Polyurethane Adhesives Gain Traction As Safer Bonding Alternative

In the fields of woodworking and construction, polyurethane (PU) adhesives, also known as foam adhesives, are gaining increasing popularity. Brands like Gorilla Glue have elevated the visibility of these adhesives in the DIY and retail sectors, often marketed as "universal glues." But is this truly the case? Before delving deeper, let’s consider a pressing question: With tightening environmental regulations and growing concerns about worker health, can traditional polyurethane adhesives still meet the demand for efficient and safe bonding solutions?

This article provides a thorough exploration of polyurethane adhesives and examines potential alternatives to help you make informed choices in 2025.

• Types of polyurethane adhesives
• Advantages of polyurethane adhesives
• Limitations and safety precautions
• Applications of polyurethane adhesives
• A unique alternative to polyurethane adhesives

Note: Since August 24, 2023, professional users in industrial and specialized applications must undergo training and certification before using polyurethane adhesives and sealants. This regulation underscores the importance of proper handling and the need for safer alternatives.

The most common types of polyurethane adhesives include:

Single-component foam adhesives do not require additional resins or hardeners. They can be applied directly from bottles or cartridges. These adhesives cure by absorbing moisture from the air and the bonded materials, causing the adhesive to foam during the process. Moisture is essential for curing.

Single-component foam PU adhesives come in two forms:

• Liquid: Ideal for long, straight seams and bonding panels.
• Thixotropic gel: Suitable for mortise and tenon joints and applications requiring a non-drip formula, such as vertical or overhead bonding.

This article focuses primarily on single-component polyurethane adhesives.

Two-component PU adhesives work by mixing two separate components:

• Resin: The adhesive component.
• Hardener: The curing agent.

These adhesives are commonly used in wood flooring but are also effective for bonding non-porous materials like plastics, metals, and composites, as they do not rely on moisture for curing. They typically offer higher bond strength and faster curing times than single-component adhesives.

While based on polyurethane, these formulations share similarities with silicone sealants but are tailored for specific bonding applications.

To provide a comprehensive guide, we outline the key strengths, weaknesses, and uses of polyurethane adhesives.

• Rapid bonding and handling strength. Compared to traditional adhesives like PVA and resin powder adhesives, polyurethane forms bonds more quickly, speeding up production. Handling time refers to the duration needed for the adhesive to reach a state where bonded materials can be moved or processed. Full curing occurs when the adhesive completes all chemical reactions and achieves maximum bond strength.
• Foaming PU adhesives have handling times ranging from 5 to 30 minutes . Slower-curing adhesives are beneficial for complex joint assemblies requiring more time for alignment and clamping.
• Once fully cured, PU adhesives exhibit excellent chemical resistance , making them ideal for environments exposed to solvents and cleaners.
• They can be painted with various types of coatings.
• Due to cross-linking during curing, polyurethane adhesives form waterproof bonds compliant with EN 204 D4 standards, making them suitable for outdoor applications. They withstand moisture and water exposure without losing bond strength.
• They operate effectively in extreme temperatures ranging from –30°C to 150°C , adapting to diverse environmental conditions.
• The foaming action provides gap-filling properties , useful for bonding irregular or uneven surfaces.
• Polyurethane adhesives offer exceptionally high bond strength once fully cured, ensuring durability and reliability.
• They are highly versatile, bonding effectively with various construction materials , including wood, metal, and masonry.

• Polyurethane adhesives contain isocyanates and diisocyanates, which may cause occupational asthma and are suspected carcinogens. This is a significant concern for worker health.
• Their chemical resistance, while advantageous, makes cleanup difficult , especially when spills occur due to the adhesive’s foaming nature.
• If the adhesive contacts skin (which should be avoided), it can only be removed by scraping unless addressed immediately. It also tends to attract dirt, often leaving unsightly stains on hands.
• Clamping is essential during curing to prevent bonded materials from being pushed apart, which could result in weak bonds and thick glue lines.
• At least one of the bonded materials must be porous , limiting their use in certain applications.

There’s no denying the excellence of foaming PU adhesives, as evidenced by their widespread use in woodworking and construction. However, advancements in polymer technology have led to the development of new products that offer superior bonding strength and safer handling.

"At Protrade, we offer a unique alternative—a non-foaming adhesive that retains all the benefits of PU adhesives without the drawbacks." —Des Duddy, Co-Managing Director

Wood Max Express, for instance, provides a bond strength approximately four times that of high-grip adhesives, effectively joining two pieces of wood into a single solid component. It can also bond wood to metal, masonry, and other construction materials.

• Non-foaming: Since the adhesive doesn’t expand during curing, substrates aren’t pushed apart, and spills are minimized.
• Minimal clamping required: Only initial positioning is needed.
• Easy cleanup: Excess adhesive can be removed with a scraper or damp cloth.
• Washable: It can be easily washed off hands and tools, making it more user-friendly.
• Gap-filling: It fills gaps up to 4 mm wide with pure adhesive—no bubbles. Unlike foaming PU adhesives, which leave air pockets, Wood Max Express provides solid material for gap filling.
• 18-month shelf life: Unlike PU adhesives, which may partially cure in containers and have shorter shelf lives, Wood Max Express reduces waste.
• Faster curing: It achieves handling strength in 30 minutes and full bond strength in 8 hours, compared to PU adhesives’ 24-hour curing time.
• Enhanced safety: Free of solvents and diisocyanates, it doesn’t require training or certification for industrial use.
• Low indoor emissions: It meets A+ indoor air quality standards, ensuring a healthier work environment.
• Waterproof: Compliant with EN 204 D4 standards, it’s ideal for outdoor joinery.
• Compatible with Accoya wood: Few adhesives work well with this highly acidic treated wood, but Wood Max Express delivers unmatched bond strength.

Single-component foaming polyurethane adhesives are versatile and durable, offering rapid bonding and high strength. They are easy to apply and deliver excellent results when proper clamping and substrate compatibility are considered.

However, the challenges associated with cleanup and safety have led to the development of innovative alternatives like Wood Max Express. This product provides a safer, more user-friendly, and equally effective solution for a wide range of bonding needs.