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.
Decreased oxygen inhibition in photopolymerized acrylate/epoxide hybrid polymer coatings as demonstrated by Raman spectroscopy
Photopolymerization systems based on hybrid monomer 3,4-epoxy-cyclohexylmethyl methacrylate (METHB) were studied to investigate the oxygen inhibition effects on the conversion and polymer properties in films and coatings. METHB contains epoxide and methacrylate functional groups, which undergo cationic and free-radical photopolymerization, respectively. The conversion of both groups was obtained by Raman confocal microscopy as a function of depth. Initiator system compositions were varied and shown to affect the depth profile. The methacrylate group conversion was low at the surface due to oxygen inhibition. When both reactions were present, a cross-linked network formed and reduced oxygen sensitivity. At depths greater than the oxygen-diffusion-affected region, both functional groups' conversions did not show depth dependence. In addition, epoxide groups continued reacting after light was shuttered and reached a higher and more homogeneous conversion. These systems exhibit lower sensitivity to oxygen and offer advantages such as increased cure speed and improved film-forming properties compared to free-radical systems.
Azo initiator selection to control the curing order in dimethacrylate/epoxy interpenetrating polymer networks
Differential Scanning calorimetry (DSC) and Fourier-transform infrared (FT-IR) spectroscopic studies have been undertaken of the cure of interpenetrating polymer networks (IPNs) formed with imidazole-cured diglycidyl ether bisphenol-A (DGEBA) and with either diethoxylated bisphenol-A dimethacrylate (DEBPADM) or bisphenol-A diglycidyl dimethacrylate (bisGMA), polymerized by a range of azo initiators (AIBN64, VAZ088, VR110 and AZO168). Due to the differing decomposition rates of the azo initiators, the neat dimethacrylate resin either cured faster than (with AIBN64 and VAZO88), or similar to (VR110), or slower than (AZO168), the neat epoxy resin. In the neat DGEBA/1-methyl imidazole (1-MeI), DEBPADM/AIBN64, DEBPADM/VAZO88 and DEBPADM/VR110 resins, close to full cure was achieved. For the neat, high-temperature DEBPADM/AZO168 resin, full cure was not attained, possibly due to the compromise between using a high enough temperature for azo decomposition while avoiding depolymerization or decomposition of the methacrylate polymer. IPN cure studies showed that, by appropriate initiator selection, it was possible to interchange the order of cure of the components within the IPN so that either the dimethacrylate or epoxy cured first. In the isothermal cure of the 50:50 DEBPADM/AIBN64:DGEBA/1-Mel IPN system, the cure rate of both species was less than in the parent resins, due to a dilution effect. For this system, the dimethacrylate cured first and to high conversion, due to plasticization by the unreacted epoxy, but the subsequent cure of the more slowly polymerizing epoxy component was restricted by the high crosslink density developed in the IPN. After post-curing, however, high conversion of both reactive groups was observed and the fully cured IPN exhibited a single high-temperature Tg, close to the Tg values of the parent resins. In the higher-temperature, isothermal cure of the 50:50 DEBPADM/VR110:DGEBA/1-Mel IPN system, the reactive groups cured at a similar rate and so the final conversions of both groups were restricted, while in the 50:50 DEBPADM/AZO168:DGEBA/1-Mel system it was the epoxy which cured first. Both of these higher-temperature azo-initiated IPN systems exhibited single Tgs, indicating a single-phase structure; however, the Tgs are significantly lower than expected, due to plasticization by residual methacrylate monomer and/or degradation products resulting from the high cure temperature. Copyright © 2004 Society of Chemical Industry