Silicone oils and polysiloxane polymers can contain residual cyclic siloxanes, low-molecular-weight linear siloxanes, water and other volatile components after polymerization.
Typical targets include D3, D4, D5, D6 and higher cyclic siloxanes, depending on the silicone chemistry and final product specification.
For low-viscosity materials with a relatively high volatile content, conventional vacuum stripping or thin-film evaporation may already be sufficient. As the silicone becomes more viscous, the residual volatile target becomes lower, or thermal exposure becomes more critical, wiped-film and short-path evaporation become more attractive.
The main process question should therefore be: Which volatile siloxanes need to be removed, how low must the final residual level be, and how viscous will the silicone become during devolatilization?
Silicone Monomer Purification or Silicone Polymer Devolatilization?
These are not the same process.
Industrial chlorosilane monomers such as methylchlorosilanes are commonly separated by multistage rectification because many of the components have relatively close boiling points.
Short-path molecular distillation is generally more relevant after silicone polymerization, when the process objective is removing volatile cyclic or linear siloxanes from a higher-molecular-weight silicone oil or polymer.
For HJCHEM molecular distillation equipment, the more realistic applications therefore include:
| Feed | Typical Devolatilization Target |
|---|---|
| Dimethyl silicone oil | D3, D4, D5, D6 and other light siloxanes |
| Vinyl silicone oil | Residual cyclic siloxanes and low-boiling fractions |
| PDMS | Low-molecular-weight cyclic and linear siloxanes |
| RTV silicone base | Residual cyclics and volatile components |
| LSR silicone | Volatile siloxanes and low-molecular-weight fractions |
| Specialty silicone polymer | Residual monomer, oligomer or light-end polishing |
Professional polymer-processing suppliers specifically list cyclic-siloxane removal and monomer/oligomer stripping from silicone polymers as wiped-film applications.
Why Are D4, D5 and D6 Removed from Silicone Oil?
Cyclic siloxanes can remain after polymerization or form as low-molecular-weight by-products.
Depending on the final application, manufacturers need to reduce these components to control: product volatility, odor, weight loss, outgassing, downstream processing behavior and final product specifications.
Published silicone-process literature specifically identifies cyclic polysiloxanes such as D3, D4, D5 and D6 as common volatile components in silicone polymers and compositions.
The target should still be defined analytically.
For example:
Reduce total D4–D6 from 1.5 wt.% to below 0.3 wt.%
is far more useful for equipment selection than:
Produce high-purity silicone oil.
Why Is Silicone Devolatilization Difficult?
The separation becomes more difficult as polymer molecular weight and viscosity increase.
A low-boiling cyclic molecule is volatile enough to evaporate under vacuum, but it still has to move from inside a viscous liquid phase to the vapor-liquid interface.
This creates several engineering challenges:
High viscosity limits natural mixing and diffusion.
Thermal sensitivity limits how long the polymer should remain at elevated temperature.
Very low residual targets require increasingly efficient mass transfer.
Higher-boiling cyclics may be more difficult to remove than lighter ones.
This is why simply heating a large batch under vacuum is not always the most efficient approach.
Why Use Wiped-Film Processing?
A wiped-film evaporator continuously spreads silicone over a heated wall as a thin, renewed liquid film.
This provides a large active surface while avoiding the long hot residence time associated with a large batch vessel.
The main benefits for silicone devolatilization are:
1. Thin-film formation
Shortens the distance volatile components must travel through the viscous polymer.
2. Continuous surface renewal
The wiper continually exposes fresh material to the evaporation surface.
3. Short residence time
Helps limit unnecessary thermal exposure.
4. Continuous processing
Feed enters while devolatilized silicone and volatile condensate leave separately.
5. Viscous-feed handling
Proper wiper, feed-pump and discharge design can handle materials that become difficult to process in ordinary evaporation equipment.
The main equipment components should therefore be selected around the actual viscosity, volatile load and final residual specification.
When Is Short-Path Molecular Distillation Needed?
Not every silicone devolatilization process requires a short-path molecular still.
A conventional wiped-film evaporator with an external condenser is sufficient when moderate vacuum already achieves the target.
Short-path molecular distillation becomes more useful when: the silicone is very viscous, deeper process vacuum is required, the residual volatile target is difficult to reach, thermal residence must be minimized, or vapor-side pressure drop begins to limit external-condensation operation.
A short-path still places the condenser inside or very close to the evaporation surface, reducing vapor travel distance and pressure drop.
This is particularly useful when operating at very low absolute pressure.
The required vacuum specifications should therefore come from the process duty rather than simply selecting the lowest pump pressure available.
Short-Path Silicone Devolatilization Parameters
A published industrial patent provides a useful reference for high-viscosity silicone-polymer devolatilization in a short-path evaporator.
The feed was a vinyl-containing PDMS silicone polymer, and the process targeted volatile cyclic siloxanes.
| Parameter | Published Example |
|---|---|
| Feed | Vinyl-containing PDMS |
| Published feed viscosity | About 10,000 Pa·s |
| Main volatile fraction | D4–D18 |
| Feed/preheat temperature | About 210°C |
| Short-path evaporator temperature | About 207°C |
| Operating absolute pressure | About 3 Pa |
| Rotor speed | About 7 rpm |
| Initial D4–D18 volatiles | 1.3% |
| Final D4–D18 volatiles | 0.3% |
| Product cooling after discharge | About 80°C |
The silicone was fed by gear pump, spread into a thin layer by the rotor/wiper system, and volatile condensable components were collected by an internal condenser.
The same patent describes a broader operating envelope of approximately:
Temperature: 150–240°C
Absolute pressure: 0.01–500 Pa
with preferred pressure ranges down to approximately 0.2–10 Pa for certain high-viscosity silicone compositions.
Do Not Copy These Settings Directly
These values apply to specific silicone formulations and equipment.
They do not mean that every silicone oil should run at:
207°C / 3 Pa / 7 rpm.
The required conditions change with:
viscosity + D4/D5/D6 content + polymer molecular weight + feed rate + thermal stability + evaporation area + target residual level.
This is why material trials are especially useful for silicone devolatilization.
A Practical Silicone Devolatilization Process
For feeds with a significant volatile load, a practical flow can be:
Polymerization → Preheating / Degassing → Primary Devolatilization → Short-Path Polishing → Product Cooling
1. Primary Devolatilization
The first stage removes the larger load of easy-to-evaporate components.
This can reduce the vapor load presented to the deeper-vacuum system.
2. Short-Path Polishing
A second stage can operate at lower pressure to remove the more difficult residual cyclics and light oligomers.
3. Product Cooling
Highly viscous silicone often needs controlled cooling after discharge so that pumping and product handling remain manageable.
This staged logic is generally more useful than forcing all volatile removal into one extreme operating point.
Which Process Parameters Matter Most?
1. Feed Temperature
High-viscosity silicone needs preheating before stable pumping and film formation are possible.
The required temperature depends on the viscosity-temperature relationship of the actual silicone.
2. Operating Pressure
Reducing pressure increases the driving force for volatile removal and allows evaporation at a lower material temperature.
However, extremely deep vacuum is only useful if the condenser, cold trap and vacuum train can handle the vapor load.
HJCHEM molecular distillation systems typically achieve a complete-system no-load vacuum of 0.1–1 Pa, with 0.01–0.1 Pa available as a high-vacuum option.
These are equipment capability figures, not recommended silicone operating pressures.
Operating pressure under feed can be much higher.
3. Feed Rate
Feed rate affects: film thickness, residence time, vapor load, throughput and final residual volatile content.
Running faster can increase gross throughput while reducing devolatilization per pass.
4. Wiper Speed
Wiper speed affects film renewal, liquid distribution and residence behavior.
The correct speed depends on rotor design and viscosity.
5. Feed and Final Viscosity
Always report viscosity together with temperature.
For example: 20,000 mPa·s at 120°C is useful. 20,000 mPa·s without a measurement temperature is much less useful.
The final devolatilized silicone also considerably harder to pump than the starting material.
D4, D5 and D6 Are Not Equally Easy to Remove
The different cyclic siloxanes do not behave identically.
A published high-viscosity PDMS devolatilization study using another intensified process reported that D4 and D5 were easier to remove than D6 under the tested conditions.
In that example:
D4 decreased from approximately 5% to 0.28%.
D5 decreased from approximately 3.5% to 0.4%.
D6 decreased from approximately 1.2% to 0.38%.
This is another reason why we should provide the individual D3/D4/D5/D6 composition, rather than only stating “total volatiles.”
What Short-Path Distillation Cannot Remove
Short-path molecular distillation is a volatility-based separation process.
It is not suitable for every silicone impurity.
It will not directly solve problems caused by: nonvolatile catalyst residues, inorganic fillers, silica particles, gels, cross-linked polymer, nonvolatile pigments or chemically bound impurities.
Filled silicone formulations also require special attention because fillers can significantly change viscosity, film behavior, pumping and mechanical wear.
The equipment must therefore be designed around the complete silicone formulation, not only the siloxane polymer.
How Should the System Be Sized?
Silicone-devolatilization equipment should not be selected from feed rate alone.
A 100 kg/h feed containing 0.5% volatiles and a 100 kg/h feed containing 10% cyclic siloxanes place completely different loads on the evaporator and condenser.
Useful sizing data include:
| Process Data | Why It Matters |
|---|---|
| Silicone type | Defines chemistry and process behavior |
| Initial D3/D4/D5/D6 | Defines volatile load |
| Target cyclic content | Defines required separation |
| Feed viscosity | Determines pumping and film behavior |
| Viscosity temperature | Makes viscosity value meaningful |
| Feed rate | Defines throughput requirement |
| Feed temperature | Affects viscosity and thermal duty |
| Maximum allowable temperature | Defines thermal limit |
| Existing vacuum data | Helps establish process window |
| Final viscosity | Determines discharge-pump requirement |
Accurate system sizing should use actual volatile-removal duty and validated operating data rather than a generic kg/h number.
Why Material Trial Testing Is Valuable for Silicone
Silicone is one of the applications where test data can greatly reduce scale-up risk.
Two silicone oils with the same nominal viscosity may still behave differently because of: molecular-weight distribution, cyclic-siloxane content, fillers, functional groups, thermal stability and required residual-volatiles target.
HJCHEM Material Trial Service
HJCHEM has test equipment available for customer material trials.
Customers can send silicone oil or polymer feed when the process parameters have not yet been established.
A paid test can be used to evaluate: evaporator temperature, operating pressure, feed rate, wiper speed, film behavior, distillate/residue ratio and achievable D3/D4/D5/D6 reduction.
The trial is chargeable because it requires equipment preparation, test operation, cleaning and engineering time.
If the customer later places an equipment order with HJCHEM, the trial fee can be credited back against the purchase order.
For high-viscosity or low-residual-target silicone projects, this approach is generally more reliable than selecting an industrial system from literature parameters alone.
What Information Should You Send for a Trial or RFQ?
For preliminary evaluation, provide:
- silicone oil or polymer type;
- D3/D4/D5/D6 analysis where available;
- total volatile content;
- feed viscosity and measurement temperature;
- final target for cyclic siloxanes;
- feed capacity;
- feed temperature;
- maximum acceptable product temperature;
- presence and percentage of fillers;
- functional groups or additives;
- current devolatilization method;
- existing process data;
- sample quantity available for trial.
These buyer checks help determine whether a conventional wiped-film evaporator, a short-path molecular still, or a staged process is the better configuration.
HJCHEM Silicone Devolatilization Equipment
LABORATORY
Laboratory Glass Molecular Distillation
0.06–0.50 m²
Borosilicate Glass
Up to 20 kg/h*
For R&D, feasibility testing, and process development.
PILOT SCALE
Pilot Stainless Steel Molecular Distillation
0.06–0.50 m²
SS316L
Up to 50 L/h*
For process scale-up, validation, and pilot production.
INDUSTRIAL
Industrial Wiped Film Molecular Distillation
1–10 m²
SS316L
Up to 600 L/h*
For continuous industrial production and higher throughput.
Depending on process requirements, HJCHEM can configure laboratory, pilot and industrial wiped-film / short-path molecular distillation systems.
A complete system can include: heated feed tank, gear-pump feeding, wiped-film evaporator, internal condenser, heated residue discharge, distillate collection, cold trap, vacuum package, heating/cooling units and PLC/HMI control.
For laboratory development, glass equipment allows operators to observe film formation and foaming directly.
For pilot and industrial production, SS316L equipment provides greater mechanical strength and easier integration with continuous feed/discharge and automation.
HJCHEM molecular distillation systems should be configured according to the actual silicone viscosity, volatile load and final residual target.
Conclusion
Silicone purification by wiped-film or short-path distillation is best understood as a devolatilization process.
The main objective is typically to remove cyclic and linear volatile siloxanes such as D3, D4, D5 and D6 from a higher-molecular-weight silicone oil or polymer.
Conventional vacuum or thin-film processing already sufficient when the volatile load is high and the target is moderate.
Short-path molecular distillation becomes more useful when the silicone is highly viscous, the residual cyclic target is low, or deeper vacuum and shorter thermal residence solve a real process limitation.
Published parameters such as 207°C / 3 Pa / 7 rpm demonstrate that short-path devolatilization is technically practical for very high-viscosity silicone, but those numbers should not be copied directly to another formulation.
The safest scale-up route is therefore:
Define the cyclic-siloxane target → establish the operating window → validate with material testing → size the production system.
Have a Silicone Oil or Polymer Sample to Test?
Send HJCHEM your silicone type, D3/D4/D5/D6 content, viscosity, target residual level, capacity and available process data.
We can first evaluate whether wiped-film evaporation or short-path molecular distillation is suitable.
If the operating window is uncertain, paid material testing can be arranged on HJCHEM test equipment, and the trial fee can be credited back if you later place an equipment order.