)
What if the key to greener, safer composites lies in replacing a single chemical?
Unsaturated polyester resins (UPRs) are widely used in sectors like automotive, marine, and construction. Yet most conventional UPRs still rely on styrene, a volatile, potentially hazardous diluent that raises environmental and health concerns. At Megara Resins, we're developing the next-generation of bio-based UPRs with reduced styrene content, combining sustainability with the performance required for demanding, real-world applications.
UPRs Explained: The Role of Styrene
UPRs are usually produced through the direct reaction (polycondensation) between diols and unsaturated dibasic acids or anhydrides, resulting in a polyester that contains double carbon–carbon bonds along its backbone. To render this pre-polymer processable and curable, styrene is added as a reactive diluent. It lowers the viscosity for easier handling and, during curing, copolymerizes with the unsaturated sites, resulting in a rigid, crosslinked structure that defines the final material’s structure and performance [1].
Seeking Safer Alternatives
Replacing styrene is no simple task, as the choice of reactive diluent affects not only the viscosity and reactivity of the resin but also key properties of the final material, such as mechanical strength and glass transition temperature. Bio-based candidates like itaconic acid derivatives and methacrylated natural compounds are being explored as safer, more sustainable alternatives that balance processability with performance [2].
Our Approach
In the framework of the SUSPENS project, Megara Resins investigated several bio-based or less toxic alternatives to styrene. Particular focus was given to isobornyl methacrylate (IBOMA), dimethyl itaconate (DMI), and an acrylated ester of glycidyl versatate (ACE), selected for their favourable reactivity and reduced hazard profile compared to styrene. Key properties of the resulting resins, comprising acid value, viscosity, gel time, glass transition temperature (Tg), and tensile strength were evaluated using ASTM and ISO standard methods.
Key findings
It was demonstrated that DMI can substitute styrene up to 50 % wt., as full replacement led to high viscosities and lower reactivity towards curing. Notably, resins diluted in DMI/STY mixtures displayed tensile properties comparable to the benchmark ones, although DMI's high cost remains a limitation. ACE was synthesized in the lab from versatic acid glycidyl ester (CARDURA™ E10P) and evaluated as a potential alternative to styrene. It exhibited excellent compatibility with the prepolymer; however, full substitution resulted in unacceptably high viscosities. Partial replacement up to 30 % wt. was feasible, yielding resins with acceptable processing behaviour and, in most cases, improved tensile properties. Last, IBOMA as styrene substitute, exhibited moderate compatibility at room temperature resulting in UPRs with increased viscosity and turbidity. However, the use of IBOMA/styrene blends resulted in resins that met processing specifications and displayed sufficient mechanical performance. Overall, the strategy of partial styrene substitution, particularly with ACE and IBOMA, proved effective in maintaining resin performance while reducing styrene content.
From Lab to Application
Τhe next step focused on evaluating the performance of these bio-based UPR formulations in advanced applications such as Sheet Moulding Compound (SMC) technology, widely used in the automotive industry for the production of structural and semi-structural composite parts. During SMC processing, the UPR resin must initially exhibit low viscosity for efficient fibre impregnation, followed by a controlled thickening phase. This is achieved through the addition of alkaline agents (e.g. MgO), which react with the resin’s terminal carboxylic groups, forming a 3D network [3]. Acid value and molecular weight are key parameters, affecting both the kinetics and quality of the thickening process [4]. According to the results, a molecular weight of 1600–1800 g/mol, with a corresponding acid value of 25–35 mg KOH/g, is required to achieve proper thickening behaviour in SMC-grade UPRs. In addition, partial substitution of styrene with IBOMA had no adverse effect on the thickening profile, maintaining the final viscosity (penetration depth) at levels suitable for SMC processing. Conversely, replacing part of the styrene with ACE resulted in slower thickening and lower final viscosity, likely due to steric hindrance from its bulky, branched structure, which may limit interactions with the thickening agent. These findings support the potential use of IBOMA as a bio-based diluent that combines processability with reliable performance in automotive composite applications.
Conclusions
This study highlights the potential use of bio-based reactive diluents as viable alternatives to styrene in UPR resins. Among the candidates, IBOMA emerged as the most promising, combining sustainability with performance. Future work will focus on scale-up and the development of demonstrators, paving the way for safer, greener composite materials.
References
[1] Athawale, A.A. & Pandit, J.A. Unsaturated Polyester Resins: Fundamentals, Design, Fabrication, and Applications. Elsevier, 2019, Ch. 1, pp. 1–42.
[2] Gandini, A. & Lacerda, T.M. "From Monomers to Polymers from Renewable Resources: Recent Advances." Prog. Polym. Sci., 2015, 48, 1–39.
[3] Han, C.D. & Lem, K.W. "Rheology of Unsaturated Polyester Resins II: Thickening Behavior of Unsaturated Polyester and Vinyl Ester Resins." J. Appl. Polym. Sci., 1983, 28(2), 763–778.
[4] Freitag, B. & Jäger, C. "Thickening Paste for Unsaturated Polyester Resin Mixtures and Its Use in Sheet Moulding Compounds." Eur. Patent EP0926188B1, 2001.
Authors
Dimitrios K. Perivoliotis, R&D scientist, Megara Resins S.A.
e-mail: d.perivoliotis@megararesins.com
Poppy Krassa, R&D Manager, Megara Resins S.A.
e-mail: p.krassa@megararesins.com