Polyurethane (PU) is an extremely versatile polymer family. Which of the following applications is generally NOT a standard, mainstream use for PU in its common commercial forms?

Introduction / Context:
Polyurethanes can be tailored into foams, elastomers, adhesives, and coatings. Exam questions often contrast typical PU applications with FRP matrices, which are most commonly polyesters, vinyl esters, or epoxies.

Given Data / Assumptions:

• Flexible PU foams: seats, cushions.
• Rigid PU foams: appliance insulation (refrigerators), building panels.
• PU coatings/adhesives: ubiquitous.
• FRP matrices: predominantly unsaturated polyester, vinyl ester, or epoxy; PU matrices exist but are not mainstream.

Concept / Approach:
The question targets the “least typical” PU use. While niche PU composite matrices exist, the canonical FRP systems are not polyurethane-based. Therefore, FRP primary matrix resin is the best “not used” choice in standard curricula.

Step-by-Step Solution:
List standard PU applications.Contrast with mainstream FRP matrices.Select FRP matrix as not typically PU-based.

Verification / Alternative check:
Industry practice and standards in boatbuilding, automotive composites, and wind blades predominantly specify polyester/vinyl ester/epoxy matrices.

Why Other Options Are Wrong:
Cushions/insulation/coatings/elastomeric wheels: hallmark PU applications.

Common Pitfalls:
Confusing occasional PU composite use with mainstream FRP matrix practice.

Final Answer:
Fibre reinforced plastic (FRP) primary matrix resin