Cure depth for photopolymerization of ceramic suspensions

https://doi.org/10.1016/j.jeurceramsoc.2010.06.004Get rights and content

Abstract

The cure depth of a series of photopolymerizable SiO2 and Al2O3 ceramic suspensions was measured as a function of energy dose to determine the sensitivity parameter Dp and its dependence on ceramic volume fraction, type and concentration of photoinitiator and inert dye. As predicted by an Absorption Model for Dp, 1/Dp is a linear function of photoinitiator concentration and dye concentration. The molar extinction coefficients derived from the cure depth measurements using an Absorption Model were compared with coefficients determined via spectrophotometry.

Introduction

Photopolymerizable suspensions have been used in a dental resins,1 patterned substrates2 and layered manufacturing of ceramics3, 4 by techniques such as stereolithography.

Photopolymerizable suspensions consist of ceramic particles dispersed in liquid monomers with photoinitiators (PIs) as the photoactive substances that initiate the polymerization reactions upon illumination. The polymerization behavior is affected by the type and concentration of photoinitiators, and the fundamental kinetic parameters of the monomer. The behavior can be modified with inert dyes that absorb photons without creating free radicals and inhibitors that destroy free radicals. Scattering of the UV due to the index of refraction contrast between the ceramic particles and the monomer also affects photopolymerization.

A dose of UV of energy E will cause polymerization to a depth Cd as described by the Jacob's equation5 as Cd = Dpln(E/Ec), where Ec is the critical energy and Dp is the sensitivity of the photopolymerizable suspension. Recently, we proposed simple predictive models for the photosensitivity in terms of absorption of UV photons and critical energy in terms of exhaustion of inhibitors.6 The Inhibitor Exhaustion Model for Ec is discussed in another paper.7 This paper concerns the Absorption Model for the sensitivity term Dp and compares some detailed data with predictions of the model. The Absorption Model6 considers the attenuation of the UV beam by absorption from dyes and photoinitiators, and by scattering of the UV by particles. These are related by:1Dp=S+AΦAwhere S is the scattering term reporting the effects of light scattering and A is the absorption parameter reporting the effect of the active photoinitiators (PIs) and inert dyes for suspensions containing ceramic at a volume fraction ϕ. The scattering parameter is defined as S = 1/lsc (reciprocal scattering length lsc) and the absorption parameter is related to the properties of the PI and dye as A = (ɛPcP + ɛDcD), where cP is the concentration of the PI in mol/unit volume and ɛP is molar extinction coefficients in L/(mol cm) of the PI. Similar terms for the inert dye is the dye concentration cD and the dye molar extinction coefficient ɛD. Eq. (1) can be written as:1Dp=S+(1Φ)(εPcP+εDcD)The Absorption Model predicts that 1/Dp plotted against cP should be a straight line. The slope of the line K3 depends on photoinitiator molar extinction coefficient as shown in Eq. (3a):d(1/Dp)dcP=K3=(1Φ)εPThe intercept of the straight line I3 involves the scattering term and the dye concentration and molar extinction coefficient as shown in Eq. (3b):1DpforcP=0=I3=S+(1Φ)εDcDNote that the scattering term depends on the refractive index8, 9 and volume fraction9, 10 of the ceramic, as discussed in more detail in the previous paper.6 The slope K3 should be larger for compositions with less ceramic volume content. The molar extinction coefficient can be calculated from the slope. The intercept I3 provides the information about two parameters, the scattering term S and the dye term. For compositions with no dye (cD = 0), the scattering term can be directly determined. The intercept should increase with decreasing ceramic volume content and with increasing dye concentration.

The model also predicts that 1/Dp vs. cD is a straight line. The slope K4 depends on the absorption coefficient of the dye as in Eq. (4a):d(1/Dp)dcD=K4=(1Φ)εDThe intercept of the 1/Dp vs. dye concentration line I4 gives another estimate of the scattering term S and the PI molar absorption coefficient.1DpforcD=0=I4=S+(1Φ)εPcPThe slope of the dye concentration line should increase with decreasing ceramic volume content. The slope reports on the molar extinction coefficient of the dye. The intercept should increase with decreasing ceramic volume concentration and increasing PI concentration. Two sets of experiments, with Dp as a function of dye concentration and as a function of PI concentration, provides two independent estimates for the parameters in the Absorption Model. The molar extinction coefficients from the cure depth measurements can be compared with values measured by spectrophotometry.

This work explores the validity of the Absorption Model for about 100 suspensions with varying ceramic volume content, photoinitiator type and concentration, the presence of UV and visible light absorber, inhibitor and light stabilizers. Most of the suspensions were prepared with silica, with more limited data for alumina suspensions. Polymerization is conducted with a 355 nm UV laser and with mercury vapor UV lamps.

Section snippets

Materials

Details of the photopolymerizable suspensions are presented elsewhere.5 Briefly, the photocurable monomer system is consists of 87.5 wt% of the bifunctional monomer 1,6-hexanediol diacrylate (SR238, Sartomer, USA) and 12.5 wt% of the tetrafunctional monomer ethoxylated pentaerythritol tetraacrylate (SR494, Sartomer, USA). Both are low viscosity fast curing acrylates. These monomers are transparent to UV for wavelengths longer than about 320 nm. The photoinitiator for most of the experiments was a

Spectrophotometry

Absorbance curves of the ketone and phosphine oxide PIs and light absorbers (the triazole dye and blue light absorber) in the wavelength range 300–440 nm are shown in Fig. 1. The molar extinction coefficients of the ketone PI and the triazole dye at 355 nm were determined to be 49 ± 1 L/(mol cm) and 13,600 ± 600 L/(mol cm), respectively. The more detailed absorbance data at 355 nm for the ketone PI and the triazole dye at 4 different concentrations appear in Table 2.

To determine the molar extinction

Conclusions

The compositional dependence of the sensitivity parameter Dp, which relates cure depth Cd to energy dose E for UV photopolymerizable ceramic suspensions, can be accurately modeled in terms of the attenuation of the UV radiation by absorption of the photoinitiator, inert dyes, and scattering by the ceramic particles.

This Absorption Model predicts that the inverse of the sensitivity parameter, 1/Dp, should be a linear function of photoinitiator concentration. This is observed, and a

Acknowledgements

This research was supported by the United States Defense Advanced Research Projects Agency (DARPA) under HR0011-07-1-0034, Program Officer W.S. Coblenz and the Office of Naval Research under N00421-06-1-002, Program Officer David Shifler.

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