Use of South African spent pulping liquor to synthesise lignin phenol formaldehyde resins

Govender, Priyashnie (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: The probable scarcity of petroleum-based products, the resultant price fluctuations, and environmental concerns have driven the need for a total or partial phenol replacement in phenol-based products. Lignin, a biopolymer found in plant biomass, has great potential as a phenol substitute due to its phenolic structure, abundance as a by-product of the paper and pulp industry, and relatively lower cost and toxicity compared to phenol. However, despite its benefits and availability, lignin is rarely exploited for higher-value applications, largely due to its structural complexity and resultant low reactivity. This study aims to investigate the potential of using lignin derived from South African spent pulping liquor as a phenol substitute in phenol formaldehyde resins (PFRs). Six South African pulping-based lignins were investigated. They were characterized in terms of structural, compositional and thermal properties. Thereafter, they were used to synthesize lignin-phenol formaldehyde resins at 100% phenol substitution, labelled as LPF100 resins. These resins were characterized according to structural, curing and shear bonding strength properties. Direct use (unmodified) of the LPF100 resins as adhesives was labelled as R0 LPF100 adhesives. To improve the shear strength properties of the unmodified LPF100 adhesives, the LPF100 resins were modified via the addition of a crosslinker (hexamine) as well as the hardener glyoxal (R1) or the hardener epichlorohydrin (R2). They were labelled as R1 LPF100 and R2 LPF100 adhesives, respectively. The objective of this study was to investigate/determine the extent to which various South African pulping lignins were suitable for replacement of phenol in PFRs for use in the wood industry, specifically plywood boards. The LPF100 resins produced in this study are not expected to fulfil commercial requirements. Instead the intent is to determine the potential of these South African pulping lignins, and show that further research is needed to elevate it to a viable level. Ultimately, from the modified LPF100 adhesives, the best performing were the R1 KF2-P-N LPF100 adhesive and the R2 SL-E-T LPF200 adhesive, both recording 1.4 MPa of shear strength, thus exceeding the GB/T 17657-2013 plywood standard of ≥0.7 MPa. The curing temperature of these two resins are 71°C and 126°C, respectively. Thus, considering both curing and bonding properties, KF2-P-N lignin is a more promising phenol substitute. However, the S-SCB-S resin was a consistent performer, even recording the highest shear strength from the unmodified adhesives (0.5 MPa). Additionally, the S-SCB-S resin had the highest curing rate from all resins samples, and the lowest curing temperature of 68°C. Thus, both the KF2-P-N (pine kraft) and S-SCB-S (bagasse soda) lignins show great potential as a phenol substitute in phenol formaldehyde resins.

AFRIKAANSE OPSOMMING: Die waarskynlike skaarsheid van petroleum-gebaseerde produkte, die resultante prysfluktuasies, en omgewingskommer, het die nood vir ’n totaal of gedeeltelike fenolvervanging in fenol-gebaseerde produkte gedryf. Lignien, ’n biopolimeer gevind in plantbiomassa, het groot potensiaal as ’n fenolplaasvervanger as gevolg van sy fenoliese struktuur, oorvloed as ’n byproduk van die papier-en-pulp-industrie, en relatiewe lae koste en toksisiteit in vergelyking met fenol. Maar ongeag sy voordele en beskikbaarheid, word lignien selde ontgin vir hoër-waarde toepassings, grootliks as gevolg van sy strukturele kompleksiteit en resulterende lae reaktiwiteit. Hierdie studie beoog om die potensiaal vir die gebruik van lignien, verkry uit Suid-Afrikaanse gebruikte pulploog, as ’n fenolplaasvervanger in fenolformaldehiedharpuis, te ondersoek. Ses Suid-Afrikaanse pulp-gebaseerde ligniene is ondersoek. Hulle is gekarakteriseer in terme van strukturele, komposisionele en termiese eienskappe. Daarna is hulle gebruik om lignien-fenolformaldehiedharpuis te sintetiseer by 100% fenolvervanging, gemerk as LPF100-harpuis. Hierdie harpuis is gekarakteriseer na aanleiding van strukturele, droging- en skuifverbindingkrageienskappe. Direkte gebruik (onveranderd) van die LPF100-harpuis as kleefstof is gemerk as R0 LPF100-kleefstowwe. Om die skuifkrageienskappe van die onveranderde LPF100-kleefstowwe te verbeter, is die LPF100-harpuis ongemodifiseer via die toevoeging van ’n kruisskakel (heksamien) sowel as die verharder glioksaal (R1) of die verharder epichloorhedrien (R2). Hulle is gemerk as R1 LPF100- en R2 LPF 100-kleefstowwe, onderskeidelik. Die doelwit van hierdie studie was om die omvang te ondersoek/bepaal waartoe Suid-Afrikaanse pulpligniene gepas is vir vervanging van fenol in fenolformaldehiedharpuis vir die gebruik in die houtindustrie, spesifiek vir saamgeperste planke. Daar word nie verwag dat die LPF100-harpuis geproduseer in hierdie studie aan kommersiële vereistes gaan voldoen nie. Die doel is eerder om die potensiaal van hierdie Suid-Afrikaanse pulpligniene te bepaal, en te wys dat verdere navorsing nodig is om dit tot ’n lewensvatbare vlak op te hef. Eindelik, uit die veranderde LPF100-kleefstowwe, was die wat die beste gedoen het die R1 KF2-P-N LPF100-kleefstof en die R2 SL-E-T LPF200-kleefstof, waar beide 1.4 MPa van skuifkrag opgeteken het, en dus die GB/T 17657-2013-saamgeperste plank standaard van >0.7 MPa oorskry. Die drogingtemperatuur van hierdie twee harpuis is 71 °C en 126 °C, onderskeidelik. Dus, as albei droging- en verbindingeienskappe oorweeg word, is KF2-P-N-lignien ’n meer belowende fenolvervanger. Die S-SCB-S-harpuis was ’n konstante presteerder, wat selfs die hoogste skuifkrag van die onveranderde kleefstowwe (0.5 MPa) aangeteken het. Daarby, het die S-SCB-S-harpuis die hoogste droogtempo van al die harpuissteekproewe gehad, en die laagste droogtemperatuur van 68 °C. Dus, beide die KF2-P-N (dennekraft) en S-SCB-S (bagasse soda)-ligniene het groot potensiaal as ’n fenolvervanger in fenolformaldehiedharpuise aangetoon.

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