PVC solvent cement is made up of PVC resin, stabilizers and fillers dissolved in a cocktail of solvents. Once applied to the PVC material surface, the solvents soften and dissolve the top layer of PVC material, essentially untangling the polymer chain.
Then, as two pieces of PVC are pressed together and the solvent evaporates, the freed molecules of one piece bond with the free molecules of the second piece.
A tapered fitting ensures good contact between the outer wall of the pipe and the inner wall of the fitting. Then, the PVC resin in the solvent cement fills in any gaps that exist between the two pieces, creating one continuous piece of PVC.