What is the difference between silicone and polyurethane sealants?

Silicone sealants offer excellent UV resistance and temperature stability, making them ideal for glazing and exposed facades. However, they are generally softer and pick up dirt. Polyurethane (PU) sealants are tougher, have better abrasion resistance, and can be painted over. PU is typically preferred for floor joints and traffic areas where durability and puncture resistance are required. In more detail: both are elastomeric joint sealants, but their chemistry gives them different strengths, and using the wrong one is a common reason joints fail early. Silicone cures to a rubber with outstanding UV and weathering resistance and stable properties across a very wide temperature range, keeping its elasticity for decades on exposed faces, which is why it is the standard for glazing and curtain walling. Its weaknesses are low tear and abrasion resistance (it is easily damaged by feet, wheels and cleaning machines), most grades cannot be painted, and many silicones can stain porous stone and concrete at the joint edges. Polyurethane has higher tear strength and abrasion resistance, bonds well to concrete, does not stain it, and can be overpainted, so it is the normal choice for movement and construction joints in floors, car park decks, pavements and façade panel joints that must be painted or survive traffic. PU's weaknesses are lower long-term UV resistance (the surface can chalk or craze after years of direct sun), sensitivity to moisture while curing (it can bubble if gunned into a damp joint), and a shorter life than silicone on exposed glazing. Hybrid MS-polymer (silyl-modified) sealants sit between the two. Performance depends as much on the joint as the product: sealants are classified by movement capability (for example class 25, meaning ±25% of joint width) under BS EN ISO 11600 and the BS EN 15651 series, BS 6213 gives UK guidance on selecting sealants for building joints, and a correctly sized joint uses a closed-cell backer rod to stop three-sided adhesion, a width-to-depth ratio typically around 2:1, and clean, dry, primed faces where the manufacturer requires it. For trafficked joints in car parks and warehouses MPS normally specifies PU or a dedicated trafficable joint system; silicone is kept for glazing and non-trafficked façade joints.