Abstract
Vat photopolymerisation describes resin-based additive manufacturing processes in which ultraviolet light is used to layer-wise solidify liquid resin into a desired 3D shape. If the starting ...resin is a dual-curing formulation the object is also thermally cured to attain its final properties, obtaining either an elastomer or a thermoset. Here, we introduce cavity vat photopolymerisation, in which one photopolymer resin produces a composite material of an elastomer and thermoset. Cavities of any geometry are purposefully designed in the solid object and then filled with liquid resin during printing due to negative pressure. Thermal curing then solidifies the resin in the cavities into an elastomer, forming a distinct interface held together by strong covalent bonds. Hybrid specimens indicate improved damping, reduced fragmentation upon fracture and increased local elasticity, and we suggest several hard-shell/soft-core applications that might benefit.
Photopolymers for 3D printing belong to the largest material group used in additive manufacturing. One suitable characterization method is the photo‐differential scanning calorimetry (DSC) method, ...since UV light is used both as a probe in photo‐DSC measurements and for the curing process in digital light synthesis (DLS). The photo‐DSC method determines the speed and heat of the photopolymerization reaction, so it is an excellent method for characterizing the chemical process in DLS. In addition, the setup has been optimized in such a way that the photo‐DSC parameters used meet typical DLS process conditions. In particular, both the overlapping of the wavelengths of the light source and the photoinitiator and the similarity of the light intensities are very important. In this study, the commonly used dual curing, urethane methacrylate, DLS resin RPU 70 is thoroughly characterized. Several photo‐DSC experiments demonstrate the influence of wavelength, light intensity, temperature, relative humidity, and oxygen concentration in the atmosphere used on the curing process. Using the typical DLS parameters for light intensity and wavelength, the chemical reaction is mainly influenced by the oxygen concentration. The exposure time for vat photopolymerization processes decreases with decreasing oxygen concentration, which is why inert printing processes could have a positive effect on printing times. Additionally, it is shown that this method is also suitable for other DLS resins.
The photo differential scanning calorimetry (photo-DSC) is an appropriate method to characterize photopolymers used in additive manufacturing (AM). Important process parameters such as optimal ...ultraviolet (UV) exposure time and reaction heat can be attained by this method. However, achieving reliable and meaningful results from photo-DSC experiments requires careful sample preparation, i.e. the selection of a suitable sample shape, sample mass and sample holder (crucible). The sample shapes drop and spread with 1.0 mg and 2.8 mg sample masses were investigated in this study. Three different times from sample preparation until the start of the measurement (0, 4 and 7 h) were tested, in order to investigate different surface effects such as oxygen-diffusion, prior UV-curing through ambient radiation and evaporation of volatile components. While the 1.0 mg spread sample shape offers the thinnest film thickness (40 μm) and thus the closest comparability to high resolution print jobs, the 2.8 mg drop shape offers a more robust sample preparation with minimized surface effects. To further reduce time-dependent surface effects, this study shows how a preexisting test protocol was shortened from 42 min to 24 min without losing measuring accuracy. Furthermore, to reduce evaporation, different covers were placed on different crucibles, which were tested over time in the device’s automated sample changer (ASC) that enables automated and consecutive measurements. The combination of a cold pressed 85 μL crucible covered with a 300 μL Al2O3 crucible, which is removed shortly before the actual measurement, provides the best sample preparation for the ASC since mass loss remains below 1% for up to 10 h. Finally, two two-part resin systems, namely a methacrylate-urethane and an acrylate-epoxy based resin that are used in Digital Light Synthesis (DLS) are characterized part by part as well as in mixed state. Together with the investigation of different temperatures and atmospheres, it was possible to identify not only the part with the photoinitiator and the type of system (radical or cationic), but also a difference between methacrylates and acrylates with the aid of the photo-DSC method.
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•The 1.0 mg spread sample shape has shortest times to peak.•The 2.8 mg drop sample shape is more robust towards time-dependent surface effects.•EPX 82 follows a radical not a cationic or hybrid reaction system.•The reaction heat of EPX 82 is almost twice as much compared to RPU 70.•Sample preparation for reduced mass loss with the ASC of the DSC is presented.
The two part urethane-methacrylate resin RPU 70, used in DLS, seems to have superior material properties. These properties are achieved by a dual curing process of ultraviolet (UV) and thermal ...curing. The temperature influence on the processability of the mixed resin and the resulting mechanical properties are still unknown. It was possible to solidify the liquid RPU 70 resin into an elastomer at 60 °C for 15 h without the influence of UV light by solely harnessing the temperature sensitive crosslinking reaction between part A and part B. The ongoing thermal conversion of the liquid resin into an elastomer was analyzed with viscosity measurements, Fourier-transform infrared spectroscopy (FT−IR), differential scanning calorimetry (DSC) and Photo-DSC. The latter method proved to be effective in tracking the thermal conversion and to identify optimal exposure times for preheated resins. Furthermore, thermogravimetric analysis (TGA) showed a linear evaporation of reactive diluents in RPU 70 over time. In order to investigate the processability and to understand the effects of the thermal curing on mechanical properties, the resin was preheated to 30 °C, 40 °C, 50 °C and 60 °C for one hour before the dual curing DLS process. Even though, the viscosity of the resin was increased by preheating and additionally by the released reaction heat of the photopolymerization during DLS, the build job could be produced with the same part quality as the unheated resin. Tensile tests were conducted on dual cured specimen and the results showed that with a preheating of the resin for one hour at 50 °C, the tensile modulus of RPU 70 increased 19.56%, while tensile strength and elongation at break values remained in the standard deviation range of the comparison group. The preheating at 60 °C improved elongation at break and tensile modulus 47.61% and 5.54% respectively, while at the same time tensile strength dropped −5.02% compared to no preheating. Apart from the possibility to slightly modify the mechanical properties by preheating the resin before DLS, there are two temperature-dependent challenges for RPU 70 during the DLS process, namely evaporation and undesirable thermal curing. However, there are also new innovative ways to utilize the result of this study that RPU 70 resin can be thermally solidified into an elastomer: the creation of parts that have a hard shell and a soft core. The hard shell can be produced by using DLS, the soft core by pouring the liquid resin in the shell and thermally curing both.
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•Mechanical properties can be modified by preheating the resin before DLS.•Reactive diluents evaporate from the resin.•Liquid RPU 70 resin solidifies into an elastomer after 15 h at 60 °C.•Photo-DSC can be used to track the thermal conversion between part A and B.
A comparative study of the performance of three rheological automotive shock absorber models as well as of an extended force-velocity relation in full vehicle simulation is performed. Simulation ...results for both the shock absorber test rig and a full vehicle crossing a single obstacle are compared with measured data. While the gain of accuracy by the extended force-velocity relation is marginal, the rheological models in general yield a noticeable improvement, which, however, in full vehicle simulation is less significant than in test rig simulation. Among the rheological models studied here, the one consisting of a nonlinear spring-dashpot element with an element modelling friction by a continuous transition from the compression to the extension range in parallel and a quadratic approximation of the static gas force exhibits the best global performance.
We present the development of novel ion‐selective electrodes for tropane, atropine, and scopolamine, as a general model for tropane alkaloids and drugs such as e. g. cocaine. These potentiometric ...sensors rely either on a metallocarborane or tetraphenylborate complex as recognition element and led to systems with a near‐Nernstian behaviour in the range of 10−5–10−2 mol/L and detection limits around 2 ⋅ 10−5 mol/L. Selectivity studies with commonly used drug additives (caffeine, phenacetin) and closely related tropane derivatives show very low influence of the additives and a structure dependent interference for the derivatives. Beside this, a hydrogel‐based concept with a similar performance was developed, which could be used for on‐site measurement of solid samples. The developed sensors therefore demonstrated their suitability and might be used to enable the development of portable sensor arrays for the quantification of a wide range of tropane alkaloids.
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•Development of a set of novel ion-selective electrodes for antipyrine and its derivatives.•Selectivity demonstration towards the target analytes.•Implementation of a SPE step for ...suitable preconcentration to reach the nM range.
In this work, a set of ion-selective electrodes for the detection of antipyrine and some of its structurally closely related derivatives is developed. These novel potentiometric sensors rely on metallocarborane, cobalt bis(dicarbollide) anion (3,3′-Co(1,2-C2B9H11)2−) complexes with the protonated antipyrine derivative as active recognition element. All complexes were characterized, embedded in a PVC membrane, and coated on top of commercial and self-fabricated electrodes. All systems were in-depth evaluated and optimized with focus on their analytical performance, leading to a set of electrodes for antipyrine, 4-aminoantipyrine and 4-(acetamido)antipyrine, which showed a near Nernstian slope (55–57 mV/dec) in the range of 10−5 to 10−2 mol/L and a limit of detection in the μM range.
To lower the detection limit and to enable environmental stream water pollution monitoring, we implemented a solid-phase extraction step which could be used to achieve a pre-concentration of a factor >300 (recovery rates: 81–84%) and to lower the detection limit from the μM to the nM range. The combination of our new and highly selective electrodes with this SPE step enables the possibility to monitor antipyrine derivatives simultaneously in either an environmental surrounding or in pharmaceutical formulations. The combination of metallocarborane based ISEs with an SPE pre-concentration step and the in-depth evaluation of the closely related derivatives finally provides a set of valuable tools as well as new insights into their selectivity and real-life applicability.
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