Mechanical properties and rheology of polyalkenoate cements using various low-cost fillers

Low-cost polyalkenoate cements analogous to dental cements, i.e., cements based on polymers of acrylic acid crosslinked via bridging metal cations, were developed with a goal of producing a more flexible alternative to Portland cement. Economic constraints necessitated the replacement of the acid-degradable glass normally used in dental cements; the purpose of the glass is to provide both a source of polyvalent cations for crosslinking and solid filler for a composite material. The dual functionality of the powdered glass was accomplished via the use of manganese tetraoxide as the filler and aluminum chloride as the cation source for the ionic crosslinks. Unlike dental cements that have a gel-like consistency before setting, low viscosity cements were produced by using acrylic acid monomer rather than low-molecular weight poly(acrylic acid). Mechanical and rheological properties were used to monitor cement characteristics. Because of the large number of formulation variables, a design of experiments (DOE) approach was used. DOE helped narrow the search for formulations that would result in hardened cements and find the optimal set of ingredients that led to cements with the best properties given the economic constraints on the ingredients. Rheology was adjusted to match that of Portland cement by altering the filler volume fraction, which was very effective since the rheology depended strongly on that variable. The most pertinent independent variables for the mechanical properties were the curing time and monomer/cation ratio in the ranges tested; however, the monomer/water ratio was fixed at the minimum level possible and not increased because of economic considerations. The best materials produced in terms of mechanical properties resulted when acrylic acid monomer was partially replaced by N,N′-methylenebisacrylamide; this substitution resulted in a stronger and tougher cement.