Science Focus (issue 24)

protruding from the flat surface [7]. As illustrated in Figure 2, when force is applied to stick a Post-it note onto a surface, the elastic microspheres are flattened and cling to the surface. The flattening of microspheres temporarily provides an increased surface area for adhesion, and hence greater van der Waal’s forces for attachment [8]. But these interactions are still not very strong, so the Post-it can be peeled off just as easily. Conveniently, resticking of the Post-it is allowed without adding extra adhesive because no significant levels of adhesive will be lost on the surface after removal [7]. The deformed microspheres are known to regain their spherical shape for another round of sticking [5]. However, Post-it notes do have a limited life span. With every attachment and removal, dust and dirt accumulate onto the sticky microspheres. This diminishes the contact area with the surface, and hence reduces van der Waal’s interactions. Therefore the Post-it note will gradually lose its stickiness. 便利貼無疑是既易用又方便的發明,它不需配件(大頭 釘、萬字夾或釘書釘)就可以貼在任何地方,既不輕易脫落, 也不會留下膠水痕跡。便利貼已成為現代生活中不可或缺 的一部分,但相信很多人未必會去想便利貼為何會擁有這 樣的特質。 便利貼的歷史 Spencer Silver是一位化學家,當時希望研究出強度 足以用於組裝飛機的黏著劑 [1]。在多次嘗試中,Silver意 外發現了一種較弱的黏著劑,它可以被反覆貼在表面再撕 走而不失其黏性 [2]。那時候許多人認為這是個無用的發 明,Silver的同事Art Fry 卻建議他用這種黏著劑為自己 的合唱團詩集製作可移除的書籤,正是這個提議令便利貼 得以面世。 如何描述黏著劑的強度? 黏著劑的黏力強度可以分為兩方面描述:剪切強度和 剝離強度。如圖一所示,剪切強度可以透過量度平行於黏 合表面的拉力得知,剝離強度則是通過量度垂直於兩個表 面的力來測定。 便利貼要擁有理想的黏貼特質,它應該擁有中等剪切 強度,使它可以牢固地暫時附著在表面上;然後應該具低 剝離強度,使它能在不把物件撕破的情況下從物件表面移 除 [3]。 為甚麼聚合物黏著劑會黏? 在解釋便利貼特性之前,讓我們先了解為甚麼它能黏 附在表面上。製作便利貼所用的黏著劑是聚合物,那是重 複單體通過聚合作用組成的巨型長鏈分子。當聚合物與 表面接觸時,聚合物和物件的分子表面之間會形成范德 華力使它們黏在一起,幾乎所有聚合物黏著劑都依靠這 種原理運作 [4]。 在生產用於特定用途的聚合物黏著劑時,科學家通常會 混合不同比例的單體以改變所得聚合物的特性,這個過程產 生的聚合物叫共聚物(copolymer),定義為由多於一種單 Believe it or not, Post-it notes have been sticking around for 43 years since its invention! From bookmarks to annotating documents and brainstorming in meetings, Post-it notes have served as an indispensable addition to our daily life, boosting productivity. The next time you grab a Postit note to mark down a thought, be sure to remember this eureka moment of Silver and Fry’s discovery. Figure 2 Representation of the forces responsible for polymeric adhesion of Post-it notes. Fun Fact: The iconic canary yellow of Post-it is actually chosen by chance as the lab next to Silver’s only had pale yellow scrap paper to use during development [9]. 圖一 剪切強度和剝離強度 剪切強度 剝離強度