Mod Podge consists mostly of water and polyvinyl acetate, a common ingredient in most white glues. The water-resistant properties of polyvinyl acetate make Mod Podge a useful top coat or varnish and an effective, multipurpose glue.
Polyvinyl acetate, sometimes called PVA, white glue or wood glue, forms from the polymerization of liquid vinyl acetate. In this process, single vinyl acetate molecules come together to form a long chain. For products such as Mod Podge, polyvinyl acetate is used in an emulsion with water.
After Mod Podge is applied, the water begins to evaporate from the emulsion, eventually leaving behind a clear coating that is capable of repelling water, oils and even some petroleum products. This makes Mod Podge a versatile product that works well on nearly any porous material. Polyvinyl acetate is not resistant to certain types of fungi, algae and bacteria.
Mod Podge, as well as other white glue products, reacts well with boron containing compounds to form a non-Newtonian fluid. The addition of products such as borax to polyvinyl acetate causes the polymer strands to cross-link. The type of polyvinyl acetate product used influences the result, but standard outcomes are slime or Silly Putty. This material molds and flows under light pressure but breaks given stronger forces.