Creating polyurethane (PU) elastomers from PU prepolymers involves a process where the prepolymer (an isocyanate-terminated polymer) is mixed with a curative (usually a chain extender containing active hydrogen, like a polyol or diamine). This reaction forms a cross-linked structure, giving the resulting material elastomeric properties.

Here’s a step-by-step overview of the process:

1. Selection of Raw Materials

• Prepolymer: Choose a PU prepolymer based on the desired properties of the final elastomer. The prepolymer’s properties, such as molecular weight and isocyanate content, influence the elastomer’s hardness, elasticity, and resistance to environmental factors.
• Curative: The most common curatives are polyols and diamines. Polyols provide a flexible structure, while diamines typically create a more rigid structure. The choice of curative affects the hardness, resilience, and overall flexibility.

2. Mixing Process

• Temperature Control: Preheat the PU prepolymer and the curative, as temperature affects the viscosity and reactivity of the components. Most processes require heating the prepolymer to 60–80°C (140–176°F).
• Mixing Ratios: Accurately measure the prepolymer and curative to ensure proper stoichiometry. An imbalance may lead to incomplete curing, affecting the material’s final properties.
• Degassing: To remove any entrapped air, degas the prepolymer and curative before or after mixing. Air bubbles can weaken the elastomer’s structure.

3. Casting and Curing

• Mold Preparation: Preheat and, if necessary, coat the molds with a release agent to make demolding easier.
• Pouring and Casting: Pour the mixed prepolymer and curative into the mold carefully to avoid air entrapment.
• Curing: Cure the elastomer in the mold at room temperature or slightly elevated temperatures (typically 80–100°C or 176–212°F). The curing time varies based on formulation but often takes several hours to ensure complete polymerization.

4. Post-Curing

• Heat Treatment: For optimal properties, elastomers often undergo post-curing at a higher temperature (e.g., 100–130°C or 212–266°F) for a specified period (2–16 hours). This enhances the cross-linking, improving mechanical properties like tensile strength and elasticity.

5. Finishing

• Demolding: Carefully remove the cured elastomer from the mold.
• Trimming and Inspection: Trim any excess material and inspect for defects. If needed, apply a finishing treatment to achieve desired surface characteristics.

Key Properties and Adjustments

• Hardness: Adjusted by varying the type and ratio of the prepolymer and curative.
• Abrasion Resistance: Enhanced by selecting a high-performance polyol and isocyanate.
• Flexibility: Controlled by choosing curatives with different chain lengths or functionalities.

By modifying the prepolymer and curative formulations, it’s possible to create a wide variety of polyurethane elastomers suited for applications like seals, gaskets, wheels, and industrial machinery components.