Choosing the right sealant can mean the difference between a weathertight, long-lasting installation and costly callbacks. While both polyurethane and silicone sealants have their place in construction, understanding when to use each type is crucial for optimal performance. In this comprehensive guide, we'll explore the key differences and help you make informed decisions for your projects.
Understanding Polyurethane Sealants
Polyurethane sealants are moisture-curing elastomeric materials that form durable, flexible bonds with a wide range of construction substrates. Unlike silicone sealants, polyurethane products can be painted over once cured.
Key Advantages of Polyurethane Sealants
• Paintable Surface: Can be painted after curing, making them ideal for visible joints that need to match surrounding finishes
• Superior Adhesion: Bonds strongly to concrete, masonry, timber, cement sheet and most building materials without primers
• Abrasion Resistance: More durable than silicone in high-traffic areas and exposed locations
• Tear Strength: Excellent resistance to tearing and mechanical stress
When to Use Polyurethane Sealant
Ideal Applications
1. Construction and Expansion Joints
Polyurethane sealants excel in horizontal and vertical construction joints where movement is expected. Their high movement capability and superior adhesion make them perfect for concrete-to-concrete, concrete-to-masonry, and similar substrate combinations.
2. Curtain Wall Systems
Modern building facades demand sealants that can handle thermal movement, wind loads, and weather exposure. Polyurethane sealants provide the necessary flexibility and durability while maintaining a professional appearance that can be painted to match architectural finishes.
3. Pre-Cast Concrete Panels
The strong bond between polyurethane and concrete substrates, combined with excellent movement accommodation, makes these sealants ideal for panel-to-panel connections in tilt-up and pre-cast construction.
4. Window and Door Perimeter Sealing
When frames need to be painted or when a more robust mechanical bond is required, polyurethane sealants outperform silicone. They're particularly effective in timber frame installations where the ability to paint over the sealant creates seamless transitions.
5. Floor Joints and Traffic Areas
The superior abrasion and tear resistance of polyurethane sealants makes them the preferred choice for horizontal joints subject to foot traffic, wheeled loads, or regular cleaning.
When Silicone Might Be Preferred
While polyurethane sealants offer many advantages, silicone sealants are better suited for certain applications:
• Wet Areas: Bathrooms, kitchens, and other permanently wet environments where constant water exposure occurs
• Glass-to-Glass Joints: Structural glazing and aquarium construction where clarity and UV resistance are critical
• High-Temperature Applications: Areas exposed to temperatures exceeding 80°C, where silicone's heat resistance provides better performance
• Sanitary Fixtures: Sealing around sinks, basins, and sanitaryware where anti-fungal properties are essential
Technical Comparison: Polyurethane vs Silicone
|
Property |
Polyurethane |
Silicone |
|
Movement Capability |
±25 to +100/-50 |
±25 to +100/-50 |
|
Paintable |
Yes (after cure) |
No |
|
Adhesion to Concrete |
Excellent (primerless) |
Good (primer often needed) |
|
Abrasion Resistance |
High |
Low to moderate |
|
Water Immersion |
Not recommended |
Excellent |
|
Temperature Range |
-40°C to +80°C |
-60°C to +200°C |
|
UV Resistance |
Low to moderate |
Excellent |
|
Tack-free Time |
30-90 minutes |
7-15 minutes |
Professional Application Tips for Polyurethane Sealants
1. Joint Design is Critical
Proper joint design ensures optimal sealant performance. The width-to-depth ratio significantly affects how well the sealant can accommodate movement without failure:
• For joints 6-10mm wide, make depth equal to width
• For joints 10-20mm wide, maintain 10mm depth
• For joints 20-30mm wide, use half the joint width as depth
• Always use closed-cell polyethylene backing rod to prevent three-sided adhesion
2. Surface Preparation Matters
While PascoFlex PU25 offers excellent primerless adhesion, surface preparation is still crucial:
• Remove all dust, dirt, oil, and loose material
• Ensure surfaces are completely dry before application
• Avoid contamination from form release agents or curing compounds
• For porous substrates in demanding applications, consider Pasco AquaPrime PU Primer or Pasco Aquaprime PVC Primer
3. Application Technique
• Apply at temperatures between 5°C and 35°C for optimal curing
• Tool immediately after application using a convex-shaped tool
• Use dry tooling methods rather than wet (soap solution) tooling
• Remove excess sealant and smears immediately
• Allow full cure before painting (cure rate: 2-3mm per day)
Common Mistakes to Avoid
6. Using Polyurethane in Permanently Wet Environments
While polyurethane sealants are weather-resistant, they're not suitable for joints with temporary or permanent water immersion. Shower enclosures, swimming pools, and water tanks require silicone sealants.
7. Inadequate Joint Depth
Shallow joints or three-sided adhesion (when sealant bonds to the bottom of the joint) can cause premature failure. Always use proper backing rod placement.
8. Painting Before Full Cure
Although PascoFlex PU25 is paintable, applying paint before the sealant has fully cured can trap solvents and inhibit proper curing. Wait for complete cure based on joint depth.
9. Excessive Moisture During Cure
While polyurethane cures through moisture reaction, excessive moisture or application in wet conditions can cause bubbling or poor adhesion. Ensure backing rod is dry and joints are protected from rain during initial cure.
Environmental & Health Considerations
Modern construction increasingly demands products that meet environmental and health standards. PascoFlex PU25 polyurethane sealant excels in this area:
• Low VOC Content: At just 29g/L, PascoFlex PU25 meets Green Star Design requirements and is safe for interior applications
• Mould Resistant: Inherent resistance to mould and mildew growth improves indoor air quality
• Durability Reduces Waste: Long service life means fewer replacements and reduced material consumption over building lifetime
• ASTM C-920 Compliance: Meets international performance standards for quality and safety
Cost Considerations: Initial Price vs Long-Term Value
While polyurethane sealants may have a higher initial cost compared to some silicone products, they often provide better long-term value:
• Primerless Application: Saves labour and material costs on most substrates
• Reduced Callbacks: Superior adhesion and durability mean fewer warranty claims
• Paintability: Eliminates the need for colour-matched sealants in painted applications
• Extended Service Life: Resistance to abrasion and weathering extends replacement intervals
Making the Right Choice for Your Project
The choice between polyurethane and silicone sealants should be based on specific application requirements rather than habit or initial cost. For the majority of construction joints, expansion joints, and building envelope applications, polyurethane sealants like PascoFlex PU25 offer superior performance, durability, and value.
Key decision factors include:
• Substrate type and compatibility
• Expected joint movement
• Water exposure level
• Temperature extremes
• Need for paintability
• Traffic and abrasion exposure
Expert Support for Your Sealant Selection
At Pasco Construction Solutions, we understand that choosing the right sealant is crucial for project success. Our technical team is available to help you select the optimal product for your specific application and provide guidance on proper installation techniques.
Contact us today for:
• Technical data sheets and product specifications
• Application guidance and best practices
• Project-specific sealant recommendations
• Product samples and testing support
Melbourne: (03) 9429 9111 | 241 Ingles Street, Port Melbourne VIC 3207
Sydney: (02) 9817 4441 | 4/14 Tennyson Road, Gladesville NSW 2111
Website: www.pasco.net.au