A potted BMS controller passes a functional test at the line, then fails in the field eleven months later when a microvoid lets moisture track to a solder joint. The result is a warranty claim, a recall investigation, and a six-figure liability exposure, all traced back to a single mixing error on the shop floor.
Eyeballed mixing ratios and improvised cure cycles let this kind of defect through, because the failure mode stays invisible until the assembly is already in service. Two-component epoxy adhesive chemistry exists to put that variable under control, separating resin and hardener into two stable parts that only begin curing once combined in a specified, repeatable ratio.
Our specialists formulate and support these systems across electronics, aerospace, medical, and automotive manufacturing, with the ratio, cure, and substrate data engineers need to specify them correctly the first time.
Get the ratio, the mixing procedure, or the cure schedule wrong, and the mechanical, thermal, and dielectric properties on the technical data sheet stay theoretical.
This guide covers the underlying chemistry, the industrial applications, the mixing and degassing procedure, cure schedules, and surface preparation practices that determine whether a two-component system performs to spec.
A] What Two-Component Epoxies Are and How They Cure
1. The Chemistry of Two-Component Systems
A system that starts curing before you finish dispensing it across the assembly has already failed the application, regardless of its ultimate bond strength. That is the risk every process engineer manages when working with fast-reacting resin hardener chemistries: match the working time to the process or scrap the batch.
Two-component epoxy adhesives split the chemistry into two separate parts that a manufacturer stores independently until use:
- Part A, the resin, typically based on bisphenol A or bisphenol F epoxy resin, and
- Part B, the curing agent, commonly an amine, anhydride, or polyamide chemistry.
Mixed in a specified ratio, the two parts undergo polymerisation, the reaction in which epoxide rings on Part A open and bond with active hydrogens in Part B to build a three-dimensional crosslinked network. Crosslink density, set by the resin backbone and the stoichiometric ratio, determines the cured system’s Tg, modulus, and chemical resistance.
2. The Storage and Control Advantages
Unlike one-part epoxy systems that require heat or moisture to initiate cure and must be refrigerated in storage, two-part epoxy systems cure at the ratio and temperature the formulator specifies, giving the process engineer direct control over pot life, cure speed, and final crosslink density. That control comes at the cost of a mixing step, but it is what makes two-component chemistry the default choice wherever the application needs a tuned CTE, dielectric strength, or thermal conductivity value rather than a generic bond.
3. Calculating the Stoichiometric Mix Ratio
Representative Kohesi Bond systems show the ratio range in practice: KB 1031 AT‑2LO is specified at approximately 1:3 (Part A: Part B) by weight in product descriptions, with some sections noting a 1:3 volume ratio; KB 1372 LP at 100:25 by weight; and KB 1040 CTE‑LO at 100:10. Always confirm whether the ratio on your specific TDS is by weight or by volume and use that consistently.
WB = WA x (RB / RA)
- WA: the weight of Part A (resin) already dispensed, in grams
- WB: the required weight of Part B (hardener) to add, in grams
- RA: the Part A portion of the specified mix ratio (for example, the “100” in a 100:25 ratio)
- RB: the Part B portion of the specified mix ratio (for example, the “25” in a 100:25 ratio)
Running this calculation before every batch, rather than estimating by pour volume, is what keeps a mix inside the ratio tolerance the crosslink density depends on. If the TDS specifies a weight ratio, use a scale; if it specifies a volume ratio, use calibrated dispensing equipment.
B] Where Two-Component Epoxies Carry the Load Across Industries
1. Industrial Thermal Challenges
A room-temperature bonding epoxy will not survive 1,000 thermal cycles from -55°C to 125°C on an automotive sensor housing. CTE mismatch at the bond line under repeated thermal cycling causes most of the field cracking our technical team sees in failure analysis, and it is a selection failure, not a workmanship failure.
Across every one of the sectors below, the underlying two-component epoxy adhesive chemistry is broadly similar; what changes is the filler package and cure schedule tuned to the application.
2. Industry-Specific Requirements
- Electronics and microelectronics: Manufacturers specify two-component systems for conformal potting of PCB assemblies, die attach, and connector sealing, where volume resistivity above 10^13 Ω·cm and dielectric strength above 20 kV/mm are typical requirements to prevent leakage current between adjacent traces.
- Aerospace: Programs require NASA ASTM E-595 qualified, low outgassing systems for composite bonding and avionics potting, where total mass loss under vacuum must stay below 1.0% to avoid contaminating optical or electronic components sharing the same enclosure.
- Medical devices: Manufacturers specify USP Class VI and ISO 10993-5 compliant systems like KB 1452 HT‑2 for catheter hub bonding, sensor encapsulation, and wearable assembly, chemistries that must hold bond integrity through repeated gamma, EtO, or autoclave sterilisation cycles.
- Automotive electronics: Automotive electronics rely on two-part metal epoxy and thermally conductive epoxy systems for sensor housings, BMS potting, and EV battery module bonding, where components are often AEC‑Q200 qualified and assemblies must withstand thermal cycling to 150°C. A thermally conductive adhesive rated above 1.0 W/m·K keeps hot spots on power modules from pushing nearby components past their Tg.
- Oil and gas: Sensor housings need chemistry stable against sour gas, hydraulic fluid, and downhole temperatures that can exceed 175°C. Such conditions disqualify most standard two-component adhesive formulations within weeks.
- General industrial assembly: Relies on the same two-part adhesive chemistry for enclosure bonding and panel assembly, just tuned toward a faster cure schedule: a fast-cure potting compound that gels in 15 minutes at 80°C can move a part to final test the same shift instead of the next day, a direct throughput gain worth quantifying against your current process.
C] The Properties That Actually Belong on a Datasheet
1. Understanding Performance Metrics
A datasheet that lists “high strength” without a shear value in MPa gives a design engineer nothing to put into a stack-up calculation. Whether an engineer specifies it as a two-part epoxy glue for a field repair or as a qualified structural adhesive for a certified assembly, the same crosslink chemistry governs the properties that matter, and every one of them should carry a number.
Engineers specify this two-part resin epoxy architecture on measurable mechanical, thermal, chemical, and electrical properties. Mechanical performance centres on lap shear strength, typically 15 to 35 MPa on aluminium per ASTM D1002, and elongation at break, which governs whether the bond line survives vibration or mechanical shock without cracking.
Glass transition temperature and CTE define thermal performance. Tg ranges from 60°C in flexibilised systems to over 200°C in high-temperature formulations. Additionally, CTE runs 30 to 80 ppm/°C unfilled, dropping below 20 ppm/°C once a formulator adds silica or alumina filler to match a metal or ceramic substrate.
Chemical sensitivity note: Chemical resistance varies by curing agent chemistry; amine-cured systems generally outperform polyamide-cured systems against solvents and fuels, a distinction worth confirming against the SDS before specifying a fuel-adjacent application.
Electrical properties span insulating, above 1013 Ω·cm volume resistivity, to conductive, below 10⁻² Ω·cm. Once a formulator loads the resin with silver or nickel past the percolation threshold, the filler loading at which conductive particles form a continuous electrical path through the cured matrix.
Curing options range from room-temperature cure, roughly 24 hours at 25°C, to accelerated schedules as fast as 30 minutes at 100°C or above, a trade-off between process speed and equipment cost covered in detail below.
| Property | Typical Range | Test Method |
|---|---|---|
| Lap shear strength | 15 to 35 MPa | ASTM D1002 |
| Volume resistivity, insulating | 10^13 to 10^15 Ω·cm | ASTM D257 |
| Volume resistivity, conductive | 10^-3 to 10^-1 Ω·cm | ASTM D257 |
| CTE, filled | 15 to 40 ppm/°C | ASTM E831 |
| Tg | 60°C to 220°C | DSC or DMA |
| Service temperature | -55°C to 200°C+ | Per TDS |
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D] Step-by-Step Mixing Guide
Epoxy mixing errors account for more field returns than any single chemistry defect, and they fail silently. Getting how to mix epoxy right is a repeatable process, not a matter of feel. A bond mixed at 100:20 instead of a specified 100:25 can pass a visual inspection and a five-minute tack test, then fail a 500-hour thermal cycling qualification because the crosslink density never reached spec.
Step 1: Understand Your Components
Confirm which container is Part A, the resin, and which is Part B, the hardener or curing agent, against the current TDS before opening either one. Mismatching the epoxy and hardener labels, especially across product lines using different colour codes, is an easy and entirely avoidable error. The TDS specifies ratios either by weight or by volume, and the two are not interchangeable; using a volume ratio on a weight-specified system introduces error proportional to the density difference between the two parts.
Step 2: Safety and Workspace Preparation
Set up in a ventilated area per the SDS, with nitrile gloves and eye protection. Amine hardeners are sensitisers, and repeated skin contact raises the risk of dermal sensitisation even at low exposure levels.
Step 3: Gather Tools and Equipment
Use a calibrated scale with 0.1 g resolution for small batches, clean mixing vessels, and a rigid spatula. Reusing a container that holds a different epoxy chemistry risks cross-contamination that can inhibit cure entirely.
Step 4: Determine the Correct Mixing Ratio
Pull the ratio directly from the current TDS revision, not from memory or a previous job traveller. Kohesi Bond ratios vary by product: KB 1031 AT‑2LO is specified at approximately 1:3 (Part A: Part B), with some sources listing this by weight and others by volume; KB 1372 LP at 100:25 by weight; KB 1040 CTE‑LO at 100:10. Always confirm on your specific TDS whether the ratio is by weight or by volume. An epoxy mixing ratio error of even 5% can shift Tg by 10°C to 20°C, enough to move a part out of spec on a thermal cycling test.
Step 5: Measure Accurately
Weigh Part A first, tare the scale, then add Part B to the target weight. Volume-based ratios need calibrated syringes or graduated dispensing equipment, since eyeballing volume in a mixing cup introduces error that compounds with vessel geometry.
Step 6: The Mixing Process
Learning how to mix resin and hardener consistently is a matter of technique.
- Slow and steady for the first three to five minutes: fast mixing entrains air and shortens working time by generating localised exotherm.
- Scrape the sides and bottom of the vessel repeatedly. Unmixed resin clinging to the container wall is the most common cause of soft, tacky patches in an otherwise cured part.
- Aim for a homogenous colour. Many of our systems use contrasting Part A and Part B pigmentation specifically so a streaked, marbled mix stays visibly obvious before an operator dispenses it; a uniform colour confirms the mixing job is complete.
- Watch the pot life, the working time after mixing before viscosity rises enough to compromise flow or wetting. Pot life ranges from 15 minutes to several hours depending on formulation and ambient temperature, and it drops as batch size and ambient temperature increase, since the exotherm from polymerisation accelerates in a larger, insulated mass.
Step 7: Avoid Common Mixing Mistakes
Three failures account for most field returns: incomplete scraping of vessel walls, rushing the mix time under production pressure, and mixing a batch larger than the pot life supports for the dispensing method in use. Any one of these produces a bond that looks acceptable at assembly and fails during qualification or in the field. On a line potting 10,000 units a year at $85 in material and labour per unit, even a 2% mixing-error rate adds roughly $17,000 in scrap before counting rework labour or a field escape.
For fast-cure or high-viscosity systems, a metered gun applicator removes ratio and mixing variability entirely by dispensing both parts through a static mixer at a fixed, mechanically set ratio. This is a worthwhile capital investment once manual mixing error starts showing up in scrap rate data.
E] Vacuum Degassing: Removing Air Bubbles
1. The Mechanics of Degassing
A single 200 µm void at a bond line stress concentration point can initiate a crack under cyclic load well before the surrounding epoxy approaches its rated shear strength. Air entrained during mixing does not disappear on its own in a viscous, room-temperature-curing system; it sits in the matrix as a permanent defect.
Vacuum degassing pulls the mixed epoxy through a reduced pressure chamber, typically around -29 inHg, which expands trapped bubbles until they rise and rupture at the surface before the mix’s viscosity climbs high enough to trap them permanently.
2. Process Implementation and Benefits
Degassing delivers three measurable benefits:
- It removes the stress concentration points that trapped voids create, cutting a common source of premature bond line failure.
- It reduces internal porosity, which matters directly for potting compounds expected to provide a continuous dielectric barrier.
- It also eliminates the pitted, cloudy surface finish that undegassed epoxy produces on optical or cosmetic parts.
The process itself is straightforward: place the mixed epoxy, uncovered, in a vacuum chamber; pull vacuum gradually to keep the mix from boiling over its container; hold until bubbling visibly stops, typically two to five minutes depending on batch size and viscosity; then release vacuum slowly.
Use degassing wherever voids compromise function:
- Potting for hermetic or near-hermetic electronics enclosures
- Optical encapsulation where clarity is a spec requirement
- Any high-reliability aerospace or medical bond where a qualification standard sets a maximum void content.
For low-criticality general bonding, the added process step usually is not justified by the marginal strength gain.
F] Choosing the Right Application Method
1. Dispensing Techniques by Production Volume
The application method, more than the epoxy chemistry itself, often limits throughput on a production line. A 30-minute pot life means nothing if manual dispensing takes 25 minutes to cover the batch.
Manual application with a spatula or syringe suits low-volume runs and prototyping, where dispensing precision matters less than flexibility across varied part geometries. Knowing how to use epoxy correctly in each context, manual, semi-automatic, or fully automated, determines whether the pot life becomes a comfortable margin or a production bottleneck.
Semi-automatic application, using pre-filled dual cartridge systems dispensed through a static mixer, is standard for mid-volume production; it delivers the mixed ratio automatically and removes operator-dependent variability from the mixing step. For assembly-line touch-up work, a 2-component epoxy glue dispensed from a dual cartridge covers most general bonding needs without a separate mixing step.
Fully automated application through metre-mix-dispense equipment is the standard for high-volume electronics and automotive lines, dispensing at controlled shot weights with repeatability typically within 1 to 2% of the target. This is what makes tight process control specs achievable at scale.
2. Operational Guidelines for Application
Regardless of method, apply within the pot life window and verify shot weight or bead dimensions against the process spec at line start. Also, purge the dispensing equipment of any prior chemistry before switching formulations, since cross-contamination between incompatible epoxy systems can inhibit cure at the interface.
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G] Curing Schedules: Room Temperature Versus Accelerated Thermal Cure
1. Room Temperature Cure vs. Thermal Cure Dynamics
Pull a part off its fixture before it reaches handling strength, and it will shift, and that shift shows up later as a dimensional or bond line defect that is expensive to trace back to an impatient cure schedule.
Room-temperature curing systems typically reach handling strength in 1 to 4 hours and full mechanical and chemical properties in 24 to 72 hours at 25°C, without additional equipment, at the cost of tying up fixtures and floor space for longer.
Accelerated thermal curing moves the same chemistry through its cure schedule faster. Low-temperature cure systems, activated between 60°C and 100°C, typically reach full cure in 1 to 4 hours, while standard-temperature systems requiring 120°C or above can reach full cure in under 30 minutes.
The trade-off is oven capacity and energy cost against fixture turnover: a line running 500 parts a day at a 4-hour room-temperature cure needs roughly eight times the fixture inventory of the same line running a 30-minute thermal schedule. At $150 per fixture, cutting that inventory from 32 units down to 4 frees close to $4,200 in tooling capital.
Representative schedules: some Kohesi Bond heat-cure systems reach handling strength in roughly 60 minutes at 120°C, while comparable room-temperature-cure systems reach handling strength overnight at 25°C. Always confirm the exact schedule for your specific product on the current TDS.
2. Phase Transitions During Polymerisation
During the cure, the mixed epoxy two-part blend passes through the gel point, the moment viscosity increases sharply and the mix stops flowing. This is followed by vitrification, where the network’s Tg rises past the cure temperature and the reaction rate drops sharply as the system moves from a liquid to a glassy solid.
Do not disturb the bond line before the gel point. Do not exceed the maximum cure temperature in the TDS, since overcuring some amine systems degrades toughness. Do not assume handling strength equals full cure; pulling a fixture at handling strength and load-testing at handling strength are two different qualification steps with two different acceptance criteria.
H] Surface Preparation Requirements Before Bonding
1. The Importance of Surface Prep
An epoxy bond that fails within its rated shear strength range almost always fails at the interface, not in the bulk adhesive. This interface failure usually traces back to surface preparation that got skipped under schedule pressure.
Surface preparation exists because epoxy bonds mechanically and chemically to a substrate surface, not to whatever contamination sits on top of it. Oils, mould release residue, and oxide layers all sit between the adhesive and the base material and cap the achievable bond strength regardless of the epoxy’s rated performance.
2. Cleaning, Roughening, and Substrate-Specific Protocols
Cleanliness starts with removing visible contamination. Degreasing with isopropyl alcohol or a specified solvent removes the oils and residues a visual inspection misses. In surface energy testing, a water break test is a fast proxy that confirms the surface will wet out properly; target above 40 mJ/m² for reliable adhesion on most substrates.
Surface roughening by abrasion or media blasting to a Ra of roughly 1 to 3 µm increases mechanical interlock and effective bond area without over-roughening to the point of trapping air during application.
Substrate-specific preparation matters. Aluminium benefits from a chromate or phosphate conversion coating or, at minimum, abrasion plus a solvent wipe. Stainless steel needs abrasion to break through the passive oxide layer. Many engineering plastics need a corona or plasma treatment to raise surface energy above the epoxy’s wet-out threshold, since as-moulded plastic surface energy often sits below 30 mJ/m², which is too low for reliable adhesion without treatment.
I] Practices That Improve Bond Strength in Production
1. Adhesion vs. Cohesion
Bond strength on paper and bond strength in the field diverge most often at the two properties formulators separate deliberately: adhesion, the epoxy’s grip on the substrate, and cohesion, the epoxy’s internal strength once cured. A bond can show excellent adhesion and still fail if cohesive strength cannot support the load or the reverse. Matching both properties to the application, not just selecting the highest shear-value product on the shelf, is what determines field performance.
2. Practical Production Tips
Mixing and preparation: Weigh to the ratio precision the TDS specifies, not to the nearest gram if the spec calls for 0.1 g resolution, and run the full mix time even when the blend looks visually homogeneous early on.
Application technique: Control bond line thickness, typically 50 to 250 µm for structural bonds. A bond line that is too thin starves the joint of adhesive, and one that is too thick reduces shear strength by concentrating stress at the edges.
Curing optimisation: Where the process allows, a post-cure step, an additional 1 to 2 hours at elevated temperature after the primary cure schedule, can raise Tg by 10°C to 15°C in some amine-cured systems. This is worth validating against your specific formulation before adding it to a standard process.
3. Thermal Stress and Modulus Management
Substrate-specific strength: For dissimilar-CTE substrate pairs, such as a ceramic sensor on an aluminium housing, select a filled, lower-modulus system that absorbs thermal cycling strain rather than the highest shear-value option. This is due to the fact that a stiffer bond line transfers more stress into the weaker substrate as the assembly cycles. A simplified thermal stress estimate makes the trade-off concrete:
σ = E x ΔCTE x ΔT
- σ: the induced thermal stress at the bond line, in MPa
- E: the cured epoxy’s tensile modulus, in MPa
- ΔCTE: the CTE mismatch between the two substrates, or between substrate and adhesive, in ppm/°C
- ΔT: the temperature swing the assembly sees in service, in °C
A stiffer, higher-modulus epoxy raises E and, with it, the stress the joint carries for the same CTE mismatch and thermal swing. On a large CTE mismatch, dropping to a lower-modulus, filled system reduces σ even though the epoxy’s rated shear strength on the datasheet looks lower.
J] Troubleshooting Common Issues
1. Addressing Curing and Adhesion Failures
A soft, tacky area in an otherwise cured part points to incomplete mixing, almost always caused due to unmixed resin left clinging to the vessel wall or lid. The fix is process discipline on scraping, not a different epoxy.
A bond that cures but delaminates under load despite adequate cure time usually traces to surface contamination or a surface energy below the epoxy’s wet-out threshold. Retest with a water break test before troubleshooting the adhesive chemistry itself.
2. Resolving Physical and Structural Defects
Visible bubbles or a pitted surface in a cured part indicates the operator mixed too aggressively or skipped vacuum degassing on an application that needed it. Slow the mix rate and add a degassing step.
A batch that gels before application is complete means the mix exceeded its pot life, often because ambient temperature ran higher than the TDS assumes or the batch size outgrew the dispensing method. Mix smaller batches or move to metered dispensing.
Brittleness after cure, or a Tg lower than the TDS value, points to an off-ratio mix, most commonly excess hardener. Verify the scale calibration and the measured weights against the specified mixing ratio before requalifying the batch.
K] Selecting the Right Two-Component Epoxy
1. Key Selection Criteria
The wrong selection question is “What is the strongest 2-part epoxy?” The right one is “What does this joint need to survive, and for how long?
- Cure speed: Room-temperature cure suits low-volume or field applications where engineers have limited equipment access, while accelerated thermal cure suits production lines where fixture turnover drives throughput.
- Operating temperature: Match the service temperature range and thermal cycling profile to the epoxy’s Tg and CTE, with margin, since a bond line operating within 20°C of its Tg loses stiffness and creep resistance rapidly.
- Specialised properties: Thermal conductivity for heat-generating assemblies, electrical conductivity or insulation depending on the circuit function, and low outgassing for vacuum or optical environments are not interchangeable. Specify the property the application actually needs rather than defaulting to the highest-spec option across the board.
- Industry requirements: Confirm the qualifications the application demands before shortlisting a chemistry, whether that is component-level AEC‑Q200 qualification for automotive electronics, ASTM E‑595 for aerospace outgassing, or USP Class VI and ISO 10993‑5 for medical devices. A chemistry that fails the qualification cannot make the shortlist regardless of its mechanical data.
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Every mixing, degassing, curing, and surface preparation practice in this guide exists because a two-component epoxy system only delivers the properties on its TDS when the process controls the variables the chemistry cannot control for itself. Ratio, mix quality, degassing, cure schedule, and substrate preparation are not optional steps around the adhesive; they are the specification.
Kohesi Bond formulates two-component epoxy, conductive, and potting systems for electronics, aerospace, medical, and automotive manufacturing, with technical support for ratio selection, cure schedule optimisation, and substrate compatibility testing. If a current process is producing inconsistent bond strength or field returns tied to adhesive performance, a review of the mixing and cure process against the current TDS is often the fastest path to a fix.
Contact our technical team to discuss ratio, cure schedule, or substrate requirements for your specific application.
FAQs
Utsav Shah is a 34-year-old entrepreneur with a passion for scientific discovery. Utsav’s journey began with a deep dive into materials science, earning degrees from USC and the Institute of Chemical Technology. He’s the visionary founder of Kohesi Bond, a top-rated adhesive manufacturer, and Cenerge Engineering Solutions, a leader in heat exchangers and cryogenic pumps. With over a decade of experience, Utsav consults across various industries, ensuring they have the perfect adhesive solution for their needs. Connect with him on LinkedIn!
