Two-part adhesive is a room-temperature, fast-curing structural adhesive composed of two components, commonly identified as Component A and Component B. It is an important category of acrylic adhesives and is widely used in industrial manufacturing, construction and finishing, electronics, and electrical products because of its fast cure speed, high bonding strength, and broad material compatibility.
The following sections summarize the evaluation and verification considerations involved in introducing a two-part AB adhesive dispensing process.
1. Composition and Curing Principle of Two-Part Adhesive
Component A: Base Resin
- Resin, monomer, or prepolymer: Provides the basic molecular framework and reactive groups. These materials largely determine the final strength, toughness, and temperature resistance of the cured adhesive.
- Toughening agent or filler: Improves toughness, reduces curing shrinkage, adjusts viscosity, or provides thermal or electrical conductivity.
- Diluent or solvent: Adjusts viscosity. Some environmentally oriented adhesive formulations do not contain solvents.
- Stabilizer: Prevents premature polymerization during storage.
Common Component A materials include epoxy resin, acrylate monomers such as methyl methacrylate (MMA), polyurethane prepolymers, and silicone polymers.
Component B: Curing Agent
- Curing agent or initiator: Initiates or participates in the chemical reaction that cross-links and cures the resin.
- Accelerator or catalyst: Increases the reaction rate. Some adhesive systems require an additional accelerator.
- Filler or additive: Adjusts functional properties such as thickening and thixotropy.
Common Component B materials include amines, peroxides, isocyanates, and platinum catalysts.
Basic Curing Mechanism
When Components A and B are mixed at the specified ratio, the initiator rapidly decomposes and generates free radicals. These free radicals initiate the polymerization of the acrylate monomers or prepolymers, producing a strong cross-linked polymer network. The adhesive therefore changes from a liquid mixture into a rigid or tough solid after mixing and curing.
2. Performance Advantages of Two-Part Adhesive
Fast Curing
Fast curing is one of the most significant characteristics of two-part adhesive. At room temperature, the adhesive can reach handling strength, or initial cure, within several minutes to several tens of minutes. It typically reaches a relatively high strength within one to two hours. Structural acrylic AB adhesives are particularly known for achieving initial cure in approximately five minutes. Heating can further accelerate the curing process when the product and assembly materials permit it.
High Bonding Strength
After curing, the adhesive forms a highly cross-linked, rigid polymer. It provides high shear strength together with good impact and peel resistance, making it suitable for structural bonding applications.
Good Resistance to Impact and Vibration
Because toughening agents are incorporated into many formulations, two-part acrylic adhesives generally provide better toughness and impact resistance than two-part epoxy adhesives. They are therefore more suitable for assemblies exposed to dynamic loading or vibration.
Broad Material Compatibility
Two-part adhesives can bond many metals, including steel, aluminum, and copper. They also provide good adhesion to most plastics, including difficult-to-bond board materials such as polypropylene (PP), polyethylene (PE), and fluoroplastics, although surface treatment may sometimes be required. Other compatible materials may include fiberglass-reinforced plastic, carbon-fiber composites, ceramics, stone, and wood.
Good Oil and Contamination Tolerance
Some formulations have a relatively high tolerance for lightly contaminated surfaces, such as surfaces carrying small amounts of engine oil or cutting fluid. Certain products may even bond directly to an oily surface, but this capability must be confirmed against the product documentation and process requirements.
Weather Resistance
Two-part acrylic adhesives generally provide better ultraviolet resistance, water resistance, and aging resistance than polyurethane adhesives.
Temperature Resistance
Typical products can withstand continuous temperatures from -40 °C to 120 °C. Some high-performance products can operate at 150 °C or above. The actual temperature capability depends on the formulation, cure condition, stress state, and bonded materials.
Relatively Low Cure Shrinkage
Compared with some single-component acrylic adhesives, such as anaerobic adhesives, two-part adhesives generally have lower cure shrinkage. This helps maintain dimensional stability and reduce internal stress in the bonded assembly.
Capability to Fill Larger Gaps
Some formulations are designed to bond surfaces with a certain gap between them. The allowable gap may be 0.1–0.5 mm or greater, depending on the adhesive formulation and application requirements.
3. Two-Part Thermal-Conductive Potting Adhesive Dispensing Equipment
Types of Dispensing Machines
Two types of dispensing machines are suitable for two-part adhesive applications on mobile-device products: a motor-driven screw-rod dispensing system and a pneumatic syringe dispensing system. Based on a comparison of their performance, a pneumatic syringe dispensing machine can satisfy the dispensing requirements for mobile-device side strips, touch panels, and similar applications.
Dispensing Components: Y-Valve and Needle
The needle assembly of a two-part dispensing machine consists of three main parts: the Y-valve, the mixer head, and the dispensing needle.
- Y-valve operating principle: A valve is integrated into the lower end of the Y-valve needle. The dispensing machine clamps the Y-valve needle and moves it vertically. This movement opens or closes the mixer head to control adhesive dispensing.
- Y-valve selection: The integrated Y-valve needle is available in two diameters, 0.8 mm and 1 mm. The diameter of the integrated Y-valve needle must be greater than the diameter of the dispensing needle selected for the outlet.
- Dispensing needle: A plastic-hub stainless-steel dispensing needle is used. Most AB adhesives can accommodate a No. 22 blue needle with an internal diameter of 0.41 mm.
Cooling Equipment
The cooling equipment is installed around the mixer head. Its function is to lower the temperature of the adhesive inside the mixer head, extend the adhesive open time, reduce purge requirements, and improve production continuity.
Cooling equipment can be divided into air-cooled and electrically powered systems. Air cooling uses compressed air to lower the temperature, with temperatures reaching below 0 °C. Electrically powered cooling uses electrical energy and can reduce the temperature to approximately -30 °C.
4. Key Process Parameters for Two-Part Thermal-Conductive Potting Adhesive
Purge and Adhesive Waste
Once the two-part adhesive begins mixing inside the mixer head, the chemical reaction starts. During production, the mixed adhesive must be discharged from the mixer head within its open time. Otherwise, the adhesive may cure inside the mixer head and Y-valve, blocking the flow path and requiring both components to be discarded and replaced. Regular purging is therefore required to reduce mixer-head and Y-valve replacement.
Three factors determine the purge requirement:
- The open time of the adhesive
- The adhesive consumption per machine cycle
- The operating cycle time
A cooling unit is generally recommended because it extends the adhesive open time and reduces purge frequency. Without cooling, a considerable amount of adhesive may be wasted during repeated purging.
The cooling unit should be added at an early stage of process development. If the equipment remains idle for an extended period, timed and quantitative purging is still required. The adhesive inside the mixer head must be discharged before the open time expires.
The need for a cooling unit can be estimated as follows:
- If the adhesive consumption per machine cycle is greater than the adhesive volume inside the mixer head, approximately 1 g, and the dispensing time plus the subsequent operating time is shorter than the adhesive open time, a cooling unit may not be required.
- If the adhesive consumption per machine cycle is lower than the adhesive volume inside the mixer head, the required purging frequency must be calculated according to the operating cycle. In practice, this calculation often indicates substantial adhesive waste. Therefore, when the adhesive consumption per machine cycle is lower than the mixer-head volume, installing a cooling unit is generally recommended.
For example, assume that the adhesive open time is three minutes, the dispensing cycle is 60 s, the subsequent operation cycle is 30 s, the mixer head contains 1 g of adhesive, and the adhesive consumption per machine cycle is 0.2 g.
Pressure Holding and Dwell
When a heated compression fixture is used, holding pressure for 30–60 s can achieve approximately 50% of the final strength. At room temperature, holding pressure for 10 minutes can achieve approximately 35% of the final strength.
No additional dwell time is required before assembly after pressure holding is completed. Reliability testing should be performed after 24 hours.
Removal of Excess Adhesive
Before initial cure, excess adhesive can be wiped away directly with alcohol. Once the adhesive has fully cured, removing the residue becomes considerably more difficult.
Ventilation
The AB adhesives currently used are acrylic-based two-part adhesives. They have an irritating odor before curing, but are described as non-toxic in the stated application. After curing, they are odorless. Even when a production facility does not have a dedicated exhaust system, adequate ventilation must be maintained in the dispensing area.
Repair Considerations
Assemblies bonded with AB adhesive are generally difficult to repair. Several risks must be considered during repair:
- Repair work may damage the touch panel or liquid-crystal module.
- The housing surface may be damaged. The structural adhesive bonds through chemical interaction with the housing material, so removing the bonded structure may result in housing rejection.
- The adhesive may chemically interact with the touch-panel ink. If the ink quality is insufficient, removing the adhesive bead and cleaning the residue may damage the printed ink layer.
- Residual adhesive is difficult to remove. A typical approach is to heat the residue, soak it with alcohol, and then wipe it away.
5. Application of Two-Part Thermal-Conductive Potting Adhesive
The adhesive has been evaluated for bonding side strips in a product application.
Successful application depends on maintaining the correct mixing ratio, controlling the open time, selecting a suitable dispensing needle, and managing the adhesive temperature inside the mixer head. Purge frequency, pressure-holding time, cleaning method, ventilation, and repairability must also be evaluated together because these factors affect material consumption, production continuity, assembly quality, and service risk.
