Entering into the 21st century, a movement toward the formation of ”the resource circulation type society” from “the fossil resource dependence society” is being accelerated due to the growing sense of impending crisis for the global warming as well as the depletion of fossil resources. Toward the formation and realization of the resources circulation type society, various actions have been taken in all quarters in order to reduce the discharge of greenhouse effect gas including carbon dioxide. In the chemical industry, especially in the field of plastic production, fossil resources such as oil and coal have long been used as raw materials. However, the trend of preventing the global warming activates the actions to replace these fossil resources with the biomasses as circulative resources. As a result, a lot of plastics, including polylactic acid, nylon, polyester, and polyurethane, have been developed using various kinds of biomasses [1–4].
The epoxy compounds based on vegetable oil have long been known as the epoxy resins of biomass origin. One of them is the epoxidized vegetable oil, which is synthesized from soybean oil, linseed oil, or palm oil by the epoxidation of double bonds with active oxygen such as hydrogen peroxide or peracid. The chemical structure of an example of the epoxidized soybean oil is illustrated in Fig. 1. The other type is the epoxide compound, which is produced by epoxidizing the hydroxyl group of vegetable oil, such as castor oil, with epichlorohydrin (ECH). These compounds have the positive effect on the biodegradation and have resulting advantages in terms of the environmental protection. It is, however, difficult for them to be applied in the industrial fields where epoxy resins are generally used in combination with curing agents, because they usually have no aromatic rings in their backbone structures and have resulting disadvantages in heat endurance, mechanical, and other performance properties. Therefore, they are applicable only in the limited fields; the former is used as the stabilizer/the plasticizer (typically the epoxidized soybean or linseed oil) for food wrap films or polyvinyl chloride compounds and the modifier (typically the epoxidized palm oil) for automotive tire, and the latter is used as the reactive diluent/the flexibilizer for epoxy resin or in a narrow field of paint application. In the recent years, a new look is taken at these epoxidized compounds based on vegetable oils from the standpoint of breakaway from the fossil resource dependency. For example, the property improvement and the function enhancement are studied with taking the procedures including the combination with clay and/or plant fiber [5–9] and the introduction of organic–inorganic hybrid material . Furthermore, in the electrical insulation field where there were almost no past results to use vegetable oil-based epoxy resins heretofore, the product, such as the resin Ngk Insulator, has been tried to be commercialized with taking full advantage of conventional compounding technology to improve the performance properties of vegetable oil-based products .
When we pay more attention to wood biomass, on the other hand, there is the dimer acid (namely the dimer of C18 nonsaturation fatty acid), which is a typical example of woody material applicable to epoxy resin systems. Although the dimer acid can also be obtained from animal fats or vegetable oils, most of the dimer acids (accounting for 80–85%) appearing in the market are synthesized from the crude tall oil provided as a byproduct of kraft pulp. The commercially available dimer acid usually contains monomer (1–5%) and trimer or more (14–16%) in addition to dimer. Figure 1 shows the chemical structure of diglycidylester of a monocyclic type dimer acid. The dimer acid-based epoxy resin also has disadvantages in heat resistance, mechanical, and electrical properties after curing due to the nonaromatic backbone structure and the long side chains. In present circumstances, therefore, this type of epoxy resin is applied only in the limited fields of applications as same as the case of the vegetable oil-based epoxy resin.
Generally, the heat resistance and the mechanical and electrical performance properties of organic compounds are attributed to the aromatic ring structure. Most of the naturally occurring aromatic compounds are said to be originated from lignin. The main constituents of wood (i.e., cell-wall components) are cellulose, hemicellulose (one of polysaccharides), and lignin. The content of the lignin having aromatic ring structures is said to be 20–30% in the wood although the content varies depending on the type of wood. The lignin contained in wood is a macromolecular compound, which is generated through the intricate and irregular copolymerization between three kinds of monolignols shown in Fig. 2 and has the resulting three-dimensional network structures. Although the chemical structure of lignin is still not entirely clarified, an example of the possible lignin chemical structure is illustrated in Fig. 2. This is in cases where some of monolignols copolymerize each other to generate three-dimensional networks. When we appropriately use the lignin as a raw material and take a full advantage of the aromatic structure of lignin, it is expected that the lignin-based epoxy resin can express the performance properties equivalent to those of epoxy resin based on petrochemistry. Therefore, many researchers have made their efforts to apply wood biomass to the production of epoxy resin. The author tries to overview here the development trend of wood biomass origin epoxy resin system in Japan, at first investigating the research and development works open to the public in technical papers and patents, followed by summarizing these works in the forms as much organized as possible for deepening the understanding of readers.
WOOD BIOMASS ORIGIN EPOXY RESINS
Around a dozen of universities and public research institutes in Japan have studied to develop epoxy resin systems based on wood biomass since 1960s. As far as the author knows, some technical papers written in Japanese were open to the public earlier than the related publications appeared in Europe or America. Many of the research and development works in Japanese universities are financially supported with various fellowship grants, which are sponsored by some government jurisdictional authorities. The methods of using wood biomass for epoxy resin systems are classified into the following three categories; one is the method of preparing lignin-epoxy resins after applying treatments on industrial lignin, which has been disposed in large volume, another is that of using wood as the raw material of epoxy resin system after applying treatments directly on wood, and the other is that of composing epoxy resin and/or curing agent from woody raw materials (except the industrial lignin), which are isolated and refined from wood prior to the treatments.
EPOXY RESIN SYSTEMS BASED ON INDUSTRIAL LIGNIN
Table 1 summarizes the development works to use industrial lignins as raw materials for epoxy resin systems. The works in the table are divided into several researcher groups according to the order of the first publication year of technical article on the lignin-based epoxy resin. Brief explanations are added on the wood-based epoxy resin systems from Methods A to G listed in Table 1.
Table 1. Lignin treatment methods applied to lignin-based epoxy resin systems
Research group of university and/or public institute
Woody raw material
Treatment method for woody raw material
Functional group generated
Treatment procedure (conditions)
Year, the first year of publication; ECH, epichlorohydrin; FFPRI, Forestry and Forest Products Research Institute; AIST, The National Institute of Advanced Industrial Science and Technology; DMF, dimethylformamide; Fukui UT, Fukui University of Technology; BDMA, benzyldimethylamine.
Acid (hydrochloric or sulfuric acid) and phenol derivatives are added to kraft lignin to cause the cleavage of lignin intermolecular bond at the same time to generate the phenolic hydroxyl group in the molecule, followed by epoxidizing the phenolic hydroxyl group with ECH to provide the lignin-based epoxy resin. The epoxy resin is subsequently crosslinked with diethylenetriamine (DETA) or phthalic anhydride for using it such as adhesives (Column A of Table 1 and Fig. 3).
This method was developed by Migita and coworkers [12–14] who also carried out the following two different epoxidation methods; one is the method of directly epoxidizing the phenolic hydroxyl group in the lignin structure with ECH without using phenol derivatives [12, 13] and the other is that of generating bisguaiacyl structure by the treatment with ketone compound followed by the epoxidation [12, 13] (Column B of Table 1 and Fig. 4). In addition, Shiraishi and coworkers [15, 16] developed the technique using bisphenol-A as a phenol derivative. Two catalysts, hydrochloric acid and BF3-ethyl etherate, are used to phenolize the lignin with bisphenol-A. The method of using BF3-ethyl etherate is summarized in the column C of Table 1 and illustrated in Fig. 5. The bisphenol-A is introduced into the side chain of the lignin as the result of cleaving the ether bondage of the lignin. The lignin-epoxy resin prepared is reported to be soluble in organic solvent such as acetone due to the contribution of bisphenol-A. The lignin-epoxy resin, especially based on BF3 catalyst, provides better water-proof adhesion strength with plywood when cured with toriethylenetetramine (TETA) in the hot-press condition at 140°C.
Kraft lignin is dissolved in the dioxane/water mixture, and then the ozone-containing oxygen is injected into the mixture. After drying the product obtained, further dissolution with dioxane and the ether extraction results in providing the ozone oxidation kraft lignin. The ozonized lignin is then dissolved in an alkali water solution and crosslinked with the water-soluble epoxy resin, glycerol polyglycidylether, in order to use the product as waterborne type wood adhesives (Column D of Table 1 and Fig. 6).
It is well known that the ozone oxidization treatment cleaves the aromatic ring of lignin and generates the muconic acid derivative, which has the muconic acid residue with carboxyl groups on both ends of the conjugate double bond. Lignin is generally soluble in alkali water solution. Tomita and coworkers [17, 18] use the alkaline solution to dissolve and hydrolyze the lignin, and the resulting carboxyl group and the sodium salt are crosslinked with the water-soluble type epoxy resin. The crosslinked resin obtained has the viscoelastic absorption over a wide range of temperature and is considered to form a kind of intermolecular penetrating network (IPN) structure. The lignin-epoxy resin is reported to have the superior adhesion ability with the wood from a practical point of view, even if the ozone oxidation kraft lignin is contained up to 80wt% in the crosslinked system.
Kraft lignin or the ozone oxidation kraft lignin is dissolved in the 1% sodium hydroxide water solution at 60°C and subsequently mixed with the water-soluble epoxy resin, polyethylene glycol diglycidylether (PEGDGE), and/or the emulsified bisphenol-A type epoxy resin. The resulting mixture is crosslinked with TETA to obtain the cured lignin-epoxy resin (Column E of Table 1 and Fig. 7).
Nonaka et al. [19, 20] report that there are little differences in performance properties between the epoxidized lignin systems with and without ozone oxidation lignin, and the heat curing at 150°C provides a good adhesion performance for the system even if the system contains the lignin up to 50 wt%. Although depending on the type of curing agent combined with, the epoxidized lignin system is able to have the Tg below the ambient temperature. In this case, each lignin-based system has higher loss tangent values in a wide temperature range around the ambient temperature and has a lower elastic modulus at the ambient temperature. This indicates that the damping material can be recommended as one of the applications for this type of lignin-epoxy resin.
The epoxy resin, PEGDGE, is reacted at 80°C with the hydroxyl group of kraft lignin dissolved in dimethylformamide. The resulting lignin adducted with epoxy resin is crosslinked with a curing agent, poly(azelaic anhydride)  (Column F of Table 1 and Fig. 8).
Alcoholysis lignin or lignin sulfuric acid is dissolved in ethylene glycol and/or glycerin. Subsequently, the hydroxyl group in the lignin molecule is reacted with succinic acid to convert the lignin into multiple carboxylic acid derivatives. The derivatives obtained are then reacted with epoxy compound to provide the crosslinked epoxidized lignin resin (Column G of Table 1 and Fig. 9).
The above epoxy resin system, developed by Hirose et al. [22–28], is able to enhance the heat resistance (namely the thermal degradation temperature in this case) by converting existing hydroxyl groups to ester groups in the biomass component molecule. Besides, natural origin compounds including tartaric acid and citric acid can be used as the poly carboxylic acid for the modification. In addition, the diglycidylester of dimer acid, a biomass origin epoxy resin previously mentioned in this article, can also be combined with as an epoxy resin component. According to the descriptions in the patent, Japanese Unexamined Patent Application Publication (JP-A) No. 2002-284791, it is expected that the increase in the content of biomass origin component accelerates the biodegradation of the lignin-epoxy system.
Method Based on Lignin Related Material
Although it is not the case to directly use such as the above-mentioned industrial lignin derivatives, let us talk here about the epoxy resin synthesized from vanillin. Vanillin is an aromatic aldehyde, which is contained as a natural product mainly in vanilla (Orchidaceae), benzoin (Styracaceae), and Peruvian balsam (Leguminosae). Natural “vanilla extract” is a very complicated mixture comprising several hundred kinds of compounds, but the dominant compound causing the flavor peculiar to vanilla is vanillin. The vanillin based on the industrial wood biomass is called “lignin vanillin,” which is obtained from the lignin sulfonic acid in sulfurous acid pulp waste liquor through the oxidation decomposition process in alkali solution. Figure 10 shows the synthetic method and the chemical structure of the difunctional epoxy resin derived from the vanillin. The raw material of the epoxy resin is the dihydric phenol derivative, a white crystallization with the melting point from 174 to 175°C, which is the reaction product obtained from the dehydration condensation of vanillin and pentaerythritol. The epoxy resin obtained is a light yellow solid with the epoxide equivalent, 270 g/equiv, as described in the patent, Japan Patent (JP) No. Hei 2-45632. The epoxy resin crosslinked with diaminodiphenylmethane is reported to have several relaxations including the β-relaxation caused by the micro-Brownian motion of aromatic methoxy group at around 60°C and the relaxation caused by the hydrogen bonding between the methoxy and the hydroxyl groups at around 0°C as shown in Fig. 11. Ochi et al. [29–31] report that the impact strength, the tensile strength, and elongation are improved by the contribution of two above-mentioned relaxations in addition to that of the spiroacetal ring structure itself in the vanillin-based epoxy resin. This kind of methoxy group as an aromatic ring side chain can be found everywhere in the epoxy resin based on the lignin backbone structure. It is previously mentioned that some crosslinked lignin-epoxy resins have relatively wide range of relaxations around the room temperature according to the dynamic viscoelastic analysis [18, 19]. This relaxation behavior is expected to have a positive effect on the damping characteristics. The dynamic relaxation phenomenon confirmed by Ochi [29–31] provides useful information in the characteristic analysis of this kind of wood biomass origin epoxy resin after crosslinking.
WOOD-BASED EPOXY RESIN SYSTEMS
Table 2 summarizes several development works that use wood as the raw material for the epoxy resin system through the epoxidation process after taking certain treatments on the wood itself. These methods can be roughly classified into the two categories; one is the method of conducting the epoxidation as the second treatment after taking the first treatment on the wood, physically or chemically, by means of such as high-temperature steam, acid, alcohol, phenol, and/or ozone, and the other is that of obtaining epoxy resin after deriving the raw material for the epoxy resin from the wood through the chemical or biochemical refining process. Epoxy resins classified into the latter method are superior in performance properties and considered to have a wide range of applications. On the other hand, those classified into the former method are relatively low in price, because no refining processes are necessary in many cases, while the performance properties are presumed to be relatively poor. Therefore, both kinds of woody epoxy resins are considered to be applicable in the segregated fields suitable for each resin system in the market. Brief explanations are added on the wood-based epoxy resin systems from Methods H to O listed in Table 2.
Table 2. Wood treatment methods applied to wood-based epoxy resin systems
Research group of university and/or public institute
Woody raw material
Treatment method for woody raw material
Functional group generated
Treatment procedure (conditions)
Year, the first year of publication; Yokohama NU, Yokohama National Univ; ECH, epichlorohydrin; FFPRI, Forestry and Forest Products Research Institute; PEG, polyethylene glycol; TMAH, hydrated tetramethylammonium; OMTRI, Osaka Municipal Technical Research Institute; TUAT, Tokyo University of Agriculture and Technology; Nagaoka UT, Nagaoka Univ. of Technology; Bacteria, gene recombinant bacteria, SYK-6 strain; PDC, 2-pyron-4,6-dicarboxylic acid.
The wood (Betula) is treated by the steam at 180°C and then extracted by warm water. The extracted residue is further extracted by methanol and then dried. The lignin obtained is treated with ozone to generate the ozone oxidation lignin with muconic acid residues. This ozone oxidation lignin is heated and dissolved in bisphenol-A type epoxy resin. Subsequently, the reaction is carried out at 120°C between the carboxyl and the epoxide groups to make the prereacted-type ozone oxidation lignin-epoxy resin  (Column H of Table 2 and Fig. 12).
This lignin-epoxy resin has a wide range of viscoelastic dispersion after crosslinking and is considered to form a kind of IPN structure when combined with an aliphatic polyamine-type curing agent such as DETA or hexamethylenediamine. Each lignin-epoxy resin after curing tends to have wider viscoelastic dispersion and higher Tg with the increase in the ozone oxidation lignin content in the epoxy resin system. Moreover, it is considered possible for the lignin-epoxy resin to be applied as adhesives and molding products, because the Tg can be optionally designed to be in low to high-temperature regions when selecting the suitable type of polyamine compound as a curing agent .
The wood (Japanese larch or Japanese cedar) is decomposed to be the solid mixture by means of the high-pressure steam in the steam-explosion apparatus, which is designed to have the highest temperature and pressure, 275°C and 6.0 MPa, respectively. The solid mixture obtained is washed with water and filtrated to remove the water-soluble component. Hydroscopic methanol is used to extract the solid mixture to prepare the low-molecular weight methanol-soluble lignin. The lignin is then epoxidized with ECH (Column I of Table 2 and Fig. 13).
Nakamura et al. [33, 34] produced the methanol-soluble lignin with the number–average molecular weight (Mn) of ∼800 [the weight–average molecular weight (Mw) of ∼1200] using the steam-explosion apparatus under the pressure condition of 3.6 MPa for 5 min and the subsequent methanol extraction process. The size of the molecular weight can be controlled by changing the kind of solvent used for the extraction. It is mentioned in the patent, JP-A No. 2009-263549, that a lower molecular weight lignin is generated when using isopropanol instead of methanol and/or adding toluene to the extraction solvent. The epoxidized lignin soluble in organic solvents is obtained by using hydrated tetramethylammonium (TMAH) instead of NaOH as an alkali catalyst in the epoxidation process of the lignin, because TMAH prevents the polymerization of the lignin to a higher molecular compound during the epoxidation stage . To be more precise, the epoxidized lignin with the Mw of ∼2100 can be derived from the Japanese cedar origin lignin with the Mw of ∼1200. The epoxy resin obtained is soluble in general-purpose solvents including methyl ethyl ketone. This epoxidized lignin is used to experimentally prepare the copper-clad laminates for printed-circuit boards. Although the resulting laminating board has a higher water absorption compared to those prepared from the commercially available epoxy, the heat-resistance property is good (with a higher Tg), and other properties are similar. The methanol-soluble lignin can also be used as a replacement of phenol-type curing agent such as phenol novolac. Kagawa et al.  study to apply the lignin (Mw ≈ 1600), as a curing agent, to the transfer molding epoxy resin system used for electrical insulation apparatus.
The liquefied wood product is obtained by heating the wood (cedar wood powder) at 150°C together with the solvent mixture of polyethylene glycol and glycerin in the presence of sulfuric acid catalyst. The resulting product is mixed with an epoxy resin and an aliphatic amine curing agent (TETA) and is subsequently heated to prepare the cured epoxy resin (Column J of Table 2 and Fig. 14).
In this case, PEGDGE and diglycidylether of bisphenol-A (DGEBA) are chosen as a waterborne-type and an oiliness-type epoxy compounds, respectively, to be blended with the liquefied wood, which is compatible with both types of epoxy resins. The viscoelastic examination indicates that there is a single peak of loss modulus corresponding to the glass transition in every cured resin system studied, which proves that the epoxy resin system retains the homogeneous structure after crosslinking. The three-dimensional crosslinking network structure is also identified to be present in the cured system, because the flat region of storage modulus due to the rubber elasticity is clearly observed in the high-temperature region. Furthermore, the wood content is reported to be increased up to 53% in the liquefied wood-epoxy resin system when taking the procedures including the use of ozone oxidation wood and the split addition of wood during the preparation stage of the liquefied wood [36–40].
Methods K and L
To prepare the liquefied wood, the wood (German spruce) is treated for 2–4 h under pressurization as follows; with using sulfuric acid and resorcinol at 150°C or with using sulfuric acid and the solvent mixture of polyethylene glycol and glycerin at 140°C. The resulting liquefied wood, which is not further applied with any isolation and/or refining treatments, is used to produce the wood-based epoxy resin by reacting the phenolic and alcoholic hydroxyl groups with ECH (Columns K and L of Table 2, and Fig. 15).
These methods were developed by Kishi et al. [41–43]. In addition to the two methods mentioned earlier, there is the other one using only water and resorcinol without using sulfuric acid. In this case, a higher treatment temperature such as 250°C is necessary. The wood liquefaction in this case is caused by the solvolytic reaction based on resorcinol and/or multiple alcohols such as polyethylene glycol and glycerin. Although depending on the reaction time to liquefy, most of the systems studied are considered to have chemical bonds with the wood during the wood liquefaction process [41–43].
Performance properties of the resin cured with an aromatic amine are evaluated for the wood-based epoxy resin obtained by the first preparation procedure mentioned earlier. According to the viscoelastic measurement of the cured resin, the wood-based epoxy resin has a slightly low Tg and a wide glass transition region caused by the wide molecular weight distribution of the resin, while the storage modulus is almost similar in the glassy region and is slightly high in the rubbery region in comparison to the bisphenol-A-type epoxy resin. In addition, the flexural and adhesion properties are reported to be comparable to those of the bisphenol-A-type epoxy resin. These evaluation results indicate that the wood-based epoxy resin is presumed to have the chemical structure sufficiently crosslinked. Furthermore, a plant-type fiber (flax fiber) is used as a reinforcement to prepare the natural-fiber-reinforced material. The evaluation result of the reinforced material makes it clear that the wood-based epoxy resin has the superior adhesion ability with the flax fiber in comparison with commercially available epoxy resins as mentioned in the patent, JP-A No. 2006-63271 .
In case of wood liquefaction based on resorcinol, there is a problem of the insoluble residue component generated by the recondensation between wood components during the liquefaction. The recondensation of wood, however, is preventable using multiple alcohols due mainly to the contribution of glycerin added as a co-solvent. In case of using multiple alcohols, on the other hand, there is the problem of the gelation caused by the high-reactive phenolic hydroxyl groups in the wood during the epoxidation process. To prevent the gelation, a new synthesis route is innovated. The synthesis route enables to convert the reactive phenolic hydroxyl groups into the less reactive alcoholic ones by means of the prereaction with ECH before the main epoxidation process of wood [45, 46]. The technique, in which no phenol-type compounds are used for the wood liquefaction, is applied for as the patent, JP-A No. 2009-41010. It is assumed in the patent that these wood-based epoxy resins are potentially applicable not only in the use fields, where damping characteristics are necessary but also in those where the health and safety issue, seen as a problem in phenol type compounds, is taken seriously into account.
Methods M and N
The wood (Japanese cypress or Beech) powder is impregnated into p-cresol or the mixture of p-cresol/solvent (typically acetone) to solvate the lignin component in the wood with p-cresol. Subsequently, the 72% concentrated sulfuric acid is added to swell and decompose the cellulose component in the wood. The lignin component solvated with p-cresol comes in contact with the acid only in the interface, where the concentrated sulfuric acid comes in contact with p-cresol, which results in converting the Cα-position into a high-reactive site in the lignin backbone structure as shown in Fig. 16. The high-reactive site is attacked and grafted with p-cresol to generate lignocresol in the organic layer. In the next step, the aqueous layer (containing cellulose origin component) and the organic one (containing lignocresol) are separated. The lignocresol is obtained by extracting and refining the organic layer. The lignocresol is then reacted with ECH to form the epoxidized lignocresol resin (Columns M and N of Table 2, and Fig. 17).
The manufacturing process of the lignophenol developed by Funaoka [47, 48] is generally called “the phase-separation conversion method.” Some monophenol derivatives including p-cresol are applicable to the conversion method. The lignophenol obtained has the linear molecular structure in which the lignin is bonded with the phenol derivative and retains well the basic bonding structure of the native lignin. When p-cresol is applied to use as a phenol derivative in this manufacturing method, there is almost no difference in yield between different wood types used. The Mw range of the lignocresol obtained is from 3,000 to 5,000 for softwood and from 5,000 to 10,000 for hardwood-based materials, respectively. Besides, it is possible to further depolymerize the lignocresol down to the molecular weight level of dimer by alkali treatment. It is also confirmed possible that the monomerization and/or the demethylation of methoxy group of the lignocresol covert the lignocresol into lower Mw monophenol derivatives originated from wood, such as guaiacol, catechol, and cresols.
Some attempts to apply the lignocresol to epoxy resin were carried out by the group of Osaka Municipal Technical Research Institute [49, 50] and that of Yokohama National University, respectively. Table 3 lists the typical conditions applied to the epoxidation reaction along with the characteristics of the resulting epoxidized lignocresols. Because the lignocresol is decomposed with alkali during the epoxidation reaction when taking a usual temperature condition such as around 120°C, the reaction is carried out in comparatively low temperature. Besides, the two-step epoxidation procedure is also taken using both NaOH and a phase-transfer catalyst [51, 52]. These epoxidized lignocresol resins have the comparatively large molecular weights and are solid at room temperature as shown in Table 3. Therefore, Kadota et al. [49, 50] and Tsuda et al. [51, 52] evaluate the properties of epoxidized lignins after blending with liquid bisphenol-A type (DGEBA) or cycloaliphatic (ECEC)-type epoxy resins. As a result, the heat resistance and adhesion properties are confirmed to be improved due to the contribution of the rigid backbone structure of the lignocresol. On the other hand, there is also an attempt to use the lignocresol as a curing agent of epoxy resin. Tsuda et al.  study to cure the bisphenol-A type epoxy resin with the lignocresol in combination with an imidazole derivative as a catalyst. In this case, it is confirmed that the content of the biomass origin component is increased up to 49%, and the heat resistance of the cured epoxy resin is improved. Furthermore, the content of biomass origin component is increased up to 82 wt% in the cured resin system when the lignophenol is used as a curing agent in combination with the epoxidized lignocresol. The resulting cured resin has the Tg more than 200°C and is thermally stable according to the thermogravimetric analysis . As for the mechanical properties, there is a tendency that the cured resin increases the rigidity and decreases the flexural strength with the increase in the lignocresol concentration.
Table 3. Epoxidization conditions and characteristics of epoxidized lignocresols
Funaoka [47, 48], who has developed the phase-separation conversion method, also studies to apply the method to the industrial production. A small-scale production plant was already built and successfully operated, which indicates that the practical use of the lignophenol is assumed to be realized in near future. Taking into account the industrial usage of the epoxidized lignophenol resin, several resin composition patents are applied for in the following uses; the electrical insulator (JP No. 3936214), the materials for adhesives (JP-A No. 2004-210816), the copper clad laminates, and the resin encapsulation material (JP-A Nos. 2009-292884 and 2010-150298).
Low-molecular weight lignin compounds are effectively generated by dissolving wood with NaOH and nitrobenzene in an autoclave at 170°C. These lignin compounds obtained are further decomposed biochemically to reach a final uniform compound, 2-pyron-4,6-dicarboxylic acid (PDC), by the lignin-degrading bacteria “Sphingomonas paucimobilis SYK-6 strain.” The resulting PDC is refined and epoxidized to obtain a glycidylester type epoxy resin (Column O of Table 2 and Fig. 18).
Sphingomonas paucimobilis SYK-6 strain (hereafter called SYK-6 strain) was isolated from a pond for the treatment of waste liquor from a kraft pulp mill by Katayama and coworkers [53–56]. The SYK-6 strain is reported to be capable of completely metabolizing the low-molecular weight lignins including dimeric lignin compounds by the aids of various and specific enzymes contained in this strain [53–56]. As a result, the SYK-6 strain enables to establish the epoch-making generation route through which every low-molecular weight lignin compound is degraded to reach PDC. Typical examples of the low-molecular weight lignin compounds are vanillin, vanillate, syringaldehyde, and syringate. Metabolic pathways from these lignins to PDC are shown in Fig. 19. Genes of SYK-6 strain participating in these metabolic pathways are analyzed in detail by Katayama et al. [57, 58] and Masai et al. [59–62], and the subsequent degradation pathways from PDC to carbon dioxide and water are also confirmed to exist. Katayama  already constructed a middle scale of “the genetic recombination bioreactor,” which was able to efficiently produce PDC from low-molecular weight lignins in a 150-L fermentation vessel by means of the recombinant bacteria originated from SYK-6 strain. PDC obtained in this bioreactor is quite difficult to be synthesized through the usual chemical reaction route and has the localization of electron and the anisotropy in the molecular structure, which is the reason why PDC draws much attention as a unique compound from the aspect of the physicochemical property.
There are some attempts of polymerizing PDC to form macromolecular compounds such as polyester, polyamide, and polyurethane . The application of PDC to epoxy resin is also studied as well as the polymers based on PDC. The following two kinds of methods are proposed for the preparation of the PDC-based epoxy resin in the work  and the patent, JP-A No. 2010-59095; one is the method of performing the epoxidation with glycidol at low temperature (0–5°C) in tetrahydrofuran, and the other is that of performing the dehydration of PDC with nonsaturation alcohol such as allyl alcohol in the presence of an acid catalyst, followed by oxidizing the terminal double bonds. The glycidylester of PDC obtained is cured with maleic anhydride or phthalic anhydride for evaluation. In this case, the tensile adhesion strength with stainless steel or iron plate is confirmed to be remarkably improved in comparison with that of the commercially available bisphenol-A type epoxy resin. On the mechanism of this high-adhesion strength, it is speculated that the high strength is attributed to the strong interaction between the polarity surface of metal and the polar groups generated by the cleavage of α-pyron ring in the PDC molecule .
EPOXY RESIN SYSTEMS BASED ON WOODY MATERIALS EXCEPT LIGNIN
As previously mentioned, there are other woody materials except lignin, such as cellulose, hemicellulose, and vanillin. From a view point of practical usage, the author mentions here about the terpene compound and the natural rubber (NR) as examples of other woody materials. These two types of compounds are industrially produced by being isolated and refined from wood or wood-related materials.
The terpene compound, which can be obtained mainly from pine resin, pine-type trees, and orange skin, is the hydrocarbon with the isoprene constitution unit and can be classified into acyclic, monocyclic, dicyclic, and tricyclic types by chemical structure. As the monocyclic terpene compounds, there are limonene, dipentene (optical isomer of limonene), terpinolene, α-pinene, β-pinene, terpinene, and menthadiene, and these have been used as raw materials for epoxy resins and curing agents. When taking an example, there is the diepoxidized limonene, which is produced by oxidizing the double bond of limonene and used as a reactive diluent or a component in photocationic-curing systems. As another example, there is the terpene-diphenol (TDP), which is obtained by grafting phenols to the terpene molecule as described in the patent, JP-A No. Hei 8-198791. The simplified flow chart in Fig. 20 shows the synthetic method of TDP and the usages as epoxy resins and curing agents. There are two types epoxy resins based on TDP; one is the reaction product between TDP and ECH by the one-step method , and the other is prepared by the reaction between the liquid bisphenol type difunctional epoxy resin and TDP by the two-step method (i.e., the advancement process) as mentioned in the patent, JP No. 3508033. There are also two types of curing agents proposed; one is the TDP-novolac resin, which is the polycondensation product between TDP and formaldehyde , and the other is the TDP-based benzoxazine, which is prepared by reacting TDP with aniline and formaldehyde .
As one more example of cyclic terpene appeared in the market, there is the maleated allo-ocimene. This compound is produced by the addition reaction of maleic anhydride to allo-ocimene, which is a thermal isomerization product obtained from α-pinene and/or β-pinene as shown in the synthetic route of Fig. 21. Although this maleated allo-ocimene is solid at room temperature (the melting point > 70°C), it becomes stably liquid at room temperature by means of the isomerization treatment of using catalysts shown in the patent, JP No. Sho 62-5151. And the compound has a lot of past results as an anhydride-type-curing agent for electrical insulation castings due to the good water resistance performance enhanced by the alkyl substituents of the aromatic ring [7, 68].
Natural Rubber-Based Systems
Natural rubber (NR) is a wood origin macromolecule consisted of cis-polyisoprene [(C5H8)n] contained in the tree sap of rubber tree (Hevea brasiliensis) and applied mainly to the production of automotive tire. The standard automotive tire is said to contain about 44% nonoil materials including the NR. From the viewpoint of the extrication from dependence on oil, many attempts have been made to raise the ratio of nonoil material in the tire constitution component. In 2008, a Japanese tire maker made a market launch of the automotive tire labeled as “97% nonoil natural resources tire” in which the epoxidized natural rubber (ENR) was used as a substitute of the synthetic rubber. The ENR can be obtained by oxidizing the double bond of NR with peracetic acid as shown in Fig. 22. The ENR in the eco-friendly tire is crosslinked using the vulcanizing agent, and the epoxy group in the rubber molecule carries the function of improving the wet grip performance and/or the crack prevention ability . As an extension of the development, “100% nonoil natural resources tire” has been a challenging target in recent several years.
On the other hand, there are also some attempts to apply the ENR to adhesives (JP Nos. Hei 07-047723 and 2987391) or damping materials (JP-A No. Hei 06-220303 and JP No. 3059838). In these cases, epoxy groups contained in the ENR are reacted with the curing agent such as polyamine or acid anhydride types. The NR generally contains small amounts of nonrubber components such as protein and lipid. The protein in particular has harmful effects, because the protein causes side reactions in chemical synthesis and provokes latex allergies for human. Therefore, the protein stands in the way of the NR expanding the application range as a replacement of the synthetic rubber. To reduce the unsuitable impurities in the NR, several modification methods are developed; for example, one is the biochemical method of using enzyme (JP Nos. 2905005 and 4102499) and the other is the chemical one of using urea type compound (JP No. 3581866). In combination with these modifications, there are proposed the practical technologies by which almost all of proteins are removed (down to the nitrogen content ≤ 0.02 wt%) from the NR through the separation processes such as washing and/or multiple-time centrifugations. These modification methods are roughly illustrated in the synthesis route from the NR to the ENR in Fig. 22. The NR with negligible amount of protein is also used to produce the ENR, which is now available in the market. In addition, the Mn of the ENR is reduced to around 103–105 by the oxidation decomposition technique of using the combination of radical initiator and aldehyde, which results in providing the low-molecular weight liquefied ENR as mentioned in the patent, JP-A No. 2004-176013.
Because of the allergy issue and the concern over supply shortage, alternative sources of NR have been explored worldwide [70–72]. It is known in Japan that Tanaka et al. [73–75] isolated low-molecular weight cis-polyisoprene from some wild mushrooms in 1980s. Mushroom is generically called “kinoko” in Japanese, which is literally translated into English as “child of tree.” Although the mushroom cannot be called wood biomass, many of them are said to have close relationships with trees in terms of natural ecosystem. Mitomo et al.  have produced the NR from extracted substances of the “chichitake” mushroom (Lactarius volemus). This mushroom rubber has none of the proteins due to the nature of the mushroom and is reported to be vulcanized by the irradiation of low energy γ-ray in combination with a suitable vulcanization accelerator, such as nonane-diol-diacrylate, in n-heptane solution [76–78]. The mushroom rubber, however, has the disadvantages; one is the low-molecular weight (Mn ≤ ∼5.2 × 104) and the other is the low yield (≤∼5.8%) in comparison with those of the conventional NR, Mn ≈ 3.5 × 105 and ∼37% yield. This is an obstacle to the commercial production of the mushroom rubber at the present time. In spite of the high-production cost and the small harvest amount of the mushroom, researchers are continuously making their efforts to take advantages of the protein-free and the radiation vulcanization of the mushroom rubber. As shown in Fig. 22, the low-molecular weight mushroom rubber is considered to be applicable to the protein-free and low-molecular weight ENR. In addition, other alternative sources of NR are also studied, and the next possible candidates are said to be wild grasses, for example, “dandelion” .
As the result of various technology developments mentioned earlier, it is possible for the “natural resin” origin cis-polyisoprene to be used as epoxy resins as well as the raw materials to synthesize or modify other macromolecules. This suggests a strong possibility that the ENR can be applied extensively as a “green polymer” in the future market.
The author tried to summarize the developmental studies of applying wood biomass to epoxy resin in Japan. It was found that some of Japanese researchers established unprecedentedly unique technologies and made the steady progress in the preparation for the practical use of the wood-based epoxy resin. This is considered one of the outcomes arising from various financial research supports sponsored by some government jurisdictional authorities with long-term perspective. Although Japan is said to have little natural resources, the renewable wood resources can be obtained sustainably and abundantly as far as without breaking the balance of ecosystem or biodiversity. The full-fledged development of epoxy resin based on wood biomass has just started when viewed as a whole. In the future, the development will be further accelerated toward the construction of the environment harmony type and the resources circulation type societies based on the independent of fossil resources. As the result of continuous developmental efforts, it is expected that we can look at the scene where epoxy resins are sustainably supplied in various forms even after the depletion of oil resources.