Non-uniform Curing Research Articles

The determination of the curing induced, nonlinear elastic field of an inclusion in photo-cured materials

A wide variety of photo-cured materials have recently been developed with the rapid advancement of three-dimensional (3D) printing technology. However, most of these materials are designed as soft functional materials, and their failure mechanisms have received little attention. This work studies the mesoscale residual stress defects of photo-cured materials that are generated due to non-uniform curing and volume shrinkage during the manufacturing process. The defects are simplified as uncured inclusions embedded within a fully cured, infinite matrix which are then additionally cured. A large deformation model, validated against finite element analysis, is established to determine the nonlinear elastic field induced by the curing of inclusions with different Poisson’s ratio and shape (i.e., sphere and prolate spheroid), and is shown to outperform the infinitesimal strain model. This large deformation model, which includes a phase evolution constitutive model to describe the inclusion’s behavior and a compressible neo-Hookean model to describe the matrix’s behavior, is derived based on the same elastic field distribution as that in the infinitesimal strain solution and an ellipsoid-ellipsoid transformation kinematics assumption. The final solution is expressed in a discrete formulation and is exact for the spherical inclusion and approximate for the prolate spheroid inclusion. The prediction results of the theoretical model are compared with the finite element analysis, and reasonable agreement is obtained. This study may lay a foundation for investigating the effects of mesoscale residual stress defects on the mechanical behavior of 3D printing materials.

Curing analysis of rubber film on a Hand-Shaped former in the manufacturing of rubber gloves through novel OpenFOAM solver

Vulcanization, or curing, is a crucial aspect of rubber- or polymer-product manufacturing. Determining the optimal curing time for rubbery components is a significant challenge in the rubber industry. This study focuses on enhancing a computational fluid dynamics code used for simulating the conjugate heat transfer of nitrile-butadiene rubber (NBR) glove curing. Non-isothermal differential scanning calorimetry was employed to analyze the NBR glove-curing process, utilizing constant heating rates ranging from 2.5 to 20 °C/min. Activation-energy calculations employing four isoconversional methods and multivariate nonlinear regression played a pivotal role in determining the most appropriate curing-kinetics model. The multivariate nonlinear regression yielded well-fitted outcomes with a residual sum of squares ranking from 0.44 × 10−3 to 1.60 × 10−3, indicating the suitability of the novel curing-kinetics model for the NBR curing process. After obtaining the appropriate model, adjustments were made to the novel conjugate heat-transfer solver in OpenFOAM to forecast the degree of curing during the heating process. Experimental results from a hot-air counter-flow NBR film in the tray affirmed the accuracy of the solver, demonstrating an impressive R2 value of 0.98. Consequently, the novel solver was applied to simulate the curing process of the NBR film coating on a hand-shaped former surface, offering a visual representation of the degree of curing distribution on the NBR glove. The non-uniform curing and weak curing surface of the NBR glove were revealed, which can be used for estimating the optimal conditions for the curing process or designing an appropriate curing oven for rubber glove manufacturing in the future.

Quantitative evaluation of embedding resins for volume electron microscopy.

Optimal epoxy resin embedding is crucial for obtaining consistent serial sections from large tissue samples, especially for block faces spanning >1 mm2. We report a method to quantify non-uniformity in resin curing using block hardness measurements from block faces. We identify conditions that lead to non-uniform curing as well as a procedure to monitor the hardness of blocks for a wide range of common epoxy resins used for volume electron microscopy. We also assess cutting repeatability and uniformity by quantifying the transverse and sectional cutting forces during ultrathin sectioning using a sample-mounted force sensor. Our findings indicate that screening and optimizing resin formulations is required to achieve the best repeatability in terms of section thickness. Finally, we explore the encapsulation of irregularly shaped tissue samples in a gelatin matrix prior to epoxy resin embedding to yield more uniform sections.

Strong-field THz radiation-induced curing of composite resin materials in dentistry.

Dental caries is listed as one of the three major non-communicable diseases by the World Health Organization, and its main treatment method is to restore it by filling it with resin. At present, the visible light-cure method has the problems of non-uniform curing and low penetration efficiency, which makes the bonding area easy to develop marginal leakages, thus leading to secondary caries and requiring repeated treatment. In this work, through the strong terahertz (THz) irradiation-weak THz detection technique, it is found that the strong THz electromagnetic pulses can accelerate the curing process of the resin, and the weak-field THz spectroscopy can be used to monitor this dynamic change in real time, which will greatly promote the potential application of THz technology in dentistry.

Evolution of material properties during the solvent-assisted recycling of thermosetting polymers: to reduce the residual stress and material inhomogeneity

Primary recycling of thermosetting polymers is shown to be enabled by depolymerizing the material in a suitable organic solvent and then re-polymerizing it into near-identical networks with equivalent mechanical properties. The re-polymerization process involves sophisticated coupling among solvent diffusion, chemical reaction, and stress field development. Specifically, solvent evaporation leads to restrained volume shrinkage of the resin and residual stress development within the re-polymerized materials. Nonuniform curing degree and material properties are also developed in the direction of solvent diffusion. These impose grand challenges to identify the optimal processing conditions to recycle thermosets with high quality and high speed. In this paper, using an integrated experimental and theoretical approach, the evolutions of the stress field and material properties during the re-polymerization of thermosetting polymers are studied. It shows how to tune the processing temperature to mitigate the residual stress and material inhomogeneity of recycled thermosets. The influencing mechanisms of sample thickness and catalyst content are also revealed. The fundamental understanding is finally extended to study the manufacturing of composites with particle reinforcements using the depolymerized solution, with a particular emphasis on the minimization of the stress concentration around particles. Overall, this study provides a guideline to design material and processing conditions for homogeneous network structure and mechanical properties of recycled thermosets, which will promote the applications of the green recycling method for thermoset wastes and benefit society.

Confined-Direct Electric Curing of NaOH-activated fly ash based brick mixtures under free drainage conditions: Part 2. Confined-DEC versus oven curing

A novel accelerated curing technique – confined-DEC (Direct Electric Curing) – was compared with oven curing. Microstructural investigations revealed that under free drainage conditions non-uniform curing conditions were induced within NaOH-activated fly ash based brick mixture during confined-DEC. Strength development of the outer-core (surface) regions of the mixture was impaired by diminished alkali content due to movement of pore solution, resulting in lower compressive strength at later ages compared to identical mixtures cured in an oven for 16 h. Nevertheless, confined-DEC is a promising technique for brick mass production as the requirements of several strength classes of the South African concrete masonry units’ standard SANS 1215:2008 were met in 7 min.

Origami by frontal photopolymerization.

Origami structures are of great interest in microelectronics, soft actuators, mechanical metamaterials, and biomedical devices. Current methods of fabricating origami structures still have several limitations, such as complex material systems or tedious processing steps. We present a simple approach for creating three-dimensional (3D) origami structures by the frontal photopolymerization method, which can be easily implemented by using a commercial projector. The concept of our method is based on the volume shrinkage during photopolymerization. By adding photoabsorbers into the polymer resin, an attenuated light field is created and leads to a nonuniform curing along the thickness direction. The layer directly exposed to light cures faster than the next layer; this nonuniform curing degree leads to nonuniform curing-induced volume shrinkage. This further introduces a nonuniform stress field, which drives the film to bend toward the newly formed side. The degree of bending can be controlled by adjusting the gray scale and the irradiation time, an easy approach for creating origami structures. The behavior is examined both experimentally and theoretically. Two methods are also proposed to create different types of 3D origami structures.

Process control for thermal-spray deposition of thermoset coatings using computer simulation

Thermal spraying is a green solvent-free process with the potential of applying polymer coatings to large components in-house or on-site without the need for prolonged drying. Almost no systematic research has been undertaken on thermally spraying thermoset coatings owing to the complexity and difficulty of managing the curing process. An adequately cured thermoset coating could not be deposited by thermal spraying owing to insufficient cumulative time above the cure temperature. Preheating and post-heating the substrate under a constant heat source were not successful as they led to non-uniform curing, residual stress and the risk of overheating. This study develops and validates a computer model that simulates the deposition of thermoset coatings on metal substrates using thermal spraying and high-energy infrared irradiation. The model uses readily-available commercial software and enables precise control of the coating process to improve energy efficiency and coating quality. Further research showed that evenly cured coatings could be achieved by using variable heat fluxes and controlled utilization of inward conduction from the outer surface layers. Self curing during cooling was significant and maybe employed to increase energy efficiency. The thickness of the metal substrate was shown to be an important variable as it acts as a heat sink and, for heavy sections, can substantially increase energy consumption. The results indicate a need for sufficiently accurate process control and provide a suitable methodology for the deposition of thermoset coatings.

Polymer microfabrication by scanning based microstereolithography: Optical design and material functionality

Several research groups have attempted to optimize photopolymerization parameters to increase the throughput of scanning based microstereolithography (MSL) systems through modified beam scanning techniques. Efforts in reducing the curing line width have been implemented through high numerical aperture (NA) optical setups. However, the intensity contour symmetry and the depth of field of focus have led to grossly non-vertical and non-uniform curing profiles. This work tries to review the photopolymerization process in a scanning based MSL system from the aspect of material functionality and optical design. The focus has been to exploit the rich potential of photoreactor scanning system in achieving desired fabrication modalities (minimum curing width, uniform depth profile, and vertical curing profile) even with a reduced NA optical setup and a single movable stage. The present study tries to manipulate to its advantage the effect of optimized lower [c] (photoinitiator (PI) concentration) in reducing the minimum curing width to ~10-20 μm even with a higher spot size (~21.36 μm) through a judiciously chosen "monomer-PI" system. Optimization on grounds of increasing E(max) (maximum laser exposure energy at surface) by optimizing the scan rate provides enough time for the monomer or resin to get cured across the entire resist thickness (surface to substrate ~10-100 μm), leading to uniform depth profiles along the entire scan lengths.

Exact, integrated and complete solutions for composite flywheels

An integrated general formulation for the analysis and design of flywheel rotors made out of composite material is developed. To have an accurate estimation of stresses in the rotor, all the effects present in the literature and some new ones have been included: axial length (disks vs. cylinders), acceleration (sudden energy releases), hub stiffness and mass (attachment), non-uniform curing (different temperature inside, outside rotor) and moisture absorption (transient, permanent). Through pertinent transformations, these effects are described by a single differential equation that can be directly integrated, giving closed-form expressions for displacements, stresses and failure factors. The new formulation is applied to the study of a standard rotor of the type commonly analyzed in the literature, uni- and multilayer, made out of glass and carbon fiber/epoxy materials. The results are compared with those corresponding to each of the effects, drawing conclusions and design recommendations.

Prediction of Process-induced Geometrical Deformations for Stiffened Thermosetting Composite Panels

Stiffened thermosetting composite panels are primary structure of aircraft and fabricated with co-curing processing. The process-induced residual stresses resulted in geometrical deformation when the stiffened composite panel was fully cured and removed from the tool. An investigation into the three-dimensional cure simulation of T-shape stiffened thermosetting composite panels was presented to predict the process-induced deformation during co-curing. And flexible tools and locating tools were considered in the cure simulation. Simulation examples were presented to demonstrate use of the present finite element procedure for predicting composite parts deformations. The T300/BMI structure was analyzed provide insight into the non-uniform curing process. The temperature was higher in the interface between web and flange during the entire cure cycle. And the distortions of skin and webs for T-shape stiffened thermosetting composite panel were remarkable.

Microwave preheating of anisotropic conductive adhesive films for high-speed flip chip on flex bonding

Anisotropic conductive adhesive film (ACF) can be preheated by microwave (MW) radiation in order to reduce the bonding time for flip-chip technology. Due to sluggish and nonuniform curing kinetics at the beginning of the curing reaction, thermal curing of epoxy is more time consuming. Therefore, MW radiation may be more effective, due to its uniform heating rate during the cycle. In this paper, MW preheating (for 1–4 sec) of ACF prior to final bonding has been applied to determine the electrical and mechanical performance of the bond. Powers of 80 and 240 W MW were used to study the effect of the MW preheating. A final bonding time of 6–7 sec can be used for flip chip on flex bonding instead of 10–15 sec (standard time for flip chip bonding) for MW preheating time and power used in this study. The contact resistance (as low as 0.01) is low in these samples, whereas the standard resistance is 0.017 ohm (bonded at 180°C for 10 sec without prior MW preheating). The shear forces at breakage were satisfactory (0.167–0.183 KN) for the samples bonded for 6–7 sec with MW preheating. This is very close and even higher than the standard sample (0.173 KN). For MW preheating power of 80 W and sweeping time of 2 sec, final bonding at 6 sec can also be used because of its low contact resistance (0.019 ohm). Scanning electron microscope (SEM) investigation of microjoints and fracture surface shows uneven distribution of conductive particles and thick bond lines in samples bonded for 5 sec (with MW preheating). Samples treated with MW radiation (80 W and 2–3 sec time) serve as evidence that well-distributed particles along with thin bond lines cause low contact resistance and high joint strength.

Light intensity effect on UV cured FRP coupled composite pipe joints

In this study, 36 composite pipes were joined with an eight-layer chopped mat and woven fabric that was adhered with UV curing vinylester resin. The joined composite pipes were cured vertically with UV lamps at three different light intensities: 80, 35, and 15 m W/cm 2. The mechanical properties of the cured pipes were evaluated by conducting internal pressure testing and simply supported 4-point bending testing. The effect of UV light intensity on the internal pressure rating, the ultimate bending load, and stiffness was evaluated based on the test results. A finite element analysis, which considered the under-curing along the axial direction and radial direction, was conducted to validate the test results. There was a direct correlation observed between increased light intensity and increased residual mechanical properties such as internal pressure rating, stiffness and peak failure load. The mechanisms for variation in the system properties were found to be under-curing and non-uniform curing in the FRP joint resulting in a loss of ability to effectively transfer load from the pipe to the joint. Additional observed phenomenon was gravity leaching due to vertical curing. All mechanisms were simulated by the introduction of a sliding modulus technique in the conducted finite element analysis. FEA results agreed well with the observed failure modes.

Two-Dimensional Cure Simulation of Thick Thermosetting Composites

An investigation into the two-dimensional cure simulation of thick thermosetting composites is presented. Temperature and degree of cure distributions within arbitrary cross-sectional geometries are predicted as a function of the autoclave temperature history. The heat conduction equation for two-dimensional, transient anisotropic heat transfer is coupled to the cure kinetics of the thermosetting composite material. A heat generation term, expressed as a function of cure rate and the total heat of reaction, is introduced to account for the heat liberated during the curing process. A generalized boundary condition formulation is employed, enabling arbitrary temperature boundary conditions to be enforced straightforwardly. An incremental, transient finite difference solution scheme is implemented to solve the pertinent governing equations and boundary conditions. The boundary-fitted coordinate system (BFCS) transformation technique is combined with the Alternating Direction Explicit (ADE) finite difference method in the solution strategy. Complex gradients in temperature and degree of cure are predicted and the influence of the tool on the curing process is demonstrated. Correlation between experimentally measured and predicted through-the-thickness temperature profiles in glass/polyester laminates are presented for various arbitrary temperature cure cycle histories. Several typical glass/polyester and graphite/epoxy structural elements of arbitrary cross-section (ply-drop and angle bend) are analyzed to provide insight into the non-uniform curing process unique to thick-sections. Spatial gradients in degree of cure are shown to be strongly dependent on part geometry, thermal anisotropy, cure kinetics and the temperature cure cycle. These spatial gradients directly influence the quality and in-service performance of the finished component by inducing warpage and residual stress during the curing process.