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Current CAPSR projects involve investigations into the chemistry and technology of ultraviolet (UV) polymerization processes. This field of polymer chemistry offers manufacturers and users of coatings, inks, adhesives, and other related products the opportunity to significantly improve their productivity while reducing the air pollution normally associated with the conventional thermal curing (crosslinking) of solvent-based materials. It also allows them to greatly reduce the amount of energy they consume during polymerization. Thus, UH-Downtown students who are involved in this research have the opportunity to get direct "hands-on" experience with a developing “green” technology that can have a significant positive impact on the environment. They also have the opportunity to develop project planning and laboratory skills that will be beneficial to them as chemists in academic or industrial laboratories.
For several years, a project has been underway in the CAPSR laboratory that was designed to evaluate the type and level of solvent-extractable materials present in UV-polymerized films. We are interested in determining the effects of the total UV energy (UV dose) and peak irradiance used to polymerize the materials on these extractable components. Since virtually all UV-polymerized systems are crosslinked, by definition, they are not "soluble" in anything. Were they to go into solution, they would actually be "decomposing" rather than "dissolving". Thus, by measuring the type and level of solvent-extractables in a UV-polymerized film, information is obtained that relates to the non-crosslinked components in the polymer film.
Initial work on this project involved the development and optimization of laboratory techniques for conducting the solvent extraction process. Through this method, ethyl acetate was chosen as the solvent and a set extraction procedures was developed. Having established these extraction protocols, the next phase of the work involved the use of size exclusion chromatographic (SEC) methodology to separate the extracted components of the UV-polymerized films from each other according to their relative molecular masses (molecular sizes). Finally, these techniques were utilized to determine the effects of the UV dose and peak irradiance on these extractable components. Progress on this project was reported in a poster presentation during the RadTech 2002 Conference in Indianapolis, IN by Ms. Teresa Martinez. A further progress report was given by Ms. Martinez in a technical presentation at the 2004 e/5 UV & EB Technology Expo & Conference in Charlotte, NC during May 2004. The results to date indicate that the amount of extractable material is inversely related to the UV dose applied during polymerization.
Current work on this project involves the development and optimization of laser light scattering (LSS) methodology. This technique will be used in conjunction with SEC to determine the absolute molecular weight distributions of the solvent extractable components.
The second major research effort in the CAPSR laboratory was initiated several years ago and involves an investigation of the effects on reactivity, shelf-life stability, and polymer film properties of adding thiol-functional monomers to acrylated urethane-based, UV-polymerizable formulations. Early work indicated that when multifunctional thiols are added to acrylate-functional formulations, the resulting shelf-life stabilities are very poor and the relative reactivities are not as high as those reported in the literature.
To address the shelf-life stability issue, a variety of different experiments were run. One of the most significant ones involved the complete removal of the oligomer from the formulations in an attempt to ascertain whether or not the oligomer was primarily responsible for the lack of shelf-life stability. Thus, formulations containing only one, two, or three acrylate-functional monomers, along with varying levels of a trifunctional thiol monomer were prepared and evaluated for their shelf-life stabilities and their relative reactivities.
Once again, these experiments indicated an inherent instability in the thiol-acrylate systems and failed to show any significant reactivity advantage over UV-polymerizable formulations containing no thiol-functionality. Thus, the previously observed effects were not attributable to the presence of the oligomer. The results of the earlier work were presented at RadTech 2002 in Indianapolis, IN. Subsequent results were reported in a technical paper presented at e/5 UV & EB Technology Expo and Conference in Charlotte, NC in May of 2004.
More recent efforts on this project indicate that the most effective way to improve the shelf-life stability of these thiol-acrylate-functional materials is to add a relatively low concentration of a free radical scavenger to the formulations. Work is continuing to determine the overall effects of this procedure on the relative reactivity of the formulations, as determined by differential photocalorimetric (DPC) techniques.
One of the earliest project conducted in the CAPSR laboratory involved an evaluation of the effects of UV dose and peak irradiance on the Instron tensile properties of UV-polymerized films. Later work was done to evaluate the effects of these processing parameters on the thermomechanical properties, as measured using dynamic mechanical analysis (DMA) techniques. In all of these studies a range of peak irradiance and UV dose levels were investigated. In all cases, the UV dose had a measurable and predictable effect on tensile and thermomechanical properties. For example, the tensile strength of the films increased with increasing UV dose. In contrast, none of the dependent variables investigated were affected significantly by changes in the peak irradiance. This was unexpected and seemed to be true for peak irradiance values as low as 450 mW/cm^2 and as high as 5000 mW/cm^2 and higher.
The most recent work on this project involved an assessment of the effects of relatively low levels of peak irradiance (as low as 80 mW/cm^2) on the thermomechanical properties of UV-polymerized films. Again, no significant effect of peak irradiance was observed. These results were reported by Ms. Mai Lam for Ms. Janeth Sanchez, who conducted the experimental project, in a poster presentation at the 2004 e/5 UV & EB Technology Expo and Conference in Charlotte, NC.
During the Spring of 2004, a feasibility study was initiated that involved dispersing single-walled carbon nanotubes (SWNT) in acrylate-functional, aliphatic urethane-based, UV-polymerizable formulations. The SWNTs were furnished by Carbon Nanotechnologies, Inc., Houston, TX. It was expected that the thermomechanical and other properties of the resulting polymer film composites would be enhanced by the presence of the nanotubes. However, it was also expected that there would be difficulty in dispersing the nanotubes in the acrylate-functional formulations and that the nanotubes might interfere with the UV-polymerization process due to absorption of significant amounts of the UV radiation.
The bulk of the experimental work performed to date has been directed toward getting uniform dispersions of the nanotubes. This proved to be less difficult than at first expected. A procedure is now in place that involves dispersing the nanotubes in the acrylate-functional monomer mixture, followed by several hours of sonication at an elevated temperature. This monomer-nanotube mixture is then added to the oligomer (a very high viscosity component) and further sonication is done. Using this approach, we have been able to get dispersions that seem quite uniform, visually, in composition. These formulations all polymerize effectively under moderate UV irradiation. Thus, both of the initial concerns seem to have been overcome. Work is currently underway to evaluate the effects on film properties of incorporating the carbon nanotubes into the formulations.
This project began in December 2003 as a scouting project designed to see if reducing the amount of isobornyl acrylate (IBOA) in a UV-polymerizable formulation would cause the alpha-transitions in the dynamic mechanical analysis (DMA) data to move closer together in temperature. It was expected that as the amount of IBOA, a monofunctional monomer, is increased, the amount of crosslink density of the polymer film would decrease. This, in turn, was expected to reduce the solvent resistance of the resulting polymer films and to cause the thermomechanical (DMA) properties to be degraded slightly.
Initial work on this project did show clear trends – some expected and some not – and the results have been published iin RadTech Report, a bi-monthly publication of RadTech International North America.
Do you have research ideas that you just haven't had the time to pursue? Perhaps CAPSR personnel could do the work for you!
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Last updated or reviewed on 10/28/10