What Makes Adhesives Sticky?

Adhesives are all around us. They are in your walls, under your floor, even on your roof. If you have worked with adhesives very long, you know that they can get quite sticky. Adhesives in general are “any substance that is capable of holding materials together in a functional manner by surface attachment that resists separation,” (Pike). This article is going to focus on what makes adhesives stick.

At the heart of an adhesive’s ability to stick is the concept of adhesion, a phenomenon that occurs at the molecular level. The “stickiness” we experience with adhesives is this adhesion at work. Adhesion is the result of various forces, including mechanical interlocking, and physical and chemical bonds, that work together to create a bond between the adhesive and the substrate. The effectiveness of these bonds is influenced by the nature of materials being joined, the surface preparation, and the adhesive’s formulation.

The stickiness of an adhesive can be attributed to molecular interactions at the interface between the adhesive and the surface it is applied to. Consider the image below.

These interactions include van der Waals forces, hydrogen bonding, and sometimes covalent bonding. Van der Waals forces, though weak individually, can collectively contribute significantly to the adhesive strength when the adhesive molecules are in close proximity to the substrate molecules. Hydrogen bonding, on the other hand, offers a stronger interaction through the sharing of hydrogen atoms between molecules, contributing to the adhesive’s cohesive strength.

One study I would recommend checking out surrounding adhesive applications and the role of wetting is from Procedia Engineering. It explores the role of surface energy in adhesion, highlighting how adhesives must wet the surface effectively to form strong bonds. This wetting process is critical for maximizing the contact area between the adhesive and the substrate, thus enhancing the potential for molecular interactions.

Mechanical interlocking plays a pivotal role in the effectiveness of adhesives, especially on rough or porous surfaces. This mechanism involves the adhesive penetrating into the microscopic pores and crevices of the substrate, where it then hardens or cures, creating a mechanical lock that enhances the bond strength. This principle is particularly relevant in the context of our Tytan Professional polyurethane adhesives, which are known for their excellent penetration and filling capabilities.

This below image shows a close-up look of how our adhesive operate:

Polyurethane adhesives are celebrated for their exceptional strength, flexibility, and resistance to environmental factors such as moisture, heat, and chemicals. These adhesives are polymers formed by the reaction of a polyol with a diisocyanate or a polymeric isocyanate in the presence of suitable catalysts and additives.

The secret to the stickiness of polyurethane adhesives lies in their chemical structure. The urethane linkages (-NH-CO-O-) in these polymers allow for a balance of flexibility and rigidity, providing strong adherence to a wide range of construction materials, including plastics, metals, wood, and rubber. The chemical cross-linking that occurs during the curing process further enhances their strength and durability.

A study in Thermochima Acta delves into the cross-linking mechanisms of polyurethane adhesives, highlighting how the formation of hard segments through the reaction of isocyanate groups with polyols contributes to their high tensile strength and elasticity. This cross-linking not only ensures a strong bond but also imparts resistance to thermal and chemical degradation, making polyurethane adhesives suitable for challenging applications.

Check out the below image, which shows the reactions that happen during this cross-linking process:

Polyurethane adhesives’ ability to bond dissimilar materials is a large part of what makes them important in construction. The majority of our customer base are bonding varying building materials for a myriad of reasons. It could be bonding subfloor panels to joists, drywall sheets to studs, foam shapes, roof tiles, etc. This list could go on and on! Many of these components also have to withstand high stress and temperature variations. Polyurethane adhesives offer a solution that is both strong and flexible. The safety and structure of homes and commercial buildings often depend on the adhesives that we manufacture.

Understanding what makes adhesives stick, particularly the science behind polyurethane adhesives, reveals a world where chemistry and physics converge to create real solutions that are integral to construction. The strength, flexibility, and resistance offered by polyurethane adhesives make them a preferred choice for professionals seeking reliable and durable bonds. As our research in adhesive technology advances, the potential for new formulations and applications continues to grow, promising even stronger and more versatile adhesives in the future. We are excited to continue to showcase these marvels of material science and highlight the indispensable role adhesives plan in building and sustaining the world around us.

If you are interested in learning more about our polyurethane foam adhesives or have questions about a specific application, please reach out to your local Tytan Professional Representative. You can also give us a call at (817) 381-4427. Check out more information at tytan.com/us.

1. “Adhesion.” Merriam-Webster.com Dictionary, Merriam-Webster.

2. Britannica, The Editors of Encyclopaedia. “polyurethane”. Encyclopedia Britannica, 25 Jan. 2024.

3. Fritzsche, Jorg. “Wetting and Adhesive Forces on Rough Surfaces – An Experimental and Theoretical Study.” Procedia Engineering. 09 Apr. 2015.

4. Olejnik, Adrian. “Kinetics of cross-linking processes of fast-curing polyurethane system”. Thermochimica Acta. 09 Nov. 2019.

5. Pike, Roscoe A.. “adhesive”. Encyclopedia Britannica, 25 Apr. 2022.

6. Van Der Waals Forces. 30 Jan. 2023, //chem.libretexts.org/@go/page/1511.