Cohesion: This refers to the internal strength of an adhesive, which arises from the molecular attraction between the adhesive’s particles. It is the force that holds the adhesive’s molecules together, ensuring the material remains intact under stress.
Adhesion: This term describes the process by which an adhesive bonds to the surface of a substrate. Adhesion involves the attractive forces between the adhesive and the substrate’s surface, allowing them to stick together.
The adhesion of adhesives to paper substrates is influenced by several key mechanisms, each contributing to the overall bond strength. The primary mechanisms include:
1. Mechanical Interlocking: This occurs when the adhesive flows into the microscopic pores and irregularities of the paper surface, creating a physical lock. As the adhesive dries or cures, it hardens, forming a strong mechanical bond with the paper substrate.
2. Chemical Adhesion: This mechanism involves the formation of chemical bonds between the adhesive and the paper. At the molecular level, functional groups in the adhesive interact with the molecules on the surface of the paper, creating strong primary (covalent) or secondary (van der Waals, hydrogen) bonds.
3. Diffusion Adhesion: In this process, the adhesive polymer penetrates into the paper fibers, allowing the polymer chains to entangle with the substrate. This intermingling of molecules enhances the bond strength, especially in cases where the paper has a polymeric coating.
4. Electrostatic Adhesion: Although less common in paper substrates compared to other materials, electrostatic forces can play a role, especially in thin films or coatings where charged particles in the adhesive are attracted to opposite charges on the paper surface.
Factors influencing these mechanisms include:
Surface Roughness: A coarser paper surface allows for better mechanical interlocking, which enhances adhesion. Smoother surfaces may require adhesives with stronger chemical bonding capabilities. From the early 1900s to the 1960s, roughening devices were used to damage the surface of glue flaps in folding box gluing machines, enabling faster penetration of water-based adhesives. However, with advancements in adhesive technology, this is no longer necessary today.
Surface Energy: The efficacy of adhesive bonding is influenced by the surface energy of the paper. Higher surface energy generally improves adhesive spread and bond formation. In cases where substrates are challenging to bond, such as those coated with UV varnish, plasma pretreatment facilitates bonding even with dispersion adhesives [2],[3]. Additionally, it’s important for the adhesive itself to have appropriate surface tension. For example, water, a key component of dispersion adhesives, has a surface tension of 72.8 mN/m at 20°C. At this level, it would form droplets when applied to a paper surface and be absorbed slowly by the substrate. However, if the surface tension matches or is lower than that of the paper substrate, water is absorbed rapidly, significantly expediting the film formation process.
Hydropilicity of the solid phase: In the case of dispersion adhesives, the film formation process requires the solid components (disperse phase) and water components (dispersion medium) to be separated. This separation is primarily achieved through the absorption of water into the substrate. The speed of this separation is influenced by the degree of hydrophilicity of the solid materials (disperse phase). For instance, many types of starch are notably more hydrophilic than synthetic solids, necessitating modifications to enhance their hydrophobic properties.
Moisture Content: Paper’s hydrophilic nature means that moisture content can significantly affect adhesion. Too much moisture can weaken adhesive bonds, while controlled moisture levels can sometimes enhance diffusion and mechanical interlocking. It is important to note that the moisture affects the necessary wettability of the substrate. Experimental studies have shown that the total surface energy, comprising the dispersive and polar components, is significantly influenced by relative humidity. This increase in relative humidity leads to a higher moisture content on the coated sheet, thereby increasing the hydrophilicity of the surface. As a result, the dispersive component decreases and the polar component increases, a finding that underscores the relationship between moisture and surface energy [4].
Thomas Walther - Baumer hhs GmbH
[1] EN 923:2015, https://dx.doi.org/10.31030/2400712.
[2] Pykönen, M., Johansson, K.S., Dubreuil, M., Vangeneugden, D., Ström, G., Fardim, P., & Toivakka, M. (2010). Evaluation of Plasma-Deposited Hydrophobic Coatings on Pigment-Coated Paper for Reduced Dampening Water Absorption. Journal of Adhesion Science and Technology, 24, 511 - 537.
[3] World of Print 2009, https://www.worldofprint.de/2009/04/23/kombination-der-uv-lackieranlage-colibri-von-steinemann-technology-und-der-bobst-openairpenair-plasmabehandlung-ermoeg-licht-wirtschaftlichste-uv-lackierungen-auf-faltschachteln/.
[4] Bohra, Hemant & Fleming, P.D. & Joyce, Margaret. (2009). Surface energy of coated paper: Effect of calendering conditions and relative humidity. TAPPI 2nd Annual PaperCon’09 Conference - Solutions for a Changing World.