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Since the lignin-carbohydrate complex of lignocellulosic biomass has a recalcitrant
character that is not easy to break down into components, it is necessary to degrade
and/or separate the components by a pretreatment method before used as a source
of lignin or cellulose (Agbor et al. 2011). The pretreatment process to break up the
lignin-carbohydrate complex is an important first step for the effective biochemical
conversion of lignocellulosic biomass into biomaterial or biofuel (Mosier et al.
2005). For the effective pretreatment of lignocellulosic biomass, the increased surface area and porosity, the total or partial modification of the lignin structure, the
depolymerization and removal of hemicellulose, the reduction of cellulose crystallinity, among others, should be performed (Potumarthi et al. 2013; Asada et al.
2015b). In general, pretreatment methods are classified into biological, chemical,
physical and multiple or combinatorial pretreatments (Kurosumi et al. 2008;
Yamashita et al. 2010; Asada et al. 2011; Asada et al. 2015c; Asakawa et al. 2016;
Zhang et al. 2016).
Kishi et al. (2006) studied liquefaction as a wood treatment to recover the lignin
content of the German spruce (Picea abies) for the manufacture of epoxy resins.
First the liquefaction of the wood was carried out, and then the synthesis of epoxy
resins from liquefied wood was made. The authors evaluated the resorcinol (catalyst)/wood ratio and the time of liquefaction at various treatment times, observing
that the liquefaction time did not provide any significant effect on the flexural
strength and resin modulus. However, the average molecular weight (Mw) after the
liquefaction process was increased, and also the flexural strength and the resin modulus was increased using the catalyst. These authors concluded that flexural strength
and modulus of wood-based epoxy resins is equivalent to bisphenol-A epoxy resin
at room temperature. Kishi et al. (2006) also indicated that the epoxy function of the
wood-based resin is controlled by the concentration of phenolic OH groups in wood
liquids, which is the dominant factor for crosslinking density and the properties of
cured epoxy resins.
Muranaka et al. (2015) studied the process of extracting lignin from different
lignocellulosic plants such as cedar, rice straw and beech (with particle sizes of
150 μm), using a water acetone mixture as a medium of extraction for the development of cured epoxy resins. These authors determined that lignin was depolymerized using a flow reaction system, and the Mw of these materials was low and had
many active sites for the curing process, resulting in a uniform curing activity.
Muranaka et al. (2015) also confirmed that this resinification method of lignin is
adaptable to some types of biomass, and the resins obtained are competitive with a
conventional phenolic novolac resin.
Ferdosian et al. (2015) studied the synthesis of lignin epoxy resin from DOL and
DKL, which were depolymerized by ECH under alkaline conditions in the presence
of a phase transfer catalyst. The Mw of the DOL/epoxy mixture (≈ 790–2800 g/
mol) was greater than the DKL/epoxy mixture (Mw ≈ 500–1400 g/mol). The lignin
epoxy resin was also cured with DDM and DETA to produce a three-dimensional
crosslinking structure with the following formulations: DKL-DETA, DOL-DETA,
DKL-DDM and DOL-DDM. The results obtained from this study showed that
lignin- based epoxy samples cured with DDM curing agent had greater thermal
9 Lignin as a Coating and Curing Agent on Biodegradable Epoxy Resins
Since the lignin-carbohydrate complex of lignocellulosic biomass has a recalcitrant
character that is not easy to break down into components, it is necessary to degrade
and/or separate the components by a pretreatment method before used as a source
of lignin or cellulose (Agbor et al. 2011). The pretreatment process to break up the
lignin-carbohydrate complex is an important first step for the effective biochemical
conversion of lignocellulosic biomass into biomaterial or biofuel (Mosier et al.
2005). For the effective pretreatment of lignocellulosic biomass, the increased surface area and porosity, the total or partial modification of the lignin structure, the
depolymerization and removal of hemicellulose, the reduction of cellulose crystallinity, among others, should be performed (Potumarthi et al. 2013; Asada et al.
2015b). In general, pretreatment methods are classified into biological, chemical,
physical and multiple or combinatorial pretreatments (Kurosumi et al. 2008;
Yamashita et al. 2010; Asada et al. 2011; Asada et al. 2015c; Asakawa et al. 2016;
Zhang et al. 2016).
Kishi et al. (2006) studied liquefaction as a wood treatment to recover the lignin
content of the German spruce (Picea abies) for the manufacture of epoxy resins.
First the liquefaction of the wood was carried out, and then the synthesis of epoxy
resins from liquefied wood was made. The authors evaluated the resorcinol (catalyst)/wood ratio and the time of liquefaction at various treatment times, observing
that the liquefaction time did not provide any significant effect on the flexural
strength and resin modulus. However, the average molecular weight (Mw) after the
liquefaction process was increased, and also the flexural strength and the resin modulus was increased using the catalyst. These authors concluded that flexural strength
and modulus of wood-based epoxy resins is equivalent to bisphenol-A epoxy resin
at room temperature. Kishi et al. (2006) also indicated that the epoxy function of the
wood-based resin is controlled by the concentration of phenolic OH groups in wood
liquids, which is the dominant factor for crosslinking density and the properties of
cured epoxy resins.
Muranaka et al. (2015) studied the process of extracting lignin from different
lignocellulosic plants such as cedar, rice straw and beech (with particle sizes of
150 μm), using a water acetone mixture as a medium of extraction for the development of cured epoxy resins. These authors determined that lignin was depolymerized using a flow reaction system, and the Mw of these materials was low and had
many active sites for the curing process, resulting in a uniform curing activity.
Muranaka et al. (2015) also confirmed that this resinification method of lignin is
adaptable to some types of biomass, and the resins obtained are competitive with a
conventional phenolic novolac resin.
Ferdosian et al. (2015) studied the synthesis of lignin epoxy resin from DOL and
DKL, which were depolymerized by ECH under alkaline conditions in the presence
of a phase transfer catalyst. The Mw of the DOL/epoxy mixture (≈ 790–2800 g/
mol) was greater than the DKL/epoxy mixture (Mw ≈ 500–1400 g/mol). The lignin
epoxy resin was also cured with DDM and DETA to produce a three-dimensional
crosslinking structure with the following formulations: DKL-DETA, DOL-DETA,
DKL-DDM and DOL-DDM. The results obtained from this study showed that
lignin- based epoxy samples cured with DDM curing agent had greater thermal
9 Lignin as a Coating and Curing Agent on Biodegradable Epoxy Resins