Curing (chemistry) | EPFL Graph Search

Curing is a chemical process employed in polymer chemistry and process engineering that produces the toughening or hardening of a polymer material by cross-linking of polymer chains. Even if it is strongly associated with the production of thermosetting polymers, the term "curing" can be used for all the processes where a solid product is obtained from a liquid solution, such as with PVC plastisols. Curing process During the curing process, single monomers and oligomers, mixed with or without a curing agent, react to form a tridimensional polymeric network. In the very first part of the reaction branches of molecules with various architectures are formed, and their molecular weight increases in time with the extent of the reaction until the network size is equal to the size of the system. The system has lost its solubility and its viscosity tends to infinite. The remaining molecules start to coexist with the macroscopic network until they react with the

Thermophysical and Thermomechanical Behavior of Cold-Curing Structural Adhesives in Bridge Construction

Omar Moussa

The use of the structural adhesive bonding technique is well established in the aircraft and automotive industries where, in most cases, joints can be fabricated indoors under controlled conditions. In the civil engineering domain, however, load-bearing structures are normally erected on-site in outdoor conditions, i.e. joint fabrication is exposed to varying outdoor temperature and humidity profiles and particularly in winter, when long periods of very low temperatures are frequent, the on-site joining of structural components must remain possible. Consequently, and due to the generally large scale of the structural components, cold-curing adhesives are used, unlike in other fields where hot-curing adhesives are used. Although it offers many potential advantages in bridge construction, structural adhesive bonding is not yet widely used in civil engineering structures. This can be attributed, amongst other things, to the lack of knowledge regarding adhesive behavior during exposure to varying environmental conditions. Despite significant research efforts concerning the characterization of structural adhesives, the question of the changes that occur in the thermophysical and thermomechanical properties of cold-curing structural adhesives during their service life, and particularly at early age and low temperatures, is yet to be addressed. The objective of this work was therefore, based on experimental and analytical investigations, to understand the thermophysical and thermomechanical behavior of cold-curing structural adhesives from the time of the mixing of the different components and throughout the long-term service life. Investigations of curing at early age and low temperatures (during winter) showed that curing already takes place at temperatures slightly above 0 °C. The process is very slow however and several days of curing are required to attain significant curing degrees. The glass transition temperature, Tg, develops even more slowly due to early initiation of vitrification. A new and practical experimental method was developed to establish the relationship between glass transition temperature and curing degree for different types of cold-curing adhesives. Furthermore, cure kinetics (developed for hot-curing adhesives) proved its applicability for cold-curing adhesives at low temperatures. In contrast to thermophysical properties, mechanical properties start developing significantly only after the onset of material vitrification. Lower curing temperatures also significantly decelerate the development process. An empirical model to predict strength and stiffness as a function of simple or complex curing temperature profiles particularly for curing at low temperatures was developed, taking the temperature-dependent vitrification into account. During summer, the temperature at specific locations of certain joints (e.g. joints below the asphalt) may exceed Tg, which may lead to a significant drop in mechanical properties. The investigations showed that when cooled to temperatures below Tg, the mechanical properties fully recovered and even a significant increase in properties is achieved due to post-curing. An existing model to predict temperature-dependent mechanical properties was extended to predict the change in stiffness and strength resulting from the exceeding of Tg and subsequently after recovery. Over the long term and during ambient curing, a significant increase in mechanical and thermophysical properties occurs over the years and decades due to the completion of curing. A model based on the similar long-term increase of concrete properties was revised to also predict the long-term strength and stiffness of cold-curing structural adhesives. Finally several case studies demonstrate how the outcome of this research can be applied, particularly to predict mechanical properties at early age and low temperatures. Such results can be used to plan construction stages or estimate the required waiting periods prior to a bridge being brought into service.

EPFL2011

UV-cured Epoxy for Adhesion in Steel Reinforced Thermoplastic Composites

Basile Golaz

This work addresses the development of composite materials that contain metallic load-bearing cords with the aim of combining outstanding mechanical performance with added functionality, including electrical conductivity, data transmission, and temperature and strain sensing. The corresponding elements are impregnated with a thermoplastic elastomer for protection, and to provide mechanical integrity and surface traction. However, in order to render the resulting smart multifunctional systems sufficiently robust and cost effective, it is necessary to find innovative solutions for the rapid establishment of a strong and durable interface between the metallic reinforcements and the surrounding elastomer. In order to ensure effective transfer of mechanical loads, the interface between metal oxides and polymers is typically modified using organosilane primers. These provide strong and durable bonds, but imply a sol-gel condensation at the interface characterized by relatively slow, temperature dependent reaction kinetics, which often precludes their application on-line. Additional processing and storage steps increase costs and may result in surface contamination or premature reaction of functional groups. It is to some extent possible to resolve these latter problems by surface activation with heat or solvent treatments prior to bonding, but such treatments may be unacceptable in terms of energy consumption and/or pollution. One of the main goals of the present work has therefore been to identify a suitable primer that can be deposited and cured on-line at high processing speeds. The first phase of the study focused on the adhesion phenomena associated with the interface of a thermoplastic polyurethane elastomer (TPU) co-moulded with galvanized steel wire, and their sensitivity to hydrothermal aging. Various surface treatments were investigated for the purposes of benchmarking and to assess their efficiency at the TPU-metal interface. γ-aminopropyltriethoxysilane (γ-APS) was deposited on the steel using water (pH5 or pH10), isopropanol or ethanol as a solvent and other surface treatments included degreasing, corona discharge, acid etching, gas flame and use of a zinc phosphate conversion coating. Adhesion was characterized using a single wire pull-out test based on the microbond test geometry on unaged specimens, after 2 months of dry aging or after 24h of hydrothermal aging in water at 50°C. γ-APS was found to increase adhesion between the TPU and the substrate, outperforming the other surface treatments in terms of strength and durability. The shear strength increased from 5-11 MPa to more than 20 MPa for the unaged specimens, and from ~5 MPa to more than 15 MPa after hydrothermal aging. However, effective treatment with γ-APS required careful optimization of the deposition parameters and a 1 hour thermal cure. At the same time, a finite element (FEM) approach was used to simulate an idealized wire-reinforced thermoplastic elastomer composite, taking into account the non-linear viscoelastic response of the matrix, and the results were compared with results from an analytical elastic shear-lag model for the single filament pull-out test. The analytical model was found to be sufficiently accurate for stress analysis, but was unsuitable for fracture properties. FEM analysis allowed evaluation of the build-up in internal stresses during processing, showing that these may reach levels of 3.3 MPa, with important consequences, particularly for treatments leading to low intrinsic interfacial shear strength. The next phase of the study involved the development of an innovative fast-curing coupling method using a cationic UV-curable primer with an optimized formulation based on an epoxy monomer. The approach was first to determine the curing kinetics and the final degree of cure of the epoxy resin by photo-DSC in order to optimize the processing conditions. The processing parameters studied were the curing time, temperature, UV intensity and thermal post-curing. The influence of the primer on the interfacial mechanical properties was then investigated using the single wire pull-out test in order to tune the formulation with respect to interfacial stress transfer through the primer before and after hydrothermal aging. The formulations comprised a hyperbranched monomer for cross-linking and flexibility, an oxetane monomer to increase curing speed and reduce viscosity, and an epoxysilane to improve the adhesion to the substrate. The proposed UV-cured epoxy primer outperformed the conventional methods in terms of processing time, adhesion and durability. The processing time was reduced from 1h for the silane primer to 5s for the UV-cured epoxy primer, and the interfacial shear strength reached more than 25 MPa before aging and more than 20 MPa after hydrothermal aging. In the final phase of the work, the fast-curing adhesion promotion system was applied to systems based on other metallic substrates and thermoplastic matrices (flame retardant TPU (FR-TPU), polyamide 12, high density polyethylene). The new primer increased adhesion to stainless steel and was also effective with FR-TPU and polyamide 12. Primer application was then successfully scaled up to the cord geometry and the interfacial adhesion tested by cord pull-out tests, as opposed to single wire pull-out tests, again before and after hydrothermal aging. In this configuration, the UV-cured primer increased the maximum pull-out force by 40% before aging and 20% after aging, even with non-optimized formulations. The primer was also found to reduce the fretting wear of steel cords, which is also a considerable advantage for long term composite properties. Finally, optical fibres sensors were successfully integrated in the TPU with sufficient interfacial stress transfer to provide the multifunctional coated transmission elements with accurate temperature and strain sensing capabilities. Use of the UV-cured epoxy primer was again shown to be highly beneficial in this case. Sensing accuracies of ±1 μstrain and ±0.2°C were achieved, with a spatial resolution of 0.10 mm.

EPFL2012

Dark curing kinetics of UV printing inks by real-time ATR-FTIR spectroscopy

Eric Jacques Nouzille

In UV-curable printing technology, an ink or a varnish is exposed to the UV-light source within a fraction of a second. Subsequent polymerization therefore proceeds in the dark, referred to as dark curing. Because dark curing can be critical and contributes to a large part of the overall conversion and to the final properties of the printed ink, the aim of this work was to get a better understanding of the dark curing process. Because an ink is a complex mixture of components, simpler systems representative of free radical and cationic chemistry were studied. Special attention was given to free radical polymerization with a detailed kinetic modeling. For cationic systems, the study was more application oriented. Attenuated-Total-Reflection Fourier-Transform Infrared (ATR-FTIR) spectroscopy was found to be the method of choice to follow in real-time the progress of the reaction under UV irradiation and in the dark. This technique allowed the study of curing conditions close to those encountered in industry. Dark curing of free radical was examined in systems consisting of a monomer (multifunctional acrylate) and a photoinitiator. The reactivities of different monomers were investigated and it was found that the chemistry of the spacer between the acrylate functional groups had an influence on the extent of dark curing. For example, TPGDA (tripropylene glycol diacrylate) led to higher overall conversion than HDDA (hexanediol diacrylate) although both are diacrylate monomers. Several explanations were put forward, such as the effect of dissolved oxygen, primary cyclization and initiation efficiency. The efficiency of photoinitiators (PIs) was examined and different behaviours were highlighted. For example, it was observed that DMPA (dimethoxyphenyl acetophenone) has a better quantum yield for α-cleavage compared to HCPK (1-hydroxycyclohexylphenyl ketone), whereas HCPK is more efficient in initiating the reaction, resulting in a better overall conversion in the dark. It is suggested that the two primary radicals resulting from the cleavage of the PI behave differently for DMPA and HCPK. One radical effectively initiates the reaction and the second is mainly involved in the termination process. These two different behaviours were observed under specific experimental conditions: (i) by covering the sample with a quartz plate and taking into account dissolved molecular oxygen and (ii) by curing under inert conditions with nitrogen. Photo-crosslinking polymerization reactions are strongly influenced by diffusion effects due to the formation of an infinite network. Therefore, it was proposed that a redistribution of reactivity within the system occurs, such as the diffusion of unreacted photoinitiators towards concentrations of unreacted monomers. This diffusion process has two positive effects: (i) for the same UV dose, but different intensities, a higher extent of conversion was reached with longer irradiation times under the condition investigated; (ii) multiple exposures led to a higher overall conversion than for a single exposure, for a fixed overall UV dose. Other parameters were explored, such as temperature and diffusing oxygen from air. With temperature, different trends were observed depending on the photoinitiator and the start of dark curing, either starting before the maximum rate of polymerization (Rp,max), or after Rp,max. The development of a kinetic model to describe dark curing of free radical polymerization was proposed. Existing models based on physical parameters, such as those incorporating diffusion-controlled phenomena and free-volume are relatively complex and require extensive information on the system studied. Therefore, a semi empirical approach was chosen in order to study the variation of fitting parameters when changing curing parameters, such as the photoinitiator concentration and type, the irradiation time, the UV light intensity and the temperature. The model was derived from kinetic data and it was assumed that rate coefficients are time-dependent and that termination, which was considered to be a first-order process, is propagation dependent. The kinetic equation has the form of a stretched exponential function and incorporates four fitting parameters. The model was evaluated for a large number of experimental data and very good fits were obtained with a random distribution of residuals. The exponent β was found to be sensitive to the termination process and reflected different reactivities in the dark as in the case of DMPA and HCPK. Dark curing of cationic systems continues until complete monomer conversion but at a slower rate. Thus systems with high reaction rates, both during UV irradiation and during the first seconds of the dark period, are of most importance for UV-curable printing technology. The most widely used diepoxide monomer in cationic ink formulation is 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (DECC). In order to improve its reactivity during the first seconds of dark curing, the influence of several parameters was examined, such as the addition of an oxetane co-monomer (TMPO), the influence of relative humidity, UV dose, light intensity, temperature, and film thickness. The enhanced reactivity of DECC with TMPO was confirmed. During the cationic curing process, two reaction mechanisms compete; an activated chain end (ACE) mechanism and an activated monomer (AM) mechanism. It was observed that these two processes depend on the presence of hydroxyl containing compounds such as TMPO monomer, polyol compounds, and, most important, water. When the concentration of these components is increased the AM mechanism is enhanced, leading to higher conversion but at a slower curing rate, and a degradation of the mechanical properties. The presence of water is unavoidable and its concentration in the ink varies in presence of hydrophilic compounds, such as polyol. Furthermore, the water content increases with increasing relative humidity in air. It was shown that the reactivity of the system decreased as the percent of relative humidity increased. Moreover, although a higher overall conversion is achieved, the crosslink density of the film was severely reduced. This lower crosslink density was the result of the predominance of chain transfer reactions, leading to more pendant and short chains which were hydroxyl terminated. Moreover, the extent of conversion was thickness dependent since less water diffuses into the deeper layers occurred in thicker films.

EPFL2007