# Sticky, Tiny Plastic Balls Made from Acrylates > This 1972 patent describes how to make tiny, sticky, and durable plastic balls (microspheres) using a specific mix of acrylate chemicals and a special water-based process. - **Patent:** US 3691140 - **Original title:** Acrylate copolymer microspheres - **Owner:** Individual - **Granted:** 1972 - **Status:** Public domain (expired) - **Times cited:** 382 - **Field:** materials, consumer_electronics, chemicals, adhesives ## What it does This patent details the creation of microscopic, solid balls, called microspheres, that are inherently sticky and can be dispersed in solvents but don't dissolve in them. They are made from a blend of acrylate esters (like iso-octyl acrylate) and a small amount of another chemical, either an ionic monomer or maleic anhydride. The process involves suspending these ingredients in water and using a specific amount of emulsifier, a substance that helps mix oil and water, to create the microspheres. The resulting tiny balls are described as infusible, meaning they can't be melted down, and elastomeric, meaning they can stretch and return to their original shape. ## What it does NOT cover - Microspheres made from monomers other than alkyl acrylates and specific ionic monomers or maleic anhydride. - Microspheres that are soluble in organic solvents. - Microspheres created using polymerization processes that don't involve an aqueous suspension. - Microspheres made with emulsifier amounts below the critical micelle concentration. - Microspheres that are not inherently tacky or elastomeric. ## The clever bit The key innovation was creating microspheres that were simultaneously sticky yet removable, and stable in solvents, by precisely controlling the ratio of acrylate monomers to a small percentage of ionic or maleic anhydride monomers and using a specific aqueous suspension polymerization technique. ## Real-world examples 1. 3M Post-it Notes 2. Repositionable adhesives 3. Specialty coatings ## Why it matters These microspheres, particularly those with inherent tackiness, became foundational for the development of Post-it Notes by 3M. The unique combination of stickiness without permanent adhesion allowed for repositionable notes, a significant innovation in office supplies and organization. ## Frequently asked questions ### What does Sticky, Tiny Plastic Balls Made from Acrylates cover? This 1972 patent describes how to make tiny, sticky, and durable plastic balls (microspheres) using a specific mix of acrylate chemicals and a special water-based process. ### Who owns patent US 3691140? Individual owns this patent, granted in 1972. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 3691140 cited by? This patent has been cited by 382 later patents that build on its ideas. ### What problem does this patent solve? These microspheres, particularly those with inherent tackiness, became foundational for the development of Post-it Notes by 3M. The unique combination of stickiness without permanent adhesion allowed for repositionable notes, a significant innovation in office supplies and organization. ### What does this patent NOT cover? Microspheres made from monomers other than alkyl acrylates and specific ionic monomers or maleic anhydride. **Full plain-English explainer:** https://patentbrief.org/patent/us/3691140/post-it-repositionable-adhesive **Original patent:** https://patents.google.com/patent/US3691140 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [The Sticky Microscopic Beads Behind Post-it Notes](https://patentbrief.org/patent/us/4166152/post-it-note-adhesive) — 3M's 1977 patent on tiny, naturally sticky plastic beads that can stick to a surface, peel off easily without leaving residue, and be reused over and over again. - [How Soft Contact Lenses Were Invented Using Hydrogels](https://patentbrief.org/patent/us/3220960/soft-contact-lens-hydrogel) — This patent describes the chemical recipe for soft, water-absorbing plastic materials that form the basis of modern soft contact lenses. - [Tiny Capsules for Electronic Paper Displays](https://patentbrief.org/patent/us/5961804/e-ink-electronic-paper-display) — MIT's 1999 patent on a special ink made of tiny capsules that can change color when an electric field is applied, forming the basis for early e-readers. - [How to Make Durable Non-Slip Surfaces Using Electron Beam Curing](https://patentbrief.org/patent/us/5401560/polymer-backed-material-with-non-slip-surface-using-e-beam-cured-urethane-binder) — A method for creating flexible, non-slip materials by bonding mineral grit to plastic sheets using a special radiation-cured glue that stays strong even when stretched. - [How Super Glue Bonds Acidic Surfaces Like Wood](https://patentbrief.org/patent/us/2768109/super-glue-cyanoacrylate) — A 1954 method for using alcohol to help super glue stick to acidic surfaces like wood, which normally prevent the glue from hardening properly.