyrene nanocomposites
Chem Mater
Poly-1-butene/clay nanocomposite effect of compatibilizers on thermal and fire retardant properties
Polymers Advan Technol
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