Dissertation
Quantifying the effects of radical formation and secondary radical reactions on electron-beam polymerization
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2019
DOI: 10.17077/etd.005199
Abstract
EB polymerization uses ionizing radiation to create millions of metric tons of polymer products each year. Using ionizing radiation to initiate polymerization leads to random formation of active centers that is difficult to characterize and predict. Secondary reactions further complicate the ability to easily understand polymerization reactions initiated with ionizing radiation. The polymerization process is governed by radical reactions, and thus, understanding radical formation and secondary reactions is critical to developing relationships between formulation chemistry, processing conditions, and final polymer properties. These structure/processing conditions/properties relationships are the key to expanding EB polymerizations beyond current market applications.
In the past, researchers have established protocols to quantify radical and/or ion formation in pure monomers during exposure to ionizing radiation, termed radiation chemical yields or G-values. Each method of determining radiation chemical yield is indirect, and thus, requires a unique set of assumptions. This thesis begins with a summary and critical analysis of each protocol. Once the protocols were analyzed, two methods were chosen and adapted to measure G-values of monomers during exposure to EB. One protocol measured the total number of primary radicals generated during EB exposure (G(R^• )), while the other was designed to measure only those radicals that react to initiate polymerization (G(M^• )).
Using these protocols, the G(R^• ) and G(M^• ) values of various acrylate and methacrylate monomers were determined at different processing conditions. Monomer chemistry and dose rate was shown to impact primary radical formation. Some monomer chemistries are better at forming radicals than others. Monomers that are good radical formers are less likely to experience changes in polymer properties that result from changes in dose rate, termed dose rate effects.
In addition to monomer chemistry and processing conditions, initiation mechanism was shown to have an impact on radical formation. UV- and EB-polymerized samples initiated with equivalent radical concentration were studied to provide insight into how the two initiation mechanisms are different. EB films achieve lower ultimate conversion and have slower rates of polymerization compared to UV films polymerized with the same primary radical concentration. The lower EB conversions indicate that primary radicals generated by the EB are not as efficient at initiating polymerization compared to primary radicals generated using UV light.
Finally, the depth at which radicals form within a sample during UV and EB initiation is different and known to affect conversion depth profiles. EB conversion is low at the surface of the sample and increases well into the depth. In contrast, UV conversion is highest at the surface and decreases rapidly below the surface of the sample. Combining UV and EB initiation in a dual-cure process led to homogeneous conversion profiles throughout the depth of samples that did not achieve full conversion with UV or EB initiation alone.
The results of the experiments in this thesis aim to understand how initiation mechanism, monomer chemistry, and processing conditions affect polymerization and final polymer properties. Achieving this goal will lead to a better understanding of the fundamentals of EB polymerization. The results will in the development of necessary chemistry/processing condition/properties relationships that are currently not established for EB polymerization.
Details
- Title: Subtitle
- Quantifying the effects of radical formation and secondary radical reactions on electron-beam polymerization
- Creators
- Nicole Lynn Kloepfer Thiher
- Contributors
- Julie L P Jessop (Advisor)C Allan Guymon (Committee Member)Syed Mubeen (Committee Member)Sage M. Schissel (Committee Member)Scott Shaw (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Chemical and Biochemical Engineering
- Date degree season
- Autumn 2019
- DOI
- 10.17077/etd.005199
- Publisher
- University of Iowa
- Number of pages
- xviii, 198 pages
- Copyright
- Copyright 2019 Nicole Lynn Kloepfer Thiher
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references.
- Public Abstract (ETD)
Electron-beam (EB) curing is used to create millions of tons of plastics, inks, and adhesives each year. However, the relationship between the starting materials, processing conditions, and product properties is not well understood. This thesis explores what happens to the starting material during the curing process.
When the starting materials are exposed to the EB, a reactive species called a radical can form. Radicals are required to transform the liquid starting materials into solid products. Experiments were conducted to quantify how many radicals are formed during exposure to the EB and how radical formations is affected by the chemistry of the starting materials and the process settings of the EB. In addition to quantifying radical formation, this thesis also explored how radicals react after they are created. Once again, the chemistry of the starting material was shown to have a large impact on the EB curing process.
This investigation into the EB curing process helped to establish relationships between the chemistry of starting materials, EB processing conditions, and product properties. Developing these relationships will help guide best practices for industrial processes. Formulators can use the results of this thesis to aid in the development of new starting materials in order to achieve desirable final product properties that are currently unattainable.
- Academic Unit
- Chemical and Biochemical Engineering
- Record Identifier
- 9983779799302771
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