Unveiling of mechanical, morphological, and thermal characteristics of alkali-treated flax and pine cone fiber-reinforced polylactic acid (PLA) composites: fabrication and characterizations

Recently, there has been a resurgence of interest in biomaterials with an emphasis on biodegradability due to the crucial conversation regarding recycling and the preservation of natural resources. In this research work, the natural fiber reinforced with polylactic acid (PLA) composites was fabricated using the compression molding technique. The flax and pine cone fibers are the reinforcements, whereas PLA is the binding material. In this investigation, composites were fabricated in three combinations, 90:10, 80:20, and 70:30 matrix-reinforcement ratios. An alkaline treatment was carried out to get rid of lignin—hemicellulose and cellulose from the outer surface of the natural fibers. The FTIR and TGA studies revealed that the alkaline treatment was successful. The presence of cellulose with free OH groups is substantially evidenced by the broad band that arose at approximately 3300 cm⁻¹. Conversely, the C-H absorption band can be observed at 2900 cm⁻¹. The NaOH treatment of the treated fibers results in the disappearance of the peak at 1800 cm⁻¹, which is indicative of a reduction in lignin, and hemicellulosic content. Thermal degradation of both fibers occurred from 40 to 700 °C. In Fig. 9a, the flax and pine cone TGA plot shows two major degradation temperature peaks or two crucial degradation phases of natural fibers. When closely examined, stage 1 is 260–290 °C, and stage 2 is 380–400 °C. The second shoulder peak reflects cellulose, and lignin breakdown, whereas the first shoulder peak represents hemicellulose degradation. There were 72 and 76 wt% weight losses from flax and pine cone fiber degradation. Both samples decompose completely at 490–510 °C, as shown in Fig. 9a. The samples exhibited considerable deterioration beyond 250 °C at the start. The thermometric graphs revealed a continual reduction in sample thermal stability above this temperature. The initial step (260–290 °C) involved thermal degradation of hemicellulose, and pectin, comparable to fiber loading but with a 35 °C heat absorption increase. Stage 2 occurred at 294–390 °C. The third stage (390–429 °C) involved lignin breakdown and illustrated a 28 °C rise in heat absorption relative to unmodified samples. Complete combustion and ash conversion occurred in stage four. The scanning electron microscope analysis reveals that the alkaline treatment has produced a better surface roughness for the fibers. As far as mechanical properties are concerned, the composite, a combination with an 80:20 matrix-reinforcement ratio having 14% flax and 6% pine cone fibers, has exhibited higher tensile strength as compared to others.