On-Line Force Measurement to Monitor Contact Lens Polymerization

Polymerization often causes a decrease in material density as monomers are cross-linked to form a dense matrix. This decrease in volume can induce anisotropic stress and strain in the polymerized material leading to defects in the final product, especially if the strain is not dissipated. The measurement of polymerization stress can help in improving quality of the final product through changes in lens design, curing conditions, or formulation. This article focuses on development of an in situ force monitoring method, which monitors the polymerization force as the contact lens is formed between two mold halves. This force is an accumulation of multiple stresses such as prepolymer shrinkage and polymer-mold interactions. If polymerization stresses could be controlled during manufacturing, product yield could be improved. Well-established techniques, such as rheometry, were also used for further understanding of polymer curing.