The Impact of Substrate Porosity on Adhesive Performance

When selecting an adhesive, it is easy to focus on the properties of the adhesive itself: its chemistry, viscosity, and strength. However, the nature of the substrates being bonded is equally, if not more, important. One of the most critical substrate characteristics is porosity—the presence of tiny voids or pores in a material. The difference between a porous substrate, like uncoated paper or corrugated board, and a non-porous substrate, like plastic film, glass, or aluminum foil, fundamentally changes how an adhesive sets and cures, and therefore dictates the type of adhesive that should be used. Ignoring this can lead to complete bond failure.

For porous substrates, the mechanism of adhesion is twofold. Firstly, the adhesive mechanically 'bites' into the surface, with the liquid flowing into the microscopic pores and creating a physical anchor as it solidifies. This is known as mechanical interlocking. Secondly, and just as importantly for water-based adhesives, the porosity of the substrate is essential for the setting process. The paper or board acts like a sponge, drawing water out of the adhesive and into its own fibers. This rapid removal of water allows the polymer solids in the adhesive to coalesce and build 'green strength' or initial tack very quickly. This wicking action is why water-based adhesives are so effective and fast-setting on paper and board applications.

Non-porous substrates present a completely different challenge. Materials like PET film, metal, or glass have no pores for the adhesive to penetrate, so the bond is based purely on chemical adhesion—the intermolecular forces (like Van der Waals forces) between the adhesive and the substrate surface. More importantly, when bonding two non-porous substrates together (e.g., film-to-film lamination), a standard water-based adhesive will not work effectively. With nowhere for the water to escape, it becomes trapped between the two non-porous layers, preventing the adhesive from drying and curing. The result is a weak, soupy bond that will never achieve proper strength, a phenomenon known as a 'wet-out' failure.

This is why bonding non-porous materials requires a different adhesive strategy. One common approach is to use a solvent-based adhesive, where the volatile solvent is evaporated in a drying tunnel *before* the two substrates are nipped together in a dry lamination process. Another approach is to use a 100% solids adhesive, such as a hot-melt or a two-part reactive adhesive (like an epoxy or polyurethane), which does not contain water or solvent that needs to be removed. Understanding this fundamental interaction between adhesive and substrate is the first and most important step in any successful application. At Kubron, our technical team works closely with clients to analyze their substrates and recommend the perfect adhesive chemistry, whether it's a water-based solution for porous materials or a specialized formulation for non-porous lamination, ensuring optimal performance every time.