A Mini Review on Bio-Epoxy Resins as Industrial Products from Algae

Particularly when considered in terms of clean production, the need for water, energy, minerals and other natural resources is increasing alongside population growth and improvements in living standards and welfare levels. Intensive resource use consumes and damages numerous ecosystem services, increases the risk of sudden and destructive environmental change, causes significant reductions in biodiversity, and simultaneously intensifies poverty for certain groups. What is important here is to address the rising energy demand resulting from the industrial revolution without bringing natural resources to the limit of sufficient use, through sustainable methods such as heat recovery and energy production from waste. In our work, a brief review will be provided on both the rationale and current technology for the biotechnological production of epoxy resin, an industrial intermediate, from algae and seawater that are abundantly found in nature, even in urban treatment systems, coastal areas, and unrestored riverbanks.

Section 1: Introduction

Microalgae are a potential renewable energy source, but interestingly have become an alternative raw material source for the production of ole-chemicals (See Figure 1). Unsaturated oils produced from Schizochytrium microalgae can also be used in polymer synthesis. After structural analysis of algal oil, it is converted to an active form containing 10 epoxy radicals in one molecule through a cation exchange resin. Subsequently, fully bio-based epoxy networks are synthesized with priamine® (oleic acid dimer derivative amine) as a hardener. These new algal epoxy materials are characterized and compared with bio-based epoxy networks derived from vegetable oils (linseed or soybean oils). Epoxy co-monomers are added to the formulation to improve material properties. Both trimethylolpropane triglycidyl ether, a classical cross-linking agent, and triepoxidized phloroglucinol derived from algal phlorotannins are added.

The addition of epoxy co-monomers allows obtaining materials with various properties. Thermal and mechanical properties emphasize the interesting superiority of algal oils due to their high reactive content in reactive functions.

Finally, the addition of a blowing agent (MH15 polysiloxane) leads to the synthesis of algal oils resulting in foams with interesting properties. Thus, algal oil shows interesting potential as a suitable starting material for fully bio-based thermoset materials and foams. Although the typical synthesis example given above appears simple, it is important that each stage is validated with various quality control methods.

Figure 1. Synthesis of various biopolymers from algae

Section 2: Practical Applications The main reactant exemplified above, namely epoxidized algal oil, can provide bio-based aliphatic building blocks for other epoxy foam syntheses by reacting with amines derived from vegetable oils and another aromatic epoxy co-monomer (see Figure 2). The resulting epoxy materials and foams are bio-based with various hardnesses and high cross-linking ratios (see Figure 3). Materials can be used in electronic and electrical systems or as high-performance adhesives. Foams can be applied as thermal and vibration insulation and for the production of lightweight materials or for the transportation industry such as aircraft-interior panels and impact cushions (see Figure 4). Medical applications can also be considered for more flexible foams. Figure 2. Example of produced bio-based macromolecule Figure 3. Example of produced bio-based macromolecule  

Figure 4. Bio-based macromolecule

Section 3: Detailed Analysis Heat-cured epoxy materials are widely used in a broad range of applications including aircraft, automotive or electronic components due to their high mechanical strength, thermal and chemical stability, and excellent dielectric properties (see Figure 6). Global epoxy thermoset polymer production was estimated at 2 million tons per year in 2010 and was projected to reach 3 million tons by 2017 (see Figure 7 for test results of produced WNDR algal polymer). More than 60% of global production is used in the coatings sector and also as composites and foams. Indeed, epoxy foams are showing increasing interest in industry due to their insulation and lightweight properties (see Figure 8).

Figure 6. Ski equipment manufactured with coating material derived from algae

Figure 7. WNDR test results

Figure 8. Application process of WNDR algal polymer to ski equipment

Currently, 95% of heat-cured epoxy materials are derived from the diglycidyl ether of bisphenol A (DGEBA). The aromatic rings of bisphenol A (BPA) are particularly interesting because they provide good thermal resistance to epoxy resins.

However, this endocrine disruptor can mimic the body's own hormones and cause various adverse health effects including changes in brain chemistry and structure, behavior, immune system, enzyme activity, and alterations in male and female reproductive systems in various animals such as fish, frogs, and mammals, including snails. Therefore, there is growing interest from the chemical industry in non-hazardous reactants that allow the synthesis of BPA-free epoxy resins. Uncertainty in the price and availability of petroleum, in addition to global trends toward sustainable development principles, is encouraging the chemical industry toward sustainable production and, in particular, the use of renewable resources to synthesize bio-based chemicals and products. Thus, partially or fully bio-based epoxy polymers are a real objective today in both academic research and industrial terms, but also present a real challenge.

With the exception of the most commonly used bio-based monomers, epoxidized vegetable oils (such as soybean oil or linseed oil) and cardanol, there are few commercial bio-based epoxy reactants available.

However, polyepoxide networks derived from vegetable oils generally exhibit low glass transition temperatures due to the presence of long aliphatic chains (such as Tg=-38°C for epoxidized linseed oil cured with an amine derived from grape seed oil). Some researchers were among the first teams to synthesize polyepoxide foams with 25% epoxidized soybean oil (ESO), but poor mechanical properties were achieved. Subsequently, other researchers reported the synthesis of soybean-containing polyepoxide foams with high mechanical properties. On the other hand, researchers also evaluated the acrylation of the epoxide ring of various epoxidized triglycerides (obtained in two stages) and the effect on the glass transition temperature of the polymers obtained from the acrylate content (0 to 9 acrylates per triglyceride). Their methodology showed that increasing reactive functions within a triglyceride led to increasing thermal and mechanical properties for polymers. Therefore, the use of algal triglycerides with higher double bond content could be a very interesting alternative for vegetable oil-based polyepoxide networks. Algal oil and more specifically microalgal oil is a triglyceride source that cannot be used as a nutrient source for polymer materials. Microalgae can be cultivated on uncultivated land, in raceway ponds, or in photobioreactors. Additionally, costly nutrients such as glucose or peptone are not required. Most of these microalgal species contain unique products such as carotenoids, antioxidants, fatty acids, enzymes, polymers, peptides, toxins, and sterols. And more specifically, the amount of fatty acids in microalgae can be significant, up to 20-50% by weight of dry matter, with high yields. Algal oils depend on the algae species, but in any case present interesting structures enriched with long-chain fatty acids with high unsaturation content, such as docosahexaenoic acid (DHA) with 22 carbons and 6 double bonds. Recently, some studies report the synthesis of polymers from algal oils.

For example, in one study, stepwise polymerization of algal oil-derived diols with dicarboxylic acids was achieved. New polyesters with advantageously high melting and crystallization temperatures were obtained.

In another case, the extraction of algal oil with high yield compared to vegetable data and various cultivation conditions showed interest as polyol precursors for polyurethanes. In another study, flexible polymer materials were synthesized from epoxidized algal oil. The resulting polyurethane materials and foams exhibit good thermal and mechanical properties similar to petrochemical polyurethanes. In another study, rigid polyurethane foams were synthesized from Chlorella algal oil. These algal polyols were prepared through oxidation and epoxide ring-opening reactions using lactic acid or ethylene glycol. The final polyols were reacted with methylene diphenyl diisocyanate (MDI) using cyclopentane as a blowing agent to give PU rigid foams. In the same study, a process for the preparation of biopolymer composites from algal oil and natural fibers was patented.