Inherently flame-retardant biopolymer materials via reactive processing

A novel and promising route towards inherently flame-retardant polymeric materials has recently been developed by the Laboratory of Polymeric and Composite Materials (Belgium). The process is here discussed as a positive response to fire sector pressure and expectations with providing a solution to both industrial and societal concerns driven by climate changes.

The climate-change related world-wide pressure for biobased materials brings to the market new polymers and polymer materials which mechanical properties are equal or superior when compared to commodity plastics. However, their flame-retardancy is still far from being aligned. In addition, the pressure on the fire sector to change additives and adapt compositions with replacing conventional additives with less or non-toxic and of biobased origin has never been greater. These all create the need to provide new and innovative methods adapted to both industrial and societal needs.

With this respect, the Laboratory of Polymeric and Composite Materials (LPCM) at the University of Mons and Materia Nova Research center (Belgium), develops sustainable solutions for flame-retardant polymer and composite materials for over 20 years. The team invests in developing (1) polymerization and modification routes adapted to molecules of biological origin, (2) intensification of these routes, and (3) offers new biobased materials with improved flame-retardant properties and biocompatibility; and provides a globalized response to universal fare-safety problems [1-7].

The LPCM route towards inherently flame-retardant PLA.

A very recent example [1], able to satisfy almost all requirements:

Biobased origin;
Non-toxic compounds;
Innovative and industrially relevant continuous processes;

reflects the development of inherently flame-retardant biopolymers and materials therefrom via intensified (continuous) production pathway.

Poly(lactide) (PLA) – the first synthetic biopolymer on the market – was the model polymer of biobased origin. Indeed, nowadays PLA is produced from annually renewable resources in large volumes at relatively low cost and finds applications from disposable to durable plastic materials [8]. However, much like other synthetic plastics, PLA has its own inherent weaknesses, more particular its flame-retardant behavior, that prevents it from being widely adopted in many durable applications. Currently, the PLA additivation with various flame-retardant additives (organic phosphorus, intumescent systems, and mineral fillers) is the most used technique.

Amongst all, the organic phosphorus compounds, such as 9,10-dihydro-oxa-10-phosphaphenanthrene-10-oxide (DOPO) active at low phosphorus content of 1 to 2 wt.%, present an efficient way to enhance the flame retardancy of PLA, while keeping low the environmental impact of the system. Indeed, phosphorous presence allows the formation of an insulating layer of crosslinked or carbonized structures during material combustion and protects the underneath from the heat transfer and release of toxic compounds. However, the incorporation of at least 20 wt.% of the additive is required to obtain good flame-retardant performances (V0 classification being the target). However, the polymer itself remains sensitive to thermal degradation during material processing and an important reduction of polymer molecular weight significantly affects mechanical properties.

The LPCM team therefore developed a new technique for DOPO incorporation directly in the structure of the PLA polymer and created an inherently flame-retardant PLA [1]. The process comprises a three-step procedure (see Figure), involving:

The modification of DOPO by a simple reaction with an aromatic ketone, bearing additional functions;
The ring-opening polymerization of lactide upon using the modified DOPO as a difunctional initiator in order to obtain short PLA oligomers (OLA);
The reactive extrusion of the OLAs for chain-extension via isocyanate chemistry to inherently flame-retardant PLA.

The modification of DOPO is relatively fast (3h) and industrially relevant, although some work still needs to be done for improving product yield (give it a rise above 55 %). The new molecule, bearing two primary amines (-NH2) is able to induce simultaneous ring-opening polymerization of two DOPO-connected PLA chains via appropriate catalysis and polymerization conditions. At this second step, “Controlling the length of these oligomers allows modulating the content of phosphorus atoms in the PLA chains in order to control the flame-retardant properties of the final material.”, the Authors say. The use of laboratory-scale miniatures of industrial batch reactors facilitates process up-scaling and ensures connectivity with a continuous flow extruder for the performance of the chain-extension reaction. The only inconvenience at this stage is the low molecular mass of the OLA. Indeed, for inherent flame-retardancy, short macromolecules are needed to ensure enough phosphorus content, but reduce material mechanical strength below the acceptable limit.

Therefore, the well-known and largely used no-residue reaction of diols with diisocyanates is used to extend the length of the inherently flame-retardant OLAs to an acceptable value. At this step, reactive extruder has been used to ensure good blending, reactive cites proximity and heat-transfer at short reaction times (below 30 min).

The inherently flame-retardant PLA is characterized with good transparency, oppositely to the common simple blend with DOPO which is completely opaque. This transparence is related to (1) the amorphous nature of the final material, and (2) the complete absence of DOPO particles, as demonstrated by different analytical tools [1].

Flame-retardant properties demonstrate significantly lower pHRR levels for the new material when compared to a DOPO/PLA blend (with similar DOPO concentration) and the classification of the material as V0 according to the UL-94 test on films of 0.8 mm thickness. However, some more work is needed in order to improve the decrease in thermal stability of the new PLA with comparison to commercial PLA. Nevertheless, this problem can easily be solved in the near future as Authors trust.

For more information, go to https://www.mdpi.com/1996-1944/13/1/13

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