Bio-based reactive diluents: Focus on supply security and regulatory requirements

How does the geographical dependence on specific vegetable oil sources (e.g. soybean, rapeseed, linseed) affect the security of supply and the carbon footprint variability of reactive diluents? What requirements should coatings manufacturers impose on supply chain transparency?

Marcel Butschle: The current geopolitical tensions around the Strait of Hormuz are a reminder that petrochemical raw materials are anything but supply-secure. While some petrochemical intermediates have roughly doubled in price over the past few months, vegetable oil prices have risen by only 5–10 %. On top of that, the raw material base for Hobum’s epoxy diluents (rapeseed, soybean and sunflower oil) can be sourced entirely from European production, which is a clear advantage for security of supply.

The variability of the carbon footprint is driven, among other things, by land-use change, in particular deforestation. With European sourcing this effect barely plays a role, and transport distances are short as well. The real problem is soy and other plant oils that are sourced from regions with slash-and-burn agriculture practices. One hectare of rainforest stores around 600 t of CO₂, which is released when the land is cleared. Allocated pro rata over a 100-year cultivation period and per kg of oil, this amounts to roughly 4 kg CO₂ equivalents. By comparison, cultivation, transport, oil extraction and chemical processing together account for only about 1 kg CO₂ (Alcock et al. 2022).

With the Deforestation Regulation (EU) 2023/1115 and 2025/2650, the European Union is taking responsibility for its consumers and, from 30/12/2026, will require solid evidence that raw materials placed on the market are deforestation-free. For coatings manufacturers, this means that alongside the bio-based content, origin is just as important.

Supply chain transparency should cover not only the bio-based content but also the region of origin and certification. For soy, recognised certifications such as RTRS (Round Table on Responsible Soy) or Donau Soja (used by Hobum in the Neo product lines) provide solid evidence of deforestation-free supply chains.

What methodological requirements would a full life cycle assessment (LCA, cradle-to-gate) for epoxidised fatty acid esters versus petrochemical glycidyl ethers need to meet in order to be decision-relevant for formulators?

Butschle: First, complete coverage of the value chain. For the petrochemical C12–C14 glycidyl ether, this means crude oil extraction, refining and chemical synthesis. For the epoxidised vegetable oil ester, it means the agricultural phase (tractors, fertilisers, land-use change), oil extraction and epoxidation.

Second, the correct allocation for co-products: in the case of soy, the bulk of the plant is used as animal feed, so only part of the environmental burden can be allocated to the oil. Third, the correct functional unit must be defined. Laboratory trials show that about 2 % more of the epoxidised vegetable oil ester is needed to achieve the same viscosity reduction, so a plain kilogram-for-kilogram comparison would be misleading.

For the Hobum products, PCF values are available. However, good data quality is often a challenge. Identical raw materials from different suppliers can show markedly different PCF values. PCF and LCA are topics that regularly fill multi-day expert workshops. A proper assessment takes a lot of effort, but for the sake of transparency it is essential in the long run. At the moment many companies are working on PCF documents, which is pushing data quality upwards.

Incidentally, the bio-based carbon content serves as an excellent complement to PCF and LCA calculations because it is, in essence, incorruptible. Unlike PCF calculations, which often require interpretation, there is no room for methodological leeway here. The value can be measured using the radiocarbon method and provides clear comparability for formulators. It directly documents that the carbon originates from atmospheric CO₂ that plants have sequestered during their growth. Even when released at the end of a product’s life, this merely closes the natural cycle without introducing additional fossil CO₂ into the atmosphere.

Could epoxidised fatty acid esters prospectively be derived from industrial side streams (such as used cooking oils or tall oil fatty acids)? What quality requirements would such secondary raw materials have to fulfil for use in high-performance coatings?

Butschle: Tall oil fatty acids are only of limited suitability for epoxidised fatty acid esters. They would first have to be esterified, which is considerably more complex and more expensive than the simple transesterification of triglycerides. In the production of epoxy hardeners, on the other hand, tall oil fatty acids are well established and are used successfully.

Used cooking oils, mostly based on sunflower oil, are technically an option. The key issue is purity: the REACH registration defines narrow limit values that a secondary raw material also has to comply with. A realistic approach would be to blend them in proportionally with fresh oils in order to stay safely within the specification.

The actual hurdle is an economic one. Used cooking oils are not waste materials. They compete directly with energy recovery, and once upgraded to industrial-grade quality, their price often exceeds that of the virgin material. In summary, yes, using side streams is technically feasible. However, the economic viability is largely dictated by the energy market.

For users, long-term planning certainty is essential. Epoxidised fatty acid esters are currently classified only as H317 (skin sensitising). Do coatings manufacturers have to worry that the classification regarding reproductive toxicity will in the future turn out to be as critical as with the glycidyl ethers, or is their safety already sufficiently supported by data? And what is the mild classification based on?

Butschle: Epoxidised fatty acid esters are currently registered under REACH in the 100–1000 t/a tonnage band. For this band, the mandatory reproductive toxicity tests have been carried out. The quoted result is:“The available data are reliable and suitable for classification purposes under Regulation 1272/2008. No adverse effects on fertility or development were observed. As a result, the substance is not considered to be classified for fertility or developmental toxicity […].”

However, full long-term and carcinogenicity studies only become mandatory under REACH at tonnages of ≥ 1000 t/a (Annex X). So the same depth of data as for the C12–C14 glycidyl ether is not yet available. However, since the starting materials used in the production of epoxidised fatty acid esters are less hazardous (for instance, no epichlorohydrin), a stricter classification is not to be expected in the long term either.

How can coatings manufacturers methodically document the transition from CMR-classified reactive diluents to bio-based alternatives in their CSRD-compliant sustainability reporting? Which KPIs would be suitable for this purpose?

Butschle: CSRD requires reporting along the European Sustainability Reporting Standards (ESRS). The switch from CMR-classified to bio-based reactive diluents touches several of these standards and can be reflected consistently across the framework, both as a qualitative action and through quantitative metrics.

• ESRS E2 – Pollution. This is the central standard for the removal of the CMR classification. CMR substances are among the substances of very high concern (SVHC); their production, use and distribution have to be reported. The substitution is captured as a concrete action, supported by an absolute or percentage reduction in CMR use compared to the previous year.
• ESRS E5 – Resource Use and Circular Economy. This is where the move to the bio-based alternative directly kicks in. ESRS E5 requires disclosures on the share of renewable materials in the material inflows. A measurable increase in the bio-based raw material share of the total purchasing volume can be reported here.
• ESRS E1 – Climate Change. If the new reactive diluent has a lower PCF, this is reflected in the Scope 3.1 emissions (purchased goods and services).
• ESRS S1 – Own Workforce. Eliminating CMR substances significantly reduces exposure risks in production, filling and mixing. This can be reported as a strategic action to prevent occupational diseases under the health & safety metrics.
• ESRS S4 – Consumers and End-Users. For applicators (painters, tradespeople, end customers), the health risk is reduced through product safety based on low-hazard formulations — a reportable positive impact.
• ESRS 2 – Overarching Strategy. The switch reduces regulatory risk by getting ahead of anticipated REACH restrictions on CMR substances. It can also remove certain hazardous-substance reporting obligations from scope. For example, ESRS E2-5 disclosures may no longer be deemed material once these substances are eliminated. At the same time, it opens up new market opportunities through green building certifications such as DGNB.

Green building certifications such as DGNB increasingly demand low-emission and less hazardous building materials. Is replacing the reactive diluent alone sufficient to significantly improve the overall rating of an epoxy floor coating system, or do additional formulation components need to be addressed?

Butschle: The main criterion here is hazardous and risk substances (ENV1.2). Within this criterion, four quality levels apply to interior construction, from QS1 (10 points) up to QS4 (60 points). Each level imposes progressively stricter requirements on hazardous-substance labelling, emission behaviour and ingredients.

Because conventional C12–C14 glycidyl ethers are classified as reproductive toxicants Cat. 1B (H360F), coatings containing them fall into GISCODE RE90. DGNB has reacted to this and introduced a transitional rule that temporarily still allows RE90 for the highest quality level (QS4) — but that is not future-proof. Once this exemption expires, the coating no longer meets even the basic requirements (QS1) because of the CMR substance and can no longer be credited under ENV1.2.

Replacing it with an epoxidised fatty acid ester (without CMR classification) shifts the coating back into GISCODE RE30. As a result, the coating can again be rated up to QS4 (60 points) in a future-proof way. That is on the condition, of course, that the rest of the formulation is also low in hazardous substances and emissions.

How can epoxy coatings containing bio-based reactive diluents be treated at the end of their service life in terms of a circular economy? Are there differences compared to conventionally formulated systems with regard to thermal recovery, chemical recycling or landfill behaviour?

Butschle: The key difference is that bio-based products act as CO₂ storage during their service life. On combustion, the bound CO₂ returns to the atmosphere, but unlike petrochemical raw materials no additional, fossil CO₂ is released.

Regarding degradability, no significant differences compared to conventionally formulated systems are expected. Epoxy coatings are highly cross-linked thermosets. The bulk of the formulation consists of base resin and hardener, often amine-based. This network is chemically and mechanically extremely resistant, and that is what largely determines the degradation behaviour. There is the option of breaking down anhydride-cured epoxies by boiling them with glycols and reusing the resulting polyol solutions, but this is primarily relevant in the field of fibre-reinforced composites.

If sustainability is considered holistically: how could an epoxy coating system be designed that targets maximum bio-based content and minimum toxicity not only in the reactive diluent but also in the resin, hardener, pigments and fillers? Where do the current technological limitations lie?

Butschle: The hardener technology is the most mature. Phenalkamines from cashew nut shell liquid (CNSL) and polyaminoamides from vegetable oil fatty acids reach bio-based contents of 60 % or more, with performance that fully meets industrial requirements. There are hardly any real limitations left in this segment.

The technological limit at the moment sits with the epoxy resin itself. Bisphenol-A resins with around 30 % bio content are commercially available. This is achieved by using bio-based epichlorohydrin from glycerol. The hard limit is the bisphenol-A backbone: fully bio-based resins with comparable mechanical and chemical resistances are not available on the market. Bio-aromatics from lignin are a promising route. The hurdles lie in industrial scalability and in the heterogeneity of the lignin starting fractions, which vary strongly with origin and pulping process.

Epoxidised vegetable oils as a sole resin component are too soft and too slow in reactivity for heavy-duty industrial floors. Isosorbide from starch would offer a fully bio-based alternative, but it is prone to pronounced water uptake.

For pigments and fillers, lignin seems to be promising again. However, the biggest impact comes from avoiding energy-intensive primary raw materials rather than from switching to bio-based alternatives. High-volume industrial by-products such as recycled glass powder, electric arc furnace slag or granulated blast furnace slag could reduce the PCF compared to freshly mined mineral raw materials. A forward-looking approach would be to grind up cured epoxy coatings at end of life and reuse the powder as a filler in new systems. But of course this assumes that the waste can be collected and processed economically.

You can find Marcel Butschle’s article on bio-based reactive diluents in the May issue of European Coatings Journal.

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