Improved Impact Properties in Poly(lactic acid) (PLA) Blends Containing Cellulose Acetate (CA) Prepared by Reactive Extrusion

Abstract

Poly(lactic acid)/triacetine plasticized cellulose acetate (PLA/pCA) blends were prepared by extrusion at two different temperatures and tetrabutylammonium tetraphenyl borate (TBATPB) was added as a transesterification catalyst to reactively promote the formation of PLA-CA copolymer during the reactive extrusion. The occurrence of chain scission in the PLA phase and branching/crosslinking in the CA phase in the presence of TBATPB, resulting also in a darkening of the material, were demonstrated by studying torque measurements and by performing proper thermogravimetric tests on CA with the different additives. Tensile and impact tests onto the blends prepared at the lower temperature showed better properties than the ones obtained at a higher temperature. Then, the mechanical properties of PLA/plasticized cellulose acetate (pCA) blends prepared at the lower temperature were investigated as a function of the content of plasticized CA in the blend. A range of compositions was observed where blends exhibited improved impact properties with respect to pure PLA without a significant decrease in their elastic modulus. The study of the phase morphology of the blends revealed that the occurrence of reactive compatibilization did not significantly affect the phase distribution. In general, fibrillar CA particles were formed in the PLA matrix during extrusion, thus allowing the preparation of CA fibre reinforced composites. The trend of morphology as a function of the composition and processing conditions was then discussed by considering the evolution of phase morphology in immiscible polymer blends.

Keywords: cellulose acetate; impact properties; phase morphology; poly(lactic acid), reactive extrusion.

Figure 1

Trends of torque as a function of time recorded for PLA/pCA blends prepared at 230 °C and 197 °C with or without TBATPB. N*cm = N·cm.

Figure 2

Specimens’ color after extrusion and injection moulding.

Figure 3

Torque as a function of time: (a) for the blends prepared with pure PLA-based samples; (b) for the blends prepared with pCA-based samples. N*cm = N·cm.

Figure 4

Thermogravimetry experiments on CA samples: (a) mass loss as a function of temperature; (b) enlargement in the 50–325 °C range of the derivative curve to show mass loss inflection points.

Figure 5

Torque measurements of blends prepared at 197 °C (a) as a function of time; (b) torque at 180 s as a function of pCA content. N*cm = N·cm.

Figure 6

Charpy impact strength as a function of pCA content in the blends. The phase morphology as a function of pCA content in PLA/pCA blends is also indicated.

Figure 7

Tensile properties of blends as a function of content of pCA: (a) elastic modulus; (b) tensile strength; (c) elongation at break.

Figure 8

Scanning electron micrographs related to the PLA/pCA blends prepared at 197 °C with and without TBATPB.

Figure 9

Scanning electron micrographs where single fibres are indicated by circles. The micrographs are related to the PLA/pCA 85/15 and 75/25 blends prepared at 197 °C.

Figure 10

Pictures of residues after dissolution tests carried out on extruded strands of PLA/pCA blends obtained with or without TBATPB.

Figure 11

Dissolved cellulose acetate ratio as a function of the percentage of pCA in the blends.

Figure 12

Scanning electron micrographs related to the PLA/pCA blends prepared at 230 °C with or without TBATPB.