Depth and width of cured lines in photopolymerizable ceramic suspensions

When a photopolymerizable ceramic suspension is illuminated, the cured region is characterized by the cure width and cure depth. The cure depth follows a semilogarithmic behavior with increasing energy dose, as expected for Beer–Lambert absorption, and is described by the depth sensitivity (S_d) and depth critical energy dose (E_d). The excess cure width, which is the cured width beyond the incident illumination width, is also found to increase with the logarithm of energy dose. This quasi-Beer–Lambert behavior can be described by a width sensitivity (S_w) and width critical energy dose (E_w). The semilogarithmic dose dependence is demonstrated for ceramic suspensions containing silica, mullite, alumina, and zircon powders. Broadening can be quantified by the broadening depth (D_b), which is the cure depth at which broadening begins to occur. It is shown that the broadening depth decreases with the logarithm of the normalized refractive index contrast between the powder and monomer solution.     

Cure depth for photopolymerization of ceramic suspensions

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 D_p and its dependence on ceramic volume fraction, type and concentration of photoinitiator and inert dye. As predicted by an Absorption Model for D_p, 1/D_p 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.     

Predictive models for the photopolymerization of ceramic suspensions

The photopolymerization of suspensions of ceramic powders in monomer solutions is the fundamental step in several rapid prototyping forming techniques based on photocuring. Exposure to an UV dose of energy E causes the monomer to be polymerized to a certain cure depth, with the relation between cure depth and energy dose dependant on two properties of the photosuspension, the sensitivity and the critical energy dose. We present simple models for sensitivity in terms of the attenuation of the UV beam by absorption and scattering, and demonstrate the applicability of the model with experimental data for a UV laser, UV lamps and direct spectrophotometry. We also present a model for the critical energy dose in terms of the relative number of photo-generated radicals and the concentration of inhibitors.     

Photopolymerization of acrylate suspensions with visible dyes

Polymerization behavior of silica loaded diacrylate suspensions in the presence of color changing dyes was studied via cure depth measurements and photorheology. The dyes have their maximum absorption peak in the visible range; however, spectrophotometry revealed significant absorption also in the UV range where the photoinitiators are active. Thus, the dyes behave as other inert dyes and they affect the curing behavior. The effect of the dye concentration on the sensitivity D_p, critical energy dose E_c and time parameters determined via photorheology was studied and the data was fit to theoretical models. The measured values of 1/D_p, E_c and the photorheology time parameters linearly increase with increasing dye concentration c_D as predicted by the models.     

Investigation of influence factors in electron beam curing of epoxy resins using a calorimetry technique

Because of the complexity of the electron beam (EB) curing process, current understanding of EB curing of polymer resins and composites is limited. This article describes an investigation of different factors affecting EB curing of epoxy resin such as dose rate, time interval between irradiation doses, moisture, and photoinitiator concentration using a calorimetry technique. Results show that higher dose rate resulted in a higher and faster temperature increment in the uncured resin samples, and thus a higher degree of cure. In the multiple‐step EB irradiation, a shorter time interval between irradiation doses resulted in higher temperature in the resin samples and therefore higher degree of cure. Results indicate that moisture could delay crosslinking reaction in the early stages of the cure reaction, but accelerates it later in the curing process. Given a reasonable percentage of photoinitiator, experiments confirmed that samples with higher photoinitiator concentration reach higher degree of cure under same EB irradiation conditions. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Rheological measurements of cure rate of UV inks

The dynamic viscosity of UV inks during curing was measured at various film thickness using an oscillating plate rheometer. Logarithmic plots of the dynamic viscosity versus exposure energy are closely related by two-straight lines. A useful concept in discussing the curability is the minimum exposure energy difined as the exposure energy at the intersection of two lines. On exposure to a monochromatic light of 365 nm, the minimum exposure energy increases exponentially with film thickness; this resulting from an exponential decay of the light intensity with depth in the film. Magenta ink shows the fatest cure rate at a given thickness, followed by yellow, cyan, and black inks in that order. The difference in cure rate is attributed to different absorbances of pigments. The UV curability on direct exposure to UV lamp is predicted from the spectral sensitivity of the photoinitiator and the viscosity data obtained during curing under 365 nm light. The model calculation shows good agreement with the experimental results.     

Critical energy for photopolymerization of ceramic suspensions in acrylate monomers

The critical energy dose E_c required for photopolymerization of silica and alumina suspensions in acrylate monomers was measured for a large series of suspensions, and contrasted with a model that based on the assumption that E_c is the dose necessary to exhaust the polymerization inhibitors. The major predictions of the model were in good agreement with the data: E_c is inversely proportional to photoinitiator concentration, directly proportional to inert dye concentration, and directly proportional to the concentration of inhibitors.     

Research on the effects of surface modification of ceramic powder on cure performance during digital light processing (DLP)

Digital light processing (DLP) is one of the most important additive manufacture technologies to fabricate ceramic parts with complex geometries. Compared with pure photosensitive resin, the cure performance of ceramic suspensions is obviously different due to the optical property change after the addition of ceramic powders. In this paper, a unique oxidation process was used to modify the optical properties of nitride powders including AlN and Si₃N₄. The properties of oxidized ceramics were investigated and the cure performance of ceramic suspensions was then characterized. The effect of oxidation time on cure performance was evaluated. The results showed that for AlN, oxidation process leads to the smaller cure depth and smaller excess cure width as compared with non-oxidized AlN and for Si₃N₄, oxidation process leads to the larger cure depth and larger excess cure width as compared with non-oxidized Si₃N₄, indicating that both refractive index and light absorbance of ceramic powders have obvious effects on cure behaviors. Additionally, the cure behavior of oxidized ceramic suspension in this study shows that the relationship of cure depth vs. incident energy agrees well with Beer- Lambert model, but the excess cure width vs. incident energy is not consistent with quasi Beer-Lambert model due to the nature of digital micromirror device (DMD).     

不饱和聚酯树脂光固化工艺的研究-光引发剂浓度

本文选用TPO、651两种光引发剂，分别配制引发剂浓度为0．25、0．5、1、2wt％的不饱和聚酯体系。通过测定光固化试样的显微硬度分析光引发剂浓度对固化程度的影响。结果表明，提高引发剂浓度能抑制氧阻聚的作用，但对于较厚的试样，引发剂浓度过高会导致固化不完全。     

Photopolymerization of powder suspensions for shaping ceramics

The photopolymerization of suspensions of ceramic powders in monomer solutions is the fundamental step for forming techniques where a liquid suspension is solidified with UV light. The photopolymerization behavior is determined by the properties of the monomer and photoinitiator and by the transport of photons in the suspension. Photon transport in suspensions involves scattering from the particle and absorption by photoinitiators and inert dyes. The photopolymerization behavior is related to the formulation of the suspensions. Proper formulations are discussed for several applications. Techniques for characterization include cure depth measurements, photocalorimetry, and photorheology.     

Cure behavior of colorful ZrO2 suspensions during Digital light processing (DLP) based stereolithography process

In this study, the light absorption of the pure ZrO₂ and three types of colorful ZrO₂ mixtures were investigated. It was found that the absorbance of colored ZrO₂ powder increases with the colorant content, and the yellow-colored powder has the strongest absorbance at the wavelength of 405 nm compared with the other two types of colored ZrO₂ powder. The cure behavior investigation on the ceramic suspensions of the above four types of ZrO₂ powders during DLP process shows that as the colorant content increases, the cure depth and excess cure width both decrease due to the ceramic absorbance. The cure depth of colored suspensions is linear with logarithm of incident energy, consistent with Beer-Lambert model, while the further research into the cure width shows that the excess cure width increases nonlinearly with logarithm of incident energy, which is inconsistent with Quasi-Beer-Lambert model. Additionally, colorful ZrO₂ accessories were successfully fabricated.     

Stereolithography process: Influence of the rheology of silica suspensions and of the medium on polymerization kinetics – Cured depth and width

UV laser stereolithography is a rather new shaping technique that makes it possible the fabrication of complex 3D ceramic structures with a high dimensional accuracy. The green part is built through layer by layer photopolymerization of a light sensitive suspension.Polymerization is thus a critical step to control in this shaping technique. Photopolymerization, with the initiation, propagation and termination reactions, involves the mobility of reactive species and is then sensible to the rheology of the media. This study investigated the influence of the rheology of suspensions of silica particles in an acrylate oligomer and of the intergranular curable organic phase on the UV polymerization. In this respect, the effects of the powder concentration, the state of dispersion and of the dilution of the reactive oligomer on polymerization, are measured.In addition, the influence of the powder loading on the cure depth and cure width, which are respectively pertinent indicators of the reactivity of the suspension and of the dimensional accuracy of the green part, is evaluated.     

UV Curable Systems for Tape Casting

The substitution of solvents by photopolymerizable binders in the tape casting process allows to achieve high ceramic loading and to eliminate the drying stage which is a critical step of the tape casting process. After the rapid UV polymerization of the resin, the high strength green tapes can be debinded and sintered. Ceramic suspensions containing alumina or zirconia powder, dispersant, UV curable binder and photoinitiator have been prepared. The use of a low viscosity organic vehicle allows to prepare low viscosity ceramic suspensions, containing about 50 vol% powder, which have a shear thinning behaviour. Because of the rapid attenuation of the incident light in UV curing systems containing ceramic particles it is important to estimate the thickness of the tape that can be treated. The effect of incident energy, of photoinitiator concentration and of powder volume fraction was studied. There is an optimal photoinitiator concentration which maximizes the cured depth and which depends on the volume fraction of solid. A theoretical model based on the Beer–Lambert’s law enables the prediction of cured depth for any volume fraction of solid. To prove the ability to manufacture ceramic sheets by tape casting, some suspensions were tape cast.     

Different measures of sensitivity of recipe colour to random and proportional dye concentration error. Part 2: An example of target-position dependence and span

A regularity of the predicted sensitivities to random and proportional dye concentration errors in regard to the position of target colour has been observed for the case of dyeing acrylic with basic dyes. The sensitivity of the recipe colour to random dye concentration errors is highest for light neutral target colours and is almost negligible for dark-shade recipes. Recipes for less saturated targets are slightly more sensitive than recipes for more saturated targets of equal lightness. The span of the sensitivities to weighing error of the recipes matching a given target varies with the position of the target in colour space. By contrast, the sensitivity of recipes to dye strength error is the highest at medium to low lightness for neutral and near neutral target shades. The span of sensitivities of the recipes for any such particular target is broad, with some having low sensitivity to strength errors. Recipes for the target colours at the 'lighter' part of the gamut border were the least sensitive to strength errors. Received: 10 June 2005; Accepted: 11 July 2005.     

The effects of photocrosslinking parameters on films made from mixtures of maleate and vinyl ether functional oligomers: a factorial model

Two and three level factorial modeling has been used to determine the effects that photocrosslinking parameters such as ultraviolet (UV) dose and peak irradiance (peak lamp intensity) have on the tensile strength, surface hardness, secant modulus, elongation and relative conversion of films made from mixtures of maleate and vinyl ether (MA/VE) functional oligomers. The photoinitiator concentration and average molecular weight per unsaturation (WPU) used in the mixtures were also varied in an attempt to optimize film properties. The factorial models indicate that a binary interaction exists between UV dose and photoinitiator concentration. Small changes in peak irradiance and WPU did not show any measurable effect on the studied responses. The three level factorial model suggests that the optimal photoinitiator concentration for these mixtures is 3.0 to 3.5% (post-mass basis).     

Electron Beam Curing of the System Cycloaliphatic Diepoxide‐Epoxidized Natural Rubber‐Glycidyl Methacrylate in Presence of Cationic Initiators

Electron beam curing of the system cycloaliphatic diepoxide‐epoxidized natural rubber‐glycidyl methacrylate containing a cationic initiator was carried out. Storage modulus, glass transition temperature and pendulum hardness were measured as function of EB dose, photoinitiator concentration, content of epoxidized natural rubber, post cure temperature and post cure time. At electron beam doses larger than 100 kGy a highly cross‐linked polymer network is generated which shows a two phase morphology. Microscale elastomeric domains are incorporated into a continuous epoxy resin phase. Dynamical mechanical analysis and pendulum hardness measurement show that an increase of the ENR ratio leads to a more elastic polymer network. Post curing results in increased glass transition temperatures. This EB cured polymer system is believed to provide both toughness and favorable viscoelastic properties to be used as component of EB curable composites.     

Parametric sensitivity in tubular polymerization reactors

A recent technique for studying the parametric sensitivity of chemical reactors is applied to tubular chain homopolymerization reactors. The sensitivities of the temperature maxima with respect to various parameters of the model are computed. Using conditions typically encountered for high-pressure polyethylene systems, it is found that the temperature sensitivities with respect to all the nine parameters have their maxima at approximately the same value of the feed initiator concentration, thus leading to a generalized sensitivity-based constraint for design. It is also found that, under usual conditions of operation, no significant design constraints on the feed temperature are indicated. Detailed sensitivity plots are presented, which could be used to obtain “safe” operating conditions. The effects of changing the most important parameters, the dimensionless heat of reaction and the dimensionless activation energy (ϵ), on the sensitivity envelope are also investigated. Our studies reveal that better estimates of ϵ than are available presently are required. Sensitivities of the number-average chain length maxima with respect to the same nine parameters are also computed. Under conditions where the steady-state hypothesis applies, estimates of these sensitivities can be obtained analytically. However, for the usual values of the parameters, and close to “sensitive” values of the feed initiator concentration, this hypothesis does not apply, and the chain length sensitivities need to be obtained numerically. In the absence of the gel effect, chain length sensitivities do not usually provide design constraints because of the very low monomer conversions encountered.     

Critical Conditions of Spark Ignition of a Mixture of Gases (Oxidizer and Fuel) and Reactive Particles

The problem of spark ignition of a gas suspension including a fuel, an oxidizer, and reactive particles is considered using a two-temperature thermodiffusion model of combustion. Dependences of the minimum ignition energy on disperse-phase parameters are obtained. It is shown that the presence of exothermically reacting disperse-phase particles has an ambiguous effect on the minimum spark energy leading to ignition of the gas suspension. The values of the lower and upper concentration limits for ignition of a methane–air mixture with suspended coal dust particles are in good agreement with experimental data. Key words: hybrid gas suspension, spark ignition, critical conditions, concentration limits.     

Development of a high solid loading β-TCP suspension with a low refractive index contrast for DLP -based ceramic stereolithography

Digital light processing (DLP)-based ceramic stereolithography has recently been regarded as a promising method to produce bioceramic scaffolds because of its high processing accuracy. However, this process suffers from light scattering that can reduce its resolution. This study shows that a reactive monomer OPPEA (2-([1,1′-biphenyl]-2-yloxy) can weaken the light scattering in β-tricalcium phosphate (β-TCP) suspensions and increase the broadening depth by decreasing the refractive index contrast. Moreover, a β-TCP suspension with a low viscosity (5.76 Pa·s at 100 s^?1), high solid loading (52 vol%) and high cure depth (>200 μm) without any broadening was developed by investigating the rheological behavior and cure behavior of the β-TCP suspensions. β-TCP scaffolds with a triply periodic minimal surface structure were also fabricated and exhibited a submicron-scale structure and low shrinkage rate (8%) after sintering. This suspension holds great promise for fabricating controllable β-TCP scaffold structures for bone tissue engineering by ceramic stereolithography.     

Low VOC, low viscosity UV cationic radiation-cured ink-jet ink system

Ink-jet inks need to be very low in viscosity to transport the ink through the jet nozzle as well as for drop formation and integrity. The objective of this project was to demonstrate UV-cured systems for ink-jet inks and coatings with zero volatile organic compounds and very low viscosity. Cycloaliphatic epoxy systems were selected as binders of choice for this project due to their low viscosity and ability to be cured using UV-cationic photoinitiators. UV-curable coatings and inks with zero VOC and less than 10 cPs viscosities were formulated using exempt solvent acetone and vinyl cyclohexene diepoxide. UV-curable inks with up to 10% black dye were formulated. Rapid cure was achieved through the use of aryl sulfonium salts of hexafluorophosphoric acid. The presence of dye inhibited the reaction, but adequate cure of dyed formulations was achieved by increasing the concentration of the photoinitiator. Coatings Research Institute, 430 W. Forest Avenue, Ypsilanti, MI 48197.