Stereolithography Cure Process Modeling Using Acrylate Resin

In this paper, a complex stereolithography (SL) cure process model is presented that incorporates transient thermal and chemical effects which influence final part shape and properties. The model incorporates photopolymerization, mass diffusion, and heat transfer. Material properties are characterized and a comprehensive kinetic model parameterized for a model compound system. SL process simulations are performed using finite element methods with the software package FEMLAB, and validated by the capability of predicting the fabricated part dimensions. A degree of cure (DOC) threshold model is proposed which can predict the cure line size within 15% error, comparing with 30% prediction error by the exposure threshold model currently used in SL. Furthermore, through the sensitivity analysis conducted by the process model presented here, the sensitive parameters are identified and the SL bath temperature, photointiator absorptivity and concentration are found to be the most sensitive factors that affect the SL fabrication results. The sensitive variables will be the focus of further research meant to improve SL process speed and resolution.Mechanical Engineerin

Plasma-initiated polymerization and its applications

Plasma initiated polymerization is discussed. Topics include: polymerization of a vinyl monomer, solid phase polymerization, and inorganic ring compound polymers

Utilizing Acoustic Levitation to Determine Ionic Liquid Effects on Enthalpy of Polymerization

Ionic liquids (ILs) are emerging as an eco-friendlier alternative to traditional organic compounds. The hope with ILs in this project is to incorporate them with resins used in 3D printing to act as plasticizers which will in turn help prolong the life of the printed object. A common issue with plasticizers like phthalate in particular is they can leach out of plastics due to their vapor pressure which leads to decreased performance and can also pose a health risk being released into its surroundings or atmosphere. A major benefit of incorporating ILs in with resins comes from their low volatility which allows it to remain in the plastic and can prevent the printed object from becoming prematurely brittle. In order to move forward in attempting to integrate ILs in with resin, one needs to understand what effect it has on the heat capacity and polymerization of the resin when combined is the next step. A possible means to study this effect is by measuring the change in temperature of a reaction that is container-less which can be accomplished through acoustic levitation. The aim for this research is to levitate droplets of ILs and methyl methacrylate-based resin in an acoustic levitator, expose the droplet to lasers in order to induce photoinitiated polymerization, and record the temperature change using a FLIR camera. Coupling this data with heat capacity measurements from the DSC will allow us to calculate the heat of polymerization of PMMA and to observe how different concentrations of IL affect the heat capacity and heat of polymerization

Frontal Polymerizations: From Chemical Perspectives to Macroscopic Properties and Applications

The synthesis and processing of most thermoplastics and thermoset polymeric materials rely on energy-inefficient and environmentally burdensome manufacturing methods. Frontal polymerization is an attractive, scalable alternative due to its exploitation of polymerization heat that is generally wasted and unutilized. The only external energy needed for frontal polymerization is an initial thermal (or photo) stimulus that locally ignites the reaction. The subsequent reaction exothermicity provides local heating; the transport of this thermal energy to neighboring monomers in either a liquid or gel-like state results in a self-perpetuating reaction zone that provides fully cured thermosets and thermoplastics. Propagation of this polymerization front continues through the unreacted monomer media until either all reactants are consumed or sufficient heat loss stalls further reaction. Several different polymerization mechanisms support frontal processes, including free-radical, cat- or anionic, amine-cure epoxides, and ring-opening metathesis polymerization. The choice of monomer, initiator/catalyst, and additives dictates how fast the polymer front traverses the reactant medium, as well as the maximum temperature achievable. Numerous applications of frontally generated materials exist, ranging from porous substrate reinforcement to fabrication of patterned composites. In this review, we examine in detail the physical and chemical phenomena that govern frontal polymerization, as well as outline the existing applications

Quantifying the Reactivity of Photolytically Generated Radicals Towards Vinylic Monomers via Electrospray Ionization-Mass Spectrometry

Exact knowledge of the reactivity of photolytically generated radicals towards initiating free radical polymerization processes is still scarce. In here, quantitative coupled size-exclusion chromatography (SEC) - electrospray ionization-mass spectrometry (ESI-MS) as well as direct infusion ESI-MS was employed to obtain the propensity of variable photolytically generated radical species to initiate macromolecular growth

Snell\u27s Law of Refraction Observed in Thermal Frontal Polymerization

We demonstrate that Snell’s law of refraction can be applied to thermal fronts propagating through a boundary between regions that support distinct frontal velocities. We use the free-radical frontal polymerization of a triacrylate with clay filler that allows for two domains containing two different concentrations of a peroxide initiator to be molded together. Because the polymerization reaction rates depend on the initiator concentration, the propagation speed is different in each domain. We study fronts propagating in two parallel strips in which the incident angle is 90°. Our data fit Snell’s law vr/vi = sin θr/sin θi, where vr is the refracted velocity, vi is the incident velocity, θr is the angle of refraction, and θi is the incident angle. Further, we study circular fronts propagating radially from an initiation point in a high-velocity region into a low-velocity region (and vice versa). We demonstrate the close resemblance between the numerically simulated and experimentally observed thermal reaction fronts. By measuring the normal velocity and the angle of refraction of both simulated and experimental fronts, we establish that Snell’s law holds for thermal frontal polymerization in our experimental system. Finally we discuss the regimes in which Snell’s law may not be valid

The enhancement of weakly exothermic polymerization fronts

Abstract The propagation of one-dimensional waves resulting from chemical reactions in a sandwich-type two-layer setting is considered. One layer, termed the polymerization layer, contains the monomer and initiator molecules needed for the initiation of a self-propagating polymer front. The other layer will be referred to as the enhancement layer, and it contains the necessary reactants to support a highly exothermic self-propagating reaction wave. Heat exchange occurs between the layers, and as a result, there is a net diffusion of heat away from the region undergoing the more exothermic reaction. As frontal polymerization (FP) reactions are known not to be very exothermic, an overall transfer of heat from the enhancement layer into the polymerization layer takes place. An analysis of the basic state of the system is carried out to investigate the effect of heat transfer on the polymerization reaction. An enhancement layer is shown to promote FP. This analysis is applicable to the manufacture of thin polymer films by FP

Study of Thermal Frontal Polymerization Utilizing Reactive and Non-Reactive Additives

Thermal frontal polymerization is a process that involves a propagating front travelling through a monomer/initiator solution and converting monomer into polymer. The effects of different reactive and non-reactive additives on front temperature, front velocity, and pot life were studied in a thermal frontal polymerization system of multifunctional acrylates. One issue with thermal frontal polymerization of acrylate monomers is the amount of smoke and fumes produced due to high front temperatures. The effect of thermally-expandable microspheres was studied utilizing a variety of monomers. Solid additives including fillers, inert phase changer materials, and high thermal conductive fillers were investigated. The addition of liquid additives such as trithiol and plasticizer were also evaluated for their impact on front temperature, front velocity, and pot life. Most of the tested additives lowered front temperature and front velocity and were added until they caused the propagating front to quench. Only thiol affected pot life. Lowering front temperature reduced the amount of smoke produced, thus allowing these systems to be used in commercial settings. Of all of the tested additives, thiol worked best for lowering front temperature and reducing the amount of smoke produced. The behavior of fronts propagating in bifurcated media in which the front had different velocities was studied and compared to behavior predicted by Snell\u27s law. The spatial inhomogeneities of frontal polymerization were studied using Snell\u27s law, and it was demonstrated for the first time that thermal frontal polymerization systems follow Snell\u27s law of refraction