# Evaluation of UV-Curable Solid Rocket Propellants' Properties for Advanced 3D Printing Technologies
## 2 Materials and Devices
### Isothermal Test Bench  
The determination of pseudoplasticity and pot-life properties requires isothermal measurements over extended time intervals. Isothermal hotplates, commonly used for this type of testing, provide a contact surface at the base of the beaker. Heat then gradually diffuses through the solution under examination. However, the poor thermal properties of the propellant can lead to significant temperature stratification. To address this issue, a system was engineered to deliver uniform heat only through the container's side walls, significantly reducing stratification effects. An isothermal test bench consists of two components: the viscometer and the heating system. The viscometer measures the resisting torque applied to an element using a spindle or rotor, which rotates at a constant speed, \(\omega_{s}\), while immersed in the fluid whose rheological properties are being studied. These kinds of measurements are also known as controlled shear rate tests. They are defined by selecting a rotor speed at the start of the experiment, which remains constant throughout the test, and evaluating the resisting torque on the rotor using the viscometer's torque transducer. The shear stress is estimatedusing a simple equation, based on the specific geometric characteristics of the standard rotor [25]. Then, the viscosity is calculated using [PERSON]'s law:  
\[\tau=\eta\dot{\gamma} \tag{1}\]  
where \(\tau\) represents the shear stress in [Pa] and \(\eta\) and \(\dot{\gamma}\) denote the measured viscosity in [Pa-s] and the shear rate in [1/s]. To ensure measurement repeatability, it is mandatory to specify the following characteristics:  
* The type of viscometer;
* The used rotor;
* The rotation speed of the spindle;
* The sample temperature;
* The measurement time;
* The dimensions of the beaker and the presence or absence of a clamp;
* The production process history. Meeting all the standard criteria is challenging because of practical constraints, such as a limited solution quantity or difficulties in handling highly viscous liquids. As was also explained in a previous study [20], the ASTM D1296-20 has been adopted as the standard, though with a smaller vessel than originally specified. This modification does not pose any issues because, as [PERSON] et al. (2020) describe, the chosen vessel dimensions still produce nearly identical results for high-viscosity materials like the slurry [26]. The viscometer used to analyze the rheological properties of the slurry was the SAVISC-152/2 (Figure 1a). This instrument is equipped with four rotors from the LV category, capable of measuring the viscosity in a range of 10 to 2,000,000 mPa-s. In addition, it offers seven different rotation speeds, ranging from 0.3 to 60 RPM. The SAVISC152-2 viscometer has an error margin of 1% relative to the maximum measurable viscosity at the chosen rotor speed [27; 28; 29; 30]. The heating system, as shown in Figure 1b, employs a submerged hot water pump within a metal vessel, directly interfacing with a hotplate. The pump drives water through a red silicone tube, which terminates at the upper end of a copper coil wrapped around the black PPCF vessel. This vessel contains the propellant slurry, which is analyzed using a viscometer. After passing through the copper coil and exchanging heat with the propellant,  
Figure 1: Rotary viscometer (**a**) and isothermal test bench (**b**). Not to scale. (**a**) Rotary viscometer SAVISC 152-2 and standard LV spindles (purchased from SAMA tools, Viareggio, Italy). (**b**) (1) Sub-merged pump; (2) water supply line; (3) copper coil; (4) water return line; (5) support; (6) viscometer without spindle; (7) hotplate. the water returns to the tank through the blue silicone tube. Temperature control is achieved by modulating the flow of hot water sent to the copper coil, adjusting the pump power via a PID circuit, shown in Figure 2. All pump components are made of PPCF, which maintains its mechanical properties even at elevated temperatures.