Extreme wet adhesion of a novel epoxy-amine coating on aluminium alloy 2024-T3

Amine-epoxy polymer systems are widely used, for example as matrix materials for structural composites employed in aerospace industry and in industrial coatings on metal substrates for corrosion protection. This work focuses on the investigation of different epoxy-amine coatings on the adhesion performance on aluminum AA-2024 substrates. Two different epoxies (Epikote 828 (aromatic) and Eponex 1510 (aliphatic)) and four different amines (1,8-diaminooctane, Dytek A, Jeffamine EDR148 and Jeffamine D230) as curing agent were used in different stoichiometric ratios. These different epoxy-amine coatings were characterized using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), tensile tests (pull-off) and water-uptake measurements. Pull-off tests in dry conditions showed comparable adhesion of the coatings. Surprisingly, pull-off results showed after water soaking a higher wet adhesion of the coatings prepared with Eponex 1510 as compared to coatings prepared with Epikote 828. Moreover, the combination of Eponex 1510-Jeffamine EDR148 coatings resulted in high adhesion values (~7 MPa) with pull-off tests and these values did not change after immersion for two weeks in water. This combination shows extreme good wet adhesion performance as compared to any other epoxy-amine coating. Complete recovery was demonstrated of the adhesion of Eponex 1510-Jeffamine D230 coating after being immersed for two weeks in water and dried for two weeks. Furthermore, in contrast with Epikote 828 water uptake measurements showed almost nil water uptake for all coatings prepared with Eponex 1510. Optical microscopy investigations on the residues of the coatings after pull-off tests revealed adhesive failure in wet condition for Epikote 828, while coatings prepared with Eponex 1510 showed cohesive failure

Single-Walled Carbon Nanotube Networks:The Influence of Individual Tube–Tube Contacts on the Large-Scale Conductivity of Polymer Composites

\u3cp\u3eOver two decades after carbon nanotubes started to attract interest for their seemingly huge prospects, their electrical properties are far from being used to the maximum potential. Composite materials based on carbon nanotubes still have conductivities several orders of magnitude below those of the tubes themselves. This study aims at understanding the reason for these limitations and the possibilities to overcome them. Based on and validated by real single-walled carbon nanotube (SWCNT) networks, a simple model is developed, which can bridge the gap between macroscale and nanoscale down to individual tube–tube contacts. The model is used to calculate the electrical properties of the SWCNT networks, both as-prepared and impregnated with an epoxy-amine polymer. The experimental results show that the polymer has a small effect on the large-scale network resistance. From the model results it is concluded that the main contribution to the conductivity of the network results from direct contacts, and that in their presence tunneling contacts contribute insignificantly to the conductivity. Preparing highly conductive polymer composites is only possible if the number of direct, low-resistance contacts in the network is sufficiently large and therefore these direct contacts play the key role.\u3c/p\u3

NMR imaging of water uptake in multilayer polymeric films : stressing the role of mechanical stress

The penetration of water into two-layer polymeric films of a hydrophilic base layer and hydrophobic top layer plays an important role in their performance. Little is known about the coupled effects of water uptake and stress in such films. To study such interactive phenomena, time-dependent distributions in the film are needed, which cannot be provided by traditional techniques. In this study, high-resolution NMR imaging was used to measure water profiles, showing that applied stress increased both the uptake rate and the amount of absorbed water. A model was formulated to describe the process, using the barrier properties of and the chemical potential differences over the top coat. On the basis of this, the diffusivity in the top layer and stress contribution were estimated. The results show that external stresses significantly influence water penetration into multilayer films

Tip-enhanced Raman mapping of single-walled carbon nanotube networks in conductive composite materials

\u3cp\u3eIdentifying and characterizing the structural integrity of single-walled carbon nanotubes (SWCNTs) that are fully embedded in a polymer matrix without causing any damage to them is a difficult task to achieve for bulk samples. Using tip-enhanced Raman spectroscopy, the surface of a polymer-embedded conductive network of SWCNTs was mapped underneath a thin layer of pure polymer. The technique was also used to detect tube-breaking within the composite sample caused by mechanical stress, beyond the 'visual' capabilities of scanning electron microscopy techniques. Results show that tip-enhanced Raman mapping can be used to successfully identify and characterize SWCNTs even underneath a layer of polymer.\u3c/p\u3

Magnetic resonance imaging of the drying and wetting process of coatings on wood

As a result of the VOC regulations, a transition towards waterborne coatings has been made. Generally, waterborne coatings are intrinsically more sensitive to water. This higher sensitivity to water may negatively affect the durability of wood, due to microbiological growth. To prevent this, knowledge on the barrier properties of coatings is needed. Our studies focus on understanding the moisture transport through alkyd (solvent and waterborne) and acrylic model coating formulations applied on teak, oak and pine. One of the key research question focuses on the importance of the interface on the barrier properties (both wetting and drying) of the coating, e.g. due to grains raising and penetration. To answer this question, Magnetic Resonance Imaging on both the uncoated and the coated wood is applied. MRI signal relaxation analysis (T2) unravels whether water is free in the lumen or bound to the cell wall. Our results show that the moisture permeability depends on the specific combination of wood and coating, since the coating influences the moisture sorption of wood in different ways. For example, on pine sapwood, the acrylic coating increases the total drying time seven times compared to uncoated wood by sealing the surface

Global and local conductivity in percolating crosslinked carbon black/epoxy–amine composites

\u3cp\u3eConductive particle-filled polymer composites are promising materials for applications where both the merits of polymer and conductivity are required. The electrical properties of such composites are controlled by the particle percolation network present in the polymeric matrix. In this study, the electrical properties of crosslinked carbon black–epoxy–amine (CB-EA) composites with various CB concentrations are studied at room temperature as a function of the AC frequency f. A transition at critical frequency f\u3csub\u3ec\u3c/sub\u3e from the DC plateau σ\u3csub\u3eDC\u3c/sub\u3e to a frequency-dependent part was observed. Conductivity mechanisms for f > f\u3csub\u3ec\u3c/sub\u3e and f < f\u3csub\u3ec\u3c/sub\u3e were investigated. By considering the fractal nature, conduction for f > f\u3csub\u3ec\u3c/sub\u3e was verified to be intra-cluster charge diffusion. For f < f\u3csub\u3ec\u3c/sub\u3e, with the assistance of conductive atomic force microscopy (C-AFM), the conduction behavior of individual clusters can be observed, revealing both linear and nonlinear I–V characteristics. By combining microtoming and C-AFM measurements, 3D reconstructed images offer direct evidence that the percolating network of these materials consists of both a low-conductivity part, in which the charge transports through tunneling, and a high-conductivity part, which shows ohmic electrical properties. Nevertheless, for these CB-EA composites, the presence of these non-ohmic contacts still leads to Arrhenius-type behavior for the macroscopic conductivity.\u3c/p\u3

Bound and free water distribution in wood during water uptake and drying as measured by 1D magnetic resonance imaging

Knowledge on moisture transport in wood is important for understanding its utilization, durability and product quality. Moisture transport processes in wood can be studied by Nuclear Magnetic Resonance (NMR) imaging. By combining NMR imaging with relaxometry, the state of water within wood can be identified, i.e. water bound to the cell wall, and free water in the cell lumen/vessel. This paper presents how the transport of water can be monitored and quantified in terms of bound and free water during water uptake and drying. Three types of wood from softwood to hardwood were selected covering a range of low to high density wood; pine sapwood and oak and teak. A calibration is performed to determine the different water states in each different wood type and to convert the NMR signal into moisture content. For all wood types, water transport appeared to be internally limited during both uptake and drying. In case of water uptake, free water was observed only after the cell walls were saturated with bound water. In case of drying, the loss of bound water starts only after vanishing of free water, irrespective of the position. Obviously, there is always a local thermodynamic equilibrium of bound and free water for both uptake and drying. Finally, we determined the effective diffusion coefficient (Deff). Experimentally determined diffusion constants were compared with those derived by the diffusion models for conceptual understanding of transport mechanism. We found that diffusion in the cell wall fibers plays a critical role in the transport process

On the action of hydrotalcite-like clay materials as stabilizers in polyvinylchloride

\u3cp\u3eHydrotalcites with different counterions were tested with respect to their ability to act as heat stabilizers in PVC. The stabilization activity results from the capacity of the layered double hydroxides (LDHs) to react with the HCl formed during degradation of PVC. The experiments described are heat stability tests and experiments for a determination of the capacity of the hydrotalcites to react with HCl gas. They show a linear relationship between HCl capacity of LDHs with different counterions and their action as (heat) stabilizer. The reaction between the LDHs and the HCl occurs in a two-step process, firstly (if possible), the counterions between the LDH layers tend to react with the HCl gas, and, secondly, the LDHs themselves react with the HCl under complete destruction of the LDH structure and formation of metal chlorides.\u3c/p\u3

Effect of a set of acids and polymerization conditions on the architecture of polycarbonates obtained via ring opening polymerization

\u3cp\u3ePolycarbonate-based polymers with a well-defined architecture have become interesting materials due to their large range of applications. Ring opening polymerization (ROP) has been largely applied to make branched polycarbonates. The polymer architectures obtained via this method are strictly related with the polymerization mechanisms involved which depend on the polymerization conditions chosen. Hereby, we evaluate the catalytic activity of three acids, fumaric, trifluoroacetic, and methanesulfonic on the Cationic ROP of trimethylene carbonate (TMC) over a trifunctional initiator, trimethylol propane (TMP), under different reaction conditions. In-detail characterization of the polymers showed the co-existence of two polymerization mechanisms: the activated monomer (AM), which produces a tri-armed branched polycarbonate with inclusion of the TMP initiator (TMP-PTMC), and a combined AM/Activated Chain End (ACE) mechanism, which produces a linear polycarbonate (L-PTMC). Such mixtures were identified for nearly all the reaction variables investigated, together with other side reactions. Upon optimization of the synthesis, the polymerizations in toluene with TFA at 35 °C and equimolar acid/initiator ratio were optimal, avoiding side reactions, but still resulting in a polymer mixture composed of ∼69% TMP-PTMC and 31% of a polycarbonate linear polymer. The occurrence of such mixed polymer architectures is commonly overlooked in literature regarding CROP of branched polycarbonates. We demonstrate the importance of performing a full characterization for a successful detection of polymer mixtures having different (number of) end-functionalities, which are critical for further use in advanced applications, such as in the biomedical or pharmaceutical filed.\u3c/p\u3