Open Access Artikel verfügbar: Tack of epoxy resin films for aerospace-grade prepregs: Influence of resin formulation, B-staging and toughening

Aerospace-grade prepreg resin films based on multifunctional tetraglycidyl-4,4′-methylenedianiline (TGMDA), triglycidyl p-aminophenol (TGAP), Bisphenol A diglycidyl ether (DGEBA) and curing agent 4,4’diaminodiphenyl sulfone (DDS) are investigated in terms of tackiness by probe testing. The model epoxy systems are modified regarding the thermoplastic toughener content (polyethersulfone, PES) and the B-stage level, which is adjusted by cure prediction based on a model-free isoconversional method (Flynn-Wall-Ozawa). Additional DSC and rheological analysis are performed to study the thermal and viscoelastic material behavior in conjunction to its impact on temperature-dependent tack. Maximum achievable tack is found to decrease as a function of both degree of conversion and toughener content. Meanwhile, both influencing factors shift the tack maximum towards higher temperatures corresponding to increased flow characteristics attributed to evolving network formation and the incorporation of high molecular weight PES. In terms of absolute tack level and corresponding temperature, probe tack values similar to commercial prepreg systems (∼100 μJ mm2) are recorded for TGMDA-based formulations containing 10 wt% PES at 20% pre-cure. Model formulations, which have neither been exposed to B-staging nor toughened, show exceptionally high tack below room temperature for all investigated epoxy prepolymers and are therefore not considered processable by automated fiber placement.

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Open Access Artikel verfügbar: Adhesion-cohesion balance of prepreg tack in thermoset automated fiber placement. Part 1: Adhesion and surface wetting

The constitution of prepreg tack in automated fiber placement (AFP) is affected by a sensitive balance between adhesive interfacial bond strength and cohesive strength of the prepreg resin. In an effort to explore the role of interfacial liquid-solid interaction on the tack of commercial aerospace-grade epoxy prepreg, a surface wetting analysis was performed on AFP-related substrates. The standard test liquid combination water/diiodmethane and extracted neat epoxy resin were used for contact angle measurement employing the sessile drop method and the Owens-Wendt-Rabel-Kaelble (OWRK) model. Additional rheological and topographical analyses were carried out to account for viscous resin flow on surfaces of different roughness. The results from the material characterization are discussed against the background of tack measurement by probe tack testing utilizing a rheometer. Significant differences between the investigated surfaces in terms of both the maximum tack level and the onset temperatures of adhesion were found as a function of test parameters relevant for contact formation. General agreement with the experimental tack results was observed employing a topographically extended version of the Dahlquist criterion. For each substrate, a temperature-dependent critical storage modulus could be determined that conforms to the onset temperature of tackiness. Contact angle measurements revealed a correlation between the thermodynamic work of adhesion and maximum tack and, moreover, the tack onset in the adhesive regime when additionally incorporating surface topography. Matching ratios of polar and dispersive surface free energy and surface tension components were found to favor the molecular interaction at the interface between prepreg resin and substrate.

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Open-Acces-Artikel über Prepreg Tack erschienen: A review of mechanisms, measurement, and manufacturing implication

The stickiness of prepregs (tack) is considered a decisive material property for the success of high‐quality composite manufacturing by automated lay‐up processes such as automated fiber placement (AFP) or automated tape laying (ATL). Adverse control of prepreg tack can easily result in laminate defects or machine breakdown, which are highly undesirable considering the tremendous machinery and material costs of these processes. Prepreg tack is governed by a complex interaction of adhesive and cohesive phenomena that are influenced by machine and environmental parameters of the production process as well as by intrinsic properties of the prepreg material itself. This review aims at providing a condensed insight into the current state of research on prepreg tack. Therefore, experimental studies including the discussion of utilized tack measurement methods as well as model approaches to prepreg tack are reviewed. The findings are discussed against the background of fundamental mechanisms, the strong interdependency of influencing parameters and the challenge of translating measured tack data into an enhanced AFP/ATL process stability by process adjustment. 

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HPCFK stellt auf dem 23. Int. Dresdner Leichtbausymposium aus

Vom 27. bis 28. Juni 2019 fand in Dresden das 23. Leichtbausymposium statt. Alexander Herwig stellte in einem Vortrag “Konzepte zur Lasteinleitung in Faserverbundstrukturen für Hochleistungsanwendungen” vor. Darüber hinaus präsentierte unsere Forschungskooperation den aktuellen Forschungsstand unseres Multilayer-Inserts auf dem Gemeinschaftsstand des SPP 1712

Weitere Impressionen zum Dresdner Leichtbausymposium finden Sie auf unserem Twitter-Account.

Neues Youtube-Video: Pneumatisch aktuiertes Festkörpergelenk in Multi-Matrix-Bauweise

Der Material-Mix des Aktuators basiert auf einem Ansatz, der in unserem kürzlich erfolgreich abgeschossenen Projekt Multi-Matrix-Prepreg verfolgt wurde: Durch die gezielte Wahl unterschiedlicher Matrixwerkstoffe (Duroplast und Elastomer) und deren Anteile lassen sich für faserverstärkte Bauteile die Eigenschaften wie Impact-Resistenz, Rissfortschrittswiderstand, Schweißbarkeit, Steifigkeiten und Festigkeiten lokal beeinflussen.

Das aktuierte Festkörpergelenk macht sich diesen Ansatz zunutze, indem es eine durchgehende duroplastische Kohlenstofffaserstruktur aufweist, die im Zentrum durch den lokalen Einsatz einer Elastomermatrix unterbrochen wird. Dadurch wirkt dieser Bereich biegeelastisch. Mit Hilfe der strukturinhärenten Aktuierung durch fluidgefüllte Druckkammern lässt sich die Struktur gezielt knicken.

Einen Eindruck des Festkörpergelenks in Aktion vermittelt unser neustes Youtube-Video. Wir danken der Volkswagenstiftung für die Förderung des Vorhabens Multi-Matrix-Prepreg.

David Christian Berg promoviert mit Auszeichnung

Am 11.02.2019 verteidigte David Christian Berg, ehemaliger Mitarbeiter unserer Forschergruppe, erfolgreich seine Dissertation mit dem Titel “An Innovative Approach for Simultaneous Measurement of Cure Shrinkage and Thermal Expansion of Reactive Liquids”. Herr Berg entwickelte im Rahmen seiner Promotion ein Messsystem, das es ermöglicht, die aushärtungsbedingte Schwindung von reaktiven Flüssigkeiten bei gleichzeitiger Messung der Wärmeausdehnung zu erfassen.

Wir gratulieren ganz herzlich zu diesem Erfolg!

Kevin Engel verteidigt Dissertation erfolgreich

Am 17.12.2018 hat Kevin Engel, ehemaliger Mitarbeiter der Forschergruppe HPCFK, die Prüfung zum Dr.-Ing. bestanden.

Im Rahmen seiner wissenschaftlichen Tätigkeit beschäftigte er sich vorwiegend mit der Analyse des Einflusses von Fertigungsfehlern in Faserkunststoffverbunden und entwickelte Strategien, diese bereits in die Bauteilauslegung miteinzubeziehen. Die Dissertation trägt den Titel „Berücksichtigung prozessinduzierter Effekte in der Auslegung von Faser-Kunststoff-Verbunden“.

Wir gratulieren herzlich zu diesem Erfolg!