Understanding Adheseal Polyurethanes: PU15, PU25, and PU50 Explained

Choosing the Right Polyurethane: From Flexibility to Strength

As a builder, tradesperson, or specifier, you routinely encounter different product numbers for sealants and adhesives. Polyurethane (PU) products, in particular, often use numerical designations like ‘15’, ‘25’, or ‘50’. These numbers are not arbitrary; they directly correlate to the product’s Shore A hardness – a critical indicator of its modulus and, consequently, its joint movement capability and overall performance.

Understanding these differences is crucial for preventing common job site failures, ensuring the longevity of your work, and specifying with confidence. This guide will walk you through the practical implications of Shore A hardness within our Adheseal PU range, helping you select the right product for the job at hand.

What is Polyurethane and Why Does it Matter?

Polyurethane is a highly versatile polymer celebrated for its elastomeric properties, durability, and excellent adhesion across a wide range of substrates. In construction, polyurethanes serve a dual purpose: as high-performance sealants for movement joints and as robust adhesives for bonding various materials. They are known for their resistance to weathering, abrasion, and chemicals, making them suitable for demanding indoor and outdoor applications.

The flexibility and bond strength of polyurethanes make them ideal for accommodating dynamic conditions – such as thermal expansion and contraction – while maintaining structural integrity. However, not all polyurethanes are created equal, and their specific properties are fine-tuned for different applications.

The Core Difference: Shore A Hardness, Modulus, and Movement

The primary distinction between our Adheseal PU15, Adheseal PU25, and Adheseal PU50 lies in their Shore A hardness. Shore A is a scale used to measure the hardness of rubbers and soft plastics. For sealants, a lower Shore A value indicates a softer, more flexible material, while a higher value signifies a harder, more rigid material.

This hardness directly correlates to the product’s modulus. Modulus, in the context of sealants, refers to the stress (force) required to produce a given strain (deformation). A low modulus sealant requires less force to stretch, making it highly flexible and able to accommodate significant joint movement. A high modulus sealant, conversely, requires more force to stretch, resulting in greater strength and rigidity but less overall movement capability.

Adheseal PU15: Our Low-Modulus Movement Specialist

Our Adheseal PU15 is engineered as a low-modulus elastomeric polyurethane sealant. It is specifically formulated for applications requiring high flexibility and significant joint movement accommodation.

• Shore A Hardness: 15 (after 14 days) (TDS)
• Joint Movement Capability: ±30% of the original joint width (TDS)
• Modulus: 0.2 MPa @ 100% elongation (TDS)

This low modulus makes PU15 ideal for dynamic movement and connection joints where components are expected to move considerably, such as those found in fenestration details, precast or tilt-up panel expansion joints, and metal cladding. Its high elasticity ensures that the sealant can stretch and compress without tearing or losing adhesion, making it a preferred choice for perimeter sealing of windows and sealing mechanically fixed joints.

Adheseal PU25: Our Versatile Workhorse Sealant and Adhesive

When you need a balanced performance – combining good movement capability with moderate strength – our Adheseal PU25 (also known as Adheseal PU25FC for ‘Fast Cure’) steps in as our medium-modulus, versatile sealant/adhesive. It is designed for a broad range of general construction applications.

• Shore A Hardness: 25 (after 14 days) (TDS)
• Joint Movement Capability: ±25% of the original joint width (TDS)
• Modulus: 0.4 MPa @ 100% elongation (TDS)

With its balanced properties, PU25 excels in general construction sealing for common interfaces like masonry, brick, and various metals. It also functions effectively as an adhesive for bonding lightweight components, floor mouldings, and skirting boards. Its ±25% movement capability is suitable for many expansion and control joints, including those in pavements, car parks, and driveways, where a degree of flexibility is needed without compromising bond strength.

Adheseal PU50: Our High-Modulus Adhesive and Durable Sealant

For applications demanding exceptional toughness, high mechanical resistance, and robust adhesion with controlled movement, our Adheseal PU50 (also available as PU50FC for ‘Fast Cure’) is our high-modulus polyurethane adhesive/sealant. It is built to withstand aggressive environments and constant stresses.

• Shore A Hardness: 45 (after 14 days) (TDS)
• Joint Movement Capability: ±25% of the original joint width (TDS)
• Modulus: 0.8 MPa @ 100% elongation (TDS)

The high Shore A hardness and modulus of PU50 make it particularly effective for structural bonding of construction elements, such as bonding XPS foam core panels or manufacturing curtain walls. Its exceptional tear and abrasion resistance also positions it as an anti-pick sealant for public buildings, schools, and prisons, where durability is paramount. While it still offers ±25% movement, its primary strength lies in its ability to handle high loads, constant movement, and vibration, making it ideal for heavy-duty pavement joints and vehicular manufacturing.

Polyurethane as an Adhesive: Beyond Sealing

While all polyurethanes offer adhesive properties to some degree, products like our Adheseal PU25 and especially Adheseal PU50 are frequently chosen specifically for their bonding capabilities. When is a polyurethane adhesive the right choice?

• High Bond Strength: Polyurethanes form extremely strong, long-lasting bonds with many common construction materials, including concrete, metals, timber, and various plastics.
• Flexibility with Strength: Unlike rigid adhesives, polyurethane adhesives retain a degree of flexibility. This is crucial for bonding materials that may experience differential movement or vibration, preventing brittle failure.
• Vibration and Impact Resistance: The elastomeric nature of polyurethane allows it to absorb shock and vibration, making it invaluable in applications like vehicular manufacturing or bonding elements in high-traffic areas.
• Weather and Chemical Resistance: Polyurethane adhesives maintain their performance under harsh environmental conditions, including exposure to UV, moisture, and common chemicals.

For structural bonding where a high-strength yet flexible bond is required, such as fabricating large panels or securing components that will undergo dynamic stress, a product like PU50 is often specified.

Matching the Product to the Job

The choice between Adheseal PU15, PU25, and PU50 should always begin with a clear understanding of the application’s specific demands:

• For maximum movement and lowest stress: Opt for Adheseal PU15. This is your go-to for joints that will experience significant expansion and contraction, preventing stress on delicate substrates or joint edges.
• For balanced flexibility and moderate strength: Adheseal PU25 is the versatile choice for general construction sealing and bonding, offering a reliable balance of movement accommodation and durability.
• For high strength, rigidity, and abrasion resistance: Choose Adheseal PU50. This product is ideal for structural bonding, heavy-duty applications, and environments where toughness and resistance to wear are paramount, even if it means slightly less movement capability compared to lower modulus options.

Regardless of the chosen product, proper joint design and substrate preparation are essential. Always ensure joints are clean, dry, and free of contaminants. For sealants, using a backer rod is typically recommended to control sealant depth and achieve the correct ‘width-to-depth’ ratio, allowing the sealant to perform optimally without three-sided adhesion.

The Right Choice, Every Time

By understanding the direct relationship between Shore A hardness, modulus, and joint movement capability, you can confidently specify the correct polyurethane for your project. Our Adheseal PU15, PU25, and PU50 range offers tailored solutions – from highly flexible sealants to robust structural adhesives – ensuring optimal performance and durability in every application.