Reactive Diluents in Epoxy Systems — Types, EEW and Formulation Guide
Reactive diluents (RDs) are low-viscosity epoxide-functional compounds added to epoxy resin systems to reduce application viscosity without using volatile solvents. Unlike inert diluents or solvents, reactive diluents contain one or more epoxide groups that participate in the cure reaction, becoming chemically bound into the crosslinked network. This guide covers the main reactive diluent types, their epoxy equivalent weight (EEW), effect on cured film properties, and how to calculate the correct hardener stoichiometry when using them.
Why Use Reactive Diluents?
The primary reasons for incorporating reactive diluents into an epoxy system are:
- Viscosity reduction — Standard bisphenol A epoxy resin (YR-128/828) has viscosity of 11,000–14,000 mPas at 25°C. Even at 10 phr loading, C12-C14 glycidyl ether (RD12) reduces this to 4,000–6,000 mPas, enabling spray or brush application without solvent addition
- Solvent-free systems — Replacing volatile solvents in floor coatings and industrial maintenance systems to reduce VOC emissions
- Pot life extension — Some monofunctional diluents slightly extend pot life by diluting reactive epoxide concentration
- Flexibility improvement — Long-chain aliphatic glycidyl ethers add chain length between crosslinks, improving impact resistance and flexibility
Reactive Diluent Types — Comparison
| Diluent | CAS | Functionality | EEW (g/eq) | Viscosity (mPas, 25°C) | Effect on Tg |
|---|---|---|---|---|---|
| RD12 — C12-C14 AGE | 68609-97-2 | Monofunctional | 285–310 | 5–12 | Moderate reduction |
| RD8 — C8-C10 AGE | 68609-97-2 | Monofunctional | 205–240 | 2–5 | Greater reduction |
| BGE — Butyl glycidyl ether | 2426-08-6 | Monofunctional | 130–145 | 2–4 | Significant reduction |
| BDGE — Butanediol DGE | 2425-79-8 | Difunctional | 110–130 | 15–25 | Maintains crosslink density |
| CHDM DGE | 14228-73-0 | Difunctional | 145–165 | 50–100 | Minimal — maintains Tg |
| Benzyl alcohol | 100-51-6 | Non-reactive (OH) | N/A | 5–6 | Significant — acts as plasticiser |
EEW and Stoichiometry — Calculation When Using RD
When a reactive diluent is added to an epoxy resin, the effective EEW of the blend changes. The hardener quantity must be calculated based on the blend EEW, not the resin EEW alone. Incorrect stoichiometry leads to either under-cured (soft, tacky) or over-cured (brittle) films.
Blend EEW Formula
Blend EEW = 100 / [(% resin / EEW resin) + (% diluent / EEW diluent)]
Example: 90 parts YR-128 (EEW 185) + 10 parts RD12 (EEW 295)
Blend EEW = 100 / [(90/185) + (10/295)] = 100 / [0.4865 + 0.0339] = 192 g/eq
Use 192 as EEW when calculating polyamide or amine hardener PHR.
Effect on Film Properties
| RD Type | Loading (phr) | Viscosity reduction | Tg change | Flexibility | Recommended use |
|---|---|---|---|---|---|
| RD12 (C12-C14 AGE) | 5–10 | 30–50% | -5 to -10°C | Improved | Floor coatings, industrial maintenance |
| RD12 | 10–20 | 50–70% | -10 to -20°C | Good flexibility | Adhesives, flexible coatings |
| BGE | 5–10 | 40–60% | -15 to -25°C | Moderate | Only where flexibility matters more than Tg |
| BDGE (difunctional) | 5–15 | 20–40% | Minimal | No loss of hardness | When Tg and hardness must be maintained |
Skin Sensitisation — Critical Handling Note
All C8–C14 alkyl glycidyl ethers are classified as skin sensitisers (H317) under GHS/CLP. First exposure rarely causes reactions, but repeated skin contact without PPE can lead to occupational contact dermatitis and permanent sensitisation. Once sensitised, even trace exposure causes reactions.
- Always wear nitrile gloves (≥0.1mm) when handling RD12 or BGE
- Safety glasses or goggles for splash protection
- Ensure good ventilation — low vapour pressure but enclosed spaces accumulate vapour at elevated temperatures
- Wash skin immediately with soap and water if contact occurs — do not use solvents to clean skin
Source Reactive Diluent from Raykem
RAYCURE RD12 — C12-C14 alkyl glycidyl ether. EEW 285–310. Stocked in Dubai. UAE and Saudi Arabia supply. SDS and CoA included.
View RD12 Product → Request a Quote