Science Focus (issue 24)

By April Lam 林芷因 15 How Do Sticky Notes Work? 便利貼的原理是? Post-it notes are undoubtedly easy and convenient to use. Not only can they be placed anywhere without fasteners (i.e. tacks, paper clips, or staples), they do not easily fall off, nor leave any glue stains. Accepted as part of our many modern day conveniences, one rarely thinks about why this is possible in the first place. History of Sticky Notes Spencer Silver was a research chemist developing adhesives strong enough for aircraft construction [1]. In his attempts to invent such an adhesive, Silver instead discovered a weak adhesive that could be peeled on and off without losing its “stickiness [2].” While many may have thought this to be a useless invention, Silver’s colleague Art Fry gave him the idea of using the adhesive to create removable paper bookmarks for his choir hymn book. This moment in history led to the invention of Post-it notes. How to Describe the Strengths of Adhesives? Adhesives have two kinds of strengths that make up the adhesive bond: shear strength and peel strength. As illustrated in Figure 1, shear strength is measured by the forces pulling in a direction parallel to the two surfaces joined by the adhesive. Peel strength, on the other hand, is measured by the forces perpendicular to the two surfaces. For a Post-it note to be useful, it should have moderate shear strength to allow firm yet temporary attachment to a surface, while also having low peel strength to allow removal without tearing [3]. Why Are Polymer Adhesives Sticky? Before looking into the unique properties of sticky notes, let’s try to understand why they can stick to surfaces in the first place. Polymers are used as adhesives in the making of Post-it notes. These are large, long-chain molecules formed by repeating monomer units through a process called polymerization. When polymers come into contact with a surface, van der Waal’s forces will form between the polymers and the molecular surface of the object, causing them to stick together. Almost all polymer adhesives work using these interactions [4]. When producing polymer adhesives for a specific purpose, scientists typically mix different ratios of monomers to modify the properties of the resulting polymer. This process results in a copolymer, defined as polymers formed from more than one type of monomer. To produce the adhesive mixture to be sprayed on sticky notes, Silver found the ideal ratio to be 95 to 99 percent of acrylate monomer, and 1 to 5 percent of mixtures containing ionic monomers and maleic anhydride [5]. In other words, small amounts of acrylate monomers are substituted with ionic monomers and maleic anhydride to form long, crosslinked chains. Such modifications can improve the elastic properties of the copolymer, and allow the adhesive to be produced as an aerosol spray [5]. How Do Sticky Notes Work? But why can you stick and unstick a Post-it effortlessly? The copolymer is actually produced in the form of microspheres with diameters as small as 50 to 75 µm [6]. A single layer of sparsely spaced microspheres are sprayed on the paper of the Postit note, which looks like the bumpy surface of a basketball under a microscope, with little glue bubbles Figure 1 Shear Strength and Peel strength. Shear Strength Peel Strength