Why Does Polyurethane Sealant Have Good Adhesion?

Why is it that some sealants seem to stick to surfaces better than others? Whether it’s filling a gap in a wooden floor, sealing a joint in a concrete wall, or fusing substrates in a construction application, polyurethane sealant is typically the choice of professionals. Polyurethane sealants have the reputation of being extremely durable and most importantly, provides a strong bond to a wide variety of surfaces. From a scientific standpoint, what is the rationale for this strong adhesion?

Polyurethane sealants unlike other generic sealants that only provide a surface layer bond, chemically bond at a sub-surface level forming a much more powerful bond. Rather than being considered like a regular double sided tape seal, rather, think of a polyurethane sealant like a custom formed bond at the molecular level using a very high strength adhesive. This fact about sealants and their bond forming ability has everything to do with the chemical construction of the sealant itself as well as the construction of the materials that are in contact with the sealant. After decades of experience in sealant and adhesive manufacturing, a company like Juhuan has a sophisticated understanding of these concepts. They engineer their polyurethane sealants to optimally take advantage of chemical adhesion resulting in their sealants being used in some of the most challenging sealing situations. So, let us dive into the science of why this stuff holds on so well.

It is All About the Chemistry: The Power of Urethane

At its core, a polyurethane sealant is a polymer, which is a chain of countless repetitions of a single molecular unit, also known as a monomer. Polyols and isocyanates undergo a chemical reaction to form a polyurethane polymer. Once the sealant has been deposited, it will interact with the ambient moisture and the surface it has been applied to. This interaction is referred to as curing, which results in the chemical bond of the polymer chain and the formation of a solid three dimensional structure.

But the real secret to its adhesion lies in the curing process when the polyurethane molecules are still chemically active and can form bond not only with themselves but with the surface they are attached to. Polyurethane can form true chemical bonds with many substrates, particularly those that are porous or possess certain chemical functional groups on their surface. It is as if the sealant is extending hands and “hand-shaking” with the atoms of the wood, concrete or metal. This is a lot stronger than just relying on surface tension, as some primitive adhesives do.

Sealing porous surfaces.

Consider some common construction materials: wood, concrete, masonry, and even particleboard. What do they have in common? They are all porous. These materials have surface pores and capillaries. When a polyurethane sealant is applied to a construction material, it does not sit on its surface. Because sealants have a relatively low viscosity before they cure, it means it will 'mimic' and, therefore, fill all imperfections, pits and pores of the construction material. The polyurethane sealant will, for example, fill to the bottom of a hole if there is any. It will then cure and provide a bonded fill to the bottom of the hole. This mechanical 'locking' of the curing sealant, in addition to the aforementioned, will solidify itself 'anchor like' to the bottom like cement fill in the hole.

The combination of the interlocking and chemical bond of the polyurethane sealant offers a unique advantage. It is the reason polyurethane works in the first place. The sealant, once cured, will then become a part of the surface it is bonded to and is its reason for how effective it is for sealing foundation and seams, bonding wood structural components and holding things in place where a simple surface bond would fail.

The Differentiating Factor Behind Polyurethane's Adhesion

Another great feature about polyurethane sealants for adhesion longevity is their flexibility. Many sealants and adhesives are stiff and hard when cured. This presents a problem since materials and structures move. They can expand or contract depending on the humidity or temperature. With time, materials settle and move, and a sealant can pull away from a surface due to this movement. This will break seals and the adhesion is lost.

Once a polyurethane sealant hardens, it remains elastic and flexible, meaning that it can stretch or contract despite joint movement. Imagine this to be a really stretchable rubber band that is holding the gap. It can stretch due to this movement and then return to its original shape. Constant movement due to flexibility means that the adhesive bond is under less stress. This means that the polyurethane sealants, unlike adhesives and sealants, do not fight with the materials. This is the main feature behind urethanes for adhesives or sealants. Most sealants do not maintain great adhesion under harsh outer weather, unlike polyurethanes which will maintain adhesion under any tough weather.

Adhesion on Metals and Plastics

Almost all adhesives will have difficulty adhering to and curing on metal and plastic surfaces. These surfaces are non-porous and are chemically less reactive than other surfaces, and so will result in less adhesion formation. Polyurethanes, however, will actually adhere to and cure on these non-porous surfaces, and in the case of metals, adhesion formation will occur because of the urethane bond formation to the metal, which forms an adhesion bond to the metal (metal-oxide) layer of the metal surface. This is the reason that polys and polyurethanes are so widely used in the automotive and marine industries to seal joints on metal vehicle bodies and to bond parts on plastic and composite boats, respectively.

Adhesion to plastics, particularly low and ultra low surface energy (LS / USE) plastics, is notoriously difficult. Most adhesives and sealants will simply

be repelled by these surfaces instead of adhere to them. Plastics are, however, usually less of an adhesion problem for polys and polyurethanes. In fact, many polys and polyurethanes will adhere to low surface energy plastics. The difficulty, instead, is that in many polys and polyurethanes, adhesion is a surface phenomenon, and if the surface is not well prepared (cleaned) and if the surface is not coated (primed) with a substance that is reactive with the poly urethane system, adhesion will not occur. Because of this, polys and polyurethanes are used as sealants in highly complex applications where many types of materials are used.

Real World Performance: From Foundations to Framing

You can see adhesion proof in the real world. Think of below grade foundation walls. These walls are always surrounded by wet soil and hydrostatic pressure. A crack in the foundation wall can be sealed with polyurethane sealant. Polyurethane sealant will boldly stick to the wet concrete, adjust when the ground shifts, and keep the seal waterproof. Now think of a wooden frame building. Polyurethane sealant expands and contracts with the seasons and will stick to the wood fibers of the big wood beams and will not let air pass through the seal.

In a testing facility for manufacturers, like the ones high quality manufacturers have, these sealants are tested for their limits. These sealants undergo testing for their tensile strength, elongation, and peel adhesion to various substrates. They also undergo testing for extreme heat, extreme cold, and extreme immersion in water. These rigorous tests are done so the sealant can perform as the professionals expect and provide reliable and long lasting adhesion.

Formulation Matters: Not All Polyurethanes Are Equal

The sealant's recipe is critical. The manufacturer's know-how is fundamental because they can modify the formulation to accentuate specific traits. For instance, a polyurethane sealant intended for bonding glass may be engineered for clarity and resistance to UV rays. One intended for use in roofing may be designed for resistance to extreme weather and the ability to stick to surfaces that are slightly wet. A company with extensive experience in specializing in the formulation of adhesives and sealants for a specific application has a good grasp of the interplay between adhesive strength, flexibility, speed of cure, and useable life of the sealant.

Ultimately, the polyurethane sealant's excellent adhesion is a combination of factors rather than a single trait. It is the highly adhesive chemical bonds, the ability to penetrate and lock mechanically to porous surfaces, the lasting flexibility, and the compatibility with a variety of materials. In the home, in the construction industry, and in versatile industrial applications, it is an essential element that solves many adhesive and sealing problems, and it exemplifies the power of smart chemistry.