Journal of Dental Research, Vol 55, 299-308, Copyright © 1976 by International & American Associations for Dental Research Online Journals
Zinc polycarboxylate cements: a chemical study of erosion and its relationship to molecular structure
S. Crisp, B. G. Lewis and A. D. Wilson
Experimental results show that the rate of elution of Zn2+ and Mg2+ ions
from polycarboxylate cements decrease with the cement's age. This
phenomenon is explained by the setting reaction in which these cations are
liberated from the oxide powder by an exchange with protons from the
polyacid and in this state are vulnerable to water leaching. Subsequently,
these cations become bound to the polyanion chain and become more resistant
to water leaching. Magnesium ions are more easily hydrolyzed than zinc ions
because they are less strongly bound to the polyanion chain. Zinc ions are
apparently more strongly bound to a polyacid which is a copolymer of
acrylic and itaconic acids, than to the homopolymer of acrylic acid. The
pattern of elution of ions differs between the different cements, so that
measurements after 24 hours cannot be used to predict long-term durability
on a comparative basis. This coupled with the slow but continuous loss of
ions makes a 24-hour test of doubtful validity. Polycarboxylate cements
absorb water that is present in the matrix in loosely bound and tightly
bound forms. The ionization of the polyacid to a polyanion during the
course of the reaction apparently creates a demand for water, and the
cement where the COOH:C ratio in the polyacid is highest absorbs the most
water.
