Novel pathway to produce high molecular weight kraft lignin-acrylic acid polymers in acidic suspension systems

Abstract

Kraft lignin (KL) produced in kraft pulping process has a low molecular weight and solubility, which limits its application in industry. For the first time, KL was polymerized with acrylic acid (AA) in an acidic aqueous suspension system to produce a water soluble lignin-AA polymer with a high molecular weight in this work. The polymerization reaction was carried out using K₂S₂O₈ as an initiator, and the influence of reaction conditions on the carboxylate group content and molecular weight of resultant lignin polymers was systematically investigated. The mechanism of polymerization of KL and AA was discussed fundamentally. The resulting lignin-AA polymer was characterized by Fourier Transform Infrared spectrophotometry (FTIR), proton nuclear magnetic resonance (¹H-NMR) and elemental analyses. The results showed that the phenolic hydroxyl group (Ph-OH) content of KL promoted the polymerization under an acidic environment. Under the conditions of 1.5 wt% of initiator, 3.5 of pH, 10.0 of AA/lignin molar ratio, 0.15 mol L^-1 of lignin concentration, 3 h and 80 °C, the carboxylate group content and the molecular weight of the polymer were 7.37 mmol g^-1 and 7.4 × 10⁵ g mol^-1, respectively. The lignin-AA polymer was water soluble at a 10 g L^-1 concentration and a pH higher than 4.5. Furthermore, the flocculation performance of lignin-AA polymer in an aluminium oxide suspension was evaluated. Compared with polyAA, the lignin-AA polymer was a more efficient flocculant for aluminium oxide suspension, which shows its potential to be used as a green flocculant in industry.

Fig. 1. AA conversion in to produce PAA at pH 3.5 and 10.5 as function of reaction time.

Fig. 2. The distribution of molecular weight of PAA from KL–AA in the presence and absence of KL.

Fig. 3. AA conversion in KL–AA system and AA system as function of reaction time.

Scheme 1. Proposed reaction scheme of polymerization of KL and AA initiated by K₂S₂O₈ under acidic conditions.

Fig. 4. Relationship between Ph-OH group of KL and increased carboxylate group of KL–AA polymer.

Fig. 5. Carboxylate group content, molecular weight and hydrodynamic diameter of lignin–AA polymer as a function of initiator dosage (pH 3.5, lignin concentration 0.15 mol L⁻¹, AA/lignin molar ratio 5.5, 80 °C, 3 h).

Fig. 6. Carboxylate group content and molecular weight of lignin–AA polymer as function of reaction time (pH, 3.5, initiator, 1.5 wt%, lignin concentration, 0.15 mol L⁻¹, AA/lignin molar ratio, 5.5, 80 °C).

Fig. 7. Carboxylate group content and molecular weight of lignin–AA polymer as a function of AA/lignin molar ratio (pH, 3.5, initiator, 1.5 wt%, lignin concentration, 0.15 mol L⁻¹, 3 h, 80 °C).

Fig. 8. Carboxylate group content and molecular weight of lignin–AA polymer as a function of temperature (pH, 3.5, initiator, 1.5 wt%, lignin concentration, 0.15 mol L⁻¹, AA/lignin molar ratio, 8.0, 3 h).

Fig. 9. Carboxylate group content and molecular weight of lignin–AA polymer as a function of lignin concentration (pH, 3.5, initiator, 1.5 wt%, AA/lignin molar ratio, 10.0, 3 h, 80 °C).

Fig. 10. Relationship between carboxylate group content of lignin–AA polymer with its molecular weight (based on 1.5 wt% initiator).

Fig. 11. The FTIR spectra of KL–AA polymers and KL.

Fig. 12. H-NMR spectra of KL and KL–AA polymer.

Fig. 13. The solubility of KL–AA polymer and KL as a function of pH at 10 g L⁻¹ concentration.

Fig. 14. Flocculation performance of KL–AA polymer at different concentrations and pHs for aluminium oxide suspension.