Rheological and Tribological Properties of Hydroxypropyl Methylcellulose-Fucoidan Mixtures: Effect of Fucoidan Concentration and Salt

Our study sought to investigate the effect of fucoidan concentration (0–2.0%) on rheological and tribological properties of hydroxypropyl methylcellulose (HPMC) at 2.0%. Additionally, to examine the effect of salts (NaCl, KCl, and CaCl₂) on the tribo-rheological properties of HPMC-fucoidan mixtures, a mixture containing equal concentrations of each biopolymer (2.0%) was used. A mixture with 0.5% fucoidan exhibited a lower flow behavior index value (n, 0.62) than HPMC alone but the n value increased with increasing fucoidan concentration. The apparent viscosity at 10 s⁻¹ (η_a,10) for the mixture at 0.5% fucoidan was higher than HPMC alone but decreased at a higher fucoidan concentration (0.5–2.0%). Conversely, the η_a,100 value increased as fucoidan concentration increased. In contrast, regardless of salt type, apparent viscosity decreased with salt addition. All mixtures exhibited tan δ values greater than 1, indicating liquid-like properties. With increasing fucoidan concentration, elastic (G') and viscous (G") moduli increased and the tan δ value decreased. The mixtures with monovalent salts exhibited lower viscoelastic moduli values than that without salt, whereas the mixture with CaCl₂ presented a higher G' value and lower G'' and tan δ values. The mixtures exhibited a lower maximum friction coefficient (μ₁) value with a higher fucoidan concentration, in addition to a shortening of the mixed regime. Conversely, the mixed regime was extended with salt addition, and the monovalent salt induced an increase in the μ₁ value. Our findings indicated that there was viscoelastic and lubrication synergism between the polymers, which was influenced by salt addition.