Quantifying the modulus development during cure of adhesives and prepregs is complex due to the resin's behavior. First, each phase and transition need a careful selection of frequency, strain, and thermal cycles. Second, between the gel point and the degree of cure at which resins are typically demolded, the material has a high compliance, causing excessive noise in data. Third, samples are made using multiple layers, which can cause delamination. Lastly, the data analysis and finding of trends is difficult because most tests are performed at discrete cure values. A testing protocol to detect the moduli changes in one experiment is presented. Adhesive DA 409 was analyzed using the kinetic, viscosity, gel point, and shrinkage information to delimit and select the parameters for the moduli tests. The frequency and strain of each property were selected based on dynamic strain and frequency sweeps. First, the gel point was measured using three methods; the cross-over of storage and loss moduli, the point where the complex viscosity reached industry accepted values, and the intersection of the dissipation factor at multiple frequencies. The gel point was found between 56 and 63% cure. Second, the viscosity was measured in a parallel plate rheometer, under isothermal and non-isothermal cycles, and fitted using the Castro-Macosko model. Third, the linear shrinkage was measured in a DMA in compression using zero strain to eliminate displacement due to compression loads. The total shrinkage was 4.34%, with most of the shrinkage occurring in the liquid viscoelastic region. A linear relationship between shrinkage and cure was observed. Lastly, the modulus was measured in a DMA in a three-point bending test. With this new protocol, the modulus was measured from the gel point up to the fully cured state by sandwiching a partially cured beam between two fully cured plates. The high compliance and noise were mitigated with this sandwich sample. An effective modulus of the three plates was obtained. The modulus of the partially cured beam was extracted using the elastic sandwich beam theory. This method shows potential in the testing of thermoset composites and films.

ABSTRACTThe characterization of film adhesives is challenging because they required freezer storage, contain an inseparable filler—thermoplastic knit or fiber‐reinforcement, and are heat activated systems with a pre‐cure and unknown chemistry. A testing protocol that eliminates these sources of error is proposed. This study presents a method to generate time–temperature‐transformation (TTT) diagrams of epoxy film adhesives via differential scanning calorimetry (DSC). Non‐isothermal and isothermal DSC scans are used to capture the reaction and the glass transition temperature. The use of an initial fast ramp—up to 500 K/min—in the isothermal scans is explored for the first time. This technique shows the potential to produce a quasi‐isothermal cycle, eliminating the loss of data in the initial stage of the reaction. The total heat released, the activation energy, and the fractional kinetic parameter, are estimated via model‐free methods. The Kamal–Sourour model and the formal kinetic model are fit to model the rate of cure. The simplest model that accurately captures the reaction, a parallel two‐step model, A , is outlined. The glass transition temperature is modeled via DiBenedetto's equation to include the diffusion‐controlled mechanism. The TTT‐diagrams of two commercial adhesives, DA 408 and DA 409, are shown with an analysis of processing optimization. The use of quasi‐isothermal scans with initial fast ramps combined with the correction for filler, moisture, and pre‐curing history can be applied to characterize fast curing thermosets, complex B‐stage resins, and thermosetting composites. The modeling results can also be used in numerical studies of residual stresses and dimensional stability in the manufacturing of thermosetting composites. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45791.

This paper presents an analytical melting model inside the nozzle of a fused filament fabrication process. The model presents the limiting case scenario where the maximum melting rate is controlled by the applied force. Here, instead of having a nozzle filled with polymer melt, the melt is reduced to a melt film at the tip of the filament as it is pushed against the exit of the nozzle. The model uses a mode of melting that is governed by melting with pressure flow melt removal. The model includes effects of initial filament temperatures, heater temperature, applied force, nozzle tip angle, capillary diameter and length as well as rheological and thermal properties. The analytical solution is compared to controlled experiments done on a specially designed set-up. Furthermore, the model is used to assess the effect of nozzle tip angle, heater temperature and initial filament temperature on the melting rate within the nozzle. The comparison between the experiments and the model show that assumptions used for the model development are plausible, and that the model can be used to optimize the melting within a material extrusion additive manufacturing process, as well as predicting the performance of new materials.

ABSTRACTThree major factors decrease the accuracy of the cure measurement in standard‐isothermal testing using differential scanning calorimetry (DSC). First, cure occurs during the heating step. Second, data are lost during the stabilization period between the dynamic and isothermal step. Third, the baseline selection requires a modification to the protocol. An alternative, which is explored in this study, is the use of fast ramps, which decrease the heating time, but this has been avoided due to overshoot that occurs between the dynamic and isothermal step, which is troublesome for systems with autocatalytic kinetics. By mitigating these factors, a quasi‐isothermal protocol was developed. Therefore, more complete cure kinetics were captured with the implementation of fast DSC to decrease the ramp time and through the optimization of furnace parameters to decrease stabilization time and temperature overshoot. The data suggested this quasi‐isothermal analysis more accurately measured the isothermal curing kinetics of a commercial epoxy adhesive at 110, 115, and 120 °C for fast ramps of 175, 350, and 500 K/min compared to the traditional ramp of 5 K/min. The enthalpy spike at the dynamic to isothermal transition remains an issue; however, an empirical shift can be used to compensate for the enthalpy signal lag. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45425.

This study characterizes the degree of cure c, of a two‐part epoxy/acrylic photopolymer used in stereolithography, SLA. The use of solid free form fabrication has added a whole new list of thermosets with a chemical reaction—curing reaction—targeted for these specific manufacturing processes. The studied thermosetting system exhibits a fast curing reaction triggered by a UV‐light source. An innovative stepwise method to quantify the degree of cure using Raman spectroscopy is reported. This method abates the Raman noise and compensates for the weak signal of fast measurements. The Raman data processing algorithm BEADS was implemented. The technique presented here was capable of following the degree of cure of the individual components of the photopolymer. Based on the epoxy/acrylic monomers concentration (75%/25%), the degree of cure for the whole system was quantified. The experimental results were modeled using a modified phenomenological kinetics equation. This model includes the dependency of the rate of cure on the light intensity, and cure state. Based on the analysis reported here, the proposed model accurately fits the initiation, propagation, and termination phases of the cure cycle. The technique and methods presented here can potentially be integrated to in situ process monitoring with instant parameter feedback, and numerical modeling of residual stresses in additive manufacturing of thermosets. POLYM. ENG. SCI., 58:228–237, 2018. © 2017 Society of Plastics Engineers

The plastics industry currently faces a variety of obstacles related to sustainability, quality and process efficiency. The demand for large scale production and implementation of new technologies on an industrial level requires advances in modeling tools and process simulation in order to overcome present-day challenges. This work presents three study cases of plastics industry in which modeling and simulation of diverse manufacturing processes is key in both the evaluation and availability of new technologies.

La industria de los plasticos actualmente se enfrenta a una variedad de retos relacionados con sostenibilidad, calidad y eficiencia de procesos. La demanda de produccion a gran escala e implementacion de nuevas tecnologias a nivel industrial requiere avances en herramientas de modelado y simulacion de procesos para la solucion de problemas y superacion de retos actuales. Este trabajo presenta tres casos practicos en la industria de plasticos donde el modelado y simulacion en el procesamiento representan un papel fundamental en la evaluacion y disponibilidad de nuevas tecnologias