Under conventional production and identical devolatilization conditions: silicone oil produced via the DMC route has substantially higher cyclic siloxane content than silicone oil from the linear oligomer route.
Differences in Reaction Mechanism (Root Cause)
DMC Process (Ring-Opening Equilibrium Polymerization of Cyclosiloxanes)
DMC itself is a mixture of cyclic siloxanes D3/D4/D5/D6, which undergoes reversible equilibrium reactions under acid or alkali catalysts.
Thermodynamic equilibrium dictates that 10%–15% cyclics inherently remain in the system once the reaction completes. This fraction cannot be eliminated by polymerization alone even with extended reaction time; cyclics can only be removed by subsequent high-temperature vacuum devolatilization.
- Crude product: Total cyclics 10~15 wt%
- Finished product after standard single-stage devolatilization: Total cyclics typically 5000 ~ 20000 ppm
- Deep thin-film devolatilization: Cyclics can be reduced to below 1000 ppm, but energy consumption and cost rise sharply.
Linear Oligomer Process (Polycondensation of Hydroxyl-Terminated Linear Oligomers)
The raw material is α,ω-dihydroxypolydimethylsiloxane (linear oligomer). The reaction is dehydration polycondensation instead of equilibrium rearrangement.
An ideal catalytic system (e.g., phosphazene catalyst) only induces hydroxyl condensation to extend molecular chains and will not generate large quantities of new D4/D5 cyclics. Cyclics in the product originate from only two sources:
① Trace cyclics contained in the linear oligomer feedstock (Qualified linear oligomer raw materials are pre-devolatilized with very low background cyclic levels);
② Backbiting chain scission of the polymer backbone under strong alkali and prolonged high-temperature conditions, forming a small amount of cyclics.
Well-controlled finished products: Total cyclics generally ＜3000 ppm; premium grades can reach levels of several hundred ppm.
Important Boundary Conditions (Common Pitfalls)
Silicone oil made from linear oligomers is not absolutely cyclic-free. If strong alkali catalysts such as KOH are adopted with prolonged high-temperature reaction, backbone backbiting will also take place, leading to a marked increase in cyclic content — which may even approach that of conventional DMC silicone oil.
Low-cyclic silicone oil can also be manufactured via the DMC route: deep devolatilization with multi-stage thin-film evaporators can meet low-cyclic requirements, yet the cost is far higher than that of the linear oligomer route.
Industry Application Selection: The linear oligomer polycondensation route is preferred for textile amino silicone oils, low-volatility electronic silicone oils and REACH-compliant products, primarily because of its inherently low baseline cyclic content.
One-Sentence Summary
DMC ring-opening polymerization is an equilibrium reaction that naturally yields abundant cyclics; linear oligomer polycondensation is a non-equilibrium reaction. With proper catalyst and temperature control, cyclic content can be kept far lower.
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