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Reactive diluent dosage in epoxy adhesive is not a free parameter you can set by feel. Every 10 phr you add shifts the mix viscosity, the epoxy equivalent weight, and the hardener ratio at once. Yolatech supplies two diluents for this work: YLD-6009 and YLD-6021. Both stay reactive inside the film.
| Product | Chemistry | CAS | EEW (g/eq) | Viscosity @25 °C (mPa·s) |
| YLD-6009 | Neopentyl glycol diglycidyl ether (difunctional) | 17557-23-2 | 136–148 | 10–25 |
| YLD-6021 | Benzyl glycidyl ether (monofunctional) | 89616-40-0 | 208–228 | 2–8 |
YLD-6009 carries two epoxy groups and an EEW of 136–148 g/eq, lower than a standard bisphenol A resin at roughly 180 g/eq. YLD-6021 is monofunctional with an EEW of 208–228 g/eq, higher than the base resin.
That difference decides which direction your hardener ratio moves.
Take 90 parts of a base resin at EEW 180 and 10 parts diluent, cured with DACH at AHEW 28.5 g/eq.
With YLD-6009 the blend EEW is 100 ÷ (90 ÷ 180 + 10 ÷ 142) = 175 g/eq. DACH requirement becomes 28.5 ÷ 175 × 100 = 16.3 phr, up from 15.8 phr for the undiluted resin.
With YLD-6021 the blend EEW is 100 ÷ (90 ÷ 180 + 10 ÷ 218) = 183 g/eq. DACH requirement drops to 15.6 phr.
So the same 10 phr of diluent moves the required hardener in opposite directions. Formulators who dose diluent by feel and never recalculate end up with either unreacted epoxy or free amine, and both show up later as a joint that softens in humid service.
Viscosity falls steeply at first. Going from 0 to 10 phr of YLD-6021 typically takes a standard laminating resin from around 10,000 mPa·s down to workable spreadable range, and past that the curve flattens — you pay in heat deflection temperature for very little further thinning.
Practical ceilings are 15 phr for monofunctional grades and 25 phr for difunctional ones. Past that, reactive diluent dosage costs more heat deflection temperature than it buys back in flow. YLD-6009 is the better choice when you cannot afford to lose crosslink density, because its two epoxy groups re-enter the network rather than capping it.
Set the target viscosity first, pick the diluent type second, then recalculate the hardener ratio from the blended EEW. Doing it in that order keeps the adhesive's mechanical properties where you designed them.
Neopentyl glycol diglycidyl ether YLD-6009 — specification, and benzyl glycidyl ether YLD-6021. Send us your base resin EEW and target viscosity and we will work the dosage out with you.
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