Epoxy Resin Purification: How Short-Path Distillation Reduces Chlorine and Oligomers
High-purity epoxy resin is increasingly required for electronic encapsulation, adhesives, optical materials, coatings and other applications where chlorine content, viscosity, color and molecular-weight distribution matter.
For epichlorohydrin-derived epoxy resins, the crude product contains residual epichlorohydrin, solvent, chlorine-containing by-products, low- and high-molecular-weight fractions, salts and other process impurities.
The difficult part is that these impurities are not all removed by the same separation method.
Bulk epichlorohydrin and solvent can often be removed by conventional vacuum distillation. Ionic salts require washing or filtration. For the final purification of high-boiling epoxy products, however, wiped-film or short-path molecular distillation can become an important polishing step.
Published epoxy-resin processes have used short-path evaporation to reduce chlorine-related impurities, control oligomer content and produce lower-viscosity epoxy fractions for demanding electronic applications.
The key is to first identify which impurity must be removed and what final specification is required.
Why Does High-Purity Epoxy Resin Require Additional Purification?
Many epoxy resins are produced through reactions involving epichlorohydrin.
After reaction and initial separation, the crude resin can still contain:
- residual epichlorohydrin;
- residual organic solvent;
- water;
- hydrolyzable chlorine-containing compounds;
- organically bound chlorine-containing by-products;
- low-molecular-weight fractions;
- dimers and higher oligomers;
- inorganic salts or catalyst residues.
These components can affect final resin properties differently.
Low-chlorine epoxy grades are particularly important in electronics because chlorine-related impurities can contribute to corrosion problems in sensitive electronic components.
Commercial epoxy manufacturers therefore offer dedicated low-chlorine / high-purity epoxy grades for semiconductor, PCB, adhesive and encapsulation applications.
But “low chlorine” does not describe a single impurity.
That distinction is important when selecting the purification process.
Total Chlorine, Hydrolyzable Chlorine and Ionic Chloride Are Not the Same
Before choosing molecular distillation equipment, buyers should define exactly which chlorine specification they need to improve.
1. Ionic Chloride
Inorganic chloride and salts are generally better addressed by washing, phase separation and filtration.
They are not ideal targets for molecular distillation because they are nonvolatile.
2. Hydrolyzable Chlorine
Hydrolyzable chlorine comes from certain chlorine-containing organic structures that can release chloride during hydrolysis.
Specific epoxy-resin processes combine chemical treatment with molecular distillation to reduce this fraction significantly.
3. Total Chlorine
Total chlorine can include several chemically different chlorine-containing species.
Reducing total chlorine to a very low level therefore requires more than one purification step.
This is why a request such as: “We need electronic-grade epoxy resin.” is not enough for process design.
A more useful specification is:
Initial total chlorine: X ppm
Initial hydrolyzable chlorine: X ppm
Target total chlorine: X ppm
Target hydrolyzable chlorine: X ppm
Where Does Molecular Distillation Fit in the Process?
Molecular distillation is normally more useful as a final or near-final purification stage than as the first step after epoxy synthesis.
A practical process look like:
Reaction → Neutralization / Washing → Salt Filtration → Bulk ECH & Solvent Removal → Wiped-Film / Short-Path Purification → Final Epoxy Product
Each stage has a different job.
1. Bulk ECH and Solvent Removal
If the crude resin still contains large amounts of epichlorohydrin or solvent, these relatively volatile components should normally be removed before the deepest-vacuum polishing stage.
Sending a very high solvent load directly into a molecular still unnecessarily increases:
- vapor load;
- condenser duty;
- cold-trap load;
- vacuum-pump load.
2. Filtration
Nonvolatile salts, particles and reaction residues should normally be removed before short-path processing.
3. Short-Path Purification
The short-path stage can then focus on the difficult separation between the desired epoxy fraction and unwanted volatile or different-molecular-weight fractions.
Professional wiped-film processing is widely used for polymer purification, including the removal of residual monomers, dimers, solvents and other contaminants from epoxy and other polymer products.
Why Use Short-Path Distillation for Epoxy Resin?
Epoxy resin purification can become difficult because the material will be:
- high boiling;
- viscous;
- thermally sensitive;
- difficult to process under conventional batch distillation conditions.
A short-path molecular still addresses these problems in two ways.
First, a rotating wiper continuously distributes the resin over the evaporator wall as a thin film.
Second, the condenser is positioned very close to the evaporation surface.
The vapor therefore does not need to travel through a long pipe to an external condenser.
This reduces vapor-side pressure drop and allows operation at much lower pressure.
Professional short-path evaporators can operate around 0.001 mbar under suitable conditions, while maintaining very short thermal residence times.
The detailed working principle of molecular distillation explains how the wiped film, internal condenser and high-vacuum environment work together.
Published Operating Parameters for Epoxy Resin Short-Path Distillation
This is where this page should be much stronger than the old version.
There are actual published epoxy-resin short-path operating parameters.
A patent describes purification of cyclohexyl-containing glycidyl ether epoxy compositions using thin-film and short-path evaporators.
The ranges include:
| Parameter | Published Range |
|---|---|
| General distillation temperature | 150–270°C |
| Preferred process range | 150–230°C |
| More specific preferred range | 170–210°C |
| General pressure | 0.001–1 mbar |
| Preferred pressure | 0.005–0.2 mbar |
| More specific range | 0.007–0.1 mbar |
| Short-path example | 170–210°C / 0.007–0.01 mbar |
The patent also describes industrial operation around 207–240°C and 0.02–0.13 mbar for one specific hydrogenated bisphenol-A-derived epoxy composition.
Converted to Pa:
- 0.007 mbar ≈ 0.7 Pa
- 0.01 mbar ≈ 1 Pa
- 0.02 mbar ≈ 2 Pa
- 0.13 mbar ≈ 13 Pa
These are very useful engineering references because they also show an important point:
Actual epoxy operating pressure does not have to equal the deepest no-load vacuum capability of the equipment.
Published Epoxy Result: Chlorine and Viscosity Reduction
In the same project reference, one industrial epoxy-resin process reported:
| Property | Feed | Purified Fraction |
|---|---|---|
| Total chlorine | 4.9 wt.% | 1.3 wt.% |
| Dynamic viscosity at 25°C | 2290 mPa·s | 1100–1200 mPa·s |
The distillation also shifted the molecular-weight distribution toward the desired lower-molecular-weight epoxy fraction and substantially reduced higher oligomer fractions.
This was a specific hydrogenated epoxy chemistry.
It should not be interpreted as a universal result for all bisphenol A, bisphenol F, novolac or specialty epoxy resins.
Another Published Low-Chlorine Epoxy Process
A separate published process used chemical pretreatment followed by rolled-film molecular distillation for epichlorohydrin-derived epoxy resins.
Several reported distillation conditions included approximately:
- 155°C / 0.001 mmHg
- 170°C / 0.001 mmHg
- 185°C / 0.001 mmHg
- 190°C / 0.001 mmHg
0.001 mmHg corresponds to approximately 0.13 Pa.
One example treated a resin containing approximately 407 ppm hydrolyzable chloride and, after the complete chemical-treatment-plus-distillation sequence, reported approximately 13.9 ppm.
Another example reduced approximately 917 ppm to 32 ppm after the combined process.
The important wording here is: combined process
It would be incorrect to claim that molecular distillation alone caused the entire chlorine reduction.
The published process included chemical conversion, neutralization or filtration steps together with rolled-film distillation.
That is exactly why feed chemistry must be understood before selecting the equipment.
What Do These Published Parameters Tell Us?
They do not tell us to set every epoxy molecular still to: 185°C / 0.13 Pa.
They tell us that high-purity epoxy purification can require:
high temperature + very low absolute pressure + very short thermal exposure.
The correct operating point depends on:
- epoxy chemistry;
- epoxide equivalent weight;
- chlorine species;
- viscosity;
- molecular-weight distribution;
- desired distillate/residue fraction;
- thermal stability;
- feed rate;
- evaporation area.
Published references should be treated as starting information for process development, not universal operating recipes.
Which Fraction Is the Product?
This is another important question that generic molecular-distillation articles often miss.
The desired epoxy product is not automatically always the residue.
In some epoxy purification processes, the desired lower-molecular-weight epoxy fraction is collected as the distillate, while higher-molecular-weight oligomers remain in the residue.
HJCHEM short-path process, for example, increased the proportion of the lower molecular-weight epoxy fraction while significantly reducing higher oligomer fractions in the distillate.
Therefore, before designing the equipment, the customer should clarify: Is the valuable epoxy product expected in the distillate, the residue, or are both fractions useful?
That decision directly affects condenser design, collection, discharge and yield calculation.
Which Operating Parameters Matter Most?
1. Evaporator Temperature
Higher temperature increases vapor pressure and can improve evaporation.
But epoxy resin should not simply be heated as much as possible.
The goal is to use the lowest practical temperature that achieves the required separation at the selected pressure and residence time.
2. Operating Pressure
Pressure strongly affects the temperature required for separation.
For epoxy resin purification, published conditions range from several Pa down toward approximately 0.1 Pa depending on the chemistry and separation target.
HJCHEM standard molecular distillation systems typically provide a complete-system no-load vacuum of 0.1–1 Pa, while an optional high-vacuum configuration can reach approximately 0.01–0.1 Pa no-load vacuum.
These figures describe equipment capability.
They are not guaranteed epoxy operating pressures.
Actual pressure under feed depends on vapor load, feed rate, temperature, condenser duty, cold trap, leakage and vacuum-system configuration.
The difference between pump ultimate vacuum, no-load system vacuum and operating pressure is explained in our molecular distillation vacuum specifications.
3. Feed Rate
Feed rate changes both throughput and separation.
Increasing feed rate can:
- increase vapor load;
- change film thickness;
- shorten effective residence;
- increase condenser loading;
- change the distillate/residue split.
Maximum kg/h is therefore not the same thing as optimum kg/h.
4. Wiper Speed
The wiper distributes the epoxy as a continuously renewed film.
Its optimum speed depends on viscosity, rotor geometry, film behavior and feed rate.
Higher rpm is not automatically better.
The objective is a stable, uniform film without excessive entrainment or mechanical stress.
5. Resin Viscosity
Viscosity should always be provided together with measurement temperature.
For example: 3,000 mPa·s at 80°C is useful. 3,000 mPa·s alone is incomplete.
The feed pump and residue discharge system also need to handle the resin at the actual operating and discharge temperatures.
Molecular Distillation Cannot Remove Every Epoxy Impurity
Short-path molecular distillation is a volatility-based separation.
It should not be presented as a universal purification method.
It cannot directly remove all:
- inorganic salts;
- ionic chloride;
- solid particles;
- gel;
- cross-linked polymer;
- nonvolatile pigments;
- catalyst residues;
- chemically bound species with insufficient volatility difference.
For example, ionic contaminants is better removed by washing and filtration before distillation.
Other ultra-low-chlorine processes use extraction, adsorption, crystallization or chemical treatment precisely because molecular distillation alone may not economically achieve every chlorine specification.
Single-Stage or Multi-Stage Purification?
For a relatively clean epoxy feed, one wiped-film or short-path stage is sufficient.
For a feed containing significant solvent, residual ECH and difficult high-boiling impurities, staged treatment may be more practical.
For example:
Stage 1: Bulk volatile removal
Stage 2: Short-path epoxy fractionation / polishing
This prevents the deepest-vacuum stage from being overloaded by components that could have been removed more easily upstream.
A similar staged approach is commonly used in high-vacuum polymer purification, where an earlier wiped-film stage removes dissolved gases and lighter solvents before deeper-vacuum molecular distillation.
How Should an Epoxy Purification System Be Sized?
A molecular distillation unit should not be selected only from a requested feed rate such as: “We need 100 kg/h.”
Two 100 kg/h feeds can require completely different systems.
Consider:
Feed A
- 1% volatile fraction
- 1,500 mPa·s
- moderate separation target
versus:
Feed B
- 15% volatile fraction
- 10,000 mPa·s
- very low chlorine target
They have the same feed capacity but very different evaporation, condenser and vacuum loads.
Proper molecular distillation system sizing should consider:
| Required Data | Why It Matters |
|---|---|
| Epoxy resin type | Defines chemistry |
| Feed composition | Identifies separation targets |
| Initial total chlorine | Defines starting specification |
| Hydrolyzable chlorine | Helps define purification route |
| Residual ECH/solvent | Defines vapor load |
| Epoxide equivalent weight | Helps characterize resin |
| Molecular-weight distribution | Defines fractionation duty |
| Viscosity + temperature | Determines film/pump behavior |
| Target specification | Defines required separation |
| Feed rate | Determines equipment size |
| Maximum allowable temperature | Defines thermal limit |
Why Trial Testing Is Recommended for Epoxy Resin
Epoxy resin is exactly the type of material where literature data alone should not be used to purchase a full industrial system.
Different epoxy formulations can behave very differently.
A customer may know: feed rate = 50 kg/h
but still not know:
- which fraction should be collected;
- achievable chlorine reduction;
- appropriate evaporator temperature;
- actual operating pressure;
- distillate yield;
- residue viscosity;
- whether one stage is sufficient.
For this reason, process testing before scale-up can be valuable.
HJCHEM Material Trial Service
HJCHEM has molecular distillation test equipment available for customer material trials.
Customers can send epoxy resin samples to HJCHEM for a paid trial when the process window has not yet been established.
During testing, we can evaluate variables such as:
- evaporator temperature;
- operating pressure;
- feed rate;
- wiper speed;
- condenser temperature;
- distillate/residue ratio;
- material flow behavior;
- achievable separation under the tested conditions.
The trial service is chargeable, because sample testing requires equipment preparation, operation, cleaning and engineering time.
If the customer later purchases the corresponding equipment from HJCHEM, the trial fee can be refunded or credited against the purchase order.
This is especially useful when the target is an electronic-grade or specialty epoxy resin and published literature does not represent the actual customer formulation.
HJCHEM Molecular Distillation Options for Epoxy Resin
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.
Once the process has been validated, HJCHEM can scale the configuration according to the required throughput.
Laboratory glass equipment can be used for preliminary process development and observation of film behavior.
Pilot SS316L systems are suitable for continuous-process verification and scale-up.
Industrial SS316L systems can be configured for validated production processes.
Depending on the project, a complete system include:
- heated feed tank;
- metering feed pump;
- wiped-film short-path evaporator;
- internal condenser;
- heated residue discharge;
- distillate collection;
- cold trap;
- backing and high-vacuum system;
- heating and cooling circulators;
- PLC/HMI control.
The appropriate molecular distillation system should be selected only after the separation duty is defined.
What Should You Send for an Epoxy Resin Trial or RFQ?
Before equipment selection, please provide as much of the following information as possible:
- epoxy resin type;
- feed composition;
- initial total chlorine;
- initial hydrolyzable chlorine;
- target chlorine specification;
- residual epichlorohydrin content;
- solvent type and content;
- viscosity and measurement temperature;
- epoxide equivalent weight;
- feed capacity;
- maximum acceptable product temperature;
- desired product fraction;
- existing vacuum/distillation data;
- available sample quantity.
These buyer checks allow us to determine whether conventional vacuum evaporation, wiped-film processing or short-path molecular distillation is the more appropriate process.
Conclusion
Short-path molecular distillation has a real role in high-purity epoxy resin production, particularly when the goal involves controlling chlorine-related fractions, oligomers, molecular-weight distribution and other high-boiling impurities.
Published epoxy-resin processes demonstrate operation from approximately 150–240°C and from several Pa down toward sub-Pa pressure, but the exact conditions vary significantly with epoxy chemistry and purification target.
Molecular distillation should therefore not be treated as a universal “high-purity machine.”
A better engineering approach is:
Define the impurity → remove bulk ECH/solvent → remove nonvolatile contaminants → establish the short-path operating window → validate by trial → scale up.
Need to Test Your Epoxy Resin?
Send HJCHEM your epoxy type, chlorine data, residual ECH/solvent content, viscosity, target specification and required capacity.
If the suitable molecular-distillation conditions are not yet known, HJCHEM can arrange a paid material trial using our test equipment.
If you subsequently place the equipment order, the trial fee can be credited back against the purchase.