The Use of South African Spent Pulping Liquor to Synthesize Lignin Phenol-Formaldehyde Resins

Abstract

This study aims to investigate the potential of using lignin sourced from South African black liquor as a total phenol substitute in phenol-formaldehyde resins (PFRs), with a particular focus on bonding strength and curing properties. Four South African pulping-based lignins were used to synthesize these lignin-phenol formaldehyde resins (LPF100 resins), namely Eucalyptus Kraft lignin, Pine Kraft lignin, Bagasse Soda lignin, and Bagasse Steam Exploded lignin. Fourier-transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry were used to determine structural and curing properties. These resins were then used directly (unmodified) as adhesives to test shear bonding strength (R0 LPF100 adhesives). To improve the bonding properties of the unmodified LPF100 adhesives, the LPF100 resins were modified via the addition of a crosslinker (hexamine) as well as a hardener (either glyoxal, R1, or epichlorohydrin, R2). All R0 LPF100 adhesives fell below the GB/T 17657-2013 plywood standard of ≥0.7 MPa, with the Bagasse Soda LPF100 adhesive recording the highest bonding performance of 0.5 MPa, and the lowest curing temperature of 68°C. From the modified adhesives, the best performing were the Pine Kraft (R1) and the Eucalyptus Kraft (R2) LPF100 adhesives, recording 1.4 and 1.3 MPa, respectively. The curing temperatures of both these resins were 71°C and 80°C, respectively. Ultimately, the results of this study indicated that favorable adhesive properties may be obtained with the use of South African pulping-based lignins as a 100 percent phenol substitute in PFRs.

Lignin Phenol Formaldehyde Resins Synthesised Using South African Spent Pulping Liquor

“…The best NaOH/PL and F/PL ratios were 0.124 and 1 for LPF60 and LPF80, as seen in Figure 7a and Figure 7b. These values were lower compared to LPF100 (0.477 and 3 as shown in Figure 7c), and also low compared to previous reports [20,21,24,38]. This was attributed to phenolation causing an apparent increase in reactive sites [9], adding phenolic hydroxyl groups (Table 1), and cleaving weak ether bonds (Figure 4), thereby requiring less catalyst and formaldehyde to promote the forward reaction [28].…”

“…Sodium lignosulphonate was received as a spray-dried powder (labelled L-S). Previous studies [24] using the same lignosulphonate source have shown that the material had a high content of impurities. It was therefore puri ed using a dialysis membrane aiming to remove the hemicelluloses, simple sugars, and inorganic contaminants present.…”

“…Mid-infrared (mid-IR) spectroscopy analysis was performed on the lignin in powdered form, operated in Attenuated Total Re ectance (ATR) mode at a resolution of 4 cm -1 , with 32 scans per sample within the band region of 4000 to 400 cm -1 [24]. The powder was pressed against the diamond crystal surface, using a Bruker Alpha P ATR-IR instrument.…”

“…Approximately 5 ± 0.5 mg of sample was loaded onto an aluminium crucible which was placed in a TGA pan. A heating rate of 10 °C/min was used to heat the sample from around 20 °C up to 595 °C under argon atmosphere at 70 ml/min [24]. TRIOS software was used to plot the TGA and derivative thermogravimetric (DTG) curves.…”

“…Several studies have investigated the substitution of phenol by lignin at substitution levels below 100% [10,15,[18][19][20][21]. Recently, studies [22][23][24][25] have succeeded to replace phenol to produce lignin formaldehyde (LF) resins at a 100% substitution. However, additives were required during the adhesive formulation to be able to achieve acceptable properties for plywood applications.…”