Quasi‐isothermal DSC testing of epoxy adhesives using initial fast heating rates

Polyetheramine (PEA)-modified epoxies with various types of PEAs were prepared and respective effects on characteristics of epoxy networks were studied. The used PEAs were polyethylene glycol diamine (PEG-amine) and polypropylene glycol diamine (PPGamine) with two different molecular weights (i.e., 200 and 400 g mol −1 ). According to mechanical tests, the structural parameters of PEAs played an important role in final properties of epoxy/amine systems. PEG 400 -amine and PPG 200 -amine had the highest and lowest effects on the properties of epoxy networks, respectively. Whereas 10 phr PEG 400 -amine increased critical stress intensity factor (K IC ) and critical strain energy release rate (G IC ) of the epoxy up to 82 and 294%, the same number of PPG 200 -amine chains caused to increase the K IC and G IC up to 11 and 34%. This discrepancy could be assigned to higher flexibility index (φ = 26.22), longer chain length (~27 atoms), and higher secondary interactions [δ = 9.69 (cal cm −3 ) 0.5 ] of PEG 400 -amine in comparison with PPG 200 -amine [with φ = 8.08,~10 atoms in chain, and δ = 8.98 (cal cm −3 ) 0.5 ]. Shear yielding as a toughening mechanism was proposed based on microscopy of the crack tips. These in-depth studies could uncover underlying structure-property relationships in a relevant class of PEA-like modifiers, shedding light on the future design of top-performing homogeneous tough polymer networks.

Section: Introductionmentioning

confidence: 99%

Systematic investigation of mechanical properties and fracture toughness of epoxy networks: Role of the polyetheramine structural parameters

Abdollahi

Salimi

Barikani

et al. 2018

“…The same equation was valid to model the data with initial slow and fast ramp. The reader is encouraged to review a complete analysis of the fast ramp method reported in another article …”

Section: Resultsmentioning

confidence: 99%

TTT‐diagram for epoxy film adhesives using quasi‐isothermal scans with initial fast ramps

Puentes

Chaloupka

Rudolph

et al. 2017

Self Cite

The 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 ⇉ B, 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.

“…33 Isothermal DSC scans may be impaired by problems in defining a correct base line, which led to specific test methods containing a comprehensive combination of dynamical and isothermal tests. 37,38 The determination of partial DOC and T g by DSC scans is more flexible than mechanical tests like DMA as less effort for sample preparation is required. Furthermore, the DSC signal is more sensitive to minor changes of the thermodynamic properties like aging effects.…”

Section: Reaction Kinetic Model and Phase Transitionsmentioning

confidence: 99%

Aspects of reproducibility and stability for partial cure of epoxy matrix resin

Müller

Winkler

Gude

et al. 2019

Partial cure of thermosets is a promising approach to enhance manufacturing possibilities of reinforced and unreinforced polymers. If partial cure is taken into consideration as a genuine process parameter, novel manufacturing technologies can be developed by exploiting the specific properties of incomplete polymer networks. A main concern in this context is to control the kinetic reaction avoiding inhomogeneous or instable degrees of cure. Based on a combination of numerical simulations and experiments, a methodology is presented that enables a systematic assessment of the reproducibility and stability of partial cure. Special attention is paid to the interaction of thermal boundary conditions and the cure kinetic of thick samples as well as the storability of partially cured resin under different conditions. Guidelines for cure cycle selection, mold design, and storage are derived. The possibility to use complex multistep cure schedules and extended storage periods is demonstrated for an unmodified noninhibited epoxy resin.