Doctor of Philosophy
School of Mechanical, Materials and Mechatronics Engineering - Faculty of Engineering
Ali, Haider K, Toughening polymer surfaces, PhD thesis, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, 2006. http://ro.uow.edu.au/theses/567
The thermoset resin poly diglycol carbonate, commercially called CR-39 has excellent optical properties, is cheaper than other ophthalmic materials and is considered one of the best plastic materials for the industry. CR 39 is known to be a brittle, highly cross-linked polymer. Applying coating layers significantly affects the toughness of ophthalmic lenses; a crack will first start on the surface of the coating and propagate through to the lens. One procedure to stop cracking, although not favoured by the industry because of its cost and detrimental effect on the optical properties, is to place a thin, rubbery layer between the coating and CR-39 ophthalmic lens.
An alternative method to stop the cracking is to toughen the lens material itself by placing the upper and lower surfaces under compressive stress. Swelling the lens surface can generate compressive stress and generating a multi-composite stressed layer lens can significantly improve fracture toughness.
An axisymmetric model of the spherical lens was built and a static load was applied on the central region in order to analyse stress distribution on the surfaces of the lens. It was found that tensile stress dominates the lower surface when the load was applied on the top surface. A volumetric swelling was introduced into the axisymmetric model to generate compressive stress onto the swollen surface while the tensile stress region on the lower surface was moved towards the central region of the spherical lens. The volumetric swelling transferred the stress in the horizontal axis from the tension to the compression region.
More than one system has been designed to evaluate the best swelling agent; chloroform was the best solvent and a mixture of chloroform with acrylic acid (monomer) was found to be the best swelling agent for the CR-39 ophthalmic lens. Ultra Violet (UV) light initiated polymerisation was used to polymerise the monomer within the surface of CR-39 ophthalmic lens. The temperature during this process remained below the glass transition temperature (Tg) of CR-39 polymer.
Raman spectroscopy was used to examine the residual vinyl group in CR-39 polymer and monitor the diffusion process of the monomer in the CR-39 lens surface and the polymerisation process of the diffused monomer. The depth of this treatment was measured by using the mapping technique in Raman spectroscopy. The stress generated from swelling the lens surface was measured by photoelasticity. A 3-point bending device was developed and attached to a circular polariscope to measure the optical stress coefficient of CR-39 because it is a transparent material.
Fracture energy was evaluated using the static impact and dynamic tests and significant improvements from treating both upper and lower surfaces and applying a hard coating to the treated lenses were observed. Surface characterisation techniques were used to determine the effect of the treatment applied to the CR-39 ophthalmic lenses. Ultra-Micro Indentation System (UMIS) analysis measurements using Berkovich and spherical indenters showed a decrease in the elastic modulus. Dynamic Mechanical Analysis (DMA) measurements using the penetration and single cantilever modes showed an increase in loss modulus and a decrease in storage modulus accompanied by a lower compression modulus for the treated surfaces. Atomic force microscopy (AFM) studies revealed that the treated surface of a CR-39 ophthalmic lens was smoother than an untreated surface.
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