Formulation development and cure kinetics of UV curing vinyl ester photopolymers towards application in large-format additive manufacturing

Large-Format Additive Manufacturing (LFAM) has emerged as an effective method for rapidly building tooling from engineering polymers. While most current LFAM systems utilize thermoplastic polymers, photopolymers are advantageous because they process in ambient conditions and react within seconds upon exposure to ultraviolet (UV) light. One key application for LFAM is tooling for oven curing composites, but few thermoplastics can withstand the required 120 °C to 180 °C cure conditions. Bisphenol A ethoxylate dimethacrylate (Bis-EMA) is a suitable photopolymer oligomer for oven curing applications. However, the typical LFAM layer thickness (2 mm to 4 mm) restricts photopolymer cure kinetics.

The objectives of this work were to study: (1) thermal cycling effects and the influence of pentaerythritol triacrylate (PETIA) on through-thickness cure response, (2) effects of photoinitiator selection and concentration, and (3) multi-layer irradiation response in both unreinforced and fiberglass-reinforced photopolymers. In objective 1, 20 wt% PETIA content provided the ideal combination of improved cure response while retaining a T_g of 150 ºC. This 80:20 blend of Bis-EMA: PETIA was then used to produce a composite tool, and after five part cycles at 135 ºC, the tool had dimensional changes < 0.1 mm. In objective 2, a concentration of 0.1 wt% bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (BAPO) achieved a rapid cure response, with over 45% degree of conversion (DC) at 3 mm depth after 1.0 s irradiation. In objective 3, multi-layer effects were analyzed by irradiating up to five layers and measuring the level of cure across the depth profile. Cure gradients only improved with multiple irradiations at 0.8 s irradiation times, but the recommended minimum irradiation time to achieve >65% DC was < 2 s. Multiple layers of fiberglass composite were also tested. At a higher irradiation time of 3.5 s, the bond strength between layers was equal to or greater than the fiber-matrix shear strength within a layer.

This work demonstrated that oven-cure capable photopolymers could be formulated to cure rapidly under conditions suitable for LFAM.