Depth / Conversion Characterization of Electron-beam Initiated Polymerization
Confocal Raman microscopy was used to determine polymer conversion as a function of depth into electron-beam cured coating samples. Samples were irradiated in atmospheres with varying levels of oxygen. As expected, conversion near the surface increased as the level of oxygen was decreased. Samples were also irradiated under well-inerted conditions while varying the accelerating potential (voltage) of the electron beam. The results show low conversion near the sample bottom at the lower voltages. This lower conversion is consistent with inadequate energy penetration predicted by Monte Carlo simulations.
Predicting Dose Rate Effects in EB Polymerizations Based on Monomer Structure
Dose rate effects (DREs), or changes in polymer properties due to changes in dose rate, can be problematic during scale-up of electron-beam-initiated polymerization. DREs are not apparent for all formulation chemistries and processing parameters, and predicting when they will occur is challenging. Using glass transition temperature as measured by dynamic mechanical analysis, DREs were characterized for various acrylate monomers polymerized at different dose rates. Results show monomers with more easily abstractable hydrogens have smaller DREs. Introduction Formulations for electron-beam (EB) polymerization are often perfected on a small lab unit or pilot line before being scaled to an industrial beam. The dose (i.e., the energy absorbed by the sample) delivered by each EB unit is often comparable, but the speed at which the dose is delivered – dose rate – changes. Altering the dose rate can cause property changes in the cured polymer, known as dose rate effects (DREs). Previous work has established a protocol for measuring DREs in the glass transition temperature (Tg) for a polymer system by determining the difference in Tg at two different line speeds at a constant dose (Equation 1). T"DRE dose = T",,-/0/234 − T",,/0/234 (1) The previous study compared the DREs of five phenyl acrylate monomers and determined DREs were minimized as dose and monomer size increased across the five monomer series. It was hypothesized that the decrease in DREs for the larger monomers could be attributed to the greater number of labile bonds (primarily abstractable hydrogens) on the molecule, which increases the likelihood of chain transfer. The chain transfer mechanism provides a means of freeing trapped radicals by transferring the tethered radical to a more mobile chain and thus increasing propagation. Involving both chain transfer and propagation helps to balance radical consumption by termination at high radical concentrations (i.e., high dose rates), promoting polymer conversion and lessening DREs. This work is a follow-up to studies presented at RadTech 2016 and published in 2017. Here, we further confirm that DRE is a function of the number of abstractable hydrogens by comparing additional monomers. These additional monomers were selected to complement data from the initial series, as well as to further investigate the importance of bond energy. Dynamic mechanical analysis was used to determine the Tg of samples polymerized at different doses and dose rates, from which data the Tg DREs were calculated. Experimental Materials Six acrylate monomers were chosen to study the effect of dose rate on Tg: phenyl acrylate (PA, MP Biomedical); cyclohexyl acrylate (CHA, TCI America); benzyl acrylate (BA, MP Biomedical); tetrahydrofurfuryl acrylate (THFA, Aldrich); dodecyl acrylate (DDA, TCI America); and 2-(2ethoxyethoxy)ethyl acrylate (EEEA, TCI America) (Figure 1). Monomers PA and BA were a part of the previous study and were initially chosen because the phenyl ring provided a stable bond in Raman spectroscopy measurements for determining polymer conversion. CHA and THFA were selected for this study because of their structural similarity to PA and BA, respectively; the major difference between the two sets being the conjugation of their aromatic ring structures. DDA and EEEA were chosen to examine the impact of electron-withdrawing groups (here, the ether groups).
Novel hybrid monomers bearing cycloaliphatic epoxy and 1-propenyl ether groups
The synthesis of a novel series of hybrid monomers containing cationically polymerizable cycloaliphatic epoxide and 1-propenyl ether functional groups in the same molecule has been conducted. Detailed structure−reactivity studies of the diaryliodonium salt-induced cationic photopolymerizations of these monomers indicate that the rate of epoxide ring-opening polymerization is markedly enhanced by the presence of the 1-propenyl ether group. At the same time, the polymerization of the 1-propenyl ether groups in such hybrid monomers is retarded. A mechanism involving the free-radical-induced decomposition of the photoinitiator has been proposed which serves to amplify the rate of the photoinitiated cationic epoxide ring-opening polymerization.