Evaluation of Curing Shrinkage of the Photo-Curable Material and its Application for Photo-Curable Adhesives

Abstract

In this research, I designed measuring system for shrinkage of UV (ultraviolet) curable materials and carried out analyses of experimental factors to achieve the high reliability of the system. Throughout the process of system designing, I measured shrinkage of monomers that have different structures respectively. Based on the evaluation of monomer, shrinkage of composite materials whose properties were imitated from final products was measured along with the evaluation of factors of shrinkage. As for internal factors, the geometry of specimens had a great impact on shrinkage. Particularly, significant reduction of axial shrinkage was observed when they had a low aspect ratio. By modeling, I confirmed that the shrinkage at the flanking side of material would influence its axial shrinkage. In order to minimize the impact, a specimen that have an aspect ratio over 40 is needed.

The shrinkage of specimens varied according to contents of photo-initiator and UV intensity to affect the reactivity. The contents of photo-initiator and UV intensity could change the depth of curing in complex ways, causing shrinkage delay phenomena.

The properties of UV curing depending on system temperature, which was measured by photo-differential scanning calorimetry (DSC), varied according to glass transition temperature of specimens. It resulted from declining mobility caused by partial cross-linking through a curing process. Reactivity tended to increase up to a certain temperature and then decrease, which is because thermodynamic reaction outdo kinetic reaction as the temperature rises.

Although it is known that the reactivity of UV curing increases as the number of functionalities of monomers increases, the opposite outcome was obtained in this research. It was confirmed that the shrinkage and reactivity of di-functional monomer were relatively higher than those of hexa-functional monomer were. It would be related to the fact that materials composed of di-functional monomer has more flexible molecular structure and can maintain its mobility within the curing process.

Acrylate and methacrylate are the most structured to take advantage of the photo-curing system. The reactivity of both structures are excellent, and reactivity of acrylate is known to be higher than that of methacrylate. In the mono-acrylate system, methacrylate had an excellent reactivity, which could maintain the mobility of the polymer associated with the structure of methacrylate. Both shrinkage and reactivity of the materials were higher in multi-functional system. In particular, we are able to verify that the patterns of the results obtained from photo-DSC and from the shrinkage test are the same, which means that the evaluation using photo-DSC can predict the shrinkage evolution. Therefore, the complementary application of two methods is expected to be possible at measurement in curing system.

In the shrinkage evaluation using the isomer system, I confirmed that the reactivity was changed depending on the core structure. The molecules with a linear core structure and side chains expressed different curing characteristics according to properties such as the mobility and density of reactive site.

UV-polymerized adhesive was materialized by blending a pre-polymer that was pre-synthesized and multi-functional acrylate as a reactive diluent. The reaction was delayed in the case that the size of reactive diluent was small while its contents were high.

In curing materials such as dental adhesive, since their dimensional stability in the curing process is important and hardness after curing should be excellent, the system was designed to contain filler. UV curable adhesive system containing filler are implemented by manufacturing the acrylate including a nano-silica particle in the core structure. The shrinkage varies depending on blended materials. A nano-porous effect presented in the condition that molecules with different sizes were mixed with each other. The effect can improve the reactivity of the system. In this study, I propose the system to estimate volumetric shrinkage through measurements for axial shrinkage by designing the device to measure axial shrinkage. I analyzed internal and external factors that would affect shrinkage. The patterns of shrinkage can be predicted more accurately by controlling the factors. Various structural characteristics of materials have an effect on shrinkage, which was evaluated as the results for the reactivity and mobility of the materials. In the evaluation for products, it is possible to analyze the shrinkage change in photo-curing process through the simplified curing system and investigate the main factors influencing each system.