Cold Sintering Process (CSP) was employed to densify hydroxyapatite (HAp) using phosphoric acid (H₃PO₄) as a transient liquid phase at low temperature. HAp powders synthesized by aqueous precipitation were CSP-processed at 200 °C under 600 MPa for 30 min with H₃PO₄ contents of 5 or 10 wt% at 1 or 2 M. Apparent density (Archimedes), biaxial flexural strength (three-ball method, ABNT NBR ISO 6872), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to correlate processing, microstructure, and properties. Despite the low thermal budget, CSP achieved apparent densities of 2.44–2.55 g cm⁻³, corresponding to 77.64–84.21% of the theoretical density. The 5%–2 M condition reached the highest densification (84.21%), whereas 10%–1 M delivered the best mechanical performance (σ_f = 36.08 ± 8.88 MPa), indicating that strength is not governed by densification alone. XRD confirmed predominance of the HAp phase (ICDD 00-009-0432) for all groups; average crystallite sizes ranged from 34.35 to 56.92 nm, with specific surface area increasing as crystallite size decreased (up to 87.53 m² g⁻¹). SEM revealed a microstructural evolution consistent with dissolution–reprecipitation: from porous, weakly coalesced networks (5%–1 M) to denser, better-bridged grains (10%–1 M), while excessive acidity (10%–2 M) promoted local fragility. Overall, tailoring the chemistry of the transient liquid phase enables efficient, phase-preserving, and energy-saving densification of HAp via CSP, offering a viable route for bioceramics where low processing temperatures and controlled microstructures are required.

For composite production, commercial corn starch plasticized with 30% glycerol was used. Ubim fibers were sourced from the local market in Belém (PA) and subjected to peeling and milling processes to optimize adhesion to the polymer matrix. The composites were processed using a single-screw extruder in five TPS/fiber ratios (0, 5, 10 and 15 wt.%). Films and test specimens were molded by hot pressing under standardized parameters. The composites were characterized through density, hardness (ASTM D2240), tensile strength (ASTM D638), and impact tests, as well as microstructural analyses by scanning electron microscopy (SEM) and phase evaluation by X-ray diffraction (XRD).

The results showed that the addition of ubim fibers to the thermoplastic starch composites significantly increased tensile strength, demonstrating the effectiveness of natural reinforcement in enhancing the mechanical properties of the polymer matrix. SEM analyses revealed morphological changes, highlighting good interfacial adhesion between the ubim fibers and TPS, which is essential for efficient stress transfer. XRD indicated the presence of semi-crystalline structures influenced by fiber incorporation. These findings confirm that the use of natural fibers, such as ubim, is a promising strategy for developing biodegradable composites with improved performance. Such materials exhibit high potential for sustainable plastic packaging applications, combining mechanical performance with reduced environmental impact.