Dirk Dijkstra

Adhesive Interactions of Polyurethane Monomers with Native Metal Surfaces

Understanding of fundamental adhesion is a key issue for bonding, polymer composites, and coatings. In this study, diluted solutions of polyurethane monomers (methylene diphenyl diisocyanate isomer mixture—MDI, polypropylene ether triol—PPET) are exposed to the native surfaces of Au, Al, and Cu. The adsorption state of the molecules (from multi- to sub-monolayers) is investigated in situ by infrared external reflection absorption spectroscopy (IR-ERAS). A sequence of solvent rinsing steps helps to distinguish physisorbed from chemisorbed molecules. PPET sticks very weakly to the metals. The spectra reveal a bulk-like state. MDI adsorbs weakly on Au but the spectra differ significantly from the bulk state. On Al and Cu, MDI establishes strong adhesion. Rich new spectral features indicate a probably chemical nature of adhesion, and a specific structure in the neighboring adsorbate layer. However, detailed final conclusions on the adhesion mechanisms must not be derived without appropriate quantum mechanical modeling.

Guidelines for checking performance and verifying accuracy of rotational rheometers: viscosity measurements in steady and oscillatory shear (IUPAC Technical Report)

Abstract The paper addresses techniques for checking the performance of rotational rheometers with cone–plate, plate–plate, or concentric cylinder geometry. We focus on the determination of the viscosity as a function of the shear rate and of the magnitude of the complex viscosity as a function of the angular frequency. After summarizing the relevant definitions and test modes, we show examples of measurements in the linear viscoelastic range, and applications of the Cox–Merz relationship. Sources of reference fluids with defined viscosities are presented, and their use in tests for verification of accuracy is demonstrated. Relevant issues, predominantly for Newtonian reference liquids, are the exploration of measurement limits, related either to the shear rate range or to reliably accessible viscosity levels. Viscoelastic reference samples are also discussed. Prerequisites for sample preparation and loading are addressed. In particular, we present recommendations based on experience from various laboratories. Finally, we discuss the problem of temperature calibration, presenting techniques that allow the determination of the true sample temperature for a given set temperature of the rheometer. This paper summarizes contributions from various industrial and academic laboratories.

Liquid composite molding-processing and characterization of fiber-reinforced composites modified with carbon nanotubes

The increasing demand in fiber-reinforced plastics (FRPs) necessitates economic processing of high quality, like the vacuum-assisted resin transfer molding (VARTM) process. FRPs exhibit excellent in-plane properties but weaknesses in off-plane direction. The addition of nanofillers into the resinous matrix phase embodies a promising approach due to benefits of the nano-scaled size of the filler, especially its high surface and interface areas. Carbon nanotubes (CNTs) are preferable candidates for resin modification in regard of their excellent mechanical properties and high aspect ratios. However, especially the high aspect ratios give rise to withholding or filtering by fibrous fabrics during the impregnation process, i.e. length dependent withholding of tubes (short tubes pass through the fabric, while long tubes are restrained) and a decrease in the local CNT content in the laminate along the flow path can occur. In this study, hybrid composites containing endless glass fiber reinforcement and surface functionalized CNTs dispersed in the matrix phase were produced by VARTM. New methodologies for the quantification of the filtering of CNTs were developed and applied to test laminates. As a first step, a method to analyze the CNT length distribution before and after injection was established for thermosetting composites to characterize length dependent withholding of nanotubes. The used glass fiber fabric showed no perceptible length dependent retaining of CNTs. Afterward, the resulting test laminates were examined by Raman spectroscopy and compared to reference samples of known CNT content. This Raman based technique was developed further to assess the quality of the impregnation process and to quantitatively follow the local CNT content along the injection flow in cured composites. A local decline in CNT content of approx. 20% was observed. These methodologies allow for the quality control of the filler content and size-distribution in CNT based hybrid composites.