The Sticky Microscopic Beads Behind Post-it Notes

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.

What it covers

This patent describes microscopic, rubbery polymer spheres (microspheres) made from acrylate or methacrylate chemicals. These spheres are designed to be 'infusible' (they won't melt) and 'solvent-insoluble' (they won't dissolve), but they can be suspended in a liquid to be coated onto paper. Because they have a glass transition temperature below -20 degrees Celsius, they remain soft and sticky at room temperature. When coated onto a substrate like paper, these tiny spheres act as pressure-sensitive adhesive dots. Because they are spheres, they only touch a wall or desk at tiny contact points, creating a light bond that can be easily peeled off and restuck without leaving sticky residue behind.

What it doesn't cover

• —Does not cover adhesives made of continuous, flat polymer films instead of discrete microspheres.
• —Does not cover microspheres made using non-ionic emulsifiers or ionic emulsifiers below their critical micelle concentration.
• —Does not cover microspheres with a glass transition temperature above -20 degrees Celsius, which would be rigid and non-tacky at room temperature.
• —Does not cover microspheres that dissolve completely in organic solvents.

The clever bit

Instead of making a flat sheet of glue, the inventors created microscopic, rubbery balls that only touch surfaces at their curved edges. This limited surface contact creates a weak physical bond that is strong enough to hold paper to a wall, but weak enough to peel away cleanly without tearing the paper.

Why it matters

This chemical process solved a major manufacturing hurdle for 3M, allowing them to reliably produce the low-tack, repositionable adhesive used on Post-it Notes. While Spencer Silver famously discovered the first sticky microspheres, this patent refined the suspension polymerization process to make the beads highly stable and commercially viable, securing 3M's dominance in the office supply market.

Real-world examples

• 1.Post-it Notes
• 2.Scotch WallSafe Tape
• 3.Repositionable paper labels
• 4.Sticky flags and page markers