In the mid-1970's, the Minnesota Manufacturing and Mining Corp. (3M) introduced a product called POST-IT.TM. which is made by coating pressure sensitive microsphere adhesives on a sheet material, such as paper, which can be repeatedly adhered to and removed from a substrate without damaging the surface of the substrate. The microspheres are prepared by suspension polymerization and employ oil soluble initiators (U.S. Pat. No. 3,691,140 issued to Silver and U.S. Pat. No. 4,166,152 issued to Baker). Emulsion polymerization employing water-soluble initiators, according to Silver, can result in a substantial amount of latex having extremely small particle size which are difficult to recover and, further, have undesirable solvent solubility for many applications as adhesives. Other processes for making microspheres by suspension polymerization techniques using oil-soluble initiators are described in U.S. Pat. Nos. 4,495,318 and 4,598,112 issued to Howard, U.S. Pat. No. 4,810,783 issued to Mallya, and U.S. Pat. No. 4,839,416 issued to Orenstein.
In most cases, the adhesive microspheres must first be recovered from liquid medium before they can be re-dispersed in proper organic solvents and coated onto an appropriate surface. The suspension polymerization process described in the above patents produces a finely dispersed suspension of adhesive microspheres which are difficult and labor intensive to recover from the liquid medium. Recovery techniques typically require a compatible solvent, such as methanol, to coagulate before filtration, or mechanical force, such as centrifuge, to remove the microspheres from suspension.
Recently, Young (U.S. Pat. No. 4,833,179) reported a method of producing polymeric adhesive particles by suspension polymerization which could be easily collected by gravity filtration. The adhesive particles have additional modifying moieties, such as macromolecular monomers, hydrophobic silicas, or reactive zinc salts, which distort the properties of the adhesive.
It would therefore be desirable to develop a process for producing elastomeric microparticles that can be easily recoverable from a polymer suspension, in a form suitable for use as an adhesive.