Is Fogging After Thermal Aging in Electronic or Optical Equipment Caused by Volatile Silicone Materials?-Iota

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News > Dynamic industry > Is Fogging After Thermal Aging in Electronic or Optical Equipment Caused by Volatile Silicone Materials?

Is Fogging After Thermal Aging in Electronic or Optical Equipment Caused by Volatile Silicone Materials?

Possibly, but the appearance of the deposited film alone cannot confirm that the contamination comes from a silicone material. When enclosed equipment is heated, volatile substances from silicone oils, adhesives, sealants, lubricating greases, release agents, plastic additives, or cleaning residues may be released and condense on cooler lenses, windows, or sensor surfaces.

The correct approach is to identify the composition of the fogging film, the contamination source, and the migration path before deciding whether low-volatility silicone materials are required.

What Is the Basic Mechanism of Fogging?

When equipment is heated, solvents, unreacted components, low-molecular-weight substances, or thermal-decomposition products may enter the gas phase. If a lens or transparent window is cooler than the area surrounding the contamination source, these substances may condense on the cooler surface, forming an oily film or white haze and reducing optical transmittance.

The severity of fogging is generally affected by several factors:

The concentration of volatile and condensable components in the material
Continuous operating temperature, peak temperature, and heating duration
Distance between the contamination source and the lens
Degree of enclosure and internal air exchange
Temperature difference between the lens and the contamination source
Material quantity, exposed surface area, and degree of cure

Therefore, the absence of visible contamination when a material is used in open equipment does not prove that it will remain non-contaminating in an enclosed optical device.

Is Fogging Always Caused by Silicone Oil?

No. The investigation should include at least:

Silicone oils, silicone greases, and thermal-interface materials
Silicone and non-silicone adhesives and sealants
Insufficiently cured potting compounds and coatings
Plastics, rubbers, and their plasticizers
Release agents used during manufacturing
Residues from cleaning agents, fluxes, and processing oils
Non-silicone materials that degrade at elevated temperatures

If only the silicone oil is replaced without examining adhesives, plastics, greases, and cleaning residues, the fogging problem may continue.

Which Properties Should Be Evaluated for Low-Volatility Silicone Oil?

Volatile Content

Volatile-content data must be compared together with the test temperature, test duration, sample mass, and test method. Values obtained under different conditions cannot be compared directly.

Two materials may both be described as “low volatility,” but their data may not be comparable if they were tested at different temperatures or for different periods.

Low-Molecular-Weight Components

Silicone oils with similar conventional viscosity may still have different low-molecular-weight compositions.

In silicone materials, D3–D10 generally refers to cyclic siloxanes with different degrees of polymerization. For electronic, optical, and enclosed equipment, the total volatile content alone may be insufficient. Depending on the contamination risk, D3–D10 cyclic siloxanes and other potentially volatile or condensable components should also be evaluated.

A lower content of low-molecular-weight substances can help reduce certain volatile-contamination risks, but it cannot independently prove that the material will not cause fogging in the assembled device.

Condensable Volatile Materials

In optically sensitive environments, two concepts must be distinguished:

How much material volatilizes into the surrounding environment
How much of the released material condenses on sensitive surfaces such as lenses

The first reflects material mass loss, while the second is more directly related to optical contamination. A low total volatile loss does not necessarily mean that deposition on a lens will also be low.

NASA outgassing evaluations separately record Total Mass Loss (TML) and Collected Volatile Condensable Materials (CVCM). This distinction demonstrates that “low total volatility” and “low contamination risk to sensitive surfaces” are not identical concepts. Relevant data can be consulted in the NASA Outgassing Database.

ASTM E595 is primarily used to screen material outgassing under vacuum conditions. It cannot directly replace an actual fogging test for enclosed electronic or optical equipment operating at atmospheric pressure.

What Operating Conditions Must Be Confirmed Before Material Selection?

At minimum, confirm:

Whether the equipment is open, partially enclosed, or fully sealed
Continuous operating temperature and short-term peak temperature
Temperature distribution across the lens, light source, circuit board, and sealing area
Time, location, and appearance of the fogging
All adhesives, oils, greases, plastics, and cleaning materials used inside the equipment
Whether adhesives, sealants, and potting materials are fully cured
Requirements for optical transmittance, haze, surface residues, and electrical performance
The current volatility, outgassing, or fogging test method

A specific silicone oil, adhesive, or potting-material grade should not be recommended when these conditions are unknown.

How Can the Actual Contamination Source Be Identified?

Step 1: Establish a Complete Material List

Record every non-metallic material used inside the equipment, including small quantities of release agents, lubricants, tapes, fluxes, and cleaning residues.

A material should not be excluded as a possible source merely because only a small amount is used.

Step 2: Prepare Single-Material Control Groups

Place each candidate material separately in a container under identical temperature, time, and enclosure conditions. Include a cleaned glass plate or an actual lens as the condensation-collection surface.

The test conditions should reproduce the actual operating environment of the equipment as closely as possible.

Step 3: Establish a Blank Control

The blank container must undergo the same cleaning, assembly, and heating procedures as the test groups. This helps identify contamination originating from the container, operating environment, or test process itself.

Without a blank control, background contamination may be incorrectly attributed to the test material.

Step 4: Analyze the Composition of the Deposited Film

When possible, use infrared spectroscopy, chromatography–mass spectrometry, or another appropriate analytical method to identify the composition of the deposit.

The color, odor, or feel of the film is generally insufficient to determine reliably whether it came from silicone oil, an adhesive, or another material.

Step 5: Conduct Combined-Material and Full-Device Retesting

After individual materials pass their respective tests, combined-material testing and full-device thermal-aging testing should still be conducted.

When several materials coexist, they may change the system’s volatilization, migration, adsorption, and condensation behavior. Passing a single-material test does not guarantee that the assembled equipment will remain free of fogging.

What Material Options Can Be Considered?

Low-Volatility Silicone Oil

Low-volatility silicone oil is suitable for applications requiring lubrication, damping, electrical insulation, or a formulation base oil while also being sensitive to volatility and optical contamination.

Selection should be based on volatile content, low-molecular-weight composition, and condensable-material data obtained under specified test conditions. Viscosity or product name alone is insufficient.

Low-Volatility Silicone Rubber or Adhesive

If the contamination originates from a sealing, bonding, or potting material, check:

Low-molecular-weight components in the raw materials
Mix ratio of two-component materials
Curing temperature and curing time
Whether the interior of the material is fully cured
Whether post-curing is required

Replacing silicone oil alone cannot solve contamination caused by an adhesive or potting compound.

Silicone Resin or Other Curable Materials

If the application requires a fixed protective layer rather than liquid lubrication or damping, an appropriate silicone resin or another curable material may be evaluated.

Its adhesion, degree of cure, thermal stability, and volatile-contamination risk must still be verified.

Common Mistakes

Evaluating Only Viscosity

Comparing room-temperature viscosity without comparing volatile content, low-molecular-weight components, and high-temperature stability is insufficient. Silicone oils with the same viscosity may present different fogging risks.

Equating Low Odor with Low Fogging

A material with little odor does not necessarily contain fewer condensable volatile substances. Odor and optical contamination are not directly equivalent.

Inspecting Only the Silicone Oil

Testing only the silicone oil while ignoring adhesives, greases, plastics, tapes, fluxes, and cleaning residues can result in the actual contamination source being missed.

Conducting Only Single-Material Tests

A material that passes an individual test may still fail when used together with other materials. Optically sensitive equipment also requires combined-material and full-device validation.

Misusing Vacuum-Outgassing Data

Vacuum-outgassing data such as ASTM E595 can support preliminary material screening, but they cannot directly prove that fogging will not occur in enclosed equipment operating at atmospheric pressure.

Testing Before Full Cure

A touch-dry adhesive is not necessarily fully cured internally. Insufficiently cured materials may continue to release solvents, reaction by-products, or residual low-molecular-weight components.

Recommended Selection Procedure

Step 1: Define the equipment structure, internal temperature distribution, and complete list of non-metallic materials.

Step 2: Establish blank, single-material, and combined-material test groups to identify the contamination source progressively.

Step 3: Based on the actual function of each material, screen low-volatility silicone oils, silicone rubbers, adhesives, potting compounds, or silicone resins.

Step 4: Conduct volatility, condensation, and thermal-aging tests under specified or simulated operating conditions.

Step 5: Complete combined-material and full-device validation before determining the final material grade and processing conditions.

As a full-chain silicone solutions provider, IOTA SILICONE OIL (Anhui) CO., LTD. can assist in screening silicone oils, silicone rubbers, silicone resins, and related silicone additives.

The specific grade should be selected only after the contamination source has been identified, the test conditions have been defined, and the relevant product data have been verified.

Frequently Asked Questions

Is Lens Fogging Always Caused by Volatile Silicone Oil?

No. Adhesives, sealants, greases, plastic additives, release agents, fluxes, and cleaning residues may also release condensable substances.

Does Higher Silicone-Oil Viscosity Mean a Lower Fogging Risk?

Viscosity alone is insufficient. Low-molecular-weight content, volatile content, test conditions, operating temperature, and thermal stability must also be compared.

Does Low Volatility Mean That Fogging Will Not Occur?

No. Fogging also depends on the condensability of the volatile substances, lens temperature, degree of enclosure, material quantity, and internal temperature differences.

Can Low D3–D10 Content Guarantee That Optical Equipment Will Not Fog?

No. D3–D10 cyclic siloxanes are only some of the possible contamination sources. Other volatile components must also be examined, and combined-material and full-device tests are still required.

What Is the Difference Between TML and CVCM?

TML represents the total mass lost by a material under specified conditions. CVCM represents the portion of the released volatile substances that condenses on a collection surface.

For optical contamination, CVCM is generally more closely related to deposition risk on sensitive surfaces, but it still cannot replace testing in the actual equipment.

Can ASTM E595 Data Be Applied Directly to Electronic Equipment at Atmospheric Pressure?

No. ASTM E595 is primarily intended for screening material outgassing under vacuum conditions. Enclosed equipment operating at atmospheric pressure must still be validated using its actual temperature, structure, and material quantities.

Can Enclosed Thermal-Aging Testing Begin Once an Adhesive Is Touch-Dry?

Touch dryness does not indicate complete internal curing. Insufficiently cured materials may still release reaction by-products, solvents, or residual low-molecular-weight substances. Complete the specified cure or post-cure before testing.

How Can the Material Responsible for the Fogging Film Be Identified?

Establish blank, single-material, and combined-material control groups. When possible, analyze the composition of the deposited material and confirm the result through full-device retesting.

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