2.5. Characterization Methods

Disks of a precise layer thickness (100 μm) were printed on the ibox Mono at 10 mm diameter starting at 10 ss, increasing to 60 s at 10 s intervals to compare curing behavior. Cured samples were cleaned with IPA before the thickness was measured using a micrometer.

The viscosity of formulations was measured using a Brookfield RST-CPS-P rheometer (Ametek, Braunstone Town, UK). Tests were carried out at a constant temperature of 25 °C, a shear rate of 800 s⁻¹ for 120 s, creating 60 data points. Then, 0.2 ml of binder was deposited on the sampling surface for reading. For binders, an average of 60 data points was taken for fluid viscosity due to the Newtonian fluid behavior exhibited by homogenous systems. However, for slurries containing Fe-6.5 wt%Si particles suspended in the binder, the viscosity was taken at 100 s⁻¹ due to potential non-Newtonian behavior causing variance in viscosity with applied shear.

A Multitest- dV(u) (Mechmesin, West Sussex, UK) with 2.5 kN capacity was used to perform tensile tests at 1 mm/min. For each post-curing setting, including green bodies, seven bars were tested to gain an average for Ultimate Tensile Strength (UTS), MPa, Young’s Modulus (E), and strain at break, %. Stress at 5% strain, MPa, was also added as a mechanical property to study the binder’s predicted tensile strength at the point failure predicted for composite Fe-6.5 wt%Si flexural bars.

Specimens were printed in the geometry set by ASME B46.1 standard [46] for hardness tested on a Shore D durometer (Sauter AG, Basel-Stadt, Switzerland). An average was taken for the center point of seven samples. The samples tested were of both green body and post-cured states to observe the effect of post-curing conditions on the hardness of the polymer.

The same specimen geometry was used to measure the surface roughness of the green bodies and post-cured ones following ISO 4287 [47]. An area surface roughness was determined along the Z axis (printing direction). Figure 5 shows the dimensions and geometry of the bars. A numbering of 1 mm indentation was added to the bar to ensure surface roughness and hardness measurements were taken on the same face. Testing was conducted on a Bruker Alicona Infinite Focus (Bruker, Billerica, MA, USA).

Drafting of hardness/surface roughness bars.

FTIR-ATR was used to study the degree of conversion and define the characteristics of formulations. Absorbance peaks were studied, and the area of peaks was used to determine the conversion of alkene bonds to an alkane, a sign of polymerization. To determine the degree of conversion, the area of C=C (alkene stretching) peaks at 1635 cm⁻¹ were measured before and after exposure to UV. The area of the C=O vibration at 1720 cm⁻¹ peak was taken as the baseline, as the peak is well-defined and uninfluenced by the photopolymerization in this system. The conversion was determined using Equation (1) [48]. Cured disks of 10 mm diameter and 100 µm thickness were created using a standard 5 s exposure time across the three top-performing formulations to test each binder’s reactivity comparatively.

(ξ) Conversion; (A) Area of peak; (P) Polymerized sample; (U) Uncured sample.

A Bruker Alpha Platinum-ATR (Bruker, MA, USA) was used to carry out FTIR-ATR on samples.

For uncured resin, a small amount was deposited directly on the measuring zone of the spectrometer, and for cured samples, the same disk geometry was used as the cure depth study. Before each test, the measurement surface was cleaned with IPA. A background measurement was taken to omit any noise produced by the surrounding atmosphere. FTIR-ATR was also used to estimate the functionality of each formulation by determining the peak area of uncured samples at 810 cm⁻¹ to determine the abundance of alkene bonds available for conversion. This value was obtained through the peak area analysis of the 810 cm⁻¹ absorption band for each binder at the uncured state and defined as the Maximum Saturation Potential (MSP).

To prepare the magnetic composite test samples for the VSM tests, the magnetic slurry was deposited on OHP film and exposed to UV light to achieve a cure depth of 20 μm. The excess material was wiped off, and a new layer of the uncured slurry was deposited over the previously cured layer. This process was repeated to achieve an accumulated thickness of 100 μm. Then, 2 mg of the magnetic composite was placed into a gelatin capsule, and the remaining space was filled with cotton wool to prevent the composite from moving. The sample was placed into the vibrator of a Cryogen-free measurement system (Cryogenic, London, UK) run on a helium cooling system with a magnetizing field of up to 2 T.