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In the early stages of the e-cigarette industry, product failures caused by material compatibility issues were not uncommon. A crack caused by incompatibility between an e-liquid and a reservoir material could result in losses running into millions of dollars. As such incidents came to light, material compatibility became a critical risk to control in prefilled e-cigarette development.
Today, many of these issues are better recognised and can often be prevented through established experience. However, new e-liquid formulations, material grades and device designs continue to introduce combinations that have not been encountered before. Effective control requires accumulated experience and an understanding of failure mechanisms, together with a structured and practical process for risk identification, validation and root-cause analysis.
E-liquids may remain in long-term contact with reservoirs, pod chambers, seals, adhesive joints, heating assemblies and other internal materials. Compatibility failures may include cracking, swelling, leakage, loss of bond strength, corrosion or metal release.
Some changes are difficult to identify by appearance alone, even in short laboratory tests conducted under simulated storage conditions. Short-term testing can screen for obvious risks, but the absence of an abnormality does not demonstrate long-term storage stability. Time-dependent risks should also be assessed using actual parts in their assembled condition, supported by justified accelerated testing and longer-term retention samples.
Actual failures rarely arise from a single factor. They may result from the combined effects of e-liquid formulation, material properties, processing conditions, product design, assembly stress, contact time and storage conditions. A material may tolerate direct contact with a given e-liquid, while the finished and assembled product still fails later in use.
For prefilled products, the key question is not simply whether a material appears unchanged. It is whether the reservoir, seal and adhesive structure can continue to perform their intended functions after long-term contact with the e-liquid.
Polycarbonate (PC) offers good transparency and mechanical strength, making it widely used in transparent reservoirs and other liquid-contact components. In finished products, compatibility problems may not become visible immediately after filling. Slight whitening, crazing or fine cracks can be easily overlooked, then develop into serious reservoir failures during storage.
Such failures are often associated with environmental stress cracking (ESC). Injection moulding can leave residual stress within a part. Snap-fit features, press-fitting, local wall-thickness changes and other assembly conditions can further alter the stress state in service. When PC remains in contact with certain e-liquid formulations, the liquid environment and stress may act together, causing crazing, microcracking or complete component failure.
For this reason, PC compatibility cannot be determined solely by immersing flat material specimens in e-liquid. Material grade, processing history, residual stress, structural stress and contact time must all be considered.
Validation should use actual injection-moulded parts wherever possible, with the target e-liquid, realistic assembly conditions, relevant storage duration and temperature conditions. Testing should not stop at visual inspection for obvious reservoir rupture. Whitening, crazing, stress marks and fine-crack locations should be recorded over time. Where early anomalies or suspect areas are found, microscopic examination can help assess crack-initiation sites and propagation features. Comparing results with unfilled equivalent parts or parts exposed to a control formulation can help determine whether the issue is primarily related to the material, structural stress, or a specific formulation-material combination.
Silicone and rubber components rely on stable dimensions, hardness and elastic recovery to maintain an effective seal. After prolonged e-liquid exposure, a seal may show no obvious surface damage while still undergoing slight swelling, shrinkage, softening or hardening. If these changes move the part outside its designed compression range, or create excessive compression set, leakage may result.
Adhesive joints follow the same principle. Certain adhesive systems may lose bond strength, partially debond or allow component movement after prolonged e-liquid exposure. Material integrity is only one part of the assessment. The more important question is whether sealing or structural performance has been compromised.
For seals, evaluation should cover dimensions, hardness, elastic recovery, compression set and performance under actual compression. For adhesive joints, it should cover long-term bond strength, bond integrity, component stability and leak resistance. Results should be compared with the initial condition and product design requirements, then rechecked after the relevant storage periods. The absence of leakage immediately after assembly does not demonstrate long-term stability during e-liquid contact.
Long-term contact between e-liquid and heating assemblies or other metal components may cause corrosion and metal-ion leaching into the e-liquid. The extent of leaching can be influenced by metal grade, the stability of the surface oxide or passive film, e-liquid formulation, contact time, temperature and actual operating conditions. Results from a specific ingredient, e-liquid system and metal-material combination under a limited test scenario may not be sufficient to identify all compatibility risks that could arise under other conditions.
E-liquid pH, formulation components that may affect metal dissolution, and heating and aerosolisation can all alter the release profile of metal elements. Some issues that appear to originate on the hardware side may therefore also be influenced by e-liquid ingredients or formulation design. Comparative testing of the interaction between the formulation and the metal material is needed to establish the true cause.
Not every compatibility risk presents as cracking, swelling or corrosion. Substances from plastics, elastomers or adhesives may migrate into the e-liquid during prolonged contact, while metal components may release substances under certain conditions.
Metal-release risk should be assessed separately for storage and use. During storage, immersion testing of metal-contact components and retention testing of assembled devices can screen for changes under static e-liquid contact. For heating assemblies and adjacent metal components, heating, aerosolisation and in-use ageing should also be considered. Where relevant, device-operation testing can supplement the assessment through analysis of aerosol or residual e-liquid. Target metal elements in e-liquid or aerosol can be quantified when confirmation is required.
Material compatibility control does not need to begin from zero in every project. Historical data and project experience can help R&D teams identify known high-risk formulation, material and structural combinations early, allowing validation resources to focus on genuinely new combinations.
Historical risk identification → High-risk combination screening → New-combination validation → Root-cause analysis → Formulation, material, process or structural optimisation → Revalidation
When a project introduces a new e-liquid formulation, material grade, liquid-contact structure or manufacturing condition, targeted validation is required. Where a new formulation, material grade or liquid-contact structure combination is not supported by sufficient historical data, it should undergo as much longer-term compatibility validation as reasonably possible before market launch. Accelerated testing can help screen high-risk combinations earlier in development, but it should not fully replace continued observation over the intended storage period.
Validation plans should define functional failure criteria in advance. These may include allowable dimensional change, sealing performance, bond strength, crack rating or changes in target metal-element content. This avoids drawing conclusions solely from the fact that no obvious abnormality was observed.
When a problem is identified, the next step should not be to conclude simply that “the materials are incompatible.” The source should be narrowed down further: formulation, ingredient, material grade, processing state, assembly stress or product structure. Comparing control formulations, alternative material grades or different assembly conditions can help locate the source of failure and prevent premature attribution to a single e-liquid ingredient when the underlying cause is structural stress or processing.
E-liquid manufacturers are generally more familiar with formulations, flavourings and ingredients, while e-cigarette hardware manufacturers tend to focus on materials, structures and manufacturing processes. Material compatibility sits at the interface of e-liquid and device development. When each side evaluates the project separately, gaps can arise in risk identification and validation.
Hangsen’s R&D team covers both e-liquid development and hardware engineering and is led by multiple PhD scientists with extensive industry experience. By combining chemical engineering and hardware engineering, the team can assess compatibility issues across formulation, materials, structure, manufacturing conditions and actual product performance, identify the source of risk and support subsequent optimisation.
If you are developing a new prefilled e-cigarette product, or are encountering reservoir cracking, seal failure, leakage, heating-component corrosion or other material compatibility issues, contact Hangsen at [email protected] to discuss your project with our product and R&D teams.
• Environmental stress cracking (ESC): Failure in which a material develops crazing, cracking or fracture through the combined action of a liquid environment and internal or external stress.
• Residual stress: Stress retained within a component after processing, such as injection moulding and cooling.
• Crazing: Fine, typically whitening microvoid or microcrack structures in a plastic. It can be an early indication of ESC.
• Compression set: The extent to which a seal fails to return to its original shape after prolonged compression.
• Metal-ion leaching: The release of metal elements from a component into e-liquid or aerosol as ions or other detectable species during liquid contact or heating.
• Passive film: A thin protective oxide layer formed on certain metals that can reduce further corrosion or elemental release. Its stability depends on the material, contacting medium and temperature.
• Failure criteria: Predetermined acceptance or rejection limits used to determine whether a product continues to meet its design and use requirements.
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