Surface characteristics of phenolic resin coatings

Phenolic resins are commonly used as polymer binders for metal, paper and mineral wool substrates. For such applications, mechanical, adhesive and thermal properties are considered most important, and the effect of synthesis and structural parameters on such end-use characteristics are well-documented. However, surface characteristics of cured phenolic resins can be equally relevant and are often overlooked. Widely used resins are phenol-urea-formaldehyde (PUF) and phenol-formaldehyde (PF). It is believed that the inherent chemistry and curing procedure of these resins result in coatings with distinct surface properties and wettability. To gain more insight into surface characteristics such as morphology, chemical composition and wettability of cured PUF and PF resins, different binder formulations were applied on glass substrates and subsequently characterised by Scanning Electron Microscopy (SEM), Contact Angle Goniometry (CAG) and X-Ray Photoelectron Spectroscopy (XPS). The effect of catalyst, chemical composition and curing conditions on surface characteristics of various PUF and PF coatings were investigated. The curing temperature was found to have a strong influence on surface properties; curing at 200 °C yields a surface with varying degrees of oxidation, differences in linkages between phenolic and urea species, and a lower overall nitrogen content in case of urea-containing coatings, resulting in stronger fluctuations in water-wettability compared to surfaces hardened at lower temperatures.</p

Influence of experimental parameters on the formation and stability of silica-wax colloidosomes

Hypothesis: Silica-wax colloidosomes find application in various fields, for instance through their use as microencapsules for triggered release of chemical components or as precursors for the production of Janus particles. The characteristics of these colloidosomes are highly dependent on the particles/water-oil system composition and experimental parameters. Experiments: Different colloidosomes were prepared using silica particles (D¯ ≈ 295 nm) and a positively charged surfactant (cetyltrimethylammonium bromide, CTAB) as co-stabilizers of a wax in water. The CTAB concentration, type of stirring and wax addition procedure were systematically varied. The silica particles and colloidosomes formed were analysed by Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS). The final percentage of the silica particles embedded on the wax colloidosomes (embedding yield) was estimated by a gravimetric method and the formation of monolayer or multilayer/clusters of silica particles at the wax surface was inspected with SEM. Findings: The CTAB concentration and the wax addition procedure play a major role in obtaining an embedding yield close to 100% and a monolayer coverage of the colloidosomes surface. The results indicate the existence of a mechanism consisting of a dynamic redistribution of the surfactant between the interfaces present in the emulsion. The practical and theoretical insights provided can be used towards an efficient production and scale-up of silica-wax colloidosomes

Self replenishing polymeric coatings : repairing surface functionalities

During the last decade, extensive research has been carried out on functional coatings with easy-to-clean/self-cleaning, anti-bacterial or anti-fouling properties, mainly driven by industrial demand but also by academic interest. Such properties are strongly related to the surface characteristics, in particular chemical composition and topography. Since damage of coatings can never be totally avoided, the introduction of self-replenishing mechanisms is one way to repair the surface properties and maintain a high performance of the coatings' functionality throughout an extended service life-time.Recently, we reported self-replenishing low surface energy (hydrophobic) polymer coatings which “self-repair” damaged surfaces by replenishing it with new low surface energy groups, e.g., fluorinated-dangling ends. These low surface energy dangling ends, covalently bonded to a cross-linked network, re-orient spontaneously from the bulk towards the new air-interfaces created by the damage.In our research, we pursued a dual experimental-simulation approach to understand in-depth this self-replenishing mechanism. First, we investigated the influence of different system parameters, such as the concentration of the low surface energy component and the molecular mobility span of the dangling ends, on the recovery of the coatings' hydrophobicity. The combined approach revealed the possibility of multiple healing events, the self-replenishing efficiency and the minimum “healing agent” concentration for a maximum recovery.Thereafter, we developed robust and easy processing self-replenishing superhydrophobic coatings. By incorporating inorganic nanoparticles in the polymer system, we designed surface-structured coatings which can spontaneously recover a low surface energy, partially responsible for the superhydrophobic behavior, at new structured surfaces created upon damage. The parallel simulations revealed the minimum thickness of the polymer layer for optimal self-replenishing ability and the distribution profile of the dangling ends at the various interfaces.The dual approach outlined above has been shown to be a fertile route to understand and design these complex materials and is expected to be applied in the future to other self-healing materials as well.<br/

Mesoscopic simulations of hydrophilic cross-linked polycarbonate polyurethane networks: Structure and morphology

Polyurethane (PU) cross-linked networks are frequently used in biomedical and marine applications, e.g., as hydrophilic polymer coatings with antifouling or low-friction properties and have been reported to exhibit characteristic phase separation between soft and hard segments. Understanding this phase-separation behavior is critical to design novel hydrophilic polymer coatings. However, most of the studies on the structure and morphology of cross-linked coatings are experimental, which only assess the phase separation via indirect methods. Herein we present a mesoscopic simulation study of the network characteristics of model hydrophilic polymer networks, consisting of PU with and without methyl-polyethylene glycol (mPEG) dangling chains. The systems are analyzed using a number of tools, such as the radial distribution function, the cross-link point density distribution and the Voronoi volume distribution (of the cross-linking points). The combined results show that the cross-linked networks without dangling chains are rather homogeneous but contain a small amount of clustering of cross-linker molecules. A clear phase separation is observed when introducing the dangling chains. In spite of that, the amount of cross-linker molecules connected to dangling chains only, i.e., not connected to the main network, is relatively small, leading to about 3 wt% extractables. Thus, these cross-linked polymers consist of a phase-separated, yet highly connected network. This study provides valuable guidelines towards new self-healing hydrophilic coatings based on the molecular design of cross-linked networks in direct contact with water or aqueous fluids, e.g., as anti-fouling self-repairing coatings for marine applications

One-pot, solvent-free, metal-free synthesis and UCST-based purification of poly(ethylene oxide)/poly-ε-caprolactone block copolymers

A fast, one pot, solvent-free and metal-free synthesis of poly-ε-caprolactone and poly(ethylene oxide) block copolymers is reported. Copolymers with different molar mass, different hydrophilic to lipophilic balance, high degree of conversion and narrow molar mass dispersity have been obtained by organocatalyzed ring opening polymerization of ε-caprolactone in presence of mono- or diol-poly(ethylene oxide) as initiator and fumaric acid as catalyst. A new biocompatible and environmental friendly purification method is presented, exploiting the upper critical solution temperature of these class of copolymers in ethanol. The phase diagrams of the synthesized copolymers in ethanol are also reported. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2992–2999

Modelling of self-healing surface structures coatings

For many current engineering applications the performance of materials depends strongly on the surface properties of the top layer. In many cases a hydrophobic/superphydrophobic top surface is desired (for example for easy-toclean/ self-cleaning applications). The durability of coatings will be substantially extended if the layer which provides the hydrophobic/superhydrophobic property will have ability to self-heal. Previously, hydrophobic coatings with a self-healing surface were reported by our group. These coatings can recover a sufficiently high concentration of the lowsurface-energy groups at the air/polymer interface. The bulk material serves as reservoir of the low-surface energy component (fluorinated polymer dangling chains). A mechanism of self-replenishing involves the reorientation of the dangling chains which carry the fluorinated group. Silica particles where incorporated into the polymer system in order to introduce the surface roughness leading eventually to higher contact angles of the polymer coating. Our goal is to create a model of the a selfhealing superhydrophobic coating. In this work we use the mesoscopic modelling technique (dissipative particle dynamics) in order to study several aspects of these polymer/particle coating: 1) the segregation of the low surface energy groups at the top surface of the coating.; 2) the self-healing response of the system and 3) the dynamics and distribution of crosslinks in the polymer system in presence of relatively large silica particles. For these studies we considered the distribution of the low surface energy groups in a confined geometry at the interfaces available. The minimal thickness of the polymer layer which provides self-healing ability was also calculated. All the relevant parameters (crosslinking conditions, polymer precursor and dangling chain length and distance between particles) were changed systematically. The simulations give a valuable insight into the details of microstructures and dynamics and guide experiments towards the choice of the system with the maximal selfhealing efficiency

Self-healing adhesion on polymer coatings

Supramolecular self assembling hydrogen bonded polymeric materials are of great importance due to its practical relevance. Well designed molecules based on these concepts can produce polymeric materials with \u93responsive\u94 properties like self healing even in coating application. The reversible, non-covalent hydrogen bonding interactions are a recurring design principle for these materials. Here, we report a concept for repairing mechanical damage at the coating/substrate interface. A dopamine based hydrogen bonded self-healing system was investigated and its use for self-healing adhesion purposes will be discussed. The presence of catechol bonds in the dopamine molecules promotes strong adhesion with the metal surface whereas flexible hydrogen bonding motifs ensure inter/intra molecular hydrogen bonding at the interface with polymer coating. The reversible character of the hydrogen bonds will ensure the repair of the adhesive bonds between the coatings and the substrate upon re-contact at the interface

Design of self-dispersible charged-polymer building blocks for waterborne polyurethane dispersions

Waterborne polyurethane dispersions (PUDs) currently have a wide spectrum of applications as coating resins for biomedical products, food packaging, cosmetics and traditional coatings. At present, PUDs are commonly prepared by the “prepolymer extension” method in which isocyanate terminated self-dispersing prepolymers are dispersed in water, followed by chain extension via the terminal isocyanate groups. The preparation of such self-dispersing prepolymers from diisocyanates, ionizable diols and non-ionizable diol blocks is a stochastic process with poor control over the prepolymer molecular weight and functionality distribution, which may result in sub-optimal properties of the PUDs. We investigated a new route in which we prepare ionizable macro-diol blocks, out of hydrophobic polycaprolactone (PCL) blocks, covalently bonded to the ionizable diol dimethylolpropionic acid (DMPA) via Cationic Ring Opening Polymerization (CROP), which subsequently are coupled via isocyanate chemistry to prepare the PUDs. During the preparation of the pre-polymer the reaction was steered towards a DMPA-diblock-PCL macro-emulsifier, with controlled molecular weight and molecular weight distribution. The waterborne PUDs prepared afterwards with the designed DMPA-PCL building block showed the desired particle size, Zeta-potential and improved stability over time, when compared to a highly analogous traditional system. Furthermore, the preparation of the DMPA-PCL self-dispersing block allowed reducing up to 63% the amount of diisocyanate used compared to the commonly used “pre-polymer extension” method, without sacrificing long-term colloidal stability. The interesting features of the new DMPA-PCL self-dispersible building blocks ensure a wide range of applications, via a safer, more efficient and economic preparation route

Surface repair: Self-replenishing functional polymer coatings

Previously, we reported a self-replenishing polymer surfaces which recover their lowsurface energy (i.e. hydrophobicity), upon surface damage[1]. Following up, we used a dual experimental-simulation approach to understand in-depth the surface segregation of low surface energy components[2], the self-healing mechanism involved on the surface recovery and the influence of different parameters (e.g. mobility of the polymer healing components) on the self-replenishing behaviour. Herein we report our further studies on self-replenishing structured-surfaces. The model self-replenishing polymer system was used to develop robust and easy processing hydrophobic surface-structured coatings, which are able to recover the low surface energy groups at new structured surfaces, created after damage

Healable dual organic–inorganic crosslinked sol–gel based polymers : crosslinking density and tetrasulfide content effect

In this article, the first generation of healable sol–gel based polymers is reported. A dual organic–inorganic crosslinked network is developed containing non-reversible crosslinks and reversible (tetrasulfide) groups. The designed polymer architecture allows thermally induced mesoscale flow leading to damage closure followed by interfacial strength restoration due to reformation of the reversible groups. While the reversible bonds are responsible for the flow and the interface restoration, the irreversible crosslinks control the required mechanical integrity during the healing process. The temperature dependent gap closure kinetics is strongly affected by the crosslinking density and tetrasulfide content. Raman spectroscopy is used to explain the gap closure kinetics in air and dry nitrogen. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 201