The effect of densification temperature and paulownia wood proportion on mechanical and physical properties of pellets

Paulownia wood (PW) is a potential raw material for the production of densified products, such as pellets, provided that its influence on their physical and mechanical properties is well understood. This study evaluated the effects of PW proportion and densification temperature on pellet density, compression ratio (CR), spring-back (SB), diametrical compression strength (DCS), and extraction load, using Abies borisii-regis wood (AW) as a reference material. A full-factorial experimental design was employed with five PW:AW mixtures (0:100, 25:75, 50:50, 75:25, and 100:0) and three densification temperatures (90°C, 120°C, and 150°C). PW proportion had a stronger effect on pellet density than temperature, with density increasing progressively as PW content increased. At the highest tested densification temperature (150°C), a higher PW proportion is needed to maintain high density. Pellet CR increased consistently with PW proportion across all temperatures, while temperature was the most significant factor for the pellets’ mechanical strength. PW proportion also had a significant, but much smaller effect, while the interaction of temperature and PW proportion was also significant. The use of PW may increase energy demand during densification. At lower temperatures, a higher force is required to overcome die-wall friction, whereas higher temperatures reduce this resistance and improve process efficiency. Furthermore, spring-back decreased with increasing PW proportion, while temperature had no significant effect, and both factors acted largely independently. Overall, the enhanced compressibility, increased density, and reduced spring-back highlight the potential of PW for use in pellets or other densified products, although further validation under standardised conditions and industrial-scale processing is recommended.