ReviewEpoxy thermosets and materials derived from bio-based monomeric phenols: Transformations and performances
Graphical abstract
Introduction
In 1900, 41% of the materials used in the US were renewable, but by 1995 only ∼6% of the materials consumed were renewable. To foster and promote sustainability, developing renewable bio-based polymers and derived materials from biosources such as plant-derived feed stocks and natural monomers become a focus of intensive research. In this filed, three major challenges should be well addressed: (1) relatively high cost of certain biomass feedstocks and related biorefinery technologies; (2) mechanical and thermal properties, processability, barrier properties, durability, and other physical properties that are often not competitive with fossil-fuel derived plastics; and (3) the lack of universally accepted quantitative metrics on economical sustainability [1], [2], [3], [4]. Thermosetting polymers and related materials, such as polyurethanes, phenolic resins, urea-formaldehyde resins, unsaturated resins, amino resins, epoxy resins, etc. are produced in huge volume worldwide. Among these, epoxy resins have many advantages, such as high corrosive resistance and adhesion strength, favorable processing ability, low curing shrinkage, excellent mechanical, thermal and electrical properties, versatile formulation flexibilities, and good processing ability. Epoxy thermosets and downstream materials are extensively used in protective coatings, adhesives, constructions, high-performance composites, electrical engineering, electronic encapsulation, and so forth. Their global market, about 21.5 billion dollars in 2016, will expand to 27.5 billion dollars by 2020, with epoxy composites and adhesives in the strongest demand [5].
Approximately 90% of the epoxy monomer/prepolymer production is based on bisphenol A (BPA) and epichlorohydrin (ECH), followed by BPA-based epoxy monomer/prepolymers (DGEBA) with different molecular weight ranges. BPA is mainly produced from petroleum-derived phenol and benzene according to the cumene process, accounting for ∼20% of global benzene demand [6]. Phenol and BPA are a petrochemical compound. Renewability is of utmost importance among the fundamental 12 principles of green chemistry [7]. BPA is known as a metabolism disruptor, with ingestion the primary source of human exposure to BPA since BPA and its derivatives may leach from plastic packaging materials [8]. BPA has been banned in baby bottles in many countries and regions [9]. Crosslinked epoxy thermosets are also somewhat sensitive to hydrolysis in an outdoor environment for a long period of time, thereby increasing risks of the human exposure to BPA and derivatives [10]. On the other hand, ECH is mainly produced from 1-chloropropane, with a minor fraction produced from glycerol (a byproduct of biodiesel production, ∼10 wt%). Moreover, since the increasing fraction of biodiesel production leads to excess availability of glycerol, bio-based ECH seems attractive in the future with the ever-increasing petroleum price [11]. As a result, it is critical to seek BPA replacements to synthesize bio-based epoxy thermosets with desired properties or interesting functions. However, so far, only a few bio-based monomers, such as some epoxidized soybean oils and cardanol-based epoxy monomers, prepolymers and curing agents, have been commercialized with very limited applictions.
So far, a few excellent reviews [10], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] address bio-based epoxy thermosets derived from a large spectrum of bioresources. A general conclusion reached is that different biomass starting materials strongly affect final propropeteis of obtained biobased thermosets, by creating many different, highly versatile thermosetting networks. For instance, some biomass starting materials like vegetable oils provided their derived epoxy thermosets with the highly flexible backbone and may be more suitable as toughening agents for epoxy materials. Nevertheless, regarding advanced composites, epoxy thermosets that feature a highly crosslinked aromatic backbone and high functionalities (leading to high crosslink density) are extremely important [10], [14]. Therefore, considering structural similarities, naturally occurring or bio-transformed monomeric phenols hold great promises as building blocks to synthesize sustainable high-performance thermoplastic and thermosetting polymers [22], [23].
Hydroxyl functionalities as substituents and aromatic rings available in naturally occurring phenolic derivatives may be tailored to design new monomeric structures [22]. In the recent decade, bio-based monomeric phenol-derived epoxy thermosets based on monomeric phenols as cardanol, eugenol, vanillin, tannin acid, gallic acid, and other bio-transformed small molecule phenols have played a vital role in the direction of high-performance bio-based epoxy materials [12], [16], [23], [24]. So far, however, there is no comprehensive, critical review on this topic and a timely review is needed, so that herein we intend to provide a profile of relevant research activities and reflect future tendencies.
In principle, bio-based monomeric phenols bear at least one phenolic hydroxyl groups that can be converted to epoxy groups through O-glycidylation by reacting with ECH under an alkaline condition. O-glycidylation is also a paramount approach to producing most commodity epoxy monomers and oligomers. As shown in Scheme 1, with Method A, NaOH acts as the catalyst and reactant and ECH as the reactant and solvent, by which epoxy monomers and oligomers produce simultaneously. With method B, a phase transfer catalyst (QX) is required, to better control epoxy values, reduce organochlorine, oligomer contents and side reactions likely by decreasing the reaction temperature and reaction time [12], [25]. Aouf et al., [25], [26] studied the phase-transfer-catalytic O-glycidylation of serval natural phenolic compounds, such as methyl catechol, gallic acid, protocatechuic acid, pyrogallol and resorcinol. Two main competitive mechanisms were established: (1) the straightforward nucleophilic substitution (SN2) of phenolate ion (ArO−) by ECH accompanying with the cleavage of C-Cl bond, and (2) the attack of ArO− causes the opening of ECH ring followed by the protonation of resulting alcoholate and ring closing. Note that O-glycidylation is strongly affected by specific chemical environments depending on the phenol, catalyst, reaction temperature, reaction time, ratios of -OH to ECH, solvents, water, bases, etc. O-glycidylation is also accompanied with some side-reactions, for example, the reactions leading to six-number-ring benzodioxane derivates [24], [25].
This review pertains mainly to bio-based epoxy thermosets and related materials based on naturally occurring or bio-transformed monomeric phenols. Our particular attention is on epoxy thermosets and related materials derived from cardanol, eugenol, vanillin, tannin acid, gallic acid, tannin-depolymerized phenols, other bio-based monomeric phenols, and synthetic phenols from mixed bioresources. Discussions will be given on the derived epoxy monomers, prepolymers, curing agents, additives, obtained bulk epoxy thermosets, and related thermosetting materials including some applications. Nevertheless, this review will not present much information about biorefinery of monomeric phenols and their toxicological studies. Finally, the challenges and opportunities for the further studies in this field are outlooked.
Section snippets
Bio-based epoxy thermosets from cardanol
Cardanol is an important bio-based building block used to synthesize epoxy thermosets with low cost compared with fossil-based phenols. Cardanol is a renewable C15-chain substituted phenol derived from a non-edible byproduct of cashew nut shell liquid (CNSL), a rich mixture of the non-isoprenoic phenolic compounds, with potential production of about 450 000 metric tons per year; CNSL is one of the few major, economic sources of naturally occurring phenols [27]. Cardanol, a clear-to-pale yellow
Bio-based epoxy thermosets from eugenol
As discussed above, flexible cardanol-based epoxy monomers, prepolymers and curing agents not only display potentials, but also find practical applications, such as anti-corrosive coatings. However, cardanol epoxy thermosets always exhibit much lower Tg than those based on DGEBA do. In contrast, such naturally occurring and bio-transformed monomeric phenols of higher aromatic contents are able to endow their derived epoxy thermoset with the desired thermomechanical properties. As an example,
Epoxy thermosets derived from vanillin
Vanillin is an aldehyde and methoxy co-substituted phenol extractable from plants, such as vanilla bean, and is widely used as flavor for food, such as chocolate. Also, vanillin can be produced from lignin, accounting for ∼15% of its overall production (∼20 000 ton per year) [111], [112]. It is likely economically favorable to produce vanillin from abundant lignin [113]. Presently, vanillin serves as a very promising bio-based building block to create new monomers for polymerization. For
Bio-based epoxy thermosets derived from tannin-related phenol monomers
After lignin, tannins are the most abundant natural phenolic resources distributing in many plants, and can be classified into hydrolysable and condensed tannins. The former are the mixture of simple phenol moieties connected to glucose chains, with the main repeating unit of pentagalloyl glucose. The latter are oligomeric phenolic compounds composed of flavan-3-ol repeating units widely distributed in softwoods and hard-woods, account for ∼90% global production of tannins. Tannins may be used
Bio-based epoxy thermosets derived from other bio-based monomeric phenols
Apart from the bio-based epoxy thermosets derived from cardanol, eugenol, vanillin, tannins and the related depolymerized phenols, there are increased concerns about bio-based epoxy thermosets based on other natural phenols, such as guaiacol, ferulic acid, acetovanillone, resveratrol, syringaldehyde, quercetin, salicylic acid (ESA), 4-hydroxybenzoic acid, carvacrol, dopamine, daidzein, etc. These phenols have widely distributed molecular structures, topologies and constituents. For example,
Bio-based epoxy thermosets based on phenolic adducts from mixed bioresources
From Sections 2 to 7, we have mainly discussed bio-based epoxy thermosets derived from the same kind of bio-based monomeric phenols. Nevertheless, two different bio-based monomeric phenols can be integrated into phenolic adducts, diphenols or polyphenols (some of them can be used as a epoxy curing agent), and further glycidylated to produce bio-based epoxy monomers and prepolymers. Moreover, petroleum-based phenols can be used to bond with other bio-based molecules to produce intermediates
Conclusions
The advances in bio-based epoxy thermosets and materials derived from naturally occurring and bio-transformed monomeric phenols have been reviewed, by highlighting the molecular transformations and ultimate properties of obtained thermosets. Many bio-based monomeric phenols have been used to develop unprecedented epoxy thermosets. Certain epoxy thermosets derived from bio-based phenols have been endowed with superior and more advanced properties not easily obtainable from traditional
Challenges and opportunities
Several advances in bio-based epoxy thermosets derived from naturally occurring or bio-transformed monomeric phenols can be envisioned. At the same time, some grand challenges and opportunities are ahead for the field. From the perspective of chemical transformations, converting bio-based monomeric phenols into corresponding epoxy monomers, prepolymers and curing agents should be highly efficient, cost-effective, easy to scale up, and environmentally friendly. To this end, during the course of
Additional information
It is time to close our discussion on this interesting topic. But the advances are rapid. Serval interesting research appears in recent months, in particular, the epoxy thermosets and applied materials based on (iso)eugenol [233], [234], [235], [236], [237], [238], [239], [240], [241], vanillin [242], [243], [244], [245], [246], phloroglucinol [247], [248], [249], resveratrol [250], [251], [252], cardanol [253], magnolol [254], and more [255], [256], [257], [258], [259], [260], [261], [262],
Conflict of interest statement
The authors declare that they have no conflicts of interest to this work.
Declaration of Competing Interest
The authors report no declaration of competing interest.
Acknowledgements
This work is subsidized by the National Natural Science Foundation of China (No. 21875131 and No. 21773150), the National Key R&D Program of China (No. 2017YFE0116000), and the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2020JM-283). The funding support from Fundamental Research Funds for the Central Universities (GK202003044, GK201902014 and GK201803039) is also acknowledged.
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