Mercury has properties of a bacteriostatic agent whereas certain methacrylate polymers (for example TEGMA, triethylene glycol methacrylate) composing the matrix of resin composites "encourages the growth of microorganisms". In the Casa Pia study in Portugal (1986–1989), 1,748 posterior restorations were placed and 177 (10.1%) of them failed during the course of the study. Recurrent marginal decay was the main reason for failure in both amalgam and composite restorations, accounting for 66% (32/48) and 88% (113/129), respectively. Polymerization shrinkage, the shrinkage that occurs during the composite curing process, has been implicated as the primary reason for postoperative marginal leakage.