Cellulose/phosphorus combinations for sustainable fire retarded polylactide

• Effect of cellulose/phosphorus combination on the flame retardant properties of poly(l-lactide).
• Phosphorus was grafted on cellulose surface or added into the matrix as aluminum phytate.
• The charring effect of cellulose was enhanced in the presence of phosphorous.
• The flame retardant effect of cellulose depend on it particles size i.e. micronic vs. nanometric cellulose particles.

In the present work, sustainable flame retardant polylactide (PLA) compositions, containing cellulose–phosphorus combinations, have been studied. The effect of combining MicroCrystalline Cellulose (MCC) or NanoCrystalline Cellulose (NCC) with phosphorus, on thermal stability and flame retardant properties of PLA was investigated by thermogravimetric analysis (TGA), cone calorimeter and UL-94 tests. Phosphorus was introduced either by chemical grafting on cellulose or by coadditive melt blending by using a bio-based phosphorous agent, i.e., aluminum phytate. In both cases, the charring effect of cellulose was enhanced.The phosphorylation process used allowed grafting 16.5 wt% of P as determined by Inductively Coupled Plasma analysis (ICP). However, this process led to the generation of water soluble cellulose and to the loss of the nanometric particle shape. For this reason, the phosphorylation of cellulose has been performed only on the microcrystalline particles. Using 20 wt% of phosphorylated MCC (MCC-P) allowed reaching V0 at UL-94 test but did not result in significant reduction of pHRR. Significant pHRR reduction was obtained only when aluminum phytate was combined with phosphorylated MCC (P-MCC). As far as NCC was concerned, the use of a phosphorylated-NCC was not required since the simple combination of aluminum phytate and NCC allowed reaching significant decrease of the pHRR actually at the level of the value found when MCC-P was used in combination with aluminum phytate. The high specific surface area of NCC proved very useful to promote the formation of a better insulating charred layer.

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