Gel Time Prediction in Multifunctional Acrylates

Gel time prediction in multifunctional acrylates

The photopolymerization kinetics of multifunctional acrylates have been studied extensively since these polymers are used in a wide range of applications from lithography and coatings, to biologically related uses such as dental composites and contact lenses¹. The vinyl bonds on acrylates react readily in the presence of radicals, and in the case of multifunctional acrylates, which have multiple vinyl groups per monomer, reactions between distinct chains are also possible. These types of reactions, known as cross-linking, bind different polymer chains in the reaction volume into an insoluble network. Cross-linking does not occur during photopolymerization of monofunctional monomers, thus resulting in a soluble network of linear chains. In contrast, the cross-linked networks formed by multifunctional monomers are insoluble and this is known as the gel state. It is important to correctly identify the point at which gel occurs because it is an important parameter which will determine the durability of the resulting polymer^2-4.

The time at which the liquid resin transitions to a crosslinked gel is called the gel time, and it can be quantified experimentally using microrheology⁵.  In this work we study gelation behavior of the trifunctional monomer trimethylolpropane triacrylate with the photoinitiator DMPA.  To predict the gel time and thus the part height, we construct a reaction-diffusion model to predict gel time based on resin composition, exposure intensity, and exposure time.  Unknown coefficients in the model are estimated using experimental data from microrheology^{6, 7} and Fourier transform infrared spectroscopy (FTIR).  Specifically, the FTIR data is used to build the reaction-diffusion model for the polymerization kinetics, and the microrheology data is used to estimate a single critical value of double bond conversion that indicates the onset of gelation. 

Across the entire broad range of polymerization conditions considered, the experimentally measured gel time occurs at a double bond conversion of 12%.  Moreover, we find that the local rate of radical generation is the single model variable needed to predict the gel time.  Because our model is based on polymerization kinetics, rather than empirical fits that are typically used in stereolithography, it also can be modified for gel time predictions with different resin formulations or different exposure intensities. 

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2.         Lee, J. H.; Prud'homme, R. K.; Aksay, I. A., Cure depth in photopolymerization: Experiments and theory. Journal of Materials Research 2001, 16, (12), 3536-3544.

3.         Tang, Y. Y.; Henderson, C.; Muzzy, J.; Rosen, D. W., Stereolithography cure modelling and simulation. International Journal of Materials & Product Technology 2004, 21, (4), 255-272.

4.         Zhang, X.; Jiang, X. N.; Sun, C., Micro-stereolithography of polymeric and ceramic microstructures. Sensors and Actuators a-Physical 1999, 77, (2), 149-156.

5.         Winter, H. H.; Chambon, F., Analysis of linear viscoelasticity of a cross-linking polymer at the gel point. Journal of Rheology 1986, 30, (2), 367-382.

6.         Slopek, R. P.; McKinley, H. K.; Henderson, C. L.; Breedveld, V., In situ monitoring of mechanical properties during photopolymerization with particle tracking microrheology. Polymer 2006, 47, (7), 2263-2268.

7.         Slopek, R. In-Situ  Monitoring of the Mechanical Properties During the Photopolymerization of Acrylate Resins Using Particle Tracking Microrheology. Georgia Institute of Technology, Atlanta, 2008.