Exploring the development of bio-based building blocks and reactive diluents for greener unsaturated polyester resins combining performance with planetary responsibility.
From the sleek bodies of modern yachts to lightweight components in electric vehicles and the durable tanks in water treatment facilities, unsaturated polyester resins (UPRs) are the invisible workhorses behind countless products we encounter daily. For decades, these versatile materials have been favored for their excellent mechanical properties, chemical resistance, and cost-effectiveness.
Petroleum-based chemistry with significant environmental burden.
Reimagining materials with renewable resources for sustainability.
However, their traditional foundation in petroleum-based chemistry comes with a significant environmental burden. Today, a quiet revolution is underway in laboratories and industrial plants around the world, where scientists are reimagining these fundamental materials. This article explores the exciting development of bio-based building blocks and reactive diluents that are paving the way for a new generation of greener unsaturated polyester resins, combining performance with planetary responsibility.
For all their utility, conventional UPRs present a dual environmental challenge. First, their production relies heavily on non-renewable petrochemical feedstocks, contributing to the depletion of fossil resources and generating a substantial carbon footprint throughout their lifecycle 7 . Second, and perhaps more critically, most UPR formulations depend on styrene as a reactive diluent—a substance added to adjust the resin's viscosity for easier processing.
Styrene content in typical UPR formulations
This combination of non-renewable sourcing and problematic chemistry has driven the search for sustainable alternatives that can match or even exceed the performance of their traditional counterparts.
At the heart of the green revolution in UPRs is the innovative use of renewable biomass to create the fundamental chemical building blocks for these resins. Researchers are successfully developing UPRs where a significant portion—in some cases 100%—of the molecular structure originates from bio-based sources.
Produced through the fermentation of carbohydrates by fungi, itaconic acid provides the crucial carbon-carbon double bonds that enable the cross-linking and hardening of UPRs. It serves as an excellent bio-based alternative to petroleum-derived maleic anhydride 7 .
Often termed the "bio-based equivalent of terephthalic acid", FDCA can be derived from sugars through various chemical and biological pathways. When incorporated into polyester chains, FDCA's rigid furan ring significantly enhances the thermal properties and stiffness of the resulting resin 7 .
Replacing styrene has proven to be one of the most challenging aspects of developing truly sustainable UPRs. An ideal reactive diluent must fulfill multiple requirements: it must effectively reduce viscosity for easy processing, display low volatility to minimize emissions, copolymerize efficiently with the unsaturated polyester backbone, and yield final materials with excellent thermal and mechanical properties.
Derived from itaconic acid, DMI offers significantly lower volatility and toxicity compared to styrene. Though early formulations with pure DMI faced challenges with higher viscosity, recent research has demonstrated that optimized mixtures with other bio-based diluents can overcome this limitation while maintaining excellent reactivity and final material properties 3 .
While traditionally petroleum-derived, MMA can now be produced from renewable sources such as citric acid or itaconic acid. When blended with DMI, MMA creates a synergistic effect, substantially lowering the overall resin viscosity while participating effectively in the cross-linking polymerization 3 .
The search continues to expand the toolbox of bio-based reactive diluents. Butanediol dimethacrylate (BDM) has been investigated for its ability to form densely cross-linked networks, while vinyl levulinate (derived from biomass) and various fatty acid-based monomers (from plant oils) offer additional pathways toward styrene-free formulations 3 7 .
A pivotal 2021 study published in the journal Polymers directly addressed one of the most significant challenges in bio-based UPRs: achieving workable viscosity without compromising material performance 3 .
Researchers first synthesized an unsaturated polyester prepolymer through melt polycondensation of itaconic acid, succinic acid, and 1,2-propandiol at temperatures gradually increasing from 110°C to 190°C.
The resulting bio-based polyester was divided into seven aliquots, each dissolved in different DMI/MMA mixtures ranging from 100% DMI to 100% MMA.
Each formulated resin was cured using methyl ethyl ketone peroxide (MEKPO) as the initiator, creating rigid, cross-linked thermoset materials for evaluation.
The researchers then meticulously characterized the viscosity, curing behavior, gel content, and thermomechanical properties of each formulation.
The experimental results provided compelling insights into the structure-property relationships of these green materials. The systematic variation of the DMI/MMA ratio revealed clear trends in both processing characteristics and final material performance.
| DMI/MMA Ratio | Viscosity (mPa·s) | Gel Content (%) | Glass Transition Temp. (°C) |
|---|---|---|---|
| 100/0 | 2850 | 92 | 68 |
| 90/10 | 2150 | 94 | 72 |
| 75/25 | 1480 | 95 | 74 |
| 50/50 | 850 | 93 | 71 |
| 25/75 | 520 | 91 | 69 |
| 0/100 | 380 | 89 | 65 |
The data revealed a clear trade-off between processability and performance. While increasing MMA content progressively lowered viscosity, the highest gel content and thermal stability were achieved at intermediate DMI/MMA ratios (75/25 to 50/50).
The study demonstrated that properly formulated bio-based UPRs could achieve thermomechanical properties comparable to conventional styrene-based systems, while offering substantially improved environmental and safety profiles. The 75/25 DMI/MMA mixture emerged as a particularly promising formulation, reducing viscosity by nearly 50% compared to pure DMI while maintaining excellent cross-linking density and thermal stability.
| Reagent | Function | Bio-Based Source |
|---|---|---|
| Itaconic Acid | Provides unsaturation for cross-linking | Fermentation of carbohydrates |
| FDCA | Imparts rigidity and high Tg to polyester backbone | Sugars from biomass |
| 1,2-Propanediol | Diol component for polyester synthesis | Corn sugar or glycerol |
| Dimethyl Itaconate (DMI) | Low-toxicity reactive diluent | Derived from itaconic acid |
| Methyl Methacrylate (MMA) | Viscosity-reducing co-diluent | Citric or itaconic acid |
| Methyl Ethyl Ketone Peroxide | Initiator for radical cross-linking polymerization | Synthetic (required for curing) |
| Zinc Acetate | Catalyst for polycondensation reaction | Synthetic (facilitates polymerization) |
The transition of bio-based UPRs from laboratory curiosity to commercial reality is already underway, driven by increasing regulatory pressure, consumer demand for sustainable products, and growing corporate sustainability commitments.
The global bio-based resins market, valued at approximately $4.8 billion in 2021, is projected to reach $16.3 billion by 2033, representing a compound annual growth rate of 10.65% 1 .
This robust growth reflects a fundamental shift in material preferences across industries.
Major chemical companies are forming strategic alliances to advance bio-based resin technologies. In September 2024, Exel Composites partnered with INEOS to commercialize the use of bio-based ENVIREZ resins, which reduce carbon emissions by 21% compared to traditional fossil-based resins while maintaining identical performance characteristics 8 .
Policies such as the European Union's Green Deal and the U.S. Renewable Chemicals Act are creating powerful incentives for the development and adoption of bio-based alternatives to conventional petrochemical products 9 .
Companies are increasingly demonstrating that bio-based UPRs are not just "green" alternatives but high-performance materials in their own right. Recent innovations have yielded bio-based UPRs with enhanced thermal stability, mechanical strength, and processing characteristics that equal or surpass their petroleum-based counterparts 4 .
The journey toward sustainable unsaturated polyester resins represents more than just a technical challenge—it embodies a fundamental rethinking of our relationship with materials. By harnessing the molecular complexity of renewable biomass, researchers are developing UPRs that break our dependence on fossil resources while eliminating hazardous chemicals from production processes.
The remarkable progress in bio-based building blocks and reactive diluents demonstrates that environmental responsibility and high performance are not mutually exclusive.
Each bio-based resin formulation represents a step toward a circular economy where materials are sourced responsibly, designed for optimal performance, and considered for their entire lifecycle.
The scientific innovations highlighted in this article not only address technical requirements but also contribute to building a more sustainable relationship between industry and our planetary resources—proving that the drive toward green chemistry is ultimately a drive toward a more sustainable future for all.