Stefan Michel

Surface tension studies of levelling additives in powder coatings

Abstract In powder coatings, melt surface tension plays a decisive role in the film formation. The surface tension is controlled and adjusted by adding levelling additives to powder coating formulations. The influence of additive properties like molecular weight and chemical structures (homopolymeric and copolymeric polyacrylates and polyestermodified polysiloxanes) on surface tension of a typical powder coating binder (DER 664 UE, an epoxy resin) is studied by means of axisymmetric drop shape analysis in a temperature range from 138 to 184°C. The high accuracy of the method allows small differences in surface tension values to be distinguished. It is shown that all additives considerably decrease the surface tension of the epoxy resin. In addition, notable differences in the effect of the two groups of additives (acrylates and siloxanes) were found. Consequently, differences are expected in levelling and wetting of powder coatings formulated with polyacrylate or polysiloxane additives. Moreover, viscosity measurements have been performed in order to investigate whether these additives also affect the viscosity of the epoxy resin.

A new method for the simultaneous determination of surface tension and density of polymer melts

By employing a new strategy presented recently by Wulf etal., axisymmetric drop shape analysis (ADSA) can be used to determine simultaneously the surface tension and the density of polymer melts from sessile drops at elevated temperatures. This required the modification of the ADSA algorithm to replace the density by the mass of the drop as input parameter and the development of a closed high temperature chamber whose temperature can be precisely controlled. In addition, special sample holders for the formation of pendant and sessile drops at elevated temperatures were needed. Recently, their design has been improved, which is described in this paper. For a commercial epoxy resin (DER 664 UE), it is shown that measurements with sessile drops yield essentially the same surface tension values and temperature coefficients as with pendant drops. The densities determined with ADSA are comparable to independent PVT results.

Surface tension studies of additives in acrylic resin-based powder coatings using the Wilhelmy balance technique

A modified Wilhelmy balance technique using thin fibers as solid probes has been applied to study the effect of silicone additives in acrylic resin-based powder coatings on the surface tension of non-reactive binder systems. By measuring the temperature dependence of the surface tension in the temperature range between 140 and 180C, it could be shown that the silicone additives investigated had a very different surface activity in the molten acrylic resin. Due to the high accuracy of the measuring technique and the good reproducibility of the experiments the influence of different additive concentrations on the surface tension was detectable even at very low concentrations (below 1 wt.%). Compared to the pure powder coating binder which has a surface tension of about 30 mN/m the values decreased between 2‐15 mN/m depending on the type of the silicone additive. In addition, the temperature coefficient (d /dT) of the surface tension of the binder melt was changed. In the case of two additives, the surface tension of the powder coating and its temperature coefficient were lowered considerably. This effect of additives is desirable to reach good wetting and leveling of the powder coating.

Influence of Chemical Interactions on the Macroscopic Spreading of a Maleic Anhydride Copolymer Melt

We report on wetting experiments of a maleic anhydride copolymer melt on smooth horizontal silicon wafers which were either cleaned or coated with a crosslinked network of a poly(aminosiloxane). The surface properties of the solid substrates have been controlled using contact angle and zeta potential measurements, FT- IR attenuated total reflection (ATR) spectroscopy, and atomic force microscopy. Compared to the bare silicon wafer surface, which had a surface free energy of 61 mJ.m -2 and a weakly acidic surface character due to silanol groups, the poly(aminosiloxane) layer is characterized by basic amino groups at the outermost surface (pH IEP > 9) and a lower surface free energy (γ sv = 47 mJ.m -2 ). The results of the wetting experiments indicate clearly that macroscopic spreading of the maleic anhydride copolymer melt can be influenced by strong interactions at the solid-liquid interface near the triple line. As could be shown by FT-IR microscopy and spectroscopic ellipsometry, an interfacial chemical reaction takes place between the amino groups available on the solid poly- (aminosiloxane) surface and the anhydride groups of the copolymer melt to form imide structures at the solid- liquid interface during wetting. Due to this interfacial chemical reaction, the spreading process of the maleic anhydride copolymer melt was slowed down and the contact angle was time-dependent indicating a non-equilibrium system. It is also remarkable that lower contact angles were obtained after a certain time of contact compared to the non-reactive system, though the surface free energy of the solid substrate was lower in the reactive system. We assume that the increase of the spreading force at a given spreading speed is due to the release of the free energy during the interfacial reaction, indicating that the chemical reaction is an additional driving force within the spreading process.

ADSA-TRIS: a new method to study interfacial phenomena at polymer–aqueous solution interfaces

AbstractAxisymmetric drop shape analysis-profile (ADSA-P) was combined with total reflectometric interference spectroscopy (TRIS) in one experimental setup to study the interfacial phenomena at solid–liquid and liquid–vapor interfaces caused by adsorption/desorption (dissolution) of surface-active substances. Using sessile liquid droplets on polymer film/chromium-coated glass substrates that were optically matched with an immersion oil to a TRIS reflection prism, the optical thickness (product of physical thickness d and refractive index n) of the polymer film can be estimated by evaluating the wavelength-dependent intensity of reflected light. The sessile droplet is analyzed simultaneously by an ADSA setup arranged in a transverse direction to the path of the white-light beam of TRIS. From this analysis, the solid–vapor interfacial tension γlv(t), contact angle θ(t), contact radius r(t), drop volume V(t), and solid–liquid interfacial tension γsl(t) can be monitored as a function of time. The new method was applied to study polystyrene and poly(4-hydroxystyrene) surfaces in contact with aqueous buffer solutions and with protein solutions. The time-dependent changes in the optical film thickness caused by the adsorption of human serum albumin (HSA) and lysozyme (LSZ) were accompanied by changes in the solid–liquid interfacial tension. From the detailed study of both parameters, conclusions can be drawn with regard to the adsorption kinetics of the proteins on the hydrophobic polystyrene surfaces and to conformational changes occurring within the adsorbed protein layers. FigurePhoto of the ADSA-TRIS setup

High-Strength, Strain-Hardening Cement-Based Composites (HS-SHCC) Made with Different High-Performance Polymer Fibers

This article presents an investigation on the tensile behavior of high-strength, strain-hardening cement-based composites (HS-SHCC) made with four different types of high-performance polymer microfibers. In particular, high-density polyethylene (HDPE), poly(p-phenylene-terephthalamide) (aramid), as-spun poly(p-phenylene-2,6-benzobisoxazole) (PBO), and high-modulus PBO fibers were examined in respect of their reinforcing effect in a high-strength, finely grained, cementitious matrix. Moreover, microscopic investigations were carried out to assess the fibers’ ability to be wetted and to explain their interaction with the cementitious matrix. It was shown that the HS-SHCC made with PBO and aramid fibers yielded increased first crack stress and tensile strength, but also a considerably reduced crack width compared to the HS-SHCC reinforced with HDPE fibers. This was traced back to the considerably higher wettability of these fibers compared to the hydrophobic HDPE fibers, ensuring a stronger interfacial bond with the cementitious matrix, but also to their superior mechanical properties, such as tensile strength and Young’s modulus.

Oscillating Streaming Potential and Electro-osmosis of Multilayer Membranes

Artificial membranes consist of multilayers that have different physical or chemical properties. They are often considered as an equivalent single-layer membrane without taking into account the detail inside. Based on the Debye−Huckel approximation, we have provided analytical solutions of oscillating electrokinetic flow in multilayer membranes. Both pressure-driven flow (streaming potential) and electric-field-driven flow (electro-osmosis) were studied. The pressure and electric-field distributions in each layer can be obtained from our analytical solutions. This allows a better understanding of electrokinetic flow in multilayer membranes and benefits the design and selection of artificial membranes. The derived analytical solutions are useful for more-general time-dependent problems through a superposition of time-harmonic solutions weighted by appropriate Fourier coefficients. The properties of each membrane layer are reflected as complex quantities; therefore, a method is proposed to determine the ele...