Poly(lactic acid)/pulp fiber composites : The effect of fiber surface modification and hydrothermal aging on viscoelastic and strength properties

Poly(lactic acid) (PLA)/kraft pulp fiber (30 wt%) composites were prepared with and without a coupling agent (epoxidized linseed oil, ELO, 1.5 wt%) by injection molding. The non-annealed composite samples, along with lean PLA, were exposed to two hydro-thermal conditions: cyclic 50% RH/90% RH at 23 and 50°C, both up to 42 days. The aging effects were observed by size exclusion chromatography, differential scanning calorimetry, dynamic and tensile mechanical analysis, and fracture surface imaging. ELO temporarily accelerated the material's internal transition from viscous to an increasingly elastic response during the aging at 50°C. ELO also slowed down the tensile strength reduction of the composites at 50°C. These observations were explained with the hydrophobic ELO molecules' coupling and plasticizing effects at fiber/matrix interfaces. No effects were observed at 23°C.CODEN: JAPNA; Funding details: Teknologian Tutkimuskeskus VTT; Funding text 1: The authors would like to thank Mr. Upi Anttila (VTT) and Mr. Eino Sivonen (VTT) for the preparation of the composite samples, and a number people at RISE Bioeconomy; Dr. Kristoffer Segerholm, Dr. Prashanth Srinivasa, Mr. Henrik Pettersson, Mr. Jens Haraldsson, Ms. Anna Östberg, Ms. Erika Back, for their help executing the experimental work in Stockholm in the spring 2019. The authors gratefully acknowledge the funding from VTT and RISE.