Chemorheological Changes Research Articles

Infusion Simulation of Graphene-Enhanced Resin in LCM for Thermal and Chemo-Rheological Analysis.

The present numerical study proposes a framework to determine the heat flow parameters-specific heat and thermal conductivity-of resin-graphene nanoplatelets (GNPs) (modified) as well as non-modified resin (with no GNPs). This is performed by evaluating the exothermic reaction which occurs during both the filling and post-filling stages of Liquid Composite Moulding (LCM). The proposed model uses ANSYS Fluent to solve the Stokes-Brinkman (momentum and mass), energy, and chemical species conservation equations to a describe nano-filled resin infusion, chemo-rheological changes, and heat release/transfer simultaneously on a Representative Volume Element (RVE). The transient Volume-of-Fluid (VOF) method is employed to track free-surface propagation (resin-air interface) throughout the computational domain. A User-Defined Function (UDF) is developed together with a User-Defined Scaler (UDS) to incorporate the heat generation (polymerisation), which is added as an extra source term into the energy equation. A separate UDF is used to capture intra-tow (microscopic) flow by adding a source term into the momentum equation. The numerical findings indicate that the incorporation of GNPs can accelerate the curing of the resin system due to the high thermal conductivity of the nanofiller. Furthermore, the model proves its capability in predicting the specific heat and thermal conductivity of the modified and non-modified resin systems utilising the computed heat of reaction data. The analysis shows an increase of ∼15% in the specific heat and thermal conductivity due to different mould temperatures applied (110-170 °C). This, furthermore, stresses the fact that the addition of GNPs (0.2 wt.%) improves the resin-specific heat by 3.68% and thermal conductivity by 58% in comparison to the non-modified thermoset resin. The numerical findings show a satisfactory agreement with and in the range of experimental data available in the literature.

Carbon fibres reinforced composites. Electrical impedance analysis: a gateway to smartness

ABSTRACT For more than 7 years, our research work is focussed upon the electrical behavior of carbon fiber reinforced composites. Effectively carbon/polymeric matrix laminates can be considered as an electrically conductive network (i.e. carbon fibers) embedded in an insulating medium (i.e. the polymeric matrix). Consequently, from an electrical point of view laminated composites have been modelled owing to combinations of electrical resistances (i.e. fibers themselves and the contact points between the fibers) and capacitances (i.e. polymeric matrix). During their manufacturing, composite material such as thermoset matrix composites for instance, undergo various changes in their physical (including electrical and mechanical) properties and in their geometrical characteristics. In fact, in the case of manufacturing processes such as oven curing (vacuum bag), autoclave curing or heating plate press, beside the chemo-rheological changes, the laminated parts are the place of a compaction phenomenon inducing changes in fiber volume fraction and thicknesses. The idea developed in this paper is that since the carbon/polymeric matrix laminate can be considered as an electrical network, it can then be view and used as a sensor for following the material state during the manufacturing process enabling thus defects to be detected and this without any additional sensing material or device.

Time–temperature-transformation (TTT) and temperature–conversion-transformation (TxT) cure diagrams by RheoDSC: Combined rheometry and calorimetry on an epoxy-amine thermoset

A novel hyphenated RheoDSC combines rheological and calorimetric analyses of samples by performing rheological experiments inside a DSC cell. This enables direct studies of the chemorheological changes of epoxy-amine thermosets during crosslinking. The rheological and calorimetric information can be presented in time–temperature-transformation (TTT) cure diagrams, without the need for making interpretations based on experiments performed on different samples in the different thermal environments of individual rheometers and calorimeters.

Dynamic inflation of elastomeric spherical membranes undergoing time dependent chemorheological changes in microstructure

When an elastomeric object is subjected to loads while at a sufficiently high temperature, it deforms and undergoes changes in its macromolecular microstructure consisting of time dependent scission of macromolecular network junctions, recoiling of the affected molecules and crosslinking to form new networks with new reference configurations. A constitutive theory has been developed in previous work that accounts for the interaction of deformation and chemorheological changes. This constitutive theory is used to study the dynamical response of a spherical elastomeric membrane under internal pressure at a temperature high enough for the scission/cross-linking process to occur. Quasi-static response at a constant pressure and temperature, studied in previous work, showed that, the membrane radius increases with time due to creep that results from the scission. There can be a finite time when the membrane diameter undergoes a very rapid, or even infinite, rate of increase. This event depends on the properties of the molecular networks, the rate of formation of new networks, and the history of the increase in the membrane diameter. This present work investigates this effect when membrane inertia is included. It is found that the inclusion of inertia eliminates the sudden rapid increases in radius. The motion is also shown to depend on a characteristic time for scission relative to a characteristic time that represents the effects of inertia. Numerical results are presented for the case when the networks act as Mooney–Rivlin materials.

Novel phosphorus-modified polysulfone as a combined flame retardant and toughness modifier for epoxy resins

A novel phosphorus-modified polysulfone (P-PSu) was employed as a combined toughness modifier and a source of flame retardancy for a DGEBA/DDS thermosetting system. In comparison to the results of a commercially available polysulfone (PSu), commonly used as a toughness modifier, the chemorheological changes during curing measured by means of temperature-modulated DSC revealed an earlier occurrence of mobility restrictions in the P-PSu-modified epoxy. A higher viscosity and secondary epoxy-modifier reactions induced a sooner vitrification of the reacting mixture; effects that effectively prevented any phase separation and morphology development in the resulting material during cure. Thus, only about a 20% increase in fracture toughness was observed in the epoxy modified with 20 wt.% of P-PSu, cured under standard conditions at 180 °C for 2 h. Blends of the phosphorus-modified and the standard polysulfone (PSu) were also prepared in various mixing ratios and were used to modify the same thermosetting system. Again, no evidence for phase separation of the P-PSu was found in the epoxy modified with the P-PSu/PSu blends cured under the selected experimental conditions. The particular microstructures formed upon curing these novel materials are attributed to a separation of PSu from a miscible P-PSu–epoxy mixture. Nevertheless, the blends of P-PSu/PSu were found to be effective toughness/flame retardancy enhancers owing to the simultaneous microstructure development and polymer interpenetration.

Influence of thermally induced chemorheological changes on the torsion of elastomeric circular cylinders

When an elastomeric material is deformed and subjected to temperatures above some characteristic value T cr (near 100∘C for natural rubber), its macromolecular structure undergoes time and temperature-dependent chemical changes. The process continues until the temperature decreases below T cr. Compared to the virgin material, the new material system has modified properties (reduced stiffness) and permanent set on removal of the applied load. A new constitutive theory is used to study the influence of the changes of macromolecular structure on the torsion of an initially homogenous elastomeric cylinder. The cylinder is held at its initial length and given a fixed twist while at a temperature below T cr. The twist is then held fixed and the temperature of the outer radial surface is increased above T cr for a period of time and then returned to its original value. Assuming radial heat conduction, each material element undergoes a different chemical change. After enough time has elapsed such that the temperature field is again uniform and at its initial value, the cylinder properties are now inhomogeneous. Expressions for the time variation of the twisting moment and axial force are determined, and related to assumptions about material properties. Assuming the elastomeric networks to act as Mooney-Rivlin materials, expressions are developed for the permanent twist on release of torque, residual stress, and the new torsional stiffness in terms of the kinetics of the chemical changes.

Influence of Thermally Induced Chemorheological Changes on the Inflation of Spherical Elastomeric Membranes

When an elastomeric material is deformed and subjected to temperatures above some chemorheological value Tcr (near 100°C for natural rubber), its macromolecular structure undergoes time and temperature dependent chemical changes. The process continues until the temperature decreases below Tcr. Compared to the virgin material, the new material system has modified properties (often a reduced stiffness) and permanent set on removal of the applied load. A recently proposed constitutive theory is used to study the influence of chemorheological changes on the inflation of an initially isotropic spherical rubber membrane. The membrane is inflated while at a temperature below Tcr. We then look at the pressure response assuming the sphere's radius is held fixed while the temperature is increased above Tcr for a period of time and then returned to its original value. The inflation pressure during this process is expressed in terms of the temperature, representing entropic stiffening of the elastomer, and a time dependent property that represents the kinetics of the chemorheological change in the elastomer. When the membrane has been returned to its original temperature, it is shown to have a permanent set and a modified pressure-inflated radius relation. Their dependence on the initial inflated radius, material properties and kinetics of chemorheological change is studied when the underlying elastomeric networks are neo-Hookean or Mooney–Rivlin.

The kinetics and mechanism of cure of an amino-glycidyl epoxy resin by a co-anhydride as studied by FT-Raman spectroscopy

The thermal curing behavior of tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and a co-anhydride mixture consisting of maleic anhydride (MA) and hexahydrophthalic anhydride (HHPA) was studied from 55 to 100 °C by real-time FT-Raman spectroscopy. The quantitative changes in concentrations of anhydride, epoxy, and new-formed ester were measured and empirical reaction rate curves were constructed reflecting the kinetics of the curing process. After an induction period a simple kinetic scheme that is first order in both epoxy and anhydride monomer consumption described the reaction profile until the reaction was influenced by chemo-rheological changes due to vitrification transition. FT-Raman analysis revealed that curing propagation mainly occurs by polyesterification between epoxide and anhydride. Possible side reactions including the homopolymerization of MA are considered. The main side reaction is decarboxylation of MA that may produces some autocatalysis, but this is a minor contribution to the kinetics of cure. No conclusive evidence has been found for homopolymerization of MA or initiation of the curing reaction by the reaction product of TGDDM and MA, compared to the polyesterification.

Rheokinetic studies for the reaction injection molding of polydicyclopentadiene

AbstractDicyclopentadiene (DCPD) based liquid engineered resins are emerging among the newest commercial reaction injection molding (RIM) systems. Since the filling stage is coupled with chemical reaction, an understanding of the chemorheological changes is critical for establishing moldability criteria. An adiabatic reactive viscometry technique which involves the simultaneous measurement of viscosity and temperature changes was used to investigate the chemorheological changes during the adiabatic copolymerization of a DCPD based RIM system. The influence of impingement mixing conditions and initial material temperature on the reaction kinetics, rheology, as well as the resulting final physical properties of the system were investigated. The “apparent gel time” under adiabatic conditions is estimated from the intersection of the tangents to the initial and final portions of the viscosity curve. An autocatalytic kinetic model with an Arrhenius rate constant was used to model the reaction kinetics.

Chemorheology of unfilled and filled epoxy resins

AbstractFilled thermosetting resins used for adhesives, sealants, and coatings are two‐phase systems whose rheological behavior is determined by the properties of both phases as well as by the interactions between the matrix and the filler. The chemorheological changes during the crosslinking of an epoxy system (EPON 828 with curing agent Y from Shell Chemical Co.) were investigated. The influence of an untreated silica filler on the reaction kinetics and the Rheological behavior of the system were also analyzed. The concentration of the filler was varied between 0 and 17.25 vol%. The influence of the filler on the gel point was explained in terms of the accelerating effect of the filler on the reaction kinetics.

Monitoring the composite curing process with a fluorescence‐based fiber‐optic sensor

AbstractA novel cure sensor, based on the combination of fiber‐optic fluorometry and the viscosity/degree‐of‐cure dependence of the epoxy resin fluorescence, was developed to provide a reliable low‐cost cure‐monitoring sensor for control of composite manufacturing. The capability of the first‐generation sensor to monitor chemorheological changes that occur during the autoclave cure of carbon‐epoxy laminates was successfully demonstrated. An improved second‐generation sensor, which simultaneously monitors the changes in the epoxy fluorescence intensity and the wavelength of maximum emission, was also developed. The changes in the sensor intensity and wavelength signals, as a function of cure time, provide characteristic profiles that reveal the main chemorheological events of the cure cycle; these signals follow the changes in the degree‐of‐cure to completion. The configuration of the optrode‐laminate interface strongly affects the signal profiles and their reproducibility. A “resin cavity” optrode‐laminate interface, which improves reproducibility, was developed and tested.

Chemorheology of crosslinked epoxy resins subjected to large strains

AbstractThe previously reported work on chemorheological changes occurring in two epoxy resins when aged under large strains at temperatures above their Tg was extended. Ageing times up to 100 h were applied and the phenomenon of orientation occurring in these samples was investigated, mainly by measurements of their birefringence index. It was found that the orientation of the air‐aged samples, when studied at room temperature, depended on the ageing conditions (strain, temperature, time) and was not uniform throughout the bulk of the samples, being higher in two narrow external layers and lower in the core layer. After reheating oriented samples in air without strains for different lengths of time at temperatures above Tg, disappearance of the orientation in the external layers was observed. This is probably due to oxidation and degradation occurring in the network and scission of part of the network links, mainly those strained due to orientation. The decrease of orientation, especially in the inner layers of the polymer samples, was much slower when the reheating of the oriented‐ and unstrained samples took place in an inert atmosphere, this showing again the great influence of oxygen when it participates in ageing and heating processes of epoxy resins.

Chemorheological changes in crosslinked epoxy resins subjected to large strains

AbstractChemorheological changes in epoxy resins subjected to large strains while heated were observed earlier, but the subject was not treated quantitatively. This paper describes the investigation of an epoxy resin—Epon 826, of known chemical, structure, crosslinked with two different amines. By means of a simple calibrated apparatus, the epoxide samples were subjected to large strains in a non‐oxidizing atmosphere and in air. They were heated to different temperatures for various times; the results were compared with data obtained from unstrained samples kept in otherwise the same conditions. The results show a linear‐logarithmic relation between the torsion modulus, G(10) and the time of applied strain at a certain temperature and also a linear relation between G(10), and 1/T°K, both above T(10). The results obtained in swelling experiments support the data from 10 sec torsional modulus vs temperature measurements. An increase in the amount of solubles and in Mc is observed on extending the time of heating. A clear difference in properties between the strained and unstrained samples, kept in otherwise the sane conditions, is observed and the contribution of the applied strain to chemorheology has been shown. Both systems of crosslinked Epon 826 showed the same general behavior, although the specific data were different.