Two-part adhesive systems — epoxies, polyurethanes, methacrylates, and other reactive systems — require precise mixing of resin and hardener components in a specific ratio for the chemical reaction to proceed correctly. When the mixing ratio deviates from the specified value, the result is a cured adhesive with off-specification properties: lower strength, reduced heat resistance, altered surface tack, or sometimes no cure at all. Mixing ratio errors are a significant production failure mode that can be extremely difficult to detect without destructive testing of the cured joint.
Why Mixing Ratio Is Critical
Two-part adhesive cure is a stoichiometric chemical reaction. The resin component contains reactive groups (typically epoxy groups, isocyanate groups, or vinyl groups) that react one-to-one with complementary groups in the hardener (amines, hydroxyls, or other reactive species). The correct mixing ratio provides the stoichiometrically balanced quantities of each component so that all reactive groups can react.
When the ratio deviates:
Excess hardener (hardener-rich mix): Unreacted hardener components remain in the cured adhesive as plasticizers and low-molecular-weight contaminants. These reduce Tg, reduce modulus, and reduce long-term stability. Some hardener types in excess also create reactive end groups in the cured network that absorb moisture or degrade over time.
Excess resin (hardener-lean mix): Unreacted resin groups remain, producing a softer, tackier, lower-strength cured material. Epoxy adhesives with insufficient hardener remain sticky, fail to achieve full hardness, and may continue reacting slowly over time with absorbed moisture or atmospheric moisture.
Severe ratio errors: If the ratio is very far from specification — such as using only one component, or accidentally swapping components — no cure or only partial cure may result. The joint may appear to set initially (partial reaction or physical hardening), then lose all strength as un-reacted or partially reacted adhesive separates from the substrate.
The sensitivity to ratio varies by adhesive type and hardener chemistry. Some systems are forgiving of ±10% ratio error; others require ±2% for adequate properties. The specification sheet should identify the acceptable ratio tolerance for each product.
Sources of Mixing Ratio Errors in Production
Manual Mixing by Weight or Volume
When operators mix two-part adhesives by weighing components on a balance or measuring by volume with syringes, the accuracy depends entirely on the operator’s technique and the accuracy of the weighing equipment. Small-batch manual mixing is prone to:
- Reading scale at the wrong viewing angle (parallax error in volume measurement)
- Using tared weights incorrectly
- Not completely transferring all weighed material to the mixing vessel
- Contaminating components by using the same measuring vessel for both components without cleaning
Process specification for manually mixed adhesives must define the accuracy requirement (typically ±2–5% by weight), the weighing equipment required (resolution, calibration), and the mixing procedure.
Meter-Mix Dispensing System Errors
In production environments, meter-mix dispensing systems pump both components simultaneously in the correct ratio and mix them in a static or dynamic mixer before dispensing. These systems eliminate manual mixing errors but introduce their own failure modes:
Pump calibration drift. The pumping elements (gear pumps, piston pumps) that deliver each component wear over time, changing their displacement per stroke or per revolution. If both components wear at the same rate, the ratio is maintained. If one wears faster than the other, the ratio drifts from specification. Regular calibration — weighing the output of each component separately over a fixed number of cycles — detects this drift.
Component viscosity changes. Many adhesive components are temperature-sensitive in viscosity. Cold components from refrigerated storage are more viscous than warm components at room temperature. If the components are at different temperatures — perhaps the hardener was brought out of refrigeration more recently than the resin — their viscosities may be mismatched from the calibration conditions. Some dispensing systems have temperature-sensitive flow characteristics that produce ratio errors when component viscosity changes.
Air in pumps. Air bubbles in pump chambers produce incomplete strokes — a stroke that should pump 1 mL of component pumps less because part of the stroke volume is occupied by compressible air. Priming dispensing systems at the start of the shift and after any component container change removes air. Failure to prime causes systematic ratio errors in early-shift or post-change joints.
Component container changes and start-up. When a component container is empty and replaced, the pump must be reprimed and the new component run through the system until fresh material reaches the mixing head. Any joints dispensed during this transition period may have off-ratio composition from mixing of old and new material or from incompletely primed pumps.
Email Us to discuss mixing ratio control for your two-part adhesive process.
Cartridge System Issues
Pre-measured two-component cartridges use piston displacement to deliver both components simultaneously in the designed ratio. The plungers on each side are mechanically linked to maintain ratio. Despite this design, ratio errors can occur:
Leaking component seals. If the seal on one component cylinder leaks, that component drains ahead of the other. The first material dispensed from the leaking system is component-lean or component-rich.
Blockage in one component flow path. Partial blockage from crystallized or gelled material in the mixing nozzle or cartridge orifice restricts one component’s flow, changing the effective ratio.
Dispensing from a nearly empty cartridge. As one cartridge side empties faster than the other (from manufacturing tolerances in fill amounts), the last material dispensed may be off-ratio. Most manufacturers recommend discarding the last 5–10% of cartridge material for critical applications.
Detection of Mixing Ratio Errors
Hardness measurement — Shore hardness testing of the cured adhesive provides a quick indicator of whether the cure reached the expected hardness for the correct ratio. Under- or over-hardened material deviates from the specification hardness.
DSC residual exotherm — for laboratory analysis, DSC measures unreacted groups remaining in a cured adhesive sample. Samples with off-ratio cure show residual exotherm corresponding to unreacted components.
Gel time monitoring — for production process monitoring, measuring the gel time of a drop of mixed adhesive on a hot plate provides a real-time indication of proper mixing. Consistent gel time within established limits indicates correct ratio; deviations indicate ratio or component temperature issues.
Weight ratio verification — periodic weighing of the two components dispensed separately from the meter-mix system verifies actual delivered ratio versus set ratio. This is the most direct process control measurement.
Incure’s Mixing Guidance
Incure specifies mixing ratios and ratio tolerances for two-part products, along with recommended dispensing equipment and process controls for production applications.
Contact Our Team to discuss mixing ratio control requirements for your two-part Incure adhesive product and identify appropriate dispensing equipment and process controls.
Conclusion
Mixing ratio errors in two-part adhesive systems produce cured material with off-specification properties — reduced strength, lower Tg, surface tack, or no cure — depending on the direction and severity of the error. Errors arise from manual measuring inaccuracies, meter-mix pump calibration drift, viscosity-related dispensing issues, air in pumps, and cartridge system failures. Preventing ratio errors requires calibrated dispensing equipment, regular calibration verification, operator training for manual processes, and process monitoring through hardness, gel time, or direct ratio weighing.
Visit www.incurelab.com for more information.