Two-Part Polyurethane Adhesive Cure
Two component polyurethanes (also called 2C PUR) have the advantages of single component polyurethane adhesives but are able to cure more quickly through larger gaps and are not dependent on moisture or heat to cure. Formulations can create flexible, tough materials that absorb considerable expansion, contraction stresses.
Mixing Two Component Polyurethanes
Similar to two component epoxies, ratios of component A and component B can vary, the proper mix ratio and complete mixing is vital to maintain consistent properties. Many industrial two component polyurethanes are available in side by side cartridges such that product can be dispensed through a static mix tip. This eliminates possible errors in weighing or measuring.
To ensure that the polyurethane adhesive is properly mixed:
- Attach the correct static mix nozzle for the material. Static mix tips vary in shape, length and number of elements. If there is any doubt contact the adhesive manufacturer to ensure you are using the correct tip. Incomplete mixing will adversely affect performance.
- Dispense a bead of the adhesive about an inch or two long onto a paper towel as the first bit coming out of the nozzle may not be completely mixed. Often component A is one color and component B another to visualize good mixing.
Pot Life for Polyurethane Adhesives
Pot Life of the adhesive is loosely defined as the amount of time required for the polyurethane adhesive to double in viscosity. Full cure is generally 24 hours or more. In most cases the critical piece of information, is not the full cure but the handling strength – because handling strength indicates the time needed before assemblies can be moved or packed. Ensure the assembly is not unduly stressed until sufficient strength is achieved. The pot life will provide some indication of how quickly the two component polyurethane sets and also indicates the length of time possible between mixing and use.
Two component polyurethanes are used in thin films for bonding metals, plastics and composites. They are also used for potting components. Thick layers of adhesive or potted sections may cure more quickly than adhesive squeezed into a thin layer. This is because of heat generated by exothermic reaction.