Effect of water content on the physical properties of model dentine primerand bonding resins

Objectives: Primers and adhesives containing hydroxyethyl methacrylate (HEM A) are placed on moist dentine even though several studies indicate that wa ter may interfere with the polymerization reaction. The purpose of this stu dy was to evaluate the influence of increasing amounts of water on the phys ical properties of a model dentine primer resin (HEMA) and model dentine bo nding resin (a mixture of HEMA and Bis-GMA). Methods: Miniature (ca 10 x 0.5 mm (long x thick)) hour-glass shaped and pa rallel-sided specimens were created by casting the monomer or comonomer mix tures into appropriately shaped moulds. The water content was either 0, 5, 9, 17 or 29 vol%. One hour after polymerization, half the specimens were su bjected to physical testing under dry conditions. The other half were immer sed in water for 24 h and then tested while wet. The ultimate tensile stren gth (UTS), modulus of elasticity (E), percent elongation at failure, and to ughness were calculated. Results: The properties of the dry-stored primer and bonding resins were no t altered by water incorporation up to 9 vol%. Higher water content lowered the physical properties (P < 0.05). Immersion of primer polymers in water for 24 h significantly decreased their properties. Water immersion of the b onding resins was unaffected only for specimens containing 0 or 5 vol% wate r. Conclusions: The plasticizing effects of extrinsic water are far more impor tant than the effects of intrinsic water in poly-HEMA resin, Hybrid layers composed primarily of poly-HEMA would be expected to be mon elastic than th ose made with bifunctional, cross-linked polymer chains. (C) 1999 Elsevier Science Ltd. All rights reserved.