Development of acrylate-based material using a multivariable approach: additive manufacturing applications
Authors: Wesley Machado Cunico Marlon , de Carvalho Jonas 
Published: March 2014

Purpose

– Over the last several years, the range of applications for the photopolymerisation process has been steadily increasing, especially in such areas as rapid prototyping, UV inks, UV coats and orthodontic applications. In spite of this increase, there are still several challenges to be overcome when the application concerns materials formulation and their mechanical properties. In this context, the main aim of this work is to outline the contribution of the formulation components for the parameters of the photopolymerisation process and the resultant mechanical properties of the material.

Design/methodology/approach

– For this research, the authors have applied multivariable analysis methods, which allow the identification of principal conclusions based on experimental results. For the experimental analysis, the authors applied design of experiment, while the material formulation was based onmethyl methacrylateas a monomer, Omnrad 2500 as a photoinitiator andtrimethylolpropane triacrylateas an oligomer. The authors analysed the photopolymerisation rate, viscosity, mechanical tensile strength, flexural stiffness and softening. These results comprise a multiobjective optimisation study to identify the ideal material formulation for additive manufacturing applications. The values chosen for the materials were the following: the initiator concentration was 2 and 5% wt., the monomer volume was 5 and 10 ml and the oligomer volume was 3 and 5 ml. To analyse the system kinetics and the photopolymerisation rate, the authors identified the polymer conversion rate through a photometric-cum-gravimetric method with a wavelength of 390 nm at the peak intensity. For the softening test, the authors identified the stiffness of the material as a function of temperature, characterising the thermal-mechanical behaviour of the material and determining its degree of crystallinity (cross-linking). Additionally, the authors performed an optimisation to maximise the mechanical tensile strength, flexural stiffness, softening temperature and photopolymerisation rate while minimising the viscosity.

Findings

– Based on these studies, it was possible to identify the influence of the monomer/oligomer ratio and the initiator concentration as function of polymerisation rate, viscosity, mechanical tensile strength, stiffness and softening of the material. It was also possible to determine the photopolymerisation rate in addition to the constants of propagation and termination. As a result of these studies, the authors identified a material formulation that resulted in a softening temperature greater than 70°C, while the viscosity of material remained lower than 3 cP. The mechanical ultimate tensile strength was between 10 and 50 MPa, and the stiffness was between 1.6 and 5.8 GPa. The effect of cross-linking on the process highlighted the interaction between the monomer/oligomer ratio and the initiator. The contribution of the initiator and the inhibitor to the polymerisation rate was identified via a numerical model, which allows the prediction of the material's behaviour in different process conditions, as such curing time and penetration depth.

Originality/value

– The main value of this work is to show the possibility of optimized photopolymerizable systems through an experimental approach as a function of the mechanical properties of material. In addition, it emphasised the possibility of predicting the material behaviour in front of different situations.